Metal-ion metathesis in metal-organic frameworks: a synthetic route to new metal-organic frameworks.

A porous metal-organic framework, Mn(H(3)O)[(Mn(4)Cl)(3)(hmtt)(8)] (POST-65), was prepared by the reaction of 5,5',10,10',15,15'-hexamethyltruxene-2,7,12-tricarboxylic acid (H(3)hmtt) with MnCl(2) under solvothermal conditions. POST-65(Mn) was subjected to post-synthetic modification with Fe, Co, Ni, and Cu according to an ion-exchange method that resulted in the formation of three isomorphous frameworks, POST-65(Co/Ni/Cu), as well as a new framework, POST-65(Fe). The ion-exchanged samples could not be prepared by regular solvothermal reactions. The complete exchange of the metal ions and retention of the framework structure were verified by inductively coupled plasma-atomic emission spectrometry (ICP-AES), powder X-ray diffraction (PXRD), and Brunauer-Emmett-Teller (BET) surface-area analysis. Single-crystal X-ray diffractions studies revealed a single-crystal-to-single-crystal (SCSC)-transformation nature of the ion-exchange process. Hydrogen-sorption and magnetization measurements showed metal-specific properties of POST-65.

Synthesis of phase-pure interpenetrated MOF-5 and its gas sorption properties.

For the first time, phase-pure interpenetrated MOF-5 (1) has been synthesized and its gas sorption properties have been investigated. The phase purity of the material was confirmed by both single-crystal and powder X-ray diffraction studies and TGA analysis. A systematic study revealed that controlling the pH of the reaction medium is critical to the synthesis of phase-pure 1, and the optimum apparent pH (pH*) for the formation of 1 is 4.0-4.5. At higher or lower pH*, [Zn(2)(BDC)(2)(DMF)(2)] (2) or [Zn(5)(OH)(4)(BDC)(3)] (3), respectively, was predominantly formed. The pore size distribution obtained from Ar sorption experiments at 87 K showed only one peak, at ~6.7 Å, which is consistent with the average pore size of 1 revealed by single crystal X-ray crystallography. Compared to MOF-5, 1 exhibited higher stability toward heat and moisture. Although its surface area is much smaller than that of MOF-5 due to interpenetration, 1 showed a significantly higher hydrogen capacity (both gravimetric and volumetric) than MOF-5 at 77 K and 1 atm, presumably because of its higher enthalpy of adsorption, which may correlate with its higher volumetric hydrogen uptake compared to MOF-5 at room temperature, up to 100 bar. However, at high pressures and 77 K, where the saturated H(2) uptake mostly depends on the surface area of a porous material, the total hydrogen uptake of 1 is notably lower than that of MOF-5.

Chiral metal-organic porous materials: synthetic strategies and applications in chiral separation and catalysis.

In the light of growing demand for chiral purity in biological and chemical processes, the synthesis of chiral metal-organic porous materials (CMOPMs) have become immensely important because of their potential applications in chiral separation and asymmetric catalysis. In this chapter, the synthetic strategies for CMOPMs are discussed briefly keeping the focus on their applications. Two distinct approaches have been taken to synthesize a wide variety of chiral structures with different topologies and accessible cavities. Several CMOPMs have shown catalytic activity and enantioselectivity toward a number of chemical transformations. On many occasions, the chiral pores of the MOPMs have been utilized in order to achieve separation of enantiomers from racemates. Recent applications of homochiral MOPMs in heterogeneous asymmetric catalysis and chiral separations are also presented here.

Metathesis in single crystal: complete and reversible exchange of metal ions constituting the frameworks of metal-organic frameworks.

