Synthesis, characterization, and explosive properties of the nitrogen-rich borazine [H3N3B3(N3)3]

Article abstract of DOI:10.1021/ic990316b

[Me₃N₃B₂(Me)₂B(N₃ )] and [H₃N₃B₃(N₃)₃] were prepared from the combination of Me₃SiN₃ and the chloroborazines [Me₃N₃B₂(Me)₂BCl] and (H₃N₃B₃Cl₃). The compounds have been characterized by analytical and spectroscopic methods. The molecular structure of [H₃N₃B₃(N₃)₃] has been confirmed by single-crystal X-ray diffraction techniques: triclinic space group P1 with a = 8.579(1) A?, b = 9.182(1) A?, c = 12.043(1) A?, a = 104.62(1)°, β= 90.26(1)°, γ = 110.74(1)°, and Z = 4. The thermal behavior of the azides have been examined by TGA/DTA. The explosion sensitivity of [H₃N₃B₃(N₃)₃], which contains 82.58percent nitrogen by weight, has been accessed by drop weight impact, spark, and friction tests. These measurements indicate that [H₃N₃B₃(N₃)₃] is a relatively sensitive explosive. The Staudinger reactivity of the azides with Ph₃P was examined, and [H₃N₃B₃(N₃)₃] was found to produce [Ph₃P=NH₂](N₃). The molecular structure of this compound was determined by single-crystal X-ray diffraction techniques: orthorhombic space group P2₁2₁2₁ with a = 10.649(2) A?, b = 11.737(3) A?, c = 13.364(4) A?, α = β= γ = 90°, Z = 4.

Full text of DOI:10.1021/ic990316b

3738
Inorg. Chem. 1999, 38, 3738-3743
Synthesis, Characterization, and Explosive Properties of the Nitrogen-Rich Borazine
[H3N3B3(N3)3]
Robert T. Paine,*,† Wolfgang Koestle,† Theodore T. Borek,† Gary L. Wood,†,‡
Eugene A. Pruss,† Eileen N. Duesler,† and Michael A. Hiskey§
Department of Chemistry, University of New Mexico, Albuquerque, New Mexico 87131, and
Group DX-2, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
ReceiVed March 19, 1999
The azidoborazines [Me3N3B2(Me)2B(N3)] and [H3N3B3(N3)3] were prepared from the combination of Me3SiN3
and the chloroborazines [Me3N3B2(Me)2BCl] and (H3N3B3Cl3). The compounds have been characterized by
analytical and spectroscopic methods. The molecular structure of [H3N3B3(N3)3] has been confirmed by single-
crystal X-ray diffraction techniques: triclinic space group P1h with a ) 8.579(1) Å, b ) 9.182(1) Å, c ) 12.043(1)
Å, R ) 104.62(1)°, â ) 90.26(1)°, γ ) 110.74(1)°, and Z ) 4. The thermal behavior of the azides have been
examined by TGA/DTA. The explosion sensitivity of [H3N3B3(N3)3], which contains 82.58% nitrogen by weight,
has been accessed by drop weight impact, spark, and friction tests. These measurements indicate that [H3N3B3-
(N3)3] is a relatively sensitive explosive. The Staudinger reactivity of the azides with Ph3P was examined, and
[H3N3B3(N3)3] was found to produce [Ph3PdNH2](N3). The molecular structure of this compound was determined
by single-crystal X-ray diffraction techniques: orthorhombic space group P212121 with a ) 10.649(2) Å, b )
11.737(3) Å, c ) 13.364(4) Å, R ) â ) γ ) 90°, Z ) 4.
Introduction
A few borazines containing B-azido substituents, 1, have been
reported;3 however, limited characterization or reactivity data
are available for these compounds. In particular, Muszkat and
Azide compounds of group 13 elements are enjoying a
renaissance in interest despite a reputation for fickle and
nefarious behavior on the part of some derivatives.1,2 With
proper experimental care and attention, it appears that unpredict-
able explosive and poison hazards can be minimized and the
chemistry of most compounds can be effectively studied. In fact,
several derivatives are found to be useful starting materials for
the formation of new ring compounds as well as for the synthesis
of solid-state materials. Boron azides3 appear to be particularly
useful for these purposes. For example, Paetzold and co-
workers4 have described thermally promoted decompositions
of simple azidoboranes, R2BN3 (R ) alkyl, aryl, amino), that
produce iminoboranes, RBtNR. These compounds are stable
where sufficient steric protection of the coordinatively unsatur-
ated boron and nitrogen atoms is provided. Alternatively, the
reactive iminoboranes form diazadiboretidine, (RBdNR)2, or
borazine, (RBdNR)3, condensation products.5
co-workers,6 in 1963, briefly noted the formation of tri-B-
azidoborazines, (HNBN3)3 (2), (CH3NBN3)3 (3), and (C6H5-
NBN3)3 (4) from metathesis reactions of NaN3 with the
respective tri-B-chloroborazines. Compound 2 was reported to
be a white solid that did not melt below 300 °C. It contained
about 10% chlorine impurity, and it exploded when attempts
were made to purify it. This molecule was subsequently
described in a patent,7 and it was obtained chlorine free by
extensive reflux of the reactants in chlorobenzene. The only
additional characterization datum provided was the azide
infrared stretching frequency. Meller and Wechsberg8 also
reported the synthesis of 3, (CD3NBN3)3, and (EtNBN3)3 (5)
by similar reactions.
