Light-activated generation of nitric oxide (NO) and sulfite anion radicals (SO3˙−) from a ruthenium(ii) nitrosylsulphito complex

Abstract

This manuscript describes the preparation of a new Ru(II) nitrosylsulphito complex, trans-[Ru(NH₃)₄(isn)(N(O)SO₃)]⁺ (complex 1), its spectroscopic and structural characterization, photochemistry, and thermal reactivity. Complex 1 was obtained by the reaction of sulfite ions (SO₃²⁻) with the nitrosyl complex trans-[Ru(NH₃)₄(isn)(NO)]³⁺ (complex 2) in aqueous solution resulting in the formation of the N-bonded nitrosylsulphito (N(O)SO₃) 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, ¹H- and ¹⁵N-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(NH₃)₄(isn)(NO₂)]⁺ 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)SO₃). The pK_a value of the –SO₃ moiety of the N(O)SO₃ 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)SO₃⁻ ligand followed by formation of nitric oxide (NO˙) and sulfite radicals (SO₃˙⁻, sulfur trioxide anion radical).