Synthesis and mutual transformations of nitronium tetrakis(nitrooxy)- And tetrakis(2,2,2-trifluoroacetoxy)borates

Article abstract of DOI:10.1039/d0nj02835e

Nitronium borates of NO2[B(OX)4] type with X = CF3CO and NO2 as ligands were synthesized, and their structures were determined by powder X-ray diffraction. Crystalline NO2[B(ONO2)4] is relatively stable at room temperature and undergoes transformation to B2O3 upon heating. The [B(CF3COO)4]- anion was obtained for the first time and characterized as a structural fragment of a solid compound. The possibility of reversible modification of the complex anion by replacement of the nitrate or trifluoroacetate ligand was demonstrated for the first time by the mutual transformations of NO2[B(ONO2)4] and NO2[B(CF3COO)4].

Full text of DOI:10.1039/d0nj02835e

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Synthesis and mutual transformations
of nitronium tetrakis(nitrooxy)- and
Cite this:DOI: 10.1039/d0nj02835e
tetrakis(2,2,2-trifluoroacetoxy)borates†
a
ab
Victor P. Zelenov, *a Evgeny Yu. Gorshkov,
Mikhail V. Zavaruev,
b
c
d
Artem O. Dmitrienko,
Dmitry V. Khakimov
Ivan A. Troyan,
Alla N. Pivkina,
and Alexander V. Pavlikov
a
e
Nitronium borates of NO2[B(OX)4] type with X = CF3CO and NO2 as ligands were synthesized, and their
structures were determined by powder X-ray diffraction. Crystalline NO2[B(ONO2)4] is relatively stable at
room temperature and undergoes transformation to B2O3 upon heating. The [B(CF3COO)4]À anion was
obtained for the first time and characterized as a structural fragment of a solid compound. The possibility of
reversible modification of the complex anion by replacement of the nitrate or trifluoroacetate ligand was
demonstrated for the first time by the mutual transformations of NO2[B(ONO2)4] and NO2[B(CF3COO)4].
Received 4th June 2020,
Accepted 8th July 2020
DOI: 10.1039/d0nj02835e
rsc.li/njc
were synthesized from the corresponding tetrachloroborates and
N2O4 (Scheme 1.1).17–20 However, alkali metal tetranitratoborates19,20
1. Introduction
Nitrate and trifluoroacetate moieties are used in the synthesis appeared to be very unstable in contrast to alkylammonium salts17,18
of various complex compounds. Nitronium salts available at the and complexes of B(NO3)3 with amines.21
moment include a large number of metal–nitrate structures,
such as [Fe(NO3)4]À,1,2 [Zr(NO3)5]À,2 [Al(NO3)4]À,3–5 [Au(NO3)4]À,6,7 borates (X = ClO3) have also been synthesized previously.16,20,22
[Ga(NO3)4]À,8,9 [Hf(NO3)5]À,10 [Th(NO3)6]2À 11,12 [UO2(NO3)3]À,13 and For example, the nitronium salt NO2[B(ClO4)4] was obtained
Among the borates of M+[B(OX)4]À type, tetraperchlorato-
,
[In(NO3)4]À.12 However, only one nitronium salt is known, which from BCl3 and NO2ClO4 in carbonyl dichloride at À78 1C
has the trifluoroacetate ligands at the central metal atom in the (Scheme 1.2).16 This compound is of interest from the stand-
coordination sphere of the anion, namely, [NO2]2[Sn(CF3COO)6].14,15 point of its practical application as a propellant oxidizer.
Nitronium salts containing [B(NO3)4]À and [B(CF3COO)4]À anions The addition of the corresponding (alkyl)ammonium chloride
are unknown. Since nitronium tetranitroborate NO2[B(NO3)4] (1) to the reaction mass made it possible to obtain tetraperchlor-
contains a large excess of oxygen, it seems to be potentially atoborates containing (alkyl)ammonium cations (Scheme 1.3).
