Controlled synthesis of hydroxyapatite-supported palladium complexes as highly efficient heterogeneous catalysts

Achieving precise control of active species on solid surfaces is one of the most important goals in the development of highly functionalized heterogeneous catalysts. The treatment of hydroxyapatites with PdCl2(PhCN)2 gives two new types of hydroxyapatite-bound Pd complexes. Using the stoichiometric hydroxyapatite, Ca10(PO4)6(OH)2, we found that monomeric PdCl2 species can be grafted on its surface, which are easily transformed into Pd0 particles with narrow size distribution in the presence of alcohols. Such metallic Pd species can effectively promote alcohol oxidation using molecular oxygen and are shown to give a remarkably high TON of up to 236000. Another monomeric PdII phosphate complex can be generated at a Ca-deficient site of the nonstoichiometric hydroxyapatite, Ca9(HPO4)(PO4)5(OH), affording a catalyst with PdII structure and high activity for the Heck and Suzuki reactions. To the best of our knowledge, the PdHAP are one of the most active heterogeneous catalysts for both alcohol oxidation under an atmospheric O2 pressure and the Heck reaction reported to date. These Pd catalysts are recyclable in the above organic reactions. Our approach to catalyst preparation based on the control of Ca/P ratios of hydroxyapatites represents a particularly attractive method for the nanoscale design of catalysts. Copyright © 2002 American Chemical Society.