Urea/thiourea/selenourea-anion host compounds

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. 

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

The purpose of this review is to assemble and assess currently available information about structural modification of urea/thiourea inclusion compounds. We concentrate here on the structural aspects of new inclusion compounds with urea/thiourea/selenourea-anion host lattices, most of which were prepared and structurally analyzed in our laboratory. The versatility of urea or thiourea as a key component in the construction of novel anionic host lattices is clearly demonstrated by occurrence of many new types of linkage modes. The results show that co-molecular 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. Comments on structure-property relationship for these inclusion compounds are made wherever appropriate. 

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. 

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. 

---