Comparison of the regiochemical behavior of zirconium and hafnium in the polyinsertion of styrenes

Article abstract of DOI:10.1021/om1001342

The hafnocene-based catalyst ethylenebis(1-indenyl)hafnium dichloride/methylalumoxane, as well as its zirconium analogue, is able to oligomerize styrene, p-methylstyrene, and p-tert-butylstyrene in the presence of hydrogen to produce hydrooligomers. The c

Full text of DOI:10.1021/om1001342

4434 Organometallics 2010, 29, 4434–4439
DOI: 10.1021/om1001342
Comparison of the Regiochemical Behavior of Zirconium and Hafnium
in the Polyinsertion of Styrenes
Nunzia Galdi, Lorella Izzo, and Leone Oliva*
ꢀ
Dipartimento di Chimica, Universita degli Studi di Salerno, Via Ponte don Melillo, I-84084, Fisciano, Italy
Received February 22, 2010
The hafnocene-based catalyst ethylenebis(1-indenyl)hafnium dichloride/methylalumoxane, as
well as its zirconium analogue, is able to oligomerize styrene, p-methylstyrene, and p-tert-butylstyr-
ene in the presence of hydrogen to produce hydrooligomers. The composition of the product mixture
compared to that obtained using zirconium-based catalysts indicates that the primary insertion
occurs with greater frequency. This difference in regioselectivity is likely to be related to the electronic
differences between the two metal centers. Previous experimental and theoretical evidence suggests
that increasing the electron density at the incoming metal-carbon bond decreases the preference for
the secondary insertion; so by exploiting the contribution from the electron-releasing substituent of
the monomer and changing the metal center, from zirconium to hafnium, the regiochemistry of
insertion can be inverted from secondary to prevailingly primary. The styrene regiochemistry of
insertion is engraved into the structure of ethylene-styrene copolymers. Some relevant differences
can be observed by comparing the 13C NMR spectra of copolymer obtained with zirconium- and
hafnium-based catalysts. In fact the structure of the chains obtained with hafnium, compared to
zirconium, shows higher styrene uptake and the presence of styrene homosequences and of
regioirregularly arranged units.
1. Introduction
the chiral 1,3-diphenylbutane by using styrene hydrooligo-
merization promoted by the optically active ethylenebis-
(4,5,6,7-tetrahydro-1-indenyl)zirconium dichloride (R,R-
EBTHIZrCl2) activated by methylalumoxane (MAO). Both
the turnover and the enantiomeric excess are interesting, but
a drawback of this reaction lies in the regiochemistry of the
styrene insertion, preferentially yielding 1,4-diphenylbutane
(head-to-head insertion) rather than the desired regioregular
asymmetric hydrodimer (Scheme 1).
While the encumbrance at the catalytic site clearly plays
a role in regulating the regiochemistry of the monomer
insertion,1f experimental evidence for the relationship be-
tween the electronic features of the site and its regioselectivity
has been more difficult to collect. In particular, this can be
obtained by comparing systems in which there is a clear-cut
separation between steric and electronic factors, a prerequi-
site fulfilled in the author’s recent kinetic study comparing
the tendencies of styrene and two substituted styrenes,
p-chlorostyrene and p-methylstyrene,3 to give primary and
secondary insertion into the Zr-C bond. The results led to a
discussion of the possible effect that electron-withdrawing
and electron-releasing substituents of the monomer aromatic
ring may have on the relative rate of insertion with the two
possible regio-orientations. With the additional support of
computational modeling, it was concluded that, generally
speaking, increasing the electron density at the incoming
metal-carbon bond increases the tendency toward primary
insertion.
The catalysts promoting styrene insertion into the metal-
carbon and metal-hydrogen bond are many, with different
regiochemical behavior.1
In addition to being interested in enlightening the relation-
ship between the features of the catalytic site and the styrene
insertion regiochemistry, it is worth noting that the prefer-
ential formation of a metal-methylene rather than a metal-
methyne bond can be crucial when the catalysis aims at
promoting the synthesis of low molecular weight products.
This is the case in the recently reported2 asymmetric C-C
bond formation resulting from styrene polyinsertion
in the presence of molecular hydrogen (so-called hydro-
oligomerization). It was shown that it is possible to obtain
*Corresponding author. Tel: þ39 089969573. Fax: þ39 089 969603.
E-mail: loliva@unisa.it.
