Convenient Preparation, Thermal Properties and X-Ray Structure Determination of 2,3-Dihydro-5,6,7,8-tetranitro-1,4-benzodioxine (TNBD): a Promising High Energy Density Material (HEDM)

1. Introduction

Aromatic polynitroderivatives are among the most popular high energy materials (HEMs) [1,2,3,4,5]. From the practical standpoint, HEM must hold not only high energetic properties but also satisfy other strict requirements [4,5,6,7,8]. The most significant are chemical stability, shock-insensitivity, high density and thermostability [5,9,10,11,12].

The most well-known "old" energetic materials are mostly benzene derived polynitrocompounds. Later, N-heterocyclic high energy materials (HEMs) began to be considered as perspective candidates for replacing conventional high explosives, such as TNT, RDX, HMX, and PETN [9,10,13,14,15].

Althought mixed O,N-heterocycles, such as 1,2,5-, 1,2,4- and 1,3,4-oxadiazoles [16,17,18,19,20], as well as oxadiazole N-oxides (furoxans) [21,22,23,24,25,26] , are now amongst the most popular newly designed energetic compounds [27], derivatives of "pure" O-heterocycles as energetic materials have hardly been mentioned in scientific literature (except for unstable cyclic peroxides) [28]. 1,4-Benzodioxane (also known as 1,2-ethylenedioxybenzene, benzo-1,4-dioxane (old chemical names) or 2,3-dihydro-1,4-benzodioxin (new systematic name)) is one of many benzene annulated oxygen O-heterocycles [29].

It should be noted that there are three known structural isomers of benzodioxane (BD): unstable 1,2- benzodioxane [30,31,32], 1,3-benzodioxane [33] and most stable1,4-benzodioxane [34]:

Figure 1. Three structural isomers of benzodioxane (1,2- , 1,3- and 1,4 BD).

Figure 1. Three structural isomers of benzodioxane (1,2- , 1,3- and 1,4 BD).

It is worth noting that the latest isomer, 1,4-benzodioxane (2,3-dihydro-1,4-benzodioxine), originally synthesized by German chemist Vorländer [35] possessed the highest chemical and thermal stability [36].

Current practical applications of 1,4-benzodioxane include its use in medicinal chemistry and pharmacy, where it serves as an excellent core for structurally diverse pharmacologically active agents [37,38,39].

From 1960 onwards, 1,4-benzodioxane derivatives were extensively synthesised and investigated as biologically active compounds for their wide-spectrum pharmacological activity (e.g., anti-inflamatory, anti-histaminic, local anaesthetic, among others) at our University under the direction of Prof. Hab. Dr. Daukšas V. K. (1935-2013) [40,41,42].

Later, in the same laboratory, nitro and amino derivatives of 1,4-benzodioxane were used as a core structural fragment for the synthesis of novel annelated O, N-heterocyclic derivatives [43,44,45,46,47].

Surprisingly, nitro derivatives of 1,4-benzodioxane have not received much attention for their perspective use as energetic materials. One of the primary reasons is probably the relatively high cost of 1,4-benzodioxane as a starting material (31.40 Eu per 10 g, according to Merck Company).

In this work, we expanded our previous investigation [48] of 1,4-benzodioxane polynitro derivatives and chose one of them, tetranitro-1,4-benzodioxane (TNBD), for a a more detailed study of crystal structure, thermal stability, and other properties, with the goal of perspective using it as thermally stable high-energy high-density material.

The current research focuses on TNBD compound, an annulated O-heterocycle 1,4-benzodioxan derivative that has been completely nitrated in the aromatic benzene ring.

A more convenient, optimized synthesis of TNBD substance and its structural characterization via X-ray crystal diffraction analysis are reported below.

