Synthesis, characterization and application of metal-organic frameworks

Metal-organic frameworks (MOFs), also known as porous coordination polymers, are porous materials in which metal-containing nodes are connected by organic bridges. A larger variety of inorganic and organic components can be used to construct MOFs, and this versatility has enabled the rational design and assembly of materials having novel topologies and exceptional properties. Luminescent MOFs are potentially useful as chemically-selective sensors. We have discovered a new luminescent MOF synthesized by treating Eu(III) ions with 2,2′-bipyridine-5,5′-dicarboxylic acid in N,N-diethyl¬formamide (DEF). The X-ray crystal structure of the resulting material, Eu2(C12H6N2O4)3(DEF)4(H2O)5, shows that it is a MOF with large channels (25 × 15 Å), and that the carboxylate groups but not the nitrogen atoms of the bipyridine units are bonded to the europium centers. Surface area measurements on the desolvated material confirmed that the material shows permanent porosity. When this material is exposed to 1,3,5-trinitrotoluene (TNT) or smaller nitroaromatics, the fluorescence of this MOF was significantly quenched. In contrast, 1,1-diphenyl-2-picrylhydrazine, a larger nitroaromatic, do not cause quenching. Treatment of Zn(II) ions with 2,2′-bipyridine-5,5′-dicarboxylic acid and formic acid gives a different MOF, Zn3(C12H6N2O4)2(O2CH)2•2DEF. Crystallographic studies show that this compound crystallizes in a chiral space group P41212 by spontaneous resolution, although the specimen we examined was a racemic twin. Two of the zinc atoms are five-coordinate, whereas the other zinc atom has an octahedral coordination environment. The nano-pores, which have cross sections of 0.78 × 1.53 nm, are arranged in a herringbone fashion along the c axis. The ability of MOFs to withstand high pressures is necessary for many of the most interesting potential applications of MOFs. We carried out a high pressure study of the metal organic framework Zn4O(1,4-benzenedicarboxylate)3 (IRMOF-1) up to 8.93 GPa, using a synchrotron radiation source and a diamond anvil cell. Both as-synthesized and desolvated samples of IRMOF-1 retained some crystallinity to 6.57 GPa (65 700 atm) and 4.32 GPa (43 2 00 atm), respectively. Both begin to convert to a new material even at pressures as low as 0.21 GPa; for the as-synthesized and desolvated material, this process is essentially complete at 8.33 and 5.17 GPa. The diffraction pattern suggests that pressure promotes a hydrolysis reaction; the water molecules necessary for this reaction were absorbed from the atmosphere during sample handling after desolvation. Some amorphization occurs along with the formation of the new phase