Electrocatalytic Reduction of CO₂ to CO by Molecular Cobalt-Polypyridine Diamine Complexes

Cobalt complexes have previously been reported to exhibit high faradaic efficiency in reducing CO₂ to CO. Herein, we synthesized capsule-like cobalt-polypyridine diamine complexes [Co(L¹)](BF₄)₂ (1) and [Co(L²) (CH₃CN)](BF₄)₂ (2) as catalysts for the electrocatalytic reduction of CO₂. Under catalytic conditions, complexes 1 and 2 demonstrated the electrocatalytic reduction of CO₂ to CO in the presence or absence of CH₃OH 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 CO₂ to form a metallocarboxylate intermediate [Co^II(L)-CO₂^2-]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)-CO₂^2-]⁰ have suggested that the doubly reduced species attacks CO₂ on the C atom through the d_z² orbital, while the interaction with CO₂ 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.