[Li(XeF2)n](AF6) (A = P, As, Ru, Ir), the first xenon(II) compounds of lithium. Synthesis, Raman spectrum, and crystal structure of [Li(XeF2)3](AsF6)

Article abstract of DOI:10.1021/ic302323j

The reactions between compounds of the type MAF₆ (M = alkali metal; A = P, As, V, Ru, Ir, Sb, Nb, Ta) and xenon difluoride were studied in anhydrous hydrogen fluoride solvent. The coordination products [M(XeF ₂)_n]AsF₆ were only observed in the case of LiAF₆ (A = P, As, Ru, Ir), and the crystal structure of [Li(XeF ₂)₃]AsF₆ was determined (monoclinic space group P2₁ with a = 6.901(9) A, b = 13.19(2) A, c = 6.91(1) A, β = 91.84(2), and Z = 2). The coordination sphere of lithium is comprised of six F atoms. The compound series was also characterized by Raman spectroscopy.

Full text of DOI:10.1021/ic302323j

Article
pubs.acs.org/IC
[Li(XeF2)n](AF6) (A = P, As, Ru, Ir), the First Xenon(II) Compounds of
Lithium. Synthesis, Raman Spectrum, and Crystal Structure of
[Li(XeF2)3](AsF6)
̌
Gasper Tavcar* and Boris Zemva
̌
̌
Department of Inorganic Chemistry and Technology, Jozef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia
̌
S
* Supporting Information
ABSTRACT: The reactions between compounds of the type MAF6 (M = alkali metal; A
= P, As, V, Ru, Ir, Sb, Nb, Ta) and xenon difluoride were studied in anhydrous hydrogen
fluoride solvent. The coordination products [M(XeF2)n]AsF6 were only observed in the
case of LiAF6 (A = P, As, Ru, Ir), and the crystal structure of [Li(XeF2)3]AsF6 was
determined (monoclinic space group P21 with a = 6.901(9) Å, b = 13.19(2) Å, c = 6.91(1)
Å, β = 91.84(2)°, and Z = 2). The coordination sphere of lithium is comprised of six F
atoms. The compound series was also characterized by Raman spectroscopy.
in aHF were decanted from one arm of the crystallization vessel to the
other. A temperature gradient was maintained between both arms in
order to induce crystal growth. Alternatively, smaller vessels for crystal
growth were fabricated from FEP tubing using 10 cm and 4-mm-i.d.
INTRODUCTION
■
Complexes of the alkali-metal fluorides with xenon fluorides are
known only for XeIV and XeVI, as exemplified by the salts
MXeF5 (M = Na, K, Rb, Cs),1 MXeF7 (M = Rb, Cs),2−5 and
lengths.
M2XeF8 (M = Na, K, Rb, Cs).2−4 Until now, compounds
Reagents. Lithium fluoride was obtained from British Drug
between alkali-metal fluorides and XeF2 were unknown.
Xenon difluoride, as a ligand bonded directly to a metal ion,
was first observed in [Ag(XeF2)2]AsF6,6 and a variety of further
metal salts with XeF2 coordinated to metal ions have been
discussed in a recent review.7 However, the only complexes of
XeF2 with M+ ions are those of AgI.6,8
In this paper, we report the first coordination compound in
which XeF2 functions as a ligand toward the Li+ ion in the
presence of different AF6 (A = P, As, Ru, Ir) anions.
Houses Ltd. (99% purity). Xenon difluoride was prepared by the
photochemical reaction of xenon with difluoride at room temperature.9
aHF (Fluka, purum) was treated with K2NiF6 (Ozark-Mahoning, 99%)
for several days prior to use. Arsenic pentafluoride and PF5 were
synthesized by pressure fluorination of As2O3 (Alfa Aesar, 99%) or
P2O5 (Sigma-Aldrich, ≥98.0%) with difluoride in a nickel reactor at
300 °C as previously described.10
Caution! Reactions with aHF, AF5 (A = P, V, As, Ru, Ir, Sb, Nb, Ta),
and XeF2 must be carried out in a well-ventilated hood, and protective
clothing must be worn at all times. The experimentalist must become
familiar with these reagents and the hazards associated with them. Fresh
tubes of calcium gluconate gel should always be on hand for the immediate
treatment of skin exposed to these reagents. For a full protocol for the
treatment of HF exposure, see ref 11.
