Influence of Solvents on Catalytic C-H Bond Oxidation by a Copper(II)-Alkylperoxo Complex

Abstract

Oxidation of unactivated alkanes, which requires substantial energy for conversion to valuable organic chemicals, is a major challenge in both industry and academia. Herein, we describe how solvents affect and improve the catalytic oxidation ability of a mononuclear copper(II)-alkylperoxo complex, [Cu-II(Pr-i(3)-tren)(OOC(CH3)(2)Ph)](+) (<bold>1</bold>, Pr-i(3)-tren = tris[2-(isopropylamino)ethyl]amine), toward hydrocarbon substrates. <bold>1</bold> was prepared by adding cumene hydroperoxide and triethylamine to the solution of [Cu(Pr-i(3)-tren)(CH3CN)](2+), which was characterized using various physicochemical methods. Product analyses, along with theoretical calculations, indicate that homolytic O-O bond cleavage occurs during the thermal decomposition of <bold>1</bold> at 60 degrees C in various solvents such as CH3CN, CH3COCH3, C6H5CF3, and C6H6. Both experimental results and density functional theory (DFT) calculations supported variations in the catalytic activity of <bold>1</bold> depending on solvents. In CH3CN and CH3COCH3, <bold>1</bold> activates weak C-H bonds (bond dissociation energy (BDE) <= similar to 81.6 kcal mol(-1)), while <bold>1</bold> in C6H5CF3 and C6H6 can oxidize slightly stronger C-H bonds with a BDE of up to 84.5 kcal mol(-1). In supercritical carbon dioxide (SC-CO2), <bold>1</bold> can oxidize alkanes with strong C-H bonds, such as cyclohexane (99.5 kcal mol(-1)). The enhanced C-H bond oxidation of <bold>1</bold> in C6H5CF3, C6H6, and SC-CO2 was generally attributed to two different factors: (a) the nonpolarity of the solvent and (b) the absence of C(sp(3))-H bonds in the solvent. Interestingly, in CH2Cl2, a nonpolar solvent with C(sp(3))-H bonds, <bold>1</bold> exhibited similar reactivity to that in C6H5CF3, indicating that nonpolar solvents enhance the catalytic ability of copper(II)-cumylperoxo complex to abstract hydrogen atoms from substrates, regardless of the presence of C(sp(3))-H bonds in solvent molecules. DFT calculations employing an implicit solvent model further supported the enhanced reactivity, without the need to account for the presence of a C(sp(3))-H bond. The reactivity of the different possible reactive intermediates arising from the catalytic oxidation was also explored using DFT calculations. This study provides a perspective on how solvents can be utilized to modulate the catalytic effects on C-H bond activation.