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Synthesis and Magnetic Properties of Six New Trinuclear Oxo-Centered Manganese Complexes of General Formula [Mn3O(X-benzoato)6L3] (X = 2-F, 2-Cl, 2-Br, 3-F, 3-Cl, 3-Br; L = Pyridine or Water) and Crystal Structures of the 2-F, 3-Cl, and 3-Br Complexes

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journal contribution
posted on 21.05.1997, 00:00 by Joan Ribas, Belen Albela, Helen Stoeckli-Evans, George Christou
The reaction of N-n-Bu4MnO4 or NaMnO4 with appropriate reagents in ethanol−pyridine leads to the high-yield formation of new mixed-valence trinuclear oxo-centered MnIII,III,II complexes of general formulation [Mn3O(X-benzoato)6L3] (1, X = 2-fluoro, L = pyridine; 2, X = 2-chloro, L = pyridine; 3, X = 2-bromo, L = pyridine; 4, X = 3-fluoro, L = 2 pyridine + 1 H2O; 5, X = 3-chloro, L = 2 pyridine + 1 H2O; 6, X = 3-bromo, L = 2 pyridine + 1 H2O). The crystal structures of 1, 5, and 6 were determined. Complex 1 crystallizes in the monoclinic system, space group C2/c with a = 15.774(2) Å, b = 17.269(2) Å, c = 21.411(2) Å, β = 91.11(1)°, and Z = 4. Complex 5 crystallizes in the monoclinic system, space group P21/n with a = 15.172(2) Å, b = 17.603(2) Å, c = 21.996(3) Å, β = 106.300(10), and Z = 4. Complex 6 crystallizes in the monoclinic system, space group P21/n with a = 15.533(3) Å, b = 17.884(2) Å, c = 21.997(4) Å, β = 106.95(1)°, and Z = 4. The three complexes are neutral and possess an oxo-centered Mn3O unit with peripheral ligands provided by bridging carboxylate and terminal pyridine or H2O groups. Each manganese ion is distorted octahedral, and consideration of overall charge necessitates a mixed-valence MnIIMnIII2 description. In 1, the presence of a C2 axis through the central O atom and one of the manganese atoms (MnII) and the absence of imposed symmetry elements in 5 and 6 (they have the two MnIII with a terminal pyridine group and the MnII with a H2O terminal molecule) suggest a trapped-valence situation in all three cases. The MnII is assigned on the basis of its longer metal−ligand distances. Variable-temperature magnetic susceptibility studies were performed on 16 in the temperature range 2−300 K. Satisfactory fits to the observed susceptibility data were obtained by assuming isotropic magnetic exchange interactions and using the appropriate spin Hamiltonian and susceptibility equation. The derived J and J* exchange parameters are all relatively small in magnitude, |J| < 10 cm-1. J characterizes the MnII···MnIII interactions and J* the MnIII···MnIII interaction. Magnetization measurements at 2 K up to 50 kG indicate the variability of the ground state: S = 3/2 for 2 and 3; S = 1/2 for 1, 4, and 5; and S = 3/2, 1/2 for 6. X-band EPR spectra measured from 4 K to room temperature on polycrystalline samples of 16 show highly significant differences when the ground state is 3/2 or 1/2. For S = 3/2 complexes (2 and 3), there is a transition centered at g ≈ 4, which decreases in intensity with increasing temperature. For S = 1/2 complexes, this g ≈ 4 band does not appear but instead there are broad bands centered at g ≈ 2. These results are discussed in terms of spin frustration within the Mn3O core, which produces different spin ground states and susceptibility values.