Syntheses and characterization of mercapto-hydroxyl-palladium macromolecular chelates and their catalytic properties

An unusual macromolecular mercapto-hydroxyl bidentate ligand has been synthesized by the reaction of epoxy-phenolic oligomer with hydrogen sulfide. The palladium complexes are incorporated into the macromolecular ligand. This catalyst precursor was characterized by means of IR, XPS, UV-vis and elemental analysis. The macromolecular palladium chelates were absorbed onto a silica support. The catalytic behaviour of the silica-supported palladium complexes was investigated by the Heck reaction. The activation energy, 67 kJ mol−1, is different from that of the same reaction catalyzed homogeneously by palladium acetate. The activity of the chelates depends on the molar ratio of sulfur to palladium; thus, an active catalyst is obtained at a S:Pd ratio of 2 and the catalyst is inactive at ratios of 6 or greater. The catalyst has a better stability than the other polymeric palladium catalysts reported in the literature. A study was made of the loss of palladium from the supported macromolecule under reaction conditions. The results showed that the elution of palladium could not be explained on the basis of coordination equilibrium. A mechanism for metal elution is proposed accordingly.