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

Theoretical Investigation on One-Electron ϕ···ϕ Bonding in Diuranium Inverse Sandwich U2B6 Complex Enabled by a B6 Ring

Traditional σ, π, and δ types of covalent chemical bonding have been extensively studied for nearly a century. In contrast, ϕ-type bonding involving nf (n = 4, 5) orbitals has received less attention due to their high contraction and minimal orbital overlap. Herein, we theoretically predict a singly occupied ϕ···ϕ bonding between two 5f orbitals, facilitated by B6 group orbitals in the hexa-boron diuranium inverse sandwich structure of U2B6. From ab initio quantum chemical calculations, the global minimum structure has a septuplet state with D6h symmetry. Chemical bonding analyses reveal that the 5f and 6d atomic orbitals of the two uranium atoms interact with the ligand orbitals of the central B6 ring, exhibiting favorable energy matching and symmetry compatibility to form delocalized σ-, π-, δ-, and ϕ-type bonding orbitals. Notably, even though the ϕ···ϕ bonding orbital is singly occupied, it still has a significant role in stability and cannot be overlooked. Furthermore, the U2B6 cluster model can be viewed as a building block of UB2 solid materials from both geometric and electronic perspectives. This work predicts the first example of ϕ···ϕ bonding, highlighting the complexity and diversity of chemical bonds formed in actinide boride clusters.

Probing the Structures and Lanthanum-Lanthanum Bonding in La2Bn- (n = 4-6) Clusters

We report an investigation on the structures and chemical bonding in a series of di-lanthanum boron clusters, La2Bn- (n = 4-6), using photoelectron spectroscopy and theoretical calculations. Well-resolved photoelectron spectra are obtained and used to verify the global minima of the lanthanide boron clusters. The structures of La2B4- and La2B5- are found to consist of open B4 and B5 rings, respectively, around the La2 dimer equatorially. Theoretical evidence of La-La σ bonding is obtained in La2B4-, whereas the bonding in La2B5- is similar to that of an incomplete inverse sandwich without real La-La bonding. The global minimum of La2B6- is completely different, where one of the La atoms can be viewed as substituting a B atom of the B7 cluster due to the high electronic stability of the B73- borozene. The resulting lanthaborozene [LaB6]3- forms a half-sandwich structure with the second La atom, with evidence of La-La σ bonding. Lanthanide-lanthanide bonds are relatively rare in chemistry. The current work suggests that binary lanthanide boron clusters provide interesting systems to study lanthanide-lanthanide bonding.

Stability and chemical bonding in a series of inverse sandwich actinide boride clusters (An2B8) with δ bonding

An inverse sandwich structure has been computationally predicted for uranium boride and extended to the series of actinide elements (An) from Th to Cm. The electronic structure and chemical bonding of these novel compounds have been analyzed using density functional theory and multireference wave-function based methods. We report the trends in electronic structure and bonding for An2B8, and found that (d-π)π and (d-p)δ are the most important factors in the stability of An2B8. The (f-p)δ bond provides extra stabilization for Pa2B8 and U2B8, owing to the extensive interactions of An-B8-An, resulting in a short distance for the Pa-Pa and U-U bonds.

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.

Boron-based ternary MgTa2B6 cluster: a turning nanoclock with dynamic structural fluxionality

Boron-based complex clusters are a fertile ground for the exploration of exotic chemical bonding and dynamic structural fluxionality. Here we report on the computational design of a ternary MgTa2B6 cluster via global structural searches and quantum chemical calculations. The cluster turns out to be a new member of the molecular rotor family, closely mimicking a turning clock at the subnanoscale. It is composed of a hexagonal B6 ring with a capping Ta atom at the top and bottom, whereas the Mg atom is linked to one Ta site as a radial Ta-Mg dimer. These components serve as the dial, axis, and hand of a nanoclock, respectively. Chemical bonding analyses reveal that the inverse sandwich Ta2B6 motif in the cluster features 6π/6σ double aromaticity, whose electron counting conforms to the (4n + 2) Hückel rule. The Ta-Mg dimer has a Lewis-type σ bond, and the Mg site has negligible bonding with B6 ring. The ternary cluster can be formulated as an [Mg]0[Ta2B6]0 complex. Molecular dynamics simulations suggest that the cluster is structurally fluxional analogous to a nanoclock, even at a low temperature of 100 K. The Ta-Mg hand turns almost freely around the Ta2 axis and along the B6 dial. The tiny intramolecular rotation barrier is less than 0.3 kcal mol-1, being dictated by the bonding nature of double 6π/6σ aromaticity. The present system offers a new type of molecular rotor in physical chemistry.

