Metal-templated systems

Perhaps the simplest way to begin understanding some synthetic self-assembled systems is to look at assemblies that contain regular arrays of templating metal ions. Metal-templated systems have the potential to be more predictable than... 

Somebody will have to check that I m not making all this up" was one of Ron Breslow s (Columbia) comments during his high-speed presentation on a novel catalytic process he has developed. The remark was justified by his report of catalytic turnover numbers in the order of 10 2 Specific 9-chlorination of a steroid nicotinate ester (1) in >90% yield in 5 min. was achieved using metal/template systems like 2 to direct the reaction. 

Apart from this one-reaction type, the routine use of metal template procedures for obtaining a wide range of macrocyclic systems stems from 1960 when Curtis discovered a template reaction for obtaining an isomeric pair of Ni(n) macrocyclic complexes (Curtis, 1960). Details of this reaction are discussed later in this chapter. The template synthesis of these complexes marked the beginning of renewed interest in macrocyclic ligand chemistry which continues to the present day. 

Template Polymers. Template effects in chelating polymers constitute an interesting development in the field of metal containing polymers. The Template effects are interpreted by the fact that the small molecule is templating a pattern in the macromolecule which can be recognized by the same molecule in a subsequent process. The idea is to prepare a polymer from the metal-chelated monomer, to remove the metal ion, and then to measure the selectivity of the prepared polymer for the metal ion of the template [36]. Typical examples of template systems are 4-vinyl-4 -methylbipyridine (Neckers [36]) and 1-vinyl-imidazole (Tsuchida [37]). These are polymerized in presence of divinylbenzene [36] and appropriate metal salts (Co2+, Cu2+, Ni2+, Zn2+). The template metal ions are removed by acid leaching and the polymer subsequently used for metal ion absorption studies (Fig. 16). 

The complexes of octaphenyltetraazaporphyrin were obtained by heating (270-280°C) diphenylmaleinodinitrile and metal chlorides (MC1 n = 3, 4 M = Ga, In, V, Lu, Sn, Ti, Zr) [335], We note that the application of template synthesis allowed us to obtain the vitamin B12 and a series of other metal-corrinoide systems [318,336], The template syntheses found application to prepare ICC of Schiff bases and their analogues with coordination units MN4 (3.156) [337-339] ... 

The above mechanism is novel in that it does not require the interaction of a carbonyl moiety with the metal center. Neither a ketone/Ru complex nor a Ru alkoxide is involved in the mechanism, and the alcohol forms directly from the ketone. This non-classical mechanism also explains the high functional selectivity for the C=0 group. When the chiral molecular surface of the Ru hydride recognizes the difference of ketone enantiofaces, asymmetric hydrogenation is achieved. This is different from the earlier BINAP Ru chemistry where the enantio-face differentiation is made within the chiral metal template with the assistance of heteroatom/metal coordination. Similar heterolyses of H2 ligands have been shown by Morris and others (92) to be the critical step in the mechanism of reaction processes related to the Noyori systems. 

An analogous system involved the condensation of pentane-2,4-dione (127) with 6,6 -6w(jV-methylhydrazino)-2,2 -dipyridine (126) in the presence of nickel(II) to generate the thirteen-membered complex 128. Treatment of the complex with sodium cyanide does not release the ligand, nor does the complex form when copper is used as the metal template U0). 

At present, the known catenanes can be divided into two categories - those prepared by metal template synthesis and those synthesised in the absence of a metalion influence. A considerable number of catenanes of the first type have been prepared by Sauvage et al. as well as by a number of other workers. However, discussion on these important metal-ion-directed systems is deferred to the next chapter in which particular supramolecular assemblies produced by metal-ion-controlled procedures are discussed. 

Reports of double-helical complexes have appeared in the literature since the sixties. Despite the early interest, it is only more recently that emphasis has been given to the use of metal template synthesis for obtaining a wide range of doubly-and triply-stranded systems. In part, this attention has had its origins in an early report by Lehn et al. in which the spontaneous assembly of a dicopper(I)-containing double helix was described. 

One final example of a cryptand-like expanded porphyrin is the niobium(IV) bicyclophthalocyaninato system 9.132 reported by Gingl and Strahle in 1990. Interestingly, 9.132 was isolated as a by-product of a standard phthalocyanine-form-ing reaction (i.e., metal-templated cyclocondensation of phthalonitrile 9.102) in which NbOCls was used as the catalyst (Scheme 9.2.10). As such, it represents the fourth type of system to be prepared using this type of procedure (the other three being the parent phthalocyanines, the subphthalocyanines discussed in Chapter 2, and the superphthaolcyanines discussed in Section 9.2.1 of this chapter). 

In spite of their highly open structures, both materials may be synthesized without structure directing agents. Since most transition metal phosphates containing template molecules typically cannot be calcined without pore collapse, template-free synthesis appears to be the most promising direction for the formation of new porous materials. The exciting properties observed in the VSB family are likely glimpses of what may occur in other transition metal phosphate systems once thermal stability problems are overcome. 

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