Multifunctional ligands with podand topology provide intrinsically well-defined coordination geometries. Several subgroups of multidentate ligand systems comprising dual functionality have been developed so far. Recent advances made in this research area reflect its topicality. Work on metal complexes of ambiphilic ligands consisting of Lewis-acidic Group 13 element bridgehead atoms and additional donor functionalities is in the current focus of interest. The intrinsic topology of tetradentate ligands may introduce a fascinating reactivity and interesting electronic properties to the metal complexes. Janus-head type ligands are very promising candidates for the preparation of multimetallic complexes.

Multidentate ligand systems featuring dual functionality

New perspectives in the tailoring of hetero (bi- and tri-) metallic alkoxide derivatives

Bis(benzene)chromium. 2. Its Discovery by E. O. Fischer and W. Hafner and Subsequent Work by the Research Groups of E. O. Fischer, H. H. Zeiss, F. Hein, C. Elschenbroich, and Others†

Recent Trends in Metal Alkoxide Chemistry

Aromatic hydrocarbons as ligands. Recent advances in the synthesis, the reactivity and the applications of bis(η6-arene) complexes

Dreikomponenten-Cokondensation von Benzol, Pyridin und Chrom liefert (η6-Benzol)(η6-pyridin)chrom (5), den ersten Bis-(aren)metall-Komplex mit unsubstituiertem Pyridin. Der binare Komplex Bis(η6-pyridin)chrom (7) wird durch Desilylierung der durch Cokondensation zuganglichen Verbindung Bis[2,6-bis(trimethylsilyl)-η6-pyridin]chrom6) erhalten, wobei die sterische Abschirmung der N-Atome durch die Me3Si-Gruppen Bis-η6-Koordination im ersten Syntheseschritt gewahrleistet. Die neuen Verbindungen 5–7 werden durch 1H- und 13C-NMR, MS, ESR (Radikalkationen) und Cyclovoltammetrie charakterisiert. Gemas Rontgenstrukturanalyse kristallisiert 7 fehlgeordnet, die individuellen Molekule liegen im Gitter als synclinale oder/und als antiperiplanare Rotamere vor. 5 last sich am N-Atom protonieren. Die Bestimmung des pKs-Wertes (5-H+) aus der pH-Abhangigkeit des UV-Spektrums bzw. des Redoxpotentials fur 50/+ erweist, das η6-Pyridin in 5 2.5–3.0 pK-Einheiten basischer ist als freies Pyridin.



Metal π Complexes of Heteroarenes, II. – η6-Coordination of Unsubstituted Pyridine: (η6-Benzene)(η6-pyridine)chromium and Bis(η6-pyridine)chromium



Three-component cocondensation of benzene, pyridine, and chromium affords (η6-benzene)(η6-pyridine)chromium (5), the first bis-(arene)metal complex of unsubstituted pyridine. The binary complex bis(η6-pyridine)chromium (7) is obtained by desilylation of bis[2,6-bis(trimethylsilyl)-η6-pyridine]chromium (6), the latter being accessible through cocondensation techniques. The function of the trimethylsilyl groups is to sterically block the pyridine N atoms thereby assuring η6-coordination in the first step. The new compounds 5–7 are characterized by means of 1H and 13C NMR, MS, ESR (radical cations), and cyclic voltammetry. According to X-ray diffraction, crystals of 5 are disordered such that individual molecules exist in the lattice as synclinal or/and antiperiplanar rotamers. 5 can be protonated at nitrogen. The value of pKs (5-H+), determined from the pH dependence of the UV spectrum and of the redox potential for the couple 50/+, reveals that η6-pyridine in 5 is more basic by 2.5–3.0 pK units than free pyridine.

Metall-π-Komplexe von Heteroarenen, II. η6-Koordination von unsubstituiertem Pyridin: (η6-Benzol)(η6-pyridin)chrom und Bis(η6-pyridin)chrom

Alkoxygermanates(II), -stannates(II), and -plumbates(II) of Divalent Metal Ions, II[1]. – Compounds of the Formula M2El2(OtBu)8



