Geminal phosphorus/aluminum-based frustrated lewis pairs: C-H versus C≡C activation and CO2 fixation

Article abstract of DOI:10.1002/anie.201006901

Catch it! Geminal phosphorus/aluminum-based frustrated Lewis pairs (FLPs) are easily obtained by hydroalumination of alkynylphosphines. These FLPs can activate terminal acetylenes by two competitive pathways, which were analyzed by DFT calculations, and they can bind carbon dioxide reversibly. Therefore, alongside polyfluorinated boranes, alanes are also ideal Lewis acids for FLP chemistry.

Full text of DOI:10.1002/anie.201006901

DOI: 10.1002/anie.201006901
Frustrated Lewis Pairs
Geminal Phosphorus/Aluminum-Based Frustrated Lewis Pairs:
À
ꢀ
C H versus C C Activation and CO2 Fixation**
Christian Appelt, Hauke Westenberg, Federica Bertini, Andreas W. Ehlers, J. Chris Slootweg,
Koop Lammertsma,* and Werner Uhl*
Frustrated Lewis pairs (FLPs),[1] which have donor and
acceptor sites in close proximity, are of considerable interest
for the dipolar activation of small molecules, such as hydro-
gen,[2] alkynes,[3] and the greenhouse gases carbon dioxide[4]
and nitrous oxide.[5] So far, phosphines,[1,6] amines,[7] thio-
ethers,[3c] and carbenes[2e,8] have been applied as Lewis base,
while polyfluorinated boranes are the common Lewis acid in
FLP chemistry. Surprisingly, alanes have only been rarely
used,[3a,c,9] although these are generally better Lewis acids[10]
that can circumvent the need for decorating the acceptor site
with electron-withdrawing fluorinated substituents. Herein
we present a simple one-step synthesis of geminal phospho-
rus/aluminum-based FLPs by hydroalumination[11,12] of read-
ily available alkynylphosphines,[13] as well as on their propen-
sity to activate small molecules.[14]
couplings, indicating different conformations of the double
bond. The molecular structures of 3 and 4 were established
unequivocally by X-ray crystal structure determinations
(Figure 1),[15] which showed that 3 is the expected cis hydro-
To illustrate the diversity of our general method,
Figure 1. Molecular structures of 3 and 4. Ellipsoids set at 30%
probability; hydrogen atoms are omitted for clarity. Selected average
bond lengths [pm] and angles [8] for 3: Al1–C111 198.0, P1–C111
183.0, C111–C112 133.6, Al1···P1 315.3; Al1-C111-P1 111.0. 4: Al1–C1
199.2, P1–C1 182.2, C1–C2 134.3, Al1···P1 328.7; Al1-C1-P1 119.1.
ꢀ
Mes2PC CtBu (1; Mes = 2,4,6-Me3C6H2) was treated with
ꢀ
di(neopentyl)aluminum hydride, and Mes2PC CC6H5 (2) was
treated with di(tert-butyl)aluminum hydride at room temper-
ature, which afforded the respective adducts 3 and 4 as sole
products (Scheme 1). Interestingly, phosphinoalanes 3 (d-
(31P) = À32.0 ppm, 3J(H,P) = 37.9 Hz) and
4
(d(31P) =
À14.2 ppm, 3J(H,P) = 17.7 Hz) both have distinct 3J(H,P)
alumination adduct with a Z configured C C bond, while a
=
cis/trans isomerization occurred in the case of E-4.[16] Such an
isomerization requires intermolecular activation,[17] which is
evidently impeded by the bulky tert-butyl group on the double
bond in 3. The highly selective attack of the positively charged
aluminum atom at the a-carbon atom of the alkynylphos-
phines 1 and 2 is in accord with the charge separation in the
starting alkyne (NBO values of 1: a-C À0.49, b-C + 0.03; 2: a-
C À0.35, b-C À0.004). Expectedly, the phosphorus atoms in 3
and 4 have a pyramidal coordination and the aluminum atoms
are trigonal-planar. Furthermore, both heteroatoms are
perfectly oriented for small-molecule activation.
Scheme 1. Synthesis of FLPs 3 and 4 by hydroalumination of 1 and 2.
[*] C. Appelt, Dr. H. Westenberg, Prof. Dr. W. Uhl
Institut fꢀr Anorganische und Analytische Chemie der
Westfꢁlischen Wilhelms-Universitꢁt Mꢀnster
Corrensstrasse 30, 48149 Mꢀnster (Germany)
Fax: (+49)251-833-6610
To investigate the reactivity of our new FLPs, we
examined the reaction of 4 with terminal acetylenes, CO2,
and H2 and we also analyzed the reaction profiles computa-
tionally. Treating FLP 4 with phenylacetylene in toluene at
room temperature gave a 3:1 mixture of 5a (d(31P) =
À5.1 ppm) and 6a (d(31P) = 24.2 ppm, 3J(H,P) = 60.1 Hz;
Scheme 2), while at 508C a 1:1 mixture was obtained. Heating
the mixture at 708C for 1 hour resulted in the full conversion
of 5a into 6a, which after crystallization was isolated in 91%
yield. Single-crystal X-ray diffraction analysis revealed that
6a is formed by P/Al addition to the alkyne[3a,b] (Figure 2).[15]
E-mail: uhlw@uni-muenster.de
F. Bertini, Dr. A. W. Ehlers, Dr. J. C. Slootweg,
Prof. Dr. K. Lammertsma
Department of Chemistry and Pharmaceutical Sciences
VU University Amsterdam
De Boelelaan 1083, 1081 HV Amsterdam (The Netherlands)
E-mail: k.lammertsma@few.vu.nl
[**] This work was supported by the Deutsche Forschungsgemeinschaft
(IRTG 1444) and the Council for Chemical Sciences of the
Netherlands Organization for Scientific Research (NWO/CW).
Whereas 5a could not be obtained in pure form, its large
1
À
J(H,P) coupling of 503.5 Hz is characteristic for a P H
species, indicating that it is formed by C–H activation of the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201006901.
alkyne.[3a] This hypothesis was confirmed by the reaction of 4
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3925

