Metall‐π‐Komplexe von Benzolderivaten, 50 . Arylborane als Sandwichliganden: Darstellung von [( i PrO) 2 B(η 6 ‐Ph)] 2 M und [Mes 2 B(η 6 ‐Ph)] 2 M (M = V, Cr). Redoxeigenschaften und EPR Untersuchung M(d 5 )‐konfigurierter Neutralkomplexe sowie Bor‐zentrierter Radikalanionen

Metal π Complexes of Benzene Derivatives, 50. – Arylboranes as Sandwich Ligands: Preparation of [( i PrO) 2 B(η 6 ‐Ph)] 2 M and [Mes 2 B(η 6 ‐Ph)] 2 M (M = V, Cr). Redox Properties and EPR Study of M(d 5 ) Neutral Complexes and Boron‐Centered Radical Anions Bis(η 6 ‐arene)metal complexes bearing peripheral boryl substituents have been prepared by means of metal‐atom ligand‐vapor cocondensation leading to [( i PrO) 2 B‐η 6 ‐C 6 H 5 ] 2 Cr ( 7 ) and [( i PrO) 2 B‐η 6 ‐C 6 H 5 ] 2 V ( 8 ) as well as by lithiation and subsequent reaction with Mes 2 BF with formation of [Mes 2 B‐η 6 ‐C 6 H 5 ] 2 Cr ( 11 ) and [Mes 2 B‐η 6 ‐C 6 H 5 ] 2 V ( 12 ). According to the NMR spectra, rotation about the η‐C–BR 2 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 ( i PrO) 2 B‐ and Mes 2 B, the hyperfine coupling constants a ( 51 V) 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 Δ E 1/2 = 570 mV. This value, which is a measure of electronic communication between the two Mes 2 B groups, lies between those of 1,4‐bis(dimesitylboryl)benzene (δ E 1/2 = 690 mV) and 4,4′‐bis(dimesitylboryl)biphenyl (Δ E 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 Mes 2 BC 6 H   5 −• . 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 ( 53 Cr) and a ( 1 H), oxidation of 11 is metal‐centered; relative to parent bis(η 6 ‐benzene)chromium ( 9 ), the two Mes 2 B 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.