Radical intermediates in the reductive isomerisation of octahedral manganese carbonyl bipyridyl complexes; electrochemistry, electron spin resonance spectroscopy and molecular-orbital calculations

Abstract

The complexes fec-[Mn(CO)3L(bipy)]+[L = CNBut or P(OMe)3, bipy = 2,2′-bipyridyl], cis,trans-and cis,cis-[Mn(CO)2L(L′)(bipy)]+[L = L′= CNBut, P(OEt)3, P(OPh)3 or PPh3; L = CNBut, L′= P(OMe)3], fac-[Mn(CO)(CNBut)3(bipy)]+ and mer-[Mn(CO)L3(bipy)]+[L = CNBut or P(OR)3, R = Me or Et] underwent one-electron oxidation at a platinum-disc electrode in CH2Cl2 and/or one-electron reduction in tetrahydrofuran (thf). The complexes mer-[Mn(CO)L3(bipy)]+[L = P(OEt)3 or CNBut] were oxidised by [N2C6H4F-p][PF6] to give dications with ESR spectra characteristic of low-spin d5 transition-metal complexes. The ESR spectroscopic studies of the sodium amalgam reduction of mer- and fac-[Mn(CO)(CNBut)3(bipy)]+, cis,trans-and cis,cis-[Mn(CO)2(CNBut)2(bipy)]+, mer-[Mn(CO){P-(OMe)3}(bipy)]+, or cis,trans-[Mn (CO)2{P(OMe)3}2(bipy)]+ in thf provide evidence for the intermediacy of neutral bipyridyl ligand-based radicals in the reductive isomerisation of the cis,cis to the cis,trans, and of the fac to the mer, cations. Extended-Huckel molecular-orbital calculations on the mer and fac isomers of [Mn(CO)L3(bipy)]+ and the cis,trans and cis,cis isomers of [Mn(CO)2L2(bipy)]+(L = CNH or PH3) provide support for the formation of metal- and ligand-based radicals on oxidation and reduction respectively.