γ-Agostic interactions stabilize the propagating species in the vinyl addition polymerization of norbornene

Article abstract of DOI:10.1039/b916457j

Cationic [(tmeda)Pd(OEt₂)(Me)][B(Ar_F)₄] was synthesized by protonation of (tmeda)PdMe₂ and allowed for the first time the nature of the propagating species in the vinyl addition polymerization of norbornene to b

Full text of DOI:10.1039/b916457j

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c-Agostic interactions stabilize the propagating species in the vinyl
addition polymerization of norbornenewz
Marc D. Walter, Peter S. White and Maurice Brookhart*
Received (in Austin, TX, USA) 10th August 2009, Accepted 3rd September 2009
First published as an Advance Article on the web 28th September 2009
DOI: 10.1039/b916457j
Cationic [(tmeda)Pd(OEt2)(Me)][B(ArF)4] was synthesized by
protonation of (tmeda)PdMe2 and allowed for the first time the
nature of the propagating species in the vinyl addition polymer-
ization of norbornene to be discerned. In this system c-agostic
interactions stabilize the propagating species and permit
stepwise monomer insertions to be observed by 1H NMR
spectroscopy. This and earlier studies provide a new proposal
for the propagating species in ligand-less late transition metal-
catalyzed norbornene polymerizations.
achieved (Rinit { Rprop) similar to the catalyst systems
investigated by Goodall.9 However, mechanistic studies
revealed the shortcomings of this catalyst architecture. For
Ni the resting state was the mesitylene-capped, first-insertion
product (1) that suffered from slow mesitylene displacement
and reversible first NB insertion; for Pd the g-agostic
intermediate (2) is formed from the mesitylene complex and
chelate opening becomes rate-limiting (Chart 1).
In neither case spectroscopic information on the
propagating species was obtained. This prompted us to seek
a new catalyst system which is readily synthesized, overcomes
the limitations of the cationic (allyl)M systems and allows the
characterization of the propagating species. To this end we
turned our attention to (tmeda)PdMe2 (3).12 Reaction of 3
with [H(OEt2)2][B(ArF)4] (ArF = 3,5-(F3C)2C6H3) gave the
cationic complex [(tmeda)Pd(OEt2)(Me)][B(ArF)4] (4) in good
yield (67%).z Complex 4 has been fully characterized by
various NMR techniques and single crystal X-ray diffraction
experiments.y The molecular structure of 4 is shown in Fig. 1
and selected bond distances are provided in the figure caption.
Exposure of 4 to 2 equiv. of NB at ꢀ75 1C leads to double
NB insertion into the Pd–Me bond to give the rac-g-agostic
intermediate (5) which serves as the first model for the growing
polymer chain in late metal-catalyzed vinyl addition NB
polymerization processes. The mono-insertion product could
not be observed even when only 1 equiv. of NB was added at
low temperature (oꢀ130 1C). In this case 50% of 4 remained
unreacted while 50% was converted to 5.y Resting state 5 has
been identified by extensive NMR studies and single X-ray
diffraction analysis.y The molecular structure is shown in
Fig. 2 and selected bond distances are provided in the figure
caption.
Intensive research efforts have been devoted to the polymer-
ization of cyclic olefins due to the attractive properties of the
resulting polymers, including high glass transition temperatures,
high optical transparency, low dielectric constants and low
birefringence.1 Norbornene (bicyclo[2.2.1]hept-2-ene, NB) and
its derivatives may be polymerized via ring-opening metathesis
polymerization (ROMP), cationic or radical polymerization,
and vinyl addition polymerization. The vinyl addition
polymerization of norbornene is especially attractive since it
overcomes the limitations of ROMP (unsaturation in the
polymer backbone) and of cationic or radical polymerizations
(oligomers and rearrangements). Therefore late transition
metal catalysts based on Ni and Pd have been studied by
many groups.1–8 However, despite significant research efforts,
little is known about the propagating species in these NB
polymerizations using cationic Pd and Ni catalysts.9 In
this context the cationic ‘‘ligand-less’’ [(allyl)M(COD)][PF6]
(M = Ni or Pd) systems are the most active catalysts for
norbornene polymerization to date. However, NMR studies
revealed a major problem of these catalyst systems: polymer is
rapidly formed on exposure of norbornene to the catalyst, but
only very little catalyst is consumed indicating a slow initiation
step followed by very rapid propagation (Rinit { Rprop) and
no molecular weight control is possible.9
Spectroscopically, the most distinct feature of 5 is the
high-field shifted resonance at d ꢀ4.15 which is associated
with the H-atom participating in the g-agostic interaction with
We have recently studied the NB polymerization process
using cationic [(allyl)M(mesitylene)]+ (M = Ni,10 Pd11)
catalysts. While both systems are highly active for NB
polymerizations, again no molecular weight control could be
1
the cationic Pd center (H101ꢁ ꢁ ꢁPd 1.89(6) A). The JCH
coupling constant shows the characteristic reduction for
agostic interactions13 of 87 Hz, which is significantly smaller
than was previously observed for 2 of 110 Hz (R = Me).11
Department of Chemistry, University of North Carolina at Chapel Hill,
Chapel Hill, NC 27599-3290, USA. E-mail: mbrookhart@unc.edu;
Fax: +1 919 962 2476; Tel: +1 919 962 0362
w This article is part of a ChemComm ‘Catalysis in Organic Synthesis’
web-theme issue showcasing high quality research in organic
chemistry. Please see our website (http://www.rsc.org/chemcomm/
organicwebtheme2009) to access the other papers in this issue.
z Electronic supplementary information (ESI) available: Experimental
details and crystallographic data. CCDC 743856 and 743857. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/b916457j
Chart 1
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This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 6361–6363 | 6361