Transition Metal Behavior of Heavier Alkaline Earth Elements in Doped Monocyclic and Tubular Boron Clusters

Quantum chemical calculations are carried out to design highly symmetric-doped boron clusters by employing the transition metal behavior of heavier alkaline earth (Ae = Ca, Sr, and Ba) metals. Following an electron counting rule, a set of monocyclic and tubular boron clusters capped by two heavier Ae metals were tested, which leads to the highly symmetric Ae2B8, Ae2B18, and Ae2B30 clusters as true minima on the potential energy surface having a monocyclic ring, two-ring tubular, and three-ring tubular boron motifs, respectively. Then, a thorough global minimum (GM) structural search reveals that a monocyclic B8 ring capped with two Ae atoms is indeed a GM for Ca2B8 and Ba2B8, while for Sr2B8 it is a low-lying isomer. Similarly, the present search also unambiguously shows the most stable isomers of Ae2B18 and Ae2B30 to be highly symmetric two- and three-ring tubular boron motifs, respectively, capped with two Ae atoms on each side of the tube. In these Ae-doped boron clusters, in addition to the electrostatic interactions, a substantial covalent interaction, specifically the bonding occurring between (n - 1)d orbitals of Ae and delocalized orbitals of boron motifs, provides the essential driving force behind their highly symmetrical structures and overall stability.

Transition Metal Triple-decker Sandwich Complexes Containing Group 13 Elements

Transition metal triple-decker complexes are an interesting class of sandwich complexes that engrossed great attention due to their structures and properties. Over the decades, synthesis of triple-decker complexes featuring homocyclic, heterocyclic or π-conjugated rings as middle decks have been abundantly reported. In this regard, the chemistry of such complexes bearing boron in the middle deck are well explored due to the ability of boron-containing cycles to readily coordinate bifacially with metal atoms thereby forming triple-decker complexes. On the other hand, electron counting rules and theoretical calculations have strengthened our knowledge of the structure and bonding in these complexes. Further, these complexes can be used as synthons to generate organometallic polymers having interesting electronic, optical and magnetic properties that can be appropriately tuned to cater to a wide range of applications. In our quest for novel metallaboranes and metallaheteroboranes, we have been successful in isolating various triple-decker complexes that feature boron in the middle deck. This review explained elaborately the synthesis, structures, and bonding in such complexes reported by us and others.

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.

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.

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.

A plier-shaped binary molecular wheel B7Mg4+ cluster: hybrid in-plane heptacoordination, double π/σ aromaticity, and electronic transmutation

A plier-shaped charge-transfer [Mg2]2+[Mg2B7]− complex cluster exhibits double 6π/6σ aromaticity, whose hybrid molecular wheel structure is rationalized using the concept of electronic transmutation.

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.

