Tetrafluorophosphate anion

Article abstract of DOI:10.1021/ja963421i

The elusive POF₄^- anion has been characterized for the first time. It is formed from N(CH₃)₄F and POF₃ in CHF₃ solution at -140 °C and can be observed at this temperature by ¹⁹F and ³¹P NMR spectroscopy. Between - 140 and -100 °C it reacts with POF₃ forming the OF₂P-O-PF₅^- anion, which, at higher temperatures, reacts with F- anions to give PO₂F₂^- and PF₆^-. This reaction sequence provides a low activation energy barrier pathway for the highly exothermic dismutation of pseudo-trigonal-bipyramidal POF₄^- to tetrahedral PO₂F₂^- and octahedral PF₆^- and explains the previous failures to isolate POF₄^- salts. In addition to POF₄^-, OF₂POPF₅^-, and PO₂F₂^-, the protonated form of OF₂POPF₅^-, i.e., HOF₂POPF₅, was also identified as a by product by NMR spectroscopy. The structure, vibrational spectra, force field, NMR parameters, and dismutation energy of POF₄^- were calculated by ab initio electronic structure methods using, where required, the closely related and well-known PF₄^- ion and POF₃, PF₃, SOF₄, and SF₄ molecules for the determination of scaling factors. Although the structures and dismutation energies of isoelectronic POF₄^- and SOF₄ are very similar, their dismutation behavior is strikingly different. While POF₄^- dismutates rapidly at low temperatures, SOF₄ is kinetically stable toward dismutation to SO₂F₂ and SF₆. This fact is attributed to the lack of a low activation energy barrier pathway for SOF₄. Furthermore, the dismutation energy calculations for POF₄^-, SOF₄, and ClOF₃ revealed very large errors in the previously published thermodynamic data for the heats of formations of SO₂F₂ and SOF₄ and the dismutation reaction energy of POF₄^-. Energy barriers and the C(4v) transition states for the Berry-style pseudorotational exchange of equatorial and axial fluorines in POF₄^-, SOF₄, PF₄^-, and SF₄ were also calculated and can account for the observation that on the NMR time scale the exchange in PF₄^- and SF₄ can be frozen out at about -40 °C, while in POF₄^- and SOF₄ it is still rapid at -140 and -150 °C, respectively.

Full text of DOI:10.1021/ja963421i

3918
J. Am. Chem. Soc. 1997, 119, 3918-3928
Tetrafluorophosphate Anion
Karl O. Christe,*,1,2 David A. Dixon,3 Gary J. Schrobilgen,*,4 and
William W. Wilson1
Contribution from Hughes STX, Propulsion Directorate, Phillips Laboratory,
Edwards Air Force Base, California 93524, Loker Hydrocarbon Research Institute, UniVersity of
Southern California, Los Angeles, California 90089, Pacific Northwest National Laboratory,
Richland, Washington 99352, and Department of Chemistry, McMaster UniVersity, Hamilton,
Ontario L8S 4MI, Canada
ReceiVed September 30, 1996X
Abstract: The elusive POF4- anion has been characterized for the first time. It is formed from N(CH3)4F and POF3
in CHF3 solution at -140 °C and can be observed at this temperature by 19F and 31P NMR spectroscopy. Between
-
-140 and -100 °C it reacts with POF3 forming the OF2P-O-PF5 anion, which, at higher temperatures, reacts
with F- anions to give PO2F2- and PF6-. This reaction sequence provides a low activation energy barrier pathway
-
-
for the highly exothermic dismutation of pseudo-trigonal-bipyramidal POF4 to tetrahedral PO2F2 and octahedral
PF6- and explains the previous failures to isolate POF4- salts. In addition to POF4-, OF2POPF5-, and PO2F2-, the
protonated form of OF2POPF5-, i.e., HOF2POPF5, was also identified as a byproduct by NMR spectroscopy. The
structure, vibrational spectra, force field, NMR parameters, and dismutation energy of POF4- were calculated by ab
-
initio electronic structure methods using, where required, the closely related and well-known PF4 ion and POF3,
PF3, SOF4, and SF4 molecules for the determination of scaling factors. Although the structures and dismutation
energies of isoelectronic POF4- and SOF4 are very similar, their dismutation behavior is strikingly different. While
-
POF4 dismutates rapidly at low temperatures, SOF4 is kinetically stable toward dismutation to SO2F2 and SF6.
This fact is attributed to the lack of a low activation energy barrier pathway for SOF4. Furthermore, the dismutation
energy calculations for POF4-, SOF4, and ClOF3 revealed very large errors in the previously published thermodynamic
data for the heats of formations of SO2F2 and SOF4 and the dismutation reaction energy of POF4-. Energy barriers
-
and the C4V transition states for the Berry-style pseudorotational exchange of equatorial and axial fluorines in POF4
,
SOF4, PF4-, and SF4 were also calculated and can account for the observation that on the NMR time scale the
exchange in PF4- and SF4 can be frozen out at about -40 °C, while in POF4- and SOF4 it is still rapid at -140 and
-150 °C, respectively.
Introduction
only in equimolar mixtures of CsPO2F2 and CsPF6.12
On the basis of ion cyclotron resonance measurements5,6 and
ab initio calculations,7 the F- affinity of POF3 exceeds that of
PF3 by about 8 and 16 kcal mol-1, respectively. Our recent
synthesis of N(CH3)4PF4 from N(CH3)4F and PF3 in CH3CN
solution8 and its surprisingly high thermal stability up to 150
°C strongly suggested that the closely related, but yet unknown,
POF4- anion could also be isolated. This suggestion was also
supported by the fact that the corresponding isoelectronic sulfur
compounds, SF4 and SOF4, are both well-known and stable.9
Consequently, one would predict that POF3 should readily add
2CsF + 2POF3 f CsPO2F2 + CsPF6
(2)
In a recent 19F and 31P NMR study of the hydrolysis of AgPF6
1
in CD2Cl2, signals (19F: doublet at -85.8, JP-F ) 1016 Hz;
31P: triplet at -2.4, -10.9, -19.2 with intensities characteristic
of a quintet, rather than a triplet) were observed which, as will
be obvious from our results, were erroneously attributed to
- 13
POF4
.
The free gaseous POF4- ion has been observed as a
stable intermediate in an ion cyclotron resonance (ICR) study6
-
in which PF5 was reacted with tert-butoxide to give POF4
(reaction 3),
-
a fluoride ion to form a pseudo-trigonal-bipyramidal POF4
anion with a C2V structure similar to that of SOF4.10,11
t-C4H9O- + PF5 f POF4- + t-C4H9F
(3)
-
which then transferred F- to PF5 to give PF6 and POF3
(reaction 4).
-
POF4- + PF5 f POF3 + PF6
(4)
Previous attempts to prepare CsPOF4 from CsF and POF3
either without solvent at 130 °C or in CH3CN at 50 °C resulted
X Abstract published in AdVance ACS Abstracts, April 15, 1997.
(1) Hughes STX.
The POF3 can also react with the tert-butoxide anion to give
PO2F2 (reaction 5).
-
(2) University of Southern California.
(3) Pacific Northwest Laboratories.
(10) Oberhammer, H.; Boggs, J. E. J. Mol. Struct. 1979, 56, 107 and
references cited therein.
(4) McMaster University.
(11) Hargittai, I. J. Mol. Struct. 1979, 56, 301.
(5) Larson, J. W.; McMahon, T. B. J. Am. Chem. Soc. 1983, 105, 2944.
(6) Larson, J. W.; McMahon, T. B. Inorg. Chem. 1987, 26, 4018.
(7) Dixon, D. A.; Christe, K. O. Unpublished results.
(8) Christe, K. O.; Dixon, D. A.; Mercier, H. P. A.; Sanders, J. C. P.;
Schrobilgen, G. J.; Wilson, W. W. J. Am. Chem. Soc. 1994, 116, 2850.
(9) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements;
Pergamon Press: Oxford, 1986.
(12) (a) Lustig, M.; Ruff, J. K. Inorg. Chem. 1967, 6, 2115. (b) Note
Added in Proof: Prof. R. Schmutzler has brought a British Patent
(1,104,244, Feb 21, 1968), which was issued to Barry Tittle, to our attention,
in which KPOF4 and CsPOF4 were claimed as stable salts. This is an
erroneous claim, as shown by our study.
(13) Fernandez-Galan, R.; Manzano, B. R.; Otero, A.; Lanfranchi, M.;
Pellinghelli, M. A. Inorg. Chem. 1994, 33, 2309.
S0002-7863(96)03421-X CCC: $14.00 © 1997 American Chemical Society

Tetrafluorophosphate Anion
POF3 + t-C4H9O- f PO2F2- + t-C4H9F
J. Am. Chem. Soc., Vol. 119, No. 17, 1997 3919
enhancement prior to zero filling the free induction decay into a 128K
memory. Spectral width settings of 50 (19F) and 30 kHz (31P) were
employed, yielding data point resolutions of 1.53 (19F) and 1.80 Hz or
0.45 Hz for resolution enhanced spectra (31P) and acquisition times of
0.328 (19F) and 0.557 s (31P), respectively. Relaxation delays were
not applied. Typically, 10 000 transients were accumulated. Pulse
widths were 1.0 (19F) and 3.0 µs (31P). Line broadening parameters
used in the exponential multiplication of the free induction decays were
0.5 (19F) and -4.0 Hz with a Gaussian block parameter of 0.5 for
Gaussian multiplication (31P). Temperatures were measured with a
copper-constantan thermocouple inserted directly into the probe, are
considered accurate to (1 °C, and were constant to less than (0.1 °C.
The spectra were referenced to neat external samples of CFCl3 (19F)
and 85% H3PO4 (31P) at ambient temperature, and the IUPAC sign
convention for chemical shifts was used.
