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Inclusion Compounds


Inclusion Compounds
Clathrates, or inclusion compounds, are divided into two major types according to relative topological relationship between host and guest:
Molecular clathrates form after inclusion of guest species in cavities of host species. Host molecule (or a capsule created by several host molecules) already has a cavity. Sometimes host is called receptor. Molecular clathrates are discrete species.   Cage clathrates (or just clathrates) have guest species included in cavities of host crystal or crystal-like 3D framework. Host molecules typically do not have cavities but the crystal they form does. If host porous crystal is stable without guest, it is also called sorbent. Cage clathrates are infinite systems.

Molecular clathrates
Molecular clathrates: inclusion inside intramolecular cavities

 

Cage clathrates
Cage clathrates: inclusion inside extramolecular cavities

Inclusion compounds were discovered by Herbert Powell. In 1947-1948 he reported the crystal structures of hydroquinone inclusions with sulfur dioxide and other gases. In 1948 he introduced the term clathrate (derived from the Latin word "clathratus": "enclosed") and stated that new matter may be created not only through the creation of new molecules, but also through the inclusion and formation of host-guest materials. Now such materials are called supramolecular.
In classical clathrates, the only interaction between the host and guest molecules is van der Waals. In contrast, the assembly of host molecules into a host framework may be facilitated by either van der Waals, H-bonds, coordination, or the whole framework may be a covalently bonded giant molecule.
Some examples of inclusion compounds: Iodine forms channel-type inclusions with the components of starch ("blue reaction of iodine"); gaseous hydrocarbons form giant deposits of ice-like inclusions with water near the sea-floor and in the permafrost regions ("gas hydrates"); inclusions of various drugs, food additives and cosmetic ingredients with cyclodextrins may be found in many products we consume; inclusion of lithium in graphite is used in lithium batteries; thousands and thousands of new host molecules have been designed or found by chance over the last ~60 years.
Inclusion Compounds vs Co-crystals
Inclusion chemistry stimulated research on various applications of non-valent interactions in chemical science and materials engineering. Co-crystals arise when two or more molecular components, which are solids in their pure forms, produce a new crystal structure. Co-crystals display many similarities to inclusion compounds however their components cannot be labeled as host and guest. In the inclusion compound, the host molecules build the crystal framework, while the guest molecules play a secondary role filling the cavities. In co-crystals both components are essential in producing the new structure and none is more important than the other.
One important application of co-crystals is the production of new forms of pharmaceuticals, where the active ingredient is co-crystallized with another component. Co-crystallization changes bioavailability, dissolution rate, solubility and shelf life of the drug, as well as other pertinent properties.
Mixed crystals 
Types of mixed molecular crystals (the term Mixed Crystals was used by Kitaigorodsky)