Syntheses of LiAF6 (A = P, V, As). LiAF6 was synthesized from
LiF (0.1202 g, 4.64 mmol) and a excess of gaseous AF5 (PF5; 0.7610 g,
6.04 mmol) in aHF solvent.12 After the reaction was completed, the
excess MF5 and solvent were pumped off on the vacuum line. The
product was verified by mass balance, powder X-ray diffraction, and
Raman spectroscopy (Figure 1).
Syntheses of LiAF6 (A = Sb, Ru, Ir, Nb, Ta). LiAF6 was
synthesized from LiF (0.0623 g, 2.405 mmol) and stoichiometric
amounts of either elemental A or AF3 (Ru: 0.2393 g, 2.369 mmol),
which were subsequently fluorinated with difluoride in aHF solvent.
The reactions were monitored by mass balance, and the product was
EXPERIMENTAL SECTION
■
General Experimental Procedures. A nickel vacuum line
coupled to a mechanical pump and an oil diffusion pump was used
to carry out the reactions. Fluorine and volatile fluorides were removed
with a soda lime scrubber and liquid-nitrogen cold traps. Volatile AF5
compounds and anhydrous hydrogen fluoride (aHF) were handled on
a part of the vacuum line made from Teflon, polyfluoroethylene, and
hexafluoropropylene−tetrafluoroethylene copolymer (FEP) in order
to diminish corrosion. Pressures were measured by means of a Monel
Helicoid pressure gauge (0−3000 Torr
0.25%) connected to the
vacuum manifold with a Teflon valve. Moisture-sensitive materials
were handled in the dry argon atmosphere of a glovebox (water
content of ≤0.1 ppm; M. Braun, Garching, Germany). Reaction
vessels, made of PFA and equipped with Teflon valves and Teflon-
coated stirring bars, were used for syntheses. Crystals were grown in a
crystallization vessel, made from a T-shaped FEP reaction vessel
constructed from a 16-mm-i.d. length of the FEP tube and a length of
4-mm-i.d. FEP tubing connected to a Teflon valve. Saturated solutions
Received: October 22, 2012
Published: March 29, 2013
© 2013 American Chemical Society
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Table 1. Crystal Data and Structure Refinement of
a
[Li(XeF2)3]AsF6
a (Å)
6.901(9)
13.19(2)
6.91(1)
91.84(2)
628(2)
2
b (Å)
c (Å)
β (deg)
V (Å3)
Z
fw
703.76
P21
space group
T (°C)
λ (Å)
ρcalcd (g/cm3)
μ (mm−1
−73(1)
0.71069
3.720
)
10.778
0.0531
0.1324
R1
wR2
a
2
2
R1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = [∑(w(Fo − Fc2)2)/∑w(Fo )2]1/2
.
143-mm Debye-Scherer camera with X-ray film and Cu Kα radiation.
The intensities were visually estimated.
RESULTS AND DISCUSSION
■
Synthesis. Only the coordination compounds [Li(XeF2)n]-
(AF6) (A = As, Ru, Ir) were isolated from mixtures of MAsF6
(M = Li, Na, K, Rb, Cs) and XeF2 in aHF solvent. They slowly
lost XeF2 at room temperature, yielding LiAF6. However,
[Li(XeF2)n]PF6 can be isolated at −30 °C, but at ca. −14 °C, it
rapidly decomposes, yielding LiPF6 and XeF2. Among the other
alkali metals, only sodium indicated formation of a XeF2
coordination compound.
Figure 1. Comparison of the Raman spectra of LiAF6 (A = P, As, V,
Ru, Ir, Sb, Nb, Ta).
characterized by powder X-ray diffraction and Raman spectroscopy
(Figure 1).
Syntheses of [Li(XeF2)n]AF6 (A = P, As, Ru, Ir). The syntheses
were carried out using LiAF6 salts (see above) and excess amounts of
XeF2 in aHF solvent. The [Li(XeF2)n]AF6 (A = As, Ru, Ir) complexes
were isolated at room temperature, where they slowly lost XeF2,
yielding LiAF6. The compound [Li(XeF2)n]PF6 was isolated at −30
°C, but at ca. −14 °C, it rapidly decomposed to LiPF6 and XeF2, as can
be seen from its Raman spectrum at that temperature.