† University of New Mexico.
‡ On sabbatical leave from Valdosta State College, Valdosta, GA.
§ Los Alamos National Laboratory.
(1) (a) Scriven, E. F. V. Azides and Nitrenes: ReactiVity and Utility;
Academic Press: New York, 1984. (b) Patai, S. The Chemistry of the
Azido Group; Wiley-Interscience: New York, 1971. (c) Lwowski, W.
Nitrenes; Wiley-Interscience: New York, 1970.
(2) (a) Bertrand, G.; Majoral, J. P.; Baceiredo, A. Acc. Chem. Res. 1986,
19, 17. (b) Bock, H.; Dammel, R. Angew. Chem., Int. Ed. Engl. 1987,
26, 504. (c) Evans, B. L.; Yoffe, A. D.; Gray, P. Chem. ReV. 1959,
59, 515. (d) Tornieparth-Oetting, I. C.; Klapo¨tke, T. M. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 511. (e) Klapo¨tke, T. M. Chem. Ber. 1997,
130, 443.
We have a general interest in the chemistry of group 13 azides
as solid-state materials precursors, and we report here investiga-
tions of two azidoborazines. The synthesis and characterization
of the previously unknown 1-B-azidopentamethylborazine
(3) Steinberg, H.; Brotherton, R. J. Organoboron Chemistry; John
Wiley: New York, 1966; Vol. II, Chapter 7.
(4) (a) Paetzold, P. Fortschr. Chem. Forsch. 1967, 8, 437. (b) Paetzold,
P.; Maier, G. Chem. Ber. 1970, 103, 281. (c) Pieper, W.; Schmitz,
D.; Paetzold, P. Chem. Ber. 1981, 114, 3801. (d) Paetzold, P.; Truppat,
R. Chem. Ber. 1983, 116, 1531. (e) Meier, H. U.; Paetzold, P.;
Schro¨der, E. Chem. Ber. 1984, 117, 1954.
(5) Paetzold, P. AdV. Inorg. Chem. Radiochem. 1987, 31, 123.
(6) Muszkat, K. A.; Hill L.; Kirson, B. Isr. J. Chem. 1963, 1, 27.
(7) Keith, J. N.; Park, V.; Musket, S. F.; Klein, M. J. U.S. Patent 3,394,999,
July 30, 1968.
(8) Meller, A.; Wechsburg, M. Monatsh. Chem. 1967, 98, 513, 690.
10.1021/ic990316b CCC: $18.00 © 1999 American Chemical Society
Published on Web 07/23/1999

The Nitrogen-Rich Borazine [H3N3B3(N3)3]
Inorganic Chemistry, Vol. 38, No. 16, 1999 3739
N1,5-CH3), 2.62 (3H, N3-CH3), 0.32 (6H, B2,4-CH3); 13C{1H} δ 34.46
(N3CH3), 32.48 (N1,5-CH3), -0.5 (BCH3). Anal. Calcd for C5H15N6B3
(Mr ) 191.65): C, 31.34; H, 7.89; N, 43.85. Found: C, 31.42; H, 7.93;
N, 41.59.
[Me3N3B2(Me)2B(N3)] (6) are described along with an improved
synthesis for triazidoborazine 2. In addition, detailed charac-
terization data are provided along with a description of the
explosive sensitivity of 2.
Reaction of (HNBN3)3 with Ph3P. A sample of 2 (0.72 g, 3.54
mmol) and Ph3P (0.93 g, 3.54 m mol) were dissolved in THF (50 mL),
and the mixture was refluxed (12 h). A colorless solid (1.3 g) formed
that was insoluble in THF, toluene, Et2O, and hexane. The solid was
then refluxed with CH3CN (20 mL) for 30 min, and the mixture was
filtered while hot. The filtrate was slowly concentrated to 5 mL, and
colorless crystals of 7 were deposited: yield 0.47 g (41.6%); mp 183
°C dec.
Pyrolysis of (HNBN3)3, 2. The bulk pyrolysis of 2 was examined
by using a standard horizontal tube furnace. A sample of 2 (0.67 g,
3.3 mmol) was placed in a Pt crucible, and this was loaded, under dry
nitrogen, into a quartz pyrolysis tube. The tube was purged with a slow
stream of N2 gas and heated to 1200 °C (8 h). The solid remaining in
the crucible (0.1 g, 15%) was bone-colored. Infrared spectrum (KBr;
cm-1): 1390 (vs), 791 (m). XRD (d, Å): 3.47 (002), 2.12 (100). Anal.
