The Molybdenum(V) and Tungsten(VI) Oxoazides [MoO(N3)3], [MoO(N3)3·2 CH3CN], [(bipy)MoO(N3)3], [MoO(N3)5]2-, [WO(N3)4], and [WO(N3)4·CH3CN]

Article abstract of DOI:10.1002/anie.201505418

A series of novel molybdenum(V) and tungsten(VI) oxoazides was prepared starting from [MOF₄] (M=Mo, W) and Me₃SiN₃. While [WO(N₃)₄] was formed through fluoride-azide exchange in the reaction of Me₃SiN₃ with WOF₄ in SO₂ solution, the reaction with MoOF₄ resulted in a reduction of Mo^VI to Mo^V and formation of [MoO(N₃)₃]. Carried out in acetonitrile solution, these reactions resulted in the isolation of the corresponding adducts [MoO(N₃)₃·2 CH₃CN] and [WO(N₃)₄·CH₃CN]. Subsequent reactions of [MoO(N₃)₃] with 2,2′-bipyridine and [PPh₄][N₃] resulted in the formation and isolation of [(bipy)MoO(N₃)₃] and [PPh₄]₂[MoO(N₃)₅], respectively. Most molybdenum(V) and tungsten(VI) oxoazides were fully characterized by their vibrational spectra, impact, friction and thermal sensitivity data and, in the case of [WO(N₃)₄·CH₃CN], [(bipy)MoO(N₃)₃], and [PPh₄]₂[MoO(N₃)₅], by their X-ray crystal structures.

Full text of DOI:10.1002/anie.201505418

.
Angewandte
Communications
International Edition: DOI: 10.1002/anie.201505418
German Edition: DOI: 10.1002/ange.201505418
Azide Complexes
The Molybdenum(V) and Tungsten(VI) Oxoazides [MoO(N3)3],
[MoO(N3)3·2CH3CN], [(bipy)MoO(N3)3], [MoO(N3)5]2À, [WO(N3)4],
and [WO(N3)4·CH3CN]
Ralf Haiges,* Juri Skotnitzki, Zongtang Fang, David A. Dixon, and Karl O. Christe
In memory of Howard S. Taylor
Abstract: A series of novel molybdenum(V) and tungsten(VI)
oxoazides was prepared starting from [MOF4] (M = Mo, W)
and Me3SiN3. While [WO(N3)4] was formed through fluoride–
azide exchange in the reaction of Me3SiN3 with WOF4 in SO2
solution, the reaction with MoOF4 resulted in a reduction of
MoVI to MoV and formation of [MoO(N3)3]. Carried out in
acetonitrile solution, these reactions resulted in the isolation of
the corresponding adducts [MoO(N3)3·2CH3CN] and [WO-
(N3)4·CH3CN]. Subsequent reactions of [MoO(N3)3] with 2,2’-
bipyridine and [PPh4][N3] resulted in the formation and
isolation of [(bipy)MoO(N3)3] and [PPh4]2[MoO(N3)5],
respectively. Most molybdenum(V) and tungsten(VI) oxoa-
zides were fully characterized by their vibrational spectra,
impact, friction and thermal sensitivity data and, in the case of
[WO(N3)4·CH3CN], [(bipy)MoO(N3)3], and [PPh4]2[MoO-
(N3)5], by their X-ray crystal structures.
as very treacherous, extremely shock- and temperature-
sensitive compounds that explode violently when warmed
towards room temperature.[5] The only structurally charac-
terized polyazides of molybdenum and tungsten are [W(N3)6]
and [NMo(N3)4]À.[5,6] To the best of our knowledge, no
molybdenum(V), molybdenum(VI), or tungsten(VI) oxopo-
lyazides have been structurally characterized so far.
In analogy with our previously reported syntheses of
[Mo(N3)6], [W(N3)6], [MoO2(N3)2], and [WO2(N3)2],[5] the
reaction of molybdenum or tungsten oxotetrafluoride with an
excess of trimethylsilyl azide in acetonitrile solution at À208C
resulted in a complete fluoride–azide exchange and the
formation of dark maroon to black (M = Mo) or yellow-
orange (M = W) solutions. When the volatile compounds
(CH3CN, Me3SiF, and excess Me3SiN3) were pumped off
from the WOF4 reaction mixture, first at À208C and then
at ambient temperature, the acetonitrile adduct [WO-
(N3)4·CH3CN] was isolated as an orange solid [Eq. (1)].
