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

Novel mono- and dinuclear manganese(III) complexes: [Mn-III(acac)(PA)(2)] ( 1), [Mn-III(acac)(N-OPh-sal)(EtOH)] (2), [Mn-2(III)(PA)(4)(sal-m-xylylene)] (3), [Mn-2(III)(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 [Mn-2(III)(N-OPh-sal)(2)(X-salp entn)] (X = H (5a) and 5-MeO (5b)) have been prepared by ligand substitutio n reactions and characterized, where Hacac, HPA, N-HOPh-Hsal. H2X-sal-m-xyl ylene, and H2X-salpentn denote acetylacetone, picolinic acid, N-hydroxyphen yl-salicylideneamine, N, N'-di-substituted-salicylidene-m-xylylenediamine, and N, N'-di-substituted-salicylidene-1,5-pentanediamine, respectively. Sin gle crystals of complexes 1, 2, 4a, and 5a were used for X-ray crystallogra phic determination. Complexes I and 2 have a mononuclear structure, in whic h the central manganese(III) ions adopt a distorted octahedral geometry hav ing an elongated axial bond compared to the equatorial bonds due to the Jah n-Teller effect. For complexes 4a and 5a, their detailed structures could n ot be clarified owing to a disorder of their molecules and quick degradatio n of the crystals in air. However, the results suggested that the two manga nese(III) ions in these complexes are bridged by one sal-m-xylylene or salp entn ligand to form a dinuclear complex, and each complex has an arrangemen t similar to that of the mononuclear complex 2. The reactivities of these m anganese(III) complexes toward H2O2 have been found that the dinuclear comp lexes 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 s ubstituents on the phenyl rings of the bridging ligands, and the solvents u sed. On the basis of EPR spectroscopic studies, a series of complexes 4a-e, 5a, and 5b cycle their oxidation levels between Mn,(III, III) and Mn,(II, II) with dioxygen evolution during the decomposition of H2O2. (C) 2001 Else vier Science B.V. All rights reserved.