Synthesis and characterization of LnF(HF)(BF4)2 (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy), and crystal structures of LnF(HF)(BF4)2 (Ln = Pr, Nd) and La(BF4) 3

Article abstract of DOI:10.1002/zaac.200900123

Rare earth trifluorides (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy) react with boron trifluoride in anhydrous hydrogen fluoride (aHF) at room temperature. Products were found to be only sparingly soluble in aHF where the solubility decreases from lantha

Full text of DOI:10.1002/zaac.200900123

ARTICLE
DOI: 10.1002/zaac.200900123
Synthesis and Characterization of LnF(HF)(BF ) (Ln = La, Ce, Pr, Nd, Sm,
4
2
Eu, Gd, Tb, and Dy), and Crystal Structures of LnF(HF)(BF ) (Ln = Pr,
4
2
Nd) and La(BF )
4
3
[a]
[a]
[b]
[b]
Zoran Mazej,* Evgeny Goreshnik, Kohei Hironaka, Yasushi Katayama, and
[c]
Rika Hagiwara
Keywords: Rare earth compounds; Fluorine; Vibrational spectroscopy; Boron; Coordination chemistry
Abstract. Rare earth trifluorides (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, ces are significantly shorter [2.2489(6) Å for Pr and 2.234(1) Å for
–
Tb, and Dy) react with boron trifluoride in anhydrous hydrogen fluor�
ide (aHF) at room temperature. Products were found to be only spar�
Nd] than Ln–F(BF
4
) and Ln–F(HF) ones [Pr: 2.461(4)–2.528(5) Å,
, the lanthanum atoms are coor�
4 3
Nd : 2.446(7)–2.511(9) Å]. In La(BF )
ingly soluble in aHF where the solubility decreases from lanthanum to dinated by nine fluorine atoms with seven shorter La–F distances
dysprosium. After an excess of BF and aHF were pumped away at [2.410(2)–2.485(2) Å] and two elongated ones [2.600(2) Å]. The vi�
ambient temperature, LnF(HF)(BF
lated. Single crystals of LnF(HF)(BF
were grown for 3–5 months from solutions of LnF
Nd in aHF under pressure of BF (4 bar). Structures consist from modes are observed indicating rather covalent character of the bonding
the zig�zag [–Ln–F–Ln–] (Ln = Pr, Nd) chains connected via BF units between LnF and BF
and HF molecules into tridimensional network. The Ln–F(Ln) distan�
3
4
)
2
type of compounds were iso�
(Ln = Pr, Nd) and La(BF
brational spectra of isolated rare earth tetrafluoroborates show strong
–
4
)
2
)
4 3
deviations of the [BF
4
] anions from ideal T
d
symmetry. Because of the
3
(Ln = La, Pr) and strong cation�anion interactions, the splittings of the anion vibrational
O
2 3
3
4
3
3
.
1
. Introduction
mation about the composition of the products isolated from
reactions of LnF (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, and
3
Early attempts to use the BF /aHF medium system as reac�
3
Dy) with BF in aHF as a solvent. In this work the synthesis
3
tion medium with rare earth (III) fluorides go back to 1957
and the infrared spectra of LnF(HF)(BF ) (Ln = La–Dy), and
4
2
[1]. In their qualitative study authors said that rare earth fluor�
crystal structures of LnF(HF)(BF4)2 (Ln = Pr, Nd) and
La(BF ) are reported.
