3,6-Diazaoctane-1,8-diaminium diiodide

The asymmetric unit of the title salt, C6H20N4 2+·2I−, comprises half a 3,6-diazaoctane-1,8-diaminium dication plus an I− anion. The dications are symmetrical and lie across crystallographic centres of inversion. In the crystal, the ions form a network involving mainly weak N—H⋯I intermolecular interactions: two H atoms of the ammonium group form interactions with two I− anions and the H atom of the secondary amine forms a weak interaction with a third I− cation. The third ammonium H atom is hydrogen bonded to a secondary amine of an adjacent cation. The backbone of the cation does not form a uniformly trans chain, but is ‘kinked’ [C—N—C—C torsion angle = 71.5 (2)°], probably to accommodate the direct hydrogen bond between the ammonium group and the secondary amine in an adjacent cation.


Related literature
For the structure of a dihydrate of the title compound, together with its isostructural Cl À and Br À analogues, see Ilioudis et al. (2000).
The crystal structures of a dihydrate of 1 [C 6 H 20 N 4 2+ 2(I -) 2(H 2 O)], together with the isostructural Cland Branalogues, have been reported (Ilioudis, et al., 2000). In these structures the C 6 H 20 N 4 2+ cations again lie across centres of inversion, but they form uniformly trans chains with the magnitudes of the backbone torsion angles all in the range 177.0 (2) -180°. The presence of water in these hydrates changes the hydrogen bonding / intermolecular interaction pattern compared with 1. Two ammonium and the secondary amine H atoms interact with halide anions as in 1, but there is no direct hydrogen bond between the ammonium group and the secondary amine of a neighbouring cation. Instead, the water molecule acts as an acceptor for a hydrogen bond involving the third ammonium H atom and, in turn, acts as a donor to the secondary amine N atom of a second cation. The second H atom of the water molecule forms a fourth hydrogen bond to a halide anion.
The "kink" in the backbone chain of the cation in 1 probably occurs to accommodate the direct hydrogen bond between the ammonium group and the secondary amine in an adjacent cation, whereas in the hydrates the intervening water molecule in the hydrogen bonding network allows more flexibility and allows the cations to adopt a uniformly trans configuration. Refinement H1A, H1B, H1C and H4 were located by a difference map and their coordinates were freely refined, except that the N4-H4 bond length was restrained (target: 0.91 (2) Å, refined length: 0.833 (14) Å) as unrestrained refinement led to an unacceptably short N-H bond length. All of the remaining H atoms were placed in their calculated positions and then refined using the riding model with Atom-H lengths of 0.95 Å, (CH) or 0.99 Å (CH 2 ). Isotropic displacement parameters for all hydrogen atoms were set to 1.20 times U eq of the parent atom.

Figure 1
Molecular structure of the title compound showing the atom labeling scheme and 50° probability displacement ellipsoids.
Broken lines indicate the N-H···N hydrogen bond and the weak N-H···I intermolecular interactions. Symmetry codes: (ii) -x + 1/2, y -1/2, z; (iii) -x, y + 1/2, -z + 1/2; (iv) x + 1/2, y, -z + 1/2; (v) -x + 1/2, y + 1/2, z. 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.  (2)