Crystalline metal-organic frameworks (MOFs) are formed by reticular synthesis, which creates strong bonds between inorganic and organic units. Careful selection of MOF constituents can yield crystals of ultrahigh porosity and high thermal and chemical stability. These characteristics allow the interior of MOFs to be chemically altered for use in gas separation, gas storage, and catalysis, among other applications. The precision commonly exercised in their chemical modification and the ability to expand their metrics without changing the underlying topology have not been achieved with other solids. MOFs whose chemical composition and shape of building units can be multiply varied within a particular structure already exist and may lead to materials that offer a synergistic combination of properties.

The Chemistry and Applications of Metal-Organic Frameworks

J. Appl. Cryst.の発刊に際して

The long-standing challenge of designing and constructing new crystalline solid-state materials from molecular building blocks is just beginning to be addressed with success. A conceptual approach that requires the use of secondary building units to direct the assembly of ordered frameworks epitomizes this process: we call this approach reticular synthesis. This chemistry has yielded materials designed to have predetermined structures, compositions and properties. In particular, highly porous frameworks held together by strong metal-oxygen-carbon bonds and with exceptionally large surface area and capacity for gas storage have been prepared and their pore metrics systematically varied and functionalized.

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Reticular synthesis and the design of new materials

Adsorptive separation is very important in industry. Generally, the process uses porous solid materials such as zeolites, activated carbons, or silica gels as adsorbents. With an ever increasing need for a more efficient, energy-saving, and environmentally benign procedure for gas separation, adsorbents with tailored structures and tunable surface properties must be found. Metal–organic frameworks (MOFs), constructed by metal-containing nodes connected by organic bridges, are such a new type of porous materials. They are promising candidates as adsorbents for gas separations due to their large surface areas, adjustable pore sizes and controllable properties, as well as acceptable thermal stability. This critical review starts with a brief introduction to gas separation and purification based on selective adsorption, followed by a review of gas selective adsorption in rigid and flexible MOFs. Based on possible mechanisms, selective adsorptions observed in MOFs are classified, and primary relationships between adsorption properties and framework features are analyzed. As a specific example of tailor-made MOFs, mesh-adjustable molecular sieves are emphasized and the underlying working mechanism elucidated. In addition to the experimental aspect, theoretical investigations from adsorption equilibrium to diffusion dynamics via molecular simulations are also briefly reviewed. Furthermore, gas separations in MOFs, including the molecular sieving effect, kinetic separation, the quantum sieving effect for H2/D2 separation, and MOF-based membranes are also summarized (227 references).

Selective gas adsorption and separation in metal–organic frameworks

Open metal–organic frameworks are widely regarded as promising materials for applications1,2,3,4,5,6,7,8,9,10,11,12,13,14,15 in catalysis, separation, gas storage and molecular recognition. Compared to conventionally used microporous inorganic materials such as zeolites, these organic structures have the potential for more flexible rational design, through control of the architecture and functionalization of the pores. So far, the inability of these open frameworks to support permanent porosity and to avoid collapsing in the absence of guest molecules, such as solvents, has hindered further progress in the field14,15. Here we report the synthesis of a metal–organic framework which remains crystalline, as evidenced by X-ray single-crystal analyses, and stable when fully desolvated and when heated up to 300?°C. This synthesis is achieved by borrowing ideas from metal carboxylate cluster chemistry, where an organic dicarboxylate linker is used in a reaction that gives supertetrahedron clusters when capped with monocarboxylates. The rigid and divergent character of the added linker allows the articulation of the clusters into a three-dimensional framework resulting in a structure with higher apparent surface area and pore volume than most porous crystalline zeolites. This simple and potentially universal design strategy is currently being pursued in the synthesis of new phases and composites, and for gas-storage applications.

