Interrupted zeolite LTA and ATN-type boron imidazolate frameworks

Zeolite A (LTA) is of much interest in zeolite family because of its large-scale industrial applications. Making Zeolite A (a typical 4-connected tetrahedral framework material) with a lower connectivity (3-connected) might lead to new open architecture with expanded ring size and enhanced functionality. The first interrupted Zeolite A with 3-connected network has been experimentally realized here by a boron imidazolate (im) framework material (BIF-20) with 3-coordinate BH(mim)(3)(-) building units. Additionally, a new strategy toward the construction of functional microporous metal-organic frameworks with interrupted zeolite-type topologies is presented by both 3-connected boron imidazolate frameworks (BIF-20 and BIF-21). BIF-20 has an unusual tetrahedral framework with both debonded α and β cages, and exhibits high H(2) uptake capacity.     

Crystal engineering of binary metal imidazolate and triazolate frameworks

This article summarizes the recent advances in the crystal growth, structural control strategies and diverse structures of the binary metal imidazolate and triazolate frameworks, which are the simplest systems for crystal engineering of two-, three- and four-connected coordination polymers.     

A bioinspired synthetic approach for building metal-organic frameworks with accessible metal centers

We demonstrate how a bioinspired synthetic approach can help organic linkers distinguish between different types of metal centers in metal-organic frameworks (MOFs). Modification of an organic building unit with methyl groups enables the unit to selectively coordinate to one of the two metal sites present in the MOFs. We report four new porphyrin-based, pillared-paddlewheel frameworks: PPF-11-Zn/Zn, -Co/Co, -Mn/Zn, and -Fe/Zn, where the first and second metals indicate the metal center for the porphyrin core and paddlewheel cluster, respectively. These compounds exhibit 3D MOFs in which 2D layers are pillared by a sterically controlled bipyridine, leaving the metal centers inside the porphyrin structurally unconnected.     

Anchoring metal ions in amine-functionalized boron imidazolate framework for photocatalytic reduction of CO2

The photocatalytic reduction of CO₂ to energy-rich chemicals is highly appealing for alleviation of energy crisis and environment pollution.The introduction of different active sites is a key factor to determine the reaction activity and selectivity.Here,we demonstrate the metal ion-dependent performance for photocatalytic CO₂ reduction by anchoring transition metal ions (Co²⁺and Ni²⁺) in an amine-functionalized boron imidazolate framework （BIF-43）.As ...     

Increasing the density of adsorbed hydrogen with coordinatively unsaturated metal centers in metal-organic frameworks

Storing molecular hydrogen in porous media is one of the promising avenues for mobile hydrogen storage. In order to achieve technologically relevant levels of gravimetric density, the density of adsorbed H2 must be increased beyond levels attained for typical high surface area carbons. Here, we demonstrate a strong correlation between exposed and coordinatively unsaturated metal centers and enhanced hydrogen surface density in many framework structures. We show that the MOF-74 framework structure with open Zn(2+) sites displays the highest surface density for physisorbed hydrogen in framework structures. Isotherm and neutron scattering methods are used to elucidate the strength of the guest-host interactions and atomic-scale bonding of hydrogen in this material. As a metric with which to compare adsorption density with other materials, we define a surface packing density and model the strength of the H(2-)surface interaction required to decrease the H(2)-H(2) distance and to estimate the largest possible surface packing density based on surface physisorption methods.     

Structural stability of boron clusters with octahedral and tetrahedral symmetries

The structural stability of cagelike boron clusters with octahedral and tetrahedral symmetries has been investigated by means of first-principles calculations. Twenty-eight cluster models, ranging from B(10) to B(66), were systematically constructed using regular and semiregular polyhedra as prototypes. The binding energies per atom were, on the whole, slightly lower than those of icosahedral clusters B(80) and B(100), which are supposed to be the most stable in the icosahedral group. The larger clusters did not always have higher binding energies. Isothermal molecular dynamics simulations were performed to determine the deformation temperatures at which clusters began to break or change their structures. We found eight clusters that had nonzero deformation temperatures, indicating that they are in metastable states. The octahedral cluster B(18) had the highest deformation temperature among these, similar to that of icosahedral B(80) and B(100). The analysis of the electronic structure of B(18) showed that it attained this high stability owing to Jahn-Teller distortion.     

