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
journal contributionposted 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-O − dMnIII-O: the smaller this difference, the larger −J. Electrochemical studies show that the mixed-valence state is favored in 1−3 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.