Chiral polyoxometalate-based materials: From design syntheses to functional applications

Prompted by our interest in new photochromic organic-inorganic hybrid materials, the reactivity of [Mo7O24]6- toward a structure-directing reagent diamine such as 1,4-diazabicyclo[2.2.2]octane (DABCO) and piperazine (pipz) has been investigated, and three new molybdenum(VI)-containing compounds, namely, (H2DABCO)3[Mo7O24].4H2O (1), (H2DABCO)[Mo3O10].H2O (2), and (H2DABCO)2(NH4)2[Mo8O27].4H2O (3), have been synthesized and characterized. New synthetic routes to achieve the known compounds (H2DABCO)2(H2pipz)[Mo8O27] (4), (H2pipz)3[Mo8O27] (5), and (H2DABCO)2[Mo8O26].4H2O (6) are also reported. All of these compounds contain different poly(oxomolybdate) clusters, i.e., discrete [Mo7O24]6- blocks in 1, infinite polymeric chains 1/infinity[Mo3O10]2- in 2, 1/infinity[Mo8O27]6- in 3-5, and 1/infinity[Mo8O26]4- in 6, associated in a tridimensional assembly by hydrogen bonds with H2DABCO2+ and/or H2pipz2+ cations. Interconversion pathways and chemical factors affecting the stabilization of the different species are highlighted and discussed. At the opposite of 6, compounds 1-5 show photochromic behavior under UV excitation. Namely, compounds 1-5 shift from white or pale yellow to pale pink, reddish brown, or purple under UV illumination depending on the chemical nature of the mineral framework, with the kinetics of the color change being dictated by the nature of the organic component and by the organic-inorganic interface.

Synthesis, Characterization, and Photochromic Properties of Hybrid Organic -Inorganic Materials Based on Molybdate, DABCO, and Piperazine

Combustion synthesis in nanostructured reactive systems

Reactive and energetic materials are typically metastable and are expected to transform into thermodynamically favorable reaction products with substantial energy release. Preparation of such materials by mechanical milling is challenging: They are easily initiated by impact or friction. At the same time, milling offers a simple, scalable, and controllable technology capable of mixing reactive components on the nanoscale. In most cases, for reactive materials milling should be interrupted or arrested to preserve the metastable phases. Arrested reactive milling was exploited to prepare many inorganic reactive materials, including nanocomposite thermite, metal–metalloid, and intermetallic systems. Prepared materials are fully dense composites with unique properties, combining high density with extremely high reactivity. Different milling devices were used to prepare reactive materials and an approach was developed to transfer the process conditions between different mills. Different milling protocols, such as milling at cryogenic temperatures or staged milling can be used to prepare hybrid reactive materials with different components mixed on different scales; it was also used to tune the particle size distributions of metal-based reactive material powders. Metal–halogen composites were prepared, with metal matrix stabilizing a halogen (e.g., iodine) at temperatures substantially exceeding its boiling point. Mechanochemically prepared reactive materials can be classified based on the energy of reaction between components and the energy of oxidation of the bulk material composition. Work on mechanochemical preparation of reactive and energetic materials is reviewed with the focus on unique properties and ignition and combustion mechanisms of the mechanochemically prepared reactive materials. An ignition mechanism for nanothermites involving preignition reaction leading to a gas release preceding rapid temperature rise is discussed. A combustion mechanism is also discussed, in which the nanostructure of the mechanochemically prepared material is preserved despite the very high combustion temperatures.

Mechanochemically prepared reactive and energetic materials: a review

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Dynamics Of Heterogeneous Materials

A systematic investigation of the factors governing the reaction product composition, hydrogen bonding, and symmetry was conducted in the MoO3/3-aminoquinuclidine/H2O system. Composition space analysis was performed through 36 individual reactions under mild hydrothermal conditions using racemic 3-aminoquinuclidine. Single crystals of three new compounds, [C7H16N2][Mo3O10]·H2O, [C7H16N2]2[Mo8O26]·H2O, and [C7H16N2]2[Mo8O26]·4H2O, were grown. The relative phase stabilities for these products are dependent upon the reactant mole fractions in the initial reaction gel. This phase stability information was used to direct the synthesis of two new noncentrosymmetric compounds, using either (S)−(−)-3-aminoquinuclidine dihydrochloride or (R)-(+)-3-aminoquinuclidine dihydrochloride. [(R)-C7H16N2]2[Mo8O26] and [(S)-C7H16N2]2[Mo8O26] both crystallize in the noncentrosymmetric space group P21 (No. 4), which has the polar crystal class 2 (C2). The second-harmonic generation activities were measured on sieved powders. T...

Directed synthesis of noncentrosymmetric molybdates using composition space analysis.

