γ-Agostic species as key intermediates in the vinyl addition polymerization of norbornene with cationic (allyl)Pd catalysts: Synthesis and mechanistic insights

Article abstract of DOI:10.1021/ja9025598

Several cationic (allyl)Pd(II) complexes were synthesized and shown to be highly active for (2,3)-vinyl addition polymerization of norbornene (NB) to yield polymers with low molecular weight distributions (MWDs) ranging from 1.2-1.4. Despite the low MWDs,

Full text of DOI:10.1021/ja9025598

Published on Web 05/27/2009
γ-Agostic Species as Key Intermediates in the Vinyl Addition
Polymerization of Norbornene with Cationic (allyl)Pd
Catalysts: Synthesis and Mechanistic Insights
Marc D. Walter, Rebecca A. Moorhouse, Stephanie A. Urbin, Peter S. White, and
Maurice Brookhart*
Department of Chemistry, UniVersity of North Carolina at Chapel Hill, Chapel Hill,
North Carolina 27599-3290
Received April 6, 2009; E-mail: mbrookhart@unc.edu
Abstract: Several cationic (allyl)Pd(II) complexes were synthesized and shown to be highly active for (2,3)-
vinyl addition polymerization of norbornene (NB) to yield polymers with low molecular weight distributions
(MWDs) ranging from 1.2-1.4. Despite the low MWDs, slow initiation was followed by rapid propagation
preventing molecular weight control of the poly(norbornene). Several intermediates in these polymerizations
initiated with [(2-R-allyl)Pd(mesitylene)]+ complexes were fully characterized (NMR and X-ray diffraction).
Consistent with previous observations the allyl and NB units couple in cis-exo fashion to yield a σ,π-complex
capped by mesitylene. Mesitylene is readily displaced by NB to form an agostic intermediate in which NB
acts as a bidentate ligand and binds to the cationic Pd center via the π-system and a γ-agostic interaction
with the syn hydrogen at C7. The identity of this complex was established by NMR spectroscopy and
single-crystal X-ray diffraction. It is significant since it suggests bidentate binding of NB in the propagating
species, which cannot be observed by NMR spectroscopy. The NMR studies suggest that the second
insertion, i.e., insertion of NB in the agostic intermediate, is the slow initiation step and the subsequent
insertions are extremely fast. Therefore, slow chelate opening is the major limitation preventing a living
polymerization. This hypothesis was explored using a series of cationic substituted π-allyl complexes;
significantly increased reactivity was observed when electron-withdrawing groups were introduced into the
allyl moiety. However, despite these modifications initiation remained slow relative to chain propagation.
NB units.6 Vinyl addition polymerization circumvents both
problems and yields a completely saturated high-molecular-
Introduction
Intensive research efforts have been devoted to the polym-
weight polymer with no rearranged NB units. Thus coordination-
insertion polymerization of NB and NB derivatives has been
examined by many groups, and a number of catalysts based on
early and late transition metals have emerged. Addition of
cocatalysts (usually methylaluminoxane (MAO) or B(C6F5)3)
is often required for most early and late transition metal
complexes. However, late metal nickel and palladium complexes
have been reported which do not require cocatalyst addition.2-4
Included in this category are cationic “naked”-type nickel or
palladium complexes or multicomponent systems able to
generate similar species, neutral nickel or palladium complexes
bearing highly electrophilic groups, and cationic palladium
complexes.7 In this context the cationic “ligandless” [(allyl)M-
(COD)][PF6] (M ) Ni or Pd) systems are the most active
catalysts for NB polymerization to date. However, NMR studies
revealed a major problem of this catalyst system: polymer was
rapidly formed on exposure of NB to the catalyst, but only very
little catalyst was consumed, indicating a slow initiation step
erization of cyclic olefins mainly motivated by the useful
properties of the resulting materials which include high glass
transition temperatures, high optical transparency, low dielectric
constants, and low birefringence. Various classes of polymers
may be obtained from norbornene (NB, bicyclo[2.2.1]hept-2-
ene) and its derivatives by a suitable choice of catalysts that
operate via ring-opening metathesis polymerization (ROMP),
cationic or radical polymerization, and vinyl addition polymeri-
zation.1-4 A disadvantage of ROMP-derived poly(norbornene)
is the unsaturation in the backbone which makes hydrogenation
or vulcanization of the polymer necessary for some applica-
tions.5 Cationic and radical polymerizations normally provide
low-molecular-weight polymers which incorporate rearranged
(1) Goodall, B. L. In Late Transition Metal Polymerization Catalysis;
Rieger, B., Saunders Baugh, L., Kacker, S., Striegler, S., Eds.; Wiley-
VCH: Weinheim, 2003; pp 101-154.
(2) Janiak, C.; Lassahn, P. G. J. Mol. Catal. A: Chem. 2001, 166, 193–
209.
(6) Karafilidis, C.; Hermann, H.; Rufin˜ska, A.; Gabor, G.; Mynott, R. J.;
Breitenbruch, G.; Weidenthaler, C.; Rust, J.; Joppek, W.; Brookhart,
M. S.; Thiel, W.; Fink, G. Angew. Chem., Int. Ed. 2004, 43, 2444–
2446.
(3) Janiak, C.; Lassahn, P. G. Macromol. Rapid Commun. 2001, 22, 479–
492.
(4) Blank, F.; Janiak, C. Coord. Chem. ReV. 2009, 253, 827–861.
(5) Ohm, R.; Stein, C. In Kirk-Othmer Encyclopedia of chemical technol-
ogy; Wiley: New York, 1982; Vol. 18, pp 436-442.
(7) Kaminsky, W.; Arndt-Rosenau, M. In StereoselectiVe Polymerization
with Single-Site Catalysts; Baugh, L. S., Canich, J. A. M., Eds.; CRC
Press: Boca Raton, FL, 2008, p 412-444.
9
10.1021/ja9025598 CCC: $40.75  2009 American Chemical Society
J. AM. CHEM. SOC. 2009, 131, 9055–9069 9055

A R T I C L E S
Walter et al.
Scheme 1
followed by very rapid propagation. This is also reflected in
the lack of molecular weight control and the very broad
molecular weight distributions (MWDs) for the poly(nor-
bornene) polymers synthesized by these catalysts ranging from
2 to 3. Despite the fact that no spectroscopic information on
the “naked” Ni(II)/Pd(II) active species is available, it was
suggested to contain only the growing polymer chain and two
NB monomers. While the high activity Ni system prevented
the detection of any intermediates, the first insertion product
was observed for the analogous Pd system1 in which the Pd-C
allyl bond had added in cis-exo fashion across the double bond
of NB, consistent with earlier observations by Hughes and
Powell.8 These observations highlighted an important challenge,
namely the development of ligandless catalysts in which
initiation is rapid and molecular weight can be controlled. Under
these conditions, the structure of the propagating species might
also be discerned.
readily in a cis-exo fashion to yield a σ,π-complex capped by
mesitylene. However, NMR studies showed that this coupling
event is reversible and also suggested that the second insertion
is the slow initiation step, while propagation following the
second insertion is extremely fast, preventing a living polym-
erization. One of the reasons for the slow second insertion may
be attributed to slow mesitylene displacement.14 In order to
overcome this limitation we investigated the analogous [(2-R-
allyl)Pd(mesitylene)]+ complexes in which the mesitylene ligand
is very labile and can readily be displaced by other arenes,
olefins, and alkynes.13 The reactivity of these Pd catalysts in
NB polymerization is reported herein.
Results and Discussion
Catalyst Preparation. Complex 2-Cl has been reported
previously,13 and complex 2-CO2Me was prepared following
a similar procedure (Scheme 1, see Experimental Section for
details).
More recently, two cationic Pd complexes have been intro-
duced which appear to catalyze the polymerization of NB or
Complex (3-Me)2 is an excellent starting material for the
synthesis of compounds 4-Me and 5-Me, and it is readily
prepared by heating a benzene solution of (1-Me)2 to 50 °C
overnight in the presence of NB to yield colorless crystals of
(3-Me)2 (Scheme 2).15 A single crystal X-ray diffraction study
of (3-Me)2 confirmed coupling of NB to the allyl fragment in
cis-exo fashion, as was also observed for [(2-R-allyl)Pd(hfacac)]
(hfacac ) hexafluoroacetylacetonato) complexes8 and (3-H)2.16
Selected bond distances for (3-Me)2 are given in the figure
caption (Figure 1) and they compare well to those reported for
(3-H)2.16 While (3-Me)2 is only slightly soluble in chlorinated
hydrocarbons such as CHCl3 and CH2Cl2, it is moderately
soluble in pyridine and forms a monomeric, neutral pyridine
adduct 3-Me(py). Metathesis of (3-Me)2 with AgSbF6 in the
presence of pyridine or mesitylene yields 4-Me and 5-Me,
respectively. Both molecules have been characterized by various
NMR techniques and X-ray crystallography (Figures 2 and 3).
Selected bond distances are given in the figure captions (see
Supporting Information for details). Note the significant ring-
slippage of the mesitylene ligand in 5-Me from idealized η6-
coordination (Pd-Carene range of distances: 2.450(6)-2.677(6)
Å) which is observed for the analogous Ni complex (Ni-Carene
range of distances: 2.149(6)-2.293(6) Å). This might rationalize
the lability of mesitylene with respect to exchange with excess
of mesitylene, other arenes and ethers. No reversible C-C bond
formation was observed for (3-Me)2, 4-Me, or 5-Me in contrast
to their analogue Ni complexes14 which might be due to reduced
steric strain compared to their Ni congeners.
NB derivatives in
a controlled manner. The cationic
[Pd(CH3CN)4][BF4]2 complex generates poly(norbornene) with
9
MWDs of 1.07-1.34 and Mn of 11 000-36 000 g mol-1
.
However, the mechanistic details, e.g., the catalytic active
species, are not very well established and the insensitivity of
this catalytic system to water addition suggests that an in situ
generated cationic “Pd-H” intermediate as the initiating species.
The other example is (tBu3P)Pd(Me)(Cl) which, on addition of
1 equiv of NaB(ArF)4, forms the cationic bimetallic species
{[(tBu3P)(Me)Pd]2(µ-Cl)}{B(ArF)4} that polymerizes NB and
functionalized NBs.10-12 Although the low solubility of poly(nor-
bornene) prevented the determination of its molecular weight,
it is noteworthy that in this case decreased catalyst loading
results in a dramatic increase in apparent turnover frequency,
arguing for slow initiation followed by rapid propagation.10
However, a living polymerization was achieved for the more
soluble polymers derived from the methyl ester of norbornene
carboxylic acid with Mn’s reaching 12 000 g mol-1 at 0.86 mol
% catalyst loading and a MWD of 1.2.10
We have recently reported the synthesis and NB polymeri-
zation activity of [(2-R-allyl)Ni(mesitylene)]+ complexes.13,14
For these complexes the only detectable polymerization inter-
mediate fully characterized (by NMR and X-ray diffraction) is
the first insertion product in which the allyl and NB units couple
(8) Hughes, R. P.; Powell, J. J. Organomet. Chem. 1973, 60, 387–407.
(9) Mehler, C.; Risse, W. Macromolecules 1992, 25, 4226–4228.
(10) Yamashita, M.; Takamiya, I.; Jin, K.; Nozaki, K. Organometallics
2006, 25, 4588–4595.
Polymerizations Results. The cationic Pd complexes described
above have been investigated with respect to NB polymerization
(Chart 1), and the results are summarized in Tables 1-4.
(11) Takamiya, I.; Yamashita, M.; Nozaki, K. Organometallics 2008, 27,
5347–5354.
(12) Takamiya, I.; Yamashita, M.; Murotani, E.; Morizawa, Y.; Nozaki,
K. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 5133–5141.
(13) O’Connor, A. R.; Urbin, S. A.; Moorhouse, R. A.; White, P. S.;
Brookhart, M. Organometallics 2009, 28, 2372–2384.
(14) Walter, M. D.; Moorhouse, R. A.; White, P. S.; Brookhart, M. J. Polym.
Sci., Part A: Polym. Chem. 2009, 47, 2560–2573.
(15) Gallazzi, M. C.; Hanlon, T. L.; Porri, L. J. Organomet. Chem. 1971,
33, C45–C46.
(16) Zocchi, M.; Tieghi, G. J. Chem. Soc., Dalton 1979, 944–947.
9
9056 J. AM. CHEM. SOC. VOL. 131, NO. 25, 2009

