Rhenium , in oxidation

A series of polyoxotungstate anions which contain high valent rhenium, e.g. [Si-W10Re2O40]2 /3 /4, have been synthesized in the form of Bu"N+ salts.639 The anions contain rhenium in oxidation states VII, VI and V. Cyclic voltammograms, ESR and electronic absorption spectra have been used to characterize these complexes. 

Triangular clusters come in a great variety of structural types. For rhenium in oxidation state III there is a type of cluster not known for any other element... 

The first rhenium dithiocarbamate complexes, [ReClaCSaCNRa)], were not reported until 1960 (1181), and even then, it was not until the 1970s that substantial inroads were made. Now, dithiocarbamates are known to stabilize rhenium in oxidation states +1 to +5, with the latter being the most common, and the chemistry is dominated by strong jt-donor ligands such as oxo and imido, and especially prevalent are nitride complexes. 

The oxidation potentials for rhenium in aqueous acidic solution are summarized in the following diagram (6). 

Rhenium oxides have been studied as catalyst materials in oxidation reactions of sulfur dioxide to sulfur trioxide, sulfite to sulfate, and nitrite to nitrate. There has been no commercial development in this area. These compounds have also been used as catalysts for reductions, but appear not to have exceptional properties. Rhenium sulfide catalysts have been used for hydrogenations of organic compounds, including benzene and styrene, and for dehydrogenation of alcohols to give aldehydes (qv) and ketones (qv). The significant property of these catalyst systems is that they are not poisoned by sulfur compounds. 

This reaction is cataly2ed in solution by complexes of tungsten, molybdenum, or rhenium in high oxidation states, eg, Re ". Examples of active catalysts are... 

The tube is positioned in a tube furnace so that the boat is near the center of the furnace, a thermocouple is introduced in the open end, and the open end of the furnace is plugged with Pyrex wool. The tube is evacuated and the temperature raised to 450° at about 3° per minute. A transitory blue deposit is formed in the cool portion of the tube at 200° at 300°, a green ring forms in the tube just inside the furnace at 300 to 350°, a blue-black deposit, presumably a mixture of rhenium (V) bromide and rhenium (VI) oxide tetrabromide,2 forms in the portion of the tube just outside the furnace at 350 to 370°, a blue-black... 

The full exploitation of the wide range of oxidation states of rhenium in radiopharmaceutical design requires not only a wide range of ligand donor types with varied n-donor and acceptor properties, but also suitable reducing agents to provide access to oxidation levels below (VII). This is because for radiopharmaceutical purposes the most conveniently man-factured form of rhenium radionuclides is perrhenate. The traditional routes to reduced rhenium complexes are not particularly amenable to use in a radiopharmaceutical context, where mild, rapid, and preferably... 

Bifunctional Chelators for Rhenium in Lower Oxidation States... 

The second attribute of the catalyst concerns its electronic structure, or more simply the valence electron count. Effective catalysts must, it seems, have < 18 VE, such that coordination of a substrate or the departure of a product does not itself pose a major kinetic barrier. Furthermore, it happens that the most stable valence states of the metal will differ by two units. Thus not only will the stoichiometry of atom transfer be supported, but also the mechanism. In the case of rhenium, the oxidation states are Re(V) and Re(VII) indeed scant indication of Re(VI) has been found in this chemistry, especially in a mononuclear species. Likewise, there is no indication of the involvement of free radical chemistry. 

Independent of this study, it has been shown that PPh3 is oxidized by Eh OOH more rapidly than MeSTol, when 1 catalyzes both reactions. Nonetheless, addition of 1.0 mM PPh3, along with 34 mM MeSTol, reduces t-i/2 from 3900 to 1900 s. Owing to these complications it was not possible to present a closed-form expression for the reaction rate. We do note the strong inhibition by phosphine, which inevitably remains coordinated to rhenium in a generally less active form, at least until oxidized. 

The use of /r-hydroxo or ju-alkoxo bridged polynuclear complexes of chromium, molybdenum, tungsten, or rhenium in this route leads to the formation of monomeric bis(NHC) complexes, to the elimination of hydrogen, and to the partial oxidation of the metal [Eq.(ll)]. Chelating and nonchelating imidazolium salts as well as benzimidazolium and tetrazolium salts can be used. 

The terminal nitrido-ligands are nucleophilic and react with Lewis acids. The isolated bimetallic products which contain rhenium in the 5+ oxidation state are treated in Section 5.3.2.3.3. Rhenium(VI) compounds are obtained when [Bu4N][ReNCl4] reacts with B(C6F5)3 or BBr3. 

Complexes containing rhenium in the oxidation state +IV are comparatively rare. There is no extended chemistry in aqueous media and many rhenium(IV) complexes tend to hydrolyze when exposed to water. The stabilization of rhenium(IV) centers requires a well-balanced donor-acceptor behavior of the ligands and, thus, none of the classical rr-donor ligands such as O or TT-acceptors such as carbonyls or nitrosyls are characteristic for this oxidation state... 

As with their trans isomers, treatment of the nitrile complexes cw-[ReCl(NCR)(dppe)2] (R = aryl) with [Et20H][BF4] or TMSCF3SO3 in CH2CI2 leads to the formation of the methylenamide compounds cA-[ReCl NC(E)R (dppe)2] (E = H, TMS) and tra x-[ReCl(NCHR)-(dppe)2]. The products undergo deprotonation by bases such as [NBu4]OH to form the trans isomers of the corresponding nitrile complexes [ReCl(NCR)(dppe)2]. Reactions of cis-[ReCl(NCR)(dppe)2] complexes with [Et30][PFg] result in oxidation and isomerization to afford the rhenium(II) complexes tra 5-[ReCl(NCR)(dppe)2][PF6]. ... 

The tricarbonyl rhenium(I) core is extremely inert and does not take part in ligand exchange reactions under mild conditions, but cleavage of a Re—C bond can be observed when fac-[Re(CO)3Br3] (288a) is treated with elemental Br2 which results in oxidation and the formation of the rhenium(III) anion c x-[Re(CO)2Br4]... 

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