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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]

Background information, procedures used, typical results obtained, and calculations are provided for a short (4-6 hours) kinetic experiment which includes different methods both in performance and in analysis of the results. The experiment can be modified and extended depending on the level of students and the available laboratory time. (JN)

A cyclic voltammetry study of a series of iron(III) TAML activators of peroxides of several generations in acetonitrile as solvent reveals reversible or quasireversible FeIII/IV and FeIV/V anodic transitions, the formal reduction potentials (E°′) for which are observed in the ranges 0.4–1.2 and 1.4–1.6 V, respectively, versus Ag/AgCl. The slope of 0.33 for a linear E°′(IV/V) against E°′(III/IV) plot suggests that the TAML ligand system plays a bigger role in the FeIII/IV transition, whereas the second electron transfer is to a larger extent an iron‐centered phenomenon. The reduction potentials appear to be a convenient tool for analysis of various properties of iron TAML activators in terms of linear free energy relationships (LFERs). The values of E°′(III/IV) and E°′(IV V−1) correlate 1) with the pKa values of the axial aqua ligand of iron(III) TAMLs with slopes of 0.28 and 0.06 V, respectively; 2) with the Stern–Volmer constants KSV for the quenching of fluorescence of propranolol, a micropollutant of broad concern; 3) with the calculated ionization potentials of FeIII and FeIV TAMLs; and 4) with rate constants kI and kII for the oxidation of the resting iron(III) TAML state by H2O2 and reactions of the active forms of TAMLs formed with donors of electrons S, respectively. Interestingly, slopes of log kII versus E°′(III/IV) plots are lower for fast‐to‐oxidize S than for slow‐to‐oxidize S. The log kI versus E°′(III/IV) plot suggests that the manmade TAML catalyst can never be as reactive toward H2O2 as a horseradish peroxidase enzyme. [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]

Bis‐sulfonamide bis‐amide TAML activator [Fe{4‐NO2C6H3‐1,2‐(NCOCMe2NSO2)2CHMe}]− (2) catalyzes oxidative degradation of the oxidation‐resistant neonicotinoid insecticide, imidacloprid (IMI), by H2O2 at pH 7 and 25 °C, whereas the tetrakis‐amide TAML [Fe{4‐NO2C6H3‐1,2‐(NCOCMe2NCO)2CF2}]− (1), previously regarded as the most catalytically active TAML, is inactive under the same conditions. At ultra‐low concentrations of both imidacloprid and 2, 62 % of the insecticide was oxidized in 2 h, at which time the catalyst is inactivated; oxidation resumes on addition of a succeeding aliquot of 2. Acetate and oxamate were detected by ion chromatography, suggesting deep oxidation of imidacloprid. Explored at concentrations [2]≥[IMI], the reaction kinetics revealed unusually low kinetic order in 2 (0.164±0.006), which is observed alongside the first order in imidacloprid and an ascending hyperbolic dependence in [H2O2]. Actual independence of the reaction rate on the catalyst concentration is accounted for in terms of a reversible noncovalent binding between a substrate and a catalyst, which usually results in substrate inhibition when [catalyst]≪[substrate] but explains the zero order in the catalyst when [2]>[IMI]. A plausible mechanism of the TAML‐catalyzed oxidations of imidacloprid is briefly discussed. Similar zero‐order catalysis is presented for the oxidation of 3‐methyl‐4‐nitrophenol by H2O2, catalyzed by the TAML analogue of 1 without a NO2‐group in the aromatic ring. [ABSTRACT FROM AUTHOR]

TAML complexis oxidized by H2O2ortBuOOH in water at pH < 10 into the corresponding iron(IV) μ-oxo-bridged dimer2, which oxidizes readily ring-substituted thioanisolesp-XC6H4SMe (X=H, MeO, Me, Cl, CN) into the corresponding sulfoxides with regeneration of1. The oxidation studied under pseudo-first-order conditions using the stopped-flow technique by monitoring the fading of the 420-nm band of2follows hyperbolic kinetics according to the rate lawkobs = ab[p-XC6H4SMe]/(1 + b[p-XC6H4SMe]) at pH 8 and 25 °C. Parametersa,b, andaball decrease for electron-poorer thioanisoles and the Hammett valueρ ~ 1 has been found forab, which can be associated with the second-order rate constants for oxidation of thioanisoles by2. The kinetics of oxidation ofp-NO2C6H4SMe by H2O2catalyzed by1has been studied under steady-state conditions. Covering the concentration of1in a 100-fold range has revealed that though first-order kinetics in1is observed at low catalyst concentrations (below 10−6 M), there is a significant negative deviation from linearity at [1] > 10−6 M. The latter was rationalized by the equilibrium between the monomeric and dimeric FeIVspecies 2 M ⇌ M–M (Kd), both being able to oxidizep-NO2C6H4SMe with rate constantskmandkdwhich were found to be (13 ± 1) × 104and (0.32 ± 0.01) × 104 M−1 s−1, respectively. The difference in the rate constants is the key for resolving the dilemma of faster catalysisversusslower single-turnover reactivity of TAML activators in water. [ABSTRACT FROM AUTHOR]