oborates were prepared by “suspending the simple fluorides in
ethereal boron fluoride”. They also claimed that these salts are
insoluble in anhydrous hydrogen fluoride (aHF). On the con�
trary, other authors [2] claimed for better solubility of praseo�
4
3
2. Experimental Section
dymium(III) and neodymium(III) fluoroborates and electronic 2.1. Apparatus and Reagents
spectra of solutions containing these rare earth ions were meas�
ured. The preparation of solid LnF ·nBF was reported first in
Volatile materials (anhydrous HF, BF
uum line and an all PTFE vacuum system equipped with PTFE valves
991 and later in 1995 [3, 4]. Although the extensive research as described previously [8]. The manipulation of the non�volatile mate�
3
) were handled in a nickel vac�
3
3
1
of LnF /BF /aHF system has been done by different investiga� rials was done in a dry�box (M. Braun, Germany or Miwa Seisaku�
3
3
sho, Japan). The residual water in the atmosphere within the dry�box
M. Braun) never exceeded 2 ppm. The reactions were carried out in
tors [5–7], the proper composition of isolated products stayed
(
unknown. The reported molar ratios LnF :BF of isolated sol�
3
3
FEP (tetrafluoroethylene�hexafluoropropylene copolymer) reaction
vessels (V = 35 mL) equipped with PTFE valves [9] and PTFE coated
stirring bars. For some syntheses smaller reaction vessels (V = 14 mL)
were used. They were equipped with stainless steel valves (Whitey,
SS�1KS4, with Kel�F tips) connected to them by reducing union with
Swagelok PTFE fittings. The maximum working pressure in smaller
reaction vessels was always less than 4 bar. Prior to their use all reac�
tion vessels were passivated with elemental fluorine (Solvay Fluor and
Derivate GmbH, Germany). Rare earth trifluorides purchased from
Johnson Matthey GmbH, Alfa Products, 99.9% (REO) or Wako Chem�
ids were approximate equal to 1:2.
The preparation of single crystals and structure determina�
tion of LnF(HF)(BF ) (Ln = Pr, Nd) provided the key infor�
4
2
*
Dr. Z. Mazej
E�Mail: zoran.mazej@ijs.si
[
[
[
a] Department of Inorganic Chemistry and Technology
Jožef Stefan Institute
Ljubljana, Slovenia
b] Graduate School of Engineering
Kyoto University
Kyoto, Japan
icals (99.5%) and BF
pon Sanso, 99.8%) were used as supplied. Some LnF
3
(Union Carbide Austria, GmbH, 99.5% or Nip�
were prepared
c] Graduate School of Energy Science
Kyoto University
3
by the reaction of aHF with their trichloride hexahydrates (Wako
Chemicals, 99.5%). The hexahydrates were first carefully dehydrated
under vacuum at lower temperatures less than 200 °C and then heated
Kyoto, Japan
Supporting information for this article is available on the WWW
under www.zaac.wiley�vch.de or from the author.
Z. Anorg. Allg. Chem. 2009, 635, 2309–2315
© 2009 Wiley�VCH Verlag GmbH & Co. KGaA, Weinheim
2309

Z. Mazej et al.
ARTICLE
Table 1. Reaction conditions for the synthesis of LnF(HF)(BF
)
4 2
(Ln = La–Dy).
Starting materials
V(HF)
V(reaction ves� p(BF
sel)
3
)
Reaction time Mass balance
found
calcd.
/g
LnF
LaF
CeF
3
/mmol
/g
/ml
/ml
/bar
/d
/g
b)
3
0.72
2.49
2.90
0.60
1.03
1.35
2.30
1.26
3.10
0.57
0.95
5.58
4.51
0.60
1.21
4,09
3.67
0.141
0.491
0.574
0.119
0.203
0.268
0.463
0.254
0.643
0.117
0.197
1.166
0.966
0.127
0.259
0.883
0.806
13
6
35
35
35
35
14
14
35
14
35
35
14
35
35
35
14
35
35
4
1
10
10
/
0.253
0.878
1.026
0.212
0.364
0.477
0.820
0.449
1.124
0.207
0.447
2.034
1.666
0.222
0.447
1.520
1.378
3
4
0.938
1.067
/
PrF
PrF
PrF
PrF
3
3.5
7
2
b)
3
5
4
b)
3
3
2
<4
<4
4
3
0.364
0.458
0.809
0.460
1.042
/
b)
2
5
NdF
3
3
3
10
a)
b)
NdF
2
<4
4.5
8
2
SmF
SmF
SmF
3
3.5
14
2
10
b)
3
3
2
a)
<4
4
1
7
3
0.444
1.974
1.599
/
0.444
1.502
1.401
EuF
GdF
GdF
GdF
3
8
8
3
4
9
b)
3
3
14
2
3
a)
<4
11
4
1
TbF
DyF
3
10
10
100
108
3
a) Prepared from the corresponding trichloride. b) Clear solution was obtained.
