Two-Electron-Induced Reorganization of Cobalt Coordination and Metal–Ligand Cooperative Redox Shifting Co(I) Reactivity toward CO₂ Reduction

Abstract

Electrochemical reorganization of complex structures is directly related to catalytic reactivity; thus, the geometric changes of catalysts induced by electron transfer should be considered to scrutinize the reaction mechanism. Herein, we studied electron-induced reorganization patterns of six-coordinate Co complexes with neutral N-donor ligands. Upon two-electron transfer into a Co center enclosed within a bulky π-acceptor ligand, the catalytic site exhibited different reorganization patterns depending on the ligand characteristics. While a bipyridyl ligand released Co-bound solvent (CH3CN) to open a reaction site, a phenanthroline ligand caused Co–Narm (side “arm” of NNN–ligand) bond dissociation. The first electron transfer occurred in the Co(II/I) reduction step and the second electron entered the bulky π-acceptor, of which redox steps were assigned from cyclic voltammograms, magnetic moment measurements, and DFT calculations. In comparison, the Co complex of [NNNNCH3 –Co(CH3CN)3](PF6)2 ([1-(CH 3 CN) 3 ](PF6)2) showed a high H2 evolution reactivity (HER), whereas a series of Co complexes with bulky π-acceptors such as [NNNNCH3 –Co(L)(CH3CN)](PF6)2 (L = phen ([2-CH 3 CN](PF6)2), bpy ([3-CH 3 CN](PF6)2), [NNNNCH3 –Co(tpy)](PF6)2 ([4](PF6)2), and [NNNCH2 –Co(phen)(CH3CN)](PF6)2 ([5-CH 3 CN](PF6)2)) suppressed the HER but rather enhanced the CO2 reduction reaction. The metal–ligand cooperative redox steps enabled the shift of Co(I) reactivity toward CO2 reduction. Additionally, the amine pendant attached to the NNNNCH3 –ligand could stabilize the CO2 reduction intermediate through the hydrogen-bonding interaction with the Co–CO2H adduct.