1,2-Diphosphonium Dication: A Strong P-Based Lewis Acid in Frustrated Lewis Pair (FLP)-Activations of B-H, Si-H, C-H, and H-H Bonds

Article abstract of DOI:10.1021/jacs.5b04109

A highly Lewis acidic diphosphonium dication [(C₁₀H₆)(Ph₂P)₂]²⁺ (1), in combination with a Lewis basic phosphine, acts as a purely phosphorus-based frustrated Lewis pair (FLP) and abstracts hydride from [HB(C₆F₅)₃]^- and Et₃SiH demonstrating the remarkable hydridophilicity of 1. The P-based FLP is also shown to activate H₂ and C-H bonds.

Full text of DOI:10.1021/jacs.5b04109

Communication
pubs.acs.org/JACS
1,2-Diphosphonium Dication: A Strong P‑Based Lewis Acid in
Frustrated Lewis Pair (FLP)-Activations of B−H, Si−H, C−H, and H−H
Bonds
Michael H. Holthausen, Julia M. Bayne, Ian Mallov, Roman Dobrovetsky, and Douglas W. Stephan*
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario Canada, M5S 3H6
S
* Supporting Information
stoichiometric E−H bond activation reactions (E = H, B, C,
ABSTRACT: A highly Lewis acidic diphosphonium
dication [(C10H6)(Ph2P)2]2+ (1), in combination with a
Lewis basic phosphine, acts as a purely phosphorus-based
frustrated Lewis pair (FLP) and abstracts hydride from
[HB(C6F5)3]− and Et3SiH demonstrating the remarkable
hydridophilicity of 1. The P-based FLP is also shown to
activate H2 and C−H bonds.
Si), highlighting its remarkable hydridophilicity.
To synthesize the diphosphonium dication, 2 equiv of
[Et3Si][B(C6F5)4]·2(C7H8) were added to a C6H5F solution of
difluorophosphorane (C10H6)(Ph2PF2)(Ph2P),8 resulting in a
double fluoride abstraction (Scheme 1). The obtained
Scheme 1. Synthetic Route to Diphosphonium Dication Salt
1 and Reactions of 1 and Ph3P with B−H and Si−H Bonds
he past two decades have witnessed fundamental advances
Tin the area of Lewis acid chemistry, particularly due to the
development of increasingly electron-deficient molecules. For
instance, fluorinated organoboranes have been widely used as
Lewis acids in small molecule activation and catalysis.1 In this
context, the heterolytic splitting of dihydrogen (H2) by
sterically frustrated Lewis pairs (FLPs), composed of a Lewis
acidic borane and a suitable Lewis base, was a landmark
achievement.2 In addition to boranes and alanes,3 tricoordi-
nated carbon-4 or silicon-centered5 cations have been used as
main-group based Lewis acids in FLP-type small molecule (in
particular H2) activation. Exploring electron-deficient com-
pounds of group 15, we found that [(C6F5)3PF]+ is a highly
electrophilic phosphonium cation (EPC) that exhibits remark-
able Lewis acidity derived from strongly electron-withdrawing
substituents and an energetically accessible LUMO (opposite to
P−F bond) acceptor orbital.6 Pursuing alternative approaches
to EPCs, we showed that carbene-based fluorophosphonium7
[(SIMes)PPh2F]2+ (SIMes = 1,3-bis(2,4,6-trimethylphenyl)-2-
imidazolidinylidene) or bidentate bis(fluorophosphonium)
dications8 reveal comparable Lewis acidities, highlighting the
impact of an additional positive charge. These EPCs were
shown to catalyze a range of Lewis acid mediated trans-
formations including hydrodefluorination,6−8 hydrosilyla-
tion,7−9 dehydrocoupling, and transfer hydrogenation.10 Strong
Lewis acids based on heavier group 15 elements (As,11 Sb,12
Bi13) have been explored recently, and in some cases, the
activation of Si−H or Si−Cl bonds was observed. The
activation of H2 in reactions with group 15 Lewis acids is
rare. Indeed we reported the direct reaction of H2 with a
triphosphabenzene derivative, and computational studies
confirmed the role of phosphorus as a Lewis acid in an FLP-
type mechanism.14 Herein, we present a naphthalene-bridged
P−P bonded 1,2-diphosphonium dication which demonstrates
high Lewis acidity without a polarized P−F bond. The
electrophilicity of the dication is exploited in several
