Semi-Rigid (Aminomethyl) Piperidine-Based Pentadentate Ligands for Mn(II) Complexation

Two pentadentate ligands built on the 2-aminomethylpiperidine structure and bearing two tertiary amino and three oxygen donors (three carboxylates in the case of AMPTA and two carboxylates and one phenolate for AMPDA-HB) were developed for Mn(II) complexation. Equilibrium studies on the ligands and the Mn(II) complexes were carried out using pH potentiometry, H-1-NMR spectroscopy and UV-vis spectrophotometry. The Mn complexes that were formed by the two ligands were more stable than the Mn complexes of other pentadentate ligands but with a lower pMn than Mn(EDTA) and Mn(CDTA) (pMn for Mn(AMPTA) = 7.89 and for Mn(AMPDA-HB) = 7.07). H-1 and O-17-NMR relaxometric studies showed that the two Mn-complexes were q = 1 with a relaxivity value of 3.3 mM(-1) s(-1) for Mn(AMPTA) and 3.4 mM(-1) s(-1) for Mn(AMPDA-HB) at 20 MHz and 298 K. Finally, the geometries of the two complexes were optimized at the DFT level, finding an octahedral coordination environment around the Mn2+ ion, and MD simulations were performed to monitor the distance between the Mn2+ ion and the oxygen of the coordinated water molecule to estimate its residence time, which was in good agreement with that determined using the O-17 NMR data.

Automated summary

Two pentadentate ligands based on the 2-aminomethylpiperidine structure with two tertiary amino and three oxygen donors were developed for Mn(II) complexation. The Mn complexes formed by these ligands were found to be more stable but with lower pMn values than other pentadentate ligands.