Role of a Metal–Metal Bonded Dimer Dication in the One-Electron Oxidation of Rh(η5-C5H5)(CO)(PPh3) and Related Compounds

Abstract

The anodic oxidation mechanism of RhCp(CO)(PPh3), 1, has been studied in CH2Cl2/0.1 M [NBu4][PF6]. This complex and its analogue RhCp(PPh3)2 had been previously shown to form the fulvalenyl dirhodium complexes [Rh2FvL2(PPh3)2]2+ (Fv = (η5,η5-C10H8), L = CO (22+) or PPh3) upon chemical oxidation. The present work investigated the reaction of 1 by variable-temperature electrochemistry and IR spectroelectrochemistry. The radical cation 1+ initially undergoes a radical–radical coupling reaction, giving the metal–metal bonded dimer dication [Rh2Cp2(CO)2(PPh3)2]2+ (52+), which dominates at low temperatures. The room-temperature products are best accounted for by hydrogen atom transfer reactions of the dimer dication, affording 22+ and the metal hydride [RhCp(CO)(PPh3)H]+. The dimetalate complex [Rh2(σ:η5-C5H4)2(CO)2(PPh3)2]2+ (7) may also be formed. The radical cation of the analogue RhCp(CO)(PPh2Me) (3) undergoes very rapid formation of a similar metal–metal bonded dimer. A derivative with a large cone angle phosphine, RhCp(CO)(PiPr3) (4), does not show the same tendency toward oxidative dimerization. The monomer/dimer equilibrium [RhCp(CO)L]+ ⇌ 1/2 [Rh2Cp2(CO)2L2]2+ increasingly favors the dimer in the sequence L = PiPr3 < PPh3 < PPh2Me < PMe3, P(OPh)3, the latter two being based on earlier work. The implied dinuclear hydrogen atom transfer reactions are not mechanistically well understood, but find analogies in the chemistry of second- and third-row early transition metal complexes.