Photocatalytic CO₂ Reduction by Trigonal-Bipyramidal Cobalt(II) Polypyridyl Complexes: The Nature of Cobalt(I) and Cobalt(0) Complexes upon Their Reactions with CO₂, CO, or Proton

The cobalt complexes Co^IIL1(PF₆)₂ (1; L1 = 2,6-bis[2-(2,2′-bipyridin-6′-yl)ethyl]pyridine) and Co^IIL2(PF₆)₂ (2; L2 = 2,6-bis[2-(4-methoxy-2,2′-bipyridin-6′-yl)ethyl]pyridine) were synthesized and used for photocatalytic CO₂ reduction in acetonitrile. X-ray structures of complexes 1 and 2 reveal distorted trigonal-bipyramidal geometries with all nitrogen atoms of the ligand coordinated to the Co(II) center, in contrast to the common six-coordinate cobalt complexes with pentadentate polypyridine ligands, where a monodentate solvent completes the coordination sphere. Under electrochemical conditions, the catalytic current for CO₂ reduction was observed near the Co(I/0) redox couple for both complexes 1 and 2 at E_1/2 = -1.77 and -1.85 V versus Ag/AgNO₃ (or -1.86 and -1.94 V vs Fc^+/0), respectively. Under photochemical conditions with 2 as the catalyst, [Ru(bpy)₃]²⁺ as a photosensitizer, tri-p-tolylamine (TTA) as a reversible quencher, and triethylamine (TEA) as a sacrificial electron donor, CO and H₂ were produced under visible-light irradiation, despite the endergonic reduction of Co(I) to Co(0) by the photogenerated [Ru(bpy)₃]⁺. However, bulk electrolysis in a wet CH₃CN solution resulted in the generation of formate as the major product, indicating the facile production of Co(0) and [Co-H]ⁿ⁺ (n = 1 and 0) under electrochemical conditions. The one-electron-reduced complex 2 reacts with CO to produce [Co⁰L2(CO)] with ν_CO = 1894 cm^-1 together with [Co^IIL2]²⁺ through a disproportionation reaction in acetonitrile, based on the spectroscopic and electrochemical data. Electrochemistry and time-resolved UV-vis spectroscopy indicate a slow CO binding rate with the [Co^IL2]⁺ species, consistent with density functional theory calculations with CoL1 complexes, which predict a large structural change from trigonal-bipyramidal to distorted tetragonal geometry. The reduction of CO₂ is much slower than the photochemical formation of [Ru(bpy)₃]⁺ because of the large structural changes, spin flipping in the cobalt catalytic intermediates, and an uphill reaction for the reduction to Co(0) by the photoproduced [Ru(bpy)₃]⁺.