Boron: Elementary Challenge for Experimenters and Theoreticians

Many of the fundamental questions regarding the solid‐state chemistry of boron are still unsolved, more than 200 years after its discovery. Recently, theoretical work on the existence and stability of known and new modifications of the element combined with high‐pressure and high‐temperature experiments have revealed new aspects. A lot has also happened over the last few years in the field of reactions between boron and main group elements. Binary compounds such as B₆O, MgB₂, LiB_1?x, Na₃B₂₀, and CaB₆ have caused much excitement, but the electron‐precise, colorless boride carbides Li₂B₁₂C₂, LiB₁₃C₂, and MgB₁₂C₂ as well as the graphite analogue BeB₂C₂ also deserve special attention. Physical properties such as hardness, superconductivity, neutron scattering length, and thermoelectricity have also made boron‐rich compounds attractive to materials research and for applications. The greatest challenges to boron chemistry, however, are still the synthesis of monophasic products in macroscopic quantities and in the form of single crystals, the unequivocal identification and determination of crystal structures, and a thorough understanding of their electronic situation. Linked polyhedra are the dominating structural elements of the boron‐rich compounds of the main group elements. In many cases, their structures can be derived from those that have been assigned to modifications of the element. Again, even these require a critical revision and discussion.     

Density functional theory study of BnC clusters

B_nC clusters (n = 3–10) were studied at the density functional theory (DFT) (B3LYP)/6‐311G** level of theory. The calculations predicted that the most stable configurations of the B_nC clusters are the (n + 1)‐membered cyclic structures. For boron–carbon clusters, the configurations containing greater numbers of three‐membered boron rings are more favorable, except for the B₇C and B₉C clusters. Through molecular orbital analysis of these B_nC clusters, we have concluded that π‐electron delocalization plays a crucial role in the stability of n + 1‐membered cyclic structures. In this paper, the relative stability of each cluster is discussed based on their single atomic‐binding energies. The capability of clusters to obtain or lose an electron was also discussed, based on their vertical electron detachment energies (VDEs), adiabatic electron detachment energies (ADEs), vertical electron affinities (VEAs) and adiabatic electron affinities (AEAs). Copyright © 2011 John Wiley & Sons, Ltd.     

Near‐Infrared Quadrupolar Chromophores Combining Three‐Coordinate Boron‐Based Superdonor and Superacceptor Units

Herein, two new quadrupolar acceptor‐π‐donor‐π‐acceptor (A‐π‐D‐π‐A) chromophores have been prepared featuring a strongly electron‐donating diborene core and strongly electron‐accepting dimesitylboryl (BMes₂) and bis(2,4,6‐tris(trifluoromethyl)phenyl)boryl (B^FMes₂) end groups. Analysis of the compounds by NMR spectroscopy, X‐ray crystallography, cyclic voltammetry, and UV/Vis‐NIR absorption and emission spectroscopy indicated that the compounds have extended conjugated π‐systems spanning their B₄C₈ cores. The combination of exceptionally potent π‐donor (diborene) and π‐acceptor (diarylboryl) groups, both based on trigonal boron, leads to very small HOMO–LUMO gaps, resulting in strong absorption in the near‐IR region with maxima in THF at 840 and 1092 nm and very high extinction coefficients of ca. 120 000 m ^?1 cm^?1. Both molecules also display weak near‐IR fluorescence with small Stokes shifts.     

Extensive Structural Rearrangements upon Reduction of 9H‐9‐Borafluorene

Common wisdom has it that organoboranes are readily oxidized. Described herein is that also their reduction can result in remarkable chemistry. Treatment of dimeric 9H‐9‐borafluorene with Li metal in toluene yields two strikingly different classes of compounds. One part of the sample reacts in a way similar to B₂H₆, thus affording an aryl(hydro)borane cluster reminiscent of the [B₃H₈]^? anion. The other part furnishes a dianionic boron‐doped graphene flake devoid of hydrogen substituents at the boron centers and featuring a central B?B bond. A change in the solvent to THF allows an isolation of this dibenzo[g,p]chrysene analogue in good yields.     

