Planar Tricyclic B₈O₈ and B₈O₈^<b>–</b> Clusters: Boron Oxide Analogues of <i>s</i>-Indacene C₁₂H₈

Abstract

Boron clusters and their oxides are electron-deficient species with (π and σ) aromaticity and antiaromaticity, enabling a structural and bonding analogy between them and the aromatic hydrocarbons. s-Indacene C₁₂H₈ is normally considered as a border system between the classes of aromatic and antiaromatic hydrocarbons. We show herein, via computer global-minimum searches and B3LYP and single-point CCSD(T) calculations, that boron oxide clusters D_2h B₈O₈ (1, ¹A_g) and D_2h B₈O₈^- (2, ²B_2g) adopt planar tricyclic structures, which feature fused heterocyclic B₃O₂/B₄O₂/B₃O₂ rings and two boronyl (BO) terminals, a structural pattern analogous to the C₅/C₆/C₅ rings in s-indacene. Bonding analyses indicate that B₈O₈ (1) is a formally antiaromatic 12π system, the molecular orbitals of which are largely similar to those of s-indacene. Infrared and ultraviolet-visible spectra of B₈O₈ (1) neutral, as well as the photoelectron spectrum of B₈O₈^- (2) anion, are predicted computationally. The latter spectrum shows a sizable energy gap of 3.5 eV for 2, demonstrating the electronic robustness of 1. Our bonding analyses also shed critical light on the nature of bonding in s-indacene.