Titania Nanorod-Supported Mercaptoundecanoic Acid-Grafted Palladium Nanoparticles as a Highly Reusable Heterogeneous Catalyst for Substrate-Dependent Ullmann Coupling and Debromination of Aryl Bromides.

Abstract

Herein, by implanting palladium nanoparticles (Pd NPs) onto titanium dioxide (TiO2) nanorods (NRs) through 11-mercaptoundecanoic acid (MUA), we devised a robust heterogeneous catalyst. The formation of Pd-MUA-TiO2 nanocomposites (NCs) was authenticated using Fourier transform infrared spectroscopy, powder X-ray diffraction, transmission electron microscopy, energy-dispersive X-ray analysis, Brunauer-Emmett-Teller analysis, atomic absorption spectroscopy, and X-ray photoelectron spectroscopy techniques. Pd NPs were synthesized directly onto TiO2 nanorods without the MUA support for comparative studies. As a means of evaluating the endurance and competency of Pd-MUA-TiO2 NCs compared to their counterpart (Pd-TiO2 NCs), both were used as the heterogeneous catalyst for Ullmann coupling of a wide variety of aryl bromides. When Pd-MUA-TiO2 NCs were used, the reaction produced high yields of homocoupled products (54-88%), whereas the yield was only 76% when Pd-TiO2 NCs were used. Moreover, Pd-MUA-TiO2 NCs impressed with their outstanding reusability property, allowing over 14 reaction cycles without losing efficiency. On the flip side, just after seven reaction cycles, the productivity of Pd-TiO2 NCs dropped around 50%. Presumably, the strong affinity of Pd for the thiol groups of MUA allowed for the substantial control of leaching out of Pd NPs during the reaction. Nonetheless, another crucial feature of the catalyst is that the di-debromination reaction took place with an excellent yield of 68-84% from di-aryl bromides with long alkyl chains instead of macrocyclic or dimerized products. It is worth mentioning that AAS data confirmed that only 0.30 mol % catalyst loading was sufficient to activate a broad substrate scope with large functional group tolerance.