Macropolycyclic cryptands

In other sections in this chapter, we have referred to a variety of macropolycyclic structures which are more elaborate than the simple three-stranded bicyclic cryptands. This includes bridged double-macrocycles , in-out bicyclic amines and the macrotricyclic quaternary ammonium salts of Schmidtchen. In addition to these, there are two other types of compounds which deserve special note. The first of these is a stacked twin-ring cryptand, but it is a hybrid molecule rather than a double-cryptand . The species shown below as 20 is a crowned porphyrin, and was designed to provide a pair of metal cation binding sites similar to those which might be available in natural biological systems . 

A somewhat different approach to providing tailored cavities for metal cations was taken by the groups of Cram and Lehn °. Graf and Lehn prepared the spheroidal molecule 21 which has an interesting molecular architecture. The molecule has ten coordination sites within it, six which form an octahedral array and four which are in a tetrahedral arrangement. This remarkable compound is soluble in all solvents from petroleum 

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The various possible strategies for the synthesis of macropolycyclic cryptands are outlined in Figure 14. 

Fig. 5. Some dinuclear cryptates of macropolycyclic cryptands resulting from connection of chelating, tripodal, and macrocyclic subunits [3.24].

The synthesis of macropolycyclic cryptands generally involves stepwise, straightforward pathways (18, 20, 33) based on the successive construction of systems of increasing cyclic order macrocyclic, macrobicyclic, and so on. Newkome has recently reported a satisfactory quaternization-dealkylation procedure, facilitating the synthesis of 8 (34). Unlike the synthetic approaches to simple crown ethers (10,... 

The hrst opticaUy active macrobicycles and macropolycycles were repotted by Lehn and co-woricers (187) in 1974. Employing the synthetic methodology developed in Strasbourg for the preparation of cryptands, chiral molecular receptors, such as (5)-191 and (5)-192, have been isolated and characterized. 

Crown ethers and cryptands show much of the same functional group chemistry as simple ether- or amine-containing molecules. The remarkable reactivity of these macropolycyclic species is primarily derived not from the composition of functional groups but from their three-dimensional arrangement. The important property of strong cation complexation is determined by the topology of the cavity defined by the ether and amine groups in the molecular superstructure. 

Interest in crown ethers has exploded since the initial discovery by Pedersen. With the addition of cryptands and other macropolycycles to the armory of coordination chemists there has been feverish activity to seek out new areas in which new and original uses can be devised. Now all branches of chemistry, polymer science, pharmaceutics and industrial processes have been touched by the crown ether revolution. 

A second important advance was made with the availability of cryptands (73), a family of bicyclic polyoxadiamines which have available a three-dimensional cavity for complexation.6 Again many structural modifications are possible and lead to numerous macropolycyclic ethers. 

Macropolycyclic ligands containing intramolecular cavities of a three-dimensional nature are referred to as cryptands. The bicyclic cryptands (73) exist in three conformations with respect to the terminal nitrogen atoms, exo-exo, endo-exo and endo-endo 6 these forms can rapidly interconvert via nitrogen inversion but only the endo-endo form has been found in the crystal structures of a variety of complexes372 and for the free ligand ([2.2.2], 73, m = n = / = l).449 In their complexes with alkali and alkaline earth cations, the cryptands exhibit an enhanced stability over the crown ethers and coronands dufe to the macrobicyclic, or cryptate, effect.33 202... 

Cryptands (16) and (17) are macropolycyclic receptor molecules which provide a cavity for inclusion of a variety of substrates. Cryptate refers to the complexes. 

Although some scattered examples of binding of alkali cations (AC) were known (see [2.13,2.14]) and earlier observations had suggested that polyethers interact with them [2.15], the coordination chemistry of alkali cations developed only in the last 30 years with the discovery of several types of more or less powerful and selective cyclic or acyclic ligands. Three main classes may be distinguished 1) natural macrocycles displaying antibiotic properties such as valinomycin or the enniatins [1.21-1.23] 2) synthetic macrocyclic polyethers, the crown ethers, and their numerous derivatives [1.24,1.25, 2.16, A.l, A.13, A.21], followed by the spherands [2.9, 2.10] 3) synthetic macropolycyclic ligands, the cryptands [1.26, 1.27, 2.17, A.l, A.13], followed by other types such as the cryptospherands [2.9, 2.10]. 

Cryptands 7-9 thus function as receptors for spherical cations. Their special com-plexation properties result from their macropolycyclic nature and define a cryptate effect characterized by high stability and selectivity, slow exchange rates, and efficient shielding of the bound ion from the environment [2.17,2.27]. 

Numerous macrocyclic and macropolycyclic ligands featuring subheterocyclic rings such as pyridine, furan or thiophene have been investigated [2.70] among which one may, for instance, cite the cyclic hexapyridine torands (see 19) [2.39] and the cryptands containing pyridine, 2,2 -bipyridine (bipy), 9,10-phenanthroline (phen) etc. units [2.56,2.57,2.71-2.73]. The [Na+ c tris-bipy] cryptate 20 [2.71] and especially lanthanide complexes of the same class have been extensively studied [2.74, 2.75] (see also Sect. 8.2). 

Since the pioneering work of Pedersen (1), Lehn (2), and Cram (3) on synthetic macrocyclic and macropolycyclic host systems such as the crown ethers, cryptands, and spherands, there has been an enormous development of the field of host-guest or supramolecular chemistry. Molecular hosts designed to bind inorganic and organic, charged and neutral guest species via cumulative, noncovalent interactions have all been reported and extensive reviews on this subject have appeared (4-8). 

Undoubtly, cryptands remain the class of macrobicyclic and macropolycyclic receptors looking forward to a great future. The number of possible structures is practically unlimited and depends on a researcher imagination and creativity. Initially, the chemistry of cryptands had focused on the development of new synthetic strategies, either to improve the yields or to built up more complicated molecular architectures, like cylindrical, spherical or lateral macropolycycles, bi- or polynucleating, with hard and soft binding sites, etc. 

Macrocycle obtained by the condensation of phenol rings with formaldehyde and amenable to substitution both on the phenolic functions and on the para positions of the phenol rings Macropolycyclic ligand forming a cage Metal ion complex with a cryptand... 

Macropolycyclic polyamine ligand (a form of cryptand leading to extremely inert metal ion complexes)... 

Related macropolycycles are the cryptatesm or cryptands, which are N,0 compounds such as N[CH2CH2OCH2CH2OCH2CH2]3N, the structure of whose complex with Rb+ is shown in Fig. 3-2. 

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