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(II)L1(PF6)(2) (1; LI = 2,6-bis[2-(2,2'-bipyridin-6'-yeethyl]pyridine) and Co(II)L2(PF6)2 (2;L2 = 2,6-bis [2-(4-methoxy-2,2'-bipyridin-6 '-y1) ethyllpyridine) were synthesized and used for photocatalytic CO, 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 CO, 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 /), 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 H-2 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 [Co L-2(CO)] with vco = 1894 cm(-1) together with [(CoL2)-L-II](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 [Co1L(2)](+) species, consistent with density functional theory calculations with CoLl complexes, which predict a large structural change from trigonal-bipyramidal to distorted tetragonal geometry. The reduction of CO, 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]+.