Approaching the free-ion limit in magnetically isotropic gadolinium(III) via borohydride ligands.

Three new crystalline phases of the borohydride complex, [n-Bu4N][RE(BH4)4] (RE = Gd (1), Gd0.097Y0.903 (2) and Gd0.017Y0.983 (3)), were obtained in pure form and characterised using EPR spectroscopy and AC/DC magnetometry. In all 1-3, unusually slow field-induced magnetic relaxation was observed up to high temperatures, which above 10 K is dominated by power law thermal dependence identified as the two-phonon Raman process. Therefore, weak-field ligands such as borohydride seem to be the perfect candidates for triggering slow magnetic relaxation of magnetically isotropic gadolinium(III) ions.

Gigantic work function in layered AgF2.

AgF2 is a layered material and a correlated charge transfer insulator with an electronic structure very similar to the parent compounds of cuprate high-TC superconductors. It is also interesting as it is a powerful oxidizer. Here we present a first principles computation of its electronic properties in a slab geometry including its work function for the (010) surface (7.76 eV) which appears to be the highest among known materials with non-dipolar surfaces, and surpassing even that of fluorinated diamond (7.24 eV). We demonstrate that AgF2 will show a "broken-gap" type alignment becoming electron doped and promoting injection of holes in many wide band gap insulators if chemical reaction can be avoided. Novel junction devices involving p type and n type superconductors have been proposed. The issue of chemical reaction is discussed in connection with the possibility to create flat AgF2 monolayers achieving high-TC superconductivity. As a first step in this direction, we studied the stability and properties of an isolated AgF2 monolayer.

Facile formation of thermodynamically unstable novel borohydride materials by a wet chemistry route.

A novel wet synthetic method utilizing weakly coordinating anions that yields LiCl-free Zn-based materials for hydrogen storage has recently been reported. Here we show that this method may also be applied for the synthesis of the pure yttrium derivatives, M[Y(BH4)4] (M = K, Rb, Cs). Moreover, it can be extended to the preparation of previously unknown thermodynamically unstable derivatives, Li[Y(BH4)4] and Na[Y(BH4)4]. Importantly, these two H-rich phases cannot be accessed by standard dry (mechanochemical) or solid/gas synthetic methods due to the thermodynamic obstacles. Here we describe their crystal structures and selected important physicochemical properties.

Hydrogen storage materials: room-temperature wet-chemistry approach toward mixed-metal borohydrides.

The poor kinetics of hydrogen evolution and the irreversibility of the hydrogen discharge hamper the use of transition metal borohydrides as hydrogen storage materials, and the drawbacks of current synthetic methods obstruct the exploration of these systems. A wet-chemistry approach, which is based on solvent-mediated metathesis reactions of precursors containing bulky organic cations and weakly coordinating anions, leads to mixed-metal borohydrides that contain only a small amount of "dead mass". The applicability of this method is exemplified by Li[Zn2(BH4)5] and M[Zn(BH4)3] salts (M=Na, K), and its extension to other systems is discussed.

MYb(BH4)4 (M = K, Na) from laboratory X-ray powder data.

Two new borohydrides, potassium ytterbium tetraborohydride, KYb(BH4)4, and sodium ytterbium tetraborohydride, NaYb(BH4)4, have been synthesized via mechanochemical reactions in the solid state. The two compounds are isostructural and both crystallize in the Cmcm space group in the structure reported previously for NaSc(BH4)4 and KY(BH4)4. This crystal structure is composed of isolated homoleptic [Yb(BH4)4](-) anions surrounded by M(+) cations (M = Na, K). The packing of the M(+) cations and [Yb(BH4)4](-) anions is a distorted variant of the hexagonal NiAs structure type, with M(+) forming distorted trigonal prisms, i.e. M6. Each second prism surrounds a [Yb(BH4)4](-) anion, while the [Yb(BH4)4](-) anions are arranged into deformed octahedra around the M(+) cations.

Synthesis Method of Unsolvated Organic Derivatives of Metal Borohydrides.

A new, scalable, wet-chemistry single-pot method of synthesising pure unsolvated organic derivatives of metal borohydrides is presented. The metathetic reaction in a weakly coordinating solvent is exemplified by the synthesis of [(n-C4H9)4N][Y(BH4)4] and [Ph4P][Y(BH4)4] systems. For the latter compound, the crystal structure was solved and described. Organic borohydride salts obtained by the new method can find various applications, e.g., can be used as precursors in synthesis of hydrogen-rich mixed-metal borohydrides-promising materials for solid-state chemical storage of hydrogen.

Synthesis, Polymorphism and Thermal Decomposition Process of (n-C4H9)4NRE(BH4)4 for RE = Ho, Tm and Yb.

