Reactions between the C,C-dicopper(l) derivative of ortho-carborane and ortho-, meta- and para-diiodobenzene are reported. The reaction with 1,2-C6H4I2 unexpectedly afforded 2,2 '-bis(1 '-ortho-carboranyl)biphenyl, HCB10H10CC6H4]2 2, whereas reactions with 1,3- or 1,4-C6H4I2 provided alternative routes to 1,3-bis(1 '-ortho-carboranyl)benzene 3 and 1,4-bis(1 '-ortho-carboranyl)benzene 4, respectively. The crystal structure of the biphenyl derivative 2 revealed significant distortions in the biphenylene framework attributable to the proximity of the two bulky carborane cages. UV absorption spectra and electrochemical data on 2 and 3 showed little electronic communication between the two carborane cages in either, and negligible pi-conjugation between the two ortho-phenylene rings in 2. However, substantial evidence was found of electronic communication between the carborane cages via the paraphenylene bridge in 4. B3LYP/6-31G* computations have been carried out on compounds 2 and 4, on 4,4 '-bis(ortho-carboranyl)biphenyl 6 and on 1,2-bis(1 '-ortho-carboranyl)benzene 7. Those on 2, 4 and 6 show the computed geometries to be in very good agreement with the experimental geometries: those on 7 allowed the reported molecular geometry of this compound to be revised and revealed a long cage C C bond of 1.725(3) angstrom. (C) 2014 Elsevier Ltd. All rights reserved.

Long ago, a global search for borane superfuels led fortuitously to the discovery of carboranes. Ken Wade recalls his own undistinguished part in the space race, and notes how carboranes revitalized boron hydride chemistry and modified our ideas of chemical bonding.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Details are reported of the preparation and X-ray structural characterisation of the new compounds (PhCbo)2X, where PhCbo=2-Ph-1,2-C2B10H10 and X=S (1) or SO (3), prepared from PhCboLi and SCl2 or SOCl2 respectively, as well as the known compound with X=S2 (2). Hydrogen in the presence of palladium/carbon reduced (3) to (1). Features of their structures (exo C–S and cage C–C distances and substituent orientations) are consistent with significant exo SC dative π-bonding from their sulfur atoms to the cage hypercarbon atoms and weak exo π-bonding from the phenyl groups. Attempts to prepare systems (CboY)2, in which two ortho-carboran-di-yl residues Cbo are linked through two bridging units Y (S, SO) are also reported, as are their calculated structures.

The structures of derivatives of phenyl-ortho-carborane bearing on the second cage hypercarbon atom a pi-donor substituent (F, OH, O-, NH2, NH- and CH2-) were investigated by NMR, X-ray crystallography and computational studies. The molecular structures of these compounds, notably their cage C1-C2 distances and the orientations of their pi-donor substituents (OH, NH2, NH- and CH2-) show remarkable and systematic variations with the degree of exo pi-bonding, which varies as expected with the pi-donor characteristics of the substituent.

Reaction of the lithium salt of 1-(2'-pyridyl)-ortho-carborane, Li[1-R-1,2-C(2)B(10)H(10)](R = 2'-NC(5)H(4)), with sulfur, followed by hydrolysis, gave the mercapto-o-carborane, 1-R-2-SH-1,2-C(2)B(10)H(10) which forms chiral crystals containing helical chains of molecules linked by intermolecular S-H...N hydrogen bonds. The cage C(1)-C(2) and exo C(2)-S bond lengths (1.730(3) and 1.775(2)[Angstrom], respectively) are indicative of exo S=C pi bonding. The tin derivative 1-R-2-SnMe(3)-1,2-C(2)B(10)H(10), prepared from Li[1-R-1,2-C(2)B(10)H(10)] and Me(3)SnCl, crystallises with no significant intermolecular interactions. The pyridyl group lies in the C(1)-C(2)-Sn plane, oriented to minimise the NSn distance (2.861(3)[Angstrom]). The tin environment is distorted trigonal bipyramidal with axial N and Me. The gold derivative 1-R-2-AuPPh(3)-1,2-C(2)B(10)H(10), prepared from Li[1-R-1,2-C(2)B(10)H(10)] and AuCl(PPh(3)), reveals no NAu interaction in its crystal structure.

