Water Exchange on Seven-Coordinate Mn(II) Complexes with Macrocyclic Pentadentate Ligands: Insight in the Mechanism of Mn(II) SOD Mimetics

Seven-coordinate manganese(II) complexes [Mn(L)(H2O)2]2+, where L represents an equatorial pentadentate
macrocyclic ligand with five nitrogen donor atoms, were studied with regard to their acid−base properties, water-exchange rate constants, and corresponding activation parameters (ΔH⧧, ΔS⧧, and ΔV⧧). Three of the studied
complexes without imine bonds in the macrocyclic ligand are proven superoxide dismutase (SOD) mimetics. Their
water-exchange parameters were compared with those of the imino groups containing complex [Mn(L1)(Cl)2] (dichloro-2,13-dimethyl-3,6,9,12,18-pentaazabicyclo[12.3.1]-octadeca-1(18),2,12,14,16-pentaenemanganese(II)), which does
not show SOD activity. In addition the X-ray crystal structure of a new complex, dichloro-2,6-bis[1-(2-(N-methylamino)ethylimino)ethyl]pyridine-manganese(II) [Mn(L2)(Cl)2], which is the acyclic analog of [Mn(L1)(Cl)2], is reported. Stability
constants of the complexes and the pKa values of the ligands were measured by potentiometric titration. The
titrations of [Mn(L1)(H2O)2]2+ and [Mn(L2)(H2O)2]2+ led to complicated species distribution curves because of their
ligands containing imine bonds. Water exchange was measured by temperature- and pressure-dependent 17O
NMR techniques. In addition to the measurements on [Mn(EDTA)(H2O)]2- and its derivatives, this is the only study
of water exchange on seven-coordinate manganese complexes. The water exchange rate constants vary between
1.6 × 107 s-1 and 5.8 × 107 s-1 at 25 °C and are mainly controlled by the π-acceptor abilities of the ligands. The
exchange rate constant of the diaqua-1,4,7,10,13-pentaazacyclopentadecanemanganese(II) [Mn([15]aneN5)(H2O)2]2+
complex seems to be even higher but could not be exactly determined. On the basis of the obtained activation
parameters, the exchange mechanism of the studied seven-coordinate manganese(II) complexes follows a dissociative
pathway (Id mechanism). DFT calculations (UB3LYP/LANL2DZp) were performed to obtain the energy required for
the dissociation of the coordinated water molecule, that is, the energy difference between the starting seven-coordinate complex and a six-coordinate intermediate. The results have been discussed in terms of the catalytic
mechanism of the proven SOD mimetics.