Light-activated generation of nitric oxide (NO) and sulfite anion radicals (SO 3 ˙ - ) from a ruthenium(ii) nitrosylsulphito complex.

This manuscript describes the preparation of a new Ru(ii) nitrosylsulphito complex, trans-[Ru(NH ₃ ) ₄ (isn)(N(O)SO ₃ )] ⁺ (complex 1), its spectroscopic and structural characterization, photochemistry, and thermal reactivity. Complex 1 was obtained by the reaction of sulfite ions (SO ₃ ^2- ) with the nitrosyl complex trans-[Ru(NH ₃ ) ₄ (isn)(NO)] ³⁺ (complex 2) in aqueous solution resulting in the formation of the N-bonded nitrosylsulphito (N(O)SO ₃ ) ligand. To the best of our knowledge, only four nitrosylsulphito metal complexes have been described so far (J. Chem. Soc., Dalton Trans., 1983, 2465-2472), and there is no information about the photochemistry of such complexes. Complex 1 was characterized by spectroscopic means (UV-Vis, EPR, FT-IR, ¹ H- and ¹⁵ N-NMR), elemental analysis and single-crystal X-ray diffraction. The X-ray structure of the precursor complex 2 is also discussed in the manuscript and is used as a reference for comparisons with the structure of 1. Complex 1 is water-soluble and kinetically stable at pH 7.4, with a first-order rate constant of 3.1 × 10 ^-5 s ^-1 for isn labilization at 298 K (t _1/2 ∼ 373 min). Under acidic conditions (1.0 M trifluoroacetic acid), 1 is stoichiometrically converted into the precursor complex 2. The reaction of hydroxide ions (OH ^- ) with 1 and with 2 yields the Ru(ii) nitro complex trans-[Ru(NH ₃ ) ₄ (isn)(NO ₂ )] ⁺ with second-order rate constants of 2.1 and 10.5 M ^-1 s ^-1 (at 288 K), respectively, showing the nucleophilic attack of OH ^- at the nitrosyl in 2 (Ru-NO) and at the nitrosylsulphito in 1 (Ru-N(O)SO ₃ ). The pK _a value of the -SO ₃ moiety of the N(O)SO ₃ ligand in 1 was determined to be 5.08 ± 0.06 (at 298 K). The unprecedented photochemistry of a nitrosylsulphito complex is investigated in detail with 1. The proposed mechanism is based on experimental (UV-Vis, EPR, NMR and Transient Absorption Laser Flash Photolysis) and theoretical data (DFT) and involves photorelease of the N(O)SO ₃ ^- ligand followed by formation of nitric oxide (NO˙) and sulfite radicals (SO ₃ ˙ ^- , sulfur trioxide anion radical).