Elisa Llopis

Model compounds for the active sites of oxo-transfer molybdoenzymes. Synthesis, structural characterization, and electrochemical properties of [NH4]2[MoO2{O2CC(S)Ph2}2]

[NH4]2[MoO2{O2CC(S)Ph2}2]middot;2H2O has been prepared and its crystal structure solved, providing the first example of a co-ordinatively saturated molybdenum complex which reacts readily and reversibly with organophosphines at room temperature in water or methanol; its variable-temperature 13C{1H} NMR spectra and electrochemical properties are presented.

Compounds of tungsten(VI) with citric acid: A spectrophotometric, polarimetric and hydrogen-1, carbon-13 N.M.R. study of the formation and interconversion equilibria in aqueous solution

SummaryTungsten(VI)-citrate complexes, which occur in aqueous solution, were studied by polarimetric and absorbance measurements. The pH of the medium is the principal variable controlling complex formation and interconversion equilibria. At high pH (>6), the stable complexes are monomers with 1∶2 and 1∶1 stoichiometry, depending on the tungsten(VI)-citrate ratio, while at lower pH two different dinuclear complexes are formed. The intervals of existence of these species with the pH, the number of equivalents of acid necessary for their formation, as well as the conditional stability constant, have also been calculated.The structure of these compounds have been investigated by1H and13C-n.m.r. spectroscopy. The n.m.r. results indicate the existence in all these species of strong, covalent metal-toligand oxygen bondingvia the carboxylic oxygen atom and the adjacent hydroxylic oxygen.

On the atomic environment and the mode of action of the catalytic centre in an intercalated oxo–molybdenum complex [MoO2{O2CC(S)Ph2}2]2– for oxygen-transfer reactions

The title anion accommodated as a pillar in the interlamellar space of a Zn(II)AlIII double hydroxide(hydrotalcite-like) host is shown by X-ray absorption spectroscopy, supplemented by other studies, to possess three terminal MoO groups, in contrast to the two that exist in the dispersed, catalytically active anion.

Molybdenum(VI)-dioxo complexes with sterically bulky thiocarboxylate ligands. Reactions with aliphatic thiols and electrochemical properties

Abstract Under acid conditions, the reaction of (Bu n 4 N)2[Mo VI O 2 (O 2 CC(S)Ph 2 ) 2 ] with aliphatic thiols yields the monomeric [Mo V O(O 2 CC(S)Ph 2 ) 2 ] − as unique complex product. The experimental pseudo-first order rate constant with respect to the Mo(VI) complex was found to be K =6.1 × 10 −5 s −1 . At neutral pH, however, an unstable Mo(IV) species was formed which was also electrochemically detected in a reversible Mo(V,IV) couple. [Mo V O(O 2 CC(S)Ph 2 ) 2 ] − appears to be obtained by the reaction of [Mo IV O(O 2 CC(S)Ph 2 ) 2 ] 2− with unreacted [Mo VI O 2 (O 2 CC(S)Ph 2 ) 2 ] 2− . Steric features on the ligand (gemdiphenyl groups) explain that the latter reaction does not lead to the expected formation of μ-oxo Mo(V) dimers. [Mo V O(O 2 CC(S)Ph 2 ) 2 ]− undergoes a one-electron reversible reduction on the cyclic voltammogram time scale, being also oxidized by nitrate ions to the starting Mo(VI) complex. The relation of these results to enzymatic systems is briefly discussed.

Intercalation of the oxo-transfer molybdenum(VI) complex [MoO2{O2CC(S) Ph2}2]2– into a zinc(II)–aluminium(III) layered double hydroxide host. Catalysis of the air oxidalton of thiols

