Substantial Improvement of Pyridine-Carbene Iridium Water Oxidation Catalysts by a Simple Methyl-to-Octyl Substitution

The substitution of a methyl to an octyl group in the ancillary triazolylidene ligand an apparently simple variation induces a more than 10-fold increase of activity of the corresponding iridium complex in water oxidation catalysis when using cerium(IV) as sacrificial oxidant. Detailed NMR studies suggest that various different molecular species form, all bearing the intact triazolylidene ligand. The octyl substituent is essential for inducing the association of the iridium species, thus generating extraordinarily active multimetallic catalytic sites. Their accessibility and steady-state concentration is critically dependent on the type of sacrificial oxidant and specifically on the cerium ammonium nitrate versus catayst ratio.