Synthesis, Structure, and Olefin Polymerization Catalytic Behavior of Aryl-Substituted Zirconocene Dichlorides

Article abstract of DOI:10.1021/om034280d

Standard ligand-substitution reactions afforded moderate yields of (PhC₅H₄)₂ZrCl₂ (1), (1,3-Ph ₂C₅H₃)CpZrCl₂ (2), [1,3,-(C ₆F₅)₂C

Full text of DOI:10.1021/om034280d

Organometallics 2004, 23, 1333-1339
1333
Syn th esis, Str u ctu r e, a n d Olefin P olym er iza tion
Ca ta lytic Beh a vior of Ar yl-Su bstitu ted Zir con ocen e
Dich lor id es
Matthew P. Thornberry, Nathan T. Reynolds,† and Paul A. Deck*
Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061-0212
Frank R. Fronczek
Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803-1804
Arnold L. Rheingold§ and Louise M. Liable-Sands
Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716
Received November 5, 2003
Standard ligand-substitution reactions afforded moderate yields of (PhC5H4)2ZrCl2 (1), (1,3-
Ph2C5H3)CpZrCl2 (2), [1,3,-(C6F5)2C5H3]CpZrCl2 (5), and [1,2,4-(C6F5)2C5H3]CpZrCl2 (6).
Crystal structures of 1, 2, 6, and [(C6F5)C5H4]2HfMe2 (7) were obtained. Physicochemical
studies (CV, IR) show that C6F5 groups are strongly electron-withdrawing compared to phenyl
groups. In ethylene-1-hexene copolymerization reactions using 1, 2, (C6F5C5H4)CpZrCl2 (3),
(C6F5C5H4)2ZrCl2 (4), and 5 (toluene solution, methylalumoxane cocatalyst), C6F5 substituents
decrease catalytic activity but increase comonomer incorporation compared to phenyl
substituents.
In tr od u ction
Electronic substituent effects in metallocene-catalyzed
olefin polymerization are not as well understood.14-19
Strongly electron-donating (OMe, NMe2)20-23 or electron-
withdrawing substituents (e.g., F,22,24-27 CF3,28-32
Structure-activity relationships in olefin polymeri-
zation processes catalyzed by soluble group 4 metal
complexes, especially zirconocene dichloride (Cp2ZrCl2,
Cp ) η5-C5H5) and its derivatives, have been analyzed
extensively.1,2 Subtle changes in ancillary ligand struc-
ture can influence stereoregularity, comonomer distri-
bution, and other polyolefin properties, in addition to
catalytic activity.3,4 Empirical trends and theoretical
modeling have illuminated the polymerization mecha-
nism in considerable detail.5-13
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† Undergraduate research participant.
§ Present address: Department of Chemistry and Biochemistry,
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Publication on Web 02/12/2004

1334 Organometallics, Vol. 23, No. 6, 2004
CnF2n+1
33 BR2,34-36 and acetyl15,37-40) are often difficult
Thornberry et al.
Ch a r t 1
,
to attach to the cyclopentadienyl ligands, or they may
be incompatible with the conditions of metallocene
synthesis or olefin polymerization catalysis. For these
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usually studied over a relatively narrow range in which
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uents such as CH3 or SiMe3 is varied.41-47 The resulting
trends are difficult to uncouple from the steric effects
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corporation and polymer tacticity. Recent reports have
also pointed to the importance of ion pairing and
solvation effects in polymerization processes.6,11,48,49
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stituent effects on the copolymerization of ethylene and
1-hexene catalyzed by a series of substituted zirconocene
dichlorides and methylalumoxane (MAO).50 Our ap-
proach uses pentafluorophenyl (C6F5) as a highly elec-
tron-withdrawing substituent, as in complexes 1-6
(Chart 1). We showed in earlier reports that C6F5 is
readily attached to cyclopentadienyl (Cp) or indenyl
ligands by nucleophilic aromatic substitution reactions
of C6F6 and Cp or indenyl anions51-54 and that these
ligands can be used to prepare a wide range of transition
metal Cp complexes.55-60 Meanwhile, the phenyl (C6H5
or Ph) group is similar in overall size and shape to the
C6F5 group, but, as we will show, phenyl is weakly
electron-donating. The phenyl substituent therefore
serves as a “steric control”. This report describes the
synthesis and characterization of C6F5- and C6H5-
substituted group 4 metallocenes and the results of our
copolymerization experiments. We find that electron-
withdrawing substituents on the Cp ligands signifi-
cantly increase the incorporation of 1-hexene while
decreasing both activity and molecular weight of poly-
ethylenes produced, compared to using Cp2ZrCl2/MAO.
