Energetic materials containing fluorine. Design, synthesis and testing of furazan-containing energetic materials bearing a pentafluorosulfanyl group

Article abstract of DOI:10.1016/j.jfluchem.2012.03.010

The advantageous impact of a pentafluorosulfanyl substituent on the properties of furazan-containing energetic materials was demonstrated by the synthesis and study of the energetic properties of ten new compounds. The thermal stability of these compounds was evaluated by DSC and TGA, whereas densities, heats of formation, pressures of detonation and speeds of detonation were obtained computationally. On the basis of these data, it was concluded that the combination of the SF₅ substituent with the furazan ring led to materials of higher density and predicted detonation properties than other known furazans or SF₅-containing materials. In addition, the synthetic studies provided insight regarding the electron-withdrawing nature of the furazan ring, in particular its effect on the basicity and nucleophilic reactivity of amino furazans.

Full text of DOI:10.1016/j.jfluchem.2012.03.010

Journal of Fluorine Chemistry 143 (2012) 112–122
Contents lists available at SciVerse ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Energetic materials containing fluorine. Design, synthesis and testing of
furazan-containing energetic materials bearing a pentafluorosulfanyl group
*
Henry Martinez, Zhaoyun Zheng, William R. Dolbier Jr.
Department of Chemistry, University of Florida, PO Box 117200, Gainesville, FL 32611-7200, United States
A R T I C L E I N F O
A B S T R A C T
Article history:
The advantageous impact of a pentafluorosulfanyl substituent on the properties of furazan-containing
energetic materials was demonstrated by the synthesis and study of the energetic properties of ten new
compounds. The thermal stability of these compounds was evaluated by DSC and TGA, whereas densities,
heats of formation, pressures of detonation and speeds of detonation were obtained computationally. On
the basis of these data, it was concluded that the combination of the SF5 substituent with the furazan ring
led to materials of higher density and predicted detonation properties than other known furazans or SF5-
containing materials. In addition, the synthetic studies provided insight regarding the electron-
withdrawing nature of the furazan ring, in particular its effect on the basicity and nucleophilic reactivity
of amino furazans.
Received 16 February 2012
Received in revised form 7 March 2012
Accepted 8 March 2012
Available online 17 March 2012
Keywords:
Pentafluorosulfanyl group
Furazan
Energetic materials
Basicity
ß 2012 Elsevier B.V. All rights reserved.
Nucleophilicity
1. Introduction
is released, the pressure and speed of detonation, and the thermal
and chemical stability of the material. The carbon–oxygen balance
Energetic materials can be considered compounds or mixtures
of compounds that constitute a source of massive controllable
energy. Interest in the design of new materials with higher energy
content, better performance, lower cost, less sensitivity to impact
and with less danger to synthesize and process has allowed this
research area to remain active. Also, continued concerns related to
the potential use of chemical and biological weapons (CW/BW) has
necessitated the development of new mechanisms of defense [1].
The synthesis of new high energy materials has been focused on
use of heterocyclic compounds with high nitrogen content such as
tetrazines, furazans, triazoles and tetrazoles due to the relatively
(OB) and the density are directly related to a compound’s
performance. According to the semi-empirical equations devel-
oped by Kamlet and Jacobs, the square of the density is directly
proportional to the performance of the compound (Eqs. (1)–(3))
[3].
P
¼ 15:58r2f
(1)
ðCJÞ
D ¼ Af1=2ð1 þ B
r
Þ
(2)
(3)
f
¼ NM1=2Q1=2
positive heats of formation (DHf8) of these compounds [2].
Recently, the introduction of fluorine into such compounds has
resulted in a boost in their performance, mainly due to the increase
in their density and their thermal and chemical stability.
By definition, an energetic material is a compound or a mixture
of compounds that, after an initiation process, undergoes very
rapid self-propagating decomposition, producing gases at tremen-
dous pressure and with the evolution of a lot of heat. Temperatures
ideally can reach up to 6000 K and the pressure up to 40 GP [2].
The performance of an energetic material is mainly evaluated
on the basis of the type of products that are formed, the energy that
P is the detonation pressure (GPa), D is the speed of detonation (m/
s), A and B are constants, N is the number of moles of gaseous
products of detonation per gram of explosive, M the average
molecular weight of the gases, Q is the heat of detonation in
calories per gram of explosive and
r is density. The higher the
density is, the better the performance will be. Although the
increase of oxygen balance results in a more sensitive material, it
also increases the performance due to the ‘‘complete’’ oxidation of
all carbons and hydrogens. Most energetic materials, however, are
‘‘oxygen-deficient’’ [4].
In evaluating new energetic materials, they are generally
compared to three of the most important known energetic
materials: cyclotrimethylenetrinitramine (RDX), cyclotetra-
methylene-tetranitramine (HMX) and trinitrotoluene (TNT).
* Corresponding author.
E-mail address: wrd@chem.ufl.edu (W.R. Dolbier Jr.).
0022-1139/$ – see front matter ß 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2012.03.010

H. Martinez et al. / Journal of Fluorine Chemistry 143 (2012) 112–122
113
Table 1
formation of COS, inhibiting the formation of COF2, which is
considered a waste of fluorine [9].
Performance data of three common energetic materials.
d (g/cm3)a
P (Gpa)b
vD (m/s)c
1.1. Furazan-containing energetic materials
RDX
HMX
TNT
1.81
1.91
1.65
33.8
39.3
20.0
8750
9100
6900
1,2,5-Oxadiazoles (commonly known as furazans) have been
included in several publications as great building blocks for the
generation of insensitive High Performance Energetic Materials
(HEM) [4,10–19]. The aromaticity present in the ring increases the
thermal stability, while the planarity increases the density. That
a
Density.
b
c
Detonation pressure.
Velocity of detonation.
combined with their positive heat of formation (DHf8) makes
furazans an excellent building block for the generation of HEM
[10].
