Stability and Three-Dimensional Aromaticity of closo-Monocarbaborane Anions, CB(n)()(-)(1)H(n)(-), and closo-Dicarboranes, C(2)B(n)()(-)(2)H(n)()

A Pd-catalyzed route to carborane-fused boron heterocycles

Due to the expanding applications of icosahedral carboranes in medicinal and materials chemistry research, their functionalizations have become one of the central themes in boron-rich cluster chemistry. Although several strategies for incorporating nitrogen-containing nucleophiles on a single boron vertex of the icosahedral carboranes (C2B10H12) have been developed, methods for preparing clusters with vicinal B-N moieties are still lacking. The steric bulk of icosahedral carboranes and disparate electronic and steric nature of the N-containing groups have rendered the vicinal diamination challenging. In this article, we show how a developed Pd-catalyzed process is used to incorporate an array of NH-heterocycles, anilines, and heteroanilines with various electronic and steric profiles onto the vicinal boron vertices of a meta-carborane cluster via sequential or one-pot fashion. Importantly, oxidative cyclizations of the cross-coupling products with indoles and pyrroles appended to boron vertices generate a previously unknown class of all-boron-vertex bound carborane-fused six- and seven-membered ring heterocycles. Photophysical studies of the meta-carborane-fused heterocycles show that these structures can exhibit luminescence with high quantum yields and are amenable to further manipulations.

Selective B(3)-H Activation Affording Multinuclear Ir(III) Complexes with (o-Carboranyl)dithioester Ligands

A method was developed to link two or three o-carborane moieties to form a series of carboranyl dithioester bridging ligands via in situ substitution of haloalkanes by tetraphenylphosphonium carboranyldithiocarboxylates. Based on these ligands, direct B-H activation without the assistance of Ag(I) and alkali was successfully achieved with half-sandwich Ir(III) substrate [Cp*IrCl2]2 to yield corresponding bimetallic or trimetallic complexes. Single crystal structure analyses of the B-H activated complexes and corresponding SnCl2-inserted derivatives confirm the selective B(3)-H activation in these complexes.

Chlorinated polyhedral selenaboranes revisited by joint experimental/computational efforts: the formation of closo-1-SeB9Cl9 and the crystal structure of closo-SeB11Cl11

The recent success in the formation of chlorinated telluraboranes and the reactivities of pnictogenaboranes prompted us to re-examine the vacuum co-pyrolysis of B2Cl4 with Se2Cl2 at various molar ratios and temperatures in order to search for the generation of other polyhedral selenaboranes than closo-SeB5Cl5 (1a) and closo-SeB11Cl11 (1b), the latter being observed earlier. Interestingly, a new compound with the elemental composition SeB9Cl9 (2) was detected, this time by high- and low-resolution mass spectrometry. Further characterization by 1- and 2-D 11B-NMR spectroscopy suggests that 2 should adopt a closed bicapped square-antiprismatic geometry with selenium at the apical position. Moreover, vacuum sublimation gave suitable crystals of 1b, which were subjected to single-crystal X-ray structure determination. Crystallographic data analysis confirmed that 1b, consistent with its 26 skeletal electron count, adopts a distorted icosahedral structure close to the symmetry of C5v. Computations at the DFT-D3 level have revealed that 33% of the total computed binding motifs in the grown 1b crystals are due to the very strong chalcogen bonding. Moreover, SAPT decomposition has shown that the bonding motifs in the crystals are stabilized mainly by dispersion and electrostatic terms. Homodecoupling and high resolution 11B NMR and 77Se NMR experiments have resolved both coupling constants 1J(11B11B) and 1J(77Se11B) as well as the 77Se chemical shift of 1a and 1b, which are in reasonable agreement with the corresponding computed values. The computed 11B chemical shifts of 2 were determined by the well-established DFT/GIAO/NMR structural tool based on its B3LYP/6-311+G** internal coordinates. They agree well with the experimental values and provide a good representation of the molecular structure of 2 in solution. The extraordinary downfield 11B NMR chemical shift of B(10) in 2 has been ascribed to the intensive paramagnetic contribution to the shielding tensor in this bicapped square-antiprismatic motif. Calculations of the synproportionation free energies of smaller (n - 1) closo-selenaboranes with larger-sized (n + 1) ones support the extraordinary stability of octahedral, bicapped square-antiprismatic and icosahedral closo motifs in the SeBnCln family (n = 4-12).

