Dioxolene ligands

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). 

Transition metal complexes with dioxolene ligands may also give rise to interesting magnetooptical properties. Dei et al reported that in [Cr(TMC)SQ], where TMC= 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, the forbidden transition which is responsible for the... 

It is commonly accepted that the C-O distance of the quinoidal ligand is an important parameter suggestive of the charge of the ligand in metal-dioxolene complexes, namely ... 

We finally point out that heteroleptic-dioxolene complexes with different redox non-innocent ligands can exhibit even further difficulty assignable redox changes.60... 

Let us now illustrate some significant examples of the redox propensity of related (but less widely studied61) quinone diimines. As happens for 1,2-dioxolenes, such ligands also exhibit the reversible electron transfer sequence o-benzoquinone diiminejo-benzosemiquinone diimine monoanion/ o-phenylendiamine dianion illustrated in Scheme 4. 

Other mixed-valence metal complexes have been published that display electrochromic behavior, among them bipyridyl ruthenium complexes linked through dioxolene containing bridging ligands, such as 3,3, 4,4 -tetrahydroxybi-phenyl, 3,3",4,4"-tetrahydroxy-p-terphenyl, and 9-phenyl-2,3,7-trihydoxy-6-flu-orone [115,116] (Chart 3). Many of the redox processes that lead to frequently... 

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. 

Among the noninnocent ligands, the 1,2-dioxolene (o-quinone/catecholate) redox system has been favored because of its stability (via chelate binding) and variability (via substitution of O by NR functions and of H by substituents). While the aromatic catecholate forms are a- and tt-electron rich, the semiquinones and especially the quinones are tt-electron accepting. 

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