Mild oxidative C−H functionalization of alkanes and alcohols using a magnetic core-shell Fe3O4@mSiO2@Cu4 nanocatalyst

Abstract

A new hybrid Fe₃O₄@mSiO₂@Cu₄ material was constructed by loading a bio-inspired tetracopper(II) coordination compound [Cu₄(μ₄-O){N(CH₂CH₂O)₃}₄(BOH)₄][BF₄]₂ (Cu₄) onto the Fe₃O₄@mSiO₂ core-shell nanoparticles (NPs) composed of a magnetite (Fe₃O₄) core and mesoporous silica (mSiO₂) shell with perpendicularly aligned channels. The obtained Fe₃O₄@mSiO₂@Cu₄ magnetic nanoparticles were characterized by transmission electron microscopy (TEM), FT-IR spectroscopy, thermogravimetric analysis (TGA), powder X-ray diffraction (PXRD), and field-dependent magnetization. This hybrid material acts as a magnetically recoverable C−H functionalization nanocatalyst, namely for the mild oxidation, by t-butyl hydroperoxide at 50–70 °C in acetonitrile medium, of cycloalkanes (cyclopentane, cyclohexane, cycloheptane, and cyclooctane) to the corresponding alcohols and ketones (with up to ∼15% yields based on cycloalkane; TON 335). A related oxidation process using cyclohexanol as a more reactive substrate leads to the formation of cyclohexanone in up to ∼25% yield (TON 570). The Fe₃O₄@mSiO₂@Cu₄ nanocatalyst can be recycled five times without an appreciable loss of activity. The bond-, regio-, and stereoselectivity parameters were investigated in the oxidation of different alkane substrates (n-hexane, n-heptane, n-octane, methylcyclohexane, adamantane, cis- and trans-1,2-demethylcyclohexane), and the obtained results were compared with the homogeneous systems based on the Cu₄ catalyst. In particular, the high bond selectivity parameters detected in the oxidation of methylcyclohexane (1°:2°:3° of 1:8:142) and adamantane (2°:3° of 1:21) catalyzed by Fe₃O₄@mSiO₂@Cu₄ suggest that the reactions possibly occur in hydrophobic pockets of the nanocatalyst.