Cláudia C. L. Pereira

Chemistry and Catalytic Activity of Molybdenum(VI)-Pyrazolylpyridine Complexes in Olefin Epoxidation. Crystal Structures of Monomeric Dioxo, Dioxo-μ-oxo, and Oxodiperoxo Derivatives

The dioxomolybdenum(VI) complexes [MoO2Cl2(PzPy)] (1) and [MoO2(OSiPh3)2(PzPy)] (5) (PzPy = 2-[3(5)-pyrazolyl]pyridine) were synthesized and characterized by vibrational spectroscopy, with assignments being supported by DFT calculations. Complex 5 was additionally characterized by single crystal X-ray diffraction. Recrystallization of 1 under different conditions originated crystal structures containing either the mononuclear [MoO2Cl2(PzPy)] complex co-crystallized with 2-[3(5)-pyrazolyl]pyridinium chloride, binuclear [Mo2O4(μ2-O)Cl2(PzPy)2] complexes, or the oxodiperoxomolybdenum(VI) complex [MoO(O2)2Cl(PzPyH)], in which a 2-[3(5)-pyrazolyl]pyridinium cation weakly interacts with the Mo(VI) center via a pyrazolyl N-atom. The crystal packing in the different structures is mediated by a variety of supramolecular interactions: hydrogen bonding involving the pyridinium and/or pyrazolyl N-H groups, weak CH · · · O and CH · · · π contacts, and strong π-π stacking. Complexes 1 and 5 are moderately active catalysts for the epoxidation of cis-cyclooctene at 55 °C using tert-butylhydroperoxide as oxidant, giving 1,2-epoxycyclooctane as the only reaction product. Insoluble materials were recovered at the end of the first catalytic runs and characterized by IR spectroscopy, elemental analysis, scanning electron microscopy (SEM)-energy dispersive spectroscopy (EDS), and powder X-ray diffraction. For complex 5 the loss of the triphenylsiloxy ligands during the catalytic run resulted in the formation of a tetranuclear complex, [Mo4O8(μ2-O)4(PzPy)4]. The recovered solids could be used as efficient heterogeneous catalysts for the epoxidation of cyclooctene, showing no loss of catalytic performance between successive catalytic runs.

Infrared Spectra and Quantum Chemical Calculations of the Uranium Carbide Molecules UC and CUC with Triple Bonds.

Laser evaporation of carbon-rich uranium/carbon alloys followed by atom reactions in a solid argon matrix and trapping at 8 K gives weak infrared absorptions for CUO at 852 and 804 cm(-1). A new band at 827 cm(-1) becomes a doublet with mixed carbon 12 and 13 isotopes and exhibits the 1.0381 isotopic frequency ratio, which is appropriate for the UC diatomic molecule, and another new band at 891 cm(-1) gives a three-band mixed isotopic spectrum with the 1.0366 isotopic frequency ratio, which is characteristic of the linear CUC molecule. CASPT2 calculations with dynamical correlation find the C[triple bond]U[triple bond]C ground state as linear 3Sigma(u)+ with 1.840 A bond length and molecular orbital occupancies for an effective bond order of 2.83. Similar calculations with spin-orbit coupling show that the U[triple bond]C diatomic molecule has a quintet (Lambda = 5, Omega = 3) ground state, a similar 1.855 A bond length, and a fully developed triple bond of 2.82 effective bond order.

Infrared spectra and quantum chemical calculations of the uranium-carbon molecules UC, CUC, UCH, and U(CC)2.

Laser evaporation of carbon rich uranium/carbon alloy targets into condensing argon or neon matrix samples gives weak infrared absorptions that increase on annealing, which can be assigned to new uranium carbon bearing species. New bands at 827.6 cm(-1) in solid argon or 871.7 cm(-1) in neon become doublets with mixed carbon 12 and 13 isotopes and exhibit the 1.0381 carbon isotopic frequency ratio for the UC diatomic molecule. Another new band at 891.4 cm(-1) in argon gives a three-band mixed isotopic spectrum with the 1.0366 carbon isotopic frequency ratio, which is characteristic of the anti-symmetric stretching vibration of a linear CUC molecule. No evidence was found for the lower energy cyclic U(CC) isomer. Other bands at 798.6 and 544.0 cm(-1) are identified as UCH, which has a uranium-carbon triple bond similar to that in UC. Evidence is found for bicyclic U(CC)(2) and tricyclic U(CC)(3). This work shows that U and C atoms react spontaneously to form the uranium carbide U≡C and C≡U≡C molecules with uranium-carbon triple bonds.

Europium(III) tetrakis(β-diketonate) complex as an ionic liquid: a calorimetric and spectroscopic study.

