Investigation of the effect of metallic lewis acid on the nitrolysis of [3,7-diacetyl-1,3,5,7-tetraazabicyclo(3.3.1)-nonane] and hexamine

Article abstract of DOI:10.1002/cjoc.201190079

The effect of metallic ions on the nitrolysis of DAPT [3,7-diacetyl-1,3,5, 7-tetraazabicyclo(3.3.1)nonane] and HA (hexamine) was investigated by experimental and theoretical approaches. The combinatorial reagent, M(NO ^-₃)_n/Ac₂/NH₄NO ₃ (M=Mg²⁺, Cu²⁺, Pb²⁺, Bi ³⁺, Fe³⁺ and Zr⁴⁺), was found to be efficient in the experiment of the nitrolysis of DAPT. A key intermediate during the nitrolysis of DAPT was detected by ¹H NMR. The formation mechanism of the intermediate was proposed and analyzed. Some discrepant results for the nitrolysis of DAPT and HA catalyzed by different metallic nitrates were explained based on hard-soft and acid-base principle and stabilized energy of ion-complex. From the latter point of view, some cations with high polarizable ligands, e.g., OSO₂CF₃^-, (CF₃SO ₂)₂N^-, and (C₄F₉SO ₂)₂N^-, can increase the yields. Two newly designed catalysts, Cu[(CF₃SO₂)₂N]₂ and Cu[(C₄F₉SO₂)₂N]₂, were tested to be highly efficient. Copyright

Full text of DOI:10.1002/cjoc.201190079

FULL PAPER
Investigation of the Effect of Metallic Lewis Acid on the
Nitrolysis of [3,7-Diacetyl-1,3,5,7-tetraazabicyclo(3.3.1)-
Shi, Liangweia(史良伟n) onane] and Hexamine
Yu, Menglonga(俞梦龙)
Zhang, Yazhua(张亚竹)
Zhao, Gang*,a(赵刚)
Qin, Guangmingb(覃光明)
Lü, Jianb(吕剑)
a Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China
b Xi'an Modern Chemistry Research Institute, Xi'an, Shaanxi 710065, China
The effect of metallic ions on the nitrolysis of DAPT [3,7-diacetyl-1,3,5,7-tetraazabicyclo(3.3.1)nonane] and
HA (hexamine) was investigated b＋y exp＋erimental and theoretical approaches. The combinatorial reagent,
＋
＋
＋
＋
M(NO3− )n/Ac2O/NH4NO3 (M＝Mg2 , Cu2 , Pb2 , Bi3 , Fe3 and Zr4 ), was found to be efficient in the experi-
1
ment of the nitrolysis of DAPT. A key intermediate during the nitrolysis of DAPT was detected by H NMR. The
formation mechanism of the intermediate was proposed and analyzed. Some discrepant results for the nitrolysis of
DAPT and HA catalyzed by different metallic nitrates were explained based on hard-soft and acid-base principle
and stabilized energy of－ion-complex. From the latter point of view, some cations with high polarizable ligands, e.g.,
OSO2CF−, (CF3SO2)2N , and (C4F9SO2)2N－, can increase the yields. Two newly designed catalysts, Cu[(CF3SO2)2N]2
3
and Cu[(C4F9SO2)2N]2, were tested to be highly efficient.
Keywords metallic ions, nitrolysis, 3,7-diacetyl-1,3,5,7-tetraazabicyclo(3.3.1)nonane
Introduction
Scheme 1 Synthesis of RDX and HMX via the nitrolysis of
DAPT or HA
RDX (1,3,5-trinitro-1,3,5-triazacyclohexane) and
HMX (1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane) are
two famous munitions. Their production processes in-
clude Bachmann approach using hexamine (HA) as ma-
terial and some indirect processes, using DAPT [3,7-di-
acetyl-1,3,5,7-tetraazabicyclo(3.3.1)nonane],1 DADN
(1,5-diacetyl-3,7-dinitro-1,3,5,7-tetraazacyclooctane),2
DPT
[3,7-dintro-1,3,5,7-tetraazacyclooctane](3.3.1)-
nonane],3,4 TAT (1,3,5,7-tetraacetyloctahydro-1,3,5,7-
tetrazocine),5 SEX (1-acetyl-octahydro-3,5,7-trinitro-
1,3,5,7-tetrazocine),2 and TAX(1-acetylhexahydro-3,5-
dinitro-1,3,5-triazine), etc., as reactants. Among these
synthesis routes, an important precursor DAPT synthe-
sized by acetylation of HA, is a suitable material to syn-
thesize HMX and RDX. DAPT is firstly converted to
DADN using a mixture of H2SO4 (or polyphosphoric acid)
and HNO3, and then DADN is nitrified to HMX or RDX
by N2O5/Ac2O/HNO3. Although the nitrolysis of DADN
or TAT can all produce HMX with higher yield through
the use of HNO3/P2O5 or HNO3/N2O5,5,6 a large amount
of N2O5 or P2O5 included in these techniques increase the
cost and trouble to use. Hence, some green and economi-
cal approaches for the synthesis of RDX and HMX
should arouse out attentions on these reaction processes.
