Synthesis and structural studies of polymer-supported transition metal complexes: Efficient catalysts for oxidation of phenol

Abstract

In this work, a new simple and reproducible method for preparation of a series of novel recyclable complexes is described. The series includes unsupported 3-MHBdMBn–Met complexes (Met = iron(III), cobalt(II), nickel(II), copper(II) and zinc(II) ions), and polystyrene anchored (P-3-MHBdMBn–Met) metal ion complexes, supported by—ONNO-tetradentate Schiff-base ligand. These free and polymer anchored metal complexes are prepared by the reactions of metal solution with one molar equivalent of unsupported 3-MHBdMBn (N,N'-bis(2-hydroxy-3-methylbenzaldehyde)-4-methylbenzene-1,2-diamine) or polymer-supported (P-3-MHBdMBn) Schiff-base ligands in methanol under nitrogen atmosphere. The greatest advantage of this catalyst system over other polymer-supported catalysts is a remarkably low cost and the quality of being recycled up to six times. These benefits are due to the easily accessible materials and the simple synthetic route. A higher efficiency of complexation of metal ions on the polymer anchored 3-MHBdMBn Schiff base compared to that shown by an unsupported analogue is another advantages of this catalyst system. The structural study reveals that iron(III) complex is octahedral in geometry, whereas cobalt(Il), nickel(II) and copper(II) complexes are square planar and zinc(II) complex is of tetrahedral geometry. The catalytic activities of polystyrene-supported metal complexes toward the oxidation of phenol are investigated. Experimental results indicate that the reactivity of P-3-MHBdMBn–Met is dramatically affected by the polymer support and iron complexes are more active than other complexes. The rate of phenol conversion is high (2.41 × 10–6 mol dm–3 s–1) in the presence of polystyrene-supported 3-MHBdMBn Schiff base complexes of iron(III) ions and low in the presence of zinc(II) analogue (1.36 × 10–6 mol dm–3 s–1).