Aquabis(4-nitrobenzoato)-κ2 O,O′;κO-(piperidinium-4-carboxylato-κ2 O,O′)cadmium(II)

In the mixed ligand title compound, [Cd(C6H11NO2)(C7H4NO4)2(H2O)], which exhibits a discrete mononuclear structure, the CdII atom is in a distorted octahedral geometry, surrounded by five carboxylate O atoms and one coordinated water molecule. The piperdinium ring adopts a chair conformation and the two 4-nitrobenzoate rings are oriented at a dihedral angle of 75.8 (1)°. Intermolecular O—H⋯O and N—H⋯O hydrogen bonds link the mononuclear entities into a three-dimensional supramolecular network.

In the mixed ligand title compound, [Cd(C 6 H 11 NO 2 )(C 7 H 4 -NO 4 ) 2 (H 2 O)], which exhibits a discrete mononuclear structure, the Cd II atom is in a distorted octahedral geometry, surrounded by five carboxylate O atoms and one coordinated water molecule. The piperdinium ring adopts a chair conformation and the two 4-nitrobenzoate rings are oriented at a dihedral angle of 75.8 (1) . Intermolecular O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds link the mononuclear entities into a three-dimensional supramolecular network.

Comment
Recently, the rational design and skillful construction of the coordination complexes with mixed ligands have aroused great interest due to their intriguing framework topologies and potential applications in ino-exchange (Muthu et al., 2002), catalysis (Fujita et al., 1994), luminescence (Zheng et al., 2004), and gas storage (Rosi et al., 2003). Bearing two potential binding sites (-NH-and -COOH) capable of coordination with transition metal atoms, 4-piperidinecarboxylic acid exists as a zwitterion with the amino group protonated and the carboxylic group deprotonated in aqueous solution (Mora et al., 2002).
Thus, by carefully control the degree of the protonation/deprotonation of carboxylic and/or amino groups, it can selectively act as a bridge ligand linking different metal atoms into an infinite high-dimensional framework or as a terminal ligand forming a discrete complex (Inomata et al., 2002). However, to the best of our knowledge, only a few examples involved in 4-piperidinecarboxylic acid have been reported by far (Adams et al., 2006a,b;Podesta et al., 2002). Thus, to continue to explore the coordination behavior of 4-piperidinecarboxylic acid, herein, we report the crystal structure of a Cd II complex with 4-piperidinecarboxylate and 4-nitrobenzoxylate anion.
As shown in Fig. 1, the mononuclear structure of the title complex, (I), consists of one crystallographic independent Cd II atom, two separate 4-nitrobenzoxylate anions, one zwitterionic 4-piperidinecarboxylate molecule, and one coordinated water molecule. The sole Cd II center is six-coordinated by one coordinated water molecule (O11), and five carboxylate O atoms from two independent 4-nitrobenzoxylate (O1, O2 and O5) and one 4-piperidinecarboxylate molecule (O9 and O10), exhibiting a distorted octahedral geometry (Table 1.). The Cd-O bond distances are in the range of 2.1914 (16)-2.3684 (17)Å, which are comparable to those previously reported values (Inomata et al., 2004;Wang et al., 2008). The carboxylate group of 4-nitrobenzoxylate anion presents two different coordination modes: monodentate and asymmetric chelating bidentate fashions. In contrast, the carboxylate group of 4-piperidinecarboxylate only adopts an asymmetric chelating bidentate coordination mode. Additionally, the N atom of piperidine ring does not coordinate to a metal atom, and the protonated piperidine ring of 4-piperdinecarboxylate is in a chair conformation significantly resulted from the relatively lower energy (Delgado et al., 2001). To an aqueous solution (5 ml) of CdCl 2 . 2.5H 2 O (45.7 mg, 0.2 mmol) was slowly added a methanol solution (5 ml) containing 4-piperidinecarboxylic acid (12.9 mg, 0.1 mmol) and 4-nitrobenzoic acid (16.7 mg, 0.1 mmol) with constant stirring. And the pH value of the mixture was adjusted to 6 by NaOH solution (0.1 M). The resulting colorless solution was further stirred for half an hour and filtered. The filtrate was allowed to evaporate at room temperature. Pale-yellow block-shaped crystals were obtained within two weeks (yield 70% based on Cd II salt).

Refinement
The H atoms of the water molecule were located in the final difference Fourier map, their positions were refined and their isotropic displacement parameters were set to 1.5 times the equivalent displacement parameter of the O atom. H atoms of the NH 2 + and the piperidine ring were placed in geometrically calculated positions and their isotropic displacement parameters were set to 1.2 times the equivalent displacement parameter of their parent atoms.
Figures Fig. 1. The molecular structure of (I). Displacement ellipsoids are drawn at the 30% probability level.    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 Rfactors(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.