The versatility of solid-state metathesis reactions: From rare earth fluorides to carboiimides

Article abstract of DOI:10.1002/zaac.200801324

The new carbodiimide compounds LaF(CN₂) and LiPr ₂F₃(CN₂)₂ were obtained as crystalline powders by solid-state metathesis reactions from 1:1 molar ratios of REF₃ (RE = rare earth) and Li

Full text of DOI:10.1002/zaac.200801324

ARTICLE
DOI: 10.1002/zaac.200801324
The Versatility of Solid-State Metathesis Reactions: From Rare Earth
Fluorides to Carbodiimides
Leonid Unverfehrt,[a] Jochen Glaser,[a] Markus Ströbele,[a] Sonja Tragl,[a]
Katharina Gibson,[a] and H.-Jürgen Meyer*[a]
Keywords: Lanthanides; Fluorides; Carbodiimide; Metathesis
Abstract. The new carbodiimide compounds LaF(CN2) and
LiPr2F3(CN2)2 were obtained as crystalline powders by solid-state
metathesis reactions from 1:1 molar ratios of REF3 (RE ϭ rare
earth) and Li2(CN2) at temperatures between 500 and 700 °C over
4 days in fused copper ampoules. Higher proportions of Li2(CN2)
in reactions yielded RE2(CN2)3 compounds, including the new
Eu2(CN2)3. Single-crystal structure refinements were performed for
LaF(CN2) and LiPr2F3(CN2)2. Homologous LiRE2F3(CN2)2 com-
pounds with RE ϭ Ce, Pr, Nd, Sm, Eu, Gd were assigned by iso-
typic indexing of their XRD patterns. The crystal structure of
LaF(CN2) contains planar [La3F3] layers being interconnected by
carbodiimide ions, and is related with that of the mineral bastnäs-
ite (REF(CO3)).
of LiF, which could be formed in a metathesis reaction
starting from rare earth fluorides.
A comparative study of the employment of rare earth
fluoride instead of rare earth chloride with Li2(CN2) was,
therefore, a likely task for the exploration of metathesis re-
actions, and for the search of missing compounds [7].
Introduction
During the past years several carbodiimide compounds
of rare earth (RE) metals have been established by rational
approach using solid-state metathesis reactions. Reactions
between RECl3 and Li2(CN2) were performed to synthesize
compounds of the families RE2(CN2)3 [1], RECl(CN2) [2],
and RE2Cl(CN2)N [3] just by changing the molar ratios of
starting materials and the reaction temperature, respec-
tively. RE2(CN2)3 compounds with RE ϭ Y, C e ϪTm (ex-
cept Pm, Eu) adopt the monoclinic structure type (C2/m),
whereas the RE2(CN2)3 compounds with RE ϭ Tm, Yb, Lu
crystallize in the rhombohedral structure type (R3c).
Among these, Tm2(CN2)3 occurs dimorphic adopting both
structure types [7].
Other carbodiimide compounds were obtained by using
additives such as oxides [4] or silicates [5] in multilateral
Experimental Section
Synthesis
The rare earth trifluorides were synthesized by reacting rare earth
oxides such as RE2O3, CeO2, Pr6O11, Tb4O7 (La2O3 from Merck,
99.99 %; other RE oxides from Ventron, 99.99 %) with NH4F in
1:10 molar ratio (with respect to RE2O3) at 100 °C in a corundum
beaker in air. The excess of NH4F was removed by sublimation and
the resulting complex ammoniumfluoridometallate was decom-
¯
metathesis reactions, in which the choice of appropriate re- posed at 380 °C under vacuum to obtain pure REF3 [8]. Li2(CN2)
was synthesized by treating Li2(CO3) (Alfa, ultrapure) in flowing
ammonia at 600 °C (ca. 18 h) several times, inclusive regrinding,
until a single phase product was obtained [9].
action partners appears to be important. Metathesis reac-
tions with two or more reaction partners are exothermic
reactions, in which the exothermicity is driven not only by
the stability of the desired product but also by the lattice
energy of the co-produced salt [6]. Most metathesis reac-
tions in the field of rare earth carbodiimides have been per-
formed by using Li2(CN2) and rare earth chlorides leading
to LiCl as a co-product, in spite of the higher lattice energy
A copper tube (inner diameter ca. 5 mm, thickness of the wall ca.
