Chemistry and Physics of the Novel Molecular-Based Compound Exhibiting a Spontaneous Magnetization Below Tc = 14 K, MnCu(obbz)*1H2O (obbz = Oxamidobis(benzoato)). Comparison with the Antiferromagnet MnCu(obbz)*5H2O. Crystal structure and Magnetic Properties of NiCu(obbz)*6H2O

Article abstract of DOI:10.1021/ja00197a036

The three compounds MnCu(obbz)*5H2O, MnCu(obbz)*1H2O, and NiCu(obbz)*6H2O, hereafter abbreviated as MnCu*5H2O, MnCu*1H2O, and NiCu respectively, have been synthesized. obbz stands for oxamidobis(benzoato).The crystal structure of NiCu has been determined.NiCu crystallizes in the monoclinic system, space group P2₁/n, with a = 7.764 (1) Angstroem, b = 12.559 (3) Angstroem, c = 20.083 (3) Angstroem, β = 97.56⁰, and Z = 4.The structure consists of isolated heterodinuclear units with octahedral Ni(II) and square-planar Cu(II) ions bridged by an oxamido group.NiCu isnot isomorphous to MnCu*5H2O or to MnCu*1H2O, neither of which has been obtained in the form of single crystals suitable for X-ray work.To get information on their structure, we have studied their XANES and EXAFS spectra at both Mn and Cu edges, as well as their powder X-ray patterns.MnCu*5H2O and MnCu*1H2O have very similar structures, with four noncoordinated water molecules in the former compound.Cu(II) is in elongated tetragonal surroundings and Mn(II) in octahedral surroundings.The radius of the first shell around Mn(II) is equal to 2.17 (2) Angstroem for MnCu*5H2O and 2.16 (2) Angstroem for MnCu*1H2O.Those data are consistent with the following structure: Mn(II) would be surrounded by six oxygen atoms; two of these atoms MnCu*1H2O belong to an oxamido group, one to a carboxylato group, and one to a water molecule in the equatorial plane; the oxygen atoms in the apical positions would belong to other carboxylato groups, forming a two- or three-dimensional network.The magnetic properties of NiCu were studied in the 4.2-300 K temperature range.They are typical of an isolated heteropair with a doublet-quartet energy gap 3J/2equal to -156 (3) cm^-1.The magnetic properties of MnCu*5H2O, down to 4 K, are characteristics of alternating bimetallic chains with a minimum in the χ_MT versus T plot, χ_M being the molar magnetic susceptibility per MnCu unit and T the temperature.At 2.3 K, χ_MT for MnCu*5H2O shows a sharp maximum due to a three-dimensional antiferromagnetic ordering.In contrast, the χ_MT versus T plot for MnCu*1H2O diverges when T approaches the critical temperature T_c = 14 K.Below T_c, MnCu*1H2O exhibits a spontaneous magnetization.The field dependences of the magnetization M for MnCu*5H2O and MnCu*1H2O have been compared.For both compounds, the saturation magnetization corresponds to a S = 2 resulting spin per MnCu unit.Moreover, MnCu*1H2O at 4.2 K exhibits an M = f(H) hysteresis loop characteristic of a soft magnet.The powder X-band EPR spectra of MnCu*5H2O and MnCu*1H2O have been recorded at various temperatures down to 15 K.The comparison of the line widths suggests that the mechanism of the magnetic ordering observed for MnCu*1H2O is of exchange interaction nature rather than of dipolar nature.The perspectives in the field of the molecular-based ferromagnets ar discussed.