Single─Not Double─3D-Aromaticity in an Oxidized Closo Icosahedral Dodecaiodo-Dodecaborate Cluster

3D-aromatic molecules with (distorted) tetrahedral, octahedral, or spherical structures are much less common than typical 2D-aromatic species or even 2D-aromatic-in-3D systems. Closo boranes, [BnHn]2- (5 ≤ n ≤ 14) and carboranes are examples of compounds that are singly 3D-aromatic, and we now explore if there are species that are doubly 3D-aromatic. The most widely known example of a species with double 2D-aromaticity is the hexaiodobenzene dication, [C6I6]2+. This species shows π-aromaticity in the benzene ring and σ-aromaticity in the outer ring formed by the iodine substituents. Inspired by the hexaiodobenzene dication example, in this work, we explore the potential for double 3D-aromaticity in [B12I12]0/2+. Our results based on magnetic and electronic descriptors of aromaticity together with 11B{1H} NMR experimental spectra of boron-iodinated o-carboranes suggest that these two oxidized forms of a closo icosahedral dodecaiodo-dodecaborate cluster, [B12I12] and [B12I12]2+, behave as doubly 3D-aromatic compounds. However, an evaluation of the energetic contribution of the potential double 3D-aromaticity through homodesmotic reactions shows that delocalization in the I12 shell, in contrast to the 10σ-electron I62+ ring in the hexaiodobenzene dication, does not contribute to any stabilization of the system. Therefore, the [B12I12]0/2+ species cannot be considered as doubly 3D-aromatic.

DFT Surface Infers Ten-Vertex Cationic Carboranes from the Corresponding Neutral closo Ten-Vertex Family: The Computed Background Confirming Their Experimental Availability

Modern computational protocols based on the density functional theory (DFT) infer that polyhedral closo ten-vertex carboranes are key starting stationary states in obtaining ten-vertex cationic carboranes. The rearrangement of the bicapped square polyhedra into decaborane-like shapes with open hexagons in boat conformations is caused by attacks of N-heterocyclic carbenes (NHCs) on the closo motifs. Single-point computations on the stationary points found during computational examinations of the reaction pathways have clearly shown that taking the "experimental" NHCs into account requires the use of dispersion correction. Further examination has revealed that for the purposes of the description of reaction pathways in their entirety, i.e., together with all transition states and intermediates, a simplified model of NHCs is sufficient. Many of such transition states resemble in their shapes those that dictate Z-rearrangement among various isomers of closo ten-vertex carboranes. Computational results are in very good agreement with the experimental findings obtained earlier.

Shedding light on the physical nature of ion pair interactions involving carba-closo-dodecaborate anions. Insights from computation

closo-Carboranes are structures that have been studied for many decades due to their vast applicability in medicine, catalysis, and energy storage. In order to go deeper into the physics behind the interactions of oppositely charged ions, which have potential applications in electrical energy storage and conversion devices, the present work aims to shed light on the physical nature of the interactions involving (R-CB₁₁H₁₁^-, R = H, F, CH₃, CF₃) and M^q+ = Li⁺, Na⁺, Mg²⁺, Zn²⁺ ions. The bonding situations are evaluated in light of both canonical Kohn-Sham energy decomposition, EDA-NOCV, and local energy decomposition, LED, analyses. Electron and hole transports are also evaluated through charge transfer integrals. The findings reveal that such complexes present not only more significant electrostatic, but also non-negligible orbital contributions. Both energy decomposition analyses EDA-NOCV and DLPNO-LED confirm that the strength of ionic pair interactions (R-CB₁₁H₁₁^- ↔ M^q+) is much more dependent on the nature of the cation employed than on the substituent R used. The results also reveal that charge transfers are not significant in such interacting systems.

Advanced Tool for Chiral Separations of Anionic and Zwitterionic (Metalla)carboranes: Supercritical Fluid Chromatography

The continuous expansion of research in the field of stable carboranes and their wide potential in the drug design require carrying out fundamental studies regarding their chiral separations. Although supercritical fluid chromatography (SFC) is a viable technique for fast enantioseparations, no investigation concerning boron cluster compounds has been done yet. We aimed at the development of a straightforward method enabling chiral separations of racemic mixtures of anionic cluster carboranes and metallacarboranes that represent an analytical challenge. The fast gradient screening testing nine polysaccharide-based columns was used. The key parameters affecting the selectivity were the type of chiral selector, the type of alcohol, and the base in cosolvent. Moreover, the addition of acetonitrile or water to the cosolvent was identified as an effective tool for decreasing the analysis time while preserving the resolution. After the optimization, the chiral separations of 19 out of 20 selected compounds were achieved in less than 10 min. These results demonstrate the clear advantage of SFC over chiral separations using HPLC in terms of both analysis time and structural variety of successfully separated compounds.

3D and 2D aromatic units behave like oil and water in the case of benzocarborane derivatives

A large number of 2D/2D and 3D/3D aromatic fusions that keep their aromaticity in the fused compounds have been synthesized. In addition, we have previously proven the electronic relationship between the 3D aromaticity of boron hydrides and the 2D aromaticity of PAHs. Here we report the possible existence of 3D/2D aromatic fusions that retain the whole aromaticity of the two units. Our conclusion is that such a 3D/2D aromatic combination is not possible due to the ineffective overlap between the π-MOs of the planar species and the n + 1 molecular orbitals in the aromatic cage that deter an effective electronic delocalization between the two fused units. We have also proven the necessary conditions for 3D/3D fusions to take place, and how aromaticity of each unit is decreased in 2D/2D and 3D/3D fusions.

