Catalytic Hydrogenation of Carbon Dioxide to Methanol

Catalytic Hydrogenation of Carbon Dioxide to Methanol using

Molecular Catalysts

Von der Fakultät für Mathematik, Informatik und Naturwissenschaften der RWTH Aachen

University zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften

genehmigte Dissertation

vorgelegt von

Master of Science

Sebastian Wesselbaum

aus Recklinghausen

Berichter: Univ.-Prof. Dr. rer. nat. Walter Leitner

Univ.-Prof. Dr. rer. nat. Jürgen Klankermayer

Univ.-Prof. Dr. rer. nat. Sonja Herres-Pawlis

Tag der mündlichen Prüfung: 16.12.2016

Diese Dissertation ist auf den Internetseiten der Universitätsbibliothek online verfügbar.

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Für meine liebe Familie.

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I hereby declare that this thesis is my own original work, and that it has not been submitted

anywhere else for any award. Wherever contributions of others are involved, every effort

was made to indicate this clearly with due reference to the literature and acknowledgement

of collaborative research.

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The experimental part of this thesis has been carried out at the chair for Technische Chemie

und Petrolchemie at the Institut für Technische und Makromolekulare Chemie (ITMC) of the

RWTH Aachen University between October 2011 and March 2015 under the supervision of

Prof. Dr. Walter Leitner and Prof. Dr. Jürgen Klankermayer.

Parts of this thesis have already been published in:

Hydrogenation of Carbon Dioxide to Methanol by Using a Homogeneous Ruthenium-

Phosphine Catalyst

S. Wesselbaum, T. vom Stein, J. Klankermayer, W. Leitner, Angew. Chem. Int. Ed.

2012, 51, 7499-7502.

Hydrogenation of carbon dioxide using a homogeneous ruthenium-Triphos catalyst:

from mechanistic investigations to multiphase catalysis

S. Wesselbaum, V. Moha, M. Meuresch, S. Brosinski, K. M. Thenert, J. Kothe, T. vom

Stein, U. Englert, M. Hölscher, J. Klankermayer, W. Leitner, Chem. Sci. 2015, 6, 693-

704.

Selective Catalytic Synthesis using the Combination of Carbon Dioxide and Hydrogen -

Catalytic Chess at the Interface of Energy and Chemistry

J. Klankermayer, S. Wesselbaum, K. Beydoun, W. Leitner, DOI:

10.1002/anie.201507458 and 10.1002/ange.201507458.

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Acknowledgements

First of all, I would like to thank Prof. Dr. Walter Leitner for the challenging and very

interesting topic, the many fruitful scientific discussions, and the outstanding working

conditions. I also thank Prof. Dr. Jürgen Klankermayer for his guidance, his constant scientific

input, and for the many things he taught me.

I thank Prof. Dr. Sonja Herres-Pawlis for reviewing this thesis.

I would like to thank Dr. Markus Hölscher, Dr. Verena Moha, and Jens Kothe for the

successful cooperation and for performing the computational chemistry of this thesis.

I want to thank Dr. Giancarlo Franciò for scientific discussions and for educating me in

teaching students.

I thank Daniel Geier for his help with the continuous-flow equipment and for programming

the process control system.

I am grateful for the help of many colleagues, especially of Dr. Thorsten vom Stein, Markus

Meuresch, Dominik Limper, and Marcus Suberg.

I thank my former lab colleagues Dr. César A. Urbina-Blanco, Thomas Hermanns, and Dr.

Marcel Picard for teaching me practical skills in the lab, and for the nice working atmosphere

and fun we had. I also thank Dr. César A. Urbina-Blanco for proofreading this thesis.

I would also like to thank my skillful students who contributed to this work: Bernhard

Barwinski, Julian Kleemann, Dominik Schauenburg, and Katharina Thenert.

I thank Sandra Brosinski for her help with NMR measurements, and for her skillful help in the

lab.

Many thanks go to the staff of the analytical departments: Hannelore Eschmann, Julia

Wurlitzer, Elke Biener, and Wolfgang Falter of the GC/MS department, as well as Ines

Bachmann-Remy of the NMR department.

I thank Prof. Dr. Ulrich Englert for X-ray analysis.

I would like to thank Ralf Thelen and his crew from the mechanical workshop, as well as

Stefan Aey and Thomas Müller from the electrical workshop for taking care of the high-

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pressure equipment. I would also like to thank Günter Wirtz, Henning Kayser, Marcello

Padaro, Laurent Weisgerber, and Jendrik Wülbern for their help with computer problems.

I want to thank the whole Leitner group for the cooperativeness and wonderful working

atmosphere.

I want to thank all the partners of the SusChemSys project for financial support and for

giving me a lot of opportunities to train my softskills. The project “Sustainable Chemical

Synthesis (SusChemSys)” is co-financed by the European Regional Development Fund (ERDF)

and the state of North Rhine-Westphalia, Germany, under the Operational Programme

“Regional Competitiveness and Employment” 2007-2013.

This work was supported in part by the Cluster of Excellence “Tailor-Made Fuels from

Biomass”, which is funded by the Excellence Initiative by the German Federal and State

Governments to promote science and research at German universities.

Generous allocation of computer time by the Computation and Communication Centre of

RWTH Aachen University is gratefully acknowledged.

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Abstract

This thesis deals with the development and detailed investigation of the very first

organometallic catalyst for the selective hydrogenation of CO2 to methanol.

Organometallic catalysts were believed to allow the conversion of CO2 to methanol only via

1) the intermediate formation of CO via reverse watergas shift reaction (RWGS), leading to

an unselective raction, or via 2) the intermediate formation of alkyl formates as stable

intermediates, requiring a mixture of catalysts.

The neutral complex [Ru(TMM)(Triphos)] (2) (TMM = trimethylenmethane, Triphos = 1,1,1-

tris(diphenylphosphinomethyl)ethane) was shown to be active in the selective

hydrogenation of CO2 to methanol without the need for stable intermediates, when applied

in combination with an acidic additive (e.g. HNTf2 = bis(trifluoromethane)sulfonimide). The

activity per metal centre of this system was comparable with the activity per metal centre of

classical heterogeneous methanol catalysts. Based on mechanistic investigations, the

cationic complex [Ru(2-OAc)(Triphos)(S)]NTf2 (14) (S = solvent or free coordination site) was

developed as molecular catalyst for a system free of any additives. Under reaction

conditions, complex 2/HNTf2 as well as complex 14 formed the cationic formate complex

[Ru(2-O2CH)(Triphos)(THF)]NTf2 (8a) which was characterised spectroscopically and shown

to be an intermediate.

DFT calculations supported the assumption of a stepwise reduction of CO2 within the

coordination sphere of a ruthenium-Triphos centre: A series of hydride transfer and

protonolysis steps lead to the reduction of CO2 to formic acid, formaldehyde, and finally

methanol. The facial coordination of the Triphos ligand as well as the high thermal stability

of Ru-Triphos complexes are crucial factors for the unprecedented activity of the

investigated complexes.

Recycling of the catalytic system 2/HNTf2 was demonstrated using a biphasic system based

on 2-methyltetrahydrofurane and water. Finally, immobilisation of 2/HNTf2 in an ionic liquid

allowed its application in a continuous-flow process for the hydrogenation of CO2 to

methanol.

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Zusammenfassung

Die vorliegende Arbeit beschreibt die Entwicklung und Untersuchung der ersten homogenen

Übergangsmetall-Katalysatoren für die selektive Hydrierung von CO2 zu Methanol.

Bisher wurde davon ausgegangen, dass die Umsetzung von CO2 zu Methanol an homogenen

Katalysatoren nur über die intermediäre Bildung von CO durch reverse watergas shift

reaction (RWGS) oder die Bildung von Alkylformiaten als stabilen Intermediaten möglich sei,

was zu unselektiver Reaktion führte oder den Einsatz einer komplexen Mischung mehrerer

Katalysatoren nötig machte.

Der neutrale Komplex [Ru(TMM)(Triphos)] (2) (TMM = Trimethylenmethan, Triphos = 1,1,1-

tris(diphenylphosphinomethyl)ethan) erwies sich als aktiv in der selektiven Hydrierung von

CO2 zu Methanol ohne die Notwendigkeit stabiler Intermediate, wenn er in Kombination mit

einem sauren Additiv (z. B. HNTf2 = Bis(trifluoromethane)sulfonimide) eingesetzt wurde. Die

erzielte Aktivität pro Metallzentrum war vergleichbar mit der Aktivität pro Metallzentrum

klassischer heterogener Methanolkatalysatoren. Basierend auf Untersuchungen des

Mechanismus konnte folglich der kationische Komplex [Ru(2-OAc)(Triphos)(S)]NTf2 (14) (S =

Lösungsmittel oder freie Koordinationsstelle) als molekularer Katalysator für ein System

ohne weitere Additive entwickelt werden. Sowohl Komplex 2/HNTf2 als auch Komplex 14

bildeten unter Reaktionsbedingungen den kationischen Formiatkomplex [Ru(2-

O2CH)(Triphos)(THF)]NTf2 (8a), welcher spektroskopisch charakterisiert wurde.

DFT-Rechnungen unterstützten die Annahme einer stufenweisen Reduktion von CO2 in der

Koordinationssphäre eines Ruthenium-Triphos Zentrums: Eine Serie von Hydridtransfer- und

Protonolyseschritten führt zu einer Reduktion von CO2 zu Ameisensäure, Formaldehyd und

schließlich Methanol. Die faciale Koordination des Triphos Liganden sowie die hohe

Temperaturbeständigkeit von Ru-Triphos Komplexen spielen eine entscheidende Rolle für

die einzigartige Reaktivität der untersuchten Komplexe.

Die Rezyklierung des Katalysatorsystems 2/HNTf2 in einem 2-Methyltetrahydrofuran/H2O

Zweiphasensystem wurde demonstriert. Schließlich ermöglichte die Immobilisierung von

2/HNTf2 in einer Ionischen Flüssigkeit den Einsatz in einem Prozess zur kontinuierlichen

Hydrierung von CO2 zu Methanol.

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Table of contents

1 Introduction ........................................................................................................................ 1

1.1 Harvesting renewable energy for the chemical supply chain ..................................... 1

1.1.1 Current raw material situation ............................................................................. 1

1.1.2 CO2 as raw material............................................................................................... 1

1.1.3 Catalytic hydrogenation of CO2 – catalytic chess ................................................. 4

1.1.4 Sources for H2........................................................................................................ 7

1.2 Methanol – A basic chemical ....................................................................................... 9

1.2.1 Background: Properties and applications of methanol ........................................ 9

1.2.2 Methanol production from conventional carbon sources ................................. 11

1.3 Methanol production from CO2 and H2 ..................................................................... 11

1.3.1 Heterogeneous catalysis for CO2 hydrogenation to methanol ........................... 12

1.3.2 Homogeneous catalysis for CO2 hydrogenation to methanol ............................ 13

1.3.3 Homogeneous catalysis for methanol reforming ............................................... 20

2 Aim of the thesis .............................................................................................................. 22

3 Results & Discussion ......................................................................................................... 23

3.1 Hydrogenation of CO2 to MeOH in the presence of alcohol additives ..................... 23

3.1.1 Identifying a suitable catalytic system ................................................................ 23

3.1.2 Reaction cascade for the hydrogenation of CO2 to methanol ............................ 27

3.2 Investigating the mechanism of the CO2 hydrogenation to MeOH .......................... 31

3.2.1 Identification of the organometallic species in solution .................................... 31

3.2.2 In situ NMR-spectroscopic investigations ........................................................... 40

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3.2.3 Proposal for a catalytic cycle............................................................................... 48

3.2.4 DFT-calculations .................................................................................................. 49

3.3 Hydrogenation of CO2 to MeOH in the absence of alcohol additives ....................... 56

3.3.1 The role of the acidic additive - Development of a catalytic system with no

need for an acidic additive .................................................................................. 56

3.3.2 Parameter variations........................................................................................... 73

3.3.3 Investigations concerning catalyst deactivation ................................................. 81

3.3.4 Comparison of the reaction in presence of alcohol additive with the reaction in

absence of alcohol additive ................................................................................ 85

3.3.5 Test of different catalyst precursors ................................................................... 87

3.4 Catalyst recycling and immobilisation ....................................................................... 88

3.4.1 Catalyst recycling by distillation .......................................................................... 89

3.4.2 Catalyst recycling in the biphasic system 2-MTHF/H2O ...................................... 90

3.4.3 Catalyst immobilisation for continuous-flow application ................................... 94

4 Summary & Conclusion .................................................................................................. 113

5 Experimental .................................................................................................................. 116

5.1 General .................................................................................................................... 116

5.2 Solvents and Chemicals ........................................................................................... 116

5.3 Analysis .................................................................................................................... 117

5.3.1 NMR Spectroscopy ............................................................................................ 117

5.3.2 IR Spectroscopy ................................................................................................. 117

5.3.3 Mass Spectrometry ........................................................................................... 118

5.3.4 Gas Chromatography ........................................................................................ 118

5.4 Catalysis ................................................................................................................... 118

5.4.1 General procedure for batch catalysis .............................................................. 118

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5.4.2 General procedure for high pressure NMR experiments ................................. 121

5.4.3 Continuous catalysis ......................................................................................... 121

5.5 Synthesis .................................................................................................................. 126

5.5.1 Synthesis of [Ru(TMM)(Triphos)] (2) ................................................................ 126

5.5.2 Synthesis of [Ru(2-OAc)(Triphos)(S)]NTf2 (14) ................................................. 127

5.5.3 Synthesis and characterisation of [Ru(2-O2CH)(Triphos)(THF)]NTf2 (8a) in

solution ............................................................................................................. 129

5.5.4 Synthesis of [Ru(H)(CO)2(Triphos)]NTf2 (4NTf2) ................................................ 130

5.5.5 [Ru(2-OAc)Cl(Triphos-anisyl)] (15) ................................................................... 130

5.5.6 Synthesis of SILP catalyst .................................................................................. 131

5.6 DFT-calculations....................................................................................................... 132

6 References ...................................................................................................................... 133

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Abbreviations

2-MTHF 2-methyltetrahydrofuran

acac acetylacetonat

AEL alkaline electrolysis

ATP attached proton test

BMIM 1-butyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)imide

BPR back pressure regulator

br broadened signal

BV ball valve

CT cooling trap

d doublet

DCM dichloromethane

DEA diethanolamine

DFT density functional theory

DIPA diisopropanolamine

DMC dimethylcarbamate

DME dimethylether

DMF dimethylformamide

DMFA Dimethylammonium formate

DMSO dimethylsulfoxide

EI electron ionisation

EMIM 1-ethyl-3-methylimidazolium

EOR enhanced oil recovery

eq. equivalents

ESI-MS electrospray ionisation mass spectrometry

Et ethyl

FT-IR Fourier transform infrared spectroscopy

HMBC heteronuclear multiple bond correlation

HMQC heteronuclear multiple quantum coherence

HNTf2 bis(trifluoromethane)sulfonimide

HP high purity

HRMS High resolution mass spectrometry

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HSQC heteronuclear single quantum coherence

HTEL high-temperature electrolysis

HyS hybrid sulphur cycle

IC ion chromatography

IL ionic liquid

LFM liquid flow meter

LV lock valve

m multiplet

MDEA methyldiethanolamine

Me methyl

MEA monoethanolamine

MeCN acetonitrile

MFC mass flow controller

MSA methylsulfonic acid

MTBE methyl tertiary-butyl ether

MTG methanol-to-gasoline

MTO methanol-to-olefins

MTP methanol-to-propylene

MTV magnetic trigger valve

MV metering valve

NMR nuclear magnetic resonance

NTf2- bis(trifluoromethylsulfonyl)imide

PEMEL proton-exchange membrane electrolysis

PNV pneumatically actuated needle valve

ppm parts per million

p-TsOH p-toluenesulfonic acid monohydrate

PV proportional valve

qua quadruplet

r.t. room temperature

rWGS reversed water-gas-shift

s singlet

scCO2 supercritical CO2

SGFLUO perfluoro-alkyl functionalised silica

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SILP supported ionic liquid phase

SIMS secondary ion mass spectrometry

SOEC solid-oxide electrolysis cell

SUP super-ultra-high purity

t triplet

TAME tertiary-amylmethylether

TDI TOF determining intermediate

TDTS TOF determining transition state

TEA triethanolamine

THF tetrahydrofuran

tmeda tetramethylethylenediamine

TMM trimethylenemethane

TOF turnover frequency

TON turnover number

TPP triphenylphosphine

Triphos 1,1,1-tris(diphenylphosphinomethyl)ethane

Triphos-anisyl

1,1,1-tris{bis(4-methoxyphenyl) phosphinomethyl}ethan

Triphos-tolyl 1,1,1-tris(bis(3-methylphenyl)phosphinomethyl)ethane

TW three way valves

UP ultra-high purity

vol. volume

WGS water-gas-shift

wt.-% weight percent

INTRODUCTION

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1 Introduction

Parts of this chapter have been published: J. Klankermayer, S. Wesselbaum, K. Beydoun, W.

Leitner, DOI: 10.1002/anie.201507458 and 10.1002/ange.201507458.

1.1 Harvesting renewable energy for the chemical supply chain

1.1.1 Current raw material situation

Today, fossil raw materials contribute with around 80 % to the world’s energy supply.[1] In

2013, crude oil accounted for 33 %, coal for 30 %, natural gas for 24 %, hydro power for 7 %,

uranium for 4 %, and renewables for 2 % of the global primary energy consumption.[2] Only

around 10 % of the fossil raw materials are used to produce materials.[1] More than 105

products are produced starting mainly from the fossil carbon sources oil, natural gas, and

coal, and from the non-fossil biomass.[3] The timeframes in which the fossil resources will be

depleted were estimated by dividing the known global reserves by the annual global

consumption.[1, 4] The numbers in 2010 were 42 years for crude oil, 63 years for natural gas,

159 years for hard coal, and 227 years for lignite.[1, 4] However, the availability of fossil raw

materials is dependent not only on the amount of raw materials left, but also on the political

situation in the countries controlling these materials, as around 90 % of the oil reserves and

85 % of the natural gas reserves are owned by states.[1] Due to these reasons, and because

CO2 is nontoxic and can easily be stored and transported compared to other C1 building

blocks (e.g. CO, COCl2, HCN), the use of CO2 as alternative carbon resource has been and is

currently intensively investigated and discussed.[1, 3, 5-10]

1.1.2 CO2 as raw material

In 2012, the annual anthropogenic CO2 emission was around 32 Gt,[11] showing that CO2 is in

principle available in large quantities as carbon resource. 44 % of these emissions were from

coal combustion, 35 % from oil, and 20 % from gas.[11] The availability of cheap and clean CO2

depends on the CO2 source and the necessary/applied capture and cleaning technologies.[9]

INTRODUCTION

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Most of the CO2 used today stems from natural wells containing quite pure CO2.[9] However,

to create an economy based on CO2, CO2 from anthropogenic sources should be used.[9] In

this respect, CO2 captured from power stations is often discussed. However, the presence of

SOx and NOx makes the separation and purification sometimes costly (15 to 90 US$/t).[9] A

very interesting approach is the direct CO2 capture from air, as this would make the CO2

capture independent from the place of its emission.[9, 12] The cost for this technology was

estimated to be 600-800 US$/ton and has still to be developed further.[12] Aresta et al. listed

industrial sectors which produce relatively pure or highly concentrated CO2 streams:[9]

Ethylene oxide production (10-15 Mt/a), sweetening of liquefied natural gas (25-30 Mt/a),

ammonia production (160 Mt/a), petrochemical processes (155-300 Mt/a), fermentation

processes (>200 Mt/a), iron and steel industries (ca. 900 Mt/a), oil refineries (850-900 Mt/a),

and cement industry (>1000 Mt/a).[9] These sources account for around 10 % (ca. 3000 Mt/a)

of the total anthropogenic CO2 emissions and can be regarded as “easily available”.[9]

Finding technical solutions for easy and cheap separation of pure CO2 is crucial for the use of

CO2 as alternative carbon source. A widely applied strategy for removing CO2 from natural

gas, syngas and flue gas is absorption with the help of a solvent.[1] Here, physical solvents (no

chemical reaction with CO2), hybrid solvents (mixture of physical and chemical solvents) and

chemical solvents (chemical reaction with CO2) can be discriminated.[1] Crucial factors for

optimising absorption/desorption costs are the absorption kinetics, the regeneration energy

(desorption) and minimum solvent flow rate.[1] Physical solvents, such as methanol used in

the Rectisol process, are applicable for higher CO2 partial pressures, at which physical

solvents have higher solubilities for CO2.[1] This reduces the minimum solvent flow rate.[1]

Moreover, due to the low enthalpy of absorption less regeneration energy is needed

compared to chemical solvents.[1] However, the selectivity for CO2 absorption is not perfect

and therefore the purity of the obtained CO2 is not very high.[1] For lower CO2 partial

pressures and for meeting tighter CO2 specifications chemical solvents are the absorbents of

choice.[1] Typically employed absorbents are amino alcohol solutions containing

triethanolamine (TEA), diethanolamine (DEA), diisopropanolamine (DIPA),

monoethanolamine (MEA), or methyldiethanolamine (MDEA).[1] Absorption of CO2 in

primary/secondary amines or sterically hindered/tertiary amines follows different

mechanisms (Scheme 1).[1] Primary amines absorb 0.5 mol CO2 per mol amine due to

carbamate formation, whereas sterically hindered and tertiary amines absorb 1 mol CO2 per

INTRODUCTION

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mol amine. The regeneration energy is lower for sterically hindered amines/tertiary amines,

however, absorption rates are slower.[1] Overall, sterically hindered/tertiary amines are

favoured.[1] An alternative to amine solutions are K2CO3 solutions as used in the Benfield

process, which require the presence of V2O5 as corrosion inhibitor.[1] The development of

optimized CO2 absorbents will be crucial for the cheap availability of CO2 as carbon resource.

Scheme 1: Reactions of CO2 absorption using primary amines (1), tertiary amines (2), and K2CO3 (3).

CO2, besides water, is the end product of every combustion process of materials containing

carbon and hydrogen because of its high thermodynamic stability (ΔGf° = -396 kJ/mol).[9] Due

to this high stability, often high amounts of electrical or thermal energy, or reaction partners

having a high energy content (e.g. H2, epoxides) are necessary for the conversion of CO2.[10]

The development of catalysts lowering the high kinetic barriers of envisaged reactions is a

crucial factor.

Around 110 Mt/a CO2 are converted to chemicals in existing processes:[8, 13] The biggest

applications in industry are urea production (consuming 70 Mt CO2/a), production of

inorganic carbonates and pigments (ca. 30 Mt CO2/a), methanol production (addition of CO2

to the CO/H2 stream to balance the C/H ratio, 6 Mt CO2/a), salicylic acid production

(20 kt CO2/a), and propylene carbonate production (“a few kt per year”).[8, 13]

Other applications of CO2 make use of its physico-chemical properties.[3] Around 18 Mt/a of

CO2 are used for these purposes.[9, 14-15] The critical conditions for CO2 are relatively mild (Tc

= 31.1 °C, pc = 73.9 bar).[3] In its supercritical state CO2 (scCO2), has a high solubility strength

for hydrophobic and/or volatile substances.[3, 14, 16] Moreover, dissolving of CO2 in viscous

media like oil or ionic liquids (ILs, salts with melting points <100 °C) substantially lowers their

viscosity.[17-18] Based on these properties CO2 is used for enhanced oil recovery (EOR),[3, 19] as

well as several extraction processes. Based on the pioneering work by Kurt Zosel scCO2 is

used to decaffeinate coffee and to extract hops aroma.[14, 20] Besides the extraction of

INTRODUCTION

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natural products, also hazardous contaminants can be extracted to clean contaminated

soils[21] or cardboard cartons.[22]

scCO2 can also be used as environmentally friendly solvent to substitute organic solvents in

reactions.[14] The use of scCO2 as mobile phase in continuous-flow catalysis was successfully

demonstrated.[23-33] In these applications, scCO2 dissolves the substrates of the reaction,

carries them into the phase containing the catalyst (typically an ionic liquid phase or a

heterogeneous/heterogenised catalyst bed) and extracts the products from the catalyst

containing phase. Separation of the product from the scCO2 phase is achieved by changing

the CO2 density (variation of T, p), leaving the product with no solvent contaminations.

Leitner et al. could also show the applicability of an ionic liquid/scCO2 biphasic system for the

continuous-flow hydrogenation of CO2 to formic acid.[27] In this process CO2 is reactant and

mobile phase at the same time.

1.1.3 Catalytic hydrogenation of CO2 – catalytic chess

An intensively investigated strategy for the transformation of CO2 into valuable chemicals is

its catalytic hydrogenation using molecular hydrogen.[3, 6-7, 9-10, 34-35] The reaction of CO2 with

one equivalent H2 leads to the formation of formic acid or to the formation of CO (reversed

water-gas-shift reaction, rWGS), depending on the catalysts and conditions used (Scheme 2,

equations 1 & 2). Further reduction of CO2 beyond the “formic acid stage” by reaction with

an increasing number of H2 equivalents leads, in principle, to the formation of formaldehyde

(2 H2 eq.), methanol (3 H2 eq.), and methane (4 H2 eq.) (Scheme 2, equations 3-5).

Scheme 2: Reactions of CO2 with different numbers of H2 equivalents.

In Figure 1 the number of H2 equivalents used for CO2 reduction is displayed on the ordinate.

With an increasing number of H2 equivalents (vertical up on the ordinate) the energy

contents of the produced molecules increase: Neat formic acid has an energy density of

INTRODUCTION

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2.086 kWh L-1 or ca. 6.2 MJ kg-1,[36] methanol has an energy density of 22.7 MJ kg-1,[1] and

methane has an energy density of 55.5 MJ kg-1.[1] Therefore, this strategy can be used to

store energy in chemical bonds. In the aforementioned reactions to CO, formic acid,

formaldehyde, methanol, and methane the carbon centre is reduced without increasing the

molecular complexity of the resulting molecule. On the “chessboard” shown in Figure 1 the

resulting molecules are placed on the fields with the coordinates (0/y).

To cover the whole range of conceivable CO2 transformation reactions not only reduction of

the carbon centre, but also bond forming reactions have to be considered.[37] This was

achieved in Figure 1 by counting the total number of newly formed C-C and C-hetero bonds

(not only the bonds formed to the CO2 carbon!) on the abscissa of the “chessboard”,

enabling a quantification of the molecular complexity. A typical example for increasing the

molecular complexity of CO2 without concomitant reduction of the carbon centre is the

synthesis of urea. In this case, zero H2 equivalents are needed as no reduction of CO2 takes

place and two new C-N bonds are formed, placing urea on the field with the coordinates

(2/0) on the “chessboard”.

INTRODUCTION

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Figure 1: Classification of CO2 hydrogenation products according to the H2 equivalents needed for CO2 reduction (ordinate) and number of newly formed C-hetero or C-C bonds (abscissa) in a “catalytic chess game”.

By combining the reduction of CO2 to the “formic acid/CO stage” (1 H2 eq.), the

“formaldehyde stage” (2 H2 eq.), the “methanol stage” (3 H2 eq.), or the “methane stage”

(4 H2 eq.) (vertical up on the “chessboard”) with the formation of new bonds (rightward on

the “chessboard”) the scope of compounds available from CO2 becomes much larger.[37] This

strategy can be used to “harvest” renewable energy for the chemical value chain. Some

examples of molecules becoming available by this strategy are placed on the chessboard

shown in Figure 1 according to their classification:

Alkyl formates are obtained by a combination of CO2 reduction with one H2 equivalent to the

“formic acid/CO stage” and the formation of a new C-O bond, placing it on the field with the

coordinates (1/1) on the chessboard.[7, 10, 38-40] Carboxylic acids produced by

hydrocarboxylation require reduction of the CO2 with one H2 equivalent to CO as

intermediate, and successive formation of a new C-C bond,[41] placing it again on field (1/1).

Hydroaminomethylation for the production of tertiary amines from olefin, secondary amine,

INTRODUCTION

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CO2 and H2 again proceeds via CO2 reduction with one H2 equivalent to CO as

intermediate.[42-43] As one new C-C and one new C-N bond are formed in this reaction the

product is placed on field (2/1). The additional two H2 equivalents needed for the

hydroformylation-step und hydrogenation of the carbon double bond are not counted in this

classification.

No combination of CO2 reduction to the “formaldehyde stage” with bond forming reactions

is known today,[44-48] showing that there is still a wide field for future research.

Only recently, combinations of CO2 reduction to the “methanol stage” with bond forming

reactions have been reported: The direct N-methylation of primary and secondary amines

with CO2 and H2 was demonstrated by Klankermayer and Leitner et al. in 2013 and shortly

after by Beller et al.[49-51] In Figure 1 these products are placed on field (1/3) as three H2

equivalents are used for CO2 reduction to the “methanol stage” and one new C-N bond is

formed. Even more interestingly, N-methylated tertiary amines could be produced in a one-

pot reaction as follows:[50] In the first step of this cascade reaction, a primary amine reacts

with an aldehyde to give an imine. This imine is hydrogenated to the corresponding

secondary amine with one H2 equivalent in the second step, and the resulting secondary

amine is methylated with CO2 and three H2 equivalents in the third step. On the

“chessboard” the pharmaceutical ingredient butenafine, which could be produced by this

route, is placed on field (2/3) as three H2 equivalents are used for CO2 reduction and two

new C-N bonds are formed. The fourth H2 equivalent needed for the intermediate

hydrogenation of the imine to the secondary amine is not counted in this classification. A

combination of CO2 reduction to the “methane stage” with bond forming reactions is as yet

unknown.[52-55]

If one sticks to the chessboard picture, one could regard the catalysts needed to allow the

transformations as the chess pieces needed to move from one field to another (Figure 1).

1.1.4 Sources for H2

For the production of one metric tonne of methanol 1.38 t CO2 and 0.19 t H2 are needed.[3]

Today, the production of methanol is the third major hydrogen consumer (9 % of global

consumption) after ammonia production and crude oil refining.[1] In order to close the

INTRODUCTION

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carbon cycle when using CO2 as resource, the production of hydrogen gas must not be

accompanied by CO2 production or use of fossil fuels.[1, 3] Therefore, regenerative energy

sources (like wind, solar, geothermal energy, hydroelectric power, tide power) should be

used for water splitting into H2 and O2 (Scheme 3).[1, 3] Several electrochemical,

photochemical, and thermochemical water splitting methods have been developed,

however, electrolysis is the only method which is operating on an industrial scale at the

moment (up to 4 % of the hydrogen production in 2009).[1, 56] Existing technologies for the

coupling of photovoltaic with electrolysis have solar-to-hydrogen conversion efficiencies in

the range 5-20 % (around 20 % efficiency for photovoltaic and 80 % for electrolysis).[9] An

area of approximately 10-40 km² is needed for the production of 1 t H2 per day.[9]

Scheme 3: Water splitting into hydrogen and oxygen.

The advantage of the electrochemical methods is that they can be used in conjunction with

all kinds of regenerative energy sources. The classical method is alkaline electrolysis (AEL) of

a 30 % KOH solution at 80-90 °C.[1] Cheap electrodes based on nickel can be used for this

process.[1] However, for electrolysis using strongly fluctuating energies like wind and solar

power, the startup and shutdown behaviour of the electrolysis cell is of utmost

importance.[1] Here, the AEL has some disadvantages compared to newer methods, such as

proton-exchange membrane electrolysis (PEMEL).[1] Using PEMEL a compact setup with

higher power efficiency can be achieved, which, however, has higher investment costs.[1]

Importantly, the startup and shutdown behaviour is better suitable for fluctuating power.[1]

Many researchers focus on high-temperature electrolysis (HTEL) at temperatures >800 °C in

solid-oxide electrolysis cells (SOECs), because at these temperatures the decomposition

voltage of water and the overvoltage at the electrodes is decreased.[1, 57] Setups have been

constructed which use sunlight to generate the necessary heat as well as electricity at the

same time.[58]

In regions with much sunlight also photochemical and thermochemical water-splitting

methods can be of interest. Photochemical water splitting is in principle an interesting

alternative to electrolysis, as sunlight could be used directly.[1, 58] However, problems such as

conversion rates below 1 % and the fact that hydrogen and oxygen are not produced

separately still have to be solved in the future.[1, 58]

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Promising technologies are being developed in the field of thermochemical water splitting.[1,

58] To allow the separation of hydrogen and oxygen generation and to lower the splitting

temperature (>2500 °C for direct water splitting) processes using supporting reagents such

as the hybrid sulphur cycle (HyS) have been developed:[1, 58] Hydrogen is generated by

electrolysis of an aqueous SO2 solution, thereby generating sulphuric acid. The sulphuric acid

is decomposed at 800-1000 °C to restore SO2. Though not being used commercially today,

these solar thermal water splitting technologies use solar energy much more efficiently

compared to a combination of photovoltaic and electrolysis, making this an interesting

option for the future.[1, 58]

1.2 Methanol – A basic chemical

1.2.1 Background: Properties and applications of methanol

An impressive amount of 53 million tonnes of methanol were consumed in 2011, making it

one of the most important bulk chemicals of the chemical industry.[1] The largest producer of

this colourless liquid (mp = -97.6 °C, bp = 64.6 °C) is Methanex.[59]

Some important applications of methanol are shown in Scheme 4.[1, 59-60] In 2011, Methanol

was mainly used to produce formaldehyde (32 %), dimethyl ether (DME, 11 %), methyl

tertiary-butyl ether (MTBE) and tertiary-amylmethylether (TAME) (together 10 %), and acetic

acid (10 %).[1] Other important products are olefins (MTO and MTP processes, 6 %),

methylamines (4 %), methyl methacrylate (2 %), and chloromethane (1 %). Already in 1986,

Friedrich Asinger discussed the wide range of possible applications based on methanol in his

book “Methanol – Chemie und Energierohstoff” (translation: “Methanol – Chemical and

energy resource”).[61] Due to the raw-material situation in China today, his ideas are more

up-to-date than ever: In 2013, methanol based on coal derived synthesis gas was used to

produce around 1 million tonnes of propylene via the “methanol-to-propylene” process

(MTP), and the demand for propylene is still rising.[1] In 2006, George A. Olah published a

book extending Asinger’s vision and ever since coined the phrase “methanol economy”.[62]

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Scheme 4: Some important applications of methanol.[1, 59-60]

In addition to the use for chemical production, methanol can also serve as an energy vector.

