Judicious Ligand Design in Ruthenium Polypyridyl CO2 Reduction Catalysts to Enhance Reactivity by Steric and Electronic Effects

Abstract

A series of Ru^II polypyridyl complexes of the structural design [Ru^II (R-tpy)(NN)(CH₃ CN)]²⁺ (R-tpy=2,2':6',2''-terpyridine (R=H) or 4,4',4''-tri-tert-butyl-2,2':6',2''-terpyridine (R=tBu); NN=2,2'-bipyridine with methyl substituents in various positions) have been synthesized and analyzed for their ability to function as electrocatalysts for the reduction of CO₂ to CO. Detailed electrochemical analyses establish how substitutions at different ring positions of the bipyridine and terpyridine ligands can have profound electronic and, even more importantly, steric effects that determine the complexes' reactivities. Whereas electron-donating groups para to the heteroatoms exhibit the expected electronic effect, with an increase in turnover frequencies at increased overpotential, the introduction of a methyl group at the ortho position of NN imposes drastic steric effects. Two complexes, [Ru^II (tpy)(6-mbpy)(CH₃ CN)]²⁺ (trans-[3]²⁺ ; 6-mbpy=6-methyl-2,2'-bipyridine) and [Ru^II (tBu-tpy)(6-mbpy)(CH₃ CN)]²⁺ (trans-[4]²⁺ ), in which the methyl group of the 6-mbpy ligand is trans to the CH₃ CN ligand, show electrocatalytic CO₂ reduction at a previously unreactive oxidation state of the complex. This low overpotential pathway follows an ECE mechanism (electron transfer-chemical reaction-electron transfer), and is a direct result of steric interactions that facilitate CH₃ CN ligand dissociation, CO₂ coordination, and ultimately catalytic turnover at the first reduction potential of the complexes. All experimental observations are rigorously corroborated by DFT calculations.

Keywords: carbon dioxide; electrocatalysis; ligand design; ruthenium; steric hindrance.