The highly lewis acidic dicationic phosphonium salt: [(SIMes)PFPh 2][B(C6F5)4]2

Article abstract of DOI:10.1002/anie.201403693

The dicationic imidazolium-phosphonium salt [(SIMes)PFPh ₂][B(C₆F₅)₄]₂ has been prepared and shown to exhibit remarkable Lewis acidity in stoichiometric reactions and acting as an effective Lewis acid catalyst for the hydrodefluorination of fluoroalkanes and the hydrosilylation of olefins.

Full text of DOI:10.1002/anie.201403693

Angewandte
Chemie
DOI: 10.1002/anie.201403693
P Dication
The Highly Lewis Acidic Dicationic Phosphonium Salt:
[(SIMes)PFPh2][B(C6F5)4]2**
Michael H. Holthausen, Meera Mehta, and Douglas W. Stephan*
Abstract: The dicationic imidazolium-phosphonium salt
[(SIMes)PFPh2][B(C6F5)4]2 has been prepared and shown to
exhibit remarkable Lewis acidity in stoichiometric reactions
and acting as an effective Lewis acid catalyst for the hydro-
defluorination of fluoroalkanes and the hydrosilylation of
olefins.
dication. Oxidation of an N-heterocyclic carbene (NHC)
derived cationic phosphine[10] provides a cationic difluoro-
phosphorane, while subsequent fluoride abstraction affords
a phosphonium dication salt. This latter species is shown to be
more electrophilic than 1 or 2 in stoichiometric reactions and
exhibits Lewis acidity in catalytic reactions.
The reaction of Ph2PCl, 1,3-dimesitylimidazolidin-2-yli-
dene (SIMes) and [Et3Si][B(C6F5)4]·2(C7H8) afforded the
cationic phosphine [(SIMes)PPh2][B(C6F5)4] (3) in almost
quantitative yield (Scheme 1). The 31P{1H} NMR spectrum of
H
istorically, P-compounds have been widely exploited as
Lewis donor ligands in transition metal and organometallic
chemistry. Such ancillary roles have been critical to a number
of landmark advances in catalysis. While studied to a lesser
extent, P-based compounds have also been shown to exhibit
Lewis acidity.[1] For example, PIII di-coordinated phosphe-
nium ions have been shown to be ambiphiles and the Lewis
acidity[2] of these species has been shown to prompt activation
[3]
[4]
À
À
of C C/H or P P bonds. Similarly, the Lewis acidity of
tetra-coordinated PV-phosphonium cations has been
exploited to capture fluoride ions in sensor applications[5]
and to act as catalysts for additions to polar unsaturates[6] and
in Diels–Alder reactions.[7] In a similar manner, the classical
addition of P-based ylides to ketones is facilitated by the
electrophilicity of the phosphorus center.[8] More recently, we
have reported the preparation and Lewis acidity of the
fluorophosphonium cations [(C6F5)2PhPF]+ (1) and
[(C6F5)3PF]+ (2) and the application of these species in the
hydrodefluorination of fluoroalkanes, the isomerization of
terminal olefins and the hydrosilylation of alkenes and
alkynes.[9] This reactivity has been attributed to their energeti-
Scheme 1. Preparative route to 5.
3 in CD2Cl2 shows a singlet resonance at À1.5 ppm. This
compares with a shift of À12.9 ppm observed for the related
carbene-phosphine salt [(IDipp)PPh2]+.[11] The molecular
structure of 3 was confirmed crystallographically, revealing
À
a P Ccarbene bond length of 1.861(4) ꢀ. The remaining metric
parameters were unexceptional (see Supporting Informa-
tion).
À
cally accessible s*(P F) acceptor orbitals.
