Electrophilic bis-fluorophosphonium dications: Lewis acid catalysts from diphosphines

Article abstract of DOI:10.1039/c5sc00051c

Stepwise oxidation of 1,8-bis(diphenylphosphino)naphthalene and a series of (oligo)methylene-linked diphosphines with XeF₂ followed by fluoride abstraction yields a family of compounds featuring phosphine, phosphonium and phosphorane moieties in close proximity. The bisphosphonium ions [(C₁₀H₆)(Ph₂PF)₂]²⁺ (5) and [CH₂(Ph₂PF)₂]²⁺ (9a) exhibit remarkable Lewis acidity arising from the proximity of the phosphonium centers. The effectiveness of bisphosphonium dications (5, 9a-e) is examined in a series of Lewis acid catalysed transformations.

Full text of DOI:10.1039/c5sc00051c

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Electrophilic bis-fluorophosphonium dications:
Lewis acid catalysts from diphosphines†
Cite this: Chem. Sci., 2015, 6, 2016
a
a
ab
*
Michael H. Holthausen, Rashi R. Hiranandani and Douglas W. Stephan
Stepwise oxidation of 1,8-bis(diphenylphosphino)naphthalene and a series of (oligo)methylene-linked
diphosphines with XeF2 followed by fluoride abstraction yields a family of compounds featuring
phosphine, phosphonium and phosphorane moieties in close proximity. The bisphosphonium ions
[(C10H6)(Ph2PF)2]2+ (5) and [CH2(Ph2PF)2]2+ (9a) exhibit remarkable Lewis acidity arising from the
proximity of the phosphonium centers. The effectiveness of bisphosphonium dications (5, 9a–e) is
examined in a series of Lewis acid catalysed transformations.
Received 6th January 2015
Accepted 27th January 2015
DOI: 10.1039/c5sc00051c
www.rsc.org/chemicalscience
[(SIMes)PPh2F]2+ were shown to be effective catalysts for the
hydrodeuorination of uoroalkanes and the hydrosilylation or
transfer-hydrogenation of olens and alkynes.8–10
Introduction
Main group based Lewis acids are used as co-catalysts in poly-
merization catalysis or as Lewis acid catalysts in a wide range of
organic transformations.1 As such reactivity is predicated only
on being electron decient, group XIII compounds such as BH3,
BF3, or AlMe3 are the quintessential examples of main-group
Lewis acids. Group XIV-centered Lewis acids (C,2 Si3) have also
been exploited although as these compounds are generally very
strong Lewis acids and coordinatively unsaturated, they tend to
be employed as stoichiometric reagents. Moving further to the
right to group XV, derivatives of these elements are most
commonly Lewis bases and the electrophilic character of group
XV species has drawn lesser attention. Nonetheless, P(III)
phosphenium ions [R2P]+ have been shown to be ambiphilic
effecting C–C, C–H4 and P–P5 bond activation. P(V) phospho-
nium cations of the form [R4P]+ (R ¼ aryl, alkyl) are weakly Lewis
acidic and have been exploited as uoride ion sensors6 or as
catalysts for (cyclo)addition reactions to polar or activated
unsaturates.7 Exploring more electron decient phosphonium
cations, we found that [(C6F5)3PF]+ is a highly electrophilic
phosphonium cation (EPC). This Lewis acidity is attributed to
the presence of the strongly electron-withdrawing substituents
and the energetically accessible s*(P–F) orbital.8 In an alterna-
tive approach towards EPCs, the dicationic phosphonium salt
In group XIII11 and group XIV chemistry,12 the proximity of
two Lewis acidic centers has been shown to enhance the Lewis
acidity, providing even more potent Lewis acids and thus
impact on reactivity. Only recently, a group XV-based bidentate
stiborane was introduced as strong, chelating Lewis acid for
uoride complexation.13 Exploring these notions in phospho-
nium chemistry prompted us to target bidentate bis-phospho-
nium salts. In this manuscript we prepare a series of such
compounds exploiting a naphthalene backbone which locks
two P moieties in the close vicinity of the peri-positions. In
addition a series of more exible (oligo)methylene-linked
derivatives ((CH2)n, n ¼ 1–5) are prepared. The impact of the
proximity of two EPC centers is gauged by an examination of
these species in a series of Lewis acid catalyses.