Samples for NMR spectroscopy were prepared in medium-wall Pyrex
glass NMR tubes (Wilmad). The tubes were fused to 3-cm lengths of
0.25-in. o.d. glass tubing and joined to J. Young glass/Teflon valves
through 0.25-in. 316 stainless steel Cajon Ultra Torr unions and dried
overnight by pumping on a glass vacuum line. Weighed amounts of
N(CH3)4F were loaded into the NMR tubes in the drybox and then
transferred to a calibrated metal vacuum manifold where CHF3 solvent
(ca. 0.25 mL) followed by a known pressure of POF3 were condensed
at -196 °C onto N(CH3)4F, before flame sealing the tubes. The
following sample compositions, N(CH3)4F:POF3 ) 0.495, 0.899, 0.957,
and 3.56, were studied.
Attempted Bulk Synthesis of N(CH3)4POF4. A weighed amount
of anhydrous N(CH3)4F was placed inside the drybox into a prepassi-
vated (with ClF3) Teflon-FEP container which was closed by a stainless
steel valve. On the vacuum line, POF3 and CHF3 (large excess) were
added to the Teflon container at -196 °C. The mixture was warmed
to -140 °C with agitation and kept at this temperature for 2 h. Attempts
to isolate solid N(CH3)4POF4 by removal of the CHF3 solvent under
vacuum at -126 °C produced a white solid of the correct weight
expected for N(CH3)4POF4, but its vibrational spectra recorded at room
temperature showed an equimolar mixture of N(CH3)4PO2F2 and
N(CH3)4PF6. When this reaction was repeated either in other solvents,
such as SO2 at -78 °C, POF3 at -64 °C, or CH3CN at -31 °C, or
without a solvent at 25 °C with use of 2 Torr of gaseous POF3 in a
flamed out glass bulb, again only the dismutation products N(CH3)4-
PO2F2 and N(CH3)4PF6 were observed as final room-temperature stable
products.
Computational Methods. A variety of electronic structure calcula-
tions were performed in order to calculate the geometries, relative
energies, and vibrational frequencies of the phosphorus and sulfur
species. The electronic structure calculations were done at a number
of different levels. The first set of calculations were done at the
Hartree-Fock (HF) level with the program GRADSCF on Cray YMP
and C90 computer systems.25 A polarized double-ú valence basis set
(DZP) from Dunning and Hay26 was used for F and O, and the McLean
and Chandler sets27 (6s/4p) were used for S and P augmented by d(P)
) 0.5 and d(S) ) 0.6. Subsequently, the DZP basis set was augmented
by a set of diffuse p functions on all atoms (DZP+) with the following
exponents: p(O) ) 0.059, p(F) ) 0.074, p(P) ) 0.035, and p(S) )
0.041. The geometries and frequencies of the lowest energy species
were calculated at this level by using analytic derivative methods.28,29
The HF/DZP+ second derivative results were used to calculate the
molecular force fields with the program BMATRIX.25
(5)
Although the final products of these ion cyclotron resonance
-
reactions, PO2F2 and PF6-, were the same as those in the
dismutation reaction 2, their formation involved the tert-butoxide
ion and not two POF4- anions, which would be unlikely to react
with each other in the gas phase. Based on the relative F- ion
affinities, determined from the ICR reactions, the dismutation
reaction 6
-
2POF4- f PO2F2- + PF6
(6)
-
was estimated6 to be exothermic by 76 kcal mol-1, with PO2F2
and PF6- being the thermodynamically favored products. This
seemed in sharp contrast to isoelectronic SOF4 for which, based
on the published thermodynamic values for SOF4,14,15 SO2F2,16-18
and SF6,16 the analogous dismutation reaction 7
2SOF4 f SO2F2 + SF6
(7)
would be about thermally neutral. This huge energy difference
for two isoelectronic systems was puzzling and needed either
verification or rationalization.
In view of these challenges, it was interesting to attempt the
synthesis and characterization of a POF4- salt, to elucidate the
mechanism and energy of its dismutation reaction 6, and to study
its fluxionality and axial and equatorial fluorine ligand ex-
change.8
Experimental Section
Apparatus and Materials. Volatile materials were handled on
either a flamed-out Pyrex glass vacuum line equipped with Kontes glass-
Teflon valves and a Heise pressure gauge or a nickel/stainless steel
vacuum line equipped with MKS type 122A pressure transducers (0-
1000 Torr, (0.5% of reading) having fluorine passivated Inconel as
the wetted surface and a model PDR-5B five-channel digital readout
and power supply. Nonvolatile materials were handled in the dry
nitrogen atmosphere of a glovebox. The infrared and Raman spec-
trometers have previously been described.19 Literature methods were
21,22
used for the syntheses of N(CH3)4F20 and POF3
and the drying of
CH3CN.23,24 CHF3 (The Matheson Co. or Canadian Liquid Air) and
SO2 (The Matheson Co.) were purified by fractional condensation prior
to their use.
Nuclear Magnetic Resonance Spectroscopy. The 19F (282.409
MHz) and 31P (121.497 MHz) NMR spectra were recorded unlocked
(field drift <0.1 Hz h-1) without spinning on a Bruker AM 300
1
spectrometer equipped with a 7.0463 T cryomagnet and a 5-mm H/
13C/19F/31P combination probe. Free induction decays were typically
accumulated in 64K (19F) and 32K (31P) memories. In the case of 31
P
spectra, a Gaussian line shape function was applied for resolution
(14) Dittmer, G.; Niemann, U. Philips J. Res. 1982, 37, 1.
(15) Herron, J. T. J. Phys. Chem. Ref. Data 1987, 16, 1.
(16) Stull, D. R.; Prophet, H. JANAF Thermochemical Tables, 2nd ed.;
U.S. GPO, Washington, DC, 1971; 1978 Supplement. J. Phys. Chem. Ref.
Data 1978, 7, 793.
(17) Chase, M. W., Jr.; Curnett, J. L.; Downey, J. R., Jr.; McDonald, R.
A.; Syverud, A. N.; Valenzuela, E. A. J. Phys. Chem. Ref. Data 1982, 11,
695.
(18) Reese, R. M.; Dibeler, V. H.; Franklin, J. L. J. Chem. Phys. 1958,
29, 880.
(19) Christe, K. O.; Wilson, W. W.; Bau, R.; Bunte, S. W. J. Am. Chem.
Soc. 1992, 114, 3411.
(25) GRADSCF is an ab initio program system designed and written by
A. Komornicki at Polyatomics Research, Mountain View, CA. BMATRIX
is an auxiliary program used for force constant analysis.
(26) Dunning, T. H., Jr.; Hay, P. J. In Methods of Electronic Structure
Theory; Schaefer, H. F., III, Ed.; Plenum Press: New York, 1977; Chapter
1.
(20) Christe, K. O.; Wilson, W. W.; Wilson, R. D.; Bau, R.; Feng, J. J.
Am. Chem. Soc. 1990, 112, 7619.
(21) Christe, K. O.; Gnann, R.; Wagner, R. I.; Wilson, W. W. Eur. J.
Solid State Inorg. Chem. 1996, 33, 865.
(22) Kwasnik, W. In Handbook of PreparatiVe Inorganic Chemistry, 2nd
ed.; Brauer, G., Ed.; Academic Press: New York, 1963; Vol. 1, Section 4,
pp 193-194.
(23) Christe, K. O.; Dixon, D. A.; Mahjoub, A. R.; Mercier, H. P. A.;
Sanders, J. C. P.; Seppelt, K.; Schrobilgen, G. J.; Wilson, W. W. J. Am.
Chem. Soc. 1993, 115, 2696.
(27) McLean, A. D.; Chandler, G. S. J. Chem. Phys. 1980, 72, 5369.
(28) (a) Komornicki, A.; Ishida, K.; Morokuma, K.; Ditchfield, R.;
Conrad, M. Chem. Phys. Lett. 1977 45, 595. (b) McIver, J. W., Jr.;
Komornicki, A. Chem. Phys. Lett. 1971, 10, 202. (c) Pulay, P. In
Applications of Electronic Structure Theory; Schaefer, H. F., III, Ed.; Plenum
Press: New York, 1977; p 153. (d) Breidung, J.; Thiel, W.; Komornicki,
A. Chem. Phys. Lett. 1988, 153, 76.
(29) (a) King, H. F.; Komornicki, A. J. Chem. Phys. 1986, 84, 5465. (b)
King, H. F.; Komornicki, A. In Geometrical DeriVatiVes of Energy Surfaces
and Molecular Properties; Jorgenson, P., Simons, J., Eds.; NATO ASI Series
C. Vol. 166; D. Reidel: Dordrecht, 1986; p 207.
(24) Winfield, J. M. J. Fluorine Chem. 1984, 25, 91.

3920 J. Am. Chem. Soc., Vol. 119, No. 17, 1997
Christe et al.
Figure 1. NMR spectra (7.0463 T) of a 0.90:1.00 molar ratio of POF3:N(CH3)4F recorded in CHF3 solvent at -140 °C: (a) 31P spectrum (121.497
MHz) and (b) 19F spectrum (282.409 MHz). The labeling scheme is as follows: (A) POF4-, (B) PO2F2-, (C) OF2P-O-PF5-, (D) HOF2P-O-PF5,
-
(E) POF3, (S) CHF3 solvent. Peaks denoted by (*), (†), and (‡) are unassigned. Expansions in the 31P spectrum of OF2P-O-PF5 (C) represent
multiplet fine structures visible under high resolution on each branch of the triplet and pseudosextet (doublet of quintets) arising from the PA and
PB environments, respectively, in structure A.
Subsequent calculations on various conformers of the four molecular
species were done with the program Gaussian94 on an SGI Indigo2.30
The basis set was of polarized double-ú valence quality with the same
basis set for F and O given above and the (5s/3p) set of McLean and
Chandler augmented by the same d functions given above. The
optimized HF geometries were used to calculate the MP2 energies31 at
the frozen core level. The HF/DZVP geometries were used to calculate
NMR chemical shifts at the nonlocal BLYP level with the nonlocal
exchange potential of Becke32 and the nonlocal correlation potential
of Lee, Yang, and Parr.33 The basis set for the NMR calculations was
of triple-ú form27,34 augmented by two sets of d polarization functions
each formed from two Gaussian functions and an f polarization function.