The discovery of these electrophilic phosphonium cations
provides a new avenue to Lewis acid catalysts. However, the
initial systems described above, require strongly electron-
withdrawing substituents, thus limiting potential structural
variations. Herein, we report a new strategy that avoids the
use of fluoroarene substituents by enhancing the Lewis
acidity by targeting an electrophilic fluorophosphonium
The phosphine salt 3 is cleanly oxidized with XeF2 to the
cationic difluorophosphorane [(SIMes)PF2Ph2][B(C6F5)4] (4)
and was isolated in 81% yield (Scheme 1) This species gives
rise to a 31P{1H} NMR signal at À62.9 ppm with a 1JPF coupling
constant of 733 Hz consistent with the presence of two
equivalent F atoms. This chemical shift is downfield of that
observed for the neutral phosphorane (C6F5)3PF2 at
À48.0 ppm (1JPF = 694 Hz).[9b]
The molecular structure of 4 shows a distorted trigonal
bipyramidal arrangement at the P atom (Figure 1). The
fluoro-substituents occupy the axial positions with a F-P-F
angle of 168.8(2)8 while the carbene and the phenyl-substitu-
ents occupy equatorial positions. While carbenes have been
exploited to stabilize low valent phosphorus species,[10c,12]
compound 4 is the first cationic halophosphorane derivative
to be structurally characterized.[13]
One of the fluoro-substituents on the cation of 4 is
removed upon treatment with Et3Si[B(C6F5)4]·2(C7H8) giving
rise to the dication [(SIMes)PFPh2][B(C6F5)4]2 (5) and the
expected by-product Et3SiF. While Burford and co-workers
have recently described dicationic Sb and Bi species,[14] 5 is
[*] Dr. M. H. Holthausen, M. Mehta, Prof. Dr. D. W. Stephan
Department of Chemistry, University of Toronto
80 St. George St, Toronto Ontario M5S3H6 (Canada)
E-mail: dstephan@chem.utoronto.ca
Prof. Dr. D. W. Stephan
Chemistry Department-Faculty of Science
King Abdulaziz University
Jeddah 21589 (Saudi Arabia)
[**] D.W.S. gratefully acknowledges the financial support of the NSERC
of Canada and the award of a Canada Research Chair. M.H.H.
thanks the Alexander von Humboldt Foundation for a Feodor Lynen
Research Fellowship.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201403693.
Angew. Chem. Int. Ed. 2014, 53, 1 – 5
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
These are not the final page numbers!

.
Angewandte
Communications
ents are shortened to 1.753(3) and 1.756(3) ꢀ in 5 in
comparison to those in 3 and 4 which range from 1.799(6) ꢀ
to 1.866(5) ꢀ).
To probe the Lewis acidity of 5 the Gutmann–Beckett
method was employed.[16] Compound 5 was combined with
Et3PO in CD2Cl2. 31P NMR spectroscopy revealed the for-
mation of a mixture containing fluorophosphonium 6 and
oxophosphorane 7 salts (Scheme 2). This was evidenced by
Figure 1. POV-ray depiction of the cation of 4. Hydrogen atoms have
been omitted for clarity. Selected bond distances and angles: C1–N1
1.322(6), C1–N2 1-325(6), P–C1 1.866(5), P–C4 1.799(6), P–C10
1.815(5), P–F1 1.628(4), P–F2 1.660(4); N1-C1-N2 111.5(5), C1-P-C4
121.2(2), C1-P-C10 121.2(2), C4-P-C10 117.6(3), F1-P-F2 168.8(2), F1-
P-C1 85.0(2), F1-P-C4 92.8(3), F1-P-C10 93.4(2), F2-P-C1 83.8(2), F2-P-
C4 93.0(3), F2-P-C10 92.4(2).
a rare example of an isolable dicationic phosphonium salt
(Scheme 1).[15] Compound 5 exhibits a dramatic 31P downfield
À
shift to 78.1 ppm with a P F coupling of 1040 Hz. The
corresponding 19F resonance is observed at À131.7 ppm.
Interestingly, the 31P and 19F chemical shifts are reminiscent
of those observed for the very Lewis acidic phosphonium ions
Scheme 2. Reactions of 5 with Et3PO and (C6F5)3PF2.
