Planar Tricyclic BO and BO Clusters: Boron Oxide Analogues of s-Indacene CH.

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 CH 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 BO (1, A) and D BO (2, B) adopt planar tricyclic structures, which feature fused heterocyclic BO/BO/BO rings and two boronyl (BO) terminals, a structural pattern analogous to the C/C/C rings in s-indacene. Bonding analyses indicate that BO (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 BO (1) neutral, as well as the photoelectron spectrum of BO (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.