INFLUENCE OF THE ANION ON THE STABILITY OF 2ND-SPHERE COORDINATION OF FERRIOXAMINE B WITH CIS-DICYCLOHEXANO-18-CROWN-6 IN CHLOROFORM

The influence of the anion X(-) on the stability of the supramolecular assembly FeHDFB+,CE,X(-) (I) where FeHDFB+ is ferrioxamine B, CE is dicyclohexano- 18-crown-6, and X(-) is ClO4-, NO3-, Cl-, or picrate was investigated. The formation constants for these host-guest adducts in wet chloroform at 25 degrees C were determined as follows: K-a(ClO4-) = 1.77 x 10(4) M(-1), K-a(NO3-) = 1.34 x 10(2) M(-1), K-a(Cl-) = 1.00 x 10(1) M(-1), K-a(picrate) = 4.68 x 10(3) M(-1). Distribution constants (K-d) for the ion pairs between water and chloroform, as well as extraction constants (K-ex) for the corresponding crown ether separated ion pairs, were determined: K-d(ClO4-) = 3.10 x 10(-4) M(-1), K-d(NO3-) = 2.91 x 10(-4) M(-1), K-d(Cl-) = 3.00 x 10(-4) M(-1),K-d(picrate) = 0.24; K-ex(ClO4-) = 5.50 M(-2), K-ex(NO3-) = 3.91 x 10(-2) M(-2), K-ex(Cl-) = 3.00 10(-3) M(-2), K-ex(picrate) = 1.12 x 10(3) M(-2). Data are presented which support the assertion that crown ether intercalation into the FeHDFB+,X(-) ion pair to form the supramolecular assembly in I provides a mechanism for the chloroform extraction process which is different from the distribution process. The importance of anion solvation in determining the stability of I is illustrated by a linear plot of log K-a vs anion hydration enthalpy. Comparison with our previously published data enable us to conclude that the stability of I is equally sensitive to hydration of the cation and the anion. We conclude that matching crown ether properties to the {cation,anion} combination in an ion pair may enhance host-guest interactions and optimize aqueous/organic phase extractions.