Recent Advances in the Synthesis of High Explosive Materials

Review Recent Advances in the Synthesis of High Explosive Materials

Jesse J. Sabatini 1,* and Karl D. Oyler 2,*

Received: 31 July 2015; Accepted: 22 December 2015; Published: 29 December 2015 Academic Editor: Thomas M. Klapötke

1 US Army Research Laboratory Weapons & Materials Research Directorate (WMRD) Lethality Division, Energetics Technology Branch, Aberdeen Proving Ground, MD 21005, USA 2 US Army Armaments Research, Development & Engineering Center (ARDEC) Energetics, Warheads, & Manufacturing Directorate Explosives Development Branch, Picatinny Arsenal, NJ 07806-5000, USA * Correspondence: [email protected] (J.J.S.); [email protected] (K.D.O.); Tel.: +1-410-278-0235 (J.J.S.); +1-973-724-4784 (K.D.O.)

Abstract: This review discusses the recent advances in the syntheses of high explosive energetic materials. Syntheses of some relevant modern primary explosives and secondary high explosives, and the sensitivities and properties of these molecules are provided. In addition to the synthesis of such materials, processing improvement and formulating aspects using these ingredients, where applicable, are discussed in detail.

Keywords: energetic materials; explosives; synthesis; organic chemistry; processing

1. Introduction There is an ever-increasing need for the development of new energetic materials for explosive applications. This includes, but is not limited to, the area of primary explosives and secondary high explosives. Primary explosives (or “primaries” as they are colloquially called) are defined as energetic materials that possess an exceptionally high initiation sensitivity to impact, friction, electrostatic discharge, heat, and shock. Primaries are known to reach detonation very quickly after such an initiation event. The large amount of energy released upon initiation of a primary—typically in the form of heat or a shockwave—is used to initiate less sensitive energetic materials, including secondary explosives, propellants, and pyrotechnics. Despite their highly sensitive nature, primaries are, in general, less powerful than secondary explosive materials [1]. In other words, primaries typically possess lower detonation velocities, detonation pressures, and a lower heat of detonation than a given secondary explosive. Despite the lower energy of a primary, extreme care is urged when handling these materials. Primaries are ubiquitous in both military munitions and commercial items, where their main function is to serve as the key ingredient in initiators such as detonators, blasting caps, and pyrotechnic percussion primer formulations. By far, the most commonly used primary explosives by the U.S. military are lead azide (used most often in detonators and blasting caps) and lead styphnate (most often found in percussion primers). There has been significant interest in recent years to develop new primary explosive materials, as the lead content of these materials is highly objectionable from both a toxicological and environmental standpoint [2]. Secondary explosives (colloquially known as “secondaries”) differ from primaries in that they are much less sensitive to impact, friction, electrostatic discharge, heat, and shock. Instead, they are intended to be initiated by the heat and shockwave generated from a detonating primary explosive charge. There is a tremendous need for the development of secondary explosives that have higher performance, lower sensitivity, and lower toxicity than the currently fielded explosive compounds.

Crystals 2016, 6, 5; doi:10.3390/cryst6010005 www.mdpi.com/journal/crystals Crystals 2016, 6, 5 2 of 22

Higher performing secondaries are classified as those having higher detonation velocities, higher detonation pressures, higher crystalline densities, and a higher heat of detonation. Secondary explosives with a higher performance typically correlate to an increase in brisance, which is defined as the fragmentation ability or shattering effect of an explosive charge within a specific vicinity. As a rule of 0 thumb, explosives with higher crystal densities and higher enthalpies of formation (∆fH ) directly correlate to higher detonation pressures, detonation velocities, and overall greater energy release. Secondaries possessing a lower sensitivity are crucial for the development of insensitive munitions (IM). In the IM sub-area, the development of explosive materials with higher thermal stability and secondary explosive-based formulations that have a low sensitivity to bullet/fragment impact are critical. Minimizing sympathetic detonation—the unintended detonation of a nearby explosive charge which has been caused by a deliberate explosion of another explosive charge, as well as fast- and slow cook-off (the exploding of a munition due to excessive heat in the vicinity)—is key to those interested in IM-based materials. The ideal secondary explosive will have a low toxicity profile of the explosive itself, its degradation products via natural degradation pathways, as well as its detonation products. Although the field of explosives chemistry has existed for well over a century and produced a wealth of published compounds and their performance data, general literature reviews on explosive compounds [3–5] are relatively small in number and infrequently published. To supplement this list, herein we seek to provide an overview of the more significant recent advances in the area of explosive molecules. This summary is by no means exhaustive, given the wide scope of the explosives field, and the topics covered will focus largely on relatively recently developed compounds which are thought to have the highest likelihood of seeing practical application in the future.

2. Primary Explosives

2.1. Legacy Primary Explosives While primary explosives have historically consisted of materials that exhibit reasonable explosive power and a high sensitivity, these legacy materials also possess a high density as a result of the presence of heavy metals within their respective structures. The structures of some historically used primary explosives are given in Figure1. It is commonly agreed that the highly toxic mercury fulminate (1), adopted for use in some of the first percussion primer formulations in the early 1800s [6] and later by Alfred Nobel for some of the first blasting caps in 1867 [7], is the first primary explosive to see practical application. The high costs associated with mercury, the high sensitivity of mercury fulminate, and its ability to lose performance under high loading pressures paved the way for lead azide (2) to replace the compound starting in the early 1900s [8]. Today, lead azide (including its less sensitive dextrin-coated form) finds wide-ranging application in energetic materials as the main primary explosive ingredient in blasting caps and detonators. A few years after lead azide began to find applications is the energetics field, the first practical use of lead styphnate was described in a British patent [9]. Lead styphnate comes in two forms; normal lead styphnate (lead(II) 2,4,6-trinitro-benzene-1,3-diolate hydrate) (3) and basic lead styphnate (di(lead(II) hydroxide)-2,4,6-trinitro-benzene-1,3-diolate) (4). Although less powerful than lead azide, lead styphnate is easier to initiate. For this reason, basic lead styphnate is present in some lead azide-based stab detonator formulations such as NOL-130. The use of basic lead styphnate instead of normal lead styphnate in stab detonator mixtures assists in preventing the decomposition of lead azide via a hydrolytic pathway. Normal lead styphnate is commonly encountered as the main primary explosive ingredient in primer applications. Crystals 2016, 6, 5 3 of 22 Crystals 2016, 6, 5

Crystals 2016, 6, 5

Figure 1. Chemical structure of the primary explosives; mercury fulminate (1), lead azide (2) and normal lead styphnate (3) and basic lead styphnate (4).(4).

2.2. Why “Green” Figure 1. PrimaryChemical Explosives?structure of the primary explosives; mercury fulminate (1), lead azide (2) and normal lead styphnate (3) and basic lead styphnate (4). Over the past two decades, there has been a significantsignificant interest amongst the US Department of Defense2.2. Why (DoD) “Green” community, Primary Explosives? in particular, toto addressaddress environmentalenvironmental issues associated with military explosives,Over propellants, the past two and decades, pyrotechnics. there has Today,been Today, a significant lead azide interest and leadamongst styphnate the US Department comprise the of vast majorityDefense of military (DoD) community, and civilian in primaryparticular, explosive to address use. environmental The obvious issues issue associated with these with two military materials is thetheexplosives, presencepresence propellants, ofof lead,lead, whichwhich and ispyrotechnics.is anan establishedestablished Today, heavyheavy lead metalazidemetal and toxin.toxin. lead Until styphnate the 1960s, comprise these the materialsvast were majority notnot believedbelieved of military toto significantlysignificantly and civilian impact impactprimary health health explosive [ 10[10]]. use. Over. Over The the theobvious past past five issuefive decades, decawith des,these however, however, two materials the the view view of is the presence of lead, which is an established heavy metal toxin. Until the 1960s, these materials lead-basedof lead-based primary primary explosives explosives has has changed changed significantly. significantly. Ingestion Ingestion and/or and/or inhalation inhalation of lead of lead is now is were not believed to significantly impact health [10]. Over the past five decades, however, the view knownnow known to affect to theaffect central the nervouscentral nervous system, thesystem renal, the system, renal the system, blood, the and blood, has been and a contributinghas been a contributingof lead-based factor primary in mental explosives impairment has changed and behavioral significantly. issues Ingestion [11]. Lead and/or has inhalationbecome one of leadof the is most factornow in mentalknown impairmentto affect the andcentral behavioral nervous issuessystem, [ 11the]. Leadrenal hassystem, become the blood, one of and the has most been regulated a regulated chemical substances on an international scale. It is classified as a toxic pollutant in the USA, chemicalcontributing substances factor on in an mental international impairment scale. and It behavioral is classified issues as a[11]. toxic Lead pollutant has become in the one USA, of the and most efforts and efforts have been underway for decades to remove lead from Federal facilities, including DoD haveregulated been underway chemical for substances decades on to an remove international lead from scale. Federal It is classified facilities, as a toxic including pollutant DoD in the facilities USA, [12]. Infacilities 2012,and the[12].efforts National In have 2012, been Research the underwayNational Council Researchfor decades recommended Co touncil remove recommended that lead the from DoD Federal reviewthat thefacilities, its DoD 30 yearsincludingreview old its policyDoD 30 years on protectingold policyfacilities on workers [12].protecting In from2012, workers the lead National exposure from Research lead in firing exposure Council ranges inrecommended firing [13]. Inranges Europe, that [13]. the the In DoD Registration,Europe, review the its Registration,30 Evaluation, years Authorization,Evaluation,old policy Authorization, on and protecting Restriction workersand ofRestriction Chemicals from lead of exposure (REACH)Chemicals in firing program (REACH) ranges has program[13]. called In Europe, for has the called the specific Registration, for the regulation specific ofregulation leadEvaluation, azide of andlead Authorization, lead azide styphnate. and and lead Restriction Therefore, styphnate. of aChemicals clear Ther needefore, (REACH) exists a clear forprogram theneed successful has exists called for for development thethe specific successful and regulation of lead azide and lead styphnate. Therefore, a clear need exists for the successful replacementdevelopment of and these replacement two aforementioned of these two primary aforementioned explosives. primary explosives. development and replacement of these two aforementioned primary explosives. Aside from the environmental concerns associated with lead azide, a further complication exists in that theAside material from isthe known environmental to slowly concerns decompose associated under with ambient lead azide, conditions a further (Scheme complication 1). Lead exists azide in thatin thethatmaterial the material is known is known to to slowly slowly decompose decompose under ambient ambient conditions conditions (Scheme (Scheme 1). Lead1). Lead azide azide reacts with CO2 and H2O to produce basic lead carbonate and highly toxic hydrazoic acid (HN3). HN3 reactsreacts with with CO CO2 and2 and H H2O2O toto produce basic basic lead lead carbonate carbonate and highly and highly toxic hydrazoic toxic hydrazoic acid (HN acid3). HN (HN3 3). can react with exposed copper metal surface in pipes and wires, thus generating the highly sensitive HN3 cancan react react with with exposed exposed copper copper metal metalsurface surface in pipes in and pipes wires, and thus wires, generating thus the generating highly sensitive the highly and deadly copper azide (Cu(N3)2). Several fatal military accidents over the years have been sensitiveand anddeadly deadly copper copper azide azide (Cu(N (Cu(N3)2). Several3)2). Several fatal fatalmilitary military accidents accidents over the over years the yearshave havebeen been associatedassociated with with the the accidental accidental generation generation of of coppercopper copper azideazide [14]. [[14].14].

SchemeScheme 1. 1. Decomposition Decomposition reaction reaction of of lead lead azide. azide.

2.3. “Green” Primary Explosive Candidates 2.3. “Green” Primary Explosive Candidates 2.3.1. Metal-Free Primary Explosives 2.3.1. Metal-Free Primary Explosives 2.3.1.1. Cyanuric Triazide (CTA) 2.3.1.1. Cyanuric Triazide (CTA) 2.3.1.1. Cyanuric Triazide (CTA) CTA (6), prepared in a single, efficient, and eco-friendly step by treatment of cyanuric trichloride CTA (6), prepared in a single, efﬁcient, and eco-friendly step by treatment of cyanuric trichloride CTA(5) with (6), sodium prepared azide in a(Scheme single, 2),efficient, has more and explosiv eco-friendlye power step than by lead treatment azide, making of cyanuric it a suitable trichloride (5) with sodium azide (Scheme2), has more explosive power than lead azide, making it a suitable (5) withreplacement sodium forazide lead (Scheme azide in large-sized2), has more detonato explosivrs. Unfortunately,e power than there lead are azide, several making complications it a suitable replacement for lead azide in large-sized detonators. Unfortunately, there are several complications replacementwith the formaterial, lead azidein particular in large-sized its tendency detonato to undergors. Unfortunately, sublimation at there elevated are severaltemperatures complications [15]. withFurther, the material, the earlier in particular syntheses itsof tendencyCTA reported to undergo an extreme sublimation sensitivity atof elevatedthe material temperatures to ignition [15]. with the material, in particular its tendency to undergo sublimation at elevated temperatures [15]. Further,stimuli the [16]. earlier The crystalline syntheses material of CTA from reported the earlier an extremesyntheses sensitivityexhibited a needle-like of the material morphology, to ignition Further, the earlier syntheses of CTA reported an extreme sensitivity of the material to ignition which broke easily, thus increasing the risk of accidental detonation. stimuli [16]. The crystalline material from the earlier syntheses exhibited a needle-like morphology, which broke easily, thus increasing the risk of accidental3 detonation.

3 Crystals 2016, 6, 5 4 of 22

Crystals 2016, 6, 5 stimuli [16]. The crystalline material from the earlier syntheses exhibited a needle-like morphology, whichCrystals 2016 broke, 6, 5 easily, thus increasing the risk of accidental detonation.

Scheme 2. Synthesis of cyanuric triazide (CTA, 6).

