Coordination Chemistry of Quinolyloxy Terminated Podands in the Solid and Solution States: The Crystal and Molecular Structures of Potassium Tosylate and Potassium Isothiocyanate Complexes of 1,8-Bis(Quinolyloxy)-3,6-Dioxaoctane

Abstract

The crystal structures of 1,8-bis(quinolyloxy)-3,6-dioxaoctane complexes of potassium tosylate (I) and potassium isothiocyanate (II) have been determined and compared. Both complexes form monoclinic crystals belonging to the P2₁/c space group with a = 8.152(2), b = 23.097(6), c=16.076(5) Å, β=102.14(2)[ddot], Z=4 for (I), and a = 11.172(2), b = 22.177(2), c = 9.960(2) Å, β = 93.51(2)[ddot], Z=4 for (II). In both cases the hexadentate podand wraps around the potassium ion in a helical fashion, while the isothiocyanate and tosylate anions strongly coordinate to the metal ion (K⁺ …O(3) = 2.620(2)Å (I); K⁺ …N(3) = 2.737(7): (II)), the potassium ion having a coordination number of seven. As a consequence of strong ion-pair formation between the potassium ion and the isothiocyanate and tosylate anions, the podand adopts a subtly different conformation around the metal ion in the two complexes, presumably accommodating the different steric demands of the two anions. The marked differences in the ¹H NMR spectra of (I) and (II) in the non-coordinating solvent CDCl₃,suggests that the contact ion-pair between the complexed cation and anion persists in this solvent system. In contrast, in DMSO-d ₆ the ¹H NMR spectra of (I) and (II) are essentially identical, showing that the DMSO-d ₆ effectively displaces the anion from the coordination sphere of the potassium ion.