Synthesis of alkaline earth diazenides MAEN2 (M AE = Ca, Sr, Ba)by Controlled Thermal Decomposition of Azides under High Pressure

Article abstract of DOI:10.1021/ic2023677

The alkaline earth diazenides M_AEN₂ with M _AE = Ca, Sr andBa were synthesized by a novel synthetic approach, namely, a controlleddecomposition of the corresponding azides in a multianvil press at highpressure/high-temperature conditions. The crystal structure of hithertounknown calcium diazenide (space group I4/mmm (no. 139), a = 3.5747(6)A, c = 5.9844(9)A, Z = 2, wRp = 0.078)was solved and refined on the basisof powder X-ray diffraction data as well as that of SrN ₂ and BaN₂.Accordingly, CaN₂ is isotypic with SrN₂ (space group I4/mmm (no. 139), a =3.8054(2)A, c = 6.8961(4)A, Z = 2, wRp = 0.057)and the correspondingalkaline earth acetylenides (M_AEC2)crystallizing in a tetragonally distortedNaCl structure type. In accordance with literature data, BaN₂ adopts a moredistorted structure in space group C2/c (no. 15)with a = 7.1608(4)A, b =4.3776(3)A, c = 7.2188(4)A, β = 104.9679(33)°, Z = 4 and wRp = 0.049).The N-N bond lengths of 1.202(4)A in CaN₂ (SrN₂ 1.239(4)A, BaN₂1.23(2)A)correspond well with a double-bonded dinitrogen unit confirming a diazenide ion [N ₂]2-. Temperature-dependent insitu powder X-ray diffractometry of the three alkaline earth diazenides resulted in formation of the corresponding subnitridesM_AE2N (M_AE = Ca, Sr, Ba)at higher temperatures. FTIR spectroscopy revealed a band at about 1380 cm-1 assigned to the N-Nstretching vibration of the diazenide unit. Electronic structure calculations support the metallic character of alkaline earthdiazenides.

Full text of DOI:10.1021/ic2023677

Article
pubs.acs.org/IC
Synthesis of Alkaline Earth Diazenides M N (M = Ca, Sr, Ba) by
AE 2
AE
Controlled Thermal Decomposition of Azides under High Pressure
Sebastian B. Schneider, Rainer Frankovsky, and Wolfgang Schnick*
Department Chemie, Lehrstuhl fu
Germany
̈
r Anorganische Festko
̈
rperchemie, Ludwig-Maximilians-Universitat
̈
Mu
̈
nchen, 81377 Mu
̈
nchen,
*
S Supporting Information
ABSTRACT: The alkaline earth diazenides M N with M = Ca, Sr and
AE
2
AE
Ba were synthesized by a novel synthetic approach, namely, a controlled
decomposition of the corresponding azides in a multianvil press at high-
pressure/high-temperature conditions. The crystal structure of hitherto
unknown calcium diazenide (space group I4/mmm (no. 139), a = 3.5747(6)
Å, c = 5.9844(9) Å, Z = 2, wR = 0.078) was solved and refined on the basis
p
of powder X-ray diffraction data as well as that of SrN2 and BaN2.
Accordingly, CaN is isotypic with SrN (space group I4/mmm (no. 139), a =
2
2
3
.8054(2) Å, c = 6.8961(4) Å, Z = 2, wR = 0.057) and the corresponding
p
alkaline earth acetylenides (M C ) crystallizing in a tetragonally distorted
AE
2
NaCl structure type. In accordance with literature data, BaN adopts a more
2
distorted structure in space group C2/c (no. 15) with a = 7.1608(4) Å, b =
4
.3776(3) Å, c = 7.2188(4) Å, β = 104.9679(33)°, Z = 4 and wR = 0.049).
p
The N−N bond lengths of 1.202(4) Å in CaN (SrN 1.239(4) Å, BaN
2
2
2
2−
1.23(2) Å) correspond well with a double-bonded dinitrogen unit confirming a diazenide ion [N ] . Temperature-dependent in
2
situ powder X-ray diffractometry of the three alkaline earth diazenides resulted in formation of the corresponding subnitrides
−1
MAE2N (MAE = Ca, Sr, Ba) at higher temperatures. FTIR spectroscopy revealed a band at about 1380 cm assigned to the N−N
stretching vibration of the diazenide unit. Electronic structure calculations support the metallic character of alkaline earth
diazenides.
INTRODUCTION
and their remarkable properties (e.g., superconductivity,
photoluminescence, magnetism and low compressibility com-
■
The synthesis and characterization of novel binary metal
nitrides have a significant impact on solid-state and materials
chemistry due to the numerous applications for such
15−24
parable to that of c-BN)
justify the investigation of the
crystalline structure, stability, elasticity and electronic structures
of the diazenides. However, except for M = Sr, Ba, Os, Ir, Pd
and Pt, no other metal diazenides or pernitrides of formula type
1
−9
compounds.
In 2001, the first members of the novel class
of nitrogen-rich alkaline earth compounds M N2 were
discovered and structurally characterized by Kniep et al.
AE
2−
MN have been synthesized in crystalline form as yet, but have
revealing dinitrogen anions [N2] with NN double bonds.