interesting as a rocket propellant oxidizer. The main advant- Various tetraperchloratoborates M+[B(ClO4)4]À were synthe-
age of compound 1 over the known tetraperchloratoborate sized using a modified method from BCl3 and metal,23 ammo-
NO2[B(ClO4)4]16 is that it is halogen-free and thus quite attractive nium, or nitronium24 perchlorates in anhydrous HClO4
from the standpoint of green applications. Related tetranitro- (Scheme 1.4). In addition, alkali metal tetraperchloratoborates
borates with alkylammonium or alkali metal cations M+[B(NO3)4]À
a N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
47 Leninsky prosp., 119991 Moscow, Russian Federation. E-mail: zelenov@ioc.ac.ru
b Department of Chemistry, M. V. Lomonosov Moscow State University, Leninskie Gory,
Moscow 119991, Russian Federation
c FSRC ‘‘Crystallography and Photonics’’, Russian Academy of Sciences, 17A Butlerova
str., 119333, Moscow, Russian Federation
d N. N. Semenov Federal Research Center for Chemical Physics, Russian Academy of
Sciences, 4 Kosygina st., Building 1, 119991, Moscow, Russian Federation
e Department of Physics, M. V. Lomonosov Moscow State University, Leninskie Gory,
Moscow 119991, Russian Federation
† Electronic supplementary information (ESI) available: The spectral data, crystal-
lographic data and refinement parameters. CCDC 1980143 and 1985253. For ESI
Scheme 1 Known methods for the synthesis of compounds of M+[B(OX)4]À
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
type (X = ClO3, NO2).
d0nj02835e
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Scheme 2 Synthesis of NO2[B(NO3)4] (1).
were synthesized via the reaction of M[BCl4] with anhydrous believe that an isolated molecule of NO2[B(NO3)4] cannot be
HClO4 (Scheme 1.5).23
stable.
It should be noted that the [B(CF3COO)4]À anion was never
In the Raman spectrum of solid compound 1, symmetric
obtained as a structural fragment of an individual crystalline vibrations nsNO of linear nitronium cations appear as a strong
2
compound, although it was detected by mass spectrometry.25
The aim of this work was to develop methods for the synthesis
of previously unknown nitronium borates NO2+[B(NO3)4]À (1)
and NO2+[B(CF3COO)4]À (2) and to study their properties.
line at 1400 cmÀ1 (Fig. S1, ESI†). The IR spectrum of solid 1 also
+
comb
NO2
exhibits the corresponding bands of the NO2 ion, viz., n
3748 cmÀ1 and nas at 2296–2388 cmÀ1 (Fig. S2, ESI†). It may
at
NO2
be emphasized that asymmetric vibrations of the nitro ester
moieties appear in the vibrational spectra of the [B(NO3)4]À
as
NO2
anion of compound 1 as four n
bands in the range of 1599–
À1
2. Results and discussion
2.1 Nitronium tetrakis(nitrooxy)borate: synthesis and
1653 cm
(Raman spectrum, see ref. 26 and 33) and in the
range of 1610–1675 cmÀ1 (IR spectrum). Symmetric vibrations
nsNO of nitro ester groups appear in the Raman spectrum of
properties
2
compound 1 as two strong broad bands at 1311 and 1325 cmÀ1
,
Complexation between Lewis acids (LAs) including metal nitrates
and N2O5 results in the formation of [NO2 ]n[M(NO3)m]À species
+
the latter being more intense (Fig. S1, ESI†). As for the IR
spectrum, the n
s
NO2
vibrations appear as bands in the range of
(see the Introduction and ref. 26). We believe that the addition of
N2O5 to B(NO3)3 treated as an LA is the simplest and most
convenient method for the synthesis of compound 1. However,
B(NO3)3 is very unstable under normal conditions. Attempts to
synthesize this compound by the reaction of BCl3 with N2O5
(resulting in B2O3)27 or by the reaction of BCl3 with NO3Cl28
were unsuccessful. We also failed to synthesize boron trinitrate
or nitronium salt 1 by the reaction of BBr3 with an excess of
N2O5 (Scheme 2.1). Even at À70 1C, the reaction proceeded very
vigorously with the elimination of NO2Br. It seems that the
monomolecular elimination of NO2Hal from the intermediate
B(NO3)2Hal leads to the formation of OQBONO2,28 and then
B2O3 is formed.
À1
1240–1320 cm
(Fig. S2, ESI†).
