Stabilizer polyoxoanions

Ionic compounds such as halides, carboxylates or polyoxoanions, dissolved in (generally aqueous) solution can generate electrostatic stabilization. The adsorption of these compounds and their related counter ions on the metallic surface will generate an electrical double-layer around the particles (Fig. 1). The result is a coulombic repulsion between the particles. If the electric potential associated with the double layer is high enough, then the electrostatic repulsion will prevent particle aggregation [27,30]. 

Scheme 4 Hydrogenation of anisole by polyoxoanion-stabilized Rh(0) nanocluster catalyst...

In summary, partial hydrogenation of anisole to 1-methylcyclohexene by polyoxoanion-stabilized Rh(0) nanoclusters is very modest but could in the future be an interesting additive study in the monocyclic arene hydrogenation research area. 

Finke s system discovered in 1994 represents a special subclass of transition metal nanocluster [167,288,289]. A zerovalent transition metal core consisting of, e.g., 300-900 atoms is doubly stabilized by a cationic surfactant (e.g., and by an assembly of polyoxoanions... 

Electrosteric stabilization can be also obtained from the couple ammonium (Bu4N+)/polyoxoanion (INWnNb C>62)- The significant steric repulsion of the bulky Bu4N+ countercation, when associated with the highly charged polyoxo-anion (coulombic repulsion), provides efficient electrosterical stability towards agglomeration in solution of the resultant nanocatalysts [2, 5, 6]. 

An alternative approach to stabilize nanoparticles is to use polyoxoanions (see Scheme 9.5). Finke and coworkers described polyoxoanion- and ammonium-stabilized rhodium zerovalent nanoclusters for the hydrogenation of classical benzene compounds [95, 108]. This organometallic approach allows reproducible preparation of stable nanoparticles starting from a well-defined complex in terms of composition and structure (see Section 9.3.5). 

The polyoxoanion-stabilized Rh(0) nanoclusters were investigated in anisole hydrogenation [6,95]. The catalytic reactions were carried out in a single phase... 

Lin, Y. and Finke, R.G., Novel polyoxoanion- and Bu4N+-Stabilized, Isolable, and Redissolvable, 20-30 nm Ir300 900 nanoclusters the kinetically controlled synthesis, characterization, and mechanism of formation of organic solvent-soluble, reproducible size, and reproducible catalytic activity metal... 

Aiken, III, J.D. and Finke, R.G., Nanocluster formation synthetic, kinetic, and mechanistic studies. The detection of, and then methods to avoid, hydrogen mass-transfer limitations in the synthesis of polyoxoanion- and tetrabutylammonium-stabilized, near-monodisperse 40 6 A Rh(0) nanoclusters, J. Am. Chem. Soc., 120, 9545, 1998. 

Using the well-defined system of polyoxoanion/Bu4N -stabilized iridium nanoparticles [9, 29] as a model for the studies, Finke and coworkers [30] proposed a method that attempted to explain the formation and growth of transition-metal nanoparticles. This indirect method is based on an autocatalytic mechanism that considers a nudcation step in which a precursor A is converted to a zero-valent nuclei B with a rate constant fej, and a second step that considers the autocatalytic surface growth of the metal nanoparticles where species B catalyzes its own formation with a rate constant tc2 (Scheme 15.5). 

Moreover, the effects of other added substances such as water, acetic acid, olefin, polyoxoanion stabilizer [Bu4N]9P2WisNb3062, temperature and hydrogen pressure were also investigated for the formation of iridium nanoclusters. 

In fact, partial hydrogenations are rarely described with soluble nanoparticle catalysts. Two examples are explained in the Uterature, one reported by Finke and coworkers in the hydrogenation of anisole with polyoxoanion-stabilized Rh(0) nanoclusters [26] and one reported by Dupont and coworkers in the hydrogenation of benzene with nanoscale ruthenium catalysts in room temperature imidazolium ionic Uquids [69]. In these two cases, the yields are very modest. 

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