The Lewis acidity of fluorophosphonium salts: Access to mixed valent phosphorus(iii)/(v) species

Article abstract of DOI:10.1039/c2dt32711b

Oxidative fluorination of the electron-deficient phosphine Ph ₂P(C₆F₅) using XeF₂, followed by fluoride ion abstraction from the resulting difluorophosphorane Ph ₂P(F)₂(C₆F₅), produces electrophilic fluorophosphonium salts [Ph₂P(F)(C₆F₅)][X] (X = FB(C₆F₅)₃ or O₃SCF₃). Variable temperature NMR spectroscopic analysis of [Ph₂P(F)(C ₆F₅)][FB(C₆F₅)₃] demonstrates a fluxional process attributed to fluoride ion exchange between B(C₆F₅)₃ and [Ph₂P(F)(C ₆F₅)]⁺, suggesting that these species have comparable Lewis acidities. This exchange can also be illustrated by adding phosphine Ph₃P to [Ph₂P(F)(C₆F ₅)][FB(C₆F₅)₃] at ambient temperature to produce Ph₂P(F)₂(C₆F ₅) and Ph₃P-B(C₆F₅)₃, while heating this mixture results in thermally induced para-substitution of Ph₃P at the C₆F₅ group of the phosphonium ion to generate [Ph₃P(C₆F₄)P(F)₂Ph ₂][FB(C₆F₅)₃]. Such frustrated Lewis pair reactivity also can be exploited by reacting [Ph₂P(F)(C ₆F₅)][O₃SCF₃] with silylphosphine Ph₂PSiMe₃ to afford the unique mixed-valent salt [Ph ₂P(C₆F₄)P(F)Ph₂][O ₃SCF₃], which upon the addition of fluoride is converted to Ph₂P(C₆F₄)P(F)₂Ph₂. XeF₂ reacts with [Ph₂P(C₆F₄)P(F) Ph₂][O₃SCF₃] at ambient temperature, producing equal proportions of the dicationic salt [Ph₂P(F)(C₆F ₄)P(F)Ph₂][O₃SCF₃]₂ and the bis(difluorophosphorane) Ph₂P(F)₂(C₆F ₄)P(F)₂Ph₂, the latter of which can then be quantitatively converted to the former by adding one equiv of Me ₃SiO₃SCF₃.

Full text of DOI:10.1039/c2dt32711b

Dalton
Transactions
View Article Online
View Journal | View Issue
PAPER
The Lewis acidity of fluorophosphonium salts: access to
mixed valent phosphorus(III)/(V) species†
Cite this: Dalton Trans., 2013, 42, 2629
Lindsay J. Hounjet, Christopher B. Caputo and Douglas W. Stephan*
Oxidative fluorination of the electron-deficient phosphine Ph2P(C6F5) using XeF2, followed by fluoride
ion abstraction from the resulting difluorophosphorane Ph2P(F)2(C6F5), produces electrophilic fluorophos-
phonium salts [Ph2P(F)(C6F5)][X] (X = FB(C6F5)3 or O3SCF3). Variable temperature NMR spectroscopic
analysis of [Ph2P(F)(C6F5)][FB(C6F5)3] demonstrates a fluxional process attributed to fluoride ion exchange
between B(C6F5)3 and [Ph2P(F)(C6F5)]+, suggesting that these species have comparable Lewis acidities.
This exchange can also be illustrated by adding phosphine Ph3P to [Ph2P(F)(C6F5)][FB(C6F5)3] at ambient
temperature to produce Ph2P(F)2(C6F5) and Ph3P–B(C6F5)3, while heating this mixture results in thermally
induced para-substitution of Ph3P at the C6F5 group of the phosphonium ion to generate [Ph3P(C6F4)-
P(F)2Ph2][FB(C6F5)3]. Such frustrated Lewis pair reactivity also can be exploited by reacting [Ph2P(F)(C6F5)]-
[O3SCF3] with silylphosphine Ph2PSiMe3 to afford the unique mixed-valent salt [Ph2P(C6F4)P(F)Ph2]-
[O3SCF3], which upon the addition of fluoride is converted to Ph2P(C6F4)P(F)2Ph2. XeF2 reacts with
[Ph2P(C6F4)P(F)Ph2][O3SCF3] at ambient temperature, producing equal proportions of the dicationic salt
[Ph2P(F)(C6F4)P(F)Ph2][O3SCF3]2 and the bis(difluorophosphorane) Ph2P(F)2(C6F4)P(F)2Ph2, the latter of
which can then be quantitatively converted to the former by adding one equiv of Me3SiO3SCF3.