Results and discussion
The naphthalene-based diphosphine (C10H6)(PPh2)2 reacts with
XeF2 at low temperatures (ꢀ30 ꢁC) affording the selective
formation of the phosphine–phosphorane species (C10H6)-
(Ph2PF2)(PPh2) (1) in high yields (87%, Scheme 1). The 31P{1H}
NMR spectrum of 1 shows two triplet of doublet resonances at
9
[(SIMes)PPh2F][B(C6F5)4]2 was shown to be even more Lewis
ꢀ55.5 ppm (1JPF ¼ 742 Hz, 4JPP ¼ 10 Hz) and ꢀ18.5 ppm (5JPF
¼
acidic than [(C6F5)3PF]+, demonstrating the impact of the
additional positive charge. Nonetheless both [(C6F5)3PF]+ and
17 Hz, 4JPP ¼ 10 Hz) which are assigned to the phosphorane and
phosphine moieties respectively.14 The associated doublet of
doublet resonance of the uorine atoms shows a chemical shi
aDepartment of Chemistry, University of Toronto, 80 St. George St, Toronto, Ontario of ꢀ42.0 ppm in the 19F{1H} NMR spectrum.
M5S3H6, Canada. E-mail: dstephan@chem.utoronto.ca
bChemistry Department-Faculty of Science, King Abdulaziz University, Jeddah 21589,
Reacting (C10H6)(PPh2)2 with XeF2 in a 1 : 2 stoichiometry at
ambient temperature yields quantitatively the bis-diuor-
ophosphorane (C10H6)(Ph2PF2)2 (2, 95%). An AA0X2X20 spin
system is observed in the 31P{1H} (dA ¼ ꢀ49.4 ppm) and 19F{1H}
NMR (dX ¼ ꢀ34.3 ppm) spectra of 2 and its parameters were
evaluated by full line shape iteration.15 Both resonances of 2 are
Saudi Arabia
† Electronic supplementary information (ESI) available: Preparative,
spectroscopic and analytical details for the experiments described herein. CCDC
1041558–1041565. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c5sc00051c
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˚
squares C10 plane of 0.120(2) A. The corresponding deviation in
18
˚
the parent diphosphine (C10H6)(Ph2P)2 is 0.006 A. The steric
strain also causes the phosphorus moieties of 2 to exhibit a strong
˚
bending out of the naphthalene plane (av. 0.73(1) A vs.
˚
(C10H6)(Ph2P)2 av. 0.42 A). The shortest P/F interactions between
˚
the phosphorane moieties is 2.825(4) A which is well within the
19
˚
sum of the respective van der Waal's radii (PF: 3.27 A). The
˚
corresponding P/P distance of 3.823(3) A is signicantly longer
18
˚
than that in the parent diphosphine (3.052 A). Earlier reports
have described difficulties in effecting the quaternisation of both
P atoms in (C10H6)(Ph2P)2 due to steric congestion,20 and, to the
best of our knowledge, 2 represents the rst compound featuring
two penta-coordinated P atoms in peri-positions on a naphtha-
lene framework.
The reactions of phosphoranes 1 and 2 with [Et3Si]
[B(C6F5)4]$2(C7H8) in 1 : 1 stoichiometries yielded the
respective salts [(C10H6)(Ph2PF)(PPh2)][B(C6F5)4] (3) and
[(C10H6)(Ph2PF2)(Ph2PF)][B(C6F5)4] (4) in good to excellent
yields (Scheme 1). The 31P{1H} NMR spectrum of 3 shows
doublet of doublet resonances at ꢀ17.4 and ꢀ5.5 ppm. The
former is assigned to the Ph2PF-moiety on basis of its 1JPF value
of 783 Hz. A very large coupling constant between both P atoms
(138 Hz) indicates coordination of the lone pair of the phos-
phine-moiety to the uorophosphonium centre.21 This also
results in a large coupling constant between the uorine atom
5
and the distal P atom (164 Hz; vs. JPF ¼ 17 Hz in 1). The indi-
Scheme 1 Synthetic routes to 1–5.
cated PP interaction was conrmed by single crystal X-ray
crystallography (Fig. 2) as the P–P bond length was found to be
22
˚
˚
2.530(1) A. This is longer than the P–P distance in P4 (2.2 A)
indicating that the interaction in 3 is weak. The uoro-
substituted P atom shows a trigonal bipyramidal bonding
environment with the uorine and the coordinating phosphine-
moiety adopting axial positions. The P–F bond distance
shied to lower eld compared to the phosphorane moiety in 1.
