The process of carbon capture and sequestration has been proposed as a method of mitigating the build-up of greenhouse gases in the atmosphere. If implemented, the cost of electricity generated by a ...fossil fuel-burning power plant would rise substantially, owing to the expense of removing CO2 from the effluent stream. There is therefore an urgent need for more efficient gas separation technologies, such as those potentially offered by advanced solid adsorbents. Here we show that diamine-appended metal-organic frameworks can behave as 'phase-change' adsorbents, with unusual step-shaped CO2 adsorption isotherms that shift markedly with temperature. Results from spectroscopic, diffraction and computational studies show that the origin of the sharp adsorption step is an unprecedented cooperative process in which, above a metal-dependent threshold pressure, CO2 molecules insert into metal-amine bonds, inducing a reorganization of the amines into well-ordered chains of ammonium carbamate. As a consequence, large CO2 separation capacities can be achieved with small temperature swings, and regeneration energies appreciably lower than achievable with state-of-the-art aqueous amine solutions become feasible. The results provide a mechanistic framework for designing highly efficient adsorbents for removing CO2 from various gas mixtures, and yield insights into the conservation of Mg(2+) within the ribulose-1,5-bisphosphate carboxylase/oxygenase family of enzymes.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SBMB, SIK, UILJ, UKNU, UL, UM, UPUK
Metal–organic frameworks (MOFs) constructed from Zr6-based nodes have recently received considerable attention given their exceptional thermal, chemical, and mechanical stability. Because of this, ...the structural diversity of Zr6-based MOFs has expanded considerably and in turn given rise to difficulty in their precise characterization. In particular it has been difficult to assign where protons (needed for charge balance) reside on some Zr6-based nodes. Elucidating the precise proton topologies in Zr6-based MOFs will have wide ranging implications in defining their chemical reactivity, acid/base characteristics, conductivity, and chemical catalysis. Here we have used a combined quantum mechanical and experimental approach to elucidate the precise proton topology of the Zr6-based framework NU-1000. Our data indicate that a mixed node topology, Zr6(μ3–O)4(μ3–OH)4(OH)4 (OH2)48+, is preferred and simultaneously rule out five alternative node topologies.
The catalytic properties of the metal–organic framework Fe2(dobdc), containing open Fe(II) sites, include hydroxylation of phenol by pure Fe2(dobdc) and hydroxylation of ethane by its ...magnesium-diluted analogue, Fe0.1Mg1.9(dobdc). In earlier work, the latter reaction was proposed to occur through a redox mechanism involving the generation of an iron(IV)–oxo species, which is an intermediate that is also observed or postulated (depending on the case) in some heme and nonheme enzymes and their model complexes. In the present work, we present a detailed mechanism by which the catalytic material, Fe0.1Mg1.9(dobdc), activates the strong C–H bonds of ethane. Kohn–Sham density functional and multireference wave function calculations have been performed to characterize the electronic structure of key species. We show that the catalytic nonheme-Fe hydroxylation of the strong C–H bond of ethane proceeds by a quintet single-state σ-attack pathway after the formation of highly reactive iron–oxo intermediate. The mechanistic pathway involves three key transition states, with the highest activation barrier for the transfer of oxygen from N2O to the Fe(II) center. The uncatalyzed reaction, where nitrous oxide directly oxidizes ethane to ethanol is found to have an activation barrier of 280 kJ/mol, in contrast to 82 kJ/mol for the slowest step in the iron(IV)–oxo catalytic mechanism. The energetics of the C–H bond activation steps of ethane and methane are also compared. Dehydrogenation and dissociation pathways that can compete with the formation of ethanol were shown to involve higher barriers than the hydroxylation pathway.
In the field of metal–metal bonding, the occurrence of stable, multiple bonds between different transition metals is uncommon, and is largely unknown for different first-row metals. Adding to a ...recently reported iron–chromium complex, three additional M–Cr complexes have been isolated, where the iron site is systematically replaced with other first-row transition metals (Mn, Co, or Ni), while the chromium site is kept invariant. These complexes have been characterized by X-ray crystallography. The Mn–Cr complex has an ultrashort metal–metal bond distance of 1.82 Å, which is consistent with a quintuple bond. The M–Cr bond distances increases across the period from M = Mn to M = Ni, as the formal bond order decreases from 5 to 1. Theoretical calculations reveal that the M–Cr bonds become increasingly polarized across the period. We propose that these trends arise from increasing differences in the energies and/or contraction of the metals’ d-orbitals (M vs Cr). The cyclic voltammograms of these heterobimetallic complexes show multiple one-electron transfer processes, from two to four redox events depending on the M–Cr pair.
