Synthesis, characterisation and catalytic potential of oxovanadium(IV) based coordination polymers having a bridging methylene group

Abstract

Three polymeric oxovanadium(IV) complexes: [ CH 2{VO(salen)} ] n , [ CH 2{VO(sal-1,2-pn)} ] n and [ CH 2{VO(sal-1,3-pn)} ] n of the polymeric Schiff bases derived from 5,5′-methylenebis(salicylaldehyde) [CH 2(Hsal) 2] and 1,2-diaminoethene (en), 1,2-diaminopropane (1,2-pn) or 1,3-diaminopropane (1,3-pn) have been isolated and characterised by various physico-chemical techniques. All these complexes are insoluble in common solvents and are paramagnetic. IR spectral data confirm the coordination of ligands through the azomethine nitrogen and the phenolic oxygen atoms to the metal ion. EPR spectra of these complexes reveal the paramagnetic nature of the complexes, and anisotropic axial patterns and the Hamiltonian parameters predict a square pyramidal geometry around vanadium ions in these complexes. These complexes exhibit good catalytic activity towards the liquid-phase hydroxylation of phenol into a mixture of catechol and hydroquinone using H 2O 2 as an oxidant. Various parameters, such as concentrations of substrate, catalyst and oxidant, as well as reaction time and temperature, have been taken into consideration for the maximum hydroxylation of phenol. Under the optimised conditions, the selectivity values towards the formation of catechol and hydroquinone are about 90–98 and 2–10%, respectively. These complexes also exhibit good catalytic activity towards the oxidative bromination of salicylaldehyde in water in the presence of H 2O 2/KBr to 5-bromosalicylaldehyde and 3,5-dibromosalicylaldehyde.