Proton-Coupled, Low-Energy Pathway for Electrocatalytic CO 2 Reduction at Re(Diimine) Complexes with a Conjugated Pyrazinyl Moiety.

Large conjugated carbon framework has been incorporated as the diimine ligand for Re(α-diimine)(CO) 3 Cl complexes with a pyrazinyl linkage, either to increase energy efficiency or to turn them into heterogeneous catalysts for selective electrocatalytic CO 2 reduction. However, there exists a nonmonotonic dependence of CO 2 reduction overpotential on the conjugation size of the ligands. Understanding its origin could facilitate heterogenization of molecular catalysts with improved energy efficiency. Here, we show that the conjugated pyrazinyl moiety plays a crucial role in catalysis by enabling a proton-coupled, lower-energy pathway for CO 2 reduction. With ligands of moderate size, the pathway leads to previously unknown intermediates and decreases CO 2 reduction overpotential. Because the pathway hinges on the basicity of the pyrazinyl nitrogen, we propose that it imposes a limit on the conjugation size of the ligand for the pathway to be effective.