The first structural characterization of a binary P-N molecule: The highly energetic compound P3N21

Article abstract of DOI:10.1002/anie.200601670

(Chemical Equation Presented) An explosive mixture: The first structural characterization of a binary molecule of phosphorus and nitrogen is described. With the help of a new synthetic strategy, the phosphorus azide P ₃N₂₁ is prepared in pure form, and single crystals are isolated, enabling a structure determination by X-ray diffraction (see structure; P orange, N blue).

Full text of DOI:10.1002/anie.200601670

Angewandte
Chemie
Phosphorus Nitrides
DOI: 10.1002/anie.200601670
The First Structural Characterization of a Binary
P–N Molecule: The Highly Energetic Compound
P N **
3
21
Michael Göbel, Konstantin Karaghiosoff, and
Thomas M. Klapötke*
Dedicated to Professor Karl Otto Christe
on the occasion of his 70th birthday
Compounds of the elements phosphorus and nitrogen can
exist either as molecular species or as three-dimensional
polymeric solids. Examples of known solid-state compounds
include the structurally well-characterized phases of the
binary compound P N ,which were reported by Schnick
3
5
[
1]
et al. In contrast,none of the four binary P–N molecules
described in the literature,namely P N , P(N ) , P(N ) ,
P N , and the ionic compound (N )P(N ) ,have been
structurally characterized. As shown by Christe et al. for
[
2]
[3]
[4]
4
4
3
3
3 5
[
5]
3
21
5
3 6
[
6]
[
6]
(
N )P(N ) , the difficulties in the isolation and handling of
5 3 6
these compounds arise from their highly endothermic char-
acter and their extremely low energy barriers,which lead to
[
7]
an often uncontrollable,explosive decomposition.
Herein,we report the single-crystal X-ray structure of
[
8]
P N (1) and,thereby,the first structural characterization of
3
21
a binary P–N molecule. Although the synthesis of this
compound was first reported over 50 years ago through the
[
5a]
reaction of hexachlorophosphazene with sodium azide,
compound 1 has only been characterized by elemental
[
5a]
[5b,c]
[5c]
analysis,
and vibrational
and NMR spectroscopy.
The experimental difficulties involved in the structural
characterization of P N are a consequence of its high
3
21
energy content; our calculated gas-phase enthalpy of forma-
ꢀ1
tion is + 341.4 kcalmol .
To obtain as pure a product as possible,a new synthetic
strategy for P N ,in which the azide (N ) group is introduced
3
21
3
using trimethylsilylazide,was chosen [Eq. (1)]. The trime-
[
*] M. Göbel, K. Karaghiosoff, Prof. Dr. T. M. Klapötke
Department of Chemistry and Biochemistry
Ludwig-Maximilians-Universität Munich
Butenandtstrasse 5–13 (Haus D), 81377 Munich (Germany)
Fax: (+49)89-2180-77492
E-mail: tmk@cup.uni-muenchen.de
[
**] We are grateful to Dr. G. Fischer for the mass spectrometry
measurements, T. Kerscher for support with the calculations and Dr.
Habil M.-J. Crawford for the English translation of the manuscript.
The Cusanuswerk is gratefully acknowledged for the award of a PhD
scholarship (M.G.) and the Ludwig-Maximilians-Universität and the
Fonds der chemischen Industrie (FCI) are thanked for generous
financial support. The Federal Republic of Germany, as well as the
Free State of Bavaria, are thanked for providing a single-crystal X-ray
diffractometer within the HBFG programme.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Communications
P N Cl þ 6 ðCH Þ SiN ! P N ð1Þ þ 6 ðCH Þ SiCl
ð1Þ
3
3
6
3
3
3
3
21
3 3
Table 1: Comparison of the experimental and calculated (C point group)
1
ꢀ1
[a]
vibrational frequencies [cm ] and intensities of P N .
3
21
thylsilylchloride by-product formed was continuously
removed from the equilibrium under the reaction conditions.
Owing to its volatility,excess trimethylsilylazide could also be
easily removed from the reaction mixture. After sublimation
of the product, 1 was obtained in high purity.
