Ruthenium(III) Cyclometalates Obtained by Site-Specific Orthometallation and Their Reactivity with Nitric Oxide: Photoinduced Release and Estimation of NO Liberated from the Ruthenium Nitrosyl Complexes

Cyclometalated RuIII complexes [Ru(LSB1)(PPh3)2Cl] (1), [LSB1H2 = 2-(3-nitrobenzylideneamino)phenol], and [Ru(LSB2)(PPh3)2Cl] (2), [LSB2H2 = 4-methyl-2-(3-nitrobenzylideneamino)phenol and H = dissociable protons], were synthesized by site-specific orthometallation and characterized by spectroscopic and electrochemical studies. Complexes 1 and 2 were treated with in situ generated nitric oxide (NO), derived from an acidified nitrite solution, which afforded the ruthenium nitrosyl complexes [Ru(LSB3H)(PPh3)2(NO)Cl](ClO4) (1a) and [Ru(LBOX)(PPh3)2(NO)Cl](ClO4) (2a), respectively, [LSB3H2 = 4-nitro-2-(3-nitrobenzylideneamino)phenol, LBOXH = 5-methyl-7-nitro-2-(3-nitrophenyl)benzoxazole and H = dissociable protons]. Complexes 1a and 2a were found to be diamagnetic and were characterized by 1H NMR and 31P NMR spectral studies. Both 1a and 2a exhibited νNO in the range 1800–1835 cm–1 in the IR spectra. Molecular structures of σ-aryl ruthenium nitrosyl complexes 1a·CH3OH·2CH2Cl2·H2O and 2a·2CH2Cl2 were determined by X-ray crystallography. Nitrosylation at the metal centre, oxidative cyclization and ligand nitration were authenticated from the crystal structures. The redox properties of the metal centre were investigated. In both the nitrosyl complexes 1a and 2a, coordinated NO was found to be photolabile. The amount of photoreleased NO was estimated by using the Griess reagent and the data was compared with that obtained from sodium nitroprusside (SNP). The role of the nitro group in the ligand frame was discussed with regard to orthometallation, NO reactivity and photolability.