A FAMILY OF OXAMIDO-BRIDGED MN(II)CU(II) BIMETALLIC MOLECULAR-BASED FERRIMAGNETS - SYNTHESIS, EXAFS STRUCTURAL CHARACTERIZATION, AND MAGNETIC-PROPERTIES

A family of related compounds of general formula (PPh(4))(2)Mn-2[Cu(L) ](3) . nH(2)O have been synthesized, where PPh(4)(+) is the tetrapheny lphosphonium cation and L stands for o-phenylenebis(N'-benzyloxamidate ) (PhMe(2)opbox) (1), o-phenylenebis[N'-(3-phenylpropyl)oxamidate] (Ph Pr(2)opbox) (2), and o-phenylenebis[N'-(4-phenylbutyl)oxamidate] (PhBu (2)opbox) (3). The analysis of the XANES and EXAFS spectra of 3 at bot h Mn and Cu K-edges reveals the occurrence of an oxamidato-bridged (Mn CuII)-Cu-II two- or three-dimensional structure with Mn ... Cu separat ion of 5.4 Angstrom. Each manganese atom is surrounded by. three Cu(L) bisbidentate entities which define an octahedral MnO6 environment, wh ile each copper atom is in an essentially square-planar CuN4 surroundi ng. The magnetic properties of the three compounds have been investiga ted in the 1.8-300 K temperature range. Compounds 1-3 show characteris tic ferrimagnetic behavior with a minimum in the chi T 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 magnet ization versus magnetic field curve for 3 indicates that in the magnet ically ordered phase, all the manganese local spins are aligned along one direction and the copper local spins along the opposite one, confi rming thus the antiferromagnetic nature of the interaction between man ganese(II) and copper(II) ions through the oxamidato bridge.