Dendritic Polynitrato Energetic Motifs: Development and Exploration of Physicochemical Behavior through Theoretical and Experimental Approach.

Considering the fundamental and most desirable characteristics of energetic materials, a series of 1,2,3-triazole-based heterocyclic energetic motifs nicely tuned with nitrato (-ONO2) functionality were synthesized by a microwave-assisted environmental friendly synthetic approach with good yields. Thermal stability and the nature of evolved gases on decomposition of structurally characterized energetic motifs were analyzed by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) analysis and Fourier transform infrared coupled with TGA-DSC. The explosiveness of these motifs was explored by calculation of enthalpy of formation and density employing density functional theory, and the detonation performances (detonation pressure and velocity) were explored using EXPLO5_V6.03. All of these compounds were calculated to have better oxygen balance (-36 to -52%) as compared to that of trinitrotoluene (-74%). Most of the nitrate ester derivatives were found to exhibit low impact sensitivities, high densities, good thermal stabilities, and promising detonation properties, and PN 3 was observed to be a superior candidate in terms of its energetic characteristics. Hence, the experimental and theoretical outcomes strongly reflect that the present approach of developing dendritic high energetic materials bearing green explosive characteristics might be a potential pathway for designing and synthesizing green explosives with desired characteristics.

Introduction

Energetic materials (explosives, propellants, and pyrotechnics) are used for a variety of military purposes and industrial applications such as cutting labor costs, speeding up strenuous processes in mining, tunneling, construction, and agriculture, and thus making a tangible input to the world economy.[1] Recently, high energetic materials have gained a lead position in the framework of material science. Therefore, applied and fundamental aspects of current research have been focused on the development of high energetic materials with desirable characteristics such as high thermal stability, impact insensitivity, high energy and density, high enthalpy of formation, and most importantly, minimal environmental impact, which should be considered intensely.[2−7] To fulfill such characteristic demands of energetic materials, complete combustion of the carbon backbone and the generation of N2 as an end product on explosion are highly desirable. Hence, high nitrogen content and recommended oxygen balance are the key concerns in designing high energetic materials. As the nitrogen rich motifs fulfill standards like ecofriendliness and high heat of formation with exothermic combustion,[8,9] heterocycle-based energetic materials have gained immense attention from the advanced material and chemical research community.[10] In this context, triazole, tetrazole, and triazine have been synthesized and studied in recent years as a fascinating class of heterocyclic cores.[11,12] Further, the introduction of functionalities like nitro (−NO2), nitrate (−ONO2) ester, nitramine (NH–NO2), and nitromine (=NNO2) has enhanced the energetic properties of these compounds.[13,14] Although, the presence of nitro functionality decreases the heat of formation, it leads to an enhancement in oxygen balance, which enables oxidation of all nonoxidizing components of motifs to their respective oxides and hence increases their energetic performance (detonation pressure and velocity).[15] Moreover, the salts of above mentioned heterocyclic cores with ammonia, hydrazine, guanidine, nitrate, perchlorate, and picrate have the advantage of low vapor pressure over nonionic motifs, which eliminates the risk of exposure on inhalation, provides a high heat of formation, and high thermal stability, and interestingly, these desirable traits can be tuned via the choice of counter ions.[16,17] Although a lot of azole-based salts and polynitro compounds have been developed that are comparable to the well-known energetic materials such as research department explosive (RDX), trinitrotoluene (TNT), 4,10-dinitro-2,6,8,12-tetraoxa-4,10-diazatetracyclo[5.5.0.05,9.03,11]-dodecane, and hexanitrohexaazaisowurtzitane, unfortunately, these are highly environmental hazardous as the release of these explosives to the environment leads to the contamination of surface and ground water, soils, and sediments.[18,19] Therefore, more efforts are still needed to design and synthesize green explosives with tremendous energetic parameters such as high enthalpy of formation, stability, low impact sensitivity, and high detonation performance.[20−23]

