3D and 2D aromatic units behave like oil and water in the case of benzocarborane derivatives

2D/2D fusion of aromatic halves leading to a global aromatic is found in many polycyclic aromatic hydrocarbons, whereas 2D/3D aromaticity is difficult to achieve. Here the authors report a computational chemistry investigation showing that 3D/2D aromatic combination is not possible. A large number of 2D/2D and 3D/3D aromatic fusions that keep their aromaticity in the fused compounds have been synthesized. In addition, we have previously proven the electronic relationship between the 3D aromaticity of boron hydrides and the 2D aromaticity of PAHs. Here we report the possible existence of 3D/2D aromatic fusions that retain the whole aromaticity of the two units. Our conclusion is that such a 3D/2D aromatic combination is not possible due to the ineffective overlap between the pi-MOs of the planar species and the n + 1 molecular orbitals in the aromatic cage that deter an effective electronic delocalization between the two fused units. We have also proven the necessary conditions for 3D/3D fusions to take place, and how aromaticity of each unit is decreased in 2D/2D and 3D/3D fusions.

Automated summary

This study investigates the possibility of 3D/2D aromatic fusions and their relationship to electronic delocalization. While 2D/2D and 3D/3D aromatic fusions have been synthesized, the ineffective overlap between the pi-MOs of planar species and the n + 1 molecular orbitals in the aromatic cage makes 3D/2D aromatic combination impossible.