Preparation of Active Metals

The reactions starting from metals can be divided into two main categories, one in which a preliminary activation is performed in the absence of the substrate(s), and a second in which a substrate is present during the activation step and reacts immediately with the metal. In this part, activation corresponding to the first type is presented. 

Metals can be activated directly under the form of fine dispersions or colloids by sonication in suspension. Thus, mercury emulsions can be formed in various solvents. Their reactions with a,a dibromoketones were described by Fry et al. (p. 231). The effects of sonication in this system were interpreted as a result of the large surface area of mercury droplets, and comparable results were obtained by high-speed stirring. The electrification of the particles was not envisaged. 

Molten sodium is emulsified in an inert solvent. A blue-purple suspension of particles less than 1 [im in average size is produced by sonication with a magneto-strictive generator. In comparison, stirring at 10,000 rpm produces coarser (3 -15 m) particles. The experimental set-up is described in the paper. The dispersion reacts with chlorobenzene with no induction period in a steadily progressing, easily controlled reaction, in contrast with the mechanical dispersion. 

Margulis described similar results and evidenced a frequency effect. The particles are smaller at 44 kHz as compared to 22 kHz and a steady state in particle-size distribution is reached after ca. 20 min. Reactions of these dispersions with chlorosilanes are discussed below (p. 210). 

An experimental set-up was described for the production of zinc powders, less than 100 pm in size, up to 1 kg per hour by sonication of the molten metal. 3 

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Preparation of Active Metals from Metals their Salts and Complexes... 

Figure 8 The cell and the procedure for the in situ preparation of active metal electrodes such as Li and magnesium. The metal rod is pressed through the C.E. into the W.E. and a SS wire is wrapped around it. The three parts are placed in the polyethylene bath, which is filled with solution. Once in solution the SS wire is pulled to cut the rod and prepare fresh surfaces in solution. The last stage includes rearrangement of the electrodes in the bath [17]. (Reprinted with copyright from Elsevier Science Ltd.)...

After decomposition of the aluminum alloys there is still residual aluminum in the metal, and this residue seems to be in part responsible for the activity of the catalyst. If this residue is largely removed by continued extraction, the catalyst becomes inactive.157 Alloys containing more than 75% of nickel are only partly attacked by aqueous sodium hydroxide, or are not attacked at all then decomposition has to be undertaken with very concentrated sodium hydroxide solution or by adding solid sodium hydroxide to the molten mass.158 Alloys containing between 30% and 50% of active metal are most suitable for preparation of active metal catalysts. 

The usual methods for the preparation of active metals fall into three groups, which are characterized by common preparative methodology and the same type of defect structure of the products. 

PREPARATION OF ACTIVE METALS BY DEPOSITION FROM A HOMOGENEOUS MEDIUM... 

Literature references dealing with the preparation of active metals by reduction with gaseous agents ... 

Literature references for the preparation of active metal oxides by dehydration of hydroxides ... 

Fig. 340. Preparation of active metal oxides by oxidation of metal vapor, a funnel for addition of metal b observation port c side port d circular nozzle for air intake e first chamber with lateral observation ports (these are not shown) f illuminating device g glass tubes (the remaining parts of the apparatus are made from sheet iron) h carbon electrodes i flow meter activated by differential pressure n precipitation cell s movable carbon electrode.

Although the study of fused or molten salts in principle might be dated back to the time the first investigator melted a salt preparation, the extensive and systematic study of these fluids is a relatively recent extension of chemistry. During the nineteenth century comparatively few investiga tions of fused salts were reported, and this aspect was frequently incidental to the primary purpose of the research, as in the preparation of active metals by Davy and by Bunsen, the classic studies of Faraday on the laws... 

In general, the reaction can be performed only with organometallics of active metals such as lithium, sodium, and potassium, but Grignard reagents abstract protons from a sufficiently acidic C—H bond, as in R—C=C—H —> R—C=C—MgX. This method is best for the preparation of alkynyl Grignard reagents. ... 