Received 13th November 2012,
Accepted 30th November 2012
DOI: 10.1039/c2dt32711b
www.rsc.org/dalton
Apart from the above applications in organocatalysis, the
reactivity of electron-deficient phosphonium salts has
Introduction
Phosphorus compounds are well known as ligands in tran- remained largely underexplored. The chemistry of frustrated
sition metal chemistry. Indeed these Lewis bases are ubiqui- Lewis pairs (FLPs) has thus far focused principally on the reac-
tously employed in the stoichiometric and catalytic processes tivity of systems derived from sterically encumbered group 13
familiar to synthetic inorganic, organic and organometallic Lewis acids in combination with a variety of group 15
chemists. On the other hand, the Lewis acidity of phos- bases.6–15 Indeed many researchers have exploited these FLPs
phonium salts has garnered much less attention.1,2 Nonethe- to activate a wide variety of small molecules, perhaps most
less, diverse applications of phosphonium salts as Lewis acid notably H2.16–19 Some efforts have extended FLPs to group 14
catalysts for C–C, C–O, C–N bond formations and asymmetric Lewis acids,20–24 however our interests, together with the pre-
transformations have been reviewed.3 Gabbaï and coworkers4 cedent of electrophilic phosphorus chemistry, prompted us to
have exploited the Lewis acidity of proximal phosphonium turn our attention to the Lewis acidity of high-valent phos-
centres to enhance the fluoride affinity of boranes, generating phorus compounds. We have previously utilized the Lewis
highly sensitive fluoride sequestering agents. While the acidity of P(V) in strained amidophosphoranes for the capture
majority of these studies employed alkyl or aryl phosphonium of CO2.25 Herein, we demonstrate that fluorophosphonium
salts, Terada and Kouchi5 reported that the presence of elec- salts exhibit Lewis acidity comparable to B(C6F5)3, while high-
tron-withdrawing substituents at
a phosphonium centre lighting reactions with phosphine bases that provide unique
enhances the energetic accessibility of σ* orbitals to provide routes to mixed-valent P(III)/P(V) compounds or phosphorus-
an effective Lewis acid catalyst for Diels–Alder reactions.
based FLPs.
Experimental section
General procedures
Department of Chemistry, University of Toronto, Toronto, Ontario, Canada,
M5S 3H6. E-mail: dstephan@chem.utoronto.ca
†Electronic supplementary information (ESI) available: Selected NMR spectra
All preparations and manipulations were carried out under an
anhydrous N2 atmosphere using standard Schlenk and glove-
box techniques. All glassware was oven-dried and cooled under
have been deposited in supplementary data. CCDC 904900–904902 and 910420.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c2dt32711b
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 2629–2635 | 2629

View Article Online
Paper
Dalton Transactions
1
vacuum before use. Me3SiO3SCF3, Ph3P, Ph2PSiMe3 and δ 148.4 (dm, JFC = 240 Hz, B(o-C6F5)), 140.0 (s, p-C6H5), 139.2
1
1
[NBu4][Ph3SiF2] were purchased from Aldrich and used (dm, JFC = 206 Hz, B(p-C6F5)), 136.9 (dm, JFC = 231 Hz, B(m-
2
3
without further purification. XeF2 and Ph2P(C6F5) were pur- C6F5)), 134.2 (d, JPC = 15 Hz, o-C6H5), 131.5 (d, JPC = 16 Hz,
chased from Apollo Scientific and used without further purifi- m-C6H5), broad, unresolved B(i-C6F5), P(C6F5) and P(i-C6H5)
cation. B(C6F5)3 was purchased from Boulder Chemicals and signals not observed.
used without further purification. All glassware was oven-
dried and cooled under vacuum before use. CH2Cl2, Et2O,
Preparation of [Ph2P(F)(C6F5)][O3SCF3] (3)
n-pentane, and toluene were dried using an Innovative In a glovebox, a 20 mL flask was charged with Ph2P(C6F5)
Technologies solvent purification system. CD2Cl2 (Aldrich) was (221 mg, 626 μmol), a stir bar and 10 mL of CH2Cl2. XeF2
deoxygenated, distilled over CaH2, then stored over 4 Å mole- (106 mg, 626 μmol) was weighed in a separate flask. The XeF2
cular sieves before use. C6D5Br (Aldrich) was deoxygenated solution was carefully transferred to the Ph2P(C6F5) solution
and stored over 4 Å molecular sieves before use. NMR spectra and the colourless effervescing mixture was stirred for 30 min.
were obtained on a Bruker AvanceIII-400 MHz spectrometer.
The solution was then transferred to a vial containing
Me3SiO3SCF3 (140 mg, 626 mmol) and the colourless efferves-
