Chromium complexes catecholates

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). 

Rodriguez, J. H., Wheeler, D. E., McCusker, J. K., 1998, Density Functional Studies of a Heisenberg Spin Coupled Chromium-Semiquinone Complex and its Chromium-Catechol Analog , J. Am. Chem. Soc., 120, 12051. 

The chemistry of the transition metals including chromium(III) with these ligands has been the subject of a recent and extensive review,788 with references to the early literature. The close relationship between the catechol (180), semiquinone (181) and quinone (182) complexes may be appreciated by considering the redox equation below (equation 44). 789 The formal reduction potentials for the chromium(III) complexes (183-186 equation 45) are +0.03, -0.47 and -0.89 V (vs. SCE in acetonitrile) respectively. 

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... 

Complexation of chromium(III) by nitrilotriacetic add (H2nta) has been investigated. Values of equilibrium constants (/ = 0, 25 °C) are log K = 10.66 and log K2 — 8.73.1234 A large number of mixed complexes with nta/chromium(III) have been prepared, including mixed aminoad-dates,1235 acetylacetonates, catecholates, oxalates and complexes with 1,10-phenanthroline and 2,2 -bipyridyl.1236 1238... 

While the absolute configuration of transition metal catecholates has not been determined by x-ray diffraction, the assignments are based on several lines of reasoning. In particular, for both the rhodium(III) and chromium(III) complexes, that isomer precipitated by A-tris(ethylenediamine)cobalt(III) is assigned the A configuration. 

Finally, an analogy is drawn between the chromium and rhodium catecholate complexes. The salts [Co(en)3][M(cat)3] (M = Rh, Cr), prepared from A-[Co(en)3]I3H20 have been shown by x-ray powder patterns to be isostructural and therefore of the same absolute configuration. Thus the catecholate systems are at least self-consistent. 

In addition to preparing the model catecholate complexes of rhodium and chromium, the analogous enterobactin complexes were also prepared and their CD spectra recorded (62). From examination of molecular models it is apparent that either the A-cis or A-cis diastereomers of a metal enterobactin complex are structurally possible. In theory, these diastereomers should be separable by chromatographic techniques analogous to those used for the hydroxamates (vide supra) however, under a variety of conditions only one chromatographic fraction is obtained. We conclude that one isomer predominates to the exclusion of the other. 

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