Europium reduction and lanthanide coordination in polyoxometalates
Item
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Title
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Europium reduction and lanthanide coordination in polyoxometalates
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Identifier
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d_2009_2013:b0a34ff1f7eb:10609
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identifier
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10524
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Creator
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Jing, Jing,
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Contributor
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Lynn C. Francesconi
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Date
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2010
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Language
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English
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Publisher
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City University of New York.
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Subject
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Analytical chemistry | Inorganic chemistry | Lanthanide | Polyoxometalates | SAXS(small-angle X-ray scattering ) | XAFS (X-ray absorption fine structure)
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Abstract
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Polyoxometalates (POMs) are a unique class of metal-oxygen cluster anions in which the early transition metals are in their highest oxidation states. POMs have applications in quite diverse disciplines including catalysis, medicine and material sciences, many of which are based upon their reduction-oxidation (redox) properties. The variation of metal coordination environments and metal-oxygen framework architectures influences redox properties of POMs, and the extent of that influence depends on the nature of the incorporated metal ion. When combined with lanthanide (Ln) ions, Ln-POMs form new structures, exhibit catalytic properties, and offer unique functionality, such as the creation of luminescent and Lewis acid catalytic centers. Because of this, Ln-POM derivatives have attracted increasing attention in recent years.;The first project focused on Europium (Eu) and Wells-Dawson Heteropolytungstates because of their prospects in electroanalytical chemistry. Eu is redox-active. Wells-Dawson Heteropolytungstates contain electroactive, lacunary P-W-O anions that, upon reduction in aqueous and organic media, form heteropoly blue species that retain the same structures as the parent anions. The combination of the electrochemical properties of both the Eu and the ligands in one molecule produces a two-center, multielectron redox. We wish to investigate multi-electron redox processes so as to derive some information about the interaction between the Ln f orbitals and the tungsten d-band LUMO. To this end, electrochemical techniques (cyclic voltammetry and bulk electrolysis), in situ XAFS (X-ray absorption fine structure) spectroelectrochemistry, NMR spectroscopy and optical luminescence were used. The cyclic voltammograms show concentration dependence. The reduced form of K7[(H2O)4Eu(alpha-1-P 2W17O61)] was probed by Eu LIII edge XANES and confirmed that Eu(III) is reduced to Eu(II). EXAFS data for the reduced Eu(II)-POM shows the average Eu-O bond length is 2.55(4) A, which is 0.17 A longer than that for the oxidized anion, and consistent with the 0.184 difference between the Eu(II) and Eu(III) ionic radii.;In the second project, the Nd3+, Sm3+, Eu3+, Tb3+, Dy3+, and Yb 3+ complexes with alpha-1-[P2W17O61] 7- of 1:1 Ln:alpha-1-ligand stoichiometries, [(H2O) nLn(alpha-1-P2W17O61)] 7-, as the tetra-n-butylammonium solid salts as well as their solutions in dry and wet acetonitrile were probed through use of voltammetry, electrolysis, and EXAFS. The comparative metrical data obtained about the inner-sphere Eu coordination environments in the solid salt of TBA +-Eu-alpha-1 and upon its dissolution in dry MeCN with a 0.1 M TBAPF 6 electrolyte suggest that MeCN binds to Eu(III) in part (0 < delta < 4) and in exchange with H2O, forming a mixed H2O-MeCN solvate. These results, in combination with the EXAFS results for the TBA +-Ln-alpha-1 systems across the 4f period provide a fresh perspective on the variation of Ln(III) coordination in MeCN, including the effects of the lanthanoid contraction as well as known variations of MeCN and H 2O residence times.;The third project, the SAXS measurements for potassium salts of [alpha-1-P 2W17O61]10-, [Eu(alpha-1-P 2W17O61)]7- and [Yb(alpha-1-P 2W17O61)]7- at different concentrations, is presented in Appendix. The Guinier plots showing linear and parallel response suggest that the cluster size for each compound is independent of concentration and the clusters remain monodisperse without aggregation at high concentrations. The Rgs obtained from the Guinier fit over the low Q range and P(r) analysis over the entire Q rang exhibit good consistence with each other, which again indicates that the clusters remain intact and monodisperse with no interparticle interactions and no aggregation at high concentrations.
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Type
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dissertation
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Source
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2009_2013.csv
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degree
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Ph.D.
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Program
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Chemistry
