Tetramethylammonium Difluorobromate(I), (CH3)4N+BrF2-

Article abstract of DOI:10.1021/ic961283s

The thermolysis of (CH₃)₄N⁺Br(OCF₃)₂ ^- between -70 and -10°C, evolving F₂CO during the decomposition, gave (CH₃)₄N⁺BrF₂^-. The characterization of (CH₃)₄N⁺BrF₂^- was carried out by IR, Raman, and ¹⁹F- and ¹³C-NMR. For BrF₂^- two infrared absorption bands at 236 and 450 cm^-1 and one band in the Raman spectrum at 460 cm^-1 were observed in accord with a centrosymmetric structure. Ab initio calculations are presented for BrF₂^- the isoelectronic KrF₂, and other related compounds with D_∞h symmetry.

Full text of DOI:10.1021/ic961283s

4280
Inorg. Chem. 1997, 36, 4280-4283
Tetramethylammonium Difluorobromate(I), (CH3)4N+BrF2
-
Rolf Minkwitz* and Raimund Bro1chler
Anorganische Chemie, Fachbereich Chemie der Universita¨t Dortmund, 44221 Dortmund, Germany
Ralf Ludwig
Physikalische Chemie, Fachbereich Chemie der Universita¨t Dortmund, 44221 Dortmund, Germany
ReceiVed October 24, 1996X
-
The thermolysis of (CH3)4N+Br(OCF3)2 between -70 and -10 °C, evolving F2CO during the decomposition,
gave (CH3)4N+BrF2-. The characterization of (CH3)4N+BrF2- was carried out by IR, Raman, and 19F- and 13C-
-
NMR. For BrF2 two infrared absorption bands at 236 and 450 cm-1 and one band in the Raman spectrum at
460 cm-1 were observed in accord with a centrosymmetric structure. Ab initio calculations are presented for
-
BrF2 the isoelectronic KrF2, and other related compounds with D∞h symmetry.
-
Introduction
attributed to the BrF2 anion, while the IR-active deformation
frequency was not observed.
Although many attempts to prepare BrF2 were made since
1965, the anion could never be isolated and its symmetry is
still unclear because of conflicting spectroscopic data.
For the preparation of BrF2-, alkali-metal cations should not
be used as counterions to avoid distortions of the anion which
are well described in the literature.2-4
Fluorohalogenates [XF2]- (X ) Cl, Br, I) are well-known
from the literature. However, a controversy about their
vibrational spectroscopy is still going on.1-6
-
The first triatomic fluorohalogenate, ClF2-, was prepared by
Christe and Guertin in 1965.1 It was based on the reaction of
ClF with NOF and identified by infrared spectroscopy. Later
the full characterization of ClF2- was achieved via Raman and
IR spectra with cesium, rubidium, and potassium as cations.2
The theoretically predicted linear structure with D∞h symmetry
was apparently not observed. The IR spectra showed an intense
band attributed to νs(Cl-F), which is forbidden for a linear
centrosymmetric structure. The difference between the theoreti-
cally predicted D∞h symmetry and the experimental results was
explained as a result of crystal field effects.
Recently we reported the preparation of the new bromate(I)
salt (CH3)4N+(CF3O)2Br-.10 The compound was synthesized
-
by the reactions of (CH3)4N+Br- or (CH3)4N+BrCl2 with an
excess of CF3OCl at -78 °C. It was assumed, that the salt
-
might be a suitable precursor for (CH3)4N+BrF2
.
Experimental Section
First attempts to prepare IF2- were described by Meinert and
Klamm.7 Unfortunately no spectroscopic data were reported.
Further attempts to synthesize IF2- failed.8,9 Only disproportion
products such as I2 and Cs3IF6 were obtained.8
All synthetic work and sample handling were performed by
employing standard Schlenk techniques and a standard glass vacuum
line. The glass vacuum line and the reaction vessels were dried by
vacuum and treated with CF3OCl. Nonvolatile materials were handled
under dry nitrogen. CF3OCl was prepared, according to the literature
method, from F2CO with ClF and a CsF catalyst.22,23 The infrared
spectra were recorded on a Bruker IFS 113v FT-IR spectrometer in a
cooled cell24 equipped with CsBr plates.
