Small-pore hydridic frameworks store densely packed hydrogen.

Nanoporous materials have attracted great attention for gas storage, but achieving high volumetric storage capacity remains a challenge. Here, by using neutron powder diffraction, volumetric gas adsorption, inelastic neutron scattering and first-principles calculations, we investigate a magnesium borohydride framework that has small pores and a partially negatively charged non-flat interior for hydrogen and nitrogen uptake. Hydrogen and nitrogen occupy distinctly different adsorption sites in the pores, with very different limiting capacities of 2.33 H2 and 0.66 N2 per Mg(BH4)2. Molecular hydrogen is packed extremely densely, with about twice the density of liquid hydrogen (144 g H2 per litre of pore volume). We found a penta-dihydrogen cluster where H2 molecules in one position have rotational freedom, whereas H2 molecules in another position have a well-defined orientation and a directional interaction with the framework. This study reveals that densely packed hydrogen can be stabilized in small-pore materials at ambient pressures.

Towards pump-probe single-crystal XFEL refinements for small-unit-cell systems.

Serial femtosecond crystallography for small-unit-cell systems has so far seen very limited application despite obvious scientific possibilities. This is because reliable data reduction has not been available for these challenging systems. In particular, important intensity corrections such as the partiality correction critically rely on accurate determination of the crystal orientation, which is complicated by the low number of diffraction spots for small-unit-cell crystals. A data reduction pipeline capable of fully automated handling of all steps of data reduction from spot harvesting to merged structure factors has been developed. The pipeline utilizes sparse indexing based on known unit-cell parameters, seed-skewness integration, intensity corrections including an overlap-based combined Ewald sphere width and partiality correction, and a dynamically adjusted post-refinement routine. Using the pipeline, data measured on the compound K4[Pt2(P2O5H2)4]·2H2O have been successfully reduced and used to solve the structure to an R1 factor of ∼9.1%. It is expected that the pipeline will open up the field of small-unit-cell serial femtosecond crystallography experiments and allow investigations into, for example, excited states and reaction intermediate chemistry.

From Metal Hydrides to Metal Borohydrides.

Commencing from metal hydrides, versatile synthesis, purification, and desolvation approaches are presented for a wide range of metal borohydrides and their solvates. An optimized and generalized synthesis method is provided for 11 different metal borohydrides, M(BH4) n, (M = Li, Na, Mg, Ca, Sr, Ba, Y, Nd, Sm, Gd, Yb), providing controlled access to more than 15 different polymorphs and in excess of 20 metal borohydride solvate complexes. Commercially unavailable metal hydrides (MH n, M = Sr, Ba, Y, Nd, Sm, Gd, Yb) are synthesized utilizing high pressure hydrogenation. For synthesis of metal borohydrides, all hydrides are mechanochemically activated prior to reaction with dimethylsulfide borane. A purification process is devised, alongside a complementary desolvation process for solvate complexes, yielding high purity products. An array of polymorphically pure metal borohydrides are synthesized in this manner, supporting the general applicability of this method. Additionally, new metal borohydrides, α-, α'- ß-, γ-Yb(BH4)2, α-Nd(BH4)3 and new solvates Sr(BH4)2·1THF, Sm(BH4)2·1THF, Yb(BH4)2· xTHF, x = 1 or 2, Nd(BH4)3·1Me2S, Nd(BH4)3·1.5THF, Sm(BH4)3·1.5THF and Yb(BH4)3· xMe2S (" x" = unspecified), are presented here. Synthesis conditions are optimized individually for each metal, providing insight into reactivity and mechanistic concerns. The reaction follows a nucleophilic addition/hydride-transfer mechanism. Therefore, the reaction is most efficient for ionic and polar-covalent metal hydrides. The presented synthetic approaches are widely applicable, as demonstrated by permitting facile access to a large number of materials and by performing a scale-up synthesis of LiBH4.

Fluoride substitution in LiBH4; destabilization and decomposition.

Fluoride substitution in LiBH4 is studied by investigation of LiBH4-LiBF4 mixtures (9 : 1 and 3 : 1). Decomposition was followed by in situ synchrotron radiation X-ray diffraction (in situ SR-PXD), thermogravimetric analysis and differential scanning calorimetry with gas analysis (TGA/DSC-MS) and in situ infrared spectroscopy (in situ FTIR). Upon heating, fluoride substituted LiBH4 forms (LiBH4-xFx) and decomposition occurs, releasing diborane and solid decomposition products. The decomposition temperature is reduced more than fourfold relative to the individual constituents, with decomposition commencing at T = 80 °C. The degree of fluoride substitution is quantified by sequential Rietveld refinement and shows a selective manner of substitution. In situ FTIR experiments reveal formation of bands originating from LiBH4-xFx. Formation of LiF and observation of diborane release implies that the decomposing materials have a composition that facilitates formation of diborane and LiF, i.e. LiBH4-xFx (LiBH3F). An alternative approach for fluoride substitution was performed, by addition of Et3N·3HF to LiBH4, yielding extremely unstable products. Spontaneous decomposition indicates fluoride substitution to have occurred. From our point of view, this is the most significant destabilization effect seen for borohydride materials so far.

