Is There BN Bond-Length Alternation in 1,2:3,4:5,6-Tris(biphenylylene)borazines?

The alternation of the BN bonds in the central borazine ring of the overcrowded 1,2:3,4:5,6-tris(biphenylylene)borazine (2 a) and its tribromo derivative (2 g) is investigated by computational methods and compared with their experimentally obtained crystal structures. The calculations are performed with a meta-generalized-gradient-approximation (GGA) density functional (Tao-Perdew-Staroverov-Scuseria (TPSS)) without and with dispersion corrections, including Becke-Johnson damping, in conjunction with a polarized triple-ζ basis set. These data show a small bond-length alternation (BLA) of around 0.01 Å in 2 a and 2 g. This outcome is in good agreement with X-ray diffraction data for 2 g, but at variance with earlier X-ray diffraction measurements that gave a BLA of 0.06 Å for 2 a. A re-investigation of the crystal structure of 2 a reveals a positional disorder that precludes a discussion of the BN bond lengths. The synthesis of 2 g is the first example of an electrophilic aromatic substitution of an aryl borazine with elemental bromine. Successful bromination was also demonstrated for hexaphenylborazine.