Effect of temperature on the protonation constants of 2,2′:6′,2″-terpyridyl and the stability constant of the bis(2,2′:6′,2″-terpyridyl)iron(II) cation

Abstract

The concentration-based association constants (pK_a1 and pK_a2) on the molar scale between 2,2′:6′,2-terpyridyl (terpy) and hydrogen ion, giving the cations [terpy H]⁺ and [terpy H₂]²⁺, have been determined in aqueous solution at various temperatures between 298 and 472 K and at an ionic strength 0.05 mol dm^–3 from spectrophotometric data. The computer program SQUAD was used to give overall equilibrium constants for the association reactions between protons and free base yielding the base cations. The constants for these reactions (log₁₀K^H₁ and log₁₀β^H₂) are related to pK_a values through pK_a1= log₁₀K^H₁ and pK_a2= log₁₀β^H₂–pK_a1. Formation of the triprotonated species [terpy H₃]³⁺ was negligible under the prevailing experimental conditions. The overall concentration stability constant (log₁₀β₂) of the bis(2,2′:6′,2″-terpyridyl)iron(II) cation has been determined over the same temperature range with I= 0.1 mol dm^–3. The constants decrease regularly with temperature and plots of pK_a1, pK_a2 and log₁₀β₂ against 1/T were linear within experimental error. The derived thermodynamic quantities for the association reactions, ΔH and ΔS, are quoted over the experimental temperature ranges.