Formation and structures of hafnocene complexes in MAO- and AlBu 3i/CPh3[B(C6F5) 4]-Activated systems

Article abstract of DOI:10.1021/om800664p

The formation of cationic species relevant to olefin polymerization based on (SBI)HfCl₂, Me₂C(C₅H₄)(Hu) HfCl₂, Ph₂C(C₅H₄)(Flu)HfCl ₂, and L′HfCl₂

Full text of DOI:10.1021/om800664p

Organometallics 2008, 27, 6333–6342
6333
Formation and Structures of Hafnocene Complexes in MAO- and
AlBui3/CPh3[B(C6F5)4]-Activated Systems
Konstantin P. Bryliakov,*,† Evgenii P. Talsi,† Alexander Z. Voskoboynikov,‡
Simon J. Lancaster,§ and Manfred Bochmann*,§
BoreskoV Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, 630090,
NoVosibirsk, Russian Federation, Department of Chemistry, Moscow State UniVersity, LomonosoVsky
Prospect 1, 113000 Moscow, Russian Federation, and Wolfson Materials and Catalysis Centre, School of
Chemical Sciences and Pharmacy, UniVersity of East Anglia, Norwich NR4 7TJ, United Kingdom
ReceiVed July 12, 2008
The formation of cationic species relevant to olefin polymerization based on (SBI)HfCl2,
Me2C(C5H4)(Flu)HfCl2, Ph2C(C5H4)(Flu)HfCl2, and L′HfCl2 activated by MAO, AlMe3/CPh3[B(C6F5)4],
and AlBui3/CPh3[B(C6F5)4] (SBI ) rac-Me2Si(Ind)2; L′ ) C2H4(Flu)(5,6-C3H6-2-MeInd)) was studied
by 1H, 13C, and 19F NMR spectroscopy. Thermally stable heterobinuclear intermediates of the type [LHf(µ-
Me)2AlMe2]+[MeMAO]- and [LHf(µ-Me)2AlMe2]+[B(C6F5)4]- were identified when using MAO and
AlMe3/CPh3[B(C6F5)4] as activators, respectively. The stability of these species explains the low
productivity of hafnocene catalysts in the presence of AlMe3-containing activators, compared to
zirconocenes. By contrast, in the ternary systems LHfCl2/AlBui3/CPh3[B(C6F5)4] hydride species were
detected that must be responsible for the formation of the highly active sites in olefin polymerization.
The ionic hydrido species differ significantly in stability. The formation of the mixed-alkyl complex
L′Hf(Me)CH2SiMe3 proceeds with surprisingly high diastereoselectivity; the sterically more hindered
isomer is produced preferentially. It reacts with CPh3[B(C6F5)4] to afford the ion pair [L′Hf-
CH2SiMe3]+[B(C6F5)4]- as two diastereomers that exist in dynamic equilibrium. The rates of site
epimerization of this ion pair indicate only small energy differences between the two isomers.
thylalumoxane (MAO) appeared to be an unexpectedly poor
activator.1-6 Similar differences in the catalytic productivity of
AlBui3/CPh3[B(C6F5)4]– and MAO-activated systems were
previously reported for some other C1-, C2-, and Cs-symmetric
hafnocenes.8-10 Determining the origin of these differences is
self-evidently an important task.
Previously, some of us have shown that heterobinuclear ion
pairs [(SBI)Zr(µ-Me)2AlMe2]+[Me-MAO]- are formed upon the
activation of the C2-symmetric zirconocene (SBI)ZrCl2 (SBI )
rac-Me2Si(Ind)2) with MAO,11,12 whereas in the ternary catalytic
system (SBI)ZrCl2/AlBui3/[CPh3][B(C6F5)4] the last confidently
assigned precursor of the active species of polymerization was
the binuclear complex [(SBI)Zr(µ-Cl)2Zr(SBI)][B(C6F5)4]2.13 In
addition, at high Al/Zr ratios (g20), the hydride- and allyl-
bridged heterobinuclear complex [(SBI)Zr(µ-H)(µ-C4H7)-
AlBui2][B(C6F5)4] was identified, which is probably the ther-
modynamic sink of this catalyst system.13 A similar hydride-
Introduction
In the past few years, interest in hafnium-based metallocene
and post-metallocene olefin polymerization catalysts has risen
sharply, resulting in promising new findings.1-6 It was shown
that ultrahigh molecular weight polypropylene elastomers could
be prepared using highly active C1-symmetric hafnocene
catalysts.1-3 Pyridyl-amide hafnium complexes were recently
optimized for commercial high-temperature propylene polymeriza-
tion.4-6 In addition, the latter catalysts have been applied in
the high-volume preparation of olefin block copolymers through
chain shuttling polymerization.7
Significantly, the aforementioned catalysts are effectively
activated by mixtures of triisobutyl aluminum and cation-
generating borates (e.g., AlBui3/CPh3[B(C6F5)4]), whereas me-
* Corresponding authors. (K.P.B.) Fax: +7 383 3308056. E-mail:
bryliako@catalysis.ru. (M.B.) Fax: (+44) 1603 592044. E-mail:
m.bochmann@uea.ac.uk.
(8) Seraidaris, T.; Kaminsky, W.; Seppälä, J.; Löfgren, B. Macromol.
Chem. Phys. 2005, 206, 1319.
(9) Yano, A.; Sone, M.; Yamada, S.; Hasegawa, S.; Akimoto, A.
Macromol. Chem. Phys. 1999, 200, 924.
† Boreskov Institute of Catalysis.
‡ Moscow State University.
§ University of East Anglia.
(1) Rieger, B.; Troll, C.; Preuschen, J. Macromolecules 2002, 35, 5742.
(2) Hild, S.; Cobzaru, C.; Troll, C.; Rieger, B. Macromol. Chem. Phys.
2006, 207, 665.
(3) Cobzaru, C.; Rieger, B. Macromol. Symp. 2006, 236, 151.
(4) Boussie, T. R.; Diamond, G. M.; Goh, C.; Hall, K. A.; LaPointe,
A. M.; Leclerc, M. K.; Murphy, V.; Shoemaker, J. A. W.; Turner, H.; Rosen,
R. K.; Stevens, J. C.; Alfano, F.; Busico, V.; Cipullo, R.; Talarico, G. Angew.
Chem., Int. Ed. 2006, 45, 3278.
(5) Froese, R. D. J.; Hustad, P. D.; Kuhlman, R. L.; Wenzel, T. T. J. Am.
Chem. Soc. 2007, 129, 7831.
(6) Boussie, T. R.; Diamond, G. M.; Goh, C.; Hall, K. A.; LaPointe,
A. M.; Leclerc, M. K.; Lund, C.; Murphy, V. U.S. Pat. 7.018.949, 2006.
(7) Arriola, D. J.; Carnahan, E. M.; Hustad, P. D.; Kuhlman, R. L.;
Wenzel, T. T. Science 2006, 312, 714.
(10) Seraidaris, T.; Puranen, A.; Karessoja, M.; Löfgren, B.; Repo, T.;
Leskelä, M.; Seppälä, J. J. Polym. Sci. A: Polym. Chem 2006, 44, 4743.
(11) Bryliakov, K. P.; Semikolenova, N. V.; Yudaev, D. V.; Zakharov,
V. A.; Brintzinger, H. H.; Ystenes, M.; Rytter, E.; Talsi, E. P. J. Organomet.
Chem. 2003, 683/1, 92.
(12) For related studies on the zirconocene/MAO system see also: (a)
Tritto, I.; Donetti, R.; Sacchi, M. C.; Locatelli, P.; Zannoni, G. Macromol-
ecules 1997, 30, 1247. (b) Babushkin, D. E.; Semikolenova, N. V.;
Zakharov, V. A.; Talsi, E. P. Macromol. Chem. Phys. 2000, 201, 558. (c)
Babushkin, D.; Brintzinger, H. H. J. Am. Chem. Soc. 2002, 124, 12869. (d)
Babushkin, D.; Brintzinger, H. H. Chem. Eur. J. 2007, 13, 5294.
(13) Bryliakov, K. P.; Talsi, E. P.; Semikolenova, N. V.; Zakharov,
V. A.; Brand, J.; Alonso-Moreno, C.; Bochmann, M. J. Organomet. Chem.
2007, 692, 859.
10.1021/om800664p CCC: $40.75
2008 American Chemical Society
Publication on Web 10/31/2008

6334 Organometallics, Vol. 27, No. 23, 2008
BryliakoV et al.
