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

Photocatalytic reduction of CO to formic acid (HCOOH) was investigated in either organic or aqueous/organic media by employing three water-soluble [RhCp*(LH)Cl] (LH = n,n'-dihydroxy-2,2'-bipyridine; = 4, 5, or 6) in the presence of [Ru(bpy)], 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[]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 CO 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 H selectivity shows a reverse phenomenon.