Water-Induced\nSwitching in Selectivity and Steric\nControl of Activity in Photochemical CO<sub>2</sub> Reduction Catalyzed\nby RhCp*(bpy) Derivatives

Photocatalytic\nreduction of CO<sub>2</sub> to formic acid (HCOOH)\nwas investigated in either organic or aqueous/organic media by employing\nthree water-soluble [Rh<sup>III</sup>Cp*­(LH<sub>2</sub>)Cl]<sup>+</sup> (LH<sub>2</sub> = n,n’-dihydroxy-2,2′-bipyridine; <i>n</i> = 4, 5, or 6) in the presence of [Ru(bpy)<sub>3</sub>]<sup>2+</sup>, 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[<i>d</i>]imidazole (BIH) and triethanolamine (TEOA). Through studying the\nelectron-donating effects of two hydroxyl groups introduced into the\nbipyridyl ligand, we found that the substituent positions greatly\naffect both the catalytic efficiency and selectivity in CO<sub>2</sub> reduction. More importantly, the HCOOH selectivity shows a dramatic\nincrease from 14 to 83% upon switching the solvent media from pure\norganic to an aqueous/organic mixture, where the H<sub>2</sub> selectivity\nshows a reverse phenomenon. The enhanced HCOOH selectivity and the\ndrastic decrease in the H<sub>2</sub> yield are well rationalized\nby the fact that the catalytic CO<sub>2</sub> hydrogenation is not\nonly driven photochemically via the attack of Rh<sup>III</sup>(H)Cp*­(LH<sub>2</sub><sup>–·</sup>) on CO<sub>2</sub> but also partly\nbypassed by a dark H<sub>2</sub> addition reaction yielding [Rh<sup>III</sup>(H)­Cp*(L)]<sup>−</sup> from [Rh<sup>III</sup>Cp*­(L)Cl]<sup>+</sup>, which was also separately investigated\nunder dark conditions. A combination of experimental and theoretical\napproaches was made to clarify the p<i>K</i><sub>a</sub> values of catalyst intermediates together with the abundant species\nresponsible for the major catalytic processes. Our DFT studies unveil\nthat the exceptionally large structural strain given by the steric\ncontacts between the 6,6′-dihydroxyl groups and the Cp* moiety\nplays a significant role in bringing about an outstanding catalytic\nperformance of the 6,6′-substituted derivative. The intrinsic\nreaction coordinate calculations were carried out to clarify the mechanism\nof hydride transfer steps leading to generating formate together with\nthe heterolytic H<sub>2</sub> cleavage steps leading to afford the\nkey hydridorhodium intermediates. This study represents the first\nreport on the water-induced high selectivity in CO<sub>2</sub>-to-HCOOH\nconversion, shedding new light on the strategy to control the efficiency\nand selectivity in the catalysis of CO<sub>2</sub> reduction.