To improve the production process of HMX or RDX
from DAPT or HA (Scheme 1), a combination method
through experimental exploration and theoretical
calculation could be potent. Owing to the sensitivity of
this reaction to acid, a mixed reagent of metallic salts
and Ac2O is usually used in highly efficient nitration or
nitrolysis reactions. Herein, a batch of new＋reagents
Bi , Fe , and Zr ) were firstly studied in the nitroly-
sis reaction of DAPT. Some of these ternary mixed re-
agents tested to be of high efficiency were also used in
the nitrolysis of HA. Theoretically, on the basis of
hard-soft and acid-base principle7,8 and the theoretical
stabilized energy of metallic Lewis salts with NO3− , a
reasonable explanation for the different experimental
results was made. We mainly elaborated the key roles of
2＋
2＋
M(NO3− )n/Ac2O/NH4NO3 (M＝Mg , Cu2 , Pb
,
3＋
3＋
4＋
*
E-mail: zhaog@mail.sioc.ac.cn
Received July 19, 2010; revised September 20, 2010; accepted November 15, 2010.
Chin. J. Chem. 2011, 29, 283— 287
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Shi et al.
FULL PAPER
1
the cation and anion of metallic Lewis salt. Moreover,
based on the experimental results and theoretical analy-
sis achieved, a newly developed reaction of catalytic
nitrolysis was also performed. The object in this re-
search is to find not only potentially green routes of the
synthesis of RDX or HMX but also new catalytic reac-
tions that exceed current procedure.
(Scheme 2), which is supported by H NMR analysis
shown in Figure 1. Two possible mechanisms for the
formation of M_1 are proposed in Figure 2. There are
two different bond-cleaving modes of C—N bonds
shown in route_1 and route_2. The bond-cleaving mode
in route_1 makes for the products of HMX and M_1,
and that in route_2 makes for the products of RDX and
M_1. Similar to the electrophilic nitration process of
aromatic compound, the nitrolysis of DAPT in Figure 2
also undergoes a process attacked by NO+2 through
electrostatic interaction. According to DAPT structure
and its calculated electrostatic potential (ESP) isosur-
face shown in Figure 3, the metallic ion is speculated to
affect the negative ESP region. When NO+2 attacks
one of the electrophilic positions located at N(1) and
N(2) atoms of DAPT, the C—N bond in tetraazacyclic
structure cleaves more easily than the C—N bond of the
bridge ring. The main nitrolysis product RDX obtained
in our experiment supported the conclusion. Therefore,
the mechanism proposed in route_2 is prevailing. It
should be pointed out that less HMX (less 3%) was also
found when the nitrolysis reagent of M(NO3)n/Ac2O/
NH4NO3 was used. As for the regulation role of
NH4NO3, it has been discussed in the nitrolysis mecha-
nism of hexamine recently.10
Experimental and theoretical study
Two procedures have been used to synthesize HMX
from DAPT, one with NH4NO3 and the other without it.
The procedure without NH4NO＋3 using Ac2O (10 e＋quiv.)
＋
＋
＋
and M(NO3)n (M＝Mg2 , Cu2 , Pb2 , Bi3 , Fe3 and
＋
Zr4 ) (2.5 equiv.) gave the RDX only with ＜5% yield.
