Tetraethylammonium l-malate 1.36-hydrate

The asymmetric unit of the title compound, C8H20N+·C4H5O5 −·1.36H2O, contains two independent ion pairs, with similar conformations, and three water molecules of crystallization, one water molecule haing a site-occupancy factor of 0.721 (5). Intramolecular O—H⋯O hydrogen bonds, involving the hydroxy groups and an O atom of each carboxylate anion, generate five-membered rings involving S(5) ring motifs. In the crystal structure, molecules are linked together by water molecules through four-membered O—H⋯O—H⋯O—H interactions to form one-dimensional infinite chains along the a axis. Since the molecules are also linked into one-dimensional infinite chains along the b axis, molecular sheets parallel to the (001) plane are created. Overall, the crystal structure is stabilized by two intramolecular O—H⋯O hydrogen bonds, nine intermolecular O—H⋯O and ten C—H⋯O hydrogen bonds.

Data collection: APEX2 (Bruker, 2005); cell refinement: APEX2; data reduction: SAINT (Bruker, 2005); program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL and PLATON (Spek, 2003 (Rahman et al., 2008). In this study, we used a different anion in order to compare the interaction between the tartarate and malate ions. Generally, organic molecules contain substituents with the ability to form inter-and intramolecular hydrogen bonding. In this work, tetraethylammonium L-malate [C 2 H 5 ) 4 N] + [C 4 H 5 O 5 ] -, was synthesized by neutralization reaction of tetraethylammonium hydroxide with L-malic acid. Related compounds containing the same anion have been prepared (Allen et al., 2006, Ying-Ying et al., 2007. Tetraethylammonium hydroxide is a strong base, which easily deprotonates the carboxylic acid moiety of L-malic acid to form carboxylate anion and water as a by-product (Allen et al., 2006). The reaction between tetraethylammonium hydroxide and L-malic acid forms a weak bond. It seems that the bond formed between tetraethylammonium and L-malic acid is weaker than a covalent bond but may still contribute to the achieved minimum energy configuration (Anandha et al., 2008).
In the title compound I, Fig. 1, the asymmetric unit is composed of two crystallographically independent ion pairs (A and B), with similar conformations and three water molecules of crystallization. One of the water molecule (O1W) is partially occupied with a site-occupancy factor of 0.721 (5). The bond lengths (Allen et al. 1987) and angles are within normal ranges. Intramolecular O3A-H3OA···O5A and O3B-H3OB···O5B hydrogen bonds form S(5) ring motifs (Table 1) (Bernstein et al., 1995). In the crystal structure, the molecules are linked together by water molecules through directed four-membered O-H···O-H···O-H interactions to form 1-D infinite chains along the a-axis (Fig. 2). Since the molecules are also linked into 1-D infinite chains along the b-axis, molecular sheets parallel to the (001)-plane are created (Fig. 2). The crystal structure is stabilized by intramolecular O-H···O (x 2) hydrogen bonds, intermolecular O-H···O (x 9) and C-H···O (x 10) hydrogen bonds (Table 1).

Experimental
The synthetic procedure is similar to the previous one (Abdul Rahman et al., 2008) except that L-malic acid (6.704 g, 0.05 mole) was used. Single crystals suitable for X-ray diffraction were obtained by slow evaporation at room temperature.

Refinement
The H atoms bound to O1W and O2W were located from the difference Fourier map and constrained to ride on the parent atom. The hydrogen atoms of O3W were also located from the difference Fourier map and refined freely. The hydrogen of the hydroxy groups were positioned using a freely rotating O-H bond and constrained with a fixed disatnce of 0.82 Å. The rest of the hydrogen atoms were positioned geometrically and refined as a riding model. A rotating group model was used for the methyl group. One of the water molecule (O1W) is partially occupied with a site-occupancy factor of 0.721 (5). In the absence of significant anomalous dispersion effects, the Friedel pairs (6331) were averaged. Only the relative configuration supplementary materials sup-2 is known. The highest peak (0.51 e. Å -3 ) is located 0.35 Å from H6BC and the deepest hole (-0.46 Å -3 ) is located 0.67 Å from O1W. Fig. 1. The molecular structure of (I) with atom labels and 40% probability ellipsoids for non-H atoms. The hydrogen atoms of the cations were omitted for clarity. Intramolecular interactions are shown as dashed lines.

Special details
Experimental. The low-temperature data was collected with the Oxford Cyrosystem Cobra low-temperature attachment. 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.