P
olyazides are highly energetic compounds that have
attracted considerable interest as potential high-energy-
density materials (HEDM).[1] Because of their highly endo-
thermic nature, polyazido compounds are usually explosive
and very shock sensitive, rendering the synthesis of molecules
with a large number of azido groups difficult. The chemistry
of high-oxidation-state metal polyazides is more challenging
than the one of lower-oxidation-state metal polyazides, owing
to the fact that the increased oxidation potential of the central
metal atom results in an increased sensitivity and explosive-
ness.[2] The stabilization of neutral polyazides by either anion
or adduct formation has been well-established in recent
years.[3] Another less common approach for the stabilization
of high-oxidation state metal azides is the introduction of
oxygen atoms to reduce the oxidation potential of the central
metal atom.[4] The binary polyazides Mo(N3)6, W(N3)6,
[PPh4][Mo(N3)7], and [PPh4][W(N3)7] have been described
CH3CN
½WOF4Š þ 4 Me3SiN3
½WOðN Þ Á CH CNŠ
ð1Þ
ƒƒƒƒƒ!
3
4
3
À4 Me3SiF
Single crystals of [WO(N3)4·CH3CN] were grown from
acetonitrile solution by slow evaporation of the solvent in
vacuo. The compound was characterized by its X-ray crystal
structure, vibrational and NMR spectra, as well as the
observed reaction stoichiometry (see the Supporting Infor-
mation). In the case of the [MoOF4] reaction, MoVI was
reduced to MoV by Me3SiN3 under evolution of N2 and very
dark maroon-colored [Mo(N3)3·2CH3CN] was isolated when
the volatile compounds were pumped off [Eq. (2)].
CH3CN
2 ½MoOF4Š þ 8 Me3SiN3
2 ½MoOðN Þ Á 2 CH CNŠ
ð2Þ
ƒƒƒƒƒƒƒƒ!
3
3
3
À8 Me3SiF, À3 N2
Attempts of growing single crystals of the acetonitrile
adduct [MoO(N3)3·2CH3CN] that were suitable for X-ray
crystal structure determination were unsuccessful. The com-
pound was identified and characterized by the observed
reaction stoichiometry, its vibrational spectra (Supporting
Information), and its conversion into the adduct [(bipy)MO-
(N3)3] and the anion [MO(N3)5]2À. The reaction of the metal
oxotetrafluorides with trimethylsilyl azide in SO2 solution
at À208C resulted in the formation of the corresponding
unsolvated oxoazides [MoO(N3)3] and [WO(N3)4], respec-
tively. Both compounds are stable at ambient temperature but
are treacherous and explode violently upon slight provocation
(friction and impact). The composition of the compounds was
[*] Prof. Dr. R. Haiges, J. Skotnitzki, Prof. Dr. K. O. Christe
Loker Hydrocarbon Research Institute and Department of Chemistry,
University of Southern California
Los Angeles, CA 90089-1661 (USA)
E-mail: haiges@usc.edu
Z. Fang, Prof. Dr. D. A. Dixon
Department of Chemistry, The University of Alabama
Tuscaloosa, AL 35487 (USA)
Supporting information and ORCID(s) from the author(s) for this
article are available on the WWW under http://dx.doi.org/10.1002/
anie.201505418.
15550
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 15550 –15555

Angewandte
Chemie
established by their vibrational spectra and the observed
reaction stoichiometry (Supporting Information). The adduct
[(bipy)MoO(N3)3] was formed quantitatively when [MoO-
(N3)3] was reacted with 2,2’-bipyridine (bipy) in acetonitrile
solution [Eq. (3)]. It was isolated as a brown crystalline solid
and characterized by its vibrational spectra and X-ray crystal
structure.
3CN
½MoOðN Þ Š þ bipy CH
½ðbipyÞMoOðN Þ Š
ð3Þ
ƒƒƒ!
3
3
3 3
When a solution of [(bipy)MoO(N3)3] in acetonitrile was
kept in an evacuated thin-walled (0.5 mm wall thickness) FEP
reactor for a period of several days, a dark heterogeneous
mixture with some light-brown crystals was obtained once the
solvent was removed. An X-ray crystal structure determina-
tion identified the crystalline material as [((bipy)MoON3)2O2]
by X-ray crystal structure determination (Supporting Infor-
mation). This doubly O-bridged molybdenum oxoazide was
presumably formed through the hydrolysis of [(bipy)MoO-
(N3)3] with moisture that had diffused into the reactor. No
further attempts were made to characterize this compound or
to identify other hydrolysis products. The metal oxoazides
[MoO(N3)3] and [WO(N3)4] were quantitatively converted
into salts containing the pentaazido anions [MoO(N3)5]2À and
[WO(N3)5]À, respectively, through reactions with [PPh4][N3]
in acetonitrile solution [Eq. (4) and (5)].
Figure 1. Crystal structure of A) [(bipy)MoO(N3)3], B) [WO-
(N3)4·CH3CN], and C) the anion in [PPh4]2[MoO(N3)5]. Ellipsoids are
set at 50% probability; hydrogen atoms have been omitted for clarity.
[MoOF4] (1.64(1) Š),[9] which contains F-bridges. The aver-
À
age N N distances in the azido ligands of 1.143(3) Š for the
terminal azide bonds and 1.212(2) Š for the internal bonds
are typical for covalent azides. The tungsten oxoazide [WO-
CH3CN
½MoOðN Þ Š þ 2 ½PPh Š½N Š
½PPh Š ½MoOðN Þ Š
ð4Þ
ð5Þ
ƒƒƒ!
