Inclusion compound

Example of an inclusion complex consisting of a p-xylylenediammonium bound within a cucurbituril

In host–guest chemistry, an inclusion compound is a complex in which one chemical compound (the "host") has a cavity into which a "guest" compound can be accommodated. The interaction between the host and guest involves purely van der Waals bonding. The definition of inclusion compounds is very broad, extending to channels formed between molecules in a crystal lattice in which guest molecules can fit.

IUPAC definition

Inclusion Compound: A complex in which one component (the host) forms a cavity or, in the case of a crystal, a crystal lattice containing spaces in the shape of long tunnels or channels in which molecular entities of a second chemical species (the guest) are located. There is no covalent bonding between guest and host, the attraction being generally due to van der Waals forces.

Examples and case studies

Calixarenes

Calixarenes and related formaldehyde-arene condensates are one class of hosts that form inclusion compounds. One famous illustration is the adduct with cyclobutadiene, which otherwise is unstable.

Cyclodextrins

Cyclodextrins are well established hosts for the formation of inclusion compounds.^[1][2][3] Illustrative is the case of ferrocene which is inserted into the cyclodextrin at 100 °C under hydrothermal conditions.

Cyclodextrin also forms inclusion compounds with fragrances. As a result, the fragrance molecules have a reduced vapor pressure and are more stable towards exposure to light and air. When incorporated into textiles the fragrance lasts much longer due to the slow-release action.

Non-examples

Cryptands and crown ethers typically do not form inclusion complexes since the guest is bound by forces stronger than van der Waals bonding. If the guest is enclosed on all sides so that it is 'trapped', the compound is known as a clathrate, not an inclusion complex. In molecular encapsulation, a guest molecule is trapped inside another molecule.