| Abstract
| - A new Cr(V) complex, K[CrVO(qaH3)2]·H2O (I), and its Na+ analog have been characterized by a range of spectroscopic techniques. Compound I is the first isolated and fully characterized Cr(V) complex with a naturally occurring optically active ligand, which can be used as a model compound of a new type in the studies of the role of Cr(V) in Cr-induced carcinogenesis. The likely structure of I in the solid state (14 K) has been determined from the multiple-scattering modeling of the XAFS spectra.
- A new Cr(V) complex, K[CrVO(qaH3)2]·H2O (Ia; qaH3 = quinato = (1R,3R,4R,5R)-1,3,4,5-tetrahydroxycyclohexanecarboxylato(2−)), synthesized by the reaction of K2Cr2O7 with excess qaH5 in MeOH (Codd, R.; Lay, P.A. J. Am. Chem. Soc.1999, 121, 7864−7876), has been characterized by microanalyses, electrospray mass spectra,and UV−visible, CD, IR, EPR, and X-ray absorption spectroscopies. This complex is of interest because of itsability to act as both a structural and a biomimetic model for a range of Cr(V) species believed to be generatedin vivo during the intracellular reduction of carcinogenic Cr(VI). The Na+ analogue of Ia (Ib) has also beenisolated and characterized by microanalyses and IR and X-ray absorption spectroscopies. The reaction of Cr(VI)with MeOH in the presence of qaH5 that leads to I proceeds via a Cr(IV) intermediate (observed by UV−visiblespectroscopy), and a mechanism for the formation of I has been proposed. DMF or DMSO solutions of I arestable for several days at 25 °C, while I in aqueous solution (pH = 4) disproportionates to Cr(VI) and Cr(III) inminutes. The likely structures in the solid state for Ia (14 K) and Ib (∼293 K) have been determined using bothsingle-scattering (Ia,b) and multiple-scattering (Ia) analyses of XAFS data. These analyses have shown thefollowing: (i) In agreement with the results from the other spectroscopic techniques, the quinato ligands arebound to Cr(V) by 2-hydroxycarboxylato moieties, with Cr−O bond lengths of 1.55, 1.82, and 1.94 Å for theoxo, alcoholato, and carboxylato O atoms, respectively. (ii) The position of an oxo O atom is somewhat disordered.This is consistent with molecular mechanics modeling of the likely structures. The XAFS, EPR, and IR spectroscopicevidence points to the existence of hydrogen bonds between the oxo ligand and the 3,4,5-OH groups of thequinato ligands in the solid state of I.
|
