Stable Mn2+, Cu2+ and Ln3+ complexes with cyclen-based ligands functionalized with picolinate pendant arms

Abstract

In this study we present the results of the equilibrium, dissociation kinetics, DFT and X-ray crystallographic studies performed on the complexes of metal ions of biomedical importance (Mn²⁺, Cu²⁺ and Gd³⁺) formed with octadentate ligands based on a cyclen platform incorporating two picolinate pendant arms (dodpa²⁻ and Medodpa²⁻). The stability constants of the complexes were accessed by multiple methods (pH-potentiometry, direct and competition UV-vis spectrophotometry and ¹H-relaxometry). The stability constants of the complexes formed with dodpa²⁻ and Medodpa²⁻ do not differ significantly (e.g. log K_[Mn(dodpa)] = 17.40 vs. log K_{[Mn(Medodpa)]} = 17.46, log K_[Cu(dodpa)] = 24.34–25.17 vs. log K_{[Cu(Medodpa)]} = 24.74 and log K_{[Gd(dodpa)]⁺} = 17.27 vs. log K_{[Gd(Medodpa)]⁺} = 17.59), which indicates that the steric hindrance brought by the methyl groups has no significant effect on the stability of the complexes. The stability constants of the Mn²⁺ complexes formed with the cyclen dipicolinates were found to be ca. 3 log K units higher than those determined for the complex of the cyclen monopicolinate (dompa⁻), which indicates that the second picolinate moiety attached to the backbone of the macrocycle is very likely coordinated to the Mn²⁺ ion. However, the stability of the [Cu(dodpa)] and [Cu(Medodpa)] complexes agrees well with the stability constant of [Cu(dompa)]⁺, in line with the hexadentate coordination around the metal ion observed in the X-ray structure of [Cu(Medodpa)]. The [Gd(dodpa)]⁺ and [Gd(Medodpa)]⁺ complexes display a fairly high kinetic inertness, as the rate constants of acid catalysed dissociation (k₁ = 2.5(4) × 10⁻³ and 8.3(4) × 10⁻⁴ M⁻¹ s⁻¹ for [Gd(dodpa)]⁺ and [Gd(Medodpa)]⁺, respectively) are smaller than the value reported for [Gd(do3a)] (k₁ = 2.5 × 10⁻² M⁻¹ s⁻¹). The [Mn(dodpa)] complex was found to be more inert than [Mn(Medodpa)]. The results of the diffusion-ordered NMR spectroscopy (DOSY) and DFT calculations of diamagnetic [La(dodpa)]⁺ and [Lu(dodpa)]⁺ complexes indicate the formation of a trinuclear entity of the La complex in aqueous solution.