The B35 cluster with a double-hexagonal vacancy: a new and more flexible structural motif for borophene

Structures and Properties of CoB19 +/0/- Clusters

A global search for the lowest energy structure of Co atom-doped boron clusters (CoB19 +, CoB19, and CoB19 - clusters) was conducted. The lowest energy structures of them are remarkably different from those of B20 and CoB18 - clusters. CoB19 + clusters have a bowl-shaped geometry, where the Co atom is at the bottom of the bowl and is coordinated with eight B atoms. The CoB19 cluster presents seven- and eight-membered B rings. The CoB19 - cluster can be viewed as a structure that evolves from a Co-doped boron plane. The coordination number of CoB19 and CoB19 - clusters are 16 and 14, respectively. Several low-lying isomers have quasi-planar structures for the CoB19 - cluster. Some properties including charge transformation and distribution, HOMO-LUMO gaps, molecular orbital distribution, and stability of neutral CoB19 are discussed. CoB19 + and CoB19 - exhibit magnetism with a net moment of 1.0 and 0.94 μB because of odd number of electrons.

Theoretical prediction of chiral actinide endohedral borospherenes

Actinide encapsulation can form chiral borospherenes, and the covalent character of An–B bonds dominates the formation of these actinoborospherenes.

TM4B180/− (TM = Hf, Ta, W, Re, Os): new structure construction with TM doped B wheel units

The lowest energy structure of Ta₄B₁₈ shows a conflicting aromaticity and is assembled from four planar molecular Ta@B₉ units.

Anchoring a bow-shaped boron single chain in binary Be6B7- cluster: hybrid octagonal ring, multifold π/σ aromaticity, and dual electronic transmutation

Elemental boron clusters do not form linear chain or monocyclic ring structures, which is in contrast to carbon. Based on computer global searches and quantum chemical calculations, we report on the viability of a curved boron single chain in binary Be6B7- cluster. The boron motif assumes a bow shape, being anchored on a Be6 prism. Such a motif, which appears to be highly strained in its free-standing form, is exotic in boron-based clusters and nanostructures. Chemically, the cluster is analogous to a "clam-and-pearl-chain" system at the nanoscale (about 1 nm in size), in which a Be6 clam moderately opens its mouth, except that a B7 pearl chain is too large to be encapsulated inside. The picture differs from a three-layered sandwich. This cluster features a hybrid Be2B7 monocyclic ring, which is octagonal in nature and supports double 10π/6σ aromaticity. The number of π bonds substantially surpasses that in bare boron clusters of similar sizes. Two Be3 rings in the prism are also σ aromatic, albeit with effective 1σ/1σ electron-counting only. The unique multifold 1σ/10π/6σ/1σ aromaticity governs the geometry of the Be6B7- cluster, which can also be rationalized using the concept of dual electronic transmutation.

Are all planar and quasi-planar boron clusters aromatic? Counter examples of island or global π antiaromaticity from chemical bonding analysis

Boron is an electron-deficient element. The flatland of planar or quasi-planar (2D) boron clusters is believed to possess aromaticity for all members, which remains a fundamental issue in debate in boron chemistry. Using a selected set of D_2h B₆^2-, C_2h B₂₈^2-, and C_2v B₂₉^- clusters as counter examples, we shall present computational evidence for global or island π antiaromaticity in 2D boron clusters. The latter two are flattened for the purpose of clarity, which model their quasi-planar C₂ or C_s monoanion clusters observed in prior gas-phase experiments. Chemical bonding in the clusters is elucidated collectively on the basis of canonical molecular orbital (CMO) analysis, adaptive natural density partitioning (AdNDP), electron localization functions (ELFs), and localized molecular orbital (LMO) analysis. These results are complementary to each other and yet highly coherent. As a quantitative indicator, nucleus-independent chemical shifts (NICSs) are calculated at selected specific points in the clusters, which help differentiate between π aromaticity and antiaromaticity. Intriguingly, triangular sites in the same boron cluster can be aromatic, antiaromatic, or nonaromatic, despite the fact that they are physically indistinguishable. The phenomenon is understood in analogy to hydrocarbons and polycyclic aromatic hydrocarbons (PAHs). Even perfect sheet-like boron clusters are convertible to the PAH analogous systems. This work provides compelling examples for global and island π antiaromaticity in the 2D boron clusters.

