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

The anodic oxidation mechanism of RhCp(CO)(PPh 3 ), 1, has been studied in CH 2 Cl 2 /0.1 M [NBu 4 ][PF 6 ]. This complex and its analogue RhCp(PPh 3 ) 2 had been previously shown to form the fulvalenyl dirhodium complexes [Rh 2 FvL 2 (PPh 3 ) 2 ] 2+ (Fv = (η 5,η 5 -C 10 H 8 ), L = CO ( 2 2+ ) or PPh 3 ) 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 [Rh 2 Cp 2 (CO) 2 (PPh 3 ) 2 ] 2+ ( 5 2+ ), which dominates at low temperatures. The room-temperature products are best accounted for by hydrogen atom transfer reactions of the dimer dication, affording 2 2+ and the metal hydride [RhCp(CO)(PPh 3 )H] + . The dimetalate complex [Rh 2 (σ:η 5 -C 5 H 4 ) 2 (CO) 2 (PPh 3 ) 2 ] 2+ ( 7 ) may also be formed. The radical cation of the analogue RhCp(CO)(PPh 2 Me) ( 3 ) undergoes very rapid formation of a similar metal–metal bonded dimer. A derivative with a large cone angle phosphine, RhCp(CO)(P i Pr 3 ) ( 4 ), does not show the same tendency toward oxidative dimerization. The monomer/dimer equilibrium [RhCp(CO)L] + ⇌ 1/2 [Rh 2 Cp 2 (CO) 2 L 2 ] 2+ increasingly favors the dimer in the sequence L = P i Pr 3 < PPh 3 < PPh 2 Me < PMe 3, 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.