Departamento de Química, Facultad de Ciencias Bioquímicas y Farmacéuticas, UNR, Suipacha 531, 2000, Rosario, Argentina.
Dalton Trans. 2011 Jul 14;40(26):7033-45. doi: 10.1039/c1dt10214a. Epub 2011 Jun 1.
The pollutant Cr(VI) is known to be very carcinogenic. In conditions of excess of Cr(VI), oxidation of D-galacturonic acid (Galur), the major metabolite of pectin, yields d-galactaric acid (Galar) and Cr(III). The redox reaction takes place through a multistep mechanism involving formation of intermediate Cr(II/IV) and Cr(V) species. The mechanism combines one- and two-electron pathways for the reduction of Cr(IV) by the organic substrate: Cr(VI)→ Cr(IV)→ Cr(II) and Cr(VI)→ Cr(IV)→ Cr(III). This is supported by the observation of the optical absorption spectra of Cr(VI) esters, free radicals, CrO(2)(2+) (superoxoCr(III) ion) and oxo-Cr(V) complexes. Cr(IV) cannot be directly detected; however, formation of CrO(2)(2+) provides indirect evidence for the intermediacy of Cr(II/IV). Cr(IV) reacts with Galur much faster than Cr(V) and Cr(VI) do. The analysis of the reaction kinetics via optical absorption spectroscopy shows that the Cr(IV)-Galur reaction rate inversely depends on [H(+)]. Nevertheless, high [H(+)] still does not facilitate accumulation of Cr(IV) in the Cr(VI)-Galur mixture. Cr(VI) and the intermediate Cr(V) react with Galur at comparable rates; therefore the build-up and decay of Cr(V) accompany the decay of Cr(VI). The complete rate laws for the Cr(VI), Cr(V) and Cr(IV)-Galur redox reaction are here derived in detail. Furthermore, the nature of the five-co-ordinated oxo-Cr(V) bischelate complexes formed in Cr(VI)-Galur mixtures at pH 1-5 is investigated using continuous-wave and pulsed electron paramagnetic resonance (EPR) and density functional theory (DFT).
污染物 Cr(VI) 已知具有很强的致癌性。在 Cr(VI) 过量的情况下，果胶的主要代谢物 D-半乳糖醛酸 (Galur) 会被氧化生成 D-半乳糖酸 (Galar) 和 Cr(III)。该氧化还原反应通过多步机制发生，涉及中间 Cr(II/IV) 和 Cr(V) 物种的形成。该机制结合了单电子和双电子途径，用于有机底物还原 Cr(IV)：Cr(VI)→Cr(IV)→Cr(II) 和 Cr(VI)→Cr(IV)→Cr(III)。这一点得到了 Cr(VI) 酯、自由基、CrO2(2+)（超氧 Cr(III) 离子）和氧代-Cr(V) 配合物的光吸收光谱观察结果的支持。Cr(IV) 不能直接检测到；然而，CrO2(2+) 的形成为 Cr(II/IV) 的中间产物提供了间接证据。Cr(IV) 与 Galur 的反应速度比 Cr(V) 和 Cr(VI) 都快得多。通过光吸收光谱法对反应动力学进行分析表明，Cr(IV)-Galur 反应速率与 [H(+)] 呈反比。然而，高 [H(+)] 并不能促进 Cr(IV) 在 Cr(VI)-Galur 混合物中的积累。Cr(VI) 和中间 Cr(V) 与 Galur 的反应速率相当；因此，Cr(V) 的积累和衰减伴随着 Cr(VI) 的衰减。本文详细推导了 Cr(VI)、Cr(V) 和 Cr(IV)-Galur 氧化还原反应的完整速率定律。此外，使用连续波和脉冲电子顺磁共振 (EPR) 和密度泛函理论 (DFT) 研究了在 pH 值为 1-5 的 Cr(VI)-Galur 混合物中形成的五配位氧代-Cr(V) 双螯合配合物的性质。