By simple salt-exchange processes the starting materials Na2El2(OtBu)6 (El = Ge, Sn, Pb) can be transformed to germanates, stannates and plumbates of divalent magnesium and divalent transition metals. Two types of compounds are formed in these reactions: MEl2(OtBu)6 [El = Ge, M = Mg (1A), Cr (1B), Mn (1C), Zn (1F); El = Pb, M = Mn (3C), M = Zn (3F)] and M2El2(OtBu)8 [El = Ge, M = Co (1d), Ni (1e); El = Sn, M = Mg (2a), Cr (2b), Mn (2c), Co (2d), Ni (2e); El = Pb, M = Co (3d)]. Single-crystal X-ray diffraction studies have been performed on 1C, 1d, 2a, 2b, 2c, 2d, and 2e, and the structures have been solved. In 1C the Mn atom occupies the center of an elongated O6 octahedron, the germanium(II) atoms displaying pyramidal coordination by three oxygen atoms. The central molecular cage can be described as two MnO3Ge trigonal bipyramids sharing the common central Mn atom and being wrapped by tert-butyl groups linked to the oxygen atoms. The other compounds of the MEl2(OtBu)6 formula seem to be isostructural with the exception of 3F, which displays a 1H-NMR spectrum which is not compatible with this structure. All X-ray structures of the compounds M2El2(OtBu)8 show the same feature: to a central M2(OtBu)2 four-membered ring are spirocyclically connected two M(OtBu)2El rings through the common metal atoms M. The structure is completed by the coordination of an exocyclic tert-butoxy group to the terminal El atoms. The metal atoms M are therefore quasi tetrahedrally coordinated while the Ge and Sn atoms are in pyramidal threefold oxygen atom environments. All molecules display an El M M El one-dimensional arrangement. From susceptibility measurements it is apparent, that in the compounds MEl2(OtBu)6 and M2El2(OtBu)8 the transition metal atoms are in high-spin configurations, which is also supported by the UV spectra. Analysis of the structural data of the series 2a–2e reveal important contributions of the electronic environments of the transition metal atoms to the M M and M Sn distances. A qualitative MO description is used to explain these features. Again it has been shown that the, „geometrical softness” of Ge(OtBu)3 and Pb(OtBu)3 is greater than of Sn(OtBu)3, as the former two can accomodate Cr2+ and Mn2+ in a sixfold coordination site by two units, while Sn(OtBu)3 coordinates Cr2+ and Mn2+ with only two alkoxy groups. When 1C and 2d are allowed to react with nonacarbonyldiiron Mn-Ge2(OtBu)6 · 2 Fe(CO)4 (4) and Co2Sn2(OtBu)8 · 2 Fe(CO)4 (5), respectively, are formed. Compound 4 displays presumably five metal atoms in a linear arrangement while 5 has six metallic elements arranged in one dimension. The latter fact has been unambigously proved by an X-ray structure determination.

Alkoxygermanate(II), -stannate(II) und -plumbate(II) zweiwertiger Metall-Ionen, II. Verbindungen des Formeltyps M2El2(OtBu)8

Metal π Complexes of Benzene Derivatives, 50. – Arylboranes as Sandwich Ligands: Preparation of [(iPrO)2B(η6-Ph)]2M and [Mes2B(η6-Ph)]2M (M = V, Cr). Redox Properties and EPR Study of M(d5) Neutral Complexes and Boron-Centered Radical Anions



Bis(η6-arene)metal complexes bearing peripheral boryl substituents have been prepared by means of metal-atom ligand-vapor cocondensation leading to [(iPrO)2B-η6-C6H5]2Cr (7) and [(iPrO)2B-η6-C6H5]2V (8) as well as by lithiation and subsequent reaction with Mes2BF with formation of [Mes2B-η6-C6H5]2Cr (11) and [Mes2B-η6-C6H5]2V (12). According to the NMR spectra, rotation about the η-C–BR2 bonds in 7 and 11 is free at 25°C, restrictions setting in at –85°C. The neutral radicals 8• and 12• have been studied by EPR spectroscopy: despite of the profound difference in the electron-accepting properties of the groups (iPrO)2B- and Mes2B, the hyperfine coupling constants a(51V) differ by 5% only; the extent of perturbation of the electronic structure manifests itself more clearly in the g tensor, which is tetragonal for 8 but orthorhombic for 12, the degeneracy of the LUMO's e1g being raised in the latter. Cyclovoltammetry on 11 exhibits reversible oxidation as well as reduction processes. Reduction which, according to EPR, is ligand-centered, occurs in two steps which are separated by the redox splitting ΔE1/2 = 570 mV. This value, which is a measure of electronic communication between the two Mes2B groups, lies between those of 1,4-bis(dimesitylboryl)benzene (δE1/2 = 690 mV) and 4,4′-bis(dimesitylboryl)biphenyl (ΔE1/2 = 270 mV). The EPR spectrum of the radical anion 11−• reveals hyperfine coupling to one boron nucleus of a magnitude very similar to that of the free ligand radical anion Mes2BC6H. Therefore, reduction of 11 is ligand-centered, and an intramolecular electron exchange between the boron centers is slow on the EPR time scale. As inferred from the observation of the hyperfine interactions a(53Cr) and a(1H), oxidation of 11 is metal-centered; relative to parent bis(η6-benzene)chromium (9), the two Mes2B groups cause an anodic shift of 290 mV for the couple 11+/0. The radical cations 11+• are prone to protodeborylation, in this aspect resembling the respective silyl derivatives.

Metall-π-Komplexe von Benzolderivaten, 50. Arylborane als Sandwichliganden: Darstellung von [(iPrO)2B(η6-Ph)]2M und [Mes2B(η6-Ph)]2M (M = V, Cr). Redoxeigenschaften und EPR Untersuchung M(d5)-konfigurierter Neutralkomplexe sowie Bor-zentrierter Radikalanionen

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Metall-π-Komplexe von Heteroarenen, II. η6-Koordination von unsubstituiertem Pyridin: (η6-Benzol)(η6-pyridin)chrom und Bis(η6-pyridin)chrom

Alkoxygermanate(II), -stannate(II) und -plumbate(II) zweiwertiger Metall-Ionen, II. Verbindungen des Formeltyps M2El2(OtBu)8

Metall-π-Komplexe von Benzolderivaten, 50. Arylborane als Sandwichliganden: Darstellung von [(iPrO)2B(η6-Ph)]2M und [Mes2B(η6-Ph)]2M (M = V, Cr). Redoxeigenschaften und EPR Untersuchung M(d5)-konfigurierter Neutralkomplexe sowie Bor-zentrierter Radikalanionen