The synthesis and full characterization of (C2H5)4N+IF2- was
recently performed by Naumann and Meurer, by the direct
reaction of IF with (C2H5)4N+F-.3 Two IR- and one Raman-
active frequencies were observed in the vibrational spectra. This
is consistent with a linear structure of symmetry D∞h.
(10) Minkwitz, R.; Bro¨chler, R. Z. Anorg. Allg. Chem. 1997, 623, 487.
(11) Wilson, W. W.; Christe, K. O. Inorg. Chem. 1989, 28, 4172.
(12) Lustig, M.; Ruff, J. K. Inorg. Chem. 1966, 5, 2124.
(13) Claassen, H. H.; Goodman, G. L.; Malm, J. G.; Schreiner, F. J. Chem.
Phys. 1965, 42, 1229.
(14) Herzberg, G. Infrared and Raman Spectra of Polyatomic Molecules;
D. Van Nostrand Co., Inc.: Princeton, NJ, 1945; S. 274, 311.
(15) Overend, J. Infrared Spectra and Molecular Structure; Davies, M.,
Ed.; Amer. Elsevier: New York, 1963, S. 352.
-
The first attempts to prepare BrF2 from CsF and BrF were
described by Surles et al.4 The Raman spectrum of the reaction
product showed four frequencies instead of the one frequency
expected for D∞h symmetry. Surles et al. explained their data
with symmetry reduction from D∞h to C2V. The synthesis of
-
Cs+BrF2 by codepositing CsBr salt with an argon/fluorine
mixture was described by Miller and Andrews.6 In the IR
spectrum of the deposit, two bands at 533 and 527 cm-1 were
(16) Christe, K. O.; Naumann, D. Inorg. Chem. 1973, 12, 59.
(17) Claassen, H. H.; Chernick, C. L.; Malm, J. G. J. Am. Chem. Soc. 1963,
85, 1927.
(18) Kalinowski, H.-J.; Berger, S.; Braun, S. 13C-NMR Spektroskopie; Georg
Thieme Verlag: Stuttgart, New York, 1984; p 201.
(19) Simons, W. W., Ed. The Sadtler Guide to Carbon-13 NMR-Spectra;
Sadtler-Heyden: London, 1983; S. 279.
X Abstract published in AdVance ACS Abstracts, August 15, 1997.
(1) Christe, K. O.; Guertin, J. P. Inorg. Chem. 1965, 4, 905.
(2) Christe, K. O.; Sawodny, W.; Guertin, J. P., Inorg. Chem. 1967, 6,
1159.
(3) Naumann, D.; Meurer, A. J. Fluor. Chem. 1995, 70, 83.
(4) Surles, T.; Quarterman, L. A.; Hyman, H. H. Z. Anorg. Allg. Chem.
1973, 35, 668.
(20) Hesse, M.; Meier, H.; Zeeh, B. Spektroskopische Meth. in der
organischen Chemie; Georg Thieme Verlag: Stuttgart, New York,
1987; Vol. 3, p 179.
(21) Wilson, W. W.; Christe, K. O. Inorg. Chem. 1989, 28, 4172.
(22) Gould, D. E.; Anderson, L. R.; Young, D.; Fox, W. B. J. Am. Chem.
Soc. 1969, 91, 1310.
(23) Schack, C. J.; Maya, W. J. Am. Chem. Soc. 1969, 91, 2902.
(24) Bayersdorfer, L.; Minkwitz, R.; Jander, J. Z. Anorg. Allg. Chem. 1976,
392, 137.
(5) Baran, E. J. Z. Naturforsch. 1973, 28b, 502.
