Metals, activated anions

Magnesium is a relatively reactive metal, and can be chromated in nearly neutral solutions as well as in acid solutions. The range of treatments possible illustrates well the role of pH, activating anion, temperature and duration of treatment in promoting the breakdown of passivity in the chromate solution and the consequent formation of a chromate him. 

Anodic dissolution reactions of metals typically have rates that depend strongly on solution composition, particularly on the anion type and concentration (Kolotyrkin, 1959). The rates increase upon addition of surface-active anions. It follows that the first step in anodic metal dissolution reactions is that of adsorption of an anion and chemical bond formation with a metal atom. This bonding facilitates subsequent steps in which the metal atom (ion) is tom from the lattice and solvated. The adsorption step may be associated with simultaneous surface migration of the dissolving atom to a more favorable position (e.g., from position 3 to position 1 in Fig. 14.1 la), where the formation of adsorption and solvation bonds is facilitated. 

All factors influencing the potentials of the inner or outer Helmholtz plane will also influence the zeta potential. For instance, when, owing to the adsorption of surface-active anions, a positively charged metal surface will, at constant value of electrode potential, be converted to a negatively charged surface (see Fig. 10.3, curve 2), the zeta potential will also become negative. The zeta potential is zero around the point of zero charge, where an ionic edl is absent. 

These examples serve to illustrate several general points about use of chiral catalysts for D-A reactions. A cationic metal center is present in nearly all of the catalysts developed to date and has several functions. It is the anchor for the chiral ligands and also serves as a Lewis acid with respect to the dienophile. The chiral ligands establish the facial selectivity of the complexed dienophile. There are several indications of the importance of the anions to catalytic activity. Anions, in general,... 

Once the optimal metal/ligand/anion combination has been determined, the choice of the metal complex which is actually put into the reaction solution will become of importance. Active hydrogenation catalysts are metal-ligand complexes which can either be prepared in situ simply by mixing a suitable precur-... 

Thallium(i) salts have long been used in reactions with organic and organometallic halide complexes as a means of activating the halide by removal as insoluble T1X. However, the thallium ions proved not to be innocent bystanders, and numerous examples were reported in COMC (1995) where the metal-bound thallium complexes were formed. Deliberate reactions of thallium(i) and thallium(m) salts with metal carbonyl anions have yielded a variety of complexes of the form T1 MLJ3. In the past decade, new examples of metal carbonyl derivatives of thallium have been prepared (see Table 2). In addition, the propensity for Tl+ to form adducts with 16-electron noble metal complexes has been exploited. 

Our efforts in this area of catalysis began in 1980. Our initial emphasis was on the preparation of supported phase transfer catalysts. We later became interested in the chemistry of anioni-cally activated alumina(25) and the reactivity of metal carbonyl anions prepared under these conditions. A brief description of our work in the preparation of these materials and their synthetic applications follows. 

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-... 

Recently the unprecedented example of stereoselective C—Si bond activation in cu-silyl-substituted alkane nitriles by bare CQ+ cations has been reported by Hornung and coworkers72b. Very little is known of the gas-phase reactions of anionic metal complexes with silanes. In fact there seems to be only one such study which has been carried out by McDonald and coworkers73. In this work the reaction of the metal-carbonyl anions Fe(CO) (n = 2, 3) and Mn(CO) (n = 3, 4) with trimethylsilane and SiH have been examined. The reactions of Fe(CO)3 and Mn(CO)4 anions exclusively formed the corresponding adduct ions via an oxidative insertion into the Si—H bonds of the silanes. The 13- and 14-electron ions Fc(CO)2 and Mn(CO)3 were observed to form dehydrogenation products (CO) M(jj2 — CH2 = SiMe2) besides simple adduct formation with trimethylsilane. The reaction of these metal carbonyl anions with SiFLj afforded the dehydrogenation products (CO)2Fe(H)(SiII) and (CO)3Mn(II)(SiII). ... 

A similar hybrid type of radical/anionic reactions can be effected, when manganese metal, activated by catalytic amounts of lead dichloride and trimethylchlorosi-lane, is employed instead of zinc, which makes the original process synthetically more reliable and attractive by reducing the amounts of reagents (RX and ketone) needed to a 1.5 molar excess over the alkenes (Scheme 6.36) [57]. 

Heterocyclization reactions with saturated moieties (alcohols, amines, thiols, etc.) or acids on unsaturated counterparts (alkenes, allenes, alkynes, etc.) are not covered in this chapter since they are addition, and not isomerization, reactions. Silver is also widely used as an activating agent for producing highly reactive metallic cations (anion metathesis), which, in turn, may catalyze cycloisomerization reactions. This aspect is covered only when the silver control experiments give substantial positive results. 

Metal-14 anions can add to activated ethylenic bonds8. Thus, the reaction of Ph3ELi (E = Ge, Sn) with cobaltocenium or decamethylcobaltocenium salts resulted in a... 

Generally, most oxygen transfer reactions employing alkyl hydroperoxides require transition metal activation, since the alkoxide anions are relatively poor leaving groups, even poorer than the hydroxide anion. The reasons why they are sometimes employed in preference to hydrogen peroxide are as follows ... 

Chemistry is different synthesising a molecular cage and putting into it a chemical entity (metal ion, anion or molecule) is considered a deserving and admirable action. The design and synthesis of cages at a molecular level has become so common and distinguished an activity to be mentioned in the... 

Tn transition Group Villa, complexes of iron, cobalt, nickel and rhodium are active in CO3 dcoxygenation. The reactivity of a series of metal carbonyl anions with carbon dioxide has been found to parallel their relative nucleophilicities. The highly nucleophilic (CpFefCO) ] reacts to form the dimer [CpFefCO) ] 2 and carbonate, whereas (CofCO) ]" is unrcactivc [284]. 

Compound (4) reacts with halides, preferably their alkylanunonium salts, in acetone or DME with exchange of one CO for a halide, giving anionic complexes of the type Co(CO)2(NO)X (X = Cl, Br, I, CN) (equation 29). Owing to the high charge density at the metal, these anions can act as sources for the hyponitrous ion, NO, which is transferred in nucleophihc substitutions at activated (benzyl) halides, giving aldoximes (e.g. PhCH=NOH from PhCH2Y) (equation 30). ... 

Because of the larger ionic radius of the actinide metals versus the group 4 metals (Th + = 1.08 A vs. Zr + = 0.79 A), organoactinides make good test compounds for the relative noncoordination of ionic activator anions... 

Li and Wong reported a Tb + complex Tb-26 with pendant aza-15-crown-5 (Figure 13.12) that showed recognition of two important bio-active anions, lactate and salicylate, in aqueous solution [48]. The luminescence lifetime of Tb-26 in H2O was dependent on the nature of the ratio in solution. Upon addition of S.Oequiv of lactate, the luminescence lifetime of Tb-26 increased from 1.45 ms to its maximum of 2.15 ms. Moreover, Tb-26 shows a high selectivity of lactate over other anions such as HCOJ, H2PO4, AcO , and Cl . In addition, two coordi-natively unsaturated terbium complexes Tb-27 and Tb-28 (Figure 13.12) could be displaced upon metal chelation to aromatic carboxylic anions, such as salicylate, in water [49]. 

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