Jeffrey R. Long

Carbon Dioxide Capture in Metal–Organic Frameworks

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: Prospects for New Materials

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.

Hydrocarbon Separations in a Metal-Organic Framework with Open Iron(II) Coordination Sites

An Iron Separator Petroleum processing initially yields a mixture of saturated and unsaturated hydrocarbons—the feedstocks for fuels, plastics, pharmaceuticals, and a wide range of other commercial products. At present, distillation is the primary means of separating the components of this mixture. A sorbent or membrane-based approach to separation could reap substantial energy savings. Bloch et al. (p. 1606) found that an iron-based metal organic framework material shows promise for very efficient sorption-based separation of ethane and ethylene, propane and propylene, and several other light hydrocarbon mixtures. Neutron diffraction directly revealed the binding motifs at the iron centers that selectively pinned down the olefins while the saturated hydrocarbons passed by. Tighter binding of olefins to iron centers in a sorbent facilitates their efficient separation from saturated hydrocarbons. The energy costs associated with large-scale industrial separation of light hydrocarbons by cryogenic distillation could potentially be lowered through development of selective solid adsorbents that operate at higher temperatures. Here, the metal-organic framework Fe2(dobdc) (dobdc4– : 2,5-dioxido-1,4-benzenedicarboxylate) is demonstrated to exhibit excellent performance characteristics for separation of ethylene/ethane and propylene/propane mixtures at 318 kelvin. Breakthrough data obtained for these mixtures provide experimental validation of simulations, which in turn predict high selectivities and capacities of this material for the fractionation of methane/ethane/ethylene/acetylene mixtures, removal of acetylene impurities from ethylene, and membrane-based olefin/paraffin separations. Neutron powder diffraction data confirm a side-on coordination of acetylene, ethylene, and propylene at the iron(II) centers, while also providing solid-state structural characterization of the much weaker interactions of ethane and propane with the metal.

Exploiting single-ion anisotropy in the design of f-element single-molecule magnets

Scientists have long employed lanthanide elements in the design of materials with extraordinary magnetic properties, including the strongest magnets known, SmCo5 and Nd2Fe14B. The properties of these materials are largely a product of fine-tuning the interaction between the lanthanide ion and the crystal lattice. Recently, synthetic chemists have begun to utilize f-elements—both lanthanides and actinides—for the construction of single-molecule magnets, resulting in a rapid expansion of the field. The desirable magnetic characteristics of the f-elements are contingent upon the interaction between the single-ion electron density and the crystal field environment in which it is placed. This interaction leads to the single-ion anisotropies requisite for strong single-molecule magnets. Therefore, it is of vital importance to understand the particular crystal field environments that could lead to maximization of the anisotropy for individual f-elements. Here, we summarize a qualitative method for predicting the ligand architectures that will generate magnetic anisotropy for a variety of f-element ions. It is hoped that this simple model will serve to guide the design of stronger single-molecule magnets incorporating the f-elements.

Kondo resonance in a single-molecule transistor

When an individual molecule, nanocrystal, nanotube or lithographically defined quantum dot is attached to metallic electrodes via tunnel barriers, electron transport is dominated by single-electron charging and energy-level quantization. As the coupling to the electrodes increases, higher-order tunnelling and correlated electron motion give rise to new phenomena, including the Kondo resonance. To date, all of the studies of Kondo phenomena in quantum dots have been performed on systems where precise control over the spin degrees of freedom is difficult. Molecules incorporating transition-metal atoms provide powerful new systems in this regard, because the spin and orbital degrees of freedom can be controlled through well-defined chemistry. Here we report the observation of the Kondo effect in single-molecule transistors, where an individual divanadium molecule serves as a spin impurity. We find that the Kondo resonance can be tuned reversibly using the gate voltage to alter the charge and spin state of the molecule. The resonance persists at temperatures up to 30 K and when the energy separation between the molecular state and the Fermi level of the metal exceeds 100 meV.

Strong CO2 Binding in a Water-Stable, Triazolate-Bridged Metal−Organic Framework Functionalized with Ethylenediamine

Reaction of CuCl(2) x 2 H(2)O with 1,3,5-tris(1H-1,2,3-triazol-5-yl)benzene (H(3)BTTri) in DMF at 100 degrees C generates the metal-organic framework H(3)[(Cu(4)Cl)(3)(BTTri)(8)(DMF)(12)] x 7 DMF x 76 H(2)O (1-DMF). The sodalite-type structure of the framework consists of BTTri(3-)-linked [Cu(4)Cl](7+) square clusters in which each Cu(II) center has a terminal DMF ligand directed toward the interior of a large pore. The framework exhibits a high thermal stability of up to 270 degrees C, as well as exceptional chemical stability in air, boiling water, and acidic media. Following exchange of the guest solvent and bound DMF molecules for methanol to give 1-MeOH, complete desolvation of the framework at 180 degrees C generated H(3)[(Cu(4)Cl)(3)(BTTri)(8)] (1) with exposed Cu(II) sites on its surface. Following a previously reported protocol, ethylenediamine molecules were grafted onto these sites to afford 1-en, featuring terminal alkylamine groups. The N(2) adsorption isotherms indicate a reduction in the BET surface area from 1770 to 345 m(2)/g following grafting. The H(2) adsorption data at 77 K for 1 indicate a fully reversible uptake of 1.2 wt % at 1.2 bar, while the CO(2) isotherm at 195 K shows a maximal uptake of 90 wt % at 1 bar. Compared to 1, the alkylamine-functionalized framework 1-en exhibits a higher uptake of CO(2) at 298 K and pressures up to ca. 0.1 bar, as well as a higher CO(2)/N(2) selectivity at all measured pressures. Significantly, 1-en also exhibits an isosteric heat of CO(2) adsorption of 90 kJ/mol, which is much higher than the 21 kJ/mol observed for 1. This chemisorption interaction is the strongest reported to date for a metal-organic framework and points toward the potential utility of alkylamine-appended frameworks for the postcombustion capture of CO(2) from low-pressure flue gas streams.

