Urea and Thiourea Clathrates

Harris, K. D. M., Fundamental and applied aspects of urea and thiourea inclusion compounds , Supramol. Chem. 2007,19,47-53. 

Single crystal of urea inclusion compound (originaily wHh 1.0-dibromooctane guest j 

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At present, clathrate compounds are considered as both ideal [l] and nonideal solid solutions [2-3] of the guest component in a crystalline framework of the host component. In the absence of the guest component, clathrate 3-modification is considered to be metastable with respect to the stable a-modification under ordinary conditions, i.e. Ap.= i - i <0, where [x, is the chemical potential of the corresponding modification. A number of experimental data on water, urea and thiourea clathrates, hardly consistent with the classical theory, made us study the formation of clathrates with unstable (labile) host frameworks. A comprehensive motivation of such possibility is given in [4]. 

The importance of intermolecular effects can be appreciated for urea and thiourea from the observation of the nitrogen resonance when these molecules are in different molecular environments, in clathrates or in complex salts, for example (see Polymolecular systems ). 

The most common hosts for inclusion polymerization are urea, thiourea, perhydrotriphenylene (PHTP), deoxycholic acid (DCA), apocholic acid (ACA) and tris(o-phenylenedioxy)cyclotriphosphazene (TPP)(Fig. 2). They have the common feature of forming channel-like clathrates, but differ in many specific properties. For instance, urea and thiourea have a rigid structure in which the host molecules are connected by hydrogen bonds and possess a high selectivity towards the guests. In urea channels are rather narrow whereas in thiourea they are wider as a consequence, linear molecules include only in urea and branched or cyclic molecules in thiourea. On the contrary, chainnels existing in PHTP clathrates are very flexible and can accomodate linear, branched and cyclic molecules. 

The ability of a molecule to act as a guest depends primarily on how it fills the clathrate cavities. In this respect the chemical nature of the guest and host is only of limited importance polar hosts, such as urea and thiourea, and apolar ones, such as PHTP, can include hydrocarbons, alcohols and ethers, carboxylic acids and esters, etc., i.e. a number of molecules of very different polarity. 

Chekhova, G.N. Dyadin, Yu.A. Rodionova, T.U. Clath-rates of urea and thiourea. Part IV. Stoichiometry of thiourea clathrates with small molecules and the nature of their packing in the matrix channels. Izv. Sibirskogo Otd. Akad. Nauk SSSR, Ser. Khim. Nauk 1979, N5 (78). 

Qathrate compounds are of this type molecules of one substance are trapped in the open structure of molecules of another. Hydroquinone forms clathrate compounds with SO2 and methanol, for example. Urea and thiourea also have the property of forming complexes, known as adducts, with certain types of hydrocarbons. In these cases molecules of the hydrocarbons fit into holes or channels in the crystals of urea or thiourea the shape and size of the molecules determine whether they will be adducted or not. 

The separation of n-alkanes by clathration techniques is well documented. Methods such as urea and thiourea adduction as well as molecular sieves have been employed ... 

The examples mi t have illistrated that functional grou (e.g. OH, COOH, NH,), as they are a component of classical crystal inclusion compounds are usually used for construction, cross-linking, and stabilization of the host lattice (Fig. 6a), and are not used, as could have been, for direct binding of guest molecules, e.g. via coordination or H-bonding (Fig. 6b). To speak with a newly developed classification system on inclusion compounds (see Chapter 1 of Vol. 140), tho are true clathrates and i t coordinatoclathrates (cf. Fig. 6, for a more detailed sj fication see Fig. 15 in Chapter 1 of Vol. 140). As in the case of urea and thiourea, a rather stable, but nearly invariable host lattice with rigidly... 

Of those compounds (clathrates) which during crystallization occlude other compounds in the cavities of their crystals, those with urea and thiourea (8, 9) have a certain importance for the identification of hydrocarbons. 

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. 

A somewhat unique situation has been studied in the free radical polymerization in urea 70) and thiourea (77) clathrates. Several monomers have been found for which all conditions for solid state polymerization to a stereoregular equilibrium crystal outlined above are fulfilled. The radicals are protected from termination by neighboring radicals by the urea or thiourea walls of the canals. The proper approach of the monomer molecules is achieved in thiourea by stacking the monomer... 

The conditions of the formation of the solution of such kind for channel inclusion compounds are simplified substantially and seem to be reduced to those of the formation of the individual clathrates, i.e. to the conformity of the guest molecules dimensions to the cross-section of the clathrate framework channel (for urea for thiourea 6,9-7j4A). 

We must stress the fact that the formation of clathrates is not specific for hydrocarbons. A large number of monofunctional and difunctional compounds are known which form clathrates with urea or thiourea. 

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