1,2-Dioxolene complexes catecholates

A series of catecholate derivatives 189 and 190 were prepared from the parent dichloro complexes by reaction with o-catechol or tetrachloro-o-catechol and Me3N in hot methanol.83 The corresponding reaction with an unsymmetrical catechol, 3,5-di-ferf-butylcatechol produced a mixture of isomers 191a and 191b. The related dioxolene complexes 192 and the amidophenolate complexes 193 were also prepared by the same method. An X-ray crystal structure on the latter complex shows that the phenolate oxygen is trans to the aryl carbon atom. 

Synthesis and molecular structure of two bis(zinc-dioxolene) complexes have been also described.186 The [ Zn(TpCum,Me) 2(L-H2)] derivative in Fig. 3.39 has two protonated catecholate ligands, whereas [ Zn(TpCum,Me) 2(L-H)] has one protonated catecholate and one semiquinone ligand. 

Such a complex is constituted not only by the redox-active V(IV) centre, but also by the redox-active catecholate ligand. In fact, as we will see in more detail in Chapter 6, Section 3, o-dioxolene ligands can undergo the redox processes illustrated in Scheme 4. 

Transition metal complexes with o-dioxolene ligands constitute one of the most intriguing classes of complexes as far as their electrochemical behaviour is concerned, in that, as already mentioned in Chapter 5, Section 1, such ligands are able to shuttle through the oxidation states o-benzoquinone)o-benzosemiquinone/catecholate illustrated in Scheme 250 (a process carried out in nature by the dicopper (I I)-based enzyme catechol oxidase through a single two-electron step see Chapter 9, Section 1.2). 

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