Two-Electron-Induced Reorganization of Cobalt Coordination and Metal-Ligand Cooperative Redox Shifting Co(I) Reactivity toward CO 2 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 (CH 3 CN) to open a reaction site, a phenanthroline ligand caused Co-N arm (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 NCH 3 -Co(CH 3 CN) 3 ](PF 6 ) 2 ([ 1-(CH 3 CN) 3 ](PF 6 ) 2 ) showed a high H 2 evolution reactivity (HER), whereas a series of Co complexes with bulky π-acceptors such as [NNN NCH 3 -Co(L)(CH 3 CN)](PF 6 ) 2 (L = phen ([ 2-CH 3 CN ](PF 6 ) 2 ), bpy ([ 3-CH 3 CN ](PF 6 ) 2 ), [NNN NCH 3 -Co(tpy)](PF 6 ) 2 ([ 4 ](PF 6 ) 2 ), and [NNN CH 2 -Co(phen)(CH 3 CN)](PF 6 ) 2 ([ 5-CH 3 CN ](PF 6 ) 2 )) suppressed the HER but rather enhanced the CO 2 reduction reaction. The metal-ligand cooperative redox steps enabled the shift of Co(I) reactivity toward CO 2 reduction. Additionally, the amine pendant attached to the NNN NCH 3 -ligand could stabilize the CO 2 reduction intermediate through the hydrogen-bonding interaction with the Co-CO 2 H adduct.