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 Mn-III complex [Mn-2(mu-OAc)(mu-OMe)(5-Br-salpentO)(MeOH)(2)]Br has been prepared and its structure and reactivity toward H2O2 studied in comparison with [Mn-2(mu-OAc)(mu-OMe)(salpentO)(MeOH)(2)]Br (salpentOH = 1,5-bis(salicylidenamino)pentan-3-ol and 5-Br-salpentOH = 1,5-bis(5-bromesalicylidenaminopentan-3- ol). The X-ray diffraction analysis of [Mn-2(mu-OAc)mu-OMe)(5-Br-salpentO)(MeOH)(2)]Br (monoclinic, P2(l)/n, a = 13.081(2) Angstrom, b = 13.429(2) Angstrom,c = 17.375(2) Angstrom, beta = 102.31(1)degrees, V = 2982.0 Angstrom(3), Z = 4) revealed a mu-alkoxo, mu-acetatodimanganese(III) core with a Mn ... Mn separation of 2.932(1) Angstrom. 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 [Mn-2(mu-OAc)(mu-OMe)(5-Br-salpentO)(MeOH)(2)]Br or [Mn-2(mu-OAc)(mu-OMe)(salpentO)(MeOH)(2)]-Br is first order in catalyst concentration. The two complexes show saturation kintics in methanol, with the higher k(cat) = 0.98 s(-1) and k(cat)/K-M = 70 M-1 s(-1) observed for [Mn-2(mu-OAc)(mu-OMe)(salpentO)(MeOH)(2)]Br.