Cyclopentadienyl and olefin substituent effects on insertion and β-hydrogen elimination with group 4 metallocenes. kinetics, mechanism, and thermodynamics for zirconocene and hafnocene alkyl hydride derivatives

Article abstract of DOI:10.1021/om0580351

Reactions of group 4 metallocene dihydrides, (R_nCp) ₂MH₂ (R_nCp = alkyl-substituted cyclopentadienyl; M = Zr, Hf), with olefins afford stable metallocene alkyl hydride complexes of the general formula (R_nCp)₂M(CH ₂CHR′₂)(H) (R′ = H, alkyl). For sterically crowded, monomeric dihydrides, Cp*₂ZrH₂ (Cp* = η⁵-C₅Me₅), Cp*(η⁵- C₅Me₄H)ZrH₂, Cp*(η⁵-C ₅Me₄Et)-ZrH₂, Cp*₂HfH ₂, and Cp*(η⁵-C₅H₃-1,3- (CMe₃)₂)HfH₂, second-order rate constants for olefin insertion have been measured. For Cp*₂HfH₂, the relative rates of olefin insertion have been found to be 1-pentene> styrene ? cis-2-butene > cyclopentene > trans-2-butene > isobutene. The rate of isobutene insertion into Cp*(η⁵-C ₅Me₄H)ZrH₂ is 3.8 × 1Q³ times greater than that for Cp*₂ZrH₂ at -63°C, demonstrating the striking steric effect for isobutene insertion imposed by a tenth methyl substituent on the two cyclopentadienyl ligands. A primary k _H/k_D of 2.4(3) at 23°C and a linear free energy correlation to σ (ρ = -0.46(1)) for para-substituted styrene insertion indicate that insertion into a Zr-H bond proceeds via rate-determining hydride transfer to coordinated olefin, with small positive charge buildup at the β-carbon of the inserting styrene. The rates of β-H elimination for the series (R_n-Cp)₂Zr(CH₂CHR′)(H) have been measured via rapid trapping of the intermediate zirconocene dihydride with 4,4-dimethyl-2-pentyne. Key observations for β-H elimination are (a) primary kinetic deuterium isotope effects (k_H/k_D = 3.9-4.5) and (b) a linear free relationship for the phenethyl hydride series Cp*(η⁵-C₅Me₄H)Zr(CH ₂CH₂-p-C₆H₄-X)(H) (X = H, CH ₃, CF₃, OCH₃), which correlates better to o than σ⁺; p = -1.80(5). The rate of β-H elimination slows with more substituted, hence more sterically crowded, cyclopentadienyl ligands. Equilibration of a series of Cp*(CpR_n)Zr(CH ₂CHMe₂)(H) and Cp*(CpR_n)Zr(CH ₂CH₂CH₂CH₃)(H) with free isobutene and 1-butene has established the relative ground-state energies of isobutyl and n-butyl complexes. These data, in combination with the free energies of activation for β-H elimination, allow free energy profiles to be constructed for insertion and β-H elimination for each olefin.

Full text of DOI:10.1021/om0580351

Organometallics 2005, 24, 5407-5423
5407
Cyclopentadienyl and Olefin Substituent Effects on
Insertion and â-Hydrogen Elimination with Group 4
Metallocenes. Kinetics, Mechanism, and
Thermodynamics for Zirconocene and Hafnocene Alkyl
Hydride Derivatives
Paul J. Chirik† and John E. Bercaw*
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of
Technology, Pasadena, California 91125
Received June 21, 2005
Reactions of group 4 metallocene dihydrides, (RnCp)2MH2 (RnCp ) alkyl-substituted
cyclopentadienyl; M ) Zr, Hf), with olefins afford stable metallocene alkyl hydride complexes
of the general formula (RnCp)2M(CH2CHR′2)(H) (R′ ) H, alkyl). For sterically crowded,
monomeric dihydrides, Cp*2ZrH2 (Cp* ) η5-C5Me5), Cp*(η5-C5Me4H)ZrH2, Cp*(η5-C5Me4Et)-
ZrH2, Cp*2HfH2, and Cp*(η5-C5H3-1,3-(CMe3)2)HfH2, second-order rate constants for olefin
insertion have been measured. For Cp*2HfH2, the relative rates of olefin insertion have been
found to be 1-pentene > styrene . cis-2-butene > cyclopentene > trans-2-butene > isobutene.
The rate of isobutene insertion into Cp*(η5-C5Me4H)ZrH2 is 3.8 × 103 times greater than
that for Cp*2ZrH2 at -63 °C, demonstrating the striking steric effect for isobutene insertion
imposed by a tenth methyl substituent on the two cyclopentadienyl ligands. A primary kH/
kD of 2.4(3) at 23 °C and a linear free energy correlation to σ (F ) -0.46(1)) for
para-substituted styrene insertion indicate that insertion into a Zr-H bond proceeds via
rate-determining hydride transfer to coordinated olefin, with small positive charge buildup
at the â-carbon of the inserting styrene. The rates of â-H elimination for the series (Rn-
Cp)2Zr(CH2CHR′)(H) have been measured via rapid trapping of the intermediate zirconocene
dihydride with 4,4-dimethyl-2-pentyne. Key observations for â-H elimination are (a) primary
kinetic deuterium isotope effects (kH/kD ) 3.9-4.5) and (b) a linear free relationship for the
phenethyl hydride series Cp*(η5-C5Me4H)Zr(CH2CH2-p-C6H4-X)(H) (X ) H, CH3, CF3, OCH3),
which correlates better to σ than σ+; F ) -1.80(5). The rate of â-H elimination slows with
more substituted, hence more sterically crowded, cyclopentadienyl ligands. Equilibration of
a series of Cp*(CpRn)Zr(CH2CHMe2)(H) and Cp*(CpRn)Zr(CH2CH2CH2CH3)(H) with free
isobutene and 1-butene has established the relative ground-state energies of isobutyl and
n-butyl complexes. These data, in combination with the free energies of activation for â-H
elimination, allow free energy profiles to be constructed for insertion and â-H elimination
for each olefin.
Introduction
tion determine polymer chain length and hence influ-
ence many important polyolefin properties.
Olefin insertion into a metal-hydrogen bond and its
microscopic reverse, â-H elimination from a transition
metal alkyl, represent fundamental transformations in
organometallic chemistry and constitute key steps in a
variety of catalytic processes.1 Olefin insertion and â-H
elimination have special relevance to metallocene-
catalyzed olefin polymerization; â-H elimination is a
major chain transfer pathway, and insertion of an olefin
into a metal-hydrogen bond is an important (re)-
initiation step.2 Under typical polymerization conditions
the relative rates of chain propagation and â-H elimina-
Although the effect of metallocene symmetry on
polymer stereochemistry is fairly well-understood,3 a
predictive correlation between cyclopentadienyl substi-
tution pattern and polymer molecular weight has not
yet been established. Commercial application of current
metallocene catalysts is often hampered by the low
polymer molecular weights, especially when the polym-
erization reaction is carried out at plant operating
temperatures (normally greater than 60-80 °C for
polypropylene).4 End group analysis of these polymers
reveals that chain termination occurs primarily by â-H
and â-methyl elimination pathways. Hence, mechanistic
understanding of each of the fundamental transforma-
tions comprising olefin polymerization, and correlation
* To whom correspondence should be addressed. E-mail:
bercaw@caltech.edu.
† Current address: Department of Chemistry and Chemical Biology,
Cornell University, Ithaca, NY 14853-1301.
(1) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals, 2nd ed.; Wiley: New York, 1994; Chapters 7, 9.
(2) Brintzinger, H. H.; Fischer, D.; Mu¨lhaupt, R.; Rieger, B.;
Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143.
(3) (a) Gilchrist, J. H.; Bercaw, J. E. J. Am. Chem. Soc. 1996, 118,
12021. (b) Herzog, T. A.; Zubris, D. L.; Bercaw, J. E. J. Am. Chem.
Soc. 1996, 118, 11988, and references therein.
(4) Resconi, L.; Camurati, I.; Sudmeijer, O. Top. Catal. 1999, 7, 145.
10.1021/om0580351 CCC: $30.25 © 2005 American Chemical Society
Publication on Web 09/23/2005

5408 Organometallics, Vol. 24, No. 22, 2005
Chirik and Bercaw
of their rates as the cyclopentadienyl ligand substituents
are varied (albeit a daunting prospect!), may lead to the
development of metallocene catalysts that can be tai-
lored to produce polymers with desired stereochemistry
and molecular weight.
zirconocenedimethyl/methylalumoxane mixtures.11 There
were few direct kinetic studies employing relevant
models for the propagating species,12,13 until reports by
Landis and co-workers, using quenched-flow kinetics
and direct NMR methods, have measured elementary
rate laws and rate constants for an ansa-zirconocene
catalyst [rac-C2H4(1-indenyl)2Zr(CH3)][CH3B(C6F5)3],14
and by Casey, using direct NMR methods to establish
the relative rates for R-olefin insertion for Cp*2Y-CH2-
(CH2)nCHMe2 (n ) 0, 1).15 Our group has very recently
completed an investigation of chain initiation and
propagation kinetics for [Cp*CpZr(CH2CMe3)][CH3B-
(C6F5)3].16
These measurements have provided important infor-
mation concerning the rate laws and rates for initiation,
propagation, and chain transfer for these catalyst
systems. Additionally, numerous theoretical studies
have been aimed toward understanding the rates and
mechanisms of these fundamental transformations.17
Although the effect of cyclopentadienyl substitution on
the stereo- and regiochemical outcome of a metallocene-
catalyzed polymerization has been well documented,18
less clear is how the catalyst structure influences the
rates of olefin insertion and â-H elimination. Using 16-
electron zirconocene and hafnocene dihydride and alkyl
hydride complexes as models for the active polymeri-
zation catalyst (currently presumed to be a 16-electron,
cationic polymeryl-solvento adduct or -contact ion pair),
we have measured the rates and investigated mecha-
nisms of olefin insertion and â-H elimination in order
to assess the effects of ancillary cyclopentadienyl ligand
substitution on the rates of these processes. As part of
these investigations we found it possible to cleanly
equilibrate a series of Cp*(CpRn)Zr(CH2CHMe2)(H) and
Cp*(CpRn)Zr(CH2CH2CH2CH3)(H) complexes with free
isobutene and 1-butene, allowing thermodynamic and
kinetic data to be extracted. These establish relative
ground-state energies and thus, in combination with the
free energies of activation for â-H elimination, permit
free energy profiles to be constructed for insertion and
â-H elimination for 1-butene versus isobutene.
Olefin insertion and â-H elimination reactions for
metallocenes have been the subject of several experi-
mental and theoretical investigations.5-7 Previous in-
vestigations from our research group have focused on
understanding the kinetics and mechanism of olefin
insertion and â-H elimination for metallocene catalysts.
Using both coalesence and magnetization transfer NMR
techniques, the kinetics of olefin insertion into a metal-
hydrogen bond has been examined with a series of group
5 metallocene complexes of the general formula (η5-
C5R5)2M(η2-CH2dCHR′)(H) (M ) Nb, Ta; R ) H, Me;
R′ ) H, CH3, C6H5).8 For the series of ethylene hydride
compounds, increased rates of insertion were observed
for niobium relative to tantalum and for less congested
cyclopentadienyl relative to Cp* ligands (Cp* ) η5-C5-
Me5). Similar effects are also observed in the propylene
hydride and styrene hydride complexes, although the
steric effects dominate the electronic contributions.
Permethylscandocene complexes, Cp*2ScR (R ) H,
alkyl), insert ethylene rapidly at low temperature (-80
°C) and without complication from chain transfer by â-H
elimination. Hence, the kinetics of ethylene insertion
has been measured for a series of scandium alkyls. The
rates of ethylene insertion are affected by the strength
of the scandium-carbon bond, where generally stronger
bonds result in slower rates.9 The rates of â-H elimina-
tion for a variety of scandium alkyls and phenethyl
complexes have been determined via rapid trapping of
the scandium hydride with 2-butyne. The relative free
energies of the transition states for â-H elimination, and
hence that for olefin insertion, have been probed by
systematically varying the substituent at the â-carbon.
As with the niobocene styrene hydride complexes, a
linear free energy correlation was found for this scan-
dium-phenethyl series, indicating a quite polar transi-
tion state with (in the limiting picture) the electropos-
itive scandium center abstracting hydride of the positively
charged â-carbon atom.
(10) Karl, J.; Dahlmann, M.; Erker, G.; Bergander, K. J. Am. Chem.
Soc. 1998, 120, 5643.
(11) Siedle, A. R.; Lamanna, W. M.; Newmark, R. A.; Schroepfer, J.
N. J. Mol. Catal. A 1998, 128, 257.
(12) Beswick, C. L.; Marks, T. J. J. Am. Chem. Soc. 2000, 122, 10358.
(13) (a) Casey, C. P.; Carpenetti, D. W., II; Sakurai, H. J. Am. Chem.
Soc. 1999, 121, 9483-9484. (b) Casey, C. P.; Carpenetti, D. W., II.
Organometallics 2000, 19, 3970-3977. (c) Brandow, C. G.; Mendiratta,
A.; Bercaw, J. E. Organometallics 2001, 20, 4253. (d) Casey, C. P.;
Carpenetti, D. W.; Sakurai, H. Organometallics 2001, 20, 4262-4265.
(14) (a) Sillars, D. R.; Landis, C. R. J. Am. Chem. Soc. 2003, 125,
9894. (b) Landis, C. R.; Rosaaen, K. A.; Sillars, D. R. J. Am. Chem.
Soc. 2003, 125, 1710.
More recent studies have focused on elucidating the
rates of chain propagation and transfer in cationic
zirconocene catalysts. Erker has reported an experi-
mental estimate for olefin insertion into cationic zirco-
nium allyl fragments,10 whereas Siedle has determined
the relative rates of chain propagation and transfer for
(15) Casey, C. P.; Tunge, J. A.; Lee, T. Y.; Fagan, M. A. J. Am. Chem.
Soc. 2003, 125, 2641-2651.
(16) Klamo, S. K.; Bercaw, J. E. J. Am. Chem. Soc., submitted for
publication.
(17) (a) Woo, T. K.; Margl, P. M.; Lohrenz, J. C. W.; Blo¨chl, P. E.;
Ziegler, T. J. Am. Chem. Soc. 1996, 118, 13021. (b) Richardson, D. E.;
Alameddin, N. G.; Ryan, M. F.; Hayes, T.; Eyler, J. R.; Siedle, A. R. J.
Am. Chem. Soc. 1996, 118, 11244. (c) Froese, R. D. J.; Musaev, D. G.;
Matsubara, T.; Morokuma, K. J. Organometallics 1997, 16, 2514, and
references therein.
(18) (a) Toto, M.; Cavallo, L.; Corradini, P.; Moscardi, G.; Resconi,
L.; Guerra, G. Macromolecules 1998, 31, 3431. (b) Ro¨ll, W.; Brintzinger,
H. H.; Rieger, B.; Zolk, R. Angew. Chem., Int. Ed. Engl. 1990, 29, 279.
(c) Gilchrist, J. H.; Bercaw, J. E. J. Am. Chem. Soc. 1996, 118, 12021.
(d) Coates G. W. Chem. Rev. 2000, 100, 1223. (e) Resconi, L.; Cavallo,
L.; Fait, A.; Piemontesi, F. Chem. Rev. 2000, 100, 1253.
(5) (a) Jordan, R. F.; Dasher, W. E.; Echols, S. F. J. Am. Chem. Soc.
1986, 108, 1718. (b) Jordan, R. F.; Bajgur, C. S.; Willet, R.; Scott, B. J.
Am. Chem. Soc. 1986, 108, 7410.
(6) Watson, P. L. J. Am. Chem. Soc. 1982, 104, 337.
(7) Jeske, G.; Lauke, H.; Mauerman, H.; Swepston, P. N.; Shuman,
H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8091.
(8) (a) Doherty, N. M.; Bercaw, J. E. J. Am. Chem. Soc. 1985, 107,
2670. (b) Burger, B. J.; Santarsiero, B. D.; Trimmer, M. S.; Bercaw, J.
E. J. Am. Chem. Soc. 1988, 110, 3134.
(9) Burger, B. J.; Thompson, M. E.; Cotter, W. D.; Bercaw, J. E. J.
Am. Chem. Soc. 1990, 112, 1566.

