Spying on the boron-boron triple bond using spin-spin coupling measured from 11 B solid-state NMR spectroscopy.

There is currently tremendous interest in the previously documented example of a stable species exhibiting a boron-boron triple bond ( Science , 2012, 336 , 1420). Notably, it has recently been stated using arguments based on force constants that this diboryne may not, in reality, feature a boron-boron triple bond. Here, we use advanced solid-state NMR and computational methodology in order to directly probe the orbitals involved in multiple boron-boron bonds experimentally via analysis of ¹¹ B- ¹¹ B spin-spin ( J ) coupling constants. Computationally, the mechanism responsible for the boron-boron spin-spin coupling in these species is found to be analogous to that for the case of multiply-bonded carbon atoms. The trend in reduced J coupling constants for diborenes and a diboryne, measured experimentally, is in agreement with that known for alkenes and alkynes. This experimental probe of the electronic structure of the boron-boron multiple bond provides strong evidence supporting the originally proposed nature of the bonds in the diboryne and diborenes, and demonstrates that the orbitals involved in boron-boron bonding are equivalent to those well known to construct the multiple bonds between other second-row elements such as carbon and nitrogen.