Nitrosyl-Centered Redox and Acid–Base Interconversions in [Ru(Me₃[9]aneN₃)(bpy)(NO)]^3,2,1+. The p<i>K</i>_a of HNO for its Nitroxyl Derivative in Aqueous Solution

This work reports the preparation of a new 6-coordinated nitrosyl compound and its use as a model to explore the redox and acid–base properties of the three redox states of bound nitrosyl (formally NO⁺, NO^•, NO^–/HNO) in {RuNO}^6,7,8 species. We prepared the octahedral {RuNO}⁶ complex [Ru­(Me₃[9]­aneN₃)­(bpy)­(NO)]³⁺ (Me₃[9]­aneN₃: 1,4,7-trimethyl-1,4,7-triazacyclononane; bpy = 2,2′-bipyridine), and the related [Ru­(Me₃[9]­aneN₃)­(bpy)­(NO₂)]⁺ nitro derivative. The compounds were characterized by chemical analysis, X-ray diffraction, NMR, IR, and UV–vis spectroscopies, cyclic voltammetry (CV), UV–vis/IR spectroelectrochemistry, and theoretical calculations (DFT, (TD)­DFT). The reaction kinetics between the {RuNO}⁶ complex and the nucleophile OH^– is also presented. The incorporation of tridentate and bidentate ligands in the coordination sphere prevents labilization issues associated with the trans effect when attaining the reduced states of the nitrosyl group. This allows for a consistent interpretation of the changes in the main geometrical parameters: Ru–N and N–O distances, Ru–N–O angle, and the ν_NO frequency and electronic transitions. We explore the redox properties in acetonitrile and aqueous solutions, and provide a potential (<i>E</i>_1/2) − pH (Pourbaix) diagram for the three diatomic nitrosyl-bound species, as well as for HNO and NO₂^–, including the report of the p<i>K</i>_a of the [Ru­(Me₃[9]­aneN₃)­(bpy)­(HNO)]²⁺ ion, 9.78 ± 0.15 at 25.0 °C