Two-Electron-Induced Reorganization of Cobalt Coordination and Metal-Ligand Cooperative Redox Shifting Co(I) Reactivity toward CO 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 (CHCN) to open a reaction site, a phenanthroline ligand caused Co-N (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 [NNN-Co(CHCN)](PF) ([](PF)) showed a high H evolution reactivity (HER), whereas a series of Co complexes with bulky π-acceptors such as [NNN-Co(L)(CHCN)](PF) (L = phen ([](PF)), bpy ([](PF)), [NNN-Co(tpy)](PF) ([](PF)), and [NNN-Co(phen)(CHCN)](PF) ([](PF))) suppressed the HER but rather enhanced the CO reduction reaction. The metal-ligand cooperative redox steps enabled the shift of Co(I) reactivity toward CO reduction. Additionally, the amine pendant attached to the NNN-ligand could stabilize the CO reduction intermediate through the hydrogen-bonding interaction with the Co-COH adduct.