Stable Hydride Research Articles

Effect of hydrogen absorption/desorption cycling on hydrogen storage properties of a LaNi3.8Al1.0Mn0.2 alloy

The effect of long-term hydrogen absorption/desorption cycling up to 3500 cycles on the hydrogen storage properties of LaNi3.8Al1.0Mn0.2 alloy was investigated. The pressure-composition (PC) isotherms for absorption/desorption and the absorption kinetics were measured at 433 K, 453 K and 473 K. X-ray diffraction analysis revealed that the alloy had a homogeneous hexagonal CaCu5 type structure and kept this structure even after 3500 cycles, but the diffraction peaks were broadened. The degree of peak broadening was increased with increase of the cycle number, but exhibiting a maximum after initial activation. The shapes of PCT curves after 300, 2000 and 3500 cycles were similar to that after initial activation. It was found that the alloy subjected to 300 cycles did not exhibit significant changes in hydrogen storage capacity, but the long-term cycling up to 2000 and 3500 cycles resulted in obvious decrease in hydrogen storage capacity. The degradation of the hydrogen capacity might be resulted from the formation of the irreversible sites and more stable hydride phase, though no new phase was found after absorption/desorption cycling from XRD pattern as shown in Fig. 6 because of the limitation of XRD analysis sensibility. The hydrogen absorption kinetics after 300 cycles was deteriorated but improved again after 2000 and 3500 cycles compared with that of after initial activation. The changes in hydrogenation properties of the alloy induced by cycling were discussed by considering the crystal structure, lattice strain and pulverization of the sample.

Local metal and deuterium ordering in the deuterated ZrTiNi C14 Laves phase

Local structure of the hexagonal C14 Laves phase in an annealed approximately equiatomic ZrTiNi alloy was studied before and after deuteration using neutron total scattering. Rietveld refinements of the P6 3 / mmc AB 2 model demonstrate that the A-sites are shared by Zr and Ti, whereas the two non-equivalent B-sites are occupied by a mixture of Ti and Ni. Reverse Monte Carlo refinements using a joint fit of the neutron total scattering data in real and reciprocal spaces revealed significant short-range ordering of Ti and Ni on the B-sites. The refined Ti–Ni nearest-neighbor distances are appreciably shorter than the corresponding Ti–Ti and Ni–Ni distances. Nevertheless, the differences in the effective sizes of chemically distinct tetrahedral interstices (i.e. [Zr 2Ti 2], [Zr 2TiNi], [Zr 2Ni 2], etc.), which are important for hydrogen absorption, remain rather small. Rietveld refinements of the structure of the deuterated sample TiZrNiD 2.14 confirmed that deuterium occupies various [A 2B 2] tetrahedral interstices. RMC refinements suggested a strong preferential occupation of deuterium in the [Zr 2Ti 2] and [Zr 2TiNi] tetrahedra whereas the [Zr 2Ni 2] sites remained nearly empty. These results support earlier models which predicted preferential occupancy by hydrogen in sites coordinated by metals that form the most stable binary hydrides. The deuterium atoms in the [Zr 2TiNi] tetrahedra are displaced toward Ni. X-ray absorption near-edge structure measured for Ti, Zr, and Ni K-edges demonstrated that deuteration is accompanied by the reduction in the density of unoccupied electronic states (just above the Fermi level) associated with Ti and Zr.

The mechanism of hydrogen uptake in [NiFe] hydrogenase: first-principles molecular dynamics investigation of a model compound

The recent discovery of a model compounds of [NiFe] hydrogenase that catalyzes the heterolytic cleavage of the H(2) molecule into a proton and a stable hydride in water solution under room conditions opened up the possibility to understand the mechanism of H(2) uptake by this peculiar class of enzymes. The simplest model compound belongs to the class of NiRu bimetallic cationic complexes mimicking, in water solution and at room conditions, the hydrogenase active site. By using first-principles molecular dynamics computer simulations, in the Car-Parrinello scheme, we investigated models including the water solvent and nitrate counterions. Several simulations, starting from different initial configurations, provided information on the first step of the H(2) cleavage: (1) the pathway of H(2) approach towards the active site; (2) the role of the ruthenium-bonded water molecule in providing a base that extracts the proton from the activated H(2) molecule; (3) the minor role of Ni in activating the H(2) molecule and its role in stabilizing the hydride produced.

