In JoVE (1)

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Articles by Terrence J. Collins in JoVE

Other articles by Terrence J. Collins on PubMed

Electron-Transfer Oxidation by Phase-Separating Reagents

Inorganic Chemistry. Sep, 1998  |  Pubmed ID: 11670633

Rapid Total Destruction of Chlorophenols by Activated Hydrogen Peroxide

Science (New York, N.Y.). Apr, 2002  |  Pubmed ID: 11951040

A practical, inexpensive, green chemical process for degrading environmental pollutants is greatly needed, especially for persistent chlorinated pollutants. Here we describe the activation of hydrogen peroxide by tetraamidomacrocylic ligand (TAML) iron catalysts, to destroy the priority pollutants pentachlorophenol (PCP) and 2,4,6-trichlorophenol (TCP). In water, in minutes, under ambient conditions of temperature and pressure, PCP and TCP are completely destroyed at catalyst:substrate ratios of 1:715 and 1:2000, respectively. The fate of about 90% of the carbon and about 99% of the chlorine has been determined in each case. Neither dioxins nor any other toxic compounds are detectable products, and the catalysts themselves show low toxicity.

TAML Oxidant Activators: a New Approach to the Activation of Hydrogen Peroxide for Environmentally Significant Problems

Accounts of Chemical Research. Sep, 2002  |  Pubmed ID: 12234208

In the Institute for Green Oxidation Chemistry, we develop oxidation catalysts with useful lifetimes via an iterative design process in which oxidation-sensitive ligand groups are identified and replaced. The process has led to catalysts with greatly enhanced stability toward oxidative and hydrolytic degradation. The iron catalysts described herein can be comprised exclusively of biochemically common elements. They are efficient and selective peroxide activators for many areas of technology. They are water-soluble and are effective in minute quantities (nanomolar to low micromolar) over a broad pH range. Various green chemistry applications are sketched.

Understanding the Mechanism of H(+)-induced Demetalation As a Design Strategy for Robust Iron(III) Peroxide-activating Catalysts

Journal of the American Chemical Society. Oct, 2003  |  Pubmed ID: 14531659

The FeIII-TAML (tetra-amido macrocyclic ligand) activators 1 (Y = Cl) and 2 (Y = H2O), a (R = Me, X = H), b (Me, Cl), c (Me, MeO), d (Et, Cl), e (F, H), f (F, Cl), are five-coordinated in the solid state (X-ray crystallography) but are six-coordinated species in water with two H2O axial ligands. The first pKa's of aqueous ligands are in the range of 9.5-10.5. The acid-induced demetalation of 2 follows the equation kobs = k1*[H+] + k3*[H+]3. The rate constants k1* and k3* vary by 5 and 11 orders of magnitude depending on the nature of substituents R. The highest stabilization against the demetalation is achieved for R = F.

XANES Evidence Against a Manganyl Species in the S3 State of the Oxygen-evolving Complex

Journal of the American Chemical Society. Jul, 2004  |  Pubmed ID: 15225020

Manganyl Mn=O species have been suggested as possible intermediates in photosynthetic water oxidation and as the reactive species in asymmetric olefin epoxidation. The first X-ray absorption spectrum for a MnV=O complex is reported. Comparison of the EXAFS data for Na[MnV=O(HMPAB) with those for lower-valent Mn complexes suggests that EXAFS measurements and edge-energy measurements are unlikely to have sufficient sensitivity to reliably reflect the presence of Mn=O species. In contrast, the preedge transition for Na[MnV=O(HMPAB) is 4-fold more intense than the most intense preedge transition observed for nonmanganyl complexes. This increase in intensity is shown to be sufficiently sensitive to allow detection of a manganyl species if it is formed. These data together with published data for the photosynthetic oxygen-evolving complex provide strong evidence against the presence of manganyl Mn=O species in the oxygen-evolving complex through the S3 state.

Catalytically Active Mu-Oxodiiron(IV) Oxidants from Iron(III) and Dioxygen

Journal of the American Chemical Society. Mar, 2005  |  Pubmed ID: 15725005

The reaction between an Fe(III) complex and O(2) to afford a stable catalytically active diiron(IV)-mu-oxo compound is described. Phosphonium salts of orange five-coordinated Fe(III)-TAML complexes with an axial aqua ligand ([PPh(4)]1-H(2)O, tetraamidato macrocyclic Fe(III) species derived from 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) react rapidly with O(2) in CH(2)Cl(2) or other weakly coordinating solvents to produce black mu-oxo-bridged diiron(IV) complexes, 2, in high yields. Complexes 2 have been characterized by X-ray crystallography (2 cases), microanalytical data, mass spectrometry, UV/Vis, Mossbauer, and (1)H NMR spectroscopies. Mossbauer data show that the diamagnetic Fe-O-Fe unit contains antiferromagnetically coupled S = 1 Fe(IV) sites; diamagnetic (1)H NMR spectra are observed. The oxidation of PPh(3) to OPPh(3) by 2 was confirmed by UV/Vis and GC-MS. Labeling experiments with (18)O(2) and H(2)(18)O established that the bridging oxygen atom of 2 derives from O(2). Complexes 2 catalyze the selective oxidation of benzylic alcohols into the corresponding aldehydes and bleach rapidly organic dyes, such as Orange II in MeCN-H(2)O mixtures; reactivity evidence suggests that free radical autoxidation is not involved. This work highlights a promising development for the advancement of green oxidation technology, as O(2) is an abundant, clean, and inexpensive oxidizing agent.

