[B(C6F5)4]: An air stable, lewis acidic stibonium salt that activates strong element-fluorine bonds

Article abstract of DOI:10.1021/ja505214m

As part of our ongoing interest in main group Lewis acids for fluoride anion complexation and element-fluorine bond activation, we have synthesized the stibonium borate salt [Sb(C₆F₅)₄][B(C ₆F₅)₄] (3). The perfluorinated stibonium cation [Sb(C₆F₅)₄]⁺ present in this salt is a potent Lewis acid which abstracts a fluoride anion from [SbF ₆]^- and [BF(C₆F₅)₃] ^- indicating that it is a stronger Lewis acid than SbF₅ and B(C₆F₅)₃. The unusual Lewis acidic properties of 3 are further reflected by its ability to polymerize THF or to promote the hydrodefluorination of fluoroalkanes in the presence of Et ₃SiH. While highly reactive in solution, 3 is a perfectly air stable salt, making it a convenient Lewis acidic reagent.

Full text of DOI:10.1021/ja505214m

Communication
pubs.acs.org/JACS
[Sb(C6F5)4][B(C6F5)4]: An Air Stable, Lewis Acidic Stibonium Salt That
Activates Strong Element-Fluorine Bonds
Baofei Pan and Franco̧ is P. Gabbaï*
Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, United States
S
* Supporting Information
Chart 1
ABSTRACT: As part of our ongoing interest in main
group Lewis acids for fluoride anion complexation and
element-fluorine bond activation, we have synthesized the
stibonium borate salt [Sb(C6F5)4][B(C6F5)4] (3). The
perfluorinated stibonium cation [Sb(C6F5)4]+ present in
this salt is a potent Lewis acid which abstracts a fluoride
anion from [SbF6]− and [BF(C6F5)3]− indicating that it is
a stronger Lewis acid than SbF5 and B(C6F5)3. The
unusual Lewis acidic properties of 3 are further reflected
by its ability to polymerize THF or to promote the
hydrodefluorination of fluoroalkanes in the presence of
Et3SiH. While highly reactive in solution, 3 is a perfectly air
stable salt, making it a convenient Lewis acidic reagent.
dicationic antimony species,11 we have now decided to
investigate the synthesis and properties of pentafluorophenyl-
antimony(V) derivatives. In this paper we describe the
synthesis and unusual Lewis acidic properties of the tetrakis-
(pentafluorophenyl)stibonium cation ([Sb(C6F5)4]+), a com-
pound related to highly acidic fluorophosphonium cations12
such as [FP(C6F5)3]+ recently described by Stephan.13
ver the past decades, the field of Lewis acid chemistry has
Owitnessed some important developments marked by the
advent of electron-deficient molecules such as fluorinated
organoboranes.1 These compounds have become ubiquitous
electrophilic activators widely used for applications in
catalysis1a−c and small-molecule activation.1d−h Strong Lewis
acidity is also observed for silylium derivatives2 which have
emerged as one of the best platforms for the activation of
strong C−F bonds.3 While these advances unambiguously
illustrate the Lewis acidity of these tricoordinate group 13 and
14 species, the pioneering work of Olah shows that strong
Lewis acidity can also be expressed by group 15 elements,
especially antimony.4 Early experimental work by Gutmann
shows that the chloride ion affinity of SbCl5 exceeds that of
BCl3, suggesting that the former is a stronger Lewis acid.5 A
similar conclusion can be derived from a comparison of the
computed gas phase fluoride ion affinity of SbF5 (489 kJ/mol)
and BF3 (338 kJ/mol), with the former exceeding the latter by
more than 150 kJ/mol.6 However, like BF3 and BCl3, the use of
these antimony pentahalides is complicated by their highly
reactive and corrosive nature. In particular, both SbF5 and
SbCl5 react violently with water, releasing the corresponding
hydrohalic acid. An extension of the lessons learned in the
chemistry of fluorinated boranes1c would suggest that the
introduction of organic substituents in antimony(V) com-
pounds may afford strong Lewis acids that do not display the
inconvenient corrosive properties of the pentahalide derivatives.