up to ~300 °C with intermittent treatment by dry HCl to convert con�
taminated LnOCl to LnCl . Anhydrous HF (Fluka, Purum) was treated box. aHF (10 mL) was condensed onto the starting material at 77 K.
with K NiF (Ozark Mahoning) for several hours prior to use. Afterwards, the reaction mixtures were brought up to room tempera�
ture and BF (~ 4 bar) was added till clear solution was obtained. After
Pr) (0.5 mmol) was loaded into the wider of the two tubes in a dry�
3
2
6
3
2
.2. Techniques
one day, the clear solution was transferred into the narrower tube. The
evaporation of the solvent from these solutions was carried out by
maintaining a temperature gradient corresponding to about less then
–1
The infrared spectra, with a resolution of 4 cm , of powdered samples
sandwiched between AgCl windows in a leak tight brass�cell were
recorded (10 scans) with a Perkin–Elmer FTIR 1710 spectrometer. Ra�
man spectra with a resolution of 2 cm were recorded on samples in
sealed quartz capillaries (10–20 scans) with a Renishaw Raman Imag�
ing Microscope System 1000, with the 632.8 nm exciting line of a He�
Ne laser.
1
0 K between both tubes for 4–5 months. The effect of this treatment
was to enable aHF and the excess of BF to be slowly evaporated from
a narrower into a wider tube leaving colorless crystals of La(BF
and green crystals of PrF(HF)(BF . Single violet crystals of
NdF(HF)(BF were grown from the solution, obtained by reacting of
Nd with BF in aHF and slowly evaporated for 3 months.
–1
3
4 3
)
4 2
)
4 2
)
2
O
3
3
X�ray powder diffraction patterns were obtained using the Debye–
Scherrer technique with Ni�filtered Cu�K radiation. Samples were
α
loaded into quartz capillaries (0.3 mm) in a dry�box. Intensities were
estimated visually.
The grown crystals were very small and were of block (La) or thin
plate shape (Pr, Nd). Single crystals were immersed in perfluorinated
oil (ABCR, FO5960) in the dry�box, selected under the microscope
and afterwards transferred into the cold nitrogen stream of the diffrac�
4 2 4 3
tometer. Some crystals of LnF(HF)(BF ) (Ln = Pr, Nd) and La(BF )
were selected in the dry�box and placed inside 0.3 mm quartz capilla�
ries and heat�sealed. Raman spectra could be obtained only for
2
.3 Synthesis of LnF(HF)(BF4)2
LnF
aHF was condensed onto LnF
tion vessel was warmed up and BF
reaction vessel reaches 4 bar. To get a higher pressure of BF
appropriate amount of the latter was condensed into reaction vessel
at 77 K. The stirring of reaction mixture was carried out at ambient
temperature. The supernatant solution above an insoluble solid has
been only slightly colored, typical for particular Ln3+ cation, thus
3
(Ln = La–Dy) was loaded in a dry�box into reaction vessel and
at 77 K on vacuum system. The reac�
was added till the pressure in
, the
3
La(BF
LnF(HF)(BF
fluorescence of the measured samples.
4 3
) . Attempts to get Raman spectra of single crystals of
3
4 2
)
(Ln = Pr, Nd) were unsuccessful due to the strong
3
2.5. Crystal Structure Determination.
showing a low solubility of LnF
tions were obtained only if small amounts of LnF
3
in the aHF/BF
3
system. Clear solu�
Data were collected on a Rigaku AFC7 diffractometer with Mercury
3
were used. After CCD area detector using graphite monochromated Mo�K radiation at
α
excess of BF
LnF(HF)(BF
conditions are given in Table 1.
3
and aHF were pumped away at room temperature 200 K. The data were corrected for Lorentz and polarization effects.