Polymerization of Norbornene with (allyl)Pd Catalysts
A R T I C L E S
Scheme 2
All catalysts exhibit good to high activity in the polymeri-
zation of NB. In addition, low MWDs ranging from 1.2 to 1.4
were obtained independent of conversion, but unfortunately, the
molecular weight is observed to be nearly independent of
conversions and the Mn is larger than expected for living
polymerization based on the moles monomer/moles catalyst
ratio, e.g., a ratio of 200:1 should give polymer with an Mn of
ca. 20 000 g mol-1 at 100% conversion. These observations
indicate that the investigated catalyst systems suffer from slow
initiation and rapid propagation, and only a small percentage
of the catalyst is active for polymerization. However, the catalyst
reactivity can be ordered in the following manner: 2-Cl >
2-CO2Me . 4-Me ≈ 5-Me. Most apparent is the fact that the
catalyst bearing electron-withdrawing groups (R ) Cl and
CO2Me) exhibit higher activity than the system with an electron-
donating substituent (R ) Me). This implies a more rapid
initiation for 2-Cl and 2-CO2Me compared to 4-Me and 5-Me
since after 2-3 insertions all species should show the same
propagating species. Considering that previous studies in our
laboratory have shown that the mesitylene displacement in 2-R
(R ) H, Me and Cl) was rapid when exposed to olefins and
alkynes,13 we decided to investigate the reason for the substantial
Figure 1. ORTEP diagram of (3-Me)2 (50% probability ellipsoids).
Hydrogen atoms have been omitted for clarity. Selected bond distances (Å):
Pd1-C1 2.152(3), Pd1-C2 2.187(3), Pd1-C9 2.042(3), Pd1-C l1
2.3754(8), Pd1-Cl_2 2.5793(8). Atoms with labels bearing “_2” are
generated by the symmetry operation: -x, -y, -z.
Figure 2. ORTEP diagram of the first insertion product 4-Me (50%
probability ellipsoids). Hydrogen atoms and the [SbF6]- anion have been
omitted for clarity. Selected bond distances (Å): Pd1-C1 2.166(6), Pd1-C2
2.234(6), Pd1-C10 2.045(6), Pd1-N12 2.191(5), Pd1-N18 2.090(5).
9
J. AM. CHEM. SOC. VOL. 131, NO. 25, 2009 9057

A R T I C L E S
Walter et al.
Reactivity of 4-Me with B(C6F5)3 and the Generation of the
Active Catalyst Species. We have recently used [(2-R-
allyl)Ni(NCMe)2][B(ArF)4] (R ) H or Me and [B(ArF)4]-
)
[B(C6H3(CF3)2)4]-, as a precursor to generate “ligand-free”
complexes of the type [(2-R-allyl)Ni+(ArF)B-(ArF)3] on addition
of a Lewis acid, B(C6F5)3. In these complexes one of the aryl
rings of the [B(ArF)4]- ligand is weakly coordinated to the
cationic Ni(II) center and it is readily displaced to generate
highly active catalysts for the polymerization of 1,3-dienes such
as butadiene and isoprene and NB to yield highly cis enchained
polymer18 and high-molecular-weight poly(norbornene), respec-
tively.14
No polymerization activity is observed for 4-Me in the
absence of B(C6F5)3 similar to [(2-R-allyl)Ni(NCMe)2][B(ArF)4].
However, on addition of B(C6F5)3 the activated complex
polymerizes NB at 23 °C with 150 TOs within 1 h. Although
the polymer samples exhibited narrow MWDs of about 1.4, no
molecular weight control was achieved (Table 4).
Figure 3. ORTEP diagram of the first insertion product 5-Me (50%
probability ellipsoids). Hydrogen atoms and the [SbF6]- anion have been
omitted for clarity. Selected bond distances (Å): Pd1-C1 2.145(6), Pd1-C2
2.180(5), Pd1-C10 2.063(5), Pd1-C12 2.450(6), Pd1-C14 2.331(6),
Pd1-C15 2.435(6), Pd1-C17 2.607(6), Pd1-C18 2.677(6), Pd1-C20
2.594(6).
In an attempt elucidate the identity of the active species in
the vinyl addition polymerization of NB, NMR studies were
undertaken. Colorless CD2Cl2 solutions of 4-Me immediately
undergo a color change to yellow on addition of B(C6F5)3 (4
equiv) and the 19F NMR spectrum shows resonances attributable
to the py ·B(C6F5)3 adduct (δ -131.9, -157.4, -164.0), free
borane (δ -127.2, -142.9, -160.4), and a third species that
Chart 1
3
shows resonances at δ -139.2 (d, JFF ) 19 Hz), -154.5 (t,
3JFF ) 21 Hz), and -162.7 (dd, 3JFF ) 19 and 21 Hz). The 19
F
NMR chemical shifts of the py ·B(C6F5)3 adduct are in good
agreement with the ones reported for the DMAP·B(C6F5)3
adduct of δ -131.7, -157.6, and -163.5.19 The last set of
resonances at δ -139.1, -154.5, and -162.7 may be attributed
to a py·B(C6F5)3 adduct which coordinates to the cationic [(2-
Me-propenyl-norbornyl)Pd]+ species in analogy to [(2-R-
allyl)Ni+(ArF)B-(ArF)3] (Scheme 4). The precise coordination
mode of this adduct to the cationic Pd center is hard to elucidate
with certainty, and it might occur via the π-system of either
the pyridine ring or of a fluorinated aryl ring of the borane.
However, coordination to the π-system of the pyridine moiety
seems to be the most reasonable assumption consistent with
the NMR spectroscopic results. On cooling of the reaction
mixture in the NMR probe in an attempt to freeze out any
fluctionality, precipitation occurred most likely due to the low
solubility of the pyridine-derived B(C6F5)3 adducts19 which
makes low-temperature NMR studies to either elucidate dynam-
ics or the reactivity toward NB very challenging. Regardless
of the nature of the coordination, the pyridine-borane adduct
serves as a capping ligand for the cationic [(2-Me-propenyl-
norbornyl)Pd]+ species similar to [(2-R-allyl)Ni+(ArF)B-(ArF)3]
and therefore activating complex 4-Me for the polymerization.
Since the catalytic activity of 4-Me/B(C6F5)3 is similar to 5-Me
but requires the addition of cocatalyst, the reactivity of 5-Me
toward NB will be considered in the following section.
difference in polymerization activity using low-temperature
NMR spectroscopy.
Mechanistic Studies. Insertion of NB into the Pd-Allyl
Bond in (1-Me)2. We have recently demonstrated that the
coupling event between NB and the allyl fragment in [(2-Me-
allyl)Ni(µ-Cl)]2 is a reversible process which occurs at low
temperatures. No intermediates were detected by NMR spec-
troscopy during the insertion and deinsertion process (Scheme
3a).14 However, insertion for the Pd system (1-Me)2 occurs at
higher temperatures compared to the analogous Ni system which
might allow intermediates to be discerned by NMR spectroscopy.
When 2 equiv of NB were added to a CD2Cl2 solution of
(1-Me)2 at -94 °C, vinylic resonances due to both free (δ 5.93)
3
and bound (δ 5.26 and 5.14, JHH ) 3.5 Hz) NB are detected.
All four allyl resonances in the NB π-adduct are inequivalent
and resonate at δ 4.43, 4.35, 3.47, and 2.90 (Figure S1,
Supporting Information). As the temperature is raised, these
signals diminish and are nearly undetectable at -29 °C as the
equilibrium shifts torward (1-Me)2 and NB at higher temper-
atures. Broadening of the NB resonance at -20 °C suggests
rapid exchange between NB and the minor amount of 1-Me(NB)
present. Above -20 °C only resonances due to (1-Me)2 and
free NB are observed (Figure S2a, Supporting Information).
However, at room temperature and above, resonances corre-
sponding to the insertion product (3-Me)2 start to appear
(Scheme 3b, also see Figure S2b in the Supporting Informa-
tion).17 The exchange between free and bound NB to form
1-Me(NB) was confirmed by NOESY NMR techniques at low
temperatures (-92 °C). The free energy for this equilibrium
process has been evaluated to be ∆G (-92 °C) ) -1.7 kcal
mol-1 (see Supporting Information for details, Figure S3).
(17) The interaction of (1-Cl)2 toward NB was also investigated, and the
results are similar to (1-Me)2 (see Experimental Section for details).
However, on the experimental scale the reaction did not proceed in
benzene at 50 °C, and in refluxing CH2Cl2 only incomplete conversion
was achieved. Furthermore, prolonged heating did not improve the
conversion; instead polymer and palladium metal formation was
observed.
(18) O’Connor, A. R.; White, P. S.; Brookhart, M. J. Am. Chem. Soc. 2007,
129, 4142–4143.
(19) Lesley, M. J. G.; Woodward, A.; Taylor, N. J.; Marder, T. B.;
Cazenobe, I.; Ledoux, I.; Zyss, J.; Thornton, A.; Bruce, D. W.; Kakkar,
A. K. Chem. Mater. 1998, 10, 1355–1365.
9
9058 J. AM. CHEM. SOC. VOL. 131, NO. 25, 2009

