Homogeneous Hydrogenation Catalysis with Monodisperse, Dendrimer-Encapsulated Pd and Pt Nanoparticles**

In this report we show that composite materials that consist of noble metal nanoparticles stabilized within dendrimer interiors are suitable for use as homogeneous hydrogenation catalysts. These interesting new materials are prepared by sorbing PdII or PtII ions into hydroxyl-terminated poly(amidoamine) (PAMAM) dendrimers (Gn-OH, where Gn represents the nth generation) where they complex with interior amine groups. Subsequent chemical reduction of the metal ions with BH4ÿ yields dendrimer-encapsulated metal nanoparticles that contain the same number of atoms as were preloaded into the dendrimer initially. The resulting composites are soluble in water and stable, either dry or solvated, for at least several months. Thus, the dendrimer acts as both a template for the preparation of monodisperse nanoparticles and a porous stabilizer. Dendrimer-encapsulated Pd clusters exhibit high catalytic activity for the hydrogenation of alkenes in water. Importantly, the catalytic activity can be controlled by adjusting the size (generation) of the dendrimer; that is, the dendrimer acts as a ananofiltero with a synthetically controllable mesh. Stabilized noble metal nanoparticles have been used as catalysts and photocatalysts in solutions for many years.[1±5] The activity of these materials is controlled by their size, crystal structure, and the nature of the stabilizer. Synthetic routes to soluble metal nanoparticles include chemical or electrochemical reduction of metal salts in the presence of stabilizers.[2, 3, 6] The purpose of the stabilizer is to control the particle size and prevent agglomeration, but strong adsorption of the stabilizers on the active sites may result in a loss of catalytic activity. Ideally, a metal-particle catalyst should be surrounded by a nanofilter or weak adsorbents that permit passage of the substrate and product of the catalytic reaction, but which prevent agglomeration. The dendrimer plays just this role in the experiments described here. Our approach for the preparation of dendrimer-encapsulated Pd and Pt metal particles is similar to that we,[7] and later others,[8] reported previously for Cu. As indicated above, the experiment is quite simple: first, a predetermined quantity of metal ions are extracted into the interior of a Gn-OH dendrimer, and second the ions are chemically reduced with BH4ÿ to yield zerovalent metal particles. Figure 1 shows absorption spectra of dendrimer-encapsulated PdII and PtII. For example, a 2 mm solution of PtCl42ÿ has a strong ligand ± metal charge-transfer band at lmax(e)ˆ 214 nm (8000).[9] After addition of 0.05 mm G4-OH to this solution, the band at 214 nm decreases and shifts to 221 nm and a new band at 249 nm appears.[10] These changes are a consequence of the encapsulation of metal ions by the dendrimer. After reduction of the composite the spectrum changes significantly: there is now a much higher absorption intensity at low energy, which results from the interband transition of the encapsulated zero-valent metal particles.[4, 11] Similar spectroscopic results are observed for the G4-OH/PdII composites. The bands at 217 and 276 nm that result from PdCl42ÿ disappear after the addition of G4-OH and new bands are evident at 225 and 300 nm.[10] A new interband transition is observed after reduction.[6] Microscopy confirms that the chemical reduction of PtII or PdII encapsulated within G4-OH (G4-OH(Pt)n and G4-OH(Pd)n, respectively) yields intradendrimer metal nanoparticles (G4-OH(Ptn) and G4-OH(Pdn), respectively).[12] High-resolution transmission electron microscopy (HRTEM) images (Figure 2) clearly show that dendrimer-encapsulated particles are quite monodisperse and that their shape is roughly spherical. The diameters of the metal particles for G4OH(Pt40), G4-OH(Pt60), and G4-OH(Pd40) particles are 1.4 0.2, 1.6 0.2, and 1.3 0.3 nm, respectively, which are slightly larger than the theoretical values of 1.1, 1.2, and 1.1, respectively, which are calculated using the as[*] Prof. R. M. Crooks, M. Zhao Department of Chemistry P.O. Box 30012, Texas A&M University College Station, TX 77842 ± 3012 (USA) Fax: (‡1) 409-845-1399 E-mail : [email protected] [**] This work was supported by the National Science Foundation and the Robert A. Welch Foundation. M.Z. acknowledges fellowship support from the Electrochemical Society, The Eastman Chemical Company, and Phillips Petroleum. TEM results are based upon research conducted at the Transmission Electron Microscopy Laboratory in the Department of Earth and Planetary Science of the University of New Mexico, which is supported by NSF, NASA, and the State of New Mexico. We also acknowledge Professor David E. Bergbreiter and his research group for assistance with the hydrogenation catalysis measurements. Figure 1. a) Absorption spectra of solutions containing 2.0 mm K2PtCl4 and 2.0mm K2PtCl4‡ 0.05 mm G4-OH before and after reduction with a fivefold molar excess of NaBH4. Curves in b) were obtained under the same conditions as those in a) except 2.0mm Na2PdCl4 was used in place of the K2PtCl4. The optical path length for all UV/Vis measurements was 0.1 cm.