Bis(N-heterocyclic silylene)xanthene in Transition-Metal Catalysis and Main Group Chemistryvorgelegt von
der Technischen Universität Berlin
Doktor der Naturwissenschaften
- Dr. rer. nat. -
Gutachter: Prof. Dr. Matthias Driess (Technische Universität Berlin)
Gutachter: Prof. Dr. Christian Müller (Freie Universität Berlin)
Tag der wissenschaftlichen Aussprache: 27.01.2020
Berlin 2020
Yuwen Wang
Die vorliegende Arbeit entstand in der Zeit von Sep. 2019 bis Nov. 2019 unter der Betreuung von Prof.
Dr. Matthias Driess am Institut für Chemie der Technischen Universität Berlin.
Von Herzen kommend gilt mein Dank meinem verehrten Lehrer
Herrn Professor Dr. Matthias Driess
für die Aufnahme in seinen Arbeitskreis, für seine engagierte Unterstützung,
und für die Forschungsfreiheit.
Acknowledgements
First and foremost, I would like to express my heartfelt and sincere gratitude to my supervisor Prof.
Matthias Driess for his continuous support of my Ph.D. study. His immense knowledge, warmly
encouragement, and endless support greatly promoted my research. Without his guidance and
invaluable input, this research work would not have been possible. I could not have imagined having
a better Doktorvater for my Ph.D. study.
I am grateful to Professor Dr. Christian Müller for acting as the external referee for this thesis, and
Professor Dr. Maria Andrea Mroginski for accepting the invitation to be the chairman of the doctoral
committee.
I would like to express my special gratitude to Dr. Shenglai Yao and Dr. Yun Xiong, who helped me
in numerous ways during various stages of my Ph.D. which is not only limited to scientific research
but also to life. Their valuable suggestions, insightful discussions, and positive feedback have
significantly contributed to the success of my Ph.D. research. Thank you again for being so supportive
all the time.
I would also like to take this opportunity to thank Professor Dr. Yitzhak Apeloig, Dr. Miriam Karni
(Technion-Israel Institute of Technology, Israel), Dr. Tibor Szilvasi (University of Wisconsin−Madison,
United State) and Dr. Arseni Kostenko for carrying out the DFT calculations for the research results of
this thesis. Their excellent theoretical calculation work has largely deepened the understanding of the
silyene chemistry.
The analytic centers of the Institut für Chemie, Technische Universität Berlin are acknowledged for
their outstanding service and invaluable discussions. I would like to especially thank Paula Nixdorf
for the assistance in the XRD measurement, Dr. Jan Dirk Epping and Dr. Sebastian Kemper for the
NMR measurement, Juana Krone for the elemental analysis measurement, and Dr. Maria Schlangen-
Ahl and Marc Griffel for the HRMS measurement.
I gratefully acknowledge the China Scholarship Council (CSC) for financial support. I would also
like to thank all the members of the Berlin International Graduate School of Natural Sciences and
Engineering (BIG-NSE).
I am indebted to all the lab members of the Driess group for their kind help and friendship. These
include the current members Dr. Shenglai Yao, Dr. Yun Xiong, Dr. Jan Dirk Epping, Dr. Prashanth
Menezes, Dr. Bochao Su, Dr. Arseni Kostenko, Dr. Stephan Kohl, Dr. Biswarup Chakraborty, Andrea
Rahmel, Stefan Schutte, Changkai Shan, Jian Xu, Marcel-Philip Lücke, Carsten Walter, Alexander
Burchert, André Hermansdorfer, Frank Czerny, Christopher Eberle, Viktoria Forstner, Niklas J.
Hausmann, Rodrigo Beltran Suito, Shweta Kalra, Noah Subat and as well as the former group members
Dr. Zhenbo Mo, Dr. Yupeng Zhou, Dr. Xiaohui Zhao, Dr. Ernesto Ballestero, Dr. Terrance Hadlington,
Dr. Jan Paulmann, Min Ha Kim and Mandy Prillwitz.
I would like to acknowledge Dr. Shenglai Yao for proof-reading and correcting this thesis. I thank
Marcel-Philip Lücke for translating the abstract.
I would like to express my gratitude to Professor Dr. Huadong Wang for his guidance and kind help
during my Master study at Fudan University.
Last but not least, I would like to take this opportunity to express my heartfelt gratitude to my parents
and family for supporting and encouraging me to pursue my dreams. My special gratitude goes to my
sister Yuxiao Wang and brother Qiang Kou who have kept supporting me through years. Finally, I owe
thanks to my beloved husband, Xichang for his continued and unfailing love, support and
encouragement during my pursuit of Ph.D. degree. I will be forever grateful for your presence in my
life.
Nickel(0) Complex for Efficient Catalytic Hydrogenation of Olefin”.
Yuwen Wang, Arseni Kostenko, Shenglai Yao, Matthias Driess*,
J. Am. Chem. Soc., 2017, 139, 13499–13506. (DOI: https://doi.org/10.1021/jacs.7b07167)
2) “Silicon-Mediated Selective Homo- and Heterocoupling of Carbon Monoxide”.
Yuwen Wang, Arseni Kostenko, Terrance J. Hadlington, Marcel-Philip Luecke, Shenglai Yao,
Matthias Driess*,
3) “An Isolable Bis(silylenes)-Stabilized Germylone and Its Reactivity”.
Yuwen Wang, Miriam Karni, Shenglai Yao, Yitzhak Apeloig,* Matthias Driess*,
J. Am. Chem. Soc., 2019, 141, 1655−1664. (DOI: https://doi.org/10.1021/jacs.8b11605)
4) “Synthesis of an Isolable Bis(silylenes)-Stabilized Silylone and Its Reactivity Towards Small
Gaseous Molecules”.
Yuwen Wang, Miriam Karni, Shenglai Yao, Alexander Kaushansky, Yitzhak Apeloig,* Matthias
Driess*,
J. Am. Chem. Soc., 2019, 141, 12916−12927. (DOI: https://doi.org/10.1021/jacs.9b06603)
5) “Silicon−Mediated Coupling of Carbon Monoxide, Ammonia, and Primary Amines to Form
Acetamides”.
Marcel-Philip Luecke, Arseni Kostenko, Yuwen Wang, Shenglai Yao, Matthias Driess*,
Angew. Chem. Int. Ed., 2019, 58, 12940–12944. (DOI: https://doi.org/10.1002/anie.201904361)
6) “N-heterocyclic Silylenes as Powerful Steering Ligands in Catalysis”.
Saeed Raoufmoghaddam, Yu-Peng Zhou, Yuwen Wang, Matthias Driess*,
J. Organomet. Chem., 2017, 829, 2−10. (DOI: https://doi.org/10.1016/j.jorganchem.2016.07.014)
Presentations and Posters
1) “The Application of Bis(silylenes) in Main Group Chemistry and the RCO− (R = P, As) as Building
Blocks for Germanium Phosphorus Compounds and Material Precursors”.
Yuwen Wang, Matthias Driess*,
2) “Bis(N-heterocyclic silylene)xanthene as a Powerful Tool in Transition Metal Catalysis and Main-
Group Chemistry”.
May 10−11, 2019.
Yuwen Wang, Miriam Karni, Shenglai Yao, Yitzhak Apeloig,* Matthias Driess*,
Poster, 2019 Reaxys PhD Prize Symposium, Amsterdam, The Netherlands, October 3−4, 2019.
4) “Silicon-Mediated Selective Homo- and Heterocoupling of Carbon Monoxide”.
Yuwen Wang, Arseni Kostenko, Terrance J. Hadlington, Marcel-Philip Luecke, Shenglai Yao,
Matthias Driess*,
Poster, 9th European Silicon Days, Saarbrücken, Germany, September 9−12, 2018.
5) “Divalent Silicon-Assisted Activation of Dihydrogen in a Bis(N-heterocyclic silylene)xanthene
Nickel(0) Complex for Efficient Catalytic Hydrogenation of Olefins”.
Yuwen Wang, Arseni Kostenko, Shenglai Yao, Matthias Driess*,
Poster, 18th International Symposium on Silicon Chemistry (ISOS XVIII) in conjunction with
the 6th Asian Silicon Symposium (ASiS-6), Jinan, China, August 6−11, 2017.
ZUSAMMENFASSUNG
[Xant(SiIIL)2] [Xant = 9,9-Dimethyl-Xanthene-4,5-diyl, L = PhC(NtBu)2] sowie seiner Anwendung in
der Übergangsmetallkatalyse und Hauptgruppenchemie gewidmet.
Zunächst, wurde das neuartige Bis(silylen) [Xant(SiIIL)2] erfolgreich synthetisiert und als starker σ
Donorligand in der Übergangsmetallchemie eingesetzt. Die Reaktion von [Xant(SiIIL)2] mit Ni(cod)2
(COD = cycloocta-1,5-dien) führte zur Bildung des Nickel-Olefin-Komplexes [Xant(SiIIL)2]]Ni(η2-
1,3-cod). Durch einen Ligandenaustausch mit PMe3 gelang die Isolation des Ni-Komplexes
[Xant(SiIIL)2]Ni(PMe3)2. Beide Ni-Komplexe konnten H2 unter Ausbildung zweier Nickelhydrid-
Komplexe aktivieren. Bemerkenswerterweise ist, der Nickel-Olefin-Komplex [Xant(SiIIL)2]Ni(η2-1,3-
cod) ein chemoselektiver, effizienter Präkatalysator für die homogene Hydrierung von Olefinen mit
einem breiten Substratspektrum bei 1 bar H2-Druck und Raumtemperatur. Berechnungen des
katalytischen Hydrierungsprozesses, basierend auf der Dichtefunktionaltheorie (DFT), ergaben einen
neuen Modus der H2-Aktivierung, bei dem die SiII-Atome im [Xant(SiIIL)2] Ligand eine Kooperative
Rolle in den Schritten der H2-Spaltung und Hydrid-Übertragung auf das Olefin besitzen.