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atmosphere procedures and crystallizations refers to n-hexane.
(C6F5C5H4)CpZrCl2 (3),62 (C6F5C5H4)2ZrCl2 (4),51 (C6F5C5H4)2-
HfMe2 (7),62 (C6F5C5H4)Na,51 [1,3-(C6F5)2C5H3]Na,51 1,2,4-
(C6F5)3C5H2]Na,54 ZrCl4(THF)2,63 and CpZrCl3(THF)64 were
prepared by published methods. Phenylcyclopentadiene65 and
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methods and converted to their corresponding sodium salts
by treatment with NaH in THF. (The latter deprotonations
were slow and proceeded with some decomposition; KH would
probably have been a better reagent.) MAO was obtained as a
10% solution in toluene as a gift from Albemarle Corporation
and dried under high vacuum (10-5 Torr, 15 h) to remove the
solvent and “free” trimethylaluminum.68,69 NMR experiments
used a Varian Unity 400 instrument. NMR solvents were
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Aryl-Substituted Zirconocene Dichlorides
Organometallics, Vol. 23, No. 6, 2004 1335
Ta ble 1. Cr ysta llogr a p h ic Da ta
2
1
6
7
empirical formula
fw
diffractometer
cryst dimens (mm)
cryst syst
a (Å)
C
22H18Cl2Zr
C22H18Cl2Zr
444.48
C28H7Cl2F15Zr
790.46
C24H14F10Hf
670.84
444.48
Nonius Kappa CCD
0.03 × 0.08 × 1.00
orthorhombic
11.5690(13)
23.3310(18)
6.755(3)
90
Nonius Kappa CCD
0.10 × 0.25 × 0.35
monoclinic
14.8140(2)
29.8600(9)
8.6230(3)
90
99.7980(14)
90
3758.71(18)
P21/n (No. 14)
8
Nonius Kappa CCD
0.20 × 0.23 × 0.28
monoclinic
11.2970(2)
14.0810(3)
16.4470(4)
90
90.6920(14)
90
2616.08(10)
P21/n (No. 14)
4
Siemens P4
0.30 × 0.30 × 0.30
triclinic
10.0776(2)
10.4687(2)
10.6314(2)
86.6088(6)
74.0965(4)
80.2345(7)
1062.95(4)
P1h (No. 2)
2
b (Å)
c (Å)
R (deg)
â (deg)
90
90
γ (deg)
V (Å)3
1823.3(8)
Fmm2 (No. 42)
4
1.619
0.9
896
space group
Z
D
calc (Mg m-3
)
1.571
0.9
1792
2.007
0.8
1536
2.096
5.0
640
abs coeff (mm-1
F000
)
λ(Mo KR) (Å)
0.71073
150
0.71073
120
0.71073
150
0.71073
198
temp (K)
θ range
3.5-30.3
2.6-30.0
2.6-35.0
2.0-28.4
no. of reflns colld
no. of indep reflns
abs corr method
no. of data/params
R [I > 2σ(I)]
9620
18 512
20 309
6958
1308
10 906
11 399
4587
multiscan
1308/64
0.061
multiscan
10 906/451
0.033
multiscan
11 399/415
0.032
DIFABS
4587/316
0.045
Rw [I > 2σ(I)]
0.16
1.09
0.94, -1.07
0.074
0.87
0.75, -0.73
0.081
0.98
0.48, -0.84
0.120
0.97
1.41, -1.85
GoF on F2
peak, hole (e Å-3
)
vacuum-transferred from appropriate drying agents. 19F NMR
spectra were referenced to external C6F6 in CDCl3 (-163 ppm).
Quantitative 13C experiments were conducted using Randall’s
published method.70 GPC analyses were carried out at Dow
Chemical Company (Midland, Michigan) using a Waters 150C
instrument. Elemental analyses were performed by Desert
Analytics (Tucson, AZ) or Oneida Research Services (Whites-
boro, NY). The Cambridge Structural Database (CSD) was
searched using the ConQuest software package provided by
Cambridge.
with a thermocouple, an ethylene inlet-port, a septum-sealed
injection port, and a Teflon-sealed overhead paddle stirrer
fitted through a condenser. After drying in an oven for several
hours, the reactor was evacuated to below 0.1 Torr while still
warm and then flushed thorougly with nitrogen. Purified61,71
toluene (400 mL) was added, followed by 50 mg of MAO to
scavenge adventitious moisture. Comonomer (1-hexene, 20 mL,
freshly distilled from calcium hydride) was then added using
a syringe. The reactor was lowered into a vigorously agitated
water bath interfaced to an Omega temperature controller.
