Bis(3-butylamino-4-nitro)piperazine

: 2,4-Diﬂuoronitrobenzene is treated sequentially with butylamine and then piperazine to prepare a building block for forming a crystalline lattice by non-covalent interactions. A single crystal X-ray structure determination was performed.


Introduction
In compound 1, the butyl side chains pack forming a channel stabilised by two different hydrogen bonds. One intramolecular bifurcated hydrogen bond forms from the butylamino group to the nitro group and an intermolecular bifurcated hydrogen bond forms to the nitro group from the NH hydrogen atom of the diaminobutane side chain. These hydrogen bonds stabilise the packing.
Compound 2 is stabilised by bifurcated hydrogen bonds formed between the nitro group and two PhNH hydrogen atoms. The structure is unusual because, instead of forming infinite chains, it has a hexameric motif. The work reported in this paper enhances our knowledge and understanding of how different, but related, compounds crystallise.  In compound 1, the butyl side chains pack forming a channel stabilised by two different hydrogen bonds. One intramolecular bifurcated hydrogen bond forms from the butylamino group to the nitro group and an intermolecular bifurcated hydrogen bond forms to the nitro group from the NH hydrogen atom of the diaminobutane side chain. These hydrogen bonds stabilise the packing.

Discussion
Compound 5 is reported in this paper ( Figure 3) and the spectroscopic data is in the Supplementary Materials. It was made by the sequential treatment of 2,4-difluoronitrobenzene 3 with butylamine and then piperazine in a digestion bomb at 150 • C. Three steps were combined into one step. The fluorine atom ortho to the nitro group displaces first with good selectivity. Unlike other similar building blocks, the crystal structure of compound 5 is closely packed, rather than porous ( Figure 4). However, the knowledge gained helped us to construct a picture of what types of molecule to make using this approach, which is likely to lead to open framework porous materials. Firstly, diversity is possible here as there are two sites, two amines, where the molecular structure can be varied. Secondly, compound 5 is a more rigid molecule, which packs closely compared to the more flexible framework of compound 1, which packs forming channels. Further studies will continue to investigate flexibility versus rigidity as a predictive way of uncovering open framework channels in crystal structures.
The asymmetric unit of compound 5 consists of a complete C 24 H 34 N 6 O 4 molecule (containing C1) and a half-molecule (containing C25), with the latter completed by crystallographic inversion symmetry at the centre of the piperazine ring, which results in the uncommon situation of Z = 12 in the orthorhombic space group Pbca.
In the extended structure of compound 5, some weak C-H . . . O interactions with H . . . O separations in the range 2.45-2.60 Å and very weak aromatic π-π stacking contacts (shortest centroid-centroid separation = 3.9458 (9) Å) may help to consolidate the packing, which is otherwise presumably dominated by Van der Waals' interactions. A PLATON [5] analysis of the packing indicated that there was no free space and the structure may, therefore, be regarded as 'closely packed'.

Experimental Procedure
IR spectra were recorded on a diamond Attenuated Total Reflection (ATR) Fourier transform infrared (FTIR) spectrometer (ThermoScientific, Hemel Hempstead, UK); Ultraviolet (UV) spectra were recorded using a PerkinElmer Lambda 25 UV-Vis spectrometer (PerkinElmer, Coventry, UK) with EtOH as the solvent. The term sh means shoulder. 1 H and 13 C nuclear magnetic resonance (NMR) spectra were recorded at 400 and 100.5 MHz, respectively, using a Bruker 400 spectrometer (Bruker, Oxford, UK). Chemical shifts, δ, are given in ppm and measured by comparison with the residual solvent. Coupling constants, J, are given in Hz. High-resolution mass spectra were obtained at the University of Wales, Swansea, using an Atmospheric Solids Analysis Probe (ASAP) (Positive mode) Instrument: Xevo G2-S ASAP (SpectraLab, Markham, ON, Canada). Melting points were determined with a Dynalon (Raptor Supplies, London, UK).

Single-Crystal Diffraction
The crystal structure of compound 5 (yellow block, 0.14 × 0.09 × 0.04 mm, recrystallized from dichloromethane/light petroleum ether) was established using intensity data collected on a Rigaku AFC11 CCD diffractometer (Cu Kα radiation, λ = 1.54178 Å) at 100 K. An analytical absorption correction was applied, and the structure was routinely solved by dual-space methods using SHELXT and the structural model completed and optimized by refinement against |F| 2 with SHELXL-2018. The N-bound H atoms were located in difference maps and their positions were freely refined. The C-bound H atoms were placed geometrically (C-H = 0.95-0.99 Å) and refined as riding atoms: the methyl groups were allowed to rotate, but not to tip, to best fit the electron density. The constraint U iso (H) = 1.2U eq (carrier) or 1.5U eq (methyl carrier) was applied in all cases. Full details of the structure and refinement are available in the deposited cif.

Conclusions
The sequential reaction of 2,4-difluoronitrobenzene with butylamine and then piperazine for rapid molecular construction in a digestion bomb at 150 • C was illustrated and the product was characterised by an X-ray single crystal structure determination as a closely packed structure. The two-steps in one only worked in the digestion bomb at 150 • C and failed in EtOH under reflux (24 h reflux for each step). The higher temperature is needed to drive the first step to completion.