Cationic hafnium alkyl complexes that are stable toward β-hydride elimination below 10 °C and active as initiators for the living polymerization of 1-hexene [5]

Full text of DOI:10.1021/ja0114198

10746
J. Am. Chem. Soc. 2001, 123, 10746-10747
Cationic Hafnium Alkyl Complexes that Are Stable
toward â-Hydride Elimination below 10 °C and
Active as Initiators for the Living Polymerization of
1-Hexene
Parisa Mehrkhodavandi and Richard R. Schrock*
Department of Chemistry
Massachusetts Institute of Technology
77 Massachusetts AVenue
Cambridge, Massachusetts 02139
ReceiVed June 11, 2001
Recently we reported the synthesis and activation of zirconium
dialkyl complexes bearing arylated diamido pyridine ligands,
[(MesNCH2)2C(CH3)(2-C5H4N)]2- ([MesNpy]2-), and their use
as initiators for the polymerization of 1-hexene.1 Use of {[Mes-
Npy]ZrMe}[B(C6F5)4] as an initiator was complicated by forma-
tion of unreactive {[MesNpy]2Zr2Me3}[B(C6F5)4] and by 2,1-
insertion into the ZrMe bond followed by â-hydride elimination
to give inactive species. {[MesNpy]Zr(CH2CHMe2)}[B(C6F5)4]
was found to be an excellent initiator at 0 °C in bromobenzene;
only 1,2-insertion of 1-hexene into Zr-C bonds appeared to take
place, and cationic intermediates appeared to be stable toward
â-hydride elimination. However, {[MesNpy]Zr(CH2CHMe2)}-
[B(C6F5)4] itself was unstable toward â-hydride elimination. We
now find that cationic hafnium complexes of the type {[MesNpy]-
HfR}[B(C6F5)4] (R ) Et, n-Bu, i-Bu, n-Pr, i-Pr) that are formed
upon treating [MesNpy]HfR2 with [Ph3C][B(C6F5)4] can be
characterized readily by NMR methods, are stable toward
â-hydride elimination at 10 °C or below, and are active initiators
for the living polymerization of 1-hexene via 1,2-insertion into
the Hf-C bond at a rate that is approximately half that estimated
for the Zr-catalyzed process.
Figure 1. 13C{1H} NMR spectra (0 °C, C6D5Br) for addition of 1-hexene
to 2*; (a) 1-2 equiv of 1-hexene, (b) 45 equiv of 1-hexene.
at 93.3 ppm corresponding to the isobutyl methylene carbon atom
in 2* along with a resonance for 13CH2dCMe2 at 111.1 ppm.
The 1H NMR spectrum (-20 °C, C6D5Br) for 2 shows resonances
for the isobutyl methyl, methylene, and methine protons at 0.55,
0.44, and 1.73 ppm, respectively. Unlike its zirconium analogue,1
[(MesNpy)Hf(i-Bu)][B(C6F5)4] shows no sign of decomposition
over a period of 2 h at 0 °C.
The dichloride complex, [MesNpy]HfCl2, was prepared by
methods analogous to those used to prepare [MesNpy]ZrCl2.1
Dialkyl complexes of the type [MesNpy]HfR2 (R ) Et, n-Bu,
i-Bu, n-Pr, i-Pr) were prepared by treating [MesNpy]HfCl2 with
Grignard reagents. All dialkyl complexes are stable at room
temperature in the solid state or in solution (benzene, toluene,
ether, etc.). Perhaps the most unusual is [MesNpy]Hf(i-Pr)2, whose
structure was shown by X-ray crystallography to be similar to
that of [MesNpy]ZrMe2.1 (The structure will be described later
in a full paper.) There is a good deal of evidence in the literature
that dialkyl group 4 metal complexes can be relatively resistant
toward â-hydride abstraction, especially those of hafnium. For
example, Cp2MEt2 is relatively stable only when M ) Hf.2 A
variety of isopropyl complexes are known for metals outside of
group 4.3-8
Addition of 1-2 equiv of 1-hexene to a solution of 2* led to
a decrease in the intensity of the isobutyl methylene resonance
in 2* at 93.3 ppm, appearance of a new 13C{1H} NMR resonance
at 49.0 ppm for the first insertion product, and appearance of
other resonances near 45 ppm for the second, third, and so forth
insertion products (Figure 1a). Addition of 44 more equiv of
1-hexene led to the appearance of a resonance at 44.3 ppm for
the labeled methylene in the intermediate formed by multiple 1,2-
insertions plus the usual resonances (natural 13C abundance) for
atactic poly(1-hexene) formed via a 1,2-insertion process (Figure
1b). The living polymer is formed quantitatively (vs Ph3CH), and
there is no sign of decomposition by â-hydride elimination over
a period of 2-3 hours at 0 °C.