2
The latter anions represent deprotonated diazene N H units
been predicted by density-functional calculations to form under
2
2
24−29
and were therefore named diazenides. In the past SrN and
HP/HT conditions.
2
BaN have been synthesized from the corresponding metals at
2
In order to extend the class of nitrogen rich metal diazenides
or pernitrides, we have targeted new synthetic approaches for
these compounds, and we were successful using controlled
decomposition of highly reactive precursors like the corre-
sponding azides. In this contribution, we present our novel
synthesis route for the alkaline earth diazenides SrN and BaN .
6
20 °C under nitrogen pressure (0.55−0.56 GPa) in a
10−14
specialized autoclave system.
Recently, related noble
metal nitrides with analogous formula type MNMN (M
=
2
NM
Os, Ir, Pd and Pt) have been synthesized in laser-heated
diamond anvil cells in combination with cryogenically loaded
nitrogen at high pressure of 50 GPa and temperatures around
2
2
1
5−23
2
000 °C.
According to structural and electronic
In addition, we report on the synthesis, structural, spectroscopic
and electronic characterization of the novel alkaline earth
considerations, these noble metal compounds are assumed to
contain 4-fold negatively charged nitrogen dimers having the
4
+
4− 15−24
diazenide CaN and compare its structure to the predicted
2
composition M [N ] .
Unlike the alkaline earth
2
2
6,29
1
5−24
model.
diazenides, the latter anions (bond length about 1.41 Å)
are derived from diazane, N H , containing N−N single bonds
2
4
2−
and are isosteric with peroxide [O ] . Accordingly, these
Received: November 1, 2011
Published: January 11, 2012
2
4+
4−
15−24
compounds M [N ] have been classified as pernitrides,
2
©
2012 American Chemical Society
2366
dx.doi.org/10.1021/ic2023677 | Inorg. Chem. 2012, 51, 2366−2373

Inorganic Chemistry
Article
EXPERIMENTAL SECTION
■
Synthesis of NH N . Phase-pure NH N was obtained by the
4
3
4
3
metathesis reaction of NH NO (3.99 g, 50 mmol, Sigma-Aldrich,
4
3
9
9.0%) and NaN (3.25 g, 50 mmol, Acros Organics, Geel, Belgium,
3
99%) in a silica tube at elevated temperatures. By heating from room
temperature to 200 °C within 0.5 h, annealing for 12 h and cooling
3
0
down again to room temperature within 6 h, NH N3 was
4
precipitated at the cold end of the silica tube separated from
NaNO , which crystallized at the hot end during the reaction.
3
Synthesis of Ca(N ) . Aqueous solutions of calcium azide were
3
2
obtained from the reaction of Ca(OH) with excess of NH N (1:4)
2
4
3
31
according to the procedure reported in the literature. First Ca(OH)2
1.5 g, 20.1 mmol, Sigma-Aldrich, 99.995%) was dissolved in 200 mL
of water. Then an excess of NH N (4.8 g, 80.5 mmol) was added
(
4
3
generating aqueous Ca(N ) , NH and H O. Ammonia was boiled off,
3
2
3
2
and calcium azide was precipitated by evaporating the water. The
calcium azide obtained was dried over CaCl (Sigma-Aldrich, 99.99%)
2
using a vacuum desiccator.
Synthesis of Sr(N ) and Ba(N ) . Strontium and barium azide
Figure 1. Observed (crosses) and calculated (gray line) powder
diffraction pattern as well as difference profile of the Rietveld
3
2
3 2
were obtained by the reaction of the corresponding hydroxide (Sigma-
Aldrich, 99.995%) with an aqueous solution of HN , as reported in the
refinement of CaN ; peak positions of CaN (first row) and of CaO
2
2
3
3
2,33
literature.
The extremely dangerous HN was distilled from NaN3
(second row; side phase) are marked by vertical lines.
3
(
Acros Organics, Geel, Belgium, 99%) and H SO . The solid azides
2
4
were dried over P O using a vacuum desiccator. A general procedure
for the synthesis of the azides of the heavier alkaline earth metals has
been described in the literature.
with a spherical harmonics of fifth order. The relevant crystallographic
data for the three alkaline earth diazenides and further details of the
data collection are summarized in Tables 1 and 2. Further information
of the crystal structures may be obtained from the Fachinformations-
zentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax,
(49) 7247-808-666; e-mail, crysdata@fiz-karlsruhe.de) on quoting the
depository numbers CSD-423721 (CaN ), CSD-423722 (SrN ) and
4
10
31
Caution! Due to the very low thermal and mechanical shock resistance
of HN , hydrazoic acid should always be handled with maximum caution.
Therefore, whenever working with HN , efficient protective clothing such as
face protections, a leather coat and steel reinforced gloves must be worn.
3
3
2
2
Synthesis of M N (M = Ca, Sr, Ba). The three alkaline earth
CSD-423723 (BaN2).
