Paramagnetic Polyoxometalate Research Articles

Polyoxometalate-based high-spin cluster systems: a NMR relaxivity study up to 1.4 GHz/33 T.

Paramagnetic polyoxometalates [RE30Co8Ge12W108O408(OH)42(OH2)30]56- (Rare Earth (RE): Gd, Dy, Eu, and Y) are of special interest with regard to their application as alternative contrast agents in non-human magnetic resonance imaging which is increasingly used in materials science and process engineering. This class of new paramagnetic materials promises detailed findings in the magnetic resonance images due to their rather large total electron spin on the one hand, i.e. large, field-dependent relaxivities up to the highest magnetic fields, and due to their relatively large cluster sizes with an impact on adsorption and penetration on the other hand. Apart from the magnetic field dependence, the sensitivity of relaxivities to motional correlation times will be shown for these polyoxometalates which is a prerequisite for modelling and understanding the physical behaviour of this new class of polyoxometalates in MRI. Also for the qualitative and quantitative interpretation of MR images, the knowledge of transverse and longitudinal relaxivities of the paramagnetic clusters in a given environment is mandatory. Examples considered in this publication are proteins in milk fractionation, the deposit of which was measured by MRI.

A Dendritic Supramolecular Complex as Uniform Hybrid Micelle with Dual Structure for Bimodal In Vivo Imaging

In this study, a uniform complex dual micelle bearing a gadolinium-substituted polyoxometalate core, a hydrophobic middle, and a biocompatible organic shell, which exhibits a uniform size and definite molecular weight and chemical composition, is constructed through a simple two-step process for use as a fluorescence and magnetic resonance imaging (MRI) contrast agent. A paramagnetic polyoxometalate cluster, K13 [Gd(β2 -SiW11 O39 )2 ], is first encapsulated with a cationic 3-generation dendron (D-3) bearing triethylene glycol monomethyl ether terminal groups through ionic substitution. The formed electrostatic complex, (D-3)13 [Gd(β2 -SiW11 O39 )2 ] (D-GdSiW), which displays an ambivalent micellar structure, is then demonstrated to act as a suitable carrier for the uptake of cationic fluorescent dyes, forming a loaded complex. The dye-loaded complex D-GdSiW-D is thus proven to display high relaxivity as a contrast agent, maintaining its structural stability under physiological conditions with low cell cytotoxicity. Confocal laser scanning microscopic observation also demonstrates the fluorescence imaging property of the prepared complex upon consumption by MCF-7 cells. Both in vivo fluorescence imaging and MRI further confirm that the complex micelle with dual micellar structure may be a suitable liver-specific contrast agent for clinical diagnosis.

Solution and solid-state 31P NMR study of paramagnetic polyoxometalates

Abstract The “lacunary” sandwich polyoxometalate αβ-[Co 3 Na(H 2 O) 2 (P 2 W 15 O 56 ) 2 ] 17− was characterized by solution and solid-state 31 P NMR. The solution 31 P NMR spectrum shows two relatively broad and two narrow signals, assigned to P atoms close and far from the cobalt paramagnetic nuclei, respectively. Solid-state 31 P NMR spectra enable to obtain supplementary data, like chemical shift anisotropy values. The CSA of P atoms' nuclei close to the metallic centres are very large (∼3300 ppm) as compared to those of P nuclei far from the metallic centres (∼500 and 700 ppm). Cumbersome temperature effects were observed at high spinning speeds. In order to quantify these effects, NMR experiments at variable temperature were performed on another paramagnetic polyoxometalate α 2 -[Ce(III)(P 2 W 17 O 61 )] 7− .

Photochromic Molecular Material Based on Anderson–Evans Polyoxometalate $$\hbox{[}\hbox{Cr}\hbox{(OH)}_{6}\hbox{Mo}_{6}\hbox{O}_{18}\hbox{]}^{3-}$$ and Ruthenium Mononitrosyl Complexes $$\hbox{[}\hbox{RuNO}\hbox{(}\hbox{NH}_{3}\hbox{)}_{4}\hbox{(X)}\hbox{]}^{n+}\hbox{(}\hbox{X}=\hbox{NH}_{3}, \hbox{OH}; n=3, 2\hbox{)}$$