1 mm) was cut into 4 cm long pieces and each piece was sealed
from one side by with an arc melter under vacuum. A copper con-
tainer was charged with a carefully homogenized 1:1 molar mixture
of REF3 and Li2(CN2) (total amount about 250 mg) under dry ar-
gon inside a glovebox. Afterwards, the copper container was evacu-
ated and sealed from the other side with an arc melter. The copper
ampoule was jacketed into an evacuated silica ampoule in order to
prevent oxidation with air during the heating process in the fur-
nace. Afterwards, the ampoule was placed into a tube furnace and
heated at 500Ϫ700 °C for 4 d. After reaction, the copper container
was allowed to cool down to room temperature and opened in air.
The products were washed with water, dried with acetone, and in-
spected by powder XRD.
* Prof. Dr. H.-J. Meyer
E-Mail: juergen.meyer@uni-tuebingen.de
[a] Abteilung für Festkörperchemie und Theoretische
Anorganische Chemie
Institut für Anorganische Chemie
Ob den Himmelreich 7
Eberhard-Karls-Universität Tübingen
72074 Tübingen, Germany
Z. Anorg. Allg. Chem. 2009, 635, 479Ϫ483
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
479

L. Unverfehrt, J. Glaser, M. Ströbele, S. Tragl, K. Gibson, H.-J. Meyer
ARTICLE
Table 1. Lattice parameters /pm, monoclinic angles /°, unit cell volumes /106 pm3, and number of single indexed reflections of LaFCN2
(Cmcm) and LiRE2F3(CN2)2 (C2/c).
a
b
c
β
V
indexed reflections
LaFCN2
388.64(4)
1166.2(1)
1159.7(1)
1155.2(2)
1146.5(3)
1141.7(4)
1140.1(3)
872.37(9)
664.16(7)
658.3(1)
653.7(1)
645.6(2)
642.2(3)
638.9(2)
782.48(8)
855.51(9)
849.8(1)
845.5(1)
838.1(2)
834.3(3)
832.2(3)
Ϫ
265.29(7)
598.4(2)
586.17(2)
577.2(2)
560.9(4)
552.7(6)
548.2(3)
40
40
43
47
33
30
30
LiCe2F3(CN2)2
LiPr2F3(CN2)2
LiNd2F3(CN2)2
LiSm2F3(CN2)2
LiEu2F3(CN2)2
LiGd2F3(CN2)2
115.431(5)
115.381(6)
115.330(8)
115.31(1)
115.377(2)
115.28(1)
LaF(CN2) was synthesized at 500 °C. Single crystals of LaF(CN2)
were obtained after reaction at 750Ϫ800 °C (1:1 molar ratio).
Single crystals of LiPr2F3(CN2)2 were found after the reaction was
performed at 750 °C.
have been proven to be efficient in syntheses of rare earth
compounds containing nitridoborate [12], carbodiimide
[1Ϫ3, 7], and tetracyanamidosilicate [5] ions. During the
past, most reactions of this type were performed starting
from rare earth trichlorides.
Reactions between REF3 compounds and Li2(CN2) in 1:1
molar ratio resulted in three distinct main products, as
shown in Equations (1)Ϫ(3). Formations of crystalline
powders of new compounds LaF(CN2) and LiRE2F3(CN2)2
may be described according to Equations (1) and (2), with
respect to the results of indexed powder patterns.
