Crystal structure of catena-poly[[cadmium(II)-di-μ2-bromido-μ2-l-proline-κ2 O:O′] monohydrate]

In the title salt, crystalline water molecules serve as donors for the weak intermolecular O—H⋯O and O—H⋯Br hydrogen bonds which link adjacent polymeric chains.


Chemical context
The characterization of second-order non-linear optical (NLO) materials is important because of their potential applications such as frequency shifting, optical modulation, optical switching, telecommunication and signal processing. It is known that the chiral amino acids and their complexes are potential materials for NLO applications (Eimerl et al., 1989;Pal et al., 2004;Srinivasan et al., 2006). This study is a part of an ongoing investigation of the crystal and molecular structures of a series of amino acid-metal complexes (Sathiskumar et al., 2015;Balakrishnan et al., 2013).
In (I), as observed in the chloride analogue (Yukawa et al., 1983), there is an intramolecular N1-H1AÁ Á ÁO2 hydrogen bond between the amino group and the carboxylate fragment.

Figure 2
The crystal packing of (I) viewed along the a axis. Dashed lines denote intermolecular hydrogen bonds. C-bound H atoms have been omitted for clarity.

Figure 3
A portion of the crystal packing viewed along the a axis and showing hydrogen bonds (dashed lines) between two neighbouring polymeric chains.

Synthesis and crystallization
To prepare the title compound, l-proline (Loba) and cadmium bromide tetrahydrate (Loba) in an equimolar ratio were dissolved in double-distilled water. The obtained solution of the homogeneous mixture was evaporated at room temperature to afford the white crystalline title compound, which was then recrystallized by slow evaporation from an aqueous solution.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. As the title compound is isotypic with its chlorido analogue (Yukawa et al., 1983), the atomic coordinates of the latter were used as starting values in the initial cycles of the refinement. The positions of water hydrogen atoms were calculated by method of Nardelli (1999). Further, the O-H and H1WÁ Á ÁH2W distances of the water molecules were restrained to 0.85 (2) and 1.38 (2) Å , respectively, using the DFIX option and included in the structurefactor calculations with U iso (H1W/H2W) = 1.1U eq (O1W). The remaining hydrogen atoms were placed in geometrically idealized positions (C-H = 0.97-0.98 Å and N-H = 0.89 Å ) with U iso (H) = 1.2U eq (C/N) and were constrained to ride on their parent atoms. Reflections 110 and 020 were partially obscured by the beam stop and were omitted.  Data collection: APEX2 (Bruker, 2008); cell refinement: APEX2 and SAINT (Bruker, 2008); data reduction: SAINT and XPREP (Bruker, 2008); program(s) used to solve structure: atomic coordinates of chlorido analogue (Yukawa et al., 1983) used as starting values in the initial cycles of the refinement; program(s) used to refine structure: SHELXL2014/6 (Sheldrick, 2015); molecular graphics: PLATON (Spek, 2009) and Mercury (Macrae et al., 2008).

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.