Structures, Electronic, and Spectral Properties of Doped Boron Clusters MB12 0/- (M = Li, Na, and K)

Structures and electronic properties of alkali metal atom-doped boron clusters MB12 0/- (M = Li, Na, K) are determined using the CALYPSO method for the global minimum search followed by density functional theory. It is found that the global minima obtained for the neutral clusters correspond to the half-sandwich structure and those of the monoanionic clusters correspond to the boat-shaped structure. The neutral MB12 (M = Li, Na, K) can be considered as a member of the half-sandwich doped B12 clusters, and the geometrical pattern of anion MB12 - (M = Li, Na, K) is a new structure that is different from other doped B12 clusters. Natural population and chemical bonding analyses reveal that the alkali metal atom-doped boron clusters MB12 - are characterized as charge transfer complexes, M+B12 2-, resulting in symmetrically distributed chemical bonds and electrostatic interactions between cationic M+ and boron atoms. The calculated spectra indicate that MB12 0/- (M = Li, Na, K) has meaningful spectral features that can be compared with future experimental data. Our work enriches the varieties of geometrical structures of doped boron clusters and can provide much insight into boron nanomaterials.

Crystalline Semiconductor Boron Quantum Dots

Zero-dimensional boron structures have always been the focus of theoretical research owing to their abundant phase structures and special properties. Boron clusters have been reported extensively by combining structure searching theories and photoelectron spectroscopy (PES) experiments; however, crystalline boron quantum dots (BQDs) have rarely been reported. Here, we report the preparation of large-scale and uniform crystalline semiconductor BQDs from the expanded bulk boron powders via a facile and efficient probe ultrasonic approach in the acetonitrile solution. The obtained BQDs have 2.46 nm average lateral size and 2.81 nm thickness. Optical measurements demonstrate that a strong quantum confinement effect occurs in the BQDs, implying the increase of the band gap from 1.80 eV for the corresponding bulk to 2.46 eV for the BQDs. By injecting the BQDs into poly(vinylpyrrolidone) as an active layer, a BQD-based memory device is fabricated that shows a rewriteable nonvolatile memory effect with a low transition voltage of down to 0.5 V and a high on/off switching ratio of 10³ as well as a good stability.

Trishomoaromatic (B3 N3 Ph6 ) Dianion: Characterization and Two-Electron Reduction

Benzene, a common aromatic compound, can be converted into an unstable antiaromatic 8π-electron intermediate through two-electron reduction. However, as an isoelectronic equivalent of benzene, borazine (B₃ N₃ Ph₆ ), having weak aromaticity, undergoes a totally different two-electron reduction to afford (B₃ N₃ R₆ )^2- homoaromatic compounds. Reported here is the synthesis of homoaromatic (B₃ N₃ Ph₆ )^2- by the reduction of B₃ N₃ Ph₆ with either potassium or rubidium in the presence of 18-crown-6 ether. Theoretical investigations illustrate that two electrons delocalize over the three boron atoms in (B₃ N₃ Ph₆ )^2- , which is formed by the geometric and orbital reorganization and exhibits (π,σ)-mixed homoaromaticity. Moreover, (B₃ N₃ Ph₆ )^2- can act as a robust 2e reductant for unsaturated compounds, such as anthracene, chalcone, and tanshinones. This 2e reduction is of high efficiency and selectivity, proceeds under mild reaction conditions, and can regenerate neutral borazine.

The Rise of 2D Photothermal Materials beyond Graphene for Clean Water Production

Water shortage is one of the most concerning global challenges in the 21st century. Solar-inspired vaporization employing photothermal nanomaterials is considered to be a feasible and green technology for addressing the water challenge by virtue of abundant and clean solar energy. 2D nanomaterials aroused considerable attention in photothermal evaporation-induced water production owing to their large absorption surface, strong absorption in broadband solar spectrum, and efficient photothermal conversion. Herein, the recent progress of 2D nanomaterials-based photothermal evaporation, mainly including emerging Xenes (phosphorene, antimonene, tellurene, and borophene) and binary-enes (MXenes and transition metal dichalcogenides), is reviewed. Then, the optimization strategies for higher evaporation performance are summarized in terms of modulation of the intrinsic photothermal performance of 2D nanomaterials and design of the complete evaporation system. Finally, the challenges and prospective of various kinds of 2D photothermal nanomaterials are discussed in terms of the photothermal performance, stability, environmental influence, and cost. One important principle is that solutions for water challenges should not introduce new environmental and social problems. This Review aims to highlight the role of 2D photothermal nanomaterials in solving water challenges and provides a viable scheme toward the practical use in photothermal materials selection, design, and evaporation systems building.