Chart 1
Scheme 1
bridged complex was earlier observed by Go¨tz and co-workers
in the reaction of the Cs-symmetric Ph2C(Cp)(Flu)ZrCl2 with
AlBui3 and [PhNMe2H][B(C6F5)4].14
For comparison, 1 was reacted with AlMe3/CPh3[B(C6F5)4]
(Hf:Al:B ) 1:30:1.1) to give [(SBI)Hf(µ-Me)2AlMe2]+-
[B(C6F5)4]- (5-BX4) (Scheme 1). 5-BX4 is stable at room
temperature for days. Addition of 3 equiv of propene to the
[(SBI)Hf(µ-Me)2AlMe2]+[B(C6F5)4]- species at low temperature
(-40 °C) resulted in no monomer incorporation or pro-
pene consumption. Warming the sample in 10 °C steps led to
the disappearance of the propene signals only at +20 °C, the
spectrum of 5-BX4 remaining unchanged. Injection of 3 equiv
of 13C2H4 into the sample containing [(SBI)Hf(µ-
Me)2AlMe2]+[B(C6F5)4]- at -10 °C showed slow ethylene
consumption at this temperature; there was complete 13C2H4
conversion within ca. 2 h. Again the spectrum of 5-BX4
remained essentially unchanged, a typical behavior of these
catalysts where the first insertion is rate limiting.19-21
The Cs-symmetric complexes 2 and 3 react with MAO and
AlMe3/CPh3[B(C6F5)4] in a similar manner: the monomethylated
derivatives Me2C(Cp)(Flu)HfMeCl and Ph2C(Cp)(Flu)HfMeCl
were detected as the main species at AlMAO:Hf ratios of 90:1,
while increasing the AlMAO:Hf ratio to 300:1 led to quantitative
formation of [Me2C(Cp)(Flu)Hf(µ-Me)2AlMe2]+[MeMAO]- (6-
MAO) and [Ph2C(Cp)(Flu)Hf(µ-Me)2AlMe2]+[MeMAO]- (7-
MAO), respectively. Activation with AlMe3/CPh3[B(C6F5)4]
gave [Me2C(Cp)(Flu)Hf(µ-Me)2AlMe2]+[B(C6F5)4]- (6-BX4)
and [Ph2C(Cp)(Flu)Hf(µ-Me)2AlMe2]+[B(C6F5)4]- (7-BX4). In
these Cs-symmetric examples, the two terminal Al-Me groups
are inequivalent. All these heterobinuclear intermediates are
thermally stable for at least 1 day at room temperature.
In this work, the structures of the cationic metallocene species
formed upon activation of C2-, Cs-, and C1-symmetric hafnocenes
1-4 (Chart 1) with MAO and AlBui3/CPh3[B(C6F5)4] were
compared. Earlier studies on the use of Zr- and Hf-CH2SiMe3
complexes have established the role of this alkyl ligand as
models for the metal-bound polymeryl chain and provided
detailed information on the solution structure, ion aggregation,
and dynamics of metallocene catalysts.15-17 We report here the
synthesis of mixed-alkyl derivatives of the C1-symmetric
complex 4, together with the ligand dynamics of the ion pair
derived from this ligand system on activation with AlBui3/
CPh3[B(C6F5)4]. Under closely analogous conditions, hafnium
was found to produce different resting states from zirconium.18
Results and Discussion
Activation of Hafnocene Dichlorides with MAO and
AlMe3/CPh3[B(C6F5)4]. On reacting (SBI)HfCl2 (1) with an
excess of MAO in toluene-d8, two types of intermediates were
identified. The first one, predominating at Al/Hf ratios of 30-60,
was identified as (SBI)HfMeCl. The second one (5-MAO)
becomes the predominant species at an Al/Hf ratio of g240.
On the basis of its NMR data, 5-MAO was identified as the
heterobinuclear outer-sphere ion pair [(SBI)Hf(µ-Me)2Al-
Me2]+[MeMAO]- (Scheme 1). This compound proved ther-
mally stable at room temperature for at least 1 day. It showed
quite low activity for ethylene polymerization; only traces of
polyethylene were detected in the sample after injecting 9 equiv
of 13C2H4 and storing at -10 °C for 2 h.
The spectra of the C1-symmetric complex 4 with MAO or
AlMe3/CPh3[B(C6F5)4] are more complicated. At Al/Hf ratios
of 30-70, several (three or four) types of Hf complexes were
found to coexist in comparable concentrations, whereas at Al/
Hf ratios of 90:1 or higher, only one species, identified as
[(L′)Hf(µ-Me)2AlMe2]+[MeMAO]-(8-MAO)(L′)C2H4(Flu)(5,6-
C3H6-2-MeInd)), was found to prevail and appeared to be stable
for days in an NMR tube at room temperature. The interaction
(14) Go¨tz, C.; Rau, A.; Luft, G. J. Mol. Catal. A: Chem. 2002, 184, 95.
(15) Song, F.; Cannon, R. D.; Bochmann, M. Chem. Commun. 2004,
542.
(16) Song, F.; Lancaster, S. J.; Cannon, R. D.; Schormann, M.;
Humphrey, S.; Zuccacia, C.; Macchioni, C.; Bochmann, M. Organometallics
2005, 24, 1315.
(17) (a) Alonso-Moreno, C.; Lancaster, S. J.; Zuccaccia, C.; Macchioni,
A.; Bochmann, M. J. Am. Chem. Soc. 2007, 127, 9282. (b) Alonso-Moreno,
C.; Lancaster, S. J.; Wright, J. A.; Hughes, D. L.; Zuccaccia, C.; Correa,
A.; Macchioni, A.; Cavallo, L.; Bochmann, M. Organometallics,in press.
(c) For DFT calculations on the ion pair [(SBI)Zr-CH2SiMe3]+[B(C6F5)4]-
see: (d) Duce`re, J. M.; Cavallo, L. Organometallics 2006, 25, 1431.
(18) For the reactions of Cp2HfEt2 with CPh3[B(C6F5)4] see: Bochmann,
M.; Lancaster, S. J. J. Organomet. Chem. 1995, 497, 55.
(19) Review: Bochmann, M. J. Organomet. Chem. 2004, 689, 3982.
(20) Bochmann, M.; Jaggar, A. J.; Nicholls, J. C. Angew. Chem., Int.
Ed. Engl. 1990, 29, 780.
(21) (a) Fink, G.; Zoller, W. Makromol. Chem. 1981, 182, 3265. (b)
Fink, G.; Rottler, R. Angew. Makromol. Chem. 1981, 94, 25. (c) Fink, G.;
Schnell, D. Angew. Makromol. Chem. 1982, 105, 31. (d) Mynott, R.; Fink,
G.; Fenzl, W. Angew. Makromol. Chem. 1987, 154, 1. (e) Fink, G.; Fenzl,
W.; Mynott, R. Z. Naturforsch. Teil B 1985, 40b, 158.

Hafnocene Complexes in MAO-ActiVated Systems
Organometallics, Vol. 27, No. 23, 2008 6335
1
of 4 with AlMe3/CPh3[B(C6F5)4] (Hf:Al:B ) 1:50:1.1) resulted
in the formation of a brown oil containing cationic species
identified as [L′Hf(µ-Me)2AlMe2]+[B(C6F5)4]- (8-BX4). The C1
symmetry of 8-MAO is reflected in the four separate 1H NMR
signals seen for the bridging and terminal Al-Me groups.
The H NMR spectrum is shown in Figure 1. According to
the 13C NMR spectra of 9, Hf-CH2- groups were absent.
However, there are two hydride signals marked as H1 (1H, t,
2JHH ) 5 Hz) and H2 (2H, d, JHH ) 5 Hz), which display
2
correlation peaks in the 1H COSY spectra. On the basis of this
information, 9 is formulated as a hydride-bridged binuclear
species, [(SBI)Hf(µ-H)2Al(H)Bui]+[B(C6F5)4]- (Scheme 2).
1
Raising the temperature leads to the coalescence of the H
peaks of the terminal AlMe2 groups (at 343 K, apparent ∆Gex
) 17.5 kcal mol-1), the bridging methyls remaining nonequiva-
lent (see Supporting Information). It is poorly reactive toward
propene: when ca. 3.5 equiv of propene was added to the
solution of 8-BX4 at low temperature, complete consumption
was achieved only after warming the sample above +20 °C.
The results confirm that the alkylation of hafnocenes with
AlMe3 proceeds in stages, first forming the monomethyl
chloride, followed by dechlorination and formation of the
heterobinuclear cation [LHf(µ-Me)2AlMe2]+. We note that
higher AlMAO:Hf ratios are generally needed for complete
The facile formation of hydrido-bridged species on activation
of hafnocene dichlorides with AlBui3/CPh3[B(C6F5)4] is in
contrast to the reaction of related zirconocenes with AlBui3-
enriched MAO, which produces predominantly methyl-bridged
complexes of the type [(SBI)Zr(µ-Me)2Al(R)Bui]+ (R ) Me,
Bui), whereas hydride-bridged products are only barely detect-
able at elevated AlBui3/Zr ratios.12d By contrast, the addion of
HAlBui2 to [(L)Zr(µ-Me)AlMe2]+[Me-MAO]- was found to
lead to the formation of neutral zirconocene hydrides
LZrH2 · n(AlR2X).25
We had found earlier that zirconocenes react with AlBui3/
CPh3[B(C6F5)4] in several steps to give eventually a hydrido
species, [(SBI)Zr(µ-H)(µ-C4H7)AlBui2][B(C6F5)4], a likely ther-
modynamic sink.13 The reactivity of 9 toward olefins was
therefore tested to probe its ability to act as direct catalyst
precursor. The µ-H product 9 is certainly more reactive than
the µ-Me compound 5-BX4, and 3 equiv of propene added at
-30 °C was rapidly consumed on warming to 0 °C, although
the 1H NMR spectrum of the hydride species remained
essentially unchanged and the formation of a Hf-propyl species
could not be detected. On the other hand, more bulky olefins,
which polymerize only slowly, led to the complete disappear-
ance of the Hf-hydride signals. For example, whereas 3 equiv
of 2,4,4-trimethyl-1-pentene added to a toluene-d8/1,2-difluo-
robenzene solution of 9 (ca. 0.1 M) did not react at -10 °C,
warming to room temperature led to the nearly complete
disappearance of the peaks of 9, accompanied by a correspond-
ing drop in the trimethylpentene concentration. The signals of
9 were replaced by those of a new C1-symmetric hafnocene
species (revealed by nonequivalent SiMe2 peaks in the 13C NMR
spectrum); however, once again signals assignable to the Hf-
CH2- fragment could not be identified, and it remains uncertain
whether the observed intermediate is a product of a single-
insertion reaction.