cing solution was stirred for another 30 min. The solvent
Preparation of Ph2P(F)2(C6F5) (1)
A solution of Ph2P(C6F5) (48 mg, 136 μmol) in 5 mL of CH2Cl2 volume was then reduced to approx. 2 mL in vacuo and 10 mL
was added to a solution XeF2 (23 mg, 136 μmol) in 5 mL of of n-pentane was added to the solution resulting in the for-
CH2Cl2, resulting immediately in a colourless, effervescing mation of a white, oily precipitate. The mixture was stirred for
solution. After approx. 1 min, effervescence had ceased and 5 minutes before the precipitate was allowed to settle. The
the solvent volume was reduced to approx. 1 mL in vacuo, then supernatant was decanted and discarded, and the solid was
2 mL of n-pentane were added. Slow evaporation of solvent dried in vacuo and isolated as a white powder (243 mg, 75%,
from the colourless solution yielded diffraction-quality crystals Calcd for C19H10F9O3PS: C, 43.86; H, 1.94%. Found: C, 43.59;
(53 mg, >99%, Anal. Calcd for C18H10F7P: C, 55.40; H, 2.58%. H, 2.23%). 1H NMR (CD2Cl2, Me4Si, 400 MHz): δ 8.14–8.02
Found: C, 55.50; H, 2.82%). 1H NMR (CD2Cl2, 400 MHz, (6H, o,p-C6H5), 7.87 (m, 4H, m-C6H5). 19F NMR (CD2Cl2,
1
Me4Si): δ 8.12 (m, 4H, o-C6H5), 7.61 (m, 2H, p-C6H5), 7.52 (m, 377 MHz, CFCl3): δ −79.1 (s, 3F, CF3), −123.0 (dt, JPF = 1016
4H, m-C6H5). 19F NMR (CD2Cl2, 377 MHz, CFCl3): δ −34.5 (dt, Hz, 4JFF = 17 Hz, 1F, PF), −125.2 (m, 2F, o-C6F5), −133.5 (m, 1F,
4
1JPF = 688 Hz, JFF = 14 Hz, 2F, PF2), −134.2 (m, 2F, o-C6F5), p-C6F5), −155.8 (m, 2F, m-C6F5). 31P{1H} NMR (CD2Cl2, H3PO4,
3
1
3
−153.3 (t, JFF = 20 Hz, 1F, p-C6F5), −162.6 (m, 2F, m-C6F5). 81 MHz): δ 87.1 (dt, JPF = 1016 Hz, JPF = 6 Hz, PF). 13C{1H}
31P{1H} NMR (CD2Cl2, 162 MHz, H3PO4): δ −57.3 (td, JPF
=
NMR (CD2Cl2, 100 MHz, Me4Si): δ 149.4 (dm, JFC = 273 Hz,
1
1
687 Hz, JPF = 14 Hz, PF2). 13C{1H} NMR (CD2Cl2, 100 MHz, p-C6F5), 149.3 (dm, 1JFC = 261 Hz, o-C6F5), 139.6 (dm, 1JFC = 257
3
1
1
4
5
Me4Si): δ 145.8 (dm, JFC = 245 Hz, o-C6F5), 142.5 (dm, JFC
=
Hz, m-C6F5), 139.6 (dd, JPC = 3 Hz, JFC = 2 Hz, p-C6H5), 134.6
272 Hz, p-C6F5), 137.9 (dm, JFC = 250 Hz, m-C6F5), 135.8 (dt, (d, 2JPC = 15 Hz, o-C6H5), 131.4 (d, 3JPC = 16 Hz, m-C6H5), 121.0
1
2JPC = 14 Hz, JFC = 11 Hz, o-C6H5), 133.5 (dt, JPC = 182 Hz, (q, JFC = 321 Hz, CF3), 115.8 (dd, JPC = 114 Hz, JFC = 14 Hz,
3
1
1
1
2
2JFC = 25 Hz, i-C6H5), 133.2 (dt, JPC = 4 Hz, JFC = 1 Hz, i-C6H5), i-C6F5 signal not observed.
4
5
3
4
p-C6H5), 129.1 (dt, JPC = 17 Hz, JFC = 2 Hz, m-C6H5), i-C6F5
signal not observed.
Preparation of [Ph3P(C6F4)P(F)2Ph2][FB(C6F5)3] (4)
A solution of Ph3P (12 mg, 46 μmol) in 2 mL of CH2Cl2 was
added to a solution of 2 (43 mg, 47 μmol) in 2 mL of CH2Cl2
Preparation of [Ph2P(F)(C6F5)][FB(C6F5)3] (2)
A solution of 1 (137 mg, 351 μmol) in 5 mL CH2Cl2 was added instantly producing a white precipitate. The mixture was
to a vial containing a solution of B(C6F5)3 (179 mg, 350 μmol) heated to 40 °C for 15 min, over which time the precipitate
in 5 mL of CH2Cl2, producing a colourless solution. The gradually dissolved to form a colourless solution. The solvent
solvent volume was reduced to approx. 1 mL, then 5 mL of volume was reduced to approx. 0.5 mL in vacuo and 1 mL of
n-pentane were added, resulting in a white precipitate, which Et2O was added, followed by 1 mL of n-pentane. Slow evapo-
was allowed to settle before decanting the supernatant. The ration of solvents from the solution produced colourless,
solid was dried in vacuo and isolated as a white powder diffraction-quality crystals (53 mg, 99%, Calcd for C54H25-
(307 mg, 97%, Anal. Calcd for C36H10BF22P: C, 47.92; H, BF22P2: C, 55.66; H, 2.16%. Found: C, 55.16; H, 2.58%).
1.12%. Found: C, 47.19; H, 1.38%). 1H NMR (CD2Cl2, 1H NMR (CD2Cl2, Me4Si, 400 MHz): δ 8.17 (m, 4H, C6H5), 7.92
400 MHz, Me4Si): δ 8.10 (m, 2H, p-C6H5), 7.97–7.79 (8H, o, (m, 3H, C6H5), 7.79–7.61 (14H, C6H5), 7.55 (m, 4H, C6H5).
1
m-C6H5). 19F NMR (CD2Cl2, 377 MHz, CFCl3): δ −123.4 (dt/br, 19F NMR (CD2Cl2, 377 MHz, CFCl3): δ −36.8 (dt, JPF = 699 Hz,
4
1JPF = 1020 Hz, JFF = 17 Hz, 1F, PF), −124.1 (s/br, 2F, 4JFF = 12 Hz, 2F, PF2), −123.5 (m, 2F, P(C6F4)), −128.5 (m, 2F,
3
P(o-C6F5)), −131.3 (s/br, 1F, P(p-C6F5)), −135.6 (d/br, JFF
=
P(C6F4)), −136.6 (m/br, 6F, B(o-C6F5)), −163.5 (t, 3JFF = 20 Hz, 3F,
16 Hz, 6F, B(o-C6F5)), −153.9 (m/br, 2F, P(m-C6F5)), −161.6 (s/ B(p-C6F5)), −167.9 (m, 6F, B(m-C6F5)), −191.4 (m/br, 1F, BF).
