5
A Tetranuclear Manganese Cluster with a Star-Shaped Mn 4 O 6 Core Motif: Directed Synthesis using a Mononuclear Precursor Complex Nicole Reddig, Michael U. Triller, Daniel Pursche, Annette Rompel* and Bernt Krebs** Münster, Institut für Anorganische und Analytische Chemie und Institut für Biochemie der Westfälischen Wilhelms-Universität Münster Received May 17th, 2002. Dedicated to Professor Dieter Fenske on the Occasion of his 60 th Birthday Abstract. The tetranuclear manganese(II) complex [Mn 4 (ppi) 6 ](BPh 4 ) 2 (2) (Hppi 2-pyridylmethyl-2-hydroxy- phenylimine) is prepared by using the precursor complex [Mn(ppi) 2 ]·H 2 O(1). Based on UV/Vis- and IR-spectroscopy data in combination with mass spectrometry it has been concluded that 1 is a mononuclear neutral Mn II complex, in which two ppi ligands chelate the manganese atom. Compound 2 crystallizes in the tri- clinic space group P1 ¯ (no. 2), with a 17.500(3), b 17.955(4), c 19.101(4) A ˚ , α 113.79(3)°, β 111.33(3)°, γ 93.91(3)°, V 4950(2) A ˚ 3 and Z 2. In the tetranuclear [Mn 4 (ppi) 6 ] 2 com- Ein vierkerniger Mangancluster mit einer sternförmigen Mn 4 O 6 -Kernstruktur: Gezielte Synthese mit einem mononuklearen Precursorkomplex Inhaltsübersicht. Mit der Darstellung von [Mn 4 (ppi) 6 ](BPh 4 ) 2 (2) (Hppi 2-Pyridylmethyl-2-hydroxyphenylimin) unter Verwendung des Precursorkomplexes [Mn(ppi) 2 ]·H 2 O(1) ist die gezielte Syn- these eines vierkernigen Mangan(II)-Clusters gelungen. Anhand von UV/Vis- und IR-spektroskopischen Daten sowie des Massen- spektrums wurde 1 als ein mononuklearer Mn II -Neutralkomplex identifiziert, in dem zwei ppi-Liganden das Manganion umgeben. Die Verbindung 2 kristallisiert im triklinen Kristallsystem in der Raumgruppe P1 ¯ (Nr. 2) mit a 17.500(3), b 17.955(4), c 19.101(4) A ˚ , α 113.79(3)°, β 111.33(3)°, γ 93.91(3)°,V Introduction Besides several mono- and dinuclear manganese containing metalloenzymes like manganese superoxide dismutase (MnSOD) [1], manganese dioxygenase [2], manganese catal- ase (MnCat) [37] and manganese peroxidase [8, 9], manganese is also present in the active site of the oxygen evolving complex (OEC) of Photosystem II (PS II) [9]. Re- cently, an X-ray crystal structure analysis of a dimeric PS II core complex from the thermophilic cyanobacterium Synechococcus elongatus with a resolution of 3.8 A ˚ became available [10]. In the OEC the exact orientation of the four * PD. Dr. A. Rompel Institut für Biochemie Wilhelm-Klemm-Str. 2 D-48149 Münster Fax: 49 (0)251/8338366 e-mail: [email protected] 2458 WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002 00442313/02/628/24582462 $ 20.00.50/0 Z. Anorg. Allg. Chem. 2002, 628, 24582462 plex cation Mn(1), Mn(2), and Mn(3) are equivalently coordinated by two deprotonated Hppi ligands leading to a N 4 O 2 donor set. The environment of the central Mn(4) is formed by coordination of three [Mn(ppi) 2 ] fragments resulting in a phenoxo bridged star- shaped Mn 4 O 6 core motif. The average distance of directly adjacent manganese ions is 3.310 A ˚ , whereas the average distance of Mn(1), Mn(2), and Mn(3) among each other is 5.732 A ˚ . Keywords: Manganese; Mn 4 O 6 core; Precursor; Phenoxo bridge 4950(2) A ˚ 3 und Z 2. In diesem vierkernigen [Mn 4 (ppi) 6 ] 2 -Kom- plexkation werden die äquivalenten N 4 O 2 -Donorsets um Mn(1), Mn(2) und Mn(3) von je zwei deprotonierten Hppi-Liganden gebil- det. Durch Koordination dreier [Mn(ppi) 2 ]-Fragmente wird das zentrale Manganion Mn(4) so umgeben, dass eine ausschließlich phenoxo-verbrückte sternförmige Mn 4 O 6 -Kernstruktur entsteht. Der mittlere Abstand direkt benachbarter Manganionen ist 3.310 A ˚ , während der Abstand von Mn(1), Mn(2) und Mn(3) zueinander im Durchschnitt 5.732 A ˚ beträgt. manganese ions and the essential cofactors Ca 2 and Cl is still subject to controversy. Different topological models (cubane [11], “dimer-of-dimers” [12], “dangling-model” [13], butterfly [14], tetrahedron [14]) have been discussed based on Mn X-ray absorption spectra (XAS) and EPR/ ENDOR studies. Among the proposed structures for the Mn 4 O x core also Y-shaped and funnel arrangements have been subject to intensive investigations as possible struc- tural motifs for the active site of PS II [12, 14]. Synthetic clusters containing three or four metal centers are often obtained by self assembly reactions. Small varia- tions of the reaction conditions have great influence on the ** Prof. Dr. B. Krebs Institut für Anorganische und Analytische Chemie Wilhelm-Klemm-Str. 8 D-48149 Münster Fax: 49 (0)251/8338366 e-mail: [email protected]

