?ukasz Gutowski,Stanis?aw Cudzi?o

Military University of Technology,Faculty of Advanced Technologies and Chemistry,Gen.Sylwestra Kaliskiego 2 Street,00-908,Warsaw,Poland

Keywords:Heat resistant explosives Detonation parameters Synthesis Nitration

ABSTRACT This paper reviews the achievements in the field of synthesis of new thermally resistant explosive compounds in the years 2009 through 2019.The performance characteristics of these compounds(sensitivity,thermal decomposition parameters,and detonation parameters)were compared with those of 1,3,5-triamino-2,4,6-trinitrobenzene,which still seems to be an unrivalled model of a thermally resistant and generally low-sensitivity explosive material.New thermally stable explosives(TSEs)were found among macromolecular compounds with tri-and dinitrophenyl groups,nitro and amine-nitro derivatives of azoles,and polynitro derivatives of calixarenes.Some of them match TATB in terms of thermal resistance and additionally have higher detonation parameters.

1.Introduction

The search for new energy-rich explosives with increasing thermal stability has been ongoing since the 1960s[1-5].Since then,several dozen compounds have been obtained and characterized with rapid decomposition above 300°C.This is a widely recognized criterion for the durability and thermal stability of explosive compound molecules[6].Due to their unique properties(high durability,operational reliability,and safety),they have been found numerous applications in both civil and military technology.They are used in blasting operations at large depths and with temperatures of 200°C-300°C.Spacecraft(e.g.,space rockets)uses TSE in systems for disconnecting worn-out components,which is necessary due to the high temperatures reached by its fuselages when moving in the earth’s atmosphere.The heat-resistant explosives are also used for nuclear bomb detonators(Table 1).

A common strategy for the structure design and synthesis of the new TSE is to synthesise a chemically and thermally stable precursor(core)into which explosophores are introduced directly(nitration)or indirectly(e.g.,oxidation of the amine group).This strategy was used for the synthesis of 2,6-bis(picrylamino)-3,5-dinitropiridine(PYX).In the first step the conjugation ofπ-πand n-πelectron is expanded via reaction of 2,6-diaminopiridine with 2-chloro-1,3,5-trinitrobenzene leading to 2,6-bis(picrylamino)piridine(Pre-PYX).After reaction of Pre-PYX with fuming nitric acid,PYX is obtained[3].It is easy to notice that there are nitro groups in 2,6-bis(picrylamino)piridine.In some cases it is no longer necessary to introduce new explosophoric groups into a molecule and a thermally stable explosive can be obtained from a compound having explosive properties.Addition of amino groups to 1,3,5-trinitrobenzene introduces strong intermolecular interactions affecting on the decomposition point[2,4].Oxidizing 2,4,6-trinitrotoluene with sodium hypochlorite,2,2′,4,4′,6,6′-hexanitrostilbene,which is one of the standard thermostable explosives,can be obtained[1].Aromatic azoles and azines are excellent for building-blocks,which can be used in designing of new TSE.Due to the fact that they are aromatic compounds,They increase the range of electronic couplings.Their positive enthalpy of formation means that they can act as explosophores.One of the first examples of usage of this strategy are obviously tetranitro-2,3,5,6-dibenzo-1,3a,4,6a-tetraazapentalene(TACOT)and 3-picrylamino-1,2,4-triazole(PATO).Having studied the papers published during the last decade we state,that all of the above mentioned methods have been using so far for the synthesis of new heat resistant energetic compounds.

2.TATB and TACOT homologue explosives

The best-known thermostable explosive is undoubtedly 1,3,5-triamino-2,4,6-trinitrobenzene(TATB).It owes its fame and its broad applicability in extreme conditions(e.g.,a wide range of temperature and pressure changes,ionizing radiation,electrical discharges,and intensive shock)mainly to its high melting and decomposition temperature(about 350°C),extremely low sensitivity to mechanical stimuli,lack of reactivity in relation to other substances(chemical inertia),and low solubility,even in the most aggressive organic solvents[2,7-10].

