Syntheses and structural properties of rare earth carbodiimides

Article abstract of DOI:10.1021/ic0608952

Crystalline samples of rare earth carbodiimides were synthesized by solid-state metathesis reactions of rare earth trichlorides with lithium cyanamide in sealed silica ampules. Two distinct structures were determined by single-crystal X-ray diffraction. The structure determined for Sm ₂(CN₂)₃ [C2/m, Z = 2, a = 14.534(2) A, b = 3.8880(8) A, c = 5.2691(9) A, β = 95.96(2)°, R ₁ = 0.0267, and wR₂ = 0.0667] was assigned for RE ₂(CN₂)₃ compounds with RE = Y, Pr, Nd, Sm, Gd, Tb, Dy, Ho, and Er, and the structure determined for Lu₂(CN ₂)₃ [R32, Z = 3, a = 6.2732(8) A, c = 14.681 (2) A, R₁ = 0.0208, and wR₂ = 0.0526] was assigned for the smallest rare earth ions with RE = Tm, Yb, and Lu by powder X-ray diffraction. Both types of crystal structures are characterized by layers of [NCN]^2- ions whose arrangements can be derived from the motif of a closest packed layer of sticks. These layers alternate with layers of rare earth ions in a one-by-one sequence. Different tilting arrangements of the N-C-N-axes relative to the stacking directions (c) and different arrangements of RE ³⁺ ions within metal atom layers account for the two distinct structures in which Sm³⁺ and Lu³⁺ ions adopt the coordination numbers 7 and 6, respectively.

Full text of DOI:10.1021/ic0608952

Inorg. Chem. 2006, 45, 8188−8193
Syntheses and Structural Properties of Rare Earth Carbodiimides
Michael Neukirch, Sonja Tragl, and H.-J u1 rgen Meyer*
Abteilung f u¨ r Festk o¨ rperchemie und Theoretische Anorganische Chemie, Institut f u¨ r Anorganische
Chemie, UniVersit a¨ t T u¨ bingen, Auf der Morgenstelle 18, D-72076 T u¨ bingen, Germany
Received May 23, 2006
Crystalline samples of rare earth carbodiimides were synthesized by solid-state metathesis reactions of rare earth
trichlorides with lithium cyanamide in sealed silica ampules. Two distinct structures were determined by single-
crystal X-ray diffraction. The structure determined for Sm
5.2691(9) Å, â ) 95.96(2) , R 0.0267, and wR
Y, Pr, Nd, Sm, Gd, Tb, Dy, Ho, and Er, and the structure determined for Lu
.2732(8) Å, c 14.681(2) Å, R 0.0208, and wR 0.0526] was assigned for the smallest rare earth ions with
2
(CN
0.0667] was assigned for RE
(CN
2
)
3
[C2/m, Z
)
2, a
)
14.534(2) Å, b
(CN compounds with
[R32, Z 3, a
) 3.8880(8) Å,
c
RE
)
°
1
)
2
)
2
2 3
)
)
2
2
)
3
)
)
6
)
1
)
2
)
RE
)
Tm, Yb, and Lu by powder X-ray diffraction. Both types of crystal structures are characterized by layers of
2-
[
NCN] ions whose arrangements can be derived from the motif of a closest packed layer of sticks. These layers
alternate with layers of rare earth ions in a one-by-one sequence. Different tilting arrangements of the N
−
C
−
N-
3+
axes relative to the stacking directions (c) and different arrangements of RE ions within metal atom layers account
for the two distinct structures in which Sm3 and Lu3 ions adopt the coordination numbers 7 and 6, respectively.
+
+
Introduction
After the discovery of nitridoborate compounds with linear
[
valent rare earth elements. Especially, lanthanum compounds
have been synthesized and structurally characterized with
n-
NBN]3 ions, their structural chemistry has been widely
-
different types of nitridoborate anions, including [BN] ,
8
-
9-
6- 12-17
[B
2
N
4
] , [B
3
N
6
] , and [BN
3
] .