Synthesis of Globular Precursors

o‐Carborane (C₂B₁₀H₁₂) was adapted to perform as the core of globular macromolecules, dendrons or dendrimers. To meet this objective, precisely defined substitution patterns of terminal olefin groups on the carborane framework were subjected to Heck cross‐coupling reactions or hydroboration leading to hydroxyl terminated arms. These led to new terminal groups (chloro, bromo, and tosyl leaving groups, organic acid, and azide) that permitted ester production, click chemistry, and oxonium ring opening to be performed as examples of reactions that demonstrate the wide possibilities of the globular icosahedral carboranes to produce new dendritic or dendrimer‐like structures. Polyanionic species were obtained in high yield through the ring‐opening reaction of cyclic oxonium compound [3,3′‐Co(8‐C₄H₈O₂‐1,2‐C₂B₉H₁₀)(1′,2′‐C₂B₉H₁₁)] by using terminal hydroxyl groups as nucleophiles. These new polyanionic compounds that contain multiple metallacarborane clusters at their periphery may prove useful as new classes of compounds for boron neutron capture therapy with enhanced water solubility and as cores to make a new class of high‐boron globular macromolecules.     

Formation of an Isolable Divinylborinium Ion through Twofold 1,2‐Carboboration between a Diarylborinium Ion and Diphenylacetylene

Borinium ions, that is, two‐coordinate boron cations, are the most electron‐deficient isolable boron compounds. As borinium ions have only four formal valence electrons on boron, they should show a strong tendency to accept electron pairs on the boron atom to fill its valence shell. Thus chemical reactions of borinium ions are expected to give products in which the coordination number of boron is increased from two to three or four. However, contrary to this expectation, we found that the dimesitylborinium ion (Mes₂B⁺) undergoes twofold 1,2‐carboboration reactions with two equivalents of diphenylacetylene to yield an unprecedented borinium ion ( 1 ⁺) with two substituted vinyl groups on the boron center. NMR spectroscopy and X‐ray diffraction analysis of 1 ⁺, together with electronic‐structure calculations, revealed that the positive charge is delocalized over the entire π‐conjugated system. The fact that the chemical transformation of a borinium ion gives rise to a different borinium ion without a change in the coordination number is remarkable and should provide new insight into the chemistry of the Group 13 elements.     

Hexagonal Bipyramidal [Ta2B6]−/0 Clusters: B6 Rings as Structural Motifs

It has been a long‐sought goal in cluster science to discover stable atomic clusters as building blocks for cluster‐assembled nanomaterials, as exemplified by the fullerenes and their subsequent bulk syntheses. ¹ , ² Clusters have also been considered as models to understand bulk properties, providing a bridge between molecular and solid‐state chemistry. ³ Because of its electron deficiency, boron is an interesting element with unusual polymorphism. While bulk boron is known to be dominated by the three‐dimensional (3D) B₁₂ icosahedral motifs, ⁴ new forms of elemental boron are continuing to be discovered. ⁵ In contrast to the 3D cages commonly found in bulk boron, in the gas phase two‐dimensional (2D) boron clusters are prevalent. ⁶ – ⁸ The unusual planar boron clusters have been suggested as potential new bulking blocks or ligands in chemistry. ^6a Herein we report a joint experimental and theoretical study on the [Ta₂B₆]⁻ and [Ta₂B₆] clusters. We found that the most stable structures of both the neutral and anion are D_6h bipyramidal, similar to the recently discovered MB₆M structural motif in the Ti₇Rh₄Ir₂B₈ solid compound. ⁹      

Interpenetration of a 3D Icosahedral M@Ni12 (M=Al,Ga) Framework with Porphyrin‐Reminiscent Boron Layers in MNi9B8

