Synthesis and structure of the new compound La2O(CN 2)2 possessing an interchanged anion proportion compared to the parent La2O2(CN2)

Article abstract of DOI:10.1002/zaac.200400450

La₂O(CN₂)₂ was synthesized from a 1:1:2 molar reaction mixture of LaCl₃, LaOCl, and Li₂(CN ₂) at 650°C. Well developed single crystals were grown from a LiCl-KCl flux. The crystal structure was refined as monoclinic (space group C2/c, Z = 2, a = 13.530(2) A, b = 6.250(1) A, c = 6.1017(9) A, β = 104.81(2)°) from single crystal X-ray diffraction data. The La³⁺ and (CN₂)^2- ions in the crystal structure of La₂O(CN₂)₂ can be compared to Fe ³⁺ and S₂^2- ions in the cubic pyrite structure, being arranged like in a distorted NaCl type structure with their centers of gravity. In addition, the O^2- ions in La₂O(CN ₂)₂ are occupying 1/4 of the tetrahedral voids formed by the arrangement of metal ions.

Full text of DOI:10.1002/zaac.200400450

Synthesis and Structure of the New Compound La O(CN ) Possessing an
2
2 2
Interchanged Anion Proportion Compared to the Parent La O (CN )
2
2
2
Synthese und Kristallstruktur der neuen Verbindung La O(CN ) mit einem
2
2 2
vertauschten Anionenverhältnis gegenüber der Bezugsverbindung La O (CN )
2
2
2
Radhakrishnan Srinivasan, Sonja Tragl, and H.-Jürgen Meyer*
Tübingen, Abteilung für Festkörperchemie und Theoretische Anorganische Chemie am Institut für Anorganische Chemie der Universität
Received October 14th, 2004.
2
Ϫ
Abstract. La
mixture of LaCl
2
O(CN
2
)
2
was synthesized from a 1:1:2 molar reaction
(CN ) at 650 °C. Well developed
and S
2
ions in the cubic pyrite structure, being arranged like in
3
, LaOCl, and Li
2
2
a distorted NaCl type structure with their centers of gravity. In
addition, the O ions in La O(CN ) are occupying 1/4 of the
2 2 2
2
Ϫ
single crystals were grown from a LiCl-KCl flux. The crystal struc-
ture was refined as monoclinic (space group C2/c, Z ϭ 2, a ϭ
tetrahedral voids formed by the arrangement of metal ions.
˚
˚
˚
1
3.530(2) A, b ϭ 6.250(1) A, c ϭ 6.1017(9) A, β ϭ 104.81(2)°) from
3ϩ 2Ϫ
single crystal X-ray diffraction data. The La and (CN
2
)
ions
3
ϩ
in the crystal structure of La
2
O(CN
2
)
2
can be compared to Fe
Keywords: Lanthanum; Oxides; Cyanamides; Carbodiimides
Introduction
originate from the graphite boat. All the homologous
Ln O (CN ) compounds were synthesized following the
2
2
2
Research interest in C-N containing compounds of the lan-
thanides has been growing recently with the discovery of
new (NCN)
same procedure, and their crystal structures have been re-
ported as tetragonal for Ln ϭ La and trigonal for Ln ϭ
Ce, Pr, Nd, Sm, Eu, and Gd [9]. Both types of structures
2
Ϫ
compounds such as Eu(CN2) [1],
LiEu (CN )I , LiEu (CN ) I [2], LnCl(CN ) with Ln ϭ
2
2
3
4
2 3 3
2
2ϩ
are represented by an alternating sequence of [La O ] and
2
2
La, Ce, Pr [3], and Ln Cl(CN )N with Ln ϭ La, Ce [4].