A key for the develop-
explored for alkali (A), alkali earth (AE), and divalent rare
1
-3
ment of the chemistry of rare earth nitridoborates was the
successful implementation of solid-state metathesis reactions
earth (RE) nitridoborates with formulas A
3
(BN
, respectively. Great progress has
been achieved by extending this chemistry toward the tri-
2
),
3
AE -
4-10
11
2 2
(BN ) ,
3 2 2
and RE (BN )
3
between rare earth trichloride and lithium nitridoborate [Li -
1
8,19
2
(BN )].
*
To whom correspondence should be addressed. Phone: 49-7071-29-
7
6226. Fax: 49-7071-29-5702. E-mail: juergen.meyer@uni-tuebingen.de.
(
(
(
(
(
(
(
(
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629, 1863-1870.
(
(
10) H a¨ berlen, M.; Glaser, J.; Meyer, H.-J. J. Solid State Chem. 2005, 178,
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132.
8188 Inorganic Chemistry, Vol. 45, No. 20, 2006
10.1021/ic0608952 CCC: $33.50
© 2006 American Chemical Society
Published on Web 09/07/2006

Syntheses and Properties of Rare Earth Carbodiimides
Compounds with [NCN]2 ions have been known much
-
high-yield synthesis of trivalent pseudo-binary rare earth
carbodiimides RE (CN , their crystal structures, and their
structural properties.
3-
longer than those with [NBN] ions. Examples of well-
2
2 3
)
20-23
known A
and alkali earth
2
(CN
2
) and AE(CN
2
) compounds include alkali
2
4-30
27
2
elements, with Ca(CN ) being presum-
Experimental Section
ably the most striking example, because of its applications,
for example, in organic synthesis and in soil treatments. Com-
pounds with analogous compositions to those of the alkali
or alkali earth elements are known for group 11 and 12
Synthesis. All manipulations for the synthesis of RE (CN ) were
2
2 3
performed in an Ar-filled glovebox (Braun LabMaster 130, M.
Braun GmbH) with commercial or synthesized starting materials.
SmCl was purchased (ABCR, 99.9%), and LuCl was synthesized
3
1-34
elements,
the Ca(CN
and recently, Mn(CN
2
) was reported to adopt
3
3
) structure.35
from Lu
2
O
3
(Rhone Poulenc, 99.99%) and NH
4
Cl (Merck, p.a.) as
2
43
described in the literature. The obtained LuCl
20-850 °C under a dynamic vacuum below 1 ×10 mbar. Li
CN ) was made from Li (CO ) (Alfa, ultrapure) under flowing
ammonia at 650 °C (12 h).
Reactions were performed with RECl
3
was sublimed at
The chemistry of rare earth carbodiimide compounds is
not yet well-established. The successful synthesis of RE
CN compounds with RE ) La and Ce was already
-3
8
2
-
2
-
(
2
2
3
(
2 3
)
2 3
reported long ago, departing from reactions of RE O with
3
2 2
and Li (CN ) in a 2:3
HCN.3 Products were, however, not well-characterized at
6,37
molar ratio together with KCl/LiCl as a flux. Mixtures with total
masses around 400 mg were carefully homogenized in an agate
mortar under argon. Each mixture was sealed into an evacuated
silica ampule and then placed into a tube furnace.
Reactions at lower temperatures (e.g., 500 °C) and with shorter
reaction times (e.g., 48 h) yielded light gray to yellow crystalline
powders, later identified as RE (CN ) for RE ) Sm and Lu,
2 2 3
according to the indexed powder X-ray diffraction (XRD) patterns.
When samples were treated at 650 °C for 3 weeks, some gray and
that time, and no structural or spectroscopic data are avail-
able. The structure of Eu(CN
single crystal, being obtained from a reaction mixture of EuN,
reacted at 1300 K.38 In addition, rare earth
C, and NaN
dicyanamides RE(N(CN) for RE ) La, Ce, Pr, Nd, Sm,
and Eu were recently synthesized from aqueous solutions.
