An Efficient Synthesis and Preliminary Investigation of Novel 1,3-Dihydro-2H-benzimidazol-2-one Nitro and Nitramino Derivatives

The preparation and properties of a series of novel 1,3-dihydro-2H-benzimidazol-2-one nitro and nitramino derivatives are described. A detailed crystal structure of one of the obtained compounds, 4,5,6-trinitro-1,3-dihydro-2H-benzimidazol-2-one (TriNBO), was characterized using low temperature single crystal X-ray diffraction, namely an orthorhombic yellow prism, space group ‘P 2 21 21′, experimental crystal density 1.767 g/cm3 (at 173 K). Methyl analog, 5-Me-TriNBO-monoclinic red plates, space group, P 21/c, crystal density 1.82 g/cm3. TriNBO contains one activated nitro group at the fifth position, which was used for the nucleophilic substitution in the aminolysis reactions with three monoalkylamines (R=CH3, C2H5, (CH2)2CH3) and ethanolamine. The 5-R-aminoderivatives were further nitrated with N2O5/ HNO3 and resulted in a new group of appropriate nitramines: 1,3-dihydro-2H-5-R-N(NO2)-4,6-dinitrobenzimidazol-2-ones. Thermal analysis (TGA) of three selected representatives was performed. The new compounds possess a high melting point (200–315 °C) and thermal stability and can find a potential application as new thermostable energetic materials. Some calculated preliminary energetic characteristics show that TriNBO, 5-Me-TriNBO, 5-methylnitramino-1,3-dihydro-2H-4,6-dinitrobenzimidazol-2-one, and 5-nitratoethylnitramino-1,3-dihydro-2H-4,6-dinitrobenzimidazol-2-one possess increased energetic characteristics in comparison with TNT and tetryl. The proposed nitrocompounds may find potential applications as thermostable high-energy materials.


Introduction
Various types of nitrocompounds contain appropriate structural fragments: (a) C-NO 2 ; (b) O-NO 2 , and (c) N-NO 2 which are differentiated into three groups (nitrocompounds, organic nitrates, and nitramines) [1]. The representatives of the latter group, organic nitramines, are widely used as high-energy materials (HEMs) and components of explosive compositions, propellants, or solid rocket fuel [2,3]. Heterocyclic nitroderivatives containing a nitramino group have been insufficiently investigated when compared to benzene analogs (tetryl, pentryl, etc.) and alicyclic (RDX, HMX, CL-20, and some others that are similar) nitramine compounds, nowadays produced in the industry in bulk quantities [4,5]. Some representatives of pyridine nitramines have been investigated in the published work of Polish scientists [6][7][8]. A few 1,2,4-triazole and tetrazole nitramines have been mostly studied during the last decade by researchers originating from Germany [9][10][11] and Russia [12,13]. An interesting work dedicated to energetic nitroimidazole nitramines has recently been published by USA authors [14].
In continuation to our previous studies [15][16][17][18], herein we describe the synthesis, LC-MS analysis, X-ray structure, and preliminary investigation of properties of some new nitro and nitramino benzimidazole derivatives. Most of the nitration procedures, which have been previously applied for the nitration of benzimidazole derivatives [19][20][21][22][23][24][25][26][27][28][29][30],were carried out in a step-by-step procedure, introducing one nitro group in one reaction. In the present work we successfully introduced three nitro groups in one synthetic procedure by our modified efficient nitration methodology (Figure 1

General Methods and Materials
All chemical reagents and solvents were purchased from commercial suppliers (Sigma-Aldrich (St. Louis, MO, USA), TCI-Europe (Zwijdrecht, Belgium) and Merck (Darmstadt, Germany)) and were used without purification unless otherwise specified. The N2O5/HNO3 solution was prepared from HNO3 and P2O5 according to the known method [31]. The purity of the compounds was monitored by TLC, using silica gel 60 F254 aluminum plates (Merck, Darm-stadt, Germany). Visualization was performed under UV light. The flash column chromatography was carried out by using a Wakogel C-200 (Wako Chemical, Osaka, Japan) silica gel. The calculated densities and other molecular characteristics of the investigated compounds (3)(4)(5)(6) were obtained using ACD Labs software (Toronto), vers.11. The melting points were determined in open capillaries using a MEL-TEMP apparatus (Barn-stead Thermolyne Corp., Dubuque, IA, USA). UV-VIS spectra were recorded using a Lambda 25 UV-VIS spectrophotometer (PerkinElmer, Waltham, MA, USA). IR spectra were recorded in KBr on a PerkinElmer spectrophotometer (FT-IR Spectrum BX II). NMR spectra were obtained with Bruker spectrometer (400 MHz) in d6-dimethylsulfoxide, using a residual solvent signal as an internal standard. Liquid chromatography-mass spectrometry (LC-MS) analyses were