The origin problem for the NMR chemical shift calculations was
handled by using the GIAO method.35 In order to compare to
experiment, the following standards were used: CCl3F for F, [PO4]3-
for P, and H2O for O. The difference in the shift from the standard is
reported.
Density functional calculations were done with the program DGauss36
at the local (LDFT) level with a polarized double-ú basis set (DZVP2).37
The local potential fit of Vosko, Wilk, and Nusair38 was used.
Geometries were optimized at this level and frequencies were calculated
by analytic derivative methods.39 These geometries were used to
calculate the NMR chemical shifts40 at the IGLO41 and LORG levels.42
In order to calculate the dismutation reaction energies, additional
calculations were done. All calculations were done with the HF
geometries. The additional calculations were done with the correlation-
consistent basis sets43,44 at the cc-VDZ and aug-cc-VTZ levels at the
(35) Cheeseman, J. R.; Trucks, G. W.; Keith, T. A.; Frisch, M. J. J.
Chem. Phys. 1996, 104, 5497. London, F. J. Phys, Radium (Paris) 1937,
8, 397. Ditchfield, R. Mol. Phys. 1974, 27, 789. Wolinski, K.; Hinton, J.
F.; Pulay, P. J. Am. Chem. Soc. 1990, 112, 8251.
(36) (a) Andzelm, J.; Wimmer, E.; Salahub, D. R. In The Challenge of
d and f Electrons: Theory and Computation; Salahub, D. R., Zerner, M.
C., Eds.; ACS Symposium Series, No. 394; American Chemical Society:
Washington, DC, 1989; p 228. (b) Andzelm, J. In Density Functional Theory
in Chemistry; Labanowski, J., Andzelm, J., Eds.; Springer-Verlag: New
York, 1991; p 155. (c) Andzelm, J. W.; Wimmer, E. J. Chem. Phys. 1992,
96, 1280. DGauss is a density functional program available from Oxford
Molecular.
(37) Godbout, N.; Salahub, D. R.; Andzelm, J.; Wimmer, E. Can. J.
Chem. 1992, 70, 560.
(38) Vosko, S. J.; Wilk, L.; Nusair, W. Can J. Phys. 1980, 58, 1200.
(39) Komornicki, A.; Fitzgerald, G. J. Phys. Chem. 1993, 98, 1398 and
references therein.
(30) Gaussian 94: 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.;
Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.;
Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov,
B. 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.; Stewart, J. J. P.;
Head-Gordon, M.; Gonzalez, C.; Pople, J. A., Gaussian, Inc.; Pittsburgh,
PA, 1995.
(31) (a) Møller, C.; Plesset, M. S. Phys. ReV. 1934, 46, 618. (b) Pople,
J. A.; Binkley, J. S.; Seeger, R. Int. J. Quantum Chem. Symp. 1976, 10, 1.
Pople, J. A.; Krishnan, R.; Schlegel, H. B.; Binkley, J. S. Int. J. Quantum
Chem. Symp. 1979, 13, 325. Handy, N. C.; Schaefer, H. F., III J. Chem.
Phys. 1984, 81, 5031.
(32) (a) Becke, A. D. Phys. ReV. A 1988, 38, 3098. (b) Becke, A. D. In
The Challenge of d and f Electrons: Theory and Computation; Salahub,
D. R., Zerner, M. C., Eds.; ACS Symposium Series, No. 394; American
Chemical Society: Washington, DC, 1989; p 166. (c) Becke, A. D. Int. J.
Quantum Chem. Symp. 1989, 23, 599.
(33) Lee, C.; Yang, W.; Parr, R. G. Phys, ReV. B 1988, 37, 785.
(34) Dunning, T., Jr. J. Chem. Phys. 1971, 55, 716.
(40) Arduengo, A. J., III; Dixon, D. A.; Kumashiro, K. K.; Lee, C.;
Power, W. P.; Zilm, K. W. J. Am. Chem. Soc. 1994, 116, 6361.
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Tetrafluorophosphate Anion
J. Am. Chem. Soc., Vol. 119, No. 17, 1997 3921
-
Table 1. NMR Parameters for the POF4-, PO2F2-, and OF2POPF5 Anions and POF3 and HOF2POPF5 in CHF3 solution at -140 °C
coupling constants, Hz
δ(31P), ppm
δ(19F), ppm
1J
1J
1J
2J
2J
2J
3J
3J
4J
(31PA- (31PB- (31PB- (31PA- (31PA- (19Fb- (31PA- (31PA- (19Fa-
speciesa
PA
PB
Fa
Fb
Fc
19Fa) 19Fb) 19Fc) 31PB)
1H)
19Fc) 19Fb)
19Fc) 19Fb)
-
POF4
POF3
-75.1 (quin)b
-33.9 (quar)
-14.2 (tr)
-61.6 (d)
-92.1 (d)
-84.4 (d)
819
1061
948
-
PO2F2
-
OF2POPF5 -28.9
-148.1
-87.3 -63.0
-79.8
(d, quin)d
not obsde
979
736
706
729
614
3
55.3
41.0
7.4
3.9
3
(tr, quin, d)
(d, quin, d, tr)c
(d)
(d, d)
HOF2POPF5 -27.4
-131.9
(d, quin, d, tr)
-86.9 -63.9
968
12
(tr, d, m)
(d)
(d, d)
a For a definition of the individual atoms see structures A and B. b d ) doublet, tr ) triplet, quar ) quartet, quin ) quintet, m ) unresolved.
c The similarity of 1J(31PB-19Fb) and 1J(31PB-19Fc) causes the expected doublet of quintets to have the appearance of a sextet. d Only one of the two
quintets was observed; the second one was obscured by the solvent peak; the listed δ value was obtained from the position of the observed quintet
1
and the known J(31PB-19Fc) value. e Probably obscured by the CHF3 solvent peak.
MP2 level. In addition, higher levels of correlation were examined at
not observed at -140 °C. Rather, the spectra show that the
POF4- anion, like SOF4,47 is fluxional on the NMR time scale
the coupled cluster single and double excitation with a correction for
triples (CCSD(T)) level45 with the cc-VDZ basis set. The CCSD(T)
(see below).
calculations were done with the program MOLPRO.46
A second major species (peaks B in Figure 1) exhibits a triplet
at -14.2 ppm in the 31P NMR spectrum and a doublet at -84.4
Results and Discussion
ppm in the 19F NMR spectrum [1J(31P-19F) ) 948 Hz] and is
-
assigned to the previously characterized PO2F2 anion.13,48,49
Resonances assigned to unreacted POF3 (19F, -92.1 ppm,
doublet (peaks E in Figure 1); 31P, -33.9 ppm, quartet (not
shown in Figure 1); 1J(31P-19F) 1061 Hz)50,51 were also
observed.
Synthesis and Properties of N(CH3)4POF4. The synthesis
-
of POF4 salts is very difficult and was achieved only from a
soluble F- ion source20 and POF3 in an inert solvent, such as
CHF3, at temperatures of about -140 °C.
CHF3
A fourth set of intense resonances (peaks C in Figure 1) is
assigned to the oxygen-bridged OF2P-O-PF5- anion (structure
A). The 19F resonance of the POF2 group occurs at -87.3 ppm
N(CH3)4F + POF3 -140 °C8 N(CH3)4POF4
(8)
Even at this low temperature, the POF4- anion already starts to
dismutate according to (6), and at elevated temperatures
-
-
equimolar mixtures of PO2F2 and PF6 salts are observed as
the only products. Thus, our attempts to isolate solid N(CH3)4-
POF4 from solutions of N(CH3)4F and POF3 in CHF3 at -126
°C, SO2 at -78 °C, POF3 at -64 °C, or CH3CN at -31 °C
produced only the dismutation products. Similarly, a solvent-
free reaction of N(CH3)4F with 2 Torr of gaseous POF3 at 35
and is comprised of a doublet arising from coupling to the
directly bonded phosphorus atom [1J(31PA-19Fa) ) 979 Hz].
The PF5 group displays an AX4 pattern in the 19F NMR spectrum
with the doublet at -63.0 ppm and the quintet at -79.8 ppm
[2J(19Fb-19Fc) ) 55.3 Hz]. Both multiplets are split into
doublets by their respective one-bond couplings to phosphorus
-
°C did not result in POF4 formation. In view of these
difficulties, N(CH3)4POF4 could not be isolated as a stable solid
and was characterized by its multinuclear NMR spectra in CHF3
solution at -140 °C (see below). These results are in accord
with the earlier report12 that CsF and POF3 in CH3CN at 50 °C
produced only CsPO2F2 and CsPF6.
1
[1J(31PB-19Fb) ) 736 Hz and J(31PB-19Fc) ) 729 Hz] (one
19F and 31P NMR Spectroscopy. The 19F and 31P NMR
spectra of about equimolar mixtures of N(CH3)4F and POF3
recorded at -140 °C in CHF3 (Figure 1 and Table 1) consist of
several first-order multiplet patterns. An intense quintet centered
set of the quintets is masked by the intense CHF3 solvent doublet
at -81.62 ppm). The 31P NMR spectrum consists of two sets
of intense complex multiplets. A triplet of quintets of doublets
centered at -29.0 ppm is assigned to PA and arises from
1J(31PA-19Fa), 3J(31PA-19Fb), and 3J(31PA-19Fc) ) 3.9 Hz. The
multiplet at -148.1 ppm is assigned to PB, which appears to be
a sextet under low resolution conditions but is shown actually
to be a severely overlapping doublet of quintets at higher
resolution that arises from two nearly equal one-bond 31P-19F
couplings, namely 1J(31PB-19Fb) and 1J(31PB-19Fc). Resolution
enhancement using a Gaussian line fit reveals that each line of
the doublet of quintets pattern is further split into an overlapping
doublet of triplets arising from 2J(31PA-31PB) ) 3 Hz and
4J(19Fa-19Fb) ) 3 Hz to give a pseudoquartet. The OF2P-O-
-
at -75.1 ppm in the 31P NMR spectrum is assigned to the POF4
anion and has its doublet counterpart in the 19F NMR spectrum
at -61.6 ppm with J(31P-19F) ) 819 Hz (peaks A in Figure
1
1). Although POF4- is expected to have a trigonal-bipyramidal
geometry like the isoelectronic SOF4 molecule, with the oxygen
and two fluorines occupying the equatorial plane and two
fluorines occupying the axial positions,10,11 the limiting spectrum
in which the axial and equatorial fluorines are inequivalent was
(43) Dunning, T. H., Jr. J. Chem. Phys. 1989, 90, 1007.
