Crystal structure of catena-poly[[[triaquastrontium]-di-μ2-glycinato] dibromide]

The characteristic structural feature of the title compound is the formation of cationic chains extending parallel to [001], with the Br− counter-anions located in between. Intermolecular N—H⋯O, N—H⋯Br, O—H⋯O and O—H⋯Br hydrogen bonds stabilize the structure.

In the title coordination polymer, {[Sr(C 2 H 5 NO 2 ) 2 (H 2 O) 3 ]Br 2 } n , the Sr 2+ ion and one of the water molecules are located on twofold rotation axes. The alkaline earth ion is nine-coordinated by three water O atoms and six O atoms of the carboxylate groups of four glycine ligands, two in a chelating mode and two in a monodentate mode. The glycine molecule exists in a zwitterionic form and bridges the cations into chains parallel to [001]. The Br À counter-anions are located between the chains. Intermolecular hydrogen bonds are formed between the amino and carboxylate groups of neighbouring glycine ligands, generating a head-to-tail sequence. Adjacent head-to-tail sequences are further interconnected by intermolecular N-HÁ Á ÁBr hydrogen-bonding interactions into sheets parallel to (100). O-HÁ Á ÁBr and O-HÁ Á ÁO hydrogen bonds involving the coordinating water molecules are also present, consolidating the threedimensional hydrogen-bonding network.

Chemical context
Research in the field of coordination polymers has undergone rapid development in recent years due to their interesting structures and their wide range of applications as functional materials (Lyhs et al., 2012). One of the simplest amino acids is glycine and some glycine-metal complexes have been reported previously (Fleck et al., 2006 and references therein). The crystal structures of strontium combined with anions of amino acids are rare. As part of our ongoing investigations of the crystal and molecular structures of a series of metal complexes derived from amino acids (Sathiskumar et al., 2015a,b;Balakrishnan et al., 2013), we report here the crystal structure of a polymeric strontium-glycine complex, {[Sr(C 2 H 5 NO 2 ) 2 (H 2 O) 3 ]Br 2 } n , (I).

Structural commentary
The asymmetric unit of (I) contains one Sr 2+ ion, one glycine ligand, one and a half water molecules and one bromide anion (Fig. 1). The Sr 2+ cation and one of the water molecules (O4) are located on special positions with site symmetry 2. The bond lengths involving the carboxylate atoms and the protonation of the amino group reveal a zwitterionic form for the glycine ligand in (I). The Sr 2+ ion is nine-coordinated by three oxygen atoms [Sr-O = 2.526 (4)-2.661 (2) Å ] of water molecules and six carboxylate oxygen atoms of four glycine ligands [Sr-O = 2.605 (2)-2.703 (2) Å ]. The glycine ligands coordinate each cation in a bis-bidentate and bis-monodentate way and simultaneously bridge two alkaline earth cations. As shown in Fig. 2, this coordination mode leads to the formation of polymeric chains running parallel to [001]. Adjacent Sr 2+ ions are separated by 4.3497 (3) Å within a chain and the shortest SrÁ Á ÁSr distance between neighbouring chains is 9.4960 (3) Å .

Supramolecular features
The crystal structure of (I) contains an intricate network of intermolecular N-HÁ Á ÁO, N-HÁ Á ÁBr, O-HÁ Á ÁO and O-HÁ Á ÁBr hydrogen bonds (Table 1). The protonated N atom of the glycine molecule is capable of forming three hydrogenbonding interactions. One of them is the characteristic headto-tail sequence in which amino acids are self-assembled through their amino and carboxylate groups (Sharma et al., 2006;Selvaraj et al., 2007;Balakrishnan et al., 2013). In (I), the zwitterionic glycine molecules are arranged in linear arrays that run parallel to the [110] direction (Fig. 3), and adjacent glycine molecules are interconnected by an intermolecular N1-H1AÁ Á ÁO1 hydrogen bond. This interaction can be described as a head-to-tail sequence having a C(5) graph-set motif (Bernstein et al., 1995). In each array, the Br À counter anions bridge neighbouring glycines. Taken together, these three interactions form a hydrogen-bonded sheet extending parallel to (100). One of the water molecules (O3) acts as a donor for two different Br À anions. These intermolecular O-HÁ Á ÁBr interactions result in a cyclic dibromide motif as observed in the crystal structure of N,N 0 -dibenzyl-N,N,N 0 ,N 0tetramethylethylenediammonium dibromide dihydrate (Srinivasan et al., 2006). Within this motif, the distance between Br anions is 5.3398 (3) Å , and the distance between The coordination environment of Sr 2+ in the crystal structure of (I). Displacement ellipsoids are drawn at the 40% probability level.

Synthesis and crystallization
Crystals of (I) were grown from an aqueous solution by slow solvent evaporation at room temperature. Analytical grade reagents glycine (Merck) and strontium bromide hexahydrate (Sigma-Aldrich) were taken in a 2:1 molar ratio, dissolved in double-distilled water and stirred well for 4 h using a temperature-controlled magnetic stirrer to yield a homogeneous mixture. The solution was finally filtered using Whatman filter paper. The beaker containing the solution was closed with a polythene sheet with two (or) three perforations and kept in a dust-free atmosphere for slow evaporation. Single crystals were harvested after a growth period of 20 days.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The positions of the amino and water H atoms were located from difference Fourier maps. The O3-H3B and O4-H4 distances of the water molecules were restrained to 0.85 (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 atoms.

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.