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

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 Zn^II, Cu^II, Cd^II, Pb^II and Ba^II 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 Ba^II co-ordinated additionally to a ClO₄⁻ anion in a bidentate fashion to give a 10-co-ordinate metal centre. The binding of L to Zn^II, Cu^II, Cd^II and Pb^II 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 Zn^II, Cd^II, Ba^II and Pb^II of L reveal a rigidity which is lost on increasing temperature.