Monopicolinate cyclen and cyclam derivatives for stable copper(II) complexation

Abstract

The syntheses of a new 1,4,7,10-tetraazacyclododecane (cyclen) derivative bearing a picolinate pendant arm (HL1), and its 1,4,8,11- tetraazacyclotetradecane (cyclam) analogue HL2, were achieved by using two different selective-protection methods involving the preparation of cyclen-bisaminal or phosphoryl cyclam derivatives. The acid-base properties of both compounds were investigated as well as their coordination chemistry, especially with Cu 2+, in aqueous solution and in solid state. The copper(II) complexes were synthesized, and the single crystal X-ray diffraction structures of compounds of formula [Cu(HL)](ClO 4) 2· H 2O (L = L1 or L2), [CuL1](ClO 4) and [CuL2] Cl·2H 2O, were determined. These studies revealed that protonation of the complexes occurs on the carboxylate group of the picolinate moiety. Stability constants of the complexes were determined at 25.0 °C and ionic strength 0.10 M in KNO 3 using potentiometric titrations. Both ligands form complexes with Cu 2+ that are thermodynamically very stable. Additionally, both HL1 and HL2 exhibit an important selectivity for Cu 2+ over Zn 2+. The kinetic inertness in acidic medium of both complexes of Cu 2+ was evaluated by spectrophotometry revealing that [CuL2] + is much more inert than [CuL1] +. The determined half-life values also demonstrate the very high kinetic inertness of [CuL2] + when compared to a list of copper(II) complexes of other macrocyclic ligands. The coordination geometry of the copper center in the complexes was established in aqueous solution from UV-visible and electron paramagnetic resonance (EPR) spectroscopy, showing that the solution structures of both complexes are in excellent agreement with those of crystallographic data. Cyclic voltammetry experiments point to a good stability of the complexes with respect to metal ion dissociation upon reduction of the metal ion to Cu + at about neutral pH. Our results revealed that the cyclam-based ligand HL2 is a very attractive receptor for copper(II), presenting a fast complexation process, a high kinetic inertness, and important thermodynamic and electrochemical stability. © 2012 American Chemical Society.