Exchange chemistry of tBu3P(CO2)B(C6F 5)2Cl

Article abstract of DOI:10.1039/c2dt30206c

Halide exchange from the species tBu₃P(CO₂)B(C ₆F₅)₂Cl 1 with Me₃SiOSO ₂CF₃ gave tBu₃P(CO₂)B(C ₆F₅)₂(OSO₂

Full text of DOI:10.1039/c2dt30206c

View Online / Journal Homepage / Table of Contents for this issue
This article is published as part of the Dalton Transactions themed
issue entitled:
Frustrated Lewis Pairs
Guest Editor: Douglas Stephan
Published in issue 30, 2012 of Dalton Transactions
Articles in this issue include:
Communication
Hydrogen activation by 2-boryl-N,N-dialkylanilines: a revision of Piers’ ansa-aminoborane
Konstantin Chernichenko, Martin Nieger, Markku Leskelä and Timo Repo
Paper
Frustrated Lewis pair addition to conjugated diynes: Formation of zwitterionic 1,2,3-butatriene
derivatives
Philipp Feldhaus, Birgitta Schirmer, Birgit Wibbeling, Constantin G. Daniliuc, Roland Fröhlich,
Stefan Grimme, Gerald Kehr and Gerhard Erker
Paper
Fixation of carbon dioxide and related small molecules by a bifunctional frustrated pyrazolylborane
Lewis pair
Eileen Theuergarten, Janin Schlösser, Danny Schlüns, Matthias Freytag, Constantin G. Daniliuc,
Peter G. Jones and Matthias Tamm
Visit the Dalton Transactions website for more cutting-edge inorganic chemistry research

Dynamic Article Links
Dalton
Transactions
Cite this: Dalton Trans., 2012, 41, 9016
www.rsc.org/dalton
COMMUNICATION
Exchange chemistry of tBu3P(CO2)B(C6F5)2Cl†
Rebecca C. Neu, Gabriel Ménard and Douglas W. Stephan*
Received 27th January 2012, Accepted 7th February 2012
DOI: 10.1039/c2dt30206c
Halide exchange from the species tBu3P(CO2)B(C6F5)2Cl 1
with Me3SiOSO2CF3 gave tBu3P(CO2)B(C6F5)2(OSO2CF3) 2.
Similarly, Lewis acid exchange occurs in reactions of 1 with
Al(C6F5)3 and [Cp2TiMe][B(C6F5)4] affording the products,
tBu3P(CO2)Al(C6F5)3 3 and [tBu3P(CO2)TiCp2Cl][B(C6F5)4] 4.
The dramatic increase in atmospheric CO2 levels has been
shown to be a major contributing factor in global climate
change.1 Efforts to mitigate emissions have focused on reduced
consumption, improved efficiencies and new technologies to
capture the carbon products of combustion.2 In our efforts we
have been exploring fundamentally new reactivity of main group
systems toward this end. In these efforts, we have uncovered the
notion of “frustrated Lewis pairs” (FLPs) and have demonstrated
that such systems react with a variety of small molecules.3–6 In
particular, combinations of sterically demanding phosphines and
boranes have been shown to reversibly bind CO2.7 The O’Hare
and Piers groups have since extended this chemistry to effect the
reduction of CO2.8,9 We have also examined CO2 insertions into
B-amidinates10 as well as explored bis-borane-containing
systems. These latter systems demonstrate binding of both O-
atoms of the CO2 moiety to boron with phosphine binding to the
central C-atom (Fig. 1).11 Nonetheless, a common feature of
these P/B CO2 systems is their inherent thermal instability result-
ing in CO2 loss and regeneration of the FLP thus, precluding
further reactivity of the “activated” CO2 fragment. Recent efforts
have also explored related Al–P FLP systems and demonstrated
that the species Mes3PC(OAlX3)2 (X = Cl, Br)12 are readily
formed and stable species capable of further reactivity of the
CO2 fragment. In this case, reduction employing ammonia
borane with subsequent hydrolysis resulted in the stoichiometric
reduction of CO2 to methanol.12 More recently, we reported that
Fig. 1 Examples of FLP-CO2 derivatives.
Scheme 1 Synthesis of compounds 2–4.
chemistry is of interest as many of these CO2-complexes are
thermally unstable and in extreme cases liberate CO2 above
−20 °C.7 Moreover, reactions of 1 with stronger Lewis acids
provide facile pathways to access new main group- and transition
metal-CO2 derivatives.
Mes3PC(OAlX3)2 reacts further with CO2 to liberate CO,
Mes3PC(OAlX2)2OAlX3 and [Mes3PX][AlX4].13
Recent studies have probed the impact of substituent modifi-
cation on the stability of boron- and phosphorus-based FLP-CO2
and formate derivatives.14 In that work, the species tBu3P(CO2)
B(C6F5)2Cl 1 was prepared. Herein, we explore the further reac-
tivity of 1, demonstrating that exchange of Cl for triflate is poss-
ible without loss of CO2. The ability to perform such exchange
Initial attempts to abstract chloride from tBu3P(CO2)B
(C6F5)2Cl 1 via reaction with NaBPh4 and Ag(OSO2CF3)
resulted in no reaction. In contrast, stoichiometric reactions of 1
with K[B(C6F5)4] or [Et3Si][B(C6F5)4] appeared to react,
however the respective products could not be readily isolated.
However, reaction of 1 with a stoichiometric equivalent of
Me3SiOSO2CF3 cleanly led to the facile and near quantative iso-
Department of Chemistry, 80 St. George St.University of Toronto,
Toronto, Ontario, Canada, M5S 3H6. E-mail: dstephan@
chem.utoronto.ca
†Electronic supplementary information (ESI) available. CCDC
864916–864918. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c2dt30206c
lation of a new product 2 (Scheme 1). This product exhibited 31
P
{1H} and 11B{1H} NMR signals at 48.88 and 3.76 ppm, respect-
ively. These values are similar to those seen for 1. The 1H NMR
9016 | Dalton Trans., 2012, 41, 9016–9018
This journal is © The Royal Society of Chemistry 2012