¯
3
3
4
3
4
2
3 5
(N3)4·CH3CN] crystallizes in the triclinic space group P1 with
two molecules per asymmetric unit (Z = 4, Z’ = 2). The crystal
structure of this metal oxoazide contains isolated molecules
and the closest Nazide···Nazide distances (2.978(9) Š) are again
comparable to the sum of van der Waals radii. The closest
intermolecular W···Nazide and W···O distances are 3.997(4) Š
and 4.267(4) Š, respectively. In both nonequivalent molecules
of the unit cell, the coordination geometry around the central
tungsten atom is derived from a distorted pseudo-octahedron
with four azido groups in the equatorial positions (Fig-
ure 1B). The oxygen atom and the acetonitrile molecule
occupy the axial positions. All four azido ligands are oriented
away from the oxygen atom and pointing towards the
acetonitrile ligand, resulting in a molecular structure resem-
CH3 CN
½WOðN Þ Š þ ½PPh Š½N Š
½PPh Š½WOðN Þ Š
ƒƒƒ!
3
4
4
3
4
3 5
Both oxopentaazidometalate salts were isolated as
orange-brown (Mo) or bright orange solids that were
identified and characterized by the observed reaction stoi-
chiometry and their vibrational and NMR spectra. The salt
[PPh4]2[Mo(N3)5] was also characterized by its X-ray crystal
structure. Attempts to obtain single crystals of [PPh4][WO-
(N3)5] were unsuccessful.
Details of the crystallographic data collection and refine-
ment parameters for the structurally characterized com-
pounds [(bipy)MoO(N3)3], [PPh4]2[MoO(N3)5], [((bipy)-
MoON3)2O2], and [WO(N3)4·CH3CN] are given in the Sup-
porting Information. The bipyridine adduct [(bipy)MoO-
(N3)3] crystallizes in the monoclinic space group P21/n with
four symmetry-related molecules per unit cell. While the
solid-state structure consists of isolated and separated mol-
ecules, the closest intermolecular Nazide···Nazide distances are
just 2.996(2) Š, which is about the sum of van der Waals radii
(3.0 Š).[7] The closest intermolecular Mo···Nazide and Mo···O
distances are 3.839(2) Š and 5.154(2) Š, respectively. The
structure of the [(bipy)MoO(N3)3] molecule is derived from
a pseudo octahedral geometry with the bipyridine ligand, the
oxygen atom and one azido group in the equatorial positions
and two azido groups occupying the axial positions (Fig-
À
bling an umbrella. The W Nazide distances range from 1.956-
(4) Š to 2.019(4) Š with an average value of 1.989(4) Š that is
shorter than the one observed for ([bipy)WO2(N3)2] (2.049-
(3) Š)[8] but slightly longer than the one in [W(N3)6] (1.978-
(2) Š).[5] The average W O distance of 1.697(4) Š is 0.032 Š
À
shorter than the one found in [(bipy)WO2(N3)2][8] and much
shorter than in tetrameric and O-bridged [WOF4] (2.11-
(4) Š).[10]
The salt [PPh4]2[MoO(N3)5] crystallizes in the triclinic
¯
space group P1 (Z = 2). The solid-state structure consists of
isolated and well-separated [PPh4]+ cations and [MoO(N3)5]2À
anions. The closest intermolecular N···N and Mo···N distances
are 4.471(2) Š and 6.480(2) Š, respectively. The closest
intermolecular Mo···O distance is 9.985(1) Š. The structure
of the [MoO(N3)5]2À anion is again derived from a distorted
octahedron with the oxygen and one azido group occupying
the axial positions and the remaining four N3 ligands in the
equatorial positions (Figure 1C). The average Mo-Nazide
À
ure 1A). The average Mo Nazide distance of 2.051(2) Š is
about 0.01 Š shorter than that in the related molybdenum-
(VI) dioxodiazide [(bipy)MoO2(N3)2].[8] The observed Mo O
À
distance of 1.691(1) Š in [(bipy)MoO(N3)3] is 0.025 Š shorter
than in [(bipy)MoO2(N3)2][8] but longer than in polymeric
Angew. Chem. Int. Ed. 2015, 54, 15550 –15555
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 15551

.
Angewandte
Communications
distance of 2.112(2) Š is significantly longer than that of
À
[bipy(MoO(N3)3] (2.051(2) Š). The Mo O distance of 1.684-
(1) Š is somewhat shorter than the one in [(bipy)MoO(N3)3].