Growth of Single Crystals of [Li(XeF2)3]AsF6. Stoichiometric
amounts of LiAsF6 and XeF2 were placed in a narrow FEP tube and
dissolved in aHF. The vessel was cooled to −50 °C, and aHF solvent
was slowly pumped off on the vacuum line at a rate of approximately 1
mL/day. The resulting powder contained small crystals and was
immersed in perfluorinated oil (ABCR, FO5960) in a drybox. A
suitable crystal was selected under the microscope and transferred into
the cold nitrogen stream of the X-ray diffractometer.
Crystal Structure Determination. A single-crystal X-ray data set
was collected for [Li(XeF2)3]AsF6 using a Mercury CCD area detector
coupled to a Rigaku AFC7 diffractometer using graphite-monochro-
mated Mo Kα radiation (λ = 0.71069 Å). The data were corrected for
Lorentz and polarization effects. A multiscan absorption correction
was applied to the data sets. All calculations during the data processing
were performed using the CrystalClear software suite.13 Structures
were solved using direct methods14 and expanded Fourier techniques.
Full-matrix least-squares refinement of F2 against all reflections was
performed using the SHELX 97 program.15
Sodium hexafluoroarsenate does not completely dissolve in
aHF but dissolves after the addition of XeF2, indicating that a
coordination compound is probably formed. It was not possible
to establish the existence of this compound by Raman
spectroscopy after the solvent was removed at −50 °C. The
only vibrational bands seen correspond to free XeF2 and
NaAsF6.
In general, it is noted that the smaller the formula unit
volume of the salt, the greater the lattice energy. Therefore,
small cations should be more favorable than larger. Moreover, a
small cation has a greater polarizing power than a larger one.
Thus, Li+ polarizes the nonbridging XeF2 more than Na+,
providing a further explanation for the existence of the lithium
compound. Another factor contributing to the exclusion of the
other alkali metals from XeF2 complex formation is the absolute
electronegativities of these cations, which are significantly
higher in the case of Li+ (40.52 eV) than those of the other
alkali-metal ions (Na+ = 26.21 eV; K+ = 17.99 eV; Cs+ = 14.5
eV), suggesting that electron density donation from XeF2 to Li+
will be higher than that in the case of the heavier alkali-metal
cations.16
The coordination number of Li+ will tend to be lower than
those for the heavier cations. The LiAF6 salts have NaCl-type
lattices in which Li+ is octahedrally coordinated by F− ligands,
whereas for the K+, Rb+, and Cs+ salts, the lattice type is
commonly CsCl, albeit rhombohedral, with eight or more F
atoms in the cation coordination sphere.
Slow crystallization (temperature gradient 10 °C) from
saturated solutions of [Li(XeF2)3]AsF6 in aHF always resulted
in single crystals of LiAsF6. The formation of [Li(XeF2)3]AsF6
is therefore most likely kinetically favorable, while the
formation of LiAsF6 is thermodynamically preferred. Therefore,
Crystal structure data for the compound was collected at −73 °C.
Details of the data collection and structure refinement are given in
Table 1 and the Supporting Information.
Raman Spectroscopy. Raman spectra of powdered samples in
sealed quartz capillaries or in an FEP reaction vessel ([Li(XeF2)n]PF6)
were recorded on a Renishaw Raman Imaging Microscope System
1000 using the 632.8 nm line of a He−Ne laser for excitation. The
geometry for all of the Raman experiments was 180° backscattering
with a laser power of 25 mW.
Powder X-ray Diffraction Patterns. Diffraction data were
recorded for powdered samples in sealed quartz capillaries using a
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only a few suitable crystals of [Li(XeF2)3]AsF6 were obtained
during rapid crystallization from saturated solutions.