Boron-Based Chiral Helix Be6 B10 2- and Be6 B11 - Clusters: Structures, Chemical Bonding, and Formation Mechanism

Boron forms a rich variety of low-dimensional nanosystems, including the newly discovered helix Be₆ B₁₀ ^2- (1) and Be₆ B₁₁ ^- (2) clusters. We report herein on the elucidation of chemical bonding in clusters 1/2, using the modern quantum chemistry tools of canonical molecular orbital analyses and adaptive natural density partitioning (AdNDP). It is shown that clusters 1/2 contain a chiral helix Be₂ B₁₀ Be₂ or Be₂ B₁₁ Be₂ skeleton with a total of 11 and 12 segments, respectively, which effectively curve into "helical pseudo rings" and chemically consist of two "quasicircles" as defined by their anchoring Be centers. The helix skeleton is connected via Lewis-type B-B and Be-B-Be σ bonds, being further stabilized by island π/σ bonds and a loose π bond at the junction. The Be₆ component in 1/2 assumes a distorted prism shape only physically, and it is fragmented into four parts: two terminal Be₂ dimers and two isolated Be centers. A Be₂ dimer at the far end manages to bend over and cap a quasicircle from one side of B plane. Consequently, each quasicircle of a helical pseudo ring is capped from opposite sides by two Be₂ /Be units, facilitating intramolecular charge-transfers of 5 electrons from Be to B. Overall, the folding of B helix involves as many as 10 electrons. The enormous electrostatics offers the ultimate driving forces for B helix formation.

Borophene Concentric Superlattices via Self-Assembly of Twin Boundaries

Due to its in-plane structural anisotropy and highly polymorphic nature, borophene has been shown to form a diverse set of linear superlattice structures that are not observed in other two-dimensional materials. Here, we show both theoretically and experimentally that concentric superlattice structures can also be realized in borophene via the energetically preferred self-assembly of coherent twin boundaries. Since borophene twin boundaries do not require the creation of additional lattice defects, they are exceptionally low in energy and thus easier to nucleate and even migrate than grain boundaries in other two-dimensional materials. Due to their high mobility, borophene twin boundaries naturally self-assemble to form novel phases consisting of periodic concentric loops of filled boron hexagons that are further preferred energetically by the rotational registry of borophene on the Ag(111) surface. Compared to defect-free borophene, concentric superlattice borophene phases are predicted to possess enhanced mechanical strength and localized electronic states. Overall, these results establish defect-mediated self-assembly as a pathway to unique borophene structures and properties.

A quasi-plane IrB18− cluster with high stability

A quasi-plane anionic IrB₁₈⁻ cluster with high stability is uncovered by a CALYPSO structural search method.

Analysis of the structures, stabilities and electronic properties of MB16− (M = V, Cr, Mn, Fe, Co, Ni) clusters and assemblies

Stacking of lowest-energy structures of Fe₂B₂₄⁻ and Co₂B₂₄⁻ dimers.

Competition between tubular, planar and cage geometries: a complete picture of structural evolution of Bn (n = 31–50) clusters

Stimulated by the early theoretical prediction of B₈₀ fullerene and the experimental finding of the B₄₀ cage, the structures of medium-sized boron clusters have attracted intensive research interest during the last decade, but a complete picture of their size-dependent structural evolution remains a puzzle.

Study on spheroidization and the growth mechanism of eutectic boron in Cu–B alloys

The formation process of a solid boron sphere, hollow boron sphere and cladding structure.

The structure and chemical bonding in inverse sandwich B6Ca2 and B8Ca2 clusters: conflicting aromaticity vs. double aromaticity

Boron-based B₆Ca₂ and B₈Ca₂ clusters adopt unique inverse sandwich architectures, which are stabilized by interesting conflicting aromaticity and double aromaticity, respectively.

