Polyazidopyrimidines: High-energy compounds and precursors to carbon nanotubes

Article abstract of DOI:10.1002/anie.200602778

(Figure Presented) An additional azidomethyl group dramatically decreases the melting points of the corresponding azidopyrimidines. Theoretical calculations show that these polyazido compounds (see picture; N light blue, C dark blue, H white) exhibit high

Full text of DOI:10.1002/anie.200602778

Communications
Polyazido Compounds
DOI: 10.1002/anie.200602778
Polyazidopyrimidines: High-Energy Compounds
and Precursors to Carbon Nanotubes**
Chengfeng Ye, Haixiang Gao, Jerry A. Boatz,
Gregory W. Drake, Brendan Twamley, and
Jeanꢀne M. Shreeve*
Organic polyazido-substituted compounds are at the fore-
front of high-energy research. Polyazido organic compounds
have high relative heats of formation as one azido group adds
ꢀ1
about 87 kcalmol of endothermicity to a hydrocarbon
[
1]
compound. In this family of compounds, 3,6-diazido-
,2,4,5-tetrazine (1) has the highest reported heat of forma-
1
ꢀ1
ꢀ1 [2]
tion, about 1101 kJmol (6709 kJkg ). The compound
4
,4’,6,6’-tetraazido-2,2’-azo-1,3,5-triazine (2) has a heat of
ꢀ1
ꢀ1
[3]
formation of 2171 kJmol
(6164 kJkg ; Scheme 1).
Recently it was demonstrated that 1 and 2 are good
precursors to carbon nitride nanomaterials. Thermal decom-
[
4]
[5]
position of 1 and 2 yields nitrogen-rich nanolayered,
Scheme 1. Examples of reported polyazido compounds.
[*] Dr. C. Ye, Dr. H. Gao, Dr. B. Twamley, Prof. J. M. Shreeve
Department of Chemistry
University of Idaho
Moscow, ID 83844-2343 (USA)
Fax: (+1)208-885-9146
E-mail: jshreeve@uidaho.edu
Dr. J. A. Boatz
Space and Missile Propulsion Division
Air Force Research Laboratory
10 East Saturn Blvd.
Edwards AFB, CA 93524 (USA)
Dr. G. W. Drake
NASA Solid Propellants Branch ER22
Marshall Space Flight Center
Huntsville, AL 34812 (USA)
[
**] J.M.S., H.G., and C.Y. gratefully acknowledge the financial support
of the AFOSR (Grant F49620-03-1-0209), NSF (Grant CHE0315275),
and ONR (Grant N00014-02-1-0600). The Bruker (Siemens) SMART
APEX diffraction facility was established at the University of Idaho
with the assistance of the NSF-EPSCoR program and the M. J.
Murdock Charitable Trust, Vancouver, WA (USA).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Chemie
nanoclustered, and nanodendritic carbon nitrides, depending
on the different heating processes.
is not as detonation-sensitive as 1. However, extreme care is
absolutely necessary!
To date, many of the reported polyazido compounds are
based on C/N heteroaromatic cycles, including tetrazine (as in
For purposes of comparison, triazido- and tetraazidopyr-
imidine derivatives 9 and 11 were prepared as outlined in
Scheme 2. The solid-state structures of these two compounds
[
4]
[6]
[7]
[8]
1
), triazine (as in 3), heptazine, triazole, and tetra-
[
9]
ꢀ
6
[10]
[19]
zole , and inorganic anions, including [P(N ) ] ,
were established by single-crystal X-ray analysis (Figure 1).
3
ꢀ
[11]
2ꢀ [12]
ꢀ
[13]
ꢀ
[14]
[
[
B(N ) ] ,
[Si(N ) ] ,
[Te(N ) ] ,
[Ti(N ) ] ,
3
3
4
3
6
3
5
5
ꢀ
[15]
3ꢀ [16]
Sb(N ) ] , and [U(N ) ] , as well as triazidocarbenium
3
6
3
7
[
1]
perchlorate and dinitramide (4; Scheme 1). Few reports
focus on pyrimidine as a basic building block. Although 2,4,6-
[
17]
triazidopyrimidine (5) has been known since 1979,
it has
mainly been used for photochemical reactions. Few physical
properties of this compound are known.
Preliminary calculations show that 5 has a positive heat of
formation comparable to that of 2,4,6-triazido-1,3,5-triazine
(
3). Herein, we report the synthesis of a series of polyazido-
