Rhenium-oxide compounds

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. 

The multiple bond to the metal in a rhenium(V) compound is best represented by the MO description of it being a triple bond, consistent with the symmetry of the px and p orbitals on the ligand see 1 (1). In keeping with that, these bonds are found to be relatively short and quite strong. The structure has also been presented as 1 a valence bond formulation that facilitates the counting of oxidation states but does not provide an accurate representation. 

As noted previously, the rhenium-catalyzed reactions dealt with to this point can be carried out in vessels open to the atmosphere. This is advantageous because of the convenient working procedures it allows. On the other hand, it means that molecular oxygen is not available as the stoichiometric oxidizing reagent (53-55). Three rhenium(V) compounds, 24-26, have been prepared that do activate 02. 

Perrhenate and related building blocks are constituents of several cluster compounds where they act as terminal groups in organometallic rhenium oxides such as in [(cp Re)3(//2-0)3(/U3-0)3Re03]+ (49)21 Qj. jjj heterometallic clusters such as the structurally related [(Re)3(//f dppm)3(/u -0)3Re03]+ (dppm = bis(diphenylphosphino)methane) and Pt4 P(C6H 11)3)4 (//-C0)2(Re04)2]. A series of platinum-rhenium and platinum-rhenium-mercury clusters with Pt-Re multiple bonds has been isolated from reactions of Pt3 precursors with Rc207 or perrhenate. " ... 

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. 

ReOCU reacts with PhNCO to give the rhenium(VI) compound [Re(NPh)Cl4] which forms adducts with donor solvents such as TFIF or acetonitrile. Anionic [Re(NPti)Cl5] is obtained when [Re(NPh)Cl4] is treated with [Mc4N]Cl. Two other approaches to rhenium(VI) arylimido compounds include an azo splitting reaction starting from 2-(arylazo)pyridines and the oxidation of rhenium(V) imides by nitric acid. ... 

Rhenium complexes containing the metal in the oxidation state +III are comparatively numerous. This may be ascribed to the fact that the d configuration of the rhenium center can readily be stabilized by ligands with pronounced donor and rr-acceptor properties. Most of the rhenium(III) compounds are stable against hydrolysis rendering them suitable for nuclear medical applications. 

The main routes for the synthesis of rhenium(II) compounds involve oxidation of rhenium(I) or the reduction of rhenium(III) complexes. Additionally a few examples are known where Re" species are formed during the cleavage of Re—Re bonds of bimetallic units. 

Polypyridyl ligands are able to stabilize electron-rich metal centers as has been shown for the d -systems Ru and Os. This suggests this class of ligands also to be appropriate for rhenium(I) compounds, and this will be discussed more in detail in Section 5.3.2.7.2, and rhenium(II) complexes which represent intermediates in the syntheses of the Re species or are accessed by oxidation of Re. ... 

The ability of polypyridyl ligands to accept electron density from electron-rich rhenium centers and, thus, to contribute to the stabilization of rhenium complexes with the metal in low oxidation states has already been discussed for rhenium(II) compounds. Only small modifications to the polypyridyl ligand or the metal center can create dramatic differences in the properties of the resulting complexes. Generally, the starting materials which have been introduced as precursors for rhenium(II) polypyridyl complexes in Section 5.3.2.6.2, are also appropriate for the synthesis of related rhenium(l) compounds. 

ReCl3(PPh3)(benzil)] reacts with bipy and related ligands or terpy to form a number of rhe-nium(III) and rhenium(II) compounds which are useful precursors for the synthesis of lower-valent rhenium complexes. " Thus, reduction of [Re(bipy)3][PF6]2 with zinc amalgam results in the rhenium(I) compound [Re(bipy)3][PF6] in excellent yields. The corresponding terpyridyl bis-chelate [Re(terpy)2][PF6] has been prepared in a similar manner. " The electrochemistry of the products provides a convenient measure of the chemical reactivity associated with the redox processes. Thus, the one-electron oxidation of [Re(bipy)3]" is reversible at -0.33 V, whereas the Re"/Re" redox couple is irreversible and occurs at relatively low potentials (-1-0.61 V) which is consistent with the instability of [Re(bipy)3] + in solution. However, in the presence of a small coordinating molecule such as CNBu, oxidation to the rhenium(III) state is readily available by the formation of seven-coordinate complexes of the composition [Re(bipy)3(L)]. " ... 

Mol, Visser, and Boelhouwer 741 subjected 1,2-dimethyl-butane (ring structure of the suggested transition state for disproportionation of propylene) to rhenium oxide-alumina catalyst under conditions which propylene gives high disproportionation conversions. This compound was stable only at high temperatures (730 °C), where thermal cracking occurred, were olefins found. 

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