Corannulene anions

The electronic state of corannulene anion with C5v symmetry is 2El. The symmetric product representation of Ex is decomposed as follows ... 

Substituting the magnitude of spin density pc of the corannulene anion and the coronene anion obtained by MO calculation into equation (10), the calculated aH is tabulated in Table 1. 

From the present calculation, we can conclude that the upper botmd of the energy barrier for the corannulene anion and coronene anion is 7.9 and 0.2 meV, respectively. 

It should be noted that the quadratic vibronic interaction is vanishing in the corannulene anion, since E2 X E2 does not contain E2 representation in equation (16). The anharmonic potentials are expressed by... 

For the corannulene anion, complicated hyperfine structures due to the JT effect has been observed at low temperatures. The potential barrier between the JT-distorted minima has been estimated to be 2.2 meV. On the other hand, for... 

For the corannulene anion, the quadratic vibronic interaction is vanishing due to its symmetry, and it is necessary to include the fifth-order vibrational anharmonicity and fourth-order vibronic interaction to explain the five equivalent minima. For coronene, on the other hand, if the quadratic vibronic interaction is considered, the interaction gives rise to three minima on the JT surface of the coronene anion. However, this is not the case. It is necessary for the sixth-order vibrational anharmonicity and fourth-order nonlinear vibronic interaction to give rise to the six equivalent minima. 

The new carbon allotropes, fullerenes [14] and nanotubes [15], can accept a large number of electrons to their -system however, the neutral compounds are hardly soluble, especially in THF. Because of the low solubility of fullerenes, their complete reduction to hexaanions with Li is possible only under extreme conditions, such as the use of an ultrasound bath [16], When the reduction of fullerenes is performed in the presence of a small amount of 2, moderate conditions are required, e.g., low temperature and without sonication [17]. Corannulene, which is a fragment of fullerenes, is highly soluble in THF and is easily reduced. The resulting corannulene anion serves as an efficient electron shuttle, which assists in complete reduction of the practically insoluble fullerenes. In a similar way, single walled nanotubes were reduced by charge transfer by the use of small aromatic compounds, such as naphthalene, fluorenone and anthraquinone [18]. 

An early electrochemical study of corannulene revealed the presence of two well-defined polarographic waves with half-wave potentials of-1.88 and -2.36 V (r-butylammonium perchlorate in acetonitrile). The first wave represented a reversible, one-electron reduction leading to radical anion formation (emerald green solution) further characterized by UV-VIS and ESR. The second wave was reported to be associated with the formation of a bright red species which is not paramagnetic, but it is not believed to be the dianion, but rather some decay product of it. Treatment of THF solutions of 8 with sodium and potassium metals also led to the formation of the same species. ... 

There have been several developments in this area since this manuscript was prepared. The heat of combustion of corannulene was determined by microbomb combustion calorimetry and its gas-phase enthalpy of formation was estimated at 110.8 kcal/mol. All anionic oxidation states of corannulene were observed by optical absorption, EPR, and NMR spectroscopies. More support for the an-nulene-within-annulene model of the corannulene tetraanion was presented. An alternative pyrolysis route to corannulene was reported, as well as some attempts toward the synthesis of bowl-shaped subunits of fullerenes. And in contrast with previous semiempirical studies," ab initio calculations predicted a general concave preference for the metal cation binding to semibuckminsterfullerene 2%. ... 

The present HF calculations show that there are five and six equivalent minima for the corannulene and the coronene anion, respectively. The symmetry of the stable minima is Cs and C2/l, respectively. The JT potentials are also obtained. The calculated energy barrier between the distorted minima is 7.9 and 0.2 meV for the corannulene ion and the coronene ion, respectively. This is the reason why JT distortion could not be observed in the coronene ion. 

Y. Bando, T. Sakurai, S. Seki, H. Maeda, Corannulene-fused anion-responsive rt-conjugated molecules that form self-assemblies with unique electronic properties. Chem. Asian J. 8, 2088-2095 (2013)... 

Bowl-shaped carbocations (33) were obtained by alkylation of the parent corannulene. ° The cations were obtained as stable salts and characterized by X-ray analysis. Phenalenyl cations (e.g. (34)) were obtained as air-stable BPh4 salts. Observations made in a selective synthesis of l,2-di(organo)fullerenes led to a mechanism whereby a cationic fullerene intermediate is generated by Cu(ll) oxidation of a fullerene radical (or anion). ... 

