Israel Martyr Mbomekalle

Iron Polyoxometalate Single‐Molecule Magnets

Iron sandwich on a tungstate bun: Two new polyoxotungstates with paramagnetic iron(III) heteroatoms (see structure, W blue, Fe yellow, O red) possess S=15/2 and S=5 ground states. Both compounds are single-molecule magnets, and the hexairon species shows large hysteresis (see picture) and quantum tunneling effects at low temperature. Electrochemical studies indicate that these species are stable in solution for a wide range of pH values.

Structural, Magnetic, EPR, and Electrochemical Characterizations of a Spin-Frustrated Trinuclear CrIII Polyoxometalate and Study of Its Reactivity with Lanthanum Cations

The asymmetric Cr(III) polyoxometalate complex Cs(10)[(gamma-SiW(10)O(36))(2)(Cr(OH)(H(2)O))(3)] x 17 H(2)O (1) has been synthesized in water under atmospheric pressure from the trinuclear precursor [Cr(3)(CH(3)COO)(7)(OH)(2)] and the divacant ligand [gamma-SiW(10)O(36)](8-). Complex 1 is built up of two [gamma-SiW(10)O(36)](8-) Keggin units sandwiching a trinuclear {(Cr(III)(OH)(H(2)O))(3)} fragment where the paramagnetic centers are bridged by three mu-OH ligands forming a nearly isosceles triangle. The magnetic properties of this spin-frustrated system have thus been interpreted considering a 2-J Hamiltonian showing that the Cr(III) ions are antiferromagnetically coupled and that 1 possesses an S = 3/2 ground state with an S = 1/2 first excited state located at 11 cm(-1). These results have been confirmed by EPR spectroscopy measurements (Q-band), which have also enabled the quantification of the electronic parameters characterizing the quadruplet spin ground state. The magnitude of the magnetic exchange interactions and the nature of the ground state are discussed in light of previously reported isosceles triangular S = 3/2 clusters. UV-visible and electrochemical studies have shown that 1 is stable in aqueous media in a 1-7 pH range. This stability is chemically confirmed by the study of the reactivity of 1 with La(III) cations, which has allowed the isolation of the Cs(4)[(gamma-SiW(10)O(36))(2)(Cr(OH)(H(2)O))(3)(La(H(2)O)(7))(2)] x 20 H(2)O compound (2). Indeed, during the synthetic process of this 3d-4f system, the integrity of the [(gamma-SiW(10)O(36))(2)(Cr(OH)(H(2)O))(3)](10-) building unit constituting 1 is maintained despite the high oxophilic character of the La(III) ions. The single crystal X-ray diffraction study of 2 has revealed that in the solid state the rare earth cations connect these subunits, affording a 3d-4f double-chain monodimensional system.

[H4AsW18O62]7−, A novel Dawson heteropolyanion and two of its sandwich-type derivatives [Zn4(H2O)2(H4AsW15O56)2]18−, [Cu4(H2O)2(H4AsW15O56)2]18−: cyclic voltammetry and electrocatalytic properties towards nitrite and nitrate

Abstract The cyclic voltammetry behaviours of the new Dawson heteropolyanion (HPA) octadecatungstomonoarsenate [H 4 AsW 18 O 62 ] 7− were studied as a function of pH. The electrochemistry of the fully symmetrical octadecatungstodiarsenate [As 2 W 18 O 62 ] 6− in a pH 0.3 medium is constituted mainly by four reduction steps, the first two featuring reversible one-electron processes. The comparison of the cyclic voltammetry behaviours of two complexes reveals, interestingly, that the merging of the first two waves of the monoarsenate occurs in a less acidic pH medium than for the diarsenate. The voltammograms of the two corresponding trivacant species, [H 4 AsW 15 □ 3 O 56 ] 13− and [As 2 W 15 □ 3 O 56 ] 12− , in which the symbol □ represents the vacancy, were also studied and compared. The comparison was extended to the Zn- and Cu-sandwich-type derivatives, with particular emphasis on the compounds of the monoarsenate series, [Zn 4 (H 2 O) 2 (H 4 AsW 15 O 56 ) 2 ] 18− and [Cu 4 (H 2 O) 2 (H 4 AsW 15 O 56 ) 2 ] 18− . All these species prove efficient in the electrocatalytic reduction of nitrite, but only copper-substituted derivatives are active for the electrocatalytic reduction of nitrate. To our knowledge, this observation constitutes the first example of electrocatalysis of nitrate reduction by a sandwich complex of the polyoxometalate family.