2. Results and Discusion

2.1. Materials and Methods

All chemicals and solvents for the synthesis of the benzodioxane derivatives were from Sigma Aldrich (St. Louis, MO, USA). NMR spectra were recorded on Varian Unity Inova 300 spectrometer (300MHz) with tetramethylsilane (TMS) as internal standard. Chemical shifts were reported in ppm (δ). Infrared spectra recorded for KBr discs on Perkin-Elmer Spectrum GX FT-IR. Raman spectra registered on FT-Raman spectrometer Horiba HR800 for the 633-nm-excited spectra. Melting points determined on capillary melting point apparatus Melt-Temp. Thermogravimetric analysis performed on LINSEIS STA PT 1600, Thermogravimetric Analyzer. Quantum mechanical calculation was carried out using PC Spartan ‘10 (Wavefunction Inc., version 1.1.0). Single crystal X-ray diffraction evaluated using instrument “Bruker-Nonius KappaCCD” (Computing data collection 'KappaCCD') and computing data reduction “Denzo and Scalepak (Otwinowski & Minor, 1997)” were applied. All diagrams and calculations performed using maXus (Bruker Nonius, Delft & MacScience, Japan).

2.2. Optimized Synthesis of TNBD by Nitration of 6,7-Dinitro-1,4-benzodioxane (6,7-dinitro-2,3-dihydro-1,4-benzodioxine):

6,7-Dinitro-2,3-dihydro-1,4-benzodioxine (prepared according to the procedure [49]) (25 g, 0.11 M) was added in small portions to the stirred 100% HNO₃ (100 ml, 1.05 M), cooled up to 10 ^oC. The reaction mixture was allowed to warm by gradually increasing the temperature from 10 to 70 ^oC through external heating in a water bath. After cooling to 40–50 ^oC, 50 ml of 98% H₂SO₄ was added by drops, and stirring was continued for 30 min. at 70 ^oC. The reaction was finalized by adding 60 ml of fuming sulfuric acid, containing 30% SO_3, and stirring for 2 hours at 80 ^oC. After cooling to room temperature, the reaction mixture was poured into 0.5 kg of broken ice.

Figure 1. Synthesis scheme of TNBD.

Figure 1. Synthesis scheme of TNBD.

The supernatant was filtered, washed several times with cold distilled water, and dried at room temperature. Yield: 28g, 81%. The analytical sample of TNBD was obtained after two re-crystallizations with acetone. M. p. 286-288 ^oC (lit. m.p. 286 ^oC [50]). It must be noted that this optimized synthesis gave a better yield than our two previously tested variants: starting from 6-nitro-2,3-dihydro-1,4-benzodioxine (63%), or from 5,7-dinitro-2,3-dihydro-1,4-benzodioxine (71%), and using analogous reaction conditions.

2.1. Structure and Properties Investigation

2.1.1. Physicochemical Properties

Molecular weight – 316.14, molecular formula = C₈H₄N₄O₁₀, molecular composition (C 30.39% H 1.28% N 17.72% O 50.61% ). Oxygen balance OB_CO2= - 40.48 %.

TNBD is a white powder in a dispersed state, colorless prisms in crystalline state, crystals almost insoluble in water and conc. sulfuric acid. TNBD is well soluble and crystallizes from methylacetate, acetone, and 2-butanone, but is significantly less soluble in benzene and toluene and almost insoluble in other solvents such as hexane, dichloroethane or CCl₄. Many solvents were tested for the TNBD solubility but only 2-butanone (methylethyl ketone) was found to be the best for growing of high quality crystals, suitable for X-ray diffraction analysis. It is worth noting, that TNBD is substantially more hydrolyticaly stable than analogous structured compounds such as 1,2,3,4-TetrNB (tetranitrobenzene) and HNB (hexanitrobenzene) (further details are discussed below).

2.1.2. Chemical/Hydrolytic Stability

It is generally known that more nitro groups affording benzenes with higher energy suffering from lesser chemical stability [51]. Similarly, higher levels of nitration also induce reduced hydrolytic stability. 1,2,3,4-Tetranitrobenzene is an impact sensitive explosive that easely decomposes in the presence of moisture to 2,4-dinitro-1,3-dihydroxy benzene. Hexanitrobenzene (HNB), another well-known and extremely potent nitroarene explosive [3,5,52], easily hydrolyzes to trinitrophloroglucine when exposed to air humidity [53,54]. It is worth noting that among various poly-substituted benzene nitroderivatives, TNBD possess much superior chemical and thermal stability than 1,2,3,4-tetranitrobenzene and hexanitrobenzene (Figure 3):

Under normal conditions, TNBD has a good hydrolytic stability. Surprisingly, we found no any degradation products after 20 years of storing TNBD samples in our laboratory at normal temperature. An additional dioxan ring in the structure of the TNBD molecule appears to have a strong stabilizing effect. Hydrolytic lability, light sensitivity, and other undesirable properties of hexanitrobenzene (HNB) limited its practical application [55]. Contrary to these HNB properties, synthesized in this work TNBD compound exhibited good chemical stability under normal conditions.

