James L. Dye

Electrides: Early Examples of Quantum Confinement

Electrides are ionic solids with cavity-trapped electrons, which serve as the anions. Localization of electrons in well-defined trapping sites and their mutual interactions provide early examples of quantum confinement, a subject of intense current interest. We synthesized the first crystalline electride, Cs(+)(18-crown-6)(2)e(-), in 1983 and determined its structure in 1986; seven others have been made since. This Account describes progress in the synthesis of both organic and inorganic electrides and points to their promise as new electronic materials. Combined studies of solvated electrons in alkali metal solutions and the complexation of alkali cations by crown ethers and cryptands made electride synthesis possible. After our synthesis of crystalline alkalides, in which alkali metal anions and encapsulated alkali cations are present, we managed to grow crystalline electrides from solutions that contained complexed alkali cations and solvated electrons. Electride research is complicated by thermal instability. Above approximately -30 degrees C, trapped electrons react with the ether groups of crown ethers and cryptands. Aza-cryptands replace ether oxygens with less reactive tertiary amine groups, and using those compounds, we recently synthesized the first room-temperature-stable organic electride. The magnetic and electronic properties of electrides depend on the geometry of the trapping sites and the size of the open channels that connect them. Two extremes are Cs(+)(15-crown-5)(2)e(-) with nearly isolated trapped electrons and K(+)(cryptand 2.2.2)e(-), in which spin-pairing of electrons in adjacent cavities predominates below 400 K. These two electrides also differ in their electrical conductivity by nearly 10 orders of magnitude. The pronounced effect of defects on conductivity and on thermonic electron emission suggests that holes as well as electrons play important roles. Now that thermally stable organic electrides can be made, it should be possible to control the electron-hole ratio by incorporation of neutral complexant molecules. We expect that in further syntheses researchers will elaborate the parent aza-cryptands to produce new organic electrides. The promise of electrides as new electronic materials with low work functions led us and others to search for inorganic electrides. The body of extensive research studies of alkali metal inclusion in the pores of alumino-silicate zeolites provided the background for our studies of pure silica zeolites as hosts for M(+) and e(-) and our later use of nanoporous silica gel as a carrier of high concentrations of alkali metals. Both systems have some of the characteristics of inorganic electrides, but the electrons and cations share the same space. In 2003, researchers at the Tokyo Institute of Technology synthesized an inorganic electride that has separated electrons and countercations. This thermally stable electride has a number of potentially useful properties, such as air-stability, low work function, and metallic conductivity. Now that both organic and inorganic electrides have been synthesized, we expect that experimental and theoretical research on this interesting class of materials will accelerate.

Electrides: Ionic Salts with Electrons as the Anions

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)ze-, 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.

Pulse Radiolysis Studies. XVIII. Spectrum of the Solvated Electron in the Systems Ethylenediamine–Water and Ammonia–Water

The pulse radiolysis technique with fast infrared detection was used to determine the optical absorption spectrum of the solvated electron in the systems ethylenediamine–water and ammonia–water. These spectra, determined over the entire concentration range, show the following: In ammonia and in ethylenedamine, the bands at 1550 and 1350 nm, respectively, have the same shape and position as those attributed to the solvated electron in alkali metal solutions. In each of the two‐component systems a single band is seen with the peak position intermediate to those in the pure solvents. For ethylenediamine–water mixtures, the band shape (normalized) and half‐width (energy scale) are invariant with composition, while for ammonia–water mixtures the ratio of the peak position to the half‐width is invariant. These observations suggest a delocalized electron with optical characteristics determined by the aggregate properties of the solvent. Models which require that the optical properties be strongly influenced by s...

Determination of stability constants of cesium [2]-cryptand complexes in nonaqueous solvents by cesium-133 NMR

Cesium complexes with four diazapolyoxamacrobicyclic ligands ([2]-cryptands), C211, C221, C222, and C222B, were investigated in pyridine, acetone, propylene carbonate, N,N-dimethylformamide, acetonitrile, and dimethyl sulfoxide solutions by cesium-133 NMR. The relative stabilities of the complexes are in the order Cs+·C221≥Cs+·C222>Cs+·C222B≫Cs+·C211. The formation constants are strongly influenced by the solvating abilities of the solvents. The NMR data and stereochemical considerations indicate that in the C222−Cs+ system there is an equilibrium between “exclusive” and “inclusive” conformations of the complex. The other three ligands must form only the “exclusive” complex.