High-valent Iron Complexes with Tetraamido Macrocyclic Ligands: Structures, Mössbauer Spectroscopy, and DFT Calculations

Journal of Inorganic Biochemistry. Apr, 2006  |  Pubmed ID: 16464502

Iron complexes of tetraamido macrocyclic ligands (TAML) are unique synthetic oxidation catalysts. In general, the central Fe(III) ion (S=3/2) is surrounded by four, almost planar, deprotonated amide-N sigma-donors although the full suite with new generation systems includes some substitution of amides with related donor functionalities. Oxidation under different conditions affords a variety of high-valent forms of iron-TAMLs. This review provides a summary and discussion of structural and spectroscopic features of complexes oxidized by one equivalent above the ferric state. These comprise Fe(IV)-TAML high spin (S=2) and intermediate spin (S=1) systems, wherein the oxidation equivalent can be taken from the metal (Fe(IV)) or the ligand (TAML radical-cation Fe(III)), and coupled spin (S=0) systems of mu-oxoiron(IV) dimers. The discussion is principally based on data obtained by X-ray crystallography, Mössbauer spectroscopy, and density functional theory calculations.

Little Green Molecules

Scientific American. Mar, 2006  |  Pubmed ID: 16502615

"Green" Oxidation Catalysis for Rapid Deactivation of Bacterial Spores

Angewandte Chemie (International Ed. in English). Jun, 2006  |  Pubmed ID: 16673442

Total Degradation of Fenitrothion and Other Organophosphorus Pesticides by Catalytic Oxidation Employing Fe-TAML Peroxide Activators

Journal of the American Chemical Society. Sep, 2006  |  Pubmed ID: 16967942

A Fe-TAML/H2O2 catalytic oxidation process achieves facile in-solution total degradation of fenitrothion and two other organophosphorus (OP) pesticides. Degradation products have been identified and quantified providing evidence for oxidative hydrolysis, oxidative desulfuration, perhydrolysis, and deep oxidation. Degradation pathways can be selected by pH control to completely obviate all toxic residuals. Aquatic toxicity assays support the environmental compatibility of the degradation process.

Activity-stability Parameterization of Homogeneous Green Oxidation Catalysts

Chemistry (Weinheim an Der Bergstrasse, Germany). Dec, 2006  |  Pubmed ID: 17029311

Small-molecule synthetic homogeneous-oxidation catalysts are normally poorly protected from self-destruction under operating conditions. Achieving design control over both activity and half-life is important not only in advancing the utility of oxidation catalysts, but also in minimizing hazards associated with their use and disposal. Iron(III)-TAML (tetraamido-macrocyclic ligand) oxidant catalysts rapidly activate H(2)O(2) for numerous significant processes, exhibiting high and differing activity and varying half-lives depending upon the TAML design. A general approach is presented that allows for the simultaneous determination of the second-order rate constant for the oxidation of a targeted substrate by the active catalyst (k(II)) and the rate constant for the intramolecular self-inactivation of the active catalyst (k(i)). The approach is valid if the formation of the active catalyst from its resting state and the primary oxidizing agent, measured by the second-order rate constant k(I), is fast and the catalyst concentration is very low, such that bimolecular inactivation pathways can be neglected. If the oxidation process is monitored spectrophotometrically and is set up to be incomplete, the kinetic trace can be analyzed by using the equation ln(lnA(t))/A(infnity)=ln(k(II)/k(i)[Fe(III)](tot)-k(i)t, from which k(II) and k(i) can be determined. Here, A(t) and A(infinity) are absorbances at time t and at the end of reaction (t=infinity), respectively, and [Fe(III)](tot) is the total catalyst concentration. Several tools were applied to examine the validity of the approach by using a variety of different Fe(III)-TAML catalysts, H(2)O(2) and tBuOOH as oxidizing agents, and the dyes safranine O and orange II as target substrates. Learning how catalyst activities (k(II)) and catalyst half-lives (k(i)) can be controlled by ligand design is an important step in creating green catalysts that will not persist in the environment after they have achieved their purpose.

Chemical and Spectroscopic Evidence for an FeV-oxo Complex

Science (New York, N.Y.). Feb, 2007  |  Pubmed ID: 17185561

Iron(V)-oxo species have been proposed as key reactive intermediates in the catalysis of oxygen-activating enzymes and synthetic catalysts. Here, we report the synthesis of [Fe(TAML)(O)]- in nearly quantitative yield, where TAML is a macrocyclic tetraamide ligand. Mass spectrometry, Mössbauer, electron paramagnetic resonance, and x-ray absorption spectroscopies, as well as reactivity studies and density functional theory calculations show that this long-lived (hours at -60 degrees C) intermediate is a spin S = 1/2 iron(V)-oxo complex. Iron-TAML systems have proven to be efficient catalysts in the decomposition of numerous pollutants by hydrogen peroxide, and the species we characterized is a likely reactive intermediate in these reactions.

Human Pharmaceuticals in the Aquatic Environment: a Challenge to Green Chemistry

Chemical Reviews. Jun, 2007  |  Pubmed ID: 17530905

Polarized X-ray Absorption Spectroscopy of Single-crystal Mn(V) Complexes Relevant to the Oxygen-evolving Complex of Photosystem II

Journal of the American Chemical Society. Oct, 2007  |  Pubmed ID: 17918832

High-valent Mn-oxo species have been suggested to have a catalytically important role in the water splitting reaction which occurs in the Photosystem II membrane protein. In this study, five- and six-coordinate mononuclear Mn(V) compounds were investigated by polarized X-ray absorption spectroscopy in order to understand the electronic structure and spectroscopic characteristics of high-valent Mn species. Single crystals of the Mn(V)-nitrido and Mn(V)-oxo compounds were aligned along selected molecular vectors with respect to the X-ray polarization vector using X-ray diffraction. The local electronic structure of the metal site was then studied by measuring the polarization dependence of X-ray absorption near-edge spectroscopy (XANES) pre-edge spectra (1s to 3d transition) and comparing with the results of density functional theory (DFT) calculations. The Mn(V)-nitrido compound, in which the manganese is coordinated in a tetragonally distorted octahedral environment, showed a single dominant pre-edge peak along the MnN axis that can be assigned to a strong 3d(z(2))-4p(z) mixing mechanism. In the square pyramidal Mn(V)-oxo system, on the other hand, an additional peak was observed at 1 eV below the main pre-edge peak. This component was interpreted as a 1s to 3d(xz,yz) transition with 4px,y mixing, due to the displacement of the Mn atom out of the equatorial plane. The XANES results have been correlated to DFT calculations, and the spectra have been simulated using a TD (time-dependent)-DFT approach. The relevance of these results to understanding the mechanism of the photosynthetic water oxidation is discussed.

