Rhenium(I) Phosphazane Complexes for Electrocatalytic CO2 Reduction

A library of 11 rhenium phosphazane complexes was synthesized and characterized for fundamental studies of electrochemical CO2 reduction. Because they contain chelating phosphorus donors, these complexes prevent deleterious dimerization of the precatalyst similarly to their monodentate analogues. These chelating species open a coordination site without complete ligand loss, which is hypothesized to increase catalyst longevity. Furthermore, the phosphazane ligands provided sites for synthetic modifications to control electronic structure, steric bulk, and install hydrogen bond donors. The effects of these parameters on the formation of CO from CO2 were investigated in the context of Faradaic efficiencies, catalytic current responses (i cat/i p), and catalyst longevity. Full structural characterization provided insight into the differences in catalytic performance between the complexes, including a dirhenium variant. Re­(PNP H )­(bpy)­(CO)2OTf (bpy = 2,2′-bipyridine, PNP H = (Ph)2PNHP­(Ph)2) showed high catalytic performance with an (i cat/i p)2 value of 795 and was active for >24 h with a 94% Faradaic efficiency. This library of electrocatalysts establishes a new class of tunable Re complexes from which a deeper mechanistic understanding of the CO2 reduction reaction emerges and underscores the efficacy of exploiting ligand-based participation in catalytic processes.