Detailed spectroscopic, thermodynamic, and kinetic studies on the protolytic equilibria of FeIII cydta and the activation of hydrogen peroxide

Ariane Brausam, Joachim Maigut, Roland Meier, Petra Á Szilágyi, Hans Jürgen Buschmann, Werner Massa, Z. Homonnay, Rudi Van Eldik

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Abstract

The crystal structure of the as-yet-unknown salt K[FeIII(cydta) (H2O)]·3H2O, where cydta = (±)-trans-1,2- cyclohexanedia-minetetraacetate, has been resolved: orthorhombic space group Pbca with R1 = 0.0309, wR2 = 0.0700, and GOF = 0.99. There are two independent [FeIII(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 [FeIIIcydta)(H2O)]- in water was studied in detail by a combination of techniques: (i) Measurements of the pH dependence of the FeIII/II cydta redox potentials by cyclic voltammetry enabled the estimation of the stability constants (0.1 M KNO3, 25 °C) of [FeIII(cydta)(H2O)] - (log βIII 110 = 29.05 ± 0.01) and [FeII(cydta)(H2O)]2- (log β II 110 = 17.96 ± 0.01) as well as pK III a1OH = 9.57 and pKII a1H = 2.69. The formation enthalpy of [FeIII(cydta)(H2O)]- (ΔHo = - 23 ± 1 kJ mol-1) was measured by direct calorimetry and is compared to the corresponding value for [Fe III(edta)(H2O)]- (ΔHo = - 31 ± 1 kJ moP1). (ii) pH-dependent spectro-photometric titrations of FeIIIcydta lead to pKIII a1OH = 9.54 ± 0.01 for deprotonation of the coordinated water and a dimerization constant of log Kd = 1.07. These data are compared with those of FeIIIpdta (pdta = 1,2-propanediaminete-traacetate; pK III a10H = 7.70 ± 0.01, log Kd = 2.28) and Fe IIIedta (pKIII a10H = 7.52 ± 0.01, log Kd = 2.64). Temperature- and pressure-dependent 17O NMR measurements lead to the following kinetic parameters for the water-exchange reaction at [FeIII(cydta)(H2O)]- (at 298 K): Kex = (1.7 ± 0.2) x 107 s-1, ΔH = 40.2 ± 1.3 kJ moF1, ΔSΔ = +28.4 ± 4.7 J mol-1 K-1, and ΔVΔ=+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 [FeIII(cydta)(H 2O)]- was performed using stopped-flow techniques. The reaction was found to consist of two steps and resulted in the formation of a purple FeIII side-on-bound peroxo complex [FeIII(cydta) (η2-O2)]3-. The peroxo complex and its degradation products were characterized using Mössbauer 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 [FeIII(cydta)(η2-O 2)]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.

Original languageEnglish
Pages (from-to)7864-7884
Number of pages21
JournalInorganic Chemistry
Volume48
Issue number16
DOIs
Publication statusPublished - Aug 17 2009

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hydrogen peroxide
Hydrogen Peroxide
Chemical activation
Thermodynamics
activation
thermodynamics
Deprotonation
Kinetics
Water
kinetics
Interchanges
Catalysis
Buffers
Entropy
Ligands
Acids
Dimerization
Calorimetry
Prisms
Titration

ASJC Scopus subject areas

  • Inorganic Chemistry
  • Physical and Theoretical Chemistry

Cite this

Brausam, A., Maigut, J., Meier, R., Szilágyi, P. Á., Buschmann, H. J., Massa, W., ... Van Eldik, R. (2009). Detailed spectroscopic, thermodynamic, and kinetic studies on the protolytic equilibria of FeIII cydta and the activation of hydrogen peroxide. Inorganic Chemistry, 48(16), 7864-7884. https://doi.org/10.1021/ic900834z

Detailed spectroscopic, thermodynamic, and kinetic studies on the protolytic equilibria of FeIII cydta and the activation of hydrogen peroxide. / Brausam, Ariane; Maigut, Joachim; Meier, Roland; Szilágyi, Petra Á; Buschmann, Hans Jürgen; Massa, Werner; Homonnay, Z.; Van Eldik, Rudi.

In: Inorganic Chemistry, Vol. 48, No. 16, 17.08.2009, p. 7864-7884.

