Cryptophanes are a class of organic

Organic compound

An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

 supramolecular compound

Supramolecular assembly

A supramolecular assembly or "supermolecule" is a well defined complex of molecules held together by noncovalent bonds. While a supramolecular assembly can be simply composed of two molecules , it is more often used to denote larger complexes of molecules that form sphere-, rod-, or sheet-like...

s studied and synthesized primarily for molecular encapsulation

Molecular encapsulation

Molecular encapsulation in supramolecular chemistry is the confinement of a guest molecule inside the cavity of a supramolecular host molecule...

 and recognition. One possible noteworthy application of cryptophanes is encapsulation and storage of hydrogen

Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 gas for potential use in fuel cell

Fuel cell

A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. Hydrogen is the most common fuel, but hydrocarbons such as natural gas and alcohols like methanol are sometimes used...

 automobiles. Cryptophanes can also serve as containers in which organic chemists can carry out reactions

Chemical reaction

A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...

 that would otherwise be difficult to run under normal conditions. Due to their unique molecular recognition

Molecular recognition

The term molecular recognition refers to the specific interaction between two or more molecules through noncovalent bonding such as hydrogen bonding, metal coordination, hydrophobic forces, van der Waals forces, π-π interactions, electrostatic and/or electromagnetic effects...

 properties, cryptophanes also hold great promise as a potentially new way to study the binding of organic molecules with substrates, particularly as pertaining to biological and biochemical applications.

Discovery

Cryptophanes were discovered by André Collet and Jacqueline Gabard in 1981 when these researchers created, using template-directed synthesis, the first cryptophane, now known as cryptophane-A.

Structure

Cryptophane cages are formed by two cup-shaped [1.1.1]orthocyclophane

Cyclophane

A cyclophane is a hydrocarbon consisting of an aromatic unit and an aliphatic chain that forms a bridge between two non-adjacent positions of the aromatic ring. More complex derivatives with multiple aromatic units and bridges forming cagelike structures are also known...

 units (see cyclotriveratrylene

Cyclotriveratrylene

Cyclotriveratrylene is a macrocycle and used in host-guest chemistry as a molecular host . The compound can be synthesised from veratrole alcohol by addition of a suitable acid which can be perchloric acid in methanol or formic acid or sulfuric acid in acetic acid.An alternative is synthesis from...

), connected by three bridges (denoted Y). There are also choices of the peripheral substitutes R1 and R2 attached to the aromatic rings of the units. Most cryptophanes exhibit two diastereomer

Diastereomer

Diastereomers are stereoisomers that are not enantiomers.Diastereomerism occurs when two or more stereoisomers of a compound have different configurations at one or more of the equivalent stereocenters and are not mirror images of each other.When two diastereoisomers differ from each other at...

ic forms (syn and anti), distinguished by their symmetry

Symmetry

Symmetry generally conveys two primary meanings. The first is an imprecise sense of harmonious or aesthetically pleasing proportionality and balance; such that it reflects beauty or perfection...

 type. This general scheme offers a variety of choices (Y, R1, R2, and symmetry type) by which the shape, the volume, and the chemical properties of the generally hydrophobic pocket inside the cage can be modified, making cryptophanes suitable for encapsulating many types of small molecules and even chemical reactions.

General classification

Depending on their structure, cryptophane cages are classified according to the following table.

Classification of known cryptophane structures

Structure			Name
Bridges Y	R1	R2	anti	syn	described in
3 × O(CX2)2O, where X is H Hydrogen Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...  or D Deuterium Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...	OCX3	OCX3	A		Brotin et al.
3 × O(CX2)2O	OCH2CO2H	OCH2CO2H	A3
3 × O(CX2)2O	OCH3	H	C	D
3 × O(CH2)3O	OCH3	OCH3	E	F
3 × O(CH2)3O	OCH2CO2H	OCH2CO2H	E3
3 × O(CH2)5O	OCH3	OCH3	O	P
3 × O(CH2)5O	OCH2CO2H	OCH2CO2H	O3
3 × OCH2C≡CC≡CH2O	CH3	CH3	γ (gamma)	δ (delta)
2 × O(CH2)2O, 1 × O(CH2)3O	OCH3	OCH3	223
2 × O(CH2)3O, 1 × O(CH2)2O	OCH3	OCH3	233
2 × O(CH2)2O, 1 × O(CH2)4O	OCH3	OCH3	224