Polymer and non-polymer-grafted dioxidomolybdenum(VI) complexes having ONO donor ligand and their catalytic activities for the oxidative bromination of organic substrates

Abstract

The ligand H2sal-iah (I), derived from salicylaldehyde and indole-3-acetic hydrazide, reacts with [MoVIO2(acac)2] in methanol to give dioxidomolybdenum(VI) complex [MoVIO2(sal-iah)(MeOH)] (1). Drop wise addition of 30% aqueous H2O2 to the methanolic solution of 1 yields complex [MoVIO(O2)(sal-iah)(MeOH)] (2). Complex 1 has been grafted via covalent bonding through imino nitrogen of the indole to chloromethylated polystyrene cross-linked with 5% divinylbenzene {now abbreviated as PS-[MoVIO2(sal-iah)(MeOH)] (3)}. All these complexes are characterized by various spectroscopic techniques (IR, electronic, 1H and 13C NMR) and thermal as well as field-emission scanning electron micrographs (FE-SEM) studies. The crystal structure of 1 has been determined, confirming the ONO binding mode of I. The polymer-grafted complex 3 catalyzes the oxidative bromination, by H2O2, of styrene and trans-stilbene. Under the optimized reaction conditions, the oxidative bromination of styrene gave 96% conversion in 2h time where 2-bromo-1-phenylethane-1-ol and 1,2-dibromo-1-phenylethane are the main products and 1-phenylethane-1,2-diol is the product obtained by the attack of nucleophile water on the α-carbon of 2-bromo-1-phenylethane-1-ol. Oxidative bromination of trans-stilbene gave 96% conversion with 2,3-diphenyloxirane (trans-stilbene oxide), 1,2-dibromo-1,2-diphenylethane and 2-bromo-1,2-diphenylethanol as the products. Suitable reaction mechanisms for both reactions have been suggested. Neat complex [MoVIO2(sal-iah)(MeOH)] (1) is equally active but the recyclability and heterogeneity tests of polymer-grafted complex makes it better over neat analog.