SF5
NO2
CF3
Since the first synthesis of 3,4-diaminofurazan 1 by Coburn in
1968 [20], a vast number of furazan derivatives with good densities
and energetic properties have been prepared.
Studies on the thermal decomposition of amino furazan
derivatives at different temperatures have shown that the major
products are CO, CO2, H2O, HCN, N2 (when the azo or hydrazine
moieties are present) and NO2 (when present as a nitro group in
the molecule). Minor products include HCNO, NH3, HNO3 and N2O.
The same studies indicate that most aminofurazans have a higher
thermal stability than HMX or RDX [19,21,22].
1.19 g/ml
1.20 g/ml
1.49 g/ml
N
N
N
N
N
R
N
R
OH
Alternatively, the high nitrogen content and positive
tetrazole also makes this heterocycle an excellent building block
for the synthesis of HEM. Although tetrazole has a positive Hf8, it
DHf8 of
R =
CF3 - 1.69 g/mL
SF5 - 1.90 g/mL
D
exhibits good thermal stability [23–25]. Theoretical and experi-
mental studies have shown that the thermal decomposition of 5-
aminotetrazole goes through two different retro [3+2] mecha-
nisms with relatively close activation energies. The first mecha-
nism and most favorable generates NH2CN and HN3, while the
second one produces CH3N3 and N2. However, when the amino
group of the aminotetrazole has been functionalized, the second
mechanism is the most favorable [23–27].
Fig. 1. Higher density of SF5 compounds.
The performances of these three materials are given in Table 1 [1].
The development of new energetic materials with better
performance than RDX, HMX or TNT constitutes an important
area of current active research.
In order to achieve better performance than RDX or HMX, new
energetic materials have often incorporated fluorine into the
system [1,3,4], and during recent decades there has been increased
interest regarding the use of the SF5 group in energetic materials
[4–9]. Incorporation of the SF5 group gives rise to compounds of
higher density (Fig. 1), and the high fluorine content along with the
presence of hydrogen leads to the formation of hydrogen fluoride
(HF) upon detonation, generating a large amount of energy. The S–
F Bond Dissociation Energy (BDE) is 79 kcal/mol, while the BDE of
H–F is 136 kcal/mol [5,6,8,9].
The possibility of a higher density, larger energy release, and
better thermal and chemical stability without increasing the
sensitivity make the SF5 group attractive in the synthesis of high
energy materials (HEM).
Fig. 2 provides an example of a polynitro-SF5 energetic material,
which upon detonation produces HF as the only product contain-
ing fluorine, while the presence of sulfur (S) leads into the
A combination of tetrazole and furazan rings has been
previously reported [28], and recently Shreeve et al. reported
furazan tetrazolate-based salts as highly insensitive energetic
materials [29].
A number of SF5-containing energetic materials with predicted
performance close to those for HMX, RDX and TNT, and having the
benefit of lesser or no impact sensitivity, have been previously
synthesized [5,30]. Fig. 3 shows five such SF5-containing energetic
materials that have densities ꢀ1.85 g/cmꢁ3, and which have
predicted pressures and velocities of detonation of approximately
17.8–20.5 GPa and 6900–7100 m/s, respectively.
Despite all the potential benefits provided by the SF5 moiety,
the number of SF5-containing energetic materials has been limited
by the few available sources of the aliphatic SF5 group. SF5Br and
SF5Cl are the main ultimate building blocks for preparation of
aliphatic SF5 compounds, and the chemistry of these two species is
limited to free radical chemistry. For this reason, the synthesis of
F
O2N
NO2
O
NO2
NO2
O
O
F
O
SF5
O2N
NO2
F
28 HF + 7 CO + 22 CO2 + 2.5 H2O +10.5 N2 +2.5 COS + H2S
3.5 FDEFTEC
Fig. 2. Polynitro-SF5 energetic material.

114
H. Martinez et al. / Journal of Fluorine Chemistry 143 (2012) 112–122
conditions. Scheme 1 shows the synthetic pathway that was
utilized in this work for the synthesis of some common
aminofurazan building blocks [10,14–17].
The furazan ring possesses an electron withdrawing ability that
makes the amino group(s) in aminofurazans a challenge to
derivatize [31,32]. Nevertheless most of the productive synthetic
chemistry involving aminofurazans involves nucleophilic attack
and condensation reactions of the amino groups or oxidative
reactions of the amino groups. Ring opening reactions are also
common, but of course such reactions destroy the energetic
properties of the material [31].
2.2. The SF5-acetyl building block
In order to achieve an efficient and in principle ‘‘easy’’
incorporation of the SF5 group into aminofurazans, methyl 2-
pentafluorosulfanylacetate 5 was chosen as a potentially useful
SF5-containing building block, and it was prepared by the method
that was developed in our lab in 2006 (Scheme 2) [33].
3,4-Diaminofurazan 1 was chosen as our initial starting furazan
substrate, and it was used to establish conditions for preparation of
SF5-acetamidofurazan derivatives. Various reaction conditions
were attempted, including refluxing the two reactants in toluene
or methanol, and using methoxide or NaH to promote the
substitution, but all attempts proved unsuccessful in obtaining
the desired product from ester 5 (Scheme 3).
Fig. 3. Examples of SF5-containing energetic materials.
organic SF5 building blocks that would allow the ‘‘easy’’ incorpo-
ration of this group into HEM remains a significant challenge.
In the current paper we will be reporting the synthesis of
several SF5-building blocks and evaluation of their potential utility
in the synthesis of some new high-density SF5-furazan-based
energetic materials with relatively low hydrogen content, and
which therefore have the potential to release ‘‘free fluorine’’ upon
detonation. The thermal and chemical stability of these new
energetic materials will be evaluated and insight provided by
Cheetah calculations regarding their possible energetic perfor-
mance.
