On the structure and bonding in the B4O4+ cluster: a boron oxide analogue of the 3,5-dehydrophenyl cation with π and σ double aromaticity

Abstract

Boron oxide clusters offer intriguing molecular models for the electron-deficient system, in which the boronyl (BO) group plays a key role and the interplay between the localized BO triple bond and the multicenter electron delocalization dominates the chemical bonding. Here we report the structural, electronic, and bonding properties of the B₄O₄⁺ cationic cluster on the basis of unbiased Coalescence Kick global-minimum searches and first-principles electronic structural calculations at the B3LYP and single-point CCSD(T) levels. The B₄O₄⁺ cluster is shown to possess a C_s (1, ²A′) global minimum. It represents the smallest boron oxide species with a hexagonal boroxol (B₃O₃) ring as the core, terminated by a boronyl group. Chemical bonding analyses reveal double (π and σ) aromaticity in C_s B₄O₄⁺, which closely mimics that in the 3,5-dehydrophenyl cation C₆H₃⁺ (D_3h, ¹A₁′), a prototypical molecule with double aromaticity. Alternative D_2h (2, ²B_3g) and C_2v (3, ²A₁) isomeric structures of B₄O₄⁺ are also analyzed, which are relevant to the global minima of B₄O₄ neutral and B₄O₄⁻ anion, respectively. These three structural motifs vary drastically in terms of energetics upon changing the charge state, demonstrating an interesting case in which every electron counts. The calculated ionization potentials and electron affinities of the three corresponding neutral isomers are highly uneven, which underlie the conformational changes in the B₄O₄^+/0/− series. The current work presents the smallest boron oxide species with a boroxol ring, establishes an analogy between boron oxides and the 3,5-dehydrophenyl cation, and enriches the chemistry of boron oxides and boronyls.