Electron Transfer Reactions Involving Transition Metal Ions

Electron-Transfer Reactions Involving Transition-Metal Ions 

Most of the free-radical mechanisms discussed thus far have involved some combination of homolytic bond dissociation, atom abstraction, and addition steps. In this section, we will discuss reactions that include discrete electron-transfer steps. Addition to or removal of one electron fi om a diamagnetic organic molecule generates a radical. Organic reactions that involve electron-transfer steps are often mediated by transition-metal ions. Many transition-metal ions have two or more relatively stable oxidation states differing by one electron. Transition-metal ions therefore firequently participate in electron-transfer processes. 

The decomposition of peroxyesters has been shown to be strongly catalyzed by Cu(I). The process is believed to involve oxidation of the copper to Cu(II)  

An example of this reaction is the reaction of cyclohexene with t-butyl perbenzoate, which is mediated by Cu(I). The initial step is the reductive cleavage of the perester. The t-butoxy radical then abstracts hydrogen from cyclohexene to give an allylic radical. The radical is oxidized by Cu(II) to the carbocation, which captures benzoate ion. The net effect is an allylic oxidation. 

The reactions of copper salts with diacyl peroxides have been investigated quite thoroughly, and the mechanistic studies indicate that both radicals and carbocations are involved as intermediates. The radicals are oxidized to carbocations by Cu(II), and the final products can be recognized as having arisen from carbocations because characteristic patterns of substitution, elimination, and rearrangement can be discerned  

SECTION 12.8. ELECTRON-TRANSFER REACTIONS INVOLVING TRANSITION-METAL IONS 

Electron Transfer Reactions Involying Transition Metal Ions 

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Rate constant data for several homonuclear electron transfer reactions involving transition metal complex ions in water are summarized in table 7.1. The striking feature of the results is that the rate constants vary over a very wide range from a low 2x 10 s to a high of 4 x lO M s. Since these... 

Table 7.1 Kinetic Data for Homonuclear Electron Transfer Reactions Involving Transition Metal Complex Ions in Water at 25°C [5] ...

Because of their high negative normal redox potentials, Sm + and Yb + are able to reduce many ions. This property has not yet been used for preparative purposes but it has been exploited in mechanistic studies of electron transfer reactions involving transition metal complexes. Rate constants of reactions between Yb + ions (prepared by electrolytic reduction of Yb " ) and some Co " " complexes were measured (Christensen et al., 1970). More accurate kinetic measurements, with the pulse radiolysis technique, have been made (Faraggi et al., 1973) on the reactions of Yb + and Sm + with [Co(NH3)5Xp+ (X = F, Cl, Br, I, N3, NCS, OH, CN), [Co(NH3)d +, [Ru(NHj)5X]2+ (X = Cl, Br, I, OH) and [Ru H3)J +. The reactions were performed in deaerated aqueous solutions containing tert-butyl alcohol. 

Much of the early work on electron transfer reactions focused on systems involving transition metal ions. Many redox couples based on the Fe(III)/ Fe(II), Co(III)/Co(II), and Cr(III)/Cr(II) can be prepared in the laboratory by changing the ligands coordinated to the central metal ion. From these studies it is possible to distinguish between two types of reactions which would otherwise be classified as simple. A classic example from the work of Taube [3] is the reaction... 

This first decade of crystallographic study of enzymes helped to create two new fields bio-inorganic chemistry and mechanistic bio-organic chemistry. Harry B. Gray (1935- ), at Caltech, began to make fundamental contributions to the chemistry and chemical physics of metal-loproteins. Unlike reactions between transition metal ions and their complexes, which involve intimate contact and electron-transfer upon collision, electron transfer between metals on different proteins is not so intimate and may require several collisions and may proceed through... 

JmoP K similar in magnitude to that measured for d-transition metals. The authors concluded that there was no compelling reason to invoke a nonadiabatic path for electron transfer reactions involving Euf, ions. 

Studies of redox reactions involving transition metals in solution have established that there are basically two types of mechanism for electron transfer, outer sphere and inner sphere mechanisms. In addition to this distinction, redox reactions are divided into self exchange reactions between two different oxidation states of the same metal ion and redox reactions between complexes with different metal centres. In the following sections the mechanisms of these reactions will be discussed in conjunction with a discussion of the factors which influence the rates of redox reactions. 

Reactions involving the peroxodisulfate ion are usually slow at ca 20°C. The peroxodisulfate ion decomposes into free radicals, which are initiators for numerous chain reactions. These radicals act either thermally or by electron transfer with transition-metal ions or reducing agents (79). 

Acid-base (neutralization) reactions are only one type of many that are applicable to titrimetric analysis. There are reactions that involve the formation of a precipitate. There are reactions that involve the transfer of electrons. There are reactions, among still others, that involve the formation of a complex ion. This latter type typically involves transition metals and is often used for the qualitative and quantitative colorimetric analysis (Chapters 8 and 9) of transition metal ions, since the complex ion that forms can be analyzed according to the depth of a color that it imparts to a solution. In this section, however, we are concerned with a titrimetric analysis method in which a complex ion-forming reaction is used. 

The best known and most nsefnl of the chemiluminescent reactions involving electron transfer is the oxidation of luminol (3.100) or its derivatives in alkaline medium. The oxidant can be hydrogen peroxide, sodium ferricyanide or hypochlorite, usually with a catalyst that can be a transition metal ion, such as Cu " Co +, Fe + and Mtf+, or haem and haemproteins, e.g. peroxidases. The reaction mode is shown in Figure 3.22.4"... 

The reactions leading to the formation of these polymers—except polyphenylene—have one feature in common, although they otherwise differ greatly in mechanism the crucial step in the reaction sequence is a one-electron transfer from the monomer to a transition metal ion serving as an electron acceptor. In addition to being an electron acceptor the transition metal ion is probably also involved in the coupling reaction by complexation of radical-like intermediates produced. 

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