Silyl-migrations in frustrated Lewis pair chemistry: Reactions of ((CH 3)3Si)3P and B(C6F 4H)3 with H2 and CO2

Article abstract of DOI:10.1039/c2cc36470k

The 1:1 mixture of tris(trimethylsilyl)phosphine and tris(2,3,5,6- tetrafluorophenyl)borane behaves as frustrated Lewis pair (FLP) and shows H ₂ and sequential two-step double CO₂ activation with subsequent Me₃Si group tra

Full text of DOI:10.1039/c2cc36470k

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Silyl-migrations in frustrated Lewis pair chemistry: reactions of
((CH3)3Si)3P and B(C6F4H)3 with H2 and CO2w
Katsuhiko Takeuchi and Douglas W. Stephan*
Received 5th September 2012, Accepted 9th October 2012
DOI: 10.1039/c2cc36470k
The 1 : 1 mixture of tris(trimethylsilyl)phosphine and tris(2,3,5,6-
tetrafluorophenyl)borane behaves as frustrated Lewis pair (FLP)
and shows H2 and sequential two-step double CO2 activation
with subsequent Me3Si group transfer.
that the (Me3Si)3P–B(p-C6F4H)3 mixture is expected to behave
as a FLP.
To further probe this notion, the reaction of the (Me3Si)3P–
B(p-C6F4H)3 mixture with H2 (5 atm) in toluene was probed.
Monitoring the reaction by NMR spectroscopy indicated
the products were a 1 : 1 mixture of the salt, [(Me3Si)4P]-
[HB(p-C6F4H)3] (1), and the phosphine–borane adduct, (Me3Si)2-
(H)P–B(p-C6F4H)3 (2) (Scheme 1). This stands in contrast to the
corresponding reactions of the mixture of alkyl- or aryl-substituted
phosphines R3P ((R = Cy, t-Bu, i-Pr, 2,4,6-Me3C6H2, and
2-Me-C6H4)) and B(p-C6F4H)3 with H2 which produce salt of
the form [R3PH][HB(p-C6F4H)3].16 In the present case, the salt
[(Me3Si)3PH][HB(p-C6F4H)3] is thought to form initially but
undergoes a rapid disproportionation in the presence of the
(Me3Si)3P–B(p-C6F4H)3 mixture to give 1 and 2.
The 11B NMR spectrum of 1 shows a sharp doublet signal at
ꢀ24.7 ppm (1JHB = 91 Hz), consistent with the presence of the
anion [HB(p-C6F4H)3].16 In the 31P{1H} NMR spectrum, 1 shows a
sharp singlet signal at ꢀ201.2 ppm, similar to that reported for
[(Me3Si)4P][B(C6F5)4] (ꢀ200.1 ppm).17 On the other hand, the
11B{1H} and 31P NMR spectra of 2 showed broad signals at
ꢀ14.5 ppm and ꢀ139.3 ppm, respectively. In addition, the 31P
NMR resonance of 2 at ꢀ139.3 ppm shows a large 1JPH coupling
constant of 322 Hz, similar to described for the phosphine–borane
adduct, t-Bu2(H)P–B(p-C6F4H)3 (397 Hz).16b These data prompt
the formulation of 2 as the phosphine–borane adduct.
Sterically encumbered pairs of Lewis acids and bases, or
‘‘frustrated Lewis pairs (FLPs)’’ retain high reactivity derived
from unquenched Lewis acidity and basicity.1 For example,
FLPs as typified by intra- and intermolecular combinations of
bulky phosphines with electrophilic boranes have been
exploited to activate a variety of small molecules including
H2,2 CO2,3 N2O,4 CO,5 NO,6 B–H bonds,7 alkenes,8 dienes,9
alkynes,10 diynes,11 disulfides,12 and azides.13 Although various
phosphines were exploited in these reactions, to date the
substituents have been largely limited to alkyl and aryl groups.
