Lewis Base/Acid Coordination Stabilizes Planar 6π Aromatic Si6/P6

Bond-alternated and bond-equalized hexazine derivatives

Planar cyclic conjugated molecules satisfying the Hückel rule are generally classified as aromatics and π delocalization is the key feature of aromatic compounds. It was shown that the π system of benzene prefers bond-alternation and that the delocalization observed is a consequence of bond-equalization, which is a σ effect. For systems wherein the π bonds are strong, such as those between N atoms, the π-distortivity may outweigh the σ preference for bond-equalization. Thus, one anticipates a bond-alternated structure for N6; however, neither the bond-equalized structure (D6h) nor the bond-alternated one (D3h) is a minimum as evident from the previous literature; calculations have shown that cyclic N6 is non-planar. Herein we show that a Lewis acid coordination strategy can be employed to stabilize the planar structure of N6. We show that the structure can be bond-alternated or bond-equalized depending on the strength of the Lewis acid. Kinetic stability with respect to concerted decomposition to three N-N triple bonded systems was assessed, and a few bond-equalized N6 systems were found to be potential candidates for synthesis.

Aromatic nature of neutral and dianionic 1,4-diaza-2,3,5,6-tetraborinine derivatives

The aromatically relevant parameters of boron-rich inorganic benzenes-neutral and dianionic 1,4-diaza-2,3,5,6-tetraborinine derivatives (B4N2R6)-have been computationally estimated and evaluated from geometric, electronic, magnetic, and energetic points of view. The majority of the criteria (ASE, NICSzz, ELF, and PDI) indicate that the aromaticity of the neutral B4N2 benzene analogue stabilized by Lewis bases lies in between those of benzene and borazine. On the other hand, the aromaticity of the dianionic B4N2 benzene analogue 4' is controversial. The pronounced aromatic nature of 4' is supported by ELFπ, PDI, and NICSπzz, but ASE, the FiPC-NICS plot, and ACID oppose this. These data confirm that even with the same B4N2-skeletal framework of a 6π-system, the aromatic feature varies depending on the overall charge of the B4N2 systems.