Bis(l-serinium) oxalate dihydrate: polymorph II

A corrected and improved structure of the polymorph II of 2C3H8NO3 +·C2O4 2−·2H2O, based on single-crystal data, is presented. The structure is refined with anisotropic displacement parameters for all non-H atoms and all H atoms are located. Due to the charged moieties, the structure is classified as a molecular salt. Intermolecular O—H⋯O−, O—H⋯O and N+—H⋯O−hydrogen bonds link the components of the structure. The l-serinium cations and oxalate anions form a network of channels in [100] direction, filled with the water molecules of crystallization. The dihedral angle between the CO2 units of the oxalate dianion is 10.2 (3)°


Comment
Divers crystal forms of molecular salts of L-serine with oxalic acid can be obtained by grinding or kneading powders of both compounds, as decribed earlier (Braga et al., 2013). However, previously reported structure of form II of 2C 3 H 7 NO 3 + .C 2 O 4 2-.2H 2 O was based on powder diffraction X-ray data and therefore hydrogen atoms were positioned geometrically (including -NH 3 + and -COOH groups) and the structure refined with isotropic displacement parameters for for L-serinium cations. Moreover, the H atoms of water molecules were not located. Crystallization of the polymorph II of 2C 3 H 7 NO 3 + .C 2 O 4 2-.2H 2 O is also possible by slow evaporation from water solution and it results in crystals of the size sufficient to perform single-crystal X-ray diffraction experiment. Hence, the proper H-atom positions can be found. Both hydrate polymorphs have different unit-cell dimensions, whereas crystal packing remains virtually very similar, with a characteristic motif of the zigzag chains formed by hydrogen bonds between the water molecules along the [100] direction. The location of hydrogen bond network surrounding the oxalate anion allows for discrimination between the polymorphic forms. The form I contains 8 hydrogen bonds between the oxalate anion and 6 neighbouring serine cations.
In the polymorph II, obtained from powder data, due to the lack of proper positions of H-atoms, one can suspect the presence of 11 hydrogen bonds around the oxalate anion, located within the donor-acceptor distance in the range from 2.4 to 3.0 Å. In the polymorph II structure derived from single-crystal X-ray diffraction data, 9 hydrogen bonds are formed between the oxalate anion to the 6 neighbouring serine cations. This difference results from a wrong assignment of the carboxylic H-atom in one of the serinium cations in the structure obtained from powder data (see Fig.1, atoms O2 and O12).Both structures of the form II have different distances between the oxalate anions. The structure of Braga et al. (2013) presents a denser arrangement between the oxalate anions. The distances between oxygen atoms from neighbouring anions for the structure obtained by single crystal X-ray measurement range from 3.237 (2) to 4.975 (2) Å.
The corresponding values in the structure derived from the powder data are between 2.63 (1) to 4.97 (1) Å.

Experimental
A mixture of L-serine (0.158 mg) with oxalic acid (0.068 mg) was dissolved in water (15 mL) in the 2:1 stoichiometric ratio and set aside to crystallize by slow evaporation at 309 K. Little needle-shaped crystals of bis(L-serinium) oxalate dihydrate form II (m.p. 357 K) have grown on bigger needle-shaped crystals of oxalic acid.

Refinement
All H-atoms bound to C were placed at the calculated positions and were treated as riding on the parent atom with U iso (H) = 1.2U eq (C) and C-H distances of 0.98 Å for methine and 0.97 Å for methylene groups. N + -bound H atoms were placed supplementary materials sup-2 Acta Cryst. (2013). E69, o1667-o1668 in locations indicated by a difference Fourier synthesis and were refined using a riding model, with U iso (H) = 1.5U eq (C) and N-H distance of 0.89 Å. H atoms attached to O were placed in locations indicated by a difference Fourier synthesis and were refined using a riding model with U iso (H) values set at 1.5 U eq (O) and with a distance restraint of O-H = 0.82 Å, except for the water molecules for which O-H distances were constrained to 0.840 (5) Å. An absolute structure has been assigned by reference to an unchanging chiral centre in the crystallization procedure. For the refinement 1367 Friedel pairs were not merged.

Computing details
Data collection: CrysAlis PRO (Agilent, 2012); cell refinement: CrysAlis PRO (Agilent, 2012); data reduction: CrysAlis PRO (Agilent, 2012) and SORTAV (Blessing, 1995); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and DIAMOND (Brandenburg, 1999).  The crystal packing in polymorph II viewed along the a axis. Dashed lines indicate hydrogen bonds. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.17 e Å −3 Δρ min = −0.17 e Å −3 Absolute structure: Flack (1983), 1367 Friedel pairs Absolute structure parameter: −0.4 (9) 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. 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 > 2σ(F 2 ) is used only for calculating R-factors(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.