l-2-Nitrimino-1,3-diazepane-4-carboxylic acid

The cyclic form of l-nitroarginine, C6H10N4O4, crystallizes with two independent molecules in the asymmetric unit. According to the geometrical parameters, similar in both molecules, the structure corresponds to that of l-2-nitrimino-1,3-diazepane-4-carboxylic acid; there are, however, conformational differences between the independent molecules, one of them being close to a twisted chair while the other might be described as a rather flattened boat. All six active H atoms in the two molecules are involved in hydrogen bonds, two of which are intramolecular and four intermolecular, forming an infinite chain of molecules along the b axis.


Comment
The salts of the L-arginine have been intensively investigated as non-linear optical materials [Petrosyan et al.(2005) and Karapetyan et al.(2007)]. Recently, reports about L-nitroarginine [Apreyan et al.(2008)] and its crystalline salts [Apreyan et al.(2007)] (a promising line of non-linear optical materials) have appeared.
We present herein a structural study of the cyclic form of L-nitroarginine, C 6 H 10 N 4 O 4 (I), which crystallizes with two independent molecules in the unit cell. The molecule was reported for the first time by (Paul et al., 1961) where it was suggested to be 2-nitro-4-carboxy-1,3--diazacycloheptane, on the basis of chemical properties and IR spectra. According to the present single-crystal X-ray diffraction results the L-2-nitrimino-1,3-diazepane-4-carboxylic acid (L-NIDCA) form is suggested instead. A view of the H-bonded pair of crystallographically independent molecules is shown in Fig. 1. The values of bond distances and angles are in agreement with common accepted values which lead to the proposed structural interpretation. In spite of the metric similarities there are conformational differences between the independent moieties, one of them being close to a twist-chair while the other may be described as an essentially flattened boat. All six active H atoms in the crystal are involved in hydrogen bonding (Table 1), two of them being intra-and four inter-molecular, linking crystallographically independent units and by way of which an infinite chain of molecules along the b axis is formed (Fig. 2).

Experimental
The obtainement of crystals of the title compound consisted of a two step process.First of all, the potassium salt of (I) was obtained by the reaction of L-nitroarginine with KOH. Afterwards, by the interaction of this potassium salt with HBF4 and further separation of the poorly soluble KBF4 salt, single crystals of (I) were obtained by slow evaporation at room temperature. The compound obtained is more correctly named L-2-nitrimino-1,3-diazepane-4-carboxylic acid (L-NIDCA).
Details of the obtainment of L-NIDCA and L-NIDCA . H 2 O, as well as vibrational spectra, thermal properties and SHG will be reported soon separately [Apreyan and Petrosyan, 2008].

Refinement
In spite of a pronounced centrosymmetric statistics of intensities, non-centrosymmetric P2(1)2(1)2(1) was chosen as the space group, on the basis of second harmonic generation. The statistics was latter justified by the structure resolution, which presents a strong pseudo centrosymmetric character. All the H atoms were placed in geometrically calculated positions and included in the refinement in a riding model approximation, with U iso (H): 1.5U eq (of hydroxyl O atoms) and 1.2U eq (other carrier atoms). The positional as well as anisotropic thermal parameters of non-hydrogen atoms were refined without restraints. In the absense of any significant anomalous effect, Friedel pairs were merged.
supplementary materials sup-2 Figures   Fig. 1. A perspective view of the crystallographically independent molecules paired via intermolecular H-bonds showing atomic numbering and displacement ellipsoids at the 50% probability.