Biphenylene is an alternant, polycyclic hydrocarboncomposed of two benzene rings joined together by a pair of mutual attachments, 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.
Preparation
The biphenylene structure can also be understood as a dimer of the reactive intermediatebenzyne, which in fact serves as a major synthetic route, by heating the benzenediazonium-2-carboxylate zwitterion prepared from 2-aminobenzoic acid. Another approach is by N-amination of 1H-benzotriazole with hydroxylamine-O-sulfonic acid. The major product, 1-aminobenzotriazole, forms benzyne in an almost quantitative yield by oxidation with lead acetate, which rapidly dimerises to biphenylene in good yields.
Properties
Biphenylene, a pale yellowish solid with a hay-like odor, was first synthesized by Lothrop in 1941. The chemistry of biphenylene is extensive, and has been the subject of two major reviews. Biphenylene is quite stable both chemically and thermally, and behaves in many ways like a traditional polycyclic aromatic hydrocarbon. However, both the spectral and chemical properties show the influence of the central ring, leading to considerable interest in the system in terms of its degree of lessened aromaticity. Questions of bond alternation and ring currents have been investigated repeatedly. Both X-ray diffraction and electron diffraction 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 through the central ring. However, more sensitiveNMR evidence, and particularly the shifting of proton resonances to high field, does indicate the existence of electron delocalization in the central ring. 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 ring. Magnetic susceptibility measurements also show a diminishing of both diamagnetic exaltation and diamagnetic anisotropy, relative to comparable pure systems, which is also consistent with a reduction of ring current diamagnetism. The electronic structure of biphenylene in the gas phase has the HOMO at a binding energy of 7.8 eV.
Higher biphenylenes
A fair number of higher 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 rings, 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 gas sensor.