While the procedure with NH4NO3 using NH4NO3/98＋%
HNO3 (2.5 equiv.) and M(NO3)n/Ac2O (M＝Mg2
,
＋
＋
＋
＋
＋
Cu2 , Pb2 , Bi3 , Fe3 and Zr4 ) (2.5 equiv.) obtained
a better result.
For the determination of the ratio of RDX and HMX,
the reaction solvent is always removed under vacuum.
The crude product was then purified by column chro-
matography to make product as a white solid. The ratio
1
of RDX and HMX was determined by H NMR. The
mixture was recrystallized in acetone to give pure RDX
1
and HMX. RDX: m.p. 201 ℃; H NMR (CD3SOCD3,
300 MHz) δ: 6.09 (s, 6H); 13C NMR (CD3SOCD3, 75
－1
Scheme 2 Ring-opening reaction to form M_1 via the nitrolysis
of DAPT
MHz) δ: 40.05; IR (KBr) ν: 3067, 1573, 1269, 916 cm .
HMX: m.p. 285 ℃; 1H NMR (CD3SOCD3, 300 MHz) δ:
－1
6.02 (s, 8H); IR (KBr) ν: 3055, 1570, 1273, 910 cm .
The calculation is based on density functional theory
(DFT). The generalized gradient corrected exchange
correlation function (GGA) has been used in our study.
The functional formalism in GGA is PW91. All the at-
oms in the model molecules or ions are relaxed to their
equilibrium positions so that the energy threshold is 1.0
－5
×10 eV/atom, the maximum force and the displace-
ment of atoms converge at 0.002 Hartree and 0.005 Å,
respectively. The H, N, C and O atoms are treated as all
electron calculations, while the Cu, Fe, Bi, Zr and Pb
elements are calculated using DFT semi-local pseudo-
potential (DSPP)9 which replaces the effects of core
electrons with a simple potential. Such calculation has
less computational cost and includes some degree of
relativistic effects. In order to speed up SCF conver-
gence, the iterative subspace (DIIS) and thermal smear-
ing techniques have been used. 0.016 au/Å (about ±10
kcal/mol) as the isosurface is chosen in the calculation
of electrostatic potential.
It is interesting that the yield of the main product
RDX increases along with the adding of Ac2O and
＋
2 ＋
NH4NO3 into M(NO3− )n/DAPT (M＝Mg , Cu2
,
＋
＋
＋
＋
Pb2 , Bi3 , Fe3 and Zr4 ). Usually, the reacti＋on for the
＋
M(NO3− )n/Ac2O/NH4NO3/DAPT (M＝Mg2 , Cu2
,
＋
＋
＋
＋
Pb2 , Bi3 , Fe3 and Zr4 ) system finished within 8 h
at room temperature, but within 4 h at 65 ℃. The yield
increased appreciably when the molecular sieve was
added. It is known that DAPT is synthesized from HA,
so a good yield of RDX can also be obtained when the
reaction system is changed int＋o M(NO3− )n/NH4NO3
Results and discussion
＋
＋
＋
＋
＋
Ac2O/HA (M＝Mg2 , Cu2 , Pb2 , Bi3 , Fe3 and Zr4 ).
This method is more efficient than those previonsly re-
ported.11 The detailed experimental results for the ni-
trolysis of DAPT and＋HA using ＋M(N＋O3− )n/NH4NO3
The nitrolysis of DAPT initially performed using
Bi(NO3)3•5H2O/Ac2O as the catalyst gave a product
without HMX. The results were the same for those reac-
tions using catalysts, such as Cu(NO3)3•3H2O,
Fe(NO3)3•9H2O, and Zr(NO3)4•5H2O, even with mo-
lecular sieve. The reason may be attributed to the
ring-opening of DAPT to form an intermediate, M_1
＋
＋
＋
Ac2O (M＝Mg2 , Cu2 , Pb2 , Bi3 , Fe3 and Zr4 ) are
summarized in Table 1. It is evident that the result using
Fe(NO3)3•5H2O or Cu(NO3)2•5H2O is preferable.
284
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Chin. J. Chem. 2011, 29, 283— 287

Nitrolysis of [3,7-Diacetyl-1,3,5,7-tetraazabicyclo(3.3.1)nonane] and Hexamine
Figure 1 1H NMR spectrum of M_1.