The influence of the AF6− anion on the structural diversity of
coordination compounds of the type [Li(XeF2)n](AF6) is also
important but to a lesser extent than that of the cation. The
reactions of LiAF6 (A = P, V, Ru, Ir, Sb, Nb, Ta) salts and XeF2
in aHF solvent were also studied. Coordination compounds of
the type [Li(XeF2)n](AF6) were only obtained in the case
where A is P, Ru, and Ir. On the basis of their Raman spectra,
they are most likely isostructural with [Li(XeF2)3]AsF6. Raman
spectroscopy shows that all compounds in this series are
unstable and decompose with time at room temperature to
LiAF6 and XeF2. The most stable compound is [Li(XeF2)3]-
(AsF6), which is still possible to detect by Raman spectroscopy
after several weeks at room temperature. To account for this
−
behavior, the Lewis basicity of AF6 , the size of the anion, the
−
charge on the F− ligands of AF6 , and the solubilities of the
Figure 2. Coordination sphere of Li in [Li(XeF2)3]AsF6. Thermal
parent LiAF6 salts should be considered. Because the lattice
energy (U) is roughly proportional to the inverse of the cube
root of the formula unit volume, the U term is not likely to be
ellipsoids are drawn at the 50% probability level.
Bridging XeF2 molecules link the Li atoms, forming infinite
layers perpendicular to the b axis. The layers are shifted
diagonally along the ac plane, placing AsF6 anions and
nonbridging XeF2 molecules in the middle of the Li4(XeF2)4
squares of the neighboring layers (Figure 3). The layers are
−
much diminished for the larger AF6 relative to the smaller
−
anion. The greater difference could derive from the differing F−
−
ligand charges. For example, in SbF6 , the charge on the F atom
−
must be greater than that in PF6 . So, when SbF6− approaches a
cation, it competes more effectively in displacing XeF2 than
PF6− does. On the other hand, a larger anion size could increase
repulsion forces between F atoms in the crystal structure.
The structure of [Li(XeF2)3](AsF6) consists of Li4(XeF2)4
squares connected to form layers, with the XeF2 molecule and
AsF6 unit from two neighboring layers pushed toward this
square. If the volume of the anion inserted into the square
would increase, so would the repulsive forces between the F
−
atoms. Changing the anion to SbF6 , for instance, should
increase the A−F distance from approximately 1.74(4) Å in
LiAsF617 to 1.877(6) Å in LiSbF6,18 an increase of 0.13 Å. This
could decrease the shortest F(As) to F(Xe) distance to
approximately 2.57 Å, which may be enough to shift the
equilibrium toward the formation of LiSbF6.
Figure 3. Packing of the layers in the structure of [Li(XeF2)3]AsF6.
There are other factors, such as the relative solubilities of the
parent LiAF6 salts in aHF, that also affect the success or failure
of the synthetic approach that has been used. The solubility
data for LiAF6 salts are incomplete. Heavier LiAF6 (A = Sb, Bi,
Ta, Nb) salts often have lower solubilities, which could also
account for the nonexistence of these coordination compounds.
Similar behavior was observed in the case of silver(I)
coordination compounds with XeF2, where it was possible to
prepare [Ag(XeF2)2]AF6 (A = P, As), but no indication of XeF2
coordination was observed in the cases of A = Sb, Nb, and Ta.8
Most probably, the reasons for such behavior in the Li+ series of
XeF2 complexes are the same.
One must take care not to overgeneralize based on the
apparent nonexistence of these compounds. It may simply be
that one has not found the correct conditions for kinetic
stabilization of what are, in most cases, thermodynamically
unstable salts.
linked by electrostatic interactions between the positively
charged Xe atoms of the XeF2 molecules and negatively
charged F atoms of the XeF2 molecules and AsF6 anions.
−
There are seven Xe1···F contacts in the range 3.18(3)−3.60(1)
Å, five Xe2···F contacts in the range 3.28(1)−3.40(2) Å, and
four Xe3···F contacts in the range 3.32(1)−3.61(3) Å.
All known isostructural LiAF6 (A = P, As, Ru, Ir, Sb, Nb,
Ta)18,19 compounds for which crystal structures have been
determined crystallize in the space group R3. The Li cation is
̅
coordinated to six F atoms of the AF6 units. The previously
reported Li−F(AF6) distances obtained from single-crystal
17
structures are 2.047(1) Å (PF6 ) and 2.035 Å (IrF6 ),20 while
the corresponding distance determined by Rietveld refinement
for Li(AsF6) powder is 2.04(5) Å, which is longer than the Li−
F(AsF6) distance in [Li(XeF2)3](AsF6), which is 1.92(3) Å.
