Boron-based inorganic heterocyclic clusters: electronic structure, chemical bonding, aromaticity, and analogy to hydrocarbons

This Perspective article deals with recent computational and experimental findings in boron-based heterocyclic clusters, which focuses on binary B-O and B-S clusters, as well as relevant ternary B-X-H (X = O, S, N) species. Boron is electron-deficient and boron clusters do not form monocyclic rings or linear chains. Boron-based heterocyclic clusters are intuitively even more electron-deficient and feature exotic chemical bonding, which make use of O 2p, S 3p, or N 2p lone-pairs for π delocalization over heterocyclic rings, facilitating new cluster structures and new types of bonding. Rhombic, pentagonal, hexagonal, and polycyclic clusters are discussed herein. Rhombic species are stabilized by four-center four-electron (4c-4e) π bonding, that is, the o-bond. An o-bond cluster differs from a typical 4π antiaromatic system, because it has 4π electrons in an unusual bonding/nonbonding combination, which takes advantage of the empty 2p_z atomic orbitals from electron-deficient boron centers. A variety of examples (notably including boronyl boroxine) possess a hexagonal ring, as well as magic 6π electron-counting, making them new members of the inorganic benzene family. Pentagonal clusters bridge rhombic o-bond systems and inorganic benzenes, but they do not necessarily favor 6π electron-counting as in cyclopentadienide anion. In contrast, pentagonal 4π clusters are stable, leading to the concept of pentagonal o-bond. One electron can overturn the potential energy landscape of a system, enabling rhombic-to-hexagonal structural transition, which further reinforces the idea that 4π electron-counting is favorable for rhombic systems and 6π is magic for hexagonal rings. The bonding analogy between heterocyclic clusters and hydrocarbons goes beyond monocyclic species, which allows rational design of boron-based inorganic analogs of polycyclic aromatic hydrocarbons, including s-indacene as a puzzling aromatic/antiaromatic system. Selected linear B-O clusters are also briefly discussed, featuring dual 3c-4e π bonds, that is, ω-hyperbonds. Dual ω-hyperbonds, rhombic or pentagonal o-bond, and inorganic benzenes share a common chemical origin. The field of boron-based heterocyclic clusters is still in its infant stage, and much new chemistry remains to be discovered in forthcoming experimental and theoretical studies.

A photoelectron spectroscopy and quantum chemical study on ternary Al–B–O clusters: AlnBO2− and AlnBO2 (n = 2, 3)

Dictated by sequential and competitive oxidation of B versus Al centers, ternary Al₂BO₂^−/0 and Al₃BO₂^−/0 clusters do not possess a BO₂ unit despite its structural robustness.

Pentagonal five-center four-electron π bond in ternary B3N2H5 cluster: an extension of the concept of three-center four-electron ω bond

Ternary B₃N₂H₅ (C_2v, ¹A₁) cluster has a heteroatomic B₃N₂ ring, with 4π electrons in a robust bonding/nonbonding combination, which is proposed as a five-center four-electron o-bond.

On the nature of bonding in binary Be2O2 and Si2O2 clusters: rhombic four-center four-electron π and σ bonds

The structural and electronic properties and chemical bonding of binary Be2O2 and Si2O2 clusters have been studied using quantum chemical calculations at the B3LYP level. For the Be2O2 cluster, the potential energy surface is probed by unbiased structural searches and the global-minimum structure was established using the B3LYP calculations, complemented by PBE0 and single-point CCSD(T) calculations for top isomers. The perfectly planar D2h Be2O2 ((1)Ag) global minimum is well defined, being at least 3.64 eV lower in energy than alternative structures at the CCSD(T)//B3LYP/aug-cc-pVTZ level. Chemical bonding analyses show that D2h Be2O2 and Si2O2 clusters possess the rhombic four-center four-electron (4c-4e) π bond, that is, the o-bond, a conception derived from electron-deficient boron oxide clusters lately. Furthermore, the Be2O2 and Si2O2 clusters also exhibit rhombic 4c-4e σ bonds, both for the radial and tangential σ frameworks (σr and σt). The σt framework is classified as an o-bond only formally, due to the secondary contribution from the Be/Si s component. The three-fold (π, σr, and σt) o-bonds in Be2O2 and Si2O2 are considered to resemble the three-fold aromaticity in all-metal Al4(2-) dianions. A 4c-4e o-bond makes use of four O 2p electrons, which would otherwise be two lone-pairs, for a delocalized and completely bonding orbital, as well as a residual nonbonding orbital. Three-fold o-bonds thus greatly stabilize the binary Be2O2 and Si2O2 clusters. We anticipate that the bonding concept should be applicable to additional molecular systems, including those with larger heterocyclic rings.

Planar B3S2H3−and B3S2H3clusters with a five-membered B3S2ring: boron–sulfur hydride analogues of cyclopentadiene

Boron–sulfur hydride clusters,C_2vB₃S₂H₃⁻and B₃S₂H₃, possess a five-membered B₃S₂ring as the core, which is analogous to cyclopentadiene in terms of π bonding.

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

The B₄O₄⁺cluster has a hexagonal structure with a boroxol ring. It features double (p and s) aromaticity, akin to the 3,5-dehydrophenyl cation.