Fig. 2 POV-Ray depiction of 2, B: yellow-green, C: black, O: red, F:
pink, P: orange, S: yellow, H-atoms are omitted for clarity.
Fig. 3 POV-Ray depiction of 3, C: black, O: red, F: pink, Al: blue-
grey, P: orange. H-atoms are omitted for clarity.
data were consistent with the presence of tBu moieties while the
19F NMR spectrum showed resonances at −78.59 and −134.40,
−157.34 and −164.94 ppm attributable to the presence of triflate
and chemically equivalent fluoroarene rings on B. The IR spec-
trum exhibited a CvO stretch at 1706 cm−1 further supporting
the retention of the CO2 moiety. Repetition of the reaction
employing 1-13C showed P–C coupling of 93 Hz, reaffirming
the presence of the P–CO2 bond. These data led to the formu-
lation of 2 as the product of Cl-triflate metathesis tBu3P(CO2)B
(C6F5)2(OSO2CF3). Crystals suitable for X-ray diffraction were
grown from a layered solution of CH2Cl2 and hexanes and the
crystal structure confirmed the proposed formulation (Fig. 2).‡
The geometry about B is pseudo-tetrahedral with a B–O bond
distances of 1.498(2) Å and 1.545(2) Å to the CO2 and triflate
fragments, respectively. The corresponding C–O bond distance
in the CO2 moiety is 1.318(2) Å while the B–O–C angle is
118.7(1)°. The terminal CvO bond distance was found to be
1.199(2) Å. The P–C bond distance to the CO2-carbon was
found to be 1.892(1) Å. While this latter metric parameter is
similar to that seen in tBu3P(CO2)B(C6F5)3 (P–C: 1.893(1) Å,
B–O: 1.547(2) Å)7 and (C6H2Me3)2PCH2CH2B(C6F5)2(CO2)
(P–C: 1.900(3) Å, B–O: 1.550(4) Å),7 the B–O bond distance in
2 is significantly shorter, consistent with the stronger Lewis
acidity of B(C6F5)2(OSO2CF3) versus B(C6F5)3. The P–C bond
of 2 is also longer than that reported for Me2CvC(BCl2)2(CO2)
PtBu3 (P–C: 1.874(3) Å, B–O: 1.577(4) Å, 1.585(3) Å)14 and
slightly shorter than that in Me2CvC(B(C6F5)2)2(CO2)PtBu3
(P–C: 1.896(2) Å, B–O: 1.647(3) Å, 1.672(3) Å). It should be
noted, that the B–O distance in 2 is significantly shorter than
those seen in these bisborane compounds.
band at 1686 cm−1 was consistent with the retention of CO2 in
3. Reaction of 1-13C with 0.5tol·Al(C6F5)3 yielded 3-13C which
displayed a resonance in the 31P{1H} NMR at 47.72 ppm with a
P–C coupling constant of 89 Hz, corresponding to a one bond
phosphorous–carbon coupling. Previously reported activation of
CO2 by aluminum-based FLPs involved the Lewis acid in a 2 : 1
ratio relative to the Lewis base. However, in the exchange chem-
istry of 1 with a single equivalent of 0.5tol·Al(C6F5)3, there
proved to be complete conversion to 4 with no evidence of
unreacted 1 thereby confirming the presence of a 1 : 1 Lewis
base to Lewis acid CO2 activation product. Collectively, the
spectroscopic data in addition to elemental analysis were consist-
ent with the formulation of 3 as tBu3P(CO2)Al(C6F5)3. This was
subsequently confirmed crystallographically (Fig. 3).‡ There are
two molecules in the asymmetric unit where both phosphines are
bound to the C of CO2 with P–C distances of 1.884(2) Å and
1.885(2) Å, respectively. A single oxygen atom of the CO2 frag-
ment was found to be bound to Al with a O–Al distance aver-
aging 1.828(2) Å. The terminal CvO bond distances were
found to be 1.208(2) Å and 1.211(2) Å.
To probe the exchange capabilities with a transition metal-
based Lewis acid, compound 1 was reacted with a solution in
which the Ti-cationic salt [Cp2TiMe][B(C6F5)4] was preformed
via the reaction of Cp2TiMe2 and [Ph3C][B(C6F5)4]. The deep
red–brown solution of the Ti cation progressively and quickly
lightened in colour to a vibrant red. After stirring for an hour,
manipulation afforded an orange solid
4 in 96% yield
(Scheme 1). The 31P{1H} NMR spectrum of 4-13C revealed the
P–C coupling of 86 Hz, inferring that the phosphine–CO2 frag-
ment was successfully exchanged between the Lewis acid
centers. This was further supported by the observation of the
infrared absorption at 1670 cm−1, assignable to the carbonyl
stretching frequency. 11B{1H} and 19F NMR spectra confirmed
that the [B(C6F5)4] anion was a component of 4 with no evi-
Compound 1 was also observed to react with an equivalent of
(0.5tol)·Al(C6F5)3 at room temperature to give a faintly yellow
solution which following manipulation yielded a white solid 3 in
65% isolated yield (Scheme 1). This species gave rise to a 31P
{1H} signal at 46.65 ppm, however no 27Al NMR signal was
discernible at room temperature. The 19F data consisted of
signals at −123.12, −155.83 and −163.39 ppm supporting the
inclusion of the Al(C6F5)3 moiety. An infrared absorption at
1
dence of residual ClB(C6F5)2. The H NMR spectrum showed a
resonance at 6.65 ppm attributable to the cyclopentadienyl rings
on Ti as well as a signal at 1.66 ppm attributable to the tert-butyl
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 9016–9018 | 9017

