Selective nitrogen capture by porous hybrid materials containing accessible transition metal ion sites.

Selective dinitrogen binding to transition metal ions mainly covers two strategic domains: biological nitrogen fixation catalysed by metalloenzyme nitrogenases, and adsorptive purification of natural gas and air. Many transition metal-dinitrogen complexes have been envisaged for biomimetic nitrogen fixation to produce ammonia. Inspired by this concept, here we report mesoporous metal-organic framework materials containing accessible Cr(III) sites, able to thermodynamically capture N2 over CH4 and O2. This fundamental study integrating advanced experimental and computational tools confirmed that the separation mechanism for both N2/CH4 and N2/O2 gas mixtures is driven by the presence of these unsaturated Cr(III) sites that allows a much stronger binding of N2 over the two other gases. Besides the potential breakthrough in adsorption-based technologies, this proof of concept could open new horizons to address several challenges in chemistry, including the design of heterogeneous biomimetic catalysts through nitrogen fixation.

Discovering the active sites for C3 separation in MIL-100(Fe) by using operando IR spectroscopy.

A reducible MIL-100(Fe) metal-organic framework (MOF) was investigated for the separation of a propane/propene mixture. An operando methodology was applied (for the first time in the case of a MOF) in order to shed light on the separation mechanism. Breakthrough curves were obtained as in traditional separation column experiments, but monitoring the material surface online, thus providing evidences on the adsorption sites. The qualitative and quantitative analyses of Fe(II) and, to some extent, Fe(III) sites were possible, upon different activation protocols. Moreover, it was possible to identify the nature and the role of the active sites in the separation process by selective poisoning of one family of sites: it was clearly evidenced that the unsaturated Fe(II) sites are mainly responsible for the separation effect of the propane/propene mixture, thanks to their affinity for the unsaturated bonds, such as the C=C entities in propene. The activity of the highly concentrated Fe(III) sites was also highlighted.

Why hybrid porous solids capture greenhouse gases?

Hybrid porous solids, with their tunable structures, their multifunctional properties and their numerous applications, are currently topical, particularly in the domain of adsorption and storage of greenhouse gases. Most of the data reported so far concern the performances of these solids in this domain, particularly in terms of adsorbed amounts of gas but do not explain at the atomic level why and how adsorption and storage occur. From a combination of structural, spectroscopic, thermodynamic experiments and of molecular simulations, this tutorial review proposes answers to these open questions with a special emphasis on CO(2) and CH(4) storage by some rigid and flexible hybrid porous materials.

Cathode composites for Li-S batteries via the use of oxygenated porous architectures.

Li-S rechargeable batteries are attractive for electric transportation because of their low cost, environmentally friendliness, and superior energy density. However, the Li-S system has yet to conquer the marketplace, owing to its drawbacks, namely, soluble polysulfide formation. To tackle this issue, we present here a strategy based on the use of a mesoporous chromium trimesate metal-organic framework (MOF) named MIL-100(Cr) as host material for sulfur impregnation. Electrodes containing sulfur impregnated within the pores of the MOF were found to show a marked increase in the capacity retention of Li-S cathodes. Complementary transmission electron microscopy and X-ray photoelectron spectroscopy measurements demonstrated the reversible capture and release of the polysulfides by the pores of MOF during cycling and evidenced a weak binding between the polysulphides and the oxygenated framework. Such an approach was generalized to other mesoporous oxide structures, such as mesoporous silica, for instance SBA-15, having the same positive effect as the MOF on the capacity retention of Li-S cells. Besides pore sizes, the surface activity of the mesoporous additives, as observed for the MOF, appears to also have a pronounced effect on enhancing the cycle performance. Increased knowledge about the interface between polysulfide species and oxide surfaces could lead to novel approaches in the design and fabrication of long cycle life S electrodes.

How linker's modification controls swelling properties of highly flexible iron(III) dicarboxylates MIL-88.

