Catalysis and Kinetics of Hydrogen Peroxide Disproportionation by Dinuclear Manganese(III) Complexes of 1,5-Bis(salicylidenamino)pentan-3-ol and the 5-Bromophenyl-Substituted Derivative

Abstract

The dinuclear MnIII complex [Mn2(mu-OAc)(mu-OMe)(5-Br-salpentO)(MeOH)2]Br has been prepared and its structure and reactivity toward H2O2 studied in comparison with [Mn2(mu-OAc)(mu-OMe)(salpentO)(MeOH)2]Br (salpent-OH = 1,5-bis(salicylidenamino)pentan-3-ol and 5-Br-salpentOH = 1,5-bis(5-bromesalicylidenaminopentan-3-ol). The X-ray diffraction analysis of [Mn2(mu-OAc)(mu-OMe)(5-Br-salpentO)(MeOH)2]Br (monoclinic, P21/n, a = 13.081(2) A, b = 13.429(2) A, c = 17.375(2) A, beta = 102.31(1) degrees, V = 2982.0 A3, Z = 4) revealed a mu-alkoxo, mu-acetatodimanganese(III) core with a Mn...Mn separation of 2.932(1) A. The ligand lies in the meridional plane, and the sixth coordination position of each manganese atom is occupied by a methanol molecule providing two substitution-labile sites in the cis position. The two complexes showed catalytic activity toward disproportionation of H2O2 in methanol and dimethylformamide in the 0-25 degrees C temperature range. The initial rate of oxygen evolution in the presence of [Mn2(mu-OAc)(mu-OMe)(5-Br-salpentO)(MeOH)2]Br or [Mn2(mu-OAc)(mu-OMe)(salpentO)(MeOH)2]-Br is first order in catalyst concentration. The two complexes show saturation kinetics in methanol, with the higher kcat = 0.98 s-1 and kcat/KM = 70 M-1 s-1 observed for [Mn2(mu-OAc)(mu-OMe)(salpentO)(MeOH)2]Br.

New dimanganese(III) complexes of pentadentate (N2O3) Schiff base ligands with the [Mn2(μ-OAc)(μ-OR)2]3+ core: Synthesis, characterization and mechanistic studies of H2O2 disproportionation

“…3) was obtained with the following parameters: J = À13.1 cm À1 , g = 1.90 and p = 0.2%. The calculated exchange parameter J falls in the range observed for other dialkoxo bridged diMn III complexes with short MnÁ Á ÁMn distances (2.98-3.00 Å ) [18,19,27,[34][35][36][37] that allow direct overlap between the d xy orbitals.…”

“…The initial rate method was used to determine the rate constants (for experimental details see Ref. [18]). Each rate constant reported here represents the mean value of multiple determinations that fall within ±5%.…”

“…The evolved dioxygen was measured volumetrically as previously described [18], and H 2 O 2 was determined by titration with horseradish peroxidase [12]. Turnovers as high as 2000 have been measured for complexes 1-2 in DMF.…”

“…In previous work, we have studied the catalase-like activity of a series of diMn III complexes of salpentOH (1,5-bis(salicylidenamino)pentan-3-ol) and its phenyl-ring substituted derivatives, and we have found that some minor changes in the Schiff base ligand have incidence on the catalytic activity of the complexes [12,18,19]. Steric effects determine the optimal approach of the substrate, but also electronic properties (electron-donor or withdrawing character) of the substituents in the aromatic ring of the ligand induce variation of the k cat value.…”

Synthesis, characterization and antioxidant activity of water soluble MnIII complexes of sulphonato-substituted Schiff base ligands

“…The H 2 O 2 disproportionation catalyzed by 1-2 was measured by volumetric determination of the evolved O 2 from reaction mixtures in water, as previously described [31].…”