Reassignment of the crystal space group of crystalline Pr[M(CN)6] · 5H2O (M = Cr, Fe, Co)

Article abstract of DOI:10.1016/j.poly.2007.05.002

Refinement of the X-ray crystal structures of Pr[M(CN)₆] · 5H₂O (M = Cr, Fe, Co) enables their space group to be reassigned to P6₃/mmc. Spectral characteristics are reported for M = Cr and the distinction between the penta

Full text of DOI:10.1016/j.poly.2007.05.002

Polyhedron 26 (2007) 4019–4023
www.elsevier.com/locate/poly
Reassignment of the crystal space group of
crystalline Pr[M(CN) ] Æ 5H O (M = Cr, Fe, Co)
6
2
a,b
c,d
b
d,
*
Lin-Ping Zhang , Xian-ju Zhou , Thomas C.W. Mak , Peter A. Tanner
a
Key Laboratory of Science and Technology of Eco-Textiles, Ministry of Education, College of Chemistry and Chemical Engineering,
Donghua University, Shanghai 200051, PR China
b
Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong S.A.R., PR China
c
Institute of Modern Physics, Chongqing University of Post and Telecommunications, Chongqing 400065, PR China
Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong S.A.R., PR China
d
Received 19 March 2007; accepted 1 May 2007
Available online 10 May 2007
Abstract
Refinement of the X-ray crystal structures of Pr[M(CN)
6 2 3
] Æ 5H O (M = Cr, Fe, Co) enables their space group to be reassigned to P6 /
mmc. Spectral characteristics are reported for M = Cr and the distinction between the pentahydrate and tetrahydrate series is clearly
made from assignments of the infrared spectra.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: Cyanide bridge; Lanthanide ion; Bimetallic; Crystal structure; Vibrational spectra
1
. Introduction
parameters of all Ln[Fe(CN) ] Æ nH O (n = 4, 5) com-
6 2
pounds according to the hexagonal model proposed by
Bailey et al. In 1976, the tetrahydrate and the pentahydrate
complexes were classified as hexagonal and orthorhombic,
respectively, by Hulliger et al. [12,13].
The lanthanide(III) hexacyanometalate(III) hydrates,
III
III
Ln [M (CN) ] Æ nH O (Ln = lanthanide; M = transition
6
2
metal; n = 4, 5) are precursors for perovskite-type oxides
which have a variety of applications such as chemical sen-
sors and catalysts [1–4]. Their crystal structures consist of
alternating cyanide-bridged MC and LnN O (y = 2, 3)
However, there was still a lack of agreement regarding
the crystal system of the tetrahydrates. In 1988, Mullica
et al. [14,15] reported that Sm[Co(CN) ] Æ 4H O belongs
6
6
y
6
2
polyhedra in an orthorhombic or hexagonal lattice,
depending on the degree of hydration at the lanthanide
center [5–7].
to the monoclinic space group P2 /m, just like Sm[Fe-
1
(CN) ] Æ 4H O. But in 1989, Petter et al. [16] stated that
6
2
both compounds belong to the orthorhombic space group
Cmcm. This dispute was settled through a careful reinvesti-
gation on the crystal structures, showing that both com-
pounds belong to the orthorhombic system in space
group Cmcm [17]. Finally in 1999, Wang et al. [18] studied
the properties of a series of lanthanide hexacyanofer-
rate(III) n-hydrates and reported that crystalline Ln[Fe-
(CN) ] Æ 4H O (Ln = Sm–Lu) are all orthorhombic with
In 1973, Bailey et al. [8] determined the crystal structure
of La[Fe(CN) ] Æ 5H O using single crystal X-ray diffrac-
6
2
tion analysis. They showed that it belongs to the hexagonal
space group P6 /m (no. 176) and that the nine-coordinate
3
lanthanum ion is located at a site of D3h symmetry. This
structure was confirmed by Kietaibl and Petter [9]. Bonnet
and Paris [10,11] subsequently reported the unit-cell
6
2
the same space group Cmcm.
Previous reports in the literature have assigned the pen-
*
tahydrate compounds to the hexagonal space group P6 /m
Corresponding author. Fax: +852 2788 7406.
3
E-mail address: bhtan@cityu.edu.hk (P.A. Tanner).
