Metal complexes of dipyridine hexaaza macrocycles. Structural differences between 18- and 20-membered macrocycles on complexation

Abstract

The hexaaza macrocycles 3,6,14,17,23,24-hexaazatricyclo[17.3.1.1^8,12]tetracosa-1(23),8,10,12(24),19,21-hexaene ([18]py₂N₄) and 3,7,15,19,25,26-hexaazatricyclo[19.3.1.1^9,13]hexacosa-1(25),9,11,13(26),21,23-hexaene ([20]py₂N₄) were synthesised. The protonation constants of both compounds and the stability constants of their complexes with a wide range of metal ions (Mn²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Pb²⁺, Mg²⁺, Ca²⁺, Ba²⁺, La³⁺ and Gd³⁺) were determined at 25 °C with ionic strengths of 0.10 mol dm⁻³ in KNO₃. The overall basicity of [20]py₂N₄ is 3.20 log units larger than that of [18]py₂N₄ due to the weaker repulsion between the contiguous protonated ammonium sites, which are separated by propyl chains in [20]py₂N₄ rather than ethyl chains in [18]py₂N₄. The stability constants of complexes of each metal decrease as the cavity size of the macrocycle is increased, taking into account the difference in basicity of the ligands; the values for the Cu²⁺, Ni²⁺ and Zn²⁺ complexes of both ligands are exceptionally high. Single crystal structures of complexes [Ni([18]py₂N₄)](ClO₄)₂·2CH₃CN (1), [Cu([18]py₂N₄)](ClO₄)₂·2CH₃CN (2), [Co([20]py₂N₄)][Co(H₂O)₆]_0.5(SO₄)₂·CH₃OH·4H₂O (3), [Ni([20]py₂N₄)](ClO₄)₂(4) and [Cu([20]py₂N₄)](ClO₄)₂ (5) were determined. In all complexes, the metal centre exhibit a hexaco-ordinate environment and the macrocycle adopts a twisted helical topology. The effect of the cage sizes of [18]py₂N₄ and [20]py₂N₄ on the molecular dimensions of metal complexes of both macrocycles is evaluated and a significant decrease in the helicity is observed in complexes of the 18-membered macrocycle compared to complexes of the 20-membered ring. The X-ray structural results, together with molecular mechanics calculations and NMR studies performed for metal complexes of both macrocycles, indicate that both ligands have enough flexibility to encapsulate smaller and larger metal ions, although it is clear that [20]py₂N₄ is more flexible than [18]py₂N₄.