Solvates — The Simplest Example of Lattice Inclusion

At one extrme of the inclusion compounds variety are complexes, usually classified as solvates, in whkh molecules of the solvent are enclosed within intermolecular voids of the crystal to fill empty space or to complete a coordination spl re aroimd a functional moiety. Quite often, it is difficult to control such a co-crystallization 

Stereographic projection of the crystal structure of the 2 1 inclusion compound between CSj and cavitand 1. One CSj ( guest ) molecule is encapsulated within the host cavity, the second CSj ( solvent ) being located between the oomplexed entities (taken from Ref. ) 

Stereoview of the intermoiecular arrangement in the crystal structure of 2,6 4,8-bis(ruthoto-cenyl)-2,4,6,8-tetrafluoro-cyclotetraphosphazene toluene solvate 

An enormous variety of solvates associated with many different kinds of compounds is reported in the literature. In most cases this aspect of the structure deserved little attention as it had no effect on other properties of the compound under investigation. Suitable examples include a dihydrate of a diphosphabicyclo[3.3.1]nonane derivative benrene and chloroform solvates of crown ether complexes with alkyl-ammonium ions and acetonitrile (Fig. 4) and toluene (Fig. 5) solvates of organo-metallic derivatives of cyclotetraphosphazene In most of these structures the solvent entities are rather loosely held in the lattice (as is reflected in relatively high thermal parameters of the corresponding atoms), and are classified as solvent of crystallization or a space filler However, if the geometric definition set at the outset is used to describe clathrates as crystalline solids in which guest molecules 

In selected cases, the effect of solvation on the crystalline structure form is, however, consideraUy more pronounced. For example, the observed packing in the crystal of 2,4,6-tris(l,3-propylenediamiiw-N,N -)cyclotriphosphazene (4) di-hydrate (Fig. 6) is due to strong intermolecular hydrogen bonds between molecules of water and suitable couples of N-H groups on the host moiety The H O species form also continuous H-bonded layers of solvation around the cyclophosphazene derivatives, thus stabilizing the crystal lattice. 

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