Synthesis, Spectroscopic Investigation and Immobilization Of Copper(II) Complexes as Oxidation Catalysts

Abstract

Schiff-base complexes that function as catalysts in aqueous media have been developed. The design of environmentally benign metal catalysts with multidentate ligands is a field of intense current interest. Transition metal complexes with Schiff-base ligands offer a promising route towards discovery of novel catalysts due to the ease of their synthesis, which allows structural modification for optimization of the catalytic sites. Bis(salicylaldehyde)-based complexes, for example, can be modified by derivatizing the ligand backbone to present desirable properties like enhanced solubility in water. Derivatization of 2,4-dihydroxybenzaldehyde at the more reactive 4-position has generated structurally diverse precursor aldehydes for synthesis of pentadentate Schiff-base ligands. Polyethylene glycol side chains attached at the 4-hydroxyl group of 2,4-dihydroxybenzaldehydes has resulted in new water-soluble ligands. These ligands coordinate to copper(II) acetate resulting in binuclear copper(II) Schiff-base complexes that have been shown to catalyze the aerobic oxidation of model catechol, 3,5-ditertbutyl catechol, faster in aqueous methanol than in pure methanol. Derivatization of 2,4-dihydroxybenzaldehyde with p-vinylbenzyl group resulted in a polymerizable precursor aldehyde that led to synthesis of a polymerizable Schiff-base ligand. A homogeneous catalytic system was developed in which this polymerizable ligand was anchored on a polymer support derived from styrene and butyl acrylate by miniemulsion polymerization. The macromolecular polymeric catalyst was found to have superior catalytic properties over small molecular weight analogues. Reduced Schiff-base complexes based on bis-pyridyl ligands were also explored as catalysts because they provide flexible metal-binding properties due to reduction of the rigid azomethine bond to a less constrained amine moiety. These bis(pyridyl)-based catalysts were found to have higher glycosidase activity than bis(salicyaldehyde)-based catatalysts at pH 10.5.