Organorhenium chemistry

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.^[1] 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.^[2]

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[Re(CO)₅] with rhenium in the formal oxidation state −1. Bromination of dirhenium decacarbonyl gives bromopentacarbonylrhenium(I),^[3] then reduced with zinc and acetic acid to pentacarbonylhydridorhenium:^[4]

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:^[5]

Re(CO)₅Br + 3 H₂O → [Re(CO)₃(H₂O)₃]Br + 2 CO

With tetraethylammonium bromide Re(CO)₅Br reacts to give the anionic tribromide:^[6]

Re(CO)₅Br + 2 NEt₄Br → [NEt₄]₂[Re(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

Structure of methylrhenium trioxide.

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:^[7]

Re₂O₇ + (CH₃)₄Sn → CH₃ReO₃ + (CH₃)₃SnOReO₃

Analogous alkyl and aryl derivatives are known. Although PhReO₃ is unstable and decomposes at –30 °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.^[8] MTO and other organylrhenium trioxides catalyze oxidation reactions with hydrogen peroxide as well as olefin metathesis in the presence of a Lewis acid activator.^[9] 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.^[10]

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)₄ReO]₂Mg(thf)₂, Re₂O₃Me₆ and Re₂O₃(CH₂SiMe₃)₆" J. Chem. Soc., Dalton Trans., 1985, pp. 2167-2175. doi:10.1039/DT9850002167