The Hexahydro-closo-hexaborate Dianion [B6H6]2– and Its Derivatives

Electrochemical deconstruction of alkyl substituted boron clusters to produce alkyl boronate esters

Closo-Hexaborate (closo-B6H62-) can engage in nucleophilic substitution reactions with a wide variety of alkyl electrophiles. The resulting functionalized boron clusters undergo oxidative electrochemical deconstruction, selectively cleaving B-B bonds while preserving B-C bonds in these species. This approach allows the conversion of multinuclear boron clusters into single boron site organoboranes. Trapped boron-based fragments were isolated from the electrochemical cluster deconstruction process, providing further mechanistic insights into the developed reaction.

Bilayer Kagome Borophene with Multiple van Hove Singularities

The appearance of van Hove singularities near the Fermi level leads to prominent phenomena, including superconductivity, charge density wave, and ferromagnetism. Here a bilayer Kagome lattice with multiple van Hove singularities is designed and a novel borophene with such lattice (BK-borophene) is proposed by the first-principles calculations. BK-borophene, which is formed via three-center two-electron (3c-2e) σ-type bonds, is predicted to be energetically, dynamically, thermodynamically, and mechanically stable. The electronic structure hosts both conventional and high-order van Hove singularities in one band. The conventional van Hove singularity resulting from the horse saddle is 0.065 eV lower than the Fermi level, while the high-order one resulting from the monkey saddle is 0.385 eV below the Fermi level. Both the singularities lead to the divergence of electronic density of states. Besides, the high-order singularity is just intersected to a Dirac-like cone, where the Fermi velocity can reach 1.34 × 106 m s-1. The interaction between the two Kagome lattices is critical for the appearance of high-order van Hove singularities. The novel bilayer Kagome borophene with rich and intriguing electronic structure offers an unprecedented platform for studying correlation phenomena in quantum material systems and beyond.

Redox-Active Boron Clusters

ConspectusIn this Account, we discuss our group's research over the past decade on a class of functionalized boron clusters with tunable chemical and physical properties, with an emphasis on accessing and controlling their redox behavior. These clusters can be thought of as three-dimensional aromatic systems that have distinct redox behavior and photophysical properties compared to their two-dimensional organic counterparts. Specifically, our lab has studied the highly tunable, multielectron redox behavior of B12(OR)12 clusters and applied these molecules in various settings. We first discuss the spectroscopic and electrochemical characterization of B12(OR)12 clusters in various oxidation states, followed by their use as catholytes and/or anolytes in redox flow batteries and chemical dopants in conjugated polymers. Additionally, the high oxidizing potential and visible light-absorbing nature of fluoroaryl-functionalized B12(OR)12 clusters have been leveraged by our group to generate weakly coordinating, photoexcitable species that can promote photooxidation chemistry.We have further translated these solution-phase studies of B12(OR)12 clusters to the solid state by using the precursor [B12(OH)12]2- cluster as a robust building block for hybrid metal oxide materials. Specifically, we have shown that the boron cluster can act as a thermally stable cross-linking material, which enhances electron transport between metal oxide nanoparticles. We applied this structural motif to create TiO2- and WO3-containing materials that showed promising properties as photocatalysts and electroactive materials for supercapacitors. Building on this concept, we later discovered that B12(OCH3)12, the smallest of the B12(OR)12 family, could retain its redox behavior in the solid state, a previously unseen phenomenon. We successfully harnessed this unique behavior for solid-state energy storage by implementing this boron cluster as a cathode-active material in a Li-ion prototype cell device. Recently, our group has also explored how to tune the redox properties of clusters other than B12(OR)12 species by synthesizing a library of vertex-differentiated clusters containing both B-OR and B-halogen groups. Due to the additive qualities of different functional groups on the cluster, these species allow access to a region of electrochemical potentials previously inaccessible by fully substituted closo-dodecaborate alkoxy-based derivatives.Lastly, we discuss our research into smaller-sized redox-active polyhedral boranes (B6- and B10-based cluster cores). Interestingly, these clusters show significantly less redox stability and reversibility than their dodecaborate-based counterparts. While exploring the functionalization of closo-hexaborate to create fully substituted derivates (i.e., [B6R6Hfac]-), we observed unique oxidative decomposition pathways for this cluster system. Consequently, we leveraged this oxidative instability to generate useful alkyl boronate esters via selective chemical oxidation. We further explored a closo-decaborate cluster as a platform to access electrophilic [B10H13]+ species capable of directly borylating arene compounds with unique regioselectivity. Upon chemical oxidation of the arylated decaborate clusters, we successfully synthesized various aryl boronate esters, establishing the generality of the oxidative cluster deconstruction concept.Overall, our work shows that boron clusters are an appealing class of redox-active molecules, and this fundamental and understudied property can be leveraged for constructing novel materials with tunable physical and electrochemical properties, as well as producing unique chemical reagents for small molecule synthesis.

