The electronic structure of several mono(1,1′-diamidoferrocene) uranium complexes (NNR)UX2 (NNR = fc(NR) 2, fc = 1,1′-ferrocenediyl, R = SiMe3, Si tBuMe2, SiMe2Ph, X = I, CH2Ph), (NNTBS)UI(OAr) (OAr = 2,6-di-tert-butylphenoxide), and (NN TBS)U(CH2Ph)(OAr) was investigated by electrochemistry, electronic absorption and vibrational spectroscopy, and DFT calculations. Similar metrical parameters were observed for (NNTBS)U(CH 2Ph)2 and (NNDMP)U(CH2Ph) 2 (and also for the previously reported (NNTMS)UI 2(THF), (NNTBS)UI2(THF), and (NN TBS)U(CH2Ph)(OAr)) that translate in similar DFT parameters (bond orders, metal charges) despite some small differences observed by electrochemistry and IR or electronic absorption spectroscopy. © 2013 American Chemical Society.

The Zr6 nodes of the metal-organic frameworks (MOFs) UiO-66 and UiO-67 are metal oxide clusters of atomic precision and can be used as catalyst supports. The bonding sites on these nodes - that is, hydrogen-bonded H2O/OH groups on UiO-67 and non-hydrogen-bonded terminal OH groups on UiO-66 - were regulated by modulation of the MOF syntheses. Ir(C2H4)2(C5H7O2) complexes reacted with these sites to give site-isolated Ir(C2H4)2 complexes, each anchored to the node by two Ir-Onode bonds. The supported iridium complexes on these sites have been characterized by infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectroscopies and density functional theory calculations. The ethylene ligands on iridium are readily replaced by CO, and the νCO frequencies of the resultant complexes and those of comparable complexes reported elsewhere show that the support electron-donor tendencies increase in the order HY zeolite 蠐 UiO-66 < UiO-67 (= NU-1000) < ZrO2 < MgO. The sharpness of the IR νCO bands shows that the degree of uniformity of the support bonding sites decreases in the order ZrO2 ≈ UiO-67 ≈ NU-1000 < MgO < UiO-66 蠐 HY zeolite. The reactivity of supported Ir(CO)2 complexes with C2H4 to form Ir(C2H4)(CO) and Ir(C2H4)2(CO) is influenced by the support electron-donor properties, with the reactivity increasing in the order MgO = ZrO2 = NU-1000 (not reactive) < UiO-66 < UiO-67 蠐 HY zeolite. Density functional theory calculations characterizing the complexes supported on NU-1000, UiO-66/67, and HY zeolite concur with the use of the calculated νCO bands as indicators of electron-donor properties of the supported metal catalysts. Our calculations also show that the reactivity of the supported Ir(CO)2 complexes with C2H4 is correlated with the electron-donor properties of the iridium center. The supported Ir(C2H4)2 samples are precatalysts for ethylene hydrogenation and ethylene dimerization, with the activity for each reaction increasing with increasing electron-withdrawing strength of the support.

Metal-organic frameworks with Zr6 nodes, UiO-66 and NU-1000, were investigated as supports for Ir(CO)2 and Ir(C2H4)2 complexes. A single bonding site for the iridium is identified on the nodes of NU-1000, whereas two sites are identified on UiO-66, although at low iridium loadings only one site is occupied. Density functional theory calculations provide structural results that are in good agreement with infrared and X-ray absorption fine-structure spectra. The reactivity of node-supported Ir(CO)2 with C2H4 and the catalytic activity and selectivity of the species initially present as Ir(C2H4)2 for ethylene hydrogenation and dimerization were investigated both experimentally and computationally and shown to be strongly influenced by the node.

Using a combination of density functional theory and lattice models, we study the effect of CO₂ adsorption in an amine functionalized metal-organic framework. These materials exhibit a step in the adsorption isotherm indicative of a phase change. The pressure at which this step occurs is not only temperature dependent but is also metal center dependent. Likewise, the heats of adsorption vary depending on the metal center. Herein we demonstrate via quantum chemical calculations that the amines should not be considered firmly anchored to the framework and we explore the mechanism for CO₂ adsorption. An ammonium carbamate species is formed via the insertion of CO₂ into the M-N_amine bonds. Furthermore, we translate the quantum chemical results into isotherms using a coarse grained Monte Carlo simulation technique and show that this adsorption mechanism can explain the characteristic step observed in the experimental isotherm while a previously proposed mechanism cannot. Furthermore, metal analogues have been explored and the CO₂ binding energies show a strong metal dependence corresponding to the M-N_amine bond strength. We show that this difference can be exploited to tune the pressure at which the step in the isotherm occurs. Additionally, the mmen-Ni₂(dobpdc) framework shows Langmuir like behavior, and our simulations show how this can be explained by competitive adsorption between the new model and a previously proposed model.

Diamine-appended metal-organic frameworks display great promise for carbon capture applications, due to unusual step-shaped adsorption behavior that was recently attributed to a cooperative mechanism in which the adsorbed CO2 molecules insert into the metal-nitrogen bonds to form ordered ammonium carbamate chains [McDonald et al., Nature, 2015, 519, 303]. We present a detailed study of this mechanism by in situ X-ray absorption spectroscopy and density functional theory calculations. Distinct spectral changes at the N and O K-edges are apparent upon CO2 adsorption in both mmen-Mg2(dobpdc) and mmen-Mn2(dobpdc), and these are evaluated based upon computed spectra from three potential adsorption structures. The computations reveal that the observed spectral changes arise from specific electronic states that are signatures of a quasi-trigonal planar carbamate species that is hydrogen bonded to an ammonium cation. This eliminates two of the three structures studied, and confirms the insertion mechanism. We note the particular sensitivity of X-ray absorption spectra to the insertion step of this mechanism, underpinning the strength of the technique for examining subtle chemical changes upon gas adsorption.

Recent accidents resulting in worker injury and radioactive contamination occurred due to pressurization of uranium yellowcake drums produced in the western U.S.A. The drums contained an X-ray amorphous reactive form of uranium oxide that may have contributed to the pressurization. Heating hydrated uranyl peroxides produced during in situ mining can produce an amorphous compound, as shown by X-ray powder diffraction of material from impacted drums. Subsequently, studtite, [(UO2)(O2)(H2O)2](H2O)2, was heated in the laboratory. Its thermal decomposition produced a hygroscopic anhydrous uranyl peroxide that reacts with water to release O2 gas and form metaschoepite, a uranyl-oxide hydrate. Quantum chemical calculations indicate that the most stable U2O7 conformer consists of two bent (UO2)(2+) uranyl ions bridged by a peroxide group bidentate and parallel to each uranyl ion, and a μ2-O atom, resulting in charge neutrality. A pair distribution function from neutron total scattering supports this structural model, as do (1)H- and (17)O-nuclear magnetic resonance spectra. The reactivity of U2O7 in water and with water in air is higher than that of other uranium oxides, and this can be both hazardous and potentially advantageous in the nuclear fuel cycle.

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.