Oxygen atom exchange between H2O and non-heme oxoiron(IV) complexes: Ligand dependence and mechanism

Mayank Puri, Anna Company, Gerard Sabenya, Miquel Costas, Lawrence Que

Research output: Contribution to journalArticle

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Abstract

Detailed studies of oxygen atom exchange (OAE) between H2 18O and synthetic non-heme oxoiron(IV) complexes supported by tetradentate and pentadentate ligands provide evidence that they proceed by a common mechanism but within two different kinetic regimes, with OAE rates that span 2 orders of magnitude. The first kinetic regime involves initial reversible water association to the FeIV complex, which is evidenced by OAE rates that are linearly dependent on [H2 18O] and H2O/D2O KIEs of 1.6, while the second kinetic regime involves a subsequent rate determining proton-transfer step between the bound aqua and oxo ligands that is associated with saturation behavior with [H2 18O] and much larger H2O/D2O KIEs of 5-6. [FeIV(O)(TMC)(MeCN)]2+ (1) and [FeIV(O)(MePy2TACN)]2+ (9) are examples of complexes that exhibit kinetic behavior in the first regime, while [FeIV(O)(N4Py)]2+ (3), [FeIV(O)(BnTPEN)]2+ (4), [FeIV(O)(1Py-BnTPEN)]2+ (5), [FeIV(O)(3Py-BnTPEN)]2+ (6), and [FeIV(O)(Me2Py2TACN)]2+ (8) represent complexes that fall in the second kinetic regime. Interestingly, [FeIV(O)(PyTACN)(MeCN)]2+ (7) exhibits a linear [H2 18O] dependence below 0.6 M and saturation above 0.6 M. Analysis of the temperature dependence of the OAE rates shows that most of these complexes exhibit large and negative activation entropies, consistent with the proposed mechanism. One exception is complex 9, which has a near-zero activation entropy and is proposed to undergo ligand-arm dissociation during the RDS to accommodate H2 18O binding. These results show that the observed OAE kinetic behavior is highly dependent on the nature of the supporting ligand and are of relevance to studies of non-heme oxoiron(IV) complexes in water or acetonitrile/water mixtures for applications in photocatalysis and water oxidation chemistry.

Original languageEnglish (US)
Pages (from-to)5818-5827
Number of pages10
JournalInorganic Chemistry
Volume55
Issue number12
DOIs
StatePublished - Jun 20 2016

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oxygen atoms
Ion exchange
Oxygen
Ligands
Atoms
ligands
Kinetics
kinetics
Water
water
Entropy
Chemical activation
activation
entropy
saturation
Proton transfer
Photocatalysis
acetonitrile
Association reactions
dissociation

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Oxygen atom exchange between H2O and non-heme oxoiron(IV) complexes : Ligand dependence and mechanism. / Puri, Mayank; Company, Anna; Sabenya, Gerard; Costas, Miquel; Que, Lawrence.

In: Inorganic Chemistry, Vol. 55, No. 12, 20.06.2016, p. 5818-5827.

Research output: Contribution to journalArticle

Puri, Mayank ; Company, Anna ; Sabenya, Gerard ; Costas, Miquel ; Que, Lawrence. / Oxygen atom exchange between H2O and non-heme oxoiron(IV) complexes : Ligand dependence and mechanism. In: Inorganic Chemistry. 2016 ; Vol. 55, No. 12. pp. 5818-5827.
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title = "Oxygen atom exchange between H2O and non-heme oxoiron(IV) complexes: Ligand dependence and mechanism",
abstract = "Detailed studies of oxygen atom exchange (OAE) between H2 18O and synthetic non-heme oxoiron(IV) complexes supported by tetradentate and pentadentate ligands provide evidence that they proceed by a common mechanism but within two different kinetic regimes, with OAE rates that span 2 orders of magnitude. The first kinetic regime involves initial reversible water association to the FeIV complex, which is evidenced by OAE rates that are linearly dependent on [H2 18O] and H2O/D2O KIEs of 1.6, while the second kinetic regime involves a subsequent rate determining proton-transfer step between the bound aqua and oxo ligands that is associated with saturation behavior with [H2 18O] and much larger H2O/D2O KIEs of 5-6. [FeIV(O)(TMC)(MeCN)]2+ (1) and [FeIV(O)(MePy2TACN)]2+ (9) are examples of complexes that exhibit kinetic behavior in the first regime, while [FeIV(O)(N4Py)]2+ (3), [FeIV(O)(BnTPEN)]2+ (4), [FeIV(O)(1Py-BnTPEN)]2+ (5), [FeIV(O)(3Py-BnTPEN)]2+ (6), and [FeIV(O)(Me2Py2TACN)]2+ (8) represent complexes that fall in the second kinetic regime. Interestingly, [FeIV(O)(PyTACN)(MeCN)]2+ (7) exhibits a linear [H2 18O] dependence below 0.6 M and saturation above 0.6 M. Analysis of the temperature dependence of the OAE rates shows that most of these complexes exhibit large and negative activation entropies, consistent with the proposed mechanism. One exception is complex 9, which has a near-zero activation entropy and is proposed to undergo ligand-arm dissociation during the RDS to accommodate H2 18O binding. These results show that the observed OAE kinetic behavior is highly dependent on the nature of the supporting ligand and are of relevance to studies of non-heme oxoiron(IV) complexes in water or acetonitrile/water mixtures for applications in photocatalysis and water oxidation chemistry.",
author = "Mayank Puri and Anna Company and Gerard Sabenya and Miquel Costas and Lawrence Que",
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T1 - Oxygen atom exchange between H2O and non-heme oxoiron(IV) complexes