Aminofurazan
1 was therefore, in practice, insufficiently
nucleophilic to replace the methoxy group of 5. Known pKa values
of various substituted aminofurazans, which are given in Table 2
[32], are consistent with this conclusion. Protonated diaminofur-
azan 1 is only slightly less acidic than protonated methanol
(CH3OH2+) (pKa’s of ꢁ1.94 and ꢁ2.2, respectively), which means
that the amino groups of 1 are only slightly more electron donating
than the oxygen of methanol. A SciFinder search revealed that the
more basic/nucleophilic p-nitroaniline (pKa ꢀ +1) is also not
2. Results and discussion
2.1. Synthesis of SF5-furazan-based energetic materials
The synthesis of aminofurazans generally starts with the
synthesis of aminoglyoximes, followed by dehydration under basic
Scheme 1. Synthesis of common furazan building blocks.

H. Martinez et al. / Journal of Fluorine Chemistry 143 (2012) 112–122
115
F5S
O
F5S
OMe
OMe
1.SF5Cl - Et3B, -40oC
2.MeOH / 50oC
Caro's acid
MeOH
OMe
71%
OAc
92.7%
5
Scheme 2. Synthesis of methyl 2-pentafluorosulfanylacetate 5.
Scheme 3. Attempted reactions between diaminofurazan 1 and SF5-ester 5.
Scheme 6. Synthesis and use of the benzotriazole derivative of SF5-acetic acid.
Scheme 4. Synthesis of pentafluorosulfanylacetyl chloride (7).
converted to its acetamide derivative by reaction with esters, but
requires the more reactive acetyl chloride or acetic anhydride.
Indeed, most previous amide-forming reactions with 1 have
required acyl chlorides as acylating agents [34]. Thus 5 was
converted via the carboxylic acid (6), into its respective acid
chloride, pentafluorosulfanylacetyl chloride (7), the latter step of
Scheme 4 using methodology virtually identical to that reported by
Sitzmann for the preparation of SF5CF2COCl [35]. In contrast to
attempts to use SOCl2 or oxalyl chloride in this preparation, the use
of benzoyl chloride allowed easy isolation of almost pure 7 via its
distillation from the reaction mixture into an ice-cooled trap.
Pentafluorosulfanylacetyl chloride 7 proved sufficiently reac-
tive with diaminofurazan 1 to allow formation of a mixture of the
mono and bis-amide products, 8 and 9, as shown in Scheme 5.
Nevertheless, the low yield for preparation of acyl chloride 7
encouraged us to seek a more efficient method for this conversion.
The reaction between amines and N-acylbenzotriazoles has
been widely used for the synthesis of amides [36], with the
reaction often providing better yields and cleaner reactions than
could be obtained from the reactions of amines and acyl chlorides.
Acylbenzotriazole 10 was readily prepared, in higher yield than the
acid chloride, but unfortunately its reaction with diaminofurazan 1
(Scheme 6) proved not nearly as efficient as that of the acid
chloride, so there was no overall advantage to using the
acylbenzotriazole method.
Fig. 4. Other synthesized, but unreactive aminofurazans.
carbodiimide (EDC). As can be seen from the second sequence
given in Scheme 7, both the mono- or the bis-amide were able to be
obtained relatively free of the other by control of the reaction
stoichiometry.
This carbodiimide methodology was then used to prepare four
additional pentafluorosulfamylacetamide HEMs using three ami-
nofurazans and aminotetrazole (Scheme 8).
Product 14 was contaminated by 5% of the isomeric product
with the amide derived from the other amino group. Neither 12 nor
14 could be converted to their bis-acetamides. Two other
aminofurazans that we had prepared, 3-amino-4-nitrofurazan
(2) and azoxy-bis-aminofurazan (3) were not sufficiently reactive
to form SF5-acetamides under these conditions. The former was
not surprising considering its reported pKa value of ꢁ4.46 (Table 2).
Two other aminofurazans compounds, the azo and hydrazino-bis-
aminofurazanes, 18 and 19, which had been prepared from azoxy
precursor 3 by literature methods [16], were also unreactive under
the EDC reaction conditions (Fig. 4).
During the process of making new SF5-energetic materials, it
was noted that the synthesis of 4-(1H-tetrazol-5-yl)-3-amine-N-
(2,4,6-trinitrophenyl)furazan (20) had not been reported. Due to
the high energy that the three rings present on this compound, it
was expected that this material should have good energetic
properties. Therefore 20 was prepared as shown in Scheme 9.
Ultimately, the process that proved most efficient for prepara-
tion of both the mono- and the bis-amide from acid 6 was its direct
condensation with the diaminofurazan in the presence of a
carbodiimide [37]. Various carbodiimides were tried, with the best
results being obtained with 1-ethyl-3-(3-dimethylaminopropyl)
Scheme 5. First synthesis of pentafluorosulfanylacetamidofurazans.

116
H. Martinez et al. / Journal of Fluorine Chemistry 143 (2012) 112–122
SF5
F5S
F5S
H2N
NH2
NH
N
NH2
NH HN
F5S
O
EDC
O
O
O
N
N
N
N
N
THF, DMAP
O
OH
O
O
9
8
6
1
13%
52%
2.2 equiv
F5S
SF5
F5S
NH
N
NH2
NH HN
H2N
NH2
F5S
O
EDC
O
O
O
EDC
N
N
N
N
N
THF, DMAP
70%
THF, DMAP
>90%
O
O
O
OH
8
9
6
1
0.67 equiv
Scheme 7. Synthesis of mono and bis-amides from SF5-acetic acid using EDC.