2.1.3. Spectral Properties

NMR spectra: ¹³C spectrum ( δ, [ppm, d₆-DMSO, freq. – 75 MHz): 143,91; 132,43; 129,02; 66,31; ¹H spectrum (d₆-DMSO, freq.- 300MHz): 4,73 (s, 4H).

FT-IR spectrum (cm^-1): 2967, 2889, 1597, 1573, 1566,1558, 1492, 1442, 1382, 1342,1335, 1309, 1249, 1230, 1078, 995, 951, 902, 860, 838, 797, 778, 726, 710, 654, 628, 438.

Figure 4. FT-IR spectrum of TNBD registered in KBr disc.

Figure 4. FT-IR spectrum of TNBD registered in KBr disc.

Raman shift spectra measurements were conducted using instrument FT-Raman spectrometer “Horiba HR800” for the 633-nm-excited spectra. The laser beam was focused to a spot of ca. 1-mm² area (laser power – 300 mW).

Figure 5. FT-Raman spectrum (100 scans) of TNBD (5,6,7,8-tetranitro-2,3-dihydro-1,4-benzodioxine) powder.

Figure 5. FT-Raman spectrum (100 scans) of TNBD (5,6,7,8-tetranitro-2,3-dihydro-1,4-benzodioxine) powder.

2.1.4. Thermal Properties

Analysis of thermal properties of TNBD was performed on LINSEIS STA PT 1600, Thermogravimetric analyzer (at 5 ⁰C/min.):

Figure 6. Thermogravimetric analysis (DTA) of TNBD sample (23 mg, at 5 ^oC/min).

Figure 6. Thermogravimetric analysis (DTA) of TNBD sample (23 mg, at 5 ^oC/min).

The differential thermal analysis (DTA) of twice re-crystallized material revealed onset at 247 and isotherm at 288 ⁰C. The investigated material degraded completely after reaching 300 ^oC, demonstrating thermostability of TNBD being close to that of HNS (hexanitrostilbene), also known as thermostable HEM [56].

Thermal stability of TNBD was found to be significantly higher than that of its analogues, 1,2,3,4-tetranitrobenzene (m.p. 120-121 ⁰C) and hexanitrobenzene (HNB) (m.p. 245-262 ⁰C, with decomposition) [53].

2.1.5. Crystal Properties and Structure Factors Leading to High Density

It was undertaken to establish TNBD three dimensional structure and density. The crystals of TNBD for its X-ray diffraction was obtained by slow evaporation at room temperature of solution in 2-butanone. The geometries of TNBD are tabulated below. All diagrams and calculations were performed using maXus (Bruker Nonius, Delft & MacScience, Japan).

Figure 7a. TNBD - crystal cell package (OrtepPack).

Figure 7a. TNBD - crystal cell package (OrtepPack).

The main parameters of unit cell are listed as following:

a = 10.9946(4),	A alpha = 90.0 deg.
b = 10.1532(4),	A beta = 94.415(2) deg.
c = 10.1814(3),	A gamma = 90.0 deg.
Cell volume	1133.18(7) Å3

Remarkably, only a half of this compound molecule is located in a separate crystal cell unit. A full molecule structure is shown below in Figure 7b:

TNBD - 5,6,7,8-tetranitro-2,3-dihydro-1,4-benzodioxine

Figure 7b. ORTEP representation of TNBD. The thermal ellipsoids are presented with 50% probability (CCDC 2320894) (crystal was grown from 2-butanone).

Figure 7b. ORTEP representation of TNBD. The thermal ellipsoids are presented with 50% probability (CCDC 2320894) (crystal was grown from 2-butanone).

Table 1. The main crystallographic data for TNBD:.