Ni:Al laminate composites were fabricated by repeatedly cold-rolling Al and Ni foils that were stacked together with initial thicknesses of 25 and 18 μm, respectively. The rolling process consisted of multiple 50 % thickness reductions wherein the first reduction was followed by cutting, restacking, and rerolling to achieve a total of three, six or nine 50 % thickness reductions. However, some of the laminates also received a more mild series of six 20 % thickness reductions without restacking. An analysis program was written and used to quantify the distribution of layer thicknesses, bilayer thicknesses and local chemistries for the complex laminate microstructures, while also preserving positional information for the constituent layers. The resulting distributions show that while we see no clustering of very large bilayers in any of the composites, the heavily rolled laminates with only 50 % thickness reductions have a higher percentage of very large bilayers, relative to the volume mean bilayer, compared to laminates with the additional 20 % thickness reductions. This phenomenon is attributed to less uniform layer deformation and more layer pinch-off with 50 % thickness reductions compared to the more gradual 20 % thickness reductions. Differential scanning calorimetry was performed on the laminates to determine the exothermic peak temperatures and the total energy released during controlled heating. Peak temperatures correlate with the volume average bilayer thickness, while the energy release correlates with the bilayer thickness distribution. The velocity and maximum temperature of self-propagating reactions were measured for the laminates and were found to vary according to processing conditions but not according to the volume average bilayer thickness. Foils with 20 % thickness reductions have both hotter and faster reactions compared to samples with only 50 % thickness reductions. The distributions of layer thicknesses, bilayer thicknesses, and local chemistries within the laminates are used to predict the maximum temperature during reaction. The velocities of the unsteady reaction propagations, though, could not be predicted effectively, at least with current analytical models.

An analysis of the microstructure and properties of cold-rolled Ni:Al laminate foils

Single crystals of a new β-octamolybdate salt containing protonated 1,4-diazabicyclo[2.2.2]octane cations were prepared under mild hydrothermal conditions. This compound, [C6H13N2]2[C6H14N2][Mo8O26], was then used as a starting material in the synthesis of [C6H13N2]6[Mo16O53F2]·4H2O, which contains previously unreported [Mo16O53F2]12- anions. The structure-directing properties of γ-[Mo8O26]4-, a likely intermediate in this pH-dependent transformation, are responsible for the site selection of the fluoride incorporation. [Mo16O53F2]12-, the largest reported polyoxofluoromolybdate cluster, expands upon the limited number of such anions in the literature. The structures of both compounds were determined using single-crystal X-ray diffraction.

[Mo16O53F2]12-: a new polyoxofluoromolybdate anion.

Reaction in Ni-Al laminates by laser-shock compression and spalling

A new mechanical method for creating reactive laminate powders has been developed using a two-step process; in the first step bulk reactive materials are created by cold-rolling stacks of alternating sheets of nickel and aluminum into foils with bilayer thicknesses ranging from 2.9 to 1.8 μm. This step establishes the average reactant spacing and, hence, the reactivity of the material. In the second step the rolled foils are then ground into laminate powders and sieved based on their diameters, which range from 850 to 53 μm. Our processing methodology allows the particle size and the reactant spacing to be varied independently. Powders made by this method have heat releases within a differential scanning calorimeter (DSC) that vary with the average reactant spacing, similar to rolled and sputter deposited foils. However, the measured heats also vary with the average diameter of the powders, as smaller particles show a systematic decrease in heat. Furthermore, this effect is magnified for the powders with the coarsest microstructure, as they show the largest drop in DSC heat release. The physical densities also vary as a function of particle size. The powders with the largest average bilayer thickness become Ni-rich at the smallest particle sizes, powders with the next finest average bilayer thickness become Al-rich, and powders with the smallest average bilayer thickness show little variation. We attribute the particle size dependence of the DSC heats to small powders being broken from regions of the original rolled foils that contain a high volume fraction of Ni-rich and Al-rich bilayers. These microstructural and chemical variations alter the exothermic reactions that are seen during slow heating in a DSC, as well as the heats of reaction that are measured in the DSC as a function of powder size. We support this hypothesis of non-random breakup during grinding by simulating bimodal distributions of bilayer chemistry within powders and modeling their densities and heats of reaction. The simulations are compared with measured values. Finally, we normalize the effects of particle size and bilayer thickness by plotting the measured DSC heats of reaction versus the number of bilayers per particle; the values merge toward one curve, with the largest decrease in heat occurring when there are fewer than 150 bilayers in each particle. This ratio proves useful when selecting particles for particular applications.

Adam K. Stover

Papers

Directed synthesis of noncentrosymmetric molybdates using composition space analysis.

An analysis of the microstructure and properties of cold-rolled Ni:Al laminate foils

[Mo16O53F2]12-: a new polyoxofluoromolybdate anion.

Reaction in Ni-Al laminates by laser-shock compression and spalling

Mechanical fabrication of reactive metal laminate powders