Comprehensive Mechanisms of Electrocatalytic CO2 Reduction by [Ir(bip)(ppy)(CH3CN)](PF6)2

Examples of transition metal complexes capable of the dual roles of light harvesting and catalysis of CO₂ reduction are rare. This self-sensitized approach simplifies systems and efficiencies; therefore complete understanding of mechanistic principles is essential for improving catalysts. Here we present a comprehensive study of dark reactions using electrochemical techniques to understand the multiple pathways for the selective reduction of CO₂ to CO by an example self-sensitized photocatalyst: [Ir(bip)(ppy)(CH₃CN)]²⁺. (bip = 2,6-bis(benzimidazole)pyridine, ppy = 2-phenylpyridine). Cyclic voltammetry (CV) in acetonitrile under anhydrous conditions reveals electrocatalysis by a two electron cycle at -1.7 V vs Fc^+/0 (denoted the cat-1 region) in which the metallocarboxylate formed by binding of Ir(I) to CO₂ is cleaved by CO₂ as the oxide acceptor. At -1.9 V (denoted the cat-2 region) the Ir(CO₂) intermediate is reduced and catalysis is accelerated. In the presence of water, the Ir(CO₂) is protonated to Ir(CO₂H) which is reduced at a potential less negative than -1.7 V and then the oxide acceptor is either CO₂ to form HCO₃- or protons to release H₂O and the conjugate base of the acid source. Further reduction of Ir(CO₂H) at cat-2 again accelerates catalysis. Rates vary widely in these various regimes with the minimum kobs of 0.3 s^-1 for anhydrous cat-1 to a maximum cat-2 rate of 2100 s^-1 with 1% water. Competitive deactivation pathways were discovered as Ir-Ir dimerization without reacting with CO₂ or the formation of a hydride-bridged dinuclear complex during extended electrolysis at high water concentration. The Ir-Ir dimer was characterized by high-resolution mass spectrometry and X-ray absorption spectroscopy (XAS).