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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 Cu(II)-alkoxide (Cu(II)-OR) intermediate is believed to modulate the ?C-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 Tp(tBu)Cu(II)(OCH2CF3) (Tp(tBu) = 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 Cu(II)-thiolate and Cu(II)-alkylperoxo complexes from the literature, we found an increased ? interaction in the Cu(II)-OR bond that results in the spectroscopic features. These insights lay the basis for further elucidating the mechanism of copper-catalyzed alcohol oxidations.

SUBMITTER: Hayes EC

PROVIDER: S-EPMC4988936 | biostudies-literature | 2016 Mar

REPOSITORIES: biostudies-literature

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Electronic Structure of a Cu(II)-Alkoxide Complex Modeling Intermediates in Copper-Catalyzed Alcohol Oxidations.

Hayes Ellen C EC Porter Thomas R TR Barrows Charles J CJ Kaminsky Werner W Mayer James M JM Stoll Stefan S

Journal of the American Chemical Society 20160318 12

In the copper-catalyzed oxidation of alcohols to aldehydes, a Cu(II)-alkoxide (Cu(II)-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 Tp(tBu)Cu(II)(OCH2CF3) (Tp(tBu) = hydro-tris(3-tert-butyl-pyrazolyl)borate) and investigated the influence of the trifluoroethoxide ligand on the electronic structure of the compl ...[more]

PMID: 26907976

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Project description:Two new monomeric Cu(II) alkoxide complexes were prepared and fully characterized as models for intermediates in copper/radical mediated alcohol oxidation catalysis: Tp(tBuR)Cu(II)OCH2CF3 with Tp(tBu) = hydro-tris(3-tert-butyl-pyrazol-1-yl)borate 1 or Tp(tBuMe) = hydro-tris(3-tert-butyl-5-methyl-pyrazol-1-yl)borate 2. These complexes were made as models for potential intermediates in enzymatic and synthetic catalytic cycles for alcohol oxidation. However, the alkoxide ligands are not readily oxidized by loss of H; instead, these complexes were found to be hydrogen atom acceptors. They oxidize the hydroxylamine TEMPOH, 2,4,6-tri-t-butylphenol, and 1,4-cyclohexadiene to the nitroxyl radical, phenoxyl radical, and benzene, with formation of HOCH2CF3 (TFE) and the Cu(I) complexes Tp(tBuR)Cu(I)-MeCN in dichloromethane/1% MeCN or 1/2 [Tp(tBuR)Cu(I)]2 in toluene. On the basis of thermodynamics and kinetics arguments, these reactions likely proceed through concerted proton-electron transfer mechanisms. Thermochemical analyses give lower limits for the "effective bond dissociation free energies (BDFE)" of the O-H bonds in 1/2[Tp(tBuR)Cu(I)]2 + TFE and upper limits for the free energies associated with alkoxide oxidations via hydrogen atom transfer (effective alkoxide ?-C-H BDFEs). These values are summations of the free energies of multiple chemical steps, which include the energetically favorable formation of 1/2[Tp(tBuR)Cu(I)]2. The effective alkoxide ?-C-H bonds are very weak, BDFE ? 38 ± 4 kcal mol(-1) for 1 and ?44 ± 5 kcal mol(-1) for 2 (gas-phase estimates), because C-H homolysis is thermodynamically coupled to one electron transfer to Cu(II) as well as the favorable formation of the 1/2[Tp(tBuR)Cu(I)]2 dimer. Treating 1 with the H atom acceptor (t)Bu3ArO(•) did not result in the expected alkoxide oxidation to an aldehyde, but rather net 2,2,2-trifluoroethoxyl radical transfer occurred to generate an unusual 2-substituted dienone-ether product. Treating 2 with (t)Bu3ArO(•) gives no reaction, despite evidence that overall ligand oxidation and formation of 1/2[Tp(tBuMe)Cu(I)]2 is significantly exoergic. The origin of this lack of reactivity may be due to insufficient weakening of the alcohol ?-C-H bond upon complexation to copper.

Project description:A recent report established that the tetrabutylammonium (TBA) salt of hexavanadopolymolybdate TBA4H5[PMo6V6O40] (PV6Mo6) serves as the redox buffer with Cu(II) as a co-catalyst for aerobic deodorization of thiols in acetonitrile. Here, we document the profound impact of vanadium atom number (x = 0-4 and 6) in TBA salts of PVxMo12-xO40(3+x)- (PVMo) on this multicomponent catalytic system. The PVMo cyclic voltammetric peaks from 0 to -2000 mV vs Fc/Fc+ under catalytic conditions (acetonitrile, ambient T) are assigned and clarify that the redox buffering capability of the PVMo/Cu catalytic system derives from the number of steps, the number of electrons transferred each step, and the potential ranges of each step. All PVMo are reduced by varying numbers of electrons, from 1 to 6, in different reaction conditions. Significantly, PVMo with x ≤ 3 not only has much lower activity than when x > 3 (for example, the turnover frequencies (TOF) of PV3Mo9 and PV4Mo8 are 8.9 and 48 s-1, respectively) but also, unlike the latter, cannot maintain steady reduction states when the Mo atoms in these polyoxometalate (POMs) are also reduced. Stopped-flow kinetics measurements reveal that Mo atoms in Keggin PVMo exhibit much slower electron transfer rates than V atoms. There are two kinetic arguments: (a) In acetonitrile, the first formal potential of PMo12 is more positive than that of PVMo11 (-236 and -405 mV vs Fc/Fc+); however, the initial reduction rates are 1.06 × 10-4 s-1 and 0.036 s-1 for PMo12 and PVMo11, respectively. (b) In aqueous sulfate buffer (pH = 2), a two-step kinetics is observed for PVMo11 and PV2Mo10, where the first and second steps are assigned to reduction of the V and Mo centers, respectively. Since fast and reversible electron transfers are key for the redox buffering behavior, the slower electron transfer kinetics of Mo preclude these centers functioning in redox buffering that maintains the solution potential. We conclude that PVMo with more vanadium atoms allows the POM to undergo more and faster redox changes, which enables the POM to function as a redox buffer dictating far higher catalytic activity.

Dataset Information

Electronic Structure of a Cu(II)-Alkoxide Complex Modeling Intermediates in Copper-Catalyzed Alcohol Oxidations.

Publications

Electronic Structure of a Cu(II)-Alkoxide Complex Modeling Intermediates in Copper-Catalyzed Alcohol Oxidations.

Similar Datasets

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