Design and Synthesis of a Thermally Stable Organic Electride Mikhail Y. Redko, ² James E. Jackson, ² Rui H. Huang, ² and James L. Dye* ,²,‡ Contribution from the Department of Chemistry, Michigan State UniVersity, East Lansing, Michigan 48824, and SiGNa Chemistry, LLC, 530 East 76th Street, Suite 9E, New York, New York 10021 Received May 17, 2005; E-mail: dye@msu.edu Abstract: An electride has been synthesized that is stable to auto-decomposition at room temperature. The key was the theoretically directed synthesis of a per-aza analogue of cryptand[2.2.2] in which each of the linking arms contains a piperazine ring. This complexant was designed to provide strong complexation of Na + via pre-organization of a “crypt” that contains eight nonreducible tertiary amine nitrogens. The structure and properties indicate that, as with other electrides, the “anions” are electrons trapped in the cavities formed by close-packing of the complexed cations. The isostructural sodide, with Na - anions in the cavities, is also stable at and above room temperature. Introduction Organic electrides are crystalline salts in which alkali cations (Li + through Cs + ) are complexed by organic molecules that serve to isolate the cations from electrons trapped in intermo- lecular cavities and channels. 1-7 Electrides provide unique examples of low-density electron gases confined to cavities and channels of known geometry, which makes them related to both salts and plasmas. Therefore, they are of interest in theories of electron-spin interactions and the insulator-metal transition. Because the electrons are weakly bound, electrides also exhibit low-energy electron emission 8,9 that could be useful in devices such as IR-sensitive photomultipliers and thermoelectric power or refrigeration sources. Thermal instability above about -40 °C of the seven previously synthesized electrides that contain oxa-based complexants presented major difficulties in their study and use. For example, electrides made with crown ethers and cryptands, such as cryptand[2.2.2] (1), spontaneously decom- posed at ambient temperatures to ethylene and an alkoxide. 10 Here we describe the design, synthesis, structure, and properties of an electride and a sodide that are stable (in the absence of air and moisture) to this destructive process up to and above room temperature. When alkali metals can be dissolved in pure or mixed amine or ether solvents without a complexant, such solutions generally contain, in addition to alkali cations, both solvated electrons (e - solv ) and alkali metal anions (M - ). 11,12 Removal of the solvent, or the addition of a less polar solvent, results in re-formation of the metal. The addition of a powerful complexant for alkali metal cations, such as a cryptand, a crown ether, or an aza analogue, greatly increases metal solubility by complexing the cations, and prevents re-formation of the metal upon solvent removal. The synthesis of pure electrides rather than mixtures with alkalides requires suppression of the concentration of alkali metal anions in solution. In favorable cases, a slight excess of a strong enough complexant, L, for the cations can suppress alkali metal anion formation through the reaction, For a given solvent, the equilibrium position of reaction 1 depends on both the alkali metal and the complexant. At one extreme, Li - has never been observed in solution, while Na - is so thermodynamically favored in amines and ethers that, until the present work, no electride with Na + L as the cation had been synthesized. The previous synthesis of a thermally stable sodide and a potasside 13 exploited the ability of the fully methylated aza- cryptand, 2, to complex K + . Its thermal stability confirmed earlier conclusions that tertiary nitrogen aza analogues of crown ethers and cryptands are robust toward reductive bond cleavage. For example, sodides that contained Li + in a smaller aza- cryptand 14 and those that had K + or Cs + complexed by an aza- crown ether 15 were stable to decomposition at and above room ² Michigan State University. ‡ SiGNa Chemistry, LLC. (1) Dawes, S. B.; Ward, D. L.; Huang, R. H.; Dye, J. L. J. Am. Chem. Soc. 1986, 108, 3534-3535. (2) Dye, J. L. Prog. Inorg. Chem. 1984, 32, 327-441. (3) Dye, J. L. Nature 1993, 365, 10-11. (4) Dye, J. L.; Wagner, M. J.; Overney, G.; Huang, R. H.; Nagy, T. F.; Tomanek, D. J. Am. Chem. Soc. 1996, 118, 7329-7336. (5) Dye, J. L. Inorg. Chem. 1997, 36, 3816-3826. (6) Dye, J. L. Science 2003, 301, 607-608. (7) Wagner, M. J.; Dye, J. L. In Molecular Recognition: Receptors for Cationic Guests, 1st ed.; Gokel, G. W., Ed.; Pergamon Press: Oxford, U.K., 1996; Vol. 1, pp 477-510. (8) Huang, R. H.; Dye, J. L. Chem. Phys. Lett. 1990, 166, 133-136. (9) Phillips, R. C.; Dye, J. L. Chem. Mater. 2000, 12, 3642-3647. (10) Cauliez, P. M.; Jackson, J. E.; Dye, J. L. Tetrahedron Lett. 1991, 32, 5039- 5042. (11) Matalon, S.; Golden, S.; Ottolenghi, M. J. Phys. Chem. 1969, 73, 3098- 3101. (12) DeBacker, M. G.; Dye, J. L. J. Phys. Chem. 1971, 75, 3092-3096. (13) Kim, J.; Ichimura, A. S.; Huang, R. H.; Redko, M.; Phillips, R. C.; Jackson, J. E.; Dye, J. L. J. Am. Chem. Soc. 1999, 121, 10666-10667. (14) Eglin, J. L.; Jackson, E. P.; Moeggenborg, K. J.; Dye, J. L.; Bencini, A.; Micheloni, M. J. Inclusion Phenom. 1992, 12, 263-274. (15) Kuchenmeister, M. E.; Dye, J. L. J. Am. Chem. Soc. 1989, 111, 935-938. M - + L h M + L + 2e - solv (1) Published on Web 08/10/2005 12416 9 J. AM. CHEM. SOC. 2005, 127, 12416-12422 10.1021/ja053216f CCC: $30.25 © 2005 American Chemical Society