The Preparation, Characterization, and Pressure-Influenced Dihydrogen Interactions of Tetramethylphosphonium Borohydride.

Tetramethylphosphonium borohydride was synthesized via an ion metathesis reaction in a weakly-coordinating aprotic environment. [(CH3)4P]BH4, in contrast to related [(CH3)4N]+ compounds which tend to crystallize in a tetragonal system, adopts the distorted wurtzite structure (P63mc), resembling some salts containing analogous ions of As and Sb. [(CH3)4P]BH4 decomposes thermally in several endo- and exothermic steps above ca. 240 °C. This renders it more stable than [(CH3)4N]BH4, with a lowered temperature of decomposition onset by ca. 20 °C and solely exothermic processes observed. Raman spectra measured at the 0-10 GPa range indicate that a polymorphic transition occurs within 0.53-1.86 GPa, which is further confirmed by the periodic DFT calculations. The latter suggests a phase transition around 0.8 GPa to a high-pressure phase of [(CH3)4N]BH4. The P63mc phase seems to be destabilized under high pressure by relatively closer dihydrogen interactions, including the C-H…H-C contacts.

Towards hydrogen-rich ionic (NH4)(BH3NH2BH2NH2BH3) and related molecular NH3BH2NH2BH2NH2BH3.

Attempts of the synthesis of ionic (NH4)(BH3NH2BH2NH2BH3) via a metathetical approach resulted in a mixture of the target compound and partly dehydrogenated molecular NH3BH2NH2BH2NH2BH3 product. The mixed specimen was characterised by NMR and vibrational spectroscopies, and the cocrystal structure was analyzed from powder X-ray diffraction data supported by theoretical density functional theory calculations. The compound crystallises in a P21/c unit cell with the lattice parameters of a = 13.401(11) Å, b = 13.196(8) Å, c = 17.828(12) Å, ß = 128.83(4)°, V = 2556(3) Å3 and Z = 16. Despite their impressive hydrogen content, similar to ammonia borane, both title compounds release hydrogen substantially polluted with borazine and traces of ammonia and diborane.

Synthesis, Structure, and Electric Conductivity of Higher Hydrides of Ytterbium at High Pressure.

While most of the rare-earth metals readily form trihydrides, due to increased stability of the filled 4f electronic shell for Yb(II), only YbH2.67, formally corresponding to YbII(YbIIIH4)2 (or Yb3H8), remains the highest hydride of ytterbium. Utilizing the diamond anvil cell methodology and synchrotron powder X-ray diffraction, we have attempted to push this limit further via hydrogenation of metallic Yb and Yb3H8. Compression of the latter has also been investigated in a neutral pressure-transmitting medium (PTM). While the in situ heating of Yb facilitates the formation of YbH2+x hydrides, we have not observed clear qualitative differences between the systems compressed in H2 and He or Ne PTM. In all of these cases, a sequence of phase transitions occurred within ca. 13-18 GPa (P3̅1m-I4/m phase) and around 27 GPa (to the I4/mmm phase). The molecular volume of the systems compressed in H2 PTM is ca. 1.5% larger than of those compressed in inert gases, suggesting a small hydrogen uptake. Nevertheless, hydrogenation toward YbH3 is incomplete, and polyhydrides do not form up to the highest pressure studied here (ca. 75 GPa). As pointed out by electronic transport measurements, the mixed-valence Yb3H8 retains its semiconducting character up to >50 GPa, although the very low remnant activation energy of conduction (<5 meV) suggests that metallization under further compression should be achievable. Finally, we provide a theoretical description of a hypothetical stoichiometric YbH3.

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.

Extending the chemistry of weakly basic ligands: solvates of Ag+ and Cu+ stabilized by [Al{OC(CF3)3}4]- anion as model examples in the screening of useful weakly interacting solvents.