RE2(CN2)3 compounds given in Equation (3) were already
known from previous experiments, originally synthesized
from 2:3 molar ratios of RECl3 and Li2(CN2) [Equation (4)]
X-ray Powder Diffraction
The XRD patterns of the reaction products were collected with
a Stadi-P (Stoe, Darmstadt) powder diffractometer using Cu-Kα1
radiation (germanium monochromator). After the single crystal
refinements were successfully performed for LaF(CN2) and
LiRE2F3(CN2)2 their calculated X-ray patterns were used to inspect
unknown reflections in the XRD powder patterns resulting from
analogous reactions of other rare earth elements. Lattice param-
eters of homologous RE compounds were indexed and calculated
with the aid of the program system WinXPow [10] with the results [1, 7]. All reported carbodiimide products behave stably in
listed in Table 1. The powder pattern of the new Eu2(CN2)3 was
indexed isotypic to monoclinic RE2(CN2)3 (C2/m) compounds
revealing lattice parameters of a ϭ 1450.4(2) pm, b ϭ 387.08(6) pm,
c ϭ 526.8(1) pm, and β ϭ 95.815(9)°.
air and can be purified by washing with water.
LaF3 ϩ Li2(CN2) Ǟ LaF(CN2) ϩ 2 LiF
(1)
(2)
2 REF3 ϩ 2 Li2(CN2) Ǟ LiRE2F3(CN2)2 ϩ 3 LiF
X-ray Single-Crystal Diffraction
with RE ϭ Ce, Pr, Nd, Sm, Eu, Gd
Transparent needle shaped single-crystals of LaF(CN2) and trans-
parent green rodlike crystals of LiPr2F3(CN2)2 were selected and
mounted on the tips of glass fibers for single-crystal X-ray diffrac-
tion measurements (Stoe, IPDS I diffractometer, graphite monoch-
romated Mo-Kα radiation) at room temperature (Table 2). Inten-
sities were corrected for Lorentz factors, polarization and
absorption effects by using the IPDS software X-Red/X-Shape. The
crystal structure solutions and anisotropic refinements of all atoms
were obtained by using the SHELX-97 program package [11].
3 REF3 ϩ 3 Li2(CN2) Ǟ RE2(CN2)3 ϩ LiREF4 ϩ 5 LiF
with RE ϭ Tb, Dy, Ho, Er, Tm, Yb, Lu
2 REF3 ϩ 3 Li2(CN2) Ǟ RE2(CN2)3 ϩ 6 LiF
with RE ϭ Y, C e Ϫ Lu (except Pm)
(3)
(4)
Rare earth trifluorides appear to be easier in synthesis
(from oxides) and handling than the corresponding chlo-
Further details on the crystal structure investigation can be
obtained from Fachinformationszentrum Karlsruhe, 76334 Eggen-
stein-Leopoldshafen, Germany (Fax: (ϩ49)7247-808-666; E-Mail: rides. In the course of our reactions with Li2(CN2) we noted
crysdata@fiz-karlsruhe.de on quoting the CSD number 419916 of
LaF(CN2) and CSD number 419925 of LiPr2F3(CN2)2.
that the co-produced LiCl (from RECl3 reactions) can be
easily washed away with water from air stable products.
This is not the case with LiF (from REF3 reactions) as the
solubility of LiF in H2O is only about 0.15 % at 20 °C,
making a separation process, if desired, more difficult.
The synthesis of the LaF(CN2) parallels the already re-
ported formation of RECl(CN2) with RE ϭ La, Ce, Pr from
reactions of RECl3 and Li2(CN2) [2]. In contrast, the for-
mation LaF(CN2) was obtained only for RE ϭ La.
Infrared Spectroscopy
Infrared spectra were recorded from KBr pellets in the spectral
region between 4000 and 350 cmϪ1 with a Perkin-Elmer FT-IR
spectrometer (SPEKTRUM 1000).
Reactions of rare earth trifluorides with RE ϭ Ce, Pr,
Nd, Sm, Eu, Gd lead to the formation of compounds with
Results and Discussion
Solid-state metathesis reactions may be regarded as a the formula LiRE2F3(CN2)2. The employment of the se-
straightforward tool for target orientated syntheses, which cond half series of rare earth elements with RE ϭ Tb, Dy,
480
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Z. Anorg. Allg. Chem. 2009, 479Ϫ483

The Versatility of Solid-State Metathesis Reactions: From Rare Earth Fluorides to Carbodiimides
Table 2. Crystal data from single crystal structure refinements on LaF(CN)2 and LiPr2F3(CN2)2.