Polymerization of Norbornene with (allyl)Pd Catalysts
A R T I C L E S
Table 1. Polymerization of NB with [(2-Cl-allyl)Pd(mesitylene)][SbF6] (2-Cl)a
b
Mw
entry
time (min)
temp (°C)
cat (mmol)
NB/Ni mol ratio
% conversionb
Mnb
MWDb
TONb
1
2
3
4
5
6
7
2
5
0
0
0
0
0
2.0 × 10-5
2.0 × 10-5
2.0 × 10-5
2.0 × 10-5
2.0 × 10-5
2.0 × 10-5
2.0 × 10-5
234
234
234
234
234
234
467
2
10
58
79
87
77 600
99 200
1.3
1.4
1.3
1.3
1.4
1.2
1.1
5
23
112 000
184 100
231 000
215 300
255 100
165 700
153 000
240 000
310 000
297 100
316 600
202 000
10
15
20
25
5
136
185
204
234
467
0
23
100
100
a TON, (moles monomer)(conversion)/moles catalyst. Polymerization conditions: 10 mL total volume, CH2Cl2 (2 mL); methylcyclohexane (7 mL),
NB stock solution (4.67 M) in methylcyclohexane (1 mL). b Averaged over several trials.
Table 2. Polymerization of NB with [(2-CO2Me-allyl)Pd(mesitylene)][SbF6] (2-CO2Me)a
entry
time (min)
temp (°C)
cat (mmol)
NB/Ni mol ratio
% conversion
Mn
Mw
MWD
TON
1
2
3
4
10
20
25
30
40
50
50
0
0
0
0
0
0
0
2.0 × 10-5
2.0 × 10-5
2.0 × 10-5
2.0 × 10-5
2.0 × 10-5
2.0 × 10-5
2.0 × 10-5
234
234
234
234
234
234
234
3
27
69
71
94
96
90
126 200
232 000
273 000
295 000
274 700
288 400
103 400
176 500
300 000
357 000
367 800
370 000
393 800
135 600
1.4
1.3
1.3
1.3
1.4
1.4
1.3
7
63
161
166
220
225
211
5
6
7b
a TON, (moles monomer)(conversion)/moles catalyst. Polymerization conditions: 10 mL total volume, CH2Cl2 (2 mL), methylcyclohexane (7 mL),
NB stock solution (4.67 M) in methylcyclohexane (1 mL). b Addition of B(C6F5)3 (2 equiv) before NB addition.
a
Table 3. Polymerization of NB with [(2-Me-propenylnorbornyl)Pd(py)2][SbF6] (4-Me)/B(C6F5)3
b
Mw
entry
time (min)
temp (°C)
cat (mmol)
NB/Ni mol ratio
% conversion
Mnb
MWDb
TONb
1
2
3
4
15
30
45
60
23
23
23
23
1.68 × 10-5
1.68 × 10-5
1.68 × 10-5
1.68 × 10-5
150
150
150
150
8
35
75
98
86 000
123 000
150 000
206 000
263 000
1.4
1.4
1.3
1.3
12
53
113
147
106 000
155 000
209 000
a TON, (moles monomer)(conversion)/moles catalyst. Polymerization conditions: 10 mL total volume, CH2Cl2 (2 mL), methylcyclohexane (6 mL),
NB stock solution (1.26 M) in methylcyclohexane (2 mL). b Averaged over several trials.
Table 4. Polymerization of NB with [(2-Me-propenylnorbornyl)Pd(mesitylene)][SbF6] (5-Me)a
b
Mw
entry
time (min)
temp (°C)
cat (mol)
NB/Ni mol ratio
% conversion
Mnb
MWDb
TONb
1
2
3
4
5
6
7
8
9
2
5
7
10
15
20
30
45
60
23
23
23
23
23
23
23
23
23
1.6 × 10-5
1.6 × 10-5
1.6 × 10-5
1.6 × 10-5
1.6 × 10-5
1.6 × 10-5
1.6 × 10-5
1.6 × 10-5
1.6 × 10-5
200
200
200
200
200
200
200
200
200
3
10
18
30
37
41
48
85
100
n/d
n/d
n/d
1.2
1.2
1.2
1.3
1.3
1.3
1.4
1.4
6
20
36
60
74
82
96
170
200
380 000
430 000
466 000
526 000
564 000
616 000
656 000
663 000
460 000
496 000
560 000
660 000
723 000
791 000
907 000
917 000
a TON, (moles monomer)(conversion)/moles catalyst. Polymerization conditions: 10 mL total volume, CH2Cl2 (2 mL), methylcyclohexane (6 mL),
NB stock solution (1.60 M) in methylcyclohexane (2 mL). b Averaged over several trials.
Reactivity of 2-Me and 5-Me toward NB. Addition of 1 equiv
of NB at -120 °C to a CDCl2F solution of 2-Me immediately
forms the first insertion product 5-Me, and the chemical shifts
agree with independently synthesized 5-Me (Figure S4 in
Supporting Information). This observation indicates that the first
insertion is not rate-determining under polymerization conditions
and thus 2-Me and 5-Me will exhibit similar catalytic activities.
Addition of another equivalent of NB to this reaction mixture
or to solutions of isolated 5-Me leads to instantaneous displace-
ment of mesitylene to yield 7-Me (Scheme 5). This behavior is
consistent with our reactivity studies on 2-Me in which the
mesitylene ligand is rapidly displaced by olefins and alkynes
to form bis(olefin) and bis(alkyne) adducts, respectively.13
However, 5-Me behaves differently relative to the analogous
Ni species for which the mesitylene exchange is too slow to be
observed on the NMR time scale.14 Furthermore, the NMR data
obtained for 7-Me are inconsistent with a 2:1 adduct such as
[(2-Me-propenylnorbornyl)Pd(π-norbornene)2]+ species which
might be expected based on our studies of the reactions of olefins
and alkynes with 2-Me13 but are more consistent with a 1:1
adduct. The addition of two equivalents of NB relative to Pd
induces a rapid exchange between free and bound NB that
cannot be completely frozen out at temperatures as low as -140
°C, and the formation of a 2:1 adduct cannot be observed.
The best resolved spectra of 7-Me were obtained when only
ca. 0.9 equiv of NB was added to 5-Me at -120 °C (Figure
S5, Supporting Information). Various 1D and 2D NMR tech-
niques were employed to gain structural information on 7-Me.
The vinyl protons of the coordinated NB are inequivalent and
resonate at δ 6.19 and 5.81. However, the most notable feature
1
in the resulting H NMR spectrum of 7-Me is a high-field
doublet at δ -1.28 (2JHH) 11.5 Hz) of intensity 1 which
correlates in the 1H-1H COSY NMR spectrum to a doublet at
δ 1.52 (2JHH) 11.5 Hz), also of intensity 1 (Figure S6,
Supporting Information). Additionally, both protons are attached
to the same carbon atom resonating at δ 42.5 as confirmed by
9
J. AM. CHEM. SOC. VOL. 131, NO. 25, 2009 9059

A R T I C L E S
Walter et al.
Scheme 3
Me2C6H3)NC(Me)CHC(Me)N(C6H3Me2-2,6))23 and a cationic [(di-
en)Cu(norbornene)]+ (dien ) HN(CH2CH2NH2)2) complex.24 The
constrained bicyclic structure of NB aligns C15-H15b nearly
perfectly to fill the fourth coordination site around the square-planar
cationic Pd center leading to a short Pd1-C15 distance of 2.756(6)
Å and a Pd1-H15b distance25 of 2.186(73) Å.
Scheme 4
The observation of 7-Me as an intermediate in the NB
polymerization process is significant since a similar agostic
interaction has been invoked to explain the 2-exo,7′-syn linkage
in NB oligomers generated by a cationic zirconocene catalyst,6
and it also suggests an important structural motif with respect
to the catalytically active species during polymerization. Al-
though we were unable to detect other intermediates in the
polymerization process by NMR spectroscopy, we suggest that
the bidentate coordination mode of NB and/or an agostic
interaction involving the growing polymer chain also prevail
in the propagating species. This contradicts Goodall’s proposed
active site in which the “naked” Ni(II)/Pd(II) species is believed
to contain only the growing polymer chain and two NB
monomers bound exclusively via the π-system to form an
unlikely 14 valence electron intermediate.1
a
1H-13C HSQC NMR experiment (Figure S7a and S7b,
Supporting Information). These spectroscopic data support the
formulation of 7-Me as a species in which one NB molecule
binds via the π-system and a γ-agostic interaction to the syn
hydrogen atom at C7. Therefore, NB can act as a bidentate
ligand (Scheme 6). This interaction is further supported by the
1JCH coupling constants for the agostic and nonagostic hydrogen
atom attached to C7 of 110 and 160 Hz, respectively (Figure
S8, Supporting Information).20,21
However, the final piece of evidence came from a crystal
structure analysis of 7-Me isolated from the NMR sample of 5-Me
and NB (∼0.9 equiv) in CDCl2F. For this purpose, the yellow
CDCl2F solution was layered with pentane and kept at -80 °C
for 3 days to yield light yellow crystals of 7-Me (Figure 4).
The structural motif of a NB molecule acting as a bidentate
ligand binding via the olefin funtionality and an agostic interaction
to the methylene bridge is rare, but has been observed in a neutral
trinuclear Ru cluster,22 [(nacnac)Rh(norbornene)] (nacnac ) (2,6-
5-Me and NB (ratio ≈ 1:1.1) were mixed at -140 °C
followed by warming and monitoring 7-Me by NMR: At -130
°C, the exchange between bound and the small fraction of free
NB is nearly frozen out and the vinylic resonances of the bound
NB, as well as its agostic CH resonance, are observed at the
(23) Budzelaar, P. H. M.; Moonen, N. N. P.; de Gelder, R.; Smits, J. M. M.;
Gal, A. W. Eur. J. Inorg. Chem. 2000, 753–769.
(24) Pasquali, M.; Floriani, C.; Gaetani-Manfredotti, A.; Chiesi-Villa, A.
J. Am. Chem. Soc. 1978, 100, 4918–4919.
(20) Brookhart, M.; Green, M. L. H. J. Organomet. Chem. 1983, 250, 395–
408.
(25) The “X-ray determined” bond length is a measure of the distance
between the centroids of the electron density of the two atoms
concerned, and the centroid of the electron density around a covalently
bonded hydrogen atom is not coincident with its nuclear position but
is displaced significantly in the direction of the hydrogen-(other atom)
σ-bond. (Churchill, M. R. Inorg. Chem. 1973, 12, 1213–1214.)
(21) Brookhart, M.; Green, M. L. H.; Parkin, G. Proc. Natl. Acad. Sci.
U.S.A. 2007, 104, 6908–6914.
(22) Brown, D. B.; Cripps, M.; Johnson, B. F. G.; Martin, C. M.; Braga,
D.; Grepioni, F. Chem. Comm. 1996, 1426–1427.
9
9060 J. AM. CHEM. SOC. VOL. 131, NO. 25, 2009

Polymerization of Norbornene with (allyl)Pd Catalysts
A R T I C L E S
Scheme 5
Scheme 6
As mentioned above, coordinated NB in 7-Me exchanges
rapidly on the NMR time scale. In the presence of a significant
excess of free NB, the exchange is fast even at temperatures as
low as -150 °C, and on warming, rapid polymer formation is
observed above -20 °C. While no additional intermediates can
be detected by NMR spectroscopy, free mesitylene never
competes with free NB as a ligand for the cationic Pd center.
These observations are consistent with a slow second insertion,
viz., the opening of the olefin-chelate is rate-determining, relative
to chain propagation.
On the basis of these studies, we reasoned if indeed chelate
opening is the rate-determining step dictating slow insertion of
a second NB monomer, the introduction of electron-withdrawing
or sterically demanding groups into the allyl fragment should
destabilize the chelate. The hypothesis regarding the influence
of electron-withdrawing groups on the polymerization process
is indirectly supported by the bulk polymerization results
described for catalysts 2-Cl and 2-CO2Me which are both
significantly more active than 5-Me and 4-Me/B(C6F5)3 (Tables
1-4). However, the decrease in reactivity moving from 2-Cl
to 2-CO2Me remains unexplained and appears to be counter-
intuitive: The ester functionality is a stronger electron-withdraw-
ing group than chloride, and therefore, an increased reactivity
might be expected.26 To address this discrepancy we investigated
the interaction of 2-Cl and 2-CO2Me with NB by low-
temperature NMR experiments.
same positions as described above when a 1:0.9 ratio was used.
On warming, the two vinylic resonances of the bound NB begin
to broaden, reach coalescence at -86 °C, and then sharpen with
increasing temperature to a single resonance. Loss of NB from
Pd renders the vinylic hydrogens equivalent, and thus, the
coalesced signal represents the weighted average of the two
bound vinyl signals and the vinyl signals of the small fraction
of free NB present. Loss of NB from 7-Me and recoordination
will result in site exchange of the agostic CH7-syn proton of 7-Me
with the H7-syn proton of free NB (present in very low
concentration relative to 7-Me). This exchange process results
in the δ -1.28 resonance of 7-Me at -120 °C (slow exchange)
moving downfield to δ -0.65 at -12 °C (weighted average
resonance of agostic H7-syn of 7-Me plus H7-syn of free NB) (see
Supporting Information, Figure S9 for details). Addition of
variable concentrations of NB suggests that this exchange
process is associative in nature. Further evidence for this
assumption is provided when a more bulky substituted allyl
ligand is used (see below).
Influence of Electron-Withdrawing Groups on the Allyl
Fragment (R ) Cl and CO2Me). The bulk polymerization
experiments described above clearly showed that the NB
polymerization initiated by 2-Cl exhibited the highest reactivity
within the group of catalysts investigated. Therefore, 1 equiv
of NB was added to a CDCl2F solution of 2-Cl below -120
°C, and monitored by NMR spectroscopy. The insertion into
the allyl-Pd bond was instantaneous and 5-Cl formed im-
mediately under these conditions (Scheme 6). Details regarding
the NMR characterization are described in the Experimental
Section. On further addition of ca. 1 equiv of NB to 5-Cl, the
agostic intermediate 7-Cl was formed quantitatively and identi-
fied by NMR spectroscopy. However, in the presence of a slight
excess of NB in solution, the exchange process of free and
bound NB is more rapid than in 7-Me. Consequently, an NMR
spectrum in which the NB exchange is completely frozen out
on the NMR time scale is difficult to obtain even at temperatures
as low as -140 °C. Therefore, 7-Cl was generated on addition
of only 1.6 equiv of NB to 2-Cl. This lead to a 3:4 mixture of
5-Cl and 7-Cl, but the line shapes were sharp and allowed the
complete characterization of 7-Cl at -120 °C using a variety
of 1D and 2D NMR techniques (see Experimental Section for
details). The most noteworthy feature is the high field resonance
in the 1H NMR spectrum at δ -0.94 (2JHH ) 11.5 Hz)
Figure 4. ORTEP diagram of the agostic complex 7-Me (50% probability
ellipsoids). All hydrogen atoms were located in the difference Fourier map.
Hydrogen atoms were included in calculated positions using a riding model,
but not refined, except the geminal hydrogen atoms on C15, which have
been refined isotropically. The hydrogen atoms and the [SbF6]- anion have
been omitted for clarity, except the ones attached to C15. Selected bond
distances (Å): Pd1-C1 2.218(6), Pd1-C2 2.286(6), Pd1-C6 2.042(6),
Pd1-C12 2.257(6), Pd1-C13 2.236(6), Pd1-C15 2.756(6), Pd1-H15b
2.186(73).
(26) March, J. AdVanced Organic Chemistry: Reactions, Mechanisms and
Structure; 2nd ed.; McGraw-Hill: New York, 1977.
9
J. AM. CHEM. SOC. VOL. 131, NO. 25, 2009 9061

