Double aromaticity in the triplet ground state of Be2©Be6H6: A 4π Baird and 6σ Hückel electron system

Abstract

The lowest-energy structure of Be8H6 adopts a distinctive star-like configuration (Be2©Be6H6), consisting of a Be2 dimer oriented orthogonally to a planar hexagonal Be6H6 ring, with hydrogen atoms bridging the outer edges. This system, characterized by D6h symmetry and a triplet ground state (3A1g), features an exceptionally short Be–Be bond length of 1.874 Å. Chemical bonding analysis reveals a rare dual aromaticity: six delocalized σ electrons conform to the 4n + 2 Hückel rule, while four delocalized π electrons satisfy the 4n Baird rule. Notably, Be2©Be6H6 represents the first example exhibiting both π-Baird and σ-Hückel dual aromaticity. Covalent dative interactions between the s/p orbitals of the Be2 dimer and the delocalized Be6H6 framework contribute to the exceptional stability of this system. This study demonstrates the effectiveness of integrating multiple aromaticity frameworks, specifically Hückel and Baird aromaticity, in designing and stabilizing novel inorganic systems.