Initiation of the Electrochemical Reduction of CO₂ by a Singly Reduced Ruthenium(II) Bipyridine Complex

The one-electron reduction of [CpRu(bpy)NCCH₃]PF₆ (Cp = cyclopentadienyl; bpy = 2,2'-bipyridine), abbreviated as [Ru-S]⁺, where S = CH₃CN, in CO₂-saturated acetonitrile initiates a cascade of rapid electrochemical and chemical steps (ECEC) at an electrode potential of ca. 100 mV positive of the first reduction of the ruthenium complex. The overall two-electron process leads to the generation of a CO-bound ruthenium complex, [Ru-CO]⁺, and carbonate, as independently confirmed by NMR spectroscopy. Simulations of the cyclic voltammograms using DigiElch together with density functional theory based calculations reveal that the singly reduced ruthenium complex [Ru-S]⁰ binds CO₂ at a rate of ca. 10⁵ M^-1 s^-1 at almost zero driving force. Subsequent to CO₂ binding, all of the steps leading up to deoxygenation are highly exergonic and rapid. A model of the potential energy profile of the CO₂ approach to the Ru center in the singly reduced manifold reveals a direct correlation between the reactivity toward CO₂ and the nucleophilicity at the metal center influenced by different ligand environments. Through the binding of CO₂ after the first reduction, overpotentials associated with consecutive electrochemical reductions are avoided. This work therefore provides an important design principle for engineering transition-metal complexes to activate CO₂ under low driving forces.