Weakly Coordinating Anions (WCAs) facilitate the formation of exotic "naked" cationic species. However, the feasibility of the respective synthesis approaches may be limited by the basicity of the solvent utilized, as the latter is one of the most important factors determining the solvation ability. In this work, we focus on a series of novel complexes of Ag(i) and Cu(i) with weakly basic ligands such as CH2Cl2, Cl3CCN and SO2 stabilized by perfluorinated alkoxyaluminate, Al[(ORF)4]-, RF = C(CF3)3. The discussion includes their synthesis protocols, crystal structures, vibrational spectra and thermal stability (TGA/DSC/EGA). We show that the Cu-SO2 adducts present exceptional stability in relation to other metal-SO2 complexes. To broaden the scope of weakly basic ligands which could prove helpful in the development of chemistry with WCAs, the screening of potential candidates based on DFT calculations is presented.

Building blocks for the chemistry of perfluorinated alkoxyaluminates [Al{OC(CF3)3}4]-: simplified preparation and characterization of Li+-Cs+, Ag+, NH4+, N2H5+ and N2H7+ salts.

Advanced weakly coordinating anions (WCAs) significantly facilitate synthesis of various exotic chemical compounds and novel, potentially useful materials. One of such anions - [Al{OC(CF3)3}4]-, denoted [Al(ORF)4]-, appears particularly convenient, as it can be easily prepared from the commercially available alanates and HOC(CF3)3. Here we present a thorough characterization of a series of solvent-free M[Al(ORF)4] salts, M = Li-Cs, Ag, NH4, N2H5 and N2H7, and related compounds of monovalent cations, which are crucial starting materials for further work with these species. Notably, the corresponding synthetic protocols are updated by an improved method for fast, facile and easily scalable synthesis of Li[Al(ORF)4], which remains the most useful primary source of the anion. The physico-chemical properties of these salts including crystal structures, thermal stability by TG/DSC, vibrational spectra as well as solubility are discussed in a systematic fashion.

Two new derivatives of scandium borohydride, MSc(BH4)4, M = Rb, Cs, prepared via a one-pot solvent-mediated method.

Two new derivatives of scandium borohydride, MSc(BH4)4, M = Rb, Cs, were prepared via two different synthetic methodologies - mechanochemical and solvent-mediated. The latter led to products free from the commonly present halide contamination, as evidenced by powder X-ray diffraction, FTIR spectroscopy and TGA/DSC/MS. The rubidium derivative crystallizes in an orthorhombic unit cell of the Pbcm space group in the structure which can be derived from ht-CrVO4, while CsSc(BH4)4 adopts a monoclinic (P21/c) unit cell which has monazite (CePO4) as a structural aristotype. Thermal decomposition of the samples obtained using the two methods was compared, evidencing the influence of lithium chloride on the decomposition reactions as well as chemical identity of the decomposition products. Uncontaminated MSc(BH4)4 salts decompose thermally yielding nearly pure hydrogen with the maximum decomposition rate at 230 °C and 235 °C, for M = Rb and Cs, respectively. Among the by-products of the solvent-mediated synthesis, a new cubic crystalline phase of M3ScCl6, M = Rb, Cs, has been detected.

Silver route to cuprate analogs.

The parent compound of high-[Formula: see text] superconducting cuprates is a unique Mott insulator consisting of layers of spin-[Formula: see text] ions forming a square lattice and with a record high in-plane antiferromagnetic coupling. Compounds with similar characteristics have long been searched for without success. Here, we use a combination of experimental and theoretical tools to show that commercial [Formula: see text] is an excellent cuprate analog with remarkably similar electronic parameters to [Formula: see text] but larger buckling of planes. Two-magnon Raman scattering and inelastic neutron scattering reveal a superexchange constant reaching 70% of that of a typical cuprate. We argue that structures that reduce or eliminate the buckling of the [Formula: see text] planes could have an antiferromagnetic coupling that matches or surpasses the cuprates.

Amidoboranes of rubidium and caesium: the last missing members of the alkali metal amidoborane family.

We report the synthesis and physicochemical characterization of two ammonia borane derivatives: rubidium amidoborane (RbNH2BH3) and caesium amidoborane (CsNH2BH3). Both compounds undergo solid-solid phase transition upon heating and then evolve pure hydrogen at temperatures lower than 125 °C. The phase transition is clearly seen in the Raman spectra. We present crystal structures of both low- and high-temperature forms of the title compounds which were solved from powder X-ray data.

