Pentalene

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ImageFileL1 = Pentalene.svg
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IUPACName = Pentalene
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Section1 = Chembox Identifiers
CASNo = 250-25-9
PubChem = 5460726
SMILES = c1cc2cccc2c1
InChI = 1/C8H6/c1-3-7-5-2-6-8(7)4-1/h1-6H
ChEBI = 33074
Section2 = Chembox Properties
Formula = C₈H₆
MolarMass = 102.13 g/mol
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Pentalene is a polycyclic hydrocarbon composed of two fused cyclopentadiene rings. It has chemical formula C₈H₆. It is antiaromatic, because it has 4n pi electrons. For this reason it dimerizes even at temperatures as low as -100°C [Carey, Francis A.; Sundberg, Richard J.; (1984). "Advanced Organic Chemistry Part A Structure and Mechanisms" (2nd ed.). New York N.Y.: Plenum Press. ISBN 0-306-41198-9.] [cite journal | author = Bally T., Chai S., Neuenschwander M., Zhu Z. | date = 1997 | title = Pentalene: Formation, Electronic, and Vibrational Structure | journal = J. Am. Chem. Soc. | volume = 119 | issue = | pages = 1869–1875(7)| doi = 10.1021/ja963439t | url = http://www-chem.unifr.ch/tb/bally/pdf/pentalene.pdf | format = reprint]

Dilithium pentalenide was isolated (1962) long before pentalene itself (1997) ["The Pentalenyl Dianion" Thomas J. Katz, Michael Rosenberger J. Am. Chem. Soc.; 1962; 84(5); 865-866. DOI|10.1021/ja00864a038] 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 10 pi aromatic species and is thus a bicyclic analogue of the cyclooctatetraene (COT) dianion C₈H₈^2-.

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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 ["The Organometallic Chemistry of Pentalene" Owen T. Summerscales, F. Geoffrey N. Cloke Coord. Chem. Rev.; 2006; 250; 1122-1140.]

See also

* Benzocyclobutadiene

References