Assignment
Experiment
Raman
Calculation
BLYPB3LYP
IR
n N + n N
3
3406 (w)
2511 (w)
as
3
s
2nsN3
n N
2162 (vs) 2181 [26] 2189 (128) [196] 2318 (172)
2181 (701) [30] 2308 (1016)
as
3
The appearance of only one signal at d = 13.6 ppm (Dn
1
/2
3
1
2180 (567) [72] 2306 (624)
2178 (940) [50] 2304 (1101)
=
10 Hz) in the P NMR spectrum of 1 indicates that a
1
4
complete chloride–azide exchange occurred. The N and
2
2
175 (303) [25] 2303 (251)
1
5
15
N NMR spectra of 1 are shown in Figure 1. The N NMR
173 (83) [8]
2300 (37)
nsN3
nsN3
1291 [21] 1275 (13) [51]
257 (22) [18]
1268 (175) [1]
1
1256 (s)
1336 (191)
1333 (103)
1324 (848)
1
1
265 (99) [6]
258 (661) [3]
nas(PN)Ring
1201 (vs)
1149 (1536) [1] 1220 (1793)
114 (1069) [2] 1185 (1191)
1
nas(PN)Ring
dN3
dN3
910 (w)
786 (m)
739 (m)
1048 (88) [0]
727 (421) [0]
679 (175) [0]
1120 (96)
779 (517)
729 (232)
721 (239)
672 (191) [1]
[
b]
dbend(PN)ring
dwag(PN)ring
drock(PN)ring
712 [100] 638 (1) [63]
582 (222) [1]
[
b]
,
dN3 614 (m)
dN3 564 (m)
617 (222)
567 (137)
558 (99)
[
b]
1
4
15
,
562 (11) [15]
Figure 1. N NMR (top) and N NMR (bottom) spectra of P N in
3
21
533 (148) [0]
C D . An enlargement of the broad signal at d=ꢀ300 ppm in the
6
6
1
4
526 (116) [1]
454 [63] 410 (0) [47]
N NMR spectrum is also shown.
[
b]
dbend(NPN)
456 (w)
2
20 [21] 298 (2) [1]
[
b]
dtwist(NPN)
160 [26] 215 (1) [11]
signal at d = ꢀ305.4 ppm is assigned to the ring nitrogen
[
a] The intensities of the calculated IR and Raman spectra are given in
ꢀ1
4
ꢀ1
atoms,by comparison with the chemical shifts of hexasub-
(kmmol ) and [Š amu ], respectively. The IR frequencies calculated
using the B3LYPmethod have very low intensities and are not given.
[
9]
15
stituted cyclophosphazenes. The N NMR signals of the
covalent azide groups are assigned according to the typical
[
10]
chemical shifts reported for covalent azides:
d = ꢀ152.7
14
(
N ), ꢀ166.8 (N ),and ꢀ291.3 ppm (N ). In the N NMR
b
g
a
spectrum,the signals of the ring nitrogen atoms and the N
31G(d): 0.9940; B3LYP/6-31G(d): 0.9613),good agreement
between the calculated and experimental frequencies is
obtained. The vibrational frequencies observed for the
liquid phase were assigned by comparison with the frequen-
cies calculated for a molecule of C1 point symmetry,in
contrast to the approach reported in the literature,
a
atoms appear as a broadened peak at ꢀ300 ppm.
The Raman and IR spectra of 1 are shown in Figure 2,and
a comparison of the experimental and calculated (unscaled)
vibrational frequencies is given in Table 1. If standard scaling
[
11]
[5b,c]
factors are applied for both calculation methods (BLYP/6-
where
a D3h molecular symmetry was assumed.
The lowest-energy conformation calculated for a free
P N molecule in the gas phase corresponds to that detected
3
21
in the solid state by single-crystal X-ray diffraction (Figure 3).
The relative arrangement of the azide groups and the
(noncrystallographic) C1 symmetry of the molecule are
consistent in both structures. P N crystallizes in the space
3
21
¯
group P1 with two formula units per unit cell. Three azide
groups (N4-N5-N6,N10-N11-N12,and N19-N20-N21) are
nearly parallel to the phosphazene ring,as is also found in the
calculated gas phase structure. The three remaining azide
groups (N7-N8-N9,N13-N14-N15,and N16-N17-N18) are
nearly perpendicular to the ring. The N ꢀN /N ꢀN bond
a
b
b
g
lengths and the N -N -N angles are in good agreement with
a
b
g
[
10]
those of other covalently bound azides. The six-membered
ring in 1 is nearly planar,with a slight chair conformation,as
observed for the phosphazene ring of the P N Cl starting
3
3
6
[
12]
material.
The P-N-P/N-P-N angles (120.8(2)–122.9(2)8/
Figure 2. IR (top) and Raman (bottom) spectra of P N .
117.0(2)–118.2(1)8),as well as the P ꢀN distances (1.556(2)–
3
21
6
038 www.angewandte.org
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protection from electrostatic charge,is mandatory. Ignoring these
safety precautions can result in serious injury!
P N Cl and trimethylsilylazide were purchased from Aldrich.