The intercalation of bis(2.2-diphenyl-2-sulfidoacetato)dioxomolybdate(VI)[MoO2{O2CC(S)Ph2}2]2–1 into an hydrotalcite-like zinc(II)-aluminium(III) layered double hydroxide by anion exchange yielded [Zn3–xAlx(OH)6]x+[{MoO2[O2CC(S)Ph2]2}y//2(NO3)x–y]x–·H2O (x= 0.75). Data from powder X-ray diffraction, IR and diffuse reflectance spectroscopy and thermogravimetnc analysis confirmed intercalation within a 13 A-height gallery, indicating that the molybdenum anion probably lies with its cis-dioxomolybdenum(VI) plane parallel to the hydroxide layers. Complex 1, immobilized in the layered double hydroxide, operates as an effective heterogeneous catalyst for the oxiDalton of thiols (PhSH) by either dioxygen or air as evaluated by gas chromatography. The active oxidant is [MoO2{O2CC(S)Ph2}2]2–, while a molybdenum(VI) species acts as the reductant by interaction with dioxygen to regenerate the molybdenum(VI) oxidant This constitutes the first reported heterogeneous catalyst containing an oxo-transfer MoVIO2complex capable of activating dioxygen. Oxidalion proceeds in ethanolic solutions without deactivation by water or excess thiol even at relatively high temperatures (up to 80°C). It is, therefore, concluded that intercalation of [MoO2{O2CC(S)Ph2}2]2– into the layered double hydroxide host inhibits the formation of a catalytically inactive molybdenum(V) species which was previously observed under homogeneous conditions.

Tungsten(VI) complexes with citric acid (H4cit). Structural characterisation of Na6[{WO2(cit)}2O]·10H2O

The complex anion [W2O5(cit)2]6– was obtained by crystallisation from an equimolar aqueous solution of sodium tungstate and citric acid (H4cit). It has been characterised by elemental analysis, IR and 1H and 13C NMR spectroscopy. The IR spectrum is consistent with a monooxo-bridged dinuclear structure as revealed by a single-crystal X-ray diffraction study of Na6[W2O5(cit)2]·10H2O. The crystals are monoclinic, space group P21/a, a= 17.708(3), b= 9.908(2) and c= 9.018(2)A, β= 91.57(2)°, R= 0.025 for 3101 observed [I/σ(I) 2.5] reflections. The complex anion contains a O2WOWO2 core with the bridging oxo group lying at a crystallographic centre of symmetry (W–Ob–W 180°). Each citrate ligand is three co-ordinated to one tungsten atom through the deprotonated hydroxy, α-carboxylate, and one β-carboxylate group. Principal dimensions are: W–Ob 1.893(1), (W = Ot)av 1.757(2), W–Ohydroxy 1.958(2), W–Oα-carboxy 2.195(3) and W–Oβ-carboxy 2.289(2)A. All methylene and β-carboxylate groups become magnetically equivalent upon redissolution in water as a result of the dissociation of the less strongly bonded β-carboxylate group.

Potentiometric study of the molybdenum(VI)–benzilic acid system. Structural characterisation and electrochemical properties of [NH4]2[MoO2{O2CC(O)Ph2}2]·2H2O

The formation of complexes between molybdate and benzilate (2-hydroxy-2,2-diphenylacetate), Hbza–, ions has been investigated in the range pH 2–7 by potentiometric measurements. Computer treatment of the potentiometric data revealed the formation of two complexes the [MoO4]2–, Hbza–, H+ stoichiometries of which are [1,2,2] and [2,2,4]; log β122= 17.35, log β224= 29.07. Compared to other less bulky hydroxycarboxylate ligands previously studied, this result indicates that the phenyl groups of benzilate only prevent the formation of [2,2,5] dimeric complex which presumably has a double oxo bridge. The mononuclear [1,2,2] complex has been isolated and its structure determined by X-ray analysis of its ammonium salt, [NH4]2[MoO2{O2CC(O)Ph2}2]·2H2O; monoclinic, space group P21/n, a= 8.106(2), b= 25.702(8), c= 13.628(4)A, β= 90.62(2)° and Z= 4. The redox properties of this complex salt were also investigated and compared with those previously found for an analogous complex containing two thiobenzilate ligands.

