Hypercoordinated carbon has fascinated chemists since the development of the concept of the tetravalent carbon. The advent of superacids has opened up the world of hypercoordinated species and now a crystal structure of a hexacoordinated carbon has been reported for the C₆(CH₃)₆²⁺ species 1.¹

The molecule is prepared by first epoxidation of hexamethyl Dewar benzene, followed by reaction with Magic acid, and crystallized by the addition of HF. The crystal structure shows a pentamethylcyclopentadienyl base capped by a carbon with a methyl group. The x-ray structure is well reproduced by the B3LYP/def2-TZVP structure shown in Figure 1. (While this DFT method predicts a six-member isomer to be slightly lower in energy, MP2 does predict the cage as the lowest energy isomer.)

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Figure 1. B3LYP/def2-TZVP optimized geometry of 1.

The Wiberg bond order for the bond between the capping carbon and each carbon of the five-member base is about 0.54, so the sum of the bond orders to the apical carbon is less than 4. The carbon is therefore not hypervalent, but it appears to truly be hypercoordinate. (A topological electron density analysis (AIM) study would have been interesting here.) NICS analysis indicates the cage formed by the apical carbon and the five-member ring expresses 3-D aromaticity. This can be thought of as coming from the C₅(CH₃)₅⁺ fragment with its 4 electrons and the CCH₃⁺ fragment with two electrons, providing 4n + 2 = 6 electrons for the aromatic cage.

References

1) Malischewski, M.; Seppelt, K., "Crystal Structure Determination of the Pentagonal-Pyramidal Hexamethylbenzene Dication C₆(CH₃)₆²⁺" Angew. Chem. Int. Ed. 2017, 56, 368-370, DOI: 10.1002/anie.201608795.

Wentrup and co-workers examined the strained, non-planar aromatic 1.¹

The UKS-BP86-D3BJ/def2-TZVP optimized geometry of the singlet 1 is shown in Figure 1. The molecule is decidedly twisted, with an angle of about 52°. This large twist, weakening the π-bond between the two aromatic fragments, suggests that the triplet state of 1 might be easily accessible. The geometry of ³1 is also shown in Figure 1, and the two aromatic portions are orthogonal.

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Figure 1. UKS-BP86-D3BJ/def2-TZVP optimized geometries of ¹1 and ³1.

The proton and ¹³C NMR studies of 1 show increasing paramagnetism, observed as line broadening, with increasing temperature. Confirming this is ESR which shows increasing signal with increasing temperature. The triplet state is clearly present. The experimental ΔE_ST=9.6 kcal mol^-1 and the computed singlet-triplet gap is 9.3 kcal mol^-1. This is in excellent agreement, and much better than previous computations which predict a gap of 3.4 kcal mol^-1, but omitted the D3 correction. This dispersion correction stabilizes the singlet state over the triplet state, as might be expected. (The triplet has the two aromatic components orthogonal and so they have minimal dispersion interactions, while the aromatic planes are much closer in the singlet state.)

For comparison, the computed ΔE_ST of isomer 2 is much larger: 17.9 kcal mol^-1. The energies of the triplet states of 1 and 2 are nearly identical. Both of these structures have orthogonal, non-interacting aromatic moieties. However, the energy of ¹2 with the twist angles of ¹1 is 8.2 kcal mol^-1 lower than that of ¹1. This the twisting causes a significant strain to the singlet state, but not to the triplet, and that gives rise to its small singlet-triplet gap.

References

1) Wentrup, C.; Regimbald-Krnel, M. J.; Müller, D.; Comba, P., "A Thermally Populated, Perpendicularly Twisted Alkene Triplet Diradical." Angew. Chem. Int. Ed. 2016, 55, 14600-14605, DOI: 10.1002/anie.201607415.

InChIs

1: InChI=1S/C42H24/c1-5-13-29-25(9-1)17-21-33-34-22-18-26-10-2-6-14-30(26)38(34)41(37(29)33)42-39-31-15-7-3-11-27(31)19-23-35(39)36-24-20-28-12-4-8-16-32(28)40(36)42/h1-24H
InChIKey=YEHKZURNXPRJHP-UHFFFAOYSA-N

2: InChI=1S/C42H24/c1-5-13-29-21-37-33(17-25(29)9-1)34-18-26-10-2-6-14-30(26)22-38(34)41(37)42-39-23-31-15-7-3-11-27(31)19-35(39)36-20-28-12-4-8-16-32(28)24-40(36)42/h1-24H
InChIKey=PKXAAFWZKNGAED-UHFFFAOYSA-N