Syntheses and Structures of F6XeNCCH3 and F6Xe(NCCH3)2

Article abstract of DOI:10.1002/anie.201507635

Acetonitrile and the potent oxidative fluorinating agent XeF₆ react at -40 C in Freon-114 to form the highly energetic, shock-sensitive compounds F₆XeNCCH₃ (1) and F₆Xe(NCCH₃)₂CH₃CN (2CH₃CN). Their low-temperature single-crystal X-ray structures show that the adducted XeF₆ molecules of these compounds are the most isolated XeF₆ moieties thus far encountered in the solid state and also provide the first examples of Xe^VI-N bonds. The geometry of the XeF₆ moiety in 1 is nearly identical to the calculated distorted octahedral (C_3v) geometry of gas-phase XeF₆. The C_2v geometry of the XeF₆ moiety in 2 resembles the transition state proposed to account for the fluxionality of gas-phase XeF₆. The energy-minimized gas-phase geometries and vibrational frequencies were calculated for 1 and 2, and their respective binding energies with CH₃CN were determined. The Raman spectra of 1 and 2CH₃CN were assigned by comparison with their calculated vibrational frequencies and intensities.

Full text of DOI:10.1002/anie.201507635

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201507635
German Edition: DOI: 10.1002/ange.201507635
Xenon Fluorides Very Important Paper
Syntheses and Structures of F6XeNCCH3 and F6Xe(NCCH3)2
Kazuhiko Matsumoto, Jamie Haner, HØl›ne P. A. Mercier, and Gary J. Schrobilgen*
In memory of Henry Selig
Abstract: Acetonitrile and the potent oxidative fluorinating
agent XeF6 react at À408C in Freon-114 to form the highly
energetic, shock-sensitive compounds F6XeNCCH3 (1) and
F6Xe(NCCH3)2·CH3CN (2·CH3CN). Their low-temperature
single-crystal X-ray structures show that the adducted XeF6
molecules of these compounds are the most isolated XeF6
moieties thus far encountered in the solid state and also
the highest energy conformer is 3.[12] Calculations with the
spin-free exact two-component theory in its one-electron
variant (SFX2C-1e) indicate that both 3 and 4 are minima[13]
on the potential energy surface. Structure 5 is a transition
state at both levels of theory. Although XeF6 has been
extensively studied by single-crystal X-ray diffraction, none of
its seven known crystal modifications[14] contain XeF6 (C3v)
molecules that are well-isolated from one another. Five
provide the first examples of XeVI N bonds. The geometry of
À
+
the XeF6 moiety in 1 is nearly identical to the calculated
distorted octahedral (C3v) geometry of gas-phase XeF6. The C2v
geometry of the XeF6 moiety in 2 resembles the transition state
proposed to account for the fluxionality of gas-phase XeF6.
The energy-minimized gas-phase geometries and vibrational
frequencies were calculated for 1 and 2, and their respective
binding energies with CH3CN were determined. The Raman
spectra of 1 and 2·CH3CN were assigned by comparison with
their calculated vibrational frequencies and intensities.