Figure 1. Thermal ellipsoid plot (ellipsoids at 30% probability) of 9 (a)
and 11 (b). Hydrogen atoms have been included but not labelled.
pyrimidine compounds, their calculated heats of formation,
and their role as a source of carbon nanotubes by detonation
generation.
[
1]
Currently, except for the triazidocarbon cation in 4,
Compound 9 is planar and packs in layers, in a fashion similar
[
6a]
molecules with three or four azido groups on the same
carbon atom have not been available because of their
instability, high sensitivity, and difficulty in synthesis. Only a
few geminal diazido compounds have been prepared from the
SnCl - or ZnCl -catalyzed reaction of substituted benzalde-
hyde or acetophenone with TMSN3. However, heterocycles
based on geminal diazido compounds have not been reported.
We have prepared pentaazidopyrimidine 7 starting from 5-
carboxyaldehyde-2,4,6-triazidopyrimidine (6; Scheme 2). The
best yield of 35% could be achieved by first substituting the
three chlorine atoms on the pyrimidine ring with azido groups
and subsequently transforming the aldehyde group into a
geminal diazido functionality by using TMSN /SnCl .
to the packing of 3.
However, in 9 the molecules have
significant overlap with the adjacent layer. The layers are also
farther apart (3.19 Š) as the steric bulk of the methyl group
prevents closer association. There are no significant intermo-
lecular interactions in either 3 or 9. The structure of 11 is quite
different. The packing no longer displays flat, planar sheets,
but stacks along the a axis. These stacks are composed of two
alternating molecules. The gap between the molecules in
these stacks is about 3.26 Š at the maximum separation with
108 angles between the heterocycles. There are also weak
nonclassical hydrogen bonds between the methylene group
and terminal azido nitrogen atoms, both within the stacks
(C7–N13, ca. 3.31 Š) and between the stacks (C7–N19,
ca. 3.41 Š). This compound was difficult to crystallize and
exhibits disorder in the azidomethyl group as well as rota-
2
2
[
18]
3
2
Remarkably, this pentaazido compound is a liquid at room
temperature with a melting point at about ꢀ488C and has
good thermal stability up to about 1798C. It is noteworthy
that 7 can be purified by column chromatography and routine
handling while avoiding external heating. This suggests that 7
[
19]
tional twinning.
The melting point of the triazido analogue 9 is 1038C,
while the addition of one extra azido group reduces the
melting point drastically by about 808C to 22.58C in 11
(Table 1). The introduction of this fourth azido group does
not result in any obvious decrease in thermal stability. The
presence of a fifth azido group in 7 led to a decrease in the
melting point by an additional 708C with just a slight decrease
in thermal stability (Table 1).
A possible rationale for the lower melting point observed
for the (azidomethyl)pyrimidine compound is the inefficient
packing caused by the azidomethyl group. The azido group in
1
1 is disordered (see above) and exhibits free rotation around
Table 1: Physical properties of polyazido compounds.
ꢀ3
o
o
o
298
[a]
ꢀ1
Compd. Density [gcm
]
M.p. [ C] T (DSC) [ C] D H
[kJmol
]
d
f
[
b]
c]
1
3
7
9
1
1
1.72
1.72
1.71
1.55
1.65
130
94
130
180
179
195
193
1121.7
1136.0
1807.1
1087.4
1452.7
2192.0
[
ꢀ48
103
1
2
22.5
o
ꢀ1
[
a] Calculated value in the gas phase. [b] D H
(solid)=1101 kJmol
,
f
298
ꢀ1
o
Scheme 2. Synthesis of polyazidopyrimidines; TMS=SiMe3.
reference [2]. [c] D H
(solid)=1050 kJmol , reference [6b].
f
298
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7263

