A Flexible Fluorescent Zr Carboxylate Metal-Organic Framework for the Detection of Electron-Rich Molecules in Solution.

A novel Zr(IV) dicarboxylate metal organic framework (MOF) built up from an s-tetrazine derived ligand was prepared. This solid, which exhibits a diamond type network, combines a good stability in water, a structural flexibility, and fluorescence properties thanks to the organic ligand. It is noteworthy that this fluorescence is quenched when exposed to electron-rich molecules in solution, such as amines or phenol, this phenomenon being associated with the adsorption of the quencher, as unambiguously proven by X-ray diffraction (XRD) analysis. Finally, the quenching efficiency is shown to be governed not only by electronic and steric factors but also by the relative polarity of the solvent, the MOF, and the quencher. This work thus suggests that it is possible to develop new MOF-based sensors presenting in a given medium (such as water) highly selective responses.

Adaptability of the metal(iii,iv) 1,2,3-trioxobenzene rod secondary building unit for the production of chemically stable and catalytically active MOFs.

The use of a 5,10,15,20-tetrakis(3,4,5-trihydroxyphenyl)porphyrin has yielded a new MOF based on M-1,2,3-trioxobenzene chains that can be made of M = Zr(iv) or RE(iii) (RE = rare earth), showing a very high and limited chemical stability, respectively. The robust metallated Zr-analogue, Co-MIL-173(Zr), has proven to be a heme-like heterogeneous catalyst suitable for aerobic oxidation of hydrocarbons.

Structure determination of Ba5AlF13 by coupling electron, synchrotron and neutron powder diffraction, solid-state NMR and ab initio calculations.

The room temperature structure of Ba5AlF13 has been investigated by coupling electron, synchrotron and neutron powder diffraction, solid-state high-resolution NMR (19F and 27Al) and first principles calculations. An initial structural model has been obtained from electron and synchrotron powder diffraction data, and its main features have been confirmed by one- and two-dimensional NMR measurements. However, DFT GIPAW calculations of the 19F isotropic shieldings revealed an inaccurate location of one fluorine site (F3, site 8a), which exhibited unusual long F-Ba distances. The atomic arrangement was reinvestigated using neutron powder diffraction data. Subsequent Fourier maps showed that this fluorine atom occupies a crystallographic site of lower symmetry (32e) with partial occupancy (25%). GIPAW computations of the NMR parameters validate the refined structural model, ruling out the presence of local static disorder and indicating that the partial occupancy of this F site reflects a local motional process. Visualisation of the dynamic process was then obtained from the Rietveld refinement of neutron diffraction data using an anharmonic description of the displacement parameters to account for the thermal motion of the mobile fluorine. The whole ensemble of powder diffraction and NMR data, coupled with first principles calculations, allowed drawing an accurate structural model of Ba5AlF13, including site-specific dynamical disorder in the fluorine sub-network.

Solving the Hydrogen and Lithium Substructure of Poly(triazine imide)/LiCl Using NMR Crystallography.

Poly(triazine imide) with incorporated lithium chloride has recently attracted substantial attention due to its photocatalytic activity for water splitting. However, an apparent H/Li disorder prevents the delineation of structure-property relationships, for example, with respect to band-gap tuning. Herein, we show that through a combination of one- and two-dimensional, multinuclear solid-state NMR spectroscopy, chemical modelling, automated electron diffraction tomography, and an analysis based on X-ray pair distribution functions, it is finally possible to resolve the H/Li substructure. In each cavity, one hydrogen atom is bound to a bridging nitrogen atom, while a second one protonates a triazine ring. The two lithium ions within each cavity are positioned between two nitrogen atoms of neighbouring triazine rings. The thereby induced local dipole moments cause slight buckling of the framework and lateral displacements of the Cl- ions at a coherence length below 2 nm. Nevertheless, the average structure conforms to space group P21 21 21 . In this way, we demonstrate that, in particular, the above-mentioned techniques allow for smart interplay in delineating the real structure of PTI/LiCl.

Quantitative (13)C Solid-State NMR Spectra by Multiple-Contact Cross-polarization for Drug Delivery: From Active Principles to Excipients and Drug Carriers.