A Tetranuclear Manganese Cluster with a Star-Shaped Mn4O6 Core Motif: Directed Synthesis using a Mononuclear Precursor Complex

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Page 1: A Tetranuclear Manganese Cluster with a Star-Shaped Mn4O6 Core Motif: Directed Synthesis using a Mononuclear Precursor Complex

A Tetranuclear Manganese Cluster with a Star-Shaped Mn4O6 Core Motif:Directed Synthesis using a Mononuclear Precursor Complex

Nicole Reddig, Michael U. Triller, Daniel Pursche, Annette Rompel* and Bernt Krebs**

Münster, Institut für Anorganische und Analytische Chemie und Institut für Biochemie der Westfälischen Wilhelms-Universität Münster

Received May 17th, 2002.

Dedicated to Professor Dieter Fenske on the Occasion of his 60th Birthday

Abstract. The tetranuclear manganese(II) complex[Mn4(ppi)6](BPh4)2 (2) (Hppi � 2-pyridylmethyl-2-hydroxy-phenylimine) is prepared by using the precursor complex[Mn(ppi)2]·H2O (1). Based on UV/Vis- and IR-spectroscopy datain combination with mass spectrometry it has been concluded that1 is a mononuclear neutral MnII complex, in which two ppi ligandschelate the manganese atom. Compound 2 crystallizes in the tri-clinic space group P1 (no. 2), with a � 17.500(3), b � 17.955(4),c � 19.101(4) A, α � 113.79(3)°, β � 111.33(3)°, γ � 93.91(3)°,V � 4950(2) A3 and Z � 2. In the tetranuclear [Mn4(ppi)6]2� com-

Ein vierkerniger Mangancluster mit einer sternförmigen Mn4O6-Kernstruktur:Gezielte Synthese mit einem mononuklearen Precursorkomplex

Inhaltsübersicht. Mit der Darstellung von [Mn4(ppi)6](BPh4)2 (2)(Hppi � 2-Pyridylmethyl-2-hydroxyphenylimin) unter Verwendungdes Precursorkomplexes [Mn(ppi)2]·H2O (1) ist die gezielte Syn-these eines vierkernigen Mangan(II)-Clusters gelungen. Anhandvon UV/Vis- und IR-spektroskopischen Daten sowie des Massen-spektrums wurde 1 als ein mononuklearer MnII-Neutralkomplexidentifiziert, in dem zwei ppi-Liganden das Manganion umgeben.Die Verbindung 2 kristallisiert im triklinen Kristallsystem in derRaumgruppe P1 (Nr. 2) mit a � 17.500(3), b � 17.955(4), c �

19.101(4) A, α � 113.79(3)°, β � 111.33(3)°, γ � 93.91(3)°, V �

Introduction

Besides several mono- and dinuclear manganese containingmetalloenzymes like manganese superoxide dismutase(MnSOD) [1], manganese dioxygenase [2], manganese catal-ase (MnCat) [3�7] and manganese peroxidase [8, 9],manganese is also present in the active site of the oxygenevolving complex (OEC) of Photosystem II (PS II) [9]. Re-cently, an X-ray crystal structure analysis of a dimeric PSII core complex from the thermophilic cyanobacteriumSynechococcus elongatus with a resolution of 3.8 A becameavailable [10]. In the OEC the exact orientation of the four

