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 CO ₂ 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 H ₂ formation forming [Mn ₂ (H _-1 L ¹ )(CO) ₆ Br] and [Mn(H _-1 L ³ )(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 [Mn ₂ (L ² )(CO) ₆ ] ₂ , but 1 and 3 do not. Reduction of 1 , 2 , and 3 is accompanied by bromide loss, and the final species represent [Mn ₂ (H _-1 L ¹ )(CO) ₆ ] ^3- , [Mn ₂ (L ₂ )(CO) ₆ ] ^2- , and [Mn(H _-1 L ³ )(CO) ₃ ] ^2- , respectively. 1 and 2 are active catalysts in the electrochemical CO ₂ reduction reaction, whereas 3 decomposes quickly under an applied potential. Thus, the second redox active unit is crucial for enhanced stability. The proton relay in 1 alters the kinetics for the 2H ⁺ /2e ^- reduced products of CO ₂ in dmf/water mixtures. For 2 , CO is the only product, whereas formate and CO are formed in similar amounts, 40% and 50%, respectively, in the presence of 1 . Thus, the reaction rate for the internal proton transfer from the phenol moiety to the metal center forming the putative Mn-H species and subsequent CO ₂ insertion as well as the reaction rate of the reduced metal center with CO ₂ forming CO are similar. The overpotential with regard to the standard redox potential of CO ₂ to CO and the observed overall rate constant for catalysis at scan rates of 0.1 V s ^-1 are higher with 1 than with 2 , that is, the OH group is beneficial for catalysis due to the internal proton transfer.