Allylammonium hydrogen oxalate hemihydrate

In the title hydrated molecular salt, C3H8N+·C2HO4 −·0.5H2O, the water O atom lies on a crystallographic twofold axis. The C=C—C—N torsion angle in the cation is 2.8 (3)° and the dihedral angle between the CO2 and CO2H planes in the anion is 1.0 (4)°. In the crystal, the hydrogen oxalate ions are linked by O—H⋯O hydrogen bonds, generating [010] chains. The allylammonium cations bond to the chains through N—H⋯O and N—H⋯(O,O) hydrogen bonds. The water molecule accepts two N—H⋯O hydrogen bonds and makes two O—H⋯O hydrogen bonds. Together, the hydrogen bonds generate (100) sheets.

In the title hydrated molecular salt, C 3 H 8 N + ÁC 2 HO 4 À Á0.5H 2 O, the water O atom lies on a crystallographic twofold axis. The C C-C-N torsion angle in the cation is 2.8 (3) and the dihedral angle between the CO 2 and CO 2 H planes in the anion is 1.0 (4) . In the crystal, the hydrogen oxalate ions are linked by O-HÁ Á ÁO hydrogen bonds, generating [010] chains. The allylammonium cations bond to the chains through N-HÁ Á ÁO and N-HÁ Á Á(O,O) hydrogen bonds. The water molecule accepts two N-HÁ Á ÁO hydrogen bonds and makes two O-HÁ Á ÁO hydrogen bonds. Together, the hydrogen bonds generate (100) sheets.
Supporting information for this paper is available from the IUCr electronic archives (Reference: HB7243).

Comment
Oxalic acid, together with its anions, is one of the best building blocks for the construction of supramolecular structures based on hydrogen bonds. The adducts of oxalic acid and aliphatic amines have been examined by single-crystal X-ray diffraction and other techniques. Three types of characteristic structural motifs are present: (i) linear chains of dicarboxylic acids formed by strong hydrogen bonds; (ii) dimers of dicarboxylic acid molecules; (iii) isolated oxalate monoanions or dianion units (MacDonald et al., 2001;Vaidhyanathan et al., 2001Vaidhyanathan et al., , 2002; Ejsmont, 2006Ejsmont, , 2007Ejsmont & Zaleski 2006a, 2006b).
The crystal structure of the title salt, (I), consists of allyloammonium cations, hydrogen oxalate anions and water molecules (Fig. 1). A search of the Cambridge Structural Database (CSD; CONQUEST Version 1.16; Allen, 2002) afforded that the geometrical parameters of the allyloammonium cation (Table 1) Table 2).

Experimental
Colourless prisms of (I) were grown at room temperature by slow evaporation of an aqueous solution of allylamine and oxalic acid in a 1:1 stoichiometric ratio.

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. 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 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.