An OCO3- trianionic pincer tungsten(VI) alkylidyne: Rational design of a highly active alkyne polymerization catalyst

Article abstract of DOI:10.1021/ja2117975

Synthesis, characterization, and catalytic alkyne polymerization results for the first trianionic pincer alkylidyne complex, [^tBuOCO]W ≡ CC(CH₃)₃(THF)₂ (6), are described. Complex 6 is a highly active catalyst for the polymerization of acetylenes and exhibits a high turnover number (4371), activity (1.05 × 10⁶ g _PPA mol_cat^-1 h^-1),and yield (87%) for the polymerization of 1-ethynyl-4-fluorobenzene.

Full text of DOI:10.1021/ja2117975

Communication
pubs.acs.org/JACS
An OCO3− Trianionic Pincer Tungsten(VI) Alkylidyne: Rational Design
of a Highly Active Alkyne Polymerization Catalyst
Soumya Sarkar, Kevin P. McGowan, Subramaniam Kuppuswamy, Ion Ghiviriga, Khalil A. Abboud,
and Adam S. Veige*
Center for Catalysis, Department of Chemistry, University of Florida, Gainesville, Florida 32611, United States
S
* Supporting Information
ization catalysts.11 Each mechanism shares a common feature: a
second substrate must bind to a metallocyclobutadiene
intermediate. Thus, ligands capable of forcing open or labile
ABSTRACT: Synthesis, characterization, and catalytic
alkyne polymerization results for the first trianionic pincer
alkylidyne complex, [tBuOCO]WCC(CH3)3(THF)2
metal coordination sites should promote alkyne polymerization.
Trianionic pincer ligands12 constrain three anionic donors
(6), are described. Complex 6 is a highly active catalyst
for the polymerization of acetylenes and exhibits a high
to a meridional plane and are capable of supporting high-
turnover number (4371), activity (1.05 × 106 gPPA molcat−1 h−1),
oxidation-state metal complexes with vacant coordination sites
and M−X multiple bonds (X = N, O, C).13 Thus, coordination
and yield (87%) for the polymerization of 1-ethynyl-
4-fluorobenzene.
of a second alkyne to a metallacyclobutadiene intermediate bearing
a trianionic pincer should be feasible (Figure 1). Building on initial
solable high-oxidation-state alkylidyne complexes continue
I
to receive significant attention for applications in catalysis,1
especially now that some are commercially available and syn-
thetic protocols are established for easily accessible ver-
sions.2 One application is the polymerization of unsaturated
substrates. Polyacetylenes (PAs) and polyphenylacetylenes (PPA)
are an important class of functional materials that possess unique
properties, including electrical conductivity, paramagnetic sus-
ceptibility, optical nonlinearity, photoconductivity, gas perme-
ability, liquid crystallinity, and chain helicity.3 Catalysts
containing a metal−alkyl bond polymerize alkynes via an
insertion mechanism.3a−d Alternatively, transition-metal alkyli-
denes4 and metal halide multicomponent catalysts that form a
MC bond in situ3c operate via a metathesis mechanism,
including examples capable of living polymerization.5
Figure 1. Illustration of how trianionic pincer ligands can facilitate
coordination of a second alkyne.
progress in synthesizing trianionic pincer alkylidene complexes,13e
we now describe the synthesis of the first OCO3− trianionic
pincer alkylidyne and its catalytic application in PPA synthesis.
Alcoholysis between 1 equiv of (tBuO)3WCtBu (2) and
terphenyl diol 1 (denoted as [tBuOCO]H3) in THF affords
[tBuOCHO]WCC(CH3)3(OtBu)(THF) (3) as a yellow cry-
stalline solid in 72% yield (eq 1). Figure 2 depicts the structure
Alkylidyne complexes also polymerize alkynes via metathesis6
and insertion7 mechanisms. One proposed insertion pathway
involves ring expansion (Scheme 1).7d,8 Initial alkylidyne−alkyne
Scheme 1. Proposed Ring Expansion Polymerization of
Alkynes
of 3 determined by single-crystal X-ray diffraction (XRD). The
pincer ligand binds to the metal ion in the diphenolate form,
creating a five-coordinate W(VI) ion in a distorted trigonal-
bipyramidal (tbp) geometry with an Addison parameter τ = 0.85.14
The pincer ligand chelates through the phenolate donors in
the tbp equatorial sites, creating a large O1−W1−O2 angle of
cycloaddition provides the prototypical metallacyclobutadiene
intermediate. Next, a second acetylene monomer co-
ordinates and inserts to expand the ring.9 The subsequent
steps involve similar ring expansion to generate larger MCx
rings (x odd) with alternating single and double bonds.7b,d
Schrock and co-workers proposed an alternative ring expansion
step via an associative mechanism for catalysts bearing smaller
fluorinated alkoxides.10 Also, deprotonation of the metal-
locyclobutadiene intermediate or protonation of the alkylidyne
can lead to alkylidene species that are active alkyne polymer-
t
135.65(9)°. A BuO− ligand occupies the remaining equatorial
Received: December 18, 2011
Revised: February 17, 2012
Published: February 21, 2012
© 2012 American Chemical Society
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Journal of the American Chemical Society
Communication
and the W−Cpincer carbon, respectively. Consistent with
documented examples such as {[tBuOCO]MoCHC-
(CH3)2Ph(NAr)}{Ph3PCH3}18 and {MoCC(CH3)3(NAr)-
(OCCH3(CF3)2)2}{Ph3PCH3},19 the 31P{1H} NMR spectrum
of 4 reveals a singlet at 21.6 ppm for the phosphonium
countercation.
