Selective CO2 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 Co-I center and a dianionic ligand, producing CO as the major product and trace amount of H-2. 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.