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.

/pdf/reticular-synthesis-and-the-design-of-new-materials-lazbx7tdux.pdf

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

From molecules to crystal engineering: supramolecular isomerism and polymorphism in network solids.

The crystal and molecular structure of PF2(NH2) has been determined at 95 K. A single crystal was grown in situ on a Weissenberg goniometer and transferred to a CAD4 diffractometer for data collection. Crystals of PF2(NH2) are monoclinic, space group P21/c, with a= 5.2125(26), b= 12.830(5), c= 4.896(4)A, β= 115.07(6)°, and Z= 4. Two quadrants of data were collected and merged to give 1 300 reflections, of which 1 243 were considered to be above background. The final R was 0.0376. The P–N bond length is 1.6384(9)A, while the P–F(1)[1.6083(11)A] and P–F(2)[1.5899(6)A] distances are inequivalent and reflect the involvement of F(1) in intermolecular hydrogen bonding. The N is also hydrogen bonded and the intermolecular bonding gives rise to a corrugated layer of PF2(NH2) molecules. Infrared and Raman spectra of PF2(NH2) in the liquid, unannealed solid, and crystalline phases have also been recorded and analysed. Spectra of the crystalline solid, formed by a spontaneous annealing process at 110 K, are consistent with the crystal structure, showing shifts in N–H stretching frequencies, and at least one band, absent for non-crystalline phases, assigned to deformations of the hydrogen-bonding network. Stretching frequencies for N–H bonds in NHD species have been used to give accurate N–H bond lengths, and the combined X-ray and spectroscopic data allow the hydrogen-bonding contacts within the crystal, which determine the overall crystal packing, to be fully characterised.

Aminodifluorophosphine: crystal and molecular structure at 95 K, and vibrational spectra of condensed phases

In the title compound, trans-dichloro(1,5,9,13-tetrathiacyclohexadecane-κ 4 S)rhodium(III) pentaiodide diiodine, [RhCl 2 (C 12 H 24 S 4 )](I 5 ).I 2 the complex cations are embedded in a three-dimensional polyiodide matrix composed of cages of I 5 - ions and I 2 molecules interacting via I...I contacts which range from 3.336 (2) to 4.133 (2) A.

trans-dichloro([16]aneS4)rhodium(III) pentaiodide diiodine

Bis(1,4,7-trithiacyclononane)gold Dication: A Paramagnetic, Mononuclear Au(II) Complex.

In the title complex, [Ru(C 12 H 24 O 4 S 2 ) 3 ](PF 6 ) 2 .2H 2 O.CH 3 OH, the Ru II ion lies on a crystallographic threefold axis and is coordinated to three symmetry-equivalent macrocyclic ligands via their S donor atoms [Ru-S 2.3620(9) and 2.3859(9) A]. The macrocycle adopts a typical bidentate chelating coordination mode with S donors in exo and O donors in endo orientations.

Tris(1,4,7,10-tetraoxa-13,16-dithiacyclooctadecane-S,S')ruthenium(II) Bis(hexafluorophosphate)-Water-Methanol (1/2/1)

In the title compound, [Pb(ClO(4))(2)(C(15)H(11)N(3))](n), two molecules occupy general positions while the third lies on a crystallographic twofold axis, giving a total of two and a half molecules per asymmetric unit. Each metal centre is coordinated equatorially by three 2,2':6',2''-terpyridine (terpy) N-donor atoms and axially by two perchlorate O-donor atoms. The distorted pentagonal bipyramidal geometry is completed by two equatorial O-donor atoms from two perchlorate anions which bridge to two different adjacent metal centres. The coordination about each metal centre is very similar to that seen at the unique Pb(II) centre in the previously published polymorph [Engelhardt, Harrowfield, Miyamae, Patrick, Skelton, Soudi & White (1996). Aust. J. Chem. 49, 1135-1146], but the new polymorph differs from it by the insertion on each side of an existing [bis(perchlorato)(terpy)lead(II)] molecule of two additional such units. Pairs of asymmetrically bridging perchlorate anions link irregularly spaced Pb(II) centres into undulating chains parallel to [201] which exhibit a repeat distance of 26.280 (4) A. The significance of this new polymorph lies in the fact that, while it is chemically identical to the known polymorph, it is structurally distinct from it.

Alexander J. Blake

Papers

Aminodifluorophosphine: crystal and molecular structure at 95 K, and vibrational spectra of condensed phases

trans-dichloro([16]aneS4)rhodium(III) pentaiodide diiodine

Bis(1,4,7-trithiacyclononane)gold Dication: A Paramagnetic, Mononuclear Au(II) Complex.

Tris(1,4,7,10-tetraoxa-13,16-dithiacyclooctadecane-S,S')ruthenium(II) Bis(hexafluorophosphate)-Water-Methanol (1/2/1)

A new polymorph of poly[bis(μ2‐perchlorato‐κ2O:O′)(2,2′:6′,2′′‐terpyridine‐κ3N,N′,N′′)lead(II)] with a greatly extended chain repeat distance