engleski

Structural and magnetic investigations of [CrIIINbV] compounds ; antiferromagnetic couplinf of CrIII through –O–NbV–O– bridges

With the aim to control the magnetic properties of the heteropolynuclear complexes, Lt–M–Lb–M'–Lt (Lb = bridging ligand, Lt = terminal ligand), many strategies have been considered focusing on alternating the chemical nature of the metal center (M and/or M') and bridging ligands (Lb) and their intramolecular arrangement (Lb–M and Lb–M' bond lengths and M–Lb–M' bond angles). In particular, the delocalization between the magnetic centers can be improved by using long conjugated bridges, which lead to low energy ππ* excitations, small diamagnetic spin linkers such as Cl−, O2−, or closed-shell metal ions, which allow a direct M–M magnetic coupling. Thus, the main parameter which enables the control of magnetic properties is the strength of the orbital overlap between the metal and ligand. For the synthesis of novel and more diversified polynuclear species we often use the tris(oxalato)oxoniobate(V) anion, [NbO(C2O4)3]3− as a building block toward other metal centers. A special feature of this anion is that it can form heterometallic complexes through the bridging oxalate (μ-C2O4) and/or oxo (μ-O) group. In our previous work we reported compound [Cr2(bpy)4(μ-O)4Nb2(C2O4)4]·3H2O (bpy = 2, 2'-bipyridine) which crystallizes in the orthorhombic Pcab space group and contains a square shaped {; ; ; Cr2(μ-O)4Nb2}; ; ; core, exhibiting relatively strong antiferromagnetic coupling between two CrIII ions mediated by two –O–NbV–O– diamagnetic bridges [1]. The occurrence of this unexpected magnetic exchange interaction between two paramagnetic centres (separated more than 7.4 Å) inspired us to investigate how the change of the aromatic N-donor ligands will affect the structural and magnetic properties of new compounds. From the reaction of aqueous solutions of tris(oxalato)oxoniobate(V) anions and bis(phenathroline)chromium(III) cations, compound [Cr2(phen)4(μ-O)4Nb2(C2O4)4]·2H2O (1) has been obtained (phen = 1, 10-phenanthroline), crystallizing in tetragonal P42212 space group. Using tridentate 2, 2′:6′, 2′′-terpyridine (terpy) instead of bidentate ligands in similar procedure, compounds [Cr2(terpy)2(H2O)2(μ-O)4Nb2(C2O4)4]·4H2O (2) and [Cr(terpy)(C2O4)(H2O)][Cr2(terpy)2(C2O4)2(μ-O)2Nb(C2O4)2]·3H2O (3) have been isolated (monoclinic P21/n and triclinic space groups, respectively). Heteropolynuclear compounds 1–3 have been investigated by IR spectroscopy, single-crystal X-ray diffraction and magnetization measurements.

Oxo-Bridged ; Chromium(III) ; Niobium(V) ; Antiferromagnetic Coupling

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