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Mn catalases catalyze disproportionation of H2O2 into H2O and O2 by using a Mn2(m-O2CR)(m-O/OH/H2O)2 structural unit that cycles between the MnII2 and MnIII2 oxidation states. Because of the fast kinetics of this enzymatic reaction each independent step of the catalytic cycle has not yet been characterized. In this context, biomimetic compounds provide a unique way for testing mechanisms in these enzymes. It is known that the reaction rate and even the mechanism change with the solvent. However, most of these studies have been performed in non-aqueous solvents (DMF, acetonitrile, methanol) [1]. For this reason, there is great interest in obtaining information on the mechanism of the catalytic disproportionation of H2O2 in aqueous medium. DiMn complexes of 1,5-bis (salicylidenamino)pentan-3-ol (salpentOH) and its phenyl-ring substituted derivatives X-salpentOH (X = NO2, Br, Cl, H, Me, OMe) posses catalase activity with saturation kinetics and are able to disproportionate more than 1000 equivalents of H2O2 [2]. However, the activity of these complexes was evaluated in DMF and methanol because of their slight solubility in water. These observations prompted us to obtain the diMn complexes of 5-SO3-salpentOH and 3-Me-5-SO3-salpentOH, and to study the kinetics of H2O2 disproportionation upon reaction of H2O2 with [Mn2(m-OMe)(m-OAc)(3-X-5-SO3-salpentO)]Na, in aqueous medium. These complexes show catalase activity, with first-order dependence on catalyst and saturation kinetics on [H2O2], in water. Upon addition of base the complexes show improved efficiency (kcat/KM) and are able to disproportionate more than 1500 eq. of H2O2 without decomposition. The electron-donating methyl group lowers the redox potential of the complex and accelerates the turnover rate of H2O2 dismutation, placing this complex as the most efficient water soluble MnCAT mimic known up to date [1,3].