Boron-based Clusters Research Articles

CB11S3+: A Triangular Boron-Based Cluster with One Planar Tetracoordinate Carbon at Its Edge.

Boron-based clusters containing planar tetracoordinate carbon (ptC) are unique and scarce. Isoelectronic-replacing after proper vulcanization is an effective strategy to obtain the ptC based on the B12 cluster. Herein we report computational evidence for a ternary CB11S3+ (C2v, 1A1) cluster, which possesses a concentric double-triangle structure containing one ptC atom at the peripheral edge. The unbiased structural explorations of potential energy surfaces and high-level CCSD(T) calculations indicate that the ptC CB11S3+ cluster is a true global minimum. Born-Oppenheimer molecular dynamics (BOMD) simulations reveal that it is dynamically stable against isomerization and decomposition. Chemical bonding analysis reveals that three delocalized π bonds endow the π aromaticity to the CB11 unit of CB11S3+. In addition, the strong S → B π back-bonding is also conducive to the stability of CB11S3+. The current findings offer opportunities for further boron-based ptC clusters.

Structural evolution and electronic properties of medium-sized boron clusters doped with selenium

The exploration of boron-doped clusters' structures and properties significantly advances our understanding of nanomaterials at the microscopic level. This study employs density functional theory to systematically investigate the structural evolution and electronic properties of singly Se-doped boron-based clusters, SeBn− (n = 16–24). The dopant atom exhibits a distinct preference for positioning outside the Bn−/0 framework, mirroring the arrangement found in the corresponding pure boron skeleton. Predicting the structural and electronic properties of doped clusters using simulated photoelectron spectra. Notably, magic clusters SeB19− and SeB23− are identified, showcasing remarkable relative stability. Furthermore, SeB24− is suggested to potentially exist as a pseudo-tubular isomer, representing a unique three-dimensional structure. The natural population analysis uncovers an electron transfer from Se to B atoms. The electronic localization function highlights substantial electron delocalization in SeBn−. Additionally, isochemical shielding surface analysis reveals pronounced aromaticity in the pseudo-tubular SeB24− isomer, thereby significantly enhancing the overall stability of the clusters.

Geometry and electronic structures of rare earth-doped boron-based clusters <inline-formula><tex-math id="M2">\begin{document}$ {\text{REB}}_n^ - $\end{document}</tex-math></inline-formula> (RE = La, Sc; <i>n</i> = 6, 8)

Rare earth doped boron clusters have attracted much attention due to their special optical, electrical and magnetic properties. The geometric structures, stability, electronic properties and aromaticity of negative rare earth doped boron clusters <inline-formula><tex-math id="M11">\begin{document}$ {\text{REB}}_n^ - $\end{document}</tex-math></inline-formula> (RE = La, Sc; <i>n</i> = 6, 8) are investigated with the artificial bee colony algorithm combined with density functional theory calculations at the PBE0/RE/SDD//B/6-311+G* level of theory. Calculations show that the ground state structures of <inline-formula><tex-math id="M12">\begin{document}$ {\text{REB}}_n^ - $\end{document}</tex-math></inline-formula> (RE = La, Sc; <i>n</i> = 6, 8) are all of <i>C</i><sub>2</sub> symmetry, and the doped lanthanide atom is located in a “boat-shaped” structure at the top center. By comparing with the experimental photoelectron spectra, it is confirmed that the ground state structure of <inline-formula><tex-math id="M13">\begin{document}$ {\text{LaB}}_{8}^ - $\end{document}</tex-math></inline-formula> is a “zither-like” three-dimensional structure, and the ground state structure of <inline-formula><tex-math id="M14">\begin{document}$ {\text{ScB}}_{8}^ - $\end{document}</tex-math></inline-formula> is an “umbrella” structure with <i>C</i><sub>7<i>V</i></sub> symmetry formed by the scandium atom at the “umbrella handle”. The electron localization between RE—B is not as good as that between B—B. The simulated photoelectron spectra have similar spectral characteristics to the experimental results. The lowest energy structures of <inline-formula><tex-math id="M15">\begin{document}$ {\text{LaB}}_{6}^ - $\end{document}</tex-math></inline-formula> and <inline-formula><tex-math id="M16">\begin{document}$ {\text{ScB}}_{6}^ - $\end{document}</tex-math></inline-formula> are <i>σ</i>-π double aromatic clusters, and the structures exhibit aromaticity. The density of states of low-energy isomers shows that the open shell <inline-formula><tex-math id="M17">\begin{document}$ {\text{ScB}}_{8}^ - $\end{document}</tex-math></inline-formula> density of states spectrum exhibits spin polarization phenomenon, which is expected to assemble magnetic material components. These studies contribute to understanding the evolution of structure and properties of nanomaterials, and provide important theoretical support for designing nanomaterials with practical value.

