Macrocyclic ligand design. Structure-function relationships underlying the interaction of zinc(II), cadmium(II), silver(I) and lead(II) with mixed-donor macrocyclic ligands

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 logK values for the respective complexes in 95% methanol (I= 0.1; Et(4)NCiO(4), 25 degrees 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)(2)SO by means of C-13 n.m.r. spin-lattice relaxation (T-1) 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.