Effect of coordination dissymmetry on the catalytic activity of manganese catalase mimics.

Two mixed-valence Mn(II)Mn(III) complexes, [Mn2L1(OAc)2(H2O)]BPh4·2.5H2O and [Mn2L2(OAc)2]·4H2O, obtained with unsymmetrical N4O2-hexadentate L1(2-) (H2L1 = 2-(N,N-bis(2-(pyridylmethyl)aminomethyl)-6-(N-(2-hydroxybenzyl)benzylaminomethyl)-4-methylphenol) and N4O3-heptadentate L2(3-) (NaH2L2 = 2-(N,N-bis(2-(pyridylmethyl)aminomethyl)-6-(N'-(2-hydroxybenzyl)(carboxymethyl)aminomethyl)-4-methylphenol sodium salt) ligands, have been prepared and characterized. Both complexes share a µ-phenolate-bis(µ-acetate)Mn(II)Mn(III) core and N3O3-coordination sphere around the Mn(II) ion, but differ in the donor groups surrounding Mn(III) (NO4(solvent) and NO5). In non-protic solvents, these two complexes are able to disproportionate at least 3600 equiv. of H2O2 without significant decomposition, with first-order dependence on catalyst and saturation kinetics on [H2O2]. Spectroscopic monitoring of the reaction mixtures revealed the two complexes disproportionate H2O2 employing a different redox cycle, with retention of dinuclearity. The higher catalytic efficiency of [Mn2L2(OAc)2] was rationalized in terms of the larger labilizing effect of the heptadentate ligand that favors the acetate-shift and the replacement of the non-coordinating benzyl arm of L1 by a carboxylate arm in L2 which facilitates the formation of the catalyst-H2O2 adduct, placing [Mn2L2(OAc)2] as the most efficient among the phenolate-bridged diMn catalysts based on the kcat/KM criterion.

Preparation, characterization and activity of CuZn and Cu2 superoxide dismutase mimics encapsulated in mesoporous silica.

Encapsulation of three superoxide dismutase (SOD) functional mimics, [CuZn(dien)2(µ-Im)(ClO4)2]ClO4 (1), [Cu2(dien)2(µ-Im)(ClO4)2]ClO4 (2) (Im = imidazolate, dien = diethylenetriamine), and [CuZn(salpn)Cl2] (3) (H2salpn = 1,3-bis(salicylideneamino)propane) in mesoporous MCM-41 silica afforded three hybrid catalysts 1@MCM-41, 2@MCM-41 and 3@MCM-41. Spectroscopic and magnetic analyses of these materials confirmed the metal centers of the complexes keep the coordination sphere after insertion into the MCM-41 silica matrix. For the imidazolate-bridged complexes the silica channels restraint the relative orientation of the two metal ions. While 3@MCM-41 shows SOD activity significantly lower than the host-free complex, insertion of the imidazolate-bridged CuZn or Cu2 complexes by ion exchange onto mesoporous MCM-41 silica affords durable and recoverable supported catalysts with much better SOD activity than the free complexes. For confined imidazolate-bridged complexes, 1@MCM-41 and 2@MCM-41, the small pore size of the silica matrix improves the SOD activity more than a host with larger pores. This high SOD activity is attributed to the close-fitting of the complexes into the nanochannels of MCM-41 silica that favors the Cu active site and HImZn(or Cu) group stay in close proximity during catalysis.