Dimeric structure, /?-CyDs

The dimeric structure of j8-CyD is most frequently observed as a building block of crystal structures. There are two types of arrangement of the dimer unit. In the head-to-head channel-type structure, the dimer unit is arranged to form an infinite linear channel. The other type is the layer-type structure, in which dimer units are arranged to form a chess-board-like pattern. Both ends of the dimer cavity are open to the intermolecular space in the adjacent layer. 

A crystal of the J-CyD hydrate shows the cage-type packing structure [21). In the solid state, d-CyD forms complexes with several macrocyclic compounds, such as cydononanone to cydopentadecanone [183]. Currently only the structure of the cydoundecanone complex has been reported. The crystal shows a typical channel-type packing structure consisting of dimeric d-CyD with the head-to-head mode [78]. Four cydoundecanone molecules are induded in the dimer cavity (Fig. 7.19B). Two guest molecules are perpendicularly located at the center of the dimer cavity and each of the other two molecules is coaxially included at the primary hydroxyl end of the cavity. 

The hydroxyl group of both isomers forms hydrogen bonds equally with secondary hydroxyl groups of a-CyD. Another example of a crystalline complex with a racemic compound is the j8-CyD complex with flurbiprofen. In the crystal, two j8-CyD molecules form a head-to-head dimer and a pair of (R)- and (S)-flurbiprofen is packed in the barrel-like cavity (Fig. 7.25A) [163]. In contrast, an excess of (S)-isomer was detected in the j8-CyD complex with racemic fenoprofen [157]. j8-CyD molecules form a same dimer structure in the crystal of the complex with each isomer, but the arrangement of guest molecules in the dimer cavity differs between the two crystals. Two (H)-isomer molecules are included in the head-to-head mode (Fig. 7.25B) while the head-to-tail arrangement is observed in the (S)-isomer complex (Fig. 7.25C) [158]. In fS-CyD complexes with N-acetylphenylalanine, the L-isomer is disordered in the dimer cavity while two molecules of D-isomer are included in a head-to-head mode [177]. 

The crystal structures of two forms of g-cyclodextrin barbital complexes (Form I and Form II) were investigated by X-ray analysis. In Form I crystal, two 3-cyclodextrin(g-CyD) molecules including two barbital molecules form a dimeric structure by the hydrogen bonds among their secondary hydroxyl groups. The unit cell volume of Form II is about twice as large as that of Form I and there exist two dimers... 

It is obvious that the barbital molecule included shallowly in the 3 CyD(2) can not escape easily because of the existence of the shifted 3 CyD(l ) as depicted in Fig. 3 (b). As the maximum molecular diameter of barbital molecule is larger than the minimum cavity diameter of 3-CyD, a barbital molecule buried in the dimeric 3 CyD also can not escape from the crystalline state. So barbital molecules are bound very tightly by 3-CyDs. However, when the crystals are dissolved in aqueous medium and the matrix structure of 3-CyD is broken, the barbital molecules included only at their ethyl groups are easily released. 

The results are in accord with the proposed supramolecular structures of diazirine 45 and its hosts (vide supra). No azine, di(3-nortricyclanylidene)hydrazine (53), was formed upon photolysis of the dimeric 45 (oc-CyD)2 complex because of a complete encapsulation of the reactive guest. Bimolecular reaction of the guests within the (45 /J-CyD)2 dimer should, of course, produce azine 53 by virtue of their geminated assemblage. Yet, the affinity of carbene 46 for the O H bonds present in both CyDs predominated, giving D-substituted CyDs (55a, 55b, iso-55a, and /.w-55b), via in-nermolecular reactions,21 that were detected using FAB MS analysis (Table 5).133... 

CyDs form crystalline complexes with metallocenes. Crystallographic study of several metallocene complexes of a-CyD and -CyD has been reported [226-228]. These complexes mostly crystallize in a head-to-head channel-type packing structure. In the a-CyD complex with ferrocene, the metal ion is located at the center of the dimer cavity and the host a-CyD molecule has direct contacts only with the cyclopentadienyl ligands [226]. Some metallocenium complexes of a-CyD exhibit a similar structure [227, 228]. Anions such as PFfi are bound at the primary hydroxyl side (Fig. 7.24). 

In the next larger cydodextrin, j8-CyD, aliphatic inserts are less popular, although the crystal structures of [3]pseudorotaxanes with aliphatic mono- [25] and diacids [18, 26, 27], diols [28], and bis-imidazolyl hydrocarbons [29] have been reported. j8-CyD/monocarboxylic acid complexes pack in channel structures and create supramolecular polymers, whereas 8-CyD/diacid complexes break the channel structure, contrary to what could be anticipated based on the propensity of acids to dimerize. With the wide y-CyD and aromatic guests, such as Congo Red [14] and ( )-stilbene [15] dimerization of the guest inside y-CyD is observed to form [4] and [3]pseudorotaxanes. Irradiation of crystalline complexes of the latter yielded syn-tetraphenylcyclobutane as a single isomer. 

The packing of each molecule in the crystal is shown in Fig. 3. The 3-CyD dimers are packed in the manner of the brickwork pattern which is one of the cage type packing structures and has been frequently seen in the 3-CyD inclusion complex structure of the triclinic space group PI. 

Fig. 1 (a),(b) The structure of 3 CyD aspirin complex (1) and (2), viewed from the primary hydroxyl side of each 3-CyD, respectively, (c) The molecular structure of (3 CyD)2(aspirin)2 salicylic acid complex. Aspirin and salicylic acid are drawn by full lines and circles. One of the statistically disordered salicylic acid at three sites is shown in the center of 3-CyD dimer. 

In the isomorphous structure of benzophenone and biphenyl complexes, the 3 cyd molecules are associated in head to head dimers all secondary hydroxyl ends of the two partners are connected by hydrogen bonds. These dimers are stacked along the c axis to produce channels... 

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