phases contain ionic oligomers, (XeF5 FÀ)n (n = 4 or 6), and
+
two of the modifications, formulated as (XeF5 FÀ)3·XeF6,
contain coordinated XeF6 molecules with local symmetries
that are close to C2v. Similarly, the [Xe2F13]À anion in its
[NO2]+ salt may be formulated as a [XeF7]À anion that is
fluorine-bridged to an XeF6 (C2v) molecule.[15] Low-temper-
ature solution 19F and 129Xe NMR spectroscopy has shown
that XeF6 exists as a fluxional (XeF6)4 tetramer in which the
four Xe and 24 F atoms are chemically equivalent on the 19F
and 129Xe NMR time scales and the 129Xe–19F couplings are
averaged.[16,17] The gas-phase 19F and 129Xe NMR spectra of
XeF6 display broad singlets.[18] Prior to this work, no examples
T
he stereochemical activity of the valence electron lone pair
(VELP) of XeF6 has been a subject of considerable interest
for over 50 years. The three possible gas-phase geometries of
XeF6 are an octahedral structure (3), where the Xe VELP is
stereochemically inactive, a monocapped octahedral (C3v)
structure (4), and the C2v structure (5). Gaseous XeF6 has
been studied by electron diffraction,[1–5] far-infrared,[6]
Raman,[7,8] and photoelectron[9,10] spectroscopy. It has been
concluded that the gas-phase structure of monomeric XeF6
has C3v symmetry (4), as predicted by the VSEPR model[11] for
an AX6E molecule and by previous quantum-chemical
calculations.[12]
of XeVI N bonds had been reported. A considerable number
À
II
2
3
À
of Xe N species in which Xe is bonded to sp-, sp -, or sp -
hybridized N atoms are known,[19] whereas only three exam-
ples of XeIV N bonds have been reported, [C6F5XeF2][BF4]·
À
(NCCH3)n (n = 1 or 2)[20] and F2OXeNCCH3.[21]
A 1:1 complex between XeF6 and CH3CN was formed by
their stoichiometric reaction in Freon-114 (1,2-dichlorotetra-
fluoroethane) at À408C [Eq. (1)]. The resulting adduct was
soluble, giving a colorless solution. Removal of Freon-114 at
À788C resulted in a white powder, which rapidly decreased in
volume under dynamic vacuum at À788C, suggesting that
1 had formed a weak Freon-114 solvate. A 1:3 complex of
XeF6 and CH3CN was prepared in a similar manner using
a 1:3 ratio of XeF6 and CH3CN [Eq. (2)]; however, there was
no evidence for the formation of a Freon-114 solvate.
Freon-114
XeF þ CH CN
F XeNCCH
3
ð1Þ
ð2Þ
ƒƒƒƒƒ!
6
3
6
À40 ꢀ
C
Explicitly correlated CCSD(T)-F12b calculations indicate
that the lowest energy conformer of gaseous XeF6 is 4 and
Freon-114
XeF þ 3 CH CN
F XeðNCCH Þ Á CH CN
6
ƒƒƒƒƒ!
6
3
3
2
3
À40 ꢀ
C
Solid samples of 1 and 2·CH3CN proved to be kinetically
stable at À7888C, but detonated when mechanically shocked at
this temperature. Both compounds slowly lost CH3CN under
dynamic vacuum at À40 to À208C. Attempts to prepare 2
using a 1:2 molar ratio of XeF6/CH3CN yielded a mixture of
1 and 2·CH3CN. The addition of one equivalent of CH3CN to
2·CH3CN led to a mixture of CH3CN and 2·CH3CN with no
evidence for F6Xe(NCCH3)3 formation.
[*] Dr. K. Matsumoto, M. Sc. J. Haner, Dr. H. P. A. Mercier,
Prof. G. J. Schrobilgen
Department of Chemistry, McMaster University
Hamilton, ON, L8S 4M1 (Canada)
E-mail: schrobil@mcmaster.ca
Supporting information and ORCID(s) from the author(s) for this
article are available on the WWW under http://dx.doi.org/10.1002/
anie.201507635.
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14169

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Communications
Little information on the solution structure of 1 could be
garnered from its 19F and 129Xe NMR spectra in SO2ClF (see
the Supporting Information for a full discussion).
Single crystals of 1 and 2·CH3CN suitable for X-ray
crystal-structure determination were grown by slowly cooling
their respective SO2ClF and CH3CN/Freon-114 solutions. The
crystal structures of the isolated adducts and their calculated
structures are in excellent agreement (Figure 1 and Figure 2,
Tables S1 and S2; see the Supporting Information for a full
discussion of the calculated geometries).
Figure 1. X-ray crystal structure of F6XeNCCH3 (top). Ellipsoids are
shown at the 50% probability level. The calculated geometry
(PBE1PBE/aug-cc-pVTZ(-PP)) for F6XeNCCH3 (bottom) is also shown.
Figure 2. X-ray crystal structure of F6Xe(NCCH3)2·CH3CN. The lattice
CH3CN molecule is not shown (top). Ellipsoids are shown at the 50%
probability level. The calculated geometry (PBE1PBE/aug-cc-pVTZ(-
PP)) for F6Xe(NCCH3)2 (bottom) is also shown.
The local symmetry of the XeF6 moiety in 1 is approx-
imately C3v (Figure S1), whereas its crystal site symmetry is Cs.
