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

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journal contribution
posted on 2022-04-26, 22:07 authored by María del Rosario Merino-García, Luis Antonio Soriano-Agueda, Juan de Dios Guzmán-Hernández, Diego Martínez-Otero, Bruno Landeros Rivera, Fernando Cortés-Guzmán, José Enrique Barquera-Lozada, Vojtech Jancik
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 B3N3H6 (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 B3N3 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.

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