The new carbodiimide Li2Gd2Sr(CN2) 5 having a crystal structure related to that of Gd 2(CN2)3

Article abstract of DOI:10.1002/zaac.201200404

The new carbodiimide compounds Li₂RE₂Sr(CN ₂)₅ (RE = Sm, Gd, Eu, Tb) were prepared by a straight forward solid state metathesis reaction of REF₃, SrF₂, and Li₂(CN₂) at around 600 °C. The crystal structure of Li₂Gd₂Sr(CN₂)₅ was solved based on X-ray single-crystal diffraction data. Corresponding Li₂RE ₂Sr(CN₂)₅ compounds were analyzed by isotypic indexing of their powder patterns. The crystal structure of Li ₂Gd₂Sr(CN₂)₅ can be well related to that of Gd₂(CN₂)₃, because both structures are based on layered structures composed of close packed layers of [N=C=N] ^2- sticks, alternating with layers of metal ions. The crystal structure of Li₂Gd₂Sr(CN₂)₅ can be considered to contain an ABC layer sequence of [N = C=N]^2- layers with the interlayer voids being occupied by (three) distinct types of cations. Copyright

Full text of DOI:10.1002/zaac.201200404

ARTICLE
DOI: 10.1002/zaac.201200404
The New Carbodiimide Li2Gd2Sr(CN2)5 Having a Crystal Structure Related
to That of Gd2(CN2)3
Leonid Unverfehrt,[a] Markus Ströbele,[a] and H.-Jürgen Meyer*[a]
Keywords: Carbodiimide; Metathesis reaction; Crystal structure; Lithium; Gadolinium
Abstract. The new carbodiimide compounds Li2RE2Sr(CN2)5 (RE = Gd2(CN2)3, because both structures are based on layered structures
Sm, Gd, Eu, Tb) were prepared by a straight forward solid state me-
tathesis reaction of REF3, SrF2, and Li2(CN2) at around 600 °C. The
crystal structure of Li2Gd2Sr(CN2)5 was solved based on X-ray single-
crystal diffraction data. Corresponding Li2RE2Sr(CN2)5 compounds
were analyzed by isotypic indexing of their powder patterns. The crys-
tal structure of Li2Gd2Sr(CN2)5 can be well related to that of
composed of close packed layers of [N=C=N]2– sticks, alternating with
layers of metal ions. The crystal structure of Li2Gd2Sr(CN2)5 can be
considered to contain an ABC layer sequence of [N = C=N]2– layers
with the interlayer voids being occupied by (three) distinct types of
cations.
RE2(CN2)3 (RE = Y, Gd, Lu) are partially substituted by tri-
valent activator ions such as Ce, Tb, or Eu. Photoluminescence
properties were reported for several homologous compounds,
and a dichromatic (In,Ga)N pumped LED was developed with
Y2–xCex(CN2)3.[5]
In the meantime, a number of new carbodiimide compounds
have been prepared starting from easy to make and easy to
handle rare-earth trifluorides. In spite of the aforementioned
difference of using RECl3 or REF3 in the preparation of
Tm2(CN2)3 phases, we experienced that the employment of a
Introduction
A number of rare-earth carbodiimides were prepared by re-
acting appropriate mixtures of RECl3 (RE = rare-earth) and
Li2(CN2) in solid state reactions at temperatures between 500
and 750 °C. The formula types RE2(CN2)3,[1] RECl(CN2),[2]
RE2Cl(CN2)N,[3] and LiRE(CN2)2[4] were obtained by this way
of synthesis just by changing the molar ratios of starting
materials and by using appropriate reaction temperatures.
RE2(CN2)3 compounds with RE = Y, Ce–Tm (except Pm)
adopt monoclinic structures (C2/m) with the coordination trifluoride or trichloride as starting material would usually lead
number (CN) of the RE3+ ions being 7, whereas the
RE2(CN2)3 compounds of the smaller members RE = Tm, Yb,
Lu crystallize in rhombohedral structures (R3c) with the CN
of the RE3+ ions being 6. Among these, Tm2(CN2)3 occurs
dimorphic, adopting both types of structures. Rhombohedral
Tm2(CN2)3 was obtained when reacting TmCl3 and Li2(CN2)
in the temperature range between 500 and 650 °C. Under me-
chanic pressure rhombohedral Tm2(CN2)3 is transformed into
its monoclinic modification under an adherent volume shrink-
age of about 20%.
