Synthesis of Neutral and Charged Trinitromethyl Borohydrides and Their Complexes

Article abstract of DOI:10.1007/s10593-015-1634-4

We propose several simple and effective methods for the synthesis of previously unknown trinitromethyl borohydrides and their complexes with simple cyclic ethers and aromatic nitrogen- containing heterocycles, whereas acyclic ethers did not form such complexes. The data from physicochemical investigations showed that these unique compounds contain directly linked oxidizing and reducing fragments. Some transformations of trinitromethylborane complexes were demonstrated, which can occur by cleavage of all types of bonds formed by boron atom in the starting compounds.

Full text of DOI:10.1007/s10593-015-1634-4

DOI 10.1007/s10593-015-1634-4
Chemistry of Heterocyclic Compounds, Vol. 50, No. 12, March, 2015 (Russian Original Vol. 50, No. 12, December, 2014)
SYNTHESIS OF NEUTRAL AND CHARGED TRINITROMETHYL
*
BOROHYDRIDES AND THEIR COMPLEXES
1
1
1**
O. P. Shitov , V. A. Tartakovskii , and S. L. Ioffe
We propose several simple and effective methods for the synthesis of previously unknown trinitromethyl
borohydrides and their complexes with simple cyclic ethers and aromatic nitrogen- containing
heterocycles, whereas acyclic ethers did not form such complexes. The data from physicochemical
investigations showed that these unique compounds contain directly linked oxidizing and reducing
fragments. Some transformations of trinitromethylborane complexes were demonstrated, which can
occur by cleavage of all types of bonds formed by boron atom in the starting compounds.
Keywords: borane complex with THF, trinitromethane, trinitromethyl borohydride anion, trinitromethyl
borohydride complexes, trinitromethyl borohydrides.
Borane ether complexes (for example, THF·BH3) smoothly react with НХ type derivatives (inorganic
acids, water, and organic hydroxy compounds) with evolution of hydrogen (1 equiv. for each В–Н bond). The
elimination of hydrogen molecule is assumed to involve a four-membered transition state I, in which the boron
atom is coordinated with atom Х, having at least one lone electron pair (Scheme 1) [2].
Scheme 1
H
H
H
+
O
H
–B
+
O
–
–B
H
+
O
.
:
+
H X
H
B
H
.
.
.
–H2
.
. .
.
H
X
X
H
I
In case the atom Х, which is bonded to an acidic proton, has no lone electron pair, such process
becomes impossible. This means that С–Н acids, regardless of their acidity, should not interact with borane
ether complexes. Indeed, aliphatic nitro compounds [2], as well as 1,1-dinitroethane* do not react with
THF·BH3.
*Unpublished results of research by O. P. Shitov and S. L. Ioffe.
_______
*Dedicated to Academician Yu. N. Bubnov on the occasion of his 80th birthday.
For Preliminary Communication, see [1].
**To whom correspondence should be addressed, e-mail: iof@ioc.ac.ru.
1N. D Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47 Leninsky Ave., Moscow
119991, Russia.
_________________________________________________________________________________________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1795-1806, December, 2014.
Original article submitted July 28, 2014.
0009-3122/15/5012-1647©2015 Springer Science+Business Media New York
1647

At the same time, trinitromethane (nitroform) gives relatively stable complexes with THF and some other
simple cyclic ethers. In this case, entirely different process is possible, which involves the transition state II
and, instead of neutral complex of substituted borane, produces the boronium salt 1. The latter apparently can
react rapidly with THF·ВН3 complex, forming the corresponding boronium salt 2 (Scheme 2). The performed
experiments showed that nitroform does indeed react with THF·ВН3 complex according to the indicated
scheme, smoothly forming the boronium salt 2, which was easily identified by physicochemical methods.
Scheme 2
H
H
–B
+
O
THF, 18°C
30 min
.
.
H
.
THF·BH3
+
THF·HC(NO2)3
.
O
.
–H2
.
.
.
.
H
C(NO2)3
II
THF·BH3
[(THF)2BH2]+[C(NO2)3]–
[(THF)2BH2]+[BH3C(NO2)3]–
–THF
1
2
The attempts at isolation of boronium salt 2 may result in explosions due to its fairly high sensitivity to
mechanical initiation. However, the salt 2 can be stabilized by changing from THF to pyridine ligand in its
cation, and the resulting salt 3а can be easily isolated as individual compound (Scheme 3).
Scheme 3
Py, 18°C, 3 h
[(Py)2BH2]+[BH3C(NO2)3]–
2
–2THF
3a
Strictly speaking, the explanation of nitroform reaction with THF·ВН3 complex does not necessarily
involve the Scheme 2. The same result may be obtained using the aci-form of trinitromethane, which is in
equilibrium with the true nitro compound HC(NO2)3
(NO2)2C=N(O)OH. However, such interpretation
does not explain why 1,1-dinitroethane and aliphatic nitro compounds do not react with the THF·ВН3 complex.
