In chemistry, cryptands are a family of synthetic,

bicyclic In chemistry, a bicyclic molecule () is a molecule that features two joined rings. Bicyclic structures occur widely, for example in many biologically important molecules like α-thujene and camphor. A bicyclic compound can be carbocyclic (a ...

and polycyclic, multidentate

ligand In coordination chemistry, a ligand is an ion or molecule ( functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's ele ...

s for a variety of

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

s. The Nobel Prize for Chemistry in 1987 was given to Donald J. Cram, Jean-Marie Lehn, and Charles J. Pedersen for their efforts in discovering and determining uses of cryptands and crown ethers, thus launching the now flourishing field of

supramolecular chemistry Supramolecular chemistry refers to the branch of chemistry concerning chemical systems composed of a discrete number of molecules. The strength of the forces responsible for spatial organization of the system range from weak intermolecular force ...

. The term ''cryptand'' implies that this ligand binds substrates in a

crypt A crypt (from Latin '' crypta'' " vault") is a stone chamber beneath the floor of a church or other building. It typically contains coffins, sarcophagi, or religious relics. Originally, crypts were typically found below the main apse of a ...

, interring the guest as in a burial. These molecules are three-dimensional analogues of

crown ether In organic chemistry, crown ethers are cyclic chemical compounds that consist of a ring containing several ether groups (). The most common crown ethers are cyclic oligomers of ethylene oxide, the repeating unit being ethyleneoxy, i.e., . I ...

s but are more selective and strong as complexes for the guest ions. The resulting complexes are lipophilic.

Structure

The most common and most important cryptand is ; the systematic

IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...

name for this compound is 1,10-diaza-4,7,13,16,21,24-hexaoxabicyclo .8.8exacosane. This compound is termed .2.2ryptand, where the numbers indicate the number of

ether In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula , where R and R′ represent the alkyl or aryl groups. Ethers can again b ...

oxygen atoms (and hence binding sites) in each of the three bridges between the amine nitrogen caps. Many cryptands are commercially available under the tradename Kryptofix. All-amine cryptands exhibit particularly high affinity for alkali metal cations, which has allowed the isolation of salts of K⁻.

Properties

Cation binding

The three-dimensional interior cavity of a cryptand provides a binding site – or host – for "guest" ions. The complex between the

ic guest and the cryptand is called a cryptate. Cryptands form complexes with many "hard cations" including , lanthanoids,

alkali In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of ...

metals, and alkaline earth metals. In contrast to crown ethers, cryptands bind the guest ions using both

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at seve ...

and

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as we ...

donors. This three-dimensional encapsulation mode confers some size-selectivity, enabling discrimination among alkali metal cations (e.g. Na⁺ vs. K⁺). Some cryptands are luminescent.

Anion binding

Polyamine-based cryptands can be converted to polyammonium cages, which exhibit high affinities for anions.

Laboratory uses

Cryptands enjoy no commercial applications but are reagents for the synthesis of inorganic and organometallic salts. Although more expensive and more difficult to prepare than

s, cryptands bind alkali metals more strongly. They are especially used to isolate salts of highly basic anions. They convert solvated alkali metal cations into lipophilic cations, thereby conferring solubility in organic solvents to the resulting salts. Referring to achievements that have been recognized in textbooks, cryptands enabled the synthesis of the alkalides and electrides. For example, addition of 2,2,2-cryptand to a solution of sodium in ammonia affords the salt

a(2,2,2-crypt) A, or a, is the first letter and the first vowel of the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is ''a'' (pronounced ), plural ''aes'' ...

sup>+e⁻, isolated a blue-black paramagnetic solid. Cryptands have also been used in the crystallization of Zintl ions such as . Although rarely practical, cryptands can serve as

phase transfer catalyst In chemistry, a phase-transfer catalyst or PTC is a catalyst that facilitates the transition of a reactant from one phase into another phase where reaction occurs. Phase-transfer catalysis is a special form of heterogeneous catalysis. Ionic ...

s by transferring ions.

References

General reading

* * * {{cite journal , journal =

Pure Appl. Chem. ''Pure and Applied Chemistry'' is the official journal for the International Union of Pure and Applied Chemistry (IUPAC). It is published monthly by Walter de Gruyter Walter de Gruyter GmbH, known as De Gruyter (), is a German scholarly publ ...

, year = 1993 , volume = 65 , issue = 3 , pages = 511–514, doi = 10.1351/pac199365030511, title = Simple methods for the preparation of cryptands, first1=K. E., last1= Krakowiak, first2= J. S., last2= Bradshaw, first3= H.-Y., last3= An , first4= R. M., last4= Izatt, doi-access= free Supramolecular chemistry Amines Chelating agents