2
2
2Ϫ
(
CN2) layers along their crystallographic c-axis. For tet-
ragonal La O (CN ) the alignment of the (CN ) ions has
Stimulated by research on lanthanide compounds with BNx
anions [5], referred as to nitridoborates (in agreement with
the designation of oxoborates) and after the discovery of
oxide nitridoborates of the formula type Ln O (BN ) with
Ln ϭ La [6], Pr, Nd [7], we have now synthesized the new
oxide di-nitridocarbonate La O(CN ) . The authors note
here that the designation of a (NCN) containing com-
pound as cyanamide or carbodiimide is sometimes ambigu-
ous. The composition of La O(CN ) is related to the
2Ϫ
2
2
2
2
been reported perpendicular to the c-axis, and for trigonal
2Ϫ
Ln O (CN ) the (CN ) ions have been refined parallel to
2
2
2
2
6
6
3
the c-axis.
Lanthanide doped La O (CN ):Ln compounds with
2
2
2
2
2 2
2ϩ
3ϩ
Ln ϭ Eu or Pr were studied for their luminescence
properties [10Ϫ12]. The presence of [La O ] layers in the
2
Ϫ
2ϩ
2
2
structure of La O (CN ):Ln compounds has been ac-
2
2
2
2
2 2
counted for the efficient luminescence behaviour [13].
In the course of our studies of metathesis reactions [14]
between lanthanide chlorides and lithium cyanamide, we
have synthesized new (NCN)2 containing compounds.
With the employment of lanthanum oxide chloride, we here
report the synthesis for the new La O(CN ) , a route that
known compound La O (CN ), marked by an interchanged
number of anions.
2
2
2
The known lanthanide (Ln) compounds with the
Ln O (CN ) composition were characterized from X-ray
Ϫ
2
2
2
powder diffraction studies reported by Hashimoto et al. [8].
La O (CN ) was originally synthesized by treating La O
3
2
2 2
2
2
2
2
has been analogously applied for the synthesis of
La O (BN ) [6].
with flowing NH at 950 °C for 12 h in a graphite boat.
The carbon in the obtained La O (CN ) was considered to
3
6
6
3
2
2
2
Experimental Section
*
Dr. H.-Jürgen Meyer
Preparation methods
Universität Tübingen
Institut für Anorganische Chemie
Abteilung für Festkörperchemie und Theoretische Anorganische
Chemie
Auf der Morgenstelle 18
D-72076 Tübingen
Tel.: 07071-29-76226
Fax. 07071-29-5702
E-mail: juergen.meyer@uni-tuebingen.de
All reactions were performed in fused silica tubes or in sealed cop-
per ampoules fused in silica tubes. Copper ampoules were obtained
from a copper tube that was cut into pieces of 5-6 cm length,
cleaned with HCl, and rinsed in distilled water before being air
dried at 100 °C in an oven. The pieces were welded under Ar in an
electric arc, first from one side, and then from the other side after
loading with the reaction mixture. The reaction containers were
Z. Anorg. Allg. Chem. 2005, 631, 719Ϫ722
DOI: 10.1002/zaac.200400450
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
719

R. Srinivasan, S. Tragl, H.-J. Meyer
fused in evacuated silica tubes and heated in a commercial (Carbol-
ite) furnace up to temperatures between 500 and 650 °C, using
heating and cooling durations of about 24 h.
Table 1 Crystallographic details and structure refinement data of
La O(CN ) .
2 2 2
Space group (no.), Z
Unit cell dimension (in A)
C2/c (15), 4
La
Li
from La
limed) [15]. LaOCl was synthesized according to [6], and Li
2
O(CN
(CN ). LaCl
(Ventron, 99.9 %) and NH
2
)
2
was synthesized from mixtures of LaCl
was obtained by the ammonium chloride route
Cl (Merck, 99.8 %, sub-
2
(CN )
3
, LaOCl, and
˚
a ϭ 13.530(2), b ϭ 6.250(1),
c ϭ 6.1017(9), β ϭ 104.81(2)°
498.9(1)
4.978
373.88
transparent colourless, needle shaped
0.1 x 0.04 x 0.04
STOE, IPDS
2
2
3
˚
3
O
3
4
Cell volume (in A )
2
Density calc. (in g cmϪ3)
Molecular weight (in g·molϪ1)
Crystal appearance
2
was synthesized as described in [4]. The purity of all phases was
controlled by X-ray powder diffraction.
