Energetic cocrystal, ionic salt and coordination polymer of a

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Energetic cocrystal, ionic salt and coordination polymer of a perchlorate free high energy density oxidizer: influence of pKa modulation on their formation Qing Ma, Shiliang Huang, Huanchang Lu, Fude Nie, Longyu Liao, Guijuan Fan, and Jinglun Huang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01293 • Publication Date (Web): 06 Jan 2019 Downloaded from http://pubs.acs.org on January 9, 2019

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Crystal Growth & Design

Energetic cocrystal, ionic salt and coordination polymer of a perchlorate free high energy density oxidizer: influence of pKa modulation on their formation Qing Ma,† Shi-Liang Huang,† Huan-Chang Lu, Fude Nie, Long-Yu Liao,* Gui-Juan Fan* and JingLun Huang* Institute of Chemical Materials, Chinese Academy of Engineering Physics, Mianyang 621900, China ABSTRACT: Cocrystal, ionic salt and coordination polymer had prompted the development of propellants, explosives and pyrotechnics. However, the difference between their formation based on the same coformer was rarely studied. 5,5’Bis(trinitromethyl)-3,3’-bi-1H-1,2,4-triazole (BTNMBT) is a perchlorate-free, favorable to scale-up and green energetic oxidizer with high physical performance (oxygen balance: +18.4 %, IS: 22.5 J, FS: 252 N, D: 9073 m s-1, P: 36.2 GPa). To investigate the influence of different coformers on the formation of BTNMBT’s cocrystal, ionic salt and metal-organic framework, organic acid as well as organic and inorganic bases with different dissociation constant (pKa) were theoretically studied. In this work, two energetic cocrystals, energetic ionic salt and energetic metal-organic framework were synthesized based on BTNMBT. For their formation, the basic principle is found as follows: i) when pKa value of organic base is far lower than both pKa values of hydrogen-protons in organic acid, the reaction systems prone forming co-crystals; ii) If pKa value of selected organic base is higher than one of hydrogen-protons in organic acid but lower than another one, 1:1 energetic ionic salt appears; iii) The 1:2 type of energetic ionic salt (or coordination polymer) can form when pKa value of corresponding base is higher than both of hydrogen-protons in organic acid. Among these shapes of derivatives, coordination polymer form of BTNMBT not only exhibits good detonation performance (D: 8872 m s-1), but also shows positive oxygen balance (+18.2%) and high thermal stability (Td: 180oC) comparable to those of AP and superior to those of ADN. These discoveries can assist the design and preparation of other promising energetic materials towards future high-performing energy applications.

INTRODUCTION In the past decades, the design and development of halogen-free, high performance and high energy dense oxidizers (HEDOs) becomes more and more significant in the solid rocket fuel industry of energetic materials. The search for a smokeless and eco-friendly propellant has encouraged many scientists to look for chlorine-free oxidizers as a substitute for replacing ammonium perchlorate (AP) because it contributes to acid rain and ozone layer depletion, in addition to having deleterious impacts on the human thyroid and being a persistent contaminant in groundwater. 1,2 Current research interests of HEDOs have focused on the design and synthesis of trinitromethyl substituted energetic materials, including C-trinitromethyl3-7 and N-trinitromethyl neutral compounds and ionic salts.8-11 Energetic cocrystal (ECC), energetic ionic salt (EIS) and energetic coordination polymer (ECP) are very promising research directions in energetic materials area. ECC technique provides a stabilizing effect for decreasing mechanical sensitivities in some extent. In the development of ECC, energetic coformers mainly referred to traditionally used explosives such as HMX, 1-methyl-

2,4,6-trinitrobenzene (TNT), 1-methyl-3,4,5trinitropyrazole (MTNP) and 1-methyl-3,5-dinitro-1,2,4triazole (MDNT).12-20 Nowadays, some other promising materials were developed expect for CL-20 based ECCs, such as the interesting melt-castable energetic cocrystal based on 3,4-diaminofurazan (DAF).21 In the recent year, the research direction of ECCs turned into other nitrogenrich heterocycles, such as 5,5’-dinitro-2H,2H’-3,3’-bi-1,2,4triazole (DNBT), 3,6-dinitropyrazole[4,3-c]-pyrazole (DNPP), 3-nitro-1,2,4-triazol-5-one (NTO), ethylenedinitramine (EDNA), 3,6-bis(1H-1,2,3,4-tetrazol5-ylamino) (BTATz), 3,5-dimethylpyrazol-1-ylsubstituted-1,2,4,5-tetrazine (BPT), 1H,1H’-5,5’bistetrazole-1,1’-diolate (BTO).22-29 Recently, cocrystals of AP with non-energetic compounds like different types of crown ethers were prepared through the solvent/antisolvent method.30 To the best of our knowledge, the energetic cocrystals via perchlorate-free oxidizers have not been reported yet. In contrast with neutral compound (mother structure) as well as ECC, EIS and ECP can increase the thermostability and provide significant promoting effect in the crystal density of energetic materials. During the

