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.[1] In 1974 he told about Rieke-magnesium.[2] A 1989 paper by Rieke lists several metals that are allowed by the
periodic table to be produced by his process:
Cd,
Zn,
Ni,
Pt,
Pd,
Fe,
In,
Tl,
Co,
Cr,
Mo,
W,
Cu, which in turn are called Rieke-nickel, Rieke-platinum, etc.[3]
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
μm down to 0.1 μm or less. Some metals like
nickel and
copper give black
colloidal suspensions 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.[3]
Rieke originally described three general procedures:
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.[3]
Reaction with an alkali metal at temperatures below its melting point, with a
catalytic amount (5-10% by mole) of an
electron carrier such as
naphthalene[3] or
biphenyl.[7] This method can be used with
lithium as the reducing agent, even at room temperature, and is therefore less hazardous than the previous method; and often results in more reactive powders.[5]
Reaction with previously prepared
lithium naphthalide[3] or
lithium biphenylide[7] 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.[3]
The alkali metal chloride
coprecipitates with the finely divided metal, which can be used in situ or separated by washing away the alkali chloride with a suitable solvent.[3]
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.[8] Rieke-Zn also reacts with bromoesters to give organozinc reagents of value for the
Reformatsky reaction.[9]
Rieke magnesium reacts with aryl halides, some even at −78 °C, to afford the corresponding
Grignard reagents, often with considerable selectivity.[10] Rieke magnesium is famous for enabling the formation of "impossible Grignard reagents" such as those derived from aryl fluorides and from 2-chloronorbornane.[5]
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,[citation needed] Timms,[11] Ozin,[citation needed] 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.[12] In contrast to previous methods, it did not require special equipment, and the main challenges were only the handling of pyrophoric reagents and/or products, and the need for
anhydrous reagents and
air-free techniques. Thus his discovery gained much attention because of its simplicity and the reactivity of the activated metals.
^
abRieke, Reuben D.; Sell, Matthew S.; Klein, Walter R.; Chen, Tian-An; Brown, Jeffrey D.; Hanson, Mark V. (1995). "Rieke Metals: Highly Reactive Metal Powders Prepared by Alkali Metal Reduction of Metal Salts". Active Metals. pp. 1–59.
doi:
10.1002/9783527615179.ch01.
ISBN978-3-527-29207-3.
^Chen, T.-A.; Wu, X.; Rieke, R. D. (1995). "Regiocontrolled Synthesis of Poly(3-alkylthiophenes) Mediated by Rieke Zinc: Their Characterization and Solid-State Properties". Journal of the American Chemical Society. 117: 233–244.
doi:
10.1021/ja00106a027.
^Rieke, R. D.; Hanson, M. V. (1997). "New Organometallic Reagents Using Highly Reactive Metals". Tetrahedron. 53 (6): 1925–1956.
doi:
10.1016/S0040-4020(96)01097-6.
^Lee, J.-S.; Velarde-Ortiz, R.; Guijarro, A.; Wurst, J. R.; Rieke, R. D. (2000). "Low-Temperature Formation of Functionalized Grignard Reagents from Direct Oxidative Addition of Active Magnesium to Aryl Bromides". Journal of Organic Chemistry. 65 (17): 5428–5430.
doi:
10.1021/jo000413i.
PMID10993378.
^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
^Reuben D. Rieke, Phillip M. Hudnall (1972). "Activated Metals. I. Preparation of Highly reactive magnesium metal". J. Am. Chem. Soc. 94 (20): 7178–7179.
doi:
10.1021/ja00775a066.
^(2018): "
About Us". Rieke Metals's website, accessed on 2019-03-19.