13-Membered macrocycles and their complexometric properties: study of 7,11-bis(carboxymethyl)-1,4-dioxa-7,11-diazacyclotridecane

Abstract

The protonation constants of L1[7,11-bis(carboxymethyl)-1,4-dioxa-7,11-diazacyclotridecane] and the stability constants of complexes formed by this ligand with the alkaline-earth metal ions, the divalent first-row transition metal ions, Cd 2+, and Pb 2+ were determined by potentiometric methods, at 25°C and ionic strength 0.10 M in tetramethylammonium nitrate. In general, the stability constant values for the complexes studied are lower than expected for a potential six-co-ordinate ligand with an N 2O 4 co-ordination sphere. The complexes of L1 with metal ions, forming mainly electrostatic interactions (the alkaline-earth metal ions, Mn 2+ and Pb 2+), have a sharp decrease of stability when compared with the corresponding complexes of 12-membered ligands, L3 and L4 [7,10-bis(carboxymethyl)-1,4-dioxa-7,10-diazacyclodecane and 4,10-bis(carboxymethyl)-1,7-dioxa-4, 10-diazacyclododecane, respectively]. However, the Cu 2+ and Ni 2+ complexes of L1 and L4 have about the same values of stability constants. Hence, the 13-membered ligand is more selective for the divalent first-row transition metal ions than the 12-membered ones are. To explain these results we suggest that probably all the donor atoms of the ligands ( L1, L3 and L4) are involved on the co-ordination to the metal ions when mainly electrostatic interactions take place (also to the Co 2+, as spectroscopic measurements have revealed) but the distances and principally the orientation of the lone-pairs of electrons of L1 are not in the better position for the co-ordination. Based on electronic and EPR spectroscopic measurements in solution and in magnetic moments of the complexes some explanations about the peculiarities of the Ni 2+ and Cu 2+ complexes of L1 and L4 were proposed. The Ni 2+ complexes of both ligands exhibit tetragonally distorted octahedral geometry, but this metal ion seems to fit better in the cage formed by L1 than that of L4, as indicated by the values of stability constants. Both ligands should adopt different arrangements around the metal ion, the distortion from the octahedron being more important for [Ni L4] reflecting the loss in terms of crystal field stabilisation energy. However, the Cu 2+ forms wit L1 a square pyramidal pentaco-ordinate complex while L4 prefers a distorted octahedral arrangement.