5
5
A relatively large JPF (5JA X ¼ JAX ¼ ꢀ18 Hz) and a compara-
0
0
tively small 4JPP coupling constant (4JAA ¼ 3 Hz) might indicated
0
a through space interaction between both phosphonium
moieties.16
˚
(1.637(2) A) is signicantly longer than in other uo-
8,9,14
The molecular structure of 2 shows two trigonal bipyramidal
phosphorus moieties with uorine atoms occupying the axial
positions (av. F–P–F: 177.0(1)ꢁ, Fig. 1). The P–F bond length (av.
˚
rophosphonium species (av. 1.54(1) A)
which is consistent
with the donation of electron density from the phosphine unit
into the s*-orbital of the P–F bond. In contrast to 2, the naph-
thalene backbone of 3 show no evidence of steric strain (mean
1.67(3) A) is typical for diuorotriarylphosphoranes.14,17 The close
˚
proximity of the phosphorane moieties in 2 leads to signicant
steric strain. This results in a buckling of its naphthalene-back-
bone as evidenced by the mean C-atom deviation from the least-
˚
C-atom deviation from the least-squares C10 plane: 0.006(1) A).
It is noteworthy that interactions between the P atoms of
adjacent phosphine and alkyl or aryl phosphonium centers in
Fig. 1 POV-ray depiction of 2. P: orange, F: yellow-green, C: black. Fig. 2 POV-ray depiction of the cation in 3. P: orange, F: yellow-
Hydrogen atoms are omitted for clarity.
green, C: black. Hydrogen atoms are omitted for clarity.
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the related cations [(C10H6)(Ph2PR)(Ph2P)]+ (R ¼ alkyl, aryl) is protonated naphthalene-based bis-phosphine was previously
not observed,17,20a,23 reecting the Lewis acidity of the uo- studied by NMR spectroscopy.25 While dications incorporating a
rophosphonium cation. While we have previously reported the P–P bond21b,26 or a bridging substituent23,27 have been previously
intramolecular coordination of an amide fragment to a uo- isolated.
rophosphonium moiety,24 compound 3 appears to be the rst
In an effort to probe the Lewis acidity of 5, it was reacted with
example of phosphine coordinating to a uoro-phosphonium an equivalent of Et3PO employing the Gutmann–Beckett
center. It is important to note that such an interaction is not method (Scheme 2).28 The 31P{1H} NMR spectrum of the reac-
observed in a 1 : 1 mixture of Ph3P and [Ph3PF][B(C6F5)4] indi- tion mixture shows a broad resonance at 60.5 ppm corre-
cating that the close proximity between Lewis acid and base sponding to Et3PO interacting with 5. This results in an acceptor
imposed by the naphthalene backbone is crucial for adduct number of 21.4, which suggests that 5 is approximately half as
formation.
