Thermodynamics and kinetics of RuIII(edta) as an efficient scavenger for nitric oxide in aqueous solution

Abstract

The edta complex of Ru^III reacts very rapidly with NO in aqueous solution at pH = 5 to form a stable nitrosyl complex. The results from FT-IR (ATR) and ¹⁵N-NMR studies clearly support the NO⁺ character of coordinated NO, such that the nitrosyl product can be formulated as [Ru^II(edta)NO]⁺. A combination of UV-Vis spectroscopy and electrochemical detection of NO was used to determine the overall equilibrium constant K_NO as (9.1 ± 1.2) × 10⁷ M⁻¹ at 25 °C and pH = 5.0. Stopped-flow kinetic studies on the reaction of acetate-buffered solutions of [Ru^III(edta)H₂O]⁻ with NO gave k_on values two orders of magnitude lower than that reported in the literature as a result of buffer effects. The values of k_on determined at low and high pH, viz. 3.8 × 10⁴ and 1.2 × 10⁵ M⁻¹ s⁻¹, respectively, are significantly smaller than that found at pH = 5.0, and in agreement with that observed for the substitution reactions of Ru^III(edta) with other entering nucleophiles. Attempts to determine k_on for the binding of NO to [Ru^III(edta)H₂O]⁻ using laser flash photolysis failed due to the occurrence of side reactions. Under specific conditions (NO in excess and NO₂⁻ as impurity), the formation of the disubstituted [Ru^II(edta)(NO⁺)(NO₂⁻)]²⁻ species was detected using ¹⁵N-NMR spectroscopy. Laser flash photolysis of this complex leads to multiple chemical reaction steps as a result of the formation of two primary photoproducts, which decay with different rate constants to the starting complex. Possible mechanisms for these photoinduced reactions are proposed and compared to related systems reported in the literature.