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Binuclear Manganese Compounds of Potential Biological Significance. 1. Syntheses and Structural, Magnetic, and Electrochemical Properties of Dimanganese(II) and -(II,III) Complexes of a Bridging Unsymmetrical Phenolate Ligand

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journal contribution
posted on 03.01.2003, 00:00 by Lionel Dubois, Dao-Feng Xiang, Xian-Shi Tan, Jacques Pécaut, Peter Jones, Stéphane Baudron, Laurent Le Pape, Jean-Marc Latour, Carole Baffert, Sylvie Chardon-Noblat, Marie-Noëlle Collomb, Alain Deronzier
Reactions of the unsymmetrical phenol ligand 2-(bis(2-pyridylmethyl)aminomethyl)-6-((2-pyridylmethyl)(benzyl)aminomethyl)-4-methylphenol with Mn(OAc)2·4H2O or Mn(H2O)6(ClO4)2 in the presence of NaOBz affords the dimanganese(II) complexes 1(CH3OH), [Mn2(L)(OAc)2(CH3OH)](ClO4), and 2(H2O), [Mn2(L)(OBz)2(H2O)](ClO4), respectively. On the other hand, reaction of the ligand with hydrated manganese(III) acetate furnishes the mixed-valent derivative 3(H2O), [Mn2(L)(OAc)2(H2O)](ClO4) 2. The three complexes have been characterized by X-ray crystallography. 1(CH3OH) crystallizes in the monoclinic system, space group P21/c, with a = 10.9215(6) Å, b = 20.2318(12) Å, c = 19.1354(12) Å, α = 90°, β = 97.5310(10)°, γ = 90°, V = 4191.7 Å3, and Z = 4. 2(H2O) crystallizes in the monoclinic system, space group P21/n, with a = 10.9215(6) Å, b = 20.2318(12) Å, c = 19.1354(12) Å, α = 90°, β = 97.5310(10)°, γ = 90°, V = 4191.7 Å3, and Z = 4. 3(H2O) crystallizes in the monoclinic system, space group P21/c, with a = 11.144(6) Å, b = 18.737(10) Å, c = 23.949(13) Å, α = 90°, β = 95.910(10)°, γ = 90°, V = 4974(5) Å3, and Z = 4. Magnetic measurements revealed that the three compounds exhibit very similar magnetic exchange interactions −J = 4.3(3) cm-1. They were used to establish tentative magneto−structural correlations which show that for the dimanganese(II) complexes −J decreases when the Mn−Ophenoxo distance increases as expected from orbital overlap considerations. For the dimanganese(II,III) complexes, crystallographic results show that the MnII−Ophenoxo and MnIII−Ophenoxo bond lengths are inversely correlated. An interesting magneto−structural correlation is found between −J and the difference between these bond lengths, δMn-O = dMnII-OdMnIII-O:  the smaller this difference, the larger −J. Electrochemical studies show that the mixed-valence state is favored in 13 by ca. 100 mV with respect to analogous complexes of symmetrical ligands, owing to the asymmetry of the electron density as found in the analogous diiron complexes.

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