Tungsten―mannitol and sorbitol complexes: structural characterization by IR and carbon-13 nuclear magnetic resonance spectroscopy

Abstract Polarimetric and spectrophotometric studies of the mannitol-tungsten(IV) system show the formation of three table complexes. Their stoichiometries, degrees of condensation and interconversion equilibria have been studied. The structure of these compounds has been investigated by 13C NMR spectroscopy in aqueous solution and by IR spectroscopy in the solid state for the two species stable at pH

The reduction of tris-dithiolene complexes of molybdenum(VI) and tungsten(VI) by hydroxide ion: kinetics and mechanism

The kinetic study of the spontaneous reduction of some neutral tris-dithiolene complexes [ML3] of molybdenum(VI) and tungsten(VI), (L = S2C6H4(2-), S2C6H3CH3(2-) and S2C2(CH3)2(2-); M = Mo or W) by tetrabutylammonium hydroxide in tetrahydrofuran-water solutions demonstrates that OH- is an effective reductant. Their reduction is fast, clean and quantitative. Depending upon both the molar ratio in which the reagents are mixed and the amount of water present, one- or two-electron reductions of these tris-dithiolene complexes were observed. If Bu4NOH is present in low concentration or/and at high concentrations of water, the total transformation of the neutral M(VI) complex into the monoanionic M(V) complex is the only observed process. Stopped-flow kinetic data for this reaction are consistent with the rate law: -d[ML3]/dt = d[ML3-]/dt = k[ML3][Bu4NOH]. The proposed mechanism involves nucleophilic attack of OH- to form a mono-anionic seven-coordinate intermediate [ML3OH]-, which interacts with another molecule of [ML3] to generate the monoanionic complex [ML3]- transfering the oxygen from coordinated OH- to water. Hydrogen peroxide was identified as the reaction product. The molybdenum complexes are more difficult to reduce than their corresponding tungsten complexes, and the values of k obtained for the molybdenum and tungsten series of complexes increase as the ene-1,2-dithiolate ligand becomes more electron-withdrawing (S2C6H4(2-) > S2C6H3CH3(2-) > S2C2(CH3)2(2-)). This investigation constitutes the only well-established interaction between hydroxide ion and a tris(dithiolene) complex, and supports a highly covalent bonding interaction between the metal and the hydroxide ion that modulates electron transfer reactions within these complexes.

Structure and electrochemical properties of the new triply bridged molybdenum(V) complex [NBun4]2[Mo2O2(µ-O)(SCH2CO2)2(µ-SCH2CO2)]

The reaction of molybdate ions with an excess of thioglycolic acid has been studied in aqueous solution at room temperature by UV/VIS and NMR spectroscopy. As previously observed, spectroscopic data suggest initial formation of the complex [MoO2(SCH2CO2)2]2–1, which is subsequently reduced by the excess of acid. Two molybdenum(V) complexes are formed in equilibrium in this redox reaction. The main product has been isolated as an orange-red solid with the formula [NBun4]2[Mo2O3(SCH2CO2)3]·2H2O 2. It crystallizes in the monoclinic space group P21/a with a= 20.761 (8), b= 17.441(7), c= 14.587(6)A, β= 101.08(6)°, and Z= 4. The structure contains two cofacial MoVO2(SCH2CO2)2 distorted octahedra sharing one oxygen atom and one thioglycolate ligand on a pseudo-symmetry plane (Ot–Mo 1.677, Mo–Mo 2.643, Mo–S1,b 2.489, Mo–Ol,b 2.337, Mo–Ob 1.929 A, Mo–Sl,b–Mo 64.1, Mo–Ol,b–Mo 68.9, Mo–Ob–Mo 86.5°; t =terminal, b = bridging). Cyclic voltammetry shows that complexes 1 and 2 undergo a two-electron irreversible reduction at –1.27 and –1.38 V vs. the saturated calomel electrode, respectively, in methanol. In both reduction processes the monooxomolybdenum(IV) species [MoO(SCH2CO2)2]2– and [MoO(SCH2CO2)(solv)2] are generated. A reaction mechanism for the oxidation of thioglycolic acid by molybdate ions is proposed from the combined analysis of spectrophotometric, NMR, structural, and voltammetric data. Dimeric molybdenum(V) products are generated from reaction of the above monooxomolybdenum(IV) species with the starting dioxomolybdenum(VI) complex 1. A particularly remarkable supporting feature is the formation of Me2S when the reaction is carried out in the presence of Me2SO.