Corannulene cyclophane 105 was prepared by treating 36 with a mixture of 1,4-bis (mereaptomethyl)benzene and potassium tert-butoxide (Scheme 31) [32]. Similarly, l,5-pentadithiol/l,6-bis(bromomethyl)corannulene cyclophane (106) was prepared from 37 and 1,5-pentadithiol in 75% yield [129]. Synthesis of [3,3]seleno/l,6-dimethylcorannulene cyclophane (107), a cyclophane from two corannulenes, was not straightforward [129]. The key step is the in situ transformation of 37 to the bis(selenide anion), which reacts with the additional 37 to afford the seleno-bridged dimer 107. 

Janata J, Gendell J, Ling CY, Barth W, Backes L, Mark HB Jr, Lawton RG (1967) Concerning the Anion and Cation Radicals of Corannulene. J Am Chem Soc 89 3056... 

Miscellaneous. The photoejection of an electron from the antiaromatic corannulene dianion has been studied by Shenhar et al. Electrostatic repulsion between the corannulene radical-anion and the ejected electron facilitates the dissociation of the cage of the photoproducts initially formed. 

The esthetically attractive an anion inside a trianion model was proposed for the tetraanion of corannulene based on the early models of electron distribution in corannulene [51], This view, however, has been a matter of some controversy, since while some of the spectral characteristics of the tetraanion seem to be explained well by this model, the charge- C NMR relations as well as the results of early ah initio calculations suggest that more than three extra electrons go to the rim of corannulene upon the tetraanion formation [52],... 

A quite different approach to the complexation of corannulene derivatives with transition metals was applied by Chin [65]. His group hydrogenated 1 to its octahy-dro derivative 67 which was subsequently deprotonated by BuLi to the respective fluorene-type anion 68. This species was then used for the formation of two complexes with (CpZrCh) and [Re(CO)3]+. In both cases X-ray structure determination showed // coordination of the metal to the central ring on the exo (convex) side of the very shallow bowl of 68. However, these complexes relate more to the fluorene anion chemistry than to buckybowls. 

The reduction using alkali metals is carried out in dry aprotic solvents, usually THF, and in an inert atmosphere (vacuum). The metals used are Li (wire), Na, K, Rb and Cs (mirror). The PAHs can accept several charges (up to 6 electrons). Consecutive paramagnetic and diamagnetic states can be observed by EPR and NMR spectroscopies, respectively. Many examples of reduction with alkali metals are discussed in depth here. Corannulene (2) offers an excellent example of the application of this reduction method. 2 was reduced by Li [6, 7], Na, K, Rb and Cs [8], and four reduction steps were observed, each having a different color (with the exception of sodium, which gave only two reduction steps). The four reduction steps yielded mono-, di-, tri- and tetra-anions, which were characterized by EPR or NMR [6-9]. 

Following the dimerization of corannulene, a system of two tethered corannulene units, 1,8-dicorannulenyloctane (38) [118], was reduced with alkali metals to study the formation of an intramolecular sandwich [8]. The reduction was done with Li, Na, K, Rb and Cs, and the two diamagnetic anions tetraanion and octa-... 

The second diamagnetic anion is the octaanion 38, which consists of two sets of H NMR peaks. One set is between = 6.7 and 7.2 ppm, in the same region as the signal of the 34 itself, therefore they were assigned to the protons of the corannulene fragments. The second set contained four broad peaks in the range = 2.2 to 3.2 ppm assigned to the protons of the tether. The Li NMR spectmm of 38 is very similar to that of 34 (dimer of 2 ). The chemical shifts of the two... 

The PAHs indenocorannulene (39), dibenzo[a,g]corannulene (40), and diben-zo[o,g]cyclopenta[h,I]corannulene (41) are three examples of extended corannulenes, which contain a central corannulene system fused to five- and six-membered rings. The reduction of these systems [119, 120] focused on the following issues. First, what is the aromaticity of these curved PAHs anions, will they behave like large polycyclic systems, or have aimulenic character Secondly, what is the possibility of aggregation and dimerization in these systems The third subject of interest was the effect of different alkali metals on the reduction process. 

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