The structures of 1-Me-2-COOH-1,2-closo-C2B10H10 (1), 1-Ph-2-COOH-1,2-closo-C2B10H10 (2) and 1-Ph-2-COPh-1,2-closo-C2B10H10 (3) have been determined by X-ray crystallography. In 1 the orientation of the COOH group is defined by θCOOH=65.0(2)syn°, and this group H-bonds to that in a second molecule via a centrosymmetric eight-membered ring, R22(8). DFT calculations on the parent species 1-COOH-1,2-closo-C2B10H11 reveal that the structure with θCOOH=90syn° is preferred, with a barrier to COOH rotation of ≈17 kJmol−1. Similar calculations on 1 yield a lower rotational barrier, ≈7 kJmol−1, because internal H-bonding is now denied. In compound 2 the COOH group is twisted to lower θ values [5.0(3)° and 39.7(3)°] but both crystallographically independent molecules exist as dimers in the solid state by virtue of R22(8) rings. Compound 2 crystallises from wet solvent as a monohydrate, the structure of which reveals a non-planar centrosymmetric R44(12) ring and near-orthogonal Ph and COOH substituents [θPh=6.7(2) and θCOOH=78.0(2)syn°]. In compound 3 molecular association by H-bonding is blocked, so 3 serves as a useful comparative structure for 2.

Four C-substituted derivatives of ortho-carborane, 1-R-1,2-C2B10H11, where R = 2'-pyridyl (1), 2'-picolyl (2), 5'-bromo-2'-pyridyl (3) or 3'-pyridyl (4) have been prepared using adaptations of standard procedures, and structurally characterised by single crystal X-ray diffraction studies in an exploration of C-H...N hydrogen bonding effects involving their carborane CH units. Calculations at the MP2/6-31G* level of theory were used to assess the strength of the hydrogen bonding detected, and calculated NMR shifts at the GIAO-B3LYP/6-311G* level were compared with measured C-H shifts to show that intramolecular C-H...N hydrogen bonding persisted in solution in the case of compound 1. The value of IR C-H stretching frequencies for probing hydrogen bonding in these systems was also studied. An unsuccessful attempt to convert compound 3 into a macrocyclic species (C2B10H10C5H3N)(3) in which three ortho-carborane units are linked through 2,5-disubstituted pyridine rings is also described.

The macrocyclic compound, [1,2-C2B10H10-1,4-C6H4-1,7-C2B10H10-1,4-C6H4]2 (5)—a novel cyclooctaphane, was prepared by condensation of the C,C′-dicopper(I) derivative of meta-carborane with 1,2-bis(4-iodophenyl)-ortho-carborane. The X-ray crystal structure of 5·C6H6·6C6H12 was determined at 150 K, revealing an extremely loose packing mode. Molecule 5 has a crystallographic Cs and local C2v symmetry; the macrocycle adopts a butterfly (dihedral angle 143°) conformation with the ortho-carborane units at the wingtips and the phenylene ring planes roughly perpendicular to the wing planes. Multinuclear NMR spectra suggest that molecule 5 in solution inverts rapidly via the planar D2h geometry, which (from ab initio HF/6-31G* calculations) is only 1 kcal mol−1 higher in energy than the C2v one. An attempt to prepare an even larger macrocycle, comprising three para-carborane and three ortho-carborane units linked by six para-phenylene units, was unsuccessful.

The synthesis and crystal and molecular structure are described Of (C2B10H11)C6H4(C2B10H10)C6H4 (C2B10H11) (4), an acyclic assembly of three ortho-carborane units connected through their carbon atoms by two para-phenylene units. For this compound, and published structures of other aryl-ortho-carboranes, correlations are noted between the orientations of aryl substituents and cage carbon-carbon distances (C1-C2). Ab initio RHF/6-31G* and MP2/6-31G* studies on 1-phenyl-ortho-carborane, PhC2B10H11, and other model systems have been used to explore the variations in their energies, C1-C2 bond lengths, and C2-C1-C-aryl bond angles with the orientation of their aryl groups, variations believed to reflect weak interactions between the aryl substituents' pi systems and the carborane cages. The synthesis of a pentafluorophenyl derivative of 4, (C2B10H11)C6H4(C2B10H10)C6H4(C2B10H10)C6F5, is also described. Copyright (C) 2003 John Wiley Sons, Ltd.