1
(d(31P) = 78 ppm, d(19F) = À122 ppm) and
2
(d(31P) =
the 31P NMR spectrum which reveals a doublet resonance at
68 ppm, d(19F) = À121 ppm) inferring similar P-environments
comparably low field (d(31P) = 147.2 ppm) with a typical JPF
1
and thus perhaps comparable Lewis acidities. The formula-
coupling constant of 989 Hz[9] attributed to 6 whereas a singlet
resonance observed at 14.0 ppm was attributed to 7.[13] These
data infer that simple coordination of Et3PO to the phospho-
nium P atom of 5 does not occur but rather an oxide–fluoride
exchange reaction takes place affording [Et3PF][B(C6F5)4] (6)
and [(SIMes)POPh2][B(C6F5)4] (7). The salts 6 or 7 could not
be separated from the reaction mixture, however, these
species were independently synthesized and fully character-
ized. The formulation of the cationic oxophosphorane
derivative featuring an imidazolium-based substituent 7 was
further confirmed by X-ray crystallography (Figure 2).[13]
Similar to 5, the cation of 7 shows a distorted tetrahedral
environment at the P atom. However, the cation of 7 exhibits
tion of
5
was further confirmed crystallographically
(Figure 2). The P F bond length (1.532(2) ꢀ) is substantially
shorter than those observed in phosphorane 4 (1.628(4) and
À
À
1.660(4) ꢀ). Similarly the P C bonds to the phenyl-substitu-
À
À
À
longer P C bonds (P C1 1.881(2) ꢀ, av. P Caryl 1.797(2) ꢀ)
consistent with the formal mono-cationic charge.
Efforts to apply the Childꢁs protocol to assess the Lewis
acidity[17] involved combination of 5 with excess crotonalde-
hyde at À208C in CD2Cl2. NMR data suggest a complex
mixture with formation of 7 and 4 in addition to other organic
products. While these Lewis acidity tests do not permit
comparison to other Lewis acids, the observed reactivity
suggests that the dication 5 is highly electrophilic. This notion
is supported by the fact that exposure of 1 mg of 5 to [D8]THF
leads to polymerization as GPC after 2 h reveals a molecular
weight of the polymer of 8.6 ꢂ 104 Da with a polydispersity of
2.6. Similar evidence of Lewis acidity is seen with the addition
of 2 mol% of 5 to a solution of 1,1-diphenylethylene in
CD2Cl2 as this effects complete Friedel–Crafts dimerization of
the olefin in less than 30 min yielding the indene product in
97% isolated yield.[18]
Figure 2. POV-ray depiction of the dication of 5 (left) and the cation of
7 (right). Hydrogen atoms have been omitted for clarity. Selected bond
distances and angles: 5: C1–N1 1.329(3), C1–N2 1.316(4), P–C1
1.852(3), P–C4 1.753(3), P–C10 1.756(3), P–F 1.532(2); N1-C1-N2
112.8(3), C1-P-C4 113.4(1), C1-P-C10 109.2(1), C4-P-C10 116.6(1), F-P-
C1 101.8(1), F-P-C4 106.4(1), F-P-C10 108.2(1); 7: C1–N1 1.328(3),
C1–N2 1-328(2), P–C1 1.881(2), P–C4 1.798(2), P–C10 1.796(2), P–O
1.480(2); N1-C1-N2 111.8(2), C1-P-C4 106.7(1), C1-P-C10 106.9(1), C4-
P-C10 109.0(1), O-P-C1 107.2(1), O-P-C4 113.7(1), O-P-C10 112.9(1).
2
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
In order to get insights into the fluorophilicity, 5 was
reacted with phosphorane (C6F5)3PF2 in CD2Cl2 (Scheme 2).
This led to the clean and complete conversion to fluorophos-
a silylium ion as a hydrodefluorination catalyst[9b] (see
Supporting Information).
