Oxo-vanadium(IV) unsymmetrical Schiff base complex immobilized on γ-Fe2O3 nanoparticles: A novel and magnetically recoverable nanocatalyst for selective oxidation of sulfides and oxidative coupling of thiols

In this research, an unsymmetrical salen-type oxo-vanadium(IV) complex, [VO(salenac-OH)] (salenac-OH = [9-(2 ',4 '-dihydroxyphenyl)-5,8-diaza-4-methylnona-2,4,8-trienato](-2)), was synthesized and covalently immobilized on the surface of magnetic gamma-Fe2O3 nanoparticles. The resulting gamma-Fe2O3@[VO(salenac-OH)] nanoparticles were characterized by several techniques including Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), scanning and transmission electron microscopies (SEM and TEM), energy-dispersive X-ray (EDX) spectroscopy, vibrating sample magnetometry (VSM), thermal gravimetric analysis (TGA), inductively coupled plasma (ICP), and elemental analysis. The prepared gamma-Fe2O3@[VO(salenac-OH)] nanoparticle was utilized as an efficient catalyst for selective oxidation of sulfides to sulfoxides using 30% H2O2 as oxidant and oxidative coupling of thiols into disulfides with urea/H2O2 (UHP) as an oxidizing reagent. The products were achieved with good to excellent yields at room temperature with no over-oxidation of sulfoxides and disulfides to unexpected by-products. This catalyst can be magnetically recovered by applying an external magnet and reused for five continuous cycles in both oxidation reactions without a significant loss in its catalytic activity. Furthermore, the FT-IR spectrum and XRD pattern of the recovered catalyst showed no critical change to those of the fresh one.