Polyoxometalates Functionalized by Bisphosphonate Ligands: Synthesis, Structural, Magnetic and Spectroscopic Characterizations and Activity on Tumor Cell Lines

We report the synthesis and characterization of eight new Mo, W, or V-containing polyoxometalate (POM) bisphosphonate complexes with metal nuclearities ranging from 1 to 6. The compounds were synthesized in water by treating Mo(VI), W(VI), V(IV), or V(V) precursors with biologically active bisphosphonates H(2)O(3)PC(R)(OH)PO(3)H(2) (R = C(3)H(6)NH(2), Ale; R = CH(2)S(CH(3))(2), Sul and R = C(4)H(5)N(2), Zol, where Ale = alendronate, Sul = (2-Hydroxy-2,2-bis-phosphono-ethyl)-dimethyl-sulfonium and Zol = zoledronate). Mo(6)(Sul)(2) and Mo(6)(Zol)(2) contain two trinuclear Mo(VI) cores which can rotate around a central oxo group while Mo(Ale)(2) and W(Ale)(2) are mononuclear species. In V(5)(Ale)(2) and V(5)(Zol)(2) a central V(IV) ion is surrounded by two V(V) dimers bound to bisphosphonate ligands. V(6)(Ale)(4) can be viewed as the condensation of one V(5)(Ale)(2) with one additional V(IV) ion and two Ale ligands, while V(3)(Zol)(3) is a triangular V(IV) POM. These new POM bisphosphonates complexes were all characterized by single-crystal X-ray diffraction. The stability of the Mo and W POMs was studied by (31)P NMR spectroscopy and showed that all compounds except the mononuclear Mo(Ale)(2) and W(Ale)(2) were stable in solution. EPR measurements performed on the vanadium derivatives confirmed the oxidation state of the V ions and evidenced their stability in aqueous solution. Electrochemical studies on V(5)(Ale)(2) and V(5)(Zol)(2) showed reduction of V(V) to V(IV), and magnetic susceptibility investigations on V(3)(Zol)(3) enabled a detailed analysis of the magnetic interactions. The presence of zoledronate or vanadium correlated with the most potent activity (IC(50)~1-5 μM) against three human tumor cell lines.

High nuclearity Ni/Co polyoxometalates and colloidal TiO2 assemblies as efficient multielectron photocatalysts under visible or sunlight irradiation

Visible or sunlight irradiation of high nuclearity cobalt-or nickel-centers containing polyoxometalates (POMs), alone or in association with colloidal TiO2, results in their neat, apparently direct, multielectron reduction. These materials prove very effective in the first example of visible or sunlight driven POM-based degradation of Acid Orange 7.

Manganous heteropolytungstates. Synthesis and heteroatom effects in Wells–Dawson-derived sandwich complexes

A tetranuclear manganous Wells–Dawson sandwich-type polyoxometalate has been synthesized by the reaction of α-Na12(As2W15O56) with an aqueous solution of MnCl2·4H2O. The structure of this complex, αββα-Na16(MnIIOH2)2MnII2(As2W15O56)2·55H2O (Na1), was determined by single-crystal X-ray crystallography (a = 14.5230(12) A, b = 14.7104(13) A, c = 19.8927(17) A, α = 84.326(2)°, β = 81.709(2)°, γ = 65.584(2)°, triclinic, R1 = 6.2%, based on 26721 independent reflections) and is similar to the phosphorus analogue, αββα-Na16(MnIIOH2)2MnII2(P2W15O56)2 (Na2). Magnetization studies confirm that both Na1 and Na2 show antiferromagnetic coupling of the four Mn(II) centers. Despite the structural similarities, electrochemical studies reveal that the presence of arsenic shifts the Mn waves to more positive potentials. Catalytic studies confirm that 1 is a significantly better catalyst than 2 for the H2O2-based epoxidation of cis-cyclooctene, cyclohexene, and 1-hexene.