In this contribution, we present an analysis of the main parameters influencing the efficiency of the (1)H â†’ (13)C multiple-contact cross-polarization nuclear magnetic resonance (NMR) experiment in the context of solid pharmaceutical materials. Using the optimum experimental conditions, quantitative (13)C NMR spectra are then obtained for porous metal-organic frameworks (potential drug carriers) and for components present in drug formulations (active principle ingredient and excipients, amorphous or crystalline). Finally, we show that mixtures of components can also be quantified with this method and, hence, that it represents an ideal tool for quantification of pharmaceutical formulations by (13)C cross-polarization under magic-angle spinning NMR in the industry as it is robust and easy to set up, much faster than direct (13)C polarization and is efficient for samples at natural abundance.

Recent advances in application of (27)Al NMR spectroscopy to materials science.

Valuable information about the local environment of the aluminum nucleus can be obtained through (27)Al Nuclear Magnetic Resonance (NMR) parameters like the isotropic chemical shift, scalar and quadrupolar coupling constants, and relaxation rate. With nearly 250 scientific articles per year dealing with (27)Al NMR spectroscopy, this analytical tool has become popular because of the recent progress that has made the acquisition and interpretation of the NMR data much easier. The application of (27)Al NMR techniques to various classes of compounds, either in solution or solid-state, has been shown to be extremely informative concerning local structure and chemistry of aluminum in its various environments. The development of experimental methodologies combined with theoretical approaches and modeling has contributed to major advances in spectroscopic characterization especially in materials sciences where long-range periodicity and classical local NMR probes are lacking. In this review we will present an overview of results obtained by (27)Al NMR as well as the most relevant methodological developments over the last 25years, concerning particularly on progress in the application of liquid- and solid-state (27)Al NMR to the study of aluminum-based materials such as aluminum polyoxoanions, zeolites, aluminophosphates, and metal-organic-frameworks.

Investigating the Case of Titanium(IV) Carboxyphenolate Photoactive Coordination Polymers.

The reactivity of 2,5-dihydroxyterephthalic acid (H4DOBDC) with titanium(IV) precursors was thoroughly investigated for the synthesis of metal-organic frameworks under solvothermal conditions. Four crystalline phases were isolated whose structures were studied by a combination of single-crystal or powder X-ray diffraction and solid-state NMR. The strong coordination ability of the phenolate moieties was found to favor the formation of isolated TiO6 octahedra bearing solely organic ligands in the resulting structures, unless hydrothermal conditions and precondensed inorganic precursors are used. It is worth noting that these solids strongly absorb visible light, as a consequence of the ligand-to-metal charge transfer (LMCT) arising from Ti-phenolate bonds. Preliminary photocatalytic tests suggest that one compound, namely, MIL-167, presents a higher activity for hydrogen evolution than the titanium carboxylate MIL-125-NH2 but that such an effect cannot be directly correlated with its improved light absorption feature.

In Situ Solid-State (13)C NMR Observation of Pore Mouth Catalysis in Etherification of ß-Citronellene with Ethanol on Zeolite Beta.

The reaction mechanism of etherification of ß-citronellene with ethanol in liquid phase over acid zeolite beta is revealed by in situ solid-state (13)C NMR spectroscopy. Comparison of (13)C Hahn-echo and (1)H-(13)C cross-polarization NMR characteristics is used to discriminate between molecules freely moving in liquid phase outside the zeolite and molecules adsorbed inside zeolite pores and in pore mouths. In the absence of ethanol, ß-citronellene molecules enter zeolite pores and react to isomers. In the presence of ethanol, the concentration of ß-citronellene inside zeolite pores is very low because of preferential adsorption of ethanol. The etherification reaction proceeds by adsorption of ß-citronellene molecule from the external liquid phase in a pore opening where it reacts with ethanol from inside the pore. By competitive adsorption, ethanol prevents the undesired side reaction of ß-citronellene isomerization inside zeolite pores. ß-citronellene etherification on zeolite beta is suppressed by bulky base molecules (2,4,6-collidine and 2,6-ditertiarybutylpyridine) that do not enter the zeolite pores confirming the involvement of easily accessible acid sites in pore openings. The use of in situ solid-state NMR to probe the transition from intracrystalline catalysis to pore mouth catalysis depending on reaction conditions is demonstrated for the first time. The study further highlights the potential of this NMR approach for investigations of adsorption of multicomponent mixtures in general.

A Breathing Zirconium Metal-Organic Framework with Reversible Loss of Crystallinity by Correlated Nanodomain Formation.