Herein, we describe the synthesis and characterization of triazole-based heterocyclic energetic motifs functionalized with nitrate ester as a new class of green energetic materials. As shown in Scheme , the molecular complexity was incorporated from a small molecular framework leading to dendritic structures by increasing the number of peripheral nitrate groups as well as triazole moieties to ensure the fundamental requisites of an energetic motif. The triazole ring was inserted as the core moiety as it not only provides the thermal stability but also couples with low impact and shock sensitivity,[24] whereas nitrato functionality was incorporated to enhance the oxygen balance as well as the density of the motifs. A microwave-assisted approach was developed to synthesize the precursors of polynitrate esters with excellent yields and purity. Next, the molecular integrity and structural analysis of the newly synthesized compounds were examined using various spectroscopic techniques such as Fourier transform infrared (FT-IR) spectroscopy, NMR spectroscopy, mass spectrometry, and single crystal X-ray diffraction (SC-XRD) analysis. Furthermore, thermal properties were evaluated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), and computational studies were done using the program package Gaussian 09.

Scheme 1

Routes for the Chemical Synthesis of Triazole-Based Nitrate Esters

(a) NaN3, H2O, 105 °C, 35 min, microwave (MW); (a′) acetonitrile/H2O (2:1), 30 min, 100 °C, MW; (a″) dimethylformamide (DMF), NaN3, 45 °C, 40 min, MW; (b) butyne-1,4-diol, 25–30 min, 85 °C, MW; (b′) propargyl alcohol, CuSO4·5H2O/Na-ascorbate (10, 30 mol %), dichloromethane (DCM)/H2O (1:2), 12 h, room temperature (RT); (c) LiNO3, (CF3CO)2O, Na2CO3, acetonitrile, RT, 6–7 h.

Results and Discussion

Synthesis and Characterization

In general, the copper catalyzed (3 + 2) cycloaddition reaction between electron rich azides (dipoles) and electron deficient alkynes (dipolarophiles) in the presence of a protic solvent is the most common synthesis strategy for preparing 1,2,3-triazoles with high regioselectivity. But, the development of green and industrially companionable synthetic methodologies for the production of the triazole skeleton in small molecular entities has been an emerging area of curiosity for synthetic chemists in recent decades. In this context, herein, we describe the microwave-assisted safe synthesis of precursor azides and polyhydroxy 1,2,3-triazoles with excellent yield (>95%) and purity (Scheme ). The azides were synthesized from the commercially available corresponding halides with NaN3 using H2O, DMF, and acetonitrile/water mixtures as solvents under microwave irradiation (Scheme ). Polyhydroxy triazoles were synthesized by heating butyne-1,4-diol and the azides in a closed vessel at 90 °C and 45 W under solvent-free conditions. Triazoles 1a and 1a′ were purified by column chromatography using 10% MeOH/DCM as dense yellow oils, whereas 2a, 3a, and 4a were obtained as pure white solids by triturating the viscous reaction mixture with methanol. Further, the series of polyhydroxy triazoles were easily converted into their corresponding nitrate ester derivatives using a mixture of lithium nitrate, trifluoroacetic anhydride, and Na2CO3 as a base in dry acetonitrile at room temperature for 6–7 h. Nitrate esters PN and PN were obtained as greenish liquids after purification. On the other hand, PN, PN, and PN were obtained as pure white solids on recrystallization with ethanol. The NMR spectra were recorded in CDCl3 for compounds PN and PN, whereas dimethyl sulfoxide (DMSO)-d6 was used as solvent for compounds PN, PN, and PN.

1H and 13C spectra of compounds PN, PN, and PN were supported by heteronuclear single quantum coherence (HSQC) spectra as well as 13C attached proton test (APT).

