Potassium l-2-nitrimino-1,3-diazepane-4-carboxylate monohydrate

The title compound, K+·C6H9N4O4 −·H2O, crystallizes with the K atoms located on special positions related by pseudocentres of symmetry. Each K atom is coordinated by six O-atom donors. The N and water H atoms are involved in inter- and intramolecular N—H⋯O, N—H⋯N and O—H⋯O hydrogen bonding. The data indicate inversion twinning.


S1. Comment
Cyclic L-2-nitrimino-1,3-diazepane-4-carboxylic acid (L-NIDCA), produced by elimination of amine from L-nitroarginine (Paul et al., 1961) , may generate new non-linear optical materials like L-nitroarginine itself [Apreyan et al. (2008a[Apreyan et al. ( , 2008b; Karapetyan et al.(2007);Petrosyan et al. (2005); the crystal structures of L-NIDCA and its monohydrate have been recently reported [Karapetyan (2008a[Karapetyan ( , 2008b]. This paper presents a structural study of the potassium salt of L-NIDCA monohydrate. The structure was solved and refined in the orthorhombic unit cell with I222 space group. The choice of the non-centric space group was based on the generation of second harmonic observed on a powder sample (YAG:Nd laser, Kurtz-Perry method [Kurtz & Perry,1968]).
In this structure, two independent potassium cations occupy special positions. These potassium atoms are located about pseudo-inversion centers, which is most likely the reason for the presence of high level pseudosymmetry in the structure.
A view of the asymmetric unit is shown in Fig. 1. The high value of U eq of atom C3 of the 1,3-diazepane ring compared to those of its neighbors indicates potential disorder of this atom. In the crystal structure, the nitrogen-bound H atoms and the water H atoms are involved in N-H···O, N-H···N and O-H···O hydrogen bonding (Table 1), one of them being intra-and the other three intermolecular, linking anions and water molecules in infinite layers parallel to the bc plane ( Fig. 2).

S2. Experimental
The title compound was synthesized from a mixture of aqueous solutions containing L-nitroarginine (2 g, Sigma-Aldrich) and KOH (0.512 g) at room temperature. Single crystals of the title compound were obtained by slow evaporation of the solution. At 97° C decomposition of the crystals was observed.

S3. Refinement
The data set was collected in a full sphere of reciprocal space. Space group I222 was chosen on the basis of the powder second harmonic of YAG:Nb laser generation property of the crystals of the title compound. In spite of the fact that all H atoms appear in difference Fourier maps in reasonable positions, they became unacceptable after refinement. Because of this, all the H atoms except those belonging to the water molecule were placed in geometrically calculated positions and included in the refinement in a riding model approximation, with U iso (H) = 1.2U eq (carrier atom). The positions of the H atoms of the water molecule were located in difference Fourier maps and included in the refinement with fixed O-H (0.85 Å), H···H (1.35 Å) distances and isotropic temperature parameters U iso (H) = 1.4U eq (O). The absolute configuration has been determined using L-nitroarginine of known absolute configuration.      (20) 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 > 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.