In the second experiment, ca. 3 equiv of allylbenzene was
added to a 0.1 M toluene-d8/1,2-difluorobenzene solution of 9
at -10 °C. After warming the sample to room temperature, 9
was found to have been consumed completely, and at least two
C1-symmetric hafnocene species were observed in the mixture.
A 13C NMR peak at δ 71.0 (at -10 °C) was detected, indicating
possibly the formation of a Hf-CH2 species.
Activation of Me2C(Cp)(Flu)HfCl2 and Ph2C(Cp)(Flu)HfCl2
with AlBui3/CPh3[B(C6F5)4]. Compounds 2 and 3 react with
AlBui3/CPh3[B(C6F5)4] in a more complex way than 1. The
reaction products of 2 with this activator system gave a mixture
of species that were too unstable at 2-5 °C to be identified. By
contrast, the reaction of 3 with AlBui3/CPh3[B(C6F5)4] depends
on the mixing temperature. Stirring the mixture (Hf:Al:B ) 1:50:
1.1) at 0 °C for 10 min resulted in a new Cs-symmetric ion pair
as the predominant species, which, with its simple NMR
spectrum, is most likely the chloro-bridged salt [{Ph2C(C5H4)-
(Flu)Hf(µ-Cl)}2]2+[B(C6F5)4-]2.26 By contrast, mixing the
conversion
of
ansa-hafnocenes
to
[LHf(µ-Me)2-
AlMe2]+[MeMAO]- (up to 240-300) than in the cases of ansa-
zirconocenes11 and ansa-titanocenes,22 where AlMAO:metal ratios
of 100-200 were enough for complete conversion to the
heterobinuclear species. The Hf ion pairs are poorly active in
ethylene and propene polymerization. This is not unexpected
taking into account the observed low productivity of the ansa-
hafnocene/MAO systems in catalytic olefin polymeriza-
tions.1-6,23,24
Activation of (SBI)HfCl2 with AlBui3/CPh3[B(C6F5)4]. Most
highly active borate-activated metallocene systems involve
AlBui3 as a component. It is therefore important to understand
the nature of the active sites formed in the systems hafnocene/
AlBui3/CPh3[B(C6F5)4] and thus answer the question of why
activation with AlBui3/CPh3[B(C6F5)4] leads to much more
productive catalysts than MAO. To this end, the reactions of
1-4 with AlBui3 in the presence and absence of CPh3[B(C6F5)4]
were studied by H and 13C NMR spectroscopy.
1
Initially the alkylation of 1 with AlBui3 in the absence of
trityl salt was probed. At room temperature, the monoalkylation
equilibrium is established within 5 min, a 30-fold excess of
AlBui3 leading to a 2:3 mixture of (SBI)HfCl2 and (SBI)Hf-
(Bui)Cl. One can see that (SBI)HfCl2 requires a much higher
Al:metal ratio for monoalkylation than (SBI)ZrCl2 (for the latter,
5 equiv of AlBui3 gave 90% monoalkyl product).13 That is why
relatively high Al:Hf ratios (generally 30-50 and 10 in some
cases) were used in subsequent experiments.
The reaction of 1 with AlBui3/CPh3[B(C6F5)4] (Hf:Al:B )
1:10-50:1.1 gave a red oil, which precipitated to the bottom
of the NMR tube. The upper (toluene) phase was removed; its
1H NMR spectrum showed the formation of Ph3CH along with
isobutene in the course of activation of 1. The oily phase was
dissolved in a mixture of toluene-d8 and 1,2-difluorobenzene
(5:1 v/v), and the NMR spectra of the resulting samples were
measured. Only one species (9) was found to prevail in solution
over a broad range of Al:Hf ratios (10-50:1). Compound 9
was stable at room temperature for hours, and complete
degradation took place only on heating to +95 °C.
(22) (a) Bryliakov, K. P.; Babushkin, D. E.; Talsi, E. P.; Voskoboynikov,
A. Z.; Gritzo, H.; Schro¨der, L.; Damrau, H.-R.; Wieser, U.; Schaper, F.;
Brintzinger, H. H. Organometallics 2005, 24, 894. (b) Talsi, E. P.; Bryliakov,
K. P.; Semikolenova, N. V.; Zakharov, V. A.; Bochmann, M. Kinet. Catal.
2007, 48, 490.
(23) Nedorezova, P. M.; Veksler, E. N.; Aladyshev, A. M.; Tsvetkova,
V. I.; Optov, V. A.; Brusova, G. P.; Lemenovskii, D. A. Kinet. Catal. 2006,
47, 245.
(24) Nedorezova, P. M.; Veksler, E. N.; Optov, V. A.; Tsvetkova, V. I.;
Sklyaruk, B. F. Polym. Sci. Ser. A 2007, 49, 99.
(25) Babushkin, D.; Panchenko, V. N.; Timofeeva, M. N.; Zakharov,
V. A.; Brintzinger, H. H. Macromol. Chem. Phys. 2008, 209, 1210.
(26) For related chloro-bridged zirconocenes [{CpR2Zr(µ-Cl)}2]2+ see
ref 13 and: Wu, F.; Dash, A. K.; Jordan, R. F. J. Am. Chem. Soc. 2004,
126, 15360.

6336 Organometallics, Vol. 27, No. 23, 2008
BryliakoV et al.
1
Figure 1. (a) H and (b) 13C NMR spectra of 9 obtained by the reaction of (SBI)HfCl2 (1) with AlBui3/CPh3[B(C6F5)4] (Hf:Al:B ) 1:50:
1.1; toluene-d8/1,2-difluorobenzene, 20 °C).
Scheme 2
reagents at 20 °C for 1.5 h (Hf:Al:B ) 1:25-50:1.1) gave rise
to a new hafnium hydride species, 10. One of the hydrides
teristic of bridging rather than terminal metallocene hydri-
des;13,14,27-31 however, the full structure could not be elucidated.
Reactions of C2H4(Flu)(5,6-C3H6-2-MeInd)HfCl2 (4). As
indicated in the Introduction, the hafnocene 4 is of interest since
it produces high molecular weight elastomeric polypropylene
displays a 1H NMR peak at δ -5.56 (at +2 °C; 2H, dd, JHH
)
13 and 3.5 Hz). 1H COSY spectra reveal that this signal shows
a weak correlation with another signal at ca. δ 2.0, which
overlaps with intense peaks of toluene-d8 and AlBui3. This
species was found to be an ion pair with the [B(C6F5)4]-
(28) Visser, C.; van den Hende, J. R.; Meetsma, A.; Hessen, B.; Teuben,
J. H. Organometallics 2001, 20, 1620.
(29) Chirik, P. J.; Bercaw, J. E. Organometallics 2005, 24, 5407.
(30) Chung, J. H.; Lee, S. M. Bull. Korean Chem. Soc. 2006, 27, 759.
(31) Garratt, S.; Carr, A. G.; Langstein, G.; Bochmann, M. Organome-
tallics 2003, 36, 4276.
1
counteranion. The position of the H hydride peak is charac-
(27) Grossman, R. B.; Doyle, R. A.; Buchwald, S. L. Organometallics
1991, 10, 1501.

Hafnocene Complexes in MAO-ActiVated Systems
Organometallics, Vol. 27, No. 23, 2008 6337
Figure 2. 1H NMR spectrum (toluene-d8/1,2-difluorobenzene, 20 °C) of 11 obtained by the reaction of 4 with AlBui3/CPh3[B(C6F5)4]
(Hf:Al:B ) 1:100:1.1). The asterisk marks the hydride peak of the minor hafnium hydrido species.
and possesses two inequivalent sites for the σ-ligands. This
complex was therefore studied in more detail.
isomers strongly predominates. There is evidence for the
presence of a minor second isomer in the crude reaction mixture,
but only the major isomer could be crystallized. The solid state
structure is shown in Figure 3.
