Urea inclusion compounds structural properties

At sufficiently low temperatu es, most conventional urea inclusion compounds undergo phase transitions associated with changes in the symmetry of the host structure and changes in the dynamic properties of the guest molecules. For alkanelurea and oc.co-dibromoalkane/urea inclusion compounds, structural and dynamic " aspects of these phase transitions have been investigated extensively using a variety of techniques. Qualitatively, the alkane/ urea and a,co-dibromoalkane/urea inclusion compounds behave in a siinilar way with respect to these transitions. 

A considerable amount of recent work has been directed towards the study of detailed molecular orientations and motions of guest molecules in urea channel inclusion compounds, as well as the generation of crystalline modifications in different space groups, such as P2i2i2i [30a,b] R3c [60] and Pbcn [67], but the principal structural characteristics of the channel-type host lattices remain virtually unaltered. Structural properties of the 1,10-dibromodecane/urea and 1,12-dibro-modecane/urea inclusion compounds have been determined by single-crystal X-ray... 

K.D.M. Structural properties of the low temperature phase of the hexadecane/urea inclusion compounds, investigated by synchrotron x-ray powder diffraction. J. Phys. Chem. 1997, 101, 9926. 

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... 

Bromine M-edge extended x-ray absorption fine structure (EXAFS) experiments were carried out to investigate a variety of local structural properties of urea inclusion compounds containing a.co-dibromoalkane guest... 

Guest molecules in solid host structures are often constrained to exhibit uncharacteristic conformational properties, which can be exploited as a means of carrying out spectroscopic characterization of such conformations. For example, guest molecules in the 1,6-dibromohexane/urea inclusion compound (a commensurate system) exist exclusively with both bromine end groups in the gauche conformation allowing a definitive characterization of the vibrational properties of this conformation. 

Papers concerning the physical properties of polymers as the guest components in urea inclusion compounds and polymerization reactions of guest monomer molecules within the urea tunnel structure have been reviewed elsewhere. The polymers studied included poly (ethylene), poly (acrylonitrile), poly (1,3-butadiene), poly(eth-ylene oxide), poly(tetrahydrofiiran), poly(acrolein), poly(vinyl chloride), poly(ethyl acrylate), poly(lactide), poIy(lactic acid), poly(ethylene adipate). poly(ethylene succinate), acrylonitrile-ethyl acrylate copolymer, and poly(hexanediol di acrylate). 

Harris, K.D.M. Hollingsworth, M.D. Structural properties of the guest species in diacyl peroxide/urea inclusion compounds An x-ray diffraction investigation. Proc. R. Soc., A 1990, 431, 245. 

Harris, K.D.M. Smart, S.P. Hollingsworth, M.D. Hollingsworth, Structural properties of a,o)-dibromoalkane/ urea inclusion compounds A new type of interchannel guest molecule ordering. J. Chem. Soc., Faraday Trans. 1991, 87, 3423. 

Shannon, I.J. Stainton, N.M. Harris, K.D.M. Structural properties of urea inclusion compounds containing carboxylic acid anhydride guest molecules Anomalous modes of guest-molecule ordering. J. Mater. Chem. 1993, 3, 1085. 

Y eo. L. Harris. K.D.M. Guillaume. F. Temperature-dependent structural properties of the 1,10-decanedicar-boxylic acid/urea inclusion compound. J. Solid State Chem. 1997. 128. 273. 

Solid-state and C NMR spectroscopy has been used to study the structural properties of 1-bromodecane in urea. It was found that the end groups of the guest molecules are randomly arranged. The dynamic characteristics of 1-bromodecane in urea inclusion compounds were probed by variable-temperature solid-state H NMR speetroseopy (line shapes, spin-spin relaxation T2, spin-lattiee relaxation Tix and Tiq) between... 

The fortuitous discovery that urea forms inclusion compounds with many unbranched organic molecules was first reported by Bengen in 1940 15). Structural studies by Schlenk 16) and by Smith 11 18> determined the now familiar canal structure of these materials. The chemistry of these compounds has been reviewed on a number of occasions 19-21), as have specific applications in chromatography22, their use in inclusion polymerisation 23,24), and studies of their thermodynamic25 and spectroscopic 26 properties. 

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. 

A brief review on the crystal structures of urea, thiourea, and selenourea is included for understanding the properties of hydrogen-bonded channel frameworks in their inclusion compounds. The N-H—E (E = O, S, Se) hydrogen bonds which play an instrumental role in the structures of urea/thiourea/selenourea inclusion compounds are discussed in this section. 

Full reviews on the formation, structure, and properties of a variety of inclusion compounds of urea and thiourea were published about 10 years ago [2S], but very significant advances have been made in the present decade with the aid of modem... 

Thiourea also forms solid inclusion compounds with a tunnel host structure (Figure 2b). However, while the urea tunnel has a fairly uniform cross-section, the thiourea tunnel instead has prominent constrictions (diameter about 5.8 A) and bulges (diameter about 7.1 A) at certain positions along the tunnel, and some properties of thiourea inclusion compounds may be better understood by regarding the host structure as cage type rather than tunnel type. 

The first step in the generation of the database of molecular properties in cocrystals was the retrieval of co-crystal structures from the CSD. Co-crystals were defined as organic structures containing at least two eharge neutral nonsolvent molecules with different structural formulas. To avoid possible statistical bias by the frequent occurrence of some eommonly used eo-formers e.g., urea, 4,4 -bipyridine, see Table 1 in ref. 7), struetures with these moleeules were excluded. Initial survey of the data showed that inelusion eompounds, which consist of a large and a small molecule, form a distinet group in the data set. Co-crystals of molecules with more than 30 and with less than six nonhydrogen atoms were thus removed and the analysis foeused on eo-erystals other than inclusion compounds. The final data set eontained 710 struetures. 

Polymers, as a component of assemblies, polymer blends, alloys, and composites, have been studied by mixing them with different species. However, in these instances, the structures and properties are statistical in nature. Polymers have numerous structural potentials in the main chains and side chains. If polymers can be used as guest molecules, the resultant complexes could provide new structures and functions. Polymeric inclusion compounds are thought to be a typical example of nanoscale composites—molecular level composites made by bottom-up approaches. Urea, thiourea, and per-hydrotriphenylene have been extensively studied as host molecules for the formation of inclusion compounds with various polymers. 

Some compounds owe their inclusion complex formation ability to the crystal structure in the solid state. One example is urea, for which an inclusion complex formation ability is not a microscopic property of a single molecule but a property of macroscopic solid matter. Therefore, urea is not able to form inclusion complexes in solution. In contrast, the... 

The inclusion properties of urea were discovered by Ben-gen in 1940 and this tubulate host has since become one of the most studied. Thiourea and selenourea form related, but slightly different, clathrate structures. Figure 1 illustrates the structure of the (thiourea)3 -(carbon tetrachloride) compound. Many 1,3-diarylurea derivatives also include guest molecules, but these produce hydrogen-bonded complexes with acceptor guest species, rather than clathrates. " ... 

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