Insights into Second-Sphere Effects on Redox Potentials, Spectroscopic Properties, and Superoxide Dismutase Activity of Manganese Complexes with Schiff-Base Ligands

Abstract

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-F 2 [ 3 ], 3,5-Cl 2 [ 4 ], 5-NO 2 [ 5 ], 3,5-(NO 2 ) 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 × 10 6 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 O 2 •– to the metal center. Even complex 5 , [Mn(5-(NO 2 )salpn) (OAc) (H 2 O)], 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 KO 2 demonstrates that in diluted solutions these complexes behave as catalysts supporting several additions of excess O 2 •– , 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 O 2 •– dismutation cycle catalyzed by complexes 1–6 and modulate the strength of the Mn-substrate adduct for electron-transfer through an inner-sphere mechanism.

Pursuing the Elixir of Life: In Vivo Antioxidative Effects of Manganosalen Complexes

“…On the other hand, supramolecular interactions also play an essential role in biological processes recognition [ 85 ]. Second-sphere effects of the substituents may also modulate the redox potentials of the metal center and the metal–ligand bond strength and may guide their reactivity with superoxide anion radical [ 86 ]. Alkoxy substituents on the phenyl rings can also participate in establishing supramolecular interactions through hydrogen bonding, which added to their mentioned effect on redox potentials could explain the high activity as enzyme mimics of alkoxy substituted manganosalen complexes [ 87 , 88 , 89 ].…”

“…The efficiency of these systems seems to be related to the presence of at least one vacancy or a labile coordination position on the manganese core. Electron donor substituents on the phenyl rings of the salen moiety also increase the catalase activity of manganosalen complexes [ 86 ]. The rates at which manganosalen complexes scavenge hydrogen peroxide are similar to those reported for metalloporphyrins.…”

Pursuing the Elixir of Life: In Vivo Antioxidative Effect of Manganosalen Complexes, a Review

“…On the other hand, supramolecular interactions also play an essential role in biological processes recognition [86]. Second-sphere effects of the substituents may also modulate the redox potentials of the metal centre and the metal-ligand bond strength and guide their reactivity with superoxide anion radical [87]. Alkoxy substituents on the phenyl rings can also participate in establishing supramolecular interactions through hydrogen bonding, which added to their mentioned effect on redox potentials, could explain the high activity as enzyme mimics of alkoxy substituted manganosalen complexes [88][89][90].…”

Copper bis(thiosemicarbazone) Complexes with Pendent Polyamines: Effects of Proton Relays and Charged Moieties on Electrocatalytic HER

“…The metal‐centered and ligand‐centered reduction potentials of molecular catalysts can be tuned through substituent effects by employing electron‐donating or electron‐withdrawing groups [1–3] . For electrocatalysts, changing the electronic structure directly influences overpotential and the turnover frequency (TOF) of catalysis [4–6] .…”