* PD. Dr. A. RompelInstitut für BiochemieWilhelm-Klemm-Str. 2D-48149 MünsterFax: �49 (0)251/8338366e-mail: [email protected]

2458 WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002 0044�2313/02/628/2458�2462 $ 20.00�.50/0 Z. Anorg. Allg. Chem. 2002, 628, 2458�2462

plex cation Mn(1), Mn(2), and Mn(3) are equivalently coordinatedby two deprotonated Hppi ligands leading to a N4O2 donor set.The environment of the central Mn(4) is formed by coordinationof three [Mn(ppi)2] fragments resulting in a phenoxo bridged star-shaped Mn4O6 core motif. The average distance of directly adjacentmanganese ions is 3.310 A, whereas the average distance of Mn(1),Mn(2), and Mn(3) among each other is 5.732 A.

Keywords: Manganese; Mn4O6 core; Precursor; Phenoxo bridge

4950(2) A3 und Z � 2. In diesem vierkernigen [Mn4(ppi)6]2�-Kom-plexkation werden die äquivalenten N4O2-Donorsets um Mn(1),Mn(2) und Mn(3) von je zwei deprotonierten Hppi-Liganden gebil-det. Durch Koordination dreier [Mn(ppi)2]-Fragmente wird daszentrale Manganion Mn(4) so umgeben, dass eine ausschließlichphenoxo-verbrückte sternförmige Mn4O6-Kernstruktur entsteht.Der mittlere Abstand direkt benachbarter Manganionen ist 3.310A, während der Abstand von Mn(1), Mn(2) und Mn(3) zueinanderim Durchschnitt 5.732 A beträgt.

manganese ions and the essential cofactors Ca2� and Cl�

is still subject to controversy. Different topological models(cubane [11], “dimer-of-dimers” [12], “dangling-model”[13], butterfly [14], tetrahedron [14]) have been discussedbased on Mn X-ray absorption spectra (XAS) and EPR/ENDOR studies. Among the proposed structures for theMn4Ox core also Y-shaped and funnel arrangements havebeen subject to intensive investigations as possible struc-tural motifs for the active site of PS II [12, 14].

Synthetic clusters containing three or four metal centersare often obtained by self assembly reactions. Small varia-tions of the reaction conditions have great influence on the

** Prof. Dr. B. KrebsInstitut für Anorganische und Analytische ChemieWilhelm-Klemm-Str. 8D-48149 MünsterFax: �49 (0)251/8338366e-mail: [email protected]

Page 2: A Tetranuclear Manganese Cluster with a Star-Shaped Mn4O6 Core Motif: Directed Synthesis using a Mononuclear Precursor Complex

A Tetranuclear Manganese Cluster

resulting structure. Therefore well-directed design is of highinterest for preparative inorganic chemistry to open newways for the synthesis of polynuclear complexes. The use ofprecursor complexes as building blocks establishes anaccessable way to design multinuclear compounds with adefined structural arrangement.

In this paper, we report the synthesis and crystal struc-ture of [Mn4(ppi)6](BPh4)2 (2), using the precursor complex[Mn2(ppi)2]·H2O (1).

Experimental Section

All chemicals were commercially available and used without furtherpurification. Physical measurements were done at the Institut fürAnorganische und Analytische Chemie, WWU Münster. Elementalanalyses were performed on a Vario EL III analyser (Elementar).Infra red spectra were recorded on an IFS 48 FTIR spectrophoto-meter (Bruker) using KBr pellets. Electronic spectra were obtainedon a Hewlett-Packard HP-Diode Array No. 8453 spectrophoto-meter in quartz cuvettes (1 cm) using CHCl3 (1) and DMF (2) assolvents. Mass spectra were recorded on a Varian MAT 212.1H- and 13C-NMR measurements were performed in CDCl3 usinga WH 300 spectrometer (Bruker) with SiMe4 as internal standardat the Organisch-Chemisches Institut, WWU Münster.