A computational study to determine the role of σ-hole in Br/OH substituted nido-carborane and its binding capabilities

A detailed investigation of the σ-hole on the halogen atom present in the nido-heteroboranes is made by employing quantum mechanical methods. The bromide and the hydroxyl groups are incorporated in the exo-substituents of the nido-boranes. The potential of the bromide σ-hole was compared to that of electrostatic potential of hydroxyl group counterpart. The presence of a carbon atom vertex, in a different position of a system, influences the σ-hole and hence its binding abilities. Bromide substituted nido-carboranes have less potential and hence weaker binding ability compared to their closo-counterparts. Binding affinity with aliphatic is found to be more compared to that of aromatic system. The presence of solvent dampened the electrostatic interactions. Apart from the neutral system, the binding capabilities of charged nido-heteroboranes were also studied. The results of this study will be further useful for several applications viz., crystal engineering, drug designing (Pharmaceuticals), medicine, material science, energy storage devices, etc.

Binding Properties of Small Electrophilic Anions [B6 X5 ]- and [B10 X9 ]- (X=Cl, Br, I): Activation of Small Molecules Based on π-Backbonding

Superelectrophilic anions constitute a special class of molecular anions that show strong binding of weak nucleophiles despite their negative charge. In this study, the binding characteristics of smaller gaseous electrophilic anions of the types [B6 X5 ]- and [B10 X9 ]- (with X=Cl, Br, I) were computationally and experimentally investigated and compared to those of the larger analogues [B12 X11 ]- . The positive charge of vacant boron increases from [B6 X5 ]- via [B10 X9 ]- to [B12 X11 ]- , as evidenced by increasing attachment enthalpies towards typical σ-donor molecules (noble gases, H2 O). However, this behavior is reversed for σ-donor-π-acceptor molecules. [B6 Cl5 ]- binds most strongly to N2 and CO, even more strongly than to H2 O. Energy decomposition analysis confirms that the orbital interaction is responsible for this opposite trend. The extended transition state natural orbitals for chemical valence method shows that the π-backdonation order is [B6 X5 ]- >[B10 X9 ]- >[B12 X11 ]- . This predicted order explains the experimentally observed red shifts of the CO and N2 stretching fundamentals compared to those of the unbound molecules, as measured by infrared photodissociation spectroscopy. The strongest red shift is observed for [B6 Cl5 N2 ]- : 222 cm-1 . Therefore, strong activation of unreactive σ-donor-π-acceptor molecules (commonly observed for cationic transition metal complexes) is achieved with metal-free molecular anions.

Measuring Electronic Structure of Multiply Charged Anions to Understand Their Chemistry: A Case Study on Gaseous Polyhedral closo-Borate Dianions

Research on multiply charged anions (MCAs) in the gas phase has been intensively performed during the past decades, mainly to understand fundamental molecular physics phenomena, for example, intramolecular Coulomb repulsion and existence of the repulsive Coulomb barrier. However, the relevance of these investigations with respect to understanding MCAs' chemistry appears often vague. Here, we discuss how insights into the electronic structure obtained from negative ion photoelectron spectroscopy (NIPES) combined with theoretical calculations and collision-induced dissociation can provide a fundamental understanding of the intrinsic chemical reactivity of MCAs and their fragments. This is exemplified in our studies on polyhedral closo-borate dianions [B_nX_n]^2- (n = 6, 10, 11, 12; X = H, F-I, CN) and their fragment ions. For example, the rational design of closo-borate dianions with specific electronic properties is described, which leads to generating highly reactive fragments. Depending on the dianionic precursor, these fragments are tuned to either bind noble gases effectively or activate small molecules like CO and N₂. The intrinsic electronic properties of closo-borate dianions are further compared to their electrochemistry in solutions, revealing solvent effects on the redox potentials. Neutral host molecules such as cyclodextrins are found to bind strongly to [B_nX_n]^2-, and gas phase NIPES provides insights into the intrinsic host-guest interactions. Finally, outlooks including the direct NIPES of molecular fragment ions that cannot be generated in the condensed phase and their utilization in preparative mass spectrometry are discussed.

