RESUMO
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 â¢- 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 â¢-, 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 â¢- dismutation cycle catalyzed by complexes 1-6 and modulate the strength of the Mn-substrate adduct for electron-transfer through an inner-sphere mechanism.
RESUMO
A new diMnIII complex, [Mn2L(OAc)2(H2O)](BPh4)·3H2O (1), obtained with the unsymmetrical N3O3-ligand H3Lâ¯=â¯1-[N-(2-pyridylmethyl),N-(2-hydroxybenzyl)amino]-3-[N'-(2-hydroxybenzyl),N'-(benzyl)amino]propan-2-ol, has been prepared and characterized. The unsymmetrical hexadentate ligand L3- leads to coordination dissymmetry (dissimilar donor atoms) around each Mn ion (N2O4 and NO4(solvent), respectively) leaving one labile site on one of the two Mn ions that facilitates interaction of the metal center with H2O2, as in Mn catalase. 1 is able to catalyze H2O2 disproportionation in acetonitrile, with second-order rate constant kcatâ¯=â¯23.9(2)â¯M-1â¯s-1. The accessibility of the MnII2 state and the closeness of the two one-electron reduction processes suggest 1 employs MnIII2/MnII2 oxidation states for catalysis.
Assuntos
Catalase/química , Peróxido de Hidrogênio/química , Manganês/química , Modelos Químicos , Catálise , Domínio Catalítico , OxirreduçãoRESUMO
A mononuclear Mn(III) complex MnL·4H2O (H3L=1-[N-(2-pyridylmethyl),N-(2-hydroxybenzyl)amino]-3-[N'-(2-hydroxybenzyl),N'-(4-methylbenzyl)amino]propan-2-ol) has been prepared and characterized. This complex catalyzes the dismutation of superoxide efficiently, with catalytic rate constant kcat=1.7×10(6)M(-1)s(-1) and IC50 1.26µM, obtained through the nitro blue tetrazolium photoreduction inhibition superoxide dismutase assay, in aqueous solution of pH7.8. MnL is also able to disproportionate more than 300 equivalents of H2O2 in CH3CN, with initial rate of H2O2 decomposition given by ri=kcat [MnL](2) [H2O2] and kcat=1.32(2)mM(-2)min(-1). The accessibility of the Mn(IV) state (E(p)=0.53V vs. saturated calomel electrode), suggests MnL employs a high-valent catalytic cycle to decompose O2(-) and H2O2.
Assuntos
Complexos de Coordenação/síntese química , Peróxido de Hidrogênio/química , Compostos de Manganês/síntese química , Compostos Organometálicos/síntese química , Propanolaminas/síntese química , Piridinas/síntese química , Catalase/química , Catálise , Complexos de Coordenação/química , Cinética , Ligantes , Compostos de Manganês/química , Compostos Organometálicos/química , Oxirredução , Propanolaminas/química , Piridinas/química , Superóxido Dismutase/químicaRESUMO
Two new tri-Mn(III) complexes of general formula [Mn3L2(µ-OH)(OAc)]ClO4 (H3L = 1-[N-(2-pyridylmethyl),N-(2-hydroxybenzyl)amino]-3-[N'-(2-hydroxybenzyl),N'-(4-X-benzyl)amino]propan-2-ol; 1ClO4, X = Me; 2ClO4, X = H) have been prepared and characterized. X-ray diffraction analysis of 1ClO4 reveals that the complex cation possesses a Mn3(µ-alkoxo)2(µ-hydroxo)(µ-phenoxo)(4+) core, with the three Mn atoms bound to two fully deprotonated N3O3 chelating L(3-), one exogenous acetato ligand, and one hydroxo bridge, the structure of which is retained upon dissolution in acetonitrile or methanol. The three Mn atoms occupy the vertices of a nearly isosceles triangle (Mn1···Mn3 = 3.6374(12) Å, Mn2···Mn3 3.5583(13) Å, and Mn1···Mn2 3.2400(12) Å), with one substitution-labile site on the apical Mn ion occupied by terminally bound monodentate acetate. Temperature-dependent magnetic susceptibility studies indicate the presence of predominant antiferromagnetic intramolecular interactions between Mn(III) ions in 1ClO4. Complexes 1ClO4 and 2ClO4 decompose H2O2 at comparable rates upon initial binding of peroxide through acetate substitution, with retention of core structure during catalysis. Kinetic and spectroscopic studies suggest that these complexes employ the [Mn-(µ-oxo/aquo)-Mn](4+) moiety to activate peroxide, with the additional (µ-alkoxo)(µ-phenoxo)Mn(µ-alkoxo) metallobridge carrying out a structural function.