Synthesis, characterization and chemistry of the fluoronitramide anion

Article abstract of DOI:10.1055/s-2007-966004

The fluoronitramide anion, as its potassium, tetraisopropyl p-phenylenediguanidinium, and tetraphenylphosphonium salts, have been synthesized. The latter salt was characterized in detail as it was by far the most stable. The free acid HN(F)(NO₂) was found to be unstable, as was the ammonium salt of fiuoronitramide. The fluorine atom was shown to be capable of displacement by nucleophiles. In contrast to dinitramide, which at room temperature is unreactive towards aqueous alkali, fiuoronitramide reacts instantly with aqueous hydroxide. Single crystal X-ray diffraction data show an unusual degree of uncertainty in the position of the fluorine atom in all of the salts that were examined. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2007-966004

SHORT PAPER
1151
Synthesis, Characterization and Chemistry of the Fluoronitramide Anion
S
ynthesis
e
andCharac
f
terizatio
f
nof Fluo
r
ronitram
e
ide y C. Bottaro,*,a Richard Gilardi,b Paul E. Penwell,a Mark Petrie,a Ripudaman Malhotraa
a
Chemical Science and Technology Laboratory, SRI International, 333 Ravenswood Avenue, Menlo Park, CA 94025, USA
Fax +1(650)8596196; E-mail: ripudaman.malhotra@sri.com
Naval Research Laboratory, Washington, DC 20375, USA
b
Received 20 December 2006; revised 26 January 2007
In fond memory of our esteemed colleague Robert J. Schmitt
fluoronitramide was obtained in nearly quantitative yield
by base-catalyzed elimination in isopropanol as solvent;
however, the crystal quality of his hygroscopic, hydrolyt-
ically unstable solid was poor. A much better crystal qual-
Abstract: The fluoronitramide anion, as its potassium, tetraisopro-
pyl p-phenylenediguanidinium, and tetraphenylphosphonium salts,
have been synthesized. The latter salt was characterized in detail as
it was by far the most stable. The free acid HN(F)(NO2) was found
to be unstable, as was the ammonium salt of fluoronitramide. The ity with a diminished surface area was derived using
fluorine atom was shown to be capable of displacement by nucleo-
ethanol as solvent with only a slight reduction in yield.
philes. In contrast to dinitramide, which at room temperature is un-
The product precipitated from the solution in 90% yield.
reactive towards aqueous alkali, fluoronitramide reacts instantly
This synthetic route is amenable to scale-up to a multi-
with aqueous hydroxide. Single crystal X-ray diffraction data show
mole scale, although this was not undertaken in our labo-
ratories.
an unusual degree of uncertainty in the position of the fluorine atom
in all of the salts that were examined.
Key words: fluoronitramide, dinitramide, difluoramide, nitrocyan-
amide
O
N
O
O
Oi-Pr
1. Et3N
SOCl2
Oi-Pr
COO–K+
O
Et3NH+N3
–
i-Pr-OH
2. KOH
The recent synthesis of the dinitramide ion (1; Figure 1)1
as well as the reputed instability of the difluoroamide an-
ion (2)2 lead directly to the question of the existence and
stability of the structurally hybrid fluoronitramide ion.
The potential uses for fluoronitramide salts in propellant
technology are numerous, as it is a source of oxidizing ox-
ygen and fluorine, giving further motivation to pursue the
synthesis of this system. We report here on the synthesis
and characterization of a few salts of the fluoronitramide
anion.
C
O
O
3
O
Oi-Pr
NO2
MeOH
AcONO2
F2, N2, H2O
K+ F-N--NO2
phosphate buffer
KOCH2CF3
N
5
H
4
Scheme 1 Synthesis of potassium fluoronitramide
The tetraphenylphosphonium salt was prepared by the
rapid preparation and mixing of aqueous solutions of tet-
raphenylphosphonium chloride and potassium fluoro-
nitramide (fluoronitramide salts decompose quickly in
aqueous solutions), which resulted in precipitation of fine
needles of the tetraphenylphosphonium salt. The crude
product was dried and crystallized from hot acetonitrile,
to obtain pale yellow prisms in ~80% yield. Attempts to
isolate the ammonium salt by analogous ion exchange
procedures were unsuccessful. The superior polarizing ca-
pacity of the ammonium ion, compared with that of the
potassium ion, arises from the localized, interactive nature
of the hydrogen bond; even though the ammonium ion has
an ostensibly larger radius than the potassium ion, its
charge is anisotropically configured by its four protons
and it also has the capacity to act simultaneously as a
Lewis and a Brønsted acid, thus expanding its mechanistic
capacity.