General Methods and Materials
All chemical reagents and solvents were purchased from commercial suppliers (Sigma-Aldrich (St. Louis, MO, USA), TCI-Europe (Zwijdrecht, Belgium) and Merck (Darmstadt, Germany)) and were used without purification unless otherwise specified. The N 2 O 5 /HNO 3 solution was prepared from HNO 3 and P 2 O 5 according to the known method [31]. The purity of the compounds was monitored by TLC, using silica gel 60 F254 aluminum plates (Merck, Darm-stadt, Germany). Visualization was performed under UV light. The flash column chromatography was carried out by using a Wakogel C-200 (Wako Chemical, Osaka, Japan) silica gel. The calculated densities and other molecular characteristics of the investigated compounds (3-6) were obtained using ACD Labs software (Toronto), vers.11. The melting points were determined in open capillaries using a MEL-TEMP apparatus (Barn-stead Thermolyne Corp., Dubuque, IA, USA). UV-VIS spectra were recorded using a Lambda 25 UV-VIS spectrophotometer (PerkinElmer, Waltham, MA, USA). IR spectra were recorded in KBr on a PerkinElmer spectrophotometer (FT-IR Spectrum BX II). NMR spectra were obtained with Bruker spectrometer (400 MHz) in d6-dimethylsulfoxide, using a residual solvent signal as an internal standard. Liquid chromatography-mass spectrometry (LC-MS) analyses were performed using an HPLC system (CBM-20A controller, two LC-2020AD pumps, SIL-30AC autosampler, and CTO-20AC column oven; Shimadzu, Japan) equipped with photodiode array (PDA) detector (SPD-M20A Prominence diode array detector; Shimadzu, Japan) and mass spectrometer (LCMS-2020, Shimadzu, Japan) equipped with an ESI source. The chromatographic separation was conducted using a Hydrosphere C18 column, 4 × 150 mm 2 (YMC, Kyoto, Japan) at 40 • C and a mobile phase that consisted of 0.1% formic acid (solvent A) and acetonitrile (solvent B) delivered in gradient elution mode at a flow rate of 0.6 mL min −1 . The elution program that was used was as follows: isocratic 0% B for 0.5 min, from 0 to 60% B over 4.5 min, isocratic 60% B for 0.1 min, from 60 to 0% B over 0.1 min, isocratic 0% B for 5 min. Mass scans were measured from m/z 10 up to 1000, at 350 • C interface temperature, 250 • C DL temperature, ±4500 V interface voltage, and neutral DL/Qarray, using N 2 as nebulizing and drying gas. Mass spectrometry data were acquired in both the positive and the negative ionization modes. The data were analyzed using Lab-Solutions LC-MS software (YMC, Japan). Thermogravimetric analysis was performed on a PerkinElmer thermogravimetric analyzer, STA6000 TGA/DSC. Density measurements were performed at 293 K using an AccuPyc 1330 Pycnometer (Micromeritics) Single crystal X-ray diffraction was evaluated using the instrument "Bruker-Nonius KappaCCD" (Computing data collection 'KappaCCD') and computing data reduction "Denzo and Scalepak (Otwinowski and Minor, 1997)" were applied. All diagrams and calculations were performed using maXus (Bruker Nonius, Delft & MacScience, Tokyo, Japan).