X-ray Crystallography

A single crystal of PN suitable for X-ray diffraction was grown in a DCM/methanol solvent system at room temperature by slow evaporation. As depicted in Table S1, compound PN crystallized in the monoclinic crystal system (space group I2/a), with cell volume 1841.3 Å3 and 1.67 g cm–3 calculated density. The thermal ellipsoid plot and packing diagram indicated the unambiguously determined molecular structure, as shown in Figure . In the triazole ring, N–N bond lengths are 1.34 and 1.30 Å for N1–N2 and N2–N3, respectively. Oxygen–nitrogen bond lengths for nitrate groups vary from 1.10 to 1.54 Å; O1–N4, N4–O2, N4–O2,O7–N5A, N5A–O8, and N5A–O9 bond lengths are 1.40, 1.19, 1.19, 1.10, 1.54, and 1.12 Å, respectively. Also, the bond distances between C3–N3 (1.36) and C2–N1 (1.35) lie within the normal range of C–N bond lengths. Whereas, the bond angles N1–N2–N3, N2–N1–C2, N2–N3–C3, O8–N5A–O9, and O2–N4–O3 are 107.79, 110.61, 108.63, 97.92, and 113.11°, respectively. In PN, very weak hydrogen bond (H-bond) interactions were observed as the length of the H-bond was found to be 3.4 Å. Also, no H-bonding interactions with bond length 2.5–2.9 Å were observed. All selected crystallographic data and parameters of the compound are detailed in Table S1.

Figure 1

(a) Oak Ridge thermal ellipsoid plot (50% ellipsoidal probability) of PN; (b) packing diagram of PN viewed along C axis.

Physical and Energetic Properties

Heat of formation is one of the most important properties of energetic compounds, and it reflects their inherent energy as well as nitrogen content. As is evident from Table , most of the motifs were found to exhibit positive gaseous heats of formation (36–96 kJ mol–1) except PN and PN. The condensed heats of formation (ΔHc) for motifs PN–PN were 2.42, −110.25, and 23.23 kJ mol–1, respectively. The considerable enthalpy of formation clearly reflected their high nitrogen richness and the presence of more nitrato groups (see Table ). The calculated densities of these energetic motifs range between 1.39 and 1.73 g cm–3, as shown in Table . The densities of the motifs PN, PN, and PN were 1.73, 1.71, and 1.66 g cm–3, whereas PN and PN were found to be less dense (1.44 and 1.39 g cm–3, as shown in Table ). Because density is a function of the packing parameter of the molecular framework, the aforementioned values of density clearly reflect that PN, PN, and PN have a highly compact molecular framework as compared to that of PN and PN.

Table 1

Physical Properties of Polynitrate Esters

compd	Tdeca (°C)	densityb (g cm–3)	OBc (%)	ΔHfd (kJ mol–1)	ΔEe (kJ mol–1)	Vdf (km s–1)	Pg (GPa)	H50h (cm)
PN2	120	1.44	–51.49	–70.94	503.65	7.17	19.86	29
PN3	155	1.73	–36.35	36.04	510.41	7.95	26.27	46
PN4	141	1.71	–48.00	73.70	520.06	7.80	25.16	34
PN5	154	1.66	–40.00	–35.77	487.25	7.68	24.36	209
PN6	150	1.39	–47.00	96.58	485.32	6.89	17.30	52

Onset decomposition temperature.

Density (g cm–3).

Oxygen balance (OB).

Calculated enthalpy of formation in gaseous phase (kJ mol–1).

Highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap.

Detonation velocity (km s–1).

Detonation pressure (GPa).

Height at which the motif detonates 50% (impact sensitivity).

The impact sensitivity plays an important role in scaling up promising energetic materials, generally expressed in the form of H50, i.e., the height from which a hammer of standard weight falling upon the explosive will lead to a 50% probability of detonation. The H50 values were calculated for the corresponding energetic compounds (see Table ) employing eq , as suggested by Cao.[25]QNO is the net Mulliken charge on the nitro group (NO2) and OB100 is the oxygen balance of an energetic motif. As tabulated in Table S1, the characteristic QNO and OB100 values of the present energetic motifs were calculated using the reported method.[25] Because H50 is the direct function of the square of the net Mulliken charge on the nitro group (QNO), the superior H50 of PN over the other motifs can be attributed to its high QNO. Moreover, as illustrated in Table , all of the motifs were found to have lower impact sensitivities as compared to those of RDX (28 cm) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (32 cm), which again supported their promising energetic candidature.