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structural construction of EIS and ECP, energetic heterocylic compounds comprised of gem-dinitromethyl and trinitromethyl moieties feature high heats of formation, high densities and a large amount of active oxygen. The synthesis and development of EIS based on gem-dinitromethyl and trinitromethyl frameworks were often a series of high energy density materials.31-33 Comparing with EIS, ECP bearing gem-dinitromethyl moieties became attractive energetic materials with multiple coordination sites, which was also considered as green candidates for lead-free primary explosive, but its synthesis was relatively difficult.34-37 The replacement of gem-dinitromethyl moieties by trinitromethyl groups in a molecule can possibly increase its density, oxygen balance and heat of formation, thereby improving the detonation performance.

Figure 1. Screening for three forms of energetic materials based on the same energetic oxidizer through pKa modulation strategy 5,5’-Bis(trinitromethyl)-3,3’-bi-1H-1,2,4-triazole (BTNMBT) is a perchlorate-free, favorable to scale-up and green energetic oxidizer with high physical performance (oxygen balance: +18.4 %, IS: 22.5 J, FS: 252 N, D: 9073 m s1, P: 36.2 GPa).38 As shown in Figure 1, this new oxidier BTNMBT can react with different organic or inorganic bases to form versatile types of energetic materials. To study this phenomenon, dissociation constant (pKa) of these coformers need to be probed. As is known to us all, pKa has close relationship with information on the chemical reactivity of organic and inorganic compounds. Meanwhile, pKa is an important energetic parameter in the predicting reaction of organic chemistry and disclosing significant insights behind the bond data.39 In this work, four different kinds of basic coformers were chosen, including organic bases with or without electronwithdrawing groups (-CH3, -NH2, etc) as well as inorganic base. At first, their pKa values were calculated. Subsequently, to investigate the influence of pKa values on the final products, they were applied in the synthesis of energetic cocrystal, salt and CP. Energetic performance such as sensitivity, detonation properties and specific impulses were measured and calculated, respectively. EXPERIMENTAL SECTION

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Caution! Though these coformers used are insensitive towards impact, friction or electrostatic discharge, 5,5’Bis(trinitromethyl)-3,3’-bi-1H-1,2,4-triazole (BTNMBT) and as-synthesized products are still energetic, especially easily to generate energetic output when they meet elevated temperature. These compounds should be handled carefully with necessary protections such as gloves, eye-protecting glasses, face shield, etc. Materials and measurements. All reagents purchased commercially were used as received (Sigma-Aldrich, TCI, Acros and J&K Organics). 5,5’-Bis(trinitromethyl)-3,3’-bi1H-1,2,4-triazole (BTNMBT) was prepared according to our previously report.38 1H and 13C NMR spectra were recorded on a 400 MHz (Bruker AVANCE 400) nuclear magnetic resonance spectrometer. Chemical shifts in the 1H and 13C spectra are reported relative to Me Si. The 4 melting point and decomposition temperature were recorded on a differential scanning calorimeter-thermal gravity (TGA/DSC2, METTLER TOLEDO, STARe system) at a heating rate of 5 oC min-1. Infrared (IR) spectra were measured on SHIMADZU IRTracer-100 FT-IR spectrometer in the range of 4000–400 cm-1 as KBr pellets at 20oC. Elemental analyses (C,H,N) were carried out on a elemental analyzer (Vario EL Cube, Germany). Impact and friction sensitivities were performed by BAM fall or friction apparatus (OZM Research, Czech Republic). All crystals were mounted on a MiteGen MicroMesh using a small amount of Cargille Immersion Oil. Data were collected on a Bruker three-circle platform diffractometer equipped with a SMART APEX II CCD detector. A kryoFlex low-temperature device was used to keep the crystals at 296 K and 173 K during data collection. Data collection was performed and the unit cell was initially refined using APEX2. Data reduction was carried out using SAINT and XPREP. Corrections were applied for Lorentz, polarization, and absorption effects using SADABS. The structures were further solved and refined with the aid of the programs using direct methods and least-squares minimization by SHELXS-97 and SHELXL-97 code. The full-matrix least-squares refinement on F2 involved atomic coordinates and anisotropic thermal parameters for all non-H atoms. The H atoms were included using a riding model. The non-H atoms were refined anisotropically. The finalized CIF files were checked with checkCIF, and deposited at the Cambridge Crystallographic Data Centre as supplementary publications (The crystalline parameters were listed in the Supporting Information). Intra- or intermolecular hydrogen-bonding interactions were analyzed with Diamond software (version 3.2 K) as well as the illustrations of molecular structures. Powder X-ray diffraction (PXRD) patterns for as-synthesized crystals were obtained via a Bruker D8 Advance instrument (Bruker AXS Inc., Madison, WI, USA) (Cu Kα radiation, voltage 40 kV and current 40 mA). The data were obtained over the angle range from 5 o to 40o at the scanning speed of 0.02o per second. Scheme 1. Pathway for preparing energetic cocrystals and ionic salts of BTNMBT in this work.


