Metal-ion Complexation of Noncyclic Poly(oxyethylene) Derivatives. II. PMR Studies of the Complexation with Alkali and Alkaline-earth Metal Cations

The complexation of homogeneous polyethylene glycols and their dimethyl ethers(glymes) with alkali and alkaline-earth metal cations was studied by PMR spectroscopy on the basis of a comparison with crown ethers. The ethylene proton signal of octaethylene glycol in methanol-d4 was split into three lines by the addition of a potassium, rubidium, caesium, strontium, or barium cation. The latter two cations also caused large downfield shifts of the signals, which are quite comparable with those of 15-crown-5 and 18-crown-6 caused by the same metal cations. As for heptaethylene glycol, a splitting was observed in the presence of a strontium ion or an alkali metal cation except for a lithium ion. A large downfield shift was also observed in the presence of a strontium or barium ion. As for hexaethylene glycol, the signal was split only by a sodium ion, and the optimal interaction with a calcium ion was confirmed on the basis of the downfield shift. In methanol-d4, lithium and magnesium ions did not cause any change in the PMR spectra of either the crown ethers or the glycols. The PMR downfield-shift method also disclosed the metal-ion complexation of the glycols in water. The terminal hydroxyl groups play an important role in the complexation.