Thermodynamic study of complexation of alkali and alkaline-earth metal ions with benzene-containing cryptands

Abstract

Log K, Δ H, and Δ S values for interactions of four benzene-containing cryptands ( 1–4) with alkali metal ions, Sr 2+, and Ba 2+ were determined at 25°C by calorimetric titration in absolute methanol. The cryptands form stable complexes with the metal ions (except Li +). Log K values range from 2.90 (2-Cs + interaction) to 5.10 (4-K + interaction). Very low reaction heats for Li + complexation with 1–4 indicate that the cavity sizes of these cryptands are too large to match the small Li +. Different structures of the benzo bridges of the cryptands have an effect on cation complexation. All cryptands studied selectively bind K + over the other cations. Cryptand 4 selects K + over Ba 2+ by a factor of 10, which is a reversal of the common Ba 2+ over K + selectivity order demonstrated by most [2,2,2]-type cryptands. Cryptands 1 and 2 show more negative (favorable) Δ H values and more negative or less positive (unfavorable) Δ S values for the interactions with the metal ions (except Li + and 2-Cs +) than cryptands 3 and 4.