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{{short description|Cyclic, multidentate ligands adept at encapsulating cations}}
[[File:Cryptate of potassium cation.jpg|thumbnail|Structure of [[2.2.2-Cryptand|[2.2.2]cryptand]] encapsulating a potassium cation (purple). At crystalline state, obtained with an X-ray diffraction.<ref>{{cite journal | last1= Alberto|first1= R.|last2= Ortner|first2= K. |last3=Wheatley|first3= N.|last4= Schibli|first4= R.|last5= Schubiger|first5= 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 |pmid= 11457025| issue = 13|bibcode= 2001JAChS.123.3135A}}</ref>]]
[[File:Cryptand.svg|thumb|[2.2.2]Cryptand]]

In [[chemistry]], '''cryptands''' are a family of synthetic, [[bicyclic]] and [[Polycyclic compound|polycyclic]],  [[Denticity|multidentate]] [[ligand]]s for a variety of [[cation]]s.<ref>{{cite book|last=Von Zelewsky|first=A.|title=Stereochemistry of Coordination Compounds|publisher=John Wiley|location=Chichester|date=1995|isbn=0-471-95057-2|url-access=registration|url=https://archive.org/details/stereochemistryo0000zele}}</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>{{cite book|last=Lehn|first= J. M.|title=Supramolecular Chemistry: Concepts and Perspectives|publisher= VCH|location= Weinheim |date=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 [[Structural analog|analogues]] of [[crown ether]]s but are more selective and strong as [[Coordination complex|complexes]] for the guest ions. The resulting complexes are [[lipophilic]].<ref>{{cite journal |doi=10.1002/(SICI)1521-3773(19990419)38:8<1018::AID-ANIE1018>3.0.CO;2-G|title=Structural Classification and General Principles for the Design of Spherical Molecular Hosts|year=1999|last1=MacGillivray|first1=Leonard R.|last2=Atwood|first2=Jerry L.|journal=Angewandte Chemie International Edition|volume=38|issue=8|pages=1018–1033|pmid=25138490}}</ref>

==Structure==
The most common and most important cryptand is {{chem2|N[CH2CH2OCH2CH2OCH2CH2]3N}}; the systematic [[IUPAC]] name for this compound is 1,10-diaza-4,7,13,16,21,24-hexaoxabicyclo[8.8.8]hexacosane.  This compound is termed [[2.2.2-Cryptand|[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 [[potasside|K<sup>−</sup>]].<ref>{{cite journal | last1= Kim|first1= J.|last2= Ichimura|first2= A. S.|last3= Huang|first3= R. H.|last4= Redko|first4= M.|last5= Phillips|first5= R. C.|last6= Jackson|first6= J. E.|last7= Dye|first7= 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 | issue = 45|bibcode= 1999JAChS.12110666K}}</ref>