High-Pressure Behavior of Silver Fluorides up to 40 GPa.

A combined experimental-theoretical study of silver(I) and silver(II) fluorides under high pressure is reported. For AgI, the CsCl-type structure is stable to at least 39 GPa; the overtone of the IR-active mode is seen in the Raman spectrum. Its AgIIF2 sibling is a unique compound in many ways: it is more covalent than other known difluorides, crystallizes in a layered structure, and is enormously reactive. Using X-ray diffraction and guided by theoretical calculations (density functional theory), we have been able to elucidate crystal structures of high-pressure polymorphs of AgF2. The transition from ambient pressure to an unprecedented nanotubular structure takes place via an intermediate orthorhombic layered structure, which lacks an inversion center. The observed phase transitions are discussed within the broader framework of the fluorite → cotunnite → Ni2In series, which has been seen for other metal difluorides.

Persistence of Mixed and Non-intermediate Valence in the High-Pressure Structure of Silver(I,III) Oxide, AgO: A Combined Raman, X-ray Diffraction (XRD), and Density Functional Theory (DFT) Study.

The X-ray diffraction data collected up to ca. 56 GPa and the Raman spectra measured up to 74.8 GPa for AgO, or AgIAgIIIO2, which is a prototypical mixed valence (disproportionated) oxide, indicate that two consecutive phase transitions occur: the first-order phase transition occurs between 16.1 GPa and 19.7 GPa, and a second-order phase transition occurs at ca. 40 GPa. All polymorphic forms host the square planar [AgIIIO4] units typical of low-spin AgIII. The disproportionated Imma form persists at least up to 74.8 GPa, as indicated by Raman spectra. Theoretical hybrid density functional theory (DFT) calculations show that the first-order transition is phonon-driven. AgO stubbornly remains disproportionated up to at least 100 GPa-in striking contrast to its copper analogue-and the fundamental band gap of AgO is ∼0.3 eV at this pressure and is weakly pressure-dependent. Metallization of AgO is yet to be achieved.

Ag2S2O8 meets AgSO4: the second example of metal-ligand redox isomerism among inorganic systems.

Valence (redox) isomerism based on electron exchange between a metal and a ligand is immensely rare in purely inorganic systems, with only one documented case, that of PbS2 which adopts two polymorphic forms corresponding to Pb(iv)(S2-)2 and Pb(ii)(S22-). Here we have taken advantage of metathetic reactions using salts of weakly coordinating anions and we have prepared for the first time Ag(i)2S2O8, silver(i) peroxydisulphate. The title compound crystallizes in the non-centrosymmetric Cc space group with partial disorder of the anionic sublattice. Ag(i)2S2O8 is a highly thermally unstable diamagnetic and colourless valence isomer of the antiferromagnetic and black Ag(ii)SO4, described by us in the past.

Local and Cooperative Jahn-Teller Effect and Resultant Magnetic Properties of M2AgF4 (M = Na-Cs) Phases.

The crystal structure, magnetic properties, heat capacity, and Raman spectra of double-perovskite M2AgF4 (M = K, K3/4Rb1/4, K1/2Rb1/2, K1/4Rb3/4, and Rb) phases have been examined, adding to the body of previous results for the M = Na, Cs derivatives. The results suggest that double-perovskite K2AgF4 adopts a disordered orthorhombic Bmab structure with an antiferrodistortive arrangement of the elongated and tilted [AgF6] octahedra rather than the structure with the ferrodistortive arrangement of compressed octahedra, as suggested previously (Mazej, Z.; Goreshnik, E.; Jaglicic, Z.; Gawel, B.; Lasocha, W.; Grzybowska, D.; Jaron, T.; Kurzydlowski, D.; Malinowski, P. J.; Kozminski, W.; Szydlowska, J.; Leszczynski, P. J.; Grochala, W. KAgF3, K2AgF4 and K3Ag2F7: important steps towards a layered antiferromagnetic fluoroargentate(II). CrystEngComm 2009, 11, 1702-1710). A re-examination of the previously collected single-crystal X-ray diffraction data confirms the current structure assignment, and it is also in agreement with recent theoretical calculations. High-field electron paramagnetic resonance spectra reaffirm the presence of elongated [AgF6] octahedra in the crystal structure of all M2AgF4 phases studied. The local structure of the M = K derivative is most complex, with regions of the sample that are quite orthorhombically distorted, whereas other regions more closely resemble the tetragonal phase. The mixed-cation K/Rb phases are also inhomogeneous, containing regions of the pure K compound and regions of another high-symmetry phase (likely tetragonal) of a mixed (Rb-richer) compound with unknown composition. The temperature-resolved phase diagram of all K/Rb phases has been established and positioned within the entire M = Na, K, Rb, Cs series.

