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

This article is dedicated to Prof. Georgiy B. Shul’pin on the occasion of his 70th birthday.
https://doi.org/10.1016/j.molcata.2016.06.028Get rights and content

Highlights

  • Magnetic core-shell nanoparticle catalyst was assembled and fully characterized.

  • This hybrid Fe3O4@mSiO2@Cu4 material bears catalytically active tetracopper(II) unit.

  • Magnetically recoverable nanocatalyst is active for mild oxidation of cycloalkanes.

  • Effects of various reaction parameters were investigated.

  • Different selectivity parameters were studied.

Abstract

A new hybrid Fe3O4@mSiO2@Cu4 material was constructed by loading a bio-inspired tetracopper(II) coordination compound [Cu44-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.

Introduction

Within the sweeping research progress in the field of nanocatalysis [1], [2], [3], the design of novel hybrid materials incorporating magnetically recoverable nanoparticles (NPs) decorated with catalytically active units has received an increased attention [4], [5], [6], [7]. In fact, magnetic separation of nanocatalysts and their easy isolation from the reaction medium and reutilization in subsequent batches constitute very powerful features, which can lead to the development of new and more efficient catalytic protocols [1], [2], [3], [4], [5], [6], [7]. The use of magnetically recoverable NPs is particularly attractive when dealing with expensive catalysts or in the catalytic processes that involve inert substrates and show low conversions, thus justifying frequent catalyst and substrate recycling procedures [5], [6], [7]. A notable example of such a class of inert substrates concerns alkanes, which are typically involved into low-conversion transformations, including various C−H functionalization processes [8], [9], [10].

Despite alkanes are very highly abundant in nature and can be considered as inexpensive substrates, an intrinsic inertness of these hydrocarbons limits their application as raw materials in different oxidative C−H functionalization processes that undergo efficiently and under mild conditions [8], [9], [10], [11], [12], [13]. However, there are a good number of homogeneous catalytic systems and protocols for the mild alkane oxidation based on different bio-inspired or bio-mimetic metal complexes, which are highly active when combined with a certain oxidant and under tuned reaction conditions [14], [15], [16]. In this regard, the heterogenization of such metal complexes on various supports or carriers and, in particular, on magnetic nanoparticles (e.g., based on magnetite, Fe3O4 [5a,d]) represents a perspective research direction [5], [6], [7]. It should be noted that the application of magnetically recoverable nanocatalysts for the mild oxidative C−H functionalization of alkanes still remains very little explored [17], thus motivating further research in this field.

Our recent studies were focused on the design of homogeneous multicopper(II) catalytic systems for the mild oxidation of alkanes [18] and on the fabrication of functional materials based on hybrid magnetic nanoparticles [19]. Aiming at merging these two research directions, the main objectives of the present work consisted of (i) assembling a novel magnetic core-shell nanoparticle catalyst bearing a loaded catalytically active unit, and (ii) evaluating the generated material as a magnetically recoverable catalyst for the mild oxidation of alkanes and alcohols. As a catalytically active unit for loading, we have selected a bio-inspired tetracopper(II) triethanolaminate [Cu44-O){N(CH2CH2O)3}4(BOH)4][BF4]2 (Cu4) complex (Scheme 1) on account of its high stability as well as remarkable and versatile catalytic behavior in the homogeneous oxidation of alkanes and other substrates [16], [20].

Hence, we report herein the fabrication, characterization, and catalytic application in an oxidative C−H functionalization of a novel hybrid Fe3O4@mSiO2@Cu4 nanocatalyst, generated via the loading of an active Cu4 unit onto the core-shell Fe3O4@mSiO2 nanoparticles that comprise a magnetite (Fe3O4) core and a mesoporous silica (mSiO2) shell with perpendicularly aligned channels. The obtained material acts as a magnetically recoverable nanocatalyst for the mild oxidation, by t-BuOOH at 50–70 °C, of various C5–C8 cycloalkanes to the corresponding alcohols and ketones and, in turn, for the oxidation of a more reactive model substrate, cyclohexanol, to give cyclohexanone.

Section snippets

Materials and methods

All chemicals were obtained from commercial sources and used as received. Compound [Cu44-O){N(CH2CH2O)3}4(BOH)4][BF4]2 (Cu4) was synthesized according to a reported procedure [20]. Fourier transform infrared (FT-IR) spectra were measured within the 4000–400 cm1 range using a Nicolet 360 FT-IR spectrometer with the KBr pellet technique. Thermogravimetric (TGA) analysis was performed on a Perkin-Elmer thermal analyzer, by heating the sample up to 800 °C at a rate of 10 °C/min under nitrogen

Preparation and characterization of Fe3O4@mSiO2@Cu4 nanocatalyst

The hybrid core-shell Fe3O4@mSiO2@Cu4 material was fabricated by loading, via a direct sorption process in an EtOH/H2O medium (v:v = 9:1), a tetracopper(II) compound [Cu44-O){N(CH2CH2O)3}4(BOH)4][BF4]2 (Cu4) onto the Fe3O4@mSiO2 matrix [19]; it is composed of a magnetite (Fe3O4) core and a mesoporous silica (mSiO2) shell with perpendicularly aligned channels (Scheme 2). After annealing and purification procedures, the resulting Fe3O4@mSiO2@Cu4 material (with a Cu4 content of ∼10 wt%) was

Conclusions

In the present work, we designed, fabricated, and fully characterized a novel hybrid material, Fe3O4@mSiO2@Cu4, as well as applied it as a magnetically recoverable and recyclable nanocatalyst for oxidative C−H functionalization, namely for the mild oxidation of cycloalkanes and an alcohol (cyclohexanol). In this nanocatalyst, the catalytic function is played by a bio-inspired tetracopper(II) triethanolaminate complex, which was loaded onto the Fe3O4@mSiO2 core-shell nanoparticles composed of a

Acknowledgements

This work was supported by the Foundation for Science and Technology (FCT) (IF/01395/2013/CP1163/CT005, UID/QUI/00100/2013, SFRH/BSAB/114190/2016), Portugal, and National Natural Science Foundation of China (21471071, 21431002).

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