Zirconocene and Hafnocene Alkyl Hydride Derivatives
Organometallics, Vol. 24, No. 22, 2005 5409
Table 1. Rate Constants for Insertion of Disubstituted Olefins CH2dC(CH3)R with Cp*2ZrH2 (1) at 23 °C
olefin substituent R
product
kins × 104 (M-1 s-1
)
CH3
CH2CH3
Cp*2Zr(CH2CHMe2)(H) (3)
65(3)
Cp*2Zr{CH2CH(CH3)(CH2CH3)}(H) (4)
Cp*2Zr{CH2CH(CH3)CH(CH3)CH2CH3}(H) (6)
Cp*2Zr{CH2CH(CH3)CH2CHMe2}(H) (5)
Cp*2Zr{CH2CH(CH3)(C6H5)}(H) (7)
24(3)
CH(CH3)CH2CH3
CH2CHMe2
C6H5
7.3(2)
6.3(4)
14(2)
CH2CMe3
no insertion
Table 2. Rate Constants as a Function of Solvent
for Reaction of Cp*2ZrH2 (1) with
Isobutene at 23 °C
Results
Reactions of the metallocene dihydride complexes
(CpRn)2MH2 (CpRn ) alkyl-substituted cyclopentadi-
19
solvent
kins × 104 (M-1 s-1
)
δ Zr-H (ppm)
ꢀa
enyl; M ) Zr, Hf) with olefins such as propylene,
1-butene, and isobutene afford stable metallocene alkyl
hydride complexes, (CpRn)2M(CH2CHR′2)(H) (R′ ) alkyl),
in quantitative yield (by NMR) (eq 1). In general, the
remaining metal-hydride bond is unreactive toward
excess olefin.20 The metallocene alkyl hydride complexes
are stable in benzene-d6 for days at 25 °C.
toluene-d8
65(3)
7.48
7.30
5.75
5.10
2.4
4.3
7.6
12.3
diethyl ether-d10
tetrahydrofuran-d8
pyridine-d5
35(2)
17(5)
no reaction
a Data taken from CRC Handbook of Chemistry and Physics,
67th ed.; CRC Press: Boca Raton, FL.
Table 3. Rate Constants for Reaction of Cp*2ZrH2
(1) with Isobutene and trans-2-Butene as a
Function of Temperature
× 104
kins
× 104
isobutene
trans-2-butene
temperature
(K)
kins
(M-1 s-1
)
(M-1 s-1
)
220
240
250
263
275
296
4.2(1)
29(2)
69(3)
3.3(1)
8.5(1)
19(3)
65(3)
Kinetics of the Reactions of Olefins with Cp*2-
MH2 Complexes. Permethylzirconocene dihydride, Cp*2-
ZrH2 (1), and permethylhafnocene dihydride, Cp*2HfH2
(2), are monomeric in benzene-d6 solution, and thus the
kinetics of their insertion reactions with olefins are
straightforward to interpret. Reactions of 1 and 2 with
1,1-disubstituted olefins (e.g., isobutene) and R-olefins
are first order in both olefin and the metallocene
dihydride (eq 2). To probe olefin steric effects on the rate
of insertion, reaction of 1 with a variety of 1,1-disub-
stituted olefins has been examined. Thus reaction of 1
with isobutene, 2-methyl-1-butene, 2,4-dimethyl-1-pen-
tene, 2,3-dimethyl-1-pentene, and R-methylstyrene
yields Cp*2Zr(CH2CHMe2)(H) (3), Cp*2Zr{CH2CH(CH3)-
(CH2CH3)}(H) (4), Cp*2Zr{CH2CH(CH3)CH2CHMe2}(H)
(5), Cp*2Zr{CH2CH(CH3)CH(CH3)CH2CH3}(H) (6), and
Cp*2Zr(CH2CH(CH3)(C6H5))(H) (7), respectively. The
rate constants for these reactions have been measured
at 23 °C, and the second-order rate constants (kins) are
presented in Table 1. Reaction of 1 with 2,4,4-trimethyl-
1-pentene produces no reaction. Likewise no alkane is
observed upon attempted hydrogenation of this olefin
with 1 as the catalyst.
extreme reactivity of the Zr-H bonds of 1 toward most
functional groups limits the range of solvents that could
be examined, however.
Addition of either cis- or trans-2-butene to a toluene-
d8 solution of 1 affords the zirconocene n-butyl hydride
complex Cp*2Zr(CH2CH2CH2CH3)(H) (8) (eq 3). A zir-
conocene sec-butyl hydride intermediate has not been
detected, even when the reaction is monitored at -90
°C. These results are contrary to those obtained with
related zirconocene hydrido chloride21a and organoac-
tinide complexes,21b where detection of the internal alkyl
hydride complex has been achieved. Monitoring the
reaction between 1 and trans-2-butene at low temper-
atures (T < 250 K) allows for measurement of the rate
constant for insertion (Table 3). Likewise, the effect of
temperature on the rate of isobutene insertion has also
been determined, and those rate constants are also
reported in Table 3. Attempts to measure similar rate
constants for the insertion of cis-2-butene, propene,
styrene, and 1-pentene have been unsuccessful due to
the rapid rates of these reactions. Trisubstituted olefins
such as 2-methyl-2-butene do not react with 1 over the
course of several days at room temperature.
Isobutene, cyclopentene, styrene, and 1-pentene react
with 2, affording Cp*2Hf(CH2CHMe2)(H) (9), Cp*2Hf-
(cyclo-C5H9)(H) (10), Cp*2Hf(CH2CH2C6H5)(H) (11), and
Cp*2Hf{CH2(CH2)3CH3}(H) (12). As with 1, addition of
(19) Chirik, P. J.; Day, M. W.; Bercaw, J. E. Organometallics 1999,
18, 1873.
(20) Addition of 1 equiv of ethylene to [(CpRn)2ZrH2]2 results in clean
formation of (CpRn)2Zr(CH2CH3)(H). However, addition of excess
ethylene results in reductive elimination of ethane and formation of
the zirconocyclopentane. McAlister, D. R.; Erwin, D. R.; Bercaw, J. E.
J. Am. Chem. Soc. 1978, 100, 5966.
(21) (a) Chirik, P. J.; Day, M. W.; Labinger, J. A.; Bercaw, J. E. J.
Am. Chem. Soc. 1999, 121, 10308. (b) Lin, Z.; Marks, T. J. J. Am. Chem.
Soc. 1990, 112, 5515.
The effect of solvent on the rate of isobutene insertion
with 1 has also been examined. The experimentally
determined rate constants for the insertion reaction
have been measured in a number of deuterated solvents
at 23 °C, and these values are contained in Table 2. The

5410 Organometallics, Vol. 24, No. 22, 2005
Chirik and Bercaw
Table 4. Rate Constants for Reaction of Cp*2HfH2
(2) with Isobutene, trans-2-Butene, and
cis-2-Butene as a Function of Temperature
isobutene
trans-2-butene
cis-2-butene
temperature kins
×
kins
106 (M-1 s-1
×
kins
×
(K)
106 (M-1 s-1
)
)
106 (M-1 s-1
)
265
275
296
320
340
345
362
1200(100)
1900(400)
6200(200)
2.5(1)
62(5)
220(50)
590(30)
cis- or trans-2-butene to 2 immediately affords Cp*2Hf-
(CH2CH2CH2CH3)(H) (13). In general, the rates of olefin
insertion with 2 are slower than those for 1. For
example, the rate constant (23 °C) for isobutene inser-
tion with 2 is 2.6 × 103 times slower than (the extrapo-
lated value) for 1. The slower rates of insertion for 2
permit quantitative determination of olefin insertion
rate constants for this series of olefins. The rate
constants for insertion of isobutene, cis-2-butene, and
trans-2-butene have been determined over a range of
temperatures (Table 4), permitting determinations of
their activation parameters (Table 5). The rate con-
stants for very fast insertions of 1-pentene and styrene
for 2 have been measured at -43 °C. Using the
experimentally determined rate constants and activa-
tion parameters, a quantitative comparison of the rates
of insertion for each olefin studied has been obtained
from extrapolation of the measured rates to a common
temperature of -43 °C.
65(3)
120(1)
Table 5. Activation Parameters for Reaction of
Cp*2ZrH2 (1) and Cp*2HfH2 (2) with Selected
Olefins
metallocene
dihydride
∆Sq (cal
∆Hq (kcal
∆Gq
296K
olefin
mol-1 K-1
)
mol-1
)
(kcal mol-1
)
Cp*2ZrH2 (1) isobutene
Cp*2ZrH2 (1) trans-2-butene
Cp*2HfH2 (2) isobutene
Cp*2HfH2 (2) trans-butene
Cp*2HfH2 (2) cis-2-butene
Cp*2HfH2 (2) cyclopentene
-38(3)
-29(3)
-43(10)
-45(6)
-48(8)
-43(8)
9.0(1)
9.9(1)
12.2(1)
9.7(1)
9.6(3)
7.7(1)
20.2(1)
18.5(1)
24.9(2)
23.1(2)
23.7(3)
20.4(5)
Table 6. Data for the Linear Free Energy
Relationship for Insertion of p-Substituted
r-Methylstyrenes with Cp*2ZrH2 (1)
para substituent
kins × 105 (M-1 s-1
)
σ
σ+
OCH3
CH3
H
F
CF3
7.72(1)
7.47(2)
6.92(1)
6.35(2)
5.45(2)
-0.25
-0.14
0
0.15
0.52
-0.78
-0.30
0
-0.07
0.08
(H) (16), and Cp*2Zr{CH2CH(CH3)-p-C6H4-CF3}(H) (17),
respectively (eq 4). The rates of insertion for each
reaction have been measured at 23 °C, and the observed
rate constants are listed in Table 6. Construction of a
Hammett plot reveals a better correlation to σ (r2 )
0.996) than σ+ (r2 ) 0.667), with F ) -0.46 (1).22
Isotope Effects on the Rate of Insertion. Prepa-
ration of Cp*2ZrD2 (1-d2) may be accomplished via
addition of deuterium gas to a solution of 1 at -5 °C.
At higher temperatures, incorporation of deuterium into
the pentamethylcyclopentadienyl ligands is observed.19
Mixing equimolar quantities of 1 and 1-d2 in toluene-
d8 followed by addition of isobutene allows for measure-
ment of the kinetic isotope effect for olefin insertion
1
using the integration of the H NMR isobutyl methine
resonance versus the isobutyl methyl groups (both
relative to an internal standard). From this analysis, a
kinetic isotope effect, kH/kD ) 2.4 (3), has been measured
at 23 °C.
Linear Free Energy Relationship for the Inser-
tion Reaction of r-Methylstyrene with 1. Addition
of R-methylstyrene to a benzene-d6 solution of 1 affords
Effect of Cyclopentadienyl Substitution on the
Rate of Isobutene Insertion. In addition to 1 and 2,
other monomeric zirconocene and hafnocene dihydrides
have been reported.19 Addition of isobutene to Cp*(η5-
C5Me4H)ZrH2 (18) results in quantitative formation of
Cp*(η5-C5Me4H)Zr(CH2CHMe2)(H) (19) with a rate con-
stant of 3.3 (4) × 10-2 M-1 s-1 at -63 °C. Attempts to
measure the rate of the reaction at higher temperatures
have been unsuccessful due to the rapid rate of conver-
sion. Extrapolating the isobutene insertion rate constant
for 1 to -63 °C, using the experimentally determined
activation parameters, yields a value of 8.7 × 10-6 M-1
s-1; that is, 18 inserts isobutene 3800 times faster than
does 1 at -63 °C.
1
Cp*2Zr{CH2CH(CH3)C6H5}(H) (7) (eq 4). The H NMR
spectrum of 7 displays two inequivalent Cp* rings and
two diastereotopic CR protons of the alkyl group. If the
reaction reverses rapidly enough, equivalencing of the
Cp* group and CR protons would be expected. However,
heating a sample of 7 does not result in coalescence of
the Cp* or alkyl resonances, and eventually reductive
elimination of alkane and decomposition of 7 is ob-
served. Because reaction of 1 with R-methylstyrene is
clean and proceeds at a convenient rate, electronic
effects were probed by varying the para position of
styrene. Addition of 4-fluoro-R-methylstyrene, 4-methyl-
R-methylstyrene, 4-methoxy-R-methylstyrene, and 4-tri-
fluoromethyl-R-methylstyrene to 1 affords Cp*2Zr{CH2-
CH(CH3)-p-C6H4-F}(H) (14), Cp*2Zr{CH2CH(CH3)-p-
C6H4-CH3}(H) (15), Cp*2Zr{CH2CH(CH3)-p-C6H4-OCH3}-
(22) Values for σ and σ+ were taken from: Isaacs, N. S. Physical
Organic Chemistry; John Wiley and Sons: New York, 1986; p 131.

Zirconocene and Hafnocene Alkyl Hydride Derivatives
Organometallics, Vol. 24, No. 22, 2005 5411
Table 7. Regiospecificity of Styrene Insertion for
a Series of Zirconocene Dihydrides
1,2-product 2,1-product
zirconocene dihydride
(%)
(%)
Cp*2ZrH2 (1)
100
100
100
60
0
0
Cp*{(η5-C5H3-1,3-(CMe3)2}ZrH2 (20)
{(η5-C5H3-1,3-(CMe3)2}2ZrH2 (21)
Cp*(η5-THI)ZrH2 (31)
0
reaction conditions. Terminal alkynes such as 3,3-
dimethyl-1-butyne produce mixtures of products arising
from insertion of the alkyne, i.e., forming the alkenyl
hydride complex, as well as those arising from σ bond
metathesis with a Zr-H, generating the alkynyl com-
plex and H2 (eq 6). Small internal alkynes such as
2-butyne undergo rearrangement to afford crotyl hy-
dride complexes.24 However, sterically demanding, in-
ternal alkynes such as 4,4-dimethyl-2-pentyne (37) react
with the series of zirconocene dihydrides to yield solely
the insertion product, (CpRn)2Zr{C(CH3)dCH(CMe3)}-
(H). In all cases examined, only one alkenyl hydride
product is observed. Addition of CD3OD to the series of
zirconocene alkenyl hydride complexes and analysis of
40
20
56
100
[(η5-C5Me4H)2ZrH2]2 (32)
80
Cp*{(η5-C5H3-1,3-(CHMe2)2}ZrH2 (33)
[(η5-C5H4-CMe3)2ZrH2]2 (30)
44
0
Attempts to measure the rate of isobutene insertion
for Cp*{η5-C5H3-1,3-(CMe3)2}ZrH2 (20) and {η5-C5H3-
1,3-(CMe3)2}2ZrH2 (21) have been unsuccessful, as the
rates of insertion are too fast to be measured by low-
temperature NMR techniques. A rate constant of 5.1 (2)
× 10-3 M-1 s-1 at 23 °C has been measured for the
reaction of Cp*(η5-C5Me4-CH2CH3)ZrH2 (22) with isobu-
tene affording Cp*(η5-C5Me4-CH2CH3)Zr(CH2CHMe2)-
(H) (23), approximately half the rate for the insertion
of isobutene with 1.
2
the resulting deuterated olefin by H{1H} NMR spec-
Preparation of Cp*{η5-C5H3-1,3-(CMe3)2}HfH2 (24)
has been accomplished via addition of n-BuLi to Cp*-
{η5-C5H3-1,3-(CMe3)2}HfCl2 under an atmosphere of H2
as reported for 2.23 Reaction of 24 with isobutene results
in clean formation of Cp*{η5-C5H3-1,3-(CMe3)2}Hf(CH2-
CHMe2)(H) (25). Rate constants for isobutene insertion
have been measured between -48 and -3 °C and yield
activation parameters of ∆Hq ) 11.8(1) kcal‚mol and ∆Sq
) -27(5) eu.
troscopy reveals that the zirconium center adds to the
less hindered C2 of the acetylene in each case (eq 7).
Effect of Cyclopentadienyl Substitution on the
Regiospecficity of Styrene Insertion. The influence
of cyclopentadienyl substitution on the regiospecificity
of olefin insertion has been examined for a series of
zirconocene dihydride complexes. Reaction of 1 with
styrene in benzene-d6 results in clean formation of Cp*2-
Zr(CH2CH2C6H5)(H) (26), arising from the 1,2-insertion
of the carbon-carbon double bond of the olefin (eq 5).
Likewise, reaction of styrene with 20 and 21 affords
solely primary insertion products, Cp*{η5-C5H3-(CMe3)2}-
Zr(CH2CH2C6H5)(H) (27) and {η5-C5H3-(CMe3)2}2Zr-
(CH2CH2C6H5)(H) (28), respectively (Table 7). Reaction
of the less substituted zirconocene dihydride, [Cp*(η5-
C5H4-CMe3)ZrH2]2 (30), with styrene yields only the
secondary insertion product, Cp*(η5-C5H4-CMe3)Zr{CH-
(CH3)C6H5}(H), as a 1:1 mixture of diastereomers.
Confirmation of the regioselectivity of these reactions
has been assayed via addition of CH3OD to a benzene
solution of the zirconocene phenethyl hydride. The
resulting ethylbenzenes have been analyzed by 2H{1H}
NMR spectroscopy, and the signals of 1-deutero-ethyl-
benzene and 2-deutero-ethylbenzene determined by
integration versus an internal standard. Addition of
styrene to [Cp*(THI)ZrH2]2 (31) (THI ) tetrahydroin-
denyl), [(η5-C5Me4H)2ZrH2]2 (32), and Cp*{η5-C5H3-1,3-
(CHMe2)2}ZrH2 (33) results in a mixture of regioisomers,
depending on the substitution of the metallocene (Table
7). For 31 and 33 the 2,1-insertion products are formed
as a ca. 1:1 mixture of diastereomers.
Reaction of 37 with the zirconocene alkyl hydride
complexes also forms the alkenyl hydride complex and
free olefin. These products are consistent with â-H
elimination from the alkyl hydride liberating free olefin,
and the zirconocene dihydride subsequently trapped by
the acetylene in solution. Under the appropriate condi-
tions (vide infra), this reaction allows for measurement
of the rates of â-H elimination from a series of zir-
conocene alkyl hydride complexes.
A series of zirconocene isobutyl hydride complexes has
been prepared from addition of isobutene to a benzene-
d6 solution of the zirconocene dihydride. In this manner,
Cp*2Zr(CH2CHMe2)(H) (3), Cp*(η5-C5Me4H)Zr(CH2-
CHMe2)(H) (19), Cp*(η5-C5H5)Zr(CH2CHMe2)(H) (38),
Cp*(η5-C5H4-CMe3)Zr(CH2CHMe2)(H) (39), Cp*(THI)-
Zr(CH2CHMe2)(H) (40), Cp*{η5-C5H2-1,3-(CHMe2)2}Zr-
(CH2CHMe2)(H) (41), Cp*{η5-C5H3-1,3-(CMe3)2}Zr(CH2-
CHMe2)(H) (42), and (η5-C5Me4H)2Zr(CH2CHMe2)(H)
(43) have been prepared.
â-H Elimination Promoted by Internal Alkynes.
Reaction of the zirconocene dihydride complexes with
alkynes leads to a variety of products depending on the
The observed rate constants for reaction of 19 and
38-43 with 4,4-dimethyl-2-pentyne (37) have been
(23) Roddick, D. M.; Fryzuk, M. D.; Seidler, P. F.; Hillhouse, G. L.;
Bercaw, J. E. Organometallics 1985, 4, 97.
(24) McDade, C.; Bercaw, J. E. J. Organomet. Chem. 1985, 279, 281.