(44) Kendall, R. A.; Dunning, T. H., Jr.; Harrison, R. J. J. Chem. Phys.
1992, 96, 6796.
(45) (a) Bartlett, R. J. J. Phys. Chem. 1989, 93, 1697. (b) Kucharski, S.
A.; Bartlett, R. J. AdV. Quantum Chem. 1986, 18, 281. (c) Bartlett, R. J.;
Stanton, J. F. In ReViews of Computational Chemistry; Lipkowitz, K. B.,
Boyd, D. B., Eds.; VCH Publishers: New York, 1995; Vol. V, Chapter 2,
p 65.
(46) MOLPRO is a package of ab initio programs written by Werner,
H.-J. and Knowles, P. J. with contributions from Almlof, J., Amos, R. D.,
Deegan, M. J. O., Elbert, S. T., Hampel, C., Meyer, W., Peterson, K. A.,
Pitzer, R. M., Stone, A. J., Taylor, P. R., and Lindh, R.
(47) Christe, K. O.; Schack, C. J.; Curtis, E. C. Spectrochim. Acta, Part
A 1977, 33A, 323.
-
PF5 anion has been previously characterized by 19F and 31P
NMR spectroscopy in CH3CN solution,48 and in view of the
different solvent and large temperature difference, the NMR
(48) Il’in, E. G.; Meisel, M.; Shcherbakova, M. N.; Wolf, G. U.; Buslaev,
Yu. A. Dokl. Akad. Nauk SSSR 1982, 266, 878.
(49) Reddy, G. S.; Schmutzler, R. Z. Naturforsch. Teil B 1970, 25b, 1199.
(50) Moedritzer, K.; Maier, L.; Groenweghe, L. C. D. J. Chem. Eng.
Data 1962, 7, 307.
(51) Gutowsky, H. S.; McCall, D. W.; Slichter, C. P. J. Chem. Phys.
1953, 21, 279.

3922 J. Am. Chem. Soc., Vol. 119, No. 17, 1997
Christe et al.
parameters obtained in the present study are in good agreement
with the published values (two spin-spin couplings not reported
for the OF2P-O-PF5- anion in the previous study are reported
for this ion. The 19F shielding decreases upon going from POF3
-
[δ(19F), -92.1 ppm] to POF4 [δ(19F), -61.6 ppm] and from
-
PF3 [δ(19F), -34.3 ppm52] to PF4 [average δ(19F), -18.7
2
3
in this study, i.e., J(31PA-31PB) and J(31PA-19Fc)).
ppm8], and is a well-established trend for neutral fluorides and
their fluoroanions.53 This shielding trend is also found for
neutral fluorides and their cations, as in the structurally related
Several weak multiplets also appear in the 19F and 31P NMR
spectra at -140 °C. One set of multiplets (peaks D in Figure
1) is assigned to the protonated OF2P-O-PF5- anion, namely,
HOF2P-O-PF5 (structure B). In the 31P spectrum, a triplet
+
pairs SOF3 [δ(19F), 32 ppm54] > SOF4 [δ(19F), 75 ppm54],
+
SF3 [δ(19F), 25.5 to 30.5 ppm55] > SF4 [average δ(19F), 95
ppm56], and SeF3+ [δ(19F), 6.0 ppm57] > SeF4 [average δ(19F),
24.8 ppm57]. On the basis of the simple “atom-in-a-molecule”
approach to the paramagnetic contribution to nuclear shielding,58
the lowest molecular excitation energy is expected to increase
with increasing positive charge so that the paramagnetic
contribution to the shielding increases. Apparently, the effect
of increased E-F bond order in the more positively charged
species, which causes greater deviations from the spherical
symmetry of F- and a larger paramagnetic contribution, is offset
by the larger molecular excitation term. The opposite trend is
noted in the 31P shieldings, which increase in the order POF3
[1J(31PA-19Fa) ) 968 Hz] of doublets [2J(31PA-1H) ) 12 Hz]
of unresolved multiplets due to PA occurs at -27.4 ppm, and a
doublet [2J(31PB-19Fc) ) 614 Hz] of quintets [2J(31PB-19Fb) )
706 Hz] is observed for PB. In the 19F NMR spectrum, a doublet
[1J(31PA-19Fa)] at -86.9 ppm due to the two Fa atoms and a
doublet [1J(31PB-19Fb)] of doublets [2J(19Fb-19Fc) ) 41 Hz] at
-63.9 ppm due to the four Fb atoms are observed. The doublet
of quintets, expected for Fc, has not been observed because of
its relative broadness, low intensity, and/or probable overlap
with the strong CHF3 solvent peak. Although the 2J(31PA-31PB),
3J(31PA-19Fb), and 3J(31PA-19Fc) couplings were not observed
under our resolution conditions because of their expected small
magnitudes (cf. corresponding coupling constants for structure
A in Table 1), the given assignments are tied together through
either common coupling constants or constant relative intensities
and growth patterns in different samples. The NMR parameters
of the HOF2PO group are in good agreement with those reported
for HOPOF2 in CH3CN solvent at room temperature,48 although
2J(31P-1H) was not observed for HOPOF2 in the previous study.
In that study, the 31P resonance of HPO2F2 occurred to slightly
-
[δ(31P), -33.9 ppm] < POF4 [δ(31P), -75.1 ppm] and PF3
-
[δ(31P), 103.5 ppm59] < PF4 [δ(31P), 42.3 ppm8], and is also
+
observed for the 77Se shieldings SeF3 [δ(77Se), 1122 ppm60]
< SeF4 [δ(77Se), 1082 ppm60]. Decreases in central element
shielding with decreasing coordination number are general and
correlate with increasing p-orbital populations associated with
a more highly positive (electronegative) central atom.58
The 1J(31P-19F) coupling decreases in the order POF3 (1061
-
Hz) > POF4 (819 Hz). Analogous trends are observed for
-
PF3 (1401 Hz59) > PF4 (average, 1032 Hz8) and for the
1J(77Se-19F) couplings of SeF3+ (1213 Hz57) > SeF4 (average,
745 Hz61), which are consistent with a decrease in the bond
-
order for the more polar P-F and Se-F bonds of POF4-, PF4
,
and SeF4 relative to those of POF3, PF3, and SeF3+, respectively.
The magnitudes of the scalar couplings can be correlated to
the s-electron densities at the spin-coupled nuclei for a Fermi
contact dominated spin-spin coupling mechanism.62 The
decrease in scalar coupling is consistent with the anticipated
decrease in s-character with increasing bond polarity and
coordination number.
-
higher frequency of PO2F2 with 19F shifts being almost the
same, and similar trends are noted for HOF2P-O-PF5 and
OF2P-O-PF5-. The remaining very weak spectral features
appearing in the 19F and 31P NMR spectra have not been
assigned.
-
-
-
Fluxionality in POF4 and SOF4. The POF4 anion, like
its isoelectronic analog, SOF4,47,63 undergoes rapid intramo-
lecular exchange of its axial and equatorial fluorines. The
mechanism by which the intramolecular ligand exchange process
Warming a CHF3 solution containing POF4 from -140 to
-
-90 °C resulted in loss of the 19F and 31P resonances of POF4
with only the PO2F2- and OF2P-O-PF5- resonances remain-
ing. The decomposition is in accord with global reaction 6 with
POF4 dismutating to PO2F2 and PF6 by means of the
-
-
-
-
occurs in POF4 and SOF4 is most likely the classical Berry
pseudorotation mechanism64 involving a pyramidal C4V transition
-
intermediate dimeric OF2P-O-PF5 anion. Although no
-
resonances attributed to the other dismutation product, PF6
,
(52) Muetterties, E. L.; Phillips, W. D. J. Am. Chem. Soc. 1957, 79,
322.
(53) Jameson, C. J. In Multinuclear NMR; Mason, J., Ed.; Plenum
Press: New York, 1987; Chapter 16, p 441.
(54) Brownstein, M.; Dean, P. A. W.; Gillespie, R. J. Chem. Commun.
1970, 9.
(55) Azeem, M.; Brownstein, M.; Gillespie, R. J. Can. J. Chem. 1969,
47, 4159.
(56) Muetterties, E. L.; Phillips, W. D. J. Am. Chem. Soc. 1959, 81,
1084.
were observed in the 19F and the 31P spectra, the solution at
-90 °C contained a white precipitate of what is presumably
N(CH3)4PF6. The formation of N(CH3)4PF6 as a main product
under these conditions was established by vibrational spectros-
copy of the solid products.
-
A recent paper reports the preparation of POF4 by the
hydrolysis of AgPF6 in CD2Cl2 at room temperature.13 The
reported NMR parameters consist of a doublet in the 19F NMR
(57) Brownstein, M.; Gillespie, R. J. J. Chem. Soc., Dalton Trans. 1973,
67.
1
spectrum at -85.8 ppm, J(31P-19F) ) 1016 Hz, and a triplet
in the 31P NMR spectrum, unconventionally reported in ppm at
-2.4, -10.9, and -19.2, with relative intensities characteristic
of a quintet rather than a triplet. The 31P chemical shift and
scalar coupling differ significantly from those determined for
POF4- in the present work, and in fact, the 31P-19F coupling is
more in line with that for tetracoordinate phosphorus.49 More-
over, the reported stability of the species at ambient temperature
(58) Jameson, C. J.; Mason, J. In Multinuclear NMR; Mason, J., Ed.;
Plenum Press: New York, 1987; Chapter 3, pp 63-67.