The utility of 5 in catalytic hydrosilylation of alkenes and
alkynes was also investigated. Within 14 to 24 h at 458C
complete hydrosilylation of 1-hexene (8a), cis-2-hexene (8b),
cyclohexene (8c) and 1-methylcyclohexene (8d) was
observed in the presence of Et3SiH and 2 mol% of 5
(Table 2) affording the corresponding hydrosilylation prod-
ucts 9a–d in high isolated yields (80–90%). In the case of 8d,
phonium salt 2 and difluorophosphorane 4 as revealed by 31
P
and 19F NMR spectroscopy. This infers that 5 is more
fluorophilic than 2. The fluorophilicity of 5 was further
demonstrated in the 1:1 reaction of 5 with Ph3CF. This led to
the conversion of 5 to 4 and trityl cation as evidenced by 31P,
19F and 13C NMR spectra.
This evidence of Lewis acidity and fluorophilicity suggests
that 5 could act as an effective Lewis acid catalyst. This
prompted an investigation of 5 as a catalyst for hydrode-
fluorination reactions. A series of fluoroalkanes was com-
bined with Et3SiH in the presence of 5 as a catalyst (Table 1).
Table 2: Catalytic hydrosilylation of alkenes and alkynes.
Table 1: Catalytic hydrodefluorination of fluoroalkanes.
[b,c]
[b,d]
À
À
%C F
Substrate
Cat.
t [h] %Si F
TON
(mol%)[a]
1-fluoroadamantane
fluorocyclohexane
1-fluoropentane
(CF3)C6H5
1
1
5
1
5
10
5
10
1
1
1
24
24
24
24
24
>99
92
>99
53
31
66
100
99
100
88
44
72
100
100
100
88
8.8
7.2
4.0
2.8
1,4-C6H4(CHF2)2
perfluorotoluene
14
29
20
28
[a] Relative to fluoroalkane substrate. [b] Conversions were determined
by 19F NMR spectroscopy using fluorobenzene as an internal standard.
À
[c] Calculated from the proportion of C F bonds originally present
À
À
relative to Si F bonds formed. [d] Calculated from the proportion of C F
bonds consumed after time t (h).
1
[a] Conversions were determined by means of H NMR spectroscopy
using, when necessary, a toluene standard, isolated yields are given in
parenthesis. [b] Two equivalents Et3SiH were utilized.
At ambient temperature, almost complete conversion of
1-fluoroadamantane, 1-fluorocyclohexane and 1-fluoropen-
tane was observed within one hour. The consumption of
substrate and growth of the resonance attributable to Et3SiF
was evidenced by the 19F NMR spectra of the reaction
mixtures. Trifluorotoluene is almost consumed within 24 h
while higher catalyst loadings are required (10 mol%) to
achieve significant conversion of 1,4-C6H4(CHF2)2. In the case
À
exclusively anti 1,2-addition of the Si H bond to the olefin
gives rac-9d whereas norbornene is selectively transformed to
the exo-9e in a fashion similar to other Lewis acid catalysts
B(C6F5)3[23] or 2.[9a] Styrene substrates are also hydrosilylated
to the respective products 9g–k. It is noteworthy, that
silylether (9 f) and chloro functional groups (9j) are tolerated
by this catalyst. In the case of p-methoxy-a-methylstyrene
also conversion of the methoxy group to silylether function-
alized product 9k was observed. Finally, alkyne 8l was
selectively converted to the cis-substituted alkene 9l.
À
of perfluorotoluene, incomplete reaction of the aliphatic C F
À
bonds is achieved after 24 h. The aryl C F bonds remain
untouched. Mechanistically, these hydrodefluorinations are
thought to proceed via a mechanism analogous to those
described for other strong, main-group based Lewis acids,
including B(C6F5)3,[19] 2,[9b] silylium[20] and carbenium ions[21]
and aluminium species.[22] This would involve an abstraction
of fluoride by 5 from the fluoroalkane to give 4 and
a carbocation. The latter intermediate reacts with silane to
give alkane and generates silylium cation which abstracts F
from 4 to regenerate 5. This mechanism is supported by the
observation that 5 does not react with Et3SiH even after
several days reaction time. In addition, a further competition
experiment confirmed the reaction of fluoroalkane with 5 in
the presence of silylium cation precluding the involvement of
In summary, we have described the conversion of
a cationic phosphine salt to a cationic difluorophosphorane
and on to a dicationic fluorophosphonium derivative. The
species 5 is shown to be highly Lewis acidic in stoichiometric
reactions and to act as a Lewis acid catalyst in hydrodefluori-
nation and hydrosilylation reactions. It is noteworthy that the
previously reported electrophilic boranes or fluorophospho-
nium monocations exploit highly fluorinated, strongly elec-
tron-withdrawing substituents. In contrast, the phosphonium
Angew. Chem. Int. Ed. 2014, 53, 1 – 5
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3
These are not the final page numbers!