4
)
2
(Ln = La–Dy) were isolated. The details of reaction A multi�scan absorption correction was applied to all data sets. Both
structures were solved by direct methods using SIR�92 [10] and refined
with SHELXL�97 [11] software, implemented in program package
WinGX [12]. In the structure of PrF(HF)(BF ) the position of hydro�
4 2
gen atom was found in a difference Fourier map. Structure drawings
were generated by Balls & Sticks, freely available software [13].
2
.4. Preparation of Single Crystals of LnF(HF)(BF ) (Ln =
4 2
Pr, Nd) and La(BF4)3
Preparations were carried out in a double T�shaped apparatus consist�
ing of two FEP tubes (19 mm. o.d., and 6 mm. o.d.). LnF
3
(Ln = La, X�ray crystallographic files in CIF format have been sent to Fach�
2310
www.zaac.wiley�vch.de
© 2009 Wiley�VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2009, 2309–2315

4 2 4 2 4 3
PrF(HF)(BF ) , NdF(HF)(BF ) and La(BF )
informationzentrum Karlsruhe, Abt. PROKA, 76344 Eggenstein� reacted LnF . This is especially evident when larger amounts
3
Leopoldshafen, Germany, as supplementary material CDS�420466
of LnF (Ln = Sm, Eu, Gd) (3–6 mmol) were used. The fact
3
[
[
PrF(HF)(BF
4 2 4 2
) ], CDS�420467 [NdF(HF)(BF ) ], CDS�420465
that the experimental mass is larger than the calculated one
La(BF ], and can be obtained by contacting FIZ (quoting the article
4 3
)
(
Ln = Ce, Pr), suggests that the sample might contain some
details and corresponding SUP number). Crystal and structure refine�
ment data are given in Table 2.
Ln(BF ) . However, so far we could not get any evidence
4
3
about the existence of other Ln(BF ) (Ln = Ce–Dy) phases.
4
3
The growth of single crystals of La(BF ) confirmed the exis�
4
3
Table 2. Summary of crystal data and refinement results for
tence of other phases in the LnF /BF /aHF system as already
LnF(HF)(BF
)
4 2
(Ln = Pr, Nd) and La(BF
4 3
) .
3
3
proposed previously [6, 7]. The preparation of larger amounts
of LnF(HF)(BF ) requires much longer reaction time. The for�
PrF(HF)(BF
monoclinic
)
4 2
NdF(HF)(BF
monoclinic
)
4 2
4 3
La(BF )
4
2
crystal system
space group
a /Å
triclinic
P3c1
mation of rare earth(III) tetrafluoroborates in aHF seems to be
governed not only by thermodynamic but also by kinetic fac�
tors. During this work, it was found that the starting LnF pre�
P2
1
/m
P2
1
/m
7.203(3)
5.886(2)
8.811(3)
113.62(2)
342.2(2)
2
7.162(7)
5.847(5)
8.779(8)
113.600(13)
336.9(5)
2
10.9941(4)
10.9941(4)
11.2215(5)
�
b /Å
3
c /Å
pared from corresponding trichlorides are more reactive than
commercially supplied ones. The difference might be ex�
plained by the kinetic reason caused by the difference in the
o
β /
3
V /Å
1174.63(8)
6
Z
–1
crystallite sizes of LnF prepared from trichlorides and of com�
3
mol wt /g·mol
353.54
3.43
356.87
3.518
399.34
3.387
–3
mercially supplied LnF . The line widths in the X�ray powder
ρ
calcd. /g·cm
3
T /K
200
200
200
patterns of LnF prepared from trichlorides were broader than
3
–
1
µ /mm
7.263
7.853
5.636
those in the X�ray powder patterns of commercially supplied
trifluorides.
a)
R
1
0.036
0.054
0.024
b)
wR
GOOF
2
0.084
0.142
0.053
c)
1.169
1.095
1.112
a) R1 = ΣÀÀFoÀ – ÀFcÀÀ/ΣÀFoÀ for I > 2σ(I). b) wR2 = [Σ(w(F
Σ(w(F 2)2]1/2 for I > 2σ(I). c) GOF = [Σw(F 2 – F 2)2) / No – Np)]1/
, in which No = no. of reflns and Np = no. of refined parameters.