3
Crystal size (in·mm )
Diffractometer
Radiation, Temperature
α
Mo-K (λ ϭ 71.073 pm),
graphite monochromator, 293(2) K
Synthesis of La O(CN )
2
2 2
Range: θ
Range: h, k, l
Data correction
µ (in mm
Unique reflections
Collected reflections
0 0
(F > 2σ(F ))
Parameters refined
R indices (all data)
Final R indices [I > 2σ(I)]
GooF (all reflections)
Res. peak: max.; min. (in e/A ) 3.17; Ϫ1.19
5.27 to 30.35°
Ϫ19 Յ h Յ 19, Ϫ8 Յ k Յ 8, Ϫ8 Յ l Յ 8
La
2
O(CN
2
)
2
was first obtained as a minor phase, when mixtures
Lorentz, polarisation and absorption
16.78
734
Ϫ1
with different stoichiometries of LaCl
in silica ampoules at 500Ϫ650 °C. The oxygen contamination may
have been due to water or oxygen impurities of the employed LaCl
or from the wall of the silica tube. Yields were increased, when
oxygen sources were introduced into the reaction externally. There-
3
and Li
2
(CN
2
) were reacted
)
4289
3
42 (all atoms refined anisotropically)
R1 ϭ 0.0283, wR2 ϭ 0.0679
R1 ϭ 0.0266, wR2 ϭ 0.0671
1.141
a)
a)
fore, a homogeneous mixture of LaCl
reacted in a 1:1:2 molar ratio at 650 °C and yielded a light brown
powder, later known to be La O(CN , besides LiCl.
3 2 2
, LaOCl, and Li (CN ) was
˚
3
2
2 2
)
a)
2ϪF
2)2 / (Σw(F
2 2 1/2
R1 ϭ ΣʈF
o
Η Ϫ ΗF
c
ʈ / ΣΗF
o
Η; wR2 ϭ [Σw(F
o
c
o
) )]
LaCl
3
ϩ LaOCl ϩ 2 Li
2
(CN
2
) Ǟ La
2
O(CN
2
)
2
ϩ 4 LiCl
(1)
Table 2 Atomic coordinates and isotropic-equivalent displacement
parameters (in A ) for La O(CN ) .
2 2 2
La O(CN )
2
2 2
appeared quite stable against air and water, so that
˚
2
water could even be used to wash off LiCl from the product.
In a separate attempt, the same mixture (total mass: 225 mg) was
combined with a LiCl-KCl mixture (29 mg) and reacted similarly
as in reaction (1). The mixture was heated in a sealed copper am-
poule at 600 °C for ten days before being cooled down slowly to
a
Atom
Wyckoff
position
x
y
z
U
eq
La1
O1
N1
N2
C1
8 f
4 e
8 f
8 f
8 f
0.10072(2)
0
0.2915(3)
0.0923(3)
0.1530(3)
0.21443(3)
0.0260(6)
0.3423(7)
0.6174(6)
0.2739(6)
0.07542(4)
0.0096(1)
0.0118(8)
0.0143(6)
0.0133(7)
0.0111(8)
3
/
4
0.2473(6)
0.9698(6)
0.6112(8)
300 °C first and then to room temperature. From this reaction well
developed needle-shaped colourless crystals were grown and after-
wards selected for X-ray single crystal diffraction studies.
a
U
eq is defined as one-third of the trace of the orthogonalized Uij tensor.
X-ray Diffraction
˚
Table 3 Bond lengths (in A) and N-C-N bond angle (in °) in
La O(CN ) .
