Stepwise Nitrate → Dinitrogen Transformation Mediated by a Series of Synthetic Heme/Cobalt and Heme/Copper Assemblies: Modeling Denitrification with Cytochrome <i>c</i> Oxidase Mimics

Nitrogen oxides (NO _<i>x</i> ) and their reductive transformation pathways (NO₃ ^-→NO₂ ^-→NO→N₂O→N₂) have been implicated in a medley of important physiological, pathological, ecological, and environmental processes. Nature involves disparate enzymatic platforms to perform such NO₃ ^-→N₂ transformations; however, their mechanistic details remain enigmatic to date. Moreover, synthetic models to mimic such stepwise NO₃ ^-→N₂ processes are extremely scarce. Herein, we have employed a series of heme/nonheme (Co/Cu/Zn) assemblies that can efficiently and stoichiometrically reduce NO₃ ^-→NO₂ ^- and eventually produce NO in a stepwise fashion. Reductive coupling of NO→N₂O is further demonstrated using Lewis acid or heme/copper assemblies, while N₂O→N₂ transformation, for the first time, has been carried out successfully using reduced heme and heme/copper assemblies. In addition, the oxidative transformations of NO→NO₂ ^- and NO₂→NO₃ ^- have been manifested using μ-oxo heme-Fe^III-O-M^II (M: Co/Cu) complexes. All of the reactivities involved in this study have been scrupulously followed by a combination of UV-visible, ²H NMR, EPR, and FT-IR spectroscopies, mass spectrometry, along with ¹⁵N-labeling experiments. Such a comprehensive understanding of stepwise NO₃ ^-→N₂ transformation may provide important information about the role of heterobinuclear heme/nonheme centers in NO _<i>x</i> transformations, as well as shed light on the reactivity landscapes of other NO _<i>x</i> converting enzymes in biology.