Aluminum aminoterephthalate Al(OH)[H(2)N-BDC] x 0.3 (H(2)N-H(2)BDC (denoted MIL-53-NH(2)(as)) was synthesized under hydrothermal conditions. The activation of the compound can be achieved in two steps. The treatment with DMF at 150 degrees C leads to Al(OH)[H(2)N-BDC] x 0.95 DMF (MIL-53-NH(2)(DMF)). In the second step, DMF is thermally removed at 130 degrees C. Upon cooling in air, the hydrated form Al(OH)[H(2)N-BDC] x 0.9 H(2)O (MIL-53-NH(2)(lt)) is obtained. The dehydration leads to a porous compound that exhibits hysteresis behavior in the N(2) sorption experiments. The MIL-53-NH(2)(lt) can be modified by postsynthetic functionalization using formic acid, and the corresponding amide Al(OH)[HC(O)N(H)-BDC] x H(2)O (MIL-53-NHCHO) is formed. All four phases were thoroughly characterized by X-ray powder diffraction, solid-state NMR and IR spectroscopy, and sorption measurements, as well as thermogravimetric and elemental analysis. Based on the refined lattice parameter similar breathing behavior of the framework as found in the unfunctionalized MIL-53 can be deduced. Solid-state NMR spectra unequivocally demonstrate the presence of the guest species, as well as the successful postsynthetic functionalization.

Five new flexible functionalized aluminum hydroxo terephthalates [Al(OH)(BDC-X)]·n(guests) (BDC = 1,4-benzene-dicarboxylate; X = -Cl, 1-Cl; -Br, 2-Br; -CH(3), 3-CH(3); -NO(2), 4-NO(2); -(OH)(2), 5-OH(2)) were synthesized under solvothermal conditions. The as synthesized (Al-MIL-53-X-AS) as well as the activated compounds were characterized by X-ray powder diffraction (XRPD), IR spectroscopy, thermogravimetric (TG), and elemental analysis. Activation, that is, removal of unreacted H(2)BDC-X molecules and/or occluded solvent molecules, followed by hydration in air at room temperature, led to the narrow pore (NP) form of the title compounds [Al(OH)(BDC-X)]·n(H(2)O) (Al-MIL-53-X). Thermogravimetric analysis (TGA) and temperature-dependent XRPD (TDXRPD) experiments performed on the NP-form of the compounds indicate high thermal stability in the range 325-500 °C. As verified by N(2), CO(2), or H(2)O sorption measurements, most of the thermally activated compounds exhibit significant microporosity. Similar to pristine Al-MIL-53, the present compounds retain their structural flexibility depending on the nature of guest molecules and temperature, as verified by cell parameter determination from XRPD data. The breathing behavior of the functionalized frameworks upon dehydration-rehydration, investigated by temperature and time-dependent XRPD measurements, differs significantly compared to parent Al-MIL-53.

Al together now! A new stable aluminum aminoterephthalate system contains octameric building blocks that are connected by organic linkers to form a 12-connected net (see picture). The structure adopts a cubic centered packing motive in which octameric units replace individual atoms, thus forming distorted octahedral (red sphere) and tetrahedral cages (green spheres) with effective accessible diameters of 1 and 0.45 nm, respectively.

The dehydrated aluminum form of the metal−organic framework compound MIL-53 shows a temperature-driven phase transition with pronounced structural hysteresis as recently shown by neutron diffraction and scattering experiments (J. Am. Chem. Soc.200813011813). Thereby, the structure of the corner-sharing metal AlO4(OH)2 octahedra differs in terms of local symmetry for the dehydrated MIL-53(Al) material in its high- and low-temperature form with open or closed pore structure, respectively. In this work, some of the framework aluminum ions were exchanged by chromium(III) to introduce an electron spin resonance (ESR) active probe ion. The resulting material was investigated by means of ESR and electron nuclear double resonance (ENDOR) spectroscopy to verify the incorporation of chromium at the octahedral framework sites. In addition, variable-temperature ESR measurements were performed to analyze the temperature-dependent phase behavior of the doped MIL-53 material. The Cr(III) ions have an electron spin S = 3/2 which shows a characteristic fine structure interaction depending very sensitively on the local symmetry of the chromium site. Therefore, the fine structure splitting of the Cr(III) probe ions allows for a concise elucidation of changes in the local symmetry at the CrO4(OH)2 octahedra which occur upon structural transitions. In agreement with results from neutron experiments, the transformation from the open to the closed pore structure was found to occur in the temperature range between 150 and 60 K whereas the back transformation is taking place within a smaller temperature interval between 330 and 375 K.