Large conjugated carbon framework has been incorporated as the diimine ligand for Re(α-diimine)(CO)3Cl complexes with a pyrazinyl linkage, either to increase energy efficiency or to turn them into heterogeneous catalysts for selective electrocatalytic CO2 reduction. However, there exists a non-monotonic dependence of CO2 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 the conjugated pyrazinyl moiety plays a crucial role in the catalysis by enabling a proton-coupled, lower-energy pathway for CO2 reduction. With ligands of moderate size, the pathway leads to previously unknown intermediates and decreases CO2 reduction overpotential. Because the pathway hinges on 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.

Proton-coupled, low-energy pathway for electrocatalytic CO2 reduction at Re(diimine) complexes with a conjugated pyrazinyl moiety

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