Its very high energy density of 22.7 MJ/kg makes it suitable for energy storage and for the

use as fuel.[1] This energy density is much higher compared to the energy density of Li-ion

batteries (0.5-3.6 MJ/kg).[1] Because of its physico-chemical properties MeOH can be easily

stored and transported using existing technologies.[1] In contrast to oil, methanol is water

mixable and biodegradable.[1] Methanol can be used as a fuel additive or pure in modified

engines and direct methanol fuel cells.[1] The materials used in the fuel system have to be

resistant towards methanol. Moreover, methanol can be converted to conventional fuels by

the “methanol-to-gasoline” process (MTG).[1, 59] In 2011, already 11 % of the produced

methanol were used in gasoline/fuel applications, and this sector is growing fast.[1] A very

detailed, exhaustive discussion of methanol utilisation technologies and a methanol based

economy can be found in excellent books.[1, 61-62]

Another potential use of methanol might be the safe storage and transportation of hydrogen

and CO.[1] The gravimetric storage capacity in methanol is 87.4 wt.-% for CO and 12.5 wt.-%

for H2. Methanol can be catalytically split or reformed to different CO, H2, and CO2 gas-

mixtures.[1] Thus, methanol can serve as a liquid form of synthesis gas in the stoichiometric

ratio required for many industrial applications.[1] Large amounts of H2 are generated from

INTRODUCTION

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methanol reforming (Scheme 5 b, back reaction). This process is typically catalyzed by

copper/zinc catalysts at 180-300 °C.[1] In 2013, it was shown that aqueous methanol can be

reformed to CO2 and H2 at much lower pressures and temperatures using homogeneous

organometallic catalysts (for details see 1.3.3).[63-66] Compared to formic acid (4.3 wt.-%),

methanol has a higher gravimetric storage capacity for H2 of 12.5 wt.-%. Interestingly, the

equimolar methanol/water mixture produced by CO2 hydrogenation to methanol (Scheme

5 b) has a similarly high H2 storage capacity of 12.0 wt.-%. Therefore, the separation of

methanol and water would be unnecessary for hydrogen storage applications.

1.2.2 Methanol production from conventional carbon sources

Today, methanol is produced on large scale by conversion of fossil-fuel derived synthesis gas

(CO/H2) in the presence of heterogeneous catalysts (e.g. Cu/Zn/Al-oxide) at elevated

pressures (50-250 bar) and temperatures (200-350 °C) (Scheme 5, a).[1, 59] For hydrogen-rich

synthesis gas mixtures (e.g. from methane steam reforming), CO2 is added as it consumes

more H2 than CO does (Scheme 5, b). Both reactions are tied through the WGS reaction

which is also catalysed by the typical heterogeneous methanol catalysts under reaction

conditions (Scheme 5, c).[1]

Scheme 5: Production of Methanol from synthesis gas.[1]

1.3 Methanol production from CO2 and H2

As envisioned by Asinger and Olah, it might be possible to build up an economy based on

methanol.[1, 61-62] Consequently, if CO2 and H2 could be efficiently converted to methanol, an

economy based on CO2 and H2 could be imagined. In fact, the conversion of CO2 and H2 to

methanol by heterogeneous catalysts is known from the classical methanol production

processes, in which CO2 is added to the synthesis gas stream (chapter 1.2.2).[1]

INTRODUCTION

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1.3.1 Heterogeneous catalysis for CO2 hydrogenation to methanol

Heterogeneous catalysts are known to catalyse the hydrogenation of CO and CO2 to

methanol, as well as the WGS reaction (Scheme 5).[1] Especially Cu/Zn-oxide based catalysts

were investigated in this respect.[67-70] It was found that the addition of small amounts (up to

3 %) of CO2 to the synthesis gas enhances the yield of methanol.[1, 71] However, high amounts

of CO2 or pure CO2 led to the formation of water as byproduct which reduced the rate of

methanol formation.[1, 67-68]

Nevertheless, Lurgi demonstrated the conversion of CO2 to methanol using a Cu/Zn/Al-

catalyst by Süd-Chemie (now Clariant) in a pilot plant in 1994.[1, 70, 72-73] At 60 bar pressure

and ca. 260 °C methanol was obtained with per-pass conversions around 35-45 %. A slight

catalyst deactivation was observed. The selectivity was as high as 99.96 % (excluding water).

Starting in 1996, NIRE and RITE of Japan built a pilot plant (50 kg/day) based on a new

Cu/ZnO/ZrO2/Al2O3/SiO2 catalyst, which showed slow deactivation over time.[74-76] Another

interesting approach is the CAMERE process of the Korean Institute of Science and

Technology.[1, 77-79] In this process a rWGS reactor is coupled to a methanol formation

reactor. In the rWGS reactor, CO2/H2 is partly converted to CO and H2O. After water removal,

the resulting CO/CO2/H2 stream is fed to the methanol reactor. The production capacity of

the pilot plant is 100 kg methanol per day. Another pilot plant has been operated since 2009

by Mitsui Chemicals with a capacity of 100 tonnes per year.[1, 80-82] In this process, the

Cu/ZnO/ZrO2/Al2O3/SiO2 catalyst developed in a joint research with RITE (vide supra) is used.

CRI operates a plant in Iceland that produces methanol from geogenic CO2 and hydrogen

produced by water electrolysis.[83-85] The process is powered by geothermal energy,

rendering the process economic. The “George Olah Renewable Methanol Plant” has been

operated since 2011 and has a production capacity of 5 million litres per year. In December

2014, CRI announced collaboration with industrial partners, universities, and research

institutions to implement its technology in Germany, with the goal to recycle carbon-dioxide

emissions from a coal-fired powerplant.[83]

In 2012, Behrens et al. elucidated a detailed picture of the elementary steps and the role of

the multi-component catalyst material Cu/ZnO/Al2O3 in the hydrogenation of CO and CO2 to

methanol.[69] The seemingly simple overall transformation of CO or CO2 and H2 to methanol

proceeds through a complex series of bond cleavage and bond forming processes on the

INTRODUCTION

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catalyst surface involving the intermediates HCOO, HCO, HCOOH, H2COOH, H2CO, and CH3O.

The active site consists of Cu steps with Zn substituted into the Cu steps. For a typical syngas

mixture (59.5 % H2, 8 % CO2, 6 % CO, rest inert) at 60 bar and 210 °C-250 °C a TOF of 75.6 h-1

(mol methanol/mol Cu sites) was calculated.[69] This was the benchmark activity at the time

the research for this thesis was started. There is strong evidence that CO2 is directly

converted on Cu/ZnO catalysts rather than being transformed to CO first.[69, 86]

While this thesis was in preparation, some advances in the field of heterogeneous catalysts

were made: In 2014, Graciani et al. demonstrated that the metal-oxide interface in

Cu/CeOx/TiO2 is highly active for CO2 conversion to methanol.[87] At 300 °C, they estimated

the TOF to be as high as 29160 h-1. However, one has to be careful when comparing this TOF

value for Cu/CeOx/TiO2 with the one reported by Behrens et al. for Cu/ZnO/Al2O3, as they

were calculated based on different models.[86] For the Cu/CeOx/TiO2 catalysts Graciani et al.

proposed a mechanism via consecutive rWGS and hydrogenation of CO to methanol.[87]

1.3.2 Homogeneous catalysis for CO2 hydrogenation to methanol

In contrast to heterogeneous catalysts, much less reports exist about homogeneous catalysts

for the CO2 hydrogenation to methanol. This seems to be somewhat surprising, given that

homogeneous organometallic catalysts have been known to activate CO2 for its

hydrogenation to formic acid since Inoue’s discovery in 1976.[88] In 2007, Philip G. Jessop

speculated that the reduction of CO2 beyond the formic acid level typically requires much

higher temperatures, and that only few catalysts are both kinetically capable and stable at

these reaction conditions.[7]

The first reports of methanol formation from CO2 and H2 in the presence of organometallic

catalysts stem from Tominaga et al. from the NIRE and RITE institutes in Japan.[53, 55] In 1993,

they reported the hydrogenation of CO2 to methane via successive formation of CO and

methanol as intermediates in the presence of a Ru3(CO)12/KI catalytic system under harsh

reaction conditions (240 °C, 90-140 bar).[53, 55] The homogeneous catalytic system was active

in the rWGS reaction, converting CO2 and H2 to CO and H2O, and in the successive

hydrogenation of CO to methanol. Methane formation was found to be mainly catalysed by

deposited ruthenium metal. Consequently, the selectivity could be shifted towards methanol

by addition of KI which prevented deposition of metallic ruthenium.[53, 55] In a typical

INTRODUCTION

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experiment in the presence of KI, methanol was obtained with a TON (based on the number

of ruthenium atoms) of up to 32, besides CO with a TON of 11, methane with a TON of 8, and

traces of ethanol.[53, 55] Interestingly, Tominaga et al. showed that by using a similar

Ru3(CO)12/KI catalytic system in the presence of Co2(CO)8 as cocatalyst ethanol was produced

by homologation of methanol.[89-90] At 200 °C, CO2 could be converted to CO (TON = 8),

methanol (TON = 31), ethanol (TON = 12), methyl formate (TON = 1), and methane (TON =

13).

However, for almost another 20 years, no organometallic catalyst was found for the

selective hydrogenation of CO2 to methanol. It seemed that the key to selective methanol

formation at milder reaction conditions was to find a catalyst which catalyses the direct

hydrogenation of CO2 to methanol without catalysing the rWGS leading to the formation of

CO.

In 2010, the catalytic reduction of CO2 with stoichiometric amounts of boranes instead of

molecular H2 has been achieved using nickel-pincer complexes.[91-92] Metal-free catalytic

systems employing frustrated lewis pairs (FLP)[93-99] N-heterocyclic carbenes (NHC),[100] and

silyl-cations[101] have been shown to be active for the CO2 reduction to the methanol stage.

However, these systems are as yet limited by the use of stoichiometric amounts of boranes

and silanes as reduction agents, by the necessity to hydrolyse the formed intermediates with

H2O and/or NaOH to release the methanol product, and by the destruction of the FLP

systems upon the hydrolysis step. Metal-free catalytic systems for the reduction of CO2 were

discussed in detail in two comprehensive reviews.[102-103]

Due to the lack of organometallic catalysts being capable of transforming CO2 to methanol,

indirect routes from CO2 to methanol via CO2 derived intermediates were proposed by

Milsteins’s group in 2011.[104-106] Milstein et al. developed Ru-PNN pincer complexes

(Scheme 8, C) for the efficient hydrogenation of methyl formate,[104] dimethyl carbonate,[104]

methyl carbamates,[104] urea derivatives,[107] and formamides[108] to methanol. As those

substrates can be produced from CO2, two-step production processes were envisioned for

the indirect production of methanol from CO2 by organometallic catalysis (Scheme 6). Using

the Ru-PNN complex, dimethyl carbonate could be quantitatively hydrogenated to methanol

with a TOF up to 2500 h-1 (60 bar H2, 145 °C).[104] Methyl formate could be hydrogenated

using the same complex with a TOF up to 531 h-1 (50 bar H2, 110 °C).[104] Various alkyl and

INTRODUCTION

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aryl urea derivatives, such as 1,3-dihexylurea, could be hydrogenated at 110 °C and 13.6 bar

H2 to yield methanol in 46-94 %.[107] Formylmorpholine hydrogenation at a H2 pressure of

10 bar and a temperature of 110 °C resulted in the formation of methanol in 97 % yield.[108]

Scheme 6: Indirect routes for methanol production from CO2.[104-108]

Ding’s group reported an interesting method for indirect methanol production as coproduct

by modification of the Shell omega process (Scheme 7).[109] In the omega process, ethylene

glycol (EG) is produced by hydrolysis of ethylene carbonate, which is produced from

ethylene oxide and CO2 in the first step of the reaction. Ding et al. proposed to replace the

ethylene carbonate hydrolysis by the ethylene carbonate hydrogenation to ethylene glycol

and methanol.[109] Using the Ru-PNP catalyst shown in Scheme 7, a TON of up to 87000 and a

TOF of up to 1200 h-1 could be obtained.[109]

Scheme 7: Methanol production by a modified omega process as proposed by Ding et al.[104, 109]

Milstein’s Ru-PNN complexes were not able to catalyse the hydrogenation of CO2 directly,

i.e. they could only catalyse the hydrogenation of the CO2 derived intermediates, making

INTRODUCTION

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two-step procedures necessary to produce methanol. In 2011, Huff and Sanford tackled this

problem by setting up an integrated one-pot cascade reaction:[110] They used three different

catalysts (Scheme 8, A, B, C) in one reaction mixture to catalyse the three steps of the

cascade shown in Scheme 9, which were (a) hydrogenation of CO2 to formic acid, (b)

esterification of formic acid to methyl formate, and (c) hydrogenation of methyl formate to

methanol.[110] The combination of steps (a) and (b) of the cascade reaction had been known

to result in the formation of methyl formate.[7] In most cases, this reaction had been carried

out in the presence of a base (e.g. NEt3).[7] However, Huff and Sanford found that by adding

the Lewis acid Sc(OTf)3 (B) as co-catalyst to [RuCl(OAc)(PMe3)4] (A) in the absence of base,

the esterification step was significantly enhanced, giving a TON of 40 after 16 hours and a

TOF of 32 h-1 in the first hour.[110] The last step of the cascade, the hydrogenation of methyl

formate to methanol (c), had little precedent in the literature.[104, 110] Sanford and Huff used

the Ru-PNN complex (C) developed by Milstein et al.[104] to accomplish this reaction step

(Scheme 8).[110] Using a mixture of these three catalysts A, B, and C in CD3OH at 135 °C under

a CO2 pressure of 10 bar and a H2 pressure of 30 bar, a TON of 2.5 was achieved.[110] The

main factors hampering higher TONs were the inhibition/deactivation of the Ru-PNN catalyst

(C) by CO2 and by Sc(OTf)3 (B).[110] By spatial separation of catalysts A/B from catalyst C

inside one reactor a higher TON of up to 21 could be obtained.[110]

Scheme 8: Catalysts used for indirect hydrogenation of CO2 to methanol.[104, 110]

Scheme 9: Indirect hydrogenation of CO2 to methanol via methyl formate (lower pathway) as shown by Sanford/Huff.[110]

Transferred to the “catalytic chess” model introduced in chapter 1.1.3, Huff and Sanford

needed a combination of pawn, rook, and knight to “walk” from CO2 to methanol (Figure 2,

INTRODUCTION

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blue pathway). Based on this eye-opening strategy the aim of the present work was to find

the “organometallic queen” which is able to walk directly from CO2 to methanol (green

pathway).

Figure 2: In a “catalytic chess” game, Huff and Sanford need three different chess pieces to walk from CO2 to MeOH.[104, 110]

The aim of the present work was to find the “organometallic queen” which can walk directly from CO2 to MeOH.

Only very recently, while this thesis was in the writing process, Huff/Sanford et al. described

a cascade reaction process for the hydrogenation of CO2 to methanol.[111] In contrast to their

previous report, methanol formation via intermediate formation of dimethylformamide

(DMF) was envisaged this time (Scheme 10).[111] The hydrogenation of CO2 in the presence of

HNMe2 has been known to lead to the formation of DMF with very high activities and

selectivities (steps (a) and (b) of the cascade shown in Scheme 10).[7, 10] Step (c) of the

cascade reaction, the hydrogenation of amides to methanol, had much less precedent in the

literature.[108, 112] Huff/Sanford et al. showed that the Ru-MACHO-BH4 complex shown in

Scheme 10 together with 50 equivalents of the base K3PO4 is capable of selectively

hydrogenating DMF to methanol at 50 bar H2 pressure and 155 °C. As catalyst deactivation

became apparent at 155 °C, a temperature ramp as well as a huge excess of H2 was used to

achieve high CO2 conversions in the overall cascade reaction to methanol of up to 96 %: CO2

hydrogenation to DMF was first carried out at 95 °C, and after 18 hours the temperature was

raised to 155 °C to achieve hydrogenation of the DMF intermediate to methanol. A TON of

INTRODUCTION

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up to 550 could be obtained using this strategy. However, the selectivity to methanol was

only about 30 % with DMF and dimethylammonium formate (DMFA) as coproducts. Using

the same temperature ramp and the same catalytic system also dimethylammonium

dimethylcarbamate (DMC), which forms upon reaction of HNMe2 and CO2, could be used as

substrate instead of CO2. In this case, DMC initially reacts to CO2 and HNMe2 at 95 °C before

the cascade proceeds through DMF formation as described above. Based on this example,

combined capture of CO2 in the form of compounds like DMC and conversion to methanol

was proposed.

Scheme 10: Indirect hydrogenation of CO2 to methanol DMF (lower pathway) as shown by Sanford/Huff.[111]

An interesting method for combining low pressure CO2 capture with subsequent

hydrogenation to methanol was published by Milstein’s group shortly after (Scheme 11).[113]

In this approach CO2 is captured by an aminoalcohol at low pressures (1-3 bar) at 150 °C in

the presence of Cs2CO3 as catalyst, which leads to the formation of the corresponding

oxazolidone as intermediate.[113-114] The resulting oxazolidone solution could be

hydrogenated at 135 °C and 60 bar H2 to give methanol and to restore the aminoalcohol

after addition of the Ru-PNN pincer complex shown in Scheme 11 and tert-BuOK (10 mol-%).

Excess CO2 had to be removed after the CO2 capture step as the Ru-PNN pincer complex was

deactivated by the presence of CO2. Using valinol in DMSO as capture medium, CO2 could be

converted to methanol in about 50 % yield by this procedure. The allure of these combined

CO2 capture/conversion concepts is that energy costs associated with CO2 release from

capture solutions could be avoided.

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Scheme 11: CO2 capture and subsequent hydrogenation to MeOH as demonstrated by Milstein et al.[113]

Two years after first results of this thesis concerning the CO2 hydrogenation to methanol

using the catalytic system Ru/Triphos/acid (Triphos = 1,1,1-

tris(diphenylphosphinomethyl)ethane) had been published,[115] Cantat et al. showed that the

same catalytic system Ru/Triphos/acid could also be used for the disproportionation of

formic acid to methanol, CO2, and water.[116] This reaction was shown the first time by

Goldberg et al. in 2013.[117] Using [Ru(COD)(methylallyl)2]/Triphos/methylsulfonic acid at

150 °C, formic acid could be decomposed leading to the formation of methanol with a yield

of up to 50 % (TON = 83).[116] Cantat et al. found that this catalytic mixture catalysed the

decomposition of formic acid to CO2 and H2, as well as the transfer hydrogenation of formic

acid to methanol.[116] Together with the findings described in the excerpt from the present

thesis which had already been published at this time[115] a network of reactions leading to

methanol formation could be established (Scheme 12).[116]

Scheme 12: Proposed pathways for the disproportionation of formic acid to methanol according to [116]

.

The production of methanol by disproportionation of formic acid is an interesting variation,

however, as pure formic acid itself is a valuable chemical which cannot easily be prepared

from CO2 in pure form[7] the direct hydrogenation of CO2 to methanol still appears to be

much more promising for the production of methanol from alternative carbon sources.

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1.3.3 Homogeneous catalysis for methanol reforming

The back-reaction of the CO2 hydrogenation to methanol, the catalytic dehydrogenation of

aqueous methanol to CO2 and H2 (methanol reforming), was the first time reported using

homogeneous organometallic catalysts by Cole-Hamilton et al. in 1987.[118] They used

[Rh(bipy)2]Cl as catalyst in the presence of NaOH at 120 °C to decompose a 95/5 v/v solution

of methanol/H2O at a TOF of 7 h-1.

In 2013, while this thesis was in preparation, more efficient dehydrogenation of aqueous

methanol was reported by Grützmacher’s and Beller’s groups.[63-65, 119] Beller’s group showed

that with the catalyst [RuHCl(CO)PNP] (PNP = HN(CH2CH2PiPr2)2) methanol could be

decomposed to CO2 and three equivalents of H2 with a TOF (based on moles of methanol) of

up to 1573 h-1 in the presence of potassium hydroxide at 95 °C.[63] A 3/2 mixture of methanol

and water gave a TOF of up to 244 h-1 and a 9/1 MeOH/H2O mixture gave a TOF of 88 h-1.[63]

In a long-term stability test a 9/1 mixture of MeOH/H2O was decomposed over a period of

23 days giving a TON of >116667.[63]

Grützmacher’s group used [K(dme)2][Ru(H)(trop2dad)] (trop2dad = 1,4-bis(5H-

dibenzo[a,d]cyclohepten-5-yl)-1,4-diazabuta-1,3-diene) for the decomposition of a 1/1.3

MeOH/D2O mixture at 90 °C in the presence of THF solvent under base free conditions.[64]

After 10 hours, 78 % of the methanol was decomposed to CO2 and H2 (TON = 156). During

the catalytic cycle, the non-innocent azadiene ligands reversibly store molecular hydrogen.

Shortly after, Beller’s group reported the use of the iron pincer complex [FeH(BH4)(CO)(PNP)]

(PNP = HN(CH2CH2PiPr2)2) for methanol reforming.[119] Pure methanol was decomposed to

CO2 and H2 in the presence of KOH with a TOF of up to 245 h-1 at 91 °C. A mixture of 4/1

MeOH/H2O was decomposed with a TOF of 137 h-1 under the same conditions.

Beller et al. reported in 2014 that by using a mixture of Ru-MACHO-BH4 (shown in Scheme

10) and [Ru(H)2(dppe)2] (dppe = 1,2-bis(diphenylphosphino)ethane) aqueous methanol could

be dehydrogenated in the absence of base.[120] The catalysts were reported to operate in a

synergistic manner, i. e. their combined activity was higher than the sum of the activities of

each single catalyst. At 93.5 °C an average TOF of 93 h-1 was obtained over 7 hours and a

total TON of 4286 was obtained after 8 days.

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In 2014, Milstein’s group used the Ru-PNN pincer complex which was employed before for

the hydrogenation of alkyl formates to methanol (Scheme 8, C) for methanol reforming to

CO2 and H2 in the presence of NaOH or KOH and toluene solvent.[65] The released CO2 was

trapped as carbonate. Methanol was converted with 77 % within 9 days with an average TOF

of 14.3 h-1 based on methanol. Interestingly, the organic layer could be separated from the

aqueous layer and reused without addition of new catalyst. The procedure could be

repeated over a period of 1 month giving a TON of around 9667 based on methanol.

Interestingly, all reported catalytic systems active in the dehydrogenation of aqueous

methanol solutions possess multidentate ligands. None of these catalysts was, however,

reported to be active for the hydrogenation of CO2 to methanol.

AIM OF THE THESIS

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2 Aim of the thesis

When the experimental work for this thesis was started, organometallic catalysts were

believed to allow the hydrogenation of CO2 to methanol only via formate esters as stable

intermediates,[104, 110] or via the intermediate formation of CO by rWGS (vide supra).[53, 55]

Whereas the cascade reaction via formate esters suffered from its complexity and partial

incompatibility of the three different catalysts used leading to limited turnover numbers, the

approach via rWGS suffered from its bad selectivity, leading to the formation of CO and

methane as side products.

The aim of the present thesis was the development of the very first catalytic system based

on a single organometallic complex for the selective hydrogenation of CO2 to methanol.

After identification of a suitable catalytic system, the mechanism of this system should be

investigated in detail. Reaction schemes should be developed and realised which allow for

easy recycling of the homogeneous catalyst in repetitive batch as well as in continuous-flow

mode.

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3 Results & Discussion

3.1 Hydrogenation of CO2 to MeOH in the presence of alcohol additives

Parts of this chapter have been published in: S. Wesselbaum, T. vom Stein, J. Klankermayer,

W. Leitner, Angew. Chem. Int. Ed. 2012, 51, 7499-7502.[115]

3.1.1 Identifying a suitable catalytic system

As a starting point, a catalytic system should be identified which could integrate all three

reaction steps of the cascade reaction established by Huff and Sanford for the CO2

hydrogenation to methanol.[110] This reaction cascade consisted of three consecutive

reactions:[110] (a) the hydrogenation of CO2 to formic acid, (b) the esterification of formic acid

with methanol to methyl formate, and (c) the hydrogenation of methyl formate to methanol

(vide supra).[110] The hydrogenation of CO2 in the presence of methanol or ethanol using

organometallic catalysts was well known to lead to the formation of the corresponding alkyl

formates (combination of steps (a) and (b) of the cascade).[7] The reaction pathway is

believed to be CO2 hydrogenation to formic acid followed by thermal esterification of formic

acid with the present alcohol.[7] Several organometallic catalysts were known to catalyse this

reaction, with ruthenium-phosphine complexes being the most active ones.[7] Therefore, a

catalytic system based on a ruthenium phosphine complex seemed to be the obvious choice

to integrate all three reaction steps.

However, the last step of the cascade, the hydrogenation of alkyl formates to methanol (c),

had little precedent in the literature.[104, 110] Milstein et al. developed Ru-PNN pincer

complexes which made the efficient and selective hydrogenation of methyl formate to

methanol possible for the first time.[104] Due to a lack of alternatives, Huff and Sanford used

this Ru-PNN complex for step (c) of their cascade reaction, despite its deactivation by CO2

which led to limited turnover numbers (TON).[110]

A catalytic system that had been investigated already earlier by Klankermayer and Leitner et

al. and other groups was identified as promising candidate for the integration of steps (a),


-24-

(b), and (c) of the cascade.[121-127] This catalytic system was based on ruthenium as the

central metal and the tridentate ligand Triphos (Triphos = 1,1,1-

tris(diphenylphosphinomethyl)ethane). The active catalyst could be formed either in situ

from Ru(acac)3 and Triphos 1 or from the readily accessible ruthenium(II)-complex

[Ru(TMM)(Triphos)] (TMM = trimethylenemethane) 2 (Scheme 13),[128] and provided an

excellent catalyst for the hydrogenation of carboxylic esters, amides, lactones, and

carboxylic acids.[121-127] While this thesis was in progress, the scope of this catalytic system

was extended to the hydrogenation of carbonates, acid anhydrides, imides and ureas by

Klankermayer and Leitner et al.[129] Distinct reactivities of this catalytic system were

observed dependent on whether an acidic additive was present or not.[126-127, 129]

Mechanistic investigations by Klankermayer and Leitner et al. suggested that species of type

3, in which the Triphos ligand is coordinated in a facial fashion, facilitate hydride transfer and

protonolysis as key steps for the addition of hydrogen to the carboxylate functional group in

the presence of acidic additives (Scheme 13).[127]

Therefore, the Ru/Triphos catalytic system showed two characteristics which made it a very

promising candidate as catalyst for the whole cascade reaction: it was based on a Ru-

phosphine complex, which might favour CO2 activation by facile insertion into the Ru-H

hydride bond,[7, 130] and it proved to facilitate the hydrogenation of carboxylate groups.

Scheme 13: In situ catalytic system 1, preformed catalyst precursor 2, and active species 3 (S=solvent or substrate) for the hydrogenation of carboxylate functional groups.


-25-

However, the hydrogenation of formate esters, step (c) of the cascade reaction, had not yet

been demonstrated using this catalytic system. In a first set of experiments, both catalytic

systems 1 and 2 were tested in the hydrogenation of formate esters to methanol (Table 1).

Table 1: Catalytic hydrogenation of formate esters.[a]

Entry Cat. Acid

R pH2[b]

[bar]

TON[c]

1 1[d]

MSA Et 50 75

2 1[d]

MSA Me 50 74

3 1[d]

‒ Et 30 72

4 1[d]

MSA Et ‒ 0[e]

5 ‒ MSA Et 30 0[e]

6 2 ‒ Et 30 5

7 2 MSA Et 30 77

8[f]

1[d]

MSA Et 30 79

[a] Reaction conditions: Ru-complex (25 µmol), substrate (2.5 mmol), 2.0 mL THF, 140 °C, 24 h; [b] at room temperature; [c]

TON = mol MeOH/mol catalyst; [d] 50 µmol (2 eq.) Triphos; [e] no methanol but traces of ethanol (about 4 % yield) were

observed; [f] 12.5 mmol substrate.

Indeed, the hydrogenation of ethyl formate was possible at 140 °C and 50 bar H2 pressure

using the catalytic system 1 together with 1.5 eq. (equivalents) of methylsulfonic acid (MSA)

as acidic additive. Methanol was yielded in 75 % corresponding to a TON of 75 (Table 1,

entry 1). In an analogous manner, methyl formate could be hydrogenated giving only

methanol as product with a TON of 74 (Table 1, entry 2). Interestingly, omitting the acidic

additive had no significant influence on the reactivity in the case of catalytic system 1,

whereas it had a strong detrimental influence in the case of catalyst 2 (Table 1, entries 3 &

6). In the presence of 1.5 eq. MSA also catalyst 2 enabled the hydrogenation of ethyl

formate to methanol with a TON of 77 (Table 1, entry 7). The positive influence of the acid in

the case of catalyst 2 suggested that structures of type 3 which form in presence of acid play

an important role in this catalytic transformation.[127] The effect of the omitted acidic

additive might not be observable in the case of catalytic system 1, as the reaction solution

most probably contains the weak acid acetylacetone from hydrogenolysis of the

acetylacetonate ligands under H2 pressure.[131]

Control experiments in the absence of metal-precursor or hydrogen did not show the

formation of methanol (Table 1, entries 4 & 5). However, in both cases small amounts of

ethanol (around 4 % yield) were found in the reaction solution, which might stem from slow


-26-

decarbonylation of ethyl formate to CO and ethanol under reaction conditions, or from ethyl

formate hydrolysis.

Using higher concentrations of the substrate ethyl formate did not lead to increased TONs,

indicating that the catalyst was deactivated after around 80 turnovers (Table 1, entry 8).

Already after 6 hours reaction time the 31P{1H}-NMR spectrum of the reaction solution in d8-

THF showed exclusively a characteristic set of a doublet (δ = 18.2 ppm, JP-P = 28.7 Hz) and

triplet (δ = 6.4 ppm, JP-P = 28.7 Hz) (Figure 3). The 1H-NMR spectrum showed a characteristic

hydride signal (δ = -6.8 ppm, dt, JH-P = 63.9 Hz, JH-P = 15.3 Hz). Based on literature data, these

signals were ascribed to the cationic carbonyl complex [Ru(H)(CO)2(Triphos)]+ (4).[132] The

carbonyl ligands were most likely formed by decarbonylation of alkyl formate.[127, 133] Based

on this observation, a new synthesis route for [Ru(H)(CO)2(Triphos)]NTf2 (4NTf2) could be

established (Scheme 14): Stirring complex 2 together with 1 eq. of

bis(trifluoromethane)sulfonimide (HNTf2) in ethyl formate under 60 bar H2 pressure for 24

hours at 140 °C led to the exclusive formation of 4NTf2 which could be isolated in 97 % yield

by simply removing all volatiles in vacuo.

The results summarised in Table 1 proved that catalytic systems 1 and 2 were indeed able to

catalyse the hydrogenation of alkyl formates to methanol in the presence of an acidic

additive. This fact made these catalytic systems suitable candidates to be tested in the CO2

hydrogenation reaction.


-27-

Figure 3: 31

P{1H}-NMR spectrum (121 MHz, d8-THF, r.t.) of the reaction solution of an ethyl formate hydrogenation reaction

using catalyst 2 after 6 hours at 60 bar H2, 140 °C.

Scheme 14: Synthesis of the cationic carbonyl complex 4NTf2 by heating a solution of 2 in ethyl formate under H2 pressure.

3.1.2 Reaction cascade for the hydrogenation of CO2 to methanol

Encouraged by these results, the hydrogenation of CO2 to methanol with catalytic systems 1

and 2 was tested. Ethanol was added to the reaction solution in order to allow the formation

of ethyl formate as intermediate in the cascade reaction and to allow direct identification of

any methanol formed in solution by NMR and GC.

