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

Nitrite (NO ) 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 NO reduction chemistry was modeled on mono-metallic 3d-metals, generating metal-nitrosyls or NO with HO or HO products. Herein, to understand the relative potency of a bimetallic system, we report the acid-induced reductive conversion of η-bound NO to NO on Cu-Co centers of a hetero-bimetallic Cu-nitrito-Co complex, [(LNH)Cu-NO -Co] (Cu-NO -Co, 2) bearing an octadentate N-cryptand ligand (LNH). The Cu-NO -Co generates [Cu(LNH)Co] (1) upon reaction with one equiv. acid (HClO, H ions source) with NO via a presumed transient nitrousacid (ONOH) intermediate species. Likewise, this NO reduction was found to form HO, which is believed to be from the decomposition of HO, an intermediate species. In addition, complex 2, in the presence of more than one equiv. H ions also showed the formation of NO with HO. Mechanistic investigations, using N-labeled-NO , O-labeled-ONO and H-labeled-DClO (D source), revealed that the N-atom and O-atom in the NO and NO gases are derived from NO ligand and H-atom in HO derived from H-source, respectively.