Reversible transformation of ZnII coordination geometry in a single crystal of porous metal-organic framework [Zn3(ntb)2(EtOH)2].4 EtOH

A 3D porous metal-organic framework [Zn3(ntb)2(EtOH)2]n.4nEtOH (1) that generates 1D channels of honeycomb aperture has been prepared by the solvothermal reaction of Zn(NO3)(2).6 H2O and 4,4',4'-nitrilotrisbenzoic acid (H3NTB) in EtOH at 110 degrees C. Framework 1 exhibits reversible single-crystal-to-single-crystal transformations upon removal and rebinding of the coordinating EtOH as well as the EtOH guest molecules, which give rise to desolvated crystal [Zn3(ntb)2]n (1') and resolvated crystal [Zn3(ntb)2-(EtOH)2]n.4nEtOH (1'). The X-ray structures indicate that 3D host framework is retained during the transformations from 1 to 1' and from 1' to 1', but the coordination geometry of ZnII ions changes from/to trigonal bipyramid to/from tetrahedron, concomitant with the rotational rearrangement of a carboxylate plane of the NTB3- relative to its associated phenyl ring. To retain the single crystal integrity, extensive cooperative motions must exist between the molecular components throughout the crystal. Framework 1' exhibits permanent porosity, thermal stability up to 400 degrees C, and blue luminescence, and high storage capabilities for N2, H2, CO2, and CH4.     

Selective CO2 adsorption by a triazacyclononane-bridged microporous metal-organic framework

Metal-organic frameworks constructed by self-assembly of metal ions and organic linkers have recently been of great interest in the preparation of porous hybrid materials with a wide variety of functions. Despite much research in this area and the large choice of building blocks used to fine-tune pore size and structure, it remains a challenge to synthesise frameworks composed of polyamines to tailor the porosity and adsorption properties for CO(2). Herein, we describe a rigid and microporous three-dimensional metal-organic framework with the formula [Zn(2)(L)(H(2)O)]Cl (L=1,4,7-tris(4-carboxybenzyl)-1,4,7-triazacyclononane) synthesised in a one-pot solvothermal reaction between zinc ions and a flexible cyclic polyaminocarboxylate. We have demonstrated, for the first time, that a porous rigid framework can be obtained by starting from a flexible amine building block. Sorption measurements revealed that the material exhibited a high surface area (135 m(2) g(-1)) and was the best compromise between capacity and selectivity for CO(2) over CO, CH(4), N(2) and O(2); as such it is a promising new selective adsorbent for CO(2) capture.     

Probing the Lewis acidity and catalytic activity of the metal-organic framework [Cu3(btc)2] (BTC=benzene-1,3,5-tricarboxylate)

An optimized procedure was designed for the preparation of the microporous metal-organic framework (MOF) [Cu3(btc)2] (BTC=benzene-1,3,5-tricarboxylate). The crystalline material was characterized by X-ray diffraction, optical microscopy, SEM, X-ray photoelectron spectroscopy, N2 sorption, thermogravimetry, and IR spectroscopy of adsorbed CO. CO adsorbs on a small number of Cu2O impurities, and particularly on the free CuII coordination sites in the framework. [Cu3(btc)2] is a highly selective Lewis acid catalyst for the isomerization of terpene derivatives, such as the rearrangement of alpha-pinene oxide to campholenic aldehyde and the cyclization of citronellal to isopulegol. By using the ethylene ketal of 2-bromopropiophenone as a test substrate, it was demonstrated that the active sites in [Cu3(btc)2] are hard Lewis acids. Catalyst stability, re-usability, and heterogeneity are critically assessed.     

Structure and charge control in metal-organic frameworks based on the tetrahedral ligand tetrakis(4-tetrazolylphenyl)methane

Use of the tetrahedral ligand tetrakis(4-tetrazolylphenyl)methane enabled isolation of two three-dimensional metal-organic frameworks featuring 4,6- and 4,8-connected nets related to the structures of garnet and fluorite with the formulae Mn(6)(ttpm)(3)5 DMF3 H(2)O (1) and Cu[(Cu(4)Cl)(ttpm)(2)](2)CuCl(2)5 DMF11 H(2)O (2) (H(4)ttpm=tetrakis(4-tetrazolylphenyl)methane). The fluorite-type solid 2 displays an unprecedented post-synthetic transformation in which the negative charge of the framework is reduced by extraction of copper(II) chloride. Desolvation of this compound generates Cu(4)(ttpm)(2)0.7 CuCl(2) (2 d), a microporous material exhibiting a high surface area and significant hydrogen uptake.     

