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

Abstract

The synthesis of a series of 1,2‐diamino‐o‐carboranes (1–4) is reported. The molecular structures of these diamino‐o‐carboranes are remarkable as the inner‐cluster C−C bonds are all ultra‐long (162.7–193.1 pm) and vary substantially with small variations in the substituents. The results of quantum mechanical investigations suggest that the origin of the bond elongation is significant in‐plane negative hyperconjugation of lone pairs of the nitrogen substituents with the σ* orbitals of the C−C bonds in o‐carboranes.

Flexible C−C Bonds: Reversible Expansion, Contraction, Formation, and Scission of Extremely Elongated Single Bonds

“…Since deviation from the standard causes al arge loss of bonding energy,s tructural parameters such as bond length and bond angle are nearly constant among carbon atoms.H owever, strained molecules such as sterically congested polycyclica romatic hydrocarbons [1][2][3][4][5][6][7][8] as well as cyclic p-conjugated compounds [9][10][11][12][13][14][15] exhibit unusual parameters,a nd thus have attracted much attention due to their potential applications.W ith regard to the C À C single bond, which has as tandard length of 1.54 ,t hree approaches have been taken to increase the bond length in neutral organic compounds to beyond 1.7 :( i) diamondoid dimers, [16] (ii)f used or clamped hexaphenylethanes (HPEs), [17][18][19] and (iii)d iaminocarboranes (Figure 1). [20] Among these,w ef ocused on the second approach (ii)w hile adopting the core-shell strategy,w hich enables the isolation of stable dispiro[dibenzocycloheptatriene (DBCHT)]-type HPEs 1 with an extremely elongated C(sp 3 )ÀC(sp 3 )b ond [1.806(2) for 1cat 400 K].W eenvisaged that such a"hyper covalent bond" should be weak enough to exhibit reversible expansion, contraction, formation, and scission, which could be visualized by X-ray analyses.…”

Thiaboranes on Both Sides of the Icosahedral Barrier: Retaining and Breaking the Barrier with Carbon Functionalities

“…It should be also mentioned that the IBO results have revealed a 3c2e B−B−B bond in the B−B−B triangle with the elongated B−B separation of 2.19 Å, not drawn as a bond in Figure . Such a long B−B vector is not very surprising considering the fact that even the C−C distance in a substituted o ‐carbaborane can reach more than 1.9 Å …”

“…Such a long BÀ B vector is not very surprising considering the fact that even the CÀ C distance in a substituted o-carbaborane can reach more than 1.9 Å. [33] In summary, both methylation chemistry and the concept of icosahedral barrier have been expanded to the area of thiaboranes. The computations have been used to estimate the height of the barrier as well as to examine nido-thiaboranes with a larger number of vertices than eleven.…”

Flexible C−C Bonds: Reversible Expansion, Contraction, Formation, and Scission of Extremely Elongated Single Bonds

“…Since deviation from the standard causes al arge loss of bonding energy,s tructural parameters such as bond length and bond angle are nearly constant among carbon atoms.H owever, strained molecules such as sterically congested polycyclica romatic hydrocarbons [1][2][3][4][5][6][7][8] as well as cyclic p-conjugated compounds [9][10][11][12][13][14][15] exhibit unusual parameters,a nd thus have attracted much attention due to their potential applications.W ith regard to the C À Cs ingle bond, which has as tandard length of 1.54 , three approaches have been taken to increase the bond length in neutral organic compounds to beyond 1.7 :( i) diamondoid dimers, [16] (ii)f used or clamped hexaphenylethanes (HPEs), [17][18][19] and (iii)d iaminocarboranes (Figure 1). [20] Among these,w ef ocused on the second approach (ii)w hile adopting the core-shell strategy,w hich enables the isolation of stable dispiro[dibenzocycloheptatriene (DBCHT)]-type HPEs 1 with an extremely elongated C(sp 3 ) À C(sp 3 )b ond [1.806(2) for 1cat 400 K].W eenvisaged that such a"hyper covalent bond" should be weak enough to exhibit reversible expansion, contraction, formation, and scission, which could be visualized by X-ray analyses.…”

“…With regard to the C−C single bond, which has a standard length of 1.54 Å, three approaches have been taken to increase the bond length in neutral organic compounds to beyond 1.7 Å: (i) diamondoid dimers, [16] (ii) fused or clamped hexaphenylethanes (HPEs), [17–19] and (iii) diaminocarboranes (Figure 1). [20] Among these, we focused on the second approach (ii) while adopting the core–shell strategy, which enables the isolation of stable dispiro[dibenzocycloheptatriene (DBCHT)]‐type HPEs 1 with an extremely elongated C(sp 3 )−C(sp 3 ) bond [1.806(2) Å for 1 c at 400 K]. We envisaged that such a “hyper covalent bond” should be weak enough to exhibit reversible expansion, contraction, formation, and scission, which could be visualized by X‐ray analyses.…”