Active catalytic centers in silica-supported Cu(II) and Mn(II) biomimetic complexes: Correlation between catalytic and EPR data

Two polydentate ligands represented as LA and LB have been synthesized and subsequently grafted on a silica surface via covalent bonds. The organic ligands LA and LB as well as the heterogenized ligands LA·SiO2 and LB·SiO2 reacted with manganese(II) and copper(II) ions leading to the formation of the corresponding metal complexes. Catalytic epoxidations of simple olefins with hydrogen peroxide were studied using the above manganese complexes in the presence of a cocatalyst. The major products of the oxidations were the epoxides and the studied manganese complexes showed significant catalytic activities. The copper(II) complexes have been evaluated for the catalytic oxidation of 3,5-di-t-butylcatechol (DTBC) by dioxygen. Based on a combination of structural information, obtained form EPR spectroscopy, and the catalytic activity we suggest that the catalysis of DTBC oxidation by copper complexes is critically determined by two factors (a) for dinuclear complexes, it is enhanced when the Cu···Cu distance approaches the OC···CO distance in the catechol molecule (2.7 Å), and (b) immobilization on silica support results in enhancement of the catalytic activity in cases where even monomeric complexes are located at relative distances approaching the critical OC···CO distance (2.7 Å). This provides a practical rule for design and synthesis of more efficient copper-catalysts for the catechol oxidation.

Graphical abstractTwo polydentate ligands have been synthesized and immobilized on a silica surface via covalent bonds. The silica-supported Mn(II) complexes catalyze efficiently the epoxidation of several alkenes by hydrogen peroxide in the presence of ammonium acetate. The corresponding copper(II) complexes have been evaluated for the catalytic oxidation of 3,5-di-t-butylcatechol by dioxygen. Catalytic data and structural information obtained by EPR spectra are correlated and discussed.Figure optionsDownload full-size imageDownload as PowerPoint slide