Enzyme Mimetic Active Intermediates for Nitrate Reduction in Neutral Aqueous Media

Abstract

Nitrate is a pervasive aquatic contaminant of global environmental concern. In nature, the most effective nitrate reduction reaction (NRR) is catalyzed by nitrate reductase enzymes at neutral pH, using a highly-conserved Mo center ligated mainly by oxo and thiolate groups. Mo-based NRR catalysts mostly function in organic solvents with a low water stability. Recently, an oxo-containing molybdenum sulfide nanoparticle that serves as an NRR catalyst at neutral pH was first reported. Herein, in a nanoparticle-catalyzed NRR system a pentavalent Mo^V (=O)S₄ species, an enzyme mimetic, served as an active intermediate for the NRR. Potentiometric titration analysis revealed that a redox synergy among Mo^V -S, S radicals, and Mo^V (=O)S₄ likely play a key role in stabilizing Mo^V (=O)S₄ , showing the importance of secondary interactions in facilitating NRR. The first identification and characterization of an oxo- and thiolate-ligated Mo intermediates pave the way to the molecular design of efficient enzyme mimetic NRR catalysts in aqueous solution.

Keywords: EPR spectroscopy; enzymes; molybdenum; reaction mechanisms; reduction.