Extending the π-System in Mn^I Diimine Tricarbonyl Complexes: Impacts on Photochemistry, Electrochemistry, and CO₂ Catalytic Reduction Activity

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.