Catalytic investigations of carbon–carbon bond-forming reactions by a hydroxyapatite-bound palladium complex

Abstract

A new type of hydroxyapatite-bound palladium complex (PdHAP-1) was synthesized by treatment of a nonstoichiometric Ca-deficient hydroxyapatite, Ca₉(HPO₄)(PO₄)₅(OH), with PdCl₂(PhCN)₂ in acetone solution. Characterization by means of physicochemical methods revealed that a monomeric PdII phosphate complex could be generated at a Ca-deficient site, which displayed outstanding catalytic activities for the Mizoroki–Heck reaction and Suzuki–Miyaura coupling reaction of aryl bromides. The remarkably high catalytic activity of the hydroxyapatite catalyst is ascribed to the exceptionally robust monomeric Pd structure, in which Pd is surrounded by anionic phosphate ligands, as confirmed by XAFS analysis. It is also proven that, upon adjustment of the solvent system, the PdHAP-1 was able to catalyze the Suzuki–Miyaura coupling of activated aryl chlorides in the presence of TBAB. Under such conditions, the in situ generated Pd nanocluster on the surface of hydroxyapatite was effective as a catalytically active species.