Crown ethers: sensors for ions and molecular scaffolds for materials and biological models.

Cryptates: the chemistry of macropolycyclic inclusion complexes

Looking back to the introductory section, it appears that in recent years appreciable progress has been made in understanding and controlling the process of spherical recognition of alkali and alkaline-earth cations by organic ligand molecules. Information stored in various structural features allows the thermodynamic and kinetic features of the complexation process to be monitored. Further progress may be achieved by making use of the feedback from present results to adjust and improve the model and to analyze the information-property relationship.

Design of organic complexing agents Strategies towards properties

Electrides are ionic compounds that have alkali metal cations complexed by a crown ether or cryptand, with trapped electrons as counterions. The crystal structures and properties of two electrides illustrate the diversity that is encountered. One Cs(+) (18-crown-6)(z)e(-), has relatively isolated, trapped electrons apparently centered at each anionic site. It has a low conductivity consistent with electron localization, with an activation energy for conductivity of at least 0.45 electron volt. The other, K(+) (cryptand[2.2.2])e(-), has electron pairs trapped in an elongated cavity in a singlet ground state, but there is also a thermally accessible paramagnetic state available. This electride is much more conducting, with an activation energy of only 0.02 electron volt.

https://science.sciencemag.org/content/sci/247/4943/663.full.pdf

Electrides : ionic salts with electrons as the anions

Dynamic NMR studies of supramolecular complexes

Crystalline salt of the sodium anion (Na

The NMR chemical shift and line width were measured for /sup 23/Na/sup -/ in tetrahydrofuran (THF), ethylamine (EA), and methylamine (MA), for /sup 87/Rb/sup -/ in THF and EA, and for /sup 133/Cs/sup -/ in THF. In all cases, the counterion was the 2,2,2 cryptate complex of the corresponding cation. The chemical shift of Na/sup -/ is, within experimental error, the same as that calculated for the gaseous anion (based upon the measured value for the gaseous atom) and is independent of solvent. Comparison with the solvent-dependent chemical shift of Na/sup +/ provides conclusive evidence that Na/sup -/ is a ''genuine'' anion with two electrons in a 3s orbital which shield the 2p electrons from the influence of the solvent. (auth)

Nuclear magnetic resonance studies of alkali metal anions

Sodium-23 nuclear magnetic resonance study of exchange rates. Sodium cryptate in ethylenediamine

The measurement conditions and data treatment used to obtain accurate rate constants and activation energies from /sup 23/Na pulsed Fourier transform data are described in detail. Application is made to the exchange of Na/sup +/ between the free state and the complex with 2,2,2-cryptand in four solvents. The exchange proceeds via dissociation of the complex. Activation enthalpies of 12.3, 13.6, 13.8, and 16.1 kcal mol/sup -1/ and activation entropies of -7.6, -12.6, -8.1, and +5.3 cal mol/sup -1/ deg/sup -1/ were obtained for the release of Na/sup +/ from the 2,2,2-cryptate complex in ethylenediamine, pyridine, tetrahydrofuran, and water, respectively.

A sodium-23 nuclear magnetic resonance study of the exchange kinetics of sodium(1+) ion with 2,2,2-cryptate complexes in water, ethylenediamine, tetrahydrofuran, and pyridine

Alkali metals can be made to dissolve in solvents in which they are normally insoluble or only slightly soluble, by using cyclic polyethers to complex the cations. Although the use of the cyclic polyether, dicyclohexyl-18-crown-6 results mainly in the formation of M− as the reducing species, the bicyclic polyether, “cryptate” yields in addition, substantial concentrations of solvated electrons. It also increases the solubility compared with the “crown” compound.



Durch zyklische Polyather, die Kationkomplexe zu binden vermogen, konnen Alkalimetalle auch in solchen Losungsmitteln gelost werden. in denen sie sonst nicht oder nur wenig loslich sind. Wahrend Dicyklohexyl-18-crown-6 bevorzugt zur Bildung von M− (M Metallatom) fuhrt, ergibt der bicyklische Polyather “Cryptat” bei erhohter Metall-Loslichkeit zusatzlich betrachtliche Konzentrationen an solvatisierten Elektronen.

Joseph M. Ceraso

Papers

Crystalline salt of the sodium anion (Na

Nuclear magnetic resonance studies of alkali metal anions

Sodium-23 nuclear magnetic resonance study of exchange rates. Sodium cryptate in ethylenediamine

A sodium-23 nuclear magnetic resonance study of the exchange kinetics of sodium(1+) ion with 2,2,2-cryptate complexes in water, ethylenediamine, tetrahydrofuran, and pyridine

Alkali Metal solutions Effect of Two Cyclic Polyethers on solubility and Spectra