Attaining Control by Design over the Hydrolytic Stability of Fe-TAML Oxidation Catalysts

Journal of the American Chemical Society. Apr, 2008  |  Pubmed ID: 18335938

The iron(III) complexes of tetra amidato macrocyclic ligands (TAMLs) ([Fe{1-X1-2-X2C6H2-4,5-(NCOCMe2NCO)2CR2}(OH2)]- , 1: X1 = X2 = H, R2 = Me2 (a), R2 = (CH2)2 (b); X1 = X2 = Cl, R2 = F2 (c), etc.), which the proton is known to demetalate at pH < 3, are also subject to catalyzed demetalation by Brønsted acid buffer components at pH 4-9 such as H2PO4-, HSO3-, and CH3CO2H, HO2CCH2CO2-. Buffers based on pyridine (py) and tris(hydroxymethyl)aminomethane (TRIS) are catalytically inactive. Where reactions proceed, the products are demetalated TAMLs and iron species of variable composition. Pseudo-first-order rate constants for the demetalation (kobs) are linear functions of the acid concentrations, and the effective second-order rate constants k1,eff have a hyperbolic dependence on [H+] (k1,eff = a1[H+]/(b1+[H+]). The rate of demetalation of 1a in H2PO4-/HPO42- buffer is appreciable, but the kobs values for 1b and 1c are immeasurably low, showing that the rates are strongly affected by the CR2 or "tail" fragments, which are known to potently affect the TAML basicity. The reactivities of 1 depend insignificantly on the aromatic ring or "head" group of 1. The proposed mechanism involves precoordination of the acidic buffer species followed by hydrolysis. The demetalating abilities of buffer species depend on their structures and acidities. Thus, although pyridine-2-carboxylic (picolinic) acid catalyzes the demetalation, its 3- and 4-isomers (nicotinic and isonicotininc acids) are inactive. The difference is rationalized to result from the ability that only coordinated picolinic acid has to deliver a proton to an amidato nitrogen in an intramolecular manner. The reaction order in picolinic acid equals one for 1a and two for 1b. For 1b, "inactive" pyridine and nicotinic acid speed up the demetalation in the presence of picolinic acid, suggesting that the second order arises from the axial binding of two pyridine molecules, one of which must be picolinic acid. The binding of pyridine- and imidazole-type ligands was confirmed by UV/vis equilibrium measurements and X-ray crystallography. The implications of these mechanistic findings for designing superior Fe-TAML oxidation catalysts and catalyst formulations are discussed using the results of DFT calculations.

Destruction of Estrogens Using Fe-TAML/peroxide Catalysis

Environmental Science & Technology. Feb, 2008  |  Pubmed ID: 18351108

Endocrine disrupting chemicals (EDCs) impair living organisms by interfering with hormonal processes controlling cellular development Reduction of EDCs in water by an environmentally benign method is an important green chemistry goal. One EDC, 17alpha-ethinylestradiol (EE2), the active ingredient in the birth control pill, is excreted by humans to produce a major source of artificial environmental estrogenicity, which is incompletely removed by currenttechnologies used by municipal wastewater treatment plants (MWTPs). Natural estrogens found in animal waste from concentrated animal feeding operations (CAFOs) can also increase estrogenic activity of surface waters. An iron-tetraamidomacrocyclic ligand (Fe-TAML) activator in trace concentrations activates hydrogen peroxide and was shown to rapidly degrade these natural and synthetic reproductive hormones found in agricultural and municipal effluent streams. On the basis of liquid chromatography tandem mass spectrometry, apparent half-lives for 17 alpha- and 17 beta-estradiol, estriol, estrone, and EE2 in the presence of Fe-TAML and hydrogen peroxide were approximately 5 min and included a concomitant loss of estrogenic activity as established by E-Screen assay.

(TAML)FeIV O Complex in Aqueous Solution: Synthesis and Spectroscopic and Computational Characterization

Inorganic Chemistry. May, 2008  |  Pubmed ID: 18380453

Recently, we reported the characterization of the S = (1)/ 2 complex [Fe (V)(O)B*] (-), where B* belongs to a family of tetraamido macrocyclic ligands (TAMLs) whose iron complexes activate peroxides for environmentally useful applications. The corresponding one-electron reduced species, [Fe (IV)(O)B*] (2-) ( 2), has now been prepared in >95% yield in aqueous solution at pH > 12 by oxidation of [Fe (III)(H 2O)B*] (-) ( 1), with tert-butyl hydroperoxide. At room temperature, the monomeric species 2 is in a reversible, pH-dependent equilibrium with dimeric species [B*Fe (IV)-O-Fe (IV)B*] (2-) ( 3), with a p K a near 10. In zero field, the Mössbauer spectrum of 2 exhibits a quadrupole doublet with Delta E Q = 3.95(3) mm/s and delta = -0.19(2) mm/s, parameters consistent with a S = 1 Fe (IV) state. Studies in applied magnetic fields yielded the zero-field splitting parameter D = 24(3) cm (-1) together with the magnetic hyperfine tensor A/ g nbeta n = (-27, -27, +2) T. Fe K-edge EXAFS analysis of 2 shows a scatterer at 1.69 (2) A, a distance consistent with a Fe (IV)O bond. DFT calculations for [Fe (IV)(O)B*] (2-) reproduce the experimental data quite well. Further significant improvement was achieved by introducing hydrogen bonding of the axial oxygen with two solvent-water molecules. It is shown, using DFT, that the (57)Fe hyperfine parameters of complex 2 give evidence for strong electron donation from B* to iron.