2 2 2
All phase analyses were done by X-ray powder diffraction. The
powder patterns were recorded on a StadiP diffractometer (STOE,
Darmstadt), using germanium monochromated Cu-Kα1 radiation
La1ϪO1
La1ϪO1
La1ϪN1
La1ϪN1
La1ϪN1
La1ϪN2
2.409(2)
2.448(2)
2.650(4)
2.756(3)
2.811(4)
2.595(4)
La1ϪN2
La1ϪN2
C1ϪN1
C1ϪN2
N1ϪC1ϪN2
2.655(3)
2.762(4)
1.227(6)
1.233(6)
176.1(5)
˚
(
λ ϭ 1.540598 A) and a position sensitive X-ray detector (opening
angle: 2θ ϭ 6°). The powdered samples of La O(CN were
2
2 2
)
washed in ethanol, dried overnight, and placed in between two my-
lar foils for the X-ray powder pattern measurement. Routine analy-
ses were done in the 2θ range between 10 and 60°. As a new struc-
ture was evident from the powder pattern, a measurement in the
2
θ range of 5Ϫ130° was performed with step increments of 0.2°
Some crystallographic data and measurement parameters are pro-
vided in Table 1. The atomic positions along with the isotropic-
equivalent displacement parameters are given in Table 2. Selected
bond distances and the N-C-N bond angle are shown in Table 3.
Details on the crystal structure investigations can be obtained from
the Fachinformationszentrum Karlsruhe, Germany, D-76344 Egg-
enstein-Leopoldshafen (fax: (ϩ49)7247-808-666; e-mail: crysdat-
a@fiz-karlsruhe.de) on quoting the depository number 391255 for
and exposure periods of 120 seconds. The powder pattern was inde-
xed using Louer’s algorithm (DICVOL) consistent with a C-centred
monoclinic cell (a ϭ 13.507(2) A, b ϭ 6.2432(9) A, c ϭ 6.1002(7) A,
β ϭ 104.833(8)°, V ϭ 497.3(2) A ) for 75 selected peaks. The com-
position of the title compound was first derived from a Rietveld
˚
˚
˚
˚
3
2 2 2
refinement. The refined composition of La O(CN ) was the key
information for the straight forward synthesis. In favour of the bet-
ter accuracy, we here present the single crystal structure refinement
studies only.
2 2 2
La O(CN ) .
2 2 2
Suitable colourless transparent single crystals of La O(CN ) were
selected under a microscope and mounted on the tips of glass fibres
for X-ray diffraction studies. Single crystal measurements were per-
Results and Discussion
formed on an IPDS (STOE, Darmstadt) X-ray diffractometer
As La
O (CN ) is known for a long time, the new La2-
2 2 2
˚
α
using graphite monochromated Mo-K radiation (λ ϭ 0.71073 A).
O(CN ) represents the second composition in this system.
2
2
720
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
zaac.wiley-vch.de
Z. Anorg. Allg. Chem. 2005, 631, 719Ϫ722

2 2 2
Synthesis and Structure of the New Compound La O(CN )
2 2 2
Fig. 1 Unit cell of La O(CN ) . Lanthanum atoms are shown
black, carbon atoms white, nitrogen atoms dark grey, and oxygen
atoms light grey.
Fig. 3 Perspective view of edge sharing [OLa4/2] tetrahedral chains
in the structure of La O(CN running along the c axis.
2
2 2
)
Fig. 2 Relation of cubic FeS
2
(Pyrite) with a substructure of
1
La O(CN in which the oxide ions occupy / of the tetrahedral
2
2
)
2
4
voids. Metal atoms are shown black, carbon atoms white, nitrogen
atoms dark grey, sulphur and oxygen atoms light grey.
The synthesis route employed here to obtain
La O(CN ) (1) may be also considered for (doped)
2
2 2
La O (CN ) compounds following (2) instead of the former
2
2
2
synthesis where graphite serves as a carbon source for the
2
Ϫ
(
CN2) formation.