2
) was recently refined from a
3
2 3
)
3
9
Attempts are being undertaken in our laboratory to syn-
thesize rare earth nitridocarbonate compounds by using a
similar synthetic approach to that employed earlier for the
development of the rare earth nitridoborates. Solid-state
brown powders were obtained, containing colorless crystals of RE
CN . The silica ampules were opened in the air, and the products
were washed twice with water and then with acetone to remove
the coproduced LiCl and the flux. Homologous RE (CN com-
2
-
(
2 3
)
metathesis reactions between rare earth trichloride and Li
CN ) have been already successfully applied to synthesize
members of the rare earth series RECl(CN
2
-
2
2 3
)
(
2
pounds for RE ) Y and Pr-Lu (except Pm and Eu) were synthe-
sized under analogous conditions and investigated by powder XRD.
X-ray Diffraction Studies. Single crystals of RE (CN ) with
4
0
2
), RE
2
Cl-
4
1
42
2
2 3
(
CN )N, and RE O(CN ) . Here, we report on a reliable
2 2 2 2
RE ) Sm and Lu were selected and mounted on the tips of glass
fibers for intensity measurements using a single-crystal X-ray
diffractometer (Stoe, IPDS, Darmstadt, Germany), equipped with
(graphite) monochromated Mo KR radiation (λ ) 0.71073 Å). The
intensity data were corrected for Lorentz, polarization, and absorp-
tion effects by the IPDS software X-Red/X-Shape. The crystal
(
20) Pulham, R. J.; Hubberstey, P.; Down, M. G.; Thunder, A. E. J. Nucl.
Mater. 1979, 85-86 (A), 299-303.
(
21) Down, M. G.; Haley, M. J.; Hubberstey, P.; Pulham, R. J.; Thunder,
A. E. J. Chem. Soc., Dalton Trans. 1978, 10, 1407-1411.
22) Harper, A.; Hubberstey, P. J. Chem. Res., Synop. 1989, 7, 194-195.
23) Becker, M.; Nuss, J.; Jansen, M. Z. Anorg. Allg. Chem. 2000, 626,
(
(
2505-2508.
structure solutions and refinements were obtained with the program
(
(
(
24) Dehlinger, U. Z. Kristallogr. 1927, 65, 286-290.
44
package SHELX-97. Crystals of Lu
2
2
(CN )
3
were obtained as
25) Bredig, M. A. J. Am. Chem. Soc. 1942, 64, 1730-1731.
26) Yamamoto, Y.; Kinoshita, K.; Tamaru, K.; Yamanaka, T. Bull. Chem.
Soc. Jpn. 1958, 31, 501-502.
racemic twins. Therefore, the structure of the Lu compound was
refined using the TWIN instruction with the matrix (-1 0 0 1 0
-1 0 1 0 0 -1). Anisotropic refinements were performed for
all atoms. Selected data of the crystal structures and refinement
parameters are given in Table 1. Atom positions and isotropic
displacement parameters are provided in Table 2, and selected
interatomic distances and angles are given in Table 3. More detailed
crystallographic data and the anisotropic displacement parameters
are provided in the Supporting Information.
(27) Vannerberg, N. G. Acta Chem. Scand. 1962, 16, 2263-2266.
(28) Berger, U.; Schnick, W. J. Alloys Compd. 1994, 206, 179-184.
(29) Reckeweg, O.; DiSalvo, F. J. Angew. Chem. 2000, 39, 412-414.
(30) Liao, W.; Dronskowski, R. Acta Crystallogr., Sect. E 2004, 60, 124-
126.
(
(
31) Bowden, F. P.; Montagu-Pollock, H. M. Nature 1961, 191, 556-559.
