Crystalline complexes

Urea has the remarkable property of forming crystalline complexes or adducts with straight-chain organic compounds. These crystalline complexes consist of a hoUow channel, formed by the crystallized urea molecules, in which the hydrocarbon is completely occluded. Such compounds are known as clathrates. The type of hydrocarbon occluded, on the basis of its chain length, is determined by the temperature at which the clathrate is formed. This property of urea clathrates is widely used in the petroleum-refining industry for the production of jet aviation fuels (see Aviation and other gas-TURBINE fuels) and for dewaxing of lubricant oils (see also Petroleum, refinery processes). The clathrates are broken down by simply dissolving urea in water or in alcohol. 

Crystallization and Purification Solvent. Dimethylacetamide is useful ia the purification by crystallization of aromatic dicarboxyHc acids such as terephthahc acid [100-21-0] and/vcarboxyphenylacetic acid [501-89-3]. These acids are not soluble ia the more common solvents. DMAC and dibasic acids form crystalline complexes containing two moles of the solvent for each mole of acid (16). Microcrystalline hydrocortisone acetate [50-03-3] having low settling rate is prepared by crystallization from an aqueous DMAC solution (17). 

Complexes of DMAC and many inorganic haHdes have been reported (20). These complexes are of iaterest because they catalyze a number of organic reactions. Complexes of DMAC and such heavy metal salts as NiBr2 exert a greater catalytic activity than the simple salts (21). The crystalline complex of SO and dimethylacetamide has been suggested for moderating the reaction conditions ia sulfation of leuco vat dyestuffs (22). 

Anhydrous silver hexafluorophosphate [26042-63-7] AgPF, as well as other silver fluorosalts, is unusual in that it is soluble in ben2ene, toluene, and xylene and forms 1 2 molecular crystalline complexes with these solvents (91). Olefins form complexes with AgPF and this characteristic has been used in the separation of olefins from paraffins (92). AgPF also is used as a catalyst. Lithium hexafluorophosphate [21324-40-3] LiPF, as well as KPF and other PF g salts, is used as electrolytes in lithium anode batteries (qv). 

A yellow crystalline complex (2) melting at 198°C is formed from sorbic acid and iron tricarbonyl (10) ... 

Chlorinated Trisodium Phosphate. Chlorinated trisodium phosphate [11084-85-8] is a crystalline complex of hydrated trisodium orthophosphate and sodium hypochlorite that releases hypochlorite when mixed with water. Its formula is (Na PO 11H20)4 NaOCl. Commercial... 

Unit cells of pure cellulose fall into five different classes, I—IV and x. This organization, with recent subclasses, is used here, but Cellulose x is not discussed because there has been no recent work on it. Crystalline complexes with alkaU (50), water (51), or amines (ethylenediamine, diaminopropane, and hydrazine) (52), and crystalline cellulose derivatives also exist. Those stmctures provide models for the interactions of various agents with cellulose, as well as additional information on the cellulose backbone itself. Usually, as shown in Eigure la, there are two residues in the repeated distance. However, in one of the alkah complexes (53), the backbone takes a three-fold hehcal shape. Nitrocellulose [9004-70-0] heUces have 2.5 residues per turn, with the repeat observed after two turns (54). 

In 1975, Weber and Vogtle showed that open-chained polyethers, 2,6-pyridine-dimethanol, and ortho-xylene derivatives terminated in an 8-quinolinyl group could form stable, 1 1, crystalline complexes with a variety of metal salts. The podands were prepared from 8-hydroxyquinoline and the corresponding dihalides. A typical example is shown in Eq. (7.10). 

However, it should be mentioned that the dissolution process of a solid, crystalline complex in an (often relatively viscous) ionic liquid can sometimes be slow. This is due to restricted mass transfer and can be speeded up either by increasing the exchange surface (ultrasonic bath) or by reducing the ionic liquid s viscosity. The latter is easily achieved by addition of small amounts of a volatile organic solvent that dissolves both the catalyst complex and the ionic liquid. As soon as the solution is homogeneous, the volatile solvent is then removed in vacuo. 

The need of the acylurea site participating in intermolecular hydrogen bonding (cf. Figs. 11 and 12) for the complex formation is exemplified by the fact that a 1 1 mixture of JV-(p-dimethylaminophenyl)phenylacetamide (21) and JV-isobutyl-p-nitro-benzamide (22) gives no crystalline complexes under the same conditions as with 19 and 20. The trend of the complex formation often changes, when the combinations of R7 and R8 are reversed 35). 