AE
2
AE
diazenides were synthesized under high-pressure/high-temperature
conditions in a modified Walker-type module in combination with a
Temperature-Dependent in Situ X-ray Diffractometry. In situ
X-ray powder diffraction data were collected with a STOE Stadi P
1000 t press (Voggenreiter, Mainleus, Germany). As pressure medium,
powder diffractometer (Mo Kα radiation (0.7093 Å)) equipped with
1
precastable MgO-octahedra (Ceramic Substrates & Components, Isle
of Wight, U.K.) with edge lengths of 14 or 18 mm (14/8 or 18/11
assembly) were employed. Eight tungsten carbide cubes (Hawedia,
Marklkofen, Germany) with truncation edge lengths of 8 or 11 mm
compressed the octahedra. The three alkaline earth azides were
carefully ground, filled into a cylindrical boron nitride crucible (Henze
BNP GmbH, Kempten, Germany) and sealed with a fitting boron
a computer controlled STOE resistance graphite furnace. Enclosed in a
silica glass capillary under argon, the samples were heated from room
temperature to 900 °C at a rate of 5 °C/min in steps of 25 °C. At each
heating step (after holding the temperature for 1 min), a diffraction
pattern was recorded with an IP-PSD in the range of 2° ≤ 2θ ≤ 80°.
Infrared Spectroscopy. Fourier transform infrared (FTIR)
measurements were carried out on a Bruker IFS 66v/S spectrometer.
Spectra of the samples were recorded in an evacuated cell at ambient
3
4−38
nitride plate. Details of the setup are described in the literature.
CaN (SrN , BaN ) was synthesized in a 14/8 (18/11, 18/11)
−1
2
2
2
conditions between 400 and 4000 cm after diluting the samples in
KBr pellets (2 mg sample, 300 mg KBr, pressed with 10 kN).
assembly which was compressed up to 12 (9, 3) GPa at room
temperature within 325 (214, 68) minutes, then heated up to 800
Calculation of the Electronic Structure. The LMTO method in
(
550, 450) °C in 300 (30, 30) minutes, kept at this temperature for 20
41
its scalar relativistic version (TB-LMTO-ASA program) was used to
(
15, 15) minutes and cooled down to room temperature in 15 (10, 10)
perform the calculation of the band structure, electronic density of
minutes. Subsequently, the pressure was released over a period of 958
623, 183) minutes. The recovered MgO-octahedron was broken apart
states (DOS) and crystal orbital Hamiltonian population (COHP) of
(
42
CaN2 and SrN . Detailed descriptions of the method are given
2
in a glovebox and the sample carefully isolated from the surrounding
boron nitride crucible. A black metallic powder of alkaline earth
diazenides was obtained, which is extremely sensitive to moisture.
Powder X-ray Diffraction (PXRD), Structure Solution and
Rietveld Refinement. The crystal structures of the three alkaline
earth diazenides were analyzed on the basis of powder X-ray diffraction
data. The powder diffraction patterns were recorded with a STOE
Stadi P powder diffractometer (STOE, Darmstadt, Germany) in
Debye−Scherrer geometry using Ge(111) monochromated Mo and
43,44
elsewhere.
The k point set was extended to a 24 × 24 × 24 array to
properly describe the metallic compounds. The basis sets consisted of
Ca, 4s/{4p}/3d, Sr, 5s/{5p}/4d/{4f}, and N, 2s/2p/{3d}, where
4
5
orbitals given in brackets were downfolded. The electronic structure
calculations converged when the change of the total energy was
−5
smaller/equal to 10 Ry. The COHP method was used for bond
analysis. COHP gives the energy contributions of all electronic states
for a selected bond. The values are negative for bonding and positive
for antibonding interactions. With respect to the COHP diagrams, we
plot −COHP(E) to obtain positive values for bonding states. The
structural parameters were taken from the Rietveld refinements. The
orbital character of the bands was analyzed using the so-called fat-band
plots. Magnetic susceptibility measurements and electronic structure
Cu Kα radiation (0.7093 Å and 1.54056 Å). For the alkaline earth
1
diazenides of strontium and barium the resulting powder X-ray
diffraction patterns fit perfectly the ones reported in the
10−14
literature.
In the case of the novel diazenide of calcium, the
indexing resulted unambiguously in a tetragonal unit cell with a = 3.57
Å and c = 5.98 Å. A Rietveld refinement (Figure 1) was performed
calculations for BaN
already revealed that the compound exhibits
2
39
with the TOPAS package using the structure of SrN as starting
Pauli paramagnetism and should be metallic, despite the paramagnetic
2
1
1
model. The reflection profiles were determined using the fundamental
triplet state of the diazenide ion. Therefore, a test spin-polarized
calculation with nonzero magnetic moments artificially placed on the
nitrogen atoms of CaN and SrN was performed and converged also
40
parameter approach by convolution of appropriate source emission
profiles with axial instrument contributions and crystalline micro-
structure effects. Preferred orientation of the crystallites was described
2
2
back to the metallic state.
2
367
dx.doi.org/10.1021/ic2023677 | Inorg. Chem. 2012, 51, 2366−2373

Inorganic Chemistry
Article
up by the strontium ions with [N ]2 units aligned along [001].