[8–13,18–21]. In 1996, Yukawa et al. [21] reported that
0
277-5387/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2007.05.002

4020
L.-P. Zhang et al. / Polyhedron 26 (2007) 4019–4023
Pr[Co(CN) ] Æ 5H O belongs to P6 /m just as for other pen-
tahydrates reported before. In the present study, we have
were subjected to anisotropic refinement. The aqua ligand
O1W and lattice water molecule O2W respectively occupy
6
2
3
reinvestigated the crystal structures of Pr[M(CN) ] Æ 5H O
sites of symmetry m and 3 in space group P6 /m, and mm
6
2
3
(
M = Cr, Fe, Co) and established a reassignment of the
and 3m in space group P6 /mmc. The hydrogen atoms,
3
space group to P6 /mmc (no. 194). The spectral character-
which are necessarily disordered when the oxygen atoms
occupy the latter three sites, cannot be reliably located in
the structure refinement.
3
istics of these compounds have been interpreted and a clear
spectral distinction is made between the tetrahydrate and
pentahydrate series.
3. Results and discussion
2
. Experimental
3.1. Crystal structures
PrCl Æ xH O was prepared from Pr O (Strem, 99.9%)
3
2
6
11
by repeated evaporation with concentrated HCl (AR
grade, Reidel-de-Haen). K M(CN) (M = Cr, Fe, Co)
Table 1 presents experimental and statistical summaries
for compounds Pr[M(CN) ] Æ 5H O (M = Co, Cr, Fe)
3
6
6
2
was purchased from Strem Chemicals.
refined in space groups P6 /mand P6 /mmc. The structural
3 3
parameters refined in both space groups show insignificant
differences, and consequently the space group of higher
2
.1. Syntheses
symmetry, namely P6 /mmc, should be assigned according
to convention. Interestingly, Pretsch et al. [7] reported that
3
III
III
Pr [M (CN) ] Æ 5H O (M = Cr, Fe, Co) were prepared
6
2
by slow evaporation of aqueous solutions of PrCl Æ xH O
when orthorhombic Cmcm Er[Co(CN) ] Æ 4H O loses all its
3
2
6
2
and K M(CN) containing stoichiometric amounts of
bound and unbound water molecules, it is transformed to
3
6
M:Pr. The crystals were dried by tissue and used
immediately.
hexagonal Er[Co(CN) ] which belongs to space group
6
P6 /mmc.
3
The crystal and molecular structure of Pr[M(CN) ] Æ
6
2
.2. Spectral measurements
5H O herein are almost identical to those reported for
2
La[Co(CN) ] Æ 5H O [19] and La[Fe(CN) ] Æ 5H O [8]. The
6
2
6
2
The UV–Visible absorption spectra of aqueous solutions
structural characteristics of Pr[Cr(CN) ] Æ 5H O are speci-
6 2
of the complexes were recorded using a HP UV-8453 spec-
trophotometer to measure the range between 200 and
fied by listing their Wyckoff positions and site symmetries:
ꢀ
ꢀ
Pr(1) in 2(c), 6m2; Cr(1) in 2(a), 3m; C(1) in 12(k), m; N(1)
in 12(k), m; O(1 W) in 6(h), mm; and O(2 W) in 4(f), 3m (see
1
100 nm in H O at 298 K. Room temperature and 10 K
2
3
+
luminescence spectra were excited by the 355 nm line of a
Continuum Nd-YAG laser. The emission was dispersed
through an Acton 0.5 m monochromator with an 1800
Fig. 1 for the atom labeling). The Pr ion is nine-coordi-
3
+
nated in the form of a PrN (H O) group; the Cr ion is
6
2
3
six-coordinated in the form of an octahedral CrC group;
6
ꢀ1
g mm grating blazed at 250 nm and detected by a back-
illuminated SpectruMM CCD detector. The sample was
housed in an Oxford Instruments closed cycle cryostat,
with a base temperature of 10 K. The emission from
Pr[Co(CN) ] Æ 5H O was very weak with two broad fea-
and cyanide linkages between PrN (H O) and CoC6
6
2
3
groups build an infinite polymeric array (see Fig. 2). Two
uncoordinated water molecules occupy zeolitic holes on a
3
+
6-fold rotatory-inversion axis above and below the Pr
3
+
ion. The Pr ion is coordinated to three aqua ligands in
the mirror plane and to the N atoms of six cyanides, with
three on each side of the mirror plane. The coordination
6
2
ꢀ
1
3+
tures at 16257, 16361 cm due to Pr emission, and a
ꢀ
1
lower energy band at 14488 cm . Infrared absorption
spectra were recorded at 295 K using KBr discs. A Per-
kin–Elmer 2000 FTIR spectrometer was employed with
3
+
geometry of Pr is thus approximately D . The uncoordi-
6
h
nated water molecule O(2W) is linked to the coordinated
water molecule O(1W) through a weak hydrogen bond,
as in Pr[Co(CN) ] Æ 5H O [21], La[Co(CN) ] Æ 5H O [19]
ꢀ
1
resolution 2 cm
.