The Reaction of Hydrogen Halides with Tetrahydroborate Anion and Hexahydro-closo-hexaborate Dianion

The mechanism of the consecutive halogenation of the tetrahydroborate anion [BH₄]⁻ by hydrogen halides (HX, X = F, Cl, Br) and hexahydro-closo-hexaborate dianion [B₆H₆]²⁻ by HCl via electrophile-induced nucleophilic substitution (EINS) was established by ab initio DFT calculations [M06/6-311++G(d,p) and wB97XD/6-311++G(d,p)] in acetonitrile (MeCN), taking into account non-specific solvent effects (SMD model). Successive substitution of H⁻ by X⁻ resulted in increased electron deficiency of borohydrides and changes in the character of boron atoms from nucleophilic to highly electrophilic. This, in turn, increased the tendency of the B–H bond to transfer a proton rather than a hydride ion. Thus, the regularities established suggested that it should be possible to carry out halogenation more selectively with the targeted synthesis of halogen derivatives with a low degree of substitution, by stabilization of H₂ complex, or by carrying out a nucleophilic substitution of B–H bonds activated by interaction with Lewis acids (BL₃).

Effects of Glymes on the Distribution of Mg(B10H10) and Mg(B12H12) from the Thermolysis of Mg(BH4)2

We examined the effects of concentrations and identities of various glymes, from monoglyme up to tetraglyme, on H2 release from the thermolysis of Mg(BH4)2 at 160–200 °C for 8 h. 11B NMR analysis shows major products of Mg(B10H10) and Mg(B12H12); however, their relative ratio is highly dependent both on the identity and concentration of the glyme to Mg(BH4)2. Selective formation of Mg(B10H10) was observed with an equivalent of monoglyme and 0.25 equivalent of tetraglyme. However, thermolysis of Mg(BH4)2 in the presence of stoichiometric or greater equivalent of glymes can lead to unselective formation of Mg(B10H10) and Mg(B12H12) products or inhibition of H2 release.

Properties of gaseous closo-[B6X6]2- dianions (X = Cl, Br, I)

Electronic structure, collision-induced dissociation (CID) and bond properties of closo-[B₆X₆]^2- (X = Cl-I) are investigated in direct comparison with their closo-[B₁₂X₁₂]^2- analogues. Photoelectron spectroscopy (PES) and theoretical investigations reveal that [B₆X₆]^2- dianions are electronically significantly less stable than the corresponding [B₁₂X₁₂]^2- species. Although [B₆Cl₆]^2- is slightly electronically unstable, [B₆Br₆]^2- and [B₆I₆]^2- are intrinsically stable dianions. Consistent with the trend in the electron detachment energy, loss of an electron (e^- loss) is observed in CID of [B₆X₆]^2- (X = Cl, Br) but not for [B₆I₆]^2-. Halogenide loss (X^- loss) is common for [B₆X₆]^2- (X = Br, I) and [B₁₂X₁₂]^2- (X = Cl, Br, I). Meanwhile, X˙ loss is only observed for [B₁₂X₁₂]^2- (X = Br, I) species. The calculated reaction enthalpies of the three competing dissociation pathways (e^-, X^- and X˙ loss) indicated a strong influence of kinetic factors on the observed fragmentation patterns. The repulsive Coulomb barrier (RCB) determines the transition state for the e^- and X^- losses. A significantly lower RCB for X^- loss than for e^- loss was found in both experimental and theoretical investigations and can be rationalized by the recently introduced concept of electrophilic anions. The positive reaction enthalpies for X^- losses are significantly lower for [B₆X₆]^2- than for [B₁₂X₁₂]^2-, while enthalpies for X˙ losses are higher. These observations are consistent with a difference in bond character of the B-X bonds in [B₆X₆]^2- and [B₁₂X₁₂]^2-. A complementary bonding analysis using QTAIM, NPA and ELI-D based methods suggests that B-X bonds in [B₁₂X₁₂]^2- have a stronger covalent character than in [B₆X₆]^2-, in which X has a stronger halide character.

