Synthesis, redox chemistry, molecular and electronic structure of some cyclopentadienylcobalt pentafulvene complexes

Abstract

The pentafulvene complexes [(C 5R 5)Co(C 6H 4R′R′′)] 3d (R=H, R′=Me, R′′=Ph), 3f (R=H, R′=Ph, R′′=C 6H 4NO 2-4), 3g (R=H, R′=R′′=SMe) and 9 (R=Me, R′=R′′=Ph) were prepared from the respective fulvenes and [(C 5R 5)Co(C 2H 4) 2] 8a, b. Protonation of 3 occurs at the fulvene C-α to give substituted cobaltocenium cations. The crystal and molecular structures of 3g, 9 and of the protonation product [ 3e+H] + (R=R′=Ph) were determined. In the neutral complexes the non-planar fulvene ligands are essentially η 4-coordinated with short uncoordinated exocyclic carbon–carbon double bonds. In contrast, the protonated species exhibit η 5-coordinated planar five-membered rings. Using cyclic voltammetry, the complexes 3e, f and 9 were shown to reversibly undergo one-electron oxidation and reduction reactions to give the cations [ 3e] +, [ 3f] +, [ 9] + and anions [ 3e] −, [ 3f] −, [ 9] −. Complex 3g also forms an anion [ 3g] −, but oxidation is irreversible. The X-band ESR spectra of [ 3e] −, [ 3e] + and of [ 9] −, [ 9] + were recorded. In marked contrast to the cations, the anion radicals exhibit considerable anisotropy of the g and A tensors. d-Electron spin densities ρ d=0.5 for the radical anions were derived from a detailed analysis and simulation of the ESR spectra. Using extended Hückel and Fenske–Hall MO calculations for the model complex [(C 5H 5)Co(C 5H 4CH 2)] 3h, the ESR and electrochemical properties were explained by the different metal participation in the HOMO (=SOMO of the cation radicals) and LUMO (=SOMO of the anion radicals).