CuO+ and CuOH2+ Complexes: Intermediates in Oxidation Catalysis?

Characterization of monocopper intermediates in enzymes and other catalysts that attack strong C-H bonds is important for unraveling oxidation catalysis mechanisms and, ultimately, designing new, more efficient catalytic systems. Because initially formed 1:1 Cu/O2 adducts resulting from reactions of Cu(I) sites with O2 react relatively sluggishly with substrates with strong C-H bonds, it has been suggested that reductive O-O bond scission might occur instead to yield more reactive CuO(+) or protonated CuOH(2+) cores. Experimental and theoretical studies of CuO(+) species in the gas phase have provided key insights into the possible reactivity of these species, but detailed information is lacking for discrete complexes with the CuO(+) or CuOH(2+) core in solution or the solid state. We describe herein our recent efforts to address this issue through several disparate approaches. In one strategy based on precedent from studies of enzymes and synthetic compounds with iron-α-ketocarboxylate motifs, reactions of O2 with Cu(I)-α-ketocarboxylate complexes were explored, with the aim of identifying reaction pathways that would implicate the intermediacy of a CuO(+) species. A second approach focused on the reaction of N-oxides with Cu(I) complexes, with the goal being to elicit O-N bond heterolysis to yield CuO(+) complexes. For both strategies, the course of the reactions depended on the nature of the supporting bidentate N-donor ligand, and indirect evidence in support of the sought-after CuO(+) intermediates was obtained in some instances. In the final approach discussed herein, strongly electron donating and sterically encumbered pyridine-dicarboxamide ligands (L) enabled the synthesis of LCu(II)OH(-) complexes, which upon one-electron oxidation formed complexes with the CuOH(2+) core that were characterized in solution. Rapid hydrogen atom abstraction (HAT) from dihydroanthracene (DHA) was observed, yielding LCu(II)OH2. The O-H bond dissociation enthalpy (BDE) of ∼90 kcal/mol for this complex was determined through evaluation of its pKa (∼19) and the LCu(II)OH(-)/LCu(III)OH reduction potential (approximately -0.08 V vs Fc/Fc(+)). Thus, the poor oxidizing power of the complex is offset by the high basicity of the hydroxide moiety to yield a strong O-H bond. This high BDE provided a thermodynamic rationale for the rapid HAT rate from DHA and suggested that stronger C-H bonds could be attacked. Indeed, using an inert solvent (1,2-difluorobenzene), substrates with C-H bond strengths as high as 99 kcal/mol were shown to react with the CuOH(2+) complex, and a linear log k vs C-H BDE plot supported similar HAT pathways across the series. Importantly, these results provided key evidence in favor of the possible intermediacy of this core in oxidation catalysis, and we suggest that because it is a more energetically accessible intermediate than the CuO(+) moiety, it should be considered as an alternative in proposed mechanisms for oxidations by enzymes and other synthetic systems.