The metallocene geometry resembles that of (SBI)Hf-
(Me)(CH2SiMe3).16 The bond lengths to the trimethylsilylmethyl
ligands and the angles to the second ligands in both complexes
are experimentally indistinguishable. The difference in bonding
to the fluorenyl and indenyl parts of the ligand is reflected in
the Hf-CtInd and Hf-CFlu distances of 2.22 and 2.30 Å,
respectively. The fluorenyl also exhibits greater variation in the
Hf-C bond lengths, between 2.428(8) and 2.605(8) Å, but they
are all sufficiently short to designate η5 hapticity.
The most striking feature of the molecular structure of 12 is
that this major product is not the one predicted based on steric
considerations: the trimethylsilylmethyl ligand is bound on the
apparently more hindered side of the metallocene wedge,
adopting a conformation whereby the alkyl ligand is located in
close proximity to both the fluorenyl and indenyl six-membered
rings. This isomer is produced with high stereoselectivity,
presumably under kinetic control. A possible explanation is that
in the precursor 4 the same steric interactions slightly weaken
the Hf-Cl bond in that position, such that substitution of that
chlorine atom is kinetically favored.32 This finding urges caution
when predicting reaction outcomes based on prima facie steric
considerations.
The mixed-alkyl complex L′Hf(Me)CH2SiMe3 (13) was
prepared in a two-step, two-pot procedure from 4, but without
isolating the intermediate 12. Complex 13 is very poorly soluble
in light petroleum, but highly soluble in toluene. Attempts to
crystallize 13 from toluene/light petroleum mixtures led only
to the precipitation of a fine solid, which trapped impurities. A
sample pure enough for subsequent spectroscopic studies was
isolated by extracting the crude product with toluene/light
petroleum, followed by removal of the solvents and washing
the residue with light petroleum.
Mixing 4 with AlBui3/CPh3[B(C6F5)4] (Hf:Al:B ) 1:40-100:
1) in toluene generates two types of hafnium hydride complexes
that settle as a violet oil to the bottom of the NMR tube. As
before, the upper phase was removed and the oil was dissolved
in toluene-d8 containing ca. 15 vol-% 1,2-difluorobenzene. Using
a higher excess of AlBui3 along with longer reaction times leads
to the formation of predominantly one hydride species, 11,
identified as [L′Hf(µ-H)2AlBui2]+[B(C6F5)4]-. A typical 1H
NMR spectrum is shown in Figure 2.
1
The H NMR spectrum of 11 shows a hydride resonance at
δ -2.11 (1H, d, H1, JHH ) 6 Hz), which correlates with a second
peak at δ 1.65. Apparently, due to the C1 symmetry, two
nonequivalent bridging hydrides are found at two very different
chemical shifts. The Al-CH2 peaks at δ -0.02 (2H, dd, JHH
)
14 and 7.2 Hz) and -0.33 (2H, dd, JHH ) 14 and 7.8 Hz)
correlate with a signal at δ ca. 1.6, which apparently belongs
to AlCH2CHMe2 groups. The peaks listed above appear and
degrade in parallel with the Ind-H peak at δ 5.88 and Ind-CH3
peak at δ 1.64; thus they can be ascribed to one and the same
species 11, the anionic part of which displays 19F NMR signals
at δ -132.5, -163.0, and -166.5. Ion pair 11 is unstable at
room temperature and slowly decomposes (within 36 h).
Warming the sample to 60 °C resulted in the loss of the hafnium
hydride resonances. Species 11 reacts with 3 equiv of propene
on warming from -20 to +10 °C.
Like 9, species 11 reacts with 2,4,4-trimethyl-1-pentene: after
the addition of 10 equiv of the alkene to the solution of 11 in
1
toluene-d8/1,2-difluorobenzene ([11] ) 0.05 M), the H NMR
signals of 11 had disappeared after 1 h at room temperature,
while the minor hydride species (marked with an asterisk in
Figure 2) remained unchanged.
In order to evaluate the ligand dynamics of C1-symmetric
systems and evaluate the energy difference between the two
possible alkyl ligand binding sites, we proceeded to prepare the
mixed-alkyl
complex
C2H4(Flu)(5,6-C3H6-2-MeInd)-
Surprisingly, while the diastereotopic Hf-CH2 group of the
chloro trimethylsilylmethyl complex gives the expected pair of
doublets in the chloroform-d1 1H NMR spectrum, the Hf-CH2
signal of the methyl trimethylsilylmethyl appears as a singlet.
Hf(Me)CH2SiMe3 and the corresponding [L′Hf-CH2SiMe3]+
cation.
There was no significant reaction between the dichloride
complex 4 and Me3SiCH2MgCl until the mixture was heated
to 80 °C. The preferred solvent for this reaction is toluene. Under
these conditions, the reaction is selective for the monoalkylation
product L′Hf(Cl)CH2SiMe3 (12). One of the two possible
(32) Evidence that the coordination site occupied by Cl in 12 is the less
hindered one is also provided by the equilibrium between cations 14a and
14b, where 14a is the dominant species.

6338 Organometallics, Vol. 27, No. 23, 2008
BryliakoV et al.
Figure 3. Left: Molecular structure of L′Hf(Cl)CH2SiMe3 (12), showing the atomic numbering scheme. Displacement ellipsoids are drawn
at the 30% probability level. Hydrogen atoms have been omitted for clarity. Selected bond length (Å) and angles (deg), esd’s are in parentheses:
Hf(1)-C(1) 2.224(8); Hf(1)-Cl(1), 2.4143(19), Hf(1)-C(21) 2.428(8), Hf(1)-C(41) 2.473(8), Hf(1)-C(42) 2.488(9), Hf(1)-C(43) 2.511(9),
Hf(1)-C(22) 2.547(7), Hf(1)-C(52) 2.547(8), Hf(1)-C(33) 2.605(8), Hf(1)-C(44) 2.623(8), Hf(1)-C(27) 2.699(7), Hf(1)-C(28) 2.737(8),
Hf(1)-CtFlu 2.304, Hf(1)-CtInd 2.22; C(1)-Hf(1)-Cl(1) 101.27(19), CtInd-Hf(1)-CtFlu 128.4, Si(1)-C(1)-Hf(1) 122.6(4) (CtFlu ) centroid
of the fluorenyl ring; CtInd ) centroid of the indenyl ring). Right: View of 12 perpendicular to the Cl-Hf-C(1) plane, illustrating the steric
environment of the CH2SiMe3 ligand.
Scheme 3
The identity of the resonance was confirmed by HETCOR and
DEPT experiments. It therefore appears that in chloroform-d1
both Hf-CH2 hydrogen atoms have coincidentally identical
chemical shifts. In toluene-d8 the diastereotopic Hf-CH2 protons
appear separately in the 1H NMR spectra at temperatures up to
+40 °C.
Hf-CHH-Si and the Cp-CH3 protons was observed, thus leading
to a conclusion that the structure of the major isomer corre-
sponds to that of 14a (Scheme 3 and Supporting Information).
The most thermodynamically stable cation therefore appears to
be that where the trimethylsilylmethyl ligand is located in the
alternative position to the chloro trimethylsilymethyl complex
12.
Treatment of 13 with CPh3[B(C6F5)4] in toluene affords
[L′HfCH2SiMe3]+[B(C6F5)4]- (14) as a red-brown oil (Scheme
3).33 At low temperatures (-40 to 0 °C), the 13C and 1H NMR
spectra show two diastereomers, 14a and 14b, which differ in
the site of CH2SiMe3 coordination (Figure 4). At -20 °C, the
ratio of major and minor isomers was found to be 1.6:1. In both
The interconversion of 14a and 14b leads to a broadening of
the spectra with increasing temperature; coalescence is reached
at +25 °C. Given the low ligand symmetry, the site epimer-
ization is characterized by two different exchange rate constants
(such that ka[14a] ) kb[14b]), and the characteristic time of
the overall exchange process τex ) (ka + kb)-1. This leads to
estimates for ka ) 1.1 × 102 s-1 and kb ) 1.8 × 102 s-1 and
1
structures, three separate H NMR signals were found for the
SiMe3 groups, with the high-field resonances (at δ -1.58 for
the minor isomer and δ -2.58 for the major one) indicating
the γ-agostic bonding of one of the CH3 groups.15-17 The
position of the CH2SiMe3 ligand in 14a and 14b was determined
from the 1H,1H NOESY spectrum: in the major isomer, a NOE
correlation peak (at 300 ms mixing time) between one of the
‡
‡
for ∆Ga ex and ∆Gb ex (at the coalescence temperature of 298
K) ) 14.7 and 14.4 kcal mol-1, respectively.34 These values
are close to that observed for the [(SBI)Hf(CH2-
SiMe3)]+[B(C6F5)4]- ion pair.16
(33) For the related activation of mixed-alkyl Zr and Hf complexes with
B(C6F5)3 see: Beswick, C. L.; Marks, T. J. J. Am. Chem. Soc. 2000, 122,
10358. For the reaction of [(SBI)ZrCH2SiMe3][B(C6F5)4] with 1-hexene
see: Landis, C. R.; Christianson, M. D. Proc. Natl. Acad. Sci. 2006, 103,
15349.
