Electride

An electride is an ionic compound in which an electron serves the role of the anion.[1] Solutions of alkali metals in ammonia are electride salts.[2] In the case of sodium, these blue solutions consist of [Na(NH3)6]+ and solvated electrons:

Na + 6 NH3 → [Na(NH3)6]+ + e−

The cation [Na(NH3)6]+ is an octahedral coordination complex.

Solid salts

Addition of a complexant like crown ether or [2.2.2]-cryptand to a solution of [Na(NH3)6]+e− affords [Na (crown ether)]+e− or [Na(2,2,2-crypt)]+e−. Evaporation of these solutions yields a blue-black paramagnetic solid with the formula [Na(2,2,2-crypt)]+e−.

Most solid electride salts decompose above 240 K, although [Ca24Al28O64]4+(e−)4 is stable at room temperature.[3] In these salts, the electron is delocalized between the cations. Electrides are paramagnetic, and are Mott insulators. Properties of these salts have been analyzed.[4]

ThI2 and ThI3 have also been reported to be electride compounds.[5] Similarly, CeI
2, LaI
2, GdI
2, and PrI
2 are all electride salts with a tricationic metal ion.[6][7]

Other kinds of electrides

Magnesium reduced nickel(II)-bipyridyl (bipy) complex have been labeled organic electrides. An example is [(THF)4Mg4(μ2-bipy)4]–, in which the electride ion is centered within a square formed by four Mg2+ centers within the larger complex.[8]

"Inorganic electrides" have also been described.[9]

Reactions

Eelectride salts are powerful reducing agents, as demonstrated by their use in the Birch reduction. Evaporation of these blue solutions affords a mirror of Na metal. If not evaporated, such solutions slowly lose their colour as the electrons reduce ammonia:

2[Na(NH3)6]+e− → 2NaNH2 + 10NH3 + H2

This conversion is catalyzed by various metals.[10] An electride, [Na(NH3)6]+e−, is formed as a reaction intermediate.

High-pressure elements

Theoretical evidence supports electride behaviour in insulating high-pressure forms of potassium, sodium, and lithium. Here the isolated electron is stabilized by efficient packing, which reduces enthalpy under external pressure. The electride is identified by a maximum in both the electron localization function and the charge density distribution,[11] which distinguishes the electride from pressure-induced metallization. Electride phases are typically semiconducting or have very low conductivity,[12][13][14] usually with a complex optical response.[15] A sodium compound called disodium helide has been created under 113 gigapascals (1.12×10^6 atm) of pressure.[16]

The intrinsic polarization between atomic nucleus and the electron anion in these high pressure electrides will lead to unique properties, such as the splitting of the longitudinal and transverse acoustic modes (i.e., LA-TA splitting, an analogue to the LO-TO splitting in ionic compound),[17] the universal but robust gapless surface state in insulating electride that forming a de facto real space topological distribution of charge carriers,[18] and the colossal charge state of some impurities in them.[11]

Layered electrides (Electrenes)

Layered electrides or electrenes are single-layer materials consisting of alternating atomically thin two-dimensional layers of electrons and ionized atoms.[19][20] The first example was Ca2N, in which the charge (+4) of two calcium ions is balanced by the charge of a nitride ion (-3) in the ion layer plus a charge (-1) in the electron layer.[19]

