X-ray absorption spectroscopic studies of chromium(V/IV/III)- 2-ethyl-2-hydroxybutanoato(2-/1-) complexes

Structures of the complexes Cr(V)O(ehba)(2)](-), Cr(IV)O(ehbaH)(2)](0), and Cr(III)(ehbaH)(2)(OH(2))(2)](+) (ehbaH(2) = 2-ethyl-2-hydroxybutanoic acid) in frozen aqueous solutions (10 K, Cr] = 10 mM, 1.0 M ehbaH(2)/ehbaH, pH 3.5) have been determined by single- and multiple-scattering fitting of X-ray absorption fine structure (XAFS) data. An optimal set of fitting parameters has been determined from the XAFS calculations for a compound with known crystal structure, NaCr(V)O(ehba)(2)] (solid, 10 K). The structure of the Cr(V) complex Cr(V)O(ehba)(2)](-) does not change in solution in the presence of excess ligand. Contrary to the earlier suggestions made from the kinetic data (Ghosh, M. C.; Gould, E. S. J. Chem. Soc., Chem. Commun. 1992, 195-196), the structure of the Cr(IV) complex (generated by the Cr(VI) + As(III) + ehbaH(2) reaction) is close to that of the Cr(V) complex (five-coordinate, distorted trigonal bipyramidal) and different from that of the Cr(III) complex (six-coordinate, octahedral). For both Cr(V) and Cr(IV) complexes, some disorder in the position of the oxo group is observed, which is consistent with but not definitive for the presence of geometric isomers. The structure of the Cr(IV) complex differs from that of Cr(V) by protonation of alcoholato groups of the ligands, which leads to significant elongation of the corresponding Cr-O bonds (2.0 vs 1.8 A). This is reflected in the different chemical properties reported previously for the Cr(IV) and Cr(V) complexes, including their reactivities toward DNA and other biomolecules in relation to Cr-induced carcinogenicity.