After establishing an internal temperature of 50 °C, the stirred
reactor was purged for 5 min with ethylene (purified by
passage over Engelhard Q5 oxygen scavenger and Davison 10X
molecular sieves) through an immersed gas dispersion tube,
allowing the exhaust to escape via the condenser to a mineral
oil bubbler. During reactor temperature equilibration, a
cocktail containing MAO (300 mg) and metallocene precatalyst
(2.6 µmol) in 10 mL of toluene was stirred for about 20 min at
25 °C to preactivate the catalyst. An aliquot of the catalyst
cocktail (typically 1-5 mL depending on anticipated activity)
was injected into the reactor to commence polymerization.
Reactions in which the internal temperature varied by more
than 1 °C were quenched and discarded. After 5 min, 20 mL
of acidified methanol (prepared by diluting 50 mL of concen-
trated hydrochloric acid to 1 L with methanol) was added to
quench the reaction. The reactor was vented, and the contents
were poured into 200 mL of rapidly stirred acidified methanol.
The solvent volume was reduced to about 20 mL by evapora-
tion, and another 200 mL of acidified methanol was added to
induce precipitation. The polymer was collected on a filter,
washed with methanol, and dried in a vacuum oven at 60 °C
for at least 12 h. We conducted several control experiments
with zirconocene dichloride to ensure that activity (gPE mol-1
atm-1 h-1) was invariant with metallocene concentration and
with a 2-fold increase in reaction time.
Cr ysta llogr a p h ic Stu d ies. Colorless prisms of 1 were
obtained by cooling a concentrated solution of 1 in hexane/
toluene to 25 °C. Cooling a hexane/toluene solution of 2 to 25
°C afforded colorless crystals that were coated with an
unidentified dark residue. Pale yellow crystals of 6 were
obtained by cooling a warm toluene/hexane solution slowly to
-10 °C. Colorless crystals of 7 were discovered in a hexane
solution of the complex in a Schlenk flask that had been left
in a refrigerator for several months. Relevant crystallographic
data are assembled in Table 1. Complete data tables are
included in the Supporting Information. The crystals used for
the structural characterization of 1 (and the ensuing data set)
were of marginal quality, and an ambiguity arose in the
assignment of the space group. We initially solved the struc-
ture in C2 (monoclinic), but systematic absences were equally
consistent with Fmm2 (orthorhombic), so we defaulted to the
higher symmetry. Thermal ellipsoids looked about the same
in both solutions. We deduce that even if 1 does not have
crystallographic mm2 symmetry, the molecular geometry must
still be very close to C2v. An initial structure of 2 obtained at
200 K showed only one molecule per asymmetric unit, but
some of the ellipsoids for the carbon atoms of the unsubstituted
Cp ligand were elongated, suggesting a positional disorder. A
superior data set was subsequently obtained at 120 K, showing
two molecules per independent unit, differing most notably
by rotation about the Cp-Zr axis of the unsubstituted Cp
ligand. The solutions and refinements of the structures of 6
and of 7 were routine.
1,1′-Dip h en ylzir con ocen e Dich lor id e (1). A solution of
ZrCl4 (0.29 g, 1.2 mmol) and sodium phenylcyclopentadienide
(0.40 g, 2.4 mmol) in THF (50 mL) was stirred at room
temperature for 24 h. After evaporating the solvent, the green
residue was recrystallized from hot toluene to afford 0.25 (0.56
mmol, 47%) of green-yellow needles. A second crystallization
P olym er iza tion Stu d ies. The reactor used for polymeri-
zation experiments was a 1.5 L flanged glass kettle equipped
(69) Imhoff, D. W.; Simeral, L. S.; Blevins, D. R.; Beard, W. R. ACS
Symp. Ser. 2000, 749, 177-191.
(70) Hsieh, E. T.; Randall, J . C. Macromolecules 1982, 15, 1402-
1406.
(71) Evans, W. J .; Pedersen, S.; Farina, J .; Meyer, J .; Staunton, G.
Chem. Eng. News 2002, 80, 8-12.