Ab Initio SCF MO Study of the Interaction of Lithium with Ammonia

An ab initio molecular orbital study of the stability and electronic structure of lithium–ammonia complexes was made. A complex of lithium with one ammonia molecule is predicted to be stable by 20 kcal/mole relative to a free lithium atom and a free ammonia molecule. The bonding in the complex is primarily through sharing of the lone pair of electrons on ammonia between the ammonia molecule and the lithium atom. The unpaired electron orbital is rearranged upon formation of the complex so that it has very low density in the region of the ammonia with the exception of a sharp peak in density near the nitrogen nucleus. The complex with two ammonia molecules is nearly isoenergetic with the one ammonia complex plus an ammonia molecule. Addition of a second ammonia causes an increase in the preferred lithium–nitrogen distance and an increase in the occupancy of the nitrogen orbitals by the unpaired electron. Ammonia appears repulsive to the unpaired electron in the diammonia complex, also. The implications of t...

Alkali metals in silica gel (M-SG): A new reagent for desulfonation of amines

A novel method for the desulfonation of secondary amines is described. Alkali metals absorbed into nanostructured silica (M-SG) were found to be useful solid-state reagents for the desulfonation of a range of N,N-disubstituted sulfonamides. M-SG reagents are room-temperature-stable free-flowing powders that retain the chemical reactivity of the parent metal, decreasing the danger and associated cost of using reactive metals.

Hyperfine Interactions in Solutions of Cs and Rb in Methylamine

Of all the alkali metals studied in methylamine and ethylenediamine, only Rb and Cs in methylamine showed hyperfine splitting in their ESR spectra. Rb shows a sixline spectrum and Cs an eight-line spectrum. The splitting indicates an appreciable electron density at the metai nucleus. The electron densities of Cs and Rb⁸⁷ in methylamine at 25 deg C are calculated to be 6.9 and 3.5%, respectively, of that for the free atoms. (D.L.C.)

Optical spectra of alkali metal anion and ’’electride’’ films

The optical absorption spectra of thin films produced by evaporation of methylamine from solutions which contain cryptated alkali cations and either alkali metal anions or solvated electrons have been examined over the region 4 000–40 000 cm−1. Films prepared from solutions of Na− show a strong absorption band at 15 400 cm−1, a pronounced shoulder at 18 900 cm−1, and a small but distinct peak at 24 500 cm−1. Films prepared from solutions of K−, Rb−, and Cs− have maxima at 11 900, 11 600, and 10 500 cm−1, respectively, while those prepared from solutions which contain e−solv have maxima at 7400 cm−1 independent of whether the solution is made by contacting K or Rb metals. Films made by evaporating solutions which contain Na−, K−, Rb−, and Cs− are gold colored by reflectance and blue by transmittance and are presumed to be similar to the crystalline salt Na+C⋅Na−, in which C is 2,2,2‐cryptand. The principal absorption maxima occur at wavelengths very close to those of M− in the corresponding metal solutions...

Spectroscopic studies of ionic solvation. XX. Cesium-133 NMR study of cesium salts in different solvents

Cesium-133 chemical shifts were measured in a number of solvents as a function of salt concentration and of the counterion. Infinite-dilution chemical shifts (vs. aqueous Cs+ ion at infinite dilution) ranged from +59.8 ppm for nitromethane solutions to −29.4 ppm for pyridine. In general, the magnitude of the downfield chemical shift reflected the donor ability of the solvents. Ion-pair formation constants were calculated from the concentration dependences of133Cs chemical shifts in several nonaqueous solvents.

Correlated EPR—Optical Spectra of Potassium in Ethylamine—Ammonia Mixtures

Extensive electron paramagnetic resonance (EPR) studies of potassium in ethylamine—ammonia mixtures were made over a temperature range of −180° to +100°C and at varying ammonia concentrations up to 68 mole %. Hyperfine splittings, g values, linewidths, and spin concentrations were measured. Optical spectra were measured from −65° to +35°C for a number of the solutions.Correlation of optical and EPR results rules out the assignment of the visible absorption to the monomeric species responsible for hyperfine splitting. The unusual temperature dependence of the EPR spectra cannot be explained in the light of current models, nor can the ammonia dependence be quantitatively interpreted. A model involving equilibrium between atoms and monomers quantitatively describes the temperature dependence. To explain the dependence upon ammonia concentration, it is necessary to consider two monomeric species, one fully solvated by ethylamine and the other involving replacement of ethylamine by one molecule of ammonia.Abov...