Organometallic immobilized

The first example of purification by selective labelling was described in 1968 for a peptide that was s)mthesized in solution (2). In 1976, Merrifield and co-workers (3) applied this concept to SPPS and demonstrated that peptides bearing an intentionally introduced Cys-Met N-terminal dipeptide can be purified by absorption onto an organometallic immobilized matrix. The native sequence lacking Cys-Met was recovered through a CNBr-mediated Met-X specific cleavage. 

As outlined above, immobilization in a fluorinated liquid phase demands the functionahzation of the ligand with perfluoroalkyl chains and, even then, the solubihty is strongly influenced by the nature of the complex. Ionic hquids of the alkylmethyhmidazolium type (Fig. 4) have been recently developed as alternative solvents for organometallic catalysis and have the practical advantage of using directly the commercially available chiral hgands and complexes. 

The above example outlines a general problem in immobilized molecular catalysts - multiple types of sites are often produced. To this end, we are developing techniques to prepare well-defined immobilized organometallic catalysts on silica supports with isolated catalytic sites (7). Our new strategy is demonstrated by creation of isolated titanium complexes on a mesoporous silica support. These new materials are characterized in detail and their catalytic properties in test reactions (polymerization of ethylene) indicate improved catalytic performance over supported catalysts prepared via conventional means (8). The generality of this catalyst design approach is discussed and additional immobilized metal complex catalysts are considered. 

During the catalytic cycle, surface intermediates include both the starting compounds and the surface metal atoms. This working site is a kind of supramolecule that has organometallic character, and, one hopes, the rules of the organometallic chemistry can be valid for this supramolecule. The synthesis of molecular models of these supramolecules makes it possible to study the elementary steps of the heterogeneous catalysis at a molecular level. Besides similarities there are, of course, also differences between the reactivity of a molecular species in solution and an immobilized species. For example, bimo-lecular pathways on surfaces are usually prohibited. 

We consider, primarily, events in solids since most e.s.r. studies have been carried out on radicals trapped in solids. Only relatively persistent organometallic radicals have been studied by liquid-phase e.s.r. with in situ radiolysis, because of the technical difficulties involved. In most solid systems at low temperature radical centres are physically trapped in the rigid matrix and hence can be studied by e.s.r. without difficulty. However, although radicals as such may be immobile, this does not necessarily apply to electron-gain or -loss centres, particularly if these are charged, since electron-transfer may be facile. 

Chemoselectivity describes the preferred formation of one out of several products due to the selective interaction of a reagent with the substrate. In electroorganic conversion, the electrode is the reagent that can influence the reaction course in several ways The electrode material can form immobilized organometallics or oxides that can shift the conversion like mediators from an outer sphere electron transfer to a more selective inner sphere electron transfer [134]. Overvoltages can suppress the hydrogen evolution in cathodic reduction... 

The method of catalyst immobilization is one for the reasons for the success of the SAPC approach. Rather than covalently linking an organometallic complex to a support—which usually leads to loss of catalytic efficiency and leaching of the metal—it is the catalyst-philic phase that is immobilized. 

One essential aspect of the pubhcly funded project Smart Solvents/Smart Ligands was the development of new and useable techniques for the isolation and reuse of organometallic catalysts. The work of Leitner and Dinjus shows various possibilities for the immobilization of modified and conventional catalytic systems. 

The same authors compared catalysts prepared from these precursors and [Ru(BINAP)Cl2]2 adsorbed on MCM-41 (with 26 and 37 A pores) and an amorphous mesoporous silica (with 68 A pores) all treated with combinations of SiPh2Cl2 and Si(CH2)3X (X = NH2, CO2H). Catalysts were also prepared in which the organometallic precursors were immobilized by entrapment into silica (using sol-gel techniques). This is one of the few studies in which the performance of chiral phosphine catalysts immobilized by covalent and noncovalent procedures are compared directly. The materials were examined as catalysts for the hydrogenation of sodium a-acetamidocinnamate and of a-acetamidocinnamic acid under similar conditions to those used for the catalysts on unmodified MCM-41. The catalysts... 

A variant on the ship-in-a-bottle approaches described above is to react a preformed catalyst with the zeolite framework. This is analogous to procedures developed for immobilizing organometallic and coordination compounds on surfaces, but occurs within zeolite pores. These compounds that have reacted with the external surface must be removed, along with those adsorbed on the surface, to produce a true single-site catalyst. Two examples, illustrating different synthetic approaches, are discussed below. 

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