A comparison of cationic zirconium methyl and isobutyl initiators that contain an arylated diamido-pyridine ligand for polymerization of 1-hexene. Elucidation of a dramatic 'initiator effect' [16]

Full text of DOI:10.1021/ja000772v

J. Am. Chem. Soc. 2000, 122, 7841-7842
7841
yield employing Me3SiCl (3 equiv, ether, 1 h at 22 °C). Subse-
quent alkylation of [MesNpy]ZrCl2 with RMgBr (R ) Me or i-Bu)
yielded the dialkyl derivatives, [MesNpy]ZrR2 (R ) Me (1a) or
i-Bu (1b)). The diisobutyl derivative is thermally stable for days
at room temperature in the dark in the solid state, but appears to
decompose slowly in fluorescent room light. Slow rotation of the
mesityl rings on the NMR time scale about the N-Cipso bond is
observed in all compounds at low temperatures, and in [MesNpy]-
ZrCl2 even at 22 °C.
An X-ray study of 1a (Figure 1; see Supporting Information)
showed it to have approximately a trigonal bipyramidal structure
with planar equatorial amido nitrogens (N(1) and N(2)), one which
closely resembles the structures found for [Me3SiNpy]TiBr2 and
[Me3SiNpy]Ti(CH2SiMe3)Br.13 The plane of each mesityl ring is
roughly perpendicular to the Zr-N-Cipso plane, while the N(1)-
Zr-N(2) angle (102.24(12)°) is significantly smaller than the
analogous angle found in (for example) [(MesNCH2CH2)2NMe]-
ZrMe2 (140.5(2)°).5
A Comparison of Cationic Zirconium Methyl and
Isobutyl Initiators that Contain an Arylated
Diamido-Pyridine Ligand for Polymerization of
1-Hexene. Elucidation of a Dramatic “Initiator
Effect”
Parisa Mehrkhodavandi, Peter J. Bonitatebus, Jr., and
Richard R. Schrock*
Department of Chemistry, Massachusetts Institute
of Technology, Cambridge, Massachusetts 02139
ReceiVed March 3, 2000
We are developing the chemistry of d0 zirconium complexes
that contain diamido/donor ligands,1 e.g., [(t-Bu-N-o-C6H4)2O]2-,2-4
[(MesNCH2CH2)2NR]2-,5 and others,6-10 in particular chemistry
that concerns cationic monoalkyl complexes as initiators for olefin
polymerization. In the case of the [Ph3C][B(C6F5)4]-activated [(t-
BuN-o-C6H4)2O]ZrMe2 system, polymerization of 1-hexene takes
place in a living fashion in chlorobenzene below 10 °C via 1,2-
insertions into the Zr-Me bond, as shown by 13C labeling3 and
kinetic11 studies. Until now we have employed ligands that have
a central donor and two atoms in the two arms that connect the
amido atom to the central donor, and which, with two excep-
tions,10,12 are relatively flexible. In this communication we report
a variation of the relatively rigid diamido pyridine derivative
reported by Gade,13 and elucidate a striking difference between
two observable cationic alkyl initiators in 1-hexene polymerization
reactions and dramatically different stabilities of cationic alkyls
toward â elimination.