An intrinsic photoluminescent ionic liquid based on europium(III) tetrakis(β-diketonate) complex with a tetraalkylphosphonium as counterion was synthesized. Calorimetric measurements showed a melting point at 63 °C, which allows the ionic liquid classification. When cooling the material from the liquid state, metastable supercooled ionic liquid is obtained, as seen from NMR spectroscopy as well. Eu(III) photoluminescence is clearly observed while the absorption spectra of the ligand is dominant, showing the antenna effect. This was confirmed with submicrosecond time scale luminescence spectroscopy, where a rise of Eu(III) emission is observed with the correspondent decay of the ligand excited state. Temperature effects in the photoluminescence are also shown, being prominent above the melting point where the intensity decreases with Arrhenius behavior. Eu(III) luminescence decays also show features characteristic of energy migration between homologue Eu(III) species. Solvent effects were also studied by NMR and Luminescence spectroscopies, highlighting that the nucleophilicity of organic solvents such as n-alcohols leads to a coordination with Eu(III), which ultimately compromises the stability of the complex.

Actinide sulfides in the gas phase: experimental and theoretical studies of the thermochemistry of AnS (An = Ac, Th, Pa, U, Np, Pu, Am and Cm).

The gas-phase thermochemistry of actinide monosulfides, AnS, was investigated experimentally and theoretically. Fourier transform ion cyclotron resonance mass spectrometry was employed to study the reactivity of An(+) and AnO(+) (An = Th, Pa, U, Np, Pu, Am and Cm) with CS(2) and COS, as well as the reactivity of the produced AnS(+) with oxidants (COS, CO(2), CH(2)O and NO). From these experiments, An(+)-S bond dissociation energies could be bracketed. Density functional theory studies of the energetics of neutral and monocationic AnS (An = Ac, Th, Pa, U, Np, Pu, Am and Cm) provided values for bond dissociation energies and ionization energies; the computed energetics of neutral and monocationic AnO were also obtained for comparison. The theoretical data, together with comparisons with known An(+)-O bond dissociation energies and M(+)-S and M(+)-O dissociation energies for the early transition metals, allowed for the refining of the An(+)-S bond dissociation energy ranges obtained from experiment. Examination of the reactivity of AnS(+) with dienes, coupled to comparisons with reactivities of the AnO(+) analogues, systematic considerations and the theoretical results, allowed for the estimation of the ionization energies of the AnS; the bond dissociation energies of neutral AnS were consequently derived. Estimates for the case of AcS were also made, based on correlations of the data for the other An and the electronic energetics of neutral and ionic An. The nature of the bonding in the elementary molecular actinide chalcogenides (oxides and sulfides) is discussed, based on both the experimental data and the computed electronic structures. DFT calculations of ionization energies for the actinide atoms and the diatomic sulfides and oxides are relatively reliable, but the calculation of bond dissociation energies is not uniformly satisfactory, either with DFT or CCSD(T). A key conclusion from both the experimental and theoretical results is that the 5f electrons do not substantially participate in actinide-sulfur bonding. We emphasize that actinides form strikingly strong bonds with both oxygen and sulfur.

Synthesis of ferrocenyldiimine metal carbonyl complexes and an investigation of the Mo adduct encapsulated in cyclodextrin

Cr and Mo tetracarbonyl complexes bearing the diimine ligand N,N′-bis(ferrocenylmethylene)ethylenediamine (FcNN) have been prepared from the ligand and M(CO)6. Two isomeric forms of (FcNN)Mo(CO)4, corresponding to the cis,cis and cis,trans geometries with respect to the CN bonds of the free ligand, were shown to exist in a 1∶12.5 ratio by 1H NMR (NOE experiments). By contrast, only the trans,trans isomer is observed for the Cr complex (FcNN)Cr(CO)4. The compounds (FcNN)M(CO)4 show ferrocene-based irreversible oxidation processes that lead to the deposition of a film at the electrode surface. (FcNN)Mo(CO)4 was immobilised in permethylated β-CD (TRIMEB) by addition of the guest to a solution of TRIMEB in dichloromethane. Removal of the solvent led to the isolation of an inclusion compound with a 2∶1 host∶guest stoichiometry, as evidenced by powder X-ray diffraction, thermogravimetric analysis, FTIR and 13C CP MAS NMR spectroscopy. The electrochemical properties of (FcNN)Mo(CO)4 upon encapsulation are discussed.