1336 Organometallics, Vol. 23, No. 6, 2004
Thornberry et al.
from hexane/toluene afforded colorless needles. 1H NMR
An a lysis of Electr on ic Su bstitu en t Effects. Nu-
merous methods have been developed for quantifying
the effects of ligand substituents, and differences among
these methods are notable. Using oxidation potentials
of arylated ferrocenes, one would conclude that both
C6F5 and C6H5 are electron-withdrawing, although the
effect of C6H5 is much smaller in magnitude (about 5
mV per C6H5 group compared to 170 mV per C6F5
group).72 Using infrared spectroscopic analysis of
arylated CpMn(CO)3 complexes, C6F5 is strongly electron-
withdrawing, increasing νCO(A) by 4 cm-1 per C6F5
group, whereas each C6H5 group has a roughly opposite
effect.67 Core electron binding energies are arguably the
most direct probe of metal-centered electron de-
ficiency,73-76 especially for complexes lacking useful
ligands for IR spectroscopy (such as carbonyl)77,78 or
highly metal-centered HOMOs needed for voltammetric
measurements.42 Unfortunately, the XPS method also
suffers from poor sensitivity and resolution, and we lack
the special equipment needed to mount air-sensitive
samples. However, iron core electron (Fe 2p) binding
energies correlated well with δCp for a series of [(η6-
arene)FeCp][PF6] complexes,79 and in that series, the
C6H5 subsituent (arene ) biphenyl) is about as electron-
donating as two methyl groups. We likewise thought of
using the chemical shifts (δCp) of the unsubstituted Cp
ligands of the unsymmetrical metallocenes (2, 3, 5, and
6) to probe relative metal-centered electrophilicities. An
upfield shift of about 0.16 ppm in δCp was observed for
each of two phenyl groups in 1 (relative to Cp2ZrCl2),
signifying a relatively strong electron-donating effect of
the phenyl groups. In contrast, each C6F5 group in 3
and 5 effected a downfield shift in δCp of only about 0.03
ppm. The triarylated complex (6) deviated significantly
from this trend, showing an upfield shift in δCp of 0.07
ppm relative to Cp2ZrCl2. However, with increasing
arylation, more conformations bring the aryl groups in
close proximity to the opposing Cp ligand, which could
result in transannular local magnetic field anisotropies
(“ring currents”). In the case of 6, structural distortions
may also be significant. These issues are much less
important in metallocenes having parallel ligands. In
summary, we do not yet have an independent quantita-
tive measure of the electronic effects of Ph versus C6F5
in group 4 complexes, but where good comparisons can
be made in other complexes, the Ph group appears to
be neutral or weakly electron-donating, whereas C6F5
is moderately electron-withdrawing. Importantly C6F5
is always found to be more electron-withdrawing than
C6H5, which probably surprises no one.
(CDCl3): δ 7.53 (m, 4 H), 7.15 (t, 3J HH ) 7.6 Hz, 4 H), 7.32 (tt,
4
3
3J HH ) 7.4 Hz, J HH ) 1.5 Hz, 2 H), 6.71 (t, J HH ) 4J HH ) 2.8
3
Hz, 4 H), 6.29 (t, J HH
)
4J HH ) 2.6 Hz, 4 H). 13C NMR
(CDCl3): δ 133.0, 129.2, 128.4, 128.3, 126.3, 115.8, 115.6. Anal.
Calcd for C22H18Cl2Zr: C, 59.45; H, 4.08. Found: C, 59.33; H,
3.76.
1,3-Dip h en ylzir con ocen e Dich lor id e (2). A mixture of
CpZrCl3(DME) (0.514 g, 1.46 mmol), sodium 1,3-diphenylcy-
clopentadienide (0.30, 1.2 mmol), and toluene (50 mL) was
stirred at reflux for 20 h. The reaction mixture was cooled and
filtered through a glass frit. The toluene was stripped, and
the crude solid was recrystallized from toluene/hexane to afford
1
0.13 g (0.29 mmol, 23%) of gray needles. H NMR (CDCl3): δ
3
3
7.69 (m, 4 H), 7.46 (t, J HH ) 7.8 Hz, 4 H), 7.34 (tt, J HH ) 7.4
4
4
Hz, J HH ) 1.4 Hz, 2 H), 7.24 (t, J HH ) 2.6 Hz, 1 H), 6.88 (d,
4J HH ) 2.4 Hz, 2 H), 6.18 (s, 5 H). 13C NMR (CDCl3): δ 133.1,
129.2, 128.5, 127.3, 126.2, 117.2, 113.8, 112.9. Anal. Calcd for
C
22H18Cl2Zr: C, 59.45; H, 4.08. Found: C, 59.13; H, 3.85.
1,3-Bis(p en ta flu or op h en yl)zir con ocen e Dich lor id e (5).
