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Stability and Ligand Exchange Reactions of Chromium(IV) Carboxylato Complexes in Aqueous Solutions1

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journal contribution
posted on 19.11.1997, 00:00 by Rachel Codd, Peter A. Lay, Aviva Levina
The quantitative in situ generation of a range of Cr(IV) carboxylato complexes in aqueous media has been achieved by a combination of the newly-developed Cr(IV) ligand exchange chemistry together with the existing methods of reduction of Cr(VI) or Cr(V) complexes. The reactions Cr(VI) + As(III) and Cr(V) + V(IV) in buffer solutions of the corresponding ligands were used for generation of Cr(IV) complexes with 2-hydroxy-2-methylbutanoate (hmba), 2-ethyl-2-hydroxybutanoate (ehba), and (−)-quinate (qa) ligands. Addition of oxalate (ox), malonate (mal), or 2-picolinate (pic) to the generated Cr(IV) complexes led to the quantitative formation of the new Cr(IV) species. Spectral and chemical properties of Cr(IV) complexes with the mentioned ligands have been described for the first time (except for the known Cr(IV)−ehba complexes). In excess ligand, Cr(IV) appears to exist mainly as bis-chelated oxo complexes, on the basis of UV−visible and CD spectroscopic data. All of the studied Cr(IV) complexes exhibit bell-shaped pH dependences of their stabilities. The regions of maximum stabilities and maximal half-lives ([Cr(IV)]0 = 0.1 mM; 25 °C) are as follows:  pH ∼ 3 and ∼30 min for Cr(IV)−hmba and Cr(IV)−ehba; pH ∼ 5 and ∼1.5 h for Cr(IV)−ox; pH ∼ 5 and ∼1.5 min for Cr(IV)−mal; pH ∼ 5 and ∼20 min for Cr(IV)−pic; pH ∼ 6 and ∼1 h for Cr(IV)−qa. The stabilities of Cr(IV) complexes have been compared with those of the corresponding Cr(V) complexes (studied by EPR spectroscopy). The results are discussed in terms of the possible roles of Cr(IV) and Cr(V) complexes as the DNA-damaging agents in chromium-induced genotoxicities.