Metal redox active centres

In 1990 we reported the synthesis of new redox-responsive crown ether molecules that contain a conjugated link between the crown ether unit and a ferrocene redox-active centre (Beer et al., 1990a). Examples of some of the species synthesized are shown in Fig. 5. The electrochemical behaviour of these species was investigated and also the electrochemical behaviour of their analogues with a saturated link between the ferrocene unit and the crown ether. The changes in the CVs of [2a] upon addition of magnesium cations are shown in Fig. 6. The metal cation-induced anodic shifts of [2a], [2b] and also their saturated analogue [3] and vinyl derivatives [4a], [4b] are shown in Table 1. 

Compounds [54] and [55] have been shown to complex group 1 and 2 metal cations and also ammonium and alkylammonium cations by nmr and UV/Vis spectroscopies and also by a number of solid-state X-ray crystallographically determined structures. The quinone moieties in these molecules constitute not only the coordination site but also the redox-active centre. The complexation... 

In this section we focus on the Zn(II)-containing enzymes carbonic anhydrase II and carboxypeptidases A and G2. These are somewhat different from other systems so far described. Zinc(II) is not a redox active centre, and so cannot take part in electron-transfer processes. It is, however, a hard metal centre (see Table 6.9) and is ideally suited to coordination by N- and O-donors. It is also highly polarizing, and the activity of Zn(II)-containing metallo-enzymes depends on the Lewis acidity of the metal centre. 

More recent approaches to the effects of the ligands on the redox activity of metal complexes are based upon the assumption that the electrode potential of a redox change involving a metal complex is determined by the additivity of the electronic contribution of all the ligands linked to the metal centre, or to the overall balance between the c-donor and the 7r-acceptor capability of each ligand.3 In particular two ligand electrochemical parameters have gained popularity ... 

Mammalian cytochrome oxidase has attracted particular attention. It contains four redox-active transition metal centres, two type a cytochromes (a and a3) and two copper ions. Other oxidases... 

It has long been established that the functional unit of cytochrome oxidase contains four redox-active metal centres. Two of these, cytochromes a and a3, contain heme A (Fig. 5-23) coordinated in different ways in subunit I. The heme group is not covalently linked to the protein. The main structural features are the carbonyl group at position 8 and the isoprenoid chain at position 2 of the porphyrin ring. 

A series of heterotrinuclear complexes of the tetraketone 134 has been characterized. In such complexes the two outer O4 compartments are occupied by a [U02] + ion, whereas the central O4 compartment is occupied by a M(II) ion (M = Mn, Co, Ni, Cu, Zn). As a typical example of the electrochemical behaviour of such complexes, Figure 9 compares the cyclic voltammetric response of (U02)2Zn(dbba)2(py)4 with that of (U02)2Ni(dbba)2(py)4. The complex containing the redox-inactive zinc(II) centre, (U02)2Zn(dbba)2(py)4, affords two separate reductions at about —1.1 and —1.3 V (vs. SCE), which are assigned to the U(VI) U(V) reduction of each uranyl ion. The insertion of the redox-active nickel(II) as central metal ion causes the appearance of the further Ni(II) Ni(I) reduction at about —1.5 V. A similar behaviour is exhibited by (U02)2Cu(dbba)2(py)4, with the only difference that the Cu(II) Cu(I) reduction occurs at about —0.5 V, thus preceding the two separate U02 + reductions. 

A variation of the above strategy has been used to produce the related [2]-rotax-ane 16 in 15% yield. In this case, two potentially redox-active fullerenes act as the stoppers. CgQ was chosen for inclusion in this product because of its interesting electrochemical and electronic properties and, in particular, because it is a strong electron acceptor. Although the substituted fullerene stoppers appear to have a significant influence on the redox properties of the metal centre (an anodic shift occurs), the converse is not true and the metal does not significantly affect the redox properties of the fullerene groups. 

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