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 (pKa1 and pKa2) 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 (log10KH1 and log10βH2) are related to pKa values through pKa1= log10KH1 and pKa2= log10βH2–pKa1. 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 pKa1, pKa2 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.