Recognition spherical substrates

Beyond the spherical substrate, the next step in the control of molecular recognition and in the strategic elaboration of molecular systems displaying highly specific properties resides in the design of synthetic ligands for organic molecules, i. e. of synthetic molecular receptors 165). 

The simplest recognition process is that of spherical substrates these are either positively charged metal cations (alkali, alkaline-earth and lanthanide cations) or the negative halide anions (see Chapt. 3). 

Another motivation for this work is the development of copolymers that are tuned to a certain surface, that is, copolymers that have a memor/ of the preparation conditions and are able to reproduce their specific conformation in the vicinity of the surface with predefined chemical heterogeneity. It is believed that copolymers designed in this way could have potential for the recognition of patterned substrates through the formation of stable adsorption complexes with planar or spherical substrates composed of two chemically distinct sites, one of which has a preferential affinity for one of the... 

Linear recognition is displayed by the hexaprotonated form of the ellipsoidal cryptand bis-tren 33, which binds various monoatomic and polyatomic anions and extends the recognition of anionic substrates beyond the spherical halides [3.11, 3.12]. The crystal structures of four such anion cryptates [3.11b] provide a unique series of anion coordination patterns (Fig. 4). The strong and selective binding of the linear, triatomic anion N3" results from its size, shape and site complementarity to the receptor 33-6H+. In the [N3 pyramidal arrays of +N-H "N- hydrogen bonds, each of which binds one of the two terminal nitrogens of N3-. 

The spherically shaped cryptophanes are of much interest in particular for their ability to bind derivatives of methane, achieving for instance chiral discrimination of CHFClBr they allow the study of recognition between neutral receptors and substrates, namely the effect of molecular shape and volume complementarity on selectivity [4.39]. The efficient protection of included molecules by the carcerands [4.40] makes possible the generation of highly reactive species such as cyclobutadiene [4.41a] or orthoquinones [4.41b] inside the cavity. Numerous container molecules [A.38] capable of including a variety of guests have been described. A few representative examples of these various types of compounds are shown in structures 59 (cyclophane) 60 (cubic azacyclophane [4.34]), 61a, 61b ([4]- and [6]-calixa-renes), 62 (cavitand), 63 (cryptophane), 64 (carcerand). 

Thus, cryptands (1-3) and (5) display spherical recognition towards spherical cations and anions. The special complexation properties encountered clearly result from the macropolycyclic nature of these ligands. They define a cryptate effect characterized by high stability and selectivity of complexation, slow exchange rates and efficient shielding of the bound substrate from the environment. 

Considering together the three cryptates [NH4 c (5)] (10), [H2O e (5)-2H ] (11) and [Cr cz (5)-4H ] (7), it may be concluded that the spherical macrotricycle (5) is a molecular receptor possessing a tetrahedral recognition site in which the substrates are bound in a tetrahedral array of hydrogen bonds. It represents a state of the art illustration of the molecular engineering involved in abiotic receptor chemistry. 

In order to further develop the coordination chemistry of anions and to extend recognition of anionic substrates beyond the spherical halides, an ellipsoidal macro-bicyclic cryptand Bis-Tren (14) was designed, whose hexaprotonated form was expected to bind various anions [9, 10]. Indeed, potentiometric and spectroscopic measurements showed that (14)-6H complexes a number of monovalent and polyvalent anions. The strong and selective binding observed for the linear triatomic anion NJ may be attributed to its complementarity to the molecular cavity of (14)-6H . As confirmed by crystal structure determination, NJ forms the cryptate [N c (14)-6H ] (15), in which the substrate is bound inside the cavity by two pyramidal arrays of three hydrogen bonds, which hold the two terminal... 

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