Synthesis, characterization, and catalytic activity for thioanisole oxidation of homogeneous and heterogeneous binuclear manganese(II) complexes with amino acid-based ligands

Abstract

A binuclear Mn(II) complex [Mn2(HL)2(H2O)4], HL = 2-[(2-Hydroxy-benzylidene)-amino]-3-(4-hydroxy-phenyl)-propionic acid, has been prepared and characterized by physico-chemical and spectroscopic methods. The results suggest that the amino acid Shiff base ligand is coordinated as a bivalent anion with a tridentate NO2 donor set involving the phenolic and carboxylic acid oxygens and azomethine nitrogen. A heterogenized binuclear manganese catalyst was synthesized by the covalent anchoring of [Mn2(HL)2(H2O)4] onto a modified silica gel surface, through the reactive (3-chloropropyl)-trimethoxysilane group. The surface properties of the functionalized catalyst were analyzed by physico-chemical and spectroscopic methods. The silica-supported metal complex, ([Mn2L(HL)(H2O)4]/SiO2), catalyzes the oxidation of thioanisole with H2O2, to give the sulfoxide and sulfone. [Mn2L(HL) (H2O)4]/SiO2 shows lower catalytic activity and turnover number compared to the homogeneous catalyst [Mn 2 II –(HL)2(H2O)4]. However, the activity of the immobilized catalyst remains nearly unchanged after five cycles, indicating that it is truly heterogeneous.