Planar B41- and B42- clusters with double-hexagonal vacancies

Since the discovery of the B₄₀ borospherene, research interests have been directed to the structural evolution of even larger boron clusters. An interesting question concerns if the borospherene cages persist in larger boron clusters like the fullerenes. Here we report a photoelectron spectroscopy (PES) and computational study on the structures and bonding of B₄₁^- and B₄₂^-, the largest boron clusters characterized experimentally thus far. The PE spectra of both clusters display broad and complicated features, suggesting the existence of multiple low-lying isomers. Global minimum searches for B₄₁^- reveal three low-lying isomers (I-III), which are all related to the planar B₄₀^- structure. Isomer II (C_s, ¹A') possessing a double hexagonal vacancy is found to agree well with the experiment, while isomers I (C_s, ³A'') and III (C_s, ¹A') both with a single hexagonal vacancy are also present as minor isomers in the experiment. The potential landscape of B₄₂^- is found to be much more complicated with numerous low-lying isomers (VII-XII). The quasi-planar structure VIII (C₁, ²A) containing a double hexagonal vacancy is found to make major contributions to the observed PE spectrum of B₄₂^-, while the other low-lying isomers may also be present to give rise to a complicated spectral pattern. Chemical bonding analyses show isomer II of B₄₁^- (C_s, ¹A') and isomer VIII of B₄₂^- (C₁, ²A) are π aromatic, analogous to that in the polycyclic aromatic hydrocarbon C₂₇H₁₃⁺ (C_2v, ¹A₁). Borospherene cage isomers are also found for both B₄₁^- and B₄₂^- in the global minimum searches, but they are much higher energy isomers.

Crown-like charge-transfer lithium-doped boron oxide complexes B8O2Li+/0

A variety of researches on boron oxide clusters have indicated the key role of boronyl (BO) group in the structures and bonding. Based upon global structural searches and electronic structure calculations at the B3LYP and single-point coupled cluster single double (triple) (CCSD(T)) levels, we present the possibility of construction of lithium-doped boron oxide B₈O₂Li^+/0 clusters (1-2). Different from the structures of pure B₆^+/0/- and B₆(BO)₂^0/-, the B₈O₂Li^+/0 which can be formulated as B₆(BO)₂Li^+/0 are not the double-chain structures, they are the crown-like structure, and the Li is like a diamond that links the crown. Detailed AdNDP analyses indicate the π aromaticity of B₈O₂Li⁺ (1). The results obtained in this work reveal that the metal could influence the structures and properties of boron oxides significantly.

One-dimensional nearly free electron states in borophene

Two-dimensional boron (borophene) features structural polymorphs and distinct in-plane anisotropy, opening opportunities to achieve tailored electronic properties by intermixing different phases. Here, using scanning tunneling spectroscopy combined with first-principles calculations, delocalized one-dimensional nearly free electron states (NFE) in the (2,3) or β₁₂ borophene sheet on the Ag(111) surface were observed. The NFE states emerge from a line defect in borophene, manifested as a structural unit of the (2,2) or χ₃ sheet, which creates an in-plane potential well that shifts the states toward the Fermi level. The NFE states are held near the 2D plane of borophene, rather than in the vacuum region as observed in other nanostructures. Furthermore, borophene can provide a rare prototype to further study novel NFE behaviors, which may have potential applications in transport or field emission nanodevices based on boron.

Flat Boron: A New Cousin of Graphene

The mechanical exfoliation of graphene from graphite provides the cornerstone for the synthesis of other 2D materials with layered bulk structures, such as hexagonal boron nitride, transition metal dichalcogenides, black phosphorus, and so on. However, the experimental production of 2D flat boron is challenging because bulk boron has very complex spatial structures and a rich variety of chemical properties. Therefore, the realization of 2D flat boron marks a milestone for the synthesis of 2D materials without layered bulk structures. The historical efforts in this field, particularly the most recent experimental progress, such as the growth of 2D flat boron on a metal substrate by chemical vapor deposition and molecular beam epitaxy, or liquid exfoliation from bulk boron, are described.

