The race for the first generation of the pentazolate anion in solution is far from over

Article abstract of DOI:10.1039/b417010e

The previous claim for the first generation of the pentazolate anion in solution was carefully reexamined; no evidence for the formation of cyclo-N ₅^- was found under the reported conditions. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b417010e

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The race for the first generation of the pentazolate anion in solution is
far from over{
a
Thorsten Schroer,* Ralf Haiges, Stefan Schneider and Karl O. Christe*
a
a
ab
Received (in Cambridge, UK) 11th November 2004, Accepted 10th January 2005
First published as an Advance Article on the web 27th January 2005
DOI: 10.1039/b417010e
2
2 9
The previous claim for the first generation of the pentazolate
of N , the expected decomposition product of N . Because the
3 5
anion in solution was carefully reexamined; no evidence for the
2
synthesis of the arylpentazole starting material was carried out
15
with azide, singly N-labeled only in the terminal position, the
formation of cyclo-N
conditions.
5
was found under the reported
1
5
appearance of the N-label in all positions of the azide
decomposition product would be strong evidence for the
2
1
The successful synthesis of N AsF in 1999 has stimulated
formation of an intermdiate N
anion in which all nitrogens
5
6
5
worldwide interest in the preparation of new homonuclear
have become equivalent (see Scheme 1).
2
polynitrogen species. One of the most promising polynitrogen
2
Due to the potential of cyclo-N
5
salts as high energy density
candidates is the pentazolate anion, cyclo-N . Theoretical
material (HEDM), we have pursued their bulk synthesis over the
past three years. In the course of these studies, we have also
reinvestigated the published reaction in more detail.
5
2
calculations predict it to be vibrationally stable, and potential
starting materials, e. g., arylpentazoles, have been known for over
2
3
0 years. In 2002, the cyclo-N
5
5
anion was detected in the gas
The 4-MeOC
the method described by Ugi et al,
MeOC ][BF ] with NaN in a mixture of MeOH and
n-hexane at 240 uC. The only impurities found in the product
were small amounts of [4-MeOC ][BF ] and 4-MeOC
The 4-MeOC H N was suspended in a solution of CAN and
6 4 5
H N starting material was prepared, similar to
10
phase, using the para-pentazolylphenolate anion as starting
by reacting [4-
material and electrospray negative-ion mass spectrometry as
4
the analytical method. This result was confirmed by laser
6
H
4
N
2
4
3
desorption ionization, time-of-flight mass spectrometry of
5
N,N-dimethylamino-phenylpentazole. However, attempts to pre-
2
6
H
4
N
2
4
6 4 3
H N .
6
4
5
3 2 3 2
Zn(NO ) in CD OD/H O (77 : 23 v/v) at 278 uC. The
7
concentrations of the salts were the same as previously described.
pare cyclo-N
5
salts in bulk by either the oxidative cleavage of the
6a,b
C–N bond using ozone or the reductive one using alkali metals
6a
The mixture was warmed to 240 uC and shaken for 2 h at this
temperature. We could not obtain a clear solution, and the
resulting suspension was centrifuged at 278 uC to compact the
solid colorless phase, the Raman spectrum of which at 270 uC
in liquid ammonia, were unsuccessful. There is only one claim
2
for the preparation of the cyclo-N anion in solution and its
7
5
15
identification by N NMR
spectroscopy.
Using
(
NH Ce(IV)(NO ) (CAN) in aqueous methanol at 240 uC,
4
)
2
3 6
showed only signals belonging to the starting material
2
the para-methoxyphenyl group in para-methoxyphenylpentazole
was oxidized to para-benzoquinone, supposedly yielding the zinc
salt of the pentazolate anion as coproduct (see Scheme 1).
4
-MeOC H N and the NO anion.
