Pentalene
Encyclopedia
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.
Dilithium pentalenide was isolated (1962) long before pentalene itself (1997). 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 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.
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
composed of two fused cyclopentadiene
Cyclopentadiene
Cyclopentadiene is an organic compound with the formula C5H6. This colorless liquid has a strong and unpleasant odor. At room temperature, this cyclic diene dimerizes over the course of hours to give dicyclopentadiene via a Diels–Alder reaction...
rings. It has chemical formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
C8H6. It is antiaromatic
Antiaromaticity
Antiaromatic molecules are cyclic systems containing alternating single and double bonds, where the pi electron energy of antiaromatic compounds is higher than that of its open-chain counterpart. Therefore antiaromatic compounds are unstable and highly reactive; often antiaromatic compounds...
, because it has 4n
Hückel's rule
In organic chemistry, Hückel's rule estimates whether a planar ring molecule will have aromatic properties. The quantum mechanical basis for its formulation was first worked out by physical chemist Erich Hückel in 1931...
pi electrons. For this reason it dimerizes even at temperatures as low as -100°C.
Dilithium pentalenide was isolated (1962) long before pentalene itself (1997). It is prepared from reaction of dihydropentalene (pyrolysis
Pyrolysis
Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures without the participation of oxygen. It involves the simultaneous change of chemical composition and physical phase, and is irreversible...
of an isomer of dicyclopentadiene
Dicyclopentadiene
Dicyclopentadiene, abbreviated DCPD, is a chemical compound with formula C10H12. At room temperature, it is a white crystalline solid with a camphor-like odor. Its energy density is 10,975 Wh/l....
) with n-butyllithium
N-Butyllithium
n-Butyllithium is an organolithium reagent. It is widely used as a polymerization initiator in the production of elastomers such as polybutadiene or styrene-butadiene-styrene...
in solution and forms a stable salt. In accordance with its structure proton NMR
Proton NMR
Proton NMR is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen-1 nuclei within the molecules of a substance, in order to determine the structure of its molecules. In samples where natural hydrogen is used, practically all of the hydrogen consists of the...
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
Cyclooctatetraene
1,3,5,7-Cyclooctatetraene is an unsaturated derivative of cyclooctane, with the formula C8H8. It is also known as [8]annulene. This polyunsaturated hydrocarbon is a colorless to light yellow flammable liquid at room temperature...
(COT) dianion C8H82-.
The dianion can also be considered as two fused cyclopentadienyl
Cyclopentadienyl complex
A cyclopentadienyl complex is a metal complex with one or more cyclopentadienyl groups . Based on the type of bonding between the metals and the cyclopentadienyl]] moieties, cyclopentadienyl complexes are classified into the following three categories: a) π-complexes, b) σ-complexes, and c) ionic...
rings, and has been used as a ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
in organometallic chemistry
Organometallic chemistry
Organometallic chemistry is the study of chemical compounds containing bonds between carbon and a metal. Since many compounds without such bonds are chemically similar, an alternative may be compounds containing metal-element bonds of a largely covalent character...
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.