New 2-methylenepropylene-bridged cryptands with high sodium ion selectivity: A thermodynamic study of complexation

Abstract

Thermodynamic quantities for the interactions of mono- and tri(2-methylenepropylene)-bridged cryptands, cryptand [3.3.1], cryptand [2.2.2], and 18-crown-6-with Na⁺, K⁺, Rb⁺, and Cs⁺ have been determined by calorimetric titration in an 80:20 (v/v) methanol: water solution at 25°C. Incorporation of the 2-methylenepropylene (−CH₂C(=CH₂)CH₂−) bridge(s) into cryptand [2.2.2] results in a large change in the ligand-cation binding properties. Tri(2-methylenepropylene)-bridged cryptand [2.2.2] (2) shows high selectivity factors for Na⁺ over K⁺ and other alkali cations, while 2-methylenepropylene-bridged cryptand [2.2.2.] (1) selects K⁺ over Na⁺, as does cryptand [2.2.2]. The K⁺/Na⁺ selectivity is reversed with increasing number of 2-methylenepropylene bridges. This observation indicates that increasing the number of 2-methylenepropylene bridges on cryptand [2.2.2] favors complexation of a small cation over a large one. The logK values for the formation of 1 and 2 complexes (except 1-Cs⁺ and 2-Na⁺) decrease as compared with those for the corresponding [2.2.2] complexes. Formation of six-membered chelate ring(s) by the propyleneoxy unit(s) of 1 and 2 with a cation stabilizes the cryptate complexes of the small Na⁺ and destabilizes the complexes of large alkali metal cations. Thermodynamic data indicate that the stabilities of the cryptate complexes studied are dominated mostly by the enthalpy change. In most cases, both stabilization of Na⁺ complexes and destabilization of the complexes of large alkali metal cations by six-membered chelate ring(s) also result from an enthalpic effect. Cryptand [3.3.1] shows a selectivity for K⁺ over Cs⁺, despite its two long CH₂(CH₂OCH₂)₃CH₂ bridges. The [3.1] macroring portion of [3.3.1]may be too small to effectively bind the Cs⁺, resulting in the low stability of the Cs⁺ complex.