A Comparative ¹H‐ and ¹³C‐NMR Study of the Dianion and Dication of Biphenylene

Abstract

Biphenylene dianion has been prepared by Li reduction of the parent hydrocarbon. It is stable below -30' and was characterized by its 'H-and I3C-NMR spectra. A comparison of these data with those of the dication indicates he existence of ion pairs in the dianion case which are responsible for a different charge distribution. The iiamagnetic ring currents of both ions, however, are of comparable magnitude, and both are more diatropic than he parent hydrocarbon. Predictions of the n-charge-density effect on 'H chemical shifts are improved by :alculations that use the linear and quadratic electric-field effect equation instead of the simple Spiesecke-Schneider .elation.Cyclic conjugated n systems containing [4n]n electrons can be transformed to -4n + 2177 systems either by oxidation to the dication or by reduction to the dianion [l]. In Zeneral, such transformations should be accompanied by a gain in delocalization energy. rhus, they are of particular interest in the case of the unstable cyclobutadiene [2], the smallest member of the neutral [4n]annulene family.While experiments to generate substituted cyclobutadiene dications were successful 31, analogous attempts to synthesize substituted cyclobutadiene dianions were less rewarding [4] [S]. For example, in the case of 3,4-diphenylbenzocyclobutadiene dianion as Nell as tetraphenylcyclobutadiene dianion, it was concluded -mainly on the basis of the NMR data -that the negative charge is predominantly residing in the phenyl substitumts, thereby rendering olefinic character to the central four-membered ring [S]. Similar results seemed to hold for biphenylene (l), where 12' was well-characterized by 'H-and 'C-NMR measurements [6], while equally clear data for 12-were still missing [7]. On the 3ther hand, however, strong support for the diatropic nature of 12-came from 'Li-NMR neasurements [8], where an upfield shift of 6.1 ppm relative to external aq. LiCl was 3bserved.We now conducted a clean reduction of biphenylene with Li sand in THF at -78" by dtrasonic treatment. A quantitative conversion to the dianion, formed as a deep copperxown suspension, was achieved in less than 2 h. We could then determine the complete H-and 13C-NMR data for 12-, and these results are reported in the present communication. In addition, we compare the results for the dianion with those obtained earlier for the dication [6], which we supplemented by an analysis of the 'H-NMR-AA'XX' system.

Selected topics from recent NMR studies of organolithium compounds

“…This technique was also successful in the case of biphenylene, were we had found 109 that the dianion 29, formed initially via reduction by potassium in THF, reacts with protic solvents to yield benzocyclo-octatetraene (31) via the Woodward-Hoffmann allowed ring opening reaction of the primarily formed 4a,8b-dihydrobiphenylene (30). In the absence of proton donors, 29 has a half-life of 1.7 h and opens the four-membered ring to yield o,o'dilithiodiphenyl (32) 110 , a compound with a lithium double bridge 111 .…”

Electropolymerization of Bis(2-cyano-2-α-thienylethenyl)arylenes

“…Nevertheless, the possibility of reductive ring-opening of the central biphenylene unit could not be eliminated. It has been reported that the biphenylene dianion is unstable above 0 °C, giving the dilithiobiphenyl as the final product …”

“…However, due to the effects of conjugation, monomer 2 has an absorption maximum shifted to 395 nm (ε = 1.8 × 10 4 ). On the other hand, monomer 3 shows two absorption bands, which are characteristic for biphenylene and its derivatives, , with the maxima at 337 nm (band I, ε = 4.1 × 10 4 ) and 465 nm (band II, ε = 3.0 × 10 4 ), respectively.…”

“…Our present results demonstrated the importance of the biphenylene unit in the oxidative electropolymerization. Previous work on oligo(2,7-biphenylenylene-( E )-vinylene)s indicated that biphenylene-containing conjugated molecules tend to delocalize positive charges over the conjugated π-systems. , Since typical mechanisms for electropolymerization of thiophenes involve coupling of the corresponding cation radicals, effective dispersion of the positive charge over the cation-radical of 3 may largely reduce the intermolecular electrostatic repulsion between the radical cations, increasing the tendency toward polymerization.…”

The Radical Anion and Dianion of Benzo[3,4]cyclobuta[1,2-b]phenazine

“…The potassium salt of its dianion was stable with respect to ring opening, in contrast to biphenylene, which ring-opens upon reduction with lithium (sand). 34,35 The dianion of P3 is stable due to charge delocalization over its 9ring system. Cyclobutadiene linked (aza)acenes can be attractive n-and p-type semiconductor materials.…”