In a series of recent papers, we have shown that
organostiboranes7 such as A8 and tetraarylstibonium ions9
such as B10 are sufficiently Lewis acidic to complex fluoride in
aqueous media (Chart 1). Encouraged by these results and
inspired by the recent work of Burford on highly Lewis acidic
The reaction of C6F5Li with SbCl5 in diethyl ether/hexane
afforded SbCl(C6F5)4 (1) along with the known Sb(C6F5)5
(Scheme 1).14 Compound 1 can be easily separated from the
Scheme 1
mixture by fractional crystallization from Et2O. This colorless,
air stable compound has been fully characterized. Its 19F NMR
spectrum in dichloromethane at −80 °C shows two sets of
C6F5 resonances whose 1:3 integration ratio suggests that the
molecule has trigonal bipyramidal geometry. Upon elevation of
the temperature, the two sets of C6F5 resonances coalesce,
indicating a fluxional behavior by which the equatorial and axial
C6F5 groups exchange their positions. A line shape analysis
indicates an activation energy of 33( 1) kJ/mol for this
process, which we propose occurs via a square pyramidal
transition state as in the case for Sb(C6F5)5.14 This value is 9
kJ/mol higher than that reported for Sb(C6F5)5, a factor that
we assigned to the decreased steric bulk around antimony in 1
as well as the apicophilicity of the chloride ligand. Solutions of
1 in MeCN are not conductive, indicating the tight
coordination of the chloride anion. This is in contrast to
Received: May 24, 2014
Published: June 19, 2014
© 2014 American Chemical Society
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Figure 1. Left: 19F NMR spectra of 1, 2, and 3 in CH2Cl2. The ortho, meta, and para resonances of the C6F5 groups bound to antimony are
identified by the corresponding letters. Right: Solid state structure of 1 and 2. Ellipsoids are drawn at a 50% probability level. Selected bond lengths
(Å) and angles (deg) for 1: Sb1−Cl1 = 2.4509(11), Sb1−C1 = 2.130(3), Sb1−C7 = 2.144(3) Å, Sb−C13 = 2.143(3), Sb1−C19 2.197(3) Å; C1−
Sb1−C13 115.56(12), C1−Sb1 C7 119.49(12), C13−Sb1−C7 124.16(12), C19−Sb1−Cl1 176.09(8). Selected bond lengths (Å) and angles (deg)
for molecule I of 2 with the corresponding metrical parameter for molecule II in brackets: Sb1−C1 2.108(3) [2.090(3)], Sb1−C7 2.100(4)
[2.089(3)], Sb1−C13 2.103(3) [2.094(3)], Sb1−C19 2.146(3) [2.145(3)], Sb1−O1 2.377(2) [2.471(2)], C7−Sb1−C13 112.03(14) [113.28(13)],
C7−Sb1−C1 117.98(14) [114.01(13)], C13−Sb1−C1 118.76(13) [116.27(13)], C19−Sb1−O1 177.91(11) [174.36(10)].
Ph4SbCl which adopts a partially ionic structure in MeCN.15
This strong coordination of the chloride anion can be explained
by the electron-withdrawing properties of the C6F5 group. The
crystal structure of 1 (Figure 1) supports this interpretation
with the Sb−Cl bond distance of 1 (Sb1−Cl1 = 2.451(1) Å)
being much shorter than that reported for Ph4SbCl (2.686(1)
Å).16 The Sb−C bonds in 1 are well differentiated with the axial
Sb−C (2.197(3) Å) bond distance being longer than the
equatorial ones (av. 2.14 Å).
toluene.17 This reaction proceeded smoothly to afford the
stibonium borate salt [Sb(C6F5)4][B(C6F5)4] (3) (Scheme 2).
