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

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.