Figure 2 Two possible mechanisms for the formation of M_1.
Chin. J. Chem. 2011, 29, 283— 287
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285

Shi et al.
FULL PAPER
Lewis acid, which also explains the considerabl＋e yield
＋
when Fe(NO3)3•5H2O catalyst is added. Thus, Cu2 , Bi3 ,
＋
＋
Fe3 and Zr4 ions can be attributed to soft Lewis acid,
＋
＋
while Mg2 and Pb2 ions are hard Lewis acid. In brief,
more soft Lewis acid gives higher yield in this reaction.
Besides the relationship established between the yield
of RDX and the hardness of model cations, the role of
cations in the nitrolysis reaction was further speculated to
regulate the production of NO+2 . So six theoretical com-
plex models with＋ a str＋uctural formula, M(NO3− )n (NO3− )
Figure 3 DAPT structure and its electrostatic potential surface.
Table 1 Nitrolysis results using different metallic nitrates
＋
＋
＋
＋
(M＝Mg2 , Cu2 , Pb2 , Bi3 , Fe3 and Zr4 ), were pro-
posed and calculated. These optimized model structures
and the stabilized energies ＋calcul＋ated through M(NO3)n
Metallic salt
Mg(NO3)2•6H2O
Cu(NO3)2•3H2O
Pb(NO3)4
Yield/%_DAPTa
Yield/%_HAb
11
62
36
54
59
43
＜ 5
＋
＋
＋
4＋
(M＝Mg2 , Cu2 , Pb2 , Bi3 , Fe3 and Zr ) and NO3−
63
were illustrated together in Figure 4. The stabilized ener-
45
58
69
47
gies order is not correlative to the ionic rad＋ii (0.7＋2, 0.7＋3,
＋
1.19, 1.03, 0.6＋5 and 0.72 Å for Mg2 , Cu2 , Pb2 , Bi3 ,
Bi(NO3)3•5H2O
Fe(NO3)3•9H2O
Zr(NO3)4•5H2O
＋
Fe3 and Zr4 , respectively), but the ordering of the
hardness of corresponding monatomic cations has similar
rank to the stabilized energies, and therefor＋e the s＋tabilized
a The reactant is DAPT. b The reactant is HA.
2＋
energies of M(NO3− )n (NO3− ) (M＝Mg2 , Cu2 , Pb ,
＋
＋
＋
Bi3 , Fe3 and Zr4 ) can be also used to explain the ex-
perimental yield partly. It is obvious that the anions have
effects on the role of Lewis acid of model cations and the
formation of NO+2 . But it should be noted that the stabi-
lized energies of the ion-complexes can drop when the
NO3− ligand is changed to some polarizable ligands.
Compared with the stabilized energy of Cu(NO3− )2 with
NO3− , － 53.68 kcal/mol, the stabilized energies of
To further explain the results obtained in experiment,
theoretical calculation based on DFT method was ap-
plied. First of all, based on hard-soft and acid-base prin-
ciple, the ionization potential (I), electron affinity (A),
absolute electronegativity (χ), and hardness ＋(η) of
2＋
2＋
2＋
3＋
monatomic metallic ions (Mg , Cu , Pb , Bi3 , Fe
4＋
and Zr ) were calculated and listed in Table 2. The
calculated results are generally consistent with the ex-
perimental data achieved. Large values of χ for the
model ions are characteristic of Lewis acids, so all cata-
lysts herein are Lewis acid type of metallic salts. The
last column of data η in Table 2 is found to be associ-
ated with the reaction yield listed in Table 1. The hard-
Cu[OSO2CF3]2 with OSO2CF− , Cu[(CF3SO2)2N]2 with
3
(CF3SO2)2N－, and Cu[(C4F9SO2)2N]2 with (C4F9SO2)2N－
were calculated to be －43.48, －40.01 and －22.10
kcal/mol, respectively. On the basis of the relationship
between the yield and stabilized energy, the yields will
increase when the an－ionic species are changed to
2＋
ness value of metallic ion Mg is the largest, accord-
OSO2CF− , (CF3SO2)2N , (C4F9SO2)2N－, etc.