Zudem wurde das Bis(silylen) [Xant(SiIIL)2] eingesetzt, um Kohlenmonoxid zu aktivieren, welches
eine extrem starke C-O-Dreifachbindung enthält. Unerwarteterweise fungierte das Bis(silylen) als
Vier-Elektronen-Reduktionsmittel bereits unter milden Reaktionsbedingungen und ermöglichte die
selektive desoxygenative Homokupplung von CO zu dem entsprechenden Disilylketen
[Xant(SiIIL)2](µ-O)(µ-CCO). Im Gegensatz dazu war das Dibenzofuran-Analogon von [Xant(SiIIL)2]
mit einem größeren Abstand zwischen den beiden SiII-Atomen gegenüber CO-inert. Dies deutet auf
die entscheidende Rolle des SiSi-Abstand bei der kooperativen CO-Bindung und -Aktivierung hin.
Theoretischen Untersuchungen zur CO-Homokupplung mit [Xant(SiIIL)2] ergaben, dass im ersten
Schritt der CO-Bindung und –Spaltung CO als Lewis-Säure (Vier-Elektronen-Akzeptor) dient. Dies
steht im Gegensatz zur CO-Koordination durch Übergangsmetalle, bei der CO als Lewis-Basis (Zwei-
Elektronen-Donor) fungiert.
Darüber hinaus war das Bis(silylen) [Xant(SiIIL)2] in der Lage, hochreaktives monoatomares und
nullvalentes Silizium- und Germanium zu stabilisieren, wodurch die Verbindungen [Xant(SiIIL)2]E 0
(E = Ge oder Si) erhalten und strukturell, sowie spektroskopisch charakterisiert werden konnten. Die
elektronische Struktur von [Xant(SiIIL)2]E 0 (E = Ge oder Si) wurde durch DFT-Berechnungen
untersucht, die eindeutig zwei freie Elektronenpaare auf dem zentralen E0 (E = Ge oder Si) Atom
zeigten. In seiner Reaktivität zeigt [Xant(SiIIL)2]E 0 (E = Ge oder Si) eine auffällige Chemie gegenüber
kleinen Molekülen. Die [Xant(SiIIL)2]Ge0 Verbindung konnte ein oder zwei AlBr3-Moleküle
koordinieren, wobei die Lewis-Addukte [Xant(SiIIL)2]Ge(AlBr3) bzw. [Xant(SiIIL)2]Ge(AlBr3)2
erhalten wurden. Durch Reaktion von [Xant(SiIIL)2]Ge0 mit 9-Borabicyclo[3.3.1]nonan (9-BBN) als
potentielle Lewis-Säure, gelang die Isolierung des ersten Boryl-Germylen-Komplexes mit einer
heteroallylischen GeSiSi π-Konjugation. Interessanterweise wurde durch Reaktion von
[Xant(SiIIL)2]Ge0 mit Ni(cod)2 der einzigartige {[Xant(SiIIL)2]GeI}2NiII Komplex mit einem
dreigliedrigen Ge2Ni-Ring erhalten. Im Vergleich zu [Xant(SiIIL)2]Ge0, war die [Xant(SiIIL)2]Si0-
Verbindung oxophiler und konnte daher mit dem milden Oxidationsmittel N2O kontrollierbar zur
Reaktion gebracht werden. Durch die Regulierung der eingesetzten Molmenge des zugesetzten N2O
konnten verschiedene Produkte erhalten werden. Darüber hinaus wurde die Spaltung der starken N-H-
Bindung in Ammoniak durch [Xant(SiIIL)2]Si0 ermöglicht, welches das erste Beispiel der NH3-
Aktivierung in der Ylidon-Chemie darstellt. Herausragend ist, dass [Xant(SiIIL)2]E 0 (E = Ge oder Si)
geeignet war, ein frustriertes Lewis-Paar (FLP) mit BPh3 zu bilden, so dass die kooperative Spaltung
von Wasserstoff sowie die Addition von Ethylen gelang. Diese waren die ersten Beispiele und zeigen
auf, dass Ylidone in der FLP-Chemie eingesetzt werden können.
Abstract
This dissertation is devoted to the synthesis of the first bis(N-heterocyclic silylene)xanthene
compound [Xant(SiIIL)2] [Xant = 9,9-dimethyl-xanthene-4,5-diyl, L = PhC(NtBu)2] and its application
in transition-metal catalysis and main group chemistry.
At first, the novel bis(silylenes) compound [Xant(SiIIL)2] was successfully synthesized and utilized
as a strong σ donor ligand in transition-metal chemistry. The reaction of [Xant(SiIIL)2] with Ni(cod)2
(COD = 1,5-cyclooctadiene) led to the formation of the intriguing Ni-olefin complex
[Xant(SiIIL)2)]Ni(η2-1,3-cod) which could undergo ligand exchange with PMe3 to afford the Ni
complex [Xant(SiIIL)2]Ni(PMe3)2. Both Ni compounds can activate H2 to generate two hydrido Ni
complexes. Remarkably, [Xant(SiIIL)2]Ni(η2-1,3-cod) was a strikingly efficient precatalyst for the
homogeneous hydrogenation of olefins with a wide substrate scope at 1 bar H2 pressure and room
temperature. Density Functional Theory (DFT) calculations on the catalytic hydrogenation process
revealed a new mode of H2 activation, in which the SiII atoms from the [Xant(SiIIL)2] ligand play a
cooperative role in the steps of H2 cleavage and hydride transfer to the olefin.
Then the bis(silylenes) compound [Xant(SiIIL)2] was utilized to activate carbon monoxide which
has an extremely strong C≡O bond. Unexpectedly, under mild reaction conditions, the [Xant(SiIIL)2]
compound acts as a four-electron reduction reagent to facilely achieve the selective deoxygenative
homocoupling of CO, affording the corresponding disilylketene [Xant(SiIIL)2](µ-O)(µ-CCO)
compound. In contrast, the dibenzofuran analogue of [Xant(SiIIL)2] with a longer SiIISiII distance
was inert towards CO, indicating the crucial role of the SiSi distance in cooperative CO binding and
activation. The theoretical investigations on CO homocoupling with [Xant(SiIIL)2] revealed that the
initial step of CO binding and scission involves CO acting as a Lewis acid (four-electron acceptor), in
sharp contrast to CO coordination mediated by transition-metals where CO serves as a Lewis base
(two-electron donor).
Moreover, comprising two strong σ donating SiII atoms, the bis(silylenes) [Xant(SiIIL)2] was capable
to stabilize highly reactive monatomic zero-valent silicon and germanium to give [Xant(SiIIL)2]E 0 (E
= Ge or Si) species which were structurally and spectroscopically characterized. The electronic
structure of [Xant(SiIIL)2]E 0 (E = Ge or Si) was investigated by DFT calculations which
unambiguously exhibited two lone-pairs of electrons on the central E0 (E = Ge or Si) atom. In its
reactivity, [Xant(SiIIL)2]E 0 (E = Ge or Si) shows a striking chemistry towards small molecules. The
[Xant(SiIIL)2]Ge0 compound could coordinate one or two AlBr3 to generate the Lewis adducts
[Xant(SiIIL)2]Ge(AlBr3) and [Xant(SiIIL)2]Ge(AlBr3)2, respectively. [Xant(SiIIL)2]Ge0 could also
react with 9-borabicyclo[3.3.1]nonane (9-BBN) as a potential Lewis acid to furnish the first
boryl(silyl)germylene complex possessing a heteroallylic BGeSi π-conjugation. Interestingly,
when [Xant(SiIIL)2]Ge0 was mixed with Ni(cod)2, the unique {[Xant(SiIIL)2]GeI}2NiII complex with a
three-membered Ge2Ni ring was obtained. Compared with [Xant(SiIIL)2]Ge0, [Xant(SiIIL)2]Si0 is more
oxophilic and therefore could react with the mild oxidant N2O in a controllable way. By changing the
molar amount of the added N2O, distinct products could be obtained. In addition, the cleavage of the
strong N−H bond in ammonia was also accomplished by [Xant(SiIIL)2]Si0, which was the first case of
NH3 activation in ylidone chemistry. Remarkably, [Xant(SiIIL)2]E 0 (E = Ge or Si) was suitable to form
a frustrated Lewis pair (FLP) with BPh3 to cooperatively achieve the heterolytic dihydrogen cleavage
or ethylene addition, representing, for the first time, that a metallylone could be applied in FLP
chemistry.
