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)(6)L-3] (X = Cl, Br; L = pyridine) with 2,2'-bipyridine in CH2Cl2 leads to the high-yield formation of new mixed-valence tetranuclear (MnMn3III)-Mn-II 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 P2(1)/n with a = 19.849(8) Angstrom, b = 13.908(5) Angstrom, c = 30.722(19) Angstrom, beta = 107.35(2)degrees, Z = 4. Complex 1 is neutral, and consideration of overall charge necessitates a mixed-valence (MnMn3III)-Mn-II description. Each manganese ion is distorted octahedral, especially the three Mn-III ions, owing to a first-order Jahn-Teller effect. The Mn-II 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, J(bb) between the two-body Mn-III ions and two J(wb) ("wing-body") between the Mn-III ions of the "body" of the butterfly and the Mn-II or Mn-III 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 J(wb) are identical, it is then possible to solve the problem numerically by applying the Kambe method. With both methods, the derived J(bb) and J(wb) exchange parameters are very similar for the 2-Cl and 2-Br complexes. The best R factors [Sigma (i)(chi (Mcalc) - chi (Mobs))(2)/Sigma (i)(chi (Mobs))(2)] (similar to 10(-6)) were obtained from 300 to 40 K. The J values are, thus, as follows. For 1, J(bb) = -23.2 cm(-1), J(wb) = -4.9 and -4.8 cm(-1), and g = 1.93. For 2, J(bb)= -22.8 cm(-1), J(wb) = -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 J(wb)), 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).