Empirical formula
LaF(CN)2
LiPr2F3(CN2)2
Formula weight /g·molϪ1
Crystal system
Space group
197.94
orthorhombic
Cmcm (No. 63)
425.82
monoclinic
C 2/c (No. 15)
Unit cell dimensions /pm
a ϭ 387.72(8), b ϭ 869.3(2),
c ϭ 780.3(2)
263.00(9)
a ϭ 1156.2(2), b ϭ 656.3(1),
c ϭ 847.1(1), β ϭ 115.32(2)
581.1(2)
Volume /106 pm3
Z
4
4
Density (calculated) /g·cmϪ3
Absorption coefficient /mmϪ1
F(000)
4.999
15.973
344
4.867
16.560
752
Crystal description
Crystal size /mm
θ for data collection /°
Index range
Colorless needle
0.34 ϫ 0.10 ϫ 0.06
4.69 to 30.26
Ϫ5 Յ h Յ 5, Ϫ12 Յ k Յ 12, Ϫ11 Յ l Յ 11
Green block
0.18 ϫ 0.14 ϫ 0.07
3.67 to 32.84
Ϫ17 Յ h Յ 17, Ϫ9 Յ k Յ 9, Ϫ12 Յ l Յ 12
Reflections collected
Independent reflections
Rint
2153
238
0.0571
238 /20
Numerical
0.1186 and 0.4338
0.108(8)
6182
1028
0.0372
1028 /57
Numerical
0.1902 to 0.4047
0.0151(4)
Data /parameters
Absorption correction
Min. and max. transmission
Extinction coefficient
Goodness-of-fit on F2
R indices [I > 2σ(I)]
R indices (all data)
Largest diff. peak and hole eϪ /106 pm3
Temperature /K
1.356
1.060
R1 ϭ 0.0231, wR2 ϭ 0.0651
R1 ϭ 0.0231, wR2 ϭ 0.0651
1.545 and Ϫ1.522
293(2)
R1 ϭ 0.0172, wR2 ϭ 0.0427
R1 ϭ 0.0223, wR2 ϭ 0.0438
1.947 and Ϫ0.799
293(2)
λ (Mo-Kα) /pm
Diffractometer
CSD-Number
71.073
STOE IPDS I
419916
71.073
STOE IPDS I
419925
Ho, Er, Tm, Yb, Lu yielded the already known RE2(CN2)3 obtained as a high yield product according to Equation (4),
compounds plus LiREF4 [13, 14] as side phases.
starting from TmF3 and Li2(CN2).
Pure RE2(CN2)3 compounds were already obtained pre-
The crystal structure of LaF(CN2) is shown in
viously, by using the appropriate 2:3 molar ratio of starting Figure 1. A common pattern of the crystal structures of
materials. From these results we note that RE2(CN2)3 com- LaCl(CN2) and LaF(CN2) is their arrangement of [LaX]2ϩ
pounds can be synthesized by using rare earth chlorides or (X ϭ Cl, F) and (CN2)2Ϫ layers alternating along the stack-
fluorides [Equation (4)]. This is also the case for the first ing directions. The carbodiimide groups in the structure of
half series of RE elements when the appropriate 2:3 molar LaCl(CN2) are not fully symmetrically with respect to their
ratio of reactants is employed. However, our experiments coordination environments with lanthanum atoms, yielding
suggested that there were some differences with respect to CϪN distances of d ϭ 125.1(6) and 120.2(6) pm, respec-
products and yields of reactions, sometimes favoring a tively. Carbon atoms of the carbodiimide group in the
metathesis reaction with rare earth fluoride and sometimes
with chloride. For example, Ce2(CN2)3 was obtained as a
low yield product when synthesized from CeCl3, but ap-
peared in high yields when synthesized from CeF3. Mono-
clinic Eu2(CN2)3 was a missing member of the RE2(CN2)3
[1, 7] series, which was successfully synthesized as a single
phase product (besides LiF) by metathesis reaction starting
from EuF3 and identified from indexed XRD patterns
[Eu2(CN2)3: C2/m, a ϭ 1450.4(2) pm, b ϭ 387.08(6) pm,
c ϭ 526.8(1) pm, β ϭ 95.815(9)°]. The unit cell volume of
Eu2(CN2)3 [V ϭ 294.3(1)·106 pm3] nicely fits into the series
of RE2(CN2)3 compounds [7].