Geometries were optimized at the B3LYP//DZVP2/cc-
pVDZ-PP and SVWN5//DZVP2/cc-pVDZ-PP levels[11] of
density functional theory for the metal oxoazide species
[MO(N3)3], [MO(N3)4]À, [MO(N3)5]2À, [MO(N3)4], and [MO-
(N3)5]À, as well as the adducts [MO(N3)3·CH3CN], [MO-
(N3)3·2CH3CN], [MO(N3)4·CH3CN], and [(bipy)MO(N3)3]
(M = Mo, W). The obtained geometries and calculated vibra-
tional frequencies and intensities are given in the Supporting
Information. The local DFT functional was included as it
often gives better geometries for transition metal compounds
than do hybrid functionals such as B3LYP. Relative energies
were calculated at the B3LYP/aug-cc-pVDZ(-PP), MP2/
ROMP2/aug-cc-pVDZ(-PP),[12] and R/UCCSD(T)/aug-cc-
pVDZ(-PP)[13] levels, the latter two at the SVWN5 geo-
metries.[14] The results are shown in Figure 2 for [MO(N3)4]
and in Figure 3 for [MO(N3)3].
Figure 3. Optimized structures of the metal oxoazide species derived
from [MO(N3)3] (M=Mo, W) and reaction energies [kcalmolÀ1] at the
B3LYP, ROMP, and CCSD(T) levels.
due to the lack of counterions in the gas-phase calculations.
À
The averaged Mo N bond distance of 2.068 Š is still shorter
than experiment. In case of the doublet states of oxidation
state + V, the spin on the metal is somewhat delocalized onto
the azide nitrogen atoms (Supporting Information). The
electron affinities of [MO(N3)4] were calculated at all three
levels, as the ROMP2/MP2 and B3LYP levels did not agree
well. The best results are at the CCSD(T) level and show that
[MoO(N3)4] is much easier to reduce than is [WO(N3)4] by
0.67 eV, and the electron affinity (EA) of 4.13 eV for MoO-
(N3)4 is comparable to that for [MoF6].[15] The B3LYP EAs are
higher than the CCSD(T) values and the ROMP2/MP2 values
À
are lower than the CCSD(T) EAs. The binding energy of N3
to [WO(N3)4] of 70 kcalmolÀ1 is 6 kcalmolÀ1 higher than the
corresponding value for [MoO(N3)4], showing that the former
is a better Lewis acid. The N3 affinity for WO(N3)4 is
À
comparable to the FÀ affinity of [WF6].[15] The addition of
CH3CN to [Mo(N3)4] results in a pseudo-octahedral [Mo-
(N3)4·CH3CN]. The conformation shown in Figure 2 with
CH3CN occupying an axial position is prefered over the
conformation with CH3CN and three N3 groups in equatorial
positions by 8 kcalmolÀ1 for W and 7 kcalmolÀ1 for Mo.
The CH3CN binding energy of 28 kcalmolÀ1 for [WO-
(N3)4] is 3 kcalmolÀ1 larger than for [MoO(N3)4]. This pattern
of the W compounds being better Lewis acids than the Mo
compounds that are being more easily reduced is well-
established in transition-metal oxide clusters and plays an
important role in their catalytic acitivity.[16] The addition of
Figure 2. Optimized structures of the metal oxoazide species derived
from closed shell [MO(N3)4] (M=Mo, W) and relative energies
[kcalmolÀ1] at the CCSD(T) level.
À
The calculated Mo O bond distance in [(bipy)MoO(N3)3]
with the SVWN5 functional is 1.670 Š, 0.021 Š shorter than
À
the experimental value. The averaged Mo N bond distance is
2.000 Š, which is also shorter, by 0.051 Š compared to the
experiment. For the external and internal azide bonds, the
À
N3 to [WO(N3)3] is 3 kcalmolÀ1 more exoergic than to
À
averaged N N bond distances are predicted to be 1.166 Š and
1.220 Š, respectively, slightly longer than experiment. For
[MoO(N3)3] and the value is larger than for the addition of
N3 to [WO(N3)4]. This is consistent with the larger steric
À
[WO(N3)4·CH3CN], the W O bond and the averaged W N
bond distances are calculated to be 1.685 Š and 1.986 Š,
which are also slightly shorter than the experimental values.
constraints for the latter. The addition of the second N3À ion is
endothermic in the gas phase. To model this process, we
performed single-point calculations using a self-consistent
reaction field (SCRF) approach with CH3CN as the solvent
(e = 35.7).[17] The SCRF calculations were done with the
COSMO approach and parameters at the ROMP2/aug-cc-
À
À
À
In contrast, the predicted Mo O bond distance in [MoO-
(N3)5]2À with the SVWN5 functional is 1.710 Š, slightly longer
than in the crystal by 0.026 Š. This difference could easily be
15552 www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 15550 –15555

Angewandte
Chemie
pVDZ level.[18] The solvent-included calculations correspond
to the free energy at 298 K obtained by adding the gas-phase
and solvent-free energies. The free energies in CH3CN for the
addition of the first N3À ion are about half the exothermicity
of the gas phase values but are still substantially exothermic.
with 2,2’-bipyridine or salt formation with [PPh4][N3]
increases the stability of the metal oxoazides, but only the
salt [PPh4]2[MoO(N3)5] can be considered insensitive. All of
the other investigated metal oxoazides must be treated as
explosion hazards.