The 19F NMR resonances of the [Sb(C6F5)4]+ unit are
distinctly more downfield that those of 1 and 2, thus supporting
an ionic structure in solution (Figure 1). This salt can be
crystallized from CH2Cl2. Salt 3 crystallizes in the tetragonal
space group I4 (Figure 2). The stibonium and the borate units
̅
With compound 1 in hand, we became eager to investigate
its conversion into a stibonium cation by abstraction of the
chloride ligand. To this end, 1 was allowed to react with
trimethylsilyltriflate (TMSOTf) in acetonitrile. This reaction
proceeded smoothly to afford the antimony triflate derivative 2
(Scheme 2). The 19F NMR spectrum of this compound
Scheme 2
Figure 2. Right: Solid state structure of the [Sb(C6F5)4]+ cation in 3 as
determined by X-ray diffraction. Ellipsoids are drawn at the 30%
probability level. Selected bond lengths and angles: Sb−C1 2.095(2);
C1′−Sb1−C1″ = C1−Sb1−C1‴ = 107.78(13)°, C1−Sb1−C1′ = C1−
Sb1−C1″ = C1′−Sb−C1‴ = C1″−Sb−Cl‴ = C1−Sb1−C1
110.32(7). Left: LUMO [Sb(C6F5)4]+ overlaid on the optimized
geometry (isovalue =0.05).
displays a single set of C6F5 resonances. These resonances
which show no sign of decoalescence upon cooling are notably
more downfield that those of 1 (Figure 1). This downfield shift
suggests that the triflate ligand of 2 is more weakly coordinated
to antimony than the chloride ligand of 1, leading to a more
electrophilic antimony moiety. Compound 2 crystallizes with
two independent molecules (I and II) in the asymmetric unit.
Coordination of the triflate ligand is confirmed by the Sb−O
bonds of 2.377(2) Å (molecule I) and 2.471(2) Å (molecule II)
(Figure 1). Both molecules adopt a distorted trigonal
bipyramidal geometry (Σ∠(Ceq−Sb−Ceq) = 348.7° and
343.5° for molecule I and II, respectively) with the triflate
acting as an axial ligand. As in 1, the axial Sb−C bond (av. 2.14
Å) is longer than the equatorial ones (av. 2.10 Å). The
observed coordination of the triflate anion prompted us to
investigate even less coordinating anions. To this end,
compound 1 was treated with [Et3SiHSiEt3][B(C6F5)4] in
show no unusually short contacts. The C−Sb−C angles of
107.78(13)° of 110.32(7)° as well as the short Sb−C bonds
(2.095(2) Å) serve as an additional confirmation that the
antimony atom is free of interaction with external donors. It is
also interesting to note that, as a result of the similar structure
and volume of the [Sb(C6F5)4]+ cation and [B(C6F5)4]− anion,
the cell parameters of 3 are close to those of group 14
tetrakis(pentafluorophenyl) derivatives which also crystallize in
tetragonal space groups.18 The LUMO of [Sb(C6F5)4]+
calculated for the DFT optimized geometry of the cation can
be viewed as the linear combination of the four Sb−C σ*-
orbitals (Figure 2). The largest lobe of this orbital is spherically
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distributed about the antimony center making the heavy group
15 atom the most acidic site of the molecule.
acids, namely SbF5 and B(C6F5)3 for which fluoride ion
affinities of 489 and 444 kJ/mol, respectively, have been
calculated.6 Compound 2 reacted slowly with [SbF6]− (as a
TBA salt) in THF with only a small amount of 4 (<15%)
present in the reaction mixture after 30 min. Compound 2
showed no sign of reaction with [BF(C6F5)3]− (as a TBA salt).
Compound 3 showed a drastically different behavior, activating
both [SbF6]− and [BF(C6F5)3]− very rapidly in THF, with both
reactions being completed within 5 min, the time necessary to
record the NMR spectrum (Scheme 3). While free B(C6F5)3
was readily detected in the reaction mixture of 3 and
[BF(C6F5)3]−, SbF5, the byproduct of [SbF6]− activation,
could not be observed in the reaction of 3 and [SbF6]−
(Scheme 3). Our inability to observe SbF5 in THF is certainly
concerning. However, we note that no 19F NMR spectrum for
SbF5 in THF is available. Instead, measurements have been
carried out for the neat substance or in noninteracting solvents.
The SbF5 resonances are extremely broad, spanning a 70 ppm
range,20 and often complicated by aggregation.21 Upon
standing, the THF solvent used in the reaction of 3 with
[SbF6]− polymerized thus complicating analysis of the mixture.
