Colloidal Organometallic Catalysts

As mentioned in the previous section, there are good reasons to search for new reaction conditions for Heck and related reactions, which permit catalyst recovery, the use of less toxic solvents, and simpler product recovery. The use of liquid or supercritical (SC) CO2 addresses all of these issues [171]. Until recently, however, the use of supercritical COj had been limited to organometallic Pd complexes functionalized with perfluorinated ligands [172-174], due to the limited solubility of metal colloids in CO2, and often required the use of water as a co-solvent [175]. The work described here shows that dendrimers can be used to solubilize Pd nanoclusters in liquid and SC CO2. This new finding opens the door to the combined benefits of a catalyst that promotes Heck couplings, but without the need for toxic ligands or solvents. 

A preparation and characterization of new PtRu alloy colloids that are suitable as precursors for fuel-cell catalysts have been reported [43cj. This new method uses an organometallic compound both for reduction and as colloid stabilizer leading to a Pt/Ru colloid with lipophilic surfactant stabilizers that can easily be modified to demonstrate hydrophilic properties. The surfactant shell is removed prior to electrochemical measurements by reactive annealing in O2 and H2. This colloid was found to have nearly identical electrocatalytic activity to several other recently developed Pt/Ru colloids as well as commercially available Pt/Ru catalysts. This demonstrates the potential for the development of colloid precursors for bimetallic catalysts especially when considering the ease of manipulating the alloy composition when using these methods. 

In addition to the synthesis of specific ligands and the resulting catalysts new ideas and concepts begin to combine with transition metal chemistry to open up new areas in homogeneous catalysis, like catalysis under supercritical conditions, colloidal catalysis , organometallic electrocatalysis and multi-metallic catalysis " ( cooperative catalysis ). 

Despite their catalytic (preparative) efficiency similar colloidal systems will be only occasionally included into the present description of aqueous organometallic catalysis although it should be kept in mind that in aqueous systems they can be formed easily. Catalysis by colloids is a fast growing, important field in its own right, and special interest is turned recently to nanosized colloidal catalysts [62-64], This, however, is outside the scope of this book. 

Bonnemann H et al 1996 Nanoscale colloidal metals and alloys stabilized by solvents and surfactants preparation and use as catalyst precursors J. Organometall. Chem. 520 143... 

The mechanism for this ostensibly homogeneous process, the Chalk-Harrod mechanism, [264] was based on classical organometallic synthetic and mechanistic research. Its foundation lies in the oxidative addition of the silane Si-H bond to the low oxidation state metal complex catalyst, a reaction which is well established in the organometallic literature. Lewis reported in 1986 that the catalyti-cally active solutions contained small (2.0 nm) platinum particles, and demonstrated that the most active catalyst in the system was in fact the colloidal metal. [60, 265] Subsequent studies established the relative order of catalytic activity for several precious metals to be platinum > rhodium > ruthenium = iridium > osmium. [266] In addition, a dependence of the rate on colloid particle morphology for a rhodium colloid was observed. [267]... 

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