Oxidatively induced isomerisation of vinylidene ligands to alkynes: ESR spectra of paramagnetic vinylidene and alkyne arene metal complexes

Abstract

UV irradiation of [M(CO)₃(η-arene)] and Me₃SiCCSiMe₃ gives [M(CO)₂{CC(SiMe₃)₂}(η-arene)] (M = Cr, arene = C₆H₂Me₄-1,2,3,5 2V, C₆H₃Me₃-1,2,3 3V or C₆H₆ 4V; M = Mo, arene = C₆Me₆ 5V or C₆H₃Me₃-1,3,5 6V). The crystal structure of [Cr(CO)₂{CC(SiMe₃)₂}(η-C₆H₆)] 4V confirms the presence of the vinylidene ligand; the complex has approximate C_s symmetry with the C(SiMe₃)₂ plane orthogonal to the arene_centroid–Cr–C_α–C_β plane. Voltammetry and IR and NMR spectroscopy show that in solution [Mo(CO)₂{CC(SiMe₃)₂}(η-C₆H₃Me₃-1,3,5)] 6V thermally equilibrates with the alkyne isomer [Mo(CO)₂(η-Me₃SiCCSiMe₃)(η-C₆H₃Me₃-1,3,5)] 6A. The vinylidene complexes [M(CO)₂{CC(SiMe₃)₂}(η-arene)] 2V–6V undergo one-electron oxidation to the alkyne cations [M(CO)₂(η-Me₃SiCCSiMe₃)(η-arene)]⁺ 2A⁺–6A⁺via fast, redox-induced vinylidene-to-alkyne isomerisation. These cations are reduced to the neutral alkyne complexes [M(CO)₂(η-Me₃SiCCSiMe₃)(η-arene)] 2A–6A which slowly isomerise thermally to the neutral vinylidene complexes 2V–6V. Paramagnetic vinylidene and alkyne complex cations have been characterised by ESR spectroscopy; unpaired electron density is extensively delocalised from the metal centre to the C₂ ligand, in agreement with the results of EHMO calculations.