After treatment of salt 2 with methanol (3 equiv.), the neutral complex THF·BH2C(NO2)3 (4а) was
formed in a good yield (Scheme 4).
Scheme 4
2MeOH
MeOH
THF
–H2
2
[(MeO)2B]+[BH3C(NO2)3]–
H+[BH3C(NO2)3]–
–2H2
–B(OMe)3
–2THF
THF·BH2C(NO2)3
87%
4a
Thus, the reaction of THF·ВН3 complex with trinitromethane, presented in Schemes 2 and 4, consumed
starting reagents in a different stoichiometry, as well as gave a different type of products than the standard
reactions of THF·ВН3 complex with НХ type compounds containing acidic hydrogen, presented in Scheme 1.
The complex 4а is a unique product, as the only known borane complex with an ether, which is stable
in pure form at ambient conditions. This complex contains both reducing (ВН2) and oxidizing (С(NO2)3)
fragments directly bonded to each other, and is a true organoboron compound with a В–С bond (the structure is
discussed in more detail below).
The aforementioned transformations are quite generally applicable. Borane complexes with other cyclic
ethers react with trinitromethane analogously (Scheme 5). At the same time, ВН3 solutions in acyclic ethers do
1648

not form any stable reaction products with trinitromethane, which may be explained by increased steric
obstacles in the intermediates or final products.
Scheme 5
1) HC(NO2)3, THF
5–7°C, 30 min
(CH2)n
(CH2)n
O ·BH3
O ·BH2C(NO2)3
4a–c
2) MeOH, THF
20°C, 40 min
a n = 4, b n = 5, c n = 6
Trinitromethane is not the only compound interacting with THF·ВН3 according to Schemes 2, 4, and 5.
A similar reaction of this borane complex has been observed also with hydrogen cyanide [3], but then the
second ligand in the cation of obtained type 2 salt is cyano group of the [BH3CN]- anion (Scheme 6).
Scheme 6
H
H
–B
+
O
HCN
THF
.
.
H
.
THF·BH3
THF·BH2+ ···NC–
.
.
.
–H2
–THF
..
. . .
O
NC
H
THF·BH3
Py
–THF
THF·BH2+ ···NCBH3
[PyBH2]+[NCBH3]–
–
–THF
Another important feature should be noted with regard to the boronium salt 2. It was proposed by
Brown [2] that the reducing properties of THF·BH3 type complexes are caused by equilibrium: 2THF·BH3
[(THF)2BH2]+BH4 , in other words, the THF·BH3 complex may act as a hydride type reducing agent. In this
-
investigation, the [(THF)2BH2]+ cation involved in such equilibrium was reliably identified by spectral analysis
of salt 2 (Scheme 2). At the same time, NMR study of THF·BH3 complex over the temperature interval from
-70С to 20С did not reveal signals corresponding to the cation [(THF)2BH2]+. This clearly indicated that the
THF·BH3 complex can not be a hydride type reducing agent, similar to NaBH4.
Apparently, the interaction of THF·BH3 complex with trinitromethane is the most interesting transformation
leading to trinitromethyl-substituted borohydrides (Schemes 2 and 4), but other routes for the synthesis of these
compounds are also possible. In particular, THF·BH3 complex reacts smoothly with potassium salt of
trinitromethane, giving a good yield of potassium trinitromethyl borohydride 5а, which is quite sensitive to
mechanical initiation (Scheme 7). Careful acidification of this compound with anhydrous HCl solution in ether
Scheme 7
THF
H+[BH3C(NO2)3]–
4a
–H2
HCl, Et2O, THF
20°C, 2.5 h
–KCl
[(Py)2BH2]+Cl–
THF·BH3
THF, 20°C, 1.3 h
K+[C(NO2)3]–
K+[BH3C(NO2)3]–
3a
THF, 20°C
40 min
–KCl
5a
Py·HCl, CHCl3
THF, 20°C, 16 h
[PyH]+[BH3C(NO2)3]–
–KCl
5b
1649

leads to the neutral complex 4а already mentioned before, but reaction with bis(pyridyl)boronium chloride – to
the boronium salt 3а. Treatment of the hydride 5а with pyridinium hydrochloride results in cation exchange and
the formation of trinitromethyl borohydride 5b.
