Acid-catalyzed Transformation of Nitrite to Nitric Oxide on Copper(II)-Cobalt(II) Centers in a Bimetallic Complex

Nitrite (NO₂ ^-) serves as a pool of nitric oxide (NO) in biological systems under hypoxic conditions, and it is transformed to NO by nitrite reductase (NiR) enzyme in the presence of acid (H⁺ ions). However, NO synthases (NOSs) generate NO via L-arginine oxidation in normoxic conditions. Previously, acid-induced NO₂ ^- reduction chemistry was modeled on mono-metallic 3d-metals, generating metal-nitrosyls or NO_(g) with H₂O or H₂O₂ products. Herein, to understand the relative potency of a bimetallic system, we report the acid-induced reductive conversion of η²-bound NO₂ ^- to NO on Cu^II-Co^II centers of a hetero-bimetallic Cu^II-nitrito-Co^II complex, [(LN₈H)Cu^II-NO₂ ^--Co^II]³⁺ (Cu^II-NO₂ ^--Co^II, 2) bearing an octadentate N₈-cryptand ligand (LN₈H). The Cu^II-NO₂ ^--Co^II generates [Cu^II(LN₈H)Co^II]⁴⁺ (1) upon reaction with one equiv. acid (HClO₄, H⁺ ions source) with NO_(g) via a presumed transient nitrousacid (ONOH) intermediate species. Likewise, this NO₂ ^- reduction was found to form H₂O, which is believed to be from the decomposition of H₂O₂, an intermediate species. In addition, complex 2, in the presence of more than one equiv. H⁺ ions also showed the formation of NO_(g) with H₂O. Mechanistic investigations, using ¹⁵N-labeled-¹⁵NO₂ ^-, ¹⁸O-labeled-¹⁸O¹⁴N¹⁶O^- and ²H-labeled-DClO₄ (D⁺ source), revealed that the N-atom and O-atom in the ^14/15NO and ¹⁴N¹⁸O gases are derived from NO₂ ^- ligand and H-atom in H₂O derived from H⁺-source, respectively.