The 14N NMR spectra of solutions of nitronium tetranitra-
toborate 1 demonstrate an interesting feature. The signals d(N)
at À45 Æ 2 ppm are typical of nitrate groups, but the shift from
À90 ppm (in a sulfolane solution) to À63 ppm (MeNO2) is far from
the shifts characteristic of nitronium salts (À130 Æ 3 ppm),26,34,35
and are close to the shifts of covalent nitro esters (N2O5,
CF3COONO2 and C2F5COONO2).36–38 We suppose that dis-
solved nitronium tetranitroborate 1 can reversibly decompose into
covalent N2O5 and boron trinitrate. Both NMR data and the results
of calculations (see above) confirm that the isolated molecule of
NO2[B(NO3)4] is unstable similar to the isolated N2O5 molecule.39–41
The structure of NO2[B(NO3)4] (1) was determined by powder
We developed a ‘‘one-pot’’ method for the synthesis of
compound 1 from boric acid and N2O5 (Scheme 2.2). Gradual
nitration of hydroxyl groups was followed by liquefaction of the
solid mixture. The nitro ester moieties of the intermediate
dinitrate B(OH)(ONO2)2 as electron-withdrawing groups
strongly deactivate the unreacted hydroxyl substituent. To
increase the electrophilicity of the reactant, nitric acid was
preliminarily added to the reaction mixture in order to trans-
form covalent dinitrogen pentoxide into the ionic form
[NO2]+[NO3]À 29–31 and thus facilitate nitration. Since the inter-
action of boric acid with N2O5 produces HNO3, no additional
nitric acid may be needed, but in this case the reaction
proceeds slower and with a lower yield. Boron trinitrate reacts
with N2O5 and successfully transforms into the crystalline salt
%
X-ray diffraction analysis (Fig. 1). The space group is I4, and the
+
NO2 [B(NO3)4]À (1) (Scheme 2.2).
Since the structure of compound 1 was previously unknown,
we carried out B3LYP/6-311++g(d,p) calculations,32 but a stable
gas-phase configuration of molecule 1 was not determined. The
calculations unambiguously led to the decay of compound 1
with elimination of either N2O5, or N2O4 and O2. Therefore, we
Fig. 1 General view of compound 1 in a crystal. Isotropic displacement
parameters for atoms are drawn at 50% probability.
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Table 1 Characteristics of compound 1 and other oxidizers
Compound
Gross formula
[O]a (%)
a
Tdec (1C)
c
NO2[B(ClO4)4]b
NO2[B(NO3)4]
NH4ClO4
NH4N(NO2)2
NH4NO3
(CH3)4N[B(NO3)4]
B1Cl4N1O18
B1N5O14
ClH4N1O4
H4N4O4
H4N2O3
B1C4H12N5O12
B1C3H9N4O9
65.1
65.6
34
25.8
20
13.3
9.3
2.7
2.0
1.5
0.77
0.58
—
Scheme 3 Thermolysis of NO2[B(NO3)4] (1).
B83
b
B150
B15042
B170
B7518
lattice constants are a = 9.10760(9) Å and c = 5.86902(8) Å. In the
crystal, all NO3 groups of the tetranitratoborate anion are symme-
À16.8
trically equivalent. The NO2+ cation forms four short contacts NÁÁÁO (CH3)3N B(NO3)3
À18.8
—
21
c
(NÁÁÁO distance 2.697 Å) with the oxygen atoms of the anion.
a
[O] is the oxygen balance, a is the oxidizer excess ratio and Tdec is the
temperature of the onset of intense decomposition. The oxidizer isn’t
b
It is known that at room temperature, symmetric sulfate
(NO2)2SO4 undergoes very fast dimerization to the relatively
stable disulfate (NO2)2S2O7 with the elimination of N2O5.26
Unlike the unstable dinitronium sulfate26 and alkali metal
tetranitratoborates,19 thermal decomposition of solid nitratoborate
1 occurs at higher temperatures with the formation of nitrogen
oxides (Scheme 3 and Fig. 2). Thermolysis of borate 1 features a
one-step mass loss (Dm = À82%, extrapolated onset temperature
Teo = 83 1C). The process is accompanied by an endothermic heat
effect of À530 J gÀ1. After heating to 300 1C (Fig. 2), complete
conversion of compound 1 to B2O3 occurs (Scheme 3).
In Table 1, the characteristics of nitronium borate 1 are
compared with those of known oxidizing agents and related
structures.
c
‘‘green’’. No data are available.
involves nitration of hydroxyl groups followed by replacement of
nitro groups by trifluoroacetyl ones. Nitronium cation is the most
electrophilic particle; therefore, it is obvious that B(NO3)3 and/or
compound 1 were formed as intermediates. Besides, we established
that the interaction of hydroxyl groups of boric acid with excess of
TFAA leads to dehydration of the acid to give B2O3 rather than
B(CF3COO)3 (3).
