Deliberate Synthesis for Magnetostructural Study of\nLinear Tetranuclear Complexes B^IIIMn^IIMn^IIB^III, Mn^IIIMn^IIMn^IIMn^III,\nMn^IVMn^IIMn^IIMn^IV, Fe^IIIMn^IIMn^IIFe^III, and Cr^IIIMn^IIMn^IICr^III.\nInfluence of Terminal Ions on the Exchange Coupling

Abstract

As part of an ongoing effort to deliberate synthesis of polynuclear heterometal complexes, we are exploring synthetic\nroutes to high-nuclearity complexes using “metal oximates” as building blocks. Series of tetranuclear linear complex\nions of the general types M_AM_BM_BM_A, where M_A is a trivalent or tetravalent metal ion and M_B is a divalent metal\nion, e.g., Mn(II), have been synthesized by using the dimetal(II) anionic cores, [(M^II_B)₂(dfmp)₃]^5- as a bridging\nligand for the terminal LM_A fragments where H₃dfmp is a dinucleating phenol-oxime ligand, 2,6-diformyl-4-methylphenol\noxime, and L denotes a facially coordinating cyclic tridentate amine, 1,4,7-trimethyl-1,4,7-triazacyclononane. The\nfollowing combinations are reported here, B^IIIMn^IIMn^IIB^III (<b>1</b>), Mn^IIIMn^IIMn^IIMn^III (<b>2</b>), Mn^IVMn^IIMn^IIMn^IV (<b>3</b>), Fe^IIIMn^IIMn^IIFe^III (<b>4</b>), and Cr^IIIMn^IIMn^IICr^III (<b>5</b>). The compounds have been characterized spectroscopically and by magnetic\nsusceptibility measurements in the temperature range 2.0−290 K at different field strengths. Complexes <b>1</b>−<b>4</b> have\nalso been structurally characterized by single-crystal X-ray diffraction techniques at 100 K. The magnetic behaviors\nof the compounds indicate weak antiferromagnetic coupling between the manganese(II) centers in the central\ntrisphenoxo-bridged dimanganese(II) core, whereas the coupling between the terminal M_A and its neighboring Mn(II) center varies and is weak ferromagnetic or antiferromagnetic. The relative interaction intensity in such a series\nof complexes is discussed. Finally, a profound influence of the charge on the terminal metal ions on the strength\nof the exchange coupling in the central dimanganese(II) core has been observed and discussed in relation to the\ncovalency of the metal−ligand bonding.