−
Table 1. Synthesis Conditions and Crystallographic Data for
CaN , SrN and BaN
2
2
2
2
Figure 2 represents a top view of the resulting unit cell of
CaN2
SrN2
BaN2
synthesis
conditions
12 GPa @ 800 °C
9 GPa @
550 °C
3 GPa @
450 °C
multianvil
assembly
14/8
18/11
18/11
fw/g·mol−
1
68.0914
115.6334
165.3534
space group
I4/mmm (no. 139)
I4/mmm (no. C2/c (no. 15)
39)
1
cell params /Å,
deg
a = 3.5747(6)
c = 5.9844(9)
a = 3.8054(2) a = 7.1608(4)
c = 6.2961(4) b = 4.3776(3)
c = 7.2188(4)
β = 104.9679
(
33)
V/Å3
76.47(3)
2
91.17(1)
2
218.61(2)
4
Z/cell
−
3
ρcalc/g·cm
μcalc/mm
2.9572(10)
30.622(11)
1.202(4)
4.212(1)
37.068(1)
1.239(4)
5.0237(5)
17.73(2)
1.23(2)
−
1
dNN/Å
Data Collection
type of
diffractometer
STOE Stadi P
geometry
Debye−Scherrer
Figure 2. Top view of the unit cell of CaN and SrN along [010]
2
2
radiation,
monochromator
Cu Kα (λ = 1.54056
Mo Kα (λ = 0.7093 Å),
2+
2+
1
1
(Ca /Sr , orange; nitrogen, blue).
Å), Ge(111)
Ge(111)
T/°C
detector
25(2)
M N (M = Ca, Sr) in the (101) plane. The alkaline earth
ion is coordinated “side-on” by four and “end-on” by two
diazenide units (Figure 3), giving a coordination number of 8 +
AE
2
AE
linear PSD (Δ2θ = 5°)
2θ range/deg
10−90
5−90
19
2−80
690
no. of obsd reflns 17
Structure Analysis and Refinement
fundamental parameter model
meth of
refinement
program package
atomic parameters
TOPAS Academic
3
3
6
background
function/
params
shifted Chebyshev/9
shifted
Chebyshev/
12
shifted
Chebyshev/
15
2
2
2
R indices
GoF(χ ) = 1.200
GoF(χ ) =
.740
GoF(χ ) =
1.495
1
Rp = 0.062
Rp = 0.044
Rp = 0.037
wR = 0.078
p
wR = 0.057
p
wR = 0.049
p
2
+
2+
RESULTS AND DISCUSSION
Figure 3. Coordination spheres of Sr /Ca on the left, and of the
■
2+
2+
diazenide on the right (Ca /Sr , orange; nitrogen, blue).
Crystal Structure Elucidation of CaN and SrN . Both
2
2
alkaline earth diazenides crystallize in the same structure type,
which is isotypic with the corresponding alkaline earth
acetylenides (M C ), namely, in a tetragonally distorted
2
= 10. The dinitrogen unit occupies the octahedral voids and is
coordinated by six MAE2+ ions. The occupied octahedrons are
AE
2
connected to each other by shared edges and faces. The
NaCl-type. Therefore, they can be topologically derived from
Sr N (anti-CdCl -type) by occupying all octahedral voids made
experimentally found N−N bond length of SrN synthesized by
2
2
2
2
Table 2. Refined Atomic Coordinates and Isotropic Displacement Parameters (in 0.01 Å ) for CaN , SrN and BaN
2
2
2
a
atoms
Wyckoff
x
y
z
Uiso × 100
CaN2
SrN2
BaN2
Ca1
N1
2a
4e
0
0
0
0
0
3.926(96)
6.15(15)
0.3995(5)
Sr1
N1
2a
4e
0
0
0
0
0
3.733(57)
2.60(13)
0.4016(4)
Ba1
N1
4e
8f
0
0.2006(2)
1/4
1.413(21)
1.99(13)
0.3023(18)
0.1547(39)
0.0508(19)
a
2
Uiso = B /(8π ).
eq
2
368
dx.doi.org/10.1021/ic2023677 | Inorg. Chem. 2012, 51, 2366−2373

Inorganic Chemistry
Article
1
0
closed packing, in which the [N2]2− ions occupy all octahedral
voids (Figure 4, left). Again, edge and face sharing of octahedra
Kniep et al. was 1.224(2) Å. Here, we find a distance between
the nitrogen atoms of 1.202(4) Å in CaN and 1.239(4) Å in
2
SrN , which is in good agreement with the former results.
results. However, due to the lower symmetry of BaN
compared to CaN and SrN , the octahedral voids are occupied
2
2
As already mentioned, Wessel et al. have predicted the crystal
2
2
29
with nitrogen dimers aligned horizontally along the diagonals of
the Ba2+ octahedra with alternating orientation within each
layer. Only along [100], the same orientation of dinitrogen
structures of all alkaline earth diazenides theoretically. For
their theoretical investigation, 15 possible structural models
with atomic ratio cation to anion of 1:2 were taken into account
and thereby the ground-state structures predicted. The
calculations confirm that the experimentally found structures
for SrN and BaN are minimum energy structures, thus being
2
+
units is found. Therefore, the coordination sphere of Ba is
quite similar but slightly more distorted (Figure 4, right). Two
2−
diazenide units coordinate in an “end-on” and four [N2] ions
2
2
in a “side-on” fashion (with two different sets of distances to
the thermodynamically most stable structures. For CaN2,
however, the search for a plausible thermodynamic ground-
state structure resulted basically in five possible structure types
2+
Ba ), thus resulting in a coordination number of 2 + 4 + 4 =
1
0. As expected, the experimentally found N−N bond distance
of the diazenide ion 1.204(3) Å is intermediate between those
of a N−N single-bond and triple-bond.