Table 1Properties of the compounds described in this manuscript.

These unique properties and applications result from the molecular and crystalline structure of TATB[11].The complete substitution of hydrogen atoms in the benzene ring with alternately distributed amine and nitro groups favours the formation of very strong intramolecular hydrogen bonds NH··O and provides the almost ideal flatness of the molecule.In contrast,moderately strong intermolecular interactions,also through hydrogen bonds,cause the molecules to arrange in parallel layers bound in the crystal structure by weak van der Waals forces.The extensive network of hydrogen bonds in the layers significantly increases the number of vibratory and rotational energy states of the molecules,enabling the effective dissipation of energy supplied by heating or shock.The build-up of energy from mechanical interactions(i.e.,generation of“hot spots”)is also prevented by the graphite-like crystal lattice of TATB,as the layers move relatively easily against each other,reducing internal friction during rapid deformation of the sample[2,12].

From the viewpoint of thermal stability,the molecular and electronic structure of an organic compound seems even more important[13].TATB is an exceptional molecule even at this level of matter organization.The C-NO2and C-NH2bonds are shorter and the C-C bonds are longer than in other aromatic amine and nitro compounds.The high dissociation energy values of the bonds are critical for the stability of the molecule(C-NH2,C-NO2).As a result,during slow heating,the decomposition reactions with the release of gaseous products start only above 300°C,at which point TATB will also sublimate[14,15].

Numerous studies on the kinetics of TATB decomposition,employing various methods,have shown that,in the temperature range 297°C-382°C,the apparent activation energy of thermal decomposition is 250 kJ/mol to 260 kJ/mol and the frequency factor value is between 1018.6s-1to 1019s-1[15].The main products of the initial stages of TATB decomposition are aminonitrobenzofurazanes[16].This means that the thermal decomposition of TATB starts from the intramolecular reaction of adjacent nitro and amino groups to form a furazane ring and release water molecule,and not from the homolytic decomposition of the C-NO2bond.Thus,even in the case with TATB below the temperature required for direct rupture of the C-NO2bond in nitrobenzene derivatives,the rearrangement of atoms is possible and results in energy release and loss of a low-molecular weight,thermodynamically stable compound,290 kJ/mol to 300 kJ/mol[17].

In heat-resistant explosives,both the release of gases and the release of energy should occur at the highest possible temperature.However,according to Boltzmann’s energy distribution law,at any temperature higher than absolute zero,some of the molecules in the macroscopic sample have an energy higher than the energy of their decomposition.Stability and heat resistance are therefore not absolute-it is rather a question of a different way of reducing the reaction rate constantkof the ongoing decomposition reactions than by lowering temperatureT:

where,Ais frequency factor related to the frequency of bond vibration in the molecule[s-1],Eis energy required to rupture the weakest bond in the molecule[J/mol],Ris universal gas constant[J/mol·K],andTis absolute temperature[K].

Thus,for a compound to be stable and thermally resistant,the rate of decomposition must be as low as possible,which can be achieved by minimizing factorAand maximizingE.Resonant stabilisation and extensiveπ-πand n-πelectron conjugations increase the energy of bondsE(in conjugated systems,the multiplicity of bonds is greater than one),while atoms bonding with more than one other atom decreasesA.Spreading the range of conjugation can be obtain by replacing a hydrogen atom by a halogen atom[18].Multiple bonds stiffen the structure and make vibrations more difficult,and furthermore,the energy supplied to the particle can be effectively scattered in many directions.Similar,although weaker,stabilizing effects include intra-and intermolecular hydrogen bonds,transverseπ-πstacking interactions between extensive aromatic structures,and dipole-dipole type interactions appearing when stacked one above the other layers made of heterocyclic aromatic compounds.However,the multiple and spatial characteristics of these interactions make them an effective way of reducing the rate of decomposition at any temperature.