Treating 4 with methyl triflate in Et2O results in O-alkylation
of the alkoxide, providing the coordinatively un-
saturated neutral trianionic pincer alkylidyne [tBuOCO]W
CC(CH3)3(Et2O) (5) as a red-orange solid (eq 3). Phosphonium
Figure 2. Structure of [tBuOCHO]WCC(CH3)3(OtBu) (THF) (3).
coordination site with correspondingly smaller angles [O3−
W1−O2 = 103.51(8)° and O3−W1−O1 = 104.21(9)°]. THF
and the alkylidyne fragment occupy axial sites with a near-linear
C35−W1−O4 bond angle of 173.30(1)° and a W1−C35 bond
length of 1.773(4) Å. To the best of our knowledge, the
W1−O4 bond length of 2.458(3) Å is the longest reported for a
THF coordinated to W(VI); such bond elongation is
attributable to the trans influence of the tungsten−alkylidyne.
The longest crystallographically characterized W−THF bond
found for W(IV) [2.571(3) Å] is in the paddlewheel complex
W2(μ2-O2CAr)4(THF)2 [Ar = p-(OMe)C6H4].15 Presumably,
the W−W quadruple bond imparts an even stronger trans
influence on the W−THF bond.
triflate ([Ph3PCH3][OSO2CF3]) formed during the alkylation
partially precipitated as a white solid but could not be
completely removed. The 1H NMR spectrum of the bulk
material reveals resonances associated with 5 and a doublet at
1.85 ppm (JP−H = 12.8 Hz) attributable to the methyl protons
of residual phosphonium triflate. Resonances consistent with
Cs-symmetric 5 appear at 0.67 and 1.95 ppm for the alkylidyne
and Bu protons of the ligand, respectively. Signals for the
coordinated diethyl ether appear at 1.12 and 3.27 ppm.
Single crystals amenable for XRD interrogation deposit from
a concentrated Et2O solution of 5 at −35 °C. Figure 3 depicts
the solid-state molecular structure of 5, in which the trianionic
t
1
In solution, complex 3 exhibits a H NMR spectrum indic-
t
ative of Cs symmetry. Three singlets attributable to the Bu
protons of the pincer, the tert-butoxide, and the alkylidyne
appear at 1.67, 1.49, and 0.86 ppm, respectively. Consistent
with documented examples of metal complexes bearing a
diphenolate, the Cipso−H appears downfield at 8.74 ppm
(toluene-d8, −65 °C).12a,16 The most salient feature in the
13C{1H} NMR spectrum is a resonance at 291.7 ppm for the
alkylidyne carbon.
Treating 3 with methylene(triphenyl)phosphorane (Ph3P
CH2) in Et2O at 25 °C precipitates the analytically pure
trianionic pincer alkylidyne salt {[tBuOCO]WCC-
(CH3)3(OtBu)}{PPh3CH3} (4) as a canary-yellow micro-
crystalline solid in 83% yield (eq 2). Complex 4 is the first
Figure 3. Structure of [tBuOCO]WCC(CH3)3(Et2O) (5).
pincer ligand is bound to the W(VI) ion in a tridentate
meridional fashion. The W(VI) ion is in a square-pyramidal
geometry with a calculated Addison parameter τ = 0.14.14 The
W(VI) ion resides 0.49 Å above the O1−O3−O2−C12 basal
plane. The W−Cpincer bond length of 2.134(5) Å is normal
compared to other trianionic pincer tungsten complexes.13e The
coordinated Et2O is rather weakly bound to the metal, as
evidenced by the long W1−O3 bond of 2.185(2) Å. The most
comparable complex is W(OAr)4Cl(Et2O)20 (Ar = 2,6-
Cl2C6H3), with a W(V)−OEt2 bond length of 2.104(13) Å.
