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Terrence J Collins

Publications and source records attributed to Terrence J Collins.

11 recordsLinked to original sources
PubMed2006

Chemical and spectroscopic evidence for an FeV-oxo complex.

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.

Catalysis↗
PubMed2006

Activity-stability parameterization of homogeneous green oxidation catalysts.

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.

Journal Article↗
PubMed2006

Total degradation of fenitrothion and other organophosphorus pesticides by catalytic oxidation employing Fe-TAML peroxide activators.

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.

Journal Article↗
PubMed2006

High-valent iron complexes with tetraamido macrocyclic ligands: structures, Mössbauer spectroscopy, and DFT calculations.

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.

Iron↗
PubMed2005

Catalytically active mu-Oxodiiron(IV) oxidants from Iron(III) and dioxygen.

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.

Journal Article↗
PubMed2004

XANES evidence against a manganyl species in the S3 state of the oxygen-evolving complex.

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.

Manganese↗
PubMed2003

Understanding the mechanism of H(+)-induced demetalation as a design strategy for robust iron(III) peroxide-activating catalysts.

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.

Biomimetic Materials↗
PubMed2002

Rapid total destruction of chlorophenols by activated hydrogen peroxide.

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.

Catalysis↗
PubMed2002

TAML oxidant activators: a new approach to the activation of hydrogen peroxide for environmentally significant problems.

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.

Catalysis↗