Content
Content
1.1.1 Isolable NHSis……………………………………………………………………...………..4
1.1.2 Isolable bis(NHSis)………………………………………………...………………………...7
1.1.2.2 Isolable bis(NHSis) synthesized via reduction of bis(chlorosilane)……………………....8
1.1.2.3 Isolable bis(NHSis) synthesized via introduction of amidinato silylene into chelating
scaffold…………………………………………………………………………………...9
1.2.1 Bidentate bis(NHSis) ligands in transition-metal catalysis……………………..…………..11
1.2.2 Tridentate bis(NHSis) ligands in transition-metal catalysis……………..……………….…13
1.3 Low-valent group 14 element compounds for small molecules activation……..…….………….15
1.3.1 Low-valent group 14 element compounds for carbon monoxide activation……..…….…....15
1.3.1.1 Low-valent carbon compounds for carbon monoxide activation………..….….………...15
1.3.1.2 Low-valent silicon compounds for carbon monoxide activation……………..……..…...17
1.3.1.3 Low-valent germanium compounds for carbon monoxide activation…….……………..19
1.3.2 Low-valent silicon compounds for carbon dioxide activation…..…..…………………...….20
1.3.2.1 Divalent silicon compounds for carbon dioxide activation………..…….…….………...20
1.3.2.2 Zero-valent silicon compounds for carbon dioxide activation………...……….………...22
1.4 Isolable zero-valent group 14 element E compounds (E = Si or Ge)…...………………………….23
1.4.1 Isolable triatomic zero-valent silicon compounds…………………………………………..23
1.4.2 Isolable diatomic zero-valent group 14 element E compounds (E = Si or Ge)………………24
1.4.2.1 Isolable diatomic zero-valent silicon compounds…………………………………….....24
1.4.2.2 Isolable diatomic zero-valent germinium compounds…………………………………...25
1.4.3 Isolable monatomic zero-valent group 14 element E compounds (E = Si or Ge)…..………..26
Content
1.4.3.1 Cyclic alkyl silylenes supported monatomic zero-valent group 14 element E compounds
(E = Si or Ge)…………………………………………….…………………...…………26
1.4.3.2 Stable carbenes supported monatomic zero-valent group 14 element E compounds (E =
Si or Ge)….………………………………………………………………………..…….27
1.4.3.3 Other examples of isolable monatomic zero-valent group 14 element E compounds (E =
Si or Ge)….…………………………………………………………...…………………29
3.RESULTS AND DISCUSSION……………………………………………………………………33
3.1.1 Background…………………………………………………..……………………………..33
3.1.3 Synthesis of bis(NHSis)dibenzofuran compound 1b…………………….…………………35
3.1.4 Comparison of 1a and 1b…………………………………………………………………...36
3.2 Synthesis of bis(NHSis)xanthene-coordinated Ni complexes and their applications in the catalytic
hydrogenation of olefins………………………………………………………………………...39
3.2.3 Stoichiometric H2 activation by the bis(NHSis)xanthene-coordinated Ni complexes 2 and
3…………………………………………………………………………………………....43
3.2.5 Bis(NHSis)-stabilized Ni complex 2 for catalytic hydrogenation of olefins……..……..…...48
3.2.6 Mechanism of the catalytic hydrogenation process…………………………..……….…….50
3.3 Bis(NHSis)xanthene-mediated small molecules activation……………………..………………55
3.3.1 Bis(NHSis)xanthene-mediated reductive homocoupling of carbon monoxide……...……...55
3.3.1.1 Background...….………………………………………....……………………………...55
3.3.2 Bis(NHSis)xanthene-mediated carbon dioxide activation…………………..……………...66
3.3.2.1 Background...…………….……………………………....……………………………...66
3.3.2.3 DFT-derived mechanism of bis(NHSis)xanthene-mediated carbon dioxide activation.....68
3.4 Synthesis of the first bis(NHSis)-stabilized monatomic zero-valent germanium compound and its
reactivity towards small molecules……………………..……………………………………….71
3.4.1 Background...…………..…………………………………………………………………….71
3.4.3 Electron structure of compound 15………………………….……………….…….………...74
3.4.4 Reactivity of compound 15……………………………………………….………………….76
3.4.4.1 Reactivity of compound 15 towards AlBr3…………………………..………..................76
3.4.4.2 Reactivity of compound 15 towards 9-BBN……………………………...…...…………78
3.4.4.3 Reactivity of compound 15 towards Ni(cod)2………………………..…………..…...…81
3.4.4.4 Heterolytic H2 cleavage with compound 15 in the presence of BPh3…………………….83
3.5 Synthesis of the first bis(NHSis)-stabilized monatomic zero-valent silicon compound and its
reactivity towards small gaseous molecules…………………………...….……………………87
3.5.1 Background………………………………..…………………..……………………...….…87
3.5.2 Synthesis of the first bis(NHSis)-stabilized Si0 compound 23……...…….………...…….…88
3.5.3 Electron structure of compound 23………………………….……………..……………….92
3.5.4 Reactivity of compound 23……………………………………..……..…………………....93
3.5.4.1 Reactivity of compound 23 towards N2O………………………….…..………………...93
3.5.4.2 Reactivity of compound 23 towards NH3…………………………..……………..……..97
3.5.4.3 H2 and ethylene activation by compound 23 with the assistance of BPh3………………..99
3.5.4.4 Mechanism of H2 and ethylene activation processes………..……………….…….…...102
4. SUMMARY……………………………………………………………………………………....107
5.4.1 Synthesis of compound 1a…………………………..……………………………….…..…115
5.4.2 Synthesis of compound 1b…………………………..…………………………………...…116
Content
5.4.8 Synthesis of 13C labeled compound [Xant(SiIIL)2](µ-O)(µ-13C13CO) 8…………………….122
5.4.9 Synthesis of 18O labeled compound [Xant(SiIIL)2](µ-18O)(µ-CC18O) 8…………………….123
5.4.10 Synthesis of 13C and 18O labeled compounds [Xant(SiIIL)2](µ-O)(µ-13C13CO), [Xant(SiIIL)2]
(µ-O)(µ-13CC18O), [Xant(SiIIL)2](µ-18O)(µ-CC18O) and [Xant(SiIIL)2](µ-18O)(µ-C13CO)]
8…………………………………………………………………...…………………….....123
5.4.13 Synthesis of 13C labeled compound 13………………..…………………...………………126
5.4.14 Synthesis of compound 14………………………………………………...………………127
5.4.15 Synthesis of compound 15………………………………………………...………………128
5.4.16 One-pot synthesis of compound 15………………………………………………………..129
5.4.17 Synthesis of compound 16………………………………………………………………...129
5.4.18 Synthesis of compound 17………………………………………………………………...130
5.4.19 Synthesis of compound 18………………………………………………………………...131
5.4.20 Synthesis of compound 19………………………………………………………………...132
5.4.21 Synthesis of compound 20………………………………………………………………...133
5.4.22 Synthesis of deuterated compound 20-d2………………………………………………….134
5.4.23 Synthesis of compound 21………………………………………………………………...135
5.4.24 Synthesis of compound 22………………………………………………………………...136
5.4.25 Synthesis of compound 23………………………………………………………………...137
5.4.26 Synthesis of compound 24………………………………………………………………...138
5.4.27 Synthesis of compound 25...................................................................................................139
5.4.30 Synthesis of deuterated compound 27-d2………………………………………………….142
Content
5.5 Additional experiments………………………………………………………………………...144
6. REFERENCES…………………………………………………………………………………...147
7. APPENDIX………………………………………………………………………………………167
7.1 Crystal data and structure refinement…………………………………………………………..167
Table 7-1. Crystal data and structure refinement for compounds 1a, 1b and 2……………......…...167
Table 7-2. Crystal data and structure refinement compounds 3~5…………………...………..…..168
Table 7-3. Crystal data and structure refinement compounds 8, 12 and 13…….....………...……..169
Table 7-4. Crystal data and structure refinement compounds 14~16……………………………...170
Table 7-5. Crystal data and structure refinement compounds 17~19……………………………...171
Table 7-6. Crystal data and structure refinement compounds 20~22……………………………...172
Table 7-7. Crystal data and structure refinement compounds 23~25……………………………...173
Table 7-8. Crystal data and structure refinement compound 26~28………………………..……...174
7.2 DFT calculations……………………………………………………………………………….175
7.3 Abbreviations…….…………………………………………………………………………….181
1. INTRODUCTION
It is well-known that transition-metal complexes serve as powerful tools in catalytic transformations
and small molecule activation. Recent research has shown that low-valent main group element
compounds can behave like transition-metals.1 The last three decades have witnessed spectacular
achievements in the field of low-valent group 14 element chemistry, which is particularly evident by
the successful isolation of previously elusive divalent2 and zero-valent3 group 14 element compounds.
Some of the low-valent group 14 element compounds have acted as excellent ligands in transition-
metal catalysis,4 in view of their strong σ donating and π accepting group 14 element centers.5
Moreover, the low-valent group 14 element compounds, mimicking transition-metals,1 are capable of
small molecules activation,6 e.g., H2 activation by the GeI compound ArGeGeAr [Ar = 2,6-(2,6-
iPr2C6H3)2C6H3], 7 and H2 or NH3 activation by a stable singlet carbene.8
The introduction of my dissertation focuses on the advances of fours subtopics in the low-valent
group 14 element chemistry, i.e., the developments of isolable silylenes [especially of N-heterocyclic
silylenes (NHSis)], the applications of bis(NHSis) in transition-metal-based catalysis, the utilization
of low-valent group 14 element compounds in small molecules activation, and the synthesis and
reactivity of isolable zero-valent Ge and Si compounds (Figure 1-1). In the following subchapters, the
state-of-the-art knowledge related to these four sections will be discussed in details.
Figure 1-1. Overview of the introduction.
Introduction
2
1.1 Isolable divalent silicon compounds
The chemistry of divalent silicon compounds is mainly represented by the dicoordinate neutral
silylene species, heavy analogues of carbenes. However, in stark contrast to the parent carbene (H2C:)
A (Figure 1-2), in which the carbon atom forms a sp2-hybrid orbital and is in a triplet state,9 the parent
silylene (H2Si:) B (Figure 1-2) possesses singlet ground state adopting a (3s)2(3p)2 valence electron
configuration due to the dramatically different size of the 3s and 3p orbitals.10,11,12 Involving one lone-
pair of electrons and one empty p orbital, silylenes therefore are typically highly reactive species which
could only be isolated in cryogenic matrices (≤ 77 K) during the 1980s.13,14 Above this temperature,
the reactive silylenes will immediately undergo dimerization or further polymerization.15
Figure 1-2. The different electronic ground states of the parent carbene A and silylene B.
In order to access the isolable silylenes at room temperature, two marvelous stabilization strategies
could be applied: thermodynamic and/or kinetic stabilizations both aiming at an efficient protection of
the highly reactive vacant p orbitals of silylenes (Figure 1-3).12
Figure 1-3. The strategies for the stabilization of silylenes.
Introduction
3
The thermodynamic stabilization strategy generally utilizes heteroatom substitutes to partially fill
the empty p orbitals of silylenes via the mesomeric effect and the intramolecular coordination of the
heteroatoms (Figure 1-3, X). The kinetic stabilization employs extremely bulky substitutes, providing
steric hindrance, to prevent silylenes from self oligomerization or attacking by external nucleophiles
(Figure 1-3, Y).
