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

The complex [Mn(bpy)(CO)(3)Br], 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 p-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(a-diimine)(CO)(3)Br] complexes. It is found that the propensity of these complexes to photochemically dimerize trends with the energy of the a- diimine p* energy. Electrochemically, it is observed that the second reduction event in these systems becomes increasinglythermodynamically favorable and approaches the potential of the first reduction event as the p-system expands. In fac- [Mn(bqn)(CO)(3)Br], 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)(3)Br] confirms that the dimer, [Mn(bqn)(CO)(3)Br]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.

Automated summary

This study investigates the relationship between the catalytic activity of [Mn(bpy)(CO)(3)Br] and the steric and electronic properties of the aromatic diimine ligand. It is found that as the p-system in the ligand expands, the propensity of the complexes to dimerize photochemically increases. Additionally, the second reduction event becomes more thermodynamically favorable as the p-system expands, approaching the potential of the first reduction event.