Table 3. Total Energies and Molar Volumes of Predicted
Vibrational Spectroscopy. For SrN and SrN
2
2+
3−
2−
Ground-State Structures for CaN Taken from Wessel et
2
(=(Sr ) (N ) ([N ] ), a nitride-diazenide) inelastic neutron
4 2 2
29
al.
scattering (INS) spectroscopy already revealed that an observed
feature at 1380 cm− (SrN) and 1307 cm (SrN ) can be
1
−1
3
structure type
Etotal (kJ/mol)
V (cm /mol)
2
assigned to the N−N stretching vibration of the diazenide
ZnCl2
ZrO2
−1950.0
−1950.0
−1949.8
−1949.8
−1949.0
25.04
25.07
23.54
23.57
23.63
46
ion. Remaining features at considerably lower wavenumbers
−1
(
<350 cm ) were assigned to acoustic, optic translational and
PbCl2
2−
librational modes of the [N2] ion.
Concerning a free nitrogen molecule (point symmetry D ),
TiO (brookite)
2
∞
h
CaC2
the only fundamental vibration (Σ ) is Raman-active and IR-
g
47
inactive due to the rule of mutual exclusion. However, the
infrared spectra of M N (M = Ca, Sr, Ba), which are shown
(
Table 3) within a tolerance of ΔEtotal = 1 kJ/mol relative to
ZnCl .
AE
2
AE
2
in Figure 5, exhibit clear features at 1376.9 (CaN ), 1378.9
For the reported ZnCl2 structure type, the authors
additionally analyzed the common crystallographic details
space group, lattice parameters, atomic positions, interatomic
2
−
1
(
SrN ) and 1375.9 cm (BaN ), which are assigned to the N−
2
2
2
−
N stretching vibration of the [N2]
ion. Therefore, in
(
comparison to the free dinitrogen molecule with only one
Raman-active mode, the corresponding site symmetry of the
diazenide ion in the solid has to be reduced to observe IR-active
modes. This condition can be verified using the correlation
N−N distance). In this structural model, the cations are
coordinated by four dinitrogen units in an “end-on” and to
another two units in a “side-on” mode which results in an
effective coordination number of 4 + 4 = 8. The coordination
sphere of the diazenide unit again is octahedral. The resulting
N−N distance of 1.255 Å is in good agreement with the one
47
method. The reduced site symmetry for the dinitrogen unit in
CaN and SrN is C4v, resulting in two possible IR-active modes
(A , E). In BaN , there is a D∞h → C reduction in symmetry
with one IR- and Raman-active vibration (A). Applying the
2
2
experimentally found. Synthesized CaN however, is of the
1
2
1
2
M C structure type. Nevertheless, the calculations reveal that
AE
2
47
the M C structure type is close to the ZnCl type in total
correlation method, the only IR-active modes for CaN
SrN (factor symmetry D4h) are A2u and E , and for BaN
A and B . Therefore, in the corresponding crystal, the solely
2
and
(C2h)
2
AE
2
2
energy (ΔE = 1 kJ/mol).
2
u
total
Crystal Structure Elucidation of BaN . BaN shows a
2
2
u
u
close structural relationship to the structure type of SrN and
Raman-active mode in the free dinitrogen molecule is split into
either one of those IR-active modes for the dinitrogen units,
which is clearly observed in the FTIR spectra.
2
CaN , although the strontium and barium compounds do not
2
crystallize isotypically. In BaN , the metal ions also form a
2
Figure 4. Unit cell of BaN with coordination of the diazenide unit on the left; Ba2+ coordination sphere on the right (Ba , light purple; nitrogen,
2+
2
blue; nitrogen not in unit cell, turquoise).
2
369
dx.doi.org/10.1021/ic2023677 | Inorg. Chem. 2012, 51, 2366−2373

Inorganic Chemistry
Article
Temperature-Dependent in situ X-ray Diffractometry.
The temperature-dependent in situ X-ray diffraction patterns
for CaN and BaN are depicted in Figure 6. The temperature-
2
2
dependent in situ powder X-ray diffraction patterns of SrN are
2
given in Figure S1 in the Supporting Information. It is clearly
seen that each alkaline earth diazenide transforms into the
subnitride MAE2N upon heating. For CaN , the temperature of
2
transformation is about 100 °C lower (at approximately 230
°
C) than for SrN and BaN (both at approximately 325 °C).
2 2
These measurements are in good agreement with the reported
10,14
value for SrN of 320 °C upon heating SrN in vacuum.
2
2
It is noteworthy that all three alkaline earth diazenides react
analogously forming the respective subnitrides MAE2N upon
heating. In the case of strontium and barium diazenide this
behavior seems reasonable as the stoichiometric nitrides
48
MAE3N do not seem to exist. In contrast, Ca N is a stable
2
3
2
Figure 5. FTIR spectra of CaN (black), SrN (blue) and BaN2
(
2
2
green). The region of the [N2]2 feature is enlarged (bottom left).
