Catechols metal complexes

Figure 9 Possible protonation schemes of tris(catecholate) metal complexes. In path 1, the metal complex undergoes a series of two overlapping one-proton steps to generate a mixed salicylate-catecholate coordination. Further protonation results in the precipitation of a tris(salicylate) complex (e.g. enterobactin, MECAM). This differs from path 2, in which a single two-proton step dissociates one arm of the ligand to form a bis(catecholate) chelate. Path 3 incorporates features of paths 1 and 2. In this model, the metal again imdergoes a series of two overlapping one-proton reactions. However, unlike the case of path 1, the second proton displaces a catecholate arm, which results in a bis(catecholate) metal complex...

Transition metal complexes of o-benzoquinone, o-semiquinone and catecholate ligands. C. G. Pier-pontand R. M. Buchanan, Coord. Chem. Rev., 1981, 38, 45-87 (72). 

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). 

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 ... 

Kruger, H.-J. Iron-containing models of catechol dioxygenases, Biomimetic Oxidations Catalyzed by Transition Metal Complexes , Ed. Meunier, B. Imperial College Press London, 2000, pp. 363—413. 

Tire early-late transition metal complex of Raymond et al. is interesting in that it requires both metal atoms to form the basic C3 structure (ideally D i, may form). Titanium complexation to three catechol units leads to a tridentate ligand, and, only when palladium bromide is added, trans coordination to palladium gives the agglomerate 31 [78]. 

Terminally metallizable dyes (30) are obtained by the interaction of a diazonium salt and a coupling component containing a chelating system, for example salicylic acid, catechol, salicyl-aldoxime or 8-hydroxyquinoline, and their coordination chemistry is typical of these compounds. Such dyes were rarely used as preformed metal complexes but were usually applied to cotton and then converted to their copper complexes on the fibre to improve their fastness to wet treatments. A typical example is the blue dyestuff (31). 

Although they do not appear to have found application, lanthanide metal complexes of 8-hydroxyquinoline, 2-picolinic acid and catechol have been found to exhibit fungicidal properties.155... 

Karpishin, T. B. Stack, T. D. P. Raymond, K. N. Octahedral versus trigonal prismatic geometry in a series of catechol macrobicyclic ligand-metal complexes, J. Am. Chem. Soc. 1993,115, 182-192. 

Metal Complexes Containing Catechol and Semiquinone ligands. 41 331... 

Due to some special structural and magnetic peculiarities, in particular, free-radical properties, the quinone ligands and their metal complexes are apart from the other kinds of coordination compounds [125a, 138a], as will be shown below. Here we present an overview of the main methods for the synthesis of complexes containing benzoquinone, semiquinone, and catecholate ligands, and peculiarities of the products. 

Adamic and Bartak [6] used high pressure aqueous size exclusion chromatography with reverse pulse amperometric detection to separate copper(II) complexes of poly(amino carboxylic acids), catechol and fulvic acids. The commercially available size exclusion chromatography columns were tested. Columns were eluted with copper(II) complexes of poly(aminocarboxylic acids), citric acids, catechol and water derived fulvic acid. The eluent contained copper(II) to prevent dissociation of the labile metal complexes. Reverse pulse electrochemical measurements were made to minimise oxygen interferences at the detector. Resolution of a mixture of DTP A, EDTA and NTA copper complexes was approximately the same on one size exclusion chromatography column as on Sephadex... 

Lanthanide(III) catecholates exhibit a complex coordination chemistry in aqueous solution [154], While at neutral pH, 1 1 complexes are present in solution, higher pH values (pH > 11) favor 2 1, 3 1 and 4 1 catechol metal stoichiometries as shown in Na[Gd(cat)2] 10H2O , Na6[Ln(cat)3]2-20H2O (Ln = Gd, Ho) and Na5[Gd(cat)4] 19.2H20. The latter monogadolinium complex is isomorphous with the cerium(IV) complex (Table 11). The change of the metal charge from Ln(IV) to Ln(III) now requires a fifth sodium cation which is disordered over several sites in the cell. 

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