Synthesis and characterisation of polymer-anchored oxidovanadium(IV) complexes and their use for the oxidation of styrene and cumene

Abstract

The Schiff bases H2fsal-ea (I), H2fsal-pa (II) and H2fsal-amp (III), derived from 3-formylsalicylic acid and 2-aminoethanol, 3-aminopropanol and 2-amino-2-methylpropanol, respectively, have been connected, by means of covalent bonds, to chloromethylated polystyrene cross-linked with 5% divinylbenzene. On treatment with [VO(acac)2] in dimethylformamide (DMF), these polymer-anchored ligands PS-H2fsal-ea (IV), PS-H2fsal-pa (V) and PS-H2fsal-amp (VI) gave the oxidovanadium(IV) complexes, PS-[VO(fsal-ea)·DMF] (4), PS-[VO(fsal-pa)·DMF) (5) and PS-[VO(fsal-amp)·DMF] (6), respectively. The corresponding neat complexes [VO(fsal-ea)]2 (1), [VO(fsal-pa)]2 (2) and [VO(fsal-amp)]2 (3) have also been similarly prepared. These complexes all exhibit a medium intensity band between 964 and 993 cm-1 in their IR spectra resulting from the V=O stretch. The EPR spectra of the polymer-anchored complexes are characteristics of monomelic VIV centres with a simple S = 1/2 electronic spin and with an axial pattern typical of square pyramidal geometry. Broad features for the neat complexes along with magnetic susceptibility studies suggest the presence of antiferromagnetic exchange interactions between two vanadium centres in close proximity. These catalysts have been tested for the oxidation of styrene and cumene and were found to be efficient. Styrene gives five reaction products namely styrene epoxide, benzaldehyde, 1-phenylethane-1,2-diol, benzoic acid and phenylacetaldehyde, whereas cumene gives acetophenone, 2-phenylpropanal, α-methyl styrene epoxide, 2-phenyl-2-propanol, 2-isopropyl-1,4- benzoquinone and α-methyl styrene. The polymer-anchored heterogeneous catalysts are recyclable. The catalytic activities of the neat complexes have also been examined and compared with the corresponding anchored analogues. © Wiley-VCH Verlag GmbH & Co. KGaA, 2007.