(1) (a) Ammendola, P.; Tancredi, T.; Zambelli, A. Macromolecules
1986, 19, 307–310. (b) Pellecchia, C.; Longo, P.; Grassi, A.; Ammendola, P.;
Zambelli, A. Makromol. Chem., Rapid Commun 1987, 8, 277–279. (c)
Oliva, L.; Caporaso, L.; Pellecchia, C.; Zambelli, A. Macromolecules 1995,
28, 4665–4667. (d) Kretschmer, W. P.; Troyanov, S. I.; Meetsma, A.; Hessen,
B.; Teuben, J. H. Organometallics 1998, 17, 284–287. (e) Caporaso, L.; Izzo,
L.; Oliva, L. Macromolecules 1999, 32, 7329–7331. (f) Izzo, L.; Napoli,
M. G.; Oliva, L. Macromolecules 2003, 36, 9340–9345. (g) Guo, N.; Li, L.;
Marks, T. J. J. Am. Chem. Soc. 2004, 126, 6542–6543. (h) Capacchione, C.;
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Mitsudo, T. Angew. Chem. 2007, 46, 5958–5961. (j) For a recent review
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(2) Galdi, N.; Della Monica, C.; Spinella, A.; Oliva, L. J. Mol. Catal.
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r
pubs.acs.org/Organometallics
Published on Web 09/21/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 20, 2010 4435
Scheme 1. Regiochemistry of Styrene Insertion into the Zr-Hydrogen and Zr-Carbon Bond
Evidence supporting this idea was put forward by Scott
et al., who reported how an electron-releasing substituent in
ethylene-styrene copolymerization can reactivate a catalyst
by shifting the vinylaromatic monomer insertion from sec-
ondary to primary.4
In addition to these kinetic aspects, the regiochemistry of
styrene insertion in ethylene-styrene copolymerization could
determine the microstructure of the macromolecules and con-
sequently also the material properties. In fact, blockwise
copolymerizations5 have been reported when the styrene inser-
tion is primary, while, more in general, secondary insertion
produces alternating ethylene-styrene copolymers.6
2. Results and Discussion
Figure 1. Plot of [1,3-dim.]/[1,4-dim.] ratio versus feed compo-
sition: (A) styrene-[(rac-EBIZrCl2)/MAO]; (B) styrene-[(rac-
EBTHIZrCl2)/MAO] (from ref 2); (C) styrene-[(rac-EBIHfCl2)/
MAO]; (D) p-tert-butylstyrene-[(rac-EBIHfCl2)/MAO]; (E)
p-methylstyrene-[(rac-EBIHfCl2)/MAO].
2.1. Hydrooligomerizations. In light of the evidence on the
influence of the electron density of the incoming metal-
carbon bond on the regiochemistry of styrene insertion with
zirconium-based catalysts, we devised to extend the investi-
gation to a hafnium-based catalyst. In the present work we
report the comparison of the behavior of the dichloride
complexes of hafnium and zirconium bearing the
ethylenebis(1-indenyl) ancillary ligand, so that the effect of
the electronic features of the metallic center on the regio-
chemistry of insertion could be explored. As a matter of fact
the literature reports that these two ansa metallocenes have
substantially identical geometry7 and in particular indicates
that the metal-ring centroid distance in the rac-ethylenebis-
or secondary insertion into the Zr-C bond, the secondary
one giving an intermediate more prone to attack by hydrogen
rather than further styrene insertion.
Figure 1 proposes the comparison among the experimen-
tal results obtained with several hafnium- and zirconium-
based catalysts. In agreement with the reasoning above,2,3
the ratio between primary and secondary insertion is quite
accurately represented by the ratio between the 1,3-dimer
and 1,4-dimer extrapolated at a hydrogen pressure high
enough to inhibit the production of trimers and heavier oligo-
mers. So lines A and C show for both Zr and Hf a prevailing
secondary styrene insertion into the metal-carbon bond, this
preference being less relevant for the hafnium. A deeper insight
into the influence of the electronic factors on the regiochem-
istry is given by the comparison with the behavior of styrenic
monomers with increased electron density at the vinyl double
bond. The results of the p-methylstyrene hydrooligomerization
are represented by line E in Figure 1, which shows that the 1,3/
1,4-dimer ratio reaches and plateaus at a value greater than 1.
The behavior of p-tert-butylstyrene seems to be similar (line D).