By taking advantage of the aforementioned strategies, Jutzi and co-workers reported the first
isolable silylene I-1, stabilized by two bulky pentamethylcyclopentadienyls, via the reduction of the
corresponding dichlorosilane precursor (Chart 1-1a).16 In 1994, the first NHSi was reported by West
et al.,17 which represents a landmark in NHSi chemistry.
Up to now, remarkable advances have been achieved in the flourishing stable silylene chemistry,
which is evident by the successful isolation of various silylenes, e.g., the cyclic alkyl silylenes I-2~3
(Chart 1-1b),18 the cyclic (alkyl)(amino)silylene I-4 (Chart 1-1c),19 phosphine-stabilized silylene I-5
(Chart 1-1d)20 and the diverse acyclic silylenes I-6~11 (Chart 1-1e).21
Chart 1-1. Selected examples of isolable silylenes.
Given the significant role of NHSis in the silylene chemistry, the recent fruitful reactivity will be
discussed in detail in the following sections 1.1.1 and 1.1.2.
Introduction
4
In 1991, Arduengo and coworkers reported the first stable crystalline N-heterocyclic carbene
(NHC).22 Three years later, the first isolable NHSi I-12a, a heavier analogue of the “Ardueugo type”
carbene, was synthesized by West and Denk et al. through the reduction of the N-heterocyclic silicon
dichloride precursor with potassium (Scheme 1-1).23a This seminal work opened the avenue to stable
NHSis and fueled tremendous interest of chemists for the synthesis of silylene species.
Scheme 1-1. Synthesis of the first isolable NHSi I-12a.
Chart 1-2. Examples of five-membered NHSis.
Recently, a series of unsaturated five-membered NHSis I-12b~f featuring new alkyl23b and aryl23c,d
substitutes on the N atoms were reported (Chart 1-2a). Interestingly, the analogue of I-12a with a
saturated backbone (i.e., I-13a) was also reported by West and Denk et al.,24a which was more reactive
than its thermally stable unsaturated analog I-12a and could reversibly oligomerize to disilene in the
solid state or in concentrated solutions.24b This tendency to oligomerization could be prevented when
there are methyl or t-butyl substitutes present at the ring carbon atoms (Chart 1-2b, I-13b~e).24c,d
Introduction
5
Remarkably, the benzo- or pyrido-fused five-membered NHSis I-14a~d, including a bis(silylenes) I-
14d, were also isolated (Chart 1-2c).25
1.1.1.2 Isolable six-membered NHSis
The six-membered NHSi chemistry started with the isolation of the modified β-diketiminate
(Nacnac) ligand supported NHSi I-18a reported by our group in 2006.26 The dibromosilane precursor
I-17 was obtained by a one-pot synthesis in which the Nacnac scaffold I-15 was firstly lithiated by n-
BuLi to generate I-16 followed by the reaction with SiBr4 in the presence of N,N,N’,N’-
tetramethylethylenediamine (TMEDA) (Scheme 1-2). It is worth noting that, in the absence of
TMEDA, the reaction of I-16 with SiBr4 resulted in a mixture of products instead of the compound I-
17. Therefore, TMEDA is considerably significant in both activating SiBr4 by forming a complex and
promoting the dehydrohalogenation to give I-17 instead of the Nacnac-SiBr3. Ultimately, the reduction
of I-17 with potassium graphite led to the desired six-membered NHSi I-18a as yellow crystals
(Scheme 1-2).
Scheme 1-2. Synthesis of the six-membered NHSi I-18a26.
The two resonance structures I-18a and I-18b indicate that there are two possible nucleophilic
centers in this NHSi compound: one is at the silicon atom with a lone-pair of electrons and the other
is at the exocyclic methylene group of the backbone (Scheme 1-3).26 Consequently, NHSi I-18
exhibited unique reaction modes compared with the aforementioned NHSis. For instance, compound
I-18 reacted with trimethylsilyl trifluoromethanesulfonate (MeOTf) to give the 1,4-addition product I-
19 with the SiMe3 unit bonded at the exocyclic methylene group and the OTf moiety coordinated to
the SiII atom (Scheme 1-3).26 Compound I-19 was not stable in solution and isomerized to the 1,1-
product I-20 gradually at room temperature. The reaction of I-18 with Brønsted acid
Introduction
6
[H(OEt2)2] +[B(C6F5)4]
− led to the formation of the silyliumylidene species I-21 stabilized by a planar
aromatic 6π-electron delocalization via the protonation of the nucleophilic exocyclic methylene part
(Scheme 1-3).27,28 When B(C6F5)3 was mixed with I-18, the zwitterionic product I-22 was isolated.27
Interestingly, the exposure of I-18 to water vapor furnished a siloxy silylene I-23 containing both a SiII
and SiIV atoms (Scheme 1-3).29
Scheme 1-3. Resonance structures of I-1826 and synthesis of compounds I-19~2327~29.
1.1.1.3 Isolable four-membered NHSis
The first isolable four-membered NHSi is the amidinate ligand stabilized chlorosilylene I-25
(Scheme 1-4) synthesized by Roesky et al. in 2006.30 The reduction of the trichloride precursor I-24
with two molar equivalents of potassium resulted in the target silylene I-25 in a mere 10 % isolated
yield (Scheme 1-4). Three years later, the same group modified this synthesis method by utilizing NHC
(i.e., 1,3-di-tert-butylimidazol-2-ylidene) or LiN(SiMe3)2 as reductive reagents and the dichlorosilane
I-26 as the precursor (Scheme 1-4).31 Consequently, the isolated yields were improved to 35% (for the
NHC case) and 90% [for the LiN(SiMe3)2 case], respectively.
Introduction
7
Interestingly, one of the three chlorine substituents in compound 1-24 could be replaced by NMe2,
OtBu, OiPr or PiPr2 groups leading to the compounds I-27.32 Then the reduction of I-27 by potassium
generated the four novel heteroleptic NHSis I-28 (Scheme 1-4).
Scheme 1-4. Synthesis of the four-membered NHSis I-2530,31 and I-2832.
1.1.2 Isolable bis(NHSis)
Recently, bis(NHSis) compounds, containing two NHSi moieties in one molecule, have attracted
increasing attentions owing to the enhanced σ-donating property compared with the NHSis involving
one silylene site. To date, the strategies to access such isolable bis(NHSis) species could be classified
into three categories.
The first method represents the reduction of trichlorosilane precursors to generate the bis(NHSis)
compounds. For instance, the treatment of the trichloride precursor I-24 with three molar equivalents
of KC8 resulted in the isolation of the bis(silylenes) I-29 featuring a SiI−SiI bond (Scheme 1-5).33 The
molecular structure of I-29, determined by an X-ray diffraction analysis, demonstrated that each Si
atom adopted a disordered tetrahedral geometry with one lone-pair of electrons occupying the apex.
In 2011, Kato et al. reported the intriguing phosphine-stabilized bis(NHSis) I-31 through an
analogous procedure, in which the trichlorosilane precursor I-30 was dechlorinated by three molar
equivalents of lithium (Scheme 1-5).20c
Introduction
8
1.1.2.2 Isolable bis(NHSis) synthesized via reduction of bis(chlorosilanes)
The second strategy to obtain isolable bis(NHSis) is the reduction of bis(chlorosilanes) precursors.
In 2005, Gehrhus and Lappert et al. synthesized the first biphenyl-bis(NHSis) I-33 via the reductive
dehalogenation of the bis(dichlorosilane) precursor I-14d (Scheme 1-6).25c
Later, an oxygen-bridged bis(NHSis) I-34 was isolated by Driess et al. through the
dehydrochlorination of disiloxane I-33 (Scheme 1-6).34 Notably, while the two silicon atoms of I-14d
pointed in opposite directions, the two divalent silicon atoms of I-34 oriented in the same direction
providing the possibility to serve as a chelating ligand in transition metal chemistry.
Scheme 1-6. Synthesis of bis(NHSis) compounds I-14d25c and I-3434.
Introduction
9
scaffold
The chlorosilylene I-2530 (Scheme 1-4), stabilized by an amidinate ligand, contains a Si−Cl bond
and therefore can be facilely attached to the dilithiated chelating backbone via a salt metathesis reaction
to access isolable bis(NHSis) compounds.
By taking advantage of this strategy, our group reported the first chelating bis(NHSis) I-37 with a
central phenyl group in 2012.35 The bis(NHSis) I-37 was synthesized by the initial reaction of 4,6-di-
tert-butylresorcinol I-35 with n-BuLi leading to the dilithiated compound I-36 followed by the salt
metathesis reaction with two molar equivalents of chlorosilylene I-25 (Scheme 1-7).
Scheme 1-7. Synthesis of bis(NHSis) I-3735.
Inspired by the successful isolation of the first chelating bis(NHSis) I-37, the same group utilized
ferrocene as a backbone and synthesized the chelating bis(NHSis) I-40 via a similar reaction procedure
(Scheme 1-8).36
Introduction
10
In 2014, the first pyridine-based chelating bis(NHSis) I-42 was synthesized through a one-pot
procedure, in which the dilithiated product was generated in-situ and two molar equivalents of
chlorosilylene I-25 were subsequently added (Scheme 1-9).37 The composition and molecular structure
of I-42 were unambiguously confirmed by multinuclear NMR spectroscopy and a single-crystal X-ray
diffraction analysis.
Recently, employing the similar one-pot synthesis process, Driess et al. also isolated the chelating
bis(NHSis) I-44 with a carborane scaffold (Scheme 1-9).38
Scheme 1-9. One–pot synthesis of bis(NHSis) compounds I-4237 and I-4438.
