Aromatic Metal-Centered Monocyclic Boron Rings: Co©B8− and Ru©B9−

D 4h H©K4H4-: a planar tetracoordinate hydrogen global minimum

Herein we report a square-like D4h H©K4H4- anion with one planar tetracoordinate hydrogen (ptH) center, which is the global minimum (GM) structure and possesses good dynamic stability. The planar structure of the system is preserved by four peripheral K-H-K three-center two-electron (3c-2e) σ bonds together with one 5c-2e σ bond over the HK4 core. The multicenter ionic bonds dominate the stability of ptH, while the contribution of qualitative σ aromaticity is extremely limited.

Exploring the Use of "Honorary Transition Metals" To Push the Boundaries of Planar Hypercoordinate Alkaline-Earth Metals

The quest for planar hypercoordinate atoms (phA) beyond six has predominantly focused on transition metals, with dodecacoordination being the highest reported thus far. Extending this bonding scenario to main-group elements, which typically lack d orbitals despite their larger atomic radius, has posed significant challenges. Intrigued by the potentiality of covalent bonding formation using the d orbitals of the heavier alkaline-earth metals (Ae = Ca, Sr, Ba), the so-called "honorary transition metals", we aim to push the boundaries of planar hypercoordination. By including rings formed by 12-15 atoms of boron-carbon and Ae centers, we propose a design scheme of 180 candidates with a phA. Further systematic screening, structural examination, and stability assessments identified 10 potential clusters with a planar hypercoordinate alkaline-earth metal (phAe) as the lowest-energy form. These unconventional structures embody planar dodeca-, trideca-, tetradeca-, and pentadecacoordinate atoms. Chemical bonding analyses reveal the important role of Ae d orbitals in facilitating covalent interactions between the central Ae atom and the surrounding boron-carbon rings, thereby establishing a new record for coordination numbers in the two-dimensional realm.

Searching for stable copper borozene complexes in CuB7- and CuB8. Phys Chem Chem Phys 2024;26:12928-12938. [PMID: 38456623 DOI: 10.1039/d4cp00296b] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Copper has been shown to be an important substrate for the growth of borophenes. Copper-boron binary clusters are ideal platforms to study the interactions between copper and boron, which may provide insight about the underlying growth mechanisms of borophene on copper substrates. Here we report a joint photoelectron spectroscopy and theoretical study on two copper-doped boron clusters, CuB7- and CuB8-. Well resolved photoelectron spectra are obtained for the two clusters at different wavelengths and are used to understand the structures and bonding properties of the two CuBn- clusters. We find that CuB8- is a highly stable borozene complex, which possesses a half-sandwich structure with a Cu+ species interacting with the doubly aromatic η8-B82- borozene. The CuB7- cluster is found to consist of a terminal copper atom bonded to a double-chain B7 motif, but it has a low-lying isomer composed of a half-sandwich structure with a Cu+ species interacting with an open-shell η7-B72- borozene. Both ionic and covalent interactions are found to be possible in the binary Cu-B clusters, resulting in different structures.

Electronic Control of the Position of the Pb Atom on the Surface of B8 Borozene in the PbB8 Cluster

Spontaneous symmetry-breaking is common in chemical and physical systems. Here, we show that by adding an electron to the C7v PbB8 cluster, which consists of a planar B8 disk with the Pb atom situated along the C7 axis, the Pb atom spontaneously moves to the off-axis position in the PbB8- anion. Photoelectron spectroscopy of PbB8- reveals a broad ground-state transition and a large energy gap, suggesting a highly stable closed-shell PbB8 borozene complex and a significant geometry change upon electron detachment. Quantum chemistry calculations indicate that the lowest unoccupied molecular orbital of the C7v PbB8 cluster is a degenerate π orbital mainly consisting of the Pb 6px and 6py atomic orbitals. Occupation of one of the 6p orbitals spontaneously break the C7v symmetry in the anion due to the Jahn-Teller effect. The large amplitude of the position change of Pb in PbB8- relative to PbB8 is surprising owing to bonding interactions between the Pb 6p orbital with the π orbital of the B8 borozene.

Metal-Centered Boron-Wheel Cluster of Y©B112- with Rare D11h Symmetry

Molecules with high point-group symmetry are interesting prototype species in the textbook. As transition metal-centered boron clusters tend to have highly symmetric structures to fulfill multicenter bonding and high stability, new boron clusters with rare point-group symmetry may be viable. Through in-depth scrutiny over the structures of experimentally already observed transition metal-centered boron-wheel complexes, geometric and electronic design principles are summarized, based on which we studied M©B11k- (M = Y, La; Zr, Hf; k = 1, 2) clusters and found that a Y©B112- boron-wheel complex has an unprecedented D11h point-group symmetry. The remarkable stability of the planar Y©B112- complex is illustrated via extensive global-minimum structural search as well as comprehensive chemical bonding analyses. Similar to other boron-wheel complexes, the Y©B112- complex is shown to possess σ and π double aromaticity, indeed following the electronic design principle previously summarized. This new compound is expected to be experimentally identified, which will extend the currently known largest possible planar molecular symmetry and enrich the metal-centered boron-wheel class.

Size effects and electronic properties of zinc-doped boron clusters Zn B n (n = 1-15)

CONTEXT

To gain a deeper understanding of zinc-doped boron clusters, theoretical calculations were performed to investigate the size effects and electronic properties of zinc-doped boron clusters. The study of the electronic properties, spectral characteristics, and geometric structures of Zn B n (n = 1-15) is of great significance in the fields of semiconductor materials science, material detection, and improving catalytic efficiency. The results indicate that Zn B n (n = 1-15) clusters predominantly exhibit planar or quasi-planar structures, with the Zn atom positioned in the outer regions of the B n framework. The second stable structure of Zn B 3 is a three-dimensional configuration, indicating that the structures of zinc-doped boron clusters begin to convert from the planar or quasi-planar structures to the 3D configurations. The second low-energy structure of Zn B 15 is a novel configuration. Relative stability analyses show that the Zn B 12 has better chemical stability than other clusters with a HOMO-LUMO gap of 2.79 eV. Electric charge analysis shows that part electrons on zinc atoms are transferred to boron atoms, and electrons prefer to cluster near the B n framework. According to the electron localization function, it gets harder to localize electrons as the equivalent face value drops, and it's challenging to see covalent bond formation between zinc and boron atoms. The spectrograms of Zn B n (n = 1-15) exhibit distinct properties and notable spectral features, which can be used as a theoretical basis for the identification and confirmation of boron clusters doped with single-atom transition metals.

