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

The one-electron reduction of [CpRu(bpy)NCCH]PF (Cp = cyclopentadienyl; bpy = 2,2'-bipyridine), abbreviated as [Ru-S], where S = CHCN, in CO-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] binds CO at a rate of ca. 10 M s at almost zero driving force. Subsequent to CO binding, all of the steps leading up to deoxygenation are highly exergonic and rapid. A model of the potential energy profile of the CO approach to the Ru center in the singly reduced manifold reveals a direct correlation between the reactivity toward CO and the nucleophilicity at the metal center influenced by different ligand environments. Through the binding of CO 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 CO under low driving forces.