6 4 5 3
The suspension was warmed from 240 uC to room temperature
over a period of 2 days. We could not find any evidence for the
1
5
7
The following N NMR evidence was presented for the
2
2
2
formation of cyclo-N5 , the N3 anion or p-benzoquinone by
Raman and NMR spectroscopy. The only identifiable compound
was 4-MeOC H N which originated from the decomposition of
formation of N5 : (i) The observation of a signal at 210 ¡ 2 ppm
2
which is close to the value predicted for N ; and (ii) the
5
7,8
6
4
3
observation of two signals at 2283 and 2147 ppm which were
4
-MeOC
6
H
4
N
5
.
attributed to the terminal and central nitrogen atoms, respectively,
In another experiment the suspension was stirred for 2 h at
2
40 uC. After the solid had settled, a small amount of dark red
{ Electronic supplementary information (ESI) available: Experimental
details and NMR spectra. See http://www.rsc.org/suppdata/cc/b4/
b417010e/
*schroer@usc.edu (Thorsten Schroer)
kchriste@usc.edu (Karl O. Christe)
mother liquor could be separated and was transferred into an
NMR tube at 240 uC. The only species, observable by nitrogen
NMR spectroscopy, at 240 and 220 uC were the NO anion and
2
3
+
the NH
4
cation.
Scheme 1
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2
Most interestingly, the nitrogen signal of the NO
3
anion was
oxidizes the azide ion under formation of nitrogen, even at
240 uC.{
observed in this solvent system at the same chemical shift as the
7 2
one previously attributed to N . Furthermore, it was shown that
5
ðMeOHÞ
ð{40 0CÞ
1
4z
3z
at the given concentrations, this signal was also observable in the
Ce zNaN3 DCCA Ce z1 N2
2
1
5
3 2 3
N NMR spectrum of a solution of CAN/Zn(NO ) in CD OD/
15
15
H
2
O without N enrichment (the natural abundance of N is
.37%) and in the absence of any other nitrogen containing
compounds (Fig. 1).
Therefore, the signal previously observed in the N NMR
This is in accord with previous observations, that Ce(IV) salts
1
1
rapidly oxidize HN in aqueous solution, and was verified by us
3
0
in control experiments. Actually, CAN can be used for the
2
1
5
quantitative analysis of N
Secondly, 4-MeOC
3
in solution.
7
2
5
spectrum at 210 ppm and previously attributed to N
is most
6
H
4
N
5
decomposes in MeOH under forma-
15
3
tion of 4-MeOC H N . For 4-MeOC H N , which is N-labeled
likely due to the natural abundance signal of the nitrate anion. If
2
6
4
3
6
4
5
the 210 ppm signal were indeed due to N , two signals should be
2
at N2 or N3, the resulting decomposition product,
5
1
5
1
5
observed in this region, one for N-labeled N
2
5
and another one
. In view of the narrow line widths
4-MeOC
6
H
4
N
3
, is N-labeled at either N
b
or N
c
(see Scheme 1).
N-labeled at N or
, and recorded its N NMR spectrum in CD OD. The
chemical shift for N was found at 2135.50 ppm and that for N
at 2148.16 ppm (Fig. 2).
1
5
15
for naturally abundant NO
3
We prepared a sample of 4-MeOC
6
H
4
N
3
,
b
15
of these signals, an accidental coincidence of their chemical shifts
N
c
3
must be considered highly unlikely. The assignment of the
2
b
c
2
10 ppm signal to NO3 is further supported by the surprising
7
persistence of this signal on warm up and the reported changes in
relative intensities of the signals on going from 240 to 220 uC.
Although no integral values were reported in ref. 7, the peak with
the highest intensity in both spectra is the one at d 5 210 ppm.
While the intensity of the signals for 4-MeOC H N seem to
The chemical shift of 2148.16 ppm is very close to that
of2147.2 ppm, attributed by the authors of ref. 7 to the central
1
5
nitrogen of the azide anion. Furthermore, the N chemical
shifts of the central and terminal nitrogens of NaN in an aqueous
3
solution were observed by us at 2133.65 and 2281.45 ppm,
respectively (Fig. 2). Therefore, the signal at 2147.2 ppm
6
4
5
decrease, the intensities of the signals of the decomposition
products increase. This pattern indicates decomposition of
should be assigned to N of 4-MeOC H N , while that at
c 6 4 3
4
-MeOC H N rather than that of the compound with a chemical
5
2283 ppm belongs to the terminal nitrogen of the azide anion.
The presence of some azide, which is only terminally labeled,
could stem from some unreacted azide starting material. The
6
4
shift at d 5 210 ppm.