The synthesis and characterization of two novel complexes formed by the photochromic mononitrosyl cations \([\hbox{RuNO}(\hbox{NH}_{3})_{5}]^{3+}\) and \([\hbox{RuNO}(\hbox{NH}_{3})_{4}\hbox{OH}]^{2+}\), and the paramagnetic polyoxometalate (POM) anion \([\hbox{Cr}(\hbox{OH})_{6}\hbox{Mo}_{6}\hbox{O}_{18}]^{3-}\) are reported. Compound \([\hbox{RuNO}(\hbox{NH}_{3})_{4}\hbox{OH}]_{3}[\hbox{Cr}(\hbox{OH})_{6} \hbox{Mo}_{6}\hbox{O}_{18}]_{2}\cdot15\hbox{H}_{2}\hbox{O}\) (1) was characterized by X-ray diffraction, it crystallizes in the triclinic space group, P −1, a = 10.5858(2), b = 11.0407(2), c = 16.8077(4) A, \(\alpha=76.9192(7), \beta = 73.7736(7), \gamma = 84.090(1)^{\circ}\), R=0.0636 for 16046 reflections with I \( > 2 \sigma\) (I). Compound 1 and compound \([\hbox{RuNO}(\hbox{NH}_{3})_{5}][\hbox{Cr}(\hbox{OH})_{6}\hbox{Mo} _{6}\hbox{O}_{18}]\cdot8\hbox{H}_{2}\hbox{O}\), (2) are paramagnetic in the whole temperature range 2–300 K. The metastable state (MSI) was detected in 1 by differential scanning calorimetry (DSC). The decay curve shows two peaks in the heat release at about 245 and 265 K with activation energies and frequencies values equal to 0.72 eV and 3 × 1011 s−1 and 0.67 eV and 7 × 1011 s−1 for the high- and low-temperature peak, respectively [see F. Bottomley (J. Chem. Soc. Dalton Trans. 1600, 1997)].

1H NMR study of pyridine-type ligands coordinated to the paramagnetic polyoxometalates, [Co IIW 11MO 39] n− (M=Co II, Ni II, Cu II, or Fe III)

1H NMR spectra of pyridine, 3-, and 4-methylpyridine coordinated to the paramagnetic polyoxometalates, [Co IIW 11Co IIO 39] 8− (CoCo), [Co IIW 11Ni IIO 39] 8− (CoNi), [Co IIW 11Cu IIO 39] 8− (CoCu), and [Co IIW 11Fe IIIO 39] 7− (CoFe) are reported. For these complexes the ligand exchange is slow on the NMR time-scale, and separate resonance lines have been observed for the complexes and the free ligands. Since no NMR signal is observed for [SiW 11Cu II(ptl)O 39] 6− and [SiW 11Fe III(ptl)O 39] 5− (ptl=pyridine-type ligand), appearance of good NMR spectra for the CoCu and CoFe complexes indicates that magnetic coupling with the fast relaxing Co II ion shortens the electronic relaxation times of the Cu II and Fe III ions by one or two orders of magnitude. The isotropic shifts for the CoNi and CoCu complexes parallel those of the [SiW 11Ni IIO 39] 6− (SiNi) complexes which come mainly from the contact shifts. CoNi and CoCu complexes, which exhibit sharper NMR lines than SiNi complexes, may be useful in measuring the contact shifts for complex systems. The isotropic shifts for the CoCo and CoFe complexes contain both contact and pseudocontact shifts. Analysis of the isotropic shifts for the CoCo complex of pyridine shows that exchange coupling between the two Co II ions reduces the contact shifts and reverses the signs of the pseudocontact shifts.

Preparation, Crystal Structures, and Magnetic and ESR Properties of Molecular Assemblies of Ferrocenium Derivatives and Paramagnetic Polyoxometalates

The synthesis, X-ray crystal structures, and ESR and magnetic properties of the anhydrous salt Na3[Cr(OH)6Mo6O18]·6(CH3)2SO (CrDMSO) containing the Anderson−Evans polyanion, of two new molecular compounds based on this polyanion ([Fe(C5Me5)2]3[Cr(OH)6Mo6O18]·20H2O [FeCr(1)], Na[Fe(C5H5)2]2[Cr(OH)6Mo6O18]·3H2O [FeCr(2)]), and of another based on a Keggin polyanion, [Fe(C5Me5)2]4[HPCu(H2O)W11O39]·6CH3CN (FeCu), are reported. CrDMSO: monoclinic, C2/c, a = 25.37(2) A, b = 14.632(2) A, c = 15.455(6) A, β = 123.17(4)°, Z = 4. FeCr(1): triclinic, P1, a = 12.489(3) A, b = 14.113(3) A, c = 15.662(4) A, α = 101.55(2)°, β = 105.82(2)°, γ = 110.23(2)°, Z = 1. FeCr(2): monoclinic, P21/n, a = 10.015(2) A, b = 7.722(6) A, c = 25.137(5) A, β = 90.626(9)°, Z = 2. FeCu: monoclinic, P21/n, a = 15.596(7) A, b = 15.138(3) A, c = 25.921(12) A, β = 97.53(2)°, Z = 2. In the compound FeCr(1) the Fe(C5Me5)2 units are perpendicular to each other and form a centrosymmetrical group of three cations, which alternates with the pla...

Coexistence of Mobile and Localized Electrons in Bis(ethylene)dithiotetrathiafulvalene (BEDT‐TTF) Radical Salts with Paramagnetic Polyoxometalates: Synthesis and Physical Properties of (BEDT‐TTF)8[CoW12O40]· 5.5 H2O

Alternating layers of radicals of the π-electron donor 1 and of polyoxonions [CoW12O40]6− (2, polyhedral representation) are contained in the title compound. Thus, organic charge carriers and inorganic electron-acceptor components lie next to each other. Since the polyoxometalates can accommodate different metal ions at specific sites, such anions are attractive precursors for the preparation of radical cation salts with new electrical and magnetic properties.