Download citation
Acta Cryst. (2014). A70, C656
Download citation

link to html
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 vertex-shared 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].

Download citation
Acta Cryst. (2014). A70, C1232
Download citation

link to html
By applying soft synthesis conditions, in presence of a mixture that contains malonic acid, (3 mmol), lanthanide salt CeCl3.7H2O (1 mmol), alkaline-earth hydroxide Ca(OH)2 (1 mmol), and at pH below 4, we have obtained single crystals of a novel rare-earth complex which includes the dianion [L2-], {[Ce2(C3H2O4)3(H2O)3].2H2O}.The given formula was deduced from the single crystal structure analysis. The asymmetric unit corresponds to chemical formula (Figure 1). This novel polymeric compound is a three-dimensional MOF, built up from cross-linked infinite chains of one-edge-shared CeO7(H2O)3 and CeO9 polyhedrons embedding solvent molecules. Each crystallographically different metal is connected to two neighbouring ones, through four μ2-oxo bridges, to form repeat four-membered typical Ce/O/Ce/O rings. The three independant ligands show different conformations of their end functional groups: syn-syn, syn-anti, anti-anti, giving explanation for the concomitant magnetic interactions and interesting magneto-structural data. These studies are still in progress. The TG curve shows five successive weight losses. The material begins losing weight directly upon starting the thermal gravimetric experiments. It is one of the more unstable lanthanide malonates known. The correlative thermal behaviour data and X-Ray structural sub-features of the novel Ce(III)-based MOF obtained, brought out the great supramolecular effects: decarboxylation process, crystalline, and thermal stabilities are much more related to the specific infinite hydrogen patterns, than the size, shape, and embedding capability of voids accommodated in the 3D packing.
Follow Acta Cryst. A
Sign up for e-alerts
Follow Acta Cryst. on Twitter
Follow us on facebook
Sign up for RSS feeds