Nickel(II) unsymmetrical Schiff base complex immobilized into three dimensional mesoporous silica KIT-6: a novel and efficient catalyst for selective oxidation of sulfides to sulfoxides

In this work, a new Ni(II) unsymmetrical Schiff-base complex, Ni(salenac), was successfully synthesized and immobilized into three-dimensional (3D) mesoporous silica KIT-6 (Korean Institute of Technology No. 6). The novel composite KIT-6-pr-NH2-Ni(salenac) was characterized by various techniques and utilized to selectively oxidize different aromatic and aliphatic sulfides to the corresponding sulfoxides with yields of 80-95% by using hydrogen peroxide (30% H2O2) as an environmentally friendly oxidant in ethanol solution at 25 degrees C. This oxidizing system allowed the chemoselective oxidation of sulfides containing other functional groups. The results showed that the OH and C=C groups of the compounds such as 2-hydroxyethyl phenyl sulfide and allyl methyl sulfide remained intact during sulfide oxidation. The catalyst was stable in the reaction conditions, and could be recused at least five times without significant decline in catalytic activity. Moreover, Fourier transform infrared (FTIR) spectrum of the recovered catalyst matches that of the fresh catalyst, and the catalyst structure remains unchanged after recovery.