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(μ-OAc)(μ-OMe)(5-Br-salpentO)(MeOH)2]Br has been prepared and its structure
and reactivity toward H2O2 studied in comparison with [Mn2(μ-OAc)(μ-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 [Mn2(μ-OAc)(μ-OMe)(5-Br-salpentO)(MeOH)2]Br (monoclinic, P21/n,
a = 13.081(2) Å, b = 13.429(2) Å, c = 17.375(2) Å, β = 102.31(1)°, V = 2982.0 Å3, Z = 4) revealed a μ-alkoxo,
μ-acetatodimanganese(III) core with a Mn···Mn separation of 2.932(1) Å. 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 °C temperature range. The initial rate of oxygen evolution
in the presence of [Mn2(μ-OAc)(μ-OMe)(5-Br-salpentO)(MeOH)2]Br or [Mn2(μ-OAc)(μ-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(μ-OAc)(μ-OMe)(salpentO)(MeOH)2]Br.