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Crystal Growth & Design O2N O2N

NO2

H N N

N

Organic acid

N N H

N 1

N

Organic or inorganic bases

N

NH

O2N

NO2 NO2

NH2 HN N N NH2 H 2N N N N N

H 3C

2

KOH 5

4

3

H2O

MeOH

rt, 10min

50oC, 2h

K

NO2 O2 N N O2 N NN

O2 N O2 N

N

NO2 N

N

O2 N O2 N

H NN

NN H

N

NH

N

N

6

O2 N NH

NO2 NO2

K

NO2

H N N

N

N N NO2 N H NO2 NH2 O2N H3 C N N N N

7

O2 N O2 N

NO2

HN N H2 N

N H

N N N NH2

NN NO2 N NO2 O2 N 9

H N N N O2 N

NO2 NO2

8

Synthesis of energetic cocrystal BTNMBT: 1,2,3triazole-2H (1:2) (6). As shown in Scheme 1, to a solution of anhydrous methanol (10 mL), BTNMBT (1) (0.432 g, 1 mmol) and 1,2,3-triazole-2H (2) (0.138 g, 2 mmol) were added in portions. Then the mixture was heated to 50 oC and reacted for another 2 h. Subsequently, the clear solution was filtered and left to crystallize at ambient temperature. Colorless cocrystals were then obtained, yield of 0.589 g (83%). Td: 118 oC; Tp: 142oC. 1H NMR ([D6]DMSO, 25oC): σ=11.94 (br, s, 4H), 7.98 (s, 4H) ppm; 13C NMR ([D ]DMSO, 25oC): σ=148.89, 148.19, 130.74 ppm. 6 IR: ν=3349(m), 3163(s), 2758 (m), 2625 (m), 1959 (w), 1597 (s), 1519 (w), 1479 (w), 1382 (m), 1284, 1195 (w), 1163 (w), 1078 (m), 923 (m), 796 (m), 673 (w), 623 (m) cm-1. Elemental analysis calcd: C 20.99, H 1.41, N 44.06; found: C 20.93, H 1.43, N 45.93. Synthesis of energetic cocrystal BTNMBT: 1-methyl5-amino-tetrazole-1H (1:1) (7). To a solution of anhydrous methanol (10 mL), BTNMBT (1) (0.432 g, 1 mmol) and 1-methyl-5-amino-tetrazole-1H (3) (0.1 g, 1.1 mmol) were added in portions. Then the mixture was heated to 50 oC and reacted for another 2 h. Subsequently, the clear solution was filtered and left to crystallize at ambient temperature. Colorless cocrystals were then obtained, yield of 0.417 g (78%). Td: 107 oC; Tp: 131oC. 1H NMR ([D6]DMSO, 25oC): σ=9.98 (br, s, 4H), 3.75 (s, 3H) ppm; 13C NMR ([D6]DMSO, 25oC): σ=155.58, 148.89, 148.25, 32.11 ppm. IR: ν=3465 (s), 3344 (s), 3278 (w), 3224 (w), 3170 (w), 3035 (m), 2954 (w), 2883 (w), 2798 (m), 2667 (w), 2069 (w), 1896 (w), 1698 (m), 1651 (m), 1589 (s), 1519 (w), 1490 (w), 1409 (w), 1381 (w), 1340 (m), 1284 (s), 1179 (m), 1075 (m), 949 (s), 848 (m), 794 (m), 724 (w), 667 (w), 621 (m) cm-1. Elemental analysis calcd: C 18.02, H 1.32, N 44.65; found: C 18.38, H 1.34, N 45.29.