(34) To estimate the exchange rate constants, the Cp-H 1H NMR peaks
were considered, assuming the ∆ω0 ) 290 Hz (at-40 °C) and coalescence
temperature +25 °C. At temperatures lower than-40 °C, the 1H NMR peaks
of 7a and 7b become too broad due to the increased viscosity of the toluene/
difluorobenzene solvent.

Hafnocene Complexes in MAO-ActiVated Systems
Organometallics, Vol. 27, No. 23, 2008 6339
first-order conditions one can derive the propagation rate
obs
constants kp (estimated from the polymer yield) of 150 s-1
at 20 °C and 330 s-1 at 50 °C. These values are very comparable
to the site epimerization rates observed here for 14 and thus
support the conclusion that the back-skip and chain propagation
processes proceed with comparable rates.35,36
Conclusions
The interaction of hafnocenes 1-3 with MAO, AlMe3/
CPh3[B(C6F5)4], and AlBui3/CPh3[B(C6F5)4] lead to different
types of cationic intermediates with different reactivities. The
methyl-bridged heterobinuclear intermediates [LHf(µ-Me)2Al(µ-
Me)2][MeMAO] and [LHf(µ-Me)2Al(µ-Me)2][B(C6F5)4] are
thermally stable at room temperature but display only low olefin
polymerization productivities; that is, they polymerize propene
only on warming to room temperature. The stability of these
µ-Me products is thought to be responsible for the comparatively
low catalytic activity of hafnocenes when AlMe3 is present (i.e.,
with AlMe3-rich MAO). Activation with AlBui3/CPh3[B(C6F5)4],
on the other hand, generates heterobinuclear hydrido-bridged
species of the type [LHf(µ-H)2AlBui2]+ or [LHf(µ-H)2Al-
(H)Bui]+, which are much more reactive in olefin polymeriza-
tions. The hafnium hydride species were completely consumed
in reactions with a few equivalents of 2,4,4-trimethyl-1-pentene
and allylbenzene, thus confirming their role as the active site
precursors.
Similar reactions were found for the C1-symmetric hafnocene
4. The two chloride ligands show surprisingly different reactiv-
ity, as shown by the stereoselective synthesis of L′Hf-
(Cl)CH2SiMe3. The ion pair [C2H4(Flu)(5,6-C3H6-2-
MeInd)HfCH2SiMe3]+[B(C6F5)4]- produced on trityl borate
activation exists as a mixture of two diastereomers, which are
in equilibrium. The site epimerization rates indicate that the two
possible alkyl ligand coordination sites are of very comparable
energy; these rates are also of the same order of magnitude as
propagation rates for propene polymerizations with this system.
Experimental Section
1
Methylaluminoxane (MAO) was purchased from Crompton
GmbH (Germany) as a toluene solution (total Al content 1.8 M,
Al as AlMe3 0.5 M). Ethylene-13C2 (99% 13C) was purchased from
Aldrich. Triisobutylaluminum (AlBui3) was purchased from Aldrich
and either diluted to obtain a solution of [Al] ) 1 M or used as
received for experiments with high Al/Hf ratios. Trimethylaluminum
(AlMe3) was purchased from Aldrich as a 2 M solution in toluene.
Toluene was purified by refluxing over sodium metal and distilled
under dry nitrogen. 1,2-Difluorobenzene was dried over molecular
sieves (4 Å) and degassed. All operations were carried out under
dry nitrogen (99.999%) by standard Schlenk techniques. Solids and
toluene were transferred and stored in a glovebox. (SBI)HfCl2 (1),37
Me2C(Cp)(Flu)HfCl2 (2),38 Ph2C(Cp)(Flu)HfCl2 (3),39 and CPh3-
[B(C6F5)4]40 were prepared according to literature procedures.
Complex C2H4(Flu)(5,6-C3H6-2-MeInd)HfCl2 (4) was kindly do-
nated by Prof. B. Rieger.
Figure 4. H NMR spectrum (toluene-d8/1,2-difluorobenzene) of
the cationic hafnium species obtained by the reaction of 14 with
CPh3[B(C6F5)4] (Hf:B ) 1:1.1) at differing temperatures. Asterisks
mark the Hf-CHH-Si peaks of both major (14a) and minor (14b)
isomers.
Of particular interest is the question of the relation between
the chain propagation rate and the observed site epimerization
rate. For the system 4/AlBui3/CPh3[B(C6F5)4], Rieger and co-
workers reported an increase in isotacticity of propene polym-
erization with temperature ([mmmm] fraction rising from 17
to 34% upon increasing the polymerization temperature from 0
to 50 °C) and explained this by a favored back-skip of the
growing polymer chain before monomer coordination.1-3 One
should expect then that the back-skip and chain propagation
processes should have comparable rates. The observed site
epimerization in 14 can be regarded as a model of this back-
skip process, providing the kinetic parameters of this exchange.
From the reported polymerization data of Rieger under pseudo-
1H, 13C{1H}, and 19F NMR spectra were recorded in standard 5
mm NMR tubes on either Bruker Avance DPX-300 or Avance 400
spectrometers. Typical operating conditions for 13C NMR measure-
obs
(35) One should be aware, however, that kp
estimated from the
(37) Herrmann, W. A.; Rohrmann, J.; Herdtweck, E.; Spaleck, W.;
Winter, A. Angew. Chem., Int. Ed. Engl. 1989, 28, 1511.
(38) Ewen, J. A.; Jones, R. L.; Razavi, A.; Ferrara, J. D. J. Am. Chem.
Soc. 1988, 110, 6255.
polymer yield might be ca. 1 order of magnitude lower than that determined
from the time dependence of the number-average molecular weight at the
initial stages of polymerization.15,36
(36) (a) Song, F.; Cannon, R. D.; Bochmann, M. J. Am. Chem. Soc.
2003, 125, 7641. (b) Song, F.; Cannon, R. D.; Lancaster, S. J.; Bochmann,
M. J. Mol. Catal. 2004, 218, 21.
(39) Mise, T.; Miya, S.; Yamazaki, H. Chem. Lett. 1989, 1853.
(40) Bochmann, M.; Lancaster, S. J. J. Organomet. Chem. 1992, 434,
C1.

6340 Organometallics, Vol. 27, No. 23, 2008
BryliakoV et al.
3
ments were the following: spectral width 20 kHz; spectrum
accumulation frequency 0.1 Hz; 100-5000 transients, 90° pulse at
6.7 µs. Correct integral values were obtained from inverse gated
spectra. 13C, 1H correlations were established by using the standard
Bruker HXCOBI (for direct C-H interactions) pulse program.
Operating conditions for 1H NMR measurements: spectral width 5
kHz; spectrum accumulation frequency 0.2-0.5 Hz; number of
transients 32-64, ca. 30° pulse at 2 µs. Typical operating conditions
for 19F NMR measurements: spectral width 30 kHz; spectrum
accumulation frequency 0.1-0.5 Hz; number of transients 32-64.
3.0 Hz), 6.49 (d, 1H, Cp-H, JHH ) 3.0 Hz), 5.94 (d, 1H, Cp-H,
JHH ) 3.0 Hz), 5.42 (d, 1H, Cp-H, JHH ) 3.0 Hz), 0.64 (s, 3H,
SiMe), 0.54 (s, 3H, SiMe), -0.82 (s, 3H, HfMe). 13C{1H} NMR
(75.47 MHz, toluene-d8/1,2-difluorobenzene, 20 °C, Al/Hf ) 30,
13C-enriched MAO used): δ 41.2 (1C, Hf-Me, J1 ) 124 Hz).
CH
[(SBI)Hf(µ-Me)2AlMe2][MeMAO] (5-MAO). 1H NMR (300.13
MHz, toluene-d8/1,2-difluorobenzene, 20 °C, Al/Hf ) 240): δ 6.17
(2H, Cp-H), 5.05 (2H, Cp-H), 0.80 (s, 6H, SiMe2), -0.60 (s, 6H,
AlMe2), -0.90 (s, 3H, Hf(µ-Me)2Al). 13C{1H} NMR (75.47 MHz,
toluene-d8/1,2-difluorobenzene, 20 °C, Al/Hf ) 240, 13C-enriched
1
For calculations of H and 13C chemical shifts, the resonance of
MAO used): δ 35.7 (2C, Hf(µ-Me)2Al, J1 ) 113 Hz).
CH
1
the CH3 group of toluene solvent was taken as δ 2.09 and 20.40,
respectively. 19F chemical shifts were referenced externally to
CFCl3. Sample temperature measurement uncertainty and temper-
ature reproducibility were less than (1 °C.
