Effect of Ionic Liquids on the synthesis and reactivity of manganese complexes with carboxylate ligand

Abstract

In order to mimic the active site of the Mn-catalase enzyme, the syntheses of three manganese(II) (1-3) and four manganese(III) (4-7) compounds have been studied, and the effect that an Ionic Liquid (1-Butyl-3-methylimidazolium Chloride, BmimCl) and another chloride salt (Bu4NCl) had caused on the syntheses. For manganese(II) compounds, stoichiometric amounts of Mn(2-NO2C6H4COO)2 (previously synthesized) and 2,2'-bipyridine were used, and different reaction conditions (solvent and presence of BmimCl or Bu4NCl in solution) were tested. The dinuclear compounds obtained 1 and 2 have general formula [{Mn(bpy)(2-NO2C6H4COO)(L1)}(μ-2-NO2C6H4COO)2{Mn(bpy)(2-NO2C6H4COO)(L2)}], where for 1 L1 = EtOH, L2 = H2O and for 2 L1 = L2 = H2O. Analogous compounds had been synthesized by the research group previously. Compound 3 [Mn(bpy)(H2O)2(2-NO2C6H4COO)2]n could be a more complex system that will need to be studied in more detail. Manganese(III) compounds were obtained from the comproportionation reaction between a manganese(II) salt (nitrate or perchlorate) and Bu4NMnO4. The reaction conditions were also modified by adding BmimCl or Bu4NCl to the solutions, which led to the obtention of surprising compounds. Dinuclear compounds 4 and 7 (general formula [{Mn(H2O)(bpy)}2 (μ-2-NO2C6H4COO)2(μ-O)](X)2, where X = NO3- (4) or ClO4- (7)) had already been obtained as well by the research group. The mixed-valence compounds [Mn(bpy)Cl2(H2O)2] [{Mn(bpy)Cl}(μ-2-NO2C6H4COO)(μ-O)2{Mn(bpy)(H2O)}] (5) and [Mn(bpy)2Cl2][{Mn(bpy)(H2O)}2 (μ-2-NO2C6H4COO)2(μ-O)]Cl3(ClO4) (6) were only obtained by adding BmimCl or Bu4NCl to the solutions. These could also be more complex systems. In this work, any crystal of good quality for X-ray diffraction has been obtained. However, the different compounds were characterized by IR spectroscopy, conductivity measurements, study of the magnetic behaviour and EPR spectroscopy. The molecular formulas of compounds 1-7 have been proposed from these studies.Finally, an attempt to measure the catalase activity of the manganese(III) compounds 4-7 was performed, by the volumetric measurement of the O2 evolved by the reaction of disproportion of H2O2. The compounds were poorly active, and the results have not been included in the memory.