Thioureas host-lattice structures

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. 

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. 

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. 

The characteristic of crystal lattices is a strict periodical succession of structurally identical molecular units, in the sense of an inclusion lattice also of holes, channels, layers etc. which may include guest molecules in an oriented fashion. This organizing principle makes topochemistry possible. One of the early studies in this area was the inclusion polymerization of dienes in the channels of urea, respectively thiourea, leading to stereoregular polymers (Eq. 1) Although stereodifferentiating inclusion polymerization/co-polymerization has been performed in other host lattices, too, e.g. in the channels of the perhydrotriphenylene host (6) it is still a problem of actual interest... 

Figure 8.35 Perspective view of the crystal structure of [(C2H5)4lsP]2C404 6(NH2)2CS (17) showing individual columns of well-ordered tetraethylammonium cations enclosed in the channel-type host lattice built of squarate-thiourea species. The origin of the unit cell lies at the upper left corner, with a pointing towards the reader, b downward and c from left...

Thiourea forms a layered lattice able to act as host for some organometallic guest molecules. The ferrocene-thiourea inclusion compound has been investigated by spectroscopic methods (NMR, Mossbauer) and it has been shown that the thiourea host structure is preserved [492-502]. Other inclusion compounds of thiourea with organometallics such as ( /" -l,3-cyclohexadiene)Mn(CO)3, (7/ -C6H6)Cr(CO)3, (/ -l,3-cyclohexadiene)Fe(CO)3, and t/ -C(CH2)3Fe(CO)3 have been reported [503, 504]. 

Takemoto, K., and Sonoda, N., 1984, htclnsion compoimds in urea, thiourea and selenourea, in Inclusion Compounds Structural Aspects of Inclusion Compounds Formed by Organic Host Lattices, Vol. 2 (J. L. Atwood, J. E. D. Davies, and D. D. MacNicol, eds.). Academic Press, New York, pp. 47-67. 

Achiral objects can be assembled into chiral solid-state structures, and this is frequently the case for urea 1 when it encloses guests. Other compounds adopt a chiral conformation in solution and therefore may ultimately produce either chiral or achiral host structures. On the other hand, thiourea 2 forms an inclusion lattice that is achiral. This arrangement is nonetheless very effective in enclosing guest molecules. 

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