Correlative studies of three oxalato-bridged polymers, obtained under hydrothermal conditions for the two isostructural compounds {Rb(HC2O4)(H2C2O4)(H2O)2}∞1, 1, {H3O(HC2O4)(H2C2O4).2H2O}∞1, 2, and by conventional synthetic method for {Rb(HC2O4)}∞3, 3, allowed the identification of H-bond patterns and structural dimensionality. Ferroïc domain structures are confirmed by electric measurements performed on 3. Although 2 resembles one oxalic acid sesquihydrate, its structure determination doesn’t display any kind of disorder and leads to recognition of a supramolecular network identical to hybrid s-block series, where moreover, unusual H3O+ and NH4+ similarity is brought out. Thermal behaviors show that 1D frameworks with extended H-bonds, whether with or without a metal center, have the same stability. Inversely, despite the dimensionalities, the same metallic intermediate and final compounds are obtained for the two Rb+ ferroïc materials.

The title compound, [CrRb(C(2)O(4))(2)(H(2)O)(2)](n), obtained under hydrothermal conditions and investigated structurally at 100 K, is a three-dimensional supramolecular isomer of the layered structure compound studied at room temperature. This novel polymer is built up from crosslinked heterobimetallic units. The linkage of alternating edge- and vertex-shared RbO(7)(H(2)O)(2) and CrO(4)(H(2)O)(2) polyhedra running along three different directions gives a dense packing. The two independent ligands display two η(4)-chelation modes and two conventional carboxylate bridges. However, the pentadentate ligand connects the Cr(III) and Rb(I) ions through one O-atom bridge, while the hexadentate ligand exhibits an additional η(3)-chelation and two O-atom bridges. Each coordinated water molecule forms an O-atom bridge between the two metals. Moreover, in the absence of protonated ligands, these water molecules act as donors through their four H atoms in strong-to-weak hydrogen bonds. This results in zigzag chains of alternating oxalate and aqua ligands parallel to the twofold screw axis. The six double oxalates known to date containing an alkali and Cr(III) all present layered two-dimensional structures. In the series, this supramolecular isomer is the first three-dimensional framework.

In the title compound, [CaCs2(C2O4)2(H2O)4]n, the Ca2+ ion, lying on a twofold rotation axis, is coordinated by four O atoms from two oxalate ligands and two bridging water mol­ecules in an octa­hedral geometry. The Cs+ ion is coordinated by seven O atoms from six oxalate ligands, one bridging water and one terminal water mol­ecule. The oxalate ligand displays a scarce high denticity. The structure contains parallel chain units runnig along [10-1], formed by two edge-sharing Cs polyhedra connected by CsO9 polyhedra connected by a face-sharing CaO6 octahedron. These chains are further linked by the oxalate ligands to build up a three-dimensional framework. O—H⋯O hydrogen bonds involving the water mol­ecules and the carboxyl­ate O atoms enhance the extended structure.

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].

Single crystals of two new bimetallic oxalate compounds with the formula [ACr(C 2 O 4 ) 2 (H 2 O) 4 ] n (A = Li or Na), namely catena-poly [[diaqualithium(I)], (II), have been synthesized, characterized and their crystal structures elucidated by X-ray diffraction analysis and compared. The compounds crystallize in the monoclinic space group C2/m for (I) and in the triclinic space group P1 for (II); however, they have somewhat similar features. In the asymmetric unit of (I), the Li and Cr atoms both have space-group-imposed 2/m site symmetry, while only half of the oxalate ligand is present and two independent water molecules lie on the mirror plane. The water O atoms around the Li atom are disordered over two equivalent positions separated by 0.54 (4) Å . In the asymmetric unit of (II), the atoms of one C 2 O 4 2À ligand and two independent water molecules are in general positions, and the Na and Cr atoms lie on an inversion centre. Taking into account the symmetry sites of both metallic elements, the unit cells may be described as pseudo-facecentred monoclinic for (I) and as pseudo-centred triclinic for (II). Both crystal structures are comprised of one-dimensional chains of alternating trans-Cr(CO) 4 (H 2 O) 2 and trans-A(CO) 4 (H 2 O) 2 units 2 -bridged by bis-chelating oxalate ligands. The resulting linear chains are parallel to the [101] direction for (I) and to the [111] direction for (II). Within the two coordination polymers, strong hydrogen bonds result in tetrameric R 4 4 (12) synthons which link the metal chains, thus leading to two-dimensaional supramolecular architectures. The two structures differ from each other with respect to the symmetry relations inside the ligand, the role of electrostatic forces in the crystal structure and the molecular interactions of the hydrogen-bonded networks. Moreover, they exhibit the same UV-Vis pattern typical of a Cr III centrosymmetric geometry, while the IR absorption shows some differences due to the oxalate-ligand conformation. Polymers (I) and (II) are also distinguished by a different behaviours during the decomposition process, the precursor (I) leading to the oxide LiCrO 2 , while the residues of (II) consist of a mixture of sodium carbonate and Cr III oxide.