METHODS

The calculations were performed using the ABCluster global search technique combined with density functional theory (DFT) methods. The selected low-energy structures were subjected to geometric optimization and frequency calculations at the PBE0/6-311 + G(d) level to ensure structural stability and eliminate any imaginary frequencies. To acquire more precise relative energies, we performed single-point energies calculations for the low-lying isomers of Zn B n (n = 1-15) at the CCSD(T)/6-311 + G(d)//PBE0/6-311 + G(d) level of theory. All calculations were performed using Gaussian 09 software. To facilitate analysis, we utilized software tools such as Multiwfn, and VMD.

Investigation of Pb-B Bonding in PbB2(BO)n- (n = 0-2): Transformation from Aromatic PbB2- to Pb[B2(BO)2]-/0 Complexes with BB Triple Bonds

Boron has been found to be able to form multiple bonds with lead. To probe Pb-B bonding, here we report an investigation of three Pb-doped boron clusters, PbB2-, PbB3O-, and PbB4O2-, which are produced by a laser ablation cluster source and characterized by photoelectron spectroscopy and ab initio calculations. The most stable structures of PbB2-, PbB3O-, and PbB4O2- are found to follow the formula, [PbB2(BO)n]- (n = 0-2), with zero, one, and two boronyl ligands coordinated to a triangular and aromatic PbB2 core, respectively. The PbB2- cluster contains a BB double bond and two Pb-B single bonds. The coordination of BO is observed to weaken Pb-B bonding but strengthen the BB bond in [PbB2(BO)n]- (n = 1, 2). The anionic [PbB2(BO)2]- and its corresponding neutral closed-shell [PbB2(BO)2] contain a BB triple bond. A low-lying Y-shaped isomer is also observed for PbB4O2-, consisting of a central sp2 hybridized B atom bonded to two boronyl ligands and a PbB unit.

Boron-based Pd3B26 alloy cluster as a nanoscale antifriction bearing system: tubular core-shell structure, double π/σ aromaticity, and dynamic structural fluxionality

Boron-based nanoclusters show unique geometric structures, nonclassical chemical bonding, and dynamic structural fluxionality. We report here on the theoretical prediction of a binary Pd3B26 cluster, which is composed of a triangular Pd3 core and a tubular double-ring B26 unit in a coaxial fashion, as identified through global structural searches and electronic structure calculations. Molecular dynamics simulations indicate that in the core-shell alloy cluster, the B26 double-ring unit can rotate freely around its Pd3 core at room temperature and beyond. The intramolecular rotation is virtually barrier free, thus giving rise to an antifriction bearing system (or ball bearing) at the nanoscale. The dimension of the dynamic system is only 0.66 nm. Chemical bonding analysis reveals that Pd3B26 cluster possesses double 14π/14σ aromaticity, following the (4n + 2) Hückel rule. Among 54 pairs of valence electrons in the cluster, the overwhelming majority are spatially isolated from each other and situated on either the B26 tube or the Pd3 core. Only one pair of electrons are primarily responsible for chemical bonding between the tube and the core, which greatly weaken the bonding within the Pd3 core and offers structural flexibility. This is a key mechanism that effectively diminishes the intramolecular rotation barrier and facilitates dynamic structural fluxionality of the system. The current work enriches the field of nanorotors and nanomachines.

Geometric and electronic structures of medium-sized boron clusters doped with plutonium

Doping metal heteroatoms is an effective strategy to regulate the geometric and electronic structure of boron based nanoclusters. However, the exploration of the ground state structures of metal-boron-based nanoclusters is still a challenge duo to the complexity of the bonding interactions between heterogeneous atoms and boron cluster and the number of isomers on the potential energy surface increases exponentially with cluster size. Here, we use the CALYPSO cluster structural search method in combination with density functional theory calculations to study the geometries and electronic properties of anionic boron clusters doped with plutonium (PuBn-,n= 10-20). Our results show that the medium-sized PuB14-cluster exhibits excellent stability with highest occupied molecular orbital-lowest unoccupied molecular orbital energy gap of 2.30 eV. The remarkable stability of the anionic PuB14-cluster is due to the robust interactions between the Pu metal and the B14skeleton, along with the strong covalent interactions between the B atoms. These findings enrich the geometric structure database of metal doped clusters and provide valuable insights for the future synthesis of boron based nanomaterials.

[Cs©C18]+ and [Na©C14]+: perfect planar alkaline-metal-centered polyynic cyclo[n]carbon complexes with record coordination numbers

Searching for the maximum coordination number (CN) in planar species with novel bonding patterns has fascinated chemists for many years. Using the experimentally observed polyynic cyclo[18]carbon D9h C18 and theoretically predicted polyynic cyclo[14]carbon D7h C14 as effective ligands and based on extensive first-principles theory calculations, we predict herein their perfect planar alkaline-metal-centered complexes D9h Cs©C18+ (1) and D7h Na©C14+ (4) which as the global minima of the systems possess the record coordination numbers of CN = 18 and 14 in planar polyynic species, respectively. More interestingly, detailed energy decomposition and adaptive natural density partitioning bonding analyses indicate that the hypercoordinate alkaline-metal centers in these complexes exhibit obvious transition metal behaviors, with effective in-plane (π-6s)σ, (π-7p)σ, and (π-5d)σ coordination bonds formed in Cs©C18+ (1) and (π-3s)σ, (π-3p)σ, and (π-3d)σ coordination interactions fabricated in Na©C14+ (4) to dominate the overall attractive interactions between the metal center and its cyclo[n]carbon ligand. Similarly, alkaline-metal-centered planar Cs Cs©C17B (2), C2v Cs©C17- (3), C2v Na©C13B (5), and C2v Na©C13- (6) have also been obtained with CN = 18, 17, 14, and 13, respectively.

