Partially Fluoride-Substituted Hydroxyapatite as a Suitable Support for the Gold-Catalyzed Homocoupling of Phenylboronic Acid: An Example of Interface Modification

Interface modification of hydroxyapatite-supported nanogold by fluoride ion improved the activity of the catalyst toward aerobic homocoupling of phenylboronic acid. In the aerobic homocoupling reaction of PhBF3K catalyzed by hydroxyapatite-supported gold nanoclusters (Au:HAP), a study on the reactivity and reusability of the Au:HAP catalyst showed an unusual yield profile for the reaction. Intensive characterization of the catalyst by X-ray diffraction, transmission electron microscopy, and energy-dispersive Xray spectroscopic analysis exhibited that the structure of the metal oxide support undergoes transformation from HAP to fluorapatite (FAP) and finally to calcium fluoride (CaF2) during the course of these cycles of homocoupling of PhBF3K. Investigations of the interactions of Au:HAP with PhBF3K and of the fluoride ion induced structural changes in both the support and the AuNCs, indicating that partially fluoride-substituted HAP (F-HAP) is the optimal support for the homocoupling of PhB(OH)(2). This is effective both as a stabilizer for AuNCs through the phosphate moiety and in activation of C-B bond transmetalation through B-F interactions. The results strongly suggest that fine-tuning of the structure of the interface between metal clusters and their support (namely, surface modification) might be important in developing chemo-selective catalysts.