Tris-catecholate complexes

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... 

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. 

The chelation of Pu(IV) and Am(III) by the LICAM series has been studied in detail at neutral pH by electrochemical and spectrophotometric procedures268). The Pu(IV) chelate of 3,4,3-LICAMS appears to be a tris (catecholate) complex, indicating that the full denticity of the ligand is not utilized in vivo. Investigation of the complexation of Pu(IV) by 3,4,3-LICAMC establishes a complexation involving the carboxylate para to the carbonyl. Spectroscopic evidence of the complexation of Am(III) by 3,4,3-LICAMS and 3,4,3-LICAMC was also obtained. Significant differences in the spectra of the two complexes were noted. The authors did not exclude complexation through the C-4 car-boxylates. 

It can be seen from molecular models that two diastereoisomers are possible for the ferric enterobactin complex, A-cis and A-cis. These are not mirror images because of the optical activity of the ligand. The similarity of the roles played by the ferrichromes and enterobactin lent additional speculative interest to the preferred absolute configuration of the iron complex (20). The structural studies of the tris catechol complexes (vide infra) and the spectroscopic properties of the chromic... 

Carbonyl-substituted catechols (Fig. 5.21) react with Ti(IV) salts under slightly basic conditions to yield tris-catecholates in a first assembly step. In the presence of Li+ ions (Na and K+ don t do the trick because their ion sizes are larger ), two of these tris-catecholate complexes form dimers bridged by three Li" " ions [52]. The crystal structures of three representative examples only differing with respect to the... 

Fig. 5.21. Top Self-assembly drives the formation of helical, homochiral dimeric titanium tris-catecholate complexes. Dimerization is only mediated by LC, while Na and K" " do not lead to comparable products. Bottom Crystal structures of the dimers formed from the aldehyde (left, R = H), the ethyl ketone (centre, R = C2H5), and the methyl ester (right, R = OCH3). Shown is a side view in space-filling representation and a ball-and-stick model with a view along the Ti—Ti...

Fig. 3 (a)The coordinate vector for catechol coordination to a metal center, (b) The chelate plane defined by a metal ion and the coordinate vectors in a tris-catechol complex, (c). The approach angle for each chelate of the tris-catechol complex, (d) A M4L6 tetrahedral cluster (adaptedfrom Ref. [24]). (View this art in color at www.dekker.com.)... 

The racemization of dinuclear triple-stranded helicates has been studied. These complexes adopt either AA or AA configurations arising from the tris-bidentate coordinated metal centers. A mononuclear tris-catecholate complex ML3 racemizes via a Bailar twist mechanism. Linked tris-catecholate metal centers would be expected to racemize at the same rate as that of the mononuclear complex in the absence of mechanical coupling. However, when two tris-catecholate complexes are linked in a helicate, the racemization of the M2L3 structure (from AA to AA) slows by a factor of one hundred, while racemization of the four tris-catecholate metal centers in an M4L6 tetrahedron M4L6 is not observed. Although the components of these assemblies are labile, the chiral tetrahedral structure displays structural inertness. 

The Al(III) and Ga(III) complexes of the unsymmetrical derivative of acetylacetonate (see Figure 4.7), with Rj = CFj and Rj = 2-C4H4S, undergo isomerization and racemization by a common pathway. A dissociative mechanism was suggested from the 10-fold increase in rate on changing the solvent from (CDCl2)2 to DMSO. The solvent effect criterion is a subjective one and an 8-fold change in rate between H2O and DMSO was taken to be minor for a tris-catecholate complex of Ga(in). ... 

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