Tris(trimethylsilyl)phosphine, (Me3Si)3P, has electropositive
silyl groups on the phosphorus atom and thus highly reactive
Si–P bonds. These bonds easily reacts with various element
halides, carboxylic acid chlorides, and carboxylic esters, as
well as other organic electrophiles.14 While sterically analogous
to the quintessential FLP-phosphine t-Bu3P, the reactivity of the
P–Si bonds in (Me3Si)3P is expected to result in altered FLP
reactivity. In this regard, it is noteworthy that the silylphosphine
R2PSiMe3 have been shown to react with B(C6F5)3 to effect para-
attack affording a facile route to compounds of the form,
R2PC6F4B(C6F5)2.15 We have previously shown that use of
tris(2,3,5,6-tetrafluorophenyl)borane, B(p-C6F4H)3, effectively
precludes such nucleophilic displacement.16 Thus, in the present
report herein the FLP chemistry of the mixture of (Me3Si)3P and
B(p-C6F4H)3 in reactions with H2 or CO2 is described.
The molecular structure of 1 was confirmed by X-ray
analysis (Fig. 1). The [(Me3Si)4P]+ unit of 1 has tetrahedral
geometry with Si–P–Si bond angles of 109.26(5) and 109.68(5)1.
The average of the P–Si bond lengths of 1 is 2.306(3) A. These
structural features are similar to those previously reported by
Driess and coworkers for the salt [(Me3Si)4P][B(C6F5)4].17
The formation of 1 provides a facile route to salts containing
The combination of 1 equiv. of (Me3Si)3P and B(p-C6F4H)3
in C6D5Br solution results in a color change to pale
yellow. However, the 1H, 11B, 19F, and 31P NMR spectra
of the mixture showed no evidence for adduct formation
between (Me3Si)3P and B(p-C6F4H)3, even after it had
been left for 2 weeks at room temperature. This result indicates
Department of Chemistry, University of Toronto, 80 St. George St.,
Toronto, Ontario, Canada M5S 3H6.
E-mail: dstephan@chem.utoronto.ca
w Electronic supplementary information (ESI) available: Synthetic
and spectroscopic details. CCDC 899513–899515. For ESI and crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/
c2cc36470k
Scheme 1 Synthesis of 1 and 2.
c
11304 Chem. Commun., 2012, 48, 11304–11306
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Fig. 1 POV-ray depiction of 1. (C: black, F: pink, B: yellow-green, Si:
blue, P: orange.)
the [(Me3Si)4P] cation as the only previously known route to
such salts require generation of silylium cation reagents.17
In an analogous fashion the reaction of the FLP mixture
of (Me3Si)3P–B(p-C6F4H)3 was also reacted with CO2. The
course of the reactions was shown to depend on the reaction
solvent. In pentane solution, the mixture reacted with CO2
Fig. 2 POV-ray depictions of 3. (C: black, F: pink, B: yellow-green,
O: red, Si: blue, P: orange.)
features of 3 indicate a contribution of zwitterionic resonance
structure, 30 (Scheme 2).
The corresponding reaction of (Me3Si)3P/B(p-C6F4H)3
and CO2 in CH2Cl2 solution results in the formation of the
bis-insertion product adduct (Me3SiO)2CQP–C(OSiMe3)QO -
B(p-C6F4H)3 (4) (Scheme 2). The 31P{1H} and 11B{1H} NMR
spectra of 4 exhibited shape signals at 2.4 and ꢀ0.7 ppm,
respectively. The 13C{1H} NMR spectrum of 4 showed two
doublets consistent with the presence of two CO2 units at
202.0 and 205.5 ppm, although assignment of these signals to
the specific CO2 unit remains unclear.
to give the mono-adduct, (Me3Si)2P–C(OSiMe3)QO
-
B(p-C6F4H)3 (3) (Scheme 2). Again in contrast to the corres-
ponding reactions of alkyl-phosphine and B(p-C6F4H)3,16 the
species (Me3Si)3P–C(QO)O–B(p-C6F4H)3 was not observed.