A mixture of CpZrCl3 (1.04 g, 3.97 mmol), sodium 1,3-bis-
(pentafluorophenyl)cyclopentadienide (1.72 g, 4.09 mmol), and
toluene (100 mL) was stirred at 110 °C for 2 h. The hot mixture
was filtered, and the precipitate was washed with an ad-
ditional 50 mL of hot toluene. The filtrate was cooled to 25
°C, and the resulting microcrystalline precipitate was collected
on a filter, washed with pentane (25 mL), and dried under
1
vacuum to afford 1.91 g (3.06 mmol, 77%) of a white solid. H
NMR (CDCl3): δ 7.35 (s, 1 H), 6.93 (s, 2 H), 6.55 (s, 5 H). 19F
3
NMR (CDCl3): δ -139.65 (m, 4 F), -153.51 (tt, J FF ) 21 Hz,
4J FF ) 2.6 Hz, 2 F), -161.65 (m, 4 F). Anal. Calcd for C22H8-
Cl2F10Zr: C, 42.32; H, 1.29. Found: C, 42.55; H, 1.03.
1,2,4-Tr is(p en ta flu or op h en yl)zir con ocen e Dich lor id e
(6). A mixture of CpZrCl3(DME) (0.138 g, 0.391 mmol), [1,2,4-
(C6F5)3C5H2]Na (0.222 g, 0.379 mmol), and toluene (25 mL)
was stirred at 22 °C for 20 h and then filtered. The filtrate
was evaporated, and the residue was triturated with hexane
(15 mL, 0 °C) to afford 0.132 g (0.167 mmol, 44%) of a tan
solid. An analytically pure sample was obtained by crystal-
lization from toluene/hexane. 1H NMR (CDCl3): δ 7.35 (s, 2
3
H), 6.42 (s, 5 H). 19F NMR (CDCl3): δ -134.98 (d, J FF ) 19
3
3
Hz, 4 F), -138.96 (d, J FF ) 18 Hz, 2 F), -151.77 (tt, J FF
)
)
4
3
4
21 Hz, J FF ) 2.8 Hz, 2 F), -152.46 (tt, J FF ) 21 Hz, J FF
3.0 Hz, 1 F), -160.50 (m, 2 F), -160.93 (m, 4 F). Anal. Calcd
for C28H7Cl2F15Zr: C, 42.55; H, 0.89. Found: C, 42.77; H, 0.77.
Resu lts
Meta llocen e Syn th esis. Several metallocenes (1-
6) desired for the catalytic studies described here were
prepared in moderate yields by standard ligand-
substitution reactions (eqs 1-6). Complexes 3 and 4
were reported earlier.51 The synthesis and purification
of the thermally unstable triarylated metallocene (6)
were carried out below 25 °C. We speculate that poor
ligand basicity accounts for the thermal instability of
6.
Str u ctu r a l Stu d ies. Three metallocene dichlorides
(1, 2, and 6) were structurally characterized. We also
(72) Lu, S.; Strelets, V. V.; Ryan, M. F.; Pietro, W. J .; Lever, A. B.
P. Inorg. Chem. 1996, 35, 1013-1023.
(73) Gassman, P. G.; Macomber, D. W.; Hershberger, J . W. Orga-
nometallics 1983, 2, 1470-1472.
2 [PhC5H5]Na + ZrCl4(THF)2 f 1
[1,3-Ph2C5H3]Na + CpZrCl3(DME) f 2
[C6F5C5H4]Na + CpZrCl3(DME) f 3
2 [C6F5C5H4]Na + ZrCl4(THF)2 f 4
(1)
(2)
(3)
(4)
(74) Gassman, P. G.; Winter, C. H. Organometallics 1991, 10, 1592-
1598.
(75) Gassman, P. G.; Deck, P. A.; Winter, C. H.; Dobbs, D. A.; Cao,
D. H. Organometallics 1992, 11, 959-960.
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Curnow, O. J .; Zheng, X. M. Organometallics 1997, 16, 5209-5217.
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1992, 441, 285-294.
(78) Graham, P. B.; Rausch, M. D.; Taschler, K.; Vonphilipsborn,
W. Organometallics 1991, 10, 3049-3052.
(79) Gassman, P. G.; Deck, P. A. Organometallics 1994, 13, 2890-
2894.
[1,3-(C6F5)2C5H3]Na + CpZrCl3(DME) f 5 (5)
[1,2,4-(C6F5)3C5H2]Na + CpZrCl3(DME) f 6 (6)

Aryl-Substituted Zirconocene Dichlorides
Organometallics, Vol. 23, No. 6, 2004 1337
F igu r e 1. Thermal ellipsoid plot of 1 shown at 50%
probability. Hydrogen atoms were removed for clarity.