(6) Miller, J. H.; Andrews, L. Inorg. Chem. 1979, 18, 988.
(7) Meinert, H.; Klamm, H. Z. Chem. 1965, 5, 468.
(8) Meinert, H. Habilitation, Humboldt-Universita¨t Berlin, 1968.
(9) Schmeisser, M.; Sartori, P.; Naumann, D. Chem. Ber. 1970, 103, 880.
S0020-1669(96)01283-9 CCC: $14.00 © 1997 American Chemical Society

-
(CH3)4N+BrF2
Inorganic Chemistry, Vol. 36, No. 19, 1997 4281
The Raman spectra were recorded on a T 64000 Jobin Yvon, using
the 514.5 nm exciting line of an Ar+ laser tube (Spectra Physics). Owing
-
to their hygroscopic nature, the BrF2 salts were measured in the dry
nitrogen atmosphere of a duran glass cell.25
-
The NMR spectra of (CH3)4N+BrF2 in CH3CN solution were
recorded at -38 °C on a Bruker AM 300 with CFCl3 as external
reference.
Preparation of (CH3)4N+BrF2-. A dry 35 mL cap glass vessel
containing 1 g (3.08 mmol) of tetramethylammonium bis(trifluo-
romethoxy)bromate(I) (CH3)4N+(CF3O)2Br- 10 was slowly warmed from
-78 to -10 °C. Carbonyl difluoride, which is formed during the
thermolysis, was removed continously. The completion of the reaction
-
was checked by weighing the reaction product. (CH3)4N+BrF2 is a
colorless solid which is extremely sensitive toward hydrolysis and can
be stored without decomposition at -70 °C for several weeks. At room
temperature in a disproportionation reaction the solid decomposes into
Br- and BrF4-, whereas the solution decomposes at -35 °C.
-
Ab Initio Calculations. Ab initio calculations for BrF2 were
performed with the Gaussian 94 program27 at the Hartree-Fock level
using standard 3-21G*, 6-31*, 6-31+G*, and 6-311G** basis sets.
Including electron correlation, the anion was also calculated at the MP2
level of theory and with a 6-31+G* basis set. Harmonic vibrational
frequencies were computed for the minimum-energy structures and
scaled by empirical factors to maximize their fit with the experimentally
observed frequencies. The calculated frequencies for all basis sets are
corrected throughout by 10% on the Hartree-Fock level28,29 and by
5% when electron correlation is taken into account.30
For a consistent comparison with the isoelectronic KrF2 and other
related compounds, only calculations on a small 3-21G* basis set could
be performed. However, it is well-known that a 3-21G* basis set is
already quite successful in accounting for hypervalent structures. The
mean absolute deviations of calculated from experimental bond lengths
between heavy atoms are only 1.5 pm for this basis set which is the
same as for the larger 6-31G* basis. The same is true for calculated
frequencies where the scatter in the 3-21G* data is much less than that
resulting from the 3-21G basis.31
Figure 1. Vibrational spectra of (CH3)4N+BrF2-: Trace A, spectrum
of the solid on CsBr plates at -70 °C; trace B, Raman spectrum of the
solid at -78 °C (cation bands marked with an asterisk).
-
Table 1. Raman and Infrared Frequenciesa of (CH3)4N+BrF2
Results and Discussion
Raman
IR
assgnt
Formation and Stability. The thermolysis of (CH3)4-
N+(CF3O)2Br- in the temperature range -70 to -10 °C (eq 1)
470 (sh)
460 (100)
2δ(F-Br-F)
νs(Br-F)
νas(Br-F)
δ(F-Br-F)
lattice
450 (vs, br)
236 (m)
(CH3)4N+(CF3O)2Br- -70 to -10 °C8
(CH3)4N+BrF2- + 2F2CO (1)
79 (96)
66 (70)
lattice
3048 (w)
3035 (w)
2964 (w)
ν(CH3)
3036 (69)
2958 (43)
2913 (14)
2829 (11)
1474 (27)
1449 (8)
ν(CH3)
yields tetramethylammonium difluorobromate(I), (CH3)4N+BrF2-.