5412 Organometallics, Vol. 24, No. 22, 2005
Chirik and Bercaw
Table 8. Observed First-Order Rate Constants for the Reaction of Zirconocene Isobutyl Hydride
Complexes with 4,4-Dimethyl-2-pentyne (37)
kobs × 106 (s-1
)
kobs × 105 (s-1
)
zirconocene isobutyl hydride
Cp*2Zr(CH2CHMe2)H (3)
[37] (M)
2 °C
23 °C
0.244a
0.650a
1.10a
0.244
1.05
2.8(4)
2.5(6)
3.1(5)
Cp*(η5-C5Me4H)Zr(CH2CHMe2)H (19)
Cp*CpZr(CH2CHMe2)H (38)
1.34(5)
1.23(3)
1.46(7)
2.44
0.244
0.672
0.244
0.244
1.37
1060(20)
1120(50)
1030(40)
335(4)
340(5)
382(9)
Cp*(η5-C5H4-CMe3)Zr(CH2CHMe2)H (39)
Cp*(THI)Zr(CH2CHMe2)H (40)
4.88
0.152
0.908
1.32
108(2)
108(4)
107(3)
284(6)
298(8)
270(5)
33.5(5)
34.0(2)
38.2(8)
54.4(2)
54.3(5)
52.6(3)
Cp*{η5-C5H3-1,3-(CHMe2)2}Zr(CH2CHMe2)H (41)
Cp*{(η5-C5H3-1,3-(CMe3)2}Zr(CH2CHMe2)H (42)
(η5-C5Me4H)2Zr(CH2CHMe2)H (43)
1.22
0.244
1.22
0.122
1.37
4.88
0.061
0.244
0.976
a 2-Butyne was used in place of 37.
Scheme 1
zirconocene isobutyl hydrides as a function of ligand.
Their values, normalized (or extrapolated) to that for 3
at 23 °C, are shown in Scheme 2.
Kinetic Isotope Effects for â-H Elimination.
Preparation of isotopically labeled zirconocene isobutyl
hydride complexes of the general formula (CpRn)2Zr-
(CH2CDMe2)(D) has been accomplished via addition of
isobutene to (CpRn)2ZrD2.21a The isotopic purity of all
of the compounds has been determined to be >95%
based on 1H and 2H NMR spectroscopy. The rates of â-D
elimination for Cp*(THI)Zr(CH2CDMe2)(D) (40-d2), Cp*-
{η5-C5H3-1,3-(CHMe2)2}Zr(CH2CDMe2)(D) (41-d2), Cp*-
{η5-C5H3-1,3-(CMe3)2}Zr(CH2CDMe2)(D) (42-d2), Cp*-
(η5-C5Me4H)Zr(CH2CDMe2)(D) (19-d2), and (η5-C5Me4H)2-
Zr(CH2CDMe2)(D) (43-d2) have been determined at 23
°C by employing 37 as the trap for the intermediate
zirconocene dideuteride. Comparison of these rate con-
stants yields the kinetic isotope effect for â-H elimina-
tion for this series of isobutyl hydride complexes (kH/kD
) 3.9 to 4.5) (Table 10).
Linear Free Energy Relationship for â-H Elimi-
nation. Reaction of 18 with p-substituted styrenes
affords the phenethyl hydride complexes Cp*(η5-C5-
Me4H)Zr(CH2CH-p-C6H4-X)(H) (X ) H, 44; OCH3, 45;
CH3, 46; CF3, 47) (Scheme 3). Reaction of each of the
phenethyl hydride complexes with 37 as the dihydride
trap affords rates of â-H elimination for each complex
(Table 11). From these data a better linear free energy
correlation to σ (r2 ) 0.992) than to σ+ (r2 ) 0.922) is
observed, displaying a F value of -1.80(5).
determined and found to be independent of the concen-
tration of 37 over the range of 0.244 to 4.88 M (Table
8). For 3, 2-butyne was used as the dihydride trap, since
the reaction of 1 and 37 was not straightforward.
Because the reverse of â-H elimination (k-1) does not
effectively compete with trapping (k2) when several
equivalents of alkyne are used, the rate of formation of
the alkenyl product is effectively the rate of â-H
elimination (k1 ) kobs) (Scheme 1). Further evidence for
the dissociative nature of the reaction on the acetylene
concentration has been obtained through the use of two
different acetylene traps. Reaction of 42 with 5 equiv
of 37 or 2,2-dimethyl-1-butyne at -5 °C forms the
alkenyl hydride complexes with observed rate constants
of 3.3(4) × 10-4 and 2.7(3) × 10-4 s-1, respectively.
Activation parameters derived from the experimentally
determined rate constants at various temperatures are
given in Table 9. In all cases, the entropy of activation
(∆Sq) is small, further supporting dissociative substitu-
tion of the alkyl by the alkenyl as shown in Scheme 1.
Rates of â-H Elimination with Different Alkyls.
The effect of alkyl group on the rate of â-H elimination
has been examined with a series of Cp*(η5-C5H4-CMe3)-
Zr(R)(H) complexes. Addition of the appropriate olefin
to the hydride dimer 29 affords the zirconocene alkyl
hydride complexes Cp*(η5-C5H4-CMe3)Zr(R)(H) (R )
CH2CH2CMe3, 48; CH2(CH2)2CHMe2, 49; CH2CH2CH2-
CH3, 50; cyclopentyl, 51). As with the zirconocene
isobutyl hydride complexes, the rates of conversion of
These data provide an interesting comparison of
relative rates of â-H elimination for this series of

Zirconocene and Hafnocene Alkyl Hydride Derivatives
Organometallics, Vol. 24, No. 22, 2005 5413
Table 9. Activation Parameters for â-H Elimination for Zirconcene Isobutyl Hydride Complexes 38-42
zirconocene isobutyl hydride
∆Sq (eu)
∆Hq (kcal mol-1
)
∆Gq296K (kcal mol-1
)
Cp*CpZr(CH2CHMe2)H (38)
7.7(8)
-4.4(4)
3.1(8)
-4.1(9)
-5.2(1)
21.9(4)
18.8(6)
23.3(3)
19.6(7)
20.5(3)
19.6(4)
20.1(6)
21.3(4)
20.8(8)
22.0(3)
Cp*(η5-C5H4-CMe3)Zr(CH2CHMe2)H (39)
Cp*(THI)Zr(CH2CHMe2)H (40)
Cp*{η5-C5H3-1,3-(CHMe2)2}Zr(CH2CHMe2)H (41)
Cp*{(η5-C5H3-1,3-(CMe3)2}Zr(CH2CHMe2)H (42)
Scheme 2
energy changes for these reactions are reported in Table
14.
Discussion
Olefin Insertion for (CpRn)2MH2 (M ) Zr, Hf).
The metallocene dihydride complexes (CpRn)2MH2 (M
) Zr, Hf) undergo facile reaction with unhindered
R-olefins and internal olefins, affording the correspond-
ing alkyl hydride complexes (CpRn)2M(R′)(H). On the
basis of kinetic measurements of olefin insertion, con-
veniently measured using 1H NMR spectroscopy, we
have been able to elucidate the mechanism of olefin
insertion into neutral, d0 metallocene dihydride com-
plexes. These experiments reveal the influence of olefin
and cyclopentadienyl sterics on the rates of olefin
insertion. The mechanistic sequence that is consistent
with our evidence is shown in Scheme 4. The reaction
proceeds via reversible coordination of the alkene fol-
lowed by insertion through the normal four-centered
transition structure. Although the geometry of the olefin
dihydride adduct is unknown, we propose that olefin
insertion takes place when the olefin occupies the lateral
position (i.e., cis to only one zirconium hydride), based
on preliminary density functional theory calculations
with ethylene and Cp2ZrH2.25 Reaction of isobutene with
an equimolar mixture of Cp*2ZrH2 and Cp*2ZrD2 indi-
cates a kinetic isotope effect of 2.4(3) at 23 °C. This value
is indicative of a normal, primary isotope effect and is
consistent with a pre-equilibrium involving rapid coor-
dination and dissociation of olefin, followed by rate-
determining hydride transfer. A similar kinetic profile
has been proposed for reactions of olefins with Cp*2M-
(OR)(H) (M ) Th, U), where primary kinetic isotope
effects of 1.4(1) and 1.3(2) have been measured for the
insertion of cyclohexene and 1-hexene at 60 °C.21b These
conclusions regarding rapid, reversible olefin binding
with rate-determining insertion into the Zr-H bond are
also in accord with reversible olefin binding and rate-
limiting insertion into M-C bonds required to explain
the secondary deuterium isotope effects that support
R-agostic assistance for (1) hydrocyclization of R,ω-
dienes by scandocene hydrides,26 (2) hydrodimerization
of 1-hexene by zirconocenium catalysts,27 (3) the isotope
effect on propylene chain propagation reported by
Brintzinger and co-workers,28 and (4) chain propagation
the alkyl to alkenyl (Table 12) are independent of the
concentration of 4,4-dimethyl-2-pentyne (37); hence kobs
) k1 ) kâ-H
.
Because the steric effects for â-H elimination for Cp*-
(η5-C5H4-CMe3)Zr(R)(H) were much smaller than for
olefin insertion with 2, we wished to examine the effect
of ancillary ligand substitution on the relative rates of
â-H elimination for a series of n-butyl and isobutyl
hydrides to establish whether insertion is more sensitive
to steric effects more generally. The observed rate
constants for these reactions are contained in Table 13.
From these data, a general trend is observed: more
substituted metallocenes have a ratio of â-H elimination
rate constants (kâ-Hn-Bu/kâ-Hi-Bu) greater than 1, whereas
n-Bu
less sterically hindered metallocenes have kâ-H
kâ-H
/
i-Bu
less than 1, implying that electronic effects
(slightly) favor â-H elimination from isobutyl, but steric
effects override this preference with the more crowded
Cp*(η5-C5Me4H)Zr(R)H.
Relative Ground-State Energies of Zirconocene
Alkyl Hydride Complexes. Addition of 5 equiv of
1-butene to the series of zirconocene isobutyl hydride
complexes results in formation of an equilibrium mix-
ture of the zirconocene isobutyl hydride and the zir-
conocene n-butyl hydride complexes along with the free
olefins (eq 8). Likewise, the equilibrium may be ap-
proached from addition of isobutene to the zirconocene
n-butyl hydride complexes. The experimentally deter-
mined equilibrium constants and the corresponding free
(25) Brandow, C. G.; Bercaw, J. E. Unpublished results.
(26) Piers, W. E.; Bercaw, J. E. J. Am. Chem. Soc. 1990, 112, 9406.
(27) Krauledat, H.; Brintzinger, H. H. Angew. Chem., Int. Ed. Engl.
1990, 29, 1412.
(28) LeClerc, M. K.; Brintzinger, H. H. J. Am. Chem. Soc. 1995, 117,
1651.

5414 Organometallics, Vol. 24, No. 22, 2005
Table 10. Kinetic Isotope Effects for â-H Elimination for
Chirik and Bercaw
Isobutyl Hydride Complexes 19 and 40-43 at 23 °C
zirconocene isobutyl hydride
kH (s-1
)
kD (s-1
)
kH/kD
Cp*(η5-C5Me4H)Zr(CH2CDMe2)D (19-d2)
Cp*(THI)Zr(CH2CDMe2)D (40-d2)
1.34(9) × 10-5
1.06(6) × 10-3
2.8(8) × 10-3
3.52(9) × 10-4
5.38(6) × 10-4
3.29(12) × 10-6
2.64(1) × 10-4
7.10(7) × 10-4
7.81(10) × 10-5
1.34(8) × 10-4
4.1(3)
4.1(2)
3.9(3)
4.5(1)
4.0(2)
Cp*{η5-C5H3-1,3-(CHMe2)2}Zr(CH2CDMe2)D (41-d2)
Cp*{(η5-C5H3-1,3-(CMe3)2}Zr(CH2CDMe2)D (42-d2)
(η5-C5Me4H)2Zr(CH2CDMe2)D (43-d2)
Scheme 3
significant amounts of the formally 18-electron solvento
complex, although separate NMR signals for 1 and
1-solvento are not observed, and low-temperature NMR
experiments have not been attempted. If significant
concentrations of 18-electon 1-solvento do, in fact, form
in the more polar solvents, olefin coordination would be
impeded. A similar solvent effect has been noted in the
related organoactinide complexes, Cp*2M(OR)(H) (M )
Th, U), for which kTHF/ktoluene ) 0.59.21b
Addition of para-substituted R-methylstyrenes, p-CH2d
C(CH3)(C6H4-X) (X ) H, CH3, CF3, OCH3), to 1 affords
the zirconocene alkyl hydrides 7 and 14-17. A better
linear free energy correlation of the observed rate
constants is obtained with the Hammett constant σ (vis-
a`-vis σ+) with F ) -0.46(1). Better correlation to σ has
also been observed with permethylniobocene and per-
methyltantalocene styrene hydride complexes, where
X-ray crystal structures indicate that the p orbital of
the â-carbon and the π system of the phenyl ring are
twisted out of resonance by approximately 30°. In the
transition state for insertion the pseudo-five-coordinate
carbon does not allow for full overlap of the â-carbon
and the phenyl ring π system.8b The relatively small,
negative F value of -0.46(1) is indicative of a cyclic
transition state with little charge separation, with only
a slight positive charge development at the â-carbon of
the coordinated olefin, thus implicating transfer of
hydrogen as closer to H• than H-. Also consistent with
these findings are the highly negative entropies of
activation (-30 to -48 eu) indicating a highly ordered
transition structure for olefin insertion. Therefore, the
transition-state model developed for the niobocene and
tantalocene olefin hydride complexes may be extended
to Cp*2MH2.
Table 11. Linear Free Energy Relationship Data
for Phenethyl Complexes 44-47,
Cp*(C5Me4H)Zr(CH2CH2-p-C6H5-X)(H)
k-H × 104 (s-1
)
phenethyl substituent
23 °C
σ
σ+
X ) CF3 (47)
X ) H (44)
X ) CH3 (46)
X ) OCH3 (45)
2.04(15)
4.87(9)
6.94(14)
8.53(6)
0.53
0
-0.14
-0.25
0.08
0
-0.30
-0.78
in 1-hexene polymerization as established by Landis and
co-workers using heavy atom kinetic isotope effects.29
If insertion into a Zr-H bond is slower than olefin
binding or loss for 16-electron Cp*2ZrH2, then one might
reasonably assume that insertion into a M-R bond (M
) Sc, Zr; R ) alkyl, polymeryl) would also be slower for
14-electron Cp2Sc-R(olefin) or 16-electron [Cp2Zr-R(sol-
vento)]+, given the universally slower olefin insertion
into M-C versus M-H.
While there is not a large effect of solvent on the rate
of isobutene insertion, the reaction proceeds more
rapidly in solvents with lower dielectric constants than
in those that have higher donicity (Table 2). This
decrease appears to correlate with the chemical shift of
the zirconium hydride. It has been shown previously
that the 1H NMR chemical shift of a group 4 metallocene
hydride resonance may be used as a measure of elec-
tronic and coordinative unsaturation.30 For the 16-
electron dihydride complex Cp*2ZrH2 (1), the zirconium
hydride resonance appears at 7.48 ppm (toluene-d8),
substantially downfield from 0.55 and 1.07 ppm ob-
served for the 18-electron complexes Cp*2ZrH2(PF3) and
Cp*2ZrH2(CO) (both in toluene-d8).31 In donor solvents
such as THF-d8, the zirconocene hydride resonance for
1 shifts to 5.75 ppm, possibly indicating a buildup of
The effect of olefin substitution on the rate of insertion
has also been examined with 1 and 2. Not surprising is
the observed decrease in rate of insertion for more
hindered olefins (eq 2); however, the effect for 1 is rather
small, at least for the range of olefins examined (Table
1). Placing a methyl at either the 3 or 4 position of the
olefin has little effect on the rate of insertion for
2-methyl-substituted olefins. For example, the rate of
isobutene insertion is approximately 10 times faster
than that for 2,4-dimethyl-1-pentene. On the other
hand, further increasing the substitution of the 4-posi-
tion of olefin, as in the case of 2,4,4-trimethyl-1-pentene,
produces no reaction with the zirconium hydride. At-
tempts to hydrogenate this olefin in the presence of 1
(29) Landis, C. R.; Rosaaen, K. A.; Uddin, J. J. Am. Chem. Soc. 2002,
122, 10358.
(30) (a) Bercaw, J. E. In Transition Metal Hydrides; Advances in
Chemistry Series 167; 1978; Chapter 10, p 136. (b) Hillhouse, G. L.;
Bulls, A. R.; Santarsiero, B. D.; Bercaw, J. E. Organometallics 1988,
7, 1309.
(31) Manriquez, J. M.; McAlister, D. R.; Sanner, R. D.; Bercaw, J.
E. J. Am. Chem. Soc. 1978, 100, 2716.