Controlling structural topology of metal-organic frameworks with a desymmetric 4-connected ligand through the design of metal-containing nodes

Two new metal-organic frameworks(MOFs),[Cu2(H_2O)_2(BCPIA)](BUT-20)and(Me_2NH_2)[In(BCPIA)](BUT-21)were designed and synthesized through the solvothermal reaction between a newly created desymmetric 4-connected ligand,5-(2,6-bis(4-carboxyphenyl)pyridin-4-yl)isophthalic acid(H_4BCPIA)and Cu(NO_3)2 2.5H_2O or In(NO_3)_3·5H_2O,respectively,and characterized by single-crystal and powder Xray diffraction,thermogravimetric analysis,infrared spectroscopy,and elemental analysis.The two MOFs have three-dimensional structures,in which both the BCPIA 4 ligand and metal-containing entities,Cu_2(COO)_4(H_2O)_2 and In(COO)_4 act as 4-connected nodes.However,different linkage configurations of the two metal-containing nodes,quadrilateral Cu_2_TD_2(COO)_4(H_2O)_2and tetrahedral In(COO)_4,lead to distinct structural networks of BUT-20 and 21,with Nbo and Unc topologies,respectively.     

Metal-organic frameworks from zinc sulfite clusters, chains, and sheets: 4-connected, (3,4)-connected 3-D frameworks and 2-D arrays of catenane-like interlocking rings

Even though open-framework solids have been made in a variety of compositions such as silicates, phosphates, germanates, borates, and phosphites, few are known that are based on trigonal-pyramidal sulfite anions. We report here the first synthetic and structural studies of metal-organic framework materials in the zinc sulfite composition. It is demonstrated here that Zn2+ and SO32- can form various neutral inorganic subunits that can be 0-D clusters, 1-D chains, or 2-D sheets. These inorganic subunits of different dimensionality can subsequently be connected into extended frameworks of higher dimensionality through bifunctional ligands. In (ZnSO3)2en, infinite corrugated ZnSO3 layers are pillared by ethylenediamine (en) molecules into a 3-D network that can be classified as a (3,4)-connected net based on tetrahedral Zn nodes and trigonal-pyramidal S nodes. In (ZnSO3)pip, infinite ZnSO3 chains are cross-linked with piperazine molecules into a 3-D framework that can be classified as 4-connected net based on tetrahedral Zn nodes only. In (ZnSO3)2(TMDPy)2, (ZnSO3)2 dimers are doubly bridged by trimethylenedipyridine molecules into an infinite chain with a string of circles. Each circle along the chain is interlocked with another circle from a chain in the perpendicular direction, creating a 2-D pattern with an infinite-square array of catenane-like units.     

Non-3d metal modulated zinc imidazolate frameworks for CO2 cycloaddition in simulated flue gas under ambient condition

Cycloaddition of CO₂ and epoxide into cyclic carbonate is one of the most efficient ways for CO₂ conversion with 100% atom-utilization. Metal–organic frameworks are a kind of potential heterogeneous catalysts, however, high temperature, high pressure, and high-purity CO₂ are still required for the reaction. Here, we report two new Zn(II) imidazolate frameworks incoporating MoO₄^2– or WO₄^2– units, which can catalyse cycloaddition of CO₂ and epichlorohydrin at room temperature and atomospheric pressure, giving 95% yield after 24 h in pure CO₂ and 98% yield after 48 h in simulated flue gas (15% CO₂ + 85% N₂), respectively. For comparison, the analogic Zn(II) imidazolate framework MAF-6 without non-3d metal oxide units showed 71% and 33% yields under the same conditions, respectively. The insightful modulation mechanisms of the MoO₄^2– unit in optimizing the electronic structure of Zn(II) centre, facilitating the rate-determined ring opening process, and minimizing the reaction activation energy, were revealed by X-ray photoelectron spectroscopy, temperature programmed desorption and computational calculations.     

