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A Comparative XAS and X-ray Diffraction Study of New Binuclear Mn(III) Complexes with Catalase Activity. Indirect Effect of the Counteranion on Magnetic Properties

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posted on 18.10.2004, 00:00 by Gema Fernández, Montserrat Corbella, Montserrat Alfonso, Helen Stoeckli-Evans, Isabel Castro
Four new binuclear Mn(III) complexes with carboxylate bridges have been synthesized:  [{Mn(nn)(H2O)}2(μ-ClCH2COO)2(μ-O)](ClO4)2 with nn = bpy (1) or phen (2) and [{Mn(bpy)(H2O)}2(μ-RCOO)2(μ-O)](NO3)2 with RCOO = ClCH2COO (3) or CH3COO (4). The characterization by X-ray diffraction (1 and 3) and X-ray absorption spectroscopy (XAS) (14) displays the relevance of this spectroscopy to the elucidation of the structural environment of the manganese ions in this kind of compound. Magnetic susceptibility data show an antiferromagnetic coupling for all the compounds:  J = −2.89 cm-1 (for 1), −8.16 cm-1 (for 2), −0.68 cm-1 (for 3), and −2.34 cm-1 (for 4). Compounds 1 and 3 have the same cation complex [{Mn(bpy)(H2O)}2(μ-ClCH2COO)2(μ-O)]2+, but, while 1 shows an antiferromagnetic coupling, for 3 the magnetic interaction between Mn(III) ions is very weak. The four compounds show catalase activity, and when the reaction stopped, Mn(II) compounds with different nuclearity could be obtained:  binuclear [{Mn(phen)2}(μ-ClCH2COO)2](ClO4)2, trinuclear [Mn3(bpy)2(μ-ClCH2COO)6], or mononuclear complexes without carboxylate. Two Mn(II) compounds without carboxylate have been characterized by X-ray diffraction:  [Mn(NO3)2(bpy)2][Mn(NO3)(bpy)2(H2O)]NO3 (5) and [Mn(bpy)3](ClO4)2·0.5 C6H4-1,2-(COOEt)2·0.5H2O (8).