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Issue 11, 2015
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Stable Mn2+, Cu2+ and Ln3+ complexes with cyclen-based ligands functionalized with picolinate pendant arms

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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.

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

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Publication details

The article was received on 26 Sep 2014, accepted on 20 Jan 2015 and first published on 20 Jan 2015


Article type: Paper
DOI: 10.1039/C4DT02985B
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Dalton Trans., 2015,44, 5017-5031

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

    A. Rodríguez-Rodríguez, Z. Garda, E. Ruscsák, D. Esteban-Gómez, A. de Blas, T. Rodríguez-Blas, L. M. P. Lima, M. Beyler, R. Tripier, G. Tircsó and C. Platas-Iglesias, Dalton Trans., 2015, 44, 5017
    DOI: 10.1039/C4DT02985B

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