1
br, 3H, B(p-C6F5)), −166.7 (m/br, 6F, B(m-C6F5)), −190.4 (s/br, 11B NMR (CD2Cl2, 128 MHz, BF3·OEt2): δ −0.6 (d, JFB = 65 Hz,
3
1F, BF). 11B NMR (CD2Cl2, 128 MHz, BF3·OEt2): δ 1.9 (s/br, BF). BF). 31P{1H} NMR (CD2Cl2, 162 MHz, H3PO4): δ 16.1 (t, JPF
=
1
31P{1H} NMR (CD2Cl2, 162 MHz, H3PO4): δ 87.2 (d/br, JPF
=
6 Hz, 1P, Ph3P), −58.4 (tm, 1JPF = 699 Hz, 1P, PF2). 13C{1H} NMR
1020 Hz, PF). 13C{1H} NMR (partial, CD2Cl2, 100 MHz, Me4Si): (CD2Cl2, 100 MHz, Me4Si): δ 136.8 (d, JPC = 3 Hz, P(p-C6H5)),
4
2630 | Dalton Trans., 2013, 42, 2629–2635
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
Table 1 Crystallographic data
3
4·0.25C6H14
6
8
Formula
Formula wt
Cryst. Sys.
Space group
a (Å)
b (Å)
c (Å)
C
19H10F9O3PS
C55.5H28.5BF22P2
1186.03
Triclinic
ˉ
P1
10.7377(6)
13.3608(8)
18.5330(11)
85.615(3)
89.546(4)
71.427(3)
2512.6(3)
2
C32H20F12O6P2S2
854.54
Monoclinic
P21/n
14.5862(6)
14.4109(6)
17.6835(8)
90
110.851(2)
90
3473.6(3)
4
C30H20F6P2
556.40
Monoclinic
P21/c
16.4223(17)
8.9674(10)
18.0179(17)
90
104.440(5)
90
2569.6(5)
4
520.30
Orthorhombic
Pbca
17.5800(11)
12.4210(7)
18.6519(12)
90
90
90
4072.9(4)
8
1.697
α (°)
β (°)
γ (°)
V (Å3)
Z
d (g cm−3
)
1.568
0.208
1.634
0.354
1.438
0.231
μ (mm−1
)
0.339
Total data
Unique data
I > 2σ (I2)
Variables
R1
wR2
GOF
34 987
5704
3355
47 216
12 691
9213
739
0.0534
0.1673
1.029
32 487
8380
4555
21 903
5797
3673
298
487
343
0.0446
0.1051
1.000
0.0739
0.2478
1.037
0.0465
0.1139
0.983
136.2 (dt, 2JPC = 14 Hz, 3JFC = 11 Hz, P(o-C6H5)), 134.3 (d, 3JPC
=
C6H5), 134.4 (s, C6H5), 131.4 (d, JPC = 16 Hz, C6H5), 130.7 (s,
4
11 Hz, P(m-C6H5)), 133.8 (d, JPC = 4 Hz, P(p-C6H5)), 131.2 (d, C6H5), 129.4 (d, JPC = 8 Hz, C6H5), 121.0 (q, 1JFC = 321 Hz, CF3),
3
4
1
2
1
2JPC = 14 Hz, P(o-C6H5)), 129.5 (dt, JPC = 18 Hz, JFC = 2 Hz, 116.4 (dm, JPC = 113 Hz, JFC = 12 Hz, i-C6H5), 115.2 (d, JPC
=
1
2
P(m-C6H5)), 129.4 (dt, JPC = 65 Hz, JFC = 16 Hz, P(i-C6H5)), 14 Hz, i-C6H5), i-C6F4 signals not observed.
116.1 (d, JPC = 92 Hz, P(i-C6H5), signals for broad, unresolved
1
Preparation of [Ph2P(F)(C6F4)P(F)Ph2][O3SCF3]2 (6)
C6F5 groups are not reported.
A colourless solution of XeF2 (40 mg, 236 μmol) in 5 mL of
CH2Cl2 was added to a yellow solution of 5 (160 mg, 233 μmol)
in 5 mL of CH2Cl2, and agitation of the mixture yielded a
colourless solution. The solvent volume was reduced to
approx. 1 mL in vacuo and 10 mL of n-pentane was added to
the white slurry. The mixture was vigorously agitated for 2 min
and the precipitate was allowed to settle. The supernatant was
decanted and the solid was dried in vacuo and isolated as a
white powder (162 mg, 81%, Calcd for C32H20F12O6P2S2: C,
44.98; H, 2.36%. Found: C, 44.43; H, 2.89%). 1H NMR (CD2Cl2,
Me4Si, 400 MHz): δ 8.40–7.68 (20H, C6H5). 19F NMR (CD2Cl2,
377 MHz, CFCl3): δ −78.8 (s, 6F, CF3), −121.6 (m, 4F, C6F4)
Preparation of [Ph2P(C6F4)P(F)Ph2][O3SCF3] (5)
A colourless solution of Ph2PSiMe3 (80 mg, 310 μmol) in 2 mL
of C6H5Br was added to a flask containing solid 3 (150 mg,
288 μmol). Agitation of the mixture for several minutes pro-
duced a yellow solution containing trace particles, which was
then filtered through a Kimwipe® plug in a glass pipette. The
solvent and volatile Me3SiF were removed in vacuo, resulting in
a clear, viscous, yellow oil. Et2O (10 mL) was then added, and
the mixture was agitated thoroughly for 5 min. A yellow oil
settled out from the colourless supernatant, which was then
decanted, and the residue was dried in vacuo. n-Pentane
(10 mL) was then added, and the mixture was triturated to a
fine suspension before the precipitate was allowed to settle.