À
The addition of the second N3 ion in CH3CN to form the
The thermal stabilities of the oxoazides were determined
through differential thermal analysis (DTA) scans with
heating rates of 58CminÀ1. The resulting decomposition
onset temperatures are included in Table 1. Only the bipyr-
idine adduct [(bipy)MoO(N3)3] as well as the salts [PPh4]2-
[MoO(N3)5] and [PPh4][WO(N3)5] showed smooth decompo-
sitions. The compounds [MoO(N3)3], [MoO(N3)3·2CH3CN],
[WO(N3)4], and [WO(N3)4·CH3CN] exploded violently upon
heating at a rate of 58CminÀ1. It is interesting to note that the
molybdenum(V) azides are thermally less stable than the
tungsten(IV) azides and that the solvate [MoO-
(N3)3·2CH3CN] has a higher decomposition temperature
than the unsolvated [MoO(N3)3] while [WO(N3)4·CH3CN]
and [WO(N3)4] decompose at similar temperatures. As can be
expected, the salts [PPh4]2[MoO(N3)5] and [PPh4][WO(N3)5]
show the highest decomposition temperatures of 1808C and
1908C, respectively, among the azido compounds of this study.
In conclusion, a series of novel molybdenum(V) and
tungsten(VI) oxoazides has been prepared and characterized.
The reaction of [WOF4] with Me3SiN3 results in a complete
fluoride–azide exchange and formation of [WO(N3)4]. The
reaction of molybdenum oxotetrafluoride, [MoOF4], with
Me3SiN3 results in the reduction of MoVI to MoV under N2
evolution and formation of the novel molybdenum(V) azide
[MoO(N3)3]. The solvent-free metal oxoazides could be
isolated as explosive and highly sensitive solids when SO2
was used as solvent. The solvated adducts [MoO-
(N3)3·2CH3CN] and [WO(N3)4·CH3CN] were obtained when
the reactions of the metal oxofluorides with Me3SiN3 were
carried-out in acetonitrile solution. The reactions of [MoO-
(N3)3] with 2,2-bipyridine and [PPh4][N3] resulted in the
formation of [(bipy)MoO(N3)3] and [PPh4]2[MO(N3)5],
respectively. The hydrolysis of [(bipy)MoO(N3)3] resulted in
the formation and isolation of [((bipy)MoON3)2O2]. The
molybdenum and tungsten oxoazides were characterized by
their vibrational spectra, impact, friction and thermal sensi-
tivity data, and, in the cases of [WO(N3)4·CH3CN],
[(bipy)MoO(N3)3], [PPh4]2[MoO(N3)5], and [((bipy)-
MoON3)2O2], by their X-ray crystal structures. Most com-
pounds of this work, the CH3CN and N3À addition reactions of
[MO(N3)3] and [MO(N3)4], as well as the reduction reactions
of [MO(N3)4] (M = Mo, W) were studied by computational
methods.
dianion is also exothermic but the exothermicity is substan-
tially smaller. Note that there are no counterions present in
our SCRF model, which could further stabilize the formation
of the dianion. The exothermicity for the addition of CH3CN
is essentially the same for [MO(N3)3] and [MO(N3)4]. The
addition of the second CH3CN is less exothermic than that of
the first CH3CN.
The observed and calculated vibrational data of the
investigated molybdenum and tungsten oxoazides are listed in
the Supporting Information. The vibrational assignments are
supported by the DFT calculations. The IR spectra of the
compounds are dominated by bands that are due to the
nas(N3) vibration modes at about 2000–2200 cmÀ1. The n-
(MoO) mode is observed at about 940–960 cmÀ1 for the
molybdenum compounds and at about 960–980 cmÀ1 for the
tungsten compounds. Furthermore, the strong bands of the
nas(N3) modes in the region 2000–2200 cmÀ1 are the dominat-
ing features in the Raman spectra. The much weaker bands of
the ns(N3) modes are observed at about 1200–1350 cmÀ1. They
are characteristic for the presence of covalently bound azido
À
groups. The M Nazide stretching modes are observed at about
420–470 cmÀ1. The presence of covalent azides was also
confirmed by the 14N NMR spectra. Solutions of all com-
pounds in SO2, CD3CN, or CDCl3 exhibited resonances with
chemical shifts of about À280 ppm, À140 ppm, and À200 ppm
for Na, Nb, and Ng, respectively, characteristic for covalent
azido compounds.