Multifunctionality of silver closo-boranes.

Silver compounds share a rich history in technical applications including photography, catalysis, photocatalysis, cloud seeding and as antimicrobial agents. Here we present a class of silver compounds (Ag2B10H10 and Ag2B12H12) that are semiconductors with a bandgap at 2.3 eV in the green visible light spectrum. The silver boranes have extremely high ion conductivity and dynamic-anion facilitated Ag+ migration is suggested based on the structural model. The ion conductivity is enhanced more than two orders of magnitude at room temperature (up to 3.2 mS cm-1) by substitution with AgI to form new compounds. Furthermore, the closo-boranes show extremely fast silver nano-filament growth when excited by electrons during transmission electron microscope investigations. Ag nano-filaments can also be reabsorbed back into Ag2B12H12. These interesting properties demonstrate the multifunctionality of silver closo-boranes and open up avenues in a wide range of fields including photocatalysis, solid state ionics and nano-wire production.

Variable-temperature structural studies on valence tautomerism in cobalt bis(dioxolene) molecular complexes.

A variable-temperature single-crystal structural study of five valence tautomeric cobalt molecular complexes, CoII(3,5-DBSQ)2(DBPy)2 (1), CoII(3,5-DBSQ)2(DBPy)2·1.33C7H8 (1S), CoII(3,5-DBSQ)2(DCPy)2·C7H8 (2S), CoII(3,5-DBSQ)2(TBPy)2 (3) and CoII(3,5-DBSQ)2(TCPy)2 (4) (S = toluene, 3,5-DBSQ = 3,5-di-tert-butylsemiquinonate, DBPy = 3,5-dibromopyridine, DCPy = 3,5-dichloropyridine, TBPy = 3,4,5-tribromopyridine and TCPy = 3,4,5-trichloropyridine) is reported. The re-crystallization of (1S) in toluene at 277 K resulted in a concomitant formation of a solvent-free polymorph, CoII(3,5-DBSQ)2(DBPy)2 (1). Thermally induced valence tautomerism (VT) is observed only in (1S), (1) and (2S) [hs-CoII(3,5-DBSQ)2L2 ↔ ls-CoIII(3,5-DBSQ)(3,5-DBCat)L2 (hs = high spin, ls = low spin, 3,5-DBCat = 3,5-di-tert-butylcatecholate)], whereas (3) and (4) remain locked in the hs-CoII(3,5-DBSQ)2 state during cooling of the sample. Multi-temperature single-crystal studies demonstrate the change in cobalt coordination environment during the VT conversion. The non-solvated compound (1) shows a sharp VT transition (T1/2 ∼ 245 K with ΔT ∼ 10 K) from hs-CoII(3,5-DBSQ)2(DBPy)2 to ls-CoIII(3,5-DBSQ)(3,5-DBCat)(DBPy)2 oxidation state, whereas the other polymorph with lattice solvent (1S) results in a broad transition (T1/2 ∼ 150 K with ΔT ∼ 100 K). This increase in the VT transition temperature for (1) relative to (1S) illustrates the effect of lattice solvent on the VT transition mechanism. Additionally, the influence of halogen substitutions on the pyridine ring is discussed with respect to observed VT behaviour in the studied compounds.

Challenges in the synthetic routes to Mn(BH4)2: insight into intermediate compounds.

We have studied the reaction of MnCl2 with MBH4 (M = Li(+), Na(+), K(+)) in Et2O. Crystal structures of two new intermediates, named [{M(Et2O)2}Mn2(BH4)5] (M = Li(+), Na(+)), were elucidated by X-ray diffraction. Mn(BH4)2 in a mixture with LiBH4 or NaBH4 forms upon the solvent removal in a vacuum. [{M(Et2O)2}Mn2(BH4)5] contains 2D layers formed by Mn and BH4 groups, linked through the alkali metal atoms coordinated to Et2O. The loss of the solvent molecules leads to the segregation of the partially amorphous or nanocrystalline LiBH4/NaBH4 and a creation of the 3D framework of the crystalline Mn(BH4)2. While using LiBH4 led to Mn(BH4)2 contaminated with LiCl, presumably due to an efficient trapping of the latter salt by the [Mn(BH4)2-Et2O] system, the reaction with NaBH4 produced chlorine-free Mn(BH4)2 accompanied with NaBH4. Using KBH4 led to the formation of K2Mn(BH4)4 as a second phase. Two pyridine-containing solvomorphs, [Mn(py)3(BH4)2] and [Mn(py)4(BH4)2]·2py, were isolated in pure form. However, Mn(BH4)2 partly decomposes upon removal of pyridine molecules.

