Probing the electronic factors responsible for the cyclic electron-transfer induced isomerism fac⇄mer: Synthesis, electrochemical and spectroscopic studies of fac-[Mn(CO)3(L′–L′)L]0/+ complexes

Spectroscopic (IR, 31P NMR and UV–Vis) and electrochemical studies on fac-[Mn(CO)3(L′–L′)(L)]0/+,where L′–L′ = 1,2-bis(diphenylphosphino)ethane (dppe) or 1,10-phenanthroline (phen) and L = bromide, triflate, imidazole (im), isonicotinamide (isn) or N-(2-hydroxyethyl)isonicotinamide (heisn), were undertaken to understand the effect of various ligands on the CO–Mn–L and CO–Mn–(L′–L′) bonding characteristics of these complexes. Crystal structures for L = triflate/L′–L′ = dppe, L = triflate/L′–L′ = phen and L = isn/L′–L′ = phen are reported and they show that the two Mn–O(OSO2CF3) and Mn–N(isn) distances are similar. The tricarbonyl complexes exhibit two major bands in the 250–300 and 350–450 nm region of the UV–Vis spectrum. The lowest energy bands have been assigned as a contribution from both the metal-centered (MC) and metal to ligand (dπ → L′–L′) charge transfer (MLCT) transitions. The energy of this maximum absorption decreases in the order Br− ∼ triflate > im > isn ∼ heisn. The cyclic four-component mechanism fac - Mn ( I ) → - e fac - Mn ( II ) → mer - Mn ( II ) → + e mer - Mn ( I ) → fac - Mn ( I ) was observed at room temperature by voltammetric techniques for all the cases. On the basis of d metal orbital splitting, an electronic molecular orbital diagram is proposed. In this model, the ligands along the z-axis play a relevant role in the reverse of the HOMO energies of the fac/mer isomers by stabilizing the metal d z 2 orbital relative to dxy in mer-Mn(II).