The reaction between [MesNpy]ZrMe2 and [Ph3C][B(C6F5)4]
in bromobenzene-d5 results in the formation of Ph3CCH3 and what
appears to be an orange species that is stable for days at -30 °C
(under dinitrogen, according to NMR spectra) that we formulated
initially as “{[MesNpy]ZrMe}[B(C6F5)4]”. However, we find that
addition of only 0.5 equiv of [Ph3C][B(C6F5)4] gives a yellow
species that has virtually identical proton and carbon NMR spectra
as that formed by adding 1 equiv of [Ph3C][B(C6F5)4]. At 22 °C
the Zr-Me resonance is found at 0.66 ppm in the proton NMR
spectrum, while in the Zr13CH3 analogue a broad Zr-Me carbon
resonance is found at ∼38 ppm. At -60 °C in a mixed solvent
(1:1 C6D5Br:C6D5CD3) two methyl resonances are observed in a
ratio of ∼2:1 at 32.1 and 41.9 ppm; JCH was found to be 113 Hz
for the downfield resonance. All data are consistent with formation
of a dimeric monocationic species having the formula {[Mes-
Npy]2Zr2Me3}[B(C6F5)4] (2) that is relatively unreactive toward
additional [Ph3C][B(C6F5)4] at room temperature at the concentra-
tions employed. (The orange color that is present upon addition
of 1 equiv of [Ph3C][B(C6F5)4] is ascribed to excess [Ph3C]-
[B(C6F5)4].) One possible formulation for 2 is analogous to the
crystallographically characterized monocationic dimer formed
from a zirconocene dimethyl species and 0.5 equiv of activator
in which one of the three methyl groups bridges the two zirconium
centers symmetrically.16
Addition of 80 equiv of 1-hexene to a 1:1 mixture of 2 and
[Ph3C][B(C6F5)4] (formed from 1a and 1 equiv of [Ph3C]-
[B(C6F5)4], 0.01 M Zr, 22 °C, C6D5Br) leads to formation of poly-
(1-hexene) over a period of 3-4 h. A plot of ln[1-hexene] versus
time (Figure 2) shows significant curvature, and a significant
amount (at least 50%) of 2 remains (according to proton NMR)
after all 80 equiv of 1-hexene has been consumed. The most linear
portion of the plot (>100 min) yields a kobs of 0.016 min-1 at
294 K ([Zr]o ) 10 mM). Similar behavior is observed at higher
temperatures (kobs ) 0.021 min-1 at 298 K, 0.034 min-1 at 303
K, and 0.074 min-1 at 313 K). An Eyring plot yielded ∆H‡ )
14.3(5) kcal/mol and ∆S‡ ) -26(2) cal/(mol K). Addition of a
second 80 equiv of 1-hexene to the 294 K sample after
consumption of the first 80 equiv led to further polymer formation
with a kobs ) 0.018 min-1 derived from a now somewhat more
linear ln[1-hexene] versus time plot. However, a significant
amount of the initiator is still present even after 160 equiv of
1-hexene have been consumed. GPC analysis of two different
poly(1-hexene) samples formed from the methyl initiator showed
them to have relatively high Mn values (approximately 10× theory
based on all Zr present), but a surprisingly narrow molecular
weight distribution (PDI ) 1.02, 1.08).
The pyridyl diamine shown in eq 1 was prepared according to
the method reported by Gade.13 Arylation under conditions
reported by Buchwald14,15 gave the dimesityl derivative, H2-
[MesNpy], in good yield. Addition of H2[MesNpy] to Zr(NMe2)4
gave [MesNpy]Zr(NMe2)2, in which the dimethylamido ligands
1
are inequivalent, according to H and 13C{1H} NMR spectra.
[MesNpy]Zr(NMe2)2 was converted into [MesNpy]ZrCl2 in high
(1) Gade, L. H. Chem. Commun. 2000, 173.
(2) Baumann, R.; Davis, W. M.; Schrock, R. R. J. Am. Chem. Soc. 1997,
119, 3830.
(3) Baumann, R.; Schrock, R. R. J. Organomet. Chem. 1998, 557, 69.
(4) Schrock, R. R.; Baumann, R.; Reid, S. M.; Goodman, J. T.; Stumpf,
R.; Davis, W. M. Organometallics 1999, 18, 3649.
(5) Liang, L.-C.; Schrock, R. R.; Davis, W. M.; McConville, D. H. J. Am.
Chem. Soc. 1999, 121, 5797.
(6) Schrock, R. R.; Schattenmann, F.; Aizenberg, M.; Davis, W. M. Chem.
Commun. 1998, 199.
(7) Graf, D. G.; Schrock, R. R.; Davis, W. M.; Stumpf, R. Organometallics
1999, 18, 843.
(8) Aizenberg, M.; Turculet, L.; Davis, W. M.; Schattenmann, F.; Schrock,
R. R. Organometallics 1998, 17, 4795.
(9) Schrock, R. R.; Seidel, S. W.; Schrodi, Y.; Davis, W. M. Organome-
tallics 1999, 118, 428.