Boron-Based Inverse Sandwich V2B7- Cluster: Double π/σ Aromaticity, Metal-Metal Bonding, and Chemical Analogy to Planar Hypercoordinate Molecular Wheels

Inverse sandwich clusters composed of a monocyclic boron ring and two capping transition metal atoms are interesting alloy cluster systems, yet their chemical bonding nature has not been sufficiently elucidated to date. We report herein on the theoretical prediction of a new example of boron-based inverse sandwich alloy clusters, V₂B₇^-, through computational global-minimum structure searches and quantum chemical calculations. This alloy cluster has a heptatomic boron ring as well as a perpendicular V₂ dimer unit that penetrates through the ring. Chemical bonding analysis suggests that the inverse sandwich cluster is governed by globally delocalized 6π and 6σ frameworks, that is, double 6π/6σ aromaticity following the (4n + 2) Hückel rule. The skeleton B-B σ bonding in the cluster is shown not to be strictly Lewis-type two-center two-electron (2c-2e) σ bonds. Rather, these are quasi-Lewis-type, roof-like 4c-2e V-B₂-V σ bonds, which amount to seven in total and cover the whole surface of inverse sandwich in a truly three-dimensional manner. Theoretical evidence is revealed for a 2c-2e Lewis σ single bond within the V₂ dimer. Direct metal-metal bonding is scarce in inverse sandwich alloy clusters. The present inverse sandwich alloy cluster also offers a new type of electronic transmutation in physical chemistry, which helps establish an intriguing chemical analogy between inverse sandwich clusters and planar hypercoordinate molecular wheels.

Photoelectron Spectroscopy and Theoretical Study of Di-Copper-Boron Clusters: Cu2B3- and Cu2B4. J Phys Chem A 2023. [PMID: 37235389 DOI: 10.1021/acs.jpca.3c02417] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Copper has been found to be able to mediate the formation of bilayer borophenes. Copper-boron binary clusters are ideal model systems to probe the copper-boron interactions, which are essential to understand the growth mechanisms of borophenes on copper substrates. Here, we report a joint photoelectron spectroscopy and theoretical study on two di-copper-doped boron clusters: Cu₂B₃^- and Cu₂B₄^-. Well-resolved photoelectron spectra are obtained, revealing the presence of a low-lying isomer in both cases. Theoretical calculations show that the global minimum of Cu₂B₃^- (C_2v, ¹A₁) contains a doubly aromatic B₃^- unit weakly interacting with a Cu₂ dimer, while the low-lying isomer (C_2v, ¹A₁) consists of a B₃ triangle with the two Cu atoms covalently bonded to two B atoms at two vertexes. The global minimum of Cu₂B₄^- (D_2h, ²A_g) is found to consist of a rhombus B₄ unit covalently bonded to the two Cu atoms at two opposite vertexes, whereas in the low-lying isomer (C_s, ²A'), one of the two Cu atoms is bonded to two B atoms.

Sc@B28-, Ti@B28, V@B28+, and V@B292-: Spherically Aromatic Endohedral Seashell-like Metallo-Borospherenes

Transition-metal-doped boron nanoclusters exhibit unique structures and bonding in chemistry. Using the experimentally observed seashell-like borospherenes C₂ B₂₈^-/0 and C_s B₂₉^- as ligands and based on extensive first-principles theory calculations, we predict herein a series of novel transition-metal-centered endohedral seashell-like metallo-borospherenes C₂ Sc@B₂₈^- (1), C₂ Ti@B₂₈ (2), C₂ V@B₂₈⁺ (3), and C_s V@B₂₉^2- (4) which, as the global minima of the complex systems, turn out to be the boron analogues of dibenzenechromium D_6h Cr(C₆H₆)₂ with two B₁₂ ligands on the top and bottom interconnected by four or five corner boron atoms on the waist and one transition-metal "pearl" sandwiched at the center in between. Detailed molecular orbital, adaptive natural density partitioning (AdNDP), and iso-chemical shielding surface (ICSS) analyses indicate that, similar to Cr(C₆H₆)₂, these endohedral seashell-like complexes follow the 18-electron rule in bonding patterns (1S²1P⁶1D¹⁰), rendering spherical aromaticity and extra stability to the systems.

Geometric and electronic diversity of metal doped boron clusters

Being intermediate between small compounds and bulk materials, nanoparticles possess unique properties different from those of atoms, molecules, and bulk matter. In the past two decades, a combination of cluster structure prediction algorithms and experimental spectroscopy techniques was successfully used for exploration of the ground-state structures of pure and metal-doped boron clusters. The fruitfulness of this dual approach is well illustrated by the discovery of intriguing microstructures and unique physicochemical properties such as aromaticity and bond fluxionality for both boron and metal-doped boron clusters. Our review starts with an overview of geometrical configurations of pure boron clusters B_n, which are presented by planar, nanotube, bilayer, fullerene-like and core-shell structures, in a wide range ofnvalues. We consider next recent advances in studies of boron clusters doped with metal atoms paying close and thoughtful attention to modifications of geometric and electronic structures of pure boron clusters by heteroatoms. Finally, we discuss the possibility of constructing boron-based nanomaterials with specific functions from metal-boron clusters. Despite a variety of fruitful results obtained in numerous studies of boron clusters, the exploration of boron-based chemistry has not yet reached its peak. The intensive research continues in this area, and it should be expected that it brings exciting discoveries of intriguing new structures.

TMB12: a newly designed 2D transition-metal boride for spintronics and electrochemical catalyst applications

Exploring two-dimensional (2D) ferromagnetic materials with a high transition temperature and large magnetic anisotropy is extremely essential for highly efficient spintronic applications. With the density functional theory method, we predicted planar hypercoordinate transition-metal borides, TMB₁₂ (TM = Ti, V, Cr, Mn, Fe; B = boron), by the condensation of TM@B₈ and B₄ units. The results showed that these transition-metal borides possess superior thermal, dynamic and mechanical stabilities. Interestingly, the TMB₁₂ monolayer with TM = (V, Cr) is confirmed as a robust ferromagnetic metal with a high Curie temperature of ∼335 K and ∼221 K, respectively. In addition, the system with TM = (Mn, Fe) is found to be an antiferromagnetic metal with a Néel temperature of ∼173 K and ∼91 K, respectively. In particular, large perpendicular magnetic anisotropy is identified for CrB₁₂, MnB₁₂, and FeB₁₂ monolayers, around 198-623 μeV. Furthermore, four TMB₁₂ (TM = Ti, V, Cr, Mn) systems are determined to be candidate catalysts for the hydrogen evolution reaction, with nearly zero free energy of hydrogen adsorption (ΔG_H = -0.0003 to -0.03 eV). Our study highlighted potential 2D metal borides for spintronic devices and high efficiency electrochemical catalysts.

