Cryptand

  Cryptands are a family of synthetic bi- and polycyclic multidentate ligands for a variety of cations.^[2] 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.^[3] 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 ethers but are more selective and complex the guest ions more strongly. The resulting complexes are lipophilic.

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Structure

The most common and most important cryptand is N[CH₂CH₂OCH₂CH₂OCH₂CH₂]₃N; 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." All-amine cryptands exhibit particularly high affinity for alkali metal cations, which has allowed the isolation of salts of K^-.^[4]

Properties

The three-dimensional interior cavity of a cryptand provides a binding site - or nook - for "guest" ions. The complex between the cationic guest and the cryptand is called a cryptate. Cryptands form complexes with many "hard cations" including NH₄⁺, lanthanides, 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⁺ vs. K⁺).

Uses

Cryptands although they are more expensive and more difficult to prepare offer much better selectivity and strength of binding^[5] than other complexants for alkali metals, such as crown ethers. They are able to extract otherwise insoluble salts into organic solvents. Cryptands increase the reactivity of anions in salts since they effectively break up ion-pairs. They can be also be used as phase transfer catalysts by transferring ions from one phase to another.^[6] Cryptands enabled the synthesis of the alkalides and electrides. They have also been used in the crystallization of Zintl ions such as Sn₉²⁻.

References

1. ^ Alberto, R.; Ortner, K.; Wheatley, N.; Schibli, R.; Schubiger, A. P. "Synthesis and properties of boranocarbonate: a convenient in situ CO source for the aqueous preparation of [^99mTc(OH₂)₃(CO)₃]⁺." J. Am. Chem. Soc. 2001, 121, 3135-3136. doi:10.1021/ja003932b
2. ^ von Zelewsky, A. Stereochemistry of Coordination Compounds; John Wiley: Chichester, 1995. ISBN 0-471-95057-2.
3. ^ Lehn, J. M. Supramolecular Chemistry: Concepts and Perspectives; VCH: Weinhiem, 1995.
4. ^ Kim, J.; Ichimura, A. S.; Huang, R. H.; Redko, M.; Phillips, R. C.; Jackson, J. E.; Dye, J. L. "Crystalline Salts of Na⁻ and K⁻ (Alkalides) that Are Stable at Room Temperature." J. Am. Chem. Soc. 1999, 121, 10666–10667. doi:10.1021/ja992667v
5. ^ Dietrich, B. "Cryptands" in Comprehensive Supramolecular Chemistry; Gokel, G. W. Ed; Elsevier: Oxford, 1996; Vol. 1, pp 153–211. ISBN 0080406106.
6. ^ Landini, D.; Maia, A.; Montanari, F.; Tundo, P. "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." J. Am. Chem. Soc. 1979, 101, 2526–2530. doi:10.1021/ja00504a004

General reading

Lee, J.D. (1991). Concise Inorganic Chemistry, 4th edition, New York: Chapman & Hall, 306-08 & 353. ISBN 0-412-40290-4.