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

Nitrogen oxides (NO x ) 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-FeIII-O-MII (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 x transformations, as well as shed light on the reactivity landscapes of other NO x converting enzymes in biology.