The supernatant was decanted and the solid was dried
in vacuo yielding a light-yellow powder (147 mg, 74%, Calcd for
C31H20F8O3P2S: C, 54.24; H, 2.94%. Found: C, 52.53; H,
3.05%). Although elemental data are insufficient to support
the compound’s formulation, NMR spectra (see ESI,† Fig. 3–5)
1
−123.2 (dm, JPF = 1033 Hz, 2F, PF). 31P{1H} NMR (CD2Cl2,
1
162 MHz, H3PO4): δ 86.7 (dm, JPF = 1033 Hz, PF). 13C{1H}
NMR spectra could not obtained due to the exceedingly low
solubility of the compound in all common NMR solvents.
Diffraction-quality crystals were grown by slow evaporation of a
saturated CD2Cl2 solution.
1
Preparation of Ph2P(C6F4)P(F)2Ph2 (8)
reveal the presence of 5 along with minor impurities. H NMR
(CD2Cl2, Me4Si, 400 MHz): δ 8.18–8.03 (6H, X6H5), 7.92 (m, 4H, A colourless solution of [NBu4][Ph3SiF2] (36 mg, 67 μmol) in
C6H5), 7.63 (m, 4H, C6H5), 7.51–7.42 (6H, C6H5). 19F NMR 2 mL of CH2Cl2 was added to a yellow solution of 5 (46 mg,
(CD2Cl2, 377 MHz, CFCl3): δ −78.9 (s, CF3, 3F), −123.1 (m, 67 μmol) in 2 mL of CH2Cl2, instantly producing a colourless
1
4
C6F4, 2F), −124.4 (dt, JPF = 1011 Hz, JFF = 14 Hz, PF, 1F), solution. The solvent volume was reduced to approx. 0.5 mL
−125.9 (m, C6F4, 2F). 31P{1H} NMR (CD2Cl2, 162 MHz, H3PO4): in vacuo and 10 mL of n-pentane were added. The mixture was
1
3
3
δ 86.8 (dt, JPF = 1011 Hz, JPF = 7 Hz, PF, 1P), −15.0 (t, JPF
=
agitated vigorously for 5 min and was left to settle for 10 min,
23 Hz, Ph2P, 1P). 13C{1H} NMR (CD2Cl2, 100 MHz, Me4Si): δ 148.5 after which point a light-pink oil had settled out from the
(dm, JFC = 260 Hz, C6F4), 147.7 (dm, JFC = 267 Hz, C6F4), colourless supernatant. The supernatant was decanted and the
139.6 (dd, JPC = 3 Hz, JFC = 2 Hz, C6H5), 134.3 (d, JPC = 38 Hz, solvent was allowed to evaporate slowly. A white, crystalline
1
1
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 2629–2635 | 2631

View Article Online
Paper
Dalton Transactions
substance, covered in a colourless oil, remained after solvent
evaporation was complete. The crystalline residue was tritu-
rated with 10 mL of cold n-pentane (to remove residual
Ph3SiF), dried in vacuo, and isolated as a white, microcrystal-
line powder (23 mg, 62%, Calcd for C30H20F6P2: C, 64.76; H,
3.62%. Found: C, 64.78; H, 3.99%). 1H NMR (CD2Cl2, Me4Si,
400 MHz):
δ
8.09–7.25 (20H, C6H5). 19F NMR (CD2Cl2,
1
4
377 MHz, CFCl3): δ −34.6 (dt, JPF = 684 Hz, JFF = 12 Hz, PF2,
2F), −128.1 (m, C6F4, 2F), −133.3 (m, C6F4, 2F). 31P{1H} NMR
Fig. 1 POV-ray depiction of the cation of 3.
3
(CD2Cl2, 162 MHz, H3PO4): δ −23.6 (t, JPF = 36 Hz, Ph2P, 1P),
−57.2 (tm, JPF = 684 Hz, PF2, 1P). 13C{1H} NMR (CD2Cl2,
1
100 MHz, Me4Si): δ 148.1 (dm, 1JFC = 246 Hz, C6F4), 145.2 (dm, generate electrophilic phosphonium salts of the formula
1JFC = 260 Hz, C6F4), 135.8 (dt, JPC = 15 Hz, JFC = 9 Hz, C6H5), [Ph2P(F)(C6F5)][X] (2: X = FB(C6F5)3; 3: X = O3SCF3). The
133.8 (dm, JPC = 11 Hz, C6H5), 133.6 (dt, JPC = 182 Hz, 31P{1H} NMR spectrum of 3 shows a sharp doublet of triplets
1
1
i-C6H5), 133.4 (dt, JPC = 21 Hz, JFC = 1 Hz, C6H5), 133.1 (dt, at δ 87.1 (1JPF = 1016 Hz, JPF = 6 Hz) resulting from coupling
3
JPC = 4 Hz, JFC = 1 Hz, C6H5), 131.5 (d, JPC = 11 Hz, C6H5), 129.1 to the P-bound fluoride and the ortho-fluorines of the C6F5
(dt, JPC = 17 Hz, JFC = 2 Hz, C6H5), 129.1 (d, JPC = 7 Hz, C6H5), ring, while the 19F spectrum reveals similar multiplicity for the
4
i-C6F4 signals not observed.