–
N
–
N
O2N
NO2
F
F
1
2
Figure 1 Dinitramide (1) and difluoroamide (2) anions
The potassium salt of fluoronitramide ion was synthesized
by the route shown in Scheme 1. The synthesis begins
with converting succinic anhydride into its isopropyl half
ester. The free acid chloride is then converted into the
acylazide, which is then transformed by Curtius rear-
rangement to the isocyanate. Methanolysis of the isocyan-
ate produces the methyl carbamate, which in turn can be
converted into the nitramine function by treatment with
acetyl nitrate, followed by a brief and quantitative am-
monolysis. The resulting isopropyl b-nitraminopropi-
onate was fluorinated on the nitrogen in a buffered
aqueous media in 90% yield and the potassium salt of
Dependency of Stability of Fluoronitramide Salts on
the Counterion
SYNTHESIS 2007, No. 8, pp 1151–1153
x
x
.
x
x
.2
0
0
7
The thermal stability of the fluoronitramide salts was in-
vestigated by differential scanning calorimetry (DSC).
Advanced online publication: 28.03.2007
DOI: 10.1055/s-2007-966004; Art ID: M07606SS
© Georg Thieme Verlag Stuttgart · New York

1152
J. C. Bottaro et al.
SHORT PAPER
+
–
Figure 2 shows the DSC scans for the potassium and the
tetraphenylphosphonium salts. They both display a sharp
exothermic decomposition. Evidently, the decomposition
is strongly dependent upon the polarizing capability of the
counterion. Thus, the potassium salt decomposed at
145 °C and the tetraphenylphosphonium salt at 215 °C;
the ammonium salt could not be isolated at room temper-
ature (20 °C).
+
O2N-N-F⋅⋅⋅⋅M
O2N-N M-F
NO + NO
Scheme 2 Decomposition of fluoronitramide salts
Preliminary Assessment of the Chemistry of Fluoro-
nitramide Anion
Fluoronitramide behaves as an electrophilic nitroaminat-
ing agent. In an attempted ion exchange of potassium flu-
oronitramide with ammonium ions, using an ammonia-
charged ABERLYST®15 polysulfonic acid resin in meth-
anol, only ammonium methoxynitramide was isolated, in-
dicating methanolysis of the N–F bond had occurred.
Heating potassium fluoronitramide to 85 °C with sodium
nitrite in dimethylformamide (DMF) gave a 20% (unopti-
mized) yield of potassium dinitramide.
No nitrocyanamide ion3 was observed to evolve from the
attempted reaction of potassium cyanide with potassium
fluoronitramide in DMF. The study of the reactivity of
fluoronitramide with other nucleophilic reagents is cur-
rently underway. In the reaction of potassium fluoronitra-
mide with quinuclidine, the most nucleophilic amine
known, there was TLC evidence of a trace of quinuclid-
ine-N-nitro imine ylide, by comparison to the known
ylide, (CH3)3N+N–NO2. The dissolution of fluoronitra-
mide salts in basic aqueous media results in rapid hydrol-
ysis, whereas dinitramide salts are not visibly reactive in
alkaline solutions at room temperature.
One attractive mechanistic possibility for the decomposi-
tion is the counterion-mediated dissociation of the fluo-
ride ion to give the singlet nitronitrene, which undergoes
migratory rearrangement to the nitric oxide dimer which,
in turn, dissociates to NO (see Scheme 2). This hypothesis
was tested by decomposing approximately two milli-
grams of the potassium salt in an evacuated 10-cm path-
length IR cell and analyzing the gases. The IR spectrum
clearly showed the PQR branches, characteristic of NO,
centered around 1880 cm–1. This band was accompanied
by a slightly weaker band due to NO2 between 1750 and
1800 cm–1. The formation of NO was further confirmed
by letting in air, which converted the resulting NO into
NO2 and dramatically increased the intensity of the NO2
band to more than fifty times its original intensity. In view
of the substantially stronger absorption coefficient of
NO2, we can surmise that greater than 95% of the decom-
position gas was NO.
X-ray Crystallography
X-ray diffraction studies of the potassium, tetraphe-
nylphosphonium, and tetraisopropyl p-phenylenediguani-
dinium salts yielded an anomalous uncertainty in the
position of the fluorine atom.