First, catalytic system 1 with 2 eq. Triphos was tested in ethanol/THF. After 24 hours at

140 °C, 10 bar CO2 and 30 bar H2 only very low quantities of methanol corresponding to a


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TON of 2 were detected (Table 2, entry 1). However, in the presence of MSA (1.5 eq.) as

acidic additive, methanol was formed with a TON of 52, which was already the best TON

ever obtained using an organometallic complex in this transformation (Table 2, entry 2). An

even slightly higher TON of 63 could be achieved with complex 2. Like catalytic system 1,

also 2 displayed a much higher productivity in the presence of 1.5 eq. of MSA compared to

the absence of acid (Table 2, entries 3 & 4).

Table 2: Hydrogenation of carbon dioxide to methanol in the presence of alcohol additive.[a]

Entry Cat. Acid[b]

pH2[c]

[bar]

pCO2[c]

[bar]

TON[d]

1 1[e]

‒ 30 10 2

2 1[e]

MSA 30 10 52

3

2 ‒ 30 10 8

4 2

MSA 30 10 63

5 ‒ MSA 30 10 0

6

1[e]

MSA 30 ‒ 0

7 1[e]

MSA ‒ 10 0

[a] Reaction conditions: catalyst (25 µmol), THF (1.5 mL), EtOH (10 mmol), 140 °C, 24 h; [b] 38 µmol (1.5 eq.) methane

sulfonic acid; [c] at room temperature; [d] TON = mmol MeOH/mmol catalyst; [e] 50 µmol (2 eq.) Triphos.

GC-analysis of the gas phase showed no formation of CO, indicating that the reaction did not

proceed via rWGS.[53, 55] In the 1H-NMR spectrum of the reaction solution small amounts of

ethyl formate and traces of methyl formate could be detected (Figure 4). The methyl

formate traces could only be detected in the 1H-NMR spectrum by extensive zooming into

the formate region and were too small to be quantified. This supported the assumption that

methanol formation proceeded at least partly via a cascade reaction and alkyl formate

intermediates. GC-analysis of the liquid phase corroborated the formation of methanol and

ethyl formate (Figure 5). However, at this stage a stepwise reduction of CO2 to methanol via

the formate anion in the coordination sphere of the metal could not be ruled out. The

1H-NMR spectrum in Figure 4 also showed a small singlet at 8.8 ppm, which could stem from

traces of free formic acid as well as from formate coordinated to the ruthenium metal

centre.[134-136]


-29-

Figure 4: Representative 1H-NMR spectrum (300 MHz, d6-dmso, r.t.) of the reaction solution of a CO2 hydrogenation

reaction using catalytic system 1 in the presence of MSA (1.5 eq.) in ethanol/THF; mesitylene was added as internal

standard.

Figure 5: Representative gas chromatogram of the reaction solution of a CO2 hydrogenation reaction using catalytic system 1 in the presence of MSA (1.5 eq.) in ethanol/THF; heptane was added as internal standard.


-30-

Control experiments were performed to confirm the origin of the observed methanol from

homogeneously catalysed CO2 hydrogenation. As expected, in the presence of acid alone no

CO2 reduction products were formed (Table 2, entry 5). Also the reaction in the absence of

either CO2 or H2 did not lead to methanol formation (Table 2, entries 6 & 7). Reaction

solutions containing metal and ligand were yellow and clear after performing the reactions.

No metal deposition was visible to the naked eye. An experiment with ruthenium supported

on carbon (5 wt.-% Ru/C) as catalyst led to no detectable formation of methanol

(0.025 mmol Ru, 1.0 eq. HNTf2, 10 mmol EtOH, 1.5 mL THF, 10 bar CO2, 30 bar H2, 140 °C,

24 h). The molecular nature of the catalytically active metal species was further supported

by a mercury poisoning experiment. A CO2 hydrogenation reaction was performed

(0.025 mmol catalyst 2, 1.0 eq. HNTf2, 10 mmol EtOH, 1.5 mL THF, 20 bar CO2, 60 bar H2,

140 °C, 24 h) in the presence of a large excess of mercury (0.1 mL, 6.75 mmol).[137-138] The

amount of methanol determined in this experiment was about 60 % of the amount formed

in an experiment using the same reaction conditions in the absence of mercury. This

difference can be explained by losses of the reaction solution during the separation of the

reaction solution from the added mercury before analysis. The fact that mercury did not lead

to a shutdown of the catalysis supported the assumption of a molecular catalyst species.

Using the so far most productive complex 2, a first screening of reaction parameters was

carried out to identify key parameters for future optimisation (Table 3). By changing the acid

from MSA comprising a coordinating anion[125] to HNTf2 comprising a weakly coordinating

anion the TON could be increased from 63 (Table 2, entry 4) to 86 (Table 3, entry 1). Raising

the CO2 pressure to 20 bar and the H2 pressure to 60 bar further increased the TON to 221

(Table 3, entry 2). Variation of the amount of HNTf2 showed the best result for the 1 : 1

stoichiometric ratio (Table 3, entries 2-4). Interestingly, in the case of the sulfonic acids MSA

and p-toluenesulfonic acid monohydrate (p-TsOH) the highest TONs were observed in the

presence of a slight excess (1.5 eq.) of the acid (Table 3, entries 5-10). However, both acids

led to lower TONs as compared to HNTf2.

For a more detailed analysis of the influence of the acid the reader is referred to chapter

3.3.1, and for a more detailed analysis of other parameters like temperature and partial

pressures the reader is referred to chapter 3.3.2. The next chapter will focus on the

identification of the active catalyst species and the understanding of the reaction

mechanism as basis for further investigations concerning reaction parameters.


-31-

Table 3: Variation of some reaction parameters in the hydrogenation of carbon dioxide to methanol in the presence of alcohol additive.

[a]

Entry Cat. Acid (eq.)[b]

pH2[c]

[bar]

pCO2[c]

[bar]

TON[d]

1

2 HNTf2 (1.0) 30 10 86

2 2

HNTf2 (1.0) 60 20 221

3 2

HNTf2 (1.5) 60 20 201

4 2

HNTf2 (3.0) 60 20 186

5 2

MSA (1.0) 60 20 100

6 2

MSA (1.5) 60 20 149

7 2

MSA (3.0) 60 20 39

8 2

p-TsOH (1.0) 60 20 128

9 2

p-TsOH (1.5) 60 20 193

10 2

p-TsOH (3.0) 60 20 72

[a] Reaction conditions: catalyst (25 µmol), THF (1.5 mL), EtOH (10 mmol), 140 °C, 24 h; [b] equivalents to catalyst; [c] at

room temperature; [d] TON = mmol MeOH/mmol catalyst.

3.2 Investigating the mechanism of the CO2 hydrogenation to MeOH

Parts of this chapter have been published: S. Wesselbaum, V. Moha, M. Meuresch, S.

Brosinski, K. M. Thenert, J. Kothe, T. vom Stein, U. Englert, M. Hölscher, J. Klankermayer, W.

Leitner, Chem. Sci. 2015, 6, 693-704.[139]

The results shown in the previous chapter demonstrated the possibility to hydrogenate

carbon dioxide to methanol using a single homogeneous transition-metal catalyst under

relatively mild reaction conditions. This chapter focuses on the identification of the active

catalyst species and the detailed reaction mechanism.

3.2.1 Identification of the organometallic species in solution

NMR experiments were carried out in order to identify organometallic intermediates of the

catalytic reaction sequence with catalyst precursor 2 upon stepwise addition of the required

components. Therefore, an experiment was carried out in the absence of an alcohol

additive: A solution of catalyst precursor 2 and HNTf2 (1 eq.) in d8-THF was pressurised with

CO2 (20 bar at r.t.) and H2 (60 bar at r.t.), stirred for 1 h at 140 °C, and the resulting clear and

yellow reaction solution was analysed by NMR spectroscopy. The 1H-NMR spectrum showed

a sharp signal at 3.27 ppm which indicated the catalytic formation of MeOH with a TON of

35. This was surprising, as the reaction was carried out in the absence of any alcohol

additive. Thus, one of the species formed under these conditions must have been able to


-32-

serve as catalyst for the hydrogenation of CO2 to methanol without the intermediate

formation of the formate ester. The 31P{1H}-NMR spectra of the reaction solution recorded

at room temperature and at -40 °C are depicted in Figure 6. The [1H,31P]-HMBC-NMR

spectrum recorded at -40 °C is shown in Figure 7.

Figure 6: 31

P{1H}-NMR spectra (top: at r.t., bottom: at -40 °C) of the reaction solution of a CO2 hydrogenation to methanol

(20 bar CO2, 60 bar H2, 140 °C, 1 h) with catalyst 2 (50 μmol) and HNTf2 (1 eq.) in d8-THF (2.0 mL).


-33-

Figure 7: [1H,

31P]-HMBC (600 MHz, d8-THF, -40 °C) spectrum of the reaction solution after CO2 hydrogenation to methanol

(20 bar CO2, 60 bar H2, 140 °C, 1 h) with catalyst 2 (50 μmol) and HNTf2 (1 eq.) in d8-THF (2.0 mL).

The 31P{1H}-NMR spectrum showed a characteristic set of a doublet (δ = 18.2 ppm, JP-P =

28.7 Hz) and a triplet (δ = 6.4 ppm, JP-P = 28.7 Hz). This signal was correlated with the hydride

signal (δ = -6.7 ppm, dt, JH-P = 63.9 Hz, JH-P = 15.3 Hz) in the [1H,31P]-HMBC-NMR spectrum.

Based on literature data these signals could be assigned to the cationic carbonyl complex

[Ru(H)(CO)2(Triphos)]+ (4).[132] ESI-MS analysis (m/z = 783.1) further confirmed this

assignment. According to the integral ratios in the 31P{1H}-NMR spectrum, the content of

complex 4 in solution was about 4 %. The formation of complex 4 was well in line with the

assumption of cationic complexes of type 3 as catalytically active species. The carbonyl

ligands in complex 4 most likely formed by decarbonylation of aldehyde intermediates or

methanol.[127, 140-141] This hypothesis was supported by the fact that complex 4 was the only

complex observed during the studies concerning alkyl formate hydrogenation with complex

2 (vide supra). Formation of complex 4 was revealed as possible deactivation pathway by

testing its catalytic activity in the CO2 hydrogenation in the absence of alcohol (standard

conditions: V(THF) = 2.08 mL, c(Ru) = 6 mmol L-1, HNTf2 (1 eq.), p(CO)2 = 20 bar at r.t., p(H2) =

60 bar at r.t., T = 140 °C, t = 24 h), which resulted in a TON of only 9.


-34-

A [1H,31P]-HMBC-NMR experiment revealed a correlation of the sharp singlet at 43.3 ppm in

the 31P{1H}-NMR showed the broad hydride signal at -8.7 ppm in the 1H-NMR spectrum. By

comparison with literature data these signals were originally assigned to the neutral dimeric

complex [Ru2(-H)2(Triphos)2].[125] However, recent analysis by Meuresch et al. suggested

that the signals more likely result from a cationic dimer with the formula [Ru2(-

H)3(Triphos)2]+ (5).[142-143] In the following, 5 will be referred to as “dimer”. According to the

integral ratios in the 31P{1H}-NMR spectrum 5 formed in about 2 %. Formation of 5 was

revealed as second deactivation pathway, as the isolated dimer proved to be inactive in the

CO2 hydrogenation to methanol.

The small sharp singlet at 36.7 ppm in the 31P{1H}-NMR spectrum showed the formation of

another phosphor containing species in about 6 % (based on the integral ratios in the

31P{1H}-NMR spectrum). Analysis of the mixture by ESI-MS (+) showed a mass of m/z =

1557.9. Based on this data and by comparison with literature data the signals were assigned

to the chloro-bridged dimer [Ru2(Cl)3(Triphos)2]+ (6).[144] The chloro ligands could only stem

from chloride traces in the starting materials or solvents. To check if chloride containing

complexes are active in the CO2 hydrogenation to methanol, a reaction under standard

conditions using complex 2 with the addition of 1-butyl-3-methylimidazolium chloride (3 eq.)

was performed. In this experiment only traces of methanol (TON = 1) were observed after

24 hours. The 31P{1H}-NMR spectrum of the reaction solution showed formation of the

already identified complexes 4 (δ = 16.4 ppm, d, JP-P = 28.7 Hz; δ = 4.6 ppm, t, JP-P = 28.7 Hz)

and 6 (δ = 35.0 ppm, s) (Figure 8). The signals at 46.5 ppm (dd, JP-P = 39.4 Hz, JP-P = 16.9 Hz),

9.5 ppm (dd, JP-P = 39.4 Hz, JP-P = 31.9 Hz), and -2.2 ppm (m) could be assigned to

[RuCl(H)CO(Triphos)] (7) by comparison with literature data.[145] In the 1H-NMR spectrum the

corresponding hydride signal was observed at -5.8 ppm (ddd, JH-P = 95.0 Hz, JH-P = 19.7 Hz, JH-P

= 15.0 Hz). These results confirmed that Ru-Triphos complexes bearing chloro ligands are

inactive in the CO2 hydrogenation to methanol.


-35-

Figure 8: 31

P{1H}-NMR spectrum (121 MHz, d8-THF, r.t.) of the reaction solution after CO2 hydrogenation to methanol

(20 bar CO2, 60 bar H2, 140 °C, 1 h) with catalyst 2 (25 μmol) and HNTf2 (1 eq.) in d8-THF (2.0 mL) in the presence of 1-butyl-3-methylimidazolium chloride (3 eq.).

The main species in the 31P{1H}-NMR spectrum shown in Figure 6 accounting for over 85 % of

the total signal intensity gave rise to a broad singlet at 44.2 ppm. The shape of the signal

indicated fluxional behaviour of this species at room temperature. Therefore, a low-

temperature NMR measurement was performed at 233 K. The broad singlet split into a

doublet (46.3 ppm, 2P, JP-P = 42.5 Hz) and a triplet (43.9 ppm, 1P, JP-P = 42.5 Hz). In the

[1H,31P]-HMBC-NMR spectrum recorded at 233 K a coupling of these signals to a proton

signal at 8.7 ppm (br) was observed, which was well in the range of ruthenium coordinated

formate (Figure 7).[134-136] The corresponding [1H,13C]-HMBC-NMR experiment showed the

coupling of the proton signal at 8.7 ppm to a singlet at 178.8 ppm in the 13C{1H}-NMR,

further corroborating the formation of a formate complex.[110, 146-147] Hydride signals

corresponding to this species were not detected in the [1H,31P]-HMBC-NMR.

The same formate complex could be generated independently starting from complex 2 as

follows: HNTf2 (7.0 mg, 0.025 mmol) was dissolved in d8-THF (0.5 mL) and added to complex


-36-

2 (19.5 mg, 0.025 mmol, 1 eq.) in d8-THF (0.5 mL) at room temperature giving a deep red

coloured solution. HCO2H (0.9 μL, 0.025 mmol, 1 eq.) was then added via micro-syringe and

the solution turned orange. NMR analysis of the crude reaction mixture at 233 K showed the

generation of the same formate complex as observed in the reaction solution of a CO2

hydrogenation reaction (compare Figure 7 and Figure 9).

Figure 9: [1H,

31P]-HMBC spectrum (600 MHz, d8-THF, -40 °C) after addition of 1 eq. of HNTf2 and 1 eq. of HCO2H to complex

2 in d8-THF.

The 31P{1H}-NMR spectrum showed the characteristic set of doublet (δ = 47.1, JP-P = 42.4 Hz,

2P) and triplet (δ = 44.2, JP-P = 42.4 Hz, 1P) (Figure 9), the 1H-NMR spectrum showed the

formate signal at 8.8 ppm (Figure 9), and the 13C{1H}-NMR showed the formate signal at

178.4 ppm. According to the integrals in the 31P{1H}-NMR spectrum the formate complex

formed in about 70 % besides some other, yet unidentified phosphor containing species. The

yield of the formate complex could be increased to 86 % by adding HNTf2 and HCO2H more

carefully to complex 2 while vigorously stirring the solution. FT-IR analysis of the crude

solution at room temperature showed a νCO stretching mode at 1543 cm-1, which is a typical

value for 2-coordinated formate (Figure 10).[135-136, 146-147] Based on these data and on the

basis of literature precedence,[135] the structure of this complex was assigned as

[Ru(2-O2CH)(Triphos)(THF)]+ (8a) (Scheme 15). As a formate exchange between 2- and 1-


-37-

coordination could be excluded based on the IR-data,[135] the fluxional behaviour of complex

8a observed on the NMR time scale could be most plausibly explained as follows: The labile

THF ligand temporarily dissociates to form a five-coordinated intermediate. Subsequent

association of the THF ligand in another position, thereby changing places with a

coordinated formate-oxygen, leads to exchange of the axial and equatorial phosphorus

positions on the NMR time scale. Therefore, axial and equatorial positions cannot be

discriminated by NMR-spectroscopy. Cooling of the solution slows down the exchange, thus,

axial and equatorial positions become distinguishable.

Figure 10: FT-IR spectrum (transmission) after addition of 1 eq. of HNTf2 and 1 eq. of HCO2H to complex 2 in d8-THF.


-38-

Scheme 15: Formation of the same formate complex 8a from catalyst precursor 2 in either the presence of HNTf2 (1 eq.) and H2/CO2 under reaction conditions (upper pathway), or by addition of HNTf2 (1 eq.) and HCO2H (1 eq.) in THF (lower pathway).

This interpretation was supported by the formation of a non-fluxional formate complex

upon addition of the less labile ligand acetonitrile (0.1 mL) to the freshly prepared solution

of 8a in THF (0.5 mL) at room temperature: The signals due to 8a disappeared completely

and the 31P{1H}-NMR spectrum measured directly after addition of the acetonitrile showed

new signals due to the formation of complexes bearing the MeCN ligand (Figure 11, bottom).

The main newly formed species accounting for 74 % of the total signal intensity in the

31P{1H}-NMR spectrum gave rise to a doublet (42.8 ppm, 2P, JP-P = 42.2 Hz) and a triplet

(29.6 ppm, 1P, JP-P = 42.2 Hz). In the 1H-NMR spectrum the formate signal was still apparent

at 8.6 ppm (br) (Figure 12, bottom). FT-IR analysis of this solution at room temperature again

showed a νCO stretching mode at 1544 cm-1, consistent with the structure [Ru(2-

O2CH)(Triphos)(MeCN)]+ 8b. Interestingly, the signals of 8b decreased over a period of 5

hours at room temperature at the expense of a new doublet (47.6 ppm, 2P, JP-P = 20.6 Hz)

and triplet (5.5 ppm, 1P, JP-P = 20.6 Hz) (Figure 11, top). At the same time, the formate signal

at 8.6 ppm disappeared with concomitant formation of an upfield hydride signal (dt,

-5.5 ppm, JH-P = 105.0 Hz, JH-P = 19.3 Hz) in the 1H-NMR spectrum (Figure 12, top).


-39-

Figure 11: 31

P{1H}-NMR spectrum (121 MHz, d8-THF, r.t.) after addition of 1 eq. of HNTf2, 1 eq. of HCO2H, and 0.1 mL

acetonitrile to complex 2 in d8-THF measured directly (bottom) and again after 5 hours at room temperature (top).

Figure 12: 1H-NMR spectrum (300 MHz, d8-THF, r.t.) after addition of 1 eq. of HNTf2, 1 eq. of HCO2H, and 0.1 mL acetonitrile

to complex 2 in d8-THF measured directly (bottom) and again after 5 hours at room temperature (top).


-40-

These NMR data indicated formation of the literature known complex

[Ru(H)(MeCN)2(Triphos)]+ (9) by decarboxylation of 8b.[148] Therefore, the formation of

formate complex 8 from complex 2 could be most plausibly explained via reversible CO2-

insertion into the analogous solvent-coordinated cationic Ru-hydride complex (Scheme 16).

Scheme 16: Addition of acetonitrile to a solution of 8a in THF leads to the formation of the acetonitrile complex 8b; complex 9 is formed by decarboxylation of 8b at room temperature.

3.2.2 In situ NMR-spectroscopic investigations

Ruthenium-formate complexes are very common intermediates in the hydrogenation of CO2

to formic acid.[35, 149-150] Therefore, high-pressure NMR experiments were carried out to

probe if the formate complex 8a is a kinetically competent intermediate also in the

hydrogenation of CO2 to methanol: As described above, a solution of 8a was prepared by

adding HNTf2 (1 eq.) and HCO2H (1 eq.) to complex 2 in d8-THF. The orange solution (0.3 mL)

was transferred to a high-pressure NMR tube made of sapphire (inner volume = 0.93 mL),

and 1H-NMR and 31P{1H}-NMR spectra of this solution were recorded at room temperature.

Then, the NMR tube was pressurised with hydrogen (60 bar), and carefully heated at 140 °C

by dipping the lower 4 cm of the tube into an oil-bath. After 40 minutes at 140 °C, 1H-NMR

and 31P{1H}-NMR spectra were recorded at 80 °C in the spectrometer. Figure 13 shows both

1H-NMR spectra. The spectrum recorded directly after preparing the solution showed the

broad singlet of formate species 8a at 8.62 ppm (Figure 13, bottom). In the spectrum

recorded after heating the solution at 140 °C under H2 pressure, the formate signal had

nearly disappeared (Figure 13, top). Instead, a singlet at 3.27 ppm was observed, showing

the formation of methanol. Integration of the formate signal and the methanol signal

relative to the aromatic protons of Triphos revealed a nearly complete conversion (ca. 97 %)

of the formate-ligand to methanol. The corresponding 31P{1H}-NMR spectra are shown in

Figure 14. The spectrum recorded directly after preparing the solution showed the broad

singlet at 42.9 ppm in about 83 % of the total intensity corresponding to the formate

complex 8a (Figure 14, bottom). Besides this signal, a sharp singlet was observed at


-41-

57.0 ppm indicating the presence of a symmetric, yet unidentified phosphor containing

species (ca. 17 % of total intensity). No correlation of this signal to ligands other than Triphos

were observed in a [1H,31P]-HMBC-NMR measurement. One might speculate that this signal

belongs to a solvato complex in which three THF molecules are coordinated to the

ruthenium-Triphos fragment. In the spectrum recorded after heating the solution at 140 °C

under H2 pressure the signals due to 8a and the unknown complex had disappeared (Figure

14, top). Instead, the formation of the hydride dimer 5 in around 44 % yield was observed

besides some yet unidentified species (Figure 14, bottom).

Figure 13: 1H-NMR spectra (300 MHz, d8-THF) of the reaction mixture after addition of 1 eq. HNTf2 and 1 eq. HCO2H to

complex 2 in d8-THF measured in a high-pressure NMR tube at r.t. before pressurising with H2 (bottom), and at 80 °C after pressurising with 60 bar H2 and heating at 140 °C in an oil-bath (top).


-42-

Figure 14: 31

P{1H}-NMR spectra (121 MHz, d8-THF) of the reaction mixture after addition of 1 eq. HNTf2 and 1 eq. HCO2H to

complex 2 in d8-THF measured in a high-pressure NMR tube at r.t. before pressurising with H2 (bottom), and at 80 °C after pressurising with 60 bar H2 and heating at 140 °C in an oil-bath (top).

In summary, this experiment showed that methanol could be formed directly by

hydrogenation of the coordinated formate ligand in complex 8a (Scheme 17). No

intermediate stabilisation of the formate as alkyl formate was necessary.

Scheme 17: Hydrogenation of the formate complex 8a leads to the formation of methanol in high yield.

In situ high-pressure NMR studies were carried out to elucidate the behaviour of complex 2

directly under turnover conditions. Complex 2 (0.0125 mmol) was dissolved in d8-THF

(0.25 mL), and HNTf2 (1 eq.) dissolved in d8-THF (0.25 mL) was added to give a deep red

solution. 0.3 mL of this solution were transferred to a high-pressure NMR tube (inner volume

= 0.93 mL), and 1H-NMR and 31P{1H}-NMR spectra were recorded at 25 °C. After pressurising

with CO2 (20 bar) and H2 (60 bar) again spectra were recorded at 25 °C. The NMR tube was

heated at 80 °C by a hot air stream inside the NMR machine, and 1H-NMR spectra were

recorded directly, after 1 hour, after 2 hours, and after 4 hours. 31P{1H}-NMR spectra were


-43-

always recorded after the corresponding 1H-NMR spectra with a delay of 30 minutes due to

the time for measuring the 1H-NMR spectra and due to shimming. After cooling to room

temperature again a 1H-NMR spectrum was recorded.

The 31P{1H}-NMR spectra are depicted in Figure 15. The spectrum recorded at 0 bar

overpressure and 25 °C showed no signal due to the starting complex 2 and two new sharp

singlets at 48.6 ppm and 58.8 ppm. The sharp singlets indicated the formation of two

symmetric species. In an attempt to identify these species, a separate experiment was

carried out: Complex 2 (0.025 mmol) was dissolved in dichloromethane (DCM, 0.5 mL) and

HNTf2 (0.025 mmol, 1 eq.) in d8-THF was added at -78 °C. The deep red solution was

transferred to a NMR tube, and a [1H,31P]-HMBC-NMR experiment was carried out at -40 °C

(Figure 16). The 31P{1H}-NMR spectrum showed again mainly the formation of the two sharp

singlets at 49.4 ppm (74 % of total intensity in 31P{1H}-NMR) and 59.5 ppm (10 %).

Additionally, a singlet at 33.9 ppm (13 %) due to unreacted starting complex 2 was observed.

In the [1H,31P]-HMBC-NMR experiment no correlation of the singlet at 59.5 ppm to ligands

other than Triphos was apparent. As inferred from previous experiments (vide supra), one

might speculate that this signal belongs to a cationic solvato complex forming after complete

protonation of the TMM ligand to isobutene. The main signal at 49.4 ppm showed, besides

the coupling to the Triphos ligand (δ = 2.6 ppm, s, 6H, CH2; δ = 1.8 ppm, s, 3H, CH3), a

coupling to two further signals at 1.9 ppm (s) and 1.6 ppm (s). One might speculate that

these signals stem from a coordinated methylallyl ligand in [Ru(methylallyl)(Triphos)]+ (11),

which was reported to form upon protonation of the TMM ligand.[151] More details on this

reaction will be published in D. Limper’s PhD-Thesis.*

In the 31P{1H}-NMR spectrum recorded at 25 °C after pressurising with CO2/H2 the sharp

singlets at 48.6 ppm and 58.8 ppm had almost disappeared (Figure 15). Instead, a broad

singlet at 43.8 ppm with a shoulder was observed. After heating up to 80 °C solely this broad

singlet remained. Based on the previous investigations concerning formate complex 8a (vide

supra) this signal was assigned to 8a. In the 31P{1H}-NMR spectrum recorded after 90

minutes the formation of a sharp singlet at 43.2 ppm was observed, whose integral

increased upon longer reaction times. This signal was assigned to the dimeric complex 5

which was identified as a deactivation product earlier (vide supra). No formation of the

* Lehrstuhl für Technische Chemie und Petrolchemie, RWTH Aachen University, Germany.


-44-

second deactivation product [Ru(H)(CO)2(Triphos)] (4) was observed under these mild

conditions and short reaction time.

Figure 15: 31

P{1H}-NMR spectra (121 MHz, d8-THF) of a CO2 hydrogenation reaction carried out in a high-pressure NMR

tube. Reaction conditions: complex 2 (25 μmol/mL), HNTf2 (1 eq.), d8-THF (0.3 mL), p(CO2) = 20 bar at r.t., p(H2) = 60 bar at r.t.


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Figure 16: [1H,

31P]-HMBC spectrum (600 MHz, d8-THF, -40 °C) after addition of HNTf2 (1 eq.) to complex 2 in d8-THF/DCM.

In Figure 17 magnifications of the formate areas of the recorded 1H-NMR spectra are

depicted. Directly after pressurising the solution with CO2/H2 at room temperature a broad

singlet at 8.7 ppm appeared. Together with the 31P{1H}-NMR data formation of a certain

amount of the formate complex 8a already at room temperature seemed to be likely. The

formate signal was strongly broadened at 80 °C indicating fluctuating formate species. At the

same time a singlet at 3.29 ppm (not shown) showed the formation of methanol. After 4

hours the NMR tube was cooled to room temperature, and again a 1H-NMR spectrum was

recorded which showed the signal due to formate complex 8a at 8.7 ppm.

Figure 18 shows magnifications of the hydride areas of the recorded 1H-NMR spectra. The

spectra recorded after 1 hour and after 2 hours at 80 °C showed a very small signal at

-6.7 ppm. However, this hydride species is unlikely to play a role in the catalytic

transformation of CO2 to methanol as the signal was not observed in the spectra recorded

thereafter. In the spectra recorded after 1, 2, and 4 hours a broad hydride signal was

observed at -8.8 ppm with increasing integrals indicating the formation of dimeric species 5.


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Figure 17: 1H-NMR spectra (300 MHz, d8-THF) of a CO2 hydrogenation reaction carried out in a high-pressure NMR-tube.

Magnification of the formate area. Reaction conditions: complex 2 (25 μmol/mL), HNTf2 (1 eq.), d8-THF (0.3 mL), p(CO2) = 20 bar at r.t., p(H2) = 60 bar at r.t.


-47-

Figure 18: 1H-NMR spectra (300 MHz, d8-THF) of a CO2 hydrogenation reaction carried out in a high-pressure NMR-tube.

Magnification of the hydride area. Reaction conditions: complex 2 (25 μmol/mL), HNTf2 (1 eq.), d8-THF (0.3 mL), p(CO2) = 20 bar at r.t., p(H2) = 60 bar at r.t.

To assess the reversibility of the CO2 hydrogenation reaction to methanol the CO2/H2

pressure was released and the reaction solution was heated for 2.5 days at 80 °C. However,

no decrease of the methanol signal at 3.29 ppm could be observed in the 1H-NMR spectrum.

In summary, these NMR studies suggested that the formate complex 8a was formed directly

after pressurising a solution of complex 2 and HNTf2 with CO2/H2 at room temperature and

remained the main detectable species under reaction conditions. Throughout the reaction

no hydride species were detected which could be ascribed to active species, indicating that

the presumed cationic hydride intermediates are too short-lived to be observed on the NMR

time scale.[135] It is likely that the short-lived hydride species are rapidly and reversibly

converted to the observable formate complex 8a as resting state by CO2 insertion into the

ruthenium-hydride bond.[35, 136, 149-150]


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3.2.3 Proposal for a catalytic cycle

The experimental results described above strongly suggested that CO2 can be reduced

stepwise via the formate anion in the coordination sphere of a cationic, homogeneous

organometallic complex based on Ru-Triphos. However, as the formate complex 8a was

identified as the resting state under turnover conditions, further spectroscopic insight into

the reaction mechanism could not be obtained directly. Therefore, DFT calculations were

performed in order to get a more detailed picture of this multistep transformation. A basic

catalytic cycle for the stepwise hydrogenation of CO2 to methanol through the formic acid

and formaldehyde stages could be formulated based on previous mechanistic investigations

on the Ru-Triphos system by Klankermayer and Leitner et al.,[127] and based on recent work

on hydrogenation of CO2/CO2-derivatives,[104, 110, 152-153] formic acid decomposition,[135, 154]

formaldehyde decomposition,[155] and methanol reforming (Scheme 18):[63] A plausible

starting point of this catalytic cycle is the cationic ruthenium-hydride species 3/I, which most

likely forms from complex 2 by protonation of the TMM ligand by the acidic additive and

hydrogenolysis. Migratory insertion of CO2 results in the formation of the spectroscopically

observed formate complex (8/V). Reaction with one equivalent H2 leads to the reduction

beyond the formic acid stage to Ru-hydroxymethanolate species (IX). Via intermediate

formation of formaldehyde and consumption of a second equivalent of hydrogen IX is

transformed to the Ru-methanolate complex (XVIII). The product methanol is finally

liberated and the cycle is closed by hydrogenolysis of the Ru-OMe unit with a third

equivalent of hydrogen.


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Scheme 18: Proposed basic catalytic cycle for the hydrogenation of CO2 to methanol.

3.2.4 DFT-calculations

The DFT-calculations presented in this chapter have been performed in cooperation with

Verena Moha, Jens Kothe, and Markus Hölscher at the Institut für Technische und

Makromolekulare Chemie, RWTH Aachen University.

3.2.4.1 Calculation of the catalytic cycle

Generation of formate complex 8/V

The first step of the basic catalytic cycle is the insertion of CO2 into the Ru-H bond of

complex I to form the spectroscopically observed formate complex 8/V. This step was widely

investigated in the context of formic acid production. The system most closely resembling

the Ru-Triphos system was a ruthenium complex bearing three phosphine ligands, which

was investigated by Sakaki’s group.[152] Therefore, a similar reaction pathway was

investigated for the first step of the catalytic cycle (Figure 19, red pathway): The structure


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with the lowest energy II (9.5 kcal mol-1) is obtained by replacing the THF molecule in I by

CO2. In the next step, the hydride ligand is transferred to the carbon atom of the CO2

molecule via transition state II-III (21.2 kcal mol-1). Rotation of the Ru-O and Ru-C bonds in III

leads to the formation of the stable 2-formate IV (3.3 kcal mol-1). Subsequent exchange of

the H2 in IV by THF leads to the even more stable resting state V/8a (-3.2 kcal mol-1). This

result was in agreement with the fact that only V/8a was observed during high-pressure

NMR investigations. Moreover, the small energetic span of 21.2 kcal mol-1 between I and

V/8a was in agreement with the observation of V/8a under CO2/H2 pressure already at room

temperature.

