1,1-Dimethylbiguanidium(2+) dinitrate

In the crystal structure of the title compound, C4H13N5 2+·2NO3 −, the main intermolecular interactions are the N—H⋯O hydrogen bonds between the cationic amino groups and the O atoms of the nitrate ions. All amino H atoms and nitrate O atoms are involved in the three-dimensional hydrogen-bond network. There are two graph-set motifs R 2 2(8), which include the amino groups connected to the N atoms in the biguanide 3-, 4- and 5-positions, and the O atoms of a nitrate ion. They are extended along the a axis. An O atom of the second nitrate ion is involved in a graph-set motif C(4) that is a part of a helix-like N—H⋯O⋯H—N—H⋯O⋯ chain oriented along the b axis. There are also two weak C—H⋯O interactions in the crystal structure.

In the crystal structure of the title compound, C 4 H 13 N 5 2+ Á-2NO 3 À , the main intermolecular interactions are the N-HÁ Á ÁO hydrogen bonds between the cationic amino groups and the O atoms of the nitrate ions. All amino H atoms and nitrate O atoms are involved in the three-dimensional hydrogen-bond network. There are two graph-set motifs R 2 2 (8), which include the amino groups connected to the N atoms in the biguanide 3-, 4-and 5-positions, and the O atoms of a nitrate ion. They are extended along the a axis. An O atom of the second nitrate ion is involved in a graph-set motif C(4) that is a part of a helix-like N-HÁ Á ÁOÁ Á ÁH-N-HÁ Á ÁOÁ Á Á chain oriented along the b axis. There are also two weak C-HÁ Á ÁO interactions in the crystal structure.
The title compound was prepared during the primary research oriented on salt materials with delocalized π electrons for applications in non-linear optics. For comparison, see similar salt materials (Fridrichová et al., 2010;Matulková et al., 2011).

Experimental
A solution of the title compound was prepared by neutralization of stoichiometric amounts of dimethylbiguanidium hydroxide and nitric acid. The hydroxide was prepared from dimethylbiguanide hydrochloride (97%, Sigma-Aldrich) by the exchange reaction on an anex. Small transparent colourless crystals were obtained from water solution first after two years of crystallization. They were stable on air and non-hygroscopic.

Refinement
The primary amino H atoms were located in a difference Fourier map and refined with a N-H distance restraint equal to 0.88 Å and with U iso (H) = 1.2U eq (N). All other H positions were calculated after each cycle of refinement using a riding model, with C-H = 0.98 Å for methyl H atoms and U iso (H) = 1.5U eq (C). Fig. 1. Atom-labelling scheme of the title compound. Displacement ellipsoids are drawn at the 50% probability level.

Special details
Experimental. Vibrational spectra were recorded on a Nicolet Magna 760 FTIR spectrometer: IR spectra using DRIFTS technique in the 100 -4000 cm -1 region, with 2 cm -1 resolution and Happ-Genzel apodization, Raman spectra using Nicolet Nexus FT Raman 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.