Electrocatalytic CO2 Reduction to C1 and C2 Oxygenates by Involving an Organic Radical-Coordinated Zn(II) Complex...

A four-coordinate Zn(II) complex (1) containing two ligand-based radicals was synthesized by reacting ligand H4Sar(AP/AP) with Zn(ClO4)2·6H2O, and the complex was structurally characterized by X-ray single crystal measurement. The variable-temperature magnetic susceptibility measurement in solid and magnetic moment analysis at 298 K in solution using the Evans method revealed that in the complex, the radical-based unpaired electrons remained uncoupled in solution, while intramolecular antiferromagnetic coupling was realized in the solid state, leading to a diamagnetic ground state. The ligand-based unpaired electrons underwent two one-electron oxidations and two one-electron reductions in the potential range of 0.10 to -1.20 V vs. Fc/Fc+. To leverage ligand-based electron-transfer processes for CO2 reduction, the complex was coated on a graphite electrode surface using Nafion-117, and electrocatalytic CO2 reduction was carried out within the potential range of -0.79 to -1.39 V vs RHE in 0.5 M KHCO3 aqueous solution. This provided value-added C1 (CH3OH and HCOO-) and C2 (CH3COO-, CH3CHO, and CH3CH2OH) products with moderate FE (30-41%), while continuous electrocatalysis at -0.79 V for 8 h rendered mainly CH3CH2OH. Thus, without the involvement of metal-based redox processes and using only the ligand-based electron-transfer events of complex 1, electrocatalytic CO2 reduction was achieved and reported.