B3N3 Borazine Substitution in Hexa-peri-Hexabenzocoronene: Computational Analysis and Scholl Reaction of Hexaphenylborazine

The doping of graphene molecules by borazine (B3N3) units may modify the electronic properties favorably. Therefore, the influence of the substitution of the central benzene ring of hexa-peri-hexabenzocoronene (HBC, C42H18) by an isoelectronic B3N3 ring resulting in C36B3N3H18 (B3N3HBC) is investigated by computational methods. For comparison, the isoelectronic and isosteric all-B/N molecule B21N21H18 (termed BN) and its carbon derivative C6B18N18H18 (C6BN), obtained by substitution of a central B3N3 by a C6 ring, are also studied. The substitution of C6 in the HBC molecule by a B3N3 unit results in a significant change of the computed IR vibrational spectrum between 1400 and 1600 cm-1 due to the polarity of the borazine core. The properties of the BN molecule resemble those of hexagonal boron nitride, and substitution of the central B3N3 ring by C6 changes the computed IR vibrational spectrum only slightly. The allowed transitions to excited states associated with large oscillator strengths shift to higher energy upon going from HBC to B3N3HBC, but to lower energy upon going from BN to C6BN. The possibility of synthesis of B3N3HBC from hexaphenylborazine (HPB) using the Scholl reaction (CuCl2/AlCl3 in CS2) is investigated. Rather than the desired B3N3HBC an insoluble and X-ray amorphous polymer P is obtained. Its analysis by IR and 11B magic angle spinning NMR spectroscopy reveals the presence of borazine units. The changes in the 11B quadrupolar coupling constant CQ, asymmetry parameter ?, and isotropic chemical shift diso(11B) with respect to HPB are in agreement with a structural model that includes B3N3HBC-derived monomeric units in polymer P. This indicates that both intra- and intermolecular cyclodehydrogenation reactions take place during the Scholl reaction of HPB.