Mechanistically Inspired Design of Fe(III)-TAML Peroxide-activating Catalysts

Journal of the American Chemical Society. Sep, 2008  |  Pubmed ID: 18722448

Recent broad-ranging mechanistic studies of FeIII-TAML peroxide activators enable a strategy for designing catalysts with improved (i) hydrolytic and (ii) operational stabilities, (iii) faster activation of H2O2 and other peroxides, and (iv) a pH of highest activity closer to 7. Combining all items of insight leads to [Fe{1-NO2C6H3-3,4-(NCOCMe2NCO)2CF2}(OH2)]- (1a) which exhibits the most desirable technical performance in its class.

Density Functional Theory Study of the Structural, Electronic, and Magnetic Properties of a Mu-oxo Bridged Dinuclear Fe(IV) Complex Based on a Tetra-amido Macrocyclic Ligand

Inorganic Chemistry. Oct, 2008  |  Pubmed ID: 18817379

Recently, the synthesis, crystallographic structure, and Mossbauer characterization of the first example of an [(Fe(IV)TAML)2O](2-) (TAML = tetra-amido microcyclic ligand) complex were reported. Here, we elucidate the prominent structural, electronic, and magnetic properties of this complex on the basis of density functional theory (DFT) calculations. While the torsion between the molecular halves is caused by hydrogen bonding between the TAML moieties, the bending of the Fe-O-Fe unit is an intrinsic property of the bridge. The values for the (57)Fe isomer shift and quadrupole splitting obtained with DFT are in good agreement with experimental results and indicate that the irons have intermediate spin states (S1 = S2 = 1). The iron spins are coupled by strong antiferromagnetic exchange to yield a ground state with system spin S = 0. The Fe-O distances in the excited S > 0 states are significantly longer than in the ground state. Since the wave function of the ground state, in which the iron spins are antiferromagnetically coupled to give system spin S = 0, is a linear combination of Slater determinants that cannot be treated with existing DFT codes, the Fe-O distance for the S = 0 state has been estimated by extrapolation from the optimized geometries for the ferromagnetic state (S = 2) and the broken symmetry state to be 1.748 A, in good agreement with the crystallographic distance 1.728 A. To accommodate the spin-dependent reorganization energies, the conventional bilinear spin Hamiltonian has been extended with a biquadratic coupling term: H(ex) = c' + j0S1 x S2 + j1(S1 x S2)(2). A computational scheme is presented for estimating the exchange parameters, yielding the values j0 = 199 cm(-1) and j1 = -61 cm(-1) for [(Fe(IV)B*)2O](2-). Two mechanisms for biquadratic exchange are discussed.

Catalase-peroxidase Activity of Iron(III)-TAML Activators of Hydrogen Peroxide

Journal of the American Chemical Society. Nov, 2008  |  Pubmed ID: 18928252

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.

Persuasive Communication About Matters of Great Urgency: Endocrine Disruption

Environmental Science & Technology. Oct, 2008  |  Pubmed ID: 18983074

The Impact of Surfactants on Fe(III)-TAML-catalyzed Oxidations by Peroxides: Accelerations, Decelerations, and Loss of Activity

Chemistry (Weinheim an Der Bergstrasse, Germany). Oct, 2009  |  Pubmed ID: 19711381

Iron(III) complexes of tetraamidato macrocyclic ligands (TAMLs), [Fe{4-XC(6)H(3)-1,2-(NCOCMe(2)NCO)(2)CR(2)}(OH(2))](-), 1 (1 a: X = H, R = Me; 1 b: X = COOH, R = Me); 1 c: X = CONH(CH(2))(2)COOH, R = Me; 1 d: CONH(CH(2))(2)NMe(2), R = Me; 1 e: X = CONH(CH(2))(2)NMe(3) (+), R = Me; 1 f: X = H, R = F), have been tested as catalysts for the oxidative decolorization of Orange II and Sudan III dyes by hydrogen peroxide and tert-butyl hydroperoxide in the presence of micelles that are neutral (Triton X-100), positively charged (cetyltrimethylammonium bromide, CTAB), and negatively charged (sodium dodecyl sulfate, SDS). The previously reported mechanism of catalysis involves the formation of an oxidized intermediate from 1 and ROOH (k(I)) followed by dye bleaching (k(II)). The micellar effects on k(I) and k(II) have been separately studied and analyzed by using the Berezin pseudophase model of micellar catalysis. The largest micellar acceleration in terms of k(I) occurs for the 1 a-tBuOOH-CTAB system. At pH 9.0-10.5 the rate constant k(I) increased by approximately five times with increasing CTAB concentration and then gradually decreased. There was no acceleration at higher pH, presumably owing to the deprotonation of the axial water ligand of 1 a in this pH range. The k(I) value was only slightly affected by SDS (in the oxidation of Orange II), but was strongly decelerated by Triton X-100. No oxidation of the water-insoluble, hydrophobic dye Sudan III was observed in the presence of the SDS micelles. The k(II) value was accelerated by cationic CTAB micelles when the hydrophobic primary oxidant tert-butyl hydroperoxide was used. It is hypothesized that tBuOOH may affect the CTAB micelles and increase the binding of the oxidized catalysts. The tBuOOH-CTAB combination accelerated both of the catalysis steps k(I) and k(II).

Design of More Powerful Iron-TAML Peroxidase Enzyme Mimics

Journal of the American Chemical Society. Dec, 2009  |  Pubmed ID: 19928965

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.