Sterically Unprotected Nucleophilic Boron Cluster Reagents

A cornerstone of modern synthetic chemistry rests on the ability to manipulate the reactivity of a carbon center by rendering it either electrophilic or nucleophilic. However, accessing a similar reactivity spectrum with boron-based reagents has been significantly more challenging. While classical nucleophilic carbon-based reagents normally do not require steric protection, readily accessible, unprotected boron-based nucleophiles have not yet been realized. Herein, we demonstrate that the bench stable closo-hexaborate cluster anion can engage in a nucleophilic substitution reaction with a wide array of organic and main group electrophiles. The resulting molecules containing B‒C bonds can be further converted to tricoordinate boron species widely used in organic synthesis.

Elusive Zintl Ions [μ-HSi4 ]3- and [Si5 ]2- in Liquid Ammonia: Protonation States, Sites, and Bonding Situation Evaluated by NMR and Theory

The existence of [μ-HSi₄ ]^3- in liquid ammonia solutions is confirmed by ¹ H and ²⁹ Si NMR experiments. Both NMR and quantum chemical calculations reveal that the H atom bridges two Si atoms of the [Si₄ ]^4- cluster, contrary to the expectation that it is located at one vertex Si of the tetrahedron. The calculations also indicate that in the formation of [μ-HSi₄ ]^3- , protonation is driven by a high charge density and an increase of electron delocalization compared to [Si₄ ]^4- . Additionally, [Si₅ ]^2- was detected for the first time and characterized by NMR. Calculations show that it is resistant to protonation, owing to a strong charge delocalization, which is significantly reduced upon protonation. Thus, our methods reveal three silicides in liquid ammonia: unprotonated [Si₅ ]^2- , terminally protonated [HSi₉ ]^3- , and bridge-protonated [μ-HSi₄ ]^3- . The protonation trend can be roughly predicted by the difference in charge delocalization between the parent compound and the product, which can be finely tuned by the presence of counter ions in solution.

Interaction of Hydrogen with MB6 (M = Ba, Ca, La, and Sr) Surfaces from First Principles

We show results of basic energetics and interacting behavior of hydrogen with metal hexaboride surfaces using a combination of self-consistent density functional calculations and dynamics based on the Car-Parrinello method. Our results show that hydrogen is strongly attracted to localized exposed boron atoms and interactions with the terminal cations are strictly repulsive. From these, preliminary local adsorption energy calculations suggest that a single hydrogen molecule per surface unit-cell is possible (one ML). Strongest bonds are found when hydrogen is above the terminal boron atoms affected by reduced coordination and dangling bonds. This location serves to restore the hexaboride unit to a more stable structure by providing electronic density to the deficient surface octahedra. Additionally, trajectories from dynamic simulations provide insight into how hydrogen recombination reactions occur on the surface through dissociative adsorption and the method of travel prior to recombination to be along the octahedral face and bridging sites connecting separate unit cells on the surface. Upon adsorption, a single hydrogen atom becomes localized at the dangling bond site while the second interacts with the surface along a weaker potential energy path. Desorption at lower temperatures occurs when migrating atoms from separate adsorption sites intersect to form a new pair.

Improvement in hydrogen binding ability of closo-dicarboranes via functionalization and designing of extended frameworks

