Molecular water oxidation catalysis by zwitterionic carboxylate bridge-functionalized bis-NHC iridium complexes

Abstract

Zwitterionic water-soluble [Cp*Ir^IIICl{(MeIm)₂CHCOO}] and [Ir^I(cod){(MeIm)₂CHCOO}] complexes featuring a carboxylate bridge-functionalized bis-N-heterocyclic carbene ligand efficiently catalyzed water oxidation using ammonium cerium(IV) nitrate (CAN) or sodium periodate as sacrificial oxidants. Excellent yields with TOF₅₀ numbers up to 1000 h⁻¹ have been achieved using CAN as electron acceptor at [CAN]/[Ir] ratios higher than 700. The investigation of the reaction mechanism by UV-vis seems to evidence that both catalyst precursors are transformed into the same active molecular species resulting from the degradation of the hydrocarbon ligands which is partially supported by the similarity of the oxygen evolution profiles at moderate oxidant/catalyst ratios for both catalyst precursors and the same chemical oxidant. In addition, DLS studies provide evidence for the participation of homogeneous iridium molecular species as intermediates likely stabilized by the carboxylate-functionalized bis-NHC ligand.