Innentitelbild: Exceptionally Long C−C Single Bonds in Diamino‐o‐carborane as Induced by Negative Hyperconjugation (Angew. Chem. 5/2019)

Abstract

1,2‐Diamino‐o‐carborane mit außergewöhnlich langen C‐C‐Bindungen (bis 193 pm) wurden synthetisiert. In ihrer Zuschrift auf S. 1411 erklären Z. Li, X.‐Q. Xiao et al. die beobachtete Streckung mit einer negativen Hyperkonjugation zwischen den freien Elektronenpaaren der Stickstoffatome und dem σ*C‐C‐Orbital des o‐Carborans. Im Bild halten zwei Kraniche ein solches Molekül, wobei sie dessen C‐C‐Bindung bis fast zum Zerreißen dehnen. (Passenderweise fliegen sie dabei über die “Broken Bridge” bei Hangzhou.)

Exceptionally Long Covalent CC Bonds—A Local Vibrational Mode Study

“…Li, Müller at al. synthesized a series of 1,2-diamino-o-carboranes with exceptionally long C−C bonds ranging between 1.627 and 1.931 Å depending on the amino substituents [ 5 ]. The longest C−C bond of 1.931 Å was found for the diphenyl compound.…”

“…The longest C−C bond of 1.931 Å was found for the diphenyl compound. As a major factor for the CC bond elongation negative hyperconjucation between the nitrogen lone pair and the C−C orbitals was identified [ 5 ]. Recently, Mandal et al calculated an even longer C−C bond of 2.011 Å using a B3PW91-D3/cc-pVTZ model chemistry [ 12 ] which seems to overshoot the C−C bond length [ 137 ].…”

“…Molecules 1-53 are categorized into seven groups Group I-Group VII and targeted CC bonds are indicated by red coloration. The C−C bonds of Group I molecules (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13) and Group II molecules (14-18) are exposed to increasing steric repulsion as the bulkiness of substituents increases. Group III molecules (19)(20)(21)(22)(23)(24) represent different diamondoid configurations.…”

“…The typical length of a C(sp 3 )−C(sp 3 ) bond is around 1.54 Å [ 1 ]. However, it may exceed this standard value considerably, which has led to a competition of pushing the CC bonds to their limits by finding the best strategy for synthesizing the longest but still intact CC bond [ 2 , 3 , 4 , 5 , 6 , 7 ]. Whereas the question of the practical use of such ultra-long C−C bonds is still open, the study of these extreme cases including novel CC bonding situations such as e.g., the recently established CC tetrel bonding [ 8 , 9 , 10 ] strongly contributes to enriching our understanding of the CC bond and the concept of the chemical bond in general [ 11 , 12 , 13 ].…”

“…The D symmetrical ethane radical cation is a classic example with calculated C−C bond lengths of 1.915 Å or more, depending on the level of theory used [ 33 , 34 , 35 ]. A series of diamino-o-carboranes have been synthesized with inner-cluster C−C bond lengths between 1.990 Å [ 4 ] and 1.931 Å [ 5 ] for which negative hyperconjugation between the nitrogen lone pairs and the (C−C) orbital has been held primarily responsible for causing CC bond elongation [ 3 ]. Recently, Mandal et al demonstrated in a B3PW91-D3/cc-pVTZ study how fine tuning of negative hyperconjugation effects can result in even longer C−C bonds as in the case of amino oxide-o-carborane and di-N,N-dimethylamino-o-carborane [ 12 ].…”