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Covalent Inorganic Azides.

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Covalent Inorganic Azides
Inis C. Tornieporth-Oetting* and Thomas M. Klapotke"
The chemistry of covalent inorganic
azides originated with the synthesis of
aqueous HN, solutions by Tony Curtis
in 1890. A little later, in 1900, it proved
possible to prepare iodine azide, IN,, as
the first member of the meanwhile complete series of halogen azides. Since then
it has been possible to synthesize, in
addition to HN, and the stable salt
azide compounds of elements from Groups 13 to 17. In these
compounds the N, moiety acts as a
pseudohalogen and is primarily covalently coordinated to the nonmetal. Only a few organic azides, however, as well
as HN,, H,N:, and all halogen azides
have been thoroughly studied with respect to structure and bonding. The
combined application of diffraction
methods (X-ray and electron diffraction) and microwave spectroscopy together with quantum chemical approaches such as ab initio SCF and
density functional calculations have led
Keywords: ab initio calculations . azides
. nitrogen compounds
2. Synthesis and Spectroscopic Properties
1. Introduction
Covalent azides have been known for more than 100 years."]
Table 1 contains a--by no means complete-list of main group
azides in which the N, group acts as a pseudohalogen and is
primarily covalently bound to the corresponding nonmetal. Until now. however, only very few of these compounds could be
definitively structurally characterized. Examples include
HN,,"' H,N:,[31 CF,N,,L41Te(N,):,l5I XN, (X = F,16] Cl,[']
Br,''] If'* ''I), and I(N& .I' If one considers the N, fragment
as a pseudohalogen (absolute electronegativity according to
Mulliken: C1 8.3, Br 7.5, N, 7.7 eV),17b.' I the hydrogen compound HN, and the pseudohalogen -halogen compounds XN,
(X = F. C1, Br, I) are the simplest members of this class of
substances. This simplicity, combined with the strong tendency
to detonate and the, in some cases, considerable toxicity, presents a significant challenge to the experimental chemist."3J In
this article, we wish to illuminate the interaction of experimental, preparative. structural, and theoretical studies in this area of
chemistry, using examples of hydrogen and halogen azides with
structures that, in most cases, have been definitively determined
only over the last five
l 5 I Afterwards, the structure and
bonding of species related to halogen azides, namely 1,N: ,[16]
I(N3)l,I1'' ONN,,"'] and O,NN, ,I1'] will be discussed, and an
outlook for future preparative studies in this field will be given.
[*] Dr. I. C . Tornieporth-Oetting
Institut f u r Anorgdnische und Analytische Chemie der Technischen Universitit
StrdsSe des 17 Juni 135, D-10623 Berlin (Germany)
Telefax. Int. code + (30)314-26468
Prof Dr. T. M. Klapotke
Chemistry Department, University of Glasgow
Glasgow G12 8QQ ( U K )
Telefdx' Int. code +(41)3304888
Angrs. ('hem. hi.Ed. EngI. 1995, 34. 511 -520
in the last few years to an improved understanding of the molecular properties
of numerous nonmetal azides, almost all
of which are explosive. This interaction
of theory and experiment has greatly enhanced the development of azide chemistry and has led to realistic expectations
for the synthesis of as yet unknown nonmetal azides.
2.1. Synthesis
Covalent azides are highly explosive, especially in pure form.
It is therefore imperative that work on a preparative scale is
performed under appropriate safety precautions, such as face
shields, protective leather clothing, gloves, and ear protection.
Experiments should also be performed on as small a scale as
feasible. As an example, the explosive decomposition of less
than a gram of bromine azide kept in a glass flask which in turn
was placed inside a metal-shielded glass Dewar, led to the complete pulverization of the glass and tearing of the metal shielding.[". 301 All halogen azides are very sensitive in this respect.
Chlorine azide and bromine azide are particularly sensitive to
small pressure variations and regularly explode when Ap 2
0.05 Torr.[sl Characteristic detonation data for some halogen
azides and nitrosyl azide, N,O, have been reported (Table 2).['01
These compounds are potentially better explosives than lead
azide (see Table 2 ) , which is used as a detonator. However, they
are by far not kinetically stable enough to find use as detonators,[31- 3 3 1
Phosphorus nitrile azide, [PN(N,),], ,which can be isolated in
pure form, is one of the most nitrogen-rich inorganic compounds (see Table
This material is also very shock-sensitive in the solid state and detonates when mechanically stressed
and upon exposure to an electric discharge. According to the
results of ab initio calculations, the compound possesses
approximate C , symmetry (Fig. I)
its structure, however,
has not yet been experimentally determined by diffraction methods. While the nitrogen-rich cation As(N,)f is "stable" in SO,
Ver1agsgesellschqft mbH, 0-69451 Weinheim,1555
S 10.00+ .25/0
51 1
I. C. Tornieporth-Oetting and T. M. Klapotke
Table 1. Selected covalent nonmetal azides and azide ions (the compounds marked by a n asterisk have been structurally characterized).
