All-boron aromatic clusters as potential new inorganic ligands and building blocks in chemistry

Silicon doped boron clusters: how to make stable ribbons?

A doping of small boron clusters with silicon atoms leads to the formation of stable boron nanoribbon structures. We present an analysis on the geometric and electronic structure, using MOs and electron localization function (ELF) maps, of boron ribbons represented by the dianions B₁₀Si₂^2- and B₁₂Si₂^2-. The effect of Si dopants and the origin of the underlying electron count [π²⁽ⁿ⁺¹⁾σ²ⁿ] are analyzed. Interaction between both systems of delocalized π and σ electrons creating alternant B-B bonds along the perimeter of a ribbon induces its high thermodynamic stability. The enhanced stability is related to the self-locked phenomenon.

Thermal activation of methane by vanadium boride cluster cations VBn+ (n = 3–6)

A new type of active species, transition metal boride cluster cations [VB_n⁺ (n = 3–6)], has been experimentally identified to dehydrogenate methane under thermal collision conditions. The B₃ unit in VB₃⁺ cluster is polarized by the V⁺ cation to activate CH₄.

Planarization of B20 clusters by Si and C atom substitution

An optimization strategy combining a global semi-empirical quantum mechanical search and all-electron density functional theory was adopted to determine the lowest energy structures of B₁₉Si and B₁₉C clusters. The planarization of a B₂₀ cluster by Si and C atom substitution was observed. The structural transition was from the double-ring tubular B₂₀ to an almost perfect planar B₁₉Si and a quasi-planar bowl B₁₉C. B₁₉Si possessed a geometry with a central B atom surrounded by a six-membered ring and a 13-atom outer ring. B₁₉C adopted a geometry with a B₅C six-membered hole. Both Si and C atoms occupied peripheral positions. The observed planarization may be attributed to sp² hybridization, changes in the peripheral bonding, and structural mechanics. Some properties, including the HOMO-LUMO gaps, on-site charge on Si and C atoms, and deformed charge distribution, were discussed.

Recent progress in boron nanomaterials

Various types of zero, one, and two-dimensional boron nanomaterials such as nanoclusters, nanowires, nanotubes, nanobelts, nanoribbons, nanosheets, and monolayer crystalline sheets named borophene have been experimentally synthesized and identified in the last 20 years. Owing to their low dimensionality, boron nanomaterials have different bonding configurations from those of three-dimensional bulk boron crystals composed of icosahedra or icosahedral fragments. The resulting intriguing physical and chemical properties of boron nanomaterials are fascinating from the viewpoint of material science. Moreover, the wide variety of boron nanomaterials themselves could be the building blocks for combining with other existing nanomaterials, molecules, atoms, and/or ions to design and create materials with new functionalities and properties. Here, the progress of the boron nanomaterials is reviewed and perspectives and future directions are described.

Long-range magnetic response of toroidal boron structures: B16 and [Co@B16]-/3- species

A correlation between the long-range characteristics of the magnetic response of toroidal boron-based structures is given, involving the uncoordinated B₁₆ cluster and the hypercoordinated [Co@B₁₆]^-/3- counterparts. It is found that the perfectly symmetrical doubly aromatic systems share common features, involving a continuous shielding region for the orientation-averaged response (isotropic), and a long-ranged shielding cone under a perpendicularly oriented applied field (B). In contrast, the conflicting aromatic structure given by the slightly distorted species, exhibits an enhanced deshielding cone under B, which dominates the isotropic character of the response. In addition, [Mn@B₁₆]^- and [Cu@B₁₆]^- clusters were evaluated, denoting the role of the coordinated metal atom in such property. This information is valuable to account for a global magnetic response driven by the bonding pattern acting in each respective compound, and for the possible characterization of intermolecular aggregates or extended structures via NMR experiments.