Design parameters of the anode catalyst for the polymer electrolyte membrane fiiel cells were investigated in the aspect of active metal size and inter-metal distances. Various kinds of catalysts were prepared by using pretreated Ketjenblacks as support materials. The prepared electro-catalysts have the morphology such as the sizes of active metal are in the range from 2.0 to 2.8nm and the inter-metal distances are 5.0 to 14.2nm. The electro-catalysts were evaluated as an electrode of PEMFC. In Fig. 1, it looked as if there was a correlation between inter-metal distances and cell performance, i.e. the larger inter-metal distances are related to the inferior cell performance. 

Before discussing the preparation of late transition metal complexes resulting from the activation of O-H bonds by late transition metal complexes, we wbl describe metathesis methods for the preparation of hydrido(hydroxo), hydrido(alkoxo), and hydrido(carboxylato) complexes. Though many methods of preparation of transition metal hydroxides, alkoxides, etc. by a metathesis reaction have been reported [1], only a limited number of examples of the preparation of hydrido(hydroxo), hydri-do(alkoxo) complexes etc. by metathesis are available. 

In marked contrast to the majority of activated metals prepared by the reduction process, cobalt showed limited reactivity toward oxidative addition with carbon halogen bonds. Iodopentafluorobenzene reacted with 2 to give the solvated oxidative addition products CoL and Co(C,F5)2 or Co(C F )L The compound CoiOJF 2PEt, was isolated in 54% yield by addition of triethylphosphine to tne solvated materials. This compound was also prepared in comparable yield from 1 by a similar procedure. This compound had previously been prepared by the reaction of cobalt atom vapor with C6F5I(81). 

The preparation of catalysts usually involves the impregnation of a support with a solution of active metal salts. The impregnated support is then dried, calcined to decompose the metal salt and then reduced (activated) to produce the catalyst in its active form. Microwaves have been employed at all stages of catalyst preparation. Beneficial effects of microwave heating, compared with conventional methods, have been observed especially in the drying, calcination, and activation steps. 

An alternative approach for the preparation of supported metal catalysts is based on the use of a microwave-generated plasma [27]. Several new materials prepared by this method are unlikely to be obtained by other methods. It is accepted that use of a microwave plasma results in a unique mechanism, because of the generation of a nonthermodynamic equilibrium in discharges during catalytic reactions. This can lead to significant changes in the activity and selectivity of the catalyst. 

H. Bonnemann, W. Brijoux, R. Brinkmann, E. Dinjus, T. Jouben, R. Fretzen, and B. Korall, Highly dispersed metal clusters and colloids for the preparation of active liquid-phase hydrogenation catalysts, J. Mol. Catal. 74,323-333 (1992). 

In many syntheses activation is not effected by sonochemical preparation of the metal alone but rather by sonication of a mixture of the metal and an organic reagent(s). The first example was published many years ago by Renaud, who reported the beneficial role of sonication in the preparation of organo-lithium, magnesium, and mercury compounds [86]. For many years, these important findings were not followed up but nowadays this approach is very common in sonochemistry. In another early example an ultrasonic probe (25 kHz) was used to accelerate the preparation of radical anions [87]. Unusually for this synthesis of benzoquinoline sodium species (5) the metal was used in the form of a cube attached to the horn and preparation times in diethyl ether were reduced from 48 h (reflux using sodium wire) to 45 min using ultrasound. 

Both macrocyclic and macrobicyclic ligands allow preparation of alkali metal solutions, but the former yield mainly M-, whereas solvated electrons are obtained with the [2]-cryptates (162,163). The enhancement of anion reactivity should also useful for activating anionic polymeriza-... 

Preparation of alkenylcarbene metal complexes was reported by Le Bozec and Dixneuf et al. in 1991 by activation of propargylic alcohols in the presence of methanol [13[. Thus, photolysis of M(CO)6 (M = Cr, W) in the presence of 2-propyn-l-ol derivatives 30 in the presence of MeOH gave the corresponding... 

A recent development of work with metal vapors, which lies between atom chemistry and conventional synthetic chemistry, is the preparation of reactive metal slurries. When a metal vapor is condensed with an inert organic compound, e.g., an alkane or sometimes an ether, and the condensate is allowed to warm to room temperature, the resultant slurry contains metal in a reactive form. It is less reactive than the metal atoms because aggregation of the atoms has occurred and is comparable in reactivity to active forms of metal produced by other methods, e.g., Raney nickel. The catalytic and synthetic potential of these metal slurries is being explored (55, 60). 

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