Diaquabis(2-oxo-2H-chromene-3-carboxylato-κ2 O 2,O 3)cadmium

In the title mononuclear cadmium complex, [Cd(C10H5O4)2(H2O)2], the CdII atom, located on a crystallographic inversion center, exhibits a slightly distorted octahedral geometry and is six-coordinated by two O atoms from water molecules in the axial positions and four O atoms from two deprotonated coumarin-3-carboxylic acid ligands in the equatorial plane. Angles around the CdII atom vary between 81.00 (5) and 99.00 (0)°. The Cd—O bond lengths vary between 2.1961 (13) and 2.3360 (13) Å. O—H⋯O hydrogen bonds between the H atoms of coordinated water molecules and the O atoms of carboxylate groups link the complex molecules into layers parallel to the ab plane.

In the title mononuclear cadmium complex, [Cd(C 10 H 5 O 4 ) 2 -(H 2 O) 2 ], the Cd II atom, located on a crystallographic inversion center, exhibits a slightly distorted octahedral geometry and is six-coordinated by two O atoms from water molecules in the axial positions and four O atoms from two deprotonated coumarin-3-carboxylic acid ligands in the equatorial plane. Angles around the Cd II atom vary between 81.00 (5) and 99.00 (0) . The Cd-O bond lengths vary between 2.1961 (13) and 2.3360 (13) Å . O-HÁ Á ÁO hydrogen bonds between the H atoms of coordinated water molecules and the O atoms of carboxylate groups link the complex molecules into layers parallel to the ab plane.

Comment
In the past decades, much attention has been paid to the design and synthesis of self-assembling metal complex systems with organic ligands containing O donors due to their fascinating structural diversity (Lin et al., 2010) and potential applications in the areas of catalysis (Bischof et al., 2010), magnetism (Ghoshal et al., 2007), gas adsorption (Li & Zhou, 2009), and luminescence (Chen et al., 2008). Coumarin-3-carboxylic acid is such a ligand and complexes containing it have been reported (Georgieva et al., 2007). Herein, we report the synthesis and crystal structure of a new mononuclear cadmium complex coordinated by coumarin-3-carboxylic acid.
The molecule of the title mononuclear cadmium(II) complex, [Cd(C 10 H 5 O 4 ) 2 (H 2 O) 2 ], occupies a special position with the metal center being located on a crystallographic inversion center. Each Cd II atom exhibits a slightly distorted octahedral geometry and is six-coordinated by two O atoms from water molecules in the axial positions and four O atoms from two deprotonated coumarin-3-carboxylic acid ligands in the equatorial plane. Angles around the Cd II atom vary between 81.02 (6)° and 98.98 (8)°. The Cd-O bond distances between the Cd II atom and the O atoms vary between 2.196 (2) and 2.336 (2) Å, all of which are comparable to those reported for other cadmium-oxygen donor complexes (e.g., ).
The (C1C2C3C4C5C6) ring and the (C6C5C7C8C9O1) ring are almost coplanar, and the dihedral angles is 1.673 (5)°. The dihedral angle between The C8C9C10O2 plane and the O2O3Cd1 plane is 28.541 (7)°. O-H···O hydrogen bonds between the hydrogen atoms of coordinated water molecules and the O atoms of carboxyl groups joins the complexes into two-dimensional layers parallel the ab plane (Table 1, Fig. 2).

Experimental
The title complex was synthesized by carefully layering a solution of Cd(NO 3 ) 2 .4H 2 O (30.8 mg, 0.1 mmol) in ethanol (10 ml) on top of a solution of coumarin-3-carboxylic acid (19.0 mg, 0.1 mmol) and LiOH (8.4 mg, 0.2 mmol) in H 2 O (10 ml) in a test-tube. After about one month at room temperature, colorless block-shaped single crystals suitable for X-ray investigation appeared at the boundary between the ethanol solution and the water layer with a yield of 23% (12.1 mg).

Refinement
Carbon H atoms were placed geometrically (C-H = 0.93 Å) and treated as riding with U iso(H) = 1.2U eq (C). Water H atoms

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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 > 2sigma(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.