DESIGN, SYNTHESIS AND CHARACTERIZATION OF THERMALLY STABLE ENERGETIC MATERIALS BASED ON VERSATILE FUNCTIONALIZATION OF 3,5-DINITROPYRAZOLES

Abstract

The thesis entitled "Design, Synthesis and Characterization of Thermally Stable Energetic Materials Based on Versatile Functionalization of 3,5-dinitropyrazoles" presents the results obtained from the research work carried out on the synthesis, characterization and energetic properties evaluation of thermally stable energetic materials based on various derivatives of 4-substituted-3,5-dinitropyrazoles. Different derivatives of 3,5-dinitropyrazoles with varying substituents [amino (-NH2), nitro (-NO2), azido (-N3), hydroxy (-OH), carboxyl (-COOH) and polynitrobenzene] at the 4th position are used as starting materials in this work. Further, different N-functionalization approaches like, energetic salt formation, N-methylation and connection with another energetic ring via C-C bonds, N-methylene-C bridges, 1,3,4-oxadiazole bridges, etc., are explored to obtain novel thermally stable energetic materials (EMs) with detonation performance similar or surpassing conventionally used EMs. The work carried out has been broadly divided into eight parts. CHAPTER 1 describes salient features of high energy density materials (HEDMs). After a brief discussion on the history of HEDMs, their classification based on performance and safety into primary, secondary and tertiary explosives, along with their applications and commonly used examples has been done. Various parameters used to evaluate the energetic and physical properties of energetic materials are discussed. The notions behind designing new HEDMs are stated by taking examples of some conventionally used energetic materials. The drawbacks of the continued usage of the currently used explosives have been explained. The requirements of modern energetic materials, along with the challenges faced by the researchers in the field, are also described. The primary focus of all the work described in this thesis is on synthesizing thermally stable EMs. The necessity of thermally stable EMs is discussed concisely. Some known common approaches reported in the literature for the design and synthesis of thermally stable EMs, along with their salient aspects, are consolidated in this chapter. As most of the work described in this thesis is based on 3,5-dinitropyrazole based compounds, the importance of azole based energetic materials with particular emphasis on 3,5-dinitropyrazole-based energetic compounds is discussed. The chapter ends with the scope of the present of work carried out and reported in the thesis. The objective and scope of the present work are • Synthesis of new thermally stable energetic materials based on C-C connected 3,5-dinitropyrazole and polynitrobenzene derivatives. • Employing 4-bromo-3,5-dinitropyrazole derivatives as organohalides in Suzuki coupling with various phenylboronic acid derivatives for making C-C connected thermally stable energetic materials. • Synthesis of thermally stable energetic materials based on underestimated 4-carboxylic-3,5-dinitropyrazoles (CDNP). • Exploring N-acetonitrile derivatives of nitropyrazoles for the synthesis of thermally stable and insensitive compounds based on N-methylene-C linked 4-substituted -3,5-dinitropyrazoles and 1,2,4-triazole-3-one. • Synthesis of thermally stable primary explosive with positive oxygen balance based on benzofuroxan framework, as a substitute for toxic lead azide. • Synthesis of compounds having energetic properties similar to or surpassing TNT/RDX while tailoring them for high personal and environmental safety. • Complete characterization of the new compounds using various spectroscopic techniques and determination of their energetic and physical properties. CHAPTER 2 describes the general experimental procedures adopted in the synthesis of the new compounds and details of various characterization techniques utilized. Specific synthetic details of some of the starting materials used in the work described in the thesis are also presented. CHAPTER 3 outlines a novel approach for the synthesis of thermally stable energetic materials by connecting N-methyl-3,5-dinitropyrazole with polynitrobenzene frameworks through carbon-carbon bonds using Pd(0)-mediated Suzuki cross-coupling reactions. Coupling reaction followed by nitration, amination and oxidation leads to C-C connected pentanitro energetic derivatives 6 and 10. Various other energetic derivatives such as amino (5), azido (7), nitramino (8) and energetic salts (11 – 14) were also explored to fine-tune the properties (scheme 1 and 2). Overall, this research introduces a novel method for synthesizing C-C coupled energetic materials that are not accessible by any other routes. All the compounds were thoroughly characterized using IR, NMR [1H, 13C{1H}], differential scanning calorimetry (DSC), elemental analysis, and HRMS studies. Compounds 5, 10 and 13 were further characterized through 15N NMR, and 6 and 14 were confirmed through single-crystal X-ray diffraction studies. The physicochemical and energetic properties of all the energetic compounds were explored. Most of the synthesized compounds demonstrated high thermal stability (Tdec > 250 °C), among which compounds 5 and 6 showed excellent thermal stability having decomposition temperatures above 300 °C. Excellent thermal stability, acceptable sensitivity, and good