The trifluoroacetyl moiety can replace the nitrate anion of
inorganic nitrates, the covalent nitro ester group and even one
of two nitronium cations in dinitronium sulfates.14,26,37,43,44
Therefore, replacement of the nitrate moieties of B(NO3)3 and
[B(NO3)4]À by the trifluoroacetyl group is real, and we proved it
via the direct synthesis of borate 2 from borate 1 (Scheme 4E). It
2.2 Nitronium tetrakis(trifluoroacetato)borate
Nitronium borate NO2[B(CF3COO)4] (2) was synthesized by the should be noted that the reverse reaction, namely, substitution
reaction of boron tris(trifluoroacetate) 3 with TFAN (Scheme 4A of trifluoroacetate groups in the coordination sphere of the
and B) and from boric acid (Scheme 4C). In the former case, central boron atom of compound 2 by nitrate groups also
mixing N2O5 and trifluoroacetic acid anhydride led to the gradual successfully led to the formation of compound 1 (Scheme 4D).
formation of trifluoroacetyl nitrate36 that added to LA 3, and
The Raman (Fig. S3, ESI†) and IR spectra (Fig. S4, ESI†) of
crystals of the nitronium salt 2 precipitated. Preliminary addition crystalline compound 2 exhibit clearly seen bands in the range
of trifluoroacetic acid to B(CF3COO)3 (3) (see Scheme 4A) catalyzed from 1757 to 1792 cmÀ1 corresponding to carbonyl groups.
the formation of compound 2 probably due to the formation of Quantum chemical calculations demonstrated that the experi-
intermediate 4. The yield of compound 2 was 72–91% (Scheme 4C). mental (Fig. S3, ESI†) and calculated (Fig. S5, ESI†) Raman
We believe that the synthesis of salt 2 from boric acid (Scheme 4C) frequencies of the [B(CF3COO)4]À ion are close. In the Raman
Fig. 2 Thermogravimetry (TG, green curve) and differential scanning calorimetry (DSC, blue curve) traces for sample 1 (conditions: linear heating at 10 K minÀ1
in argon flow).
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Scheme 4 Methods of synthesis of NO2[B(NO3)4] (1) and NO2[B(CF3COO)4] (2).
Scheme 6 Reaction of B(CF3COO)3 (3) with N2O5.
the crystal, all CF3COO groups adopt a staggered conformation
relative to the B–O bond, thus indicating a large contribution
from the repulsion between carboxyl groups. The NO2+ cation forms
four short contacts NÁÁÁO (average distance NÁÁÁO 2.675 Å) with
CQO groups of the tetrakis(2,2,2-trifluoroacetoxy)borate anion.
It was previously shown that anions of the [BCl3X]À type (X =
ONO2, C(NO2)3) readily decomposed due to disproportionation.20
Reaction of BF3 with N2O5 resulted in the formation of NO2[BF4]45
instead of complex BF3ÁN2O5 (NO2+[BF3(NO3)]À).46,47 This
transformation occurred due to the elimination of NO2F and its
subsequent interaction with other BF3 molecules (Scheme 5).
Hence, anions with heterotypic ligands in the coordination sphere
of the central boron atom are probably unstable. The reaction
of LA B(CF3COO)3 (3) with N2O5 confirmed this assumption
(Scheme 6). According to the spectral data, the interaction led to
a mixture of salts 1 and 2 rather than the expected nitronium
Fig. 3 General view of NO2[B(CF3COO)4] (2) in a crystal. Isotropic dis-
placement parameters for atoms are drawn at 50% probability. The
disordered fluorine atoms were omitted for clarity.
spectrum of compound 2 the characteristic frequency of sym-
metric vibrations nNs O of the nitronium cation (B1408 cmÀ1
)
2
is somewhat higher than that of NO2BF4 (1405 cmÀ1), but the
comb
NO2
n
frequencies in the IR spectra of NO2[B(CF3COO)4] (2) and
35
NO2BF4 are very close (3769 and 3767 cmÀ1, respectively).
Unlike borate 1, the d(N) shifts in the 14N NMR spectra of
borate 2 are BÀ132 ppm (sulfolane, CF3COOD), which is
typical of nitronium salts.