Group 13 [I]
Group 1
Group 14 [ l 131
Group 15 [I]
Table 2. Characteristic detonation data for IN,, BrN,, N,O. and Pb(N,), [lo]
IN, (s)
A H explosion [kcal kg- '1
standard gas volume [Lkg-'1
detonation temperature T,, [K] [a]
moles of gas per kg ( n )
specific energyJ'[mtkg-'1 [b]
lead block expansion per l O g [ ~ m - ~ ]140
A@ [kcalmol-'1
104 ( s )
BrN, (1)
N.8 (B) Pb(N,), is)
102 (1)
110 (6)
101 (s)
[a] T,. estimated for the isochoric process [33]. [b] f = nRT,, (R
1 0 ~ 4 m t K - 1 m o l ~1 mt=l000kpm=2.3423kcal)[33].
8.478 x
solutions and has been characterized by multinuclear
NMR (I4N, 75As) and Raman spectroscopy, the compound
[As(N,),]AsF, is very explosive in the solid state.[281
Group 16
Group 17 [lo, 16,171
Hydrazoic acid and many of its covalent inorganic (e.g.,
CIN,, Me,SiN,) and organic derivatives (e.g., CH,N,) are also
very toxic. HN, is as poisonous as hydrogen cyanide.['"'
In spite of these difficulties (or maybe even because of them),
several research groups did early work on the synthesis of hydrazoic
its organic derivatives,[' 3 , 351 and halogen
azides.[I4.361 Even though HN, can be prepared by the action of
dilute sulfuric acid on sodium azide or the reaction of nitrous
acid and hydrazine, it is most readily synthesized on the preparative scale by the reaction of stearic acid with sodium azide at
100- I30 "C in the melt [Eq. (a)].[31The preparation of the haloCH,(CH,),,COOH
+ NaN,
melt. 110 130'C
+- -
+ HN,
gen azides is best done according to the Equations (b)-(e).[I4l
We recently discovered an easy access to chlorine azide in a
Fig. 1. Calculated molecular structure
of [PN(N,),], (ab initio methods,
RHF/6-31 G*); d(NlLN2) =1.16,
d(N2-N3) =1.32.
d(N3-P) =1.79,
d(P-N4) = 1.73 A.
gas phase
+ CI,
+ Br,
20 C/O C.pure
+ NaCl
h i s C. Tornieporth-Oetting was born on March 19, 1963 in Hamburg, Germany. She studied at the Universitiit Oldenburg and
at the Technische Universitat ( T U ) Berlin, where she also received her doctorate degree in 1992, working under the supervision
of Thomas M . Klapotke. She is currently employed as a senior scientist at the TU Berlin. She is the author of more than forty
publications and two chapters of books, and together with Klapotke is the coauthor of a textbook on nonmetal chemistry. Her
research interests in the area of nitrogen chemistry stretch,fiorn the purely inorganic halogen chemistry ofthis element, including
its Jluorine chemistry, to the coordination chemistry of organic amino compounds and the study of biologically relevant amino
acid model complexes.
Thomas M . Klapotke was born on February 24, 1961 in Gottingen, Germany. He studied at the Technische Universitat Berlin
and received his doctorate degree there in 1986, working under the supervision of Hartmut Kopj Afier postdoctoral work with
Jack Passmore at the University of New Brunswick, he completed his habilitation in 1990 at the TU Berlin. In 1995 he was
appointed to the Ramsay Chair of Chemistry at the University ofGlasgow9. He is the holder of the Karl- Winnacker-Stipendium
(1994) and has been awarded, amongst others, the Schering-Preis (1987) and the Maier-Leibnitz-Preis (1994). Furthermore,
he is the author of more than 120publications,six chapters of books, and with Tornieporth-Oetting is the coauthor o f a textbook
on nonmetal chemistry. His research interests encompass the preparative synthesis of unstable nonmetal compounds in ihe areas
o f nitrogen- halogen and nitrogen -chalcogen chemistry, including the chemistry of elementaljluorine, as well as the theoretical
characterization of the bonding in these compounds.
Angew. Chem. Int. Ed. Engl. 1995, 34, 511-520
Covalent Inorganic Azides
solvent-free system by the reaction of trimethylsilyl azide and
nitryl chloride [Eq. (f)]. After fractional condensation, it was
possible to isolate pure liquid CIN, .[''I
3 Me,SiN,
+ 3 CINOz
2 CIN,
+ Me,SiCI + Me,SiOSiMe,
+ 2 N,O + N,O,
2.2. I4N NMR Spectroscopy
3. Stability and Thermodynamics
Ab initio calculations have been used increasingly in recent
years to extract bond energies and thermodynamic stabilities of
covalent azides from calculated total energies." '* 431 These data
are, for obvious reasons, difficult to obtain experimentally.
