Diaza-18-crown-6 ethers containing partially-fluorinated benzyl sidearms: Effects of covalently bonded fluorine on the alkali metal complexation

Abstract

The stability constants for the diaza-18-crown-6 ethers 2-6 and alkali metal cations (Na+, K+, Rb+ and Cs+) were determined using potentiometry in 95% methanol. For each metal ion the stability constants of the partially-fluorinated ligands 3-6 were larger than that of the non-fluorinated ligand 2, which might reflect an interaction between fluorine atoms and alkali metal cations. The stability constant of the ligand 4 was larger than that of the ligand 5 for each metal cation tested. This finding was also supported by the results of cation-induced chemical shifts in 1H-, 19F-NMR and extraction experiment. The potentiometry and NMR results as well as the X-ray crystal structures revealed that the position and number of fluorine atoms in the benzyl side arms was crucial for the enhanced interaction between a ligand and an alkali metal.