Aromatic cage-like B34 and B35+: new axially chiral members of the borospherene family

Abstract

Shortly after the discovery of all-boron fullerenes D_2d B₄₀^−/0 (borospherenes), the first axially chiral borospherenes C₃/C₂ B₃₉⁻ were characterized in experiments in 2015. Based on extensive global minimum searches and first-principles theory calculations, we present herein two new axially chiral members to the borospherene family: the aromatic cage-like C₂ B₃₄(1) and C₂ B₃₅⁺(2). Both B₃₄(1) and B₃₅⁺(2) feature one B₂₁ boron triple chain on the waist and two equivalent heptagons and hexagons on the cage surface, with the latter being obtained by the addition of B⁺ into the former at the tetracoordinate defect site. Detailed bonding analyses show that they follow the universal bonding pattern of σ + π double delocalization, with 11 delocalized π bonds over a σ skeleton. Extensive molecular dynamics simulations show that these borospherenes are kinetically stable below 1000 K and start to fluctuate at 1200 K and 1100 K, respectively. The IR, Raman, and UV-vis spectra of 1 and 2 are computationally simulated to facilitate their experimental characterization.