Designing a Four-Ring Tubular Boron Motif through Metal Doping

Tubular boron clusters represent a class of extremely unusual geometries that can be regarded as a key indicator for the 2D-to-3D boron structural evolution as well as the embryos for boron nanotubes. While a good number of pure boron or metal-doped boron tubular clusters have been reported so far, most of them are two-ring tubular structures, and their higher-ring analogues are very scarce. We report herein the first example of a four-ring tubular boron motif in the cagelike global minimum of Be₂B₂₄⁺. Global-minimum searches of MB₂₄^q and M₂B₂₄^q (M = alkali/alkaline-earth metals; q = 1+, 0, 1-) reveal that the most stable structure of Be₂B₂₄⁺ is a C_2v-symmetric cage having a four-ring tubular boron moiety, whereas it is a high-lying isomer for those having a two/three-ring tubular boron motif for all other systems. The B₂₄ framework in Be₂B₂₄⁺ can be viewed as consisting of two two-ring B₁₂ tubular structures linked together at one side of the B₆ rings along the high-symmetry axis and two offside B₆ rings capped by two Be atoms. The Be₂-B₂₄ bonding is best described as Be₂²⁺ in an excited triplet state, forming two highly polarized covalent bonds with B₂₄^- in a quartet spin state. The unique ability of beryllium to make strong covalent and electrostatic interactions makes the Be₂B₂₄⁺ cluster stable in such an unusual geometry.

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.

Structure and Chemical Bonding in Medium-Size Boron Clusters Doped with Praseodymium

After reports of unusually low oxidation states of lanthanide elements in Ln-B clusters and their inverse sandwich geometrical topologies, the interest shifted from boride clusters doped with transition metal (TM) elements to the boride clusters doped with lanthanide atoms. In this work, the results obtained by a combined approach consisting of CALYPSO structure predictions and density functional theory (DFT) calculations for the neutral and anionic PrB_n series, n = 7-16, are reported. A close agreement between our calculated vertical detachment energies and experimental data supports the accuracy of the results obtained. Contrary to the medium-size TM-doped medium boron clusters, which prefer three types of structural configurations, all lowest-energy states of the medium-size Pr-doped boron clusters have half-sandwich geometries. An interesting structural evolution pattern was found for both neutral and anionic PrB_n clusters at n = 7, 10, 13, and 16, which includes quasi-planar B₇ units half-sandwiching the Pr atom. Unusual oxidation numbers of +2 and +1 were found for the Pr atom in the PrB₇^- and PrB₈^- anions, respectively. Chemical bonding analysis for the neutral PrB₇ and PrB₁₃ clusters revealed that their high stability stems from interactions between Pr 5d and B 2p orbitals. A stable tubular-shaped PrB₃₀ cluster is proposed as a promising building block for boron-based nanotubes.

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.

Highly stable actinide(III) complexes supported by doubly aromatic ligands

Owing to the electron-deficient nature of boron atom, the structures and properties of boron clusters can be enriched by doping various metal atoms, including lanthanide metal atoms. Nevertheless, the viability...

La@[La5&B30]0/−/2−: endohedral trihedral metallo-borospherenes with spherical aromaticity

Extensive first-principles theory calculations predict the perfect endohedral metallo-borospherene D3h La@[La5&B30] (1) and its monoanion Cs La@[La5&B30]− (2) and dianion D3h La@[La5&B30]2− (3) which appear to be spherically aromatic in nature.

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.

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.

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.

The unified quantum mechanical structure of tubular molecular rotors with multiple equivalent global minimum structures: the 18*C2h → D9d case of La-[B2@B18]-La

La-[B₂@B₁₈]-La demonstrates decisive changes of the properties of molecular rotors, from multiple (here 18) equivalent individual global minimum structures to a quantum mechanical unified structure. This affects their geometries, their symmetries (18*C_2h → D_9d), the generation of energy bands, and high-resolution spectroscopy.

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.

Insights into the Structures and Bonding of Medium-Sized Cerium-Doped Boron Clusters

Since the discovery of metal-doped boron clusters attracted great significance to create a new class of materials, research interests have been directed to chemical bonding and structural evolution of lanthanide boride clusters. Here, we perform an extensive ground-state structure search for the CeB_n and CeB_n^- clusters in the size range from 9 to 18 using the Crystal structure AnaLYsis by Particle Swarm Optimization method and density functional theory optimization. It is found that the ground-state structures in both neutral and anionic series possess half-sandwich geometry. The host boron moiety in neutral series has a tendency to form borophene-like geometry. The pentagonal and hexagonal holes are more common in the larger anionic CeB_n^- series. The theoretical photoelectron spectroscopy has been simulated by applying time-dependent density functional theory calculations. The neutral CeB₁₄ cluster is identified as a magic cluster on the basis of its robust relative stability with respect to its neighbors. Electronic structure and chemical bonding analyses reveal that the CeB₁₄ cluster possesses a large HOMO-LUMO gap and enhanced stability with strong delocalized π and δ bonding via interactions between the Ce 5d- and B 2p-AOs.