In conclusion, a novel inorganic anion (the fluoronitra-
mide anion) has been synthesized and characterized. Al-
though it does not possess the stability to a variety of
environmental challenges displayed by the dinitramide
anion, it has a well-defined profile of reactivity, and can
undergo displacement of fluorine by active nucleophiles
such as water or methanol to give Angeli’s salt, (trioxo-
dinitrate), or methoxy nitramide, respectively.
19F NMR spectra were recorded on a Varian Gemini 300 NMR
spectrometer at 282 MHz in benzene-d6 using 2,2,2-trifluoroethanol
(0 ppm) as an external reference. Thermal stability measurements
were carried out on a DuPont Model DSC2920 differentially scan-
ning calorimeter, and the data were analyzed using TA Instruments
Universal Analysis version 4.0D. IR spectra were recorded on a
Perkin Elmer Model 1420 infrared spectrometer. The samples were
sent to Galbraith Laboratories for elemental microanalyses.
Isopropyl b-Nitraminopropionate (4)
To a solution of Et3N (10 g, 100 mmol) in i-PrOH (100 mL) was
added succinic anhydride (10 g, 100 mmol). The resulting mixture
was stirred for 12 h, concentrated in vacuo, redissolved in MeOH
(100 mL) and treated with a solution of KOH (2 M, 100 mL) in
Figure 2 DSC scans for the decomposition of fluoronitramide salts:
tetraphenylphosphonium (top panel) and potassium (bottom panel)
Synthesis 2007, No. 8, 1151–1153 © Thieme Stuttgart · New York

SHORT PAPER
Synthesis and Characterization of Fluoronitramide
1153
MeOH (50 mL). The resulting solution was mixed with toluene
(200 mL) and the resulting suspension was concentrated in vacuo.
The solid residue was dried in vacuo at 100 °C then added to a so-
lution of SOCl2 (200 mmol) in chlorobenzene (100 mL). The mix-
ture was stirred for 4 h, filtered and the filtrate was concentrated in
vacuo to give the crude isopropyl b-(chlorocarbonyl)propionate
(not isolated).
tative. The product was always kept at zero degrees centigrade or
lower.
IR (KBr): 3300 (broad, adventitious H2O), 1390 (strong, sharp;
probable N–O stretching mode), 1300 (poorly resolved, but strong
and of equal intensity), 1020 (sharp, weak), 870 (strong singlet),
690 (strong singlet) 740 (strong singlet) cm–1.
The entirety of the crude acid chloride was added to a preformed
suspension of triethyl ammonium azide [110 mmol; prepared by
mixing Et3N (110 mmol), EtOH (110 mmol) and trimethylsilyl
azide (110 mmol) in EtOAc (200 mL)] and the mixture was stirred
for 1 h, filtered and concentrated to 100 mL in vacuo. The resulting
concentrate was eluted through a SiO2 column (4¢¢ × 1¢¢; EtOAc)
and the fractions eluting with Rf = 0.8 were collected. The effluent
from the column (~150 mL in all) was mixed with chlorobenzene
(150 mL), concentrated in vacuo to 100 mL and heated to 90 °C for
1 h, at which point nitrogen evolution ceased. The crude isopropyl
b-isocyanatopropionate was dissolved in MeOH (100 mL) then
treated with Et3N (1 g, 10 mmol) and allowed to stand at r.t. for 12
h. The solution was then concentrated and distilled under high vac-
uum (bp 120 °C, 100 mTorr) to give crude isopropyl b-(methoxy-
carbonylamino)propionate (17 g). This was added to a solution of
acetyl nitrate (100 mmol) in CHCl3 (100 mL) and stirred at r.t. for
12 h. The resulting solution was washed with K2HPO4 (2 M,
2 × 200 mL), dried (MgSO4), concentrated in vacuo and eluted
through a SiO2 column (1¢¢ × 4¢¢; EtOAc) with the fractions eluting
with Rf = 0.9 collected. The combined fractions were treated with
excess NH3 and the resulting solid was partitioned between H3PO4
(1 M, 100 mL) and toluene (100 mL). The organic layer was dried
(MgSO4), concentrated and passed through a SiO2 column (1¢¢ × 2¢¢;
EtOAc) with the fractions eluting with Rf = 0.6 collected. The com-
bined fractions were concentrated and the residue was stirred for 1
h at 50 °C, under high vacuum, to give isopropyl b-nitraminopropi-
onate (4; 14 g, 80% overall from 3) of sufficient purity to be used
for further synthesis.