Magnetic Properties of the Layered Lanthanide Hydroxide Series YxDy8-x(OH)20Cl4·6H2O: From Single Ion Magnets to 2D and 3D Interaction Effects

The magnetic properties of layered dysprosium hydroxides, both diluted in the diamagnetic yttrium analogous matrix (LYH:0.04Dy), and intercalated with 2,6-naphthalene dicarboxylate anions (LDyH-2,6-NDC), were studied and compared with the recently reported undiluted compound (LDyH = Dy8(OH)20Cl4·6H2O). The Y diluted compound reveals a single-molecule magnet (SMM) behavior of single Dy ions, with two distinct slow relaxation processes of the magnetization at low temperatures associated with the two main types of Dy sites, 8- and 9-fold coordinated. Only one relaxation process is observed in both undiluted LDyH and intercalated compounds as a consequence of dominant ferromagnetic Dy-Dy interactions, both intra- and interlayer. Semiempirical calculations using a radial effect charge (REC) model for the crystal field splitting of the Dy levels are used to explain data in terms of contributions from the different Dy sites. The dominant ferromagnetic interactions are explained in terms of orientations of easy magnetization axes obtained by REC calculations together with the sign of the superexchange expected from the Dy-O-Dy angles.

NEW LIPOPHILIC COMPONENTS OF PITCH DEPOSITS FROM AN EUCALYPTUS GLOBULUS ECF BLEACHED KRAFT PULP MILL

ABSTRACT The chemical composition of three pitch samples coming from an Eucalyptus globulus ECF bleached kraft pulp mill was studied by GC-MS. All samples are rich in fatty acids, including several ω- and α-hydroxyfatty acids and several oxidation products of β-sitosterol, such as 5,6-epoxy-24-ethylcholestane-3-ol and 24-ethylcholestane-3,5,6-triol. One oxidation product of oleic acid, namely 9,10-dihydroxyoctadecanoic acid was also identified. The α-hydroxyfatty acids, the sterols oxidation products and 9,10-dihydroxyoctadecanoic acid are reported here for the first time as components of pitch issued from ECF bleached kraft pulp mills.

Metal-organic frameworks based on uranyl and phosphonate ligands

Three new crystalline metal-organic frameworks have been prepared from the reaction of uranyl nitrate with nitrilotris(methylphosphonic acid) [H6nmp, N(CH2PO3H2)3], 1,4-phenylenebis(methylene)diphosphonic acid [H4pmd, C6H4(PO3H2)2], and (benzene-1,3,5-triyltris(methylene))triphosphonic acid [H6bmt, C6H3(PO3H2)3]. Compound [(UO2)2F(H3nmp)(H2O)]·4H2O (I) crystallizes in space group C2/c, showing two crystallographically independent uranyl centres with pentagonal bipyramidal coordination geometries. While one metal centre is composed of a {(UO2)O3(μ-F)}2 dimer, the other comprises an isolated {(UO2)O5} polyhedron. Compound [(UO2)(H2pmd)] (II) crystallizes in space group P21/c, showing a centrosymmetric uranyl centre with an octahedral {(UO2)O4} coordination geometry. Compound [(UO2)3(H3bmt)2(H2O)2]·14H2O (III) crystallizes in space group P\bar 1, showing two crystallographically independent uranyl centres. One uranyl centre is a {(UO2)O5} pentagonal bipyramid similar to that in (I), while the other is a {(UO2)O4} centrosymmetric octahedron similar to that in (II). Compounds (I) and (III) contain solvent-accessible volumes accounting for ca 23.6 and 26.9% of their unit-cell volume, respectively. In (I) the cavity has a columnar shape and is occupied by disordered water molecules, while in (III) the cavity is a two-dimensional layer with more ordered water molecules. All compounds have been studied in the solid state using FT-IR spectroscopy. Topological studies show that compounds (I) and (III) are trinodal, with 3,6,6- and 4,4,6-connected networks, respectively. Compound (II) is instead a 4-connected uninodal network of the type cds.

Thorium and Uranium Carbide Cluster Cations in the Gas Phase: Similarities and Differences between Thorium and Uranium

Laser ionization of AnC4 alloys (An = Th, U) yielded gas-phase molecular thorium and uranium carbide cluster cations of composition An(m)C(n)(+), with m = 1, n = 2-14, and m = 2, n = 3-18, as detected by Fourier transform ion-cyclotron-resonance mass spectrometry. In the case of thorium, Th(m)C(n)(+) cluster ions with m = 3-13 and n = 5-30 were also produced, with an intriguing high intensity of Th13C(n)(+) cations. The AnC13(+) ions also exhibited an unexpectedly high abundance, in contrast to the gradual decrease in the intensity of other AnC(n)(+) ions with increasing values of n. High abundances of AnC2(+) and AnC4(+) ions are consistent with enhanced stability due to strong metal-C2 bonds. Among the most abundant bimetallic ions was Th2C3(+) for thorium; in contrast, U2C4(+) was the most intense bimetallic for uranium, with essentially no U2C3(+) appearing. Density functional theory computations were performed to illuminate this distinction between thorium and uranium. The computational results revealed structural and energetic disparities for the An2C3(+) and An2C4(+) cluster ions, which elucidate the observed differing abundances of the bimetallic carbide ions. Particularly noteworthy is that the Th atoms are essentially equivalent in Th2C3(+), whereas there is a large asymmetry between the U atoms in U2C3(+).