F igu r e 4. Thermal ellipsoid plot of 7 shown at 50%
probability. Hydrogen atoms were removed for clarity.
ligand of 6 shows a “slip” distance (∆, defined according
to ref 81) of 0.074 Å, whereas the corresponding
parameter for the unsubstituted Cp ligand of 6 is 0.023
Å. As a reference, the Cp ligands of Cp2ZrCl2 show ∆ )
0.031-0.036 Å.80 We conclude that the “slip” distortions
observed for 6 are not especially noteworthy and may
not contribute substantially to the instability of this
complex. The Zr-Cl bond lengths in the zirconocenes
were invariant to ancillary ligand substitution (entries
1-4).
F igu r e 2. Thermal ellipsoid plot of 2 showing one of two
nearly identical molecules in the asymmetric unit at 50%
probability. Hydrogen atoms were removed for clarity.
A search of the Cambridge Structural Database (CSD)
revealed several crystallographically characterized group
4 metallocene complexes bearing isolated aryl substit-
uents. We are interested in the native conformational
preferences of aryl substituents, so we excluded from
our analysis several structures having vicinal rather
than isolated aryl groups.86-88 Aryl conformational
preferences may be described by two parameters (Figure
5, Table 2): the absolute Cp-aryl interplanar angle (0°
< R < 90°) and the absolute C(aryl)-Cp(centroid)-M-
X2(centroid) conformational angle (0° < â < 180°). In
the symmetrical, 1,1-diarylated metallocene (1), both
C6H5 groups are directed exactly between the two chloro
ligands, although the ambiguity in the space group
assignment means that the trivial torsion angle (â )
0°) may be inexact. Both C6H5 groups of 1 are nearly
coplanar with their respective Cp ligands (R ) 5°),
although the deviation from coplanarity is mostly due
to a slight splaying of the phenyl substituents away
from the chlorine atoms. Ten zirconocene dichloride
structures in the CSD having aryl substituents with
only hydrogens in the positions adjacent to the aryl
F igu r e 3. Thermal ellipsoid plot of 6 shown at 50%
probability. Hydrogen and fluorine atoms were removed
for clarity. Fluorine atoms are numbered the same as the
carbons to which they are attached.
discovered crystals of the hafnocene dimethide 7 in a
flask that had been abandoned in a refrigerator for
several months. Crystallographic data are collected in
Table 1. Thermal ellipsoid plots are shown in Figures
1-4. The metric data for the metallocene cores of 1 and
2 (Table 2, entries 1 and 2) were nearly identical to
corresponding data previously reported for Cp2ZrCl2
(entry 4).80 The triarylated zirconocene complex (6)
showed a lengthening of the Cp-Zr distance for the
substituted Cp ligand and widening of the Cp-Zr-Cp
angle in 6 (Table 2, entry 3 vs entry 4). In an effort to
understand the thermal instability of 6, we also exam-
ined the Zr-C bond distances for the triarylated Cp
ligand for “slip” distortions.81-85 The triarylated Cp
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Organometallics 1987, 6, 2012-2014.
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Marder, T. B. J . Organomet. Chem. 1990, 394, 777-794.
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4, 929-935.
(84) Carl, R. T.; Hughes, R. P.; Rheingold, A. L.; Marder, T. B.;
Taylor, N. J . Organometallics 1988, 7, 1613-1624.
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(86) Zhang, F.; Mu, Y.; Zhao, L.; Zhang, Y.; Bu, W.; Chen, C.; Zhai,
H.; Hong, H. J . Organomet. Chem. 2000, 613, 68-76.
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Chem. 2003, 682, 8-13.
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Polamo, M. Acta Chem. Scand. 1996, 50, 1116-1120.

1338 Organometallics, Vol. 23, No. 6, 2004
Thornberry et al.
