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 [Co^II]⁺/[Co^I] and [Co^I]/([Co^I]^•– and/or [Co^II]⁻) ([Co^II] = 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.