A series of organically modified iron(III) terephthalate MIL-88B and iron(III) 4,4'-biphenyl dicarboxylate MIL-88D flexible solids have been synthesized and characterized through a combination of X-ray diffraction, IR spectroscopy, and thermal analysis (MIL stands for Material from Institut Lavoisier). The swelling amplitude of the highly flexible MOFs tuned by introducing functional groups onto the phenyl rings shows a clear dependence on the steric hindrance and on the number of groups per aromatic ring. For instance, while the introduction of four methyl groups per spacer in dried MIL-88B results in a large permanent porosity, introducing two or four methyl groups in MIL-88D allows an easier pore opening in the presence of liquids without drastically decreasing the swelling magnitude. The influence of the degree of saturation of the metal center and the nature of the solvent on the swelling is also discussed. Finally, a computationally assisted structure determination has led to a proposal of plausible structures for the closed (dried) and open forms of modified MIL-88B and MIL-88D and to evaluation of their framework energies subject to the nature of the functional groups.

Porous metal-organic-framework nanoscale carriers as a potential platform for drug delivery and imaging.

In the domain of health, one important challenge is the efficient delivery of drugs in the body using non-toxic nanocarriers. Most of the existing carrier materials show poor drug loading (usually less than 5 wt% of the transported drug versus the carrier material) and/or rapid release of the proportion of the drug that is simply adsorbed (or anchored) at the external surface of the nanocarrier. In this context, porous hybrid solids, with the ability to tune their structures and porosities for better drug interactions and high loadings, are well suited to serve as nanocarriers for delivery and imaging applications. Here we show that specific non-toxic porous iron(III)-based metal-organic frameworks with engineered cores and surfaces, as well as imaging properties, function as superior nanocarriers for efficient controlled delivery of challenging antitumoural and retroviral drugs (that is, busulfan, azidothymidine triphosphate, doxorubicin or cidofovir) against cancer and AIDS. In addition to their high loadings, they also potentially associate therapeutics and diagnostics, thus opening the way for theranostics, or personalized patient treatments.

Infrared study of the influence of reducible iron(III) metal sites on the adsorption of CO, CO2, propane, propene and propyne in the mesoporous metal-organic framework MIL-100.

The present study illustrates the importance of the oxidation state of iron within the mesoporous iron trimesate [{Fe(3)O(H(2)O)(2)F(0.81)(OH)(0.19)}{C(6)H(3)(CO(2))(3)}(2)] denoted MIL-100(Fe) (MIL= Material from Institut Lavoisier) during adsorption of molecules that can interact with the accessible metal sites through π-back donation. Adsorption of CO has been first followed by FTIR spectroscopy to quantify the Lewis acid sites in the dehydrated Fe(III) sample, outgassed at 150 °C, and on the partially reduced Fe(II/III), outgassed at 250 °C. The exposure of MIL-100(Fe) to CO(2), propane, propene and propyne has then been studied by FTIR spectroscopy and microcalorimetry. It appears that π-back donating molecules are strongly adsorbed on reduced iron(II) sites despite the weaker Lewis acidity of cus Fe(2+) sites compared to that of Fe(3+) ones, as shown by pyridine adsorption.

129Xe NMR study of the framework flexibility of the porous hybrid MIL-53(Al).

The metal-organic framework MIL-53 exhibits a structural transition between two possible porous structures, so-called large-pore (lp) and narrow-pore (np) forms, depending on the temperature or when guest molecules are adsorbed. (129)Xe NMR has been used to study the lp --> np transition induced by the adsorption of xenon as revealed by the adsorption isotherms. The NMR spectra show that the two structures, characterized by two distinct lines, coexist for xenon pressures above 5 x 10(4) Pa at room temperature, but a complete transformation is achieved when the temperature is decreased. An original interpretation of the NMR results allowed us to quantify the rate of the structural transformation. In particular, at room temperature, we have shown that 28% of the channels remain open. Two possible interpretations of the hysteresis observed in the chemical shift variation versus xenon pressure are proposed.

Pd nanoparticles embedded into a metal-organic framework: synthesis, structural characteristics, and hydrogen sorption properties.

The metal-organic framework MIL-100(Al) has been used as a host to synthesize Pd nanoparticles (around 2.0 nm) embedded within the pores of the MIL, showing one of the highest metal contents (10 wt %) without degradation of the porous host. Textural properties of MIL-100(Al) are strongly modified by Pd insertion, leading to significant changes in gas sorption properties. The loss of excess hydrogen storage at low temperature can be correlated with the decrease of the specific surface area and pore volume after Pd impregnation. At room temperature, the hydrogen uptake in the composite MIL-100(Al)/Pd is almost twice that of the pristine material. This can be only partially accounted by Pd hydride formation, and a "spillover" mechanism is expected to take place promoting the dissociation of molecular hydrogen at the surface of the metal nanoparticles and the diffusion of monatomic hydrogen into the porosity of the host metal-organic framework.

