On the structure and bonding in the B₄O₄⁺cluster: a boron oxide analogue of the 3,5-dehydrophenyl cation with p and s 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 Cs (1, (2)A') 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 Cs B4O4(+), which closely mimics that in the 3,5-dehydrophenyl cation C6H3(+) (D3h, (1)A1'), a prototypical molecule with double aromaticity. Alternative D2h (2, (2)B3g) and C2v (3, (2)A1) 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.

Double aromaticity arising from σ- and π-rings

“…For a long time, aromaticity has been considered to emerge exclusively from interactions between π-symmetric orbitals; however, this symmetry constraint has meanwhile been expanded to include interactions of σ-type 1-3 , δ-type [4][5][6] , and φ-type symmetry 7 . Since double aromaticity arising from σ-orbital and π-orbital interactions has been proposed by Schleyer 8 , compounds exhibiting double aromaticity have been predicted theoretically [9][10][11][12][13][14][15][16][17] , and metal clusters and compounds with monocyclic carbon/boron rings that bear double aromaticity, generated in the gas phase, have already been characterized [18][19][20][21][22][23] . The concept of the σ-double and π-double aromaticity has also been discussed in the synthetic investigation of an anionic C 2 B 3 ring compound 24 .…”

Preparation and characterization of chemically bonded argon–boroxol ring cation complexes

“…For each boroxol ring cation complex, there are three 3c–2e delocalized π bonds within the boroxol ring, satisfying the 4 n + 2 rule for aromaticity, similar to the B 4 O 4 + cations reported previously. 50 Thus, all three boroxol ring cation clusters are π aromatic.…”

Structures and chemical bonding of B3O3−/0 and B3O3H−/0: A combined photoelectron spectroscopy and first-principles theory study

“…The O4= =B1≡ ≡O6 structural block possesses a typical 3c-4e π hyperbond. 13,23,48,49 Specifically, the nonbonding/bonding combination of HOMO-4 and HOMO-8 constitutes such a 3c-4e π hyperbond. HOMO-4 is essentially nonbonding between the B1-O4 and B1-O6 segments, with p z of O4 (9%), p z of O6 (48%), and p z of B1 (5%); HOMO-8 is strongly π bonding between the O4/B1/O6 centers, with the p z atomic orbitals (AOs) of B1 (15%), O4 (69%), and O6 (11%).…”