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-[VIVO(sal-his)(acac)] 1, which upon oxidation yielded the dioxidovanadium(v) PS-[VVO 2(sal-his)] 2 complex. The corresponding non polymer-bound complexes [VIVO(sal-his)(acac)] 3 and [VVO2(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. © 2009 The Royal Society of Chemistry.