Perfect cubic metallo-borospherenes TMB (TM = Ni, Pd, Pt) as superatoms following the 18-electron rule.

Transition-metal (TM)-doped metallo-borospherenes exhibit unique structures and bonding in chemistry which have received considerable attention in recent years. Based on extensive global minimum searches and first-principles theory calculations, we predict herein the first and smallest perfect cubic metallo-borospherenes TMB (TM = Ni (1), Pd (2), Pt (3)) and NiB (1) which contain eight equivalent TM atoms at the vertexes of a cube and six quasi-planar tetra-coordinate face-capping boron atoms on the surface. Detailed canonical molecular orbital and adaptive natural density partitioning bonding analyses indicate that TMB (1/2/3) as superatoms possess nine completely delocalized 14c-2e bonds following the 18-electron principle (1S1P1D), rendering spherical aromaticity and extra stability to the complex systems.

Heptacoordinate transition-metal-decorated metallo-borospherenes and multiple-helix metallo-boronanotubes.

The recent discovery of lanthanide-metal-decorated metallo-borospherenes LMB (LM = La, Tb) marks the onset of a new class of boron-metal binary nanomaterials. Using the experimentally observed or theoretically predicted borospherenes as ligands and based on extensive first-principles theory calculations, we predict herein a series of novel chiral metallo-borospherenes Ni ∈ B (1), Ni ∈ B (3), Ni ∈ B (4), Ni ∈ B (5), and Ni ∈ B (6) as the global minima of the systems decorated with quasi-planar heptacoordinate Ni (phNi) centers in η-B heptagons on the cage surfaces, which are found to be obviously better favoured in coordination energies than hexacoordinate Ni centers in previously reported Ni ∈ B (2). Detailed bonding analyses indicate that these phNi-decorated metallo-borospherenes follow the σ + π double delocalization bonding pattern, with two effective (d-p)σ coordination bonds formed between each phNi and its η-B ligand, rendering spherical aromaticity and extra stability to the systems.

Lanthanide/actinide boride nanoclusters and nanomaterials based on boron frameworks consisting of conjoined B rings ( = 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 LaB (1), LaB (3), LaB (7) can be extended to their actinide counterparts AcB (1'), AcB (3'), AcB (7') with a B monocyclic ring ( = 7-9) sandwiched by two Ac dopants. Such MB inverse sandwiches (1/1', 3/3', 7/7') can be used as building blocks to generate the ground-state LaB (2)/AcB (2'), LaB (4)/AcB (4'), / LaB (5)/AcB (5'), AcB (6'), AcB, AcB (8'), LaB (9)/AcB (9'), and AcB (10') which are based on boron frameworks consisting of multiple conjoined B rings ( = 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 rings and metal doping centers, conferring three-dimensional aromaticity and extra stability to the systems.

A bottom-up approach from medium-sized bilayer boron nanoclusters to bilayer borophene nanomaterials.

Inspired by the experimentally observed bilayer B and theoretically predicted bilayer B-B and based on extensive density functional theory calculations, we report herein a series of novel medium-sized bilayer boron nanoclusters B (I), B (II), B (III), B (IV), B (V), B (VI), B (VII), and B (VIII) which are the most stable isomers of the systems reported to date effectively stabilized by optimum numbers of interlayer B-B σ bonds between the inward-buckled atoms on top and bottom layers. Detailed bonding analyses indicate that these bilayer species follow the universal bonding pattern of σ + π double delocalization, rendering three-dimensional aromaticity in the systems. More interestingly, the AA-stacked bilayer structural motif in B (VII) with a B bilayer hexagonal prism at the center can be extended to form bilayer B (IX), B (X), B (XI), B (XII), and B (XIII) which contain one or multiple conjoined B bilayer hexagonal prisms sharing interwoven zig-zag boron triple chains between them.

La@[La&B]: endohedral trihedral metallo-borospherenes with spherical aromaticity.

It is well-known that transition-metal-doping induces dramatic changes in the structures and bonding of small boron clusters, as demonstrated by the newly observed perfect inverse sandwich [La(η-B)La] and [La(η-B)La]. Based on extensive global minimum searches and first-principles theory calculations, we predict herein the possibility of perfect endohedral trihedral metallo-borospherene La@[La&B] (1, A') and its monoanion La@[La&B] (2, A') and dianion La@[La&B] (3, A'). These La-doped boron clusters are composed of three inverse sandwich La(η-B)La on the waist and two inverse sandwich La(η-B)La on the top and bottom which share one apex La atom at the center and six periphery B units between neighboring η-B and η-B rings, with three octo-coordinate La atoms and two nona-coordinate La atoms as integrated parts of the cage surface.

