Channel-type packing

Fig. I8.2a-c. Packing of cyclodextrin molecules in crystal lattices. There are two cage packing patterns, in herringbone (a) and in brick-wall-type arrangement (b). In the channel-type packing (c), cyclodextrins are stacked like coins in a roll. In the illustrations, cyclodextrins are seen from the side, cavities are indicated by shading [S6S]...

Fig. 7.15. Channel-type packing structures observed in a-CyD complexes A/ith benzenesulfonate (A) and iodine-iodide (B).

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. 

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). 

Fig. 5. The packing status of Form II complex (a) Both channel and cage type packing systems of 3 CyD molecules. Barbital and water molecules are drawn by full lines and circles, (b) The stacking of B CyD(2) and B--CyD(3) molecules in the channel type packing area, (c) The stacking of 3 CyD(4) and shifted 3-CyD(l ) molecules in the cage type packing area. Barbital molecule included in 3-CyD(4) is denoted by an asterisk.

Form II constructs a g-CyD tetramer-like structure, which form the channel type packing. 

Packing of the cyclodexthn molecules (a, P, P) within the crystal lattice of iaclusion compounds (58,59) occurs in one of two modes, described as cage and channel stmctures (Fig. 7). In channel-type inclusions, cyclodextrin molecules are stacked on top of one another like coins in a roU producing endless channels in which guest molecules are embedded (Fig. 7a). In crystal stmctures of the cage type, the cavity of one cyclodextrin molecule is blocked off on both sides by neighboring cyclodextrin molecules packed crosswise in herringbone fashion (Fig. 7b), or in a motif reminiscent of bricks in a wall (Fig. 7c). 

Plate type packing to separate the phases is discussed by Carlsson et al. (1983) and by Hatziantoniu etal. (1986). De Vos et al. (1982,1986) describe use of a monolithic porous catalyst with vertical and horizontal channels. The liquid phase flows downward through an array of parallel channels in the monolith, while gas moves in cross flow through a separate set of channels. Another approach treats the catalyst to make part of the surface hydrophobic or lyophobic (Berruti et aL, 1984). The gas phase has direct access to the surface on these unwetted portions of the surface, resulting in partial, spatial segregation of the phases. 

Crystallization of 5 in the open air from an initially aprotic solvent (N,N-dimethyl-acetamide) led to a non-layered structure which is characterized by a three-dimensional lattice of loosely-packed host species interspaced by channel-type zones accommodating the solvent guest components (Fig. 9). 

Figure 7. Schematic representation of the packing arrangement in cyclodextrin complexes (a) channel type, (b) cage or herringbone type, and (c) brick type. (Reproduced with permission from W. Sanger in Inclusion Compounds, Vol. 2, J. L. Atwood, J. E. D. Davies, and D. D. MacNicol, Eds., Academic Press, New York, 1984, p. 231.)...

Figure 8. A stereoscopic view of the packing in the inclusion compound of 1,1,6,6-tetraphenylhexa-2,4-dyne-1,6-diol, 2, with (a) chalcone and (b) 9-anthraldehyde. 9-Anthraldehyde gives cage-type and chalcone gives channel-type structures. [Reproduced with permission from F. Toda, Topics Curr. Chem. 140, 43 (1987).]...

Figure 6.23 Schematic representation of the packing of cyclodextrin structures, (a) Head-to-head channel type (b) head-to-tail channel type (c) cage type (d) layer type and (e) layer type composed of /TCD dimers. (Reproduced from [24] with permission of Elsevier).

Fig. 1 (a) y-CD chemical structure and approximate dimensions of a-, P-, and y- CDs schematic representation of packing structures of (c) cage-type, (d) layer-type, and (e) head-to-tail channel-type CD crystals and (f) CD-IC channels containing included polymer guests... 

When complexes are prepared in the presence of iodide, four different complex types can be obtained, depending on the conditions. Here we will only discuss the blackish-brown complex (a-cyclodextrin)2-Cdo.5-l5-27H2O. On the Cf, axis of the molecule are found four of the five iodine atoms of Is", the central fifth one being disordered. The disordered iodine atom is located between the two a-cyclodextrin molecules which face each at their wide bases, 11.18. Structures of 11.18 are packed one on top of the other in a way as to create a continuous channel filled with iodine atoms. This is a channel-type complex (Noltemeyer and Saenger 1980). With guest molecules other than iodine, either one or the other of the two types of crystalline structures discussed here, i.e. cage or channel, can be observed. 

The cage-type packing structure is frequently observed for relatively small guest molecules which can be enclosed in the host cavity. CyD molecules are arranged in a herring-bone fashion and both ends of the host cavity are closed by adjacent molecules to create an isolated "cage (Fig. 7.13A). The channel-type structure is... 

Fig. 7.13. Schematic drawing of crystal packings cage-type observed in the fi-CyD complex with hexamethylenetetramine (A), head-to-head channel-type observed in the a-CyD complex with iodine-iodide (B), and layer-type observed in the a-CyD complex with p-nitrophenol (C).

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