Characterization and X-ray absorption spectroscopic studies of bis[quinato(2-)]oxochromate(V)

A new Cr(V) complex, K[(CrO)-O-V(qaH(3))(2)]. H2O (Ia; qaH(3) = quinato = ( 1R,3R,3R,5R)-1,3,4,5-tetrahydroxycyclo-hexanecarboxylato(2-)), synthesized by the reaction of K2Cr2O7 with excess qaH(5) in MeOH (Codd, R. Lay, P. A. J. Am. Chem. Sec. 1999, 121, 7864-7876), has been characterized by microana lyses, electrospray mass spectra, and UV-visible, CD, IR, EPR, and X-ray ab sorption spectroscopies. This complex is of interest because of its ability to act as both a structural and a biomimetic model for a range of Cr(V) sp ecies believed to be generated in vivo during the intracellular reduction o f carcinogenic Cr(VI). The Na+ analogue of Ia (Ib) has also been isolated a nd characterized by microanalyses and IR and X-ray absorption spectroscopie s. The reaction of Cr(VI) with MeOH in the presence of qaH(5) that leads to I proceeds via a Cr(IV) intermediate (observed by UV-visible spectroscopy) , and a mechanism for the formation of I has been proposed. DMF or DMSO sol utions of I are stable for several days at 25 degrees C, while I in aqueous solution (pH = 4) disproportionates to Cr(VI) and Cr(III) in minutes. The likely structures in the solid state for Ia (14 K) and Ib (similar to 293 K ) have been determined using both single-scattering (Ia,b) and multiple-sca ttering (Ia) analyses of XAFS data. These analyses have shown the following : (i) In agreement with the results from the other spectroscopic techniques , the quinato ligands are bound to Cr(V) by 2-hydroxycarboxylato moieties, with Cr-O bond lengths of 1.55, 1.82, and 1.94 Angstrom for the ore, alcoho lato, and carboxylato O atoms, respectively. (ii) The position of an oxo O atom is somewhat disordered. This is consistent with molecular mechanics mo deling of the likely structures. The XAFS, EPR, and IR spectroscopic eviden ce points to the existence of hydrogen bonds between the oxo ligand and the 3,4,5-OH groups of the quinato ligands in the solid state of I.