Direct Detection of Oxygen Ligation to the Mn(4)Ca Cluster of Photosystem II by X-ray Emission Spectroscopy

Angewandte Chemie (International Ed. in English). 2010  |  Pubmed ID: 20017172

Designing Green Oxidation Catalysts for Purifying Environmental Waters

Journal of the American Chemical Society. Jul, 2010  |  Pubmed ID: 20565079

We describe the synthesis, characterization, aqueous behavior, and catalytic activity of a new generation of Fe(III)-TAML (tetraamido macrocycle ligand) activators of peroxides (2), variants of [Fe{(OC)(2)(o,o'-NC(6)H(4)NCO)(2)CMe(2)}(OH(2))(-)] (2d), which have been designed to be especially suitable for purifying water of recalcitrant oxidizable pollutants. Activation of H(2)O(2) by 2 (k(I)) as a function of pH was analyzed via kinetic studies of Orange II bleaching. This was compared with the known behavior of the first generation of Fe(III)-TAMLs (1). Novel reactivity features impact the potential for oxidant activation for water purification by 2d and its aromatic ring-substituted dinitro (2e) and tetrachloro (2f) derivatives. Thus, the maximum activity for 2e occurs at pH 9, the closest yet to the EPA guidelines for drinking water (6.5-8.5), allowing 2e to rapidly activate H(2)O(2) at pH 7.7. In water, 2e has two axial water ligands with pK(a)'s of 8.4 and 10.0 (25 degrees C). The former is the lowest for all Fe(III)-TAMLs developed to date and is key to 2e's exceptional catalytic activity in neutral and slightly basic solutions. Below pH 7, 2d was found to be quite sensitive to demetalation in phosphate buffers. This was overcome by iterative design to give 2e (hydrolysis rate 2d > 100 x 2e). Mechanistic studies highlight 2e's increased stability by establishing that to demetalate 2e at a comparable rate to which H(2)PO(4)(-) demetalates 2d, H(3)PO(4) is required. A critical criterion for green catalysts for water purification is the avoidance of endocrine disruptors, which can impair aquatic life. Fe(III)-TAMLs do not alter transcription mediated by mammalian thyroid, androgen, or estrogen hormone receptors, suggesting that 2 do not bind to the receptors and reducing concerns that the catalysts might have endocrine disrupting activity.

Fast Water Oxidation Using Iron

Journal of the American Chemical Society. Aug, 2010  |  Pubmed ID: 20698652

Photolysis of water, a long-studied strategy for storing solar energy, involves two half-reactions: the reduction of protons to dihydrogen and the oxidation of water to dioxygen. Proton reduction is well-understood, with catalysts achieving quantum yields of 34% when driven by visible light. Water oxidation, on the other hand, is much less advanced, typically involving expensive metal centers and rarely working in conjunction with a photochemically powered system. Before further progress can be made in the field of water splitting, significant developments in the catalysis of oxygen evolution are needed. Herein we present an iron-centered tetraamido macrocyclic ligand (Fe-TAML) that efficiently catalyzes the oxidative conversion of water to dioxygen. When the catalyst is combined in unbuffered solution with ceric ammonium nitrate, its turnover frequency exceeds 1.3 s(-1). Real-time UV-vis and oxygen monitoring of the active complex give insights into the reaction and decay kinetics.

Thermodynamic, Electrochemical, High-pressure Kinetic, and Mechanistic Studies of the Formation of Oxo Fe(IV)-TAML Species in Water

Inorganic Chemistry. Dec, 2010  |  Pubmed ID: 21086984

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).

Rapid, Biomimetic Degradation in Water of the Persistent Drug Sertraline by TAML Catalysts and Hydrogen Peroxide

Environmental Science & Technology. Sep, 2011  |  Pubmed ID: 21823671

Iron TAML activators (oxidation catalysts based upon tetraamido macrocyclic ligands) at nanomolar concentrations in water activate hydrogen peroxide to rapidly degrade sertraline, the persistent, active pharmaceutical ingredient (API) in the widely used drug Zoloft. Although all the API is readily consumed, degradation slows significantly at one intermediate, sertraline ketone. The process occurs from neutral to basic pH. The pathway has been characterized through four early intermediates which reflect the metabolism of sertraline, providing further evidence that TAML activator/peroxide reactive intermediates mimic those of cytochrome P450 enzymes. TAML catalysts have been designed to exhibit considerable variability in reactivity and this provides an excellent tool for observing degradation intermediates of widely differing stabilities. Two elusive, hydrolytically sensitive intermediates and likely human metabolites, sertraline imine and N-desmethylsertraline imine, could be identified only by using a fast-acting catalyst. The more stable intermediates and known human metabolites, desmethylsertraline and sertraline ketone, were most easily detected and studied using a slow-acting catalyst. The resistance of sertraline ketone to aggressive TAML activator/peroxide treatment marks it as likely to be environmentally persistent and signals that its environmental effects are important components of the full implications of sertraline use.

Prediction of High-valent Iron K-edge Absorption Spectra by Time-dependent Density Functional Theory

Dalton Transactions (Cambridge, England : 2003). Nov, 2011  |  Pubmed ID: 21956429

In recent years, a number of high-valent iron intermediates have been identified as reactive species in iron-containing metalloproteins. Inspired by the interest in these highly reactive species, chemists have synthesized Fe(IV) and Fe(V) model complexes with terminal oxo or nitrido groups, as well as a rare example of an Fe(VI)-nitrido species. In all these cases, X-ray absorption spectroscopy has played a key role in the identification and characterization of these species, with both the energy and intensity of the pre-edge features providing spectroscopic signatures for both the oxidation state and the local site geometry. Here we build on a time-dependent DFT methodology for the prediction of Fe K- pre-edge features, previously applied to ferrous and ferric complexes, and extend it to a range of Fe(IV), Fe(V) and Fe(VI) complexes. The contributions of oxidation state, coordination environment and spin state to the spectral features are discussed. These methods are then extended to calculate the spectra of the heme active site of P450 Compound II and the non-heme active site of TauD. The potential for using these methods in a predictive manner is highlighted.