[(SBI)Hf(µ-Me)2AlMe2][B(C6F5)4] (5-BX4). H NMR (300.13
MHz, toluene-d8/1,2-difluorobenzene, 20 °C): δ 6.10 (d, 2H, Cp-
H, JHH ) 3.2 Hz), 5.00 (d, 2H, Cp-H, JHH ) 3.2 Hz), 0.75 (s, 6H,
SiMe2), -0.70 (s, 6H, AlMe2), -0.93 (s, 6H, Hf(µ-Me)2Al); 2.00
(s, 3H, Ph3CCH3). 13C NMR (75.47 MHz, toluene-d8/1,2-difluo-
Preparation of MAO and Al2Me6 Samples. Solid MAO was
prepared from commercial MAO (Crompton) by removal of the
solvent under vacuum at 20 °C. The solid product obtained
(polymeric MAO with total Al content 40 wt % and Al as residual
AlMe3 ca. 5 wt %) was used for the preparation of the samples.
13CH3-enriched MAO was prepared by ligand exchange of 99%
13CH3-enriched Al2Me6 (70 mol % of total Me groups) and solid
MAO (30 mol % of total Me groups) in toluene solution followed
by subsequent removal of the liquid fraction of Al2Me6 under
vacuum to give a sample of 13C-enriched MAO (65-70% 13C)
with desired Al2Me6 content (polymeric MAO with total Al content
of 40% and Al as residual AlMe3 ca. 5 wt %). A more detailed
description is presented in ref 41.
Preparation of NMR Samples. The appropriate amounts of
hafnium complex (typically, 0.01-0.06 mmol) and MAO or
CPh3[B(C6F5)4] were placed in 5 mm NMR tubes in a glovebox
and capped with rubber septa. Further addition of AlMe3 solution
(if necessary, Al/Hf ) 30-50) to the tube with the hafnium complex
was performed outside the glovebox with gastight microsyringes.
For hafnocenes/MAO samples, the spectra were recorded in toluene.
robenzene, 20 °C): δ 115.8 (2C, Cp, J1 ) 179 Hz), 113.9 (2C,
CH
Cp, J1CH ) 179 Hz), 86.4 (2C, Si-C(Cp)), 35.2 (2C, Hf(µ-Me)2Al,
J1 ) 113 Hz), -3.2 (2C, SiMe2, J1 ) 122 Hz), -7.2 (2C,
CH
CH
AlMe2, J1 ) 115 Hz).
CH
Me2C(Cp)(Flu)HfMeCl. 1H NMR (400.13 MHz, toluene-d8/
1,2-difluorobenzene, 20 °C, Al/Hf ) 90): δ 6.10 (1H, Cp-H), 5.75
(1H, Cp-H), 5.45 (1H, Cp-H), 4.95 (1H, Cp-H), 1.85 (s, 3H, CMe),
1.77 (s, 3H, CMe), -1.06 (s, 3H, HfMe).
[Me2C(Cp)(Flu)Hf(µ-Me)2AlMe2][MeMAO] (6-MAO). 1H NMR
(400.13 MHz, toluene-d8, 20 °C, Al/Hf ) 300): δ 5.68 (2H, Cp-
H), 4.76 (2H, Cp-H), 1.98 (s, 6H, C(CH3)2). AlMe2 and Hf(µ-
Me)2Al peaks not observed (hidden by broad and intense MAO
signal).
[Me2C(Cp)(Flu)Hf(µ-Me)2AlMe2][B(C6F5)4] (6-BX4). 1H NMR
(400.13 MHz, toluene-d8/1,2-difluorobenzene, 20 °C): δ 5.63 (2H,
Cp-H), 4.74 (2H, Cp-H), 1.90 (s, 6H, CMe2), -0.72 (s, 3H, AlMe),
-0.81 (s, 3H, AlMe), -1.11 (s, 6H, Hf(µ-Me)2Al). 13C NMR
(100.614 MHz, toluene-d8/1,2-difluorobenzene, 20 °C): δ 118.0 (2C,
Cp), 102.2 (2C, Cp), 28.1 (2C, Hf(µ-Me)2Al), 27.7 (2C, CMe2),
-6.5 (2C, AlMe), -9.0 (2C, AlMe).
For hafnocenes/AlMe3/CPh3[B(C6F5)4] systems, the resulting
cationic species were poorly soluble in toluene but formed oily
precipitates in the lower part of the NMR tube. Normally, after
shaking the samples at room temperature, they were allowed to
stand to achieve the phase separation, then the upper (toluene) phase
was removed and the oily residue was dissolved in 0.5 mL of
toluene-d8 and 0.1 mL of 1,2-difluorobenzene to obtain a homo-
geneous solution.
For hafnocenes/AlBui3/CPh3[B(C6F5)4] systems, in a glovebox
the appropriate amounts of hafnium complex (typically, 0.03-0.06
mmol) and CPh3[B(C6F5)4] were placed in a Schlenk tube equipped
with a magnetic stirrer and capped with a rubber septum. Further
addition of AlBui3 solution was performed outside the glovebox
with gastight microsyringes at 0 °C, and the mixture was then stirred
at room temperature (typically 1-2 h). The oily phase was allowed
to settle and the upper toluene phase was decanted. This removed
most (but not all) of the excess AlBui3. The oil was dissolved in
toluene-d8 and 1,2-difluorobenzene (5:1 v/v), and the resulting
samples were taken for the NMR measurements.
Ph2C(Cp)(Flu)HfMeCl. 1H NMR (400.13 MHz, toluene-d8, 20
°C, AlMAO/Hf ) 90): δ 6.50 (1H, Cp-H), 6.30 (1H, Cp-H), 5.90
(1H, Cp-H), 5.45 (1H, Cp-H), -1.08 (s, 3H, HfMe).
[Ph2C(Cp)(Flu)Hf(µ-Me)2AlMe2][MeMAO] (7-MAO). 1H NMR
(400.13 MHz, toluene-d8, 20 °C, Al/Hf ) 300): δ 6.15 (2H, Cp-
H), 5.15 (2H, Cp-H), -1.05 (6H, Hf(µ-Me)2Al).
[Ph2C(Cp)(Flu)Hf(µ-Me)2AlMe2][B(C6F5)4] (7-BX4). 1H NMR
(400.13 MHz, toluene-d8/1,2-difluorobenzene, 20 °C): δ 6.05 (2H,
Cp-H), 5.12 (2H, Cp-H), -0.64 (s, 3H, Al(CH3)), -0.79 (s, 3H,
AlMe), -1.09 (s, 6H, Hf(µ-Me)2Al). 13C NMR (100.614 MHz,
toluene-d8/1,2-difluorobenzene, 20 °C): δ 123.1 (2C, Cp), 111.4
(2C, Cp), 99.0 (C, C(Cp)), 63.3 (1C, CPh2), 30.6 (2C, Hf(µ-Me)2Al),
-6.3 (2C, AlMe), -9.1 (2C, AlMe).
[C2H4(Flu)(5,6-C3H6-2-MeInd)Hf(µ-Me)2AlMe2][MeMAO] (8-
MAO). 1H NMR (300.13 MHz, toluene-d8, 20 °C, Al/Hf ) 90): δ
5.41 (s, 1H, Cp-H), 1.79 (s, 3H, Cp-CH3), -0.6 (s, 3H), -0.75 (s,
3H, AlMe), -1.94 (s, 3H, Hf(µ-Me)2Al). 13C NMR (75.47 MHz,
toluene-d8/1,2-difluorobenzene, 20 °C, 13C-enriched MAO was
Propene and ethene (13C-ethene) were injected in the NMR tubes
via gastight syringes upon appropriate cooling (below -30 °C).
Typical NMR Data for the [B(C6F5)4]- Counteranion. 13C{1H}
NMR (75.47 MHz, toluene-d8, 1,2-difluorobenzene, 20 °C): δ 148.9
(8C, o-C, J1 ) 246 Hz), 138.7 (d, 4C, p-C, J1 ) 246 Hz),
used): δ 103.5 (1C, Cp), 103.2 (1C, Cp-H, J1 ) 178 Hz), 36.7
CH
(1C, Hf(µ-Me)2Al, J1 ) 112 Hz), 36.3 (1C, Hf(µ-Me)2Al, J1
CH
CH
) 112 Hz), 14.2 (1C, Cp-CH3, J1CH ) 130 Hz), -6.1 (1C, AlMe,
J1 ) 113 Hz), -7.8 (1C, AlMe, J1 ) 113 Hz).
CH
CH
CF
CF
136.9 (d, 8C, m-C, J1 ) 245 Hz). 19F NMR (282.40 MHz,
[C2H4(Flu)(5,6-C3H6-2-MeInd)Hf(µ-Me)2AlMe2]+[B(C6F5)4]- (8-
CF
1
toluene-d8, 1,2-difluorobenzene, 20 °C): δ -132.5 (2F, o-F), -163.0
BX4). H NMR (300.13 MHz, toluene-d8/1,2-difluorobenzene, 20
3
(t, 1F, p-F, JFF ) 19 Hz), -166.5 (2F, m-F).
