Transition-metal alkyl ions

The strongest evidence in favor of propagation at the transition metal-alkyl bond is the existence of one-component, that is, metal-alkyl-free polymerization catalysts. Of these systems the Phillips catalyst was studied most thoroughly because of its commercial importance. Originally it was believed that Cr(VI) ions stabilized in the form of surface chromate and perhaps dichromate resulting from the interaction of Cr03 with surface hydroxyl groups above 400°C are the active species in polymerization 286,294... 

The syntheses of transition metal alkyls can be accomplished in several ways. A common approach is to take advantage of Ihe nucleophilicity of a carbonylale ion.59 For example ... 

The highly electronegative metal ion affords the appropriate empty orbitals for the stabilization of the complex anion in the transition state. Both late and early transition metal alkyls are prone to this reaction, but its occurrence had to be particularly, invoked in the case of the early transition metals. Many similar reactions, such as the reaction of metal alkyls with other FIX compounds, could be described as if they followed this pathway, but the use of the term a-bond metathesis is restricted to those reactions in which one reacting species is a metal hydrocarbyl or metal hydride and the other reactant is a hydrocarbon or dihydrogen. Two reactions have been depicted in Fig. 4.34. There are, of course, borderline cases when the reacting hydrocarbon is acidic, as in the case of 1-alkynes, a direct attack of the proton at the carbanion can be envisaged. It has been proposed that acyl metal complexes of the late transition metals may also... 

Evidence for the above reactions comes from kinetic studies (ISO), and from the thermal decomposition of ethyltetracarbonylcobalt which gives ethylene and tetracarbonylcobalt hydride (130). Earlier studies have eliminated possible free radical mechanisms (4). Supporting evidence arises from the demonstration of the reversible addition of ethylene to a platinum hydride (see Eq. 27) and from the observation that a number of transition metal alkyls have been shown to lose readily a hydride ion from the 2-carbon of the alkyl group (101). 

Electrophilic metal ions, notably, Hg + can cleave transition metal alkyl bonds relatively easily. Two main pathways have been identified, one of which is attack at the a carbon of the alkyl, which can lead to inversion of configuration... 

Electrophilic metal ions, notably can cleave transition metal alkyl bonds relatively easily. Two main pathways have been identified, one of which is attack at the a carbon of the alkyl, which can lead to inversion of configuration at (bat carbon (Eq. 8.3S). In the other, attack occurs at the metal or at the M—C bond and retention of configuration is found (Eq. 8.36). The difference has been ascribed to the greater basicity of the metal in the iron case. ... 

Historically, cationic transition metal alkyls have been suspected to be the active species in many types of homogeneous olefin polymerizations for almost three decades. The ion pair model was Inferred from a number of different observations, such as solvent effects, electrochemical measurements, theoretical calculations, model ionization reactions, in situ syntheses of alkylated cations, the... 

Transition-metal-alkyl bonds can be formed by a variety of reactions that include metathetical replacement of a halide ion, oxidative addition, and insertion of an alkene into a metal-hydride bond. " A similar set of reactions is available for the synthesis of transition-metal-aryl bonds, although the analogous insertion of a benzyne intermediate into a metal-hydride bond is not particularly viable as a synthetic route. For alkyl complexes that have longer chains than methyl, thermal decomposition to give the metal-hydride complex by a j5-hydrogen transfer reaction is frequently observed at ambient temperature. 

The reactions of alkyl hydroperoxides with ferrous ion (eq. 11) generate alkoxy radicals. These free-radical initiator systems are used industrially for the emulsion polymerization and copolymerization of vinyl monomers, eg, butadiene—styrene. The use of hydroperoxides in the presence of transition-metal ions to synthesize a large variety of products has been reviewed (48,51). 

Alkyl hydroperoxides are among the most thermally stable organic peroxides. However, hydroperoxides are sensitive to chain decomposition reactions initiated by radicals and/or transition-metal ions. Such decompositions, if not controlled, can be auto accelerating and sometimes can lead to violent decompositions when neat hydroperoxides or concentrated solutions of hydroperoxides are involved. 

Multilayers of Diphosphates. One way to find surface reactions that may lead to the formation of SAMs is to look for reactions that result in an insoluble salt. This is the case for phosphate monolayers, based on their highly insoluble salts with tetravalent transition metal ions. In these salts, the phosphates form layer stmctures, one OH group sticking to either side. Thus, replacing the OH with an alkyl chain to form the alkyl phosphonic acid was expected to result in a bilayer stmcture with alkyl chains extending from both sides of the metal phosphate sheet (335). When zirconium (TV) is used the distance between next neighbor alkyl chains is - 0.53 nm, which forces either chain disorder or chain tilt so that VDW attractive interactions can be reestablished. 

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. ... 

Two-component systems are obtained by the interaction of transition metal compounds of groups IV-VIII of the periodic system with or-ganometallic compounds of groups I-III elements (Ziegler-Natta catalysts). An essential feature of the formation of the propagation centers in these catalysts is the alkylation of the transition metal ions by an organo-metallic cocatalyst. 

The radicals formed in these reactions may also participate in the alkylation of ions of the transition metal ... 

However, these reactions remain hypothetical, and the mechanism of alkylation of low-valent coordinatively insufficient ions during their interaction with hydrocarbons calls for a detailed study. When the activation by some additives is performed the formation of the active transition metal-carbon bond by oxidative addition is also possible, e.g. in the case of such additives as alkylhalogenides or diazocompounds according to the schemes ... 

Variable valence transition metal ions, such as Co VCo and Mn /Mn are able to catalyze hydrocarbon autoxidations by increasing the rate of chain initiation. Thus, redox reactions of the metal ions with alkyl hydroperoxides produce chain initiating alkoxy and alkylperoxy radicals (Fig. 6). Interestingly, aromatic percarboxylic acids, which are key intermediates in the oxidation of methylaromatics, were shown by Jones (ref. 10) to oxidize Mn and Co, to the corresponding p-oxodimer of Mn or Co , via a heterolytic mechanism (Fig. 6). 

Castro CE, WC Kray (1963) The cleavage of bonds by low-valent transition metal ions. The homogeneous reduction of alkyl halides by chromous sulfate. J Am Chem Soc 85 2768-2773. 

These reactions are subject to catalysis by certain transition metal ions and with small amounts of MnBr2 or CuCl the reaction proceeds satisfactorily with alkyl bromides.134... 

Peroxyl radicals with a strong oxidative effect along with ROOH are continuously generated in oxidized organic compounds. They rapidly react with ion-reducing agents such as transition metal cations. Hydroxyl radicals react with transition metal ions in an aqueous solution extremely rapidly. Alkyl radicals are oxidized by transition metal ions in the higher valence state. The rate constants of these reactions are collected in Table 10.5. 

The aqua ion is not easily reduced nor oxidized. It is the slowest reacting of the bivalent transition metal ions with e " k = 7.7 X 10 M s ) and the product Mn+q is very reactive. However Mn(CNR)5 (R = a variety of alkyl and aryl groups) is stable and the selfexchange in the Mn(l,ll) hexakis(isocyanide) system has been studied by Mn and H nmr line broadening. The effects of solvent, temperature, pressure and ligand have been thoroughly explored. 

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