STABILITY AND LIGAND-EXCHANGE REACTIONS OF CHROMIUM(IV) CARBOXYLATO COMPLEXES IN AQUEOUS-SOLUTIONS

The quantitative in situ generation of a range of Cr(IV) carboxylato c omplexes in aqueous media has been achieved by a combination of the ne wly-developed Cr(IV) ligand exchange chemistry together with the exist ing methods of reduction of Cr(VI) or Cr(V) complexes. The reactions C r(VI) + As(III) and Cr(V) + V(IV) in buffer solutions of the correspon ding Ligands were used for generation of Cr(TV) complexes with 2-hydro xy-2-methylbutanoate (hmba), 2-ethyl-2-hydroxybutanoate (ehba), and (- )-quinate (ga) ligands, Addition of oxalate (ox), malonate (mal), or 2 -picolinate (pie) to the generated Cr(IV) complexes led to the quantit ative formation of the new Cr(IV) species. Spectral and chemical prope rties of Cr(IV) complexes with the mentioned ligands have been describ ed for the first time (except for the known Cr(TV)-ehba complexes). In excess Ligand, Cr(TV) appears to exist mainly as bis-chelated oxo com plexes, on the basis of UV-visible and CD Spectroscopic data. All of t he studied Cr(IV) complexes exhibit bell-shaped pH dependences of thei r stabilities. The regions of maximum stabilities and maximal half-liv es ([Cr(IV)](0) = O.1 mM; 25 degrees C) are as follows: pH similar to 3 and similar to 30 min for Cr(IV)-hmba and Cr(IV)-ehba; pH similar to 5 and similar to 1.5 h for Cr(IV)-ox; pH similar to 5 and similar to 1.5 min for Cr(IV)-mal; pH similar to 5 and similar to 20 min for Cr(I V)-pic; pH similar to 6 and similar to 1 h for Cr(IV)-qa. The stabilit ies of Cr(IV) complexes have been compared with those of the correspon ding Cr(V) complexes (studied by EPR spectroscopy). The results are di scussed in terms of the possible roles of Cr(IV) and Cr(V) complexes a s the DNA-damaging agents in chromium-induced genotoxicities.