Enhanced reversal of ABCG2-mediated drug resistance by replacing a phenyl ring in baicalein with a meta-carborane.

Success of chemotherapy is often hampered by multidrug resistance. One mechanism for drug resistance is the elimination of anticancer drugs through drug transporters, such as breast cancer resistance protein (BCRP; also known as ABCG2), and causes a poor 5-year survival rate of human patients. Co-treatment of chemotherapeutics and natural compounds, such as baicalein, is used to prevent chemotherapeutic resistance but is limited by rapid metabolism. Boron-based clusters as meta-carborane are very promising phenyl mimetics to increase target affinity; we therefore investigated the replacement of a phenyl ring in baicalein by a meta-carborane to improve its affinity towards the human ABCG2 efflux transporter. Baicalein strongly inhibited the ABCG2-mediated efflux and caused a fivefold increase in mitoxantrone cytotoxicity. Whereas the baicalein derivative 5,6,7-trimethoxyflavone inhibited ABCG2 efflux activity in a concentration of 5 μm without reversing mitoxantrone resistance, its carborane analogue 5,6,7-trimethoxyborcalein significantly enhanced the inhibitory effects in nanomolar ranges (0.1 μm) and caused a stronger increase in mitoxantrone toxicity reaching similar values as Ko143, a potent ABCG2 inhibitor. Overall, in silico docking and in vitro studies demonstrated that the modification of baicalein with meta-carborane and three methoxy substituents leads to an enhanced reversal of ABCG2-mediated drug resistance. Thus, this seems to be a promising basis for the development of efficient ABCG2 inhibitors.

Structural evolution and bonding characteristics of neutral Cs2B n clusters

In this study, the unbiased crystal structure analysis by particle swarm optimisation (CALYPSO) combined with the density functional theory (DFT) method were used to perform a structure search for neutral Cs2B n (n = 1–12) clusters, as well as the corresponding global minimum of the potential energy surface were determined. The ground-state structures of the Cs2B n (n = 1–12) clusters have four types of geometries: planar, arch bridge, double pyramid, and shield shape. Subsequently, the relative stability of the ground-state structures was systematically analysed based on three effective rules, which indicates that the C7V Cs2B8 with a double pyramid structure has unusual stability. The results of the molecular orbitals (MOs) and density of states analysis (DOS) show that the B-2s and 2p atomic orbitals of Cs2B8 are the main components of MOs. In addition, from the results of adaptive natural density partitioning (AdNDP) and localised orbital locator (LOL), it can be seen that the excellent stability of Cs2B8 originates from the delocalised large π bonds with aromaticity and the s–p bonding of B–B. Our results reveal the structural growth mode of Cs2B n clusters and enrich the chemical bonding properties of boron-based clusters.

Electron-compensation: a valid strategy for chemically stabilizing boron-based clusters with hypercoordinate centres.

To eliminate the chemical instability caused by the electron-deficiency of boron, two types of usual dative π bonds are recommended for compensating boron atoms in the computational design of boron-based clusters having hypercoordinate centres, which can lead to unusually stable targets for synthesis in the condensed phase.