.
Angewandte
Communications
W. Thiel, M. Alcarazo, J. Am. Chem. Soc. 2012, 134, 16753 –
16758; c) M. H. Holthausen, S. K. Surmiak, P. Jerabek, G.
Frenking, J. J. Weigand, Angew. Chem. 2013, 125, 11284 –
11288; Angew. Chem. Int. Ed. 2013, 52, 11078 – 11082; d) J.
dication 5 achieves similar levels of Lewis acidity and
reactivity without highly electron withdrawing substituents.
Moreover, the synthetic route to 5 is facile, high yielding and
amenable to structural modifications, offering the potential of
a large family of highly Lewis acidic compounds and catalysts.
The impact of structural variations on the reactivity and Lewis
acidity of such dicationic phosphonium species is expected to
provide new avenues to both Lewis acid catalysis and FLP
chemistry. These aspects are the subjects of on-going studies.
ˇ
Petuskova, M. Patil, S. Holle, C. W. Lehmann, W. Thiel, M.
Alcarazo, J. Am. Chem. Soc. 2011, 133, 20758 – 20760.
[11] D. Mendoza-Espinosa, B. Donnadieu, G. Bertrand, J. Am.
Chem. Soc. 2010, 132, 7264 – 7265.
[12] a) T. Bçttcher, O. Shyshkov, M. Bremer, B. S. Bassil, G.-V.
Rçschenthaler, Organometallics 2012, 31, 1278 – 1280; b) K.
Schwedtmann, M. H. Holthausen, K. O. Feldmann, J. J. Wei-
gand, Angew. Chem. Int. Ed. 2013, 52, 14204 – 14208; Angew.
Chem. 2013, 125, 14454 – 14458.
[13] C. Maaliki, C. Lepetit, C. Duhayon, Y. Canac, R. Chauvin,
Chem. Eur. J. 2012, 18, 16153 – 16160.
[14] A. P. M. Robertson, N. Burford, R. McDonald, M. J. Ferguson,
Angew. Chem. Int. Ed. 2014, 53, 3480 – 3483; Angew. Chem.
2014, 126, 3548 – 3551.
[15] a) S. Conejero, M. Y. Song, D. Martin, Y. Canac, M. Soleil-
havoup, G. Bertrand, Chem. Asian J. 2006, 1, 155 – 160; b) S. M.
Huber, F. W. Heinemann, P. Audebert, R. Weiss, Chem. Eur. J.
2011, 17, 13078 – 13086.
[16] V. Gutmann, Coord. Chem. Rev. 1976, 18, 225 – 255.
[17] R. F. Childs, D. L. Mulholland, A. Nixon, Can. J. Chem. 1982, 60,
801 – 808.
[18] a) A. G. Evans, E. D. Owen, J. Chem. Soc. 1959, 4123 – 4125;
b) L. J. Hounjet, C. Bannwarth, C. N. Garon, C. B. Caputo, S.
Grimme, D. W. Stephan, Angew. Chem. Int. Ed. 2013, 52, 7492 –
7495; Angew. Chem. 2013, 125, 7640 – 7643.
[19] a) C. B. Caputo, D. W. Stephan, Organometallics 2012, 31, 27 –
30; b) T. Stahl, H. Klare, M. Oestreich, ACS Catal. 2013, 3, 1578 –
1587.