Table 1 Crystal data, details of the measurement and structuresolution for [Mn4(ppi)6](BPh4)2 (2)

2

Formula C120H94N12B2Mn4O6

Crystal dimensions 0.24 � 0.20 � 0.16 mm3

Formula weight 2041.45 g·mol�1

Crystal system triclinicSpace group P1 (Nr. 2)Lattice parameters a � 17.500(3) A

b � 17.955(4) Ac � 19.101(4) Aα � 113.79(3)°β � 111.33(3)°γ � 93.91(3)°

Cell volume 4950(2) A3

Formla units per cell 2Calculated density 1.370 g·cm�3

Diffractometer type Stoe IPDSMeasurement temperature 213(2) KRadiation (wavelength) Mo-Kα, (λ � 0.71073 A)Scan type ϕ-scan, 200°, �ϕ � 0.9°θ range for data collection 4.05° � θ � 26.05°Range in h k l �20 � h � 20

�22 � k � 22�23 � l � 23

Observed reflections 39539Unique reflections 18116 (Rint � 0.0965)Reflections with [I > 2σ(I)] 9777Data/restraints/parameters 18116/0/1297Structure solution SHELXS-97,Refinement program SHELXTL-97,

all reflections R1 � 0.1257Residual- wR2 � 0.1200a

Indices reflections with I > 2σ(I) R1 � 0.0557wR2 � 0.0981a

Absorption coefficient 0.564 mm�1

Goodness-of-fit 0.969Largest residual peak and hole 0.318 and �0.252 eA�3

aw � 1/[σ2(Fo2) � (0.0370 P)2] with P � (Fo

2 � 2 Fc2)/3

Z. Anorg. Allg. Chem. 2002, 628, 2458�2462 2459

Table 2 Selected distances /A and angles /° for 2

atom distance atom distance

Mn(1)�Mn(2) 5.720(3) Mn(2)�N(6) 2.230(3)Mn(1)�Mn(3) 5.797(2) Mn(2)�N(7) 2.311(3)Mn(1)�Mn(4) 3.331(2) Mn(2)�N(8) 2.250(4)Mn(2)�Mn(3) 5.679(2) Mn(3)�O(5) 2.138(3)Mn(2)�Mn(4) 3.285(2) Mn(3)�O(6) 2.144(3)Mn(3)�Mn(4) 3.313(1) Mn(3)�N(9) 2.332(4)Mn(1)�O(1) 2.158(3) Mn(3)�N(10) 2.245(3)Mn(1)�O(2) 2.139(3) Mn(3)�N(11) 2.350(4)Mn(1)�N(1) 2.331(3) Mn(3)�N(12) 2.240(3)Mn(1)�N(2) 2.230(3) Mn(4)�O(1) 2.178(3)Mn(1)�N(3) 2.328(3) Mn(4)�O(2) 2.162(3)Mn(1)�N(4) 2.233(3) Mn(4)�O(3) 2.198(3)Mn(2)�O(3) 2.134(3) Mn(4)�O(4) 2.173(3)Mn(2)�O(4) 2.143(3) Mn(4)�O(5) 2.170(3)Mn(2)�N(5) 2.309(3) Mn(4)�O(6) 2.195(3)

atom angle atom angle

O(1)�Mn(1)�O(2) 79.5(1) O(1)�Mn(4)�O(2) 78.6(1)O(1)�Mn(1)�N(1) 108.8(1) O(1)�Mn(4)�O(3) 95.2(1)O(1)�Mn(1)�N(2) 128.1(1) O(1)�Mn(4)�O(4) 93.0(1)O(1)�Mn(1)�N(3) 140.1(1) O(1)�Mn(4)�O(5) 169.2(1)O(1)�Mn(1)�N(4) 73.0(1) O(1)�Mn(4)�O(6) 93.6(1)O(2)�Mn(1)�N(1) 140.4(1) O(2)�Mn(4)�O(3) 92.4(1)O(2)�Mn(1)�N(2) 73.5(1) O(2)�Mn(4)�O(4) 168.2(1)O(2)�Mn(1)�N(3) 109.0(1) O(2)�Mn(4)�O(5) 94.3(1)O(2)�Mn(1)�N(4) 127.7(1) O(2)�Mn(4)�O(6) 96.3(1)N(1)�Mn(1)�N(2) 71.3(1) O(3)�Mn(4)�O(4) 80.1(1)N(1)�Mn(1)�N(3) 89.4(1) O(3)�Mn(4)�O(5) 93.2(1)N(1)�Mn(1)�N(4) 91.1(1) O(3)�Mn(4)�O(6) 168.7(1)N(2)�Mn(1)�N(3) 91.0(1) O(4)�Mn(4)�O(5) 95.1(1)N(2)�Mn(1)�N(4) 155.5(1) O(4)�Mn(4)�O(6) 92.4(1)N(3)�Mn(1)�N(4) 71.4(1) O(5)�Mn(4)�O(6) 79.0(1)