We also observed that the use of a donor solvent plays a key
role in these reactions. Indeed when 3 was mixed with [SbF6]−
(as a TBA salt) in CH2Cl2, a broadening of the [Sb(C6F5)4]+
and [SbF6]− 19F NMR resonances is observed without any
evidence for the formation of 4. Addition of THF to this
solution results in the fast formation of 4 which is detected as
the main product of the reaction after 10 min. The synergy
observed between THF and [Sb(C6F5)4]+ in the activation of
[SbF6]− is reminiscent of that observed in frustrated Lewis
pairs.1d−h It is also reminiscent of the C−F bond activation of
CH3F by SbF5, which occurs only in solvents of sufficient
donor abilities.22
Compounds 2 and 3 show no sign of decomposition when
stored in air for several days. This stability was not anticipated,
especially for compound 3 which contains a base-free
[Sb(C6F5)4]+ unit. Surprised by the stability of these
complexes, we decided to carry out additional tests. While
sharp 19F NMR resonances are observed for the [Sb(C6F5)4]+
cation when 3 is dissolved in CH2Cl2, much broader resonances
are observed in THF, suggesting the possible dynamic
coordination of solvent molecules. On standing, the resulting
solution undergoes polymerization, indicating that [Sb-
(C6F5)4]+ is a potent activator (Scheme 3). No THF
Scheme 3. Reactivity of 3 with Selected Substrates
To complete this initial survey of the unusual properties of 3,
we decided to test its ability to promote the hydro-
defluorination (HDF) of simple fluoroalkanes.3,13b Rapid
dehydrofluorination of 1-C8H17F and PhCF3 was observed
upon combination of these fluoroalkanes with Et3SiH (2.3
equiv for 1-C8H17F, 9.2 equiv for PhCF3) and 1 mol % of 3 in
CH2Cl2 (Scheme 3). The progress of the reaction could be
conveniently followed by the disappearance of the fluorooal-
kane starting material and the appearance of Et3SiF in the 19F
NMR spectrum of the reaction mixture. While we were
tempted to propose that [Sb(C6F5)4]+ is involved in the C−F
bond activation step, we found that 3 does not react with 1-
C8H17F and PhCF3. Instead it quickly reacts with Et3SiH to
form C6F5H and Sb(C6F5)3 as confirmed by 19F NMR
spectroscopy. Formation of these two products is consistent
with the generation of Et3Si+ as the active hydrodefluorination
species. The formation of the silylium ion has been confirmed
by measuring the 29Si{1H} NMR spectrum of a concentrated
MeCN solution containing 3 and 2 equiv of Et3SiH. After an
overnight data acquisition, the spectrum reveals the presence of
solvated Et3Si+ at 36.6 ppm17a along with a resonance assigned
to the hydrolysis product (Et3Si)2O (9.0 ppm).23 The
activation of Et3SiH by 3 is interesting because hydride
abstraction from the silicon atom is followed by an irreversible
reductive elimination step thereby preventing hydride back-
transfer to the silylium center. It also suggests that Sb(C6F5)3 is
not sufficiently basic to quench the activity of the catalyst.
In summary, we describe the synthesis of [Sb(C6F5)4]+, a
new perfluorinated organostibonium ion. This “SbArF” cation is
isolated as a [B(C6F5)4]− salt which presents the advantage of
polymerization was observed when 2 was dissolved in THF.
The higher Lewis acidity of 3 was probed in greater details
using the Gutmann−Beckett method.19 We first studied the
formation of the Et3PO adduct by ESI mass spectrometry
which showed the corresponding molecular ion ([(C6F5)4Sb-
OPEt3]+, m/z = 922.96) for CH2Cl2 solutions containing 2 or 3
and the phosphine oxide. Next, we measured the 31P chemical
shifts of the bound Et3PO which was detected at 73.0 ppm for 2
and 74.6 ppm for 3. The value obtained for 3 is lower than that
reported for the strong Lewis acid B(C6F5)3 (76.6 ppm).19b
When the same measurement was carried out with 1, the
chemical shift of Et3PO (51 ppm) appeared unperturbed,
suggesting that 1 does not display any acidity toward this Lewis
base.