(59) Christe, K. O.; Dixon, D. A.; Sanders, J. C. P.; Schrobilgen, G. J.;
Wilson, W. W. Inorg. Chem. 1994, 33, 4911.
(60) Birchall, T.; Gillespie, R. J.; Vekris, S. L. Can. J. Chem. 1965, 43,
1672.
(61) Damarius, R.; Huppmann, P.; Lentz, D.; Seppelt, K. J. Chem. Soc.,
Dalton Trans. 1984, 2821.
(62) Jameson, C. J. In Multinuclear NMR; Mason, J., Ed.; Plenum
Press: New York, 1987; Chapter 4, pp 102-104.
(63) Dudley, F. B.; Shoolery, J. N.; Cady, G. H. J. Am. Chem. Soc. 1956,
78, 568.
is not in agreement with the observed thermal instability of
-
POF4
.
-
The NMR parameters for POF4 follow several previously
(64) (a) Berry, R. S. J. Chem. Phys. 1960, 32, 933. (b) Wasada, H.;
Hirao, K. J. Am. Chem. Soc. 1992, 114, 16.
established trends49-51 and are in accord with our expectations

Tetrafluorophosphate Anion
J. Am. Chem. Soc., Vol. 119, No. 17, 1997 3923
-
Table 2. Calculated and Predicted Geometries of POF4 Compared to Calculated and Observed Geometries of Isoelectronic SOF4
-
POF4
SCF/DZ+P+
SOF4
SCF/DZ+P+
SCF/DZ+P
LDF/DZVP2
pred
1.47
1.59
1.65
98.6
113.4
SCF/DZ+P
LDF/DZVP2
obsda
r(X-O) (Å)
1.465
1.583
1.648
98.3
1.464
1.579
1.646
97.93
112.4
1.493
1.630
1.677
99.4
1.406
1.529
1.583
97.2
1.403
1.528
1.581
97.36
111.8
1.440
1.592
1.634
98.2
1.409
1.539
1.596
97.7
r(X-Feq) (Å)
r(X-Fax) (Å)
O-X-Fax (deg)
Feq-X-Feq (deg)
113.0
114.5
111.4
111.7
112.8
a Values from refs 10 and 11.
state with four equivalent fluorine atoms.65 NMR spectroscopy
alone cannot distinguish between this and other mechanisms
that result in the permutation of fluorine nuclei.66,67 Other
mechanisms have been favored for compounds such as ClF3,68
which possesses two free valence electron pairs on the central
atom, or SiH4F- 65 and PH4F,69 where in the minimum energy
structures, the two axial positions are occupied by one fluorine
and one hydrogen ligand. Failure to slow the exchange process
in POF4- at -140 °C and in SOF447 at -150 °C indicates that
both species have very low activation barriers to intramolecular
bipyramidal structure of C2V symmetry as the energy minimum,
in accord with the experimentally known10,11 structure of SOF4.
By analogy with our previous study8 of the closely related PF3
and PF4- species, the SCF calculations gave better results than
those at the DFT level. By using the calculated and the
experimentally known10,11 SOF4 results as a basis for correcting
-
the calculated POF4 values, the following geometry, which
we expect to be very close to the true geometry, is predicted
-
for POF4
.
-
exchange. This behavior contrasts with the fluxionality of PF4
and SF4 for which limiting spectra have been obtained and
activation barriers of 10.3 kcal mol-1 for PF4- 8 and 11.8 (neat
liquid)70 and 12.4 kcal mol-1 (gas phase)71 for SF4 have been
determined by variable-temperature NMR experiments. The
ν4(A1) and ν5(A1) antisymmetric combination of axial and
equatorial scissoring bends in PF4-/SF4 and POF4-/SOF4,
This geometry is very similar to that established10,11 for SOF4.
The bond angles are almost identical and all bonds in POF4
respectively, correspond to the motions involved in the Berry
-
- 8
pseudorotation exchange mechanism. In PF4
and SF4,72
are somewhat longer (0.05-0.07Å) than the corresponding ones
in SOF4, as expected for an increased polarity of the bonds due
to the formal negative charge in POF4-. The compression of
the F-P-F angles by the oxygen ligand, which causes a
deviation from the ideal trigonal-bipyramidal geometry with
180° and 120° bond angles, is in accord with the electron domain
of the P-O double bond being larger than those of the P-F
single bonds.73
ν4(A1) occurs at 210 and 228 cm-1, respectively, paralleling
the experimentally determined activation barriers. In POF4
-
and SOF4,47 the respective ν5(A1) modes are very similar to
each other but occur at significantly lower frequencies than their
tetrafluoride counterparts, i.e., 169 and 174 cm-1, respectively,
suggesting that the activation barriers for POF4- and SOF4 are
significantly lower than those in PF4- and SF4. These conclu-
sions are borne out in more detail in our theoretical calculations
of the transition states and energy barriers involved in this
exchange (see below).
Vibrational Spectra and Force Field. The vibrational
spectra of POF4- and SOF4 were also calculated at the different
levels of theory. As with the geometry calculations, the SCF
values, after appropriate scaling, duplicated best the experi-
mentally observed47 spectra of SOF4 and, therefore, were used
also for POF4- with the scaling factors taken from SOF4. The
results are summarized in Table 3. As can be seen from this
-
Theoretical Calculations. Since the isolation of solid POF4
salts, particularly in a reasonably pure state for vibrational
spectroscopy or as single crystals for X-ray diffraction, was
frustrated by extreme experimental difficulties, the geometry,
force fields, and some spectroscopic properties were calculated
by density functional theory and molecular orbital theory at the
self-consistent field (SCF) level. To test the adequacy of these
calculations, the NMR shifts of POF4- were also calculated and
compared to the observed ones. As an additional quality check
and for obtaining realistic scaling factors for our methods, we
have also calculated these data for the well-characterized
isoelectronic SOF4 and other closely related molecules and ions.
-
table, the spectra of POF4 and SOF4 are very similar, except
-
for the expected lowering of the frequencies in POF4 due to
the formal negative charge which increases the polar character
of the bonds in POF4-. The same trend was observed for the
structurally closely related pair PF4-/SF4 (see Table 4).
Furthermore, the calculated frequencies, IR and Ra intensities,
and 32S-34S isotopic shifts (see below) require the following
corrections of the assignments previously proposed47 for SOF4.
The very weak and questionable 447-cm-1 IR and 455-cm-1
Raman bands cannot be due to ν4(A1), which should be of
medium intensity in the IR, and probably belong to a combina-
tion band, such as (ν5 + ν12)(B2) ) 444 cm-1. Based on the
calculated IR intensities, the medium strong 567-cm-1 IR band
cannot be due to ν9(B1), which should be of almost zero IR
intensity, and is reassigned to ν4(A1). Since ν4(A1), ν6(A2),
ν9(B1), and ν11(B2) should all exhibit significant Raman intensi-
ties and occur within 12 cm-1 of each other, but only one Raman
band at 566 cm-1 is observed in this region, a quadruple
coincidence of Raman bands at 566 cm-1 must be assumed.
The revised assignments are given in Table 3 and satisfy all
-
Geometry of POF4-. The geometries of POF4 and iso-
electronic SOF4 were calculated at the SCF/DZ+P, SCF/
DZ+P+, and LDF/DZVP2 levels, and the results are summa-
rized in Table 2. Both calculations resulted in a pseudo-trigonal-
(65) Windus, T. M.; Gordon, M. S.; Burggraf, L. W.; Davis, L. P. J.
Am. Chem. Soc. 1991, 113, 4356.
(66) Steigel, A. In NMRsBasic Principles and Progress; Dynamic NMR
Spectroscopy; Diel, P., Fluck, E., Kosfeld, R., Eds.; Springer-Verlag: New
York, 1978; Vol. 15, p 1 and references therein.
(67) Eisenhut, M.; Mitchell, H. L.; Traficante, D. D.; Kaufman, R. J.;
Deutch, J. M.; Whitesides, G. M. J. Am. Chem. Soc. 1974, 96, 5385.
(68) Minyaev, R. M. Chem. Phys. Lett, 1992, 196, 203.
(69) Windus, T. M.; Gordon, M. S. Theor. Chim. Acta 1992, 83, 21.
(70) Gombler, W. Diploma Thesis, Saarbru¨cken, FRG, 1974. Gombler,
W. Personal communication.
(71) Spring, C. A.; True, N. S. J. Am. Chem. Soc. 1983, 105, 7231.
(72) Sawodny, W.; Birk, K.; Fogarasi, G.; Christe, K. O. Z. Naturforsch.,
B 1980, 35B, 1137 and references therein.
(73) (a) Gillespie, R. J.; Robinson, A. E. Angew. Chem., Int. Ed. Engl.
1996, 35, 495. (b) Gillespie, R. J.; Bytheway, I.; DeWitte, R. S.; Bader, R.
F. W. Inorg. Chem. 1994, 33, 2115.

3924 J. Am. Chem. Soc., Vol. 119, No. 17, 1997
Christe et al.
