Inclusion compounds, urea/thiourea

Selenourea [630-10-4] ]5k.e urea and thiomea can form channel inclusion compounds (87) with a variety of hydrocarbons. Though the difference in channel diameter between thiourea and selenourea is small, selenourea seems to be much more selective for the inclusion of certain guest molecules (eg, cis/trans isomers). 

The complexes are solids but are not useful as derivatives, since they melt, with decomposition of the complex, at the melting point of urea. They are useful, however, in separating isomers that would be quite difficult to separate otherwise. Thiourea also forms inclusion compounds though with channels of larger diameter, so that n-alkanes cannot be guests but, for example, 2-bromooctane, cyclohexane, and chloroform readily fit. 

Polar and coordinatively active functional groups are structural elements frequently found in the constitution of crystal inclusion hosts, mainly including conventional host molecules7 . Typical examples are urea (2), thiourea (5), hydroquinone (4), Dianin s compound (5), deoxycholic add (6) or simply water (Fig. 1). This was the reason to assume that functional groups play an important part in the construction of crystal inclusion compounds. 

Thiourea and selenourea both form canal inclusion compounds. The selenourea compounds, reported only briefly 37), appear to be isostructural with the rhombo-hedral thiourea compounds, although with larger dimensions which are more susceptible to the sizes of guest molecules. For urea, thiourea and selenourea the crystal densities of the compounds are significantly less than those of the hosts values (g cm - 3) for the pairs host/complex are 1.30/1.20 1.40/1.10 2.08/1.60-1.65 for urea, thiourea, selenourea, respectively37). 

Description of the Canal Structures of Urea and Thiourea Inclusion Compounds... 

Thiourea canal inclusion compounds 19 26) have a wider diameter than those formed by urea, such that n-alkanes are not included but that molecules of cross-section approximately 5.8-6.8 A are trapped 64). Thus many inclusion compounds have been reported between thiourea and branched alkanes or cyclic molecules. Of special interest are the inclusion compounds with cyclohexane derivatives and the recent studies carried out on the preferred conformation(s) of the ring in the restricted environment of the thiourea canal. 

Pedersen (1971) reported on a number of crystalline complexes with thiourea and similar molecules and found that the solubility of dibenzo-18-crown-6 [11] in methanol is enhanced by urea and thiourea. The latter observation precludes the possibility of the solid complexes being urea or thiourea inclusion compounds. Factors that are of importance in the complex formation are ring size (dibenzo-12-crown-4 did not form complexes), and steric hindrance in the complex caused by the guest molecule. Thiourea, N-phenylthiourea, and 2-thiazolidinethione form complexes but N,N,N -trimethylurea, thiocarbanilide and N-methylthiazolidinethione do not. 

For a review of urea and thiourea inclusion compounds, sec Takemolo Sonoda. in Atwood Davies MacNicol, Ref. 60. vol. 2, pp. 47-67. 

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. 

Another type of inclusion compd is the channel or canal compound. Here the straight chain compds, such as hydrocarbons, acids,esters , alcohols, aldehydes, ketones, etc are enclosed in the channels formed by compds, such as urea, thiourea, choleic acids, cyclodextrins, etc. As examples of channel compds may be cited, the urea-decone compd, [CO(NH2)2] g.C, 2H26, and various zeolites. (See also Ref 10, pp431 ... 

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

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. 

Host — A - molecular entity that forms an -> inclusion complex with organic or inorganic -> guests, or a - chemical species that can accommodate guests within cavities of its crystal structure. Examples include cryptands and crowns (where there are -> ion-dipole interactions between heteroatoms and positive ions), hydrogen-bonded molecules that form clathrates (e.g., hydroquinone and water), and host molecules of inclusion compounds (e.g., urea or thiourea). The - van der Waals forces and hydrophobic interactions (- hydrophobic effect) bind the guest to the host molecule in clathrates and inclusion compounds. 

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