Two-layer molecular rotors: A zinc dimer rotating over planar hypercoordinate motifs

Multi-layer molecular rotors represent a class of unique combination of topology and bonding, featuring a barrier-free rotation of one layer with respect to other layers. This emerging fluxional behavior has been found in a few doped boron clusters. Herein, we strongly enrich this intriguing family followed by an effective design strategy, summarized as essential factors: i) considerable electrostatic interactions originated from a strong charge transfer between layers; ii) the absence of strong covalent bonds between layers; and iii) fully delocalized σ/π electrons from at least one layer. We found that planar hypercoordinate motifs consisting of monocyclic boron rings and metals with σ + π dual aromaticity can be regarded as one promising layer, which can support the suspended X₂ (X = Zn, Cd, Hg) dimers. By detailed investigations of thermodynamic and kinetic stabilities of 60 species, eventually, MB₇ X₂ ^- and MB₈ X₂ (X = Zn, Cd; M = Be, Ru, Os; Be works only for Zn-based cases) clusters were verified to be the global-minimum two-layer molecular rotors. Especially, their electronic structure analyses vividly confirm the practicability of the electronic structure requirements mentioned above for designing multi-layer molecular rotors.

Be3B11- cluster: a dynamically fluxional beryllo-borospherene

The beryllium-doped Be₃B₁₁^- cluster has two nearly isoenergetic isomers, adopting the smallest trihedral spherical geometries with a boron single-chain skeleton. The B₁₁ skeleton in the global minimum (C_2v, ¹A₁) comprises three conjoined boron rings (one B₈/two B₇) on the waist, sharing two B₃ equilateral triangles at the top and bottom, respectively. However, the local minimum (C_s, ¹A') has one deformed B₄ pyramid at the top. The drastic structural transformation of B₁₁ skeletons from perfectly planar B₁₁ clusters mainly profited from robust electrostatic interaction between Be atoms and B₁₁ skeletons. The dynamic simulations suggest that two species can interconvert via a novel mechanism, that is "triangle-pyramid-triangle", which facilitates the free migration of boron atoms in the B₁₁ skeleton, thereby showing the fascinating dynamic fluxionality. The chemical bonding analyses reveal that the B₁₁ skeleton is covered by two types of delocalized π bonds in an orthogonal direction, which leads to its spherical aromaticity.

The dodeca-coordinated La©B8C4 +/0/- molecular wheels: conflicting aromaticity versus double aromaticity

The transition-metal centered boron molecular wheels have attracted the attention of chemists. The highest deca-coordination number for central metal atoms was observed in D _10h Ta©B₁₀ ^- and Nb©B₁₀ ^- molecular wheels. Here, we report a theoretical study of La©B₈C₄ ^q (q = +1, 0, -1) clusters with the dodeca-coordinated La atom. The La©B₈C₄ ^q clusters adopt fascinating molecular wheel structures, showing a La atom enclosed by a perfect B₈C₄ monocyclic ring. The cationic La©B₈C₄ ⁺ cluster has a C _4v symmetry with the distinctly out-of-plane distortion of the La atom (0.70 Å), which is gradually flattened by the sequential reduction reaction. The distortion of the La atom from the plane in the neutral La©B₈C₄ cluster decreases to 0.46 Å. The La©B₈C₄ ^- species turns out to be perfectly planar. Chemical bonding analyses indicate that the neutral La©B₈C₄ and anionic La©B₈C₄ ^- possess 10σ and 9π/10π double aromaticity, respectively, obeying the principle of double aromaticity. However, the cationic La©B₈C₄ ⁺ has 10σ and 8π conflicting aromaticity, representing a counterexample in planar hyper-coordinated molecular wheels. The dodeca-coordination number in La©B₈C₄ ^q (q = +1, 0, -1) clusters is unprecedented, which provides a new idea and concept for searching planar hyper-coordinated systems.

σ-Aromatic MAl6S6 (M = Ni, Pd, Pt) Stars Containing Planar Hexacoordinate Transition Metals

Hypercoordinate transition-metal species are mainly dominated by the 18-valence-electron (18ve) counting. Herein, we report ternary MAl₆S₆ (M = Ni, Pd, Pt) clusters with the planar hexacoordinate metal (phM) centers, which feature 16ve counting instead of the classic 18ve rule. These global-minimum clusters are established via unbiased global searches, followed by PBE0 and single-point CCSD(T) calculations. The phM MAl₆ units are stabilized by six peripheral bridging S atoms in these star-like species. Chemical bonding analyses reveal that there are 10 delocalized electrons around the phM center, which can render the aromaticity according to the (4n + 2) Hückel rule. It is worth noting that adding an (or two) electron(s) to its π-type lowest unoccupied molecular orbital (LUMO) will make the system unstable.

Quasi-planar Co atom-doped boron cluster: CoB192

PURPOSE AND METHODS

A global search for the lowest energy structure of CoB₁₉^2- clusters was conducted.  RESULTS: Its ground state is a quasi-planar structure with the Co atom surrounded by a B₈ ring. The central Co atom has an oxidation state of +1 with d⁸ electron configuration. The wave function analysis showed that the Co-B interaction is not a covalent bond. The bonding strength of peripheral B-B bonds is stronger than that of inner ones. The inner B₈ ring bonds with outer boron atoms via σ- and π-type bonds.

CONCLUSION

CoB₁₉^2- shows remarkable aromatic character.