−
−
−
The large octahedral AsF6 ion pushes the equatorial bridging
XeF2 molecules toward the nonbridging XeF2, as indicated by
the (XeF2)F−Li−F(AsF6) angles, which are generally greater
than 90° [89(1), 93(1), 94(1), and 98(1)°].
Crystal Structure of [Li(XeF2)3]AsF6. The coordination
−
sphere around the Li atom consists of six F atoms (one AsF6
unit and five XeF2 molecules; Figure 2). There is only one
nonbridging XeF2 molecule, coordinated to the Li atom
through an F ligand, with a Li−F(Xe) distance of 2.07(3) Å.
In the four bridging XeF2 molecules, the bridging Li−F(Xe)
distances range from 1.97(2) to 2.03(3) Å.
The (Li)F−Xe bridging distances are in the range 1.976(8)−
2.005(8) Å, which is in a range similar to that reported for
(Ag)F−Xe [1.979(3) Å] and similar to all bridging distances in
coordination compounds of M2+ containing hexafluoroanions
and bridging XeF2 ligands. The real difference can be seen in
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the nonbridging XeF2 molecule, where the (Li)F−XeF distance
is not elongated [2.00(1) Å], whereas the nonbridging FXe−F
distance is shortened to 1.95(1) Å, which is the usual distance
for nonbridging XeF2 molecules coordinated to metal centers.
The (M)F−XeF distances for M = Cd [2.043(6)−2.079(6)
Å],21,22 Mg [2.051(4)−2.079(9) Å],23 Zn [2.078(5) Å],24 and
Cu [2.083(3)−2.102(5) Å]24,25 are actually longer than 2.00(1)
Å, as seen in the lithium coordination compound. The
[Ca(XeF2)4](AsF6)2 compound is the only compound with
XeF2 coordinated to the metal center in which similar short
(M)F−XeF [2.000(9) Å] distances are found.26
symmetric stretching mode, resulting in an increase in the
stretching frequency. The decomposition products “free” XeF2
[497 (sh) cm−1] and LiAsF6 [707, 572 (sh) and 376 cm−1]
could also be observed in the Raman spectrum.
−
The AsF6 modes of LiAsF6 are ν1 (707 cm−1), ν2 (572
cm−1), and ν5 (376 cm−1). The corresponding modes for the
−
ideal octahedral Oh symmetry of AsF6 occur at ν1, 689 cm−1;
ν2, 573 cm−1; and ν5, 375 cm−1.29 Modes at 698, 677, 584, and
−
366 cm−1 are assigned to the AsF6 unit in [Li(XeF2)3]AsF6.
−
The Oh symmetry of AsF6 in the coordination compound is
reduced to lower symmetry, resulting in more Raman-active
Raman Spectroscopy. The Raman spectrum of [Li-
(XeF2)3](AsF6) is shown in Figure 4. The high polarizability
modes.30−32
The Raman spectra of [Li(XeF2)n]PF6, [Li(XeF2)n]RuF6,
and [Li(XeF2)n]IrF6 show Xe−F stretching bands that are
similar to those of [Li(XeF2)3]AsF6 (Figure 5 and Tables 2 and
Figure 4. Raman spectrum of [Li(XeF2)3]AsF6: *, LiAsF6; **, free
XeF2.
of xenon usually results in intense Raman bands for the
symmetric Xe−F stretching modes. Modes involving As−F and
Li−F vibrations are usually far less intense and broader.
Because of thermal decomposition of the compound to LiAsF6
and XeF2, both decomposition products are also present in the
Raman spectrum of [Li(XeF2)3](AsF6) (Figure 4). Four XeF2
(Xe1 and Xe2) are bridging, while one XeF2 (Xe3) is
nonbridging (Figure 2). The totally symmetric (Σg+) stretching
mode for XeF2 occurs at 497 cm−1.27 When the nonbridging
XeF2 molecule is distorted by interaction of one F atom with
the metal ion, the band at 497 cm−1 is replaced by two bands.