PF resonance at δ −123.0 (1JPF = 1016 Hz, JFF = 17 Hz). The
X-ray structure of 3 (Fig. 1) shows the expected tetrahedral geo-
metry at phosphorus with a P–F bond length of 1.547(2) Å.
In contrast to the sharp NMR signals observed for 3, the
31P{1H} and 19F resonances of 2 are somewhat broad at
ambient temperature, with no discernible coupling. These
signals sharpen with decreasing temperature so that the P–F
couplings are resolved at −39 °C, and broaden dramatically
upon increasing the temperature to 60 °C. This temperature
dependent behaviour, which is not observed for the triflate
derivative 3, suggests fluoride delivery from the [FB(C6F5)3]−
anion to the Lewis acidic fluorophosphonium cation. This
interpretation is further supported by the ambient tempera-
ture addition of Ph3P to 2 in CD2Cl2, which immediately gen-
erated a white precipitate, identified as the adduct, Ph3P–
B(C6F5)327 (Scheme 1). 31P{1H} NMR analysis of the supernatant
confirmed the presence of difluorophosphorane 1. Transfer of
fluoride from the fluoroborate anion to the fluorophospho-
nium cation, suggests that this cation has similar fluorophili-
city to B(C6F5)3.
X-Ray data collection, reduction, solution and refinement
Single crystals were coated in Paratone-N oil in the glovebox,
mounted on a MiTegen Micromount and placed under an N2
stream. The data were collected on a Bruker Apex II diffracto-
meter. The data were collected at 150( 2) K for all crystals.
Data reduction was performed using the SAINT software
package and an absorption correction applied using SADABS.
The structures were solved by direct methods using XS and
refined by full-matrix least-squares on F2 using XL as
implemented in the SHELXTL suite of programs.26 All non-
hydrogen atoms were refined anisotropically. Carbon-bound
hydrogen atoms were placed in calculated positions using an
appropriate riding model and coupled isotropic temperature
factors (see Table 1 and ESI†).
Results and discussion
Reaction of XeF2 with the commercially available, electron-
deficient phosphine, Ph2P(C6F5), cleanly generates the difluoro-
phosphorane, Ph2P(F)2(C6F5) 1 (Scheme 1), and subsequent
fluoride ion abstractions with either B(C6F5)3 or Me3SiO3SCF3
Interestingly, heating the mixture of Ph3P–B(C6F5)3 and 1 to
reflux for 10 min resulted in the immediate formation of a
new species 4. The 31P{1H} NMR spectrum of this product
shows two new triplet resonances at δ 16.1 (3JPF = 6 Hz) and
−58.4 (1JPF = 699 Hz). The 19F NMR spectrum of 4 exhibited
seven signals, consistent with attack of Ph3P at the para-posi-
tion of the phosphonium ion’s C6F5 group. The resulting phos-
phonium–phosphorane, [Ph3P(C6F4)P(F)2Ph2][FB(C6F5)3]
4
(Scheme 1), was crystallized and characterized by X-ray struc-
tural analysis (Fig. 2). The axially disposed fluorine atoms of
the phosphorane moiety give rise to an F(1)–P(2)–F(2) angle of
175.20(14)°. The P(2)–F(1) and P(2)–F(2) bond lengths of 1.656(3)
and 1.657(3) Å, respectively, are typical of difluorotriarylphos-
phorane species.25,28 The reaction producing 4 is thought to
proceed via initial fluoride abstraction from 1 by thermally
liberated B(C6F5)3. Subsequent attack of Ph3P at the fluoro-
phosphonium cation occurs with concomitant migration of
fluoride ion to the Lewis acidic fluorophosphonium centre,
producing 4. This reactivity is analogous to the thermally-
Scheme 1 Synthesis of 1–5.
2632 | Dalton Trans., 2013, 42, 2629–2635
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
Scheme 2 Reactivity of 5.
Fig. 2 POV-ray depiction of 4.
induced para-substitution reaction of Ph3P with B(C6F5)3
affording Ph3P(C6F4)B(F)(C6F5)2,27 thus the formation of 4
further illustrates the Lewis acidity of the phosphonium
cation, [Ph2P(F)(C6F5)]+.
Fig. 3 POV-ray depiction of the dication of 6.
A related species is derived from the ambient temperature
reaction of 3 with the silylphosphine Ph2PSiMe3 in C6D5Br.
This reaction produced a brilliant, yellow solution of 5
(Scheme 1), which exhibits a 31P{1H} NMR spectrum with a
parameters are unexceptional. The second product from the
reaction of 5 with XeF2 was observed in the supernatant by its
31P{1H} NMR signal at δ −57.2, and formulated as the bis-
(difluorophosphorane), Ph2(F)2P(C6F4)P(F)2Ph2 7. Support for
this observation was obtained by the reaction of 7 with
Me3SiO3SCF3, which quantitatively produced the dicationic
salt 6.
The addition of [NBu4][Ph3SiF2] to 5 in CH2Cl2 also
instantly resulted in disappearance of the solution’s yellow
colour, and a 31P{1H} NMR spectrum of the mixture reveals
phosphine–phosphorane 8 (Scheme 2) with two new triplet
resonances at δ −23.6 (3JPF = 36 Hz) and −57.2 (1JPF = 684 Hz).
