Redox-active ligand

The coordination of redox-active ligands such as 1,2-bis-dithiolates, to the M03Q7 cluster unit, results in oxidation-active complexes in sharp contrast with the electrochemical behavior found for the [Mo3S7Br6] di-anion for which no oxidation process is observed by cyclic voltammetry in acetonitrile within the allowed solvent window [38]. The oxidation potentials are easily accessible and this property can be used to obtain a new family of single-component molecular conductors as will be presented in the next section. Upon reduction, [M03S7 (dithiolate)3] type-11 complexes transform into [Mo3S4(dithiolate)3] type-I dianions, as represented in Eq. (7). 

In addition, Bryce and Chesney have developed chiral oxazolines linked to tetrathiafulvalene in order to use these ligands as redox-active ligands. When applied to the test reaction, these ligands gave only low enantioselec-tivities (<21% ee), as shown in Scheme 1.32. 

In addition, Bryce et al. have studied the binding of palladium to other S/N-ferrocenyloxazoline ligands by cyclic voltammetry and proved that it was reversible.These redox-active liganding systems were successfully used in the test reaction, providing the product in both high yield and enantioselectivity of up to 93% ee, as shown in Scheme 1.70. 

The incorporation of radicals is a highly promising way to construct larger magnetically coupled clusters in 4f chemistry [44, 64], but this is yet to happen for the actinides [43]. A promising way forward seems to be the use of redox-active ligands [65]. 

A similar behaviour is exhibited by the isostructural [Zn(phen)3]2 +. 192 In MeCN solution it exhibits an irreversible reduction (Ep = — 1.40 V vs. SCE) followed by further ligand-centred irreversible processes (we must in fact remind that polypyridines are redox active ligands).193... 

Just zinc complexes bearing redox-active ligands sometimes display apparently reversible redox processes. Really, in these cases the electron transfer processes are centred on the ligand. This is, for example, the case of [Zn(papm)Cl2] (papm = 2-(phenylazo)pyrimidine), the (distorted) trigonal bipyramidal molecular structure of which is illustrated in Figure 134.194... 

Other, even more significant, examples of Zn(II) complexes with redox-active ligands will be discussed in Chapter 6, Section 5. 

Metal Complexes Containing Redox-active Ligands... 

LESS KNOWN REDOX-ACTIVE LIGANDS IN METAL COMPLEXES... 

As briefly alluded to, there are different classes of redox-active ligands in addition to the above mentioned ones. For example, we have seen in Chapter 5, Section 8, that azo-groups (in particular, 2-(phenylazo)pyr-imidine) are able to undergo two separate one-electron reduction processes. Conjugated polynitriles (mnt, tcne, tcnq) also constitute an important class of redox-active molecules and the electrochemical behaviour of their metal complexes has been reviewed.107 The same holds as far as alkyldithiocarbamates (Rdtc) and their metal complexes are concerned,108 or nitrosyl complexes in their possible NO+[NO fNO redox sequence.109 Thus, we would like to conclude the present Chapter by discussing a few less known redox non-innocent ligands. 

These complexes combine a central metal in a high oxidation state with a redox active ligand (catechol). This combination arises from the idea that the electronic perturbation induced in the metal complex by reaction with dioxygen can discharge itself ... 

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