Abstract
The reaction of the Schiff base (3,5-di-tert-butyl-2-hydroxybenzylidene)-2-hydroxybenzohydrazide (H3L) with a copper(II) salt of a base of a strong acid, i.e., nitrate, chloride or sulphate, yielded the mononuclear complexes [Cu(H2L)(NO3)(H2O)] (1), [Cu(H2L)Cl]·2MeOH (2) and the binuclear complex [{Cu(H2L)}2(µ-SO4)]·2MeOH (3), respectively, with H2L− in the keto form. Compounds 1–3 were characterized by elemental analysis, Infrared (IR) spectroscopy, Electrospray Ionisation Mass Spectrometry (ESI-MS) and single crystal X-ray crystallography. All compounds act as efficient catalysts towards the peroxidative oxidation of cyclohexane to cyclohexyl hydroperoxide, cyclohexanol and cyclohexanone, under mild conditions. In the presence of an acid promoter, overall yields (based on the alkane) up to 25% and a turnover number (TON) of 250 (TOF of 42 h−1) after 6 h, were achieved.
Highlights
Alkanes are highly abundant, cheap and very attractive potential substrates for the direct synthesis of added value functionalized products
We have recently synthesized [17] trinuclear aroylhydrazone Cu(II) complexes derived from the study, we have further extended the work, by synthesizing two mononuclear and one binuclear
We have synthesized to the Cu(II) ions in the enol form when the Cu(II) source is a salt of a base of a weak acid, or when
Summary
Cheap and very attractive potential substrates for the direct synthesis of added value functionalized products. Their chemical inertness requires harsh temperatures leading to low conversions, limiting their wide usage as raw materials for atom efficient and selective oxidation processes [1,2,3,4]. Like the active centres of some enzymes (e.g., particulate methane monooxygenase, a multi-copper enzyme that catalyzes the hydroxylation of alkanes) [10], transition metal complexes can potentially catalyse the functionalization of the non-activated C-H bonds of hydrocarbons under mild reaction conditions [11]. The promising approach of design bio-inspired catalysts has been pursued [11,12,13,14,15] and several multi-copper(II) complexes have been found to exhibit a high catalytic activity in the oxidation of cycloalkanes by hydrogen peroxide to the corresponding alkyl hydroperoxides, alcohols and ketones [1,5,11,16,17,18,19,20,21].
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