Empirical formula	C8 H4 N4 O10
Formula weight	316.138
Temperature	173(2) K (-100 0C)
Wavelength	0.71073 Å
Crystal system	Monoclinic
Space group	C 2/c
Unit cell dimensions	a = 10.9946(4), A alpha = 90.0 deg.
	b = 10.1532(4),A beta = 94.415(2) deg.
	c = 10.1814(3), A gamma = 90.0 deg.
Volume	1133.18(7) Å3
Z	4
Density (calculated)	1.853 g/cm3
Density (measured, pycnom.)	1.84 g/cm3
Absorption coefficient	0.176 mm-1
F(000)	640
Crystal size	0.31 x 0.22 x 0.21 mm
Two theta max. for data	58.0 deg.
Reflections collected	2368
Independent reflections	1464 [R(int) = 0.020]
Refinement method	Full-matrix least-squares on F2
Data / restraints / parameters	1292 / 0 / 108
Final R indices [I>3sigma(I)]	R1 = 0.032, wR2 = 0.077
R indices (all data)	R1 = 0.041, wR2 = 0.116
Largest diff. peak and hole	0.41 and -0.37 e.Å-3

Table 1. The main crystallographic data for TNBD:.

Empirical formula	C8 H4 N4 O10
Formula weight	316.138
Temperature	173(2) K (-100 0C)
Wavelength	0.71073 Å
Crystal system	Monoclinic
Space group	C 2/c
Unit cell dimensions	a = 10.9946(4), A alpha = 90.0 deg.
	b = 10.1532(4),A beta = 94.415(2) deg.
	c = 10.1814(3), A gamma = 90.0 deg.
Volume	1133.18(7) Å3
Z	4
Density (calculated)	1.853 g/cm3
Density (measured, pycnom.)	1.84 g/cm3
Absorption coefficient	0.176 mm-1
F(000)	640
Crystal size	0.31 x 0.22 x 0.21 mm
Two theta max. for data	58.0 deg.
Reflections collected	2368
Independent reflections	1464 [R(int) = 0.020]
Refinement method	Full-matrix least-squares on F2
Data / restraints / parameters	1292 / 0 / 108
Final R indices [I>3sigma(I)]	R1 = 0.032, wR2 = 0.077
R indices (all data)	R1 = 0.041, wR2 = 0.116
Largest diff. peak and hole	0.41 and -0.37 e.Å-3

(Full geometry tables for the compound TNBD are presented below in Apendix A)

(Apendix A: Geometry tables for the compound TNBD:

A.1 INTRAMOLECULAR BOND LENGTHS

A.2 INTRAMOLECULAR BOND ANGLES

A.3 INTRAMOLECULAR TORSION ANGLES (H omitted)

A.4 INTERMOLECULAR NON-BONDED DISTANCES

A.5 DIHEDRAL ANGLES FORMED BY LSQ PLANES)

It is worth noting, that nitrogroups in positions 7 and 8 of TNBD are twisted out of the benzene ring plane by different angles, 46.143 and 68.706 degrees (because symmetry, the similar angles are observed for another two NO₂ groups in positions 5 and 6 correspondingly.

For comparison, in HNB (hexanitrobenzene), nitrogrups are rotated at 53^o [52,53] .

In the crystal structure of TNBD, the molecules are in special positions; each molecule lies on a symmetry axis of order 2. Therefore, atoms O1 and O4, C2 and C3, C9 and C10, etc. are symmetrically equivalent. Figure 8 shows a projection of the molecule along the axis of symmetry. This figure clearly indicates that the dioxane ring in TNBD is characterized by a twist conformation. The exit of the C2 atom from the planar phenyl ring of the molecule is equal to 0.384(2) Å (correspondingly, the exit of the C3 atom is −0.384(2) Å). The dihedral angle between the phenyl ring of the molecule and the “triangle” O1−C2−C3 is 41.8(1)° (due to symmetry, the same angle is observed between the phenyl ring and the “triangle” C2−C3−C4).

Figure 8. A projection of the molecular structure of TNBD along the twofold axis.

Figure 8. A projection of the molecular structure of TNBD along the twofold axis.

