The Impact of a Proton Relay in Binuclear α‑Diimine-Mn(CO) 3 Complexes on the CO 2 Reduction Catalysis

Herein, we describe the redox chemistry of bi- and mononuclear α-diimine-Mn(CO)3 complexes with an internal proton source in close proximity to the metal centers and their catalytic activity in the electrochemically driven CO2 reduction reactions. In order to address the impact of the two metal sites and of the proton source, we investigate a binuclear complex with phenol moiety, 1, a binuclear Mn complex with methoxyphenol unit instead, 2, and the mononuclear analogue with a phenol unit, 3. Spectroelectrochemical investigation of the complexes in dmf under a nitrogen atmosphere indicates that 1 and 3 undergo a reductive H2 formation forming [Mn2(H–1L1)(CO)6Br] and [Mn(H–1L3)(CO)3], respectively, which is redox neutral for the complex and equivalent to a deprotonation of the phenol unit. The reaction likely proceeds via internal proton transfer from the phenol moiety to the reduced metal center forming a Mn–H species. 2 dimerizes during reduction, forming [Mn2(L2)(CO)6]2, but 1 and 3 do not. Reduction of ...