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"Metal-organic coordination systems may exert a variety of specific properties including porosity, conductivity, or luminescence and have potential applications in various fields, from heterogeneous catalysis to supramolecular magnetism. Recently, these systems are exploited as molecular precursors by thermal decomposition for the preparation of mixed-metal oxide [1-3]. Since this synthetic route is relatively new and unexplored, there are few reported examples of nanomaterials obtained in this way. The heterobimetallic polymeric compounds [CaCr2(bpy)2(C2O4)4]·0.83H2O}n (1) and {[CaCr2(phen)2(C2O4)4]·0.33H2O}n (2) (bpy = 2,2'-bipyridine; phen = 1,10-phenanthroline) were synthesized and characterized. The crystal structure of 2 is reported: the [Cr(phen)(C2O4)2]– unit, through the two oxalate groups, acts as a chelating ligand towards Ca cations, resulting in one-dimensional (1D) double zigzag chains, formed of diamond-shaped units. The ability of compounds 1 and 2 to act as the single-source precursors for the formation of the CaII–CrIII oxides was explored by thermal analysis (TGA and DTA) in the stream of the synthethic air or nitrogen, powder X-ray diffraction and IR spectroscopy. The heat treatment of 1 and 2 resulted in the formation of mixed-metal oxide phases, CaCrO4, α-CaCr2O4 and β-CaCr2O4. Thermal processing of compounds at 1100 0C in the stream of air, led to the appearance of the orthorhombic β-CaCr2O4 phase as the major crystalline oxide, together with CaCrO4. By heating the samples up to 1100 0C in nitrogen flow, only β-CaCr2O4 phase was formed, whereas the heating at 1400 0C caused the crystallization of two polymorphs, α-CaCr2O4 and β-CaCr2O4. J.P. acknowledge financial help from the Croatian Academy of Sciences and Arts. This work has been supported by HAZU grant "XBroad – program za brzo i točno određivanje mikrostrukturnih parametara iz proširenja difrakcijskih linija polikristalnih uzoraka" and ""Nanokristalni oksidi prijelaznih metala: Mikrovalna sinteza i strukturna svojstva"""
abstracts
When dealing with the crystal structure solving of low-crystalline materials, it is of paramount importance to combine DFT geometry optimization with direct-space methods in order to find the correct structural model. Particularly, in the case of inorganic-organic hybrids prepared by solvothermal route, finite dimensional single crystal is very difficult to obtain. Hence, we present several successfully elucidated crystal structures from the powder XRD of the low-crystalline inorganic-organic hybrids: Cr(HL)Cl3, HL=Hbdmpza (1), [1] VO(C10H7COO)2 (2), VO(C14H9COO)2 (3). All of these structures have been solved from powder XRD data, either from laboratory or synchrotron source, by combining direct-space methods (simulated annealing), DFT geometry optimization, and constraint Rietveld refinement. The solid compound 1 was isolated in acetonitrile by starting CrCl3·9H2O and HL only in a concentrated solution, from which microcrystalline aggregates precipitated. Additionally, the presence of a carboxylate proton in HL stabilizes the structure by H-bonds, but also enables adsorption of moisture. Other two hybrids were synthesized in o-xylene by reacting vanadium (V) triisopropoxide with 1-naphthalenecarboxylic [2] or 9-anthracenecarboxylic acid. [2] They contain one-dimensional chain of corner-sharing tetrahedra in the case of VO(C10H7COO)2 (Figure 1(a), (c)), and corner-sharing octahedra for VO(C14H9COO)2 (Figure 1(b), (d)) oriented along orthorhombic/monoclinic c-axis, respectively. While VO(C14H9COO)2 exhibits bidentate bridging binding of organic moiety to the metal center, VO(C10H7COO)2 shows monodentate mode as evidenced from DFT and infrared spectroscopy. The obtained crystal structures were further verified by direct methods (ab initio approach). I. D. acknowledges financial support from the Unity through Knowledge Fund (www.ukf.hr) of the Croatian Ministry of Science, Education and Sports (Grant Agreement No. 7/13).
