Water-induced Switching in Selectivity and Steric Control of Activity in Photochemical CO2 Reduction Catalyzed by RhCp*(bpy) Derivatives

Photocatalytic reduction of CO2 to formic acid (HCOOH) was investigated in either organic or aqueous/organic media by employing three water-soluble [RhIIICp*(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 to 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 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 RhIII(H)Cp*(LH2-•) on CO2 but also partly bypassed by a dark H2 addition reaction yielding [RhIII(H)Cp*(L)]- from [RhIIICp*(L)Cl]+, which was also separately investigated under the dark conditions. Combination of experimental and theoretical approaches were made to clarify the pKa 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’-subsituted derivative. The intrinsic reaction coordinate calculations were carried out to clarify the mechanism of hydride transfer steps leading to generate formate together 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 a new light on the strategy to control the efficiency and selectivity in the catalysis of CO2 reduction.