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 = CNBu^t or P(OMe)₃, bipy = 2,2′-bipyridyl], cis,trans-and cis,cis-[Mn(CO)₂L(L′)(bipy)]⁺[L = L′= CNBu^t, P(OEt)₃, P(OPh)₃ or PPh₃; L = CNBu^t, L′= P(OMe)₃], fac-[Mn(CO)(CNBu^t)₃(bipy)]⁺ and mer-[Mn(CO)L₃(bipy)]⁺[L = CNBu^t 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 CNBu^t] 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)(CNBu^t)₃(bipy)]⁺, cis,trans-and cis,cis-[Mn(CO)₂(CNBu^t)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-Hückel 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.