dicationic species, [(C10H6)(Ph2P)2]2+ (1), is stabilized by
intramolecular coordination of the phosphine moiety to the
P(V) atom which renders 1 a rare example of a dicationic, P−P-
bonded diphosphonium species.15 Compound 1 as a [B-
(C6F5)4]− salt was isolated in high yields (95%) and fully
characterized. The diphosphonium salt 1 is relatively stable and
does not decompose in coordinating solvents such as
acetonitrile. In the solid state, it can be exposed to air for 24
h without significant decomposition. The 31P NMR spectrum
of 1 shows a singlet resonance at 23.2 ppm (CD2Cl2)
comparable to the related phosphonium center in [Ph3P−
PPh2]+ (δ(31P) = 15 ppm).16
Received: April 28, 2015
© XXXX American Chemical Society
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DOI: 10.1021/jacs.5b04109
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Journal of the American Chemical Society
Communication
In an effort to probe the Lewis acidity of 1 the Gutmann−
Beckett method was employed.17 Reaction with 1 equiv of
Et3PO in CD2Cl2 revealed an acceptor number (AN) of 30
which is significantly higher than that of the corresponding
bis(fluorophosphonium) dication ([(C10H6)(Ph2PF)2]2+: AN =
21).8 Mixtures of dicationic fluorophosphonium species and
Et3PO were reported to react rapidly via a fluoride−oxide
exchange reaction;7,8 however, species 1 exhibits increased
stability showing no degradation even in the presence of Et3PO
after 24 h at 50 °C. It is interesting to note that NMR
spectroscopic investigations indicated no adduct formation
between diphosphonium species 1 and t-Bu3P, while Ph3P
displays only a weak interaction with 1, inferring the presence
of unique all-phosphorus-based FLPs.
To gain insight into the hydridophilicity of 1, a 1:1 mixture of
1 and Ph3P was reacted with [t-Bu3PH][HB(C6F5)3]2b in
CD2Cl2 (Scheme 1). The 11B{1H} NMR spectrum of the
reaction mixture revealed two resonances assigned to the
[B(C6F5)4]− anion (δ(11B) = −16.6 ppm) and the phosphine
borane adduct Ph3P−B(C6F5)3 (3, δ(11B) = −0.3 ppm).18
Adduct 3 was identified in the 31P{1H} NMR spectrum as well,
with a resonance at −4.4 ppm. Two additional 31P resonances
Figure 1. LUMO of the cation of 1 overlaid on its optimized geometry
(hydrogens were omitted for clarity) and equilibrium between 2 and
2′.
Species 1 also reacts with Ph3P and Et3SiH in C6D5Br
(Scheme 1). After 24 h of reaction time at ambient
temperature, almost quantitative conversion to 2 was observed
by means of 31P{1H} NMR spectroscopy. A singlet resonance
at −3.3 ppm was assigned to the cation of [Ph3PSiEt3][B-
(C6F5)4] (5), and its formulation was subsequently confirmed
by independent synthesis (see SI). The stronger Lewis base t-
Bu3P (2 equiv) reacts with 1 to give 2 and [t-Bu3PSiEt3][B-
(C6F5)4] (6). Afterward, deprotonation of 2 by the phosphine
yields [t-Bu3PH]+ and 1,8-diphenylphosphinonaphthalene (1,8-
dppn). Indeed, reaction of the [N(SO2CF3)2]−-salt of 2 with t-
Bu3P revealed quantitative formation of 1,8-dppn and [t-
Bu3PH][N(SO2CF3)2] (see SI). The formation of 6 was
affirmed crystallographically (Figure 2), and the comparably
large P−Si bond length (2.388 Å) is likely a result of steric
congestion.5c
1
revealed JPH coupling and were assigned to the cations of [t-
Bu3PH][B(C6F5)4] (4, δ(31P) = −59.8 ppm, JPH = 429 Hz)2a
1
and [(C10H6)(Ph2P)2H][B(C6F5)4] (2, δ(31P) = −4.1 ppm,
1JPH = 284 Hz).19 The H NMR spectrum of the reaction
1
mixture shows a low field triplet resonance at 10.2 ppm
corresponding to the P−H fragment of 2. Collectively, these
NMR data indicate a fast exchange of the H atom between the
P centers in 2. The [N(SO2CF3)2]−-salt of 2 was independently
prepared (see Supporting Information (SI)). X-ray crystallog-
raphy of 2 revealed that in the solid state the H atom is bonded
to one P atom with P−H distances of 1.32(3) and 2.76(1) Å.