32) Becker, M.; Nuss, J.; Jansen, M. Z. Naturforsch. B 2000, 55, 383-
385.
(
(
(
(
(
(
(
(
(
(
33) Baldinozzi, G.; Malinowska, B.; Rakib, M.; Durand, G. J. Mater.
Chem. 2002, 12, 268-272.
The carefully washed RE
an X-ray powder diffractometer (Stoe, StadIP, Darmstadt, Germany)
using monochromatic Cu KR radiation (λ ) 1.54051 Å). The
powder XRD patterns of RE (CN with RE ) Sm and Lu were
2 2 3
(CN ) powders were inspected with
34) Becker, M.; Jansen, M. Acta Crystallogr., Sect. C 2001, 57, 347-
348.
35) Liu, X.; Krott, M.; M u¨ ller, P.; Hu, C.; Lueken, H.; Dronskowski, R.
Inorg. Chem. 2005, 44, 3001-3003.
36) Hartmann, H.; Eckelmann, W. Z. Anorg. Allg. Chem. 1948, 257, 183-
1
2
2 3
)
45
indexed with the aid of the program system WinXPow, and the
194.
lattice parameters were refined therefrom. The powder pattern of
37) Hartmann, H.; Dobek, G. Z. Anorg. Allg. Chem. 1953, 271, 138-
43.
38) Reckeweg, O.; DiSalvo, F. J. Z. Anorg. Allg. Chem. 2003, 629, 177-
79.
39) J u¨ rgens, B.; Irran, E.; Schnick, W. J. Solid State Chem. 2005, 178,
2-78.
40) Srinivasan, R.; Glaser, J.; Tragl, S.; Meyer, H.-J. Z. Anorg. Allg. Chem.
005, 631, 479-483.
41) Srinivasan, R.; Str o¨ bele, M.; Meyer, H.-J. Inorg. Chem. 2003, 42,
406-3411.
42) Srinivasan, R.; Tragl, S.; Meyer, H.-J. Z. Anorg. Allg. Chem. 2005,
31, 719-722.
1
Sm
monoclinic cell yielding a ) 14.578(2) Å, b ) 3.8978(5) Å, c )
.2781(7) Å, and â ) 95.883(8)°. The pattern of Lu (CN was
2 2 3
(CN ) was indexed (using 33 single indexed lines) with a
1
5
2
2 3
)
7
(43) Meyer, G.; Morss, L. R. Synthesis of Lanthanide and Actinide Com-
pounds; Kluwer Academic Publishers: Hannover, Argonne, 1990.
(44) Sheldrick, G. M. SHELX-97: Program Package for Crystal Structure
Determination; University of G o¨ ttingen: G o¨ ttingen, Germany, 1997.
(45) WinXPow, Version 1.10: Diffractometer Software; Stoe & Cie
GmbH: Darmstadt, Germany, 2001.
2
3
6
Inorganic Chemistry, Vol. 45, No. 20, 2006 8189

Neukirch et al.
Table 1. Crystal Data from the Structure Refinements of Sm2(CN2)3 and Lu2(CN2)3
formula
fw (g/mol)
Sm2(CN2)3
420.79
Lu2(CN2)3
470.03
syst, space group, Z
unit cell dimensions (Å, deg)
monoclinic,C2/m (No. 12), 2
a ) 14.534(2)
b ) 3.8880(8)
c ) 5.2691(9)
â ) 95.96(2)
V ) 296.1(1)
4.719
trigonal, R32 (No. 155), 3
a ) 6.2732(8)
c ) 14.681(2)
3
unit cell vol (Å )
V ) 500.3(1)
4.680
29.35
3
dcalcd (g/cm )
µ (mm 1) (Mo KR)
-
19.55
0.0267, 0.0667
a
R1, wR2 (all data)
0.0208, 0.0526
a
R1 ) ∑hkl||Fo| - |Fc|∑hkl|Fo||. wR2 ) ∑hklw(|Fo| - |Fc| ) /[∑hklw(Fo )]1/2.