Two ternary borides MNi₉B₈ (M=Al, Ga) were synthesized by thermal treatment of mixtures of the elements. Single‐crystal X‐ray diffraction data reveal AlNi₉B₈ and GaNi₉B₈ crystallizing in a new type of structure within the space group Cmcm and the lattice parameters a=7.0896(3) Å, b=8.1181(3) Å, c=10.6497(4) Å and a=7.0897(5) Å, b=8.1579(4) Å, c=10.6648(7) Å, respectively. The boron atoms build up two‐dimensional layers, which consist of puckered [B₁₆] rings with two tailing B atoms, whereas the M atoms reside in distorted vertices‐condensed [Ni₁₂] icosahedra, which form a three‐dimensional framework interpenetrated by boron porphyrin‐reminiscent layers. An unusual local arrangement resembling a giant metallo‐porphyrin entity is formed by the [B₁₆] rings, which, due to their large annular size of approximately 8 Å, chelate four of the twelve icosahedral Ni atoms. An analysis of the chemical bonding by means of the electron localizability approach reveals strong covalent B?B interactions and weak Ni?Ni interactions. Multi‐center dative B?Ni interaction occurs between the Al–Ni framework and the boron layers. In agreement with the chemical bonding analysis and band structure calculations, AlNi₉B₈ is a Pauli‐paramagnetic metal.     

Computational Studies on Non‐covalent Interactions of Carbon and Boron Fullerenes with Graphene

First‐principles DFT calculations are carried out to study the changes in structures and electronic properties of two‐dimensional single‐layer graphene in the presence of non‐covalent interactions induced by carbon and boron fullerenes (C₆₀, C₇₀, C₈₀ and B₈₀). Our study shows that larger carbon fullerene interacts more strongly than the smaller fullerene, and boron fullerene interacts more strongly than that of its carbon analogue with the same nuclearity. We find that van der Waals interactions play a major role in governing non‐covalent interactions between the adsorbed fullerenes and graphene. Moreover, a greater extent of van der Waals interactions found for the larger fullerenes, C₈₀ and B₈₀, relative to smaller C₆₀, and consequently, results in higher stabilisation. We find a small amount of electron transfer from graphene to fullerene, which gives rise to a hole‐doped material. We also find changes in the graphene electronic band structures in the presence of these surface‐decorated fullerenes. The Dirac cone picture, such as that found in pristine graphene, is significantly modified due to the re‐hybridisation of graphene carbon orbitals with fullerenes orbitals near the Fermi energy. However, all of the composites exhibit perfect conducting behaviour. The simulated absorption spectra for all of the graphene–fullerene hybrids do not exhibit a significant change in the absorption peak positions with respect to the pristine graphene absorption spectrum. Additionally, we find that the hole‐transfer integral between graphene and C₆₀ is larger than the electron‐transfer integrals and the extent of these transfer integrals can be significantly tuned by graphene edge functionalisation with carboxylic acid groups. Our understanding of the non‐covalent functionalisation of graphene with various fullerenes would promote experimentalists to explore these systems, for their possible applications in electronic and opto‐electronic devices.     

Analysis of Why Boron Avoids sp2 Hybridization and Classical Structures in the BnHn+2 Series

We performed global minimum searches for the B_nH_n+2 (n=2‐5) series and found that classical structures composed of 2c–2e B? H and B? B bonds become progressively less stable along the series. Relative energies increase from 2.9 kcal mol^?1 in B₂H₄ to 62.3 kcal mol^?1 in B₅H₇. We believe this occurs because boron atoms in the studied molecules are trying to avoid sp² hybridization and trigonal structure at the boron atoms, as in that case one 2p‐AO is empty, which is highly unfavorable. This affinity of boron to have some electron density on all 2p‐AOs and avoiding having one 2p‐AO empty is a main reason why classical structures are not the most stable configurations and why multicenter bonding is so important for the studied boron–hydride clusters as well as for pure boron clusters and boron compounds in general.     