In summary,TATB is an aromatic nitro-amino compound in which the nitro groups are permanently bonded to the carbon atoms of the benzene ring and linked by numerous strong hydrogen bonds to the amine groups of the same molecule and other molecules surrounding it in a given layer.Flat particles from successive layers(arranged in front of each other)optimally fill the space,reducing the free space in the elementary cell to a minimum.

During slow heating,the sublimation and decomposition of the TATB start when temperatures exceed 300°C,and above 350°C,the pyrolysis already proceeds at a high rate.A question is how to change the molecular and crystalline structure of a nitro compound to withstand heating,say up to 500°C?The Arrhenius formula for the rate constant of a decomposition reaction may be helpful in answering this question.

If we assume that the frequency factorAfor the sought compound will be the same as for TATB(this assumption is valid because of the strong hydrogen bonds in TATB),then the dissociation energy of the bond that is critical for stability of the compound(E)can be calculated from the following equation:

TheEvalue calculated in this way is about 7% higher than the homolysis energy of the C-NO2bond in nitrobenzene.In order to obtain a new thermo-resistant nitro compound that is more energy rich and more stable than TATB,it is necessary to eliminate other decomposition paths than NO2radical separation(in the case of TATB,it is a furazane reaction path),which is even more effective than stabilisation of TATB structure due to sharing of mobile electrons.Moreover,the new compound should be able to create networks of strong hydrogen bonds and other intermolecular interactions.

Such a compound seems to be tetranitro-2,3,5,6-dibenzo-1,3a,4,6a-tetraazapentalen(TACOT,Fig.1),which,during heating at rates higher than 5°C/min,decomposes at temperatures exceeding 400°C[19].The activation energy of TACOT thermal decomposition,determined from Kissinger’s method using the results from dynamic scanning microcalorimetry,is 292 kJ/mol to 330 kJ/mol,and the frequency factorA=5.2×1019s-1.TATB tests,performed under identical conditions,showed that its decomposition occurs at temperatures lower by 24°C-28°C,while the activation energy is 200 kJ/mol to 221 kJ/mol and the frequency factor is 1.9×1013s-1.

The clear advantage of TACOT over TATB is a result of the absence of substituents other than the nitro group in the benzene rings and a polycyclic and cumulative structure,which means multiple bonds of atoms forming the molecule.Also important is the high degree of aromaticity of the initial dibenzo-1,3a,4,6atetraazapentene,in which as many as 18 electrons are coupled(6 electrons are provided by nitrogen atoms from a pentalene-like connector having an isoelectronic structure with naphthalene).The dibenzotetraazapentalene core is modified by replacing the carbon atoms by nitrogen atoms which increases density,oxygen balance and nitrogen content.It has a positive effect on detonation parameters[20].

3.Thermo-resistant explosives from the group of conjugated nitroarenes

It has long been known that explosive compounds of high molecular weight,composed of nitroarenes separated with bridges containing mobile electrons and forming extensive systems of coupled n-πandπ-πelectron bonds,show high thermal resistance[21].Prominent representatives of this group of compounds are especially 2,2′,4,4′,6,6′-hexanitrostilben (HNS) and 2,6-bis(picrylamino)-3,5-dinitropyridine(PYX)(Fig.2).For many years,both have been produced on an industrial scale and used in military technology,space exploration,and the prospecting and extraction of oil and gas from deposits located at large depths[4].

Fig.1.Structure of tetranitro-2,3,5,6-dibenzo-1,3a,4,6a-tetraazapentalene.

3.1.5,5′-Bis(2,4,6-trinitrophenyl)-2,2′-bi(1,3,4-oxadiazole)(5)

In 2016,T.Klap¨otke and T.Witkowski reported for the first time receiving a new thermostable explosive compound(5),in which trinitrophenyl(picryl)groups are combined with two rings of 1,3,4-oxadiazole[22].The substrate in the synthesis of bis(2,4,6-trinitrophenyl)-2,2′-bi(1,3,4-oxadiazole)is the low-cost and commercially available 2,4,6-trinitrotoluene(TNT).The synthesis process comprises four stages,but each of them has a yield exceeding 80%(Fig.3).