O
O
N
N
N
N
N
N
N
SF5
H2N
NH2
H2N
HN
N
F5S
F5S
O
EDC
O
THF, DMAP
N
N
N
N
OH
O
O
O
O
O
O
11
12
45%
O
O
O
O
N
N
N
N
N
N
H
N
H2N
NH2
H2N
O
SF5
EDC
O
THF, DMAP
N
N
N
N
N
N
OH
O
O
14
35%
13
N
N
N
N
N
N
H
HN
NH2
SF5
F5S
O
HN
N
EDC
O
THF, DMAP
N
N
4
OH
N
N
O
O
15 35%
F5S
O
N
N
EDC
N
N
N
N
NH2
NH SF5
O
THF, DMAP
OH
N
H
N
H
16
17
80%
Scheme 8. Reaction between various amines and SF5-acetic acid in the presence of EDC.
2.3. The pentafluorosulfanyl isocyanate building block
The high reactivity of SF5NCO toward nucleophiles makes it
potentially a great building block. However, it is this same high
reactivity that limits the possible solvents in which it can be used. A
solvent screen carried out with SF5NCO indicated that it reacts with
acetone rapidly and slowly with THF and acetonitrile. Although the
isocyanate is unreactive with solvents such as dichloromethane and
1,2-dichloroethane, unfortunately most of the furazan substrates
had virtually no solubility in these non-polar solvents.
The high reactivity of SF5–N55C55O toward several nucleophiles
has been already carefully studied [38,39], including its addition to
polynitro alcohols to yield the respective carbamates [7,40].
Scheme 10 shows the synthesis of bis-SF5-carbamate 21, which
was previously prepared by Sitzmann [40], and which will be used
as a ‘‘baseline’’ material in evaluating our new compounds.
Scheme 9. 4-(1H-tetrazol-5-yl)-3-amine-N-(2,4,6-trinitrophenyl)furazan (20).

H. Martinez et al. / Journal of Fluorine Chemistry 143 (2012) 112–122
117
Scheme 10. Synthesis of a bis-SF5-carbamate energetic material.
Scheme 13. Attempted oxidation of amino urea 22.
HN
O
reactions conditions, and the product was amino nitro furazan 2
(Scheme 13).
SF5
H2N
NH2
H2N
HN
N
SF5NCO
All synthesized materials exhibited good thermal and chemical
stability, with the exception of 3-N-pentafluorosulfanylacetamide-
4-(1H-tetrazol-5-yl)-furazan 15, which was stable in the solid
state, but decomposed rapidly in solution. This is probably due to
the enhancement of the electron-withdrawing ability of the
furazan by the presence of the highly electron-withdrawing
tetrazole group, which makes the carbonyl group of 15 more
electron deficient and thus more susceptible to nucleophilic attack.
N
N
N
CH2Cl2, RT
99%
O
O
22
1
HN
O
SF5
O2N
NH2
O2N
HN
SF5NCO
N
N
N
N
20% + 80%
CH2Cl2. r.t.
O
O
2
23
2.5. Reactivity of aminofurazans
Scheme 11. Reactions of SF5NCO with aminofurazans 1 and 2.
There is little significant information related to the reactivity of
aminofurazans other than the few reported pKa values of some
derivatives (Table 2). In order to have a better understanding of the
lack of reactivity of other aminofurazan derivatives that were
prepared and used in this project, the ground states of some of
these materials were calculated at B3LYP/6ꢁ31+G(d,p) level using
Gaussian 03 Rev. E.01 [43], and the calculated C–NH2 bond lengths
were correlated with the pKa values reported by Tselinskii (Table 2)
[32].
Due to this solvent limitation, only two urethane compounds
were able to be prepared via the reaction of SF5NCO with furazan
derivatives, with the latter one (23) only able to be obtained as a
mixture with starting furazan. Obviously, this was very disap-
pointing. Any attempt to react the remaining NH2 group of 22 with
a second equivalent of SF5NCO was prevented by the insolubility of
22 (Scheme 11).
The equation derived from the plot of was found to be
y = 71.67x ꢁ 101.11, with a correlation coefficient of R2 = 0.9914.
This correlation coefficient indicates that there is strong correla-
tion between these two parameters. Using the equation from the
plot in Table 3 and the calculated C–NH2 bond distances from the
ground states of some aminofurazans used in this project, the pKa
values (NH3+) were estimated in order to understand and establish
an order of reactivity (Table 4). Combining our observed
experimental results with the calculated pKa values, it can be
2.4. Attempts to carry out N-nitration of acetamides
Although all of the newly synthesized SF5-furazan-based
materials should themselves have energetic properties, an increase
in C/O balance through incorporation of nitro groups should
improve their energetic performance. However, using bis-acet-
amide 9 as test substrate, all attempts to accomplish N-nitration of
acetamides failed. Three mild nitration conditions were used: (1)
H2SO4 (98%)–HNO3 (70%), (2) NH4NO3–CF3CO3H–HNO3 (70%) cat.,
and (3) NO2BF4–CH3CN or THF. In all of these cases, starting
materials were recovered. Stronger conditions for nitration were
attempted: (1) HNO3 (>99%), (2) NO2NO3–HNO3 (>99%), (3) TFAA–
HNO3 (>99%), and (4) NO2BF4–NaH or Py–THF. However, under
these conditions, the starting materials were either partially or
completely destroyed.
Table 2
R
NH3
pKa values of some aminofurazan derivatives.
N
N
O
R
pKa
Another approach to adding additional oxygen to the synthe-
sized SF5-furazan materials could be through the oxidation of
residual amino groups to nitro groups. Indeed, the oxidation of
mono acetamide 8 was successful using three different methods:
(1) H2O2 (50%)–H2SO4 (98%)–Na2WO4 in 50% yield; (2) HOFꢂCH3CN
[41,42] in 65% yield; (3) H2O2 (50%)–H2SO4 (98%) in 30% yield
(Scheme 12).
