Cryptands selective binding

Glasses exist that fnnction as selective electrodes for many different monovalent and some divalent cations. Alternatively, a hydrophobic membrane can be made semiper-meable if a hydrophobic molecnle called an ionophore that selectively binds an ion is dissolved in it. The selectivity of the membrane is determined by the structnre of the ionophore. Some ionophores are natnral products, such as gramicidin, which is highly specific for K+, whereas others such as crown ethers and cryptands are synthetic. Ions such as, 1, Br, and N03 can be detected using quaternary ammonium cationic surfactants as a lipid-soluble counterion. ISEs are generally sensitive in the 10 to 10 M range, but are not perfectly selective. The most typical membrane material used in ISEs is polyvinyl chloride plasticized with dialkylsebacate or other hydrophobic chemicals. 

The field, as we now know it, started with the selective binding of alkali metal cations by natural [1.21-1.23] as well as by synthetic macrocyclic and macropoly-cyclic ligands, the crown ethers [1.24,1.25] and the cryptands [1.26,1.27]. The out-... 

The macrobicyclic cryptands also bind AEC very strongly. Ligand 9 displays a unique and very high preference for Sr2+ and Ba2+ over Ca2+. Suitable structural modifications allow control over the M2+/M+ selectivity from preference for AEC to preference for AC binding [2.31]. 

Linear recognition is displayed by the hexaprotonated form of the ellipsoidal cryptand bis-tren 33, which binds various monoatomic and polyatomic anions and extends the recognition of anionic substrates beyond the spherical halides [3.11, 3.12]. The crystal structures of four such anion cryptates [3.11b] provide a unique series of anion coordination patterns (Fig. 4). The strong and selective binding of the linear, triatomic anion N3" results from its size, shape and site complementarity to the receptor 33-6H+. In the [N3 pyramidal arrays of +N-H "N- hydrogen bonds, each of which binds one of the two terminal nitrogens of N3-. 

As we have implied, the ability of these host molecules to bind guests is often very specific, often linked to the hydrogen-bonding ability of the host, enabling the host to pull just one molecule or ion out of a mixture. This is called molecular recognition In general, cryptands, with their well-defined 3D cavities, are better for this than monocyclic crown ethers or ether derivatives. An example is the host 30, which selectively binds the dication 31 ( = 5) rather than 31 ( = 4), and 31 n = 6) rather than 31 (n = 7). The host 32, which is water soluble, forms 1 1 complexes with neutral aromatic hydrocarbons, such as pyrene and fluoranthene. 

An attempt to confer selectivity for K+ upon reduction succeeded by using the azocryptand pictured in Scheme II. Upon electrochemical reduction of the azocryptand portion of the cryptand, enhanced binding by more than a hundredfold was measurable for K+ although no enhancement with Na+ was observed (52). Fine tuning of molecular parameters should afford both binding enhancement and selectivity. 

A rational approach to developing a fluorescent chemosensor for potassium (K ) is presented. In this approach, J-M. Lehn s [222] cryptand, which selectively binds K, is covalently attached to coumarin at positions 6 and 7. In this hybrid system, coumarin plays the role of a transducer translating the free energy of supramolecular interaction between the cryptand and into measurable enhancement of its fluorescence. By Immobilizing this system on an optical fiber, the continuous monitoring of in patients undergoing open-heart surgery can be achieved. 

The work of Jean-Marie Lehn has led to a new subfield of chemistry called supramolecular chemistry. He studied the binding between metal atoms and organic molecules and eventually created cryptands to bind with metal centers (see Chapters 9 and 15 for more details). In 1987 he shared the Nobel Prize with Donald J. Cram and Charles J. Pedersen for their development and use of molecules with structure-specific interactions of high selectivity. ... 

In order to further develop the coordination chemistry of anions and to extend recognition of anionic substrates beyond the spherical halides, an ellipsoidal macro-bicyclic cryptand Bis-Tren (14) was designed, whose hexaprotonated form was expected to bind various anions [9, 10]. Indeed, potentiometric and spectroscopic measurements showed that (14)-6H complexes a number of monovalent and polyvalent anions. The strong and selective binding observed for the linear triatomic anion NJ may be attributed to its complementarity to the molecular cavity of (14)-6H . As confirmed by crystal structure determination, NJ forms the cryptate [N c (14)-6H ] (15), in which the substrate is bound inside the cavity by two pyramidal arrays of three hydrogen bonds, which hold the two terminal... 

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