Scheme 2. SynthesisSynthesis of of cyanuric cyanuric triazide (CTA, 6). Although CTA was first patented in 1921 [17], a newer synthetic procedure for the material developed by ARDEC in 2009 resulted in a reduced particle size of the material [18]. As a result of Although CTA was firstfirst patented in 1921 [[17],17], a newer synthetic procedure for the material the new synthesis procedure, CTA has been further investigated by researchers at ARDEC, and has developed byby ARDECARDEC inin 20092009 resulted resulted in in a a reduced reduced particle particle size size of of the the material material [18 [18].]. As As a result a result of the of found successful use as a replacement for both lead azide and lead styphnate in the NOL-130 stab thenew new synthesis synthesis procedure, procedure, CTA CTA has beenhas been further further investigated investigated by researchers by researchers at ARDEC, at ARDEC, and has and found has detonator mix, as well as some more recent work as a potential lead styphnate replacement in foundsuccessful successful use as ause replacement as a replacement for both for lead both azide lead and azide lead and styphnate lead styphnate in the NOL-130 in the stabNOL-130 detonator stab percussion primer formulations (NB: The lead-based NOL-130 formulation consists of 40% lead detonatormix, as well mix, as someas well more as recent some workmore as recent a potential work lead as a styphnate potential replacement lead styphnate in percussion replacement primer in styphnate, 20% lead azide, 20% barium nitrate, 15% antimony trisulfide, and 5% tetrazene). The percussionformulations primer (NB: Theformulations lead-based (NB: NOL-130 The lead-b formulationased NOL-130 consists formulation of 40% lead consists styphnate, of 40% 20% lead sublimation of CTA, and thus its looming safety issues, however, remains an Achilles heel for this styphnate,azide, 20% barium20% lead nitrate, azide, 15% 20% antimony barium trisulfide,nitrate, 15% and antimony 5% tetrazene). trisulfide, The sublimationand 5% tetrazene). of CTA, The and material if it is to ever gain serious traction in having widespread serious military and civilian sublimationthus its looming of CTA, safety and issues, thus however,its looming remains safety anissues, Achilles however, heel for remains this material an Achilles if it is heel to ever for gainthis applications. materialserious traction if it is into havingever gain widespread serious serioustraction militaryin having and widespread civilian applications. serious military and civilian applications. 2.3.1.2. 2-Diazo-4,6-Dinitrophenol (DDNP) 2.3.1.2.The 2-Diazo-4,6-Dinitrophenol synthesis of DDNP (9) is descri described(DDNP)bed in Scheme 3 3.. TreatmentTreatment ofof picricpicric acidacid (7)(7) withwith aa controlledcontrolled simultaneous addition of aqueous sodium sulfide yields sodium picramate (8), which is then simultaneousThe synthesis addition of DDNP of aqueous (9) is descri sodiumbed sulfide in Scheme yields 3. sodium Treatment picramate of picric (8), acid which (7) with is then a controlled converted converted to DDNP via the Griess diazotization. DDNP has found explosive use since the early 20th simultaneousto DDNP via theaddition Griess diazotization.of aqueous sodium DDNP hassulfide found yields explosive sodium use sincepicramate the early (8), 20thwhich century is then [1]. century [1]. Although old enough to be considered a legacy primary explosive, it tends to fall into a convertedAlthough oldto DDNP enough via to the be Griess considered diazotization. a legacy DD primaryNP has explosive, found explosive it tends use to fallsince into the aearly different 20th different category because of its heavy metal-free nature. centurycategory [1]. because Although of its old heavy enough metal-free to be considered nature. a legacy primary explosive, it tends to fall into a different category because of its heavy metal-free nature.

Scheme 3. Synthesis of DDNP (9).

Scheme 3. Synthesis of DDNP (9). At thisthis time,time, DDNP DDNP is generallyis generally considered considered to be to an be unsuitable an unsuitable candidate candidate for demanding for demanding military military primer applications. The material is reputed to suffer from reliability issues in extreme cold primerAt applications.this time, DDNP The material is generally is reputed considered to suffer to from be an reliability unsuitable issues candidate in extreme for cold demanding weather weather climates. DDNP’s low sensitivity to friction, its low flame temperature, and its militaryclimates. primer DDNP’s applications. low sensitivity The material to friction, is reputed its low ﬂame to suffer temperature, from reliability and its issues incompatibility in extreme withcold incompatibility with lead azide also plague this compound [19]. Due to different requirements weatherlead azide climates. also plague DDNP’s this compound low sensitivity [19]. Due toto differentfriction, requirements its low flame necessary temperature, for the production and its necessary for the production of commercial primer formulations, DDNP has found recent usage and incompatibilityof commercial primerwith lead formulations, azide also DDNPplague has this found compound recent [19]. usage Due and to interest different from requirements the civilian interest from the civilian sectors as a “green” replacement for lead styphnate in primer applications necessarysectors as afor “green” the production replacement of commercial for lead styphnate primer formulations, in primer applications DDNP has [20 found,21]. recent usage and [20,21]. interest from the civilian sectors as a “green” replacement for lead styphnate in primer applications 2.3.1.3. Peroxide-Based Explosives [20,21]. 2.3.1.3. Peroxide-Based Explosives Although primary explosives are designed with the intent to be sensitive to various ignition 2.3.1.3.stimuli,Although Peroxide-Based it has primary always Explosives beenexplosives believed are thatdesigned peroxide-based with the intent explosives to be sensitive are too sensitiveto various to ignition garner practicalstimuli, it use has for always even primarybeen believed explosive that applications. peroxide-based Chemically, explosives the are high too degreesensitive of instabilityto garner Although primary explosives are designed with the intent to be sensitive to various ignition ofpractical the oxygen-oxygen use for even primary bond is explosive the culprit applicatio behindns. this Chemically, sensitivity. the Two high peroxide-based degree of instability materials of stimuli, it has always been believed that peroxide-based explosives are too sensitive to garner thatthe oxygen-oxygen have been studied bond extensively is the culprit are triacetone behind this triperoxide sensitivity. (TATP) Two andperoxide-based hexamethylenetriperoxide materials that practical use for even primary explosive applications. Chemically, the high degree of instability of diaminehave been (HMTD). studied Their extensively syntheses are are triacetone remarkably tr simpleiperoxide and (TATP) are performed and hexamethylenetriperoxide from cheap, commercially the oxygen-oxygen bond is the culprit behind this sensitivity. Two peroxide-based materials that diamine (HMTD). Their syntheses are remarkably simple and are performed from cheap, have been studied extensively are triacetone triperoxide (TATP) and hexamethylenetriperoxide commercially available starting materials, as summarized in Scheme 4. TATP (10) and HMTD (11) diamine (HMTD). Their syntheses are remarkably simple and are performed from cheap, are synthesized from acetone and hexamine, respectively, in the presence of acidic water and commercially available starting materials, as summarized in Scheme 4. TATP (10) and HMTD (11) are synthesized from acetone and hexamine, respectively,4 in the presence of acidic water and 4 Crystals 2016, 6, 5 5 of 22 Crystals 2016, 6, 5 availablehydrogenCrystals 2016 starting,peroxide. 6, 5 materials, The historically as summarized negative in Schemesafety profile4. TATP of (10)peroxide-based and HMTD (11)primary are synthesized explosives, fromcoupled acetone with the and blatant hexamine, terroristic respectively, use of materi in theals presence such as ofTATP acidic and water HMTD and have hydrogen discouraged peroxide. the hydrogen peroxide. The historically negative safety profile of peroxide-based primary explosives, Theinvestigation historically and negative use of organic safety proﬁle peroxides of peroxide-based as a viable energetic primary material explosives, for practical coupled withuse. the blatant coupled with the blatant terroristic use of materials such as TATP and HMTD have discouraged the terroristic use of materials such as TATP and HMTD have discouraged the investigation and use of investigation and use of organic peroxides as a viable energetic material for practical use. organic peroxides as a viable energetic material for practical use.

Scheme 4. Synthesis of TATP (10) and HMTD (11). Scheme 4. Synthesis of TATP (10) and HMTD (11). Work spearheaded byScheme Winter 4. inSynthesis 2015, howeveof TATP r,(10) has and shown HMTD that(11). organic peroxides can be designed that exhibit high performance, yet reasonable sensitivities [22]. As summarized in Scheme Work spearheadedspearheaded by by Winter Winter in 2015,in 2015, however, howeve hasr, shown has shown that organic that organic peroxides peroxides can be designed can be 5, a given diketone or dialdehyde was reacted with hydrogen peroxide in the presence of a catalytic thatdesigned exhibit that high exhibit performance, high performance, yet reasonable yet reasonable sensitivities sensitivities [22]. As summarized [22]. As summarized in Scheme 5in, aScheme given amount of iodine to afford the bis-geminal hydroperoxy derivatives 12–15. Despite the presence of diketone5, a given or diketone dialdehyde or dialdehyde was reacted was with reacted hydrogen with peroxidehydrogen in peroxide the presence in the of presence a catalytic of amounta catalytic of normally labile peroxy groups, it is believed that the lability is suppressed due to the presence of iodineamount to of afford iodine the to bis-geminalafford the bis-geminal hydroperoxy hydr derivativesoperoxy derivatives 12–15. Despite 12–15. the Despite presence the ofpresence normally of intermolecular O–H…O hydrogen bonds, as well as several O…O and C…H close contacts which labilenormally peroxy labile groups, peroxy it isgroups, believed it thatis believed the lability that is the suppressed lability is due suppresse to the presenced due to of the intermolecular presence of was determined to occur in the crystalline lattice of 15 upon confirmation of the structure by single O–Hintermolecular . . . O hydrogen O–H…O bonds, hydrogen as well as bonds, several as O well . . . Oas and several C . . . O…O H close and contacts C…H which close wascontacts determined which crystal X-ray diffraction. towas occur determined in the crystalline to occur lattice in the of crystalline 15 upon conﬁrmation lattice of 15 of upon the structure confirmation by single of crystalthe structure X-ray diffraction. by single crystal X-ray diffraction.

Scheme 5. Synthesis of stable peroxi peroxide-basedde-based primary explosives.

The sensitivitiessensitivitiesScheme and and performance performance5. Synthesis of of geminal stableof geminal peroxi peroxidesde-based peroxides 12–15, primary compared12–15, explosives. compared to TATP, areto summarizedTATP, are insummarized Table1. Remarkably, in Table 1. geminalRemarkably, peroxy geminal compounds peroxy 12–15compounds are more 12–15 insensitive are more to insensitive impact and to The sensitivities and performance of geminal peroxides 12–15, compared to TATP, are frictionimpact and than friction TATP, withthan comparableTATP, with comparable or better ESD or sensitivities. better ESD sensitivities. This phenomenon This phenomenon occurs despite occurs the summarized in Table 1. Remarkably, geminal peroxy compounds 12–15 are more insensitive to betterdespite oxygen the better balances, oxygen higher balances, crystalline higher crystalline densities, higher densities, detonation higher detonati velocities,on velocities, and total energy and total of impact and friction than TATP, with comparable or better ESD sensitivities. This phenomenon occurs detonationenergy of detonation values compared values compared to TATP. Theto TATP. Winter The approach Winter approach is unique is because unique itbecause describes it describes the ﬁrst despite the better oxygen balances, higher crystalline densities, higher detonation velocities, and total knownthe first examples known examples of stabilized of stabilized peroxide-based peroxide primary-based explosives. primary explosives. Although the Although thermal stabilitiesthe thermal of energy of detonation values compared to TATP. The Winter approach is unique because it describes thisstabilities class ofof compounds this class of could compounds use some could improvement, use some the improvement, area of peroxide-based the area primaryof peroxide-based explosives, the first known examples of stabilized peroxide-based primary explosives. Although the thermal inprimary light of explosives, this discovery, in oughtlight of to bethis investigated discovery, further ought forto possiblebe investigated application. further for possible stabilities of this class of compounds could use some improvement, the area of peroxide-based application. primary explosives, in light of this discovery, ought to be investigated further for possible

application.

5 Crystals 2016, 6, 5 Crystals 2016, 6, 5 6 of 22 Table 1. Sensitivities and performance of geminal peroxides 12–15 and TATP.

TableData Category 1. Sensitivities and 12 performance 13 of geminal 14 peroxides 15 12–15 and TATP TATP. IS [J] a 2 1 2 3 0.3 Data Category 12 13 14 15 TATP FS [N] b 5 5 5 <5 0.1 a ISESD [J] c [J] c 20.2 0.5 1 0.1 2 0.25 3 0.16 0.3 FS [N] b 5 5 5 <5 0.1 Tdec [°C] d 117 98 100 105 150–160 ESD c [J] c 0.2 0.5 0.1 0.25 0.16 e Ω˝ [%]d −126.2 −114.18 −100.76 −88.83 −151.19 Tdec [ C] 117 98 100 105 150–160 e f ΩCOρ2 [g/cc][%] ´126.21.35 ´1.375114.18 ´1.6100.76 1.6´88.83 1.18´151.19 f ρ P[g/cc]cj [kbar] g 1.35117 1.375126 195 1.6 195 1.6 – 1.18 P [kbar] g 117 126 195 195 – cjVdet [m/s] h 6150 6250 6428 7130 5300 h Vdet [m/s]0 i 6150 6250 6428 7130 5300 Δ0fH [kJ/mol]i −703.6 −660.8 −418.2 −418.2 −583.8 ∆fH [kJ/mol] ´703.6 ´660.8 ´418.2 ´418.2 ´583.8 Δex0U0 [kJ/kg]j j −4636 −4875 −5083 −5498 −2745 ∆exU [kJ/kg] ´4636 ´4875 ´5083 ´5498 ´2745 a b c d ISa = impact sensitivity; bFS = friction sensitivity; c ESD = electrostatic discharge;d Tdec = Temperature IS = impact sensitivity; FS = friction sensitivity; ESD = electrostatic discharge; Tdec = Temperature e f g h of decomposition; eΩ = CO2 oxygen balance; fρρ = crystalline density; g Pcj = detonation pressure; of decomposition; ΩCO 2 = CO2 oxygen balance; = crystalline density; Pcj = detonation pressure; h i i 0 0 j j 0 0 VdetV =det detonation= detonation velocity; velocity; Δ∆fHfH = =molar molar enthalpy enthalpy ofof formation; formation;∆ ex ΔUexU= total= total energy energy of detonation. of detonation.

2.3.1.4. Tetrazene and MTX-1 1-Amino-1-(1H-tetrazol-5-yl)-azo-guanidine-tetrazol-5-yl)-azo-guanidine hydrate, hydrate, known known colloquially colloquially as as tetrazene tetrazene (16), (16), is isa awell-known well-known sensitizer sensitizer in in percussion percussion primer primer compos compositionsitions (Figure (Figure 2)2)[ [8].8]. Without Without its its presence in primers, initiation is known to be dreadfully unreliable,unreliable, or fails toto functionfunction altogether.altogether. Despite its reliability and widespread use in primers, tetraz tetrazeneene suffers from a high degree of hydrolytic and thermal instability. It It has been found to rapidlyrapidly decomposedecompose at temperaturestemperatures ofof ca.ca. 90 ˝°C,C, and this temperature is commonly encountered during handlinghandling andand storagestorage ofof thethe material.material. Addition of boiling water to tetrazene is also known to rapidly decompose thethe material.material.