°C): δ 5.38 (s, 1H, Cp-H), 1.71 (s, 3H, Cp-CH3), -0.69 (s, 3H,
AlMe), -0.74 (s, 3H, Hf(µ-Me)2Al), -0.82 (s, 3H, AlMe), -1.88
(s, 3H, Hf(µ-Me)2Al); 2.00 (s, 3H, Ph3CCH3). 13C NMR (75.47
MHz, toluene-d8/1,2-difluorobenzene, 10 °C): δ 103.5 (1C, Cp),
(SBI)HfMeCl. 1H NMR (300.13 MHz, toluene-d8/1,2-difluo-
robenzene, 20 °C, AlMAO/Hf ) 30): δ 6.62 (d, 1H, Cp-H, JHH
)
103.2 (1C, Cp-H, J1CH ) 178 Hz), 36.2 (1C, Hf(µ-Me)2Al, J1
)
CH
(41) Bryliakov, K. P.; Talsi, E. P.; Bochmann, M. Organometallics 2004,
23, 149.
112 Hz), 36.1 (1C, Hf(µ-Me)2Al, J1 ) 112 Hz), 14.2 (1C, Cp-
CH

Hafnocene Complexes in MAO-ActiVated Systems
Organometallics, Vol. 27, No. 23, 2008 6341
CH3, J1 ) 130 Hz), -6.1 (1C, AlMe), J1 ) 113 Hz), -7.8
mixture was heated to 80 °C for 12 h, during which time the
suspended dichloride complex dissolved. The volatiles were
removed under vacuum and the magnesium salts separated by
extraction with a mixture of 20 mL of light petroleum and 20 mL
of toluene. The resulting yellow solution was concentrated to
approximately 20 mL under reduced pressure to remove the light
petroleum. To this solution was added MeMgCl (2.5 mmol, 0.83
mL, 3 M in tetrahydrofuran) at room temperature and the reaction
mixture stirred for 16 h. Removal of the volatiles gave a yellow
oil, which solidified on addition of 20 mL of light petroleum. The
addition of a further 20 mL of toluene was necessary to dissolve
the product, and the insoluble residues were separated by filtration.
The crude product crystallized from a yellow oil only after complete
distillation of the solvent and was washed with 20 mL of light
petroleum (0.9 g, 1.41 mmol, 57%). In order to prevent further
losses, the compound was not recrystallized, and no elemental
analysis was obtained. 1H NMR (300.13 MHz, toluene-d8, 20 °C):
δ 7.79 (dt, JHH ) 8 Hz, JHH ) 1 Hz, Ar), 7.75 (dt, JHH ) 8 Hz, JHH
CH
CH
(1C, AlMe, J1 ) 113 Hz).
CH
(SBI)Hf(Bui)Cl. 1H NMR (300.13 MHz, toluene-d8, 20 °C, Al/
Hf ) 90): δ 5.88 (d, 1H, Cp-H, JHH ) 3.0 Hz), 5.36 (s, 1H, Cp-H,
JHH ) 3.0 Hz), 0.62 (s, 3H, SiMe), 0.51 (s, 3H, SiMe), -0.72 (dd,
1H, Hf-CH2), JHH ) 14 Hz, JHH ) 7 Hz), -1.11 (dd, 1H, Hf-CH2,
JHH ) 14 Hz, JHH ) 5 Hz). Isobutene: δ 4.65 (s, 2H, CH2), 1.59
(s, 6H, Me). 13C NMR (75.47 MHz, toluene-d8, 1,2-difluoroben-
zene, 20 °C): δ 80.4 (1C, Hf-CH2, J1 ) 113 Hz), -1.8 (1C,
CH
SiMe), -2.4 (1C, SiMe).
[(SBI)Hf(µ-H)2Al(H)(Bui)]+[B(C6F5)4]- (9). 1H NMR (300.13
2
MHz, toluene-d8, 20 °C, Al/Hf ) 90): δ 6.38 (d, 2H, Cp-H, JHH
) 2.8 Hz), 5.64 (s, 2H, Cp-H, JHH ) 2.8 Hz), 1.40 (t, 1H, hydride,
JHH ) 5), 0.75 (s, 6H, SiMe), -1.13 (d, 2H, hydride, JHH ) 5 Hz).
Isobutene: δ 4.69 (s, 2H, CH2), 1.60 (s, 6H, Me). 13C NMR (75.47
MHz, toluene-d8, 1,2-difluorobenzene, 20 °C): δ 113.4 (2C, C-H
(Cp), J1CH ) 172 Hz), 103.1 (2C, C-H (Cp), J1CH ) 179 Hz), 87.9
(2C, C (Cp)), -3.2 (2C, SiMe2).
[{Ph2C(C5H4)(Flu)Hf(µ-Cl)}2]2+ · 2[B(C6F5)4]-. 1H NMR (400.13
MHz, toluene-d8/1,2-difluorobenzene, 20 °C): δ 6.16 (4H, Cp-H),
5.08 (4H, Cp-H). 13C NMR (100.614 MHz, toluene-d8/1,2-
difluorobenzene, 20 °C): δ 126.6 (4C, C-H, (Cp)), 112.0 (4C, C-H,
(Cp)), 100.5 (2C, C(Cp)), 63.3 (2C, CPh2).
) 1 Hz, Ar), 7.35-6.9 (overlapped with toluene), 6.77 (dd, JHH
)
8.7 Hz, JHH ) 1 Hz, Ar), 6.75 (dd, JHH ) 8.7 Hz, JHH ) 1 Hz, Ar),
5.84 (s, 1H, Cp-H), 3.77-3.26 (m), 2.93-2.62 (m), 1.92 (s, 3H,
Cp-CH3), -0.05 (s, 9H, SiMe3), -1.24 (s, 3H, Hf-Me), -3.03 (m,
2H, diastereotopic Hf-CH2). 13C NMR (75.47 MHz, toluene-d8, 20
°C): δ 142.2, 140.8, 127.2, 124.1, 122.7, 122.5, 121.9, 119.3, 116.5,
103.4 (1C, C-H (Cp)), 95.6, 58.4 (1C, JCH ) 106 Hz, Hf-CH2),
45.5 (1C, JCH ) 115 Hz, Hf-Me), 32.6, 32.5, 27.7, 26.9, 26.4, 25.2,
14.4 (1C, J1 ) 127 Hz, Cp-CH3), 2.9 (3C, J1 ) 116 Hz,
Activation of Ph2C(Cp)(Flu)HfCl2 with AlBui3/CPh3[B(C6F5)4]
(Hf:Al:B ) 1:25-50:1.1): Species 10. 1H NMR (400.13 MHz,
toluene-d8/1,2-difluorobenzene, 20 °C): δ 6.22 (1H, Cp-H, JHH
)
CH
CH
2 Hz), 5.37 (1H, Cp-H, JHH ) 2 Hz), 5.08 (1H, Cp-H, JHH ) 2
Hz), 2.0, -5.56 (1H, JHH ) 13 Hz, JHH ) 3.5 Hz). 13C NMR
(100.614 MHz, toluene-d8/1,2-difluorobenzene, 20 °C): δ 123.0 (1C,
quaternary C), 118.6 (1C, C-H, (Cp)), 113.6 (1C, C-H, (Cp)),
112.2 (1C, Cp), 97.6 (1C, C(Cp)), 59.9 (1C, CPh2).
SiMe3).
[C2H4(Flu)(5,6-C3H6-2-MeInd)HfCH2SiMe3]+[B(C6F5)4]- (14).
In a glovebox, the appropriate amounts of C2H4(2-Me-5,6-
(C3H6)Ind)Hf(Me)CH2SiMe3 (typically, 0.01-0.06 mmol) and
CPh3[B(C6F5)3] (1.1 equiv) were placed in a 5 mm NMR tube
capped with a rubber septum. Toluene-d8 (0.5 mL) was added with
appropriate cooling (<0 °C). After mixing the sample, the latter
was allowed to stand until the phases had separated. The upper
(organic) phase was removed; the oily phase was washed with 0.3
mL of toluene-d8 and dissolved in 0.3 mL of toluene-d8 and 0.1
mL of 1,2-difluorobenzene. A trace of SiMe4 due to hydrolysis was
observed. Since the compound is an equilibrium mixture of two
diastereomers, the spectra are sensitive to the measurement tem-
perature, so the NMR data are presented for -20 °C (slow
exchange) and +25 °C (fast exchange).