T2 - Ligand dependence and mechanism

AU - Puri, Mayank

AU - Company, Anna

AU - Sabenya, Gerard

AU - Costas, Miquel

AU - Que, Lawrence

PY - 2016/6/20

Y1 - 2016/6/20

N2 - Detailed studies of oxygen atom exchange (OAE) between H2 18O and synthetic non-heme oxoiron(IV) complexes supported by tetradentate and pentadentate ligands provide evidence that they proceed by a common mechanism but within two different kinetic regimes, with OAE rates that span 2 orders of magnitude. The first kinetic regime involves initial reversible water association to the FeIV complex, which is evidenced by OAE rates that are linearly dependent on [H2 18O] and H2O/D2O KIEs of 1.6, while the second kinetic regime involves a subsequent rate determining proton-transfer step between the bound aqua and oxo ligands that is associated with saturation behavior with [H2 18O] and much larger H2O/D2O KIEs of 5-6. [FeIV(O)(TMC)(MeCN)]2+ (1) and [FeIV(O)(MePy2TACN)]2+ (9) are examples of complexes that exhibit kinetic behavior in the first regime, while [FeIV(O)(N4Py)]2+ (3), [FeIV(O)(BnTPEN)]2+ (4), [FeIV(O)(1Py-BnTPEN)]2+ (5), [FeIV(O)(3Py-BnTPEN)]2+ (6), and [FeIV(O)(Me2Py2TACN)]2+ (8) represent complexes that fall in the second kinetic regime. Interestingly, [FeIV(O)(PyTACN)(MeCN)]2+ (7) exhibits a linear [H2 18O] dependence below 0.6 M and saturation above 0.6 M. Analysis of the temperature dependence of the OAE rates shows that most of these complexes exhibit large and negative activation entropies, consistent with the proposed mechanism. One exception is complex 9, which has a near-zero activation entropy and is proposed to undergo ligand-arm dissociation during the RDS to accommodate H2 18O binding. These results show that the observed OAE kinetic behavior is highly dependent on the nature of the supporting ligand and are of relevance to studies of non-heme oxoiron(IV) complexes in water or acetonitrile/water mixtures for applications in photocatalysis and water oxidation chemistry.

AB - Detailed studies of oxygen atom exchange (OAE) between H2 18O and synthetic non-heme oxoiron(IV) complexes supported by tetradentate and pentadentate ligands provide evidence that they proceed by a common mechanism but within two different kinetic regimes, with OAE rates that span 2 orders of magnitude. The first kinetic regime involves initial reversible water association to the FeIV complex, which is evidenced by OAE rates that are linearly dependent on [H2 18O] and H2O/D2O KIEs of 1.6, while the second kinetic regime involves a subsequent rate determining proton-transfer step between the bound aqua and oxo ligands that is associated with saturation behavior with [H2 18O] and much larger H2O/D2O KIEs of 5-6. [FeIV(O)(TMC)(MeCN)]2+ (1) and [FeIV(O)(MePy2TACN)]2+ (9) are examples of complexes that exhibit kinetic behavior in the first regime, while [FeIV(O)(N4Py)]2+ (3), [FeIV(O)(BnTPEN)]2+ (4), [FeIV(O)(1Py-BnTPEN)]2+ (5), [FeIV(O)(3Py-BnTPEN)]2+ (6), and [FeIV(O)(Me2Py2TACN)]2+ (8) represent complexes that fall in the second kinetic regime. Interestingly, [FeIV(O)(PyTACN)(MeCN)]2+ (7) exhibits a linear [H2 18O] dependence below 0.6 M and saturation above 0.6 M. Analysis of the temperature dependence of the OAE rates shows that most of these complexes exhibit large and negative activation entropies, consistent with the proposed mechanism. One exception is complex 9, which has a near-zero activation entropy and is proposed to undergo ligand-arm dissociation during the RDS to accommodate H2 18O binding. These results show that the observed OAE kinetic behavior is highly dependent on the nature of the supporting ligand and are of relevance to studies of non-heme oxoiron(IV) complexes in water or acetonitrile/water mixtures for applications in photocatalysis and water oxidation chemistry.

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