==Properties==
===Cation binding===
The three-dimensional interior cavity of a cryptand provides [[Host–guest chemistry|a binding site – or host – 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 {{chem|NH|4|+}}, [[lanthanoids]], [[alkali]] 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<SUP>+</SUP> vs. K<SUP>+</SUP>). Some cryptands are luminescent.<ref>{{cite journal |doi=10.1016/S0010-8545(00)00246-0|title=Design principles of fluorescent molecular sensors for cation recognition|year=2000|last1=Valeur|first1=B.|journal=Coordination Chemistry Reviews|volume=205|pages=3–40}}</ref>
===Anion binding===
[[Polyamine]]-based cryptands can be converted to polyammonium cages, which exhibit high affinities for anions.
<ref>{{cite journal |doi=10.1002/1521-3773(20010202)40:3<486::AID-ANIE486>3.0.CO;2-P|title=Anion Recognition and Sensing: The State of the Art and Future Perspectives|year=2001|last1=Beer|first1=Paul D.|last2=Gale|first2=Philip A.|journal=Angewandte Chemie International Edition|volume=40|issue=3|pages=486–516|pmid=11180358|s2cid=22334242 }}</ref>
==Laboratory uses==
Cryptands enjoy some commercial applications (e.g. in homogenous-time-resolved-fluorescence, HTRF, technologies using Eu3+ as central ion). More importantly, they are reagents for the synthesis of inorganic and organometallic salts. Although more expensive and more difficult to prepare than [[crown ether]]s, cryptands bind alkali metals more strongly.<ref>{{cite journal |last1=Danil de Namor |first1=Angela |last2=Ghousseini |first2=Lily |last3=Lee |first3=Walter |title=Stability constants and free energies of complexation of metal-ion cryptates in nitromethane. Derived parameters for the extraction of cations by cryptand 222 from water to pure nitromethane |journal=Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases |date=1985 |volume=81 |issue=10 |pages=2495–2502 |doi=10.1039/F19858102495 |url=https://pubs.rsc.org/en/content/articlelanding/1985/f1/f19858102495|url-access=subscription }}</ref> They are especially used to isolate salts of highly basic anions.<ref>{{cite encyclopedia|last=Dietrich|first= B. |title=Cryptands|encyclopedia=Comprehensive Supramolecular Chemistry|editor-last= Gokel|editor-first= G. W.  |publisher=Elsevier|location= Oxford|date= 1996| volume= 1| pages= 153–211| isbn=0-08-040610-6}}</ref> 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 [[alkalide]]s and [[electride]]s. For example, addition of 2,2,2-cryptand to a solution of sodium in ammonia affords the salt [Na(2,2,2-crypt)]<sup>+</sup>e<sup>−</sup>, isolated a blue-black paramagnetic solid.<ref>{{cite journal | author = Dye, J. L. | title = Electrons as Anions | journal = [[Science (journal)|Science]] | year = 2003 | volume = 301 | pages = 607–608 | doi = 10.1126/science.1088103 | pmid = 12893933 | issue = 5633| s2cid = 93768664 }}</ref><ref>Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. {{ISBN|0-12-352651-5}}</ref> Cryptands have also been used in the crystallization of [[Zintl ion]]s such as {{chem|Sn|9|4−}}.<ref>{{cite journal |doi=10.1021/ic50170a036|title=Stable Homopolyatomic Anions. Synthesis and Crystal Structures of Salts Containing the Pentaplumbide(2-) and Pentastannide(2-) Anions|year=1977|last1=Edwards|first1=Paul A.|last2=Corbett|first2=John D.|journal=Inorganic Chemistry|volume=16|issue=4|pages=903–907}}</ref>

Although rarely practical, cryptands can serve as [[phase transfer catalyst]]s since their cationic complexes are lipophilic.<ref>{{cite journal | last1= Landini|first1= D.|last2= Maia|first2= A.|last3= Montanari|first3= F. |last4=Tundo|first4= P. | title = Lipophilic [2.2.2]cryptands as phase-transfer catalysts. Activation and Nucleophilicity of Anions in Aqueous-Organic Thirteen-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 | issue = 10}}</ref>

==See also==
* [[Tris(2-(2-methoxyethoxy)ethyl)amine]], an acyclic amine-polyether ligand that is less expensive than cryptands
* [[Clathrate]], a compound that encapsulates ions or molecules
* [[Clathrochelate]], a [[ligand]] that encapsulated metal ions
* [[Cryptophane]], a family of organic compounds that encapsulates other molecules
* [[Cyclodextrin]], a family of organic compounds consists of ring of glucose subunits, also used for [[host-guest chemistry]]

==References==
{{reflist}}

==General reading==
* {{GoldBookRef | file = C01426 | title = cryptand}}
*{{cite book | first = J.D.| last =Lee| date= 1991 | title =Concise Inorganic Chemistry  | edition = 4th| pages = 306–308 & 353| publisher = Chapman & Hall| location = New York| isbn = 0-412-40290-4 }}
* {{cite journal | journal = [[Pure Appl. Chem.]] | 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}}

[[Category:Supramolecular chemistry]]
[[Category:Amines]]
[[Category:Chelating agents]]