o c
2 – F 2)2/
o
o
c
3
.2. Crystal Structures of LnF(HF)(BF ) (Ln = Pr, Nd)
4 2
2
and La(BF4)3
Single crystals of LnF(HF)(BF4)2 (Ln = Pr, Nd) and
La(BF ) were prepared by slow evaporation of volatiles from
4
3
Supporting Information (see footnote on the first page of this arti�
cle): X�ray powder diffraction data of LnF(HF)(BF (Ln = La, Ce,
Pr, Nd, Sm, Eu, Gd, Tb and Dy) and additional figures of crystal
structures of LnF(HF)(BF (Ln = Pr, Nd) and La(BF
saturated solutions of corresponding LnF (Ln = La, Pr) or
3
4 2
)
Nd O in aHF acidified with excess of BF . The crystallization
2
3
3
was an extremely slow process (3–5 months). Attempts to
speed the crystallization (for example, higher temperature gra�
dient) resulted in immediate precipitation of powdered non�
crystalline products.
)
4 2
4 3
) .
3. Results and Discussion
LnF(HF)(BF ) (Ln = Pr, Nd) crystallizes in the monoclinic
4
2
3.1. Synthesis
space group P2 /m (No. 11) with two formula units in the unit
1
Previous reports about the solubility of LnF in anhydrous cell. The basic motif of the structure is an infinite zig�zag chain
3
hydrogen fluoride (aHF) acidified with BF are contradictory. (oriented along [010] direction) comprised of lanthanide atoms
3
Meanwhile some authors claimed that these salts were insolu� connected by fluorine (F2) bridges located at a center of sym�
ble in anhydrous hydrogen fluoride (aHF) [1], others claimed metry (Figure 1).
for better solubility of praseodymium(III) and neodymium(III)
fluoroborates [2]. In this study it was found that LnF is only
3
sparingly soluble in aHF acidified with BF , where the solubil�
3
ity decreases from lighter to heavier rare earth metals. Remov�
ing of the solvent (aHF) and an excess of BF yielded the
3
3+
solids with colors typical for the particular Ln cation. As
found by gravimetric measurements (Table 1), vibrational
spectroscopy (Paragraph 3.3.) and by comparison of X�ray
powder diffraction data of the isolated products with X�ray
powder diffraction data calculated from the crystal structure of
PrF(HF)(BF ) (Paragraph 3.4.), the composition of the pro�
4
2
Figure 1. Infinite [–Ln–F–Ln–] zig�zag chain in the crystal structure
ducts isolated after reaction of LnF with BF in aHF corre� of LnF(HF)(BF ) (Ln = Pr, Nd).
3
3
4 2
sponds to LnF(HF)(BF ) (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd,
4
2
Tb and Dy). The mass balances shown in Table 1, indicate
some variations. An experimental mass smaller than the calcu�
The coordination of each lanthanide atom is further extended
lated one implies that the reactions have not been completed by six BF units and completed by one HF molecule. Rare
4
and that the final products could contain small amounts of un� earth atoms are nonacoordinated with fluorine atoms 2 × F2,
Z. Anorg. Allg. Chem. 2009, 2309–2315
© 2009 Wiley�VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley�vch.de
2311

Z. Mazej et al.
ARTICLE
2
× F11, 2 × F21 at the apexes of slightly distorted trigonal and 1.388(13)–1.396(18) Å for Nd. The fourth fluorine atom
prism (Figure 2). Three additional ligands F1, F12 and F23 are (F22) of B(2)F unit is involved in F22···H–F1 hydrogen bond.
4
capping rectangular faces of the prism.