Calcd for BN (24.82): B, 43.56; N, 56.44. Found: B, 43.10; N, 55.75;
C, 0.93.
Explosive Sensitivity Characterization of (HNBN3)3, 2. Three
small-scale tests have been designed to evaluate the sensitivity of
energetic materials to stimuli which may cause accidental initiation.
The drop-weight machine or drop hammer evaluates impact sensitivity.
In the test, a 2.5 kg weight is dropped from a set height onto a 40 mg
sample of the explosive on 150 grit garnet sandpaper. A series of drops
is made from different heights and explosion or nonexplosion is
recorded. The criterion for explosion is a sound level of 120 dB recorded
from a microphone set 33 in. from the point of initiation. The test results
are summarized as H50, the height in centimeters at which the probability
of explosion is 50%. The H50 of 2 was determined to be 16.2 cm.
The second test evaluates the susceptibility to initiation by human
electrostatic discharge. The test is performed by placing approximately
10 mg of energetic material in an insulating plastic cup with a
conductive steel base. The sample is covered with Pb foil and the entire
assembly held in place by an insulating plastic cover containing a hole
to expose the Pb foil. This assembly is centered below a brass
dressmaker’s needle, and the steel base and needle are charged to a
difference of 15.24 kV. The needle is then lowered, piercing the foil,
and a spark is discharged through the energetic material when the needle
is 0.085 in. above the steel base. The total energy of the spark is 0.36
J and is a conservative value for the amount of electrostatic energy
discharged by the human body. The test is interpreted by examining
the Pb foil for tears or other damage as evidence of reaction. Compound
2 was determined not to be sensitive to human electrostatic discharge.
The final test determines the initiation sensitivity of an energetic
material to friction. This test is performed by placing approximately 1
mg of material on a porcelain plate underneath a rounded porcelain
striker. The striker is pivotal to an arm of variable length to which
weights are hung, allowing different loads to be achieved between the
porcelain grinding surfaces. The plate is then mechanically driven
underneath the striker at a given load. The criterion for reaction is an
audible or visual reaction or both as detected by the operator. The test
results are reported as a 50% load, the weight in kilogram where the
probability of initiation is 50%. The 50% load for compound 2 was
determined to be 10.6 kg.
Experimental Section
General Information. Standard inert-atmosphere techniques were
used for the manipulation of all reagents and products. Solvents were
dried and degassed by standard methods, vacuum-distilled, and stored
over an appropriate drying agent. All solvent transfers were ac-
complished by vacuum distillation. Azidotrimethylsilane was purchased
from Aldrich and used as received. 2,4,6-Trichloroborazine and
6-chloro-1,2,3,4,5-pentamethylborazine were prepared as described in
the literature.9,10 Infrared spectra were obtained on a Nicolet 6000 FT-
IR instrument. NMR spectra were recorded on Bruker 250 MHz and
JEOL 400 MHz spectrometers. Spectral standards were Me4Si (1H, 13C)
and F3B‚OEt2 (11B). Resonances downfield of the standards were
assigned as +δ. Mass spectra were measured on a Finnegan model
4500 GC/MS instrument with use of a solids probe. Elemental analyses
were obtained from the UNM Microanalytical Services facility and
Galbraith Laboratories. Thermogravimetric (TGA) and differential
thermal analyses (DTA) were obtained from a Delta Series TGA7
analyzer (range 50-1400 °C, 10° min-1, Ar atmosphere, 25 cm3 min-1).
X-ray powder diffraction analyses were obtained with a Scintag model
PAD-V diffractometer (Cu KR radiation).
Caution! All of the azidoborazine materials described here should
be handled with extreme caution. All may vigorously explode under
various conditions.
Synthesis and Characterization of Compounds. 2,4,6-Triazi-
doborazine (2). A solution of Me3SiN3 (5.3 mL, 4.60 g, 40.0 mmol)
in diethyl ether (40 mL) was added to a solution of (HNBCl)3 (2.45 g,
13.3 mmol) in diethyl ether (40 mL) held at -78 °C. The mixture was
allowed to warm slowly, and it was stirred at 23 °C (12 h). The solvent
and volatile residues were removed by vacuum evaporation leaving a
white solid. The solid was redissolved in fresh Et2O (40 mL), the
mixture was filtered, and the solvent was evaporated, leaving white
solid 2: yield 2.1 g (97%). The solid could be further purified by
recrystallization or sublimation. Recrystallizations could be accom-
plished from a minimum volume of cold (-20 °C) Et2O or tetrahy-
drofuran (THF). Sublimation was accomplished with an ice-cooled
coldfinger in a standard vacuum sublimator with an oil bath temperature
of ∼110 °C or with a simple evacuated glass tube placed in an oven
(107 °C) with a small heat gradient. Sublimations were complete and
thin platelet crystals formed. Purified samples displayed sharp melting
points: 152-153 °C. Soluble: Et2O, THF, CH3CN, CH2Cl2. Sparingly
soluble: benzene, toluene, hexane. Mass spectrum (30 eV) [m/e (%)]:
204, M+ (100). Infrared spectrum (KBr; cm-1): 3424 (s), 3409 (m),
3363 (s), 2144 (s), 1465 (s), 1382 (m), 1310 (m), 1069 (m). NMR
spectra (C6D6): 11B{1H} δ 26.5; H δ 3.43. Anal. Calcd for H3B3N12
(Mr ) 203.57): H, 1.48; N, 82.6; B, 15.93. Found: H, 1.56; N, 79.89;
B, 15.21; C, 1.57.