Figure 2. Molecular structure of tetrazene (16) and MTX-1 (17).

In light of of these these concerns, concerns, Fronabarger Fronabarger and and Will Williamsiams reacted reacted tetrazene tetrazene in inaqueous aqueous solution solution to toproduce produce MTX-1, MTX-1, which which has haspromise promise for serving for serving as a more as a morethermally thermally stable stablereplacement replacement [23,24]. [ 23Initial,24]. Initialstudies studies have shown have shown that thatwhile while tetrazene tetrazene decomposes decomposes exothermically exothermically at at144 144 °C,˝C, MTX-1 MTX-1 undergoes undergoes exothermic decomposition at 214 ˝°C.C. Holding tetrazene at 90 °C˝C for for 120 120 h h results results in in 36% 36% weight weight loss, loss, while MTX-1 MTX-1 experiences experiences just just 4% 4% weight weight loss loss after after the thesame same time time periods. periods. Exposure Exposure of MTX-1 of MTX-1 to water to waterfor 120 for days 120 has days virtually has virtually no effect, no effect,as MTX-1 as MTX-1 can be canre-isolated be re-isolated in nearly in nearlyquantitative quantitative yield. It yield. has Itbeen has stated been stated that MTX-1 that MTX-1 has hascomparable comparable impact, impact, friction friction and and ESD ESD sensitivities sensitivities to to tetrazene, tetrazene, thus adding to its appeal as a potential replacement to this traditionally used organic sensitizing agent in primer compositions.

2.3.2. Metal-Organic Primary Explosives

2.3.2.1. Potassium 4,6-Dinitrobenzofuroxan (KDNBF) KDNBF (21)(21) isis synthesizedsynthesized in in a a three-step three-step procedure, procedure, as as summarized summarized in in Scheme Scheme6[ 625 [25].]. Exposure Exposure of oof-nitroaniline o-nitroaniline to NaOClto NaOCl and baseand base affords affords benzofuroxan benzofuroxan 19, which 19, iswhich subjected is subjected to mixed to acid mixed nitration acid tonitration yield dinitrobenzofuroxanto yield dinitrobenzofuroxan 20. Treatment 20. Trea of dinitrobenzofuroxantment of dinitrobenzofuroxan with potassium with bicarbonatepotassium furnishesbicarbonate KDNBF. furnishes KDNBF.

6 Crystals 2016, 6, 5 7 of 22 Crystals 2016, 6, 5 Crystals 2016, 6, 5

Scheme 6. Synthesis of KDNBF. Scheme 6. Synthesis of KDNBF.

Potassium is generally considered an innocuous and much safer alternative to lead. Thus, Potassium isis generally generally considered considered an innocuousan innocuous and muchand much safer alternativesafer alternative to lead. to Thus, lead. KDNBF, Thus, KDNBF, much like DDNP, has found use as a suitable “green” primary explosive alternative to lead KDNBF,much like much DDNP, like has DDNP, found has use found as a suitable use as a “green” suitable primary “green” explosive primary alternativeexplosive alternative to lead styphnate to lead styphnate in civilian primer compositions. While lead styphnate has a decomposition temperature of styphnatein civilian primerin civilian compositions. primer compositions. While lead While styphnate lead hasstyphnate a decomposition has a decomposition temperature temperature of 280–290 ˝ ofC, 280–290 °C, KDNBF’s decomposition temperature is much lower (217 °C). Also like DDNP, KDNBF 280–290KDNBF’s °C, decomposition KDNBF’s decomposition temperature temperature is much lower is much (217 ˝ C).lower Also (217 like °C). DDNP, Also KDNBF like DDNP, has notKDNBF been has not been adopted for widespread military use; the lower thermal stability of KDNBF may be one hasadopted not been for widespread adopted for military widespread use; themilitary lower use; thermal the lower stability thermal of KDNBF stability may of beKDNBF one of may the reasons be one of the reasons as to why this material has not been used as a universal alternative to lead styphnate ofas the to whyreasons this as material to why hasthis notmaterial been has used not as been a universal used as alternativea universalto alternative lead styphnate to lead in styphnate primary in primary explosive compositions. inexplosive primary compositions. explosive compositions.

2.3.2.2. Potassium 4,6-Dinitro-7-Hydroxybenzofuroxan (KDNP) Scheme7 7details details the the synthesis synthesis of KDNP of KDNP (25) [ 26(25)]. Mixed[26]. Mixed acid nitration acid nitration of 3-bromoanisole of 3-bromoanisole delivered deliveredtrinitro derivative trinitro derivative 23. This material 23. This wasmaterial treated was with treated potassium with potassium azide in reﬂuxingazide in refluxing methanol methanol to effect toconcomitant effect concomitant substitution substitution at C–3 and at cleavage C–3 and of cleava the methylge of the ether, methyl thus yieldingether, thus azide yielding 24. Treatment azide 24. of Treatmentthis azide withof this diethylcarbonate azide with diethylcarbonate in heat produces in he KDNPat produces (25). TheKDNP approach (25). The detailed approach in Scheme detailed6 isin Schemean improvement 6 is an improvement over previous over syntheses previous that synthe requiredses that the userequired of water, the whichuse of was water, retained which in was the product.retained in This the ledproduct. to the This formation led to ofthe needles formation that of were needles capable that were of breaking, capable and of breaking, thus dramatically and thus dramaticallyincreased the increased sensitivity the of sensitivity the material. of the The materi newal. synthetic The new synthetic procedure procedure allowed forallowed a free-ﬂowing for a free- powderflowing powder to be obtained, to be obtained, and particle and sizeparticle could size be could varied be by varied simple by variations simple variations in temperature in temperature and the andaddition the addition rate of the rate solvent of the duringsolvent recrystallization. during recrystallization.

Scheme 7. Synthesis of KDNP (25). Scheme 7. Synthesis of KDNP (25).

Though originally synthesized over 30 years ago, KDNP has drawn renewed interest as a lead Though originally synthesized over 30 years ago, KDNP has drawn renewed interest as a lead styphnate replacement, due to its comparable explosive performance in military-relevant primers styphnate replacement, due to its comparable explosive performance in military-relevant primers and its thermal stability, as provided in Table 2 [22,26]. Note the structural similarities that exist and its thermal stability, as provided in Table2[ 22,26]. Note the structural similarities that exist between KDNBF and KDNP; though KDNP possess a single hydrogen atom less than KDNBF, the between KDNBF and KDNP; though KDNP possess a single hydrogen atom less than KDNBF, the absence of this hydrogen allows for the restoration of aromaticity in the benzofuroxan ring system. absence of this hydrogen allows for the restoration of aromaticity in the benzofuroxan ring system. This phenomenon likely serves to explain the increased thermal stability of KDNP compared to This phenomenon likely serves to explain the increased thermal stability of KDNP compared to KDNBF. KDNBF.

7 Crystals 2016, 6, 5 Crystals 2016, 6, 5 8 of 22 Table 2. Sensitivities and performance of KDNP (25).

TableData 2. Sensitivities Category and performance KDNP of KDNP (25) (25). IS [J] a 0.047 DataFS Category [N] b KDNP9.81 (25) c c ESDIS [J] a[µJ] >6750.047 FSTdec [N] [°C]b d 2859.81 ESD c [µJ] c e >675 Ω [%] −34.3 ˝ d Tdecρ [[g/cc]C] f 1.982285 e ΩCO [%] ´34.3 ΔfH0 2[kJ/mol] g −197.07 ρ [g/cc] f 1.982 Δex0 U0 [kJ/kg]g h 3280 ∆fH [kJ/mol] ´197.07 a b 0 h c d IS = impact sensitivity; FS ∆=ex frictionU [kJ/kg] sensitivity; ESD = electrostatic3280 discharge; Tdec = Temperature e f g 0 ofa decomposition; Ω b = CO2 oxygen balance;c ρ = crystalline density; ΔfdH = molar enthalpy of IS = impact sensitivity; FS = friction sensitivity; ESD = electrostatic discharge; Tdec = Temperature of h ex e 0 f ρ g 0 formation;decomposition; Δ UΩ =CO total2 = CO energy2 oxygen of detonation. balance; = crystalline density; ∆fH = molar enthalpy of formation; h 0 ∆exU = total energy of detonation. 2.3.2.3. Potassium 1,1'-dinitramino-5,5'-Bis(tetrazolate) (K2DNABT) 2.3.2.3. Potassium 1,1'-dinitramino-5,5'-Bis(tetrazolate) (K2DNABT) In 2014, Klapötke reported the synthesis and performance of K2DNABT (31) [27]. This material is preparedIn 2014, in Klapötke a seven step reported sequence, the synthesis starting fr andom performance dimethyl carbonate of K2DNABT (Scheme (31) 8). [27 Desymmetrization]. This material is preparedof dimethyl in acarbonate, seven step followed sequence, by starting condensation from dimethyl with glyoxal carbonate afforded (Scheme bis(hydrazone)8). Desymmetrization 27. Treatment of dimethylwith N-chlorosuccinimide carbonate, followed (NCS), by condensation followed by with displace glyoxalment afforded of the bis(hydrazone) chlorines with 27. azide, Treatment afforded with Ndiazido-chlorosuccinimide bis(hydrazone) (NCS), intermediate followed by(29). displacement Acid-induced of the cyclization chlorines withto give azide, bis(tetrazole) afforded diazido (30), bis(hydrazone)nitration of this intermediatespecies with dinitrogen (29). Acid-induced pentoxide, cyclization and formation to give of bis(tetrazole)the resultant (30),dipotassium nitration salt of thisafforded species K2 withDNABT. dinitrogen pentoxide, and formation of the resultant dipotassium salt afforded K2DNABT.

Scheme 8. Synthesis of K2DNABT (31). Scheme 8. Synthesis of K2DNABT (31).

K2DNABT was prepared based on the premise that it could serve as a potential replacement for K DNABT was prepared based on the premise that it could serve as a potential replacement for lead lead azide2 in detonator applications; it is particularly intriguing in that it incorporates two 1- azide in detonator applications; it is particularly intriguing in that it incorporates two 1-nitraminotetrazole nitraminotetrazole moieties, each consisting of six consecutive nitrogen atoms which contribute moieties, each consisting of six consecutive nitrogen atoms which contribute greatly to the high greatly to the high explosive output of the molecule. Compared to lead azide, K2DNABT was found explosive output of the molecule. Compared to lead azide, K DNABT was found to have comparable to have comparable sensitivities to impact, friction and ESD, a2 significantly larger detonation velocity, sensitivities to impact, friction and ESD, a signiﬁcantly larger detonation velocity, and a similar and a similar detonation pressure (Table 3). K2DNABT was subjected to 100 °C for 48 h and was found detonation pressure (Table3). K DNABT was subjected to 100 ˝C for 48 h and was found to undergo to undergo no mass loss or decomposition2 at this temperature. Undoubtedly, this material holds no mass loss or decomposition at this temperature. Undoubtedly, this material holds promise promise and should be investigated further as a potential alternative to lead azide. However, its and should be investigated further as a potential alternative to lead azide. However, its current current synthesis does suffer from a lengthy synthesis route, and low yields were encountered in synthesis does suffer from a lengthy synthesis route, and low yields were encountered in some of the some of the synthetic transformations; most notably, the conversion of dichloro bis(hydrazone) (28) synthetic transformations; most notably, the conversion of dichloro bis(hydrazone) (28) to diazido to diazido bis(hydrazone) (29), which proceeded in only 38% yield. bis(hydrazone) (29), which proceeded in only 38% yield.

8 Crystals 2016, 6, 5 Crystals 2016, 6, 5 9 of 22 Table 3. Sensitivities and performance of K2DNABT as compared to Pb(N3)2.

Data Category K2DNABT (31) Pb(N3)2 Table 3. Sensitivities and performance of K2DNABT as compared to Pb(N3)2. IS [J] a 1 2.5–4 b DataFS Category [N] K2DNABT≤ (31)1 Pb(N30.1–1)2 c c ESDIS [J] [J]a 10.003 2.5–4<0.005 TFSdec [N][°C]b d ď1200 0.1–1 315 c ce ΩESD [%][J] 0.003−4.8 <0.005−11 ˝ d Tρdec [g/cc][ C] f 2002.11 315 4.8 e ΩCO2 [%] ´4.8 ´11 Pcj [kbar] g 5920 8330 ρ [g/cc] f 2.11 4.8 gh VPdetcj [kbar] [m/s] 592033.8 8330 31.7 0 h i ΔVfHdet [kJ/mol][m/s] 33.8326.4 31.7450.1 0 i ∆ΔfexHU0[kJ/mol] [kJ/kg] j 326.41036.1 450.11569 0 j ∆exU [kJ/kg] 1036.1 1569 a IS = impact sensitivity; b FS = friction sensitivity; c ESD = electrostatic discharge; d Tdec = Temperature a b c d IS = impact sensitivity;e FS = friction sensitivity; f ESD = electrostatic discharge;g T = Temperature of decomposition; Ω = CO2 oxygen balance; ρ = crystalline density; Pcj = detonationdec pressure; of decomposition; e Ω = CO oxygen balance; f ρ = crystalline density; g P = detonation pressure; h CO2 i 20 j 0 cj Vh det = detonation velocity; i ΔfH0 = molar enthalpy of formation;j Δ0exU = total energy of detonation. Vdet = detonation velocity; ∆fH = molar enthalpy of formation; ∆exU = total energy of detonation. 2.3.2.4. 1,5-(dinitramino) Tetrazole Dipotassium Salt 2.3.2.4. 1,5-(dinitramino) Tetrazole Dipotassium Salt In 2015, Klapötke went on to synthesize the similarly structured dipotassium salt of 1,5- In 2015, Klapötke went on to synthesize the similarly structured dipotassium salt of dinitramino tetrazole (33), as described in Scheme 9 [24,28]. Treatment of methyl carbazate 26 with 1,5-dinitramino tetrazole (33), as described in Scheme9[ 24,28]. Treatment of methyl carbazate 26 with cyanogen azide produced tetrazole (32), which was subjected to nitration, followed by basic cyanogen azide produced tetrazole (32), which was subjected to nitration, followed by basic conditions conditions to give the dipotassium salt of 1,5-Dinitramino tetrazole. to give the dipotassium salt of 1,5-Dinitramino tetrazole.

