Metal π complexes of benzene derivatives, 50[]. - Arylboranes as sandwich ligands: Preparation ol [(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

Article abstract of DOI:10.1002/cber.19961290721

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 e_1g 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 δE_1/2 = 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 (δE_1/2 = 690 mV) and 4,4′-bis(dimesitylboryl)biphenyl (δE_1/2 = 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 Mes₂BC₆H₅^-. 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. VCH Verlagsgesellschaft mbH, 1996.