Then the mixture was heated to 50 oC and reacted for another 2 h. Subsequently, the clear solution was filtered and dried under nitrogen gas. Yellow-brown crystalline solid was then obtained, yield of 0.482 g (90%). To desolvate, it was kept in the oven under vacuum at 50 oC for 24 h. It should be noted that the compound was extremely unstable when it was desolvated and must be manipulated in ambient temperature avoiding heat. Td: 104 oC; Tp: 119 oC, 138oC. 1H NMR ([D6]DMSO, 25oC): σ=8.45 (br, s, 2H), 7.15 (J1C-H=48 MHz, t, 3H), 6.48 (br, s, 1H), 3.96 (s, 1H) ppm; 13C NMR ([D6]DMSO, 25oC): σ=162.75, 158.31, 153.30, 148.88, 58.00 ppm. IR: ν=3329 (s), 3176 (s), 1698 (m), 1591 (s), 1386 (m), 1277 (m), 1214 (w), 1133 (w), 1093 (w), 984 (w), 845 (w), 794 (m), 719 (w), 623 (w) cm-1. Elemental analysis calcd: C 18.02, H 1.32, N 44.65; found: C 18.05, H 1.29, N 44.72. Synthesis of energetic coordination polymer bipotassium 5,5’-bis(trinitromethyl)-3,3’-bi-1H-1,2,4triazolate (9). As shown in Scheme 1, to a suspension of deionic water (10 mL) BTNMBT (1) (0.432 g, 1 mmol) and potassium hydroxide (5) (0.112 g, 2 mmol) were added in portions. Then the mixture was stirred for another 30 min. Subsequently, the clear solution was filtered and dried under nitrogen gas. Yellow crystalline solid was then obtained, yield of 0.468 g (92%). To desolvate, it was kept in the oven under vacuum at 50 oC for 24 h. Td: 180 oC; Tp: 188oC. 13C {1H} NMR ([D6]DMSO, 25oC): σ=153.41, 151.75, 148.88 ppm. IR: ν=1585 (s), 1375 (m), 1294 (m), 1179 (w), 1121 (w), 1099 (w), 978 (w), 845 (w), 799 (m), 719 (w), 627 (w) cm-1. Elemental analysis calcd: C 14.12, H 0.00, N 32.94; found: C 13.67, H 0.27, N 32.14. RESULTS AND DISCUSSION Computational details. In this work, we used (SMD) M06-2X/6-311++G(2df,2p)///B3LYP/6-31+G(d) for the calculations of pKa values or adopted from the database of IBonD 2.0 (website version), equations (1) and (2) are listed as follows:40-42 ― ― HA(solv) + B(solv) →A(solv) + HB(solv)

∆G(solv)

p𝐾a(HA) = p𝐾a(HB) + 2.303RT

(1) (2)

As shown in the Scheme 2 and Scheme 3, pKa values of anion, cation and coformers were calculated. Scheme 2. Calculated and experimental pKa values of the title coformers in methanol.

Synthesis of energetic ionic salt 3,5-diaminotriazolium-1H 5,5’-bis(trinitromethyl)-3,3’-bi-1H-1,2,4triazolate (8). To a solution of anhydrous methanol (10 mL), BTNMBT (1) (0.432 g, 1 mmol) and 3,5-diamino-1,2,4triazole-1H (4) (0.1 g, 1 mmol) were added in portions.