Structural evolution in boron-based clusters B5Aln0/-/+ (n = 1–4): Al atoms transition from the periphery of the planar W-shaped B5 ring to the vertex of the bipyramid

The structures and properties of binary boron-aluminum B5Aln0/-/+ (n = 1–4) clusters have been systematically explored using the density functional theory method at the B3LYP/6–311+G(d) level and the coupled cluster method at the CCSD(T)/6–311+G(2df)//B3LYP/6–311+G(d) level. Lowest-energy structures, stabilities, growth behaviors and chemical bonding of these clusters were analyzed. Our results show that when the number of doped Al atoms is one or two, the Al atoms are located at the periphery, and the host B5 cluster preferentially forms a W-shape core, which is only slightly affected by the Al atoms. When there are three or four Al atoms, the B5Aln0/-/+ (n = 3, 4) clusters have their lowest energy structures preferentially in capped bipyramid configurations. Neutral B5Aln (n = 1, 3) clusters are somewhat more stable than their neighboring n clusters, while anionic and cationic B5Aln-/+ (n = 2, 4) clusters tend to be somewhat more stable. We also simulated the infrared (IR) spectrum and photoelectron spectroscopy (PES) of these clusters for future experimental comparison. Adaptive natural density partitioning (AdNDP) analysis shows that a variety of delocalized multicenter bonds appear in these clusters, which may enhance the stability of the clusters.

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.

Impressive capacity of the B7− and V2B7 clusters for CO2 capture

A teetotum form is found as the global equilibrium structure of V2B7+/0/− clusters. Formation of a bimetallic configuration and aromaticity are found to be dominant factors stabilizing the doped V2B7+/0/− teetotum structures. Magnetic ring current maps indicate the V2B7+ cation as doubly aromatic. Exploration on the interactions of one and two CO2 molecules with the pure B7− and doped VB7 and V2B7+/0/− clusters shows that these boron-based clusters can capture CO2 and transform it through a dissociative adsorption mechanism. The pure B7− anion cluster performs even better than the doped clusters in CO2 capture ability.

AUTOMATON: A Program That Combines a Probabilistic Cellular Automata and a Genetic Algorithm for Global Minimum Search of Clusters and Molecules.

A novel program for the search of global minimum structures of atomic clusters and molecules in the gas phase, AUTOMATON, is introduced in this work. This program involves the following: first, the generation of an initial population, using a simplified probabilistic cellular automaton method, which allows easy control of the adequate distribution of atoms in space; second, the fittest individuals are selected to evolve, through genetic operations (mating and mutations), until the best candidate for a global minimum surfaces. In addition, we propose a simple way to build the descendant structures by establishing a ranking of genes to be inherited. Thus, by means of a chemical formula checker procedure, genes are transferred to the offspring, ensuring that they always have the appropriate type and number of atoms. It is worth noting that a fraction of the fittest group is subject to mutation operations. This program also includes algorithms to identify duplicate structures: one based on geometric similarity and another on the similar distribution of atomic charges. The effectiveness of the program was evaluated in a group of 45 molecules, considering organic and organometallic compounds (benzene, cyclopentadienyl anion, and ferrocene), Zintl ion clusters [Sn9- m- nGe mBi n](4- n)- ( n = 1-4 and m = 0-(9- n)), star-shaped clusters (Li7E5+, E = BH, C, Si, Ge) and a variety of boron-based clusters. The global minimum and the lowest-energy isomers reported in the literature were found for all the cases considered in this article. These results successfully prove AUTOMATON's effectiveness on the identification of energetically preferred structures of a wide variety of chemical species.

Organomimetic clusters: Precision in 3D.

Biomimetic molecules that can be easily tailored offer numerous opportunities. Now, boron-based clusters have been shown to be excellent biomimetics. The ease with which the cluster surfaces can be modified stands to change how chemists might go about preparing materials for imaging, drug delivery and other applications.