The dissociation energies for HN, and the halogen azides
have been calculated and appropriately corrected to yield dissociation enthalpies at room temperature A H [Eq. (g), X = H, F,
CI. Br, I ; see also Table 3).[43.441
One of the most suitable methods for the characterization of
covalent azides in solution is 14N N M R spectroscopy, which has
proven to be an invaluable tool for the study of small nitrogencontaining molecule^.[^^^ In addition, during the last few
years it was demonstrated that 14N N M R spectroscopy
could not only be utilized for the elucidation of structures in solution, but also for the determination of equilibrium
constants and thermodynamic parameters.[38-391 For many
covalent azides. such as the halogen azides XN, (X = CI,
Br, I). covalent phosphorus and arsenic azides, as well as covalent azide ions such as As(N3)4f, the I4N N M R spectra
show three well-resolved resonances, which have been assigned
to the chemically inequivalent nitrogen
791 Presumably owing to the large quadrupole moment of the nitrogen
nucleus. there were no observable l 4 N P t 4 N couplings. This is
not surprising when considering the very small coupling constant. estimated at about 30 Hz. Using variable temperature and variable concentration 14N N M R studies it was possible to demonstrate that iodine azide, which is monomeric in
the gas phase but polymeric in the crystalline state, as well as
alkylarsenic(1lr) azides, which are oligomeric as pure liquids,
dissolve in CDCI, to give monomolecular solvates. In the temperature and concentration ranges studied, no azide exchange
between the arsenic or iodine centers, respectively, was found.
Figure 2 shows the I4N N M R spectrum of Me,AsN, as an
example; the resonances were assigned according to the numbering shown in Figure 4.
- 1 00
Flg 2 I4N NMR spectrum of Me,AsN, (28904MH2, 1 molL-', relative to
MeNO,, 22 C , C DC I,) [42]
Angru . <'him. Inr Ed Engl. 1995, 34. 51 1- 520
+ X + AHg
Reaction (g) is strongly endothermic in all cases and the value
calculated for HN, agrees well with the experimentally obtained
data (85 kcal mol- ').I4']
According to Table 3. the X-N, bond
Table 3. Calculated reaction enthalpies A H [kcal mol- '1 and calculated standard
enthalpies of formation A H : [kcdlmol-'] for the reactions (g) (i) (MP2/6-31 G * * )
- 144.2
- 164.4
energy decreases in the order H + F > CI > Br > I . This result
should not be misinterpreted with respect to the stability of the
halogen azides, however, since the thermal decomposition of
XN, is not initiated by the breaking of the X-N, bond (X = H,
F. C1, Br, I), but, at least in the gas phase, by dissociation into
X N ( j Z - ) and N2('C;) [Eq. (h)]. The decomposition of solid
+ XN(%) + AH,,
IN, is much more complicated and not yet completely understood. Even though the gas phase decomposition reaction according to Equation (h) is spin-forbidden, pyrolysis experiments on HN, in the temperature range 285-470°C have
clearly shown that this species fragments into HN and N, .[461
(See reference [I31 regarding the gas phase pyrolysis of NC-N,
and organic azides.) The occurrence of this singlet-triplet coupling during the HN, fragmentation has been the subject of
extensive quantum chemical calculations at a very high
The calculated values for AHh (Table 3) of the halogen azides
indicate that, in the gas phase, FN, should be least stable and
IN, most stable. This is in agreement with experimental observations. In addition, the calculated dissociation enthalpy A H ,
for HN, (Table 3) is in very good agreement with the experimental values of 15.0 kcalmol-Lr481and 17.5 k ~ a l m o l - ' . [ ~The
dissociation according to Equation (h) is exothermic for FN,,
CIN,, and BrN,, and mildly endothermic for IN,. However, the
reaction enthalpies at room temperature A H , , calculated for
Equation (i), indicate that all halogen azides possess a large
2 XN,
3 N,
+ X, + AHi
I. C. Tornieporth-Oetting and T. M. Klapotke
positive enthalpy of formation (Table 3). These two sets
of data readily explain the extreme instability of all halogen
azides on the one hand, and the slight, but clearly apparent
kinetic stability of IN, on the other.[14]As was only recently
shown experimentally, the dissociation barrier (to XN
and N,) is indeed larger for chlorine azide than for fluorine
The thermodynamic stability of the very labile N-oxides ONN, (nitrosyl azide) and 0,N-N, (nitryl azide) [Eq. (j) and (k),
respectively] has also been the subject of ab initio studies. The
dissociation of N,O into N,O and N, was predicted to be very
exothermic ( A q = - 91 kcalmol-').[181 N,O,, which so far
has only been detected in solutions, also dissociates into the thermodynamically more stable N,O (A@ = - 57 kcalmol-').['91
- N,
+ N,
2 N,O
Experimentally it is possible to stabilize halogen azides, particularly iodine azide, through the formation of species that are
more highly coordinated. The reaction of IN, with numerous
nitrogen bases such as pyridine and a,d-bipyridyl leads to the
formation of stable donor -acceptor complexes, in which the
nitrogen base is coordinated to the positively polarized iodine
atom of IN, .I5'] This distinct polarization of the N-I bond also
causes IN, to act as a Lewis acid towards halogenide or pseudohalogenide ions. IN3 reacts with NMe:N;
to give the extremely
explosive compound NMe: I(N,); . [ 5 *I The analogous species
PPh: I(N,);, however, is not explosive.[521Interestingly, in
addition to the I(N,); anion, which, according to the pseudohalogen concept, corresponds to ICl; , the ICl: analogue,
namely the I(N3)Z cation has recently been prepared (see Section 4) .I'7l
Tetraazidoarsoniumhexafluoroarsenate, As(N3); AsF, ,which
can be handled well in SO, solutions, is very sensitive
in the solid state and detonates upon exposure to mechanical stress or a high frequency discharge. Using a
Born- Haber cycle, the reaction enthalpy of the decomposition
according to Equation (I) was estimated to be AH, =
- 406 kcal mol- 1.[281
4. Structure and Bonding
In contrast to the ionic azides of the type A'N; (A = metal,
all contain a linear N, unit of D,, symmetry, covalent azides present as discrete XN, species in the gas
phase (X = H, halogen, etc.) display a bent trans C, configuration with a N-N-N bond angle of approximately 172 & 3", and
two significantly different N-N bond lengths (Fig. 3).f141
Table4 contains a summary
of experimentally obtained and
quantum mechanically calculated
structural parameters for XN,
azides. They show clearly the exNI
cellent agreement between the exFig. 3. Depiction of a covalent
perimental data and those that
XN, azide in the bent
were quantum mechanically calconformation.