1
6-Azido-1,2,3,4,5-pentamethylborazine (6). A solution of Me3SiN3
(2.6 mL, 2.26 g, 19.6 mmol) in Et2O (50 mL) was added to a solution
of [Me3N3B2(Me)2B(Cl)] (3.63 g, 19.6 mmol) in Et2O (50 mL) at -78
°C. The mixture was slowly warmed to 23 °C and stirred (12 h). The
resulting solution was filtered and the solvent evaporated from the
filtrate, leaving a colorless oil, which was vacuum-distilled (75-78
°C, 74 mTorr). A colorless oil, 6, was collected: yield 3.48 g (93%).
The oil slowly crystallized upon standing: mp 70-72 °C. Soluble:
Et2O, THF, CH3CN, toluene. Mass spectrum (30 eV) [m/e (%)]: 193,
M+ (61). Infrared spectrum (KBr; cm-1): 2952 (s), 2916 (s), 2835 (m),
2154 (vs), 2147 (vs), 1472 (vs), 1457 (vs), 1416 (vs), 1376 (vs), 1279
(s), 1257 (m), 1091 (s), 1019 (s), 880 (s), 656 (m), 625 (m), 574 (m).
NMR spectra (C6D6): 11B{1H} δ 37.2 (B3,5), 25.0 (B1); 1H δ 2.70 (6H,
Crystallographic Measurements and Structure Solutions. Suitable
crystals of 2 and 7 were obtained by sublimation and slow solvent
evaporation, respectively. Crystals were mounted in glass capillaries
under nitrogen and centered on a Syntex P3/F automated diffractometer.
Determinations of the crystal class, orientation matrix, and unit cell
dimensions were performed in a standard manner. Data were collected
with use of Mo KR radiation (λ ) 0.710 73 Å), a highly oriented
graphite crystal monochromator, a scintillation counter, and a pulse
height analyzer. Selected data collection parameters are summarized
in Table 1. The space group assignments11 were made by inspection
of small data sets, and absorption corrections were made on the basis
(9) (a) Niedenzu, K.; Dawson, J. W. Inorg. Synth. 1967, 10, 139. (b)
Laubengayer, A. W.; Moews, P. C.; Porter, R. F. J. Am. Chem. Soc.
1961, 83, 1337.
(10) (a) Toeniskoetter, R. H.; Hall, F. R. Inorg. Chem. 1963, 2, 29. (b)
Meller, A.; Marecek, H. Monatsh. Chem. 1968, 99, 1666.
(11) Space group notation: International Tables for X-ray Crystallography;
Reidel: Dordrecht, Holland, 1983; Vol. I, pp 73-346.

3740 Inorganic Chemistry, Vol. 38, No. 16, 1999
Paine et al.
Table 1. Crystallographic Data for 2, [HNB(N3)]3, and 7,
[Ph3PNH2](N3)
Compound 6 displays an intense parent ion in its EI mass
spectrum along with several fragment ions: the most intense
ion corresponds to (M - N2)+ at m/e 162. The infrared spectrum
of 6 is very similar to the spectrum of (CH3NBN3)3.8 On the
basis of assignments given by Meller,8 intense bands at 2154
and 2147 cm-1 may be assigned to azide stretching frequencies.
The 11B NMR spectrum of 6 shows two resonances centered at
δ 37.2 and 25.0 in a 2:1 ratio. The lower field resonance can
be confidently assigned to the two equivalent B2,4-Me groups
based upon chemical shift16,17 and area ratio. The higher field
resonance is assigned to the B6-N3 group. This shift is in the
general region found for B-pseudohalogen-substituted bora-
2
7
chem formula
fw
cryst syst
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
H3B3N12
203.57
triclinic
P1h
8.579(1)
9.182(1)
12.043(1)
104.62(1)
90.26(1)
110.74(1)
853.8(2)
4
C18H17N4P
320.33
orthorhombic
P212121
10.649(2)
11.737(3)
13.364(4)
90
90
90
1670.4(7)
4
1.274
1
zines.17 The H NMR spectrum shows resonances at δ 2.70,
Z
D
2.62, and 0.32 in a 2:1:2 ratio that are assigned to N1,5-CH3,
N3-CH3, and B2,4-CH3, respectively. The 13C{1H} NMR
spectrum contains resonances at δ 32.48, 34.46, and -0.5 that
are assigned to N1,5-CH3, N3-CH3, and B2,4-CH3, respec-
tively.