The one at higher frequency is assigned to the stretch of the
shorter Xe−F bond [ν(Xe−F)], and the band at lower
frequency corresponds to the stretch of the longer Xe−F
bond [ν(Xe···F)]. In XeF+ salts, one F ligand is effectively
removed from the XeF2 molecule and the (Xe−F)+ stretching
frequency is normally higher than 600 cm−1.28
Figure 5. Comparison of the Raman spectra of [Li(XeF2)3]AsF6,
[Li(XeF2)n]PF6, [Li(XeF2)n]RuF6, and [Li(XeF2)n]IrF6: *, FEP
reaction vessel; **, free XeF2; ×, LiAF6.
3). This implies that all compounds in the series are most
probably isostructural. The bands assigned to nonbridging XeF2
are in range 535−530 cm−1, while the bands for bridging XeF2
Table 2. Key Raman Bands for [Li(XeF2)n](AF6) (A = As, P,
a
Ru, Ir)
In the case of the present compound, the small shoulder at
532 cm−1 can be assigned to the nonbridging Xe(3)F2 molecule
[ν(Xe−F)], whereas the Xe−F stretch [ν(Xe···F)] is expected
at approximately 476 cm−1 but is probably hidden under the
broader base of the most intense peak at 518 cm−1. The latter
peak is assigned to the Xe−F stretching modes of the four
bridging XeF2 molecules. The symmetric stretching mode of
the bridging XeF2 molecule might have been expected to be
close to the value of free XeF2. However, because the bridging
XeF2 molecules in the Li···F−Xe−F···Li moieties are anchored
between two Li+ ions, a higher energy is required for the
AF6
AsF6
PF6
RuF6
IrF6
terminal (Xe−F)
bridging (Xe−F)
532 (23)
518 (100)
698 (3)
677 (7)
584 (3)
366 (4)
530 (42)
534 (26)
515 (100)
661 (35)
640 (23)
588 (15)
260 (39)
535 (24)
514 (100)
666 (41)
516 (100)
−
AF6
571 (11)
239 (31)
a
Vibrational frequencies are in cm−1, and relative intensities are given
in parentheses.
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a
Table 3. Comparison of the Raman Bands of LiAF6 (A = P, V, As, Sb, Ru, Ir, Nb, Ta) in the Solid State
LiPF6
LiVF6
LiAsF6
LiRuF6
LiIrF6
LiSbF6
LiNbF6
LiTaF6
ν1
ν2
ν5
770 (100)
568 (21)
474 (32)
684 (100)
524 (10)
326 (34)
707 (100)
572 (27)
376 (68)
670 (95)
579 (16)
279 (100)
679 (98)
565 (17)
261 (100)
670 (100)
569 (16)
295 (79)
703 (100)
558 (8)
713 (76)
579 (7)
285 (90)
286 (100)
a
Vibrational frequencies are in cm−1, and relative intensities are given in parentheses.
are in range 518−514 cm−1. The bands for PF6 were not
−
̌
̌ ̌
k, M.; Benkic, P.; Zemva, B. Inorg. Chem. 2004, 43 (2),
(23) Tramse
699−703.
(24) Tavca
observed because of their low intensities.
̌
r, G.; Goreshnik, E.; Mazej, Z. J. Fluorine Chem. 2006, 127
(10), 1368−1373.
(25) Mazej, Z.; Goreshnik, E. Inorg. Chem. 2008, 47 (10), 4209−
ASSOCIATED CONTENT
■
S
* Supporting Information
4214.
X-ray crystallographic file in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
̌
̌ ̌
(26) Benkic, P.; Tramsek, M.; Zemva, B. Solid State Sci. 2002, 4 (11−
12), 1425−1434.
(27) Agron, P.; Begun, G.; Levy, H. Science 1963, 139, 842−843.
(28) Sladky, F. O.; Bulliner, P. A.; Bartlett, N. J. Chem. Soc. A 1969,
2179−2188.
(29) Naulin, C.; Bougon, R. J. Chem. Phys. 1976, 64, 4155−4158.
(30) Gillespie, R. J.; Landa, B. Inorg. Chem. 1973, 12, 1383−1388.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: gasper.tavcar@ijs.si. Phone: +386 1 4773225.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge the Slovenian Research
Agency (ARRS) for financial support of the Research Program
P1-0045 (Inorganic Chemistry and Technology).
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