Boron clusters with 46, 48, and 50 atoms: competition among the core-shell, bilayer and quasi-planar structures

Using a genetic algorithm combined with density functional theory calculations, we perform a global search for the lowest-energy structures of B_n clusters with n = 46, 48, 50. Competition among different structural motifs including a hollow cage, core-shell, bilayer, and quasi-planar, is investigated. For B₄₆, a core-shell B₄@B₄₂ structure resembling the larger B_n clusters with n ≥ 68 is found to compete with a quasi-planar structure with a central hexagonal hole. A quasi-planar configuration with two connected hexagonal holes is most favorable for B₅₀. More interestingly, an unprecedented bilayer structure is unveiled at B₄₈, which can be extended to a two-dimensional bilayer phase exhibiting appreciable stability. Our results suggest alternatives to the cage motif as lower-energy B_n cluster structures with n > 50.

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.

Heteroborospherene clusters Nin ∈ B40 (n = 1-4) and heteroborophene monolayers Ni2 ∈ B14 with planar heptacoordinate transition-metal centers in η7-B7 heptagons

With inspirations from recent discoveries of the cage-like borospherene B₄₀ and perfectly planar Co ∈ B₁₈⁻ and based on extensive global minimum searches and first-principles theory calculations, we present herein the possibility of the novel planar Ni ∈ B₁₈ (1), cage-like heteroborospherenes Ni_n ∈ B₄₀ (n = 1–4) (2–5), and planar heteroborophenes Ni₂ ∈ B₁₄ (6, 7) which all contain planar or quasi-planar heptacoordinate transition-metal (phTM) centers in η⁷-B₇ heptagons. The nearly degenerate Ni₂ ∈ B₁₄ (6) and Ni₂ ∈ B₁₄ (7) monolayers are predicted to be metallic in nature, with Ni₂ ∈ B₁₄ (6) composed of interwoven boron double chains with two phNi centers per unit cell being the precursor of cage-like Ni_n ∈ B₄₀ (n = 1–4) (2–5). Detailed bonding analyses indicate that Ni_n ∈ B₄₀ (n = 1–4) (2–5) and Ni₂ ∈ B₁₄ (6, 7) possess the universal bonding pattern of σ + π double delocalization on the boron frameworks, with each phNi forming three lone pairs in radial direction (3d_z2², 3d_zx², and 3d_yz²) and two effective nearly in-plane 8c-2e σ-coordination bonds between the remaining tangential Ni 3d orbitals (3d_x2−y2 and 3d_xy) and the η⁷-B₇ heptagon around it. The IR, Raman, and UV-vis absorption spectra of 1–5 are computationally simulated to facilitate their experimental characterizations.

Effects of manganese doping on the structure evolution of small-sized boron clusters

Atomic doping of clusters is known as an effective approach to stabilize or modify the structures and properties of resulting doped clusters. We herein report the effect of manganese (Mn) doping on the structure evolution of small-sized boron (B) clusters. The global minimum structures of both neutral and charged Mn doped B cluster [Formula: see text] (n  =  10-20 and Q  =  0, ±1) have been proposed through extensive first-principles swarm-intelligence based structure searches. It is found that Mn doping has significantly modified the grow behaviors of B clusters, leading to two novel structural transitions from planar to tubular and then to cage-like B structures in both neutral and charged species. Half-sandwich-type structures are most favorable for small [Formula: see text] (n  ⩽  13) clusters and gradually transform to Mn-centered double-ring tubular structures at [Formula: see text] clusters with superior thermodynamic stabilities compared with their neighbors. Most strikingly, endohedral cages become the ground-state structures for larger [Formula: see text] (n  ⩾  19) clusters, among which [Formula: see text] adopts a highly symmetric structure with superior thermodynamic stability and a large HOMO-LUMO gap of 4.53 eV. The unique stability of the endohedral [Formula: see text] cage is attributed to the geometric fit and formation of 18-electron closed-shell configuration. The results significantly advance our understanding about the structure and bonding of B-based clusters and strongly suggest transition-metal doping as a viable route to synthesize intriguing B-based nanomaterials.

