Precursor Transformation during Molecular Oxidation Catalysis with Organometallic Iridium Complexes

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• 作者：Ulrich Hintermair ; Stafford W. Sheehan ; Alexander R. Parent ; Daniel H. Ess ; David T. Richens ; Patrick H. Vaccaro ; Gary W. Brudvig ; Robert H. Crabtree
• 刊名：The Journal of the American Chemical Society
• 出版年：2013
• 出版时间：July 24, 2013
• 年：2013
• 卷：135
• 期：29
• 页码：10837-10851
• 全文大小：867K
• 年卷期：v.135,no.29(July 24, 2013)
• ISSN：1520-5126

文摘

We present evidence for Cp* being a sacrificial placeholder ligand in the [Cp*Ir^III(chelate)X] series of homogeneous oxidation catalysts. UV鈥搗is 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)M^I(L,L鈥?]⁺ (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-渭-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.