Dianion, catechol

Enterobactin (ent), the cycHc triester of 2,3-dihydroxy-A/-benzoyl-l-serine, uses three catecholate dianions to coordinate iron. The iron(III)-enterobactin complex [62280-34-6] has extraordinary thermodynamic stabiUty. For Fe " +ent , the estimated formal stabiUty constant is 10 and the reduction potential is approximately —750 mV at pH 7 (23). Several catecholate-containing synthetic analogues of enterobactin have been investigated and found to have lesser, but still impressively large, formation constants. 

Starting with cluster [(Cl4cat)2Mo2Fe6Sg(PEt3)6], where Cl4cat is the tetrachloro-catecholate dianion, the cluster [Mo6Fe2oS3o]8 was synthesized and characterized... 

The alkene inserts either in the metal hydride bond or in the metal silyl bond. The latter reaction leads to alkenylsilyl side products and also alkane formation may occur. Similar reactions have been observed for hydroboration, the addition of R2BH to alkenes. (R2 may be the catechol dianion). 

Tris-catecholate complexes were prepared from symmetrically and unsymmetrically substituted catechols (LH2) 4-chlorocatechol, 4,5-dichlorocatechol, 4-nitrocatechol, 3,4-dinitrocatechol and 4,5-dinitrocatechol202. All of these complexes are prepared in aqueous solution and are water-stable, down to ca pH 4. The 111 NMR signals for the free and complexed catechol moieties are well separated, and enable the determination of formation constants for each of the complexes, according to equation 46, where L is the catechol dianion. 

Impressive coordination ability of divalent phenolic ligands (Fig. 22, Table 11) was demonstrated in homoleptic catecholato derivatives [152]. Surprisingly, Ce(IV) was not reduced by the catechol dianion. In addition, the low solubility of the complex Na4[Ce(C6H402)4] 21H20 even permits its direct synthesis and crystallization from aqueous solutions (Eq. 14a, Table 11) [153]. The crystal structure consists of discrete 8-coordinate [Ce(cat)4)]4 dodecahedra. Each sodium is coordinated to two nonequivalent oxygens from two catecholato... 

DTBSQ) in the presence of an equivalent of HO-.12 The first reduction step is a reversible one-electron process that is followed by a second one-electron reduction, which can be reversible in rigorously anhydrous media to give catechol dianion (3,5-DTBC2-) ... 

HjCat represents catechol or hydroquinone, Q quinone, SQ semiquinone anion, HCat- catechol monoanion, and Cat2 catechol dianion DTBQ represents 3,5-di-rerr-butyl-o-quinone, o-Q o-benzoquinone, p-Q p-benzoquinone, TCQ tetrachloro-o-benzoquinone, and TFQ tetrafluoro-o-benzoquinone. 

Fig.4 Generation of chirality upon metal ligand coordination from a orientation of individual chelating ligands (8/k) ([Ni(en)(OH2)4]2+) b tris-bidentate octahedral coordination (A/A) ([Fe(cat2 )3]3" [88]) c coordination of chiral ligands (R/S) ([Co(NH3)4(sarcosinato)]2+) d loss of improper symmetry axes of individual ligands upon coordination ([Pt(iso-butylenediamine)(meso-stilbenediamine)]2+ [89]) en = 1,2-ethylenediamine cat2- = 1,2-catecholate dianion...

Figure 5. A schematic of the A-cis isomer of chromic and ferric enterobactin. The metal lies at the center of a distorted octahedron formed by the oxygen atoms of tne three catechol dianions.

Figure 17-H-6 A proposed catalytic cycle for the role of tyrosinase in oxidizing phenol to o-quinone. Cat = catecholate dianion.

For a recent review on the coordination chemistry of the catecholate dianion see C. G. Pierpont and C. W. Lange, Progress in Inorganic Chemistry, Wiley-Interscience, New York, 1994, vol. 41, p. 331. 

The simple model complex, tris(cateeholato)chromate(III) has been prepared, and complete resolution of the optical isomers was achieved at pH 13. The known crystal structure of [Cr(cat)3]3- and arguments similar to those for the hydroxamate chromium complexes lead to the assignments of absolute configuration of the CD spectra. It was found that the CD spectra of A-[Cr(cat)3f and [Cr(ent)3f " are essentially identical, and the mirror image of chromic desferriferrichrome (Fig. 28), which shows that enterobactin has a predominant A-cis absolute configuration 147). Unfortunately the usual oxidation sensitivity of the catechol dianion is substantially increased in the chromium complexes, which precludes their use as biological probes l47). 

Quinone / semiquinone anion / catecholate dianion Effective magnetic moment (in Bohr magnetons)... 

The most important member of the phenolate class of siderochromes, enterochelin, is a cyclic triester of 2,3-dihydroxyl-A -benzoyl-L-serine. It was isolated from E. coli by O Brien and Gibson (1970), and its mode of action established by Gibson (in Australia) and Nielands (in the United States, 1974) using a series of mutants, each one of which reliably failed to carry out one of the biochemical steps. X-ray diffraction analysis showed that each ferric cation was bound to the six oxygen atoms supplied by the three catechol dianion moieties... 

Earlier work on catechol oxidation catalyzed by metal ions [27,126] and chelates [127] led to the conclusion that a catechol dianion or monoanion is first coordinated to the metal center and this 1 1 complex reacts with 0 to produce a ternary complex, which is the active... 

BPMCN)(DTBCH)] showed that the DBCH ligand binds to the iron center asymmetrically. The complexes react with O2 or NO to afford blue-purple Fe(III)-catecholate dianion complexes, [Fe "(L)(cat)J that react further with O2 to give a high yield of cleavage products. The products are mainly derived from intradiol... 

Considerable effort has been expended in studies of the interaction of metal ions with catechols with a view to understanding oxygenase activity. In aprotic media, the electrochemical properties of substituted catechols have been examined. Reactions of 3,5-di-tert-butyl-o-quinone with manganese(II) result in stable tris-Mn(IV) or bis-Mn(III) complexes of the corresponding catecholate dianion, Bu C , depending on whether the initial ratio of reactants is 1 3 or 1 2. This flexible redox chemistry may be important for redox catalysis. The O2 oxidation of the iron-catechol complex [Fe(salen)(Bu2CH)] has also been examined in aprotic media. 

Several cerium(IV) complexes of various bidentate and tetradentate hydroxypyrodinonate (HOPO) complexes have been studied as model compounds for plutonium(IV) complexes (Xu et al., 2000). Bidentate HOPO monoanions are isolelectronic with catecholate dianions and they display a similar complex formation behavior towards cerium(IV) ions. However, HOPO ligands are more acidic and form stable complexes with cerium(IV) at lower pH values than catechol. The tetradentate ligands form more stable complexes than the corresponding bidentate ligands. New types of chelators for cerium(IV) and pluto-nium(IV) are the 2,3-dihydroxyterephthalamides (Gramer and Raymond, 2004 Xu et al., 2004). Some authors have made comparisons between the coordination chemistry and the redox behavior of cerium and berkelium (Lebedev et al., 1975 Milyukova et al., 1980 Yakovlev et al., 1982). 

In order to generate the barium salt of the titanium tris(catecholate) dianion from its ammonium salt, an acid-base reaction with barium hydroxide has been employed, Eq. 5.26 ... 

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