The impact (IS) and friction sensitivities (FS) of the
molybdenum(V) and tungsten(VI) oxoazides were deter-
mined using a BAM (Bundesanstalt für Materialforschung
und -prüfung) impact and a BAM friction tester. The obtained
sensitivity and stability data are summarized in Table 1. As
can be expected, the metal polyazides [MoO(N3)3] and
[WO(N3)4] are very sensitive to impact and friction. The
acetonitrile adducts [MoO(N3)3·2CH3CN] and [WO-
(N3)4·CH3CN] show about the same sensitivities as the
corresponding unsolvated compounds. Adduct formation
Table 1: Sensitivity data[a] for the metal oxoazides.
Compound
RDX[b]
Pb(N3)2
[MoO(N3)3]
[MoO(N3)3·2CH3CN]
[(bipy)MoO(N3)3]
[PPh4]2[MoO(N3)5]
[WO(N3)4]
[WO(N3)4·CH3CN]
[PPh4][WO(N3)5]
Tdecomp [8C]
FS [N]
IS [J]
7.5
2.5
<1
<1
2
>100
<1
<1
12
220
300
120
0.1
<5
<5
240
>360
<5
<5
[19]
104[c]
145[c,d]
145
Experimental Section
Caution! Polyazides are extremely shock-sensitive and can explode
violently upon the slightest provocation. Because of the high energy
content and the high detonation velocity of these azides, their
explosions are particularly violent and can cause, even on a one mmol
scale, significant damage. The use of appropriate safety precautions
(safety shields, face shields, leather gloves, protective clothing, such as
heavy leather welding suits and ear plugs) is mandatory. Ignoring
safety precautions can lead to serious injuries!
180[e]
171[c]
170[c]
190
>360
[a] FS=friction sensitivity, IS=impact sensitivity. [b] 1,3,5-Trinitroper-
hydro-1,3,5-triazine, hexogen. [c] Explosion. [d] Broad endotherm at 85–
1208C. [e] Endotherm at 1558C (melting).
Angew. Chem. Int. Ed. 2015, 54, 15550 –15555
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 15553

.
Angewandte
Communications
Materials and apparatus: All reactions were carried out in Teflon-
FEP ampules that were closed by stainless steel valves. Volatile
materials were handled in a Pyrex glass vacuum line. Non-volatile
materials were handled in the dry nitrogen atmosphere of a glove box.
The starting materials [MoOF4] and [WOF4] were prepared from the
corresponding hexafluorides and HF/SiO2. PPh4N3 was prepared
according to a previously reported procedure,[20] and 2,2’-bipyridine
(bipy) (Aldrich) was used without further purification. Trimethylsilyl
azide (Aldrich) was purified by fractional condensation. Solvents
were dried using standard methods and freshly distilled prior to use.
Crystal-structure determinations: The single-crystal X-ray dif-
fraction data were collected on Bruker SMART or Bruker SMART
APEX DUO diffractometers using MoKa radiation The structures
were solved by intrinsic phasing and refined on F2 using the Bruker
SHELXTL Software Package and ShelXle.[21] ORTEP drawings were
prepared using the ORTEP-3 for Windows V2.02 program.[22] Further
crystallographic details can be obtained from the Cambridge Crys-
tallographic Data Centre (CCDC, 12 Union Road, Cambridge
CB21EZ, UK (Fax: (+ 44) 1223–336–033; e-mail: deposit@ccdc.ca-
m.ac.uk) on quoting the deposition no. CCDC 1405935–1405937.
Computational methods: The geometries were optimized at the
density functional theory (DFT)[23] level with the LSDA (local spin
density approximation) SVWN5[24] and hybrid B3LYP[25] exchange-
correlation functionals with the DFT-optimized DZVP2 basis set[26]
for H, C, N, and O atoms and the cc-pVDZ-PP[27] basis set for Mo and
W using the Gaussian09 program system.[28] Vibrational frequencies
were calculated to show that the structures were minima. The
ROMP2 and CCSD(T) calculations were performed with the
MOLPRO program system.[29]
[2] a) R. Haiges, J. Boatz, A. Vij, V. Vij, M. Gerken, S. Schneider, T.
Schroer, M. Yousufuddin, K. Christe, Angew. Chem. Int. Ed.
2004, 43, 6676 – 6680; Angew. Chem. 2004, 116, 6844 – 6848; b) R.
Haiges, A. Vij, J. Boatz, S. Schneider, T. Schroer, M. Gerken, K.
Christe, Chem. Eur. J. 2004, 10, 508 – 517.
[3] R. Haiges, P. Deokar, K. O. Christe, Angew. Chem. Int. Ed. 2014,
53, 5431 – 5434; Angew. Chem. 2014, 126, 5535 – 5538.
[4] R. Haiges, M. Vasiliu, D. A. Dixon, K. O. Christe, Angew. Chem.
Int. Ed. 2015, 54, 9101 – 9105; Angew. Chem. 2015, 127, 9229 –
9233.
[5] R. Haiges, J. A. Boatz, R. Bau, S. Schneider, T. Schroer, M.
Yousufuddin, K. O. Christe, Angew. Chem. Int. Ed. 2005, 44,
1860 – 1865; Angew. Chem. 2005, 117, 1894 – 1899.
