Desolvation and Dehydrogenation of Solvated Magnesium Salts of Dodecahydrododecaborate: Relationship between Structure and Thermal Decomposition

Divalent closo-monocarborane solvates for solid-state ionic conductors

Li-ion batteries have held the dominant position in battery research for the last 30+ years. However, due to inadequate resources and the cost of necessary elements (e.g., lithium ore) in addition to safety issues concerning the components and construction, it has become more important to look at alternative technologies. Multivalent metal batteries with solid-state electrolytes are a potential option for future battery applications. The synthesis and characterisation of divalent hydrated closo-monocarborane salts - Mg[CB₁₁H₁₂]₂·xH₂O, Ca[CB₁₁H₁₂]₂·xH₂O, and Zn[CB₁₁H₁₂]₂·xH₂O - have shown potential as solid-state electrolytes. The coordination of a solvent (e.g. H₂O) to the cation in these complexes shows a significant improvement in ionic conductivity, i.e. for Zn[CB₁₁H₁₂]₂·xH₂O dried at 100 °C (10^-3 S cm^-1 at 170 °C) and dried at 150 °C (10^-5 S cm^-1 at 170 °C). Solvent choice also proved important with the ionic conductivity of Mg[CB₁₁H₁₂]₂·3en (en = ethylenediamine) being higher than that of Mg[CB₁₁H₁₂]₂·3.1H₂O (2.6 × 10^-5 S cm^-1 and 1.7 × 10^-8 S cm^-1 at 100 °C, respectively), however, the oxidative stability was lower (<1 V (Mg²⁺/Mg) and 1.9 V (Mg²⁺/Mg), respectively). Thermal characterisation of the divalent closo-monocarborane salts showed melting and desolvation, prior to high temperature decomposition.

A low temperature pyrolytic route to amorphous quasi-hexagonal boron nitride from hydrogen rich (NH4)3Mg(BH4)5

Pure amorphous quasi-hexagonal boron nitride with minute amounts of amorphous quasi-cubic form was obtained via thermal decomposition of a novel tri-ammonium magnesium penta-borohydride precursor, (NH4)3Mg(BH4)5, in the temperature range of 220-250 °C, which is significantly lower than 1000-1500 °C applied in industrial approaches. The (NH4)3Mg(BH4)5 precursor, the most hydrogen rich mixed-cation borohydride salt known to date (21 wt% H), was prepared via low temperature high-energy dry disc-milling. The compound adopts a tetragonal I4/mcm unit cell isostructural with Rb3Mg(BH4)5 and Cs3Mg(BH4)5. The multi-step thermal decomposition yields hydrogen contaminated with B2H6 and borazine volatiles. The solid residue rinsed with water corresponds to amorphous boron nitride of high purity as evidenced by 11B MAS NMR, PXRD, FTIR and EDX analyses.

Manifestations of Weak O-H···F Hydrogen Bonding in M(H2O) n(B12F12) Salt Hydrates: Unusually Sharp Fourier Transform Infrared ν(OH) Bands and Latent Porosity (M = Mg-Ba, Co, Ni, Zn)

Eight M(H₂O) _n(Z) salt hydrates were characterized by single-crystal X-ray diffraction (Z^2- = B₁₂F₁₂^2-): M = Ca, Sr, n = 7; M = Mg, Co, Ni, Zn, n = 6; M = Ba, n = 4, 5. Weak O-H···F hydrogen bonding between the M(H₂O) _n²⁺ cations and Z^2- resulted in room-temperature Fourier transform infrared (FTIR) spectra having sharp ν(OH) bands, with full widths at half max of 10-30 cm^-1, which are much more narrow than ν(OH) bands in room temperature FTIR spectra of most salt hydrates. Clearly resolved ν_asym(OH/OD) and ν_sym(OH/OD) bands with Δν(OH) as small as 17 cm^-1 and Δν(OD) as small as 11 cm^-1 were observed (Δν(OX) = ν_asym(OX) - ν_sym(OX)). The isomorphic hexahydrates ( R3̅) have two fac-(H₂O)₃ sets of H₂O ligands and nearly octahedral coordination spheres. They exhibited four resolvable ν(OH) bands, one ν_asym(OH)/ν_sym(OH) pair for H₂O ligands with longer O(H)···F distances and one ν_asym(OH)/ν_sym(OH) pair for H₂O ligands with shorter O(H)···F distances. The ν(OH) bands for the three H₂O molecules with shorter, slightly stronger O(H)···F hydrogen bonds were broader, more intense, and red-shifted by ca. 25 cm^-1 relative to the bands for the three other H₂O molecules, the first time that such small differences in relatively weak O(H)···F hydrogen bonds in the same crystalline hexahydrate have resulted in observable IR spectroscopic differences at room temperature. For the first time room temperature ν(OH) values for salt hexahydrates showed the monotonic progression Mg²⁺ > Co²⁺ > Ni²⁺ > Zn²⁺, essentially the same progression as the p K_a values for these metal ions in aqueous solution. A further manifestation of the weak O-H···F hydrogen bonding in these hydrates is the latent porosity exhibited by Ba(H₂O)_5,8(Z), Sr(H₂O) _n,m(Z), and Ca(H₂O)_4,6(Z). Finally, the H₂O/D₂O exchange reaction Co(D₂O)₆(Z) → Co(H₂O)₆(Z) was ca. 50% complete in 1 h at 50 °C in N₂/17 Torr H₂O( g).

Facile synthesis of anhydrous alkaline earth metal dodecaborates MB12H12 (M = Mg, Ca) from M(BH4)2

Thermal decomposition of MB₁₂H₁₂ (M = Mg, Ca) forms H-deficient monomers MB₁₂H_12−x containing icosahedral B₁₂ skeletons and is followed by the formation of (MB_yH_z)_n polymers.