Redox reactions macrocycles

Following the generation of a macrocyclic ring by a template reaction it has often been the practice to modify the ring by further reaction. Examples of such modification are given by the formation of the derivative complexes (71) and (72) discussed in Section 2.3. Other examples are described in subsequent chapters - in particular, ligand redox reactions have been widely used and a range of such reactions is presented in Chapter 8. 

As mentioned previously, a large number of redox reactions involving macrocyclic ligand complexes have resulted in discrete changes in the unsaturation pattern of a variety of macrocyclic systems. Chemical, electrochemical, and catalytic reactions have been widely used to change the level of unsaturation in such systems. Although the mechanisms of the majority of such transformations are not well understood, it is clear that the reactions tend to proceed via prior change in the oxidation state of the central metal ion. 

The reactions of azofurazans have been used to obtain the hydrazine and the amino derivatives. For example, reactions of azofurazans, including macrocyclic azofurazan 196, with BunLi and the lithium derivatives of methylfur-azans were studied. Several competitive processes were found to occur (1) the addition of a Li reagent at the N=N bond (2) the redox reaction giving rise to hydrazofurazans and (3) the reaction of the side chain of azofurazan (Scheme 44) <2004RCB615>. 

The electrochemistry of dioxoosmium(VI) complexes has also been extensively studied. The tra 5-dioxoosmium(VI) complexes of polypyridyl and macrocyclic tertiary amine ligands display very similar proton-coupled electron transfer couples. In aqueous solutions at pH < 5-7 the cyclic voltammograms of n-a i-[0s (0)2(bpy)2] show a remarkable reversible three-electron couple and a one-electron Os coimle. In the Pourbaix diagram two break points are observed in the pH dependence of the Os couple, which correspond to the pAa values of Os —OH2 and Os —(OHXOH2) (Figure 10). The redox reactions are shown in Equations (41)-(43). At pH >8 the 3e Os wave splits into a pH-independent le Os wave and a 2e/2H" Os wave (Equations (44) and (45)). 

Subsequently, Backvall and coworkers developed triple-catalysis systems to enable the use of dioxygen as the stoichiometric oxidant (Scheme 3) [30-32]. Macrocyclic metal complexes (Chart 1) serve as cocatalysts to mediate the dioxygen-coupled oxidation of hydroquinone. Polyoxometallates have also been used as cocatalysts [33]. The researchers propose that the cocatalyst/BQ systems are effective because certain thermodynamically favored redox reactions between reagents in solution (including the reaction of Pd° with O2) possess high kinetic barriers, and the cocatalytic mixture exhibits highly selective kinetic control for the redox couples shown in Scheme 3 [27]. 

Scheme 15.9 Reversible shuttling of the benzylic amide macrocycle in [2]rotaxane 53 by redox reactions.

Figure 2 Precursors of macrocyclic ligands which allow redox reactions for monomeric molybdenum centres...

Nickel(II) complexes with a variety of tetraaza macrocycles have been found to undergo facile one-electron redox reactions. Such reactions have been accomplished by means of both chemical and electrochemical procedures. The kinetic inertness and thermodynamic stability of the tetraaza macrocyclic complexes of nickel(II) make them particularly suitable systems for the study of redox processes. A very extensive summary of the potentials for the redox reactions of nickel(II) complexes with a variety of macrocycles is given in ref. 2622. 

The electrochemistry of both rans-dioxoosmium(VI) and rutheni-um(VI) macrocyclic tertiary amine complexes has been studied in great detail. Both trans-[RuVI(b)2(0)2]2+ and rans-[RuVI(14TMC)(0)2]2 + and its related complexes display similar cyclic voltammograms in aqueous solution (132, 212). At pH < 7, three reversible/quasi-reversible redox couples, corresponding to the redox reactions are observed ... 

Extended conjugated systems of jr-electrons make these compounds promising candidates for various elecfronic devices [49, 50] and solar cell compo-nenfs [51,52]. These macrocycles provide a variefy of subsfifufion sifes which can be used fo create compounds with controlled electron donor/acceptor properties [53, 54]. In nature, metal-containing macrocycles are present as central structural units in many substances involved in biochemical redox reactions [55]. 

Metal-free porphyrins can undergo several steps of reduction and oxidation at the macrocyclic ring Tr-system. Metalloporphyrins may undergo reduction and oxidation reactions both at the porphyrin Tr-system and at the central metal ion. The site and rate of such redox reactions strongly depend on the porphyrin structure, the nature of the central metal ion, and the environment. Many of the fundamental reactions of porphyrins and metalloporphyrins have been studied by radiation chemical methods these studies are reviewed in this chapter. 

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