With the distances F22···F1 = 2.475 Å for Pr and 2.483 Å for
Nd, and the angle F22–H···F1 at 178.6 °, the hydrogen bonding
has to be quite strong [19] as these values are very similar to
those in crystalline HF [F–H···F distance = 2.49(1) Å; F–H···F
angle ~ 120.1 °] [20]. The corresponding B–F22 bond length
is 1.359(11) Å for Pr and 1.37(2) Å for Nd. In the case of
second B(1)F4 unit, three bridging fluorine atoms (Fb
=
2
× F11, F12) provide common apexes with three PrF polyhe�
9
dra. The fourth fluorine atom (F13) appears to be terminal (Ft),
with markedly shortened B–F bond length [1.346(13) Å for
t
Pr, 1.342(18) Å for Nd] in comparison with B–Fb ones
[
1.404(11)–1.414(7) Å for Pr and 1.40(2)–1.409(12) Å for
Figure 2. Coordination of Ln in LnF(HF)(BF
)
4 2
(Ln = Pr, Nd).
Nd]. In both structures BF
48 to 176 °.
4
units, the B–F–Ln angles vary from
1
The reaction products of LnF (Ln = La, Ce, Pr, Nd, Sm, Eu,
3
Further association of the [–Ln–F–Ln–] zig�zag chains by
Gd, Tb and Dy) with BF in aHF have similar X�ray powder
3
BF units and HF molecules results in the formation of a 3D
network (Figure 3).
4
diffraction patterns (see Supporting Information), which match
the X�ray powder diffraction data calculated from the crystal
structure of LnF(HF)(BF ) (Ln = Pr, Nd). On the basis of this
4
2
data (and supported by infrared data, Paragraph 3.3) it could be
concluded that their composition also corresponds to general
formula LnF(HF)(BF ) . The small differences in the X�ray
4
2
powder patterns of the LnF(HF)(BF ) (Ln = La–Dy) are prob�
4
2
ably not the consequence of the differences in their crystal
structures, but of the fact that rare earth(III) tetrafluoroborates,
when isolated from aHF solution, were sometimes of low crys�
tallinity and consequently yielded poor X�ray powder diffrac�
tion patterns.
La(BF ) crystallizes in the trigonal space group P3c1 (No.
4
3
1
65) with six formula units in the unit cell. The lanthanum
cations are nonacoordinate with the fluorine ligands of nine
BF units. The coordination of fluorine atoms around lantha�
4
num is irregular and it could be hardly described with idealized
structures (tricapped trigonal prismatic array or square anti�
prism capped on one rectangular face) usual for coordination
number nine (Figure 4).
Figure 3. Arrangement of the [–Ln–F–Ln–] zig�zag chains, connected
by BF
4
units and HF molecules, in the crystal structure of
LnF(HF)(BF ) (Ln = Pr, Nd).
4 2
The Ln–F2 distance (Figure 2) is considerably shorter [Pr:
.2489(6) Å, Nd: 2.234(2) Å] than all other Ln–F distances
2
within this structure [i.e. Ln–FBF and Ln–FH distances are in
3
the range 2.461(4)–2.528(5) Å for Pr and 2.446(7)–2.511(9) Å
for Nd compound]. The Ln–F2 distance is also shorter than
Ln–F distances in LnF (Ln = Pr, Nd) [14–16]. On the other
3
hand it is similar to Ln–F distances in other compounds, in
which Ln–F–Ln bridges are also present (i.e. Ln F(AuF ) :
2
4 5
2
.283 Å for Pr and 2.260 Å for Nd [17]; PrFPtF , 2.245 and
6
2
.296 Å [18]).
Two crystallographically independent BF4 units differ on
their coordination behavior (Figure S1 in Supplementary Mate�
rial). Three fluorine atoms (2 × F21, F23) belonging to B(2)F4
unit are bound to three lanthanide (Ln = Pr, Nd) atoms with
B–F bond lengths in the range 1.385(12)–1.411(8) Å for Pr Figure 4. Coordination of lanthanum in La(BF
4 3
) .
2312 www.zaac.wiley�vch.de © 2009 Wiley�VCH Verlag GmbH & Co. KGaA, Weinheim Z. Anorg. Allg. Chem. 2009, 2309–2315

4 2 4 2 4 3
PrF(HF)(BF ) , NdF(HF)(BF ) and La(BF )
Seven La–F distances form rather narrow set of 2.409(2)–
.485(2) Å, and the other two (F23) are markedly elongated to
.600(2) Å. For comparison, in (H O) La F(AsF ) and
2
2
3
8
2
6 13
(
H O) La F(AsF ) (AsF ) , the lanthanum atoms are nonaco�
3 3 2 3 2 6 9
ordinate in a tricapped trigonal prismatic array with La–F bond
lengths from 2.2203(7) to 2.727(7) Å and 2.2236(7) to
2
.740(6) Å, respectively [8].
