The π-system in a series of three fac-[Mn(α-diimine)(CO)3Br] complexes is extended from 2,2′-bipyridine (bpy) to 2-(2-pyridyl)quinoline (pq) to 2,2′-biquinoline (bqn). The effects on photochemical mechanism, electrochemistry, and electroreduction of CO2 is observed via cyclic voltammetry, bulk electrolysis, and UV-vis and IR spectroscopies.
Abstract
The complex [Mn(bpy)(CO)3Br], has been previously studied as both an electrocatalyst and a photocatalyst, in conjugation with a photosensitizer, for CO2 reduction to CO. This study considers the relationship between this catalytic activity and the steric and electronic nature of the aromatic diimine ligand. To this end, the π-system in the bidentate ligand is increased step-wise from 2,2′-bipyridine (bpy) to 2-(2-pyridyl)quinoline (pq) to 2,2′-biquinoline (bqn) in a series of three fac-[Mn(α-diimine)(CO)3Br] complexes. It is found that the propensity of these complexes to photochemically dimerize trends with the energy of the α-diimine π* energy. Electrochemically, it is observed that the second reduction event in these systems becomes increasingly thermodynamically favorable and approaches the potential of the first reduction event as the π-system expands. In fac-[Mn(bqn)(CO)3Br], the second reduction is more favorable than the first reduction, precluding the formation of a dimer intermediate; even though, chemical reduction of fac-[Mn(bqn)(CO)3Br] confirms that the dimer, [Mn(bqn)(CO)3Br]2 is able to form and not prevented by steric considerations. Though the second reduction potential is more positive for bqn and pq than for bpy, the CO2 reduction mechanism changes such that the overpotential for carbon dioxide reduction occurs at more negative potentials, leading to a decrease in overall catalytic activity.