In total, three novel organic derivatives of lanthanide borohydrides, n-But4NRE(BH4)4 (TBAREB), RE = Ho, Tm, Yb, have been prepared utilizing mechanochemical synthesis and purified via solvent extraction. Studies by single crystal and powder X-ray diffraction (SC-XRD and PXRD) revealed that they crystalize in two polymorphic forms, α- and ß-TBAREB, adopting monoclinic (P21/c) and orthorhombic (Pnna) unit cells, previously found in TBAYB and TBAScB, respectively. Thermal decomposition of these compounds has been investigated using thermogravimetric analysis and differential scanning calorimetry (TGA/DSC) measurements, along with the analysis of the gaseous products with mass spectrometry (MS) and with analysis of the solid decomposition products with PXRD. TBAHoB and TBAYbB melt around 75 °C, which renders them new ionic liquids with relatively low melting points among borohydrides.

Incorporation of expanded organic cations in dysprosium(III) borohydrides for achieving luminescent molecular nanomagnets.

Luminescent single-molecule magnets (SMMs) constitute a class of molecular materials offering optical insight into magnetic anisotropy, magnetic switching of emission, and magnetic luminescent thermometry. They are accessible using lanthanide(III) complexes with advanced organic ligands or metalloligands. We present a simple route to luminescent SMMs realized by the insertion of well-known organic cations, tetrabutylammonium and tetraphenylphosphonium, into dysprosium(III) borohydrides, the representatives of metal borohydrides investigated due to their hydrogen storage properties. We report two novel compounds, [n-Bu4N][DyIII(BH4)4] (1) and [Ph4P][DyIII(BH4)4] (2), involving DyIII centers surrounded by four pseudo-tetrahedrally arranged BH4- ions. While 2 has higher symmetry and adopts a tetragonal unit cell (I41/a), 1 crystallizes in a less symmetric monoclinic unit cell (P21/c). They exhibit yellow room-temperature photoluminescence related to the f-f electronic transitions. Moreover, they reveal DyIII-centered magnetic anisotropy generated by the distorted arrangement of four borohydride anions. It leads to field-induced slow magnetic relaxation, well-observed for the magnetically diluted samples, [n-Bu4N][YIII0.9DyIII0.1(BH4)4] (1@Y) and [Ph4P][YIII0.9DyIII0.1(BH4)4] (2@Y). 1@Y exhibits an Orbach-type relaxation with an energy barrier of 26.4(5) K while only the onset of SMM features was found in 2@Y. The more pronounced single-ion anisotropy of DyIII complexes of 1 was confirmed by the results of the ab initio calculations performed for both 1-2 and the highly symmetrical inorganic DyIII borohydrides, α/ß-Dy(BH4)3, 3 and 4. The magneto-luminescent character was achieved by the implementation of large organic cations that lower the symmetry of DyIII centers inducing single-ion anisotropy and separate them in the crystal lattice enabling the emission property. These findings are supported by the comparison with 3 and 4, crystalizing in cubic unit cells, which are not emissive and do not exhibit SMM behavior.

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.

Ferromagnetic ground state for a hypothetical iron-based extended metal atom chain.

Theoretical calculations for the first tri-iron-based extended metal atom chain (EMAC) molecule are reported. The studied triple-high-spin (S = 6) complex exhibits ferromagnetic ordering (according to Ising and spin-projection approximations), which renders it unique among all previously prepared and theoretically calculated EMAC compounds. This ordering originates from the prevailing ferromagnetic nearest-neighbor interactions, while the magnetic superexchange between terminal Fe(2+) sites is weaker and antiferromagnetic. Calculations indicate that this linear chain system based on a tri-iron core shows potential for the development of spin-frustrated behavior, which could be achieved through rational modification of the equatorial and axial ligands.

M[Y(BH4)4] and M2Li[Y(BH4)(6-x)Clx] (M = Rb, Cs): new borohydride derivatives of yttrium and their hydrogen storage properties.

Two novel bimetallic borohydrides, Rb[Y(BH4)4] and Cs[Y(BH4)4], have been synthesised via mechanochemical reactions between MBH4, M = Rb or Cs and Y(BH4)3. Both are ionic compounds comprising complex anions [Y(BH4)4](-) and alkali metal cations, and they adopt crystal structures which were not observed previously for quasi-ternary borohydrides. LiCl, a by-product from the synthesis of Y(BH4)3, is not an unreactive dead-weight but rather it promotes formation of trimetallic borohydride-chlorides, M2Li[Y(BH4)(6-x)Cl(x)], which crystallise in the elpasolite-type structures. Formation of Cs2Li[Y(BH4)(6-x)Cl(x)] takes place during the mechanochemical synthesis of Cs[Y(BH4)4] at room temperature, while that of Rb2Li[Y(BH4)(6-x)Cl(x)] only after heating of the sample above the melting point. The M[Y(BH4)4]/M2Li[Y(BH4)(6-x)Cl(x)] mixtures slowly release H2 when melted (T(m) = 170 °C for M = Rb, 210 °C for M = Cs) but the main stage of thermal decomposition occurs at a T(dec) of ~270 °C, similarly to what is observed for K[Y(BH4)4] and NaBH4/Y(BH4)3 composites. We also demonstrate how the T(dec) of M[Y(BH4)4] salts may be tuned by choosing the MBH4 precursor of the appropriate Lewis basicity.