The oxidation of SF5-urea-amino-furazan 22 was attempted
using these same three methods, plus a fourth method using
Ph3PCH2Ph+H2SO5ꢁ–CH3CN. In all cases, the amino group
had been oxidized, but the urea had proved unstable to the
NH2
CH3
OCH3
N3
ꢁ1.94
ꢁ2.15
ꢁ2.51
ꢁ2.88
ꢁ4.46
NO2
Table 3
Furazan pKa vs. C–NH2 bond distance.
˚
R
pKa
C–NH2 (A)
NH2
CH3
OCH3
N3
ꢁ1.94
ꢁ2.15
ꢁ2.51
ꢁ2.88
ꢁ4.46
1.38516
1.38093
1.37397
1.36974
1.34938
NO2
Scheme 12. Oxidation of amino group of 8.

118
H. Martinez et al. / Journal of Fluorine Chemistry 143 (2012) 112–122
Table 4
moiety should be more reactive than the –NH2 group, but if it
reacts with the SF5-acetyl building block, the product must be
unstable, perhaps due to the adjacent NH group.
Calculated pKa values using the plot from Table 3.
˚
R
C–NH2 (A)
pKa
1
2
1.38516
1.34938
ꢁ1.94a
ꢁ4.46a
2.6. Properties of the synthesized SF5-furazan-based energetic
materials
3b
1.35792c
1.35014d
ꢁ3.77c
ꢁ4.33d
For the convenience of the reader, Fig. 5 summarizes the ten
new energetic materials that were prepared within this project.
These materials were characterized by three methods: shock
sensitivity, thermal stability, and performance as determined by
Cheetah calculations.
4
1.35872
1.3625
ꢁ3.73
ꢁ3.45
ꢁ3.50c
ꢁ2.95d
ꢁ4.31
11
13b
1.36186c
1.36957d
1.35046
18
The impact sensitivity was evaluated qualitatively by placing
5–10 mg of sample in a flat polished area, and hitting the sample
with a flat-head hammer. With the exception of compound 20, all
of the synthesized materials exhibited no sensitivity to impact
under the conditions of this test. Compound 20 showed partial
decomposition. By comparison, when nitroguanidine was tested in
the identical manner, detonation occurred. Although there is a
more technical procedure to evaluate the impact sensitivity, this
particular test serves to exemplify the intrinsic ability of the SF5
group to reduce the impact sensitivity of high energy materials.
In order to evaluate the thermal stability of the new materials,
thermogravimetrical analysis (TGA) and Differential Scanning
Calorimetry (DSC) were utilized for all compounds in Fig. 5 using a
TA instrument SDT Q600, TGA/DSC combo instrument, which uses
a ventilated system (open-pan). An example of a DSC/TGA
thermogram is shown is Fig. 6, with all other DSC/TGA thermo-
grams being given in Supporting information. Detailed information
for the thermograms is presented in Table 5.
19b
1.37266
1.39755
ꢁ2.71 (NH2)
ꢁ0.94 (NH)
a
Exp. value Ref. [32].
b
c
Unsymmetrical amino groups, two values are calculated.
NH2 group far from the 5N–Oꢁ moiety.
NH2 group closer from the 5N–Oꢁ moiety.
d
+
seen that for any furazan–NH3 that has a pKa greater or equal to
ꢁ3.72, that amino furazan will form a SF5-acetamide in the EDC
reaction. This is the case for the 3-amino-4-(tetrazo-5-yl-furazan)
(4), which gives a product that is not stable in solution. The high
electron-withdrawing ability of the tetrazole added to that innate
in the furazan ring pushes the amino group to the limit of its
nucleophilic reactivity.The azoxy-bis-aminofurazan 3 did not yield
any product, and it has calculated pKa values of ꢁ3.77 and ꢁ4.33
for its unsymmetrical amino groups. There is a possibility that a
product is formed, but that it is unstable under the reaction
conditions.Interestingly, hydrazino-bis-aminofurazan 19 has cal-
culated pKa values for its two types of nitrogens of greater than
ꢁ3.72 (ꢁ2.71 and ꢁ0.93) but did not give products using either the
SF5-acetyl chloride or the SF5-acetic acid–EDC method. Based on
the calculated C–N distances (Table 4), the NH from the hydrazine
Fig. 6 shows in green the loss of weight vs. the temperature.
Compound 28 has a 5% weight loss at 243.73 8C which indicates
high thermal stability. At 293 8C it has lost almost 80% of its weight
and remains the same even at temperature up to 1000 8C. This
indicates that there is a remaining mass that did not convert into
O
N
N
N
N
F5S
SF5
F5S
F5S
NH
N
NH2
NH
N
NH2
NH HN
O
O
O
O
N
N
N
N
O
N
O
O
O
12
9
8
N
O
O
SF5
O
N
N
N
NH
N
N
N
N
F5S
F5S
NH
N
N
N
NH
NH2
NH
O
O
N
H
N
N
O
O
O
O
17
15
14
NO2
NH
H
H
N
N
F5S
N
F5S
O2N
N
N
N
F5S
NH
NO2
NH
NH2
NH
NO2
NH
O
O
O
N
N
N
N
N
N
NO2
O
N
O
O
24
20
22
23
NO2
O2N
H
H
N
N
O
O
F5S
SF5
O
O
21
Baseline material
Fig. 5. New energetic materials prepared in this work.

H. Martinez et al. / Journal of Fluorine Chemistry 143 (2012) 112–122
119
Fig. 6. DSC/TGA thermogram of compound 9.
gas. This is probably due to the lack of oxygen in the molecule. This
problem is usually addressed in the final explosive material by
mixing the desired compound with some oxidizers, which is
essentially a material that will provide ‘‘oxygen’’ during the
combustion process. The blue line shows the heat flow with an
exotherm (energy release) at the maximum point of 282.43 8C with
an energy release of 247.2 J/g, indicating that this and the other
derivatives are promising as energetic materials.
trend, since diaminofurazan (1) starts to decompose at 200 8C,
whereas its mono-SF5-acetamide (8) begins decomposition at
169.01 8C.