This monomer was recently scrutinized by Scott et al.,4 who
observed that the tert-butyl substituent had a powerful ability
to shift the insertion from secondary to primary. The picture
proposed by the results of Figure 1 is that the preference for the
secondary styrene insertion into the metal-carbon bond
decreases on passing from Zr to Hf, as well as decreases by
introducing electron-releasing substituents into the monomer
aromatic ring. By combining these two observations it is
possible to obtain a prevailing primary insertion into the
metal-carbon bond when the metal is hafnium and the
monomer is an alkyl-substituted styrene.
˚
(1-indenyl)hafnium dichloride (rac-EBIHfCl2) is only 0.002 A
shorter than in the rac-ethylenebis(1-indenyl)zirconium
dichloride (rac-EBIZrCl2) due to the contraction of the 4f
lanthanide. Consequently, it can be assumed that the pockets
for the incoming substrate have a very similar shape.
The regiochemistry can be studied by keeping in mind that
the styrene polyinsertion can be regulated by feeding the
reaction mixture with H2. The molecular hydrogen breaks
the metal-carbon bond, inducing the production of oligo-
mers, thus giving also a picture of the regiochemistry of
insertion. We showed that, in the presence of zirconocenes,
increasing the ratio between hydrogen and styrene in the feed
increases the ratio between the 1,3-dimer and 1,4-dimer in
the product mixture.2 This observation is a strong clue of
primary insertion into the Zr-H bond followed by primary
(4) (a) Theaker, G. W.; Morton, C.; Scott, P. Dalton Trans. 2008, 48,
6883–6885. (b) Theaker, G. W.; Morton, C.; Scott, P. J. Polym. Sci., Part A:
Polym. Chem. 2009, 47 (12), 3111–3117.
(5) Caporaso, L.; Izzo, L.; Sisti, I.; Oliva, L. Macromolecules 2002, 35,
4866–4870.
(6) (a) Oliva, L.; Longo, P.; Izzo, L.; Di Serio, M. Macromolecules
1997, 30, 5616–5617. (b) Arai, T.; Ohtsu, T.; Suzuki, S. Macromol. Rapid
Commun. 1998, 19, 327–331.
(7) (a) Ewen, J. A.; Haspeslagh, L.; Atwood, J. L.; Zhang, H. J. Am.
Chem. Soc. 1987, 109, 6544–6545. (b) Piemontesi, F.; Camurati, I.; Resconi,
L.; Balboni, D.; Sironi, A.; Moret, M.; Zeigler, R.; Piccolrovazzi, N.
Organometallics 1995, 14, 1256–1266.
An analogous difference in the regiochemistry has been
observed in the couple of complexes methylenebis(1-indenyl)-
zirconium dichloride and methylenebis(1-indenyl)hafnium

4436 Organometallics, Vol. 29, No. 20, 2010
Galdi et al.
Table 1. 1H NMR Data for L2M(Me)2 þ B(C6F5)3 in C6D6
at 25 °C
δ CH3-M þ
(ppm)
δ CH3-M
(neutral)
-
δ CH3-B(C6F5)3
(ppm)
complex
rac-EBIZrMe2
rac-EBIHfMe2
-0.61
-0.35
-0.44
-0.66
-0.97
-1.16
Table 2. Polymerization Conditions and Composition
of Copolymers
Figure 2. Ionic pair generated by reaction of the rac-EBIMMe2
complex (M = Zr or Hf) with tris(pentafluorophenyl)borane.
precatalysta
T (°C)
% styrene (mol)c
Mw
Mw/Mn
b
fE
Scheme 2. (1) Metal Methylation; (2) Methyl Abstraction
rac-EBIHfCl2
rac-EBIHfCl2
rac-EBIZrCl2
rac-EBIZrCl2
20
50
20
50
0.045
0.029
0.045
0.029
44
32
21
14.5
24 900
16 700
44 750
30 000
1.3
1.5
1.8
1.6
a All of the runs were carriedout in the presence of metallocene (5 mg),
MAO (Al/M = 1000), styrene (12 mL), toluene (8 mL), and ethylene
pressure suited to producing the desired monomer feed composition.
The catalyst activity is respectively 6.8, 6.5, 15, 181 kg/mol h. b Mole
3
ratio of ethylene to styrene in the feed. c Content of styrene units in the
copolymer determined from the 13C NMR spectra.
dichloride activated by MAO. These catalytic systems produce
the hydrodimers with a ratio 1,3/1,4 respectively of 0.05 and 1,
supporting the conclusion that hafnium-based catalysts are
more prone to give primary styrene insertion into the carbon-
metal bond (see Experimental Section).