Consumption of 1-hexene by [(MesNpy)Hf(i-Bu)][B(C6F5)4]
in C6D5Br at 0 °C follows first-order kinetics. Two consecutive
additions of 60 equiv of 1-hexene to a solution of 2 (0.015 M) in
C6D5Br at 0 °C yielded identical linear plots of ln[1-hexene]
versus time with kobs ) 0.059 min-1 or kp ) 0.067 M-1 s-1. No
induction period was observed. Addition of 1-hexene to this
solution after it had been stored at room temperature for 24 h led
to first-order olefin consumption of 1-hexene at 0 °C, but at a
Activation of [MesNpy]Hf(i-Bu)2 (1) and [MesNpy]Hf(13CH2-
CHMe2)2 (1*) in C6D5Br with 1 equiv of [Ph3C][B(C6F5)4] at -20
°C led to formation of Ph3CH, isobutene, and the isobutyl cations,
[(MesNpy)Hf(i-Bu)][B(C6F5)4] (2 and 2*; eq 1). The 13C{1H}
NMR spectrum (0 °C, C6D5Br) of activated 1* shows a singlet
(1) Mehrkhodavandi, P.; Bonitatebus, P. J., Jr.; Schrock, R. R. J. Am. Chem.
Soc. 2000, 122, 7841.
1
rate ∼ /3 of that observed originally. Resonances for vinylic
(2) Negishi, E.; Takahashi, T. Acc. Chem. Res. 1994, 27, 124.
(3) Chisholm, M. H.; Haitko, D. A.; Folting, K.; Huffman, J. C. J. Am.
Chem. Soc. 1981, 103, 4046-4053.
protons at 4.84 and 4.82 ppm indicated that some â-hydride
elimination had taken place in 24 h at room temperature; we
propose to yield catalytically inactive metal-containing products.
The value of kp ) 0.067 M-1 s-1 should be compared with kp )
0.044 M-1 s-1 for polymerization of 1-hexene under identical
conditions with the analogous Zr species, assuming all Zr is active.
(4) Jaffart, J.; Mathieu, R.; Etienne, M.; McGrady, J. E.; Eisenstein, O.;
Maseras, F. Chem. Commun. 1998, 2011-2012.
(5) Jones, W. D.; Wick, D. D. Inorg. Chem. 1997, 36, 2723-2729.
(6) Pahor, N. B.; Dreos-Garlatti, R.; Geremia, S.; Randaccio, L.; Tauzher,
G.; Zangrando, E. Inorg. Chem. 1990, 29, 3437-3441.
(7) Randaccio, L.; Bresciani-Pahor, N.; Toscano, P. J.; Marzilli, L. G. J.
Am. Chem. Soc. 1980, 102, 7372-7373.
1
However, only ∼ /3 of the Zr was active in that particular run so
that the true value for the Zr system is ∼0.13 M-1 s-1
,
(8) Reger, D. L.; Garza, D. G.; Lebioda, L. Organometallics 1991, 10,
902-906.