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.

Cage-like La4B24 and Core-Shell La4B290/+/- : perfect spherically aromatic tetrahedral metallo-borospherenes

Cage-like and core-shell metallo-borospherenes exhibit interesting structures and bonding. Based on extensive global searches and first-principles theory calculations, we predict herein the perfect tetrahedral cage-like T_d La₄B₂₄ (1) and core-shell T_d La₄B₂₉ (2), T_d La₄B₂₉⁺ (3), and T_d La₄B₂₉^- (4) which all possess the same geometrical symmetry as their carbon fullerene counterpart T_d C₂₈, with four equivalent interconnected B₆ triangles on the cage surface and four nona-coordinate La centers in four conjoined η⁹-B₉ rings. In these tetra-La-doped boron complexes, La₄[B@B₄@B₂₄]^0/+/- (2/3/4) in the structural motif of 1 + 4 + 28 contain a B-centered tetrahedral T_d B@B₄ core in a La-decorated tetrahedral La₄B₂₄ shell, with the negatively charged tetra-coordinate B^- at the center being the boron analog of tetrahedral C in T_d CH₄ (B^- ~ C). Detailed orbital and bonding analyses indicate that these T_d lanthanide boride complexes are spherically aromatic in nature with a universal La--B₉ (d-p) σ and (d-p) δ coordination bonding pattern. The IR, Raman, and UV-Vis or photoelectron spectra of these novel metallo-borospherenes are computationally simulated to facilitate their spectral characterizations. Graphical abstract.

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.

Boron-based Be2B5+/0/− alloy clusters: inverse sandwiches with pentagonal boron ring and reduction-induced structural transformation to molecular wheel structure

Boron-based Be₂B₅^+/0/− alloy clusters feature inverse sandwich versus molecular wheel structures, which sensitively depend on their charge states and show distinct π/σ aromaticity.

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.

Planar Pentacoordinate Nitrogen in a Pseudo-Double-Aromatic NBe5H4+ Cluster

High-level quantum-chemical calculations have been used to predict a cationic ternary NBe₅H₄⁺ cluster containing a planar pentacoordinate nitrogen atom. The proposed cluster has pseudo dual aromaticity and is kinetically and thermodynamically very stable.

Distinct electronic structures and bonding interactions in inverse-sandwich samarium and ytterbium biphenyl complexes

Inverse-sandwich samarium and ytterbium biphenyl complexes were synthesized by the reduction of their trivalent halide precursors with potassium graphite in the presence of biphenyl. While the samarium complex had a similar structure as previously reported rare earth metal biphenyl complexes, with the two samarium ions bound to the same phenyl ring, the ytterbium counterpart adopted a different structure, with the two ytterbium ions bound to different phenyl rings. Upon the addition of crown ether to encapsulate the potassium ions, the inverse-sandwich samarium biphenyl structure remained intact; however, the ytterbium biphenyl structure fell apart with the concomitant formation of a divalent ytterbium crown ether complex and potassium biphenylide. Spectroscopic and computational studies were performed to gain insight into the electronic structures and bonding interactions of these samarium and ytterbium biphenyl complexes. While the ytterbium ions were found to be divalent with a 4f¹⁴ electron configuration and form a primarily ionic bonding interaction with biphenyl dianion, the samarium ions were in the trivalent state with a 4f⁵ electron configuration and mainly utilized the 5d orbitals to form a δ-type bonding interaction with the π* orbitals of the biphenyl tetraanion, showing covalent character.

Inverse-sandwich samarium and ytterbium biphenyl complexes were synthesized and characterized by X-ray crystallography. Combined experimental and computational studies indicated that they have distinct electronic structures and bonding interactions.

