Synthesis and Characterization of meso-Substituted Cobalt Tetradehydrocorrin and Evaluation of Its Electrocatalytic Behavior Toward CO2 Reduction and H2 Evolution.

A meso-aryl substituted cobalt(II) tetradehydrocorrin complex (Co(II)TDHC) has been synthesized and investigated. The corrin framework, determined by X-ray crystallographic analysis, is found to be relatively planar except at the C1 and C19 positions. Cyclic voltammetry (CV) measurements indicate two positively shifted reversible redox couples at -0.53 and -1.70 V vs Fc/Fc+ for [CoII]+/[CoI] and [CoI]/([CoI]•- and/or [CoII]-) ([CoII] = Co(II)TDHC), respectively, compared with the previously reported cobalt porphyrin complex, because the tetradehydrocorrin ligand efficiently promotes the formation of low-valent metal species due to its monoanionic character. Furthermore, it is found that the current in the CV measurement is significantly enhanced upon addition of H2O under a CO2 atmosphere, indicating the progression of electroreductive catalysis by Co(II)TDHC. However, controlled-potential electrolysis (CPE) using Co(II)TDHC under the same conditions shows generation of H2 as a major product and only a small amount of CO as a CO2 reduction product; Faradaic efficiencies are calculated to be 66.8 and 4.5%, respectively. The CPE with a buffer solution under an N2 atmosphere reveals that the selective H2 generation is promoted by the moderate acidification of the solution under CO2 saturation conditions. The present study demonstrates that the significantly stabilized Co(I) species with the monoanionic ligand framework preferentially catalyzes the thermodynamically favored H2 evolution rather than CO2 reduction.