New, systematic syntheses of boron hydrides via hydride ion abstraction reactions: preparation of B2H6, B4H10, B5H11, and B10H14

Interplay between the Reorientational Dynamics of the B3H8 - Anion and the Structure in KB3H8

The structure and reorientational dynamics of KB₃H₈ were studied by using quasielastic and inelastic neutron scattering, Raman spectroscopy, first-principles calculations, differential scanning calorimetry, and in situ synchrotron radiation powder X-ray diffraction. The results reveal the existence of a previously unknown polymorph in between the α'- and β-polymorphs. Furthermore, it was found that the [B₃H₈]^- anion undergoes different reorientational motions in the three polymorphs α, α', and β. In α-KB₃H₈, the [B₃H₈]^- anion performs 3-fold rotations in the plane created by the three boron atoms, which changes to a 2-fold rotation around the C ₂ symmetry axis of the [B₃H₈]^- anion upon transitioning to α'-KB₃H₈. After transitioning to β-KB₃H₈, the [B₃H₈]^- anion performs 4-fold rotations in the plane created by the three boron atoms, which indicates that the local structure of β-KB₃H₈ deviates from the global cubic NaCl-type structure. The results also indicate that the high reorientational mobility of the [B₃H₈]^- anion facilitates the K⁺ cation conductivity, since the 2-orders-of-magnitude increase in the anion reorientational mobility observed between 297 and 311 K coincides with a large increase in K⁺ conductivity.

Thermodynamic Hydricity of Small Borane Clusters and Polyhedral closo-Boranes

Thermodynamic hydricity (HDA^MeCN) determined as Gibbs free energy (ΔG°[H]⁻) of the H⁻ detachment reaction in acetonitrile (MeCN) was assessed for 144 small borane clusters (up to 5 boron atoms), polyhedral closo-boranes dianions [B_nH_n]²⁻, and their lithium salts Li₂[B_nH_n] (n = 5–17) by DFT method [M06/6-311++G(d,p)] taking into account non-specific solvent effect (SMD model). Thermodynamic hydricity values of diborane B₂H₆ (HDA^MeCN = 82.1 kcal/mol) and its dianion [B₂H₆]²⁻ (HDA^MeCN = 40.9 kcal/mol for Li₂[B₂H₆]) can be selected as border points for the range of borane clusters’ reactivity. Borane clusters with HDA^MeCN below 41 kcal/mol are strong hydride donors capable of reducing CO₂ (HDA^MeCN = 44 kcal/mol for HCO₂⁻), whereas those with HDA^MeCN over 82 kcal/mol, predominately neutral boranes, are weak hydride donors and less prone to hydride transfer than to proton transfer (e.g., B₂H₆, B₄H₁₀, B₅H₁₁, etc.). The HDA^MeCN values of closo-boranes are found to directly depend on the coordination number of the boron atom from which hydride detachment and stabilization of quasi-borinium cation takes place. In general, the larger the coordination number (CN) of a boron atom, the lower the value of HDA^MeCN.

Li(NH3)B3H8: a new ionic liquid octahydrotriborate

We report a strategy of using small cations to construct ionic liquid octahydrotriborate. The novel liquid Li(NH3)B3H8, prepared by a facile reaction of NH4B3H8 with LiH, froze below -33.4 °C and crystallized into a monoclinic unit cell with lattice parameters of a = 8.813(1) Å, b = 8.8626(1) Å, c = 8.2076(1) Å, and β = 110.1046(5)°, providing a promising functional liquid octahydrotriborate with the highest B3H8- content and lowest freezing temperature.