2.2. NMR Evidence from the Ion Pair. Experimental evi-
dence for the difference in the acidity of the two catalytic
centers can be obtained by synthesis and NMR analysis of
the cationic species in a well-characterized form. In fact, the
catalytic site in the systems described above is produced by
methylalumoxane following metal methylation and methyl
abstraction (see Scheme 2).8 But the features of this ion pair
are difficult to study because of the complicated and variable
composition of MAO.
However, a useful model of the catalytic center can be
obtained by reacting the dimethyl derivatives rac-EBIZrMe2
and rac-EBIHfMe2 with the stoichiometric abstractor
tris(pentafluorophenyl)borane.9 Consequently, the same cat-
ionic complex active in the catalysis is generated, but in this
case it is paired with a well-defined counterion, methyltris-
(pentafluorophenyl)borate.
Figure 3. Qualitative schematic energy profiles for insertion of
monomer into catalyst.
center is less acidic than the corresponding zirconium center,
being less attracted toward the counteranion.
Many studies show that this kind of ion pair is not sepa-
rated, as some interaction between the positively charged
center and the negatively charged one persists (Figure 2).
However this conclusion should be handled with caution
because, while it is in accordance with the statements by
Ewen et al.,7a by Cardin et al.,11a and by Felten et al.,11b
it disagrees with the findings of Luo and Marks12 in their
calorimetric study. These authors, for a series of metallorganic
compounds of Zr and Hf, observed that the detachment of the
methide by B(C6F5)3 from the zirconium compounds is generally
more exothermic than from the hafnium compounds.
1
According to Brintzinger et al., the H NMR chemical
shift of a Me-B(C6F5)3- anion can be regarded as a measure
of its “freedom”, with a limit value of δ = 0.8 ppm being
found for the methyl group of a loose ion pair.10 Table 1
reports the chemical shifts observed in the ionic pair obtained
from the zirconium and hafnium complexes. Their compar-
ison seems to support the idea that the hafnium cationic
2.3. Ethylene-Styrene Copolymerization. The develop-
ment of the homogeneous polyinsertion catalysis made it
(8) (a) Bochmann, M. J. Organomet. Chem. 2004, 689, 3982–3998. (b)
Babushkin, D. E.; Semikolenova, N. V.; Panchenko, V. N.; Sobolen, A. P.;
Zakharov, V. A.; Talsi, E. P. Macromol. Chem. Phys. 1997, 198 (12), 3845–
3854. (c) Bryliakov, K. P.; Talsi, E. P.; Bochmann, M. Organometallics
2004, 23, 149–152.
(9) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1991, 113,
3623-3625. Several other stoichiometric activators have been reported.
See e.g.: Zhou, J.; Lancaster, S. J.; Walker, D. A.; Beck, S.; Thornton-
Pett, M.; Bochmann, M.J. Am. Chem. Soc. 2001, 123, 223-237 and Metz,
M. V.; Schwartz, D. J.; Stern, C. L.; Nickias, P. N.; Marks, T. J. Angew.
Chem., Int. Ed. 2000, 39, 1312-1316, and references therein.
(10) Beck, S.; Lieber, S.; Schaper, F.; Geyer, A.; Brintzinger, H. H.
J. Am. Chem. Soc. 2001, 123, 1483–1489.
(11) (a) Cardin, D. J.; Lappert, M. J.; Raston, C. L. In Chemistry of
Organo-Zirconium and Hafnium Compounds; John Wiley and Sons: New
York, 1986. (b) Felten, J. J.; Anderson, W. P. J. Organomet. Chem. 1972, 36,
87–92.
(12) Luo, L.; Marks, T. J. Top. Catal. 1999, 7, 97–106.
(13) (a) Stevens, J. C.; Timmers, F. J.; Wilson, D. R.; Schmidt, G. F.;
Nickias, P. N.; Rosen, R. K.; Knight, G. W.; Lai, S. (The Dow
Company) Eur. Patent 416815, 1991. (b) Longo, P.; Grassi, A.; Oliva, L.
Makromol. Chem. 1990, 191, 2387–2396. (c) Pellecchia, C.; Oliva, L.