À
The Xe N distance (2.762(2) Š) is significantly longer than
II
distances in [C6F5XeNCCH3]+ (2.640(6)–
À
the Xe
N
2.610(11) Š),[22] but similar to the XeIV N bond lengths in
F2OXeNCCH3 (2.808(5), 2.752(5) Š)[21] and [C6F5XeF2]-
[BF4]·NCCH3 (2.742(4) Š).[20] The bond lengths are signifi-
À
À
À
those of the closed face [Xe F3/F3A: 1.8560(9) Š; Xe F4:
1.8601(12) Š)]. At this time, it is not clear whether the C3v
geometry of 1 is imposed by CH3CN or the presence of
a sterically active Xe VELP. In the latter scenario, the Xe and
N VELPs would oppose one another. Calculations are
currently underway to ascertain the location of the Xe
cantly shorter than the sum of the Xe and N van der Waals
II
radii (3.71 Š),[23] but notably longer than Xe N bonds
À
(2.02(1)–2.236(4) Š).[19] The N1 atom of 1 is located on
a
pseudo-C3 axis. The non-linear Xe1-N1-C1 angle
À
(160.31(17)8) is likely the result of crystal packing and the
deformability of this angle. The F atoms of the XeF6 moiety
comprise two groups, where F1, F2, and F2A form the most
open trigonal face proximate to N1 and opposite to the most
closed face, F3, F3A, and F4. The Xe VELP presumably
VELP. The average Xe F bond length of 1 (1.907(2) Š) is
comparable to those of other neutral XeVI species [XeOF4:
1.890(2) Š;[24] XeO2F2: 1.899(3) Š[25]].
Compound 2·CH3CN contains lattice CH3CN molecules
that lie in channels along the b axis and are twofold
disordered along the a axis with a 50:50 population ratio
(Figure S2). The crystal site symmetry of the XeF6 unit is Cs;
however, the isolated unit closely approximates C2v symmetry.
À
À
À
resides between the Xe F1, Xe F2, and Xe F2 bonds of the
more open face. Correspondingly, the F-Xe-Fangles [F1-Xe1-
F2 and F1-Xe1-F2A: 115.65(4)8; F2-Xe1-F2A: 105.92(6)8]
proximate to N1 are considerably more open than those of the
opposite triad [F3-Xe1-F4 and F3A-Xe1-F4: 79.93(4)8; F3-
Xe1-F3A: 82.59(8)8]. The difference between the average
F-Xe-F angles of the open and closed faces is 29.53(6)8. The
À
The Xe N bond lengths (2.785(2) Š) of 2 are very similar to
that of 1. The N and C atoms of both coordinated CH3CN
molecules are approximately located on the pseudo-mirror
plane bisecting the F1-Xe1-F2 and F4-Xe1-F5 angles. The
À
À
Xe F bonds of the open face are significantly longer
Xe F bonds of F6Xe(NCCH3)2 can be classified into three
groups: two equatorial F atoms adjacent to the presumed
À
À
[Xe1 F1: 1.9769(13) Š; Xe1 F2/F2A: 1.9451(8) Š] than
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Table 1: Selected experimental and calculated Raman frequencies (cmÀ1
)
À
À
location of the Xe VELP [Xe1 F1: 1.981(2) Š; Xe1 F2:
and assignments for F6XeNCCH3.[a]
À
À
1.9876(13) Š], two axial F atoms [Xe1 F3 and Xe1 F3A:
Exp.[b]
Calculated[c]
Assignment
1.8936(11) Š], and two equatorial F atoms opposite to the Xe
À
À
VELP [Xe1 F4: 1.868(2) Š; Xe1 F5: 1.873(2) Š]. The
geometry of the XeF6 moiety may be viewed as a C2v-
distorted AX6E VSEPR arrangement (5) of the Xe VELP
624.1(4)
609.1(100)
632.5(75)[184] A1, ns(XeF3b)
À
and six Xe F bond pair domains, where the Xe VELP
605.7 sh
599.5(13)
620.0(8)[211]
557.5(18)[78]
E, nas(XeF3a)+nas(XeF3b)
occupies a position between F1 and F2 in the equatorial Xe1–
F1/F2–F4/F5 plane, causing the axial F3/F3A atoms to bend
away from the equatorial VELP position towards F4/F5,
resulting in an F3-Xe1-F3A angle of 156.90(7)8. When the
548.4(1)
540.2(11)
A1, ns(XeF3a)
À
Xe N bonds (2.785(2) Š) are included in the description of
the Xe coordination sphere, the F3/F3A, N1/N1A, and Xe1
491.0(12)
477.5(9)
504.5(14)[110] E, nas(XeF3a)Ànas(XeF3b)
atoms are seen to be coplanar within Æ 0.110 Š, with the
335.3(1)[10]
334.2(1)[1]
A1, ds(XeF3b)
E, das(XeF3a)+das(XeF3b)
À
Xe N bonds avoiding the Xe VELP position.