2
2 2
2
Table 2. Atom Positions and Isotropic Displacement Parameters for
Sm2(CN2)3 and Lu2(CN2)3
Table 4. Lattice Parameters (in Å and deg), Unit Cell Volumes
(in Å ), and Number of Single Indexed Reflections of
3
RE2(CN2)3 Compounds
multiplicity,
symmetry
atom
x
y
z
Ueqa
indexed
reflns
RE
Y
a
b
c
â
V
Sm2(CN2)3
Sm
4, m
4, m
2, 2/m
4, m
4, m
4, m
0.1348(1)
0.3283(5)
0
0
0
0
0.0705(1) 0.0063(2)
14.278(9) 3.781(6)
5.209(3) 95.73(3) 279.8(5)
29
32
32
33
27
31
23
28
46
30
21
23
C(1)
C(2)
N(1)
N(2)
N(3)
Lu2(CN2)3
Lu
0.404(1)
/2
0.362(1)
0.009(1)
0.011(2)
0.010(1)
Pr 14.853(2) 3.9907(6) 5.3243(9) 96.125(8) 313.8(1)
Nd 14.754(2) 3.9563(5) 5.3084(6) 96.027(5) 308.15(9)
Sm 14.578(2) 3.8978(5) 5.2781(7) 95.883(8) 298.3(1)
Gd 14.453(2) 3.8472(5) 5.2586(6) 95.762(7) 290.92(8)
Tb 14.369(3) 3.8201(9) 5.243(1) 95.70(2) 286.4(2)
Dy 14.287(5) 3.7901(9) 5.224(1) 95.66(1) 281.5(2)
Ho 14.214(9) 3.766(2)
Er 14.184(2) 3.7471(3) 5.2051(5) 95.559(6) 275.34(7)
Tm 6.3393(7)
Yb 6.294(2)
Lu
1
1
0.1590(4) - /2
1
0.1901(4) - /2
-0.170(1) 0.009(1)
-0.266(1) 0.014(1)
0.118(5)
0
6, 3
9, 2
18, 1
0
0
0.1716(1) 0.0080(1)
5.210(3) 95.60(4) 277.5(4)
1
1
C
N
0.369(2)
0.338(1) 0.264(1) 0.255(5)
/3
/3
0.013(2)
0.018(1)
14.766(1)
14.723(3)
14.708(2)
513.9(1)
505.1(3)
501.1(1)
a
2
Ueq (Å ) is defined as one-third of the trace of the orthogonalized Uij
6.2723(8)
tensor.
the group of already known rare earth carbodiimides. The
Table 3. Selected Interatomic Distances (in Å) and Angles (in deg) in
the Crystal Structures of Sm2(CN2)3 and Lu2(CN2)3
40
41
carbodiimides RE
2
(CN
2 3 2 2 2
) , RECl(CN ), RE Cl(CN )N, and
4
2
2 2 2
RE O(CN )
were all obtained by the same general syn-
Sm2(CN2)3
Sm-N(3)
Lu2(CN2)3
thesis route, following eqs 1-4.
2.382(6) 1x
2.460(7) 1x
2.479(4) 2x
2.498(4) 2x
2.544(6) 1x
1.227(9) 1x
1.235(9) 1x
1.229(7) 2x
Lu-N
2.282(7) 3x
High purity of the starting materials, the absence of oxygen
or associated traces of water, and strict handling under a dry,
inert atmosphere turned out to be important for successful
Sm-N(3)
Sm-N(1)
Sm-N(2)
Sm-N(2)
C(1)-N(1)
C(1)-N(2)
C(2)-N(3)
Lu-N
C-N
2.325(6) 3x
1.216(7) 2x
and high-yield syntheses of RE
eq 1. Because of the stability of the produced RE
2
(CN
2
)
3
compounds following
(CN
2
2 3
)
against air and moisture, the coproduced LiCl and the flux
can be easily washed out with water.