Compound 8 and Ph3SiF were simultaneously extracted from
the concentrated reaction mixture with n-pentane, and slow
evaporation of the solvent produced diffraction-quality crystals
of the mixed-valent product. The by-product Ph3SiF could then
be removed by washing the crystals with n-pentane to afford 8
as an analytically pure solid. X-ray structural analysis of 8
(Fig. 4) clearly shows the para-disposed phosphorane and
phosphine functionalities of the fluorinated arene. The
respective geometries of the two phosphorus centres are
3
doublet of triplets at δ 86.8 (1JPF = 1011 Hz, JPF = 7 Hz) and a
triplet at δ −15.0 (3JPF = 23 Hz). These signals are consistent
with the presence of Ph2P and PF fragments occupying
mutually para positions of a C6F4 ring, and thus the formu-
lation of 5 as [Ph2P(C6F4)P(F)Ph2][O3SCF3]. The bright yellow
colour of 5 can presumably be attributed to charge transfer
from the phosphine through the arene to the para-disposed
phosphonium electrophile further supporting Lewis acidity of
the fluorophosphonium cation. The formation of 5 is a rare
example of a reaction between a phosphonium cation and
neutral base.29 In addition, compound 5 is analogous to R2P-
(C6F4)B(C6F5)2 in that it contains both Lewis acidic and basic
centres. As such 5 can be described as an all-phosphorus intra-
molecular FLP.
Compound 5 reacts with XeF2 in CH2Cl2 solution with
immediate loss of colour, generating two new 31P{1H} NMR
1
signals of equal intensity at δ 86.7 (dm, JPF = 1033 Hz) and
−57.2 (tm, 1JPF = 685 Hz). These signals are consistent with the
presence of fluorophosphonium and difluorophosphorane
fragments. Upon standing, colourless crystals of a species 6
were obtained from the reaction mixture. This product
accounts for the 31P{1H} NMR signal at δ 86.7. X-ray structural
analysis revealed 6 to be the symmetrical bis(phosphonium)
salt, [Ph2P(F)(C6F4)P(F)Ph2][O3SCF3]2 (Scheme 2). Not surpris-
ingly, this dicationic salt is quite insoluble in all common sol-
vents, although NMR spectroscopic analysis could be carried
out on a dilute CD2Cl2 solution. The X-ray structure of 6
(Fig. 3) shows the presence of the two para-disposed fluoro-
phosphonium functionalities about the C6F4 group with an
average P–F bond length of 1.54 Å. The remaining metric Fig. 4 POV-ray depiction of 8.
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 2629–2635 | 2633

View Article Online
Paper
Dalton Transactions
fluorophosphonium cations are of comparable Lewis acidity to
B(C6F5)3 augurs well for applications of P-based Lewis acids in
synthesis and catalysis. Such developments are the subject of
on-going efforts in our laboratories.
Acknowledgements
Financial support from NSERC of Canada is gratefully
acknowledged. DWS is grateful for the award of a Canada
Research Chair. CBC is grateful for the award of an NSERC
Alexander Graham Bell Graduate Doctoral Scholarship.
Scheme 3 Proposed pathways for the reaction of 5 with XeF2.
pseudo-trigonal bipyramidal and pseudo-trigonal pyramidal,
with the P–F bond distances of the former averaging 1.66 Å.
The formation of 8 affirms the Lewis acidic nature of the
fluorophosphonium centre of 5. Moreover, this observation
supports the description of 5 as an FLP.
Notes and references
1 D. Uraguchi, S. Sakaki and T. Ooi, J. Am. Chem. Soc., 2007,
129, 12392–12393.
2 X. Wang and S.-K. Tian, Tetrahedron Lett., 2007, 48,
6010–6013.
3 T. Werner, Adv. Synth. Catal., 2009, 351, 1469–1481.
4 T. W. Hudnall, Y. M. Kim, M. W. P. Bebbington,
D. Bourissou and F. P. Gabbaï, J. Am. Chem. Soc., 2008, 130,
10890–10891.
5 M. Terada and M. Kouchi, Tetrahedron, 2006, 62, 401–409.
6 D. W. Stephan and G. Erker, Angew. Chem., Int. Ed., 2010,
49, 46–76.
7 K. Unverhau, G. Lubbe, B. Wibbeling, R. Frohlich, G. Kehr
and G. Erker, Organometallics, 2010, 29, 5320–5329.
8 C. M. Mömming, E. Otten, G. Kehr, R. Fröhlich,
S. Grimme, D. W. Stephan and G. Erker, Angew. Chem., Int.
Ed., 2009, 48, 6643–6646.
9 H. D. Wang, R. Fröhlich, G. Kehr and G. Erker, Chem.
Commun., 2008, 5966–5968.
10 P. Spies, S. Schwendemann, S. Lange, G. Kehr, R. Fröhlich
and G. Erker, Angew. Chem., Int. Ed., 2008, 47, 7543–7546.
11 G. Ménard and D. W. Stephan, J. Am. Chem. Soc., 2010, 132,
1796–1797.
Two possible reaction pathways can be envisioned for the
formation of 6 and 7 from 5 and XeF2 (Scheme 3). Direct oxi-
dation of the phosphine centre in 5 with XeF2 could generate
the intermediate [Ph2PF(C6F4)P(F)2Ph2]+. In this species, Lewis
acidity of the phosphonium centre should be enhanced by the
para-disposed phosphorane, prompting intermolecular fluo-
ride redistribution to form 6 and 7 (Scheme 3). Alternatively,
one can consider that activation of XeF2 between the acidic
and basic functionalities of two independent cations would
generate 6 and Ph2P(C6F4)P(F)2Ph2 8. Subsequent oxidation of
8 by XeF2 would produce 7. Efforts to discern the operative
pathway via low temperature reactions were unsuccessful. It is
noteworthy that in the former mechanism the proposed inter-
mediate is analogous to the isolated species 4, whereas in the
latter pathway the proposed intermediate 8 was prepared and
isolated independently. These observations suggest that the
independent pathways may be competitive.
