Phosphorus as a lewis acid: CO2 sequestration with amidophosphoranes

Article abstract of DOI:10.1002/anie.201201422

CO₂ snapper: Compounds containing both acidic and basic P,N functionalities have been prepared. Of these, two amidophosphoranes containing highly reactive P-N bonds within four-membered rings react rapidly with CO ₂, resulting in rel

Full text of DOI:10.1002/anie.201201422

.
Angewandte
Communications
DOI: 10.1002/anie.201201422
Carbon Dioxide Capture
Phosphorus as a Lewis Acid: CO2 Sequestration with
Amidophosphoranes**
Lindsay J. Hounjet, Christopher B. Caputo, and Douglas W. Stephan*
The role of CO2 as a greenhouse gas has prompted wide-
spread efforts for carbon capture. To date, a number of
strategies have been developed to sequester this gas using
materials such as alumina, silica, zeolites, activated carbon,
and metal–organic frameworks (MOFs).[1] While some of
these systems are being applied on increasing scale, efforts to
discover fundamentally unique strategies for CO2 capture
continue. In this regard, the use of metal-free, frustrated
Lewis pairs (FLPs) has received recent attention.[2] In 2009,
we reported the reversible binding of CO2 by borane/
phosphine-based FLPs.[3] Subsequent work by the groups of
OꢀHare[4] and Piers,[5] as well as ourselves,[6] has demonstrated
the use of B/P, B/N, and Al/P-based FLPs for the conversion
of CO2 into methanol, methane, or CO.
Scheme 1. Synthesis of 2–4.
While Group 13/15 Lewis acid/base combinations con-
tinue to reveal new aspects of FLP chemistry, less attention
has been paid to expanding the variety of FLP systems
available. A creative departure from the original systems has
been described by Alcarazo and co-workers.[7] In that work,
all-carbon-based FLPs of N-heterocyclic carbenes and Lewis
acidic allenes were described. Another alternative has been
developed by Wass and co-workers,[8] who exploited Group 4
metal Lewis acids with bulky phosphines to activate a variety
of small molecules. While considering new approaches, we
noted the innovative examples of organic transformations and
anion capture that are facilitated by Lewis acidic phospho-
nium cations[9] and queried the viability of such species in FLP
chemistry. Herein, we describe the syntheses of aminophos-
phonium salts, which incorporate Lewis basic and acidic
pnictogen functionalities within an intramolecular system.
These species serve as hypothetical intermediates for the
generation of ring-strained amidophosphoranes, which are
shown to sequester CO2.
methyl signal at d = 2.67 ppm. With the amine functionality
unaffected by fluorination, facile FÀ abstraction from 1 by
Me3SiO3SCF3 yields [Ph2PF(o-C6H4NHMe)][O3SCF3] (2),
a rare example of a compound containing both acidic
phosphonium and pendant amine functionalities. The
31P{1H} NMR spectrum of 2 reveals a strongly deshielded P
nucleus with a doublet resonance at d = 94.4 (1JPF = 980 Hz),
while the 1H spectrum is largely unchanged, except for
a downfield shift of the NH signal to d = 5.20.
The combination of unquenched Lewis acidic and basic
pnictogens within 2 prompted our investigation of its
potential behavior as an FLP in reactions with CO2. Exposure
of 2 to an overpressure of 13CO2 in CD2Cl2 resulted in no
reaction, and attempts to deprotonate the amine with
one equiv of Et3N or nBuLi in THF yielded a mixture of
products that could not be separated. However a new, bright-
yellow species (3) was cleanly prepared from 1 in good yields
by the slow addition of an n-pentane solution of tBuLi to
Reaction
of
the
o-phosphinoaniline
Ph2P(o-
[11]
C6H4NHMe)[10] with XeF2 at À358C in CH2Cl2 produces
the off-white difluorophosphorane, Ph2PF2(o-C6H4NHMe),
1 (Scheme 1). The 31P{1H} NMR spectrum of 1 reveals a high-
field triplet at d = À45.6 ppm (1JPF = 625 Hz), while the
1H NMR spectrum depicts the NH resonance as a broad
quartet at d = 4.68 ppm (3JHH = 5.0 Hz) that couples to the N-
1
a THF solution of the compound at À788C. The H NMR
À
spectrum of 3 suggests the presence of a P N bond, as the N-
methyl signal is split into a doublet of doublets (3JPH = 4.4 Hz
and 4JFH = 2.4 Hz). The 31P{1H} NMR signal of 3 was observed
as a doublet at d = À44.6 ppm (1JPF = 679 Hz), and the
corresponding 19F resonance was seen at d = À44.4 ppm.