Rubber Chem. Technol. 1999, 72, 553–558. (d) Rodrigues, A. S.; Carpentier,
J .F. Coord. Chem. Rev. 2008, 252, 2137–2154. (e) Inoue, N.; Shiomura, T.;
Kouno, M. (Mitsui Toatsu Chemicals Inc) Eur. Pat. Appl. 108824, 1993;
Chem. Abstr. 1994, 121, 58212.

Article
Organometallics, Vol. 29, No. 20, 2010 4437
Figure 4. Aliphatic region of the 13C NMR spectrum of the ethylene-styrene copolymer obtained at 20 °C using rac-EBIHfCl2-based
catalyst.
possible to synthesize previously unknown polymer materi-
als. Of these, the ethylene-styrene (E-S) copolymers13 are
of particular interest, as, depending on their composition,
they exhibit13e either elastomeric or thermoplastic behavior.
The ansa zirconocenes are also known to promote this
copolymerization, so the catalytic properties of the two ansa
metallocenes EBIZrCl2 and EBIHfCl2, after activation using
MAO, could be compared in ethylene-styrene copolymeri-
zation in order to observe how regiochemical features affect
the catalytic behavior and the properties of the material.
The results of the E-S copolymerizations in the presence
of metallocenes are summarized in Table 2, and it can be
observed that the hafnocene also is able to copolymerize
these monomers but with lower activity.
contrasts sharply with what is observed with the Zr complex,
whose relevant activity at -20 °C gives isotactic alternating
ethylene-styrene copolymer,6,15 but it is not surprising
because of the reported low ability of MAO to activate the
hafnocenes at low temperature.16
Moreover, the copolymers obtained with Hf show a sig-
nificant presence of regioirregularities (the peaks around
35 ppm in Figure 4) as a result of several primary inserted
styrene units in addition to the secondary ones.
Due to the two possible regiochemical orientations, it is
worth noting that the kinetic picture of the ethylene-styrene
copolymerization in the presence of the hafnium-based
catalyst is more complex than that in the presence of the
zirconium-based catalyst; thus the copolymerization should
be treated as terpolymerization.17 Furthermore, the NMR
spectrum reveals a peak around 41 ppm assigned to the
styrene homosequences in the copolymer chains that could
be related to the occasional primary polyinsertion.5
The narrow polydipersity indicates a single-site catalyst
produces these macromolecules and allows us to discuss the
relationship between catalyst feature and macromolecule
microstructure.
The styrene content in the copolymer increases remarkably
when the reaction temperature is decreased, showing a de-
crease of the ethylene-styrene reactivity ratio. The same trend
is well known for the ansa zirconocenes and justified by
assuming different pathways for ethylene and styrene inser-
tions.6 While the coordination of ethylene should be the rate-
determining step, the following insertion having a lower energy
barrier, the coordination of the styrene, should be an equilib-
rium step forming a barrier to the insertions (Figure 3).
A similar picture was produced also by DFT calculations
on the mechanism of E-S copolymerization when con-
strained geometry catalysts were used.14
Since the styrene coordination is stronger than that of
ethylene, the availability of the catalytic site for ethylene
coordination decreases with decreasing temperature. The
result is that reactivity toward the less reactive monomer
increases as the temperature decreases.
3. Conclusion
The styrene insertion into the metal-carbon bond with
hafnocene-based catalysts is scarcely regioregular, showing
to a similar degree 1,2- and 2,1-insertion. In this respect the
analogous zirconium-based catalysts show higher tendency
toward secondary insertion. It is unlikely that this different
behavior could be ascribed to geometric differences of the
catalytic site; more probably differences in the electronic
features of the metallic center must be invoked.
The different regiochemistry is also recorded by the micro-
structure of the ethylene-styrene copolymers, with the pres-
ence of styrene homosequences and of a relevant amount
of regioirregularly enchained units in the macromolecules
obtained with the hafnium-based catalyst. On the contrary,
(15) Oliva, L.; Izzo, L.; Longo, P. Macromol. Rapid Commun. 1996,
17, 745–748.
(16) Bryliakov, K. P.; Talsi, E. P.; Voskoboynikov, A. Z.; Lancaster,
S. J.; Bochmann, M. Organometallics 2008, 27, 6333–6342.
(17) Arriola, D. J.; Bokota, M.; Campbell, R. E.; Klosin, J., Jr.;
LaPointe, R. E.; Redwine, O. D.; Shankar, R. B.; Timmers, F. J.;
Abboud, K. A. J. Am. Chem. Soc. 2007, 129 (22), 7065–7076.
