## Saturday, January 26, 2019

### Exceptionally Long C−C Single Bonds in Diamino-o-carborane as Induced by Negative Hyperconjugation

Li, J.; Pang, R.; Li, Z.; Lai, G.; Xiao, X.-Q.; Müller, T., Angew. Chem. Int. Ed. 2019, 58, 1397-1401
Contributed by Steven Bacharach
Reposted from Computational Organic Chemistry with permission

Chemists are constantly checking the limits of theories, and the limits of bonding is one that has been subject to many tests of late. I have posted on two recent papers (herehere) that probe just how long a C-C bond can be, and now Li, Müller, and co-workers report a structure that pushes that limit even further out.1

They prepared and obtained the x-ray structure of five derivatives of o-carborane, namely compounds 12a3a3b and 4. In all of these, the C-C bond in the carborane is stretched well beyond that of a typical C-C bond (see Table 1). The longest case is in 3b where the C-C bond length is a whopping 1.931 Å (see Figure 1), which obliterates the previous record holder at 1.798 Å.2 B3PW91-D3/cc-pVTZ computations corroborate these structures and the long C-C bond.

Scheme 1: Carboranes with long C-C bonds (highlighted in blue)
Table 1. C-C bond distance (Å)
 cmpd r(C-C) expt r(C-C) DFT 1 1.829 1.839 2a 1.720 1.710 3a 1.893 1.917 3b 1.931 1.936 4 1.627 1.607
Figure 1. B3PW91-D3/cc-pVTZ optimized structure of 3b.

Topological electron density analysis locates a bond path between the two carbons in all five structures. The Wiberg bond index is small, with a value of only 0.34 in 3b. Natural bond orbital (NBO) analysis identifies a negative hyperconjugation interaction between the nitrogen lone pair and the σ*C-C orbital. This rationalizes both the very long C-C bond and the very short C-N bonds, and the trends associated with the variation between 1° amine, 2° amine and imine.

### References

1. Li, J.; Pang, R.; Li, Z.; Lai, G.; Xiao, X.-Q.; Müller, T., “Exceptionally Long C−C Single Bonds in Diamino-o-carborane as Induced by Negative Hyperconjugation.” Angew. Chem. Int. Ed. 201958, 1397-1401, DOI: 10.1002/anie.201812555.
2. Ishigaki, Y.; Shimajiri, T.; Takeda, T.; Katoono, R.; Suzuki, T., “Longest C–C Single Bond among Neutral Hydrocarbons with a Bond Length beyond 1.8 Å.” Chem 20184, 795-806, DOI: 10.1016/j.chempr.2018.01.011.

### InChIs

3b: InChI=1S/C22H28B10N2/c1-13-7-15(3)19(16(4)8-13)11-33-21-22(34-12-20-17(5)9-14(2)10-18(20)6)25(21)23-27(21)24-30(23,25)28(22,25)29(22)26(21,22,27)31(24,27,29)32(24,28,29)30/h7-10,33-34H,11-12H2,1-6H3
InChIKey=UEZUONSMPNIZRQ-UHFFFAOYSA-N

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