Synthetic and structural studies of phosphine coordinated boronium salts

The reaction of BrH₂B·SMe₂ with primary and secondary phosphines affords a range of boronium salts of the form [H₂B(PR₃)₂]Br (PR₃ = PHCy₂1; PHPh₂, 2; PH₂Cy, 3), which have been fully characterised including solid-state determinations. Reactions of bulky tertiary phosphines, e.g., PCy₃ and PPh₃, with BrH₂B·SMe₂ do not proceed beyond the phosphine-stabilised bromoborane adducts, however, the smaller tertiary phosphine PMe₂Ph readily proceeds to form [H₂B(PMe₂Ph)₂]Br (4). The formation of the unsymmetrical boronium salts [H₂B(PH₂Cy)(PHCy₂)]Br (5) and [H₂B(PHCy₂)(PHPh₂)]Br (6) was observed by in situ NMR spectroscopy, however, the compounds were found to spontaneously disproportionate to their respective homophosphine boronium cations, even on prolonged storage in solution at -78 °C. Di- and triphosphines were found to form ring-closed boronium salts to afford [H₂B(κ²-P,P'-diphosphine)]Br (diphosphine = dppe, 7; dcpe, 8; dmpe, 9; dppf, 10; dppf with [AsF₆]^- counterion, 11; amphos, 12). The analogous methodology with Br₂HB·SMe₂ proved less generally applicable due to an accessible decomposition pathway being available to boronium salts bearing primary and secondary phosphines, leading to the formation of phosphonium salts, although [BrHB(dcpe)]Br (13), [BrHB(PMe₂Ph)₂]Br (14) and [BrHB(amphos)]Br (15) were synthesised with varying degrees of success. Reaction of BrH₂B·SMe₂ with diphars afforded the boronium salt [H₂B(κ²-P,P'-diphars)]Br (16), which featured two pendant arsine arms. Similarly, triphos was found to react with BrH₂B·SMe₂ to give [H₂B(κ²-P,P'-triphos)]Br (17), which featured a pendant phosphine arm. Substitution of the bromide counter anion with either hexafluoroarsenate or hexfluoroantimonate anions revealed weak hydrogen bonds between the P-H bonds of the boronium cations and the anions, that appeared through NMR studies to be retained in solution (where hydrogen bonding order was determined to be Br^- > [SbF₆]^-/[AsF₆]^-). This was further demonstrated by comparison of solid-state structures and solution NMR data of 1 with [H₂B(PHCy₂)₂][SbF₆] (18), 4 with [H₂B(PMe₂Ph)₂][AsF₆] (19) and 17 with [H₂B(triphos)][AsF₆] (20).

Facile preparation and dehydrogenation of unsolvated KB3H8

A convenient route was developed to produce unsolvated KB₃H₈. This compound can release hydrogen and minor boranes by subsequent cleavage of its B–H and B–H–B bonds in the 150–250 °C temperature range. And pure K₂B₁₂H₁₂ can be prepared through KB₃H₈ pyrolysis, which is an optional approach to produce dodecaborate compounds.

A novel method for the synthesis of solvent-free Mg(B3H8)2

A novel and solvent-free method for the synthesis of Mg(B₃H₈)₂ by a gas–solid reaction between B₂H₆ and Mg₂NiH₄ is presented.

Unexpected formation of new fluoroboranes from the reaction of NMe4B3H8 with BF3 and MeC identical with CH: exo-2-FB4H9 and trans-MeCH=CHBF2

The new fluoroboranes exo-2-FB4H9 1 and trans-MeCH=CHBF2 2 have been obtained unexpectedly and in good yield from the reaction of tetramethylammonium octahydrotriborate (NMe4B3H8) with boron trifluoride and propyne (MeC identical to CH).

Formation of metallaboranes from the group IV transition metals and pentaborane(9): crystal and molecular structure of [(Cp(2)Zr)(2)B(5)H(8)][B(11)H(14)]

The reactions between [(C(5)H(5))(2)MCl(2)] (where M = Ti, Zr, Hf) and Li[B(5)H(8)] in a variety of solvents have been investigated. In the case of Zr, a pale orange solid, mu-(Cp(2)ClZr)B(5)H(8) (1), is formed in 70% yield. Compound 1 exists as a B(5)H(9) cage with a Cp(2)ClZr moiety replacing a bridging H atom. The variable temperature NMR spectra of 1 reveal two fluxional processes, one (DeltaG(++) = 54 kJ mol(-1)) which renders a plane of symmetry in the molecule and a higher temperature one (DeltaG(++) = 48 kJ mol(-1)) which renders all the basal B atoms equivalent. Dynamic processes are suggested to account for these observations. Passage of a CH(2)Cl(2) solution of 1 through silica gel affords 2, [(Cp(2)Zr)(2)B(5)H(8)][B(11)H(14)], a yellow, air-stable, crystalline solid, in 14% yield. The cation in 2, [(Cp(2)Zr)(2)B(5)H(8)](+), consists of a distorted spiro[2.2]pentane-like B(5) moiety comprising two B(3) triangles sharing a naked boron vertex. The two triangles are twisted 73 degrees with respect to each other, and the two [Cp(2)Zr] groups bond in a trihapto arrangement to the two opposite B-B-B edges. Each exterior B-Zr edge is H-bridged, and the B atoms possess terminal hydrogens. Reactions of Cp(2)HfCl(2) with Li[B(5)H(8)] lead to the formation of the analogue of 2, [(Cp(2)Hf)(2)B(5)H(8)][B(11)H(14)] (3). The precursor to 3, that is, the Hf analogue of 1, is not observed. Reaction between Li[B(5)H(8)] and Cp(2)TiCl(2) afforded no identifiable products, but reaction with CpTiCl(3) resulted in cage coupling and the formation of B(10)H(14).