calcd, g cm-3
1.584
T, °C
20
0.120
0.710 73
2.89/5.76
20
0.169
0.710 73
3.34/5.09
µ, cm-1
λ(Mo KR), Å
RF/RwF,a %
a R ) Σ||Fo| - |Fc||/Σ|Fo|; RwF ) Σw(|Fo| - |Fc|)2/ΣwFo ], w-1
)
2
Triazidoborazine, 2, as noted in the Introduction, has been
previously reported to form from combination of (HNBCl)3 and
NaN3.6,7 On the basis of the descriptions in the literature, the
product is not obtained in a pure state, and this may be
responsible for the reported tendency for samples to easily
detonate. Paetzold and co-workers4 have noted that azidoboranes
can be conveniently prepared by reactions of B-Cl groups with
Me3SiN3. The obvious benefits of this reagent include the facts
that Me3SiN3 is readily soluble in organic solvents and the
elimination product, Me3SiCl, is volatile and easily removed
along with solvent by vacuum evaporation at 23 °C. No salt
byproducts are formed that require separation from reaction
mixtures. In practice, the 1:3 combination of (HNBCl)3 with
Me3SiN3 in Et2O results in ready elimination of Me3SiCl at 23
°C with formation of the target species 2 in high yield (eq 2).
σ2(F) + gF2.
of ψ scans.12 Redundant and equivalent data were averaged and
converted to unscale |Fo| values following corrections for Lorentz and
polarization effects. There was a small (∼2%) decrease in the intensities
of the standard reflections of 2, but no corrections were applied.
All calculations were performed on Siemens SHELXTL PLUS
(Microvax II and PC versions)13 structure determination systems.
Solutions of the data sets were by direct methods, and full-matrix least-
squares refinements were employed.14 Neutral-atom scattering factors
and anomalous dispersion terms were used for all non-hydrogen atom
coordinates during the refinements. Listings of non-hydrogen atom
coordinates, anisotropic thermal parameters, hydrogen atom coordinates,
and bond distances and angles are provided in the Supporting
Information.
Results and Discussion
The existence of 6-azido-1,2,3,4,5-pentamethylborazine, 6,
apparently has not been reported previously. The compound is
obtained in high yield as a colorless crystalline solid from the
1:1 combination of Me3SiN3 and 6-chloro-1,2,3,4,5-pentameth-
ylborazine in Et2O (eq 1). The compound is indefinitely stable15
Compound 2 is relatively pure when recovered by vacuum
evaporation of the solvent and Me3SiCl, although elemental
analysis suggests a small retention of organic solvent in the
crude product. Samples were further purified by recrystallization
from cold Et2O or THF or by vacuum sublimation at ∼110 °C.
Given the earlier reports,6 it was surprising that no evidence
for detonation of samples of 2 during workup were encountered.
Nonetheless, as described below, this compound does detonate
under some conditions and care should be exercised in working
with the compound. Unlike previous reports, pure samples of 2
display a sharp melting point at 152-153 °C. Compound 2 is
easily introduced into a mass spectrometer; it shows a parent
ion envelope in the EI mass spectrum (m/e 204-202), and the
ion at m/e 204 is the base peak in the spectrum. The fragment-
atom pattern is rich in species, and it is noted that ions
corresponding to (M - N2)+ and (M - 3N2)+ are observed.
The infrared spectrum of 2 contains a strong band at 2144 cm-1
that is assigned to the azide stretching mode. The 11B NMR
in the absence of moisture; however, it slowly hydrolyzes in
damp air. The compound is analytically pure, and it melts
sharply without decomposition. Upon cooling, it recrystallizes.
(12) The empirical absorption corrections use an ellipsoidal model fitted
to azimuthal scan data that are then applied to the intensity data:
SHELXTL Manual, Revision 4; Nicolet XRD Corp.: Madison, WI,
1983.
(13) Sheldrick, G. M. Nicolet SHELXTL Operations Manual; Nicolet XRD
Corp.: Cupertino, CA, 1981. SHELXTL uses absorption, anomalous
dispersion, and scattering data compiled in: International Tables for
X-ray Crystallography; Kynoch: Birmingham, England, 1974; Vol.
IV, pp 55-60, 99-101, 149-150. Anomalous dispersion terms were
included for all atoms with atomic numbers greater than 2.
(14) A general description of the least-squares algebra is found in:
Crystallographic Computing; Ahmed, F. R., Hall, S. R., Huber, C.
P., Eds.; Munksgaard: Copenhagen, 1970; p 187. The least-squares
refinement minimizes Σw(|Fo - Fc|)2, where w ) 1/[σ(F)2 + gF2].
(15) Samples have been stored under dry nitrogen in our laboratory for
more than 4 years with no visible decomposition.
(16) Maringgele, W. In The Chemistry of Inorganic Homo- and Hetero-
cycles; Haiduc, I., Ed.; Academic Press: London, 1987; Vol. I, p 17.
