Ion-Pair Formation of 3m-Crown-mEther (m= 5, 6) and Its Monobenzo Derivative Complex Ions with Several Pairing Anions in Water

Abstract

Ion-pair formation constants (KMLX in mol-1·dm3) for M(B18C6)+, M(15C5)+, or M(B15C5)+ with pairing anions (X-) in water were determined by potentiometry with ion-selective electrodes at 25 °C over wide ranges of the ionic strength (I). The symbols B18C6, 15C5, and B15C5 denote benzo-18-crown-6 ether, 15-crown-5 ether, and benzo-15C5, respectively; metal salts, MX, used are sodium picrate (NaPic), KPic, NaBPh4, NaMnO4, and KMnO4. The KMLX values at I = 0 mol dm-3 (KMLX°) were evaluated from analyzing the I dependence of the log KMLX values determined. Then, effects of shapes of X-, the benzo group of the ethers, their ring sizes, and the shielding of the metal ions, M+, by the ethers on these KMLX° values were discussed in comparison with the values previously reported on M(18C6)X, M(15C5)Pic, and M(B15C5)Pic. Also, center-to-center distances in the M(18C6 derivatives)+-X- or M(15C5 ones)+-X- pairs were estimated from Bjerrum's equation.