Macrocyclic Ligand Design. Structure–Function Relationships Underlying the Interaction of Zinc(II), Cadmium(II), Silver(I) and Lead(II) with Mixed-Donor Macrocyclic Ligands

Abstract

An investigation of the interaction of zinc(II), cadmium(II), silver(I) and lead(II) with a series of mixed-donor dibenzo-substituted macrocyclic ligands, incorporating O₂N₃-, O₂N₄-, O₃N₂-, O₃N₃-, O₄N₂- and O₂N₃S-donor sets, has been carried out. The log K values for the respective complexes in 95% methanol (I = 0·1; Et₄NClO₄, 25°C) have been determined potentiometrically. All ligands form 1 : 1 (metal/ligand) complexes with the above metal ions although in isolated cases species of type MLH²⁺ were also observed. An investigation of the effect of variation of ligand structure on the respective complex stabilities has been carried out and the results compared with those obtained previously for closely related mixed-donor macrocyclic systems.

The binding of silver(I) and lead(II) to the 17- and 18-membered O₃N₂-ligand systems has been assessed in (CD₃)₂SO by means of ¹³C n.m.r. spin–lattice relaxation (T₁) studies as well as by observation of the induced chemical shifts of particular ligand resonances on complex formation. Overall, the n.m.r. results correlate well with the observed thermodynamic stabilities of the individual complexes.