Zn(II)-Siloxane Clusters as Versatile Building Blocks for Carboxylate-Based Metal-Organic Frameworks.

Siloxanes have long been known for their highly desirable properties suited for a wide range of practical applications; however, their utilization as modular building blocks for crystalline open frameworks has been limited. In this study, a simple solvothermal pathway has been found to synthesize unprecedented Zn(II)-siloxane clusters supported by acetate ligands, [(RSiO2)8Zn8(CH3CO2)8] (R = Me or Ph). The same reaction using a dicarboxylate ligand such as 1,4-benzenedicarboxylate or 2,6-naphthalenedicarboxylate produces a new type of metal-organic framework, named SiMOF here, based on the [Si8Zn8] units. With the maximum connectivity of 8, the building block is shown to form topologically interesting structures such as octahedral supercages or uninodal 8-connected frameworks. All SiMOFs synthesized possess permanent porosity and high thermal stability and are naturally hydrophobic, as demonstrated by adsorptions of toluene, ethanol, methanol, and water vapor as well as water contact angle measurements. These promising characteristics for well-defined porous solids are attributed to metal-bound siloxane groups in the structural building units.

Sonochemical Preparation of a Magnet-Responsive Fe3O4@ZIF-8 Adsorbent for Efficient Cu2+ Removal.

This work presents a novel approach to synthesizing magnetic core-shell nanocomposites, consisting of magnetic nanoparticles and a metal-organic framework, for environmental applications. The synthesis is based on the encapsulation of magnetic Fe3O4 nanoparticles with microporous zeolitic imidazolate framework-8 (ZIF-8) nanocrystals via ultrasonic activation under a continuous supply of precursor solutions. This sonochemical approach is proven to be a fast, cost-effective, and controllable route for the preparation of magnet-responsive Fe3O4@ZIF-8 nanoparticles with a core-shell structure. The functional nanomaterial possesses a high content of ZIF-8 and combined micro/mesoporosity, and thus can be used as adsorbents that can be easily separated using a magnet. In particular, the sonochemically prepared Fe3O4@ZIF-8 exhibits significant adsorption performance for the removal of copper ions from water: a short adsorption time (10 min), high maximum uptake capacity (345 mg g-1), and excellent removal efficiency (95.3%). These performances are interpreted and discussed based on the materials characteristics of Fe3O4@ZIF-8 established by microscopy, gas sorption, X-ray diffraction, and thermal analysis.

Zirconium-Formate Macrocycles and Supercage: Molecular Packing versus MOF-like Network for Water Vapor Sorption.

In systematic efforts toward a new type of molecule-based porous materials, facile and efficient synthetic methods have been established to obtain macrocyclic [Zr6]6 and supercage-like {[Zr6]6}8, where [Zr6] represents [Zr6O4(OH)4(CO2) n] building unit commonly found in Zr-based metal-organic frameworks. The reactions involve in situ hydrolysis of DMF solvent to produce formate linkers and thus do not require any organic ligand. A minor variation in the composition of two cyclic hexamers thus obtained results in dramatic differences in crystal packing which in turn lead to distinctive and selective sorption behavior for water vapor. It is shown that the high heat of water adsorption and unrestricted uptake under high humidity are consequences of the highly polar surface and flexible crystal packing. The reversibility of water adsorption is demonstrated by cyclic measurements of uptake and regeneration under dynamic flow conditions.

Facile synthesis of metal/metal oxide nanoparticles inside a nanoporous carbon matrix (M/MO@C) through the morphology-preserved transformation of metal-organic framework.

A facile method to transform metal-organic frameworks (MOFs) into metal/metal oxide@carbon (M/MO@C) composites with well-defined shapes is reported. The porosity of carbon and the particle sizes of M/MO are readily controlled by a simple two-step process that includes impregnation of the polymer precursors and a thermolysis reaction.

Robust molecular crystals of titanium(IV)-oxo-carboxylate clusters showing water stability and CO2 sorption capability.

A series of new Ti(IV)-oxo clusters supported by carboxylate ligands are synthesized by solvothermal reactions, and their crystal structures and gas sorption properties are studied in detail. Depending on the reaction medium, two types of Ti6O6 clusters are formed with benzoate-derived ligands whose 2- or 4-position is substituted by a hydrophobic moiety. The molecular packing structures of these Ti clusters are found stable against water- and heat-treatments. Gas sorption studies reveal that a dense-packed Ti-carboxylate cluster can adsorb almost the same number of CO2 molecules as the one with considerable intermolecular voids in the packing structure. The effect of benzoate substituents on the gas sorption behavior is discussed.

Unprecedented and highly symmetric (6,8)-connected topology in a porous metal-organic framework through a Zn-Ti heterometallic approach.