(10) Flores, M. A.; Manzoni, M.; Baumann, R.; Davis, W. M.; Schrock,
R. R. Organometallics 1999, 18, 3220.
(11) Goodman, J. T. Unpublished observations.
(12) Schrock, R. R.; Liang, L.-C.; Baumann, R.; Davis, W. M. J.
Organomet. Chem. 2000, 591, 163.
(13) Friedrich, S.; Schubart, M.; Gade, L. H.; Scowen, I. J.; Edwards, A.
J.; McPartlin, M. Chem. Ber. Rec. 1997, 130, 1751.
(14) Wolfe, J. P.; Wagaw, S.; Marcoux, J. F.; Buchwald, S. L. Acc. Chem.
Res. 1998, 31, 805.
(15) Wolfe, J. P.; Wagaw, S.; Buchwald, S. L. J. Am. Chem. Soc. 1996,
118, 7215.
(16) Chen, Y.-X.; Stern, C. L.; Yang, S.; Marks, T. J. J. Am. Chem. Soc.
1996, 118, 12451.
10.1021/ja000772v CCC: $19.00 © 2000 American Chemical Society
Published on Web 07/29/2000

7842 J. Am. Chem. Soc., Vol. 122, No. 32, 2000
Communications to the Editor
85% yield versus an internal diphenylmethane standard. At 0 °C
the isobutene begins to disappear (we propose that it is polym-
erized), and the amount of {[MesNpy]Zr(i-Bu)}[B(C6F5)4] de-
creases to ∼65% of theory. {[MesNpy]Zr(i-Bu)}[B(C6F5)4]
decomposes with time in a smooth first-order manner (kobs
)
0.0017 min-1; t1/2 ) 40 min) at 0 °C to give isobutene (which is
observed to increase in concentration and then ultimately decrease
as it is polymerized) and a Zr species that we believe to be
catalytically inactive. Addition of 100 equiv of 1-hexene at 263
K to {[MesNpy]Zr(i-Bu)}[B(C6F5)4] led to quantitative formation
of poly(1-hexene) over a period of 2 h, complete consumption of
{[MesNpy]Zr(i-Bu)}[B(C6F5)4], a plot of ln[1-hexene] vs time
that was linear throughout the polymerization time period (Figure
2), and no formation of any significant olefinic byproducts, in
particular none with olefinic resonances near 5.36 ppm. The
observed rate constant was found to be 0.019 min-1 at [Zr]o )
7.3 mM, or kobs/[Zr]o ) k ) 0.0436 M-1 s-1. Polymerizations at
253 (k ) 0.0268 M-1 s-1), 273 (k ) 0.0833 M-1 s-1), and 283 K
(k ) 0.164 M-1 s-1) were similarly well-behaved. An Eyring plot
yielded ∆H‡ ) 8.1(7) kcal/mol and ∆S‡ ) -33(3) cal/(mol K).
GPC analysis of the poly(1-hexene) formed from the isobutyl
initiator at 0 °C was found to have an Mn value approximately
3× theory based on all Zr present and a PDI of 1.03. The initial
rate of consumption of 100 equiv of 1-hexene ([Zr] ) 7.3 mM)
was found to be approximately 1000 times more rapid than the
rate of decomposition of {[MesNpy]Zr(i-Bu)}[B(C6F5)4] at 0 °C,
which accounts for complete initiation to give the (much more
stable) propagating cation.
Figure 2. Circles: Plot of ln[1-hexene/standard] vs time after addition
of 80 equiv of 1-hexene to the activator formed from [MesNpy]ZrMe2
and 1 equiv of [Ph3C][B(C6F5)4] (0.01 M in C6D5Br) at 25 °C. Squares:
Plot of ln[1-hexene/standard] vs time after addition of 100 equiv of
1-hexene to the activator formed from [MesNpy]Zr(i-Bu)2 and 1 equiv
of [Ph3C][B(C6F5)4] (0.00073 M in C6D5Br) at -10 °C.
