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

Abstract

Recently, the NHC→B[triple bond, length as m-dash]B←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(C6H3Pri 2-2,6)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 1 via 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.