XeF4 as a ligand for a metal ion

Article abstract of DOI:10.1002/anie.200803365

Noble molecule: [Mg(XeF₂)(XeF₄)](AsF ₆)₂ is the first coordination compound, in which XeF ₂ XeF₄ are simultaneously coordinated to the same metal center (see structure; purple Xe, green F,

Full text of DOI:10.1002/anie.200803365

Communications
DOI: 10.1002/anie.200803365
Noble Gas Compounds
XeF4 as a Ligand for a Metal Ion**
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Gasper Tavcar* and Boris Zemva
In memory of Neil Bartlett
[1]
Soon after the first noble-gas compound XePtF6 was
published, many laboratories around the world commenced
research in the field of noble-gas chemistry. Soon the first
binary fluoride of xenon, XeF4,[2] was prepared. Xenon
tetrafluoride is a weak Lewis base that reacts only with
strong Lewis acids yielding [XeF3]+ salts with appropriate
anions, such as [SbF6]À, [Sb2F11]À[3] or [BiF6]À, and [Bi2F11]À.[4]
Some molecular adducts with XeF4 were also described that
resulted from the semi-ionic bonding in the molecule.[5] The
first coordination compound with a noble-gas fluoride acting
as a ligand to a metal ion was [Ag(XeF2)2]AsF6.[6] A series of
such coordination compounds has since been prepared,[7] but
coordination compounds with XeF4, (a poorer Lewis base or
less thermally stable than XeF2[8]) as a metal ion ligand, were
not known.
distances are elongated. These elongations, therefore distort
the [AsF6]À octahedra and the As2 center is notably non-
centric.
The first attempt to coordinate XeF4 to a metal center was
made using Mg(AsF6)2 because of the strong interaction of
Mg2+ with XeF2.[9] Excess XeF4 was used to favor the
formation of a coordination compound. After the removal
of excess XeF4 under vacuum at room temperature, Raman
spectroscopy of the product showed that XeF4 was coordi-
nated to the metal center. Crystallization of the product gave
single crystals of composition [Mg(XeF2)(XeF4)](AsF6)2.[10]
The synthesis of [Mg(XeF2)(XeF4)](AsF6)2 was repeated
using the molar ratio XeF2:XeF4 = 1:1. The product obtained
had the same Raman spectrum as the isolated crystal.
Subsequently, preparations with higher amounts of XeF2
(XeF2:Mg > 1) were made and the intensity of the Raman
band at 575 cmÀ1 increased, indicating that this band is
associated with coordinated XeF2. At the molar ratio
XeF2:Mg = 2:1 or higher the only product was [Mg(XeF2)2]-
(AsF6)2, which is in accord with the lower Lewis basicity of
XeF4.[8]
Figure 1. Coordination of Mg atom in the crystal structure of
À
[Mg(XeF2)(XeF4)](AsF6)2. Interatomic distances [pm]: Xe1 F1 205.9(7),
À
À
À
À
Xe1 F2 193.6(7), Xe2 F3 208.3(6), Xe2 F4 187.1(7), Xe2 F5 191.6(7),
À
À
À
À
Xe2 F6 190.6(7), As1 F11 176.1(6), As1 F12 176.4(6), As1 F13
167.4(7), As1 F14 168.2(7), As1 F15 170.2(6), As1 F16 169.4(8),
As2 F21 175.1(6), As2 F22 168.8(7), As2 F23 169.1(7), As2 F24
170.9(7), As2 F25 168.7(7), As2 F26 176.8(6), Mg F1 193.5(7), Mg
À
À
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À
À
À
À
À
À
À
À
À
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F3 195.6(7), Mg F11 199.6(7), Mg F12 201.4(7), Mg F21 201.9(7),
À
Mg F26 201.9(7).
The magnesium ion is surrounded by six fluorine atoms,
four from [AsF6]À units, one from XeF2, and one from XeF4
(Figure 1). Both xenon fluorides are nonbridging ligands,
whereas the surrounding [AsF6]À units connect magnesium
atoms forming layers along the b axis (Figure 2). The
[As1F6]À unit is trans bridging, whereas the [As2F6]À unit is
À
a cis bridging in the complex. In both cases, the bridging As F
ˇ
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[*] Dr. G. Tavcar, Prof. Dr. B. Zemva
ˇ
Department of Inorganic Chemistry and Technology, “Jozef Stefan”
Institute
Jamova 39, 1000 Ljubljana (Slovenia)
Fax: (+386)1-477-3155
E-mail: gasper.tavcar@ijs.si
[**] The authors gratefully acknowledge to the Slovenian Research
Agency (ARRS) for the financial support of the Research Program
P1-0045 (Inorganic Chemistry and Technology).
Figure 2. View of the structure of [Mg(XeF2)(XeF4)](AsF6)2 along the
b axis.
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There are eight electrostatic interactions in the range
329.3(8)–380(3) pm between the positively charged xenon
atom of XeF2 and negatively charged FÀ ligands of the XeF4
and [AsF6]À units. In accordance with the higher positive
charge at the Xe atom in XeF4, this molecule forms six
electrostatic interactions with FÀ ligands in the range of
310.3(8)–357.7(11) pm. The shorter XeF4···XeF2 interaction of
310.3(8) pm is especially important in connecting the layers
(XeF4)](AsF6)2 because two XeF4 molecules are now coordi-
nated to the same metal center (Figure 4). Efforts to prepare
single crystals of this compound failed. Although large excess
of XeF4 was used, the only crystalline product obtained
(Figure 3). In the XeF2·XeF4 adduct,[5] this Xe Fe distance is
À
335.9 pm. The bridge angle, F-Xe-F, between XeF4 and XeF2
is 134.2(4)8.
Figure 4. Comparison of the Raman spectra of different products in