The Zn(II)-Ti(IV) heterometallic approach is successfully adopted to produce a highly symmetric, binodal metal-organic framework with unprecedented topology in network solids. This material, ZTOF-2, possesses permanent porosity as shown by reversible sorption of various gases.

Nonporous titanium-oxo molecular clusters that reversibly and selectively adsorb carbon dioxide.

Nonporous titanium(IV) clusters supported by carboxylate ligands show selective sorption of carbon dioxide even in the amorphous phase. A ring-type Ti8 cluster with a permanent inner pore also adsorbs gases, but not selectively.

Unique coordination-based heterometallic approach for the stoichiometric inclusion of high-valent metal ions in a porous metal-organic framework.

A heterometallic metal-organic framework incorporating Zn(2+) and Ti(4+) is successfully synthesized. The unique coordination mode of each metal ion to 2-oxido-1,4-benzenedicarboxylate leads to Zn6Ti2 building blocks, which extend to form a double-walled primitive cubic net. This material possesses a permanent porosity and long-term stability.

Topologies of metal-organic frameworks based on pyrimidine-5-carboxylate and unexpected gas-sorption selectivity for CO(2).

A simple and multitopic ligand, pyrimidine-5-carboxylate (pmc), has been used to obtain a series of metal-organic frameworks (MOFs) based on Co(2+), Cd(2+), and Cu(2+). The networks possess well-defined topologies of body-centered-cubic, rutile, and interpenetrated NbO structures, respectively. Among those, [Cu(pmc)(2)] possesses a permanent porosity resulting from straight one-dimensional channels of 5.5 Å free passages. Unexpectedly, this porous MOF displays a highly selective sorption behavior for CO(2), and the sorptions of N(2), Ar, O(2), H(2), and CH(4) at two different temperatures are found to be negligible. The results of diffraction and spectroscopic analyses exclude framework dynamics or incomplete evacuation as the origin of the gas-sorption selectivity.

Discrimination of small gas molecules through adsorption: reverse selectivity for hydrogen in a flexible metal-organic framework.

Zn(2+) ions react with 3,5-pyridinedicarboxylate (pydc) to form a new metal-organic framework (MOF), [Zn(pydc)(dma)] (dma = N,N'-dimethylacetamide), based on a noninterpenetrating (10,3)-a topology. The framework possessing narrow one-dimensional channels is highly flexible, and as a result, guest-dependent breakthrough-like adsorptions occur under atmospheric pressure. This "gate opening" requires strong interactions between gas molecules and the adsorbent, and therefore [Zn(pydc)(dma)] shows a reverse selectivity for H(2) at 77 K, which is very unusual in MOFs. At 195 K, only CO(2) is selectively adsorbed by this material because of the temperature dependence of the gated adsorption.

Isoreticular metal-organic polyhedral networks based on 5-connecting paddlewheel motifs.

A metal-organic polyhedral network with a unique 5-connected topology is expanded into a series using different metal ions or dicarboxylate ligands. The prototype material (ZmID), [Zn(4)(mip)(4)(dabco)(OH(2))(2)] (mip = 5-methylisophthalate, dabco = diazabicyclo[2.2.2]octane), is based on 5-connecting paddlewheel motifs and possesses large cage-like pores (8-20 A diameter). The metal ion is replaced by Co(2+) and/or the dicarboxylate by isophthalate (ip) or 2,7-naphthalenedicarboxylate (2,7-ndc) to give isoreticular frameworks [Zn(4)(ip)(4)(dabco)(OH(2))(2)] (ZID), [Co(4)(ip)(4)(dabco)(OH(2))(2)] (CID), and [Zn(4)(2,7-ndc)(4)(dabco)(OH(2))(2)] (ZND). X-ray powder diffraction and gas sorption studies reveal that ZID and CID have sustainable pore structures and show higher N(2) uptakes than ZmID. ZND is found unstable with respect to the removal of guest solvents. ZmID, ZID, and CID are all similar in terms of the H(2) sorption capacities (1.4-1.5 wt % at 77 K and 1 bar) and isosteric heat of H(2) adsorption (6-7 kJ/mol at low coverage).

Targeted synthesis of a prototype MOF based on Zn4(O)(O2C)6 units and a nonlinear dicarboxylate ligand.

A surprisingly small number of topologies are known in metal-organic frameworks (MOFs) that possess Zn(4)(O)(O(2)C)(6) building units. One of such MOFs having a simple, nonlinear dicarboxylate ligand, 5-methylisophthalate (mip), is targeted and successfully synthesized. The framework, [Zn(4)(O)(mip)(3)], is based on a nonuniform, uninodal six-connected net and is featured by complicated meshlike pores with narrow passages. Gas sorption studies reveal the potentials as a prototype from which new MOFs having interesting properties can be derived.