Anionic cobalt and iron metallalumatranes that bind dinitrogen in an end‐on fashion were prepared and characterized by X‐ray crystallography. Along with literature‐known neutral cobalt and iron ...metallalumatranes, they form a quartet of low‐valent coordination complexes for comparing dinitrogen activation and functionalization at cobalt versus iron centers. In the anionic metallalumatranes, the metal atoms are proposed to have subvalent oxidation states of –1. The electronic structure of the anionic iron alumatrane, which was probed by electron paramagnetic resonance spectroscopy, Mössbauer spectroscopy, and density functional theory, is most consistent with Fe(–1)→Al(+3). Functionalization of dinitrogen was achieved by reaction of the ferrate alumatrane with 1,2‐bis(chlorodimethylsilyl)ethane and KC8 (1 equiv.) to generate an iron(II) disilylhydrazido complex. The transformation of dinitrogen to disilylhydrazido(2–) is an overall four‐electron process.
An alumatrane ligand is used to support electron‐rich ferrate(1–) and cobaltate(1–) centers within anionic metallalumatrane complexes. These complexes activate dinitrogen weakly. In the case of ferrate, the bound dinitrogen can be converted into to a disilylhydrazido(2–) ligand.
An end-on superoxido complex with the formula {CoIII(OH2)(trpy)CoIII(OO•)(trpy)(μ-bpp)}4+ (3 4+) (bpp– = bis(2-pyridyl)-3,5-pyrazolate; trpy = 2,2′;6′:2″-terpyridine) has been characterized by ...resonance Raman, electron paramagnetic resonance, and X-ray absorption spectroscopies. These results together with online mass spectrometry experiments using 17O and 18O isotopically labeled compounds prove that this compound is a key intermediate of the water oxidation reaction catalyzed by the peroxido-bridged complex {CoIII(trpy)2(μ-bpp)(μ-OO)}3+ (1 3+). DFT calculations agree with and complement the experimental data, offering a complete description of the transition states and intermediates involved in the catalytic cycle.
New mononuclear ruthenium complexes with general formula Ru(bid)(B)(Cl) (bid is (1Z,3Z)-1,3-bis(pyridin-2-ylmethylene)isoindolin-2-ide; B = bidentate ligand 2,2'-bipyridine or R(2)-bpy, where R = ...COOEt or OMe) were synthesized and tested as precatalysts for the hydrogenative reduction of CO(2) in 2,2,2-trifluoroethanol (TFE) as solvent with added NEt(3). Significant amounts of formic acid were produced by these catalysts and a kinetic analysis based on initial rate constants was carried out. The potential mechanisms including intermediate species for these catalytic systems were investigated by means of quantum chemical calculations to gain deeper insight into the processes. The effect of electron-donating and electron-withdrawing groups on catalyst performance was studied both experimentally and theoretically.
Molecular twisting: A family of dinuclear Ru complexes of general formula {Ru(T)(L)}2(μ‐bpp)(n+1)+ (T=tridentate meridional ligand; bpp=tetradentate bridging ligand and L=monodentate ligand, n=1 or ...2) have been prepared and thoroughly characterized. In solution these complexes display a global dynamic behavior in which the monodentate ligands undergo a synchronized twisting motion.
Dinuclear ruthenium complexes (Ru(bid))2(μ‐bpp)(μ‐OAc) and (Ru(trpy))2(μ‐bpp)(μ‐X)2+ X=Cl−, OAc−, and OCHO−; bpp=3,5‐bis(2‐pyridyl)pyrazolato; ...bid−=(1Z,3Z)‐1,3‐bis(pyridin‐2‐ylmethylene)isoindolin‐2‐ide; trpy=2,2':6',2“‐terpyridine were tested as catalysts for the hydrogenative reduction of carbon dioxide in the solvent 2,2,2‐trifluoroethanol in the presence of excess amine (triethylamine). Significant amounts of formic acid were produced by these catalysts, and a kinetic analysis based on initial rate constants was performed. These catalytic systems were investigated by using DFT calculations to elucidate the hydrogenative reduction mechanism. The results are compared with those obtained with previously reported mononuclear catalyst counterparts.
Carbon footprints: New dinuclear ruthenium complexes that act as efficient catalysts for the hydrogenative reduction of carbon dioxide to formic acid have been prepared and fully characterized. The kinetics associated with the catalytic cycle were studied experimentally and a full catalytic cycle was determined on the basis of DFT calculations, which complement the experimental results.
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