Supramolecular entities

A particular point of interest included in these hehcal complexes concerns the chirality. The heUcates obtained from the achiral strands are a racemic mixture of left- and right-handed double heUces (Fig. 34) (202). This special mode of recognition where homochiral supramolecular entities, as a consequence of homochiral self-recognition, result from racemic components is known as optical self-resolution (203). It appears in certain cases from racemic solutions or melts (spontaneous resolution) and is often quoted as one of the possible sources of optical resolution in the biological world. On the other hand, the more commonly found process of heterochiral self-recognition gives rise to a racemic supramolecular assembly of enantio pairs (204). 

The mechanism of recognition of most supramolecular entities (such as abiotic receptors) is the formation of several hydrogen bonds. Since heterocyclic tautomers possess both strong HBA and HBD properties (see Sections III,G, V,D,2, and VI,G), they are often used for this purpose. For instance, the hydrogen bond network formed by 5,5 -linked bis(2-pyridones) has been used by Dickert to obtain sensors (96BBG1312). 

Methods to determine and control the properties of individual surfactant molecules and to determine the conditions needed to produce well-defined molecular assemblies are just beginning to emerge. We are at the threshold of being able to produce dehberately stmctured supramolecular entities with properties tailored to meet special applications. Some additional examples of problems that will have significant impacts over the next one or two decades follow ... 

Supramolecular Entities of Trinuclear Cold(l) Complexes Sandwiching Small Organic Acids... 

Rawashdeh-Omary, M.A., Omary, M.A., Fackler, J.P. Jr, Galassi, R., Pietroni, B.R. and Burini, A. (2001) Chemistry and optoelectronic properties of stacked supramolecular entities of trinuclear Gold(I) complexes sandwiching small organic acids. Journal of the American Chemical Society, 123, 9689. 

The noncovalent interactions or van der Waals forces involved in supramolecular entities may be a combination of several interactions, e.g., ion-pairing, hydrophobic, hydrogen bonding, cation-Tr, tt-tt interactions, etc. They comprise interactions between permanent multipoles, between a permanent multipole and an induced multipole, and between a time-variable multipole and an induced multipole. 

Dunitz JD (1996) The crystal as a supramolecular entity. In Desiraju GR (ed) Perspectives in supramolecular chemistry. Wiley, Chichester... 

The species and properties defining a given level of complexity result from and may be explained on the basis of the species belonging to the level below and of their multibody interaction, e.g. supramolecular entities in terms of molecules, cells in terms of supramolecular entities, tissues in terms of cells, organisms in terms of tissues and so on up to the complexity of behavior of societies and ecosystems. For example, in the self-assembly of a virus shell, local information in the subunits is sufficient to tell the proteins where to bind in order to generate the final polypro-teinic association, thus going up a step in complexity from the molecular unit to the... 

Dunitz, J. D. In Perspectives in Supramolecular Chemistry. The Crystal as a Supramolecular Entity, Desiraju, G. R., Ed. Wiley Chichester, 1996. 

This section will examine selected examples of supramolecular entities built by means of contacts in which gold is the acceptor of electron density, and which can have different classes of donors non-metals different from hydrogen, which may be some of the atoms in Groups 17,16,15 or 14 a hydrogen atom of a covalent NM—H bond, where the non-metal is in most cases a carbon or a cloud of electron density. 

In this section I will comment on some examples of gold complexes that display supramolecular entities in the solid state, although in these cases the metal atom is not directly involved in the van der Waals interactions. As in the previous section, the electron density donor can be a non-metal different from hydrogen, which can be a halogen or a chalcogen a hydrogen atom of a covalent NM — H bond or a doud of n electron density. 

On other occasions, both types of secondary interactions are present in the same compound and can afford supramolecular entities via cooperative interactions. Table 5.3 shows examples of gold complexes containing N—H X bonds in addition to or instead of Au- Au contacts. 

As regards the heterometals that, together with gold, form supramolecular entities, there are not very many. It would be tempting to think that the group congeners silver... 

Of the metals that form luminescent supramolecular entities with gold, that for which most complexes are known is thallium in its +1 oxidation state. As described below, in recent years the contributions of several laboratories have been reported. Nevertheless, in some cases, the papers also report similar reactions with other metals, leading to similar structures. In order to maintain a congruent synthetic description, those examples will be discussed together as they appear in the original work. 

A complex is an entity consisting of two or more molecular species which are interacting in such a manner that they are being held together in a physically characterizable structural relationship. It may thus be considered as a supramolecular entity, which implies use of covalent bonds within its components and of intermolecular bonds for holding them together. 

One point to address concerns the use of the words s pramolecular and supermolecule. The concept of supramolecular chemistry has become a unifying attractor, in which areas that have developed independently have spontaneously found their place. The word supramolecular has been used in particular for large multiprotein architectures and organized molecular assemblies [1.16]. On the other hand, in theoretical chemistry, the computational procedure that treats molecular associations such as the water dimer as a single entity is termed the supermolecule approach [1.34,1.35]. Taking into account the existence and the independent uses of these two words, one may then propose that supramolecular chemistry be the broader term, concerning the chemistry of all types of supramolecular entities from the well-defined supermolecules to extended, more or less organized, polymolecular associations. The term super molecular chemistry would be restricted to the specific chemistry of the supermolecules themselves. 

Bond-making processes such as those described above extend supramolecular reactivity to cocatalysis, mediating synthetic reactions within the supramolecular entities formed by coreceptor molecules. 

The formation of supramolecular entities from photoactive components may be expected to pertub the ground-state and excited-state properties of the individual species, giving rise to novel properties that define a supramolecular photochemistry [8.2, A. 10, A.20J. 

The photophysical and photochemical features of supramolecular entities form a vast area of investigation into processes occurring at the level of intermolecular organization. They may depend on recognition events and then occur only if the correct selective binding of the complementary active components takes place, as illustrated for a two-component case in Figure 17. 

Self-assembly may occur with self-recognition, mixtures of components yielding defined superstructures without interference or crossover (see Section 9.5). A special case is self-resolution which results in the formation of homochiral supramolecular entities from racemic components (see Sections 9.4.2, 9.4.4 and 9.7). 

The mixed sites species TPU would represent a self-complementary component capable of homo-self-assembly into a (TPU) supramolecular entity. 

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