ν(CH3)
-
(CH3)4N+BrF2 can be stored at -70 °C for several weeks
ν(CH3)
without decomposition. The salt disproportionates at room
2δs(CH3)
δas(CH3)
δas(CH3)
δs(CH3)
F(CH3)
-
temperature to (CH3)4N+BrF4 and Br2.
1483 (sh)
1455 (vs)
1424 (m)
1399 (vw)
1288 (w)
1181 (w)
1033 (m)
953 (vs)
750 (w)
Vibrational Spectra. The Raman and infrared spectra of
(CH3)4N+BrF2- are shown in Figure 1. The observed frequen-
cies are listed in Table 1 along with their assigments. Figure 2
1412 (8)
1290 (5)
1178 (6)
F(CH3)
F(CH3)
(25) Werner, A. Dissertation, Universita¨t Dortmund, 1988.
(26) Hyman, H. H. Science 1964, 145, 773.
F(CH3)
950 (26)
759 (32)
νas(C4-N)
νs(C4-N)
(27) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson,
B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T. A.; Peterson, G. A.;
Montgomery, J. A.; Raghavachari, K.; Laham, M. A.; Zakrzewski,
V. G.; Ortiz, J. V.; Foresman, J. B.; Ciolowski, J.; Stefanov, B.;
Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen,
W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.;
Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.;
Steward, J. J. P.; Head-Gordon, M.; Gonzales, C.; Pople, J. A. G94
Gaussian 94 (reVision A.1) A; Gaussian, Inc.: Pittsburgh, PA, 1995.
(28) Pulay, P.; Forgarasi, G.; Pang, F.; Boggs, J. E. J. Am. Chem. Soc.
1979, 101, 2550.
(29) Pople, J. A.; Schlegel, H. B.; Krishnan, R.; Frees, D. J.; Binkley, J.
S.; Frisch, M. J.; Whiteside, R. A.; Hout, R. F.; Hehre, W. J. Int. J.
Quant. Chem. 1981, 15, 269.
(30) Yamaguchi, Y.; Schaeffer, H. F. J. Chem. Phys. 1980, 73, 2310.
(31) For an authoritative description of the ab initio methods and basis
sets employed here, see: Hehre, W. G.; Radom, L.; Schleyer, P. v.
R.; Pople, J. A. Ab initio Molecular Orbital Theory; Wiley: New York,
1986.
709 (m)
470 (sh)
455 (m)
δas(C4-N)
δs(C4-N)
δ(C4-N)
377 (2)
a Frequencies in cm-1
.
shows expanded sections of the Raman spectrum of (CH3)4-
N+BrF2- (A) as well as of the decomposition spectrum of BrF2
(B). Figure 3 depicts a correlation diagramm of the experi-
mental and calculated frequencies.
-
The assignment for the normal modes of the tetramethyl-
ammonium cation is obtained from comparison with tetra-
methylammonium tetrafluorohalogenates(III) (CH3)4N+ClF4
-

4282 Inorganic Chemistry, Vol. 36, No. 19, 1997
Minkwitz et al.
deformation mode whose intensity is enhanced by Fermi
resonance (Figure 2A). Under the assumption of Fermi
resonance,14 the unperturbated frequencies of ν1 and 2ν2 are
expected at 463 and 467 cm-1, respectively.15 In contrast
Claassen et al. observed no Fermi resonance for the isoelectronic
krypton difluoride and explained their findings by the large
difference of about 16 cm-1 between ν1 and 2ν2.13 The intense
lines at 77 and 66 cm-1 in the Raman spectrum are attributed
to lattice vibrations.
