Tetraaquabis(2,3-dihydro-1,4-benzodioxine-2-carboxylato)calcium(II)

In the title coordination compound, the metal ion is bonded to two monodentate carboxylate groups and four water molecules.

In this work, we report the synthesis and the structure of the coordination properties of 1,4-benzodioxane 2-carboxylic acid toward calcium carbonate to afford the title compound Ca(C 9 H 7 O 4 ) 2 (H 2 O) 4 .
The crystal structure of the title compound has monoclinic symmetry with half a molecule in the asymmetric unit, the other half being generated by a crystallographic inversion center. The calcium ion is bonded to four aqua ligands and two 1,4-benzodioxane 2-carboxylate ligands, whose carboxylate groups link to the central atom in data reports monodentate mode (Fig. 1). The Ca1-O1, Ca1-O5, and Ca1-O6 bond lengths are 2.304 (2), 2.358 (2) and 2.317 (2) Å , respectively. The dioxane ring adopts a half-chair conformation with the pendant carboxylate group in an axial orientation. In the arbitrarily chosen asymmetric unit, C2 has an R configuration but crystal symmetry generates a racemic mixture.
In the crystal, O-HÁ Á ÁO hydrogen bonds link the molecules into (010) sheets with the acceptor O atoms being parts of carboxylate groups (O1 and O2) and the dioxane ring (O4) and the packing is consolidated by weak C-HÁ Á ÁO interactions (Fig. 2, Table 1).

Synthesis and crystallization
In a 100 mL two-necked flask, anhydrous CaCO 3 (0.0100 g, 0.100 mmol) was dissolved in deionized water (20 mL) by heating to 338 K, and a solution of 1,4-benzodioxane-2-carboxylic acid (0.0560 g, 0.200 mmol) dissolved in distilled water (10 mL) was added dropwise at 353 K. The reaction mixture was refluxed for 2 h and then concentrated under vacuum to 10 mL. The precipitate obtained upon cooling overnight was filtered off and washed with cold distilled water. Colorless crystals suitable for X-ray analysis were grown from a warm water-methanol mixed solvent mixture (1:1) at room temperature. Yield: 0.0362 g (55%), m.p. 501-505 K.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2.

Figure 1
The molecular structure of the title compound with displacement ellipsoids drawn at the 50% probability level. Unlabeled atoms are generated by the symmetry operation 1 À x, 1 À y, 2 À z.

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