Single Crystal Structure Analysis

Single X-ray diffraction studies of complex 2 were performed ona Stoe IPDS diffractometer with graphite-monochromated Mo-Kα

radiation (λ � 0.71073 A) at 213 K. Crystal data and experimentalconditions are given in Table 1. Intensity data were collected in theϕ-scan mode. The structure was solved with the Patterson methodand refined by full matrix least squares procedure on Fo

2 values(SHELXTL-Plus-97 [15]). Anisotropic displacement parameterswere used to refine the positions of all non-hydrogen atoms. Hydro-gen atoms were placed on calculated positions according to theriding model with group isotropic temperature factors. The residualvalue wR2 in Table 1 is defined as [Σw(Fo

2�Fc2)2 / Σw(Fo

2)2]1/2.Selected distances and angles are listed in Table 2. Crystal data forthe structure of 2 have been deposited with the Cambridge Crystal-lographic Data Centre, CCDC 185225. Copies of the data can beobtained free of charge on application to The Director, CCDC,12 Union Road, Cambridge CB21EZ, UK (Fax int.code�(1223)336-033; e-mail for inquiry: [email protected]).

Synthesis

2-Pyridylmethyl-2-hydroxyphenylimine (Hppi)

Hppi was synthesized according to a modified procedure [16, 17].In a Schiff base reaction stoichiometric amounts of pyridine-2-carbaldehyde (10.7 g, 0.10 mol) and 2-aminophenol (10.9 g,0.10 mol) in absolute ethanol (100 mL) were refluxed for 30 min.The solvent was removed under vacuum and the remaining yellowprecipitate was recrystallized from diethyl ether to yield 12.5 g

Page 3: A Tetranuclear Manganese Cluster with a Star-Shaped Mn4O6 Core Motif: Directed Synthesis using a Mononuclear Precursor Complex

N. Reddig, M. U. Triller, D. Pursche, A. Rompel, B. Krebs

(63 mmol, 63%). C12H10N2O (Mr � 198.2 g/mol): C: 72.05 (calc.72.71)%, H: 4.87 (5.08)%, N: 13.70 (14.13)%.Melting point: 107 °C (109 °C [16])1H NMR (300 MHz, CDCl3): 6.91 ppm (ddd, 1H, pyridine-CH), 7.02 ppm(ddd, 1H, phenol-CH), 7.23 ppm (ddd, 1H, phenol-CH), 7.36 ppm (ddd, 2H,phenol-CH), 7.80 ppm (ddd, 1H, pyridine-CH), 8.17 ppm (ddd, 1H, pyri-dine-CH), 8.70 ppm (ddd, 1H, pyridine-CH), 8.82 ppm (s, 1H, imine-CH).13C NMR (75 MHz, CDCl3): 115.4 ppm (phenol-C), 116.2 ppm (pyridine-C), 120.2 ppm (pyridine-C), 121.6 ppm (phenol-C), 125.2 ppm (pyridine-C), 129.9 ppm (phenol-C), 134.7 ppm (phenol-C), 136.7 ppm (pyridine-C),149.8 ppm (pyridine-C), 152.8 ppm (phenol-C), 154.3 ppm (pyridine-C),157.2 ppm (imine-C).IR (KBr, 4000 � 400 cm�1): ν(OH) 3386 cm�1 s, ν(OH) 3364 cm�1 s,ν(CHArom.) 3049 cm�1 w, ν(CHImin) 2911 cm�1 w, ν(CNImin) 1627 cm�1 m,ν(CCArom.) 1587 cm�1 s, ν(CCArom.) 1488 cm�1 s, 1469 cm�1 s, 1439 cm�1

m, 1358 cm�1 s, 1285 cm�1 m, 1245 cm�1 s, 1205 cm�1 m, 1176 cm�1 m,1152 cm�1 s, 1033 cm�1 w, 993 cm�1 w, 972 cm�1 w, 951 cm�1 w, 927 cm�1

w, 878 cm�1 w, 856 cm�1 w, δ(CHArom.) 783 cm�1 s, δ(CHArom.) 740 cm�1 s,598 cm�1 w, 511 cm�1 w.FIR (PE, 400 � 80 cm�1): 435 cm�1 s, 404 cm�1 vs, 359 cm�1 w, 303 cm�1

w, 257 cm�1 s, 249 cm�1 s, 221 cm�1 s, 212 cm�1 s, 142 cm�1 w.MS (70 eV): m/z � 198 ([Hppi]�, 100 %), 181 ([Hppi � OH]�, 24%), 169(35 %), 120 ([Hppi�C5H4N]�, 81 %), 79 ([C5H4N]� 41 %).

