Mn(II), Cu(II), and Zn(II) Complexes Based on H3nota Derivatives Possessing Picolyl and Picolinate Pendants: Structural and Equilibrium Studies ─ Kinetic Inertness, Electrochemical Properties, and Relaxivity of the Mn Complexes
Enikő Madarasi, Axia Marlin, Ferenc Krisztián Kálmán, David Esteban-Gómez, Marie Cordier, Raphaël Tripier, Carlos Platas-Iglesias, Gyula Tircsó, Véronique Patinec
Inorg. Chem. 2025, 64, 32, 16594–16607, DOI: 10.1021/acs.inorgchem.5c02705, hal-05217573
 

Two derivatives of tacn (1,4,7-triazacyclononane) are investigated for their complexation with Cu(II), Zn(II), and Mn(II). The different denticities of macrocyclic ligands Hnoapy and H2noapa offer the possibility to modulate the coordination environment and stability of the complexes. [Cu(noapy)]ClO4(H2O)2 exhibits a square-pyramidal penta-coordinated geometry in the solid state, while UV–visible and EPR spectroscopies and DFT are consistent with a hexa-coordinated species in solutions. The hexa-coordination of zinc(II) in the [Zn(noapy)]ClO4 complex was evidenced in the solid state, thanks to the formation of a 1D coordination polymer through a bridging carboxylate coordinating group. The [Mn2(noapy)2](ClO4)2·H2O complex presents a trigonal prismatic geometry around the cation similar to that of the Zn(II) complex. The stability constants of Hnoapy and H2noapa complexes were measured by pH-potentiometric titrations. The inertness of [Mn(noapa)] and [Mn(noapy)]+ complexes was assessed through Zn(II) exchange reactions, revealing that decomplexation occurs via both spontaneous and proton-assisted pathways. The electrochemical studies in aqueous solutions revealed their resistance toward oxidation at pH 6.7. Relaxivity measurements, supported by DFT calculations, confirmed the presence of one inner-sphere water molecule coordinated to Mn(II) in both [Mn(noapa)] and [Mn(noapy)]+ complexes. Furthermore, 17O NMR studies of water exchange kinetics suggested an associative activation mechanism for [Mn(noapy)]+ and a dissociative activated one for [Mn(noapa)].