Rhombohedral Tm2(CN2)3 (II) was previously synthe-
sized from TmCl3 and Li2(CN2). Pressure experiments have
shown that Tm2(CN2)3 is dimorphic, and that monoclinic
Tm2(CN2)3 (I) is obtained as moderately crystalline powder
after applying pressure on rhombohedral Tm2(CN2)3 (II)
[7]. Crystalline powder of monoclinic Tm2(CN2)3 (I) was structure of LaF(CN2).
Figure 1. View along the approximate [100] direction of the crystal
Z. Anorg. Allg. Chem. 2009, 479Ϫ483
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481

L. Unverfehrt, J. Glaser, M. Ströbele, S. Tragl, K. Gibson, H.-J. Meyer
ARTICLE
Figure 2. Section of [LaF] layers in the structures of LaF(CN2) (left) and LiPr2F3(CN2)2 (right). Note that the layer at right is not
fully planar.
LaF(CN2) structure occupy positions at the center of inver-
sion and the CϪN distances are refined at d ϭ 122.0(6) pm.
The linear [NϭCϭN] unit is surrounded by six La atoms
in a nearly trigonal antiprismatic fashion. Infrared spectra
of LaCl(CN2) revealed a strong asymmetrical vibration
signal (νasym
)
at 2033 cmϪ1 and strong deformation
vibration signals (δ) at 660 and 648 cmϪ1. For LaF(CN2)
the corresponding signals were obtained at 1981 cmϪ1
(νasym), 661, and 685 cmϪ1 (δ). These values are closely re-
lated to many other (CN2)2Ϫ containing compounds [15].
A striking feature in the crystal structure of LaF(CN2) is
the planar arrangement of [LaF] layers, constructed from
six membered [La3F3] rings, displayed in Figure 2 (left),
which can be related to the arrangement of boron and
nitrogen atoms in the structure of hexagonal boron nitride.
These layers are interconnected by nitrogen atoms of the
carbodiimide ions. Lanthanum atoms in the structure are
surrounded in a nine fold [LaN6F3] environment formed
by six nitrogen atoms [dLaϪN ϭ 259.7(4)Ϫ267.8(5) pm]
of [NϭCϭN] groups forming a trigonal prism, which is
Figure 3. Coordination environment of La in the structure of
LaF(CN2).
formula unit (Figure 2, right). These layers are intercon-
nected by [NϭCϭN] and the remaining two-thirds of fluor-
ine atoms. The [NϭCϭN] group in LiPr2F3(CN2)2 is not
fully symmetrically having an NϪCϪN angle of 178.3(3)°
and CϪN distances of d ϭ 123.4(4) and 121.7(4) pm,
respectively. The monitored infrared spectra of
tricapped by fluorine atoms [dLaϪF
ϭ
258.7(4)Ϫ
LiPr2F3(CN2)2 revealed frequencies of 2024 cmϪ1 (νasym
)
263.3(6) pm] as shown in Figure 3. A related structure
pattern is known from the structure of the most common
fluoride carbonate mineral bastnäsite [16]. The relation
between these two compounds, LaF(CN2) and REF(CO3),
exists also in a chemical matter, because (CN2)2Ϫ ions can
hydrolyze to form (CO3)2Ϫ, and vice-versa, (CO3)2Ϫ can be
converted into (CN2)2Ϫ by ammonolysis reaction, as shown
for Li2(CO3) in the synthesis part.
and 682, 673, 646 cmϪ1 (δ) but also the frequency of the
pseudo symmetrical vibration at 1260 cmϪ1 indicating the
small asymmetry of the [NϭCϭN] group.
RE atoms in the bastnäsite structure (usually combi-
nations of cerium, lanthanum and yttrium) have a nine fold
[REO6F3] coordination as well, resulting from six oxygen
atoms of carbonate groups, and three fluorine atoms con-
necting the RE atoms into [REF] layers. However, adjacent
layers are shifted relative to each other, resulting in a
trigonal prismatic environment for the carbonate groups.