(17) No¨th, H.; Wrackmeyer, B. Nuclear Magnetic Resonance Spectroscopy
of Boron Compounds; NMR Basic Principles and Progress; Springer-
Verlag: Berlin, 1978; Vol. 14.

The Nitrogen-Rich Borazine [H3N3B3(N3)3]
Inorganic Chemistry, Vol. 38, No. 16, 1999 3741
Figure 1. Molecular structure and atom-labeling scheme for [HNB-
(N3)]3, 2. The view shows both unique molecules in the asymmetric
unit.
Table 2. Bond Lengths (Å) and Angles (°) for 2 [HNB(N3)]3
Figure 2. Unit cell projection along the b axis for [HNB(N3)]3, 2.
(a) Bond Lengths
molecule 1
molecule 1, the mean deviation, including all atoms, is 0.004
Å, with the largest deviations occurring for N(9), -0.156 Å,
N(11), 0.150 Å, and N(12), 0.269 Å. In molecule 2, the mean
deviation is 0.003 Å, with the largest deviations appearing for
N(18), 0.111 Å, and N(24), 0.106 Å. The two rings lie in parallel
planes with an interplanar angle of <2°. This arrangement is
nicely illustrated in a unit cell view shown in Figure 2. The
average B-N(ring) bond length, 1.418 Å, is comparable with
the average B-N(ring) distance in [(HN)3(HB)2BNH2],18 1.418
Å, while these are shorter than the average B-N(ring) distances
in (HNBH)319, 1.436 Å, and (HNBNMe2)3,20 1.433 Å. As
expected, the average B-N(ring) distance in 2 is shorter than
the average exo B-N3 distance, 1.459 Å. The average internal
borazine ring angles are B-N-B ) 121.8° and N-B-N )
118.2°. The exo azido groups are slightly bent, N-N-N(av)
) 174.4°, and they form an average B-N-N angle of 118.5°
with the borazine ring. The average BN-N distance in the exo
azido groups, 1.234 (2) Å, is clearly shorter than the typical
N-N single-bond distance (1.45 Å)2e and more comparable to
an NdN double-bond distance (1.25 Å).2e This distance in 2
also is shorter than the comparable distance in cyanuric azide
(NCN3)3, 1.266 (2) Å.21 The average terminal N-N distance
in the azido groups, 1.127 (2) Å, is longer than a typical Nt N
distance, 1.098 Å,2e and it is also longer than the related distance
in (NCN3)3, 1.116 (2) Å. These data suggest that the charge
distribution in the B-N-N-N fragments of 2 may be some-
what less asymmetric than found in the C-N-N-N fragments
of (NCN3)3. That is, the classical valence bond representation
depicted by 8 is favored over that of 8′.21 One mechanism that
B(1)-N(1)
B(1)-N(2)
B(2)-N(2)
B(2)-N(3)
B(3)-N(3)
B(3)-N(1)
B(1)-N(4)
B(2)-N(7)
1.411(2)
1.420(2)
1.419(2)
1.419(2)
1.423(2)
1.416(2)
1.467(2)
1.453(2)
B(3)-N(10)
N(4)-N(5)
N(5)-N(6)
N(7)-N(8)
N(8)-N(9)
N(10)-N(11)
N(11)-N(12)
1.455(2)
1.242(2)
1.125(2)
1.229(2)
1.127(2)
1.230(2)
1.129(2)
molecule 2
B(4)-N(13)
B(4)-N(14)
B(5)-N(14)
B(5)-N(15)
B(6)-N(15)
B(6)-N(13)
B(4)-N(16)
B(5)-N(19)
1.412(2)
1.424(2)
1.420(2)
1.412(2)
1.427(2)
1.414(2)
1.460(2)
1.460(2)
B(6)-N(22)
N(16)-N(17)
N(17)-N(18)
N(19)-N(20)
N(20)-N(21)
N(22)-N(23)
N(23)-N(24)
1.458(2)
1.225(2)
1.129(2)
1.234(2)
1.128(2)
1.245(2)
1.124(2)
(b) Bond Angles
B-N-B(ring)av
N-B-N(ring)av
N-N-Nav
B-N-B(ring)av
N-B-N(ring)av
N-N-Nav
(c) Hydrogen-Bonding Parameters
N-H‚‚‚Nx
sym x H‚‚‚Nx N‚‚‚Nx N-H-Nx H-Nx-N
Type 9 H-Bonds
x, 1 + y, 2.474 3.244
z
N(1)-H(1a)‚‚‚N(9)
N(2)-H(2a)‚‚‚N(18)
149.4
168.2
152.6
173.2
152.9
139.4
152.9
142.0
-x, -y, 2.386 3.233
1 - z
N(13)-H(13a)‚‚‚N(21) x, 1 + y, 2.466 3.253
z
N(15)-H(15a)‚‚‚N(12) 1 - x,
2.3328 3.184
-y, -z
Type 9′ H-Bonds
1 - x, 2.735 3.557
-y, -z
N(3)-H(3a)‚‚‚N22
160.4
157.0
102.3
104.1
N(14)-H(14a)‚‚‚N(4) -x, -y, 2.776 3.583
1 - z
spectrum displays a single resonance, δ 26.5, and this resonance
position is comparable to that assigned to the B-N3 group in
1
6. The H NMR spectrum contains a broad singlet at δ 3.43
might contribute to this structural feature is hydrogen bonding.