In the infrared spectrum of (CH3)4N+BrF2- the antisymmetric
Br-F stretching mode appears at 450 cm-1 and the bending
mode was observed at 236 cm-1. The bending frequency of
236 cm-1 is in good agreement with that of 232.6 cm-1 observed
for the isoelectronic krypton difluoride, whereas the antisym-
metric stretching mode exhibits a significant shift toward smaller
wavenumbers.13
This effect is well-known for pentatomic fluorohalogenates.
- 16
17
A comparison of IF4
with the isoelectronic XeF4 shows
that the antisymmetric stretching mode of the anion is 138 cm-1
lower than that of the isoelectronic, neutral compound. The
antisymmetric stretching mode also has a lower frequency than
the symmetric stretching mode.
-
The vibrational spectroscopic data of BrF2 clearly suggest
a linear anion with D∞h symmetry. The large tetramethyl-
ammonim cations are preventing interanionic contacts and
distortion of the linear structure. This observation is in
agreement with the results of Naumann and Meurer, who
Figure 2. Trace A: Scale expansion of the Raman spectrum of
-
(CH3)4N+BrF2 and assignment of the resonances. Trace B: Partial
decomposition of (CH3)4N+BrF2-. Beside νs(Br-F) and the first
-
overtone δ(F-Br-F) of BrF2-, disproportionation product BrF4 is
seen (cation bands marked with an asterisk).
-
obtained a linear IF2 anion with tetraethylammonium as
counterion.3
Ab Initio Calculations. Ab initio calculations are of great
-
help for the assignment of the BrF2 resonances in the
experimental infrared and Raman spectra. In Table 2 and Figure
-
3, BrF2 is compared to isoelectronic KrF2 and other related
species with D∞h symmetry. Unfortunately structures with
atoms larger than Br could be calculated only with a small
3-21G* basis set. However, this basis set is known to yield
quite reasonable geometries and frequencies for hypervalent
structures. For consistency, all species were calculated with
this basis set. For all species only positive force constants were
obtained (see Table 3). This result is in contradiction to
calculated force constants for BrF2- 5 and KrF213 using the FG-
matrix method, which due to the underdetermined nature of the
problem could not yield well-determined interaction constants.
As can be seen from Figure 3, the δ(F-X-F) (X ) Cl, Br,
I) frequencies are in acceptable agreement with the available
experimental data. The trend of increasing frequencies on going
from IF2- to ClF2- allows the prediction of the δ(F-Cl-F) in
ClF2-, which could not be observed by Christe et al. because
the spectra were recorded only for wavenumbers larger than
Figure 3. Correlation diagram of the experimental frequencies of
BrF2-, KrF2, and other related compounds with D∞h symmetry (A)
compared with the frequencies calculated with a HF-3-21G* basis set
(B).
400 cm-1
.
The calculated symmetric and antisymmetric
stretching modes are in agreement with the experimental spectra
-
of BrF2 and IF2-. In particular the calculations indicate that
both frequencies lie close together within 30 cm-1 and νas(Br-
F) may be smaller than νs(Br-F). Naumann and Meurer3 found
the opposite behavior for IF2-, with νas(I-F) being observed
at higher frequency than νs(I-F). On the basis of our own
observations for BrF2- and other fluorohalogenates, we should
point out that the accurate determination of the experimental
frequencies of the antisymmetric stretching mode is difficult
because of the large bandwidths in the IR spectra. This may
cause uncertainties of about 20 cm-1 in the experimental values
of ν3.
- 11
and (CH3)4N+BrF4
,
as well as (CH3)4N+I(NO3)2-, (CH3)4-
N+I(NO3)4-, and (CH3)4N+Br(NO3)2
For a linear, centrosymmetric triatomic ion such as XY2
.
- 12
-
+
three normal modes of vibration, classified as (∑g + Πu +
∑
u
+), are expected, where ν2 (Πu) and ν3 (∑u+) are only infrared
active and ν1 (∑g+) is only Raman active.
