Polyhedral Borane Research Articles

Closomers: Versatile Monodisperse Molecular Nanoparticles

This review is dedicated to a relatively new, but nevertheless very quickly evolving field of chemistry – closomer chemistry. Closomers are complex organoelement molecules built on a [B12]2– icosahedral platform that have the combined benefits of the symmetry and stability of polyhedral boranes and the diversity of organic chemistry. On a material level, closomers can be considered as monodisperse molecular nanoparticles because of their size, spherical symmetry, and ease of adjusting the arm length. The ease of twelvefold side‐chain functionalization and the possibility of heterosubstitution makes closomers very useful medical tools for targeted and traceable imaging and drug delivery applications.

Nitrogen heterocyclic salts of polyhedral borane anions: from ionic liquids to energetic materials

Synthesis, structure, and physical properties of nitrogen heterocyclic salts of the polyhedral borane anions (closo-decaborate [B10H10]2–, closo-dodecaborate [B12H12]2–, carba-closo-dodecaborate [CB11H12]–, and 3,3'-cobaltabis(1,2-dicarbollide) [3,3'-Co(1,2-C2B9H11)2]–) and their derivatives, including their potential application as ionic liquids and energetic materials, are reviewed.

Carborane Derivative Conjugated with Gold Nanoclusters for Targeted Cancer Cell Imaging.

Polyhedral borane derivatives have been utilized in the treatment of boron neuron capture therapy (BNCT) for brain glioma, and much attention has been paid to search excellent biocompatible boron-rich composites for effective cancer BNCT therapy. In this study, we have exploited the self-assembly of the gold nanoclusters with carborane amino derivatives (GNCs-CB) for the precise bioimaging of cancer cells and targeted delivery of this carborane compound to the tumors. Our observations demonstrate that the GNCs-CB can readily realize accurate tumor imaging and long-term accumulation in tumor sites by EPR effect and nanometer size effect, and thus efficiently implement tumor-targeting delivery of the carborane derivative and facilitate the real-time fluorescent visualization monitoring of the carborane targeted delivery process. This makes it possible to realize the accurate location of the tumor by the carborane derivative and reduce the damage to normal tissues in the process of boron neutron capture therapy through imaging guided treatment.

Catalyst‐Free Polyhydroboration of Dodecaborate Yields Highly Photoluminescent Ionic Polyarylated Clusters

AbstractThe incorporation of polyhedral boranes into novel photoluminescent materials is an area with increasing interest. While the neutral carboranes have been widely investigated for this purpose, the dodecaborate ion has received much less attention. Herein we report a significant expansion to the scope of substitution reactions for the dodecaborate ion, whereby this cluster was observed to react directly with a wide range of arenes in a step‐wise and controlled manner. In the products of these reactions, the dodecaborate ion serves as a core upon which up to nine mono‐ or polycyclic aromatic hydrocarbon ligands are exohedrally bonded. Spectroscopic evidence suggests the presence of conjugation between the π systems of the aryl ligands and the dodecaborate core, resulting in materials which exhibit high solution‐phase photoluminescence, as well as molar absorptivities and Stokes shifts that are significantly larger than those of the free arenes from which they were derived. We propose that this broad reactivity is a valuable synthetic tool for the incorporation of polyhedral boron into novel organic structures.

Catalyst-Free Polyhydroboration of Dodecaborate Yields Highly Photoluminescent Ionic Polyarylated Clusters.

The incorporation of polyhedral boranes into novel photoluminescent materials is an area with increasing interest. While the neutral carboranes have been widely investigated for this purpose, the dodecaborate ion has received much less attention. Herein we report a significant expansion to the scope of substitution reactions for the dodecaborate ion, whereby this cluster was observed to react directly with a wide range of arenes in a step-wise and controlled manner. In the products of these reactions, the dodecaborate ion serves as a core upon which up to nine mono- or polycyclic aromatic hydrocarbon ligands are exohedrally bonded. Spectroscopic evidence suggests the presence of conjugation between the π systems of the aryl ligands and the dodecaborate core, resulting in materials which exhibit high solution-phase photoluminescence, as well as molar absorptivities and Stokes shifts that are significantly larger than those of the free arenes from which they were derived. We propose that this broad reactivity is a valuable synthetic tool for the incorporation of polyhedral boron into novel organic structures.

