Cryptand
1904052
225750040
2008-07-15T06:06:03Z
Rifleman 82
1255637
identical image on commons
[[Image:Cryptate of pottasium cation.jpg|thumbnail|250px|Structure of [2.2.2]cryptand encapsulating a potassium cation (purple). At crystalline state, obtained with an X-ray diffraction.<ref>{{cite journal | author = Alberto, R.; Ortner, K.; Wheatley, N.; Schibli, R.; Schubiger, A. P. | title = Synthesis and properties of boranocarbonate: a convenient in situ CO source for the aqueous preparation of [<sup>99m</sup>Tc(OH<sub>2</sub>)<sub>3</sub>(CO)<sub>3</sub>]<sup>+</sup> | journal = [[J. Am. Chem. Soc.]] | year = 2001 | volume = 121 | pages = 3135-3136 | doi = 10.1021/ja003932b}}</ref>]]
'''Cryptands''' are a family of synthetic bi- and polycyclic multidentate [[ligand]]s for a variety of cations.<ref>von Zelewsky, A. ''Stereochemistry of Coordination Compounds''; John Wiley: Chichester, 1995. ISBN 0-471-95057-2.</ref> 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]].<ref>Lehn, J. M. ''Supramolecular Chemistry: Concepts and Perspectives''; VCH: Weinhiem, 1995.</ref> The term cryptand implies that this ligand binds substrates in a [[crypt]], interring the guest as in a burial. These molecules are three dimensional analogues of [[crown ether]]s but are more selective and complex the guest ions more strongly. The resulting complexes are lipophilic.
==Structure==
The most common and most important cryptand is N[CH<sub>2</sub>CH<sub>2</sub>OCH<sub>2</sub>CH<sub>2</sub>OCH<sub>2</sub>CH<sub>2</sub>]<sub>3</sub>N; the formal [[IUPAC]] (International Union of Pure and Applied Chemistry) name for this compound is 1,10-diaza-4,7,13,16,21,24-hexaoxabicyclo[8.8.8]hexacosane. So it is easy to see why the common name of "cryptand" was preferable. This compound is termed '''[2.2.2]cryptand''', 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."<ref>{{cas | 23978-09-8}}</ref> All-amine cryptands exhibit particularly high affinity for alkali metal cations, which has allowed the isolation of salts of K<sup>-</sup>.<ref>{{cite journal | author = Kim, J.; Ichimura, A. S.; Huang, R. H.; Redko, M.; Phillips, R. C.; Jackson, J. E.; Dye, J. L. | title = Crystalline Salts of Na<sup>−</sup> and K<sup>−</sup> (Alkalides) that Are Stable at Room Temperature | journal = [[J. Am. Chem. Soc.]] | year = 1999 | volume = 121 | pages = 10666–10667 | doi = 10.1021/ja992667v}}</ref>
==Properties==
The three-dimensional interior cavity of a cryptand provides a binding site - or nook - for "guest" ions. The complex between the [[cation]]ic guest and the cryptand is called a cryptate. Cryptands form complexes with many "hard cations" including NH<sub>4</sub><sup>+</sup>, [[lanthanoids]], [[alkali]] metals, and [[alkaline earth metals]]. In contrast to typical crown ethers, cryptands bind the guest ions using both [[nitrogen]] and [[oxygen]] donors. Their three-dimensional encapsulation mode confers some size-selectivity, enabling discrimination among alkali metal cations (e.g. Na<SUP>+</SUP> vs. K<SUP>+</SUP>).
==Uses==
Cryptands are more expensive and difficult to prepare, but offer much better selectivity and strength of binding<ref>Dietrich, B. "Cryptands" in ''Comprehensive Supramolecular Chemistry''; Gokel, G. W. Ed; Elsevier: Oxford, 1996; Vol. 1, pp 153–211. ISBN 0080406106.</ref> than other complexants for alkali metals, such as [[crown ether]]s. They are able to extract otherwise insoluble salts into organic solvents. They can be also be used as [[phase transfer catalyst]]s by transferring ions from one phase to another.<ref>{{cite journal | author = Landini, D.; Maia, A.; Montanari, F.; Tundo, P. | title = Lipophilic [2.2.2] cryptands as phase-transfer catalysts. Activation and nucleophilicity of anions in aqueous-organic two-phase systems and in organic solvents of low polarity | journal = [[J. Am. Chem. Soc.]] | year = 1979 | volume = 101 | pages = 2526–2530 | doi = 10.1021/ja00504a004}}</ref> Cryptands enabled the synthesis of the alkalides and [[electride]]s. They have also been used in the crystallization of [[Zintl ion]]s such as Sn<sub>9</sub><sup>2−</sup>.
==See also==
* [[Clathrochelate]]
==References==
<references />
==General reading==
* {{GoldBookRef | file = C01426 | title = cryptand}}
*{{cite book | first = J.D.| last =Lee| authorlink = | coauthors = | year = 1991| month = | title =Concise Inorganic Chemistry | chapter = | editor = | others = | edition = 4th edition| pages = 306-08 & 353| publisher = Chapman & Hall| location = New York| id = ISBN 0-412-40290-4| url = }}
[[Category:Supramolecular chemistry]]
[[Category:amines]]
[[Category:chelating agents]]
[[fr:Cryptand]]
[[it:Criptando]]
[[ja:クリプタンド]]
[[pl:Kryptandy]]
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