Microstructure and hydrogen storage properties of porous Ni@Mg

A nanoporous Ni layer on the surface of Mg was prepared utilizing the reduction reaction of Ni2+ by Mg in NiCl2.12H2O solution. XRD result showed that the product was composed of cubic Ni, hexagonal Mg and little Mg(OH)2. SEM and TEM results suggested that a nanoporous Ni layer microencapsulated Mg and formed Ni@Mg. The specific surface area of Ni@Mg formed in this reaction was 65.3 m2/g. Hydrogen uptake increased as the temperature increased. Moreover, Ni@Mg showed a reversible hydriding-dehydriding process without formation of the stable hydrides, which indicated that the nanoporous Ni layer on the surface of Mg showed an obvious physisorption characteristics rather than hydrogen storage behavior of Mg.

Design of Molecular Chains Based on the Planar Tetracoordinate Carbon Unit C2Al4

B3LYP DFT study revealed that the planar tetracoordinate carbon (ptC) molecules, C2Al4E8 (E = CH3, NH2, and OH), could be condensed to one-dimensional molecular chains containing planar or quasi-planar C2Al4 units by eliminating CH4, NH3, and H2O molecules. Natural bond orbital analyses indicate that the electronic structures of C2Al4 moieties in the chains, including ionic and covalent bonding, do not change significantly in comparison with those of the free C2Al4E8 (E = CH3, NH2, and OH) ptC species. The reason for the structural evolution from ptC units to the molecular chains is discussed. Because of the eliminations of stable hydride molecules (CH4, NH3, and H2O), the condensations are thermodynamically favorable, which implies the promise to synthesize the materials experimentally.

Synthesis, characterization and hydrogen storage behaviour of AB2 (ZrFe2, Zr(Fe0.75V0.25)2, Zr(Fe0.5V0.5)2 type materials

In this paper, we describe and discuss the synthesis, structural-microstructural and hydrogen storage behaviour of three AB2 type storage materials namely (a) ZrFe2, (b) Zr(Fe0.75V0.25)2 and (c) Zr(Fe0.5V0.5)2. These alloys were synthesied by radio frequency induction melting in argon atmosphere. X-ray diffraction and transmission electron microscope have been employed for structural and microstructural characterizations. The XRD study reveals that the lattice constants and the unit cell volume of ZrFe2, Zr (Fe0.75V0.25)2, Zr(Fe0.5V0.5)2 alloys, which has C14 type hexagonal Laves phase. The Surface morphology and elemental composition of these alloys were investigated by scanning electron microscope and energy dispersive X-ray analysis. The pressure composition isotherms of these alloys were investigated at room temperature and pressure ranges of 0–100 atm respectively, measured through a fully computerized PCI apparatus. As we increase the concentration of V (substituted for Fe), the total hydrogen storage capacities increased up to 1.45 wt%. This capacity is achieved in Zr(Fe0.5V0.5)2 alloy, while the reversible hydrogen storage capacity decreases due to the formation of a stable hydride phase. It has been found that the lattice constants increase with higher vanadium concentration. This is indicating that the majority of vanadium atoms reside in the B-site. The broader X-ray diffraction peaks observed in Zr(Fe0.5V0.5)2 alloy indicates a higher degree of disorder for alloys with the higher V-content. The yet another interesting feature observed in our present study is that the plateau pressure remains well below 1 atm for all the compositions.

Structure and Thermodynamic Properties of the NaMgH3 Perovskite: A Comprehensive Study

