How long a C–C bond can be? An example of extraordinary long C–C single bond in 1,2-diarylamino-o-carborane

The longest CC single bond of 1.990(4) Å known thus far is observed in the single crystal X-ray structure of 1,2-(NHMes)2-o-carborane (Mes = 2,4,6-trimethylphenyl), which is readily synthesized via a one-pot process. [Display omitted] How long a CC bond can be? A question has long fascinated chemists. This work reports an example of extraordinary long CC bond distance of 1.990(4) Å observed in single-crystal X-ray structure of 1,2-(NHMes)2-o-carborane (2; Mes = 2,4,6-trimethylphenyl). DFT calculations show that hyperconjugation of lone pairs of the nitrogen atoms into the empty σ* orbital of the cage CC bond is the origin of the bond elongation. Such hyperconjugation can be suppressed if the two nitrogen atoms in 2 are linked to a Lewis acidic germanium (II) center.