Fig. 1 Displacement ellipsoid plot (50% probability) of [(tmeda)-
Pd(OEt2)(Me)][B(ArF)4] (4). The counteranion [B(ArF)4]ꢀ was omitted
for clarity. Selected bond distances (A): Pd1–N1 2.088(10), Pd1–N2
2.210(10), Pd1–C11 2.030(12), Pd1–O1 2.116(3).
Fig. 3 1H NMR spectra (ꢀ75 1C) of the high-field region depicting
the growing polymer chain stabilized by g-agostic interactions as a
function of time.
Noteworthy is the fact that the tmeda ligand is not replaced
during chain growth. We are currently actively pursuing
alternative catalyst systems to overcome this limitation.
In summary, we have provided the first structural and
spectroscopic evidence for the nature of the propagating
species in the vinyl addition polymerization of norbornene
catalyzed by late transition metal complexes. exo-Insertion of
norbornene into the growing polymer chain does not provide a
species with a b-hydrogen that is appropriately positioned to
form a b-agostic CH interaction as is observed for ethylene
and a-olefin polymerizations.14,15 However, the penultimately
enchained monomer is perfectly positioned to stabilize the
unsaturated metal center through an agostic interaction of the
endo g-H as illustrated by the structure of 5 (Fig. 2) and
through NMR analysis of the growing chain. In combination
with our previous studies on [(allyl)Pd(mesitylene)]+ catalysts
we suggest a new possible structure for the as yet unobserved
Fig. 2 Displacement ellipsoid plot (50% probability) of [(tmeda)-
Pd(Me-(NB)2)][B(ArF)4] (5). The counteranion [B(ArF)4]ꢀ was omitted
for clarity. Selected bond distances (A): Pd1–N1 2.243(4), Pd1–N2
2.101(3), Pd1–C16 2.018(4), Pd1–C9 2.469(4), Pd1–H101 1.89(6).
Complex 5 is a competent catalytic species for the vinyl
addition polymerization of NB. Addition of 5 equiv. of NB
allows the chain growth (insertion) process to be monitored by
NMR spectroscopy. The insertion process is slow at ꢀ50 1C
and shows that the 3rd and higher insertions occur at roughly
the same rate, while the g-agostic feature prevails in the
1
growing polymer chain. Curiously, the H NMR resonances
of the H-atom participating in the g-agostic interaction shifts
upfield for the 3rd and 4th insertions, while the 5th and all
1
other insertions occur downfield at about the same H NMR
1
chemical shift at d ꢀ4.5 ppm.z The JCH coupling constant
varies only slightly for each species (87–89 Hz) suggesting a
similar structural feature as observed in 5. The sudden down-
field shift might be related to a change in enchainment mode of
the NB polymer, e.g. changing from rac to meso and therefore
influencing the strength of the g-agostic interaction. The
polymerization reinitiates on addition of more NB equivalents
(Fig. 3). However, on the bulk scale it becomes obvious that
after about 10 insertions the polymer chain combined with the
TMEDA ligand shield the Pd center sufficiently that chain
growth slows to non-viable rates, rendering the system
Fig. 4 Proposed propagating species in the ligand-less norbornene
unsuitable for accomplishing
a
living polymerization.
polymerization.
ꢂc
This journal is The Royal Society of Chemistry 2009
6362 | Chem. Commun., 2009, 6361–6363

