Host-guest inclusion compounds

The formation of host-guest inclusion compounds in crystals (or in highly ordered membranes or other biological structures) is a phenomenon more organised than simple co-crystallisation of a pair of substances. The special characteristic of inclusion crystallisation is a constant identity of one member in a series of pairs, this member thereby being identified as host, and a constancy of structure type (but not necessarily dimensions) for the host. The higher molecular organisation involved in inclusion... 

Dipolar Alignment for Second Harmonic Generation Host-Guest Inclusion Compounds... 

We highlight here a few studies in which the synthesis of chiral molecules has been achieved through the use of organic crystals in the hopes that this will prove a useful incentive and review. The reported studies fall into two natural categories. In the one case one starts with racemic mixtures or optically inactive compounds, crystallize these materials into chiral crystals and finally by subsequent reactions, trap this chirality in the final chemical products. In the second category one forms host-guest inclusion compounds in which the host is already an optically resolved compound. This in turn leads to the formation of optically active guest molecules. 

While these chiral host-guest inclusion compounds have been demonstrated to produce excellent ee s it has proven difficult to predict in an apriori fashion the direction of enantiomeric preference. For example in the cyclization reaction of 21, host 16b produces one enantiomer in 98% ee while 16c produces the opposite enamtioner with 95% ee. It should be noted that 16b and 16c are rather similar and it is therefore difficult without further work to ascribe the exact cause for the different product outcome. ... 

Palladium, rhodium and ruthenium complexes of the Schiff base salen are synthesized in the supercages of zeolite Y. The existence of intracrystalline transition metal-salen complexes is verified by a detailed physicochemical characterization. The catalytic properties of the prepared host/guest inclusion compounds are explored in the hydrogenation of hexene-(l) or an equimolar mixture of hexene-(l) and 2,4,4-trimethylpentene-(l). 

Despite this, X-ray analyses were very helpful in the discovery of host/guest inclusion compounds and certainly they are in most cases also instructive for the conditions in solution. 

It is essential that no covalent bonds are formed between host and guest molecules, as, e.g., when diastereomeric salts are used for separation. The differentiation of the enantiomers is effected only by the chiral (spatial) environment in the crystal or in the interior of the host molecule, respectively, whereby hydrogen bridges and dipol-dipol interactions may increase the stability of the inclusion compounds ( coordinato-clathrates ). The different energy constants of the diastereomeric host/guest inclusion compounds which result, e.g., in different solubilities, can be used to isolate one of the enantiomers. 

Fig. 1 Definitions and overall scheme of supramolecular events (a) chemical host synthesis and formation of a supramolecule (host-guest inclusion compound) (b) endo- and exosupramolecular receptor-substrate (host-guest) chemistry and (c) formation of mono-and multimolecular inclusion compounds.

Already, more than 25 years ago, Toda et al. reported the mechanochemical preparation of crystalline host-guest inclusion compounds and of charge-transfer (CT) systems. CT systems have also been studied by Kuroda et al., who were able to obtain multicomponent systems based on racemic bis-)8-naphtol, benzoquinone, and anthracene the cocrystal formation could be easily followed by the change in color of the solid mixtures. Importantly, this material was not accessible from conventional solution crystallization procedures and required structure determination from X-ray powder diffraction. ... 

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