Synergistic flame‐retardant effect and mechanisms of boron/phosphorus compounds on epoxy resins

To explore the component synergistic effect of boron/phosphorus compounds in epoxy resin (EP), 3 typical boron compounds, zinc borate (ZB), boron phosphate (BPO₄), and boron oxide (B₂O₃), blended with phosphaphenanthrene compound TAD were incorporated into EP, respectively. All 3 boron/phosphorus compound systems inhibited heat release and increased residue yields and exerted smoke suppression effect. Among 3 boron/phosphorus compound systems, B₂O₃/TAD system brought best flame‐retardant effect to epoxy thermosets in improving the UL94 classification of EP composites and also reducing heat release most efficiently during combustion. B₂O₃ can interact with epoxy matrix and enhance the charring quantity and quality, resulting in obvious condensed‐phase flame‐retardant effect. The combination of condensed‐phase flame‐retardant effect from B₂O₃ and the gaseous‐phase flame‐retardant effect from TAD effectively optimized the action distribution between gaseous and condensed phases. Therefore, B₂O₃/TAD system generated component synergistic flame‐retardant effect in epoxy thermosets.     

Structures and bonding of B4O5 and B4O5− clusters: Emergence of boroxol ring and competition between rhombic B2O2 and hexagonal B3O3 cores

Boron oxide clusters are electron-deficient species with novel structures and bonding, in which the emergence of rhombic and boroxol rings is of interest. We report on computational prediction of the global-minimum structures for two boron oxide clusters: B₄O₅ and B₄O₅⁻. These structures differ distinctly, as established through global machine searches and electronic structure calculations at B3LYP and single-point CCSD(T) levels. While B₄O₅ neutral cluster has a rhombic B₂O₂ core, the B₄O₅⁻ anion features a boroxol B₃O₃ ring. One electron completely changes the potential landscapes. Bonding analyses show that the 4π electron-counting is crucial for a rhombic B O cluster, in contrast to π sextet for a boroxol ring, which underlies the competition between rhombic and boroxol rings in B₄O₅/B₄O₅⁻ clusters. A possible pathway for rhombic-to-hexagonal transformation is proposed based on intrinsic reaction coordinate calculations. Anion B₄O₅⁻ cluster, a new member of the inorganic benzene family, is among the smallest B O species with a free-standing boroxol ring, governed collectively by composition and electron-counting.     

(Dibenzoylmethanato)boron Difluoride Derivatives Containing Triphenylamine Moieties: A New Type of Electron‐Donor/π‐Acceptor System for Dye‐Sensitized Solar Cells

(Dibenzoylmethanato)boron difluoride derivatives containing triphenylamine moieties were synthesized as a new type of electron‐donor/π‐acceptor system. These new compounds exhibited long‐wavelength absorptions in the UV/Vis spectra, and reversible oxidation and reduction waves in cyclic voltammetry experiments. Their amphoteric redox properties are based on their resonance hybrid forms, in which a positive charge is delocalized on the triphenylamine moieties and a negative charge is localized on the boron atoms. Molecular orbital (MO) calculations indicate that their HOMO and LUMO energies vary with the number of phenylene rings connected to the difluoroboron‐chelating ring. This is useful for optimizing the HOMO and LUMO levels to an iodine redox (I^?/I₃^?) potential and a titanium dioxide conduction band, respectively. Dye‐sensitized solar cells fabricated by using these compounds as dye sensitizers exhibited solar‐to‐electric power conversion efficiencies of 2.7–4.4 % under AM 1.5 solar light.     

The Boron Conundrum: Which Principles Underlie the Formation of Large Hollow Boron Cages?

