Manganese complexes with planar or tridimensional acyclic or cyclic Schiff base ligands

The coordination properties toward manganese(II) salts of a variety of [1 + 2], [2 + 1], [2 + 2] and [3 + 1] acyclic and cyclic ligands with different shapes and coordinating moieties have been studied. These ligands, obtained by condensation of suitable formyl- and primary amine precursors, give rise to mono- or dinuclear manganese(II) or manganese(III) complexes, characterized by elemental analyses, IR spectroscopy, ESI-MS spectrometry together with X-ray structural determinations. In particular, in the structure of [Mn(H3–LC2)(H2O)2](Cl)(H2O), where the planar, potentially dinucleating ligand H5–LC2 forms by the [2 + 1] condensation of pyridoxal hydrochloride and 1,3-diamino-2-propanol, the central octahedral manganese(III) ion resides into the N2O2 Schiff base moiety of [H3–LC2]2−, which does not behave as a compartmental system as the central alcoholic group does not take part in the coordination. In [Mn(H3–LA5)]·0.25H2O·CH3OH, where the potentially dinucleating ligand H6–LA5 forms by the [3 + 1] condensation of 3-formylsalicylic acid and tris(aminoethyl)amine, the central octahedral manganese(III) ion resides into the external O3O3 site while the inner N4O3 site does not participate in the coordination because of the protonation of the imine groups.Although the formulation and properties in the solid state of the prepared manganese complexes can be recognized, also with the aid of the related zinc(II) analogs, by usual physico-chemical measurements, their solubility and stability in organic solvents or in water is not predictable in advance, because of the occurrence of demetalation–metalation reactions, mainly detected by ESI-MS experiments. These metal encapsulation–release processes, however, represent a necessary prerequisite for further investigations aimed at the use of these complexes in medicine, based on metal ion release in consequence of specific external stimuli.

Planar end-off/side-off acyclic, tripodal end-off acyclic and planar cyclic compartmental Mn(II) and Mn(III) Schiff base complexes, derived from pyridoxal and salicylaldehyde, were studied by IR, HPLC/ESI-MS and X-ray methods.Figure optionsDownload as PowerPoint slideHighlights
► Demetalation–transmetalation, site migration in Mn compartmental complexes.
► Possible use as MRI contrast agents of compartmental manganese complexes.
► Manganese complexes stable in water.
► Behavior of the manganese complexes by ESI-MS measurements.