Cryptands spherands

Cross-fertilisation between the crown ethers (or, more generally, corands), cryptands, spherands and podands has produced an enormous range of hybrid hosts such as cryptaspherands and hemispherands, many exhibiting all the useful features of the parent materials (Figure 3.16). 

In general, there are two types of macrocycles or cryptands containing the ferrocene unit. The first is that in which the ferrocene is appended to either a macrocycle — represented schematically by Fig. 6-1, or to a cryptand, spherand or cavitand (Fig. 6-2). The second class of compounds has ferrocene incorporated... 

Fig. 6-2. Schematic representation of a ferrocene-containing macrocycle in which the ferrocene moiety is attached to a cryptand, spherand or cavitand. Fc = 1-substituted or 1,1 -disubstituted ferrocene jc = O, S, or NR m,n,p = 1,2, or 3 y = number of ferrocene units attached, usually 1 to 4.

Soon after crown ethers came on the scene as the first synthetic host molecules capable of binding guests, cryptands followed, and soon thereafter, the spherands. The preorganization of these three classes of molecules follows the order of their invention, as does overall binding affinity, i.e., crownsNobel Laureate, was the creator of the family of hosts that he named spherands. The spherand story began shortly after the genesis of supramolecular chemistry, and it demonstrates how quickly the field matured, as complex syntheses and methods of characterization enhanced the rapid sophistication of host-guest chemistry. 

Fig. 7. Crown type and analogous receptor molecules of different varieties (1) crown ethers (2) cryptands (3) a podand (4) a spherand and (5) the natural...

Fig. 3. Crown compounds/cryptands and analogous inclusion hosts. (1 4) Crown macro rings bicyclic cryptands (5) [37095-49-17, (6) [31250-06-3J, (7) [31364-42-8] (8) [23978-09-8]-, (9) spherical cryptand [56698-26-1]-, (10) cylindrical cryptand [42133-16-4]-, (11) apodand [57310-75-5]-, and (12) a spherand...

The spherand prepared by Cram and coworkers was designed to have a relatively small molecular cavity and appeared to prefer complexation with Li and Na over larger cations like K", Rb, etc. Tlie spheroidal cryptand prepared by Lehn ° involved strategy employed previously but the spherand 24 was prepared by quite a different approach. 

X 10 forNa+,2.8 X 10 for, and 1.3 x 10 for Li These constants were of a magnitude that persuaded the authors to liken the binding properties of these calixar-enes to cryptands and spherands. 

The problem of molecular recognition has attracted biologically oriented chemists since Emil Fischer s lock-and-key theory l0). Within the last two decades, many model compounds have been developed micelle-forming detergents11, modified cyclodextrins 12), many kinds of crown-type compounds13) including podands, coronands, cryptands, and spherands. Very extensive studies using these compounds have, however, not been made from a point of view of whether or not shape similarity affects the discrimination. 

In addition to macrocyclic hosts discussed above, many other molecules capable of selective complexation have been synthesized. They belong to so-called macrocyclic chemistry [30] encompassing crown ethers discussed in this Chapter, cryptands 61-63 [21], spherands 70 [31], cyclic polyamines 71 [32], calixarenes 18 [5], and other cyclophane cages such as 72 [33] to name but a few. Hemicarcerand 5 [2b] discussed in Chapter 1 and Section 7.3 also belongs to this domain. Typical macrocyclic host molecules are presented in Chapter 7. 

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]. 

An additional nuance in the nomenclature of these compounds concerns their complexes. The open-chained compounds are often referred to as podands and their complexes as podates. The cyclic ethers may also be called coronands and their complexes are therefore coronates. Complexed cryptands are cryptates. The even more complicated structures known as spherands, cavitands, or carcerands are called spherates, cavitates, or carcerates, respectively, when complexed. The combination of a macrocycle (crown ether or coro-nand) and a sidechain (podand) is typically called a lariat ether. 

The AMBER-based approach used to model cyclic polyethers and cryptands has also been applied to the study of the Li+, Na+, and K+ complexes of three spherands (Fig. 14.3)12641. Experimentally determined metal ion selectivities were successfully reproduced. A similar AMBER-based model, used for molecular mechanics and dynamics of a cyclic urea-based spherand, was also successful in reproducing its metal ion selectivity12651. A number of new conformations of the spherand, including the global energy minimum, were located using molecular dynamics12651. 

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