1.2 Bis(NHSis) as chelating ligands in transition-metal catalysis
The design of novel ligands to steer the reactivity of transition-metal complexes is crucial to the
development of organometallic chemistry.39 Among the diverse ligand systems, chelating ligands are
prominent owing to their significant advantages in controlling the electronic and geometric properties
of metal complexes.40 The well-established transition-metal complexes supported by chelating ligand
have been applied to a variety of research areas ranging from catalytic and stoichiometric chemical
transformations to material chemistry.41
Introduction
11
Since the first isolation of the NHSi-stabilized iron complex by Welz and Schmid in 1977,42 NHSi
ligands with strong σ-donating nature5 have fueled tremendous interest in organometallic chemistry.4
Recently, the bis(NHSis) compounds with two strong σ-donating SiII atoms have been utilized as
bidentate (Chart 1-3a) or tridentate (Chart 1-3b) chelating ligands in transition-metal chemistry.4 Their
transition-metal complexes show excellent catalytic performance in various homogeneous catalytic
transformations,4 which will be exhibited in detail in the following sections 1.2.1 and 1.2.2.
Chart 1-3. Examples of chelating bis(NHSis) ligands.
1.2.1 Bidentate bis(NHSis) ligands in transition-metal catalysis
The oxygen bridged bis(NHSis) I-34, serving as a bidentate ligand, reacted with Ni(cod)2 (COD =
1,5-cyclooctadiene) to generate the Ni complex I-45, in which the two SiII atoms and one COD were
coordinated to the Ni center.34 Then Enthaler and Inoue et al. showed that this bis(NHSis)-supported
Ni complex I-45 was an effective precatalyst in the C−C cross-coupling reaction of aryl halides with
organometallic zinc or Grignard reagents (Scheme 1-10).43
Scheme 1-10. C-C cross-coupling catalyzed by bis(NHSis)-Ni(cod) I-45.
Introduction
12
Another intriguing bidentate bis(NHSis) ligand is the one with a ferrocene backbone (i.e., I-40) 36
and its coordination behavior towards the transition-metal cobalt was firstly investigated. Treatment
of the ferrocene-based bis(NHSis) I-40 with the CpCoI (Cp = Cyclopentadienyl) precursor prepared in
advance led to the successful isolation of the bis(NHSis) coordinated CpCoI complex I-46 (Scheme 1-
11a).36 This complex could act as a precatalyst in the [2+2+2] cyclotrimerization reactions of
phenylacetylene to give two isomers of triphenylbenzene (Scheme 1-11a).36 Interestingly, the
cyclotrimerization of phenylacetylene and acetonitrile catalyzed by I-46 resulted in the substituted
pyridines (Scheme 1-11a). In 2017, the ferrocene-based bis(NHSis) ligand I-40 was also utilized to
stabilize Fe0 complex I-47 with a η6-benzene ring which could achieve the catalytic hydrogenation of
ketones with a wide substrate scope under 50 bar H2 pressure at 50 oC (Scheme 1-11b).44
Scheme 1-11. a) [2+2+2] cyclotrimerizations catalyzed by bis(NHSis)-CoCp I-46. b) Hydrogenation
reaction catalyzed by bis(NHSis)-Fe(η6-benzene) I-47.
The bis(NHSis) I-44 with a carborane backbone could also act as a bidentate chelating ligand in
transition-metal chemistry. Its coordination ability towards NiII was studied leading to the isolation of
the bis(NHSis)-supported NiBr2 complex I-48 (Scheme 1-12).38 Remarkably, the catalytic amination
of various aryl halides and aryl triflates, i.e., Buchwald-Hartwig cross-coupling reaction, could be
Introduction
13
achieved by bis(NHSis)-NiBr2 I-48 in moderate to good yield (Scheme 1-12).38
Scheme 1-12. Buchwald-Hartwig cross-coupling catalyzed by bis(NHSis)-NiBr2 I-48.
1.2.2 Tridentate bis(NHSis) ligands in transition-metal catalysis
The bis(NHSis) I-37 with a benzene ring can serve as a tridentate ligand and react with various
transition-metal precursors to afford pincer-type transition-metal complexes I-49~52 (Chart 1-
4).35,45,46 Treatment of bis(NHSis) I-37 with Ir and Rh precursors afforded the corresponding pincer-
type Ir-, Rh-based complexes I-49~51.45 Moreover, a Ni pincer complex I-52 could be obtained by the
reaction of tridentate ligand I-37 with NiBr(dme) (dme = 1,2-dimethoxyethane) in the presence of an
excess amount of NEt3. 46
Chart 1-4. Bis(NHSis) I-37 stabilized transition-metal complexes I-49~52.
Then the catalytic performances of these typical pincer-type transition-metal complexes I-49~52
were investigated. The in-situ generated complex I-50 successfully catalyzed the C–H borylation of
arenes with pinacolborane (Scheme 1-13a).45 The complex bis(NHSis)-NiBr I-52 could act as a
catalyst for the Sonogashira cross-coupling reaction between phenylacetylene and 1-octenyl iodide
(Scheme 1-13b).46 The mechanism investigations also shed light on the elementary steps in the cross-
Introduction
14
coupling process.
Scheme 1-13. a) C-H borylation of benzene catalyzed by in-situ generated bis(NHSis)-IrHCl(coe) I-
50 (COE = cyclooctene). b) Sonogashira cross-coupling reaction catalyzed by the bis(NHSis)-NiBr
I-52.
Besides the bis(NHSis) I-37 featuring a benzene scaffold, the pyridine-based bis(NHSis) I-42 is
also a versatile tridentate ligand in transition-metal chemistry. The reaction of the tridentate ligand I-
42 with Fe(PMe3)4 afforded the pincer-type bis(NHSis)-Fe0(PMe3)2 complex I-53 with the
coordination of the N atom of the pyridine backbone (Scheme 1-14a).37 The Fe0 pincer complex I-53
could also be obtained by the reduction of the bis(NHSis)-FeIICl2 precursor with potassium graphite
in THF in the presence of an excess of PMe3.
The bis(NHSis)-stabilized Fe0 complex I-53, with an electron-rich Fe center and two strong σ
donating Si atoms, served as a good precatalyst in the hydrosilylation of acetophenone and its
derivatives (Scheme 1-14a).37
The coordination capability of the bis(NHSis) I-42 towards cobalt complex was also investigated
by Cui and our group. The pincer-type bis(NHSis)-CoBr2 complex I-54, which was synthesized by the
reaction of tridentate ligand I-42 with the CoBr2 precursor, could effectively catalyze the regioselective
C−H borylation of pyridines, furans and fluorinated arenes (Scheme 1-14b).47
Introduction
15
Regioselective borylation reaction catalyzed by bis(NHSis)-CoBr2 I-54.
1.3 Low-valent group 14 element compounds for small molecules activation
Low-valent group 14 element compounds with highly reactive group 14 centers are capable of
activating numerous small molecules, such as H2, CO, CO2, NH3, and various organic compounds.5~8
Among these diverse activations, the CO activation is significant and draws tremendous interest as CO
can serve as a versatile C1 building block to produce multicarbon compounds (e.g., fuels, solvents, and
organic bulk chemicals) to solve the shortage of fossil fuels.48a With the growing environmental issues
raised by the CO2 emission, the capture48b and transformation48c of CO2 to valuable chemicals has also
been a hot topic nowadays. Therefore, the recent advances regarding COx (x = 1, 2) activation mediated
by the low-valent group 14 element compounds will be discussed in section 1.3.
1.3.1 Low-valent group 14 element compounds for carbon monoxide activation
1.3.1.1 Low-valent carbon compounds for carbon monoxide activation
Introduction
16
Previous studies have shown that highly transient triplet carbenes could react with carbon monoxide
to afford ketene species.49 In 2006, Bertrand et al. showed that the stable acyclic and cyclic
(alkyl)(amino)carbenes (aAAC and cAAC) I-55 and I-57 could also achieve the fixation of CO to
generated the corresponding stable ketene compounds I-56 and I-58 (Scheme 1-15a), representing the
first examples of the stable singlet aAAC and cAAC for CO activation.50
Scheme 1-15. CO activation by stable carbenes.
Three years later, the cyclic diamidocarbene I-59, which was more electrophilic than classical NHCs,
was also shown by Bielawski et al. to react with CO to give the ketene compound I-60 in a reversible
fashion (Scheme 1-15b).51 Interestingly, the ferrocene-based NHC I-61 reacted with CO to generate
the aminoketene compound I-62, which was not stable and further reacted with one additional
molecule of NHC I-61 to give the zwitterionic compound I-63 (Scheme 1-15c).52 In 2013, Bertrand et
Introduction
17
al. reported another example that two molecules of NHC I-64 were added to one molecule of CO to
form the oxyallyl derivative I-65. When the temperature was raised to -10 oC, compound I-66 was
obtained via the migration of the Dip (Dip = 2,6-iPr2C6H3) group from the N to the O atom (Scheme
1-15c).53
1.3.1.2 Low-valent silicon compounds for carbon monoxide activation
Elemental silicon and its subvalent compounds may hold a unique position among potentially
suitable low-valent group 14 element systems for CO activation, because it has a relatively high
reduction ability in both elemental form and sub-valent states (e.g, divalent silicon in silylenes) and is
highly abundant with about 28% of the mass of the Earth’s crust. It has been shown that silicon atoms
and small clusters react with CO in cryogenic matrices to yield carbonyl complexes which can
photochemically rearrange to elusive cyclic four-membered Si2(µ-O)(µ-CSi) and Si2(µ-O)(µ-CCO)
species.54 Previous efforts towards divalent silicon (‘silylene’)-mediated CO activation included the
observation of transient silylene-CO adducts.55
More recently, the reaction of CO with 1, 4-disila (Dewar benzene) I-67 led to the formation of an
intriguing cyclic disilyl ketone I-68 (Scheme 1-16a).56 In 2013, Sekiguchi and Scheschkewitz et al.
reported the carbonylation of cyclotrisilene I-69 with CO to afford the unexpected product I-70
(Scheme 1-16b).57
Scheme 1-16. CO activation by low-valent silicon compounds I-67 and I-69.