B: a bilayer boron cluster.

Size-selected negatively-charged boron clusters (B) have been found to be planar or quasi-planar in a wide size range. Even though cage structures emerged as the global minimum at B, the global minimum of B was in fact planar. Only in the neutral form did the B borospherene become the global minimum.

Novel B-C binary fullerenes following the isolated BC hexagonal pyramid rule.

B-C binary monolayers and fullerenes (borafullerenes) have received considerable attention in recent years. Inspired by the newly reported BC semiconducting boron carbide monolayer isovalent to graphene (Tian et al., Nanoscale, 2019, 11, 11099), we predict herein at density functional theory level a new class of borafullerenes (1-8) following the isolated BC hexagonal pyramid rule.

Perfect cubic La-doped boron clusters La&[La@B] as the embryos of low-dimensional lanthanide boride nanomaterials.

La-doped boron nanoclusters have received considerable attention due to their unique structures and bonding. Inspired by recent experimental observations of the inverse sandwich LaB (1) and triple-decker LaB (2) and based on extensive global searches and first-principles theory investigations, we present herein the possibility of the perfect cubic La-doped boron clusters La&[La@B] (3, A) and La&[La@B] (4, A) which appear to be the embryos of the metallic one-dimensional LaB (5) nanowire, two-dimensional LaB (6) nanosheet, and three-dimensional LaB (7) nanocrystal, facilitating a bottom-up approach to build cubic lanthanide boride nanostructures from gas-phase clusters. Detailed molecular orbital and bonding analyses indicate that effective (d-p)σ, (d-p)π and (d-p)δ covalent coordination interactions exist in La&[La@B] (3/4) clusters, while the 1D LaB (5), 2D LaB (6), and 3D LaB (7) crystals exhibit mainly electrostatic interactions between the trivalent La centers and cubic B frameworks, with weak but discernible coordination contributions from La (5d) ← B (2p) back-donations.

Sea-shell-like B and B: two new axially chiral members of the borospherene family.

Since the discovery of the cage-like borospherenes B and the first axially chiral borospherenes / B , a series of fullerene-like boron clusters in different charge states have been reported in theory. Based on extensive global minimum searches and first-principles theory calculations, we present herein two new axially chiral members B (I) and B (VI) to the borospherene family. B (I) features two equivalent heptagons on the top and one octagon at the bottom on the cage surface, while B (VI) possesses two equivalent heptagons on top and two equivalent heptagons at the bottom.

Predicting lanthanide boride inverse sandwich tubular molecular rotors with the smallest core-shell structure.

Lanthanide-boron binary clusters possess interesting structures and bonding which may provide insights into designing new boride nanomaterials. Inspired by the recently discovered mono-decker inverse sandwich D LaB (A') (1) and based on the extensive first-principles theory calculations, we predict herein the possible existence of a series of bi-decker inverse sandwich di-lanthanide boron complexes including D La[B] (A) (2), D La[B] (A) (3), and C La[B@B] (A) (4) which all contain a tubular B ligand (n = 18, 20) sandwiched by two La atoms at the two ends. In these novel clusters, La[B@B] (4) as a tubular molecular rotor with the smallest core-shell structure reported to date in boron-based nanoclusters possesses a B-bar rotating constantly and almost freely inside the B tube around it at room temperature.

Low-dimensional functional networks of cage-like B with effective transition-metal intercalations.

As the first all-boron fullerene observed in experiments, cage-like borospherene B40 has attracted considerable attention in recent years. However, B40 has been proved to be chemically reactive and tends to coalesce with one another via the formation of covalent bonds. We explore herein the possibility of low-dimensional functional networks of B40 with effective transition-metal intercalations.

Fluxional bonds in quasi-planar and half-sandwich (M = K, Rb, and Cs).

Detailed molecular orbital and bonding analyses reveal the existence of both fluxional σ- and π-bonds in the global minima C (1) and C MB (3) and transition states C (2) and C (4) of dianion and monoanions (M = K, Rb, and Cs). It is the fluxional bonds that facilitate the fluxional behaviors of the quasi-planar and half-sandwich which possess energy barriers smaller than the difference of the corresponding zero-point corrections. © 2019 Wiley Periodicals, Inc.

Fluxional Bonds in Planar B , Tubular Ta@B , and Cage-Like B.

Based on detailed bonding analyses on the fluxional behaviors of planar B , tubular Ta@B , and cage-like B , we propose the concept of fluxional bonds in boron nanoclusters as an extension of the classical localized bonds and delocalized bonds in chemistry. © 2018 Wiley Periodicals, Inc.

