Effects of Ligand N and O Donor Content on the Structure, Aqueous Stability, and SOD Activity of a Series of Mn ²⁺ Complexes with Pyridine-Containing Tripodal Ligands

A series of six Mn²⁺ complexes of pyridine-containing, tripodal ligands were investigated in this study to examine the effect of varying ligand N and O donor content on their structural, aqueous stability and superoxide dismutase (SOD) activity. Crystalline forms of the complexes of the formula [MnL-(OAc)-(MeOH)]-BPh₄, where L is a tripodal ligand, were characterized by IR, elemental analysis, and X-ray diffraction. Crystal structures of each compound reveal a hepta-coordinated Mn²⁺ ion with distorted pentagonal bipyramidal geometry. Complex formation in aqueous solution was examined by potentiometric titration and cyclic voltammetry. Mn²⁺-binding affinities (log β) of the ligands are strongly influenced by the N and O donor content of the ligands, with greater N content resulting in higher stability. Reduction potentials, <i>E</i> _1/2 Mn-(III/II) of the complexes also correlate to the N and O donor content of the ligands, with lower O content producing higher <i>E</i> _1/2 values. The aqueous complexes catalyze the efficient disproportionation of superoxide ion, where the apparent catalytic rate constants (<i>k</i> _cat) are influenced by the nature of the O-donor moiety (-OH vs -OCH₃). For complexes with methoxy-containing ligands, increasing -OCH₃ content correlates negatively with <i>k</i> _cat values. The opposite trend is observed for complexes with hydroxy-containing ligands, suggesting the role of hydrogen bonding and/or proton transfer by -OH groups in the catalytic mechanism of these complexes.