Introduction
18
It has also been shown that a disilenyl lithium I-71 allows the scission of the CO triple bond and the
following C−C coupling to generate the product I-72 (Scheme 1-17).58a The ketenyl intermediate I-73
was proposed but could not be verified experimentally (Scheme 1-17). Interestingly, when disilenide
compound I-71 was mixed with M(CO)6 (M=Cr, Mo, W) in benzene, the compound I-74 bearing a
C=C=O ketene moiety was isolated.58a
Scheme 1-17. CO activation by the disilenyl lithium I-71.
Very recently, Apeloig et al. reported the CO activation mediated by silyl lithium and silenyl lithium
derivatives.58b The reaction of silyl lithium I-75 with CO led to the generation of the intriguing bis(silyl)
ketyl radical I-76 and tetra(silyl) di(ketyl) biradical I-77. When the silenyl lithium I-78 was exposed
to 1 bar CO, the unexpected 1-silaallenolate I-79 was obtained, providing a new strategy to synthesize
silaallenes.
Scheme 1-18. CO activation by the silyl lithium and silenyl lithium compounds I-75 and I-78.
In 2019, Aldridge et al. demonstrated that the acyclic silylene I-80 could achieve the reductive
coupling of CO to afford the dimeric product I-81 (Scheme 1-19).59 The molecular structure of I-81
exhibited that the central part was a six-membered Si2C2O2 ring with two SiIV atoms.
Introduction
19
1.3.1.3 Low-valent germanium compounds for carbon monoxide activation
The first CO activation example by the stable germylene was reported by Power and co-workers in
2009.60 Exposure of the diarylgermylene I-82a (R = H) to CO led to the isolation of CO coupling
product I-83a, in which the coupled (CO)2 moiety was inserted into the Caryl−Ge bond {aryl = 2,6-
(2,6-iPr3C6H3)2]C6H3} with a migration of a isopropyl group to form a six-membered ring (Scheme 1-
20).
However, when the bulkier germylene I-82b (R = iPr) was exposed to CO, the (CO)2 fragment was
inserted into the bond between the less bulky aryl group {i.e., [2,6-(2,4,6-Me3C6H2)2]C6H3} and Ge
atom with a migration of a methyl group, resulting in the formation of the compound I-83b (Scheme
1-20).
Introduction
20
1.3.2 Low-valent silicon compounds for carbon dioxide activation
1.3.2.1 Divalent silicon compounds for carbon monoxide activation
In 1996, Jutzi and co-workers reported the CO2 activation by the first isolable silylene I-1,16 i.e.,
decamethylsilicocene (Chart 1-1).61 Interestingly, when toluene was utilized as the solvent, compound
I-84 containing a Si(O2CO2)Si fragment could be obtained (Scheme 1-21). When pyridine was used
as the solvent, compound I-85 featuring an eight-membered ring was generated (Scheme 1-21).
Scheme 1-21. CO2 activation by the decamethylsilicocene I-1.
NHSis also exhibit high reactivity towards CO2. For instance, the siloxy silylene I-23,29 which was
obtained via the reaction of the modified Nacnac ligand supported NHSi I-18a26 with water vapor
(Scheme 1-3), could react with CO2 to give the silanoic silylester I-86 with a Si=O bond.62 The
amidinato silylene I-87 reacted with CO2 affording the silicon carbonate compound I-88 featuring a
Si[O2C=O] fragment.63,64
Scheme 1-22. CO2 activation by the NHSis I-23 and I-87.
Introduction
21
In 2014, Kira and co-workers, utilizing the cyclic alkyl silylene I-2, also achieved CO2 activation.
Exposure of compound I-2 to CO2 led to a color change of the solution, then the addition of MeOH
into the solution gave the bis(silyl) carbonate I-89 as the final product.65
Scheme 1-23. CO2 activation by the cyclic alkyl silylene I-2.
Compound I-91, which was generated through the intermolecular transfer of the transient acyclic
silylenes I-92, could behave as a masked acyclic silylene and react with CO2 to afford the silicon
carbonate compound I-93.66 The stable acyclic silylene I-621a could also react with CO2 resulting in
the compound I-90 with the elimination of CO.59
Scheme 1-24. CO2 activation by the acyclic silylenes I-6 and I-92.
Scheme 1-25. CO2 activation by the bis(silylenes) I-31.
Introduction
22
Kato and Baceiredo et al. reported that one molecule of phosphine-stabilized bis(silylenes) I-31
reacted with four CO2 molecules giving the aminosilicate compound I-94 involving two
pentacoordinate silicon atoms.20c
Notably, for silylene-mediated CO2 activations, silicon carbonate compounds containing a
Si(O2C=O) moiety were typically generated. In the reactions of disilene with CO2, the addition of two
silicon atoms to the C=O bond of CO2 was observed. For example, the disilenes I-95 and I-97 trapped
one molecule of CO2 to give the compounds I-9667 and I-9868, respectively. Interestingly, compound
I-98 could further react with one CO2 molecule to afford the compound I-99 via the insertion of one
O atom from CO2 into the Si−Si bond.
Scheme 1-26. CO2 activation by the disilenes I-95 and I-97.
1.3.2.2 Zero-valent silicon compounds for carbon dioxide activation
Besides the divalent silicon compounds, zero-valent silicon species are also capable of CO2
activation. In 2015, Robinson et al. demonstrated that the NHC-stabilized diatomic zero-valent silicon
compound I-100 reacted with CO2 in 1:5 molar ratio to give compound I-101 featuring a six-membered
Si2O3C ring.69
Scheme 1-27. CO2 activation by the diatomic zero-valent silicon compound I-100.
Introduction
23
Interestingly, the reaction of one molar equivalent of the bis(NHSis)-stabilized Si0 species I-102
with four molar equivalents of CO2 at -30 oC generated the silicon dicarbonate compound I-103.70
DFT calculations revealed that the CO2 activation process involved a silicon monoxide intermediate
bis(NHC)-Si=O and a dioxide intermediate bis(NHC)-SiO2.
Scheme 1-28. CO2 activation by the monatomic zero-valent silicon compound I-102.
1.4 Isolable zero-valent group 14 element E compounds (E = Si or Ge)
Zero-valent element compounds, although well known in transition-metal chemistry,71 are rare in
main-group chemistry, particularly the highly reactive heavy group 14 elements E0 (E = Si, Ge).2,72~87
Utilizing strongly σ donating and bulky ligands, species bearing heavy zero-valent group 14 elements
have been isolated successfully in recent years. They fall into three major categories: triatomic E0 3L3,
72
diatomic E0 2L2
73~76 and monatomic E0L2 77~87 compounds (E = Si, Ge; L = σ donor ligand), respectively.
Owing to their unique bonding motifs, structures, and reactivities, heavy group 14 E0 compounds are
of interest to both synthetic chemists2,72-87 and theoreticians.88,89 In this section, these three categories
of zero-valent silicon or germanium complex will be discussed in detail.
1.4.1 Isolable triatomic zero-valent silicon compounds
Utilizing cAAC with strong σ donating and enhanced π accepting properties, Roesky et al.
successfully synthesized the first cAAC-stabilized triatomic Si0 compound I-105 with a Si3 cyclic ring
through the reduction of (cAAC)SiCl4 I-104 precursor by potassium graphite (Scheme 1-29).72 The
molecular structure of I-105, ambitiously determined by an X-ray diffraction analysis, exhibited that
each silicon atom was three-coordinated and adopted a trigonal pyramidal geometry with one lone-
pair of electrons occupying the apex. DFT calculations of the bonding situation in I-105 revealed a
Introduction
24
partial double bond character of the Si−C bond due to the significant π accepting capability of cAAC.72
Scheme 1-29. Synthesis of triatomic zero-valent silicon compound I-10572.
1.4.2 Isolable diatomic zero-valent group 14 element E compounds (E = Si or Ge)
1.4.2.1 Isolable diatomic zero-valent silicon compounds
In 2008, Robinson and co-workers successfully reduced the NHC-coordinated SiCl4 compound I-
106 by KC8 to isolate the first NHC-stabilized diatomic zero-valent silicon compound I-100 with a
lone-pair of electrons on each silicon atom, representing a landmark and paving the way for zero-
valent silicon chemistry (Scheme 1-30a).73 An X-ray structural analysis of species I-100 shows that
the two C−Si bonds are almost perpendicular to the central Si=Si double bond with the C–Si–Si bond
angles of 93.37(5)°.
Scheme 1-30. Synthesis of diatomic zero-valent silicon compounds I-10073 and I-10874.
Introduction
25
Recently, taking advantage of the similar carbene-stabilization strategy, Roesky et al. reported the
cAACs-supported dinuclear Si0 species I-108 comprising a Si=Si double bond via the reduction of the
cAAC-stabilized tetrachlorosilane precursor I-107 (Scheme 1-30b).74 The theoretical investigations
and the experimentally Raman results both demonstrated that the lone-pair of electrons of each silicon
atom was polarized towards the C atom of carbene which is analogous to the situation in triatomic Si0
compound I-105.
1.4.2.2 Isolable diatomic zero-valent germanium compounds
Inspired by the isolation of diatomic Si0 compound I-100, Jones, Stasch and Frenking et al. reported
the first germanium analogue of compound I-100. The reduction of the NHC-stabilized GeCl2 I-109
with the Nacnac-supported Mg(I) reagent led to the desired NHC-stabilized diatomic Ge0 compound
I-110 (Scheme 1-31a).75 In 2014, the first NHSi-stabilized dinuclear Ge0 complex I-112 (Scheme 1-
31b) was investigated by So et al..76 This species was obtained through the reduction of the NHSi-
supported dichlorosilane I-111 with KC8 (Scheme 1-31b).76
Scheme 1-31. Synthesis of diatomic zero-valent germanium compounds I-11075 and I-11276.
Introduction
26
1.4.3 Isolable monatomic zero-valent group 14 element E compounds (E = Si or Ge)
In addition to the triatomic and diatomic Si0 species, another newly emerging category of low-valent
silicon chemistry is the monatomic zero-valent group 14 element compounds L:→E0 :L. Such
species with a bent geometry, in which the central E atom, stabilized by donor-acceptor interaction
between E and donor ligand L, possesses four valence electrons as two lone-pairs, are termed as
ylidones (carbone: E = C; silylone: E = Si; germylone: E = Ge; stannylone: E = Sn; plumbylone: E =
Pb) (Figure 1-4).88e This section will focus on the currently known silylone and germylone chemistry.