[Mn(ppi)2]·H2O (1)

The mononuclear precursor complex [Mn(ppi)2]·H2O (1) was pre-pared by reacting a solution of Hppi (99 mg, 0.5 mmol) in acetone(10 mL) with Mn(acac)2 (63 mg, 0.25 mmol) suspended in acetone(25 mL). Upon addition of the ligand solution to the yellow slurry,an immediate colour change to deep red was observed. After stir-ring for two hours, the red product was isolated by filtration andwashed with acetone to yield 107 mg (0.23 mmol, 92 %) of 1.[C24H18N4MnO2]·H2O (Mr � 467.1 g/mol): C: 61.74 (calc.61.66)%, H: 4.11 (4.32)%, N: 11.77 (11.99)%.Melting point: > 350°C

IR (KBr, 4000 � 400 cm�1): 3429 cm�1 br, 3128 cm�1 w, 2921 cm�1 w, 1585cm�1 s, 1559 cm�1 m, 1535 cm�1 m, 1478 cm�1 vs, 1462 cm�1 s, 1450 cm�1

s, 1438 cm�1 m, 1376 cm�1 w, 1369 cm�1 w, 1325 cm�1 s, 1301 cm�1 w,1277 cm�1 s, 1252 cm�1 m, 1179 cm�1 w, 1141 cm�1 s, 1098 cm�1 w, 1025cm�1 w, 923 cm�1 w, 902 cm�1 w, 866 cm�1 s, 851 cm�1 w, 799 cm�1 w, 777cm�1 w, 749 cm�1 s, 733 cm�1 m, 633 cm�1 w, 583 cm�1 w.UV/VIS: λmax (lg ε): 328 nm (4.234), 501 nm (4.201).MS (FAB�): m/z � 701 ([Mn2(ppi)3]�, 16 %), 450 ([Mn(ppi)(Hppi)]�, 13 %),449 ([Mn(ppi)2]�, 10 %), 252 ([Mn(ppi)]�, 69 %).

[Mn4(ppi)6](BPh4)2 (2)

[Mn4(ppi)6](BPh4)2 (2) was prepared by adding 2 eq. of sodiumtetraphenylborate (46 mg, 0.13 mmol) to a solution containing 3eq. of the precursor complex 1 (93 mg, 0.20 mmol) and 1 eq.Mn(OAc)2·4H2O (16 mg, 0.07 mmol) in a 1:1 mixture of methanoland acetonitrile (20 mL). The resulting red precipitate was filteredoff and washed with cold acetonitrile yielding 117 mg (0.06 mmol,86 %) of complex 2. Crystals suitable for single X-ray diffractionwere obtained in a DMF / methanol (7:3) mixture by vapour dif-fusion of diethyl ether. C120H94N12B2Mn4O6 (Mr � 2041.45 g/mol):C: 70.23 (calc. 70.57)%, H: 4.73 (4.64)%, N: 8.05 (8.24)%.Melting point: > 350 °C

IR (KBr, 4000�400 cm�1): 3435 cm�1 br, 3054 cm�1 w, 3038 cm�1 w,3011 cm�1 w, 1585 cm�1 s, 1565 cm�1 m, 1547 cm�1 m, 1479 cm�1 vs,1460 cm�1 s, 1441 cm�1 m, 1425 cm�1 m, 1361 cm�1 w, 1299 cm�1 s,1280 cm�1 s, 1250 cm�1 m, 1183 cm�1 w, 1146 cm�1 m, 1101 cm�1 w,1050 cm�1 w, 1009 cm�1 w, 929 cm�1 w, 900 cm�1 w, 865 cm�1 m, 851 cm�1

w, 803 cm�1 m, 775 cm�1 m, 749 cm�1 s, 733 cm�1 s, 705 cm�1 s, 635 cm�1

w, 612 cm�1 w, 589 cm�1 w, 528 cm�1 w, 516 cm�1 w.UV/Vis: λmax (lg e): 319 nm (4.758), 487 nm (4.611).

