Insights into Second-Sphere effects on redox potentials, spectroscopic properties, and superoxide dismutase activity of manganese complexes with Schiff-Base Ligands
Fecha
2019-01-02
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Editor
ACS
Resumen
Descripción
Six Mn-Schiff base complexes, [Mn(X-salpn)]0/+ (salpn =
1,3-bis(sal-ic-ylidenamino)propane, X = H [1], 5-Cl [2], 2,5-F2 [3], 3,5-
Cl2 [4], 5-NO2 [5], 3,5-(NO2)2 [6]), were synthesized and characterized
in solution, and second-sphere effects on their electrochemical and
spectroscopic properties were analyzed. The six complexes catalyze the
dismutation of superoxide with catalytic rate constants in the range 0.65 to
1.54 × 106 M−1 s
−1 obtained through the nitro blue tetrazolium
photoreduction inhibition superoxide dismutases assay, in aqueous
medium of pH 7.8. In solution, these compounds possess two labile
solvent molecules in the axial positions favoring coordination of the highly
nucleophilic O2
•− to the metal center. Even complex 5, [Mn(5-
(NO2)salpn) (OAc) (H2O)], with an axial acetate in the solid state,
behaves as a 1:1 electrolyte in methanolic solution. Electron paramagnetic
resonance and UV−vis monitoring of the reaction of [Mn(X-salpn)]0/+
with KO2 demonstrates that in diluted solutions these complexes behave as catalysts supporting several additions of excess O2
•−,
but at high complex concentrations (≥0.75 mM) catalyst self-inhibition occurs by the formation of a catalytically inactive dimer.
The correlation of spectroscopic, electrochemical, and kinetics data suggest that second-sphere effects control the oxidation
states of Mn involved in the O2
•− dismutation cycle catalyzed by complexes 1−6 and modulate the strength of the Mn-substrate
adduct for electron-transfer through an inner-sphere mechanism.
Palabras clave
Redox reactions, Ligands, Substituents, Electron paramagnetic resonance spectroscopy, Transition metals