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 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 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)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.