2D/2D fusion of aromatic halves leading to a global aromatic is found in many polycyclic aromatic hydrocarbons, whereas 2D/3D aromaticity is difficult to achieve. Here the authors report a computational chemistry investigation showing that 3D/2D aromatic combination is not possible.

Conjugation between 3D and 2D aromaticity: does it really exist? The case of carborane-fused heterocycles

Although several synthesized icosahedral carborane fused 2D π-ring systems are known, and even considerable conjugation has been noted between them in some cases, the phenomenon itself is not fully understood. Based on the results of our computational study, it can be concluded that the 2D aromatic character of the fused (exo) five-membered ring is low, even in cases where significant conjugation was proposed in previous studies. Moreover, the carborane moiety constricts the bonding properties of the exo ring, thus prohibiting or promoting different Lewis resonance structures. These results will shed further light on the design and electronic modulation of new carborane-based materials.

We have demonstrated that carborane fused heterocycles can definitely be considered as non-2D aromatic compounds; therefore, their aromatic character was significantly overestimated in earlier studies.

Thermal and Electrochemical Interface Compatibility of a Hydroborate Solid Electrolyte with 3 V-Class Cathodes for All-Solid-State Sodium Batteries

Thermal stability of solid electrolytes and their compatibility with battery electrodes are key factors to ensure stable cycling and high operational safety of all-solid-state batteries. Here, we study the compatibility of a hydroborate solid electrolyte Na₄(B₁₂H₁₂)(B₁₀H₁₀) with 3 V-class cathode active materials: NaCrO₂, NaMnO₂, and NaFeO₂. Among these layered sodium transition metal oxide cathodes, NaCrO₂ shows the highest thermal compatibility in contact with the hydroborate solid electrolyte up to 525 °C in the discharged state. Furthermore, the electrolyte remains intact upon the internal thermal decomposition of the charged, that is, desodiated, cathode (Na_0.5CrO₂) above 250 °C, demonstrating the potential for highly safe hydroborate-based all-solid-state batteries with a wide operating temperature range. The experimentally determined onset temperatures of thermal decomposition of Na₄(B₁₂H₁₂)(B₁₀H₁₀) in contact with 3 V-class cathodes surpass those of sulfide and selenide solid electrolytes, exceeding previous thermodynamic calculations. Our results also highlight the need to identify relevant decomposition pathways of hydroborates to enable more valid theoretical predictions.

Transformation of various multicenter bondings within bicapped-square antiprismatic motifs: Z-rearrangement

Reported herein are mutual rearrangements in the whole series of seven bicapped-square antiprismatic closo-C₂B₈H₁₀ by means of high-quality computations that disprove the earlier postulated dsd (diamond-square-diamond) scheme for these isomerizations. The experimentally existing closo-1,2-C₂B₈H₁₀ was able to be converted to 1,6-, and 1,10-isomers by pyrolysis, and the dsd (diamond-square-diamond) mechanism was offered as an explanation of these processes. However, these computations disprove the postulated dsd scheme for these isomerizations that take place in the ten-vertex closo series. Experimentally observed thermal rearrangements, both in the parent and substituted closo-1,2-C₂B₈H₁₀, closo-1-CB₉H₁₀^-, and closo-B₁₀H₁₀^2-, indirectly support these refined computations. All these processes are based on the new concept of the so-called Z-mechanism, being consistent with a transition state of a boat shape with an open hexagonal belt that results from the initial breakage of three bonds. Such bond breakings and the consequent bond formations bring to mind the shape of the letter Z. In effect, the pattern of multicenter bonding shifts from reactant through a transition state to product. The molecular rearrangements that are available experimentally favour either the axial or equatorial isomers, and this ratio depends on temperature and the type of cluster and its substitution.

Modulation of γ-Secretase Activity by a Carborane-Based Flurbiprofen Analogue

All over the world, societies are facing rapidly aging populations combined with a growing number of patients suffering from Alzheimer's disease (AD). One focus in pharmaceutical research to address this issue is on the reduction of the longer amyloid-β (Aβ) fragments in the brain by modulation of γ-secretase, a membrane-bound protease. R-Flurbiprofen (tarenflurbil) was studied in this regard but failed to show significant improvement in AD patients in a phase 3 clinical trial. This was mainly attributed to its low ability to cross the blood-brain barrier (BBB). Here, we present the synthesis and in vitro evaluation of a racemic meta-carborane analogue of flurbiprofen. By introducing the carborane moiety, the hydrophobicity could be shifted into a more favourable range for the penetration of the blood-brain barrier, evident by a logD7.4 value of 2.0. Furthermore, our analogue retained γ-secretase modulator activity in comparison to racemic flurbiprofen in a cell-based assay. These findings demonstrate the potential of carboranes as phenyl mimetics also in AD research.