-
Table 3. Calculated Vibrational Frequencies of POF4 Compared to Calculated and Observed Frequencies of SOF4
-
SOF4
POF4
IR and assignmt
obsd freq, cm-1 (int)
calcd SCF freq (IR/Ra int) calcd SCF freq (IR/Ra int)
Raman
in point
activity group C2V
approx mode description
IRa
Ramana
unscaled
scaledb
unscaled
scaledc
IR, RA A1 ν1
ν XdO
1380 vs
796 m
588 mw 587 (1.7)p
1380 (0.7)p 1471 (338/4.3)
1380
796
593
565
192
553
839
636
553
918
562
269
1351 (418/3.7)
832 (62/7.3)
540 (15/1.4)
566 (32/0.9)
183 (0/0.1)
1267
741
481
521
169
493
708
570
484
833
514
245
ν2
ν sym XF2eq
795 (10)p
894 (59/13.8)
666 (6.7/4.1)
614 (44/1.0)
209 (0.1/0.3)
600 (0/1.0)
942 (611/0.1)
691 (70/0.5)
600 (0.1/2.2)
1030 (319/1.3)
610 (44/2.3)
292 (0.9/1.2)
ν3
ν sym XF2ax
ν4
sym comb of eq and ax δ sciss
asym comb of eq and ax δ sciss
τ
567 ms
566 (1.7)
174 (0.4)
566 (1.7)
815 sh
ν5
-, RA A2 ν6
535 (0/0.6)
IR, RA B1 ν7
ν sym XF2ax
mix of δ wag XdO and δ wag XF2eq
mix of δ wag XdO and δ wag XF2eq
819 vs
639 ms
795 (623/0)
619 (75/0.4)
526 (0.2/1.6)
935 (368/0.3)
558 (51/1.7)
266 (2.7/0.7)
ν8
ν9
640 sh
566 (1.7)
924 (0.2)
566 (1.7)
265 (0.7)
IR, RA B2 ν10 ν asym XF2eq
926 s
558 ms
ν11 sym comb of equat rock and axial bend
ν12 asym comb of equat rock and axial bend 270 vw
a Data from ref 47, except for reassigning ν9 to ν4, eliminating the assignment of the extremely weak and questionable 447 IR and 455 cm-1 Ra
bands to ν4 (A1) and assuming a quadruple coincidence of ν4, ν6, ν9, and ν11 at 566 cm-1 in the Raman spectrum. b The stretching and deformation
frequencies were multiplied by empirical factors of 0.8908 and 0.9210, respectively, to maximize their fit with the observed frequencies, except for
-
ν1, which was multiplied by 0.9381 to exactly duplicate the observed frequency. c The empirical scaling factors from SOF4 were applied to POF4
.
- 59
- 8
Table 4. Comparison of the Calculated (SCF) Vibrational
for POF2
,
PF4
,
POF3,74 PF3,75 SF4,72 and SOF2.59 In
Frequencies (Observed Values in Parentheses) of the Isoelectronic
addition to the expected trends, i.e., increasing covalency and
bond strength with increasing oxidation state and positive
charge,8 there is one unexpected feature. When adding an
oxygen ligand to PF4-, the axial P-F bonds are strengthened
much more than the equatorial ones. Thus, fr(PFax) increases
from 1.82 to 3.47 mdyn/Å and the axial bond lengths are
reduced from 1.74 to 1.65 Å, while fr(PFeq) only increases from
3.94 to 4.96 mdyn/Å and the equatorial bond lengths are only
reduced from 1.60 to 1.59 Å. A similar, although less
pronounced, effect is observed for the SF4/SOF4 couple. It
appears that the ionic character of the axial bonds, which contain
strong ionic contributions and can be described as semi-ionic,
three-center-four-electron bonds,76 is reduced more by the
oxidative oxygenation than that of the more covalent equatorial
-
-
Pairs SOF4/POF4 and SF4/PF4
- a
a
- b
b
XOF4 in C2V
POF4
SOF4
PF4
SF4
A1
ν1
ν2
ν3
ν4
ν5
ν6
ν7
ν8
ν9
ν10
ν11
ν12
1267
741
481
521
169
493
708
570
484
833
514
245
1380 (1380)
796 (796)
593 (588)
565 (567)
192 (174)
553 (566)
839 (819)
636 (639)
553 (566)
918 (926)
562 (558)
269 (270)
795 (798)
416 (422)
464 (446)
201 (210)
392 (-)
(892)
(558)
(533)
(227)
(-)
A2
B1
523 (515)
(729)
446 (446)
746 (745)
(474)
(867)
B2
293 (290)
(353)
a Data from Table 3. b Data from ref 8.
-
bonds. The increase in the equatorial bond strength from PF4
to POF4- is as expected and is comparable to that encountered
on going from the predominantly covalent PF3 to POF3 (see
Table 8).
the frequency, intensity, and isotopic shift requirements from
our calculations.
The symmetry force constants and potential energy distribu-
C4W Transition States and Energy Barriers toward In-
tramolecular Fluorine Ligand Exchange. The ν5(A1) vibra-
tion (see above) exhibits the lowest frequency and directly leads
to the C4V transition state, which is the lowest energy pathway
in these molecules for the equatorial-axial fluorine exchange8
by the following Berry-type64 pseudorotation exchange mech-
anism.
-
tion for POF4 and SOF4 are given in Tables 5 and 6, respec-
tively. As can be seen from the potential energy distribution,
ν4 and ν5 and also ν11 and ν12 are symmetric and antisymmetric
combinations of their corresponding symmetry coordinates.
Their motions can be depicted in the following way.
To probe the agreement of our calculated force fields with
available experimental data, the 32S and 34S isotopic shifts were
calculated from the SCF second derivatives for SOF4. The
calculated shifts were scaled by the same factors as the
corresponding frequencies and are compared in Table 7 with
the reported47 experimental data. As can be seen from Table
7, our calculated isotopic shifts are in good agreement with the
experimental values and support our revised assignments for
SOF4.
Since the internal stretching force constants are a measure
for the bond strengths, these constants were calculated for POF4-
and SOF4 from the symmetry force constants of Table 5, using
the explicit F matrix previously reported47 for SOF4. The results
are listed in Table 8 and are compared to the literature values
The C4V transition states, which represent the activation energy
barriers for this exchange,8 were calculated for POF4-, SOF4,
PF4-, and SF4 to support our conclusions from the above NMR
data that the barriers in the XOF4 species are significantly lower
than those in the corresponding XF4 species. The C4V structures
were calculated at the SCF/DZ+P level, and MP2 energies were
calculated at these geometries. The vibrational frequencies were
(74) Siebert, H. Anwendungen der Schwingungsspektroskopie in der
Anorganischen Chemie, Anorganische und Allgemeine Chemie in Einzel-
darstellungen VII; Springer-Verlag: Berlin, Heidelberg, New York, 1966.
(75) Small, C. E.; Smith, J. G. J. Mol. Spectrosc. 1978, 73, 215.
(76) Pimentel, G. C. J. Chem. Phys. 1951, 19, 446. Hach, R. J.; Rundle,
R. E. J. Am. Chem. Soc. 1951, 73, 4321. Rundle, R. E. J. Am. Chem. Soc.
1963, 85, 112.

Tetrafluorophosphate Anion
Table 5. Scaleda SCF Force Fieldsb of POF4 and SOF4
J. Am. Chem. Soc., Vol. 119, No. 17, 1997 3925
-
a The calculated SCF force constants were scaled by multiplication with the square or product of the corresponding scaling factors given in
footnote b of Table 3. b Stretching constants in mdyn/Å, deformation constants in mdyn Å/rad2, and stretch-bend interaction constants in mdyn/
rad.
-
Table 6. Potential Energy Distributionsa for POF4 and SOF4 Based on Their SCF Force Fields
-
POF4
SOF4
C2V
freq, cm-1
PED (%)
freq, cm-1
PED (%)
A1
ν1
ν2
ν3
ν4
ν5
ν6
ν7
ν8
ν9
ν10
ν11
ν12
1267
741
481
521
169
493
708
570
484
833
514
245
64.3(1) + 14.5(4) + 11.5(5) + 9.4(2)
67.8(2) + 16.8(3) + 7.2(4) + 6.2(5) + 2.0(1)
44.0(3) + 28.4(4) + 14.9(2) + 12.5(5)
45.7(4)a + 27.1(5)a + 22.9(3) + 3.5(2)
73.4(5)a + 26.3(4)a
1380
796
593
565
192
553
839
636
553
918
562
269
55.4(1) + 20.0(4) + 15.7(5) + 8.6(2)
62.3(2) + 15.3(3) + 10.8(4) + 9.0(5) + 2.5(1)
65.0(3) + 14.9(2) + 11.5(4) + 7.8(5) + 0.9(1)
65.1(4) + 31.1(5) + 1.9(3) + 1.9(2)
75.3(4) + 24.5(5)
A2
B1
100(6)
100(6)
68.2(7) + 17.2(9) + 14.6(8)
66.8(7) + 17.3(9) + 15.8(8)
53.3(8) + 46.1(9)
51.7(9) + 47.6(8)
61.6(9) + 37.8(8)
58.1(9) + 41.7(8)
B2
59.2(10) + 30.4(12) + 10.4(11)
63.4(12)a + 34.6(11)a + 2.1(10)
80.5(12)a + 19.0(11)a
58.1(10) + 31.1(12) + 10.7(11)
61.1(12) + 37.1(11) + 1.8(10)
83.5(12) + 16.0(11)
a The following symmetry coordinates were used: S1, ν XdO; S2, ν sym XF2eq; S3, ν sym XF2ax; S4, δ sym XF2eq; S5, δ sym XF2ax; S6, τ; S7,
ν as XF2ax; S8, δ wag XF2eq; S9, δ wag XdO; S10, ν as XF2eq; S11, δ XF2ax out of OXF2ax plane; S12, δ rock XF2eq
.
Table 7. Calculated and Observed 32S-34S Isotopic Shifts of SOF4
the barriers for the equatorial-axial fluorine ligand exchange,
freq, cm-1
∆ν calcd, cm-1
∆ν obsd,a cm-1
∼15b
are summarized in Table 10 and are compared to the experi-
- 8
mental values for SF4 and PF4
. As can be seen from Table
A1
ν1
ν2
ν3
ν4
ν5
ν6
ν7
ν8
ν9
ν10
ν11
ν12
1380
796
588
567
174
566
819
639
566
926
558
270
16.3
2.1
0.4
3.1
0
10, the SCF values differ by only 1-2 kcal from the MP2 values
and both agree well with the experimentally available data.
Therefore, even though no experimental data are available due
∼2.3c
-
to the low barrier heights, the values for POF4 and SOF4 are
A2
B1
0
also expected to be close to the real numbers. The results from
these calculations show that the barriers to axial-equatorial
fluorine ligand exchange in the XOF4 species (3.6 ( 1 kcal
mol-1) are indeed significantly lower than those (11.3 ( 1.1
kcal mol-1) in the corresponding XF4 species and readily
account for the different exchange rates observed in the NMR
experiments (see above).