Received: March 25, 2014
Published online: && &&, &&&&
Keywords: carbene ligands · hydrodefluorination ·
.
hydrosilylation · Lewis acids · phosphonium salts
[1] a) N. Burford, P. J. Ragogna, J. Chem. Soc. Dalton Trans. 2002,
4307 – 4315; b) O. Guerret, G. Bertrand, Acc. Chem. Res. 1997,
30, 486 – 493; c) D. Gudat, Acc. Chem. Res. 2010, 43, 1307 – 1316.
[2] J. M. Slattery, S. Hussein, Dalton Trans. 2012, 41, 1808 – 1815.
[3] A. H. Cowley, R. A. Kemp, Chem. Rev. 1985, 85, 367 – 382.
[4] a) M. H. Holthausen, K. Feldmann, S. Schulz, A. Hepp, J. J.
Weigand, Inorg. Chem. 2012, 51, 3374 – 3387; b) M. H. Holth-
ausen, A. Hepp, J. J. Weigand, Chem. Eur. J. 2013, 19, 9895 –
9907; c) C. A. Dyker, N. Burford, Chem. Asian J. 2008, 3, 28 – 36.
[5] a) T. W. Hudnall, Y. M. Kim, M. W. P. Bebbington, D. Bourissou,
F. P. Gabbaꢃ, J. Am. Chem. Soc. 2008, 130, 10890 – 10891;
b) C. W. Chiu, Y. Kim, F. P. Gabbai, J. Am. Chem. Soc. 2009,
131, 60 – 61.
[20] C. Douvris, O. V. Ozerov, Science 2008, 321, 1188 – 1190.
[21] M. Alcarazo, C. Gomez, S. Holle, R. Goddard, Angew. Chem.
Int. Ed. 2010, 49, 5788 – 5791; Angew. Chem. 2010, 122, 50 – 81.
[22] W. Gu, M. R. Haneline, C. Douvris, O. V. Ozerov, J. Am. Chem.
Soc. 2009, 131, 11203 – 11212.
[23] a) S. Rendler, M. Oestreich, Angew. Chem. Int. Ed. 2008, 47,
5997 – 6000; Angew. Chem. 2008, 120, 6086 – 6089; b) J. Her-
meke, M. Mewald, M. Oestreich, J. Am. Chem. Soc. 2013, 135,
17537 – 17546; c) M. Rubin, T. Schwier, V. Gevorgyan, J. Org.
Chem. 2002, 67, 1936 – 1940.
[6] A. Werner, Adv. Synth. Catal. 2009, 351, 1469 – 1481.
[7] M. Terada, M. Kouchi, Tetrahedron 2006, 62, 401 – 409.
[8] G. Wittig, U. Schçllkopf, Chem. Ber. 1954, 87, 1318.
[9] a) M. Pꢄrez, L. J. Hounjet, C. B. Caputo, R. Dobrovetsky, D. W.
Stephan, J. Am. Chem. Soc. 2013, 135, 18308 – 18310; b) C. B.
Caputo, L. J. Hounjet, R. Dobrovetsky, D. W. Stephan, Science
2013, 341, 1374 – 1377; c) L. J. Hounjet, C. B. Caputo, D. W.
Stephan, Dalton Trans. 2013, 42, 2629 – 2635.
[10] a) O. Back, B. Donnadieu, P. Parameswaran, G. Frenking, G.
Bertrand, Nat. Chem. 2010, 2, 369 – 373; b) J. Carreras, M. Patil,
4
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
Communications
P Dication
M. H. Holthausen, M. Mehta,
D. W. Stephan*
&&&& — &&&&
The Highly Lewis Acidic Dicationic
Phosphonium Salt: [(SIMes)PFPh2]
[B(C6F5)4]2
Versatile phosphonium salt: The dication
[(SIMes)PFPh2][B(C6F5)4]2 is prepared by
oxidation of an NHC-derived cationic
phosphine followed by fluoride abstrac-
tion. This species exhibits remarkable
Lewis acidity in stoichiometric reactions
as well as in Lewis acid catalysis of
hydrodefluorination of fluoroalkanes and
the hydrosilylation of olefins and acety-
lenes.
Angew. Chem. Int. Ed. 2014, 53, 1 – 5
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!