Figure 19: Generation of the spectroscopically observed formate complex 8/V and subsequent generation of Ru-hydroxymethanolate complex IX. The energetically most favourable pathway is shown in red. The Triphos ligand is omitted for clarity, S = THF.

Generation of Ru-hydroxymethanolate complex IX

The second step of the basic catalytic cycle is the reduction of the formate beyond the

formic acid stage resulting in the Ru-hydroxymethanolate species IX (Figure 19, red

pathway). Coordinated formic acid is generated from the coordinated formate in complex

V/8a by changing the formate coordination mode from bidentate to monodentate and

association of a H2 molecule (VI, 11.8 kcal mol-1), and by subsequent proton transfer to the


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carbonyl C=O bond via a six-membered transition state (VI-VII, 13.6 kcal mol-1).[152] In the

resulting formic acid complex (VII, 12.5 kcal mol-1) the Ru-H unit is regenerated. Three other

pathways leading to VII were investigated (Figure 19, black pathways), including outer

sphere attack of CO2, but were much less energetically favourable and did not pass through

the spectroscopically observed formate complex V, making these pathways unlikely to play a

role. There was much less known about the reaction steps leading to reduction beyond the

formic acid stage.[63, 116-117, 155] For the Ru-Triphos system investigated here, formation of the

crucial Ru-hydroxymethanolate intermediate IX (16.1 kcal mol-1) could be rationalised by

exchange of the THF molecule in VII for a H2 molecule to give VIII (14.3 kcal mol-1) and

subsequent hydride transfer to the carbon atom of the formic acid via transition state VIII-IX

(29.8 kcal mol-1).

Generation of Ru-methanolate complex XVIII

The third step of the basic catalytic cycle is the reduction of the Ru-hydroxymethanolate

species IX to the Ru-methanolate complex XVIII via the formaldehyde stage. The

interconversion between free methanediol and formaldehyde has been studied.[155] Here,

the protonolysis within the coordination sphere through heterolytic cleavage of the

coordinated H2 molecule was investigated (Figure 20, the energetically most favourable

pathway is shown in red): Coordination of a THF molecule to IX results in the formation of

IXc (34.2 kcal mol-1). Via transition state IXc-XV (42.7 kcal mol-1), involving proton transfer

and C-O bond cleavage, formaldehyde complex XV (25.0 kcal mol-1) is formed directly. By

interaction with an additional THF molecule via hydrogen bonding the barrier is lowered to

40.5 kcal mol-1 (transition state IXc-XV(thf)). The other pathways shown in Figure 20 had

significantly higher energy barriers.


-52-

Figure 20: Generation of formaldehyde complex XV from methanediolate complex IX via intramolecular proton transfer. The energetically most favourable pathway is shown in red. The Triphos ligand is omitted for clarity, S = THF.

Pathways involving external medium-assisted proton transfer were also investigated (Figure

21): Carboxylate units as proton shuttles (like formic acid or acetic acid; here calculated for

the case of acetic acid, green pathway in Figure 21) may in principle lower the energy barrier

significantly to 27.1 kcal mol-1 (transition state Xa-XIa). A similar pathway using water as the

external proton shuttle gave a barrier of 41.2 kcal mol-1, which was comparable to the

barrier calculated for the intramolecular proton transfer pathways (Figure 21, blue pathway).


-53-

Figure 21: Generation of formaldehyde complex XV from methanediolate complex IX via medium-assisted proton transfer (water: blue; acetate: green). The Triphos ligand is omitted for clarity, S = THF.

The methanolate complex XVII (21.7 kcal mol-1) is formed by replacement of the THF

molecule in formaldehyde complex XV by a H2 molecule to give XVI (26.6 kcal mol-1) and

subsequent migratory hydride transfer (transition state XVI-XVII, 27.3 kcal mol-1) (Figure 22).

XVII is stabilised by an agostic C-H-Ru interaction. Association of a solvent molecule leads to

the formation of methanolate complex XVIII (22.3 kcal mol-1).

Formation of methanol and closing the catalytic cycle

The last step of the basic catalytic cycle is the formation of methanol and the closing of the

catalytic cycle by regeneration of hydride complex I (Figure 22): A proton is intramolecularly

transferred to the oxygen atom of the coordinated methanolate via the four-membered

transition state XVIII-XXIV (31.5 kcal mol-1) to give the methanol complex XXIV

(12.8 kcal mol-1). Dissociation of methanol and association of a H2 molecule regenerates

starting complex I’. I’ lies 14.1 kcal mol-1 above the reference point, indicating that the

reaction is endergonic under the boundary conditions of the calculation model. This was due

to the fact that all values shown here were computed for the gas phase, an approach

commonly applied because calculations with the inclusion of solvent effects are very time


-54-

consuming for huge organometallic complexes and gas phase calculations served sufficiently

well for the understanding of reaction mechanisms in many cases.[156] Recalculation with the

inclusion of solvent effects was therefore restricted to the net reaction, using the MN12-L

density functional in combination with the IEF-PCM (Grel(I’) = -1.9 kcal mol-1) and the CPCM

continuum model (Grel(I’) = -2.2 kcal mol-1), and the IEF-PCM additionally with a radii model

recently developed by Truhlar et al.[157] (Grel(I’) = -4.1 kcal mol-1). All results indicated the

reaction to be exergonic, in line with the experimental observations.

Figure 22: Formation of methanol and closing the catalytic cycle via methanolate complex XVIII. The Triphos ligand is omitted for clarity, S = THF.

In summary, a plausible catalytic cycle including all transition states could be found by DFT

calculations, indicating the possibility to reduce CO2 stepwise to methanol within the

coordination sphere of a single Ru-Triphos centre through a series of hydride transfer and

protonolysis steps. However, according to the energetic-span model introduced by Kozuch

and Shaik[156] complex V/8a (lying at -3.2 kcal mol-1 on the hypersurface; Figure 19)

represents the TOF determining intermediate (TDI), and IXc-XV(thf) (lying at 40.5 kcal mol-1

on the hypersurface; Figure 20) represents the TOF determining transition state (TDTS) of

the catalytic cycle. The resulting energetic span of 43.7 kcal mol-1 appeared to be high for a

reaction running at 140 °C.[158] The calculation method used (gas phase, M06-L/def2-TZVP)


-55-

was shown to give reliable results, at least for the hydrogenation of olefins.[158]

Nevertheless, recalculation of the catalytic cycle in the solvent phase might be interesting for

validation. As shown in Figure 21, external proton sources might significantly lower the

energetic span and therefore cannot be excluded to play a role in the catalytic cycle at this

stage.

3.2.4.2 The influence of the coordination geometry

As the Ru-Triphos system was the first organometallic catalyst which enabled the reduction

of CO2 to methanol within its coordination sphere, investigations concerning the influence of

the facial geometry of the Triphos ligand were carried out. A comparison of the facial

coordination with a meridional coordination could not be performed with the original

Triphos ligand because of its rigidity. Therefore, three P(Ph)2Me ligands instead of one

Triphos ligand were used to recalculate a crucial step in the catalytic cycle, the reduction of

formic acid complex VIII to methanediolate complex IX via transition state VIII-IX (Figure 23).

Complexes corresponding to the structures of VIII, IX and VIII-IX were constructed in facial

and meridional coordination mode. The meridional analogue of structure VIII was chosen as

reference point (0 kcal mol-1). The span for the facial case (Figure 23, red) was 11.7 kcal mol-1

and therefore in satisfying agreement with the barrier height of 15.5 kcal mol-1 of the

original Triphos complex, indicating that three P(Ph2)Me ligands were a suitable model for

the Triphos ligand. For the meridional case the span was 36.4 kcal mol-1 (Figure 23, black),

showing that the facial coordination of the Triphos ligand indeed was crucial for low energy

barriers.


-56-

Figure 23: Comparison of meridional with facial coordination by recalculation of the structures VIII, VIII-IX, and XI using the ligand P(Ph)2Me as model.

3.3 Hydrogenation of CO2 to MeOH in the absence of alcohol additives

Parts of this chapter have been published in: Chem. Sci. 2015, 6, 693-704.[139]

After demonstrating the possibility of CO2 reduction to methanol in the coordination sphere

of a single Ru-Triphos centre in the absence of an alcohol additive by NMR experiments and

DFT calculations further key parameters of the catalytic reaction were investigated, and the

results were analysed for consistency with the mechanistic proposal.

3.3.1 The role of the acidic additive - Development of a catalytic system with no need for

an acidic additive

Firstly, a more detailed analysis of the role of the acidic additive was carried out. In the

NMR-experiments described above the cationic formate complex 8 was identified as an

active intermediate in the catalytic transformation of CO2 to methanol. Additionally, the

DFT-calculations strongly supported that cationic species of type 3 play an important role in

the catalytic cycle. In summary, these results suggested that one role of the acid in the

catalytic system 2/HNTf2 is the generation of a cationic species as the active site upon

-2 0 2 4 6 8

10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40

DG

/ (

kcal

/mo

l)

TS

TS

fac

mer 0.0

5.6

36.4

17.3

30.7

10.5


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reductive removal of the TMM ligand. To prove this assumption, the preparation of an

isolated cationic catalyst precursor was envisaged. Attempts to isolate formate complex 8

failed due to the instability of this complex. However, synthesis of the analogous cationic

acetate complex was successful (Scheme 19): In the first step, the literature known complex

[Ru(2-OAc)Cl(Triphos)] (12) was synthesized by stirring commercially available

[Ru(2-OAc)Cl(PPh3)3] (13) together with Triphos (1 eq.) in toluene at 110 °C for 3 hours.[159]

In the second step, isolated complex 12 was stirred together with AgNTf2 (1.03 eq.) in THF or

toluene at 60 °C for 3 hours. The greyish AgCl precipitate was filtered off by passing the

yellow solution over silica. Removal of the solvent in vacuo and drying for 24 hours in vacuo

at room temperature yielded a yellow-orange powder which was analysed by 1H-, 13C-, 31P-,

and 19F-NMR, FT-IR, and ESI-HRMS.

Scheme 19: Synthesis of the acetate complex 14 (S = THF, H2O or free coordination site, depending on if and which coordinating molecules are available).

The 31P{1H}-NMR spectrum in thoroughly dried d2-DCM (r.t.) showed only one sharp singlet

at 42.5 ppm, indicating the selective formation of either a symmetrical complex or formation

of a highly dynamic complex (Figure 24). In the 1H-NMR spectrum in thoroughly dried d2-

DCM no other signals than those due to the Triphos and acetate ligands were observed

(Figure 25). These data in combination with 13C- and 19F-NMR data suggested the presence

of the cation [Ru(2-OAc)(Triphos)]+ and the anion NTf2‒. This was supported by ESI-HRMS

analysis (m/z (+) = 785.14337). FT-IR analysis of the yellow-orange powder showed very

similar absorption bands between 1400 cm-1 and 1600 cm-1 as compared to the starting

complex 12 indicating a similar 2-binding mode of the acetate ligand.[160] Yellow single

crystals suitable for X-Ray diffraction were obtained by crystallisation from DCM layered

with pentane. The X-Ray diffraction data gave the structure of complex

[Ru(2-OAc)(Triphos)(H2O)]NTf2 (14a) in which the open coordination site was saturated with

H2O from adventitious traces of water (Figure 26). Consequently, 31P{1H}-NMR spectra of 14

measured in wet d2-DCM showed splitting of the singlet at 42.5 ppm into a doublet

(41.4 ppm, JP-P = 44.5 Hz, 2P) and a triplet (43.7 ppm, JP-P = 44.5 Hz, 1P) upon cooling the


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solution to 223 K (Figure 27). However, the 1H-NMR spectrum of the dried complex in dry d2-

DCM showed no signal due to coordinated water (Figure 25), and thus the acetate complex

in solution could be formulated as [Ru(2-OAc)(Triphos)(S)]NTf2 (14) with S being either a

free coordination site or a weakly bound solvent molecule, depending on whether a

coordinating molecule is available or not.[161]

Figure 24: 31

P{1H}-NMR spectrum (243 MHz, d2-DCM, r.t.) of complex 14 in thoroughly dried d2-DCM.


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Figure 25: 1H-NMR spectrum (400 MHz, d2-DCM, r.t.) of complex 14 in thoroughly dried d2-DCM.

Figure 26: Solid state structure of the cation 14a as derived from X-ray diffraction (hydrogen atoms are omitted for clarity). Some selected bond lengths (Å): Ru‒P1 = 2.245(9); Ru‒P2 = 2.255(3); Ru‒P3 = 2.253(0); Ru‒O1 = 2.171(2); Ru‒O2 = 2.208(6); Ru‒O3 = 2.204(7).


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Figure 27: 31

P{1H}-NMR spectra (243 MHz, d2-DCM) of complex 14 measured in wet d2-DCM. Top: measured at room

temperature. Bottom: measured at 223 K. In the presence of water the coordination site in 14 is saturated by water giving

the complex [Ru(2-OAc)(Triphos)(H2O)]NTf2 (14a).

From NMR-spectroscopy and HRMS it was not possible to discriminate between monomeric

[Ru(2-OAc)(Triphos)(S)]NTf2 (14) and multinuclearic species [Ru(2-OAc)(Triphos)(S)]x[NTf2]x.

However, the discrimination was possible by using the two different Triphos derivatives

Triphos and Triphos-anisyl†[142] (1,1,1-tris{bis(4-methoxyphenyl)phosphinomethyl}ethan): An

equimolar mixture of [Ru(2-OAc)Cl(Triphos)] (12) (10.3 mg, 12.5 μmol, 1 eq.) and [Ru(2-

OAc)Cl(Triphos-anisyl)] (15) (12.5 mg, 12.5 μmol, 1 eq.) was stirred together with AgNTf2

(11.6 mg, 30 μmol, 2.4 eq.) in toluene (1.5 mL) at 60 °C for 5 hours. The toluene was

removed in vacuo and the residue dissolved in d2-DCM (0.5 mL). After passing the orange

solution through a syringe-filter to remove the AgCl precipitate the solution was analysed by

NMR-spectroscopy. The 31P{1H}-NMR spectrum showed two singlets at 43.5 and 45.3 ppm in

a ratio of nearly 1 : 1, indicating the formation of two complexes (Figure 28). The signals

were ascribed to [Ru(2-OAc)(Triphos)(S)]NTf2 (14) and [Ru(2-OAc)(Triphos-anisyl)(S)]NTf2

(16). The respective 1H-NMR spectrum confirmed the formation of 14 and 16 (Figure 29).

The absence of further signals in the 31P{1H}-NMR and 1H-NMR due to mixed multinuclearic

complex species like [Ru2(μ-OAc)2(Triphos)(Triphos-anisyl)][NTf2]2 corroborated the

monomeric structure of 14 in solution.[162]

† The ligand Triphos-anisyl was synthesised by Markus Meuresch, ITMC, RWTH Aachen University.


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Figure 28: 31

P{1H}-NMR (243 MHz, d2-DCM, r.t.) of the reaction of an equimolar mixture (12.5 μmol) of

[Ru(2-OAc)Cl(Triphos)] (12) and [Ru(

2-OAc)Cl(Triphos-anisyl)] (15) with AgNTf2 (2.4 eq.).


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Figure 29: 1H-NMR (600 MHz, d2-DCM, r.t.) of the reaction of an equimolar mixture (12.5 μmol) of [Ru(

2-OAc)Cl(Triphos)]

(12) and [Ru(2-OAc)Cl(Triphos-anisyl)] (15) with AgNTf2 (2.4 eq.).

A high-pressure NMR experiment was carried out to investigate the reactivity of acetate

complex 14 under CO2 hydrogenation conditions: Complex 14 (13.3 mg, 0.0125 mmol) was

dissolved in d8-THF (0.5 mL), and 0.3 mL of this yellow solution were transferred to a high-

pressure NMR tube (inner volume = 0.93 mL). 1H-NMR and 31P{1H}-NMR spectra were

recorded at 25 °C. After pressurising with CO2 (20 bar) and H2 (60 bar) again 1H-NMR and

31P{1H}-NMR spectra were measured at 25 °C. The NMR tube was heated at 80 °C by a hot air

stream inside the NMR machine, and 1H-NMR spectra were recorded directly, and again

after 1.5 hours. 31P{1H}-NMR spectra were always recorded after the corresponding 1H-NMR

spectra with a delay of 30 minutes due to the time for measuring the 1H-NMR and due to

shimming. The NMR tube was carefully heated for 1 hour at 140 °C in an external oil bath,

and after that again 1H-NMR and 31P{1H}-NMR spectra were measured at 80 °C and 25 °C.

Figure 30 shows the recorded 31P{1H}-NMR spectra. The 31P{1H}-NMR spectrum of the

catalyst solution at 25 °C showed the broad singlet at 42.3 ppm due to starting complex 14.

Directly after pressurising with CO2/H2 a small, broad singlet appeared at 42.9 ppm,

indicating formation of a little amount of formate complex 8a already at room temperature.

At 80 °C inside the NMR machine both signals overlapped and only one broad singlet at


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around 42 ppm was observable. No changes were observed in the spectra measured at 80 °C

after heating the solution for 2 hours at 80 °C and for 1 hour at 140 °C. Only after cooling to

25 °C again both signals at 42.3 ppm and 42.9 ppm could be observed. The signal at

42.3 ppm (about 40 % of total intensity) was related to starting complex 14. The newly

formed signal at 42.9 ppm (about 60 % of total intensity) was assigned to formate complex

8a. The formation of 8a in small amounts directly after pressurising with CO2/H2 at room

temperature and in about 60 % (according to 31P{1H}-NMR) after heating for 2 hours at 80 °C

and 1 hour at 140 °C was corroborated by the observation of a formate signal at 8.6 ppm in

the corresponding 1H-NMR spectra measured at 25 °C (Figure 31). At the same time

formation of methanol was observed (s, 3.2 ppm). In the spectra measured at 80 °C only a

broad singlet around 8.1 ppm was observed in the formate region, whereas in the last

spectrum measured after cooling to room temperature the formate signal of 8a was

observed at 8.6 ppm besides a singlet at 7.6 ppm due to traces of methyl formate. Methyl

formate can form once methanol is present in the reaction solution. However, it does not

accumulate in the reaction mixture as methyl formate is also hydrogenated to methanol

under reaction conditions (3.1.1). In the 1H-NMR spectra measured directly after pressurising

with CO2/H2 and after heating at 80 °C a very small broad hump at 10.6 ppm (not shown in

the magnification) could be observed, indicating the release of a very small amount of free

carboxylic acid into the solution. In the spectra after heating at 140 °C this signal

disappeared. Ethanol was detected in the 1H-NMR (t, 1.1 ppm) from acetate/acetic acid

hydrogenation. No hydride species was observed in any of the recorded 1H-NMR spectra,

again indicating that the presumed cationic hydride intermediates are too short-lived to be

observed on the NMR time scale.[135]

In summary, these studies demonstrated that the same formate complex 8a as observed in

the studies with complex 2/HNTf2 was formed also from acetate complex 14 in the absence

of acid or alcohol additives. Methanol was observed in solution corresponding to a TON of 5

proving that complex 14 can be used as molecularly defined precursor for the hydrogenation

of CO2 to methanol without any acidic additive.


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Figure 30: 31

P{1H}-NMR spectra (121 MHz, d8-THF) of a CO2 hydrogenation reaction carried out in a high-pressure NMR

tube. Reaction conditions: complex 14 (25 μmol/mL), d8-THF (0.3 mL), p(CO2) = 20 bar at r.t., p(H2) = 60 bar at r.t.

Figure 31: 1H-NMR spectra (300 MHz, d8-THF) of a CO2 hydrogenation reaction carried out in a high-pressure NMR tube.

Magnification of the formate area. Reaction conditions: complex 14 (25 μmol/mL), d8-THF (0.3 mL), p(CO2) = 20 bar at r.t., p(H2) = 60 bar at r.t.

Next, the performance of catalytic systems 2 and 14 in presence and absence of acid was

investigated in more detail. As suggested by the NMR studies and by DFT calculations the

hydrogenation of CO2 to methanol is possible in the absence of an alcohol additive.

Therefore, the following studies were carried out in pure THF (i.e. in the absence of any


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alcohol additive). For a more detailed analysis of the influence of the alcohol additive see

chapter 3.3.4.

Catalytic systems 2 and 14 were compared using a standard set of reaction conditions in the

absence of alcohol additives (V(THF) = 2.1 mL, c(Ru) = 12 mmol L-1, p(CO2) = 20 bar at r.t.,

p(H2) = 60 bar at r.t., T = 140 °C, t = 24 h) (Table 4). Catalyst 2 gave only a very low TON in the

absence of an acid (Table 4, entry 1). However, in the presence of an equimolar amount of

the acid HNTf2 a greatly increased TON of 228 was observed (Table 4, entry 2). Using the

cationic catalyst precursor 14 in the absence of any acidic additive a high TON of 165 was

achieved, whereas the addition of HNTf2 (0.5 eq.) did not increase the catalyst performance

(Table 4, entries 3 and 4). These results were consistent with the formation of the cationic

complex [Ru(H)(H2)(S)(Triphos)]+ (3) as catalytically active species, as suggested by DFT

calculations (3.2.4).

The lower performance of catalyst 14 compared to catalytic system 2/HNTf2 could be

explained by the more difficult hydrogenation of the acetate ligand to give the common

intermediate 8a, as observed in the in situ NMR experiments (vide supra): Whereas catalytic

system 2/HNTf2 was converted completely to formate complex 8a upon heating to 80 °C

under CO2/H2 pressure, catalytic system 14 was converted to 8a in about 60 % after 2 hours

at 80 °C and 1 hour at 140 °C.

The more difficult hydrogenation of acetic acid/acetate compared to formic acid/formate

was also evident from experiments concerning the hydrogenation of the free acids. With

catalytic system 2/HNTf2 (c(Ru) = 12.5 mmol L-1, 1 eq. HNTf2) formic acid (100 eq.) could be

fully converted to methanol at a hydrogen pressure of 60 bar (at r.t.) and a reaction

temperature of 140 °C within 24 hours (2.0 mL THF), whereas for the hydrogenation of acetic

acid (100 eq.) a reaction temperature of 180 °C was necessary to achieve full conversion. The

efficient hydrogenation of formic acid under these conditions was in line with the proposed

mechanism shown in Scheme 18.

The picture could be completed by a successful hydrogenation of paraformaldehyde

(100 eq.) to methanol using the same catalytic system 2/HNTf2 (c(Ru) = 12.5 mmol L-1, 1 eq.

HNTf2, 60 bar H2 at r.t., 2.0 mL THF, 0.2 mL H2O, 140 °C, 24 h). Interestingly, the 31P{1H}-NMR

spectrum of this reaction solution measured at 233 K showed a doublet (δ = 46.3 ppm, 2P,

JP-P = 42.4 Hz) and a triplet (δ = 43.8 ppm, 1P, JP-P = 42.4 Hz) indicating the formation of


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formate complex 8a. Formation of 8a was corroborated by the observation of a broad singlet

at 8.7 ppm in the 1H-NMR spectrum. The observation of 8a indicated reversibility of the

catalytic cycle.

Table 4: Hydrogenation of CO2 to methanol in the absence of alcohol additive: influence of the acid.[a]


pH2[c]

[bar]

pCO2[c]

[bar]

TON[d]

1 2 ‒ 60 20 8

2 2 HNTf2 (1.0) 60 20 228

3 14 ‒ 60 20 165

4 14 HNTf2 (0.5) 60 20 156

5 2 HNTf2 (0.9) 60 20 155

6 2 HNTf2 (1.5) 60 20 196

7 2 HNTf2 (2.0) 60 20 181

8 2 p-TsOH (1.0) 60 20 135

9 2 p-TsOH (1.5) 60 20 152

10 2 p-TsOH (2.0) 60 20 115

11 2 MSA (1.0) 60 20 61

12 2 MSA (1.5) 60 20 68

13 2 MSA (2.0) 60 20 20

[a] Reaction conditions: catalyst (25 µmol), THF (2.1 mL), 140 °C, 24 h; [b] equivalents to catalyst; [c] at room temperature;

[d] TON = mmol MeOH/mmol catalyst.

The lack of activity with complex 2 in the absence of an acid was investigated in more detail:

A CO2 hydrogenation was performed in a batch reactor using complex 2 in the absence of

any acidic additive and terminated after 1 hour (V(d8-THF) = 1.0 mL, c(Ru) = 25 mmol L-1,

p(CO2) = 20 bar at r.t., p(H2) = 60 bar at r.t., T = 140 °C). The 31P{1H}-NMR spectrum of the

solution showed a triplet (δ = 33.6 ppm, 1P, JP-P = 31.9 Hz) and a doublet (δ = 25.2 ppm, 2P,

JP-P = 31.9 Hz) indicating the formation of the neutral literature-known complex

[Ru(H)2CO(Triphos)] (17) in about 94 % (based on the total intensity in the 31P{1H}-NMR

spectrum) (Figure 32).[127] The formation of complex 17 was corroborated by observation of

the corresponding doublet of doublets (δ = -7.3 ppm, JH-P = 50.6 Hz, JH-P = 18.1 Hz) in the

hydride region of the 1H-NMR spectrum. Interestingly, Zanobini et al. showed that

protonation of complex 17 with strong protic acids at low temperatures leads to the

formation of [Ru(H)(H2)CO(Triphos)]+ (18), a cationic structure resembling the active hydride

species 3 which was used as the starting point for the DFT calculations of the catalytic cycle

(3.2.4).[163] Therefore, the productivity of isolated complex 17 in the presence of the strong

acid HNTf2 (1 eq.) was assessed in a batch reaction (V(THF) = 2.1 mL, c(Ru) = 6 mmol L-1,

p(CO2) = 20 bar at r.t., p(H2) = 60 bar at r.t., T = 140 °C, t = 24 h). Indeed, an active catalyst

was formed under these reaction conditions, and a TON of 256 was obtained after 24 hours,


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which was about 76 % of the TON obtained using catalyst 2/HNTf2 under the same reaction

conditions.

Figure 32: 31

P{1H}-NMR spectrum (121 MHz, d8-THF, r.t.) of the reaction solution of a CO2 hydrogenation reaction using

complex 2 in the absence of any acidic additive terminated after 1 hour. (V(d8-THF) = 1.0 mL, c(Ru) = 25 mmol L-1

, p(CO2) = 20 bar at r.t., p(H2) = 60 bar at r.t., T = 140 °C).

The reactivity of complex 17 was investigated by means of NMR spectroscopy: Addition of

HNTf2 (1 eq.) to a solution of 17 in d8-THF at room temperature led to the evolution of gas

indicating the loss of a hydride ligand by protonation to H2. In the 31P{1H}-NMR spectrum

three doublets of doublets appeared (δ = 51.4 ppm, 1P, JP-P = 41.0, JP-P = 20.9 Hz; δ =

20.5 ppm, 1P, JP-P = 41.0, JP-P = 28.7 Hz; δ = 7.8 ppm, 1P, JP-P = 28.7, JP-P = 20.9 Hz) indicating

the formation of a Ru-Triphos species with three more different ligands filling up the

coordination sphere (Figure 33). In the corresponding 1H-NMR spectra the hydride signal due

to 17 disappeared and a pseudo doublet of triplets appeared instead (δ = -5.5 ppm, JH-P =

88.0, JH-P = 17.7 Hz). These data suggested the formation of the cationic complex

[Ru(H)CO(Triphos)(S)]+ (19).

With this reaction solution a CO2 hydrogenation reaction was performed (V(d8-THF) =

1.0 mL, c(Ru) = 25 mmol L-1, p(CO2) = 20 bar at r.t., p(H2) = 60 bar at r.t., T = 140 °C, t = 1 h),

and the reaction solution was analysed by 1H- and 31P{1H}-NMR at 233 K. Once again,

formation of formate complex 8a was evident from the characteristic signals in the 31P{1H}-

NMR and 1H-NMR spectra, indicating that CO dissociated from complex 19. However,


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association of two CO ligands led to formation of the inactive biscarbonyl complex 4, which

could not be reactivated (see chapter 3.3.3). The fact that already one equivalent of CO was

present in the reactor when carbonyl complex 17 was used as catalyst precursor explained

the 24 % decrease in TON, as catalyst deactivation by formation of 4 happened faster.

In summary, these studies supported that cationic species play a major role in the catalytic

transformation of CO2 to methanol. Using neutral catalyst precursor 2 led to formation of

the inactive neutral carbonyl complex 17, whereas using neutral catalyst precursor 2 in the

presence of HNTf2 (1 eq.) or using cationic catalyst precursor 14 in the absence of acid led to

the formation of the active intermediate 8a.

Figure 33: 31

P{1H}-NMR spectra (162 MHz, d8-THF, r.t.) of complex 17 in d8-THF before (top) and after (bottom) the addition

of 1 eq. of HNTf2 at room temperature (S = solvent).

Next, the influence of the amount of HNTf2 added to complex 2 was investigated (Table 4,

entries 2, 5-7). The maximum TON of 228 was observed at a 1 : 1 molar ratio, supporting the

fact that the proton was required in stoichiometric amounts for reductive removal of the


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TMM ligand in 2 leading to 3. With only 0.9 equivalents of HNTf2 a lower TON of 155 was

achieved. Increasing the amount of HNTf2 to 1.5 and 2.0 equivalents led to lower TONs

(calculated from the amount of methanol formed) of 196 and 181, respectively. One

explanation could be formation of dimethyl ether (DME) from consecutive etherification of

methanol in the presence of excess acid. As DME is a gas at room temperature (boiling point

= -24 °C) it was not observable in the reaction solution after depressurising the reactor.

Therefore, high-pressure NMR experiments were carried out. However, one has to bear in

mind that also in these experiments formed products might partition between gas and liquid

phase and only the liquid phase was analysed by NMR spectroscopy.

In a first experiment, MeOH (81 mg, 2.5 mmol) and HNTf2 (3.5 mg, 0.0125 mmol) were

dissolved in d8-THF (1 mL), and 0.5 mL of this solution were transferred to a high-pressure

NMR tube (inner volume = 0.93 mL). The solution was heated at 140 °C in an external oil

bath for 5 hours, and afterwards quantitative 1H-NMR and 13C{1H}-NMR spectra were

recorded. In the 1H-NMR spectrum formation of DME was evident from a singlet at

3.19 ppm. This was supported by formation of a singlet at 58.4 ppm in the 13C{1H}-NMR

spectrum. As the singlet due to MeOH (3.24 ppm) overlapped with the singlet due to DME

(3.19 ppm) in the 1H-NMR spectrum quantification was done based on the integrals found in

the 13C{1H}-NMR spectrum. According to the integral ratio, conversion of methanol to DME

was about 37 %. This result proved that DME indeed easily formed under reaction conditions

in the presence of excess acid. A CO2 hydrogenation reaction using complex 2 in the

presence of 1 equivalent HNTf2 was performed inside the high-pressure NMR tube (V(d8-

THF) = 0.3 mL, c(Ru) = 25 mmol L-1, p(CO2) = 10 bar at r.t., p(H2) = 30 bar at r.t., T = 140 °C, t =

24 h). Again, quantitative 1H-NMR and 13C{1H}-NMR spectra were recorded. According to the

integral ratio in the 13C{1H}-NMR spectrum a small amount (about 3.7 %) of the formed

methanol had reacted to DME in a consecutive reaction (Figure 34, top). This was probably

caused by a very small excess of HNTf2 in the reaction solution. A second CO2 hydrogenation

experiment using complex 2 was performed in the presence of 2 equivalents HNTf2. In this

experiment about 46 % of the formed methanol had reacted to DME (according to the

integral ratio in the 13C{1H}-NMR spectrum) clearly showing that DME formation was

catalysed by excess acid (Figure 34, bottom).


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Figure 34: 13

C{1H}-NMR spectra (75 MHz, d8-THF, r.t.) of reaction solutions of CO2 hydrogenation experiments inside a high-

pressure NMR tube using complex 2 in the presence of 1 eq. HNTf2 (top) and in the presence of 2 eq. HNTf2 (bottom) (V(d8-THF) = 0.3 mL, c(Ru) = 25 mmol L

-1, p(CO2) = 10 bar at r.t., p(H2) = 30 bar at r.t., T = 140 °C, t = 24 h).

The effect of other acidic additives, namely MSA and p-toluenesulfonic acid monohydrate (p-

TsOH), on the hydrogenation of CO2 with complex 2 was investigated (Table 4, entries 8-13).

