Poly[di-μ-aqua-diaquabis(μ7-oxalato-κ9 O 1:O 1:O 1,O 2:O 2:O 2′:O 2′,O 1′:O 1′)calciumdicaesium]

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.


Comment
In  Fig. 2, the alkaline atom is nonacoordinated by seven O atoms from six oxalate ligands, one bridging water and one terminal water molecule, while the alkaline-earth atom, which is located on a twofold rotation axis, is bound to two equivalent oxalate groups and two water molecules, the later being in a cis arrangement (Fig. 2). In the CaO 4 (H 2 O) 2 octahedron, the organic ligands are coordinate with the metal centre in an expected η 4 chelation, observed usually with this rigid ligand. The two resulting fivemembered rings are perpendicular, the dihedral angle being of 89.76 (6)°. The equatorial plane is defined by two equivalent aqua ligands in cis position and two equivalent O3 atoms, while the axial positions are occupied by O1 atoms.
The maximum atomic deviation from the mean plane [O2w, Ca1, O3, O3 iii , O2w iii ; symmetry code: (iii) -x, y, -z+1/2] is 0.165 (2) Å for the O3 donor. In connecting the Cs and Ca polyhedra, the oxalate ligand acts as bis-chelate, forming fivemenbered rings with the both cations. Moreover, it bridges the metal atoms via all its O atoms, and is surrounded by six Cs and one Ca atoms through the two carboxylate groups. One group (O1, C1, O2) bridges five atoms (4 Cs and 1 Ca) and adopts an unusual µ 5 :η 3 -η 2 coordination mode; the other group (O3, C2, O4) bridges four atoms (3 Cs and 1 Ca) and adopts a common µ 4 :η 2 -η 2 coordination mode. As a result, the oxalate dianion displays an interesting nonadenticity in its chelating and bridging coordination modes, involving three triply bonding O atoms (O2, O3 and O4) and a tetrabonding one (O1). In the overall packing, the Cs polyhedra, linked alternately by one edge (O1···O1) and one corner (O2 atom) in a one-dimensional ladder running approximately along the [110] direction, are interconnected by the vertices to generate a two-dimensional network drawing small hexagonal cavities (Fig. 3). The adjacent pseudo-layers thus obtained, are  Fig. 4).

Experimental
The synthetic preparation was carried out in an aqueous solution at room temperature. The starting materials, caesium carbonate, trivalent chromium oxide and oxalic acid dihydrate, in equal stoechiometric amounts (0.5 mmol) were dissolved in deionized water (20 ml) and the resulting dark pink solution was stirred for two hours. The title compound crystallizes from the slowly evaporated solution after about one month and then filtered from chromium oxide powder. A few light pink crystals with prismatic geometry and suitable size for X-ray analysis were found trapped in colourless crystals of oxalic acid, then isolated, filtered and washed with diethyl ether. The light pink colour, which can not be removed after several washing, shows that small Cr(III) inclusion is present but not incorporated in the structure.

Refinement
The H atoms were located in a difference Fourier map and were refined isotropically. The highest electron density in the final difference Fourier map is 0.76 Å from Cs1 and the deepest hole is 0.72 Å from Cs1.

Computing details
Data

Figure 1
The chain built of Cs polyhedra and Ca octahedra.

Figure 2
The metal atom environments of the title compound. Displacement ellipsoids are drawn at the 50% probability level.