The first stage of(5)synthesis consisted of oxidation of TNT to 2,4,6-trinitro-benzoic acid(2)with a NaClO3/HNO3mixture.The product of this reaction was then chlorinated(using POCl3)to the corresponding acid chloride(3)and condensed with oxail dihydrazide(oxalic acid dihydrazide).In the final stage,bis(picryl)oxailhydrazide(4)was dehydrated in 20% oleum at room temperature.The product precipitated when the reaction mixture was poured into water[22].

Structural studies have shown that the length of the bond between the oxadiazole rings in(5)is shorter than that of a typical single carbon-carbon bond,because it is involved in theπ-electron conjugation spreading to the entire molecule.Both rings of the coupling lie in the same plane,while the trinitrophenyl groups are deflected from this plane by 70.347°[22].

The differential scanning microcalorimetry(DSC)method showed that(5)decomposes only above 335°C[20].Subsequent detailed studies of thermal properties conducted by J.Zhou et al.[23]showed that,while heating the sample at a rate of 10°C/min,the maximum decomposition peak appeared at 370°C.The activation energy of thermal decomposition determined by the Kissinger method using DSC curves,recorded at 2,5,10,and 15°C/min,is 233 kJ/mol(5)is insensitive to friction,and its impact sensitivity is typical for secondary explosives(5 J)[22].

The experimentally determined density of(5)is 1.837 g/cm3,which is higher than that of HNS(1.74 g/cm3)and PYX(1.757 g/cm3).A high density combined with the high value for the standard enthalpy of formation(197.6 kJ/mol)cause(5)to exceed HNS and PYX in terms of detonation parameters(Dmax=8030 m/s,pC-J=27.3 GPa)[22].

To summarize, 5,5′-Bis(2,4,6-trinitrophenyl)-2,2′-bi(1,3,4-oxadiazole)has all the features of a good thermostable explosive with high application potential.It can withstand heating up to 335°C,is insensitive to friction,and has a similar sensitivity to shock and electric sparks as HNS.The density,formation enthalpy,nitrogen content,velocity,and detonation pressure of(5)are higher than those parameters for the currently used HNS and PYX.It can be obtained easily using inexpensive and generally available reagents.It does not dissolve in water and is scarcely soluble in typical organic solvents[22].

3.2.4,8-Bis(2,4,6-trinitrophenyl)-difurazan[3,4-b:3′4′-e]pyrazine(7)

The oxadiazole isomer,which is even more popular among researchers involved in the synthesis of modern explosives,is 1,2,5-oxadiazole(furazan).The value of the formation enthalpy of this compound is approximately three times higher than that of 1,3,4-oxadiazole[24],which usually favourably affects the detonation parameters but negatively affects the resistance to thermal and mechanical stimuli[25-27].Nevertheless,compounds containing the furazan grouping are worth considering as potential TSE,as they may exhibit low sensitivity and at least moderate thermal resistance.A good example is 4H,8H-difurazano[3,4-b:3′,4′-e]pyrazine(6).In this connection,16 electrons are conjugated,as evidenced by the results of a crystallographic structure study indicating that the torso angle between C7-N3-C8-N5 is 179.22°,so the furazanopyrazine ring is almost perfectly flat.Due to this,among other reasons,(6)decomposes at a temperature of about 280°C.

Fig.2.Structures of HNS and PYX,commercial thermally stable explosives.

Fig.3.Synthesis scheme of(5).

N.Liu et al.[28]proved that,by substituting hydrogen atoms in the(6)pyrazine ring with nitrophenyl and/or amine groups,it is possible to obtain compounds characterized by excellent thermal stability and simultaneously higher detonation parameters.For example,by picryling(6)in acetonitrile with an addition of triethanolamine at ambient temperature for 29 h,(7)is obtained with a yield of about 80%,as shown in Fig.4.

