Rhodium nanoclusters, stabilization

A rather effective catalyst proved to be the system of rhodium nanoclusters stabilized with pol5winylpirrolidone (PVP) and supported on finely dispersed oxides (gamma-alumina, silica, or titania) and modified with Cnd (Huang et al. ). With this system EtPy was hydrogenated at 25 C and 70 bar hydrogen in THE with a TOP of 58.6 min and an ee of 65.4%. 

Figure 12. The relationship between the logarithm of the relative hydrogenation rate over CFP-supported rhodium nanoclusters, with respect to the polymer-stabilized nanostructured catalyst, for a number of a number of alkenes, as a function of their affinity to the support (expressed as the square difference of the solubility parameter of the support and of the substrate). (Reprinted from Ref [33], 1991, with permission from the American Chemical Society.)...

The asymmetric hydrogenation of prochiral ketones is often an important step in the industrial synthesis of fine and pharmaceutical products. Several noble metal nanoparticles have been investigated for asymmetric catalysis of prochiral substrates but platinum colloids have been the most widely studied and relevant enantiomeric excesses have been reported (>95%). Nevertheless, the enantioselec-tive hydrogenation of ethyl pyruvate catalyzed by PVP-stabilized rhodium nanocluster modified by cinchonidine and quinine was reported by Li and coworkers (Scheme 11.7) [68]. 

X-D M, J-Q M, li Z-C, Kou Y (2005) Rhodium nanoparticles stabilized by ionic copolymers in ionic liquids Iraig lifetime nanocluster catalysts for benzene hydrogenation. J Am Chem Soc 127 9694-9695... 

The most spectacular results, in terms of comparison between CFPs- and carbon-supported metal catalysts, were likely provided by Toshima and co-workers [33,34]. As illustrated in Section 3.3.3, they were able to produce platinum and rhodium catalysts by the covalent immobilization of pre-formed, stabilized metal nanoclusters into an amine functionalized acrylamide gel (Scheme 5). To this purpose, the metal nanopartides were stabilized by a linear co-polymer of MMA and VPYR. The reaction between its ester functions and the amine groups of the gel produced the covalent link between the support and the... 

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). 

FIGURE 7.3 Plot of mole H2/mole AB versus time (min) for the hydrolysis of lOmL lOOmM (30.9mg) H3NBH3 catalyzed by in situ formed zeohte framework stabilized rhodium(O) nanoclusters (36mg Rh +-Y with a rhodium content of 1.2 wt%, [Rh] = 0.2mM) at various temperatures [157]. 

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