Mechanistic insights into electrocatalytic CO2 reduction within [RuII(tpy)(NN)X]n+ architectures

Abstract

A series of Ru^II-polypyridyl complexes of the design [Ru^II(tpy)(NN)X]ⁿ⁺ (tpy = 2,2′:6′,2′′-terpyridine; NN = bidentate polypyridine; X = Cl⁻ or CH₃CN; n = 1 or 2) have been synthesized and analyzed for their ability to function as electrocatalysts in the reduction of CO₂ to CO. Varying the electron-donating/withdrawing character of the NN polypyridyl ligand has allowed for modification of electron density at the formally Ru^II metal center. Complexes where X = Cl⁻ display ligand substitution for CH₃CN with differing rates of Cl⁻ dissociation (k_–Cl), therefore providing a degree of insight into the electron density and thus the chemical activity at the Ru^II center. Detailed analysis of the cyclic voltammograms under argon vs. CO₂ atmospheres using multiple switching potentials and scan rates ranging from ν = 25–2000 mV s⁻¹ has painted a picture of how monodentate ligand lability due to NN polypyridyl electron-donating character is related to electrocatalytic CO₂ reduction activity of Ru^II-polypyridyl complexes. From these studies, multiple mechanistic pathways towards generating the catalytically active [Ru(tpy⁻)(NN⁻)CO₂]⁰ species are proposed and differ via the order of electrochemical and chemical processes.