Unfortunately, the Hf-based catalyst is practically inactive
at 0 °C and at temperatures lower than 0 °C. This fact
(14) Yang, S. H.; Jo, W. H.; Noh, S. K. J. Chem. Phys. 2003, 119 (3),
1824–1837.

4438 Organometallics, Vol. 29, No. 20, 2010
Galdi et al.
with the analogous zirconocene catalyst, the prevailingly
secondary insertion substantially prevents the formation of
styrene sequences and is the key factor for the alternating
stereoselective copolymerization.
were stopped after 9 h by quenching the reaction mixture in
acidified ethanol. The organic phase was separated and recovered
by shaking with water and n-pentane (3ꢀ). After desiccationof the
solution with Na2SO4, the n-pentane was removed under reduced
pressure. The composition of the mixture obtained was deter-
mined by GC-MS analysis, which showed the complete consump-
tion of the monomer in all cases.
4. Experimental Section
A further distillation under reduced pressure affords a fraction
of hydrogenated monomer, a second fraction containing the
mixture of hydrodimers, and a third containing the hydrotrimer.
The percentage of monomers converted into dimers ranges
from 31% to 4%, except for the run in the presence of rac-
methylenebis(1-indenyl)hafnium dichloride, where the amount
of dimer is around 1%.
4.5. Oligomer Analyses. The NMR spectra were recorded on
Bruker DRX 400 spectrometers at room temperature. The
sample was prepared by dissolving 30 mg of sample in 0.5 mL
of chloroform-d. The solvent peak (13CDCl3: δ = 77.23 ppm)
was used as the reference for the chemical shift, in δ units (ppm).
GC-MS measurements of the mixture of hydrooligomers
were recorded on a GC Trace 2000 Series connected to a
Finnigan Thermoquest GLQ Plus 2000 spectrometer with an
ion trap detector.
4.1. General Considerations. All the moisture-sensitive opera-
tions were carried out in an atmosphere of nitrogen using
standard Schlenk techniques. Dry solvents were freshly distilled
before use. The toluene was kept under reflux in the presence of
sodium for 48 h and then distilled in an atmosphere of nitrogen.
Styrene (99% GC, Aldrich) and 4-methylstyrene (96%, Aldrich)
were purified by stirring 1 h over calcium hydride before
distillation under nitrogen at reduced pressure. Methylalumox-
ane (MAO) was supplied by Witco as 30 wt % solution in
toluene, then dried by removing in vacuo solvent and traces of
trimethylaluminum.
1,2-Bis(3-indenyl)ethane was purchased from Aldrich, while
rac-ethylenebis(4,5,6,7-tetrahydro-1-indenyl)zirconium
dichloride (rac-EBTHIZrCl2) was purchased from MCAT.
rac-Ethylenebis(1-indenyl)zirconium dimethyl (rac-EBIZrMe2)
and rac-methylenebis(1-indenyl)hafnium dichloride were prepared
by using the procedures described in the literature.18,19
Other materials and reagents available from commercial
suppliers were generally used without further purification.
4.2. Synthesis of rac-EBIHfMe2. A 2.33 g sample of 1,2-bis-
(indenyl)ethane (98.4% GC, 9.02 mmol) was dissolved at room
temperature in 40 mL of Et2O contained in a 250 mL Schlenk
flask. Then 22.5 mL of 1.6 M MeLi in Et2O (36.1 mmol) was
added dropwise with stirring over 10 min. The mixture was
stirred for a further 3 h at room temperature under a nitrogen
atmosphere to obtain a thick yellow suspension. HfCl4 (2.91 g,
9.02 mmol) in 20 mL of hexane was added to this suspension.
The reaction mixture was stirred at room temperature for 1 h to
give a dark gray suspension. The Et2O was removed under
reduced pressure, and the resulting dark gray solid was extracted
using 2 ꢀ 100 mL of toluene at 40 °C. Finally, the toluene
solution was filtered and the solvent was evaporated under
reduced pressure to give 1.9 g of a yellow solid (45.3% yield).
After crystallization using toluene, spectroscopically pure
rac-EBIHfMe2 was obtained as demonstrated by 1H NMR analysis.
1H NMR (400 MHz, δ, ppm, C6D6): -1.16 (s, HfCH3, 6H),
2.86-2.89 (m, CH2 bridge, 4H), 5.53 (d, Cp H, 2H, J = 3.43 Hz),
6.32 (d, Cp H, 2H, J = 3.43 Hz), 6.71-7.34 (m, Ar, 8H).