−
phase. However it is not formed during thermal decomposition
of CaN2.
Electronic Structure. To better understand the electronic
structure of the alkaline earth diazenides and their bonding
situation, first principles calculations on the electronic structure
were performed. As the band structure, density of states (DOS)
However, there are still a number of unidentified features,
which might be caused by additional (translational, acoustic,
librational) modes of the diazenide unit due to lattice
distortions. On the other hand, due to the extremely high
sensitivity to moisture, these compounds tend to hydrolyze
very easily. As the KBr-sample pellet is exposed to normal
atmosphere upon transfer from the glovebox to the sample
chamber of the spectrometer, diverse intermediates could be
formed resulting in a variety of possible IR vibrations due to
diverse functional groups.
and crystal orbital Hamiltonian population (COHP) of BaN
2
1
1,24
have already been calculated,
we here report on the
electronic structure of CaN and SrN . In Figure 7 the band
2
2
structures of both diazenides are depicted. As expected, the
electronic structure calculations suggest that the compound
should be metallic, which is in good agreement with the
observed black metallic color of the samples. In both cases, the
main contributions of bands at the Fermi level comes from the
Our attempts to record Raman spectra failed. This may be
caused by reflection or absorption of the laser by the intensely
black alkaline earth diazenides.
M -d (Ca, 3d; Sr, 4d) and the N-p (2p) states. The
AE
contribution of the M -p states (Ca, 4p; Sr, 5p) is negligible.
AE
Figure 6. Temperature-dependent in situ X-ray powder diffraction patterns of CaN (bottom) and BaN (top).
2
2
2
370
dx.doi.org/10.1021/ic2023677 | Inorg. Chem. 2012, 51, 2366−2373

Inorganic Chemistry
Article
Figure 7. TB-LMTO band structures for CaN (left) and SrN (right).
2
2
To illustrate the bonding situation, Figure 8 displays a
Upon forming N by linear combination of the atomic orbitals
2
2−
scheme of the molecular-orbital diagram of N and [N ] .
of elemental nitrogen, the molecular orbitals 1σ
π * and 2σ * are formed. The bonding orbitals 2σ and 1π
u
g u g u
, 1σ *, 2σ , 1π ,
2
2
1
g
u
g
are filled up with six electrons resulting in a maximum of
bonding multiplicity of three (triple bond). Adding further two
electrons into the 1π * molecular orbital (red vertical lines in
g
2−
Figure 8) results on the one hand in [N2] and on the other
hand in a reduced bonding multiplicity of 2 which corresponds
to the double-bonded nitrogen dimer. Notice that only 50% of
2
−
the antibonding molecular orbital 1π * is occupied. The [N ]
g
2
4−
ion is isosteric to molecular oxygen and [C2] and therefore
should exhibit paramagnetic behavior. However, magnetic
susceptibility measurements and electronic structure calcula-
tions for BaN revealed that the compound exhibits Pauli
2
paramagnetism and should be metallic, despite the para-
11
magnetic triplet state of the isolated diazenide ion. Thus, a
test spin-polarized calculation with nonzero magnetic moments
artificially placed on the nitrogen atoms was conducted for
CaN and SrN and converged back to the metallic state. This
Figure 8. Molecular-orbital scheme of the dinitrogen molecule
2
−
illustrating the electron filling for N and [N ] .
2
2
2
2
implies that CaN and SrN should be metallic as well. Further
2
2
addition of two electrons into the molecular-orbital scheme
Figure 9. Total DOS (left), COHP analysis of Ca−N (middle) and N−N (right) for CaN2.
2
371
dx.doi.org/10.1021/ic2023677 | Inorg. Chem. 2012, 51, 2366−2373

Inorganic Chemistry
Article
then results in [N2]4− and another reduction in bonding
multiplicity by 1 (single bond).
material is available free of charge via the Internet at http://
pubs.acs.org.
To elucidate the bonding situation in CaN and SrN , the
2
2
DOS and COHP (M −N and N−N) were calculated and are
AE
AUTHOR INFORMATION
■
*
8
depicted in Figure 9 (CaN ) and Figure S2 in the Supporting
2
Corresponding Author
E-mail: wolfgang.schnick@uni-muenchen.de. Phone: (+49)
9-2180-77436. Fax: (+49)89-2180-77440.
Information (SrN ). In the COHP plots, the bonding states are
2
given as features to the right, whereas antibonding states show
up as features to the left. Since they share the same space
groups and crystal structures, the situation for both alkaline
earth diazenides is basically the same. The bonding states at
ACKNOWLEDGMENTS
■
about −20 eV are mainly based on the 1σ orbitals of the
s
We gratefully acknowledge financial support by the Fonds der
Chemischen Industrie FCI and the Deutsche Forschungsge-
meinschaft DFG (Project SCHN377/13-2). The authors thank
Prof. Dirk Johrendt for valuable discussion.