Lewis acidic as B(C6F5)3. In comparison, combination of the salt
The 31P{1H} NMR spectrum of [(C10H6)(Ph2PF2)(Ph2PF)]+ (4) [Ph3PF][B(C6F5)4] and Et3PO in a 2 : 1 stoichiometry showed
shows a broad triplet resonance at ꢀ51.7 ppm (1JPF ¼ 717 Hz) only a very small shi in the resonance of Et3PO. These obser-
assigned to the phosphorane moiety and a broad doublet at 96.6 vations infer that the Lewis acidity of a phosphonium center in
ppm (1JPF ¼ 1012 Hz) corresponding to the phosphonium 5 is enhanced by the proximity of a second uorophosphonium
centre. A crystallographic study revealed that the P–F bond moiety. Monitoring the 1 : 1 mixture of 5 and Et3PO over time
+
lengths at the penta-coordinated P atom of 4 (av. 1.664(2) A) are revealed slow transformation to [Et3PF] (6)9 and [(C6H10)(Ph2-
˚
longer than the P–F bond distance at the adjacent uo- PO)(Ph2PF)]+ (7). The 31P{1H} NMR spectrum of (7) shows the
rophosphonium centre (1.543(1) A; Fig. 3). Similar to 2, a weak expected doublet of doublet resonances at ꢀ34.5 ppm (1JPF
¼
˚
P/F interaction between the F on the phosphonium and P of 697 Hz, JPP ¼ 23 Hz) and 47.1 ppm (JPF ¼ 2 Hz). The high eld
˚
the phosphorane fragment is observed (2.995(2) A). Compared shi of the uorophosphonium moiety and the low eld shi of
to 2 there is a decreased buckling of the naphthalene moiety the phosphine oxide indicate some degree of donation of elec-
(mean C-atom deviation from the least-squares C10 plane: tron density from the PO fragment to the electrophilic uo-
˚
˚
0.097(1) A), a shorter P/P distance (3.668(2) A), and the phos- rophosphonium cation. A similar but faster uoride oxide
phonium P atom in 4 shows a reduced bending out of the exchange reaction was previously reported for the more Lewis
2+
naphthalene plane (0.057(1) A) compared to its phosphorane acidic dication [(SIMes)PPh2F] .9
˚
˚
neighbour (0.076(1) A).
A family of related bis-phosphonium dication salts was
The bis-diuorophosphorane 2 also reacts with two equiva- envisioned and thus reactions of (oligo)methylene-bridged
lents of [Et3Si][B(C6F5)4]$2(C7H8) to give the salt diphosphines (CH2)n(Ph2P)2 (n ¼ 1 (a), 2 (b), 3 (c), 4 (d), 5 (e))
[(C10H6)(Ph2PF)2][B(C6F5)4]2 (5) in high yields (84%, Scheme 1). and XeF2 were performed in CH2Cl2 to yield the respective bis-
The 31P{1H} and 19F{1H} NMR spectra of 5 reveal an AA0XX0 spin diuorophosphoranes (CH2)n(Ph2PF2)2 (8a–e) in high to almost
system. The 19F{1H} NMR spectrum reveals a very broad doublet quantitative isolated yields (Scheme 3). The species 8a–d have
resonance (dX ¼ ꢀ117.1 ppm, Dn1/2 ¼ 450 Hz). The 31P{1H} NMR been previously prepared by using a range of oxidation reagents
resonance (dA ¼ 96.8 ppm) shows a relatively large 1JPF coupling including F2,29 SF4,30 Me2NSF3,31 (CF3)2CO32 COF2 33 and NF3O34
constant (1JAX ¼ JA X ¼ 1004 Hz) which has also been observed or by halide exchange from the respective bromophosphor-
for the related cations [(C6F5)3PF]+ (1062 Hz),8 and [(SIMes) anes30 or by hydrouorination of the respective silyl phosphi-
PPh2F]2+ (1040 Hz 9). Similar to 2, dication 5 also features a nimines with HF.35 However, most of these methods suffer from
1
0
0
5
5
relative large JPF (5JA X ¼ JAX ¼ ꢀ13 Hz) and a comparatively low yields, laborious workup procedures and the use of
0
0
small JPP coupling constant (4JAA ¼ 3 Hz). The related doubly hazardous reagents. The reaction of phosphoranes 8a–e with
4
0
two equivalents of [Et3Si][B(C6F5)4]$2(C7H8) resulted in the
formation of corresponding bis-phosphonium dication salts
9a–e (Scheme 3). All ve derivatives were isolated and fully
characterized. It is curious to note that the salts 9b and 9d,
featuring linkers with an even number of CH2 units, are only
Fig. 3 POV-ray depiction of the cation in 4$(CH2Cl2). P: orange,
F: yellow-green, C: black. Hydrogen atoms are omitted for clarity. The
solvate molecule was removed by the squeeze routine of the program
PLATON.
Scheme 2 Reaction of 5 with Et3PO.