O 2N N pKa1

N

H N

N H

N

pKa2 N NO2 O 2N

1

Table 1. Crystal data and structure refinement parameters for cocrystals (6, 7), salt 8•CH3OH and ECP 9•H2O. 6

7

8·CH3OH

9·H2O

Empirical formula

C10H4N18O12

C8H7N17O12

C9H11N17O13

C6H2K2N12O13

Mr [g mol-1]

572.34

533.31

565.35

528.40

T [K]

298

298

298

173

Crystal system

Monoclinic

Monoclinic

Triclinic

Monoclinic

Space group

P21/c

P21/c

P-1

C2/c

a [Ǻ]

21.067(7)

9.5546(15)

9.799(4)

22.396(7)

b [Ǻ]

5.977(2)

20.695(3)

10.577(5)

6.231(2)

c [Ǻ]

20.629(6)

11.2367(18)

10.835(5)

14.043(4)

α [o]

90

90

81.381(11)

90

β [o]

116.921(7)

113.633(4)

83.527(11)

114.097(8)

γ [o]

90

90

77.342(11)

90

V [Ǻ3]

2316.1(13)

2035.5(6)

1079.7(8)

1788.9(10)

Z

4

4

2

4

λ [Ǻ]

0.71073

0.71073

0.71073

0.71073

Dc [g cm-3]

1.641

1.740

1.739

1.962

5

μ [mm ]

0.149

0.161

0.160

0.632

~15.6

F (000)

1160

1080

576

1056

Reflns

4074

3657

3802

4412

0.1382

0.0855

0.1196

0.0989

0.2781

0.2060

0.2793

0.2752

R1 (all data) a

0.1967

0.1045

0.1768

0.1545

wR2(all data) b

3075

0.2186

0.3168

0.3288

GOF on F2

1.098

1.091

1.097

1.067

CCDC number

1828751

1828750

1828752

1846361

NH2

H 3C N

N

HN

N

NH

N

NO2

pKa2= 14.9

pKa1= 8.1

pKa(calc.) in MeOH

N

N N

pKa (calc.) in MeOH

Suitable crystals of 6-8 for X-ray single diffraction were obtained from anhydrous methanol and 9 was grown slowly from water. The crystal structures of cocrystals (6, 7), salt (8) and ECP (9) were shown in Figure 2-5 and their selected crystalline parameteres were listed in Table 1.

NO2

O 2N

H 2N

N

2

3

4

3.2

3.5

8.7

NH2

Scheme 3. Calculated pKa values of the title coformers in water. NO2

pKa2

O 2N

H N

N

O 2N N

N H pKa1

N NO2

N

NO2

1 pKa(calc.) in H2O

pKa1= 5.3

O 2N

pKa2= 11.2

KOH

pKa(exp.) in H2O

-1

To make the results approach the experimental environment, we considered the effect of different solvents and calculated pKa values for both acidic hydrogen proton in BTNMBT. Based on the above-mentioned cocrystals and salts, we could use a molecular scale based on pKa values to explain their formation. For these BTNMBT derivatives, the principle is found as follows: i) when pKa value of organic base is far lower than both pKa values of hydrogenprotons in organic acid, the reaction systems prone forming co-crystals; ii) if pKa value of selected organic base is higher than one of hydrogen-protons in organic acid but lower than another one, 1:1 energetic ionic salt appears; iii) the 1:2 type of energetic ionic salt (or coordination polymer) can form when pKa value of corresponding base is higher than both of hydrogenprotons in organic acid. Expect for pKa values, other factors are equally important for the cocrystal formation such as solvent effect, intermolecular interaction and electrostatic potentials.28 Crystal structure

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R1 (I>2σ(I)) a wR2 (I>2σ(I))

a

b

2

𝑅1 = ∑(𝐹0 ― 𝐹𝑐)/∑𝐹0; b 𝑤𝑅2 = [∑𝑤(𝐹20 ― 𝐹2𝑐 ) ―

2 1/2 ∑𝑤(𝐹20) ] .

Cocrystal 6, which crystallizes as colorless blocks in the monoclinic P21/c space group consists of four molecules per unit cell. The calculated crystal density is 1.641 g cm-3 at 25oC. The molecular structure is shown in Figure 2a. It is worthy noting that two 1,2,3-triazole molecules are distributed symmericly in the neighbourhood of trinitromethyl group in cocrystal 6. Due to this special structural characteristic and extensive 2D hydrogen bond networks of cocrystal, its crystal packing assembles into a wave-like stacking which are shown in Figure 2b and 2c. Meanwhile, four


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representative hydrogen bonds are provided in Table 2. The shortest hydrogen • • • acceptor (H • • • A) in this cocrystal is H2 •••N13, and the length is 1.895 Å which is shorter than average hydrogen bond length of 2.05-2.24 Å in nitrogen-rich energetic cocrystals.23 The wave-like stacking can be usually seen in those low-sensitivity and highly energetic materials, in this case, the stability of cocrystal 6 should be also enhanced. It is suggested that these coformers self-assembled into layers enhancing the strength of crystal packing as well as stability of explosive crystal.

