| Abstract
| - The crystal structure of a tetranuclear mixed-valence manganese butterfly complex, [Mn4O2(2-Cl-benzoato)7(bpy)2], is reported for the first time. Spin frustration gives an ST = 7/2 ground state, whereas it is ST = 5/2 for the benzoato derivative. The complex has the same “butterfly” shape as the analogous Mn4III complexes reported previously. The MnII ion is clearly defined in the structure, indicating a trapped mixed-valence complex. The Jwb values obtained in the full-diagonalization method are very similar, irrespective of the oxidation state of the wingtip manganese ion (MnII or MnIII).
- The reaction of [Mn3O(2-X-benzoato)6L3] (X = Cl, Br; L = pyridine) with 2,2‘-bipyridine in CH2Cl2 leads to thehigh-yield formation of new mixed-valence tetranuclear MnIIMn3III complexes of general formulation [Mn4O2(X-benzoato)7(bpy)2] (1, X = 2-chloro; 2, X = 2-bromo). The crystal structure of 1 was determined. Complex 1crystallizes in the monoclinic system, space group P21/n with a = 19.849(8) Å, b = 13.908(5) Å, c = 30.722(19)Å, β = 107.35(2)°, Z = 4. Complex 1 is neutral, and consideration of overall charge necessitates a mixed-valenceMnIIMnIII3 description. Each manganese ion is distorted octahedral, especially the three MnIII ions, owing to afirst-order Jahn−Teller effect. The MnII is assigned on the basis of the longer metal−ligand distances. Variabletemperature magnetic susceptibility studies were performed on 1 and 2 in the temperature range 2−300 K. Thetopology of the molecule requires three J values, Jbb between the two-body MnIII ions and two Jwb (“wing-body”)between the MnIII ions of the “body” of the butterfly and the MnII or MnIII of the “wing” of the butterfly. Withoutany simplifying assumptions, a full diagonalization matrix method is necessary to solve the problem, but assumingthat both Jwb are identical, it is then possible to solve the problem numerically by applying the Kambe method.With both methods, the derived Jbb and Jwb exchange parameters are very similar for the 2-Cl and 2-Br complexes.The best R factors [∑i(χMcalc − χMobs)2/∑i(χMobs)2] (∼10-6) were obtained from 300 to 40 K. The J values are,thus, as follows. For 1, Jbb = −23.2 cm-1, Jwb = −4.9 and −4.8 cm-1, and g = 1.93. For 2, Jbb= −22.8 cm-1,Jwb = −4.8 and −4.7 cm-1, and g = 1.92. With these values, the expected ground-state spin must be 7/2, veryclose in energy to low-lying spin states of 9/2, 5/2, 3/2, and 1/2. They are all almost degenerate. By application ofKambe's method (with only one Jwb), the results are completely similar. Magnetization measurements at 2−30K from 2 to 50 kG confirm that the ground state is S = 7/2 for 1, with the D parameter equal to −0.60 cm-1.
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