We present evidence for Cp* being a sacrificial placeholder ligand in the [Cp*IrIII(chelate)X] series of homogeneous oxidation catalysts. UV-vis and 1H-NMR profiles as well as MALDI-MS data show a rapid and irreversible loss of the Cp* ligand under reaction conditions, which likely proceeds through an intramolecular inner-sphere oxidation pathway reminiscent of the reductive in-situ elimination of diolefin placeholder ligands in hydrogenation catalysis by [(diene)MI(L,L’)]+ (M = Rh and Ir) precursors. When oxidatively stable chelate ligands are bound to the iridium in addition to the Cp*, the oxidized precursors yield homogenous solutions with a characteristic blue color that remain active in both water- and CH-oxidation catalysis without further induction period. Electrophoresis suggests the presence of well-defined Ir-cations, and EDX, XPS, 17O-NMR, and resonance-Raman spectroscopy data reveal the molecular identity of the blue species to be a bis-µ-oxo di-iridium(IV) coordination compound with two waters and one chelate ligand bound to each metal. DFT calculations give insight into the electronic structure of this catalyst resting state, and time-dependent simulations confirm assignments of the experimental spectroscopic data. [(cod)IrI(chelate)] precursors bearing the same chelate ligands are shown to be equally effective pre-catalysts for both water- and CH-oxidations using NaIO4 as chemical oxidant.
Hintermair, U., Sheehan, S. W., Parent, A. R., Ess, D. H., Richens, D. T., Vaccaro, P. H., ... Crabtree, R. H. (2013). Precursor transformation during molecular oxidation catalysis with organometallic iridium complexes. Journal of the American Chemical Society, 135(29), 10837-10851. https://doi.org/10.1021/ja4048762