Z. Anorg. Allg. Chem. 2002, 628, 2458�24622460

Results and Discussion

A complete characterization of 1 by physical methods (ele-mental analysis, IR-, UV/Vis-spectroscopy and mass spec-trometry) leads to the formulation [Mn(ppi)2]·H2O for 1.The IR spectrum of Hppi exhibits two OH stretching vi-brations (3386 cm�1, 3364 cm�1). These bands can be attri-buted to different hydrogen bonds. The ortho-hydroxy pro-ton is involved in intramolecular hydrogen bonding to theimino N atom and intermolecular hydrogen bonding of theortho-hydroxy O atom of an adjacent molecule (compare[17]). These bands cannot be observed in the IR spectrumof 1, which can be explained with a coordination of depro-tonated phenol groups to MnII. In addition to this there aretwo characteristic bands at 2911 cm�1 and 1627 cm�1 inthe Hppi spectrum, effected by CH and CN streching vi-brations of the imino group, respectively. The CN strechingvibration is not present in the spectrum of 1, which iscaused by coordination of the imino N atom of 1. Theseassumptions are supported by comparison with the IRspectrum of 2, where the respective bands are also not ob-served.

The UV/Vis spectrum of 1 exhibits two bands originatingfrom LMCT transitions from a phenolate oxygen pπ orbitalto dσ* and dπ* manganese orbitals [18]. It is obvious thatthe ratio of the extinction coefficients for 1 and 2 (e.g. 17137and 57340 M�1cm�1) is approximately 1:3. This is consist-ent with the ratio of [Mn(ppi)2] fragments in the complexes1 and 2. Similar observations have been made for iron(III)complexes with phenolate ligands [19]. The mass spec-trometry fragmentation pattern (vide supra) also supportsthe formulation of 1 as [Mn(ppi)2]·H2O. Consequently, thestructure of 1 (shown in figure 1) is comparable to the re-ported iron complex [Fe(ppi)2](ClO4]·H2O [20].

Fig. 1 Proposed structure of 1 based on physical methods

The unit cell of 2 contains two [Mn4(ppi)6]2�-units andfour tetraphenylborate ions. There are only MnII ions pre-sent in 2. The coordination spheres of Mn(1), Mn(2) andMn(3) exhibit equivalent N4O2 donor sets. Therefore onlythe coordination environment of Mn(1) will be described.Two ppi ligands coordinate Mn(1) meridionally to form a[Mn(ppi)2] fragment as a subunit of [Mn4(ppi)6]2�. Thebond lengths vary depending on the type of donor atomwith the phenolate oxygen atoms being closest to the

Page 4: A Tetranuclear Manganese Cluster with a Star-Shaped Mn4O6 Core Motif: Directed Synthesis using a Mononuclear Precursor Complex

A Tetranuclear Manganese Cluster

Fig. 2 Ellipsoid plot of the cation in 2

manganese ion. The Mn(1)�O(1)- and Mn(1)�O(2)-bondswith values of 2.158(3) A and 2.139(3) A are the shortest,followed by the bond lengths of Mn(1)�N(2) andMn(1)�N(4) (2.230(3) A and 2.233(3) A, respectively) ofthe imine nitrogen atoms. The pyridine nitrogen atoms N(1)and N(3) exhibit the longest distances to Mn(1) (2.331(3)A and 2.328(3) A, respectively). The bond anglesO(1)�Mn(1)�N(4), N(3)�Mn(1)�N(4), andO(1)�Mn(1)�N(3) with values of 73.0(1)°, 71.4(1)°, and140.1(1)° are indicative of a distortion of the octahedral co-ordination environment of Mn(1). This can be attributed tothe ligand structure that only allows for the formation offive-membered chelate rings. Despite their conjugated π-systems, both ppi-ligands show a distortion from planarity.This is exemplified by a comparison of the least squaresplanes calculated for the phenol and pyridine ring withinthe N4 containing ligand. It shows that the phenol groupforms an angle of 6.3° with the pyridine ring. In addition,the overall angle of 116.6° formed by the two ligands differssignificantly from the expected value of 90° for ideal meri-dional coordination. This results in decreasing the distancesof the cis-standing phenolate oxygen atoms O(1) and O(2)and enables the [Mn(ppi)2] fragment to bind another metalion in the fashion of a chelating ligand.