A metal-organic framework with highly polar pore surfaces: selective CO2 adsorption and guest-dependent on/off emission properties

A 3D porous Zn(II) metal-organic framework {[Zn(2)(H(2)dht)(dht)(0.5)(azpy)(0.5)(H(2)O)]·4H(2)O} (1; H(2)dht=dihydroxyterphthalate, azpy=4,4'-azobipyridine) has been synthesised by employing 2,5-dihydroxyterephthalic acid (H(4)dht), a multidentate ligand and 4,4'-azobipyridine by solvent-diffusion techniques at room temperature. The as-synthesised framework furnishes two different types of channels: one calyx-shaped along the [001] direction and another rectangle-shaped along the [101] direction occupied by guest water molecules. The dehydrated framework, {[Zn(2)(H(2)dht)(dht)(0.5)(azpy)(0.5)]} (1') provides 52.7% void volume to the total unit-cell volume. The pore surfaces of 1' are decorated with unsaturated Zn(II) sites and pendant hydroxyl groups of H(2)dht linker, thereby resulting in a highly polar pore surface. The dehydrated framework 1' shows highly selective adsorption of CO(2) over other gases, such as N(2), H(2), O(2) and Ar, at 195 K. Photoluminescence studies revealed that compound 1 exhibits green emission (λ(max)≈530 nm) on the basis of the excited-state intramolecular proton-transfer (ESIPT) process of the H(2)dht linker; no emission was observed in dehydrated solid 1'. Such guest-induced on/off emission has been correlated to the structural transformation and concomitant breaking and reforming of the OH···OCO hydrogen-bonding interaction in the H(2)dht linker in 1'/1.     

Microporous metal-organic framework constructed from heptanuclear zinc carboxylate secondary building units

A novel, three-dimensional, noninterpenetrating microporous metal-organic framework (MOF), [Zn7O2(pda)5(H2O)2]5 DMF4 EtOH 6 H2O (1) (H2PDA=p-phenylenediacrylic acid, DMF=N,N-dimethylformamide, EtOH=ethanol), was synthesized by constructing heptanuclear zinc carboxylate secondary building units (SBUs) and by using rigid and linear aromatic carboxylate ligands, PDA. The X-ray crystallographic data reveals that the seven zinc centers of 1 are held together with ten carboxylate groups of the PDA ligands and four water molecules to form a heptametallic SBU, Zn7O4(CO2)10, with dimensions of 9.8 x 9.8 x 13.8 A3. Furthermore, the heptametallic SBUs are interconnected by PDA acting as linkers, thereby generating an extended network with a three-dimensional, noninterpenetrating, intersecting large-channel system with spacing of about 17.3 A. As a microporous framework, polymer 1 shows adsorption behavior that is favorable towards H2O and CH3OH, and substantial H2 uptake. In terms of the heptanuclear zinc carboxylate SBUs, polymer 1 exhibits interesting photoelectronic properties, which would facilitate the exploration of new types of semiconducting materials, especially among MOFs containing multinuclear metal carboxylate SBUs.     

Synergetic effect of host-guest chemistry and spin crossover in 3D Hofmann-like metal-organic frameworks [Fe(bpac)M(CN)4] (M=Pt, Pd, Ni)

The synthesis and characterization of a series of three‐dimensional (3D) Hofmann‐like clathrate porous metal–organic framework (MOF) materials [Fe(bpac)M(CN)₄] (M=Pt, Pd, and Ni; bpac=bis(4‐pyridyl)acetylene) that exhibit spin‐crossover behavior is reported. The rigid bpac ligand is longer than the previously used azopyridine and pyrazine and has been selected with the aim to improve both the spin‐crossover properties and the porosity of the corresponding porous coordination polymers (PCPs). The 3D network is composed of successive {Fe[M(CN)₄]}_n planar layers bridged by the bis‐monodentate bpac ligand linked in the apical positions of the iron center. The large void between the layers, which represents 41.7 % of the unit cell, can accommodate solvent molecules or free bpac ligand. Different synthetic strategies were used to obtain a range of spin‐crossover behaviors with hysteresis loops around room temperature; the samples were characterized by magnetic susceptibility, calorimetric, Mössbauer, and Raman measurements. The complete physical study reveals a clear relationship between the quantity of included bpac molecules and the completeness of the spin transition, thereby underlining the key role of the π–π stacking interactions operating between the host and guest bpac molecules within the network. Although the inclusion of the bpac molecules tends to increase the amount of active iron centers, no variation of the transition temperature was measured. We have also investigated the ability of the network to accommodate the inclusion of molecules other than water and bpac and studied the synergy between the host–guest interaction and the spin‐crossover behavior. In fact, the clathration of various aromatic molecules revealed specific modifications of the transition temperature. Finally, the transition temperature and the completeness of the transition are related to the nature of the metal associated with the iron center (Ni, Pt, or Pd) and also to the nature and the amount of guest molecules in the lattice.