The highest TONs were observed in the presence of a slight excess (1.5 eq.) of the respective

sulfonic acid. However, both acids led to lower TONs as compared to HNTf2. The obtained

maximum TONs were well in line with the expected coordination ability of the acid anions

according to their size and charge distribution (HNTf2 < p-TsOH < MSA) corroborating the

assumption of cationic complexes of type 3 as active catalytic species. In contrast to the

NTf2- anion, the anions of p-TsOH and MSA could coordinate to the ruthenium centre and

therefore interfere with the formation of active catalytic species of type 3 or 8a.[125]


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To prove this assumption, a CO2 hydrogenation experiment using catalyst 2 and p-TsOH

(1.5 eq.) was carried out (V(d8-THF) = 1.0 mL, c(Ru) = 25 mmol L-1, p(CO2) = 20 bar at r.t.,

p(H2) = 60 bar at r.t., T = 140 °C). After a reaction time of one hour the reaction was

terminated, and the reaction solution was analysed by NMR at 233 K. The 31P{1H}-NMR

spectrum showed the formation of 5 phosphor-containing species (Figure 35). The major

component in solution was the common formate intermediate 8a (ca. 52 % based on the

integral ratios in the 31P{1H}-NMR spectrum). The already known deactivation product 4 was

also formed in about 15 %. Interestingly, no formation of the inactive hydride dimer 5 was

observed, indicating that formation of 5 was supressed by the presence of additional ligands

in solution. Compared to the spectrum recorded of a reaction using complex 2 together with

HNTf2 (Figure 6, chapter 3.2.1) some new signals were observed:

The triplet (δ = 40.5 ppm, JP-P = 47.5 Hz, 1P) and doublet (δ = 36.5 ppm, JP-P = 47.5 Hz, 2P)

were assigned to [Ru(p-TsO)2(Triphos)] (20). This assignment was supported by the

independent generation of 20 by stepwise addition of 2 equivalents of p-TsOH to 2 in THF.

The 31P{1H}-NMR spectra of this stepwise addition are shown in Figure 36. Addition of 1

equivalent of p-TsOH to complex 2 led to the formation of a broad singlet at 38.7 ppm,

which split up into the same set of doublet and triplet as was assigned to 20 when measured

at 233 K. At the same time, the signal due to starting complex 2 decreased. After the

complete addition of 2 equivalents p-TsOH the signal due to complex 2 had disappeared

completely and the selective formation of 20 was observed. The assignment was further

supported by mass spectrometry (FAB (+): m/z = 1067.2).

Besides the signals due to complex 20 the 31P{1H}-NMR spectrum in Figure 35 indicated the

formation of two different complexes of the type [Ru(A)(B)(C)(Triphos)] (with A, B, and C

being different ligands) by two sets of three correlating doublets of doublets (assigned as a

and b in Figure 35) (a: δ = 54.2 ppm, JP-P = 42.4 Hz, JP-P = 17.2 Hz, 1P; δ = 14.7 ppm, JP-P =

42.4 Hz, JP-P = 30.1 Hz, 1P; δ = 0.98 ppm, JP-P = 30.1 Hz, JP-P = 17.2 Hz, 1P; b: δ = 51.2 ppm, JP-P

= 42.2 Hz, JP-P = 19.6 Hz, 1P; δ = 19.6 ppm, JP-P = 42.4 Hz, JP-P = 29.3 Hz, 1P; δ = 5.0 ppm, JP-P =

29.3 Hz, JP-P = 19.6 Hz, 1P). The signals due to a could be correlated to a doublet of doublets

of doublets in the hydride region (δ = -5.3 ppm, JH-P = 94.6 Hz, JH-P = 19.5 Hz, JH-P = 13.6 Hz) by

a [1H,31P]-HMBC-NMR experiment, indicating that one of the three ligands in a is a hydride.

In the same fashion the signals due to b could be correlated to a pseudo doublet of triplets


-72-

in the hydride region (δ = -5.6 ppm, JH-P = 88.1 Hz, JH-P = 17.1 Hz). Other ligands in a and b

might be CO, p-TsOH, THF, or H2O.

In summary, these experiments demonstrated that the presence of even weakly-

coordinating anions such as p-TsO- in the reaction mixture hampered the formation of the

formate complex 8a, explaining why HNTf2 was the preferred acidic additive.

Figure 35: 31

P{1H}-NMR spectrum (243 MHz, d8-THF, -40°C) of the reaction solution of a CO2 hydrogenation reaction using

complex 2 in the presence of p-TsOH (1.5 eq.) (V(d8-THF) = 1.0 mL, c(Ru) = 25 mmol L-1

, p(CO2) = 20 bar at r.t., p(H2) = 60 bar at r.t., T = 140 °C, t = 1 h).


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Figure 36: 31

P{1H}-NMR spectra (162 MHz, d8-THF, r.t.) of the stepwise addition of p-TsOH to complex 2 in d8-THF at room

temperature.

3.3.2 Parameter variations

In the previous chapters catalytic system 2/HNTf2 (1 eq.) was identified as the most effective

catalyst precursor for the hydrogenation of CO2 to methanol. Next, the influence of key

parameters (like catalyst concentration, H2- and CO2-pressures, reaction temperature, and

reaction time) on the TON was investigated to gain an idea of suitable starting parameters

for later catalyst-recycling and continuous-flow experiments.

First, the catalyst concentration was varied using a standard set of reaction parameters

(V(THF) = 2.08 mL, p(CO2) = 20 bar at r.t., p(H2) = 60 bar at r.t., T = 140 °C, t = 24 h). By

varying the acid concentration in parallel a stoichiometric ratio of complex 2 to HNTf2 of 1 : 1

was maintained. Reducing the concentration of 2/HNTf2 from 12 mmol L-1 to 6 mmol L-1 and

3 mmol L-1 led to a huge increase in the obtained TONs from 228 to 335 and 442,

respectively (Figure 37). Decreasing the concentration to 1.5 mmol L-1 led to a further

smaller increase of the TON to 489. This observation was in line with the formation of the

inactive dimeric complex 5 which was identified as deactivation product in chapter 3.2.1,

and corroborated that the active catalyst species contains a single ruthenium centre.

Another effect greatly contributing to the increasing TON with decreasing catalyst

concentration resulted from the pressure drop throughout the reaction. As the reactions

were performed in a closed reaction system the pressure dropped significantly due to the


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consumption of CO2 and H2 (e.g. from ca. 120 bar at 140 °C to about 40 bar at 140 °C in the

case of the highest concentration 12 mmol L-1). At a halved catalyst concentration only half

the amount of CO2 and H2 had to be converted to MeOH to obtain the same TON, i.e. the

pressure drop at a comparable TON was significantly lower. As lower pressures significantly

slowed down the reaction (vide infra) higher TONs were observed at lower catalyst

concentrations.

Figure 37: TON as a function of the concentration of complex 2/HNTf2 (1 : 1) (V(THF) = 2.1 mL, p(CO2) = 20 bar at r.t., p(H2) = 60 bar at r.t., T = 140 °C, t = 24 h).

Decreasing the reaction temperature to 120 °C, 100 °C, and 80 °C resulted in reduced TONs

of 169, 67, and 24, respectively (Figure 38). From thermodynamics a higher equilibrium

concentration of methanol would be expected at lower temperatures, showing that the

reaction was controlled rather by kinetics than by thermodynamics in the investigated

temperature range. According to Arrhenius a bigger increase in the obtained TONs with

increasing temperature would be expected. However, as deduced from high-pressure NMR

experiments (vide supra) the formation of the main catalyst deactivation product 4 also

increased with increasing reaction temperature, limiting the positive effect of higher

reaction temperatures.

0

100

200

300

400

500

12 6 3 1,5

228

335

442 489

TON

Concentration of complex 2/HNTf2 (1 : 1) / mmol L-1


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Figure 38: TON as a function of the reaction temperature using complex 2/HNTf2 (1 : 1) (c(Ru) = 12 mmol L-1

, V(THF) = 2.1 mL, p(CO2) = 20 bar at r.t., p(H2) = 60 bar at r.t., t = 24 h).

Pressure variation at a constant stoichiometric ratio of p(CO2)/p(H2) = 1/3 from 40 bar to

80 bar, and to 120 bar (all pressures at r.t.) resulted in a strong increase of the resulting TON

from 78 to 228, and to 367, respectively (Figure 39). Higher pressures have a positive effect

on the equilibrium concentration of methanol as liquid products are formed from gaseous

reactants. However, temperature variation showed that the reaction was controlled rather

by kinetics than by thermodynamics (vide supra). Therefore, the higher TONs most probably

resulted from higher reaction rates, which in turn resulted from higher reactant

concentrations in the liquid catalyst phase at higher pressures.

Figure 39: TON as a function of the total pressure at a constant ratio of p(CO2)/p(H2) = 1/3 using complex 2/HNTf2 (1 : 1) (c(Ru) = 12 mmol L

-1, V(THF) = 2.1 mL, p(CO2) = 20 bar at r.t., p(H2) = 60 bar at r.t., T = 140 °C, t = 24 h).

0

50

100

150

200

250

80 100 120 140

24

67

169

228

TON

reaction temperature / °C

0

100

200

300

400

40 80 120

78

228

367

TON

total pressure at r.t. / bar


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Next, the H2 partial pressure was varied at a constant CO2 pressure of 20 bar (at r.t.) (Figure

40). Increasing the H2 pressure from 60 bar (at r.t.) to 80 bar (at r.t.) led to an increase in

TON by 32 % to 301. A further increase in H2 pressure to 100 bar (at r.t.) led to an increase in

TON by 16 % to 348. In the latter case the conversion of CO2 to methanol was about 40 %, as

determined from the initially charged amount of CO2 (22.1 mmol, determined by weight)

and the amount of methanol formed (8.7 mmol). Interestingly, increasing the CO2 pressure

at a constant H2 pressure of 60 bar did not lead to increased TONs (Figure 41). Increasing the

CO2 pressure from 20 bar to 30 bar did not affect the TON, whereas increasing the pressure

further to 40 bar led to a slight decrease of the TON by about 10 % to 204.

Figure 40: TON as a function of the H2 pressure (at r.t.) at a constant CO2 pressure of 20 bar (at r.t.) using complex 2/HNTf2 (1 : 1) (c(Ru) = 12 mmol L

-1, V(THF) = 2.1 mL, T = 140 °C, t = 24 h).

Figure 41: TON as a function of the CO2 pressure (at r.t.) at a constant H2 pressure of 60 bar (at r.t.) using complex 2/HNTf2 (1 : 1) (c(Ru) = 12 mmol L

-1, V(THF) = 2.1 mL, T = 140 °C, t = 24 h).

0

100

200

300

400

60 80 100

228

301 348

TON

H2 pressure at r.t. / bar

0

50

100

150

200

250

20 30 40

228 226 204

TON

CO2 pressure at r.t. / bar


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From the calculated catalytic cycle (see chapter 3.2.4) it became clear that the rate

determining states were the TOF determining intermediate (TDI) 8a/V and the TOF

determining transition state (TDTS) IXc-XV(thf). Only concentrations of the reactants

involved in the steps between the TDI and the TDTS accelerate or inhibit the reaction.[156] As

no CO2 is involved in these steps but H2 is, the increase of the CO2 pressure did not show an

accelerating effect whereas the increase of the H2 pressure did. This was also in line with the

observations during high-pressure NMR experiments (see chapter 3.2.2) in which the

formation of 8a already occurred at mild reaction conditions (25 °C, 20 bar CO2, 60 bar H2)

whereas the transformation of 8a to methanol involving three equivalents of H2 needed

higher temperatures of at least 80 °C. However, this could not explain the decrease in TON

at the highest CO2 pressure of 40 bar (at r.t.). This might be explained by the phenomenon of

gas expansion of liquids by CO2.[164-165] Many properties of the liquid, like dielectric constant,

diffusion rate, viscosity, and hydrogen solubility, are changed by dissolved CO2. Diffusion

rates and H2 solubility are increased by dissolved CO2 which should lead to higher TONs.

However, Jessop et al. investigated the influence of gas pressures on the hydrogenation of

CO2 to formic acid and found that expanding the liquid MeOH/NEt3 phase by adding the

unpolar inert gas ethane led to decreased reaction rates.[165] When they repeated the same

experiment using the more polar fluoroform instead of ethane no decrease in the reaction

rate was observed. The authors concluded that ethane rendered the liquid phase less polar,

thereby causing the reaction rate to drop. Similarly, dissolving the unpolar CO2 in most

organic liquids leads to decreased Kamlet-Taft parameters π*.[164, 166] For example, methanol

had a π*-value of 0.57 (40 °C) at 1 bar CO2 pressure which decreased to 0.47 (40 °C) upon

pressurising with 41.4 bar CO2.[167] The same trend was observed for acetone showing a π*-

value of 0.70 (40 °C) at 1 bar CO2 pressure and of 0.52 (40 °C) at 41.4 bar CO2 pressure. It is

likely that this trend is also valid for THF, giving a possible explanation for the lower

observed TON of 204 at a high CO2 pressure of 40 bar (at r.t.).

Indeed, comparison of solvents with different polarities (π*-values) and basicities (ß-values)

showed a detrimental effect of lower polarities on the observed TONs: The polarities

(π*-values) of the tested solvents ranked THF > 1,4-dioxane ≈ 2-MTHF > toluene.[168] The

basicities (ß-values) ranked 2-MTHF > THF > 1,4-dioxane > toluene.[168] The observed TONs

decreased in the order of decreasing polarities from 228 in THF to 194 in 1,4-dioxane, to 186

in 2-MTHF, and to 11 in toluene. Especially the comparison between THF and 2-MTHF


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showed that the π*-value played a more important role than the ß-value for the outcome of

the reaction. Whereas the reaction solution with THF was yellow and clear after the

reaction, the reaction solutions with 1,4-dioxane and 2-MTHF were yellow and contained a

small amount of a red precipitate which was identified as the hydride dimer 5. In toluene

even more of this precipitate was observed (as judged by the naked eye) and the remaining

liquid was only slightly yellow.

In summary, these results suggested that solvents with high enough polarities and basicities

are necessary to stabilise the active catalyst species in solution. From the computed catalytic

cycle (chapter 3.2.4) it became clear that solvent molecules play an important role as labile

ligands in many transition states and intermediates. Therefore, the strong influence of the

solvent on the catalyst productivity was not surprising.

Figure 42: TON as a function of the used solvent using complex 2/HNTf2 (1 : 1) (c(Ru) = 12 mmol L-1

, V(THF) = 2.1 mL, p(CO2) = 20 bar at r.t., p(H2) = 60 bar at r.t., T = 140 °C, t = 24 h).

A TON/time profile of the CO2 hydrogenation to methanol with catalyst 2/HNTf2 (1 : 1) in

THF was mapped out by performing batch reactions with different reaction times (c(Ru) =

6 mmol L-1, V(THF) = 2.1 mL, p(CO2) = 20 bar at r.t., p(H2) = 60 bar at r.t., T = 140 °C) (Figure

43). No pronounced induction period could be observed, and after 1 hour already a TON of

70 was obtained, corresponding to a turnover frequency (TOF) of 70 h-1. This activity was –

per active site – well in the range of the activity of heterogeneous Cu/ZnO-based methanol

catalysts.[69] However, the TOF dropped to around 30 h-1 in the second hour. Methanol

formation continued reaching a TON of 258 after 16 hours. Up to this point the consumption

of reaction gases led to a pressure drop from initially 120 bar to 72 bar. The reaction slowed

0

50

100

150

200

250 228

194 186

11

TON


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down further to give a TON of 440 (c(MeOH) = 2.29 mol L-1) after 48 hours. At this point, the

reaction stopped and no increase in the TON was observed after 72 hours (TON = 439). To

check if this was due to catalyst deactivation, due to reaching the thermodynamic

equilibrium, or due to the kinetic effect of the low remaining reactant gas pressures, an

experiment was carried out as follows: The reactor was repressurised after 72 hours to the

initial pressure with p(CO2)/p(H2) = 1/3 and heated for another 24 hours. Again, no increase

in the TON was observed (TON = 443), indicating that the catalyst was deactivated at this

point. In Figure 44 31P{1H}-NMR spectra of three different reactions terminated after 1 hour,

24 hours, and 72 hours are depicted. The spectra showed the complex species as discussed

above (chapter 3.2.1). The broad singlet due to the active formate intermediate 8a

decreased strongly over time whereas the signals due to the inactive dimer 5 and the

inactive carbonyl complex 4 increased. Whereas the amount of dimer 5 did not increase

further after 24 hours, the amount of carbonyl complex 4 strongly increased. After 72 hours

nearly all of the formate complex 8a was converted to the inactive species 5 and 4,

supporting that the catalyst was deactivated after 72 hours and supporting that 5 and 4 are

the only products of catalyst deactivation.

It is likely that the pronounced catalyst deactivation was due to an increased rate of the

deactivation reactions relative to the rate of the productive catalytic cycle. To avoid slowing

down of the methanol formation due to the pressure drop, reactions were conducted with

intermittent repressurisation to the initial pressure with p(CO2)/p(H2) = 1/3 (red triangle, red

circle, and red diamond in Figure 43). In the reaction terminated after 32 hours the reactor

was repressurised to the initial pressure (80 bar at r.t.) after 16 hours reaction time, leading

to a TON of 478. In the reaction terminated after 48 hours the reactor was repressurised to

the initial pressure after 16 hours reaction time and again after 32 hours total reaction time,

yielding a TON of 603. Finally, in the reaction terminated after 64 hours, the reactor was

repressurised after 16 hours, again after 32 hours, and again after 48 hours, yielding a TON

of 895 (n(MeOH) = 11.2 mmol, m(MeOH) = 0.36 g, V(MeOH) = 0.45 mL, c(MeOH) =

4.09 mol L-1). This TON was twice as high as the TON obtained after the 72 hours reaction

time without intermediate repressurisation, showing that the catalyst was indeed much

more stable under nearly isobaric reaction conditions.

The absence of an induction period indicated that the formed methanol accumulating in

solution did not enhance the reaction rate under these reaction conditions. However, this


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observation did not exclude the possibility that the reaction partly proceeded via a cascade

reaction involving methyl formate as intermediate. In fact, it is very likely that the reaction

proceeded in parallel partly via the cascade reaction, as traces of methyl formate were

found in high-pressure NMR experiments (3.3.1) which could easily be hydrogenated to

methanol with catalyst 2/HNTf2 (3.1.1). More details on this topic are given in chapter 3.3.4.

Figure 43: TON as a function of time using complex 2/HNTf2 (1 : 1) in THF (c(Ru) = 6 mmol L-1

, V(THF) = 2.1 mL, p(CO2) = 20 bar at r.t., p(H2) = 60 bar at r.t., T = 140 °C) as obtained from batch reactions terminated at the given reaction times. (■): Batch reactions without repressurisation. (▲): The autoclave was repressurised to the initial pressure with p(CO2)/p(H2) =

1/3 after 16 h. (●): The autoclave was re-pressurised after 16 h and again after 32 h. (♦): The autoclave was re-pressurised

after 16 h, after 32 h, and after 48 h.

0 10 20 30 40 50 60 70 800

100

200

300

400

500

600

700

800

900

1000

TON

reaction time / hours


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Figure 44: 31

P{1H}-NMR spectra of the reaction solution of three different CO2 hydrogenation reactions using complex

2/HNTf2 (1 : 1) in d8-THF which were terminated after different reaction times without intermediate repressurisation

(p(CO2) = 20 bar at r.t., p(H2) = 60 bar at r.t., T = 140 °C). 4 = [Ru(H)(CO)2(Triphos)]+, 5 = [Ru2(-H)2(Triphos)2], 6 =

[Ru2(Cl)3(Triphos)2]+, 8a = [Ru(

2-O2CH)(Triphos)(THF)].

3.3.3 Investigations concerning catalyst deactivation

In the chapters above it was demonstrated that the hydride dimer 5 and the cationic

carbonyl complex 4 were the only products of catalyst deactivation, and that catalyst

deactivation was pronounced at lower pressures (i.e. slower rates of methanol formation).

After 72 hours reaction time, the 31P{1H}-NMR spectrum of a CO2 hydrogenation reaction

without intermediate repressurisation using complex 2/HNTf2 (1 : 1) in d8-THF (c(Ru) = 6

mmol L-1, p(CO2) = 20 bar at r.t., p(H2) = 60 bar at r.t., T = 140 °C) showed that 65 % of the

catalyst precursor were converted to 4 and 17 % were converted to 5, indicating that the

formation of 4 by decarbonylation reactions was the most distinctive deactivation pathway.

However, no free CO was detectable in a GC analysis of the gas phase (vide supra). Possible

decarbonylation reactions are: 1) decarbonylation of formaldehyde intermediates in the

catalytic cycle like XV,[127, 141] 2) decarbonylation of formic acid intermediates in the catalytic

cycle like VII and VIII,[169-170] 3) decarbonylation of the methanol product,[141] and 4)

decarbonylation of methyl formate traces formed in a side reaction as soon as methanol is

present.[171]


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Reactions 1) and 2) might be supressed by higher H2 pressures, as further hydrogenation of

these formaldehyde and formic acid intermediates is favoured under these conditions.

Indeed, higher H2-pressures showed greatly enhanced TONs (3.3.2). High-pressure NMR

experiments also indicated that decarbonylation was much less pronounced at 80 °C

compared to 140 °C (3.2.2), however, at the same time the rate of methanol formation

decreased strongly giving much lower TONs (3.3.2). Reaction 2) might also be supressed by

the presence of water, as CO is the anhydride of formic acid. However, in an CO2

hydrogenation experiment using complex 2/HNTf2 (1 : 1) with addition of water (0.2 mL,

500 eq.) under otherwise standard conditions (V(THF) = 2.0 mL, c(Ru) = 12 mmol L-1, p(CO2) =

20 bar at r.t., p(H2) = 60 bar at r.t., T = 140 °C, t = 24 h) no increased TON (221) as compared

to an experiment in the absence of water (TON = 228) was observed. The 31P{1H}-NMR

spectrum of this reaction solution showed again the formation of formate complex 8,

hydride dimer 5, and carbonyl complex 4, proving at the same time the stability of the

catalytic system 2/HNTf2 towards water. Reactions 3) and 4) might be supressed by the

removal of methanol, which is easily achieved in the envisaged continuous-flow process

(3.4.3).

As mentioned above, a reaction using carbonyl complex 4 as catalyst precursor resulted in a

TON of only 9 (V(THF) = 2.1 mL, c(Ru) = 6 mmol L-1, HNTf2 (1 eq.), p(CO)2 = 20 bar at r.t., p(H2)

= 60 bar at r.t., T = 140 °C, t = 24 h). An experiment in ethanol/THF under otherwise

unchanged conditions gave a TON of 4, indicating that the possible cascade reaction via

methyl formate was also not efficiently catalysed by 4. Next, experiments were conducted to

check the possibility of reactivating 4 in a convenient way: Increasing the reaction

temperature to 200 °C resulted in an only slightly increased TON of 12 (V(THF) = 2.1 mL,

c(Ru) = 6 mmol L-1, HNTf2 (1 eq.), p(CO)2 = 20 bar at r.t., p(H2) = 60 bar at r.t., t = 24 h).

Addition of HNTf2 (1 eq.) to complex 4 at 140 °C and otherwise same conditions did not

result in the formation of methanol. Removal of CO by oxidation to CO2 with

trimethylamine-N-oxide was unselective.‡

In summary, these experiments suggested that the decarbonylation reaction cannot be

avoided completely by simple tuning of the reaction conditions. Also no easy method for

reactivating 4 was found. However, enhancing the methanol formation rate relatively to the

‡ Personal communication by Markus Meuresch, ITMC, RWTH Aachen.


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rate of catalyst deactivation by higher and constant H2- and CO2-pressures proved to be an

effective way to achieve greatly enhanced TONs.

Formation of the dimeric complex 5, representing the second, less pronounced deactivation

reaction, was usually observed with Ru/Triphos complexes during reduction reactions under

neutral (i.e. in the absence of acidic additive) or cationic (i.e. in presence of acidic additive)

conditions.[125, 129, 163] The mechanism of formation and the structure of 5 remain as yet

unclear (see chapter 3.2.1). However, as the formation of the dimeric complex 5 clearly

involves a bimolecular reaction, this deactivation pathway might be supressed by working at

lower catalyst concentrations. As mentioned in chapter 3.3.2, lowering the catalyst

concentration indeed led to increased TONs. However, this effect superimposed the effect of

a smaller pressure drop at lower catalyst concentrations, making a quantification of the

concentration effect impossible. Moreover, low catalyst concentrations are generally

undesirable as they lower the space-time-yield. As it is likely to assume that the formation

mechanism of 5 involves two coordinatively unsaturated Ru-Triphos fragments,[163] it was

tested if formation of 5 could be avoided by additional ligands in solution. A CO2

hydrogenation experiment using complex 2/HNTf2 (1 : 1) under standard reaction conditions

but in the presence of 1 equivalent triphenylphosphine (TPP) yielded a lower TON of 164 as

compared to the reaction in the absence of TPP (TON = 228) (V(THF) = 2.1 mL, c(Ru) =

12 mmol L-1, HNTf2 (1 eq.), p(CO)2 = 20 bar at r.t., p(H2) = 60 bar at r.t., T = 140 °C, t = 24 h).

In Figure 45 the 31P{1H}-NMR spectra of the reactions in absence (top) and presence

(bottom) of TPP are displayed. In the spectrum of the reaction in the presence of TPP clearly

the formation the formate intermediate 8 was observed as broad singlet at 42.0 ppm. This

assignment was confirmed by a correlation of this signal with a formate signal at 8.8 ppm in

a [1H,31P]-HMBC-NMR experiment and was in line with the observed formation of methanol.

Interestingly, in the presence of TPP indeed no formation of the hydride dimer 5 was

observed in the 31P{1H}- and 1H-NMR spectra. Additionally, formation of carbonyl complex 4

was also supressed. However, two sets of doublets of doublets and two sharp singlets

indicated the formation of four yet unidentified phosphor containing species. A more

detailed investigation of the influence of coordinating additives on catalyst deactivation

might be worthwhile.


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Figure 45: 31

P{1H}-NMR spectra (162 MHz, d8-THF, r.t.) of reaction solutions of CO2 hydrogenation reactions using complex

2/HNTf2 (1 : 1) in the absence of TPP (top) and in the presence of 1 eq. TPP (bottom) (V(d8-THF) = 1.0 mL, c(Ru) = 12.5 mmol L

-1, HNTf2 (1 eq.), p(CO)2 = 20 bar at r.t., p(H2) = 60 bar at r.t., T = 140 °C, t = 24 h).

Another possibility to avoid the dimerisation reaction would be to employ a Triphos-

derivative with a higher steric hindrance compared to Triphos. In their studies concerning

hydrogenation reactions with Ru/Triphos catalytic systems, M.Meuresch et al. found that

the dimerisation reaction could be efficiently supressed if a more bulky Triphos-xylyl

derivative (1,1,1-tris(bis(3,5-dimethylphenyl)phosphinomethyl)ethane) was used instead of

Triphos.[142-143] However, [Ru(TMM)(Triphos-xylyl)] (21)§/HNTf2 (1 : 1) did not show an

increased TON (385) compared to the reaction with [Ru(TMM)(Triphos)] (2) under the same

reaction conditions (393)**, indicating that electronic and steric effects superimposed the

effect of avoiding dimerisation (V(THF) = 2.1 mL, c(Ru) = 6 mmol L-1, HNTf2 (1 eq.), p(CO)2 =

20 bar at r.t., p(H2) = 60 bar at r.t., T = 140 °C, t = 24 h).

§ This complex was synthesised by M. Meuresch, ITMC, RWTH Aachen University, Aachen.

** The value given here (393) is higher than the value stated in chapter 3.3.2 for the same reaction conditions.

Here, a hot plate stirrer equipped with an aluminium cone was used for heating of the autoclave instead of an oil bath, allowing faster heating to reaction temperature and more efficient heat transfer. To retain comparability, only experiments were compared throughout this thesis, which were performed using the same heating system. For clarification, it was always indicated if an aluminium cone was used.


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3.3.4 Comparison of the reaction in presence of alcohol additive with the reaction in

absence of alcohol additive

In this chapter, the question if the observed formation of methanol proceeded via a cascade

reaction via alkyl formate intermediates or via direct CO2 hydrogenation within the

coordination sphere of the Ru-Triphos fragment is discussed.

There were several hints that the reaction proceeded partly via a cascade reaction as soon

as alcohol was present in the reaction solution: In the initial studies using catalyst 2/acid in a

mixture of ethanol (0.58 mL) and THF (1.5 mL) as solvent, clearly the formation of ethyl

formate was observed in small amounts besides traces of methyl formate (observable only

by extensive zooming into the formate region of the 1H-NMR spectrum) and considerably

formation of methanol (chapter 3.1.2). Moreover, catalyst 2/acid was shown to be an

efficient catalyst for the hydrogenation of methyl formate and ethyl formate to methanol

(chapter 3.1.1) making it very likely that part of the observed methanol in the CO2

hydrogenation reaction was generated via the observed alkyl formate as intermediate.

In the reactions performed in absence of alcohol additive methyl formate could be observed

in trace amounts by extensive zooming into the formate region of the 1H-NMR spectrum

(singlet at around 8.1 ppm), besides even smaller traces of formic acid (singlet at around

8.2 ppm) (Figure 46). The amounts of methyl formate were too small to be quantified by GC.

However, a rough estimation based on integration of the signal in the 1H-NMR spectrum

showed around 0.2 % selectivity (TON = 0.4) to methyl formate under standard reaction

conditions besides formation of methanol with a TON of 228 (complex 2/HNTf2, V(THF) =

2.1 mL, c(Ru) = 12 mmol L-1, HNTf2 (1 eq.), p(CO)2 = 20 bar at r.t., p(H2) = 60 bar at r.t., T =

140 °C, t = 24 h). At a lower temperature of 80 °C the formation of methanol was strongly

decreased to a TON of 24, and the TON found for methyl formate increased to roughly 1.7,

corresponding to a methyl formate selectivity of around 7 %. This indicated that the

formation of methyl formate proceeded already at lower temperatures,[7] whereas the

further hydrogenation of methyl formate to methanol and the direct transformation of CO2

to methanol within the coordination sphere of the catalyst was efficiently achieved only at

higher temperatures. Methyl formate did not accumulate in the reaction solution

throughout the reaction, as judged from the integrals of methyl formate in the 1H-NMR

spectra of reactions terminated after different reaction times. These observations strongly


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suggested that the formation of methanol proceeded partly via methyl formate as

intermediate as soon as methanol was present in the reaction solution. A reaction sequence

involving CO as intermediate (e.g. formation of methyl formate decarbonylation of

methyl formate formation of methanol) was excluded as the presence of CO-gas in the

reaction mixture led to the exclusive formation of the inactive carbonyl complex 4 as judged

from the characteristic signals in the 31P{1H}-NMR spectrum and no methanol formation was

observed in this case (complex 2/HNTf2, c(Ru) = 25 mmol L-1, HNTf2 (1 eq.), V(d8-THF) = 1 mL,

p(CO)2 = 20 bar at r.t., p(CO) ≈ 1 bar at r.t., p(H2) = 60 bar at r.t., T = 140 °C, t = 24 h).

Consequently, an experiment employing only synthesis gas (CO/H2) also showed the

exclusive formation of 4 and no methanol formation (2/HNTf2, V(THF) = 2.0 mL, c(Ru) =

12.5 mmol L-1, HNTf2 (1 eq.), p(CO) = 10 bar at r.t., p(H2) = 30 bar at r.t., T = 140 °C, t = 24 h).

Figure 46: Formate region of the 1H-NMR spectrum (300 MHz, d6-dmso, r.t.) of a reaction solution of a CO2 hydrogenation

reaction performed at standard reaction conditions in the absence of alcohol additive (complex 2/HNTf2, V(d8-THF) = 2.1 mL, c(Ru) = 12 mmol L

-1, HNTf2 (1 eq.), p(CO)2 = 20 bar at r.t., p(H2) = 60 bar at r.t., T = 140 °C, t = 24 h).

There were also several hints that the formation of methanol proceeded directly by

transformation of CO2 within the coordination sphere of the Ru-Triphos fragment: In a high-

pressure NMR experiment it was shown that formate complex 8a (which could be generated

in situ from CO2/H2 or by the addition of HNTf2/HCO2H to complex 2) could be converted to

methanol in the presence of only H2 and in the absence of any alcohol additive (chapter


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3.2.2). Moreover, the TON/time curve of the reaction in the absence of alcohol additive did

not show a pronounced induction period or an autocatalytic effect (chapter 3.3.2, Figure 43),

showing that CO2 could be directly converted to methanol in the absence of alcohol and that

the presence of alcohol had no enhancing effect on the rate of methanol formation.

Consequently, the TON obtained in a CO2 hydrogenation reaction in pure THF (V = 2.1 mL)

(TON = 228) was as high as the TON obtained in the presence of alcohol additive (V(THF) =

1.5 mL, V(EtOH) = 0.58 mL) (TON = 221) under otherwise same conditions (complex 2/HNTf2,

HNTf2 (1 eq.), c(Ru) = 12.5 mmol L-1, p(CO2) = 20 bar at r.t., p(H2) = 60 bar at r.t., T = 140 °C, t

= 24 h). DFT calculations further supported that a direct catalytic transformation of CO2 to

methanol can take place directly within the coordination sphere of the Ru-Triphos fragment

(chapter 3.2.4).