Donor-acceptor duality of the transition-metal-like B2 core in core-shell-like metallo-borospherenes La3&[B2@B17]- and La3&[B2@B18]. RSC Adv 2020;10:34225-34230. [PMID: 35519037 PMCID: PMC9056772 DOI: 10.1039/d0ra06769e]

Transition-metal doping induces dramatic structural changes and leads to earlier planar → tubular → spherical → core–shell-like structural transitions in boron clusters. Inspired by the newly discovered spherical trihedral metallo-borospherene D_3h La₃&B₁₈⁻ (1) (Chen, et al., Nat. Commun., 2020, 11, 2766) and based on extensive first-principles theory calculations, we predict herein the first and smallest core–shell-like metallo-borospherenes C_2v La₃&[B₂@B₁₇]⁻ (2) and D_3h La₃&[B₂@B₁₈]⁻ (3) which contain a transition-metal-like B₂ core at the cage center with unique donor–acceptor duality in La₃&B_n⁻ spherical trihedral shells (n = 17, 18). Detailed energy decomposition and bonding analyses indicate that the B₂ core in these novel complexes serves as a π-donor in the equatorial direction mainly to coordinate three La atoms on the waist and a π/σ-acceptor in the axial direction mainly coordinated by two B₆ triangles on the top and bottom. These highly stable core–shell complexes appear to be spherically aromatic in nature in bonding patterns. The IR, Raman, and photoelectron spectra of 2 and 3 are computationally simulated to facilitate their spectroscopic characterizations.

The smallest core–shell-like metallo-borospherenes C_2v La3&[B₂@B₁₇]⁻ and D_3h La₃&[B₂@B₁₈]⁻ have been predicted at first-principles theory level which contain a transition-metal-like B₂ core with unique donor–acceptor duality.

Structural effects of alkali-metals on the B12 skeleton

After an exhaustive exploration of the potential energy surface of B₁₂E^- and B₁₂E₂ (E = Li-Cs) systems, it was found that for the anionic series, a cage-type and a quasi-planar structure (very similar to the naked B₁₂ cluster) compete to be the putative global minimum. For neutral systems, competition arises between the quasi-planar cluster and a double-ring with the alkali-metals on the highest-symmetry axis. The chemical bonding analyses show that for the entire series, the interaction, predominantly electrostatic, is essentially indistinguishable regardless of the alkali-metal and insufficient for determining the isomeric preference. The isomerization energy decomposition analysis (IEDA) reveals that in the anions, the structural change in the lighter complexes is possible because of the relatively low energy required for the boron skeleton deformation, as opposed to the case of heavy metals. In the case of the neutral systems, the factor determining one isomer over the other corresponds to that of the energy deformation of the alkali-metal dimer.

The teetotum cluster Li2FeB14 and its possible use for constructing boron nanowires

Systematic density functional theory (DFT) calculations using the TPSSh functional and the def2-TZVP basis set were carried out to identify the global energy minimum structure of the Li2FeB14 cluster. Keeping the double ring tubular shape of FeB14, capping of two Li atoms leads to a teetotum form at a low spin state, in which the Fe atom is endohedrally covered by two B7 strings, and both Li atoms are attached to Fe along the C7 axis at both sides. Calculated results show that strong electrostatic interactions between 2Li+ and Fe2- arising from Li electron transfer upon doping particularly provide a key driving force for stabilizing this charge-transfer structure. The bonding pattern of the teetotum can be understood from the hollow cylinder model (HCM). TD-DFT calculations demonstrate that this cluster can also be regarded as a useful material for transparent optoelectronic devices. Furthermore, the Li2FeB14 superatom can be used as a building block for making boron-based nanowires with metallic character. Replacement of Li atoms by Mg atoms was also found to lead to nanowires.