These data suggest the formulation of 3 as the amidophos-
phorane Ph2PF(o-C6H4NMe). The X-ray structure of 3
confirmed this formulation, demonstrating a distorted trigo-
nal bipyramidal geometry at P, which is contained within
a strained four-membered ring (Figure 1).[12] Although rare (a
few amidofluorophosphoranes have previously been pre-
pared),[13] compound 3 is the first to be crystallographically
[*] Dr. L. J. Hounjet, C. B. Caputo, Prof. Dr. D. W. Stephan
Department of Chemistry, University of Toronto
80 St. George Street, Toronto, Ontario, M5S 3H6 (Canada)
E-mail: dstephan@chem.utoronto.ca
Homepage: http://www.chem.utoronto.ca/staff/DSTEPHAN
[**] D.W.S. gratefully acknowledges the financial support of the NSERC
of Canada and the award of a Canada Research Chair.
À
characterized. The P N bond was found to be exceptionally
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201201422.
long (1.842(7) ꢁ and 1.839(6) ꢁ)[13] within each of two
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À
an elongated P O bond (1.775(1) ꢁ and 1.778(1) ꢁ) in each
of two molecules in the asymmetric unit. Within 4, the
geometry about P is trigonal bipyramidal with the carbamato
À
=
O occupying an axial position trans to F. The C O and C O
bonds in the CO2 fragment of 4 average 1.330(2) and
1.212(2) ꢁ, respectively, which are slightly longer than those
seen in tBu3PCO2B(C6F5)3 (1.299(2) ꢁ, 1.208(2) ꢁ)
and
(Me3C6H2)2PCH2CH2B(C6F5)2(CO2)
(1.284(4) ꢁ,
1.209(4) ꢁ). Compounds 3 and 4 are both rare examples of
crystallographically characterized phosphoranes, and to the
best of our knowledge, 4 is also the first known pentacoordi-
nate phosphorane containing
a carbamato substituent.
Figure 1. POV-ray depiction of 3.
Although kinetically hindered insertion of CO2 into the
À
P N bond of (CF3)3PMe(NMe2) has been previously reported
to proceed over several days to give a six-coordinate
crystallographically independent molecules. The N atom
zwitterionic carbamatophosphate,[13] the facile insertion of
À À
À
occupies an axial position opposite F, although the N P F
CO2 into the P N bond of 3, relieves strain within the four-
angles (164.4(3)8 and 165.1(3)8) are somewhat distorted from
linearity, presumably in response to constraints imposed by
the equatorially disposed anilido Cipso. The geometry of the
amido unit is nearly planar, as the sum of the angles about N is
membered ring, thereby dramatically accelerating the reac-
tion. It is noteworthy that 4 is thermally robust, as it is
unchanged on heating to 1208C in toluene for 1 h.
Related compounds incorporating a second o-N-methyl-
aniline group were prepared (Scheme 2). Oxidative fluorina-
tion of PhP(o-C6H4NHMe)2 with XeF2 produced the phos-
À
3568 and the N Cortho distances of 1.439(9) ꢁ and 1.442(10) ꢁ
are typical of single bonds. The C C bonds within the anilido
ring are similar to those within neighboring Ph rings, in
contrast to the dearomatization observed within metal com-
plexes of the amidophosphine ligand.[14]
À
The exposure of a bright-yellow THF solution of 3 to
1 atm of CO2 at ambient temperature results in its immediate
discoloration, and NMR analysis performed shortly there-
after indicates complete conversion to a new species (4). The
1H NMR spectrum of 4 shows a singlet N-methyl resonance at
d = 3.15 ppm, and the loss of its coupling to both P and F is
À
consistent with rupture of the P N bond within 3 to
accommodate CO2. The 13C{1H} NMR spectrum shows the
CO2 unit as a doublet at d = 153.1 ppm with 2JPC = 8 Hz, while
the 31P{1H} spectrum shows a doublet at d = À57.0 ppm, with
1JPF = 664 Hz. The corresponding 19F NMR signal is seen at
d = À34.2 ppm. IR data for 4 show a characteristic carbonyl
absorption band at 1696 cmÀ1. Collectively, these data suggest
Scheme 2. Synthesis of 6–8.
