chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...

, cryptands are a family of synthetic, bicyclic and polycyclic, multidentate ligands 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 convent ...

s. The Nobel Prize for Chemistry in 1987 was given to

Donald J. Cram Donald James Cram (April 22, 1919 – June 17, 2001) was an American chemist who shared the 1987 Nobel Prize in Chemistry with Jean-Marie Lehn and Charles J. Pedersen "for their development and use of molecules with structure-specific inter ...

Jean-Marie Lehn Jean-Marie Lehn (born 30 September 1939) is a French chemist. He received the Nobel Prize in Chemistry together with Donald Cram and Charles Pedersen in 1987 for his synthesis of cryptands. Lehn was an early innovator in the field of supramolec ...

, and

Charles J. Pedersen Charles John Pedersen ( ja, 安井良男, ''Yasui Yoshio'', October 3, 1904 – October 26, 1989) was an American organic chemist best known for describing methods of synthesizing crown ethers during his entire 42-year career as a chemist for D ...

for their efforts in discovering and determining uses of cryptands and crown ethers, thus launching the now flourishing field of supramolecular chemistry. 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 chur ...

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

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

electride An electride is an ionic compound in which an electron is the anion. Solutions of alkali metals in ammonia are electride salts. In the case of sodium, these blue solutions consist of a(NH3)6sup>+ and solvated electrons: :Na + 6 NH3 → a(N ...

s. 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 catalysts 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