Conclusions
12 G. Ménard and D. W. Stephan, Angew. Chem., Int. Ed., 2011,
50, 8396–8399.
In summary, Lewis acidic fluorophosphonium salts are readily
prepared by oxidation of a phosphine with XeF2 and sub- 13 G. Ménard and D. W. Stephan, Angew. Chem., Int. Ed., 2012,
sequent fluoride abstraction. Reactions of the resulting fluoro- 51, 4409–4412.
phosphonium salts with Ph3P and Ph2PSiMe3 yield 4 and 5, 14 C. Appelt, H. Westenberg, F. Bertini, A. W. Ehlers,
respectively, demonstrating the fluorophilicity of these Lewis
acids, which readily undergo para-attack at the C6F5 group.
J. C. Slootweg, K. Lammertsma and W. Uhl, Angew. Chem.,
Int. Ed., 2011, 50, 3925–3928.
This reactivity is reminiscent of that observed for related P/B 15 H. Westenberg, J. C. Slootweg, A. Hepp, J. Kosters,
FLP systems.27,30 While compound 5 reacts with XeF2 to give 6
and 7, it also reacts with a source of fluoride to give 8. Com-
S. Roters, A. W. Ehlers, K. Lammertsma and W. Uhl,
Organometallics, 2010, 29, 1323–1330.
pounds 5 and 8 represent rare examples of mixed-valent P(III)/(V) 16 G. C. Welch and D. W. Stephan, J. Am. Chem. Soc., 2007,
compounds,31–37 and 8 is, to the best of our knowledge, the
129, 1880–1881.
first crystallographically characterized species containing both 17 P. A. Chase, G. C. Welch, T. Jurca and D. W. Stephan,
phosphine and phosphorane functionalities. In addition, the Angew. Chem., Int. Ed., 2007, 46, 8050–8053.
Lewis acidity of fluorophosphonium cations provides routes to 18 G. C. Welch, R. R. S. Juan, J. D. Masuda and D. W. Stephan,
these unprecedented mixed-valent phosphorus compounds, Science, 2006, 314, 1124–1126.
thus affording all-phosphorus FLPs. The demonstration that 19 D. W. Stephan, Org. Biomol. Chem., 2012, 10, 5740–5746.
2634 | Dalton Trans., 2013, 42, 2629–2635
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
20 M. Alcarazo, C. Gomez, S. Holle and R. Goddard, Angew. 29 D. J. Burton, J. Fluorine Chem., 1983, 23, 339–345.
Chem., Int. Ed., 2010, 49, 5788–5791.
21 J. Iglesias-Siguenza and M. Alcarazo, Angew. Chem., Int. Ed.,
2012, 51, 1523–1524.
22 B. Ines, S. Holle, R. Goddard and M. Alcarazo, Angew.
Chem., Int. Ed., 2010, 49, 8389–8391.
23 B. Ines, M. Patil, J. Carreras, R. Goddard, W. Thiel and
M. Alcarazo, Angew. Chem., Int. Ed., 2011, 50, 8400–8403.
24 A. Schäfer, M. Reißmann, A. Schäfer, W. Saak, D. Haase
30 F. Schulz, V. Sumerin, M. Leskela, T. Repo and B. Rieger,
Dalton Trans., 2010, 39, 1920–1922.
31 S. M. I. Al-Rafia, M. J. Ferguson and E. Rivard, Inorg.
Chem., 2011, 50, 10543–10545.
32 T. L. Charlton and R. G. Cavell, Inorg. Chem., 1969, 8, 2436–
2440.
33 A. A. Pinkerton and R. G. Cavell, J. Am. Chem. Soc., 1971,
93, 2384–2390.
and T. Müller, Angew. Chem., Int. Ed., 2011, 50, 12636– 34 R. G. Cavell and L. F. Doty, Inorg. Chem., 1978, 17, 3086–
12638. 3093.
25 L. J. Hounjet, C. B. Caputo and D. W. Stephan, Angew. 35 H. Luo, R. McDonald and R. G. Cavell, Angew. Chem., Int.
Chem., Int. Ed., 2012, 51, 4714–4717.
Ed., 1998, 37, 1098–1099.
26 G. M. Sheldrick, Bruker AXS Inc., Madison, WI, 2000.
27 G. C. Welch, R. Prieto, M. A. Dureen, A. J. Lough, O.
A. Labeodan, T. Holtrichter-Rossmann and D. W. Stephan,
Dalton Trans., 2009, 1559–1570.
28 G. M. Sheldrick, Acta Crystallogr., Sect. B: Struct. Crystallogr.
Cryst. Chem., 1975, 31, 1776–1778.
36 G. Bettermann, H. Buhl, R. Schmutzler, D. Schomburg and
U. Wermuth, Phosphorus, Sulfur Silicon Relat. Elem., 1983,
18, 77–80.
37 G. Bettermann, P. Look, D. Schomburg and
R. Schmutzler, Phosphorus, Sulfur Silicon Relat. Elem.,
1987, 30, 499–502.
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 2629–2635 | 2635