TG/DTA/DSC thermal analyses have shown that,when(7)samples are heated at a rate of 10°C/min,decomposition starts when 350°C is reached and has its highest rate at about 413°C(maximum DSC peak).In this respect,(7)is superior to TATB.The experimentally determined density of this compound is 1.82 g/cm3,and the calculated values of velocity and detonation pressure at this density are 7874 m/s and 28.2 GPa,similar to those calculated by the same method for TATB.Unfortunately,(7)is much more sensitive to impact,friction,and electric spark(10 J,240 N,and 1.0 J respectively)[28].TATB is practically insensitive to these initiating stimuli[2].The fact that(7)at a lower density shows comparable detonation parameters to TATB is due to the presence of an endothermal oxadiazole ring,which results in a high positive formation enthalpy of(7)(894.1 kJ/mol),significantly exceeding the formation enthalpy of TATB(-139.8 kJ/mol)[28].Considering the excellent thermal resistance of the compound in question,the availability of raw materials for its synthesis,and its acceptable sensitivity,it may become a substitute for TATB or TACOT in the future.

3.3.5,5′,6,6′-tetranitro-2,2′-bibenzimidazole(10)

Commonly known cores used in synthesis of TSEs are the aforementioned dibenzo-1,3a,4,6,6a-tetraazapentene and N,N′-diphenyl-2,5-diaminopyridine.This group also includes 2,2′-bibenzimidazole(BBI),which,during heating,does not decompose but sublimates at 401°C[29].BBI can be obtained by condensation of oxalic acid with o,o’-phenylenediamine(8)in the presence of polyphosphoric acid as a catalyst(Fig.5).The reaction occurs at 165°C for 2.5 h.Subsequent nitration of the resulting product with 80%-100%nitric acid(V)at 50°C-60°C leads to(10)with a yield of 67%(see Fig.6).

Fig.4.Synthesis scheme of(7).

Fig.5.Synthesis scheme of(10).

When heated at a rate of 10°C/min,the(10)undergoes a polymorphic transformation in the temperature range 180°C-250°C.At 394°C,it decomposes at the highest rate.The apparent activation energy for the thermal decomposition of this compound,determined by the Kissinger method,is 216 kJ/mol[30].

The sensitivity of(10)to mechanical stimuli is similar to that of 2,4,6-trinitrotoluene(15 J,＞360 N).Due to its outstandingly negative oxygen balance(-88.8%),it shows relatively low detonation parameters.At the density of 1.74 g/cm3,it detonates at a velocity of about 6200 m/s,and the calculated detonation pressure is 14.5 GPa[29].Therefore,it is more appropriate to use it as an additive that increases the thermal stability of explosive compositions than as an individual explosive.

4.Thermally stable nitro derivatives of diazoles

It has long been known that,as the number of nitrogen atoms that replace carbon in the role of structure-forming compounds in explosives increases,the density and enthalpy of formation of the explosives also increases,so there is an increase in detonation parameters.It has been observed that even dinitro derivatives of diazoles(pyrazoles and imidazoles)are energetic compounds that can compete,in terms of velocity and detonation pressure,with hexogen or even octogen[31].Unfortunately,an increase in the number of nitrogen atoms in the compound skeleton results in the deterioration of stability and increased sensitivity[32].The remedy for this may be the introduction of amine groups into the ortho positions in relation to nitro groups,which would result in the formation of numerous intra-and intermolecular hydrogen bonds stabilizing the structure[33].

