Palladium-coated nickel nanoclusters: new Hiyama cross-coupling catalysts Laura Dura´n Pacho´n, Mehul B. Thathagar, Frantisˇek Hartl and Gadi Rothenberg* Received 26th September 2005, Accepted 4th November 2005 First published as an Advance Article on the web 17th November 2005 DOI: 10.1039/b513587g The advantages of bimetallic nanoparticles as C–C coupling catalysts are discussed, and a simple, bottom-up synthesis method of core–shell Ni–Pd clusters is presented. This method combines electrochemical and ‘wet chemical’ techniques, and enables the preparation of highly monodispersed structured bimetallic nanoclusters. The double-anode electrochemical cell is described in detail. The core–shell Ni–Pd clusters were then applied as catalysts in the Hiyama cross-coupling reaction between phenyltrimethoxysilane and various haloaryls. Good product yields were obtained with a variety of iodo- and bromoaryls. We found that, for a fixed amount of Pd atoms, the core–shell clusters outperform both the monometallic Pd clusters and the alloy bimetallic Ni–Pd ones. THF is an excellent solvent for this process, with less than 2% homocoupling by-product. The roles of the stabiliser and the solvent are discussed. Introduction Assembling functional nano-sized objects is one of the key objectives of nanotechnology, with applications ranging from controlled drug release to improved television screens. 1–3 Due to their size, there is no clear-cut argument for building nano- structures using either a top-down or a bottom-up approach. On one hand, top-down methods (such as chemical etching or machining), usually give more precise and controllable results. On the other hand, bottom-up protocols, especially those based on self-assembly, are attractive because they can enable, in the long run, the inexpensive manufacturing that is needed for technological applications. 4–7 One exciting area of application for metal nanoparticles is catalysis. 8 Owing to their ‘intermediate’ size, too big to be ‘homogeneous catalysts’ but too small to be ‘bulk metals’, nanoparticles (referred to also as nanoclusters 9,10 ) can some- times catalyse reactions that are not accessible to their homo- geneous or heterogeneous counterparts, as we and others recently showed in the case of Suzuki 11–14 and Sonogashira 15 cross-coupling. Moreover, the large surface area can give a catalytic advantage over conventional systems. 13,16,17 Bimetal- lic catalysts 18,19 are especially interesting for several reasons: combining two metals may provide control over the catalytic activity, selectivity and stability, and some combinations may exhibit synergistic effects. 20–22 Moreover, by controlling the type of cluster synthesised, one can improve the ‘‘catalyst atom economy’’. 23,24 Using bottom-up synthesis, one can envisage three types of cluster mixtures (Fig. 1): alloy particles, core–shell particles, and segregated particles. The core–shell species are particu- larly interesting, as catalysis occurs on the shell surface, so one can envisage a cluster where the core is an inexpensive, inactive metal, and the shell made from an active (noble) metal. 25 In this paper, we report the first combined chemical/electroche- mical cluster synthesis approach to make core–shell Ni–Pd nanoparticles. We then show that not only these clusters are active in Hiyama cross-coupling, but they are also superior to the monometallic and bimetallic alloy structures. The pros and cons of the synthesis method and the possible applications of such clusters in other catalytic systems are discussed. Results and discussion Preparation and characterization of metal clusters Homometallic cluster suspensions of Ni and Pd were electro- chemically prepared in the presence of TOAB (tetraoctyl ammonium bromide) as surfactant to avoid particle agglom- eration. Dark brown suspensions were obtained for both Ni clusters and Pd clusters, as observed previously in the litera- ture. 20 The Pd clusters were stable for long periods (several months) when kept under nitrogen, without observing any agglomeration. In contrast, the Ni clusters remained stable only for ca. two weeks, and only under a dry and non- oxidising atmosphere. The bimetallic alloy Ni–Pd clusters were prepared in a similar way by simultaneous electrolyses of the corresponding metal wire electrodes. Good yields and high stabilities were obtained. This observation already points to one advantage of the alloy preparation: it enables the study and application of Ni-containing clusters in a stable configuration. To synthesise the structured core–shell Ni–Pd particles, we combined electrochemical and ‘wet chemical’ methods. 26 First, homometallic Ni particles were prepared electrochemically as Van ‘t Hoff Institute of Molecular Sciences, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands. E-mail: gadi@science.uva.nl This journal is  c the Owner Societies 2006 Phys. Chem. Chem. Phys., 2006, 8, 151–157 | 151 PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics