A Rieke metal is a highly reactive metal powder generated by reduction of a metal salt with an alkali metal. These materials are named after Reuben D. Rieke, who first described along with an associate in 1972 the recipes for their preparation.[
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Rieke metals are highly reactive because they have high surface area and lack surface oxides that can retard reaction of bulk materials. The particles are very small, ranging from 1-2 micrometre down to 0.1 μm or less. Some metals like nickel and copper give black colloid that do not settle, even with centrifugation, and cannot be filtered. Other metals such as magnesium and cobalt give larger particles, but these are found to be composed mainly of the alkali salt by-product, with the metal dispersed in them as much finer particles or even as an amorphous phase.[
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Preparation
Rieke metals are usually prepared by a redox of an anhydrous metal chloride with an alkali metal, in a suitable solvent.[Angew Chem Int Ed Engl - February 1993 - Alois Furstner - Chemistry of and with Highly Reactive Metals][
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- MgCl2 + 2 K → Mg + 2 KCl
Rieke originally described three general procedures:
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Reaction with molten sodium or potassium in a solvent whose boiling point is higher than the metal's melting point, and which can dissolve some of the anhydrous salt, in an inert atmosphere. Suggested combinations were potassium in tetrahydrofuran (THF), sodium in 1,2-dimethoxyethane, and either metal with benzene or toluene. The exothermic reaction takes a few hours, and usually requires refluxing.
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Reaction with an alkali metal at temperatures below its melting point, with a catalysis amount (5-10% by mole) of an electron carrier such as naphthalene
[ or biphenyl.]
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Reaction with previously prepared lithium naphthalide
[ or lithiobiphenyl][ instead of lithium. This process can be carried out at even lower temperatures, below ambient. Although slower, it was found to produce even smaller particles.][
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The alkali metal chloride coprecipitation with the finely divided metal, which can be used in situ or separated by washing away the alkali chloride with a suitable solvent.[
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Uses
Rieke zinc has attracted the greatest attention of all the Rieke metals. Interest is motivated by the ability of Rieke Zn to convert 2,5-dibromothiophenes to the corresponding polythiophene.
Rieke magnesium reacts with aryl halides, some even at −78 °C, to afford the corresponding , often with considerable selectivity.[
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History
The use of highly reactive metals in chemical synthesis was popularized in the 1960s. One development in this theme is the use of metal vapor synthesis, as described by Skell, Timms,[Peter L. Timms, "New Developments in Making Compounds and Materials by Condensing Gaseous High-temperature Species at Atmospheric or Low Pressure". School of Chemistry, University of Bristol, Bristol BS8 1TS, UK] Ozin, and others. All of these methods relied on elaborate instrumentation to vaporize the metals, releasing an atomic form of these reactants.
In 1972, Reuben D. Rieke, a professor of chemistry at the University of North Carolina, published the method that now bears his name.
Rieke continued this work at the University of Nebraska–Lincoln. He and his wife Loretta founded Rieke Metals LLC in 1991, based on these materials.[(2018): " About Us". Rieke Metals's website, accessed on 2019-03-19.]
Safety
Production and use of Rieke metals often involves the handling of highly Pyrophoricity materials, requiring the use of air-free techniques.
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