Precursor Transformation during Molecular Oxidation Catalysis with Organometallic Iridium Complexes

Abstract

We present evidence for Cp* being a sacrificial placeholder ligand in the [Cp*Ir(III)(chelate)X] series of homogeneous oxidation catalysts. UV-vis and (1)H 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)M(I)(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 homogeneous 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 TEM-EDX, XPS, (17)O NMR, and resonance-Raman spectroscopy data are most consistent with 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 agree with the assignments of the experimental spectroscopic data. [(cod)Ir(I)(chelate)] precursors bearing the same chelate ligands are shown to be equally effective precatalysts for both water- and CH-oxidations using NaIO4 as chemical oxidant.

Molecular Catalysts for Water Oxidation

“…270 In accord with Macchioni's prior observations under related conditions, NMR and mass spectrometry showed a rapid and irreversible loss of Cp* in chemically driven water oxidation. When oxidatively stable chelate ligands are bound to iridium, solutions with a characteristic blue color remain active for water oxidation (or C−H oxidation) without an induction period.…”

A full set of iridium(iv) pyridine-alkoxide stereoisomers: highly geometry-dependent redox properties

“…First, the exceptional substitutional inertness that is observed in our Ir( iv ) species supports the hypothesis that our WOCs can be composed of many, non-interconverting species regardless of predicted thermodynamic energy differences. 10 Furthermore, we observe that this isomerism has implications beyond simply complicating characterization – the highly anisotropic nature of the pyalk ligand can be expected to affect the redox properties of each WOC isomer to a substantial degree, much as we see with 1–8 . For a complex oxidative process such as water oxidation, the catalyst's redox transitions are of critical importance; we would, therefore, expect that isomerism could play a significant role in the catalytic activities of individual species, which would thus be expected to differ considerably in redox and catalytic properties.…”

“…8–13 Computational and spectroscopic studies indicated that the resulting WOC resting states are much altered from the precursor structure, being oxo-bridged dinuclear compounds (Scheme 1) that retain the chelate ligand but lose Cp* to oxidative decomposition. 5,10,14–16 Despite intensive study, the precise structural identity of the catalytic resting state(s) has not yet been confirmed by small molecule crystallography because suitable crystals cannot be obtained. We suspect that this may be due to the structural variability possible in such dinuclear species; with the combination of variable ligands on the nonchelate sites and diastereomerism means that hundreds of species could form.…”

Probing the Viability of Oxo-Coupling Pathways in Iridium-Catalyzed Oxygen Evolution

“…This result shows the difficulty of obtaining a stable peroxo Ir III –Ir III species, although the active catalytic species is likely to be different electronically and structurally. 60 …”