Pd-0@Polyoxometalate Nanostructures as Green Electrocatalysts: Illustrative Example of Hydrogen Production

Green-chemistry type procedures were used to synthesize Pd0 nanostructures encapsulated by a vanadium-substituted Wells-Dawson-type polyoxometalate (Pd0@POM). The cyclic voltammogram run with the Pd0@POM-modified glassy carbon electrode shows well-defined waves, associated with Pd0 nanostructures and the VV/VIV redox couple. The Pd0@POM-modified electrode displayed remarkably reproducible cyclic voltammetry patterns. The hydrogen evolution reaction (HER) was selected as an illustrative example to test the electrocatalytic behavior of the electrode. The kinetic parameters of the HER show the high efficiency of the Pd0@POM-modified electrode. This is the first example of electrochemical characterization of a modified electrode based on a vanado-tungstic POM and Pd0 nanostructures.

One-step synthesis and stabilization of gold nanoparticles in water with the simple oxothiometalate Na2[Mo3(μ3-S)(μ-S)3(Hnta)3]

Na2[Mo3(μ3-S)(μ-S)3(Hnta)3] serves both as a reducing and a capping agent in the synthesis of Au nanoparticles in water at room temperature in a “fully green chemistry-type process”, thus avoiding the usual two-step preparation of thiolate-monolayer-coated gold nanoparticles. The nanoparticles were characterized by TEM, DLS, UV-vis spectroscopy, XPS and XRD analyses and cyclic voltammetry.

Semi-vacant Wells–Dawson anions. Synthesis of tri-tungsten-vacant derivatives and crystallographic studies of [αββα-(CuIIOH2)2(CuII)2(AsW15(OH2)3(OH)O52)2]12−

The tri-tungsten-vacant polyoxometalate, [alpha-AsW15(OH)4O52]13-, derived from the semi-vacant Wells-Dawson complex [alpha-AsW18(OH)4O58]7-, reacts with the late-transition metal cations, Cu(II) or Zn(II), to form sandwich-type species; the X-ray crystal structure of [alphabetabetaalpha]-(Cu(II)OH2)2(Cu(II))2(AsW15(OH2)3(OH)O52)2]12-, prepared by the acidification of [alphabetabetaalpha]-(Cu(II)OH2)2(Cu(II))2(AsW15(OH)4O52)2]18-, reveals that the missing heteroatoms are distal to the central Cu4 unit and the vertices of the vacant tetrahedron are occupied by one OH- and three OH2 groups.

Direct and improved synthesis of the tri-nickel sandwich-type polyoxoanion [Ni3Na(H2O)2(PW9O34)2]11−

Abstract The direct use of preformed B,α-Na 9 PW 9 O 34 was successful in improving the synthesis of Na 11 [Ni 3 Na(H 2 O) 2 (PW 9 O 34 ) 2 ]·14H 2 O. The complex was characterized by elemental analysis, IR spectroscopy, magnetic properties and cyclic voltammetry. The yield was increased to 30%, which is four times more than that obtained in the initial synthesis. The delay before the appearance of the first crystals is drastically shortened, from months to hours, and these crystals are predominantly those of the yellow compound of interest. Keys to these improvements are the use of freshly prepared B,α-Na 9 PW 9 O 34 through prolonged heating of A,β-Na 9 PW 9 O 34 and the increase of the concentrations of the reactants in a close-to-neutral pH medium. Starting the synthesis with several concentrations of preformed A,β-Na 9 PW 9 O 34 revealed the importance of the concentrations of reactants in speeding up the formation of crystals and suggested strongly that the [A-PW 9 O 34 ] 9− →[B-PW 9 O 34 ] 9− isomerization might be the slow step in the whole process. Cyclic voltammograms as well as X-ray diffraction structure determination confirm that the synthesized compound is indeed Na 11 [Ni 3 Na(H 2 O) 2 (PW 9 O 34 ) 2 ]·14H 2 O. A study of the magnetic properties of Na 11 [Ni 3 Na(H 2 O) 2 (PW 9 O 34 ) 2 ]·14H 2 O indicates that the three nickel centres exhibit ferromagnetic exchange interactions.