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

Boronyl-Based Polycyclic Superhalogens

Superhalogenity refers to the tendency of radicals to have higher electron affinity (EA) than halogens or anions to possess higher vertical detachment energy (VDE) than do halides (3.64 eV). In Srivastava, A. K. J. Phys. Chem. A 2023, 127, 4867-4872, we demonstrated a simple strategy in which some polycyclic hydrocarbons (PHs) can be turned into polycyclic superhalogens (PSs) by substituting CN groups in the place of hydrogens. We also notice that the superhalogenity of cyanide-based PSs is related to their aromaticity. Boronyl (BO) is an isoelectronic and inorganic analog of the cyano (CN) group. Therefore, we consider the substitution of BO in PHs and compare them with CN-based PSs using the density functional theory. In the case of C5H5-, we notice that the B3LYP and CCSD(T) calculated VDEs of resulting C5H5-n(BO)n- anions increase with the increase in the number of BO substituents (n) such that they become superhalogens for n ≥ 3 like C5H5-n(CN)n- anions. However, their aromaticity does not correspond to the superhalogenity, unlike C5H5-n(CN)n-. Similarly, all BO-substituted PHs possess structures similar to their CN analogs. Although their aromaticity is reduced compared to CN-based anions, the VDE of all these BO-based polycyclic anions and the EA of their radicals exceed 5 eV than those of the corresponding CN-based PSs. Therefore, this study proposes a new class of boronyl-based polycyclic superhalogens. These superhalogen anions might attract synthetic chemists and experimentalists for further explorations.

Boron Oxide B5O6− Cluster as a Boronyl-Based Inorganic Analog of Phenolate Anion

Boron oxide clusters have structural richness and exotic chemical bonding. We report a quantum chemical study on the binary B5O6− cluster, which is relatively oxygen-rich. A global structural search reveals planar C2v (1A1) geometry as the global minimum structure, featuring a heteroatomic hexagonal B3O3 ring as its core. The three unsaturated B sites are terminated by two boronyl (BO) groups and an O− ligand. The B5O6− cluster can be faithfully formulated as B3O3(BO)2O−. This structure is in stark contrast to that of its predecessors, Cs B5O5− and Td B5O4−, both of which have a tetrahedral B center. Thus, there exists a major structural transformation in B5On− series upon oxidation, indicating intriguing competition between tetrahedral and heterocyclic structures. The chemical bonding analyses show weak 6π aromaticity in the B5O6− cluster, rendering it a boronyl analog of phenolate anion (C6H5O−) or boronyl boroxine. The calculated vertical detachment energy of B5O6− cluster is 5.26 eV at PBE0, which greatly surpasses the electron affinities of halogens (Cl: 3.61 eV), suggesting that the cluster belongs to superhalogen anions.

M(BO)k+1- Anions: Novel Superhalogens Based on Boronyl Ligands

Superhalogens have been a subject of continuous attraction in the last four decades due to their unusual structures and interesting applications. In the quest for new superhalogens based on boronyl (BO) ligands, we investigate M(BO)_k+1^- anions using a high-level ab initio CCSD(T) and B3LYP density functional method for M = Li, Na, K, Be, Mg, Ca, B, and Al. Their structures are linear for M = Li, Na, K; trigonal planar for M = Be, Mg, Ca; and tetrahedral for M = B, Al. These anions are energetically and thermodynamically stable against various fragmentations. Their superhalogen property has been established due to their higher vertical detachment energy (VDE) than halogen, being in the range 4.44-7.81 eV. For a given k, the stability and VDE of M(BO)_k+1^- anions decrease with an increase in the size of M, which is due to the decrease in charge delocalization on BO moieties. There exists a linear correlation between frontier orbitals energy gap and VDE with coefficient R² = 0.93888. It was also noticed that the BO ligand, due to the lower dimerization energy, can be preferably used as an inorganic analogue of CN. However, the structural relaxation in BO-based neutral species significantly affect the VDE and, hence, their superhalogen nature. We believe that these findings should be interesting for researchers working in superatomic chemistry as well as in boron chemistry.

Electronic structure of triangular M3 (M = B, Al, Ga): nonclassical three-center two electron π bond and σ delocalization

The small molecule clusters have received more and more attention due to their widespread applications in chemical insulators, explosives, semiconductors and the high energy density materials industry. The electron deficiency of group IIIA elements endows their clusters with interesting properties. In this work, the electronic structures of M₃ (M = B, Al, Ga) have been investigated by means of a complete active space self-consistent field (CASSCF) method. The nature of the chemical bond has been analyzed using the quantum theory of atoms in molecules (QTAIM) and electron localization function (ELF) analyses. The following conclusions can be drawn: in M₃ (M = B, Al, Ga) clusters, two π electrons are shared by three atoms forming a 3c-2e delocalization π bond. Going from B₃ to Al₃ to Ga₃, more and more electrons move from the bond pair to the outside of the M atom, which leads to a gradual enhancement of the delocalization of σ electrons. Aromaticity and the adaptive natural density partitioning (AdNDP) analyses reveal the existence of the 3c-2e π bond and delocalization of σ electrons in the studied systems.

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.

Structural and bonding properties of BS−/0 and BS3−/0

The structures of BS⁻ and BS₃⁻ were determined by the combination of size-selected anion photoelectron spectroscopy and theoretical calculations.

Double-ring tubular (B2O2)n clusters (n = 6-42) rolled up from the most stable BO double-chain ribbon in boron monoxides

Based on extensive global searches and first-principles theory calculations, we present herein the possibility of double-ring tubular (B₂O₂)_n clusters (n = 6-42) (2-10) rolled up from the most stable one-dimensional (1D) BO double-chain ribbon (1) in boron monoxides. Tubular (3D) (B₂O₂)_n clusters (n ≥ 6) are found to be systematically much more stable than their previously proposed planar (2D) counterparts, with a 2D-3D structural transition at B₁₂O₁₂ (2). Detailed bonding analyses on 3D (B₂O₂)_n clusters (2-10) and their precursor 1D BO double-chain ribbon (1) reveal two delocalized B-O-B 3c-2e π bonds over each edge-sharing B₄O₂ hexagonal unit which form a unique 6c-4e o-bond to help stabilize the systems. The IR, Raman, UV-vis, and photoelectron spectra of the concerned species are computationally simulated to facilitate their experimental characterization.

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.