Lanthanide/actinide boride nanoclusters and nanomaterials based on boron frameworks consisting of conjoined Bn rings (n = 7-9)

Extensive global minimum searches augmented with first-principles theory calculations performed in this work indicate that the experimentally observed perfect inverse sandwich lanthanide boride complexes D_7h La₂B₇^- (1), D_8h La₂B₈ (3), D_9h La₂B₉^- (7) can be extended to their actinide counterparts C_2v Ac₂B₇^- (1'), D_8h Ac₂B₈ (3'), D_9h Ac₂B₉^- (7') with a B_n monocyclic ring (n = 7-9) sandwiched by two Ac dopants. Such M₂B_n^-/0 inverse sandwiches (1/1', 3/3', 7/7') can be used as building blocks to generate the ground-state C₂ La₄B₁₃^- (2)/Ac₄B₁₃^- (2'), D₂ La₄B₁₅^- (4)/Ac₄B₁₅^- (4'), C_3v/C₃ La₄B₁₈ (5)/Ac₄B₁₈ (5'), O_h Ac₇B₂₄⁺ (6'), O_h Ac₇B₂₄, T_d Ac₄B₂₄ (8'), C₁ La₅B₂₄⁺ (9)/Ac₅B₂₄⁺ (9'), and T_d Ac₄B₂₉^- (10') which are based on boron frameworks consisting of multiple conjoined B_n rings (n = 7-9). Detailed bonding analyses show that effective (d-p)σ, (d-p)π and (d-p)δ coordination bonds are formed between the B_n rings and metal doping centers, conferring three-dimensional aromaticity and extra stability to the systems. In particular, the perfect body-centered cubic O_h Ac₇B₂₄⁺ (6') and O_h Ac₇B₂₄ with six conjoined B₈ rings can be extended in x, y, and z dimensions to form one-dimensional Ac₁₀B₃₂ (11'), two-dimensional Ac₃B₁₀ (12'), and three-dimensional AcB₆ (13') nanomaterials, presenting a B₈-based bottom-up approach from metal boride nanoclusters to their low-dimensional nanomaterials.

Particle Swarm Predictions of a SrB8 Monolayer with 12-Fold Metal Coordination

Materials containing planar hypercoordinate motifs greatly enriched the fundamental understanding of chemical bonding. Herein, by means of first-principles calculations combined with global minimum search, we discovered the two-dimensional (2D) SrB₈ monolayer, which has the highest planar coordination number (12) reported so far in extended periodic materials. In the SrB₈ monolayer, bridged B₈ units are forming the boron monolayer consisting of B₁₂ rings, and the Sr atoms are embedded at the center of these B₁₂ rings, leading to the Sr@B₁₂ motifs. The SrB₈ monolayer has good thermodynamic, kinetic, and thermal stabilities, which is attributed to the geometry fit between the size of the Sr atom and cavity of the B₁₂ rings, as well as the electron transfer from Sr atoms to electron-deficient boron network. Placing the SrB₈ monolayer on the Ag(001) surface shows good commensurability of the lattices and small vertical structure undulations, suggesting the feasibility of its experimental realization by epitaxial growth. Potential applications of the SrB₈ monolayer on metal ions storage (for Li, Na, and K) are explored.

Planar Pentacoordinate Zinc Group Elements Stabilized by Multicentric Bonds

Planar pentacoordinate zinc group elements, (M = Zn, Cd, Hg) were computationally found to be at a global minimum in Li₅M⁺ clusters. The stability of these clusters is due to the presence of multicentric bonds. The central element (Zn, Cd, Hg) in each cluster features a negative oxidation state owing to the in-plane electron donation by the Li₅⁺ framework. A similar global minimum planar pentacoordinate structure is found in Na₅Zn⁺ and Na₅Cd⁺ clusters.

Probing the Nature of the Transition-Metal-Boron Bonds and Novel Aromaticity in Small Metal-Doped Boron Clusters Using Photoelectron Spectroscopy

Photoelectron spectroscopy combined with quantum chemistry has been a powerful approach to elucidate the structures and bonding of size-selected boron clusters (B_n^-), revealing a prevalent planar world that laid the foundation for borophenes. Investigations of metal-doped boron clusters not only lead to novel structures but also provide important information about the metal-boron bonds that are critical to understanding the properties of boride materials. The current review focuses on recent advances in transition-metal-doped boron clusters, including the discoveries of metal-boron multiple bonds and metal-doped novel aromatic boron clusters. The study of the RhB^- and RhB₂O^- clusters led to the discovery of the first quadruple bond between boron and a transition-metal atom, whereas a metal-boron triple bond was found in ReB₂O^- and IrB₂O^-. The ReB₄^- cluster was shown to be the first metallaborocycle with Möbius aromaticity, and the planar ReB₆^- cluster was found to exhibit aromaticity analogous to metallabenzenes. Expected final online publication date for the Annual Review of Physical Chemistry, Volume 73 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Structural transformations in boron clusters induced by metal doping

In the last decades, experimental techniques in conjunction with theoretical analyses have revealed the surprising structural diversity of boron clusters. Although the 2D to 3D transition thresholds are well-established, there is no certainty about the factors that determine the geometry adopted by these systems. The structural transformation induced by doping usually yields a minimum energy structure with a boron skeleton entirely different from that of the bare cluster. This review summarizes those clusters no larger than 40 boron atoms where one or two dopants show a radical transformation of the structure. Although the structures of these systems are not easy to predict, they often adopt familiar shapes such as umbrella-like, wheel, tubular, and cages in various cases.

The smallest 4f-metalla-aromatic molecule of cyclo-PrB2− with Pr–B multiple bonds

The concept of metalla-aromaticity proposed by Thorn–Hoffmann (Nouv. J. Chim. 1979, 3, 39) has been expanded to organometallic molecules of transition metals that have more than one independent electron-delocalized system. Lanthanides, with highly contracted 4f atomic orbitals, are rarely found in multiply aromatic systems. Here we report the discovery of a doubly aromatic triatomic lanthanide-boron molecule PrB₂⁻ based on a joint photoelectron spectroscopy and quantum chemical investigation. Global minimum structural searches reveal that PrB₂⁻ has a C_2v triangular structure with a paramagnetic triplet ³B₂ electronic ground state, which can be viewed as featuring a trivalent Pr(III,f²) and B₂⁴⁻. Chemical bonding analyses show that this cyclo-PrB₂⁻ species is the smallest 4f-metalla-aromatic system exhibiting σ and π double aromaticity and multiple Pr–B bonding characters. It also sheds light on the formation of the rare B₂⁴⁻ tetraanion by the high-lying 5d orbitals of the 4f-elements, completing the isoelectronic B₂⁴⁻, C₂²⁻, N₂, and O₂²⁺ series.

