Role of water in complexation of 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6) with Li+ and K+ in hydrophobic 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ionic liquid

Abstract

Complexation characteristics of 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6, 18C6) with Li+ and K+ in a hydrophobic ionic liquid of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide under dry and humid conditions at 298.2 K were studied by 1H and 13C NMR chemical shifts. The comparison of the 1H and 13C chemical shifts of 18C6 molecule between the dry and humid IL solutions without the alkali metal ions showed that uncomplexed 18C6 molecules are solvated by water molecules in the humid ionic liquid solution. The changes in the 1H and 13C chemical shifts of 18C6 ligand molecule with the increases in the Li+ and K+ concentrations revealed that in both dry and humid ionic liquid solutions 18C6 molecule forms 1:1 complexes with Li+ and K+. The 1H NMR data of water molecules in the humid ionic liquid solutions demonstrated that water molecules interact with Li+-18C6 complexes and free Li+, but do not with K+-18C6 complexes and free K+. The mechanisms of the formation of the Li+ and K+ complexes in the humid ionic liquid solution are different from each other due to the differences in the complex-water interactions.