Monoclinic polymorph of poly[[di-μ-aqua-triaquadi-μ-oxalato-barium(II)copper(II)] monohydrate]

A monoclinic polymorph of the title compound, {[BaCu(C2O4)2(H2O)5]·H2O}n, is reported. The structure is best described as a coordination polymer where the CuII and BaII centers are coordinated by five and nine O atoms, respectively, in capped quadratic antiprismatic and tetragonal pyramidal geometries. The polymerization arises due to the presence of bridging mono- and bidentate oxalate ligands as well as bridging water molecules. The crystal structure is consolidated by a three-dimensional network of hydrogen bonding.


Monoclinic polymorph of poly[[di-µ-aqua-triaquadi-µ-oxalatobarium(II)copper(II)] monohydrate]
Justin Nenwa, Michel M. Belombe, Boniface P. T. Fokwa and Richard Dronskowski S1. Comment 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(Bélombé et al., , 2006Belombe, Nenwa, Bebga et al., 2007;Bélombé, Nenwa, Mbiangué et al., 2007;Nenwa, 2004), and is formulated as .H 2 O} 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(C 2 O 4 ) 2 (H 2 O) 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(C 2 O 4 ) 2 (H 2 O)] 2complex 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/O15 i 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(C 2 O 4 ) 2 (H 2 O)] 2-, and variable coordination modes of the Ba II centers.

S2. Experimental
Compound (I) was obtained by mixing Ba(NO 3 ) 2 (0.31 g, 1.2 mmol, Riedel-de Haën, pure) and K 2 [Cu(C 2 O 4 ) 2 ].2H 2 O (0.18 g, 0.51 mmol), freshly prepared according to the method of Kirschner (1960), in warm water (60 °C; 100 ml). A solid precipitated immediately. The mixture was stirred for about 1 h at the same temperature and left to stand undisturbed over three days at ambient temperature. The blue prismatic crystals that formed were isolated by filtration, dried in air and one of these was used in the X-ray diffraction analysis.

S3. Refinement
All water-bound H atoms were first located in a difference Fourier map and then refined with distance restraints of O-H = 0.83 (3) Å with all U iso (H) freely refined. The highest peak and deepest hole in the final difference Fourier map are, respectively, 0.49 Å from atom H13B and 1.00 Å from Cu.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Ba 0.141583 (8)