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, 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 k_obs 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 a high water concentration. The Ir–Ir dimer was characterized by high resolution mass spectrometry and X-ray absorption spectroscopy (XAS).