Crystal structure of poly[[μ2-diaqua-diaqua-μ2-l-proline-κ2 O:O′-strontium] dibromide]

In the title polymer, zwitterionic proline and water molecules interact with the bromide counter-anions through intermolecular N—H⋯Br and O—H⋯Br hydrogen-bonding interactions, providing a novel supramolecular structure.

In the title coordination polymer, {[Sr(C 5 H 9 NO 2 )(H 2 O) 4 ]Br 2 } n , the proline molecule exists in a zwitterionic form with one of the ring C atoms disordered over two sites [site-occupancy factors = 0.57 (6):0.43 (6)]. The Sr II ion is ninecoordinated by six water O atoms, two monodentate and two 2 -bridging, and three carboxylate O atoms of the proline ligands, with two bridging [Sr-O range = 2.524 (4)-2.800 (5) Å ]. In the crystal, there is no direct interaction between the proline molecules. However, the proline and water molecules associate with the bromide counter-anions through a number of intermolecular O-HÁ Á ÁBr and N-HÁ Á ÁBr hydrogen-bonding interactions, giving a threedimensional supramolecular structure.

Chemical context
The study of coordination polymers has been an area of rapid development in recent years due to their interesting structures and their wide range of applications as functional materials (Lyhs et al., 2012). Reports of the crystal structures of alkaline earth metal ions combined with anions of amino acids are very limited. As part of our ongoing investigations of the crystal and molecular structures of a series of metal complexes generated from amino acids (Revathi et al., 2015;Sathiskumar et al., 2015a,b;Balakrishnan et al., 2013), we report here the crystal structure of a polymeric strontium-proline complex, {[Sr(C 5 H 9 NO 2 )(H 2 O) 4 ] 2+ 2(Br À )} n , (I).

Structural commentary
The asymmetric unit of the title complex (I) contains one Sr 2+ ion, one bridging proline ligand and four water molecules, two of which are monodentate and two bridging, and two bromide ISSN 2056-9890 counter-anions (Fig. 1). In (I), the bond lengths involving the carboxylate atoms and the protonation of the amino group suggest that the proline molecule exists in a zwitterionic form. The Sr II ion is nine-coordinated by six water oxygen atoms [Sr-O = 2.582 (6)-2.707 (5)Å ] and three carboxylate oxygen atoms of zwitterionic proline ligands [Sr-O = 2.524 (4)-2.800 (4) Å ; Table 1]. In the strontium-glycine complex, the Sr-O (water) and Sr-O(carboxylate) distances ranges are 2.526 (4)-2.661 (2) and 2.605 (2)-2.703 (2) Å , respectively (Revathi et al., 2015). In (I), one of the carbon atoms (C4) of the pyrrolidine ring is disordered over two sites. In the major component of the pyrrolidine ring, there is a twist conformation on the C2-C5 bond with a pseudo-rotation angle Á = 40.1 (14) and a maximum torsion angle ' m = 43.8 (10) for the atom sequence N1-C2-C5-C4A-C3 (Rao et al., 1981). In the minor component, the pyrrolidine ring exhibits an envelope conformation on N1 with a pseudo-rotation angle Á = 341.5 (19) and a maximum torsion angle ' m = 36.0 (9) for the atom sequence N1-C2-C5-C4B-C3 (Rao et al., 1981). As shown in Fig. 2, the title complex forms a coordination polymeric chain that lies parallel to the a axis. Adjacent Sr II ions are separated by 3.9387 (7) Å within a chain.

Supramolecular features
The crystal structure of (I), is stabilized by intermolecular N-HÁ Á ÁBr and O-HÁ Á ÁBr hydrogen bonds ( Table 2). One of the characteristic features observed in amino acid complexes is the head-to-tail sequence in which amino acids are self-associated through their amino and carboxylate groups (Sharma et al., 2006;Selvaraj et al., 2007;Balakrishnan et al., 2013;Revathi et al., 2015). In the crystal structure of the l-proline lithium bromide monohydrate complex, there is a head-to-tail sequence observed (Sathiskumar et al., 2015a). In contrast, there is no direct hydrogen-bonding interaction between the proline molecules in (I). The coordination sphere of Sr 2+ in the crystal structure of (I). Only the major components of the disordered proline ligands are shown. Displacement ellipsoids are drawn at the 50% probability level. For symmetry codes, see Table 1. Symmetry codes: (i) x þ 1; y; z; (ii) x À 1 2 ; Ày þ 1 2 ; Àz þ 1; (iii) x þ 1 2 ; Ày þ 1 2 ; Àz þ 1.

Figure 2
The Sr-water coordination polymeric chain substructure of (I), with peripheral water O-HÁ Á ÁBr hydrogen bonds shown as dashed lines.
As shown in Fig. 3, two water molecules and two bromide anions along with Sr 2+ ions generate a hydrogen-bonded sheet which lies parallel to the a axis. Within this sheet, two Sr 2+ ions and two water oxygens form a cyclic motif. Water molecules (O3 and O4) interconnect the bromide anions, forming a chain. In (I), two molecules (O5 and O6) act as donors for intermolecular O-HÁ Á ÁBr hydrogen bonds. These hydrogen bonds generate a cyclic dibromide motif similar to that observed in a related structure (Revathi et al., 2015). Adjacent dibromide motifs in (I), which run parallel to the b axis, are interconnected by proline ligands through intermolecular N-HÁ Á ÁBr hydrogen bonds on both sides (Fig. 3). Adjacent supramolecular arrangements of cyclic dibromide Á Á ÁprolineÁ Á Ácyclic dibromide motifs are interlinked further by water molecules (O3 and O4) through O-HÁ Á ÁBr hydrogen bonds. This entire arrangement forms a butterfly-like structure. The overall hydrogen-bonded supramolecular structure (Fig. 4) is three-dimensional.

Synthesis and crystallization
Single crystals of the title complex were obtained by slow evaporation from an aqueous solution of l-proline and The butterfly-like supramolecular arrangements generated by intermolecular N-HÁ Á ÁBr and O-HÁ Á ÁBr hydrogen bonds. Only atoms involved in hydrogen-bonding interactions are labelled.

Figure 4
The crystal packing of (I) viewed along the a axis, with hydrogen bonds shown as dashed lines. C-bound H atoms have been omitted for clarity.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. One of the carbon (C4) atoms of the pyrrolidine ring appears to be disordered over two sites. These positions were defined for this atom and constrained refinement of the site-occupation factors led to a value of 0.57 (6) for the major component. The positions of amino and water H atoms were located from difference Fourier maps. Further, the O-H distances in the water molecules were restrained to 0.85 (2) Å . The N-H distances of amino group were also restrained, to 0.89 (2) Å . The remaining hydrogen atoms were placed in geometrically idealized positions (C-H = 0.97 Å with U iso (H) = 1.2U eq (C) and were constrained to ride on their parent atom. The Flack absolute structure parameter was determined to be 0.008 (8)  Data collection: APEX2 (Bruker, 2004); cell refinement: APEX2 and SAINT (Bruker, 2004); data reduction: SAINT and XPREP (Bruker, 2004); program(s) used to solve structure: SIR92 (Altomare et al., 1993); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: PLATON (Spek, 2009) and Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015).