There are two potential idealized H-bond interaction modes,
that is assigned to the N-H group.
Sublimation of samples of 2 produced X-ray-quality single
crystals, and the molecular structure of 2 was consequently
determined by X-ray diffraction techniques. A view of the
molecule is shown in Figure 1, and selected bond lengths and
angles are summarized in Table 2. There are two unique
molecules in the asymmetric unit and four molecules in the unit
cell. Both molecule 1 and molecule 2 are completely planar. In
(18) Harshbarger, W.; Lee, G. H.; Porter, R. F.; Bauer, S. H. J. Am. Chem.
Soc. 1969, 91, 551.
(19) Harshbarger, W.; Lee, G. H.; Porter, R. F.; Bauer, S. H. Inorg. Chem.
1969, 8, 1683.
(20) Hess, H.; Reiser, B. Z. Anorg. Allg. Chem. 1971, 381, 91.
(21) Kessenich, E.; Klapo¨tke, T. M.; Knizek, J.; No¨th, H.; Schulz, A. Eur.
J. Inorg. Chem. 1998, 2013.

3742 Inorganic Chemistry, Vol. 38, No. 16, 1999
Paine et al.
organic solvent left from recrystallization of 2. Powder XRD
of the ceramic powder shows two broad reflections: 2d (Å) )
3.47 and 2.12. The breadths and positions of the reflections are
consistent with formation of turbostratic BN with small particle
size.
The explosive sensitivity properties of 2 reveal that this
material is sensitive to both impact and friction. A drop-weight-
impact test on a 40 mg sample of 2 gave an H50 value of 16.2
cm, which compares to a value of 22.9 cm for HMX (1,3,5,7-
tetranitro-1,3,5,7-tetrazocine) or 14 cm for PETN (pentaerythritol
tetranitrate). Usually any value of 14 cm or less is considered
extremely hazardous. Furthermore, a friction sensitivity test on
2 showed extreme ease of initiation, which is not uncommon
behavior for azides. A value of 10.6 kg compares with 13.0 kg
for PETN. PETN is considered a primary explosive to both
impact and friction. Electrostatically this compound is safe to
handle, as a 0.36 J spark failed to elicit any reaction. These
results confirm that 2 should be handled with considerable
caution.
Figure 3. TGA trace for 2.
Scheme 1
The reactivities of 2 and 6 are also of interest, and in this
respect, we examined the Staudinger reactivities22 of 2 and 6
toward Ph3P. Parallel chemistry with (NCN3)3 was first reported
by Kesting,23 and the suggested outcome is summarized in eq
3. This chemistry was recently reexamined by Klapo¨tke and
and they are depicted by 9 and 9′. H(1a), H(2a), H(3a), H(13a),
co-workers,21 and instead of 10, the product isolated was the
interesting tetrazole isomer 11, shown in eq 4. In the case of 6,
H(14a), and H(15a) are capable of participating in H-bonding
with neighboring azido groups in a second molecule, and
calculated distances between molecules 1 and 2 are listed in
Table 2. It is obvious that the range of observed distances of
the type depicted by 9 (2.33-2.47 Å) is shorter than the range
represented by 9′ (2.74-2.78 Å). Such interactions would be
expected to weaken and lengthen the idealized terminal NtN
triple bond interaction.
A TGA trace for 2 is presented in Figure 3. It shows an abrupt
weight loss (∼70%) in the temperature range 50-220 °C
followed by a more gradual loss (∼7%) from 220 to 460 °C.
The average (four runs) total weight loss was 77%. The DTA
shows a major endotherm centered at 155 °C that corresponds
to the melting process and two exotherms centered at ∼260
and ∼420 °C. These events appear to be consistent with the
processes summarized in Scheme 1. The theoretical weight
losses accompanying these three idealized steps are as follows:
step A, 0%; step B, 41.3%; step C, 22.1% (total 63.4%). The
observed TGA trace shows an ∼5-10% weight loss prior to
melting, which may result from loss of recrystallization solvent.
Taking this into account, the observed total weight loss is close
to that calculated. The larger observed weight loss in step B is
consistent with a fast thermolysis event.
we observe that the 1:1 reaction with Ph3P leads to no
discernible reaction, as 6 is quantitatively recovered. Reaction
of 2 with Ph3P in a 1:1 ratio in refluxing THF gives a colorless
solid (7) with limited solubility and a melting point of 183 °C.
The melting is accompanied by gas evolution.