6
2
6
2
2
.3. X-ray crystallography
and La[Fe(CN) ] Æ 5H O [8].
6 2
The bond lengths and angles in Pr[M(CN) ] Æ 5H O are
6
2
The X-ray intensity data of the three isomorphous com-
reported in Table 2. The C„N bond distances do not differ
plexes were collected on a Bruker SMART 1000 CCD dif-
from standard values [22]. The M–C bond distance is 8.5%
6
fractometer with graphite-monochromatized Mo Ka
shorter for the low-spin 3d ion compared with low-spin
3
˚
radiation (k = 0.71073 A) at 293 K. In each hkl file, reflec-
3d , and the trend with atomic number exhibits the linear
ꢀ
tions ðhh2hlÞwith l = 2n + 1 are systematically absent,
relation shown in Fig. 3. A similar linear regression, but
with a 7% more negative slope, is found for the M–C bond
distances in K M(CN) (M = Cr, Mn, Fe, Co [22]) where
indicating that the space group is P6 /mmc. In the scenario
3
that such reflections all happen to be too weak to be
3
6
observed, space group P6 /m is also possible though unli-
there is no cyanide bridging. The involvement of covalency
3
kely. The structure was solved by direct methods and
refined by full-matrix least squares based on F using the
SHELXTL program package. All the non-hydrogen atoms
in the M–C bond in Pr[M(CN) ] Æ 5H O is exhibited by its
strict directional character with the angle N–C–M being
linear.
6
2
2

L.-P. Zhang et al. / Polyhedron 26 (2007) 4019–4023
4021
Table 1
Experimental and statistical summaries for Pr[M(CN)
6 2
] Æ 5H O
Pr[Co(CN) ] Æ 5H
P6 /m
10CoN
446.04
hexagonal
6
2
O
Pr[Cr(CN)
P6 /m
10CrN
439.11
hexagonal
6
] Æ 5H
2
O
Pr[Fe(CN)
P6 /m
10FeN
442.96
hexagonal
6 2
] Æ 5H O
3
P6
3
/mmc
3
P6
3
/mmc
10CrN
3
P6
3
/mmc
Empirical formula
Formula weight
Crystal system
Space group
C
6
H
6
O
5
Pr
C
6
H
10CoN
O
6 5
Pr
C
6
H
6
5
O Pr
C
6
H
O
6 5
Pr
C
6
H
6
O
5
Pr
C
6
H
6 5
10FeN O Pr
446.04
hexagonal
439.11
hexagonal
442.96
hexagonal
P6 /mmc (no.194)
3
7.524(1)
7.524(1)
14.351(3)
90
90
120
P6
3
/m (no.176)
P6
3
/mmc (no.194)
P6
3
/m (no.176)
P6
3
/mmc (no.194)
3
P6 /m (no.176)
˚
a (A)
7.4748 (6)
7.4748(6)
14.220(3)
90
90
120
7.4748 (6)
7.4748(6)
14.220(3)
90
90
120
7.6734(5)
7.6734(5)
14.674(2)
90
90
120
7.6734(5)
7.6734(5)
14.674(2)
90
90
120
7.524(1)
7.524(1)
14.351(3)
90
90
120
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
˚
3
V (A )
688.1(1)
2
688.1(1)
2
748.2(1)
2
748.2(1)
2
703.5(2)
2
703.5(2)
2
Z
T (K)
D (mg m
293(2)
2.153
4.728
1953
293(2)
2.153
4.728
1849
293(2)
1.949
3.965
2018
293(2)
1.949
3.965
1900
293(2)
2.091
4.476
2298
293(2)
2.091
4.476
272
ꢀ3
)
ꢀ
1
l (mm
)
Total data
Unique data
699
425
759
458
242
240
Goodness-of-fit
1.083
0.0244
0.0619
0.0276
0.0634
1.104
0.0234
0.0618
0.0241
0.0620
1.134
0.0212
0.0528
0.0293
0.0557
1.173
0.0183
0.0445
0.0213
0.0456
1.102
0.0352
0.0972
0.0465
0.1042
1.061
0.0419
0.1178
0.0492
0.1235
a
1
R [I > 2r(I)]
b
wR
R
2
[I > 2r(I)]
(all data)
1
wR
a
2
(all data)
P
P
R
1
=
iF
o
jꢀjF
c
i/ jF
o
j.P
P
b
2
o
2
2
2
2 1=2
o
wR2 ¼½ ½wðF ꢀF Þꢂ= ½wðF Þꢂ
.
c
n
n
6 2
Fig. 2. Three-dimensional coordination network of Pr[Cr(CN) ] Æ 5H O
bridged by cyanide groups viewed along the a-axis. Uncoordinated water
molecules are omitted for clarity.
