Tubulate inclusion compounds

Like urea, thiourea 2 also forms tubulate inclusion compounds with a rather similar cross-sectional topology [15], However, the tubes are now differently... 

Figure 17 The hydrogen-bonded supramolecular synthon present in all helical tubulate inclusion compounds formed by diol 11 (and its family of hosts). The molecules hydrogen bond together O-H O-H O-H to form the threefold screw axis structure illustrated. Identical arrangements subtended by the second diol hydroxy groups give rise to the chiral tubular lattice structure shown in Figure 16. Hydrogen atoms are omitted for clarity.

Dance. I.G. Craig, D.C. Structure and analysis of helical tubulate inclusion compounds formed by 2,6-dimethylbi- 46. cyclo[3.3.1]nonane exo-2, xo-6-diol. J. Am. Chem. Soc. 

The racemic alicyclic diol 10 can form two types of lattice inclusion compounds on crystallisation, ellipsoidal clathrates (Section 3.1.2) or helical tubulates (Section 3.2.1). Small guests give the former, and large guests give the latter, lattice. 

An interesting inclusion compound derived from 2,6-dimethylbicyclo[3.3.1]nonane-exo-2,exo-6-diol, 2, was reported by the group led by Bishop and Dance. " The crystal structure of the alicyclic diol 2 can be ho. construed as packing of helices along the c-axis. The parallel canals that result from the helical tubules have an unobstmcted triangular cross-sectional area of roughly 20 which are H3C ... 

Figure 1.5.3 Fitting of (a) rods into tubules to form inclusion compounds and (b) right- and left-handed helices to form sheets.

Other subsidiary descriptive terms used to illustrate the spatial relationships between host and guest refer to the designations "layer-type" (two-dimensionally open), "channel-type" (one-dimensionally open), and cage-type" (totally enclosed) inclusion compound, also termed "intercalates." "tubulates." and "cryptates," respectively, which makes further coinbinations of descriptive hybrid terms possible, e.g., tubulatoclathrate indicating a channel-type clathrate and so Even more precise... 

Many further cases of inclusion materials were discovered by happy accident during the next two centuries, for example, additional clathrate hydrates, the Hofmann clathrates, phenol inclusion compounds, Dianin s compound, urea tubulates, choleic acids, cyclodextrins, aud interpenetrated hydroquinone inclusion compounds. These substances remained problematic despite being the object of much painstaking scientific study. Mauy were unstable under ambient conditions and therefore it proved difficult to determine accurate ratios of their two components A and B. Furthermore, the substances did not follow the usual rules of covalent bonding, leading to their representation in the fonn (A)x (B)y. It was surmised that one component somehow trapped the other, but no experimental methods were available to analyze this phenomenon. 

The inclusion properties of urea were discovered by Ben-gen in 1940 and this tubulate host has since become one of the most studied. Thiourea and selenourea form related, but slightly different, clathrate structures. Figure 1 illustrates the structure of the (thiourea)3 -(carbon tetrachloride) compound. Many 1,3-diarylurea derivatives also include guest molecules, but these produce hydrogen-bonded complexes with acceptor guest species, rather than clathrates. " ... 

---