The crystal structure of LiPr2F3(CN2)2, can be con-
sidered as an adduct, LiF·2PrF(CN2)(Figure 4), and is
closely related with the structure of LiLa2F3(SO4)2 [17]. The
structure is constructed from buckled 4.82 [PrF] layers of
Figure 4. View along the approximate [010] direction of the crystal
praseodymium atoms and one third of fluorine atoms per structure of LiPr2F3(CN2)2.
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The Versatility of Solid-State Metathesis Reactions: From Rare Earth Fluorides to Carbodiimides
Table 4. Atomic distances /pm, (multiplicity) and angles /° in struc-
tures of LaF(CN2) and LiPr2F3(CN2)2.
LaF(CN2)
La(1)ϪF(1)
La(1)ϪN(1)
C(1)ϪN(1)
258.7(4) (2x), 263.3(6) (1x)
259.7(4) (4x), 267.8(5) (2x)
122.0(6) (2x)
180
N(1)ϪC(1)ϪN(1)
LiPr2F3(CN2)2
Pr(1)ϪF(3)
Pr(1)ϪF(2)
Pr(1)ϪN(1)
Pr(1)ϪN(2)
N(1)ϪC(1)
N(2)ϪC(1)
Li(1)ϪF(2)
Li(1)ϪF(3)
Li(1)ϪN(2)
Li(1)ϪN(1)
N(2)ϪC(1)ϪN(1)
234.37(7)
243.4(2), 250.1(2), 256.3(2)
254.9(2), 261.4(2), 262.4(2)
256.0(2), 261.3(2)
123.4(4)
121.7(4)
195.5(2) (2x)
Figure 5. Coordination environment of Pr (left) and Li (right) in
the structure of LiPr2F3(CN2)2.
199.5(8)
Table 3. Atom positions and equivalent isotropic displacement pa-
257.7(6) (2x), 303.5(3) (2x)
316.5(4) (2x), 343.3(6) (2x)
178.3(3)
rameters /104 pm2 in structures of LaF(CN)2 and LiPr2F3(CN2)2.
x
y
z
Ueq
LaF(CN)2
La(1)
F(1)
N(1)
C(1)
0
0.2222(1)
0.4193(6)
0.1322(6)
0
1/4
1/4
0.0124(4)
0.016(1)
1/2
1/2
1/2
0.4475(7) 0.015(1)
1/2 0.011(1)
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Pr(1)
F(2)
F(3)
N(1)
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C(1)
Li(1)
0.2869(1)
0.1861(2)
1/2
0.0042(1)
0.1670(1) 0.0066(8)
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[14] R. E. Thoma, G. D. Brunton, R. A. Penneman, T. K. Keenan,
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254.9(2)Ϫ262.4(2) pm] of carbodiimide ions (Figure 5, left).
The lithium ions are localized between metal layers, in
which metal atoms from adjacent layers form a distorted
trigonal prismatic cage, bridged along the prism axes by
four [NϭCϭN] and one fluorine. Together with the fluorine
atoms from each [PrF2] layer, there are three short LiϪF
[dLiϪF ϭ 195.5(8)Ϫ199.5(8) pm] and two short LiϪN con-
tacts [dLiϪN ϭ 257.7(6) pm]. But there are more nitrogen
and even carbon neighbors from two nearly side-on
arranged [NϭCϭN] groups in the vicinity of lithium, as
shown in Figure 5.
[15] O. Reckeweg, A. Simon, Z. Naturforsch. 2003, 58b, 1097.
[16] J. D. Grice, V. Maisonneuve, M. Leblanc, Chem. Rev. 2007,
107, 114.
Acknowledgement
We gratefully acknowledge support of this research by the
Deutsche Forschungsgemeinschaft (Bonn) through the project
Nitridocarbonate.
[17] M. S. Wickleder, Z. Anorg. Allg. Chem. 1999, 625, 302.
Received: October 10, 2008
Published Online: January 13, 2009
Z. Anorg. Allg. Chem. 2009, 479Ϫ483
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
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