Catechols/catecholates/catecholato

The resolution of tris(catecholato)chromate(III) has been achieved by crystallization with L-[Co(en)3]3+ the diastereomeric salt isolated contained the L-[Cr(cat)3]3 ion.793 Comparison of the properties of this anion with the chromium(III) enterobactin complex suggested that the natural product stereospeeifically forms the L-cis complex with chromium(III) (190). The tris(catecholate) complex K3[Cr(Cat)3]-5H20 crystallizes in space group C2/c with a = 20.796, 6 = 15.847 and c = 12.273 A and jS = 91.84° the chelate rings are planar.794 Electrochemical and spectroscopic studies of this complex have also been undertaken.795 Recent molecular orbital calculations796 on quinone complexes are consistent with the ligand-centred redox chemistry generally proposed for these systems.788... 

Os(bpy)2LL]2+/1+/0 (L-L are quinone, semiquinone, or catecholato ligands derived from catechol, 3,5-di-ter<-butylcatechol, or tetrachlo-rocatechol) have been characterized by UV/Vis/NIR and EPR spectroscopies. These spectroscopic properties and the crystal structure of [0s(bpy)2(dbcat)]C104 confirm an Os( III (-catecholate ground state for the +1 ion. This contrasts with the ground state of the +1 ions of Ru analogs, which are best described as Ru(II)-semiquinone complexes... 

A crystallographic structure was reported for an oxygen-coordinated intermolecular complex, [Si(catecholato)2] 2THF (196)201a. Interestingly, the two catechol ligands in 196 are coplanar (Si-Ocat = 1.719, 1.727 A), while the THF molecules occupy trans positions (Si-OxHF = 1.930 A)... 

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... 

Catecholato complexes of the type trans-[0s(0)2(Rcat)2]2 (Rcat = catechol, 4-methylcatechol, 4-tert-butylcatechol, 4-nitrocatechol) are made from 0s04 and the catechol in alkali or from the catechol and trans-K2[0s(0)2(0H)4]. The pyridine complexes trans-[0s(0)2 (Rcat)(py)2] were obtained from 0s04, pyridine, and the catechol (Rcat = cat, 4-Me-cat) [va9(0s02), -820 cm 1 vs(0s02), —860 cm 1] (255). 

Reaction of 0s04 with catechol or substituted catechols Rcat in chloroform yields the deep blue diamagnetic Os(Rcat)3 species (Rcat = catechol, 4-rerf-octyl-, 4-terr-butyl-, 3,5-di-ferf-butyl-catech-ol).486 X-Ray crystal structures of Os(cat)3 and of the tris complex with 3,5-di-terf-butylcatechol show these to have D3 symmetry the Os—O distances fall within the range 1.947 to 1.985 A (mean 1.960 A) and the C—O distances are between 1.30 and 1.35 A the catecholato (02C6H4 or 02C6H2) rings are essentially planar.666 For Os(cat)3, IR, Raman, HNMR and electrochemical data were obtained the latter showed two one-electron reversible reductions, presumably to [Os(cat)3] and [Os(cat)3]2-. The diamagnetism of these formally osmium(VI) species probably arises from the distortion from octahedral to Z>3 symmetry.486... 

Supported oxo-rhenium catalysts in heterogeneous systems have also been reported, for example, the polystyrene-supported (catecholato)oxo-rhenium(VII) complexes (38), obtained from the reaction of polystyrene-supported catechol with [ReOCl3(PPh3)2], which catalyze alcohol oxidation to ketones or aldehydes with dimethylsulfoxide and epoxide... 

Fig. 11. Possible protonation schemes of tris catecholate metal complexes. The compound shown is [M3+(MECAMS)]3-. In scheme 1 the metal complex undergoes a series of two overlapping one-proton steps to generate a mixed salicylate-catecholate coordination about the metal ion. Further protonation results in Ihe precipitation of a tris salicylate complex (e.g. enterobactin, MECAM). This differs from scheme 2, in which a single two-proton step dissociated one arm of the ligand to form a bis(catecholate) chelate (e.g. TRIMCAM). Scheme 3 incorporates features of scheme 1 and 2. In this model the metal again undergoes a series of two overlapping one-proton reactions. However, unlike the case of scheme 1, the second proton displaces a catecholate arm, which results in a bis-(catecholato) metal complex. This scheme is discussed for Ga and In complexes of enterobactin analogs in Ref. 141)...

As with hydroxamate siderophores, simple tris(catecholato) metallate(lll) complexes have served as models for enterobactin. Unlike hydroxamate, catecholate is a symmetric, bidentate ligand. Consequently, there are no geometrical isomers of simple tris(catecholato) metal complexes, and only A and A optical isomers are possible. However all siderophore catecholates are substituted asymmetrically on the catechol ring, such that geometric isomers may in principle exist. However, in the case of enterobactin molecular models show only the more symmetric cis chelate is possible, as the A or A form. 

As with the hydroxamate siderophores, our initial approach has been to study simple tris(catecholato)metallate(III) complexes as models for the tricatecholate siderophore enterobactin. Unlike hydroxamates, catecholate is a symmetric, bidentate ligand. 

The crystal structure of this isostructural series of catechol complexes consists of discrete [M(catechol) 4 ] dodecahedra, a hydrogen bonded network of 21 waters of crystallization and sodium ions, each of which is bonded to two catecholate oxygens and four water oxygens. Of the possible eight coordinate poly-hedra, only the cube and the dodecahedron allow the presence of the crystallographic 4 axis on which the metal ion sits. As depicted in Figure 5 and verified by the shape parameters in Table IV, the tetrakis(catecholato) complexes nearly display the ideal D2d molecular symmetry of the mmmm isomer of the trigonalfaced dodecahedron. 

However, if only Domain III is important in recognition, it would be expected that the simple tris(catecholato)-rhodium(TII) complex would be an equally good inhibitor. In fact, even at concentrations in which the rhodium-catechol complex was in very large excess, no inhibition of iron uptake was observed, suggesting that Domain II is important in the recognition process. 

The role of Domain II in the recognition process was probed by using a rhodium dimethyl amide of 2,3-dihydroxybenzene (DMB) as a catechol ligand, with one more carbonyl ligand than in the tris(catecholato)-rhodium(III) complex. Remarkably, this molecule shows substantially the same inhibition of en-terobactin-mediated iron uptake in E. coli as does rhodium MECAM itself. Thus, in addition to the iron-catechol portion of the molecule, the carbonyl groups... 

---