Vanadyl Salen Complex Anchored on Polymer: An Efficient Catalyst for Selective Oxidation of Sulfides to Sulfoxides

Schiff base ligands H2L1-3 were prepared by condensation of 2-hydroxybenzaldehydes and 4-carboxy-o-phenylenediamine in ethanol under reflux conditions in presence of a few drops of formic acid. The ligands were then treated with vanadyl acetylacetonate in refluxing ethanol to prepare the corresponding vanadyl salen complexes (VOL1-3) in good yields (75-86%). The obtained complexes were then tethered on polyethyleneimine (PEI) in presence of N, N '-dicyclohexylcarbodiimide (DCC) and 4-dimethylaminopyridine (DMAP), followed by electrospinning to prepare vanadyl salen complex functionalized nanofibers (VOL1-3-PEI). FTIR, UV-Vis, and 1H NMR spectroscopy were used to ensure the authenticity of the syntheses. Scanning electron microscopy (SEM), on the other hand, showed that the obtained nanofibers were almost uniform and uni-axially aligned with their longitudinal axes oriented randomly with no entanglement between them. VOL1-3-PEI nanofibrous samples were then used as recyclable heterogeneous catalysts for selective oxidation of sulfides to sulfoxides. Effect of various parameters such as type and amount of the oxidant, solvent, and amount of the catalyst was studied. Although, all three catalysts had high activity and chemo-selectivity toward formation of sulfoxides rather than sulfones, VOL2-PEI showed superior performance than the other analogs in presence of H2O2 as an easily affordable oxidant. Moreover, VOL2-PEI showed promising recyclability and was re-used up to five cycles without significant loss of activity.