Macrocyclic ligand ring size effects. Part 1. Complexation of tetra-aza ligands containing fused dibenzo substituents with ZnII and CdII: a solution stability, X-ray crystal structure, and molecular mechanics study

Abstract

The interaction of Zn^II and Cd^II with an extensive series of dibenzo-substituted macrocyclic and open-chain tetra-amines has been investigated. All ligands form 1 : 1 (metal : ligand) complexes in solution with these metal ions; the thermodynamic stabilities of the complexes in 95% methanol (I= 0.1 mol dm^–3, NEt₄ClO₄) have been determined. Along one series of 14- to 16-membered macrocyclic systems a dislocation in the stability pattern occurs for Zn^II at the 16-membered ring complex while, for Cd^II, the dislocation corresponded to the 15-membered ring species. As a consequence there is an enhanced stability difference (Zn^II > Cd^II) between the complexes of this latter ring. The observed discrimination shown by the 15-membered ring is largely maintained when bulky alkyl substituents are appended to carbons adjacent to two of the secondary amine donors; however, the absolute magnitudes of the respective stabilities are decreased because of steric effects. In an extension of this study it has been found that open-chain analogues show no unusual discrimination between Zn^II and Cd^II. Other macrocyclic and open-chain ligands containing only anilino nitrogen donors have been shown to form only very weak complexes with Zn^II and Cd^II. The X-ray structures and molecular mechanics investigation of two free ligands and three metal complexes are reported.