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Scheme 3 Synthetic route to 8a–e and 9a–e.
sparingly soluble in CD2Cl2 whereas those with an odd numbers
of methylene carbons in the linker (9a, 9c, 9e) show good
solubility. The respective NMR spectra of 9a–e exhibit AA0XX0
spin systems (see ESI†). Compounds 9a–c show higher order
effects due to coupling between the nuclei of the magnetically
inequivalent phosphonium moieties and thus the NMR
parameters were obtained via a full line shape iteration proce-
dure.15 Interestingly 9a exhibits a large P–F coupling constant
(1JAX ¼ 1024 Hz) and signicant coupling between both uorine
Scheme 4 Reaction of 9a–e with Et3PO.
case of 5 almost quantitative Friedel–Cras dimerization was
observed within 24 h at ambient temperature and the resulting
product was isolated in 94% yield.38 Complete conversion was
also achieved using 9a–c, however, 9a required only one hour
reaction time, while 9b,c were complete in 24 h. In the case of
9d–e under the same conditions aer 24 h, the Friedel–Cras
product was formed in lesser yields (9d: 50%, 9e: 25%). The
corresponding reaction of 1,1-diphenylethylene with Et3SiH in
the presence of these Lewis acid catalysts was assessed (Scheme
5).9,10a Using 9a as the catalyst gave 89% of the hydrosilylated
product in one hour at ambient temperature whereas 9b and 5
achieve comparable yields aer 24 h at 50 ꢁC. No conversion is
observed in the case of less Lewis acidic catalysts 9c–e.
In a similar fashion treatment of benzophenone with two
equivalents of Et3SiH in the presence of Lewis acid catalysts 5
and 9a,b led to hydrodeoxygenation and complete conversion to
diphenylmethane and Et3SiOSiEt3 aer one hour (5, 9a) or three
hours (9b) respectively (Scheme 5). Compounds 9c–e required
elevated temperatures and 36 h reaction time and afforded
product in decreased yields (9c: 93%, 9d: 72%, 9e: 46%).
A fourth test involved the utility of these catalysts in the
dehydrocoupling of Si–H bonds in silanes with phenol (Scheme
5).10b Compounds 5 and 9a result in rapid dehydrogenative
coupling which gives PhOSiEt3 in high yields within one hour at
ambient temperature. In contrast, elevated temperature (50 ꢁC)
and 24 h reaction time were required to observe similar reac-
tivity for catalyst 9b. With an increase in methylene-linker
length a decrease in conversion was observed (9c: 30%, 9d: 19%)
and only traces of the coupling product are detected using 9e.
Finally, these EPCs were evaluated as catalysts for the
hydrodeuorination of uoropentane using Et3SiH as the
hydride source (Scheme 5).8,9 Compounds 9a and 5 catalyse the
hydrodeuorination affording almost complete conversion
atoms (4JXX ¼ 12 Hz) suggesting enhanced Lewis acidity.
0
Remarkable large P–P coupling constant was observed for 9b
(3JAA ¼ 70 Hz) suggesting a weak through space interaction
0
between the phosphorus centers (see ESI†).36
Lewis acidity tests of this series of bis-uorophosphonium
ion salts 9a–e by the Gutmann–Beckett method28 were per-
formed (Scheme 4). Determination of an acceptor number for
9a was hampered by deprotonation of the central CH2 moiety,
affording [Et3POH][B(C6F5)4] (10) and [(CH)(Ph2PF)2][B(C6F5)4]
(11). Compound (10) showed a broad 31P{1H} NMR resonance at
74.1 ppm, while (11) reveals an AA0XX0 spin system in the 31P
{1H} (dA ¼ 81.2 ppm) and 19F{1H} NMR spectrum (dX
¼
ꢀ99.4 ppm). The formation of 11 was further supported by its
independent synthesis from 9a and t-Bu3P.