A single-crystal X-ray diffraction analysis led to the identi-
fication of the product as depicted in eq 5. The compound
The TGA analysis data and Scheme 1 suggest that 2 should
be a precursor for BN, although with low ceramic yield. In fact,
careful pyrolysis of samples (0.5-1.0 g) of 2 using a tube
furnace (1200 °C) produces BN powder. The samples have a
bone color, instead of the expected pure white color and
elemental analyses show small, variable carbon contamination
(1-2%). The carbon likely comes from small amounts of
crystallizes in the chiral space group P212121, and the asym-
-
metric unit is composed of the [Ph3PdNH2+] cation and N3
anion. A view of the molecule is displayed in Figure 4, and
selected bond lengths and angles are summarized in Table 3.
(22) Staudinger, H.; Meyer, J. HelV. Chim Acta 1919, 635.
(23) Kesting, W. J. Prakt. Chem. 1923, 105, 242.

The Nitrogen-Rich Borazine [H3N3B3(N3)3]
Inorganic Chemistry, Vol. 38, No. 16, 1999 3743
Figure 4. Molecular structure and atom-labeling scheme for
[Ph3PdNH2](N3), 7.
Figure 5. Hydrogen-bonding chains for [Ph3PdNH2](N3), 7.
tetrazole structure represented by 12, because the compound
Table 3. Bond Lengths (Å) and Angles (deg) for 7, [Ph3PNH2](N3)
(a) Bond Lengths
P-N(1)
P-C(1)
P-C(7)
1.612(3)
1.785 (3)
1.794(3)
P-C(13)
N(2)-N(3)
N(3)-N(4)
1.803(3)
1.154(3)
1.148(4)
(b) Bond Angles
N(1)-P-C(1)
N(1)-P-C(7)
N(1)-P-C(13)
C(1)-P-C(13)
106.9(2)
C(1)-P-C(7)
C(7)-P-C(13)
N(2)-N(3)-N(4)
109.8(1)
107.3(1)
176.8(4)
109.8(2)
113.1(2)
109.9(2)
reacts slowly with Ag(I). A related trityl azide-triphenylphos-
phine adduct has been postulated;28 however, in view of the
structure determination of 7, this proposal should be revisited.
The synthesis, structure, reactivity, and explosive character-
istics of 2 described here provide interesting new information
on this known but poorly understood molecule. In addition, the
data collected here suggest that 2 is a precursor to boron nitride
and a confined-volume, high-energy synthesis may provide a
method to obtain new powder morphologies of BN. Further
studies of the explosive performance characteristics are under-
way, and they will be reported later along with results from
electronic structure calculations.
(c) Hydrogen Bonding Parameters
sym x N-H H‚‚‚N N‚‚‚N N-H‚‚‚N
N-H‚‚‚Nx
N-H(1a)‚‚‚N(2) x, y, z
N-H(1b)‚‚‚N(4) 0.5 + x,
0.919 1.957 2.785
0.918 1.964 2.865
176.9
166.6
0.5 - y, -z
There is no crystallographic symmetry on the unit. The
arrangement of groups about the phosphorus atom is tetrahedral
with average C-P-C and C-P-N angles of 108.7 and 109.9°,
respectively. The P-N(1) distance, 1.162(3) Å, is close to the
ideal double-bond distance, PdN, of 1.57 Å as compared to
the normal P-N single-bond length, 1.76 Å.24
The azide group is linear, N(2)-N(3)-N(4) ) 176.8(4)°, with
N-N bond distances of N(2)-N(3) ) 1.154(3) Å and N(3)-
N(4) ) 1.148(3) Å. These distances compare well against those
in (Me4N)N3,25, 1.155(5) and 1.176(5) Å, and other ionic
azides.26 There is hydrogen bonding between the azide and NH2
group, and the network is shown in Figure 5. The bond lengths
and angles in one unit are also listed in Table 3. It is interesting
to note that 7 was probably first observed by Staudinger in
1921;27 however, it has been assumed to have a cyclic nonionic
Acknowledgment. R.T.P. thanks the UNM Center for
Microengineered Materials (CMEM) and the National Science
Foundation (Grant CHE-9508668) for support of this research.
Supporting Information Available: Additional information on the
collection of X-ray data, final atomic coordinates, H atom coordinates,
thermal parameters, and bond lengths and angles for 2 and 7 in CIF
format. This material is available free of charge via the Internet at
http://pubs.acs.org.
IC990316B
(26) Choi, C. S. In Energetic Materials; Fair, H. D., Walker, R. F., Eds.;
Plenum Press: New York, 1977; Vol. 1, p 97.
(27) Staudinger, H.; Hauser, E. HelV. Chim Acta 1921, 4, 861.
(28) Leffler, J. E.; Honsberg, U.; Tsuno, Y.; Forsblad, I. J. Org. Chem.
1961, 26, 4810.
(24) International Tables of X-ray Crystallography; Kynoch: Birmingham,
England, 1968; Vol. III, p 270.
(25) Christe, K. O.; Wilson, W. W.; Bau, R.; Bunte, S. W. J. Am. Chem.
Soc. 1992, 114, 3411.