13.6
2.9
0.1
13.8
3.2
0
13.1
2.4
B2
14.0
∼2.5c
a Matrix isolation data from ref 47 with the revised assignments from
Table 3. b The matrix IR band for ν1(A1) at 1376 cm-1 is complicated
by Fermi resonance with several combination bands, adding uncertainty
to the reported isotopic shift. c These shifts were estimated from the
shoulders shown in Figure 3J of ref 47.
The observed differences in the fluorine ligand exchange rates
of POF4-, SOF4, PF4-, and SF4 can readily be explained from
their geometries. In SOF4 and POF4-, the Fax-X-Fax and Feq-
X-Feq bond angles are already much closer to the ideal C4V
angle of about 142° and therefore require less deformation
-
Table 8. Stretching Force Constants (mdyn/Å) of POF4 and SOF4
Compared to Those of POF2-, PF4-, POF3, PF3, SF4, and SOF2
- a
- b
- c
d
e
b
f
a
-
POF2
2.68
POF4
PF4
3.94
1.82
POF3 PF3 SOF4 SF4 SOF2
energy than those in SF4 and PF4 to reach the C4V geometry
fr, eq
fr, ax
4.96
3.47
9.76
6.35 5.49 5.40 5.36 4.06
4.46 3.25
11.38
of the transition state. The sums of the angle deformations
required for reaching the C4V transition state geometries for these
species are as follows: SF4, 71.5°; PF4-, 68.4°; SOF4, 51.8°;
POF4-, 49.4°. This order parallels the inversion barriers that
have been found (SF4, 12 kcal/mol; PF4-, 10.3 kcal/mol; SOF4,
4 kcal/mol; POF4-, 3 kcal mol-1) and may provide a useful
general relationship for the estimation of inversion barriers in
similar molecules. A brief search for other low-lying transition
states for POF4- showed that states, such as the C3V geometry,
f(XdO) 8.34
11.57
11.12
a Reference 59. b This work. c Reference 8. d Reference 74. e Refer-
ence 75. f Reference 72.
also calculated for these transition states, and each state exhibits,
as expected, one imaginary frequency ranging from 129 cm-1
-
in POF4 to 189 cm-1 in SF4. The results are summarized in
Table 9. The energies of the transition states, which represent

3926 J. Am. Chem. Soc., Vol. 119, No. 17, 1997
Christe et al.
Table 9. Vibrational Frequencies, Infrared and Raman Intensities, and Geometries of the C4V Transition States of POF4-, SOF4, PF4-, and SF4
-
-
geometry
POF4
SOF4
PF4
1.676
SF4
r(XF) (Å)
r(XO) (Å)
FXF (deg)
OXF (deg)
1.619
1.462
1.562
1.399
84.1
1.593
83.7
109.3
82.0
83.2
108.7
Vibrational Frequencies and Intensities
freq, cm-1 (IR, RA inta)
scaledb
unscaled
C4V
unscaled
scaledb
unscaled
scaledb
unscaled
scaledb
A1
B1
ν XdO
1362 (366, 4.1)
796 (55, 6.9)
552 (50, 1.0)
544 (0, 1.1)
129i
582 (0, 0.7)
879 (1000, 0.2)
597 (111, 1.5)
413 (6, 2.8)
1278
709
508
485
119
536
783
550
380
1486 (323, 4.7)
860 (553, 13.7)
604 (56, 1.7)
649 (0, 2.9)
156i
651 (0, 1.3)
992 (940, 1.0)
658 (109, 1.8)
463 (3, 4.3)
1394
766
556
578
144
600
884
606
426
ν sym XF4 in phase
δ sym XF4
790 (183, 6.0)
544 (23, 1.3)
468 (0, 2.3)
173i
531 (0, 0.6)
722 (854, 5.4)
704
501
417
159
489
643
899 (115, 13.7)
615 (43, 2.4)
614 (0, 4.9)
189i
613 (0, 1.2)
905 (847, 5.1)
801
566
547
174
565
806
ν sym XF4 out of phase
δ sym XF4 out of phase
δ sciss XF4
B2
E
ν sym XF4
δ OXF4
δ asym XF4
493 (20, 0)
454
570 (43, 0.1)
525
a Ir intensities in km/mol. b The empirical scaling factors from Table 3, footnote b, were used.
Table 10. Activation Energy Barriers (kcal/mol) for the
Berry-Type Exchange of Axial and Equatorial Fluorine Ligands in
POF4-, SOF4, PF4-, and SF4
latter method gives for all compounds the most consistent
agreement with experiment.
Reaction Energies. As already pointed out above, the
thermal stabilities and dismutation characteristics of isoelectronic
E(C2V-C4V)
+ ZPE
methoda,b species E(C2V-C4V)
exptl value
-
SOF4 and POF4 differ dramatically. It was important to
SCF
MP2
SF4
SF4
12.4
10.3
12.3
10.3
11.8 (liquid),c
12.4 (gas)d
establish whether this different behavior is due to thermodynam-
ics or kinetics. A search of the thermochemical literature
-
-
-
SCF
MP2
PF4
PF4
11.4
10.2
11.4
10.2
implied that the reaction energies of the POF4 and SOF4
10.3e
dismutation reactions differ by about 70 kcal mol-1. Such a
huge difference would be very remarkable for these isoelectronic
systems and prompted us to examine these data in more detail.
The value of -76 kcal mol-1, quoted by Larsen and
McMahon for the dismutation reaction of POF4-,6 was based
on their fluoride ion affinity measurements. However, fluoride
affinity values alone are insufficient to derive the required heats
of formation of the complex anions, and one must also know
the heats of formation of the neutral parent molecules. Unfor-
tunately, these authors quoted neither the values nor the sources
of the ∆Hf° values of the three necessary parent molecules,
POF3, PO2F, and PF5. Using the JANAF values16,17 of -295.6
and -376.9 kcal mol-1 for ∆Hf° of POF3 and PF5, respectively,
and a published value of -171.3 kcal mol-1 for ∆Hf° of PO2F,
which was derived from tungsten transport measurements,14 and
Larsen and McMahon’s fluoride ion affinities6 (Scheme 1), one
obtains a more plausible value of -36 kcal mol-1 for the
dismutation energy of POF4-, which is in excellent agreement
with our values calculated by ab initio methods (see Table 12).
The second set of data that needed scrutiny were the heats
of formation of SO2F2 and SOF4 (the ∆Hf° value of SF6 is well
determined16). Although SO2F2 has been included in the
JANAF tables, the selected values were based on percent
conversions for reaction 9 between 110 and 700 °C, which were
SCF
MP2
SOF4
SOF4
5.2
3.6
5.0
3.3
-
-
SCF
MP2
POF4
POF4
3.4
2.7
3.2
2.5
a All calculations were carried out with the same polarized double-ú
basis set. b Density Functional Theory calculations were also carried
out for these transition states with the same basis set and yielded values
that were consistently about 2-4 kcal/mol lower than the MP2 values.
c Reference 70. d Reference 71. e Reference 8.
were at least 20 kcal mol-1 higher than the C4V structure and in
some cases were not transition states.
NMR Chemical Shift Calculations. NMR chemical shift
calculations were carried out with the GIAO method35 at the
nonlocal BLYP level32,33 with use of a triple-ú basis set
augmented by two polarization functions and the HF/DZVP
geometries. The results for SF4, PF4-, SOF4, and POF4 are
-
summarized in Table 11. Our calculated chemical shift values
are consistently low by about 18 ( 7 ppm, and the chemical
shift differences within a given compound are in good agreement
with experiment. In view of the strong influence of conditions
(such as the physical state of the compound, solvent, and
temperature), the calculated values are in good agreement with
the experimental ones and lend further support to our NMR
SO2 + Cl2 + 2HF f SO2F2 + 2HCl
(9)
taken from a 1963 U.S. Patent.77 However, the listed value of
-181.3 kcal mol-1 for ∆Hf° of SO2F2 differs significantly from
an earlier value of -205 kcal mol-1, which was derived from
electron impact studies18 and was not considered for the JANAF
tables based on their adoption of a poor D(S-F) value for SF6.
Similarly, the value for ∆Hf° of SOF4 is poorly determined.
-
spectroscopic identification of the POF4 anion given above.
They also demonstrate that the species, having a 31P shift of
-10.9 ppm and previously attributed to POF4 ,
- 13 cannot be due
There is only one experimental value of -235.7 kcal mol-1
,
to this anion.
derived from tungsten transport studies,14 and one estimated
value of -228.0 kcal mol-1, which is coupled to the SO2F2
value of -181.3 kcal mol-1. The coupling of this estimate to
Chemical shift calculations were carried out also at the
LORG42 and IGLO41 levels using a TZVP basis set (see Table
2). For POF4- and SF4, the LORG 19F NMR shifts show better
agreement with experiment than the other two methods, but after
an empirical adjustment of 18 ppm to the GIAO values, the
(77) Ruh, R. P.; Davis, R. A.; Allswede, K. A. U.S. Patent 3,092,458
(1963).

Tetrafluorophosphate Anion
J. Am. Chem. Soc., Vol. 119, No. 17, 1997 3927
-
Table 11. Comparison of Calculated and Observed Chemical Shiftsa (ppm) for SF4, PF4-, SOF4, and POF4
calculated relative chemical shift
GIAO/TZ2PF/BLYP
LORG/TZVP/LDFT
unadjusted
IGLO/TZVP/LDFT
unadjusted
compd
obsd chem shift
unadjusted
adjustedf
SF4
19
19
F
F
97.73b
37.03
78.0
23.0
493.4
96
41
511.4
91
54
478
155
84
490
ax
eq
33S
-
PF4
19
19
F
F
9.3c
-46.7
40.5
-10.1
-65.0
26.8
7.9
-47
44.8
-3.1
-83
43
24
-57
46
ax
eq
31P
SOF4
19
F
64.3
37.3
268.4
232.4
133
180
ax
av 75d
50.8
68.8
126
164
}
}
}
}
19
}
}
F
118
198
287
148
215
283
eq
33S
286.4
250.4
17O
-
POF4
19
F
-87.3
-102.6
-101.5
114.9
-81
-47
ax
av -85.8e
-75.1
-94.95
-76.95
-86
-58
}
}
19
F
-90
-122
130
-68
-122
128
eq
31P
17O
-83.5
132.9
3-
a The following standards were used for the chemical shifts: 19F, CFCl3; 31P, PO4
;
17O, H2O; 33S, CS2. b Reference 71. c Reference 8. d Reference
63. e This work. f An empirical adjustment of 18 ppm was applied to the calculated values to maximize their fit with the observed shifts.
