Biomimetic Mn-Catalases Based on Dimeric Manganese Complexes in Mesoporous Silica for Potential Antioxidant Agent

Abstract

Two new structural and functional models of the Mn-catalase with formula [{Mn(III)(bpy)(H2O)}(μ-2-MeOC6H4CO2)2(μ-O){Mn(III)(bpy)(X)}]X, where X = NO3 (1) and ClO4 (2) and bpy = 2,2'-bipyridine, were synthesized and characterized by X-ray diffraction. In both cases, a water molecule and an X ion occupy the monodentate positions. The magnetic properties of these compounds reveal a weak antiferromagnetic behavior (2J = -2.2 cm(-1) for 1 and -0.7 cm(-1) for 2, using the spin Hamiltonian H = -2J S1·S2) and negative zero-field splitting parameter DMn (-4.6 cm(-1) and -3.0 cm(-1) for 1 and 2, respectively). This fact, together with the nearly orthogonal orientation of the Jahn-Teller axes of the Mn(III) ions explain the unusual shape of χMT versus T plot at low temperature. Compound 1 presents a better catalase activity than 2 in CH3CN-H2O media, probably due to a beneficial interaction of the NO3(-) ion with the Mn complex in solution. These compounds were successfully inserted inside two-dimensional hexagonal mesoporous silica (MCM-41 type) leading to the same hybrid material ([Mn2O]@SiO2), without the X group. The manganese complex occupies approximately half of the available pore volume, keeping the silica's hexagonal array intact. Magnetic measurements of [Mn2O]@SiO2 suggest that most of the dinuclear unit is preserved, as a non-negligible interaction between Mn ions is still observed. The X-ray photoelectron spectroscopy analysis of the Mn 3s peak confirms that Mn remains as Mn(III) inside the silica. The catalase activity study of material [Mn2O]@SiO2 reveals that the complex is more active inside the porous silica, probably due to the surface silanolate groups of the pore wall. Moreover, the new material shows catalase activity in water media, while the coordination compounds are not active.

New insights into the comprehension of the magnetic properties of dinuclear MnIII compounds with the general formula [{MnL(NN)}2(μ-O)(μ-n-RC6H4COO)2]X2

“…The magnetic study for compounds 1 and 2 was recently reported. 21 Both compounds present a weaker antiferromagnetic coupling than 6 and 7, and show deviations in the low temperature range of the χ M T vs. T plot, similar to 6.…”

“…Compounds 1 and 2 were recently published, but a deep discussion of their magnetic properties was not reported. 21 The crystal structures of the new compounds (3)(4)(5)(6)(7) are reported here. The magnetic coupling constant and the ZFS parameters of the Mn III ions have been determined and the limits of detection for the sign and magnitude of these parameters have been evaluated.…”

Dinuclear manganese(iii) complexes with bioinspired coordination and variable linkers showing weak exchange effects: a synthetic, structural, spectroscopic and computation study

“…7 Therefore, the coordination chemistry of manganese has been in part developed by the desire to mimic the structure and spectroscopic properties of the active sites of these enzymes, and their catalytic activity. A large number of dimanganese complexes in various oxidation states with single fluorido, 8 oxido/hydroxido/alkoxido, [9][10][11] bis-µ-oxido/hydroxido/ alkoxido, 12 µ 1,3 -carboxylato 13 and mixed oxido/hydroxido/ alkoxido-carboxylato [14][15][16] bridges and terminal carboxylato ligands 17 have been synthesised and characterised over the years revealing structural, spectroscopic, or catalytic resemblance to the native enzymes. The progress in design and synthesis of low-molecular weight antioxidant catalysts as catalasemimics has been recently reviewed.…”

Biomimetic Cu, Zn and Cu2 complexes inserted in mesoporous silica as catalysts for superoxide dismutation

“…Insertion of the a biomimetic catalyst in this inorganic matrix can be used to reproduce certain properties of the native active site, such as confinement and site isolation, conferring stability to the complex while retaining, or even enhancing, its catalytic actvity [20][21][22][23][24][25].…”