Both phases, the rhombohedral and the monoclinic modifi-
cation of Tm2(CN2)3 were obtained separately when TmF3 (in-
stead of TmCl3) was reacted with Li2(CN2). The rhombohedral
modification was obtained by heating at 400–450 °C, and the
monoclinic modification of Tm2(CN2)3 was obtained at
500–650 °C. Another interesting issue of RE2(CN2)3 com-
pounds is their luminescence behavior when RE ions of
to the same carbodiimide product in SSM reactions, unless a
fluoride or a chloride ion is contained in the resulting carbodi-
imide compound. Because of the different sizes of chloride vs.
fluoride ions, distinct crystal structures were reported, e.g. for
RECl(CN2) and REF(CN2).[6] In addition, there are examples
of fluoride containing compounds where the corresponding
chloride counterpart is missing (at least until now), like for
LiRE2F3(CN2)2,[6] Eu4F5(CN2)2,[7] or AE3REF5(CN2)2 (AE =
alkaline earth).[8] An overview of known rare earth carbo-
diimide compounds is given in Table 1.
¯
Meanwhile, solid state metathesis (SSM) reactions[9] have
been extended for the preparation of several new carbodi-
imides, mixed-anion carbodiimides,[10,11] and complex cyan-
amide compounds.[12,13] In this work we were focusing on the
development of mixed-cation carbodiimides, in particular on a
compound having the desired composition SrGd2(CN2)4, be-
cause (Ce3+ or Eu3+) doped RE2(CN2)3 compounds have
shown interesting luminescence properties, and a Ca or Sr con-
taining carbodiimide could offer an additional perspective for
Eu2+ doping.
* Prof. Dr. H.-J. Meyer
E-mail: juergen.meyer@uni-tuebingen.de
[a] Abteilung für Festkörperchemie und Theoretische Anorganische
Chemie
Experimental Section
Institut für Anorganische Chemie
Auf der Morgenstelle 18
Universität Tübingen
72076 Tübingen, Germany
Rare earth trifluorides were synthesized by reacting RE2O3 (Ventron,
99.99%) for RE = Sm, Eu, Gd, and Tb4O7 (Ventron, 99,99%) with
84
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Z. Anorg. Allg. Chem. 2013, 639, (1), 84–88

The New Carbodiimide Li2Gd2Sr(CN2)5
out to be Li2Gd2Sr(CN2)5 as the main product, besides some minor
reflections corresponding to Gd2(CN2)3, SrF2, and LiF. Reactions at
615 °C yielded Li2Gd2Sr(CN2)5 and LiF as main products, with an
estimated conversion rate of the reaction greater than 90% (as esti-
mated by XRD). The XRD pattern of a sample that was reacted at
650 °C also revealed Li2Gd2Sr(CN2)5 as the main product, besides
some unidentified reflections with less intensity. A powder pattern of
Li2Gd2Sr(CN2)5 was indexed (40 single indexed reflections) consider-
ing a C-centered monoclinic cell with a = 13.716(3) Å, b = 3.6727(7)
Å, c = 11.448(3) Å, with β = 109.18(1)°, and a unit cell volume of
V = 544.7(3) Å3. Homologous rare earth compounds (RE = Sm, Eu,
and Tb) were characterized by isotypic indexing, with the results being
presented in Table 2.
Table 1. Carbodiimide compounds containing trivalent rare-earth (RE)
elements.
Formula
Example(s)
Literature
RE2(CN2)3
RE3(CN2)3N
RECl(CN2)
Y, Ce – Lu (except Pm)
La, Ce
La, Ce, Pr
[1]
[4]
[2]
RE2Cl(CN2)N
LiRE(CN2)2
RE2Sr2(CN2)5
Li2RE2Sr(CN2)5
La, Ce
Y, La, Ce, Pr
La
[3]
[4, 17]
[8]
this work
[7]
Sm, Gd, Eu, Tb
a
Eu4F5(CN2)2
AE3REF5(CN2)2
REF(CN2)
AE = Sr; RE = La
La, Ce
[8]
[6]
LiRE2F3(CN2)2
RE2O(CN2)2
RE2O2(CN2)
RE2(CN2)(SiO4)
ARE[Si(CN2)4]
Ce, Pr, Nd, Sm, Eu, Gd
La, Ce
Y, La–Gd, Dy–Yb
La, Ce, Pr
A = K, Rb, Cs
RE = La – Lu (except Pm)
M = Sr, Eu
[6]
[10]
[15]
[11]
[12]
A
colorless single-crystal of Li2Gd2Sr(CN2)5 was selected and
mounted on the tip of a glass fiber for an intensity measurement using
an X-ray single-crystal diffractometer (Stoe, IPDS, Darmstadt, Ger-
many), equipped with (graphite) monochromated Mo-Kα radiation,
(λ = 0.71073 Å). The intensity data were corrected for Lorentz, polar-
ization by the IPDS software, and absorption effects were corrected
by X-Red/X-Shape. Further details of the measurement are presented
in Table 3. The crystal structure solution was performed with direct
methods (SHELXS) and the refinement by full-matrix least square re-
finements (SHELXL-97).[14]
b
LiM2[Al(CN2)4]
[13]
a) Mixed-valent, containing Eu2+ and Eu3+. b) divalent Eu.