Boronium salts 3 can be also synthesized by reaction of sodium borohydride with trinitromethane in
THF (Scheme 8). In this case, borohydride 5c is formed initially and reacts further without isolation in
exchange reaction with bis(pyridyl)boronium chloride, producing the boronium salt 3а. The hydride 5c was
recently obtained in this particular way by the Christe group [4]. These researchers isolated compound 5c in its
individual state and performed a comprehensive physicochemical characterization.
Scheme 8
[(Py)2BH2]+Cl–, CHCl3
THF
NaBH4 + HC(NO2)3
Na+[BH3C(NO2)3]–
3a
–NaCl
–H2
5c
Finally, hydrides containing a trinitromethyl group may be obtained by exchange reactions of the respective
chloroboranes or [A2BH2]+Cl- type boronium salts (A – amine) with trinitromethane salts (Scheme 9).
Scheme 9
n THF·BH2Cl + M+[C(NO2)3]–
n THF·BH2C(NO2)3
n
–MCln
4a
(from 6a–с)
THF·BH3
THF, CHCl3
15–17°C, 3 h
n [A2BH2]+Cl– + M+[C(NO2)3]–
n [A2BH2]+[C(NO2)3]–
6a–f
n
–MCln
THF
room temp., 1 h
[A2BH2]+[BH3C(NO2)3]–
a A = Py, b A = MeNH2, c A = Me2NH, d A = NH3,
e A = pyrazole, f A = N-methylpyrazole;
3a–c
M = K (n = 1), Hg (n = 2)
Trinitromethylborane ether complexes 4а-с may participate in exchange reaction with aromatic
nitrogen-containing heterocycles, as well as with oxaadamantanes, forming the amine complexes 7a-s or ether
complexes 4d,е, respectively. The by-products of this process are boronium salts 6 (studied in the case of
compound 6а). This transformation likely involves initial cleavage of B–C(NO2)3 bond and formation of ionic
intermediates III (Scheme 10).
Scheme 10
+
H
A
[C(NO2)3]–
4a–c
(CH2)n
O
B
H
A
A·BH2C(NO2)3
4d,e, 7a–s
benzene
room temp.
– (CH2)n
O
III
(from 4а)
Py
O
O
4 d A =
, e A =
[(Py)2BH2]+[C(NO2)3]–
O
6a
7a–s A = heterocyclic amines (Table 1)
1650

TABLE 1. Time Required for the Synthesis and Characteristics of
Products from Reactions of Oxaadamantane or Heterocyclic Amines (A)
with Complex 4а
Starting amine or oxaadamantane Reaction
Product
Mp, °С
Yield, %
(A)
time, h
A·BH2C(NO2)3
2-Oxaadamantane
2,6-Dioxaadamantane
Pyridine
254
312
1
143-144
100*
75
4d
4е
7a
7b
7c
7d
7e
7f
125-126
78-80
59*2
65
3-Methylpyridine
4-Methylpyridine
4-Ethylpyridine
1
49-50
1.5
0.7
1
104-105
70-71
63
61
3-Bromopyridine
3-Nitropyridine
79-80
64
4
109-110
127-129
85-87
55
4-Nitropyridine
23
72
70
2
76*3
60*4
63
7g
7h
7i
3-(Trinitromethyl)pyridine
4-(Trinitromethyl)pyridine
Pyrazine
149-150
125-126 (decomp.)
95-97 (decomp.)
88-89
99
7j
Pyrimidine
2
95
7k
7l
Thiazole
2
90
N-Methylimidazole
1-Methyl-5-nitroimidazole
1,2,5-Thiadiazole
1-Methyl-1,2,4-triazole
1-Methyl-1,2,3,4-tetrazole
2-Methyl-1,2,3,4-tetrazole
γ,γ'-Bipyridyl
1.5
3
81-82
63
7m
7n
7о
7р
7q
7r
7s
115-117
101-102
105-107
112-115
92-93
93
240
2
71*4
88*4
48
96
144
2
79
59*5
145-147
_______
*Recrystallized from СCl4–pentane mixture.
*2The yield of compound 7а in THF was 4%. Besides that, approximately
15% of salt [(Py)2BH2]+[C(NO2)3]- (6a) was isolated.
*3Recrystallized from benzene–C5H12 mixture.
*4Recrystallized from CH2Cl2–CHCl3 mixture.
*5The product was [γ,γ'-bipyridyl]·2[BH2C(NO2)3] complex.
We should emphasize that GLC analysis of borane ether complexes 4а-с did not reveal the presence of
ether ligands, unlike the free ethers formed in the reaction. This enabled chromatographic control of the
exchange reactions shown in Scheme 10 by quantitative determination of the eliminated cyclic ethers.