Nonetheless, it is thought to form initially and subsequent
Me3Si group migration affords the product 3 (Scheme 2).18 At
room temperature, the 31P{1H} and 11B{1H} NMR signals for
3 at ꢀ104.5 and ꢀ13.5 ppm were observed to be a very broad
peaks. In addition, the 13C{1H} NMR signal for the CO2
unit of 3 was not observed. However, preparation of the
13CO2 analog showed a very broad signal for the 13CO2 unit
at 194.2 ppm. On cooling to ꢀ40 1C, both the 31P{1H} and
13C{1H} NMR resonances sharpened and shifted slightly to
ꢀ101.6 ppm and 199.4 ppm, respectively. Moreover these
signal exhibited a P–C coupling constant of 66 Hz. These
temperature dependent NMR studies suggest an equilibrium
involving dissociation of the B–O bond of 3.
The molecular structure of 4 was determined by X-ray
crystallographic methods (Fig. 3). The structural feature of
the P–C(OSiMe3)QO - B(p-C6F4H)3 unit of 4 (B1–O1 =
1.561(2) A, P1–C1 = 1.8070(19) A, C1–O1 = 1.275(2) A,
C1–O2 = 1.296(2) A) are quite similar to the corresponding
structural features of 3. The P1–C2 bond is of appropriate
length to be considered as a double bond (1.754(2) A). The
C2–O3 and C2–O4 bond length are 1.298(2) and 1.319(2) A,
respectively. The C1–P1–C2 angle in 4 (101.08(9)1) is similar to
those exhibited by typical phosphaalkenes (100.9(6)–108.1(3)1).19 In
addition, the molecular structure of 4 is reminiscent of that of
The molecular structure of 3 was determined by X-rayz
crystallographic and NMR spectroscopic analyses (Fig. 2).
The B1–O1 bond is found to be 1.576(3) A. The P1–C1 bond
length is 1.824(2) A, which is significantly shorter than that of
t-Bu3P–C(QO)O–B(p-C6F4H)3 (1.892(2) A).3e The C1–O1
bond (1.271(3) A) is apparently shorter than the C1–O2 bond
(1.306(3) A), but is longer than the CQO double bond of
t-Bu3P–C(QO)O–B(p-C6F4H)3 (1.203(2) A).3e These structural
2-phosphaallyl cation [(Me2N)2CPC(NMe2)2][ClO4] (C–P
=
1.796(4) A, C–P–C = 103.6(2)1).20 These structural data for 4
support its zwitterionic formulation as shown in 40 (Scheme 2).
The solvent-dependence of the reactions of (Me3Si)3P/
B(p-C6F4H)3 and CO2 infers that the species 3 is insoluble in
Fig. 3 POV-ray depictions of 4. (C: black, F: pink, B: yellow-green,
Scheme 2 Synthesis of 3 and 4.
O: red, Si: blue, P: orange.)
c
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Chem. Commun., 2012, 48, 11304–11306 11305

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pentane, but is very soluble in CH2Cl2. This notion was
supported by the demonstration that addition of 3 to excess
CO2 in CD2Cl2 resulted in the quantitatively formation of 4
(Scheme 2). Furthermore, use of 13CO2 gave a 1 : 1 mixture of
isotopomers, (Me3SiO)2CQP–13C(OSiMe3)QO - B(p-C6F4H)3
(4-1-13C) and (Me3SiO)213CQP–C(OSiMe3)QO - B(p-C6F4H)3
(4-2-13C). While silyl-migrations in the reactions of (C6H2(t-Bu)3)P-
(SiMe3)2 with CO2 have been previous reported21 and show
insertion of CO2 into the P–Si bond is facile, the present
results suggest that the labile borane in 3 allows access
to the FLP derived from (Me3Si)2P–C(OSiMe3)QO and
B(p-C6F4H)3 prompting further reaction to give 4.