Zirconocene and Hafnocene Alkyl Hydride Derivatives
Organometallics, Vol. 24, No. 22, 2005 5415
Table 12. Rates of â-H Elimination as a Function of Alkyl Ligand for Cp*(η5-C5H4-CMe3)Zr(R)(H)
Complexes 39 and 48-51 at 23 °C
k-H × 104 (s-1
)
alkyl group (R)
olefin released
23 °C
relative rate
R ) CH2CH2CMe3 (48)
R ) CH2(CH2)2CHMe2 (49)
R ) CH2(CH2)2CH3 (50)
R ) CH2CHMe2 (39)
3,3-dimethyl-1-butene
3-methyl-1-pentene
1-butene
1.47(8)
17.8(6)
18.3(5)
90.6(6)
1000(25)
1
12.1
12.4
62
isobutene
R ) cyclopentyl (51)
cyclopentene
680
Table 13. Comparison of the Rates at 23 °C for â-H Elimination for a Series of Zirconocene n-Butyl and
Isobutyl Hydride Complexes
zirconocene butyl hydride
(R ) CH2CH2CH2CH3, CH2CHMe2)
i-Bu
k-Hn-Bu (s-1
)
k-Hi-Bu (s-1
)
k-Hn-Bu:k-H
Cp*(η5-C5Me4H)Zr(R)H
7.85(2) × 10-5
3.35(9) × 10-4
9.28(1) × 10-4
9.88(2) × 10-4
1.83(5) × 10-3
1.34(9) × 10-5
3.5(2) × 10-4
1.06(6) × 10-3
2.8(8) × 10-3
9.06(6) × 10-3
5.86
Cp*{(η5-C5H3-1,3-(CMe3)2}Zr(R)H
Cp*(THI)Zr(R)H
0.96
0.875
0.357
0.201
Cp*{η5-C5H3-1,3-(CHMe2)2}Zr(R)H
Cp*(η5-C5H4-CMe3)Zr(R)H
Table 14. Equilibrium Constants and Free Energy
Data for the Isobutyl Hydride:n-Butyl Hydride
Equilibrium (eq 8)
The rate of insertion for R-olefins (e.g., 1-pentene,
styrene) with 2 is approximately 108 times faster than
that for 1,1-disubstituted olefins at -43 °C. Unfavorable
steric interactions between the more substituted 1,1-
disubstituted olefins and the bulky pentamethylcyclo-
pentadienyl ligands are almost certainly responsible for
the dramatic difference in rates. These results are in
accord with previous studies that demonstrate R-olefins
undergo hydrozirconation more rapidly than 1,1-disub-
stituted olefins.33 Similar observations have been made
in metallocene-catalyzed olefin polymerization, where
it is commonly found that R-olefins undergo insertion
at a much greater rate than 1,1-disubstituted olefins.34
Casey35 has estimated the differences in binding ener-
gies for mono- and disubstituted alkenes using model
yttrocene olefin alkyl chelate complexes. Using 2,5-
dimethyltetrahydrofuran as a competing ligand, the
enthalpy for olefin binding was found to be approxi-
mately 2.0 kcal‚mol-1 stronger for the monosubstituted
alkene complex as compared to the 1,1-disubstituted
olefin chelate.
296 K
zirconocene butyl hydride
(R ) CH2CH2CH2CH3, CH2CHMe2)
Keq
∆Gq
296K
(M-1
)
(kcal mol-1
)
Cp*2Zr(R)H
51(3)
43(3)
25(4)
16(2)
9.6(3)
9.1(4)
-2.3(2)
-2.1(2)
-1.9(2)
-1.7(1)
-1.3(1)
-1.3(1)
Cp*(η5-C5Me4H)Zr(R)H
Cp*{η5-C5H3-1,3-(CMe3)2}Zr(R)H
Cp*{η5-C5H3-1,3-(CHMe2)2}Zr(R)H
Cp*(η5-THI)Zr(R)H
Cp*(η5-C5H4-CMe3)Zr(R)H
Scheme 4
These results have special relevance to the chain
epimerization mechanism proposed by Busico to account
for the tacticity dependence on monomer concentration
in metallocene-catalyzed isospecific olefin polymeriza-
tion.36 The change in olefin enantioface is accomplished
via initial â-H elimination to a 1,1-disubstituted olefin
hydride cation intermediate. A key feature of the
mechanism involves in-plane olefin rotation about the
π face of the coordinated alkene and a switch of Zr from
one enantioface to the other, presumably via weak
coordination to the σ framework. It is assumed that once
the 1,1-disubstituted olefin is lost from the metal center
and its solvent sphere, it cannot compete for reinsertion
into Zr-H with the high concentration of propylene
monomer present.37 For 2 the observed much slower
insertion of isobutene as compared with 1-pentene (a
factor of 8 × 107) supports this assertion; the observed
rate difference would not allow reinsertion of the
also results in no reaction, demonstrating its inability
to insert into the zirconium hydride bond under normal
conditions. Changing the steric disposition of the olefin
(e.g., cis-2-butene versus isobutene) has a dramatic
impact on the rate of insertion with 1 and Cp*2HfH2
(2). Internal olefins such as cis- and trans-2-butene
insert much faster than 1,1-disubstituted olefins. For
both metallocenes, the rate of cis-2-butene insertion is
faster than for trans; with 2 the cis isomer inserts
approximately 100 times faster than trans. The reason
for the faster insertion of the cis isomer is not clear, but
this preference seems general, even for late transition
metal hydrides.32 The very large difference observed for
2 likely reflects less steric hindrance in the transition
state for insertion, cis-2-butene having its substituents
placed on one side only.
(33) Labinger, J. A. In Comprehensive Organic Synthesis; Trost, B.
M., Fleming, I., Eds.; Pergamon Press: Oxford, 1991; Chapter 3.9.
(34) Busico, V.; Cippullo, R. J. Am. Chem. Soc. 1994, 116, 9329.
(35) Casey, C. P.; Fisher Klein, J.; Fagan, M. A. J. Am. Chem. Soc.
2000, 122, 4320.
(36) Busico, V.; Caporaso, L.; Cippulo, R.; Landriani, L.; Angelini,
G.; Margonelli, A.; Segre, A. L. J. Am. Chem. Soc. 1996, 118, 2105.
(37) Yoder, J. C.; Bercaw, J. E. J. Am. Chem. Soc. 2002, 124, 2548.
(32) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry;
University Science Books: Mill Valley, CA, 1987.

5416 Organometallics, Vol. 24, No. 22, 2005
Chirik and Bercaw
Scheme 5
vinylidene end group of the polymer chain, especially
in the presence of such a large excess of R-olefin.
Cyclopentadienyl ligand substitution may have a very
striking effect on the rate of olefin insertion. Comparing
the rates of isobutene insertion for 1 and 18 at -63 °C
reveals a 3.8 × 103-fold rate enhancement for the latter!
Such a large rate increase on replacing with hydrogen
only one of the 10 cyclopentadienyl methyl groups must
indicate the transition state for isobutene insertion with
1 is very hindered. Similar effects are observed with
Cp*(η5-C5H3-1,3-(CMe3)2)ZrH2 (20) and (η5-C5H3-1,3-
(CMe3)2)2ZrH2 (21), where in both cases the reduced
steric disposition of ligand vis-a`-vis 1 results in insertion
rates that are too fast to measure with NMR techniques,
even at -63 °C. On the other hand, the rate of isobutene
insertion is not changed by replacement of one of the
pentamethylcyclopentadienyl methyl groups with an
ethyl group; isobutene inserts for 22 at about half the
rate as for 1.
Cyclopentadienyl substitution also has a significant
effect on the regiospecificity of styrene insertion. More
crowded metallocenes such as 1, 18, 20, and 21 proceed
with high regioselectivity, yielding solely the 1,2-inser-
tion product. However, more open metallocenes, 31, 32,
and 33, form mixtures of both 1,2- and 2,1-insertion
products, and in one case, 29, only 2,1-insertion products
are formed. From these results, it appears that in the
absence of overriding steric effects, styrene prefers to
insert in a 2,1-fashion, presumably due to electronic
stabilization of the Zr-C bond by an R-phenyl substitu-
ent.38
The experimentally determined rate constants and
resulting activation parameters allow for direct com-
parison of the insertion barrier for zirconium and
hafnium (Table 5). For isobutene and trans-2-butene,
the ∆Gq for insertion into a Zr-H bond is approximately
4 kcal‚mol-1 lower than that for the corresponding
Hf-H bond, corresponding to an approximate insertion
rate ratio of 103. Similar differences in activity have
been observed in a variety of zirconium and hafnium
catalytic systems and may be attributed to the ground-
state stabilization due to the greater Hf-H bond dis-
sociation energy.
â-H Elimination for (CpRn)2M(R′)(H) (R, R′ )
alkyl). Zirconocene alkyl hydride complexes of the
general formula (CpRn)2Zr(R′)(H) (R, R′ ) alkyl) undergo
facile (reversible) â-H elimination as determined by
trapping with 4,4-dimethyl-2-pentyne (37). Because the
rate of trapping is faster, even with only a few equiva-
lents of 37 (Scheme 1), the rates of â-H elimination for
a series of zirconocene isobutyl hydride complexes is
readily established (Scheme 2). In general, more sub-
stituted metallocenes display slower rates of â-H elimi-
nation, although in comparison to olefin insertion, rates
of â-H elimination are much less affected by ancil-
lary ligand substitution. For example, comparing the
[Cp*2Zr] system to the only slightly less hindered
[Cp*(η5-C5Me4H)Zr] system, insertion of isobutene with
18 is 3800 times faster than for 1, whereas in the
reverse reaction, â-H elimination for 19 is only 5 times
faster than for 3. Whereas the same unfavorable steric
interactions are present in the transition state for both
directions, steric interactions increase more dramati-
cally as the reaction proceeds in the insertion direction,
beginning with separated olefin and relatively un-
crowded zirconocene dihydride. By contrast, in the â-H
elimination direction the alkyl already possesses some
unavoidable steric interactions with the cyclopentadi-
enyl ligands that increase comparatively little in ac-
cessing the transition state.
Because the rates of both olefin insertion and â-H
elimination have been measured (or extrapolated from
activation parameters) at 23 °C for the [Cp*2Zr] system
as well as for the less hindered [Cp*(η5-C5Me4H)Zr]
system, the equilibrium constants for the addition of
isobutene to the dihydrides can be computed (Scheme
5). Assuming a negative value for the reaction entropy
(∆S° ) -20 eu is assumed), the insertion of isobutene
with 1 is rather exothermic, ∆H° ≈ -10.5 kcal‚mol-1
and for 18 ∆H° ≈ -14.4 kcal‚mol-1
.
Although it is difficult to confidently compare the bis-
(pentamethylcyclopentadienyl)hafnium system (for which
we have relative insertion rates) to the (pentamethyl-
cyclopentadienyl)(tert-butylcyclopentadienyl)zirco-
nium system (for which we have relative rates of â-H
elimination), the rate constants for â-H elimination
(Table 12) appear to be influenced substantially less by
the steric demands of the olefin, as compared to olefin
insertion with Cp*2HfH2 (2). Moreover, the kâ-H values
do not follow an obvious order; less hindered R-olefins
are eliminated faster, yet the n-alkyl hydrides â-H
eliminate 50-700 times slower than the cyclopentyl
hydride. The latter order is opposite that for insertion;
R-olefins such as 1-pentene undergo insertion with 2
approximately 106 times faster than cyclopentene. These
data allow a rough comparison that indicates that the
equilibrium between dihydride + olefin and the alkyl
hydride (kins/kâ-H ) Keq) lies toward the n-alkyl hydride
by several orders of magnitude (i.e., the equilibrium lies
much more toward the n-pentyl hydride than it does
toward cyclopentyl hydride).
Cyclopentadienyl steric effects on â-H elimination
have also been observed for the polymerization of
propylene with metallocene catalysts. Polypropylene
produced from Cp2ZrCl2/MAO mixtures terminates pri-
marily by â-H elimination, whereas the polyolefin
generated from Cp*2ZrCl2/MAO mixtures contains end
groups arising from chain termination by â-methyl
elimination. The increased preference for â-methyl
elimination for the latter system is believed to be a
(38) Nelson, J. E.; Bercaw, J. E.; Labinger, J. A. Organometallics
1989, 8, 2404, and references therein.

Zirconocene and Hafnocene Alkyl Hydride Derivatives
Organometallics, Vol. 24, No. 22, 2005 5417
Scheme 6
Scheme 7
result of inhibition of â-H elimination by the sterically
demanding pentamethylcyclopentadienyl rings (Scheme
6).39
The kinetic isotope effects for â-H elimination have
been determined with a series of zirconocene isobutyl
hydride complexes, (CpRn)2Zr(CH2CLMe2)(L) (L ) H or
D). In all cases a normal, primary isotope effect has been
observed (Table 10). These data suggest that â-H
elimination proceeds via rate-determining C-H(D) bond
cleavage followed by rapid dissociation of the coordi-
nated olefin. The values obtained in this study (∼4) are
somewhat larger than those measured for related
systems. A primary kinetic isotope effect of 2.0 has been
measured for â-H elimination in the scandium phen-
ethyl complex Cp*2ScCH2CH2C6H5.9 Likewise, a smaller
kinetic isotope effect of 1.6 has been measured for the
â-H elimination in the polymerization of propene with
2-d1-propene with [(Me2Si)2{η5-C5H-3,5-(CHMe2)2}(η5-
C5H2-4-CHMe2)]ZrCl2/methyalumoxane mixtures.40 The
larger values for the alkyl hydrides in the current study
may reflect a somewhat later transition state (more
C-H bond breaking) for the less electrophilic, neutral
group 4 metallocene derivatives.
The electronic effects on the rate of â-H elimination,
probed with a series of para-substituted phenethyl
hydride complexes, Cp*(η5-C5Me4H)Zr(CH2CH2-p-C6H5-
X)(H) (X ) H, Me, OCH3, CF3), are also indicative of
positive charge development in the transition structure
(F ) -1.80(5)). The magnitude of the effects is very
similar to that found for â-H elimination for Cp*2ScCH2-
CH2-p-C6H4-X (F ) -1.87).9 Interestingly, here we have
for comparison a measurement of the electronic effects
in the olefin insertion direction (F ) -0.49), substan-
tially less than for the â-H elimination direction, once
again suggesting that the transition state for the latter
is comparatively late, with more charge developing in
that direction. Given the exothermicity of olefin inser-
tion (Scheme 5), such asymmetry is reasonable.
barriers for â-H elimination as well as the free energy
changes for equilibrating the zirconocene isobutyl hy-
dride and n-butyl hydride presents a rare opportunity
to differentiate the contributions of ground-state and
transition-state effects for â-H elimination. The free
energy change for eq 8 (Table 14) provides a basis for
assessing the relative ground-state energies of the two
zirconocene alkyl hydride complexes (Scheme 7). The
change in free energy accompanying eq 8 is not the free
energy difference between the isobutyl and n-butyl
hydride complexes, but rather a composite of the relative
zirconocene alkyl hydride ground states and the change
in free energy for the isomerization of 1-butene to
isobutene. Addition of the value41 (+3.2 kcal‚mol-1) for
the reverse of the latter to the free energy change for
eq 8 yields the relative ground-state free energy differ-
ences (∆G°296K(met)) for two zirconocene alkyl hydride
complexes (Table 15). Note that these calculations allow
ordering of the relative ground states of the n-butyl
hydride as compared to the isobutyl hydride for a given
ligand array, but do not allow comparison of the ground
states of two alkyl hydride complexes with different
cyclopentadienyl ligands.
In all cases examined, the zirconocene isobutyl hy-
dride is thermodynamically favored over the corre-
sponding n-butyl hydride complex. In an attempt to
understand the electronic contribution of this effect, the
free energies of formation of isobutane and n-butane
have been examined. At 23 °C, the branched alkane is
favored by 1.10 kcal‚mol-1 over the linear alkane,41 in
accord with the observed preference for the zirconocene.
We have previously noted a fairly general correlation
between metal-carbon and carbon-hydrogen bond
strengths, allowing the relative strengths of M-C bonds
to be predicted from the relative energies of the corre-
sponding C-H bonds.42 Accordingly, the [Cp2ZrH] moi-
ety has a thermodynamic preference for the more stable
alkyl and therefore prefers the isobutyl hydride over the
n-butyl hydride by a predicted 1.10 kcal‚mol-1. Although
electronic preferences favoring the isobutyl hydride are
important, as it is favored over the n-butyl hydride for
all, cyclopentadienyl ligand substitution does influence
the position of the equilibrium. For less substituted
zirconocenes such as [Cp*(η5-C5H4-CMe3)Zr] and [Cp*-
(THI)Zr], the electronic contributions are apparent, as
Delineation of Ground- versus Transition-State
Effects in â-H Elimination. Measurement of the
(39) (a) Resconi, L.; Piemontesi, F.; Franciscono, G.; Abis, L.; Fiorani,
T. J. Am. Chem. Soc. 1992, 114, 1025. (b) Horton, A. D. Organome-
tallics 1996, 15, 2675. (c) Chirik, P. J.; Bercaw, J. E. Organometallics
2005, 24, 2789.
(40) Veghini, D.; Henling, L. M.; Burkhardt, T. J.; Bercaw, J. E. J.
Am. Chem. Soc. 1999, 121, 564.
(41) Benson, S. W. Thermochemical Kinetics, 2nd ed.; Wiley and
Sons: New York, 1976.
(42) (a) Bryndza, H. E.; Fong, L. K.; Paciello, R. A.; Tam, W.; Bercaw,
J. E. J. Am. Chem. Soc. 1987, 109, 1444. (b) Labinger, J. A.; Bercaw,
J. E. Organometallics 1988, 7, 926.