approximately twice the value for the hafnium catalyst. Polym-
10.1021/ja0114198 CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/03/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 43, 2001 10747
Scheme 1. Activation of [MesNpy]HfEt2 with
[Ph3C][B(C6F5)4]
cationic species in solution, [(MesNpy)Hf(i-Pr)][B(C6F5)4], and
the product of 1,2-insertion of propene into the Hf-C bond,
[(MesNpy)HfCH2CH(CH3)(i-Pr)][B(C6F5)4]. (Pyridyl ortho proton
resonances belonging to what we propose to be multiple insertion
products were also observed in the aromatic region of the 1H NMR
spectrum of this solution.) A 13C DEPT experiment reveals a
resonance at 88.5 ppm corresponding to the hafnium methylene
carbon in [(MesNpy)HfCH2CH(CH3)(i-Pr)]+ and a resonance at
79.0 ppm corresponding to the hafnium methine carbon in
[(MesNpy)Hf(i-Pr)]+. The ratio of [(MesNpy)HfCH2CH(CH3)(i-
Pr)]+ to [(MesNpy)Hf(i-Pr)]+ is 4.2:1 if activation is carried out
at -20 °C. Formation of [(MesNpy)Hf(n-Pr)]+ was not observed
over a period of 1 h at 0 °C. Storage of the same solution at 10
°C for 24 h led to the disappearance of the resonances for
1
[(MesNpy)Hf(i-Pr)]+ and the appearance of two new H NMR
resonances at 4.68 and 4.73 ppm corresponding to vinylic protons
of a â-hydride elimination product. In contrast only about half
of the [(MesNpy)HfCH2CH(CH3)(i-Pr)]+ had disappeared during
this time period. Therefore, we can say with confidence that
[(MesNpy)Hf(i-Pr)]+ is significantly less stable than [(MesNpy)-
HfCH2CH(CH3)(i-Pr)]+. The greater instability of an internal alkyl
toward â-hydride elimination has been postulated for some time
in Zr and Hf olefin polymerization systems.10,11 We believe these
results to be the first direct demonstration of that proposal in
cationic group 4 alkyl complexes.
It is becoming clear that in several of the cationic zirconium
diamido donor systems that we have been investigating â-hydride
elimination is slow in part for steric reasons.12-14 We now believe
that in the (MesNpy)2- systems, in particular, the [B(C6F5)4]-
anion itself contributes significantly to steric crowding by
interacting with the metal in as yet undefined ways,15 probably
trans to the nitrogen donor. This viewpoint is inspired by recent
theoretical16,17 and experimental18,19 results on cationic metallocene
catalysts, which suggest that especially in solvents such as toluene,
even the [B(C6F5)4]- anion is a weakly bound ligand, and therefore
one must consider the ion pair as the reactive entity, not the cation
alone. Although we will now be able to measure a variety of
polymerization rates in these well-behaved systems, it is not clear
what process is actually being measured. We will try to resolve
some of these issues by varying the nature of the aryl group on
nitrogen and by measuring relative decomposition rates for
terminal and internal cationic alkyl complexes and relative rates
of reaction of cationic terminal and internal alkyl complexes with
olefins. The results of these experiments will be reported in due
course.
erization of 1-hexene at five temperatures between 5 and -20
°C yielded ∆Hq and ∆Sq values of 10.9(5) kcal mol-1 and -23-
(2) cal mol-1 K-1 respectively. These values should be compared
with those obtained for polymerization of 1-hexene with [(MesN-
py)Zr(i-Bu)][B(C6F5)4] (∆H‡ ) 8.1(7) kcal mol-1 and ∆S‡ ) -33-
(2) cal mol-1 K-1). Seven poly(1-hexene) samples (prepared at 0
°C) that contained up to 600 equiv of 1-hexene were found to
have polydispersities between 1.02 and 1.05 and molecular
weights (as determined by light scattering coupled with a
refractive index detector9) that were essentially equal to the
molecular weights expected for a well-behaved living system
(Figure 2, Supporting Information).
