James L. Dye

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For 50 years, Dye’s research has focused on the species formed when alkali metals interact with solvents or complexants. It became clear in 1969 (from his work and that of others) that alkali metal anions are present in metal-amine solutions along with the well-known solvated electrons. A milestone in this research was his discovery in 1970 that crown ether and cryptand complexants for alkali metal cations greatly enhanced the solubility of alkali metals in amine and ether solvents. This initiated a search for solvent-free salts in which the complexed alkali metal cation has as its counter-ion either an alkali metal anion or a trapped electron. In 1974, the Dye group isolated the first crystalline salt of an alkali metal anion, Na+(cryptand[2.2.2])Na-. Thus the 100-year old dogma that the alkali metals form only cations was shattered! In the years since that discovery, the Dye group has synthesized and characterized more than 40 alkalides, salts that contain the complexed alkali metal cation and one of the anions, Na-, K-, Rb-, or Cs-.

From this point on, a major dream of the group was to synthesize crystalline electrides, salts in which the anion is a trapped electron. For twelve years, only deep blue-black powders could be made. They were clearly electrides according to their optical, magnetic and EPR properties, but the extreme difficulty in handling these reactive and thermally unstable materials frustrated efforts to crystallize them. Finally in 1986 the crystalline electride Cs+(18-crown-6)2e- was synthesized and structurally characterized, followed over the years by seven additional crystalline electrides. The structures and properties of electrides show that the electrons are trapped in cavities and interact with one another through connecting channels. Thus,electrides are early examples of quatum confinement, a subject of intense current interest. To first order, electrides can be viewed as stoichiometric F-centers in which the electrons form a complex lattice gas. Recent research by others, stimulated by the discovery of electrides by the Dye group, has resulted in at least 50 research publications about electrides from other laboratories since 2003.

Studies of these reactive species are difficult because of their thermal instability. In spite of this, the Dye group has used NMR, EPR, ENDOR, magnetic susceptibility, time-resolved fluorescence, photoelectron emission, single crystal reflectance, thin film optical spectra, conductivity, impedance spectroscopy, EXAFS, pulsed radiolysis and neutron scattering to characterize alkalides and electrides.

Although alkalides have become useful two-electron reductants for organic synthesis, the reactivity and especially the thermal instability of alkalides and electrides has, until recently, prevented the exploitation of their unique electronic, magnetic and optical properties. A major focus of Dye’s research until 2004 was the synthesis of thermally stable alkalides and electrides that utilize aza- rather than oxa- crown ethers and cryptands. Thermally stable crystalline alkalides were prepared and, in 2004, a room temperature-stable crystalline electride was synthesized and structurally characterized.

A recent initiative has great potential for the development of a new class of electrides. Instead of using organic complexants to sequester the cation from reduction by trapped electrons, thermally stable 'inorganic electrides' utilize all-silica zeolites as hosts for the alkali metal. Cesium vapor can be incorporated into ITQ-4 as well as into into a 3D all-silica zeolite (ITQ-7) and all-silica zeolite beta at both room temperature and 100 ºC. Sodium, potassium and rubidium can also be incorporated from the vapor phase. Both experimental and theoretical studies have shown that cesium ionizes in the channels to produce Cs+ and trapped electrons. The synthesis by others of completely new classes of "inorganic electrides" has become a topic of great interest recently. We have recently found that liquid alkali metals and alloys can be readily incorporated into the nanoscale pores (~15 nm nominal diameter) of silica gel at loadings up to 40 wt% alkali metal. When formed at room temperature, the metal is present as small particles. Various heating protocols form other stages of included metal cations and electrons. The big advantage for chemistry is that these materials are nearly as powerful reducing agents as the parent metals and can produce clean hydrogen by reaction with water, but they are stable in dry air and easily handled. A company, SiGNa Chemistry Inc., has been formed to commercialize this material. The environmentally benign products of such reactions and the potential savings in solvents, multiple reaction steps, and hazardous wastes, resulted in the receipt by SiGNa of the 2008 Presidential Green Chemistry Challenge Award.

All of the work described here has been published.