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Electrochemical and Kinetic Insights into Molecular Water Oxidation Catalysts Derived from Cp*Ir(pyridine‐alkoxide) Complexes
Author(s) -
Sackville Emma V.,
Marken Frank,
Hintermair Ulrich
Publication year - 2018
Publication title -
chemcatchem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.497
H-Index - 106
eISSN - 1867-3899
pISSN - 1867-3880
DOI - 10.1002/cctc.201800916
Subject(s) - chemistry , redox , bulk electrolysis , catalysis , inorganic chemistry , electrochemistry , oxygen evolution , aqueous solution , kinetic isotope effect , photochemistry , cyclic voltammetry , organic chemistry , physics , electrode , deuterium , quantum mechanics
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 (NaIO 4 ) and ceric ammonium nitrate (CAN) in water and aqueous t BuOH 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 Ir IV 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.