Unlocking the Impact of Solvents on Catalytic C–H Bond Oxidation by 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, [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.

A groundbreaking discovery in the field of catalysis has emerged from the laboratories of Professor Jaeheung Cho and his team in the Department of Chemistry at UNIST. Their pioneering work has led to the development of a copper(II)–alkylperoxo complex that promises to revolutionize the realms of synthetic chemistry and industrial applications.

The key to this innovative catalyst lies in its remarkable ability to decompose strong carbon-hydrogen bonds (C−H bonds) through harnessing specific solvents. By precisely manipulating the solvent environment, the researchers have uncovered the exceptional reactivity of their copper(II)–alkylperoxo complex.

Through a meticulously designed series of experiments, the research team successfully synthesized the copper(II)–alkylperoxo complex and subjected it to supercritical carbon dioxide (SC-CO2), a fluid state of carbon dioxide that exhibit both gas and liquid properties simultaneously. This novel approach resulted in the most reactive metal–alkylperoxo peroxide compound to date.

Professor Cho highlighted, “Our comprehensive analysis of oxidation reactions and advanced theoretical calculations have introduced a new era in oxidation catalysis utilizing copper(II)–alkylperoxo as a catalyst.”

Of particular significance is the oxidation of unactivated alkanes, like methane and ethane, traditionally known for their stability and energy-intensive oxidation processes. By tailoring the copper(II)–alkylperoxo complex composition, the researchers achieved selective oxidation of unactivated alkanes, a pivotal advancement in catalytic science. Moreover, the team’s exploration of various solvents confirmed the unprecedented ability of their catalyst to break down resilient C−H bonds.

Figure 1. Schematic image, showing the comparison of reactivity of copper(II)-alkylperoxo species, depending on the solvent used.

Yuri Lee, the first author of the study, emphasized, “Our research signifies a milestone in reactivity manipulation through solvent engineering within copper(II)–alkylperoxo species.”

Professor Cho further underlined, “Our work not only showcases the exceptional oxidation capabilities of copper(II)–alkylperoxo species, but also elucidates their solvent-dependent reactivity, laying the foundation for cutting-edge metal catalysts in various scientific domains.”

The research team anticipates that this transformative research not only propels the boundaries of synthetic chemistry, but also holds immense promise for environmental and industrial applications, heralding a new era of catalytic excellence and sustainable technology. This research, with Professor Jaeheung Cho as the corresponding author, was published in the online version of ACS Catalysis on February 20, 2024. The study received funding from the National Research Foundation of Korea (NRF) and the Ministry of Science and ICT (MSIT), highlighting its importance in advancing eco-friendly technologies and catalytic innovation.

Journal Reference
Yuri Lee, Bohee Kim, Seonghan Kim, et al., “Influence of Solvents on Catalytic C–H Bond Oxidation by a Copper(II)–Alkylperoxo Complex,” ACS Catal., (2024)