| Popis: |
Crown ethers and their complexes have received a continuous attention because of their widespread chemical and biological application. It was shown that the alkali metal chemistry of biologically important transport agents, such as some naturally occurring antibiotics, parallels in many aspects to that of the crown ethers towards these metal ions. For this reason these compounds are used as synthetic model systems for examination of the selective ion-transport through the cell membranes. In this paper we report the complexation of dibenzo-24-crown-8 with potassium and ammonium hexafluorophosphates, as continuation of our systematic investigation on the alkali and ammonium salt complexes with different crown ethers both in solution and in the solid state. The new complexes were identified and characterized by elemental, spectroscopic (IR, UV/VIS, 1H NMR) and X-ray structural analyses. The potassium complex crystallizing in two polymorphic forms: one with nine and the other with eight coordinated cation. The conformation of the crown ring in one of them is identical with the conformation in ammonium complex, where cation forms hydrogen bonds with the oxygen atoms of the polyether ring. The results obtained are compared with those obtained for the corresponding hexaflurophospahate complexes of dibenzo-18-crown-6 and dibenzo-30-crown-10. The coordination behaviour of crown ethers towards cations is influenced by the size and charge of the linked cation and by the flexibility and cavity size of the macrocyclic compound, but it was shown that the cation-cavity size relation is not the crucial factor for the structure of these complexes. |