dumbbell. At about −12 eV the dinitrogen 1σ * states can be
u
found mixing with M -s, -p and -d states (Ca, 4s, 4p, 3d; Sr, 5s,
AE
5
p, 4d). Above that, around −8 to −5 eV the 2σ and the 1π
g
u
states of the dinitrogen unit show up as bonding states together
with an increased contribution of the M -d states and a
AE
REFERENCES
■
decreased one of the M -p and -s states. The antibonding
AE
(
5
1) Matoba, G. Patent JP-B 50010804, 1975. Matoba, G. Patent JP-B
0010803, 1975. Kobayashi, H. Patent JP-A 03006314, 1991. Chokei,
bands in the proximity of the Fermi level ranging from −4 to 2
eV are found to be basically built up by the 1π * states of the
g
T. Patent JP 55094432, 1980. Willners, H. Patent SE 115621, 1946.
2) Jhi, S. H.; Ihm, J.; Louie, S. G.; Cohen, M. L. Nature 1999, 399,
32−134.
dinitrogen unit, which are slightly mixed with the M -s and -d
AE
(
1
(
states. These metal−nitrogen interactions are found to be
responsible for the significant widths of the corresponding
bands in the band structure, and thus probably are also
responsible for the metallic character of these compounds. The
3) McMillan, P. F. Nat. Mater. 2002, 1, 19−25.
(4) Maruyama, T.; Morishita, T. Appl. Phys. Lett. 1996, 69, 890−891.
(5) Yamanaka, S.; Hotehama, K.; Kawaji, H. Nature 1998, 392, 580−
bands above 2 eV are predominantly formed by M -d states.
582.
AE
(
(
(
5
(
6) Zerr, A.; Miehe, G.; Boehler, R. Nat. Mater. 2003, 2, 185−189.
For the M −N combination (Figure 9, middle; Figure S2 in
AE
7) Chhowalla, M.; Unalan, H. E. Nat. Mater. 2005, 4, 317−322.
8) Leineweber, A.; Jacobs, H.; Hull, S. Inorg. Chem. 2001, 40, 5818−
the Supporting Information, middle), there are basically only
bonding states up to E . Above the Fermi level (in the region of
F
822.
the unoccupied states), the antibonding M −N interactions
AE
9) Wu, Z. G.; Chen, X. J.; Struzhkin, V. V.; Cohen, R. E. Phys. Rev. B
005, 71, 214103.
10) Auffermann, G.; Prots, Y.; Kniep, R. Angew. Chem. 2001, 113,
are observed. The COHP plots of the N−N combination
2
(
(
Figure 9, right; Figure S2 in the Supporting Information,
right) in the range of −4 to 2 eV are clearly dominated by
antibonding states where the Fermi level crosses these states at
about half of their antibonding character. This perfectly
matches with the expected 50% occupation of the antibonding
565−567; Angew. Chem., Int. Ed. 2001, 40, 547−549.
(11) Vajenine, G. V.; Auffermann, G.; Prots, Y.; Schelle, W.; Kremer,
R. K.; Simon, A.; Kniep, R. Inorg. Chem. 2001, 40, 4866−4870.
(12) Auffermann, G.; Schmidt, U.; Bayer, B.; Prots, Y.; Kniep, R.
2
−
Anal. Bioanal. Chem. 2002, 373, 880−882.
13) Auffermann, G.; Kniep, R.; Bronger, W. Z. Anorg. Allg. Chem.
006, 632, 565−571.
14) Prots, Y.; Auffermann, G.; Tovar, M.; Kniep, R. Angew. Chem.
002, 114, 2392−2394; Angew. Chem., Int. Ed. 2002, 41, 2288−2290.
15) Gregoryanz, E.; Sanloup, C.; Somayazulu, M.; Badro, J.; Fiquet,
G.; Mao, H.-K.; Hemely, R. J. Nat. Mater. 2004, 3, 294−297.
16) von Appen, J.; Lumey, M.-W.; Dronskowski, R. Angew. Chem.
006, 118, 4472−4476; Angew. Chem., Int. Ed. 2006, 45, 4365−4368.
17) Crowhurst, J. C.; Goncharov, A. F.; Sadigh, B.; Evans, C. L.;
π-states of the diazenide ion [N ] .
2
(
2
(
2
CONCLUSIONS
■
By controlled decomposition of the alkaline earth azides of
calcium, strontium and barium, the corresponding diazenides
M N (M = Ca, Sr, Ba) were synthesized in accord with the
(
AE
2
AE
(
2
(
pressure-homologue rule (isotypic structures with the heavier
homologues can be synthesized by lower pressure; lighter
homologues: higher pressure). FTIR spectroscopy of these
Morrall, P. G.; Ferreira, J. L.; Nelson, A. J. Science 2006, 311, 1275−
1278.
−1
diazenides resulted in a band at about 1380 cm assigned to
the N−N strechting vibration of the diazenide ion. Temper-
ature-dependent in situ powder X-ray diffraction revealed that
CaN , SrN and BaN transformed into the corresponding
(
18) Young, A. F.; Montoya, J. A.; Sanloup, C.; Lazzeri, M.;
Gregoryanz, E.; Scandolo, S. Phys. Rev. B 2006, 73, 153102.
19) Montoya, J. A.; Hernandez, A. D.; Sanloup, C.; Gregoryanz, E.;
Scandolo, S. Appl. Phys. Lett. 2007, 90, 011909.
