Salt-metal transition

The facile and reversible reaction of propagating species with transition metal halide complexes to form a polymeric halo-compound is one of the key steps in atom transfer radical polymerization (ATRP, see Section 9.4). 

These traps react with radicals by ligand- or electron-transfer to give products which can be determined by conventional analytical techniques. 

These reactions can be considered as electron-transfer processes, in which various atoms or molecules ionize or ions become discharged. Electron transfer processes play a very important role in the photoactivity of pigments such as titanium oxide (cf. section 5.1) and zinc oxide (cf. section 5.2), and in photo-Fenton reactions (cf. section 5.6). 

It has also been shown that iron oxides are activators of the photooxidation of nylon 6,6 the red form (Fe203) is more photo-active than the black form (Fc304) [66]. 

Inorganic salts participate in a number of photochemical reactions. The transfer of electrons from one ion to another ion, or to solvent molecules, plays an important role in the majority of inorganic photoreactions. A number of observations suggest that water of hydration participates in the electron transitions, e.g.  

It is assumed that the latter species dissociates or reaets in various ways, for example  

Cations which are photoreduced through an electron transfer process include (180nm), Cu (200nm), (308nm), Fe (230nm) and Ce  

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We have already seen that in the aquo-complex which is usually formed when a simple transition metal salt dissolves in water, the... 

Of course, these schemes indicate only that the overall reactions may be classified as nucleophilic 1,3-substitutions and, in the last case, as electrophilic 1,3-substitut ions. The reactions often proceed only in the presence of catalytic or stoichiometric amounts of transition metal salts, while in their absence 1,1--substitutions or other processes are observed. The 1,1-substitutions are also catalyzed by salts of transition metals, and it is not yet well understood, which factors influence the 1,1 to 1,3-ratio. In a number of 1,3-Substitutions there is... 

The Acetaldehyde Oxidation Process. Liquid-phase catalytic oxidation of acetaldehyde (qv) can be directed by appropriate catalysts, such as transition metal salts of cobalt or manganese, to produce anhydride (26). Either ethyl acetate or acetic acid may be used as reaction solvent. The reaction proceeds according to the sequence... 

Alkali or alkaline-earth salts of both complexes are soluble in water (except for Ba2[Fe(CN)g]) but are insoluble in alcohol. The salts of hexakiscyanoferrate(4—) are yellow and those of hexakiscyanoferrate(3—) are mby red. A large variety of complexes arise when one or more cations of the alkah or alkaline-earth salts is replaced by a complex cation, a representative metal, or a transition metal. Many salts have commercial appHcations, although the majority of industrial production of iron cyanide complexes is of iron blues such as Pmssian Blue, used as pigments (see Pigments, inorganic). Many transition-metal salts of [Fe(CN)g] have characteristic colors. Addition of [Fe(CN)g] to an unknown metal salt solution has been used as a quaUtative test for those transition metals. 

Other miscellaneous compounds that have been used as inhibitors are sulfur and certain sulfur compounds (qv), picryUiydrazyl derivatives, carbon black, and a number of soluble transition-metal salts (151). Both inhibition and acceleration have been reported for styrene polymerized in the presence of oxygen. The complexity of this system has been clearly demonstrated (152). The key reaction is the alternating copolymerization of styrene with oxygen to produce a polyperoxide, which at above 100°C decomposes to initiating alkoxy radicals. Therefore, depending on the temperature, oxygen can inhibit or accelerate the rate of polymerization. 

The discovery by Ziegler that ethylene and propylene can be polymerized with transition-metal salts reduced with trialkyl aluminum gave impetus to investigations of the polymerization of conjugated dienes (7—9). In 1955, synthetic polyisoprene (90—97% tij -l,4) was prepared using two new catalysts. A transition-metal catalyst was developed at B. E. Goodrich (10) and an alkaU metal catalyst was developed at the Ekestone Tke Rubber Co. (11). Both catalysts were used to prepare tij -l,4-polyisoprene on a commercial scale (9—19). 

Alkyl hydroperoxides give alkoxy radicals and the hydroxyl radical. r-Butyl hydroperoxide is often used as a radical source. Detailed studies on the mechanism of the decomposition indicate that it is a more complicated process than simple unimolecular decomposition. The alkyl hydroperoxides are also sometimes used in conjunction with a transition-metal salt. Under these conditions, an alkoxy radical is produced, but the hydroxyl portion appears as hydroxide ion as the result of one-electron reduction by the metal ion. ... 