Synthesis and Properties Investigation of 1,3-Dihydro-2H-benzimidazol-2-one Polynitrocompounds
Trinitrosubstituted 1,3-dihydro-2H-benzimidazol-2-one derivatives were prepared by the application of a convenient and efficient one-pot nitration reaction, which avoids the use of an aggressive fuming nitric acid and multistep procedures.
The main compound (TriNBO) (1) was prepared from 1,3-dihydro-2H-benzimidazol -2-one, by nitration reaction using an efficient system derived from potassium nitrate/sulphuric acid: A total of 13.4 g (0.1 M) of 1,3-dihydro-2H-benzimidazol-2-one was added in small portions by stirring and cooling (ice/water) to the 33.3 g (0.33 mol) KNO 3 , solution in 100 mL H 2 SO 4 (98%) (Scheme 1 below). When the whole amount of the compound was added, the reaction mixture was slowly (during 1 h) warmed up to 60 • C. Then, the reaction mixture was stirred at 50-60 • C for 4 h. Finally, the reaction mixture was allowed to cool to room temperature and then cooled to 4 • C in the freezer (12 h). The obtained yellow crystalline product was filtered off on the porous glass filter and washed several times with 50% cold H 2 SO 4 , then with cold distilled water and dried at 100 • C overnight. The yield was 22.3 g (83%), m. p. 314-315 • C (sublimation with decomp.) (lit. m.p. 313-315 • C (dec.) [32]. The obtained reaction product (TriNBO) was pure and did not contain any impurities of dinitro-or tetranitro-derivatives (LC-MS analysis data are presented below in the next section in Figure 2a  performed using an HPLC system (CBM-20A controller, two LC-2020AD pumps, SIL-30AC autosampler, and CTO-20AC column oven; Shimadzu, Japan) equipped with photodiode array (PDA) detector (SPD-M20A Prominence diode array detector; Shimadzu, Japan) and mass spectrometer (LCMS-2020, Shimadzu, Japan) equipped with an ESI source. The chromatographic separation was conducted using a Hydrosphere C18 column, 4 × 150 mm 2 (YMC, Kyoto, Japan) at 40 °C and a mobile phase that consisted of 0.1% formic acid (solvent A) and acetonitrile (solvent B) delivered in gradient elution mode at a flow rate of 0.6 mL min −1 . The elution program that was used was as follows: isocratic 0% B for 0.5 min, from 0 to 60% B over 4.5 min, isocratic 60% B for 0.1 min, from 60 to 0% B over 0.1 min, isocratic 0% B for 5 min. Mass scans were measured from m/z 10 up to 1000, at 350 °C interface temperature, 250 °C DL temperature, ±4500 V interface voltage, and neutral DL/Qarray, using N2 as nebulizing and drying gas. Mass spectrometry data were acquired in both the positive and the negative ionization modes. The data were analyzed using Lab-Solutions LC-MS software (YMC, Japan). Thermogravimetric analysis was performed on a PerkinElmer thermogravimetric analyzer, STA6000 TGA/DSC. Density measurements were performed at 293 K using an AccuPyc 1330 Pycnometer (Micromeritics) Single crystal X-ray diffraction was evaluated using the instrument "Bruker-Nonius KappaCCD" (Computing data collection 'KappaCCD') and computing data reduction "Denzo and Scalepak (Otwinowski and Minor, 1997)" were applied. All diagrams and calculations were performed using maXus (Bruker Nonius, Delft & MacScience, Tokyo, Japan).

Synthesis and Properties Investigation of 1,3-Dihydro-2H-benzimidazol-2-one Polynitrocompounds
Trinitrosubstituted 1,3-dihydro-2H-benzimidazol-2-one derivatives were prepared by the application of a convenient and efficient one-pot nitration reaction, which avoids the use of an aggressive fuming nitric acid and multistep procedures.

in One Step
The main compound (TriNBO) (1) was prepared from 1,3-dihydro-2H-benzimidazol -2-one, by nitration reaction using an efficient system derived from potassium nitrate/sulphuric acid: The obtained analysis results clearly demonstrate that nitration reaction product is pure trinitroderivative, TriNBO, which does not contain any dinitro-or tetranitroderivatives impurities.
Additionally, the main properties of this material were described.  The obtained analysis results clearly demonstrate that nitration reaction product is pure trinitroderivative, TriNBO, which does not contain any dinitro-or tetranitro-derivatives impurities.
Additionally, the main properties of this material were described. The main properties of TriNBO (C 7 H 3 N 5 O 7 ) are shortly represented below. TriNBO in its amorphous or microcrystalline state is a stable yellow powder, almost insoluble in water and concentrated sulfuric acid, but is better soluble in nitric acid. With alcohols (especially methanol) and some other polar solvents, TriNBO forms stable solvates. TriNBO is well soluble in DMSO, DMF, dimethylacetamide, sulfolane, methyl acetate, methanol, nitromethane, and nitrobenzene. It is much less soluble in benzene, toluene, and xylene and poorly soluble in other solvents such as acetone, hexane, dichloroethane, or CCl 4 . TriNBO is stable during storage at room temperature. The analysis results of a sample in our laboratory did not show any changes even after 20 years.