NH4F in 1:12 molar ratio in a corundum beaker at 100 °C in air. The
excess of NH4F was removed by sublimation, and the resulting com-
plex ammonium salt was decomposed under vacuum at 380 °C to ob-
tain pure REF3. SrF2 (Riedel de Haen 99.99%) was dried under vac-
uum at 650 °C. Li2(CN2) was synthesized as previously reported.[12]
Manipulations of the starting materials, such as weighting, mixing, and
charging the reaction vessels were carried out in a glove-box under
dry argon. Carefully homogenized mixtures of REF3, SrF2, and
Li2(CN2) with 2:1:4 molar, and later with 2:1:5 molar ratio, having
total masses of about 250 mg, were loaded into reaction vessels made
of silica or copper. Silica ampoules were used for reactions at 500 and
550 °C for durations of about 50 h. In spite of the employment of
fluoride, the silica ampoules remained fully intact, thanks to the rela-
tively low heating temperatures and short reaction duration. Reactions
at 600 and 650 °C were performed in copper tubes, which were sealed
by arc-melting and vacuum-sealed into silica tubes to prevent an oxi-
dation of the copper during the following heating process.
Table 3. Crystal data and structure refinement data for Li2Gd2Sr(CN2)5.
Formula weight
Temperature
616.15 g·mol–1
293(2) K
Wavelength
71.073 pm
Crystal system
Space group
monoclinic
C2/m
Unit cell dimensions
a = 13.712(4) Å
b = 3.6687(7) Å
c = 11.449(3) Å
β = 109.17(3)°
0.5440(2) nm3
2
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
3.761 g·cm–3
16.93 mm–1
544
Ampoules were heated in tube furnaces between 500 to 650 °C for
50 h. Afterwards the containers were allowed to cool down to room
temperature before they were opened in air. The products were washed
with water, dried with acetone, and inspected by powder X-ray diffrac-
tion.
Crystal size
Theta range for data collection
Index ranges
0.04 ϫ 0.04 ϫ 0.02 mm
3.09 to 24.95°
–16 Յ h Յ 16, –4 Յ k Յ 4,
–13 Յ l Յ 13
2345
Reflections collected
Independent reflections
Completeness to θ = 24.95°
Refinement method
Data / restraints / parameters
Goodness-of-fit on F2
Final R indices [I Ͼ 2σ(I)]
R indices (all data)
563 (Rint = 0.0387)
99.3%
X-ray Diffraction: All reaction products were inspected by X-ray
powder diffraction (StadiP, Stoe, Darmstadt, Ge-monochromated Cu-
Kα1 radiation). According to the obtained powder patterns, the forma-
tion of Li2Gd2Sr(CN2)5 can be considered to proceed in two major
steps. Reactions at 500 °C yielded Gd2(CN2)3 and LiF, besides vir-
tually unchanged SrF2. The XRD patterns of samples that were heated
at 550 °C revealed reflections of a new compound that later turned
Full-matrix least-squares on F2
563 / 0 / 64
1.133
R1 = 0.0239, wR2 = 0.0569
R1 = 0.0294, wR2 = 0.0579
1.169 and –0.569 e·Å–3
Largest diff. peak and hole
Table 2. Refined lattice constants /Å, angles /°, unit cell volumes /Å3, and the number of single indexed lines (sil) of XRD powder patterns for
Li2RE2Sr(CN2)5 compounds.
Compound
a
b
c
β
V
sil
Li2Sm2Sr (CN2)5
Li2Eu2Sr (CN2)5
Li2Gd2Sr (CN2)5
Li2Tb2Sr (CN2)5
13.692(5)
13.702(4)
13.724(4)
13.710(2)
3.710(1)
3.6936(8)
3.674(1)
3.6484(4)
11.480(3)
11.462(3)
11.461(4)
11.417(2)
109.04(2)
109.02(2)
109.17(2)
109.262(9)
551.2(3)
548.4(3)
545.9(4)
539.1(2)
28
36
28
30
Z. Anorg. Allg. Chem. 2013, 84–88
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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85

L. Unverfehrt, M. Ströbele, H.-J. Meyer
ARTICLE
Further details of the crystal structure investigation can be obtained
from the Fachinformationszentrum Karlsruhe, 73644 Eggenstein-Leo-
poldshafen, Germany (Fax: +49-7247-808-666; E-mail: crysdata@fiz-
karlsruhe.de), on quoting the depository number CSD-424485.
