Mild alkane C–H and O–H oxidations catalysed by mixed-N,S copper, iron and vanadium systems
Reactions of 1,6-bis(2′-pyridyl)-2,5-dithiahexane (Py2S2) with different sources of Cu(II), Fe(II) and V(III) afford the corresponding novel complexes [CuCl(Py2S2)]Cl (1), [CuCl(Py2S2)](CuCl2) (1′), [Cu(OTf)2(Py2S2)] (2), [Cu(Py2S2)(H2O)2](OTf)2 (2′), [FeCl2(Py2S2)] (3), [Fe(Py2S2)(CH3CN)2][SbF6]2 (4) and [VCl2(Py2S2)]Cl (5), bearing Py2S2 as a tetradentate ligand. All the compounds were characterised by IR, ESI–MS, elemental analyses and, in the cases of 1′, 2′ and 4, the molecular structures were also elucidated by single X-ray crystal diffraction analysis. Complexes 1–5 were evaluated as catalysts or catalyst precursors for the mild peroxidative oxidation of cyclohexane in acetonitrile typically at 25°C and in the solvent-free oxidation of primary and secondary alcohols under microwave (MW) irradiation. The influences of the type and amount of acid promoter, amounts of oxidant and catalyst, time and temperature, on the product yields and TONs, are investigated.The iron(II) complexes 3 and 4 are the most active catalysts in the oxidation of cyclohexane with H2O2 in a slightly acidic medium, leading to maximum overall yields (based on the alkane) of 38 and 28%, and turnover numbers (TON) up to 950 and 1450, respectively.Additionally, the Cu and Fe complexes (1–4) proved to be useful catalysts in various MW-assisted alcohol oxidations at 80°C with t-BuOOH. The oxidation of 1-phenylethanol catalysed by the Fe complex 3 in the presence of pyrazine-2-carboxylic acid (co-catalyst) is very fast, giving the acetophenone product (TOF=4470h−1) in good yield (75%) just after 5min of reaction time.