Polymer-Bound Oxidovanadium(IV) and Dioxidovanadium(V) Complexes As Catalysts for the Oxidative Desulfurization of Model Fuel Diesel

The Schiff base (Hfsal-dmen) derived from 3-formylsalicylic acid and N,N-dimethyl ethylenediamine has been covalently bonded to chloromethylated polystyrene to give the polymer-bound ligand, PS-Hfsal-dmen (I). Treatment of PS-Hfsal-dmen with [V^IVO(acac)₂] in the presence of MeOH gave the oxidovanadium(IV) complex PS-[V^IVO(fsal-dmen)(MeO)] (1). On aerial oxidation in methanol, complex 1 was oxidized to PS-[V^VO₂(fsal-dmen)] (2). The corresponding neat complexes, [V^IVO(sal-dmen)(acac)] (3) and [V^VO₂(sal-dmen)] (4) were similarly prepared. All these complexes are characterized by various spectroscopic techniques (IR, electronic, NMR, and electron paramagnetic resonance (EPR)) and thermal as well as field-emission scanning electron micrographs (FE-SEM) studies, and the molecular structures of 3 and 4 were determined by single crystal X-ray diffraction. The EPR spectrum of the polymer supported V^IVO-complex 1 is characteristic of magnetically diluted V^IVO-complexes, the resolved EPR pattern indicating that the V^IVO-centers are well dispersed in the polymer matrix. A good ⁵¹V NMR spectrum could also be measured with 4 suspended in dimethyl sulfoxide (DMSO), the chemical shift (−503 ppm) being compatible with a VO₂⁺-center and a N,O binding set. The catalytic oxidative desulfurization of organosulfur compounds thiophene, dibenzothiophene, benzothiophene, and 2-methyl thiophene (model of fuel diesel) was carried out using complexes 1 and 2. The sulfur in model organosulfur compounds oxidizes to the corresponding sulfone in the presence of H₂O₂. The systems 1 and 2 do not loose efficiency for sulfoxidation at least up to the third cycle of reaction, this indicating that they preserve their integrity under the conditions used. Plausible intermediates involved in these catalytic processes are established by UV−vis, EPR, ⁵¹V NMR, and density functional theory (DFT) studies, and an outline of the mechanism is proposed. The ⁵¹V NMR spectra recorded for solutions in methanol confirm that complex 4, on treatment with H₂O₂, is able to generate peroxo-vanadium(V) complexes, including quite stable protonated peroxo-V^V-complexes [V^VO(O)₂(sal-dmen-NH⁺)]. The ⁵¹V NMR and DFT data indicate that formation of the intermediate hydroxido-peroxo-V^V-complex [V^V(OH)(O₂)(sal-dmen)]⁺ does not occur, but instead protonated [V^VO(O)₂(sal-dmen-NH⁺)] complexes form and are relevant for catalytic action.