We report for the first time the complete and reversible exchange of metal ions constituting a robust microporous framework while maintaining not only the structural integrity of the framework but also single crystallinity. Solvothermal reaction of Cd(NO(3))(2) .4H(2)O with H(3)hett in DMF produced a new crystalline MOF 1 having a cubic network with 3D channels. The basic building unit of the framework 1 is an octahedron where the square planar tetrametallic SBUs ({Cd(4)O}(6+)) and tricarboxylate linkers (hett(3-)) occupy the vertices and triangular faces of the octahedron, respectively. The cubic framework is generated by sharing the vertices of the octahedra. The framework is exceptionally stable in open air as well as in various solutions including aqueous medium. Most interestingly, the framework constituting Cd(II) ions undergo complete and reversible exchange with Pb(II) in aqueous solution at room temperature without altering structural integrity and losing single crystallinity. We have also studied the complete exchange of framework Cd(II) ions with lanthanide ions (Dy(III) and Nd(III)) with the retention of the framework topology. The reversible exchange of framework constituting metal ions while maintaining the framework integrity and single crystallinity was confirmed by a combination of various methods including ICP-AES, in situ PXRD, TGA, IR, optical microscopy, elemental analysis, N(2) sorption and in situ single crystal X-ray structural analysis.

Postsynthetic modification switches an achiral framework to catalytically active homochiral metal-organic porous materials.

The postsynthetic modification strategy is adopted to demonstrate for the first time the syntheses of catalytically active chiral MOPMs from a preassambled achiral framework, MIL-101, by attaching L-proline-derived chiral catalytic units to the open metal coordination sites of the host framework. Various characterization techniques (including PXRD, TGA, IR, and N(2) absorption measurements) indicated that the chiral units are successfully incorporated into MIL-101, keeping the parent framework intact. The new chiral MOPMs show remarkable catalytic activities in asymmetric aldol reactions (yield up to 90% and ee up to 80%). It is interesting to note that these heterogeneous catalysts show much higher enantioselectivity than the corresponding chiral catalytic units as homogeneous catalysts. This study demonstrates a simple and efficient route for the generation of catalytically active chiral MOPMs. A variety of chiral catalytic units can be, in principle, incorporated into chemically robust achiral MOPMs with large pores by postmodification and the resulting chiral MOPMs may find useful applications in catalytic asymmetric transformations.

Methane sorption and structural characterization of the sorption sites in Zn2(bdc)2(dabco) by single crystal X-ray crystallography.

Sorption isotherms of methane in Zn(2)(bdc)(2)(dabco) are measured up to a pressure of 35 bar in the temperature range between 198-296 K. The methane sorption measurements at 296 K showed an uptake of 137 cm(3) cm(-3) at 35 bar. The enthalpy of methane adsorption for Zn(2)(bdc)(2)(dabco) estimated by the virial equation is 13.6 kJ mol(-1) at zero coverage. X-ray structure analysis of methane-adsorbed Zn(2)(bdc)(2)(dabco) by synchrotron radiation at 90 K revealed that methane molecules occupy three independent sorption sites (A, B, and C) with a stoichiometry of Zn(2)(bdc)(2)(dabco) x 6.69 CH(4), which is consistent with the results of the gas sorption measurements at 198 K. In a cavity, eight symmetry-related methane sorption sites A are located near the {Zn(2)(CO(2))(4)} paddle-wheel units, while four symmetry-related methane sorption sites B are near the center of the small windows along the a and b axes. Both A and B sites are half-occupied. Methane molecules occupying sites A are not only in van der Waals contact with the paddle-wheel units, but also interact with the phenyl rings of bdc ligands through partial pi-HC interactions. Methane molecules in B sites interact with the side of the phenyl rings through van der Waals interaction. The site C, located at the center of the cavity, is a secondary sorption site; methane molecules occupying sites C are in van der Waals contact with those in sites A and B.

Unique asymmetric (CuII4) double-stranded helicate from a hexadentate piperazine-based ligand: ligand conformation isomerism upon coordination.

In methanol, the reaction of Cu(ClO(4))(2).6H(2)O and a sterically constrained piperazine imine phenol ligand (H(2)L), in the presence of NEt(3), affords a novel tetranuclear copper(II) complex of formula [Cu(II)(4)(mu(3)-L)(2)(mu-OH)(2)(H(2)O)(2)](ClO(4))(2).H(2)O (1). The X-ray structure of this complex shows an elongated Cu(4) quasi-tetrahedron coordinated to two hexadentate chair-(e,a)-mu(3)-piperazine bridging ligands. Variable-temperature magnetic studies show an S(t) = 0 spin ground state resulting from antiferromagnetic interactions between Cu(II) ions within the complex.