N(3)-C(2)-N(3) 180.0
N(1)-C(1)-N(2) 176.2(8)
N-C-N
178.0(3)
2
RECl + 3Li (CN ) f RE (CN ) + 6LiCl
(1)
(2)
3
2
2
2
2 3
indexed trigonal-rhombohedrally (using 23 single indexed lines)
with lattice parameters of a ) 6.2723(8) Å and c ) 14.708(2) Å.
In each case, few weak lines of unknown phases were present and,
RECl + Li (CN ) f RECl(CN ) + 2LiCl
3
2
2
2
2 2 2 3
occasionally, very little Li O. Homologous RE (CN ) compounds
with RE ) Y and Pr-Lu (except Pm and Eu) were indexed corre-
spondingly with a monoclinic crystal system for Y and Pr-Er and
trigonal-rhombohedrally for Tm-Lu. All refined lattice parameters
are presented in Table 4.
2RECl
3
+ Li
3
N + Li
2
(CN
2
) f RE
2
Cl(CN
2
)N + 5LiCl (3)
RECl + REOCl + 2Li (CN ) f RE O(CN ) + 4LiCl (4)
3
2
2
2
2 2
Infrared Spectroscopy. The infrared spectra were recorded on
a Perkin-Elmer FT-IR spectrometer in a range from 400 to 4000
cm . The samples were measured as KBr pellets. Characteristic
The employment of a KCl/LiCl flux has been reported
previously for the synthesis of RECl(CN ) compounds (RE
La, Ce, and Pr) and usually allows for the obtainment of
better crystal growth and for reactions to proceed at lower
temperatures. According to our experiments and to dif-
ferential thermal analyses, reaction 1 can proceed at tem-
peratures below 400 °C in a KCl/LiCl melt.
Single-crystal structure refinements were performed for
2 2 3 2 2 3
Sm (CN ) (I) and Lu (CN ) (II). Both structures are closely
related by their nearly hexagonal [NCN] stick packing modes
2
-
1
)
2
-
-1
bands of the [NCN] ions were observed at 1954 cm (Sm νas
)
-
1
and 1967 cm (Lu νas). The C-N bending vibrations were found
at 706, 669, 635, and 616 cm for Sm (CN ) and at 681 and 641
2 2 3
-
1
-
1
cm for Lu
Results and Discussion
The solid-state metathesis reaction between RECl
CN ) was successfully applied for the synthesis of the novel
rare earth carbodiimides RE (CN . The discovery and
identification of these novel compounds allows us to extend
2 2 3
(CN ) .
3
2
and Li -
(
2
2
)
2 3
displayed in Figure 1. The monoclinic (C2/m) structure of I
3
+
2-
contains one layer of Sm ions and one layer of [NCN]
8190 Inorganic Chemistry, Vol. 45, No. 20, 2006

Syntheses and Properties of Rare Earth Carbodiimides
Figure 1. Top view of two distorted hexagonal stick packings of [NCN]2- ions (“sticks”) in the crystal structures of Sm2(CN2)3 (left) and Lu2(CN2)3
(
right). Corresponding unit cells are projected onto their ab planes. Nitrogen atoms of [NCN]2- ions are shown with blue color.
carbodiimide ions (Figure 5) with Lu-N distances ranging
between 2.28 and 2.33 Å (Table 3).
The C-N bond lengths within the [NCN]2 ions are nearly
equivalent in both of the refined crystal structures, leading
to the assignment of these anions to occur with the carbo-
diimide (NdCdN) rather than the cyanamide (N-CtN)
structure. Refined C-N distances of the two distinct [NCN]2
ions in I are at dC-N ) 1.227(9), 1.235(9), and 1.229(7) Å,
-
-
2
-
whereas for the [NCN] ion in II, they are at dC-N
)
1
.216(7) Å (Table 3). For comparison, the C-N distances
2-
of the [NCN] ion in La
2 2
Cl(CN )N amount to similar values
of dC-N ) 1.234(3) and 1.238(6) Å. A more pronounced
difference of these distances with values of dC-N ) 1.202(6)
and 1.251(6) Å was obtained in the [NCN]2 ion of
-
LaCl(CN
2
).