Fig. 1. Coordination environment of the metal atoms in
Pr[Cr(CN) ] Æ 5H O (50% thermal ellipsoids) with atom labeling.
6
2
In contrast, the C–N–Pr angle is bent, and the Pr-O dis-
tance is longer whereas the Pr–N distance is shorter for
M = Cr than for M = Co. However, these changes are
minor (<1.5%) and of similar magnitudes to measure-
ment errors. The La–O and La–N bond distances in
La[M(CN) ] Æ 5H O (M = Fe, Co) [8,19]; refined in space
Table 2
˚
Bond lengths (A) and angles (°) for Pr[M(CN)
6
] Æ 5H
2
O
6
2
M = Cr
M = Fe
M = Co
2.536(6)
2.577(4)
1.889(4)
1.145(6)
group P6 /m) are slightly longer (ꢁ2–3%) than those in
3
Pr(1)–O(1W)
Pr(1)–N(1)
M(1)–C(1)
C(1)–N(1)
2.563(4)
2.54 (1)
2.58 (1)
1.96 (1)
1.12 (2)
the corresponding Pr[M(CN) ] Æ 5H O compounds.
2.563(3)
2.061(3)
1.144(4)
6
2
3
.2. Infrared spectra
O(1W)–Pr(1)–N(1)
N(1)–C(1)–M(1)
C(1)–N(1)–Pr(1)
69.13(3)
179.1(3)
171.7(3)
69.1 (1)
179(1)
171(1)
69.13(4)
180.0(5)
170.6(4)
The vibrational mode of predominantly CN stretching
character in octahedral M(CN)6 ions in the solid state
3
ꢀ

4022
L.-P. Zhang et al. / Polyhedron 26 (2007) 4019–4023
2
2
2
1
1
1
.10
.05
.00
.95
.90
.85
hydrate but two prominent sharper bands for the tetrahy-
drate. The large breadth of this band in Pr[Cr(CN) ] Æ
6
d(M-C)=3.428-0.05686Z
2
5H O (Fig. 4) illustrates the large thermal amplitude of
(R
= 0.9966)
2
Cr
the coordinated aquo ligand. The water stretching region
ꢀ
1
(
3000–3610 cm ) is complex for both the tetrahydrate
and pentahydrate systems because bands due to both coor-
dinated and hydrogen-bonded water appear together with
vibrational overtones. The lowest energy band in Fig. 4
for the spectrum of Pr[Cr(CN) ] Æ 5H O is the Cr–C stretch
Fe
6
2
ꢀ
1
at 435 cm
.
Co
27
3.3. UV–Vis spectra of aqueous solutions of Pr[M(CN)6]
24
25
26
The UV–Vis absorption spectra of aqueous solutions of
Atomic Number, Z
Pr[M(CN) ] Æ 5H O are compared with K [Co(CN) ](aq) in
6
2
3
6
3
ꢀ
Fig. 5 and serve to show the presence of free M(CN)6
ions in solution. The sharp, weak features between
Fig. 3. Plot of M–C bond distance versus atomic number of M for
Pr[M(CN) ] Æ 5H O.
6
2
2
2
4
43 nm and 485 nm are due to 4f –f intra-configurational
3
+
transitions of Pr . Although the lowest energy band
(485 nm) clearly corresponds to the P
ꢀ
1
ꢀ1
3
3
is observed at 2142 cm
(M = Co) and at 2138 cm
H transition,
4
0
(
M = Cr) [22]. For Ln[M(CN) ] Æ nH O, members of the
the higher energy features are not clearly resolved and
too broad to permit specific assignments. The bands at
306, 379 nm for M = Cr occur at similar wavelengths
(308, 375 nm) to corresponding features in the room tem-
perature absorption spectrum of crystalline Cs KCr(CN)
6
6
2
n = 5 series easily transform at room temperature into
n = 4, but the two series are readily distinguished by the
characteristic water and CN stretching peaks in the IR
spectra, as shown for Pr[Cr(CN) ] Æ 5H O and Pr[Fe-
6
2
2
(
CN) ] Æ 4H O in Fig. 4, where the appropriate structures
[23]. These features have been assigned to spin-allowed
6
2
4
4
4
are also drawn. The inset in the figure shows the spectral
ligand field transitions corresponding to T , T
A ,
2g
1
g
2g
region of Pr[Cr(CN) ] Æ 5H O between 2100 and
respectively [23]. The much stronger bands at 262 and
ꢁ200–220 nm correspond to charge-transfer transitions.
