A Seven-Coordinate Manganese(II) Complex Formed with a Single Tripodal Heptadentate Ligand as a New Superoxide Scavenger

Abstract

A new manganese(II) complex, Mn(II)−tris[2-[N-(2-pyridylmethyl)amino]ethyl]bishexafluorophosphate (Mn−TPAA), has been synthesized, its X-ray crystal structure resolved, and its superoxide stoichiometric scavenging activity established. The complex has been formed with the tripodal potentially heptadentate ligand TPAA which turns out to be the single ligand coordinated to the metal ion and therefore achieves the seven coordination. The complex crystallizes in the space group P212121, a = 19.654(9) Å, b = 15.416(5) Å, c = 10.425(4) Å, with four formula units per unit cell. The manganese is bonded to three pyridine nitrogen atoms, to three amine groups, and to the tripodal bridging nitrogen N. The reactivity toward superoxide has been investigated using the indirect xanthine−xanthine oxidase−cytochrome c method, the electrochemical reaction of in-situ generated superoxide, and γ and pulse radiolysis measurements. From the cytochrome c assay, it has been found that the IC50 value is equal to 4.4 μM. From electrochemical experiments performed in dry acetonitrile and in the presence of oxygen, the complex appears to induce the one-electron reduction of oxygen to split into two separate steps. Electrochemistry also shows that superoxide reacts with the complex. Confirmation of this reaction is obtained with pulse radiolysis which allows the observation of a transient, visualized by its difference absorption spectrum, with a rate constant of 1.05 × 107 M-1 s-1. Inhibition of the formation of H2O2 is also found.