Supramolecular coordinative bonding

Fig. 34. Compound 129 assembles to a supramolecular structure through coordinative bonds between the cobalt(III) ions and the pyridine residues of the bridging B(py)(OMe) group...

In this review, CPOs constructed by covalent bonds are mainly focused on however, stable coordination bonds comparable to the stability of the covalent bonds have potential for future enhanced molecular design of novel CPOs. One representative is the bond between pyridine-type nitrogen and metal, which is widely used in supramolecular chemistry, that is, the cyclic supramolecular formation reaction between pyridine-substituted porphyrin and metal salts (Fig. 6d) [27,28]. Palladium salts are frequently used as the metal salts. From the viewpoint of the hard and soft acid and base theory (HSAB), this N-Pd coordination bond is a well-balanced combination, because the bonds between nitrogen and other group X metals, N-Ni and Ni-Pt coordination bonds, are too weak and too strong to obtain the desired CPOs, respectively. For the former, the supramolecular architectures tend to dissociate into pieces in the solution state, and for the latter. 

The silver(I) complexes with the tetrakis(methylthio)tetrathiafulvalene ligand have been reported, the nitrate salt presents a 3D structure with an unprecedented 4.16-net porous inorganic layer of silver nitrate,1160 the triflate salt presents a two interwoven polymeric chain structure.1161 The latter behaves as a semiconductor when doped with iodine. With a similar ligand, 2,5-bis-(5,5,-bis(methylthio)-l,3,-dithiol-2 -ylidene)-l,3,4,6-tetrathiapentalene, a 3D supramolecular network is constructed via coordination bonds and S"-S contacts. The iodine-doped compound is highly conductive.1162 (Methylthio)methyl-substituted calix[4]arenes have been used as silver-selective chemically modified field effect transistors and as potential extractants for Ag1.1163,1164... 

The network structures to be discussed will all involved hydrogen bonding as the supramolecular synthon. It should be noted however that other interactions such as coordinate bonds and host-guest interactions may also organise host molecules into network structures. Coordination polymers constructed from molecular hosts may involve functionalised calixarenes [8-11], cyclotriveratrylene [12], or cucurbituril [13]. Calixarenes have also been used to build up network structures via host-guest interactions [14,15]. It is also notable that volatile species may be trapped within the solid state lattice of calix[4] arene with a structure entirely composed of van der Waals interactions [16]. 

An obvious possibility in the design of supramolecular anion receptors is the incorporation of a metal center in with which the anion may form a coordinate bond. Some examples of this type (e.g. 36) have already been mentioned. This section discusses a number of other metal complexes capable of multipoint molecular recognition based upon binding anionic ligands to a metal center along with the incorporation of other stabilizing interactions. 

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. 

This concept may also be extended to polynuclear helicates [38]. When 2-amino-quinoline and 4-chloroaniline were mixed with the phenanthroline dialdehyde shown in Scheme 1.10, a dynamic library of potential ligands was observed to form. The addition of copper(I) causes this library to collapse, generating only dicopper and tricopper helicates. As in the mononuclear case of Scheme 1.9, the driving force behind this selectivity appeared to be the formation of structures in which all ligand and metal valences are satisfied. The use of supramolecular (coordination) chemistry to drive the covalent reconfiguration of intraligand bonds thus... 

Hydrogen-bonded supramolecular capsule I compared with coordination capsule II. The external aliphatic groups are omitted for clarity. Note that a pair of intermolecular O—H O hydrogen bonds is replaced by four square-planar Cu-O coordination bonds in the metal-ion insertion process that generates the isostructural inorganic analog. From R. M. McKinley, G V. C. Cave and J. L. Atwood, Proc. Nat. Acad. Sci. 102, 5944-8 (2005). 

The selection of one or more components occurs as function of either internal (the nature and the geometry of the binding subunits, the stoichiometry, etc.) or external factors (nature of the solvent, the presence of specific molecules or ions, etc.). In view of the lability of the reversible molecular and supramolecular interactions (H-bonding, van der Waals, coordinative bonds, etc.) the self-assembly processes may present a number of novel features such as cooperativity, diversity, selection, or adaptation. 

Coordinate bonding is another type of direction-specific interaction. This type of interaction occurs between metal ions and electron-rich atoms and is of moderate strength. Such interactions have also been utihzed in the formation of supramolecular assemblies, and several examples are given in Chap. 3. 

Hydrogen bonds and metal coordination bonds have been the mainstay of the supramolecular chemist when it comes to the intentional manipulation of noncovalent interactions. However, nature s examples teach us the power of using weak, non-directional interactions to stabilize the folded state and deepen the folding funnel. Thus, van der Waals and solvophobic interactions are crucial to foldamer design, although they have been scarcely utilized in synthetic systems. In the simplest sense, such input is realized by incorporating amphiphilic character into the chain. 

In a third approach, supramolecular polymers are based on the reversibility of metal-coordination bonding. These polymers are the closest analogues to conventional macromolecules, because most of the polymers disclosed make use of strong bonding,16 in which the reversibility can be tuned by chemical means only. However, appropriate choice of the metal ion can give rise to bonding that resembles that of the other two approaches. The DP of the polymers in the case of the coordination polymers is similar to that of the condensation polymers, and achieving exact stoichiometry is of distinct importance here. 

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