Crystal structure and Hirshfeld surface analysis of poly[[di-μ3-glycine-lithium] perchlorate]

Crystal and molecular structure of bis(glycinium)lithium perchlorate salt is reported and intermolecular N— H⋯O and C—H⋯O hydrogen bonds stabilize the salt in the crystalline state.

In the title salt, {[Li(C 2 H 5 NO 2 ) 2 ]ClO 4 } n , the Li + cation is coordinated by four carboxylate oxygen atoms of the glycine molecules with a distorted tetrahedral geometry. The glycine exists in a zwitterionic form with protonated amino and deprotonated carboxylate groups. In the crystalline state, the title salt is primarily stabilized by intermolecular N-HÁ Á ÁO and C -HÁ Á ÁO interactions which interconnect various units. Hirshfeld surface analysis indicates that the intermolecular HÁ Á ÁO/OÁ Á ÁH interactions are the most important contributors to the crystal packing.

Chemical context
As part of an ongoing effort aimed at the elucidation of the crystal and molecular structures of several metal complexes/ co-crystals originating from simple amino acids (Balakrishnan et al., 2013a,b;Revathi et al., 2015;Sathiskumar et al., 2015a,b), we report herein the crystal structure of (I), a bis-(glycinium)lithium perchlorate salt complex and discuss the hydrogen-bonding interactions it forms. The crystal packing and important molecular geometries of (I) are compared with a closely related structure bis(glycine)lithium nitrate salt complex (Baran et al., 2009).

Structural commentary
An ORTEP view of the title salt is shown in Fig. 1. The asymmetric unit contains two glycinium units, one Li cation and a perchlorate anion. Both glycine molecules exhibit a zwitterionic structure, as evident from the bond lengths involving the carboxylate atoms (Table 1) and the protonation of the N atoms of the glycine molecules. In (I), the torsion angle N1A-C2A-C1A-O1A in the one of the glycinium is À0.18 (19) , while the corresponding angle is 20.75 (18) in the ISSN 2056-9890 other glycinium. The superposition of these two glycine molecules involving non-hydrogen atoms reveals high degree of similarity with an r.m.s.d. value of 0.13 Å , the maximum deviation (0.19 Å ) being observed at the C (C2A and C2B) atom.
In the crystal, the Li cation is coordinated by four carboxylate oxygen atoms of the glycine molecules. One oxygen atom from each glycine molecule is incorporated in the Li coordination sphere with Li-O distances ranging from 1.906 (3) to 2.015 (3) Å . The geometry around the Li cation is distorted tetrahedral, as discernible from the angles around the Li cation (Table 1). The lithium coordination is extended as a layer that runs parallel to the b axis. The distance between two adjacent Li ions is 3.270 (13) Å .
In a closely related structure of the complex bis(glycine) lithium nitrate (Baran et al., 2009), the Li cation is surrounded by four carboxylate oxygen atoms in a distorted tetrahedral geometry as in (I). The distance between two adjacent Li ions is 5.034 Å .

Figure 2
The crystal structure of (I), comprising alternate layers of Li-glycinium units and perchlorate anions, viewed down the b axis. Hydrogen bonds are indicated by dashed lines.

