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 (pK_a2 = 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-effect of the hydroxo group on the water lability on any divalent metal center in general. The observed trans-labilizing effect of the hydroxo ligand is much smaller than in the case of aqua-hydroxo-M­(III) trivalent metal species. Whether this is a general property of trans-aqua-hydroxo-M­(II) species, or if it is specific for Mn­(II) and/or to the seven-coordinate structures, remains to be seen and motivates future studies. In addition, an influence of the hydroxo ligand on the SOD activity of manganese­(II) complexes could be evaluated for the first time as well. Compared with the mononuclear analogue, which is not able to form stable hydroxo species, our pH dependent studies on the SOD activity of the dinuclear amine complex have indicated that the hydroxo ligand may promote protonation and release of the product H₂O₂, especially in solutions of higher pH values, by increasing its pK_a value.