A R T I C L E S
Walter et al.
corresponding to the agostic C7-Hsyn proton, while the non-
agostic C7-Hanti proton resonates at δ 1.65 (2JHH ) 11.5 Hz).
An 1H-13C HSQC NMR experiment verified that both protons
are attached to the same carbon (C7) resonating at δ 43.2.
However, the chemical shift separation between these protons
of ∆δ ) 2.59 is less than for the analogous 7-Me complex (∆δ
) 2.80). There is also considerably less difference in the agostic
observations for this system are qualitatively similar to unac-
tivated 2-CO2Me. After warming of the reaction mixture to -64
°C in the NMR probe 2-CO2Me/B(C6F5)3) converted to
5-CO2Me/B(C6F5)3 as the only Pd-containing species detectable
in solution and insoluble poly(norbornene).
In summary, installing electron-withdrawing groups in the
2-position of the allyl fragment indeed facilitates chelate opening
in the agostic intermediate 7-R as initially hypothesized.
However, very electron-withdrawing groups such as -CO2Me
or borane-coordinated -CO2Me render the cationic Pd center
so electron-poor that the rate of mesitylene displacement
becomes slow and provides an additional barrier to insertion.
This feature demonstrates a significant drawback of this catalyst
architecture with respect to achieving living polymerization.
Influence of a Sterically Demanding Substituent in the
1-Position of the Allyl Fragment (R ) tBu, Me). Another
possible solution to influence chelate opening is to introduce
steric strain into the allyl moiety. For this purpose the dimer
(1-tBu,Me)2 was synthesized which bears a bulky tert-butyl
substituent in the 1-anti position.29-31 This 1-anti tert-butyl
group is expected to disfavor the π-coordination in the 5-tBu,Me
and 7-tBu,Me intermediates.
The synthesis of 2-tBu,Me is straightforward and follows the
procedures as described for other 2-R (R ) Me, Cl and CO2Me)
systems (Scheme 6, see Experimental Section for details).
Although no bulk polymerizations were undertaken with this
catalyst system, the reactivity of 2-tBu,Me toward NB was
studied using low-temperature NMR spectroscopy. The results
of this study were rather surprising: On addition of 3 equiv of
NB to a CDCl2F solution of 2-tBu,Me at -86 °C complete
displacement of mesitylene was observed (Scheme 7). However,
in contrast to 2-Me, the first insertion of NB into the Pd-allyl
bond was not instantaneous as anticipated. This is substantiated
by the following observations: (a) on addition of NB three high-
field resonances (which are strongly coupled to the low-field
resonances given in parentheses) at δ -0.94 (1.44), -0.30
(0.63), and -0.05 (0.76) were observed, suggesting the presence
of three distinct species containing an agostic interaction
between the syn-H7 of NB and the cationic Pd center, (b) two
of these species (δ -0.30 and -0.05) convert over time or on
further addition of NB into 7-tBu,Me which is an analogue
species to 7-R (R ) Me, Cl) as substantiated by NMR
spectroscopy and single-crystal X-ray diffraction (Figure 5), (c)
the integration of the diastereoptopic CH2 resonances (δ
2.64-2.59) which are uniquely indicative for a first insertion
product correlate to the highest field agostic resonance (δ -0.94)
corresponding to the 7-tBu,Me species, and (d) no further
intermediates were observed on warming the NMR sample to
-40 °C, but slow polymer formation occurred. Note the
coupling of NB occurs exclusively to C3, the less-substituted
carbon of the π-allyl moiety. This behavior agrees with the
observations concerning [(1-Me-allyl)Pd(COD)][PF6]1 but con-
trasts to [(1-Me-allyl)Ni(mesitylene)][B(ArF)4] in which NB
couples to the more-substituted allylic carbon.14
1
and nonagostic JCH coupling constants in 7-Cl (126 and 146
Hz) versus 7-Me (110 and 160 Hz). These observations point
toward a weaker agostic interaction for 7-Cl as compared to
7-Me. Note that for 7-Cl, polymerization of NB occurs even at
-120 °C over the course of several hours, while poly(nor-
bornene) is rapidly formed above -40 °C, but no additional
intermediates are detected. All these results are consistent with
the observed increased reactivity of 2-Cl during bulk polymer-
izations and with a slow initiation followed by rapid propagation.
They also confirm that the second insertion of NB into 7-Cl is
rate-limiting; however, once this insertion occurs, all further
insertions are rapid, consistent with our proposal of a rate-
limiting chelate opening.
The decreased activity of 2-CO2Me is inconsistent with this
proposal, since the electron-withdrawing ester functionality
should allow for a weaker coordination of the olefin arm to the
cationic Pd center and therefore a more facile chelate opening.
Low-temperature NMR studies were undertaken to investigate
this discrepancy. The reactivity of 2-CO2Me toward NB was
rather surprising. In contrast to 2/5-Me and 2/5-Cl displacement
of coordinated mesitylene is considerably slower for 2/5-
CO2Me. For example, treatment of 2-CO2Me with 5 equiv of
NB at -120 °C in CDCl2F results in formation of 5-CO2Me as
a major species with minor amounts of 2-CO2Me remaining
together with a small fraction of agostic complex 7-CO2Me.27
(Under these conditions, 2-Me or 2-Cl would have been fully
converted to 7-Me or 7-Cl). Warming to -98 °C results in
disappearance of 2-CO2Me and NMR evidence shows that the
bound NB of 7-CO2Me is in rapid exchange with free NB.
Further warming to -64 °C results in formation of poly(nor-
bornene) with 5-CO2Me as the sole observable Pd species in
solution. Signals for free mesitylene and bound mesitylene in
5-CO2Me are sharp at -64 °C indicating slow exchange on
the NMR time scale. These results indicate again that chelate
opening in the agostic intermediate (7-CO2Me) is slow relative
to propagation, but that a further barrier to initiation is formation
of 7-CO2Me from 5-CO2Me. This added barrier likely explains
the unexpected rate order 2-Cl > 2-CO2Me.
We also investigated the possibility of activating the ester
functionality on addition of B(C6F5)3 to 2-CO2Me. To this end
2-CO2Me and B(C6F5)3 (1.5 equiv) were dissolved in CDCl2F.
The resonances of 2-CO2Me/B(C6F5)3 broadened at low tem-
perature significantly compared to 2-CO2Me, but at -130 °C
the solubility of the resulting adduct is very low and it
precipitated from solution. However, relatively sharp resonances
were observed at -110 °C. In the 13C{1H} NMR spectrum, both
resonances of the ester functionality, CO2Me, shift on exposure
to B(C6F5)3 downfield by ca. 2 ppm (see Experimental Section
for details) consistent with an interaction of the ester functional-
ity with the electron-deficient borane,28 while the remaining
resonances are nearly unchanged, although broadened. NB was
added to this activated complex 2-CO2Me/B(C6F5)3 and the
These data strongly suggested that the two other agostic
species observed at δ -0.30 and -0.05 are attributable to prefirst
insertion complexes. Hence, it was of interest to characterize
the identity of these preinsertion products in more detail. To
this end, 0.6 equiv of NB was added to a frozen CDCl2F solution
(27) This sample of 2-CO2Me contained ca. 1 equiv of mesitylene which
may influence the ratio of 5-CO2Me/7-CO2Me.
(28) Parks, D. J.; Piers, W. E.; Parvez, M.; Atencio, R.; Zaworotko, M. J.
Organometallics 1998, 17, 1369–1377.
(29) Lukas, J.; Ramakers-Blom, J. E.; Hewitt, T. G.; De Boer, J. J. J.
Organomet. Chem. 1972, 46, 167–177.
(30) Volger, H. C. Rec. TraV. Chim. 1968, 87, 225–240.
(31) Lukas, J.; Coren, S.; Blom, J. E. Chem. Comm. 1969, 1303–1304.
9
9062 J. AM. CHEM. SOC. VOL. 131, NO. 25, 2009

Polymerization of Norbornene with (allyl)Pd Catalysts
A R T I C L E S
Scheme 7
of 2-tBu,Me, mixed at -140 °C, and inserted into a precooled
NMR probe (-120 °C). Under these conditions 40% of
unreacted 2-tBu,Me remained in solution (as expected), while
the two preinsertion species, 8a-tBu,Me and 8b-tBu,Me, were
formed in a ca. 1:1 ratio (>50%) and 7-tBu,Me was formed in
less than 10% (Scheme 7). Attention to detail must to be
observed, otherwise 7-tBu,Me is readily formed in significant
quantity at the expense of 8a-tBu,Me and 8b-tBu,Me. Interest-
ingly, even at -130 °C the two preinsertion species exchange
with each other as substantiated by a significant line-broadening
relative to the line shapes for 2-tBu,Me at the same temperature
(υ1/2 ) 8 vs 4 Hz). (We note that the chemical shifts of the
preinsertion complexes at δ -0.19 (0.90) and 0.22 (0.77) (low-
field resonances are given in parentheses), respectively, are
slightly different at this temperature and solvent compared to
CD2Cl2 at -86 °C.) The 1JCH coupling constants of the agostic
and nonagostic H7 resonances in both species are only slightly
different, 135 vs 138 Hz and 131 vs 139 Hz, respectively,
indicating only a very weak agostic interaction in both species
and also explaining the facile exchange of NB between these
species at this temperature.
There are a few notable features in the crystal structure of
7-tBu,Me compared to 7-Me. The Pd1-C1, Pd1-C15, and
Pd1-H15b distances are significantly elongated by ca. 0.12 Å
which reflects the steric bulk introduced by the 1-tert-butyl
group.25 The weakening of the agostic interaction is also
reflected by the reduced chemical shift separation between the
agostic and nonagostic protons of ∆δ ) 2.38. Despite the
weakened agostic interaction, the exchange between free and
bound NB is slow on the NMR time scale. However, this
discrepancy is readily explained when considering a space-filling
model of 7-tBu,Me, which shows that the Pd center is
effectively shielded by the bulky tert-butyl group, and therefore,
the rate of intermolecular associative exchange between free
and bound NB is slow.
In conclusion, the problem of slow initiation cannot to be
resolved by introducing a bulky group in close proximity to
the metal, since the first insertion step becomes rate-limiting
together with a still slow opening of the π-coordinate chelate
arm.
Conclusions
Several cationic (allyl)Pd(II) complexes have been studied
for the (2,3)-vinyl addition polymerization of NB which exhibit
good to high activity. For example the most active catalyst, 2-Cl
produces 470 TOs in less than 5 min at 23 °C while only 200
TOs are achieved within 1 h at 23 °C for 5-Me (Tables 1 and
3). In all cases the NMR and polymerization data point to a
slow initiation process coupled with a rapid propagation rate
for polymerization which prevents living polymerization and
therefore polymer molecular weight control. In this context the
remarkably narrow MWD values of 1.2-1.4 might appear
counterintuitive, but Gold has shown using statistical analyses
that such MWD values are completely consistent with a very
slow initiation event followed by very rapid propagation.32
The high polymerization rates and the fact that only a small
fraction of the Pd species is responsible for polymer formation
imply that the barrier to NB insertion in these propagating
species must be extremely low. However, the first-formed
intermediate in polymerizations initiated with [(2-R-allyl)Pd-
Figure 5. ORTEP diagram of the agostic complex 7-tBu,Me (50%
probability ellipsoids). All hydrogen atoms were located in the in the
difference Fourier map. Hydrogen atoms were included in calculated
positions using a riding model but not refined, except the geminal hydrogen
atoms on C15, which have been refined isotropically. The hydrogen atoms
and the [SbF6]- anion have been omitted for clarity, except the ones attached
to C15. Selected bond distances: Pd1-C1 2.330(4), Pd1-C2 2.326(4),
Pd1-C6 2.084(3), Pd1-C12 2.237(4), Pd1-C13 2.194(4), Pd1-C15
2.852(4), Pd1-H15b 2.33(5).
(32) Gold, L. J. Chem. Phys. 1958, 28, 91–99.
9
J. AM. CHEM. SOC. VOL. 131, NO. 25, 2009 9063

