Oxidoperoxidotungsten(VI) and dioxidotungsten(VI) complexes catalyzed oxidative bromination of thymol in presence of H2O2-KBr-HClO4

Abstract

Two oxidoperoxidotungsten(VI) complexes and the corresponding dioxidotungsten(VI) complexes of tridentate ONO donor Schiff base ligands, H2hap-nah (I) and H2hap-bhz (II) (Hhap = 2-hydroxyacetophenone, nah = nicotinoylhydrazide and bhz = benzoylhydrazide) have been reported. Complexes [WVIO(O2)(hap-nah)(MeOH)] (1), [WVIO(O2)(hap-bhz)(MeOH)] (2), [WVIO2(hap-nah)(MeOH)] (3) and [WVIO2(hap-bhz)(MeOH)] (4) have been characterized by elemental analysis, spectroscopic (IR, UV-Vis, 1H and 13C NMR) and thermogravimetric studies. Single crystal X-ray analysis of 2 and [WO(O2)(hap-inh)(MeOH)] (inh = isonicotinoylhydrazide) (whose synthesis has already been reported in the literature) confirms dibasic tridentate behavior of the ligands and their coordination to the metal ion through two oxygen and a nitrogen atoms. These complexes catalyze oxidative bromination of thymol, a model oxidative halogenation reaction normally shown by vanadium complexes, under acidic (HClO4) condition using 30% H2O2 in the presence of KBr. Under optimized reaction conditions, the selectivity of products follows the order: 4-bromothymol > 2,4-dibromothymol > 2-bromothymol. © 2015 Elsevier B.V. All rights reserved.