Bearing in mind that the Lewis acidity of [Sb(C6F5)4]+
toward relatively large bases such as Et3PO may be hampered
by steric effects, we decided to investigate the behavior of 2 and
3 toward the small fluoride anion. Both 2 and 3 react with KF
(in THF for 2 and CH2Cl2/THF for 3) to afford SbF(C6F5)4
(4), which has been characterized by NMR spectroscopy and
elemental analysis. The antimony bound fluoride anion of 4 in
CD3CN leads to a 19F NMR resonance at −25.0 ppm. The
ortho, para, and meta C6F5 resonances at −124.6, −144.6, and
−156.6 ppm, respectively, are almost identical to those of 1
(−124.5, −144.9, and −156.6 ppm). Having observed that both
2 and 3 quickly react with labile fluoride sources such as KF, we
decided to evaluate the fluoride affinity of these two species via
a series of competition experiments. Toward this end, we
selected two of the most potent and commonly used Lewis
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(12) Holthausen, M. H.; Mehta, M.; Stephan, D. W. Angew. Chem.,
Int. Ed. 2014, 53, 6538−6541.
being air stable. We attribute the air stability of this derivative to
the tetrahedral geometry of the stibonium center which
prevents an approach by nucleophiles. In solution, however,
solvent promoted reorganization processes allow access to the
electrophilic antimony center, revealing the unusually high
acidity of this derivative. We are currently probing the behavior
of this salt toward a broad range of organic and inorganic
substrates.
́
(13) (a) Perez, M.; Hounjet, L. J.; Caputo, C. B.; Dobrovetsky, R.;
Stephan, D. W. J. Am. Chem. Soc. 2013, 135, 18308−18310.
(b) Caputo, C. B.; Hounjet, L. J.; Dobrovetsky, R.; Stephan, D. W.
Science 2013, 341, 1374−1377. (c) Gabbaï, F. P. Science 2013, 341,
1348−1349.
́
(14) García-Monforte, M. A.; Alonso, P. J.; Ara, I.; Menjon, B.;
Romero, P. Angew. Chem., Int. Ed. 2012, 51, 2754−2757.
(15) Long, G. G.; Stevens, J. G.; Tullbane, R. J.; Bowen, L. H. J. Am.
Chem. Soc. 1970, 92, 4230−4235.
ASSOCIATED CONTENT
■
(16) Sharutin, V. V.; Sharutina, O. K.; Pakusina, A. P.; Platonova, T.
P.; Zadachina, O. P.; Gerasimenko, A. V. Russ. J. Coord. Chem. 2003,
29, 89−92.
S
* Supporting Information
Additional experimental and computational details. Crystallo-
graphic data in cif format. These material is available free of
charge via the Internet at http://pubs.acs.org.
(17) (a) Lambert, J. B.; Zhang, S.; Ciro, S. M. Organometallics 1994,
13, 2430−2443. (b) Nava, M.; Reed, C. A. Organometallics 2011, 30,
4798−4800. (c) Connelly, S. J.; Kaminsky, W.; Heinekey, D. M.
Organometallics 2013, 32, 7478−7481.
(18) (a) Karipides, A.; Foerst, B. Acta Crystallogr., Sect. B 1978, B34,
3494−3496. (b) Karipides, A.; Forman, C.; Thomas, R. H. P.; Reed, A.
T. Inorg. Chem. 1974, 13, 811−815.
AUTHOR INFORMATION
■
Corresponding Author
francois@tamu.edu
(19) (a) Mayer, U.; Gutmann, V.; Gerger, W. Monatsh. Chem. 1975,
106, 1235−1257. (b) Beckett, M. A.; Brassington, D. S.; Coles, S. J.;
Hursthouse, M. B. Inorg. Chem. Commun. 2000, 3, 530−533.
(20) Bock, A.; Dubois, M.; Bonnet, P.; Hamwi, A.; Avignant, D.;
Moch, L.; Morel, B. J. Fluorine Chem. 2012, 134, 24−28.
(21) Davies, T. K.; Moss, K. C. J. Chem. Soc. A 1970, 1054−1058.