propagating species in the ligand-less NB polymerization
exclusively stabilized by g-agostic interactions between
coordinated NB monomer and growing polymer chain (Fig. 4).
We thank the National Science Foundation (CHE-0615704)
for support, and the Alexander-von-Humboldt Foundation
for a Feodor-Lynen-Fellowship (MDW), and Dr David L.
Harris for his help with NMR spectroscopy.
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Notes and references
y Crystal structure data for 4 [C11H29N2OPd, C32H12BF24] (c09 219):
M = 1174.99, orthorhombic, Pbca, a = 17.9055(8), b = 19.9639(9),
c = 27.1312(13) A, Z = 8, V = 9698.4(8) A3, Dc = 1.609 Mg mꢀ3
,
T = 100(2) K, 62 819 reflections measured, 9916 unique reflections,
Rint = 0.049, 725 parameters refined, R(all) = 0.077, wR(all) = 0.166,
S = 1.02. CCDC 743857. Crystal structure data for 5 [C21H39N2Pd,
C32H12BF24
12.9847(3),
109.9810(10), Z = 4, V = 6609.4(3) A3, Dc = 1.296 Mg mꢀ3
]
(c09 205):
M
=
1289.17, monoclinic, P21/c,
a
=
b
=
31.1704(6), 17.3760(5) A,
c
=
b =
,
12 W. de Graaf, J. Boersma, W. J. J. Smeets, A. L. Spek and
G. van Koten, Organometallics, 1989, 8, 2907–2917.
13 M. Brookhart, M. L. H. Green and G. Parkin, Proc. Natl. Acad.
Sci. U. S. A., 2007, 104, 6908–6914.
14 S. A. Svejda, L. K. Johnson and M. Brookhart, J. Am. Chem. Soc.,
1999, 121, 10634–10635.
T = 100(2) K, 81 207 reflections measured, 12 241 unique reflections,
Rint = 0.039, 751 parameters refined, R(all) = 0.068, wR(all) = 0.167,
S = 1.06. CCDC 743856.
z Lowering the temperature to ꢀ120 1C results in decoalescence of
each high-field agostic signal into two closely spaced doublet of
doublets in a 1 : 5.4 ratio. These isomer pairs arise from freezing out
the ring inversion of the tmeda ligand which results in formation of
two diastereomers.
15 D. J. Tempel, L. K. Johnson, R. Leigh Huff, P. S. White and
M. Brookhart, J. Am. Chem. Soc., 2000, 122, 6686–6700.
ꢂc
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Chem. Commun., 2009, 6361–6363 | 6363