Single-cluster electronics

This article reviews the scope of inorganic cluster compounds interrogated in single-molecule break-junction measurements. This body of work lies at the intersection between the fields of inorganic cluster chemistry and single-molecule electronics, where discrete inorganic cluster molecules are used as the active components in molecular electronic circuitry. We explore the breadth of transition metal and main group cluster compounds that have been studied in single-cluster junctions, largely within the context of scanning tunnelling microscopy break-junction (STM-BJ) measurements. Our discussion centers on how the structure and bonding of inorganic cluster compounds give rise to desirable quantum transport effects such as room-temperature current blockade, sequential tunneling, voltage-gated conductance switching, destructive quantum interference, and high thermoelectric currents.

Carboranes as Lewis Acids: Tetrel Bonding in CB11H11 Carbonium Ylide

High-level quantum-chemical computations (G4MP2) are carried out in the study of complexes featuring tetrel bonding between the carbon atom in the carbenoid CB11H11—obtained by hydride removal in the C-H bond of the known closo-monocarbadodecaborate anion CB11H12(−) and acting as Lewis acid (LA)—and Lewis bases (LB) of different type; the electron donor groups in the tetrel bond feature carbon, nitrogen, oxygen, fluorine, silicon, phosphorus, sulfur, and chlorine atomic centres in neutral molecules as well as anions H(−), OH(−), and F(−). The empty radial 2pr vacant orbital on the carbon centre in CB11H11, which corresponds to the LUMO, acts as a Lewis acid or electron attractor, as shown by the molecular electrostatic potential (MEP) and electron localization function (ELF). The thermochemistry and topological analysis of the complexes {CB11H11:LB} are comprehensively analysed and classified according to shared or closed-shell interactions. ELF analysis shows that the tetrel C⋯X bond ranges from very polarised bonds, as in H11B11C:F(−) to very weak interactions as in H11B11C⋯FH and H11B11C⋯O=C=O.

Strong σ-Hole Activation on Icosahedral Carborane Derivatives for a Directional Halide Recognition

Crystal engineering based on σ-hole interactions is an emerging approach for realization of new materials with higher complexity. Neutral inorganic clusters derived from 1,2-dicarba-closo-dodecaborane, substituted with -SeMe, -TeMe, and -I moieties on both skeletal carbon vertices are experimentally demonstrated herein as outstanding chalcogen- and halogen-bond donors. In particular, these new molecules strongly interact with halide anions in the solid-state. The halide ions are coordinated by one or two donor groups (μ₁ - and μ₂ -coordinations), to stabilize a discrete monomer or dimer motifs to 1D supramolecular zig-zag chains. Crucially, the observed chalcogen bond and halogen bond interactions feature remarkably short distances and high directionality. Electrostatic potential calculations further demonstrate the efficiency of the carborane derivatives, with V_s,max being similar or even superior to that of reference organic halogen-bond donors, such as iodopentafluorobenzene.

Expanding the Scope of Palladium-Catalyzed B - N Cross-Coupling Chemistry in Carboranes

Over the past several years, a number of strategies for the functionalization of dicarba-closo-dodecaboranes (carboranes) have emerged. Despite these developments, B - N bond formation on the carborane scaffold remains a challenge due to the propensity of strong nucleophiles to partially deboronate the parent closo-carborane cluster into the corresponding nido form. Here we show that azide, sulfonamide, cyanate, and phosphoramidate nucleophiles can be straightforwardly cross-coupled onto the B(9) vertices of the o- and m-carborane core from readily accessible precursors without significant deboronation by-products, laying the groundwork for further study into the utility and properties of these new B-aminated carborane species. We further showcase select reactivity of the installed functional groups highlighting some unique features stemming from the combination of the electron-donating B(9) position and the large steric profile of the B-connected carborane substituent.

Carborane Guests for Cucurbit[7]uril Facilitate Strong Binding and On-Demand Removal

High-affinity guests have been reported for the macrocyclic host cucurbit[7]uril (CB[7]), enabling widespread applications, but hindering CB[7] materials from being returned to their guest-free state for reuse. Here, we present polyhedral boron clusters (carboranes) as strongly binding, yet easily removable, guests for CB[7]. Aided by a Pd-catalyzed coupling of an azide anion, we prepared boron-functionalized 9-amino-ortho-carborane that binds to CB[7] with a K_a ≈ 10¹⁰ M^-1. Upon basic treatment, ortho-carborane readily undergoes deboronation to yield anionic nido-carborane, a poor guest for CB[7], facilitating recovery of guest-free CB[7]. We showcase the utility of the modified ortho-carborane guest by recycling a CB[7]-functionalized resin. With this report, we introduce stimuli-responsive decomplexation as an additional consideration in the design of high-affinity host-guest complexes.