Efficient hydrogen sorption in 8-connected MOFs based on trinuclear pinwheel motifs.

Two new metal-organic frameworks based on trinuclear pinwheel motifs are prepared using dicarboxylate and diamine ligands. The structure of [Co3(bdc)3(dabco)] (1) (bdc = 1,4-benzenedicarboxylate; dabco = 1,4-diazabicyclo[2.2.2]octane) is described as pillared layers, whereas [Co3(ndc)3(dabco)] (2) (ndc = 2,6-naphthalenedicarboxylate) forms a variation of primitive cubic net with 3D connected pores. The two 8-connected MOFs are thermally stable at 160 and 250 degrees C for 1 and 2 respectively in the air and possess corrugated channels owing to the high connectivities of the secondary building unit. As a result, they show highly efficient hydrogen sorption capabilities. Especially, a high hydrogen uptake (2.45 wt % at 77 K and 1 bar) is observed for 2 that has the unique combination of high surface area and small portals.

Low-level self-assembly of open framework based on three different polyhedra: metal-organic analogue of face-centered cubic dodecaboride.

Dinuclear paddlewheels linked by 5-methylisophthalate and dabco act as 5-connecting nodes to form a highly symmetric open framework based on three different Archimedean polyhedra.

Selective gas sorption property of an interdigitated 3-D metal-organic framework with 1-D channels.

An interdigitated 3-D metal-organic framework, [Cd(3)(OH)(2)L(4)(H(2)O)(2)], with 1-D channels was prepared using 4-aminophenyl-1H-tetrazole (HL) and Cd(II) ions, where the host framework shows selective gas sorption behavior that is based on the different nature of the interactions between the gas and the framework rather than on the size-exclusion effect of the micropores.

Discovery, synthesis, and characterization of an isomeric coordination polymer with pillared kagome net topology.

A topological isomer based on Zn2 paddlewheel, dicarboxylate, and diamine ligands is synthesized by solvothermal methods after careful modulation of the reaction conditions. The new framework is characterized by a pillared Kagome net topology and possesses a sustainable pore structure with high surface area (approximately 2400 m2/g) and large hexagonal channels (approximately 15 A).

Vapor phase inclusion of ferrocene and its derivative in a microporous metal-organic porous material and its structural characterization by single crystal X-ray diffraction.

The inclusion of ferrocene and its derivative in metal-organic porous material MOF-5 is achieved by vapor diffusion; single-crystal X-ray diffraction studies using synchrotron radiation of ferrocene-loaded MOF-5 reveal well-ordered guest molecules packed into the pores.

Synthesis, X-ray crystal structures, and gas sorption properties of pillared square grid nets based on paddle-wheel motifs: implications for hydrogen storage in porous materials.

A systematic modulation of organic ligands connecting dinuclear paddle-wheel motifs leads to a series of isomorphous metal-organic porous materials that have a three-dimensional connectivity and interconnected pores. Aromatic dicarboxylates such as 1,4-benzenedicarboxylate (1,4-bdc), tetramethylterephthalate (tmbdc), 1,4-naphthalenedicarboxylate (1,4-ndc), tetrafluoroterephthalate (tfbdc), or 2,6-naphthalenedicarboxylate (2,6-ndc) are linear linkers that form two-dimensional layers, and diamine ligands, 4-diazabicyclo[2.2.2]octane (dabco) or 4,4'-dipyridyl (bpy), coordinate at both sides of Zn(2) paddle-wheel units to bridge the layers vertically. The resulting open frameworks [Zn(2)(1,4-bdc)(2)(dabco)] (1), [Zn(2)(1,4-bdc)(tmbdc)(dabco)] (2), [Zn(2)(tmbdc)(2)(dabco)] (3), [Zn(2)(1,4-ndc)(2)(dabco)] (4), [Zn(2)(tfbdc)(2)(dabco)] (5), and [Zn(2)(tmbdc)(2)(bpy)] (8) possess varying size of pores and free apertures originating from the side groups of the 1,4-bdc derivatives. [Zn(2)(1,4-bdc)(2)(bpy)] (6) and [Zn(2)(2,6-ndc)(2)(bpy)] (7) have two- and threefold interpenetrating structures, respectively. The non-interpenetrating frameworks (1-5 and 8) possess surface areas in the range of 1450-2090 m(2)g(-1) and hydrogen sorption capacities of 1.7-2.1 wt % at 78 K and 1 atm. A detailed analysis of the sorption data in conjunction with structural similarities and differences concludes that porous materials with straight channels and large openings do not perform better than those with wavy channels and small openings in terms of hydrogen storage through physisorption.