Potassium and Sodium Complexes of 1,3-calix[4]-bis-crown-6: Crystal and Molecular Structures, 1H-NMR Investigation and Molecular Dynamics Simulation

Abstract

The crystal structures of two alkali metal complexes involving the ditopic receptor 1,3-calix[4]-bis-crown-6, Bis-C6, are reported. KBis-C6(NO3)·2CH3CN (1) crystallizes in the monoclinic space group C2/c: a = 18.025(8), b = 17.672(9), c = 17.454(13) Å, β = 97.55(5)°, V = 5511(9) Å3, Z = 4. Refinement led to a final conventional R value of 0.091 for 1563 reflections. The structure of (1) is isomorphous to those previously observed for the free ligand and some of its mono- and binuclear cesium complexes: the conformation of the ligand is the same in all cases, which results in nearly identical packings and cell parameters. The potassium ion is bonded to the six oxygen atoms of the crown-ether and two from nitrate ions. Na2 Bis-C6(NO3)2(H2O)2·CH3CN (2) crystallizes in the monoclinic space group P21/c: a = 13.487(6), b = 16.758(9), c = 23.542(15) Å, β = 101.17(4)°, V = 5220(8) Å3, Z = 4. Refinement led to a final conventional R factor of 0.14 for 2546 reflections. One sodium ion and one water molecule are included in each crown-ether moiety. The sodium ion is bonded to three oxygen atoms of the crown only, two from nitrate ions and the one of the water molecule, which also forms possible hydrogen bonds with two ether oxygen atoms. 1H-NMR experiments show that 1:1 potassium or sodium complexes are formed in chloroform solution from picrate salts, they also confirm the bonding mode observed in the solid state, with no evidence of π-interactions between the cations and the aromatic rings. Molecular dynamics simulations were performed in vacuo and in an explicit water phase on Bis-C6/NaNO3 mono- and binuclear complexes to study the influence of the nitrate counter-ion on the structural features of these complexes.