The use of weakly nucleophilic heterocycles (А) for performing the exchange reaction shown in
Scheme 10 required longer reaction duration (Table 1). The complex 4b may be useful here as well, because it
reacted in this process several times faster than the complex 4а. This was apparently caused by the lower energy
of complex formation in the case of tetrahydropyran compared to THF. The reverse reaction with complexes
7a-s or 4d,e could not be achieved even with a large excess of cyclic ether (THF or tetrahydropyran).
We should emphasize that only aromatic nitrogen-containing heterocycles containing no NH protons
could undergo the exchange reaction shown in Scheme 10. The use of other amines in this reaction (for
example, ammonia or triethylamine) destroyed the starting complexes 4а or 4b. Generally, the use of all
methods described above allowed to synthesize borane complexes BH2C(NO2)3 with planar aromatic
heterocycles, but such complexes could not be obtained with other amines.
Some chemical transformations of trinitromethyl borohydrides were studied. It was shown that ether
complexes 4а-с could be transformed not only in ligand exchange reactions (by formal B–O bond cleavage), but
also other types of boron bonds could be cleaved. Thus, the complexes 4а-с were weak reducing agents (through В–
Н bond cleavage). For example, these compounds can reduce benzoic acid to benzaldehyde. The hydrolysis and
1651

alcoholysis reactions of complex 4а usually are nonselective, resulting in cleavage of both В–Н and В–С bonds. The
use of trinitroethanol in this process, however, allowed to selectively cleave only the В–Н bonds. Unfortunately, the
residual THF could not be removed from the energetic material (compound 8) formed in this process even after
prolonged exposure to vacuum (Scheme 11).
Scheme 11
OCH2C(NO2)3
2 HOCH2C(NO2)3
THF·BH2C(NO2)3
THF + (O2N)3C B
THF, room temp., 50 h
OCH2C(NO2)3
4a
8
3 ROH –2H2
OCH2C(NO2)3
THF ·B C(NO2)3
OCH2C(NO2)3
B(OR)3 + THF + HC(NO2)3
R = H, Me
Heating the complex 4а in inert solvents gave a modeate yield of α-trinitromethyl tetrahydrofuran
(preparation of this product in radical reactions of THF with nitroform derivatives has been reported [5]).
It can not be ruled out that boronium salts of type 6, which are well soluble in common organic solvents
(СH2Cl2, THF, CHCl3, МеCN, alcohol, etc.), can be convenient trinitromethylating agents. This is illustrated by
reaction shown in Scheme 12, although we did not optimize its conditions.
Scheme 12
MeI
[(Py)2BH2]+[C(NO2)3]–
unidentified products
+
MeC(NO2)3
THF, room temp., 3 days
37%
6a
Establishing the structure of organoboron hydrides containing trinitromethyl group is a separate task.
The molecular formulas of these compounds in most cases (except those particularly unstable and sensitive to
mechanical initiation) were confirmed by elemental analysis. The structures of trinitromethyl borohydrides were
proved by physicochemical methods (Table 2).
1
The Н NMR spectra of trinitromethyl borohydrides contained a set of signals characteristic of the
ligands present, but significantly shifted downfield relative to those of free ligands. The degree of downfield
shift (Δδ, ppm) may depend on the stability of complex. The signals of protons bonded to boron atoms were
identified under the conditions of double boron-proton resonance [6]. The structures of some borohydrides were
additionally confirmed by 13C and 15N NMR spetroscopy.
The presence of В–Н bonds in trinitromethyl borohydrides was reliably confirmed by IR spectroscopy.
The characteristic stretching vibrations of these bonds were observed at 2350-2520 cm-1 [7]. Specifically, the
stretching vibrations of B–H bonds in the [BH3C(NO2)3]- anion were observed at lower frequencies than the
respective vibrations in neutral complexes L·BH2C(NO2)3 and [L2BH2]+ cations (L – ligand). The latter two
types of structures were easily distinguished by comparing the integrated intensities of the characteristic
1H NMR signals from ligand L and the ВН2 group. Besides that, the IR spectra of trinitromethyl borohydrides
contained three bands characteristic of vibrations in the trinitromethyl group [8].
The main structural issue for trinitromethyl borohydrides is distinguishing whether these are true
organoboron compounds (>B–C(NO2)3) or contain a nitronate fragment (>B–O–N(O)=C(NO2)2). The
intramolecular 1,3-С,O-migration of borohydride fragment between the two aforementioned structural types is
also quite possible. The resulting structure will thus be determined by thermodynamic control, as supported by
1652

1653

recently published computational results [4], according to which the borohydride anion [H3B–C(NO2)3]- is
significantly favored over the [H3B–O–N(O)=C(NO2)3]- anion, containing the respective nitronate fragment.
