Metal co-ordination

The introduction of the metal precursor into the CFP can occur upon metal co-ordination or ion exchange (Scheme 2). 

Oxygenated Bronzes , P. Hagenmuller, Progr. Solid State Chem., 1971, 5, 71. The Photochemistry of Transition-metal Co-ordination Compounds , W. L. 

Figure 1-2. Some examples of transition metal co-ordination compounds. In each case, the metal changes co-ordination number, but not oxidation state.

The design of polydentate ligands containing imines has exercised many minds over many years, and imine formation is probably one of the commonest reactions in the synthetic co-ordination chemist s arsenal. Once again, the chelate effect plays an important role in stabilising the co-ordinated products and the majority of imine ligands contain other donor atoms that are also co-ordinated to the metal centre. The above brief discussion of imine formation will have shown that the formation of the imine from amine and carbonyl may be an intra- or intermolecular process. In many cases, the detailed mechanism of the imine formation reaction is not fully understood. In particular, it is not always clear whether the nucleophile is metal-co-ordinated amine or amide. Some intramolecular imine formation reactions at cobalt(m) are known to proceed through amido intermediates. A particularly useful intermediate (5.24) in metal-directed amino acid chemistry is... 

It is very often extremely difficult to demonstrate that a metal-co-ordinated hydroxide ion is involved in a particular reaction. Studies of kinetic behaviour provide one of the most powerful tools for the determination of reaction mechanisms. It is not, however, always easy to distinguish between intra- and intermolecular attack of water or hydroxide. The most unambiguous studies have been made with non-labile cobalt(m) complexes, and we will open this discussion with these compounds. 

In the preceding section we discussed the use of co-ordinated hydroxide as an intramolecular nucleophile. It could also act as a nucleophile to an external electrophile. Over the past few decades, there has been considerable interest in the nucleophilic properties of metal-bound hydroxide ligands. One of the principal reasons for this relates to the widespread occurrence of Lewis acidic metals at the active site of hydrolytic enzymes. There has been a lively discussion over the past thirty years on the relative merits of mechanisms involving nucleophilic attack by metal-co-ordinated hydroxide upon a substrate or attack by external hydroxide upon metal-co-ordinated substrate. As we have shown above, both of these mechanisms are possible with non-labile model systems. 

As an example, Fig. 5.6 depicts a typical diffraction spectrum. It is evident that long range order does not exist in our chalcogenide samples. However, the broad difffactrogram peak centered at 20 = 42.5° has the characteristic of a nanodivided ruthenium metal [22]. This points out that the active center in this chalcogenide materials is essentially of metallic nature. The material, either in powder or colloidal form, was analyzed by the EXAFS technique [11]. The local range order of this technique allowed for some structural determination of our samples. Thus, for example, the co-ordination distances for ruthenium-selenium and ruthenium-ruthenium are R(RU-se) = 2.43 A y R(ru.rU) = 2.64 A, respectively. The metal-metal co-ordination distance is of the same order of magnitude as that of well known cluster based materials such as the Chevrel phase [35, 37], cf. Fig. 5.2b. This testifies that the used chemical route leads to the formation of cluster-like materials. 

Three general methods have been used to enhance the nucleofugacity (i.e., leaving group ability) of sulfur metal co-ordination, oxidation, and alkylation. 

It must be emphasised that self-assembly is very far from a unique feature of supramolecular systems - it is ubiquitous throughout life chemistry. Biological systems aside, self-assembly is also commonplace throughout chemistry. The growth of crystals, the formation of liquid crystals, the spontaneous generation of synthetic lipid bilayers, the synthesis of metal co-ordination complexes, and the alignment of molecules on existing surfaces are but a few of the many manifestations of self-assembly in chemical systems. 

For the preparation of the required 1 2 [Ru ligand 47] intermediate, a pro-tection-deprotection strategy was employed. This involved reaction of 47 with trimethyloxonium tetrafluoroborate to block one metal co-ordination site in 47 to yield 48. After co-ordination of one ruthenium ion, the remaining site was unblocked by reaction with DABCO in refluxing acetonitrile. Reaction of this 1 2 (metal ligand) product with a second metal ion was then carried out. 

However, most of these difficulties should not arise in the case of template-monomer complexes formed with very high association constants. Consequently a number of laboratories, including our own, are currently involved in the design of new polymerisable monomers which can form strong complexes with some of the common structural motifs possessed by many template molecules [5-8], (See also Chapter 4 for further examples.) The use of transition metal co-ordination complexes as templates in imprinting is another particularly promising approach (see Chapter 6 for more detail). 

A novel class of nucleic acid mimics has been described which possess two ethylenediamine moieties for intermolecular metal co-ordination (25). In the presence of Zn + ions and template DNA, the analogues (25) form relatively stable structures, stabilised by the co-ordination of adjacent chelating moieties with zinc ions. It was shown that with an oligothymidine template and the adenine derivative of (25) that a 2 1 complex was formed, which showed a biphasic melting transition. Short RNA duplexes (3-4 bp) are considerably stabilised if both termini of the duplexes are bridged by non-nucleotidic linkers. For example, the pairing of rGAA with rUUC in such a cyclic system exhibits a Tm of 36°C in IM salt solution. 

The mutual influence of ligands in transition-metal co-ordination compounds with multiple metal-ligand bonds (E. M. Shustoroyich, M. A. Porai-Koshits, and Y. A. Buslaev, Co-ordination Chem. Rev., 1975,17, 1)l... 

Steric control of metal co-ordination (D. C. Bradley, Chem. in Britain, 1975,11, 393). 

In some structures, both (2) and (3) can occur together. Nedocromil sodium trihydrate, for example, shows both metal co-ordinated water and channel water. 

In many ascorbate salts, the combined effects of metal co-ordination and hydrogen-bonding apparently cause the C-6-OH to adopt a synclinal (or gauche) position with respect to the C-5-OH. 

Distribution may be affected by unresolved metal co-ordination number effects... 

Interest continues in the use of transition-metal co-ordination complexes, e.g. VO(acac)2Cl, as photoinitiators for vinyl polymerization. Note that the patent literature relating to photopolymerization is covered in Appendix Al. 

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