culated. It was necessary, however, to account for electron correlation (e.g., according to
M d l e r - P l e ~ s e t ) [in
~ ~order
to obtain good correlation between
the theoretical results and the experimental findings.['41In addition, it proved to be very helpful to introduce quasirelativistic
pseudopotentials for the heavy halogens bromine and iodine to
account for relativistic effects. For instance, calculations utilizing an effective core potential often led to better results in less
time than all-electron calculations.[54*
What additional information can be gleaned from these MO
calculations? The bent N, unit in covalent XN, azides and the
different N-N bond lengths are particularly noteworthy. One
of these bonds (NI -N2) is significantly shorter than a typical
N-N single bond (1.44 A), while the other (N2-N3) is slightly
longer than the N-N triple bond in N, (1.098 A).[561A localized orbital
of the covalent XN, azides (X = H,
F, C1, Br, I, NO, NO,) results in an
NBO (natural bond orbital) analy+
is[^'] that yields the configuration
* \.N1
shown in Figure 4 as the energetically
Fig, 4, Lewis structure of
most favorable Lewis structure. According to this, there is a single bond
covalent XN,
between N1 and N2 and a triple bond
between N2 and N3.
The total energy E = E(Lewis) + E(non-Lewis) deviates in
general only very little from .!?(Lewis) (most often E(nonLewis) < 1 % E(Lewis)). Therefore NBOs are well suited to describe the covalent effects in a molecule in accordance with its
NH4),[1a-i. 531 which
. .-
Table 4. Structural parameters of covalent XN, azides (for atom numbering scheme see Fig. 3): experimental and ab initio data [lo, 14, 431
method [a]
d ( X - N l ) [A]
exp./ab initio
Ab initio
d(Nl-N2) [A]
exp./ab initio
d(N2-N3) [A]
exp./ab initio
t ( N 1 , N2,N3) [ I
exp./ab initio
t ( X , N1, N2) ["I
exp./ab initio
1.281- 11.290
- 11.270
- 11.274
1.I 34/1 .I 58
1.09,' - 11.156
- /1.154
- j1.151
169.6117 1.4
1741- 1169.2
- 1174.3
- 1172.1
108.6/ 109.3
1141- 1111.6
- 1107.1
- j108.0
[a] MW: microwave data; ED: electron diffraction: X : X-ray diffraction. [b] The structure of solid, polymeric (IN3)=has so far not been calculated. [c] No experimental
structure data is known to date. [d] MPZILANLIDZ P. [el MP2/6-31 + G*, iodine: ECP, basis set: [5~5pld]/(3~3pld)-(DZP).
Angen. Chem. Inr. Ed. Engl. 1995, 34, 511 -520
Covalent Inorganic Azides
“natural” Lewis structure. It is best to allow for the non-covalent effects which are ignored in the Lewis structure by a secondorder perturbation calculation. The most important non-covalent contributions in the XN, system are the x-delocalization
over the entire molecule (resonance) on the one hand and a
strong intramolecular donor -acceptor interaction on the other.
This interaction leads to the transfer ofelectron density from the
tilled (X-N1 )-o orbital into the unfilled, antibonding (N2N3)-n* orbital, which weakens the X-N1 and N2-N3 bonds,
while it strengthens the N l - N 2 bond. Table 5 lists linear
NLMO (natural localized molecular orbital) bond orders for
some XN, azides, which were obtained by allowing for noncovalent effects in the NBO analysis.[s81In addition, it is noteworthy that the total bond order (XBO) at the central N atom of the
N, moiety is significantly greater than three in all cases.