5, x FOR PEER REVIEW 5 of 21
The main properties of TriNBO (C7H3N5O7) are shortly represented below. TriNBO in its amorphous or microcrystalline state is a stable yellow powder, almost insoluble in water and concentrated sulfuric acid, but is better soluble in nitric acid. With alcohols (especially methanol) and some other polar solvents, TriNBO forms stable solvates. TriNBO is well soluble in DMSO, DMF, dimethylacetamide, sulfolane, methyl acetate, methanol, nitromethane, and nitrobenzene. It is much less soluble in benzene, toluene, and xylene and poorly soluble in other solvents such as acetone, hexane, dichloroethane, or CCl4. TriNBO is stable during storage at room temperature. The analysis results of a sample in our laboratory did not show any changes even after 20 years.

Thermal Properties Investigation
Analysis of the thermal properties of TriNBO was performed on a PerkinElmer thermogravimetric analyzer, STA6000 TGA/DSC at 5 °C/min ( Figure 4):  Thermal degradation of TriNBO is a complex process. There are two separate processes that are observed simultaneously on heating, partial sublimation, and decomposition after starting mass loss at the point of 315 °C. The total mass loss reached a maximum of 339 °C (decomposition without deflagration).

Crystal Properties of TriNBO (X-ray Diffraction Structure Analysis)
This was undertaken to establish 4,5,6-trinitro-1,3-dihydro-2H-benzimidazol-2-one three-dimensional structure and density. The crystals of TriNBO (yellow prisms) for its X-ray diffraction were obtained by long-standing in the freezer (at −18 °C temperature) of its solution in 95% HNO3 (the reason for the selection of such non-ordinary solvent was to avoid solvate formation). The geometries of TriNBO are tabulated below ( Figures 5-7). The main crystallographic data are shown below in Table 1. Thermal degradation of TriNBO is a complex process. There are two separate processes that are observed simultaneously on heating, partial sublimation, and decomposition after starting mass loss at the point of 315 • C. The total mass loss reached a maximum of 339 • C (decomposition without deflagration).