Multistep N2 breathing in the metal-organic framework co(1,4-benzenedipyrazolate).

A variety of spectroscopic techniques combined with in situ pressure-controlled X-ray diffraction and molecular simulations have been utilized to characterize the five-step phase transition observed upon N(2) adsorption within the high-surface area metal-organic framework Co(BDP) (BDP(2-) = 1,4-benzenedipyrozolate). The computationally assisted structure determinations reveal structural changes involving the orientation of the benzene rings relative to the pyrazolate rings, the dihedral angles for the pyrazolate rings bound at the metal centers, and a change in the metal coordination geometry from square planar to tetrahedral. Variable-temperature magnetic susceptibility measurements and in situ infrared and UV-vis-NIR spectroscopic measurements provide strong corroborating evidence for the observed changes in structure. In addition, the results from in situ microcalorimetry measurements show that an additional heat of 2 kJ/mol is required for each of the first four transitions, while 7 kJ/mol is necessary for the last step involving the transformation of Co(II) from square planar to tetrahedral. Based on the enthalpy, a weak N(2) interaction with the open Co(II) coordination sites is proposed for the first four phases, which is supported by Monte Carlo simulations.

Explanation of the adsorption of polar vapors in the highly flexible metal organic framework MIL-53(Cr).

A comparison of the adsorption of water, methanol, and ethanol polar vapors by the flexible porous chromium(III) terephthalate MIL-53(Cr) was investigated by complementary techniques including adsorption gravimetry, ex situ X-ray powder diffraction, microcalorimetry, thermal analysis, IR spectroscopy, and molecular modeling. The breathing steps observed during adsorption strongly depend on the nature of the vapor. With water, a significant contraction of the framework is observed. For the alcohols, the initial contraction is followed by an expansion of the framework. A combination of IR analysis, X-ray diffraction, and computer modeling leads to the molecular localization of the guest molecules and to the identification of the specific guest-guest and host-guest interactions. The enthalpies of adsorption, measured by microcalorimetry, show that the strength of the interactions decreases from ethanol to water. Differential scanning calorimetry experiments on an EtOH/H(2)O mixture suggest a selective adsorption of ethanol over water.

Functionalization in flexible porous solids: effects on the pore opening and the host-guest interactions.

The synthesis on the gram scale and characterization of a series of flexible functionalized iron terephthalate MIL-53(Fe) type solids are reported. Chemical groups of various polarities, hydrophilicities, and acidities (-Cl, -Br, -CF(3), -CH(3), -NH(2), -OH, -CO(2)H) were introduced through the aromatic linker, to systematically modify the pore surface. X-ray powder diffraction (XRPD), molecular simulations, thermogravimetric analyses, and in situ IR and (57)Fe Mössbauer spectrometries indicate some similarities with the pristine MIL-53(Fe) solid, with the adoption of the narrow pore form for all solids in both the hydrated and dry forms. Combined XRPD and computational structure determinations allow concluding that the geometry of the pore opening is predominantly correlated with the intraframework interactions rather than the steric hindrance of the substituent. Only (MIL-53(Fe)-(CF(3))(2)) exhibits a nitrogen accessible porosity (S(BET) approximately 100 m(2) g(-1)). The adsorption of some liquids leads to pore openings showing some very specific behaviors depending on the guest-MIL-53(Fe) framework interactions, which can be related to the energy difference between the narrow and large pore forms evaluated by molecular simulation.

Influence of [Mo6Br8F6]2- cluster unit inclusion within the mesoporous solid MIL-101 on hydrogen storage performance.

