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Homo- and Heteroplynuclear Complexes Synthesis, Structures and Magnetism

The design and synthesis of new materials based on polynuclear transition metal complexes has been a very stimulating challenge for chemists nowadays, as the need for the materials that have more diversified and more sophisticated properties is constantly growing. A generally accepted strategy in synthesizing new materials consists in assembling (paramagnetic) metal centres in such a way that the resulting solid lattices, with their components properly arranged, exhibit desired physical characteristics: mechanical, optical, electric and especially, magnetic ones. The magnetic properties (ferro-, ferri- and antiferro-) of such systems depend mostly on the nature of the interacting metal ions and the bridging ligands, as well as on the whole structural architecture that can be created by the supramolecular arrangements of the building blocks [1, 2]. In designing homo- and heteropolynuclear complexes a very essential role belongs to the oxalate ion, for its ability to adopt bis(chelating) coordination mode as well as to mediate magnetic interactions between paramagnetic metal centres, these properties representing the basis for the intensive magneto-structural investigations of the oxalate-bridged compounds. A very frequently used approach for the synthesis of polynuclear oxalate-bridged species consists in using stable mononuclear anionic oxalate complexes as ligands toward the second metal ion, the approach known as -complexes as ligands- or -building-blocks chemistry-. In relation to the current magneto-structural studies on the homo- and heterometallic polynuclear transition-metal complexes in our laboratory, the heterotrinuclear oxalate-bridged complex [/Cu(mu-C_2O_4)(bpy)_2/_2Cr(C_2O_4)]NO_3 x H_2O (1) (bpy = 2, 2'-bipyridine) has been synthesized using the mononuclear anionic oxalate ligand [Cr(C_2O_4)_3]^3- as the building block [2]. Analysis of magnetic properties (between 1.8 and 290 K) for this compound showed the ferromagnetic exchange interaction between the Cu^II and Cr^III ions (J = +1.20 cm^-1). Two novel heterometallic complexes [/Cu(bpy)_2/_2(mu-C_2O_4)][Cu(bpy)_2(mu-C_2O_4)NbO(C_2O_4)_2]_2 x 0.5bpy x 7H_2O (2) and [/Ni(bpy)_2/_2(mu-C_2O_4)]_3[NbO(C_2O_4)_3]_2 x 24H_2O (3), with the oxalate-bridged dinuclear Cu^IICu^II and Cu^IINb^V units in 2 and the oxalate-bridged dinuclear Ni^IINi^II units in 3, have been prepared using [NbO(C_2O_4)_3]^3- as the initial reagent [3]. The magnetic susceptibility measurements (between 1.8 and 290 K) showed the ferromagnetic exchange interaction between the copper(II) atoms in the Cu^IICu^II unit in 2 (J = +7.9 cm^-1), and the antiferromagnetic interaction between the nickel(II) atoms in the Ni^IINi^II unit in 3 (J = -35.7 cm^-1). Except for the magnetic susceptibility and single crystal X-ray diffraction analysis, the novel compounds have been characterized by means of infrared spectroscopy and thermogravimetric measurements.

crystal structure; magnetism; oxalate-bridged complexes

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