We propose that the active species in each system (activated
1a or 1b) is a solvated form of {[MesNpy]Zr(R)}[B(C6F5)4] (R
) Me or i-Bu). However, for steric reasons{[MesNpy]ZrMe}-
[B(C6F5)4] reacts with 1-hexene to a significant extent by 2,1
insertion to give a 3-heptyl complex that is highly susceptible to
â hydride elimination to yield 2-heptenes; only a fraction reacts
via a 1,2 insertion to give a stable propagating species. In contrast,
{[MesNpy]Zr(i-Bu)}[B(C6F5)4] does not form a strong adduct with
[MesNpy]Zr(i-Bu)2. Therefore, virtually all the Zr is available to
react with 1-hexene, and a steric environment is created in which
only 1,2 insertion of 1-hexene can take place. In each case the
propagating alkyl for steric reasons is the most likely to react in
a 1,2 fashion and is the most stable to â elimination. The reason
the PDI of the poly(1-hexene) in the potentially complex and
dynamic methyl system is so low is not yet clear.
Figure 3. Plot of [2-heptenes]/[standard] vs time (min) for two con-
secutive additions of 80 equiv of 1-hexene to the activator formed from
[MesNpy]ZrMe2 and 1 equiv of [Ph3C][B(C6F5)4] (25 °C, C6D5Br);
standard ) hexamethylbenzene.
In the polymerization reactions described immediately above
weak resonances grow in at 5.36 ppm, rapidly at first, but then
more slowly, and finally stop when all 1-hexene has been
consumed. They grow in again upon addition of a second 80 equiv
aliquot of 1-hexene and stop again upon consumption of the
second 80 equivalents of 1-hexene (Figure 3); note that less of
this product is formed during the second phase. By proton NMR
comparison with authentic samples this product has been identified
as a mixture of trans-2-heptene and cis-2-heptene (∼3:1). Vacuum
transfer of the volatile components yields a sample with an
identical olefinic pattern near 5.36 ppm. We propose that the
2-heptenes form by selective â elimination in a 3-heptyl complex,
the 2,1 “misinsertion” product of the reaction between a cationic
zirconium methyl species and 1-hexene, that the product of â
elimination is inactive, and that the fraction of 3-heptyl complex
that reacts with 1-hexene does so in a 1,2-fashion and produces
an alkyl that is relatively stable to â elimination.
Addition of 1-hexene to 2 that is prepared by addition of 0.5
equiv of [Ph3C][B(C6F5)4] to 1a (0.01 M Zr, 22 °C, C6D5Br) also
leads to formation of poly(1-hexene) and 2-heptenes, but the rate
of polymerization is ∼25% of the rate when the initiator is pre-
pared by addition of 1.0 equiv of [Ph3C][B(C6F5)4] to 1a. Addition
of 0.4 equiv of [Ph3C][B(C6F5)4] to 1a leads to a mixture of 2
and ∼0.1 equiv of 1a that is virtually unreactive toward 1-hexene
(∼50 equiv yields a trace of poly(1-hexene) in 1 day). We propose
that {[MesNpy]ZrMe}[B(C6F5)4] is the active initiator in the
presence or absence of excess [Ph3C][B(C6F5)4], and that it and
1a are in an equilibrium with 2 that lies far to the side of 2.
Activation of 1b with [Ph3C][B(C6F5)4] in C6D5Br at -30 °C
produced Ph3CH, isobutene, and a yellow species that we
formulate as monomeric {[MesNpy]Zr(i-Bu)}[B(C6F5)4] in at least
We believe the results presented here are a clear-cut example
of an “initiator effect” in olefin polymerization by relatively well-
characterized group 4 catalysts.17,18 Further exploration of the
phenomena uncovered here and their implications for a variety
of alkyls, monomers, and catalysts that contain more bulky aryl
groups on nitrogen is under way.
Acknowledgment. We thank the Department of Energy (DE-FG02-
86ER13564) and Exxon Corporation for supporting this research and Dr.
Arturo Casado for useful discussions.
Supporting Information Available: Figure 1, experimental proce-
dures, fully labeled ORTEP drawing, crystal data and structure refinement,
atomic coordinates, bond lengths and angles, and anisotropic displacement
parameters for [MesNpy]ZrMe2 (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
JA000772V
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