Communications
molÀ1, respectively). In 5a-vdW, the acetylenic proton is
oriented towards the Lewis base, which yields 5a by
À
deprotonation after initial formation of the Al C bond and
subsequent migration of H to P (DDE° = 7.6, DDE =
À10.7 kcalmolÀ1; Figure 3). In 6a-vdW, the C C bond is
ꢀ
facing both phosphorus and aluminum atoms, facilitating an
À
À
asynchronous 1,2-addition by Al C and then P C bond
formation (DDE° = 4.5, DDE = À27.2 kcalmolÀ1; Figure 3). It
can be concluded that products 5a and 6a are formed
depending on the initial orientation of the alkyne. Upon
heating, 5a can interconvert via vdW into the thermodynami-
cally favored 6a, which corroborates with the experimental
details. Using tert-butylacetylene as substrate, 5b is the
thermodynamic sink that does not rearrange into 6b owing
to steric crowding in the transition state (compare with TS6a
Scheme 2. Reaction of FLP 4 with terminal acetylenes.
À
in Figure 3), which precludes the formation of the P C bond
under the reaction conditions.
The behavior of 4 with terminal alkynes encouraged us to
examine the FLP activation of CO2, for which there are only
limited examples.[4,9] CO2 was bubbled through a solution of 4
in toluene at room temperature for 30 seconds, which after
crystallization afforded CO2 adduct 7 in 74% yield (d(31P) =
5.6 ppm; d(13C) (CO2) = 165.1 ppm, 1J(C,P) = 106.5 Hz). A
crystal structure determination confirmed the five-membered
Figure 2. Molecular structures of 5b and 6a. Ellipsoids set at 30%
probability; hydrogen atoms except at P1 (5b) and C5 (6a) and
toluene (5b) or hexane (6a) solvent molecules are omitted for clarity.
Selected average bond lengths [pm] and angles [8] for 5b: Al1–C5
198.7(3), P1–H1 127(2), C5–C6 120.3(3), C1–C2 135.0(3); Al1-C5-C6
171.1(2). 6a: Al1–C5 202.6(3), P1–C6 181.9(3), C1–C2 134.1(4), C5–C6
134.1(4); Al1-C1-P1 106.1(1).
heterocycle (Figure 4)[15] bearing short C O (128.0 pm) and
À
[4a]
=
À
C O (120.6 pm) bonds,
but an elongated P C(O) bond
(191.9 pm). This carbon dioxide binding can be reversed:
Treatment of 7 in the solid state at 1358C under vacuum for
2 minutes resulted in the complete reformation of FLP 4.
Analysis of the formation of CO2 adduct 7 at the M06-2X/
6-31 + G(d,p) level of theory[18] showed the intermediacy of a
with tert-butylacetylene at 408C (2 h), which afforded only 5b
1
(92%; d(31P) = À5.2 ppm, J(H,P) = 507.5 Hz) and could be
characterized crystallographically (Figure 2). The molecular
structure of 5b reveals a phosphonium alkynylaluminate with
À
ꢀ
typical Al C and C C bond
lengths of d = 198.7 and
120.3 pm, respectively.
To provide insight into
the mode of activation and
to establish whether 6a is a
primary product or formed
by 5a, we resorted to M06-
2X/6-31 + G(d,p) calcula-
tions[18] on the full system.
Three van der Waals com-
plexes were observed upon
reacting
phenylacetylene
with FLP 4 of which the
most stable complex (vdW,
DE = À5.7 kcalmolÀ1
;
Figure 3) has the alkyne
positioned near the alumi-
num atom[3a,b,19] and orthog-
onal to the P-C-Al plane of
4. Rotating the alkyne into
the P-C-Al plane either
clockwise or anticlockwise
determines the reaction out-
come by bringing about the
other complexes, namely
Figure 3. Relative M06-2X/6-31+G(d,p) energies [kcalmolÀ1] for the reaction of FLP 4 with phenylacetylene,
yielding products 5a and 6a. Selected bond lengths [pm] for vdW: Al1–C35 299.8, Al1–C36 353.1, C35–C36
121.2. 5a-vdW: Al1–C35 353.8, Al1–C36 389.3, C35–C36 121.1. 6a-vdW: Al1–C35 359.3, P1–C36 317.1, C35–
C36 121.1. TS5a: Al1–C35 214.9, P1–H47 175.0, C35–H47 129.6. TS6a: Al1–C35 283.5, P1–C36 217.6, C35–
C36 124.8. 5a: Al1–C35 200.4, P1–H47 139.3, C35–C36 122.6. 6a: Al1–C35 203.4, P1–C36 183.6, C35–C36
5a-vdW
and
6a-vdW
(DDE = 1.2 and 2.2 kcal 134.8.
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Angew. Chem. Int. Ed. 2011, 50, 3925 –3928

