One of the great longstanding dreams of synthetic and theoretical organic chemists is to prepare a stable molecule containing a pentacoordinate carbon atom. Bickelhaupt and co-workers propose a novel series of compounds that hint that this might be possible.1

Their attack is to first find a CR3 radical that is stable in its planar form. The nitrile group perfectly satisfies this goal. Next they look at the series of compounds X-C(CN)3-X- (1) where X is a halogen, searching for a stable D3h structure. This is found with the halogens: Br, I, and At, at the ZORA-OLYP/TZ2P level. Seems like case closed, except that inspection of the supporting materials shows that the nature of the D3h structure is sensitive to computational method. So, with the larger basis set ZORA-OLYP/QZ4P or with ZORA-OPBE/TZ2P, only the I and At compounds are local D3h minima. And with ZORA-M06/TZ2P, only the At compound is a local minimum. The authors do mention these points at the end of the article. So, what we have here is a tantalizing suggestion for how to prepare a hypercoordinate carbon species, but further computational (and experimental) work is clearly needed.


1: X = F, Cl, Br, I, At

References

(1) Pierrefixe, S. C. A. H.; van Stralen, S. J. M.; van Strale, J. N. P.; Guerra, C. F.; Bickelhaupt, F. M., "Hypervalent Carbon Atom: "Freezing" the SN2 Transition State," Angew. Chem. Int. Ed., 2009, 48, 6469-6471, DOI: 10.1002/anie.200902125

InChIs

1(F): InChI=1/C4F2N3/c5-4(6,1-7,2-8)3-9/q-1, InChIKey=LBDHZXPWKBFYBC-UHFFFAOYAX

1(Cl): InChI=1/C4Cl2N3/c5-4(6,1-7,2-8)3-9/q-1, InChIKey=NMFGEVWEEWBFSS-UHFFFAOYAU

1(Br): InChI=1/C4Br2N3/c5-4(6,1-7,2-8)3-9/q-1, InChIKey=FHKJQJBDHAEQES-UHFFFAOYAC

1(I): InChI=1/C4I2N3/c5-4(6,1-7,2-8)3-9/q-1, InChIKey=FWKBAUUEXUSUNH-UHFFFAOYAO

1(At): InChI=1/C4At2N3/c5-4(6,1-7,2-8)3-9/q-1, InChIKey=BJQBFKCUAROAAS-UHFFFAOYAK