This paper outlines the development of our knowledge and understanding of the structures and bonding of boron cluster compounds, with particular reference to the evolving complementary roles localized bonding and molecular orbital treatments have played in providing simple rationalizations of their polyhedral molecules.

A {B20H28} species, from the oligomerisation of B5H9, has a central nido {B10H12} core with two nido {B5H8} units bound to it at mutually adjacent positions. [(CH2CH2C5H4N)-arachno-B10H10(NC5H4-closo-C2B10H10)], a by-product from the reaction of [B10H12(NCMe)2] with ortho-ethynyl-pyridine, has mutually linked closo {C2B10} and arachno {B10} cluster units conjoined to a central aromatic pyridine unit; the arachno {B10} residue is also fused to an organyl double-ring structure involving a second pyridyl moiety. Both species model possible steps to globular megaloborane synthesis via the assembly of boron hydride units around a central cluster core, as well as having other interesting features. The small size of the crystals for both compounds necessitated the use of synchrotron X-radiation for sufficient diffracted-beam intensity for crystallographic analysis.

Fourteen new derivatives of ortho-carborane, 1,2-C2B10H12, have been prepared containing functionalized aryl groups attached to one or both of the boron atoms (B9 and B12) antipodal to the carbon atoms, in order to test the suitability of such species for use in the preparation of new categories of phenylene ether carboranylene ketone (PECK) polymers. Model compounds prepared from 9-iodo-ortho-carborane 1 to test synthetic procedures and reactions included the mono-substituted carboranes 9-R1-1,2-C2B10H11 in which the boron attached group R1 was p-tolyl (2), p-HO2CC6H43, p-MeOC6H4COC6H44, p-PhOC6H45 or p-PhCOC6H4OC6H46 (C6H4 represents para-phenylene throughout). Formation of 3 from 2 by CrO3 oxidation showed the capacity of the boron–aryl link to withstand strongly oxidizing conditions. Formation of 4 from 3 and anisole, PhOMe, in trifluoromethane sulfonic acid (TFSA), and of 6 from 5 and benzoic acid, PhCO2H, in TFSA showed such systems could undergo the acylation reactions that would allow polymer formation from suitable diarylcarborane monomers. Compound 7, 9-(3-PhCO-4-PhO-C6H3)-1,2-C2B10H11 an isomer of 6, was also obtained during the synthesis of 6 from 5 and benzoic acid. The diarylcarboranes 9,12-R22-1,2-C2B10H10 with R2 = p-tolyl 9, p-HO2CC6H410 or p-MeOC6H4COC6H411, bearing identical functionalised aryl groups attached to both antipodal boron atoms, were prepared from 9,12-I2-1,2-C2B10H108. Further series of diarylcarboranes 1,9- and 1,12-R32-1,2-C2B10H10, bearing identical aryl groups R3 on one carbon atom (C1) and on the antipodal boron atom (B12), or on the boron atom (B9) antipodal to the unsubstituted carbon atom (C2), have also been prepared and characterized, with R3 = p-tolyl 12, 13, p-HO2CC6H414, 15 or p-MeOC6H4COC6H416, 17.