A R T I C L E S
Walter et al.
(mesitylene)]+ complexes has been fully characterized (NMR
and X-ray diffraction) and shown to be a species in which the
allyl and NB units couple in a cis-exo fashion to yield a σ,π-
complex capped by mesitylene, 5-R. This coupling event is very
fast and occurs at temperatures as low as -120 °C. In the
presence of free NB the mesitylene ligand is readily displaced
to form the agostic intermediate 7-R which has also been fully
characterized by various NMR techniques and in two cases
7-Me and 7-tBu,Me by X-ray diffraction. The observation of
this intermediate is significant since, to date, no group has
achieved spectroscopic characterization of these propagating
species that bear no ancillary ligands. The ability of NB to act
as a bidentate ligand is most likely an important structural
element for the stabilization of the propagating species which
we, unfortunately, cannot observe by NMR. The fact that
polymer formation occurs from the intermediate 7-R in the
presence of excess NB and no other intermediates are observed
strongly argues for the second insertion, i.e., insertion of NB
in 7-R, being the slow initiation step and, that following the
second insertion, propagation is extremely fast. This leads to
the conclusion that chelate opening in 7-R is the most severe
restriction preventing a living polymerization in these systems.
In principle, this limitation might be overcome by introducing
electron-withdrawing or sterically demanding groups on the allyl
fragment. While the reaction rate significantly increases for 2-Cl
due to a more rapid initiation (second insertion), a decrease is
observed in the case of the more electron-withdrawing 2-CO2Me
system. NMR studies show that in the 2/5-CO2Me system
displacement of mesitylene is slow compared to the 2/5-Cl
system and provides a further barrier to initiation. The introduc-
tion of a sterically demanding t-butyl group in the 1-anti position
allowed the observation of the γ-agostic preinsertion species
8a/b-tBu,Me in which the mesitylene ligand was displaced by
one NB monomer. After NB has inserted into the less-substituted
allyl carbon bond, mesitylene is readily displaced to form the
γ-agostic species 7-tBu,Me. However, chelate opening continues
to be the slow initiation step again preventing a living
polymerization. In summary, in all systems investigated slow
initiation relative to propagation was observed and could not
be circumvented by altering the electronic and steric features
of these allyl complexes. Currently we are investigating alterna-
tive systems which might overcome these restrictions and
therefore ensure a living polymerization and control of polymer
molecular weight.
Robertson Microlit Laboratories of Madison, NJ. Structure factor
tables are available from the authors upon request. Crystallographic
data were also deposited with the Cambridge Crystallographic Data
Centre. Copies of the data (CCDC No. 724661-724665) can be
obtained free of charge via http://www.ccdc.cam.ac.uk/data_request/
cif, by e-mailing data_request@ccdc.cam.ac.uk, or by contacting
The Cambridge Crystallographic Data Centre, 12, Union Road,
Cambridge CB 1EZ, U.K. (fax +44 1223 336033).
Materials. All solvents were deoxygenated and dried by passage
over columns of activated alumina.33,34 CD2Cl2, purchased from
Cambridge Laboratories, Inc., was dried over CaH2, vacuum-
transferred to a Teflon sealable Schlenk flask containing 4 Å
molecular sieves, and degassed via three freeze-pump-thaw
cycles. Freon (CDCl2F) was prepared according to a literature
procedure and stored over activated 4 Å molecular sieves at -25
°C.35 NB was purchased from Aldrich and dried by refluxing over
activated 4 Å molecular sieves, followed by distillation.36 B(C6F5)3
was purchased from Strem and sublimed prior to use.37 Mesitylene
(1,3,5-Me3C6H3) was purchased from Aldrich and used without
further purification. PdCl2 was purchased from J&J Materials and
used as received. [(2-MeO2C-allyl)PdCl]2 (1-CO2Me)2, [(1-Me3C-
2-Me-allyl)Pd(µ-Cl)]2 (1-tBu,Me)2, and [(2-R-allyl)Pd(Mesityl-
ene)][SbF6] (2-R) (R ) H, Me, Cl), were synthesized according to
literature methods.13,29-31,38
Synthesis.
Synthesis of [(2-MeO2C-allyl)Pd(mesitylene)][SbF6] Complex
38
(2-CO2Me). Under an argon atmosphere, [(2-MeO2C-allyl)PdCl]2
(0.278 g, 0.578 mmol) was dissolved in dry methylene chloride
(20 mL), and the resulting yellow solution was stirred at room
temperature for 5 min to dissolve the solid. Excess mesitylene (0.50
mL, 3.57 mmol) was then added via syringe, and the resulting
solution cooled in a dry ice/isopropanol bath to -78 °C. A solution
of AgSbF6 (0.397 g, 1.16 mmol) in 5 mL of methylene chloride
was then added dropwise at -78 °C, and the mixture was stirred
for ca. 30 min at this temperature before it was allowed to warm
slowly to room temperature. The resulting yellow solution was
cannula filtered and the solvent evaporated in vacuo. The product
was washed with 3 × 10 mL of pentane and dried under vacuum
to yield yellow powder, which was stored in the glovebox at -35
°C to prevent thermal degradation. Yield: 0.491 g. Low-temperature
1H NMR analysis indicated the presence of free mesitylene; despite
repeated attempts, free mesitylene could not be removed. 1H NMR
(500 MHz, CD2Cl2, -90 °C): δ 7.05 (s, 3H, mes-CH), 4.89 (s,
2H, Hsyn), 3.75 (s, 3H, CH3), 3.36 (s, 2H, Hanti), 2.40 (s, 9H, mes-
CH3). Free mesitylene (∼2 equiv) δ 6.92 (s, 6H, mesAr), 2.28 (s,
18H, mesCH3). 13C{1H} NMR (125 MHz, CD2Cl2, -90 °C): δ 162.9
(C3), 132.1 (C5), 115.1 (C7), 107.6 (C1), 64.8 (C2), 53.7 (C4), 20.2
(C6). Free mesitylene (∼2 equiv) δ 140.2 (s, Cipso), 123.0 (s,
Experimental Section
General Considerations. All reactions, unless otherwise stated,
were conducted under an atmosphere of dry, oxygen-free argon
using standard high-vacuum, Schlenk, or drybox techniques. Argon
was purified by passage through BASF R3-11 catalyst (Chemalog)
and 4 Å molecular sieves. All palladium catalysts were stored under
1
1
argon in an MBraun glovebox at -35 °C. H, 13C{1H}, H-1H
COSY, 1H-13C HSQC, 1H-1H NOESY, 1H-1H TOCSY, and 13
C
DEPT135 NMR spectra were recorded on a Bruker DRX 500 MHz,
a Bruker DRX 400 MHz, or a Bruker 400 MHz AVANCE
spectrometer. Chemical shifts are referenced relative to residual
CHCl3 (δ 7.24 for 1H), CH(D)Cl2 (δ 5.32 for 1H), CHCl2F (δ 7.47
(33) Alaimo, P. J.; Peters, D. W.; Arnold, J.; Bergman, R. G. J. Chem.
Educ. 2001, 78, 64.
1
for H), 13CD2Cl2 (δ 53.8 for 13C), 13CDCl3 (δ 77.0 for 13C), and
(34) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518–1520.
(35) Siegel, J. S.; Anet, F. A. L. J. Org. Chem. 1988, 53, 2629–2630.
(36) Gilliom, L. R.; Grubbs, R. H. J. Am. Chem. Soc. 1986, 108, 733–742.
(37) Driver, T. G.; Day, M. W.; Labinger, J. A.; Bercaw, J. E. Organo-
metallics 2005, 24, 3644–3654.
13CDCl2F (δ 104.2 for 13C). GPC analyses at high temperature were
conducted in HPLC grade 1,2,4-trichlorobenzene at 140 °C using
a Waters Alliance GPCV 2000 at the Cornell Center for Materials
Research Polymer Characterization Facility. Molecular weights are
reported relative to polystyrene standards. Thermal transitions were
measured with a Seiko 220C DSC on the second heat with a heating
rate of 10 °C min-1. Elemental analyses were carried out by
(38) Kurosawa, H.; Hirako, K.; Natsume, S.; Ogoshi, S.; Kanehisa, N.;
Kai, Y.; Sakaki, S.; Takeuchi, K. Organometallics 1996, 15, 2089–
2097.
9
9064 J. AM. CHEM. SOC. VOL. 131, NO. 25, 2009

