Dendritic metal-containing

One of the first results on the use of phosphine dendrimers in catalysis was reported by Dubois and co-workers [16]. They prepared dendritic architectures containing phosphorus branching points which can also serve as binding sites for metal salts. These terdentate phosphine-based dendrimers were used to incorporate cationic Pd centers in the presence of PPh3. Such cationic metalloden-dritic compounds were successfully applied as catalysts for the electrochemical reduction of C02 to CO (e.g. 9, Scheme 9) with reaction rates and selectivities comparable to those found for analogous monomeric palladium-phosphine model complexes suggesting that this catalysis did not involve cooperative effects of the different metal sites. 

The main emphasis has so far been placed on metal-containing catalysts, both in the field of dendritic catalysis and in organic catalysis in general. However, the current trend is increasingly towards catalysis with purely organic compounds. A number of example of such metal-free - and dendritic - catalysts,... 

The first of these octaalkylation reactions was reported as early as 1982, and led to a primitive dendritic core containing a metal-sandwich unit [36], Thus, like the hexafunctionalization, this reaction is very specific. Two hydrogen atoms of each methyl group are re-... 

The concept of having layers of reactive functionalities or sites in the interior of dendritic macromolecules has been elegantly demonstrated recently by Newkome s group.30 The synthesis of dendritic micelles, containing layers of triple bonds to which metal clusters can be attached,... 

Metal cluster complexes exhibit many interesting catalytic, optical, and other potentially useful properties for materials science applications, and many impressive advances have been made tvith respect to their synthesis [53-55]. The incorporation of these remarkable metal-containing units into the main chain of polymer structures clearly offers exciting possibilities [56], but this area is, in general, poorly developed. The follovring three examples of linear structures illustrate the growling interest and broad potential of this area. Dendritic structures that contain metal clusters are discussed in Chapter 8. 

A variety of other metal-containing building blocks have been used to create dendritic architecture with metals at interior sites. Porphyrins possess many of the advantages of polypyridyl complexes in terms of facile synthetic derivatization, and these have been used to construct a range of remarkable structures. For example, linear, four-directional complexes 8.38 have been prepared by means of metal-catalyzed coupling methodologies [74—76]. In addition, windmill-like lightharvesting porphyrin arrays 8.39 have been prepared [77, 78]. 

Finally, the dendrimer or star may be surface or internally functionalized or decorated with metal-containing motifs. This type of dendrimer is widely investigated with a view to preparing novel catalytic systems in which a dendritic effect results in enhanced or modified catalytic properties with respect to a similar number of mononuclear complexes. As many of these species are strictly organometaUic they do not fall properly within the scope of this volume. 

Almost 70 of the approximately 115 elements are transition metals. These are elements with electrons successively added to their d or f subshells and have both filled and unfilled orbitals suitable for bonding. They exhibit a variety of oxidation states and bond formations, such as sigma and pi bonds or coordination bonds as are found in typical chelate structures. These characteristics are the reason that transition metal-containing macromolecules exist in a wide variety of structures including linear, two-dimensional, three-dimensional, dendrite, and assemblies. Many of these polymers are often referred to as organometallic polymers. We have, somewhat arbitrarily, separated this class from coordination polymers, which have been covered in Volumes 1, 3, and 5. 

Vast arrays of metal-containing polymers have been produced that offer a wide variety of properties. Key milestones in the history of this diverse topic and a sense of its growth and importance were discussed in this chapter. While initial efforts focused on polysiloxanes, today s efforts are quite diverse and include the production of multisite catalysts, variable oxidation state materials, and smart materials where the precise structure can be changed through the introduction of different counterions. These polymers have been produced by all of the well-established polymerization methodologies. The metal atoms reside as part of the macromolecular backbone, in sidechains, coordinated to the backbone, and as integral parts of dendrites, stars, and rods. Truly, many of tomorrow s critically important materials will have metal atoms as an integral part of the polymer framework, which will allow the materials to function as demanded. 

Gravity segregation in polyphase systems is of even greater importance. If free dendrites form, their density relative to that of the unsolidified liquid will primarily determine the way in which they move. This can best be illustrated by considering the case of the tin-base bearing metals containing copper and antimony. During... 

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