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

Metal–Organic Frameworks for Separations

Kenji Sumida, David L. Rogow, Jarad A. Mason, Thomas M. McDonald, Eric D. Bloch, Zoey R. Herm, Tae-Hyun Bae, Jeffrey R. Long

Carbon Dioxide Capture in Metal–Organic Frameworks

New materials capable of storing hydrogen at high gravimetric and volumetric densities are required if hydrogen is to be widely employed as a clean alternative to hydrocarbon fuels in cars and other mobile applications. With exceptionally high surface areas and chemically-tunable structures, microporous metal–organic frameworks have recently emerged as some of the most promising candidate materials. In this critical review we provide an overview of the current status of hydrogen storage within such compounds. Particular emphasis is given to the relationships between structural features and the enthalpy of hydrogen adsorption, spectroscopic methods for probing framework–H2 interactions, and strategies for improving storage capacity (188 references).

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Hydrogen storage in metal–organic frameworks

The escalating level of atmospheric carbon dioxide is one of the most pressing environmental concerns of our age. Carbon capture and storage (CCS) from large point sources such as power plants is one option for reducing anthropogenic CO(2) emissions; however, currently the capture alone will increase the energy requirements of a plant by 25-40%. This Review highlights the challenges for capture technologies which have the greatest likelihood of reducing CO(2) emissions to the atmosphere, namely postcombustion (predominantly CO(2)/N(2) separation), precombustion (CO(2)/H(2)) capture, and natural gas sweetening (CO(2)/CH(4)). The key factor which underlies significant advancements lies in improved materials that perform the separations. In this regard, the most recent developments and emerging concepts in CO(2) separations by solvent absorption, chemical and physical adsorption, and membranes, amongst others, will be discussed, with particular attention on progress in the burgeoning field of metal-organic frameworks.

Carbon Dioxide Capture: Prospects for New Materials

A series of four isostructural microporous coordination polymers (MCPs) differing in metal composition is demonstrated to exhibit exceptional uptake of CO2 at low pressures and ambient temperature. These conditions are particularly relevant for capture of flue gas from coal-fired power plants. A magnesium-based material is presented that is the highest surface area magnesium MCP yet reported and displays ultrahigh affinity based on heat of adsorption for CO2. This study demonstrates that physisorptive materials can achieve affinities and capacities competitive with amine sorbents while greatly reducing the energy cost associated with regeneration.

Dramatic tuning of carbon dioxide uptake via metal substitution in a coordination polymer with cylindrical pores.

The designed synthesis of heterobimetallic microporous coordination polymers (MCPs) is reported by a strategy employing the selective replacement of a single metal in homometallic MCPs with two unique metal coordination environments: octahedral and tetrahedral. This strategy is successful in the preparation of six mixed-metal MCPs, where Co/Zn and Ni/Zn versions of MOF-4, MOF-39, and a Zn-BTEC MCP are reported.

Selective Metal Substitution for the Preparation of Heterobimetallic Microporous Coordination Polymers

The combination of zinc(II) nitrate with 1,3,5-(triscarboxyphenyl)benzene (H 3BTB) leads to five different microporous coordination polymers (MCPs). Two of these were previously known (MOF-177 and MOF-39), whereas polymer-induced heteronucleation was used in the discovery of three phases that have not been previously reported ( Zn/BTB ant, Zn/BTB tsx, and Zn/BTB dia). Modification of crystallization conditions allows for the bulk-scale synthesis of each of these MCPs. Zn/BTB ant and Zn/BTB tsx are each interpentrated 6,3-connected nets composed of the basic zinc carboxylate secondary building unit (SBU) and the tritopic linker BTB. The underlying noninterpenetrated net of Zn/BTB ant is derived for the net of anatase, whereas that of Zn/BTB tsx is the previously unreported "tsx" framework. Zn/BTB dia consists of an underlying diamondoid net in which four linear, trinuclear zinc hourglass SBUs are arranged about a central mu 4-oxo anion as the tetrahedral unit in the net and BTB further links the hourglass SBUs. Zn/BTB ant, Zn/BTB tsx, and MOF-177 are here defined as polymorphic frameworks in that each is composed of the same SBU and linker but differ in topology and thus pore structure. These frameworks may be called a polyreticular series by analogy to several reported isoreticular series. The effect of linker-linker interactions are discussed.