345.9(1)
The Raman spectra of 1 and 2·CH3CN are shown in
Figure 3. Selected experimental and calculated gas-phase
(PBE1PBE/aug-cc-pVTZ(-PP)) frequencies and assignments
273.5(1)
223.9(1)
246.8(1)[<1]
E, 1r(XeF3a)+1r(XeF3b)
A1, ds(XeF3a)
205.9(2)[17]
150.6(3)
147.2(<1)
149.2(<1)[1]
E, [das(XeF3a)Àdas(XeF3b)]+d(XeNC)
A1, n(XeN)
129.4(1)
106.1(1)[9]
[a] See Table S3 for a full list of frequencies and assignments for all
bands; abbreviations are given in the footnotes. [b] Values in paren-
theses denote relative Raman intensities. [c] PBE1PBE/aug-cc-pVTZ
(-PP). Values in parentheses denote Raman intensities (ŠamuÀ1). Values
in square brackets denote infrared intensities (kmmolÀ1).
2·CH3CN, respectively, were assigned to n(XeN). The low
frequencies of the coupled d(XeNC) modes (147–205 cmÀ1)
À
are also consistent with weak Xe N interactions and highly
deformable Xe-N-C angles, and likely account for the non-
linearity of this angle in the crystal structures of 1 and
2·CH3CN.
À
Binding energies for the Xe N interactions in 1 and 2
were determined at the MP2/aug-cc-pVTZ(-PP) level of
theory [Eqs. (3)–(5)]. The energy for free XeF6 was approxi-
mated from structure 3 (Table S7) and used because it was not
possible to optimize 4 at the MP2 level of theory.[26] The
energy differences between 3 and 4 are 0.79 (CCSD(T)/
CBS)[12] and 7.53 kJmolÀ1 (CCSD(T)-F12b).[13]
Figure 3. Raman spectra of a) 1 and b) 2·CH3CN recorded at À1508C
using 1064 nm excitation. Symbols denote FEP sample tube lines (*)
and an instrumental artifact (†).
À157:1 kJ molÀ1
À129:5 kJ molÀ1
À286:7 kJ molÀ1
ð3Þ
ð4Þ
ð5Þ
XeF6 þ CH3CN ! F6XeNCCH3
are listed in Table 1 and Table 2. Full lists, including the
frequencies and assignments for coordinated CH3CN, are
provided in Tables S3 and S4, along with a detailed discussion.
The frequency trends are well reproduced by the calculations.
Factor-group analyses were performed to account for split-
tings that occur on some bands (Tables S5 and S6).
F6XeNCCH3 þ CH3CN ! F6XeðNCCH3Þ2
XeF6 þ 2 CH3CN ! F6XeðNCCH3Þ2
In conclusion, the adducts 1 and 2·CH3CN have been
synthesized and structurally characterized by X-ray crystal-
lography and Raman and NMR spectroscopy. They represent
In both adducts, high-frequency shifts occur for n(CN),
n(CC), and d(NCC), which is in accordance with the weak
À
À
Xe N bonding interaction that is indicated by the crystal
the first examples of XeVI N bonds, and their structures
À
structures and quantum-chemical calculations. The Xe F
provide structural evidence that is consistent with the
stereochemical activities of their Xe VELPs. Both adducts
are well isolated from one another in their crystal lattices,
based on the long intermolecular contacts observed among
their structural units, and provide the only examples of
stretches (1: 478–624 cmÀ1; 2·CH3CN: 454–597 cmÀ1) occur in
ranges similar to those reported for gas-phase and matrix-
isolated XeF6,[6–8] but are shifted to lower frequencies. Weak
bands at 129 and 110 cmÀ1 in the Raman spectra of 1 and
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Table 2: Selected experimental and calculated Raman frequencies (cmÀ1) and
À1968C. The sample was warmed to À788C, and frozen CH3CN was
carefully leached off the walls of the reactor with Freon-114. In this
way, CH3CN was diluted in Freon-114 prior to coming into contact
with XeF6. Moreover, XeF6 has a low solubility in Freon-114 at
À788C. After most of the CH3CN had dissolved, the sample was
warmed to À408C and mixed for 10 min. Removal of Freon-114 by
pumping at À788C resulted in a white powder corresponding to
Freon-114 solvated F6XeNCCH3. Further pumping on the solvate at
À788C resulted in removal of Freon-114, yielding a white powder
corresponding to pure 1.
