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

The copper-catalyzed oxidation of alcohols to aldehydes is an important reaction in enzymes and in synthetic chemistry. In these reactions, a CuII-alkoxide (CuII-OR) intermediate is believed to modulate the αC-H bond strength of the deprotonated substrate to facilitate the oxidation. We characterized the electronic structure of the stable model 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 exceptionally large gzz value of 2.44 and a uniquely small copper hyperfine coupling zz of 40 x 10−4 cm−1 (120 MHz). Single-crystal electron nuclear double resonance (ENDOR) shows that the unpaired spin population is highly localized on the copper ion, with no more than 15 % on the ethoxide oxygen. Electronic absorption and magnetic circular dichroism (MCD) spectra show weak ligand-field transitions between 5000 cm−1 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 well-studied 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.