Diacetamidinium sulfate

In the crystal structure of the title compound, 2C2H7N2 +·SO4 2−, which contains four cations and two anions in the asymmetric unit, the ions are interconnected by an extensive hydrogen-bonding system whereby two of the O atoms of sulfate ion are hydrogen-bonded to the amidinium H atoms of two cations, leading to the formation of two eight-membered rings. The two remaining O atoms interconnect two H atoms of acetamidinium cations, forming an infinite chain. The C⋯N separations within the H2N⋯C⋯NH2 moieties are similar, with an average value of 1.305 (2) Å, which is in good agreement with a delocalization model.

In the crystal structure of the title compound, 2C 2 H 7 N 2 + ÁSO 4 2À , which contains four cations and two anions in the asymmetric unit, the ions are interconnected by an extensive hydrogen-bonding system whereby two of the O atoms of sulfate ion are hydrogen-bonded to the amidinium H atoms of two cations, leading to the formation of two eightmembered rings. The two remaining O atoms interconnect two H atoms of acetamidinium cations, forming an infinite chain. The CÁ Á ÁN separations within the H 2 NÁ Á ÁCÁ Á ÁNH 2 moieties are similar, with an average value of 1.305 (2) Å , which is in good agreement with a delocalization model.
The crystal structure of acetamidinium sulfate has been determined in order to evaluate the degree of association of these ionic species. Two crystallographically independent bisacetamidinium sulfates were found in the unit cell ( Fig. 1). The molecular structure of the title compound is made up of two mutually similar acetamidinium units and one sulfate ion. All these ions are interconnected by extensive hydrogen bonding systems where two of the oxygen atoms of sulfate ion (O1 and O2) are bonded to the exo-amidinium hydrogen atoms of two units which leading to the formation of two 8-membered rings ( Fig. 2). The S-O distances for these particular oxygen atoms O1 and O2 are slightly elongated in comparison to remaining two oxygen atoms O3 and O4 which form a chain. On the other hand, two remaining oxygen atoms interconnect two endo-hydrogen atoms of acetamidinium units forming thus an infinite chain. The C-N separations within the H 2 N···C···NH 2 fragments are mutually similar with average value of 1.305 (2)Å which is with a good agreement with a delocalization concept of the double bond in H 2 N-C(CH 3 )═NH 2 cationand the literature data, where the range 1.302-1.312Å was found.
The comparison of the title compound with the published structures can be made on the bases of two different criteria.
The first, all acetamidinium salts reveal the same geometry and structural parameters of the acetamidinium ion. The second criterion is the type of the supramolecular structure formed. There are large differences between the title compound where the two planar layers of acetamidinium ions are interconnected to the infinite double layer. Other 2D structures are found for the acetamidinium formate (Tominey et al., 2006), dinitromethanide (Jalový et al., 2009), amidinium acetate (Ferretti et al., 2004) with the stairs like layered structure and one of the polymorphs of amidinium (2-hydroxyethoxy)acetate (Ferretti et al., 2004). On the other hand, the second polymorph of amidinium (2-hydroxyethoxy)acetate (Ferretti et al., 2004), acetamidinium tetrazolate (Tominey et al., 2006) and acetamidinium chloride (Cannon et al., 1976) reveal 3D structures with large cavities. There are a couple of related acetamidinium ion containing structures which are of interest as for example bis(acetamidinium) hexafluorosilicate (Calov & Jost, 1990) where the multicentered contacts between acetamidinium hydrogen atoms and fluorine atoms were found and the selenium and rhenium containing cluster compound where the hydrogen contacts of acetamidinium ion with cyano group bonded to the rhenium atoms and a short contact between selenium and nitrogen atoms were found (Emirdag-Eanes et al., 2002). characteristic are the same as described previously by Jalový et al., (2005). The crystals suitable for X-ray were prepared by crystallization from methanol by slow cooling of the hot solution.

Refinement
All the hydrogens were discernible in the difference electron density map. However, all the hydrogens were situated into idealized positions and refined riding on their parent C or N atoms, with N-H = 0.86Å, C-H = 0.96Å for methyl, U iso (H) = 1.2U eq (N) and U iso (H) = 1.5U eq (C) for methyl H atoms, respectively. Fig. 1

Special details
Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The 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.