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

Rodríguez-Rodríguez, Aurora and Garda, Zoltán and Ruscsák, Erika and Esteban-Gómez, David and de Blas, Andrés and Rodríguez-Blas, Teresa and Lima, Luís M. P. and Beyler, Maryline and Tripier, Raphaël and Tircsó, Gyula and Platas-Iglesias, Carlos (2015) Stable Mn2+, Cu2+ and Ln3+ complexes with cyclen-based ligands functionalized with picolinate pendant arms. DALTON TRANSACTIONS, 44. pp. 5017-5031. ISSN 1477-9226

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Abstract

In this work 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 greater by ca. 3 log K units 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 coordinate 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-1s-1 for [Gd(dodpa)]+ and [Gd(Medodpa)]+, respectively) are smaller than the value reported for [Gd(do3a)] (k1=2.510 2 M-1s-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.

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