Slow reactions of isomeric metallacarboranes of general formulae [(NMe2)3TaC2B9H11] (3 isomers) and [(NMe2)3TaC2B9H10Me] (3 isomers) with CD2Cl2 afford quantitative yields of monochloro complexes [Cl(NMe2)2TaC2B9H11] and [Cl(NMe2)2TaC2B9H10Me]. Exposure to CD2Cl2 for months leads to solutions containing about 70% of the dichlorides in three cases. More prolonged exposure of these and the other monochlorides leads to a mixture of boron-substituted complexes. Hydrolysis of [3,3,3-(NMe2)3-3,1,2-TaC2B9H11] by moist toluene results in the formation of the oxo-bridged complex 3,3'-[3,3-(NMe2)2-3,1,2-TaC2B9H11]2(μ-O), characterised by single-crystal X-ray crystallography. The limited solubility of the latter complex in CD2Cl2 eliminates the presence of this compound in the reaction of [3,3,3-(NMe2)3-3,1,2-TaC2B9H11] with CD2Cl2. The reaction of [2,2,2-(NMe2)3-2,1,12-TaC2B9H11] with CH2Br2 in C6D6 quantitatively yields the monobromide [2-Br-2,2-(NMe2)2-2,1,12-TaC2B9H11]. Prolonged reaction with CH2Br2 leads directly to isomeric boron-substituted complexes with no evidence for dibromides. The influence on 11B, 13C and 1H NMR chemical shifts of replacing an amide group in [(NMe2)3TaC2B9H11] with chloride to give [Cl(NMe2)2TaC2B9H11] is also discussed.

Synthetic routes to new C–B–N-containing mesophase materials from borazarene-type precursors have been explored. Such mesophases have the potential for forming C/BN alloys which could be processed in analogous ways to carbonaceous mesophases, e.g. into cokes, fibres and composites. Carbon–boron–nitrogen-based mesophase pitches have been generated at ambient pressure by pyrolysis of borazarenes in the presence of AlCl3. The AlCl3 is essential for mesophase formation in these systems. Pyrolyses of 10-chloro-9-aza-10-boraphenanthrene and 2,2′-bis(dichloroborylamino)biphenyl have both yielded black, optically anisotropic pitches. These precursors form C/BN ceramic alloys which are turbostratic following heat treatment to 1000°C. The alloys show a clear advantage in terms of their oxidation resistance, compared with that of carbon derived from naphthalene mesophase pitch, despite contamination with AlCl3, which probably catalyses their oxidation.

A neutron diffraction study of the salt (PSH+)(nido-7 8-C2B9H12-) reveals the unsymmetrically bridging site of the endo hydrogen in the carborane anion. This is the first neutron diffraction structure determination of a cluster borane or carborane containing natural abundance boron isotopes.

The amine elimination reaction of W(NtBu)2(NHtBu)2 with the di-basic carborane acid nido-7,8-C2B9H13 generates the tungsten(VI) carborane complex, [W(NtBu)(NHtBu)2(C2B9H11)] 1, in which a C2B9H11 ligand has replaced one imido ligand. One of the remaining amido ligands can be substituted by acidic functions, thus 2,6-dimethylphenol yields [W(NtBu)(NHtBu)(2,6-Me2C6H3O)(C2B9H11)] 2, and water gives the μ-oxo complex {[W(NtBu)(NHtBu)(C2B9H11)]2(μ-O)} 3 as a mixture of racemic and meso diastereomers. Complexes 1 and 2 have been characterised by single crystal X-ray diffraction, revealing that replacing an amido ligand in 1 by a weaker π-donor phenoxide in 2 results in shorter metal–amido and metal–dicarbollide distances. Substitution of one amido ligand in 1 by a chloro ligand is achieved using Me3SiCl giving [W(NtBu)(NHtBu)Cl(C2B9H11)] 4. Insertion of acetonitrile into the tungsten amido bond, followed by proton transfer gives {W(NtBu)2[N(H)C(Me)NHtBu](C2B9H11)} 5, the N-tert-butyl acetamidine adduct of the unknown M(1σ2π)3 complex [W(NtBu)2(C2B9H11)]. A structural study by X-ray diffraction reveals that compound 5 contains one short linear (W–N 1.750(3) Å; W–N–C 173.8(3)°) and one longer bent (1.795(3) Å, 151.4(3)°) imido ligand, with a hydrogen bond between the amidine and bent imido ligand, demonstrating for the first time that a sp2 hybridised bent imido ligand can act as a hydrogen bond acceptor.

α,α′-Bis(2-phenyl-1,2-carboran-1-yl)lutidine has been prepared and the solid state structure characterised by X-ray diffraction. The structure reveals weak CH⋯π interactions between molecules, forming a herringbone motif. Surprisingly, the pyridyl nitrogen atoms take part in no long-range interactions.