The coordination sphere around Mn(4) is formed by sixphenolate oxygen atoms from the [Mn(ppi)2] fragments re-sulting in a phenoxo bridged star-shaped Mn4O6 core motif.It can be best described as distorted octahedral. TheMn�O bond lengths are within a small range between2.162(3) A for Mn(4)�O(2) and 2.198(3) A forMn(4)�O(3). The deviation from ideal octahedral structureis shown by comparing the angles O(1)�Mn(4)�O(5),

Z. Anorg. Allg. Chem. 2002, 628, 2458�2462 2461

Fig 3 Coordination of Mn(1) within the [Mn(ppi)2]-fragment of 2

O(2)�Mn(4)�O(4), and O(3)�Mn(4)�O(6). The averagevalue for these angles is 168.7°. Consequently, the valuesof the bond angles O(1)�Mn(4)�O(2), O(3)�Mn(4)�O(4)and O(5)�Mn(4)�O(6) are 78.6(1)°, 80.1(1)°, and 79.0(1)°,respectively. The corresponding oxygen atoms (e.g. O(1)and O(2)) belong to the phenoxo bridges of the same[Mn(ppi)2]-fragment. Three of the [Mn(ppi)2] fragments co-ordinate the central MnII ion, so that 2 can also be de-scribed as {[Mn(ppi)2]3Mn}(BPh4)2. The fragments are ar-ranged in a pseudo-C3-symmetry with respect to Mn(4).The average distance of Mn atoms that adjoin directly is3.310 A, which is expected for manganese(II) complexescontaining phenoxo bridged Mn ions. The noticeable higheraverage distance of Mn(1)�Mn(2), Mn(1)�Mn(3), andMn(2)�Mn(3) is 5.732 A. All four manganese ions are inplane, which is proved by calculating a least squares planefor Mn(1), Mn(2), Mn(3), and Mn(4). The distances to thiscalculated plane are 0.001 A (Mn(1), Mn(2) and Mn(3))and �0.003 A (Mn(4)).

So far, for the general core formula Mn4O6 only ada-mantane structures are known, where the oxygen atomspreponderantly belong to oxo bridges, while in 2 all oxygenatoms come from phenoxo bridges [21�24]. In the hepta-nuclear complexes NEt4[Mn7(OH)3Cl3(hmp)9]Cl[MnCl4][25] (Hhmp � 2-hydroxymethylpyridine) and[Mn7(OCH3)12(dbm)6]·CHCl3·14MeOH [26] (Hdbm � di-

Fig 4 Structure of the Mn4O6 core in 2

Page 5: A Tetranuclear Manganese Cluster with a Star-Shaped Mn4O6 Core Motif: Directed Synthesis using a Mononuclear Precursor Complex

N. Reddig, M. U. Triller, D. Pursche, A. Rompel, B. Krebs

benzoylmethane) the star-like Mn4O6 motif can be foundas a subunit. Due to the fact that in both complexes theoxidation states of the manganese ions vary between �IIand �IV a detailed comparison of structural parameters isnot possible. In addition to the two heptanuclear Mn com-plexes mentioned above there also exist several tetranuclearchromium and cobalt complexes [27�29] and one iron com-plex [30] exhibiting the described M4O6 motif. Among thosethe tetranuclear iron complex [Fe4(OCH3)6(dpm)6](Hdpm � dipivaloylmethane) shows the highest degree ofsimilarity [30]. The iron centers are connected by meth-anolate bridges. The remaining coordination sites are occu-pied by the bipodal ligand dpm. All iron ions are in theoxidation state �III. Here the average distance between twoadjacent iron centers is 3.140 A. Because of the higher nu-clear charge this distance is smaller compared to the corre-sponding Mn�Mn distance of 3.310 A in 2. This is alsoobserved for the other average metal-metal distances inboth complexes, which are 5.431 A in [Fe4(OCH3)6(dpm)6]and 5.732 A in 2.

The funnel motif has been subject to intensive investi-gations as a possible structural motif for the active site ofPS II. In analogy to reported chromium and cobalt com-plexes [28] the use of precursor complexes that still havefree metal binding sites offers a synthetic route to controlledpreparation of polynuclear metal complexes. There are sev-eral possibilities to change the composition of the designedcomplexes. It is e.g. conceivable to substitute both the cen-tral manganese ion and the manganese ions of the outersphere by various M2� ions. This is of particular interestfor the design of model compounds for higher nuclearitymetalloenzymes.

Acknowledgements. We gratefully acknowledge financial support bythe Deutsche Forschungsgemeinschaft. M.U. T. thanks the Fondsder Chemischen Industrie for a Doktorandenstipendium. A. R.thanks the Ministerium für Wissenschaft, Weiterbildung undSchule for the Benningsen-Foerder-Preis.

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