Crystal Properties of TriNBO (X-ray Diffraction Structure Analysis)
This was undertaken to establish 4,5,6-trinitro-1,3-dihydro-2H-benzimidazol-2-one three-dimensional structure and density. The crystals of TriNBO (yellow prisms) for its X-ray diffraction were obtained by long-standing in the freezer (at −18 • C temperature) of its solution in 95% HNO 3 (the reason for the selection of such non-ordinary solvent was to avoid solvate formation). The geometries of TriNBO are tabulated below (Figures 5-7). The main crystallographic data are shown below in Table 1.
An Ortep image of the TriNBO X-Ray structure is shown in Figure 7.
A detailed list of bond and angles values and other X-ray data are placed in Appendices A and B (Table A1 and Figure A1). Schematic representation of dihedral angles is shown in Figure A1 in Appendix B.      The crystal structure investigation results clearly demonstrate that dihedral angles between 4, 5, and 6 nitro groups and benzimidazol-2-one plane are very different (angles for nitrogroups: 4-NO 2 : 7.26 • 5-NO 2 : 88.85 • and for 6-NO 2 : 14.42 • ) and the longest nitrogroup bond C-N (1.495 Å) is for 5-NO 2 (activated nitro group, which can participate in substitution reactions). Other neighbor nitro group C-N bonds are substantially shorter: 4-NO 2 (1.427 Å) and 6-NO 2 (1.471 Å). We explored the activity of the 5-NO 2 group in TriNBO for the derivatization and synthesis of the new energetic nitramines, possessing 4,6-dinitrobenzimidazole moiety. The starting compound 5-methyl-1,3-dihydro-2H-benzimidazol-2-one, was obtained by the published method by heating a mixture of 4-methyl-1,2-phenylenediamine and urea at 180 • C [33].
An Ortep image of the TriNBO X-Ray structure is shown in Figure 7. A detailed list of bond and angles values and other X-ray data are placed in Appendices A and B (Table A1 and Figure A1). Schematic representation of dihedral angles is shown in Figure A1 in Appendix B.
The 5-Me-TriNBO was synthesized using a convenient one-step nitration reaction of 5-methyl-1,3-dihydro-2H-benzimidazol-2-one, by the application of the same nitration system derived from potassium nitrate/sulphuric acid, but in modified temperature conditions (Scheme 2 below): KNO3 totaling 33.3 g (0.33 mol) was added in small portions to 100 mL of conc. H2SO4 (98%) with stirring and cooling (5 °C, ice-water bath) and the homogenous nitration mixture was obtained. This was followed by the careful addition of 14.82 g (0.1 mol) 5-methyl-1,3-dihydro-2H-benzimidazol-2-one in small portions until the reaction mixture became a clear light yellow. Hereafter, the reaction temperature was gradually raised to about 60 °C for 2 h, and finally, the mixture was heated to 90 °C for 4 h. The resulting mixture was allowed to cool to room temperature and then transferred to a closed vessel and placed in the freezer (+4 °C) to stand overnight. The obtained orange-red microcrystalline supernatant was filtered using sintered glass filter. The product was washed with 50% H2SO4 and then several times with distilled H2O and finally dried at 100 °C to the constant weight. The yield was 25.4 g, 89.7%. M.p. 285-7 °C (dec.). The spectral data are summarized in Table A2 in Appendix C. The physicochemical properties (solubility, and others) of 5-methyl-1,3-dihydro-2H-benzimidazol-2-one are almost similar to TriNBO, however, 5-methyl group insertion in aromatic ring leads to the substantial decrease in thermostability. The obtained one step reaction product (5-Me-TriNBO) was pure and did not contain any dinitro-derivative impurities (LC-MS analysis data are represented below). Scheme 2. Efficient nitration reaction of 5-methyl-1,3-dihydro-2H-benzimidazol-2-one for the preparation of 5-methyl-TriNBO (2) in one step. KNO 3 totaling 33.3 g (0.33 mol) was added in small portions to 100 mL of conc. H 2 SO 4 (98%) with stirring and cooling (5 • C, ice-water bath) and the homogenous nitration mixture was obtained. This was followed by the careful addition of 14.82 g (0.1 mol) 5-methyl-1,3dihydro-2H-benzimidazol-2-one in small portions until the reaction mixture became a clear light yellow. Hereafter, the reaction temperature was gradually raised to about 60 • C for 2 h, and finally, the mixture was heated to 90 • C for 4 h. The resulting mixture was allowed to cool to room temperature and then transferred to a closed vessel and placed in the freezer (+4 • C) to stand overnight. The obtained orange-red microcrystalline supernatant was filtered using sintered glass filter. The product was washed with 50% H 2 SO 4 and then several times with distilled H 2 O and finally dried at 100 • C to the constant weight. The yield was 25.4 g, 89.7%. M.p. 285-7 • C (dec.). The spectral data are summarized in Table A2 in Appendix C. The physicochemical properties (solubility, and others) of 5-methyl-1,3-dihydro-2H-benzimidazol-2-one are almost similar to TriNBO, however, 5methyl group insertion in aromatic ring leads to the substantial decrease in thermostability. The obtained one step reaction product (5-Me-TriNBO) was pure and did not contain any dinitro-derivative impurities (LC-MS analysis data are represented below).  