NEW 2-METHYLENEPROPYLENE-BRIDGED CRYPTANDS WITH HIGH SODIUM-ION SELECTIVITY - A THERMODYNAMIC STUDY OF COMPLEXATION

Thermodynamic quantities for the interactions of mono- and tri(2-methy lenepropylene)-bridged cryptands, cryptand [3.3.1], cryptand [2.2.2], and 18-crown-6 with Na+, K+, Rb+, and Cs+ have been determined by calo rimetric titration in an 80:20 (v/v) methanol:water solution at 25 deg rees C. Incorporation of the 2-methylenepropylene (-CH2C(=CH2)CH2-) br idge(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 a lkali 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 numbers of 2-methylenepropylene bridges. This observation indicates that increasing the number of 2-methylenepr opylene bridges on cryptand [2.2.2] favors complexation of a small cat ion over a large one. The log K values for the formation of 1 and 2 co mplexes (except 1-Cs+ and 2-Na+) decrease as compared with those for t he corresponding [2.2.2] complexes. Formation of six-membered chelate ring(s) by the propyleneoxy unit(s) of 1 and 2 with a cation stabilize s the cryptate complexes of the small Na+ and destabilizes the complex es 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+ complex es 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)(3)CH2 bridges. The [3.1] macroring portion of [3.3.1 ] may be too small to effectively bind the Cs+, resulting in the low s tability of the Cs+ complex.