Research output: Contribution to journalArticle

Brausam, Ariane ; Maigut, Joachim ; Meier, Roland ; Szilágyi, Petra Á ; Buschmann, Hans Jürgen ; Massa, Werner ; Homonnay, Z. ; Van Eldik, Rudi. / Detailed spectroscopic, thermodynamic, and kinetic studies on the protolytic equilibria of FeIII cydta and the activation of hydrogen peroxide. In: Inorganic Chemistry. 2009 ; Vol. 48, No. 16. pp. 7864-7884.
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title = "Detailed spectroscopic, thermodynamic, and kinetic studies on the protolytic equilibria of FeIII cydta and the activation of hydrogen peroxide",
abstract = "The crystal structure of the as-yet-unknown salt K[FeIII(cydta) (H2O)]·3H2O, where cydta = (±)-trans-1,2- cyclohexanedia-minetetraacetate, has been resolved: orthorhombic space group Pbca with R1 = 0.0309, wR2 = 0.0700, and GOF = 0.99. There are two independent [FeIII(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 [FeIIIcydta)(H2O)]- in water was studied in detail by a combination of techniques: (i) Measurements of the pH dependence of the FeIII/II cydta redox potentials by cyclic voltammetry enabled the estimation of the stability constants (0.1 M KNO3, 25 °C) of [FeIII(cydta)(H2O)] - (log βIII 110 = 29.05 ± 0.01) and [FeII(cydta)(H2O)]2- (log β II 110 = 17.96 ± 0.01) as well as pK III a1OH = 9.57 and pKII a1H = 2.69. The formation enthalpy of [FeIII(cydta)(H2O)]- (ΔHo = - 23 ± 1 kJ mol-1) was measured by direct calorimetry and is compared to the corresponding value for [Fe III(edta)(H2O)]- (ΔHo = - 31 ± 1 kJ moP1). (ii) pH-dependent spectro-photometric titrations of FeIIIcydta lead to pKIII a1OH = 9.54 ± 0.01 for deprotonation of the coordinated water and a dimerization constant of log Kd = 1.07. These data are compared with those of FeIIIpdta (pdta = 1,2-propanediaminete-traacetate; pK III a10H = 7.70 ± 0.01, log Kd = 2.28) and Fe IIIedta (pKIII a10H = 7.52 ± 0.01, log Kd = 2.64). Temperature- and pressure-dependent 17O NMR measurements lead to the following kinetic parameters for the water-exchange reaction at [FeIII(cydta)(H2O)]- (at 298 K): Kex = (1.7 ± 0.2) x 107 s-1, ΔH‡ = 40.2 ± 1.3 kJ moF1, ΔSΔ = +28.4 ± 4.7 J mol-1 K-1, and ΔVΔ=+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 [FeIII(cydta)(H 2O)]- was performed using stopped-flow techniques. The reaction was found to consist of two steps and resulted in the formation of a purple FeIII side-on-bound peroxo complex [FeIII(cydta) (η2-O2)]3-. The peroxo complex and its degradation products were characterized using M{\"o}ssbauer 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 [FeIII(cydta)(η2-O 2)]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.",
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T1 - Detailed spectroscopic, thermodynamic, and kinetic studies on the protolytic equilibria of FeIII cydta and the activation of hydrogen peroxide

AU - Brausam, Ariane

AU - Maigut, Joachim

AU - Meier, Roland

AU - Szilágyi, Petra Á

AU - Buschmann, Hans Jürgen

AU - Massa, Werner

AU - Homonnay, Z.

