Synthesis and complexation properties of C-pivot lariat ethers containing 16-crown-5, 19-crown-6 and 22-crown-7 rings toward alkali metal cations

Abstract

A new series of C-pivot lariat ethers 1–5 having an oxyquinoline moiety as part of the electron-donating sidearm were obtained from crown ether derivatives of low symmetry such as 3n-methylene-(3n + 1)-crown-n (n = 5, 6, 7) and their complexation properties toward alkali metal cations were evaluated by measuring the extractability, stability constant in THF, characteristic absorption in the UV spectrum in THF, and passive transport ability. The difference in the skeletal structure of the pivot position of the lariat ethers 1a and 4 was found to remarkably affect their complexation properties; that is, the former having a 2-methylglycerol structure around the pivot carbon showed much higher complexing ability than did the latter. The effect of the crown ring size and the number of heteroatoms of the electron-donating sidearm on the complexation properties was also examined and discussed. As a result, K⁺ selectivity was attained by 16-crown-5 and 19-crown-6 derivatives (1 and 2), whereas 22-crown-7 derivatives 3 showed Rb⁺ selectivity.