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Novel dinuclear manganese(III) complexes with bi- or tridentate and bridging tetradentate Schiff base ligands: preparation, properties and catalase-like function

2001; Elsevier BV; Volume: 20; Issue: 3-4 Linguagem: Inglês

10.1016/s0277-5387(00)00636-7

1873-3719

Toshio Nakamura, Kenshin Niwa, Shino Usugi, Hideyuki Asada, Manabu Fujiwara, Takayuki Matsushita,

Lanthanide and Transition Metal Complexes

Novel mono- and dinuclear manganese(III) complexes: [MnIII(acac)(PA)2] (1), [MnIII(acac)(N-OPh-sal)(EtOH)] (2), [MnIII2(PA)4(sal-m-xylylene)] (3), [MnIII2(N-OPh-sal)2(X-sal-m-xylylene)] (X=H (4a), 5-MeO (4b), 3-MeO (4c), 5-Br (4d), and 5-NO2 (4e)), and [MnIII2(N-OPh-sal)2(X-salpentn)] (X=H (5a) and 5-MeO (5b)) have been prepared by ligand substitution reactions and characterized, where Hacac, HPA, N-HOPh-Hsal, H2X-sal-m-xylylene, and H2X-salpentn denote acetylacetone, picolinic acid, N-hydroxyphenyl-salicylideneamine, N,N′-di-substituted-salicylidene-m-xylylenediamine, and N,N′-di-substituted-salicylidene-1,5-pentanediamine, respectively. Single crystals of complexes 1, 2, 4a, and 5a were used for X-ray crystallographic determination. Complexes 1 and 2 have a mononuclear structure, in which the central manganese(III) ions adopt a distorted octahedral geometry having an elongated axial bond compared to the equatorial bonds due to the Jahn–Teller effect. For complexes 4a and 5a, their detailed structures could not be clarified owing to a disorder of their molecules and quick degradation of the crystals in air. However, the results suggested that the two manganese(III) ions in these complexes are bridged by one sal-m-xylylene or salpentn ligand to form a dinuclear complex, and each complex has an arrangement similar to that of the mononuclear complex 2. The reactivities of these manganese(III) complexes toward H2O2 have been found that the dinuclear complexes 4a–e, 5a, and 5b can decompose excess amounts of H2O2 (H2O2/Mn<100) almost without degradation of the complexes, whereas complex 3 decomposes with an equimolar amount of H2O2. Moreover, the disproportionation rates of H2O2 have been found to depend on the bridging tetradentate ligands, the substituents on the phenyl rings of the bridging ligands, and the solvents used. On the basis of EPR spectroscopic studies, a series of complexes 4a–e, 5a, and 5b cycle their oxidation levels between Mn2(III, III) and Mn2(II, II) with dioxygen evolution during the decomposition of H2O2.