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 pi-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 pi-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-(CH3CN)3](PF6)2) showed a high H2 evolution reactivity (HER), whereas a series of Co complexes with bulky pi-acceptors such as [NNNNCH3-Co(L)(CH3CN)](PF6)2 (L = phen ([2-CH3CN](PF6)2), bpy ([3-CH3CN](PF6)2), [NNNNCH3-Co(tpy)](PF6)2 ([4](PF6)2), and [NNNCH2-Co(phen)(CH3CN)](PF6)2 ([5-CH3CN](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.

Automated summary

The reorganization of catalysts induced by electron transfer plays a crucial role in catalytic reactivity. In this study, the electron-induced reorganization patterns of Co complexes with neutral N-donor ligands were investigated. The different ligand characteristics led to different reorganization patterns, such as the release of solvent or bond dissociation. Furthermore, the metal-ligand cooperative redox steps shifted the reactivity of Co(I) towards CO2 reduction, and the amine pendant stabilized the CO2 reduction intermediate through hydrogen bonding.