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

Two new structural and functional
models of the Mn-catalase with formula [{MnIII(bpy)­(H2O)}­(μ-2-MeOC6H4CO2)2(μ-O)­{MnIII(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 MnIII 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 MnIII 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.