Hydrogen atom abstraction from hydrocarbons by a copper(III)-hydroxide complex.

Abstract

With the aim of understanding the basis for the high rate of hydrogen atom abstraction (HAT) from dihydroanthracene (DHA) by the complex LCuOH (1; L = N,N′-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamide), the bond dissociation enthalpy of the reaction product LCu(H2O) (2) was determined through measurement of its pKa and E1/2 in THF solution. In so doing, an equilibrium between 2 and LCu(THF) was characterized by UV–vis and EPR spectroscopy and cyclic voltammetry (CV). A high pKa of 18.8 ± 1.8 and a low E1/2 of −0.074 V vs Fc/Fc+ in THF combined to yield an O–H BDE for 2 of 90 ± 3 kcal mol–1 that is large relative to values for most transition metal oxo/hydroxo complexes. By taking advantage of the increased stability of 1 observed in 1,2-difluorobenzene (DFB) solvent, the kinetics of the reactions of 1 with a range of substrates with varying BDE values for their C–H bonds were measured. The oxidizing power of 1 was revealed through the accelerated decay of 1 in the presence of the substrates, including THF (BDE = 92 kcal mol–1) and cyclohexane (BDE = 99 kcal mol–1). CV experiments in THF solvent showed that 1 reacted with THF via rate-determining attack at the THF C–H(D) bonds with a kinetic isotope effect of 10.2. Analysis of the kinetic and thermodynamic data provides new insights into the basis for the high reactivity of 1 and the possible involvement of species like 1 in oxidation catalysis.