Synthesis, crystal structures and magnetic properties of trinuclear oxo-centered manganese complexes of the general formula [Mn 3O(O 2CR) 6L 3] z+ (R=Me, Et, Ph, CH 2Cl; L=3-methylpyridine or water z=1, 0)

Abstract

Preparation and magnetic properties of the trinuclear oxo-centered manganese complexes [Mn 3O(O 2CR) 6L 3] z+ ( 1, R=Me, L=3-methylpyridine, z=1; 2, R=Et, L=3-methylpyridine, z=1; for 1 and 2 the counteranion is ClO 4 −; 3, R=Ph, L=3-methylpyridine and water, z=0; 4, R=CH 2Cl, L=3-methylpyridine and water, z=0), prepared by N-n-Bu 4MnO 4 with appropriate reagents in ethanol and 3-methelpyridine, are reported. The crystal structures of complexes 1 and 2 have been determined. It is shown that complexes 1 and 2 possess an oxo-centered Mn 3O unit with peripheral ligands provided by bridging carboxylate and terminal 3-methylpyridine groups, and an approximate three-fold symmetry axis for the whole cluster. The variable temperature magnetic susceptibility of the four title complexes in the range 1.5–300 K has been interpreted in terms of the Kambe vector-coupling method and Van Vleck equation, and the J′ (cm −1) and g values are found to be −12.87, 2.22 for complex 1 and −12.31, 2.12 for complex 2, respectively. For complexes 3 and 4 the J (cm −1), J′ (cm −1), g values are found to be −5.787, −8.706, 2.09, and −4.62, −8.198, 2.09, respectively. The derived J and J′ exchange parameters are all small in magnitude, both ∣ J∣ and ∣ J′∣<9 cm −1 where J characterizes the Mn II–Mn III interaction and J′ characterizes the Mn III–Mn III interaction. Magnetization measurements up to 65 kG at 1.5 K show the ground state S=3/2 for complexes 3 and 4. These results indicate a weakly antiferromagnetic exchange coupling among the manganese ions. The effects of the bridging and terminal ligands on J and J′ values are discussed briefly.