Deliberate synthesis for magnetostructural study of linear tetranuclear complexes BIIIMnIIMnIIBIII, MnIIIMnIIMnIIMnIII, MnIVMnIIMnIIMnIV, FeIIIMnIIMnIIFeIII, and CrIIIMnIIMnIICrIII.: Influence of terminal ions on the exchange coupling

As part of an ongoing effort to deliberate synthesis of polynuclear heterometal complexes, we are exploring synthetic routes to high-nuclearity complexes using "metal oximates" as building blocks. Series of tetranuclear linear complex ions of the general types MAMBMBMA, where M-A is a trivalent or tetravalent metal ion and M-B is a divalent metal ion, e. g., Mn(II), have been synthesized by using the dimetal(II) anionic cores, [(M-B(II))(2)(dfmp)(3)](5-) as a bridging ligand for the terminal LMA fragments where H(3)dfmp is a dinucleating phenol-oxime ligand, 2,6-diformyl-4-methylphenol oxime, and L denotes a facially coordinating cyclic tridentate amine, 1,4,7-trimethyl-1,4,7-triazacyclononane. The following combinations are reported here, (BMnMnBIII)-Mn-III-Mn-II-B-II (1), (MnMnMnMnIII)-Mn-III-Mn-II-Mn-II (2), (MnMnMnMnIV)-Mn-IV-Mn-II-Mn-II (3), (FeMnMnFeIII)-Mn-III-Mn-II-Fe-II (4), and (CrMnMnCrIII)-Mn-III-Mn-II-Cr-II (5). The compounds have been characterized spectroscopically and by magnetic susceptibility measurements in the temperature range 2.0-290 K at different field strengths. Complexes 1-4 have also been structurally characterized by single-crystal X-ray diffraction techniques at 100 K. The magnetic behaviors of the compounds indicate weak antiferromagnetic coupling between the manganese(II) centers in the central trisphenoxo-bridged dimanganese(II) core, whereas the coupling between the terminal MA and its neighboring Mn(II) center varies and is weak ferromagnetic or antiferromagnetic. The relative interaction intensity in such a series of complexes is discussed. Finally, a profound influence of the charge on the terminal metal ions on the strength of the exchange coupling in the central dimanganese(II) core has been observed and discussed in relation to the covalency of the metal-ligand bonding.