Density functional theory calculations of catalytic mechanistic pathways for the formation of O2 involving triazolylidene iridium complexes

Abstract

We have studied Ir(III)-catalysed water oxidation pathways at the CAM-B3LYP level of Density Functional Theory (DFT), involving a novel oxygen-evolving catalyst (OEC), for a low-concentration mechanism. Starting from the dichloride complex Ir^III–Cl (Ir = [IrCp*Cl(triazolylidene)], Cp* = C₅Me₅⁻), DFT predicts reaction with one water molecule and formation of a cationic solvento complex [Ir^III–OH₂]⁺, followed by two proton-coupled electron transfer (PCET) steps; one involves an [Ir^III–OH₂]⁺/[Ir^IV–OH]⁺ transition, whilst the other is an [Ir^IV–OH]⁺ to [Ir^VO]⁺ transformation. The oxidation potentials and pK_a values were calculated for the transformation of [Ir^III–OH₂]⁺ to [Ir^VO]⁺ for both sequential orders of H⁺/e⁻ and e⁻/H⁺ transfer. Deprotonation of [Ir^III–OH₂]⁺ was found to be feasible even in acidic conditions (with a pK_a of 1.08), while the deprotonation of [Ir^IV–OH]⁺ is strongly disfavoured (pK_a = 10.33). The computed oxidation potentials are rather high (1.81 V and 2.01 V for [Ir^III–OH₂]⁺ and [Ir^VI–OH]⁺, respectively); however, subsequent proton dissociation is more favoured (with pK_a values for the oxidised intermediates of −16.1 and −8.1, respectively). This indicates the possibility of concerted, as opposed to stepwise, H⁺/e⁻ transfer (PCET). Further, the [Ir^VO]⁺ intermediate was found to react with two water molecules to release O₂. This sequence involves O–O bond formation followed by one PCET step, while a further single electron transfer (oxidation) step generates an [Ir^V–O–O]⁺ species, which liberates O₂ and regenerates the [Ir^III–OH₂]⁺ complex.