supplementary materials

Bouayad et al. (1995) reported the structure of the title compound, (I), in the triclinic space group P-1. Herein, a new polymorph of (I) is reported which crystallizes in the monoclinic space group C2/c. It was obtained unintentionally from aqueous solution during an on-going study of oxalate-based multifunctional materials (Bélombé et al., 2003, 2006; Belombe, Nenwa, Bebga et al., 2007; Bélombé, Nenwa, Mbiangué et al., 2007; Nenwa, 2004), and is formulated as {[Ba(H₂O)₄][Cu(C₂O₄)₂(H₂O)].H₂O}_n. The two polymorphs structurally differ with respect to their crystal systems as well as in their coordination modes around the metal centers and in the formation of their lattice networks in the bulk.

The lattice network reported by Bouayad et al. (1995) was shown to be a coordination polymer where each oxalate ion acts as a bidentate ligand, coordinating the metal centers in three different modes: first with the "internal", then with the "external" O atoms linked, respectively to Cu^II and Ba^II centers (thus generating pentacyclic rings) and, finally, with one "internal" and one "external" oxalato-O atoms bound to a neighboring Ba atom (thus forming a tetracyclic ring). In that structure, each Cu^II ion is hexa-coordinated by six O atoms that define a highly distorted octahedral geometry. By contrast, in the monoclinic polymorph, the Cu^II atom is penta-coordinated in an approximately square pyramidal geometry defined by five O atoms, with the Cu site slightly displaced from the least-squares plane through the O1–O4 atoms towards the axial water-O10 atom (Fig. 1). Therein, the coordination sphere around each Ba^II center which assumes coordination number nine, as opposed to coordination number eleven in the triclinic polymorph, is emphasized. In the monoclinic form, the Ba site is located approximately at the center of a capped tetragonal antiprism, reminiscent of the geometry around the K⁺site in the salt K[Cr(C₂O₄)₂(H₂O)₂] (Bélombé et al., 2006). Selected geometric parameters for the monoclinic polymorph are listed in Table 1 and compare very well with the published data for the triclinic polymorph (Bouayad et al., 1995).

Taken individually, the [Cu(C₂O₄)₂(H₂O)]^2- complex anions are virtually the same but are connected differently in the triclinic and monoclinic polymorphs. In the monoclinic polymorph, these ions are interconnected into layers parallel to the (101) plane via O–H···O bridges which involve the uncoordinated water molecules (Fig. 2). The 3-D polymerization arises from the linkage of "external" oxalato-O atoms to neighboring Ba centers via mono- or bi-dentate coordination modes, and by single and double water bridges across the O10 and O15/O15ⁱ atoms, respectively (Table 2). The latter double bridge interconnects the next two neighboring Ba atoms, related by a center of inversion, with a Ba···Ba separation of 4.788 (2) Å.

In conclusion, the present study reveals that the unit cell symmetry in both structural polymorphs is basically dictated by the differing spatial orientations of the common anionic complexes, [Cu(C₂O₄)₂(H₂O)]^2-, and variable coordination modes of the Ba^II centers.