Abstract
Cobalt complexes have previously been reported to exhibit high faradaic efficiency in reducing CO(2) to CO. Herein, we synthesized capsule-like cobalt-polypyridine diamine complexes [Co(L(1))](BF(4))(2) (1) and [Co(L(2)) (CH(3)CN)](BF(4))(2) (2) as catalysts for the electrocatalytic reduction of CO(2). Under catalytic conditions, complexes 1 and 2 demonstrated the electrocatalytic reduction of CO(2) to CO in the presence or absence of CH(3)OH as a proton source. Experimental and computational studies revealed that complexes 1 and 2 undergo two consecutive reversible one-electron reductions on the cobalt core, followed by the addition of CO(2) to form a metallocarboxylate intermediate [Co(II)(L)-CO(2)(2-)](0). This crucial reaction intermediate, which governs the catalytic cycle, was successfully detected using high resolution mass spectrometry (HRMS). In situ Fourier-transform infrared spectrometer (FTIR) analysis showed that methanol can enhance the rate of carbon-oxygen bond cleavage of the metallocarboxylate intermediate. DFT studies on [Co(II)(L)-CO(2)(2-)](0) have suggested that the doubly reduced species attacks CO(2) on the C atom through the d(z)(2) orbital, while the interaction with CO(2) is further stabilized by the π interaction between the metal d(xz) or d(xz) orbital with p orbitals on the O atoms. Further reductions generate a metal carbonyl intermediate [Co(I)(L)-CO](+), which ultimately releases CO.