(Dimethylphosphoryl)methanaminium iodide–(dimethylphosphoryl)methanamine (1/1)

The asymmetric unit of the title structure, C3H11NOP+·I−·C3H10NOP, consists of one (dimethylphosphoryl)methanamine (dpma) molecule, one (dimethylphosphoryl)methanaminium (dpmaH) ion and one iodide counter-anion. In the crystal, medium–strong to weak N—H⋯O and N—H⋯N hydrogen bonds connect dpmaH cations and dpma molecules into strands along [001]. The iodide counter-anions form only very weak hydrogen bonds. The crystal used for the diffraction study was found to be an inversion twin with a ratio of 0.83 (2):0.17 (2). The title structure is isotypic to that of dpmaH[ClO4]·dpma [Buhl et al. (2013 ▶). Crystals 3, 350–362].


Experimental
As illustrated in Figure 1, the asymmetric unit of the title structure consists of one dpmaH cation, one dpma molecule, and one iodide anion in the non-centrosymmetric space group Pca2 1 . Bond lengths and angles within the dpmaH cation and in the neutral dpma molecule are in the expected ranges (Buhl et al. 2013). The dpmaH cation and the dpma molecule are each connected head to tail via one strong and one moderate N-H···O hydrogen bond constructing a ten-membered ring (second level graph-set descriptor: R 2 2 (10); Figure 1 (red numbers)). Furthermore, these primary cyclic units are connected to adjacent units by N-H···N and N-H···O hydrogen bonds ( Figure 2). These connections form onedimensional strands running along [001]. The iodide counter anions form only very weak classical and non-classical hydrogen bonds, if at all. The hydrogen bonding motif of the backbone of the afore mentioned strands is represented by the second level graph-set descriptor C 2 2 (7) (Figure 2, red numbers). The connections of the dpmaH . dpma cyclic units with adjacent ones produce one more simple ring-motif, which can be described as a third level graph-set descriptor: R 3 4 (11) (Figure 2, green numbers). To visualize the basic principles of the construction a constructor graph is shown in Fig. 3 (Grell et al. 2002). The title structure is isotypic to the structure of dpmaH[ClO 4 ] . dpma (Buhl et al. 2013).
However, there are differences in detail due to the nature of the iodide counter anion. For dpmaHI.dpma a 6% smaller volume of the unit cell has been determined. In both structures the counter anions are positioned in the vicinity of the amino group of the dpma molecule. The replacement of perchlorate by the significantly weaker hydrogen bonded iodide counter anion leads to a measurable strengthening of the hydrogen bond donating property of the amino group within the strands. The crystal which was used for the diffraction study was found to be an inversion twin with a ratio of 0.83 (2):0.17 (2).

Experimental
In a typical experiment 0.5 g dpma was dissolved in 3 ml hydroiodic acid. The solution was slowly heated to dryness.
The residual solid (dpmaHI) and an equimolar amount of dpma were dissolved im 5 ml methanol. Slow evaporation of this solution at room temperature yielded colorless crystals of the title compound.

Refinement
All hydrogen atoms were identified in difference syntheses. Hydrogen atoms at the methyl groups are idealized, were refined using rigid groups and allowed to rotate about the P-C bond (AFIX 137 option of the SHELXL97 program; U iso = 1.5U eq (C)). The hydrogen atoms at the methylene groups were included using a riding model with the U iso values set to 1.2U eq (C). The coordinates of the hydrogen atoms involved in hydrogen bonds were refined with the N-H distance restrained to one common value. For each of these hydrogen atoms an individual U iso value was refined.  The asymmetric unit of the title structure is shown with displacement ellipsoids drawn at the 70% probability level. Blue broken lines indicate hydrogen bonds. The red number marks the ring size of the second level R 2 2 (10) graph-set descriptor for this connection of the dpmaH cation with the dpma molecule.

Figure 2
The primary units are further connected via hydrogen bonds to form strands running along [001].

Figure 3
Constructor graph (Grell et al. 2002) of that part of the title structure shown in figure 2 (black dots: dpma, gray dots: dpmaH, arrows: crystallographic dependency is coded by the colours).