Electrochemical and Kinetic Insights into Molecular Water Oxidation Catalysts Derived from Cp*Ir(pyridine‐alkoxide) Complexes

Abstract

We report the solution‐phase electrochemistry of seven half‐sandwich iridium(III) complexes with varying pyridine‐alkoxide ligands to quantify electronic ligand effects that translate to their activity in catalytic water oxidation. Our results unify some previously reported electrochemical data of Cp*Ir complexes by showing how the solution speciation determines the electrochemical response: cationic complexes show over 1 V higher redox potentials that their neutral forms in a distinct demonstration of charge accumulation effects relevant to water oxidation. Building on previous work that analysed the activation behaviour of our pyalk‐ligated Cp*Ir complexes 1–7, we assess their catalytic oxygen evolution activity with sodium periodate (NaIO4) and ceric ammonium nitrate (CAN) in water and aqueous tBuOH solution. Mechanistic studies including H/D kinetic isotope effects and reaction progress kinetic analysis (RPKA) of oxygen evolution point to a dimer‐monomer equilibrium of the IrIV resting state preceding a proton‐coupled electron transfer (PCET) in the turnover‐limiting step (TLS). Finally, true electrochemically driven water oxidation is demonstrated for all catalysts, revealing surprising trends in activity that do not correlate with those obtained using chemical oxidants.

Relevance of Chemical vs. Electrochemical Oxidation of Tunable Carbene Iridium Complexes for Catalytic Water Oxidation

“…This conclusion is supported by a previous study on manganese oxides for water oxidation . and complements related work on the Crabtree–Brudvig Ir(pyalk) system …”

Evidence for tetranuclear bis-μ-oxo cubane species in molecular iridium-based water oxidation catalysts from XAS analysis

“…Recently a half order in [Ir] on the rate of O 2 evolution has been found for a range of pyalkligated Ir WOCs using NaIO 4 , implying a dominant dimeric resting state liberating small amounts of active monomer into the catalytic cycle. 41 In light of the present findings this could mean a tetramer-dimer, or tetramer-dimer-monomer equilibria to be operational instead. Ascertaining how many metal sites are involved in the turnover-limiting step will be important for deciphering the workings of these highly active catalysts.…”

Molecular and heterogenized dinuclear Ir-Cp* water oxidation catalysts bearing EDTA or EDTMP as bridging and anchoring ligands

“…Nevertheless, the aspiration of establishing clear structure/properties correlations remains largely unaccomplished, as in situ structural modifications of the ligands often occur under the strongly oxidative reaction conditions necessary for WO. A particularly interesting case is that of Ir-Cp* complexes (Cp*= pentamethylcyclopentadienyl), which include some of the known most active molecular WOCs [1,[45][46][47][48][49][50][51][52][53][54][55][56][57][58][59]. The Cp* fragment undergoes major oxidative transformations under catalytic conditions; these modifications are likely necessary, in some cases, for precatalyst activation, but complicate the identification of the true active species and, consequently, the rationalization of ancillary ligand effects on activity [20,[60][61][62][63][64][65][66].…”