Polymer anchored Schiff base complexes of transition metal ions and their catalytic activities in oxidation of phenol

Abstract

The polymer anchored transition metal complexes of N, N′-bis( o-hydroxy acetophenone)ethylene diamine (HPED) Schiff base were prepared by reacting N, N′-bis(4-amino- o-hydroxy acetophenone)ethylene diamine (AHPED) Schiff base with cross-linked chloromethylated polystyrene beads and then loading of iron(III), cobalt(II) and nickel(II) ions in methanol. The N, N′-bis (4-amino- o-hydroxy acetophenone)ethylene diamine (AHPED) Schiff base was prepared by nitrosation and reduction of nitrosated HPED Schiff base in presence of Fe/HCl catalyst. The loading of AHPED Schiff base on chloromethylated polystyrene beads was 86% (2.58 mmol g −1 of beads). The AHPED Schiff base anchored polymer beads have shown 85%, 86% and 89% comlexation for iron(III), cobalt(II) and nickel(II) ions from solution of metal salts (2.6 mmol), whereas unsupported HPED Schiff base shown 80%, 88% and 77% comlexation for iron(III), cobalt(II) and nickel(II) ions. The free and polymer supported metal complexes were analyzed for molecular weight (Mw) and composition by elemental analysis. The UV, IR and magnetic measurements of free and polymer supported metal complexes have confirmed the octahedral geometry for iron(III) and square planar geometry for cobalt(II) and nickel(II) ions complexes. The thermogravimetric analysis (TGA) of Schiff base has shown 55% weight loss at 500 °C but iron(III), cobalt(II) and nickel(II) ions complexes have shown 30%, 40% and 48% weight loss at same temperature. The iron(III), cobalt(II) and nickel(II) ions complexes have shown temperature of maximum decomposition rate ( T max) as 325, 319 and 281 °C, respectively. The unsupported HPED Schiff base complexes of metal ions were found to be less stable although the trend in their thermal stability was almost same. The catalytic activity of free and polymer anchored HPED Schiff base complexes was evaluated by studying the oxidation of phenol at 70 °C. The percent conversion of phenol and turn over number (TON) was found to be optimum at 1:1:1 molar ratio of phenol, H 2O 2 and metal ions in both free and polymer supported metal complexes. The activation energy for oxidation of phenol by polymer supported HPED Schiff base complex of iron(III) was found to be low (25 kJ mol −1) in comparison to HPED Schiff base complexes of cobalt(II) (57 kJ mol −1) and nickel(II) ions (31 kJ mol −1). On the basis of literature report, a mechanism for oxidation of phenol has been proposed.