that
4
is the carbamatofluorophosphorane Ph2PF(o-
C6H4N(Me)CO2), and this formulation was confirmed by X-
ray crystallography (Figure 2).[12] Insertion of CO2 into the
À
activated P N bond of 3 results in a six-membered ring with
phorane PhPF2(o-C6H4NHMe)2 (5), and subsequent fluoride
ion abstraction by Me3SiO3SCF3 generated the salt [PhPF(o-
C6H4NMeH)2][O3SCF3] (6). The spectral parameters of these
compounds closely matched those of their monoaniline
derivatives. Crystallographic data of 6 confirmed the formu-
lation and revealed P···N separations of 3.05–3.12 ꢁ. Inter-
estingly, both amine H atoms are oriented towards F,
suggesting hydrogen bonding, although the H···F separations
of about 2.4 ꢁ indicate that this interaction is weak.[15]
Consistent with the reactivity described above, the addition
of 2 equiv of tBuLi to 5 resulted in the diamidophosphorane,
PhP(o-C6H4NMe)2 (7), which was isolated as a yellow solid in
75% yield. The 31P{1H} NMR spectrum shows a shielded P
1
nucleus with a singlet at d = À58.7 ppm, and the H NMR
spectrum demonstrates the C2 molecular symmetry with
3
a single NMe resonance at d = 2.55 ppm (d, JPH = 6.8 Hz).
Figure 2. POV-ray depiction of 4.
Treatment of 7 with 1 atm of CO2 under ambient conditions
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Angewandte
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instantly produced PhP(o-C6H4N(Me)CO2)2 (8), which was
a possibility, it is also noteworthy that the resonance forms of
3 (Scheme 3) suggest the possibility of an FLP-type descrip-
isolated in 80% yield. The 31P{1H} NMR spectrum of 8
3
À
showed a single peak at d = À68.6 ppm, and the loss of JPH
,
tion in which a charge-separated P N bond consists of an
which was made evident by the singlet NCH3 signal in the
1H NMR spectrum, was consistent with CO2 insertion into
1
À
each of the chemically equivalent P N bonds. The H and
13C{1H} NMR data were also consistent with the formulation
of 8 as the double insertion product. Use of 13CO2 revealed
a strong signal at d = 152.8 ppm in the 13C{1H} NMR spectrum
and IR spectroscopy of 8 showed the expected carbonyl
absorption at 1697 cmÀ1
.
The formulation of 8 was also confirmed by its X-ray
structure (Figure 3),[12] which showed the pseudo-C2-symmet-
ric geometry of the dicarbamatophosphorane, wherein
Scheme 3. Resonance forms of 3.
amido donor and a phosphonium acceptor. This view of 3 is
reminiscent of a B/P FLP, (C6H2Me3)2PCH2CH2B(C6F5)2,
developed by Erker and co-workers.[16] In this regard, the CO2
insertion chemistry discussed herein is also analogous to that
reported for the four-membered rings of boron imidinates,
HC(NR)2B(C6F5)2, which are also thought to react with small
molecules via an open FLP form.[17]
In summary, amidophosphoranes 3 and 7, containing the
four-membered rings, react rapidly to capture of one and two
equivalents of CO2, respectively. These compounds bind the
substrate in a manner analogous to related FLP systems.
These findings have prompted further study of the acceptor
capabilities of electron-poor phosphonium cations to provide
PV Lewis acid centers for FLP chemistry. The results of these
efforts will be reported in due course.
Received: February 20, 2012
Published online: April 4, 2012
Figure 3. POV-ray depiction of 8.
Keywords: amidophosphoranes · carbon dioxide ·
Lewis acids and bases · phosphonium · pnictogens
.
oxygen donors occupy both axial positions, while the Cipso
atoms of the aryl groups lie in the equatorial plane. The
À
lengths of mutually trans P O bonds were found to be
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Chemie
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¯
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101.782(2)8, V= 3230.1(3) ꢁ3, Z = 2, data (> 2s) = 15134, vari-
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¯
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