Double C-H bond activation of acetylene by atomic boron in forming aromatic cyclic-HBC2BH in solid neon

The organo-boron species formed from the reactions of boron atoms with acetylene in solid neon are investigated using matrix isolation infrared spectroscopy with isotopic substitutions as well as quantum chemical calculations. Besides the previously reported single C-H bond activation species, a cyclic-HBC2BH diboron species is formed via double C-H bond activation of acetylene. It is characterized to have a closed-shell singlet ground state with planar D2h symmetry. Bonding analysis indicates that it is a doubly aromatic species involving two delocalized σ electrons and two delocalized π electrons. This finding reveals the very first example of double C-H bond activation of acetylene in forming new organo-boron compounds.

Planar B38- and B37- clusters with a double-hexagonal vacancy: molecular motifs for borophenes

Boron clusters have been found to exhibit a variety of interesting electronic, structural, and bonding properties. Of particular interest are the recent discoveries of the 2D hexagonal B₃₆^-/0 which led to the concept of borophenes and the 3D fullerene-like B₄₀^-/0 which marked the onset of borospherene chemistry. Here, we present a joint photoelectron spectroscopic and first-principles study of B₃₇^- and B₃₈^-, which are in the transition size range between the 2D borophene-type clusters and the 3D borospherenes. These two clusters are found to possess highly stable 2D global-minimum structures consisting of a double-hexagonal vacancy. Detailed bonding analyses reveal that both B₃₇^- and B₃₈^- are all-boron analogues of coronene (C₂₄H₁₂) with a unique delocalized π system, featuring dual π aromaticity. These clusters with double hexagonal vacancies can be viewed as molecular motifs for the χ3-borophene which is the most stable form of borophenes recently synthesized on an Ag(111) substrate.

Manganese-centered tubular boron cluster - MnB16 (-): A new class of transition-metal molecules

We report the observation of a manganese-centered tubular boron cluster (MnB16 (-)), which is characterized by photoelectron spectroscopy and ab initio calculations. The relatively simple pattern of the photoelectron spectrum indicates the cluster to be highly symmetric. Ab initio calculations show that MnB16 (-) has a Mn-centered tubular structure with C4v symmetry due to first-order Jahn-Teller effect, while neutral MnB16 reduces to C2v symmetry due to second-order Jahn-Teller effect. In MnB16 (-), two unpaired electrons are observed, one on the Mn 3dz(2) orbital and another on the B16 tube, making it an unusual biradical. Strong covalent bonding is found between the Mn 3d orbitals and the B16 tube, which helps to stabilize the tubular structure. The current result suggests that there may exist a whole class of metal-stabilized tubular boron clusters. These metal-doped boron clusters provide a new bonding modality for transition metals, as well as a new avenue to design boron-based nanomaterials.

Structures, stabilities and spectral properties of borospherene B44- and metalloborospherenes MB440/- (M = Li, Na, and K)

Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations are carried out to study the stabilities, photoelectron, infrared, Raman and electronic absorption spectra of borospherene B₄₄⁻ and metalloborospherenes MB₄₄^0/− (M = Li, Na, and K). It is found that all atoms can form stable exohedral metalloborospherenes M&B₄₄^0/−, whereas only Na and K atoms can be stably encapsulated inside B₄₄^0/− cage. In addition, relative energies of these metalloborospherenes suggest that Na and K atoms favor exohedral configuration. Importantly, doping of metal atom can modify the stabilities of B₄₄ with different structures, which provides a possible route to produce stable boron clusters or metalloborospherenes. The calculated results suggest that B₄₄ tends to get electrons from the doped metal. Metalloborospherenes MB₄₄⁻ are characterized as charge-transfer complexes (M²⁺B₄₄²⁻), where B₄₄ tends to get two electrons from the extra electron and the doped metal, resulting in similar features with anionic B₄₄²⁻. In addition, doping of metal atom can change the spectral features, such as blueshift or redshift and weakening or strengthening of characteristic peaks, since the extra metal atom can modify the electronic structure. The calculated spectra are readily compared with future spectroscopy measurements and can be used as fingerprints to identify B₄₄⁻ and metalloborospherenes.