Activation of [MesNpy]HfEt2 with [Ph3C][B(C6F5)4] in C6D5-
Br at -20 °C led to formation of Ph3CEt and Ph3CH and two
major cationic hafnium alkyl species, {[MesNpy]HfEt}[B(C6F5)4]
and {[MesNpy]Hf(n-Bu)}[B(C6F5)4], in approximately equal
amounts. The first is formed by direct attack on an ethyl ligand
to give Ph3CEt, while the second is formed when trityl abstracts
a â-hydride from an ethyl ligand to give intermediate “{[MesNpy]-
HfEt(C2H4)}[B(C6F5)4]” followed by insertion of ethylene into
the Hf-Et bond (Scheme 1). The assignment of resonances for
the two cations was made possible by gCOSY and 13C DEPT
experiments. The amounts of {[MesNpy]HfEt}[B(C6F5)4] (47%),
Ph3CEt (43%), {[MesNpy]Hf(n-Bu)}[B(C6F5)4] (34%), and Ph3-
CH (57%) that are formed (with the sum of Ph3CEt and Ph3CH
set equal to 100%) suggest that some ethylene is lost from
intermediate “{[MesNpy]HfEt(C2H4)}[B(C6F5)4]” to give {[Mes-
Npy]HfEt}[B(C6F5)4] and free ethylene, which is then available
to react with {[MesNpy]HfEt}[B(C6F5)4] to form {[MesNpy]Hf-
(n-Bu)}[B(C6F5)4], and with {[MesNpy]Hf(n-Bu)}[B(C6F5)4] to
form higher insertion products, {[MesNpy]Hf(CH2CH2)nEt}-
[B(C6F5)4] (∼19%), where n is greater than or equal to 2.
{[MesNpy]HfEt}[B(C6F5)4] and {[MesNpy]Hf(n-Bu)}[B(C6F5)4]
can be distinguished from each other by the chemical shift of the
ortho pyridyl proton at 8.40 and 8.51 ppm, respectively, although
the ortho pyridyl proton resonance in {[MesNpy]Hf(CH2CH2)n-
Et}[B(C6F5)4] (8.52 ppm) overlaps with that for {[MesNpy]Hf-
(n-Bu)}[B(C6F5)4] in 500 MHz proton NMR spectra (C6D5Br, -20
°C). Addition of 1-2 equiv of 1-hexene to a solution containing
{[MesNpy]Hf(CH2CH2)nEt}[B(C6F5)4] (n ) 0, 1, 2, ...) leads to
formation of two major insertion products which can be formu-
lated as 1,2-insertion products, {[MesNpy]HfCH2CH(n-Bu)(Et)}-
[B(C6F5)4] (if n ) 0) and {[MesNpy]HfCH2CH(n-Bu)2}[B(C6F5)4]
(if n ) 1), on the basis of 13C DEPT experiments. Only a
resonance at 88.7 ppm corresponding to a methylene of the type
HfCH2CR′R′′ is present, consistent with 1,2-insertion. A resonance
for a hafnium-bound methine carbon, HfCHR′R′′, was not
observed.
Acknowledgment. We thank the Department of Energy (DE-FG02-
86ER13564) for supporting this research and Exxon Corporation for a
gift of [Ph3C][B(C6F5)4].
Supporting Information Available: Experimental procedures and
Figure 2 (PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
JA0114198
(10) Busico, V.; Cipullo, R.; Monaco, G.; Vacatello, M.; Bella, J.; Segre,
A. L. Macromolecules 1998, 31, 8713-8719.
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5264.
(12) Schrock, R. R.; Bonitatebus, P. J., Jr.; Schrodi, Y. Organometallics
2001, 20, 1056.
(13) Baumann, R.; Schrock, R. R. J. Organomet. Chem. 1998, 557, 69.
(14) Schrock, R. R.; Baumann, R.; Reid, S. M.; Goodman, J. T.; Stumpf,
R.; Davis, W. M. Organometallics 1999, 18, 3649.
(15) The only interaction that has been elucidated through X-ray studies is
in a thorium species in which the anion is bound in the solid state to the
metal through the meta and para fluorides in perfluorotetraphenylborate; Yang.
X.; Stern, C. L.; Marks, T. J. Organometallics 1991, 10, 840.
(16) Vanka, K.; Chan, M. S. W.; Pye, C. C.; Ziegler, T. Organometallics
2000, 19, 1841.
Activation of [MesNpy]Hf(i-Pr)2 with 1 equiv of [Ph3C]-
[B(C6F5)4] led to formation of Ph3CH (only) and two major
(17) Vanka, K.; Ziegler, T. Organometallics 2001, 20, 905.
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(9) Molecular weights were determined by light scattering in THF with a
Wyatt Minidawn system using a dn/dc value of 0.076. The dn/dc value was
determined by Wyatt on poly(1-hexene) samples prepared with the hafnium
catalysts.