The structure of NO2[B(CF3COO)]4 (2) was determined by
powder X-ray diffraction analysis (Fig. 3).
The space group is Pbca, the lattice constants are a =
15.28529(13) Å, b = 14.44923(11) Å, and c = 15.48814(17) Å. In
Fig. 4 Thermogravimetry (TG, green curve) and differential scanning
calorimetry (DSC, blue curve) traces for sample 2 (conditions: linear
heating at 10 K minÀ1 in argon flow).
Scheme 5 Reaction of BF3 with N2O5 (see ref. 45).
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borate NO2[B(CF3COO)3(NO3)]. The Raman spectrum of the mix-
The diffraction patterns of compounds 1 and 2 were indexed
ture obtained represents a superposition of the spectra of com- using the SVD (singular value decomposition) index algorithm50
pounds 1 and 2 including the characteristic lines of nitronium as implemented in Bruker TOPAS 5.0,51 and the space group
cations at 1400 and 1408 cmÀ1 (Fig. S6, ESI†).
determination were carried out using systematic absences
Thermolysis of borate 2 features a multistep mass loss analysis. Parallel tempering, as implemented in FOX,52 was
(overall Dm = À88%, extrapolated onset temperature Teo = 153 1C). used to solve the crystal structures in direct space. The struc-
The process is accompanied by an exothermic heat effect with ture of compound 1 contains only 13 positional parameters and
two overlapping endothermic effects (Fig. 4).
allows for unrestrained Rietveld refinement. The restrained
Rietveld refinement for compound 2 (with Bruker TOPAS 5.0)
was carried out using the methodology described elsewhere.53
Agreement between the Rietveld refined structures and the
PW-DFT-D optimized ones was good.54
3. Conclusions
We developed different synthetic routes to nitronium borates
NO2[B(NO3)4] (1) and NO2[B(CF3COO)4] (2) and prepared stable
solid compounds. Nitronium borates 1 and 2 as well as the
[B(CF3COO)4]À anion were obtained for the first time and
characterized using spectral methods. Thermographic studies
showed that compounds 1 and 2 are relatively stable, and
NO2[B(NO3)4] can be considered as a potential rocket propellant
oxidizer. Thermolysis of borate 1 leads to B2O3. The possibility
of reversible substitution of acid ligands in the coordination
sphere of boron was demonstrated for the first time by the
mutual transformations of compounds 1 and 2. It was con-
firmed that anions with heterotypic acid ligands in the coordi-
nation sphere of the central boron atom are unstable.
PW-DFT-D calculations were performed using VASP 5.4.455–
58
using the PBE functional59 corrected by Grimme D3 van der
Waals correction60 with Becke-Jonson damping.61 A planewave
basis set with ‘normal’ projector augmented wave (PAW)
pseudopotentials62,63 as supplied by VASP was used. Optimiza-
tions with both fixed and optimized unitcell parameters were
performed using an energy cutoff of 600 eV.
CCDC 1985253 contains all the information about the
structure of compound 1 and the Rietveld refinement along
with the raw powder pattern. CCDC 1980143 contains all the
information about the structure of compound 2 and the
Rietveld refinement along with the raw powder pattern.
The Raman spectra of solid samples were recorded in the
range of 100–2500 cmÀ1 on an Acton Research spectrometer
with a focus distance of 500 mm on a 1200 lines per mm grid. A
Princeton Instruments CCD cooled with liquid nitrogen was
4. Experimental section
B(CF3COO)3 was synthesized following a known procedure.48 used as the light-sensitive detector of the optical signal. The
All manipulations of reagents were performed in an argon spectra were measured using the 641.7 nm line of a krypton
atmosphere. Nitrogen pentoxide was prepared using a known laser (radiation power up to 4 mW) in the Raman backscatter-
method49 modified by us (see below).
ing configuration. The spectral resolution was 1.5 cmÀ1. The
The 11B (96.29 MHz), 13C (75.47 MHz), 14N (21.69 MHz) and optical path of the instrument was designed by the confocal
19F (282.40 MHz) NMR spectra were recorded on a Bruker microscope scheme with a spatial resolution of 2 mm.