The inclusion of (TBA)(2)Mo(6)Br(8)F(6) (TBA = tetrabutylammonium) containing [Mo(6)Br(8)F(6)](2-) cluster units within the pores of the mesoporous chromium carboxylate MIL-101 (MIL stand for Materials from Institut Lavoisier) has been studied. X-ray powder diffraction, thermal analysis, elemental analysis, solid-state NMR, and infrared spectroscopy have evidenced the successful loading of the cluster. In a second step, the hydrogen sorption properties of the model cluster loaded metal organic framework (MOF) system have been analyzed and compared to those of the pure MOF sample, through a combination of adsorption isotherms (77 K, room temperature), thermal desorption spectroscopy, and calorimetry (calculated and experimental) in order to evaluate the hydrogen storage efficiency of the cluster loading.

High-throughput aided synthesis of the porous metal-organic framework-type aluminum pyromellitate, MIL-121, with extra carboxylic acid functionalization.

A new porous metal-organic framework (MOF)-type aluminum pyromellitate (MIL-121 or Al(OH)[H(2)btec]·(guest), (guest = H(2)O, H(4)btec = pyromellitic acid) has been isolated by using a high-throughput synthesis method under hydrothermal conditions. Its structure was determined from powder X-ray diffraction analysis using synchrotron radiation (Soleil, France) and exhibits a network closely related to that of the MIL-53 series. It is a three-dimensional (3D) framework containing one-dimensional (1D) channels delimited by infinite trans-connected aluminum-centered octahedra AlO(4)(OH)(2) linked through the pyromellitate ligand. Here the organic ligand acts as tetradendate linker via two of the carboxylate groups. The two others remain non-bonded in their protonated form, and this constitutes a rare case of the occurrence of both bonding and non-bonding organic functionalities of the MOF family. The non-coordinated -COOH groups points toward the channels to get them an open form configuration. Within the tunnels are located unreacted pyromellitic acid and water species, which are evacuated upon heating, and a porous MIL-121 phase is obtained with a Brunauer-Emmett-Teller (BET) surface area of 162 m(2) g(-1). MIL-121 has been characterized by IR, thermogravimetry (TG) analyses, and solid state NMR spectroscopy employing a couple of two-dimensional (2D) techniques such as (1)H-(1)H SQ-DQ BABA, (1)H-(1)H SQ-SQ RFDR, (27)Al{(1)H} CPHETCOR and (27)Al MQMAS.

Reinvestigation of the M(II) (M = Ni, Co)/tetrathiafulvalenetetracarboxylate system using high-throughput methods: isolation of a molecular complex and its single-crystal-to-single-crystal transformation to a two-dimensional coordination polymer.

A high-throughput methodology combined with X-ray powder diffraction measurements was used to investigate the reactivity of the TetraThiaFulvalene TetraCarboxylic acid ((TTF-TC)H(4)) with divalent metals (M = Ni, Co) under various reaction conditions (stoichiometry, pH, temperature). Two new crystalline phases were identified and then studied by single crystal X-ray diffraction. Whereas the first one appears to be a simple salt, the second one, formulated {[M(H(2)O)(4)](2)(TTF-TC)}·4H(2)O, is built of 2:1 M:TTF-TC molecular complexes and labeled MIL-136(Ni, Co) (MIL stands for Materials Institute Lavoisier). The combination of thermogravimetric analysis and thermodiffraction studies reveals that MIL-136(Ni) exhibits a complex dehydration behavior. Indeed, a partial dehydration/rehydration process led to the single-crystal-to-single-crystal transformation of the molecular compound in a two-dimensional coordination polymer formulated {[Ni(2)(H(2)O)(5)(TTF-TC)]}·H(2)O (MIL-136'(Ni)). Magnetic and redox properties of MIL-136(Ni, Co) were investigated. Magnetic measurements indicate that all the magnetic coupling, intra- and intermolecular, are very weak; thus, the magnetic data of MIL-136(Ni, Co) have been interpreted in term of single-ion spin orbit coupling. Solid state cyclic voltammetry of MIL-136(Ni, Co) presents three reversible waves which were assigned to the redox activity of the TTF core and the metallic cations. In contrast to solids based on TTF linkers and alkaline ions, the MIL-136(Ni, Co) complexes do not act as excellent positive electrode materials for Li batteries, but present two reversible electron oxidation of the TTF core. These observations were tentatively related to the strength of the metal-carboxylate bond.

3-D coordination polymers based on the tetrathiafulvalenetetracarboxylate (TTF-TC) derivative: synthesis, characterization, and oxidation issues.

