Investigation of the substituent effects on the oxidation of styrene derivatives by silica-supported heterogeneous oxidovanadium(V) coordination compound

Vanadium coordination compounds, [VO2L]center dot(CH3OH) (1a and 1b), were prepared from the reaction of tridentate hydrazone ligand, [HL=(E)-4-amino-N '-(1-(pyridin-2-ylethylidene)benzohydrazide] with NH4VO3 and V2O5, respectively. These compounds were characterized by various analytical methods, and the structures of 1a and 1b were determined by single crystal X-ray analysis which indicated that they have the same composition but crystallized in different unit cells and can be considered as polymorphs. The silica-supported catalyst (Si-VO2L) was obtained by immobilization of 1 on the surface of silica using propionyl chloride-functionalized silica gel. Si-VO2L was characterized by thermal gravimetric analysis (TGA), energy-dispersive X-ray spectroscopy (EDX), Fourier-transform infrared (FT-IR) spectroscopy, diffuse-reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and X-ray diffraction analysis (XRD) and was employed as heterogeneous catalyst in the oxidation of styrene derivatives, and aqueous H2O2 was used as the green oxidant. Si-VO2L was recovered several times, and characterized by various analytical methods. The results indicated its high stability. The effects of parameters such as temperature, the ratio of reagents, and nature of solvent on selectivity and activity of Si-VO2L were investigated. The results indicated that both of the steric and electronic effects of the substituents on the phenyl ring of styrene can affect the selectivity and activity of the products. Electron withdrawing groups decrease the activity of the substrate and electron donating groups increase the activity.