A Family of Oxamido-Bridged Mn^IICu^II Bimetallic Molecular-Based Ferrimagnets:  Synthesis, EXAFS Structural Characterization, and Magnetic Properties

A family of related compounds of general formula (PPh₄)₂Mn₂[Cu(L)]₃·nH₂O have been synthesized, where PPh₄⁺ is the tetraphenylphosphonium cation and L stands for o-phenylenebis(N‘-benzyloxamidate) (PhMe₂opbox) (1), o-phenylenebis[N‘-(3-phenylpropyl)oxamidate] (PhPr₂opbox) (2), and o-phenylenebis[N‘-(4-phenylbutyl)oxamidate] (PhBu₂opbox) (3). The analysis of the XANES and EXAFS spectra of 3 at both Mn and Cu K-edges reveals the occurrence of an oxamidato-bridged Mn^IICu^II two- or three-dimensional structure with Mn···Cu separation of 5.4 Å. Each manganese atom is surrounded by three Cu(L) bisbidentate entities which define an octahedral MnO₆ environment, while each copper atom is in an essentially square-planar CuN₄ surrounding. The magnetic properties of the three compounds have been investigated in the 1.8−300 K temperature range. Compounds 1−3 show characteristic ferrimagnetic behavior with a minimum in the χT versus T plot, and abrupt ferromagnetic phase transition below a critical temperature T_c equal to 12.5, 11.5, and 13.5 K for 1−3, respectively. The magnetization versus magnetic field curve for 3 indicates that in the magnetically ordered phase, all the manganese local spins are aligned along one direction and the copper local spins along the opposite one, confirming thus the antiferromagnetic nature of the interaction between manganese(II) and copper(II) ions through the oxamidato bridge.