Cyclooctatetraene dication from

The dication adopts a three-legged piano-stool configuration in which the O-U-O angles vary from 84.0° to 88.2° with a mean value of 87(3)°, and average COT-U-O angles of 127(1)°. The uranium atom is 1.92(2) A away from the planar cyclooctatetraene ring, and the mean U-G bond distance is 2.65(3) A. These values compare well with those determined in the monocationic cyclooctatetraene complexes [(COT)U(NEt2)(THF)3]+.161... 

Thus the stabilization energy calculation agrees with the deduction from the disposition of filled MOs (i.e. with the 4n + 2 rule) that the cyclobutadiene dication should be stabilized by electron delocalization, which is in some agreement with experiment [45], More sophisticated calculations indicate that cyclic 4n systems like cyclobutadiene (where planar cyclooctatetraene, for example, is buckled by steric factors and is simply an ordinary polyene) are actually destabilized by n electronic effects their resonance energy is not just zero, as predicted by the SHM, but less than zero. Such systems are antiaromatic [17,46],... 

In 1945 Michael J. S. Dewar suggested that the tropylium ion (the cation derived from cycloheptatriene) should also be aromatic (Figure 9). This was confirmed in 1954 since then, the dianion of butadiene and the dication of cyclooctatetraene have also been shown to be aromatic. Like benzene, all four of these ions are planar rings with six tt electrons. According to Hiickel s rule the cyclopropene cation should also exhibit aromaticity, and it does. (In this case n = 0, and 4n + 2 = 2.) The planar anion of cyclononatetraene and the dianion of cyclooctatetraene should also be aromatic (n = 2, and 4n + 2 = 10), and both of them are. 

It is now recognised that, having a cyclic system made up of eight TT-electrons, it would not be expected to be especially stabilised. Its structure is discussed later in this chapter. If however two electrons are added to provide a dianion or two electrons are removed to provide a dication, the resultant ionic species might be stabilised since they have cyclic systems of, respectively, ten and six electrons. These salts derived from cyclooctatetraene are considered in more detail in a later section of this chapter. 

Removal of two electrons from cyclooctatetraene should provide a dication in which six ir-electrons could be delocalised over the... 

Dis.solution of 1,4-dimethyl-, 1,3,5,7-tetramethyl, 1,3,5,7-tetra-phenyl- or S /m.-dibenzo-cyclooctatetraenes in a mixture of antimony pentafluoride and sulphonyl chloride fluoride at -78 results in the formation of species which, judged from their n.m.r. spectra, are planar dications with delocalised sets of six ir-electrons, giving rise to diamagnetic ring currents [59]. In the case of the dibenzo--derivative it appears that all fourteen ir-electrons are delocalised over the sixteen carbon atom periphery. 

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