In summary, these results showed that CO2 was directly converted to methanol at the Ru-

centre. As soon as methanol was present in the reaction solution, formation of methyl

formate took place in parallel, and the methyl formate could subsequently be hydrogenated

to methanol. However, the absence of a rate enhancing effect of alcohol indicated that the

pathway via methyl formate was insignificant compared to the direct CO2 transformation.

3.3.5 Test of different catalyst precursors

All previous experiments and DFT calculations indicated that cationic species of the type

[Ru(H)(H2)(Triphos)(S)]+ (3) play an important role in the catalytic transformation.

Consequently, neutral catalyst precursors like [Ru(TMM)(Triphos)] (2) and

[Ru(H)2CO(Triphos)] (17) were found to be active in the presence of 1 eq. of acid, the

stoichiometric amount needed for the protonation of the TMM ligand or a hydride ligand.

However, 17 was found to be less productive than 2 due to the detrimental effect of CO in

the reaction solution (chapter 3.3.1). The cationic catalyst precursor [Ru(2-

OAc)(Triphos)(S)]NTf2 (14) was found to be active in the absence of acid, however, the

obtained TON was lower compared to the TON obtained with 2/HNTf2 due to the more

difficult activation by reductive removal of the acetate ligand (chapter 3.3.1).

Other catalyst precursors were evaluated and compared to the catalytic system

complex 2/HNTf2, which gave a TON of 393 (Table 5, entry 1). First, [Ru(OC6F5)2(Triphos)]


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(22)†† was tested for its performance in the CO2 hydrogenation to methanol. In the absence

of acid a TON of 14 was obtained, whereas in the presence of 1 equivalent HNTf2 a TON of

403 was obtained, which was comparable to the TON obtained with complex 2/HNTf2 (Table

5, entries 2 & 3). The need for HNTf2 indicated that complex 22 had to be activated by

protonation of a -OC6F5-ligand to give a cationic complex. Using [Ru(OC6F2H3)2(Triphos)]

(23)††/HNTf2 (1 : 1) again gave a very similar TON of 407 (Table 5, entry 4), indicating that all

three precursors gave the same active catalyst species under reaction conditions.

Next, Ru-TMM complexes containing different Triphos-derivatives were compared. As

mentioned earlier, the complex [Ru(TMM)(Triphos-xylyl)] (21)‡‡/HNTf2 (1 : 1) did not show

an increased TON (385) (Table 5, entry 5). [Ru(TMM)(Triphos-tolyl)] (24)‡‡ (Triphos-tolyl =

1,1,1-tris(bis(3-methylphenyl)phosphinomethyl)ethane) gave an about 7 % increased TON of

419 in the presence of HNTf2 (1 eq.) (Table 5, entry 6), demonstrating that steric and/or

electronic modifications of the Triphos ligand can indeed lead to improved productivities.

Nevertheless, all subsequent experiments were carried out using complex 2/HNTf2 due to its

easier accessibility from commercially available starting materials.

Table 5: Hydrogenation of CO2 to methanol in the absence of alcohol additive: test of different catalyst precursors.[a]


pH2[c]

[bar]

pCO2[c]

[bar]

TON[d]

1 2 HNTf2 (1.0) 60 20 393

2 22 ‒ 60 20 14

3 22 HNTf2 (1.0) 60 20 403

4 23 HNTf2 (1.0) 60 20 407

5 21 HNTf2 (1.0) 60 20 385

6 24 HNTf2 (1.0) 60 20 419

[a] Reaction conditions: in all reactions an aluminium cone was used for heating instead of an oil bath; catalyst (12.5 µmol),

THF (2.1 mL), 140 °C, 24 h; [b] equivalents to catalyst; [c] at room temperature; [d] TON = mmol MeOH/mmol catalyst.

3.4 Catalyst recycling and immobilisation

All NMR studies and the DFT studies were consistent with formate complex 3 being the

resting state of the active catalyst species. This chapter deals with recycling of the catalyst in

its resting state in repetitive batch experiments as well as with the development of a

continuous-flow process.

††

Synthesised by Dominik Limper, ITMC, RWTH Aachen University, Aachen. ‡‡

This complex was synthesised by M. Meuresch, ITMC, RWTH Aachen University, Aachen.


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3.4.1 Catalyst recycling by distillation

An obvious possibility to recycle a homogeneous catalyst is separation of the volatile organic

solvents and products by distillation.[24] In continuous operation normally a part of the

bottom product of a column containing the concentrated catalyst is recycled back to the

reactor. However, in the case of the CO2 hydrogenation to methanol in THF the reaction

mixture contains THF, methanol, and water. For example, the reaction solution of the

reaction with the highest TON of 895 consisted of 76.8 wt.-% THF, 14.9 wt.-% MeOH, and

8.4 wt.-% H2O. The ternary system THF/MeOH/H2O forms two binary minimum azeotropes

under atmospheric pressure.[172] The THF/MeOH azeotrope contains 31 wt.-% methanol and

has a boiling point at 60.7 °C, the THF/H2O azeotrope contains 5 wt.-% H2O and has a boiling

point at 64.0 °C. Therefore, distillation of the reaction mixture would yield water as the

bottom product,[172] which is not suitable for recycling of the Ru-Triphos catalyst as it is

insoluble in water. Additionally, separation of the THF/MeOH/H2O mixture is tedious and

needs multi-step distillations and/or membrane separations.[172-173]

Nevertheless, an experiment was carried out to check the general stability of the catalyst

complex: After performing a CO2 hydrogenation reaction with catalyst 2/HNTf2 (1 : 1) in

ethanol/THF (c(Ru) = 12 mmol L-1, HNTf2 (1 eq.), V(THF) = 1.5 mL, V(EtOH) = 0.58 mL, p(CO2)

= 20 bar at r.t., p(H2) = 60 bar at r.t., T = 140 °C, t = 24 h), the reactor was cooled to 0 °C in an

ice bath, the remaining pressure released, and the reaction solution transferred to a

distillation apparatus via cannula under argon atmosphere. All volatiles were removed from

the reaction solution under reduced pressure at room temperature, and the condensate was

collected in a cooling trap, weighed, and analysed for its methanol content. The remaining

dry catalyst was redissolved in ethanol (0.58 mL)/THF (1.5 mL) and recycled to the reactor.

No new HNTf2 was added as all previous investigations showed that HNTf2 was only

necessary for initial catalyst activation. A batch reaction was performed under the same

conditions as before, and the catalyst was recycled a second time. In Figure 47 the TONs

obtained in each cycle as well as the summed up total TON are displayed. Already the TON

obtained in the first cycle (134) was much lower than the TON obtained in the analogous

batch reaction with direct analysis of the MeOH in the reaction solution (TON = 221). This

was due to incomplete transfer of the reaction solution to the distillation apparatus and due

to losses inside the distillation apparatus. In the second cycle, already a huge drop in


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productivity to a TON of 44 was observed. In the third cycle the TON dropped further to 28.

This could partly be explained by catalyst losses during the transfer from the reactor to the

distillation apparatus and back. Moreover, from earlier attempts to isolate the active

formate intermediate 8a it was known that 8a is not stable in solid form and under reduced

pressures. Therefore, catalyst deactivation under these recycling conditions is very likely.

These results showed that the catalytic system cannot be recycled in its dry form. However,

the system still showed a remarkable stability, as in the second cycle still one third of the

initial productivity was obtained. Next, catalyst recycling in liquid solution was attempted.

Figure 47: Recycling of catalyst 2/HNTf2 (1 : 1) by removing all volatiles from the catalyst in vacuo at r.t. after a batch reaction (cycle 1), redissolving the remaining solid catalyst in ethanol/THF, and performing the next cycle (cycle 2). (c(Ru) = 12 mmol L

-1, HNTf2 (1 eq.), V(THF) = 1.5 mL, V(EtOH) = 0.58 mL, p(CO2) = 20 bar at r.t., p(H2) = 60 bar at r.t., T = 140 °C, t =

24 h).The TONs obtained per cycle are shown in dark grey, the total TONs summing up the cycles are shown in light grey.

3.4.2 Catalyst recycling in the biphasic system 2-MTHF/H2O

As shown in chapter 3.3.2, the CO2 hydrogenation to methanol could also be carried out in 2-

MTHF instead of THF, albeit with a somewhat lower TON of 186 instead of 228. Moreover,

the catalytic system was shown to be stable towards water (chapter 3.3.3). As 2-MTHF has a

miscibility gap with water, this opened up the possibility to realise a biphasic system, where

the catalyst is retained and recycled in an organic phase and methanol is removed in an

aqueous phase.[126, 174-175] The use of 2-MTHF as organic phase is desirable as 2-MTHF is

considered to be eco-friendly because it can be produced from renewable resources and is

degraded by sunlight and air.[174] Water is suitable as it has a miscibility gap with 2-MTHF and

is a by-product of the CO2 hydrogenation reaction.

0

50

100

150

200

250

1 2 3

134

44 28

134

178

206

TON

Cycle

cycle TON

total TON


-91-

The general partitioning of methanol in a biphasic 2-MTHF/H2O mixture was assessed by

dissolving methanol (0.158 g, 4.94 mmol, ca. 0.2 mL) in 2-MTHF (0.892 g, ca. 1.0 mL) and

adding deionised water (0.996 g, ca. 1.0 mL). After shaking the mixture for about 2 minutes,

the two clear phases were separated and analysed for their contents via 1H-NMR

spectroscopy (D1 = 10 s). The aqueous phase (1.281 g) contained 9.7 wt.-% methanol

(3.89 mmol, 124.7 mg), which was about 79 % of the total methanol, besides 13.4 wt.-%

2-MTHF (1.99 mmol, 171.8 mg) and 76.9 wt.-% H2O. The organic phase (0.765 g) contained

4.1 wt.-% methanol (0.98 mmol, 31.3 mg), which was about 20 % of the total methanol,

besides 8.5 wt.-% H2O and 87.4 wt.-% 2-MTHF. The mass balance based on methanol was

closed to 99 %. These results indicated that H2O was well suitable for extraction of methanol

from 2-MTHF for catalyst recycling purposes.

Recycling of catalyst 2/HNTf2 (1 : 1) using this extraction method in repetitive batch mode

was assessed as follows (Figure 48): A batch reaction was performed in 2-MTHF and

terminated after 16 hours by cooling in an ice/water bath and subsequent venting of the

reactor to release the remaining pressure (c(Ru) = 6.3 mmol L-1, HNTf2 (1 eq.), V(2-MTHF) =

2.0 mL, p(CO2) = 20 bar at r.t., p(H2) = 60 bar at r.t., T = 140 °C). The clear, orange reaction

solution was transferred to a Schlenk-tube, water (2.0 mL) was added, and the mixture was

stirred for about 20 seconds. After 10 minutes the phases were separated. The colourless

water phase was weighed and analysed for its contents by 1H-NMR spectroscopy (D1 = 10 s)

using mesitylene as internal standard in d6-acetone, and only the methanol content of the

aqueous phase was used for the calculation of the apparent TON. The orange 2-MTHF phase

was transferred back to the reactor and fresh 2-MTHF (0.25 mL) was added to compensate

the loss of 2-MTHF with the aqueous phase. No new HNTf2 was added as all previous

experiments indicated that the acid was only needed for initial formation of the active

catalyst species. The second, third, and fourth cycle were performed in the same way.


-92-

Figure 48: Procedure for repetitive batch experiments using catalyst 2/HNTf2 (1 : 1) in 2-MTHF and H2O to extract the methanol product.

The obtained TONs per cycle as well as the summed up total TONs are shown in Figure 49,

and the compositions of the aqueous phases from the extractions as well as of the 2-MTHF

phase after the last extraction in the fourth cycle are given in Table 6. Catalytic system

2/HNTf2 (1 : 1) could be recycled three times, giving a total TON of 769 after 4 cycles. After

the fourth cycle also the 2-MTHF phase was analysed by 1H-NMR spectroscopy for its

composition, and a methanol content corresponding to a TON of 12 was detected.

Moreover, washing of the autoclave with d6-DMSO (2.0 mL) gave an additional amount of

methanol corresponding to a TON of 25. In summary, a TON of 806 was obtained

corresponding to 323 mg methanol after a total reaction time of 64 hours. In the first cycle a

TON of 247 was obtained, which was around 84 % of the TON obtained in a batch reaction

conducted under the same reaction conditions but with direct analysis of the methanol

content in the 2-MTHF phase. This indicated that around 84 % of the produced methanol

were extracted by the water phase, which was in line with the observed partition of

methanol between water and 2-MTHF (vide supra). In cycle 2 still around 90 % of the initial

productivity was remained (TON = 222). This value decreased to 77 % in cycle 3 (TON = 191)

and to 45 % in cycle 4 (TON = 110). These results indicated that in principle the recycling of

complex 2/HNTf2 (1 : 1) in its active form was possible with the biphasic system

2-MTHF/H2O.


-93-

Figure 49: Recycling of catalyst 2/HNTf2 using 2-MTHF (2.0 mL) as the solvent and water (2.0 mL) for extraction of the produced methanol as shown in Figure 48. Each cycle was run for 16 hours (p(CO2) = 20 bar at r.t., p(H2) = 60 bar at r.t., T = 140 °C). The TONs obtained per cycle are shown in dark grey, the total TONs summing up the cycles are shown in light grey.

Table 6: Compositions of the phases obtained from a repetitive batch experiment using catalyst 2/HNTf2 in 2-MTHF as the solvent and water for extraction of the produced methanol as shown in Figure 48.

Mass [g] MeOH [wt.-%] 2-MTHF [wt.-%] H2O [wt.-%]

H2O phase cycle 1 2.222 4.5 13.4 82.1

H2O phase cycle 2 2.419 3.7 13.2 83.1

H2O phase cycle 3 2.422 3.2 13.9 83.0

H2O phase cycle 4 2.421 1.8 13.9 84.3

2-MTHF phase cycle 4 0.716 0.7 89.7 9.6

Using the system 2-MTHF/H2O a process scheme for continuous-flow operation is

conceivable in which all material streams can be recycled internally (Figure 50). However, as

shown in Table 6, a typical composition of the aqueous extraction phase was 4.5 wt.-%

MeOH, 13.4 wt.-% 2-MTHF, and 82.1 wt.-% H2O because of the solubility of 2-MTHF in water

of about 14 % at room temperature. The 2-MTHF content could be reduced to about 7 % by

performing the extraction at 60 °C,[175] but still tedious downstream processing would be

needed due to the formation of the azeotropes 2-MTHF/H2O and 2-MTHF/MeOH. Therefore,

intrinsic recycling of the catalyst by immobilisation inside the reactor was envisaged.

0

100

200

300

400

500

600

700

800

1 2 3 4

247 222 191

110

247

469

660

769

TON

Cycle

cycle TON

total TON


-94-

Figure 50: Process scheme for the CO2 hydrogenation to methanol using the system 2-MTHF/H2O for catalyst recycling.

3.4.3 Catalyst immobilisation for continuous-flow application

3.4.3.1 Identification of a suitable immobilisation medium

Ionic liquids

A proven concept for immobilising an organometallic catalyst in liquid phase inside a reactor

is the immobilisation in a non-volatile ionic liquid (IL).[24, 27, 29, 176-177] As the previous

experimental and theoretical mechanistic studies indicated ionic species of the type

[Ru(H)(H2)(Triphos)(S)]NTf2 (3) as active species and [Ru(2-O2CH)(Triphos)(THF)]NTf2 (8a) as

the resting state under reaction conditions, the catalytic system seemed to be perfectly

suitable for application in ionic liquids comprising the -NTf2-anion.

First experiments were carried out using the ionic liquid 1-ethyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide ([EMIM][NTf2]) in high purity (HP) grade (Iolitec,

J00331.3.2, > 99 % (NMR), [EMIM] = 99.9 % (IC), [NTf2] = 99.9 % (IC), bromide < 10 ppm,

chloride = 40 ppm).§§ The experiments were carried out using catalytic system 2/HNTf2 (1 : 1)

and a standard set of reaction parameters (c(Ru) = 12.5 mmol L-1, HNTf2 (1 eq.), V(IL) =

2.0 mL, p(CO2) = 20 bar at r.t., p(H2) = 60 bar at r.t., T = 140 °C). Complex 2 and HNTf2 (1 eq.)

were weighed into a Schlenk-tube inside a glove box and 2.0 mL of the IL were added. After

stirring this mixture for 2 h at room temperature a very fine orange suspension resulted,

which could easily be transferred to the reactor via cannula. After pressurisation with CO2

§§

Analytical data provided by Iolitec, Germany.


-95-

and H2 the reactor was heated and stirred (500 rpm) using a magnetic stirrer and an oil bath.

After 24 hours the remaining pressure was released and the resulting clear, orange reaction

solution was analysed by quantitative 1H-NMR spectroscopy. In four independent reactions

using the above described reaction conditions TONs between 4 and 20 were obtained, which

were only about 10 % of the TONs obtained in THF using the same reaction conditions (Table

7, entries 1-4). The reasons for this low observed TONs could be: 1) insufficient stirring due

to the high viscosity of the IL causing mass transfer limitation, 2) insufficient solubility of the

catalyst in the IL hampering the formation of the active catalyst species, 3) deactivation or

stabilisation of the catalyst by impurities in the IL.

First, reason 1) was investigated: Visual inspection of an open reactor (the closure head was

removed) showed, that the ionic liquid could easily be stirred using a magnetic stirring plate

in combination with an oil bath. Moreover, a CO2 hydrogenation experiment was carried out

using a magnetic stirring plate in combination with an aluminium cone for heating and

stirring the reactor. In this setup, the bottom of the autoclave was positioned very close to

the stirring plate allowing better power transmission from the stirring plate to the magnetic

stir bar. A slightly increased TON of 36 was obtained in this experiment, supporting that

insufficient stirring was not the main reason for the low TON observed (Table 7, entry 5).

To investigate reason 2) complex 2 was stirred together with HNTf2 in the ionic liquid for 2

hours at 60 °C to give a dark-red, brownish solution, which was transferred to the reactor

afterwards. Five experiments using this protocol were carried out independently (Table 7,

entries 6-10). In one experiment a high TON of 146 was obtained, indicating that in principle

the reaction was feasible in ILs. However, in the four other reactions low TONs in the range

6-9 were obtained, indicating that insufficient solubility of the catalyst complex could not be

the main reason for the low TONs. This was validated by three further experiments, in which

complex 2 (25 µmol) and HNTf2 (1 eq.) were dissolved in DCM (0.5 mL) to give a clear, dark-

red solution prior to addition of the IL (2.0 mL). DCM was removed from this mixture in

vacuo (2 h, r.t.) to give a homogeneous, dark-red solution. The mixture was transferred to

the reactor and a CO2 hydrogenation reaction was carried out as described before (Table 7,

entries 11-13). One of the three experiments showed a high TON of 111, supporting that the

reaction was in principle feasible in ILs. However, again the reproducibility was very poor

and two further experiments gave only TONs of 8, corroborating that catalyst solubility was

not the main problem.


-96-

Next, a possible deactivation or stabilisation of the catalyst by impurities in the IL (reason 3)

was investigated.[18] Impurities in the IL could be water, chloride residues from the synthesis

of the IL, and remaining starting material of the IL synthesis, like N-methylimidazole. The

water content measured by Karl-Fischer titration was 40 ppm, corresponding to a total

amount of 6.9 μmol H2O in the reaction solution (about 0.3 eq. based on [Ru]). As described

above, addition of water to the solvent THF did not influence the performance of complex

2/HNTf2 (1 : 1), making it unlikely to have a detrimental influence on the catalyst. However,

in IL the addition of a well coordinating molecule like water might stabilise the formate

intermediate 8 and other intermediates of the catalytic cycle (see DFT calculations in chapter

3.2.4; compare also crystal structure of 14a, Figure 26). Yet, no effect of the water content of

the IL varying from 41 ppm to 115 ppm, corresponding to total amounts of water present in

the reaction solution of 6.9 μmol (0.3 eq. to [Ru]) and 19.4 μmol (0.8 eq. to [Ru]), was found

in later experiments (vide infra).

Investigations done before showed that the addition of 3 equivalents chloride (based on

[Ru]) had a strongly inhibiting effect on the catalytic activity in THF (3.2.1). The used IL had a

chloride content of 40 ppm*** (i.e. 40 μg Cl- per 1 g IL), that is, the reaction solution

contained a total amount of 3.4 μmol chloride (0.14 eq. to [Ru]). As 25 μmol complex 2 were

used in the reactions, a complete deactivation of the catalyst by the chloride impurities

seemed to be unlikely. This was supported by NMR-spectroscopic analysis of the reaction

solution of a CO2 hydrogenation using complex 2/HNTf2 (1 : 1) in this IL terminated after 4

hours. The 31P{1H}-NMR spectrum (Figure 51) showed the exclusive formation of the known

carbonyl complex 4 and hydride-dimer 5 and no signals due to complexes containing

chloride ligands like 6 (compare Figure 8). Formation of these deactivation products already

after 4 hours explained the very low productivity in this IL. However, the factors causing this

fast catalyst deactivation remained unclear. Even more important, the reaction in halide-free

[EMIM][NTf2] synthesised on a halide-free reaction pathway also gave only a low TON of 11

(Table 7, entry 14),††† indicating that other impurities than halides in the IL played a crucial

role in catalyst inhibition.

***

Analytical data provided by Iolitec, Germany. †††

Halide-free [EMIM][NTf2], synthesised by Kylie Luska, ITMC, RWTH Aachen University.


-97-

Table 7: Hydrogenation of CO2 to methanol in the ionic liquid [EMIM][NTf2] with different purities.[a]


Ionic liquid grade pH2[c]

[bar]

pCO2[c]

[bar]

t

[h]

TON[d]

1[e]

2 HNTf2 (1.0) [EMIM][NTf2] HP 60 20 24 4

2[e]

2 HNTf2 (1.0) [EMIM][NTf2] HP 60 20 24 10

3[e]

2 HNTf2 (1.0) [EMIM][NTf2] HP 60 20 24 20

4[e]

2 HNTf2 (1.0) [EMIM][NTf2] HP 60 20 24 8

5[f]

2 HNTf2 (1.0) [EMIM][NTf2] HP 60 20 24 36

6[e,g]

2 HNTf2 (1.0) [EMIM][NTf2] HP 60 20 24 146

7[e,g]

2 HNTf2 (1.0) [EMIM][NTf2] HP 60 20 24 6

8[e,g]

2 HNTf2 (1.0) [EMIM][NTf2] HP 60 20 24 9

9[e,g]

2 HNTf2 (1.0) [EMIM][NTf2] HP 60 20 24 8

10[e,g]

2 HNTf2 (1.0) [EMIM][NTf2] HP 60 20 24 8

11[e,h]

2 HNTf2 (1.0) [EMIM][NTf2] HP 60 20 24 111

12[e,h]

2 HNTf2 (1.0) [EMIM][NTf2] HP 60 20 24 8

13[e,h]

2 HNTf2 (1.0) [EMIM][NTf2] HP 60 20 24 8

14[e]

2 HNTf2 (1.0) [EMIM][NTf2] †††

60 20 24 11

15[f]

2 HNTf2 (1.0) [EMIM][NTf2] SUP 60 20 24 134

16[f]

2 HNTf2 (1.0) [EMIM][NTf2] SUP 60 20 24 205

17[f]

2 HNTf2 (1.0) [EMIM][NTf2] SUP 60 20 24 144

18[f,i]

2 HNTf2 (1.0) [EMIM][NTf2] SUP 60 20 24 176

19[f,i]

2 HNTf2 (1.0) [EMIM][NTf2] SUP 60 20 24 168

20[f,i]

2 HNTf2 (1.0) [EMIM][NTf2] SUP 60 20 24 173

21[f,i]

2 HNTf2 (1.0) [EMIM][NTf2] SUP 60 20 24 175

22[f,i]

2 HNTf2 (1.0) [EMIM][NTf2] SUP 60 20 1.5 35

23[f]

2 HNTf2 (1.0) [EMIM][NTf2] SUP 90 30 24 314

[a] Reaction conditions: catalyst (25 µmol), IL (2.0 mL), 140 °C, 24 h; [b] equivalents to catalyst; [c] at room temperature; [d]

TON = mmol MeOH/mmol catalyst; [e] stirred and heated with magnetic stirrer and oil bath; [f] stirred and heated using a

magnetic stirrer equipped with an aluminium cone; [g] complex 2 and HNTf2 were dissolved in the ionic liquid by stirring the

mixture for 2 h at 60 °C before transferring it to the reactor; [h] complex 2 and HNTf2 were dissolved in DCM, the IL was

added, and the DCM was removed in vacuo before transferring the mixture to the reactor; [i] reactions were carried out

using the same reactor no. H10/18; the reaction mixture was stirred for 30 min at r.t. after pressurisation before heating up

to 140 °C. HP = high purity grade (Iolitec, J00331.3.2, > 99 % (NMR), [EMIM] = 99.9 % (IC), [NTf2] = 99.9 % (IC),

bromide < 10 ppm, chloride = 40 ppm); †††

= halide-free [EMIM][NTf2], synthesised by Kylie Luska, ITMC, RWTH Aachen

University; SUP = super-ultra-high purity (SUP) grade, produced via halide-free synthesis route (Iolitec, I01125.1.3, > 99.5 %

(NMR), ethyl sulfate = 40 ppm (IC), [EMIM] = 99.9 % (IC), [NTf2] = 99.9 % (IC)).


-98-

Figure 51: 31

P{1H}-NMR spectrum (121 MHz, d8-THF, r.t.) of a reaction solution of a CO2 hydrogenation reaction using

complex 2/HNTf2 (1 : 1) in [EMIM][NTf2] (V(IL) = 2.0 mL, c(Ru) = 25 mmol L-1

, HNTf2 (1 eq.), p(CO)2 = 20 bar at r.t., p(H2) = 60 bar at r.t., T = 140 °C, t = 4 h).

In the course of a Master’s Thesis[178] the same ionic liquid [EMIM][NTf2] was tested in a

higher commercially available ultra-high purity (UP) grade (Iolitec, K00634.1.1, > 99.5 %

(NMR), [EMIM] = 99.9 % (IC), [NTf2] = 99.9 % (IC), bromide < 10 ppm, chloride < 10 ppm).‡‡‡

The difference between this UP grade IL and the HP grade IL tested before was the lower

chloride content (< 10 ppm instead of 40 ppm) and the higher purity based on NMR essay (>

99.5 % instead of > 99 %). Moreover, the UP grade IL was colourless, whereas the HP grade

IL was slightly yellow. The CO2 hydrogenation reaction was investigated in this UP grade IL

using complex 2/HNTf2 (1 : 1) and the same standard set of reaction conditions (c(Ru) =

12.5 mmol L-1, HNTf2 (1 eq.), V(IL) = 2.0 mL, p(CO2) = 20 bar at r.t., p(H2) = 60 bar at r.t., T =

140 °C).[178] Similar to the results described above for the reaction in HP grade IL,

reproducibility was bad and TONs were found in the range 11-254. Following the

argumentation that a lack of stabilising solvent molecules in the IL (like the THF molecule

found in 8a) might be responsible for a low catalyst stability, three reactions were

performed in which THF (0.1 mL, 1.23 mmol, 49 eq. based on [Ru]) was added to the IL.

However, the same reproducibility issue occurred again (TONs = 29, 16, and 144).[178]

‡‡‡



-99-

Next, a special test batch of the same ionic liquid [EMIM][NTf2] was tested, which was

produced by Iolitec via a halide-free synthesis route (Iolitec, I01125.1.3, > 99.5 % (NMR),

ethyl sulfate = 40 ppm (IC), [EMIM] = 99.9 % (IC), [NTf2] = 99.9 % (IC)).§§§ This IL has

meanwhile become commercially available from Iolitec as super-ultra-high purity (SUP)

grade [EMIM][NTf2]. Again, this IL was tested in the CO2 hydrogenation to methanol using

complex 2/HNTf2 (1 : 1) and the same standard set of reaction conditions. In three

independent batch reactions high and much better reproducible TONs of 134, 205, and 144

were obtained, indicating that this batch of IL had a preferential composition (Table 7,

entries 15-17). When four independent batch reactions were conducted following the exact

same procedure and using the same reactor (no. H10/18) a high arithmetic mean TON of 173

was obtained with a very good reproducibility (sample standard deviation = 3.6) (Table 7,

entries 18-21). The procedure was as follows: 1) complex 2 and HNTf2 were stirred in

[EMIM][NTf2] for 2 hours at room temperature to assure sufficient solution and sufficient

reaction, 2) after transferring the catalyst solution to the autoclave (no. H10/18) and

pressurisation of the autoclave with CO2 and H2, the mixture was stirred inside the reactor

for at least 30 minutes to assure high availability of reaction gases in solution from the

beginning, 3) the autoclave was placed inside a preheated (140 °C) aluminium cone mounted

on a magnetic stirring plate (stirring rate = 500 rpm). The water content of the IL (as

determined by Karl-Fischer titration) varied from 41 ppm in the first experiment to 115 ppm

in the last experiment. No influence of the water content on the obtained TONs was

observed in this range. The initial TOF obtained with complex 2/HNTf2 (1 : 1) was estimated

from a reaction terminated after 1.5 hours (Table 7, entry 22). The TOF of 23 h-1 was smaller

compared to the TOF of 55 h-1 obtained in THF under the same reaction conditions. This

might be attributed to the higher viscosity and slower mass-transport of the reaction gases

in the IL.[18] At higher CO2 pressure (30 bar at r.t.) and H2 pressure (90 bar at r.t.) a strongly

increased TON of 314 was observed (Table 7, entry 23), which was again somewhat lower

compared to the TON obtained in THF using the same reaction conditions (TON = 367).

A CO2 hydrogenation reaction with doubled concentration of complex 2/HNTf2 (1 : 1) (for

better signal-to-noise ratio) in the same halide-free [EMIM][NTf2] was terminated after 1.5

hours, and the reaction solution was analysed by NMR-spectroscopy. The 31P{1H}-NMR

spectrum of the reaction solution is shown in Figure 52 (bottom). The same inactive carbonyl

§§§



-100-

species 4 (about 33 % of the total intensity) and dimer 5 (about 18 % of the total intensity)

were found as in the spectrum of the reaction in THF (Figure 6). However, both inactive

species were found in higher quantities compared to the reaction in THF. This assignment

was supported by correlation of these signals to the corresponding hydride signals in the 1H-

NMR spectra. A broad singlet at 43.0 ppm in the 31P{1H}-NMR spectrum which correlated to

a broad singlet at 8.6 ppm in the [1H,31P]-HMBC-NMR experiment indicated the formation of

a fluctuating formate species of the type [Ru(2-O2CH)(Triphos)(S)]+ (8) (S = coordinating

solvent). The same species was observed as the resting state when the reaction was

performed in THF (Figure 6). As the NMR shown in Figure 52 (bottom) was measured after

the addition of d8-THF to the [EMIM][NTf2] solution, it is likely that S equals d8-THF in

complex 8. However, in pure [EMIM][NTf2] with a water content of 41 ppm (0.3 eq. to [Ru])

the complex was most probably stabilised by the surrounding IL in the beginning of the

reaction. Although the anion [NTf2]- is considered to be very weakly coordinating, the

analysis of the interaction of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

([BMIM][NTf2]) with the coordinatively unsaturated copper complex [Cu(acac)(tmeda)]+

indicated that the interaction strength of this IL is well comparable with acetone.[18, 179-180] As

water and methanol were produced in the CO2 hydrogenation reaction, these might also

have stabilised complex 8.

Acetonitrile (76 eq.) was added to the product mixture after reaction in [EMIM][NTf2] and

again NMR-spectra were recorded of the resulting solution. The 31P{1H}-NMR spectrum

(Figure 52, top) as well as the corresponding 1H-NMR spectrum showed the formation of the

species [Ru(H)(MeCN)2(Triphos)]+ (9) and [Ru(MeCN)3(Triphos)]2+ (10), which were observed

earlier when acetonitrile was added to a reaction solution of a reaction in THF (compare

Figure 11 of the reaction in THF with Figure 52 (top) of the reaction in [EMIM][NTf2]). In

contrast to the 31P{1H}-NMR spectrum of the reaction in THF (Figure 11) the spectrum of the

reaction in [EMIM][NTf2] did not show the formation of [Ru(2-O2CH)(Triphos)(MeCN)]+ (8b).

In THF complex 9 formed from 8b by decarboxylation at room temperature (Scheme 16).

Therefore it is likely that 8b also formed upon addition of MeCN in [EMIM][NTf2] but reacted

very fast to complex 9.


-101-

Figure 52: 31

P{1H}-NMR spectra (162 MHz, d8-THF, r.t.) of a reaction solution of a CO2 hydrogenation reaction performed in

halide-free [EMIM][NTf2] (Iolitec, I01125.1.3, > 99.5 % (NMR), ethyl sulfate = 40 ppm (IC), [EMIM] = 99.9 % (IC), [NTf2] = 99.9 % (IC)) using complex 2/HNTf2 (c(Ru) = 25 mmol L

-1, HNTf2 (1 eq.), V(IL) = 2.0 mL, p(CO2) = 20 bar at r.t., p(H2) = 60 bar

at r.t., T = 140 °C, t = 1.5 h). Bottom: spectrum of the reaction solution with addition of d8-THF. Top: spectrum of the reaction solution with addition of d8-THF and acetonitrile (76 eq.). 4 = [Ru(H)(CO)2(Triphos)]

+; 5 = “Hydride-Dimer”; 8 =

[Ru(2-O2CH)(Triphos)(S)]

+; 9 = [Ru(H)(MeCN)2(Triphos)]

+; 10 = [Ru(MeCN)3(Triphos)]

2+.