6643–6646; (b) X. Zhao and D. W. Stephan, Chem. Commun.,
2011, 47, 1833–1835; (c) I. Peuser, R. C. Neu, X. Zhao, M. Ullrich,
¨
B. Schirmer, G. Kehr, R. Frohlich, S. Grimme, G. Erker and
D. W. Stephan, Chem.–Eur. J., 2011, 17, 9640–9650.
4 (a) E. Otten, R. C. Neu and D. W. Stephan, J. Am. Chem. Soc., 2009,
131, 9918–9919; (b) R. C. Neu, E. Otten and D. W. Stephan, Angew.
Chem., Int. Ed., 2009, 48, 9709–9712; (c) R. C. Neu, E. Otten,
A. Lough and D. W. Stephan, Chem. Sci., 2011, 2, 170–176.
5 A. Dureen and D. W. Stephan, J. Am. Chem. Soc., 2010, 132,
13559–13568.
6 A. J. P. Cardenas, B. J. Culotta, T. H. Warren, S. Grimme,
A. Stute, R. Frohlich, G. Kehr and G. Erker, Angew. Chem., Int.
¨
Ed., 2011, 50, 7567–7571.
7 M. A. Dureen, A. Lough, T. M. Gilbert and D. W. Stephan, Chem.
Commun., 2008, 4303–4305.
8 (a) J. S. J. McCahill, G. C. Welch and D. W. Stephan, Angew. Chem.,
In summary, the (Me3Si)3P–B(p-C6F4H)3 mixture behaves
as a FLP to react with H2 affording the products 1 and 2 via
Me3Si group transfers from phosphonium to phosphine. The
FLP also reacted with CO2 to give 3 which when soluble, acts
as an intermediate for further reaction with CO2 to give 4,
which has phosphaalkene character. This latter reaction is the
first example of a sequential two-step double CO2 activation
using a phosphine/borane FLP. Moreover, the present chemistry
describes the first FLP system in which the phosphine substituent
are shown to be non-innocent. This notion provides a strategy
for the design of future systems.
Int. Ed., 2007, 46, 4968–4971; (b) C. M. Momming, G. Kehr,
¨
¨
R. Frohlich and G. Erker, Chem. Commun., 2011, 47, 2006–2007.
9 M. Ullrich, K. S. H. Seto, A. J. Lough and D. W. Stephan, Chem.
Commun., 2009, 2335–2337.
10 (a) M. A. Dureen and D. W. Stephan, J. Am. Chem. Soc., 2009,
131, 8396–8397; (b) C. M. Momming, S. Fromel, G. Kehr,
¨
¨
R. Frohlich, S. Grimme and G. Erker, J. Am. Chem. Soc., 2009,
131, 12280–12289; (c) M. A. Dureen and D. W. Stephan, Organo-
metallics, 2010, 29, 6594–6607.
11 C. M. Momming, G. Kehr, B. Wibbeling, R. Frohlich, B. Schirmer,
¨
¨
S. Grimme and G. Erker, Angew. Chem., Int. Ed., 2010, 49, 2414–2417.
12 M. A. Dureen, G. C. Welch, T. M. Gilbert and D. W. Stephan,
Inorg. Chem., 2009, 48, 9910–9917.
13 (a) M. W. P. Bebbington, S. Bontemps, G. Bouhadir and D. Bourissou,
Angew. Chem., Int. Ed., 2007, 46, 3333–3336; (b) C. M. Momming,
Notes and references
¨
G. Kehr, B. Wibbeling, R. Frohlich and G. Erker, Dalton Trans., 2010,
¨
z Crystallographic data for 1: C30H39BF12PSi4, MW = 781.75, T = 150
39, 7556–7564.