AM300 spectrometer. Capillaries with acetone-d6 as a lock
The thermal behavior of compounds was studied using a
solvent were used for recording spectra of solutions in non- Netzsch STA449 F3 thermal analyzer under linear heating at a
deuterated solvents. The 14N NMR spectra were recorded with rate of 10 K minÀ1. A weighed sample (0.7 mg) was placed in
CH3NO2 (10–15 mg) as the internal standard (high-field alumina pans and covered with pierced lids. Measurements
chemical shifts have negative values), and the 13C and the 19F were performed in argon stream (70 mL minÀ1) in the tem-
NMR spectra were recorded with TMS and with CFCl3 as the perature range from room temperature to 600 1C.
external standard, respectively.
The boron content in the sample was determined using the
The IR spectra (Fig. S2 and S4, ESI†) were recorded on a gravimetric method based on the mass of boric acid. The
Bruker Vertex 70V FTIR spectrometer equipped with a Platinum nitronium salts were completely decomposed by boiling water
ATR accessory in the range of 400–4000 cmÀ1 (resolution for 5 minutes; after evaporation of H2O and volatile compo-
2 cmÀ1) in a nitrogen atmosphere. The spectrum in Fig. S7 nents under reduced pressure, subsequent drying of the resi-
(ESI†) was recorded on a Bruker ALPHA-T spectrometer with due in air at room temperature to a constant weight gives the
CaF2 optics (Nujol mull).
mass of B(OH)3.
The technique for performing X-ray powder diffractometry was
as follows. Samples of compounds 1 and 2 were sandwiched
between Kapton films and the space between the Kapton films
Synthesis of NO2[B(NO3)4] (1)
From B(OH)3. Crystalline N2O5 (4.13 g, 38.2 mmol) and boric
was evacuated. Then the samples were mounted on a Bruker AXS acid (0.47 g, 7.64 mmol) were mixed under ice cooling, HNO3
D8 Advance Xray powder diffractometer equipped with a primary (0.24 g, 3.86 mmol) was added, and the reaction mixture was
Ge111 monochromator (Cu Ka1, l = 1.54056 Å) and a LynxEye 1D stirred for 4 h under ice cooling. Then CH2Cl2 (3 mL) was added
position sensitive detector. Data were collected at room tempera- and the mixture was stirred for 5 min. The solvent was
ture in the range 10–901 2y for 1 9–901 2y for 2 and with a 0.011 2y decanted, CH2Cl2 (3 mL) was added again, and the precipitate
step size in transmission mode.
was filtered off and washed with CH2Cl2 (3 Â 2 mL). Nitronium
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tetranitratoborate NO2[B(NO3)4] (2.06 g, 88%) was obtained as a
From B(CF3COO)3. Crystalline N2O5 (0.17 g, 1.57 mmol) was
mixed with (CF3CO)2O (0.26 mL, 0.39 g, 1.85 mmol) and the
colorless crystalline compound. IR (selected bands), n, cmÀ1
:
+
3748, 2388 (NO2 ); 1609, 1308 (ONO2). Raman (selected bands), mixture was kept in a refrigerator overnight. On the next day,
+
n, cmÀ1: 1653, 1620, 1613, 1599, 1325, 1311 (ONO2); 1400 (NO2 ). the solution was added to a suspension of B(CF3COO)3 (0.78 g,
14N NMR d, (sulfolane): À45 (Dn1/2 = 185 Hz, ONO2); À92 (Dn1/2
=
2.24 mmol) in CH2Cl2 (2 mL) under ice cooling and stirring.
325 Hz); (MeNO2): À45 (Dn1/2 = 32 Hz, ONO2); À63 (Dn1/2 = 65 Hz); Nitric acid (0.02 g, 0.35 mmol) was added; the reaction mass
(CF3COOD): À60 (Dn1/2 = 105 Hz). 11B NMR d: 1.10 (sulfolane); was stirred for 10 h under ice cooling, and then kept in a
1.03 (CF3COOD); 2.51 (MeNO2). Found, %: B, 3.79. B1N5O14. refrigerator for three days. The residue was filtered off on a
Calculated, %: B, 3.55.
Schott funnel and washed with ice-cold CH2Cl2 (3 mL).