donor site can be enhanced easily by using differently
substituted P-alkynes and aluminum hydrides.
In summary, hydroalumination of alkynylphosphines
provides a facile approach to geminal phosphorus/alumi-
num-based FLPs, which are ideally suited for small-molecule
activation. The full potential of these readily available
frustrated Lewis pairs for (catalytic) small molecule activa-
tion is currently being explored in our laboratories.
Received: November 3, 2010
Revised: December 19, 2010
Published online: March 21, 2011
Figure 4. Molecular structure of 7. Ellipsoids set at 30% probability;
hydrogen atoms and toluene molecule are omitted for clarity. Selected
average bond lengths [pm] and angles [8] for four independent
molecules: Al1–C1 205.8, Al1–O1 185.9, P1–C1 178.8, P1–C5 191.9,
C1–C2 134.6, C5–O1 128.0, C5–O2 120.6; Al1-C1-P1 108.6, O1-C5-O2
126.7.
Keywords: alanes · carbon dioxide fixation ·
.
density functional calculations · frustrated Lewis pairs ·
phosphines
van der Waals complex (7-vdW, DE = À6.52 kcalmolÀ1) with
a nonlinear CO2 fragment (171.08) positioned perpendicular
to the P-C-Al plane of 4 (Figure 5). Compound 7-vdW has a
short contact between the oxygen atom of CO2 and aluminum
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=
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