Benchmark Data Sets of Boron Cluster Dihydrogen Bonding for the Validation of Approximate Computational Methods

The success of approximate computational methods, such as molecular mechanics, or dispersion-corrected density functional theory, in the description of non-covalent interactions relies on accurate parameterizations. Benchmark data sets are thus required. This area is well developed for organic molecules and biomolecules but practically non-existent for boron clusters, which have been gaining in importance in modern drug as well as material design. To fill this gap, we have introduced two data sets featuring the most common non-covalent interaction of boron clusters, the dihydrogen bond, and calculated reference interaction energies at the "golden standard" CCSD(T)/CBS level. The boron clusters studied interact with formamide, methanol, water and methane at various distances and in two geometrical arrangements. The performance of the tested approximate methods is variable and recommendations for further use are given.

Exploring the Biochemical Foundations of a Successful GLUT1-Targeting Strategy to BNCT: Chemical Synthesis and In Vitro Evaluation of the Entire Positional Isomer Library of ortho-Carboranylmethyl-Bearing Glucoconjugates

Boron neutron capture therapy (BNCT) is a noninvasive binary therapeutic modality applicable to the treatment of cancers. While BNCT offers a tumor-targeting selectivity that is difficult to match by other means, the last obstacles preventing the full harness of this potential come in the form of the suboptimal boron delivery strategies presently used in the clinics. To address these challenges, we have developed delivery agents that target the glucose transporter GLUT1. Here, we present the chemical synthesis of a number of ortho-carboranylmethyl-substituted glucoconjugates and the biological assessment of all positional isomers. Altogether, the study provides protocols for the synthesis and structural characterization of such glucoconjugates and insights into their essential properties, for example, cytotoxicity, GLUT1-affinity, metabolism, and boron delivery capacity. In addition to solidifying the biochemical foundations of a successful GLUT1-targeting approach to BNCT, we identify the most promising modification sites in d-glucose, which are critical in order to further develop this strategy toward clinical use.

Reactions of Experimentally Known Closo-C2B8H10 with Bases. A Computational Study

On the basis of the direct transformations of closo-1,2-C2B8H10 with OH(−) and NH3 to arachno-1,6,9-OC2B8H13(−) and arachno-1,6,9-NC2B8H13, respectively, which were experimentally observed, the DFT computational protocol was used to examine the corresponding reaction pathways. This work is thus a computational attempt to describe the formations of 11-vertex arachno clusters that are formally derived from the hypothetical closo-B13H13(2−). Moreover, such a protocol successfully described the formation of arachno-4,5-C2B6H11(−) as the very final product of the first reaction. Analogous experimental transformations of closo-1,6-C2B8H10 and closo-1,10-C2B8H10, although attempted, were not successful. However, their transformations were explored through computations.

The Crystal Chemistry of Inorganic Hydroborates

The crystal structures of inorganic hydroborates (salts and coordination compounds with anions containing hydrogen bonded to boron) except for the simplest anion, borohydride BH4−, are analyzed regarding their structural prototypes found in the inorganic databases such as Pearson’s Crystal Data [Villars and Cenzual (2015), Pearson’s Crystal Data. Crystal Structure Database for Inorganic Compounds, Release 2019/2020, ASM International, Materials Park, Ohio, USA]. Only the compounds with hydroborate as the only type of anion are reviewed, although including compounds gathering more than one different hydroborate (mixed anion). Carbaborane anions and partly halogenated hydroborates are included. Hydroborates containing anions other than hydroborate or neutral molecules such as NH3 are not discussed. The coordination polyhedra around the cations, including complex cations, and the hydroborate anions are determined and constitute the basis of the structural systematics underlying hydroborates chemistry in various variants of anionic packing. The latter is determined from anion–anion coordination with the help of topology analysis using the program TOPOS [Blatov (2006), IUCr CompComm. Newsl. 7, 4–38]. The Pauling rules for ionic crystals apply only to smaller cations with the observed coordination number within 2–4. For bigger cations, the predictive power of the first Pauling rule is very poor. All non-molecular hydroborate crystal structures can be derived by simple deformation of the close-packed anionic lattices, i.e., cubic close packing (ccp) and hexagonal close packing (hcp), or body-centered cubic (bcc), by filling tetrahedral or octahedral sites. This review on the crystal chemistry of hydroborates is a contribution that should serve as a roadmap for materials engineers to design new materials, synthetic chemists in their search for promising compounds to be prepared, and materials scientists in understanding the properties of novel materials.

