Metal complexes with free radicals

Chain growth polymerization has the characteristic of having an intermediate within the process that cannot be isolated [5], The intermediate can be a metal complex, a free radical, or an ion. These intermediates are transient to the process. The terms vinyl, olefin, and addition polymerization have been associated with this process [13], Monomer units add to a chain very rapidly once it has been initiated. Initiation is the creation of an active center such as a free radical or carbanion [13], An example is the thermal decomposition of benzoyl peroxide shown in Figure 3.4. To propagate the chain, an additional monomer is added at a very rapid rate as monomer concentration is reduced. Figure 3.5 shows the propagation of polystyrene. 

Direct free radical inhibitors suppress free radical formation by reacting with free radicals to form new inactive radicals (Reactions (1) and (2)) or chelating catalytically active transition metals to form inactive complexes ... 

This time-constant rate is proportional to the a-TiCU amount which proves that, at least formally, the over-all polymerization process is really a catal3rtic one, with regard to the a-TiCh. The catalytic behavior of -TiCU is, in any case, connected with the existence on its surface of metal-lorganic complexes which act in the polymerization only if a-TiCU is present. This makes stereospecific polymerization processes (of coordinated anionic nature) very different from the better known polymerization processes, initiated with free radicals. In the latter process, the initiator is not a true catalyst, since it decomposes during the reaction, forming radicals which are bound to the dead polymer on the contrary, in the case of stereospecific polymerization, each molecule of polymer, at the end of its growing period, can be removed from the active center on the solid surface of the catalyst which maintains its initial activity. 

Reaction 11 involves hydrogen atom transfer as proposed by Halpern et al. (13) in the mechanism of formic acid oxidation by cobalt (III) in aqueous solutions. In this reaction one could consider that as peracetic acid approaches the coordination sphere of Co111 and transfers the hydrogen atom to the coordinated acetate, the Co111 atom is transformed into a Co11 complex of peracetoxy radical (or Co111 complex of peracetate anion). Complexes of free radicals with metal ions have been postulated by Kochi (16). The substitution rate in this complex could be intermediate between the rate of substitution of cobalt (III) and cobalt (II) complexes owing to the contribution of the resonance structures ... 

A recent development31 is the preparation of metal polymer complexes directly on the electrode via the electrochemically induced polymerization of the metal complex. Ruthenium(II) and osmium(II) complexes with ligands containing aromatic amines, e.g. 3- or 4-aminopyridine or 5-amino-1,10-phenanthroline, are electrochemically polymerized to yield a film of the metal polymer on the electrode surface. The polymerization involves free radicals, which are formed via the initial oxidation of the metal complex to a radical cation and subsequent reaction of the radical cation with a base to yield the free radical. 

Martini M, Termini J (1997) Peroxy radical oxidation of thymidine. Chem Res Toxicol 10 234-241 Masarwa M, Cohen H, Meyerstein D, Hickman DL, Bakac A, Espenson JH (1988) Reaction of low-va-lent transition-metal complexes with hydrogen peroxide. Are they,Fenton-like or not 1. The case of Cu+aq and Cr2+aq. J Am Chem Soc 110 4293-4297 Maskos Z, Koppenol WH (1991) Oxyradicals and multivitamin tablets. Free Rad Biol Med 11 609-610... 

In general, the introduction of spatially hindered phenols into coordination compounds may produce stable free-radical forms [138b—140]. A series of metal complexes with redox ligands, containing derivatives of 2,6-di-t-butylphenols n- or a-connected, or vicinal fragments in the coordination environment of the central metal atom, were synthesized in this way 7i-aryl [141], Tt-cr-allyl [142] compounds, nitrile complexes [143], metal glioximates [144], salicylaldiminates [145,146], por-phyrines [147-149], and phthalocyanines [150,151],... 