Mimicking the OCO arrangement of the donor ligands, the closely
related bisaryloxide alkyl complex (ArO)2(CH3)3CCH2W
CC(CH3)3 contains a W(VI)C bond length of 1.755(2) Å,2e
which is only slightly longer than the W1−C27 bond length of
1.743(3) Å in 5.
trianionic-pincer-supported metal alkylidyne.17 A combination
1
of H, 13C{1H}, 31P{1H} NMR and combustion analyses per-
mits the unambiguous assignment of 4 [see the Supporting
Information (SI)]. A distinct singlet at 1.97 ppm for the 18 tBu
protons of the OCO3− ligand indicates that 4 is Cs-symmetric in
solution. The mirror plane bisects the terphenyl pincer and
contains all three W−Cpincer, W−OtBu, and WCC(CH3)3
bonds. The tBu protons of the alkylidyne appear upfield at 0.91
t
ppm, and the Bu protons of the alkoxide appear as a broad
signal at 2.03 ppm. A doublet at 1.83 ppm (JP−H = 12.8 Hz)
corresponds to the methyl protons in the phosphonium cation.
Two distinct resonances in the 13C{1H} NMR spectrum at
305.3 and 205.3 ppm correspond to the WCC(CH3)3 carbon
Employing a modified route provides analytically pure
phosphonium triflate-free alkylidyne (eq 4). Addition of 0.1
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Journal of the American Chemical Society
Communication
a
Table 1. Results of PA Polymerization by 6
b
R
yield (%)
activity
TON
Mw (Da)
Mw/Mn
H
66
87
47
6.70 × 105
1.05 × 106
6.20 × 105
3280
4371
2346
35 188
20 480
20 637
4.28
3.65
3.33
F
mL of THF to an ether suspension of 5 and [Ph3PCH3]-
[OSO2CF3] selectively dissolves the tungsten complex over the
salt. Subsequent filtration to remove [Ph3PCH3][OSO2CF3]
and cooling of the filtrate to −35 °C overnight provides
analytically pure dark-red single crystals of [tBuOCO]W
CC(CH3)3(THF)2 (6).
MeO
a
Substrate (1000 μmol) was added to 6 (0.20 μmol) in toluene
(0.5 mL) for 30 min at 25 °C, after which the reaction was quenched
b
and polymer washed with methanol. Units: gPPA molcat h−1.
−1
polymers in CDCl3 reveal broad resonances at 6.7 and 5.8
ppm, confirming the presence of (−CHCPh−)n units.
Integrating the cis-vinyl proton relative to the trans-vinyl
proton and the aromatic protons provides an approximate
value for the cis/trans content of the double bond within
the polymers.21 The R-PPA polymers are orange and contain a
high ratio of cis double bonds (PPA, 90%; p-F-PPA and
p-MeO-PPA, 70−80%). For comparison, Rh catalysts
typically provide high-cis-content PPA with a head-
to-tail microstructure.3b To evaluate the microstructure of
the polymer, the PPA sample was heated at 215 °C and then
examined for the presence of cyclized products according to
A combination of 1H and 13C{1H} NMR spectroscopy,
single-crystal XRD, and combustion analysis permits the
unambiguous identification of 6. Figure 4 depicts the solid-
known methods.22 The H NMR spectrum of the heated PPA
1
reveals signals attributable to 1,3,5- and 1,2,4-triphenylbenzene
in a 94:6 ratio, indicative of a microstructure containing
predominately head-to-tail units. It is plausible that if the ring
expansion mechanism operates, the polymers may be cyclic;
however, we were unable to elucidate their macrostructure.
All three polymers have high Mw (PPA, 35 188 Da; p-F-PPA, 20
480 Da; p-MeO-PPA, 20 637 Da), and the polydispersities (Mw/Mn)
are modest (PPA, 4.28; p-F-PPA, 3.65; p-MeO-PPA, 3.33).3b
More striking is the activity of 6 and its high turnover number
(TON), exemplified in the polymerization of p-F-PA (activity =
1.05 × 106 gPPA molcat−1 h−1, TON = 4371). It is plausible that
metathesis products may form during the time scale of the reac-
tion, but periodic GC analysis of the polymerization solution did
not indicate the presence of diphenylacetylene or ethyne.