AU - Van Eldik, Rudi

PY - 2009/8/17

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N2 - The crystal structure of the as-yet-unknown salt K[FeIII(cydta) (H2O)]·3H2O, where cydta = (±)-trans-1,2- cyclohexanedia-minetetraacetate, has been resolved: orthorhombic space group Pbca with R1 = 0.0309, wR2 = 0.0700, and GOF = 0.99. There are two independent [FeIII(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 [FeIIIcydta)(H2O)]- in water was studied in detail by a combination of techniques: (i) Measurements of the pH dependence of the FeIII/II cydta redox potentials by cyclic voltammetry enabled the estimation of the stability constants (0.1 M KNO3, 25 °C) of [FeIII(cydta)(H2O)] - (log βIII 110 = 29.05 ± 0.01) and [FeII(cydta)(H2O)]2- (log β II 110 = 17.96 ± 0.01) as well as pK III a1OH = 9.57 and pKII a1H = 2.69. The formation enthalpy of [FeIII(cydta)(H2O)]- (ΔHo = - 23 ± 1 kJ mol-1) was measured by direct calorimetry and is compared to the corresponding value for [Fe III(edta)(H2O)]- (ΔHo = - 31 ± 1 kJ moP1). (ii) pH-dependent spectro-photometric titrations of FeIIIcydta lead to pKIII a1OH = 9.54 ± 0.01 for deprotonation of the coordinated water and a dimerization constant of log Kd = 1.07. These data are compared with those of FeIIIpdta (pdta = 1,2-propanediaminete-traacetate; pK III a10H = 7.70 ± 0.01, log Kd = 2.28) and Fe IIIedta (pKIII a10H = 7.52 ± 0.01, log Kd = 2.64). Temperature- and pressure-dependent 17O NMR measurements lead to the following kinetic parameters for the water-exchange reaction at [FeIII(cydta)(H2O)]- (at 298 K): Kex = (1.7 ± 0.2) x 107 s-1, ΔH‡ = 40.2 ± 1.3 kJ moF1, ΔSΔ = +28.4 ± 4.7 J mol-1 K-1, and ΔVΔ=+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 [FeIII(cydta)(H 2O)]- was performed using stopped-flow techniques. The reaction was found to consist of two steps and resulted in the formation of a purple FeIII side-on-bound peroxo complex [FeIII(cydta) (η2-O2)]3-. The peroxo complex and its degradation products were characterized using Mössbauer 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 [FeIII(cydta)(η2-O 2)]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.

AB - The crystal structure of the as-yet-unknown salt K[FeIII(cydta) (H2O)]·3H2O, where cydta = (±)-trans-1,2- cyclohexanedia-minetetraacetate, has been resolved: orthorhombic space group Pbca with R1 = 0.0309, wR2 = 0.0700, and GOF = 0.99. There are two independent [FeIII(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 [FeIIIcydta)(H2O)]- in water was studied in detail by a combination of techniques: (i) Measurements of the pH dependence of the FeIII/II cydta redox potentials by cyclic voltammetry enabled the estimation of the stability constants (0.1 M KNO3, 25 °C) of [FeIII(cydta)(H2O)] - (log βIII 110 = 29.05 ± 0.01) and [FeII(cydta)(H2O)]2- (log β II 110 = 17.96 ± 0.01) as well as pK III a1OH = 9.57 and pKII a1H = 2.69. The formation enthalpy of [FeIII(cydta)(H2O)]- (ΔHo = - 23 ± 1 kJ mol-1) was measured by direct calorimetry and is compared to the corresponding value for [Fe III(edta)(H2O)]- (ΔHo = - 31 ± 1 kJ moP1). (ii) pH-dependent spectro-photometric titrations of FeIIIcydta lead to pKIII a1OH = 9.54 ± 0.01 for deprotonation of the coordinated water and a dimerization constant of log Kd = 1.07. These data are compared with those of FeIIIpdta (pdta = 1,2-propanediaminete-traacetate; pK III a10H = 7.70 ± 0.01, log Kd = 2.28) and Fe IIIedta (pKIII a10H = 7.52 ± 0.01, log Kd = 2.64). Temperature- and pressure-dependent 17O NMR measurements lead to the following kinetic parameters for the water-exchange reaction at [FeIII(cydta)(H2O)]- (at 298 K): Kex = (1.7 ± 0.2) x 107 s-1, ΔH‡ = 40.2 ± 1.3 kJ moF1, ΔSΔ = +28.4 ± 4.7 J mol-1 K-1, and ΔVΔ=+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 [FeIII(cydta)(H 2O)]- was performed using stopped-flow techniques. The reaction was found to consist of two steps and resulted in the formation of a purple FeIII side-on-bound peroxo complex [FeIII(cydta) (η2-O2)]3-. The peroxo complex and its degradation products were characterized using Mössbauer 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 [FeIII(cydta)(η2-O 2)]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.

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