[6] K. Dehnicke, J. Schmitte, D. Fenske, Z. Naturforsch. B 1980, 35,
1070 – 1074.
[7] L. Pauling, The Nature of the Chemical Bond and the Structure of
Molecules and Crystals; An Introduction to Modern Structural
Chemistry, 3rd ed., Cornell University Press, Ithaca, N.Y. , 1960.
[8] R. Haiges, J. Skotnitzki, Z. Fang, D. A. Dixon, K. O. Christe,
Angew. Chem. Int. Ed. 2015, 54, 9581 – 9585; Angew. Chem.
2015, 127, 9717 – 9721.
[9] A. J. Edwards, B. R. Stevento, J. Chem. Soc. A 1968, 2503 – 2510.
[10] A. J. Edwards, G. R. Jones, J. Chem. Soc. A 1968, 2074 – 2078.
[11] a) D. Figgen, K. A. Peterson, M. Dolg, H. Stoll, J. Chem. Phys.
2009, 130, 164108 – 164112; b) R. A. Kendall, T. H. Dunning,
R. J. Harrison, J. Chem. Phys. 1992, 96, 6796 – 6806; c) K. A.
Peterson, D. Figgen, M. Dolg, H. Stoll, J. Chem. Phys. 2007, 126,
124101 – 124112.
[12] P. J. Knowles, J. S. Andrews, R. D. Amos, N. C. Handy, J. A.
Pople, Chem. Phys. Lett. 1991, 186, 130 – 136.
Further experimental details are given in the Supporting Infor-
mation.
[13] a) R. J. Bartlett, M. Musial, Rev. Mod. Phys. 2007, 79, 291 – 352;
b) M. J. O. Deegan, P. J. Knowles, Chem. Phys. Lett. 1994, 227,
321 – 326; c) P. J. Knowles, C. Hampel, H. J. Werner, J. Chem.
Phys. 1993, 99, 5219 – 5227; d) P. J. Knowles, C. Hampel, H. J.
Werner, J. Chem. Phys. 2000, 112, 3106 – 3107; e) G. D. Purvis,
R. J. Bartlett, J. Chem. Phys. 1982, 76, 1910 – 1918; f) K.
Raghavachari, G. W. Trucks, J. A. Pople, M. Headgordon,
Chem. Phys. Lett. 1989, 157, 479 – 483; g) J. D. Watts, J. Gauss,
R. J. Bartlett, J. Chem. Phys. 1993, 98, 8718 – 8733.
[14] a) C. Møller, M. S. Plesset, Phys. Rev. 1934, 46, 618 – 622; b) J. A.
Pople, J. S. Binkley, R. Seeger, Int. J. Quantum Chem. 1976, 1 –
19.
[15] a) R. Craciun, R. T. Long, D. A. Dixon, K. O. Christe, J. Phys.
Chem. A 2010, 114, 7571 – 7582; b) R. Craciun, D. Picone, R. T.
Long, S. G. Li, D. A. Dixon, K. A. Peterson, K. O. Christe, Inorg.
Chem. 2010, 49, 1056 – 1070.
[16] a) Z. T. Fang, Z. J. Li, M. S. Kelley, B. D. Kay, S. G. Li, J. M.
Hennigan, R. Rousseau, Z. Dohnalek, D. A. Dixon, J. Phys.
Chem. C 2014, 118, 22620 – 22634; b) R. Rousseau, D. A. Dixon,
B. D. Kay, Z. Dohnalek, Chem. Soc. Rev. 2014, 43, 7664 – 7680.
[17] J. Tomasi, B. Mennucci, R. Cammi, Chem. Rev. 2005, 105, 2999 –
3093.
[18] A. Klamt, G. J. Schürmann, Chem. Soc. Perkin Trans. 2 1993,
799 – 805.
[19] R. Meyer, J. Kçhler, A. Homburg, Explosives, 6th ed., Wiley-
VCH, Weinheim, 2007.
Acknowledgements
The Office of Naval Research (ONR) and the Defense Threat
Reduction Agency (DTRA) funded this work. The National
Science Foundation supported the X-ray diffractometer (NSF
CRIF 1048807). We thank the Hydrocarbon Research
Foundation for financial support and Prof. G. K. S. Prakash,
Dr. W. W. Wilson, Dr. R. I. Wagner, Dr. G. B.-Chabot, as well
as P. Deokar, A. Baxter, and T. Saal for their help and
stimulating discussions. The calculations were supported by
the Chemical Sciences, Geosciences and Biosciences Divi-
sion, Office of Basic Energy Sciences, U.S. Department of
Energy (DOE) (catalysis center program). DAD thanks the
Robert Ramsay Chair Fund of The University of Alabama for
support.