It should also be mentioned that the nitro groups attached to the diazole ring increase the proton lability in position 1,making these compounds acidic[34-41].This,however,does not preclude the possibility of their usefulness,including as low-sensitivity explosives;because of this property,nitrodiazoles are able to form salts with nitrogen-rich amines.The salts,in which both the anion and cation are rich in nitrogen,constitute a new,intensively developed class of high-energy materials.They are characterized by a high value of formation enthalpy(resulting from the presence of numerous single-and double-bond C-N and N-N),high density and stability,low volatility,and at times,a reduced sensitivity to mechanical and thermal stress resulting from the stabilizing influence of ionic interactions and multiple hydrogen bonds and some of them can be considered as heat resistant energetic compounds[38-41].Moreover,nitroazoles were also used as nucleophiles in reaction with aliphatic electrophiles to obtain explosives with increased heat resistance[39,42,43].

Fig.6.Synthesis scheme of 4,4′-dinitro-3,3′-bipyrazole.

Fig.7.Scheme of 5,5′-dinitro-3,3′-bipyrazole synthesis.

4.1.Dinitro-3,3′-bipyrazoles

Kumar et al.[44]published the results of research on the synthesis of nitro derivatives of 3,3′-bipyrazole(11)(Fig.9).The shortening of the C-C bond between diazole rings in relation to the isolated single bond connecting two carbon atoms indicates thatπ electrons of both rings are coupled.An additional argument is the comparable length of chemical bonds between the atoms forming diazole rings,which leads to nitro derivatives of bipyrazoles decomposing at higher temperatures than their equivalents with one azole ring.

In the context of this work,two of the dinitro-1,2-diazoles described in Ref.[44]are particularly interesting.The first is 4,4′-dinitro-3,3′-bipyrazole(12),which melts at 305°C and decomposes at temperatures reaching 365°C.It is obtained by a mixture of fuming nitric acid and sulphuric acid acting on bipyrazole at 80°C for 6 h.The result is an explosive compound virtually insensitive to mechanical stimuli,as it does not react when hit with an energy of 40 J or when rubbed with a friction force of 360 N.Pressed to a density of 1.83 g/cm3,it detonates at 8120 m/s with a detonation pressure of 26.9 GPa.Such high detonation parameters are the result of high density and the positive enthalpy of formation(221 kJ/mol),which compensate for the negative oxygen balance(-71.4%).Despite a decomposition temperature close to that of TATB,it is the melting point(305°C)that determines the maximum temperature of its use.

The second compound from the dinitro-1,2-diazole family,which can be considered a TSE,is 5,5′-dinitro-3,3′-bipyrazole(14).It shows as little sensitivity to impact and friction as(12)but melts with decomposition only at 374°C.It can be obtained by isomerization of 1,1′-dinitro-3,3′-bipyrazole(13),formed by nitrating 3,3′-bipyrazole with a mixture of 100% HNO3and acetic anhydride.While heating the solution(13)in benzonitrile at 140°C,migration of nitro groups takes place.Detonation parameters are comparable to those calculated for the previously discussed isomer(8026 m/s and 26.2 GPa)because of almost identical formation enthalpy(229.1 kJ/mol)and similar monocrystalline density(1.81 g/cm3)(see Fig.7).

4.2.4,7-Diamino-3,8-dinitropyrazol[5,1-c][1,2,4]triazine(18)

In 2019,another energetic compound with a pyrazole ring was obtained[45].As a result of the reaction of 3,5-diamino-4-nitropyrazole(15)with tertbutyl nitrite,one amine group can be selectively transformed into the diazo group(Fig.8).The subsequent reaction with sodium nitroacetonitrilate and the intramolecular condensation of the compound(17)led to 4,7-diamino-3,8-dinitropyrazol[5,1-c][1,2,4]triazine(18).

Under DSC measurement conditions(heating rate 5°C/min,atmosphere N2),(18)starts decomposing at 355°C.As with the previously characterized compounds,this is the result of extensive electron conjugation(flat particle)and strong intra-and intermolecular hydrogen bonds forcing the layered arrangement of the particles in the crystal lattice.Such an electron and crystalline structure obviously favours high stability and low sensitivity to thermal and mechanical stimuli.The activation energy of thermal decomposition of this compound is 306.2 kJ/mol,and it is practically insensitive to impact and friction(＞60 J,＞360 N).At the same time,it is far superior to the currently used TSE in terms of detonation parameters(8727 m/s,32.6 GPa,density 1.90 g/cm3).

