The synthesis and energetic properties of pyridinium and triazolium N -(nitrobenzoyl)-imides

Syntheses of nitrogen rich, energetic N -(nitro-substituted-benzoylimino)pyridinium and N -(nitro-substituted-benzoylimino)1,2,4-triazolium imides are reported and their energetic properties calculated/determined in terms of heats of formation (HOF), densities, detonation parameters, heats of decomposition and heats of combustion


Introduction
3][4] To this end many compounds such as PYX [3,5-dinitro-2,6-bis(picrylamino)pyridine], NTO (3-nitro-1,2,4-triazol-5-one), ONC (octanitrocubane), TATB (1,3,5-triamino-2,4,6-trinitrobenzene) and DATB (1,3-diamino-2,4,6-trinitro-benzene) have been synthesized and tested.TATB and DATB have been known since the 1880s, but ignored due to their large critical diameter and insensitivity to initiation which made efficient detonation almost impossible.The performance of TATB and DATB-based compositions may be modified by addition of more energetic explosives such as RDX, HMX and/or CL-20 thus enabling the compositions to be detonated. 4,5Another way to solve the problem is to develop PBXs (plastic-bonded explosives) for military and civilian purposes by de-sensitization of HMX and PETN with appropriate plasticizers and binders.Among approaches towards Insensitive High Explosives (IHEs), raising nitrogen content by the use of heterocyclic nuclei instead of carbocyclic structures improves oxygen balance and forms N 2 as a product of explosion.Consequently, studies based on heterocyclic nitrogen compounds for IHEs have become popular. 6enzotriazole is a synthetic auxiliary that offers many advantages in organic synthesis since it is easy to introduce, activates molecules towards numerous transformations, and can be removed easily at the end of a reaction. 7In a continuing effort to explore the chemistry of compounds that are safe to handle and more cost-efficient to produce, we investigated the synthesis of nitro-substituted N-benzoylimides as potential candidates for energetic materials using acylbenzotriazoles rather than acid chlorides as the reactive starting materials. 8

Results and Discussion
Preparation of intermediate acyl benzotriazoles 2a-e was achieved by a one-step reaction as shown in Scheme 1. 9,10 Scheme 1. Synthesis of acylbenzotriazoles 2a-e.
The target heterocyclic nitro-substituted benzoylimides were prepared via the acylbenzotriazole method, which was much more convenient than the previously used acid chloride route 8 and gave almost quantitative yields.In a typical example, potassium carbonate was added to a solution of 1-aminopyridinium iodide or 4-amino-1-methyl-1H-1,2,4-triazol-4ium iodide in acetonitrile.The reaction mixture was stirred at room temperature and then the appropriate aroylbenzotriazole was added in one portion.After one hour stirring, the inorganic salts were filtered off through celite and the filtrate was concentrated in vacuo to give the crude product, which was purified by column chromatography over silica gel with ethyl acetate/hexanes and then acetonitrile/methanol as eluents to afford pure material (Schemes 2 and 3).Table 1 shows the melting points, heats of decomposition (ΔH d ) and heats of combustion (ΔH c ) of the pyridinium (4a-e) and triazolium (6a-e) aroyl ylides as determined by differential scanning calorimetry (for ΔH d ) or adiabatically in a Parr Oxygen Bomb Calorimeter (for ΔH c ).
Table 2 shows the energetic properties of 4a-e and 6a-e including calculated heats of formation (ΔH f ) and densities (), experimental densities determined by gas pycnometer and explosion parameters as calculated by Explo5 v.6.01 software.
The data in Table 1 and 2 are insufficient to determine whether or not a correlation exists between ΔH d and ΔH f but Table 2 reveals that both ρ (calcd) and ρ (exp) increase with the number of nitro groups present in the molecules.There is however, a distinct difference between the HOF values of the pyridinium and triazolium ylides with the latter some 90 kJ mol -1 higher in energy.Although the data are limited it is clear that addition of an ortho-nitro group over meta or para analogues adds ca.16-18 kJ mol -1 in both series.In comparison with RDX however, the densities, detonation pressures (P D ), detonation velocities (V D ) and specific impulses (Isp) are all greatly inferior.Thus it is expected that none of these compounds would afford useful explosives.On the basis of the correlation between ρ (calcd) and ρ (exp) , the predicted density for 2,4,6-trinitroaroyl triazolium ylid 6f is ca.1.70 g/cm 3 and calculated HOF = 310 kJmol -1 , still well short of RDX values.Considering the hazards associated with the use of trinitrobenzoic acid we decided not to attempt a synthesis of 6f (Figure 1).Table 1.Melting points, heats of decomposition and heats of combustion of pyridinium (4a-e) and triazolium (6a-e) nitrobenzoyl ylides Pyridinium imides:   Triazolium imides: The structures of ylides 4e and 6d were confirmed by X-ray crystallography (Figure 2) and in both cases the densities as determined by X-ray crystallography at 120K (1.585 and 1.669 respectively) were in good agreement with the pycnometer results.The most noteworthy features of the structures concern the relative orientations of the rings.In the case of 4e the amide function is almost coplanar with the dinitrophenyl ring (angle between mean planes = 7.2 o ) whereas the pyridinium ring is almost orthogonal (74.6 o ) to the nitroaromatic ring.However, due to the presence of the ortho nitro group, in 6d the amide group is highly twisted (65.3 o ) out of the plane of the dinitrophenyl ring.In this case the two rings are inclined at an angle of 43.4 o .