From NO2[B(CF3COO)4]. NO2[B(CF3COO)4] (1.097 g, 2.16 mmol), Nitronium salt NO2[B(CF3COO)4] (0.85 g, 75%) was obtained
obtained from B(CF3COO)3, was mixed with N2O5 (2.65 g, as colorless crystals. The spectroscopic data corresponded to
24.5 mmol) and HNO3 (1 mL; 1.5 g, 25 mmol) at 0–5 1C. The those of the authentic sample.
mixture was stirred for 3 h at 0–5 1C and kept overnight in a
From BBr3. To a solution of BBr3 (0.82 g, 3.27 mmol) in
refrigerator. The mixture was evacuated for 10 min, then ice- CH2Cl2 (5 mL), a solution of CF3COOH (1.51 g, 13.23 mmol) in
cold CH2Cl2 (2 mL) was added to the residue and white crystals CH2Cl2 (5 mL) was added gradually under ice cooling and
were filtered off and washed with CH2Cl2 (2 mL). NO2[B(NO3)4] stirring and the mixture was stirred in an ice bath for 2 h.
(1) (0.485 g, 74%) was obtained. The spectroscopic data corre- The synthesis was accompanied by HBr evolution. The reaction
sponded to those of the authentic sample.
mass was evaporated to dryness on a rotary evaporator and a
colorless crystalline residue (1.25 g) was obtained. Crystalline
N2O5 (0.82 g, 7.57 mmol) was mixed with (CF3CO)2O (1.77 g,
Synthesis of NO2[B(CF3COO)4] (2)
From B(OH)3. (a) Crystalline N2O5 (1.73 g, 16 mmol) was 8.41 mmol) in a separate vessel, the mixture was placed in a
mixed with (CF3CO)2O (6.52 g, 31 mmol) and the mixture was refrigerator for 3 h, and then the solution obtained was added
kept in a refrigerator for 2 h. Then B(OH)3 (0.51 g, 8.2 mmol) to B(CF3COO)3 with stirring under ice cooling. After 30 min of
was added to the solution and the mixture was stirred for 8 h stirring, a solution of HNO3 (0.04 g, 0.63 mmol) in CH2Cl2
under ice cooling. The suspension thus obtained was allowed to (1 mL) was added; the suspension was stirred for an additional
stay overnight in a refrigerator. Then a CH2Cl2–MeCN (V = 4 : 1) 2 h and then placed in a refrigerator overnight. The residue was
mixture (10 mL) was added to the reaction mass under stirring filtered off and washed with CH2Cl2 (2 Â 3 mL). Nitronium
and cooling in an ice–salt bath and the reaction vessel was kept borate NO2[B(CF3COO4)] (1.16 g, 84%) was obtained as color-
overnight in a freezer. The precipitate was filtered off using a less crystals. The spectroscopic data corresponded to those of
Schott funnel and washed with cold CH2Cl2 (3 mL). Crystalline the authentic sample.
NO2[B(CF3COO)4] (1.82 g) was obtained. As the solvent was
From NO2[B(NO3)4]. NO2[B(NO3)4] (0.37 g, 1.23 mmol),
removed from the mother liquor in vacuo, an additional 1.86 g obtained from B(OH)3, was mixed with CF3COOH (0.28 g,
of the reaction product was obtained as a white powder (a total 2.5 mmol) at room temperature, then (CF3CO)2O (2 mL, 3.58 g,
of 3.72 g, 91%). IR (selected bands), n, cmÀ1: 3769, 2388 (NO2+); 20 mmol) was added, and the reaction mass was stirred for 4 h at
1777 (CQO). Raman (selected bands), n, cmÀ1: 1793, 1784, 15–20 1C. The residue was filtered off and washed with ice-cold
+
1769, 1758 (CO); 1408 (NO2 ). 14N NMR, d (sulfolane): À132 CH2Cl2 (2 mL). NO2[B(CF3COO4)] (0.48 g, 77%) was obtained as
+
(Dn1/2 = 135 Hz, NO2 ). 11B NMR, d (sulfolane): À0.5. 14N NMR, colorless crystals. The spectroscopic data corresponded to those of
d (CF3COOD): À131 (Dn1/2 = 165 Hz, NO2 ). 11B NMR, d (CF3COOD): the authentic sample.
+
6.7. 13C NMR, d (sulfolane): q, 154.7, J 40 Hz (CQO); q, 114.4, J 285
À
Hz (CF3). 19F NMR, d (sulfolane): À78.09.
