Rhenium Oxidation State Research Articles

Direct Spectroscopic Confirmation of the Oxygen‐Centered Diradical Character of the Tetraoxidorhenium(VII) Cation [Re(O)4]+

AbstractMononuclear inorganic diradical species are scarce. Here, we confirm, via X‐ray absorption spectroscopy in the gas phase combined with computational studies, the oxygen‐centered diradical character of the tetraoxidorhenium(VII) cation. A dioxido‐superoxido isomer, close in energy to the diradical, is also found, where rhenium appears in its rare oxidation state of +6. Addition of one or two hydrogen atoms to [Re,O4]+ forms hydroxido ligands, and strongly disfavors isomers with any oxygen‐oxygen bond. This adds spectroscopic characterization of the rhenium oxidation state and the nature of ligands to the known ability of [Re,O4]+ to perform two consecutive hydrogen‐atom abstraction reactions from methane, and demonstrates that pentaatomic [Re,O4]+ combines a metal center in its highest oxidation state with two oxygen‐centered radical ligands in a highly reactive species.

The Magnetism of Nonstoichiometric Sr2Cr1+xRe1-xO6 (0 < x < 0.5) Double Perovskites.

The effect of nonstoichiometry on the cation distribution, crystal structure, and magnetic properties of a series of Cr-rich Sr2Cr1+xRe1-xO6 samples has been investigated. The double perovskite structure is maintained over a wide solid solution range that extends from x = 0 to approximately x = 0.5. For most of the solid solution range, the Cr-rich octahedral site maintains a nearly constant occupancy, 87% Cr and 13% Re, that is comparable to prior studies of Sr2CrReO6, while Cr steadily replaces Re on the other octahedral site. As x approaches 0.5, long-range Cr/Re ordering drops precipitously. Analysis of X-ray powder diffraction peak shapes reveals antiphase boundaries, associated with Cr/Re ordering, the concentration of which increases steadily with increasing x. Neutron powder diffraction studies confirm antiferromagnetic coupling between antisite Cr3+ ions and Cr3+ ions that occupy the normal sites, leading to site-dependent ferrimagnetic ordering. Density functional theory calculations indicate that chromium maintains a +3 oxidation state across the series, while the oxidation state of rhenium increases with increasing x. Calculations are also used to explore the energies of competing magnetic ground states. Except for the most chromium-rich compositions (x ≈ 0.5), site-dependent ferrimagnetism is retained with only a modest reduction in TC. The saturation magnetization steadily decreases as the chromium content increases due to a combination of Cr/Re antisite disorder and antiphase boundaries.

K3[MO4][MO3N] (M=Tc, Re) – Nitridotrioxidorhenate and ‐technetate from Highly Alkaline Media

AbstractThe reactions of ammonium perrhenate and pertechnetate in highly alkaline medium led to the isotypic mixed anionic nitridotrioxidorhenate and ‐technetate K3[MO4][MO3N] (M= Tc, Re). Both compounds occur as colorless crystals, which were investigated by single crystal X‐ray diffraction. Furthermore, K3[ReO4][ReO3N] has been studied by means of X‐ray photoelectron spectroscopy to determine the oxidation state of rhenium. The obtained results have been complemented by magnetic measurements. IR and Raman spectroscopy indicated the presence of Re−O as well as Re−N bonds.

Tailoring of rhenium oxidation state in ReOx thin films during reactive HiPIMS deposition process and following annealing

Bulk rhenium trioxide (ReO3) has an unusually high electrical conductivity and, being nanosized, has promising catalytic properties. However, the production of pure ReO3 thin films is challenging due to the difficulty to stabilize rhenium in a 6+ oxidation state. Here we present a novel approach for the deposition of ReOx (x ≈ 1.6–2.9) thin films using reactive high power impulse magnetron sputtering (r-HiPIMS) from a metallic rhenium target in a mixed Ar/O2 atmosphere. The thin films were deposited in the gas-sustained self-sputtering regime, observed during r-HiPIMS process according to current waveforms. The influence of the substrate temperature, the oxygen-to-argon flow ratio and post-annealing at 250 °C in the air for 3 h on the properties of the films were studied. The as-deposited films have an X-ray amorphous structure (a-ReOx) when deposited at room temperature while a nano-crystalline β-ReO2 phase when deposited at elevated temperatures (150 or 250 °C). The amorphous a-ReOx can be converted into the crystalline ReO3 with a lattice parameter of 3.75 Å upon annealing in the air. The surface morphology of the films is dense without detectable voids when elevated substrate temperatures are used. Various Re oxidation states are observed on the surface of the films in different ratios depending on the deposition parameters. All samples exhibit electrical resistivity on the order of 10−3 Ωcm and optical properties typical for thin metallic films.

Immobilization of Rhenium as a Technetium Surrogate in Aluminum Iron Phosphate Glass

Samples of sodium aluminum iron phosphate glass were synthesized containing rhenium as a surrogate of technetium from radioactive waste. The phase composition, structure and water resistance of the obtained glasses were investigated. It was shown that the samples with the rhenium oxide inclusion up to 2.88 wt % are X-ray amorphous and homogeneous, and their anionic motif corresponds to the glassy one. It was found that the oxidation state of rhenium in the obtained glasses is Re(VII), and 97% of iron is in the form of Fe(III) and 3% in the form of Fe(II). The glass was established to be highly resistant to leaching at 90°C. The rate of Re leaching from glasses in accordance with the international product consistency test (PCT) and the semi-dynamic test of State Standard GOST R 52126-2003 is no more than 6 × 10–6 and 3 × 10–6 g/(cm2 day), respectively.

Heterobimetallic coordination polymers involving 3d metal complexes and heavier transition metals cyanometallates

The results of the first steps in the design of coordination polymers based on penta- and heptacyanometallates of heavier d transitions metals are presented. The 2D structure of the coordination polymers: [{Mn(acacen)}2Ru(NO)(CN)5]n and two complexes composed of different cyanorhenates, [Ni(cyclam)]2[ReO(OH)(CN)4](ClO4)2(H2O)1.25 and [Cu(cyclam)]2[Re(CN)7](H2O)12, was confirmed by single crystal XRD study, the rhenium oxidation state having been proved by the magnetic measurements. An amorphism of [M(cyclam)]3[Re(CN)7]2 (M=Ni, Cu) polymers does not allow to define strictly their dimensionality and to model anisotropic magnetic behavior of the compounds. However, with high probability a honey-comb like layer structure could be expected for [M(cyclam)]3[Re(CN)7]2 complexes, studied in this work, because such an arrangement is the most common among the bimetallic assemblies of hexa- and octacyanometallates with a ratio [M(cyclam)]/[M(CN)n]=3/2. For the first time was prepared and fully characterized a precursor (n-Bu4N)2[Ru(NO)(CN)5], soluble in organic media.

Redox-Active Ligands Facilitate Bimetallic O2Homolysis at Five-Coordinate Oxorhenium(V) Centers

Five-coordinate oxorhenium(V) anions with redox-active catecholate and amidophenolate ligands are shown to effect clean bimetallic cleavage of O(2) to give dioxorhenium(VII) products. A structural homologue with redox-inert oxalate ligands does not react with O(2). Redox-active ligands lower the kinetic barrier to bimetallic O(2) homolysis at five-coordinate oxorhenium(V) by facilitating formation and stabilization of intermediate O(2) adducts. O(2) activation occurs by two sequential Re-O bond forming reactions, which generate mononuclear eta(1)-superoxo species, and then binuclear trans-mu-1,2-peroxo-bridged complexes. Formation of both Re-O bonds requires trapping of a triplet radical dioxygen species by a cis-[Re(V)(O)(cat)(2)](-) anion. In each reaction the dioxygen fragment is reduced by 1e(-), so generation of each new Re-O bond requires that an oxometal fragment is oxidized by 1e(-). Complexes containing a redox-active ligand access a lower energy reaction pathway for the 1e(-) Re-O bond forming reaction because the metal fragment can be oxidized without a change in formal rhenium oxidation state. It is also likely that redox-active ligands facilitate O(2) homolysis by lowering the barrier to the formally spin-forbidden reactions of triplet dioxygen with the closed shell oxorhenium(V) anions. By orthogonalizing 1e(-) and 2e(-) redox at oxorhenium(V), the redox-active ligand allows high-valent rhenium to utilize a mechanism for O(2) activation that is atypical of oxorhenium(V) but more typical for oxygenase enzymes and models based on 3d transition metal ions: O(2) cleavage occurs by a net 2e(-) process through a series of 1e(-) steps. The implications for design of new multielectron catalysts for oxygenase-type O(2) activation, as well as the microscopic reverse reaction, O-O bond formation from coupling of two M=O fragments for catalytic water oxidation, are discussed.

甲基三氧化铼在MCM-41中的组装及其对烯烃环氧化反应的催化性能

MCM-41 functionalized by propyl amino group was obtained by the reaction of bis(3-(triethoxysilyl) propyl)amine with MCM-41, which further reacted through the amino group supported on MCM-41 with 2-pyridinecarboxaldehyde to afford the MCM-41 modified by a di-nitrogen cheland. The cheland anchored on MCM-41 coordinated with methyltrioxorhenium (MTO) to bring about assembling of MTO on MCM-41. XPS analysis displayed that the oxidation state of rhenium is mainly +7 in the material. FT-IR and DR UV-vis analysis shows that the cheland was combined with MTO by coordination bonds. XRD, TEM and nitrogen adsorption analysis results show that the hexagonal symmetry structure in single dimension of MCM-41 was maintained after the assembling, however, the surface area, pore volume and pore diameter of the material changed obviously. In addition, the composition of C, H, N and Re was analyzed by elemental analysis and atomic adsorption analysis. The catalytic performances of the material on the epoxidation of olefins with hydrogen peroxide (30%) and urea hydrogen peroxide adduct (UHP) as the oxidants were investigated, separately. When the material was applied to the epoxidation of cyclohexene with UHP as oxidant at 25°C, the conversion of cyclohexene and selectivity of epoxide reached 50% and 99%, respectively. However, in the case of hydrogen peroxide (30%) as the oxidant at the same conditions, the conversion of cyclohexene and selectivity of epoxide were only 40% and 20%, respectively. The low catalytic activity is due to the low transfer velocity of the substrate in the matrix of the material and the decomposition of the supported complex under the epoxidation conditions.

Rhenium complexes and clusters supported on γ-Al 2O 3: Effects of rhenium oxidation state and rhenium cluster size on catalytic activity for n-butane hydrogenolysis

Supported metals prepared from H 3Re 3(CO) 12 on γ-Al 2O 3 were treated under conditions that led to various rhenium structures on the support and were tested as catalysts for n-butane conversion in the presence of H 2 in a flow reactor at 533 K and 1 atm. After use, two samples were characterized by X-ray absorption edge positions of approximately 5.6 eV (relative to rhenium metal), indicating that the rhenium was cationic and essentially in the same average oxidation state in each. But the Re–Re coordination numbers found by extended X-ray absorption fine structure spectroscopy (2.2 and 5.1) show that the clusters in the two samples were significantly different in average nuclearity despite their indistinguishable rhenium oxidation states. Spectra of a third sample after catalysis indicate approximately Re 3 clusters, on average, and an edge position of 4.5 eV. Thus, two samples contained clusters approximated as Re 3 (on the basis of the Re–Re coordination number), on average, with different average rhenium oxidation states. The data allow resolution of the effects of rhenium oxidation state and cluster size, both of which affect the catalytic activity; larger clusters and a greater degree of reduction lead to increased activity.

Synthesis and crystal structure of a new rhenium scandium oxide, Sc6ReO12

A number of Ln6MO12 phases, where Ln = rare earth cation and M = Mo, W, Re or U, crystallizes in a distorted rhombohedral fluorite structure with space group R-3 [1, 2]. The same crystal structure is observed for Sc6MO12 compounds, where M = Mo and W [3]. In our work a new complex rhenium scandium oxide, Sc6ReO12, has been synthesized as a single phase in a sealed silica tube and by high-pressure high-temperature synthesis, and its crystal structure has been determined by single crystal X-ray diffraction. This compound crystallizes in a rhombohedral structure (space groupR-3, a= 9.248(2) A, c = 8.720(2) A, Z = 3). Figure 1 demonstrates the dependence of the cell parameters ’’a’’ and ’’c’’ on the ionic radius of the trivalent cations in the isostructural compounds Ln6ReO12 (Ln = Sc, Ho, Er, Tm, Yb and Lu). This structure represents a three-dimensional framework formed by ReO6 and ScO7 polyhedra connected via edges and corners. ReO6 octahedra are undistorted, the Re-O interatomic distance dRe-O= 1.926 A is in a good agreement with the Re-O distances observed for other Re containing oxides with a formal rhenium oxidation state +6. ScO7 polyhedra represent capped prisms with six close oxygens (dSc-O = 2.027 2.230 A, in agreement with average Sc-O distances in other Sc-containing oxides), and one more distant oxygen (dSc-O = 2.67 A), which is included in the coordination sphere due to calculated bond-valence parameters. The synthesis of Sc6ReO12 under high pressure indicates the stability of its structure up to 50 kbar and 1300 C. The magnetic properies of Sc6ReO12 were also investigated.

Re 4As 6S 3, a thio-spinel-related cluster system

We have synthesized a new compound with formula Re 4As 6S 3 and characterized its crystal structure by Rietveld powder diffraction methods. Re 4As 6S 3 crystallizes in an face-centered cubic unit cell, space group F 4 ̄ 3m (no. 216), with lattice constant a=9.8608(1) Å and Z=4. The rhenium atoms form tetrahedral clusters linked via tetrahedral arsenic clusters to produce an NaCl-type arrangement. The oxidation state of rhenium is IV and the number of electrons shared by the rhenium atoms in the cluster is 12. The structure is based on an ordered defect thio-spinel A (1− x) B 2 X 4 where the B-type atoms form tetrahedral clusters.

Photoluminescence of 8-quinolinolato(trioxo)rhenium(VII)

The complex Re VIIO 3(oxinate) (oxine=8-quinolinol or 8-hydroxyquinoline) shows a green fluorescence at λ max=511 nm with φ=0.012 and a very weak red phosphorescence at λ max=645 nm. Both emissions originate from the lowest-energy ππ* intraligand state of the oxinate ligand. Owing to the d 0 electron configuration and the high oxidation state of rhenium the heavy atom effect transmitted to the π-electron system of the oxinate ligand is very weak.

Experimental determination of the solubility of ReO 2 and the dominant oxidation state of rhenium in hydrothermal solutions

The dominant oxidation state of rhenium in high-temperature, chloride-bearing hydrothermal solutions at geologically reasonable oxygen fugacities has been experimentally determined to be +4, although a small percentage of dissolved rhenium (≤10%) may be present in the +5 or higher valence state. This conclusion is based on a series of solubility experiments in 1.0 molal KCl solutions at 500°C using ReO 2, the K–feldspar–muscovite–quartz assemblage as a pH buffer, and Co–CoO, Ni–NiO, Re–ReO 2, MnO–Mn 3O 4, and MoO 2–MoO 3 pairs as oxygen fugacity buffers. Experiments were also conducted on the solubility of the Re–ReO 2 oxygen fugacity buffer in 0.5 and 1.5 m KCl solutions in equilibrium with the K–feldspar–muscovite–quartz pH buffer at 500°C. The solubility of Re increases dramatically with increasing KCl concentration under these conditions, implying the formation of a Cl-bearing Re species. Because Re +4 is a hard acid owing to its high ionic potential, relatively hard ligands such as Cl −, F −, O 2− and OH − should be important in the transport of Re during geological processes at elevated temperatures. Mixing/dilution, which may induce changes in pH and/or chloride concentration, should be important for the deposition of Re because the solubility of rhenium has a strong dependence on pH and the concentration of Cl −. Our experimental results may have implications for the hydrothermal remobilization and enrichment of rhenium in high-temperature environments, for the Re–Os isotopic system, and for the rhenium budget in the continental crust. Our study may also have bearing on the aqueous geochemistry of fissiogenic 99 Tc , for which Re is an analogue. These implications are discussed in detail.

Solution structures of rhenium (V) oxo peptide complexes of glycylglycylcysteine and cysteinylglycine as studied by capillary electrophoresis and X-ray absorption spectroscopy

Rhenium (V) oxo complexes of the peptides glycylglycylcysteine (GlyGlyCys) and cysteinylglycine (CysGly) have been synthesized by ligand exchange reactions starting from rhenium (V) oxo gluconate. For the first time, the complexes were characterized in solution by extended X-Ray absorption fine structure spectroscopy (EXAFS), pH dependent capillary electrophoresis (CE) with simultaneous UV absorption spectroscopy and mass spectroscopy, performed by the coupling of CE with an electrospray mass selective detector (CE-ES-MSD). EXAFS data analysis provided information on the complex structures by single and multiple scattering analysis. The investigations confirm a rhenium oxidation state of +5 with typical rhenium (V) oxo patterns. The peptides differ in their mode to coordinate the rhenium (V) oxo metal core. GlyGlyCys forms a 1:1 rhenium (V) oxo complex with a formal SN 3 coordination, whereby the peptide backbone is involved in the coordination and showed multiple scattering signals for the peptide carbonyl oxygen atoms. The complex is neutral at low pH and anionic at neutral pH. CysGly forms a 1:2 complex with a formal S 2N 2 coordination, wherein the terminal amine nitrogen atom is involved in the rhenium coordination. A cationic complex species is detected at low pH, and increasing pH values lead to the formation of an anionic species. The complex compositions were confirmed by mass spectroscopic measurements. The results are considered to be helpful in a better understanding of the Tc and Re coordination abilities with peptides. This knowledge is of great relevance for modern nuclear medicine in its commitment to utilize radiolabeled target-specific peptide radiopharmaceuticals.

High-Valent Oxo, Methoxorhenium Complexes: Models for Intermediates and Transition States in Proton-Coupled Multi-Electron Transfer Reactions

Abstract : Dioxorhenium(V) tetrapyridyl species are currently under active investigation as model systems for interfacial two-electron, two-proton transfer reaction sequences We now find that the corresponding oxo, methoxo complexes can be prepared from dioxo species and methyl trifluoromethanesulfonate. The new complexes behave nearly identically to the analogous oxo, hydroxo complexes-with one important exception: CH3(+), unlike H(+), does not dissociate from the oxo ligand. As a direct consequence, the usually elusive rhenium oxidation state, IV, is stabilized with respect to redox disproportionation and is observable for several complexes at high pH. The ability to detect this state, in turn leads to: (1) direct access to the formal reduction potentials for the isolated 1e(-) redox couples comprising the overall two electron transfer (key information for understanding multi-ET kinetics) (2) elucidation of the profound structural and energetic consequences of the initial protonation (methylation) step in the dioxorhenium(V) reduction kinetics.

Models for bimetallic catalysts: selectivity in ligand addition as a function of rhenium oxidation state in Pt3Re clusters

The ligands CO, P(OR)3, and X–(X = Cl, Br, I) add selectively to the Re Pt3 sites in the coordinatively unsaturated clusters [Pt3{Re(CO)3}(µ-dppm)3]+ and [Pt3{ReO3}(µ-dppm)3]+ respectively, dppm = Ph2PCH2PPh2: the selectivity thus displays a remarkable dependency on the oxidation state of rhenium.

The double bond migration activity of supported rhenium catalysts for olefin metathesis

The isomerization of 1-butene into cis- and trans-2-butene on supported rhenium catalysts Re 2O 7/Al 2O 3, Re 2O 7/MgO and Re 2O 7/SiO 2 has been invest igated under conditions where neither is the catalyst active in olefin metathesis, nor does the pure carrier itself provoke double bond migration. This has been achieved by pre-reduction of the samples in a H 2 flow at appropriate temperatures. The rhenium oxidation state on the catalyst surface has been studied by means of diffuse reflectance spectroscopy and by X-ray analysis. On comparing the spectral and catalytic activity data, it can be concluded that double bond migration and metathesis take place on active sites with different oxidation states for the rhenium. Metathesis occurs for catalysts reduced in mild conditions, in which Re(VI) coexists with Re(VII) on the surface. The double bond migration activity, on the other hand, is related to the presence of the Re(IV) oxidation state, which prevails under moderate reduction conditions. After more severe reduction of the samples the isomerization activity disappears.

The Oxidation State of Rhenium and Its Role in Platinum-Rhenium Reforming Catalysts

The Oxidation State of Rhenium and Its Role in Platinum-Rhenium Reforming Catalysts

Evidence for the Existence of a Potassium Rhenium Hydride and its Bearing on the Nature of the (−1)-Oxidation State of Rhenium

THIS is a preliminary report of some observations we have made on the product of the reduction of potassium perrhenate with potassium metal in ethylenediamine–water solution. According to Bravo, Griswold and Kleinberg1, and also Floss and Grosse2, rhenium is reduced to the (−1)-oxidation state in this reaction, and the solid rhenide KRe.4H2O may be isolated from the products.

Reduction of Perrhenate by Stannous Tin

rhenium, colorimetrically, is based upon the formation of rhenium thiocyanate complex, or complexes, when acidic solutions of perrhenate are reduced by stannous chloride in the presence of excess alkali thiocyanate. The color developed in the solution varies from yellow to orange-red, and absorption or transmission measurements are usually made on the absorption band of 430 m/ wavelength. Because of the instability of the reduced aqueous perrhenate solution, the colored thiocyanate complex must be extracted into an organic solvent (usually ethyl ether) for colorimetric measurements. In this test the oxidation state of rhenium in its colored