assignments for F6Xe(NCCH3)2.[a]
Exp.[b]
Calculated[c]
Assigment
596.6(100)
621.0(80)[161]
A1, ns(XeF2c)+ns(XeF2b)small
581.1(sh)
572.5(23)
558.5(78)
604.9(6)[229]
598.7(6)[313]
565.6(28)[24]
B1, nas(XeF2a)+nas(XeF2c)
B2, nas(XeF2b)
A1, ns(XeF2a)+ns(XeF2b)Àns(XeF2c)
Synthesis of 2·CH3CN: Using a procedure similar to that for 1,
XeF6 (0.1787 g, 0.7286 mmol) and CH3CN (0.0896 g, 2.183 mmol)
were combined. Removal of Freon-114 by pumping at À788C
resulted in a white powder corresponding to pure 2·CH3CN.
497.7(sh)
494.4(29)
509.1(16)[55]
A1, ns(XeF2a)Àns(XeF2b)
454.3(13)
435.1(13)
487.1(8)[136]
407.2(1)[11]
B1, nas(XeF2a)Ànas(XeF2c)
A1, d(XeF2c)+(XeF2a)small
Acknowledgements
We thank the Natural Sciences and Engineering Research
Council of Canada for support in the form of a Discovery
Grant (G.J.S.) and the Ontario Ministry of Training, Colleges,
and Universities for graduate scholarships (J.H.).
335.9(<1)[3]
334.7(<1)[2]
B2, 1r(XeF2b)+1w(XeF2c)
B1, d(FaXeFb)Àd(Fa’XeFb’)
360.8(3)
303.4(2)
293.3(<1)
233.0(1)
278.9(1)[0]
A2, 1t(XeF2b)+1t(XeF2c)
260.3(<1)[17]
201.6(<1)[10]
A1, d(XeF2b)À[d(XeF2a)]small
Keywords: fluorine chemistry · noble-gas chemistry ·
Raman spectroscopy · xenon fluorides · X-ray crystallography
A1, d(XeF2a)+d(XeF2b)À
[1(XeF2c)+d(XeNC)A+B ip]small
How to cite: Angew. Chem. Int. Ed. 2015, 54, 14169–14173
Angew. Chem. 2015, 127, 14375–14379
150.1(<1)[3]
149.5(<0.1)[0]
144.4(<0.1)[2]
B2, 1r(XeF2b)+(d(XeNC)A-B)
ip
A2, 1t(XeF2a)+1t(XeF2c)+(d(XeNC)A-B
B1, 1r(XeF2a)+1r(XeF2c)+(d(XeNC)A+B
176.2(<1)
165.1(2)
)
oop
)
oop
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76.6(<1)[6]
B2, 1w(XeF2a)+(d(XeNC)A-B
A1, n(XeN)A+B
)
ip
109.6(1)
[a] See Table S4 for a full list of frequencies and assignments for all bands;
abbreviations are given in the footnotes. [b] Values in parentheses denote relative
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Experimental Section
Caution! The adducts F6XeNCCH3 and F6Xe(NCCH3)2·CH3CN are
highly energetic materials that may detonate when mechanically or
thermally shocked (see the Supporting Information).
Synthesis of 1: Xenon hexafluoride (0.1796 g, 0.7322 mmol) was
condensed under static vacuum at À1968C into a FEP reaction tube
(outer diameter: 0.25 inch) equipped with a Kel-F valve. Freon-114
(ca. 0.2 mL) was condensed onto the XeF6 at À1968C. Direct contact
between neat CH3CN and XeF6 was avoided by condensing
a stoichiometric amount of CH3CN (0.0298 g, 0.7259 mmol) onto
the reactor walls above the frozen Freon-114 and XeF6 layers at
[19] G. L. Smith, G. J. Schrobilgen, Inorg. Chem. 2009, 48, 7714 –
7728, and references therein.
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Received: August 14, 2015
Published online: September 21, 2015
Angew. Chem. Int. Ed. 2015, 54, 14169 –14173
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