Synthesis, Radical Reactivity, and Thermochemistry of Monomeric Cu(II) Alkoxide Complexes Relevant to Cu/Radical Alcohol Oxidation Catalysis

Two new monomeric Cu­(II) alkoxide complexes were prepared and fully characterized as models for intermediates in copper/radical mediated alcohol oxidation catalysis: Tp^<i>t</i>BuRCu^IIOCH₂CF₃ with Tp^<i>t</i>Bu = hydro-tris­(3-<i>tert</i>-butyl-pyrazol-1-yl)­borate <b>1</b> or Tp^<i>t</i>BuMe = hydro-tris­(3-<i>tert</i>-butyl-5-methyl-pyrazol-1-yl)­borate <b>2</b>. 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 <i>acceptors</i>. They oxidize the hydroxylamine TEMPOH, 2,4,6-tri-<i>t</i>-butylphenol, and 1,4-cyclohexadiene to the nitroxyl radical, phenoxyl radical, and benzene, with formation of HOCH₂CF₃ (TFE) and the Cu­(I) complexes Tp^<i>t</i>BuRCu^I-MeCN in dichloromethane/1% MeCN or 1/2 [Tp^<i>t</i>BuRCu^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^<i>t</i>BuRCu^I]₂ + TFE and upper limits for the free energies associated with alkoxide oxidations via hydrogen atom transfer (<i>effective</i> alkoxide α-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^<i>t</i>BuRCu^I]₂. The <i>effective</i> alkoxide α-C–H bonds are very weak, BDFE ≤ 38 ± 4 kcal mol^–1 for <b>1</b> and ≤44 ± 5 kcal mol^–1 for <b>2</b> (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^<i>t</i>BuRCu^I]₂ dimer. Treating <b>1</b> with the H atom acceptor ^<i>t</i>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 <b>2</b> with ^<i>t</i>Bu₃ArO^• gives no reaction, despite evidence that overall ligand oxidation and formation of 1/2­[Tp^<i>t</i>BuMeCu^I]₂ is significantly exoergic. The origin of this lack of reactivity may be due to insufficient weakening of the alcohol α-C–H bond upon complexation to copper