Boron-Based (Nano-)Materials: Fundamentals and Applications

The boron (Z = 5) element is unique. Boron-based (nano-)materials are equally unique. Accordingly, the present special issue is dedicated to crystalline boron-based (nano-)materials and gathers a series of nine review and research articles dealing with different boron-based compounds. Boranes, borohydrides, polyhedral boranes and carboranes, boronate anions/ligands, boron nitride (hexagonal structure), and elemental boron are considered. Importantly, large sections are dedicated to fundamentals, with a special focus on crystal structures. The application potentials are widely discussed on the basis of the materials’ physical and chemical properties. It stands out that crystalline boron-based (nano-)materials have many technological opportunities in fields such as energy storage, gas sorption (depollution), medicine, and optical and electronic devices. The present special issue is further evidence of the wealth of boron science, especially in terms of crystalline (nano-)materials.

Ionic borohydride clusters for the next generation of boron thin-films: Nano-building blocks for electrochemical and refractory materials

Abstract

Stealth Amphiphiles: Self-Assembly of Polyhedral Boron Clusters.

This is the first experimental evidence that both self-assembly and surface activity are common features of all water-soluble boron cluster compounds. The solution behavior of anionic polyhedral boranes (sodium decaborate, sodium dodecaborate, and sodium mercaptododecaborate), carboranes (potassium 1-carba-dodecaborate), and metallacarboranes {sodium [cobalt bis(1,2-dicarbollide)]} was extensively studied, and it is evident that all the anionic boron clusters form multimolecular aggregates in water. However, the mechanism of aggregation is dependent on size and polarity. The series of studied clusters spans from a small hydrophilic decaborate-resembling hydrotrope to a bulky hydrophobic cobalt bis(dicarbollide) behaving like a classical surfactant. Despite their pristine structure resembling Platonic solids, the nature of anionic boron cluster compounds is inherently amphiphilic-they are stealth amphiphiles.

Deltahedra with holes: Structural preferences of supraicosahedral boranes

A systematic quantum chemical inquiry on supraicosahedral boranes (BnHn2−, n>12) reveals that larger polyhedral boranes no longer prefer pure deltahedral structures. In an attempt to avoid vertices that has six polyhedral neighbors (hex-caps), inevitable for larger deltahedra; they resort to faces other than triangles (squares and pentagons). These larger faces behave like holes rather than open faces as they obey the Wade’s n+1 rule for closo boranes. Further, these holes prefer to be isolated, similar to the pentagons in carbon fullerenes. The convex polyhedron, snub dodecahedron (B60H602−, I symmetry) derived from these structural preferences, is a promising candidate for experimental characterization. The larger size of borane snub balls allows the hosting of counter ions endohedrally and this increases the rigidity of the cluster. The structural principles deduced in this paper successfully explain the mystery behind the observation of square faces in some of the known supraicosahedral carboranes, and helps in understanding the complex MB66 type borides which are characterized to have a O symmetric snub frameworks. They also provide the fundamental information towards the design of borane based nanostructures.

Boron neutron capture therapy demonstrated in mice bearing EMT6 tumors following selective delivery of boron by rationally designed liposomes

The application of boron neutron capture therapy (BNCT) following liposomal delivery of a (10)B-enriched polyhedral borane and a carborane against mouse mammary adenocarcinoma solid tumors was investigated. Unilamellar liposomes with a mean diameter of 134 nm or less, composed of an equimolar mixture of cholesterol and 1,2-distearoyl-sn-glycero-3-phosphocholine and incorporating Na3[1-(2'-B10H9)-2-NH3B10H8] in the aqueous interior and K[nido-7-CH3(CH2)15-7,8-C2B9H11] in the bilayer, were injected into the tail veins of female BALB/c mice bearing right flank EMT6 tumors. Biodistribution studies indicated that two identical injections given 24 h apart resulted in tumor boron levels exceeding 67 µg/g tumor at 54 h--with tumor/blood boron ratios being greatest at 96 h (5.68:1; 43 µg boron/g tumor)--following the initial injection. For BNCT experiments, tumor-bearing mice were irradiated 54 h after the initial injection for 30 min with thermal neutrons, resulting in a total fluence of 1.6 × 10(12) neutrons per cm(2) (±7%). Significant suppression of tumor growth was observed in mice given BNCT vs. control mice (only 424% increase in tumor volume at 14 d post irradiation vs. 1551% in untreated controls). In a separate experiment in which mice were given a second injection/irradiation treatment 7 d after the first, the tumor growth was vastly diminished (186% tumor volume increase at 14 d). A similar response was obtained for mice irradiated for 60 min (169% increase at 14 d), suggesting that neutron fluence was the limiting factor controlling BNCT efficacy in this study.