In the original process using tin amides, transmetallation formed the amido intermediate. However, this synthetic method is outdated and the transfer of amides from tin to palladium will not be discussed. In the tin-free processes, reaction of palladium aryl halide complexes with amine and base generates palladium amide intermediates. One pathway for generation of the amido complex from amine and base would be reaction of the metal complex with the small concentration of amide that is present in the reaction mixtures. This pathway seems unlikely considering the two directly observed alternative pathways discussed below and the absence of benzyne and radical nucleophilic aromatic substitution products that would be generated from the reaction of alkali amide with aryl halides. 

It may be concluded from the preceding discussion that at this juncture there is no bona fide evidence for the initiation of autoxidations by direct hydrogen transfer between metal-dioxygen complexes and hydrocarbon substrates. Although such a process may eventually prove feasible, in catalytic systems it will often be readily masked by the facile reaction of the metal complex with hydroperoxide. The choice of cumene as substrate by many investigators is somewhat unfortunate for several reasons. Cumene readily undergoes free radical chain autoxidation under mild conditions and its hydroperoxide readily decomposes by both homolytic and heterolytic processes. 

We have mentioned in Section II.B.2 studies of the oxidation of olefins by molecular oxygen in the presence of low-valent Group VIII metal complexes, with the expectation of effecting homogeneous, nonradical oxidation processes. However, these reactions were shown to involve the usual free radical chain autoxidation, and no direct transfer of oxygen from a metal-dioxygen complex to an olefin was demonstrated. 

On the basis of an EPR analysis, the reaction of M(acac) with NO2 (at room temperature it is present as N2O4) has been proposed to occur via a step-by-step oxidation of the metal complex with final formation of metal nitrates and the iminoxy free radical 111 (equation 79). The presence of a donor-acceptor complex between M(acac) and N2O4 has been postulated for the electron transfer to dinitrogen tetroxide. 

Controlled scission of the Cr-C bond in the presence of a substrate was used to determine the kinetics of reduction of a number of transition metal complexes with organic free radicals. The reaction scheme for Co(NH3)5py [65] is shown in Eqs. 53-55. 

It can, however, be questioned if the oxidation of cyclohexene in the pre.sence of several transition metals is a free-radical chain mechanism. Arzoumanian et al. have reported that the catalytic decomposition of cyclohexene hydroperoxide by a rhodium complex in benzene is not a free-radical chain reaction. They found that the decomposition of each cyclohexene hydroperoxide gives rise to only one free radical [9]. In this ca.se the decomposition products were found to be 18 % l-ol (2), 33 % 1-one (1) and 35 % polymers together with small amounts of water and oxygen. They were also able to identify two free-radical species in solution, namely the cyclohexenylperoxy radical (6) and the cyclohexenyloxy radical (7). 

The rate of polymerization of polar monomers, for example, maleic anhydride, acrylonitrile, or methyl methacrylate, can be enhanced by coraplexing them with a metal halide (zinc or vanadium chloride) or an organoaluminum halide (ethyl aluminum sesqui-chloride). These complexed monomers participate in a one-electron transfer reaction with either an uncomplexed monomer or another electron-donor monomer, for example, olefin, diene, or styrene, and thus form alternating copolymers (11) with free-radical initiators. An alternating styrene/acrylonitrile copolymer (12) has been prepared by free-radical initiation of equimolar mixtures of the monomers in the presence of nitrile-coraplexing agents such as aluminum alkyls. 

In addition to the studies on reduction and oxidation of metalloporphyrins, radiolytic methods have been used to investigate reactions of radicals with metalloporphyrins that lead to formation of metal-carbon bonds. Formation of metal-carbon bonds has been implicated in various catalytic reactions and in biological systems. Therefore, numerous studies have been carried out on the formation and decomposition of such bonds involving porphyrin complexes of Pe 38.s3,62,68-70co, ° Rh, and other metals, as well as complexes of related macrocycles, such as Co-phthalocyanine and Co-B,2. Certain oxidation states of transition metal ions react with free radicals by attachment to form organometallic products, some of which are stable but others are short-lived. Pulse radiolysis has been used to investigate the formation and decay of such species. 

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