Mechanistic Insights into CO2 Photo- and Electroreduction Using Manganese Complexes

Abstract

Photo- and electrochemical reduction is a promising approach to utilize carbon dioxide as a chemical feedstock. To this end, complexes of the form, fac-[Mn(α-diimine)(CO)3Br] were studied using spectroscopic and electrochemical methods to uncover mechanistic insights in the reduction of carbon dioxide to carbon monoxide. In Chapter 2, the unexpected role of molecular oxygen on the stability of photogenerated [(CO)3(bpy)Mn–Mn(bpy)(CO)3] (Mn-Mn), from a cyanide-bridged species, [{Mn(bpy)(CO)3]2}(µ-CN)]+ (Mn2CN+) is revealed. The photoevolution of this transformation is studied in acetonitrile (MeCN) using IR and UV-vis spectroscopy. Since the presence of Mn-Mn and light is required for CO production, [Mn(bpy)(CO)3•] is proposed to be a photochemical reagent without a photosensitizer for the transformation of CO2 to CO. Intrigued by the photochemical reactivity conferred by the cyano ligand, in Chapter 3, the monomeric complex, [Mn(bpy)(CO)3(CN)] (MnCN) was synthesized an found to photochemically form [Mn(bpy)(CO)3]-, an active species for CO2 reduction. While cases of the axial X-ligand dissociating upon irradiation of these complexes are well documented, the axial cyano ligand is retained when either [Mn(bpy)(CO)3(CN)] or [Mn(mesbpy)(CO)3(CN)], MnCN(mesbpy), are irradiated anaerobically. While, MnCN(mesbpy) is too sterically hindered to undergo the same photochemical mechanism as MnCN, MnCN(mesbpy) is found to be electrocatalytically active for CO2 reduction to CO, providing an interesting distinction between photochemical and electrochemical charge transfer. In Chapter 4, the π-system of the diimine ligand 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 was observed via cyclic voltammetry, bulk electrolysis, and UV-vis and IR spectroscopies. Finally, in Chapter 5, the interplay between an extended π-system of the diimine ligand and an addition nitrogen atom are explored in the synthesis of pynaphthyridine (pynap) and binaphthyridine (bn) Mn complexes. It was found that strategic placement of the nitrogen atom in the 8-position of the pynap or the 8,8’-positions of the bn ligands conferred greater CO2 reduction abilities than the corresponding parent complexes based on hydrogen bonding interactions that stabilize the Mn-C(O)OH intermediate.