Structures of 18-crown-6 complexes with alkali cations in methanolic solution as studied by X-ray absorption fine structure

The structures of 18-crown-6 (18C6) complexes with K+ and Rb+ in methanolic solutions have been studied by X-ray absorption fine structure (XAFS) at the Br K-edge as well as at the K and Rb K-edges. The XAFS spectrum at the K or Rb K-edge has indicated that either Br- or solvent (methanol) molecules are present in the first coordination shell of K+ or Rb+ complexed by 18C6. However, the spectra obtained at the Br-K edge have strongly suggested that the alkali cations do not exist in the vicinity of Br-, indicating that no direct ion-pairing occurs between the 18C6 complex and Br-. The 18C6-K+ complex maintains D-3d symmetry even in methanol, and two methanol molecules coordinate the cation possibly from above and below the crown plane. In contrast, the corresponding Rb+ complex possibly forms an umbrella-shaped complex, in which Rb+ is situated slightly off the crown plane and three solvent molecules bind with the cation.