Reactivity of trinuclear ruthenium acetates with nitrite and nitric oxide ligands in aqueous media

The chemical reactivity of nitrosyl- and nitrite-coordinated compounds in an aqueous environment is a vital part of understanding the action of these compounds as potential nitric oxide-releasing molecules (NORMs). This work reports the behaviour of the [Ru₃O(CH₃COO)₆(py)₂NO₂] (1) complex, which is an isomeric mixture of nitrite-N and nitrite-O, and the nitrosyl complex [Ru₃O(CH₃COO)₆(py)₂NO]PF₆ (2) in aqueous medium with and without light irradiation. NO release under light irradiation was detected through chronoamperometry, which showed that nitrite complex 1 produces NO but is less effective than nitrosyl complex 2. This difference is due to the mechanism of NO production by complex 1, which depends on the nitrite-O isomer, present in minor proportion in the synthetic sample, as shown by computational and NMR data. The reactivity of these compounds in the dark was investigated under various pH values. The nitrite complex 1 had the coordinated nitrite converted to NO⁺, with a pK = 4.2. NO⁺ was readily released, yielding the solvate species [Ru₃O(CH₃COO)₆(py)₂S]⁺. For the nitrosyl complex 2, two successive nucleophilic attacks by hydroxide ions were observed producing the [Ru₃O(CH₃COO)₆(py)₂HNO₂] (3) and [Ru₃O(CH₃COO)₆(py)₂NO₂]^- (4) compounds, with pK values of 9.8 and 12.3, respectively. In buffered solutions (TRIS.HCl and PBS), the kinetic trace for the conversion of 2 to 3 suggested an induction period followed by the complete conversion to [Ru₃O(CH₃COO)₆(py)₂HNO₂] at pH values where the nitrosyl [Ru₃O(CH₃COO)₆(py)₂NO]⁺ should be the major species. Based on these observations, our data suggest a sequence of steps in which compound 3 accumulates and then, with the aid of the buffer components, increases the rate of its own formation.