Pentalene

Pentalene is a polycyclic hydrocarbon composed of two fused cyclopentadiene rings.[1] It has chemical formula C8H6. It is antiaromatic, because it has 4n π electrons where n is any integer. For this reason it dimerizes even at temperatures as low as −100 °C.[2][3] The derivative 1,3,5-tri-tert-butylpentalene was synthesized in 1973.[4] Because of the tert-butyl substituents this compound is thermally stable. Pentalenes can also be stabilized by benzannulation for example in the compounds benzopentalene and dibenzopentalene.[1]

Pentalene
Names
Preferred IUPAC name
Pentalene
Other names
Bicyclooctatetraene
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
Properties
C8H6
Molar mass 102.136 g·mol−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Dilithium pentalenide was isolated in 1962, long before pentalene itself in 1997.[5] It is prepared from reaction of dihydropentalene (pyrolysis of an isomer of dicyclopentadiene) with n-butyllithium in solution and forms a stable salt. In accordance with its structure proton NMR shows 2 signals in a 2 to 1 ratio. The addition of two electrons removes the antiaromaticity; it becomes a planar 10π-electron aromatic species and is thus a bicyclic analogue of the cyclooctatetraene (COT) dianion C8H82−.

The dianion can also be considered as two fused cyclopentadienyl rings, and has been used as a ligand in organometallic chemistry to stabilise many types of mono- and bimetallic complexes, including those containing multiple metal-metal bonds, and anti-bimetallics with extremely high levels of electronic communication between the centers.[6]

See also

References
  1. Hopf, Henning (2013). "Pentalenes-From Highly Reactive Antiaromatics to Substrates for Material Science". Angewandte Chemie International Edition. 52 (47): 12224–6. doi:10.1002/anie.201307162. PMID 24127399.
  2. Carey, Francis A.; Sundberg, Richard J. (1984). Advanced Organic Chemistry: Part A Structure and Mechanisms (2nd ed.). New York, NY: Plenum Press. ISBN 0-306-41198-9.
  3. Bally T.; Chai S.; Neuenschwander M.; Zhu Z. (1997). "Pentalene: Formation, Electronic, and Vibrational Structure" (reprint). J. Am. Chem. Soc. 119 (8): 1869–1875(7). doi:10.1021/ja963439t.
  4. Hafner, K.; Süss, H. U. (1973). "1,3,5-Tri-tert-Butylpentalene. A Stabilized Planar 8π-Electron System". Angew. Chem. Int. Ed. Engl. 12 (7): 575–577. doi:10.1002/anie.197305751.
  5. Katz, Thomas J.; Rosenberger, Michael. (1962). "The Pentalenyl Dianion". Journal of the American Chemical Society. 84 (5): 865. doi:10.1021/ja00864a038.
  6. Summerscales, Owen T.; Cloke, F. Geoffrey N. (2006). "The organometallic chemistry of pentalene". Coordination Chemistry Reviews. 250 (9–10): 1122. doi:10.1016/j.ccr.2005.11.020.
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