title: Thermodynamic, electrochemical, high-pressure kinetic, and mechanistic studies of the formation of oxo Fe(IV)-TAML species in water.
abstract: Stopped-flow kinetic studies of the oxidation of Fe(III)-TAML catalysts, [ F e{1,2-X(2)C(6)H(2)-4,5-( NCOCMe(2) NCO)(2)CMe(2)}(OH(2))](-) (1), by t-BuOOH and H(2)O(2) in water affording Fe(IV) species has helped to clarify the mechanism of the interaction of 1 with primary oxidants. The data collected for substituted Fe(III)-TAMLs at pH 6.0-13.8 and 17-45 °C has confirmed that the reaction is first order both in 1 and in peroxides. Bell-shaped pH profiles of the effective second-order rate constants k(I) have maximum values in the pH range of 10.5-12.5 depending on the nature of 1 and the selected peroxide. The "acidic" part is governed by the deprotonation of the diaqua form of 1 and therefore electron-withdrawing groups move the lower pH limit of the reactivity toward neutral pH, although the rate constants k(I) do not change much. The dissection of k(I) into individual intrinsic rate constants k(1) ([FeL(OH(2))(2)](-) + ROOH), k(2) ([FeL(OH(2))OH)](2-) + ROOH), k(3) ([FeL(OH(2))(2)](-) + ROO(-)), and k(4) ([FeL(OH(2))OH)](2-) + ROO(-)) provides a model for understanding the bell-shaped pH-profiles. Analysis of the pressure and substituent effects on the reaction kinetics suggest that the k(2) pathway is (i) more probable than the kinetically indistinguishable k(3) pathway, and (ii) presumably mechanistically similar to the induced cleavage of the peroxide O-O bond postulated for cytochrome P450 enzymes. The redox titration of 1 by Ir(IV) and electrochemical data suggest that under basic conditions the reduction potential for the half-reaction [Fe(IV)L(=O)(OH(2))](2-) + e(-) + H(2)O → [Fe(III)L(OH)(OH(2))](2-) + OH(-) is close to 0.87 V (vs NHE).
authors.affiliation: Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States.
authors.forename: Delia-Laura
authors.initials: DL
authors.lastname: Popescu
authors.forename: Melanie
authors.initials: M
authors.lastname: Vrabel
authors.forename: Ariane
authors.initials: A
authors.lastname: Brausam
authors.forename: Peter
authors.initials: P
authors.lastname: Madsen
authors.forename: Gabor
authors.initials: G
authors.lastname: Lente
authors.forename: Istvan
authors.initials: I
authors.lastname: Fabian
authors.forename: Alexander D
authors.initials: AD
authors.lastname: Ryabov
authors.forename: Rudi
authors.initials: R
authors.lastname: van Eldik
authors.forename: Terrence J
authors.initials: TJ
authors.lastname: Collins
mesh_terms: D002384:Catalysis; D005290:Ferric Compounds / Q000737:chemistry*; D006861:Hydrogen Peroxide / Q000737:chemistry*; D058085:Iron Compounds / Q000737:chemistry*; D007700:Kinetics; D047028:Macrocyclic Compounds / Q000737:chemistry*; D008665:Metalloporphyrins / Q000737:chemistry*; D011199:Potentiometry; D013056:Spectrophotometry, Ultraviolet; D013816:Thermodynamics
chemical_list: D005290:Ferric Compounds; D058085:Iron Compounds; D047028:Macrocyclic Compounds; D008665:Metalloporphyrins; C517362:iron-3,3,6,6,9,9-hexamethyl-3,4,8,9-tetrahydro-1H-1,4,8,11-benzotetraazacyclotridecine-2,5,7,10(6H,11H)-tetraone; D006861:Hydrogen Peroxide