SYNTHESIS AND MAGNETIC-PROPERTIES OF 6 NEW TRINUCLEAR OXO-CENTERED MANGANESE COMPLEXES OF GENERAL FORMULA [MN3O(X-BENZOATO)(6)L-3] (X=2-F, 2-CL, 2-BR, 3-F, 3-CL, 3-BR, L EQUALS PYRIDINE OR WATER) AND CRYSTAL-STRUCTURES OF THE 2-F, 3-CL, AND 3-BR COMPLEXES

The reaction of N-n-Bu4MnO4 or NaMnO4 with appropriate reagents in eth anol-pyridine leads to the high-yield formation of new mixed-valence t rinuclear ore-centered Mn-III,Mn-III,Mn-II complexes of general formul ation [Mn3O(X-benzoato)(6)L-3] (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, a nd 6 were determined. Complex 1 crystallizes in the monoclinic system, space group C2/c with a = 15.774(2) Angstrom, b = 17.269(2) Angstrom, c = 21.411(2) Angstrom, beta = 91.11(1)degrees, and Z = 4. Complex 5 crystallizes in the monoclinic system, space group P2(1)/n with a = 15 .172(2) Angstrom, b = 17.603(2) Angstrom, c = 21.996(3) Angstrom, beta = 106.300(10), and Z = 4. Complex 6 crystallizes in the monoclinic sy stem, space group P2(1)/n with a = 15.533(3) Angstrom, b = 17.884(2) A ngstrom, c = 21.997(4) Angstrom, beta = 106.45(1)degrees, and Z = 4. T he three complexes are neutral and possess an ore-centered Mn3O unit w ith peripheral ligands provided by bridging carboxylate and terminal p yridine or H2O groups. Each manganese ion is distorted octahedral, and consideration of overall charge necessitates a mixed-valence (MnMn2II I)-Mn-II description. In 1, the presence of a C-2 axis through the cen tral O atom and one of the manganese atoms (Mn-II) and the absence of imposed symmetry elements in 5 and 6 (they have the two Mn-III with a terminal pyridine group and the Mn-II with a H2O terminal molecule) su ggest a trapped-valence situation in all three cases. The Mn-II is ass igned on the basis of its longer metal-ligand distances. Variable-temp erature magnetic susceptibility studies were performed on 1-6 in the t emperature range 2-300 K. Satisfactory fits to the observed susceptibi lity data were obtained by assuming isotropic magnetic exchange intera ctions and using the appropriate spin Hamiltonian and susceptibility e quation. The derived J and J exchange parameters are all relatively s mall in magnitude, \J\ < 10 cm(-1). J characterizes the Mn-II... Mn-II I interactions and J the Mn-III... Mn-III 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 po lycrystalline samples of 1-6 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 approximate to 4, which decreases in in tensity with increasing temperature. For S = 1/2 complexes, this g app roximate to 4 band does not appear but instead there are broad bands c entered at g approximate to 2. These results are discussed in terms of spin frustration within the Mn3O core, which produces different spin ground states and susceptibility values.