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Electronic Structure of a CuII–Alkoxide Complex Modeling Intermediates in Copper-Catalyzed Alcohol Oxidations

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Department of Chemistry, Box 351700, University of Washington, Seattle, Washington 98195, United States
Cite this: J. Am. Chem. Soc. 2016, 138, 12, 4132–4145
Publication Date (Web):February 23, 2016
Copyright © 2016 American Chemical Society

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    In the copper-catalyzed oxidation of alcohols to aldehydes, a CuII-alkoxide (CuII–OR) intermediate is believed to modulate the αC–H bond strength of the deprotonated substrate to facilitate the oxidation. As a structural model for these intermediates, we characterized the electronic structure of the stable compound TptBuCuII(OCH2CF3) (TptBu = hydro-tris(3-tert-butyl-pyrazolyl)borate) and investigated the influence of the trifluoroethoxide ligand on the electronic structure of the complex. The compound exhibits an electron paramagnetic resonance (EPR) spectrum with an unusually large gzz value of 2.44 and a small copper hyperfine coupling Azz of 40 × 10–4 cm–1 (120 MHz). Single-crystal electron nuclear double resonance (ENDOR) spectra show that the unpaired spin population is highly localized on the copper ion (≈68%), with no more than 15% on the ethoxide oxygen. Electronic absorption and magnetic circular dichroism (MCD) spectra show weak ligand-field transitions between 5000 and 12 000 cm–1 and an intense ethoxide-to-copper charge transfer (LMCT) transition at 24 000 cm–1, resulting in the red color of this complex. Resonance Raman (rR) spectroscopy reveals a Cu–O stretch mode at 592 cm–1. Quantum chemical calculations support the interpretation and assignment of the experimental data. Compared to known CuII-thiolate and CuII-alkylperoxo complexes from the literature, we found an increased σ interaction in the CuII–OR bond that results in the spectroscopic features. These insights lay the basis for further elucidating the mechanism of copper-catalyzed alcohol oxidations.

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    Supporting Information

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    The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.5b13088.

    • Crystal data for TptBuZnII(OCH2CF3). (CIF)

    • Crystal data for TptBuZnII(OTf). (CIF)

    • Simulation parameters for EPR and ENDOR spectra, supplemental EPR and ENDOR spectra, supplemental electronic absorption and MCD spectra, structural and spectroscopic properties of CuII complexes in the literature, model molecular orbital diagram, additional details about DFT calculations, synthesis, crystallographic information. (PDF)

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    Cited By

    This article is cited by 11 publications.

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