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

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.