A Molecular MoS2 Edge Site Mimic for Catalytic Hydrogen Generation

Edging In on MoS2 Molybdenum disulfide is a widely used catalyst in the petrochemical industry that has recently shown promise for water-splitting applications. Its activity appears to be confined to edge sites with exposed disulfide groups, although the precise geometric details underlying the chemistry remain uncertain. Karunadasa et al. (p. 698) prepared a molecular complex modeling one of these edge sites, in which a triangular Mo-S-S unit is supported by metal coordination to five tethered pyridine rings. The molecule was characterized crystallographically and proved robustly active toward electrochemical generation of hydrogen from water, even when applied to crudely filtered seawater. A small molecule functionally models the active component of an extended solid material with wide catalytic applications. Inorganic solids are an important class of catalysts that often derive their activity from sparse active sites that are structurally distinct from the inactive bulk. Rationally optimizing activity is therefore beholden to the challenges in studying these active sites in molecular detail. Here, we report a molecule that mimics the structure of the proposed triangular active edge site fragments of molybdenum disulfide (MoS2), a widely used industrial catalyst that has shown promise as a low-cost alternative to platinum for electrocatalytic hydrogen production. By leveraging the robust coordination environment of a pentapyridyl ligand, we synthesized and structurally characterized a well-defined MoIV-disulfide complex that, upon electrochemical reduction, can catalytically generate hydrogen from acidic organic media as well as from acidic water.

Strong exchange and magnetic blocking in N 2 32 -radical-bridged lanthanide complexes

Single-molecule magnets approach the ultimate size limit for spin-based devices. These complexes can retain spin information over long periods of time at low temperature, suggesting possible applications in high-density information storage, quantum computing and spintronics. Notably, the success of most such applications hinges upon raising the inherent molecular spin-inversion barrier. Although recent advances have shown the viability of lanthanide-containing complexes in generating large barriers, weak or non-existent magnetic exchange coupling allows fast relaxation pathways that mitigate the full potential of these species. Here, we show that the diffuse spin of an N(2)(3-) radical bridge can lead to exceptionally strong magnetic exchange in dinuclear Ln(III) (Ln = Gd, Dy) complexes. The Gd(III) congener exhibits the strongest magnetic coupling yet observed for that ion, while incorporation of the high-anisotropy Dy(III) ion gives rise to a molecule with a record magnetic blocking temperature of 8.3 K at a sweep rate of 0.08 T s(-1).

Size-Selective Lewis Acid Catalysis in a Microporous Metal-Organic Framework with Exposed Mn2+ Coordination Sites

Treatment of selected aldehydes and ketones with cyanotrimethylsilane in the presence of the microporous metal-organic framework Mn3[(Mn4Cl)3BTT8(CH3OH)10]2 (1, H3BTT = 1,3,5-benzenetristetrazol-5-yl) leads to rapid conversion to the corresponding cyanosilylated products. The transformation is catalyzed by coordinatively unsaturated Mn2+ ions that serve as Lewis acids and lead to conversion yields of 98 and 90% for benzaldehyde and 1-naphthaldehyde, the highest thus far for a metal-organic framework. Larger carbonyl substrates cannot diffuse through the pores of 1, and conversion yields are much lower for these, attesting to the heterogeneity of the reaction and its dependence on guest size. The Mukaiyama-aldol reaction, known to require much more active Lewis catalysts, is also catalyzed in the presence of 1, representing the first such example for a metal-organic framework. Conversion yields obtained for the reaction of selected aldehydes with silyl enolates reach 63%, on par with those obtained with zeolites. Size selectivity is demonstrated for the first time with this reaction through the use of larger silyl enolate substrates.

A knotted1-like homeobox gene in Arabidopsis is expressed in the vegetative meristem and dramatically alters leaf morphology when overexpressed in transgenic plants.

The homeobox gene knotted1 (kn1) was first isolated by transposon tagging a dominant leaf mutant in maize. Related maize genes, isolated by virtue of sequence conservation within the homeobox, fall into two classes based on sequence similarity and expression patterns. Here, we report the characterization of two genes, KNAT1 and KNAT2 (for knotted-like from Arabidopsis thaliana) that were cloned from Arabidopsis using the kn1 homeobox as a heterologous probe. The homeodomains of KNAT1 and KNAT2 are very similar to the homeodomains of proteins encoded by class 1 maize genes, ranging from 78 to 95% amino acid identity. Overall, the deduced KNAT1 and KNAT2 proteins share amino acid identities of 53 and 40%, respectively, with the KN1 protein. Intron positions are also fairly well conserved among KNAT1, KNAT2, and kn1. Based on in situ hybridization analysis, the expression pattern of KNAT1 during vegetative development is similar to that of class 1 maize genes. In the shoot apex, KNAT1 transcript is localized primarily to the shoot apical meristem; down-regulation of expression occurs as leaf primordia are initiated. In contrast to the expression of class 1 maize genes in floral and inflorescence meristems, the expression of KNAT1 in the shoot meristem decreases during the floral transition and is restricted to the cortex of the inflorescence stem. Transgenic Arabidopsis plants carrying the KNAT1 cDNA and the kn1 cDNA fused to the cauliflower mosaic virus 35S promoter were generated. Misexpression of KNAT1 and kn1 resulted in highly abnormal leaf morphology that included severely lobed leaves. The expression pattern of KNAT1 in the shoot meristem combined with the results of transgenic overexpression experiments supports the hypothesis that class 1 kn1-like genes play a role in morphogenesis.