3
ingly, the nitrolysis yield of DAPT or HA is the lowest
To confirm the theoretical conclusion and speculation
discussed above, two catalysts including high polarizable
anions, Cu[(CF3SO2)2N]2 and Cu[(C4F9SO2)2N]2, were
chosen into the two nitrolysis systems, Cu[(CF3SO2)2N]2/
HNO3/NH4NO3/Ac2O/HA (system 1) and Cu[(C4F9-
SO2)2N]2/HNO3/NH4NO3/Ac2O/HA (system 2). During
the experiments, NH4NO3 in 2.5 and 3.7 equiv. and
HNO3 in 4.8 and 7.0 equiv. were used for system 1 and
system 2, respectively. The dosages of Cu[(CF3SO2)2N]2
and Cu[(C4F9SO2)2N]2 are all 10 mol%. The nitrolysis
yield and the ratio of RDX/HMX were summarized in
Table 3. The results prove that the catalysts are feasible
and efficient. The metallic Lewis acid with high
when the Mg(NO3)2•6H2O catalyst is used. Although
＋
Pb2 has a lower value of η among the model ions in
Table 2, due to the cooperation of ligand field effect and
the inert f electrons in lead element, it st＋ill belongs to
hard Lewis acid. So the reaction with＋Pb2 ion gives a
relative lower yield. As for the Fe3 , the strong po-
larizability from d electrons makes its hardness value
turn to be lower, so it also belongs to the soft
Table 2 Calculated ionization potential (I), electron affinity (A),
absolute electronegativity (χ), and hardness (η) for monatomic
cations (eV)
Ion
I
A
χ
η
Mg2+ 67.11 (80.14) 18.45 (15.04) 42.78 (47.59) 24.33 (32.55)
Cu2+ 29.51 (36.83) 22.21 (20.29) 25.86 (28.56) 3.65 (8.27)
Pb2+ 24.13 (31.94) 17.10 (15.03) 20.62 (23.49) 3.52 (8.46)
Table 3 Nitrolysis results of HA catalyzed by Cu[(CF3SO2)2N]2
and Cu[(C4F9SO2)2N]2
Entry
Catalyst
mol% Yielda/% RDX/HMX
Bi3+
Fe3+ 45.53 (54.80) 39.94 (30.65) 42.74 (42.73) 2.80 (12.08)
Zr4+
73.53 41.58 57.56 15.98
a Experimental data in parenthesis cited from reference.12
35.68
27.34
31.51
4.17
1
2
Cu[(CF3SO2)2N]2
Cu[(C4F9SO2)2N]2
10
10
91
88
1/2.7
1/5.4
a Total yield of RDX and HMX.
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Chin. J. Chem. 2011, 29, 283— 287

Nitrolysis of [3,7-Diacetyl-1,3,5,7-tetraazabicyclo(3.3.1)nonane] and Hexamine
polarizable anion improves not only the yield of the ni-
trolysis of HA or DAPT but also the ratio of HMX.
proposed, and one mechanism that the C—N bond in
tetraazacyclic structure cleaves more easily than that in
the bridge ＋ring. When＋the M(NO3)n/A＋c2O/NH4NO3 (M＝
＋
＋
＋
Mg2 , Cu2 , Pb2 , Bi3 , Fe3 and Zr4 ) reagent was used
in the nitrolysis reaction of DAPT or HA, the main prod-
uct was RDX with high yield. The theoretical results in-
dicate that the model nitrates behave as Lewis acid type
of salts, and the cations with the largest hardness give the
＋
lowest yield, e.g., Mg2 . The order o＋f the stabilized ene＋r-
2＋
2＋
gies in M(NO3− )n (NO3− ) (M＝Mg2 , Cu , Pb , Bi3 ,
＋
＋
Fe3 and Zr4 ) can explain the difference in yield. The
model of Cu(NO3− )2 (NO3− ) has the lowest stabilized
energy, and accordingly, it gives the best catalytic per-
formance relative to other models. From the stabilized
energy point of view, a higher yield can be expected
when the NO3− ligand is changed to －some polarizab－le
ligands, e.g., OSO2CF− , (CF3SO2)2N , (C4F9SO2)2N ,
3
etc. Two newly designed catalysts, Cu[(CF3SO2)2N]2 and
Cu[(C4F9SO2)2N]2, were tested to be highly efficient in
the nitrolysis of HA.
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© 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
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