Scheme 1
a pseudooctahedron and offers the advantage that the heats of
formation of all three compounds are accurately known.78
The reaction energies for the dismutation reactions 6, 7, and
10 were calculated at different levels of theory, and the results
are summarized in Table 12. Although the reaction energies
differ somewhat with the level of theory, the following
conclusions can be reached: (1) The agreement between the
Table 12. Reaction Energiesa (kcal mol-1) for the Dismutations of
POF4 (6), SOF4 (7), and ClOF3 (10)
-
-
experimental values for ClOF3 (+3.8 kcal mol-1) and POF4
reaction energy
(-36 kcal mol-1) with the calculated ones (+1.7 and -36.4,
respectively, at the MP2/aug-cc-VTZ level) is excellent and
demonstrates that the quality of the calculated values is
comparable to good experimental data. (2) The dismutation
computational level
POF4- (6)
SOF4 (7)
ClOF3 (10)
BP/DZVP2
MP2/DZP
MP2/cc-VDZ
MP2/aug-cc-VTZ
CCSD(T)/cc-VDZ
experimental value
-26.4
-31.4
-41.5
-36.4
-41.8
-36b
-32.8
-29.4
-46.9
-45.2
-47.1
5.3
3.7
1.9
1.7
2.5
3.8c
-
reactions of POF4 and SOF4 are both exothermic and of
comparable energy, with SOF4 being somewhat more exother-
-
mic than POF4 at each level of theory, except for MP2/DZP.
Clearly, the previous literature data,6,16 which implied a differ-
ence of about 70 kcal mol-1, are badly in error, and the generally
a Electronic energy difference. HP/DZP zero-point corrections are
+0.7 kcal mol-1 for (6), +0.8 kcal mol-1 for (7), and +0.5 kcal mol-1
for (10). b See text. c Data from ref 78.
14
accepted values for the heats of formation of SO2F216 and SOF4
and the dismutation energy of POF4- 6 need revision. (3) The
significant difference between the reaction energies of isoelec-
tronic SOF4 and POF4 on one hand and ClOF3 on the other
the ∆Hf° value of -205 kcal mol-1 for SO2F2 changes the ∆Hf°
estimate for SOF4 to -216.2 kcal mol-1. Depending on the
choices of the ∆Hf° values for SO2F2 and SOF4, the dismutation
energy of SOF4 ranges from -1.9 to -64.3 kcal mol-1, with
the -1.9 kcal mol-1 value being based on the most popular
-
can be rationalized by the changes in the nature of the bonding
during dismutation. In pseudo-trigonal-bipyramidal SOF4,
POF4-, and ClOF3, the two axial fluorine bonds are weak, long,
semi-ionic, three-center-four-electron bonds.76 In the SO2F2,
PO2F2-, SF6, and PF6- dismutation products, all fluorine bonds
become strong, normal, mainly covalent bonds, which causes
the dismutation reaction to be highly exothermic. For ClOF3,
however, a free valence electron pair on chlorine is present in
the ClF5 dismutation product. This free pair seeks high
s-character and induces the formation of two semi-ionic, three-
center-four-electron bond pairs for the four equatorial fluorine
ligands.79 Therefore, for the ClOF3 dismutation, the number
of semi-ionic, three-center-four-electron bonds remains un-
changed, and hence the reaction is essentially thermally neutral.
The difference between molecules containing semi-ionic, three-
center-four-electron bonds and others that contain either a
different number or none can give rise to bad estimates for the
16
14
JANAF SO2F2 and the experimental SOF4 values.
In view of these uncertainties in the experimental values and
the general experience that reaction energies can be calculated
quite accurately by ab initio methods due to the cancellation of
systematic errors on both sides of the equations, the reaction
energies were calculated for the dismutations of SOF4 and
POF4-. As a further test for the accuracy of our calculations,
we have also computed the dismutation energy for the closely
related ClOF3 molecule (reaction 10).
As for SOF4 and POF4-, this system involves the dismutation
of two pseudotrigonal bipyramids to a pseudotetrahedron and
(78) Barberi, P. B. I. S. T. Commissariat a l’Energie Atomique (France)
1976, 7, 511.
(79) Christe, K. O. XXIVth Int. Congr. Pure Appl. Chem. 1974, IV, 115.

3928 J. Am. Chem. Soc., Vol. 119, No. 17, 1997
Christe et al.
thermodynamic properties, if only the total number and not the
types of fluorine bonds are considered.
The influence of the computational levels on the reaction
energies is relatively minor due to the cancellation of errors on
both sides of the equation. Nevertheless, the following trends
were observed. The MP2/cc-VDZ calculations result in more
negative values than those at the MP2/DZVP level. The
difference between the CCSD(T) and MP2 results with the cc-
VDZ basis set is small. Improvement of the basis set leads to
a decrease in the overall exothermicity, with the larger decrease
-
found for (6), the POF4 reaction. The results with the larger
basis sets also show that the SOF4 dismutation is somewhat
-
more exothermic than that of POF4
.
Mechanism of the Dismutation Reaction of POF4-. In the
previous section it was demonstrated that the dismutation
reactions of SOF4 and POF4 are both highly exothermic and
Contrary to the POF3 dismutation which is F- catalyzed, the
SOF4 dismutation would require very strong Lewis acids such
as SbF5 or AsF5 as catalysts. Clearly, mechanism 12 would
involve several energetically very unfavorable species, such as
SF5 or polycations containing SOF4, and, therefore, should
exhibit high activation energy barriers.
Starting the dismutation of SOF4 with two SOF4 molecules
is equally unattractive. The substitution of one doubly bonded
oxygen ligand by two singly bonded fluorine ligands would
require a dimeric intermediate with one oxygen and two fluorine
bridges and one heptacoordinated sulfur atom (reaction 13).
-
of similar magnitude. Consequently, the great difference in their
dismutation behavior cannot be due to thermodynamics but must
be ascribed to kinetic effects. In this section, therefore, the likely
reaction mechanisms of these reactions will be scrutinized.
The low-temperature NMR studies (see above) provide an
+
-
insight into the mechanism of the POF4 dismutation. With
-
increasing temperature, the intensities of the POF4 and POF3
signals decrease, while those due to the [OF2POPF5]- anion
-
increase, indicating that POF4 reacts with POF3 to yield
[OF2POPF5]-. The most likely path for this reaction is shown
in (11) and involves the formation of an intermediate oxygen
Therefore, the SOF4 dismutation to SO2F2 and SF6 is mecha-
nistically unfavorable and is not observed under normal condi-
tions.
Conclusion
The ephemeral POF4- anion has been prepared and identified
for the first time. Whereas its geometry and spectroscopic
properties mimic the isoelectronic SOF4 molecule, its chemical
behavior and, in particular, its dismutation to XO2F2 and XF6
type species are very different. The POF4- anion is thermally
highly unstable and starts to dismutate even at -140 to -130
and fluorine bridged dimeric anion, which can rearrange to
[OF2POPF5]- by allowing the bridging oxygen to form two
single bonds and breaking one original P-F bond of POF3. On
further warm up, the resulting [OF2POPF5]- anion can readily
lose its PF5 molecule to the more basic F- ion present as
N(CH3)4+F- in these solutions, resulting in the formation of
-
°C to PO2F2 and PF6-, whereas SOF4 is kinetically stable
toward dismutation to SO2F2 and SF6. Extensive use of
theoretical calculations was made to better understand the
-
properties and reactions of the highly unstable POF4 anion.
-
-
the final species, PO2F2 and PF6
.
Of particular interest in this respect were its fluxionality, i.e.,
the equatorial-axial fluorine exchange, and its dismutation
process.
This mechanism involves exclusively low activation energy
steps (F- ion transfer and donor-acceptor interactions) and thus
-
explains the great ease of the POF4 dismutation even at the
very low temperatures. Since the dismutation requires the
presence of free POF3 for the formation of the crucial dimeric
[OF2POPF5]- anion, it is predicted that in the absence of free
POF3, a well-isolated POF4- anion will be a stable species that
can be isolated at ambient temperature. Synthetic approaches
would involve the uses of excess F- and of low temperatures
Acknowledgment. This paper is dedicated to the memory
of Dr. Charles B. Lindahl, a former Rocketdyne colleague, co-
worker, and friend. The authors thank Prof. G. A. Olah and
Dr. S. L. Rodgers for their active support. The work at the
Phillips Laboratory was financially supported by the Propulsion
Directorate of the U. S. Air Force, that at USC by the National
Science Foundation, that at McMaster University by the Natural
Sciences and Engineering Research Council of Canada, and that
at Pacific Northwest National Laboratory under the auspices
of the Office of Basic Energy Sciences, U.S. Department of
Energy under Contract DE-AC06-76RL01830 with Battelle
Memorial Institute, which operates the Pacific Northwest
Laboratory, a multiprogram national laboratory operated for the
U.S. Department of Energy.
-
where all POF3 can be converted to POF4 before the dismu-
tation sets in.
The second question to be addressed is why POF4- dismutates
so readily and isoelectronic SOF4 does not. Application of
mechanism 11 to the isoelectronic sulfur species would require
+
an SOF3 cation and an SOF4 molecule for the generation of
an intermediate [OF2SOSF5]+ dimeric cation that could either
eliminate SO2F2 with formation of an energetically highly
+
unfavorable SF5 cation or undergo attack by an F- anion to
give SF6 and SO2F2 (reaction 12).
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