Tetraisopropyl p-Phenylenediguanidinium Fluoronitramide
The tetraisopropyl p-phenylenediguanidinium fluoronitramide salt
was prepared by simple ion-exchange between 5 (1 mmol) and p-
phenylenediguanidinium chloride (0.5 mmol) in dry MeCN (5 mL)
at r.t. under argon. The soluble fluoronitramide salt was separated
from the insoluble by-product KCl by filtration after 1 h of stirring.
The p-phenylenediguanidinium chloride itself was prepared by
treating p-phenylenediammonium chloride (1 mmol) with diisopro-
pyl carbodiimide (2 mmol) in MeCN at reflux for 1 h.
Tetraphenylphosphonium Fluoronitramide
Tetraphenylphosphonium chloride (375 mg, 1 mmol) was dissolved
in cold H2O (~5 °C; 50 mL). Potassium fluoronitramide (5; 120 mg,
1 mmol) was then quickly (≤30 s) dissolved in cold H2O (5 mL) and
the resulting solution was immediately added to the previously pre-
pared aqueous solution of tetraphenylphosphonium chloride. The
mixture was stirred for approximately 10 s to achieve homogeneity,
placed in an ice bath and allowed to stand for 30 min. Yellow nee-
dles, which began to precipitate almost immediately after mixing,
were collected by filtration and dried under high vacuum for 10 min
to give the product as yellow needles (200 mg, ~50% yield). Re-
crystallization from MeCN (~5 mL, heated to 70 °C) gave larger
yellow needles (150 mg).
IR (KBr): 1580 (sharp singlet), 1450 (sharp singlet), 1465 (sharp
singlet), 1380 (broad singlet), 1425, 1380 (strong, singlet), 1280
(strong, sharp), 1105 (sharp, strong), 995 (moderate, sharp), 850–
870 (moderate, sharp), 690, 720, 750 (multiplet, strong and sharp)
cm–1.
1H NMR (60 MHz, CDC13): d = 1.25 (d, J = 6 Hz, 6 H), 2.7 (t, J = 6
Hz, 2 H), 3.9 (t, J = 6 Hz, 2 H), 5.15 (sept, J = 6 Hz, 1 H).
19F NMR (benzene-F6): 74.7 2 (s).
Anal. Calcd for C24H20FN2O2P: C, 68.90; H, 4.78; N, 6.70; P, 7.42;
F, 4.55. Found: C, 68.88; H, 4.87; N, 6.59; P, 7.54; F, 4.57.
The hygroscopicity and high dissolving power of this material made
determination of IR and elemental analyses somewhat difficult, and
these data were not obtained on this intermediate.
Acknowledgment
Potassium Fluoronitramide (5)
Financial support for this work by the Office of Naval Research and
useful discussions with Dr. Judah Goldwasser are gratefully ack-
nowledged.
Isopropyl b-nitraminopropionate (4, 2 g, 11 mmol) was suspended
in H2O (~40 mL) and treated with K2HPO4 (8.5 g, 50 mmol), cooled
in an ice bath and stirred until homogeneous. This cooled solution
was treated with 10% F2 in N2 until TLC (EtOAc) showed complete
consumption of starting material and formation of the faster-eluting
product (Rf = 0.8 versus 0.5 for the starting nitramine). The reaction
mixture was extracted with EtOAc (50 mL) and the organic layer
was dried (MgSO4) and concentrated to give the crude N-fluoro-N-
nitro ester as an oil. A solution of this oil (1 M in MeCN) was con-
verted into the potassium fluoronitramide by treatment with a solu-
tion of potassium trifluoroethoxide (1 M in EtOH). The reaction
time was typically one minute and the yield was essentially quanti-
References
(1) Schmitt, R. J.; Bottaro, J. C.; Penwell, P. E.; Ross, D. S. US
Patent 5198204, 1993.
(2) Fokin, A. V.; Kosyrev, Y. M.; Shevchenko, V. I. Russ.
Chem. Bull. 1982, 31, 1626; Izv. Akad. Nauk SSSR, Ser.
Khim. 1982, 1831.
(3) Boyer, J. H.; Manimaran, T.; Wolford, L. T. J. Chem. Soc.,
Perkin Trans. 1 1988, 2137.
Synthesis 2007, No. 8, 1151–1153 © Thieme Stuttgart · New York