A detailed investigation into the geometric and electronic structures of CoBQn (n = 2–10, Q = 0, −1) clusters

The size dependence of HOMO–LUMO energy gaps of Co doped boron clusters.

Deciphering chemical bonding in BnHn2−(n = 2–17): flexible multicenter bonding

Deciphering flexible multicenter bonding incloso-borane dianions B_nH_n²⁻.

Structural transition in metal-centered boron clusters: from tubular molecular rotors Ta@B21and Ta@B22+to cage-like endohedral metalloborospherene Ta@B22−

Extensive first-principles theory investigations unveil a tubular-to-cage-like structural transition in metal-centered boron clusters at (±)-D₂Ta@B₂₂⁻, the smallest axially chiral endohedral metalloborospherenes.

Structures, stabilities and spectral properties of metalloborospherenes MB0/−40 (M = Cu, Ag, and Au)

Metalloborospherenes MB0/−40 (M = Cu, Ag, and Au) are predicted. Relative energies of these metalloborospherenes suggest that Cu, Ag and Au atoms favor the exohedral configuration.

Observation of a metal-centered B2-Ta@B18−tubular molecular rotor and a perfect Ta@B20−boron drum with the record coordination number of twenty

A B₂-Ta@B₁₈⁻tubular molecular rotor and a Ta@B₂₀⁻boron drum with the record coordination number of twenty were observedviaa joint experimental and theoretical investigation.

Aromatic cage-like B46: existence of the largest decagonal holes in stable atomic clusters

The boron cluster B₄₆has a cage-like structure containing two hexagonal, two heptagonal and two decagonal holes. This finding presents a new family of cage-like boron clusters containing large B_Nholes withN= 6–10.

Stable sandwich structures of two-dimensional iron borides FeBxalloy: a first-principles calculation

Multilayer iron borides FeB_x(x= 4, 6, 8, 10) are wide-band-gap semiconductors; the electronic and optical properties of these semiconductors may be modulated by biaxial strains.

Wheel-like, elongated, circular, and linear geometries in boron-based CnB7−n(n = 0–7) clusters: structural transitions and aromaticity

Boron–carbon mixed clusters C_nB_7−n(n= 0–7) assume wheel-like, elongated, circular, and linear geometries, dictated by (π and σ) aromaticity and antiaromaticity.

Structure and Fluxionality of B13+ Probed by Infrared Photodissociation Spectroscopy

We use cryogenic ion vibrational spectroscopy to characterize the structure and fluxionality of the magic number boron cluster B₁₃⁺ . The infrared photodissociation (IRPD) spectrum of the D₂ -tagged all-¹¹ B isotopologue of B₁₃⁺ is reported in the spectral range from 435 to 1790 cm^-1 and unambiguously assigned to a planar boron double wheel structure based on a comparison to simulated IR spectra of low energy isomers from density-functional-theory (DFT) computations. Born-Oppenheimer DFT molecular dynamics simulations show that B₁₃⁺ exhibits internal quasi-rotation already at 100 K. Vibrational spectra derived from these simulations allow extracting the first spectroscopic evidence from the IRPD spectrum for the exceptional fluxionality of B₁₃⁺ .

From Quasi-Planar B56 to Penta-Ring Tubular Ca©B56: Prediction of Metal-Stabilized Ca©B56 as the Embryo of Metal-Doped Boron α-Nanotubes

Motifs of planar metalloborophenes, cage-like metalloborospherenes, and metal-centered double-ring tubular boron species have been reported. Based on extensive first-principles theory calculations, we present herein the possibility of doping the quasi-planar C_2v B₅₆ (A-1) with an alkaline-earth metal to produce the penta-ring tubular Ca©B₅₆ (B-1) which is the most stable isomer of the system obtained and can be viewed as the embryo of metal-doped (4,0) boron α-nanotube Ca©BNT_(4,0) (C-1). Ca©BNT_(4,0) (C-1) can be constructed by rolling up the most stable boron α-sheet and is predicted to be metallic in nature. Detailed bonding analyses show that the highly stable planar C_2v B₅₆ (A-1) is the boron analog of circumbiphenyl (C₃₈H₁₆) in π-bonding, while the 3D aromatic C_4v Ca©B₅₆ (B-1) possesses a perfect delocalized π system over the σ-skeleton on the tube surface. The IR and Raman spectra of C_4v Ca©B₅₆ (B-1) and photoelectron spectrum of its monoanion C_4v Ca©B₅₆⁻ are computationally simulated to facilitate their spectroscopic characterizations.