One of the bottlenecks in the implementation of a hydrogen economy is the development of storage materials that can uptake high content of H2 and release it within a suitable temperature and pressure range. Among the proposed hydride systems, the perovskite NaMgH3 is receiving increasing attention, not only as the Mg ternary based hydride with the highest hydrogen gravimetric (6 wt %) and volumetric density (88 g L−1) but also as a stable hydride likely to be formed in the transformation reactions of mixed hydrides. However, there is a large scatter in the literature for both the structure of the NaMgH3 compound and the thermodynamics of the hydrogenation/dehydrogenation processes. In this paper a critical review of the literature data, supported by a new set of experimental (in situ synchrotron X-ray diffraction, infrared spectroscopy, high-pressure differential scanning calorimetry, pressure composition isotherms) and theoretical data is presented. The influence of ball milling on the microstructure is studied in the NaMgH3 in comparison to NaH and MgH2. The infrared spectrum of NaMgH3 compound, assigned by calculated and experimental results, is characterized by vibrational regions around 1100 and 600 cm−1. In situ synchrotron X-ray diffraction measurements show the desorption reaction of NaMgH3 into NaH and Mg at about 673 K under 0.2 MPa H2, and the successive reabsorption of NaH and Mg back to NaMgH3 at 623 K under 0.5 MPa H2. From high-pressure differential calorimetry, it was measured a formation enthalpy of 141 kJ/mol f.u for NaMgH3 compound. It was confirmed the possible reaction of NaH with Mg with observation of NaMgH3 formation in 1.0 MPa H2. Finally, this work provides a thermodynamic description of the NaMgH3 phase by a critical assessment of the available information using the CALPHAD approach and the equilibrium pressure−temperature phase diagram is presented.

Dehydrogenation of ketones by pincer-ligated iridium: Formation and reactivity of novel enone complexes

The transfer dehydrogenation of several ketones by (PCP)IrH 2 (PCP = κ 3-C 6H 3-2,6-(CH 2P tBu 2) 2) ( 1) has been observed. Catalytic turnover was inhibited in most cases by the formation of stable metallacycles or the O–H oxidative addition of phenolic products. Catalytic transfer dehydrogenation of 3,3-dimethylcyclohexanone was achieved, giving the corresponding α,β-enone. The transfer dehydrogenation reaction of cycloheptanone with 1 was found to generate a surprisingly stable PCP-iridium troponyl hydride ( 9), which is stabilized by conjugation and possibly represents an unusual bicyclo[5.2.0]troponyliridium metalloaromatic structure. Complex 9 was found to catalyze the dimerization of tropone to give a fused tricyclic dihydrodicycloheptafuranol. A mechanism for this reaction is proposed wherein the coordinated troponyl group nucleophilically attacks a free tropone molecule.

Hydrogen trapping properties of Zr-based intermetallic compounds in the presence of CO contaminant gas

Intermetallic compounds, as hydrogen getters, are considered to control the quantity of hydrogen generated in radioactive waste packaging. The compounds ZrCo, Zr 2Fe and a Zr-rich Zr–Ti–V alloy have been chosen as they form very stable hydrides at ambient temperature. However, other gases are produced in the packaging such as carbon monoxide, a gas known to poison the surface of intermetallic compounds and to hinder the hydrogen sorption reaction. The three Zr-based compounds have been first characterized regarding their metallurgical state and their gas sorption properties toward pure hydrogen. Then, the sorption properties of the activated materials have been studied using a mixture of 5 vol.% CO + 95 vol.% H 2. We demonstrated that though the presence of CO sharply slows down the reaction rate the activated compounds still show significant sorption properties. Therefore, the presence of contaminant gases is not detrimental for the target application.

A theoretical study of the atomic hydrogen binding on small Ag nCu m ( n + m ⩽ 5) clusters

Density functional theory calculations were carried out for hydrogen atom binding on small Ag n Cu m clusters ( n + m ⩽ 5). It was found that hydrogen prefers to bind with Cu atoms when both Ag and Cu site co-exist in the cluster. In general the binding energies increase with the increasing Cu content for the given cluster size. The metal–H frequencies vary according to the way the metal atoms bound with hydrogen. The most favorable dissociation channels and the corresponding dissociation energies for the most stable bare clusters and cluster hydrides are determined.

Structure and phase stability of hydrogenated first-stage alkali- and alkaline-earth metal–graphite intercalation compounds

We report the hydrogenation temperature and pressure dependence of hydrogen absorption capacity in CaC 6. In addition, the structure and phase stability of hydrogenated CaC 6, LiC 6 and KC 8 has been investigated using X-ray diffraction and Raman spectroscopy. It is found that the hydrogen chemisorption in both CaC 6 and LiC 6 leads to either metastable or higher stage intermediate compounds but eventually leaves completely deintercalated graphite by forming stable metal hydrides. In contrast hydrogenation of KC 8 generates a restaged ternary hydride compound. The phase stability of hydrogenated compounds is discussed in the text based on the observed experimental results and correlated to the thermodynamic and kinetic aspects of intercalated metal.