3
3
6
Propionitrile was dried over P O and distilled prior to use. The
4
10
Raman spectra were measured using a Perkin Elmer Spectrum 2000R
NIR FT-Raman instrument (Nd:YAG Laser (1064 nm)). The IR
spectra were recorded using a Perkin Elmer Spectrum One FT-IR
3
1
15
14
instrument. The P, N,and N NMR spectra were recorded using a
1
4
Jeol EX 400 NMR spectrometer operating at 28.9 MHz ( N),
0.6 MHz ( N),or 162.0 MHz ( P); the chemical shifts are in ppm
1
5
31
4
1
4/15
31
relative to nitromethane ( N) or 85% phosphoric acid ( P). The
mass spectra were measured using a Jeol MStation JMS-700
spectrometer. The decomposition temperature was determined
using a Pyris 6 DSC instrument. The impact sensitivity at room
temperature was determined using a Bundesanstalt für Materialfor-
schung und -prüfung (BAM) drop hammer.
P N : P N Cl (224 mg,0.644 mmol) was added to a flame-dried
3
21
3
3
6
Schlenk flask under argon and dissolved in anhydrous propionitrile
(20 mL) at room temperature. Trimethylsilylazide (893 mg,
Figure 3. ORTEPrepresentation of the molecular structure of P N ;
thermal ellipsoids are set at 50% probability. Selected bond lengths [Š]
3
21
7.750 mmol) was added dropwise to the stirred solution under a
ꢀ
N1 1.558(2), P1ꢀN2 1.576(2), P1ꢀN4 1.666(2), P1ꢀ
nitrogen purge. A bubbler was connected to the flask,and a slow
stream of nitrogen gas was passed continuously through the
apparatus. The colorless solution was warmed to 608C and stirred
for 3 h at this temperature. The now pale yellow solution was then
stirred for 19 h at room temperature. The reaction mixture was
subsequently concentrated using a rotary evaporator (308C,50 mbar)
and the remaining solvent was removed on a high-vacuum line (1 ”
and angles [8]: P1
N7 1.671(2), N4ꢀN5 1.218(3), N5ꢀN6 1.117(3), N7ꢀN8 1.203(3), N8ꢀ
N9 1.114(3); N1-P1-N2 118.2(1), P1-N1-P2 122.1(1), N4-N5-N6
1
73.4(3), N7-N8-N9 173.5(3), P1-N4-N5 117.7(2), P1-N7-N8 119.2(2),
N4-P1-N7 102.2(1), N1-P1-N4 104.5(1), N2-P1-N4 111.9(1), N1-P1-N7
08.9(1), N2-P1-N7 109.8(1).
1
ꢀ3
1
0
mbar,room temperature,several minutes). The pale yellow
ꢀ
3
liquid obtained was purified by sublimation (1 ” 10 mbar,130 8C oil
bath, ꢀ868C cold finger),yielding a colorless liquid. Single crystals of
1 were obtained by the controlled cooling and warming of the
substance near its melting point. The substance was repeatedly cooled
to ꢀ78.58C with dry ice and slowly warmed to ꢀ178C in a cold room,
while being monitored with a microscope. As soon as the liquid phase
formed,it was again cooled with dry ice. The cycle was repeated until
crystals formed. The single crystals of 1 must be handled with great
care! Raman (neat,25 8C): see Table 1; IR (nujol,KBr,background
1
1
.576(2) Š),correspond well with the average values of 121.4/
18.48 and 1.58 Š reported for P N Cl . In addition,the N -P-
N angles (99.2(1)–102.2(1)8) also correspond well with the
3
3
6
a
a
average Cl-P-Cl angle of 1028. However,the average P ꢀN
a
distance in 1 (1.67 Š) is significantly shorter than the
corresponding average PꢀCl distance in N P Cl (1.97 Š).
3
3
6
The P-N -N angles are all approximately 1208,suggesting an
a
b
2
3
1
sp hybridization of the N atoms.
subtracted): see Table 1; P NMR (C D ,25 8C): d=13.6 ppm (Dn =
10 Hz); N NMR (C
a
6
6
1
/
2
1
5
Furthermore,the identity of P N was confirmed by high-
6
D
6
,25 8C): d = ꢀ152.7 (N
b
), ꢀ166.8 (N ), ꢀ291.3
b
g
3
21
1
4
(
=
N ), ꢀ305.4 ppm (Nring); N NMR (C D ,25 8C): d=ꢀ152.7 (N , Dn
resolution mass spectrometry,which also demonstrated that
the compound can be transferred into the gas phase without
decomposition. Two signals were observed in the mass
spectrum: the first mass peak corresponds to the molecular
peak,and the second to a species in which one azide group has
been removed from the P N molecule.
The thermal stability of P N was investigated using
differential scanning calorimetry (DSC). At a heating rate of
8Cmin ,the compound decomposes explosively at an onset
temperature of 2208C. This relatively high decomposition
temperature is in contrast with the very high impact
sensitivity of the substance at room temperature (< 1 J).