The reactivity of the redox-active tetracarboxylic acid derived from the tetrathiafulvalene (TTF-TC)H(4) with alkaline cations (K, Rb, Cs) is reported. The exploration of various experimental parameters (temperature, pH) led to the formation of four crystalline three-dimensional coordination polymers formulated M(2)(TTF-TC)H(2) (M = K, Rb, Cs), denoted MIL-132(K), MIL-133(isostructural K, Rb), and MIL-134(Cs). Thermogravimetric analysis and thermodiffraction show that all of the solids are thermally stable up to 150-200 degrees C in the air. In order to exploit the possibility of oxidation of the organic linker in TTF-based compounds, they were employed as positive electrodes in a classical lithium cell. A highly reversible cyclability was achieved at high current density (10 C) with a reasonable performance (approximately 50 mAh g(-1)). Finally, combined electro-(sub)hydrothermal synthesis was used to prepare a fifth 3-D coordination polymer formulated K(TTF-TC)H(2) (denoted MIL-135(K)), this time not based on the neutral TTF-TC linker but its radical, oxidized form TTF-TC(+*). This solid is less thermally stable than its neutral counterparts but exhibits a semiconducting behavior, with a conductivity at room temperature of about 1 mS cm(-1).

Water and ethanol desorption in the flexible metal organic frameworks, MIL-53 (Cr, Fe), investigated by complex impedance spectroscopy and density functional theory calculations.

The breathing behaviour of MIL-53(Cr) and MIL-53(Fe) upon water and ethanol desorption has been investigated by combining complementary experimental techniques including ThermoGravimetry Analysis (TGA), Differential Scanning Calorimetry (DSC) and Complex Impedance Spectroscopy (CIS). It was shown that two stages of solvent departure are involved in the desorption process, as revealed by (i) a change of the weight loss gradient in the TGA curve, (ii) the existence of a second endothermic peak in the DSC signal and (iii) a sudden drop and/or profile change of the ac conductivity in CIS. All these features are observed around a typical temperature T(c), for which the framework contractions, caused by the solvent desorption, occur. Moreover, it is shown that these modifications are more pronounced when the magnitude of the breathing is higher, as illustrated by the comparison of the water/MIL-53(Cr), ethanol/MIL-53(Cr) and water/MIL-53(Fe) systems. CIS data were further analyzed in the light of DFT calculations which provided the preferential arrangements of the molecules within the pores and the resulting host/guest interactions. It could then be proposed that (i) the polarization conductivity results from the local re-orientation of the µ(2)-OH dipoles bonded to the metal atom from the hybrid solid, i.e. Fe or Cr, and (ii) that dc conductivity, which can be ascribed to a proton propagation via a Grotthus mechanism, is favoured when the solvent molecules form strong hydrogen bonds between each other.

[Hybrid nanocarriers for controlled delivery of antitumour and retroviral drugs delivery]. / Des nanovecteurs hybrides pour la restitution retard de médicaments antitumoraux et antiviraux.

The efficient delivery of drugs in the body requires the use of non-toxic nanocarriers. Most of the existing materials show poor drug loading and/or rapid release of the proportion of the drug that is simply adsorbed (or anchored) at the external surface of the nanocarrier. The new porous hybrid solids, with the ability to tune their structures and porosities are well suited to serve as nanocarriers for delivery and imaging applications. Here we show that specific non-toxic porous iron(III) - based metal - organic frameworks with engineered cores and surfaces, as well as imaging properties, function as superior nanocarriers for efficient controlled delivery of antitumour and retroviral drugs against cancer and AIDS. They also potentially associate therapeutics and diagnostics, and open the way for theranostics, or -personalized patient treatments. double dagger.

BioMOFs: metal-organic frameworks for biological and medical applications.

The class of highly porous materials called metal-organic frameworks offer many opportunities for applications across biology and medicine. Their wide range of chemical composition makes toxicologically acceptable formulation possible, and their high level of functionality enables possible applications as imaging agents and as delivery vehicles for therapeutic agents. The challenges in the area encompass not only the development of new solids but also improvements in the formulation and processing of the materials, including tailoring the morphology and surface chemistry of the frameworks to fit the proposed applications.