(22) Calves, J. Y.; Gillespie, R. J. J. Am. Chem. Soc. 1977, 99, 1788−
1792.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This article is dedicated to Hubert Schmidbaur24 on the
occasion of his 80th birthday. This work was supported by the
National Science Foundation (CHE-1300371), the Welch
Foundation (A-1423), and Texas A&M University (Arthur E.
Martell Chair of Chemistry). We thank Oleg Ozerov for useful
discussions as well as James S. Jones, Masato Hirai, and Mengxi
Yang for their help with some of the data.
(23) Schafer, A.; Saak, W.; Haase, D.; Muller, T. Angew. Chem., Int.
̈
̈
Ed. 2012, 51, 2981−2984.
(24) Schmidbaur, H.; Weidlein, J.; Mitschke, K. H. Chem. Ber. 1969,
102, 4136−4146.
REFERENCES
■
(1) (a) Chen, E. Y.-X.; Marks, T. J. Chem. Rev. 2000, 100, 1391−
1434. (b) Erker, G. Dalton Trans. 2005, 1883−1890. (c) Piers, W. E.
Adv. Organomet. Chem. 2005, 52, 1−76. (d) Stephan, D. W. Dalton
Trans. 2009, 3129−3136. (e) Stephan, D. W.; Erker, G. Angew. Chem.,
Int. Ed. 2010, 49, 46−76. (f) Stephan, D. W. Dalton Trans. 2012, 41,
9015−9015. (g) Stephan, D. W.; Greenberg, S.; Graham, T. W.;
Chase, P.; Hastie, J. J.; Geier, S. J.; Farrell, J. M.; Brown, C. C.; Heiden,
Z. M.; Welch, G. C.; Ullrich, M. Inorg. Chem. 2011, 50, 12338−12348.
(h) Piers, W. E.; Marwitz, A. J. V.; Mercier, L. G. Inorg. Chem. 2011,
50, 12252−12262.
(2) (a) Reed, C. A. Acc. Chem. Res. 2010, 43, 121−128. (b) Stahl, T.;
Klare, H. F. T.; Oestreich, M. ACS Catal. 2013, 3, 1578−1587.
(c) Klare, H. F. T.; Oestreich, M. Dalton Trans. 2010, 39, 9176−9184.
(3) (a) Douvris, C.; Ozerov, O. V. Science 2008, 321, 1188−1190.
(b) Panisch, R.; Bolte, M.; Muller, T. J. Am. Chem. Soc. 2006, 128,
̈
9676−9682.
(4) (a) Olah, G. A.; Schlosberg, R. H. J. Am. Chem. Soc. 1968, 90,
2726−2727. (b) Olah, G. A.; Klopman, G.; Schlosberg, R. H. J. Am.
Chem. Soc. 1969, 91, 3261−3268. (c) Olah, G. A. J. Org. Chem. 2005,
70, 2413−2429.
(5) Gutmann, V.; Hubacek, H.; Steininger, A. Monatsh. Chem. 1964,
95, 678−686.
(6) Krossing, I.; Raabe, I. Chem.Eur. J. 2004, 10, 5017−5030.
(7) Ke, I.-S.; Jones, J. S.; Gabbaï, F. P. Angew. Chem., Int. Ed. 2014,
53, 2633−2637.
(8) Hirai, M.; Gabbai, F. P. Chem. Sci. 2014, 5, 1886−1893.
(9) (a) Wade, C. R.; Ke, I.-S.; Gabbaï, F. P. Angew. Chem., Int. Ed.
2012, 51, 478−481. (b) Wade, C. R.; Gabbaï, F. P. Organometallics
2011, 30, 4479−4481.
(10) Ke, I.-S.; Myahkostupov, M.; Castellano, F. N.; Gabbaï, F. P. J.
Am. Chem. Soc. 2012, 134, 15309−15311.
(11) Robertson, A. P. M.; Burford, N.; McDonald, R.; Ferguson, M. J.
Angew. Chem., Int. Ed. 2014, 53, 3480−3483.
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dx.doi.org/10.1021/ja505214m | J. Am. Chem. Soc. 2014, 136, 9564−9567