On the Reactivity of Mononuclear Iron(V)oxo Complexes

Journal of the American Chemical Society. Nov, 2011  |  Pubmed ID: 21985217

Ferric tetraamido macrocyclic ligand (TAML)-based catalysts [Fe{C(6)H(4)-1,2-(NCOCMe(2)NCO)(2)CR(2)}(OH(2))]PPh(4) [1; R = Me (a), Et (b)] are oxidized by m-chloroperoxybenzoic acid at -40 °C in acetonitrile containing trace water in two steps to form Fe(V)oxo complexes (2a,b). These uniquely authenticated Fe(V)(O) species comproportionate with the Fe(III) starting materials 1a,b to give μ-oxo-(Fe(IV))(2) dimers. The comproportionation of 1a-2a is faster and that of 1b-2b is slower than the oxidation by 2a,b of sulfides (p-XC(6)H(4)SMe) to sulfoxides, highlighting a remarkable steric control of the dynamics. Sulfide oxidation follows saturation kinetics in [p-XC(6)H(4)SMe] with electron-rich substrates (X = Me, H), but changes to linear kinetics with electron-poor substrates (X = Cl, CN) as the sulfide affinity for iron decreases. As the sulfide becomes less basic, the Fe(IV)/Fe(III) ratio at the end of reaction for 2b suggests a decreasing contribution of concerted oxygen-atom transfer (Fe(V) → Fe(III)) concomitant with increasing electron transfer oxidation (Fe(V) → Fe(IV)). Fe(V) is more reactive toward PhSMe than Fe(IV) by 4 orders of magnitude, a gap even larger than that known for peroxidase Compounds I and II. The findings reinforce prior work typecasting TAML activators as faithful peroxidase mimics.

Experimental and Theoretical Evidence for Multiple Fe(IV) Reactive Intermediates in TAML-activator Catalysis: Rationalizing a Counterintuitive Reactivity Order

Chemistry (Weinheim an Der Bergstrasse, Germany). Aug, 2012  |  Pubmed ID: 22829473

TAML Activator-based Amperometric Analytical Devices As Alternatives to Peroxidase Biosensors

Analytical Chemistry. Nov, 2012  |  Pubmed ID: 23005918

The ferric TAML catalysts [Fe{C(6)H(2)-1,2-( NCOCMe(2)NCO)(2)CMe(2)}(OH(2))](-) (1) with counterions Na(+) (a) and PPh(4)(+) (b) function similar to horseradish peroxidase in the mediated electron transfer relays, which constitute a basis for amperometric biosensors. The mediators are mono- and bis-cyclometalated Ru and Os compounds of the type of [M(C∼N)(x)(N∼N)(3-x)](m+) with x = 1 and 2 (N∼N = 2,2'-bipyridine, (-)C∼N = 2-phenylpyridinato). Cyclic voltammograms of the Ru and Os compounds are not affected by 1a though cathodic currents increase drastically in the presence of hydrogen peroxide. The reduction potentials of [M(C∼N)(x)(N∼N)(3-x)](m+) complexes vary with both the nature of metal (Ru or Os) and the number of cyclometalated ligands x (1 or 2) and therefore the potential of working electrode can be set in the range of from -0.1 to +0.6 V versus the normal hydrogen electrode (NHE). A prototype of a biosensor for H(2)O(2) is described, in which the 1b catalyst and [Os(C∼N)(2)(N∼N)](+) mediator were coimmobilized on the surface of the glassy carbon electrode using a polymeric coating.

Oxidation of Ethidium Using TAML Activators: A Model for High School Research Performed in Partnership with University Scientists

Journal of Chemical Education. Mar, 2013  |  Pubmed ID: 23585695

A chemical research program at a public high school has been developed. The full-year Advanced Chemical Research class (ACR) in the high school enrolls 20 to 30 seniors each year, engaging them in long-term experimental projects. Through partnerships involving university scientists, ACR high school students have had the opportunity to explore a number of highly sophisticated original research projects. As an example of the quality of experimental work made possible through these high school-university partnerships, this article describes the development of a novel method for the oxidation of ethidium bromide, a mutagen commonly used in molecular biology. Data collected from ACR alumni show that the ACR program is instrumental in encouraging students to pursue careers in scientific fields and in creating life-long problem-solvers.

TAML Activator/peroxide-catalyzed Facile Oxidative Degradation of the Persistent Explosives Trinitrotoluene and Trinitrobenzene in Micellar Solutions

Environmental Science & Technology. May, 2013  |  Pubmed ID: 23586823

TAML activators are well-known for their ability to activate hydrogen peroxide to oxidize persistent pollutants in water. The trinitroaromatic explosives, 2,4,6-trinitrotoluene (TNT) and 1,3,5-trinitrobenzene (TNB), are often encountered together as persistent, toxic pollutants. Here we show that an aggressive TAML activator with peroxides boosts the effectiveness of the known surfactant/base promoted breakdown of TNT and transforms the surfactant induced nondestructive binding of base to TNB into an extensive multistep degradation process. Treatment of basic cationic surfactant solutions of either TNT or TNB with TAML/peroxide (hydrogen peroxide and tert-butylhydroperoxide, TBHP) gave complete pollutant removal for both in <1 h with >75% of the nitrogen and ≥20% of the carbon converted to nitrite/nitrate and formate, respectively. For TNT, the TAML advantage is to advance the process toward mineralization. Basic surfactant solutions of TNB gave the colored solutions typical of known Meisenheimer complexes which did not progress to degradation products over many hours. However with added TAML activator, the color was bleached quickly and the TNB starting compound was degraded extensively toward minerals within an hour. A slower surfactant-free TAML activator/peroxide process also degrades TNT/TNB effectively. Thus, TAML/peroxide amplification effectively advances TNT and TNB water treatment giving reason to explore the environmental applicability of the approach.

Zebrafish Assays As Developmental Toxicity Indicators in The Design of TAML Oxidation Catalysts

Green Chemistry : an International Journal and Green Chemistry Resource : GC. Sep, 2013  |  Pubmed ID: 24748850

TAML activators promise a novel water treatment approach by efficiently catalysing peroxide-based degradation of chemicals of high concern at environmental concentrations. Green design ethics demands an exploration of TAML toxicity. Exposure to high concentrations of certain activators caused adverse effects in zebrafish. At typical TAML operational concentrations, development was not perturbed.

