Stabilization of a K+-(bis-Cage-Annulated 20-Crown-6) Complex by Bidentate Picrate

Abstract

Bis-cage-annulated 18-crown-6 and 20-crown-6 macrocyclic ethers (i.e., 1 and 2, respectively) have been synthesized, and their alkali metal picrate extraction profiles have been determined. Host system 1 proved to be a significantly more avid alkali metal cation complexant than 2 and somewhat more avid than 18-crown-6. Both 1 and 18-crown-6 display modest selectivity toward K+ and Rb+. A stable host-guest complex was prepared by slow evaporation of a CH2Cl2-hexane solution of an equimolar mixture of 2 and potassium picrate. The X-ray crystal structure of this complex reveals that picrate anion functions as a bidentate ligand therein. The gas-phase interaction energy between the 2 · K+ complex and picrate anion was calculated to be ca. -64.9 kcal mol-1, thereby indicating that participation of picrate anion as an additional bidentate ligand results in significant stabilization of complex 10.