Nickel(II) unsymmetrical Schiff base complex immobilized on boehmite nanoparticles: a novel and efficient heterogeneous catalyst for selective oxidation of sulfides and thiols

In this work, first, boehmite nanoparticles were prepared by an economical and simple method using accessible materials, and functionalized with 3-chloropropyltrimethoxysilane (3-CPTMS). Then, a nickel(II) unsymmetrical salen Schiff base complex, Ni(salenac-OH), where salenac-OH = [9-(2’,4'-dihydroxyphenyl)−5,8-diaza-4-methylnona-2,4,8-trienato](−2), was synthesized and immobilized on the prepared chloro-functionalized boehmite nanoparticles. The obtained Boehmite@Ni(salenac-OH) nanoparticles were characterized using various techniques. The produced Boehmite@Ni(salenac-OH) nanoparticles were employed as an effective heterogeneous nanocatalyst for the selective oxidation of sulfides to sulfoxides using 30% H₂O₂ as a green oxidant under solvent-free conditions as well as the oxidative coupling of thiols to disulfides with 30% H₂O₂ in ethanol at room temperature (25 °C). In these protocols, the desired sulfoxides and disulfides were achieved with good to excellent yields without over-oxidation to unwanted by-products. The use of this heterogeneous nanocatalyst in the mentioned organic reaction achieved good results, including high efficiency, excellent stability, easy recovery, and reusability of the catalyst for five continuous cycles. In addition, XRD and FT-IR techniques indicated that the structure of the catalyst remained intact after the recovery process.