Be2B6 and Be2B7+: two double aromatic inverse sandwich complexes with spin-triplet ground state

Abstract

Quantum chemical calculations at both density functional theory (DFT) and single-point CCSD(T) levels suggest that Be₂B₆ (D_6h, ³A_1g) and Be₂B₇⁺ (D_7h, ³A₂′) clusters adopt interesting inverse sandwich structures with spin-triplet ground states, featuring a perfect planar B₆ (or B₇) monocyclic boron ring being sandwiched by two lone Be atoms. There is pronounced charge transfer from the Be atoms to the monocyclic boron rings, leading to [Be]²⁺[B₆]⁴⁻[Be]²⁺ and [Be]²⁺[B₇]³⁻[Be]²⁺ charge transfer complexes. Chemical bonding analysis showed that Be₂B₆ and Be₂B₇⁺ clusters have σ/π double aromaticity with 4σ and 6π delocalized electrons on the boron rings. The Be₂B₆ inverse sandwich possesses the smallest monocyclic boron ring motif in Be-doped boron clusters to date.