The only disadvantage of(18)seems to be the complexity and low yield of its synthesis process(38%)as well as the necessity to use expensive and unstable reagents.

5.Nitro derivatives of calixarenes

Calix[n]arenes are macrocyclic chemical compounds,first received by Zinke and Ziegler in 1941[46].Most of them are characterized by a high melting point,often exceeding 250°C-300°C,high decomposition temperature,and high chemical stability.Despite numerous applications in other fields,they had not been proposed as cores of new thermostable explosives until Zhang et al.obtained octanitro calixarenes in 2017[44,45].Their main disadvantage is negligible solubility in typical organic solvents,which,however,improves dramatically in the case of their nitro derivatives(nitrocalixarenes)[47].

A good candidate for the core of an explosive compound resistant to thermal stimuli is,e.g.,2,4,6,8-tetraoxa-1,3,5,7(1,3)-tetrabenzenacyclooctaphene(19).This is because replacing the methylene connector with an oxygen bridge improves oxygen balance and increases density,and this has a positive effect on detonation parameters(Fig.9).

Fig.8.Scheme of(18)synthesis.

Polynitro calixarene derivatives can be treated as cyclic condensation products of nitroresorcin,but unlike simple nitrophenols,they are macromolecular compounds,they do not have acidic properties and therefore have greater chemical stability,and they are safer to use.

14,16,34,36,54,56,74,76-octanitro-2,4,6,8-tetraoxa-1,3,5,7(1,3)-tetrabenzenacyclooctaphene(21)is a thermostable and lowsensitivity explosive compound.It is obtained by nitrating the condensation product of 1,3-dihydroxybenzene with 1,3-difluoro-4,6-dinitrobenzene(20),a mixture consisting of concentrated(98%)sulphuric acid and fuming nitric acid at a volume ratio of 1:1(Fig.10).

The substrate(19)is introduced into the acid mixture at 0°C.The nitration lasted 22 h;the first 2 h were at a temperature close to 0°C and the next 20 h were at room temperature.The total yield of small-scale synthesis was 60%,and after the increase in scale up to 1000 g of finished product,the yield rose to 70%[47].

The detonation parameters and thermal stability of(21)are similar to those of TATB.When heated at 5°C/min,the maximum peak of decomposition is located approximately at 375°C.The sample is insensitive to mechanical stimuli.The calculated maximum values of velocity and detonation pressure are 7865 m/s and 28.9 GPa,respectively[48].

X.Zhang et al.[48]demonstrated that the substitution of hydrogen atoms with amine groups in positions 15and 55does not increase the thermal stability of(21)(the temperature corresponding to the maximum decomposition peak decreases to about 256°C).The replacement of the hydrogen atom in the mentioned positions with a bromine atom,while improving thermostability(highest rate of decomposition at 388°C),has a negative influence on detonation parameters(7021 m/s and 26.6 GPa).Attempts were also made to obtain dodecanitro derivatives of(19)by nitrating with various nitrating mixtures,but none of these were fully successful,i.e.,the introduction of four additional groups-NO2was not possible.The cause was most likely steric obstacles that became more severe when each subsequent nitro group was attached to the inner sphere of the crown[47].