Communications
the methylene carbon atom C7. This phenomenon has also
Compound 3 is very detonation-sensitive, and its explo-
[
6a]
been confirmed with other examples; for example, 3,5-
diazidomethyl-4-amino-triazole (m.p. 928C) and its perchlo-
rate salt (m.p. 758C) have much lower melting points than 3,5-
sive nature increases with higher purity and crystal size. We
found 5 to be much less sensitive to detonation than 3.
Therefore, 5 was examined as the carbon source and self-
heating producer for CNTs. Among the catalysts explored, for
[
20]
dimethyl-4-amino-triazole (m.p. 1968C) and its perchlorate
salt (m.p. 948C; see the Supporting Information). Thus,
introducing azidoalkyl groups could be an efficient way to
reduce the melting point and increase the heat of formation,
both of which are essential for the preparation of energetic
[
24]
example, Cu powder, Cu(OAc)2, AgNO , and Ni(ClO ) ,
3
4 2
we found that Ni(ClO4)2 was the best catalyst for the
production of CNTs. The synthetic method is straightforward.
Compound 5 (0.3 g 1.47 mmol) and Ni(ClO4)2 (0.02 g,
0.07 mmol) were placed in a 75-mL stainless-steel vessel,
which was then closed with a valve and heated to 2508C over
the course of about 20 minutes. The vessel was then allowed
to cool to room temperature. The product was observed as a
fleecy, puffy, fiber-like black residue, which could easily be
removed from the vessel with forceps. On touching with a
glass vial, the black residue agglomerated. This material floats
and disperses well in ethanol. SEM and TEM pictures of as-
prepared samples clearly show the presence of characteristic
hollow-channel CNT structures (lengths ca. 3–20 mm, diame-
ters ca. 60–80 nm and up to 100 nm; Figure 3). The yield of
CNTs is estimated to be greater than 90%. They have a
bamboo-like morphology (Figure 3c), and these “compart-
mentalized nanotubes” are segmented with a relatively
uniform segment length of about 90 nm.
[
21]
ionic liquids.
The heats of formation of 7, 9, 11, and 5-triazidomethyl-
,4,6-triazidopyrimidine (12) were predicted using quantum
2
chemical calculations (see the Supporting Information for
computation details). The optimized geometries of pentaazi-
dopyrimidine 7 and hexaazidopyrimidine 12 are shown in
Figure 2; both geometries are proven to be at local minima in
Figure 2. Optimized geometries at the B3LYP/6-311G(d,p) level of 7
(
a) and 12 (b).
the potential-energy surface. The incremental contribution of
each additional azido ligand to the heat of formation in the
sequence 9!11, 11!7, and 7!12 is + 365, + 354, and
ꢀ
1
+
385 kJmol , respectively (Table 1). Similar behavior is
observed in the corresponding sequence of isodesmic refer-
ence compounds CH CH (N3)n (n = 0–3), in which the
3
3ꢀn
incremental changes in heats of formation as a function of n
ꢀ
1
are + 351, + 337, and + 323 kJmol , respectively. For 7, using
0 kcal as its heat of sublimation, the heat of formation for the
2
ꢀ1
ꢀ1
solid state was estimated to be 1723.2 kJmol (5764 kJkg ),
which would be the third-highest heat of formation reported
for energetic materials, just lower than those of
2
ꢀ
1
ꢀ1
(
6164 kJkg ) and 1 (6709 kJkg ).
The formation of carbon nanotubes (CNTs) was inves-
tigated by catalytic detonation of the polyazidopyrimidines
above. Since their discovery in 1990, CNTs have been shown
to be extremely promising for applications in materials
science and medicinal chemistry owing to interesting elec-
tronic, mechanical, and structural properties. Synthesis of
nanomaterials on a large scale by detonation of explosive
precursors is a new promising method owing to its low cost.
Polyazido compounds with little or no hydrogen content are
regarded as such ideal precursors for nanomaterials. Kroke
first reported the preparation of CNTs using 3 by this
[
22]
detonation method in 1999, but the yield was only about
%. Optimization of the detonation conditions in the
presence of transition metals, for example, Fe, Ni, Cu, and
2
Figure 3. SEM (a,b) and TEM (c) images of CNTs generated by
catalytic detonation of 5.
[
23]
Ti, improved the yields to greater than 60%.
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Angewandte
Chemie
In conclusion, we have successfully synthesized a series of
polyazidopyrimidine compounds, in which the azidomethyl
group dramatically decreases the melting points. Theoretical
calculations show that these compounds exhibit highly
promising energetic properties. 2,4,6-Triazidopyrimidine was
found to be a novel and less sensitive precursor for the
formation of carbon nanotubes in high yields.
[19] Crystal data for 9: C
H N11, M = 217.18., T= 89(2) K, mono-
5 3 r
clinic, space group P21/c, a = 9.838(3), b = 15.295(4), c =
3
6
1
1
.3070(16) Š, b = 100.683(4)8, V= 932.5(4) Š , Z = 4, 1
=
calcd
ꢀ3
ꢀ1
.547 gcm
,
m = 0.118 mm
,
reflections collected/unique
3723/1690, R1 = 0.0337, wR2 = 0.0932 (I > 2sI), GOF = 1.032.
Crystal data for 11: C H N , M = 258.21, T= 89(2) K, mono-
clinic, space group P21/a, a = 6.5156(14), b = 22.149(5), c =
5
2
14
r
3
7.2649(16) Š, b = 97.540(4)8, V= 1039.3(4) Š , Z = 4, 1
=
calcd
ꢀ3
ꢀ1
Caution! All polyazides must be handled with extreme
care and with appropriate safety precautions.
1.650 gcm
,
m = 0.128 mm
,
reflections collected/unique
27871/2123, R1 = 0.0370, wR2 = 0.0936 (I > 2sI), GOF = 1.055,
sample was rotationally twinned (179.88 about reciprocal (0,0,1)
with a refined BASF of 0.252(3)), and the azidomethyl group was
disordered 50%. CCDC-612107 and CCDC-612108 contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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Palmer, R. C. Millonig, J. Med. Chem. 1982, 25, 331 – 333.
Received: July 13, 2006
Published online: September 13, 2006
Keywords: azides · explosives · heats of formation · nanotubes ·
.
nitrogen
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