Ta ble 2. Selected Bon d Dista n ces (Å) a n d An gles (d eg) for Sever a l Meta llocen e Com p lexes
entry
complex
Cpa-M
2.22(1)
2.21(1), 2.19(1) 2.42, 2.46
2.22(1), 2.21(1) 2.42, 2.46
2.29, 2.20
2.20(1), 2.20(1) 2.44, 2.45
2.21(1), 2.20(1) 2.45, 2.45
M-Xb
Cp-M-Cp
X-M-X
Rc
âc
1d
2e
1
2
2.45
129.3(2)
129.8(1)
129.9(1)
131.7(1)
129.5(1)
129.1(1)
132.1(2)
95.2(1)
98.0(1)
99.6(2)
98.7(1)
97.0(1)
97.1(1)
94.8(3)
5.0(7)
0.0(8)
15.2(2), 3.4(2)
12.4(2), 4.7(2)
36.9(2), 38.6(2), 39.6(2)
123.0(2), 92.1(2)
128.2(2), 87.1(2)
59.2(2), 131.9(2), 84.2(2)
3
6
2.41, 2.40
4e,f
Cp2ZrCl2
5
7
2.22, 2.24
2.25(1), 2.23(1)
24.2(5), 3.4(5)
74.9(5), 10.2(5)
a
b
Cp refers to the cyclopentadienyl ligand centroid. Cp-M distance errors are estimated from M-C distance errors. X refers to Cl or
the methyl carbon atom, where X ) Cl; M-X distance errors were typically 0.001 Å or less. c Conformational angles (absolute values)
defined in Figure 5. Errors are estimates. Molecule exhibits ambiguously characterized crystallographic mm2 symmetry. e Two essentially
d
identical independent molecules in the asymmetric unit. f Data from ref 100.
Ta ble 3. Da ta for Eth ylen e Hom p olym er iza tion
a n d Eth ylen e-1-h exen e Cop olym er iza tion
Rea ction s
activity,b
[1-hexene] Mg mol-1 Mw
mol %
entry catalysta
(M)
atm-1 h-1 (kD) PDI hexene
1
2
3
4
5
6
7
8
9
Cp2ZrCl2
1
4
53(10)
53(10)
14(2)
26(4)
26(4)
26(6)
12(6)
10(3)
7(1)
524
730
432
130
230
265
114
38
2.2
2.1
2.4
2.5
3.0
2.2
3.1
2.6
2.4
Cp2ZrCl2
0.4
0.4
0.4
0.4
0.4
0.4
6
7
5
7
13
19
1
2
3
4
5
F igu r e 5. Definitions of conformational parameters for
arylated group 4 metallocene complexes. R is the torsion
angle between the C6 and C5 planes (∼ABCD), and â is
the torsion angle C-Cp-M-E, where E is the centroid of
X1 and X2.
17
a
Catalysts were preactivated with MAO for 20 min at 25 °C.
Reaction volume ) 400 mL of toluene. Tp ) 50(1) °C. Al:Zr ) 2300.
P(C2H4) ) 1.0 atm. [1-Hexene] ) 0.40 M. Reaction time ) 5 min.
b
Standard deviations obtained from at least three runs.
such as arene stacking or C-H‚‚‚F-C interactions that
might influence the conformational disposition of the
C6F5 groups. However these relatively soft conforma-
tional parameters could still be influenced by general-
ized weak packing interactions.
In the structures of 2 (Figure 2) positioning an aryl
substituent in the center of the metallocene wedge (â ≈
0°) is prevented by transannular steric repulsions, so
the aryl groups are directed instead toward the “sides”
of the molecule. The Cp and aryl groups are nearly
coplanar. According to the CSD, when the 3- and
4-positions of a 1-aryl-substituted Cp ligand are oc-
cupied by methyl groups or by benzo ring fusion (2-
arylindenyl complexes), the aryl group is also usually
directed toward the side or into the Cp2M hemisphere
of the complex,16,89-93 and values of R are still around
15°. Thus the two parameters R and â are not strongly
correlated.
P olym er iza tion Resu lts. Table 3 summarizes our
polymerization results. The observed trend in activity
comparing Cp2ZrCl2 and its fluoroarylated congener 4
(entries 1 and 3; 4 and 9) is qualitatively consistent with
the results of Maldanis et al.,50 who showed that 4 has
slighly lower activity than Cp2ZrCl2 in ethylene homo-
polymerization at 50 °C.50 Because of the roughly
F igu r e 6. Thermal ellipsoid plot of a partial packing
diagram of 1 (50% probability) viewed along the crystal-
lographic a axis.
groups showed an average Cp-aryl interplanar angle
of R ) 12(6)°. A partial packing diagram of 1 (Figure 6)
shows how the molecules are arrayed in a head-to-tail
fashion along the crystallographic axis. However the
distance between the phenyl mean plane and the Cp
mean plane of a neighboring molecule is 3.34 Å (distance
between centroids ) 3.82 Å), probably too long to be
considered a true stacking interaction.
