Isostructurality. inclusion compounds

Table 1, Various host molecules which form familities of isostructural inclusion compounds. Distinction is made according to topological and interactive relationships of the host matrix...

The high-temperature polymorph of TMA, hypothetical y-TMA, and the polyhalide inclusions [cf. Sect. 4.1 (2) Table 2] are isostructural inclusion compounds. We shall show later that the polyhalide inclusions are channel-type compounds and that the so-called y-polymorph is actually an interstitial (clathrate) inclusion compound with empty channels. Although y-TMA is not a true polymorph in the strict thermodynamic sense, it is more convenient at this stage to be slightly inexact in nomenclature rather than pedantic. 

Crystal isostructurality refers to identical or nearly identical packing arrays of chemically distinct compounds and is inversely related to the phenomenon of polymorphism, which, instead, refers to the ability of a single compound to crystallize in different packing arrays. A series of isostructural inclusion compounds is typically based on a common host framework with voids that may be occupied by different guest molecules. The constant guest environment provided by such a series is a factor that facilitates interpretation of the mechanisms of processes such as thermal decomposition and guest exchange. 

A series of isostructural inclusion compounds presents a unique opportunity to investigate strueture and property relationships, because one structural degree of freedom. 

The examples above serve to illustrate that both uniform and diverse behaviors may be observed in different series of isostructural inclusion compounds. In the latter case, anomalous behavior can often be traced to subtle host-guest interactions. 

The finding that a new inclusion compound has an isostructural counterpart is readily achieved by the PXRD technique and represents an enormous return in terms of structural information, because it immediately defines the host substructure and the topology of the voids. This is useful for crystal structure solution of the new species by either single crystal x-ray diffraction or high-resolution PXRD. Systematic investigation of the physical and chemical properties of isostructural inclusion compounds can reveal novel phenomena in supramolecular chemistry... 

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). 

By Methods A and B, isopropyl phenyl sulfoxide was included in crystalline 1 with high ( -enantioselectivity (86 and 87% ee, respectively). Ethyl phenyl sulfoxide formed no inclusion compound by Method A, but the inclusion compound of its (5)-enantiomer was obtained by Method B. The inclusion crystal of (.V)-e(hyl phenyl sulfoxide is isostructural with that of (S)-isopropyl phenyl sulfoxide (Figure 3). As mentioned above, (6>ethyl phenyl sulfoxide was not included by Method A. The lack of one methyl group may make enthalpy (interaction with the inclusion cavity) and entropy disadvantageous in crystal packing to result in no inclusion of ethyl phenyl sulfoxide via Method A. 

In order to find a possible disordering of the guest, the isomorphous replacement method was used. 1-methylnaphthalene was replaced with 1-bromonaphthalene, and the resulted inclusion compound was isostructural with the methylnaphthalene one. 

The examples listed in Table 4 [cf. Sect. 4.1 (3)] are isostructural channel inclusion compounds in which the essential structural element is TMA TMA alone. 

TMA forms a binary inclusion compound with dimethyl sulfoxide and a ternary with dioxane and water. Both structures have been reported (see Ref. for summary). Kapon and Reisner (unpubUshed results) have shown that in addition to an inclusion compound with ethanol which is unstable under atmospheric conditions, there is a series of stable isostructural TMA inclusion compounds with guests such as iso-octane, n-tetradecane, n-octanol, and n-decanol. Structures of these inclusion compounds are now being determined in our laboratory... 

The 2 1 inclusion compounds of DHDK with ethanol and chloroform (colourless form) were isomorphous, and these were isostructural with DHDK CHjCOOH. 

Various physicochemical characterisation techniques were used to study tris(5-acetyl-3-thienyl)methane (TATM) inclusion compounds with l, -di-chloroalkane guests (n = 1-10). Thermogravimetric analysis allowed to determine host-guest stoichiometric ratios. From the multiplicity of the host signals in the C CP MAS NMR spectrum, the number of host molecules in the asymmetric unit of each inclusion compound was determined. It was found that only 1,9-dichlorononane and 1,10-dichlorodecane form isostructural TATM inclusion compounds. The obtained results were compared with those for urea-carboxylic acid complexes. 

Inelastic Neutron Scattering, p. Ill Isostructurality of Inclusion Compounds, p. 767 The Lock and Key Principle, p. 809 Macrocycle Synthesis, p. 830 Molecular Wires, p. 925 Self-Assembling Capsules, p. 1231 Simultaneous Binding of Cations and Anions, p. 129i Simultaneous Binding of Cations and Neutral Molecules, p. 1295... 

Cavitands. p. 219 Clathrate Hydrates, p. 274 Disorder and Diffuse Scattering, p. 457 Hydrogen Bonding, p. 658 Hydrophobic Effects, p. 673 Isostructurality of Inclusion Compounds, p. 767 Neutron Diffraction, p. 959 Soft and Smart Materials, p. 1302 Solid-State Nuclear Magnetic Resonance Spectroscopy, p. 1307... 

Channel Inclusion Compounds, p. 223 Isostructurality of Inclusion Compounds, p. 767 Layered Supramolecular Solids and Their Intercalates, p. 791... 

Indices for measuring the extent of isostructurality of two or more organic crystal structures can be applied not only to homomolecular crystals but also to molecular associates. such as inclusion compounds. A qualitative summary of these descriptors follows, and the reader is referred to a recent account for explicit mathematical definitions. Earlier descriptors of isostructurality included the " d-gree of isostructurahty" Ii(n) (based on the distance differences ARj between the crystal coordinates of identical nonhydrogen atoms within the same section of the asymmetric units of two or more related structures) the packing coefficient increment. A(pc) and the unit cell similarity index... 

Recently Fabian and Kalman highlighted certain limitations in the above indices, proposing that the degree of isostructurality of two or more structures be based instead on the extent of their volume overlap.They accordingly defined a "volume isostructurality index" represented by the ratio of the volume overlap of the two compared fragments and their average volumes. This parameter, computed over the entire unit cell, has merit in that it can be used selectively on two or more related inclusion compounds to measure the levels of isostructurality between the entire crystal structures, the hosts alone, or the guests alone. 

Isostructurality occurs frequently in many classes of inclusion compounds. Here, selected examples are drawn from several classes displaying this tendency, starting with clathrate hydrates and metal-containing inclusion compounds. The inherent stabilities of many host frameworks are conducive to the formation of isostructural series. At the other extreme, however, isostructurality may also arise in situations where the host is unstable, originating only within the clathrate phase, which self-assembles in the presence of guest molecules. 

Numerous instances of isostructurality among Hofmann clathrates. M(NH3)2M (CN)4 2G, are known, the high frequency of occurrence probably owing to the fact that this family of inclusion compounds has been studied extensively over a long period. In addition, isostructurality can manifest itself in different ways, e.g., with either M. M fixed and guest G variable or with M, M variable and a common guest. An example in the former category... 

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