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.

Controlling the solid-state polymerization of organic molecules to form crystalline materials remains a challenge for the synthetic chemist. In an effort to control reaction pathways through topochemistry, we have compressed the 1:1 naphthalene–octafluoronaphthalene cocrystal, C10H8·C10F8. This starting material displays a unique structure wherein the molecules are aligned in a nearly sandwich-like π–π stacking arrangement because of the inverse polarities of naphthalene and its perfluoronated derivative. This stacking arrangement and the use of fluorine as an sp3-templating functional group creates favorable interactions between the molecules along the crystallographic a axis, providing topological control over the reaction pathway at high pressure. Reaction of C10H8·C10F8 along the molecular stacking axis to form polymerized sp3 rods with single-crystalline order was confirmed through in situ single-crystal X-ray diffraction and infrared spectroscopy, as well as GC–MS analysis of the recovered polymerized material, and supported by computational models. Polymerization occurs at room temperature under rapid compression without uniaxial stress indicating enhanced control through the topology of the molecular precursor.

A new metal-organic framework (MOF)-type aluminum pyromellitate (MIL-118) has been hydrothermally synthesized in water at 210 °C for 24 h. The crystal structure of the as-synthesized phase (Al 2 (OH) 2 (H 2 O) 2 [C 10 O 8 H 2 ] or MIL-118A) has been solved from single-crystal analysis using synchrotron radiation with a specific microdiffraction setup at ESRF ID13 beamline. It consists of infinite chains of trans-connected aluminum-centered octahedra linked to each other through the pyromellitate ligand. The organic linker is not fully connected, and one carboxyl oxygen of two carboxylate functions is hydrogen bonded to the terminal water molecule attached to aluminum cations. Upon heating of the sample, the structure analysis (from powder X-ray diffraction data) showed that the terminal water is removed and the previously nonbonded carboxylate groups are now linked to aluminum reflecting a solid-state transition with the formation of Al-OCO bonding (Al 2 (OH) 2 [C 10 O 8 H 2 ] or MIL-118B). This dried phase is able to reversibly adsorb and desorb ambient water. This hydration process induces a second phase transition with the encapsulation of water molecules within the channels delimited by the infinite chains and the aromatic ligands. In the third hydrated phase (Al 2 (OH) 2 [C 10 O 8 H 2 ] • 2.75H 2 O or MIL-118C), whose structure was solved from powder X-ray diffraction data collected at ESRF ID31), the inorganic chains are shifted to each other with a ≈ b/6 translation along the b axis, corresponding to a tilt angle of 90°b etween the benzene rings and the chains, instead of 61°in the dried phase MIL-118B. A 36.5°rotation of the benzene rings is also observed along the c axis. The occurrence of such a flexible network (6% variation for cell volume) was fully characterized by X-ray thermodiffraction, thermogravimetric analysis, and solid-state NMR ( 27 Al, 1 H).