Modeling the Hydrogen Solubility in Liquid Aluminum Alloys

The modeling of hydrogen solubility in multicomponent Al-(Li, Mg, Cu, and Si) liquid phase has been performed with a thermodynamic approach using the modified quasichemical model with the pair approximation (MQMPA). All hydrogen solubility data available in literature was assessed critically to obtain the binary parameters of the MQMPA model for the Al-H, Li-H, Mg-H, Cu-H, Zn-H, and Si-H melts. For the Li-H system, a new thermodynamic description of the stable solid lithium hydride was determined based on the cp found in literature. The thermodynamic model for the Al-Li system also was reassessed in this work to take into account the short-range ordering observed for this system. Built-in interpolation techniques allow the model to estimate the thermodynamic properties of the multicomponent liquid solution from the liquid model parameters of the lower order subsystems. A comparison of the calculated hydrogen solubility performed at various equilibrium conditions of temperature, pressure, and composition with the available experimental data found in the literature is presented in this work, as well as a comparison with some results from previous modeling.

Anionic d8 Alkyl Hydrides – Selective Formation and Reactivity of Anionic cis‐PtII Methyl Hydride

AbstractThe pincer‐type complexes [(PCN)PtR] {R = H, 2; Me, 4; PCN = C6H4[CH2P(tBu)2](CH2)2N(CH3)2} react with MeLi or Et3BHNa, to give anionic cis‐Pt(Me)H complexes [(PCN*)Pt(H)(Me)]–Li+ (Me trans to P; PCN* denotes the PCN ligand in which the amine arm is not coordinated) and [(PCN*)Pt(Me)(H)]–Na+ (H trans to P). Only the isomer in which the incoming nucleophile is situated trans to the phosphane ligand is formed. These first d8 anionic alkyl hydride complexes were fully characterized spectroscopically. The hemilabile PCN ligand allows for reversible de‐coordination of the amine arm, thereby providing a desirable balance of stability vs. reactivity. Theoretical calculations on model systems indicate a concerted mechanism in which the nucleophilic attack and the amine dissociation occur concurrently. The (unobserved) methane reductive elimination from the stable anionic methyl hydride complex [(PCN*)Pt(Me)(H)]–Li+ (3) is thermodynamically and kinetically unfavorable, as indicated by DFT. This complex reacts with electrophiles, such as water and methyl iodide, to yield exclusively methane and the corresponding organometallic product (either 2 or 4). This reactivity was also further examined by DFT.

Studies of Ti 1.5Zr 5.5V 0.5(M xNi 1− x) 9.5 (M = Cr, Mn, Fe, Co, Cu, Al): Part 2. Hydrogen storage and electrochemical properties

In Part 2 of this two part series of papers, the gas phase and electrochemical properties of the quinary alloy Ti 1.5Zr 5.5V 0.5(M x Ni 1− x ) 9.5 with M = Cr, Mn, Fe, Co, Cu, or Al and x = 0.1 or 0.2 are studied and compared to the structural properties reported in Part 1. The electrochemical properties of the alloys are strongly related to the non-Laves phases. For example, the Zr 7Ni 10 phase provides a more catalytic surface and improves the high rate discharge capability of the alloy at the expense of discharge capacity due to the higher Ni to Zr ratio; the Zr 9Ni 11 phase, on the contrary, improves the discharge capacity but hinders the high rate discharge capability. In the Laves phases, C14 provides a stable hydride with lower equilibrium pressure, higher gas phase storage capacity and lower electrochemical rate capability. The C15 phase provides a less stable hydride with higher equilibrium pressure, lower gas phase storage capacity and higher electrochemical storage capacity. The main effect of each modifying element can be summarized as follows: Mn is the most beneficial substitution element found in this study. Mn increases both the gas phase and electrochemical capacities with good reversibility; it also improves the activation, hydrogen diffusion, surface reaction, and high rate discharge capability. Co increases the degree of disorder in the alloy, reduces both gas phase and electrochemical capacities, destabilizes the hydride, improves activation, and reduces surface reactivity. Fe decreases the gas phase capacity but improves the electrochemical discharge capacity. Fe facilities activation, but its surface reaction is the least active among the substituents used in this study. Al, Cr, and Cu are less desirable because they reduce: (1) the electrochemical and gas phase capacities, (2) the reversibility, and (3) the rate capability. They also slow activation.

