Oxovanadium(IV) based coordination polymers and their catalytic potentials for the oxidation of styrene, cyclohexene and trans-stilbene

Reaction between 5,5′-methylenebis(salicylaldehyde) or 5,5′-dithiobis(salicylaldehyde) and 1,2-diaminocyclohexane in equimolar ratio leads to the formation of new polymeric chelating ligands [–CH2(H2sal-dach)–]n (I) and [–S2(H2sal-dach)2–]n (II). These ligands react with [VO(acac)2] in DMF to give coordination polymers [–CH2{VO(sal-dach)·DMF}–]n (1) and [–S2{VO(sal-dach)·DMF}–]n (2). Both complexes are insoluble in common solvents and exhibit a magnetic moment value of 1.74 and 1.78μB, respectively. IR spectral studies confirm the coordination of ligands through the azomethine nitrogen and the phenolic oxygen atoms to the vanadium. These complexes exhibit good catalytic activity towards the oxidation of styrene, cyclohexene and trans-stilbene using tert-butylhydroperoxide as an oxidant. Concentration of the oxidant and reaction temperature has been optimised for the maximum oxidation of these substrates. Under the optimised conditions, oxidation of styrene gave a maximum of 76% (with 1) or 85% (with 2) conversion having following products in order of selectivity: benzaldehyde > styreneoxide > 1-phenylethane-1,2-diol > benzoic acid. A maximum of 98% conversion of cyclohexene was obtained with both the catalysts where selectivity of cyclohexeneoxide varied in the order: 2 (62%) > 1 (45%). With the conversion of 33% (with 1) and 47% (with 2), oxidation of trans-stilbene gives benzaldehyde, benzil and trans-stilbeneoxide as major products.

Graphical abstractNew polymeric complexes [–CH2{VO(sal-dach)·DMF}–]n (1) and [–S2{VO(sal-dach)·DMF}–]n (2) with polymeric chelating ligands [–CH2(H2sal-dach)–]n (I) and [–S2(H2sal-dach)2–]n (II), derived from 5,5′-methylenebis(salicylaldehyde) or 5,5′-dithiobis(salicylaldehyde) and 1,2-diaminocyclohexane, have been prepared and used as catalyst for the oxidation of styrene, cyclohexene and trans-stilbene. Figure optionsDownload full-size imageDownload as PowerPoint slide