Of rhenium compounds

A survey of known types of rhenium compounds is presented in Table 2. The selected examples include a number of commonly encountered compounds and emphasize the diversity in stmctures. 

The catalytic capabilities of rhenium compounds burst on the scene about one decade ago, featuring MeRe03 as a catalyst for reactions of hydrogen peroxide. It was quickly verified that peroxorhenium(VII) compounds were the active intermediates. With them, practical reactions and fundamental questions of mechanism could then be resolved. 

XPS that the reducibility of rhenium compounds depends on their nature and the support used reduction was more difficult on y-alumina compared with sihca due to the stronger interaction of Re with the alumina surface. Experiments with HRe04-Al203 treated in vacuo at 773 K indicated some rhenium in an intermediate oxidation state, possibly Re", but subsequent calcination in air at 673 K resulted in re-oxidation to Re ", ... 

Even by 1989, the importance of rhenium compounds in oxidation catalysis was still minimal [1], This picture has changed dramatically since organorhenium(VII) oxides, especially the water-soluble methyltrioxorhenium (MTO, 1), have proven to be excellent catalyst precursors for a surprisingly broad variety of processes, most notably for a variety of oxidation reactions [2, 3]. This article summarizes the behavior of MTO and its peroxo derivatives in the presence of water and under catalytic condition in aqueous systems, particular attention being given to the most recent findings. 

Most of the compounds with metal-metal double bonding do not arise directly from the analogue of higher bond order discussed above. Thus for instance in the chemistry of rhenium compounds the best known and also most interesting species are the Re (III) trinuclear clusters that will be discussed in the next chapter. In this sense the chemistry of compounds with metal-metal double bonds appears as a transition from the chemistry of dinuclear metal compounds to that of higher metal aggregates. 

Rhenium hexafluoride [10049-17-9J, ReF, is a pale yeUow soHd at 0°C, but a Hquid at ambient temperature. In the presence of moisture it hydroly2es rapidly forming HF, Re02, and HRe04 (see Rheniumand rhenium compounds). It is not safe to store ReF in a glass trap or glass-lined container. Leaks in the system can initiate hydrolysis and produce HF. The pressure buildup causes the system to burst and an explosion may result. 

Propylene oxide is also produced in Hquid-phase homogeneous oxidation reactions using various molybdenum-containing catalysts (209,210), cuprous oxide (211), rhenium compounds (212), or an organomonovalent gold(I) complex (213). Whereas gas-phase oxidation of propylene on silver catalysts results primarily in propylene oxide, water, and carbon dioxide as products, the Hquid-phase oxidation of propylene results in an array of oxidation products, such as propylene oxide, acrolein, propylene glycol, acetone, acetaldehyde, and others. 

Polymers containing 8-hydroxyquinoline appear to be selective adsorbents for tungsten in alkaline brines (95). In the presence of tartrate and citrate, quinaldic acid [93-10-7] allows the separation of zinc from gallium and indium (96). Either of these compounds can selectively separate lead and zinc from oxide ores as complexes (97). It is also possible to separate by extraction micro quantities of rhenium(VII), using quinoline in basic solution (98). The... 

A particularly significant part of rhenium chemistry involves cluster compounds in which there is metal—metal bonding. This chemistry centers largely around the +3 oxidation state. 

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. 

Compounds of the formulas Re(CR]), ReO(CH3)4, Li2[Re2(CH3)g] [60975-25-9], Re02(CH3)3 [56090-011-8], and Re03CH3 [70197-13-6] have been prepared. The first two compounds were obtained from reaction of rhenium hahdes or oxyhahdes and methyllithium the last three were formed from the species by oxidation or reduction reactions. The use of these hydride and alkyl complexes as catalysts is under investigation. 

Litde is known concerning the toxicology of rhenium or its compounds. Rhenium heptoxide is said to be an irritant if inhaled. 

However, like the mp, bp and enthalpy of atomization, it also reflects the weaker cohesive forces in the metallic lattice since for Tc and Re, which have much stronger metallic bonding, the -t-2 state is of little importance and the occurrence of cluster compounds with M-M bonds is a dominant feature of rhenium(III) chemistry. The almost uniform slope of the plot for Tc presages the facile interconversion between oxidation states, observed for this element. 

A funher pnini of inieresi is ihe noiiceably greaier lendency of rhenium, as compared 10 cilher manganese or leehnenum, lo form compounds wilh high coordinalion numbers... 

Rhenium compounds as catalysts, 136, 137, 141, 144, 146, 148, 150 Ring-opening polymerization, 143 catalysts for, 140 thermodynamics of, 156 Rotary engine, 123... 

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