Selective CO ₂ Reduction by Bis(bipyridine)cobalt(II) Catalysts: The Role of Pendant Pyridine as a Proton Acceptor

Electrochemical CO2 reduction reaction (CO2RR) catalyzed by molecular earth-abundant metal catalysts is a promising strategy to convert CO2 into value-added products. One recent trend in this field has been focusing on the rational design of catalysts by incorporating redox-active ligands and modifying the secondary coordination sphere (SCS) to achieve efficient and selective CO2RR. Herein, we report a series of Co bis(bipyridine) catalysts featuring various dangling groups, such as pyridine, tertiary amine, or butyl, in the secondary coordination sphere (Co-PyMe, Co-Py, Co-PrN, and Co-Bu). Efficient, selective electrocatalytic CO2RR was achieved by the complexes after the generation of triply reduced intermediate consisting of a CoI center and a dianionic ligand, producing CO as the major product and trace amount of H2. Strong correlations with the identity of dangling groups and turnover frequency (TOF) have been observed, in which Co-PyMe displayed the highest TOF (1086 s–1 in MeCN/H2O). Mechanistic studies indicated that the acceleration of CO2RR with pyridine-functionalized catalysts were derived from the protonation of pyridine dangling groups which participated as weak acids in the H-bonding network with exogenous proton sources, stabilizing CO2-bound intermediates and facilitating proton transfer. In addition, precatalytic CO2 binding and activation at the third reduction (−2.1 V) was revealed by CV and SEC-IR studies. The resultant doubly reduced CO-bound species acted as a trapping state which inhibited CO2RR electrocatalysis. Regeneration of active species was accessed via reductive dissociation of CO at a more negative potential. This study highlights the combined effects of redox-active ligands and pyridine/pyridinium as SCS groups on CO2RR catalysis and provides design principles for future development of CO2RR catalysts utilizing pyridine/pyridiniums as SCS functional groups to fine-tune the catalytic activity.