Near-equilibrium growth from borophene edges on silver

Two-dimensional boron, borophene, was realized in recent experiments but still lacks an adequate growth theory for guiding its controlled synthesis. Combining ab initio calculations and experimental characterization, we study edges and growth kinetics of borophene on Ag(111). In equilibrium, the borophene edges are distinctly reconstructed with exceptionally low energies, in contrast to those of other two-dimensional materials. Away from equilibrium, sequential docking of boron feeding species to the reconstructed edges tends to extend the given lattice out of numerous polymorphic structures. Furthermore, each edge can grow via multiple energy pathways of atomic row assembly due to variable boron-boron coordination. These pathways reveal different degrees of anisotropic growth kinetics, shaping borophene into diverse elongated hexagonal islands in agreement with experimental observations in terms of morphology as well as edge orientation and periodicity. These results further suggest that ultrathin borophene ribbons can be grown at low temperature and low boron chemical potential.

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.

Re©B8- and Re©B9-: New Members of the Transition-Metal-Centered Borometallic Molecular Wheel Family

Transition-metal-centered monocyclic boron wheel clusters (M©B _n ^q) represent a family of interesting borometallic compounds with double aromaticity. A variety of early and late transition metal atoms have been found to form such structures with high symmetries and various B _n ring sizes. Here we report a combined photoelectron spectroscopy and quantum-chemistry theoretical study of two M©B _n^- clusters from the middle of the transition metal series: Re©B₈^- and Re©B₉^-. Global minimum structure searches revealed that ReB₈^- adopts a pseudo- C_{8 v} structure while ReB₉^- is a perfectly planar D_{9 h} molecular wheel. Chemical bonding analyses showed that both clusters exhibit σ and π double aromaticity and obey the electronic design principle for metal-centered borometallic molecular wheels. The central Re atoms are found to possess unusually low oxidation states of +I in Re©B₈^- and +II in Re©B₉^-, i.e., the Re atom behaves similarly to late transition metal elements (Ru, Fe, Co, Rh, Ir) in the M©B _n^- molecular wheels. These two clusters become new members of the family of transition-metal-centered monocyclic borometallic molecular wheels, which may be viable for chemical syntheses with appropriate ligands.

B31- and B32-: chiral quasi-planar boron clusters

Chirality plays an important role in nature. Nanoclusters can also exhibit chiral properties. We report herein a joint experimental and theoretical investigation on the geometric and electronic structures of B31- and B32- clusters, using photoelectron spectroscopy in combination with first-principles calculations. Two degenerate quasi-planar chiral C1 enantiomers (I and II, 1A) with a central hexagonal vacancy are identified as the global minima of B31-. For B32-, two degenerate boat-like quasi-planar chiral C2 structures (VI and VII, 2A) with a central hexagonal vacancy are also found as the global minima, with a low-lying chair-like Ci B32- (VIII, 2Au) also present in the experiment as a minor isomer. The chiral conversions in quasi-planar B31- and B32- clusters are investigated and relatively low barriers are found due to the high flexibility of these monolayer clusters, which feature multiple delocalized σ and π bonds over buckled molecular surfaces.

Insights into the unusual semiconducting behavior in low-dimensional boron

Elementary semiconductors are rare and attractive, especially for low-dimensional materials. Unfortunately, most of the boron nanostructures have been found to be metallic, despite their typical semiconducting bulk structure. Herein, we propose a general recipe to realize low-dimensional semiconducting boron. This unusual semiconducting behavior is attributed to charge transfer and electron localization, induced by symmetry breaking that divides boron atoms into cations and anions. In addition, it is feasible to accomplish band gap engineering by rationally designing various structures. Importantly, the low-dimensional semiconducting boron allotropes are predicted to be an excellent solar-cell material with a power conversion efficiency of up to 22%, paving the way for their promising optoelectronic application.

Strong chemisorption of CO2 on B10-B13 planar-type clusters

An ab initio density functional study was performed investigating the adsorption of CO₂ on neutral boron B _n (n  =  10-13) clusters that are characterized by planar and quasiplanar ground-state atomic structures. For all four clusters, we found large chemisorption binding energies, reaching 1.6 eV between CO₂ and B₁₂, with the adsorbed molecule oriented in the plane of the cluster and adsorbed along the cluster edge. A configuration with chemisorbed dissociated CO₂ molecule also exists for B₁₁ and B₁₃ clusters. The strong adsorption is due to the bending of the CO₂ molecule, which provides energetically accessible fully in-plane frontier molecular orbitals matching the edge states of the clusters. At the same time, the intrinsic dipole moment of a bent CO₂ molecule facilitates the transfer of excess electronic charge from the cluster edges to the molecule.