After characterizing the new compound as Li2Gd2Sr(CN2)5
by single-crystal X-ray diffraction, we adjusted the proportions
of reaction partners in accordance to this new composition:
2 GdF3 + SrF2 + 5 Li2(CN2) Ǟ Li2Gd2Sr(CN2)5 + 8 LiF
(5)
Infrared Spectroscopy: Vibration spectra were recorded with a
Bruker Tensor 27 and a Vertex 70-FT-IR spectrometer within the range
of 500–4000 cm–1 by using KBr pellets.
Crystalline samples of Li2Gd2Sr(CN2)5 were prepared in sil-
ica tubes at 550 °C and in copper ampoules at 650 °C. Heating
experiments at different temperatures revealed that the reaction
can be subdivided into two major steps. The main product of
reactions performed at 500 °C was Gd2(CN2)3 (and LiF) ac-
cording to powder XRD, reflecting a relatively poor reactivity
of SrF2. When the temperature was raised to 600 °C
Li2Gd2Sr(CN2)5 was obtained as high-yield product according
to reaction (5).
The crystal structure of Li2Gd2Sr(CN2)5 (Table 1) was re-
fined with Gd1 on a 2a (2/m) position. During the refinement
process, the elongation parameters of Gd2 on a 4i (m) position
turned out to be inadequate, until Sr1 was refined on the same
site (Table 4). During the final structure refinement cycles, the
atom positions and anisotropic displacement parameters of
Gd2 and Sr1 were restrained to be equal, and the occupation
factors perfectly converged at ½ for each, within the limits of
error. Hence, the structure can be described with the notation
Li2Gd[GdSr](CN2)5.
Results and Discussion
In this study we have investigated the formation of mixed-
metal carbodiimide compounds via solid state metathesis reac-
tion, a route which has been successfully used for the prepara-
tion of numerous carbodiimide compounds (Table 1). A typical
reaction departs from appropriate mixtures of rare-earth chlor-
ide and lithium carbodiimide, as has been demonstrated for the
preparation of RE2(CN2)3 compounds:
2 RECl3 + 3 Li2(CN2) Ǟ RE2(CN2)3 + 6 LiCl
(1)
During the past, preparations of carbodiimide compounds
departing from RECl3 were performed in fused silica am-
poules, such as for reactions (1) and (2). Reactions like these
perform under stoichiometric control, thereby avoiding the for-
mation of other compositions in the Li-RE-Cl-(CN2) system,
like RECl(CN2), LiRE(CN2)2, RE2Cl(CN2)N, or others
(Table 1). The coproduced LiCl can be easily removed from
the reaction product by extraction with water to obtain a sin-
gle-phase rare-earth carbodiimide.
Table 4. Atomic coordinates and equivalent isotropic displacement pa-
rameters (pm2 ϫ 10–1) for Li2Gd2Sr(CN2)5.
x
y
z
U(eq)
Strategies for new carbodiimides may be guided by some
already known compositions of metal oxides and sulfides when
considering the carbodiimide ion as a pseudo-chalcogenide, as Gd(2)
shown for the structural relations in the series A-RE2O3,
RE2O2S, RE2O2(CN2),[15] RE2O(CN2)2,[10] RE2(CN2)3.
Li(1)
Gd(1)
–0.217(1)
0
0
0
0
0
0
0
0
–½
–½
–½
0
0.431(2)
0
29(4)
17(1)
19(1)
19(1)
28(2)
20(2)
28(2)
20(2)
20(2)
32(2)
26(3)
39(2)
0.4069(1)
0.4069(1)
0.0351(6)
0.1217(7)
0.2055(6)
–0.1097(6)
–0.1969(7)
–0.2799(6)
0
0.2451(1)
0.2451(1)
0.2165(7)
0.2992(8)
0.3774(7)
0.0061(7)
–0.0761(9)
–0.1536(7)
½
Sr(1)
N(1)
C(1)
N(2)
N(3)
C(2)
N(4)
C(3)
N(5)
Fluorides can be used as comfortable starting materials, be-
cause they behave non-air and non-moisture sensitive. How-
ever, we should note that there can be some differences related
to the reactivity and product formations in reactions when de-
parting from rare-earth chlorides or fluorides. An example is
the formation of Tm2(CN2)3 which appears with two modifica-
tions in reactions performed at 600 °C, when departing from a
thulium chloride or fluoride:
–0.0749(6)
0
0.4059(7)
The overall crystal structure of Li2Gd2Sr(CN2)5 resembles a
distorted hexagonal layer arrangement of metal ions built up
by lithium, strontium, and gadolinium, alternating with stick-
layers of carbodiimide ions (Figure 1). This structural pattern
appears to be closely related to the crystal structures of quasi
binary rare-earth carbodiimides, e.g. to the structure of
Gd2(CN2)3. The two modifications of RE2(CN2)3 compounds
also compromise structures with layers of carbodiimide ions.