Metal-organic frameworks for electrochemical reduction of carbon dioxide: The role of metal centers

Direct electrochemical reduction of CO_2 into valuable chemicals and fuel is one of the most promising approaches to address the current energy crisis and lower CO_2 emission. Recently, numerous metal-organic framework(MOF) and their derived materials have extensively been developed as electrocatalysts for CO_2 reduction owing to their unique structure including porosity, large specific surface area, and tunable chemical structures. In this review, the recent progress of MOF-based electrocatalysts for CO_2 reduction was summarized and discussed. Detailed discussions mainly focus on the synthesis and mechanism of pristine MOFs and MOF-derived materials for electrocatalytic CO_2 reduction. These examples are expected to provide clues to rational design and synthesis of stable and high-performance MOFs-based electrocatalysts for CO_2 reduction.     

Three-dimensional (3-D) metal-organic frameworks with 3-pyridin-4-yl-benzoate defining new (3,6)-connected net topologies

Reactions of different metal salts with 3-pyridin-4-yl-benzoic acid (3,4-Hpybz) under ambient condition afford a series of 3-D metal-organic frameworks with two new types of (3,6)-connected net topologies. In the isomorphic complexes [M₂(μ-H₂O)(3,4-pybz)₄]_n (M^II=Mn^II for 1, Zn^II for 2, or Cd^II for 3), the octahedral metal nodes are extended by the 3-connected pybz tectons to constitute 3-D arrays with the Schläfli symbol of (3.4.5)(3².4⁴.5⁵.6².7²), whereas [Pb(3,4-pybz)₂]_n (4) shows a completely different 3-D (4².6)₂(4⁴.6².8⁹) framework, which represents a subnet of the (4,8)-connected fluorite lattice.     

Interaction of molecular hydrogen with open transition metal centers for enhanced binding in metal-organic frameworks: a computational study

Molecular hydrogen is known to form stable, "nonclassical" sigma complexes with transition metal centers that are stabilized by donor-acceptor interactions and electrostatics. In this computational study, we establish that strong H2 sorption sites can be obtained in metal-organic frameworks by incorporating open transition metal sites on the organic linkers. Using density functional theory and energy decomposition analysis, we investigate the nature and characteristics of the H2 interaction with models of exposed open metal binding sites {half-sandwich piano-stool shaped complexes of the form (Arene)ML(3- n)(H2)n [M=Cr, Mo, V(-), Mn(+); Arene = C6H5X (X=H, F, Cl, OCH3, NH2, CH3, CF3) or C6H3Y2X (Y=COOH, X=CF3, Cl; L=CO; n=1-3]}. The metal-H2 bond dissociation energy of the studied complexes is calculated to be between 48 and 84 kJ/mol, based on the introduction of arene substituents, changes to the metal core, and of charge-balancing ligands. Thus, design of the binding site controls the H2 binding affinity and could be potentially used to control the magnitude of the H2 interaction energy to achieve reversible sorption characteristics at ambient conditions. Energy decomposition analysis illuminates both the possibilities and present challenges associated with rational materials design.     

