Detailed Spectroscopic, Thermodynamic, and Kinetic Studies on the Protolytic Equilibria of FeIIIcydta and the Activation of Hydrogen Peroxide

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• 作者：Ariane Brausam ; Joachim Maigut ; Roland Meier ; Petra . Szilgyi ; Hans-Jrgen Buschmann ; Werner Massa ; Zoltn Homonnay ; Rudi van Eldik
• 刊名：Inorganic Chemistry
• 出版年：2009
• 出版时间：August 17, 2009
• 年：2009
• 卷：48
• 期：16
• 页码：7864-7884
• 全文大小：1441K
• 年卷期：v.48,no.16(August 17, 2009)
• ISSN：1520-510X

文摘

The crystal structure of the as-yet-unknown salt K[Fe^III(cydta)(H₂O)]·3H₂O, where cydta = (±)-trans-1,2-cyclohexanediaminetetraacetate, has been resolved: orthorhombic space group Pbca with R1 = 0.0309, wR2 = 0.0700, and GOF = 0.99. There are two independent [Fe^III(cydta)(H₂O)]⁻ anions in the asymmetric unit, and the ligand is (R,R)-cydta in both cases. The coordination polyhedron is a seven-coordinate capped trigonal prism where the quadrilateral face formed by the four ligand donor oxygen atoms is capped by the coordinated water molecule. The speciation of [Fe^III(cydta)(H₂O)]⁻ in water was studied in detail by a combination of techniques: (i) Measurements of the pH dependence of the Fe^III/IIcydta redox potentials by cyclic voltammetry enabled the estimation of the stability constants (0.1 M KNO₃, 25 °C) of [Fe^III(cydta)(H₂O)]⁻ (log β^III₁₁₀ = 29.05 ± 0.01) and [Fe^II(cydta)(H₂O)]^2- (log β^II₁₁₀ = 17.96 ± 0.01) as well as pK^III_a1OH = 9.57 and pK^II_a1H = 2.69. The formation enthalpy of [Fe^III(cydta)(H₂O)]⁻ (H° = −23 ± 1 kJ mol⁻¹) was measured by direct calorimetry and is compared to the corresponding value for [Fe^III(edta)(H₂O)]⁻ (H° = −31 ± 1 kJ mol⁻¹). (ii) pH-dependent spectrophotometric titrations of Fe^IIIcydta lead to pK^III_a1OH = 9.54 ± 0.01 for deprotonation of the coordinated water and a dimerization constant of log K_d = 1.07. These data are compared with those of Fe^IIIpdta (pdta = 1,2-propanediaminetetraacetate; pK^III_a1OH = 7.70 ± 0.01, log K_d = 2.28) and Fe^IIIedta (pK^III_a1OH = 7.52 ± 0.01, log K_d = 2.64). Temperature- and pressure-dependent ¹⁷O NMR measurements lead to the following kinetic parameters for the water-exchange reaction at [Fe^III(cydta)(H₂O)]⁻ (at 298 K): k_ex = (1.7 ± 0.2) × 10⁷ s⁻¹, H = 40.2 ± 1.3 kJ mol⁻¹, S = +28.4 ± 4.7 J mol⁻¹ K⁻¹, and V = +2.3 ± 0.1 cm³ mol⁻¹. A detailed kinetic study of the effect of the buffer, temperature, and pressure on the reaction of hydrogen peroxide with [Fe^III(cydta)(H₂O)]⁻ was performed using stopped-flow techniques. The reaction was found to consist of two steps and resulted in the formation of a purple Fe^III side-on-bound peroxo complex [Fe^III(cydta)(η²-O₂)]³⁻. The peroxo complex and its degradation products were characterized using Mssbauer spectroscopy. Formation of the purple peroxo complex is only observable above a pH of 9.5. Both reaction steps are affected by specific and general acid catalysis. Two different buffer systems were used to clarify the role of general acid catalysis in these reactions. Mechanistic descriptions and a comparison between the edta and cydta systems are presented. The first reaction step reveals an element of reversibility, which is evident over the whole studied pH range. The positive volume of activation for the forward reaction and the positive entropy of activation for the backward reaction suggest a dissociative interchange mechanism for the reversible end-on binding of hydrogen peroxide to [Fe^III(cydta)(H₂O)]⁻. Deprotonation of the end-on-bound hydroperoxo complex leads to the formation of a seven-coordinate side-on-bound peroxo complex [Fe^III(cydta)(η²-O₂)]³⁻, where one carboxylate arm is detached. [Fe^III(cydta)(η²-O₂)]³⁻ can be reached by two different pathways, of which one is catalyzed by a base and the other by deprotonated hydrogen peroxide. For both pathways, a small negative volume and entropy of activation was observed, suggesting an associative interchange mechanism for the ring-closure step to the side-on-bound peroxo complex. For the second reaction step, no element of reversibility was found.