Formation of very stable and selective Cu(ii) complexes with a non-macrocyclic ligand: can basicity rival pre-organization?

Abstract

The synthesis of ligand L based on a 2,6-bis[(N,N-bis(methylene phosphonic acid)aminomethyl] pyridine scaffold is described. Potentiometry combined with UV-Vis absorption spectrophotometric titrations were used to determine the protonation constants of the ligand and the stability constants of its corresponding Cu(II), Ni(II), Zn(II) and Ga(III) cations (0.1 M NaClO₄, 25.0 °C). The physico–chemical approach revealed very large stability constants for Cu(II) complexation (logK_CuL = 22.71(7)) reflected in a very high pCu^II value of ∼ 15.5 (pH = 7.4, [L]_tot = 10⁻⁵ M, [Cu]_tot = 10⁻⁶ M), close to those measured for the strong methylphosphonate functionalized cyclen chelators. Based on a literature survey, a correlation is proposed between the pK values of branched polyamine ligands and their stability constants for Cu(II) complexation, allowing for an estimation of the latter on the basis of the protonation constants of L. Ligand L was also shown to be very selective towards Cu(II) compared to the other cations studied (ΔlogK > 4). UV-Vis spectroscopy and kinetic measurements indicated that the formation of the cupric complexes with L is very fast, which, in combination with all other properties, makes it an excellent non-cyclic target for Cu(II) radiopharmaceutical within the frame of ⁶⁴Cu positron emission tomography imaging and radiotherapy.