The product of the nitration reaction of 5-methyl-1,3-dihydro-2H-benzimidazol-2-one, 2 mg 5-Me-TriNBO solution in 1 mL dimethyl sulfoxide was used for LC-MS analysis. The results are shown in Figure 8. This was undertaken to establish the 5-methyl-4,5,6-trinitro-1,3-dihydro-2H-benzimidazol-2-one three-dimensional structure and density. The crystals of 5-Me-TriNBO (red plates) for its X-ray diffraction were obtained by prolonged standing in the freezer (at +4 °C temperature) of the compound solution in nitromethane. The Ortep of the 5-MeTriNBO is shown below (Figure 9) and the main crystallographic data are represented in Table 2. This was undertaken to establish the 5-methyl-4,5,6-trinitro-1,3-dihydro-2H-benzimidazol-2-one three-dimensional structure and density. The crystals of 5-Me-TriNBO (red plates) for its X-ray diffraction were obtained by prolonged standing in the freezer (at +4 • C temperature) of the compound solution in nitromethane. The Ortep of the 5-MeTriNBO is shown below (Figure 9) and the main crystallographic data are represented in Table 2. Materials 2022, 15, x FOR PEER REVIEW 12 of 21 Figure 9. Ortep image at the 50% probability of thermal ellipsoids for 5-methyl-4,6,7-trinitro-1,3-dihydro-2H-benzimidazol-2-one (5-Me-TriNBO, (2))-(crystal (red plate)) was obtained from the solution in nitromethane)). Structure analysis revealed that the 5-NO2 group, with a maximal angle (88.85°) and increased bond (N-C) length (1.495 angstroms) can demonstrate a heightened chemical Figure 9. Ortep image at the 50% probability of thermal ellipsoids for 5-methyl-4,6,7-trinitro-1,3dihydro-2H-benzimidazol-2-one (5-Me-TriNBO, (2))-(crystal (red plate)) was obtained from the solution in nitromethane. Structure analysis revealed that the 5-NO 2 group, with a maximal angle (88.85 • ) and increased bond (N-C) length (1.495 angstroms) can demonstrate a heightened chemical reactivity and especially increased reactivity in various displacement reactions of this NO 2 group with various basic amino-compounds [34]. The starting 5-NH(R)-4,6dinitrobenzimidazolones (R=CH 3 , C 2 H 5 CH 3 (CH 2 ) 2 , and HOCH 2 CH 2 ) were prepared by a simple aminolysis procedure that was described in our previous work by the reaction in ethanol (70 • C, 8 h) of TriNBO with an appropriate amine [34].
By stirring and cooling (ice/ water bath) (0.01 mol) of appropriate 5-NHR 1,3-dihydro-2H-benzimidazol-2-one was added in small portions to 40% N 2 O 5 / HNO 3 solution (5 mL) in 100 mL CH 2 Cl 2 . (Scheme 3 below). When the whole compound was added, the reaction mixture was slowly (during 1 h) warmed up to 40 • C. Then, the reaction mixture was stirred with a magnetic stirrer for an additional 4 h and allowed to stand at room temperature overnight. Finally, the reaction mixture was stirred and cooled to 0-5 • C in crushed ice, and the residual nitric acid was carefully neutralized by the addition of 15 g powdered MgCO 3 . The obtained neutralized yellow solution of nitramine was filtered off on the porous glass filter, and the solid supernatant of magnesium salts were washed several times with CH 2 Cl 2 and the solvent was removed on a rotary evaporator. The resulting yellow crystalline product was recrystallized from 70% acetic acid and dried at 60 • C.
By stirring and cooling (ice/ water bath) (0.01 mol) of appropriate 1,3-dihydro-2H-benzimidazol-2-one was added in small portions to 40% N2O5 solution (5 mL) in 100 mL CH2Cl2. (Scheme 3 below). When the whole compou added, the reaction mixture was slowly (during 1 h) warmed up to 40 °C. Th reaction mixture was stirred with a magnetic stirrer for an additional 4 h and all stand at room temperature overnight. Finally, the reaction mixture was stirr cooled to 0-5 °C in crushed ice, and the residual nitric acid was carefully neutra the addition of 15 g powdered MgCO3. The obtained neutralized yellow solu nitramine was filtered off on the porous glass filter, and the solid supern magnesium salts were washed several times with CH2Cl2 and the solvent was r on a rotary evaporator. The resulting yellow crystalline product was recrystalliz 70% acetic acid and dried at 60 °C. The obtained N-nitration reaction products, nitramines, were substantially p did not contain any other nitroderivative impurities (the LC-MS analysis d presented in Table 3 in the next section). The obtained N-nitration reaction products, nitramines, were substantially pure and did not contain any other nitroderivative impurities (the LC-MS analysis data are presented in Table 3 in the next section).
The yields of synthesis, m.p., and spectral data of the prepared nitramines are presented in Table 3.