Compound 20 exhibits three stages of decomposition. This is
characteristic of compounds that contain the tetrazole group. The
retro-1,3-dipolar cycloaddition at the tetrazole group releases
nitrogen at the first stage followed by a subsequent unknown
decomposition process.
Compound 21 was taken as a baseline material. This compound
has been previously synthesized [40], and it exhibited good
thermal stability. When compared to 21, all of our new energetic
materials exhibited from good (>100 8C) to excellent (>200 8C)
thermal stability. Between compounds 8 and 9, there is a 65 8C
difference in the starting decomposition temperature, where the
bis-acetamide 9 is more stable than the mono-acetamide 8. This is
a good indication that the addition of an SF5-acetyl group increases
the thermal stability of the energetic materials. Another example
of this behavior is seen in compound 15. Even though its starting
decomposition temperature is the second lowest value (100.45 8C),
its starting material, the 3-amino-4-(tetrazol-5-yl)furazan, starts
to decompose at 45.76 8C. However there were exceptions to this
Our baseline material exhibited an endotherm at 160.56 8C,
even though it was expected to exhibit an exotherm instead. The
open-pan system used for this thermogram might not capture the
energy liberated when gases are released rapidly. Four other
materials also exhibited endotherms, whereas the other six
energetic materials exhibited exotherms. The exotherms ranged
up as high as ꢁ1758 J/g.
In addition to considering its impact sensitivity and thermal
stability, the most important way to evaluate these materials is
through their density (d), detonation pressure (P) and velocity of
detonation (vD). The heats of formation (HOF) and densities were
calculated by using the group additive method,[44,45] while the
pressure and velocity of detonation, P and vD respectively, were
calculated using Cheetah software (6.0) at the Lawrence Livermore
National Laboratory (Table 6).
Table 5
It is essential to remember that the density is one of the most
important properties according to the semi-empirical equations
suggested by Kamlet and Jacobs [3,46]. Only two compounds, 12
and 17, did not converge in the calculations when using the
Cheetah software. Because the way SF5 affects energetic
materials upon detonation is still not fully understood, it is
not certain why these two molecules did not converge during
the calculations.
All of the synthesized materials have from good to excellent
densities, and when compared with the baseline material all but
three have a greater density, and one of those does not contain the
furazan group (17), while another does not contain an SF5 group
(20). This is consistent with the fact that a combination of these
two groups has good potential to enhance the properties of
energetic materials. In spite of having a positive HOF, compound 20
does not have the SF5 group, which results in a relatively low
density and thus relatively poor performance.
Thermogram data of the synthesized energetic materials.
Compound
TGA temp. of decomp. (8C)
DSC (8C, J/g)
8
169.01–245.95
149.79, endo, 76.38
225.32, endo, 67.15
9
234.73–293.09
172.06–233.88
134.10–410.88
214.81–382.10
100.45–191.88
155.45–300.00
149.61–200.05
099.66–161.50
282.43, exo, ꢁ247.20
24
12
14
15
17
22
23
[161.98, 232.31], endo, 412.10
286.85, exo, ꢁ1023
[268.60, 325.27], exo, ꢁ1758
175.55, exo, ꢁ215.7
198.23, exo, ꢁ340.9
[159.28, 196.09], endo, 241.2
[114.46, 146.71], endo, 2481.0
20
141.00–201.46
267.78–300.00
732.75–971.64
172.58, endo, 55.52
294.32, exo, ꢁ708.4
895.99, endo, 2529
21
158.22–204.29
160.56, endo, 128.4

120
H. Martinez et al. / Journal of Fluorine Chemistry 143 (2012) 112–122
Table 6
chemical ionization (CI). Thermal analyses were taken using a
TA instrument SDT Q600, TGA/DSC combo instrument.
The SF5NCO was provided by Dr. Joseph Mannion of National
Naval Laboratories at Indian Head, whereas SF5Cl was donated by
Air Products for this project.
Calculated performance data of the synthesized energetic materials.a.
Compound
d (g/cm3)
HOF (kcal/mol)
Pb (Gpa)
vDc (m/s)
8
9
2.01
2.05
2.02
2.01
2.03
1.81
1.96
2.11
2.10
1.54
1.99
ꢁ239.05
ꢁ533.70
ꢁ251.56
ꢁ109.65
ꢁ109.17
ꢁ169.13
ꢁ174.52
ꢁ196.85
ꢁ209.36
147.35
20.22
–
6741
–
24
12
14
15
17
22
23
20
21
29.48
26.12
28.62
17.85
–
7234
7368
7430
7002
–
4.2. General method for the syntheses of
pentafluorosulfanylacetamide-furazans
To a solution of aminofurazan (7.2 mmol, 1.5 equiv.) and 1-
ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) (9.6 mmol,
2.0 equiv.) in anhydrous THF (30 mL), was added pentafluorosul-
fanylacetic acid (6) (4.8 mmol, 1 equiv.) in anhydrous THF (15 mL),
dropwise with stirring using a pressure equalizing dropping
funnel. After the addition of the acid, which took about 10 min, 4-
dimethylaminopyridine (DMAP) (0.48 mmol, 0.1 equiv.) was
added, and the stirring was continued for 16 h. The reaction
mixture was then treated with 50 mL of water, extracted with
ethyl acetate (3ꢃ 30 mL), and the organic layer dried over Na2SO4,
filtered, and the solvent removed under reduced pressure. The
resulting solid (or oil) was purified by column chromatography
(silica gel, 4:1 CH2Cl2:ethyl acetate).
27.16
30.67
18.25
23.73
8049
6862
6788
6263
ꢁ623.00
a
Calculated using Cheetah software v6.
Calculated detonation pressure.