The isoreticular analogue of the metal-organic framework UiO-66(Zr), synthesized with the flexible trans-1,4-cyclohexanedicarboxylic acid as linker, shows a peculiar breathing behavior by reversibly losing long-range crystalline order upon evacuation. The underlying flexibility is attributed to a concerted conformational contraction of up to two thirds of the linkers, which breaks the local lattice symmetry. X-ray scattering data are described well by a nanodomain model in which differently oriented tetragonal-type distortions propagate over about 7-10 unit cells.

A Robust Infinite Zirconium Phenolate Building Unit to Enhance the Chemical Stability of Zr MOFs.

A novel Zr-chain based MOF, namely MIL-163, was designed and successfully synthesized using a bis-1,2,3-trioxobenzene ligand. Endowed with large square-shaped channels of 12 Šwidth, it shows remarkable water uptake (ca. 0.6 cm(3) g(-1) at saturating vapor pressure) and a remarkable stability in simulated physiological media, where archetypical Zr carboxylate MOFs readily degrade.

Isolation of Renewable Phenolics by Adsorption on Ultrastable Hydrophobic MIL-140 Metal-Organic Frameworks.

The isolation and separation of phenolic compounds from aqueous backgrounds is challenging and will gain in importance as we become more dependent on phenolics from lignocellulose-derived bio-oil to meet our needs for aromatic compounds. Herein, we show that highly stable and hydrophobic Zr metal-organic frameworks of the MIL-140 type are effective adsorbent materials for the separation of different phenolics and far outperform other classes of porous solids (silica, zeolites, carbons). The mechanism of the hydroquinone-catechol separation on MIL-140C was studied in detail by combining experimental results with computational techniques. Although the differences in adsorption enthalpy between catechol and hydroquinone are negligible, the selective uptake of catechol in MIL-140C is explained by its dense π-π stacking in the pores. The interplay of enthalpic and entropic effects allowed separation of a complex, five-compound phenol mixture through breakthrough over a MIL-140C column. Unlike many other metal-organic frameworks, MIL-140C is remarkably stable and maintained structure, porosity and performance after five adsorption-desorption cycles.

Design of hydrophilic metal organic framework water adsorbents for heat reallocation.

A new hydrothermally stable Al polycarboxylate metal-organic framework (MOF) based on a heteroatom bio-derived aromatic spacer is designed through a template-free green synthesis process. It appears that in some test conditions this MOF outperforms the heat reallocation performances of commercial SAPO-34.

Geometry flexibility of copper iodide clusters: variability in luminescence thermochromism.

An original copper(I) iodide cluster of novel geometry obtained by using a diphosphine ligand is reported and is formulated [Cu6I6(PPh2(CH2)3PPh2)3] (1). Interestingly, this sort of "eared cubane" cluster based on the [Cu6I6] inorganic core can be viewed as a combination of the two known [Cu4I4] units, namely, the cubane and the open-chair isomeric geometries. The synthesis, structural and photophysical characterisations, as well as theoretical study of this copper iodide along with the derived cubane (3) and open-chair (2) [Cu4I4(PPh3)4] forms, were investigated. A new polymorph of the cubane [Cu4I4(PPh3)4] cluster is indeed presented (3). The structural differences of the clusters were analyzed by solid-state nuclear magnetic resonance spectroscopy. Luminescence properties of the three clusters were studied in detail as a function of the temperature showing reversible luminescence thermochromism for 1 with an intense orange emission at room temperature. This behavior presents different feature compared to the cubane cluster and completely contrasts with the open isomer, which is almost nonemissive at room temperature. Indeed, the thermochromism of 1 differs by a concomitant increase of the two emission bands by lowering the temperature, in contrast to an equilibrium phenomenon for 3. The luminescence properties of 2 are very different by exhibiting only one single band when cooled. To rationalize the different optical properties observed, density functional theory calculations were performed for the three clusters giving straightforward explanation for the different luminescence thermochromism observed, which is attributed to different contributions of the ligands to the molecular orbitals. Comparison of 3 with its [Cu4I4(PPh3)4] cubane polymorphs highlights the sensibility of the emission properties to the cuprophilic interactions.

Mechanochromic luminescence of copper iodide clusters.