The second piece of evidence presented for the formation of
2
N5 was the observation of a centrally N-labeled N3 anion as a
7
15
2
apparent absence of the 2135 ppm signal for
N
b
of
15
decomposition product. A N signal at d 5 2147.2 ppm was
attributed to this nitrogen.
4-MeOC H N in Fig. 3 of ref. 7 might be due to the relatively
6 4 3
poor signal to noise ratio in their spectrum and the fact that
t
h
e
p
e
a
k
h
e
i
g
h
t
o
f
N
i
s
c
o
n
s
i
d
e
r
a
b
l
y
l
o
w
e
r
t
h
a
n
t
h
a
t
o
f
N
We question this assignment for two reasons. First of all, the
previous authors used a 50% excess of CAN, and CAN rapidly
b
c
(see Fig. 2).
14
15
Fig. 1
3 2 3 2 3 2
N NMR (inset N NMR) spectrum of CAN/Zn(NO ) (1 : 2.1) in CD OD/H O at rt.; d (external CD NO ) 5 0 ppm.
1
608 | Chem. Commun., 2005, 1607–1609
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15
15
Fig. 2
N NMR spectrum of 4-MeOC
6 4 3 3 2
H N in MeOH (left) and N NMR (natural abundance) spectrum of NaN in H O (right). d (external
3 2
CD NO ) 5 0 ppm.
7
In summary, we conclude that the previous claim for the first
observation of the pentazolate anion in the condensed phase is
insufficiently supported. In our opinion the peak in the previously
Notes and references
1
2
3
K. O. Christe, W. W. Wilson, J. A. Sheehy and J. A. Boatz, Angew.
Chem. Int. Ed., 1999, 38, 2004.
M. T. Nguyen, Coord. Chem. Rev., 2003, 244, 93 and refs. cited
therein.
R. Huisgen and I. Ugi, Chem. Ber., 1957, 90, 2914.
7
published spectra at 210 ¡ 2 ppm is due to the nitrate anion,
while the peak at 2147.2 ppm belongs to N
c
of the decomposition
product 4-MeOC . Neither the pentazolate anion nor the
6 4 3
H N
1
5
4 V. Vij, J. G. Pavlovich, W. W. Wilson, A. Vij and K. O. Christe, Angew.
Chem. Int. Ed., 2002, 41, 3051.
important decomposition product, the azide anion, N-labeled at
the central nitrogen atom, have been observed. Therefore, the race
2
for the first successful synthesis and observation of cyclo-N5 in
¨
5
6
7
H. Ostmark, S. Wallin, T. Brinck, P. Carlqvist, R. Claridge, E. Hedlund
and L. Yudina, Chem. Phys. Lett., 2003, 379, 539.
(a) I. Ugi, Angew. Chem., 1961, 73, 172; (b) V. Benin, P. Kaszynski and
J. G. Radziszewski, J. Org. Chem., 2002, 67, 1354.
R. N. Butler, J. C. Stephens and L. A. Burke, Chem. Commun., 2003,
the condensed phase is still open.
The authors appreciate the financial support of this work by the
National Science Foundation and the Air Force Office of Scientific
Research.
1
016.
8 L. A. Burke, R. N. Butler and J. C. Stephens, J. Chem. Soc., Perkin
Trans. 2, 2001, 1679.
a
Thorsten Schroer,* Ralf Haiges, Stefan Schneider and
a
a
9
D. A. Dixon, D. Feller, K. O. Christe, W. W. Wilson, V. Vij, A. Vij,
H. D. B. Jenkins, R. M. Olson and M. S. Gordon, J. Am. Chem. Soc.,
2004, 126, 834.
ab
Karl O. Christe*
Loker Research Institute University of Southern California, Los
Angeles, CA 90089-1661, USA. E-mail: schroer@usc.edu;
a
10 I. Ugi, H. Perlinger and L. Behringer, Chem. Ber., 1958, 91,
2324.
11 A. M. M. Doherty, M. D. Radcliffe and G. Stedman, J. Chem. Soc.,
Dalton. Trans., 1999, 3311.
kchriste@usc.edu; Fax: (+1) 213-740-6679; Tel: (+1) 213-740-8957
E.R.C., Inc., Air Force Research Laboratory, Edwards AFB, CA 93524,
USA
b
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