Macrocyclic ligand design. X-Ray, DFT and solution studies of the effect of N-methylation and N-benzylation of 1,4,10,13-tetraoxa-7,16-diazacyclooctadecane on its affinity for selected transition and post-transition metal ions

Abstract

Potentiometric titration in 95% methanol (I = 0.1 mol dm⁻³, Et₄NClO₄) has been employed to investigate the effect of N-methylation and N-benzylation of 1,4,10,13-tetraoxa-7,16-diazacyclooctadecane 1 on the binding of all three rings to cobalt(II), nickel(II), copper(II), zinc(II), cadmium(II), silver(I) and lead(II). The results show that enhanced selectivity for silver(I) is exhibited by the di-N-benzylated derivative 3 while the analogous dimethylated derivative 2 discriminates for both silver(I) and lead(II). The crystal structures of [Ag(1)]PF₆, [Ag(2)]PF₆, [Ag(3)]PF₆ and [Pb(1)(NO₃)₂] have been determined. In the silver complexes the NH hydrogens of 1 and the N-methyl and N-benzyl substituents of the dialkylated derivatives 2 and 3 were found to lie on the same side of the mean donor planes of the co-ordinated macrocycles (‘cis’ arrangements). Density functional theory has been employed to model the silver complexes in both their energy-minimised ‘cis’ and ‘trans’ configurations. In accordance with the X-ray evidence, the calculations predict that the ‘cis’ arrangement is the more stable in each case. The crystal structure of the lead complex of 1 shows a ‘trans’ arrangement of its NH groups.