Figure 2. Crystal structure in ball-stick diagram (a), crystal packing (b) and wave-like assembly form (c) of energetic cocrystal BTNMBT: 1,2,3-triazole-2H (1:2) (6) (red ellipsoid indicates bis-triazole heterocyclic framework; blue column represents trinitromethyl group). Table 2. Hydrogen bonds present in cocrystal 6. D―H···A

H···A/Å D···A/Å

D-H···A/o

N2―H2···N13

1.89

2.708(9)

158.2

N8―H18···N16

1.92

2.737(9)

157.0

N15―H15···N14

2.28

3.047(11)

149.2

N18―H18···N17

2.31

3.077(12)

148.2

Cocrystal 7, which crystallizes as colorless prisms in the monoclinic P21/c space group consists of four molecules per unit cell. The calculated crystal density is 1.740 g cm-3 at 25oC. The molecular structure is shown in Figure 3a. Different from cocrystal 6, its crystal packing show sandwich-type stacking as demonstrated in Figure 3b and 3c, which would protect the energetic moeities in the interlayer and enhance the molecular stability when encountering external stimuli. The shortest hydrogen••• acceptor (H•••A) in this cocrystal is H8•••N14, and the length is 1.934 Å which is longer than the shortest hydrogen bond length of cocrystal 6. Meanwhile, it is notably that the sandwich-type stacking can establish the “bridge” between layers which intrinsically helps to hold the layers and prevent shear sliding.

Figure 3. Crystal structure in ball-stick diagram (a), crystal packing (b) and sandwich-like assembly form (c) of energetic co-crystal BTNMBT: 1-methyl-5-aminotetrazole-1H (1:1) (7) (red ellipsoid indicates bis-triazole heterocyclic framework; blue column represents trinitromethyl group). Table 3. Hydrogen bonds present in cocrystal 7. D―H···A

H···A/Å

D···A/Å

N2―H2···N15

2.04

2.879(4)


D-H···A/o 163.5


N8―H8···N14

1.94

2.798(4)

N13―H13A···O5

2.68

3.527(9)

N13―H13B···N16

2.28

3.084(5)

172.3 169.3 155.4

Different from those co-crystals, ionic salt 8 crystallizes as pale yellow blocks in triclinic P-1 space group, contains one molecule of methanol, and consists of two molecules per unit cell. The calculated crystal density is 1.739 g c m-3 at 25oC. The molecular structure is shown in Figure 4a. The crystal packing of ionic salt 8 do not pack in the wave-like, face-to-face or sandwich-like packing, therefore we predicted that 8 would be more sensitive material due to its unique pin-like crystal packing (Figure 4b and 4c). As demonstrated in Table 4, though the shortest length of hydrogen•••acceptor (H•••A) in this 1:1 molar ionic salt is H13 • • • N3 is 1.778 Å, others show longer hydrogen bonds in the range of 1.936-2.607 Å. Though hydrogen bonding interaction exists in its crystal paking, the pin-like assembly pattern makes most of nitro groups arrange into a layer, which wrecks its structural and physical stability on the contratry.

Figure 4. Crystal structure in ball-stick diagram (a), crystal packing (b) and pin-like assembly form (c) of energetic ionic salt 3,5-diamino-triazolium-1H 5,5’bis(trinitromethyl)-3,3’-bi-1H-1,2,4-triazolate (1:1) solvate co-crystallized with one molecule methanol (8 • CH3OH)

Page 6 of 18

(red ellipsoid indicates bis-triazole heterocyclic framework; blue column represents trinitromethyl group). Table 4. Hydrogen bonds present in salt 8•CH3OH. D―H···A

d(H···A)/Å

D···A/Å

N4―H4···N14

1.94

2.774(9)

N13―H13···N3

1.78

2.672(8)

N15―H15···N1

2.31

3.045(8)

N15―H15···O7

2.29

2.975(9)

N16―H16A···O5

2.57

3.235(11)

N16―H16A···O7

2.39

3.074(9)

N16―H16B···N2

2.34

3.124(9)

N17―H17A···O13

2.01

2.870(11)

N17―H17B···N6

2.61

3.453(9)

O13―H13···N5

2.12

3.011(9)