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Design and synthesis of an exceptionally stable and highly porous metal-organic framework

A solid product which is a porous chiral (10,3)-a network of 1,3,5-benzenetricarboxylate molecules of formula (I) where X is H, hydrocarbon, halide, etc, linked through metal atoms. Preferably the metal atoms are transition or lanthanide metal atoms carrying one or more coordinated ligands such as diols, preferably chiral ligands present in enantiomerically pure form

Porous solid products of 1,3,5-benzenetricarboxylate and metal ions

The expected 3D and 2D topologies resulting from combining approximately linear bis- or monopyridyl ligands with [FeIIMII(CN)4] (MII = Pt, Pd, Ni) 4,4-grid sheets are well established. We show here the magnetic and structural consequences of incorporating a bent bispyridyl linker ligand in combination with [FeIIPtII(CN)4] to form the material [Fe(H2O)2Fe(DPSe)2(Pt(CN)4)2]·3EtOH (DPSe = 4,4′-dipyridylselenide). Structural investigations reveal an unusual connectivity loosely resembling a 3D Hofmann topology where (1) there are two distinct local iron(II) environments, [FeIIN6] (Fe1) and [FeIIN4O2] (Fe2), (2) as a consequence of axial water coordination to Fe2, there are “holes” in the [FeIIPtII(CN)4] 4,4 sheets because of some of the cyanido ligands being terminal rather than bridging, and (3) bridging of adjacent sheets occurs only through one in two DPSe ligands, with the other acting as a terminal ligand binding through only one pyridyl group. The magnetic properties are defined by this unusual topology...

Thermal- and Light-Induced Spin-Crossover Bistability in a Disrupted Hofmann-Type 3D Framework

Binary metal(II)–pyromellitate coordination polymers, M 2 (pm) (M=Co, Fe, Mn): synthesis, structures and magnetic properties

The protonation of [Fe(1)2]2+, where 1 = 4′-(4-pyridyl)-2,2′:6′,2″-terpyridine, generates a trication which forms a one-dimensional, hydrogen-bonded polymer. The single crystal structures of [Fe(1)(H1)][Fe(NCS)6]·2H2O, [Fe(1)(H1)][Fe(NCS)6]·MeCN and [Fe(1)(H1)][ClO4]3·EtOH are reported and illustrate that the packing of the {[Fe(1)(H1)]3+}n chains in the solid state can be modulated by changing the steric demands of the counter-ion. Subtle changes in packing on going from [Fe(1)(H1)][Fe(NCS)6]·2H2O to [Fe(1)(H1)][Fe(NCS)6]·MeCN result in a reorientation of the pendant pyridine rings with respect to the central pyridine ring of the tpy unit in [Fe(1)(H1)]3+.

The conjugate acid of bis{4′-(4-pyridyl)-2,2′:6′,2″-terpyridine}iron(II) as a self-complementary hydrogen-bonded building block

Two new, isostructural UiO-66-type metal–organic frameworks (MOFs), Zr-PyDC (Zr6O4(OH)5(pyridine-2,5-dicarboxylate)4(formate)3(H2O)34) and Zr-PzDC (Zr6O4(OH)4.75(pyrazine-2,5-dicarboxylate)4.5(formate)2.25(H2O)33) have been synthesised via an unusual high acidity route. Despite the chelating capability of the ligands, which incorporate pyridine and pyrazine N-functionalities, these robust MOFs are linked exclusively through carboxylate donors to leave free Lewis base donor sites. Both MOFs show increased heats of adsorption and higher loadings of CO2 and H2 than UiO-66. High-conversion post-synthetic modification of Zr-PyDC and Zr-PzDC yielded charged, N-methylated frameworks with enhanced gas adsorption capacities. Further, the metal binding properties of Zr-PyDC have been probed by post-synthetic metallation with CuII.

Cameron J. Kepert

Papers

Porous solid products of 1,3,5-benzenetricarboxylate and metal ions

Thermal- and Light-Induced Spin-Crossover Bistability in a Disrupted Hofmann-Type 3D Framework

Binary metal(II)–pyromellitate coordination polymers, M 2 (pm) (M=Co, Fe, Mn): synthesis, structures and magnetic properties

The conjugate acid of bis{4′-(4-pyridyl)-2,2′:6′,2″-terpyridine}iron(II) as a self-complementary hydrogen-bonded building block

Two new porous UiO-66-type zirconium frameworks; open aromatic N-donor sites and their post-synthetic methylation and metallation