Synthesis and Magnetic Properties of Six New Trinuclear Oxo-Centered Manganese Complexes of General Formula [Mn3O(X-Benzoato)6L3] (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