Poly[triaqua(μ-butane-1,2,3,4-tetracarboxylato)dicadmium(II)]

The asymmetric unit of the title CdII coordination polymer, [Cd2(C8H6O8)(H2O)3]n, contains two crystallographically independent CdII cations, one-half each of two independent anionic butane-1,2,3,4-tetracarboxylate units (L) and three water molecules. Both anionic units lie on inversion centers. One of the CdII ions is six-coordinated by four carboxylate O atoms from four L anions and two water O atoms in a distorted octahedral coordination environment. The other CdII ion is eight-coordinated by seven carboxylate O atoms from four L anions and one water O atom. The anionic units bridge neighboring CdII centers, forming a three-dimensional framework. O—H⋯O hydrogen-bonding interactions between the water molecules and carboxylate O atoms further stabilize the structure.


S1. Comment
Over years of intensive studies on tetracarboxylate ligands, transition metals and poly-carboxylates, a vast amount of data have been acquired. As an important family of multidentate O-donor ligands, organic tetracarboxylate ligands, such as 1,2,4,5-benzenetetracarboxylate, have been extensively employed in the preparation of such metal-organic compound . In this regard, butane-1,2,3,4-tetracarboxylatic acid (H 4 L) is also a good ligand in coordination chemistry due to its strong coordination ability and versatile coordination modes, so much attention has been paid to it in recent years (Liu et al., 2008). In this contribution, H 4 L was selected as a bridging ligand, and a new cadmium coordination polymer, namely [Cd 2 (L)(H 2 O) 3 ], was obtained.
As shown in Fig. 1, the asymmetric unit of the title Cd II coordination polymer, contains two crystallographically independent Cd II cations, one-half each of two independent L anions and three water molecules. The L anions lie on inversion centers. One of the Cd II ion, Cd1, is six-coordinated by four carboxylate oxygen atoms from L anions and two water oxygen atoms in a distorted octahedral coordination environment. Atom Cd2 is eight-coordinated by seven carboxylate oxygen atoms from L anions and one water oxygen atom. The L anions bridge neighboring Cd II centers to form a complicated three-dimensional framework structure (Fig. 2). The hydrogen-bonding interactions between water molecules and carboxylate oxygen atoms further stabilize the three-dimensional framework structure of the title compound.

S2. Experimental
A mixture of CdCl 2 .2H 2 O (0.10 mmol), H 4 L (0.05 mmol) and water (12 ml) was sealed in a Teflon reactor (15 ml), which was heated at 413 K for 3 d and then gradually cooled to room temperature. Purple crystals of the title compound were isolated (yield 68% based on Cd).

S3. Refinement
H atoms bonded to C atoms were positioned geometrically (C-H = 0.97 or 0.98 Å) and refined as riding, with U iso (H) = 1.2U eq (carrier). The water H atoms were located in a difference Fourier map, and were refined with distance restraints of O-H = 0.85 (1) Å and H···H = 1.35 (1) Å; their U iso values were tied to those of parent atoms by a factor of 1.5.  Displacement ellipsoids drawn at the 30% probability level. Symmetry codes:

Figure 2
View of the three-dimensional framework structure of the title compound.

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