Table 5. Linear NLMO bond orders (BO) and overall bond orders (ZBO) in discrete
covalent XN, azides (NBO analysis) [a].
BO (X-Nl) BO (NI-N2)
BO (N2-N3) ZBO (NI) ZBO(N2) ZBO(N3)
[a] HF/6-31
1 If
+ G*. lor CI. Br, I: ECP. basis set. [5~5pld]/(3~3pld)
- (DZ + P) [59].
Among the halogen azides CIN, has by far the highest X-N1
bond order of 0.90 (Table 5). This is in agreement with the
pseudohalogen concept, according to which the azide group has
an electronegativity comparable to chlorine, which implies that
the pseudohalogen-halogen compound CIN, is best compared
to CI, .[7h1 The Fact that the stretching frequencies for both compounds are similar (V(C1-CI) = 546, S(C1-N,) = 542 cm-’)[7b1
is also in agreement with the pseudohalogen theory. The rather
high bond orders in the ON-N, and 0,N-N, bonds in the
covalent nitrosyl and nitryl azides are also noteworthy,[18. 19. 291
and in order to take the covalent character of these compounds
into account, it might be more useful to term them “tetranitrogen oxide’’ or “tetranitrogen dioxide”.
A comparison of the linear X-N1 bond orders (Table 5 )
clearly shows the expected weak covalent bonds in the strongly
polarized species HN,, FN,, and IN,. A summary of calculated
NPA (natural population analysis) charges is given in Table 6.
Table 6. NPA charges 6 in covalent XN, azides [a].
ONN, lbl
O,NN, [c]
-0 54
+ 0.40
[a] HFi6-31 G * ,forC1. Br, I: ECP. basis set: [5s5pld]/(3s3pld) - (DZ + P)[59].
[bl6 at 0 = - 0 34. [c] 6 = - 0.40 (at Or,,,.,),-0.49 (at OJ; ci.\/trans relative to the
terminal N atom.
Anjieii. Chem. I n / . Ed. Enzl. 1995, 34. 511-520
The experimentally observable weakness of the I-N bond in
IN, [14,361 and in inorganic N-I species in general[“] has been
ascribed in early, more qualitative discussions to the strong
polarity of this bond. The theoretical findings presented here
confirm this view from a quantum mechanical basis.
The crystal structure analysis of IN, gave a surprising result
that is so far unique among the halogen azides. Solid iodine
azide is polymeric and present in the form of zigzag chains, with
iodine atoms that are doubly coordinated in a linear fashion
(Fig. 5).[’] It was also possible to synthesize the doubly coordinated ions I(N& [”I
and I(N,);[521 on a
preparative scale. In
agreement with the
VSEPR theory, the coordination mode at the
central iodine atom is
bent (cation) and linear
(anion, which is isoelecFig. 5. Polymeric structure of iodine azide
in the crystal (for Structural parameters see
Ironic to the hypothetiTable 4) [9].
cal Xe(N,),), respectiveIt is noteworthy
that the linear bond order of the I-N bond increases significantly from 0.67 for discrete IN, to 0.80 for the I(N,)l cation
(Table 5). Apparently iodine in binary N - I species favors
higher coordination numbers, and forms more covalent N-I
bonds. The structures of binary bromine-nitrogen compounds
are unexplored with the exception of gaseous BrN,.[81 The determination of the structure of solid bromine azide will be a
substantial future challenge.
A comparison of the structures of the formally analogous
cations H,N:[31 and I,N:[l6I (Fig. 6) shows that according to
Fig. 6. Structures of the cations H,N:
(left, C2J and 12N: (CJ
the results of an X-ray crystal structure analysis. H,N: displays
C,, symmetry in the crystal (SbF, salt), while the I,N: cation
(in I,N:EF;,
E = As, Sb) has a chainlike planar C, structure,
as determined by Raman spectroscopy. This latter result is in
agreement with ab inito calculations for which electron correlation was taken into account, which indicated that this C, structure is only 2 kcal mol-’ lower in energy than the C, conformation, which was not observed experimentally. The two different
structures for H,N: and I,N: are in accordance with the pseudohalogen concept, in which HN, is analogous to HCI, while
IN, is more closely related to ICI. A comparison of the known
structures of H,CI+ (C,”)and I,CI+ (C,) supports the analogy
between I,N: and 1,Cl’. In both cases, like in all interhalogen
species, the larger halogen occupies the central position.[621
The structures discussed for the ions I(N,)Z[”] and
I(N& [”I also correspond with those expected by pseudo515
I. C. Tornieporth-Oetting and T. M. Klapotke
halogen theory and VSEPR
I(NJ; as AX,E, type,
bent at the central iodine; I(N3); as AX,E3 type, with linear
coordination at the central iodine (Fig. 7).
Fig. 9. Rotational barrier for the cis-cis/truns-truns
isomerization of N,O. calculated at the MP2 level. w = O-N4-Nl-N2 dihedral angle.
