Overview on the Synthesis of Activated Micro- and Nanostructurized Rieke Metals: History and Present State

Abstract

History, synthesis techniques, main properties, and some applications for Rieke metals are reviewed. Three main methods for obtaining activated Rieke metals, their characteristics, advantages and disadvantages of their synthesis, and equipment are described in detail. A definite attention is given to original equipment, ultrasonic treatment of reaction systems, process safety, and use of supported ligands in the synthesis of Rieke metals.

Keywords:

• Rieke metals
• reduction
• alkali metals
• supported ligands

BIK devotes this review to his school teacher Nadezhda Vasilievna Andreeva (Moscow, Russia).

Notes

^a Metallic Na reacts with water traces and destroys peroxides which could be generated during solvent storage.

^b AlCl ₃ is highly hygroscopic. Do not expose it to humidity and use drying material under weighting.

^c Metallic Li reacts atmospheric N ₂; minimum exposition to environment is recommended.

^dTo avoid formation of Li₃N, Ar atmosphere is recommended instead of nitrogen as reported.^{[ 22 ]}

^e 2 h are required for a complete disappearing of lithium; during the reaction of alkali metal with electron carrier, dark green color should be seen, indicating formation of lithium biphenylide.

^f Various authors recommend employing an excess of the salt to provide a total absence of reductive agent which could take part in subsequent reactions of the activated metal; a molar ratio 1.5–2.0 of salt per molar equivalent of alkali metal is recommended.

^g Naphthalene quantity is about 5–10% molar ratio in respect of alkali metal (Na/K).

^h Na/K alloys are highly reactive and do not suffer passivation during reaction. Their melting points vary in function of percent of potassium (50% wt. K +15°C, 77.8% K −12.6°C).

ⁱ Finish stirring until complete solution of Na/K alloy.

^j Ultrasonic equipment is of low power, which is usually used for washing flasks in laboratory.

^k The complex decomposes in nitrogen atmosphere.^{[ 42 ]}

^l DME should be distilled in presence of metallic sodium or sodium/potassium alloy in Ar atmosphere.

^m Due to the presence of naphthalene in the reaction medium, the potassium forms an anion which reacts with Cu ^I ; when the green color of solution (corresponding to potassium naphthalenide) disappears, it means that all potassium has been consumed and the reduction is complete.

ⁿ An indicator for formation of the electron carrier is highly colored olutions (lithium naphthalenide presents dark green color and lithium biphenylide – deep blue).

^o Maintain solutions A and B in Ar atmosphere and free of hymidity.

^pFrequently, lithium pieces are covered with metal powder. In this case, Ar flow is increased and lithium pieces are destroyed mechanically on the walls of the flask.

^q Ar atmosphere is recommended, since there are some reports on the interaction of N ₂ with Mg∗.

^r It is important to consider that non-purified pieces of K or Na generate activated powders of Mg∗ of lower reactivity; so, metallic sodium can be used together with a solvent, whose boiling point is higher that the melting point of alkali metal.

∗A = reduction in reflux in THF; B = reduction by Li powder in THF with ultrasound at r.t.