After evaluation of the heats of formation and densities, the detonation performance of the motifs in terms of detonation pressure (P, GPa) and detonation velocity (Vd, km s–1) was calculated using gaseous heat of formation for EXPLO5_V6.03. Their detonation velocities and pressures lie in the range 6.89–7.95 km s–1 and 17.30–26.27 GPa, respectively. Energetic motifs PN, PN, and PN exhibited high detonation pressures (26.27, 25.16, and 24.36 GPa) as well as detonation velocities (7.95, 7.80, and 7.68 km s–1). Hence, the detonation performance of the present compounds clearly established that owing the promising energetic nature, this class of materials could find wide applications in the defense sector.

Oxygen balance, the most requisite property of an ecocompatible energetic motif to ensure complete conversion of the energetic backbone leading to the evolution of carbon dioxide and nitrogen, i.e., smokeless explosion, was calculated using the chemical formula CHNO as (values are reported in Table ). The oxygen balance of these energetic compounds was found to be in an acceptable range from −36 to −52%, which is much better than TNT (−74%). The incorporation of nitrate groups not only improved their oxygen balance but also made them capable and potent energetic motifs with high heats of formation and detonation performance.

TGA–DSC Coupled with FT-IR

Further, thermogravimetric analysis and differential scanning calorimetric experiments were conducted to examine the thermal stability of these compounds. Their corresponding onset decomposition temperatures (120–155 °C) are shown in Table and Figure S1. From the TGA and DSC thermal curves, it appears that the liquid products follow a three-step decomposition pattern, whereas the solid compounds follow a two-step decomposition process. The rate of mass loss in the first step for the solid compounds and the second step for the liquid products was quite high, indicating their energetic nature. It is also notable that the motifs PN–PN decomposed exothermally by releasing 1447, 912, 999, and 758 J g–1 as heats of decomposition. To gain an in-depth understanding of the decomposition pathway and products of these energetic motifs, IR spectroscopic analysis was also carried out using a TGA–DSC coupled with FT-IR technique. The IR spectra peaks at 3730, 3593, and 2304 cm–1 clearly indicated the evolution of water and carbon dioxide as major decomposition products, whereas the peaks at 1509 and 1590 cm–1 indicated the formation of N2O and NO as other decomposition products (see Figure ). Although the evolution of N2 cannot be detected by FT-IR because it does not have a net dipole moment, the possibility of its evolution cannot be neglected.[26] Hence, the aforementioned findings clearly reflect the oxygen richness and ecofriendliness of these newly developed energetic motifs.

Figure 2

a) TGA–DSC profile of PN; (b) FT-IR spectra showing nature of gases evolved on decomposition.

HOMO–LUMO Calculation

HOMO–LUMO gaps (ΔE) were calculated to understand the stability of these energetic motifs. As evidenced from Figure , the HOMOs and LUMOs are localized over the triazole moieties and nitrato (ONO2) groups, respectively. The ΔE values as tabulated in Table correspond to satisfactory thermal stabilities of these compounds.

Figure 3

Pictorial presentation of PN–PN optimized geometries with their highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO).

