Superfund Research Program
Biomimetic Remediation of Hazardous Substances
Project Leader: Pericles Stavropoulos (Missouri University of Science and Technology)
Grant Number: P42ES007381
Funding Period: 2000-2012
Final Progress Reports
A series of iron-containing reagents has been synthesized, characterized, and applied to the oxidative dechlorination of chlorinated organics, a family that contains many environmental chemicals of concern, particularly the chloroethylenes. The reagents have been developed with the intention to mimic a type of bioremediation, the operation of biological oxygenation/dechlorination agents. These agents are known to generate high-valent, iron oxo species suitable for the selective oxygenation of chlorinated and non-chlorinated hydrocarbons.
Over the past year, Dr. Stavropoulos and his research team concentrated on the development of second-generation catalytic reagents that provide an electron-rich and oxidative robust environment to support the catalytically active iron center. Numerous new ligands and iron(II) complexes were synthesized and fully characterized by spectroscopic means, including X-ray diffraction analysis. These iron(II) pre-catalysts are necessary for the activation of air-derived dioxygen, leading to the generation of intermediate oxidant species that are capable of attacking chloroorganics. The oxidizing ability of these reagents is largely metal centered, but is also stored onto the ligand, generating radical sites. The partitioning of oxidizing power to both metal and ligand is a known feature of biological oxygenases. The second-generation catalysts avoid over-oxidation of the ligand and direct most of the oxidizing chemistry toward the chlorinated substrate.
The synthesized compounds have the ability to effect reductive dechlorination of chloroorganics in the absence of oxygen. Indeed, at the iron(II) level, several of the reagents at hand reacted with saturated chloroorganics to achieve abstraction of chlorine atoms and generate substrate-centered radical species. The latter collapsed via coupling and hydrogen-transfer reactions to afford products that are less chlorinated than the original substrates. Similar reductive dechlorination processes have been established for biological oxygenations and underscore the ability of the synthesized reagents to mimic natural processes. In the presence of dioxygen, the mode of substrate transformation switches from reductive to oxidative, as dioxygen is activated more rapidly than chloroorganics by iron(II) reagents. A full mechanistic analysis of the oxidative dechlorination of chloroethylenes, with emphasis on trichloroethylene (TCE), is currently in progress. TCE is oxidized to chloral and minor amounts of TCE epoxide, in the presence of the iron reagents, dioxygen and a reducing agent. Hydrogen peroxide can replace the combination of dioxygen and a reducing agent. These results are analogous to those obtained in parallel experiments with the model biological oxygenase P-450 and suggest mechanistic analogies between the biological and artificial reagents.
The project investigators are currently conducting mechanistic studies directed at identifying those metal- and ligand-centered events that are pertinent to catalysis. Low-temperature reactions between iron(II) sites and dioxygen are underway to isolate and identify intermediates (iron-peroxo and/or iron-oxo species) that may be directly responsible for the oxygenation of chloroorganics. Moreover, kinetic studies are in progress to elucidate the kinetic parameters of the reaction and associate specific intermediates with critical steps in the reaction pathway. The role of the metal environment, in terms of its ability to store oxidizing power, is also under investigation through spectroscopic and electrochemical experiments that purport to provide an understanding of the redox processes that are ligand- rather than metal-centered.