Water-Induced Switching in Selectivity and Steric Control of Activity in Photochemical CO₂ Reduction Catalyzed by...

Photocatalytic reduction of CO₂ to formic acid (HCOOH) was investigated in either organic or aqueous/organic media by employing three water-soluble [Rh^IIICp*(LH₂)Cl]⁺ (LH₂ = n,n'-dihydroxy-2,2'-bipyridine; n = 4, 5, or 6) in the presence of [Ru(bpy)₃]²⁺, 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) and triethanolamine (TEOA). Through studying the electron-donating effects of two hydroxyl groups introduced into the bipyridyl ligand, we found that the substituent positions greatly affect both the catalytic efficiency and selectivity in CO₂ reduction. More importantly, the HCOOH selectivity shows a dramatic increase from 14 to 83% upon switching the solvent media from pure organic to an aqueous/organic mixture, where the H₂ selectivity shows a reverse phenomenon. The enhanced HCOOH selectivity and the drastic decrease in the H₂ yield are well rationalized by the fact that the catalytic CO₂ hydrogenation is not only driven photochemically via the attack of Rh^III(H)Cp*(LH₂^-·) on CO₂ but also partly bypassed by a dark H₂ addition reaction yielding [Rh^III(H)Cp*(L)]^- from [Rh^IIICp*(L)Cl]⁺, which was also separately investigated under dark conditions. A combination of experimental and theoretical approaches was made to clarify the pK_a values of catalyst intermediates together with the abundant species responsible for the major catalytic processes. Our DFT studies unveil that the exceptionally large structural strain given by the steric contacts between the 6,6'-dihydroxyl groups and the Cp* moiety plays a significant role in bringing about an outstanding catalytic performance of the 6,6'-substituted derivative. The intrinsic reaction coordinate calculations were carried out to clarify the mechanism of hydride transfer steps leading to generating formate together with the heterolytic H₂ cleavage steps leading to afford the key hydridorhodium intermediates. This study represents the first report on the water-induced high selectivity in CO₂-to-HCOOH conversion, shedding new light on the strategy to control the efficiency and selectivity in the catalysis of CO₂ reduction.