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The perovskite family of oxides is very attractive area of research in material science community because of the diversity of their physical and chemical characteristics, which offers opportunity to explore new functionalities and potential applications, such as photocatalysis, light harvesting, superconductivity, magnetoresistance, ferroelectric, multiferroic or dielectric performance. In general complex perovskite oxides with A2B'B''O6 or A3B'B''B'''O9 formula, where A are alkali, alkaline earth or rare earth elements, while the B', B'' and B''' cations can be any transition element, have crystal structure that consists of the corner sharing B'O6, B''O6 and B'''O6 octahedra, and A atoms located in the space between the octahedra. A variety of devices and methods have previously been used to prepare and characterize perovskite compounds, however most common method of their preparation is solid state reaction at high temperatures. Herein we propose novel pathway for the synthesis of perovskite-type oxides through molecular precursor-to-material approach, which uses heterometallic complexes as precursors for oxides. Thermal decomposition of the mixture of the well-defined and structurally characterized heterometallic oxalate-based compounds {; ; ; Ba2(H2O)5[NbO(C2O4)3]HC2O4}; ; ; ·H2O and [Ni(bpy)3]2[TaO(C2O4)3]Cl·12H2O leads to highly crystalline Ba3(NiNbTa)O9 oxide. We have also shown that Ba2NbCrO6 perovskite-type oxide can be obtained in thermal treatment of {; ; ; Ba2(H2O)5[NbO(C2O4)3]HC2O4}; ; ; ·H2O and (NH4)3[Cr(C2O4)3]·3H2O. The phase formation and structural ordering of oxides obtained by modified molecular precursor- to-material method has been investigated through a combination of thermal analysis (TGA and DTA), powder X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectroscopy. We have shown that ratio of metals can be finely tuned with the careful choice of molecular precursors as well as by appropriate amount of initial heterometallic complexes taken for the thermal decomposition reaction. The preparation strategy described here could be extended for the synthesis of various other complex perovskites at relatively mild conditions.

The area of heteropolynuclear metal complexes has attracted considerable interest not only due to their different architectures and topologies, but also for their potential applications in various fields from heterogeneous catalysis, gas storage and separation to supramolecular magnetism. Although the most used metals for the preparation of these complexes are the 3d first row transition, there are also many systems containing two or more different 3d and 4d metal ions. Diamagnetic ions such as Cl−, O2−, due to their small size, or closed-shell metal ions are usually good spin linkers, and generally cause considerable delocalization between magnetic centres. Nevertheless, the degree of delocalization depends strongly on the overlap between the magnetic orbitals on the metal and valence orbitals of ligands. Molecular ions such as oxalate, acetate, azide, etc., and conjugated bridges display low-lying excited valence states and produce larger spin and charge polarization effects. In the search for suitable synthetic routes leading to novel and more diversified polynuclear species, we have introduced the tris(oxalato)oxoniobate(V) anion, [NbO(C2O4)3]3− as new building block toward different metal centres. Special feature of this anion is that it can form heterometallic complexes through the bridging oxo (μ-O) or oxalate group (μ-C2O4). A novel heterotetranuclear oxo-bridged compound [Cr2(bpy)4(μ-O)4Nb2(C2O4)4]·3H2O (1) (bpy = 2, 2′-bipyridine) has been prepared and, in addition to the single-crystal X-ray study, characterized by magnetization measurements supported by DFT calculation. Compound 1 containing a square shaped {; ; ; ; Cr2(μ-O)4Nb2}; ; ; ; core exhibits relatively strong antiferromagnetic coupling between two chromium(III) ions mediated by two triatomic –O–NbV–O– diamagnetic bridges. In many cases, the magnetic interaction in the polynuclear complexes, based on metal centres and bridging ligands can be predicted ; in compound 1 any significant exchange coupling has not expected due to long intramolecular distances of ~7.4 Å between CrIII centres bridged by three diamagnetic atoms. In order to examine the role that metal centres, coordination sphere and crystal packing plays in modulating the magnetic properties of compound 1, additional new systems [CrIIINbV] of similar topology containing other aromatic ligands as 1, 10-phenathroline (phen) or terpyridine (terpy) have been prepared and structurally investigated: [Cr2(phen)4(μ-O)4Nb2(C2O4)4]·2H2O (2) and [Cr2(terpy)2(C2O4)2(μ-O)2Nb(C2O4)2][Cr(terpy)(C2O4)(H2O)]·3H2O (3). Exchange interaction between paramagnetic metal centres bridged by diamagnetic metal ion are known but are very rare, so the full comprehension of how the unpaired electrons of the metal centres feel each other through diamagnetic transition metal atom is still challenge.