Scheme 9. Synthesis of the dipotassium salt of 1,5-Dinitramino tetrazole (33).

Tetrazene andand itsits organic-basedorganic-based replacement, replacement, MTX-1, MTX-1, was was discussed discussed in in Section Section 2.3.1 2.3.1.2.. Klapötke Klapötke has reasonedhas reasoned that giventhat given the high the high density, density, detonation detonation velocity, velocity, detonation detonation pressure, pressure, and impact and andimpact friction and sensitivityfriction sensitivity of (33) (Table of (33)4), (Table this compound 4), this compou could alsond servecould asalso a potential serve as replacementa potential replacement for tetrazene for as atetrazene sensitizing as a agent sensitizing in primary agent explosive in primary formulations. explosive formulations. The shockwave The produced shockwave by 50 produced mg of (33) by was 50 foundmg of (33) to easily was found detonate to easily the secondary detonate explosive the secondary RDX. explosive RDX.

Table 4.4.Sensitivities Sensitivitiesand and performance performance of of the the dipotassium dipotassium salt salt of of 1,5-Dinitramino 1,5-Dinitramino tetrazole tetrazole (33) (33) and and CL-20. CL-20. Data Category 33 CL-20 Data Category 33 CL-20 IS [J] a 1 4 IS [J] a 1 4 FS [N] b ď5 48 ˝FS [N]c b ≤5 48 Tdec [ C] 240 315 Tdec d[°C] c 240 315 ΩCO2 [%] ´12.03 ´10.95 ρ [g/cc] e d 2.177 2.04 Ω [%] −12.03 −10.95 f Pcj [kbar]ρ [g/cc] e 5222.177 2.04 444 g Vdet [m/s] f 10011 9730 0 Pcj [kbar]h 522 444 ∆fH [kJ/mol] ´112.4 365.4 0 Vdet [m/s]i g 10011 9730 ∆exU [kJ/kg] 3938 6168 0 h a ΔbfH [kJ/mol] −112.4c 365.4 IS = impact sensitivity; FS = friction sensitivity; Tdec = Temperature of decomposition; d Δe exρ U0 [kJ/kg] i f 3938 6168 g ΩCO2 = CO2 oxygen balance; = crystalline density; Pcj = detonation pressure; Vdet = detonation velocity; h 0 i 0 a ∆ H = molar enthalpyb of formation; ∆ U = totalc energydec of detonation. d 2 IS =f impact sensitivity; FS = friction sensitivity;ex T = Temperature of decomposition; Ω = CO oxygen balance; e ρ = crystalline density; f Pcj = detonation pressure; g Vdet = detonation velocity; h ΔfH0 = molar enthalpy of formation; i ΔexU0 = total energy of detonation. 9 Crystals 2016, 6, 5 Crystals 2016, 6, 5 10 of 22 2.3.2.5. Copper(I) 5-Nitrotetrazolate (DBX-1)

2.3.2.5.DBX-1 Copper(I) has been 5-Nitrotetrazolate a compound of (DBX-1)intense investigation recently, due to its similarities to lead azide with respect to its initiating ability and impact, friction and ESD sensitivity, as well as high thermal stabilityDBX-1 [29]. has Unlike been athe compound blue copper(II) of intense nitrotetrazole investigation octahedral recently, duecomplex to its reported similarities by toHuynh lead azide [30], withDBX-1 respect is the tocopper(I) its initiating salt that ability manifests and impact, as orange-red friction crystals and ESD and sensitivity, adopts the as welldimeric as high structure thermal (as stabilitydetermined [29 ].by Unlike single-crystal the blue copper(II)X-ray) shown nitrotetrazole in Scheme octahedral 10. More complex importantly, reported it bycan Huynh easily [ 30be], DBX-1synthesized is the as copper(I) a free-flowing salt that powder manifests (essential as orange-red for practical crystals applications) and adopts which the dimeric was never structure observed (as determinedfor the Huynh by single-crystalcompounds. X-ray)Further, shown DBX-1 in Schemehas been 10 .successfully More importantly, demonstrated it can easily to be be a synthesized “drop-in” asreplacement a free-flowing for lead powder azide (essential (meaning for it practical can be substituted applications) as a which direct, was volume-for-volume never observed for replacement the Huynh compounds.with no hardware Further, changes) DBX-1 has in beensome successfully existing detonator demonstrated designs. to be It ais “drop-in” also being replacement investigated for leadas a azidereplacement (meaning for itlead can styphnate be substituted in certain as a direct, applications volume-for-volume as well. DBX-1 replacement has been shown with no to hardware be more changes)resistant into someoxidation existing than detonator lead azide designs. when It subjected is also being to investigatedthermal cycling as a conditions replacement (i.e. for, high lead styphnatetemperature in certainand high applications humidity). as DBX-1 well. DBX-1 is known has beento slowly shown decompose to be more when resistant stored to oxidation in water, thanand leadshows azide incompatibility when subjected when to in thermal the presence cycling of conditions periodate-based (i.e., high oxidizers temperature [31]. andNonetheless, high humidity). due to DBX-1the aforementioned is known toslowly positive decompose qualities, DBX-1 when storedis being in sought water, as and a replacement shows incompatibility for lead-based when primary in the presenceexplosives. of periodate-based oxidizers [31]. Nonetheless, due to the aforementioned positive qualities, DBX-1The is beingtraditional sought synthetic as a replacement procedure for for lead-based making DBX-1 primary is described explosives. in Scheme 10. Exposure of 5- aminotetrazoleThe traditional to a modified synthetic Sandmeyer procedure forreaction making affords DBX-1 copper is described acid salt in 34, Scheme which 10 is. converted Exposure ofto 5-aminotetrazolethe sodium salt of to 5-nitrotetrazole a modified Sandmeyer (NaNT, reaction35) upon affords exposure copper to NaOH acid salt as 34, originally which is described converted by to the sodium salt of 5-nitrotetrazole (NaNT, 35) upon exposure to NaOH as originally described by von von Herz [32] and later modified by Gilligan [33]. Subsequently, CuCl2 is treated with sodium Herzascorbate [32] andto generate later modified the copper(I) by Gilligan species [33 ].in Subsequently, situ, which reacts CuCl with2 is treated NaNT withto yield sodium DBX-1 ascorbate (36). The to generateperformance the copper(I)and sensitivities species forin situthis, whichcompound reacts are with detailed NaNT in to Table yield 5. DBX-1 (36). The performance and sensitivities for this compound are detailed in Table5. Table 5. Performance and sensitivities of DBX-1 (36). Table 5. Performance and sensitivities of DBX-1 (36). Data Category DBX-1 (36) Data CategoryIS [J] a DBX-10.036 (36) ISFS [J] [N]a b >0.0980.036 FSESD [N] cb [µJ] c >0.09812 c c ESDTdec[µ [°C]J] d 33712 ˝ d Tdec [ C] e 337 Ω [%] −9.0 Ω [%] e ´9.0 CO2 f ρ [g/cc]ρ [g/cc]f 2.5842.584 0f 0 g g ∆fHΔ H[kJ/mol] [kJ/mol] 280.9280.9 0 0 h h ∆exΔUexU[kJ/kg] [kJ/kg] 3816.63816.6 a a bb c c d d ISIS = impact = impact sensitivity; sensitivity; FSFS = = friction friction sensitivity;sensitivity; ESD ESD = = electrostatic electrostatic discharge; discharge;T dec T=dec Temperature = Temperature of e f g 0 decomposition; ΩeCO = CO2 oxygen2 balance; ρ = crystallinef density; ∆ H = molarg f enthalpy0 of formation; of decomposition; Ω2 = CO oxygen balance; ρ = crystalline density;f Δ H = molar enthalpy of h 0 ∆exU = total energy of detonation. formation; h ΔexU0 = total energy of detonation.

Scheme 10. Traditional synthetic pathway of DBX-1.

10 Crystals 2016, 6, 5 11 of 22 Crystals 2016, 6, 5

There are several concerns with this traditional synthesis as described. Since this route has been There are several concerns with this traditional synthesis as described. Since this route has been discovered, microdetonations have been reported during the reaction; a presumed result of discovered, microdetonations have been reported during the reaction; a presumed result of generating generating the highly dangerous 5-diazotetrazole in the modified Sandmeyer reaction. On a the highly dangerous 5-diazotetrazole in the modified Sandmeyer reaction. On a laboratory-scale, laboratory-scale, some improvements have been made over time including the addition of a some improvements have been made over time including the addition of a copper(II) species copper(II) species such as CuSO4·5H2O to suppress the microdetonation events by converting 5- such as CuSO4¨ 5H2O to suppress the microdetonation events by converting 5-diazotetrazole to the diazotetrazole to the innocuous 5-hydroxytetrazole. Furthermore, employing an excess of sodium innocuous 5-hydroxytetrazole. Furthermore, employing an excess of sodium nitrite in the reaction nitrite in the reaction mixture enhances the safety of the filtration step to isolate copper acid salt (34). mixture enhances the safety of the filtration step to isolate copper acid salt (34). Unfortunately, the Unfortunately, the microdetonation and dangerous filtration of copper acid salt (34) commonly microdetonation and dangerous filtration of copper acid salt (34) commonly resurface when NaNT resurface when NaNT is synthesized on a production-sized scale. Therefore, the inability to is synthesized on a production-sized scale. Therefore, the inability to manufacture NaNT, and thus, manufacture NaNT, and thus, DBX-1 on a production-sized scale has been a major drawback that has DBX-1 on a production-sized scale has been a major drawback that has limited practical application limited practical application for the material. for the material. Fortunately, pioneering work at Nalas Engineering by Salan and co-workers have solved this Fortunately, pioneering work at Nalas Engineering by Salan and co-workers have solved this looming issue [34]. Throughout the course of their investigation, it was discovered that synthesizing looming issue [34]. Throughout the course of their investigation, it was discovered that synthesizing NaNT by the traditional method shown in Scheme 10 does not result in a pure product. NaNT is NaNT by the traditional method shown in Scheme 10 does not result in a pure product. NaNT is obtained through this method with ca. 5% 5-aminotetrazole as an impurity. Furthermore, in obtained through this method with ca. 5% 5-aminotetrazole as an impurity. Furthermore, in synthesizing copper acid salt (34), it was discovered that use of HNO3 in the modified Sandmeyer synthesizing copper acid salt (34), it was discovered that use of HNO3 in the modified Sandmeyer protocol led to the depositing of the explosive 5-aminotetrazole nitrate on the feeding lines. protocol led to the depositing of the explosive 5-aminotetrazole nitrate on the feeding lines. To mitigate these issues, HNO3 was replaced with H2SO4 in the modified Sandmeyer reaction To mitigate these issues, HNO3 was replaced with H2SO4 in the modified Sandmeyer reaction (Scheme 11). (Scheme 11).

Scheme 11. ImprovedImproved synthesis synthesis of DBX-1 as performed by Nalas Engineering.

In addition to eliminating the possibilitypossibility of formingforming 5-aminotetrazole5-aminotetrazole nitrate, 5-aminotetrazole was found to be at least twice as soluble in sulfuric acid as in nitric nitric acid. acid. The The addition addition of of diatomaceous diatomaceous earth to the reaction mixture during the modifiedmodified Sandmeyer protocol resulted in copper acid salt (34) beingbeing retainedretained inin thethe solidsolid phasephase duringduring washing/purification.washing/purification. Treatment Treatment of diatomaceousdiatomaceous earth/copper acid acid salt salt 34 34 with with NaOH NaOH resulted resulted in in the the formation formation of ofNaNT, NaNT, which which could could be beisolated isolated in relativelyin relatively pure pure solution solution by by keeping keeping the the mother mother liquor liquor following following filtration. filtration. In In converting NaNT solution to DBX-1, it was found that doping the reaction mixture with 1–25 wt % of authentic DBX-1 was extremelyextremely beneficial beneficial in in minimizing minimizing the inductionthe induction period period of forming of forming DBX-1, DBX-1, and led toand an led increased to an increasedlikelihood likelihood that a batch that of NaNTa batch would of NaNT be converted would be to converted DBX-1. Throughout to DBX-1. Throughout the process, the handling process, of handlingany sensitive of any intermediates sensitive intermedia in makingtes DBX-1in making is avoided, DBX-1 is thus avoided, increasing thus increasing the safety ofthe the safety operators of the operatorsinvolved. Ininvolved. light of In these light significant of these improvements,significant improvements, it appears that it appears DBX-1 willthat garnerDBX-1 enthusiasticwill garner enthusiasticinterest as a lead-freeinterest as primary a lead-free explosive primary in detonatorexplosive in and detonator primer applications. and primer applications.

11 Crystals 2016, 6, 5 Crystals 2016, 6, 5 12 of 22 3. Secondary Explosives 3. Secondary Explosives 3.1. Legacy Secondary Explosives and Why Interest Exists in Developing Alternatives 3.1. Legacy Secondary Explosives and Why Interest Exists in Developing Alternatives Unlike primary explosives, secondary explosives are usually harder to initiate, but typically haveUnlike higher primary performance, explosives, such secondary as higher explosives detonati areon usuallyvelocities harder and to detonation initiate, but typicallypressures. have Figure higher 3 performance,depicts some suchexamples as higher of legacy detonation secondary velocities explosives and detonation that have pressures. been adopted Figure3 fordepicts use (in some some examples cases, ofwidespread legacy secondary use) for military explosives and/or that civilian have been applications. adopted for CL-20 use (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12- (in some cases, widespread use) for militaryhexaazaisowurtzitane, and/or civilian applications.40) has a particularly CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane, high performance, but it has failed to find many 40) hasapplications a particularly in explosive high performance,formulations due but itto hasthe costs failed associated to find many in making applications this material, in explosive which formulationsinvolves palladium due to thecatalysis. costs associated There has in makingbeen interest this material, from whichthe environmental involves palladium community catalysis. to Therediscovery has metal-free been interest syntheses from the of CL-20 environmental [35], but no community successful toalternative discovery has metal-free been demonstrated syntheses ofto CL-20date. [35], but no successful alternative has been demonstrated to date. 2,4,6-Trinitrotoluene (TNT, 37)37) isis thethe mostmost commonlycommonly knownknown secondarysecondary explosive in existenceexistence today amongst the general public, and its use has historically been prevalent in military and civilian sectors. It It has has found found widespread widespread use use in in the the USA USA since since World World War War I, I,and and is isa common a common ingredient ingredient in inexplosive explosive mortar mortar and and artillery artillery shells, shells, and and bombs, bombs, for for example. example. The The melting melting point point of ofTNT TNT (80 (80 °C)˝ C)is issignificantly significantly below below its its explosive explosive decomposition decomposition temperature, temperature, which which makes makes the the material material useful useful as as a amelt-castable melt-castable explosive. explosive. The The insensitive insensitive nature nature of ofTNT TNT is beneficial is beneficial because because it minimizes it minimizes the the risk risk of ofaccidental accidental detonation. detonation. Unfortunatel Unfortunately,y, TNT TNT has has a anegative negative toxicity toxicity profile profile by by today’s increasingly stringent environmental and human health safety standards. Pink and red wastewater—the former produced duringduring the the washing washing of equipmentof equipment following following munitions munitions filling, filling, and the and latter the producedlatter produced during theduring purification the purification process process of the crude of the material—are crude material—are associated associated with TNT. with Long-term TNT. Long-term exposure exposure to TNT hasto TNT been has linked been to linked adverse tohealth adverse effects, health and effects, so a replacement and so a replacement for TNT has for been TNT sought, has been particularly sought, amongstparticularly the amongst insensitive the munitions insensitive community. munitions community.

