Electrocatalyst decomposition pathways: torsional strain in a second sphere proton relay shuts off CO2RR in a Re(2,2′-bipyridyl)(CO)3X type electrocatalyst

Abstract

Group 7 tris(carbonyl) bipyridine complexes have been well explored as important CO₂ reduction reaction (CO₂RR) electrocatalysts and now represent an excellent platform for catalyst design. Recent synthetic focus has been on the installation of proton sources/relays within the primary/secondary coordination sphere. These proton sources have been implicated in directly assisting catalysis by acting as shuttles for proton transfer or through the stabilization of transition states through hydrogen bonding. Herein, we report a new ligand system for CO₂RR electrocatalysts, which features an aryl amine appended to a quinoline–bipyridine core. While the geometrical arrangement of the aryl amine seems amenable to assisting CO₂RR electrocatalysis, we find, through spectroelectrochemical and chemical reduction studies, the torsional strain imposed on the ligand induces a structural reorganization through loss of a hydrogen atom radical. This new complex, which utilizes the anionic nitrogen as a donor atom, and other Re complexes with the same coordination motif, have been found to be entirely inactive for CO₂RR. Subsequent reduction yields hydrogenation of the complex through dearomatization of the quinoline backbone concomitant with decomposition products. While the electrocatalytic capability of the reported complexes is moderate, the study represents an important investigation into the deactivation of CO₂RR electrocatalysts as a consequence of typical proton shuttle moieties and guides future ligand design by highlighting an oft overlooked structural parameter.