Manganese borohydride; synthesis and characterization.

Solvent-based synthesis and characterization of α-Mn(BH4)2 and a new nanoporous polymorph of manganese borohydride, γ-Mn(BH4)2, via a new solvate precursor, Mn(BH4)2·1/2S(CH3)2, is presented. Manganese chloride is reacted with lithium borohydride in a toluene/dimethylsulfide mixture at room temperature, which yields halide and solvent-free manganese borohydride after extraction with dimethylsulfide (DMS) and subsequent removal of residual solvent. This work constitutes the first example of establishing a successful, reproducible solvent-based synthesis route for a pure, crystalline, stable transition metal borohydride. The new polymorph, γ-Mn(BH4)2, is shown to be the manganese counterpart of the zeolite-like compound, γ-Mg(BH4)2 (cubic, a = 16.209(1) Å, space group Id3̄a). It is verified that large pores (diameter > 6.0 Å) exist in this structure. The solvate, Mn(BH4)2·1/2S(CH3)2, is subsequently shown to be the analogue of Mg(BH4)2·1/2S(CH3)2. As the structural analogies between Mg(BH4)2 and Mn(BH4)2 became evident a new polymorph of Mg(BH4)2 was identified and termed ζ-Mg(BH4)2. ζ-Mg(BH4)2 is the structural counterpart of α-Mn(BH4)2. All synthesis products are characterized employing synchrotron radiation-powder X-ray diffraction, infrared spectroscopy and thermogravimetric analysis in combination with mass spectroscopy. Thermal analysis reveals the decomposition of Mn(BH4)2 to occur at 160 °C, accompanied by a mass loss of 14.8 wt%. A small quantity of the desorbed gaseous species is identified as diborane (ρ(m)(Mn(BH4)2) = 9.5 wt% H2), while the remaining majority is found to be hydrogen.

Novel solvates M(BH4)3S(CH3)2 and properties of halide-free M(BH4)3 (M = Y or Gd).

Rare earth metal borohydrides have been proposed as materials for solid-state hydrogen storage because of their reasonably low temperature of decomposition. New synthesis methods, which provide halide-free yttrium and gadolinium borohydride, are presented using dimethyl sulfide and new solvates as intermediates. The solvates M(BH4)3S(CH3)2 (M = Y or Gd) are transformed to α-Y(BH4)3 or Gd(BH4)3 at ~140 °C as verified by thermal analysis. The monoclinic structure of Y(BH4)3S(CH3)2, space group P21/c, a = 5.52621(8), b = 22.3255(3), c = 8.0626(1) Å and ß = 100.408(1)°, is solved from synchrotron radiation powder X-ray diffraction data and consists of buckled layers of slightly distorted octahedrons of yttrium atoms coordinated to five borohydride groups and one dimethyl sulfide group. Significant hydrogen loss is observed from Y(BH4)3 below 300 °C and rehydrogenation at 300 °C and p(H2) = 1550 bar does not result in the reformation of Y(BH4)3, but instead yields YH3. Moreover, composites systems Y(BH4)3-LiBH4 1 : 1 and Y(BH4)3-LiCl 1 : 1 prepared from as-synthesised Y(BH4)3 are shown to melt at 190 and 220 °C, respectively.

Supercritical N2 processing as a route to the clean dehydrogenation of porous Mg(BH4)2.

Compounds of interest for chemical hydrogen storage at near ambient conditions are specifically tailored to be relatively unstable and thereby desorb H2 upon heating. Their decomposition must be performed in the absence of impurities to achieve clean dehydrogenation products, which is particularly challenging for an emerging class of microporous complex hydride materials, such as γ-phase Mg(BH4)2, which exhibits high surface area and readily adsorbs (sometimes undesired) molecular species. We present a novel strategy toward the purification of γ-Mg(BH4)2 using supercritical nitrogen drying techniques, (1) showing that clean hydrogen can be released from Mg(BH4)2 under mild conditions and (2) clarifying the origin of diborane among the decomposition products of stable borohydrides, a topic of critical importance for the reversibility and practical applicability of this class of hydrogen storage compounds. This technique is also widely applicable in the pursuit of the high-purity synthesis of other porous, reactive compounds, an exciting future class of advanced functional materials.