Extensive optimisation calculations are performed for the B₈₀ isomers in order to find out which principles underlie the formation of large hollow boron cages. Our analysis shows that the most stable isomers contain triangular B₁₀ or rhombohedral B₁₆ building blocks. The lowest‐energy isomer has C_3v symmetry and is characterised by a belt of three interconnected B₁₆ units and two separate B₁₀ units. At the B3LYP/6‐31G(d) level of theory, this newly discovered isomer is 2.29, 1.48, and 0.54 eV below the leapfrog B₈₀ of Szwacki et al., the T_h‐B₈₀ of Wang, and the D_3d‐B₈₀ of Pochet et al., respectively. Our C_3v isomer is therefore identified as the most stable hollow cage isomer of B₈₀ presently known. Its HOMO–LUMO gap of 1.6 eV approaches that of the leapfrog B₈₀. The leapfrog principle still remains a reliable scheme for producing boron cages with larger HOMO–LUMO gaps, whereas the thermodynamically most stable B₈₀ cages are formed when all pentagonal faces are capped. We show that large hollow cages of boron retain a preference for fullerene frames. The additional capping is in accordance with the following rules: preference for capping of pentagonal faces, formation of B₁₀ and/or B₁₆ units, homogeneous distribution of the hexagonal caps, and hole density approaching 1/9. Although our most stable B₈₀ isomer still remains higher in energy than the B₈₀ core–shell structure, we show that by applying the bonding principles to larger structures it is possible to construct boron cages with higher stabilisation energy per boron atom than the core–shell structure; a prototypical example is B₁₆₀. This clearly shows the continuous competition between the two suggested construction schemes, namely, the formation of multiple‐shell structures and hollow cages.     

Syntheses and Crystal Structures of Brominated Polyhedral Arsa‐ and Phosphaboranes

The synthesis of the perbrominated arsaboranes closo‐1,2‐As₂B₄Br₄ ( 1 ) and closo‐1,2‐As₂B₁₀Br₁₀ ( 2 ) occurs by co‐pyrolysis of B₂Br₄ and AsBr₃ at 500 °C. Repeated fractionation of the sublimable products in vacuo yields both compounds in pure form. The X‐ray structure determination for orthorhombic closo‐1,2‐As₂B₄Br₄ ( 1 ) [space group: Pbcn, a = 2345.48(17) pm, b = 627.31(4) pm, c = 1294.02(9) pm for Z = 8] and the corresponding phosphorus compound, monoclinic closo‐1,2‐P₂B₄Br₄ ( 3 ) [space group: P2₁/n, a = 806.84(6) pm, b = 1247.96(9) pm, c = 974.91(7) pm, β = 90.493(3)° and Z = 4] confirmed that both 1 and 3 , consistent with their 14‐skeletal electron counts, adopt octahedral structures distorted from regular by two arsenic or phosphorus atoms in the 1,2‐positions. The shortest boron–boron bonds within the cluster frameworks are located between the boron atoms antipodal to the heteroatoms.     

Preparation and Redox Properties of N,N,N‐1,3,5‐Trialkylated Flavin Derivatives and Their Activity as Redox Catalysts

Eight different flavin derivatives have been synthesized and the electronic effects of substituents in various positions on the flavin redox chemistry were investigated. The redox potentials of the flavins, determined by cyclic voltammetry, correlated with their efficiency as catalysts in the H₂O₂ oxidation of methyl p‐tolyl sulfide. Introduction of electron‐withdrawing groups increased the stability of the reduced catalyst precursor.     

Coaxial Triple‐Layered versus Helical Be6B11− Clusters: Dual Structural Fluxionality and Multifold Aromaticity

Two low‐lying structures are unveiled for the Be₆B₁₁⁻ nanocluster system that are virtually isoenergetic. The first, triple‐layered cluster has a peripheral B₁₁ ring as central layer, being sandwiched by two Be₃ rings in a coaxial fashion, albeit with no discernible interlayer Be−Be bonding. The B₁₁ ring revolves like a flexible chain even at room temperature, gliding freely around the Be₆ prism. At elevated temperatures (1000 K), the Be₆ core itself also rotates; that is, two Be₃ rings undergo relative rotation or twisting with respect to each other. Bonding analyses suggest four‐fold (π and σ) aromaticity, offering a dilute and fluxional electron cloud that lubricates the dynamics. The second, helix‐type cluster contains a B₁₁ helical skeleton encompassing a distorted Be₆ prism. It is chiral and is the first nanosystem with a boron helix. Molecular dynamics also shows that at high temperature the helix cluster readily converts into the triple‐layered one.     

