Rhenocene

Organorhenium chemistry describes the compounds with Re−C bonds. Because rhenium is a rare element, relatively few applications exist, but the area has been a rich source of concepts and a few useful catalysts.

General features

Rhenium exists in ten known oxidation states from −3 to +7 except −2, and all but Re(−3) are represented by organorhenium compounds. Most are prepared from salts of perrhenate and related binary oxides.O. Glemser "Ammonium Perrhenate" in Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 1476-85. The halides, e.g., ReCl₅ are also useful precursors as are certain oxychlorides.

A noteworthy feature of organorhenium chemistry is the coexistence of oxide and organic ligands in the same coordination sphere.

Carbonyl compounds

Dirhenium decacarbonyl is a common entry point to other rhenium carbonyls. The general patterns are similar to the related manganese carbonyls. It is possible to reduce this dimer with sodium amalgam to Na e(CO)₅with rhenium in the formal oxidation state −1. Bromination of dirhenium decacarbonyl gives bromopentacarbonylrhenium(I), then reduced with zinc and acetic acid to pentacarbonylhydridorhenium:

:Re₂(CO)₁₀ + Br₂ → 2 Re(CO)₅Br
:Re(CO)₅Br + Zn + HOAc → Re(CO)₅H + ZnBr(OAc)

Bromopentacarbonylrhenium(I) is readily decarbonylated. In refluxing water, it forms the triaquo cation:
:Re(CO)₅Br + 3 H₂O → e(CO)₃(H₂O)₃r + 2 CO

With tetraethylammonium bromide Re(CO)₅Br reacts to give the anionic tribromide:
:Re(CO)₅Br + 2 NEt₄Br → Et₄sub>2 e(CO)₃Br₃+ 2 CO

Cyclopentadienyl complexes

One of the first transition metal hydride complexes to be reported was (C₅H₅)₂ReH. A variety of half-sandwich compounds have been prepared from (C₅H₅)Re(CO)₃ and (C₅Me₅)Re(CO)₃. Notable derivatives include the electron-precise oxide (C₅Me₅)ReO₃ and (C₅H₅)₂Re₂(CO)₄.

Re-alkyl and aryl compounds

Rhenium forms a variety of alkyl and aryl derivatives, often with pi-donor coligands such as oxo groups. Well known is methylrhenium trioxide ("MTO"), CH₃ReO₃ a volatile, colourless solid, a rare example of a stable high-oxidation state metal alkyl complex. This compound has been used as a catalyst in some laboratory experiments. It can be prepared by many routes, a typical method is the reaction of Re₂O₇ and tetramethyltin:
:Re₂O₇ + (CH₃)₄Sn → CH₃ReO₃ + (CH₃)₃SnOReO₃

Analogous alkyl and aryl derivatives are known. Although PhReO₃ is unstable and decomposes at –30 Â°C, the corresponding sterically hindered mesityl and 2,6-xylyl derivatives (MesReO₃ and 2,6-(CH₃)₂C₆H₃ReO₃) are stable at room temperature. The electron poor 4-trifluoromethylphenylrhenium trioxide (4-CF₃C₆H₄ReO₃) is likewise relatively stable. MTO and other organylrhenium trioxides catalyze oxidation reactions with hydrogen peroxide as well as olefin metathesis in the presence of a Lewis acid activator. Terminal alkynes yield the corresponding acid or ester, internal alkynes yield diketones, and alkenes give epoxides. MTO also catalyses the conversion of aldehydes and diazoalkanes into an alkene.Hudson, A. "Methyltrioxorhenium" Encyclopedia of Reagents for Organic Synthesis. John Wiley & Sons: New York, 2002.

Rhenium is also able to make complexes with fullerene ligands such as Re₂(PMe₃)₄H₈(η²:η²C₆₀).

Further reading

* ''Synthesis of Organometallic Compounds: A Practical Guide'' Sanshiro Komiya Ed. S. Komiya, M. Hurano 1997.
* Pericles Stavropoulos, Peter G. Edwards, Geoffrey Wilkinson, Majid Motevalli, K. M. Abdul Malik and Michael B. Hursthouse "Oxoalkyls of rhenium-(V) and-(VI). X-Ray crystal structures of (Me₄ReO)₂Mg(thf)₄, Me₃SiCH₂)₄ReOsub>2Mg(thf)₂, Re₂O₃Me₆ and Re₂O₃(CH₂SiMe₃)₆" J. Chem. Soc., Dalton Trans., 1985, pp. 2167-2175.

References

{{ChemicalBondsToCarbon

Rhenium compounds
Organometallic compounds