Like PEO-LiCl04, a 6 1 crystalline compound is formed but, in this instance, the weakened interactions between polymer chains [18] contributes to the lowest melting point for any PEO-salt crystalline complex. A eutectic with composition 0 Li = 11 1 forms, provided the PEO molecular chain length is beyond the entanglement threshold [31]. For lower molecular weights, the 6 1 compound dose not crystallize in the presence of excess PEO and a crystallinity gap exists over the range 6 l < 0 Li < 12 1 [26]. 

A new approach to the resolution of sulphoxides 242 was recently reported by T oda and coworkers282. It takes advantage of the fact that some sulphoxides form crystalline complexes with optically active 2,2 -dihydroxy-l, 1-binaphthyl 243. When a two-molar excess of racemic sulphoxide 242 was mixed with one enantiomeric form of binaphthyl 243 in benzene-hexane and kept at room temperature for 12 h, a 1 1 complex enriched strongly in one sulphoxide enantiomer was obtained. Its recrystallization from benzene followed by chromatography on silica gel using benzene-ethyl acetate as eluent gave optically pure sulphoxide. However, methyl phenyl sulphoxide was poorly resolved by this procedure and methyl o-tolyl, methyl p-tolyl, s-butyl methyl and i-propyl methyl sulphoxides did not form complexes with 243. 

Bis(dimethylamino)methylborane thus acts as a four-electron donor. The reaction takes place in dioxane at RT and the red, crystalline complex is sublimable at 10 torr/20 C in 2-4% yield. 

In spite of the numerous spectral observations of complex formation between aromatic and olefinic donors with the dihalogens, the preparations of the corresponding crystalline complexes have been hindered by their enhanced reactivity (as well as the relatively weak bonding). As such, only few examples of the X-ray structural characterization of the corresponding intermolecular associates are reported, the most notable exception being the dibromine complex with benzene. 

Liquid-crystalline complexes (metallomesogens) containing platinum(II) are new types of materials that have been the subject of several studies. These have largely included complexes of the type trans-[PtX2L2] (X = C1, L = cyanobiphenyls 229 X = C1, carboxylate, L = 4-alkoxy-4 stilbazoles 230 X = C1, L = 2,4-, 3,4-, or 3,5-dialkoxystilbazoles, 2,3,4-, 2,4,5-, or 3,4,5-trialkoxystilbazoles).231,232 Their liquid-crystalline properties have also been reported. 

Selectivity studies with DTU indicated marked discrimination in the clathrate formation 23,45). As in other types of clathrates, the steric factor is important in differentiation between compounds of similar functionality but different shape. For example, DTU forms crystalline complexes with some alcohols (methanol, ethanol, propanol, 1-butanol) but not with others (2-butanol). It complexes the ethyl esters of N-acetyl derivatives of glycine, alanine, methionine and aspartic acid, but not of proline, serine, phenylalanine and glutamic acid. 

Separation of Structural Isomers by Crystalline Complexation with l,l-Di(p-hydroxyphenvl)cydohexane... 

Fig. 32a and b. Schematic illustration of intermolecular arrangements in the crystalline complexes of host 25 (taken from Ref.25>) a the two-dimensional hydrogen bonding pattern parallel to the ab plane (the shaded area represents the 1,1-diphenylcyclohexane framework) b the van der Waals type packing of the hydrogen bonded layers along the c axis (R represents the cyclohexyl ends of the host species)... 

A similar procedure was used to separate efficiently between cyclohexanol (b.p. 161 °C) and cyclohexanone (b.p. 155 °C), two structurally similar but chemically different species, by selective crystalline complexation with host 25 50). For example, when a solution of this host and a 1 1 mixture of the two guests in ethyl acetate was kept at room temperature for 24 hours, and the colorless crystals thus obtained were subsequently heated in vacuum, the composition of the resulting guest mixture was 94.71% of cyclohexanol and 5.23% of cyclohexanone. 

The binaphthol 13 is different from 1 and 7 owing to the lower acidity of its functional groups. Therefore, crystalline complexes of 13 with amines (see Table 3) are not expected to have a salt character. The 13 imidazole 1 2 complex (Fig. 22)81) was studied in the light of the general interest in this guest partner and its relation to alcohol functions in biological ensembles. The host molecule adopts ideal twofold... 

Table 5. Yields of photoreaction products of N-methylpyridones 29 in the crystalline complex with 1 and in ethanol solution...

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