The features at ꢁ257 and 307 nm in the spectrum of aque-
6
2
ꢀ
1
2
200 cm in detail. Note that only one strong CN stretch
1
2
14
ꢀ1
band is observed (corresponding to C N at 2156 cm
)
for the pentahydrate. From their positions and relative
ous Pr[Co(CN) ] are at similar wavelengths to the spectrum
6
ꢀ1
1
intensities, the weak features at 2112, 2125 cm in the
of aqueous K Co(CN) and correspond to T ,
T1g
3
6
2g
1
3
14
12 15
1
1
inset correspond to C N and C N stretching bands,
respectively. By contrast, two prominent CN stretching
bands are observed for the tetrahydrate system Pr[Fe-
A1g transitions, respectively [22]. The sharp
increase in absorbance at ꢁ200 nm corresponds to a charge
transfer transition. This band is at higher energy than in
3
ꢀ
(
CN) ] Æ 4H O. Furthermore, the water bending mode
Cr(CN)6
due to the greater cation polarizing power
6
2
ꢀ1
region (ꢁ1610 cm ) shows one broad band for the penta-
and shorter M–C bond distance.
2
156
2
125
Pr[Fe(CN)6].4H2O
Pr[Cr(CN)6].5H2O
2
112
5
00
1000
1500
2000
2500
3000
3500
4000
-1
Wavenumber (cm )
6 2 6 2
Fig. 4. FTIR spectra of Pr[Fe(CN) ] Æ 4H O and Pr[Cr(CN) ] Æ 5H O. The inset shows the expanded CN stretch region of the latter.

L.-P. Zhang et al. / Polyhedron 26 (2007) 4019–4023
4023
Acknowledgements
We thank Dr. S.K. Mak for technical help in the early
part of this study and the Hong Kong University Grants
Council for support under CERG Grant CityU 1115/02P.
PrCr(CN) (aq)
6
Appendix A. Supplementary material
CCDC 637207, 637208 and 645301 contain the supple-
mentary crystallographic data for this paper. These data
can be obtained free of charge via http://www.ccdc.cam.
ac.uk/conts/retrieving.html, or from the Cambridge Crys-
tallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@
ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at doi:10.
1016/j.poly.2007.05.002.
3P
,
I , P - 3H
1
3
2,1
6
0
4
2
00
400
600
Wavelength (nm)
References
[
[
[
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The space group P6 /m previously reported for the crys-
3
tal structure of Pr[Co(CN) ] Æ 5H O [21] has been revised
6
2
[18] X.Y. Wang, Y. Yukawa, Y. Masuda, J. Alloys Comp. 290 (1999) 85.
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3
(
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20] D.F. Mullica, W.O. Milligan, Acta Crystallogr., Sect. B 36 (1980)
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Cr and Fe compounds. The distinction between the
[
[
Ln[M(CN) ] Æ nH O tetrahydrate (n = 4) and pentahydrate
6
2
2
(
n = 5) series has been demonstrated on the basis of IR
spectral assignments. Our spectra for the pentahydrates
are in essential agreement with that reported for La[Co-
(
[22] A.G. Sharpe, The Chemistry of the Cyano Complexes of the
Transition Metals, Academic Press, London, 1976.
23] J. Hanuza, K. Hermanowicz, B. Jezowska-Trzebiatowska, A. Pie-
traszko, S.P. Feofilov, T.J. Maksimova, S.A. Basun, Bull. Pol. Acad.
Sci. Chem. 39 (1991) 331.
(
CN) ] Æ 5H O [19], but show differences from the spectra
6 2
[
reported for Ln[Fe(CN) ] Æ 5H O (Ln = La, Pr, Nd) where
6
2
some decomposition to the tetrahydrates may have
occurred [18]. Our IR spectrum of Pr[Fe(CN) ] Æ 4H O is
6
2
[24] M.C. Navarro, E.V. Pannunzio-Miner, S. Pagola, M.I. Gomez, R.E.
Carbonio, J. Solid State Chem. 178 (2005) 847.
similar to that reported for Nd[Fe(CN) ] Æ 4H O [24].
6
2