Figure 1-4. Nomenclature for monatomic zero-valent group 14 element compounds.
1.4.3.1 Cyclic alkyl silylenes supported monatomic zero-valent group 14 element E compounds
(E = Si or Ge)
In 2003, Kira and co-workers synthesized the first cyclic alkyl silylene stabilized trisilaallene
derivative I-115a (E’ = E = Si) via a two-step synthesis procedure, in which the cyclic alkyl silylene
I-218a was firstly inserted into the Si−Cl bond of tetrachlorosilane followed by the reduction of the
generated chloro(trichlorosilyl)silane precursor I-114a with KC8 (Scheme 1-32).77 Notably, unlike its
light analogue allene, featuring a liner C=C=C structure, the compound I-115a comprised a bent
Si=Si=Si unit and two Si−Si bond lengths lied in the typical range of those in disilenes. Employing the
heavier congener of I-114a, i.e., cyclic alkyl germylene I-113, the same group reported the synthesis
of the stable 1,3-digermasilaallene I-115b (E’ = Ge and E = Si) and trigermaallene I-115c (E’ = Ge and
E = Ge) via a similar synthesis strategy (Scheme 1-32).78 Interestingly, the 2-germadisilaallene I-115d
(E’ = Si and E = Ge) was directly synthesized by mixing the silylene I-2, GeCl2-dioxane complex and
KC8 (Scheme 1-32).79
Scheme 1-32. Synthesis of the cyclic alkyl silylenes stabilized monatomic zero-valent silicon and
germanium compounds I-115 a~d78~79.
Frenking et al., based on theoretical studies, argued that compounds I-115 (Scheme 1-32) should
rather be classified as ylidones.88 Remarkably, the computational investigations by Apeloig et al.
revealed that the parent trisilaallene H2Si=Si=SiH2 which has a highly acute SiSiSi bond angle of 69.4o
and the trisilacyclopropylidene are bond-stretch isomers.89a,b
1.4.3.2 Stable carbenes supported monatomic zero-valent group 14 element E compounds (E =
Si or Ge)
The first cAACs-stabilized zero-valent Si0 was reported by Roesky, Frenking and Stalke et. al. in
2013.80 Firstly, the cAAC-stabilized dichlorosilane precursor I-118 was synthesized via the reaction of
NHC coordinated SiCl2 I-116 with three molar equivalents of cAAC I-117 (Scheme 1-33a). Notably,
the C−Si bonds in compound I-118 were electron sharing bonds instead of the donor-acceptor C→Si
bonds and compound I-118 exhibited a diradical character. Then the reduction of I-118 with KC8 led
to the formation of the cAACs-stabilized zero-valent Si0 species I-119 (Scheme 1-33a).80 The Natural
Bond Orbital (NBO) analysis of the bonding situation of the CSiC unit in I-119 revealed a σ lone-pair
orbital and a three-center C−Si−C p orbital. The cAACs were then utilized to stabilize the monatomic
zero-valent Ge compound. Mixing the cAAC I-117 or I-121, GeCl2-dioxane complex and KC8 in a
1:2:2.1 molar ratio afforded the corresponding Ge0 species I-120 or I-122 (Scheme 1-33b).81 The DFT
calculations demonstrated that compounds I-120 and I-122 are germylones featuring a partially
diradicaloid character owing to the strong π accepting nature of the cAACs.
Introduction
28
Scheme 1-33. Synthesis of the cAACs-stabilized monatomic zero-valent silicon and germanium
compounds I-11980, I-120 and I-12281.
NHCs, being strong σ donor ligands, were also applied in silylone and germylone chemistry. In 2013,
Driess and co-workers, employing the chelating bis(NHC) ligand I-123, successfully synthesized the
first bis(NHC)-stabilized zero-valent Si compound I-102 through the reduction of its SiII precursor I-
124 with sodium naphthalenide (Scheme 1-34).82 The bis(NHC)-stabilized chlorosilyliumylidene
chloride I-124 was obtained via the reaction of bis(NHC) ligand I-123 with NHC-SiCl2 [NHC =
bis(2,6-diisopropyl-phenyl)-imidazol-2-ylidene].
As the energy gap between the highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) of NHC is larger than that of cAAC, the π accepting property
of NHC is weaker than that of cAAC. Consequently, the central silicon atom of I-102 is more electron-
rich than I-119 and the C−Si bond lengths in I-102 are longer than those in I-119.82
The bis(NHC)-stabilized Ge0 compound I-126 was also isolated by the reaction of its
chlorogermyliumylidene chloride precursor I-125 with NaC10H8, which is analogous to the synthesis
of its light congener I-102 (Scheme 1-34).83
Introduction
29
Scheme 1-34. Synthesis of the NHSi-stabilized monatomic zero-valent silicon and germanium
compounds I-10282 and I-12683.
1.4.3.3 Other examples of isolable monatomic zero-valent group 14 element Ecompounds (E = Si
or Ge)
In 2014, a bis(imino)pyridine pincer ligand stabilized germylone I-12784 (Scheme 1-35) and an
imino-NHC-supported germylone I-12885 (Scheme 1-35) were reported by Nikonov et al. and et al.,
respectively. More recently, two germylene coordinated silylone I-12986 (Scheme 1-35) and a novel
Si0 species I-13087 (Scheme 1-35) with a two-NHC-stabilized four-membered Si ring were devised
and synthesized successively.
Scheme 1-35. The selected examples of isolable monatomic zero-valent silicon and germanium
compounds I-127~13084~87
2. MOTIVATION AND OBJECTIVES
In order to enrich the bis(NHSis) chemistry which is still in its infancy, my dissertation is devoted
to the design and synthesis of the novel bis(NHSis) compounds, and their utilization in small molecules
activation and transition metal chemistry.
Therefore, the first objective is the synthesis of the bis(NHSis)xanthene [Xant(SiIIL)2] (xant = 9,9-
Dimethyl-Xanthene-4,5-diyl, L = PhC(NtBu)2) and bis(NHSis)dibenzofuran compounds (Scheme 2-
1). The distances of the two SiII atoms in these bis(NHSis) compounds are expected to be distinct and
will influence their reactivity.
Scheme 2-1. Synthesis of bis(NHSis) compounds with different Si···Si distances.
Considering the good performance of the xantphos as chelating ligands in transition-metal-mediated
catalysis and the limited examples of Ni-mediated homogeneous hydrogenation of olefins, I will then
utilize the bis(NHSis)xanthene, an analogue of the xantphos, to stabilize the Ni species and employ
their Ni complexes as catalysts in homogeneous hydrogenation of olefins (Scheme 2-2).
Motivation and Objectives
32
Scheme 2-2. The utilization of [Xant(SiIIL)2] as the chelating ligand in Ni-mediated hydrogenation of
olefins.
Inspired by the high reactivity of the mono(NHSi) containing one reactive SiII center towards small
molecules, I will investigate whether the two SiII atoms in bis(NHSis)xanthene and
bis(NHSis)dibenzofuran could cooperate to activate small molecules, such as COx (x = 1 or 2), and
how the Si···Si distances influence the reactivity of these bis(NHSis) compounds (Scheme 2-3).
Scheme 2-3. The utilization of bis(NHSis) compounds in small molecules activation.
With two strong σ donating SiII centers in a suitable distance, bis(NHSis)xanthene is expected to be
able to stabilize zero-valent monatomic group 14 elements E (E = Ge or Si). Therefore, the fourth
objective is to synthesize the bis(NHSis)xanthene-stabilized monatomic Ge0 and Si0 compounds and
investigate their reactivity (Scheme 2-4).
Scheme 2-4. The utilization of bis(NHSis) compounds in the stabilization of group 14 elements E0 (E
= Ge or Si).
Results and Discussion
3.1.1 Background
As discussed in the introduction 1.2, our group introduced the amidinato silylene I-2530 into
chelating ligand scaffolds, leading to the discovery of various and versatile bis(NHSis) compounds34~38
(Chart 3-1). Bis(NHSis) compounds with enhanced σ donating nature5 have been utilized as excellent
bidentate (I-40, I-44, I-34) or tridentate (I-42, I-37) chelating ligands in transition-metal-mediated
catalysis.4
In the previously reported bis(NHSis) compounds (Chart 3-1), the backbone linking two SiII atoms
drastically influences the electron density of the silicon atoms, which is evident by the chemical shifts
of the 29Si NMR signals.
Chart 3-1. Selected examples of bis(NHSis) compounds and the hypercoordinated disilene A.
For instance, when the SiII atoms are connected to C atoms of the ferrocene36 or carborane38 scaffolds,
the 29Si NMR spectra exhibit deshielded chemical shifts, i.e., 43.3 or 18.7 ppm, respectively (Chart 3-
1). However, when the SiII atoms are bonded to N (I-42)37 or O atoms (I-3434 or I-3735), the 29Si NMR
signals are remarkably shifted downfield compared with that in I-40 and I-44. Consequently, the
bis(NHSis) compounds featuring distinct spacers demonstrate different coordination behaviors
towards transition-metals.4 Interestingly, when the two SiII atoms are placed in an acenaphthen
backbone (Chart 3-1, A), the remarkable decrease of the Si···Si distance (2.623 Å) led to the formation
of a hypercoordinated Si=Si double bond.90 The 29Si NMR signals of compound A is at δ -36.5 ppm,
Results and Discussion
34
revealing an excess of electron density on the Si atoms. Compound A could act as a disilene undergoing
[2+2] cycloaddition reactions but behaves as a bis(NHSis) ligand in the reaction with Ni(cod)2.