Alkali-metal graphites are extremely reactive in air and may explode with water. In general, reactivity decreases with ease of ionization of M in the sequence Li > Na > K > Rb > Cs. Under controlled conditions H2O or ROH produce only H2, MOH and graphite, unlike the alkali-metal carbides M2C2 (p. 297) which produce hydrocarbons such as acetylene. In an important new reaction CgK has been found to react smoothly with transition metal salts in tetrahydrofuran at room temperature to give the corresponding transition metal lamellar compounds ... 

Guo et al. [70,71,73] recently attempted to hydrogenate NBR in emulsion form using Ru-PCy complexes. However, successful hydrogenation can only be obtained when the emulsion is dissolved in a ketone solvent (2-butanone). A variety of Ru-phosphine complexes have been studied. Crosslinking of the polymer could not be avoided during the reaction. The use of carboxylic acids or first row transition metal salts as additives minimized the gel formation. The reactions under these conditions require a very high catalyst concentration for a desirable rate of hydrogenation. 

Thermolysis rates are enhanced substantially by the presence of certain Lewis acids (e.g. boron and aluminum halides), and transition metal salts (e.g. Cu ", Ag1).46 There is also evidence that complexes formed between azo-compounds and Lewis acids (e.g. ethyl aluminum scsquichloridc) undergo thermolysis or photolysis to give complexed radicals which have different specificity to uncomplexed radicals.81 83... 

The decomposition of diacyl peroxides (36) is catalyzed by various transition metal salts,46,167 for example, Cu+ (Scheme 3.28).168,169 A side reaction is oxidation of alkyl radicals by the oxidized fonn of the metal salt e.g. Cu2+). 

Various multicomponent systems have also been described. Three component systems in which a second reducing agent (eg. sulfite) acts to recycle the transition metal salt, have the advantage that less metal is used (Scheme 3.43). 

Certain transition metal salts can be used as radical traps (Scheme 3.89, Scheme 3.90).486 These include various cupric (e.g. Cu(OAc)2, CuCl , Cu(SCN)i),l8 1<,8 J< 3 432 487 ferric (e.g. FeCli),316 488 and titanotis salts (eg. TiCL,).379 These traps react with radicals by ligand- or electron-transfer to give products which can be determined by conventional analytical techniques. 

Common inhibitors include stable radicals (Section 5.3.1), oxygen (5.3.2), certain monomers (5.3.3), phenols (5.3.4), quinones (5.3.5), phenothiazine (5.3.6), nitro and nitroso-compounds (5.3.7) and certain transition metal salts (5.3.8). Some inhibition constants (kjkp) are provided in Table 5.6. Absolute rate constants (kj) for the reactions of these species with simple carbon-centered radicals arc summarized in Tabic 5.7. 

Transition metal salts trap carbon-centered radicals by electron transfer or by ligand transfer. These reagents often show high specificity for reaction with specific radicals and the rates of trapping may be correlated with the nucleophilicity of the radical (Table 5.6). For example, PS radicals are much more reactive towards ferric chloride than acrylic propagating species."07... 

Various transition metal salts have been applied in quantitative determination of initiation reactions (Section 3.5.2.2). Under some circumstances, the ligand transfer may be reversible under the polymerization conditions. This chemistry forms the basis of ATRP (Section 9.4). 

Thermochromic phase transitions in transition metal salts. D. R. Bloomquist and R. D. Willett, Coord. Chem. Rev., 1982,47,125-164 (75). 

Finally, chain polymerisation can occur via coordination, as is the case for polymerisation involving Ziegler-Natta catalysts. These catalysts are complexes formed between main-group metal alkyls and transition metal salts. Typical components are shown in Table 2.1. 

Main group metal alkyls Transition metal salts... 

Activation of Silicon Bonds by Transition Metal Salts and Complexes... 

In addition to activation of sihcon bonds by fluoride ions as discussed in Section 2.4, silicon-silicon, silicon-carbon, silicon-hydrogen, and silicon-nitrogen bonds are activated by transition metal salts and transition metal complexes. Thus, hydrolysis of silicon-carbon bonds such as in phenyltrimethylsilane 81 can be induced by... 

Apart from the hardness and softness, two reactivity-related features need to be pointed out. First, iron salts (like most transition metal salts) can operate as bifunctional Lewis acids activating either (or both) carbon-carbon multiple bonds via 71-binding or (and) heteroatoms via a-complexes. However, a lower oxidation state of the catalyst increases the relative strength of coordination to the carbon-carbon multiple bonds (Scheme 1). 

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