The true structure of trinitromethyl borohydrides was clearly proved by X-ray structural analysis, performed so
far only for compounds 5b (X = [PhP=N=PPh]+) [4] and 7о [1]. The main feature in these data is the В–С bond
(1.645 Å for compound 5b and 1.71 Å for compound 7о), which is present at least in the crystalline form of
these two studied trinitromethyl borohydrides.
Another approach to confirming the structure of trinitromethyl borohydrides can be the determination of
integrated intensity of antisymmetric vibrations of C(NO2)3 group, observed for these substrates at 1580 cm-1. In
the case of products 4а and 5а, it was about 30000 cm-2·mol-1·l, while the integrated intensity for one NO2
group, determined for common aliphatic nitro compounds, was about 10000 cm-2·mol-1·l. These measurements
indicate that trinitromethyl borohydrides exist as true organoboron compounds in solutions as well.
Thus, we have developed several methods for the synthesis of neutral and charged trinitromethyl
borohydride complexes. It was shown that, unlike the respective silyl nitronates, these compounds are true
organoboron compounds containing a В–С bond.
EXPERIMENTAL
1
IR spectra were recorded on a UR-10 spectrometer in KBr pellets. Н NMR spectra were acquired on
PerkinElmer R-12 (60 MHz, at 35°С), Varian DA-60 (60 MHz), RS-60 (60 MHz), and Varian HA-100
(100 MHz) spectrometers. The signals of В–Н protons were acquired with a suitable double boron-proton
resonance probe [6] on RS-60 and Varian DA-60 spectrometers. 13С NMR spectra were acquired on Bruker
HX-90 (22 MHz) and JEOL-PS-100 (25 MHz) spectrometers. 15N NMR spectra (9 MHz) were acquired on a
Bruker SXP-4-100 spectrometer in pulsed mode. The FID signals were accumulated and Fourier transformation
was performed on a РОР 8 computer. The accuracy of chemical shift determination was ±0.2 ppm (for 15N
nuclei). The 11В NMR spectra were acquired on an RS-56/19 spectrometer (19 MHz) in stationary mode
without signal accumulation and proton suppression. Elemental analysis was performed at the analytical
laboratory of the Institute of Organic Chemistry, Russian Academy of Sciences, by manual combustion of
micro samples. Melting points were determined on a Kofler hot bench. Solvents were dried prior to all
experiments. Commercially available аnhydrous amines and cyclic ethers were used.
Bis(pyridiniumyl)boranuide (Trinitromethyl)borohydride (3а).
А. A solution of HC(NO2)3 (4.12 g, 27.2 mmol) in THF (10 ml) was added to a stirred 1.77 М solution
of ВН3·THF in THF (30.80 ml, 54.5 mmol) at 10-15°С. The reaction mixture was stirred at 18°С for 30 min,
then cooled to 5-7°С, and treated with a solution of pyridine (4.37 ml, 54.5 mmol) in THF (20 ml), stirred for
additional 3 h at 18°С, evaporated at reduced pressure (water aspirator). The oily residue was extracted with
benzene (5×25 ml), and the residue was poured on a Petri dish. After evaporation in exaust hood, the residue
crystallized, yielding 5.23 g (57%) of crude product 3а. A part of this product (1.06 g, 3.16 mmol) in THF
(10 ml) was stirred at 18°С and treated with a saturated HCl solution in Et2O (3.1 ml). The reaction mixture was
stirred at 18-19°С for another 40 min, then the colorless solution was evaporated on a Petri dish in exaust hood.
The residue was treated with Et2O (15 ml), and 0.87 g of the pure salt 3а was collected by filtration. The yield
of purification was 82%.
B. A solution of HC(NO2)3 (0.81 g, 5.36 mmol) in THF (5 ml) was added to a suspension of NaBH4
(0.21 g, 5.36 mmol) in THF (10 ml); the mixture was stirred at 20°С for 45 min until the evolution of gas
ceased. The suspension was filtered, the filtrate was added to a solution of [(Py)2BH2]+Cl- salt (1.10 g,
5.36 mmol) in СНCl3 (20 ml) at 17-20°С, and stirred for another 30 min. The obtained suspension was
evaporated under vacuum, and the residue was treated with anhydrous НCl solution in ether. Yield 1.77 g
(99%).
1654

C. A solution of salt 6a (1 g, 3.12 mmol) in THF (10 ml) under the atmosphere of dry nitrogen was
stirred at 18°С and treated with 3.12 М ВН3 solution in THF (1 ml, 3.12 mmol). The reaction mixture was
stirred at room temperature for another 1 h, the solvent was evaporated from Petri dish in exaust hood, the
residue was treated with Et2O (15 ml), crystals of salt 3а were collected by filtration. Yield 1 g (97%). The
boronium salts 3b,c were obtained analogously.
