Françoise Villain

Molecules to build solids: high TC molecule-based magnets by design and recent revival of cyano complexes chemistry

Abstract This paper was presented as a session lecture at the XXXIII ICCC in Florence (29 August–4 September 1998). It intends to point out some recent achievements in the chemistry and physics of transition metal polycyanides in the field of molecular magnetism. Prussian blue is sometimes considered as the first coordination compound and the paper shows how it is possible to obtain brand new results with Prussian blue analogues when looking at these antique systems with fresh eyes. Hexacyanometalates revealed in the last few years as very flexible molecular precursors to build three-dimensional molecule-based magnets with tunable and high Curie temperatures or to grow high nuclearity clusters with tunable high spins and anisotropy. The use of a localized electron orbital model allowed the authors’ team to push the Curie temperatures from 5.6 K in the Prussian blue itself to above room temperature in a vanadium–chromium Prussian blue analogue. Several groups confirmed the result and are improving it. In the same way, high spin molecules with ground spin states ranging from S=3/2 to 27/2 were obtained. The paper reviews some of the steps which lead to these spectacular findings and some of the prospects opened in molecular materials by this revival of polycyanide chemistry.

Metastable solid solutions in the system ZnOCoO: synthesis by hydrolysis in polyol medium and study of the morphological characteristics

Abstract Metastable solid solutions in the system ZnOCoO have been synthesized by hydrolysis of ionic zinc and/or cobalt salts in polyol medium. The solubility of Co in the zincite oxide was significantly increased (at.% Co=65) compared to that obtained under thermodynamical equilibrium (at.% Co=6.5 at 800°C). CoO was also obtained via this route. The products are made up of sub-micrometer particles. A detailed study of their morphology along with pure ZnO particle morphology was conducted. Different growth mechanisms are evidenced.

Structural studies on TeO2-PbO glasses

Abstract Binary tellurite-based glasses in the TeO 2 –PbO system were prepared and its structure investigated by means of Raman Scattering and X-ray Absorption Spectroscopy. Both spectroscopies indicate strong modifications of the first coordination shell around tellurium atoms when the PbO content increases revealing for lead its glassy network modifier role. Also, Pb L 3 -edge EXAFS measurements reveal this structural role played by lead atoms, but the presence of a medium range order contribution indicates that lead also participates to the glassy network formation.

Size-dependent magnetic properties of CoFe2O4 nanoparticles prepared in polyol.

Highly crystalline CoFe(2)O(4) nanoparticles with different diameters ranging from 2.4 to 6.1 nm have been synthesized by forced hydrolysis in polyol. The size can be controlled through adjusting the nominal water/metal molar ratio. X-ray diffraction, transmission electron microscopy, x-ray absorption spectroscopy and (57)Fe Mössbauer spectrometry were employed to investigate the structure and the microstructure of the particles produced. Magnetic measurements performed on these particles show that they are superparamagnetic with a size-dependent blocking temperature. At 5 K, high saturation magnetization (~85 emu g(-1)) approaching that of the bulk was found for the larger particles, whereas a very large coercivity (14.5 kOe) is observed for the 3.5 nm sized particles.

Synthesis and structural investigations on TeO2-PbF2-CdF2 glasses and transparent glass-ceramics

Abstract New glasses have been prepared in the oxifluoride mixed system TeO 2 -PbF 2 -CdF 2 . Starting from pure TeO 2 the addition of the fluorides leads to a decrease in the glasses characteristic temperatures. Also from Raman scattering results a structural evolution was observed where the number of structural units described as [TeO 3 ] trigonal pyramids and [TeO 3+1 ] polyhedra increases at the expense of the [TeO 4 ] trigonal bipyramids supposed to exist in the TeO 2 rich samples. Transparent glass ceramics were obtained from the glass with composition 80TeO 2 -10PbF 2 -10CdF 2 (mol%) with the PbTe 3 O 7 crystalline phase being identified by X-ray diffraction and EXAFS measurements performed at the Te K, Cd K and Pb L III edges. Also from Exafs measurements it is proposed that cadmium ions are preferentially surrounded by oxygen atoms although they were in a fluoride anion environment in the starting material.

Free radical production by hydroxy-salen manganese complexes studied by ESR and XANES.