Thiaboranes on Both Sides of the Icosahedral Barrier: Retaining and Breaking the Barrier with Carbon Functionalities

The concept of icosahedral barrier has been expanded from the chemistry of carbaboranes to the area of thiaboranes. Both representatives of this barrier, i. e., closo-1,2-C₂ B₁₀ H₁₂ and closo-1-SB₁₁ H₁₁ , are similar in their electron distribution, which is dominated by positive charge in the midpoint of the C-C vector and on the sulfur atom with experimentally determined dipole moments of 4.50 D and 3.64 D, respectively. This is a driving force for their reactivity as exemplified by their reactions with different carbon functionalities. Icosahedral closo-1-SB₁₁ H₁₁ reacts both with an electron sextet containing carbon (in the form of N-heterocyclic carbenes), reported earlier, and with methyl iodide with an electron octet on the carbon. The latter reaction provides hexamethylated thiaborane on the basis of methylation so far unknown in this area of heteroborane chemistry. The computations of the heat of formation (ΔH_f ²⁹⁸ ) make it possible to estimate the height of the barrier as well as to propose closo-thiaboranes beyond the barrier. Eleven and twelve vertex thiaboranes with nido electron count are known experimentally for breaking the barrier. These computations also suggest that the larger nido-thiaboranes are promising candidates for the corresponding experimental availability, i. e., the ΔH_f ²⁹⁸ of a 13-vertex nido-thiaborane cluster has been computed to be more negative than that of the well-known nido-SB₁₀ H₁₁ ^- cluster (-6.7 and -5.6 kcal mol^-1 per vertex, respectively).

ReB6-: A Metallaboron Analog of Metallabenzenes

Metallabenzenes are a class of molecules in which a CH unit in benzene is replaced by a functionalized transition-metal atom. While all-boron analogues of aromatic and antiaromatic hydrocarbons are well-known, there have not been any metallaboron analogs. We have produced and investigated two metal-doped boron clusters, ReB₆^- and AlB₆^-, using high-resolution photoelectron imaging and quantum chemical calculations. Vibrationally resolved photoelectron spectra have been obtained and compared with the theoretical results. The ReB₆^- cluster is found to be perfectly planar with a B-centered hexagonal structure (C_2v, ¹A₁), while AlB₆^- is known to have a similar structure, but with a slightly out-of-plane distortion (C_s, ¹A'). Chemical bonding analyses show that the closed-shell ReB₆^- is doubly σ- and π-aromatic, while AlB₆^- is known to be σ-aromatic and π-antiaromatic. The out-of-plane distortion in AlB₆^- is due to antiaromaticity, akin to the out-of-plane distortion of the prototypical antiaromatic cyclooctatetraene. The π-bonding in ReB₆^- is compared with that in both benzene and rhenabenzene [(CO)₄ReC₅H₅], and remarkable similarities are found. Hence, ReB₆^- can be viewed as the first metallaboron analog of metallabenzenes and it may be viable for syntheses with suitable ligands.

Chalcogen Bonding due to the Exo-Substitution of Icosahedral Dicarbaborane

Chalcogen atoms are a class of substituents capable of generating inner and outer derivatives of boron clusters. It is well known that chalcogenated boron clusters can form strong σ-hole interactions when a chalcogen atom is a part of an icosahedron. This paper studies σ-hole interactions of dicarbaboranes with two exopolyhedral chalcogen atoms bonded to carbon vertices. Specifically, a computational investigation has been carried out on the co-crystal of (1,2-C₂B₁₀H₁₀)₂Se₄•toluene and a single crystal of (1,2-C₂B₁₀H₁₀)₂Te₄.

New global minima of 6-vertex dicarboranes: classical but unexpected

Dicarboranes generally adopt global minimum predicted by the well-known Wade-Mingos rules, although one classical non-closo structure in the benzvalene form has long been pursued and later synthesized. Here we predicted two new non-closo global minima for 6-vertex dicarboranes (C2B4R6), i.e., trigonal bipyramid (R = SH) and butterfly (R = Cl, NH2, OH, F). The long expected classical benzvalene-like structure, however, is not the global minimum for any of the nine substituents.

Recent Progress in the Synthesis of the Monocarba-closo-dodecaborate(-) Anions

This Concept article focuses on the rapid growth in studies of the chemistry of the monocarba-closo-dodecaborate(-) anion (C1 carborane anion). As one of the most stable anions known, the C1 carborane anion has been useful for exploring the chemistry of highly reactive cations. On the other hand, development of novel functional molecules utilizing the unique properties of C1 carborane anion (e.g., σ-aromaticity, rigid spherical skeleton) has progressed more slowly. The main reason for this is the relatively undeveloped state of synthetic chemistry in this area. Recent advances in the synthetic chemistry of C1 carborane anion are highlighted in this Concept article, focusing on cross-coupling reactions at the carbon vertex, direct conversion of B-H bonds, and the synthesis of multivalent weakly coordinating anions. These progressions move this species beyond its well-established role of highly stable "counter" monocharged anion.

