Corannulene Reduction: Spectroscopic Detection of All Anionic Oxidation States

Abstract

The reduction of corannulene (1) has been followed by parallel detection of optical absorption, electron paramagnetic resonance (EPR), and nuclear magnetic resonance (NMR) spectroscopies. The major findings are the existence of a diamagnetic dianion that can be observed in NMR and negligable counterion influences in the monoand dianion. The diamagnetic state of the dianion is also supported by vanishing EPR intensity and semiempirical calculations and exhibits one indistinguishable proton NMR resonance at -5.6 ppm for potassium and lithium as counterions, respectively. The NMR signal is strongly temperature dependent and can only be obtained at low temperature (T < 230 K) where the line width decreases with temperature. Further reduction to the trianion and tetraanion succeeds with Li as the reducing metal, but not with K even at ambient temperatures. The EPR spectra of the trianion show a pronounced temperature dependence with at least two Li cations being strongly coupled.

Dianions and Tetraanions of Bowl-Shaped Fullerene Fragments Dibenzo[a,g]corannulene and Dibenzo[a,g]cyclopenta[kl]corannulene

“…We see that the formation of a dimer in these systems is essential for the formation of a tetraanion when Li is the reducing agent. These results are in line with the reduction mechanism proposed for 1, [5] where the formation of the sandwich-type structure occurs only at the last stage of the reaction, that is, the reduction to the tetraanion, and the formation of the tetraanion is consequently dependent on the formation of a dimer with lithium cations acting as the glue.…”

“…[5] This significant paratropic shift confirms its annulene nature. [4] A reduction path for corannulene was also suggested, indicating that the formation of the dimer takes place only at the tetraanionic stage of the reduction.…”

“…[5] Attention turned to the nature of the dianion, being either para-or diatropic, since it had not been detected previously. We were also concerned with the question of the reduction stage in which the formation of dimers occurs.…”

π-Conjugated Anions: From Carbon-Rich Anions to Charged Carbon Allotropes

“…[30] Such temperature-dependent 1 H-NMR behavior has been reported previously for polycyclic anions. [32] Despite the two extra electrons in its backbone, the 13 C-NMR spectrum of 5 2-shows three single peaks (δ ϭ 120, 154, and 204) at low field relative to those of the neutral compound.…”

“…[8] However, a Jahn-Teller distortion led to the eventual detection of 1 H-and 13 C-NMR spectra of the dianion. [30] The extreme high-field shifts of the protons (δ ϭ -5.6) are typical of molecules with a paratropic ring current [31] and can be explained by the ''annulene model'', where a cyclopentadienyl anion (6e/5C) is placed in the center of an antiaromatic (16e/15C) annulene perimeter.…”