Polymerization of Norbornene with (allyl)Pd Catalysts
A R T I C L E S
mesCAr), 20.9 (s, mesCH3). 1H NMR (500 MHz, CDCl2F, -120 °C):
δ 7.10 (s, 3H, mes-CH), 4.83 (s, 2H, Hsyn), 3.80 (s, 3H, CH3), 3.34
(s, 2H, Hanti), 2.40 (s, 9H, mes-CH3). Free mesitylene (∼2 equiv)
δ 6.97 (s, 6H, mesAr), 2.32 (s, 18H, mesCH3). 13C{1H} NMR (125
MHz, CDCl2F, -120 °C): δ 165.9 (C3), 132.9 (C5), 115.8 (C7),
108.3 (C1), 64.7 (C2), 55.0 (C4), 20.6 (C6). Free mesitylene (∼2
equiv) δ 140.8 (s, Cipso), 122.7 (s, mesCAr), 21.4 (s, mesCH3).
H3), 1.58 (s, 3H, CH3), 1.38 (m, 1H, H5), 1.22 (m, 1H, H9),
1.10-1.00 (m, 3H, H6, H6′, H10). 13C{1H} NMR (125 MHz, C5D5N,
23 °C): δ 126.0 (C2), 75.0 (C1), 59.5 (C9), 47.5 (C4), 44.9 (C3),
44.8 (C8), 42.4 (C5), 34.6 (C7), 30.7 (C6), 29.3 (C10), 28.1 (Me).
See the Supporting Information for details regarding the attempted
synthesis of (3-Cl)2.
Synthesis of [(1-Me3C-2-Me-allyl)Pd(µ-Cl)]2 (1-tBu,Me)2. (1-
tBu,Me)2 was synthesized according to a literature procedure.29-31
Repeated slow crystallization from MeOH at room temperature
1
yielded (1-tBu,Me)2 as the pure anti isomer. H NMR (500 MHz,
20 °C, CDCl3): δ 4.41 (s, 1H, Hsyn,a), 3.81 (s, 1H, Hsyn,b), 3.45 (s,
1H, Hanti,b), 2.01 (s, 3H, Me), 1.11 (s, 9H, CMe3). 13C{1H} NMR
(125 Hz, 293 K, CDCl3): δ 118.3 (C2), 92.5 (C4), 62.1 (C1), 36.7
(C5), 31.3 (C6), 26.3 (C3).
Synthesis of [(2-Me-propenylnorbornyl)Pd(py)2][SbF6] (4-
Me). A Schlenk flask was charged with [(2-Me-propenylnorbornyl)Pd(µ-
Cl)]2 ((3-Me)2) (100 mg, 0.17 mmol) of and AgSbF6 (114 mg, 0.34
mmol) and the solids were dissolved in 10 mL of CH2Cl2 at 0 °C. To
this reaction mixture pyridine (0.108 g, 1.37 mmol) was added via
syringe and allowed to warm to room temperature and stirred for 2 h.
The solution was filtered and taken to dryness in dynamic vacuum.
The resulting white solid was washed with pentanes (3 × 10 mL) and
dried in vacuo. The product was purified on recrystallization from a
minimal amount of CH2Cl2 layered with an excess of pentane at -38
°C to give 0.15 g of 4-Me (0.23 mmol, 68% yield) which was stored
at -35 °C under an argon atmosphere to prevent thermal degradation.
1H NMR (500 MHz CDCl3, 23 °C): δ 8.65 (s, 4H, py-Hm), 7.73 (s,
2H, py-Hp), 7.41 (s, 4H, py-Ho), 4.74 (s,1H, Hcis), 4.06 (s, 1H, Htrans),
3.15 (d,1H, 3JHH ) 7 Hz, H8), 2.71 (dd, 2JHH ) 17.6 Hz, 3JHH ) 12
Hz, H1), 2.56 (dd, 2JHH ) 17.6 Hz, 3JHH ) 5.5 Hz, H2), 1.91 (s, 1H,
H4), 1.89 (d, 1H, 2JHH ) 10 Hz, H10), 1.70 (m, 1H, H3), 1.76 (s, 3H,
Me), 1.40 (m, 1H, H7), 1.06 (m, 2H, H5+H6), 0.90 (m, 3H, H5′, H6′,
H9). 13C{1H} NMR (125 MHz, CDCl3, 23 °C): δ 149.6 (C11, py-Hm),
136.5 (C13, py-Hp), 123.5 (C12, py-Ho), 79.5 (C1), 59.6 (C9), 46.9 (C4),
44.1 (C3), 42.3 (C8), 42.1 (C5), 33.7 (C7), 29.3 (C10), 28.2 (C6), 27.0
(Me). (The resonance due to C2 was not found and was probably
obscured by a pyridine resonance.) Anal. Calcd for C21H27F6N2PdSb:
C, 38.82; H, 4.19; N, 4.31. Found: C, 38.70; H, 4.21; N, 4.45.
Synthesis of [(2-Me-propenylnorbornyl)Pd(mesitylene)][SbF6]
(5-Me). A Schlenk flask was charged with [(2-Me-propenylnor-
bornyl)Pd(µ-Cl)]2 ((3-Me)2) (100 mg, 0.17 mmol) and AgSbF6 (114
mg, 0.34 mmol) and dissolved in 10 mL of CH2Cl2 at -78 °C.
Mesitylene (0.165 g, 1.37 mmol) was added via syringe and the
reaction mixture was allowed to warm to 0 °C and stirred at this
temperature for 2 h. The yellow suspension was filtered and the
solvent was removed under dynamic vacuum. The yellow solid was
washed with pentanes (3 × 10 mL) and dried in vacuo (165 mg,
Synthesis of [(2-Me3C-2-Me-allyl)Pd(mesitylene)][SbF6] (2-
tBu,Me). Under an argon atmosphere, a Schlenk flask was charged
with (1-tBu,Me)2 (0.341 g, 0.674 mmol) and AgSbF6 (464 mg,
mmol) and dissolved in precooled CH2Cl2 (10 mL) at -85 °C. To
this solution mesitylene (243 mg, 280 µL, 2.02 mmol) was added
and the reaction mixture was stirred at -85 °C for 10 min, and
then slowly allowed to warm to 0 °C and stirred at this temperature
for an additional 2 h. During this time the color changed to yellow-
orange. Filtration via a cannula and evaporation of the solvent in
dynamic vacuum yielded a yellow-orange powder that was washed
with pentane (3 × 10 mL) and dried in vacuo. Yield: 0.61 g (1.063
mmol, 79%). 2-tBu,Me was stored at -35 °C under an argon
1
atmosphere to prevent thermal degradation. H NMR (500 MHz,
20 °C, CDCl3): δ 7.05 (s, 3H, mes-CH), 4.98 (s, 1H, Hsyn,a), 4.25
(s, 1H, Hsyn,b), 3.65 (s, 1H, Hanti,b), 2.47 (s, 9H, mes-CH3), 1.96 (s,
3H, Me), 1.01 (s, 9H, CMe3). 13C{1H} NMR (125 Hz, 293 K,
CDCl3): δ 133.9 (C7), 118.5 (C9), 117.0 (C2), 100.1 (C4), 63.5 (C1),
38.0 (C5), 30.9 (C6), 26.3 (C3), 21.1 (C8). Anal. Calcd for
C17H27F6PdSb: C, 35.60; H, 4.74. Found: C, 35.72; H, 4.80.
Synthesis of [(2-Me-propenylnorbornyl)Pd(µ-Cl)]2 (3-Me)2.
The insertion product (1-Me)2 was synthesized from [(2-Me-
allyl)Pd(µ-Cl)]2 and NB (4 equiv) in benzene solution on heating
to 50 °C according to a literature procedure.15 Under these
conditions, (3-Me)2 precipitated slowly as colorless crystals in
nearly quantitative yield. These crystals were sparingly soluble in
chlorinated solvents like CH2Cl2 and CHCl3 but exhibited good
solubility in pyridine to yield the monomeric, monopyridine adduct
3-Me(py). 1H NMR (500 MHz, C5D5N, 20 °C): δ 4.29 (s, 1H, Hcis),
3
3.93 (s, 1H, Htrans), 3.15 (br.s, 1H), 3.14 (d, 1H, JHH) 7 Hz, H8),
2
3
2.35 (dd, 1H, JHH) 16.5 Hz, JHH) 10.5 Hz, H1), 2.19 (dd, 1H,
2JHH ) 16.5, JHH) 5.5 Hz, H2), 1.76 (s, 1H, H4), 1.68 (“m”, 1H,
3
9
J. AM. CHEM. SOC. VOL. 131, NO. 25, 2009 9065

A R T I C L E S
Walter et al.
In Situ Generation of 7-Me from [(2-Me-allyl)Pd(mesityle-
ne)][SbF6] (2-Me) or 5-Me. In a screw-cap NMR tube 2-Me (20
mg, 3.8 × 10-2 mmol) was dissolved in dry degassed CDCl2F at
-140 °C, and 1.9 equiv of a NB stock solution (130 µL, 7.2 ×
10-2 mmol) in CD2Cl2 was added. The reaction mixture was inserted
1
into a precooled NMR probe (-120 °C). The H NMR spectrum
confirmed that the NB insertion was instantaneously and one NB
molecule displaced the mesitylene ligand to form 7-Me.
Alternatively, 7-Me was generated from 5-Me on NB addition:
In a screw-cap NMR tube 2-Me (20 mg, 3.27 × 10-2 mmol) was
dissolved in dry degassed CDCl2F at -140 °C, and ca. 0.9 equiv
(50 µL, 2.94 × 10-2 mmol) of a NB stock solution in CD2Cl2 was
added. The reaction mixture was inserted into a precooled NMR
probe (-120 °C). The NMR spectra confirmed the formation of
7-Me and mesitylene displacement. Single crystals suitable for an
X-ray diffraction experiment were grown at -80 °C from this
solution on addition of a layer of pentane.
0.27 mmol, 79% yield). 5-Me was stored at -35 °C under an argon
1
atmosphere to prevent thermal degradation. H NMR (500 MHz,
3
CD2Cl2, 20 °C): δ 6.88 (s, 3H, aryl Mes), 3.88 (d, JHH ) 6 Hz,
H8), 3.80 (s, 1H, Hcis), 3.77 (s, 1H, Htrans), 2.53 (dd, 1H, 2JHH ) 17
3
2
Hz, JHH) 7 Hz, H2), 2.47 (s, 9H, me Mes), 2.28 (dd, 1H, JHH
)
3
10 Hz, JHH ) 17 Hz, H1), 2.19 (s, 1H, H7), 1.97 (s, 3H, CH3),
2
1.92 (s, 1H, H4), 1.75 (d, JHH ) 10 Hz, H10), 1.62 (ddd, 1H, H3),
2
1.5-1.4 (m, 2H, H5 + H6), 1.35 (d, JHH ) 10 Hz, H10), 1.20 (m,
1H, H5′), 1.07 (m, 1H, H6′). 13C{1H} NMR (125 MHz, CDCl3, 20
°C): δ 135.3 (C12), 130.1 (C2), 121.6 (C11), 77.0 (C9), 70.7 (C1),
49.5 (C4), 48.5 (C8), 43.6 (C3), 42.9 (C5), 33.7 (C10), 30.1 (C6),
28.5 (Me), 27.3 (C7), 20.4 (C13). Anal. Calcd for C20H29F6PdSb:
C, 39.27; H, 4.78. Found: C, 39.32; H, 4.82.
1H NMR (500 MHz, CDCl2F, -120 °C): δ 6.19 (s, 1H, H11/
H12), 5.81 (s, 1H, H12/H11), 5.02 (s, 1H, Hcis), 4.87 (s, 1H, Htrans),
4.46 (br.s, 1H, H8), 3.48 (br.s., 2H, H13+H16), 2.74 (dd, 1H, H2,
coupling not resolved), 2.63 (dd, 2JHH ) 18 Hz, 3JHH ) 10 Hz, 1H,
H1), 2.06 (s, 1H, H4), 1.93 (br.s., 2H, H3+H7), 1.77-1.71 (m, 5H,
H10, H14, H14′, H15, H15′), 1.52 (d, 2JHH ) 11.5 Hz, 1H, H17′), 1.50
2
(m, 1H, H4), 1.32-1.30 (m, 2H, H5+H6), 1.23 (d, 1H, JHH ) 10
Hz, H9), 1.10-1.07 (m, 2H, H5′+H6′), -1.28 (d, 2JHH ) 11.5 Hz,
H17). 13C{1H} NMR (125 MHz, CDCl2F, -120 °C): δ 154.6 (C2),
125.3 (C11/C12), 116.7 (C12/C11), 90.5 (C1), 84.9 (C9), 52.1 (C8),
45.3 (C13+C16), 44.9 (C4), 44.6 (C5), 42.5 (C17), 42.4 (C3), 34.5
(C10), 28.5 (Me), 27.9 (C7), 27.2 (C6), 24.0 (C14/C15), 24.1 (C15/
C14).
In Situ Generation of [(2-Cl-propenylnorbornyl)Pd(mesityl-
ene)][SbF6] (5-Cl) from [(2-Cl-allyl)Pd(mesitylene)][SbF6] (2-
Cl). In a screw-cap NMR tube, [(2-Cl-allyl)Pd(mesitylene)][SbF6]
(2-H) (0.01 g, 9.53 × 10-3 mmol) was dissolved in dry degassed
CDCl2F at -130 °C and a stock solution of NB (32 µL, 9.53 ×
10-2 mmol, ca. 1 equiv.) in CD2Cl2 was added. The reaction mixture
was inserted into a precooled NMR probe (-120 °C). The insertion
was instantaneous under these conditions and 5-Cl was character-
ized by various NMR techniques.
Active Species in the Polymerization of NB by [(2-Me-
propenylnorbornyl)Pd(py)2][SbF6] ((3-Me)2) and B(C6F5)3 by
NMR (6-Me). A screw-cap NMR tube was charged with 10 mg
(0.015 mmol) of (3-Me)2 and 4 equiv (0.036 g, 0.060 mmol) of
B(C6F5)3 and dissolved in CD2Cl2 and characterized by 19F NMR
3
(376.5 MHz CD2Cl2, 23 °C): Free B(C6F5)3: δ -127.2 (d, JFF
)
19 Hz, o-F), -142.9 (br.s, p-F), -160.4 (t, m-F). Py*B(C6F5)3:
3
3
δ-131.9 (d, JFF ) 19 Hz, o-F), -157.4 (t, JFF ) 21 Hz, p-F),
3
-164.0 (td, JFF ) 19 and 21 Hz, m-F). (borane-pyridine capped
Pd species) δ -139.2 (d, 3JFF ) 19 Hz, o-F), -154.5 (t, 3JFF ) 21
3
1
Hz, p-F), -162.7 (td, JFF ) 19 and 21 Hz, m-F). H NMR (400
MHz CD2Cl2, 23 °C): δ 8.63 (d, 3JHH ) 4.8 Hz, 2H, py-Hm), 8.24
3
3
(t, JHH ) 7.6 Hz, 1H, py-Hp), 7.74 (t, JHH ) 6.8, 2H, py-Ho),
2
4.54 (bs, 2H, Hcis+Htrans), 4.08 (bs, 1H, H8), 2.60 (dd, JHH ) 11
Hz, JHH ) 6 Hz, 1H, H2), 2.28 (dd, JHH ) 11 Hz,3JHH ) 18 Hz,
1H, H1) 2.12 (s, 3H, Me), 2.0 (s, 1H, H7), 1.93(bs, 1H, H10), 1.78
(bs, 1H, H4), 1.5 (m, 1H, H3), 1.4-1.0 (5H, H9, H6, H6′, H5, H5′).
Series of Low-Temperature VT NMR Studies of the Reaction
of Cationic (allyl)Pd(II) Catalysts with NB. In Situ Generation
of [(2-Me-propenylnorbornyl)Pd(mesitylene)][SbF6] (5-Me) from
[(2-Me allyl)Pd(mesitylene)][SbF6] (2-Me). In a screw-cap NMR
tube 2-Me (10 mg, 1.9 × 10-2 mmol) was dissolved in dry degassed
CDCl2F at -140 °C, and 1 equiv of a NB stock solution (34 µL,
1.9 × 10-2 mmol) in CD2Cl2 was added. The reaction mixture was
3
2
1
inserted into a precooled NMR probe (-120 °C). The H NMR
spectrum confirmed that the NB insertion was instantaneous and
the NMR spectra agreed with independently synthesized 5-Me.
9
9066 J. AM. CHEM. SOC. VOL. 131, NO. 25, 2009

