Highly reactive metal powders

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). 

Physical Characteristics of Highly Reactive Metal Powders... 

SmCl3 resulted in the reduction only to SmC. From NdCl3 + Ca with the addition of Fe powder, the alloy Nd2Fei7 was obtained. In a discussion of the results it was observed that the products obtained at ambient temperature by mechanical alloying are the same which result from the conventional metallothermic reduction of the rare earth halides. However, the metallothermic reduction requires a temperature of 800-1000°C for the reduction of the chlorides and 1400-1600°C for the fluorides. The products of the mechanical process, on the other hand, are fine, amorphous or microcrystalline, highly reactive metal powders mixed with CaCl2. 

Whenever metallic zinc is to be used in oxidative addition processes, results are affected by the metal surface activity. Two strategies for the production of active zinc metal surfaces can be adopted (i) chemical or physical activation of commercial zinc powders, or (ii) in situ production of highly reactive metal powders by reduction of a zinc salt with a suitable reducing agent. 

In 1973, the direct potassium metal reduction of zinc salts was reported.3 This active zinc powder reacted with alkyl and aryl bromides to form the alkyl- and arylzinc bromides under mild conditions.4 The reduction of anhydrous zinc salts by alkali metals can be facilitated through the use of electron carriers. Lithium and sodium naphthalenide reduce zinc salts to give highly reactive metal powders under milder and safer conditions. Graphite5 and liquid ammonia6 have also been employed as electron carriers in producing zinc powders. A highly dispersed reactive zinc powder was formed from the sodium metal reduction of zinc salts on titanium dioxide.7... 

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

A method of broad application in oxidative addition is to use slurries of highly reactive metal powders. Such a slurry is generated when a transition-metal salt is reduced by an alkali metal in ether . Highly active powders of Ni, Pd and Pt oxidatively add allylic halides to form j -allyl complexes . ... 

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. 

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]. 

Rieke Zinc. Highly reactive metal powders are made through the reduction of metal salts, using alkali metals in a suitable solvent [38, 39]. Rieke zinc, Zn, has... 

The soluble and homogeneous forms [2] of highly reactive metals such as Rieke calcium, barium, or strontium are effective reagents for this type of polymerization. The polymerization failed when insoluble highly reactive metal powders such as Rieke magnesium [31] were used (Equation 9.2). The soluble biphenyl complexes M(biphenyl)2 (M = Ca, Sr, or Ba) were sufficiently active to facilitate polymerization. In contrast only oxidative addition of Mg into the carbon-chlorine bond occurred, and upon workup only a-chlorotolu-ene and a,a-dichlorotoluene were recovered quantitatively as products after 24 h of reflux. No coupling reaction or polymerization occurred. 

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. 

Dissolution of the metals in liquid ammonia and subsequent removal of ammonia to give highly reactive metal powders [76, 77]. 

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