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TAML activators enable unprecedented, rapid, ultradilute oxidation catalysis where substrate inhibitions might seem improbable. Nevertheless, while TAML/H 2 O 2 rapidly degrades the drug propranolol, a micropollutant (MP) of broad concern, propranolol is shown to inhibit its own destruction under concentration conditions amenable to kinetics studies ([propranolol] = 50 μM). Substrate inhibition manifests as a decrease in the second-order rate constant k I for H 2 O 2 oxidation of the resting Fe III -TAML (RC) to the activated catalyst (AC), while the second-order rate constant k II for attack of AC on propranolol is unaffected. This kinetics signature has been utilized to develop a general approach for quantifying substrate inhibitions. Fragile adducts [propranolol, TAML] have been isolated and subjected to ESI-MS, florescence, UV-vis, FTIR, 1 H NMR, and IC examination and DFT calculations. Propranolol binds to Fe III -TAMLs via combinations of noncovalent hydrophobic, coordinative, hydrogen bonding, and Coulombic interactions. Across four studied TAMLs under like conditions, propranolol reduced k I 4-32-fold (pH 7, 25 °C) indicating that substrate inhibition is controllable by TAML design. However, based on the measured k I and calculated equilibrium constant K for propranolol-TAML binding, it is possible to project the impact on k I of reducing [propranolol] from 50 μM to the ultradilute regime typical of MP contaminated waters (≤2 ppb, ≤7 nM for propranolol) where inhibition nearly vanishes. Projecting from 50 μM to higher concentrations, propranolol completely inhibits its own oxidation before reaching mM concentrations. This study is consistent with prior experimental findings that substrate inhibition does not impede TAML/H 2 O 2 destruction of propranolol in London wastewater while giving a substrate inhibition assessment tool for use in the new field of ultradilute oxidation catalysis.

The catalytic activity of the N-tailed ("biuret") TAML (tetraamido macrocyclic ligand) activators [Fe{4-XC6 H3 -1,2-(NCOCMe2 NCO)2 NR}Cl](2-) (3; N atoms in boldface are coordinated to the central iron atom; the same nomenclature is used in for compounds 1 and 2 below), [X, R=H, Me (a); NO2 , Me (b); H, Ph (c)] in the oxidative bleaching of Orange II dye by H2 O2 in aqueous solution is mechanistically compared with the previously investigated activator [Fe{4-XC6 H3 -1,2-(NCOCMe2 NCO)2 CMe2 }OH2 ](-) (1) and the more aggressive analogue [Fe(Me2 C{CON(1,2-C6 H3 -4-X)NCO}2 )OH2 ](-) (2). Catalysis by 3 of the reaction between H2 O2 and Orange II (S) occurs according to the rate law found generally for TAML activators (v=kI kII [Fe(III) ][S][H2 O2 ]/(kI [H2 O2 ]+kII [S]) and the rate constants kI and kII at pH 7 both decrease within the series 3 b>3 a>3 c. The pH dependency of kI and kII was investigated for 3 a. As with all TAML activators studied to-date, bell-shaped profiles were found for both rate constants. For kI , the maximal activity was found at pH 10.7 marking it as having similar reactivity to 1 a. For kII , the broad bell pH profile exhibits a maximum at pH about 10.5. The condition kI ≪kII holds across the entire pH range studied. Activator 3 b exhibits pronounced activity in neutral to slightly basic aqueous solutions making it worthy of consideration on a technical performance basis for water treatment. The rate constants ki for suicidal inactivation of the active forms of complexes 3 a-c were calculated using the general formula ln([S0 ]/[S∞ ])=(kII /ki )[Fe(III) ]; here [Fe(III) ], [S0 ], and [S∞ ] are the total catalyst concentration and substrate concentration at time zero and infinity, respectively. The synthesis and X-ray characterization of 3 c are also described., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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).

Environmentally useful, small molecule mimics of the peroxidase enzymes must exhibit very high reactivity in water near neutral pH. Here we describe the design and structural and kinetic characterization of a second generation of iron(III)-TAML activators with unprecedented peroxidase-mimicking abilities. Iterative design has been used to remove the fluorine that led to the best performers in first-generation iron-TAMLs. The result is a superior catalyst that meets a green chemistry objective by being comprised exclusively of biochemically common elements. The rate constants for bleaching at pH 7, 9, and 11 of the model substrate, Orange II, shows that the new Fe(III)-TAML has the fastest reactivity at pH's closer to neutral of any TAML activator to date. Under appropriate conditions, the new catalyst can decolorize Orange II without loss of activity for at least 10 half-lives, attesting to its exceptional properties as an oxidizing enzyme mimic.

