Reactivity of Radicals Generated on Irradiation of <i>trans</i>-[Ru(NH₃)₄(NO₂)P(OEt)₃](PF₆)

The electronic absorption spectrum of trans-[Ru(NH3)4(NO2)(P(OEt)3]+ in aqueous solution is characterized by a strong absorption band at 334 nm (λmax = 1800 mol-1 L cm-1). On the basis of quantum mechanics calculations, this band has been assigned to a MLCT transition from the metal to the nitro ligand. Molecular orbital calculations also predict an LF transition at 406 nm, which is obscured by the intense MLCT transition. When trans-[Ru(NH3)4(NO2)(P(OEt)3]+ in acetonitrile is irradiated with a 355 nm pulsed laser light, the absorption features are gradually shifted to represent those of the solventocomplex trans-[Ru(NH3)4(solv)(P(OEt)3]2+ (λmax = 316 nm, ε = 650 mol-1 L cm-1), which was also detected by 31P NMR spectroscopy. The net photoreaction under these conditions is a photoaquation of trans-[Ru(NH3)4(NO2)(P(OEt)3]+, although, after photolysis, the presence of the nitric oxide was detected by differential pulse polarography. In phosphate buffer pH 9.0, after 15 min of photolysis, a thermal reaction resulted in the formation of a hydroxyl radical and a small amount of a paramagnetic species as detected by EPR spectroscopy. In the presence of trans-[Ru(NH3)4(solv)P(OEt)3]2+, the hydroxyl radical initiated a chain reaction. On the basis of spectroscopic and electrochemical data, the role of the radicals produced is analyzed and a reaction sequence consistent with the experimental results is proposed. The 355 nm laser photolysis of trans-[Ru(NH3)4(NO2)(P(OEt)3]+ in phosphate buffer pH 7.4 also gives nitric oxide, which is readily trapped by ferrihemeproteins (myoglobin, hemoglobin, and cytochrome C), giving rise to the formation of their nitrosylhemeproteins(II), (NO)Fe(II)hem.