Hexahalodiborate Dianions: A New Family of Binary Boron Halides

The electron‐precise binary boron subhalide species [B₂X₆]^2? X=F, Br, I) were synthesized and their structures confirmed by X‐ray crystallography. The existence of the previously claimed [B₂Cl₆]^2?, which had been questioned, was also confirmed by X‐ray crystallography. The dianions are isoelectronic to hexahaloethanes, are subhalide analogues of the well‐known tetrahaloborate anions (BX₄^?), and are rare examples of molecular electron‐precise binary boron species beyond B₂X₄, BX₃, and [BX₄]^?.     

Exploring the Potential Energy Surface of E2P4 Clusters (E=Group 13 Element): The Quest for Inverse Carbon‐Free Sandwiches

Inverse carbon‐free sandwich structures with formula E₂P₄ (E=Al, Ga, In, Tl) have been proposed as a promising new target in main‐group chemistry. Our computational exploration of their corresponding potential‐energy surfaces at the S12h/TZ2P level shows that indeed stable carbon‐free inverse‐sandwiches can be obtained if one chooses an appropriate Group 13 element for E. The boron analogue B₂P₄ does not form the D_4h‐symmetric inverse‐sandwich structure, but instead prefers a D_2d structure of two perpendicular BP₂ units with the formation of a double B?B bond. For the other elements of Group 13, Al–Tl, the most favorable isomer is the D_4h inverse‐sandwich structure. The preference for the D_2d isomer for B₂P₄ and D_4h for their heavier analogues has been rationalized in terms of an isomerization‐energy decomposition analysis, and further corroborated by determination of aromaticity of these species.     

Reactions of Copper(II) Chloride in Solution: Facile Formation of Tetranuclear Copper Clusters and Other Complexes That Are Relevant in Catalytic Redox Processes

Mixing CuCl₂ ? 2 H₂O with benzylamine in alcoholic solutions led to an extremely colorful chemistry caused by the formation of a large number of different complexes. Many of these different species could be structurally characterized. These include relatively simple compounds such as [Cu(L¹)₄Cl₂] (L¹=benzylamine) and (HL¹)₂[CuCl₄]. Most interestingly is the easy formation of two cluster complexes, one based on two cluster units Cu₄OCl₆(L¹)₄ connected through one [Cu(L¹)₂Cl₂] complex and one based on a cubane‐type cluster ([Cu₄O₄](C₁₁H₁₄)₄Cl₄). Both clusters proved to be highly reactive in a series of oxidation reactions of organic substrates by using air or peroxides as oxidants. Furthermore, it was possible to isolate and structurally characterize ([Cu(L¹)Cl]₃ and [Cu(benz₂mpa)₂]CuCl₂ (benz₂mpa=benzyl‐(2‐benzylimino‐1‐methyl‐propylidene)‐amine), two copper(I) complexes that formed in solution, demonstrating the high redox activity of the cluster systems. In addition, it was possible to solve the molecular structures of the compounds Cu₄OCl₆(MeOH)₄, [Cu(MeOH)₂Cl₂], [Cu(aniline)₂Cl₂], and an organic side product (HC₁₃H₁₉NOCl). In fact all determined structures are of a known type but the chemical relation between these compounds could be explained for the first time. The paper describes these different compounds and their chemical equilibria. Some of these complexes seem to be relevant in catalytic oxidation reactions and their reactivity is discussed in more detail.