Precursor transformation during molecular oxidation catalysis with organometallic iridium complexes

Ulrich Hintermair, Stafford W. Sheehan, Alexander R. Parent, Daniel H. Ess, David T. Richens, Patrick H. Vaccaro, Gary W. Brudvig, Robert H. Crabtree

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Abstract

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 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, 17O 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.

Original languageEnglish
Pages (from-to)10837-10851
Number of pages15
JournalJournal of the American Chemical Society
Volume135
Issue number29
DOIs
Publication statusPublished - Jul 24 2013

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ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

Cite this

Hintermair, U., Sheehan, S. W., Parent, A. R., Ess, D. H., Richens, D. T., Vaccaro, P. H., Brudvig, G. W., & 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