Three salen-Mn(II) complexes bearing hydroxyl groups in either the ortho, para or meta positions have been synthesized and the structures of the metal complexes and their potential to produce free radicals investigated by electron spin resonance (ESR) and X-ray absorption near edge structures (XANES) spectroscopy. All three compounds were shown to generate a high level of superoxide anions in dimethyl sulfoxide (DMSO) solution. The production of oxygen radicals results from a one electron process oxidation of Mn(II) species leading to the formation Mn(III) redox state species, as revealed by a higher XANES edge energy of 2.7 eV. The formation of superoxide anion was characterized by ESR, both directly and via the use of a spin-trapping method. Under reductive condition in the presence of ascorbic acid, the reduction of Mn(III) to Mn(II) leads to the production of hydroxyl radicals by the ortho and para compounds. The efficient production O(2)*- by such salen-Mn complexes could be useful to evaluate the scavenging properties of antioxidant molecules.

Vicinal Dinitridoruthenium‐Substituted Polyoxometalates γ‐[XW10O38{RuN}2]6− (X=Si or Ge)

Reaction of the divacant polyoxometalate K(8)[gamma-XW(10)O(36)] (X = Si, Ge) with two equivalents of the metal-nitrido precursor Cs(2)[Ru(VI)NCl(5)], at room temperature in water, produces K(2)(Me(2)NH(2))(2)H(2)[gamma-XW(10)O(38){RuN}(2)], X = Si (DMA-1 a) or Ge (DMA-1 b). The X-ray crystal structures of both complexes show monomeric complexes with highly unusual vicinal terminal metal-nitrido units. The Ru[triple bond]N bond lengths are 1.594(10) and 1.612(11) A in 1 a and 1 b, respectively. EXAFS studies confirmed the key structural assignments from X-ray crystallography. The XANES spectrum of DMA-1 a, diamagnetism, NMR ((29)Si and (183)W) chemical shifts, voltammetric behavior, reductive titrations with [PW(12)O(40)](4-), and computational data are all consistent with d(2) Ru(VI) centers in these complexes. The FT-IR and Raman spectra show the expected vibrational modes of the {gamma-XW(10)} unit and the Ru[triple bond]N stretch at 1080 cm(-1), respectively. Interestingly, reduction of DMA-1 a by 4 equivalents of [PW(12)O(40)](4-) produces NH(3) in nearly quantitative yield. Cyclic voltammetry versus pH and calculations provide the energetics for the possible two-electron reduction and two-proton addition processes in this reaction.

In situ characterization of the coordination sphere of CuII complexes supported on silica during the preparation of Cu/SiO2 catalysts by cation exchange

In situ diffuse reflectance UV/visible/near-IR and EPR spectroscopies, and also XAFS, were used to study the changes in the coordination sphere of copper complexes during the different steps of preparation by cation exchange of supported Cu/SiO2 catalysts. Three different CuII precursors were used: [Cu(NH3)4(H2O)2]2+, [Cu(en)1(H2O)4]2+ and [Cu(en)2(H2O)2]2+. Before drying, the coordination sphere of the different CuII complexes remains the same as in solution, and the complexes are in electrostatic interaction with the silica support. Depending on the strength of the ligands, the CuII complexes are grafted onto the silica support as (SiO)2Cu(NH3)2(H2O)4 or (SiO)2Cu(en)1(H2O)2, or remain in electrostatic interaction with the latter as [Cu(en)2(H2O)2]2+ when the samples are dried at 25°C. Extended drying at 100°C and re-exposure to air induces weak loss of NH3 and en ligands, and some water ligands are reversibly eliminated. When the samples are calcined, the NH3 and en ligands are completely eliminated, and the same three CuII species are identified on the three samples: (SiO)2CuII(H2O)4−x, (SiO)3CuII and pairs of octahedral CuII species. On exposure to air, the three species transform into a single species: (SiO)2CuII(H2O)4.

Solid-state magnetic switching triggered by proton-coupled electron-transfer assisted by long-distance proton-alkali cation transport.

Acidity of water molecules coordinated to Co ions in CoFe Prussian blue analogues (PBA) has been used to reversibly activate the Co(III)Fe(II) ↔ Co(II)Fe(III) electron transfer. The study of the structure and the electronic structure shows that the process implies an original PCET reaction between a solid-state porous coordination polymer and hydroxide ions in solution. The PCET reaction spreads throughout the solid network thanks to a long-range H(+) and Rb(+) transport within the pore channels of PBA taking advantage of the hydrogen-bonding network of zeolitic water molecules acting as proton wires.

Photo-Induced Electron Transfer and Magnetic Switching in CoFe Cyanides: Study of the Metastable State

Abstract Photo-induced magnetisation has been recently evidenced by Hashimoto et al. in CoFe cyanide[1]. We synthesized a diamagnetic RbFeCo cyanide able to present the same photomagnetic effect. The magnetic properties of the excited state, the electronic structure and the local structure of the ground and the excited states have been investigated. The conditions required to observe the phenomenon and the mechanism of the electron transfer are discussed.