D-H···A/o 164.2 172.8 143.7 136.9 135.0 136.5 151.3 174.7 167.9 167.5

Till now, gem-dinitromethyl substituted coordination polymers have been explored widespreadly.36,37,45 However, to the best of our knowledge, trinitromethyl substituted coordination polymers have not been released yet. Usually, trinitromethyl group would lose one nitro group under medium basic condition, which was used to introduce other energetic groups.11 It is interesting that BTNMBT did not lose one nitro group like other similar structures even under strong basic condition of potassium hydroxide. This can be explained that the acidity of NH moieties in bistriazole is strong enough to affect the selectivity of acid-base reaction owing to the electron withdrawing effect of adjacent trinitromethyl groups. It is noteworthy that ECP 9 was afforded at the ambient temperature and pressure. The single-crystal X-ray analysis reveals that ECP 9 • H2O crystallizes in the monoclinic C2/c space group. Though ECP 9 • H2O contains one molecule water, its crystal density is still 1.962 g cm-3 at 173 K and 1.936 g cm-3 at 298 K. As shown in Figure 4b, Each K(I) ion coordinates to four nitrogen atoms from two separated BTNMBT2- anions and six oxygen atoms from mostly nitro groups as well as one water molecule to form ECP 9 • H2O. The bond length of K-N and K-O fall in the range of 2.836-3.388 Å. As demonstrated in Figure 5c and 5d, this coordination environment contributes to a rare-seen “starfish”-like 3D metal-organic framework of ECP 9. These crosslinking networks help to establish strong framework and enhance its polymorph stability as well as thermodynamic stability.


Page 7 of 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Crystal Growth & Design Thermal stabilities were determined by differential scanning calorimetry (DSC) in an Al2O3 crucible at a heating rate of 5 oC min-1 under nitrogen atmosphere. DSC curves are shown in Figure 5 and their values are listed in Table 5. Cocrystal 6, 7 and ionic salt 8 all show lower onset decomposition temperature than that of BTNMBT (Td: 133oC), suggesting neither of these two forms can enhance their thermal stabilities. In contrast with BTNMBT, only ECP 9 indicated an elevated decomposition temperature (Td) at 180 oC. ECP 9 also showed a close Td to that of AP,43 and higher Td than that of ADN.44 As demonstrated in Table 5, due to the high oxygen content of trinitromethyl moiety, ECP 9 shows high oxygen balance at 18.2 %, which is higher than dipotassium 4,4’-bis(dinitromethyl)-3,3’-azofurazanate (K2DNMAF) (10.6 %)45 and dipotassium dinitraminobistetrazolate (K2DNABT) (-4.8 %).46 Because cocrystal 6,7 and ionic slat 8 are all consist of the nonenergetic nitrogen-rich molecules, their oxygen balance decrease in different extent. The sensitivities toward impact and friction for these compounds have been classified in accordance to the “UN Recommendations on the Transport of Dangerous Goods” using the measured values.47 An overview of the sensitivities is given in Table 5. Cocrystals 6 and 7 are insensitive to impact and friction which are comparable to those of BTNMBT, while ionic salt 8 and ECP 9 are sensitive to impact. It is surprised that ionic salt 8 has a lower friction sensitivity (FS: 64 N) than that of ECP 9 (FS: 144 N), which is partially explained by their crystal packing and specific coordination environment. 48-53 The detonation pressure P and velocity D were calculated with EXPLO5 v6.0254 by using the computed values of the heats of formation55,56 and the experimentally measured densities. Due to the 1:2 molar mixture and the introduction of non-energetic methyl substituents, cocrystals 6 and 7 all show comparatively low detonation performance in contrast with BTNMBT. Ionic salt 8 has a high detonation performance (D: 9094 m s-1, P: 36.6 GPa) which is comparable to those of BTNMBT. Incorporation of potassium ion decreases the heat of formation of CP 9 (D: 8872 m s-1, P: 31.1 GPa), which lead to its relatively lower detonation performance than those of BTNMBT, but still higher than those of AP and ADN.