We report the smallest 4f-metalla-aromatic molecule of PrB₂⁻ exhibiting σ and π double aromaticity and multiple Pr–B bond characters.

Ground and Electronically Excited States of Main-Group-Metal-Doped B20 Double Rings

Ab initio coupled-cluster, electron propagator, and Møller-Plesset second-order perturbation theory calculations are utilized to analyze the low-lying electronic states of several metal-doped B₂₀. In the ground state, the presently focused AB₂₀/EB₂₀ (A = Li, Na, and K; E = Mg and Ca) consist of charge-separated A⁺B₂₀^-/E²⁺B₂₀^2- frameworks. The excited electronic states of AB₂₀ and EB₂₀⁺ were analyzed by computing the vertical electron attachment energies (VEAEs) of AB₂₀⁺ and EB₂₀²⁺. In several excited states, the radical electron is predominantly localized on the B₂₀ frames, which are counterparts of the low-lying states of bare B₂₀^-. A variety of basis sets were tested on obtaining VEAEs, and the aug-cc-pVDZ/_A,E d-aug-cc-pVDZ/_B combination provided the best accuracy-efficiency compromise on them. Furthermore, this work analyzes the Rydberg-like excited states of AB₂₀ and EB₂₀⁺ and will serve as a guide for future studies on similar metal-doped boron systems.

M©B7O7+ (M = Ni, Pd, Pt): aromatic molecular stars with a planar heptacoordinate transition metal

The dynamically stable global minima M©B₇O₇⁺ (M = Ni, Pd, Pt) are interesting in that they possess σ-aromaticity alone within the B₇M core moiety, which can be attributed to the strong peripheral localized O → B π back-bonding that leads to the less favourable delocalized M → B π back-bonding over the B₇M core moiety.

Theoretical probing of twenty-coordinate actinide-centered boron molecular drums

The exploration of metal-doped boron clusters has a great significance in the design of high coordination number (CN) compounds. Actinide-doped boron clusters are probable candidates for achieving high CNs. In this work, we systematically explored a series of actinide metal atom (U, Np, and Pu) doped B₂₀ boron clusters An@B₂₀ (An = U, Np, and Pu) by global minimum structural searches and density functional theory (DFT). Each An@B₂₀ cluster is confirmed to be a twenty-coordinate complex, which is the highest CN obtained in the chemistry of actinide-doped boron clusters so far. The predicted global minima of An@B₂₀ are tubular structures with actinide atoms as centers, which can be considered as boron molecular drums. In An@B₂₀, U@B₂₀ has a relatively high symmetry of C₂, while both Np@B₂₀ and Pu@B₂₀ exhibit C₁ symmetry. Extensive bonding analysis demonstrates that An@B₂₀ has σ and π delocalized bonding, and the U-B bonds possess a relatively higher covalency than the Np-B and Pu-B bonds, resulting in the higher formation energy of U@B₂₀. Therefore, the covalent character of An-B bonding may be crucial for the formation of these high CN actinide-centered boron clusters. These results deepen our understanding of actinide metal doped boron clusters and provide new clues for developing stable high CN boron-based nanomaterials.

Monovalent lanthanide(I) in borozene complexes

Lanthanide (Ln) elements are generally found in the oxidation state +II or +III, and a few examples of +IV and +V compounds have also been reported. In contrast, monovalent Ln(+I) complexes remain scarce. Here we combine photoelectron spectroscopy and theoretical calculations to study Ln-doped octa-boron clusters (LnB₈⁻, Ln = La, Pr, Tb, Tm, Yb) with the rare +I oxidation state. The global minimum of the LnB₈⁻ species changes from C_s to C_7v symmetry accompanied by an oxidation-state change from +III to +I from the early to late lanthanides. All the C_7v-LnB₈⁻ clusters can be viewed as a monovalent Ln(I) coordinated by a η⁸-B₈²⁻ doubly aromatic ligand. The B₇³⁻, B₈²⁻, and B₉⁻ series of aromatic boron clusters are analogous to the classical aromatic hydrocarbon molecules, C₅H₅⁻, C₆H₆, and C₇H₇⁺, respectively, with similar trends of size and charge state and they are named collectively as “borozenes”. Lanthanides with variable oxidation states and magnetic properties may be formed with different borozenes.

The most common oxidation state for lanthanides is +3. Here the authors use photoelectron spectroscopy and theoretical calculations to study half-sandwich complexes where a lanthanide center in the oxidation state +1 is bound to an aromatic wheel-like B₈^2- ligand.

σ-Aromaticity Planar Pentacoordinate Beryllium Atoms

Six-valence-electron planar pentacoordinate beryllium (ppBe) is explored herein as a global minimum, which is only constructed by s-block metals in BeM₅⁺ (M = Cu, Ag, Au). The bonding in ppBe can be regarded as the excited-stated Be with a 2p_x¹2p_y¹ electronic configuration, forming electron sharing with doublet M₅⁺ motifs followed by two sets of Be(p_∥) → [M₅⁺] σ donations and one Be(s) ← [M₅⁺] σ back-donation. Thus, the σ aromaticity originating from three delocalized σ orbitals gives rise to the whole stability of the high D_5h-symmetry ppBe and strongly enriches s-block planar hypercoordinate bonding.

