Thiourea, inclusion compounds with

Structural Aspects of Some Thiourea Inclusion Compounds with Monoclinic Host Structures... 

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. 

Metallocenes (but only with unsubstituted cyclopentadienyl rings) also form thiourea inclusion compounds. X-ray diffraction and Mdssbauer data on the ferrocene inclusion compound show that at 295 K the guest molecules are orientationally disordered at the sites of 32 symmetry. A phase change at 162 K generates a more ordered structure78). 

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. 

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. 

It has long been known that highly crystalline forms of polybutadiene with melting points higher than conventional crystalline polybutadiene are available via radiation polymerization of thiourea inclusion compounds (1. Crystallographic studies of the polymerization of butadiene derivatives in inclusion complexes have been reported (53). Recently, butadiene derivatives in layer perovskite salts have been polymerized to singlecrystal polymers which have been structurally characterized (54). 

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. 

Selenourea, like urea and thiourea, can form inclusion compounds with a variety of hydrocarbons. Its inclusion properties were studied by van Bekkum and coworkers in 1969 [79], well over 20 years after the discovery of the analogous urea [54] and thiourea [55] inclusion compounds. The 1 3 adduct of adamantane with... 

There are more striking differences in the unit-cell dimensions of the selenourea inclusion compounds than are observed for the thiourea inclusion compounds, as adaptation of the selenourea lattice to the shape and size of the included molecules is apparently relatively easy [57-Si]. A theoretical study of the conformations and vibrational frequencies in urea, thiourea, and selenourea compounds has been reported [54]. Studies on the complexation of TT with I" and urea, thiourea, or selenourea (and their Me derivetives) using a horizontal zone electromigration method have indicated that the complexing capacities of the urea derivatives increase in the order urea < thiourea < iV,N, N -tetramethylthiourea < selenourea [55]. 

Comparison of the urea-anion inclusion compounds with the thiourea-anion inclusion compounds indicates that urea molecules more readily combine with other anionic moieties to generate composite dimers or ribbons. Composite dimers or ribbons exist in seven complexes among the nine urea-anion inclusion compounds studied here. For instance, both urea-bicarbonate complexes 1.6 and 1.7 (Figure 36 and Figure 37) contain a hydrogen-bonded [((NH2)2CO)2(HC03)2] ribbon built of alternating urea dimers and cyclic dimeric bicarbonate moieties (LB... 

In comparing the thiourea-nitrate inclusion compounds with the thiourea-bicarbonate inclusion compounds, it is noted that the oxygen atoms of the NOJ ion, unlike the HCOJ ion, can only form acceptor hydrogen bonds with other potential donors. Therefore, the construction of a two- or three-dimensional host framework requires a higher thiourea/nitrate molar ratio, with or without cocrystallized water molecules, as is the case in complexes 2.5 and 2.6. In the 1 1 complexes 2.7,2.8, and 2.9 only a parallel arrangement of separate ribbons can be constructed from the thiourea and nitrate building blocks. 

Several IR and Raman spectroscopy studies of the thiourea inclusion compounds of monohalocyclohexanes have been reported The predominance of axial conformers in the cavities has been demonstrated, in contrast with the preferred equatorial conformation in the liquid or gaseous phases. No X-ray structures of the thourea complexes have been reported. 

Thiourea forms channel-type inclusion compounds with globular molecules.This is partly because the space group E3c lacks a screw axis that the space group of the urea inclusion compounds P6i22 possesses. The larger diameter of the channel than in the urea compounds accommodates cyclohexane and monosubstituted cyclohexane as well as carbon tetrachloride and hexachloro-ethane in the channel. With these more or less spherical molecules as the guest species, thiourea inclusion compounds are rich in their polymorphism, most undergoing two or more phase transitions below room temperature. 

The thiourea-CCl4 compound shows that a rigid moiecule can also form an inclusion compound with thiourea. It undergoes phase transitions of an order-disorder type, as the entropy change and the crystal... 

While the tunnel in conventional urea inclusion compounds is fairly cylindrical (with fairly constant cross-sectional diameter), the thiourea tuimel structure contains bulges and constrictions at different positions along the tunnel. As a consequence, it is often more appropriate to regard the thiourea host structure as cage -type rather than "tunnel"-type, and many properties of thiourea inclusion compounds can be understood... 

Fig. I Structure of the cyclohexane/thiourea inclusion compound at ambient temperature, showing 10 complete tunnels (with wi der Waals radii) viewed along the tunnel axis. Guest molecules have been inserted into the turmels, illustrating orientational disorder. The positions cf the guest molecules are not actually determined from x-ray diffraction data at ambient temperature. (View this art in color at www.dekker.com.)...

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

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]. 

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