2
LnOCl ϩ Li
2
(CN
2
) Ǟ Ln
2
O
2
(CN
2
) ϩ 2 LiCl
(2)
This synthesis may also be useful for a reinvestigation of
the crystal structure of La O (CN ), as to take a closer look
)2Ϫ ion in
Fig. 4 Coordination environment around the (CN
2
2
2
2
La O(CN ) .
2
2 2
at the reported disorder of NCN groups in this structure.
The crystal structure of La O(CN ) shown in Figure 1
2
2 2
may occur complicated on the first glance. However, the
In a different view, the crystal structure of La O(CN )
contains one-dimensional chains composed of edge sharing
2
2 2
3
ϩ
2Ϫ
arrangements of La and (CN2) ions can be well derived
from the positions of ions in a NaCl type structure. In ad-
dition, the oxygen atoms of La O(CN ) occupy 1/4 of the
[OLa4/2] tetrahedra running along the c-axis. A perspective
view of the cyanamide ions being situated in between the
chains of tetrahedra is shown in Figure 3.
2
2 2
ϩ
3
tetrahedral voids created by the La ions. Such relations
with the NaCl structure are frequently used to explain the
structures of pyrite (FeS ) or CaC . A comparison of a sec-
The (N-C-N)2 ion in La O(CN ) has two crystallo-
Ϫ
2
2 2
graphically distinct nitrogen positions. However, with calcu-
2
2
˚
˚
tion of the La O(CN ) structure with the cubic pyrite
lated values of 1.227(6) A and 1.233(6) A, the CϪN bond
2
2 2
2
Ϫ
(
(
FeS ) structure [16] having S
CN2) ions in La O(CN ) is illustrated in Figure 2. For
ions similar to the linear
lengths do not differ much from each other, meaning that
2
2
2
Ϫ
2Ϫ
the (N-C-N) ion is clearly closer to a carbodiimide than
2
2 2
2Ϫ
better understanding the structure principle, the lanthanum
ions are connected with carbon instead of nitrogen atoms
to a cyanamide. A symmetrical (N-C-N) ion may also be
3
ϩ
regarded from the drawing in Figure 4 where three La
ions contact the (N-C-N)
2
Ϫ
2Ϫ
of the (CN2) units in the Figures 1 and 2.
ion from both sides, with
Z. Anorg. Allg. Chem. 2005, 631, 719Ϫ722
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© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
721

R. Srinivasan, S. Tragl, H.-J. Meyer
bonate”. One of the authors (R. Srinivasan) is grateful for the schol-
arship provided by the Landesgraduiertenförderung Baden-
Württemberg. We are indebted to Dr. M. Ströbele for the refine-
2 2 2
ment of the La O(CN ) structure from X-ray powder data.
References
[
[
[
[
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Fig. 5 Coordination environment of the La3ϩ ion in La
O(CN ) .
2 2 2
[
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˚
average LaϪO bond length of 2.428(5) A in La O(CN ) is
2
2 2
˚
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2
2
2
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being situated in a polyhedron formed by six nitrogen
atoms (3 x N1 and 3 x N2) of the cyanamide ions and two
oxide ions as shown in Figure 5.
The infrared spectrum (Perkin Elmer FT-IR spec-
trometer) of La O(CN ) (in a KBr pellet) also suggests the
2
2 2
[
13] P. Caro, J. Less Common. Met. 1968, 16, 367.
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Ϫ1
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peak νas at 1979 cm
6
and strong δ peaks at 661 and
Ϫ1
2Ϫ
84 cm for (CN2) . Corresponding characteristic signals
Ϫ1
Ϫ1
were reported at 1950 cm and 670 cm for La O (CN )
2
2
2
[
15] G. Meyer, L. R. Morss, Synthesis of Lanthanide and Actinide
[
8].
Compounds, Kluwer Academic Publishers, 1990, pg. 135.
Acknowledgements. This research was supported by the Deutsche
Forschungsgemeinschaft (Bonn) through the project “Nitridocar-
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A32, 529.
722
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
zaac.wiley-vch.de
Z. Anorg. Allg. Chem. 2005, 631, 719Ϫ722