Exceptionally high peroxidase-like and catalase-like activities of iron(III)-TAML activators of H 2O 2 ( 1: Tetra-Amidato-Macrocyclic-Ligand Fe (III) complexes [ F e{1,2-X 2C 6H 2-4,5-( NCOCMe 2 NCO) 2CR 2}(OH 2)] (-)) are reported from pH 6-12.4 and 25-45 degrees C. Oxidation of the cyclometalated 2-phenylpyridine organometallic complex, [Ru (II)( o-C 6H 4py)(phen) 2]PF 6 ( 2) or "ruthenium dye", occurs via the equation [ Ru II ] + 1/2 H 2 O 2 + H +-->(Fe III - TAML) [ Ru III ] + H 2 O, following a simple rate law rate = k obs (per)[ 1][H 2O 2], that is, the rate is independent of the concentration of 2 at all pHs and temperatures studied. The kinetics of the catalase-like activity (H 2 O 2 -->(Fe III - TAML) H 2 O + 1/2 O 2) obeys a similar rate law: rate = k obs (cat)[ 1][H 2O 2]). The rate constants, k obs (per) and k obs (cat), are strongly and similarly pH dependent, with a maximum around pH 10. Both bell-shaped pH profiles are quantitatively accounted for in terms of a common mechanism based on the known speciation of 1 and H 2O 2 in this pH range. Complexes 1 exist as axial diaqua species [FeL(H 2O) 2] (-) ( 1 aqua) which are deprotonated to afford [FeL(OH)(H 2O)] (2-) ( 1 OH) at pH 9-10. The pathways 1 aqua + H 2O 2 ( k 1), 1 OH + H 2O 2 ( k 2), and 1 OH + HO 2 (-) ( k 4) afford one or more oxidized Fe-TAML species that further rapidly oxidize the dye (peroxidase-like activity) or a second H 2O 2 molecule (catalase-like activity). This mechanism is supported by the observations that (i) the catalase-like activity of 1 is controllably retarded by addition of reducing agents into solution and (ii) second order kinetics in H 2O 2 has been observed when the rate of O 2 evolution was monitored in the presence of added reducing agents. The performances of the 1 complexes in catalyzing H 2O 2 oxidations are shown to compare favorably with the peroxidases further establishing Fe (III)-TAML activators as miniaturized enzyme replicas with the potential to greatly expand the technological utility of hydrogen peroxide.

AbstractThe kinetics of oxidation catalysis by TAML activators of peroxides [LFeIII(OH2)]− (TAML = tetraamido macrocyclic ligand) in water can be typically quantified by a mechanism comprised of two key steps: catalyst activation to the reactive state with rate constant kI, followed by substrate oxidation with rate constant kII. Usually, kI ≪ kII and the overall catalytic activity is limited by the first step. The second-order rate constant kI can be enhanced by changing structural features in the TAML macrocycle that make the iron site more electrophilic. Chemical interventions beyond changes at the catalyst itself should also be able to change kI. In examining potentially useful interventions, the effect of chloride on kI was explored and an intriguing fact was found. For the NewTAML catalyst, [FeIII{4-NO2C6H3-1,2-(NCOCMe2NSO2)2CHMe}(OH2)]−, increasing chloride concentration reduces the rate constant kI when the pH is below the pKa of the catalyst ([LFeIII(OH2)]− ⇄ [LFeIII(OH)]2− + H+), but increases kI when the pH > pKa. Mechanisms of the observed deceleration/acceleration effects are discussed. A linear dependence between KCl and pKa for iron(III) TAMLs is established: logKCl = 16 ± 2 – (1.6 ± 0.2) × pKa. [ABSTRACT FROM AUTHOR]

The bis-sulfonamide TAML activator of peroxides [Fe{4-NO2C6H3-1,2-(NCOCMe2NSO2)2CHMe}]- (2) catalyzes the H2O2 oxidation of microcystin-LR (m-LR), a common surface water contaminant produced by cyanobacteria that is toxic at very low concentrations. The major reaction product is by 16 Da heavier than m-LR, suggesting that the first oxidative step involves addition of oxygen to one of m-LRs three C = C units to form an epoxide. The kinetic data suggest that the m-LR → m + 16 conversion occurs without any kinetically meaningful intermediate with the catalytic rate constant kII of 2.5 × 105 M−1 s−1, which characterizes the interaction of m-LR with the oxidized, active form of the catalyst. The primary m + 16 product undergoes much slower further catalytic oxidation in the aqueous reaction medium. This communication describes a potential utility of TAML/H2O2 catalysis for microcystin detoxification that calls for further research. [ABSTRACT FROM AUTHOR]

