Clathrate inclusion complexes formed by hosts of lesser versatility
Cavities in frameworks constructed from linked host molecules can be filled by guest molecules of appropriate size and shape, thus forming clathrate inclusion complexes. In general, there are no covalent or hydrogen bonds between host and guest, and the host-guest interactions are due to dispersion and polar forces. Guest-guest interactions are through dipolar and dispersion forces. In one group of clathrates, the interaction among the host molecules is strongly directional, helping form periodic arrays of cavities. One prevalent motif is the four-connected tetrahedral node found in the polyhedral clathrates, which includes the metalloid clathrates (with covalent bonding between pairs of framework atoms (Si, Ge ...)), the clathrasils (with ionic-polar bonding between SiO2 groups), and the gas hydrates where there is hydrogen bonding between adjacent framework water molecules, and sometimes, additional linkaging. A remarkable concatenation of geometrical and chemical factors governs the formation of the frameworks of these clathrates. A second prevalent motif is the ring of six hydrogen-bonded hydroxyl groups found in the quinol and phenol clathrates, and those of Dianin’s compound. The statistical mechanics of the formation of solid solutions of guests in the host frameworks has been worked out and applied particularly to the β-quinol clathrates and, in lesser detail, to the gas hydrates; a substantial number of phase diagrams have been determined for these systems. Most of these clathrates have crystal structures different from those of the pristine hosts and are intermediate phases in the binary phase diagrams. Dianin’s compound provides a rare example of formation of a primary solid solution. Clathrates can also be formed when there are only non-directional dispersion forces between host molecules, and the inclusion complexes of tetraphenylene are used as an example. Finally, the hydrogen-bonded motifs found in the quinol, etc., clathrates have inspired the synthesis of covalently bonded analogues, and these hexahosts also form clathrates.
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