Influence of the Anion on the Stability of Second-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°C were determined as follows: Ka(ClO4−) = 1.77 × 104 M−1, Ka(NO−) = 1.34 × 102 M−1, Ka(Cl−) = 1.00 × 101 M−1, Ka(picrate) = 4.68 × 103 M−1. Distribution constants (Kd) for the ion pairs between water and chloroform, as well as extraction constants (Kex) for the corresponding crown ether separated ion pairs, were determined: Kd(ClO4−) = 3.10 × 10−4 M−1, Kd(NO3−) = 2.91 × 10−4 M−1, Kd(Cl−) = 3.00 × 10−4 M−1,Kd(picrate) = 0.24; Kex(ClO4−) = 5.50 M−2, Kex(NO3−) = 3.91 × 10−2 M−2, Kex(Cl−) = 3.0010−3 M−2, Kex(picrate) = 1.12 × 103 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 Ka 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. © 1995, American Chemical Society. All right reserved.

Duke Scholars

Author Ines Batinic-Haberle Radiation Oncology

Author Alvin L. Crumbliss Chemistry