Theoretical Design of Novel Boron-Based Nanowires via Inverse Sandwich Clusters

Borophene has important application value, boron nanomaterials doped with transition metal have wondrous structures and chemical bonding. However, little attention was paid to the boron nanowires (NWs). Inspired by the novel metal boron clusters Ln₂B_n⁻ (Ln = La, Pr, Tb, n = 7–9) adopting inverse sandwich configuration, we examined Sc₂B₈ and Y₂B₈ clusters in such novel structure and found that they are the global minima and show good stability. Thus, based on the novel structural moiety and first-principles calculations, we connected the inverse sandwich clusters into one-dimensional (1D) nanowires by sharing B−B bridges between adjacent clusters, and the 1D-Sc₄B₂₄ and 1D-Y₂B₁₂ were reached after structural relaxation. The two nanowires were identified to be stable in thermodynamical, dynamical and thermal aspects. Both nanowires are nonmagnetic, the 1D-Sc₄B₂₄ NW is a direct-bandgap semiconductor, while the 1D-Y₂B₁₂ NW shows metallic feature. Our theoretical results revealed that the inverse sandwich structure is the most energy-favored configuration for transition metal borides Sc₂B₈ and Y₂B₈, and the inverse sandwich motif can be extended to 1D nanowires, providing useful guidance for designing novel boron-based nanowires with diverse electronic properties.

OsB9 -: An Aromatic Osmium-Centered Monocyclic Boron Ring

Transition-metal-centered monocyclic boron wheels are important candidates in the family of planar hypercoordinate species that show intriguing structure, stability and bonding situation. Through the detailed potential energy surface explorations of MB₉⁻ (M = Fe, Ru, Os) clusters, we introduce herein OsB₉⁻ to be a new member in the transition-metal-centered borometallic molecular wheel gallery. Previously, FeB₉⁻ and RuB₉⁻ clusters were detected by photoelectron spectroscopy and the structures were reported to have singlet D_9h symmetry. Our present results show that the global minimum for FeB₉⁻ has a molecular wheel-like structure in triplet spin state with C_s symmetry, whereas its heavier homologues are singlet molecular wheels with D_9h symmetry. Chemical bonding analyses show that RuB₉⁻ and OsB₉⁻ display a similar type of electronic structure, where the dual σ + π aromaticity, originated from three delocalized σ bonds and three delocalized π bonds, accounts for highly stable borometallic molecular wheels.

Photoelectron Spectroscopy of Size-Selected Bismuth-Boron Clusters: BiBn- (n = 6-8)

Because of its low toxicity, bismuth is considered to be a "green metal" and has received increasing attention in chemistry and materials science. To understand the chemical bonding of bismuth, here we report a joint experimental and theoretical study on a series of bismuth-doped boron clusters, BiB_n^- (n = 6-8). Well-resolved photoelectron spectra are obtained and are used to understand the structures and bonding of BiB_n^- in conjunction with theoretical calculations. Global minimum searches find that all three BiB_n^- clusters have planar structures with the Bi atom bonded to the edge of the planar B_n moiety via two Bi-B σ bonds as well as π bonding by the 6p_z orbital. BiB₆^- is found to consist of a double-chain B₆ with a terminal Bi atom. Both BiB₇^- and BiB₈^- are composed of a Bi atom bonded to the planar global minima of the B₇^- and B₈^- clusters. Chemical bonding analyses reveal that BiB₆^- is doubly antiaromatic, whereas BiB₇^- and BiB₈^- are doubly aromatic. In the neutral BiB_n (n = 6-8) clusters, except BiB₆ which has a planar structure similar to the anion, the global minima of both BiB₇ and BiB₈ are found to be half-sandwich-type structures due to the high stability of the doubly aromatic B₇^3- and B₈^2- molecular wheel ligands.

Metal-centered monocyclic carbon wheel clusters with record coordination numbers in planar species

The highest coordination number identified to date in planar species is CN = 10 in metal-centered monocyclic boron wheel clusters D_10h M©B₁₀⁻ (M = Ta and Nb) (Galeev et. al., Angew. Chem. Int. Ed., 2012, 51, 2101). Extensive global minimum searches and first-principles theory calculations performed herein indicate that the experimentally observed LaC₁₃⁺ and LaC₁₄⁺ possess the well-defined global minima of perfect metal-centered monocyclic carbon wheel D_13h La©C₁₃⁺ (1) and slightly off-centered C_2v La©C₁₄⁺ (4) (¹A₁) with record coordination numbers of CN = 13 and 11 in planar structures, respectively, further pushing the boundary of our understanding of chemical structures and bonding. Detailed molecular orbital, nucleus-independent chemical shift, and ring current analyses indicate that D_13h La©C₁₃⁺ (1) is σ + π dually aromatic in nature, with 14 totally delocalized in-plane σ electrons and 14 totally delocalized out-of-plane π electrons each matching the 4N + 2 aromatic rule (N_σ = N_π = 3). Similar σ + π dually aromatic metal-centered monocyclic wheel clusters D_13h Ca©C₁₃ (2), C_13v Ac©C₁₃⁺ (3), C_2v Y©B₆C₆⁺ (5), and C_2v Sc©B₅C₆ (6) have also been obtained with CN = 13, 13, 12, and 11, respectively. The results obtained in this work effectively enrich the chemical structures and bonding patterns of planar hypercoordinated complexes.

First-principle theory calculations reveal record coordination numbers of CN = 13 in D_13h La©C₁₃⁺, CN = 12 in C_2v Y©B₆C₆⁺, and CN = 11 in C_2v Sc©B₅C₆ in planar species, effectively enriching the chemical structures and bonding patterns of planar hypercoordinated complexes.

Strong Binding of Noble Gases to [B12X11]-: A Theoretical Study

We systematically explore the stability and properties of [B₁₂X₁₁NG]^- adducts resulting from the binding of noble gas atoms to anionic [B₁₂X₁₁]^- clusters in the gas phase of mass spectrometers. [B₁₂X₁₁]^- can be obtained by stripping one X^- off the icosahedral closo-dodecaborate dianion [B₁₂X₁₂]^2-. We study the binding of the noble gas atoms He, Ne, Ar, Kr, and Xe to [B₁₂X₁₁]^- with substituents X = F, Cl, Br, I, and CN. While He cannot be captured by these clusters and Ne only binds at low temperatures, the complexes with the heavier noble gas atoms Ar, Kr, and Xe show appreciable complexation energies and exceed 1 eV at room temperature in the case of [B₁₂(CN)₁₁Xe]^-. The predicted B-NG equilibrium distance in the complexes with Ar, Kr, and Xe is only 0.10-0.25 Å longer than the sum of the covalent radii of the two corresponding atoms, and a significant charge transfer from the noble gas atom to the icosahedral B₁₂ cage is observed.