Each of three crystallographically independent BF units is
4
coordinated by three lanthanum atoms (Figure 5 and Figure
S2 in Supporting Information). Similar to the LnF(HF)(BF4)2
structures, the B–F distances differ strongly from B–F ones:
t
b
1
.333(2) Å for F and 1.391(3) Å for F in B(1)F ; 1.343(6) Å
t b 4
for F and 1.399(5)–1.415(5) Å for F in B(2)F , respectively.
t
b
4
–
The B(3)F4 anion exhibits an interesting orientational disor�
der: the boron (B3) and the terminal fluorine atoms (F31) alter�
nate their orientations above and below the plane containing
three bridging F32 atoms (Figure 5). The corresponding B3–
F31 and B3–F32 distances are 1.35(3) Å and 1.460(14) Å,
respectively.
4 3
Figure 6. Network in the crystal structure of La(BF ) .
Figure 5. Orientational disorder of the boron and terminal fluorine
atom above and below the plane containing three lanthanum and three
bridging fluorine atoms.
The BF units share apexes with LaF polyhedra forming a
4
9
3
D network (Figure 6).
3.3. Vibrational Spectroscopy
The infrared spectra of LnF(HF)(BF ) are assigned exclu�
4
2
–
sively to the vibrations of the deformed [BF ] anion (Table 3,
4
Figure 7). The infrared spectra show extensive splittings of ν3
(
[
degenerate stretching F in Td symmetry of undeformed
2
–
BF ] ) and of ν (degenerate bending, F ) vibrations. Similar
4
4
2
–
splittings have been observed in a number of [BF ] salts with
4
alkali cations. For instance, there are six components of ν in
3
infrared spectrum of KBF . This was accounted for by isotopic
4
shift 10B: B and the total removal of degeneracy by the inter�
11
–
action of [BF ] with crystal sites of lower symmetry [21]. The
4
possibility of Fermi resonance between ν and 2ν was also
3
4
mentioned, and the infrared forbidden symmetrical breathing
4 2
Figure 7. Infrared spectra of LnF(HF)(BF ) : a) La; b) Nd; c) Dy.
mode, ν , appeared weakly. The infrared spectra of
1
LnF(HF)(BF ) show the broad and very strong infrared com�
4
2
–1
ponents of ν . The overall splitting is approximately 300 cm .
3
The splitting of ν mode would be explained by the strong
In the Raman spectrum of La(BF ) (Figure 8) the most in�
4 3
3
–
1
interaction of the fluorine atoms in BF units with lanthanide tensive band at 805 cm with two additional ones at 787 and
4
–1
atoms. The ν is weak and ν multiply split. The former has an 767 cm could be assigned to symmetrical breathing mode
1
4
additional shoulder. Unfortunately, we were not able to get ν . Degenerate stretching ν is weak in Raman and it has two
1
3
–1
Raman spectra of LnF(HF)(BF ) compounds due to the strong components (1006 and 943 cm ). The degenerate bending ν
4
4
2
–1
fluorescence.
shows three distinct components (532, 521 and 512 cm ) and
Z. Anorg. Allg. Chem. 2009, 2309–2315
© 2009 Wiley�VCH Verlag GmbH & Co. KGaA, Weinheim www.zaac.wiley�vch.de 2313

Z. Mazej et al.
Assig.b)
ARTICLE
4 3 4 2
Table 3. Raman spectrum of La(BF ) and infrared spectra of LnF(HF)(BF )
(Ln = La, Ce, Pr, Nd, Eu, Gd, Tb, Dy)a).
