Biphenylene is an
organic compound
In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. The ...
with the formula (C
6H
4)
2. It is a pale, yellowish solid with a hay-like odor. Despite its unusual structure, it behaves like a traditional
polycyclic aromatic hydrocarbon
A polycyclic aromatic hydrocarbon (PAH) is a class of organic compounds that is composed of multiple aromatic rings. The simplest representative is naphthalene, having two aromatic rings and the three-ring compounds anthracene and phenanthrene. ...
.
Bonding
Biphenylene is a
polycyclic hydrocarbon
A polycyclic aromatic hydrocarbon (PAH) is a class of organic compounds that is composed of multiple aromatic rings. The simplest representative is naphthalene, having two aromatic rings and the three-ring compounds anthracene and phenanthrene. P ...
, composed of two
benzene
Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms, ...
rings joined by two bridging bonds (as opposed to a normal
ring fusion
In organic chemistry annulation (from the Latin ''anellus'' for "little ring"; occasionally annelation) is a chemical reaction in which a new ring is constructed on a molecule.
:
Examples are the Robinson annulation, Danheiser annulation and cert ...
), thus forming a 6-4-6 arene system. The resulting planar structure was one of the first π-electronic hydrocarbon systems discovered to show evidence of
antiaromaticity
Antiaromaticity is a chemical property of a cyclic molecule with a π electron system that has higher energy, i.e., it is less stable due to the presence of 4n delocalised (π or lone pair) electrons in it, as opposed to aromaticity. Unlike aroma ...
. The spectral and chemical properties show the influence of the central
nring, leading to considerable interest in the system in terms of its degree of lessened
aromaticity
In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to saturate ...
. Questions of
bond alternation
Bond or bonds may refer to:
Common meanings
* Bond (finance), a type of debt security
* Bail bond, a commercial third-party guarantor of surety bonds in the United States
* Chemical bond, the attraction of atoms, ions or molecules to form chemical ...
and
ring currents
Ring may refer to:
* Ring (jewellery), a round band, usually made of metal, worn as ornamental jewelry
* To make a sound with a bell, and the sound made by a bell
:(hence) to initiate a telephone connection
Arts, entertainment and media Film and ...
have been investigated repeatedly. Both
X-ray diffraction
X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
and
electron diffraction
Electron diffraction refers to the bending of electron beams around atomic structures. This behaviour, typical for waves, is applicable to electrons due to the wave–particle duality stating that electrons behave as both particles and waves. Si ...
studies show a considerable alternation of bond lengths, with the bridging bonds between the benzenoid rings having the unusually great length of 1.524 Å. The separation of the rings is also reflected by the absence of the transmission of
NMR substituent effects
Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with a ...
through the central
nring. However, more sensitive
NMR
Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with ...
evidence, and particularly the shifting of proton resonances to high field, does indicate the existence of
electron delocalization
In chemistry, delocalized electrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or a covalent bond.IUPAC Gold Boo''delocalization''/ref>
The term delocalization is general and can have slightly dif ...
in the central
nring. This upfield shift has been interpreted in terms of diminished benzenoid ring currents, either with or without an accompanying paramagnetic ring current in the central
nring.
Magnetic susceptibility
In electromagnetism, the magnetic susceptibility (Latin: , "receptive"; denoted ) is a measure of how much a material will become magnetized in an applied magnetic field. It is the ratio of magnetization (magnetic moment per unit volume) to the ap ...
measurements also show a diminishing of both
diamagnetic exaltation and
diamagnetic anisotropy, relative to comparable pure
n+2 N2 or N-2 may refer to:
* Nitrogen#Allotropes, Dinitrogen (N₂)
Arts and media
* A model number of the Yamaha AvantGrand piano
* "N2", a 2011 song by Japanese indie rock band Asian Kung-Fu Generation, on the album ''Landmark (Asian Kung-Fu Gene ...
systems, which is also consistent with a reduction of
ring current diamagnetism.
The electronic structure of biphenylene in the gas phase has the
HOMO
''Homo'' () is the genus that emerged in the (otherwise extinct) genus ''Australopithecus'' that encompasses the extant species ''Homo sapiens'' ( modern humans), plus several extinct species classified as either ancestral to or closely relate ...
at a binding energy of 7.8 eV.
Preparation
Biphenylene was first synthesized by Lothrop in 1941.
The biphenylene structure can also be understood as a dimer of the reactive intermediate
benzyne, which in fact serves as a major synthetic route, by heating the benzenediazonium-2-carboxylate
zwitterion
In chemistry, a zwitterion ( ; ), also called an inner salt or dipolar ion, is a molecule that contains an equal number of positively- and negatively-charged functional groups.
: With amino acids, for example, in solution a chemical equilibrium wil ...
prepared from
2-aminobenzoic acid. Another approach is by ''N''-amination of
1''H''-benzotriazole with
hydroxylamine-''O''-sulfonic acid. The major product, 1-aminobenzotriazole, forms benzyne in an almost quantitative yield by oxidation with
lead(IV) acetate
Lead(IV) acetate or lead tetraacetate is an organometallic compound with chemical formula . It is a colorless solid that is soluble in nonpolar, organic solvents, indicating that it is not a salt. It is degraded by moisture and is typically store ...
, which rapidly
dimerises to biphenylene in good yields.
Higher biphenylenes
Polycycles containing the biphenylene nucleus have also been prepared, some having considerable antiaromatic character. In general, additional 6-membered rings add further aromatic character, and additional 4-membered and 8-membered rings add antiaromatic character. However, the exact natures of the additions and fusions greatly affect the perturbations of the biphenylene system, with many fusions resulting in counter-intuitive stabilization by
nrings, or destabilization by 6-membered rings. This has led to significant interest in the systems by theoretical chemists and graph theoreticians. Even a complete 2-dimensional carbon sheet with biphenylene-like subunits has been proposed
and was in-depth investigated by theoretical means, finding a technologically relevant direct band gap of ca. 1 eV, excitonic binding energies of ca. 500 meV and potential as a gas sensor.
Network
Researchers synthesized a biphenylene sheet consisting of sp2-hybridized carbon atoms that formed four-, six-, and eight-membered rings on a smooth gold surface. A bottom-up two-step interpolymer
dehydrofluorination
In chemistry, dehydrohalogenation is an elimination reaction which removes a hydrogen halide from a substrate. The reaction is usually associated with the synthesis of alkenes, but it has wider applications.
Dehydrohalogenation from alkyl halid ...
of an adsorbed
halogenated
In chemistry, halogenation is a chemical reaction that entails the introduction of one or more halogens into a compound. Halide-containing compounds are pervasive, making this type of transformation important, e.g. in the production of polyme ...
terphenyl
Terphenyls are a group of closely related aromatic hydrocarbons. Also known as diphenylbenzenes or triphenyls, they consist of a central benzene ring substituted with two phenyl groups. There are three substitution patterns: ''ortho''-terphenyl ...
molecule
polymerization
In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer, monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are ...
yielded ultraflat four- and eight-membered rings. The resulting allotrope was metallic.
References
{{Reflist
Antiaromatic compounds
Hydrocarbons
Biphenylenes