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

The electron spin lifetime in an assembly of chemically synthesized graphene sheets was found to be extremely sensitive to oxygen. Introducing small concentrations of physisorbed O-2 onto the graphene surface reduced the exceptionally long 140 ns electron spin lifetime by an order of magnitude. This effect was completely reversible: Removing the O-2 by using a dynamic vacuum restored the spin lifetime. The presence of covalently bound oxygen also decreased the electron spin lifetime in graphene, although to a far lesser extent compared to physisorbed O-2. The conduction electrons in graphene were found to play a significant role by counter-balancing the spin depolarization caused by oxygen molecules. Our results highlight the importance of chemical environment control and device packing in practical graphene-based spintronic applications.

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Strong Interplay between the Electron Spin Lifetime in Chemically Synthesized Graphene Multilayers and Surface-Bound Oxygen

Cyanidometallate complexes are highly versatile building units for the generation of functional porous materials. Here we report five new pillared Hofmann layer compounds incorporating the tetracyanidometallates [MoO(CN)4]2− and [MnN(CN)4]2−. These metalloligands, which are new to this class of materials, have been combined with divalent 1st-row transition metals to produce Hofmann layers that are linked into three-dimensional frameworks by ditopic bridging dipyridyls. We report the structures and anomalous thermal expansion properties of five new materials: [Mn(H2O)(bpy)½{MoO(CN)4(bpy)½}]·2H2O (1), [Mn(H2O)(bpy)½{MnN(CN)4(bpy)½}]·2H2O (2), [Fe(H2O)(bpy)½{MnN(CN)4(bpy)½}]·2H2O (3), [Co(H2O)(bpy)½{MnN(CN)4(bpy)½}]·2H2O (4) and [{Mn(H2O)2}½{Mn(bpa)2}½{MoO(CN)4(bpa)½}]·MeOH (5), (where bpy = 4,4′-bipyridine and bpa = 4,4′-bipyridylacetylene).

A new modification of an old framework: Hofmann layers with unusual tetracyanidometallate groups.

https://pubs.rsc.org/en/content/articlepdf/2014/cc/c4cc05438e

Highly unusual interpenetration isomers of electroactive nickel bis(dithiolene) coordination frameworks

Single-crystal room-temperature X-ray structure determinations of improved precision are reported for certain higher hydrates of the rare earth trichlorides (LnCl3.7H2O, Ln = La, Pr; LnCl3.6H2O, Ln = Nd, Lu) (triclinic, Pī, and monoclinic, P 2/n, forms respectively) in order to define hydrogen-bonding arrays within the two lattices.

Structural Systematics of Rare Earth Complexes. VI The Higher Hydrates of the Rare Earth Trichlorides

The isostructural 3-D coordination frameworks M2(tpt)2(NCS)4(BzOH)2·4(BzOH)·(H2O)
				(A: M = Fe(II), B: M = Co(II), tpt = 2,4,6-tris(4′-pyridyl)-1,3,5-triazine, BzOH = benzyl alcohol) consist of doubly interpenetrated (10,3)-a nets distorted by alternating T-shaped (M) and trigonal (tpt) three-connecting nodes.

Cameron J. Kepert

Papers

Strong Interplay between the Electron Spin Lifetime in Chemically Synthesized Graphene Multilayers and Surface-Bound Oxygen

A new modification of an old framework: Hofmann layers with unusual tetracyanidometallate groups.

Highly unusual interpenetration isomers of electroactive nickel bis(dithiolene) coordination frameworks

Structural Systematics of Rare Earth Complexes. VI The Higher Hydrates of the Rare Earth Trichlorides

A highly distorted (10,3)-a coordination framework constructed from alternating T-shaped and trigonal nodes