Pentalene

Pentalene is a polycyclic hydrocarbon composed of two fused cyclopentadiene rings. It has chemical formula C₈H₆ and CAS number 250-25-9. It is antiaromatic, because it has 4n pi electrons. For this reason it dimerizes even at temperatures as low as -100°C ^{[1] [2]}

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Dilithium pentalenide was isolated (1962) long before pentalene itself (1997) ^[3] 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-.

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 ^[4]

See also

• Benzocyclobutadiene

References