5418 Organometallics, Vol. 24, No. 22, 2005
Chirik and Bercaw
Table 15. Relative Ground-State Free Energies for Zirconocene Isobutyl and n-Butyl Hydride Complexes
(see Scheme 7)
∆Gq296K(met) -
zirconocene butyl hydride
(R ) CH2CH2CH2CH3, CH2CHMe2)
∆Gq296K(met)
∆Gq296K(alkane)
(kcal mol-1
)
(kcal mol-1
)
Cp*2Zr(R)H
0.90 (2)
1.0 (2)
1.3 (2)
1.5 (1)
1.9 (1)
1.9 (1)
-0.2
-0.1
0.2
Cp*(η5-C5Me4H)Zr(R)H
Cp*{η5-C5H3-1,3-(CMe3)2}Zr(R)H
Cp*{η5-C5H3-1,3-(CHMe2)2}Zr(R)H
Cp*(η5-THI)Zr(R)H
0.4
0.8
0.8
Cp*(η5-C5H4-CMe3)Zr(R)H
Table 16. Relative Ground-State Energies, â-H Activation Free Energies, and Calculated â-H
Transition-State (TS) Free Energy Differences for Zirconocene Isobutyl and n-Butyl Hydride Complexes
isobutyl
n-butyl
isobutyl
zirconocene butyl hydride
(R ) CH2CH2CH2CH3 & CH2CHMe2)
∆Gq296K(met)
∆Gq
∆Gq
TSn-butyl - TSâ-H
â-H
â-H
(kcal mol-1
)
(kcal mol-1
)
(kcal mol-1
)
(kcal mol-1
)
Cp*(η5-C5Me4H)Zr(R)H
1.0 (2)
1.3 (2)
1.5 (1)
1.9 (1)
1.9 (1)
23.9
22.0
21.3
20.8
20.1
22.9
22.0
21.4
21.4
21.0
0
Cp*{η5-C5H3-1,3-(CMe3)2}Zr(R)H
Cp*{η5-C5H3-1,3-(CHMe2)2}Zr(R)H
Cp*(THI)Zr(R)H
1.3
1.6
2.5
2.8
Cp*(η5-C5H4-CMe3)Zr(R)H
the free energy difference exceeds that for the alkanes.43
For more crowded members of the series, [Cp*2Zr] and
[Cp*(η5-C5Me4H)Zr], steric destabilization of the isobu-
tyl hydride complex makes the isobutyl hydride prefer-
ence less than for the alkanes. The energetics of this
steric contribution from such estimates spans (only) 1.0
charge on the â-carbon. On the other hand, considering
steric considerations, the â-H elimination transition
state for the zirconocene n-butyl hydride would be
favored due to decreased steric interactions with the
bulkier cyclopentadienyl ligands.
The experimentally determined transition state en-
ergy differences indicate that for the sterically crowded
metallocene [Cp*(η5-C5Me4H)Zr] these effects almost
exactly cancel, and as a result, the isobutyl and n-butyl
hydride transition states are of equal energy (Figure 1).
Thus, the faster rate of â-H elimination for Cp*(η5-C5-
Me4H)Zr(CH2CH2CH2CH3)(H) is a result of electronic
stabilization of the Cp*(η5-C5Me4H)Zr(CH2CHMe2)(H)
in the ground state. For less substituted metallocenes,
the electronically favored isobutyl hydride transition
state becomes more and more favored over the sterically
preferred n-butyl hydride transition state. This effect
increases with decreasing substitution on the metal-
locene, and as a result, the least crowded metallocene,
[Cp*(η5-C5H4-CMe3)Zr], displays the largest transition-
state energy difference.
From these data, the relative rates of â-H elimination
can be rationalized (Table 13). For less crowded metal-
locenes such as [Cp*(η5-C5H4-CMe3)Zr] and [Cp*{η5-
C5H3-1,3-(CHMe2)2}Zr], the electronic stabilization of
the isobutyl hydride transition state overcomes its
favored ground-state stabilization relative to the n-butyl
hydride, and as a result, faster rates of isobutyl hydride
â-H elimination are observed. For intermediate metal-
locenes, these effects are similar, although the transi-
tion-state effects dominate and faster isobutyl hydride
â-H elimination rates are still observed. Only in the
most crowded member of the series, [Cp*(η5-C5Me4H)-
Zr], are the transition-state effects overcome by ground-
state effects.
kcal‚mol-1
.
Grubbs44 has found similar behavior for alkyl-
substituted titanacyclobutanes, where the more stable
titanocycle is derived from the more stable olefin,
presumably a result of the structural features of the
metallacycle. Likewise, Wolczanski has found a similar
correlation between metal-carbon bond strengths and
carbon-hydrogen bond strengths in the equilibration
of tris-amido zirconium alkyl complexes.45
From the experimentally determined rates of â-H
elimination and the relative ground-state energies of the
zirconocene isobutyl and n-butyl hydrides, the relative
transition-state energies for the â-H elimination reac-
tion can be quantitatively ordered. The activation free
energy may be calculated for each â-H elimination
reaction from the rate constant. Subtraction of the
relative ground-state contributions for each of the
zirconocene alkyl hydrides affords the relative transi-
tion-state energies for both zirconocene alkyl hydrides
for a given ligand array. The relative ground-state,
activation, and transition-state energies are contained
in Table 16.
On the basis of electronic considerations and the
model established for the transition state for â-H
elimination (vide supra), the free energy of the transi-
tion state for a zirconocene isobutyl hydride would be
expected to be lower than that for the corresponding
zirconocene n-butyl hydride, because the more substi-
tuted alkyl would better stabilize the developing positive
From these studies, a moderately comprehensive
description of the â-H elimination reaction has been
established. The reaction proceeds via rate-determining
C-H bond scission via a four-centered transition-state
structure that has a modest buildup of positive charge
on the â-carbon, resulting in transfer of Hδ- to the metal
center. The rate of â-H elimination is influenced by the
structure of the ancillary ligation. For a given alkyl, the
rate of â-H elimination decreases with increasing cy-
clopentadienyl ligand substitution. When considering
(43) Although a correlation exists between M-C and C-H bond
strengths, it is not absolute, and as a result, differences in M-C bond
dissociation energies often exceed the differences in the corresponding
C-H bond strengths.
(44) Straus, D. A.; Grubbs, R. H. Organometallics 1982, 1, 1658.
(45) Schaller, C. P.; Cummins, C. C.; Wolczanski, P. T. J. Am. Chem.
Soc. 1996, 118, 591.

Zirconocene and Hafnocene Alkyl Hydride Derivatives
Organometallics, Vol. 24, No. 22, 2005 5419
Figure 1. Free energy profiles for â-H elimination for Cp*(η5-C5Me4H)Zr(CH2CHMe2)H (19) and Cp*(η5-C5Me4H)Zr(CH2-
CH2CH2CH3)H and for Cp*(η5-C5H4-CMe3)Zr(CH2CHMe2)H (39) and Cp*(η5-C5H4-CMe3)Zr(CH2CH2CH2CH3)H, showing
the relative ground-state and transition-state free energies for each pair.
d5 were purchased from Cambridge Isotope Laboratories.
Benzene-d6 and toluene-d8 were dried over LiAlH4 and sodium
and then stored over titanocene. Tetrahydrofuran-d8 and
diethyl ether-d10 were dried over CaH2 and stored over sodium/
benzophenone ketyl. Pyridine-d5 was dried over CaH2. Prepa-
rations of 1,48 2,24 3,19 20,19 21,19 22,19 31,19 32,19 and 3319 were
accomplished as described previously.
Isobutene, 2-methyl-1-butene, 2,4-dimethyl-1-pentene, 2,3-
dimethyl-1-pentene, and propylene were purchased from Al-
drich. Isobutene and propene were dried over Al(i-Bu)3 and
distilled by vacuum transfer before use. The other olefins were
distilled from CaH2 and stored over LiAlH4. Both cis- and
trans-2-butene were purchased from Matheson and dried over
activated 4 Å molecular sieves. Styrene and R-methylstyrene
were purchased from Aldrich; p-fluoro-R-methylstyrene was
purchased from Lancaster. All were distilled under reduced
pressure from CaH2 and stored frozen in the drybox. Prepara-
tions of p-methyl-R-methylstyrene, p-trifluoromethyl-R-meth-
ylstyrene, and p-methoxy-R-methylstyrene were accomplished
by reaction of Ph3PdCH2 with the appropriate acetophenone
as described previously,49 and the resulting product was
distilled at reduced pressure from CaH2. Ferrocene used as
an internal standard was purchased from Aldrich and sub-
limed before use.
â-H elimination for two different alkyls, both ground-
state and transition-state effects must be considered.
The relative ground-state energetics may be predicted
on electronic grounds from the free energies of the
alkanes, although increased cyclopentadienyl substitu-
tion slightly perturbs this value. For less substituted
metallocenes, the electronic portion of the transition-
state effect dominates and, as a result, the more
â-substituted alkyl undergoes more facile â-H elimina-
tion.
The steric effects on the reverse process, olefin inser-
tion, are generally much more sensitive, both to olefin
and cyclopentadienyl substitution. Although chain trans-
fer processes other than â-H elimination (e.g., â-methyl
elimination, chain transfer to Al) may become favored,
so that the issue is rather more complex, we conclude
that increased molecular weight should be favored for
less crowded metallocenes and less hindered olefins:
olefin insertion responds more favorably to reduced
unfavorable steric interactions than does chain transfer
by â-H elimination.
NMR spectra were recorded on a Bruker AM500 (500.13
Experimental Section
MHz for H, 76.77 for H, 125.77 MHz for 13C) spectrometer.
All chemical shifts are relative to TMS using 1H (residual),
2H, or 13C chemical shifts of the solvent as a secondary
standard. The temperature of the NMR probe was measured
before and after each kinetic run with a standard CH3OH
reference tube.
1
2
General Considerations. All air- and moisture-sensitive
compounds were handled using standard vacuum line, Schlenk,
or cannula techniques or in a drybox as described previously.46
Argon, dinitrogen, dihydrogen, and dideuterium gases were
purified by passage over columns of MnO on vermiculite and
activated molecular sieves. Solvents for air- and moisture-
sensitive reactions were stored under vacuum over titanocene47
or sodium benzophenone ketyl. NMR solvents: benzene-d6,
toluene-d8, tetrahydrofuran-d8, diethyl ether-d10, and pyridine-
Kinetics of the Reaction of 1 or 2 with Volatile Olefins.
In the drybox, a J. Young NMR tube was charged with 0.50
mL of 0.0488 M (0.0244 mmol) stock toluene-d8 solution of
metallocene dihydride containing a known amount of fer-
rocene. The NMR tube was then attached to the vacuum line
via a calibrated gas volume. The tube was evacuated at -78
(46) Burger, B. J.; Bercaw, J. E. In Experimental Organometallic
Chemistry; ACS Symposium Series No. 357; Wayda, A. L., Darens-
bourg, M. Y., Eds.; American Chemical Society: Washington, DC, 1987;
Chapter 4.
(47) Marvich, R. H.; Brintzinger, H. H. J. Am. Chem. Soc. 1971,
93, 2046.
(48) Schock, L. E.; Marks, T. J. J. Am. Chem. Soc. 1988, 110, 7701.
(49) Vogel, A. I. Vogel’s Textbook of Practical Organic Chemistry,
5th ed.; John Wiley and Sons: New York, 1989; pp 495-498.