In-between these layers rare-earth ions occupy 2/3 of octahe-
dral voids, being situated in nearly octahedral (rhombohedral
structure) and mono-capped trigonal prismatic (monoclinic
structure) environments.
2 TmCl3 + 3 Li2(CN2) Ǟ Tm2(CN2)3 (rhombohedral) + 6 LiCl (2)
2 TmF3 + 3 Li2(CN2) Ǟ Tm2(CN2)3 (monoclinic) + 6 LiF
(3)
The preparation of carbodiimide compounds having stron-
tium and gadolinium counter cations was a goal of our present
study, because strontium is expected that it can be partly sub-
stituted by Eu2+, thereby creating a compound that could serve
as a light converter (phosphor). Hence, we tried to prepare a
compound having the composition SrGd2(CN2)4 following a
straight forward SSM reaction:
The stick packing of carbodiimide ions in the structure of
Li2Gd2Sr(CN2)5 reveals some deviation from a hexagonal
closed layer, which allows for distinct arrangements of cations
(4) in interlayer spaces of the structure. Rare-earth ions are situ-
2 REF3 + SrF2 + 4 Li2(CN2) Ǟ RE2Sr(CN2)4 + 8 LiF
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© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2013, 84–88

The New Carbodiimide Li2Gd2Sr(CN2)5
Table 5. Selected bond lengths /Å, multiplicities (ϫ) and N–C–N bond
angles /° in the structure of Li2Gd2Sr(CN2)5.
Li(1)–N(2)
Li(1)–N(2)
Li(1)–N(5)
Gd(1)–N(1)
2.107(9) (2ϫ)
2.15(2) (1ϫ)
2.06(2) (1ϫ)
2.364(8) (2ϫ)
2.388(5) (4ϫ)
2.635(6) (2ϫ)
2.670(8) (1ϫ)
2.510(5) (2ϫ)
2.553(6) (2ϫ)
125(1), 120(1)
126(1), 119(1)
122.0(8) (2ϫ)
Gd(1)–N(3)
Gd(2)/Sr(1)–N(1)
Gd(2)/Sr(1)–N(3)
Gd(2)/Sr(1)–N(4)
Gd(2)/Sr(1)–N(5)
N(1)–C(1)–N(2)
N(3)–C(2)–N(4)
N(5)–C(3)–N(5)
Є 179(1)
Є 180(1)
Є 180
version center (2/m) with its carbon atom, whereas the other
two exhibit slightly distinct N–C–N bond lengths (Table 5),
as a result of a non-symmetrical cation environments of the
[N=C=N]2– ions, displayed in Figure 3. The three crystallo-
graphically distinct carbodiimide ions in the structure of
Li2Gd2Sr(CN2)5 show three strong absorption bands related to
the asymmetric stretching within the range of 1977–2102 cm–1
and deformation vibrations at 611–644 cm–1.
Lithium ions are situated in metal layers having distorted
tetrahedral environments with Li–N distances of 206–215 pm,
which is reasonable close to corresponding values reported for
LiRE(CN2)2.[4]
Figure 1. Projected unit cell of the crystal structure of Li2Gd2Sr(CN2)5.
ated in nearly octahedral (Gd1), and mono-capped trigonal-
prismatic (Gd2/Sr1) positions (Figure 2), and lithium ions oc-
cupy tetrahedral voids (Figure 3). The expected ionic radii of
Gd3+ and Sr2+ are 100 pm and 121 pm for coordination number
(CN) seven.[16] The difference between these values makes up
for the distinct values being 238.0 pm for the average Gd1–N
distance, and 258.1 pm for the average (Gd2,Sr1)–N distance
(Table 5).
Acknowledgments
The samarium and terbium compounds were prepared by Mrs. Anchal
Garg (Indian Institute of Technology, University of Delhi) during her
summer internship at the University of Tübingen. This research was
supported by the Deutsche Forschungsgemeinschaft in the framework
of the project SSM reactions.
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Z. Anorg. Allg. Chem. 2013, 84–88
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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87

L. Unverfehrt, M. Ströbele, H.-J. Meyer
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