Xiaoping Zhang

Microporous Metal–Organic Framework Based on a Bifunctional Linker for Selective Sorption of CO2 over N2 and CH4

A bifunctional organic linker 4-(4-carboxyphenyl)-1,2,4-triazole (HCPT), incorporating both carboxylate and triazole groups, has been successfully used in the construction of a 2-fold interpenetrated dynamic metal-organic framework (MOF), {[Cu3(CPT)4(μ3-OH)]·NO3·7H2O·EtOH}n (1) based on a triangular Cu(II)-hydroxo cluster as secondary building unit (SBU). Upon solvation/desolvation and temperature, the crystal cell parameters of 1 could be fine-tuned. More importantly, a transformation from disordered phase to a more ordered phase after activation was observed via a single-crystal-to-single-crystal mode. Gas sorption studies reveal that the activated 1 exhibits highly selective sorption of CO2 over N2 and CH4 at room temperature.

Assembly of metal-organic frameworks based on 3,3',5,5'-azobenzene-tetracarboxylic acid: photoluminescences, magnetic properties, and gas separations.

We systematically studied the solvothermal reactions of transition-metal ions and H4abtc ligand and successfully isolated five metal-organic frameworks with various characterized tools, which are formulated as {[Mn2(abtc)(DMA)2.75]·1.25(DMA)}n (1), {[NH2(CH3)2][Co3(COOH)2.5(abtc)(H2O)2]2·H2O}n (2), {[Zn3(abtc)1.5(DMF)3]·1.75(DMF)}n (3), {[Zn2(abtc)(H2O)0.75(DMA)0.5]·3(DMA)·(H2O)}n (4), and {[Cd2(abtc)(DMA)2]·2(DMA)}n (5), (H4abtc = 3,3,5,5-azobenzenetetracarboxylic acid, DMF = N,N-dimethylformamide, and DMA = N,N-dimethylacetamide). 1-5 all consist of {Mn(CO2)4}-type clusters and H4abtc ligands; however, they exhibit four distinct architectures resulting from different coordinated modes of H4abtc ligand. A pair of Mn(2+)ions in 1 forms a {Mn2(CO2)4} cluster, which further results in the PtS-type three-dimensional (3D) framework. In 2, three independent Co(2+) ions are bridged by COOH(-) groups to afford a {Co3} core, and {Co3} cores are connected by abtc(2-) to generate a ZSW1-type topology. Two types of {Zn2(CO2)4} secondary building units in 3 linked by abtc(2-) give the 3D NbO-type cage. When DMF is replaced by DMA and CH3OH, the scarce nou-type topology of 4 is obtained. And 5 is isomorphous to 1. Photoluminescence properties of 1-5 were characterized. Magnetic measurements demonstrate that dominant antiferromagnetic interactions exist in 1 and 2. In addition, 3 exhibits significant adsorption capability of CO2 and highly selective sorption of CO2 over N2.

Three Cadmium Coordination Polymers with Carboxylate and Pyridine Mixed Ligands: Luminescent Sensors for FeIII and CrVI Ions in an Aqueous Medium

Three new water-stable luminescent Cd(II) coordination polymers (CPs), {[Cd2(bptc)(2,2-bipy)2(H2O)2]}n (1), {[Cd2(bptc)(phen)2]·4H2O}n (2), and {[Cd2(bptc)(4,4-bipy)(H2O)2]·4H2O}n (3), were solvothermally synthesized with mixed ligands of 3,3,5,5-biphenyltetracarboxylic acid (H4bptc) and N-donor ligands (2,2-bipy = 2,2-bipyridine; phen = 1,10-phenanthroline; 4,4-bipy = 4,4-bipyridine). The CPs 1-3 show structural diversity from a 1D ladder chain to a 2D layer to a 3D porous framework, tuned by different ancillary ligands. Topological analyses reveal that the CP 2 is a 4-connected uninodal 2D net with the Schläfli point symbol {44·62}, while the CP 3 displays a 4,6-connected 2-nodal 3D net with the point symbol {3·42·52·6}{32·42·52·64·74·8}. Luminescent property studies reveal that the CPs 1-3 are promising luminescent sensors that can highly select and sensitively detect ferric and chromate/dichromate ions, in which the CP 1 with a 1D structure showed the best performance, free from the interference of other ions present in an aqueous medium. Moreover, the mechanism for the sensing properties was studied in detail.