Conclusions
In conclusion we have developed general conditions for the synthesis of energetic, nitrogen rich N-(nitro-substituted-benzoylimino)pyridinium and 1,2,4-triazolium imides via readily available acylbenzotriazoles.This strategy provides unprecedented synthetic advantages including shorter reaction times, higher yields and easier purification than the acid chloride method.However, the energetic parameters of the compounds synthesized fall short of those required for an effective explosive.

Experimental Section
Caution.Although we encountered no difficulties with the preparation or isolation of these compounds, synthetic work was restricted to the millimolar scale and care was taken to avoid impact, friction or overheating.
General.Melting points were determined on a capillary tube melting point apparatus equipped with a digital thermometer and are uncorrected.NMR spectra were recorded in DMSO-d 6 with TMS as the internal standard for 1 H (300 MHz) or in DMSO-d 6 with the residual solvent peak as the internal standard for 13 C (75 MHz), unless otherwise specified.Chemical shifts were reported in parts per million relative to the residual solvent peak.Individual peaks were reported as multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet) and coupling constants in Hertz (Hz).Elemental analyses were performed on a Carlo Erba EA 1108 elemental analyzer, Dept. of Chem., University of Florida.All reactions were carried out under nitrogen atmosphere.All reagents were purchased from commercial sources and used without treatment unless otherwise indicated.Column chromatography was performed with S733-1 silica gel (200-425 mesh).Combustion analysis was performed using a Parr Oxygen Bomb Calorimeter, under an atmosphere of excess oxygen at approximately 25 atm.All combustion residues were titrated as nitric acid to account for the formation of NO x as a combustion by-product.Determination of heats of decomposition was conducted using a DuPont Instruments DSC 2910 differential scanning calorimeter.Samples were heated from 100 ºC-500 ºC in hermetic aluminum pans, with helium as the purge gas.All sample pan lids were perforated to allow venting of gaseous products.Densities were determined at room temperature by employing a Micromeritics AccuPyc 1330 gas pycnometer.The mass of the samples were determined on a Mettler Toledo AB104-S balance, and the volumes of the samples were measured by using the pycnometer -an average of 3 measured volumes was used to calculate the density.Sample density was calculated from the ratio of mass (m) to volume (V).(d(g/cm 3 ) = m(g)/V(cm 3 ).The detonation velocity (V D ), detonation pressure (P D ) and specific impulse (I sp ) were calculated based on the calculated heat of formation values using Explo5 v.6.01.

Calculation of heats of formation and densities.
The compounds were drawn in ChemDraw followed by 2D to 3D conversion using MM2 11 minimization in Chem3D. 12The 3D geometries were parameterized using PM6 13 for further semi-empirical calculation.Geometry optimization followed by frequency calculation was performed using AMPAC software. 14The geometries were optimized to their energy minima using Trust algorithm which reduces the number of cycles and refines the precision of the calculation at each step.The optimized geometries were loaded to Codessa III 15 and MOE-2014 16 software for the calculation of van der Waal's volume, densities and heats of formation.

General method for the preparation of (nitrobenzoyl)(pyridin-1-ium-1-yl)-imides 4a-e.
Potassium carbonate (7.5 mmol) was added to a solution of N-aminopyridinium iodide 3 (2.5 mmol) in acetonitrile (25 mL) and the reaction mixture was stirred vigorously for 20 min at room temperature to give a purple solution, to which each acylbenzotriazole (2a-e, 2.5 mmol) was added in one portion.The mixtures were stirred at 20 °C for 1 h.The inorganic salts were filtered through celite and the filtrate was evaporated to dryness to give solids, which were purified by column chromatography over silica gel using first a gradient mixture of ethyl acetate/hexanes (1/10-1/2, v/v) as eluent to remove benzotriazole and then a mixture of acetonitrile/methanol (10/1, v/v) as eluent to give the products.Samples for elemental analysis were recrystallized from a mixture of acetonitrile and ethanol (1/1, v/v).

X-Ray crystallography
The X-ray single crystal diffraction data for 4e and 6d were collected at 120 K on an Agilent SuperNova instrument with focussed microsource Cu Kα radiation (λ = 1.5418Å) and ATLAS CCD area detector.The structures were solved using direct methods with SHELXS 17 and refined on F 2 using all data by full-matrix least square procedures with SHELXL-97. 17Multiscan absorption corrections were done using SCALE3 ABSPACK.The non-hydrogen atoms were refined with anisotropic displacement parameters.All hydrogen atoms were included in calculated positions with isotropic displacement parameters 1.2 times the isotropic equivalent of their carrier atoms.

Supplementary Material
1 H, 13 C spectra, elemental analyses for all obtained compounds.CIF files for compounds 4e, 6d.