Electronic Structure of a Cu^II–Alkoxide Complex Modeling Intermediates in Copper-Catalyzed Alcohol Oxidations

Abstract

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 12000 cm−1 and an intense ethoxide-to-copper charge transfer (LMCT) transition at 24000 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.

Feasible and pure P2O5-CaO nanoglasses: An in-depth NMR study of synthesis for the modulation of the bioactive ion release

“…) was occurring [31], which is consistent with the dark orange color observed for the Ca precursor (Figure S3), characteristic of electronic orbital transitions between metal ions and alkoxyalkoxide ligands [32][33][34]. In order to better elucidate the Ca precursor structure, we complemented the NMR characterization with mass spectrometry.…”

Synthesis and Characterization of Tris(oxazolinyl)borato Copper(II) and Copper(I) Complexes

“…The spectrum is also best simulated in rhombic symmetry, with similar g values as To M CuBr, but smaller hyperfine splitting for g 1 =2.07 (A 1 =24 G) and larger splitting for g 3 =2.36 (A 3 =67 G). For comparison, the EPR spectrum of Tp t Bu CuOCH 2 CF 3 , in glassed toluene at 120 K resolved g zz =2.44 (A=120 MHz, 40×10 −4 cm −1 ), [71] while g xx (2.060) and g yy (2.093) were resolved in the spectrum acquired at 10 K in glassed toluene‐dichloromethane [72] . Although the solid‐state structures of tert ‐butoxide 2 and Tp t Bu CuOCH 2 CF 3 are similarly four‐coordinate trigonal monopyramidal (τ 4 equal to 0.68 and 0.76, resp.)…”

Asymmetric Cascade Catalysis with Chiral Polyoxometalate‐Based Frameworks: Sequential Direct Aldol and Epoxidation Reactions

“…The complex exhibited an anisotropic EPR spectrum with poorly resolved copper hyperfine features and a narrower transition at the effective g value of 2.098 (Figure c), which is consistent with a quadrilateral geometry for the copper(II) complex and in accordance with the crystal structure of CuW‐PYI 1 . The EPR parameter g >2.0023 was characteristic of d x 2 − y 2 orbital of unpaired electron ground state in axial symmetry of Cu II coordination sphere . To mimic the catalysis process, we also studied the EPR spectra of CuW‐PYIs in the catalysis process with ketones (100 mmol), aldehydes (10 mmol) and tert ‐butylhydroperoxide (TBHP) (20 mmol).…”

“…[20] The EPR parameter g > 2.0023 was characteristico fd x 2 Ày 2 orbitalo fu npaired electron ground state in axial symmetry of Cu II coordination sphere. [21] To mimic the catalysis process,w ea lso studied the EPR spectra of CuW-PYIs in the catalysis process with ketones (100 mmol), aldehydes (10 mmol) and tert-butylhydroperoxide (TBHP) (20 mmol). The EPR spectra of the recovered solids from the catalytic system seemedi dentical with that of the fresh catalyst.…”