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.
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]
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)<sub>5</sub>] 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)<sub>3</sub>(H<sub>2</sub>O)<sub>3</sub>]Br + 2 CO
With tetraethylammonium bromide Re(CO)5Br reacts to give the anionic tribromide:[6]
Re(CO)5Br + 2 NEt4Br → [NEt<sub>4</sub>]2[Re(CO)<sub>3</sub>Br<sub>3</sub>] + 2 CO
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.
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).