Figure 3. Examples of legacy secondary explosives TNT (37), RDX (38), HMX (39), and CL-20 (40).

Royal DemolitionDemolition Explosive Explosive (RDX, (RDX, 1,3,5-trinitro-1,3,5-triazacyclohexane, 1,3,5-trinitro-1,3,5-triazacyclohexane, 38), which 38), is inexpensively which is preparedinexpensively from prepared hexamine, from is hexamine, produced is efficiently produced by efficiently a method by knowna method as known the Bachmann as the Bachmann process. Thisprocess. process This also process produces also HMXproduces (1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane, HMX (1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane, 39) as a by-product 39) as a [by-36]. RDXproduct is manufactured[36]. RDX is manufactured on massive scale,on massive and has scal beene, and used has commonlybeen used commonly since World since War World II. RDX War is significantlyII. RDX is significantly higher in explosivehigher in explosive performance performance than TNT. than The TNT. material The ismaterial rather is insensitive rather insensitive at room temperature,at room temperature, and is often and the “goldis often standard” the “gold used standard” when determining used when acceptable determining sensitivities acceptable when newsensitivities secondary when explosives new secondary are being explosives designed. are Like being TNT, designed. RDX has Like come TNT, under RDX increasing has come pressure under fromincreasing the EPA pressure for regulation from the dueEPA to for concerns regulation that due the to molecule concerns does that not the bind molecule to soil does effectively, not bind and to thussoil effectively, may contaminate and thus groundwater may contaminate supplies. groundwater The EPA supplies. has classified The EPA RDX has as classified a possible RDX human as a carcinogenpossible human based carcinogen on the presence based of on liver the tumorspresence found of liver in mice tumors that found ingested in foodmice contaminatedthat ingested food with RDXcontaminated over a one with to twoRDX year over period a one [to37 two]. On year environmental period [37]. On grounds, environmenta replacementsl grounds, for RDX replacements are being sought.for RDX While are being satisfying sought. the While stringent satisfying environmental the stringent regulations, environmental however, regulations, it is also a mainhowever, objective it is inalso energetics a main objective synthesis in labs energetics to develop synthesis new secondary labs to develop explosives new that secondary have as muchexplosives power that as possible,have as whilemuch havingpower as possible, little sensitivity while having as possible. as little sensitivity as possible.

3.2. TNT-Free Formulations TNT-based explosive ﬁllsfills have historically been ubiquitous in large caliber mortar and artillery munitions. TNT, as discussed previously, is a melt-castable material with a low sensitivity to impact,

12 Crystals 2016,, 6,6, 55 Crystals 2016, 6, 5 13 of 22 friction and ESD. However, it is not the ideal explosive for insensitive munition applications. TNT is frictionknown to and exhibit ESD. violent However, behavior it is not when the ideal subjected explosive to bullet for insensitiveimpact, fire, munition or whenapplications. TNT-based artillery TNT is knownand mortar to exhibit munitions violent are behavior hit with an when improvised subjected explosive to bullet device impact, (IED) fire, oror whenrocket-propelled TNT-based grenade artillery and(RPG) mortar [38], munitionsputting those arehit in withclose an proximity improvised of explosivean exploding device TNT-based (IED) or rocket-propelled round at risk of grenade being (RPG)seriously [38 ],injured putting or those killed. in This, close coupled proximity with of an the exploding environmental TNT-based drivers round to replace at risk ofTNT, being have seriously led to injuredthe development or killed. of This, safer coupledalternatives. with the environmental drivers to replace TNT, have led to the developmentThe US Army of safer successfully alternatives. developed insensitive munitions explosive 101 (IMX-101), which has been Thefielded US Armyto replace successfully the 155 developedmm M795 artillery insensitive round, munitions with further explosive applications 101 (IMX-101), in 105 which mm and has been120 mm fielded munitions. to replace IMX-101 the 155 consists mm M795 of a melt-castable artillery round, mixture with of further 2,4-dinitroanisole applications (DNAN, in 105 mm 41) and 120nitrotriazolone mm munitions. (NTO, IMX-101 42) (Figure consists 4). of Unlike a melt-castable TNT-based mixture explosive of 2,4-dinitroanisole fills, IMX-101 undergoes (DNAN, 41) only and a rapid deflagration rather than a detonation when subjected to fast cook-off, slow cook-off, fragment nitrotriazolonerapid deflagration (NTO, rather 42) (Figurethan a 4detonation). Unlike TNT-based when subjected explosive to fast fills, cook-off, IMX-101 slow undergoes cook-off, only fragment a rapid impact, bullet impact, and sympathetic detonation. TNT failed to pass in all of these categories. deflagrationimpact, bullet rather impact, than and a detonation sympathetic when detonation. subjected toTNT fast failed cook-off, to slowpass cook-off,in all of fragmentthese categories. impact, Although TNT costs $6 per pound, and the DNAN/NTO formulation costs $8 per pound, the bulletAlthough impact, TNT and costs sympathetic $6 per pound, detonation. and TNTthe failedDNAN/NTO to pass informulation all of these categories.costs $8 per Although pound, TNT the marginal increase in cost is trivial considering how many lives are likely to be saved in future combat costsmarginal $6 per increase pound, in andcost theis trivial DNAN/NTO considering formulation how many costs lives $8 are per likely pound, to be the saved marginal in future increase combat in situations [35]. costsituations is trivial [35]. considering how many lives are likely to be saved in future combat situations [35].

Figure 4. Chemical structures of DNAN (41) and NTO (42).

A well-known TNT-based formulation, which commonly finds ﬁnds use in 60 mm, 81 mm, 120 mm and numerous submunitions submunitions is is the the composition composition B B(aka (aka Comp Comp B) B)formulation. formulation. Comp Comp B consists B consists of a ofmelt-castable a melt-castable mixture mixture of TNT of TNTand andRDX. RDX. Comp Comp B is Balso is alsoin the in process the process of being of being replaced replaced in the in theaforementioned aforementioned munitions munitions by insens by insensitiveitive munition munition explosive explosive 104 (IMX-104), 104 (IMX-104), which whichconsists consists of a melt- of acastable melt-castable mixture mixture of DNAN, of DNAN, NTO and NTO RDX. and At RDX. the time, At the the time, IMX-104 the IMX-104formulation formulation has been hasqualified been qualiﬁedfor use, and for it use, is being and it evaluated is being evaluated in the aforementioned in the aforementioned munitions munitions [39]. [39].

3.3. Potential RDX Replacements

3.3.1. TKX-50 (Dihydroxylammonium 5,5'-Bis(tetrazolate) diolate)diolate) In 2012, Klapötke reported the synthesis of of TKX- TKX-5050 (47), as as summarized in in Scheme Scheme 1212 [[36,40].36,40]. Condensation ofof glyoxal glyoxal with with hydroxylamine hydroxylamine hydrochloride hydrochloride affords affords glyoxime glyo (43),xime which (43), is convertedwhich is toconverted dichlorglyoxime to dichlorglyoxime (44) in the presence(44) in the of presence chlorine of gas chlorine in ethanol gas atin coldethano temperatures.l at cold temperatures.

Scheme 12. Synthesis of TKX-50 (47).

13 Crystals 2016, 6, 5 Crystals 2016, 6, 5 14 of 22 Treatment of (44) with sodium azide produces the highly sensitive diazidoglyoxime intermediate (45), which is treated with HCl under anhydrous conditions to effect cyclization, thus Treatment of (44) with sodium azide produces the highly sensitive diazidoglyoxime intermediate yielding bis-adjacent tetrazole (46). Exposure of this latter intermediate to dihydroxylammonium (45), which is treated with HCl under anhydrous conditions to effect cyclization, thus yielding chloride results in the formation of TKX-50. bis-adjacent tetrazole (46). Exposure of this latter intermediate to dihydroxylammonium chloride Compared to RDX, TKX-50 was found to have a lower impact sensitivity, with comparable results in the formation of TKX-50. friction and ESD sensitivities. TKX-50 was found to have a higher temperature of decomposition, Compared to RDX, TKX-50 was found to have a lower impact sensitivity, with comparable density, detonation pressure and detonation velocity compared to RDX (Table 6). Due to its reported friction and ESD sensitivities. TKX-50 was found to have a higher temperature of decomposition, performance, TKX-50 has been the subject of scale-up and compatibility studies [41]. During that density, detonation pressure and detonation velocity compared to RDX (Table6). Due to its reported time, efforts by Damavarapu, Paraskos, and Cooke resulted in producing TKX-50 on a 75 g scale per performance, TKX-50 has been the subject of scale-up and compatibility studies [41]. During that time, batch. Over the course of a year, over three pounds of the material was synthesized for testing efforts by Damavarapu, Paraskos, and Cooke resulted in producing TKX-50 on a 75 g scale per batch. purposes. Over the course of a year, over three pounds of the material was synthesized for testing purposes. Table 6. Sensitivities and performance of TKX-50 and RDX. Table 6. Sensitivities and performance of TKX-50 and RDX. Data Category TKX-50 (47) RDX DataIS [J] Category a TKX-5020 (47) RDX7.5 FS [N]IS [J] b a 12020 7.5120 ESDFS c [N][J] cb 0.1120 120 0.2 ESD c [J] c 0.1 0.2 Tdec [°C] d 221 210 ˝ d Tdec [ C]e 221 210 Ω [%] e −27.1 −21.61 ΩCO2 [%] ´27.1 ´21.61 f ρ [g/cc]ρ [g/cc] f 1.9181.918 1.8581.858 g g PcjP [kbar]cj [kbar] 424424 380 380 h VdetVdet [m/s][m/s] h 96989698 8983 8983 0 i ∆ 0H [kJ/mol]i 446.6 86.3 ΔfHf [kJ/mol] 446.6 86.3 0 j ∆exU [kJ/kg] 6025 6190 ΔexU0 [kJ/kg] j 6025 6190 a b c d a IS = impact sensitivity;b FS = friction sensitivity; c ESD = electrostatic discharge; dTdec = Temperature IS = impact sensitivity;e FS = friction sensitivity; f ESD = electrostatic discharge;g Tdec = Temperature of decomposition; ΩCO = CO2 oxygen balance; ρ = crystalline density; Pcj = detonation pressure; e 2 f g ofh decomposition; Ω = iCO2 0oxygen balance; ρ = crystallinej density;0 Pcj = detonation pressure; Vdet = detonation velocity; ∆fH = molar enthalpy of formation; ∆exU = total energy of detonation. h Vdet = detonation velocity; i ΔfH0 = molar enthalpy of formation; j ΔexU0 = total energy of detonation. TKX-50 waswas found found to passto pass the vacuumthe vacuum thermal ther stabilitymal stability (VTS) test, (VTS) and thustest, showed and thus compatibility showed whencompatibility combined when with combined a wide range with ofa metals,wide range metal of oxides metals, explosive metal oxides materials, explosive plasticizers, materials, and polymers.plasticizers, One and notable polymers. exception One notable where exceptio VTS failedn where was when VTS failed TKX-50 was was when mixed TKX-50 with thewas DEMN mixed compositionwith the DEMN (DEMN composition is composed (DEMN of 34.9% is composed diethylenetriamine of 34.9% diethylenetriamine trinitrate, 33.4% ethylenediaminetrinitrate, 33.4% dinitrate,ethylenediamine 25.5% methyl dinitrate, nitroguanidine, 25.5% methyl and nitroguanidine, 6.3% nitroguanidine and 6.3% [42 nitr]). Veryoguanidine recent work [42]). by Very Sinditskii recent haswork demonstrated by Sinditskii has that demonstrated TKX-50 has a that burn TKX-50 rate slightly has a burn higher rate than slightly HMX, higher and than that theHMX, compound and that decomposesthe compound via decomposes dissociation ofvia the dissociation salt moiety, producingof the salt diammonium moiety, producing 5,5'-bis(tetrazolate) diammonium diolate 5,5'- 0 (ABTOX,bis(tetrazolate) Figure diolate5). Sinditskii (ABTOX, has Figure also shown 5). Sinditskii that the enthalpyhas also shown of formation that the ( ∆enthalpyfH ) of TKX-50 of formation is just 0 0 0 111(ΔfH˘) 16of kJ/mol,TKX-50 whichis just 111 is signiﬁcantly ± 16 kJ/mol, lower which than is significantly the ∆fH = 446.6lower kJ/mol than the that ΔfH was = initially446.6 kJ/mol reported, that aswas shown initially in Tablereported,6[43]. as shown in Table 6 [43].

Figure 5. ABTOX (48), a decomposition product of TKX-50.

3.3.2. Dihydroxylammonium Dinitro-Bis(1,2,4-triazole-1,1'-diolate) Shortly after the initial synthesis appeared forfor TKX-50, Klapötke reported the synthesis of Dihydroxylammonium dinitro- bis-1,2,4-triazole-1,1'-diolate-1,2,4-triazole-1,1'-diolate (52) (52) [44]. [44]. As As outlined in Scheme 13,13, this compound is produced in a four-step process, first by reacting oxalic acid with aminoguanidinium 14 Crystals 2016, 6, 5 15 of 22

Crystals 2016, 6, 5 compound is produced in a four-step process, ﬁrst by reacting oxalic acid with aminoguanidinium bicarbonate inin acidicacidic solution, solution, followed followed by by cyclization cyclization upon upon addition addition of baseof base to furnish to furnish bis(triazole) bis(triazole) (49). Introduction(49). Introduction of the of nitro the groupsnitro groups via the via Sandmeyer the Sandmeyer reaction, reaction, followed followed by treatment by treatment of the resulting of the dinitroresulting bis(triazole) dinitro bis(triazole) (50) oxone, (50) afforded oxone, diolafforded (51). diol Exposure (51). Exposure of the diol of to the hydroxylamine diol to hydroxylamine afforded dihydroxylammoniumafforded dihydroxylammonium salt (52). salt (52).

