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

Abstract

A series of 1,2‐diamino‐o‐carboranes with exceptionally long C−C bonds (up to 193 pm) were synthesized. In their Communication on page 1397 ff., Z. Li, X.‐Q. Xiao et al. attribute this bond elongation to the in‐plane negative hyperconjugation between the lone pairs of the nitrogen atoms and the σ*C−C orbital in the o‐carborane. In the artwork, two red‐crowned cranes are holding this molecule, stretching the C−C bond almost to breaking point (depicted by the “Broken Bridge”, Hangzhou).

Synthesis and Structure of an o‐Carboranyl‐Substituted Three‐Coordinate Borane Radical Anion

“…[109][110][111][112][113][114] The elasticity of the C1À C2 bond of o-carborane (1.629(6) Å) was studied in the presence of bulky substituents [115][116][117] (1.712(7) to 2.156(4) Å) and as a function of the C1À C2 σ*-anti-bonding orbital population by donating groups at C1 and C2 (1.723(2) to 2.065(7) Å). [118][119][120][121][122][123][124][125][126][127] The interest in electrochemical reduction [105,[128][129][130][131][132] to populate this antibonding orbital has led to its application in reversible electrochemical uranium capture. [133] According to Wade's rules, such an increase of skeletal electrons (SE) results in a transition from a 2n + 2 closo-towards a 2n + 4 nido-structure.…”