Multidentate ligands, metal coordination

Two reviews5,6 with approximately 34 references discuss a coordination approach to three-dimensional assemblies via molecular paneling. Families of planar exo-multidentate organic ligands (molecular panels) assemble into large three-dimensional assemblies through metal-coordination. In particular, m-protected square planar metals, (en)Pd2+ or (en)Pt2+ (en = ethylene-1,2-diamine), are very useful to panel the molecules. Metal-assembled cages, bowls, tubes, capsules, and polyhedra are efficiently constructed by this approach. 

Alternatively, bi- or multidentate ligands can also be used for support. As an additional benefit, the latter offer greater stability for the coordinatively bound metal center against leaching through ligand dissociation and substitution reactions. The first, somewhat remarkable, approach to this is shown in Figure 42.11, based on numerous examples of the support of bidentate phosphines on polymers [1-5]. 

Solid evidence for this type of chain mechanism (differing only in the chain propagation steps) had been earlier obtained by Margerum and his co-workers jjj study of coordination chain reactions between two metal complexes each containing one multidentate ligand, edta, trien and so on, e.g.. 

It has been tacitally assumed in this discussion that the second-order formation rate constants measure the simple water substitution process. Although this must apply when unidentate ligands replace coordinated water, a composite process could describe the replacement by multidentate ligands. However, consideration of rate constants for successive formation and dissociation processes suggests that the overall rate of complex formation with flexible bidentate (and probably multidentate) ligands such as diamines, dipyridyl, glycine is probably determined by the rate of expulsion of the first water molecule from the metal aqua ion (56, 80, cf. 3 and 84). 

In the first step, a substrate coordinates to a metal catalyst and forms an intermediate mixed complex (LMS in Scheme 13). The substrate is then activated by metal ions and dissociates from the catalyst. The complex catalyst, having accomplished its purpose, is regenerated to the original complex. The catalytic action of a metal ion depends substantially on the nature of the ligands in the intermediate mixed complex. Certain ligands induce an increase in catalytic activity, while others, e.g. multidentate ligands such as ethylenediaminetetracetic acid, inhibit the catalytic action of a metal ion. Therefore, if a polymer ligand is used as one component of a metal-complex catalyst, its properties may affect the catalytic action of the metal ion. 

Sargeson and his coworkers have developed an area of cobalt(III) coordination chemistry which has enabled the synthesis of complicated multidentate ligands directly around the metal. The basis for all of this chemistry is the high stability of cobalt(III) ammine complexes towards dissociation. Consequently, a coordinated ammonia molecule can be deprotonated with base to produce a coordinated amine anion (or amide anion) which functions as a powerful nucleophile. Such a species can attack carbonyl groups, either in intramolecular or intermolecular processes. Similar reactions can be performed by coordinated primary or secondary amines after deprotonation. The resulting imines coordinated to cobalt(III) show unusually high stability towards hydrolysis, but are reactive towards carbon nucleophiles. While the cobalt(III) ion produces some iminium character, it occupies the normal site of protonation and is attached to the nitrogen atom by a kinetically inert bond, and thus resists hydrolysis. 

Consider a metal complex CM, where C is a multidentate ligand that leaves an empty coordination position on the metal, in the presence of a monodentate ligand L. CM could also be a metalloenzyme interacting with a substrate or an inhibitor L. The paramagnetic effects observed on a nucleus of L can then be used to obtain information on its dissociation constant ... 

The study of supramolecular complexes of metal cations is really nothing more than the coordination chemistry of relatively labile (i.e. ligand substitution is relatively rapid under ambient conditions) metal ions and relatively elaborate, usually chelating or multidentate ligands (see Section 1.5) T It is therefore worth spending a little time reviewing some basics of coordination chemistry before looking at specific supramolecular systems. Experts read no further ... 

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