Evaluation of the binding of polyhedral borane anions to representative proteins

The ability of three polyhedral borane anions, [B20H18]2−, [B20H17SH]4−, and [B20H19]3−, to bind to proteins was evaluated by measuring the total boron content using inductively coupled plasma-atomic emission spectroscopy after purification by gel electrophoresis. Results were correlated to the known chemical reactivity of the compounds as well as the reported murine biodistributions of the liposomally encapsulated sodium salts of each of the polyhedral borane anions. Qualitative reactions were performed with the [B20H18]2− anion to determine the potential reactivity with simple molecular building blocks.

Planar to 3D Transition in the B6Hy Anions

Potential energy surfaces of anionic B(6)H(y) clusters were sampled using the coalescence kick method. We found that the planar to three-dimensional transition occurs in this system when y = 4. This is an important discovery because this transition suggests a major structural change as a function of dehydrogenation for the stoichiometric B(n)H(n)(-) polyhedral boranes. We also found that the B(6)H(3)(-) global minimum structure has an optical isomer. The chemical bonding patterns revealed by the adaptive natural density partitioning (AdNDP) analysis explain the geometric structure of all clusters presented here. From our chemical bonding analysis, we concluded that the 2D-3D transition occurs at B(6)H(4)(-) because the addition of one extra hydrogen atom further destroys the network of the peripheral 2c-2e B-B σ-bonding, making planar structures less stable, and because the distorted octahedral structure provides some occupation of all s- and p-AOs of boron, avoiding the presence of any empty atomic orbitals. Theoretical vertical electron detachment energies (VDEs) were calculated for comparison with future experimental work.

Facile synthesis and regeneration of Mg(BH4)2by high energy reactive ball milling of MgB2

We report direct hydrogenation of MgB(2) in a planetary ball mill. Magnesium borohydride, Mg(BH(4))(2), and various polyhedral borane anion salts have been synthesized at pressures between 50 and 350 bar H(2) without the need for subsequent isothermal hydrogenation at elevated temperature and pressure. The obtained products release ∼4 wt% H(2) below 390 °C, and a major portion of Mg(BH(4))(2) transforms back to MgB(2) at around 300 °C, demonstrating the possibility of reversible hydrogen storage in an Mg(BH(4))(2)-MgB(2) system.

Discrete nanomolecular polyhedral borane scaffold supporting multiple gadolinium(III) complexes as a high performance MRI contrast agent.

An icosahedral closo-B(12)(2-) scaffold supports 12 copies of Gd(3+)-chelate held in close proximity with each other by suitable linkers which employ azide-alkyne click chemistry. This design is the first member of a new class of polyfunctional MRI contrast agents carrying a high payload of Gd(3+)-chelate in a sterically constrained configuration. The resulting contrast agent shows higher relaxivity values at high magnetic fields. MRI contrast agents currently in use are not as effective in this regard, presumably due to a lack of steric constraint of gadolinium centers and lower water exchange rates. In vivo MRI studies in mice show excellent contrast enhancement even at one-seventh of the safe clinical dose (0.04 mmol Gd/kg) for up to a 1 h exposure.

Quantifying the thermodynamic interactions of polyhedral boranes in solution to guide nanocomposite fabrication

The solubility of boron containing nanoparticles in a variety of solvents is quantified using static light scattering in conjunction with refractometry. Four polyhedral boranes were tested in this work, using refractometry to obtain dn/dc, while static light scattering quantifies A2. A2 obtained from these measurements was then used to calculate χ, the solute–solvent interaction parameter, and the Hildebrand solubility parameter, δ, which provides a quantifiable method to identify good solvents. Of the nanoparticles studied, 1,3-di-o-carboranylpropane is thermodynamically stable in toluene, with a χ less than 0.5, a solubility limit of 2.47 mg/mL, and all solutions remaining clear with no visible particle settling. For all of the particles tested, there was good correlation between the physical observations of the solutions, χ, and δ. For instance, lower values of χ correspond to a smaller radius of gyration (Rg). A list of suitable solvents based on δ is also presented.

