Guest molecules/species

Fig. 26. Clathrate receptor chemistry (a) a chiroselective crystalline host compound (clathrand) (b) a typical guest molecule to be included in the specified configuration and (c) the crystal stmcture of the respective clathrate (A and B denote host and C the guest species) (169).

Zinc hydroxide and alkoxide species are particularly relevant to catalytic processes, often forming the active species. The cooperative effects of more than one zinc ion and bridged hydroxides are exploited in some enzymatic systems. Zinc alkyl phosphate and carboxylate materials have been important in the formation of framework compounds, often containing large amounts of free space for the inclusion of guest molecules. Aldehyde and ketone compounds are of low stability due to the poor donor capabilities of the ligands however, a number of examples have recently been characterized. 

Notwithstanding the variety of structural patterns that characterize the crystalline inclusions referred to above, it has recently been shown that the clathrate formation can be induced in a more systematic manner. The use of the clathration phenomenon to store selected guest molecules or to separate one type of guest species from another is the subject of the following sections in this account. 

Inclusion Compounds Involving Readily Deprotonizable (Proton-Donating) Guest Molecules Alcohols and Acids as Guest Species 80... 

Inclusion Compounds Involving Aprotic-Dipolar Guest Molecules Dimethylformamide and Dimethyl Sulfoxide as Guest Species. .. 92... 

If mixtures of two or more potential guest molecules are offered, the host lattice of 1 allows the selective accommodation of solvent molecules 2). In many cases, a pra-tically 100% discrimination of one guest species (>95% by NMR integration) is achieved by a single crystallization process using /, e.g. from an equimolar two-component solvent mixture. Table 2 summarizes important results (Entries 1-15). 

Special features which characterize the prototypical roof-shaped compound 26 are a rigid molecular skeleton and the type of sensor groups used. They render 26 a potential coordinatoclathrand 50). The capability of 26 in forming inclusion compounds is evident from Table 5. Here, nearly thirty different crystal inclusions of 26 are specified including as guest molecules various alcohols, acids, aprotic dipolar, and rather apolar species. 

Materials with uniform pore structures offer a wide range of applications, including catalysis, adsorption, and separation. These materials have the benefit ofboth specific pore systems and intrinsic chemical properties [1-3]. The pores in the materials are able to host guest species and provide a pathway for molecule transportation. The skeletal pore walls provide an active and/or affinity surface to associate with guest molecules. According to the International Union of Pure and Applied Chemistry (IUPAC), porous materials can be classified into three main categories based on the diameters of their pores, that is, microporous, mesoporous, and macroporous... 

It is worth noting that TTF-based sensors for species other than metals, particularly organic species, have been well studied. The principle is, of course, the same a host group capable of recognising a guest molecule is tethered to the signaller TTF, which displays an altered physical response when a guest is bound and when the receptor is free. Systems of this type are of particular interest as biosensors, for example, in the detection of saccharides for disease... 

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