High-symmetry tubular Ta@B, Ta@B, and Ta@B as embryos of α-boronanotubes with a transition-metal wire coordinated inside.

Transition-metal doping leads to dramatic structural changes and results in novel bonding patterns in small boron clusters. Based on the experimentally derived mono-ring planar C9v Ta©B92- (1) and extensive first-principles theory calculations, we present herein the possibility of high-symmetry double-ring tubular D9d Ta@B183- (2) and C9v Ta2@B18 (3) and triple-ring tubular D9h Ta2@B27+ (4), which may serve as embryos of single-walled metalloboronanotube α-Ta3@B48(3,0) (5) wrapped up from the recently observed most stable free-standing boron α-sheet on a Ag(111) substrate with a transition-metal wire (-Ta-Ta-) coordinated inside. Detailed bonding analyses indicate that, with an effective dz2-dz2 overlap on the Ta-Ta dimer along the C9 molecular axis, both Ta2@B18 (3) and Ta2@B27+ (4) follow the universal bonding pattern of σ + π double delocalization with each Ta center conforming to the 18-electron rule, providing tubular aromaticity to these Ta-doped boron complexes with magnetically induced ring currents.

Charge-induced structural transition between seashell-like B and B in 18 π-electron configurations.

Recent joint experimental and theoretical investigations have shown that seashell-like C2 B28 is the smallest neutral borospherene reported to date, while seashell-like Cs B29- (1-) as a minor isomer competes with its quasi-planar counterparts in B29- cluster beams. Extensive global minimum searches and first-principles theory calculations performed in this work indicate that with two valence electrons detached from B29-, the B29+ monocation favors a seashell-like Cs B29+ (1+) much different from Cs B29- (1-) in geometry which is overwhelmingly the global minimum of the system with three B7 heptagonal holes in the front, on the back, and at the bottom, respectively, unveiling an interesting charge-induced structural transition from Cs B29- (1-) to Cs B29+ (1+). Detailed bonding analyses show that with one less σ bond than B29- (1-), Cs B29+ (1+) also possesses nine delocalized π-bonds over its σ-skeleton on the cage surface with a σ + π double delocalization bonding pattern and follows the 2(n + 1)2 electron counting rule for 3D spherical aromaticity (n = 2).

Aromatic cage-like B and B: new axially chiral members of the borospherene family.

Shortly after the discovery of all-boron fullerenes D2d B40-/0 (borospherenes), the first axially chiral borospherenes C3/C2 B39- were characterized in experiments in 2015. Based on extensive global minimum searches and first-principles theory calculations, we present herein two new axially chiral members to the borospherene family: the aromatic cage-like C2 B34(1) and C2 B35+(2). Both B34(1) and B35+(2) feature one B21 boron triple chain on the waist and two equivalent heptagons and hexagons on the cage surface, with the latter being obtained by the addition of B+ into the former at the tetracoordinate defect site.

Cage-like Ta@B complexes (n = 23-28, q = -1-+ 3) in 18-electron configurations with the highest coordination number of twenty-eight.

Inspired by recent observations of the highest coordination numbers of CN = 10 in planar wheel-type complexes in D10h Ta@B10- and CN = 20 in double-ring tubular species in D10d Ta@B20- and theoretical prediction of the smallest endohedral metalloborospherene D2 Ta@B22- (1) with CN = 22, we present herein the possibility of larger endohedral metalloborospherenes C2 Ta@B23 (2), C2 Ta@B24+ (3), C2v Ta@B24- (4), C1 Ta@B25 (5), D2d Ta@B26+ (6), C2 Ta@B272+ (7), and C2 Ta@B283+ (8) based on extensive first-principles theory investigations. These cage-like Ta@Bqn complexes with B6 pentagonal or B7 hexagonal pyramids on their surface turn out to be the global minima of the systems with CN = 23, 24, 24, 25, 26, 27, and 28, respectively, unveiling the highest coordination number of CN = 28 in spherical environments known in chemistry. Detailed bonding analyses show that 1-8 as superatoms conform to the 18-electron configuration with a universal σ + π double delocalization bonding pattern.

Structural transition in metal-centered boron clusters: from tubular molecular rotors Ta@B and Ta@B to cage-like endohedral metalloborospherene Ta@B.

