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

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, [CuII(iPr3-tren)(OOC(CH3)2Ph)]+ (1, iPr3-tren = tris[2-(isopropylamino)ethyl]amine), toward hydrocarbon substrates. 1 was prepared by adding cumene hydroperoxide and triethylamine to the solution of [Cu(iPr3-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 1 at 60 °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 1 depending on solvents. In CH3CN and CH3COCH3, 1 activates weak C–H bonds (bond dissociation energy (BDE) ≤ ∼81.6 kcal mol–1), while 1 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), 1 can oxidize alkanes with strong C–H bonds, such as cyclohexane (99.5 kcal mol–1). The enhanced C–H bond oxidation of 1 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(sp3)–H bonds in the solvent. Interestingly, in CH2Cl2, a nonpolar solvent with C(sp3)–H bonds, 1 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(sp3)–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(sp3)–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.