Comparative study of two oxalate isomers with different dimensionalities

Abstract

We report here two bimetallic oxalate isomers with the same chemical formula [RbCr(C2O4)2(H2O)2], which have been synthesized respectively by a slow evaporation method at room temperature (compound I) [1], and under hydrothermal conditions (compound II) [2] with the same starting salts. Their structures show a several discrepancies, due probably to the synthetic conditions. Indeed, the compound I crystallizes in space group C2/m with the Cr, Rb atoms and one oxygen from water molecule lying on special positions. Moreover, the unique oxalate ligand forms a bridge between metal centers. The Cr atom is coordinated to 2 bidentate-chelating oxalates and 2 aqua ligands in a trans-conformation and any water molecule has been found around the 8-coordinated Rb atom, leading to a layered structure consists of alternating Rb and Cr polyhedra connected via the unique organic ligand. Whereas, the compound II crystallizes in space group P21/n, with all atoms located on general positions. Furthermore, two independent oxalato ligands exhibit different configurations, which one is pentadentate and the other is hexadentate with two different chelating modes. The very slightly distorted Cr octahedra consists of 2 bidentate-chelating oxalato ligands and 2 water molecules in a cis-conformation, while the alkali metal is surrounded by seven O atoms from oxalate groups, completed with two H2O molecules which are bridging the Cr and Rb polyhedra by one common edge. This results in the formation of three different chains of alternating edge-and vertexshared polyhedra through oxalates groups and aqua ligands, running along the three space directions to build a three dimensional framework. These two compounds can be considered as supramolecular isomers [3].

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“…In the hetero-nuclear approach, with the simplest ligands (n=0, 1) stemming from a,w aliphatic dicarboxylic acids, conventional synthetic route under room temperature and hydrothermal method are currently used. They allowed single crystal's growth of hetero-bimetallic compounds, essentially when the starting materials are salts, and in the case of transition metals mixed with alkaline or alkaline-earth metals [6,8,9]. When the connectors are oxides or hydroxides, and with f, d or s block's metals, one obtains supra-molecular homo-metallic materials, as for the Babased material, and (2).It seems that the number of aqua ligands [4] affects the assembly process contrary to the temperature of Commun.…”

“…This entity, if subjected to thermal decomposition, can be a soft chemical way for replacing the ceramic method used generally in perovskite-like materials. Unfortunately, we could not achieve hetero-bimetallic complexes such as those which contain transition metals [8,9]; however, we have obtained mainly homo-metallic compounds. Herein, we present correlative thermal behaviours and specific crystalline features of new rare-…”