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Structural and Functional Models for the Molybdenum, Tungsten and Copper Enzymes: Synthesis, Properties and Reactivity

Moradi Shoeili, Zeinab | 2013

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 44369 (03)
  4. University: Sharif University of Technology
  5. Department: Chemistry
  6. Advisor(s): Mohammadi Boghaei, Davar
  7. Abstract:
  8. In order to obtain more insight into the enzymatic oxygen-atom-transfer (OAT) process and elucidate the relation between the structure and function of the synthetic model complexes, new molybdenum and tungsten complexes containing different chelating ligands were systematically prepared and investigated. Herein, the cis-dioxo-molybdenum(VI) complexes with new aromatic dithiolene ligands, formulated as [MoO2(2,3ppdt)2]NR4 and [MoO2(3,4ppdt)2]NR4, (2,3ppdt= pyrido-2,3-pyrazine-2,3-dithiolato, 3,4ppdt= pyrido-3,4-pyrazine-2,3-dithiol and R= Et or Bu) were prepared and investigated by elemental analysis, IR, 1H NMR and UV-Vis spectroscopic methods. An OAT reaction of [MoO2(2,3ppdt)2]2– with PPh3 afforded [MoO(2,3ppdt)2]2– and OPPh3, which proceeded in second order reaction as v=−d/dt [MoO2] = k[MoO2][PPh3]. The mono oxomolybdenum(IV) complexes with dithiolene ligands have also been prepared and characterized. The cis-dioxo-molybdenum(VI) analogue was quantitatively formed by clean OAT reaction with second-order rate constant in a reaction between the [MoO(2,3ppdt)2]2– and an amine N-oxide. The experimental observations indicated the absence of (μ-oxo)dimolybdenum(V) complex formation in the present reactions controlled by the unique electronic properties and the strength of chelation of the dithiolato ligands. Dinuclear cis-dioxomolybdenum(VI) complex [{MoO2(Bz2Benzenediyldtc)}2] coordinated by a quadradentate dithiocarbamate (Bz2Benzenediyldtc2– = 1,4-benzenediylbis(benzyldithiocarbamate)(2-)) has been prepared and characterized by elemental analysis, 13C NMR, IR and UV-Vis spectroscopy. The kinetics of the OAT reaction between [{MoO2(Bz2Benzenediyldtc)}2] and PPh3 was spectrophotometrically studied which follows second order kinetics. Despite the steric restrictions imposed by the ligand structure to prevent the formation of μ-oxo molybdenum(V) species as a product, experimental evidences confirmed its interference during the OAT process. New compounds of the type [MoO2(L)D] where L= N(4)-substituted thiosemicarbohydrazone ligands (H2TSC), supplying a tridentate ONS donor sets or 1,3,4-thiadiazole derivative (H2MeTDZ) as a tridentate NNO donor ligand and D= MeOH or CH3CN, were synthesized and characterized by elemental analysis, 1H NMR, IR and electronic spectroscopic studies as well as single crystal X-ray diffraction analysis. An OAT reaction of [MoO2(MeTSC)D] (MeTSC = 1-(2,4-dihydroxybenzylidene)-N4-methyl-N4-phenylthiosemicarbazone) with PPh3 follows second order rate law. In all mentioned OAT kinetic studies, an Eyring plot allows the activation parameters to be determined from the temperature dependence of the rate constant, suggesting an associative transition state for the OAT reaction. The oxo-transfer model reaction from DMSO to PPh3 has also been used to test the catalytic properties of complexes [MoO2(2,3ppdt)2]2–, [MoO2(3,4ppdt)2]2–, [{MoO2(Bz2Benzenediyldtc)}2] and [MoO2(MeTSC)D] which followed by 31P NMR spectroscopy The complexes act as good catalysts of the OAT reaction indicating their biological relevance to DMSO reductases, which are able to utilise a variety of dialkyl and alkyl aryl sulphoxides as oxidizing substrates. New mononuclear oxo–peroxo complexes of tungsten(VI) of the formula [WO(O2)(TSC)(MeOH)] have been synthesized and characterized by elemental analysis, infrared, UV–visible and 1H NMR spectroscopies. Compounds [MoO2(MeTSC)D], [MoO2(MeTDZ)D] and [WO(O2)(TSC)(MeOH)] were also tested for catalytic homogeneous epoxidation of olefins and oxidation of sulfides, respectively, which show very much efficient reactivity in the oxidation reactions giving high yield, turnover number (TON) and selectivity, in each case. Moreover, chemical approaches to study copper active sites in enzymes such as ascorbate oxidase (AO) and related model complexes had been either structural modeling or functional modeling. Here in this contribution, the synthesis, crystal structure and properties of a series of 12 new ternary mononuclear copper(II) complexes of type [Cu(L)(NN)] where the L= dianionic ONO-donor amino acid salicylaldiminates or ONS donor thiosemicarbohydrazones and NN= 2,2'-bipyridine or 1,10-phenanthroline heterocyclic bases, have been presented. These complexes have been characterized by elemental analyses, IR, UV–Vis spectroscopies, magnetic measurement and cyclic voltammetry. The complexes are one-electron paramagnetic and have been structurally characterized by X-ray diffraction technique. The reaction of these copper(II) complexes with ascorbic acid as two-electron donor, in order to elucidate the relationship between the catalytic activity for the oxidation of ascorbic acid by O2 and structure of copper(II) complexes have been investigated using UV–Vis electronic spectral studies. Catalytic activity of the compounds with a N3O2 coordination environment, in the ascorbate oxidation by molecular oxygen involves copper(I) intermediate forming, which emulates the ascorbate oxidation property of Cu(II) sites in AO. This process is effective with a mole ratio of ascorbic acid to complex as approximately 100. Complexes containing N3OS donor ligands have been found to form stable Cu(I) analogues on reduction with ascorbic acid and are catalytically inactive. These observations prove that the reversible change of oxidation state, i.e., Cu(II) Cu(I), plays an important role in the catalytic function of mononuclear copper(II) complexes
  9. Keywords:
  10. Kinetics Study ; Molybdenum ; Copper ; Oxidation ; Ascorbic Acid ; Dithiocarbamate ; Thiosemi Carbazide ; Thiadiazole ; Oxygen Atom Transfer

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