Spherical trihedral metallo-borospherenes

The discovery of borospherenes unveiled the capacity of boron to form fullerene-like cage structures. While fullerenes are known to entrap metal atoms to form endohedral metallofullerenes, few metal atoms have been observed to be part of the fullerene cages. Here we report the observation of a class of remarkable metallo-borospherenes, where metal atoms are integral parts of the cage surface. We have produced La₃B₁₈^– and Tb₃B₁₈^– and probed their structures and bonding using photoelectron spectroscopy and theoretical calculations. Global minimum searches revealed that the most stable structures of Ln₃B₁₈^– are hollow cages with D_3h symmetry. The B₁₈-framework in the Ln₃B₁₈^– cages can be viewed as consisting of two triangular B₆ motifs connected by three B₂ units, forming three shared B₁₀ rings which are coordinated to the three Ln atoms on the cage surface. These metallo-borospherenes represent a new class of unusual geometry that has not been observed in chemistry heretofore.

Borospherenes are the boron-based analogs of fullerene cages. Here, the authors report a class of Ln₃B₁₈^– metallo-borospherenes with unusual spherical trihedron geometry, in which the lanthanide atoms surprisingly form a part of the cage surface.

Structure and bonding of molecular stirrers with formula B7M2- and B8M2 (M = Zn, Cd, Hg)

In this work, we systematically explored clusters with formula B₇M₂^- and B₈M₂ (M = Zn, Cd, Hg). The putative global minima are formed by an M₂ dimer and a disk-shaped boron wheel. Moreover, the chemical bonding analysis revealed that charge transfer from the metal atoms to the boron motifs resulted in (B₇)^3-(M₂)²⁺ and (B₈)^2-(M₂)²⁺ complexes with double (σ + π) aromatic boron wheels and a single bond for the metallic dimer. Above all, the computed rotational barriers of the M-M fragment with respect to the boron disk and molecular dynamics simulations indicate a virtually barrierless spin, resembling a magnetic stirrer on a baseplate.

Perfect cubic La-doped boron clusters La6&[La@B24]+/0 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 D_8h La₂B₈ (1) and triple-decker C_2v La₃B₁₄⁻ (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 O_h La₆&[La@B₂₄]⁺ (3, ¹A_1g) and O_h La₆&[La@B₂₄] (4, ²A_2g) which appear to be the embryos of the metallic one-dimensional La₁₀B₃₂ (5) nanowire, two-dimensional La₃B₁₀ (6) nanosheet, and three-dimensional LaB₆ (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 La₆&[La@B₂₄]^+/0 (3/4) clusters, while the 1D La₁₀B₃₂ (5), 2D La₃B₁₀ (6), and 3D LaB₆ (7) crystals exhibit mainly electrostatic interactions between the trivalent La centers and cubic B₂₄ frameworks, with weak but discernible coordination contributions from La (5d) ← B (2p) back-donations. The IR and Raman spectra of La₆&[La@B₂₄]^+/0 (3/4) and band structures of La₁₀B₃₂ (5) and La₃B₁₀ (6) are computationally simulated to facilitate their future characterizations.

Perfect cubic clusters O_h La₆&[La@B₂₄]^+/0 are predicted at first-principles levels to be the embryos of 1D La₁₀B₃₂, 2D La₃B₁₀, and 3D LaB₆ lanthanide boride nanomaterials in a bottom-up approach.

Inverse sandwich complexes of B7M2−, B8M2, and B9M2+ (M = Zr, Hf): the nonclassical M–M bonds embedded in monocyclic boron rings

Three delocalized σ orbitals of the boron rings are perpendicularly mixed with one negligible σ and two π bonds of the M₂ (M = Zr, Hf) motifs, giving rise to less pronounced and nonclassical bonding interactions between two short-contact M atoms.

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

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

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_9h La₂B₉^- (¹A'₁) (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_9d La₂[B₁₈] (³A_1g) (2), D_9d La₂[B₁₈]^2- (¹A_1g) (3), and C_2h La₂[B₂@B₁₈] (¹A_g) (4) which all contain a tubular B_n ligand (n = 18, 20) sandwiched by two La atoms at the two ends. In these novel clusters, La₂[B₂@B₁₈] (4) as a tubular molecular rotor with the smallest core-shell structure reported to date in boron-based nanoclusters possesses a B₂-bar rotating constantly and almost freely inside the B₁₈ tube around it at room temperature. Detailed bonding analyses indicate that these complexes are stablized by effective (d-p)σ, (d-p)π, and (d-p)δ coordination interactions between the La centers and B_n bi-decker ligand. Six multi-center fluxional σ-bonds between the B₂-core and B₁₈ tube in La₂[B₂@B₁₈] (4) are found to be responsible for its unique fluxional behaviors. The IR and Raman spectra of the concerned species are simulated to facilitate their experimental characterization.