The cations of 9b–e interact weakly with Et3PO resulting in
acceptor numbers of ꢂ5. However, in all cases slow conversion
to the monocationic species [(CH2)n(Ph2PO)(Ph2PF)][B(C6F5)4]
(12b–e) and [Et3PF]+-salt 6 was observed.9 The shorter linkers in
the case of 12b and 12c result in a chelate interaction of the
phosphine oxide and the electrophilic uorophosphonium
center as evidenced by the high eld shi of the uoro-
substituted P atom (12b: 2.2 ppm, 1JPF ¼ 717 Hz, 12c: 45.2 ppm,
1JPF ¼ 823 Hz). This notion is also supported by the observation
of comparatively low eld 31P NMR resonances arising for the
PO fragments (12b: 53.1 ppm, 12d: 44.1 ppm).28 In contrast 12d–
e show 31P{1H} NMR resonances in the typical range for discrete
phosphine oxide37 and uorophosphonium moieties.
The utility of the dications 5 and 9a–e in a variety of Lewis
acid catalysed reactions was probed (Scheme 5). 1,1-Diphenyl-
ethylene in CD2Cl2 was treated with 2 mol% of 5 or 9a–e. In the
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bisphosphorane species. Subsequent stepwise uoride
abstraction yielded phosphine/phosphonium, phosphonium/
phosphorane and bisphosphonium species. The close spatial
proximity in the naphthalene derived compound and the mono-
methylene-linked species gives rise to Lewis acidity that is
enhanced by the neighbouring uorophosphonium moieties as
evidenced by the reactivity of these species in the Lewis acid
catalysed transformations including Friedel Cras-type dimer-
ization, hydrosilylation, dehydrocoupling, hydrodeoxygenation
and hydrodeuorination. We are continuing to explore the
facile and high yielding synthetic protocol that converts readily
available bidentate donors providing Lewis acids of tuneable
strength. New applications in Lewis acid catalysis and applica-
tions of these P(V) Lewis acids in frustrated Lewis pair (FLP)
chemistry are the subject of on-going efforts.
Acknowledgements
D.W.S. gratefully acknowledges the nancial support of the
NSERC of Canada and is grateful for the award of a Canada
Research Chair. M.H.H. thanks the Alexander von Humboldt
Foundation for a Feodor Lynen Research Fellowship. R.R.H. is
grateful for the support of an NSERC CGS-M scholarship.
Notes and references
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Scheme 5 Lewis acid catalysis by salts 5 and 9a–e as catalysts for
selected Lewis acid mediated transformations. Conversions were
1
determined by H NMR or 19F NMR spectroscopy and isolated yields
are given in parenthesis. (a) Ambient temperature and one hour
reaction time (only for 9a). (b) Ambient temperature and one hour
reaction time. (c) Ambient temperature and one hour (9a) or three
hours (9b) reaction time.
´
(d) F. J. Fernandez-Alvarez, A. M. Aitani and L. A. Oro,
´
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for (5). The less Lewis acidic species 9b–e effect incomplete to
trace conversions (9b: 18%, 9c: 4%, 9d: 2%, 9e: 2%) aer 24 h at
´
2 (a) B. Ines, S. Holle, R. Goddard and M. Alcarazo, Angew.
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ꢁ
50 C.
The above results reveal that these dications are generally
effective catalysts for a variety or Lewis acid mediated reactions.
Moreover the proximity of two uorophosphonium centers as in
5 and 9a–b acts to enhance the Lewis acidity and the resulting
reactivity by a distal P–F interaction. Indeed, the longer chain
species (9c–e) show diminished reactivity behaving more like
independent [Ph2PF(alkyl)]+ cations. In this regard it is also
noteworthy that in each of the above Lewis acid-catalysed
reactions the species [Ph3PF][B(C6F5)4] was completely ineffec-
tive, which further supports the view that Lewis acidity of the
present species is enhanced by spatial proximity.
¨
¨
¨
¨
¨
4 A. H. Cowley and R. A. Kemp, Chem. Rev., 1985, 85, 367.
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Conclusions
In summary, stepwise oxidations of 1,8-bis-(diphenylphos-
phino)naphthalene and a series of bidentate (oligo)methylene-
linked phosphines with XeF2 provide mono- and
6 (a) T. W. Hudnall, Y. M. Kim, M. W. P. Bebbington,
¨
D. Bourissou and F. P. Gabbaı, J. Am. Chem. Soc., 2008,
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¨
´
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