The catalytic activity of the N-tailed ('biuret') TAML (tetraamido macrocyclic ligand) activators [Fe{4-XC6H3-1,2-( NCOCMe2 NCO)2NR}Cl]2− ( 3; N atoms in boldface are coordinated to the central iron atom; the same nomenclature is used in for compounds 1 and 2 below), [X, R=H, Me ( a); NO2, Me ( b); H, Ph ( c)] in the oxidative bleaching of Orange II dye by H2O2 in aqueous solution is mechanistically compared with the previously investigated activator [Fe{4-XC6H3-1,2-( NCOCMe2 NCO)2CMe2}OH2]− ( 1) and the more aggressive analogue [Fe(Me2C{CON(1,2-C6H3-4-X) NCO}2)OH2]− ( 2). Catalysis by 3 of the reaction between H2O2 and Orange II (S) occurs according to the rate law found generally for TAML activators ( v= kI kII[FeIII][S][H2O2]/( kI[H2O2]+ kII[S]) and the rate constants kI and kII at pH 7 both decrease within the series 3 b> 3 a> 3 c. The pH dependency of kI and kII was investigated for 3 a. As with all TAML activators studied to-date, bell-shaped profiles were found for both rate constants. For kI, the maximal activity was found at pH 10.7 marking it as having similar reactivity to 1 a. For kII, the broad bell pH profile exhibits a maximum at pH about 10.5. The condition kI≪ kII holds across the entire pH range studied. Activator 3 b exhibits pronounced activity in neutral to slightly basic aqueous solutions making it worthy of consideration on a technical performance basis for water treatment. The rate constants ki for suicidal inactivation of the active forms of complexes 3 a- c were calculated using the general formula ln([S0]/[S∞])=( kII/ ki)[FeIII]; here [FeIII], [S0], and [S∞] are the total catalyst concentration and substrate concentration at time zero and infinity, respectively. The synthesis and X-ray characterization of 3 c are also described. [ABSTRACT FROM AUTHOR]

Oxidative degradation of pinacyanol chloride (PNC) dye by H2O2, as catalyzed by the 1 FeIII-TAML activator (TAML = tetraamidomacrocyclic ligand), occurs rapidly in water, goes to completion, and exhibits noticeably complex kinetics at pH 11. The reaction achieves a carbon mineralization of 51%. The detected products are acetate, formate, oxalate, maleate, 2-nitrobenzoate, nitrite, and nitrate. The catalytic reaction is a first-order process in 1 and the reaction rate has a Michaelis dependence in hydrogen peroxide (H2O2). The reaction rate increases sharply with increasing PNC concentration, reaches a maximal value, and then declines as the PNC concentration is further increased. The initial rate (v) versus [PNC] profile has been quantified in terms of the equation: v = (c1[PNC] + c2[PNC]2)/(c3 + c4[PNC] + [PNC]2) which accounts for the maximum and further rate decline. Kinetic analysis at a more acidic pH (9 vs. 11) revealed that there is no initial rate increase and only the hyperbolic retardation by PNC is observed, in accord with the rate law v = (b1 + b2[PNC])/(b3 + [PNC]). The kinetic data has been rationalized using the adopted mechanism of catalysis by FeIII-TAML activators, which involves the reaction between 1 and H2O2 to form reactive, oxidized TAML (kI, k-I) followed by its reaction with the dye (kII). The minimalistic addition to the scheme to account for the PNC case is the assumption that 1 may rapidly and reversibly associate with PNC (K), and the associated complex reacts also with H2O2 (kID, k-ID) to form also the oxidized TAML. Spectral evidence for this association is presented. The optimization of PNC structure by density functional theory rules out coordination of PNC to 1 via the formation of a Fe-N bond. The kinetic data indicate that the rate constant kII exceeds 1 × 105 (mol L-1)-1 s-1 at pH 11 and 25°C. [ABSTRACT FROM AUTHOR]

Studies the mechanism of H+-induced demetalation as a design strategy for robust iron (III) peroxide-activating catalysts. Properties of enzymes that activate dioxygen or hydrogen peroxide; Function of polychlorophenol mineralization.