Secondary Coordination Effect on Monobipyridyl Ru(II) Catalysts in Photochemical CO2 Reduction: Effective Proton Shuttle of Pendant Brønsted Acid/Base Sites (OH and N(CH3)2) and Its Mechanistic Investigation

While the incorporation of pendant Brønsted acid/base sites in the secondary coordination sphere is a promising and effective strategy to increase the catalytic performance and product selectivity in organometallic catalysis for CO2 reduction, the control of product selectivity still faces a great challenge. Herein, we report two new trans(Cl)-[Ru(6-X-bpy)(CO)2Cl2] complexes functionalized with a saturated ethylene-linked functional group (bpy = 2,2′-bipyridine; X = −(CH2)2–OH or −(CH2)2–N(CH3)2) at the ortho(6)-position of bpy ligand, which are named Ru-bpyOH and Ru-bpydiMeN, respectively. In the series of photolysis experiments, compared to nontethered case, the asymmetric attachment of tethering ligand to the bpy ligand led to less efficient but more selective formate production with inactivation of CO2-to-CO conversion route during photoreaction. From a series of in situ FTIR analyses, it was found that the Ru–formate intermediates are stabilized by a highly probable hydrogen bonding between pendent proton donors (−diMeN+H or −OH) and the oxygen atom of metal-bound formate (RuI–OCHO···H–E–(CH2)2–, E = O or diMeN+). Under such conformation, the liberation of formate from the stabilized RuI–formate becomes less efficient compared to the nontethered case, consequently lowering the CO2-to-formate conversion activities during photoreaction. At the same time, such stabilization of Ru–formate species prevents the dehydration reaction route (η1-OCHO → η1-COOH on Ru metal) which leads toward the generation of Ru–CO species (key intermediate for CO production), eventually leading to the reduction of CO2-to-CO conversion activity.