The Corannulene Tetraanion

On the basis of NMR results, an intriguing anion within the trianion structure was proposed for S . Such a structure would require 3.0 ee excess in the rim, and 1.0 ee excess in the hub, as compared to neutral 8. Analysis of the charge densities calculated at both semiempirical AMI and ab initio levels, however, predict the hub atoms to gain only about 0.5 ee, while the rim atoms bear more of the charge at 3.5 ee.  

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MNDO calculations suggest a stacked-bowl arrangement of the two tetraan-ionic corannulene moieties in a convex-to-concave geometry with four lithiums inside the sandwich as the global energy minimum (Figure 11 Thus, in agreement with theoretical studies on the isolated tetraanions (see above), the corannulene tetraanion also prefers a bowl-shaped conformation when the countercations are included in the calculations. 

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%. ... 

Zhou and Navangul calculated MNDO hardnesses for 14 benzenoid hydrocarbons and showed it to be a good indicator for their aromaticity.Using the hardness criterion, kekulene, coronene, and the corannulene tetraanion were all predicted to be superaromatic, also confirmed by calculations of Babic and Trinajstic and Cioslowski, O Connor, and Fleischmann in the case of kekulene. " Jiao and Schleyer, however, concluded, based on geometrical, energetic, and magnetic criteria of aromaticity, that kekulene is not superaromatic. ... 

The first evidence for the self-assembly of corannulene tetraanions into a supramolecu-lar dimer, 44, was provided by studies on derivatives of 4371. Owing to their lower symmetry, dimers of monosubstituted corannulene tetraanions are expected to exhibit supramolec-ular stereochemistry, and thus exist in meso and/or as dl dimeric forms (Figure 14a, and b respectively). Reduction of tert-butylcorannulene (45) with excess lithium metal in THF-d i leads to two sets of alkyl groups in almost equal abundance, thus pointing to the presence of tightly bound dimers. Compelling evidence for dimerization comes from the successful detection of a mixed dimer between 434 and 454. In addition, diffusion... 

FIGURE 14. The meso (a) and dl (b) dimers of monosubstitute (d) corannulene tetraanions. Reproduced by permission of Science from Reference 71... 

The chiral penta-substituted corannulene, 1,3,5,7,9-penta-terfbutylcorannulene (37), which shows anomalous dynamic behavior [116], was reduced with Li and four reduction stages were observed [117]. The behavior of the anti-aromatic dianion and the aromatic tetraanion resemble that of the parent corannulene. In the final stage of the reduction, three distinct tetraanionic species could be detected. Two of the species are sandwich-type dimers, as in the case of corannulene, and are diastereomers, due to the chirality of 37 (RR/SS and meso - RS/SR). The third species was assigned as a tetraanionic monomer, which slowly disappears. 

The two Li NMR peaks coalesce into a single peak at room temperature (T. = -8 °C), indicating that the interior and exterior cations are able to exchange places without dissociation of the dimer The Li NMR spectra of both 1-i-Pr and 1-r-Bu corannulene "/4Li exhibit quite analogous behavior. Moreover, tetraanion solutions generated from mixtures of corannulene and r-Bu-corannulene show the presence of mixed dimers in the H NMR spectrum. ... 

In 1995, the diffusion coefficients of a series of polycyclics and their respective dianions as well as corannulene (64) and its tetraanion (64 ) were measured in [DgjTHF solutions [58a]. Upon the formation of the charged species, a large decrease in the diffusion coefficients was observed and, partially, assigned to the increased solvation ofthe charged systems in the [DgjTHF solutions. However, some of the systems showed too large a decrease of their diffusion coefficients upon... 

Corannulene (1) displays interesting electrochemical properties. The reduction states of 1 lie between those of the neutral hydrocarbon and the tetraanion (1 740 ). Reduction of 1 at —78 C with excess lithium metal in [Dg]-THF over a period of several days led to a series of three color changes, first to green, then to purple, and finally to brownish-red [76]. Quenching this solution with water gave tetrahydrocorannulene as the major product accompanied by small amounts of dihydrocorannulene and 1. More recently, the structure of [Na(DME)3] [l ] was analyzed by X-ray crystallography, and its bowl depth (0.85 A) was found to be slightly shallower than the parent 1 [77]. 

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],... 

The monosubstituted corannulene derivatives tert-butylcorannulene (3S) and iso-propylcorannulene (36) have lower symmetry than corannulene therefore their reduction provided evidence for the formation of the dimers of corannulene tet-raanion. The dimers of monosubstituted coraimulene tetraanions are expected to exhibit supramolecular stereochemistry, existing in meso and d,l dimeric forms. 

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-... 

Compounds 40 and 41 do not contain the dibenzofulvene subunit, which is responsible for the dimerization process in 39 therefore, their reduction produces only monomeric anions. The LUMO level in 40 and 41 is not doubly degenerate like that of corannulene, but there is rather a small energy gap between the LUMO and the NLUMO, which indicates that the formation of tetraanions of 40 and 41 should be possible [120]. 

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