4.3. Reaction of rac-EBIHfMe2 with B(C6F5)3. A 12 mg
(0.025 mmol) sample of rac-EBIHfMe2 and 13 mg (0.025 mmol)
of B(C6F5)3 were dissolved in deuterated benzene contained in
an NMR tube.
Styrene. 1,3,6-Triphenylhexane. 13C NMR (400 MHz, δ, ppm,
CDCl3): 29.5 (C-5), 34.0 (C-1), 36.1 (C-6), 36.8 (C-4), 38.7 (C-2),
45.6 (C-3), the aromatic C atoms resonate between 125.8 and
145.5 ppm. MS: 314 (Mþ).
1H NMR (400 MHz, δ, ppm, C6D6): -0.66 (s, HfCH3, 3H),
-0.35 (bs, CH3B(C6F5)3, 3H), 2.49-2.74 (m, CH2 bridge, 4H),
4.97 (d, Cp H, 1H), 5.58 (d, Cp H, 1H), 5.72 (d, Cp H, 1H), 6.19
(d, Cp H, 1H), 6.20-7.32 (m, Ar, 8H).
1,4-Diphenylbutane. 13C NMR (400 MHz, δ, ppm, CDCl3):
31.3 (C-2), 36.0 (C-1), the aromatic C atoms resonate between
125.8 and 145.5 ppm. MS: 210 (Mþ).
1,3-Diphenylbutane. 13C NMR (400 MHz, δ, ppm, CDCl3):
22.7 (C-4), 34.1 (C-1), 39.7 (C-3), 40.2 (C-2), the aromatic
C atoms resonated between 125.8 and 147.5 ppm. MS: 210
(Mþ).
4.4. Oligomerization Procedure. Styrene, 4-methylstyrene,
and 4-tert-butylstyrene were hydrooligomerized in a 250 mL
steel autoclave, which was evacuated and then charged with a
mixture of monomer, MAO, and rac-dichloride complexes of
hafnium and zirconium (Al/M in mol = 1000) and, when
necessary, toluene. The autoclave was thermostated at 50 °C,
fed with hydrogen at constant pressure (5, 8, 20, or 40 atm), and
mechanically stirred.
Ethylbenzene. 13C NMR(400MHz, δ, ppm, CDCl3):15.8(C-2),
29.1 (C-1), the aromatic C atoms resonated between 125.8 and
147.5 ppm. MS: 106 (Mþ).
p-Methylstyrene. 1,3,6-Tri-p-tolylhexane. 13C NMR (400 MHz,
δ, ppm, CDCl3): 21.2 (CH3-Ph), 29.7 (C-5), 33.5 (C-1), 35.7 (C-6),
36.9 (C-4), 38.9 (C-2), 45.2 (C-3), the aromatic C atoms resonate
between 127.9 and 147.8 ppm. MS: 356 (Mþ).
Hydrooligomerizations at atmospheric pressure of hydrogen
were performed at 50 °C in a 100 mL glass flask. The reactions
1,4-Di-p-tolylbutane. 13C NMR (400 MHz, δ, ppm, CDCl3):
21.2 (CH3-Ph), 31.5 (C-2), 35.6 (C-1), the aromatic C atoms
resonate between 126.2 and 147.8 ppm. MS: 238 (Mþ).
1,3-Di-p-tolylbutane. 13C NMR (400 MHz, δ, ppm, CDCl3):
21.2 (CH3-Ph), 22.9 (C-4), 33.7 (C-1), 39.2 (C-3), 40.3 (C-2), the
(18) Balboni, D.; Camurati, I.; Prini, G.; Resconi, L.; Galli, S.;
Mercandelli, P.; Sironi, A. Inorg. Chem. 2001, 40, 6588–6597.
(19) Agarkov, A. Y.; Izmer, V. V.; Riabov, A. N.; Kuz’mina, L. G.;
Howard, J. A.K; Beletskaya, I. P.; Voskoboynikov, A. Z. J. Organomet.
Chem. 2001, 619, 280–286.

Article
Organometallics, Vol. 29, No. 20, 2010 4439
aromatic C atoms resonated between 126.2 and 147.8 ppm. MS:
238 (Mþ).
acidified ethanol. The polymers were recovered by filtration and
dried in vacuo. The raw polymers were washed with boiling
acetone in a Kumagawa-type extractor in order to remove traces
of atactic polystyrene. The products were fully soluble in CHCl3.