Making Sense of Boron-Rich Binary Be–B Phases

There is much uncertainty in the literature about the structure of several Be-B phases between 20 and 33 atom % Be. We clarify the structural choices in this region of the phase diagram, proposing structural candidates obtained from a combination of chemical intuition and unbiased solid-state structure searches. In particular, we discuss the structural, dynamical, and electronic properties of the ground states of the BeB(2), BeB(3), and BeB(4) phases, as well as those of the complex (and superconducting) "BeB(2.75)" phase. For the latter, we find the polyhedral borane cluster electron-counting approach very useful to explain its electronic structure. We can also make sense of the partial and mixed occupancies in the structure by looking at the cavities in a parent structure. A Be(29)B(81) stoichiometry seems most reasonable for the ground state of this phase. The electronic structure points to a region of stability for three additional electrons per unit cell, a 1% difference in total electron count. For BeB(2), which is usually studied computationally in the AlB(2) structure type, we find several other structure types that are more stable, all essentially Zintl phases with 4-connected boron networks. New structure types are also predicted for BeB(3) and BeB(4) as well.

Synthesis and Investigation of [B20H17O(CH2)5]3–, a Novel Solvent Complex of the [B20H18]4–Ion

Reaction of the [trans-B(20)H(18)](2-) ion with the n-butoxide ion, formed in situ from reaction of n-butanol and NaH, in tetrahydropyran (THP) produces in good yield an unexpected and isolable solvent-coordinated polyhedral borane anion, [ae-B(20)H(17)O(CH(2))(5)](3-). The anticipated product of nucleophilic attack, [ae-B(20)H(17)On-Bu](4-), is not observed under the reaction conditions. The solvent-coordinated product is also formed in the presence of either ethoxide or carbamate ion but is not observed if the ethoxide or carbamate ion is not present in stoichiometric amounts. In the presence of the n-butanethiol anion, the coordinated THP ring undergoes a ring-opening reaction, yielding the [ae-B(20)H(17)O(CH(2))(5)Sn-Bu](4-) anion. Ring opening is also observed in the presence of the ethoxide ion in refluxing THP. Isolation of the previously proposed analogous solvent-coordinated tetrahydrofuran (THF) product, [ae-B(20)H(17)O(CH(2))(4)](3-), was unsuccessful; however, the product resulting from ring opening of THF by the n-butanethiol anion is reported.

Polyhedral borane derivatives: Unique and versatile structural motifs

Compounds with polyhedral borane moieties have demonstrated numerous unique properties for a variety of applications, including nanoelectronics, drug delivery vehicles, and live cell imaging. Polyhedral boranes are good pharmacophore analogs of carbocycles because polyhedral boranes are inherently insensitive to many undesirable enzymatic metabolic transformations typical for a majority of aromatic compounds. The defined shape, low molecular volume, and high 3D symmetry of the surface are useful for the application of the polyhedral borane scaffolds as universal and convenient spacers for the modular assembly approach with a controllable predisposition of peripheral groups.

Polyhedral Borane Analogues of the Benzynes and Beyond: Bonding in Variously Charged B12H10 Isomers

The effect of removing two protons, hydrogen atoms, or hydrides from the stable icosahedral B(12)H(12)(2-) is investigated theoretically. The resulting B(12)H(10)(q) (q = 4-, 2-, 0) isomers show interesting and understandable bond distance and stability variations, as well as special deformations associated with the apex-ring configuration typical of the underlying polyhedron. The dianions are analogous to o-, m-, and p-benzyne and have the special feature of distinct singlet and triplet states not far removed from each other in energy.

A convenient route to diversely substituted icosahedral closomer nanoscaffolds.

The design and synthesis of icosahedral polyhedral borane closomer motifs based upon carbonate and carbamate anchoring groups for biomedical applications are described. Dodecacarbamate closomers containing easily accessible groups of interest at their linker termini were synthesized via activation of the B-OH vertices as aryl carbonates and their subsequent reaction with primary amines. Novel dodecacarbonate closomers were successfully synthesized for the first time by reacting [closo-B(12)(OH)(12)](2-) with an excess of respective aryl chloroformates, utilizing relatively short reaction times, mild conditions and simple purification strategies, all of which had previously presented difficulties in closomer chemistry. This methodology for the 12-fold degenerate synthesis of carbonate and carbamate closomers will greatly facilitate further exploration of closomers as monodisperse nanomolecular delivery platforms.