1-{2-[4-(4-Nitrophenyl)piperazin-1-yl]ethyl}-4-aza-1-azoniabicyclo[2.2.2]octane iodide

The title compound, C18H28N5O2 +·I−, was observed as a main product in an intended 1:1 reaction between 4-iodonitrobenzene and 1,4-diazabicyclo[2.2.2]octane (DABCO). In the reaction, DABCO undergoes a ring opening to yield a quaternary salt of DABCO and 1-ethyl-4-(4-nitrophenyl)piperazine with an iodide anion. The crystal structure determination was carried out as no crystal structure had been previously reported in the investigations describing the corresponding reaction with 4-chloronitrobenze. Indeed, the crystal structure of the title compound confirms the molecular composition proposed earlier for the analogous chloride salt. The cation conformation is similar to the previously reported dinitro analogue 1-{2-[4-(2,4-dinitrophenyl)piperazin-1-yl]ethyl}-4-aza-1-azoniabicyclo[2.2.2]octane chloride [Clegg et al. (2004 ▶). Acta Cryst. E60, o291–o293]. The crystal packing is dominated by cation⋯I− interactions in addition to weak intermolecular C—H⋯O2N and C—H⋯N interactions between the cations.


Related literature
For a possible route of synthesis for the chloride salt of the title compound, see: Ross & Finkelstein (1963). For a related structure, see: Clegg et al. (2004). For the synthesis of the intended 1:1 product of DABCO and 4-iodonitrobenzene, see Ibata et al. (1987).  Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) Àx þ 1; Ày þ 1; Àz þ 1; (ii) x þ 1; Ày þ 1 2 ; z À 1 2 ; (iii) Àx; Ày; Àz þ 1; (iv) Àx þ 1; y À 1 2 ; Àz þ 1 2 . The reaction between DABCO and 4-chloronitrobenzene has first been reported by Ross & Finkelstein (1963). The product obtained was identified as the chloride salt analogue of the title compound, 1-(4-nitrophenyl)-4-aza-1-azoniabicyclo[2.2.2]octane chloride, instead of the expected 1:1 product. We were interested in the synthesis of the elusive 1:1 product for its potential applications in supramolecular chemistry based on halogen bonding interactions. We tried to use 4-iodonitrobenzene in order to obtain the 1:1 product. Regardless of the milder conditions (THF at reflux for 48 h) and a different halobenzene, the corresponding reaction proceeds according to the aforementioned route yielding an analogous iodide salt, 1-(4-nitrophenyl)-4-aza-1-azoniabicyclo[2.2.2]octane iodide, to the compound described above. Nonetheless, despite the failure in the intended synthesis, the crystal structure of the title compound provides a crystallographic evidence for the previously described reaction between DABCO and a 4-halonitrobenzene. Furthermore, our investigation suggests that changing the halogen atom in the 4-halonitrobenzene from chloride to iodine has evidently no effect in the outcome of the reaction (except that of different anion). A possible route for the anticipated 1:1 product is described by Ibata et al. (1987).

Data collection
The cation of the title salt lies in a conformation similar to the previously reported dinitrobenzene analogue (Clegg et al., 2004). The labeling scheme is shown in Fig. 1. The intermolecular interactions in the crystal structure comprise mostly of weak C-H···O 2 N interactions between DABCO -CH 2 groups and the -NO 2 groups [C···O distances range

Experimental
The title compound was obtained as a major product in a reaction between DABCO (2.0 mmol) and 4-iodonitrobenzene (2.0 mmol) carried out in THF (48 h at reflux). After the removal of solvent the yellow oily residue was precipitated with dichloromethane, filtered and recrystallized from water/acetone mixture to yield a batch of yellow crystals of the title compound.

Refinement
All H atoms were refined as riding atoms with fixed isotropic displacement parameters 1.2 times larger than corresponding host carbon atoms. C-H distances were refined as 0.95 Å for aromatic and 0.99 Å for methylene H atoms. All non-hydrogen atoms were refined anisotropically.

Figure 1
Asymmetric unit and labeling scheme of the title compound. Ellipsoids are presented at the 50% probability level.

Figure 2
Anion-cation and cation-cation interactions viewed along the crystallographic a-axis. Ellipsoids are presented at the 50% probability level.  Packing of the ion pairs viewed along the crystallographic b-axis. The hydrogen atoms have been omitted for clarity. 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 cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'s involving l.s. planes. 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.