Direct heating in a flame also results in an explosive
decomposition of the compound with a loud noise and a
flash of light.
a
6
6
1/2
34 Hz), ꢀ166.8 (N , Dn =115 Hz), ꢀ300 ppm (N /N , Dn
=
g
1/2
a
ring
1/2
+
950 Hz); MS (DEI,70 eV): m/z (%): 387 (39) [M] ,345 (100)
+
[
MꢀN ] ; MS (HR): m/z calcd for P N : 386.9858; found: 386.9851.
3
3
21
ꢀ1
DSC (28Cmin ): 2208C (decomp); impact sensitivity: <1 J.
Computational methods: BLYP and B3LYP density functional
theory (DFT) calculations were carried out. The geometry,IR
spectrum,and Raman spectrum of 1 were calculated with the
3
21
3
21
[
13]
Gaussian program, using 6-31G(d) basis sets.
ꢀ
1
2
Received: April 27,2006
Keywords: ab initio calculations · azides · phosphorus nitrides ·
.
structure elucidation · X-ray diffraction
[
1] a) S. Horstmann,E. Irran,W. Schnick, Angew. Chem. 1997, 109,
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001, 113,2713; Angew. Chem. Int. Ed. 2001, 40,2643.
1
2
Experimental Section
Caution! Phosphorus azides are highly endothermic compounds and
[2] E. H. Kober,H. F. Lederle,G. F. Ottmann,USA Patent US
32918645, 1966.
decompose explosively under various conditions! P N21 is extremely
[3] X. Zeng,W. Wang,F. Liu,M. Ge,Z. Sun,D. Wang, Eur. J. Inorg.
Chem. 2006,416.
3
impact sensitive. Owing to the high energy content of P N ,
3
21
explosions can cause substantial damage,even when quantities on
[4] P. Volgnandt,A. Schmidt, Z. Anorg. Allg. Chem. 1976, 425,189.
[5] a) C. Grundmann,R. Rätz, Z. Naturforsch. B 1954, 10,116; b) F.
Räuchle,M. Gayoso, Ann. Fis. 1970, 66,241; c) J. Müller,H.
Schröder, Z. Anorg. Allg. Chem. 1979, 450,149.
[
5a]
the order of 1 mmol are used.
The use of suitable protective
clothing,in particular a face shield,ear protectors,a bullet-proof vest,
arm protectors,and kevlar gloves,as well as appropriate shoes for
Angew. Chem. Int. Ed. 2006, 45, 6037 –6040
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 6039

Communications
[
6] R. Haiges,S. Schneider,T. Schroer,K. O. Christe, Angew. Chem.
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2
[
1
b) T. M. Klapötke, Chem. Ber. 1997, 130,443; c) I. C. Tornie-
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Quality (Eds.: I. Hargittai,T. Vidoczy),Plenum,New York,
1995,p. 51.
[
8] Crystal data for P N : Xcalibur S (Oxford Diffraction),0.26 ”
3
21
¯
P1, a = 7.1953(9), b =
0
7
8
5
.22 ” 0.13 mm,triclinic,space group
.3718(9), c = 12.994(1) Š, a = 87.41(1), b = 78.95(1), g =
3
ꢀ3
9.59(1)8, V= 675.8(1) Š , Z = 2, 1calcd = 1.902 gcm , 2qmax
=
2.128,Mo Ka radiation (l = 0.71073 Š),graphite monochroma-
tor, w scans, T= 200 K,6941 measured reflections,2657 inde-
pendent reflections,2207 with F > 4s(F ),217 parameters, R =
o
o
1
ꢀ1
0
.0375, wR (all data) = 0.0944, m(MoKa) = 0.486 mm ,programs
2
used: SHELXS-97,SHELXL-97 (G. M. Sheldrick, SHELXS-97,
Program for the solution of crystal structures,Universität
Göttingen, 1997; G. M. Sheldrick, SHELXL-97,Program for
the refinement of crystal structures,Universität Göttingen,
2
1
997),refinement on F ,residual electron density: 0.334/ ꢀ0.321
3
eŠ . Further details on the crystal structure investigations may
be obtained from the Fachinformationszentrum Karlsruhe,
7
8
6344 Eggenstein-Leopoldshafen,Germany (fax: ( + 49)7247-
08-666; e-mail: crysdata@fiz-karlsruhe.de),on quoting the
depository number CSD-416415.
[
9] B. Thomas,G. Seifert,G. Großmann, Z. Chem. 1980, 20,217.
[
10] P. Geißler,T. M. Klapötke,H. J. Kroth, Spectrochim. Acta Part A
995, 51,1075.
1
[
11] J. B. Foresman,A. Frisch, Exploring Chemistry with Electronic
Structure Methods,2nd ed.,Gaussian,Philadelphia, 1996,p. 64.
12] G. J. Bullen, J. Chem. Soc. A 1971,1450.
13] Gaussian03 (RevisionA.1): M. J. Frisch et al.,see Supporting
Information.
[
[
6
040 www.angewandte.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 6037 –6040