Two New Mixed-Valence Manganese Complexes of Formula [Mn4O2(X-benzoato)7(bpy)2] (X = 2-Cl, 2-Br) and the Crystal Structure of the 2-Cl Complex: Ground-State Spin Variability in the [Mn4O2]7+Complexes

The reaction of [Mn3O(2-X-benzoato)6L3] (X = Cl, Br; L = pyridine) with 2,2'-bipyridine in CH2Cl2 leads to the high-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 1 crystallizes 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-valence MnIIMnIII3 description. Each manganese ion is distorted octahedral, especially the three MnIII ions, owing to a first-order Jahn−Teller effect. The MnII is assigned on the basis of the longer metal−ligand distances. Variable temperature magnetic susceptibility studies were performed on 1 and 2 in the temperature range 2−300 K. The topology 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. Without any simplifying assumptions, a full diagonalization matrix method is necessary to solve the problem, but assuming that 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, very close in energy to low-lying spin states of 9/2, 5/2, 3/2, and 1/2. They are all almost degenerate. By application of Kambe's method (with only one Jwb), the results are completely similar. Magnetization measurements at 2−30 K 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.