Compound 3а. Mp 71-72°С (THF–Et2O). IR spectrum, ν, cm-1: 760, 1325, 1560, 1580 (С(NO2)3),
2390 (ВН3), 2455 (BH2). 1H NMR spectrum (СН2Cl2), δ, ppm: 1.15 (3Н, s, ВН3); 3.88 (2Н, s, ВН2); 7.78-8.00
(4H, m), 8.15-8.39 (2H, m) and 8.65-8.89 (4H, m, 2С5Н5N). 11B NMR spectrum (СН2Cl2), δ, ppm (J, Hz): -24.6
(q, 1JB–H = 93.0, ВН3); 2.1 (t, 1JB–H = 119.0, ВН2). Found, %: С 39.09; Н 4.48; В 6.83; N 20.37. С11Н15В2N5О6.
Calculated, %: С 39.45; Н 4.51; В 6.46; N 20.91.
THF Complex with (Trinitromethyl)borane (4a). A solution of HC(NO2)3 (2.95 g, 19.4 mmol) in THF
(7 ml) was added to a stirred 1.77 М solution of ВН3·THF in THF (22 ml, 39 mmol) at 10-15°С. The reaction
mixture was stirred for another 30 min at 5-7°С (cooling with ice-water bath) until the vigorous evolution of gas
ceased*. A solution of MeOH (1.6 ml, 39 mmol) in THF (5 ml) was added dropwise, the mixture was stirred for
another 40 min until the vigorous evolution of gas ceased. The solvents were evaporated under vacuum, the
residue was treated with hexane (~50 ml), and the precipitate was collected by filtration*2. Yield 3.95 g (87%),
1
mp 94-95°С (Et2O). IR spectrum, ν, cm-1: 805, 1320, 1580 (С(NO2)3), 2460, 2500 (BH2). H NMR spectrum
(СН2Cl2), δ, ppm (J, Hz): 2.05-2.25 (4Н, m, 3,4-СН2 THF); 2.86 (2Н, s, ВН2); 4.42 (4Н, t, J = 7.6, 2,5-СН2
1
THF). 11B NMR spectrum (СН2Cl2), δ, ppm (J, Hz): 1.0 (t, JB–H = 117.0, ВН2). Found, %: C 25.62; H 4.01;
B 4.61; N 17.75. С5Н10ВN3O7. Calculated, %: С 25.56; Н 4.29; B 4.60; N 17.88. The complexes 4b,c were
obtained analogously.
Potassium (Trinitromethyl)borohydride (5а). A suspension of KC(NO2)3 (0.6 g, 3.17 mmol) in THF
(9 ml) was stirred at 20°С under the atmosphere of dry nitrogen and treated with 1.59 М solution of ВН3·THF in
THF (2 ml, 3.17 mmol). The mixture was stirred for another 40 min, evaporated under vacuum at 10 mmHg, and the
residue was treated with 10 ml Et2O. The product was filtered off under nitrogen flow, washed with Et2O (3×10 ml).
Yield 0.64 g (99%), white powder, spontaneously explosive upon drying, autoignition temperature 192°C, soluble in
THF and MeCN. IR spectrum, ν, cm-1: 780, 1340, 1580 (С(NO2)3), 2380-2530 (ВН3). 1H NMR spectrum (MeCN),
11
1
δ, ppm: 1.05 (3Н, s, ВН3). B NMR spectrum (MeCN), δ, ppm (J, Hz): -24.4 (q, JB–H = 92.0, ВН3). (Elemental
analysis was not performed due to the extreme sensitivity of salt 5а to mechanical initiation.)
Transformations of Potassium (Trinitromethyl)borohydride (5а). А. The salt [(Py)2BH2]+Cl-
(0.54 g, 2.64 mmol) was added at 20°С to a solution of salt 5а, obtained from KС(NO2)3 (0.50 g, 2.65 mmol)
and ВН3·THF (2.65 mmol) in THF (10 ml). The reaction mixture was stirred for 1.3 h at 20°С, evaporated
under reduced pressure (10 mmHg), the residue was treated with СН2Сl2 (15 ml), filtered, the precipitate was
washed on filter with СН2Сl2 (10 ml). The combined filtrate was evaporated under vacuum, giving the salt 3а.
Yield 0.87 g (95%), mp 71-72°С. No melting point depression was observed for a sample mixed with authentic
reference compound.
