Boron-Based Mg2Be2B7 - Ternary Cluster Stabilizes a Monocyclic [B7]5- Heptagonal Ring: Chemical Bonding, Double 6π/6σ Aromaticity, and Dynamic Structural Fluxionality.

Incorporating metals into boron clusters through doping or alloying can effectively tailor the structural, electronic, and bonding properties of boron-based compound clusters. In this contribution, we report on the computational design of a ternary Mg₂Be₂B₇ ^- cluster, which assumes a three-layered geometry between Be─Mg─Mg trimer, monocyclic B₇ ring, and an isolated Be atom. Chemical bonding analysis reveals double 6π/6σ aromaticity in the system, whose π/σ electron counting conforms to the (4n + 2) Hückel rule. Heptagonal boron ring is a nonclassical structural motif, which is formally in the [B₇]^5- charge-state, except for secondary covalent interaction with Be₂ component. The Be─Mg─Mg trimer is held together by dative σ bonds with relative low bond orders. The system shows dynamic structural fluxionality, as demonstrated by the Born-Oppenheimer molecular dynamics (BOMD) simulation at near room temperature. The intramolecular rotation barrier is only 0.15 kcal mol^-1 at the single-point CCSD(T) level. A concept is proposed that a complex molecular system with mutually inert components may facilitate dynamic structural fluxionality. This work also highlights the potential to develop a heptagonal [B₇]^5- ring as inorganic ligand in boron chemistry.