Synthesis of 9-borafluorene analogues featuring a three-dimensional 1,1'-bis(o-carborane) backbone

The synthesis of [1,1'-bis(o-carboranyl)]boranes was achieved through the deprotonation of 1,1'-bis(o-carborane) reagents followed by salt metathesis with (iPr)2NBCl2. X-ray crystallography confirms planar central BC4 rings and Gutmann-Beckett studies reveal an increase in Lewis acidity at the boron center in comparison to their biphenyl congener, 9-borafluorene.

Thermal and Photophysical Properties of Highly Quadrupolar Liquid-Crystalline Derivatives of the [closo-B12 H12 ]2- Anion

Two series of 1,12-bis-zwitterionic derivatives of the [closo-B₁₂ H₁₂ ]^2- anion (B), containing either two 4-alkoxypyridinium groups (1B[n]-p) or one 4-alkoxypyridinium and one 4-pentylthianium groups (2B[n]-p), were prepared and their structural (XRD, DFT), thermal, and photophysical properties were compared with those of the analogous derivatives of the [closo-B₁₀ H₁₀ ]^2- anion (1A[n]-p and 2A[n]-p). Some 1,7-derivatives of B were isolated and investigated. Both series 1[n] and 2[n] exhibit nematic and crystalline polymorphism; the 12-vertex derivatives (B) have higher transition temperatures than those of the 10-vertex analogues (A). All compounds fluoresce with quantum yields higher for 1B (Φ_F =0.37 for 1B[7]-p and Φ_F =0.27 for 2B[7]-p) than those for the 10-vertex analogues (Φ_F =0.04 for 2A[5]-p). DFT calculations demonstrate an order of magnitude lower first hyperpolarizability, β_(-ω,ω,0) , for 2B[7]-p than that for the 10-vertex analogue 2A[7]-p (1.7×10^-30 vs. 18.9×10^-30  esu at ω=0).

Synthesis, structure and aromaticity of carborane-fused carbo- and heterocycles

The results of molecular structures, NMR data, and NICS (nucleus-independent chemical shift) and ISE (isomerization stabilization energy) values as well as molecular orbital analyses clearly suggest the presence of considerable aromatic character in the exo five-membered ring of carborane-fused carbo- and heterocycles and considerable conjugation between a 3-D carborane and a fused 2-D π-ring system.

Conjugation between a 3-D icosahedral carborane and a fused 2-D π-ring system is ambiguous. To address this issue, we prepared several carborane-fused carbo- and heterocycles. Detailed studies on their molecular structures, NMR data, and NICS (nucleus-independent chemical shift) and ISE (isomerization stabilization energy) values as well as molecular orbital analyses clearly suggest the presence of (1) considerable aromatic character in the exo five-membered ring of carborane-fused carbo- and heterocycles and (2) considerable conjugation between a 3-D carborane and a fused 2-D π-ring system. These results will shed some light on the design of new carborane-based materials.

A systematic examination of classical and multi-center bonding in heteroborane clusters

A systematic revision of bonding types on a broad series of heteroboranes covering closo, nido, arachno and hypho architectures with incorporated tetrel, pnictogen or chalcogen heterovertices.

CarbORev-5901: The First Carborane-Based Inhibitor of the 5-Lipoxygenase Pathway

The progression of cancer is accelerated by increased proliferation, angiogenesis, and inflammation. These processes are mediated by leukotrienes. Several cancer cell lines overexpress 5-lipoxygenase, an enzyme that converts arachidonic acid into leukotrienes. An early inhibitor of the 5-lipoxygenase pathway is Rev-5901, which, however, lacks in in vivo efficacy, as it is rapidly metabolized. We investigated the introduction of carboranes as highly hydrophobic and metabolically stable pharmacophores into lipoxygenase inhibitors. Carboranes are icosahedral boron clusters that are remarkably stable and used to increase the metabolic stability of unstable pharmaceutics without changing their biological activity. By introduction of meta-carborane into Rev-5901, the first carborane-based inhibitor of the 5-lipoxygenase pathway was obtained. We report the synthesis and inhibitory and cytotoxic behavior of these compounds toward several melanoma and colon cancer cell lines and their related anticancer mechanisms.

The behavior of a paramagnetic system in electric and magnetic fields as exemplified by revisiting Li@B10H14

The electric and magnetic properties of the Li@B₁₀H₁₄ complex, considered as a novel inorganic electride-type system with potential for second-order non-linear optical (NLO) applications, have already been studied. However, the reported C_2v structure is not the global energy minimum and therefore its electronic and magnetic properties need to be revisited. Moreover, by applying more accurate computational protocols (ROHF-CCSD/CCSD(T) and larger basis sets) we show that the model chemistry used earlier (UMP2/6-31+G(d)) is not sufficient for reliable description of the NLO responses of this open-shell doublet complex. The global minimum based on the C_s symmetry is significantly (by ca. 25 kcal mol^-1) more stable than the C_2v structure and it should be viewed as a system with moderate NLO responses. An excess of unpaired electron density is also responsible for the contact and pseudo-contact contributions to the magnetic properties, which was not considered in the earlier work.