Neutral closo-dicarboboranes are reported to have very low H₂ binding ability. Herein, we report an improvement in H₂ binding energy (E_b) of C₂B₄H₆ by substituting H atoms with different functional groups like X = F, Cl, Br, and XY = BO, CN and NC via quantum-chemical density functional theory based computations. In going from B₆H₆^2- to C₂B₄H₆, the E_b value is reduced from 14.6 kJ mol^-1 to 2.7 kJ mol^-1. C₂B₄X₆ and C₂B₄(XY)₆ systems, which can bind a total of eight H₂ molecules, with one H₂ molecule occupying at each B-B-C face, possess an E_b value per H₂ in the range of 4.5 kJ mol^-1 for X = F, 3.9 kJ mol^-1 for X = Cl, 5.9 kJ mol^-1 for X = Br, 6.8 kJ mol^-1 for XY = BO, 5.8 kJ mol^-1 for XY = CN and 5.2 kJ mol^-1 for XY = NC. The improvement in E_b value is found to be the highest in case of C₂B₄(BO)₆, which has the ability to bind 6.6 gravimetric wt% of H₂. The situation can be made more favorable by applying an external electric field. Energy decomposition analysis reveals that although the dispersion interaction (ca. 55-65%) has significant role in binding H₂ with such types of molecules, contribution from electrostatic and orbital interaction is also considerable. Further, we modeled an extended system by linking C₂B₄(BO)_n through 'C ≡ C' units for H₂ storage purpose. The energy difference between the highest occupied and the lowest unoccupied molecular orbitals gradually lessens with the increase in molecular length. Therefore, it can be tuned gradually by controlling the chain length, which may further open up their potency in the field of electronics. Graphical abstract C₂B₄X₆ (X = F, Cl, Br) and C₂B₄(XY)₆ (XY = BO, CN, NC) show enhanced H₂ binding ability from C₂B₄H₆. Further, 1D, 2D and 3-D frameworks can be built by joining C₂B₄(BO)_n units via 'C ≡ C' linkage.

Synthesis and Applications of Perfunctionalized Boron Clusters

This Viewpoint describes major advances pertaining to perfunctionalized boron clusters in synthesis and their respective applications. The first portion of this work highlights key synthetic methods, allowing one to access a wide range of polyhedral boranes (B4 and B6-B12 cluster cores) that contain exhaustively functionalized vertices. The second portion of this Viewpoint showcases the historical developments in using these molecules for applications ranging from materials science to medicine. Last, we suggest potential new directions for these clusters as they apply to both synthetic methods and applications.

Diverse chemistry of the dianion [closo-B9H9]2−: synthesis and reactivity of its mono-anionic derivative [arachno-B9H12-4,8-Cl2]−

Attempted protonation of the [closo-B₉H₉]²⁻ under the moisture-free conditions did not afford the expected monoanion [closo-B₉H₁₀]⁻, while its reaction with HCl in dichloromethane afforded the first chlorine-containing monoanion [arachno-B₉H₁₂-4,8-Cl₂]⁻ in a high yield.

11B-NMR shielding effects in theclosoborane series: sensitivity of shifts and their additivity

NMR shielding effects for [X_m-closo-B_nH_n−m]²⁻anions exhibit a simple linear behaviour in all positions of the borane cluster.

The properties of substituted 3D-aromatic neutral carboranes: the potential for σ-hole bonding

The calculated properties of substituted carboranes such as dipole moment, polarisability, the magnitude of the σ-hole and the desolvation free energy are compared with these properties in comparable aromatic and cyclic aliphatic organic compounds. Dispersion and charge transfer energies are similar. However, the predicted strength of the halogen bonds with the same electron donor (based on the magnitude of the σ-hole) is larger for neutral C-vertex halogen-substituted carboranes than for their organic counterparts. Furthermore, the desolvation penalties of substituted carboranes are smaller than those of the corresponding organic compounds, which should further strengthen the halogen bonds of the former in the solvent. It is predicted that substituted carboranes have the potential to form stronger halogen bonds than comparable aromatic hydrocarbons, which will be even more pronounced in the medium. This theoretical study thus lays ground for the rational engineering of halogen bonding in inorganic crystals as well as in biomolecular complexes.

Syntheses and crystal structures of the closo-borate M[B8H9] (M = [PPh4]+ and [N(n-Bu4)]+)

Protonation of M(2)[B(8)H(8)] with HCl or NEt(3)·HCl resulted in M[B(8)H(9)] (M = [PPh(4)](+) or [N(n-Bu(4))](+)). The monoanion was isolated and characterized by (1)H, (1)H{(11)B}, (11)B, and (11)B{(1)H} NMR spectroscopy. The "protonated" form [B(8)H(9)](-) showed a dynamic behavior in solution, which was analyzed by NMR spectroscopy and compared with theoretical calculations. The structures of [B(8)H(9)](-) as well as [B(8)H(8)](2-) were determined by single-crystal X-ray diffraction.