Two cadmium(II) coordination polymers constructed by carboxylate and pyridine mixed ligands: synthesis, structure and luminescent properties

Two new cadmium(II) coordination polymers (CPs), {[Cd(BTG)(bpy)(H2O)]·0.5glycol·H2O}n (1) and {[Cd6(BTB)4(bpy)4.5(glycol)]·9H2O}n (2) (H2BTG = 1,2,3-benzene tricarboxylic acid 2-(2-hydroxy-ethyl) ester, an ester of 1,2,3-benzenetricarboxylic acid and glycol; H3BTB = 1,2,3-benzenetricarboxylic acid), have been synthesized with the same reactants but different reaction conditions. CP 1 possesses an interesting stair-shaped 2D (4,4) network that stacks in a parallel fashion with a dihedral angle of 80.599(5)°. CP 2 exhibits a complicated 3D structure which can be simplified into a 3,10-connected network with an unreported Schlafli symbol of {32·4}{34·421·513·67}. The luminescent properties of CPs 1 and 2 are studied.

Dual-Functionalized Metal–Organic Frameworks Constructed from Hexatopic Ligand for Selective CO2 Adsorption

A ligand design approach, which requires rational design of ligand based on the knowledge of specific target, was applied for the synthesis of two interesting and robust MOFs 1 and 2 containing unusual several types of copper(II) secondary building units (SBUs). Thanks to unsaturated metal centers (UMCs) and azo group, microporous material 1 exhibited not only high CO2 uptake but also an impressive selective adsorption of CO2 over CH4 and N2. Moreover, a high H2 uptake of 1 was also observed.

A family of binuclear dysprosium(III) radical compounds with magnetic relaxation in ON and OFF states.

Four binuclear dysprosium compounds incorporating the radical ligand 2-(4-oxidopyridyl)-4,4,5,5-tetramethylimidazolin-1-oxyl-3-oxide (PyNONIT) have been successfully synthesized under appropriate conditions. Centrosymmetric bimetallic Dy(2)O(2) cores in all of the compounds through double-μ(2)-oxygen atoms of the N-oxide groups are realized in a metal-radical approach for the first time. Dimers 1 and 2, of the same formula {[Dy(hfac)(3)(PyNONIT)](2)}(2) (hfac = hexafluoroacetylacetonate) but obtained by different methods, which contain almost identical local symmetry of D(4d) and Dy-(O)(2)-Dy bridging fashion, however, display no out-of-phase alternating-current (ac) signal for 1 and slow relaxation of the magnetization for 2 corresponding to the difference of the crystal packing mode. The adduct ([Dy(hfac)(3)(PyNONIT)](2)[Dy(0.5)(hfac)(1.5)(H(2)O)](2)) (3) consists of two items, the dimer [Dy(hfac)(3)(PyNONIT)](2) and the monomer [Dy(hfac)(3)(H(2)O)(2)], where the symmetry of Dy(III) ion in Dy(2)O(2) decreases to D(2d), showing slow relaxation of the magnetization at lower temperature. Interestingly, a moisture-mediated reversible solid transformation between 1 and ([Dy(hfac)(3)(H(2)O)(PyNONIT)](2)) (4) has been investigated. Spongelike 1 can undergo a transition from eight to nine coordination at room temperature through hydration. A different coordination field is mostly responsible for no ac signal noticed for 4. The structural diversity of the Dy(2) family provides an opportunity to expand the investigation on 4f single-molecule magnets. Approaches that the relaxation of the supramolecular dimer can be tuned to ON and OFF states modulated by the packing mode and ligand field are presented.