Considering the tremendous influences of the backbones on the reactivity of bis(NHSis) and the
wide utilization of xanthene and dibenzofuran backbones in chelating ligands,34~38 I herein describe
the first bis(NHSis)xanthene compound [Xant(SiIIL)2] 1a and bis(NHSis)dibenzofuran compound 1b,
featuring different Si···Si distances. Then the influence of the SiII···SiII distance on the reactivity of
these bis(NHSis) compounds towards small molecules will be discussed in the section 3.3.
3.1.2 Synthesis of bis(NHSis)xanthene compound 1a
Dilithiation of 4,5-dibromo-9,9-dimethylxanthene with 2 molar equivalents of sec-BuLi in Et2O,
followed by salt-metathesis reaction with the chlorosilylene [PhC(NtBu)2]SiCl afforded the first
bis(NHSis)xanthene compound [Xant(SiIIL)2] 1a (Xant = 9,9-dimethyl-xanthene-4,5-diyl, L =
PhC(NtBu)2) as yellow crystals in 70% isolated yields (Scheme 3-1).
Scheme 3-1. Synthesis of 1a101.
Its 1H NMR spectrum shows a singlet at δ 1.25 ppm corresponding to the t-butyl groups and one set
of resonances for the two silylene moieties [PhC(NtBu)2]Si indicating a highly symmetric structure of
1a in solution. The 29Si NMR spectrum of 1a exhibits a singlet at δ 17.3 ppm which is markedly
downfield shifted relative to that of the ferrocene-based bis(NHSis) I-40 (δ 43.3 ppm),36 but
comparable to that in the carborane-bridged bis(NHSis) I-44 (δ 18.9 ppm) (Chart 3-1).38
The molecular structure of 1a was unambiguously confirmed by an X-ray diffraction analysis
(Figure 3-1). The two silicon atoms face each other with the Si−Si distance of 4.316(1) Å, revealing a
suitable space for coordination of transition-metals. The Si1, Si2, C2, C10, and O1 atoms are almost
Results and Discussion
35
in the same plane. The N3−Si1−N4 bond angle [68.98(8)°] is similar to the N2−Si2−N1 bond angle
[68.78(9)°].
Figure 3-1. a) Front and b) top views of the molecular structure of compound 1a. Thermal ellipsoids
are drawn at 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å)
and angles (o): Si1−N3 1.882(2), Si1−N4 1.8849(19), Si2−N2 1.871(2), Si2−N1 1.875(2), Si2−C10
1.934(2), Si1−C2 1.940(2), N3−Si1−N4 68.98(8), N2−Si2−N1 68.78(9).
3.1.3 Synthesis of bis(NHSis)dibenzofuran compound 1b
4,5-Dibromodibenzofuran was choosen as precursor and the corresponding bis(NHSis)dibenzofuran
compound 1b could be obtained via an analogous procedure to that of 1a (Scheme 3-2). Its 1H NMR
spectrum shows a singlet at δ 1.18 ppm corresponding to the t-butyl groups. The 29Si NMR signal is
observed at δ 17.8 ppm, nearly identical to that of compound 1a.
Scheme 3-2. Synthesis of 1b126.
Results and Discussion
36
The molecular structure, displayed in Figure 3-2, demonstrates that the two silicon atoms are
oriented in the same directions with a significantly longer SiIISiII distance of 6.045(1) Å compared
with that in 1a [4.316(1) Å] (Figure 3-2). Similar to 1a, the Si1, Si2, C2, C10, and O1 atoms are almost
in the same plane.
Figure 3-2. a) Front and b) top views of the molecular structure of compound 1b. Thermal ellipsoids
are drawn at 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å)
and angles (o): Si1−N1 1.8752(19), Si1−N2 1.870(2), Si2−N3 1.859(2), Si2−N4 1.865(2), Si1−C1
1.926(2), Si2−C10 1.918(2), N1−Si1−N2 69.19(8), N3−Si2−N4 69.68(9).
3.1.4 Comparison of 1a and 1b
In order to predict the reactivity of bis(NHSis)xanthen 1a and bis(NHSis)dibenzofuran 1b, three
parameters of these bis(NHSis) compounds are compared (Table 3-1). The chemical shift of the 29Si
NMR signal of both 1a and 1b are quite similar, indicating that the electron densities on the Si atoms
of the two compounds are quite similar (Table 3-1). The Si−Cbackbone bond length in 1a is slightly langer
than that in 1b, revealing that the interaction between the silylene moieties and the backbone of 1b is
slightly larger than that in 1a (Table 3-1).
Although the aforementioned two parameters of 1a and 1b are quite similar, the SiII···SiII distances
are drastically different in these two compounds. The distance between two SiII atoms of
bis(NHSis)xanthene 1a is 4.316(1) ppm, while the corresponding distance in bis(NHSis)dibenzofuran
1b is 6.045(1) ppm, suggesting that the cooperation tendency of two silicon atoms in 1a is perhaps
Results and Discussion
larger than that in 1b.
Table 3-1. Comparison of key parameters of compounds 1a and 1b
Parameters Bis(NHSis)xanthene 1a Bis(NHSis)dibenzofuran
Si−Cbackbone bond length (Å) 1.934(2), 1.940(2) 1.926(2), 1.918(2)
SiIISiII distance (Å) 4.316(1) 6.045(1)
38
applications in the catalytic hydrogenation of olefins
The section 3.2 is adapted with permission from “Divalent Silicon-Assisted Activation of Dihydrogen in a Bis(N-
heterocyclic silylene)xanthene Nickel(0) Complex for Efficient Catalytic Hydrogenation of Olefin”. Yuwen Wang,
Arseni Kostenko, Shenglai Yao, Matthias Driess*, J. Am. Chem. Soc., 2017, 139, 13499–13506. (DOI:
https://doi.org/10.1021/jacs.7b07167). Copyright 2017 American Chemical Society.
3.2.1 Background
Success in homogeneous transition-metal-mediated catalysis greatly depends upon the development
of well-designed ligands.39 As discussed in the introduction 1.2, the versatile bis(NHSis) compounds
(Chart 1-3) have been utilized as strong σ donor chelating ligands in transition-metal catalysis, such as
borylation, hydrosilylation and other homogenous catalytic transformations.3 Nonetheless, thorough
catalytic and mechanistic studies upon the organometallic systems bearing diverse bis(NHSis) ligands
are still in their infancy compared with NHC analogues.
Homogeneous hydrogenation of olefins, a field mainly dominated by precious metal (Rh,91 Ir,92 Ru93)
-based catalysts, is one of the most powerful and atom economical methods in organic synthesis.7 More
recently, hydrogenation catalyzed by earth-abundant first-row transition-metal complexes is emerging
as an efficient alternative way due to their environment-friendly and cost-effective properties.94~99
While heterogeneous nickel catalysts have been widely used in hydrogenation of olefins, e.g. Raney
nickel,100 studies of homogeneous hydrogenation catalyzed by nickel complexes are still rare.95~99
Bouwman et al. pioneered studies on the Ni-catalyzed homogeneous hydrogenation of 1-octene under
high pressure (50 bar) (Scheme 3-3a).95 In 2012, Hanson et al. reported a nickel hydride complex
[(CyPNHPCy)NiH](BPh4) { CyPNHPCy = HN[CH2CH2P(Cy)2]2} catalyzed alkene hydrogenation under
4 bar H2 pressure at 80 oC (Scheme 3-3b).96 Two nickel–borane species, (MesDPBPh)Ni [MesDPBPh =
MesB(o-PPh2C6H4)2] and pincer-type (tBuPBPtBu)NiH, were synthesized by Peters and coworkers, both
of which were successfully applied as active catalysts for olefin hydrogenation under 1 bar H2 pressure
at room temperature (Scheme 3-3c).97 In 2015, Lu et al. conducted the hydrogenation of unhindered
olefins with a gallium supported bimetallic Ni(0) complex under mild conditions (Scheme 3-3d).98
Despite all these significant contributions, the substrate scope is still largely limited to less sterically
hindered terminal and internal alkenes. Therefore, the development of new ligands for Ni-catalyzed
homogeneous hydrogenation of olefins is highly desirable.
Scheme 3-3. Selected examples of homogeneous hydrogenation of olefins catalyzed by Ni complexes.
Herein, I would like to discuss the coordination behaviors of the bis(NHSis)xanthene ligand 1a
towards Ni and isolated its Ni complexes [Xant(SiIIL)2]Ni(η2-1,3-cod) 2 and [Xant(SiIIL)2]Ni(PMe3)2
3. Remarkably, H2 can be activated by 2 and 3 under very mild reaction conditions to generate the
unexpected dinuclear Ni complex 4 and the first structurally characterized dihydrido Ni complex 5
stabilized by 1a, respectively. Complex 2 is a strikingly efficient catalyst in hydrogenation of olefins
with a broad substrate scope under smooth reaction conditions (1 bar H2, RT). DFT calculations
suggest an unprecedented hydrogenation mechanism where the SiII atoms in the [Xant(SiIIL)2] ligand
Results and Discussion
41
1a play a significant role in assisting the H2 cleavage, stabilizing the dihydrido Ni intermediate and
facilitating the hydrogen transfer to the olefin.101
3.2.2 Synthesis of bis(NHSis)xanthene-coordinated Ni complexes
Treatment of [Xant(SiIIL)2] 1a with Ni(cod)2 in Et2O at RT led to the formation and isolation of
unprecedented [Xant(SiIIL)2]Ni(η2-1,3-cod) (1,3-COD = 1,3-cyclooctadiene) complex 2 as dark red
crystals with a coordinatively unsaturated 16 valence-electron Ni0 center, where isomerization of 1,5-
cyclooctadiene occurred (Scheme 3-4). While monitoring the coordination process by 1H NMR
spectroscopy, I could observe the rapid generation of the symmetrical species [Xant(SiIIL)2]Ni(η4-1,5-
cod) and ‘free’ 1,5-cyclooctadiene after 15 min. Four hours later, the 1H NMR spectrum of the reaction
mixture exhibited a new set of unsymmetrical signals which could be assigned to the final products
[X