Syntheses and crystal structures of the closo-borates M2[B7H7] and M[B7H8] (M = PPh4, PNP, and N(n-Bu4)): the missing crystal structure in the series [B(n)H(n)]2- (n = 6-12)

Oxidation of [N(n-Bu(4))](2)[B(9)H(9)] with oxygen in a mixture of dimethoxyethane and CH(2)Cl(2) leads to salts of the [B(7)H(7)](2-) dianion. This is the first convenient synthesis for a seven-vertex hydro-closo-borate anion. Protonation with NEt(3)·HCl resulted in salts of the [B(7)H(8)](-) monoanion. Both closo-borate anions were isolated and characterized by (1)H, (1)H{(11)B}, (11)B, and (11)B{(1)H} NMR spectroscopy. The temperature-dependent (1)H{(11)B}, (11)B, and (11)B{(1)H} NMR spectra of [B(7)H(8)](-) were also measured. The structure of [B(7)H(7)](2-) as well as of [B(7)H(8)](-) were determined by single-crystal X-ray diffraction.

Synthesis and characterization of synthetically useful salts of the weakly-coordinating dianion [B12Cl12]2-

The closo-dodecahydrododecaborate [NEt3H]2[B12H12] has been prepared on a lab scale by an improved synthesis from cheap and readily available starting materials Na[BH4] and I2 in diglyme (diethylene glycol dimethyl ether). Subsequent chlorination with elemental chlorine in aqueous solution at normal pressure yielded the per-chlorinated weakly coordinating [B12Cl12]2- anion. By simple metathesis reaction a variety of useful salts [cation]2[B12Cl12] (cation=[NEt3H]+, [NBu4]+, Li+, Na+, K+, Cs+) is available. These salts are useful starting materials, which have the potential to open up the chemistry of [B12Cl12]2- as a weakly coordinating dianion. Exemplarily, they were used in further reactions to prepare [NO]2[B12Cl12], [PPN]2[B12Cl12], and [CPh3]2[B12Cl12]. The crystal structures of Cs2[B12Cl12].SO2, [CPh3]2[B12Cl12].2C2H4Cl2, and [CPh3]2[B12Cl12].2SO2 and preliminary crystal structures of [NO]2[B12Cl12].SO2 and [PPN]2[B12Cl12].CH2Cl2 were determined. The crystal structure of the SO2 solvate Cs2[B12Cl12].SO2 is related to the crystal structure of solvent free Cs2[B12Cl12]. [CPh3]2[B12Cl12].2C2H4Cl2 and [CPh3]2[B12Cl12].2SO2 have very similar structures in the solid state. In both cases the [CPh3]+ cations form only very weak contacts to the [B12Cl12]2- anion and SO2 or C2H4Cl2 solvent molecules respectively. The averaged experimental B-B (178.7 pm) and B-Cl (178.9 pm) bond lengths within [B12Cl12]2- are essentially unchanged in all determined structures and are reproduced well by PBE0/TZVPP quantum chemical calculations (B-B 178.6 pm, B-Cl 179.3 pm). All results indicate that [B12Cl12]2- is a readily accessible weakly-coordinating dianion.

exo-Substituent effects in halogenated icosahedral (B12H122–) and octahedral (B6H62–) closo-borane skeletons: chemical reactivity studied by experimental and quantum chemical methods

The exo-substituent effects in halogenated icosahedral B12H122– (B12) and octahedral B6H62– (B6) closo-borane skeletons were studied both experimentally and theoretically. Firstly, the equilibrium geometries of exo-substituted B12 and B6 clusters were obtained using quantum chemical calculations at the MP2/def2-SVP level. A comparison with the available X-ray crystallographic data revealed a very good agreement between the theoretical and experimental values. Secondly, other descriptors of the molecular structure of these borane compounds – 11B NMR chemical shifts – were experimentally determined and compared with the calculated values obtained by the ab initio/GIAO approach at the MP2/def2-TZVP level. It was shown that the calculated data reproduced the experiment very closely. Thirdly, we investigated experimentally the halogenation reactions of B12 and attempted to explain the observed ratios between the two obtained disubstituted products (meta/ortho ~ 4:1) by calculating their thermodynamic stabilities using the DFT/B3LYP method. These calculations showed the enhanced stability of the meta disubstituted B12 but did not explain why the para product had not been observed in the experiment. We thus turned our attention to the kinetic aspects of exo-substitution reactions by exploring the possible reaction pathways and transition states. In spite of the complexity of the plausible reaction mechanisms, reasonable agreement was obtained between the calculated activation barriers and the experimental observations concerning the halogenation reactions of the B6 and B12 molecules. It also allowed to exclude from considerations certain reaction pathways leading to the mono- and dihalogenated B12 and B6 species.

