Tris-catecholato complexes

A few ionic bis-catecholato N-coordinated complexes 177-181 have been reported192 205 207. Crystallographic studies of 177, 180 and 181 showed all three to have a near-octahedral geometry, with Si—O bond lengths very similar to those in the tris-catecholato complexes, and typical Si—N dative bond lengths of 2.157, 2.085 and 2.173 A, respectively. The 29Si chemical shifts for 177-181, which are in line with hexacoordination, are listed in Table 22. 

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... 

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. 

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... 

The stabilities of Fe" complexes of hydroxamates are much lower than those of Fe " for the bis Fe" complex of acetohydroxamic acid = 3 x 10 ). It is thought, therefore, that the mechanism of release of iron from hydroxamate siderophores may occur via reduction of Fe " to the much more weakly bound Fe" state, as also proposed for the tris(catecholato) siderophores. Cyclic voltammetry has shown that the Fe" hydroxamate complexes undergo reversible or quasi-reversible one-electron reductions under suitable conditions. The observed formal potential E( is pH dependent, decreasing from —0.388 mV (ys. SCE) at pH 7 by 60 mV per pH unit, for the case of tris(acetohydroxamato)iron(III), consistent with progressive deprotonations of the coordinated ligand. Thus, deprotonation stabilizes the Fe " vs. the Fe state. ... 

Recent studies by Perry and Keeling-Tucker [122] using tris-catecholato silicon (IV) complexes as the source of silica extend the range of influence of silica to include calcium and iron in addition to Al. Some form of silica appears to influence the mineralization of iron oxide and calcium phosphate phases as seen by others. Silica shows an affinity for Al species comparable to organic complexing ligands. 

A closer look at the system, however, does pique curiosity. The initial pH within the chamber is not 7 but 2-3, and the reactions are non-equilibrium, often irreversible, and involve other intermediates that can become important end products. The acidic pH represents a problem in that thiolates, not thiols, are the operative reductants, thus cannot reduce at pH values below their typical i.e. 8-9. This is resolved by proteins, including mfp-6, by sequence specific effects such as flanking cationic groups that reduce the Cys pK, e.g. redox active Cys-59 in DsB-A has a p Tg of 3.5. Several Cys residues in mfp-6 are acidic, but specific p Tg values have yet to be measured. The non-equilibrium, irreversible nature of the oxidation reactions is a particular problem with Dopa and other catechols. Indeed, the chemical fate of catechols in mussel byssus is highly dependent on their location. In the cuticle, the fate of Dopa appears to be tris catecholato-Fe complexes in the thread and plaque core, Dopa forms covalent cross-links after oxidation to quinones, whereas at the plaque-substratum interface, it is some combination of metal chelates and reduced H-bonded Dopa on metal oxide surfaces. The reducing capacity of mfp-6 plays a role in maximizing the latter and is astonishingly sustained, i.e. >21 days. ... 

Levina A, Foran GJ, Pattison DI, Lay PA. 2004. x-ray absorption spectroscopic and electrochemical studies of tris(catecholato(2-))-chromate(V/IV/m) complexes. Angew Chem, IntEd43(4) 462 65. 

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... 

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