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

Abstract

Herein, we present the reduction chemistry of a [GRAPHIC] dinuclear alpha-diimine rhenium complex, 1, [Re-2(L)(CO)(6)Cl-2], 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-y1)-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 alpha-diimine donors. The complex is an active catalyst for the electrochemical CO, reduction reaction. CO is the main product after catalysis, and only small amounts of H-2 were observed, which can be related to the ligand reactivity. The i(c)/i(p) ratio of 20 in dimethylformamide (DMF) + 10% water for 1 points to a higher activity with regard to [Re(bpy)(CO)(3)Cl] 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 CO, 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)(6)Cl-2](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 [Re-2(LH-3)(CO)(6)Cl](2-). In the presence of CO2, the second reduction process initiates catalysis. The reduced species is highly nucleophilic and likely favors the reaction with CO, instead of O-H bond cleavage.