(509ap) Electrocatalytic Reduction of Nitrobenzene By Iron-Salen Ligand Complexes

Nitrobenzene is a valuable precursor to rubber, pharmaceuticals, and agriculture products, though nitroaromatics also contribute to contamination of groundwater sources. A primary route of detoxifying nitrobenzene entails reducing its nitro group to an amine to produce aniline, which is less toxic and more biodegradable. In the reduction mechanism, there are two stable intermediates: nitrosobenzene (R-NO) and phenylhydroxylamine (R-NHOH). In the solution-phase electroreduction, the rate-limiting step is the reduction of phenylhydroxylamine (R-NHOH) to the R-NH intermediate. To facilitate this step and other steps in the reduction mechanism, iron-salen ligand complexes were studied as catalysts for nitrobenzene reduction.

Iron-salen complexes feature a central iron atom coordinated to four nitrogen atoms and surrounded by ligands. They can serve as catalysts for various reactions and can undergo oxidation and reduction to modify their catalytic behavior. Density Functional Theory (DFT), an electronic structure method, was applied to analyze the catalytic activity of iron-salen complexes in the electrocatalytic reduction of nitrobenzene.

DFT results demonstrated that the fully oxidized iron-salen complex is stable at electrochemical potentials of interest for nitroaromatic reduction. However, this oxidized state has six atoms coordinated to iron, preventing nitrobenzene from interacting with the iron atom. Reducing the iron-salen complex, though unfavorable thermodynamically, frees a coordination site for nitrobenzene binding by causing reorientation of the ligand and dissociation of the iron-ligand bond. The resulting partially reduced iron-salen complex is a viable catalyst for nitrobenzene reduction. Nitrobenzene binding to this complex results in an adsorption energy of -0.73 eV. The subsequent nitrobenzene reduction reaction path, at relatively small overpotentials, shows the relative energy of the system steadily decreasing with each subsequent reduction. A microkinetic model of cyclic voltammetry is used to connect DFT results to predicted electrokinetic observations. Comparisons are made with variations in the ligands on the iron-salen complex, and among different nitroaromatic substrates.