energetic properties of compounds 5, 6, 10 and 13 make them promising heat-resistant explosives. Furthermore, these compounds were found to be more thermally stable among the known N-methyl-3,5-dinitropyrazole-based and C-N coupled 3,4,5-trinitrobenzene-azole-based energetic compounds. CHAPTER 4 focuses on the synthesis, characterization, and energetic and physicochemical properties determination of different derivatives derived from polynitrobenzene and 1-H-3,5-dinitropyrazole. This investigation demonstrated that the synthesized energetic materials with C-C connections exhibit superior thermal stability compared to their counterparts with C-N connections. Another benefit of the C-C connected compounds is the inclusion of acidic NH, allowing for further adjustment of energetic properties through the formation of energetic salts. All the compounds were fully characterized using IR, NMR [1H, 13C{1H}], differential scanning calorimetry (DSC), elemental analysis, and HRMS studies. Compounds 19 and 21 were further characterized by 15N NMR, and compounds 16 and 19 were characterized by single-crystal X-ray diffraction studies. Theoretical heats of formation and energetic performance for all the energetic compounds were calculated using Gaussian 09 and EXPLO5 v6.06 programs, respectively. Compounds 19, 20, and 22 display overall energetic properties better than the commonly used heat-resistant explosive, HNS. CHAPTER 5 explores the energetic and physicochemical properties of the overlooked 4-Carboxylic-3,5-dinitropyrazoles (CDNP). Introducing carboxylate group (-COOH) at the 4th position on 3,5-dinitropyrazole resulted in its better overall stability with respect to nitro (-NO2) and azido (-N3) derivatives due to the presence of inter/intramolecular hydrogen bonding sites, as well as stable carbonyl bond. Significant improvement in energetic performance was observed compared to amino (-NH2) and hydroxy (-OH) derivatives due to its positive oxygen balance (7.92%). CDNP showed the highest thermal stability (Tdec = 248 °C) among these derivatives. The dicationic energetic salt formation of CDNP further allowed to fine-tune the overall performance and stability (Scheme 4). Energetic salts 29, 31 and 32 were further confirmed via single-crystal X-ray Diffraction Analysis. The dihydroxylammonium salt (30) of CDNP shows the best energetic performance, comparable to well-known traditional explosive RDX with better sensitivities towards impact and friction. CDNP was further used as a precursor to develop some novel thermally stable energetic compounds, including 1,3,4-oxadiazole bridged energetic compounds (34-36) and a zwitterionic compound (37). All these compounds have high thermal stabilities, with the hydroxylammonium salts 36 showing the best energetic performance and potential to be investigated as future thermally stable energetic materials. In CHAPTER 6, a series of energetic compounds were synthesized by combining various 4-substituted-3,5-dinitropyrazoles and 1,2,4-triazole-3-one via the N-methylene-C bridges. Cyano functionality can be converted to a variety of explosophores such as tetrazole, N-hydroxytetrazole, tetrazine, 5-amino-1,2,4-oxadiazole, etc. Therefore, various N-acetonitrile derivatives of azoles have been utilized for the synthesis of many asymmetrically connected energetic materials with the purpose of maintaining a balance between energy and stability. 1,2,4-triazole-3-one based compounds have shown excellent physical and thermal stability and have been paired with other heterocyclic rings via C-C or N-C bonds for the formation of thermally stable explosives. This work synthesizes and characterizes energetic compounds, 41-43 and 46-51 and 54, having 1,2,4-triazole-3-one rings connected with various nitropyrazoles/tetrazoles via N-methylene-C bridges. Compounds 41, 46 and 54 were confirmed via single-crystal X-ray Diffraction Analysis. All the compounds were found to be more energetic than traditional explosives TNT, whereas the energetic performance of 42, 48 and 49 was comparable with the well-known secondary explosive TATB. The structure property relationship was studied for natural compounds 41, 46 and 54 using Hirshfeld surface, non-covalent interaction and electrostatic potential surface analysis. CHAPTER 7 involves the synthesis of an extremely dense, thermally stable and sensitive energetic compound based on benzofuroxan framework, Dipotassium 4,6-Dinitro-5,7-dioxidobenzo[c][1,2,5]oxadiazole 1-oxide (K2DNDP). Because of environmental and health impacts, there is an ongoing necessity to develop “sustainable” primary explosives to replace existing lead-based analogs. Potassium-based primary explosives have been considered a greener alternative due to the environmental compatibility of potassium ions. At present, the straightforward and economical synthesis of high-performing materials is a top challenge in the field of energetic materials. Now we investigate a potential primary explosive, K2DNDP, which exhibits an excellent thermal stability (Tdec = 281 °C) and a calculated crystal density of 2.20 g cm-3 at 100 K. Both the selection of a remarkably simple synthesis utilizing inexpensive raw materials and its implementation for large-scale manufacturing are the key factors for potential application in military and civilian industry. CHAPTER 8 gives the overall conclusions of the entire work carried out in the present study.