Reduction–oxidation properties of organotransition-metal complexes. Part 25. The one-electron reduction of cationic cyclopentadienyl–cobalt and –rhodium nitrosyl derivatives

Abstract

The complexes [ML(NO)(η-C₅R₅)][PF₆][M = Co or Rh, R = H, L = PPh₃; R = Ph, L = P(OMe)₃ or P(OPh)₃] undergo diffusion-controlled, one-electron reduction at a platinum-based electrode in CH₂Cl₂ or tetrahydrofuran (thf). Reduction of the cobalt complexes either chemically with [Co(η-C₅H₅)₂] or by controlled potential electrolysis leads to P-donor ligand elimination, and dimerisation, to give [{Co(µ-NO)(η-C₅R₅)}₂](R = H or Ph). The primary reduction products, namely [CoL(NO)(η-C₅Ph₅)][L = P(OMe)₃ or P(OPh)₃], have been detected by e.s.r. spectroscopy at –196 °C; the rhodium analogues, [RhL(NO)(η-C₅Ph₅)][L = P(OMe)₃ or P(OPh)₃] are stable at room temperature and have been isolated from the reaction between [RhL(NO)(η-C₅Ph₅)][PF₆] and [Co(η-C₅H₅)₂]. The dimer [{Co(µ-NO)(η-C₅Ph₅)}₂] undergoes two reversible, one-electron oxidations and one irreversible, one-electron reduction; the paramagnetic salt [{Co(µ-NO)(η-C₅Ph₅)}₂][PF₆] has been isolated via[N₂C₆H₄F-p][PF₆] oxidation of the neutral precursor. The rhodium dimer [{Rh(µ-NO)(η-C₅Ph₅)}₂], prepared by the reductive nitrosylation of [{RhBr₂-(η-C₅Ph₅)}₂] with NO gas and [Co(η-C₅H₅)₂] in thf, or by the [Co(η-C₅H₅)₂] reduction of [RhI(NO)(η-C₅Ph₅)], undergoes reversible one-electron oxidation and reduction and a second, irreversible, one-electron oxidation.