Synthesis, characterization and catalytic activity of novel monomeric and polymeric vanadyl Schiff base complexes

Abstract

This article describes the synthesis and characterization of two novel, as well as four previously reported, Schiff base complexes of oxovanadium(IV) and the study of their catalytic reactivity in the aerobic oxidation of olefins. The novel (5-Br-3-NO 2salen) and (5-Br-3-NO 2 meso-stein) ligands were, respectively, synthesized from condensation of 5-bromo-3-nitro-2-hydroxybenzaldehyde with ethylenediamine or meso-1,2-diphenyl-1,2-ethylenediamine. The reaction of these ligands with VO(acac) 2 readily resulted in the synthesis of their oxovanadium(IV) complexes [VO(5-Br-3-NO 2salen)] and [VO(5-Br-3-NO 2 meso-stein)], respectively. The novel ligands and complexes were deliberately designed to increase the V 5+/V 4+ reduction potentials and to obtain better catalytic performances. [VO(5-Br-3-NO 2salen)] complex was obtained in orange polymeric form but [VO(5-Br-3-NO 2sal- meso-stein)] complex was obtained in green monomeric form. The monomeric form of the latter complex along with its high (V 5+/V 4+) reduction potential makes it a suitable candidate for the aerobic oxidation of olefins. Systematic substitutions on the ancillary ligand have allowed V 5+/V 4+ reduction potentials to be tuned over a range of approximately 475 mV. Catalytic activity increases with increasing V 5+/V 4+ reduction potential and catalytic selectivity varies by changes in the ligands.