Preparation and catalytic evaluation of ruthenium–nickel dendrimer encapsulated nanoparticles via intradendrimer redox displacement of nickel nanoparticlesw

Dendrimer-templated nanoparticle synthesis is an efficient method for preparing a variety of monoand bimetallic nanomaterials, whose sizes are constrained by the loading capacity of the dendrimer template. A common commercially available dendrimer is composed of repeating amino-amide units (polyamidoamine, PAMAM), whose internal amines coordinate with free metal ions in solution. The metal ion affinity for these amine groups strongly influences their partitioning into the dendrimer interior; metal ions such as Cu and Pd will readily complex within dendrimers under the appropriate conditions and may be directly reduced to metal nanoparticles. However, weakly coordinating metal ions require either a dendrimer template with more appropriate interior groups for coordinating metal cations, which may not be readily available, or an indirect synthetic route. Intradendrimer redox displacement, which involves the formation of dendrimer encapsulated nanoparticles (DENs) of one metal followed by exchange for a second, more noble metal, has been previously reported. Zhao et al. synthesized Cu55 DENs using generation 6, hydroxyl-terminated (G6-OH) PAMAM dendrimers. Galvanic displacement of Cu (Eox = 0.342 V) with Ag to form Ag (Ered = 0.777 V) was then performed. Subsequent displacement of Ag with Pd, Pt and Au to form the corresponding DENs can also be accomplished. Here, we describe the preparation of ruthenium–nickel dendrimer encapsulated nanoparticles (RuNiDENs) in generation 4 hydroxylterminated (G4-OH) PAMAM dendrimers. Previously reported syntheses of Ru DENs used sodium borohydride (NaBH4) 7 as well as H2(g) 8 as reductants, following complexation of the Ru for 2–3 days. In this report, we investigated Ni as a potential partner in a displacement scheme to form Ru and RuNi DENs; the displacement was evaluated for 5–20 molar excess (ME) of Ru and Ni, and the resulting effects on the DEN catalytic efficiency for a model reduction reaction were evaluated. This method of preparation eliminates significant metal ion complexation time and results in RuNi DENs that are stable for at least a week under an inert atmosphere. The galvanic displacement of Ni with Ru is based on the following set of half-reactions: 3Ni 3Ni + 6e +0.257 V