When 1,2,3-trifluoro-4-nitrobenzene(22)is used as a starting material,nitro derivatives of calixarenes where fluorine atoms are placed in the positions of 12and 52can be obtained.After nitration of(22)by a mixture of potassium nitrite and concentrated(98%)sulphuric acid for 24 h at 80°C 1,2,3-trifluoro-4,6-dinitrobenzene(23)was obtained The product precipitated when the reaction mixture was poured into water with crushed ice.Next the reaction of(23)with resorcinol catalysed by triethylamine was performed for 8 h under reflux leading to 12,52-difluoro-14,16,54,56-tetranitro-2,4,6,8-tetraoxa-1,3,5,7(1,3)-tetrabenzenacyclooctaphene (24)which was precipitated by methanol from the reaction mixture(Yield 77%).The last reaction of(24)with the salt/acid mixture conducted for 13 h at 65°C gave 12,52-difluoro-14,16,34,36,54,56,74,76-octanitro-2,4,6,8-tetraoxa-1,3,5,7(1,3)-tetrabenzenacyclooctaphene(25)with the yield of approximately 84%after pouring the mixture on crushed ice,Fig.11.

Fig.9.Structure of 2,4,6,8-tetraoxa-1,3,5,7(1,3)-tetrabenzenacyclooctaphene.

Fig.10.Synthesis scheme of(21).

Replacing of the hydrogen atoms by the fluorine atoms to the inner sphere of the crown increases density of octanitrocalixarene(1.91 g/cm3)which affects positively on the maximum calculated detonation velocity and pressure(8070 m/s and 29.5 GPa,respectively).It is also caused by higher enthalpy of formation(1000.8 kJ/mol).It is resulted by steric obstacles which decreases heat resistivity as well.When heated at a rate of 5°C/min,(25)starts decomposition at approximately 326°C and it decomposes at 334°C at the highest rate.12,52-difluoro-14,16,34,36,54,56,74,76-octanitro-2,4,6,8-tetraoxa-1,3,5,7(1,3)-tetrabenzenacyclooctaphene is almost insensitive to the mechanical stimuli like impact or friction(61 J and 360 N,respectively).The detonation parameters are superior than those of TATB,but its thermal stability is slightly lower.Nevertheless,(25)is a potential fluoro-contained thermally stable energetic compound[49].

6.Summary

The TSEs used at present are mainly nitro derivatives of aromatic hydrocarbons with extensiveπ-πand n-πelectron cross-linkages.A relatively new trend in the search for extremely heat-resistant compounds is the replacement of carbon atoms with nitrogen atoms in the structuring role.Nitrogen-rich five-and sixmembered rings may have a high degree of aromaticity,and at the same time,the compounds containing them have a higher density,a higher value of formation enthalpy,a more favourable oxygen balance,and,consequently,higher detonation parameters in comparison with their counterparts having skeletons made exclusively of carbon atoms.Unfortunately,none of the explosive compounds obtained in the last decade that contain heterocyclic groups in their structure are much more heat resistant than TATB or TACOT.

Fig.11.Synthesis scheme of(25).

Only the dinitro derivatives of 3,3′-bipyrazoles and(18)match TATB and TACOT in this aspect.(18)also has the highest detonation parameters,but its rapid implementation into production is at least doubtful,due to the complexity and low yield of the synthesis process as well as the need to use expensive and unstable reagents.C-nitro derivatives of 3,3-bipyrazoles have acidic properties,but this does not preclude their use,including as low-sensitivity explosives,as they are capable of forming salts with nitrogen-rich amines.

The development potential of nitro crown ether derivatives is also not particularly high,as it is difficult to imagine their functionalization resulting in increased thermal resistance.Moreover,the results of the research conducted so far on 2,4,6,8-tetraoxa-1,3,5,7(1,3)-tetrabenzenacyclooctaphene indicate that the introduction of more than two nitro groups into the benzene rings that form these structures is impossible,which means low detonation parameters.It is possible to increase the detonation parameters of nitrocalixarenes by placing fluorine atoms to the inner sphere of calixarene,but it also reduces the decomposition temperature.

(5)and(7)decompose at temperatures well above 300°C(higher than 335°C and 350°C,respectively),but they give way to TATB in terms of sensitivity to mechanical stimuli.Nevertheless,they can be safely handled in applications typical for secondary explosives,i.e.,with the addition of thermally resistant polymers(PBX type compositions).

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Financial support of this work by the Military University of Technology in Warsaw is gratefully acknowledged.