This reactivity demonstrates that the combination of 1 and
Ph3P acts as an FLP, heterolytically cleaving the B−H bond in
[HB(C6F5)3]− yielding 2 and 3. Interestingly, the combination
of 1 and [t-Bu3PH][HB(C6F5)3] in the absence of base resulted
in the incomplete conversion to 2 (85%). Multinuclear NMR
investigations showed evidence of the partial sequestration of
the [HB(C6F5)3]− anion by the liberated B(C6F5)3 preventing
complete hydride abstraction by 1.20 Nonetheless, this
reactivity indicates that diphosphonium dication 1 possesses a
higher hydridophilicity than the electrophilic Lewis acid
B(C6F5)3.
In further probing the Lewis acidic nature of 1, DFT
computations were performed. The geometry of the cation was
optimized at the wB97XD/def2-TZV level of theory (see SI).21
The major component of the lowest unoccupied molecular
orbital (LUMO) of 1 is the antibonding σ* orbital of the P−P
bond (Figure 1, left). Thus, the reaction of 1 with hydride is
expected to yield 2′. The donation of electron density from the
hydride into the LUMO of 1 cleaves the P−P bond and
significantly increases the computed distances between the P
atoms (1: 2.31 Å, 2′: 3.08 Å). Interestingly, adduct formation
between 1 and a fluoride anion does not initiate a P−P bond
cleavage resulting in a phosphorane species with fluoro- and
phosphonium-substituents in axial positions (P−P: 2.530(1)
Å).8 Moreover, compound 2′ and its isomer 2 are in
equilibrium with each other, with 2′ being the minor
component. This equilibrium was previously observed by
NMR spectroscopy for other salts of the cation in 2.19 The
transformation of 2′ to 2 was computed to be slightly exergonic
Figure 2. POV-ray depiction of the cation in 6. P, orange; Si, yellow;
C, black. Hydrogen atoms are omitted for clarity.
It is noteworthy that a mixture of 1 and Et3SiH in CD2Cl2 in
the absence of base shows only slow conversion to 2 and the
products of degradation of the transient [Et3Si]+ cation (65%
after 48 h). In CD3CN 1 reacts with excess Et3SiH to give
complete conversion to 1,8-dppn and H2 as observed by 31P
1
and H NMR spectroscopy. In addition, the solvated [Et3Si]+-
species was detected via a 29Si{1H} NMR experiment (see
SI).22 The species 2 is formed by hydride abstraction from
Et3SiH, while a second equivalent of silane yields H2 giving 1,8-
dppn. Collectively, these observations demonstrate the
hydridophilicity of 1 as it competes with the strong Lewis
acid [Et3Si]+.
298
(ΔGR = −1.2 kcal/mol).
B
DOI: 10.1021/jacs.5b04109
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Journal of the American Chemical Society
Communication
This remarkable hydridophilicity of 1 prompted an
exploration of stoichiometric C−H bond activation reactions.
Mixtures of 1 and phosphine were reacted with 1,4-cyclo-
hexadiene (CHD) in C6D5Br as the solvent. While use of PPh3
showed no reaction even upon heating to 100 °C for several
days, use of the more basic phosphine t-Bu3P resulted in
complete consumption of 1 after 24 h at 90 °C (Scheme 2). A
generated 2 reacts with a second equivalent of phosphine to
liberate 1,8-dppn and the cation [t-Bu3PH]+. Although several
C−H bond activations by ambiphilic phosphenium cations
have been reported previously,26 the present examples are, to
the best of our knowledge, the first examples of FLP-type C−H
activations4b,27 involving a phosphorus-based Lewis acid.
Finally, the FLP-type heterolytic splitting of H2 utilizing 1 as
a Lewis acid was probed as a mixture of diphosphonium species
1 and t-Bu3P in 1:2 stoichiometry in C6D5Br was exposed to a
pressure of 4 atm of H2 at 100 °C (Scheme 3). Within 48 h,
Scheme 2. C−H Activations by 1 and t-Bu3P
Scheme 3. H2 Activation by 1 and t-Bu3P
1
singlet resonance was observed at 7.21 ppm in the H NMR
spectrum of the reaction mixture, which indicates the formation
of benzene. This is consistent with dehydrogenation of CHD
and formation of 4 and 1,8-dppn.
complete conversion to 1,8-dppn and 2 equiv of 4 was observed
by NMR spectroscopy. This is indicative of heterolytic H2
splitting by 1 and t-Bu3P. As observed previously, 2 is
deprotonated by the second equivalent of t-Bu3P. Performing
the same reaction under an atmosphere of D2 (4 atm) resulted
in the observation of a triplet resonance (δ(31P) = 58.9 ppm,
1JPD = 65 Hz) in the 31P{1H} NMR spectrum. This is consistent
with the formation of [t-Bu3PD][(C6F5)4] (4-d1).5c In the
presence of HD gas, a mixture of the salts 4 and 4-d1 was
obtained and scrambling of HD to D2 and H2 was not observed.