Slight deviations of the N-C-N angles in the RE
2
2 3
(CN )
Figure 2. Crystal structure of Sm2(CN2)3.
structures from linearity (see values of 176-180° in Table 3)
can be considered as matrix effects, as well-known from
many other structures containing triatomic anions.
ions within the c-axis repeat (Figure 2). The coordination
3
+
number of Sm with nitrogen atoms of the carbodiimide
After the single-crystal structures were solved for Sm
2
-
ions is seven (Figure 3), with Sm-N distances ranging from
(
CN I and Lu (CN II, corresponding syntheses were
2
)
3
2
2 3
)
2.38 to 2.54 Å (Table 3). The structure contains two different
performed in order to synthesize more RE compounds.
Inspections of their powder XRD patterns revealed that RE
Y, Pr, Nd, Sm, Gd, Tb, Dy, Ho, and Er are represented
by the structure of I, and the smallest RE ) Tm, Yb, and Lu
are represented by the structure of II, as listed in Table 4.
types of carbodiimide ions with their coordination environ-
3
+
ments by Sm displayed in Figure 3.
)
The trigonal-rhombohedral (R32) structure of II is repre-
3+
sented by single layers of Lu ions alternating with single
layers of [NCN]2 ions three times along the c-axis repeat
-
Both structures are composed of alternating layers of RE3
+
(Figure 4). The structure contains only one carbodiimide ion
3
+
2-
in the asymmetric unit, being surrounded by four Lu ions.
and [NCN] ions. The [NCN] layers can be considered as
The Lu3 ions are surrounded by six nitrogen atoms of the
+
being derived from a hexagonal closest packed layer of
Figure 3. Environments of Sm3+ with [NCN]2- ions (left) and of the two distinct [NCN]2- ions with Sm3+ ions (right).
Inorganic Chemistry, Vol. 45, No. 20, 2006 8191

Neukirch et al.
In a Gedankenexperiment, we derive a simplified structure
model in order to relate both refined crystal structures with
each other. The starting motif is an idealized hexagonal
closest packed layer of [NCN] sticks which is the prototype
of those presented in Figure 1. A primitive stacking sequence
of such layers creates trigonal-prismatic voids between the
layers which can be occupied with metal atoms. Occupations
of half of the trigonal-prismatic voids by metal atoms lead
2
to the composition “RE(CN )”, displayed in Figure 6 (center),
which merits the topology of a WC-type structure. Occupa-
1
tions of only /
allows the establishment of various structural possibilities
of RE (CN structures, depending on the ordering of metal
atoms on these sites. In our idealized structure of I, the Sm
3
of the trigonal-prismatic metal atom sites
2
2 3
)
1
3
ions are ordered over / of the voids within one layer of the
ab plane, whereas in the idealized structure of II, the ordering
of Lu ions alternates over three layers of the ab plane along
the (c-)stacking direction.
In the modeled structures, our metal atoms would have a
coordination number of six (Figure 6), which is rather low
for compounds of the rare earth elements, although known
from rock-salt structured REN compounds. Partial occupa-
tions of trigonal-prismatic voids do not only create super-
structures that are displayed in Figure 6 (left and right) but
can also explain the alignments or distortions of [NCN] ions
in the structures of I and II. However, the distinct distortions
being present in the structures of I and II do not only bring
up different coordination numbers for Sm (CN ) 7) and Lu
Figure 4. Crystal structure of Lu2(CN2)3.