Fig. 7. Structures of the ions I(NJ;
(C,) and I(N3); (C,)
Even though the chemistry of the N, group can frequently be
explained by using the pseudohalogen concept, it has not been
possible to date to observe the species analogous to CI, , namely
N,-N,. It is likely, however, that such a molecule would be in
no way kinetically stabilized. High level quantum mechanical
calculations have indicated that the most stable N, isomer
should possess a twisted chainlike C, structure, which lies
188 kcalmol-I above the energy of three N,
contrast to benzene, cyclic hexazine, N,, should assume a nonplanar twist-boat structure ( D 2 ) ,which lies 26 kcalmol- above
the energy of the C, chain structure.[631
Suitable correlations of experimental and quantum mechanical molecular data together with calculations of energy hypersurfaces make it possible to estimate the structures of short-lived
molecules that cannot be isolated on the preparative scale.r641
Every defined molecular state possesses a certain structure, and
changes in its energy or charge distribution cause structural
changes that occur through the molecule's specific molecular
dynamics. The decomposition of the unstable nitrosyl azide,
N,O, will be used as an example.
N,O was shown, on the basis of Raman spectroscopy, to have
an open-chain C, structure with a trans-trans arrangement at
N1 -N4 and N1 -N2 (Fig. 8,left).['*l This compound has there-
Fig. 10. Two-dimensional MP2 energy hypersurface for the decomposition of cis+ N,.
cis N,O: a : cis-cis N,O, b: cyclic N,O (C2J. c: N,O (CZJ
barrier of 7 kcalmol-' (Fig. 9), while the transition state for the
N1 -N2 variation lies 24 kcalmol-' in energy above the transtrans isomer. This indicates that a unimolecular fragmentation
into N,O and N, is more likely to occur via cis-cis N,O and
cyclic N,O. The calculated potential energy curve for the decomposition without the rotation (Fig. ll) shows that transbut rather to cyclic
trans N,O does not lead to linear N,O (Cmv),
N,O (C,"). The hypersurface depicted in Figure 1 1 shows three
minima: trans-trans N,O, C,,-N,O + N,, and C2"N,O + N,. Hence trans-trans N,O can decompose not only by
the pathway initiated by the rotation into the cis-cis isomer, but
also by two additional pathways. Interestingly the transition
Fig. 8. Structures of nitrosyl azide. N,O, in the Irons-frunsand cis-cb configurations. optimized at the MP2 level.
fore two possible modes of unimolecular decay.r651On the one
hand, rotation around the N1 -N4 axis can lead to the cis-cis
isomer (Figs. 8 right, and 9), which then decomposes, according
to the hypersurface shown in Figure 10,via cyclic N,O into N,O
and N,. On the other hand, the trans-trans isomer can also
fragment directly by a change in the N1 -N2 distance (Fig. 11).
The rotation into the cis-cis iqomer, however, has a calculated
're1 /kcal
Fig. 11. Two-dimensional MP2 energy hypersurface for the decomposition of
trans-lruns N,O; a : Irons-rruns N,O, b: cychc N,O (C2")+ N,, c: N,O
(Cxv) f N 2 .
Angrw. Chem. Int. Ed. Engl. 1995. 34, 511-520
Covalent Inorganic Azides
state leading to cyclic N,O (C2”)is 2 kcalmol-I lower in energy
than that for the direct conversion into linear N,O (CJ
and N z . (Quite recently the unimolecular decay of N,O has been
studied on the basis of qualitative increased valence bond representations[6”hl).
At this point it appears appropriate to mention the results of
pyrolysis experiments with azides in the gas phase.“31 These
studies demonstrated that experimental investigations of reactive intermediates are not only of interest for the discovery of
new compounds and the development of new synthetic methods. Moreover. it was possible to describe some aspects of the
“microscopic” course of azide pyrolyses in a satisfactory way
using calculated energy hypersurfaces, which allowed the first
more detailed insight into the deceptively simple azide decomposition reaction.
The unimolecular decomposition of nitrosyl azide to N, and
N,O also appears to be very simple on the “macroscopic” level.
Quantum chemical calculations of the energy hypersurfaces corresponding to the decomposition of N,O (Figs. I0 and I I ) ,
however. do not only confirm the experimental finding of the
formation of N2 and N,O, but also indicate that this fragmentation occurs in a much more complex manner. The formation of
cyclic N-oxides as reactive intermediates appears certain. To our
knowledge. such compounds have not yet been observed experimentally. therefore the results presented here might encourage
more intensive efforts to prove the existence of these species
experimentally by the methods of modern gas-phase analytical
Furthermore, all molecular nitrogen oxides have a positive
enthalpy of formation.[66’Consequently, N,O, and mixtures of
NO/NZO, and NO,/HNO, are already in use as environmentally safe (i.e., halogen-free) oxidizing agents in rocket engines,
mostly in combination with methylhydrazine as
are extensive discussions about the use of extremely unstable
N-oxides such as N,O and N,O, (which would be prepared in
situ) a s one-component fuels for the generation of hypersonic
wind tunnels.l”’hl Hence knowledge of the molecular decomposition of these unstable N-oxides (nitrosyl and nitryl azides) is
not only of academic interest, but also of fundamental, if rather
specific, interest to applied science.