respectively. The further reactivity of FLP systems with CO2 and
the subsequent chemistry of such species are the subject of on-
going efforts in our laboratory.
Acknowledgements
The financial support of NSERC of Canada is gratefully
acknowledged. DWS is also grateful for the award of a Canada
Research Chair. RCN is appreciative of the award of an Ontario
Graduate Scholarship. GM is grateful for the award of a Walter
C. Sumner Fellowship and NSERC postgraduate doctoral
scholarship.
Fig. 4 POV-Ray depiction of the cation of 4, C: black, O: red, P:
orange, Cl: green, Ti: grey. H-atoms and the B(C6F5)4 counter anion are
omitted for clarity.
Notes and references
ˉ
‡2: C26H27BF13O5PS MW = 740.32, Space group: triclinic, P1, a =
groups, however no resonance was assignable to a methyl group
bound to Ti. These data did not allow an unambiguous formu-
lation of 4, however the nature of the compound was determined
via X-ray diffraction employing crystals grown from a solution
of CH2Cl2 layered with hexanes (Fig. 4).‡ The X-ray data
confirmed 4 as the salt, [tBu3P(CO2)TiCp2Cl][B(C6F5)4]. In this
case, following liberation of the Lewis acid ClB(C6F5)2, there
was Cl–Me exchange between the free Lewis acid and titanocene
fragment. The coordination sphere of the cation was shown to
involve coordination of tBu3PCO2 to [Cp2TiCl] with a Ti–O
bond distance of 2.016(2) Å. The titanocene fragment is as
expected with a Ti–Cl bond length of 2.3171(9) Å. The O–C
and CvO bond distances to the carbon of CO2 were determined
to be 1.270(3) and 1.220(3) Å, while the P–C bond length was
found to be 1.896(3) Å. The Ti–O–C and O–C–O angles were
determined to be 135.8(2)° and 128.2(3)° while the O–C–P
angles were found to 116.3(2)° and 115.5(2)°, respectively.
The derivatives 3 and 4 are thought to form by transient inter-
action of the incoming Lewis acid with the terminal CvO
oxygen atom, affording a purported intermediate analogous to
Mes3PC(OAlX3)2.12 The congestion in the present cases, results
in the steric ejection of the borane, ClB(C6F5)2. This strategy of
Lewis acid exchange from FLP derivatives has previously found
utility in the preparation of Zn–N2O derivatives from the
FLP-N2O complex tBu3PN2OB(p-C6H4F).15
9.7343(4) Å, b = 10.7665(4) Å, c = 14.9487(6) Å, α = 98.123(1)°, β =
97.127(2)°, γ = 106.658(1)°, V = 1463.3(1) Å3, Z = 2, μ = 0.287, T =