5420 Organometallics, Vol. 24, No. 22, 2005
Chirik and Bercaw
°C, 10 equiv of olefin was charged into the gas bulb by vacuum
transfer, and the gas was condensed in the tube at -78 °C.
The NMR probe was calibrated to the desired temperature,
and the sample was shaken several times just before insertion
into the probe. Approximately 10-15 spectra were recorded
at regular intervals over the duration of 2-3 half-lives. Peak
intensities of the ferrocene and metallocene dihydride were
measured for each spectrum. The observed rate constants were
obtained from slopes of plots of ln[Cp*2MH2] versus time.
Second-order rate constants were obtained by dividing the
observed psuedo-first-order rate constants by the concentration
of olefin. Activation parameters were obtained by measuring
second-order rate constants over a temperature range. A plot
of ln(k/T) versus 1/T resulted in a slope of ∆Hq/R and an
intercept of [(∆Sq/R) + 23.76]. Reported errors in the rate
constants represent one standard deviation from the least-
squares fit of the experimental data. These errors were then
used to determine the errors in the activation parameters.
Kinetics of the Reaction of 1 with Substituted r-Me-
thylstyrenes. In the drybox, 0.25 mL of 0.096 M stock
benzene-d6 solution of 1 containing a known amount of
ferrocene was charged into a J. Young NMR tube. Via
microsyringe, 0.25 mL of a benzene-d6 stock solution of
R-methylstyrene was added to the tube, and the sample was
then immediately frozen in liquid nitrogen. The tube was then
quickly thawed and inserted into a NMR probe. Approximately
10-15 spectra were recorded at regular intervals over the
duration of 2-3 half-lives. Workup of the data was carried out
as described for the previous experiments.
CHMe2); 2.24 (m, 2H,CH2CH(CH3)CH2CHMe2); 1.12 (m, 2H,
CH2CH(CH3)CH2CHMe2); 2.16 (m, 1H, CH2CH(CH3)CH2-
CHMe2); 1.02 (d, 6.5 Hz, 3H, CH2CH(CH3)CH2CHMe2); 0.98
(d, 6.5 Hz, 6H, CH2CH(CH3)CH2CH(CH3)2). 13C NMR (benzene-
d6): δ 12.30 (C5Me5); 12.50 (C5Me5); 117.34 (C5Me5); 117.71 C5-
Me5); 66.60 (CH2CH(CH3)CH2CHMe2); 39.09 (CH2CH(CH3)CH2-
CHMe2); 20.58 (CH2CH(CH3)CH2CHMe2); 22.64 (CH2CH(CH3)-
CH2CHMe2); 35.93 (CH2CH(CH3)CH2CHMe2); 13.53, 13.62
(CH2CH(CH3)CH2CH(CH3)2).
1
Cp*2Zr(CH2CH(CH3)CH(CH3)CH2CH3)(H) (6). H NMR
(benzene-d6): δ 1.93 (s, 30H, C5Me5); 6.35 (s, 1H, ZrH); -0.386
(dd, 1H, CH2CH(CH3)CH(CH3)CH2CH3); 0.220 (dd, 1H, CH2-
CH(CH3)CH(CH3)CH2CH3); 2.21 (m, 1H, CH2CH(CH3)CH-
(CH3)CH2CH3); 1.31 (m, 1H, CH2CH(CH3)CH(CH3)CH2CH3);
1.18 (m, 2H, CH2CH(CH3)CH(CH3)CH2CH3); 1.01 (t, 7 Hz, 3H,
CH2CH(CH3)CH(CH3)CH2CH3).
Cp*2Zr(CH2CH(CH3)(C6H5))(H) (7). 1H NMR (benzene-
d6): δ 1.87 (s, 15H, C5Me5); 1.94 (s, 15H, C5Me5); 6.55 (s, 1H,
ZrH); -0.944 (dd, 1H, CH2CH(CH3)(C6H5)); -0.969 (dd, 1H,
CH2CH(CH3)(C6H5)); 1.56 (t, 6.5 Hz, 1H, CH2CH(CH3)(C6H5));
1.65 (d, 7 Hz, 3H, CH2CH(CH3)(C6H5)); 7.42 (d, 7.1 Hz, 2H,
CH2CH(CH3)(ortho-C6H5)); 7.11 (t, 7.5 Hz, 2H, CH2CH(CH3)-
(meta-C6H5)); 7.29 (t, 7.3 Hz, 1H, CH2CH(CH3)(para-C6H5)).
13C NMR (benzene-d6): δ 12.24 (C5Me5); 12.48 (C5Me5); 118.44
(C5Me5); 118.77 (C5Me5); 67.06 (CH2CH(CH3)(C6H5); 39.45
(CH2CH(CH3)(C6H5); 30.10 (CH2CH(CH3)(C6H5); 117.39, 117.65,
119.54, 123.75, 134.51, 1 not located (C6H5).
Cp*2Zr(CH2CH2CH2CH3)(H) (8). 1H NMR (benzene-d6): δ
1.89 (s, 30H, C5Me5); 6.27 (s, 1H, Zr-H); 0.159 (t, 9 Hz, 2H,
CH2 CH2CH2CH3); 1.24 (m, 2H, CH2 CH2CH2CH3); 1.39 (m, 2H,
Determination of â-H Elimination Rate Constants. In
a typical experiment, a J. Young NMR tube was charged with
0.50 mL of a 0.0488 M stock benzene-d6 or toluene-d8 solution
of zirconocene dihydride containing a known amount of fer-
rocene. On the vacuum line, the tube was frozen in liquid
nitrogen and degassed with three freeze-pump-thaw cycles.
Via a 6.9 mL calibrated gas volume, 100 Torr of the desired
olefin was added at -196 °C. The tube was thawed and
CH2 CH2CH2CH3); 1.05 (t, 7.3 Hz, 3H, CH2 CH2CH2CH3). 13
C
NMR (benzene-d6): δ 12.09 (C5Me5); 113.00 (C5Me5); 52.48
(CH2CH2CH2CH3); 31.62 (CH2CH2CH2CH3); 25.57 (CH2CH2CH2-
CH3); 11.40 (CH2CH2CH2CH3).
Cp*2Hf(CH2CHMe2)(H) (9). 1H NMR (benzene-d6): δ 1.95
(s, 30H, C5Me5); 9.76 (s, 1H, Hf-H); -0.25 (d, 6.5 Hz, 2H, CH2
CHMe2); 2.09 (m, 1H, CH2 CHMe2); 1.00 (d, 6 Hz, 6H, CH2-
CH(CH3)2). 13C NMR (benzene-d6): δ 12.24 (C5Me5); 117.52 (C5-
Me5); 76.33 (CH2CHMe2); 31.94 (CH2CHMe2); 13.63 (CH2CH-
(CH3)2).
Cp*2Hf(cyclo-C5H9)(H) (10). 1H NMR (benzene-d6): δ 1.95
(s, 30H, C5Me5); 13.31 (s, 1H, HfH); -0.17, 0.456, 1.08, 1.93,
2.27 (m, C5H9). 13C NMR (benzene-d6): δ 12.27 (C5Me5); 117.57
(C5Me5); 73.17 (C1-cyclopentene); 29.86 (C2-cyclopentene); 23.51
(C3-cyclopentene).
Cp*2Hf(CH2CH2C6H5)(H) (11). 1H NMR (benzene-d6): δ
1.88 (s, 30H, C5Me5); 12.96 (s, 1H, HfH); -0.09 (t, 6.5 Hz, 2H,
CH2 CH2Ph); 2.22 (t, 6.5 Hz, 2H, CH2 CH2Ph); 6.85 (m, 1H,
para-CH2CH2C6H5); 6.98 (m, 2H, meta-CH2CH2C6H5); 7.34 (m,
2H, ortho-CH2CH2C6H5). 13C NMR (benzene-d6): δ 12.12
(C5Me5); 118.26 (C5Me5); 62.45 (CH2CH2Ph); 30.36 (CH2CH2-
Ph); 125.52 (para-C6H5); 137.50 (ortho-C6H5); 150.07 (meta-
C6H5), 1 not located (ipso-C6H5).
1
shaken. The H NMR spectrum was then recorded to ensure
that complete conversion to the alkyl hydride complex had
occurred. The tube was reattached to the vacuum line, and
via a 56.8 mL calibrated gas bulb, the desired amount of 37
was collected in the tube at -196 °C. The tube remained frozen
in liquid nitrogen until insertion into the thermostated NMR
probe. Approximately 10-15 spectra were recorded over
regular intervals over the course of 2-3 half-lives. Workup of
the data was carried out as described for the previous
experiments.
Determination of Equilibrium Constants for Zir-
conocene Alkyl Hydride Complexes. In a typical experi-
ment, a J. Young NMR tube was charged with 0.50 mL of
0.0488 M stock benzene-d6 solution containing a known
amount of ferrocene. On the vacuum line, the tube was
degassed and the appropriate amount of isobutene and 1-butene
was then added via calibrated gas volume. The tube was
thawed and shaken thoroughly and allowed to stand at room
temperature for several hours. The 1H NMR spectrum was
then recorded and the equilibrium constant computed from
the integration of the various species in solution.
Cp*2Hf(CH2CH2CH2CH3)(H) (12). 1H NMR (benzene-d6):
δ 1.95 (s, 30H, C5Me5); 9.80 (s, 1H, HfH); -0.005 (t, 9 Hz, 2H,
CH2 CH2CH2CH3); not located (CH2 CH2CH2CH3); 0.981 (m,
2H, CH2 CH2CH2CH3); 1.06 (t, 7.4 Hz, 3H, CH2 CH2CH2CH3).
13C NMR (benzene-d6): δ 12.06 (C5Me5); 116.97 (C5Me5); 62.55
(CH2CH2CH2CH3); 31.50 (CH2CH2CH2CH3); 26.93 (CH2CH2CH2-
CH3); 14.76 (CH2CH2CH2CH3).
NMR Spectroscopic Data. Cp*2Zr(CH2CH(CH3)CH2-
1
CH3)(H) (4). H NMR (benzene-d6): δ 1.92 (s, 30H, C5Me5);
6.31 (s, 1H, ZrH); -0.531 (dd, 1H, CH2CH(CH3)CH2CH3); 0.290
(dd, 1H, CH2CH(CH3)CH2CH3); not located (CH2CH(CH3)CH2-
CH3); 1.36 (m, 2H, CH2CH(CH3)CH2CH3); 0.877 (t, 7 Hz, 3H,
CH2CH(CH3)CH2CH3); 1.76 (d, 5 Hz, 3H, CH2CH(CH3)CH2-
CH3). 13C NMR (benzene-d6): δ 12.83 (C5Me5); 118.26 (C5Me5);
70.28 (CH2CH(CH3)(CH2CH3); 31.17 (CH2CH(CH3)(CH2CH3);
22.87 (CH2CH(CH3)(CH2CH3); 13.52 (CH2CH(CH3)(CH2CH3);
29.09 (CH2CH(CH3)(CH2CH3).
Cp*2Hf(CH2CH2CH2CH2CH3)(H) (13). 1H NMR (benzene-
d6): δ 1.94 (s, 30H, C5Me5); 9.81 (s, 1H, HfH); -0.005 (t, 9 Hz,
2H, CH2 CH2CH2CH2CH3); not located (CH2 CH2CH2CH2CH3);
0.991 (m, 4H, CH2 CH2CH2CH2CH3); 1.12 (t, 7.4 Hz, 3H, CH2
CH2CH2CH2CH3).
1
Cp*2Zr(CH2CH(CH3)(C6H4-p-F))(H) (14). H NMR (ben-
zene-d6): δ 1.84 (s, 15H, C5Me5); 1.86 (s, 15H, C5Me5); 6.31 (s,
1H, ZrH); -0.984 (dd, 1H, CH2CH(CH3)(C6H4-p-F)); -0.368
(dd, 1H, CH2CH(Me)(C6H4-p-F)); 2.56 (m, 1H, CH2CH(CH3)-
(C6H4-p-F)); 1.78 (d, 3H, CH2CH(CH3)(C6H4-p-F)); 7.19 (m, 2H,
1
Cp*2Zr(CH2CH(CH3)CH2CHMe2)(H) (5). H NMR (ben-
zene-d6): δ 1.94 (s, 30H, C5Me5); 6.39 (s, 1H, ZrH); -0.669 (dd,
1H, CH2CH(CH3)CH2CHMe2); 0.492 (dd, 1H, CH2CH(CH3)CH2-

Zirconocene and Hafnocene Alkyl Hydride Derivatives
Organometallics, Vol. 24, No. 22, 2005 5421
CH2CH(CH3)(ortho-C6H4-p-F)); 6.94 (m, 2H, CH2CH(CH3)-
(meta-C6H4-p-F)).
144.70 (CMe3); 60.96 (CH2CH2C6H5); 34.29 (CH2CH2C6H5);
125.56, 126.95, 137.68, 1 not located (C6H5); 97.41, 100.53,
106.63, 114.13, 122.65 (Cp).
Cp*2Zr(CH2CH(CH3)(C6H4-p-CH3))(H) (15). 1H NMR (ben-
zene-d6): δ 1.89 (s, 15H, C5Me5); 1.94 (s, 15H, C5Me5); 6.54 (s,
1H, ZrH); 0.959 (dd, 1H, CH2CH(CH3)(C6H4-p-CH3)); 0.912 (dd,
1H, CH2CH(CH3)(C6H4-p-CH3)); 2.22 (m, 1H, CH2CH(CH3)-
(C6H4-p-CH3)); 1.19 (d, 7 Hz, 3H, CH2CH(CH3)(C6H4-p-CH3));
2.16 (s, 3H, CH2CH(CH3)(C6H4-p-CH3)); 6.99 (d, 6 Hz, 2H, CH2-
CH(Me)(meta-C6H4-p-CH3)); 7.34 (d, 6 Hz, 2H, CH2CH(CH3)-
(ortho-C6H4-p-CH3)).
Cp*(η5-C5H4-CMe3)Zr{CH(C6H5)(CH3)}(H) (30). 1H NMR
(benzene-d6): δ 1.75 (s, 15H, C5Me5); 1.83 (s, 15H, C5Me5); 1.11
(s, 9H, CMe3); 1.36 (s, 9H, CMe3);1.37 (s, 1H, ZrH); 5.40 (s,
1H, ZrH); -0.18 (d, 6.5 Hz, 3H, CH(C6H5)(CH3)); 1.86 (d, 6.5
Hz, 3H, CH(C6H5)(CH3)); not located (CH(C6H5)(CH3)); 6.85,
6.89 (m, 1H, para-C6H5); 7.19, 7.25 (m, 2H, meta-C6H5); 7.39,
7.44 (m, 2H, para-C6H5); 3.18, 3.45, 3.66, 4.48, 4.62, 4.72, 4.77,
4.78 (Cp). 13C NMR (benzene-d6): δ 12.51, 12.85 (C5Me5);
114.30, 114.68, (C5Me5); 32.84, 32.88 (CMe3); 136.62, 145.88
(CMe3); 39.09, 47.91 (CH(C6H5)(CH3)); -7.240, 17.68 ((CH-
(C6H5)(CH3)); 112.09, 123.75 (para-C6H5); 125.71, 126.73 (ortho-
C6H5); 129.16, 132.36 (meta-C6H5); 2 not located (ipso-C6H5);
99.98, 100.03, 100.10, 102.43, 104.08, 105.02, 110.04, 113.79,
120.77, 121.66 (Cp).
Cp*2Zr(CH2CH(CH3)(C6H4-p-OCH3))(H) (16). 1H NMR
(benzene-d6): δ 1.89 (s, 15H, C5Me5); 1.95 (s, 15H, C5Me5); 6.53
(s, 1H, ZrH); 0.904 (dd, 1H, CH2CH(CH3)(C6H4-p-OCH3)); 0.912
(dd, 1H, CH2CH(CH3)(C6H4-p-OCH3)); 1.62 (m, 1H, CH2CH-
(CH3)(C6H4-p-OCH3)); 1.78 (d, 6.5 Hz, 3H, CH2CH(CH3)(C6H4-
p-OCH3)); 3.34 (s, 3H, CH2CH(CH3)(C6H4-p-OCH3)); 6.92 (d, 6
Hz, 2H, CH2CH(CH3)(meta-C6H4-p-OCH3)); 7.34 (d, 6 Hz, 2H,
CH2CH(CH3)(ortho-C6H4-p-OCH3)).
1
Cp*(THI)Zr(CH2CH2C6H5)(H) (34a). H NMR (benzene-
Cp*2Zr(CH2CH(CH3)(C6H4-p-CF3))(H) (17). 1H NMR (ben-
zene-d6): δ 1.84 (s, 15H, C5Me5); 1.92 (s, 15H, C5Me5); 6.54 (s,
1H, ZrH); -1.11 (dd, 1H, CH2CH(CH3)(C6H4-p-CF3)); 0.053 (dd,
1H, CH2CH(CH3)(C6H4-p-CF3)); 3.31 (m, 1H, CH2CH(CH3)-
(C6H4-p-CF3)); 1.05 (d, 6.8 Hz, 3H, CH2CH(CH3)(C6H4-p-CF3));
7.24 (d, 6 Hz, 2H, CH2CH(CH3)(meta-C6H4-p-CF3)); 7.48 (d, 6
Hz, 2H, CH2CH(CH3)(ortho-C6H4-p-CF3)).
d6): δ 1.86 (s, 15H, C5Me5); 5.89 (s, 1H, ZrH); 2.0-2.4 (m, THI);
0.706 (m, 2H, CH2CH2Ph); 0.870 (m, 2H, CH2CH2Ph); 6.94 (m,
1H, para-CH2CH2C6H5); 7.24(m, 2H, meta-CH2CH2C6H5); 7.37
(m, 2H, ortho-CH2CH2C6H5); 4.13, 4.57, 4.99 (Cp).
Cp*(THI)Zr((CH(C6H5)(CH3))(H) (34b). 1H NMR (benzene-
d6): δ 1.79 (s, 15H, C5Me5); not located (ZrH); 2.2-2.6 (m, THI);
-0.90 (d, 8 Hz, 3H, CH(C6H5)(CH3)); 1.20 (d, 8 Hz, 3H, CH-
(C6H5)(CH3)); 7.10 (m, 1H, para-CH2CH2C6H5); 7.22 (m, 2H,
meta-CH2CH2C6H5); 7.29 (m, 2H, ortho-CH2CH2C6H5); 3.54,
4.58, 4.71, 4.90, 2 not located (Cp).
Cp*(η5-C5Me4H)Zr(CH2CHMe2)(H) (19). 1H NMR (benzene-
d6): δ 1.91 (s, 15H, C5Me5); 1.60, 1.87, 2.10, 2.33 (s, 3H,
C5Me4H); 4.52 (s, 1H, C5Me4H); 6.26 (s, 1H, ZrH); -0.445 (dd,
1H, CH2CHMe2); 0.443 (dd, 1H, CH2CHMe2); 2.15 (m, 1H,
CH2CHMe2); 0.953 (d, 7 Hz, 3H, CH2CH(CH3)2); 1.06 (d, 7 Hz,
3H, CH2CH(CH3)2). 13C NMR (benzene-d6): δ 12.73 (C5Me5);
117.98 (C5Me5); 12.17, 12.32, 13.46, 14.56 (C5Me4H); 68.38
(CH2CMe2); 31.76 (CH2CHMe2); 24.47, 29.64 (CH2CH(CH3)2);
111.51, 115.70, 119.70, 120.79, 121.63 (Cp).
(η5-C5Me4H)2Zr(CH2CH2C6H5)(H) (35a). 1H NMR (benzene-
d6): δ 1.79, 1.82, 1.91, 1.99 (s, 6H, C5Me4); 6.47 (ZrH); 0.38 (t,
6.5 Hz, 2H, CH2 CH2Ph); not located (CH2 CH2Ph); 6.96 (m,
1H, para-CH2CH2C6H5); 7.35 (m, 2H, meta-CH2CH2C6H5); 7.40
(m, 2H, ortho-CH2CH2C6H5).
(η5-C5Me4H)2Zr((CH(C6H5)(CH3))(H) (35b). 1H NMR (ben-
zene-d6): δ 1.60, 1.66, 1.73, 1.79, 2.14, 2.15, 2.18, 2.32 (s, 6H,
C5Me4H); 4.36, 1 not located (s, 1H, ZrH); 0.485, 1 not located
(q, 6 Hz, 1H, CH(C6H5)(CH3)); 1.26, 1 not located (CH(C6H5)-
(CH3); 4.84, 4.89 (Cp).
Cp*(η5-C5Me4CH2CH3)Zr(CH2CHMe2)(H) (23). 1H NMR
(benzene-d6): δ 1.92 (s, 15H, C5Me5); 1.76, 1.79, 1.94, 1.98 (s,
3H, C5Me4CH2CH3); 1.85 (q, 6.5 Hz, 2H, C5Me4CH2CH3); 1.90
(t, 7 Hz, 2H, C5Me4CH2CH3); 6.34 (s, 1H, ZrH); -0.381 (dd,
1H, CH2CHMe2); 0.053 (dd, 1H, CH2CHMe2); 2.43 (m, 1H,
CH2CHMe2); 0.82 (d, 7 Hz, 3H, CH2CH(CH3)2); 1.10 (d, 7 Hz,
3H, CH2CH(CH3)2).
Cp*{η5-C5H3-1,3-(CHMe2)2}Zr(CH2CH2C6H5)(H) (36a). 1H
NMR (benzene-d6): δ 1.82 (s, 15H, C5Me5); 0.51 (d, 7 Hz. 6H,
CHMe2); 0.84 (d, 7 Hz. 3H, CHMe2); 0.88 (d, 7 Hz. 3H, CHMe2);
1.07 (d, 7 Hz. 3H, CHMe2); 2.45 (sept, 6.5 Hz, 1H, CHMe2);
2.59 (sept, 6.5 Hz, 1H, CHMe2); 6.42 (s, 1H, ZrH); -0.179 (m,
1H, CH2 CH2Ph); 0.82 (m, 1H, CH2 CH2Ph); 2.43 (m, 1H, CH2
CH2Ph); 2.84 (m, 1H, CH2 CH2Ph); 7.06 (m, 1H, para-CH2-
CH2C6H5); 7.29 (m, 2H, meta-CH2CH2C6H5); 7.42 (m, 2H, ortho-
CH2CH2C6H5); 4.89, 5.26, 5.45 (Cp).
Cp*(η5-C5H3-(CMe3)2)Hf(CH2CHMe2)(H) (25). 1H NMR
(benzene-d6): δ 1.98 (s, 15H, C5Me5); 1.22 (s, 9H, CMe3); 1.48-
(s, 9H, CMe3); 12.98 (s, 1H, HfH); -1.06 (m, 1H, CH2CHMe2);
0.75 (m, 1H, CH2CHMe2); 2.50 (m, 1H, CH2CHMe2); 1.04 (d, 7
Hz, 3H, CH2CH(CH3)2); 1.17 (d, 7 Hz, 3H, CH2CH(CH3)2); 4.92,
4.62, 6.50 (m, 1H, Cp). 13C NMR (benzene-d6): δ 12.71 (C5Me5);
116.96 (C5Me5); 32.34 (CMe3); 32.99 (CMe3); 145.06 (CMe3);
145.96 (CMe3); 76.44 (CH2CHMe2); 32.34 (CH2CHMe2); 24.33
(CH2CH(CH3)2); 30.30 (CH2CH(CH3)2); 99.86, 104.61, 105.64,
2 not located (Cp).
Cp*{η5-C5H3-1,3-(CHMe2)2}Zr(CH(C6H5)(CH3))(H) (36b).
1H NMR (benzene-d6): δ 1.76 (s, 15H, C5Me5); 1.83 (s, 15H,
C5Me5); 0.68 (d, 7 Hz. 3H, CHMe2); 0.73 (d, 7 Hz. 3H, CHMe2);
1.04 (d, 7 Hz. 3H, CHMe2); 1.06 (d, 7 Hz. 3H, CHMe2); 1.14 (d,
7 Hz. 3H, CHMe2); 1.17 (d, 7 Hz. 3H, CHMe2); 1.21 (d, 7 Hz.
3H, CHMe2); 1.24 (d, 7 Hz. 3H, CHMe2); not located (CHMe2);
6.19, 1 not located (s, 1H, ZrH); 4.25, 4.60, 4.63, 4.83, 4.89,
5.76 (Cp).
Cp*2Zr(CH2CH2C6H5)(H) (26). 1H NMR (benzene-d6): δ
1.88 (s, 30H, C5Me5); not located (ZrH); 0.54 (t, 6.9 Hz, 2H,
CH2CH2Ph); 1.78 (t, 6.3 Hz, 2H, CH2CH2Ph); 7.08 (m, 1H,
para-CH2CH2C6H5); 7.25 (m, 2H, meta-CH2CH2C6H5); 7.40 (m,
2H, ortho-CH2CH2C6H5). 13C NMR (benzene-d6): δ 12.18
(C5Me5); 117.51 (C5Me5); 51.79 (CH2CH2C6H5); 26.49 (CH2CH2-
C6H5); 125.67, 126.15, 138.00, 1 not located (C6H5).
1
Cp*CpZr(CH2CHMe2)(H) (38). H NMR (benzene-d6): δ
1.89 (s, 15H, C5Me5); 6.22 (s, 1H, ZrH); -1.93 (dd, 1H, CH2
CHMe2); 0.25 (dd, 1H, CH2 CHMe2); 2.21 (m, 1H, CH2 CHMe2);
1.03 (d, 7 Hz, 3H, CH2CHMe2); 1.10 (d, 7 Hz, 3H, CH2CHMe2);
5.74 (s, 5H, C5H5). 13C NMR (benzene-d6): δ 12.18 (C5Me5);
117.62 (C5Me5); 71.65 (CH2CHMe2); 33.17 (CH2CHMe2); 29.91
(CH2CHMe2); 28.81 (CH2CHMe2); 111.07 (Cp).
Cp*{η5-C5H3-1,3-(CMe3)2}Zr(CH2CH2C6H5)(H) (27). 1H
NMR (benzene-d6): δ 1.89 (s, 15H, C5Me5); 1.16 (s, 9H, CMe3);
0.956 (s, 9H, CMe3); 6.49 (s, 1H, ZrH); -0.122 (td, 13.9 Hz,
3.8 Hz, 1H, CH2 CH2Ph); 0.693 (td, 13.9 Hz, 3.8 Hz, 1H, CH2
CH2Ph); 2.48 (td, 13.6 Hz, 4.8 Hz, 1H, CH2 CH2Ph); 2.86 (td,
13.6 Hz, 4.8 Hz, 1H, CH2 CH2Ph); 7.05 (m, 1H, para-CH2-
CH2C6H5); 7.25 (m, 2H, meta-CH2CH2C6H5); 7.31 (m, 2H, ortho-
CH2CH2C6H5); not located (ipso-C6H5).
Cp*(η5-C5H4-CMe3)Zr(CH2CHMe2)(H) (39). 1H NMR (ben-
zene-d6): δ 1.86 (s, 15H, C5Me5); 1.34 (s, 9H, CMe3); 6.22 (s,
1H, ZrH); -1.95 (dd, 1H, CH2CHMe2); 0.14 (dd, 1H, CH2
CHMe2); 2.41 (m, 1H, CH2CHMe2); 0.97 (d, 7 Hz, 3H, CH2CH-
(CH3)2); 0.99 (d, 7 Hz, 3H, CH2CH(CH3)2); 4.87, 4.92, 5.41, 5.90
(Cp). 13C NMR (benzene-d6): δ 12.18 (C5Me5); 110.85 (C5Me5);
2.31 (CMe3); 145.60 (CMe3); 74.39 (CH2CHMe2); 33.53 (CH2-
CHMe2); 28.20 (CH2CH(CH3)2); 89.75, 98.92, 104.04, 107.98,
109.58 (Cp).
{η5-C5H3-1,3-(CMe3)2}2Zr(CH2CH2C6H5)(H) (28). 1H NMR
(benzene-d6): δ 1.28, (s, 9H, CMe3); 1.45 (s, 9H, CMe3); 5.33
(s, 1H, ZrH); 0.55 (t, 10 Hz, 2H, CH2CH2C6H5); 2.76 (t, 10 Hz,
2H, CH2CH2C6H5); 4.85, 5.22, 6.26 (s, 1H, Cp). 13C NMR
(benzene-d6): δ 32.77 (CMe3); 32.48 (CMe3); 141.28 (CMe3);