Scheme 13. Synthesis of Dihydroxylammonium dinitro-bis(1,2,4-triazole-1,1'-diolate)dinitro-bis(1,2,4-triazole-1,1'-diolate) (52).(52).

Dihydroxylammonium salt (52) shows a remarkably low sensitivity toward impact, friction, and Dihydroxylammonium salt (52) shows a remarkably low sensitivity toward impact, friction, ESD (Table 7). The explosive performance of this compound appears to be very similar to RDX. and ESD (Table7). The explosive performance of this compound appears to be very similar to RDX. Curiously, the enthalpy of formation of dihydroxylammonium salt (52) is significantly lower than Curiously, the enthalpy of formation of dihydroxylammonium salt (52) is signiﬁcantly lower than the initial reported value for TKX-50, despite the presence of two nitro groups in the structure of the the initial reported value for TKX-50, despite the presence of two nitro groups in the structure of the former compound. With the exception of the nitro groups present on (52), these two compounds have former compound. With the exception of the nitro groups present on (52), these two compounds have many similarities in their structure. It would be expected that the presence of nitro groups should many similarities in their structure. It would be expected that the presence of nitro groups should lead to a more positive enthalpy of formation. Therefore, it is plausible that the work carried out by lead to a more positive enthalpy of formation. Therefore, it is plausible that the work carried out by Sinditskii may be valid, and that the original values for TKX-50 may need to be re-evaluated. As one Sinditskii may be valid, and that the original values for TKX-50 may need to be re-evaluated. As one would would expect, dihydroxylammonium salt (52) had a higher enthalpy of formation (ΔfH0 = 213 kJ/mol) expect, dihydroxylammonium salt (52) had a higher enthalpy of formation (∆ H0 = 213 kJ/mol) than 0 f than Sinditskii’s adjusted value for TKX-50 0(ΔfH = 111 ± 16 kJ/mol). Sinditskii’s adjusted value for TKX-50 (∆fH = 111 ˘ 16 kJ/mol). Table 7. Sensitivities and performance of Dihydroxylammonium salt 52 and RDX. Table 7. Sensitivities and performance of Dihydroxylammonium salt 52 and RDX. Data Category 52 RDX Data CategoryIS [J] a 40 52 7.5 RDX ISFS [J] [N]a b >36040 120 7.5 b FS [N]c c >360 120 ESDc [J]c 0.5 0.2 ESD [J] d 0.5 0.2 Tdec˝ [°C]d 217 210 Tdec [ C] 217 210 e Ω [%]e −19.7 −21.61 ΩCO2 [%] ´19.7 ´21.61 ρ ρ[g/cc] [g/cc]f f 1.91.9 1.858 1.858 g PcjPcj[kbar] [kbar] g 390390 380 380 h V [m/s] h 9087 8983 detVdet [m/s] 9087 8983 0 i ∆fH [kJ/mol]0 i 213 86.3 ΔfH0 [kJ/mol]j 213 86.3 ∆exU [kJ/kg] 5985 6190 ex 0 j a b Δ U [kJ/kg] c5985 6190 d IS = impact sensitivity; FS = friction sensitivity; ESD = electrostatic discharge; Tdec = Temperature a b c d IS = impact sensitivity;e FS = friction sensitivity; f ESDρ = electrostatic discharge;g Tdec = Temperature of decomposition; ΩCO2 = CO2 oxygen balance; = crystalline density; Pcj = detonation pressure; h e Ω i 20 f j 0 g cj of decomposition;Vdet = detonation velocity; = CO∆fH oxygen= molar balance; enthalpy of ρ formation; = crystalline∆ex density;U = total energyP = detonation of detonation. pressure; h Vdet = detonation velocity; i ΔfH0 = molar enthalpy of formation; j ΔexU0 = total energy of detonation.

3.3.3. Nitryl Cyanide (NCNO2) 3.3.3. Nitryl Cyanide (NCNO2) Though a simple molecule on paper, the synthesis of nitryl cyanide proved to be elusive for decades.Though However, a simple Christe molecule solved on paper, this long-standing the synthesis problem, of nitryl ﬁnallycyanide synthesizing proved to be this elusive material, for anddecades. providing However, full Christe characterization solved this upon long-standing isolation [problem,45]. However, finally the synthesizing synthesis this of nitryl material, cyanide and requiresproviding specialized full characterization equipment upon at this isolation time, which [45]. However, is a drawback. the synthesis Nitryl cyanideof nitryl wascyanide prepared requires by reactingspecializedtert equipment-butyldimethylsilyl at this time, cyanide which (TBSCN) is a drawback. with nitronium Nitryl cyanide tetraﬂuoroborate was prepared in nitromethane by reacting tert-butyldimethylsilyl cyanide (TBSCN) with nitronium tetrafluoroborate in nitromethane at −30 °C (Scheme 14). Employing TBSCN in excess resulted in dramatically reduced yields, as well as the

15 Crystals 2016, 6, 5 16 of 22 Crystals 2016, 6, 5 Crystals 2016, 6, 5 ˝ atformation´30 C (Schemeof cyanogen. 14). EmployingCyanogen is TBSCN believed in excess to form resulted from a in reaction dramatically involving reduced TBSCN yields, and as nitryl well ascyanide.formation the formation of cyanogen. of cyanogen. Cyanogen Cyanogen is believed is believed to form to from form a from reaction a reaction involving involving TBSCN TBSCN and nitryl and nitrylcyanide. cyanide.

Scheme 14. Synthesis of nitryl cyanide (53) and its proposed trimerization to trinitrotriazine (54). Scheme 14. Synthesis of nitryl cyanide (53) and its proposedproposed trimerization to trinitrotriazine (54).(54). To reduce the amount of cyanogen formed, TBSCN needs to be condensed at −196 °C above a To reduce the amount of cyanogen formed, TBSCN needs to be condensed at −196 °C above a frozenTo mixture reduce theof nitronium amount of tetrafluoroborate cyanogen formed, in TBSCNnitromethane. needs toThis be mixture condensed must at be´196 thawed,˝C above and frozen mixture of nitronium tetrafluoroborate in nitromethane. This mixture must be thawed, and aonly frozen small mixture amounts of nitronium of TBSCN tetrafluoroborate can be dissolved in in nitromethane. the mixture at This a time. mixture The mustreaction be thawed,by-products and only small amounts of TBSCN can be dissolved in the mixture at a time. The reaction by-products onlyfrom smallthe reaction amounts are ofTBSF TBSCN and boron can be trifluoride dissolved (BF in3 the), which mixture are atremoved a time. by The fractional reaction condensation by-products throughfrom the areaction series of are cold TBSF traps. and Removal boron trifluoride of BF3 is (BFcritical,3), which as it arewill removed readily react by fractional with nitryl condensation cyanide in from the reaction are TBSF and boron trifluoride (BF3), which are removed by fractional condensation through a series of cold traps. Removal of BF3 is critical, as it will readily react with nitryl cyanide in throughthe liquid a phase, series of and cold this traps. transformation Removal of dramatically BF is critical, reduces as it will yields. readily react with nitryl cyanide in the liquid phase, and this transformation dramatically3 reduces yields. the liquidPure nitryl phase, cyanide and this was transformation found to be a dramatically stable liquid reduces at room yields. temperature. It undergoes very little Pure nitryl cyanide was found to be a stable liquid at room temperature. It undergoes very little decompositionPure nitryl when cyanide heated was foundto 50 °C to or be 100 a stable °C fo liquidr an hour, at room but temperature.decomposes readily It undergoes over the very course little decomposition when heated to 50 °C or 100 °C for an hour, but decomposes readily over the course decompositionof several hours when at 140 heated °C. Nitryl to 50 cyanide˝C or 100 is not˝C forcompatible an hour, with but decomposes fluorides, Lewis readily acids, over bases, the course amines, of of several hours at 140 °C. Nitryl cyanide is not compatible with fluorides, Lewis acids, bases, amines, severalK3PO4, and hours elemental at 140 ˝ C.mercury. Nitryl cyanide is not compatible with fluorides, Lewis acids, bases, amines, K3PO4, and elemental mercury. K POAlthough, and elemental not useful mercury. on its own as an RDX replacement, trimerization of nitryl cyanide would 3 Although4 not useful on its own as an RDX replacement, trimerization of nitryl cyanide would yieldAlthough trinitrotriazine not useful (54); onan itselusive own astheoretical an RDX replacement,compound with trimerization a predicted of density nitryl cyanide of 1.99 wouldg/cc, a yield trinitrotriazine (54); an elusive theoretical compound with a predicted density of 1.99 g/cc, a yielddetonation trinitrotriazine pressure of (54); 421 an kbar, elusive and theoreticala detonation compound velocity of with 9770 a predictedm/s. Though density the sensitivity of 1.99 g/cc, of detonation pressure of 421 kbar, and a detonation velocity of 9770 m/s. Though the sensitivity of atrinitrotriazine detonation pressure is not ofknown, 421 kbar, the and high a detonationtheoretical velocityperformance of 9770 makes m/s. Thoughsynthesizing the sensitivity this target of trinitrotriazine is not known, the high theoretical performance makes synthesizing this target trinitrotriazineattractive. is not known, the high theoretical performance makes synthesizing this target attractive. attractive. 3.3.4. 2,6-Diamino-3,5-Dinitropyrazine- N-Oxide (LLM-105) 3.3.4. 2,6-Diamino-3,5-Dinitropyrazine-N-Oxide (LLM-105) LLM-105, initially initially synthesized synthesized at at Lawrence Lawrence Livermore Livermore National National Laboratories Laboratories by byPagoria Pagoria and and co- co-workersworkersLLM-105, in 1994, in 1994,initially is an is insensitive ansynthesized insensitive secondary at secondaryLawrence explosive Livermore explosive material materialNational with 80% with Laboratories of 80% the ofpower the by powerPagoria of HMX. of and HMX.LLM- co- LLM-105105workers possesses in possesses 1994, a density is an a density insensitive of 1.918 of 1.918g/cc, secondary g/cc,a detonation a explosive detonation velo materialcity velocity of 8730 with of 8730m/s, 80% m/s,a of detonation the a detonationpower pressure of HMX. pressure of LLM- 35.9 of 35.9GPa,105 possesses GPa,a high a high thermal a density thermal decomposition of decomposition 1.918 g/cc, temperaturea detonation temperature invelo excess incity excess of of 8730 350 of 350m/s,°C, ˝ andC,a detonation and very very low lowpressure sensitivities sensitivities of 35.9 to toimpact,GPa, impact, a highfriction, friction, thermal and and electrostatic decomposition electrostatic discharge. discharge.temperature The most The in excess mostefficient efficient of synthesis350 °C, synthesis and to-date very to-date developed low sensitivities developed for LLM- for to LLM-105105impact, is summarized friction, is summarized and in electrostatic Scheme in Scheme 15 discharge. [46]. 15[ Diaminopyrazine46]. The Diaminopyrazine most efficient N-oxide synthesisN-oxide (DAPO, (DAPO, to-date 57) candeveloped 57) canbe prepared be preparedfor LLM- by bytreating105 treatingis summarized N-chloroN-chloro bis(cyanomethy in bis(cyanomethyl) Scheme 15 [46].l) amine Diaminopyrazine amine (55) (55) or or NN-nitroso-nitroso N-oxide bis(cyanomethyl) bis(cyanomethyl) (DAPO, 57) can amine aminebe prepared (56) (56) with by triethylamine.treating N-chloro Reaction bis(cyanomethy of the former l) compound amine (55) pr proceeds oroceeds N-nitroso in 14% bis(cyanomethyl) yield, whereas reaction amine of (56) the latter with compoundtriethylamine. is is 66%. 66%.Reaction Mixed Mixed of acid the acid formernitration nitration compound of DAPO of DAPO pr affordsoceeds affords LLM-105 in LLM-10514% yield, in a inreasonable whereas a reasonable reaction yield yieldof of 64%. the of latterVery 64%. Veryrecentcompound recent efforts is efforts 66%.by Zuckerman byMixed Zuckerman acid nitrationhave have detailed detailedof DAPO some some affords success success LLM-105 at atsynthesizing synthesizing in a reasonable LLM-105 LLM-105 yield using of using 64%. a a Veryflow flow microreactorrecent efforts synthesisby Zuckerman [[47].47]. Although Although have detailed no no increases some successin in yield at were synthesizing realized comparedLLM-105 tousing traditionaltraditional a flow syntheticmicroreactor processes, synthesis reduced [47]. Although reaction times, no increases higher purities,pu inrities, yield and were an realizedincrease compared in safety to to the traditional operator havesynthetic been processes, demonstrated. reduced reaction times, higher purities, and an increase in safety to the operator have been demonstrated.

Scheme 15. Synthesis of LLM-105 (58). Scheme 15. Synthesis of LLM-105 (58).

