Dinuclear Rhenium Complex with a Proton Responsive Ligand as a Redox Catalyst for the Electrochemical CO2 Reduction.

Herein, we present the reduction chemistry of a dinuclear α-diimine rhenium complex, 1, [Re2(L)(CO)6Cl2], with a proton responsive ligand and its application as a catalyst in the electrochemical CO2 reduction reaction (L = 4-tert-butyl-2,6-bis(6-(1H-imidazol-2-yl)-pyridin-2-yl)phenol). The complex has a phenol group in close proximity to the active center, which may act as a proton relay during catalysis, and pyridine-NH-imidazole units as α-diimine donors. The complex is an active catalyst for the electrochemical CO2 reduction reaction. CO is the main product after catalysis, and only small amounts of H2 were observed, which can be related to the ligand reactivity. The ic/ip ratio of 20 in dimethylformamide (DMF) + 10% water for 1 points to a higher activity with regard to [Re(bpy)(CO)3Cl] in MeCN/H2O, albeit 1 requires a slightly larger overpotential (bpy = 2,2'-bipyridine). Spectroscopic and theoretical investigations revealed detailed information about the reduction chemistry of 1. The complex exhibits two reduction processes in DMF, and each process was identified as a two-electron reduction in the absence of CO2. The first 2e- reduction is ligand based and leads to homolytic N-H bond cleavage reactions at the imidazole units of 1, which is equal to a net double proton removal from 1 forming [Re2(LH-2)(CO)6Cl2]2-. The second 2e- reduction process has been identified as an O-H bond cleavage reaction at the phenol group, removal of chloride ions from the coordination spheres of the metal ions, and a ligand-centered one-electron reduction of [Re2(LH-3)(CO)6Cl]2-. In the presence of CO2, the second reduction process initiates catalysis. The reduced species is highly nucleophilic and likely favors the reaction with CO2 instead of O-H bond cleavage.