Synthesis, solution studies and structural characterisation of complexes of a mixed oxa–aza macrocycle bearing pendant amino arms

Abstract

A new mixed oxa–aza macrocycle having ethylamino pendant arms on the secondary nitrogens, namely 7,10,13-tris(2-aminoethyl)-1,4-dioxa-7,10,13-triazacyclopentadecane (L), has been synthesized. The co-ordination chemistry of L towards ZnII, CuII, CdII, PbII and BaII has been studied by X-ray crystallography, potentiometry and NMR spectroscopy. The single crystal structures of [Zn(L)][ClO4]2 and [Cu(L)][ClO4]2 confirm that the complexes are isostructural with the metal ion bound to the three pendant arm N-donors and to two N-donors of the macrocyclic core to give a slightly distorted square-pyramidal geometry. The remaining three donors, a tertiary amine and the two oxygens of the ring, remain unco-ordinated leaving one part of the macrocyclic cavity open with the metal ion bound exo to the macrocyclic cavity. The single crystal structures of [Pb(L)][ClO4]2 and [Ba(L)][ClO4]2 show the metal ion co-ordinated to all the donor atoms of the ligand, with BaII co-ordinated additionally to a ClO4− anion in a bidentate fashion to give a 10-co-ordinate metal centre. The binding of L to ZnII, CuII, CdII and PbII was investigated by potentiometric measurements in aqueous solutions. These metals form only mononuclear complexes with unusually low stability constants compared with those found for other hexaamine macrocycles. All the complexes show a marked tendency to protonation. These observations suggest the presence of unco-ordinated or weakly co-ordinated N-donors also in aqueous solution. NMR spectroscopic studies on the complexes of ZnII, CdII, BaII and PbII of L reveal a rigidity which is lost on increasing temperature.