Metal-ligand polygons

Metal polygons with pairs of doubly-bridging ligands on each edge occur with three, four, six and eight metals (Sections 4.3.2, 4.4.1.9, 4.6.1.1 and 4.8.1, respectively). 

However, herein we focus on the discrete compounds, especially the three-dimensional cages or container molecules [6-10], formed mainly by coordination of metal ions with organic ligands. On the other hand, there also exist three-dimensional compounds where the stability is partly due to hydrogen bonds, 7t-7i interactions, Van der Waals forces or dipole-dipole interactions. Beside those three-dimensional compounds, one-dimensional structures like, for instance, helicates [26-30] and two-dimensional ones, the polygons, are also known [31-36]. 

Fig. 11 Molecular panelling , the construction of 3D shapes via the use of polygonal ligands and semi-protected metals, after Fujita et al. [61]...

Among the various polygonal assemblies that inorganic architectures can adopt, molecular squares are the most common. This fact can be attributed to the tendency of transition metals to form cis complexes with L-M-L bond angles of 90° and to the widespread availability of linear-bridging ligands. In this vein, Stang and coworkers... 

The examples presented in this section illustrate an elegant approach by Stang and coworkers to the design of chiral supramolecular polygons (squares, cages) and also chiral three-dimensional polyhedrons. The combination of tailored ligand connectors with the metal corners derived from the BINAP framework should allow the preparation of a variety of different supramolecular chiral objects. 

The use of multi-bridging ligands or of metals that have three or more vacant coordination sites can bring about the assembly of larger structures that are polyhedral in nature, rather than polygonal. These structures are discussed in more detail in Section 3.5. 

In such instances, the [ +2] self-assembly is modified to an [2m+m+2] assembly. For each Lewis-basic site of the n-sided panel ligand, there will be a metal acceptor. Since there are two such n-sided polygons per prism, found at each end, 2n metal acceptors are required. For each pair of metal acceptors, a single linear donor will act as the lengthwise edge of the prism (Fig. 8). 

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