The electronic, elastic, and structural properties of Ti–Pd intermetallics and associated hydrides from first principles calculations

Using an ab initio density functional approach, we report on the ground-state phase stabilities, enthalpies of formation, electronic, and elastic properties of the Ti–Pd alloy system. The calculated enthalpies of formation are in excellent agreement with available calorimetric data. We found a linear dependence between the calculated enthalpies of formation of several intermetallic structures and the Pd-concentration, indicating that each of these compounds has a very limited composition range. The elastic constants for many of these Ti–Pd intermetallics were calculated and analyzed. The B2 TiPd phase is found to be mechanically unstable with respect to the transformation into the monoclinic B19′ structure. A series of hydrides, Ti 2PdH x ( x = 1, 1.5, 2, 3, 4), have been investigated in terms of electronic structure, enthalpies of hydrogen absorption, and site preference of H atoms. Our results illustrate the physical mechanism for hydrogen absorption in term of the charge transfer, and explain why TiPd 2 does not form a stable hydride.

Direct synthesis of pure complex aluminium hydrides by cryomilling

Simple mechanochemical procedures can be used for the solid-state preparation of stable complex aluminium hydrides as hydrogen storage materials. For the synthesis of unstable complex hydrides, cryomilling at temperatures at which product decomposition does not take place under milling conditions appears to be a viable method. To probe the potential of cryomilling for the synthesis of complex aluminium hydrides, the reactions of different alkaline hydrides with AlH 3 were tested under these conditions.

Stationary inverted Lyman populations and free-free and bound-free emission of lower-energy state hydride ion formed by an exothermic catalytic reaction of atomic hydrogen and certain group I catalysts

Abstract Rb+ to Rb2+ and 2K+ to K + K2+ each provide a reaction with a net enthalpy equal to the potential energy of atomic hydrogen. The presence of these gaseous ions with thermally dissociated hydrogen formed a plasma having strong VUV emission with a stationary inverted Lyman population. Significant Balmer α line broadening of 18 and 9 eV was observed from a rt-plasma of hydrogen with KNO3, and RbNO3, respectively, compared to 3 eV from a hydrogen microwave plasma. The reaction was exothermic since excess power of about 20 mW/cc was measured by Calvet calorimetry. We propose an energetic catalytic reaction involving a resonance energy transfer between hydrogen atoms and Rb+ or 2K+ to form a very stable novel hydride ion. Its predicted binding energy of 3.0471 eV with the fine structure was observed at 4071 Å, and its predicted bound-free hyperfine structure lines matched those observed for about 40 lines to within.01 percent. Characteristic emission from each catalyst was observed. This catalytic reaction may pump a CW HI laser.

In situ high-pressure study of diborane by infrared spectroscopy

As the simplest stable boron hydride in its condensed phase, diborane exhibits an interesting structural chemistry with uniquely bridged hydrogen bonds. Here we report the first room-temperature infrared (IR) absorption spectra of solid diborane compressed to pressures as high as 50 GPa using a diamond anvil cell. At room temperature and 3.5 GPa, the IR spectrum of diborane displays rich sharply resolved fundamentals and overtones of the IR active bands, consistent with the previous low-temperature IR measurements of condensed diborane at ambient pressure. When compressed stepwise to 50 GPa, several structural transformations can be identified at pressures of approximately 3.5 GPa, approximately 6.9 GPa and approximately 14.7 GPa, as indicated by the changes in the band profile as well as the pressure dependence of the characteristic IR modes and bandwidths. These transformations can be interpreted as being enhanced intermolecular interactions resulting from compression. The geometry of the four-member ring of B(2)H(6), however, does not seem to be altered significantly during the transformations and the B(2)H(6) molecule remains chemically stable up to 50 GPa.

