Silicon-Bridged Bis(12-crown-4) Ethers as Ionophores for Sodium Ion-Selective Electrodes

A new Na⁺ ionophore with two 12-crown-4 moieties on silicon atoms and hydrophobic hydrocarbon groups on silicon atoms has been synthesized. The silicon-bridged bis(12-crown-4)s were easily obtained in high yield by simply mixing dichlorodiorganosilane and 2-hydroxymethyl-12-crown-4 under room temperature and nitrogen atmosphere. Seven compounds with different hydrocarbon substituents were synthesized. To investigate their properties as ionophores, PVC membrane-type ion-selective electrodes incorporating them were prepared, and the ion selectivity coefficients were determined. The typical selectivity sequence is Na⁺ > K⁺ > Rb⁺ > Cs⁺ > NH₄⁺ > Li⁺ > Ca²⁺ > Mg²⁺ > H⁺. The magnitude of selectivity depends on the structures of hydrocarbon substituents on the silicon atoms. The compound with two 2-ethylhexyl groups has particularly good Na⁺ selectivity, and the performance of the electrode is equal to or better than that of an electrode using a commercially available Na⁺ ionophore, malonate-bridged bis(12-crown-4). The electrode also showed better-aging stability than that of another known Na⁺ ionophore, tetraethyl 4-tert-butylcalix[4]arene-O,O',O″,O‴-tetraacetate, indicating high utility.