150(2) K. Total data: 24 536, Rint = 0.0226, Unique Data = 6690, Vari-
ables = 444, R(>2σ) = 0.0303, R(all) = 0.0835, GOF = 1.043.3:
C31H27AlF15O2P, MW = 774.48, Space group: monoclinic, P21/n, a =
18.2162(8) Å, b = 15.2634(7) Å, c = 24.776(1) Å, β = 108.346(2)°, V =
6538.7(5) Å3, Z = 8, μ = 0.226, T = 150(2) K. Total data: 59519, Rint
=
0.0472, Unique Data = 15499, Variables = 919, R(>2σ) = 0.0464, R(all)
= 0.1219, GOF = 1.018.4: C47H37BClF20O2PTi, MW = 1138.90, Space
ˉ
group: triclinic, P1, a = 10.7349(9) Å, b = 14.415(1) Å, c = 15.088(1)
Å, α = 85.942(4)°, β = 88.205(4)°, γ = 81.850(5)°, V = 2304.8(3) Å3, Z
= 2, μ = 0.400, T = 150(2) K. Total data: 35 285, Rint = 0.0578, Unique
Data = 10 393, Variables = 707, R(>2σ) = 0.0483, R(all) = 0.1080, GOF
= 1.011.
1 IPCC, Climate Change 2007: Synthesis Report, IPCC, Geneva, 2007.
2 R. Banerjee, A. Phan, B. Wang, C. Knobler, H. Furukawa, M. O’Keeffe
and O. M. Yaghi, Science, 2008, 319, 939.
3 D. W. Stephan, Org. Biomol. Chem., 2008, 6, 1535–1539.
4 D. W. Stephan, Dalton Trans., 2009, 3129–3136.
5 D. W. Stephan, Chem. Commun., 2010, 46, 8526–8533.
6 D. W. Stephan and G. Erker, Angew. Chem., Int. Ed., 2010, 49, 46–76.
7 C. M. Mömming, E. Otten, G. Kehr, R. Fröhlich, S. Grimme, D.
W. Stephan and G. Erker, Angew. Chem., Int. Ed., 2009, 48, 6643–6646.
8 A. E. Ashley, A. L. Thompson and D. O’Hare, Angew. Chem., Int. Ed.,
2009, 48, 9839–9843.
9 A. Berkefeld, W. E. Piers and M. Parvez, J. Am. Chem. Soc., 2010, 132,
10660–10661.
10 M. A. Dureen and D. W. Stephan, J. Am. Chem. Soc., 2010, 132, 13559–
13568.
11 X. X. Zhao and D. W. Stephan, Chem. Commun., 2011, 47, 1833–1835.
12 G. Ménard and D. W. Stephan, J. Am. Chem. Soc., 2010, 132, 1796–
1797.
13 G. Ménard and D. W. Stephan, Angew. Chem., Int. Ed., 2011, 50, 8396–
8399.
Conclusions
14 I. Peuser, R. C. Neu, X. Zhao, M. Ulrich, B. Schirmer, G. Kehr,
R. Fröhlich, S. Grimme, G. Erker and D. W. Stephan, Chem.–Eur. J.,
2011, 17, 9640–9650.
15 R. C. Neu, E. Otten and D. W. Stephan, Angew. Chem., Int. Ed., 2009,
48, 9709–9712.
The FLP-CO2 species tBu3P(CO2)B(C6F5)2Cl 1 can be deriva-
tized via exchange of the halide for triflate to give 2. Alterna-
tively, the borane is readily exchanged for the alane Al(C6F5)3 or
the Ti-cation [Cp2TiCl] affording compounds
3 and 4
9018 | Dalton Trans., 2012, 41, 9016–9018
This journal is © The Royal Society of Chemistry 2012