The copolymer yield is about 130 mg in the presence of the Hf-
based catalyst, whereas in the presence of Zr it is 4.5 g at 50 °C
and 360 mg at 20 °C.
4-Ethyltoluene. 13C NMR (400 MHz, δ, ppm, CDCl3): 16.0
(C-2), 21.7 (CH3-Ph), 28.6 (C-1), the aromatic C atoms resonate
between 126.2 and 147.8 ppm. MS: 120 (Mþ).
p-tert-Butylstyrene. 1,3,6-Tris(4-tert-butylphenyl)hexane. 13
C
NMR (400 MHz, δ, ppm, CDCl3): 29.6 (C-5), 31.7 (CH3-CPh),
33.5 (C-1), 34.5 (CH3-C-Ph), 35.7 (C-6), 37.0 (C-4), 38.7 (C-2),
45.2 (C-3), the aromatic C atoms resonate between 125.3 and
148.6 ppm. MS: 482 (Mþ).
The ethylene concentration in the liquid phase was deter-
mined by using the fugacity function chart of ethylene reported
in the literature.20
1,4-Bis(4-tert-butylphenyl)butane. 13C NMR (400 MHz, δ,
ppm, CDCl3): 31.4 (C-2), 31.7 (CH3-CPh), 34.5 (CH3-C-Ph),
35.5 (C-1), the aromatic C atoms resonate between 125.3 and
148.6 ppm. MS: 322 (Mþ).
4.7. 13C NMR Analysis. The spectra were recorded at room
temperature on an AM 250 Bruker spectrometer operating at
62.89 MHz in the Fourier transform mode. The samples were
prepared by dissolving 30 mg of polymer into 0.5 mL of chloro-
form-d. The solvent peak of CDCl3 was used as internal
reference at δ = 77.23 ppm for the chemical shifts.
1,3-Bis(4-tert-butylphenyl)butane. 13C NMR (400 MHz, δ,
ppm, CDCl3): 22.7 (C-4), 31.7 (CH3-CPh), 33.6 (C-1), 34.5
(CH3-C-Ph), 39.2 (C-3), 40.2 (C-2), the aromatic C atoms
resonated between 125.3 and 148.6 ppm. MS: 322 (Mþ).
1-tert-Butyl-4-ethylbenzene. 13C NMR (400 MHz, δ, ppm,
CDCl3): 15.8(C-2), 28.5(C-1), 31.7 (CH3-CPh), 34.5 (CH3-C-Ph),
the aromatic C atoms resonate between 125.3 and 148.6 ppm. MS:
162 (Mþ).
The content of the styrene units was evaluated from the area
of methylene resonances by applying the equation
χs ¼
fAðSRγ þ Þ þ AðSRβÞ þ AðSRRÞg
fAðSγ þ γ þ Þ þ AðSββÞ þ AðSβγ þ Þ þ 1:5AðSRγ þ Þ þ 1:5AðSRβÞ þ AðSRRÞg
4.6. Ethylene-Styrene Copolymerizations. The copolymeri-
zations were carried out in a 100 mL glass flask provided with a
magnetic stirrer and thermostated at the required temperature.
The reactor was sequentially charged under a nitrogen atmo-
sphere with toluene (8 mL), styrene (12 mL), MAO, and
rac-dichloride complexes of hafnium or zirconium (Al/M in mol =
1000). The inert atmosphere was removed and the reaction
mixture was saturated with ethylene. The flask was continuously
fed with gaseous monomer at atmospheric pressure to keep the
monomer concentration constant. The copolymerizations were
stopped after 2 h, and the mixture was poured into 100 mL of
4.8. GPC Analysis. Molecular weight and molar weight dis-
tribution of the polymers were determined by gel permeation
chromatography (GPC) analysis carried out at 30 °C using THF
as solvent and narrow MWD polystyrene standards as refer-
ence. The measurements were taken on a Waters 1525 binary
system equipped with a Waters 2414 RI detector using four
˚
columns (range 1000-1 000 000 A).
Acknowledgment. The authors thank Dr. Patrizia Oliva
and Dr. Mariagrazia Napoli of the University of Salerno
for experimental support. This work was financially sup-
ported by MIUR (PRIN 07).
(20) Maxwell, J. B. Data Book on Hydrocarbons; Van Nostrand Co.:
New York, 1950; pp 45, 46, 52, 53.