Thermochemistry of icosahedral closo-dicarboranes: a composite ab initio quantum-chemical perspective

We obtained accurate thermochemical properties for the ortho-, meta-, and para-dicarborane isomers (C2B10H12) by means of explicitly correlated high-level thermochemical procedures. The thermochemical properties include heats of formation, isomerization energies, C–H and B–H bond dissociation energies (BDEs), and ionization potentials. Of these only the ionization potentials are known experimentally. Our best theoretical ionization potentials, obtained by means of the ab initio W1–F12 thermochemical protocol, was 241.50 kcal mol–1 (para-dicarborane), 238.45 kcal mol–1 (meta-dicarborane), and 236.54 kcal mol–1 (ortho-dicarborane). These values agree with the experimental values adopted by the National Institute of Standards and Technology (NIST) thermochemical tables to within overlapping uncertainties. However, they suggest that the experimental values may represent significant underestimations. For all isomers, the C–H BDEs are systematically higher than the B–H BDEs because of the relative stability of the boron-centred radicals. The C–H BDEs for the three isomers cluster within a narrow energetic interval, namely between 110.8 kcal mol–1 (para-dicarborane) and 111.7 kcal mol–1 (meta-dicarborane). The B–H BDEs cluster within a larger interval ranging between 105.8 and 108.1 kcal mol–1 (both obtained for ortho-dicarborane). We used our benchmark W1–F12 data to assess the performance of a number of lower cost composite ab initio methods. We found that the Gaussian-3 procedures (G3(MP2)B3 and G3B3) result in excellent performance with overall root-mean-square deviations (RMSDs) of 0.3–0.4 kcal mol–1 for the isomerization, ionization, and bond dissociation energies. However, the Gaussian-4 procedures (G4, G4(MP2), and G4(MP2)-6X) showed relatively poor performance with overall RMSDs of 1.3–3.7 kcal mol–1.

Blue Phosphorescent Zwitterionic Iridium(III) Complexes Featuring Weakly Coordinating nido-Carborane-Based Ligands

We report the development of a new class of phosphorescent zwitterionic bis(heteroleptic) Ir(III) compounds containing pyridyl ligands with weakly coordinating nido-carboranyl substituents. Treatment of phenylpyridine-based Ir(III) precursors with C-substituted ortho-carboranylpyridines in 2-ethoxyethanol results in a facile carborane deboronation and the formation of robust and highly luminescent metal complexes. The resulting nido-carboranyl fragments associate with the cationic Ir(III) center through primarily electrostatic interactions. These compounds phosphoresce at blue wavelengths (450-470 nm) both in a poly(methyl methacrylate) (PMMA) matrix and in solution at 77 K. These complexes display structural stability at temperatures beyond 300 °C and quantum yields greater than 40%. Importantly, the observed quantum yields correspond to a dramatic 10-fold enhancement over the previously reported Ir(III) congeners featuring carboranyl-containing ligands in which the boron cluster is covalently attached to the metal. Ultimately, this work suggests that the use of a ligand framework containing a weakly coordinating anionic component can provide a new avenue for designing efficient Ir(III)-based phosphorescent emitters.

Reaction Development on π- and σ-Conjugated Bonds and Creation of Innovative Functions

Monocarba-closo-dodecaborate (1; [closo-CB11H12](-), or C1-carborane anion) is a symmetrical, stable anionic cluster, which possesses low nucleophilicity/basicity and exhibits three-dimensional aromaticity. In contrast to the rich applications of C2-carborane molecules (C2B10H12), the chemistry of the C1-carborane anion as a platform for functional molecules has not been thoroughly studied thus far due to the lack of its efficient functionalization. In particular, no efficient general methods are available for the introduction of aryl and sp(2)/sp-carbon groups at the carbon vertex of the C1-carborane anion. The unique electronic structure and potential applications of the C1-carborane anion prompted us to investigate methods to functionalize it. We developed a general, efficient C-C cross-coupling reaction of 1 under palladium catalysis which yields a variety of 1-C-functionalized C1-carborane derivatives. The use of copper(I) or lithium species as a transmetalating partner facilitated the cross-coupling process of the sterically hindered C1-carborane anion. The potential application of 1-C-arylated C1-carborane anion derivatives thus obtained were explored, some of which showed potential as pharmacophores and ionic liquid crystal behavior. Furthermore, conjugation between σ- and π-aromatic moieties in 1-C-arylated monocarba-closo-dodecaborate anion derivatives was identified by means of kinetic experimental studies combined with theoretical calculations.