Sandwich-type Na6B7− and Na8B7+ clusters: charge-transfer complexes, four-fold π/σ aromaticity, and dynamic fluxionality

Abstract

Boron-based clusters possess unusual structural and bonding properties owing to boron's electron-deficiency. We report on the theoretical prediction of two binary B–Na clusters, Na₆B₇⁻ and Na₈B₇⁺, which assume unique sandwich geometries, featuring a perfectly planar B₇ wheel and two triangular Na₃ or quasi-tetrahedral Na₄ ligands. Despite distinct electronegativities of B/Na, the B–Na clusters do not form typical salts. Both sandwich species are dynamically fluxional at 300 K and beyond. Two dynamic modes are observed: an in-plane rotation of the B₇ wheel versus twisting of the two Na₃/Na₄ ligands. Their energy barriers are negligibly small. Natural bond orbital calculations show that the clusters are charge-transfer complexes [Na₃]⁺[B₇]³⁻[Na₃]⁺ and [Na₄]²⁺[B₇]³⁻[Na₄]²⁺, respectively. Chemical bonding analyses indicate that the B₇ wheel in the clusters has 6π/6σ double aromaticity and the Na₃/Na₄ ligands are 2σ aromatic, collectively leading to four-fold π/σ aromaticity. The quasi-tetrahedral Na₄ ligand is the simplest example of spherical aromaticity and can also be considered a superatom. Interlayer bonding in the sandwiches is greater than 20 eV, due to electrostatics, which should not be confused with weakly bound species. Four-fold π/σ aromaticity and robust interlayer ionic bonding offer uniform and dilute electron clouds over the sandwiches, facilitating their dual-mode dynamic fluxionality. The Na₈B₇⁺ cluster is also a superalkali cation.