Molecular encapsulation

Molecular encapsulation in supramolecular chemistry is the confinement of a guest molecule inside the cavity of a supramolecular host molecule (molecular capsule, molecular container or cage compounds). Examples of supramolecular host molecule include carcerands and endohedral fullerenes.

An important implication of encapsulating a molecule at this level is that the guest is prevented from contacting other molecules that it might otherwise react with. Thus the encapsulated molecule behaves very differently from the way it would when in solution. The guest molecule tends to be extremely unreactive and often has much different spectroscopic signatures. Compounds normally highly unstable in solution, such as arynes or cycloheptatetraene, have been successfully isolated at room temperature when molecularly encapsulated.

One of the first examples of encapsulating a structure at the molecular level was demonstrated by Cram and coworkers ; they were able to isolate highly unstable, antiaromatic cylobutadiene at room temperature by encapsulating it within a hemicarcerand. Isolation of cyclobutadiene allowed chemists to experimentally confirm one of the most fundamental predictions of the rules of aromaticity.

In another example the cage consists of a gallium tetrahedral cluster compound stabilized by 6 bidentate catechol amide ligands residing at the tetrahedron edges. The guest is a 16 electron and thus very reactive ruthenium metallocene (an organometallic catalyst) with a cyclopentadienyl ligand (red) and a 1,3,7-octatriene ligand (blue). The total charge for this anion is 11 and the counterions are 5 tetramethyl ammonium cations and 6 potassium cations. The ruthenium compound decomposes in water within minutes but encapsulated it survives in water for weeks.

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