In the structure of 7 (Figure 4), one aryl substituent
is directed into the center of the metallocene wedge,
while one substituent is directed toward the side. The
Cp-aryl interplanar angles in 7 are small (nearly
coplanar). We speculate that the p-π donating ability
of the aryl fluorides is maximized when R ) 0°, so
smaller values of R should be expected with increasing
electron demand. In C6F5-substituted Re(I) and Fe(II)
Cp complexes that we have characterized, the torsion
angle R typically ranges from 20 to 50°, and metal-
centered electron demand in these systems is arguably
lower. However much more data would be needed to
establish a clear trend in R. Examination of packing
diagrams for 7 does not reveal any specific interactions
(89) Kimura, K.; Takaishi, K.; Matsukawa, T.; Yoshimura, T.;
Yamazaki, H. Chem. Lett. 1998, 571-572.
(90) Hauptman, E.; Waymouth, R. M.; Ziller, J . W. J . Am. Chem.
Soc. 1995, 117, 11586-11587.
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5909-5916.

Aryl-Substituted Zirconocene Dichlorides
Organometallics, Vol. 23, No. 6, 2004 1339
isosteric relationship of C6F5 and C6H5, one could infer
that the retarding (steric-encumbering) effect of C6H5
is offset by electron-donating character, whereas the
difference in activity between corresponding C6H5- and
C6F5-substituted catalysts is largely due to electronic
effects alone.
We surmise that the observed increase in 1-hexene
incorporation contributes partly to the decrease in
molecular weight, as the rate of chain transfer to
monomer (â-hydride elimination) is known to be higher
after insertion of 1-hexene. Although the frequently
cited “comonomer effect” predicts that increasing 1-hex-
ene incorporation should lead to increased activity,97-99
others have found that 1-hexene decreases the activity
significantly and that the decrease is substituent de-
pendent.44 Again, much of the work uses a range of
substituents that offers little electronic variation. Work
is underway in our laboratories to develop further
control experiments that will extend the range of
electronic effects in group 4 metallocene olefin polym-
erization catalysts in order to understand the influence
of steric, electronic, and comonomer factors more fully.
The data in Table 3 also reveal a pronounced effect
of aryl substituents on molecular weight and on the
incorporation of 1-hexene. The electron-releasing C6H5
groups effect slightly higher molecular weights relative
to zirconocene dichloride (entries 2, 5, and 6), whereas
C6F5 groups effect a dramatic decrease in molecular
weight (entries 3 and 7-9). The PDI values ranged from
2.2 to 3.1, suggesting that the polymerizations remained
orderly despite the large variations in activity and
molecular weight. Because C6H5 groups do not lead to
increased 1-hexene incorporation, however, the in-
creased molecular weight cannot be attributed to a
comonomer effect,94 but rather the increase must derive
from steric and electronic features of the catalyst site.
In contrast, electron-withdrawing C6F5 groups lead to
significant increases in the incorporation of 1-hexene
(entries 1 and 4-6).
Con clu sion s. Electron-withdrawing substituents (a)
decrease the activity of zirconocene dichloride as a
catalyst for MAO-cocatalyzed ethylene polymerization
and (b) increase the amount of 1-hexene comonomer
that is incorporated into an ethylene-1-hexene copoly-
mer.
Importantly, previously reported attempts to measure
electronic effects on R-olefin incorporation either were
ambiguous or reached the opposite conclusion. Yano et
al. found that alkylating the Cp moiety of [Ph2C(C13H8)-
(C5H4)]ZrCl2/MAO catalysts increased 1-hexene incor-
poration,95 but no correction for steric factors was
possible. In reviewing composite trends, Karol and Kao
concluded simply that electron-donating substituents
increased comonomer incorporation.96 Mohring and
Coville studied copolymerization activities using a series
of ring-substituted Cp2ZrCl2/MAO catalysts and found
no useful correlation of activity to either steric or
electronic factors. Instead they attributed the difference
between copolymerization and ethylene homopolymer-
ization activities primarily to electronic effects. However
the substituents used (R ) H, Me, Et, iPr, tBu, Me3Si,
CMe2Ph) give only a narrow range of electronic varia-
tion.44
Ack n ow led gm en t. This project was supported by
the Petroleum Research Fund (30738-G) and the Na-
tional Science Foundation (CHE-9875446). We thank W.
F. J ackson for preliminary synthetic work. We thank
P. N. Nickias and J . Gunderson (Dow Chemical Com-
pany) for the GPC measurements. We thank F. Van
Damme and A. Mollick for the construction of our
custom kettle reactors and other glass apparatus.
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal-
lographic data for 1 (Tables S1-S8), 2 (Tables S9-S16), 6
(S17-S24), and 7. Information is available free of charge via
the Internet at http://pubs.acs.org.
OM034280D
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