Trimetallic borohydride Li3MZn5(BH4)15 (M = Mg, Mn) containing two weakly interconnected frameworks.

The compounds, Li3MZn5(BH4)15, M = Mg and Mn, represent the first trimetallic borohydrides and are also new cationic solid solutions. These materials were prepared by mechanochemical synthesis from LiBH4, MCl2 or M(BH4)2, and ZnCl2. The compounds are isostructural, and their crystal structure was characterized by in situ synchrotron radiation powder X-ray and neutron diffraction and DFT calculations. While diffraction provides an average view of the structure as hexagonal (a = 15.371(3), c = 8.586(2) Å, space group P63/mcm for Mg-compound at room temperature), the DFT optimization of locally ordered models suggests a related ortho-hexagonal cell. Ordered models that maximize Mg-Mg separation have the lowest DFT energy, suggesting that the hexagonal structure seen by diffraction is a superposition of three such orthorhombic structures in three orientations along the hexagonal c-axis. No conclusion about the coherent length of the orthorhombic structure can be however done. The framework in Li3MZn5(BH4)15 is of a new type. It contains channels built from face-sharing (BH4)6 octahedra. While X-ray and neutron powder diffraction preferentially localize lithium in the center of the octahedra, thus resulting in two weakly interconnected frameworks of a new type, the DFT calculations clearly favor lithium inside the shared triangular faces, leading to two interpenetrated mco-nets (mco-c type) with the basic tile being built from three tfa tiles, which is the framework type of the related bimetallic LiZn2(BH4)5. The new borohydrides Li3MZn5(BH4)15 are potentially interesting as solid-state electrolytes, if the lithium mobility within the octahedral channels is improved by disordering the site via heterovalent substitution. From a hydrogen storage point of view, their application seems to be limited as the compounds decompose to three known metal borohydrides.

Organocatalytic domino Michael-Knoevenagel condensation reaction for the synthesis of optically active 3-diethoxyphosphoryl-2-oxocyclohex-3-enecarboxylates.

Versatile dominoes: A novel, organocatalytic, Michael-Knoevenagel condensation domino reaction of ethyl 4-diethoxyphosphoryl-3-oxobutanoate with various aryl- and aliphatic-substituted alpha,beta-unsaturated aldehydes catalyzed by a chiral diarylprolinol ether has been successfully performed. The reaction proceeds in a highly enantio- and diastereoselective manner giving access to optically active 6-substituted-3-diethoxyphosphoryl-2-oxocyclohex-3-enecarboxylates (see scheme).A novel, organocatalytic, highly enantio- and diastereoselective synthetic approach towards optically active 6-substituted-3-diethoxyphosphoryl-2-oxocyclohex-3-enecarboxylates is presented. Our methodology utilizes a Michael-Knoevenagel domino reaction sequence of ethyl 4-diethoxyphosphoryl-3-oxobutanoate and alpha,beta-unsaturated aldehydes catalyzed by a chiral diarylprolinol ether. The cyclohexenecarboxylates obtained are particularly well suited for the preparation of highly functionalized cyclohexene and cyclohexane derivatives, with up to four chiral centers and high levels of stereocontrol.

Enantioselective organocatalytic approach to alpha-methylene-delta-lactones and delta-lactams.

We present the first enantioselective organocatalytic approach for the synthesis of alpha-methylene-delta-lactones and delta-lactams. Our methodology utilizes the Michael addition of unmodified aldehydes to ethyl 2-(diethoxyphosphoryl)acrylate as the key step affording highly enantiomerically enriched adducts, which can be transformed into the target compounds maintaining the high stereoselectivity achieved in the first step. This methodology has been shown to be general and various optically active gamma-substituted alpha-methylene-delta-lactones and delta-lactams can be easily accessed.

Organocatalytic asymmetric "anti-Michael" reaction of beta-ketoesters.

The first organocatalytic "anti-Michael" reaction of cyclic-beta-ketoesters to unsaturated double bonds is described in a highly asymmetric version leading to the synthesis of alpha,alpha'-disubstituted branched double bonds as optically active Baylis-Hillman-like adducts.

Bisoxazoline-Lewis acid-catalyzed direct-electron demand oxo-hetero-Diels-Alder reactions of N-oxy-pyridine aldehyde and ketone derivatives.

A general catalytic oxo-hetero-Diels-Alder reaction for pro-chiral aldehyde and ketone N-oxy-pyridines is presented. The catalytic and asymmetric oxo-hetero-Diels-Alder reaction of electron-rich dienes with N-oxy-pyridine-2-carbaldehyde and ketone derivatives, catalyzed by chiral copper(II)-bisoxazoline complexes, gives optically active six-membered oxygen heterocycles in moderate to good yields and with excellent enantioselectivities.