Calculated velocity of detonation.
b
c
Pressure and speed of detonation for most of our materials are
better than TNT, but below RDX and HMX. When compared to the
chosen baseline material (21), most of the compounds exhibit
better performance, which indicates once more that SF5 and
furazan groups combine to give good energetic properties. The SF5-
containing furazan compounds reported in the current paper
exhibited lower heats of formation, but higher densities than the
earlier-prepared SF5-containing 1,2-diazoles and 1,2,3-triazoles
reported by Shreeve [5,30]. When our materials are compared to
these diazole and triazole materials, they performed considerably
better with respect to pressure and velocity of detonation, as
determined by Cheetah calculations.
4.2.1. 3-Amino-4-N-(2-pentafluorosulfanylacetamide)-1,2,5-
oxadiazole (8)
White solid, 90% yield; mp (DSC) = 133.5 8C. 1H NMR (DMSO-d6),
d
4.79 (q, J = 9.0 Hz, 2H, CH2), 6.00 (b, 2H, NH2), 11.32 (b, 1H, NH); 19
F
NMR (DMSO-d6),
NMR (DMSO-d6),
dA 83.60 (m, 1F),
d C
B 71.30 (d, JAB = 149 Hz, 4F); 13
d
72.2 (m, CH2), 144.0 (s, C–NH), 156.2 (s, C–NH2),
160.0 (m, CO); HRMS (MꢁH, calc): 266.9981, found: 266.9992.
3. Conclusions
4.2.2. 3,4-N,N0-bis-(2-pentafluorosulfanylacetamide)-1,2,5-
oxadiazole (9)
The goal of this work was to combine the beneficial high density
and resulting high energy properties of the furazan ring with those
of the pentafluorosulfanyl (SF5) group to make new high energy
materials. To that end, ten new potentially high energy materials
have been synthesized and characterized, all having higher density
and predicted detonation properties than other known furazans or
SF5-containing materials. As a result of this work, insights were
also gained regarding the fundamental chemistry of furazans, as
well as the chemistry of SF5-containing building blocks. Amino-
furazans are inherently of low basicity and thus of low
nucleophilicity, which makes functionalization of aminofurazans
very challenging. On the other hand, because of the electron
withdrawing power of the SF5 group, those derivatives of
aminofurazans that could be prepared were relatively unstable
and susceptible to destruction by nucleophilic attack, a fact that,
combined with the low nucleophilicity of the aminofurazans, made
them particularly challenging to prepare.
White solid 70% yield; mp (DSC, decomp.) = 234.7 8C; 1H NMR
(DMSO-d6),
NMR (DMSO-d6),
NMR (DMSO-d6),
d
4.82 (q, J = 9.0 Hz, 4H, CH2), 11.42 (br, 2H, NH); 19F
d
A 83.57 (m, 1F),
71.8 (m, CH2), 146.4 (s, C–NH), 160.0 (m, CO);
d C
B 71.49 (d, JAB = 149 Hz, 4F); 13
d
HRMS (M+H, calc): 436.9794, found: 436.9796.
4.2.3. 4-Amino-400-N-(2-pentafluorosulfanylacetamide)-[3,30:40,300]-
Ter-[1,2,5]-oxadiazole (12)
Light yellow solid, 45% yield; mp (DSC) = 133.3 8C; 1H NMR
(DMSO-d6),
d 4.72 (q, J = 9.0 Hz, 2H, CH2), 6.66 (br, 2H, NH2), 12.44
(br, 1H, NH); 19F NMR (DMSO-d6), dA 83.53 (m, 1F), dB 71.53 (d,
JAB = 149 Hz, 4F); HRMS (M+H, calc): 405.0153, found: 405.0161.
4.2.4. 4-Amino-400-N-(2-pentafluorosulfanylacetamide)-20-oxy-
[3,30:40,300]-Ter-[1,2,5]-oxadiazole (14)
Light yellow solid, 35% yield; mp (DSC) = 125.2 8C; 1H NMR
(DMSO-d6),
d 4.73 (q, J = 9.0 Hz, 2H, CH2), 6.67 (br, 2H, NH2), 12.43
Nevertheless, examination of the high energy properties of the
ten derivatives that were able to be prepared indicates that there is
significant potential and future promise for the creation of new and
excellent furazan-based, SF5-containing high energy materials, if
these synthetic challenges can be overcome.
(br, 1H, NH); 19F NMR (DMSO-d6), dA 83.51 (m, 1F), dB 71.46 (d,
JAB = 147 Hz, 4F); HRMS (MꢁH, calc): 418.9951, found: 418.9966.
4.2.5. 3-N-(2-pentafluorosulfanylacetamide)-4-(1H-tetrazo-5-yl)-
1,2,5-oxadiazole (15)
Light yellow solid, 35% yield; mp (DSC, decomp.) = 100.5 8C; 1H
4. Experimental
NMR (DMSO-d6),
d
4.62 (q, J = 9.0 Hz, 2H, CH2), 11.03 (br, 1H, NH);
A 82.32 (m, 1F), B 71.16 (d, JAB = 149 Hz, 4F).
19F NMR (DMSO-d6),
d
d
4.1. General
HRMS (MꢁH, calc): 319.9995, found: 320.0001.
All 1H NMR (300 MHz), 19F NMR (282 MHz) and 13C NMR
(75 MHz) spectra were recorded in CDCl3, acetone-d6 or DMSO-d6
on a VXR 300 spectrophotometer. Chemical shifts were referenced
with TMS, CDCl3 and CFCl3 (0 for 1H, 77.23 for 13C and 0 for 19F).