From 1D zigzag chains to 3D chiral frameworks: synthesis and properties of praseodymium(III) and neodymium(III) coordination polymers

Reaction of H3TDA with PrIII/NdIII salts resulted in two coordination polymers, [Ln(H2O)4(HTDA)(H2TDA)]·H2O (Ln = Pr(1) and Nd(2), H3TDA = 1H-[1,2,3]-triazole-4,5-dicarboxylic acid), crystallizing in the monoclinic P21/c space group with a one-dimensional zigzag chain structure. The number of ligands and water molecules coordinated to the LnIII ions can be reduced by a hydrothermal method and the products transformed into {[Ln(TDA)(H2O)]·2.5H2O}n (Ln = Pr(3) and Nd(4)), crystallizing in the trigonal P3121 space group with a three-dimensional chiral framework structure. The compounds were characterized by elemental analysis, IR, PXRD, circular dichroism spectra and single crystal X-ray diffraction. The variable temperature magnetic susceptibility studies indicated that there are antiferromagnetic interactions between the LnIII ions in 1–4. Gas sorption and separation were studied as well.

Highly selective sorption of CO2 and N2O and strong gas-framework interactions in a nickel(II) organic material

A new robust microporous Ni-MOF with cylindrical channels based on an interesting 41 helix structure was fabricated by the reaction of biphenyl-3,3′,5,5′-tetracarboxylic acid and nickel (II) acetate via a solvothermal method. Activated Ni-MOF exhibits excellent CO2 and N2O sorption capabilities as well as high separations of CO2 and N2O over N2/O2/CH4/H2/Ar/He at 273 K, 298 K and 313 K. Moreover, the framework shows high isosteric heats of adsorption of three main greenhouse gases (CO2, CH4 and N2O).

Lanthanide metal–organic frameworks: synthesis and applications

Lanthanide metal-organic frameworks (Ln-MOFs) have attracted great attention for their potential applications in gas storage and separation, catalysis, magnetism, and luminescence [1]. Based on a coordination symmetry approach, we have synthesized a series of Ln-MOFs and studied their properties. Ln-MOFs are not very suitable for gas storage because of the high density and low stability. We have synthesized a robust microporous Tb-MOF-1 which containing 1D honeycomb-type channels. The desolvated Tb-MOF-1 shows not only excellent adsorption capabilities for N2, H2, and CO2 but also significant selective sorption of CO2 over N2 and CO2 over CH4. More importantly, due to the intrinsic property from f electron, luminescent studies of the emulsions of Tb-MOF-1 showed guestdependent luminescent properties, which demonstrated their potentials for sensing small-molecule pollutants, such as benzene and acetone. We further studied the mechanism of the guest-dependent luminescent properties of two isostructural Tb-MOF-2 and Gd-MOF-2 based on cubane-shaped [Ln4(μ3-OH)4]8+ clusters [2]. There are two kinds of micropores with dimensions of 3.0 × 7.0 Å2 along the b-axis and 4.5 × 5.5 Å2 along the c-axis in the cationic framework. Tb-MOF-2 exhibits an intensive green luminescence triggered by the efficient antenna effect of the ligands under UV light. Luminescent studies showed that Tb-MOF-2 could be an efficient multifunctional luminescent material for high-sensitivity sensing of small organic molecules, metal cations, and anions. Recently, we had applied a new strategy by using bimetallic Eu/Tb-MOFs as self-calibrating luminescent sensors. The enhancement and quenching from different luminescent centers in a singular material were well demonstrated due to the unusual self-calibrating mechanism based on the energy transfer from one lanthanide center to another one. Fast response and variable luminescent colors that are visible to the naked eye were also successfully achieved in this luminescent sensor. Selective luminescent detection of LPA, a biomarker for ovarian cancer that has the lowest survival rate of all gynecologic malignancies, in the presence of major components of blood plasma, was also reported. On the other hand, lanthanide centers that are usually weakly coupled in MOFs have shown obvious advantages with respect to the study of molecular nanomagnets [3]. Based on the results concerning constraining the coordination geometries of lanthanide ions and assembly of magnetic building blocks in MOFs, we proposed two possible approaches that could be experimentally explored for the molecular magnetism in the near future: (i) chemical modification of known SMMs to serve as “complex ligands” for the construction of magnetic MOFs; (ii) doping highly magnetic anisotropic metal ions into specific axial positions in diamagnetic frameworks and varying the doping concentration. These strategies will not only bring new samples to assist an indepth understanding of the factors governing the magnetization dynamics of molecular nanomagnets but will also produce new research interest in MOF chemistry.