A. Kornath et al.
ꢀ
ꢀ
discrepancies between Me4N+PF4 and a reported Cs+PF4
(138.6(2) pm), because the negative charge increases the
ionic character of the bond.[34]
ion pair isolated in an argon matrix.[15,28] The differences be-
tween the frequencies for these three examples of ion pairs
isolated in matrices and the calculated gas-phase frequencies
cannot be explained in terms of regular matrix effects only.
It seems more likely that the structures of the anions in the
ion pairs are strongly affected by the adjacent cation. Also
covalent bonding between cations and anions might exist in
the alleged ion pairs, as it is known for alkali metal halides
trapped in matrices.[29] Notably, in case of Cs+SO2Fꢀ isolated
in argon matrices by the same technique as described above
such discrepancies do not exist. The frequencies of the
matrix-isolated ion pair agree well with calculated gas-phase
frequencies, as well as experimental data.[13,20] Finally, more
thorough matrix experiments together with theoretical cal-
culations are required to get a deeper insight into the nature
of these ion pairs.
ꢀ
The calculated S F bonds of 168.5 and 157.9 are both
longer than in SO2F2 (151.4(2) pm)[34] and reveal the differ-
ent kinds of bonds in the trigonal bipyramidal ion, which
can be rationalized in terms of semi-ionic three-center four-
electron bonding for the axial fluorine ligands and a mainly
ꢀ
ꢀ
covalent S Feq bond. The difference in S F bond lengths of
10.6 pm is comparable to molecules, such as SF4, where dif-
ꢀ
ꢀ
ferences between S Feq and S Fax of 10.1 pm have been ob-
served.[35] Comparing molecules and isoelectronic molecules,
the formal negative charges in anions clearly enhance the
formation of semi-ionic, three-center four-electron bonds,
thereby causing their unusual lengths.
Conclusion
Ab initio calculations: The ab initio RHF/6-31+G* calculat-
ed harmonic frequencies for the [SO2F3]ꢀ anion are given in
Table 2 and compared with the
The first example of a [SO2F3]ꢀ salt has been prepared from
Me4NF and SO2F2 The successful synthesis of the [SO2F3]ꢀ
anion can be ascribed to the high reactivity of “naked fluo-
ride”, since alkali metal fluorides do not react with SO2F2.
The vibrational spectra agree well with ab initio calculations,
which predict a trigonal bipyramidal structure of C2v symme-
observed frequencies. We find
reasonable agreement for all vi-
brational modes. The calculated
geometry for [SO2F3]ꢀ is shown
try
.
in Figure 3 and listed together
with literature data for the iso-
ꢀ
G
PF5 in Table 3.[18,30,31]
Experimental Section
The energy minimum is
found for a trigonal bipyrami-
dal arrangement of C2v symme-
try. The oxygen atoms in the
Apparatus and materials: All synthetic work and sample handling were
performed employing standard Schlenk techniques and
a standard
vacuum line. Me4NF was prepared by the literature method.[23] The prep-
aration and purification of SO2F2 from SO2Cl2 and NaF is described else-
where.[36] Caution: SO2F2 is an odorless, poisonous gas.
equatorial positions cause
a
Figure 3. Calculated (RHF/6-
31+G*) structure of the
[SO2F3]ꢀ anion; bond lengths
in pm.
bending of the Fax-S-Fax axis
(165.28) and a compression of
the Feq-S-O angle (115.48) from
The infrared spectra were recorded on
AHCTUNGERTGpNNUN hotometer. Spectra of dry powders were obtained by using a CsBr plate
a Bruker ifs 113v spectro-
coated with the neat sample. The Raman spectra were recorded with an
ISA T64000 spectrometer using an argon ion laser at 514.5 nm from Spec-
tra Physics. The 19F NMR spectra were recorded with a Bruker DPX 300
spectrometer. The spectra were referenced to external samples of neat
CFCl3. The IUPAC chemical shift convention was used.
ꢀ
the ideal value of 1208. The S
O bond length of 143.2 pm is in
the range of regular sulfur–oxygen double bonds in similar
anions, such as SO2Fꢀ (calcd: 145.8 pm; exptl: 147.8(1)
+A[SO2F3]ꢀ:
Synthesis of [Me4N] CHTUNGTENRNUG A sample of dry Me4NF (210 mg,
ꢀ
pm)[20,32], SOF3 (calcd: 143.3 pm)[16], and SO3Fꢀ (exptl:
2.26 mmol) was placed into a dried 5 mL quartz ampoule, and SO2F2
(2.04 g, 20 mmol)and acetonitrile (2 mL) were condensed in at ꢀ1968C.
The sealed ampoule was warmed to room temperature. The reaction pro-
ceeds in a few hours in an ultrasonic bath. The excess of SO2F2 and the
acetonitrile were pumped off first at dry ice temperature and then at
room temperature. The weight (430 mg; calcd: 441 mg for quantitative
yield) of the colorless solid indicated almost quantitative yield of
[Me4N]+ [SO2F3]ꢀ. Further characterization of the obtained compound
was carried out by vibrational and 19F NMR spectroscopy. The trifluoro-
sulfate salt is extremely sensitive to moisture. The salt decomposes at
about 908C with the formation of sulfuryl fluoride, trimethylamine, meth-
ylfluoride, and traces of other unidentified products.
Computational methods: The ab initio calculations for [SO2F3]ꢀ were per-
formed at the restricted Hartree–Fock level of theory using the Gaussian
94 program.[37] All calculations were carried out at the 6-31+G* basis
level, which augments the standard double-z plus polarization treatment
(6-31G*) with a diffuse set of s,p functions (+) on each heavy atom and
is known to describe anionic systems in an appropriate way. Harmonic vi-
brational frequencies were computed for the minimum-energy structure
142.4(4)–145.5 pm)[33], and as expected longer than in SO2F2
Table 3. Geometry of [SO2F3]ꢀ (calcd) compared to those of isoelectron-
ic ions and molecules.
A
SOF4
PF5
calcd[b]
exptl[c]
exptl[d]
[a]
ꢀ
r
r
r
G
168.5
157.9
143.2
165.2
115.4
129.1
–
159.6
153.9
140.9
164.6
123.6
–
157.7
153.4
–
180
–
ꢀ
(X O) [pm]
](Fax-X-Fax) [8]
]
]
–
](Feq-X-Feq) [8]
](Fax-X-Feq) [8]
112.8
120
90
82.6
]
G
93.2
97.7
–
[a] X=S, P. [b] RHF/6-31+G*. [c] From ref. [30]. [d] From ref. [31].
928
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 925 – 929