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

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.