A series of new oxo-vanadium(IV) complexes with octahedral coordinated vanadium centers

A series of new oxo-vanadium(IV) complexes, [VOCl0.69(OH)0.31 (2,2′-bipy)2]Cl·2H2O (1, 2,2′-bipy?=?2,2′-bipyridine) [(VO)₂Cl₄(4,4'-bipy)₃ (H₂O)₂] (2, 4,4'-bipy?=?4,4'-bipyridine), [VO(ida)(H₂O)]_n (3, H₂ida?=?iminodiacetic acid), and [(VO)₂(oa)₄]_n·4n(H₃O)·n(H₂O) (4, H₂oa?=?oxalic acid), have been synthesized and structurally characterized. 1 contains a [VOCl_0.69(OH)_0.31(2,2′-bipy)₂]⁺ cation, Cl ^– anion and two free H₂O molecules. 2 exhibits a binuclear centrosymmetric moiety built up from two [VOCl₂(4,4'-bipy)(H₂O)] units and one bridging 4,4'-bipy ligand, which provides a rare example of a 4,4'-bipy molecule acting as monodentate ligand. 3 displays a neutral chain [VO(ida)(H₂O)]_n constructed by the linkages of [VO(H₂O)]²⁺ units and ida^2? bridging ligands, while 4 offers the only example of three kinds of oa^2- ligands coexisting within the same anionic chain [(VO)₂(oa)₄^4-]_n. Their spectroscopic properties were investigated, and the magnetic susceptibility of 4 shows antiferromagnetic behavior.     

Coordination-driven self-assembly of M3L2 trigonal cages from preorganized metalloligands incorporating octahedral metal centers and fluorescent detection of nitroaromatics

The design and preparation of novel M(3)L(2) trigonal cages via the coordination-driven self-assembly of preorganized metalloligands containing octahedral aluminum(III), gallium(III), or ruthenium(II) centers is described. When tritopic or dinuclear linear metalloligands and appropriate complementary subunits are employed, M(3)L(2) trigonal-bipyramidal and trigonal-prismatic cages are self-assembled under mild conditions. These three-dimensional cages were characterized with multinuclear NMR spectroscopy ((1)H and (31)P) and high-resolution electrospray ionization mass spectrometry. The structure of one such trigonal-prismatic cage, self-assembled from an arene ruthenium metalloligand, was confirmed via single-crystal X-ray crystallography. The fluorescent nature of these prisms, due to the presence of their electron-rich ethynyl functionalities, prompted photophysical studies, which revealed that electron-deficient nitroaromatics are effective quenchers of the cages' emission. Excited-state charge transfer from the prisms to the nitroaromatic substrates can be used as the basis for the development of selective and discriminatory turn-off fluorescent sensors for nitroaromatics.     

Altering the spin state of transition metal centers in metal-organic frameworks by molecular hydrogen adsorption: a first-principles study

Our first-principles calculation shows that molecular hydrogen (H(2)) adsorption at an exposed Fe(II) site in metal-organic frameworks could induce a spin flip in the Fe(II) center resulting in a spin-state transition from a triplet high-spin (HS) to a singlet low-spin (LS) state. The Kubas-type Fe-H(2) interaction, where H(2) coordinates onto the Fe(II) center as a σ-ligand, is found commensurate in strength with the exchange interaction of Fe 3d electrons, which is responsible for the occurrence of the spin-state transition in this system. The H(2) binding energies are 0.08 and 0.35 eV per H(2) at the HS and LS states, respectively. This effect is expected to find applications in spin-control in molecular magnets, hydrogen sensing and storage.     

Hydrogen storage in microporous metal-organic frameworks with exposed metal sites

Owing to their high uptake capacity at low temperature and excellent reversibility kinetics, metal-organic frameworks have attracted considerable attention as potential solid-state hydrogen storage materials. In the last few years, researchers have also identified several strategies for increasing the affinity of these materials towards hydrogen, among which the binding of H(2) to unsaturated metal centers is one of the most promising. Herein, we review the synthetic approaches employed thus far for producing frameworks with exposed metal sites, and summarize the hydrogen uptake capacities and binding energies in these materials. In addition, results from experiments that were used to probe independently the metal-hydrogen interaction in selected materials will be discussed.