A new 4c-2e bond in B6H7-

For N(C(4)H(9))(4)B(6)H(7) a proton is shown to be localised above one of the faces of the distorted octahedron, and rhomboid rings are formed as shown by topological analysis of charge densities.

Dianionic tetraborates do exist as stable entities

To date, B(6)H(6)(2-) and some of its derivatives are the smallest members of the closo-borates that have been synthesized and analyzed in condensed phases. In contrast, no stable dianionic tetraborate has yet been observed, either in solution or solids or in the gas phase. In this work, the gas-phase stability of dianionic tetraborates B(4)X(4)(2-) (X = H, CN, NC, or BO) is investigated with ab initio methods. For this objective, the geometries of the dianions are optimized, the electronic stability is tested, and various fragmentation channels are studied. In agreement with previous examinations, tetrahedral isomers of all examined tetraborates have been found to represent geometrically stable isomers and to exhibit a triplet electronic ground state. However, these isomers are electronically unstable, i.e., their additional electrons are not bound. Furthermore, new D(2)(d)()-symmetric isomers of B(4)X(4)(2-) (X = H, CN, NC, or BO) have been identified that have a closed-shell singlet ground state and are lower in energy than their tetrahedral counterparts. Moreover, B(4)(CN)(4)(2-) and B(4)(BO)(4)(2-) represent stable gas-phase dianions and are predicted to be observable in suitable experiments. The electronic properties and geometries of these dianions are discussed in detail and explained in terms of the electrostatic repulsion of the excess electrons and the aromaticity of the dianions.

Gas-phase stability of derivatives of the closo-hexaborate dianion B(6)H(6)(2-)

B(6)H(6)(2-) does not represent a stable gas-phase dianion, but emits spontaneously one of its excess electrons in the gas phase. In this work we address the question whether small stable gas-phase dianions can be constructed from the parent B(6)H(6)(2-) dianion by substitution of the hydrogens with appropriate ligands. Various hexa-, tetra-, and disubstituted derivatives B(6)L(6)(2-), B(6)H(2)L(4)(2-), and B(6)H(4)L(2)(2-) (L = F, Cl, CN, NC, or BO) are investigated with ab initio methods in detail. Four stable hexasubstituted B(6)L(6)(2-) (L = Cl, CN, NC, or BO) and three stable B(6)H(2)L(4)(2-) (L = CN, NC, or BO) gas-phase dianions could be identified and predicted to be observable in the gas phase. The trends in the electron-detachment energies depending on various ligands are discussed and understood in the underlying electrostatic pattern and the electronegativities of the involved elements.

Investigations of Electronic Interactions Between closo-Boranes and Triple-Bonded Substituents

Intramolecular electronic interactions were investigated in five series of 12-, 10-, and 6-vertex closo-boranes and hydrocarbons (benzene, acetylene, bicyclo[2.2.2]octane, and cubane) substituted with triple-bonded groups Y≡Z (C≡CR, C≡N, C≡O, and N≡N). Structural data (single crystal X-ray crystallography), spectroscopic properties (UV, IR, and NMR), chemical behavior (dediazoniation reactions), and electronic structure calculations (hybridization and π bond order) are all in agreement that the degree of electronic conjugation between the cluster and the Y≡Z substituent is lowest for the 12-vertex closo-borane and highest for the 6-vertex analog.

Defective vertices in closo- and nido-borane polyhedra

Boron polyhedra can be described in terms of the deviation of their local vertex environments from the degree 5 vertices found in ideal icosahedra. Vertices of degrees other than 5 can be considered to be defective vertices. The most favorable structures for borane polyhedra are those in which the defective vertices are isolated as much as possible, similar to the Frank-Kasper polyhedra found in metal alloy structures. Using this criterion, the 9- and 10-vertex borane deltahedra are seen to be more favorable than the other nonicosahedral deltahedra in the boranes B(n)H(n)(2-) (6 < or = n < or = 12) in accord with experimental observations. Extension of such ideas to neutral boron hydrides of the type B(n)H(n+4) accounts for the relatively high stability of B(10)H(14), the formation of metal complexes of B(6)H(10), and the stability of B(18)H(22). In addition, the borane B(12)H(16) is predicted to form stable transition-metal complexes.