The oxidation of TAML complex(1) by various oxidants is explored in MeCN with 0.2% water at −40 °C, where the iron(V)oxo complex is stable enough for reliable spectral identification. The iron(V)oxo state is achieved using NaClO even faster than in the case of previously exploredm-chloroperoxybenzoic acid. In contrast, H2O2and organic peroxides (benzoyl peroxide,tert-butylperoxide, andtert-butylhydroperoxide) all convert1into the corresponding diiron(IV)-μ-oxo dimer (2) under the same conditions. The latter does not form when (NH4)2[Ce(NO3)6] is employed and the FeIVproduct obtained does not seem to contain an oxo moiety. In contrast to all other oxidants, the conversion of1bytBuOOH into2is characterized by non-conventional kinetics, and therefore this reaction was explored in some detail. The evidence is presented that this light-, O2-, TEMPO-, and base-dependent reaction is a free radical process under the conditions used. [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]

The electronic properties of iron tetraamido macrocyclic ligand (TAML) activators of peroxides can be finely and coarsely tuned by varying the substituents on the 'head' and 'tail' macrocyclic components, respectively. By examining the reactivity of the TAML oxidoiron(V) complex with head-NO2 and tail-F substituents, one is able to compare the impact of significantly reduced macrocyclic tetraamide donor capacity on fundamental processes such as hydrogen-atom abstraction, oxygen-atom transfer, and electron transfer by using prior studies with more electron-rich TAML systems. Herein, we demonstrate that the oxidoiron(V) form 3c can be generated by treatment of [Fe{4-NO2C6H3-1,2-(N2COCMe2N3CO)2CF2( Fe-N2)( Fe-N3)}(OH2)]- ( 1c) with m-chloroperoxybenzoic acid ( mCPBA) in MeCN at -40 °C. The oxidation proceeds through the intermediacy of the μ-oxo[iron(IV)]2 dimer. The overall rate of the FeIII→FeV conversion by mCPBA is slightly faster for 1c than that of its less electron-rich precursor [Fe{C6H4-1,2-(N1COCMe2N2CO)2CMe2( Fe-N1)( Fe-N2)}(OH2)]- ( 1a). Nevertheless, the oxidative reactivity of 3c toward thioanisole and the hydrocarbons ethylbenzene and cyclohexane exceeds that of 3a by 4.3, 2.1, and 2.6 times, respectively. The reactivity of 3c is significantly greater towards ethylbenzene than that of the oxidoiron(V) species 3b derived from [Fe{C6H4-1,2-(N1COCMe2N2CO)2NMe( Fe-N1)( Fe-N2)}(OH2)]- ( 1b). [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]

The orthopalladation of benzylamine (baH) by Pd(II) acetate in acetonitrile occurs in two time-resolved steps. A first faster step taking place at 25 °C results in the formation of the N-bound complex [Pd(OAc)2(baH)2] and follows a second-order kinetics in the incoming amine according to the rate law kobs=k1[baH]2. The second order in baH arises from the successive substitution of the solvent in [Pd(OAc)2(MeCN)2]. The latter complex transforms slowly into the orthometalated derivative [Pd(ba)(OAc)]2 at 60 °C. The initial rate of this step is inversely proportional to the concentration of baH and the following rate law holds: V0=km Pd(II)t/[baH]. This is accounted for in terms of the mechanism involving a reversible dissociation of one baH ligand from [Pd(OAc)2(baH)2] followed by the electrophilic C&z.sbnd;H bond cleavage of N-bound baH by Pd(II). [Copyright &y& Elsevier]

Abstract: Cyclometalated derivatives of ring-substituted N,N-dimethylbenzylamines with controlled redox potentials as potent mediators of bioelectrochemical electron transport are reported. The cycloruthenation of R1R2R3C6H2CH2NMe2 (R1, R2, R3=H, Me, tBuO, MeO, NMe2, F, CF3, CN, NO2) by [(η6-C6H6)RuCl(μ-Cl)]2 in the presence of NaOH/KPF6 in acetonitrile or pivalonitrile affords cyclometalated complexes [(η6-C6H6)Ru(C6HR1R2R3-o-CH2NMe2)(RCN)]PF6 [R=Me (1) and R=CMe3 (2)] in good yields. Reactions of complexes 1 and 2 with 2,2′-bipyridine (bpy) in acetonitrile or pivalonitrile result in dissociation of η6-bound benzene and the formation of [Ru(C6HR1R2R3-o-CH2NMe2)(bpy)(RCN)2]PF6 [R=Me (3) and R=CMe3 (4)]. All new compounds have been fully characterized by mass spectrometry, 1H/13C NMR, and IR spectroscopy. An X-ray crystal structural investigation of complex 1 (R1/R2/R3=H/H/H) and two complexes of type 3 (R1/R2/R3=MeO/H/H, MeO/MeO/H) has been performed. Acetonitrile ligands of 3 are mutually cis and the σ-bound carbon is trans to one of the bpy nitrogens. Measured by the cyclic voltammetry in MeOH as solvent, the redox potentials of complexes 3 for the RuII/III feature cover the range 320–720 mV (versus Ag/AgCl) and correlate linearly with the Hammett constants. Complexes 3 mediate efficiently the electron transport between the active site of PQQ-dependent glucose dehydrogenase (PQQ=pyrroloquinoline quinone) and a glassy carbon electrode. Determined by cyclic voltammetry the second order rate constant for the oxidation of the reduced (by d-glucose) enzyme active site by RuIII derivative of 3 (R1/R2/R3=H) (generated electrochemically) is as high as 4.8×107 M−1 s−1 at 25 °C and pH 7. [Copyright &y& Elsevier]