16 16 Crystals 2016,, 66,, 55 17 of 22

3.3.5. Energetic co-Crystallization of HMX and CL-20 3.3.5. Energetic co-Crystallization of HMX and CL-20 In a recent new development in applying pharmaceutical applications to the field of energetic In a recent new development in applying pharmaceutical applications to the ﬁeld of energetic materials, Matzger pioneered the concept of developing the first known energetic co-crystals. The materials, Matzger pioneered the concept of developing the ﬁrst known energetic co-crystals. The most most successful energetic co-crystal reported to date was developed by co-crystallizing a 2:1 molar successful energetic co-crystal reported to date was developed by co-crystallizing a 2:1 molar mixture mixture of CL-20 (39) and HMX (40), whose individual structures are provided in Figure 4 [48]. of CL-20 (39) and HMX (40), whose individual structures are provided in Figure4[ 48]. Surprisingly, Surprisingly, the co-crystallized material was shown to have a low impact sensitivity comparable to the co-crystallized material was shown to have a low impact sensitivity comparable to HMX rather HMX rather than CL-20, which is believed to be attributed to the high degree of C–H bonding than CL-20, which is believed to be attributed to the high degree of C–H bonding interactions present interactions present in the co-crystal. These interactions are absent from CL-20 and HMX in their in the co-crystal. These interactions are absent from CL-20 and HMX in their individual, pure form. individual, pure form. The co-crystal has a crystalline density of 1.945 g/cc, was also theoretically The co-crystal has a crystalline density of 1.945 g/cc, was also theoretically predicted to have a detonation predicted to have a detonation velocity of 9484 m/s, which is higher than HMX (Vdet = 9100 m/s), and velocity of 9484 m/s, which is higher than HMX (V = 9100 m/s), and undergoes decomposition undergoes decomposition at 235 °C, followed by detonationdet at 240 °C. Friction sensitivity, ESD at 235 ˝C, followed by detonation at 240 ˝C. Friction sensitivity, ESD sensitivity, and detonation sensitivity, and detonation pressure values were not reported. In future years, as the technology pressure values were not reported. In future years, as the technology becomes more mature, it will be becomes more mature, it will be interesting to determine whether an energetic co-crystal, such as the interesting to determine whether an energetic co-crystal, such as the aforementioned example, will aforementioned example, will maintain its integrity, structure and power when used in explosive maintain its integrity, structure and power when used in explosive formulations, when subjected to formulations, when subjected to high loading pressures, and when stored in high temperature/high high loading pressures, and when stored in high temperature/high humidity environments for long humidity environments for long periods of time. periods of time.

3.4. High-Performing Melt-Castable Materials

3.4.1. 2,3-Dinitro-2,3-Bis((nitrooxy)methyl)Butane-1,4-Diyl DinitrateDinitrate Melt-castable materials are preferred over press-curedpress-cured materials due to their less labor-intensive processes, as as well well as as their their enhanced enhanced safety safety issu issues.es. There There is also is an also increasing an increasing interest interestin the indevelopment the development of melt-castable of melt-castable materials materialsthat demosntrate that demosntrate performance performance exceeding TNT. exceeding Work at TNT. Los WorkAlamos at National Los Alamos Laboratory National spearheaded Laboratory by spearheaded Chavez led by to Chavez the synthesis led to theof a synthesisnew melt-castable of a new melt-castablematerial that appears material to that satisfy appears these to requirements, satisfy these requirements,as summarized as in summarized Scheme 16 [49]. in Scheme Commercially 16[ 49]. Commerciallyavailable (though available expensive) (though dioxan expensive)e (59) was dioxane homocoupled (59) was homocoupledto yield dinitro to derivative yield dinitro (60). derivative Removal (60).of the Removaldiacetal, offollowed the diacetal, by exhaustive followed nitration by exhaustive of the nitrationresulting oftetrol the resultingafforded 2,3-Dinitro-2,3- tetrol afforded 2,3-Dinitro-2,3-bis((nitrooxy)methyl)butane-1,4-diylbis((nitrooxy)methyl)butane-1,4-diyl dinitrate (62). dinitrate (62).

Scheme 16. Synthesis of dinitro tetranitrate ester (62).

This material melts at 85–86 °C, but does not decompose until 141 °C (Table 8). The detonation This material melts at 85–86 ˝C, but does not decompose until 141 ˝C (Table8). The detonation velocity of tetranitrate ester 62 was calculated to be identical to HMX (9100 m/s) with a calculated velocity of tetranitrate ester 62 was calculated to be identical to HMX (9100 m/s) with a calculated detonation pressure of 400 kbar (Pcj of HMX = 393 kbar). The impact, friction, and ESD sensitivity of detonation pressure of 400 kbar (Pcj of HMX = 393 kbar). The impact, friction, and ESD sensitivity of this material was found to be comparable to pentaerythritol tetranitrate (PETN). this material was found to be comparable to pentaerythritol tetranitrate (PETN).

17 Crystals 2016, 6, 5

Crystals 2016, 6, 5 Table 8. Performance and sensitivities of tetranitrate ester 62. 18 of 22 Data Category 62 Table 8. PerformanceIS and [J] a sensitivities of tetranitrate2.7 ester 62. FS [N] b 74.5 Data CategoryESD c [J] c 0.625 62 IST [J]melta [°C] d 85–862.7 FS [N]Tdecb [°C] e 74.5140 c c f ESDΩ[J] [%] 0.6250 ˝ d Tmelt ρ[ [g/cc]C] g 85–861.917 T [˝C] e 140 dec h Pcj [kbar]f 400 ΩCO2 [%] 0 ρ [g/cc]Vdet [m/s]g i 1.9179100 0 h j PcjΔ[kbar]fH [kJ/mol] 400−371 i d melt eV dec [m/s] 9100 f 2 T = Tempterature of melting; Tdet = Temperature of decomposition; Ω = CO oxygen balance; ∆ H0 [kJ/mol] j ´371 g ρ = crystalline density; h Pcj = detonationf pressure; i Vdet = detonation velocity; j ΔfH0 = molar enthalpy d e f of formation.Tmelt = Tempterature of melting; Tdec = Temperature of decomposition; ΩCO2 = CO2 oxygen balance; g h i j 0 ρ = crystalline density; Pcj = detonation pressure; Vdet = detonation velocity; ∆fH = molar enthalpy of formation. 3.4.2. 3,4-Bis(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,5-Oxadiazole-N-Oxide (BNFF, DNTF) 3.4.2.BNFF 3,4-Bis(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,5-Oxadiazole- (aka DNTF) can be synthesized depicted in SchemeN-Oxide 17 [50]. (BNFF, Formation DNTF) of furazan (63) from malononitrile, followed by diazotization and substitution furnished chloroxime (64). Exposure of the BNFF (aka DNTF) can be synthesized depicted in Scheme 17[ 50]. Formation of furazan (63) from chloroxime to silver carbonate afforded difurazano furoxan (65), which was nitrated to yield BNFF malononitrile, followed by diazotization and substitution furnished chloroxime (64). Exposure of the (66). chloroxime to silver carbonate afforded difurazano furoxan (65), which was nitrated to yield BNFF (66).

Scheme 17. Synthesis of BNFF (66).

The performance parameters parameters of of BNFF, BNFF, as as reported reported by by Sinditskii, Sinditskii, are are provided provided in inTable Table 6 6[51].[ 51 It]. isIt isworth worth noting noting that that there there are are two two exothermic exothermic ev eventsents during during the the decompos decompositionition of of BNFF, BNFF, with the firstﬁrst taking place at 292 ˝°C,C, and the second taking place at 310 ˝°C.C. Although reported densities of BNFF are north of 1.90 g/cc, these these are are likely likely at at cold cold temperatures, temperatures, as as the the density density reported reported in in Table 66 is the the experimentally experimentally determined determined density density at at room room temperature. temperature. The The detonation detonation velocity, velocity, enthalpy enthalpy of formation,of formation, and and detonation detonation energy energy values values ofof BN BNFFFF detailed detailed in in Table Table 99 representrepresent experimentallyexperimentally determined values.

TableTable 9. Performance and sensitivities of BNFF.

DataData Category BNFF BNFF (66) (66) a Tmelt [°C]˝ a 109–111 Tmelt [ C] 109–111 Tdec[°C]˝ b 292 Tdec [ C] 292 ρ [g/cc][g/cc] cc 1.8751.875 d VVdetdet [m/s] d 89308930 H0 e 657.23 ∆ΔffH0 [kJ/mol] e 657.23 0 f ∆exU [kJ/kg] 5797 ΔexU0 [kJ/kg] f 5797 a b c Tmelt = melting temperature; Tdec = temperature of decomposition; ρ = crystalline density; d e 0 f 0 Vdet = detonation velocity; ∆fH = molar enthalpy of formation; ∆exU = total energy of detonation. 18 Crystals 2016, 6, 5 Crystals 2016, 6, 5 a Tmelt = melting temperature; b Tdec = temperature of decomposition; c ρ = crystalline density; d Vdet = e 0 f 0 Crystalsadetonation T2016melt ,=6 melting, 5 velocity; temperature; ΔfH = molar b Tdec enthalpy= temperature of formation; of decomposition; ΔexU = total c ρ energy= crystalline of detonation. density; d Vdet 19= of 22 e 0 f 0 3.4.3.detonation 3,4-Bis(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,5-Oxadiazole velocity; ΔfH = molar enthalpy of formation; Δ (LLM-172,exU = total energy BNFF-1) of detonation. 3.4.3.A 3,4-Bis(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,5-Oxadiazole 3,4-Bis(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,5-Oxadiazole close molecular derivative of BNFF is LLM-172 (aka (LLM-172, BNFF-1), as BNFF-1) shown in Figure 6. The only differenceA close existing molecular in the derivative structure ofis thatBNFF BNFF is LLM-172 has the presence(aka BNFF-1), of an asN -oxide shown moiety, in Figure whereas 66.. TheThe LLM- onlyonly difference172 does not. existing existing As summarized in in the the structure structure in Table is that is 10, that BNFF LLM-172 BNFF has hasthehas presence thea melting presence of point an of N of-oxide an 85N °C,-oxide moiety, which moiety, whereasis significantly whereas LLM- 172LLM-172lower does than not. does the As melting not. summarized As point summarized of in DNTF, Table 10, in thus TableLLM-172 shifting 10, LLM-172has the a former melting has material point a melting of further 85 °C, point whichinto of the 85 is melt-castable ˝significantlyC, which is lowersigniﬁcantlyrange than[52]. thePredictably, lower melting than point due the meltingto of the DNTF, absence point thus ofof shifting DNTF,the N-oxide the thus former shifting functionality, material the former furtherthe performance material into the further melt-castable of LLM-172 into the rangemelt-castableis lower [52]. than Predictably, rangethat of [52 DNTF,]. due Predictably, toyet the the absence material due to of the also the absence Nhas-oxide reduced of the functionality,N sensitivity-oxide functionality, th toe ignitionperformance the stimuli performance of LLM-172as well. of isLLM-172 lower than is lower that than of DNTF, that of yet DNTF, the yetmaterial the material also has also reduced has reduced sensitivity sensitivity to ignition to ignition stimuli stimuli as as well. well. O O N N N N O O N N N N O2N NO2 O N N N NO 2 O 2 N N 67O 67 Figure 6. Molecular structure of LLM-172 (67). Figure 6. MolecularMolecular structure structure of LLM-172 (67). Table 10. Performance and sensitivities of LLM-172 (67).

TableTable 10.Data Performance Category and sensitivities LLM-172 of LLM-172 (67) (67). DataIS Category [J] a LLM-17211.5 (67) Data Category LLM-172 (67) FSIS [N][J] a b 11.5188 IS [J] a 11.5 ESDFS [N]c [J] b c >1.0188 FS [N] b 188 Tmelt [°C]c cd 84 ESDESDc [J] [J]c >1.0 e Tmeltdec [°C]˝ d d 293 TTmelt [[°C]C] 84 ˝ eef TΩTdecdec [[°C]C][%] 293−27 f ΩρCO [g/cc][%] g f 1.836´27 Ω2 [%] −27 ρ [g/cc] g 1.836 Pcj [kbar]g h 340 ρ [g/cc] h 1.836 Pcj [kbar] 340 Vdet [m/s] hi 8870 VPcj [kbar][m/s] i 8870340 det0 j ΔfVH0det [kJ/mol] [m/s] i j 8870644 ∆fH [kJ/mol] 644 a b 0 j c d ISa = impact sensitivity; bFS = frictionΔfH [kJ/mol] sensitivity; c ESD = electrostatic644 discharge;d Tmelt = Tempterature IS = impact sensitivity; FS = friction sensitivity; ESD = electrostatic discharge; Tmelt = Tempterature e f g aof melting; e Tdec = Temperatureb of decomposition;cf Ω = CO2 oxygen balance;g ρd ρ = crystalline ISof = melting; impact sensitivity;Tdec = Temperature FS = friction of decomposition; sensitivity; ESDΩCO2 == electrostatic CO 2 oxygen discharge; balance; =Tmelt crystalline = Tempterature density; h h i i j 0 j 0 density;Pcj = detonation Pcje = detonationdec pressure; pressure;Vdet = detonation Vdet = detonation velocity; ∆f velocity;fH = molar Δ enthalpyf2H = molar of formation. enthalpyg of formation. of melting; T = Temperature of decomposition; Ω = CO oxygen balance; ρ = crystalline density; h Pcj = detonation pressure; i Vdet = detonation velocity; j ΔfH0 = molar enthalpy of formation. 3.4.4. 3-(4-Amino-1,2,5-Oxadiazol-3-yl)-4-(4-Nitro-1,2,5-Oxadiazol-3-yl)-1,2,5-Oxadiazole (ANFF-1)(ANFF-1) 3.4.4. 3-(4-Amino-1,2,5-Oxadiazol-3-yl)-4-(4-Nitro-1,2,5-Oxadiazol-3-yl)-1,2,5-Oxadiazole (ANFF-1) A ﬁnalfinal furazan-based compound that is of interest for melt-castable applications is ANFF-1, whoseA preferredfinal furazan-based methodmethod of compound synthesissynthesis is that given is inof Schemeinterest 18 for[ [53].53 melt-castable]. BAFF-1 (68) applications is treated with is ANFF-1, sulfuric whoseacid and preferred hydrogen method peroxide of synthesis to produce is given the asymmetrical in Scheme 18 ANFF-1[53]. BAFF-1 via the (68) conversion is treated withof one sulfuric of the acidtwo aminoand hydrogen groups totoperoxide aa nitronitro group.group.to produce the asymmetrical ANFF-1 via the conversion of one of the two amino groups to a nitro group.

Scheme 18. Synthesis of ANFF-1 (69). Scheme 18. Synthesis of ANFF-1 (69). The high density, low melting point, and low level of sensitivity, as outlined in Table 1111,, makes ANFF-1The ahigh candidate density, molecule low melting thatthat oughtoughtpoint, totoand bebe low exploredexplored level offorfor sensitivity, furtherfurther explosiveexplosive as outlined development.development. in Table 11, makes ANFF-1 a candidate molecule that ought to be explored for further explosive development. 19 19 Crystals 2016, 6, 5 20 of 22

Table 11. Performance and sensitivities of ANFF-1 (69).

Data Category ANFF-1 (69) IS [J] a 43.4 FS [N] b >353 ˝ d Tmelt [ C] 100 f ΩCO2 [%] ´27 ρ [g/cc] g 1.782 a b d f IS = impact sensitivity; FS = friction sensitivity; Tmelt = Tempterature of melting; ΩCO2 = CO2 oxygen balance; g ρ = crystalline density.