5. Reactivity
It is not our intention to present a summary of the reactivity
of covalent azides that could claim to be complete. This reactivity is extremely multifaceted, particularly in organic synthesis,
but also in main group chemistry. Using a few selected examples
from inorganic chemistry-with an emphasis on those that
maintain the structural integrity of the azide moiety -we will
present a glimpse of the chemistry of covalent azides.
The protonation of covalent azides is well known and much
studied. The example of the protonated form of hydrazoic acid,
H,N;, has already been discussed in Section 4.133681It can be
prepared in supcracidic media at low temperatures according to
Equation (m). In contrast to many other compounds containing
a covalent azide group, H,N:SbF;
is not explosive.
It has already been discussed in Section 3 that IN, forms
stable donor-acceptor complexes with many N-bases, including N; [Eq. (n)]. This leads to a stabilization due to the higher
coordination number (2) at the central iodine
During the formal addition of iodide to give 1,N;. IN, also acts as
a Lewis acid [Eq. (0)].[36C,s2a1
Analogously, the reaction of IN,
with cyanide ions leads to the anion [N,I-C=N-I-N,]-.
species is assumed to contain a linear NICNIN unit with a
bridging cyanide group.i36c,5 2 a 1
Further anions of the type [X-I-N,]- (X = F, C1, Br, OCN,
CN) have been described.[36b1Iodine azide can also react as a
base, that is, it is amphoteric in the Lewis sense. This is best
demonstrated by the reaction of IN, with “I+” [Eq. (p)]”‘] and
by the formation of the cation I(N3); [Eq. (q)][”’ (for structures see Section 4).
In aqueous media, particularly in the presence of OH- ions.
halogen azides XN, (X = CI, Br, I) hydrolyze in accordance to
their bond polarity, that is, the halogen is positively polarized
(see Section 4) [Eq. (r)].[36alIodine azide, in particular, owes its
+ XO- + H,O
extreme reactivity in spite of the relatively high stability of the
molecule (see Section 4) to the high polarity of the N-I bond.
The iodine atom has acceptor as well as donor properties; the
a-N atom (Nl) of the N, group possesses primarily donor prope r t i e ~ . ’ ~Due
~ ‘ ] to its significantly higher polarity and the resulting increased reactivity, IN, reacts much faster than CIN, with
metal iodides and metal halogenides in general; an example is
the substitution reaction of iodides of Group 13 elements
[Eq. (s), E = B, Al,
+ IN,
+ 1,
IN, has proven useful in organic synthesis for the addition to
olefinic double bonds. It was possible to demonstrate that the
tendency to homolytic cleavage increases. as expected, in
the order IN, < BrN, < ClN, .[36c,691 Equations (t) and (u)
(R = Me, Ph) show examples of oxidative additions with iodine
azide from the areas of purely inorganic chemistry and
organometallic chemistry of the elements, r e ~ p e c t i v e l y . ‘ ~ ~ ~
2 SnCI,
+ 2 IN,
+ IN,
+ “SnCI,I,”
I. C. Tornieporth-Oetting and T. M. Klapotke
There are recent reports about the successful stepwise synthesis of di- and triphosphazenes using the Staudinger reaction and
starting with (CF,),PN, [Eqs. (v) and (w)].[~’’
+ NaN,
PPh3’ - N z ,
+ NaI
‘CF3)2PN3’- N z ,
Covalent triorgano-substituted azides of Group 14 elements
have found the greatest preparative application [see also
Eq. (f)]. Trimethylsilyl azide, Me,SiN,, for instance, has been
successfully used by Kurt Dehnicke and others as nitrogen atom
donor in the chemistry of sulfur and selenium.[711In this way the
preparation of many new Se-N compounds became possible,
for example the neutral species Se,NCI, as well as the cations
(SeCI),N+ and (SeCI,),N+. Recently, the synthesis of the novel
compound Se,N,CI, was performed, but this species has not yet
been structurally characterized [Eq. (x)] .[”I Certainly the most
3 Se,CI,
+ 2 Me,SiN,
2 Se,N,CI,
+ 2 Me,SiCl + N,
spectacular success for the use of Me,SiN, in the area of nonmetal chemistry has been the safe synthesis of polymeric sulfur
nitride (SN), [Eq. (y)], which up to now had been prepared by
using the explosive compound S4N4.[73J
3 Me,SiN,
+ (NSCI),
MeCN, - 1 5
+ 3 Me,SiCI + 9/2 N,
The reaction of mesityl azide with bis[2,4,6-tris(trifluoromethyl)phenyl]stannylene, with subsequent elimination of N, ,
permitted the preparation of azadistanniridines in good yields
[Eq. (z), R* = 2,4,6-(CF,),C,H,, Mes = mesityl].[741
2 RZSn
+ MesN,
+ N,
sive species has only been feasible in the last few years. With the
aid of high-level, ab initio calculations (taking into account
electron correlation) it was possible for the first time to obtain
theoretical insight into unusual, experimentally observable
structural characteristics of covalent azides in general and halogen azides in particular. The excellent agreement between experiment (electron diffraction, microwave studies) and theory (ab
initio) with respect to structural parameters gives credence to
those calculated structures for which there are no experimental
data due to the extreme lability of the compounds in question,
for instance the novel N-oxides ONN, and O’NN,.
