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).

Electrochemical and photoelectrochemical investigation of water oxidation with hematite electrodes

“…61 This behavior of the Fe(III)-OH / Fe(IV)=O molecular complexes is therefore in good agreement with the proposed first water oxidation step on hematite electrodes. In addition, it was shown that an intermediate Fe(IV)=O complex has a broad absorption spectrum centered at 820 nm.…”

TAML Activator-Based Amperometric Analytical Devices as Alternatives to Peroxidase Biosensors

“…The formation of Fe V and Fe IV species from 1 and either H 2 O 2 or t BuOOH has been discussed in detail in a recent mechanistic publication. 19 It is worth noting here that m-chloroperbenzoic acid converts 1b into the monomeric iron(V)oxo species in organic nitriles 20 which performs a clean and very fast 2e oxidation of thioanisoles. 21 Mossbauer and EXAFS spectroscopies support the case that 1a reacts with t BuOOH in water to give Fe IV observable species, 22 a 1e oxidation performed by a 2-electron oxidant with the likely intermediacy of an iron(V)oxo complex that comproportionates rapidly with additional 1a.…”

Kinetic and mechanistic studies of the reactivity of iron(IV) TAMLs toward organic sulfides in water: resolving a fast catalysis versus slower single-turnover reactivity dilemma

“…Stopped-flow kinetic measurements were carried out using a two-syringe BioKine32 SFM-20 model (BioLogic Science Instruments) at 25 °C. Technical details are summarized elsewhere [14]. Products of the oxidation of thioanisoles were analyzed by high performance liquid chromatography (HPLC) using a Varian C18 Omnispher column with acetonitrile and water as eluting solvents, as well as by gas chromatography -mass spectrometer (GC-MS) using a ThermoDSQ instrument.…”

“…In 2008 we reported that TAML complex 1 (chart 1) is transformed by t BuOOH in water into iron(IV) derivatives 2 and/or 3 depending on reaction conditions [12,13]. Later, the kinetics and mechanism of this transformation were explored in detail in collaboration with Professor Rudi van Eldik, to whom this issue is devoted [14]. Our investigations of the reactivity of Fe V and Fe IV complexes derived from TAMLs, which were performed at -40 °C in MeCN, revealed that both forms oxidize thioanisoles, though Fe V is by several orders of magnitude more reactive than Fe IV [15].…”