Phase Selection and Discovery among Five Assembly Modes in a Coordination Polymerization

Desulfurization of the thiocarbonyl ligand in square pyramidal [Ru(CS)Cl2(PCy3)2] (1-S) via sulfur atom abstraction using [Mo(H)(eta2-Me2CNAr)(N[i-Pr]Ar)2] forms [Ru(C)Cl2(PCy3)2] (1) cleanly over several hours in benzene; isolated yield is 55%. Complex 1 is also formed in 87% isolated yield upon reaction of [Ru(CHR)(PCy3)2Cl2] (R = p-C6H4Me, 2; Ph, 3) with vinyl acetate in dichloromethane. Complex 1-S is re-formed quantitatively from 1 upon treatment with elemental sulfur in CH2Cl2, but is prepared most conveniently by treatment of crude [Ru(CS)Cl2(PPh3)2(OH2)] with excess PCy3 in toluene. Nearly quantitative conversion of 1 to [Ru(CO)Cl2(PCy3)2] (1-O) occurs upon addition of dimethyldioxirane solution in acetone to 1 dissolved in CH2Cl2 at ca. -90 degrees C.

Two Generalizable Routes to Terminal Carbido Complexes

Olefin metathesis is an important tool for organic and polymer synthesis. However, some key functional groups are not tolerated even by Ru-based catalysts. We recently showed that vinyl esters can deactivate [Ru(CHPh)(PCy3)2Cl2] (1) [3] by quantitative formation of [Ru(C)(PCy3)2Cl2] (2) [4,5] A rare neutral terminal carbido complex, 2 is surprisingly stable and has few reported reactions. However, protonation of 2 by strong acid yields catalysts that rapidly initiate olefin metathesis. Thus, 2 is both a precursor to and a decomposition product of olefin metathesis catalysts. We see 2 as a potential source of a C1 fragment. Accordingly, we describe herein the first C C bond-forming reaction of this unusual compound. The terminal carbido ligand in 2 is a poor nucleophile, as shown by its failure to react with MeI, MeCOCl, and PhCH2Br. Although 2 does not react with a variety of alkenes and alkynes (see the Supporting Information), it reacts cleanly with MeO2CC CCO2Me (dimethyl acetylenedicarboxylate, DMAD) over 4 h in C6H6. A new blue-purple complex, 3, is formed as the carbido signal for 2 (C NMR: d= 471.8 ppm) is replaced by a new signal at d= 195.7 ppm. The H NMR spectrum evinces formation of a 1:1 adduct of 2 with DMAD. Formation of the cyclopropenylidene complex [Ru{=CC2(CO2Me)2}(PCy3)2Cl2] (Scheme 1) accounts for these observations. Several cyclopropenylidene complexes exist. Unlike 3, however, the cyclopropenylidene units in these complexes are substituted by phenyl or electrondonating groups. [Ru(C)(H2IMes)(PCy3)Cl2] (4 ; H2IMes= 4,5-dihydro-1,3-bis(mesityl)imidazol-2-ylidene) reacts similarly with DMAD, but the reaction is not clean since the product reacts further with DMAD before all of 4 has been consumed. However, 4 reacts more cleanly with HC CCO2Me (see the Supporting Information). Single-crystal X-ray diffraction confirmed the structure of 3. Figure 1 depicts a thermal ellipsoid plot of one of the two chemically equivalent but crystallographically independent molecules of 3 in the crystal. The data establish the expected connectivity in 3, but the large uncertainty associated with the Ru=C bond length of 1.846(10) A precludes comparison with those in related alkylidene complexes. The cyclopropenylidene ring lies in the Cl-Ru-Cl plane. The structure shows significant bond localization in the cyclopropenylidene fragment. These distances closely resemble those observed in free Scheme 1. Formation of 3 and ring-opening reactions. HBpin=pinacolborane, Ar=3,5-Me2C6H3.

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Stephen R. Caskey

Papers

Dramatic tuning of carbon dioxide uptake via metal substitution in a coordination polymer with cylindrical pores.

Selective Metal Substitution for the Preparation of Heterobimetallic Microporous Coordination Polymers

Phase Selection and Discovery among Five Assembly Modes in a Coordination Polymerization

Two Generalizable Routes to Terminal Carbido Complexes

Carbon-carbon bond formation at a neutral terminal carbido ligand: generation of cyclopropenylidene and vinylidene complexes.