Inspired by the recent discovery of the metal-centered tubular molecular rotor C B-Ta@B with the record coordination number of CN = 20 and based on extensive first-principles theory calculations, we present herein the possibility of the largest tubular molecular rotors C B-Ta@B (1) and C B-Ta@B (2) and smallest axially chiral endohedral metalloborospherenes D Ta@B (3 and 3'), unveiling a tubular-to-cage-like structural transition in metal-centered boron clusters at Ta@Bvia effective spherical coordination interactions. The highly stable Ta@B (3) as an elegant superatom, which features two equivalent corner-sharing B boron double chains interconnected by two B units with four equivalent B heptagons evenly distributed on the cage surface, conforms to the 18-electron configuration with a bonding pattern of σ + π double delocalization and follows the 2(n + 1) electron counting rule for spherical aromaticity (n = 2). Its calculated adiabatic detachment energy of ADE = 3.

Double-ring tubular (BO) 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 (BO) 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) (BO) 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 BO (2). Detailed bonding analyses on 3D (BO) 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 BO hexagonal unit which form a unique 6c-4e o-bond to help stabilize the systems.

Heteroborospherene clusters Ni ∈ B (n = 1-4) and heteroborophene monolayers Ni ∈ B with planar heptacoordinate transition-metal centers in η-B heptagons.

With inspirations from recent discoveries of the cage-like borospherene B and perfectly planar Co ∈ B and based on extensive global minimum searches and first-principles theory calculations, we present herein the possibility of the novel planar Ni ∈ B (1), cage-like heteroborospherenes Ni ∈ B (n = 1-4) (2-5), and planar heteroborophenes Ni ∈ B (6, 7) which all contain planar or quasi-planar heptacoordinate transition-metal (phTM) centers in η-B heptagons. The nearly degenerate Ni ∈ B (6) and Ni ∈ B (7) monolayers are predicted to be metallic in nature, with Ni ∈ B (6) composed of interwoven boron double chains with two phNi centers per unit cell being the precursor of cage-like Ni ∈ B (n = 1-4) (2-5). Detailed bonding analyses indicate that Ni ∈ B (n = 1-4) (2-5) and Ni ∈ B (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, 3d, and 3d) and two effective nearly in-plane 8c-2e σ-coordination bonds between the remaining tangential Ni 3d orbitals (3d and 3d) and the η-B heptagon around it.

Cage-like B clusters with the bonding pattern of σ + π double delocalization: new members of the borospherene family.

The recently observed cage-like borospherenes D B and C/C B have attracted considerable attention in chemistry and materials science. Based on extensive global minimum searches and first-principles theory calculations, we present herein the possibility of cage-like C B (1) and C B (2) which possess five hexagonal and heptagonal faces and one filled hexagon and follow the bonding pattern of σ + π double delocalization with 12 delocalized π bonds over a σ-skeleton, adding two new members to the borospherene family. IR, Raman, and UV-vis spectra of C B (1) and C B (2) are computationally simulated to facilitate their experimental characterization.

Planar B and B 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 B which led to the concept of borophenes and the 3D fullerene-like B which marked the onset of borospherene chemistry. Here, we present a joint photoelectron spectroscopic and first-principles study of B and B, which are in the transition size range between the 2D borophene-type clusters and the 3D borospherenes.

From Quasi-Planar B to Penta-Ring Tubular Ca©B: Prediction of Metal-Stabilized Ca©B 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 B (A-1) with an alkaline-earth metal to produce the penta-ring tubular Ca©B (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 (C-1). Ca©BNT (C-1) can be constructed by rolling up the most stable boron α-sheet and is predicted to be metallic in nature.

Lithium-Decorated Borospherene B: A Promising Hydrogen Storage Medium.

The recent discovery of borospherene B marks the onset of a new kind of boron-based nanostructures akin to the C buckyball, offering opportunities to explore materials applications of nanoboron. Here we report on the feasibility of Li-decorated B for hydrogen storage using the DFT calculations. The B cluster has an overall shape of cube-like cage with six hexagonal and heptagonal holes and eight close-packing B triangles.

Saturn-like charge-transfer complexes Li₄&B₃₆, Li₅&B₃₆⁺, and Li₆&B₃₆²⁺: exohedral metalloborospherenes with a perfect cage-like B₃₆⁴⁻ core.

Based on extensive first-principles theory calculations, we present the possibility of construction of the Saturn-like charge-transfer complexes Li4&B36 (2), Li5&B36(+) (3), and Li6&B36(2+) (4) all of which contain a perfect cage-like B36(4-) (1) core composed of twelve interwoven boron double chains with a σ + π double delocalization bonding pattern, extending the Bn(q) borospherene family from n = 38-42 to n = 36 with the highest symmetry of T(h).