Inorganic Boron-Based Nanostructures: Synthesis, Optoelectronic Properties, and Prospective Applications

Inorganic boron-based nanostructures have great potential for field emission (FE), flexible displays, superconductors, and energy storage because of their high melting point, low density, extreme hardness, and good chemical stability. Until now, most researchers have been focused on one-dimensional (1D) boron-based nanostructures (rare-earth boride (REB₆) nanowires, boron nanowires, and nanotubes). Currently, two-dimensional (2D) borophene attracts most of the attention, due to its unique physical and chemical properties, which make it quite different from its corresponding bulk counterpart. Here, we offer a comprehensive review on the synthesis methods and optoelectronics properties of inorganic boron-based nanostructures, which are mainly concentrated on 1D rare-earth boride nanowires, boron monoelement nanowires, and nanotubes, as well as 2D borophene and borophane. This review paper is organized as follows. In Section I, the synthesis methods of inorganic boron-based nanostructures are systematically introduced. In Section II, we classify their optical and electrical transport properties (field emission, optical absorption, and photoconductive properties). In the last section, we evaluate the optoelectronic behaviors of the known inorganic boron-based nanostructures and propose their future applications.

Structures, Stabilities, and Spectral Properties of Endohedral Borospherenes M@B40 0/- (M = H2, HF, and H2O)

The discovery of borospherene B₄₀ leads to a new beginning for the study of boron chemistry and may lead to new boron-based nanomaterials. Based on density functional theory, the structures, electronic properties, infrared and Raman spectra, photoelectron spectra, and electronic absorption spectra of endohedral borospherenes M@B₄₀ ^0/- (M = H₂, HF, and H₂O) are investigated. It is found that H₂, HF, and H₂O monomers can form stable endohedral borospherenes M@B₄₀ ^0/- (M = H₂, HF, and H₂O). In addition, the calculated results indicate that the doped molecule at the off-center location can relax to the center location within the cage and the symcenter of the doped molecule is almost located in the center of the cage. Unlike endohedral metalloborospherene Ca@B₄₀, which is a charge-transfer complex between Ca²⁺ and B₄₀ ^2-, natural population analyses and chemical bonding analyses reveal that there is no significant charge transfer of the doped molecule. The calculated spectra indicate that doping of a molecule (H₂, HF, or H₂O) in borospherene B₄₀ can change the photoelectron spectra and doping of a polar molecule (HF or H₂O) in borospherene B₄₀ can change the spectral properties. For instance, the addition of a molecule can increase infrared and Raman-active modes and cause a red shift or blue shift of electronic spectra. These spectral features can be compared with future experimental values of endohedral borospherenes M@B₄₀ ^0/- (M = H₂, HF, and H₂O).

Formation of the quasi-planar B50 boron cluster: topological path from B10 and disk aromaticity

The lowest-lying isomer of the B50 boron cluster is confirmed to have a quasi-planar shape with two hexagonal holes. By applying a topological (leap-frog) dual operation followed by boron capping, we demonstrated that such a quasi-planar structure actually comes from the smallest elongated B102-, and its high thermodynamic stability is due to its inherent disk aromaticity arising from its 32 valent π electrons that fully occupy a disk configuration of [(1σ)2(1π)4(1δ)4(2σ)2(1φ)4(2π)4(1γ)4(2δ)4(1η)4]. The aromatic character of the quasi-planar B50 is further supported by a strong diatropic magnetic current flow. The sudden appearance of a quasi-planar B50 again points out that the growth pattern of pure boron clusters is still far from being completely understood.

Starting a subnanoscale tank tread: dynamic fluxionality of boron-based B10Ca alloy cluster