Formation of a Room Temperature Stable Fe(V)(O) Complex: Reactivity Toward Unactivated C-H Bonds

Journal of the American Chemical Society. Jul, 2014  |  Pubmed ID: 24387595

An Fe(V)(O) complex has been synthesized from equimolar solutions of (Et4N)2[Fe(III)(Cl)(biuret-amide)] and mCPBA in CH3CN at room temperature. The Fe(V)(O) complex has been characterized by UV-vis, EPR, Mössbauer, and HRMS and shown to be capable of oxidizing a series of alkanes having C-H bond dissociation energies ranging from 99.3 kcal mol(-1) (cyclohexane) to 84.5 kcal mol(-1) (cumene). Linearity in the Bell-Evans-Polayni graph and the finding of a large kinetic isotope effect suggest that hydrogen abstraction is engaged the rate-determining step.

Electrocatalytic Oxygen Evolution with an Immobilized TAML Activator

Journal of the American Chemical Society. Apr, 2014  |  Pubmed ID: 24707993

Iron complexes of tetra-amido macrocyclic ligands are important members of the suite of oxidation catalysts known as TAML activators. TAML activators are known to be fast homogeneous water oxidation (WO) catalysts, producing oxygen in the presence of chemical oxidants, e.g., ceric ammonium nitrate. These homogeneous systems exhibited low turnover numbers (TONs). Here we demonstrate immobilization on glassy carbon and carbon paper in an ink composed of the prototype TAML activator, carbon black, and Nafion and the subsequent use of this composition in heterogeneous electrocatalytic WO. The immobilized TAML system is shown to readily produce O2 with much higher TONs than the homogeneous predecessors.

Valence-to-core-detected X-ray Absorption Spectroscopy: Targeting Ligand Selectivity

Journal of the American Chemical Society. Jul, 2014  |  Pubmed ID: 24946007

X-ray absorption spectroscopy (XAS) can provide detailed insight into the electronic and geometric structures of transition-metal active sites in metalloproteins and chemical catalysts. However, standard XAS spectra inherently represent an average contribution from the entire coordination environment with limited ligand selectivity. To address this limitation, we have investigated the enhancement of XAS features using valence-to-core (VtC)-detected XAS, whereby XAS spectra are measured by monitoring fluorescence from valence-to-core X-ray emission (VtC XES) events. VtC emission corresponds to transitions from filled ligand orbitals to the metal 1s core hole, with distinct energetic shifts for ligands of differing ionization potentials. VtC-detected XAS data were obtained from multiple valence emission features for a series of well-characterized Mn model compounds; taken together, these data correspond to a VtC resonant XES (VtC RXES) plane. For comparison, standard total fluorescence yield (TFY) XAS and nonresonant XES data were obtained. Dramatic intensity variations and the appearance of new features were observed in the pre-edge region by detecting at different VtC emission energies. The TFY XAS, nonresonant XES, and VtC RXES data were all modeled within a density functional theory approach. While the TFY XAS and nonresonant XES data are readily interpreted by theory, the VtC RXES cannot be reproduced within such a simplified model. Nonetheless, dramatic changes in the experimental spectra are observed that have the potential to further the information content and selectivity of XAS. Potential applications and required theoretical developments are discussed.

Activation Parameters As Mechanistic Probes in the TAML Iron(V)-oxo Oxidations of Hydrocarbons

Chemistry (Weinheim an Der Bergstrasse, Germany). Jan, 2015  |  Pubmed ID: 25410933

The results of low-temperature investigations of the oxidations of 9,10-dihydroanthracene, cumene, ethylbenzene, [D10]ethylbenzene, cyclooctane, and cyclohexane by an iron(V)-oxo TAML complex (2; see Figure 1) are presented, including product identification and determination of the second-order rate constants k2 in the range 233-243 K and the activation parameters (ΔH(≠) and ΔS(≠)). Statistically normalized k2 values (log k2') correlate linearly with the C-H bond dissociation energies DC-H, but ΔH(≠) does not. The point for 9,10-dihydroanthracene for the ΔH(≠) vs. DC-H correlation lies markedly off a common straight line of best fit for all other hydrocarbons, suggesting it proceeds via an alternate mechanism than the rate-limiting C-H bond homolysis promoted by 2. Contribution from an electron-transfer pathway may be substantial for 9,10-dihydroanthracene. Low-temperature kinetic measurements with ethylbenzene and [D10]ethylbenzene reveal a kinetic isotope effect of 26, indicating tunneling. The tunnel effect is drastically reduced at 0 °C and above, although it is an important feature of the reactivity of TAML activators at lower temperatures. The diiron(IV) μ-oxo dimer that is often a common component of the reaction medium involving 2 also oxidizes 9,10-dihydroanthracene, although its reactivity is three orders of magnitude lower than that of 2.

Reactivity and Operational Stability of N-tailed TAMLs Through Kinetic Studies of the Catalyzed Oxidation of Orange II by H2 O2 : Synthesis and X-ray Structure of an N-phenyl TAML

Chemistry (Weinheim an Der Bergstrasse, Germany). Apr, 2015  |  Pubmed ID: 25684430

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.

Reactivity and Operational Stability of N-Tailed TAMLs Through Kinetic Studies of the Catalyzed Oxidation of Orange II by H2 O2 : Synthesis and X-ray Structure of an N-phenyl TAML

Chemistry (Weinheim an Der Bergstrasse, Germany). Apr, 2015  |  Pubmed ID: 25740016

Invited for the cover of this issue are Terrence J. Collins and co-workers at Carnegie Mellon University (USA) and the National Chemical Laboratory (India). The image depicts five generations of tetraamido macrocyclic ligand (TAML) activators, which are small molecule, full-functional mimics of oxidizing enzymes that arguably outperform the peroxidase enzymes they mimic. Read the full text of the article at 10.1002/chem.201406061.