Transition-metal-like bonding behaviors of a boron atom in a boron-cluster boronyl complex [(η7-B7)-B-BO]. Chem Sci 2021;12:8157-8164. [PMID: 34194706 PMCID: PMC8208299 DOI: 10.1039/d1sc00534k]

Boron displays many unusual structural and bonding properties due to its electron deficiency. Here we show that a boron atom in a boron monoxide cluster (B₉O⁻) exhibits transition-metal-like properties. Temperature-dependent photoelectron spectroscopy provided evidence of the existence of two isomers for B₉O⁻: the main isomer has an adiabatic detachment energy (ADE) of 4.19 eV and a higher energy isomer with an ADE of 3.59 eV. The global minimum of B₉O⁻ is found surprisingly to be an umbrella-like structure (C_6v, ¹A₁) and its simulated spectrum agrees well with that of the main isomer observed. A low-lying isomer (C_s, ¹A′) consisting of a BO unit bonded to a disk-like B₈ cluster agrees well with the 3.59 eV ADE species. The unexpected umbrella-like global minimum of B₉O⁻ can be viewed as a central boron atom coordinated by a η⁷-B₇ ligand on one side and a BO ligand on the other side, [(η⁷-B₇)-B-BO]⁻. The central B atom is found to share its valence electrons with the B₇ unit to fulfill double aromaticity, similar to that in half-sandwich [(η⁷-B₇)-Zn-CO]⁻ or [(η⁷-B₇)-Fe(CO)₃]⁻ transition-metal complexes. The ability of boron to form a half-sandwich complex with an aromatic ligand, a prototypical property of transition metals, brings out new metallomimetic properties of boron.

The global minimum of the B₉O⁻ cluster is found to have an umbrella-like structure, where the central B atom exhibits transition-metal-like bonding properties, coordinated by a η⁷-B₇ ligand on one side and a BO ligand on the other.

Perfect Spherical Tetrahedral Metallo-Borospherene Ta4B18 as a Superatom Following the 18-Electron Rule

Cage-like metallo-borospherenes exhibit unique structures and bonding. Inspired by the newly reported smallest spherical trihedral metallo-borospherene D 3h Ta3B12 - (1), which contains two equivalent B3 triangles interconnected by three B2 units on the cage surface, we present herein a first-principles theory prediction of the perfect spherical tetrahedral metallo-borospherene T d Ta4B18 (2), which possesses four equivalent B3 triangles interconnected by six B atoms, with four equivalent nonacoordinate Ta centers in four η9-B9 rings as integrated parts of the cage surface. As the well-defined global minimum of the neutral, Ta4B18 (2) possesses four 10c-2e B9(π)-Ta(dσ) and eight 10c-2e B9(π)-Ta(dδ) coordination bonds evenly distributed over four Ta-centered Ta@B9 nonagons, with the remaining 18 valence electrons in nine 22c-2e totally delocalized bonds following the 18-electron principle (1S21P61D10) of a superatom. Such a bonding pattern renders spherical aromaticity to the tetrahedral complex, which can be used as building blocks to form the face-centered cubic crystal Ta4B15 (3). The IR, Raman, and UV-vis spectra of Ta4B18 (2) are theoretically simulated to facilitate its future experimental characterizations.

Expanded Inverse-Sandwich Complexes of Lanthanum Borides: La2B10- and La2B11. J Phys Chem A 2021;125:2622-2630. [PMID: 33739102 DOI: 10.1021/acs.jpca.1c01149]

Inverse-sandwich structures have been observed recently for dilanthanide boride clusters, in which two Ln atoms sandwich a monocyclic B_x ring for x = 7-9. An interesting question is if larger B_x rings are possible to form such inverse-sandwich clusters. Here we address this question by investigating La₂B₁₀^- and La₂B₁₁^- using photoelectron spectroscopy and ab initio quantum chemical calculations. Photoelectron spectra of La₂B₁₀^- and La₂B₁₁^- show complicated, but well-resolved, spectral features that are used to compare with theoretical calculations. We have found that global minimum structures of the two clusters are based on the octa-boron ring. The global minimum of La₂B₁₀^- consists of two chiral enantiomers with C₁ symmetry, which can be viewed as adding a B₂ unit off-plane to the B₈ ring, whereas that of La₂B₁₁^- can be viewed as adding a B₃ unit in-plane to the B₈ ring in a second coordination shell. Chemical bonding analyses reveal localized B-B bonds on the edge of the clusters and delocalized bonds in the expanded boron frameworks. The interactions between the La atoms and the boron frameworks include the unique (d-p)δ bonding, which was found to be the key for inverse-sandwich complexes with monocyclic boron rings. The current study confirms that the largest monocyclic boron ring to form the inverse-sandwich structures is B₉ and provide insights into the structural evolutions of larger lanthanide boride clusters.

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.

"Bottled" spiro-doubly aromatic trinuclear [Pd2Ru]+ complexes

Following an ongoing interest in the study of transition metal complexes with exotic bonding networks, we report herein the synthesis of a family of heterobimetallic triangular clusters involving Ru and Pd atoms. These are the first examples of trinuclear complexes combining these nuclei. Structural and bonding analyses revealed both analogies and unexpected differences for these [Pd₂Ru]⁺ complexes compared to their parent [Pd₃]⁺ peers. Noticeably, participation of the Ru atom in the π-aromaticity of the coordinated benzene ring makes the synthesized compound the second reported example of ‘bottled’ double aromaticity. This can also be referred to as spiroaromaticity due to the participation of Ru in two aromatic systems at a time. Moreover, the [Pd₂Ru]⁺ kernel exhibits unprecedented orbital overlap of Ru d_z² AO and two Pd d_xy or d_x²−y² AOs. The present findings reveal the possibility of synthesizing stable clusters with delocalized metal–metal bonding from the combination of non-adjacent elements of the periodic table which has not been reported previously.

Synthesis of a triangular [Pd₂Ru]⁺ complex with delocalized metal–metal bonding between non-adjacent elements of the periodic table, double aromaticity and overlap of d-AOs with different angular momentum.