Cryptates molecular cavity

There is considerable interest in the properties of macrobicyclic cryptands, for example [78] to [81], and particularly in their ability to complex protons, metal ions, and small molecules (Lehn, 1978). In the proton cryptates there exists the possibility of intramolecular +N—H N hydrogen bonding as well as interaction of the proton with the oxygen atoms, and the properties are also strongly influenced by the size of the molecular cavity. In the [l.l.l]-cryptand [78] the molecular cavity is small (Cheney et al., 1978) and... 

For cryptands in which the molecular cavity is larger than in the case of the [l.l.l]-species [78], proton transfer in and out of the cavity can be observed more conveniently. Proton transfer from the inside-monoprotonated cryptands [2.1.1] [79], [2.2.1] [80], and [2.2.2] [81 ] to hydroxide ion in aqueous solution has been studied by the pressure-jump technique, using the conductance change accompanying the shift in equilibrium position after a pressure jump to follow the reaction (Cox et al., 1978). The temperature-jump technique has also been used to study the reactions. If an equilibrium, such as that given in equation (80), can be coupled with the faster acid-base equilibrium of an indicator, then proton transfer from the proton cryptate to hydroxide ion... 

These ligands form extremely stable cation inclusion complexes, called cryptates, In which the cation Is completely surrounded by the ligand and hidden Inside the molecular cavity, and this leads to a considerable Increase of the interionic distance In the ion pairs. It has been shown that such ligands have a marked activating effect on anionic polymerizations (4,5,6). Moreover, the aggregates are destroyed and simple kinetic results have been obtained In the case of propylene sulfide (7,8,9). ethylene oxide (9,10,11) and cycloelloxanes (12) polymerizations. Though the... 

The non-complementarity between the ellipsoidal 33-6H+ and the spherical halides results in much weaker binding and appreciable distortions of the ligand, as seen in the crystal structures of the cryptates 35 where the bound ion is F , Cl-, or Br-. In these complexes, F- is bound by a tetrahedral array of hydrogen bonds whereas Cl- and Br- display octahedral coordination (Fig. 4). Thus, 33-6H+ is a molecular receptor for the recognition of linear triatomic species of a size compatible with the size of the molecular cavity [3.11]. 

As discussed in Section II,B, the nitrogen lone pairs of the [2] cryp-tands may be turned either inward or outward with respect to the molecular cavity, leading to three possible conformations exo-exo, exo-endo, and endo-endo (Fig. 2). The most favorable conformation for complex formation is the endo-endo form, in which the nitrogen lone pairs are directed inward toward the metal ion. A wealth of crystallographic data exists for [2] cryptates, primarily from Weiss s group... 

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... 

Cylindrical macrotricyclic molecules (19) are ditopic coreceptors constructed on two binding subunits, two macrocycles, linked by two bridges. When the macrocycles chosen are able to bind -NHJ groups, diammonium substrates may be expected to form mononuclear dihapto cryptates of type (20) by inclusion into the central molecular cavity of the tricyclic receptor. 

Since binding in solution results from a compromise between interaction with the ligand and solvation, new insights into the origin of the cation recognition process and of the macrocyclic and cryptate effects can be gained from experimental gas phase studies [2.34, 2.35] as well as from computer modelling calculations in vacuo or in a solvent [1.35b, 1.42, 1.43, 1.45, 2.36, 2.37, A.37]. In particular, molecular dynamics calculations indicate that complementarity is reflected in restricted motion of the ion in the cavity [1.45, 2.36]. 

The crystal structure 57 of the strong and selective complex formed by the terephthalate dianion with a hexaprotonated macrobicyclic polyamine shows that it is a molecular cryptate 56 with the dianion tightly enclosed in the cavity and held by formation of three hydrogen bonds between each carboxylate and the ammonium groups [4.19]. Both structures 53 and 57 illustrate nicely what supermolecules really are they show two covalently built molecules bound to each other by a set of non-covalent interactions to form a well-defined novel entity of supramolecular nature. Acyclic [4.20a,b] and macrobicyclic [4.20c] hydrogen bonding receptors... 

An exclusive complex of this type was actually found also in the crystalline state for the KNCS complex of the smaller [2.2.1] cryptand From molecular models, the cavity radius of this ligand was estimated to be 1.1 A which is too small for potassium to enter. As a result, occupies a site in the 18-membered ring rather than in the central cavity (see Fig. 38), thus resembling the coordination of K" by [18]crown-6. Additionally, the potassium ion is bonded to the isocyanate anion whereas anions are generally not coordinating to alkali metal ions in inclusive cryptates. 

---