Removal of Ecotoxicity of 17α-ethinylestradiol Using TAML/peroxide Water Treatment

Scientific Reports. 2015  |  Pubmed ID: 26068117

17α-ethinylestradiol (EE2), a synthetic oestrogen in oral contraceptives, is one of many pharmaceuticals found in inland waterways worldwide as a result of human consumption and excretion into wastewater treatment systems. At low parts per trillion (ppt), EE2 induces feminisation of male fish, diminishing reproductive success and causing fish population collapse. Intended water quality standards for EE2 set a much needed global precedent. Ozone and activated carbon provide effective wastewater treatments, but their energy intensities and capital/operating costs are formidable barriers to adoption. Here we describe the technical and environmental performance of a fast- developing contender for mitigation of EE2 contamination of wastewater based upon small- molecule, full-functional peroxidase enzyme replicas called "TAML activators". From neutral to basic pH, TAML activators with H2O2 efficiently degrade EE2 in pure lab water, municipal effluents and EE2-spiked synthetic urine. TAML/H2O2 treatment curtails estrogenicity in vitro and substantially diminishes fish feminization in vivo. Our results provide a starting point for a future process in which tens of thousands of tonnes of wastewater could be treated per kilogram of catalyst. We suggest TAML/H2O2 is a worthy candidate for exploration as an environmentally compatible, versatile, method for removing EE2 and other pharmaceuticals from municipal wastewaters.

Activation of Dioxygen by a TAML Activator in Reverse Micelles: Characterization of an Fe(III)Fe(IV) Dimer and Associated Catalytic Chemistry

Journal of the American Chemical Society. Aug, 2015  |  Pubmed ID: 26161504

Iron TAML activators of peroxides are functional catalase-peroxidase mimics. Switching from hydrogen peroxide (H2O2) to dioxygen (O2) as the primary oxidant was achieved by using a system of reverse micelles of Aerosol OT (AOT) in n-octane. Hydrophilic TAML activators are localized in the aqueous microreactors of reverse micelles where water is present in much lower abundance than in bulk water. n-Octane serves as a proximate reservoir supplying O2 to result in partial oxidation of Fe(III) to Fe(IV)-containing species, mostly the Fe(III)Fe(IV) (major) and Fe(IV)Fe(IV) (minor) dimers which coexist with the Fe(III) TAML monomeric species. The speciation depends on the pH and the degree of hydration w0, viz., the amount of water in the reverse micelles. The previously unknown Fe(III)Fe(IV) dimer has been characterized by UV-vis, EPR, and Mössbauer spectroscopies. Reactive electron donors such as NADH, pinacyanol chloride, and hydroquinone undergo the TAML-catalyzed oxidation by O2. The oxidation of NADH, studied in most detail, is much faster at the lowest degree of hydration w0 (in "drier micelles") and is accelerated by light through NADH photochemistry. Dyes that are more resistant to oxidation than pinacyanol chloride (Orange II, Safranine O) are not oxidized in the reverse micellar media. Despite the limitation of low reactivity, the new systems highlight an encouraging step in replacing TAML peroxidase-like chemistry with more attractive dioxygen-activation chemistry.

Unifying Evaluation of the Technical Performances of Iron-Tetra-amido Macrocyclic Ligand Oxidation Catalysts

Journal of the American Chemical Society. Mar, 2016  |  Pubmed ID: 26886296

The main features of iron-tetra-amido macrocyclic ligand complex (a sub-branch of TAML) catalysis of peroxide oxidations are rationalized by a two-step mechanism: Fe(III) + H2O2 → Active catalyst (Ac) (kI), and Ac + Substrate (S) → Fe(III) + Product (kII). TAML activators also undergo inactivation under catalytic conditions: Ac → Inactive catalyst (ki). The recently developed relationship, ln(S0/S∞) = (kII/ki)[Fe(III)]tot, where S0 and S∞ are [S] at time t = 0 and ∞, respectively, gives access to ki under any conditions. Analysis of the rate constants kI, kII, and ki at the environmentally significant pH of 7 for a broad series of TAML activators has revealed a 6 orders of magnitude reactivity differential in both kII and ki and 3 orders differential in kI. Linear free energy relationships linking kII with ki and kI reveal that the reactivity toward substrates is related to the instability of the active TAML intermediates and suggest that the reactivity in all three processes derives from a common electronic origin. The reactivities of TAML activators and the horseradish peroxidase enzyme are critically compared.

TAML/H2O2 Oxidative Degradation of Metaldehyde: Pursuing Better Water Treatment for the Most Persistent Pollutants

Environmental Science & Technology. May, 2016  |  Pubmed ID: 27088657

The extremely persistent molluscicide, metaldehyde, widely used on farms and gardens, is often detected in drinking water sources of various countries at concentrations of regulatory concern. Metaldehyde contamination restricts treatment options. Conventional technologies for remediating dilute organics in drinking water, activated carbon, and ozone, are insufficiently effective against metaldehyde. Some treatment plants have resorted to effective, but more costly UV/H2O2. Here we have examined if TAML/H2O2 can decompose metaldehyde under laboratory conditions to guide development of a better real world option. TAML/H2O2 slowly degrades metaldehyde to acetaldehyde and acetic acid. Nuclear magnetic resonance spectroscopy ((1)H NMR) was used to monitor the degradation-the technique requires a high metaldehyde concentration (60 ppm). Within the pH range of 6.5-9, the reaction rate is greatest at pH 7. Under optimum conditions, one aliquot of TAML 1a (400 nM) catalyzed 5% degradation over 10 h with a turnover number of 40. Five sequential TAML aliquots (2 μM overall) effected a 31% removal over 60 h. TAML/H2O2 degraded metaldehyde steadily over many hours, highlighting an important long-service property. The observation of metaldehyde decomposition under mild conditions provides a further indication that TAML catalysis holds promise for advancing water treatment. These results have turned our attention to more aggressive TAML activators in development, which we expect will advance the observed technical performance.

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