Covalent, Metal-Based Assembly

Ideally, from a macromolecular subunit position control perspective, directed approaches toward network construction are desirable. Indeed, the construction of dendrimers possessing metal-ligating, 4-substituted, 2,2 6, 2 -terpyridine moieties have been reported. 82,83  

Ligand incorporation was achieved by facile alkoxylation of 4-chloro-substituted ter-pyridine by a hydroxy-terminated carboxylic acid followed by divergent dendrimer con- 

Metal connectivity is advantageous. Transition metal characterization such as with electrochemical and microscopic analysis compliments standard organic -type methods (e.g., 13C and H NMR) and further enhances characterization of the large multidendri-tic structure(s). Cyclic voltammetry data strongly suggest the connection of branched structures.1821 

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Ion-dipole interactions also include coordinative bonds, which are mostly electrostatic in nature in the case of the interactions of nonpolarisable metal cations and hard bases. Coordinate (dative) bonds with a significant covalent component, as in [Ru(bpy)3]2+, are also often used in supramolecular assembly and, as we will see in Chapters 10 and 11, the distinction between supramolecular and molecular species can become rather blurred. 

Despite their solid appearance, within the gel the liquid component is mobile and is only held by capillary forces. The solid network can be either a covalent polymer or a supramolecular assembly of small molecules. The latter class of compound, termed low molecular weight gelators (LMWG) of which 14.11 14.15 are examples, is perhaps of most interest to supramolecular chemists. Perhaps the most well known gels are metal oxide based polymeric materials produced by the sol-gel process. The sol-gel process involves the hydrolysis and polycondensation of monomeric metal salts such as early transition... 

The covalent assembly of guest-encapsulating host species has long been an area of research interest. The first examples of host species binding their guests within a three-dimensional array of interactions were the class of compounds known as cryptands [26,27]. The cryptands were designed as hosts for alkali metal cations and are based on macrobicyclic-polyethers (Fig. 2). Typically, cryptands are synthesised by the addition of a diacyl-chloride to an azacrownether (Scheme 1). 

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