Exceptionally Complex Electronic Structures of Lanthanide Oxides and Small Molecules

Lanthanide (Ln) oxide clusters and molecular systems provide a bottom-up look at the electronic structures of the bulk materials because of close parallels in the patterns of Ln 4f^N subshell occupancy between the molecular and bulk Ln₂O₃ size limits. At the same time, these clusters and molecules offer a challenge to the theory community to find appropriate and robust treatments for the 4f^N patterns across the Ln series. Anion photoelectron (PE) spectroscopy provides a powerful experimental tool for studying these systems, mapping the energies of the ground and low-lying excited states of the neutral relative to the initial anion state, providing spectroscopic patterns that reflect the Ln 4f^N occupancy. In this Account, we review our anion PE spectroscopic and computational studies on a range of small lanthanide molecules and cluster species. The PE spectra of LnO^- (Ln = Ce, Pr, Sm, Eu) diatomic molecules show spectroscopic signatures associated with detachment of an electron from what can be described as a diffuse Ln 6s-like orbital. While the spectra of all four diatomics share this common transition, the fine structure in the transition becomes more complex with increasing 4f occupancy. This effect reflects increased coupling between the electrons occupying the corelike 4f and diffuse 6s orbitals with increasing N. Understanding the PE spectra of these diatomics sets the stage for interpreting the spectra of polyatomic molecular and cluster species. In general, the results confirm that the partial 4f^N subshell occupancy is largely preserved between molecular and bulk oxides and borides. However, they also suggest that surfaces and edges of bulk materials may support a low-energy, diffuse Ln 6s band, in contrast to bulk interiors, in which the 6s band is destabilized relative to the 5d band. We also identify cases in which the molecular Ln centers have 4f^N+1 occupancy rather than bulklike 4f^N, which results in weaker Ln-O bonding. Specifically, Sm centers in mixed Ce-Sm oxides or in Sm_xO_y^- (y ≤ x) clusters have this higher 4f^N+1 occupancy. The PE spectra of these particular species exhibit a striking increase in the relative intensities of excited-state transitions with decreasing photon energy (resulting in lower photoelectron kinetic energy). This is opposite of what is expected on the basis of the threshold laws that govern photodetachment. We relate this phenomenon to strong electron-neutral interactions unique to these complex electronic structures. The time scale of the interaction, which shakes up the electronic configuration of the neutral, increases with decreasing electron momentum. From a computational standpoint, we point out that special care must be taken when considering Ln cluster and molecular systems toward the center of the Ln series (e.g., Sm, Eu), where treatment of electrons explicitly or using an effective core potential can yield conflicting results on competing subshell occupancies. However, despite the complex electronic structures associated with partially filled 4f^N subshells, we demonstrate that inexpensive and tractable calculations yield useful qualitative insight into the general electronic structural features.

La3B14-: an inverse triple-decker lanthanide boron cluster

We report the observation of the first inverse triple-decker complex in a tri-lanthanide-doped boron cluster. Photoelectron spectroscopy of La₃B₁₄^- reveals well-resolved photodetachment transitions. Quantum chemical studies show that the most stable structure of the La₃B₁₄^- cluster exhibits a tilted La-B₈-La-B₈-La inverse triple-decker structure with two conjoined B₈ rings sharing a pair of B atoms due to strong inter-layer B-B bonding. The tilted structure enhances both B-B and B-La bonding, resulting in a highly stable inverse triple-decker structure. Theoretical calculations further show that multi-decker conjoined structures are viable as a new class of 1D lanthanide boron nanostructures.

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

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