Keywords: crystal structures · molybdenum · oxoazides ·
polyazides · tungsten
How to cite: Angew. Chem. Int. Ed. 2015, 54, 15550–15555
Angew. Chem. 2015, 127, 15771–15776
[20] K. O. Christe, R. Haiges, J. A. Boatz, H. D. Brooke Jenkins, E. B.
Garner, D. A. Dixon, Inorg. Chem. 2011, 50, 3752 – 3756.
[21] a) C. B. Hübschle, G. M. Sheldrick, B. Dittrich, J. Appl. Crys-
tallogr. 2011, 44, 1281 – 1284; b) G. M. Sheldrick, Acta Crystal-
logr. Sect. A 2008, 64, 112 – 122; c) G. M. Sheldrick, Acta
Crystallogr. Sect. C 2015, 71, 3 – 8; d) G. M. Sheldrick, Acta
Crystallogr. Sect. A 2015, 71, 3 – 8; e) G. M. Sheldrick, SHELXL
2014/6; f) SHELXTL, 2014/7, Bruker AXS Madison, WI, 2014.
[22] L. Farrugia, J. Appl. Crystallogr. 1997, 30, 565 – 565.
[1] a) R. Haiges, R. J. Buszek, J. A. Boatz, K. O. Christe, Angew.
Chem. Int. Ed. 2014, 53, 8200 – 8205; Angew. Chem. 2014, 126,
8339 – 8344; b) W. P. Fehlhammer, W. Beck, Z. Anorg. Allg.
Chem. 2013, 639, 1053 – 1082; c) W. K. Seok, T. M. Klapçtke,
Bull. Korean Chem. Soc. 2010, 31, 781 – 788; d) T. M. Klapçtke,
Chem. Ber. Recl. 1997, 130, 443 – 451; e) I. C. Tornieporth-
Oetting, T. M. Klapçtke, Angew. Chem. Int. Ed. Engl. 1995, 34,
511 – 520; Angew. Chem. 1995, 107, 559 – 568.
15554 www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 15550 –15555

Angewandte
Chemie
[23] R. G. Parr, W. Yang, Density-functional theory of atoms and
molecules, Oxford University Press, Clarendon Press, New York,
Oxford, England, 1989.
[24] a) J. C. Slater, Quantum Theory of Molecules and Solids: The
self-consistent field for molecules and solids, McGraw-Hill, New
York, 1963; b) S. H. Vosko, L. Wilk, M. Nusair, Can. J. Phys.
1980, 58, 1200 – 1211.
Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo,
R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R.
Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma,
V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S.
Dapprich, A. D. Daniels, Ö. Farkas, J. B. Foresman, J. V. Ortiz, J.
Cioslowski, D. J. Fox, Gaussian, Inc., Wallingford, CT, USA,
2009.
[25] a) A. D. Becke, J. Chem. Phys. 1993, 98, 5648 – 5652; b) C. T.
Lee, W. T. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785 – 789.
[26] N. Godbout, D. R. Salahub, J. Andzelm, E. Wimmer, Can. J.
Chem. 1992, 70, 560 – 571.
[27] a) N. B. Balabanov, K. A. Peterson, J. Chem. Phys. 2005, 123,
064107 – 064115; b) N. B. Balabanov, K. A. Peterson, J. Chem.
Phys. 2006, 125, 074110 – 074116.
[28] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci,
G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P.
Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg,
M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M.
Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven,
J. A. Montgomery Jr., J. E. Peralta, F. Ogliaro, M. J. Bearpark, J.
Heyd, E. N. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobaya-
shi, J. Normand, K. Raghavachari, A. P. Rendell, J. C. Burant,
S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, N. J. Millam, M.
[29] a) MOLPRO 2012.1 is a package of ab initio programs written
by H.-J. Werner, P. J. Knowles, G. Knizia, F. R. Manby, M.
Schütz, P. Celani, T. Korona, R. Lindh, A. Mitrushenkov, G.
Rauhut, K. R. Shamasundar, T. B. Adler, R. D. Amos, A.
Bernhardsson, A. Berning, D. L. Cooper, M. J. O. Deegan,
A. J. Dobbyn, F. Eckert, E. Goll, C. Hampel, A. Hesselmann,
G. Hetzer, T. Hrenar, G. Jansen, C. Kçppl, Y. Liu, A. W. Lloyd,
R. A. Mata, A. J. May, S. J. McNicholas, W. Meyer, M. E. Mura,
A. Nicklaß, D. P. O’Neill, P. Palmieri, D. Peng, K. Pflüger, R.
Pitzer, M. Reiher, T. Shiozaki, H. Stoll, A. J. Stone, R. Tarroni, T.
Thorsteinsson, M. Wang. See http://www.molpro.net; b) H. J.
Werner, P. J. Knowles, G. Knizia, F. R. Manby, M. Schütz,
WIREs Comput. Mol. Sci. 2012, 2, 242 – 253.
Received: June 12, 2015
Published online: November 2, 2015
Angew. Chem. Int. Ed. 2015, 54, 15550 –15555
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 15555