Alloying an elongated B₁₀ cluster with Ca is shown to give rise to a dynamically fluxional B₁₀Ca cluster, the latter behaving like a tank tread at the subnanoscale. Computer global search identifies the B₁₀Ca C ₂ (¹A) global-minimum structure, which is chiral in nature and retains the quasi-planar moiety of bare B₁₀ cluster with Ca capped at one side, forming a half-sandwich. The rotation barrier of B₁₀Ca cluster is reduced with respect to B₁₀ by one order of magnitude, down to 1 kcal mol^-1 at the PBE0/6-311+G* level, which demonstrates structural fluxionality at 600 K and beyond via molecular dynamics simulations. Structurewise, the Ca alloying in B₁₀Ca cluster generates rhombic defect holes, preactivating the species and making it flexible against deformation. Chemical bonding analyses indicate that the B₁₀Ca cluster is a charge-transfer [B₁₀]^2-[Ca]²⁺ complex, being doubly π/σ aromatic with the 6π and 10σ electron-counting. Such a pattern offers ideal π/σ delocalization and facilitates fluxionality. In contrast, bare B₁₀ cluster has conflicting aromaticity with 6π and 8σ electrons, which is nonfluxional with a barrier of 12 kcal mol^-1. Double π/σ aromaticity versus conflicting aromaticity is a key mechanism that distinguishes between fluxional B₁₀Ca and nonfluxional B₁₀ clusters, offering a compelling example that the concept of aromaticity (and double aromaticity) can be exploited to design dynamically fluxional nanosystems.

[La(η x -B x )La]- (x = 7-9): a new class of inverse sandwich complexes

Despite the importance of bulk lanthanide borides, nanoclusters of lanthanide and boron have rarely been investigated. Here we show that lanthanide-boron binary clusters, La₂B _x ^-, can form a new class of inverse-sandwich complexes, [Ln(η ^x -B _x )Ln]^- (x = 7-9). Joint experimental and theoretical studies reveal that the monocyclic B _x rings in the inverse sandwiches display similar bonding, consisting of three delocalized σ and three delocalized π bonds. Such monocyclic boron rings do not exist for bare boron clusters, but they are stabilized by the sandwiching lanthanide atoms. An electron counting rule is proposed to predict the sizes of the B _x ring that can form stable inverse sandwiches. A unique (d-p)δ bond is found to play important roles in the stability of all three inverse-sandwich complexes.

Probing the structures and bonding of size-selected boron and doped-boron clusters

Because of their interesting structures and bonding and potentials as motifs for new nanomaterials, size-selected boron clusters have received tremendous interest in recent years. In particular, boron cluster anions (Bn-) have allowed systematic joint photoelectron spectroscopy and theoretical studies, revealing predominantly two-dimensional structures. The discovery of the planar B36 cluster with a central hexagonal vacancy provided the first experimental evidence of the viability of 2D borons, giving rise to the concept of borophene. The finding of the B40 cage cluster unveiled the existence of fullerene-like boron clusters (borospherenes). Metal-doping can significantly extend the structural and bonding repertoire of boron clusters. Main-group metals interact with boron through s/p orbitals, resulting in either half-sandwich-type structures or substitutional structures. Transition metals are more versatile in bonding with boron, forming a variety of structures including half-sandwich structures, metal-centered boron rings, and metal-centered boron drums. Transition metal atoms have also been found to be able to be doped into the plane of 2D boron clusters, suggesting the possibility of metalloborophenes. Early studies of di-metal-doped boron clusters focused on gold, revealing ladder-like boron structures with terminal gold atoms. Recent observations of highly symmetric Ta2B6- and Ln2Bn- (n = 7-9) clusters have established a family of inverse sandwich structures with monocyclic boron rings stabilized by two metal atoms. The study of size-selected boron and doped-boron clusters is a burgeoning field of research. Further investigations will continue to reveal more interesting structures and novel chemical bonding, paving the foundation for new boron-based chemical compounds and nanomaterials.

Structural and electronic properties of MB22− (M = Na, K) clusters: tubular boron versus quasi-planar boron forms

Electron deficiency of boron atom has led to the abundant chemical properties of boron clusters, such as intriguing structures, unique multi-center bonding and electronic properties, as well as the structural evolution from planar to three-dimensional forms.

NbB12−: a new member of half-sandwich type doped boron clusters with high stability

The global minimum structure of a NbB₁₂⁻ cluster of half-sandwich type.

Be2B6 and Be2B7+: two double aromatic inverse sandwich complexes with spin-triplet ground state

Be₂B₆ and Be₂B₇⁺ clusters adopt interesting inverse sandwich structures with double σ/π aromaticity, and the former possesses the smallest monocyclic boron ring motif.

Exhaustive exploration of MgBn (n = 10–20) clusters and their anions

An unexpected tubular-shaped MgB₁₈ cluster is identified for the first time in alkaline-earth metal-doped boron clusters.