Alex Spokoyny: Inorganic Architect

Many scientists say that as a kid they always wanted to be a scientist. That wasn’t the case for UCLA’s Alex Spokoyny. “Growing up in Russia in a family where my mom was a biologist and my dad was a physicist, there was always pressure to be a scientist,” Spokoyny says. Although he liked analyzing things, he was also a natural contrarian who would argue with his parents. Still, Spokoyny eventually compromised with his mom and dad, agreeing to attend a science high school in Russia. While there, a funny thing happened: He got hooked on chemistry. “There’s a Russian saying: ‘Sometimes the appetite comes during the meal.’ It was really the experimental side of chemistry that captured my interest.” Spokoyny has been hungry ever since. From his undergraduate days through his postdoc, Spokoyny has studied a vast array of chemical systems, including boron-based clusters with therapeutic potential, metal-organic framework

Aromatic character of planar boron-based clusters revisited by ring current calculations.

The planarity of small boron-based clusters is the result of an interplay between geometry, electron delocalization, covalent bonding and stability. These compounds contain two different bonding patterns involving both σ and π delocalized bonds, and up to now, their aromaticity has been assigned mainly using the classical (4N + 2) electron count for both types of electrons. In the present study, we reexplored the aromatic feature of different types of planar boron-based clusters making use of the ring current approach. B3(+/-), B4(2-), B5(+/-), B6, B7(-), B8(2-), B9(-), B10(2-), B11(-), B12, B13(+), B14(2-) and B16(2-) are characterized by magnetic responses to be doubly σ and π aromatic species in which the π aromaticity can be predicted using the (4N + 2) electron count. The triply aromatic character of B12 and B13(+) is confirmed. The π electrons of B18(2-), B19(-) and B20(2-) obey the disk aromaticity rule with an electronic configuration of [1σ(2)1π(4)1δ(4)2σ(2)] rather than the (4N + 2) count. The double aromaticity feature is observed for boron hydride cycles including B@B5H5(+), Li7B5H5 and M@BnHn(q) clusters from both the (4N + 2) rule and ring current maps. The double π and σ aromaticity in carbon-boron planar cycles B7C(-), B8C, B6C2, B9C(-), B8C2 and B7C3(-) is in conflict with the Hückel electron count. This is also the case for the ions B11C5(+/-) whose ring current indicators suggest that they belong to the class of double aromaticity, in which the π electrons obey the disk aromaticity characteristics. In many clusters, the classical electron count cannot be applied, and the magnetic responses of the electron density expressed in terms of the ring current provide us with a more consistent criterion for determining their aromatic character.

How can carbon favor planar multi-coordination in boron-based clusters? Global structures of CB(x)E(y)(2-) (E = Al, Ga, x + y = 4).

With the high preference in forming multi-center bonding, boron has been a miracle ligand in constructing diverse planar multi-coordinate (pM) (tetra/hyper) species. Unfortunately, the boron ligand usually dislikes encompassing a pM carbon (pMC) due to the high competition with pM boron (pMB), which makes the realization of boron-based pMC very difficult and quite challenging. Herein, we propose a strategy that by means of cooperative doping and charge-compensation, we can successfully improve and tune the stability of pMC relative to pMB for CB4(2-). In the free CBxEy(2-) (E = Al/Ga) species, ptC is thermodynamically less stable than the global ptB in mono- and di-substituted systems, in agreement with the results of Boldyrev and Wang. However, the thermodynamic preference of pMC increases along with the Al/Ga-doping. The pMC species can be further stabilized by the introduction of the alkaline-earth counterion (Mg(2+)). CB2E2Mg (E = Al, Ga) designed in the present study represents the first successful design of a boron-based planar penta-coordinate carbon (ppC) structures as the global minima. The strategy proposed in this study should be useful in the manipulation of competition between exotic pMC and pMB in B-based systems.

