Hydricity, electrochemistry, and excited-state chemistry of Ir complexes for CO₂ reduction

We prepared electron-rich derivatives of [Ir(tpy)(ppy)Cl]⁺ with modification of the bidentate (ppy) or tridentate (tpy) ligands in an attempt to increase the reactivity for CO₂ reduction and the ability to transfer hydrides (hydricity). Density functional theory (DFT) calculations reveal that complexes with dimethyl-substituted ppy have similar hydricities to the non-substituted parent complex, and photocatalytic CO₂ reduction studies show selective CO formation. Substitution of tpy by bis(benzimidazole)-phenyl or -pyridine (L3 and L4, respectively) induces changes in the physical properties that are much more pronounced than from the addition of methyl groups to ppy. Theoretical data predict [Ir(L3)(ppy)(H)] as the strongest hydride donor among complexes studied in this work, but [Ir(L3)(ppy)(NCCH₃)]⁺ cannot be reduced photochemically because the excited state reduction potential is only 0.52 V due to the negative ground state potential of -1.91 V. The excited state of [Ir(L4)(ppy)(NCCH₃)]²⁺ is the strongest oxidant among complexes studied in this work and the singly-reduced species is formed readily upon photolysis in the presence of tertiary amines. Both [Ir(L3)(ppy)(NCCH₃)]⁺ and [Ir(L4)(ppy)(NCCH₃)]²⁺ exhibit electrocatalytic current for CO₂ reduction. While a significantly greater overpotential is needed for the L3 complex, a small amount of formate (5-10%) generation in addition to CO was observed as predicted by the DFT calculations.