Polystyrene bound oxidovanadium(IV) and dioxidovanadium(V) complexes of histamine derived ligand for the oxidation of methyl phenyl sulfide, diphenyl sulfide and benzoin

Abstract

Ligand Hsal–his (I) derived from salicylaldehyde and histamine has been covalently bound to chloromethylated polystyrene cross-linked with 5% divinylbenzene. Upon treatment with [VO(acac)₂] in DMF, the polystyrene-bound ligand (abbreviated as PS–Hsal–his, II) gave the stable polystyrene-bound oxidovanadium(IV) complex PS–[V^IVO(sal–his)(acac)] 1, which upon oxidation yielded the dioxidovanadium(V) PS–[V^VO₂(sal–his)] 2 complex. The corresponding non polymer-bound complexes [V^IVO(sal–his)(acac)] 3 and [V^VO₂(sal–his)] 4 have also been obtained. These complexes have been characterised by IR, electronic, ⁵¹V NMR and EPR spectral studies, and thermal as well as scanning electron micrograph studies. Complexes 1 and 2 have been used as a catalyst for the oxidation of methyl phenyl sulfide, diphenyl sulfide and benzoin with 30% H₂O₂ as oxidant. Under the optimised reaction conditions, a maximum of 93.8% conversion of methyl phenyl sulfide with 63.7% selectivity towards methyl phenyl sulfoxide and 36.3% towards methyl phenyl sulfone has been achieved in 2 h with 2. Under similar conditions, diphenyl sulfide gave 83.4% conversion where selectivity of reaction products varied in the order: diphenyl sulfoxide (71.8%) > diphenyl sulfone (28.2%). A maximum of 91.2% conversion of benzoin has been achieved within 6 h, and the selectivities of reaction products are: methylbenzoate (37.0%) > benzil (30.5%) > benzaldehyde-dimethylacetal (22.5%) > benzoic acid (8.1%). The PS-bound complex, 1 exhibits very comparable catalytic potential. These polymer-anchored heterogeneous catalysts do not leach during catalytic action, are recyclable and show higher catalytic activity and turnover frequency than the corresponding non polymer-bound complexes. EPR and ⁵¹V NMR spectroscopy was used to characterise methanolic solutions of 3 and 4 and to identify species formed upon addition of H₂O₂ and/or acid and/or methyl phenyl sulfide.