¨
14 G. Becker, H. Schmidt, G. Uhl, W. Uhl, M. Regitz, W. Rosch and
%
K, space group, trigonal, R3, a = 14.0075(12) A, b = 14.0075(12) A, c =
34.296(4) A, V = 5827.7(10) A3, Z = 6, m = 0.272 mmꢀ1, measured
reflections = 2998, independent reflections = 2998, parameters = 153,
Rint = 0.0000, R = 0.0567 (I > 2s(I)), Rw = 0.1461 (all data), GOF =
1.061. 3: C28H30BO2F12PSi3, MW = 752.57, T = 150 K, space group,
monoclinic, P21/c, a = 9.5675(4) A, b = 19.0878(8) A, c = 18.8223(9) A,
b = 93.567(2)1, V = 3430.7(3) A3, Z = 4, m = 0.276 mmꢀ1, measured
reflections = 56 358, independent reflections = 7863, parameters = 424,
Rint = 0.0878, R = 0.0453 (I > 2s(I)), Rw = 0.0908 (all data), GOF =
0.995. 4: C29H30BO4F12PSi3, MW = 796.58, T = 150 K, space group,
U.-J. Vogelbacher, Tris(Trimethylsilyl)Phosphine and Lithium
Bis(Trimethylsilyl)Phosphide. Bis-(Tetrahydrofuran), in Inorganic
Syntheses, ed. A. P. Ginsberg, John Wiley & Sons, Inc., Hoboken,
NJ, USA, 2007, vol. 27, 10.1002/9780470132586.ch48.
15 F. Schulz, V. Sumerin, M. Leskela, T. Repob and B. Rieger,
¨
Dalton Trans., 2010, 39, 1920–1922.
16 (a) M. Ullrich, A. J. Lough and D. W. Stephan, J. Am. Chem. Soc.,
2009, 131, 52–53; (b) M. Ullrich, A. J. Lough and D. W. Stephan,
Organometallics, 2009, 29, 3647–3654.
%
triclinic, P1, a = 10.8454(4) A, b = 11.2438(4) A, c = 15.4120(6) A,
´
17 M. Driess, R. Barmeyer, C. Monse and K. Merz, Angew. Chem.,
Int. Ed., 2001, 40, 2308–2310.
a = 84.657(2)1, b = 87.6520(10)1, g = 85.639(2)1, V = 1864.72(12) A3,
Z = 2, m = 0.262 mmꢀ1, measured reflections = 32 663, independent
reflections = 8451, parameters = 451, Rint = 0.0334, R = 0.0404 (I >
2s(I)), Rw = 0.1021 (all data), GOF = 1.019.
18 Other examples of silyl-migrations to oxygen atoms have
been previously reported: (a) A. G. Brook and J. J. Chrusclelt,
Organometallics, 1984, 3, 1317–1318; (b) A. G. Brook, M. Hesse,
K. M. Baines, R. Kumarathasan and A. J. Lough, Organometal-
lics, 1993, 12, 4259–4261.
1 D. W. Stephan and G. Erker, Angew. Chem., Int. Ed., 2010, 49, 46–76.
2 (a) G. C. Welch, R. R. S. Juan, J. D. Masuda and D. W. Stephan,
Science, 2006, 314, 1124–1126; (b) G. C. Welch and D. W. Stephan,
J. Am. Chem. Soc., 2007, 129, 1880–1881; (c) P. Spies, G. Erker,
19 R. Appel, F. Knoll and I. Ruppert, Angew. Chem., Int. Ed. Engl.,
1981, 20, 731–744.
20 R. O. Day, A. Willhalm, J. M. Holmes, R. R. Holmes and
A. Schmidpeter, Angew. Chem., Int. Ed. Engl., 1985, 24, 764–765.
21 R. Appel, B. Laubach and M. Siray, Tetrahedron. Lett., 1984, 25,
4447–4448.
G. Kehr, K. Bergander, R. Frohlich, S. Grimme and D. W. Stephan,
¨
Chem. Commun., 2007, 5072–5074.
3 (a) C. M. Momming, E. Otten, G. Kehr, R. Frohlich, S. Grimme,
¨
D. W. Stephan and G. Erker, Angew. Chem., Int. Ed., 2009, 48,
¨
c
11306 Chem. Commun., 2012, 48, 11304–11306
This journal is The Royal Society of Chemistry 2012