5422 Organometallics, Vol. 24, No. 22, 2005
Chirik and Bercaw
1
Cp*(THI)Zr(CH2CHMe2)(H) (40). H NMR (benzene-d6):
(η5-C5Me5)(η5-C5Me4H)Zr(CH2CH2(C6H4-p-CF3))(H) (47).
1H NMR (benzene-d6): δ 1.84 (s, 15H, C5Me5); 1.63, 1.77, 1.95,
2.16 (s, 6H, C5Me4H); 4.82 (s, 1H, ZrH); -0.237 (m, 1H, CH2-
CH2C6H5-p-CF3); 0.334 (m, 1H, CH2CH2C6H5-p-CF3); 1.51 (m,
2H, CH2CH2C6H5-p-CF3); 7.19 (d, 8.6 Hz, 2H, CH2CH2-meta-
C6H5-p-CF3); 7.48 (d, 8.6 Hz, 2H, CH2CH2-ortho-C6H5-p-CF3),
4.48 (Cp). 13C NMR (benzene-d6): δ 12.50 (C5Me5); 116.16 (C5-
Me5); 11.71, 12.06, 13.45, 14.54 (C5Me4H); 49.30 (CH2CH2C6H4-
p-CF3); 25.23 (CH2CH2C6H4-p-CF3); 32.84 (q, 28 Hz, (CH2-
CH2C6H4-p-CF3); 106.29, 114.88, 115.87, 117.32, 119.43 (Cp);
δ 1.89 (s, 15H, C5Me5); 6.24 (s, 1H, ZrH); -1.67 (dd, 1H, CH2-
CHMe2); 0.870 (dd, 1H, CH2CHMe2); 2.17 (m, 1H, CH2CHMe2);
0.975 (d, 7 Hz, 3H, CH2CH(CH3)2); 1.04 (d, 7 Hz, 3H, CH2CH-
(CH3)2); 2.49, 2.55, 2.69, 2.78 (m, THI),4.70, 4.98, 5.61 (m, 1H,
Cp). 13C NMR (benzene-d6): δ 12.70 (C5Me5); 117.39 (C5Me5);
24.32, 25.63, 26.18 28.47 (THI), 87.95 (CH2CMe2); 34.33
(CH2CMe2); 30.21 (CH2C(CH3)2); 103.46, 105.21, 111.86, 126.45,
127.93 (Cp).
Cp*{η5-C5H3-1,3-(CHMe2)2}Zr(CH2CHMe2)(H) (41). 1H
NMR (benzene-d6): δ 1.92 (s, 15H, C5Me5); 6.34 (s, 1H, ZrH);
1.10 (d, 7 Hz. 3H, CHMe2); 1.23 (d, 7 Hz. 3H, CHMe2); 1.30 (d,
7 Hz. 3H, CHMe2); 1.34 (d, 7 Hz. 3H, CHMe2); 2.79 (sept, 6.5
Hz, 1H, CHMe2); 3.25 (sept, 6.5 Hz, 1H, CHMe2); -0.359 (dd,
1H, CH2 CHMe2); 0.220 (dd, 1H, CH2 CHMe2); 2.22 (m, 1H,
CH2 CHMe2); 0.965 (d, 7 Hz, 6H, CH2CH(CH3)2); 4.74, 4.85,
5.94 (m, 1H, C5H3-1,3-CHMe2). 13C NMR (benzene-d6): δ 12.89
(C5Me5); 117.18 (C5Me5); 71.82 (CH2CMe2); 21.89, 22.64, 26.60,
27.34, 28.29, 29.07, 29.68, 30.48, 31.28 (CHMe2, CH2CMe2);
88.31, 95.92, 100.52, 109.56, 123.00 (Cp).
125.67, 125.96, 141.44, 3 not located (CH2CH2C6H4-p-CF3). 19
NMR (benzene-d6): δ -62.19 ppm (CF3).
F
Cp*(η5-C5H4-CMe3)Zr(CH2CH2CMe3)(H) (48). 1H NMR
(benzene-d6): δ 1.89 (s, 15H, C5Me5); 1.47 (s, 9H, C5H4-CMe3);
not located (s, 1H, ZrH); -0.99 (m, 1H, CH2CH2CMe3); 0.30
(m, 1H, CH2CH2CMe3); 2.32 (m, 2H, CH2CH2CMe3); 1.04 (s,
9H, CH2CH2CMe3); 4.91 (m, 2H, Cp); 5.00 (m, 1H, Cp); 5.90
(m, 2H, Cp). 13C NMR (benzene-d6): δ 12.58 (C5Me5); 116.34
(C5Me5); 32.84 (CMe3); 29.90 (CMe3); 146.06 (CMe3); 1 not
located (CMe3); 49.98 (CH2CH2CMe3); 35.94 (CH2CH2CMe3);
102.68, 104.01, 107.25, 109.89, 120.76 (Cp).
Cp*{η5-C5H3-1,3-(CMe3)2}Zr(CH2CHMe2)(H) (42). 1H NMR
(benzene-d6): δ 1.88 (s, 15H, C5Me5); 6.48 (s, 1H, ZrH); 0.956
(s, 9H, CMe3); 1.23 (s, 9H, CMe3); -0.825 (dd, 1H, CH2 CHMe2);
-0.836 (dd, 1H, CH2 CHMe2); 2.31 (m, 1H, CH2 CHMe2); 0.956
(d, 7 Hz, 6H, CH2CH(CH3)2); 4.59, 4.75, 4.79 (m, 1H, C5H3-
1,3-(CMe3)2). 13C NMR (benzene-d6): δ 12.89 (C5Me5); 117.86
(C5Me5); 32.06 (CMe3); 33.03 (CMe3); 2 not located (CMe3);
70.44 (CH2CH2Me2); 31.90 (CH2CH2Me2); 31.32 (CH2CH2-
(CH3)2); 29.98 (CH2CH2(CH3)2); 99.87,103.77, 105.82, 106.71,
116.87 (Cp).
Cp*(η5-C5H4-CMe3)Zr(CH2CH2CH2CHMe2)(H) (49). 1H
NMR (benzene-d6): δ 1.83 (s, 15H, C5Me5); 1.41 (s, 9H, C5H4-
CMe3); 5.39 (s, 1H, ZrH); -0.450 (m, 1H, CH2CH2CH2CHMe2);
-0.450 (m, 1H, CH2CH2CH2CHMe2); 0.651 (m, 2H, CH2CH2-
CH2CHMe2); 0.589 (m, 2H, CH2CH2CH2CHMe2); 2.56 (m, 1H,
CH2CH2CH2CHMe2); 0.999 (d, 6 Hz, 6H, CH2CH2CH2CH-
(CH3)2); 4.22, 4.91, 5.34, 5.98 (m, 1H, Cp). 13C NMR (benzene-
d6): δ 12.46 (C5Me5); 112.77 (C5Me5); 32.09 (CMe3); 139.36
(CMe3); 56.14 (CH2(CH2)2CHMe2); 18.39, 22.29 (CH2(CH2)2-
CHMe2) 32.78 (CH2(CH2)2CHMe2); 23.45 (CH2(CH2)2CH(CH3)2;
103.56, 105.92, 109.53, 115.77, 118.64 (Cp).
(η5-C5Me4H)2Zr(CH2CHMe2)(H) (43). H NMR (benzene-
1
d6): δ 2.01, 2.07 (s, 12H, C5Me4H); 5.10 (s, 2H, C5Me4H); 5.96
(s, 1H, ZrH); -0.06 (d, 6.8 Hz, 2H, CH2CHMe2); 2.28 (m, 1H,
CH2CHMe2); 1.03 (d, 6 Hz, 6H, CH2CH(CH3)2). 13C NMR
(benzene-d6): δ 12.49, 13.14, 13.72, 14.22 (C5Me4H); 75.33
(CH2CHMe2); 31.17 (CH2CHMe2); 30.21 (CH2CH(CH3)2); 108.37,
116.98, 117.45, 121.25, 121.76 (Cp).
Cp*(η5-C5H4-CMe3)Zr(CH2CH2CH2CH3)(H) (50). 1H NMR
(benzene-d6): δ 1.82 (s, 15H, C5Me5); 1.40 (s, 9H, CMe3); 5.94
(s, 1H, ZrH); -0.504 (dd, 1H, CH2CH2CH2CH3); -0.02 (dd, 1H,
CH2CH2CH2CH3); 1.26 (m, 2H, CH2CH2CH2CH3); 0.650 (m,
2H, CH2CH2CH2CH3); 1.06 (t, 7 Hz, 3H, CH2CH2CH2CH3);
4.23, 4.88, 4.91, 5.33 (Cp). 13C NMR (benzene-d6): δ 12.35
(C5Me5); 115.38 (C5Me5); 32.49 (CMe3); 156.10 (CMe3); 46.58
(CH2CH2CH2CH3); 29.56 (CH2CH2CH2CH3); 18.66 (CH2CH2CH2-
CH3); 12.71 (CH2CH2CH2CH3); 101.09, 103.48, 104.00, 105.83,
109.43 (Cp).
Cp*(η5-C5Me4H)Zr(CH2CH2C6H5)(H) (44). 1H NMR (ben-
zene-d6): δ 1.85 (s, 15H, C5Me5); 1.81, 1.84 (s, 6H, C5Me4H);
5.28 (bs, 1H, ZrH); -0.131 (m, 1H, CH2CH2Ph); 0.470 (m, 1H,
CH2CH2Ph); not located (CH2 CH2Ph); 7.10 (m, 1H, para-CH2-
CH2C6H5); 7.24 (m, 2H, meta-CH2CH2C6H5); 7.42 (m, 2H, ortho-
CH2CH2C6H5); 4.48 (Cp). 13C NMR (benzene-d6): δ 12.56
(C5Me5); 116.59 (C5Me5); 11.67, 12.19, 13.33, 14.71 (C5Me4H);
47.14 (CH2CH2Ph); 22.68 (CH2CH2Ph); 96.10, 105.77, 119.62,
2 not located (Cp); 122.42 (para-C6H5); 125.86 (ortho-C6H5);
129.40 (meta-C6H5); 1 not located (ipso-C6H5).
Cp*(η5-C5H4-CMe3)Zr(cyclo-C5H9)(H) (51). 1H NMR (ben-
zene-d6): δ 1.76 (s, 15H, C5Me5); 1.40 (s, 9H, C5H4-CMe3); 5.71
(s, 1H, ZrH); -0.512, 0.105, 1.13, 1.93, 2.31 (m, C5H9); 4.73,
4.89, 4.96, 5.83 (m, 1H, Cp). 13C NMR (benzene-d6): δ 12.42
(C5Me5); 114.16 (C5Me5); 32.89 (CMe3); 146.33 (CMe3); 47.65
(ipso-C5H9); 23.51, 29.23, 34.10, 37.53 (C5H9); 99.90, 103.00,
104.41, 109.96, 125.51 (Cp).
Cp*(η5-C5Me4H)Zr(CH2CH2(C6H4-p-OCH3))(H) (45). H
1
NMR (benzene-d6): δ 1.86 (s, 15H, C5Me5); 1.71, 1.77, 2.06,
2.10 (s, 6H, C5Me4H); 5.06 (s, 1H, ZrH); 0.51 (m, 2H, CH2-
CH2C6H5-p-OCH3); 1.52 (m, 2H, CH2CH2C6H5-p-OCH3); 3.24
(s, 3H, CH2CH2C6H5-p-OCH3); 6.88 (d, 8.6 Hz, 2H, CH2CH2-
meta-C6H5-p-OCH3); 7.36 (d, 8.6 Hz, 2H, CH2CH2-ortho-C6H5-
p-OCH3), 4.49 (Cp). 13C NMR (benzene-d6): δ 12.56 (C5Me5);
114.32 (C5Me5); 11.62, 12.23, 13.26, 14.78 (C5Me4H); 55.14
(CH2CH2(C6H4-p-OCH3)); 20.35 (CH2CH2(C6H4-p-OCH3)); 45.25
(C6H4-OCH3); 105.56, 113.05, 116.18, 116.72, 117.09 (Cp);
122.01 (ortho-C6H4-p-OCH3); 127.45 (para-C6H4-p-OCH3); 130.32
(meta-C6H4-p-OCH3); 159.16 (ipso-C6H4-p-OCH3).
1
Cp*CpZr{C(CH3)dC(H)(CMe3)}(H) (52). H NMR (ben-
zene-d6): δ 1.81 (s, 15H, C5Me5); 3.51 (s, 1H, ZrH); 1.10 (s,
9H, C(CH3)dC(H)(CMe3)); 2.45 (s, 3H, C(CH3)dC(H)(CMe3);
3.45 (s, 1H, C(CH3)dC(H)(CMe3); 5.48 (Cp). 13C NMR (benzene-
d6): δ 12.57 (C5Me5); 114.39 (C5Me5); 30.77 (C(CH3)dC(CMe3)-
(H)); not located (C(CH3)dC(CMe3)(H)); 3.68 (C(CH3)dC(CMe3)-
(H)); 186.82 (C(CH3)dC(CMe3)(H)); 95.71 (Cp).
Cp*(η5-C5H4-CMe3)Zr{C(CH3)dC(H)(CMe3)}(H) (53). 1H
NMR (benzene-d6): δ 1.81 (s, 15H, C5Me5); 1.41 (s, 9H, C5H4-
CMe3); 3.54 (s, 1H, ZrH); 1.14 (s, 9H, C(CH3)dC(H)(CMe3));
2.49 (s, 3H, C(CH3)dC(H)(CMe3); 3.48 (s, 1H, C(CH3)dC(H)-
(CMe3); 4.52, 5.00, 5.08, 5.81 (Cp). 13C NMR (benzene-d6): δ
12.42 (C5Me5); 114.17 (C5Me5); 33.32 (CMe3); 32.81 (CMe3);
150.46 (CMe3); 150.59 (CMe3); 12.68 (C(CH3)dC(CMe3)(H));
190.23 (C(CH3)dC(CMe3)(H)); 20.52 (C(CH3)dC(CMe3)(H));
95.65, 95.94, 98.55, 99.07, 104.03 (Cp).
Cp*(η5-C5Me4H)Zr(CH2CH2(C6H4-p-CH3))(H) (46). 1H
NMR (benzene-d6): δ 1.86 (s, 15H, C5Me5); 1.69, 1.79, 2.04,
2.11 (s, 6H, C5Me4H); 5.62 (bs, 1H, ZrH); 0.510 (m, 2H, CH2-
CH2C6H5-p-CH3); 1.57 (m, 2H, CH2CH2C6H5-p-CH3); 6.94 (d,
8.6 Hz, 2H, CH2CH2-meta-C6H5-p-CH3); 7.37 (d, 8.6 Hz, 2H,
CH2CH2-ortho-C6H5-p-CH3), 4.48 (Cp). 13C NMR (benzene-
d6): δ 12.57 (C5Me5); 116.27 (C5Me5); 11.66, 12.19, 13.30, 14.76
(C5Me4H); 58.03 (CH2CH2(C6H4-p-OCH3)); 29.21 (CH2CH2-
(C6H4-p-OCH3)); 38.06 (C6H4-CH3); 113.04, 116.57, 116.84,
117.29, 117.82 (Cp); 122.28 (ortho-C6H4-p-CH3); 137.97 (para-
C6H4-p-CH3); 143.97 (meta-C6H4-p-OCH3); 159.03x (ipso-C6H4-
p-CH3).
Cp*(THI)Zr{C(CH3)dC(H)(CMe3)}(H) (54). 1H NMR (ben-
zene-d6): δ 1.84 (s, 15H, C5Me5); 0.80-1.10 (m, THI); 3.84 (s,
1H, ZrH); 1.24 (s, 9H, C(CH3)dC(H)(CMe3)); 2.49 (s, 3H,
C(CH3)dC(H)(CMe3); 3.70 (s, 1H, C(CH3)dC(H)(CMe3); 4.98,
5.08, 5.55 (Cp). 13C NMR (benzene-d6): δ 12.60 (C5Me5); 114.31
(C5Me5); 31.90 (CMe3); 159.11 (CMe3); 3.77 (C(CH3)dC(CMe3)-

