Ureas host-lattice inclusion compound

As mentioned above, in urea/thiourea-anion inclusion compounds both urea and thiourea molecules adopt various linkage modes not only of self-assembled hydrogen-bonded chains or ribbons but also of composite ribbons with the corresponding anions. The results show that comolecular aggregates of urea or thiourea with other neutral molecules or anionic moieties can be considered as fundamental building blocks for the constructions of various types of novel host lattices. 

Yet another type of canal structure has been reported for the urea inclusion compound of 1,4-dichlorobutane 51). Even though the canals are pseudo-hexagonal in dimension, there is a significant difference in their symmetry. The host lattice is orthorhombic, space group Pbcn. The difference lies in the directions of the six pseudo-3j helices of host molecules around the walls of each canal the sequence is cyc/o-RRRLLL, as opposed to cyc/o-RRRRRR in the hexagonal inclusion compounds and cvc/o-RLRLRL in the rhombohedral. This orthorhombic host structure probably occurs also in the urea inclusion compounds with 1,5-dichloropentane and 1,6-dibromohexane 51). 

These compounds include Flofmann and Wcrner-typc inclusion compounds. inclusion compounds of urea, thiourea and sclenourea, inclusion compounds of gossypol. inclusion compounds of phenolic hosts, inclusion compounds of denxycholic acid (choleic acids), inclusion compounds of macrocyclic and oligocyciic lattice hosts and recently designed organic host lattices. 

As mentioned in Section 13.5.1, the transient species [BO(OH)2]-has been stabilized by hydrogen-bonding interactions with the nearest urea molecules in the host framework of the inclusion compound [(CH3)4N]+[B0(0H)2]--2(NH2)2C0-H20. A perspective view of the crystal structure along the [010] direction is presented in Fig. 20.4.16. The host lattice consists of a parallel arrangement of unidirectional channels whose cross section has the shape of a peanut. The diameter of each spheroidal half is about 704 pm, and the separation between two opposite walls at the waist of the channel is about 585 pm. The well-ordered tetramethylammonium cations are accommodated in double columns within each channel. 

Q. Li and T. C. W. Mak, Novel inclusion compounds with urea/thiourea/selenourea-anion host lattices, in M. Hargittai and I. Hargittai (eds.), Advances in Molecular Structure Research, vol. 4, pp. 151-225, Stamford, Connecticut, 1998. 

One important host molecule among the inclusion compounds is urea. Ordinary crystalline urea is tetragonal, but when a guest is present, urea crystallizes in a hexagonal lattice, containing the guest in long channels (Fig. 3.1). ... 

NOVEL INCLUSION COMPOUNDS WITH UREA/THIOUREA/ SELENOUREA-ANION HOST LATTICES ... 

A considerable amount of recent work has been directed towards the study of detailed molecular orientations and motions of guest molecules in urea channel inclusion compounds, as well as the generation of crystalline modifications in different space groups, such as P2i2i2i [30a,b] R3c [60] and Pbcn [67], but the principal structural characteristics of the channel-type host lattices remain virtually unaltered. Structural properties of the 1,10-dibromodecane/urea and 1,12-dibro-modecane/urea inclusion compounds have been determined by single-crystal X-ray... 

The results of our studies on urea/thiourea/selenourea-anion inclusion compounds have demonstrated that the classical urea or thiourea hydrogen-bonded host lattice can be modified in interesting ways by the incorporation of various anionic moieties, with or without cocrystallized water or other uncharged molecules, and that novel host frameworks bearing different urea, thiourea, or selenourea/guest molar ratios are generated by variation in size of the hydrophobic, pseudo-spherical R4N guest species. The stoichiometric formulas of 46 inclusion compounds and their structural details are listed in Table 10. For convenient description of stoichiometric ratios, the letters u, a, and c are used to denote the urea/thiourea/se-lenourea molecule, the anion, and the cocrystallized neutral molecule, respectively. 

Table 10. Types of Host Lattices in Inclusion Compounds of Urea Derivatives and Peralkylated Ammonium Salts ...

We now present results illustrating that the N-H- 0 and N-H -S hydrogen bonds play an instrumental role in the structures of a novel class of urea/thiourea inclusion compounds. The average lengths of 84 measured N-H - O hydrogen bonds in 16 urea-anion inclusion complexes and 114 measured N-H—S hydrogen bonds in 23 thiourea-anion host lattices are listed in Table 11. 

Our studies on the generation of novel urea, thiourea, and selenourea-anion inclusion compounds have shown that novel anionic host lattices can be constructed from urea derivatives and many types of anions as building blocks. The versatility of urea, thiourea, or selenourea as a key component in the construction of novel host lattices is clearly demonstrated by the occurrence of many new types of linkage modes, which include various discrete units, chains or ribbons, and composite ribbons with corresponding anions. These motifs are shown in Tables 14, IS, and 16, respectively. 

In summary, we have shown that novel anionic host lattices can be constructed from urea, thiourea, or selenourea molecules and various anions as building blocks, which readily adopt different topologies for the accommodation of tetraalkylam-monium ions of various sizes. By employing organic cations as templates and suitable counter anions as an ancillary host material, with or without neutral molecules such as H2O as a third component, the lattice engineering of new urea, thiourea, and selenourea inclusion compounds by self-assembly may be further explored. 

Whereas the separation of racemates in the case of urea and TOT was achieved only by a chiral crystal lattice of the achiral or racemic host, respectively, the optically active cyclodextrins, available from the chiral pool, are able to differentiate a chiral guest within their intramolecular cavity. Therefore, they do not necessarily need the crystal lattice to form inclusion compounds. The guest is encapsulated, while is is in solution, too, if the guest by size and shape fits into the cavity of the specific cyclodextrin molecule (a- (26), P- (27), or y-cyclodextrins). 

Basically, there are two main classes of urea/thiourea inclusion compounds the classical channel-type clathrates having a host lattice constructed from urea (Figure 8.1) or thiourea molecules (Figure 8.2) [1] and those whose host lattices contain anions as... 

Inclusion Compounds with Urea/Thiourea-Anion Host Lattices... 

---