La(BF )
4 3
4 2
LnF(HF)(BF )
La
IR
Ce
IR
Pr
Nd
IR
Eu
IR
Gd
IR
Tb
IR
Dy
IR
R
IR
1325(sh)
1300(w)
1240(m)
1318(w)
1294(w)
1240(m)
ν
ν
ν
1
+ ν
1
+ ν
1
+ ν
4
4
4
1300(w)
1239(m)
1297(w)
1241(m)
1300(w)
1240(m)
1299(w)
1246(m)
1300(w)
1242(sh)
1301(w)
1244(sh)
1
190(s)
1191(s)
1104(s)
1194(s)
1067(s)
1197(s)
1101(s)
1028(sh)
1005(vs)
941(s)
1200(s)
1107(s)
1206(s)
1112(s)
1200(s)
1113(s)
1202(s)
1112(s)
ν
ν
3
3
1
101(s)
1
032(sh)
1
006(1)
43(18)
1005(vs)
942(s)
1009(vs)
941(s)
1001(vs)
938(s)
994(vs)
929(s)
995(vs)
930(s)
991(vs)
1000(s)
927(s)
ν
ν
3
3
9
805(100)
787(20)
767(9)
ν
ν
ν
1
1
1
790(sh)
775(w)
790(sh)
776(m)
769(m)
775(m)
770(m)
776(m)
780(m)
770(m)
5
97(m)
599(m)
549(w)
598(w)
598(m)
554(w)
543(sh)
524(w)
517(w)
492(m)
590(m)
555(w)
590(w)
560(w)
591(w)
560(w)
591(w)
560(w)
ν
ν
ν
ν
ν
ν
ν
4
4
4
4
4
4
4
541(sh)
532(33)
521(34)
512(33)
547(w)
554(vw)
543(vw)
524(vw)
517(vw)
488(w)
518(vw)
518(vw)
479(m)
520(w)
471(m)
497(m)
501(w)
411(w)
514(w)
430(w)
510(w)
430(w)
403(w)
3
62(7)
ν
ν
2
2
3
48(0.5)
a) Raman intesities are given in parentheses. Infrared intensities: vs (very strong), s (strong), m (medium), w (weak), vw (very weak).
b) Assignments are made for tetrahedral symmetry although in the solid state the actual symmetry is lower.
–1
a shoulder (541 cm ). The remaining two bands (362 and ion due to complex bonding. The vibrational spectra indicate
–1
3
48 cm ) belong to ν .
an appreciable covalent character of the compounds.
2
4. Conclusions
LnF (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy) are
3
only sparingly soluble in aHF acidified with BF , where the
3
solubility decreases from lighter to heavier rare earth metals.
The solubility is increased by large excess of BF . According
3
to the infrared data, X�ray powder diffraction data, and single
crystal determination of LnF(HF)(BF ) (Ln = Pr, Nd), all
4
2
isolated compounds have the same composition, i.e.
LnF(HF)(BF ) The preparation of single crystals of La(BF )
4 3
4
2
showed that other phases could also exist in LnF /BF /aHF
3
3
system. The attempts to prepare single crystals of other
LnF(HF)(BF ) (Ln = La, Ce, Sm, Eu, Gd, Tb, and Dy) com�
4
2
pounds and attempts to prepare single crystals of the products
obtained in LnF /BF /aHF (Ln = Ho, Er, Tm, Yb, Lu, and Y)
3
3
system [5–7] have been so far unsuccessful.
4 3
Figure 8. Raman spectrum of La(BF ) .
Acknowledgement
The authors gratefully acknowledge the Slovenian Research Agency
Deviations of the vibrational spectra of La(BF4)3 and (ARRS) for the financial support of the present study within the re�
LnF(HF)(BF ) from the expected T symmetry are very high, search program: P1�0045 Inorganic Chemistry and Technology.
4
2
d
as estimated by the extent of splitting of ν mode and appear�
3
ance of infrared forbidden ν1 mode in infrared spectra of References
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4
2
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2314 www.zaac.wiley�vch.de
© 2009 Wiley�VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2009, 2309–2315

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Received: February 20, 2009
Published Online: June 2, 2009
1997.
Z. Anorg. Allg. Chem. 2009, 2309–2315
© 2009 Wiley�VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley�vch.de
2315