Zirconocene and Hafnocene Alkyl Hydride Derivatives
Organometallics, Vol. 24, No. 22, 2005 5423
(H)); 187.12 (C(CH3)dC(CMe3)(H)); 20.17 (C(CH3)dC(CMe3)-
(H)); 23.94, 24.58, 24.70, 26.86 (THI); 92.75, 100.32, 100.84,
101.83, 106.21 (Cp).
(t, 7.3 Hz, 3H, CH2 CH2CH2CH3), 5.91 (s, 5H Cp). 13C NMR
(benzene-d6): δ 12.65 (C5Me5); 116.81 (C5Me5); 56.71 (CH2CH2-
CH2CH3); 31.50 (CH2CH2CH2CH3); 24.95 (CH2CH2CH2CH3);
14.33 (CH2CH2CH2CH3); 107.16 (Cp).
Cp*{η5-C5H3-1,3-(CHMe2)2}Zr(CH2CH2CH2CH3)(H) (60).
1H NMR (benzene-d6): δ 1.84 (s, 15H, C5Me5); 5.38 (s, 1H,
ZrH); 1.19, 1.24, 1.42, 1.50 (d, 7 Hz, 3H, CHMe2); 2.78, 3.20
(sept, 6.5 Hz, 1H, CHMe2); -0.401 (m, 2H, CH2 CH2CH2CH3);
not located (CH2 CH2CH2CH3); 1.06 (m, 2H, CH2 CH2CH2CH3);
0.870 (t, 7.3 Hz, 3H, CH2 CH2CH2CH3); 3.95, 4.88, 5.01 (m,
1H, Cp).
Cp*(THI)Zr(CH2CH2CH2CH3)(H) (61). 1H NMR (benzene-
d6): δ 1.86 (s, 15H, C5Me5); 5.95 (s, 1H, ZrH); -0.501 (m, 2H,
CH2CH2CH2CH3); 1.40 (m, 2H, CH2 CH2CH2CH3); 0.82 (m, 2H,
CH2CH2CH2CH3); 1.10 (t, 7.0 Hz, 3H, CH2 CH2CH2CH3), 4.75,
5.02, 5.20 (m, 1H, Cp). 13C NMR (benzene-d6): δ 12.15 (C5Me5);
117.92 (C5Me5); 23.84, 25.01, 25.25, 25.93 (THI); 53.40 (CH2-
CH2CH2CH3); 47.52 (CH2CH2CH2CH3); 31.73 (CH2CH2CH2-
CH3); 14.32 (CH2CH2CH2CH3); 107.65, 110.40, 111.60, 125.13,
125.76 (Cp).
Cp*(η5-C5Me4H)Zr(CH2CH2CH2CH3)(H) (62). 1H NMR
(benzene-d6): δ 1.88 (s, 15H, C5Me5); 1.71, 1.81, 1.91, 2.08 (s,
3H, C5Me4 H); 4.47 (s, 1H, C5Me4H); 4.76 (s, 1H, ZrH); 0.057
(dd, 1H, CH2 CH2CH2CH3); 0.161 (dd, 1H, CH2 CH2CH2CH3);
not located (CH2CH2CH2CH3); 1.43 (m, 2H, CH2 CH2CH2CH3);
1.08 (t, 6.5 Hz, 3H, CH2CH2CH2CH3). 13C NMR (benzene-d6):
δ 12.60 (C5Me5); 115.71 (C5Me5); 11.67; 12.18; 12.30; 13.12
(C Me4 H); 43.66 (CH2CH2CH2CH3); 27.86 (CH2CH2CH2CH3);
165.09 (CH2CH2CH2CH3); 15.06 (CH2CH2CH2CH3); 105.44;
115.56; 116.47; 116.89; 117.52 (Cp).
Cp*{η5-C5H3-1,3-(CHMe2)2}Zr(C(CH3)dC(H)(CMe3))-
1
(H) (55). H NMR (benzene-d6): δ 1.85 (s, 15H, C5Me5); 3.68
(s, 1H, ZrH); 1.06 (d, 7 Hz. 3H, CHMe2); 1.28 (d, 7 Hz. 3H,
CHMe2); 1.34 (d, 7 Hz. 3H, CHMe2); 1.41 (d, 7 Hz. 3H, CHMe2);
2.75 (sept, 6.5 Hz, 1H, CHMe2); 3.20 (sept, 6.5 Hz, 1H, CHMe2);
1.18 (s, 9H, C(CH3)dC(H)(CMe3)); 2.48 (s, 3H, C(CH3)dC(H)-
(CMe3); 86.25 (C(CH3)dC(CMe3)(H)); 3.34 (s, 1H, C(CH3)d
C(H)(CMe3); 4.65, 4.74, 4.98 (m, 1H, C5H3-1,3-(CMe3)2). 13C
NMR (benzene-d6): δ 12.95 (C5Me5); 117.29 (C5Me5); 25.21,
25.48, 27.05, 27.26 (CHMe2); 30.22, 21.14 (CHMe2); 30.97
(CMe3); 139.72 (CMe3); 3.73 (C(CH3)dC(CMe3)(H)); not located
(C(CH3)dC(CMe3)(H)); 21.90 (C(CH3)dC(CMe3)(H)) 88.37, 99.72,
99.42, 104.73, 114.49 (Cp).
Cp*{η5-C5H3-1,3-(CMe3)2}Zr{C(CH3)dC(H)(CMe3)}(H)
1
(56). H NMR (benzene-d6): δ 1.85 (s, 15H, C5Me5); 3.56 (s,
1H, ZrH); 1.17 (s, 9H, C5H5-1,3-(CMe3)2)); 1.55 (s, 9H, C5H5-
1,3-(CMe3)2); 1.22 (s, 9H, C(CH3)dC(H)(CMe3)); 2.55 (s, 3H,
C(CH3)dC(H)(CMe3); 3.34 (s, 1H, C(CH3)dC(H)(CMe3); 4.49,
4.68, 4.95 (m, 1H, C5H3-1,3-(CMe3)2). 13C NMR (benzene-d6):
δ 13.06 (C5Me5); 119.88 (C5Me5); 32.88 (CMe3); 32.20 (CMe3);
114.16 (CMe3); 117.44 (CMe3); 31.88 (CMe3); 136.18 (CMe3);
3.83 (C(CH3)dC(CMe3)(H)); 167.31 (C(CH3)dC(CMe3)(H)); 28.04
(C(CH3)dC(CMe3)(H)); 100.99, 105.46, 109.02, 2 not located
(Cp).
Cp*(η5-C5Me4H)Zr{C(CH3)dC(H)(CMe3)}(H) (57). 1H
NMR (benzene-d6): δ 1.85 (s, 15H, C5Me5); 1.86, 1.88, 1.91,
2.01 (s, 3H, C5Me4 H); 4.47 (s, 1H, ZrH); 4.47 (s, 1H, C5Me4
H); 1.21 (s, 9H, C(CH3)dC(H)(CMe3)); 2.46 (s, 3H, C(CH3)d
C(H)(CMe3); 3.53 (s, 1H, C(CH3)dC(H)(CMe3). 13C NMR (ben-
zene-d6): δ 12.71 (C5Me5); 114.47 (C5Me5); 11.83, 12.41, 12.89,
13.32 (C5Me4H); 31.66 (CMe3); 150.89 (CMe3); 15.45 (C(CH3)d
C(CMe3)(H)); 162.71 (C(CH3)dC(CMe3)(H)); 18.41 (C(CH3)d
C(CMe3)(H)); 98.57, 102.25, 105.49, 122.54, 1 not located (Cp).
(η5-C5Me4H)2Zr{C(CH3)dC(H)(CMe3)}(H) (58). 1H NMR
(benzene-d6): δ 2.12, 2.21 (s, 6H, C5Me4H); 5.15 (s, 2H, C5-
Me4H); 3.73 (s, 1H, ZrH); 1.24 (s, 9H, C(CH3)dC(H)(CMe3));
2.46 (s, 3H, C(CH3)dC(H)(CMe3); 3.35 (s, 1H, C(CH3)dC(H)-
(CMe3). 13C NMR (benzene-d6): δ 12.11, 12.84, 13.96, 13.54
(C5Me4H); I88.33 (Zr-C(CH3)dC(H)(CMe3)); 187.30 (Zr-C(CH3)d
C(H)(CMe3)); 31.56 (Zr-C(CH3)dC(H)(CMe3)); 143.72 (Zr-
C(CH3)dC(H)(CMe3)); 18.64 (Zr-C(CH3)dC(H)(CMe3)); 98.95,
111.28, 114.82, 116.97, 120.26 (Cp).
Cp*{η5-C5H3-1,3-(CMe3)2}Zr(CH2CH2CH2CH3)(H) (63).
1H NMR (benzene-d6): δ 1.72 (s, 15H, C5Me5); 1.10, 1.31 (s,
9H, CMe3); 5.51 (s, 1H, ZrH); 0.14 (m, 2H, CH2 CH2CH2CH3);
0.652 (m, 2H, CH2CH2CH2CH3); 0.753 (m, 2H, CH2CH2CH2-
CH3); 943 (t, 7 Hz, 3H, CH2CH2CH2CH3); 4.43, 4.60, 4.65 (m,
1H, Cp). 13C NMR (benzene-d6): δ 13.06 (C5Me5); 113.86 (C5-
Me5); 32.86 (CMe3); 35.00 (CMe3); 142.62 (CMe3); 140.92
(CMe3); 42.13 (CH2CH2CH2CH3); 31.31 (CH2CH2CH2CH3);
27.38 (CH2CH2CH2CH3); 13.61 (CH2CH2CH2CH3); 100.99;
105.52; 109.05; 110.16; 111.99 (Cp).
Acknowledgment. This work has been supported
by USDOE Office of Basic Energy Sciences (Grant
No. DE-FG03-85ER13431), Exxon Chemicals America,
and the National Science Foundation (Grant No.
CHE-0131180).
Cp*CpZr(CH2CH2CH2CH3)(H) (59). 1H NMR (benzene-
d6): δ 1.73 (s, 15H, C5Me5); 6.21 (s, 1H, ZrH); -0.09 (dd, 1H,
CH2CH2CH2CH3); 0.577 (dd, 1H, CH2CH2CH2CH3); 1.92 (m,
2H, CH2 CH2CH2CH3); 0.88 (m, 2H, CH2 CH2CH2CH3); 1.05
OM0580351