B. A solution of salt 5а, obtained from KС(NO2)3 (1.00 g, 5.3 mmol) and ВН3·THF (5.3 mmol) in THF
(15 ml), was treated with anhydrous НCl (5.3 mmol) solution in Et2O (5 ml), stirred for another 2.5 h at 20°С
until the gas evolution ceased, and evaporated under vacuum. The residue was extracted with Et2O (3×50 ml),
_______
*For spectral characterization of salt 2 at this stage, a quarter of the solution was concentrated by 2/3 under
vacuum; the residue was dissolved in СН2Cl2 (15 ml). This procedure was repeated two more times, then
cautiously evaporated under vacuum to dryness, the residue was dissolved in organic solvents and spectra were
recorded.
*2This procedure occasionally resulted in detonations. To prevent that, we recommend to concentrate the
reaction mixture by 2/3, dilute the residue with hexane (30 ml), decant the liquid from precipitate, dissolve the
precipitate in benzene, filter, and evaporate the filtrate on a Petri dish, followed by recrystallization from Et2O–
hexane mixture.
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the combined extract was filtered, the filtrate was evaporated under vacuum, giving the complex 4а. Yield
1.16 g (93%), mp 94-95°С. No melting point depression was observed for a sample mixed with authentic
reference compound. The IR spectrum of the obtained complex 4а was identical to that described above.
C. A solution of salt 5а, obtained from KС(NO2)3 (5.16 g, 27.3 mmol) and ВН3·THF (27.3 mmol) in
THF (50 ml), was treated by dropwise addition of Py·HCl (3.60 g, 27.3 mmol) in CHCl3 (10 ml). The reaction
mixture was left overnight, the precipitate that formed was filtered off, washed with THF (10 ml); the filtrate
was evaporated under vacuum, giving pyridinium (trinitromethyl)borohydride (5b). Yield 5.32 g (80%),
cream colored powder, mp 70-72°С (decomp.). IR spectrum, ν, cm-1: 750-760, 1340, 1580 (С(NO2)3), 2395
1
(ВН3). H NMR spectrum (CH2Cl2), δ, ppm: 1.20 (3Н, s, ВН3); 8.03-8.68 (5Н, m, C5H5N); 12.25 (1Н, br. s,
NH). 11B NMR spectrum (CH2Cl2), δ, ppm (J, Hz): -24.3 (q, 1JB–H = 93.0, ВН3). Found, %: С 29.67; Н 3.82; В 4.64;
N 23.08. С6Н9ВN4O6. Calculated, %: С 29.54; Н 3.72; В 4.43; N 22.96.
Bis(pyridiniumyl)boranuide Trinitromethanide (6а). A suspension of KC(NO2)3 (1.80 g, 9.43 mmol)
in THF (28 ml) was stirred at 15-17°С under the atmosphere of dry nitrogen and treated by adding the salt
[(Ру)2BH2]+Cl- (1.95 g, 9.43 mmol) in CHCl3 (19 ml). The reaction mixture was stirred for further 3 h, filtered;
the precipitate was washed with THF (3×15 ml). The combined filtrate was evaporated at reduced pressure
(water aspirator). Yield 3.02 g (close to 100%), yellow crystals, mp 102-103°С (THF–Et2O). IR spectrum, ν,
-
1
cm-1: 1280, 1500 [C(NO2)3 ], 2460, 2495 (BH2). H NMR spectrum (СН2Cl2) δ, ppm: 3.86 (2Н, s, ВН2);
7.85-8.05 (4H, m), 8.30-8.50 (2H, m) and 8.78-8.96 (4H, m, 2С5Н5N). Found, %: С 41.30; Н 4.05; В 3.49;
N 21.75. С11Н12ВN5O6. Calculated, %: С 41.15; Н 3.77; В 3.37; N 21.81. The boronium salts 6b-f were
obtained analogously.
Exchange Reactions of Complex 4а with Heterocyclic Amines and Oxaadamantanes. A solution of
complex 4а (1 g, 4.25 mmol) in benzene (10 ml) was stirred at 15-20°С and treated with a solution of
heterocyclic amine or oxaadamantane (А) (4.25 mmol) in benzene (5 ml) (Scheme 10, Table 1). The reaction
mixture was stirred for the indicated time at room temperature, with periodic control of the amount of evolved
THF by GLC (60-70°С, adsorbent – chromosorb, liquid phase – PEG monolaurate). After the increase of THF
signal stopped, the reaction mixture was evaporated from Petri dish under exaust hood, the residue was
extracted first with hexane and then with Et2O. The combined extract was evaporated under vacuum at room
temperature, the residue was recrystallized from Et2O–hexane mixture. Complexes of trinitromethylborane with
amines (compounds 7а-s) or with oxaadamantanes (compounds 4d,e) were thus obtained. The main process
characteristics, as well as yields and melting points of target complexes 4d,e and 7а-s are given in Table 1.
