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.

Fig. 1. The structure of 1 in the crystalline state and its interpretation as a donor–acceptor molecule symbolized via two arrows along the C₁B bond. The two methyl groups of each of the eight grey C-atoms are omitted for clarity. The following distances (Å) are essential for the discussion: d(BB) = 1.45; d(BC) = 1.49; d(C₁N) = 1.39.

Fig. 2. Energy diagram for solid boron, B-atoms, B₂ molecules (B₂, B₂*) and the decomposition of 1 to B₂ and 2NHC. Calculated (dashed lines) and experimentally obtained values. For 2 see ESI. Footnotes to figure: *) “+2NHC” omitted for clarity; 1) ref. 26; 2) calculated for 1a: 35.7 kJ mol^–1 and 8.23 eV; 3) ref. 27.

Scheme 1

Scheme 2