Kinetics and mechanism of the interaction of azide with [(H₂O)(tap)₂RuORu(tap)₂-(H₂O)]²⁺ ion at physiological pH

Abstract

The title reaction has been studied spectrophotometrically in aqueous medium as a function of [substrate complex], [ligand], pH and temperature at constant ionic strength. At the physiological pH (7.4) the interaction with azide shows two distinct consecutive steps, i.e., it shows a non-linear dependence on the concentration of N₃ ⁻; both processes are [ligand]-dependent. The rate constant for the processes are: k ₁∼10⁻³ s⁻¹ and k ₂∼10⁻⁵ s⁻¹. The activation parameters calculated from Eyring plots are: ΔH ₁ ^‡ = 14.8 ± 1 kJ mol⁻¹, ΔS ₁ ^‡ = −240 ± 3 J K⁻¹ mol⁻¹, ΔH ₂ ^‡ = 44.0 ± 1.5 kJ mol⁻¹ and ΔS ₂ ^‡ = −190 ± 4 J K⁻¹ mol⁻¹. Based on the kinetic and activation parameters an associative interchange mechanism is proposed for the interaction process. From the temperature dependence of the outersphere association equilibrium constant, the thermodynamic parameters calculated are: ΔH ₁ ⁰ = 4.4 ± 0.9 kJ mol⁻¹, ΔS ₁ ⁰ = 64 ± 3 J K⁻¹ mol⁻¹ and ΔH ₂ ⁰ = 14.2 ± 2.9 kJ mol⁻¹, ΔS ₂ ⁰ = 90 ± 9 J K⁻¹ mol⁻¹, which gives a negative ΔG ⁰ value at all temperatures studied, supporting the spontaneous formation of an outersphere association complex.