Complete Series of Alkali-Metal M(BH3NH2BH2NH2BH3) Hydrogen-Storage Salts Accessed via Metathesis in Organic Solvents.

We report a new efficient way of synthesizing high-purity hydrogen-rich M(BH3NH2BH2NH2BH3) salts (M = Li, Na, K, Rb, Cs). The solvent-mediated metathetic synthesis applied here uses precursors containing bulky organic cations and weakly coordinating anions. The applicability of this method permits the entire series of alkali-metal M(BH3NH2BH2NH2BH3) salts (M = Li, Na, K, Rb, Cs) to be obtained, thus enabling their comparative analysis in terms of crystal structures and hydrogen-storage properties. A novel polymorphic form of Verkade's base (C18H39N4PH)(BH3NH2BH2NH2BH3) precursor was also characterized structurally. For all compounds, we present a comprehensive structural, spectroscopic, and thermogravimetric characterization (PXRD, NMR, FTIR, Raman, and TGA/DSC/MS).

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.

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.

Probing Lewis acidity of Y(BH4)3 via its reactions with MBH4 (M = Li, Na, K, NMe4).

High-energy milling of Y(BH(4))(3) (containing LiCl as a by-product, which has not been removed) with MBH(4) (M = Li, Na, K, (CH(3))(4)N) leads to the first two examples of quasi-ternary yttrium borohydrides: KY(BH(4))(4) and (CH(3))(4)NY(BH(4))(4), while no chemical reaction is observed for LiBH(4) and NaBH(4). KY(BH(4))(4) is isostructural to NaSc(BH(4))(4) (Cmcm, a = 8.5157(4) Å, b = 12.4979(6) Å, c = 9.6368(5) Å, V = 1025.62(9) Å(3), Z = 4), while (CH(3))(4)NY(BH(4))(4) crystallises in primitive orthorhombic cell, similarly to KSc(BH(4))(4) (Pnma, a = 15.0290(10) Å, b = 8.5164(6) Å, c = 12.0811(7) Å, V = 1546.29(17) Å(3), Z = 4). The thermal decomposition of hydrogen-rich KY(BH(4))(4) (8.6 wt.% H) involves the formation of an unidentified intermediate at 200 °C and recovery of KBH(4) at higher temperatures; at 410 °C, KCl and YH(2) are observed. The thermal decomposition of (CH(3))(4)NY(BH(4))(4) occurs via two partly overlapping endothermic steps with concomitant emission of H(2) and organic compounds. Heating of a NaBH(4)/Y(BH(4))(3) mixture above 165 °C results in a mixed-cation mixed-anion borohydride, NaY(BH(4))(2)Cl(2), but not NaY(BH(4))(4). The reduced reactivity of Y(BH(4))(3) towards borohydride Lewis bases when compared to hypothetical scandium borohydride can be explained by the lower Lewis acidity of Y(BH(4))(3) than Sc(BH(4))(3).

Tetra-methyl-ammonium borohydride from powder data.

In the crystal structure of the title compound, C(4)H(12)N(+)·BH(4) (-), the tetra-methyl-ammonium cations are situated on special positions with site symmetry [Formula: see text]m2. The borohydride anions are situated on special positions with 4mm site symmetry and show rotational disorder around the fourfold axis.