Mechanistic insights into CO2 conversion to CO using cyano manganese complexes

Without the use of a photosensitizer, [Mn(bpy)(CO)(3)(CN)] (MnCN) can photochemically form [Mn(bpy)(CO)(3)](-), the active species for CO2 reduction. While cases of the axial X-ligand dissociating upon irradiation of fac-[M(N-N)(CO)(3)X] complexes (M = Mn or Re; N-N = bipyridine (bpy) ligand; X = halogen or pseudohalogen) are well documented, the axial cyanide ligand is retained when either [Mn(bpy)(CO)(3)(CN)] or [Mn(mesbpy)(CO)(3)(CN)], MnCN(mesbpy), are irradiated anaerobically. Infrared and UV-vis spectroscopies indicate the formation of [Mn(bpy)(CO)(2)(MeCN)(CN)] (s-MnCN) as the primary product during the irradiation of MnCN. An in-depth analysis of the photochemical mechanism for the formation of [Mn(bpy)(CO)(3)](-) from MnCN is presented. MnCN(mesbpy) is too sterically hindered to undergo the same photochemical mechanism as MnCN. However, MnCN(mesbpy) is found to be electrocatalytically active for CO2 reduction to CO. Thus providing an interesting distinction between photochemical and electrochemical charge transfer.