In conclusion, trianionic pincer W−alkylidyne complexes are
now synthetically accessible. The critical synthetic strategy is
first to attach the ligand onto the W ion as a diphenolate to
form 3. Prior attempts with the peralkyl Np3WCC(CH3)3
(Np = neopentyl) resulted in Cipso−H bond activation of the
pincer across the WC bond to form an alkylidene.13e Once
the ligand is attached as a diphenolate, simple addition of the
mild base Ph3PCH2 deprotonates the Cipso−H proton to
generate the anionic-pincer-supported alkylidyne {[tBuOCO]-
WCC(CH3)3(OtBu)}{PPh3CH3} (4). Addition of methyl
triflate to 4 in Et2O provides the corresponding neutral
complex [tBuOCO]WCC(CH3)3(Et2O) (5) as a mixture
containing phosphonium triflate. However, addition of THF
displaces the Et2O ligand in 5, thus subtly changing the solu-
bility and allowing the isolation of [tBuOCO]WCC-
(CH3)3(THF)2 (6) in high yield and purity.
Figure 4. Structure of [tBuOCO]WCC(CH3)3(THF)2 (6).
state molecular structure of 6. Unlike 5, the W ion now binds
two THF molecules with W1−O4 and W1−O3 bond lengths
of 2.473(2) and 2.177(2) Å, respectively. Appropriately, the
different bond lengths suggest that the alkylidyne ligand
imparts a stronger trans influence than the aryl M−Cpincer bond.
The pincer ligand is clearly bound in its trianionic form, and the
pincer phenoxide donors span an angle of 153.47(10)°. The
W1−Cpincer bond length of 2.132(3) Å is statistically identical to
the corresponding bond length found in 5 [2.134(5) Å]. The
W(VI)C distance in 6 is 1.759(4) Å, which is statistically
identical to the bond length of 1.755(2) Å found in
(ArO)2(CH3)3CCH2WCC(CH3)3 but 0.011(4) Å longer
than that in 5, perhaps due to a small contribution from the
trans influence of THF.
Broad signals in the 1H NMR spectrum of 6 (C6D6) at 4.07,
3.40, 1.45, and 1.16 ppm, each integrating to four protons,
indicate that two symmetrically unique THF molecules coordi-
t
nate to the W(VI) ion. Distinct resonances for the Bu and
alkylidyne protons appear at 1.65 and 0.61 ppm, respectively,
indicating that 6 is also Cs-symmetric. A downfield resonance at
320.7 ppm in the 13C{1H} NMR spectrum is attributable to the
WC carbon, and a resonance at 193.5 ppm corresponds to
the W−Cpincer
.
Complex 6 catalyzes PPA synthesis (Table 1). Polymer
characterization includes FTIR and 1H NMR spectro-
scopy as well as gel-permeation chromatography (GPC) to
determine Mw and Mw/Mn. Complex 6 polymerizes PA, 1-
ethynyl-4-fluorobenzene (p-F-PA), and 1-ethynyl-4-methoxy-
benzene (p-MeO-PA). The monomers all absorb IR radiation
near 3300 and 2100 cm−1, corresponding to the C−H and
CC stretching vibrations, respectively (see the SI). Both
bands disappear in the IR spectra of the PPAs and are replaced
with a new band near 1595 cm−1. The 1H NMR spectra of the
Complex 6 is a highly active catalyst for polymerization of
phenylacetylenes. Relative to other metal−alkylidyne catalysts
for the polymerization of PAs, 6 is the most active.7a−c The high
activity is attributable to the inherent ligand geometry, which
creates a coordinatively unsaturated catalyst. Constraining the
three anionic donor ligands to the meridional plane, thereby
providing easy access to an open coordination site, promotes the
coordination and insertion of PA units. Furthermore, the tridentate
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Journal of the American Chemical Society
Communication
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trianionic pincer ligand renders the precatalyst sufficiently stable to
be isolable and long-lived (TON >4000), a quality we are currently
exploiting to elucidate the polymerization mechanism.
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ASSOCIATED CONTENT
■
S
* Supporting Information
1H, 13C{1H}, 31P{1H}, and 2D NMR spectra, IR spectra, and
crystallographic data (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
(12) (a) Kuppuswamy, S.; Ghiviriga, I.; Abboud, K. A.; Veige, A. S.
Organometallics 2010, 29, 6711. (b) Golisz, S. R.; Labinger, J. A.;
Bercaw, J. E. Organometallics 2010, 29, 5026. (c) Sarkar, S.; McGowan,
K. P.; Culver, J. A.; Carlson, A. R.; Koller, J.; Peloquin, A. J.; Veige,
M. K.; Abboud, K. A.; Veige, A. S. Inorg. Chem. 2010, 49, 5143.
(d) Sarkar, S.; Carlson, A. R.; Veige, M. K.; Falkowski, J. M.; Abboud,
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AUTHOR INFORMATION
■
Corresponding Author
veige@chem.ufl.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by UF, NSF (CHE-0748408
and CHE-0821346), and the Alfred P. Sloan Foundation.
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