Luminescent mechanochromic materials are particularly appealing for the development of stimuli-responsive materials. Establishing the mechanism responsible for the mechanochromism is always an issue owing to the difficulty in characterizing the ground phase. Herein, the study of real crystalline polymorphs of a mechanochromic and thermochromic luminescent copper iodide cluster permits us to clearly establish the mechanism involved. The local disruption of the crystal packing induces changes in the cluster geometry and in particular the modification of the cuprophilic interactions, which consequently modify the emissive states. This study constitutes a step further toward the understanding of the mechanism involved in the mechanochromic luminescent properties of multimetallic coordination complexes.

The structure of the aluminum fumarate metal-organic framework A520.

The synthesis of the commercially available aluminum fumarate sample A520 has been optimized and its structure analyzed through a combination of powder diffraction, solid-state NMR spectroscopy, molecular simulation, IR spectroscopy, and thermal analysis. A520 is an analogue of the MIL-53(Al)-BDC solid, but with a more rigid behavior. The differences between the commercial and the optimized samples in terms of defects have been investigated by in situ IR spectroscopy and correlated to their catalytic activity for ethanol dehydration.

A biocompatible porous Mg-gallate metal-organic framework as an antioxidant carrier.

A microporous magnesium gallate MOF was prepared from highly biocompatible reagents under environmentally friendly conditions. Its slow degradation in physiological fluids leads to the release of gallic acid and hence a high antioxidant activity, which was illustrated in the HL-60 cell line.

Silica capsules enclosing P123 triblock copolymer micelles for flurbiprofen storage and release.

Flurbiprofen was incorporated in 200-400 nm silica capsules filled with Pluronic P123 polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer. The assembly process of the capsules and the molecular organization of drug molecule, surfactant and silica were investigated using SAXS, TGA, SEM, DLS, DSC, 13C single-pulse, CPMAS and 1H-1H two-dimensional NMR. Flurbiprofen molecules are molecularly dispersed inside polypropylene cores of P123 surfactant micelles occluded in a 20-30 nm thick silica shell. Flurbiprofen molecules in polypropylene cores of P123 micelles exhibit high mobility and are easily released after introduction in simulated gastrointestinal fluid and the solubility limit is reached within minutes. Release rates are favored at high pH due to acid dissociation of the carboxylic acid group of the flurbiprofen molecule. The molecular environment of flurbiprofen in these P123 filled silica capsules is different from ordered mesoporous silica materials synthesized using P123 as sacrificial template having the drug molecules adsorbed on the silica surface of pore walls. These findings uncover a new concept of storage and release of hydrophobic bioactive molecules.

Can one and two-dimensional solid-state NMR fingerprint zeolite framework topology?

In this contribution, we have explored the potential and strength of one-dimensional (1D) (29)Si and two-dimensional (2D) (29)S-(29)Si and (29)Si-(17)O NMR as invariants of non-oriented graph for fingerprinting zeolite frameworks. 1D and 2D (29)Si NMR can indeed provide indications on the graph vertices, edges and allow the construction of the adjacency matrix, i.e. the set of connections between the graph vertices. From the structural data, hypothetical 1D (29)Si and 2D (29)Si-(29)Si NMR signatures for 193 of the zeolite frameworks reported in the Atlas of Zeolite Structures have been generated. Comparison between all signatures shows that thanks to the 1D (29)Si NMR data only, almost 20% of the known zeolite frameworks could be distinguished. Further NMR signatures were generated by taking into account 2D (29)Si-(29)Si and (29)Si-(17)O correlations. By sorting and comparison of all the NMR data, up to 80% of the listed zeolites could be unambiguously discriminated. This work indicates that (i) solid-state NMR data indeed represent a rather strong graph invariant for zeolite framework, (ii) despite their difficulties and costs (isotopic labeling is often required, the NMR measurements can be long), (29)Si and (17)O NMR measurements are worth being investigated in the frame of zeolites structure resolution. This approach could also be generalized to other zeolite-related materials containing NMR-measurable nuclides.

NMR crystallography: Applications to inorganic materials.

Current developments of NMR crystallography as well as some recent applications to diamagnetic inorganic solids are presented. First, we illustrate how solid-state NMR data can be used in combination with diffraction data for the determination of the periodic part of the crystal structures, from the space group selection, to the structure determination over the refinement and validation processes. As ss-NMR, contrary to diffraction (powder and single-crystal), is not restricted to periodic boundary conditions, ss-NMR data can be used to further complete the structural description of materials, including studies of local order/disorder, etc. This illustrated through examples, which are shown and discussed in the second part of this review.