Correlation of molecular conformation and thermodynamic stability of metalcation complexes of 1,4,7,10,13-pentaoxa-16-thiacyclooctadecane

Abstract

The results of crystal-structure analyses of 1,4,7,10,13-pentaoxa-16-thiacyclooctadecane (I) and its complexes with NaSCN, KSCN, RbSCN and AgNO3 are discussed. In all four complexes the cation coordinates to the five O atoms of the polyether sulfide (I). The sulfur-cation interaction for the complexes depends on the cation. The sulfur-cation interaction is strong and partially covalent with the Ag ÷ complex, but weak with the K ÷ and Rb ÷ complexes. In these three structures the S atom is directed into the cavity of the ligand. There is no interaction between the S and the cation in the Na ÷ complex. In both the Na÷-complexed and uncomplexed ligand, the S atom is directed away from the cavity of the ligand. The cation-oxygen ion-dipole electrostatic interaction is the main factor in the thermodynamic stability of the alkali-metal complexes, while the Ag ÷ complex is stabilized by both the cation-oxygen and cation-sulfur interactions. Correlations are made between the thermodynamic stabilities of cation complexes of 1,4,7,10,13,16-hexaoxacyclooctadecane, 1,4,7,10,13-pentaoxacyclopentadecane, 1,4,7,10,13,16-hexaoxacyclooctadecane-2,6-dione and (I), and molecular conformation determined by X-ray diffraction. These correlations provide a basis for determining the effect various ligand parameters have on the thermodynamic stability of polyether complexes.