20) Crowhurst, J. C.; Goncharov, A. F.; Sadigh, B.; Zaug, J. M.;
(
2
2
2
subnitrides MAE2N at higher temperatures. To better under-
stand the bonding situation in the diazenides, electronic
structure calculations were performed for CaN and SrN . As
(
Aberg, D.; Meng, Y.; Prakapenka, V. B. J. Mater. Res. 2008, 23, 1−5.
(21) Chen, Z. W.; Guo, X. J.; Liu, Z. Y.; Ma, M. Z.; Jing, Q.; Li, G.;
Zhang, X. Y.; Li, L. X.; Wang, Q.; Tian, Y. J.; Liu, R. P. Phys. Rev. B
2
2
expected, the diazenides show metallic behavior (DOS). In
addition, COHP calculations revealed a 50% occupation of the
antibonding π-states of the diazenide ion and therefore support
2
(
007, 75, 054103.
22) Yu, R.; Zhan, Q.; De Jonghe, L. C. Angew. Chem. 2007, 119,
1
154−1158; Angew. Chem., Int. Ed. 2007, 46, 1136−1140.
the formulation of a double-bonded, 2-fold negatively charged
N2]2 ion.
−
(23) Chen, W.; Tse, J. S.; Jiang, J. Z. J. Phys.: Condens. Matter 2010,
2, 015404.
[
2
(
24) Wessel, M.; Dronskowski, R. J. Am. Chem. Soc. 2010, 132,
ASSOCIATED CONTENT
Supporting Information
Temperature-dependent in situ X-ray powder diffraction
2421−2429.
(
■
25) Wessel, M.; Dronskowski, R. Chem.Eur. J. 2011, 17, 2598−
*
S
2
603.
(26) Wessel, M. PhD thesis, RWTH Aachen University, Aachen,
patterns, DOS and COHP (Sr−N, N−N) of SrN . This
2010.
2
2
372
dx.doi.org/10.1021/ic2023677 | Inorg. Chem. 2012, 51, 2366−2373

Inorganic Chemistry
Article
(
27) Du, X. P.; Wang, Y. X.; Lo, V. C. Phys. Lett. A 2010, 374, 2569−
574.
28) Wang, H.; Li, Q.; Li, Y.; Xu, Y.; Cui, T.; Oganov, A. R.; Mal, Y.
Phys. Rev. B 2009, 79, 132109.
29) Wessel, M.; Dronskowski, R. J. Comput. Chem. 2010, 31, 1613−
617.
2
(
(
1
(
30) Frierson, W. J. Inorg. Synth. 1946, 8, 136−138.
(
31) Fair, H. D.; Walker, R. F. Energetic Materials Vol. 1: Physics and
Chemistry of the Inorganic Azides; Plenum Press: New York, 1977.
32) Ehrlich, P.; Seifert, H. J; Brauer, G. Handbuch der Praparativen
Anorganischen Chemie Vol. 2; Ferdinand Enke Verlag: Stuttgart, 1978.
(
̈
(
(
33) Remy-Gennete, P. Ann. Chim. 1933, 19, 289−291.
34) Walker, D.; Carpenter, M. A.; Hitch, C. M. Am. Mineral. 1990,
7
(
5, 1020−1028.
35) Walker, D. Am. Mineral. 1991, 76, 1092−1100.
(36) Huppertz, H. Z. Kristallogr. 2004, 219, 330−338.
(37) Rubie, D. C. Phase Transitions 1999, 68, 431−451.
(38) Kawai, N.; Endo, S. Rev. Sci. Instrum. 1970, 8, 1178−1181.
(39) Coelho, A. A. TOPAS-Academic, Version 4.1; Coelho Software:
Brisbane, 2007.
40) Bergmann, J.; Kleeberg, R.; Haase, A.; Breidenstein, B. Mater.
Sci. Forum 2000, 303, 347−349.
41) Andersen, O. K.; Jepsen, O. Tight-Binding LMTO Vers. 4.7; Max-
Planck-Institut fur Festkorperforschung: Stuttgart, 1994.
42) Dronskowski, R.; Blochl, P. J. Phys. Chem. 1993, 97, 8617−8624.
43) Jepsen, O.; Snob, M.; Andersen, O. K. Linearized Band Structure
(
(
̈
̈
(
(
̈
Methods and its Applications; Springer Lecture Notes; Springer-Verlag:
Berlin, 1987.
(
44) Skriver, H. L. The LMTO Method; Springer-Verlag: Berlin, 1984.
(
45) Lambrecht, W. R. L.; Andersen, O. K. Phys. Rev. B 1986, 34,
2
(
439−2449.
46) Auffermann, G.; Prots, Y.; Kniep, R.; Parker, S. F.; Bennington,
S. M. Chem. Phys. Chem. 2002, 9, 815−817.
47) Weidlein, J.; Muller, U.; Dehnicke, K. Schwingungsspektroskopie
Eine Einfuhrung; Georg Thieme Verlag: Stuttgart, New York, 1982.
48) Romer, S. R.; Dorfler, T.; Kroll, P.; Schnick, W. Phys. Status
Solidi B 2009, 246, 1604−1613.
(
−
(
̈
̈
̈
̈
2
373
dx.doi.org/10.1021/ic2023677 | Inorg. Chem. 2012, 51, 2366−2373