This indicates that heterolytic cleavage of hydrogen is not
reversible and is directly analogous to the irreversible cleavage
of H2 by the FLP t-Bu3P/B(C6F5)3.28 The present system is
also an example of an all-phosphorus based FLP effecting
stoichiometric H2 activation.14
The corresponding reaction of 1,3,5-cycloheptatriene (CHT)
gave rise to a 31P{1H} NMR spectrum exhibiting three singlet
resonances which were assigned to the cation of 4, 1,8-dppn,
and a new phosphonium ion species 7 (δ(31P) = 45.1 ppm). 1H
NMR data for 7 suggest a cyclo-propanyl motif based on the
observation of a CH resonance adjacent to the P atom (δ(1H)
= 0.09 ppm).23 These data indicate the formulation of 7 as
[C7H7Pt-Bu3][B(C6F5)4]. To support this view, the tropylium
ion salt [C7H7][BF4] was independently reacted with t-Bu3P to
give the [BF4]− salt of 7. This salt was characterized by X-ray
crystallography confirming 7 as the diastereomer in which the
phosphonium substituent adopts the exo-position to the
bicyclo[4.1.0]heptane framework (Figure 3). The remaining
metric parameters are unexceptional.
In conclusion, we have presented the facile and high-yielding
synthesis of robust but highly Lewis acidic diphosphonium
dication 1. While previously reported strong Lewis acids based
on EPCs contain reactive P−F bonds, the present diphospho-
nium dication 1 achieves similar Lewis acidity as a result of the
accumulation of positive charge on two adjacent P atoms. This
phosphorus-based Lewis acid combined with phosphines
generates the first examples of purely phosphorus-based FLP
systems. The hydridophilicity of compound 1 was exploited in
the stoichiometric FLP-type activation of B−H, Si−H, C−H,
and H−H bonds. These unique examples of FLP reactivity with
an EPC creates a new avenue for phosphorus-based Lewis
acids, thus providing new strategies to FLP chemistry and
Lewis acid catalysis. The application of related dications is the
subject of ongoing studies in our laboratories.
Figure 3. POV-ray depiction of the cation of 7. P, orange; C, black.
Hydrogen atoms are omitted for clarity.
The above reactions of the diphosphonium dication 1 with
CHD and CHT are thought to proceed via hydride abstraction
generating 2. In the case of CHD, t-Bu3P sequesters the proton
affording benzene while, for CHT, the transient tropylium
cation is in equilibrium with its isomeric, bicyclic norcaradiene
derivative.24 Capture by t-Bu3P yields 7.25 In both cases, the
ASSOCIATED CONTENT
* Supporting Information
Synthetic details and spectroscopic data for all reactions and
■
S
new compounds; crystallographic details for 2, 5, and 7. CIF
C
DOI: 10.1021/jacs.5b04109
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
(14) Longobardi, L. E.; Russel, C. A.; Green, M.; Townsend, N. S.;
Wang, K.; Holmes, A. J.; Duckett, S. B.; McGrady, J. E.; Stephan, D.
W. J. Am. Chem. Soc. 2014, 136, 13453−13457.
files for 2, 5, and 7 are also available free of charge from the
Cambridge Crystallographic Data Centre under reference
CCDC-1060485−1060487. The Supporting Information is
available free of charge on the ACS Publications website at
DOI: 10.1021/jacs.5b04109.
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Angew. Chem., Int. Ed. 1985, 24, 975−976. (b) Romakhin, A. S.;
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AUTHOR INFORMATION
■
Corresponding Author
*dstephan@chem.utoronto.ca
(f) Upulani, D. M.; Somisara, K.; Buhl, M.; Lebl, T.; Richardson, N. V.;
̈
Notes
Slawin, A. M. Z.; Woolins, J. D.; Kilian, P. Chem. Ber. 2011, 17, 2666−
2677. (g) Nenajdenko, V. G.; Shevchenko, N. E.; Balenkova, E. S.;
Alabugin, I. V. Chem. Rev. 2003, 103, 229−282 and references therein
for diphosphonium dications that were not crystallographically
characterized.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
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. J.M.B. is grateful for the
award of NSERC-CGS-M and Vanier scholarships.
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