Reduction of nitrite to nitric oxide at mononuclear iron centers: biomimetic studies and mechanistic insights

The reduction of nitrite (NO 2 − ) to nitric oxide (NO • ) at iron centers constitutes a pivotal transformation in the global nitrogen cycle and in mammalian nitric oxide homeostasis. In biological systems, iron-containing enzymes such as cytochrome cd 1 nitrite reductase ( cd 1 NiR) and certain hemoproteins catalyze the selective conversion of nitrite to nitric oxide through tightly regulated proton- and electron-transfer processes. Inspired by natural systems, extensive efforts in synthetic bioinorganic chemistry have established well-defined nonheme and heme model complexes that enable detailed mechanistic investigation of nitrite reduction reactivity of iron complexes under controlled conditions. This review summarizes recent advances in understanding nitrite reduction at mononuclear iron centers with particular emphasis on mechanistic pathways, electronic structure effects, and the role of the secondary coordination sphere. Distinct proton-coupled electron transfer (PCET) mechanisms, including 2H + /1e − heterolytic N–O bond cleavage and 1H + /1e − homolytic pathways are compared across representative systems. The influences of nitrite binding modes (nitro vs nitrito), redox-active ligands, appended Lewis acids, and pendant proton relays are highlighted as key factors governing substrate activation, intermediate stabilization, and NO • release. Heme-inspired platforms, including N-confused porphyrin frameworks and dioxoisobacteriochlorin analogues, further demonstrate how macrocyclic electronics and internal proton inventories modulate reactivity and catalytic efficiency. By integrating insights from enzymatic studies and synthetic models, this review provides a unified framework for understanding how coordination chemistry, proton management, and metal-ligand cooperativity dictate nitrite activation and NO • formation.