Co-ordination tendencies of two novel compartimental oxa-aza macrobicycles. Crystal structure of a CuII(H2O) inclusion complex

Abstract

The two novel aza-oxa cryptands 16,21-dimethyl-4,7,10-trioxa-1,13,16,21-tetraazabicyclo[11.5.5]-tricosane (L¹) and 19,24-dimethyl-4,7,10,13-tetraoxa-1,16,19,24-tetraazabicyclo[14.5.5]hexacosane (L²) can bind in aqueous solution Li⁺ or Na⁺ as well as Cu²⁺, Zn²⁺ or Cd²⁺. The co-ordination features of the above receptors have been studied in aqueous solution by potentiometric (298.1 or 318.1 K. I= 0.15 mol dm^–3) and spectroscopic (¹H and ¹³C NMR and UV/VIS) techniques. In the case of Cu²⁺, Zn²⁺ and Cd²⁺ both the thermodynamic and spectroscopic data are in accord with the metal centres co-ordinated by the tetraaza moiety of the receptor. The co-ordination sphere is completed by binding water molecules or an oxygen of the polyoxa chain. The crystal structures of [CuL¹][ClO₄]₂·0.4MeOH and [CuL²(H₂O)][ClO₄]₂ have been determined. In the former the metal ion is bound by the four nitrogens and by an oxygen donor of the ligand, giving rise to a rather distorted square-pyramidal co-ordination geometry. In the latter the four nitrogen atoms of the ligand are again involved in the co-ordination to the metal ion; the resulting geometry is fairly distorted square-pyramidal, with the fifth co-ordinative position occupied by the oxygen of a water molecule. The Cu(H₂O) unit is encapsulated inside the molecular cavity, stabilized by hydrogen bonds between the water molecule and the four oxygens of the receptor. Consequently the Cu–O distance is remarkably short (2.09 Å). This complex can lose the co-ordinated water molecule both in non-aqueous solution and in the solid state. The acidic dissociation of the Cu²⁺ complexes has also been studied in 1 mol dm^–3 HClO₄ at different temperatures and the rate constants and activation parameters determined.