Thermal Analysis of Selected Nitramines
There were two representatives of synthesized nitramine compounds (5-MeN(NO 2 )DNBO (3) and 5-O 2 NO(CH 2 ) 2 N(NO 2 )DNBO (6)) that possess more interesting energetic characteristics, so, it was important to investigate their thermostability. Table 3. The main data of the synthesized 1,3-dihydro-2H-benzimidazol-2-one nitro-and nitramino compounds. The thermal stability of synthesized compounds was investigated at 5 • C/min. The results of the thermogravimetric analysis are represented in Figures 10 and 11 below.  The yields of synthesis, m.p., and spectral data of the prepared nitramines are presented in Table 3.

Thermal Analysis of Selected Nitramines
There were two representatives of synthesized nitramine compounds (5-MeN(NO2)DNBO (3) and 5-O2NO(CH2)2N(NO2)DNBO (6)) that possess more interesting energetic characteristics, so, it was important to investigate their thermostability.  The nitration reaction yields and the main properties of the synthesized new nitrocompounds are summarized in Table 3 and Table A2 in Appendix C:

The Main Physicochemical and Spectral Properties of Synthesized 1,3-Dihydro-2H-benzimidazol-2-one Nitrocompounds and Nitramines
The nitration reaction yields and the main properties of the synthesized new nitrocompounds are summarized in Tables 3 and A2 in Appendix C:

Calculated Detonation Performance of Benzimidazol-2-one Nitro and Nitramino Derivatives
The preliminary data on the energetic properties and detonating performance of newly synthesized benzimidazol-2-one nitro and nitramino compounds are presented in Table 4. where ρ is the density of the compounds. These obtained data demonstrate that the calculated detonation velocity (D) and detonation pressure of most of the new derivatives are superior to the known standard compound TNT, however, their characteristics in general are something less when compared with other standard compound tetryl characteristics.
The crystal structure and experimental crystal densities of TriNBO (1.76 g/cm 3 ) and 5-Me-TriNBO (1.82 g/cm 3 ) were investigated. We revealed that TriNBO crystallizes as orthorhombic yellow prisms in the space group P 2 21 21, while its methyl analog, 5-Me-TriNBO as monoclinic red plates in the space group P 21/c.
The results of thermal analysis (DTA) of TriNBO showed onset at 315 • C and the highest degree of decomposition at 339 • C, which means that the thermostability of TriNBO is superior to that of HNS (hexanitrostilbene), widely known as thermostable HEM [8,13,14].
The main detonation characteristics (velocity of detonation and detonation pressure) of the new nitrocompounds were calculated. The calculated detonation velocity and detonation pressure of TriNBO, its methyl analog, 5-Me-TriNBO and benzimidazol-2-one nitramines are superior to the experimental parameters of the standard aromatic energetic trinitrocompound TNT, and in general they are about equal to the detonation parameters of standard tetranitrocompound tetryl.
Our prepared energetic nitramines, 5-CH 3 N(NO 2 )DNBO) and 5-(ONO 2 CH 2 CH 2 -N(NO 2 )DNBO possess good energetic characteristics, but their thermostability is substantially lower in comparison with that of TriNBO. Nitramines containing N-ethyl and N-propyl groups are substantially less thermostable and possess slightly worse energetic properties because of their diminished oxygen balance.
Author Contributions: Conceptualization, synthesis, and manuscript writing, J.Š.; experimental design and data curation, J.S.; computer calculations, data curation, and manuscript review and editing, J.T. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.
Data Availability Statement: Samples of the compounds are not available from the authors but can be specially prepared under request.

Acknowledgments:
The authors are grateful to G. Petraikytė (Vilnius University) for the vibrational spectra and M. Jonušis (Vilnius University) for the NMR spectra registration, and V. Plaušinaitienė (Vilnius University, Faculty of Chemistry) for the substantial help with thermal analysis. The authors are thankful for the high performance computing resources that were provided by the Information Technology Research Center of Vilnius University.

Conflicts of Interest:
The authors declare no conflict of interest.