Benchmarking Quantum Chemical Methods for Optical Absorption in Boron Wheels

We benchmark various quantum chemical methods for calculating the optical absorption in planar boron wheel clusters. The geometries of neutral planar boron wheels B7, B8, and B9 clusters are optimized at the coupled-cluster singles doubles level of theory. The optical absorption spectra of these clusters are calculated using three wave-function-based methods, namely, configuration interaction singles, random phase approximation, and equation-of-motion coupled-cluster singles doubles (EOM-CCSD) as well as using a time-dependent density-functional-theory-based method using various hybrid and long-range-corrected exchange and correlation functionals. There is an ample variation in the optical absorption spectra computed using different density functionals. When compared to the EOM-CCSD spectrum, an excellent agreement is provided by CAM-B3LYP functional, followed by ωB97xD functional. PBE0, B3LYP, and B3PW91 functionals agree among each other. However, their spectra are red-shifted with respect to the EOM-CCSD counterpart. On the basis of the natural transition orbital analysis, the nature of optical excitation is also discussed.

Dynamical behavior of boron clusters

Several of the lowest energy structures of small and medium sized boron clusters are two-dimensional systems made up of a pair of concentric rings. In some cases, the barriers to the rotation of one of those rings relative to the other are remarkably low. We find that a combination of electronic and geometrical factors, including apparently the relative sizes and symmetries of the inner and outer rings, are decisive for the diminished barriers to in-plane rotation in these two dimensional clusters. A sufficiently large outer ring is important; for instance, expansion of the outer ring by a single atom may reduce the barrier significantly. A crucial factor for an apparent rotation is that the σ-skeleton of the individual rings remains essentially intact during the rotation. Finally, the transition state for the rotation of the inner ring comprises the transformation of a square into a diamond, which may be linked to a mechanism suggested decades ago for the isomerization of carboranes and boranes.

Competition between drum and quasi-planar structures in RhB18-: motifs for metallo-boronanotubes and metallo-borophenes

Two nearly degenerate isomers, one a drum and the other quasi-planar, are discovered for the gaseous RhB₁₈^– cluster, revealing a competition between the metallo-boronanotube and metallo-borophene structures.

Metal-doped boron clusters provide new opportunities to design nanoclusters with interesting structures and bonding. A cobalt-doped boron cluster, CoB₁₈^–, has been observed recently to be planar and can be viewed as a motif for metallo-borophenes, whereas the D_9d drum isomer as a motif for metallo-boronanotubes is found to be much higher in energy. Hence, whether larger doped boron drums are possible is still an open question. Here we report that for RhB₁₈^– the drum and quasi-planar structures become much closer in energy and co-exist experimentally, revealing a competition between the metallo-boronanotube and metallo-borophene structures. Photoelectron spectroscopy of RhB₁₈^– shows a complicated spectral pattern, suggesting the presence of two isomers. Quantum chemistry studies indicate that the D_9d drum isomer and a quasi-planar isomer (C_s) compete for the global minimum. The enhanced stability of the drum isomer in RhB₁₈^– is due to the less contracted Rh 4d orbitals, which can have favorable interactions with the B₁₈ drum motif. Chemical bonding analyses show that the quasi-planar isomer of RhB₁₈^– is aromatic with 10 π electrons, whereas the observed RhB₁₈^– drum cluster sets a new record for coordination number of eighteen among metal complexes. The current finding shows that the size of the boron drum can be tuned by appropriate metal dopants, suggesting that even larger boron drums with 5d, 6d transition metal, lanthanide or actinide metal atoms are possible.