Polymerization of Norbornene with (allyl)Pd Catalysts
A R T I C L E S
1H NMR (500 MHz, CDCl2F, -120 °C): δ 6.92 (s, 3H, aryl
3
Mes), 4.21 (s, 1H, Hcis), 3.88 (d, JHH ) 9 Hz, H8), 3.87 (s, 1H,
H
trans), 2.97 (dd, 1H, H2, coupling not resolved), 2.56 (s, 9H, me
Mes), 2.73 (dd, 1H, H1), 2.33 (s, 1H, H7), 2.06 (s, 1H, H4), 1.86
2
(d, JHH ) 9 Hz, H10), 1.79 (ddd, 1H, H3), 1.53 (m, 1H, H6), 1.50
2
(m, 1H, H5), 1.48 (d, JHH ) 9 Hz, H10), 1.26 (m, 1H, H5′), 1.13
(m, 1H, H6′). 13C{1H} NMR (125 MHz, CDCl2F, -120 °C): δ 134.8
(C12), 126.8 (C2), 122.1 (C11), 77.4 (C9), 68.0 (C1), 49.4 (C4), 48.8
(C8), 46.6 (C3), 42.6 (C5), 33.5 (C10), 30.0 (C6), 26.8 (C7), 20.4
(C13).
In Situ Generation of 7-Cl from [(2-Cl-allyl)Pd(mesityle-
ne)][SbF6] (2-Cl). In a screw-cap NMR tube, [(2-Cl-allyl)Pd(mesi-
tylene)][SbF6] (2-Cl) (0.02 g, 1.91 × 10-2 mmol) was dissolved
in dry degassed CDCl2F at -130 °C and a stock solution of NB
(100 µL, 3.06 × 10-2 mmol, ca. 1.6 equiv) in CD2Cl2 was added.
The reaction mixture was inserted into a precooled NMR probe
(-120 °C). The insertion was instantaneous under these conditions
and 7-Cl was characterized by various NMR techniques.
10 Hz, 1H, H1), 2.36 (s, 9H, me Mes), 2.23 (s, 1H, H7), 1.95 (s,
2
1H, H4), 1.73 (d, JHH ) 10 Hz, H10), 1.69 (ddd, coupling not
2
resolved, 1H, H3), 1.42 (m, 2H, H5+H6), 1.35 (d, JHH ) 10 Hz,
H10), 1.19 (m, 1H, H5′), 1.07 (m, 1H, H6′). 13C{1H} NMR (125
MHz, CDCl2F, -86 °C): δ 167.4 (CO2Me), 134.4 (C12), 122.6 (C2),
121.5 (C11), 84.6 (C9), 74.6 (C1), 53.9 (CO2Me), 49.6 (C8), 49.4
(C4), 42.9 (C5), 36.1 (C3), 33.4 (C10), 30.0 (C6), 26.8 (C7), 19.8
(C13).
Active Species in the Polymerization of NB by [(2-MeO2C-
allyl)Pd(mesitylene)][SbF6] Complex (2-CO2Me) and B(C6F5)3
by NMR. In a screw-cap NMR tube, [(2-MeO2C-allyl)Pd(mesity-
lene)][SbF6] (2-CO2Me) (0.021 g, 3.74 × 10-3 mmol) and B(C6F5)3
(0.029 g, 5.66 × 10-3 mmol, 1.5 equiv) were dissolved in dry
1H NMR (500 MHz, CDCl2F, -120 °C): δ 6.37 (s, 1H, H11/
H12), 5.97 (s, 1H, H12/H11), 5.40 (s, 1H, Hcis), 5.32 (s, 1H, Htrans),
3.61 (br.s., 2H, H13+H16), 3.54 (d, 3JHH ) 9 Hz, H8), 3.18 (dd, 1H,
H2, coupling not resolved), 3.01 (dd, 1H, H1, coupling not resolved),
1
degassed CDCl2F at -130 °C to form a light yellow solution. H
NMR (500 MHz, CDCl2F, -110 °C): δ 7.11 (s, 3H, mes-CH), 4.79
(s, 2H, Hsyn), 3.86 (s, 3H, CH3), 3.32 (s, 2H, Hanti), 2.44 (s, 9H,
mes-CH3). 13C{1H} NMR (125 MHz, CD2Cl2, -110 °C): δ 167.9
(C3), 132.9 (C5), 115.9 (C7), 106.9 (C1), 64.9 (C2), 56.4 (C4), 20.7
(C6). 13C{1H} NMR chemical shifts corresponding to B(C6F5)3 have
been omitted.
2
2.07 (ddd, 1H, H3, coupling not resolved), 1.89 (d, JHH ) 9 Hz,
H10), 1.80 (s, 1H, H7), 1.65 (d, 2JHH ) 11.5 Hz, H17′), 1.60 (m, 1H,
H6), 1.58 (m, 1H, H5), 1.57 (s, 1H, H4), 1.52-1.49 (m. 4H, H14,
H14′, H15, H15′), 1.48 (d, 2JHH ) 9 Hz, H9), 1.26 (m, 1H, H5′), 1.12
(m, 1H, H6′), -0.94 (d, 2JHH ) 11.5 Hz, H17). 13C{1H} NMR (125
MHz, CDCl2F, -120 °C): δ 143.6 (C2), 126.8 (C11/C12), 118.2 (C12/
C11), 90.6 (C1), 89.4 (C9), 53.1 (C8), 48.8 (C4), 46.6 (C10), 44.8
(C13/C16), 44.7 (C3), 44.6 (C16/C13), 43.2 (C17), 42.4 (C5), 34.3 (C14/
C15), 33.5 (C15/C14), 30.4 (C7), 26.8 (C6).
In Situ Generation of [(2-MeO2C-propenylnorbornyl)Pd-
(mesitylene)][SbF6]*B(C6F5)3 (5-CO2Me/B(C6F5)3) from [(2-MeO2C-
allyl)Pd(mesitylene)][SbF6]/B(C6F5)3 (2-CO2Me/B(C6F5)3). In a
screw-cap NMR tube, [(2-MeO2C-allyl)Pd(mesitylene)][SbF6] (2-
CO2Me) (0.021 g, 3.74 × 10-3 mmol) and B(C6F5)3 (0.029 g, 5.66
× 10-3 mmol, 1.5 equiv) were dissolved in dry degassed CDCl2F
at -130 °C and a stock solution of NB (250 µL, 1.87 × 10-1 mmol,
ca. 5 equiv) in CD2Cl2 was added. The bright yellow reaction
mixture was inserted into a precooled NMR probe (-120 °C).
Similar to 2-CO2Me mesitylene displacement and insertion was
not instantaneously. The obtained reaction mixture was complex
and contained several species some of which were undergoing
exchange processes: Unreacted starting material (1-CO2Me) (as the
dominant species), free NB, the mesitylene-capped first-insertion
product (2-CO2Me) and the NB-capped-first insertion product (7-
CO2Me). After warming to 220 K, polymer formed and all free
NB was consumed. The only Pd containing species was 5-CO2Me/
B(C6F5)3, and it was characterized via a variety of 1D and 2D NMR
experiments.
In Situ Generation of [(2-MeO2C-propenylnorbornyl)Pd-
(mesitylene)][SbF6] (5-CO2Me) from [(2-MeO2C-allyl)Pd(mesi-
tylene)][SbF6] (2-CO2Me). In a screw-cap NMR tube, [(2-CO2Me-
allyl)Pd(mesitylene)][SbF6] (2-CO2Me) (0.021 g, 3.74 × 10-2
mmol) was dissolved in dry degassed CDCl2F at -130 °C and a
stock solution of NB (250 µL, 1.87 × 10-1 mmol, ca. 5 equiv) in
CD2Cl2 was added. The reaction mixture was inserted into a
precooled NMR probe (-120 °C). In contrast to the observations
for 2-Me and 2-Cl, the mesitylene displacement and insertion was
not instantaneously. Under these conditions, 5-CO2Me was the
major species in solution with minor amounts of 5-CO2Me
remaining together with a small fraction of agostic complex
7-CO2Me. Warming to -98 °C resulted in disappearance of
2-CO2Me and NMR evidence showed that the bound NB of
7-CO2Me was in rapid exchange with free NB. Further warming
to -64 °C resulted in formation of poly(norbornene) with 5-CO2Me
as the sole observable Pd species in solution which was character-
ized via a variety of 1D and 2D NMR experiments.
1H NMR (500 MHz, CDCl2F, 0 °C): δ 6.88 (s, 3H, aryl Mes),
3
4.62 (s, 1H, Hcis), 4.29 (d, JHH ) 9 Hz, H8), 4.33 (s, 1H, Htrans),
2
3
3.85 (s, 3H, OCH3), 2.73 (dd, JHH ) 17 Hz, JHH ) 10 Hz, 1H,
2
3
H1), 2.69 (dd, JHH ) 17 Hz, JHH ) 7 Hz, 1H, H2), 2.45 (s, 9H,
me Mes), 2.28 (s, 1H, H7), 1.97 (s, 1H, H4), 1.80 (d, 2JHH ) 10 Hz,
H10), 1.75 (ddd, coupling not resolved, 1H, H3), 1.47 (m, 2H,
1H NMR (500 MHz, CDCl2F, -86 °C): δ 6.79 (s, 3H, aryl Mes),
4.56 (s, 1H, Hcis), 4.19 (s, 1H, Htrans), 4.18 (d, coupling unresolved
due to signal overlap, H8), 3.80 (s, 3H, OCH3), 2.69 (dd, 1H, H2,
2
H5+H6), 1.40 (d, JHH ) 10 Hz, H10), 1.27 (m, 1H, H5′), 1.12 (m,
3
2
3
2JHH ) 17 Hz, JHH ) 7 Hz), 2.67 (dd, 1H, JHH ) 17 Hz, JHH
)
1H, H6′). 13C{1H} NMR (125 MHz, CDCl2F, -86 °C): δ 168.1
9
J. AM. CHEM. SOC. VOL. 131, NO. 25, 2009 9067

