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), <b>1</b>, 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 (<b>2</b>²⁺) or PPh₃) upon chemical oxidation. The present work investigated the reaction of <b>1</b> by variable-temperature electrochemistry and IR spectroelectrochemistry. The radical cation <b>1</b>⁺ initially undergoes a radical–radical coupling reaction, giving the metal–metal bonded dimer dication [Rh₂Cp₂(CO)₂(PPh₃)₂]²⁺ (<b>5</b>²⁺), which dominates at low temperatures. The room-temperature products are best accounted for by hydrogen atom transfer reactions of the dimer dication, affording <b>2</b>²⁺ and the metal hydride [RhCp­(CO)­(PPh₃)­H]⁺. The dimetalate complex [Rh₂(σ:η⁵-C₅H₄)₂(CO)₂(PPh₃)₂]²⁺ (<b>7</b>) may also be formed. The radical cation of the analogue RhCp­(CO)­(PPh₂Me) (<b>3</b>) undergoes very rapid formation of a similar metal–metal bonded dimer. A derivative with a large cone angle phosphine, RhCp­(CO)­(PⁱPr₃) (<b>4</b>), 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