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 (<b>1</b>; L1 = 2,6-bis­[2-(2,2′-bipyridin-6′-yl)­ethyl]­pyridine) and Co^IIL2­(PF₆)₂ (<b>2</b>; 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 <b>1</b> and <b>2</b> 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 <b>1</b> and <b>2</b> at <i>E</i>_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 <b>2</b> as the catalyst, [Ru­(bpy)₃]²⁺ as a photosensitizer, tri-<i>p</i>-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]^<i>n</i>+ (<i>n</i> = 1 and 0) under electrochemical conditions. The one-electron-reduced complex <b>2</b> 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]⁺