Reaction of B(CF3COO)3 with N2O5
À
(b) To crystalline N2O5 (7.55 g, 70 mmol), B(OH)3 (1.856 g, To B(CF3COO)3 (0.47 g, 1.35 mmol), N2O5 (0.31 g, 2.8 mmol)
30 mmol) was added, then nitric acid (0.3 mL, B17 mmol) was was added under stirring in an ice bath, and the mixture was
added at 0–5 1C, and the reaction mass was stirred for 4 h under placed in a refrigerator to stay over a period of four days. Then,
ice cooling. The reaction mass became homogeneous and a CH2Cl2 (2 mL) was added to the suspension under stirring and
precipitate formed later. Then, trifluoroacetic acid anhydride ice cooling, the reaction mass was stirred for an additional 5 h,
(16.6 mL, 118 mmol) was added dropwise to the suspension at and then the mixture was placed in a refrigerator overnight.
0–10 1C over a period of B1 h, and the mixture was stirred for The residue was filtered off and washed with CH2Cl2 (5 mL) to
1 h. The reaction mass was allowed to stay overnight in a give a mixture of NO2[B(CF3COO4)] and NO2[B(NO3)4] (0.51 g) as
refrigerator (B5 1C), then a cooled mixture of solvents (4 mL of colorless crystals in a B1 : 1 ratio (according to the Raman
CH3CN and 12 mL of CH2Cl2) was added; the overall mixture spectrum, see Fig. S7, ESI†).
was stirred at 0–5 1C for 1 h and then placed in a freezer for two
days. Crystals thus formed were filtered off on a Schott funnel
Synthesis of N2O5
under an argon atmosphere. Nitronium borate NO2[B(CF3COO)4] To white nitric acid (42 mL, 1 mol; d20 1.5 g cmÀ3), (CF3CO)2O
(11.185 g, 72%) was obtained as colorless crystals of a regular (69.5 mL, 0.49 mol) was added dropwise, the temperature was
shape. The spectroscopic data corresponded to those of the maintained at 2–10 1C (ice cooling). Immediately after addition,
authentic sample.
the solution was placed in a freezer to stay overnight, and then
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Paper
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the crystals were filtered off and dried on a Schott funnel for
5 min in an argon stream (Fig. S8, ESI†). If cooling of the
resulting solution was carried out with stirring, the crystal size
was smaller, which made filtering and drying difficult. White
crystals (Fig. S8, ESI†) of N2O5 (38.9 g, 72%) were used to
synthesize the nitronium salt 2 without purification or were
purified to prepare salt 1. Purification of N2O5 was carried out
by distillation over P2O5 with magnetic stirring, supplying a
slow argon stream into a Schlenk flask containing the molten
dinitrogen pentoxide at 40–50 1C. Dinitrogen pentoxide vapors
(22–25 1C) were condensed in a receiving Schlenk flask cooled
at BÀ40 1C. To protect against air moisture, a tube with CaCl2
was used. The Raman spectrum of N2O5 corresponded to those
of the authentic sample,35,36 and the admixture of N2O4 was
absent.
¨
12 M. Schmeisser and G. Kohler, Angew. Chem., Int. Ed. Engl.,
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Conflicts of interest
There are no conflicts to declare.
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26 V. P. Zelenov, D. V. Khakimov, I. V. Fedyanin and I. A. Troyan,
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jrs.5681.
Acknowledgements
Financial support from the Russian Foundation for Basic
Research (Project Nos 16-03-00713_a and 16-29-01042_ofi_m) is
gratefully acknowledged. A. O. Dmitrienko acknowledges finan-
cial support from the Russian Foundation for Basic Research
(Project No. 183301295). The authors express their gratitude to
Dr M. I. Struchkova (N. D. Zelinsky Institute of Organic Chem-
istry RAS), P. S. Konstantinov, N. Yu. Boldyrev (Bruker Russia
Ltd.), K. A. Monogarov (N. N. Semenov Federal Research Center
for Chemical Physics RAS) and Dr I. I. Vlasov (Prokhorov
General Physics Institute RAS) for help in obtaining some data.
I. A. Troyan acknowledges the Ministry of Science and Higher
Education for supporting within the State assignment to the
FSRC ‘‘Crystallography and Photonics’’, Russian Academy of
Sciences. The in crystal calculations were carried out on com-
putational facilities at the Center for Collective Use of Super
High Performance Computing Resources at the M. V. Lomono-
sov Moscow State University.64
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