Highly reactive metal powders preparation

Rieke Magnesium. Rieke metals are highly reactive metal powders prepared by reduction of the corresponding halide by potassium in a suitable solvent. The first publication in this field by Rieke and co-workers appeared in 1972 [38]. For reviews see [39]. 

Preparation of Highly Reactive Metal Powders Some of Their Uses in Organic and Organometallic... 

In 1972, we reported a general procedure for the preparation of highly reactive metal powders. The basic procedure involved the reduction of a metal salt in a hydrocarbon or ethereal solvent. The reductions are most generally carried out with alkali metals such as potassium, sodium, or lithium. A wide range of methods have been developed to carry out the reductions. The reactivities of these resulting black powders exceed other reports in the literature for metal powders. This high reactivity has resulted in the development of several new synthetic techniques and vast improvements in many older, well established reactions. This review concentrates on the metals Mg, Ni, Zn, Cd, Co, Cu, Fe, and U. 

The "freeing" of metals from metal salts by various reducing agents is a process which is as old as civilized man itself. In the past thirty years, several new approaches to reducing metal salts have appeared(l-13). In the past few years, several workers have shown that if care is taken regarding the reducing procedure, finely divided and highly reactive metal powders or slurries can be prepared( 16-60). 

R. D. Rieke, Preparation of Organometallic Compounds from Highly Reactive Metal Powders, Science 1989, 246, 1260-1264. 

Preparations , that start with Pd(0) complexes, e.g., Pd(PPhj)4, are less versatile (the choice of ligand is limited) and need protection from Oj. One way to maintain a free choice of ligand is to use slurries of highly reactive metal powders, a method described for the Ni case . ... 

RIeke, R. D., Rhyne, L. D. Preparation of highly reactive metal powders. Activated copper and uranium. The Ullmann coupling and preparation of organometalllc species. J. Org. Chem. 1979, 44, 3445-3446. 

Rieke, R.D. 1977. Preparation of highly reactive metal powders and their use in organic and organometallic synthesis. Acc Chem Res 10 301-306. 

Rieke, R. D. 1989. Preparation of organometallic compounds from highly reactive metal powders. Science 246 1260-1264. 

Since 1972, we have published many reports describing convenient methods for the generation of highly reactive metal powders and their use in organic as well as organometallic synthesis [28-34]. Most of the active metals prepared by our group have been prepared in ethereal solvents [35]. Our initial report on the preparation of active uranium (I) employed 1,2-dimethoxyethane (DME) as a solvent (Scheme 13.1) [31]. We have since developed a method for preparing active uranium (3) in hydrocarbon solvents, which leads to a much cleaner and more controllable chemistry compared to 1 [34, 36]. 

In conclusion, a highly reactive form of chromium metal has been prepared by the Rieke reduction approach. This highly reactive chromium will react with CO to produce Cr(CO)6 in moderate yields. At this point, no additional chemistry of these highly reactive metal powders has been attempted. However, it is likely that much remains to be discovered. 

An exhaustive review on di- and poly-alkali-metal derivatives of heterofunc-tionally substituted organic molecules contains many useful references on aromatic ring metallation, particularly of nitrogen- and sulphur-containing heteroaromatics. Other reviews include a description of the use of chromium tricarbonyl-arene complexes in organic synthesis and the preparation and use in synthesis of highly reactive metal powders. ... 

A B.E.T. surface area measurement(37) was carried out on tfie activated Ni powder showing it to have a specific surface area of 32.7 m /g. Thus it is clear that the highly reactive metals have very high surface areas which, when initially prepared, are probably relatively free of oxide coatings. 

The highly reactive cadmium can be prepared by two different methods. One approach is a room temperature reduction of CdC with lithium naphthalide in THF or DME. The second approach allows the preparation of the reactive metal in a hydrocarbon solvent. First, lithium naphthalide is prepared in benzene addition of this solution to CdC produces a highly reactive cadmium powder. 

Small quantities of Sr and Ba can be obtained by decomposition of the corresponding azides under vacuum. The metals are obtained as finely divided, highly reactive black powders. They may be used in various reactions, which can be carried out directly in the equipment used for preparation. Their isolation, i.e., removal from the apparatus, is hardly possible, because on ejqiosure to air these metals react immediately with ignition. Another problem is contamination of the metal with nitride (>10%) due to a side reaction of the typer Ba + 2Ba(N3)a = BaaNa + SNg. By subjecting the Ba(Na)a to rapid decomposition, the nitride content can be kept down to a low level. 

---