[C2H4(Flu)(5,6-C3H6-2-MeInd)Hf(µ-H)2AlBui2]+[B(C6F5)4]-
1
(11). H NMR (300.13 MHz, toluene-d8, 20 °C, Al/Hf ) 90): δ
5.88 (s, 1H, Cp-H), 1.64 (s, 1H, Cp-CH3), -0.02 (2H, dd,
Al(CHH)2, JHH ) 14 Hz, JHH ) 7.2 Hz) and -0.33 (2H, dd,
Al(CHH)2, JHH ) 14 Hz, JHH ) 7.8 Hz), -2.11 (1H, d, H1, JHH
)
6 Hz). δ -4.00 (1H, dd, H2, JHH ) 10 Hz, JHH ) 3.7 Hz). Isobutene:
δ 4.69 (s, 2H, CH2), 1.60 (s, 6H, Me). 13C NMR (75.47 MHz,
toluene-d8, 1,2-difluorobenzene, 20 °C): δ 107.4 (1C, C-H (Cp)),
32.4 (2C, Al(CH2)2, J1 ) 139 Hz), 14.5 (1C, Cp-CH3, J1
)
CH
CH
129 Hz).
C2H4(Flu)(5,6-C3H6-2-MeInd)Hf(Cl)CH2SiMe3 (12). C2H4(2-
Me-5,6-(C3H6)Ind)HfCl2 (0.75 g, 1.23 mmol) was suspended in 25
mL of toluene at room temperature. Me3SiCH2MgCl (6.6 mmol,
6.2 mL, 1.06 M in diethyl ether) was added to the suspension and
the reaction mixture warmed to 80 °C for 26 h. The solvents were
removed under vacuum. The product was extracted with a solvent
mixture comprising 10 mL of toluene and 10 mL of light petroleum.
Cooling the resulting solution to -25 °C for one week yielded small
yellow crystals suitable for X-ray crystallography (0.4 g, 0.61 mmol,
50%). Anal. Calcd for C32H35ClHfSi: C, 58.09; H, 5.33. Found: C,
58.49; H, 4.83. 1H NMR (300.13 MHz, CDCl3, 25 °C): δ 7.89-7.79
(m, 3H, Ar), 7.49 (br, 2H, Ar), 7.34-7.25 (m, 3H, Ar), 7.16 (br,
1H, Ar), 7.00-6.95 (m, 1H, Ar), 5.73 (s, 1H, Cp-H), 4.28-4.16
(m, 1H, Al), 4.06-3.89 (m, 2H, Al), 3.76-3.67 (m, 1H, Al),
2.99-2.80 (m, 4H, Al), 2.25 (s, 3H, Cp-CH3), 2.06-1.96 (m, 2H,
Al), -0.33 (s, 9H, SiMe3), -2.80 (d, 1H, 2J ) 12.4 Hz, Hf-CHH),
1H NMR (300.13 MHz, toluene-d8/1,2-difluorobenzene, -20 °C).
Major isomer 14a: δ 5.41 (s, 1H, Cp-H), 1.92 (d, 1H, 2JHH ) 12.5
Hz, Hf-CHH-Si), 1.63 (s, 3H, Cp-CH3), 0.01 (s, 3H, SiMe), -0.18
(s, 3H, SiMe), -0.26 (d, 1H, 2JHH ) 12.5 Hz, Hf-CHH-Si), -2.58
(s, 3H, Hf-Me-Si). Minor isomer 14b: δ 4.46 (s, 1H, Cp-H), 1.72
(s, 3H, Cp-CH3), 0.16 (d, 1H, 2JHH ) 12.5 Hz, Hf-CHH-Si), -0.04
2
(s, 3H, SiMe), -0.14 (s, 3H, SiMe), -0.73 (d, 1H, JHH ) 12.5
Hz, Hf-CHH-Si), -1.58 (s, 3H, Hf-CH3-Si). 13C NMR (75.47 MHz,
toluene-d8/1,2-difluorobenzene, -20 °C). Major isomer 14a: δ 103.4
(1C, Cp-H), 102.9 (1C, Cp), 66.3 (1C, Hf-CH2-Si), 19.0 (1C, Si-
Me-Hf), 16.6 (1C, Cp-CH3), -0.1 (1C, SiMe), -3.0 (1C, SiMe).
Minor isomer 14b: δ 102.0 (1C, Cp), 101.1 (1C, Cp-H), 16.5 (1C,
Si-Me-Hf), 13.7 (1C, Cp-CH3), 0.5 (1C, SiMe), -2.5 (1C, SiMe).
1H NMR (300.13 MHz, toluene-d8/1,2-difluorobenzene, +25 °C):
δ 5.25 (s, br, 1H, Cp-H), 1.77 (s, 3H, Cp-CH3), -0.11 (d, 1H,
2JHH ) 12.5 Hz, Hf-CHH-Si), -0.71 (s, br, 9H, SiMe3). One of
the Hf-CHH-Si peaks not found (overlaps with other peaks). 13C
NMR (75.47 MHz, toluene-d8/1,2-difluorobenzene, +25 °C): δ
103.9 (2C, Cp-H and Cp), 72.7 (1C, CH2), 68.9 (1C, CH2), 32.6,
32.4, 29.2, 27.9, 26.4, 25.5, 14.5 (1C, SiMe3).
2
-2.95 (d, 1H, J ) 12.4 Hz, Hf-CHH). 13C NMR (75.47 MHz,
CDCl3, 25 °C): δ 143.81, 141.76, 127.76, 127.68, 127.35, 126.98,
126.67, 126.56, 124.05, 123.76, 123.35, 122.85, 122.34, 122.05,
121.30, 120.83, 119.02, 116.80, 114.49, 104.01 (‘Cp’), 96.89, 57.76
(Hf-CH2), 32.31, 32.26, 28.42, 26.96, 26.44, 14.89 (Cp-CH3), 2.01
(SiMe3).
C2H4(Flu)(5,6-C3H6-2-MeInd)Hf(Me)CH2SiMe3 (13). To a
suspension C2H4(Flu)(5,6-C3H6-2-MeInd)HfCl2 (1.51 g, 2.48 mmol)
in 25 mL of toluene was added Me3SiCH2MgCl (5.3 mmol, 5.0
mL, 1.06 M in diethyl ether) at room temperature. The reaction
X-ray Crystallography. Crystals of compound 12 are yellow
plates. From a sample under oil, one, ca. 0.07 × 0.04 × 0.015
mm, was mounted on a glass fiber and fixed in the cold nitrogen
stream on a Bruker-Nonius diffractometer equipped with a FR591
rotating anode Mo KR radiation source (λ(Mo KR) ) 0.71073 Å)

6342 Organometallics, Vol. 27, No. 23, 2008
BryliakoV et al.
and graphite monochromator. Intensity data were measured by thin-
slice ω- and φ-scans with a Bruker-Nonius Roper CCD camera on
a κ-goniostat. Total number of reflections recorded, to θmax ) 25.0°,
was 21 382, of which 4852 were unique (Rint ) 0.088); 3784 were
observed with I > 2σI.
Crystal data: C32H35ClHfSi, M ) 661.6, monoclinic, space
group P21/c, a ) 17.2434(6) Å, b ) 10.0370(3) Å, c ) 17.8692(5)
Å, ) 116.194(2)°, V ) 2775.06(15) Å3, Z ) 4, Dcalc ) 1.584 g
cm-3, F(000) ) 1320, T ) 120(2) K, µ(Mo KR) ) 3.917 mm-1
.
Data were processed using the HKL software.42 The structure
was determined by the direct methods routines in the SHELXS
program43 and refined by full-matrix least-squares methods, on F2’s,
in SHELXL.43 The non-hydrogen atoms were refined with aniso-
tropic thermal parameters. Hydrogen atoms were included in
idealized positions, and their Uiso values were set to ride on the
Ueq values of the parent carbon atoms. At the conclusion of the
refinement, wR2 ) 0.104 and R1 ) 0.084 (2B) for all 4852
Acknowledgment. We thank the Royal Society (grant
2006/R4 IJP) and RFBR (grant 06-03-32700) for the financial
support of this work. We are grateful to Prof. Dr. Bernhard
Rieger, TU Munich, for providing a sample of C2H4(Flu)(5,6-
C3H6-2-MeInd)HfCl2, to Dr. Louise Male (National X-ray
Crystallography Service) for diffraction data collection on
C2H4(Flu)(5,6-C3H6-2-MeInd)Hf(Cl)CH2SiMe3, and to Dr.
David L. Hughes (UEA) for assistance with the crystal-
lographic analysis.
2
2
reflections weighted w ) 1/[σ2(Fo ) + 22.94P] where P ) (Fo
+
2Fc2)/3; for the observed data only, R1 ) 0.054. In the final
difference map, the highest peak (ca. 1.27 e Å-3) was 0.96 Å from
Hf(1).
Supporting Information Available: NMR spectra of the
intermediates; crystal and refinement data for C2H4(Flu)(5,6-C3H6-
2-MeInd)Hf(Cl)CH2SiMe3. Crystal data are also given as CIF files.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Scattering factors for neutral atoms were taken from the
literature.44 Computer programs used in this analysis have been
noted above and were run through WinGX45 on a Dell Optiplex
745 PC at the University of East Anglia.
OM800664P
(42) Programs: Standard Data Reduction using HKL (Denzo and
Scalepack): (a) Otwinowski, Z; Minor, W. Processing of X-ray diffraction
data collected in oscillation mode. Methods Enzymol. 1997, 276, 307. (b)
Absorption Correction: Sheldrick, G. M. SADABS, Version 2007/2; Bruker
AXS Inc.: Madison, WI, 2007.
(44) International Tables for X-ray Crystallography; Kluwer Academic
Publishers: Dordrecht, 1992; Vol. C, pp 500, 219, and 193.
(45) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.
(43) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112.