Transition states for hydride-water (H –)(H 2O) n clusters, n = 2–6, 20

Hydride anion (H –) reacts quickly in aqueous solution to form molecular hydrogen (H 2) and hydroxide ion (OH –), but (H –)(H 2O) n clusters have many PES local minima in which the hydride is solvated by two or more water molecules making dihydrogen bonds. Using ab initio methods applied to (H –)(H 2O) n clusters for n ≤ 6 and for n = 20, we explored these clusters’ reaction pathways and transition states. At the B3LYP/6−311++G∗∗ level of theory, all of the small (2 ≤ n ≤ 6) clusters are unstable or barely stable at 0 K: all activation barriers for reaction to form H 2 are under 0.5 kcal/mol, all but one are under 0.2 kcal/mol, and most are negative. In some instances the lowest-barrier reaction pathway is multi-step, requiring H-bond rearrangements first, and those rearrangements are consistent with improving the “presolvation” of the emerging (OH –) in accordance with a “solvation hierarchy.” The arrangements explored for n = 20 consist of (H –) encapsulated by a dodecahedral cage with coordination of 3, 4, 5, or 6 at the hydride anion. The lowest-energy (H –)(H 2O) 20 cluster found has 4 H-bonds to the anion. Of several reaction pathways explored, the lowest barrier height is 3.68 kcal/mol at 0 K, suggesting that a low-temperature stable solvated hydride can exist. For the (H –)(H 2O) 20 clusters at their transition states to H 2 formation, the H ––H distances are at or below the van der Waals cutoff for covalent H—H bonding (92 pm). In all, 21 transition states to (H 2)(OH –)(H 2O) n –1 and eight H-bond rearranging transition states are presented. During a 25 ps AIMD simulation of H – in a box of 32 H 2O’s at 300 K, reaction to form H 2 did not occur, the hydride ion spent 95% of the time being 4-coordinated, and two first-shell exchanges were observed. This and other calculations suggest the mean survival time of (H –) in bulk water at 300 K is likely between 20 and 100 ps.

Protonic–Hydridic Bifunctionality: The Protonic (2‐Aminoethyl)dimethylphosphane Ligand in Nitrosyl Tungsten Hydride Complexes

AbstractThe chemistry of nitrosyl tungsten hydrides bearing the (2‐aminoethyl)dimethylphosphane ligand (edmp) with an acidic NH functionality was studied. Such complexes were targeted by applying the reaction of WCl3(NO)(CH3CN)2 with edmp, which gave [WCl2(NO)(η2‐edmp)2][Cl] (1a) and [WCl2(NO)(η2‐edmp)2][BPh4] (1b). Complex 1b was treated with Zn to produce [W(H)(Cl)(NO)(η2‐edmp)2][BPh4] (2b). Subsequent attempts with the use of WCl(NO)[P(OMe)3]4 and edmp as starting materials produced {W(NO)(η2‐edmp)2[P(OMe)3]}[Cl] (3a) at 85 °C and [W(H)(Cl)(NO)(η2‐edmp)2][Cl] (2a) at 120 °C. Substitution of the P(OMe)3 ligand in {W(η2‐edmp)2(NO)[P(OMe)3]}[BPh4] (3b) with CO afforded [W(CO)(η2‐edmp)2(NO)][BPh4] (4b). Complex 4b was treated withNaHBEt3, resulting in a dihydride product [W(CO)trans‐(η1‐edmp)2(H)2(NO)]Na (4c). The chlorido complexes containing one η2‐edmp ligand WCl(NO)(η2‐edmp)(PMe3)2 (5a), WCl(NO)(η2‐edmp)(CO)(PMe3) (6a), WCl(NO)(η2‐edmp)[P(OMe)3]2 (5b), and WCl(NO)(CO)(η2‐edmp)[P(OMe)3] (6b) were prepared and treated with NaHBEt3 to obtain the corresponding hydride species WH(NO)(η2‐edmp)[P(OMe)3]2 (8b), WH(NO)(CO)(η2‐edmp)(PMe3) (7a), and WH(NO)(CO)(η2‐edmp)[P(OMe)3] (7b). The initially formed WH(NO)(η2‐edmp)(PMe3)2 (8a) was spontaneously transformed into the amide complex W(NO)(CO)(NHCH2CH2PMe2)(PMe3) (11a) with loss of H2. The anionic dihydride products [W(H)2(η1‐edmp)(NO)L2]Na [L = PMe3 (9a) and L = PMe3 (9b)] were produced from 7a and 7b by the reaction with NaHBEt3. Hydride 8b turned out to be the most stable hydride complex in the given series. NMR spectroscopic experiments and deuterium labeling showed that 8b coexists in equilibrium with corresponding dihydrogen amide complex 10b. The structures of compounds 1a, 2b, 3a, 4b, 5b, 6a, and 6b were studied by single‐crystal X‐ray diffraction.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)