DINUCLEAR MANGANESE(II) COMPLEXES OF UNSYMMETRIC PHENOL-BASED DINUCLEATING LIGANDS WITH AMINO AND IMINO CHELATING ARMS - SYNTHESIS, STRUCTURE AND CATALASE-LIKE ACTIVITY

Unsymmetric phenol-based dinucleating ligands with amino and imino che lating arms, -6-[2-(dialkylamino)ethyliminomethyl]phenolate(1-) [alkyl = methyl (L1) or ethyl (L2)], formed dinuclear manganese(II) complexe s [Mn2L(RCO2)2(NCS)] (L = L1 or L2, R = Me or Ph). The complex [Mn2L2( MeCO2)2(NCS)] crystallizes in the monoclinic space group P2(1)/n, a = 13.450(2), b = 18.743(3), c = 12.662(2) angstrom and beta = 102.08(1)- degrees. An X-ray diffraction analysis revealed that the dinuclear cor e structure is bridged by the phenolic oxygen of L = 2 and by two acet ate groups. The unsymmetric ligand provides different co-ordination ge ometries about the two manganese ions. The geometry of Mn at the imine site is distorted trigonal bipyramidal with the imine N and the two a cetate 0 atoms in the basal plane and the phenolic 0 and the terminal N at the axial sites. The geometry of the Mn at the amine site is dist orted octahedral involving also the nitrogen of the isothiocyanato gro up. Magnetic susceptibility measurements over the temperature range 4. 2-300 K indicated weak antiferromagnetic interaction (J = -3 to -5 cm- 1 based on H = -2JS1.S2). Cyclic voltammetry for [Mn2L1(MeCO2)2(NCS)] in CH2Cl2 revealed three quasi-reversible redox couples (E1/2 = 0.63, 1.07 and 1.24 V vs. saturated calomel electrode) assignable to the ste pwise oxidations Mn(II)Mn(II) --> Mn(II)Mn(III) --> Mn(III)Mn(III) --> Mn(III)Mn(IV). All the complexes show a catalase-like activity, dispr oportionating H2O2 into O2 and H2O. The presence of oxomanganese(IV) i ntermediates is suggested based on visible and mass spectrometric inve stigations.