In the course of his Master’s Thesis, Klügge used the same halide-free IL to gain more insight

into possible reasons for catalyst inhibition in the ILs used before, which gave poor TONs and

reproducibility.[178] One possible impurity in the ILs used before might have been traces of

N-methylimidazole from IL synthesis. Performing a reaction using complex 2/HNTf2 (1 : 1)

under standard conditions but in the presence of N-methylimidazole (25 eq. based on [Ru])

led to a complete deactivation of the catalyst, and no formation of methanol was

observed.[178] This indicated that basic impurities from the starting materials indeed are

detrimental to the catalyst activity and should be removed from the IL. Klügge synthesised

two batches of the structural similar IL [BMIM][NTf2] via a halide-free synthesis route

starting from N-methylimidazol and butyl-methanesulfonate.[178, 181] NMR-spectroscopy did

not show any organic impurities in both batches. Interestingly, performing a CO2

hydrogenation reaction in the IL from one batch gave a high TON of 186 comparable to the

TON obtained using the commercially available halide-free [EMIM][NTf2] (Table 7, entries 15-


-102-

21), whereas the same reaction in the IL from the other batch gave only a TON of 7

comparable to the TON in HP grade [EMIM][NTf2] (Table 7, entries 1-13). These results

stressed the fact that other impurities than halides were responsible for catalyst inhibition.

In summary, these results showed the possibility to perform the CO2 hydrogenation to

methanol in ionic liquids. Using the commercially available “super ultra pure” ionic liquid

[EMIM][NTf2] (Iolitec, SUP, I01125.1.3, > 99.5 % (NMR), ethyl sulfate = 40 ppm (IC), [EMIM] =

99.9 % (IC), [NTf2] = 99.9 % (IC))**** high TONs were obtained and reproducibility was robust.

However, much attention has to be paid to the purity of the employed ionic liquid, as

impurities in ionic liquids are a common problem.[18]

Polymer melts

Alternative reaction media with very low volatility suitable for immobilising organometallic

catalysts are polymer melts.[182-184] Especially poly(ethyleneglycol) (PEG) can be considered

as “green solvent, as it is nonvolatile, nonflammable, nontoxic to humans, animals and

aquatic life, and biodegradable by bacteria in soil and sewage.”[183] Polymers with higher

molecular weights are preferred because of their lower volatilities.[183]

A first set of experiments was performed using PEG-4000 (Fluka, PhEur, melting point = 58-

61 °C) as reaction medium in combination with complex 2/HNTf2 (1 : 1) (Table 8, entries 1-3).

As PEG-4000 was a solid at room temperature, it was weighed into a Schlenk-tube (2.252 g,

corresponding to ca. 2.0 mL in the melt) together with HNTf2 and complex 2 inside a glove

box. This mixture was stirred at 80 °C for 1 hour, resulting in a dark-red, clear solution. Upon

cooling to room temperature, the mixture became solid. Inside a glovebox, the solid was

transferred to the reactor, and the reactor was closed. Three reactions were performed

independently using standard reaction conditions (n(Ru) = 25 μmol, HNTf2 (1 eq.), m(PEG-

4000) = 2.252 g, p(CO2) = 20 bar at r.t., p(H2) = 60 bar at r.t., T = 140 °C, t = 24 h). Compared

to the reaction in THF or IL low TONs of 19, 12, and 6 were obtained (Table 8, entries 1-3). As

during the experiments using ILs a strong influence of impurities in the reaction medium was

identified, three more experiments were performed using PEG-4000 BU of high purity by

another retailer (Aldrich, BioUltra, Cl- < 50 ppm, melting point = 58-61 °C). Interestingly, in

contrast to the PEG-4000 used before, an orange, slightly turbid solution was obtained upon

dissolving complex 2/HNTf2 (1 : 1) in the PEG-4000 BU (1 h, 80 °C). The TONs obtained were

****



-103-

12, 9, and 43, showing that the CO2 hydrogenation was in principle possible in polymer

melts. However, reproducibility was bad and the achieved TONs were non-satisfying.

To avoid interference of the OH-end groups in PEG with the catalyst or the formed methanol

(etherification), methyl ether end-capped PEG-DME-2000 (Aldrich, melting point = 53 °C)

was used instead in two independent reactions at the same reaction conditions (Table 8,

entries 7-8). Upon heating PEG-DME-2000 (2.280 g, corresponding to ca. 2 mL in the melt)

together with complex 2/HNTf2 (1 : 1) at 80 °C for 1 hour, the colour of the reaction solution

changed from initially red to orange and finally yellow. Much higher TONs of 76 and 113

were obtained using the end-capped PEG instead of PEG, indicating that PEG-DME-2000 was

indeed a suitable reaction medium. However, compared to the reaction in THF and IL the

TON was still smaller and the reproducibility was still bad. Increasing the pressure from 80

bar (total pressure at r.t.) to 120 bar (total pressure at r.t.) led to an increased TON of 151,

which was much lower compared to the TON in IL under the same reaction conditions (314)

(Table 8, entry 9). Formation of the observed methanol from cleavage of the methyl-ether

end groups in PEG-DME-2000 was excluded based on the unchanged integral ratio between

the signal due to the methylene protons in -(O-CH2-CH2)- and the signal due to the -OCH3

end groups in the 1H-NMR spectra.

In summary, the results indicated that liquid polymers are in principle suitable reaction

media for the hydrogenation of CO2 to methanol. However, a more extended screening of

polymers and a more detailed analysis of the reasons for the bad reproducibility is still

needed. Because TONs were higher and better reproducible in the IL [EMIM][NTf2]

continuous-flow experiments were carried out using this IL instead of PEG-DME-2000.


-104-

Table 8: Hydrogenation of CO2 to methanol in polymer melts.[a]


Polymer pH2[c]

[bar]

pCO2[c]

[bar]

TON[d]

1 2 HNTf2 (1.0) PEG-4000 60 20 19

2 2 HNTf2 (1.0) PEG-4000 60 20 12

3 2 HNTf2 (1.0) PEG-4000 60 20 6

4 2 HNTf2 (1.0) PEG-4000 BU 60 20 12

5 2 HNTf2 (1.0) PEG-4000 BU 60 20 9

6 2 HNTf2 (1.0) PEG-4000 BU 60 20 43

7 2 HNTf2 (1.0) PEG-DME-2000 60 20 76

8 2 HNTf2 (1.0) PEG-DME-2000 60 20 113

9 2 HNTf2 (1.0) PEG-DME-2000 90 30 151

[a] Reaction conditions: in all reactions an aluminium cone was used for heating instead of an oil bath; catalyst (25 µmol),

PEG-4000/PEG-4000 BU (2.252 g) or PEG-DME-2000 (2.280 g), 140 °C, 24 h; [b] equivalents to catalyst; [c] at room

temperature; [d] TON = mmol MeOH/mmol catalyst.

3.4.3.2 Continuous-flow hydrogenation of CO2 to methanol in ionic liquid

In the previous chapter it was found that the use of complex 2/HNTf2 (1 : 1) gave robust

results in the “super ultra pure” ionic liquid [EMIM][NTf2] (Iolitec, SUP, I01125.1.3, > 99.5 %

(NMR), ethyl sulfate = 40 ppm (IC), [EMIM] = 99.9 % (IC), [NTf2] = 99.9 % (IC)). This ionic

liquid was used to explore the hydrogenation of CO2 to methanol in continuous-flow.

For continuous flow experiments the reaction rig described in chapter 5.4.3 was used (Figure

59). This setup was originally planned by Jens Theuerkauf and built by the mechanical

workshop of the ITMC, RWTH Aachen University.[185] Some modifications were made: The

dead volume of the system was reduced by exchanging the four two-way ball valves for

switching between “bypass” and “reactor”[185] with two three-way ball valves (TW 1 and

TW 2). To avoid condensation of extracted methanol and water inside the capillary between

the reactor outlet and the back pressure regulator this capillary was heated using heating

tape controlled by a PID controller (Eurotherm, model 91e, TIRC 1). A modified version of the

reactor used in all previous batch experiments served as reactor in the continuous-flow

setup. The reaction solution was stirred and heated using the same hot plate stirrer

equipped with an aluminium cone as was used for the well reproducible batch reactions in IL

(Table 7, entries 18-23). For an efficient gas transport into the IL phase and for efficient

stripping of the reaction products from the IL phase at reaction conditions (140 °C) the

reaction gases were introduced via a dip tube which immerged 0.5 cm into the reaction

solution when the glass liner was filled with 2 mL of IL. The depressurised product stream

was passed through a cooling trap (CT) at -72 °C filled with glass beads and THF. The cooling


-105-

trap was periodically exchanged and analysed for its methanol content by NMR and GC. A

sketch showing the principle of the continuous-flow setup is shown in Figure 53.

Figure 53: Sketch of the continuous-flow reaction setup.

In first experiments the applicability of the reaction setup for extraction of produced

methanol from the ionic liquid [EMIM][NTf2] was evaluated. Therefore, two independent

experiments were performed as follows: Methanol (400 mg) and [EMIM][NTf2] (2.0 mL) were

transferred to the reactor, the reactor was closed, pressurised with CO2/H2 (p(CO2)/p(H2) ≈

1/3) to 120 bar, and mounted in the reaction rig. Extraction was started at 140 °C by passing

the gas stream ( (H2) = 60 mLN min-1, (CO2) = 20 mLN min-1) through the reactor at 120 bar.

At these conditions, the density of the CO2/H2 phase was roughly one-tenth of the critical

density of CO2 (0.468 g cm-3),[186] indicating a low solubility strength of the gas phase.

Therefore, the extraction of the reaction products from the reactor will be mainly based on

product volatility. The cooling traps were periodically changed and the content was analysed

by GC using heptane as standard. Based on these results the evolution of the mass balance

(MeOH extracted [g]/MeOH initially charged [g]) with time was calculated (Figure 54, red

dots). Only a short induction period of 1 hour with an apparent average extraction rate of

11.2 mg h-1 was observed, showing a fast response time of the reaction setup. Between hour

1 and hour 5.8 an average extraction speed of 38.8 mg h-1 was obtained. After these 5.8

hours already 53 % of the initially charged MeOH had been extracted. With decreasing

amounts of MeOH remaining in the reactor, the extraction speed strongly decreased to an


-106-

average of 5.7 mg h-1 between hour 5.8 and 20.8 and further to 1.5 mg h-1 between hour 24

and 29. In the end the mass balance could be closed to about 80 %. The incomplete mass

balance was most probably due to losses of MeOH with the exhaust gases, especially when

the cooling trap remained unchanged for longer periods (over night). However, the same

continuous-flow extraction experiment was repeated and gave very similar results,

indicating a very good reproducibility using this setup (Figure 54, black squares). Therefore,

this setup was used to explore the hydrogenation of CO2 to methanol in continuous-flow.

Figure 54: Mass balance (MeOH extracted [g]/MeOH initially charged [g]) versus time of a continuous-flow extraction of

MeOH (400 mg) from [EMIM][NTf2] (2.0 mL) (T(reactor) = 140 °C, T(capillary) = 140 °C, p = 120 bar, (H2) = 60 mLN min-1

,

(CO2) = 20 mLN min-1

, stirring speed = 500 rpm). The red dots and the black squares indicate independent runs.

A CO2 hydrogenation using catalyst 2/HNTf2 (1 : 1) in [EMIM][NTf2] (c(Ru) = 12.5 mmol L-1,

HNTf2 (1 eq.), V(IL) = 2.0 mL) was performed in continuous-flow mode using the same setup

and the same reaction parameters as were applied in the extraction experiment described

before. The evolution of the summed up total TON (TTON) with the time on stream is

displayed in Figure 55. Average TOF values were calculated for the indicated periods.

After a short induction period of around 1 hour with an apparent TOF of only 2.9 h-1,

continuous methanol formation proceeded with an average TOF of 15.5 h-1 between hour 1

and 5, giving a TTON of 65 after 5 hours. From hour 5 to hour 19, methanol formation

continued smoothly with an average TOF of 13.4 h-1 giving a TTON of 253 after 19 hours on

stream. During the period from hour 19 to hour 29 the average TOF decreased to 7.3 h-1,

0 10 20 30 40 50 600

10

20

30

40

50

60

70

80

90

100

average extraction speed = 1.5 mg/h

ma

ss

ba

lan

ce

/ w

t.-%

time on stream / hours

total mass balance = 80 wt.-%

average extraction speed = 38.8 mg/h


-107-

indicating increasing catalyst deactivation after around 24 hours. Methanol formation

continued with a strongly decreased average TOF of 1.2 for the next 22 hours (hour 29-51)

and nearly stopped after 67 hours on stream (TOF = 0.2 h-1). After this time, a total TON of

365 was obtained. This experiment proved the feasibility to hydrogenate CO2 to methanol in

continuous-flow using a homogeneous catalyst for the first time.

Figure 55: Total TON (TTON) versus reaction time of a continuous-flow CO2 hydrogenation using complex 2/HNTf2 (1 : 1) in [EMIM][NTf2] (V(IL) = 2.0 mL, c(Ru) = 12.5 mmol L

-1, HNTf2 (1 eq.), T(reactor) = 140 °C, T(capillary) = 140 °C, p = 120 bar,

(H2) = 60 mLN min-1

, (CO2) = 20 mLN min-1

, stirring speed = 500 rpm).

As batch reactions with varied reaction pressures showed that much higher TONs could be

obtained at increased pressures, the next continuous-flow experiment was performed using

an increased pressure of 200 bar (Figure 56). At these conditions, the density of the CO2/H2

gas mixture was still one order of magnitude lower as compared to the critical density of

CO2.[186] Again, an induction period of 1 hour was observed with an apparent TOF of only

4.0 h-1. In the next four hours, methanol production was observed with an average TOF of

21.3 h-1 (hour 1-5). The TOF increased even further to an average value of around 30 h-1 for

the next 24 hours (hour 5-29). Maybe this further apparent increase was due to an increase

of the methanol concentration in the IL phase leading to a more efficient stripping of

methanol from the IL phase. After 29 hours on stream a total TON of 657 was obtained.

After this time, catalyst deactivation became apparent, and the average TOF between hour

29 and hour 44 decreased to 12.1 h-1 and further to an average TOF of 7.5 h-1 between hour

44 and 52. After 52 hours on stream a total TON of 1062 was obtained, which was the

0 10 20 30 40 50 60 70 800

50

100

150

200

250

300

350

400

average TOF = 0.2 h-1 (hour 66-67)



TTON


TTON = 365



-108-

highest TON observed so far. Clearly, this was due to the high pressure of 200 bar and

isobaric conditions (high pressures and isobaric conditions led to strongly increased TONs

also in batch experiments, chapter 3.3.2). Compared to the continuous-flow experiment at

120 bar nearly doubled TOFs were observed. Whereas in the experiment at 120 bar catalyst

deactivation became apparent already in the period between hour 19 and hour 29, catalyst

deactivation became apparent not until the period between hour 29 and hour 44 in the

experiment at 200 bar. However, catalyst deactivation was still a problem. After the

continuous-flow experiment was stopped, the reactor was dismounted under remaining

pressure, cooled to room temperature, the pressure carefully released, and the clear, yellow

catalyst solution was transferred to a NMR tube under inert atmosphere. The 31P{1H}-NMR

spectrum showed the formation of the inactive carbonyl complex 4 in about 93 % (according

to the integral ratios in the spectrum) besides some phosphor containing species giving rise

to singlets at 42.5 ppm, 42.2 ppm, and 35.9 ppm. Comparison with the NMR spectrum of a

reaction solution of a batch reaction using the same catalyst in the same IL (Figure 52)

suggested that the singlet at 42.5 ppm was due to an active formate complex of type 8 and

the signal at 42.2 ppm was due to the hydride dimer 5. This indicated that catalyst

deactivation by formation of carbonyl complex 4 is the major challenge to be tackled in the

future.

In summary, it was demonstrated that the continuous-flow hydrogenation of CO2 to

methanol is possible by immobilising an organometallic catalyst in a high-boiling reaction

medium (estimated boiling point of [EMIM][NTf2] = 544 °C[187]), and by stripping the reaction

products from the reaction medium by excess reaction gases.


-109-

Figure 56: Total TON (TTON) versus reaction time of a continuous-flow CO2 hydrogenation using complex 2/HNTf2 (1 : 1) in [EMIM][NTf2] at an increased pressure of 200 bar (V(IL) = 2.0 mL, c(Ru) = 12.5 mmol L

-1, HNTf2 (1 eq.), T(reactor) = 140 °C,

T(capillary) = 140 °C, p = 200 bar, (H2) = 60 mLN min-1

, (CO2) = 20 mLN min-1

, stirring speed = 500 rpm).

Figure 57: 31

P{1H}-NMR spectrum (162 MHz, CD2Cl2, r.t.) of the reaction solution after stopping the continuous-flow

experiment using complex 2/HNTf2 (1 : 1) in [EMIM][NTf2] at an increased pressure of 200 bar (V(IL) = 2.0 mL, c(Ru) =

12.5 mmol L-1

, HNTf2 (1 eq.), T(reactor) = 140 °C, T(capillary) = 140 °C, p = 200 bar, (H2) = 60 mLN min-1

, (CO2) = 20 mLN min

-1, stirring speed = 500 rpm). 4 = [Ru(H)(CO)2(Triphos)]

+.

0 10 20 30 40 50 600

200

400

600

800

1000

1200

average TOF = 7.5 h-1

(hour 44-52)


(hour 20-29)TTON


TTON = 1062



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3.4.3.3 Continuous-flow hydrogenation of CO2 to methanol using SILP catalysis

After demonstrating that continuous-flow hydrogenation of CO2 to methanol is in principle

possible using complex 2/HNTf2 (1 : 1) in a bulk phase of the ionic liquid [EMIM][NTf2], the

possibility to immobilise the same combination of catalytic system and ionic liquid on a

porous support was examined. This concept, known as Supported Ionic Liquid Phase

(SILP),[188-189] was successfully applied for e.g. the aerobic oxidation of alcohols,[190] the

hydroformylation of 1-octene,[28] and the enantioselective hydrogenation of C=C bonds.[25-26]

As water is produced as byproduct in the CO2 hydrogenation to methanol, a hydrophobic

support material was chosen to avoid accumulation of water in the support material.

Perfluoro-alkyl [-Si(Me)2CH2CH2C6F13] functionalised silica (particle size 32-63 μm, mean pore

diameter 59 Å, BET surface area 207 m2 g-1, mesopore volume 0.8 mL g-1, SGFLUO) has been

proven to be suitable for this purpose by Hintermair and co-workers.[25, 191] For the synthesis

of the SILP catalyst a pore-filling degree of α = 0.5 was chosen (2.5 g of support per mL of IL),

as studies by Hintermair showed that this was the highest possible value at which the

catalyst was still a macroscopically dry powder and showed good mass transport

behaviour.[23] For better comparison with the bulk IL system the same catalyst concentration

in IL was chosen (c(Ru) = 12.5 μmol mL-1). The synthesis of the SILP catalyst based on this

support material, [EMIM][NTf2], and complex 2/HNTf2 (1 : 1) was performed following

literature-known procedures and is described in detail in chapter 5.5.6.[25-26, 28] However, THF

was used as the solvent for this procedure instead of the commonly used DCM, as

experience from catalyst synthesis and studies indicated that complex 2/HNTf2 (1 : 1) was

much more stable in the coordinating THF compared to weakly coordinating DCM. After

drying the SILP catalyst for 1 hour at room temperature in vacuo a pale yellow, dry powder

resulted.

To test this catalyst powder in the continuous-flow setup, a stainless steel tubular reactor

(inner diameter = 0.75 cm, length = 22.5 cm) equipped with two ball valves for closing was

used instead of the stirred batch reactor. The SILP catalyst (4 g SILP catalyst, containing

12.5 μmol of [Ru], 1 eq. HNTf2, and 1.0 mL [EMIM][NTf2]) was inserted into the tubular

reactor inside a glovebox giving a packed bed of 11 cm height, the reactor was mounted in

the reaction rig, and the reaction was started by heating the reactor at 140 °C and passing

the CO2/H2 flow through the reactor (p = 120 bar, (H2) = 60 mLN min-1, (CO2) = 20


-111-

mLN min-1). The evolution of the total TON with the reaction time is displayed in Figure 58.

After a short induction period of 1 hour with an apparent TOF of 5.7 h-1, methanol

production was observed with an average TOF of 15.8 h-1 for the next 5 hours. After 6 hours

on stream a TTON of 85 was obtained. However, catalyst deactivation was fast and the

average TOF decreased to 1.4 h-1 already in the period from hour 6 to hour 21, to 2.4 h-1 in

the period from hour 21 to hour 29, and finally to 0.5 h-1 in the period from hour 45 to hour

50.5. After 50.5 hours on stream a TTON of 130 was achieved. In comparison to the

experiment in bulk IL using a stirred tank reactor at 120 bar (Figure 55) nearly the same

average TOF around 15.5 h-1 was observed up to 5-6 hours on stream. However, catalyst

deactivation was much faster using the SILP system. The TTON of 130 obtained after 50.5

hours on stream using the SILP system was much lower compared to the TTON of 361

obtained after a similar time on stream (51 h) using the bulk IL. There were two main

differences between these two experiments: Firstly, the total amount of [Ru] present in the

reactor was only 12.5 μmol in the SILP system compared to 25 μmol in the bulk IL, and

secondly, in the SILP system support material was present.

The lower total amount of [Ru] present in the reactor at a constant stream of reaction gases

might lead to faster deactivation if impurities in the reaction gases were responsible for the

deactivation. In the continuous-flow experiments CO2 with the purity grade 4.5 (Westfalen,

99.995 vol.-%) was used, which contained < 5 vol.-ppm CO. Using the ideal gas law and

assuming the highest amount of CO (5 vol.-ppm) the molar CO stream was calculated to be

0.25 μmol h-1 for the employed CO2 stream of 20 mLN min-1. Under these circumstances, a

total amount of 12.6 μmol CO passed the reactor after 50.5 hours on stream, which was 1

equivalent to the amount of catalyst present. This might have been indeed enhancing the

formation of the inactive complex [Ru(H)(CO)2(Triphos)]+ (4). Complex 4 was observed as the

main deactivation product in the continuous-flow experiment using bulk IL (Figure 57).

However, the formation of 4 needs two equivalents of CO. Moreover, the batch experiments

with [Ru(H)2CO(Triphos)] (17)/HNTf2 showed that CO could dissociate from 17 as long as

there was only one CO coordinated to the complex (vide supra). As the CO which passes the

reactor with the gas stream does not accumulate inside the reactor, clearly the

decarbonylation of catalytic intermediates still played an important role in the formation of

4.


-112-

Figure 58: Total TON (TTON) versus reaction time of a continuous-flow CO2 hydrogenation using a SILP catalyst based on complex 2/HNTf2 (1 : 1), [EMIM][NTf2], and fluorinated silica support (n(Ru) = 12.5 μmol, HNTf2 (1 eq.), T(reactor) = 140 °C, T(capillary) = 140 °C, p = 120 bar, (H2) = 60 mLN min

-1, (CO2) = 20 mLN min

-1).

To explore the influence of the support material on the catalyst activity, a batch reaction was

performed with complex 2/HNTf2 (1 : 1) in THF in the presence of SGFLUO (100 mg) under

otherwise standard conditions (c(Ru) = 12.5 mmol L-1, HNTf2 (1 eq.), V(THF) = 2.0 mL, p(CO2)

= 20 bar at r.t., p(H2) = 60 bar at r.t., T = 140 °C). For analysis, the reaction solution was

filtered, the remaining support material was washed four times with d6-DMSO (0.5 mL), and

the combined solutions were weighed. Mesitylene (ca. 25 mg) was weighed into a vial, ca.

500 mg of the solution were added, and the resulting solution was analysed by 1H-NMR.

Methanol was detected corresponding to a TON of 139 which was only about 60 % of the

TON obtained in the absence of support material, indicating a detrimental effect of SGFLUO

on the catalyst. However, it could not be excluded that a part of the formed methanol

remained on the support material.

In summary, the results indicated the possibility to use a SILP catalyst for the continuous-

flow hydrogenation of CO2 to methanol.

0 10 20 30 40 50 600

25

50

75

100

125

150



(hour 21-29)TTON


TTON = 130


SUMMARY & CONCLUSION

-113-

4 Summary & Conclusion

In this thesis the very first organometallic catalytic system for the selective hydrogenation of

CO2 to methanol was described. Using the neutral complex [Ru(TMM)(Triphos)] (2) in

combination with 1 equivalent of the acid HNTf2 resulted in turnover frequencies per Ru-

centre of up to 70 h-1.

The homogeneous nature of the catalytic system was confirmed by control experiments in

the presence of mercury, by using ruthenium on carbon, and by the unambiguous

identification of the ruthenium formate complex [Ru(2-O2CH)(Triphos)(THF)]NTf2 (8a) as

resting state of the catalytic cycle using NMR and IR spectroscopy.

A reaction pathway via intermediate formation of CO by RWGS was excluded as no CO was

detected in the gas phase and as the addition of CO to the gas mixture led to complete

deactivation of the catalyst. Experiments in which the formate ligand of independently

synthesised 8a was hydrogenated to methanol demonstrated that no intermediate

stabilisation of the formate as alkyl formate was necessary, i.e. no cascade reaction was

necessary to achieve methanol formation. Consequently, the hydrogenation of CO2 to

methanol was possible in the absence of any alcohol additive.

DFT calculations supported the possibility to reduce CO2 stepwise to methanol within the

coordination sphere of a single Ru-Triphos centre. A plausible catalytic cycle including all

transition states was found, in which a series of hydride transfer and protonolysis steps leads

to the reduction of CO2 to formic acid, formaldehyde, and finally methanol. It could be

shown that the facial coordination of the Triphos ligand imposes a favourable geometrical

arrangement for the hydride transfer to carboxylate units. Together with the high

temperature stability of Ru-Triphos complexes this feature might play an important role for

the unprecedented reactivity of this complex.

Using the neutral complex 2 the addition of an acidic additive was a prerequisite to achieve

high productivities. The obtained maximum TONs using the three acids MSA, p-TsOH, and

HNTf2 were well in line with the expected coordination ability of the acid anions, with weakly

coordinating anions leading to higher TONs. Using 1 mole of HNTf2 per mole of complex 2

gave the highest TON, supporting the fact that the proton was required in stoichiometric


-114-

amounts for the reductive removal of the TMM ligand in 2 leading to the formation of active

cationic species like [Ru(H)(H2)(Triphos)(S)]+ (3) (S = solvent). Following this argumentation,

the newly synthesised cationic complex [Ru(2-OAc)(Triphos)(S)]NTf2 (14) (S = solvent or free

coordination site) was active in the absence of an acidic additive, albeit with a somewhat

lower TON compared to complex 2/HNTf2 due to a longer activation period.

Parameter variations showed positive effects on the obtained TONs upon lowering the

catalyst concentration, increasing the reaction temperature, and increasing the total

pressure (H2/CO2 = 3/1). In accordance with the TOF determining intermediate and the TOF

determining transition state of the catalytic cycle as determined from DFT calculations,

increasing the H2 partial pressure led to strongly increased TONs whereas increasing the CO2

partial pressure did not. Solvents with high enough polarities and basicities (e.g. THF,

2-MTHF, 1,4-dioxane) were necessary to obtain considerable productivities.

NMR spectroscopic analysis revealed dimerisation to give a dimeric species (5) and

decarbonylation to give [Ru(H)(CO)2(Triphos)]+ (4) as deactivation mechanisms, with

formation of 4 being the major deactivation pathway. Enhancing the methanol formation

rate relatively to the decarbonylation rate by high and isobaric H2 and CO2 pressures proved

to be an effective way to achieve greatly enhanced TONs of up to 895 (corresponding to an

amount of 0.36 g methanol) after 64 hours.

Comparing Ru-TMM complexes bearing different Triphos-derivatives, namely

[Ru(TMM)(Triphos-xylyl)] (21) and [Ru(TMM)(Triphos-tolyl)] (24), showed that steric and/or

electronic modifications might lead to improved TONs. Using 24 a 7 % increase in TON was

obtained.

Recycling of the catalytic system 2/HNTf2 was possible in the biphasic system 2-MTHF/water.

In the fourth cycle still around 50 % of the initial productivity was retained. However, the

tedious separation of the resulting 2-MTHF/water/MeOH mixture led to the investigation of

alternative methods.

Immobilisation of the catalytic system 2/HNTf2 in the ionic liquid [EMIM][NTf2] was

successful, resulting in only moderately decreased TONs of around 173 in batch experiments

compared to the results in THF. However, usage of a commercially available “super ultra

pure” ionic liquid (Iolitec) without halide and, more importantly, virtually no organic

impurities (N-methylimidazole) was a prerequisite for achieving reproducible results.


-115-

The catalytic system 2/HNTf2 in [EMIM][NTf2] was successfully applied in a continuous-flow

experiment. After 52 hours on stream a total TON of 1062 was obtained. After around 29

hours on stream catalyst deactivation became apparent. Analysis of the catalyst solution in

the reactor after finishing the reaction showed formation of the inactive carbonyl complex

[Ru(H)(CO)2(Triphos)]+ (4) in about 93 %, indicating that avoiding decarbonylation reactions

is the main challenge to be tackled in the future.

Finally, immobilisation of catalytic system 2/HNTf2 using the SILP concept allowed using the

catalyst in a fixed bed. However, catalyst deactivation was faster compared to the system in

bulk ionic liquid.

The results of this thesis demonstrated the possibility to use organometallic catalysts for the

selective transformation of CO2 to methanol for the first time. Already at this early stage of

research the obtained TOFs were in the same range as for the active sites in traditional

heterogeneous methanol synthesis catalysts. Research should be continued to search for

catalysts with improved activity and stability suitable for industrial application. The

mechanistic insights described in this thesis might be helpful for these future developments.

EXPERIMENTAL

-116-

5 Experimental

5.1 General

Safety advice: High-pressure experiments with compressed gases represent a significant

safety risk and must be conducted following appropriate safety procedures and in

conjunction with the use of suitable equipment.

For complex synthesis and catalytic experiments, moisture and oxygen were excluded by

working in a glove box or by using Schlenk techniques. Argon 4.8 (Messer, Germany) was

used as inert gas. Glassware was dried under vacuum with a heat gun, evacuated and refilled

with argon at least three times.

5.2 Solvents and Chemicals

All solvents were purified by distillation prior to use. Tetrahydrofuran, toluene,

dichloromethane, and pentane were degassed by bubbling argon through the solvent with a

frit, dried by passing over activated aluminium in steel columns, and stored over molecular

sieves under argon. Acetonitrile, 1,4-dioxane, ethyl formate, formic acid, and 2-

methyltetrahydrofuran were degassed by distillation under argon, dried over molecular

sieves and stored under argon. Dimethyl sulfoxide was degassed by bubbling argon through

the solvent for at least 2 hours, dried over molecular sieves and stored under argon.

Deionised water was taken from a reverse-osmotic purification system (Werner EasyPure II),

degassed by bubbling argon through the water with a frit, and stored under argon.

Deuterated solvents for NMR analysis were purchased from Eurisotop, degassed by three

freeze-pump-thaw cycles and stored over molecular sieves under argon. All reagents were

commercially supplied and used as received unless stated otherwise. Solids were degassed

in vacuo for at least 24 hours and by repeatedly filling the container with argon. Ionic liquids

were purchased from Iolitec and degassed and dried by stirring for 48 hours in vacuo.

Carbon dioxide (Westfalen) was of 4.5 grade and Hydrogen (Air Products) was of 5.2 grade.

Both were used as received. [Ru(2-OAc)Cl(Triphos)] (12)[159], [Ru(H)2CO(Triphos)] (17)[127]

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and the dimeric species (5)[125] were synthesised according to literature known procedures.

[Ru(TMM)(Triphos-xylyl)] (21) and Triphos-anisyl (1,1,1-tris{bis(4-

methoxyphenyl)phosphinomethyl}ethan) were synthesised by M. Meuresch, ITMC, RWTH

Aachen University, Aachen.[142, 192] [Ru(OC6F5)2(Triphos)] (22) and [Ru(OC6F2H3)2(Triphos)]

(23) were synthesised by Dominik Limper, ITMC, RWTH Aachen University, Aachen,

according to a synthesis method he developed in collaboration with Tobias Weigand, ITMC,

RWTH Aachen University.††††

5.3 Analysis

5.3.1 NMR Spectroscopy

NMR spectra were recorded with commercial spectrometers Bruker AV-600, AV-400 or AV-

300 at room temperature unless stated otherwise. Chemical shifts are given in ppm

relative to tetramethylsilane (1H and 13C) and 85 % phosphoric acid (31P). For 1H-NMR spectra

the residual solvent was used as reference. For 13C-NMR spectra the solvent was used as

reference. First order spin multiplicities are abbreviated as singlet (s), doublet (d), triplet (t),

and quadruplet (qua). Couplings of higher order or overlapped signals are denoted as

multiplet (m), broadened signals as (br). First order coupling constants J are given in Hz.