See also

References
  1. Dye, J. L. (2003). "Electrons as Anions". Science. 301 (5633): 607–608. doi:10.1126/science.1088103. PMID 12893933. S2CID 93768664.
  2. Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5
  3. Buchammagari, H.; et al. (2007). "Room Temperature-Stable Electride as a Synthetic Organic Reagent: Application to Pinacol Coupling Reaction in Aqueous Media". Org. Lett. 9 (21): 4287–4289. doi:10.1021/ol701885p. PMID 17854199.
  4. Wagner, M. J.; Huang, R. H.; Eglin, J. L.; Dye, J. L. (1994). "An electride with a large six-electron ring". Nature. 368 (6473): 726–729. Bibcode:1994Natur.368..726W. doi:10.1038/368726a0. S2CID 4242499.{{cite journal}}: CS1 maint: multiple names: authors list (link).
  5. Wickleder, Mathias S.; Fourest, Blandine; Dorhout, Peter K. (2006). "Thorium". In Morss, Lester R.; Edelstein, Norman M.; Fuger, Jean (eds.). The Chemistry of the Actinide and Transactinide Elements (PDF). Vol. 3 (3rd ed.). Dordrecht, the Netherlands: Springer. pp. 78–94. doi:10.1007/1-4020-3598-5_3. Archived from the original (PDF) on 2016-03-07.
  6. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. pp. 1240–2. ISBN 978-0-08-037941-8.
  7. Nief, F. (2010). "Non-classical divalent lanthanide complexes". Dalton Trans. 39 (29): 6589–6598. doi:10.1039/c001280g. PMID 20631944.
  8. Day, Craig S.; Do, Cuong Dat; Odena, Carlota; Benet-Buchholz, Jordi; Xu, Liang; Foroutan-Nejad, Cina; Hopmann, Kathrin H.; Martin, Ruben (13 July 2022). "Room-Temperature-Stable Magnesium Electride via Ni(II) Reduction". J. Am. Chem. Soc. 144 (29): 13109–13117. doi:10.1021/jacs.2c01807. hdl:10037/32484.
  9. Hosono, Hideo; Kitano, Masaaki (2021). "Advances in Materials and Applications of Inorganic Electrides". Chemical Reviews. 121 (5): 3121–3185. doi:10.1021/acs.chemrev.0c01071. PMID 33606511.
  10. Greenlee, K. W.; Henne, A. L. (1946). "Sodium Amide". Inorganic Syntheses. Vol. 2. pp. 128–135. doi:10.1002/9780470132333.ch38. ISBN 9780470132333.
  11. Zhang, Leilei; Wu, Qiang; Li, Shourui; Sun, Yi; Yan, Xiaozhen; Chen, Ying; Geng, Hua Y. (2021-02-10). "Interplay of Anionic Quasi-Atoms and Interstitial Point Defects in Electrides: Abnormal Interstice Occupation and Colossal Charge State of Point Defects in Dense fcc-Lithium". ACS Applied Materials & Interfaces. 13 (5): 6130–6139. arXiv:2103.07605. doi:10.1021/acsami.0c17095. ISSN 1944-8244.
  12. Marques M.; et al. (2009). "Potassium under Pressure: A Pseudobinary Ionic Compound". Physical Review Letters. 103 (11): 115501. Bibcode:2009PhRvL.103k5501M. doi:10.1103/PhysRevLett.103.115501. PMID 19792381.
  13. Gatti M.; et al. (2010). "Sodium: A Charge-Transfer Insulator at High Pressures". Physical Review Letters. 104 (11): 216404. arXiv:1003.0540. Bibcode:2010PhRvL.104u6404G. doi:10.1103/PhysRevLett.104.216404. PMID 20867123. S2CID 18359072.
  14. Marques M.; et al. (2011). "Crystal Structures of Dense Lithium: A Metal-Semiconductor-Metal Transition" (PDF). Physical Review Letters. 106 (9): 095502. Bibcode:2011PhRvL.106i5502M. doi:10.1103/PhysRevLett.106.095502. PMID 21405633.
  15. Yu, Zheng; Geng, Hua Y.; Sun, Y.; Chen, Y. (2018). "Optical properties of dense lithium in electride phases by first-principles calculations". Scientific Reports. 8 (1): 3868. arXiv:1803.05234. Bibcode:2018NatSR...8.3868Y. doi:10.1038/s41598-018-22168-1. PMC 5832767. PMID 29497122.
  16. Wang, Hui-Tian; Boldyrev, Alexander I.; Popov, Ivan A.; Konôpková, Zuzana; Prakapenka, Vitali B.; Zhou, Xiang-Feng; Dronskowski, Richard; Deringer, Volker L.; Gatti, Carlo (May 2017). "A stable compound of helium and sodium at high pressure". Nature Chemistry. 9 (5): 440–445. arXiv:1309.3827. Bibcode:2017NatCh...9..440D. doi:10.1038/nchem.2716. ISSN 1755-4349. PMID 28430195. S2CID 20459726.
  17. Zhang, Leilei; Geng, Hua Y.; Wu, Q. (2021-04-16). "Prediction of anomalous LA-TA splitting in electrides". Matter and Radiation at Extremes. 6 (3): 038403. arXiv:2104.13151. doi:10.1063/5.0043276. ISSN 2468-2047.
  18. Wang, Dan; Song, Hongxing; Zhang, Leilei; Wang, Hao; Sun, Yi; Wu, Fengchao; Chen, Ying; Chen, Xiangrong; Geng, Hua Y. (2024-02-01). "Universal Metallic Surface States in Electrides". The Journal of Physical Chemistry C. 128 (4): 1845–1854. arXiv:2402.15798. doi:10.1021/acs.jpcc.3c07496. ISSN 1932-7447.
  19. Druffel, Daniel L.; Kuntz, Kaci L.; Woomer, Adam H.; Alcorn, Francis M.; Hu, Jun; Donley, Carrie L.; Warren, Scott C. (2016). "Experimental Demonstration of an Electride as a 2D Material". Journal of the American Chemical Society. 138 (49): 16089–16094. arXiv:1706.02774. doi:10.1021/jacs.6b10114. PMID 27960319. S2CID 19062953. Retrieved 12 October 2021.
  20. Druffel, Daniel L.; Woomer, Adam H.; Kuntz, Kaci L.; Pawlik, Jacob T.; Warren, Scott C. (2017). "Electrons on the surface of 2D materials: from layered electrides to 2D electrenes". Journal of Materials Chemistry C. 5 (43): 11196–11213. doi:10.1039/C7TC02488F. Retrieved 11 October 2021.

Further reading
  • J. L. Dye; M. J. Wagner; G. Overney; R. H. Huang; T. F. Nagy; D. Tománek (1996). "Cavities and Channels in Electrides". J. Am. Chem. Soc. 118 (31): 7329–7336. doi:10.1021/ja960548z.
  • Janesko, Benjamin G.; Scalmani, Giovanni; Frisch, Michael J. (2016). "Quantifying Electron Delocalization in Electrides". Journal of Chemical Theory and Computation. 12 (1): 79–91. doi:10.1021/acs.jctc.5b00993. PMID 26652208.
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