The boron–boron triple bond? A thermodynamic and force field based interpretation of the N-heterocyclic carbene (NHC) stabilization procedure

Abstract

Recently, the NHC→BB←NHC molecule 1 has been published in Science where it is described as a stabilized B₂ molecule in its ¹Σ excited state (B₂*) (NHC = N-heterocyclic carbene C₃N₂H₂(C₆H₃Prⁱ₂-2,6)₂). The bonding of 1 based on sophisticated calculations and the BB distances of the solid compound was discussed as the first example of a B₂ triple bond in a stable molecule. Now we present an only experimentally based interpretation of 1via detailed thermodynamic considerations, including its fragmentation to B₂ molecules. Furthermore, from the vibrational spectrum force constants (f_BB for the BB bond and f_BC for the BC bond) were extracted, which are classical examples to indicate single, double and triple CC bonds in organic chemistry. The consequence of both properties of 1 (ΔE and f) generates a new interpretation which is in contrast to the triple bond donor–acceptor description visualized by arrows and which casts a critical light on the interpretation of any NHC “stabilized” molecule.