(CN ) 6) but also create very strong differences in the molar
volumes of both structures. We note that opposite tilting of
the [NCN] sticks in the structure of II creates a significantly
larger ab plane than in I. Three [NCN] ions in the structure
of Lu
2 2 3
(CN ) are packed with each other to make up an
Figure 5. Environments of Lu with [NCN]2- ions (left) and of [NCN]2-
with Lu ions (right).
3
+
2
average ab area of 34.1 Å , which is about 21% larger than
the corresponding value of 28.1 Å obtained in Sm (CN ) .
2 2 3
3
+
2
[
NCN] sticks. In the refined crystal structures, we note
The larger ab area of II is not overcompensated when the
c lattice parameters are taken into account, for example, by
distortions. Most remarkable is the opposite tilting of the
2
-
3+
[NCN] ions in the structure of II, shown in Figure 4, and
the impact of the smaller Lu ion, and we note that the
3
deviations of layers from planarity in the structure of I,
volume of the Lu compound (V/Z ) 166.8 Å ) still remains
shown in Figure 2.
about 11% larger than that of the Sm compound (V/Z ) 148.1
Figure 6. Archetype structure of hexagonal primitive “RE(CN2)” (center). Atoms drawn in blue represent nitrogen atoms of the linear [NCN] units (top
1
view). Idealized structures with /3 of the metal positions (in red) being occupied are shown at left for Sm2(CN2)3 and at right for Lu2(CN2)3, which is
decomposed into three layers. All projections are shown onto ab planes. Unit cells are shown as dashed lines.
8192 Inorganic Chemistry, Vol. 45, No. 20, 2006

Syntheses and Properties of Rare Earth Carbodiimides
SmSI-type structure for RE ) Tm, Yb, and Lu. ErOCl is
reported to be dimorphic, occurring with both structures.46
The transition of ErOCl from the PbFCl type into the SmSI
type involves a volume increase of about 15%.
A projection of the volume per formula unit (V/Z) versus
the RE elements of the homologous RE
shown in Figure 7. The well-known trend of the lanthanide
contraction is evident for RE (CN compounds between
2 2 3
(CN ) series is
2
2 3
)
RE ) Pr and Er and between RE ) Tm and Lu. The pro-
nounced jump in volume of these compounds is related to
the structural change being obtained between RE ) Er and
Tm. Between these two structures, the volume increases
3
(
from V/Z ) 137.7 to 171.3 Å ) by a remarkable 24%. There-
fore, it may be assumed that the crystal structure of II repre-
sents the low-pressure modification, whereas the structure
of I represents the high-pressure modification of RE (CN )
2 2 3
compounds.
Figure 7. Volume per formula unit (V/Z) for various of RE2(CN2)3
compounds with RE ) Pr-Lu.
3
Å ). Considering the influence of the lanthanide contraction,
a smaller volume is usually regarded for the compound with
Acknowledgment. The authors gratefully acknowledge
the support of this research by the Deutsche Forschungs-
gemeinschaft (Bonn) within the framework of our research
project “Nitridocarbonate”.
3
+
the smaller RE ion, which would be Lu
case.
2 2 3
(CN ) (II) in this
The occurrence of more than one phase along a series of
homologous lanthanide compounds is not unexpected. Well-
known examples among many are rare earth sesquioxides
and oxide-halides. Generally, a volume change of this large
of an extent is quite seldom within a series of rare earth
compounds. One example is, however, the series of rare earth
compounds REOCl. These are reported to crystallize with
the PbFCl-type structure for RE ) La-Ho and with the
Supporting Information Available: Additional tables and
crystallographic data in CIF format. This material is available free
of charge via the Internet at http://pubs.acs.org.
IC0608952
(
46) (a) Beck, H. P. Z. Naturforsch. B 1976, 31, 1562-1564. (b) Beck, H.
P. Z. Naturforsch. B 1977, 32, 1015-1021.
Inorganic Chemistry, Vol. 45, No. 20, 2006 8193