Rhodathiaborane reaction cycles driven by C2H4 and H2: synthesis and characterization of [(H)2(PPh3)RhSB8H7(PPh3)] and [(η(2)-C2H4)(PPh3)RhSB8H7(PPh3)

New 10-vertex rhodathiaboranes are reported to exhibit reversible reaction chemistry leading to the formation of stoichiometric cycles driven by oxidation/reduction chemistry of the polyhedral boron-based clusters with ethelyne and dihydrogen.

3-Pyridylacetonitrile-ligated 11-vertex rhodathiaboranes: synthesis, characterization, and X-ray crystal structure

The reaction between the 11-vertex rhodathiaborane [8,8-(PPh3)2-nido-8,7-RhSB9H10] (1) and 3-pyridylacetonitrile affords the hydrorhodathiaborane [8,8,8-(PPh3)2H-9-(3-Py-CH2CN)-nido-8,7-RhSB9H9] (2) in good yield. Treatment of this cluster with ethylene leads to the formation of red, [1,1-(PPh3)(η2-C2H4)-3-(3-Py-CH2CN)-closo-1,2-RhSB9H8] (3). Both 11-vertex polyhedral boron-based clusters have been characterized by multielement NMR spectroscopy. In addition, (3) has been analyzed by single-crystal X-ray diffraction analysis and is only the second ethylene-ligated metalla-heteroborane to be characterized in the solid state. The molecular structure of this cluster is based on an octadecahedron. In the crystal lattice, the individual clusters form layers supported by short edge-to-face π-interactions between the phenyl rings of neighboring molecules.

B30H8, B39H9 2−, B42H10, B48H10, and B72H12: polycyclic aromatic snub hydroboron clusters analogous to polycyclic aromatic hydrocarbons

Calculations performed at the ab initio level using the recently reported planar concentric π-aromatic B(18)H(6)(2+)(1) [Chen Q et al. (2011) Phys Chem Chem Phys 13:20620] as a building block suggest the possible existence of a new class of B(3n)H(m) polycyclic aromatic hydroboron (PAHB) clusters-B(30)H(8)(2), B(39)H(9)(2-)(3), B(42)H(10)(4/5), B(48)H(10)(6), and B(72)H(12)(7)-which appear to be the inorganic analogs of the corresponding C(n)H(m) polycyclic aromatic hydrocarbon (PAHC) molecules naphthalene C(10)H(8), phenalenyl anion C(13)H(9)(-), phenanthrene/anthracene C(14)H(10), pyrene C(16)H(10), and coronene C(24)H(12), respectively, in a universal atomic ratio of B:C = 3:1. Detailed canonical molecular orbital (CMO), adaptive natural density partitioning (AdNDP), and electron localization function (ELF) analyses indicate that, as they are hydrogenated fragments of a boron snub sheet [Zope RR, Baruah T (2010) Chem Phys Lett 501:193], these PAHB clusters are aromatic in nature, and exhibit the formation of islands of both σ- and π-aromaticity. The predicted ionization potentials of PAHB neutrals and electron detachment energies of small PAHB monoanions should permit them to be characterized experimentally in the future. The results obtained in this work expand the domain of planar boron-based clusters to a region well beyond B(20), and experimental syntheses of these snub B(3n)H(m) clusters through partial hydrogenation of the corresponding bare B(3n) may open up a new area of boron chemistry parallel to that of PAHCs in carbon chemistry.

Boron in ab initio calculations

Based on ab initio quantum chemical Hartee–Fock (HF) approximations, density functional theory (DFT) and LMTO-ASA methods, we have determined the geometric and electronic structures of atomic-scaled boron clusters, surfaces and nanotubes. The starting structures were constructed with the help of an `Aufbau principle', previously proposed for boron clusters. In contrast to the semi-conducting α-boron, the boron sheets show a metallic behavior similar to graphite. Boron-based clusters could also serve as models to simulate silicon-based materials.