Conclusions

In summary, a user-friendly microwave-assisted synthetic approach was developed for laboratory-scale synthesis of precursor azides and polyhydroxy triazoles for the synthesis of polynitrate esters as a new class of green energetic motifs. The newly synthesized compounds were structurally characterized by employing various spectroscopic techniques. Thermal behavior was evaluated using TGA–DSC coupled with FT-IR to evaluate the nature of gas release on thermal decomposition. Most of the motifs decomposed in the range of 140–155 °C, which indicated the moderate thermal stability of these new energetic materials. However, all of the motifs exhibited a low impact sensitivity, high density, moderate thermal stability, and good oxygen balance (−36 to −52%) except PN, which displayed better thermal properties and detonation performance (P = 26.27 GPa and Vd = 7.95 km s–1). Hence, on the basis of comparative studies of their predicted physicochemical properties and performances, it was concluded that the motifs presented in this study not only display satisfactory energetic parameters, but also address the mandatory concept of ecofriendliness.

Experimental Section

Safety Precautions

As sodium azide and its derivatives are highly sensitive to mechanical stimuli and impact, they should always be in dilute solution, and extreme caution should be taken during their utilization as reactants. Although no difficulties were faced during the handling of these compounds, one should make all manipulations inside a hood behind the safety shield. Any mechanical action involving scratching or scraping should be avoided; all safety cautions should be exercised during handling.

General Information

Chemicals were purchased from commercially available resources and used without further purification. 1H, 13C, DEPT-135, and HSQC spectra were recorded on JEOL-JNM ECX-500 MHz NMR spectrometers by using CDCl3/DMSO-d6 as solvent. Chemical shifts are reported relative to tetramethylsilane (1H and 13C). FT-IR spectra were recorded on a PerkinElmer FT-IR spectrometer. high-resolution mass spectrometry spectra were recorded on a Bruker impact-HD spectrometer. Single crystal X-ray diffraction studies were performed on an Agilent Technologies X-ray diffractometer system. A CEM-Discover microwave synthesizer was used for synthesis. The energy release and decomposition temperature were studied on a thermogravimetric analyzer and differential scanning calorimeter TGA/DSC 1 (Mettler Toledo). The thermograms were recorded from 25 to 1000 °C at a heating rate of 10 °C min–1 under an inert atmosphere of nitrogen gas in an open alumina crucible.

A single crystal of PN suitable for X-ray diffraction was grown in DCM/methanol at room temperature by slow evaporation. Diffraction studies were performed on an Agilent Technologies X-ray diffractometer system by irradiating the crystals using Cu Kα λ radiation (λ = 1.5406 Å) at 293(2) K, which revealed that it crystallizes in the monoclinic I2/a space group. Data were collected by standard “CrysalisPro” software (online version) and reduction was undertaken with CrysalisPro software (offline version). The molecular structure was solved by direct methods OLEX2 and was refined using full-matrix least-squares (F2) on SHELXL. The positions of all nonhydrogen atoms were located and were refined anisotropically. After that, hydrogen atoms were obtained from the residual density map and refined with isotropic thermal parameters. The CCDC number 1043488 was obtained for PN from “The Cambridge Crystallographic Data Centre (CCDC)” by depositing the structure at http://www.ccdc.cam.ac.uk/. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Theoretical Study

All structures were optimized using the B3LYP functional with the 6-31g* basis set using the Gaussian 09 suite of programs.[27] The input files of molecular conformation of all compounds except PN were generated in GaussView 5.[28] The SC-XRD structure for PN was used as the input file for the density functional theory studies. Frequency calculations at the same level were performed to ensure that the structures correspond to minima. Single point energy calculations were performed at 6-31+g**. These single point energy values were used for calculation of the gas phase heat of formation. Volumes of each species were calculated using the B3LYP functional with the 6-31+g** basis set. The output files from the Gaussian package were obtained, and they were visualized in MOLDEN.[29] Then, the Z-matrix was seen and converted into an input file for MOPAC.[30] The input files for MOPAC were executed and the heats of formation for the given molecules were obtained. The detonation pressure and velocity of the compounds were calculated using EXPLO5_V6.03.[31] The condensed heat of formation (ΔHc) for PN–PN was calculated using eqs and 3:where ΔHsub is the enthalpy of sublimation and Tm is the melting point (kelvin).