An unusually large expansion upon solvent adsorption occurs without apparent bond breaking in the network of a series of isoreticular chromium(III) or iron(III) diarboxylates labeled MIL-88A to D [dicarbox = fumarate (88A); terephthalate (1,4-BDC) (88B); 2,6-naphthalenedicarboxylate (2,6-NDC) (88C); and 4-4'-biphenyldicarboxylate (4-4'-BPDC) (88D)]. This reversible "breathing" motion was analyzed in terms of cell dimensions (extent of breathing), movements within the framework (mechanism of transformation), and the interactions between the guests and the skeleton. In situ techniques show that these flexible solids are highly selective absorbents and that this selectivity is strongly dependent on the nature of the organic linker.

The unusual adsorption behavior of CO2 in a nanoporous hybrid metal– organic solid is discussed (see figure). The results indicate that the gas adsorption–desorption step is related to a breathing phenomenon. This study also suggests that the main interactions responsible for the breathing phenomenon are strong guest–framework CO2–OH interactions as well as CO2–CO2 interactions along the tunnels present in the structure.

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.

A wide variety of metal borohydrides, MBH, have been discovered and characterized during the past decade, revealing an extremely rich chemistry including fascinating structural flexibility and a wide range of compositions and physical properties. Metal borohydrides receive increasing interest within the energy storage field due to their extremely high hydrogen density and possible uses in batteries as solid state ion conductors. Recently, new types of physical properties have been explored in lanthanide-bearing borohydrides related to solid state phosphors and magnetic refrigeration. Two major classes of metal borohydride derivatives have also been discovered: anion-substituted compounds where the complex borohydride anion, BH, is replaced by another anion, i.e. a halide or amide ion; and metal borohydrides modified with neutral molecules, such as NH, NHBH, NH, etc. Here, we review new synthetic strategies along with structural, physical and chemical properties for metal borohydrides, revealing a number of new trends correlating composition, structure, bonding and thermal properties. These new trends provide general knowledge and may contribute to the design and discovery of new metal borohydrides with tailored properties towards the rational design of novel functional materials. This review also demonstrates that there is still room for discovering new combinations of light elements including boron and hydrogen, leading to complex hydrides with extreme flexibility in composition, structure and properties.

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.

This investigation is based on a combination of experimental tools completed by a computational approach to deeply characterize the unusual adsorption behavior of the flexible MIL-53(Fe) in the presence of short linear alkanes. In contrast to the aluminum or chromium analogues we previously reported, the iron MIL-53 solid, which initially exhibits a closed structure in the dry state, shows more complex adsorption isotherms with multisteps occurring at pressures that depend on the nature of the alkane. This behavior has been attributed to the existence of four discrete pore openings during the whole adsorption process. Molecular simulations coupled with in situ X-ray powder diffraction were able to uncover these various structural states.

The adsorption of C1 to C4 linear hydrocarbons in the flexible metal organic framework MIL-53(Cr) has been followed by adsorption manometry coupled with microcalorimetry and Synchrotron X-ray powder diffraction. This experimental investigation was completed by molecular modeling. In the case of methane, the solid remains rigid whatever the adsorbate amount. However for the C2-C4 series, an increasing flexibility of the structure is observed, which is ascribed first to a breathing of the material from a large pore to a narrow pore form followed by a further expansion at high pressure. The collected thermodynamic and structural information suggests that a minimum adsorption enthalpy of ca. 20 kJ mol (-1) in the initial large pore structure of MIL-53(Cr) is required to induce the structural transition "large to narrow pore". Further, the enthalpy of adsorption can be used to predict the pressure at which the structure reopens. Finally, the magnitude of the breathing can be related to the size of the probe molecule via the van der Waals volume. The above trends have been successfully verified in the case of water and carbon dioxide. This combined experimental and theoretical approach gives the first elements for the prediction of whether or not the MIL53 and similar flexible structures will respond to gas loading and what would be the pressure required and further the amplitude of the induced breathing.

One of the technological problems that face society today is the environmentally friendly and economically feasible separation, recovery, and recovery/reuse of vapors and gases. Several examples are currently of interest: the selective recovery of solvents, the recovery of greenhouse gases, and the purification of hydrogen. Many processes include an adsorption step in which microporous adsorbents, such as activated carbon and zeolites, are used. Recently a new class of porous materials have found interest: these metal organic frameworks (MOFs) [1,2] or metal coordination polymers [3] are built up from inorganic subnetworks and organic complexing molecules (phosphonates, carboxylates, sulfonates). These subnetworks often contain divalent or trivalent cations connected by organic groups such as carboxylates. Such structures possess tunnels or cavities with pore sizes between 3 and 35 . Several of these organic-inorganic hybrid porous solids [4][5][6][7][8][9][10][11] have the interesting feature of being selectively flexible during the adsorption process by means of a breathing [12] or gate-opening process, [9] which depends on the nature of the adsorptive. Examples discovered by Ferey and coworkers include flexible porous carboxylates formed by chains of metallic centers (MIL-53, -69) [13,14] and with metalcenter trimers (MIL-88A).[15] These solids have shown to "breath" with unprecedented increases in volume between 50 and 85 % depending on the nature of the fluid. A consequence of this particular property is the possibility to develop novel selective separation and storage processes that could be less expensive than existing processes.In many adsorption processes since the presence of water is detrimental, it is often removed using a precolumn. In the [*] Dr.