The packing of TNBD molecules in the crystal lattice is quite dense. The packing index, calculated using Kitaigorodsky’s approach [57,58], is equal to 0.745. The crystal structure is characterized by strong intermolecular sigma-hole interactions (see Figure 9). The distance between the O52 and O61 atoms of neighboring molecules is 2.910(2) Å (the same distance occurs between the atoms O82 and O71 due to symmetry). Also noteworthy is the strong interaction between atoms O1 and O62 (as well as atoms O4 and O72), which is characterized by a distance of 2.910(2) Å. In addition, the carbon atom C2 (like C3) has increased electronegativity because it is bonded to oxygen. This promotes the formation of intermolecular hydrogen bonds of CH···O type (see in Figure 9). The parameters of this H-bond are the following: C2···O82 = 3.321(3) Å, H2B···O82 = 2.75(2) Å, C2-H2B···O82 = 122(3)°; i.d., this bond is quite weak. Nevertheless, it participates in stabilizing the crystal structure. By means of these intermolecular interactions and hydrogen bonds, the molecular chains along the long parameter of the lattice are formed in the crystal structure. The consequence of these features of the crystal structure of TNBD is a significant packing index value and, in accordance with this, a quite high density of the substance.

Figure 9. The formation of a molecular chain in the crystal structure of TNBD shows sigma-hole interactions and H-bonds. .

Figure 9. The formation of a molecular chain in the crystal structure of TNBD shows sigma-hole interactions and H-bonds. .

2.1.6. Electrostatic Potential Map (MEP ) for TNBD Molecule

The initial refinement of TNBD geometry was performed by semi-empirical (PM6) method, and further optimization and calculation were performed by DT-B3LYP functional method in conjunction with 6-311G(d,p) basis set. The compounds' structure were globally optimized with symmetry restriction, and their local minima were characterized by the harmonic frequency analysis.

Figure 10. The surface of the electrostatic potential (MEP) of TNBD, calculated by DFT-B3LYP/6-311G(d, p) method. The colours red and blue depict the compound's negatively and positively charged regions (MEP isovalue = 0.002). .

Figure 10. The surface of the electrostatic potential (MEP) of TNBD, calculated by DFT-B3LYP/6-311G(d, p) method. The colours red and blue depict the compound's negatively and positively charged regions (MEP isovalue = 0.002). .

2.1.6. Main Calculated Detonation Performance Parameters of TNBD in Comparison with Known Energetic Materials Possessing Close Structural Patterns

The main calculated detonation parameters of TNBD (at density 1,85 g/cm³).

The calculated enthalpy of TNBD formation was estimated as (-382 kJ/M) (HF). The detonation heat -5112.9 kJ/M. The main parameters calculated for TNBD by EXPLO 5 [59,60,61] are represented below:

Table 2. The main calculated detonation parameters of TNBD (at density 1.85 g/cm³), compared to known energetic materials:.

Detonation parameters	TNBD	DNAN, 2,4-dinitro- anisole	TNAN, 2,4,6-Trinitro- anisole	TNT	Tetryl
Oxygen balance, %	-40.48	-96.90	-62.50	-73.97	-47.40
Detonation temperature (K)	3874	2743	2366	3744	3370
Detonation pressure (kbar)	277.84	159	205	202	260
Detonation velocity (m/s)	7726.9 at d=1.85 g/cm3	5706 at d=1.341 g/cm3	6800 at d=1.57 g/cm3	6950 at d=1.64g/cm3	7570 at d= 1.71 g/cm3
Volume of detonation products (L/kg)	555.8	626	760	730	800

Table 2. The main calculated detonation parameters of TNBD (at density 1.85 g/cm³), compared to known energetic materials:.

Detonation parameters	TNBD	DNAN, 2,4-dinitro- anisole	TNAN, 2,4,6-Trinitro- anisole	TNT	Tetryl
Oxygen balance, %	-40.48	-96.90	-62.50	-73.97	-47.40
Detonation temperature (K)	3874	2743	2366	3744	3370
Detonation pressure (kbar)	277.84	159	205	202	260
Detonation velocity (m/s)	7726.9 at d=1.85 g/cm3	5706 at d=1.341 g/cm3	6800 at d=1.57 g/cm3	6950 at d=1.64g/cm3	7570 at d= 1.71 g/cm3
Volume of detonation products (L/kg)	555.8	626	760	730	800

These data demonstrate that the calculated detonation velocity (D) of TNBD is somewhat greater than that of the well-known energetic tetranitrocompound tetryl [1] (7727 vs. 7570 m/s), and similarly for detonation pressure (P) (278 vs. 260 kbar (exp. [3]), whereas the energetic properties of TNBD in terms aforementioned parameters significantly outperformed DNAN, TNAN, and TNT.