Benzene and Borazine, so Different, yet so Similar: Insight from Experimental Charge Density Analysis.

Although benzene and borazine are isoelectronic and isostructural, they have very different electronic structures, mainly due to the polar nature of the B-N bond. Herein, we present an experimental study of the charge density distribution obtained from the multipole model formalism and Hirshfeld atom refinement (HAR) based on high-resolution X-ray diffraction data of borazine B 3 N 3 H 6 ( 1 ) and B , B ', B ″-trichloroborazine ( 2 ) crystals. These data are compared to those obtained from HAR for benzene ( 4 ) and 1,3,5-trichlorobenzene ( 5 ) and further compared with values obtained from density functional theory calculations in the gas phase, where N , N ', N ″-trichloroborazine ( 3 ) was also included. The results confirm that, unlike benzene, borazines are only weakly aromatic with an island-like electronic delocalization within the B 3 N 3 ring involving only the nitrogen atoms. Furthermore, delocalization indices and interacting quantum atom energy for bonded and non-bonded atoms were found to be highly suitable indicators capable of describing the origin of the discrepancies observed when the degree of aromaticity in 2 and 3 is evaluated using common aromaticity indices. Additionally, analysis of intermolecular interactions in the crystals brings further evidence of a weakly aromatic character of the borazines as it reveals surprising similarities between the crystal packing of borazine and benzene and also between B , B ', B ″-trichloroborazine and 1,3,5-trichlorobenzene.