Reactivity supported metal complexes

In the last three decades, we have designed and successfully prepared various supported metal complexes on oxide surfaces that exhibit unique catalytic activities and selectivities that are different from those of their homogeneous analogues [3,4,9, 12-15]. With the aid of several sophisticated spectroscopic techniques, the structures and roles of catalytically active species on surfaces have been characterized and identified [3, 4,9,12-25]. Chemical interactions between metal complexes and oxide surfaces can provide new reactivity of metal species by the construction of a spatially controlled reaction environment and the formation of unsaturated active metal species, leading to high catalytic activity, selectivity and durability [21-25]. 

The simplest supported catalysts are mononuclear metal complexes, exemplified by industrial supported metallocene catalysts, used (with promoters) for alkene polymerization these are the so-called single-site catalysts that are finding wide industrial applications (Kristen, 1999 Kaminsky, 1999 Roscoe et al., 1998). The most common supports are metal oxides and zeolites. The metals in these complexes range from oxophilic (e.g., Zr and Ta) to noble (e.g., Rh). Supported metal complexes are stabilized by ligands—in addition to those provided by the support—such as hydride (H), hydrocarbons, and carbonyl (CO). In a typical supported metal complex, the metal is present in a positive oxidation state. Although some such complexes are relatively stable, most are, befitting their roles as catalysts, highly reactive and air- and moisture-sensitive. 

In the following paragraphs, methods of preparation and characterization of structurally simple supported metal complexes are summarized, and examples are presented that illustrate characterization data and support general conclusions about structure, bonding, reactivity, and catalysis. 

Miessner (1994) showed that partial decarbonylation of this supported complex by treatment in H2 at temperatures of 200 to 250°C leads to complexes that are so highly reactive that they combine with N2 to give well-defined supported complexes with dinitrogen ligands. This remarkable reactivity suggests possibilities for new catalytic properties of these and related supported metal complexes. 

However, the fact that the supported metal complexes are site-isolated (and unable to react with each other) opens up possibilities for stabilization of coordinative unsaturation of the metals and new reactivities. 

Macroporous glycidyl methacrylate-ethylene glycol dimethacrylate (GMA-EGDM) copolymer beads were synthesised and characterised for pore volume and surface area. These reactive copolymers were derivatised with 2-picolyl amine and coordinated with chromium and vanadium ions. The peroxocomplexes of these supported metal complexes were generated by the addition of hydrogen peroxide / tertiary butyl hydroperoxide(tert. -BHP) and shown to catalyse a variety of oxidation reactions. 

There are several key areas that need to be developed in order to be able to prepare supported metal complexes more reliably and better understand their activity. Polymer supports are still expensive to purchase and in order for them to have more widespread use it will be necessary to develop more cost effective routes for their preparation. It is important to understand the role of the polymeric backbone in the catalytic activity of immobilized metal complexes and to realize that very different reactivities and selectivities can be found if, for example, the linker or the degree of cross-linking is altered. When using dendrimer-derived materials, particular attention needs to be focused on dendritic effects. 

Because Mo and W are usually more stable and more reactive than Re, the reactivity of the corresponding isoelectronic complexes of Mo, [( iO)Mo( NAr) (=CH Bu)(CH 2Bu)[ [70], and W, [( iO)W(=NAr)(=CH Bu)(CH)Bu)] [71], have been studied. Overall, these systems are more stable than the corresponding silica-supported Re complex, and display reachvities better than those of the well-known corresponding bis-aUcoxide homogeneous derivahves. The better performances of these systems compared to their homogeneous analogues are probably due to the optimized coordination of the metal center in combinahon with a site isolation of the... 

In this chapter, SOMC/M will be used to study the reactivity of organometallic complexes with the surface of supported metals. In 1984, Travers [31] and Margit-falvi [32] simultaneously described this application of SOMC for the preparation of bimetallic catalysts. 

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