A New Tris(phosphonomethyl) Monoacetic Acid Cyclam Derivative: Synthesis, Acid-Base and Metal Complexation Studies

A new cyclam-based ligand, [4,8,11-tris(phosphonomethyl)-1,4,8,11-tetraazacyclotetradec-1-yl] acetic acid (H(7)te3p1a) was synthesised and characterised. The potentiometric and (31)P NMR spectroscopic titrations of the complexes in an aqueous solution at an ionic strength of 0.10 or 0.50 mol dm(-3) in [N(CH(3))(4)]NO(3), respectively, indicated that the ligand is highly basic. The stepwise protonations were dominated by extensive proton relocations and were influenced by the intramolecular hydrogen bonds. The thermodynamic stability constants for the complexes formed by H7te3p1a in the presence of metal ions were determined by potentiometric titrations. The Cu(2+) complexes exhibited very high stability, those of Zn(2+), Cd(2+), Pb(2+), La(3+), Sm(3+), Gd(3+), Ho(3+) and Lu(3+) presented high stability while that of Ca(2+) had lower stability, as expected. The ligand and its complexes behaviour is similar to that of the tetrakis(phosphonomethyl)cyclam derivative, H(8)tetp. The NMR spectroscopic studies of the Zn(2+) and Cd(2+) complexes pointed to the involvement of the three phosphonate moieties in the coordination of the latter ion, while several isomer species were generally present in solution for the Zn2+ complex. A major isomer was stabilised in solution upon heating. In this isomer only one of the methylphosphonate arms was coordinated to the metal centre and this isomer probably adopts a distorted square pyramidal geometry with the macrocyclic ring in the type I conformation.