Figure 5. Crystal structure in ball-stick diagram with coordination mode of ligand (hydrogen atoms in water molecules are omitted for clarity) (a), coordination environment around K+ ion (b), 3D crystal packing (c) of bipotassium 5,5’-bis(trinitromethyl)-3,3’-bi-1H-1,2,4triazolate hydrate and (d) “starfish” like-framework diagram (red and blue color stand for frameworks consist of eight atoms assembled from C/N/O/K and yellow color represents bis-triazole organic linker)(9•H2O). Thermal properties and energetic performance



Page 8 of 18

6

7

8

9

BTNMBT

AP

ADN

ρ (g cm-3)[a]

1.64

1.74

1.83

1.98

1.89

1.73

1.81

Ω (%)[b]

-5.6

+1.5

+1.5

+18.2

+18.4

+26

+26

D (m s1)[c]

7903

8384

9094

8872

9073

6368

7860

P (GPa)[d]

24.6

29.1

36.6

31.1

36.2

15.8

23.6

Δ fHs (kJ mol-1)[e]

602.4

450.9

1124.6

-64.2

+331.9

295.8

149.8

Tm (oC)[f]

—

—

—

—

—

—

93

Tdec (oC)[g]

118

107

104

180

133

>200

159

IS (J)[h]

20

25

2

1

22.5

15

3-5

FS (N)[i]

225

240

64

144

252

>360

6472

Isp(s)[j]

240

250

277

226

260

157

202

[a] Measured density at 298 K (for clarity, all the samples were immediately tested after eliminating solvents under vacuum at 25oC for 24 h); [b] Oxygen balance (based on CO); [c] Calculated detonation velocity; [d] Calculated detonation pressure; [e] Heat of formation; [f] Melting point; [g] Onset decomposition temperature (heating rate of 5 oC min-1); [h] Impact sensitivity measured by BAM drop-hammer test; [i] Friction sensitivity measured by a BAM friction tester; [j] Specific impulse.

CONCLUSIONS This study describes how the pKa value influences the formation of energetic cocrystal, salt and coordination polymer based on the same coformer. By investigating the theoretical pKa values of different acid and bases in usage, energetic cocrystal, ionic salt and coordination polymer based on a perchlorate free oxidizer BTNMBT were synthesized through a pKa modulation strategy. Energetic coordination polymer 9 exhibits positive oxygen balance (+18.2%) and high thermal stability (Td: 180oC) comparable to those of AP (OB: +26%, Td>200oC) and superior to that of ADN (OB: +26%, Td: 156oC). Meanwhile, it shows superior detonation performance (D:8872 m s-1) to those of AP (D: 6368 m s-1) and ADN (7860 m s-1). We envisioned that such a pKa incorporation strategy may serve as the underlying principle to develop versatile energetic materials in different shapes.

ASSOCIATED CONTENT

Figure 6. DSC curves of 6, 7, 8 and 9 measured at the heating rate of 5 oC min-1. Table 5. Energetic and physical properties of energetic cocrystals, ionic salt as well as coordination polymer.

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Experimental methods, FT-IR data, NMR data, differential scanning calorimetry data, thermogravimetric analysis data, and crystallographic data for these crystals. Accession Codes. CCDC contain the supplementary crystallographic data for this paper. These data can be


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obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

AUTHOR INFORMATION Corresponding Author

Author Contributions Ma and S.-L. Huang contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We Thank the NSAF Foundation of National Natural Science Foundation of China and China Academy of Engineering Physics (Grant No. U1530262), the Science Challenge Project (Grant No. TZ2018004), the National Natural Science Foundation of China (Grant No. 11402237 and 11302200) and the Science and Technology Development Foundation of China Academy of Engineering Physics (Grant No. 2015B0302055). We also thank Dr. Jin-Dong Yang from Center of Basic Molecular Science (CBMS), Department of Chemistry, Tsinghua University for calculating pKa values of coformers used in this work.

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Page 12 of 18

For Table of Contents Use Only

Manuscript title Energetic cocrystal, ionic salt and coordination polymer of a perchlorate free high energy density oxidizer: influence of pKa modulation on their formation

Author list Qing Ma, Shi-Liang Huang, Huan-Chang Lu, Fude Nie, Long-Yu Liao, Gui-Juan Fan and Jing-Lun Huang

TOC graphic

Synopsis Energetic cocrystal, ionic salt and coordination polymer featuring trinitromethyl moiety based on the same coformer were successfully prepared through a pKa modulation strategy.


12

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Figure 1 266x206mm (150 x 150 DPI)



Figure 2 187x283mm (150 x 150 DPI)


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Figure 3 172x260mm (150 x 150 DPI)



Figure 4 180x268mm (150 x 150 DPI)


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Figure 5 181x394mm (150 x 150 DPI)



Figure 6 311x828mm (150 x 150 DPI)


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Energetic cocrystal, ionic salt and coordination polymer of a

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