4. Conclusions For many of the primary and secondary explosive materials discussed in this review, it will be interesting to follow their progress in future years, as many of these ingredients move past the basic research area, and progress into an applied research setting to be investigated in various formulations. While this review is by no means a complete list of explosive ingredients that exist with potential applications, it is felt that the materials described are the most relevant on the bases of performance, synthetic utility, cost, and environmental considerations. It is critical that new energetic materials continue to be synthesized with these aforementioned bases in mind to ensure that what is being made can be used in practical applications.

Acknowledgments: The authors are indebted to Brian D. Roos (ARL) and Noah Lieb (Hughes Associates, Inc.) for many insightful discussions pertaining to this review. Thomas M. Klapötke is gratefully acknowledged for his invitation in submitting this article. The authors thank the US Army Research Laboratory (ARL), the Armaments Research, Development & Engineering Center (ARDEC), the Research, Development & Engineering Command (RDECOM) and the Ordnance Environmental Program (OEP) for funding of this work. Author Contributions: Jesse J. Sabatini and Karl D. Oyler contributed equally to this article. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Davis, T.L. The Chemistry of Powder and Explosives; GSG & Associates: San Pedro, CA, USA, 1943. 2. Klapötke, T.M. Chemistry of High-Energy Materials, 2nd ed.; Walter de Gruyter GmbH & Co.: Berlin, Germany, 2012. 3. Pagoria, P.F.; Lee, G.S.; Mitchell, A.R.; Schmidt, R.D. A review of energetic materials synthesis. Thermochim. Acta 2002, 384, 187–204. [CrossRef] 4. Badgujar, D.M.; Talawar, M.B.; Asthana, S.N.; Mahulikar, P.P. Advances in science and technology of modern energetic materials: An overview. J. Hazard. Mater. 2008, 151, 289–305. [CrossRef][PubMed] 5. Singh, R.P.; Gao, H.; Meshri, D.T.; Shreeve, J.M. Nitrogen-rich heterocycles. Struct. Bonding 2007, 125, 35–83. 6. Brede, U.; Hagel, R.; Redecker, K.H.; Weuter, W. Primer compositions in the course of time: From black powder and sinoxid to sintox compositions and sinco booster. Propellants Explos. Pyrotech. 1996, 21, 113–117. [CrossRef] 7. Nobel, A. Improved explosive and primer for the same. Dynamite. British Patent 1345, 7 May 1867. 8. Matyáš, R.; Pachman, J. Primary Explosives; Springer: Heidelberg, Germany, 2013. 9. Herz, E. Lead salt of trinitroresorcin. British Patent 17961, 29 October 1915. 10. Federoff, B.T. Encyclopedia of Explosives and Related Items; Picatinny Arsenal: Dover, NJ, USA, 1960; Volume 1. 11. Committee on Measuring Lead in Critical Populations; Commission on Life Sciences; Division on Earth and Life Studies; National Research Council. Measuring Lead Exposure in Infants, Children, and Other Sensitive Populations; The National Academies Press: Washington, DC, USA, 1993. 12. Clinton, W.J. Executive order 12856—Federal compliance with right-to-know laws and pollution prevention requirements; Ofﬁce of the Federal Register: Washington, DC, USA, 1993. 13. Dorman, D.C.; Benoff, S.H.; Bishop, E.C.; Bleecker, M.L.; Brosseau, L.M.; Goldman, R.H.; Graziano, J.H.; Milz, S.A.; Park, S.K.; Roberts, M.A.; et al. National Research Council Report: Potential Health Risks to Dod Firing-Range Personnel from Recurrent Lead Exposure; The National Academies Press: Washington, DC, USA, 2012. Crystals 2016, 6, 5 21 of 22

14. Gilligan, W.H.; Kamlet, M.J. Synthesis of Mercuric 5-Nitrotetrazole; NSWC/WOL/TR 76–146; Naval Surface Weapons Center, White Oak Laboratory: Silver Spring, MD, USA, 1976. 15. Kast, H.; Haid, A. The explosive properties of the most important initiating explosives. Angew. Chem. 1924, 38, 43–52. [CrossRef] 16. Taylor, C.A.; Rinkenbach, W.H. Preparation and Detonating Qualities of Cyanuric Triazide; Reports of Investigation; U.S. Dept. of the Interior, Bureau of Mines: Washington, DC, USA, 1923. 17. Ott, E. Explosive and process of making same. U.S. Patent 1390378, 3 November 1921. 18. Mehta, N.; Cheng, G.; Cordaro, E.A.; Dolch, T.; Yang, K.; Stec, D. Modified Ardec Triazine Triazide (TTA) Synthesis; ARMET-TR-09019; U.S. Army ARDEC: Picatinny, NJ, USA, 2009. 19. Sandstrom, J.; Quinn, A.A.; Erickson, J. Non-toxic, heavy-metal free sensitized explosive percussion primers and methods of preparing the same. U.S. Patent 20110239887, 6 October 2011. 20. Bjerke, R.K.; Ward, J.P.; Ells, D.O.; Kees, K.P. Improved primer composition. U.S. Patent 4963201, 16 October 1990. 21. Erickson, J.A. Lead-free centerfire primer with DDNP and barium nitrate oxidizer. U.S. Patent 5831208, 3 November 1998. 22. Gamage, N.-D.H.; Stiasny, B.; Stierstorfer, J.; Martin, P.D.; Klapötke, T.M.; Winter, C.H. Less sensitive oxygen-rich organic peroxides containing geminal hydroperoxy groups. Chem. Commun. 2015, 51, 13298–13300. [CrossRef][PubMed] 23. Fronabarger, J.W.; Williams, M.D. Alternative to tetrazene. U.S. Patent 8524019, 3 September 2013. 24. Yan, Q.-L.; Zeman, S.; Zhang, J.-G.; He, P.; Musil, T.; Bartoskova, M. Multi-stage decomposition of 5-aminotetrazole derivatives: Kinetics and reaction channels for the rate-limiting steps. Phys. Chem. Chem. Phys. 2014, 16, 24282–24291. [CrossRef][PubMed] 25. Costain, T. Investigation of Potassium Dinitrobenzofuroxan (KDNBF) to Provide Data Necessary for the Preparation of a Military Specification; Picatinny Arsenal: Picatinny, NJ, USA, 1970. 26. Fronabarger, J.W.; Williams, M.D.; Sanborn, W.B.; Parrish, D.A.; Bichay, M. KDNP—A lead free replacement for lead styphnate. Propellants Explos. Pyrotech. 2011, 36, 459–470. [CrossRef] 27. Fischer, D.; Klapötke, T.M.; Stierstorfer, J. Potassium 1,1'-dinitramino-5,5'-bistetrazolate: A primary explosive with fast detonation and high initiation power. Angew. Chem. Int. Edit. 2014, 53, 8172–8175. [CrossRef][PubMed] 28. Fischer, D.; Klapötke, T.M.; Stierstorfer, J. 1,5-di(nitramino)tetrazole: High sensitivity and superior explosive performance. Angew. Chem. Int. Edit. 2015, 54, 10299–10302. [CrossRef][PubMed] 29. Fronabarger, J.W.; Williams, M.D.; Bragg, J.G.; Parrish, D.A.; Bichay, M. DBX-1—A lead-free replacement for lead azide. Propellants Explos. Pyrotech. 2011, 36, 541–550. [CrossRef] 30. Huynh, M.H.V.; Hiskey, M.A.; Meyer, T.J.; Wetzler, M. Green primaries: Environmentally friendly energetic complexes. Proc. Natl. Acad. Sci. USA 2006, 103, 5409–5412. [CrossRef][PubMed] 31. Klapötke, T.M.; Piercey, D.G.; Mehta, N.; Oyler, K.D.; Sabatini, J.J. Reaction of copper(I) nitrotetrazolate (DBX-1) with sodium m-periodate. Z. Naturforsch. B J. Chem. Sci. 2014, 69, 125–127. [CrossRef] 32. Von Herz, E. C-nitrotetrazole compounds. U.S. Patent 2066954, 5 January 1937. 33. Gilligan, W.H.; Kamlet, M.J. Method of preparing the acid copper salt of 5-nitrotetrazole. U.S. Patent 4093623, 6 June 1978. 34. Ford, D.D.; Lenahan, S.; Jorgensen, M.; Dube, P.; Delude, M.; Concannon, P.E.; Anderson, S.R.; Oyler, K.D.; Cheng, G.; Mehta, N.; et al. Development of a lean process to the lead-free primary explosive DBX-1. Org. Process Res. Dev. 2015, 19, 673–680. [CrossRef] 35. Chapman, R.D.; Hollins, R.A.; Groshens, T.J.; Nissan, D.A. Benzylamine-Free, Heavy-Metal-Free Synthesis of Cl-20; SERDP SEED Project WP-1518; Naval Air Warfare Center Weapons Division China Lake: Lake, CA, USA; 28 December 2006. 36. Bachmann, W.E.; Sheehan, J.C. A new method of preparing the high explosive RDX. J. Am. Chem. Soc. 1949, 71, 1842–1845. [CrossRef] 37. Agency for Toxic Substances and Disease Registry (ATSDR). Toxic substances portal—Cyclonite (RDX). Available online: http://www.atsdr.cdc.gov/toxfaqs/tf.asp?id=411&tid=72 (accessed on 25 July 2015). 38. Thompson, M. The 50 best inventions of 2010: Less dangerous explosives. Time Magazine, 11 November 2011. Available online: http://content.time.com/time/specials/packages/article/0,28804,2029497_2030613_2029816,00.html (accessed on 29 December 2015). Crystals 2016, 6, 5 22 of 22

39. Fung, V.; Schreiber, B.; Patel, C.; Samuels, P.; Vinh, P.; Zhao, X.-L. Process Improvement and Optimization of Insensitive Explosive IMX-101; Insensitive Munitions & Energetic Materials Technology Symposium (IMEMTS) & National Defense Industrial Association (NDIA): Las Vegas, NV, USA, 2012. 40. Fischer, N.; Fischer, D.; Klapötke, T.M.; Piercey, D.G.; Stierstorfer, J. Pushing the limits of energetic materials—The synthesis and characterization of dihydroxylammonium 5,5'-bistetrazole-1,1'-diolate. J. Mater. Chem. 2012, 22, 20418–20422. [CrossRef] 41. Nicolich, S.; Samuels, P.; Damavarapu, R.; Paraskos, A.J.; Cooke, E.; Stepanov, V.; Cook, P.; Caﬂin, K.; Duddu, R. Dihydroxylammonium 5,5'-bistetrazole-1,1' diolate (TKX-50) synthesis and lab scale characterization. In Proceedings of the Insensitive Munitions Energetic Materials Technology Symposium (IMEMTS), Rome, Italy, 18–21 May 2015; National Defense Industrial Association (NDIA): Rome, Italy. 42. Aubert, S.A.; Roos, B.D. Melt cast insensitive eutectic explosive. U.S. Patent 8663406, 3 March 2014. 43. Sinditskii, V.P.; Filatov, S.A.; Kolesov, V.I.; Kapranov, K.O.; Asachenko, A.F.; Nechaev, M.S.; Lunin, V.V.; Shishov, N.I. Combustion behavior and physico-chemical properties of dihydroxylammonium 5,5'-bistetrazole-1,1'-diolate (TKX-50). Thermochim. Acta 2015, 614, 85–92. [CrossRef] 44. Dippold, A.A.; Klapötke, T.M. A study of dinitro-bis-1,2,4-triazole-1,1'-diol and derivatives: Design of high-performance insensitive energetic materials by the introduction of n-oxides. J. Am. Chem. Soc. 2013, 135, 9931–9938. [CrossRef][PubMed]

45. Rahm, M.; Bèlanger-Chabot, G.; Haiges, R.; Christe, K.O. Nitryl cyanide, NCNO2. Angew. Chem. Int. Ed. 2014, 53, 6893–6897. [CrossRef][PubMed] 46. Pagoria, P.F.; Zhang, M.X. Process for preparation of pyrazines including 2,6-diaminopyrazine-1-oxide (DAPO) and 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105). U.S. Patent 20100267955, 21 October 2010. 47. Zuckerman, N.B.; Shusteff, M.; Pagoria, P.F.; Gash, A.E. Microreactor ﬂow synthesis of the secondary high explosive 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105). In Proceedings of the Insensitive Munitions Energetic Materials Technology Symposium (IMEMTS), Rome, Italy, 18–21 May 2015; National Defense Industrial Association (NDIA): Rome, Italy. 48. Bolton, O.; Simke, L.R.; Pagoria, P.F.; Matzger, A.J. High power explosive with good sensitivity: A 2:1 cocrystal of CL-20:HMX. Cryst. Growth Des. 2012, 12, 4311–4314. [CrossRef] 49. Chavez, D.E.; Hiskey, M.A.; Naud, D.L.; Parrish, D. Synthesis of an energetic nitrate ester. Angew. Chem. Int. Ed. 2008, 47, 8307–8309. [CrossRef][PubMed] 50. Tsyshevsky, R.; Pagoria, P.; Zhang, M.; Racoveanu, A.; DeHope, A.; Parrish, D.; Kuklja, M.M. Searching for low-sensitivity cast-melt high-energy-density materials: Synthesis, characterization, and decomposition kinetics of 3,4-bis(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,5-oxadiazole-2-oxide. J. Phys. Chem. C 2015, 119, 3509–3521. [CrossRef] 51. Sinditskii, V.P.; Burzhava, A.V.; Sheremetev, A.B.; Aleksandrova, N.S. Thermal and combustion properties of 3,4-bis(3-nitrofurazan-4-yl)furoxan (DNTF). Propellants Explos. Pyrotech. 2012, 37, 575–580. [CrossRef] 52. DeHope, A.; Zhang, M.; Lorenz, K.T.; Lee, E.; Parrish, D.; Pagoria, P.F. Synthesis and small-scale performance characterization of “new” insensitive energetic compounds. In Proceedings of the Insensitive Munitions Energetic Materials Technology Symposium (IMEMTS), Rome, Italy, 18–21 May 2015; National Defense Industrial Association (NDIA): Rome, Italy. 53. Pagoria, P.; Zhang, M.; Racoveanu, A.; DeHope, A.; Tsyshevsky, R.; Kuklja, M.M. 3-(4-Amino-1,2,5-oxadiazol-3-yl)- 4-(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,5-oxadiazole. Molbank 2014, 2014, M824. [CrossRef]

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