Why are we so strongly interested in the preparation of
a class of compounds such as the covalent azides that need
to be handled with such extreme care and precautions? We
wish to demonstrate that even, and particularly, in the area
of the main group chemistry of very simple binary compounds there are still many unanswered questions with regard to
structure and bonding. There are also many new compounds
in this area that are waiting to be synthesized on a preparative
Now that questions about structure and bonding in covalent
XN, azides appear to be answered to a certain extent, we wish
to pay particular attention in the future to comparative discussions of gas-phase and solid-state structures and to the underlying questions of bonding. Until now the pair IN, (g)/[IN,],(s)
remains a singular result. Therefore the experimental determination of further structures of covalent azides, especially in the
solid state, is necessary. In addition, the quantum mechanical
description of the molecular structures of polymeric covalent
species in the solid state, which up to now is unsatisfactory, must
be improved. Many of the azides listed in Table 1 have been
described in the literature, but in many cases the structural characterization has been incomplete or not performed at all. For
instance, how does tBu,N-N, crystallize? We will face up to
these challenges in the future.
The azide reaction known in organic chemistry can also be
applied to the chemistry of phosphorus. For example, the reaction of cyclic alkyldithiophosphonic acid anhydrides with
Me,SiN, leads to quantitative yields of N-containing P- S heter~cycles.[’~]
An example from organometallic chemistry is the
reaction of Me,SiN, with R,AI-AIR,, which leads to the formation of novel AI-N heterocycles. Depending on the reaction
conditions, the a i d e moiety can be retained intact as a building
block for the ring structure in these compounds.[761In 1990 it
was shown that sulfur azides such as MeSO,N, can be successfully employed in the 1,3-dipolar cycloadditions of electrophilic
azides to 5-aIkylidenedihydrotetra~oles.~~~~
For some time,
stannyl azides of the type R,SnN, have found use in organic
We thank our co-workers for their cooperation, especially
Dr. Axel Schulz for performing numerous quantum chemical calculations. We are indebted to Prof. Dr. Paul von R. Schleyer,
Dr. Peter Buzek, and Prof. Dr. Wolfram Koch, Erlangen, for
intensive support which enabled us to perform independent calculations in our research group. Dr. Peter S. White is acknowledged
for the collaboration in the area of X-ray crystal structure analyses ( N A T O , CRG 920034), Prof. Dr. Istvan Hargittai and his
co-workers are acknowledged for performing the electron dqfraction experiments (partnership TU BerlinlTU Budapest) . The experimental work in Berlin was generously supported by the
Deutsche Forschungsgemeinschaft, the Fonds der Chemischen Industrie. and Bundesministerium fur Bildung und Wissenschaft
(Graduiertenkolleg). We thank both referees for many critical
and stimulating comments.
Received: March 29, 1994
Revised version: August 30. 1994 [A58IE]
German version: Angew. Chem. 1995, 107. 559
6. Summary and Outlook
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over 100 years, while the isolation on a preparative scale and the
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1771 H. Quast, D. Regnat, E.-M. Peters, K. Peters. H. G . von Schnering, Angew.
Chem. 1990. 102. 724: Angm'. Chem. Ini. Ed. Engl. 1990, 29. 695.
[7S] a) G. Ossig. A. Meller, S. Freitag. R. Herbst-Irmer, G. Sheldrick. Clwm. Ber.
1993, 126.2247; b) H. R. Kricheldorf, G. Schwarz. J. Kaschig, Angcw. Clirm.
1977, 89, 570; Angebi. Ctwrn. Inr. Ed. Engl. 1977. 16. 550.
[79] Below are the I4N NMR chemical shifts and line widths ( A Y , ,in~ Hz. in parenand NaN, for comparison. [42].
theses) of covalent azides (R-N,,-N,rNJ
- 315.9 (200)
- 269.9 (240)
- 349.2 (475)
-351.2 (350)
- 312.5 (300)
-297.6 (200)
-318.0 (1 50)
- 128.1 (30)
- 141.5(35)
- i19.2 (20)
i22.0 (30)
- 121.9(20)
- 132.0(50)
- 135.4 (30)
- 277.2 (65)
-205.1 (35)
- 156.6 (65)
-162.1 (125)
[a] AsF; salt.
Ingham, J. et al.
Chemical Engineering Dynamics
Modelling with PC Simulation
1994. XX, 701 pages with experimentation with thepro- The treatment employed in
vision of 85 accompanying this book is well tried and
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Angrn. Chrm. Int. Ed. Engl. 1995, 34, 511-520
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