Synthesis and Magnetic Properties of Six New Trinuclear Oxo-Centered Manganese Complexes of General Formula [Mn(3)O(X-benzoato)(6)L(3)] (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.

The reaction of N-n-Bu(4)MnO(4) or NaMnO(4) with appropriate reagents in ethanol-pyridine leads to the high-yield formation of new mixed-valence trinuclear oxo-centered Mn(III,III,II) complexes of general formulation [Mn(3)O(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 H(2)O; 5, X = 3-chloro, L = 2 pyridine + 1 H(2)O; 6, X = 3-bromo, L = 2 pyridine + 1 H(2)O). 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, 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) Å, b = 17.603(2) Å, c = 21.996(3) Å, beta = 106.300(10), and Z = 4. Complex 6 crystallizes in the monoclinic system, space group P2(1)/n with a = 15.533(3) Å, b = 17.884(2) Å, c = 21.997(4) Å, beta = 106.95(1) degrees, and Z = 4. The three complexes are neutral and possess an oxo-centered Mn(3)O unit with peripheral ligands provided by bridging carboxylate and terminal pyridine or H(2)O groups. Each manganese ion is distorted octahedral, and consideration of overall charge necessitates a mixed-valence Mn(II)Mn(III)(2) description. In 1, the presence of a C(2) axis through the central 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 H(2)O terminal molecule) suggest a trapped-valence situation in all three cases. The Mn(II) is assigned on the basis of its longer metal-ligand distances. Variable-temperature magnetic susceptibility studies were performed on 1-6 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 Mn(II).Mn(III) 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 polycrystalline 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 approximately 4, which decreases in intensity with increasing temperature. For S = (1)/(2) complexes, this g approximately 4 band does not appear but instead there are broad bands centered at g approximately 2. These results are discussed in terms of spin frustration within the Mn(3)O core, which produces different spin ground states and susceptibility values.