the system Mg(AsF6)2/XeF2/XeF4. * Spectrum of [Mg(XeF2)2](AsF6)2
from Reference [9].
always had the composition [Mg(XeF2)(XeF4)](AsF6)2. Thor-
ough prefluorination of the reaction vessel and very careful
purification of XeF4 did not help. The most probable reason
for the formation of XeF2 is the reduction of XeF4 during the
four weeks of crystallization in a FEP reaction vessel. The
structure of the coordination compound with only XeF4
ligands is still unknown.
Figure 3. Interactions between XeF4 and XeF2 molecules in the crystal
structure of [Mg(XeF2)(XeF4)](AsF6)2.
The most intense Raman bands of the single crystal of
[Mg(XeF2)(XeF4)](AsF6)2 occurring at 605, 596 (sh), 575, and
552 cmÀ1 (see Figure 4) can be assigned to Xe F vibrations.
À
Solid molecular XeF4 has D4h symmetry with Raman bands at
551(n4), 543(n1), 506(n5), 503(n1), 238(n6), and 216(n3) cmÀ1.[11]
When the XeF4 molecule donates one fluorine atom to a
strong Lewis acid yielding the [XeF3]+ cation of C2v symmetry,
the Raman bands are found to be at higher frequencies.
In XeF3SbF6 the Raman bands assigned to [XeF3]+ are at
643(n1), 571(n2), 607(n4), 608(n4), and 316(n5) cmÀ1.[12] In the
magnesium compound, the XeF4 species is not as far along the
ionization pathway as in the [SbF6]À salt and thus the lower
Experimental Section
Mg(AsF6)2 was prepared by the reaction between MgF2 and AsF5 in
anhydrous HF (aHF) as a solvent. XeF4 was prepared by the
fluorination of XeF2 with elemental fluorine under UV light in aHFas
a solvent at room temperature. The purity of XeF4 was checked by
gas-phase IR spectroscopy and Raman spectroscopy of the solids.
Solid product showed only the vibrational bands associated with
XeF4. Reactants Mg(AsF6)2 (0.2116 g, 0.557 mmol), XeF2 (0.1074 g,
0.634 mmol), and XeF4 (0.1191 g, 0.575 mmol) were weighed out in a
dry box. The solvent, aHF, was added on a vacuum line.
À
Xe F stretching frequencies are expected. The bands at 605,
596 (sh), and 552 cmÀ1 can be attributed to the XeF4 ligand
coordinated to the Mg2+ ion. The band at 575 cmÀ1 can be
assigned to the vibration of the nonbridging XeF2 ligand, as in
the [Mg(XeF2)2](AsF6)2 compound.[9] The band at 460 cmÀ1
could be a consequence of a coupling between vibrations of
XeF2 and XeF4. The bands at 728, 704, 687, 370, and 265 cmÀ1
probably arise from the two crystallographically distinct and
distorted [AsF6]À units.
Caution: Anhydrous hydrogen fluoride, AsF5, XeF2, and XeF4
must be handled in a well-ventilated hood and protective clothing
must be worn at all times! XeF4 and all of its products are susceptible
to moisture and they react with it forming XeO3, a compound which
easily detonates.[13] The experimentalist must become familiar with
these reagents and the hazards associated with them. For treatment of
HF injuries see reference [14].
Crystals were grown in a crystallization vessel made from a T-
shaped poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP)
reaction vessel, constructed using FEP tubing of inner diameter
16 mm. A 30 cm long FEP tube of inner diameter 4 mm connected by
a teflon valve provided flexibility for decanting the aHF solution from
one arm of the T-reactor to the other. Reagents were placed in the
wider tube of a crystallization vessel in stoichiometric amounts and
dissolved in aHF. This solution was decanted into the narrower part of
the T-shaped crystallization vessel. The crystals were grown with a
temperature gradient of at least 288C and were isolated from the
The Raman spectrum of the product isolated from the
experiment using a higher molar excess of XeF4 shows the
Xe F modes at 622, 590, 575, and 549 cmÀ1 (see Figure 4). A
À
weak band at 575 cmÀ1 probably arises from some of the
coordinated XeF2 and the bands at 622, 590, and 549 cmÀ1 are
most likely the signature of coordinated XeF4 molecules.
These Raman bands are different from those of [Mg(XeF2)-
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mother liquid at À208C and immersed in perfluorinated oil (ABCR,
[9] M. Tramsek, P. Benkic, B. Zemva, Inorg. Chem. 2004, 43, 699 –
703.
FO5960) in a dry-box. A suitable crystal was selected under the
microscope and transferred into the cold nitrogen stream of the X-ray
diffractometer.
[10] Crystal data for [Mg(XeF2)(XeF4)](AsF6)2: Mr = 778.75, mono-
clinic space group P21/n, a = 1291.5(19), b = 856.0(11), c =
1325(3) pm, b = 101.558(15), V= 1.435(4) nm3, Z = 4, 1calcd
=
3.604 gcmÀ3, F(000) = 1392, T= 200(2) K, Final R indices R1 =
0.0760 and wR = 0.1605 (all). Of the 6575 measured reflections
3342 were independent. The final difference Fourier synthesis
Published online: January 12, 2009
Keywords: fluorine · Lewis acids · magnesium ·
.
gave min/max residual electron density of À2328/2545 e0 nmÀ3
.
structure elucidation · xenon
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trum Karlsruhe, D-76344 Eggenstein-Leopoldshafen, Germany,
on quoting the deposition numbers CSD-419632.
[11] H. H. Claassen, C. L. Chernick, J. G. Malm, J. Am. Chem. Soc.
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