Complex of 2λ3-Oxatricyclo[3.3.1.13,7]decane and (Trinitromethyl)borane (4d). Yield 100%, off-
white crystals with creme tinge, mp 143-144°С. IR spectrum, ν, cm-1: 800, 1325, 1585-1595 (С(NO2)3), 2455-
1
2500 (BH2). H NMR spectrum (СНCl3), δ, ppm: 1.60-2.40 (12Н, m, Н Ad); 2.68 (2Н, s, ВН2); 4.82 (2Н, s,
Н Ad). 11B NMR spectrum (CHCl3), δ, ppm (J, Hz): -0.5 (t, J = 120.0, ВН2). Found, %: С 40.47; Н 5.75;
В 3.56; N 14.10. С10Н16ВN3О7. Calculated, %: С 39.90; Н 5.36; В 3.59; N 13.96.
Complex of 2λ3,6-Dioxatricyclo[3.3.1.13,7]decane and (Trinitromethyl)borane (4е). Yield 75%, white
1
crystals, mp 125-126°С. H NMR spectrum (СНCl3), δ, ppm: 2.06-2.18 (8Н, m, Н Ad); 2.73 (2Н, s, ВН2);
4.00-4.20 (2Н, m, Н Ad); 4.85-4.95 (2Н, m, Н Ad). Found, %: С 35.66; Н 4.75; В 3.49; N 13.53. С9Н14ВN3О8.
Calculated, %: С 35.67; Н 4.66; В 3.57; N 13.87.
Complex of Pyridine and (Trinitromethyl)borane (7а). Yield 59%, cream colored powder, mp
1
78-80°С. IR spectrum, ν, cm-1: 800, 1320, 1570 (С(NO2)3), 2450-2485 (BH2). H NMR spectrum (СН2Cl2), δ,
ppm: 3.06 (2Н, s, ВН2); 8.20-8.38 (2H, m), 8.70-8.86 (1H, m) and 9.10-9.30 (2H, m, С5Н5N); 11B NMR
1
spectrum (CH2Cl2), δ, ppm (J, Hz): -6.7 (t, JB–H = 107.0, ВН2). Found, %: С 29.59; Н 2.79; В 4.49; N 23.15.
С6Н7ВN4О6. Calculated, %: С 29.79; Н 2.92; В 4.47; N 23.16.
Interaction of Complex 4а with 2,2,2-Trinitroethanol. A solution of complex 4а (1.5 g, 6.4 mmol) in
THF (5 ml) was treated by adding a solution of 2,2,2-trinitroethanol (2.3 g, 13.0 mmol) in THF (5 ml). The
1656

mixture was maintained for 50 h at room temperature, while approximately 10 mmol of Н2 slowly evolved, then
evaporated at reduced pressure (10 mmHg). The residual thick oil was additionally evaporated for 10 h at
1 mmHg, the residue was washed with heptane (3×20 ml), the residue was then dissolved in benzene, dried over
MgSO4, and evaporated under vacuum. Bis(2,2,2-trinitroethyl) ester of (trinitromethyl)boronic acid (8) that
partially existed as THF complex was obtained as oily product (1.9 g). IR spectrum, ν, cm-1: 800, 1300, 1600
1
(С(NO2)3). H NMR spectrum (benzene), δ, ppm: 1.25-1.55 (4Н, br. m, 3,4-СН2 THF); 3.99 (4Н, br. s,
2ОСН2С(NO2)3); 4.04-4.24 (4Н, m, 2,5-СН2 THF) (the signal intensity ratio was approximately 3:4:3).
Interaction of Boronium Salt 6а with MeI. Methyl iodide (2 ml, 32.10 mmol) was added to boronium
salt 6a (0.8 g, 2.49 mmol) in THF (5 ml). The mixture was left for 3 days at room temperature, cautiously
evaporated under vacuum, the residue was extracted with Et2O, ether was removed by evaporation, giving
1,1,1-trinitroethane. Yield 0.15 g (37%), white crystals, mp 56°С (mp 57°С [8]). No melting point depression
was observed for a sample mixed with authentic reference compound.
The supplementary information file containing physicochemical and spectral characteristics of the
synthesized compounds, as well as X-ray structural analysis data for compound 7о is available for authorized
users.
This work was performed with financial support of the Russian Foundation for Basic Research (Grant #
15-03-05703).
The authors would like to express their gratitude to Corresponding Member of the Russian Academy of
Sciences Yu. T. Struchkov and Candidate of Chemical Sciences A. А. Gusev for performing X-ray structural
analysis of compound 7о and solving its structure.
The authors also acknowledge the assistance by Doctor of Chemical Sciences V. S. Bogdanov and
Candidate of Chemical Sciences V. V. Negrebetskii in acquiring and interpreting the double boron-proton
resonance spectra.
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