Dinuclear seven-coordinate Mn(II) complexes: effect of manganese(II)-hydroxo species on water exchange and superoxide dismutase activity.

Two dinuclear seven-coordinate manganese(II) complexes containing two pentaazamacrocyclic subunits, with imine or amine functionalities, respectively, have been synthesized and characterized in the solid state as well as in aqueous solutions of different pH, by performing X-ray structure analyses, SQUID, potentiometric, electron spray ionization-mass spectrometry (ESI-MS), electrochemical, and 17O NMR water exchange measurements (varying temperature and pressure), and by determination of SOD activity. The two manganese(II) centers within the dinuclear structures behave independently from each other and similarly to the manganese centers in the corresponding mononuclear complexes. However, the dinuclear amine complex possesses increased complex stability and acidity of the coordinated water molecules (pKa2 = 8.92) in comparison to the corresponding mononuclear analogue. This allowed us to observe a stable trans-aqua-hydroxo-Mn(II) species in an aqueous solution and to study for the first time the trans-eff...