Tetrazole-Substituted isomeric ruthenium polypyridyl complexes for low overpotential electrocatalytic CO₂ reduction

Introducing tetrazole moiety to the ligand framework of two isomeric ruthenium catalysts, cis/trans-[Ru(tpy)(mtzp)(CH₃CN)]²⁺ (tpy = 2,2′:6′,2′'-terpyridine, mtzp = 2-(1-methyl-1H-tetrazol-5-yl)pyridine), for the electrochemical reduction of CO₂ to CO has altered the catalytic pathway with significantly low overpotential (0.37 V) compared to its analogous catalysts. Without manipulating steric effects, only the electronic nature of tetrazole moiety enables CO₂ binding to ruthenium center to form metallocarboxylate intermediate just after one-electron reduction. This is the first synthesized isomeric pair of ruthenium complex follow ECE (E = electron transfer, C = chemical reaction) mechanism for electrocatalytic reduction of CO₂. By successful characterization of the Ru–CO intermediate with the help of ¹³C NMR, spectro-electrochemical studies and analysis of byproducts formed during the electrocatalysis, a mechanism of CO₂ reduction has been established in presence of water and anhydrous conditions which is further supported by density functional theory (DFT).