Zero-valent metal, active

Sulfmyl radicals have also been invoked in the synthetically useful reaction of alkane-and arenesulfinyl chlorides with activated zero-valent metals (Ag, Cu, Zn) to give the corresponding symmetrical thiosulfonates21. 

Taken together, these initial findings may eventually lead to other recyclable DECS having either modulated activities or selectivities arising from steric effects. Moreover, other catalytic processes requiring a co-catalyst, such as the Wacker process, may be particularly amenable to dendrimer-based catalytic systems because (as discussed in Sect. 2.4.2) the dendrimer interior can accommodate two or more catalytic moieties for example, a metal particle (e. g., Pd) and a metal ion (e. g., Cu +), two different metal ions, or two different zero-valent metal. 

Surface morphology is expected to be crucial when reacting an organometallic compound with such particles. It will depend on particle size as well as other parameters such as the exposed planes of the support (e.g., by epitaxy). Metal particles of various sizes are composed of zero valent metal atoms, and these atoms have different natures, depending on their location in the core or at the surface of the particles. The active centers are located at the periphery of the particle. The... 

Precisely the last condition explains the fact that mainly ICC have been obtained by the immediate interaction of ligands and zero-valent metals. Thus, a large series of metal p-diketonates was synthesized in the absence of a solvent [513,634-638], for example, iron bis- and tra-acetylacetonates [635]. It was shown that other ligands can serve as activators or promoters in these processes. In particular, the introduction of a,a or y,y -bipy into the reaction mixture gives the possibility of isolating copper acetylacetonates and adducts of similar complexes of cobalt and nickel [636], meanwhile the p-diketonates of the metals above are not formed under conditions similar to those reported in Ref. 635. Under dissolution of more active metallic barium in the mixture of another p-dikctone - dipivaloyl-methane (DPM) - with dyglime (DG) or tetraglime (TG) in absolute pentane, the mononuclear complex [Ba(DPM)2(TG)] and binuclear complex [Ba2(DPM)4 ( t-H20)(DG)] were isolated and structurally characterized [637]. 

Palladium(0)-catalysed coupling reactions of haloarenes with alkenes, leading to carbon-carbon bond formation between unsaturated species containing sp2-hybridised carbon atoms, follow a similar mechanistic scheme as already stated, the general features of the catalytic cycle involve an oxidative addition-alkene insertion-reductive elimination sequence. The reaction is initiated by the oxidative addition of electrophile to the zero-valent metal [86], The most widely used are diverse Pd(0) complexes, usually with weak donor ligands such as tertiary phosphines. A coordinatively unsaturated Pd(0) complex with a formally d° 14-electron structure has meanwhile been proven to be a catalytically active species. This complex is most often generated in situ [87-91],... 

This reaction produces activated cobalt metal powder following decomposition of the carbonyl at 200°C in diphenyl ether (DPE). The metal carbonyls are typically very stable at room temperature at elevated temperatures they dimerize and decompose, leaving the zero-valent metal. Photochemical decomposition, or photolysis, is common with coinage metals such as silver and gold. While... 

Metal oxides are ubiquitous in catalysis and are key components in several catalytic reactions. They function directly as catalytic reactive centers or serve as high surface area supports to disperse active metal centers or as promoters to enhance the rate of catalytic reactions. Many commercial catalysts consist of zero valent metal atoms dispersed finely on a high surface area metal oxide support such as silica or alumina. 

The first section is mainly concerned with some of the emerging technologies for the production of novel activated carbons that may help to fulfill the increasing demands on performance as regulations on environmental pollution become more stringent. The next two sections deal with applications of conventional and new activated carbons in gas and liquid phase applications. The penultimate section considers the uses of carbons supports for zero-valent metal dehalogenation. The... 

Basically, we have shown thus far that we can produce a particular type of zero-valent transition metal atom. It matters little whether we call this an active zero-valent metal atom, a nacent metal, or highly excited reactive metal. The important fact remains that we may use this metal atom for (a) the catalytic formation of biphenyl, and (b) for the formation of various arene 7r-complexes. The difference is the reaction conditions and general environment surrounding its creation. We can now proceed further into the development of this concept, ultimately involving polymerization. 

The mechanism was discussed recently [15]. The zero-valent metal (produced at the interface by reduction of the metal cation) induces an oxidant addition, in principle followed by nucleophilic substitution and a reducing elimination. The scheme below exhibits a simplified catalytic mechanism with aromatic halides. Electrochemistry is involved in the activation phase (formation of zero-valent metal [16-18]). The nucleophile is obviously produced by the oxidant insertion, and in the catalytic cycle hereafter Ar—Nu has to be considered of course as the Ar—Ar dimer. 

Most crystalline aluminosilicates have little intrinsic catalytic activity for hydrogenation reactions. However, a considerable amount of data has recently accumulated on the use of zero-valent metal-containing zeolites in many hydrocarbon transformations. Thus noble and transition metal molecular sieve catalysts active in hydrogenation (7,256-760), hydroisomerization (161-165), hydrodealkylation (157, 158,165-167), hydrocracking (168,169), and related processes have been prepared. Since a detailed discussion of this class of reactions is beyond the scope of this review, only a few comments on preparation and molecular-shape selectivity will be made. 

On the other hand, Tsuji, Morikawa, and Takahashi found that ir-allyl palladium complexes react with nucleophiles, such as amines and enamines, and active methylene compounds, such as malonates, with reduction of bivalent palladium to the zero-valent metal 39>4o). This is due to the stability of the reduced species. 

The effect of the chemical properties of a support on the activity of dispersed zero-valent metals has been shown [27]. Chloromethylated polystyrene, cross-linked with 4-7% DVB with high porosity and surface area (0.1 -0.03 pm particles), was functionalized by the following groups rr-donor and 7r-acceptor groups capable of forming molecular associates with the substrate hydrophobic and hydrophilic groups... 

The origin of these enhanced activities is not clear. Some investigators argue that all cases of enhanced activity are based upon the presence of small amounts of zero-valent metal. This point is extremely difficult to prove either way. Given the number of reports of enhanced activity in a rather broad variety of reactions, using both phosphine and other ligands, it would appear that such an argument is inadequate. 

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