High-resolution mass spectra (HRMS) were obtained using a
Finnegan 4500 gas chromatograph/mass spectrometer using
4.2.6. 5-N-(2-pentafluorosulfanylacetamide)-1H-tetrazole (17)
White Solid, 80% yield; mp (DSC, decomp.) = 155.5 8C; 1H NMR
(DMSO-d6),
NMR (DMSO-d6),
NMR (DMSO-d6),
d
4.89 (q, J = 9.0 Hz, 2H–CH2), 12.75 (br, 1H, NH); 19F
d
d
A 84.00 (m, 1F),
72.0 (m, CH2), 149.5 (s,55C–NH), 159.8 (m, CO);
d C
B 71.51 (d, JAB = 149 Hz, 4F); 13
HRMS (MꢁH, calc): 251.9984, found: 251.9982.

H. Martinez et al. / Journal of Fluorine Chemistry 143 (2012) 112–122
121
4.3. Synthesis of 3-N-(2-pentafluorosulfanylacetamide)-4-nitro-
product (95% yield): 1H NMR (CDCl3),
d
5.43 (q, J = 7.2 Hz, 2H), 8.31
1,2,5-oxadiazole (24)
(d, J = 8.2 Hz, 1H), 8.19 (d, J = 8.2 Hz, 1H), 7.75 (t, J = 8.4 Hz, 1H),
7.61 (t, J = 8.4 Hz, 1H); 19F NMR (CDCl3),
dA 78.01 (m, 1F), dB 72.84
To a solution of 50% H2O2 (1.6 g) and concentrated sulfuric acid
(1.1 g) at 0 8C, was added 160 mg of Na2WO4.H2O and 30 mg of 3-
amino-4-(pentafluorosulfanylmethyl)furazan (8) in one portion.
The mixture was stirred until the staring material disappears on
the TLC (ꢀ16 h, 4:1 CH2Cl2:ethyl acetate). The mixture was then
treated with 5 mL of water and extracted with ethyl acetate. The
solvent was removed under reduced pressure and the resulting oil
purified by column chromatography (silica gel, 4:1 CH2Cl2:ethyl
acetate): light yellow solid, 50% yield. mp (DSC,
(d, JAB = 151 Hz, 4F); HRMS (calc MꢁH+): 288.0230, found:
288.0243.
4.7. Computational methods
The quantum chemical calculations were performed with a
hybrid basis set at MP2/6ꢁ31+G(d,p)-LANL2DZ level of theory. The
effective core potential (ECP) of iodide was included in the
calculations in order to minimize the time in the optimization.
DMSO was used as a solvent in all calculations using the
Polarizable Continuum Model (PCM). The transition states were
characterized by one and only one negative frequency and the
Intrinsic Reaction Coordinate (IRC) connecting both the starting
material and the product. All the calculations were performed
using Gaussian 03 Rev. E01 Software package [43].
decomp.) = 172.1 8C. 1H NMR (DMSO-d6, ppm),
d 5.11 (q,
J = 9.0 Hz, 2H, CH2), 11.87 (br, 1H, NH); 19F NMR (DMSO-d6,
ppm), dA 83.90 (m, 1F), dB 71.97 (d, JAB = 149 Hz, 4F); 13C NMR
(DMSO-d6, ppm), d 71.6 (m, CH2), 144.7 (s, C–NH), 158.2 (s, C–NO2),
160.0 (m, CO). HRMS (M+H, calc): 298.9873, found: 266.9902.
4.4. General method for the syntheses of pentafluorosulfanylurea-
furazans
Acknowledgments
To 30 ml of anhydrous CH2Cl2 in a glass heavy wall pressure
vessel, closed with a septum at ꢁ40 8C, 7.8 mmol of pentafluor-
osulfanyl isocyanate was bubbled. Then, the septum was remove
and quickly 7.8 mmol of aminofurazan were added, after which the
septum was replaced by a sealed cap. The cold bath was removed
and the reaction mixture was stirred for 24 h. After that the vessel
was opened and the solvent evaporated under reduced pressure.
The authors gratefully acknowledge the financial support of
DTRA (HDTRA1-08-0008). Cheetah calculations were carried out
by Dr. Phil Pagoria of the Lawrence Livermore National Laboratory,
who is acknowledged with thanks. The providing of chemical
samples by Dr. Joseph Mannion of National Naval Laboratories at
Indian Head is acknowledged with thanks, as was the donation of
SF5Cl by Air Products. The authors in particular appreciate the
interest shown in this project by Dr. Robert Syvret of Air Products.
Prof. Jean’ne Shreeve is thanked for her assistance regarding
methods of calculation. Lastly the authors wish to thank Dr. Joe
Flanagan for his essential and consistently beneficial discussions
and advice throughout this project.
4.4.1. 3-Amino-4-N-(3-pentafluorosulfanylurea)-1,2,5-oxadiazole
(22)
White solid, 99% yield; mp (DSC, decomp.) = 149.61 8C; 1H NMR
(acetone-d6),
(br, 2H, NH2); 19F NMR (acetone-d6),
JAB = 149 Hz, 4F); 13C NMR (acetone-d6),
d
10.55 (br, 1H, NH–SF5), 8.83 (br, 1H, NH–CO), 5.57
A 77.44 (m, 1F), B 72.40 (d,
145.0 (S, C–NH2), 148.7
d
d
d
(s, C–NH), 153.0 (m, CO); HRMS (MꢁH, calc): 267.9933, found:
Appendix A. Supplementary data
267.9941.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jfluchem.2012.03.010.
4.4.2. 3-N-(3-pentafluorosulfanylurea)-4-nitro-1,2,5-oxadiazole (23)
Yellow Solid, 20% NMR yield; mp (DSC, decomp.) = 99.7 8C; 1H
NMR (acetone-d6), d 11.18 (br, 1H, NH–SF5), 9.19 (br, 1H, NH–CO);
19F NMR (acetone-d6), dA 76.78 (m, 1F), dB 72.13 (d, JAB = 155 Hz,
4F); HRMS (MꢁH, calc): 297.9575, found: 297.9678.
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