Degradation of Toxic Organic Pollutants by Non-Heme Iron-Oxo Complexes

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Water is a source of life and regarded as the most essential of natural resources. Increasing contamination of freshwater with thousands of industrial and natural chemicals is one of the key environmental problems in today’s world. Among other pollutants, the Persistent Organic Pollutants (POPs) and herbicides contribute quite a lot to water pollutions (Chart 1). In recent years; advanced oxidation processes (AOPs) represent a powerful mean for abatement of refractory and toxic pollutants in wastewater. Recently, catalytic degradations of organic pollutants in water at neutral pH by transition metal complexes have attracted considerable attention, which essentially originated from the Fenton reaction under acidic conditions. The PCPs and other toxic pollutants are being degraded using heterogeneous catalysis based Fenton-type chemistry.

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However, the main drawbacks in these processes are the sluggishness of the reaction, uncontrolled oxidation of pollutants resulting more side products. Again, these catalysts work at low pH, and the use of expensive UV light source and the difficulty of recycling restrict their practical applications. 5 Under these circumstances, there is a need for the development of some robust catalytic systems capable of performing the degradation of pollutants using dioxygen or easy-to-use oxidants. Chart 1. Various toxic organic pollutants in water. Over the last few years, a number of water-soluble iron complexes have been reported that can generate high valent iron-oxygen intermediates.

These iron complexes have potential to oxidize organic substrates using water, photocatalyst or some peracids. 6 Moreover, the iron(IV)-oxo complexes exhibit versatile reactivity such as C-H bond activation and oxo atom transfer reactions. Nam et al. reported the generation of mononuclear non-heme iron(IV) oxo complexes and the catalytic oxygenation of organic substrates using water as an oxygen source and cerium(IV) (CAN) as a one-electron oxidant. 8 Costas et al. reported some iron(IV)-oxo species supported by tetradentate nitrogen-based ligands generated from the precursor iron(II) complexes using CAN in aqueous solution. 9 Almost a decade has been passed after the discovery of the water soluble iron(IV)-oxo complexes, but there is no report of degradation of toxic organic pollutants by these metal-based oxidants. The highly reactive iron(IV)-oxo species have been used only for the oxidation of refractory C-H bonds such as cyclohexane and/or butane or for the hydroxylation of aromatic hydrocarbon.

In this work, we report a series of iron(II) complexes supported by nitrogen rich tetradentate and pentadentate ligands which are capable of generating the corresponding iron(IV)-oxo oxidant in water. Various toxic organic pollutants like PCPs and other phenolated compounds can be oxidized as well as dehalogenated by theses iron(IV)-oxo species. This study will provide a new concept of using metal-based oxidation in the degradation of toxic pollutants in wastewater. Results and discussionSynthesis and CharacterizationThe nitrogen donor tetradentate and two pentadentate ligands shown in Chart 2 were synthesized using literature procedure. The precursor iron(II) complexes of the ligands were isolated as triflate salts from the reaction of equimolar mixtures of the respective ligands, iron(II) triflate in dichloromethane. Elemental analysis and ESI-MS data suggest the formation of the pure complexes. 8 All the complexes are water soluble and stable under ambient condition.

The U. S. Environmental Protection Agency lists the chlorinated phenols such as pentachlorophenol (PCP) as a priority pollutant. Serious attention has been focused on its removal from the environment because of the intensity of this toxicity. The Fe(IV)-oxo complexes not only oxidize the phenol but also can carry out the dechlorination reactions of the PCPs . When the reactions have been carried out using 60 equivalent of CAN, the 2,4,6-trichlorophenol has been degraded catalytically. These experiments suggest that the non-heme iron(IV)-oxo complexes reported here can be used as potential catalysts in the degradation of toxic PCPs in aqueous medium.

It is an important monomer in the polycarbonate plastics, food cans, and other daily used chemicals. Daily and worldwide usage of BPA and BPA-contained products led to its ubiquitous distribution in water, soil and atmosphere. The Fe(IV)-oxo oxidants are capable of degrading BPA to its constituent phenols in a good yield. The yield is around 40-70% depending upon the oxidizing capacity of the complexes. Nonylphenol (NP) is one of the major contaminants in the water and it is considered an endocrine disruptor due to its ability to mimic estrogen and in turn disrupt the natural balance of hormones in affected organisms. Nonylphenol has been shown to affect cytokine signaling molecule secretions in the human placenta and its exposure has also been associated with breast cancer. Nonylphenol has been degraded by these high valent iron-oxo intermediate with 30-40% yield.

The Fe(IV)-oxo complexes are also capable of degrading of tetrachloroethylene, which has been classified as Group 2A carcinogen by the International Agency for Research on Cancer. It can epoxydize alkenes and the oxidized product can be further degraded. ExperimentalMaterials and MethodsAll reagents and solvents used were of commercially available reagent quality. Solvents were purified and dried prior to use. Preparation and handling of air-sensitive materials were carried out under an inert atmosphere by in a glove box.

Fourier transform infrared spectroscopy on KBr pellets was performed on a Shimadzu FT-IR 8400S instrument. Elemental analyses were performed on a Perkin Elmer 2400 series II CHN analyzer. Electro-spray ionization (ESI) mass spectra were recorded with a Waters QTOF Micro YA263 instrument. Solution electronic spectra (single and time-dependent) were measured on an Agilent 8453 diode array spectrophotometer. All room temperature NMR spectra were collected on a Bruker DPX-500 MHz spectrometer. X-band EPR measurements were performed on a JEOL JES-FA 200 instrument. GC-MS measurements were carried out with a Perkin Elmer Clarus 600 using Elite 5 MS (30m x 0. 25mm x 0. 25μm) column with a maximum temperature of 300ºC. The ligands were synthesized according to the procedure reported in the literature. The iron(II)-triflate complexes were isolated as triflate salts from equimolar mixtures of the respective ligands, iron(II) triflate in dichloromethane similar to the reported procedures.

Generation of iron(IV)-oxo complexes

To an aqueous solution (0. 5 mM) of Fe(II) complexes, CAN (6 equiv) was added at room temperature. Immediately pale green species showing absorption maxima around680-765 nm (ε ~380 M-1cm-1) were observed.

Reactions of iron(IV)-oxo complexes with substrates

The iron(II) complex (0. 02 mmol) was dissolved in 2 mL of water. To the resulting solution, substrate (100 equiv dissolved in minimum amount of acetonitrile) was added followed by the addition of 0. 1 mL aqueous solution of CAN (6 equiv). The reaction solution was allowed to stir at room temperature. After the reaction, the organic products were extracted with diethyl ether, dried over anhydrous sodium sulfate, filtered and evaporated to dryness. The products were analyzed by 1H NMR using 1,3,5-trimethoxybenzene as an internal standard and/or GC-MS using naphthalene as an internal standard.

Control Experiments

Control experiments were performed with iron(II)-triflate and CAN in the presence of substrates following the procedure described above. Small amount of oxidized products derived from substrates were formed due to the oxidizing nature of CAN. This yield has been subtracted when calculating the independent yield for the reactivity study of complexes.

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