Figure 1
Part of the crystal structure of (I) showing the atomic labelling. Displacement ellipsoids are drawn at the 50% probability level.
between adjacent arrays formed by the first glycinium molecules (Fig. 3). Similar packing features are observed for bis(glycine)lithium nitrate (Baran et al., 2009). Furthermore, a careful examination of the crystal structure reveals that the first glycinium molecule does not selfassemble in the solid state. It interacts with the perchlorate anion through intermolecular N-HÁ Á ÁO and N-HÁ Á ÁCl interactions and with the second glycinium via intermolecular C -HÁ Á ÁO interactions. In contrast, the second glycinium molecule is able to self-associate in the crystal through N-HÁ Á ÁO interactions (involving H14Á Á ÁO2B and H16Á Á ÁO2B). The former linear hydrogen bond links the glycinium molecules in a head-to-tail fashion in which amino acids are selfassociated via their amino and carboxylate groups. This is one of the characteristic features observed in many amino acids and amino acid complexes (Sharma et al., 2006;Selvaraj et al., 2007;Balakrishnan et al., 2013a,b;Revathi et al., 2015). Moreover, this head-to-tail chain sequence extends along the b-axis direction and adjacent chains are oriented in an anti-parallel fashion. Centrosymmetrically related dimers [R 2 2 (10) motif] of the second glycinium molecules are generated through H16Á Á ÁO2B interactions. Together, the H14Á Á ÁO2B and H16Á Á ÁO2B interactions lead to alternating R 2 2 (10) and R 2 4 (8) motifs (Fig. 4).
The protonated amino group of the first glycinium (mol A) is involved in five hydrogen-bonding (N-HÁ Á ÁO and N-HÁ Á ÁCl) interactions (see Table 2). One of the bifurcated hydrogen-bonding interactions is formed between H12 and atoms O5 and O6 of the perchlorate anions. This interactions generate an R 4 2 (8) loop motif in which two glycinium and two perchlorate ions are involved [ Fig. 5(a)]. Intermolecular N1A-H13Á Á ÁO4 and C2A-H21Á Á ÁO3 interactions connect the glycinium molecules and perchlorate anions into a loop with adjacent loops being interconnected by C2A-H22Á Á ÁO5 interactions [ Fig. 5(b)]. As mentioned earlier, the second glycinium interacts with carboxylate groups through its protonated amino group (N1B) (Fig. 4). It also interacts with the perchlorate anion through C2B-H24Á Á ÁO4 interaction.

Hirshfeld surface analysis and 2D fingerprint plots
The Hirshfeld surface (HS) analysis was carried out in order to understand the nature of the intermolecular interactions present in the crystal structure. The shorter and longer contacts are indicated as red and blue spots on the HS and contacts with distances equal to the sum of the van der Waals radii are represented as white. The Hirshfeld surfaces for the cation (consisting of two glycinium molecules and a lithium ion) and anion of the title salt complex were generated and analysed separately using the program CrystalExplorer (Wolff et al., 2012). The HS of the cation mapped over the normalized distance, d norm , and the 2D fingerprint plots (Spackman & McKinnon, 2002) are illustrated in Fig. 6. In the cation, intermolecular OÁ Á ÁH/HÁ Á ÁO interactions are predominant making a 66.9% contribution to the total HS. In the twodimensional fingerprint plots, these contacts are depicted as a pair of sharp spikes with d e + d i $1.9 Å . There is a remarkable difference observed in the relative contribution of the HÁ Á ÁO Second glycinium molecules (orange) and perchlorate anions are sandwiched between arrays of the first glycinium molecules (grey).
In the perchlorate anion, the relative contributions of the OÁ Á ÁH/HÁ Á ÁO and OÁ Á ÁO contacts are 81.4 and 18.2%, respectively (Fig. 7). The OÁ Á ÁH/HÁ Á ÁO contacts visible on the HS are due to the N-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds. The OÁ Á ÁO contacts are also visible on the HS and this contact of around 2.8 Å has the shortest distances of d e and d i of around 1.4 Å (Fig. 8). This OÁ Á ÁO short contact [2.879 (2) Å ] links the anionic molecules into a chain running parallel to the b-axis direction.  Thomas et al., 1994), the N atom of the isothiocynate, which acts as a fourth ligand, participates in the Li coordination sphere.

Database survey
A detailed survey was also been conducted in the protein data bank (www.rcsb.org) to understand the Li + coordination with protein molecules. The keyword 'lithium' was used in the search, which resulted in 74 hits (up to 3.0 Å resolution). There are 30 structures found with better than 1.5 Å resolution and these structures were examined further. In this dataset, we found that Li is three-to six-coordinate with a water molecule belonging to the Li complex cation. Moreover, the residues aspartate and glutamate are included in the Li coordination in several structures.

Synthesis and crystallization
The title salt was synthesized by dissolving AR-grade glycine and lithium perchlorate in a 2:1 stoichiometric ratio in double distilled water and stirred continuously for 2 h. Slow evaporation of this aqueous solution at room temperature yielded transparent colourless single crystals of the title salt.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.