A R T I C L E S
Walter et al.
<10%. Attention to detail had to be observed, otherwise 7-tBu,Me
readily formed in significant quantity at the expense of 8a-tBu,Me
and 8b-tBu,Me. Interestingly, even at -130 °C the two preinsertion
species exchange with each other as substantiated by a significant
line broadening relative to the line shapes for 2-tBu,Me at the same
temperature.
(CO2Me), 134.3 (C12), 122.8 (C2), 121.5 (C11), 84.9 (C9), 74.5 (C1),
53.7 (CO2Me), 49.7 (C8), 49.3 (C4), 42.8 (C5), 36.2 (C3), 33.3 (C10),
30.2 (C6), 27.0 (C7), 19.8 (C13). 13C{1H} NMR chemical shifts
corresponding to B(C6F5)3 have been ommited. 19F NMR (470.6
3
MHz, CDCl2F, -86 °C): Free B(C6F5)3: δ -127.6 (d, JFF ) 21
Hz, o-F), -142.7 (t, 3JFF ) 21 Hz, p-F), -160.6 (t, 3JFF ) 21 Hz,
m-F). 5-CO2Me/B(C6F5)3: δ -135.0 (d, 3JFF ) 20 Hz, o-F), -156.7
3
3
(t, JFF ) 20 Hz, p-F), -164.0 (t, JFF ) 20 Hz, m-F).
In Situ Generation of 7-tBu,Me from [(1-tBu-2-Me-allyl)Pd-
(mesitylene)][SbF6] (2-tBu,Me). In a screw-cap NMR tube,
2-tBu,Me (0.021 g, 3.15 × 10-2 mmol) was dissolved in dry
degassed CD2Cl2 at -86 °C and a stock solution of NB (330 µL,
6.30 × 10-2 mmol, 2 equiv) in CD2Cl2 was added. The reaction
mixture was inserted into a precooled NMR probe (-86 °C). The
mesitylene displacement was instantaneous, but 7-tBu,Me was
formed besides the two preinsertion species 8a,b-tBu,Me which
converted slowly to 7-tBu,Me at this temperature. 7-tBu,Me was
characterized by various NMR techniques.
1
The H and 13C NMR resonances corresponding to the allyl-
moiety in both isomers, 8a-tBu,Me and 8b-tBu,Me, are identical
at -120 °C, although slightly broadened. H NMR (500 MHz,
1
CDCl2F, -120 °C): 8a-tBu,Me or 8b-tBu,Me: δ 6.49 (s, 1H, Hs’,a),
6.00 (s, 1H, H1a/H2a), 5.47 (s, 1H, Hs,a), 5.18 (s, 1H, H2a/H1a), 3.84
(s, 1H, Ha,a), 3.36 (s, 1H, H3a/H4a), 3.00 (s, 1H, H4a/H3a), 1.82-1.73
(m, 2H, H5a, H6a), 1.72 (s, 3H, Me), 1.40-1.30 (m, 2H, H5a′,
2
1
H6a′),1.10 (s, 9H, CMe3), 0.90 (d, JHH ) 9.0 Hz, JCH ) 138 Hz,
2
1
1H, H7′a), -0.19 (d, JHH ) 9.0 Hz, JCH ) 135 Hz, 1H, H7a).
13C{1H} NMR (125 MHz, CDCl2F, -120 °C): δ 119.1 (C1a), 117.4
(C2a), 107.7 (C4a/C5a), 100.1 (C5a/C4a), 76.6 (C3a), 44.6 (C6a/C9a),
43.5 (C9a/C6a), 41.8 (C10a), 38.1 (CMe3), 32.3 (CMe3,a), 25.5 (Mea)
25.2 (C7a/C8a), 24.4 (C8a/C7a). 8b-tBu,Me or 8b-tBu,Me: δ 6.49
(s, 1H, Hs’,b), 5.59 (s, 1H, H1b/H2b), 5.47 (s, 1H, Hs,b), 4.96 (s, 1H,
H2b/H1b), 3.84 (s, 1H, Ha,b), 3.47 (s, 1H, H3b/H4b), 3.18 (s, 1H, H4b/
H3b), 1.82-1.73 (m, 2H, H5b, H6b), 1.72 (s, 3H, Me), 1.40-1.30
2
(m, 2H, H5b′, H6b′), 1.10 (s, 9H, CMe3), 0.77 (d, JHH ) 9.5 Hz,
1JCH ) 139 Hz, 1H, H7′b), 0.22 (d, 2JHH ) 9.5 Hz, 1JCH ) 131 Hz,
1H, H7b). 13C{1H} NMR (125 MHz, CDCl2F, -120 °C): δ 119.1
(C1b), 117.4 (C2b), 107.2 (C4b/C5b), 103.0 (C5b/C4b), 76.6 (C3b), 44.7
(C6b/C9b), 44.1 (C9b/C6b), 42.9 (C10b), 38.1 (CMeb), 32.3 (CMe3,b),
25.5 (Meb) 24.9 (C7b/C8b), 24.8 (C8b/C7b).
Low-Temperature NMR Study of the Reactions of [(2-Me-
allyl)Pd(µ-Cl)]2 and [(2-Cl-allyl)Pd(µ-Cl)]2 with NB. A screw-
cap NMR tube was charged with either [(2-Me-allyl)Pd(µ-Cl)]2
(0.01 g, 1.61 × 10-2 mmol) or [(2-Cl-allyl)Pd(µ-Cl)]2 (0.01 g, 1.61
× 10-2 mmol). The complexes were dissolved in CD2Cl2 (500 µL)
at -78 °C, and 2.5 equiv (75 µL, 2.42 × 10-2 mmol) of NB stock
solution in CD2Cl2 were added to each solution. Since the formed
adducts were less soluble than the starting dimers at low temper-
ature, an internal standard (1,3,5-(F3C)3C6H3: 10 µL, 0.053 mmol)
was added to monitor changes in concentration of each species.
The interaction of NB and [(2-R-allyl)Pd(µ-Cl)]2 was monitored
1H NMR (500 MHz, CD2Cl2, -86 °C): δ 6.35 (s, 1H, H11/H12),
5.90 (s, 1H, H12/H11), 5.49 (s, 1H, Hcis), 4.81 (br.s, 1H, H8), 3.44
(s, 1H, H13/H16), 3.32 (s, 1H, H16/H13), 2.64-2.59 (m, 2H, H1+H2),
2.07 (ddd, 1H, H3, coupling not resolved), 2.04 (s, 1H, H7), 1.55
(d, 1H, H10, coupling not resolved), 1.53-1.51 (m. 4H, H14, H14′,
H15, H15′), 1.50 (m, 1H, H6), 1.48 (s, 1H, H4), 1.28 (m, 1H, H5),
1.27 (d, 1H, H9, coupling not resolved), 1.12 (m, 1H, H5′), 1.08
2
(m, 1H, H6′), -0.94 (d, JHH ) 12 Hz, H17). 13C{1H} NMR (125
MHz, CD2Cl2, -86 °C): δ 136.5 (C2), 121.3 (C11/C12), 115.4 (C12/
C11), 126.0 (C1), 95.2 (C9), 64.6 (C8), 44.9 (C13/C16), 44.1 (C16/
C13), 43.5 (C4), 42.1 (C17), 41.9 (C5), 35.8 (C3), 35.7 (CMe3), 33.6
(C10), 32.3 (CMe3), 27.4 (C7), 26.8 (C6). 25.5 (Me), 24.8 (C14/C15),
24.5 (C15/C14),
1
by H NMR spectroscopy from -86 to +5 °C (see Supporting
Information for details, Figure S2a). Adducts 1-R(NB) were
characterized at -92 °C by NMR spectroscopy.
In Situ Generation of [(2-tBu-2-Me-allyl)Pd(norbornene)]-
[SbF6] (8a-tBu,Me and 8b-tBu,Me). In a screw-cap NMR tube,
2-tBu,Me (0.021 g, 3.15 × 10-2 mmol) was dissolved in dry
degassed CDCl2F at -120 °C and then frozen in liquid N2. A stock
solution of NB (25 µL, 1.78 × 10-2 mmol, 0.6 equiv) in CD2Cl2
was added, the mixture warmed to -140 °C, quickly shaken and
inserted into a precooled NMR probe at -120 °C. Under these
conditions ca. 40% of unreacted 2-tBu,Me remained in solution,
while the two preinsertion species, 8a-tBu,Me and 8b-tBu,Me, were
formed in a ca. 1:1 ratio (>50%) and 7-tBu,Me was formed in
9
9068 J. AM. CHEM. SOC. VOL. 131, NO. 25, 2009

Polymerization of Norbornene with (allyl)Pd Catalysts
A R T I C L E S
1
NB Adduct of [(2-Me-allyl)Pd(µ-Cl)]2 (1-Me(NB)): H NMR
mol) in the drybox. To dissolve catalyst and cocatalyst 2 mL of
CH2Cl2 were added, at which point the colorless solution turned
bright yellow, followed by the polymerization solvent (methylcy-
clohexane) and NB stock solution (to form a total volume of 10
mL) at room temperature. The polymerization was quenched by
injection of MeOH then filtered. Polymer was dried overnight in a
vacuum oven at 80 °C after which conversion was determined
gravimetrically. Molecular weights and MWDs were determined
by high-temperature GPC analysis.
3
(500 MHz, CD2Cl2, -80 °C): δ 5.26 (d, JHH ) 3.5 Hz, 1H, H1/
H2), 5.13 (d, JHH ) 3.5 Hz 1H, H1/H2), 4.43 (s, 1H, Hsyn,a/Hsyn,b),
3
4.35 (s, 1H, Hanti,a/Hanti,b), 3.47 (s, 1H, Hsyn,b/Hsyn,a), 2.94 (s, 1H,
H3/H4), 2.90 (s, 1H, H3/H4), 2.76 (s, 1H, Hanti,b/Hanti,a), 1.84 (s, 3H,
Me), 1.52-0.72 (6H, H5,5′,6,6′,7,7′). 13C{1H} NMR (125 MHz, CD2Cl2,
-80 °C): 128.1 (C2), 108.1 (C4/C5), 106.0 (C5/C4), 71.9 (C1/C3),
65.5 (C3/C1), 48.4 (C10), 43.0 (C9/C6), 41.8 (C6/C9), 24.6 (Me), 24.9
(C7/C8), 24.7 (C8/C7). The thermodynamic parameters of this
equilibrium process were evaluated between 180 and 215 K; see
Supporting Information for details, Figure S3.
General Procedure for the Polymerization of NB with 5-Me.
A Schlenk tube was charged with 5-Me (0.01 g, 1.6 × 10-5 mol)
in the drybox. To dissolve the catalyst, 2 mL of CH2Cl2 was added,
followed by polymerization solvent (methylcyclohexane) and NB
stock solution (to form a total volume of 10 mL) at room
temperature. Polymerization was quenched by injection of MeOH
and filtered. Polymer was dried overnight in a vacuum oven at 80
°C after which conversion was determined gravimetrically. Mo-
lecular weights and MWDs were determined by high-temperature
GPC analysis.
NB Adduct of [(2-Cl-allyl)Pd(µ-Cl)]2 (1-Cl(NB)): 1H NMR (500
MHz, CD2Cl2, -80 °C): δ 5.40 (s, 1H, H1/H2), 5.32 (s, 1H, H1/
H2), 4.78 (s, 1H, Hsyn,a/Hsyn,b), 4.65 (s, 1H, Hanti,a/Hanti,b), 3.87 (s,
1H, Hsyn,b/Hsyn,a), 3.17 (s, 1H, Hanti,b/Hanti,a), 2.98 (s, 1H, H3/H4),
2.93 (s, 1H, H3/H4), 1.6-0.72 (6H, H5,5′,6,6′,7,7′). 13C{1H} NMR (125
MHz, CD2Cl2, -80 °C): 126.0 (C2), 109.2 (C4/C5), 107.7 (C5/C4),
77.9 (C1/C3), 65.8 (C3/C1), 48.4 (C10), 43.2 (C9/C6), 43.0 (C6/C9),
(bridge head), 24.4 (C7/C8), 24.3 (C8/C7).
Polymerization Procedures. General Procedure for the Po-
lymerization of NB with 2-Cl and 2-CO2Me. A Schlenk tube was
charged with 2-Cl or 2-CO2Me (2.0 × 10-5 mol) and dissolved in
2.0 mL of CH2Cl2. Then, 7.0 mL of methylcyclohexane was added
and the solution placed in a cold bath (see Table 1 and 2 for various
temperatures) and allowed to equilibrate at the specified temperature
for 15-20 min. A similarly cold NB stock solution (234 equiv,
1.0 mL) in a methylcyclohexane (4.67 M, 0.879 g, 4.58 mmol)
was then added for a total volume of 10.0 mL. The polymerization
was monitored at various times and quenched with acidified MeOH.
The poly(norbornene) was filtered, washed with acetone, and dried
in vacuo overnight at 50 °C after which the conversion was
determined gravimetrically. Molecular weights and MWDs were
determined by high-temperature GPC.
General Procedure for the Polymerization of NB with
2-CO2Me and B(C6F5)3. Essentially, the same procedure as
described for 2-CO2Me was used, except that the catalyst 2-CO2Me
(2.0 × 10-5 mol) and B(C6F5)3 (4. 0 × 10-5 mol) were dissolved
in 2.0 mL of CH2Cl2 and stirred at 0 °C for 10 min before NB
addition.
General Procedure for the Polymerization of NB with 4-Me
and B(C6F5)3. A Schlenk tube was charged with 4-Me (0.01 g,
1.68 × 10-5 mol) and 4 equiv of B(C6F5)3 (0.02 g, 3.3 × 10-5
Acknowledgment. We thank the National Science Foundation
(CHE-0615704) for support, the Alexander-von-Humboldt Founda-
tion for a Feodor-Lynen-Fellowship (MDW), Dr. Matthew C.
Crowe for ESI-MS spectra, and Dr. David L. Harris for his help
with NMR spectroscopy. High-temperature GPC analysis was
conducted by Mr. Anthony M. Condo, Jr. at the Cornell Center for
Materials Research Polymer Characterization Facility.
Supporting Information Available: CIF files, tables giving
crystallographic data for (3-Me)2, 4-Me, 5-Me, 7-Me, and
1
7-tBu,Me, H NMR studies on the adduct formation between
(1-Me)2 and NB, thermodynamic parameters on this adduct
formation, 1H NMR spectrum of 5-Me, 1H and 13C NMR
1
investigations on 7-Me, H NMR spectra of the preinsertion
species 8a,b-tBu,Me and the attempted independent synthesis
of (3-Cl)2 and 5-Cl. This material is available free of charge
via the Internet at http://pubs.acs.org.
JA9025598
9
J. AM. CHEM. SOC. VOL. 131, NO. 25, 2009 9069