Assignments are based on attached proton tests (ATP) and 2D-correlation spectroscopy

(HSQC, HMQC, HMBC).

5.3.2 IR Spectroscopy

IR-spectra were measured on a commercial Bruker Alpha FT-IR spectrometer inside an

argon-filled glovebox. For measuring transmission, ATR and DRIFT spectra a standard T-

module, a platinum/diamond P-module and a DR-module was used, respectively.

††††

Personal communication.

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5.3.3 Mass Spectrometry

Electrospray ionisation mass spectra (ESI-MS) were measured on a commercial Varian 500

MS mass spectrometer using direct electrospray ionisation in positive (+) or negative (−)

polarisation. Samples were dissolved in organic solvents without acidification. HRMS (EI) as

well as SIMS measurements were carried out on a Finnigan MAT 95. HRMS-ESI

measurements were carried out on a Thermo Scientific LTQ (MS/MS) system equipped with a

Orbitrap XL detector.

5.3.4 Gas Chromatography

Gas chromatography was performed on a Trace GC gas chromatograph (Thermo Scientific)

equipped with a SSL Inlet (250 °C, Split 83 mL/min), a FS-Innopeg-2000 column (60 m, inner

diameter 0.25 mm, film thickness 0.25 µm) and a flame ionisation detector (250 °C) using

helium as carrier gas and a temperature program (10 min isothermal at 50 °C, ramp to 200

°C (8 °C/min), 30 min at 200 °C). n-Heptane was used as standard. Correction factors were

2.83 for ethyl formate and 2.80 for methanol.

5.4 Catalysis

5.4.1 General procedure for batch catalysis

All high pressure batch experiments were conducted in stainless steel autoclaves (inner

volume = 13 mL) equipped with a glass inlet and a magnetic stir bar. Blank tests without

catalyst assured no background activity under typical reaction conditions.

5.4.1.1 General procedure for CO2 hydrogenation reactions in organic solvents

Prior to use, the autoclave was evacuated and repeatedly purged with argon. Under an

argon atmosphere, the catalyst together with the acidic additive and, if desired, other

additives were weighed into a Schlenk tube and dissolved in the solvent. For catalyst [Ru(2-

OAc)(Triphos)(S)]NTf2 (14), the THF solution was prepared by heating [Ru(2-

OAc)Cl(Triphos)] (12) (0.025 mmol, 20.5 mg) together with AgNTf2 (1.1 eq., 0.0275 mmol,

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10.7 mg) in THF (1 mL) for 3 h at 60 °C. The solution was filtrated over silica and the silica

washed with THF (1 mL). Alternatively, isolated catalyst 14 (0.025 mmol) was used in THF

(2.1 mL) and gave the same results.

In either case the solution was transferred via cannula to the stainless steel autoclave under

argon atmosphere. The autoclave was pressurised with carbon dioxide and then hydrogen

was added up to the desired total pressure. The reaction mixture was stirred (500 rpm) and

heated at reaction temperature in an oil bath or in an aluminium cone mounted on a

magnetic stirring hot plate. After the reaction time, the autoclave was cooled to 0 °C in an

ice bath and then carefully vented. The resulting solution was analysed by 1H-NMR (D1 = 10

s) in d6-DMSO with internal standard mesitylene: The reaction solution was transferred to a

glass vial and weighed. Mesitylene (ca. 25 mg) was weighed into a second vial and ca.

500 mg of the reaction solution were added. After adding d6-DMSO (1.0 mL) ca. 0.4 mL of

this solution were transferred to a NMR tube. For the reactions done in THF the results were

confirmed by gas chromatography using heptane as internal standard.

5.4.1.2 General procedure for CO2 hydrogenation reactions in ionic liquids


argon atmosphere, the catalyst together with the acidic additive and, if desired, other

additives were weighed into a Schlenk tube. The ionic liquid was added and the mixture was

stirred for at least 2 hours at r.t. until an almost clear solution or very fine suspension was

obtained. The solution/suspension was transferred via cannula to the stainless steel

autoclave under argon atmosphere. The autoclave was pressurised with carbon dioxide and

then hydrogen was added up to the desired total pressure. To assure a sufficient solution of

reaction gases in the ionic liquid at the start of the reaction, the autoclave was placed in an

aluminium cone mounted on a magnetic stirring hot plate and the reaction mixture was

stirred at 500 rpm for 20 minutes at r.t. After that, the reaction mixture was heated and

stirred (500 rpm) at reaction temperature. After the reaction time, the autoclave was cooled

to 0 °C in an ice bath and then carefully vented. The resulting solution was analysed by 1H-

NMR (D1 = 10 s) in d6-DMSO with internal standard mesitylene.

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5.4.1.3 General procedure for CO2 hydrogenation reactions in polymer melts


argon atmosphere, the catalyst together with the acidic additive and the polymer were

weighed into a Schlenk tube equipped with a magnetic stir bar. The mixture was heated and

stirred for 1 hour at 80 °C in an oil bath until a clear solution or very fine suspension was

obtained. After that, the mixture was cooled to r.t., thereby becoming a solid. Inside a glove

box, the solid was weighed into a glass inlet of known weight and the glass inlet placed in

the autoclave. The autoclave was closed hand-tight inside the glovebox and closed properly

outside the glovebox. The autoclave was pressurised carbon dioxide and then hydrogen was

added up to the desired total pressure. To assure a sufficient solution of reaction gases in

the polymer melt at the start of the reaction, the autoclave was placed in an aluminium cone

mounted on a magnetic stirring hot plate and the reaction mixture was stirred at 500 rpm

for 1 hour at 80 °C. After that, the reaction mixture was heated and stirred (500 rpm) at the

desired reaction temperature. After the reaction time, the autoclave was cooled to 0 °C in an

ice bath and then carefully vented. The glass liner containing the reaction mixture was

weighed and ca. 3 g of D2O were added. The glass liner was closed to avoid losses by

evaporation and stirred until the polymer was dissolved. The orange coloured catalyst did

not dissolve but was suspended in the solution. The catalyst was filtered off by passing the

solution through a syringe filter. Inaccuracies resulting from removing the catalyst were

considered small enough (typically ca. 25 mg catalyst/5.5 g solution). Of the resulting clear

solution a sample was weighed into a vial together with dry DMSO as standard, and the

mixture was analysed by 1H-NMR (D1 = 10 s). The procedure was checked with a known

amount of methanol showing an observational error of +2 %.

5.4.1.4 General procedure for the hydrogenation of substrates other than CO2


argon atmosphere, the desired amount of catalyst, the acidic additive and the substrate

were weighed into a Schlenk tube and dissolved in the solvent. The solution was transferred

via cannula to the stainless steel autoclave under argon atmosphere. The autoclave was

pressurised with hydrogen and the reaction mixture stirred and heated at the reaction

temperature in an oil bath or in an aluminium cone mounted on a magnetic stirring hot

plate. After the reaction time, the autoclave was cooled to 0 °C in an ice bath and then

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carefully vented. The resulting solution was analysed by 1H-NMR (D1 = 10 s) in d6-DMSO with

internal standard mesitylene.

5.4.2 General procedure for high pressure NMR experiments

High pressure NMR experiments were performed in a 5 mm sapphire tube with an inner

diameter of 3.4 mm and a volume of 0.93 mL, which was glued into a titanium head

equipped with an electronic pressure reader (Wika). d8-THF was used as solvent for all high

pressure NMR experiments. The d8-THF solution (0.3 – 0.4 mL) of the desired reactants (e.g.

catalyst, acidic additive etc.) was prepared and transferred to the high pressure NMR tube

inside a glovebox. After that, the desired pressures of CO2 and H2 were added. Inside the

magnet of the NMR machine, the NMR tube could be heated up to 80 °C. If the tube should

be heated at higher temperatures (e.g. 140 °C), this was achieved by heating the tube in an

external oil bath. Only the lower 4 cm of the tube were dipped into the oil bath, to avoid

overheating of the titanium head. After heating in the external oil bath, the NMR tube was

introduced into the NMR magnet which was preheated to the desired measurement

temperature. The sample was shimmed manually, the spectra were phase and baseline

corrected automatically and the peaks integrated manually.

5.4.3 Continuous catalysis

5.4.3.1 Description of the continuous flow equipment

The continuous flow reaction rig was originally planned by Jens Theuerkauf[185] and built in

the mechanical workshop of the ITMC, RWTH Aachen University. This continuous setup is

similar the one designed by Ulrich Hintermair and co-workers at the ITMC, which was

described in detail elsewhere.[193-194] Compared to the original design, some changes were

made. Flow schemes of the continuous reaction set-up with stirred tank reactor and tubular

reactor are shown in Figure 59. The maximum operating pressure of the reaction rig was

300 bar. A stainless steel autoclave (inner volume = 13 mL) equipped with a glass inlet, a

magnetic stir bar and a dip tube was used as stirred tank reactor for reactions in which a

bulk ionic liquid phase was applied. The dip tube was constructed in a way that it immerged

0.5 cm into the reaction solution when the glass liner was filled with 2 mL of liquid. For

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heating and stirring a hot plate stirrer (IKA, model RET control/t) equipped with an

aluminium cone was used (TIRC 2, Figure 59, left). The reactor was equipped with a ball

valve (BV 5, Figure 59, left) and a metering valve allowing for closing the reactor (MV 2,

Figure 59, left). For reactions with SILP catalyst a stainless steel tubular reactor (inner

diameter = 0.75 cm, length = 22.5 cm) was used. Heating of the tubular reactor was achieved

with heating tape controlled by a PID controller (Eurotherm, model 91e, TIRC 2, Figure 59,

right). At both ends of the tubular reactor ball valves (BV 5 and BV 6, Figure 59, right) were

installed, which allowed for closing the reactor. In this way, the air-sensitive catalyst could

be filled into the reactor inside a glovebox, and the reactor mounted in the continuous

reaction rig without contamination with air. Heating of the capillary between the reactor

outlet and the back pressure regulator was achieved using the same combination of heating

tape and PID controller (TIRC 1). Dosing of hydrogen gas was achieved using a commercial

mass flow controller (Brooks, Smart Mass Flow 5800, MFC 2). Dosing of CO2 was achieved by

a continuous dosing unit developed by Hintermair and co-workers (MFC 1).[193] This dosing

unit consisted of an electrically heated and pneumatically actuated needle valve (SiTec,

PNV 1), a proportional valve (Festo, VPPM, PV 1) and a high pressure liquid flow meter

(Bronkhorst, LiquiFlow L, LFM). PID regulation of the valve was realised via computer using

LabVIEW™ software. The system pressure was regulated by a back pressure regulator (BPR)

developed by Hintemair and co-workers.[193] Similar to the CO2 dosing unit, the BPR

consisted of a combination of a heated pneumatic needle valve for dosing (SiTec, PNV 2), a

proportional valve (Festo, VPPM, PV 2) and, instead of the liquid flow meter in the CO2

dosing unit, a digital pressure transducer (WIKA, PIRCA+ 3). A magnetic trigger valve

(Bürkert, MTV) was used to pulse the pneumatic signal for the needle valve and the decay of

the pulses was attenuated with the help of a metering valve (HOKE, Milli-Mite, MV 1). After

the BPR unit, the product stream was bubbled trough a cooling trap (CT) made of glass

(height = 25 cm, inner diameter = 2 cm) filled with glass beads (1 mm) and THF (8 mL). The

cooling trap was cooled to −72 °C using a dry ice/ethanol bath. MFC 1, MFC 2, and the BPR

were regulated using LabVIEW™ 8.2 (National Instruments) software on a standard PC

running under Windows XP (Microsoft).

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5.4.3.2 Preparation of the reactor for continuous catalysis in ionic liquid bulk phase

For reactions in an ionic liquid bulk phase, a stainless steel autoclave (inner volume = 13 mL)

equipped with a glass inlet, a magnetic stir bar and a dip tube was used. The catalyst solution

was prepared as described in 5.4.1, and 2 mL of this solution were transferred to the

autoclave inside a glove box. The dip tube was screwed onto the autoclave and the

autoclave was closed inside the glove box by closing MV 2 as well as BV 5. After that, the

reactor was properly closed outside the glove box and pressurised with CO2 and H2

(p(CO2)/p(H2) = 1/3) up to the pressure at which the continuous flow experiment was

conducted. The autoclave was mounted in the reaction rig, placed inside the aluminium

cone of the hot plate stirrer and the solution stirred at 500 rpm at RT for at least 20 minutes

before heating was started.

5.4.3.3 Preparation of the reactor for continuous catalysis using a SILP catalyst

For reactions with SILP catalyst a stainless steel tubular reactor (inner diameter = 0.75 cm,

length = 22.5 cm) was used. Inside a glove box, a stopper of glass wool (ca. 2 cm) was filled

into the reactor. The height of the remaining empty tube was measured with a dip stick. SILP

catalyst was weighed into the reactor and the remaining height again measured with the dip

stick (difference = SILP bed volume). The catalyst bed was fixed with another stopper of glass

wool (ca. 2 cm), the reactor closed with a wrench and both ball valves (BV 5 and BV 6)

closed. Outside the glove box, all screws were thoroughly tightened and the reactor

mounted in the reaction rig.

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Figure 59: Flow-scheme of the continuous-flow setup with stirred tank reactor (left) and tubular reactor (right).

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5.4.3.4 Conduction of a continuous flow experiment

Preparation of the continuous reaction rig started one day before the reaction was started.

The PC was rebooted and LabVIEW™ was restarted. Lock valves LV 1 (compressed air) and

LV 2 (CO2) as well as ball valve BV 2 were opened. Three way valves TW 1 and TW 2 were set

to “bypass” position. Heating of MFC 1 and BPR was started. A CO2 flow of 20 mLN/min was

started and the BPR set to the desired system pressure. After reaching system pressure,

proper working of the BPR was verified, and all connections and valves were checked for

leaks with soap water. Next morning, the prepared reactor (vide supra) was mounted in the

reaction rig and the connections purged with CO2 for at least three times. LV 3 (H2) was

opened and BV 4 remained closed while a H2 flow of 100 mLN/min was programmed. After

the pressure built up to system pressure, the H2 flow was reduced to 10 mLN/min and BV 4

was opened. The desired CO2 and H2 flows were adjusted and the system was run in this way

for at least 1 hour to achieve the desired ratio of CO2 and H2 in the reaction rig. Heating of

the capillary at 140 °C was started at this point (TIRC 1).

In the case of the stirred autoclave, the reactor was pressurised with the desired system

pressure and after that, the autoclave was mounted in the reaction rig. Therefore, TW 1 and

TW 2 were set to “reactor”, BV 5 opened, and MV 2 carefully opened to direct the flow

through the reactor. Stirring of the reaction mixture was continued at 500 rpm at r.t. until

the mixture was stirred for at least 20 minutes.

In the case of the tubular reactor, the reactor was at ambient pressure when mounted in the

reaction rig. TW 1 was switched to “reactor” and then switched back to “bypass” to

pressurise the capillary between TW 1 and BV 5 with CO2/H2 at system pressure. The

pressure was carefully vented to the reactor by slowly opening BV 5 and closing it again. This

procedure was repeated until the reactor reached system pressure. TW 1 and TW 2 were

switched to “reactor” and BV 5 and BV 6 were opened to direct the flow through the reactor.

In both cases, the first cooling trap (filled with glass beads and 8 mL of THF, cooled to −72 °C)

was now installed and the reaction started by heating of the autoclave. After ca. 15 minutes

a reaction temperature of 140 °C was reached. The cooling trap was exchanged periodically.

The content of the cooling trap was collected, the cooling trap washed with THF (2 mL) and

the combined liquids weighed. For GC analysis, n-heptane (ca. 7 mg) and ca. 500 mg of this

liquid were weighed into a GC vial.

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For stopping the reaction, heating of the reactor was switched off, BV 5 and BV 6 (tubular

reactor) or BV 5 and MV 2 (autoclave) were closed, and TW 1 and TW 2 were switched to

“bypass”. The H2 flow was stopped by setting the flow rate to 0 and closing BV 4 and LV 3.

After cooling down, the reactor was dismounted from the rig, carefully vented and the

reactor content was analysed. The continuous set-up was cleaned by purging with CO2 at

120 bar and 100 mLN/min while continuing heating the capillary at 140 °C over night. For

shutting down the continuous rig, heating of the capillary was switched off, the BPR was set

to 10 bar and the CO2 flow stopped by setting the flow rate to 0 and closing BV 2 and LV 2.

After reaching 10 bar, the BPR was switched off. The reaction rig was kept under 10 bar

overpressure to avoid contamination with oxygen.

5.5 Synthesis

5.5.1 Synthesis of [Ru(TMM)(Triphos)] (2)

Complex 2 was synthesised following the procedure developed by Thorsten vom Stein et

al.[115]

A 35 mL Schlenk tube was charged with [Ru(methylallyl)2(cod)] (159.5 mg, 0.5 mmol) and

1,1,1-tris(diphenylphosphinomethyl)ethane (Triphos) (312.0 mg, 0.5 mmol, 1 eq.) in 25 mL

toluene. After heating for 2 h at 110 °C, the resulting solution was concentrated in vacuo and

treated with pentane (10 mL). The precipitating complex was isolated and washed with

pentane (3 x 10 mL). After drying, complex 2 was obtained as a pale yellow powder

(273.0 mg, 0.35 mmol, yield = 70 %). The identity and purity of 2 was confirmed by NMR-

spectroscopy.

1H-NMR (600 MHz, d2-DCM, r.t.): δ = 7.16 - 7.07 (m, 18H, CAr-H), 6.99 (m, 12H, CAr-H), 2.28

(br, 6H, P-CH2), 1.67 (br, 6H, C-CH2), 1.44 (s, 3H, CH3).

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13C{1H}-NMR (125 MHz, d2-DCM, r.t.): δ = 141.0 (m, ipso-CAr), 132.2 (m, o-CAr-H), 127.6 (s, p-

CAr-H), 127.3 (s, m-CAr-H), 106.5 (br, C(CH2)32-), 43.2 (m, C(CH2)3

2-), 38.9 (qua, JC-P = 9.7 Hz,

CH3), 38.2 [m, (Ph2PCH2)3C-CH3], 35.6 (m, P-CH2) ppm.

31P{1H}-NMR (243 MHz, d2-DCM, r.t.): δ = 34.4 (s, 3P) ppm.

For further analytical data see reference [115].

5.5.2 Synthesis of [Ru(2-OAc)(Triphos)(S)]NTf2 (14)

[Ru(2-OAc)Cl(Triphos)] (12) (123.0 mg, 0.15 mmol) and AgNTf2 (60.0 mg, 0.155 mmol, 1.03

eq.) were stirred in tetrahydrofuran (12 mL) at 60 °C for 3 hours. The solution was filtered

over silica to remove the greyish precipitate and the solvent was removed in vacuo to give a

yellow to orange solid (84 mg, 0.08 mmol, yield = 53 %).

Characterisation of the material by 1H-, 13C- and 19F-NMR, FT-IR and ESI-HRMS revealed the

presence of the cation [Ru(2-OAc)(Triphos)]+ as well as the NTf2- anion. Crystallisation from

dichloromethane layered with pentane gave yellow single crystals of complex

[Ru(2-OAc)(Triphos)(H2O)]NTf2 (14a) where the open coordination site was saturated with

H2O from adventitious traces of water (X-Ray crystal structure of 14a is shown in Figure 26

and CIF-data is available online as ESI in ref. [115]). After drying in vacuo for about 24 h at r.t.,

the 1H-NMR spectrum in d2-DCM showed no coordinated water, THF or other coordinated

ligands. Thus, the acetate complex in solution can be formulated as

[Ru(2-OAc)(Triphos)(S)][NTf2] (14) with S being a free coordination site or weakly bound

solvent molecule.

1H-NMR (600 MHz, d8-THF, r.t.): δ = 7.49 - 7.05 (m, 30H, CAr-H), 2.51 (br, 6H, P-CH2), 2.07 (s,

3H, O2CCH3), 1.74 (br qua, JH-P = 2.5 Hz, 3H, CH3) ppm.

1H-NMR (400 MHz, d2-DCM, r.t.): δ = 7.29 - 7.05 (m, 30H, CAr-H), 2.38 (br, 6H, P-CH2), 2.11 (s,

3H, O2CCH3), 1.69 (br qua, JH-P = 2.2 Hz, 3H, CH3) ppm.

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13C{1H}-NMR (151 MHz, d8-THF, r.t.): δ = 189.0 (s, O2CCH3 ), 135.7 (m, CAr), 132.2 (s, CAr-H),

129.2 (s, CAr-H), 127.7 (s, CAr-H), 120.3 (qua, JC-F = 321 Hz, CF3), 39.0 [s, (Ph2PCH2)3C-CH3], 35.8

(qua, JC-P = 10.8 Hz, CH3), 33.6 (br, P-CH2), 24.0 (m, O2CCH3) ppm.

31P{1H}-NMR (243 MHz, d8-THF, r.t.): δ = 44.0 (s) ppm.

31P{1H}-NMR (243 MHz, d2-DCM, r.t.): δ = 42.5 (s) ppm.

19F{1H}-NMR (566 MHz, d8-THF, r.t.): δ = -79.81 (s) ppm.

ESI-HRMS (THF) positive ion: calculated for [Ru(2-OAc)(Triphos)]+: m/z = 785.14411;

determined: m/z = 785.14337.

FTIR (ATR): ῦ = 3068 (w), 2966-2870 (w), 1521 (w), 1490 (w), 1464 (m), 1439 (m), 1352 (m),

1230 (w), 1184 (s), 1136 (m), 1094 (m), 1057 (s), 1002 (w), 949 (w), 837 (m), 791 (w), 739

(w), 693 (s), 654 (w), 614 (m), 600 (m), 571 (m), 548 (w), 517 (s), 489 (s), 415 (m) cm-1.

The absorption bands between 1400 and 1600 cm-1 were very similar to the bands in the

starting complex [Ru(2-OAc)Cl(Triphos)] (12) indicating a similar chelating binding mode of

the acetate ligand.

X-Ray Analysis: X-Ray analysis and refinement of the structure was carried out by Prof. U.

Englert, Institut für Anorganische Chemie, RWTH Aachen University. Intensity data were

collected with a Bruker D8 goniometer equipped with a Bruker APEX CCD area detector and

an Incoatec microsource (Mo-K radiation, = 0.71073 Å, multilayer optics) at 100 K (Oxford

Cryostream 700 instrument).

Crystal data for 14a, C43H42O3P3Ru+C2F6NO4S2- · CH2Cl2: monoclinic space group P21, a =

12.731(2), b = 15.301(3), c = 13.247(2) Å, = 109.038(3)°, V = 109.038(3) Å3, Z = 2, 26352

reflections collected within max = 25.14 °, Rint = 0.0998.

Data were integrated with SAINT (Bruker, 2009, SAINT+, version 7.68) and corrected for

absorption by multi-scan methods (Bruker, 2001, SMART, version 5.624). The structure was

solved by direct methods (SHELXS-97) and refined by full matrix least squares procedures

based on F2 as implemented in SHELXL-97. One of the trifluoromethylsulfonate groups in the

anion was disordered over two positions, with a site occupancy of 0.599(5) for the majority

conformer. A total of 24 similarity restraints for bond distances and angles in the alternative

conformations were applied. C, F and O atoms in the minority conformer were treated as

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isotropic. Anisotropic displacement parameters were assigned to all other non-hydrogen

atoms, and hydrogen atoms were treated as riding. Refinement of 647 variables based on

8701 independent data converged for wR2 (all reflections) = 0.1212, R1 (observed

reflections) = 0.0571, GOF = 1.045 and a Flack enantiomorph polarity parameter of -0.05(4).

5.5.3 Synthesis and characterisation of [Ru(2-O2CH)(Triphos)(THF)]NTf2 (8a) in solution

HNTf2 (7.0 mg, 0.025 mmol) was dissolved in d8-THF (0.5 mL) and added to

[Ru(TMM)(Triphos)] (2) (19.5 mg, 0.025 mmol, 1 eq.) in d8-THF (0.5 mL) at room

temperature, giving a deep red coloured solution. HCO2H (0.9 L, 0.025 mmol, 1 eq.) was

added via micro-syringe, the mixture was stirred and the solution became orange. The yield

of 8a was ca. 86 % as judged from the 31P{1H}-NMR spectrum. Besides the signals due to 8a a

signal of a yet unidentified phosphor containing species (singlet at 59 ppm, ca. 14 % of total

intensity) was observed in the 31P{1H}-NMR.

1H-NMR (600 MHz, d8-THF, -40 °C): δ = 8.75 (br, 1H, O2CH), 7.60 – 6.90 (m, 30H, CAr-H), 3.60

(br, THF), 2.69 (m, 2H, P-CH2), 2.47 (m, 2H, P-CH2), 2.43 (d, JH-P = 9.6 Hz, 2H, P-CH2), 1.76 (br,

THF), 1.75 [br, 3H, (Ph2PCH2)3C-CH3] ppm.

31P{1H}-NMR (243 MHz, d8-THF, -40 °C): δ = 47.1 (d, JP-P = 42.4 Hz, 2P), 44.2 (t, JP-P = 42.4 Hz,

1P) ppm.

13C{1H}-NMR (151 MHz, d8-THF, r.t.): δ = 178.4 (s, O2CH), 135.4 (s, ipso-CAr), 132.3 (s, o-CAr-H),

129.3 (s, p-CAr-H), 127.8 (s, m-CAr-H), 120.3 (qua, JC-F = 322 Hz, CF3), 66.4 (m, THF), 39.3 [s,

(Ph2PCH2)3C-CH3], 35.7 (m, CH3), 33.8 (br, P-CH2), 24.4 (m, THF) ppm.

FTIR (Transmission, THF, r.t.): ῦ1543 (m), 1486 (w), 1435 (m), 1357 (s), 1338 (m), 1227 (m),

1206 (m), 617 (m), 601 (w), 570 (w), 547 (w), 519 (s), 493 (w), 460 (w) cm-1.

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5.5.4 Synthesis of [Ru(H)(CO)2(Triphos)]NTf2 (4NTf2)

The literature known[132] complex 4 could be synthesised in nearly quantitative yield by the

following newly developed procedure:

[Ru(TMM)(Triphos)] 2 (100 mg, 0.128 mmol) and HNTf2 (35.9 mg, 0.128 mmol, 1 eq.) were

dissolved in ethyl formate (5 mL). The deep red coloured solution was transferred to a 10 mL

stainless steel autoclave. The autoclave was pressurised with hydrogen gas (60 bar) and

stirred and heated at 140 °C for 24 hours. After cooling to ca. 0 °C in an ice bath the

colourless solution was transferred to a Schlenk tube. After removing all volatiles in vacuo

complex 4NTf2 was obtained as white solid (131.9 mg, 0.124 mmol, 97 %). The identity of

4NTf2 was confirmed by NMR and ESI-MS analysis.[132]

1H-NMR (400 MHz, d8-THF, r.t.): δ = 7.72 – 6.84 (m, 30H, CAr-H), 2.85 (d, JH-P = 9.4 Hz, 2H, P-

CH2), 2.70-2.60 (m, 4H, P-CH2), 1.87 (br qua, JH-P = 3.0 Hz, 3H, CH3), −6.68 (dt, JH-P = 63.9 Hz,

JH-P = 15.3 Hz, 1H, Ru-H) ppm.

31P{1H}-NMR (162 MHz, d8-THF, r.t.): δ = 16.3 (d, JP-P = 28.7 Hz, 2P), 4.4 (t, JP-P = 28.7 Hz, 1P)

ppm.

ESI-MS (THF) positive ion: m/z = 783.1

ESI-MS (THF) negative ion: m/z = 279.9

5.5.5 [Ru(2-OAc)Cl(Triphos-anisyl)] (15)

EXPERIMENTAL

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The complex [Ru(2-OAc)Cl(Triphos-anisyl)] (15) was synthesised in an analogous way to the

synthesis of [Ru(2-OAc)Cl(Triphos)] (12).[159]

[Ru(2-OAc)Cl(PPh3)3] (13) (98.2 mg, 0.1 mmol) and Triphos-Anisyl (80.5 mg, 0.1 mmol, 1 eq.)

were dissolved in toluene (9 mL). The solution was stirred and heated at 110 °C for 3 h. The

yellow solution was cooled to room temperature, and after addition of pentane (5 mL) a

yellow solid precipitated. After decantation of the supernatant the yellow solid was washed

with pentane (3×10 mL) and dried in vacuo (76 mg, 0.08 mmol, yield = 76 %).

1H-NMR (600 MHz, d8-THF, r.t.): δ = 7.53 – 6.58 (m, 24H, CAr-H), 3.73 (s, 18H, Ar-OCH3), 2.17

(br, 6H, P-CH2), 1.94 (s, 3H, O2CCH3), 1.46 (br qua, JH-P = 2.0 Hz, 3H, CH3) ppm.

13C{1H}-NMR (151 MHz, d8-THF, r.t.): δ = 186.2 (s, O2CCH3 ), 160.5 (s, p-CAr-OCH3), 134.2 (s, o-

CAr), 128.8 (s, ipso-CAr-H), 113.3 (s, m-CAr-H), 55.5 (s, O-CH3), 38.3 [s, (An2PCH2)3C-CH3], 37.9

[qua, JC-P = 10.2 Hz, (An2PCH2)3C-CH3], 33.5 (br, P-CH2), 25.8 (m, O2CCH3) ppm.

31P{1H}-NMR (243 MHz, d2-DCM, r.t.): δ = 38.8 (br, 2P), 34.5 (br, 1P) ppm.

ESI-HRMS (THF) positive ion: [Ru(2-OAc)(Triphos-Anisyl)]+ calculated: m/z = 965.20750;

determined: m/z = 965.20648.

ATR-IR: ῦ = 3100-2900 (w), 2834 (w), 1593 (m), 1570 (m), 1531 (w), 1500 (m), 1454 (m), 1404

(m), 1288 (m), 1245 (s), 1180 (s), 1089 (s), 1028 (m), 942 (w), 844 (w), 822 (m), 797 (s), 745

(w), 722 (w), 674 (m), 622 (m), 540 (s), 525 (m), 497 (m), 465 (m), 432 (w), 418 (m) cm-1.

5.5.6 Synthesis of SILP catalyst

The synthesis of the SILP catalyst was accomplished following literature-known

procedures.[25-26, 28] [Ru(TMM)(Triphos)] (2) (14.6 mg, 18.8 μmol) and HNTf2 (5.3 mg,

18.8 μmol, 1 eq.) were weighed into a Schlenk tube and dissolved in THF (5 mL).

[EMIM][NTf2] (1.5 mL, Iolitec, I01125.1.3, > 99.5 % (NMR), ethyl sulfate = 40 ppm (IC),

[EMIM] = 99.9 % (IC), [NTf2] = 99.9 % (IC)) was added and the mixture stirred at r.t.

Perfluoro-alkyl [-Si(Me)2CH2CH2C6F13] functionalised silica (particle size 32-63 μm, mean pore

diameter 59 Å, BET surface area 207 m2∙g-1, mesopore volume 0.8 mL g-1, SGFLUO)[25, 191] was

suspended in THF (10 mL). The catalyst/IL solution was added dropwise to this carefully

stirred silica suspension. After stirring this mixture for 1 h at r.t. THF was removed slowly in

EXPERIMENTAL

-132-

vacuo (over ca. 1 h). The remaining powder was dried in vacuo to give a pale yellow, dry

powder.

5.6 DFT-calculations

DFT-calculations were carried out by M. Hölscher, V. Moha, and J. Kothe (ITMC, RWTH

Aachen University) using the Gaussian09 program series (Revision C.01 and D.01).[195]

“Gas phase calculations

The M06-L density functional[196-200] and the def2-SVP basis set[201-205] with the associated

ECP[206-207] for ruthenium were used to calculate the optimised geometries of all structures

with no constraints or restraints. The automatic density fitting approximation was

activated.[208-209] Frequency calculations were carried out to assign structures as local minima

(i = 0) or transition states (i = 1). IRC calculations were performed for the most optimised

transition states to ensure connection of the transition state with the minima.

Thermochemical corrections were computed for standard state conditions. Single point

energies were additionally calculated using M06-L/def2-TZVP. Corrected values for the Gibbs

free energies were obtained by adding the thermochemical corrections from the lower-level

geometry optimisations to the electronic energies of the higher-level single-point

calculations. These values were used for discussion throughout this work.

Solvent phase calculations

Solvent phase calculations were performed for selected gas phase structures with no

constraints or restraints using the MN12-L density functional and the def2-TZVP basis set[201-

205] with the associated ECP[206-207] for ruthenium. The automatic density fitting

approximation was activated.[208-209] IEF-PCM and CPCM[210-211] formalisms were used to

consider solvent effects. Frequency calculations were carried out to assign structures as local

minima (i = 0) or transition states (i = 1). Thermochemical corrections were performed for

413.15 K, and entropy corrections in the condensed phase were considered by specifying a

pressure of 302 atm.[212-213]” [139]

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