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

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 π-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.This work has been supported by the Ministerio de Ciencia e Innovación of Spain (Projects PID2020-113711GB-I00 (M.S.), PID2019-106830GB-I00 (J.P.), PID2019-106832RB-100 (C.V. and F.T.) and MDM-2017-0767 (J.P.)) and the Generalitat de Catalunya (projects 2017SGR39 (M.S.), 2017SGR1720 (C.V. and F.T.) and 2017SGR348 (J.P.)). Excellent service by the Supercomputer center of the Consorci de Serveis Universitaris de Catalunya (CSUC) is gratefully acknowledged.With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe