Rhenium species

The procedure is experimentally simple, and the workup involves only the destruction of the traces of hydrogen peroxide with manganese dioxide and evaporation of the hexamethyldisiloxane. Pyridine additives serve to buffer the highly acidic rhenium species and to shut down the detrimental acid-catalyzed epoxideopening pathways. The scope of this transformation is best appreciated through the examples presented in Table 12.2 [28],... 

Koide et al. [537] have described a graphite furnace atomic absorption method for the determination of rhenium at picomolar levels in seawater and parts-per-billion levels in marine sediments, based upon the isolation of heptavalent rhenium species upon anion exchange resins. All steps are followed with 186-rhenium as a yield tracer. A crucial part of the procedure is the separation of rhenium from molybdenum, which significantly interferes with the graphite furnace detection when the Mo Re ratio is 2 or greater. The separation is accomplished through an extraction of tetraphenylarsonium perrhenate into chloroform, in which the molybdenum remains in the aqueous phase. 

According to the general postulates presented, a dioxorhenium(VII) species intervenes along the reaction pathway. Attempts to detect it under the conditions of the catalytic experiments were not successful. Indeed, species A and B were not seen either 1 was the only rhenium species found by NMR spectroscopy. In an attempt to provide other validation for a rhenium(VII) intermediate, other experiments were carried out. 

In catalyst preparation, one can use this knowledge to determine the relative contributions of various hydroxyl groups before and after application of the active phase onto the support. In this way Sibeijn etal. [31] established that rhenium oxide attached to acidic sites of the alumina support exhibits higher activity for the metathesis of olefins than rhenium oxide on neutral or basic sites. As, however, rhenium species preferentially exchange with basic hydroxyls, one needs to increase the loading above a certain value (6 wt% for an alumina of 200 m2/gram) before the catalyst exhibits appreciable activity [31]. 

One of the most important peroxo complexes synthesized after 1983 is the rhenium species formed from methyltrioxorhenium (MTO) precursor. The synthesis of this complex is achieved in the way indicated in equation 2, by reacting hydrogen peroxide with MTO . The isolated peroxo complex 1 contains in the coordination sphere two /7 -peroxide bridges, a direct metal carbon bond and a molecule of water. The crystal structure of the peroxo rhenium derivative, however, was obtained by substitution of the water molecule with other ligands " more details on this aspect are enclosed in the structural characterization paragraph. 

For Re2(02CC Hs)4Cl2 the first reduction wave near —0.3 V is quasi-reversible. The product of this one-electron oxidation was not isolated as a solid, but ESR spectra were obtained on frozen dichloromethane solutions at 77°K at both X- and Q-band frequencies. Both naturally occurring rhenium isotopes have a nuclear spin of 5/2 with slightly different nuclear magnetic moments. For dimeric rhenium species there are three distinct isotopic isomers and each generates a unique ESR spectrum as a result of differing hyperfine interactions. The observed ESR spectrum is a superposition of these lines with the intensity... 

The new compounds described in this paper have a rhenium atom incorporated into closo-l,2-dicarba-3-rhenadodecaborane or c/oro-l-carba-2-rhenadodecaboranc structures. However, to avoid a complicated nomenclature for the complexes reported, and to relate them to the many known rhenium species with 77-co-ordinated cyclopentadienyl ligands, we treat the cages as nido- 1 -vertex ligands with numbering as for an icosahedron from which the twelfth vertex has been removed. 

Rhenium oxide supported on alumina is present on the surface as a single rhenium species. Laser-Raman spectroscopy studies by Kerkhof, MouUjn, and Thomas indicate that the species consists of tetrahedral Re04 ions, which are dynamically distorted by the carrier or surface hydroxyl groups, as illustrated by the broadening of the band at 916 cm". Neither laser-Raman spectroscopy nor X-ray diffraction indicates that other rhenium species, e.g., octahedrally co-ordinated rhenium or Al-Re-0 compounds, are formed. 

In a subsequent paper, electrodeposition of rhenium species from sulfuric acid solutions of perhhenic acid onto polycrystalline Pt and Au surfaces was carried out both in underpotential and overpotential regimes. Metallic Re could be obtained by applying relatively high cathodic current densities, whereas Re(IV) compounds were more likely to form in reduction of ReO4 at low current densities. Lastly, the deposition of rhenium species on Au from Re2O7 containing sulfuric acid solutions was studied. It was shown that the chemical nature of electrodeposited rhenium... 

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