The recorded Raman spectrum of (CH3)4N+BrF2- shows an
intense line at 460 cm-1 which is assigned as the symmetric
Br-F stretch. A comparison of νs(Br-F) with νs(Kr-F) for
the isoelectronic Krypton difluoride13 at 462 cm-1 yields good
agreement.
The consistency of the calculated results definitely allows
us to rule out the experimental assigment for νas(F-Cl-F)2 at
661 cm-1. Our calculations indicate that the intense IR band
-
A shoulder on the high-frequency side of the symmetric BrF2
stretching mode is attributed to the first overtone of the

-
(CH3)4N+BrF2
Inorganic Chemistry, Vol. 36, No. 19, 1997 4283
-
Table 2. Experimental and Calculated Raman and Infrared Frequenciesa and Calculated Bond Distancesb and Energiesh of (CH3)4N+BrF2
Compared to Those of Isoelectrronic KrF2 and Other Related Compounds with D∞h Symmetry
,
- c
-
- d
e
f
Rb+ClF2
Me4N+BrF2
Et4N+IF2
KrF2
XeF2
IR
Raman
476
IR
Raman
IR
Raman
IR
Raman
IR
Raman
assgnt
+
661
450
462
588
557
νas/ν3 (Σu
2δ/2ν2
)
470i
460i
+
470
445
111
462
496
108
νs/ν1 (Σg
δ/ν2 (Πu)
lattice
)
236
206
232
213
79/66
- g
- g
- g
g
g
ClF2
BrF2
IF2
KrF2
XeF2
IR
Raman
487
IR
Raman
489
IR
Raman
501
IR
Raman
558
IR
Raman
assgnt
+
479
480
494
649
635
νas/ν3 (Σu
νs/ν1 (Σg
δ/ν2 (Πu)
)
+
583
)
317
250
219
267
236
Calculated Bond Distancesb
- g
- g
- g
g
g
ClF2
BrF2
IF2
KrF2
XeF2
182.1
190.9
202.4
182.6
194.8
Calculated Energiesh and Signs Reversed
- g
- g
- g
g
g
ClF2
BrF2
IF2
KrF2
XeF2
655.11591
2758.03025
7085.72668
2936.97222
7398.44800
a Frequencies in cm-1 b Bond distances in pm. c Data from ref 2. d Data from ref 3. e Data from ref 13. f Data from ref 26. g Ab initio calculations
.
with a 3-21G* basis set. h Energies in Hartrees. i Fermi resonance.
-
Table 3. Calculated Frequenciesa for BrF2 at the RHF and MP2 Levels Using Different Standard Basis Sets
HF-3-21G
Raman
HF-3-21G*
HF-6-31+G*
HF-6-311G**
MP2-6-31+G*
IR
IR
Raman
IR
Raman
IR
Raman
IR
Raman
assgnt
+
391
480
368
393
434
νas/ν3 (Σu )
+
446
489
454
453
441
νs/ν1 (Σg )
239
250
231
231
215
δ/ν2 (Πu)
a Frequencies in cm-1
.
at 470 cm-1 observed by Christe et al. must be the antisymmetric
stretching mode and not the symmetric one. This assignment
is in good agreement with D∞h symmetry for ClF2- and follows
the rule of mutual exclusion.
-
disproportination of BrF2 into Br- and BrF4-. The signal at
-35.9 ppm is assigned to BrF4-, in good agreement with
literature values of about -37 ppm.21 The second signal with
a chemical shift of about -41.6 ppm belongs to the BrF2- anion.
NMR Spectra. The resonance signal at 54.5 ppm in the 13C-
NMR spectrum can be assigned to the quaternary ammonium
cation. Literature data for similar quaternary ammonium
salts18-20 are between 54.3 and 54.6 ppm. No signals in the
range between 119 and 125 ppm for CF3 groups remained.
Obviously the reaction, as described in eq 1, was complete. Two
resonances were observed in the 19F-NMR spectrum due to some
Acknowledgment. The financial support of this research by
the Fonds der Chemischen Industrie is greatfully acknowledged.
IC961283S