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.


Synthesis General
Organic solvents were purified by passing over activated alumina with dry argon. All chemicals were purchased from major commercial suppliers and used as received. Syntheses were performed under an inert atmosphere of dry argon using standard Schlenk techniques. NMR spectra were recorded on either 400 or 500 MHz Bruker Avance spectrometers and referenced to residual protio-solvent signals. The chemical shift δ is reported in units of parts per million (ppm).

Electrochemical water oxidation set up
Initial testing with a three electrode set up inserted into the top of the Clark electrode chamber resulted in an overload of the Clark electrode. Within 10 seconds of immersion and before any potential had been applied, the Clark electrode reading showed a maximum value of oxygen evolution. Test experiments showed this response to originate from cross currents between the working electrode and the Clark electrode connected through the mains. Powering the potentiostat from the battery of a laptop effectively eliminated these cross currents.
A second potential problem was the possibility of hydrogen produced at the counter electrode to be registered by the Clark electrode, giving an inflated reading for the amount of oxygen evolved. As such the counter electrode (Pt wire) was encased in a glass sleeve such that any hydrogen produced would be kept separate from the rest of the solution and therefore shouldn't interfere with the oxygen measurement.

Electrode optimisation
Several electrodes were investigated for use with the Clark electrode, with geometric fit into the chamber being a restriction. In all cases a Ag/AgCl refrence was used.
For Pt and Au mesh working electrodes, significant background water oxidation was observed at the desried potentials without any Ir catalysts added (figure S11 and S12).
Eventually a suitable electrode was found in a 0.5 × 0.5 cm BDD plate with electrical wire attached via epoxy resin sealed with silicone (figure S13).