Measurement of barriers for alkene dissociation and for inversion at zirconium in a d0 zirconium-alkyl-alkene complex

Article abstract of DOI:10.1021/om9908940

The β-allyl zirconacyclobutane complex Cp*₂Zr[CH₂CH(CH₂CH double bond CH₂)CH₂] (7) reacted rapidly with B(C₆F₅)₃ in CD₂Cl₂ at -78 °C to form the zwitterionic d^o zirconium(IV) chelate complex Cp*₂Zr[η¹,η²-CH₂CH[CH₂B (C₆F₅)₃]CH₂CH double bond CH₂] (2a and 2b). Low-temperature ¹H, ¹³C, TOCSY1D, and NOESY1D NMR spectroscopy of 2 established the bonding of the tethered alkene to the d^o metal center. A dynamic NMR study of the interconversion of 2a and 2b allowed measurement of the alkene dissociation energy (ΔG^? = 10.5 (2a to 2b) and 10.3 (2b to 2a) kcal mol^-1), but the complex decomposed before the barrier for site epimerization at the zirconium center could be determined. Reaction of 7 with [(C₆H₅)₂(CH₃)NH][B (C₆F₅)₄] led to the formation of two isomeric d^o zirconium(IV)-alkyl-alkene chelates Cp*₂Zr[η¹, η²-CH₂CH(CH₃)CH₂CH double bond CH₂][B(C₆F₅)₄] (8a and 8b). This more thermally stable zirconium-alkyl-alkene complex allowed the measurement of barriers associated with decomplexation of the alkene (ΔG^? = 10.7 and 11.1 kcal mol^-1) and site epimerization at the zirconium center (ΔG^? = 14.4 kcal mol^-1) by line shape analysis of variable-temperature ¹H and ¹³C NMR spectra.

Full text of DOI:10.1021/om9908940

3970
Organometallics 2000, 19, 3970-3977
Mea su r em en t of Ba r r ier s for Alk en e Dissocia tion a n d for
In ver sion a t Zir con iu m in a d 0 Zir con iu m -Alk yl-Alk en e
Com p lex
Charles P. Casey* and Donald W. Carpenetti II
Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706
Received November 8, 1999
The â-allyl zirconacyclobutane complex Cp*2Zr[CH2CH(CH2CHdCH2)CH2] (7) reacted
rapidly with B(C6F5)3 in CD2Cl2 at -78 °C to form the zwitterionic d0 zirconium(IV) chelate
complex Cp*2Zr[η1,η2-CH2CH[CH2B(C6F5)3]CH2CHdCH2] (2a and 2b). Low-temperature 1H,
13C, TOCSY1D, and NOESY1D NMR spectroscopy of 2 established the bonding of the tethered
alkene to the d0 metal center. A dynamic NMR study of the interconversion of 2a and 2b
allowed measurement of the alkene dissociation energy (∆Gq ) 10.5 (2a to 2b) and 10.3 (2b
to 2a ) kcal mol-1), but the complex decomposed before the barrier for site epimerization at
the zirconium center could be determined. Reaction of 7 with [(C6H5)2(CH3)NH][B(C6F5)4]
led to the formation of two isomeric d0 zirconium(IV)-alkyl-alkene chelates Cp*2Zr[η1,η2-
CH2CH(CH3)CH2CHdCH2][B(C6F5)4] (8a and 8b). This more thermally stable zirconium-
alkyl-alkene complex allowed the measurement of barriers associated with decomplexation
of the alkene (∆Gq )10.7 and 11.1 kcal mol-1) and site epimerization at the zirconium center
1
(∆Gq )14.4 kcal mol-1) by line shape analysis of variable-temperature H and 13C NMR
spectra.
In tr od u ction
zwitterionic complex Cp*2Zr+[η1,η2-CH2CH{CH2B--
(C6F5)3}CH2CHdCH2] (2).5 Other reported examples of
d0 zirconium-alkene complexes include Jordan’s alkoxy-
alkene complex [Cp2Zr(η1,η2-OCMe2CH2CH2CHdCH2)]-
[MeB(C6F5)3] (3),6 Royo’s [(Cp)(η5,η2-C5H4SiMe2CH2-
CHdCH2)Zr(CH2C6H5)][B(C6F5)4] (4),7 and Erker’s
(MeCp)2Zr+CH2CH(CH3)CH2CHdCHCH2B-(C6F5)3 (5)
and related complexes (Scheme 1).8 Our studies have
shown that complexation of an alkene to a d0 yttrium
or zirconium center polarizes the π-bond, that decom-
plexation is fast at low temperature, and that there is
a significant barrier to inversion at pyramidal Cp*2YR
centers.9
The stereochemistry of metallocene-catalyzed propy-
lene polymerization is controlled by the microstructure
of the catalysts and in some cases by the ability of the
zirconium alkyl to undergo inversion of configuration
at the metal center. Highly isotactic polypropylene can
be formed using C2-symmetric metallocene catalysts,10
as the tacticity is not affected by the rate of inversion
at the metal center since the same enantioface of
propene coordinates selectively to either of the C2-
related sites. Highly syndiotactic polypropylene can be
d0 metal-alkyl-alkene complexes have been pro-
posed as crucial but unobserved intermediates1 in
metallocene-catalyzed alkene polymerizations.2 These
intermediates have eluded observation presumably
because of their high kinetic reactivity and their low
thermodynamic stability due to the absence of d to π*
back-bonding. Recently, we reported the first d0 metal-
alkyl-alkene complex, Cp*2Y(η1,η2-CH2CH2C(CH3)2-
CHdCH2) (1),3 which is stabilized by chelation. Subse-
quently, we reported detailed analyses of the kinetics
and thermodynamics of d0 yttrium-alkyl-alkene com-
plexes.4 We have also recently published a preliminary
report on a d0 zirconium-alkyl-alkene complex, the
(1) (a) Cossee, P. J . Catal. 1964, 3, 80. (b) Arlman, E. J .; Cossee, P.
J . Catal. 1964, 3, 99. (c) Brookhart, M.; Green, M. L. H.; Wong, L.-L.
Prog. Inorg. Chem. 1988, 36, 1. (d) Brookhart, M.; Volpe, A. F.; Lincoln,
D. M.; Horva´th, I. T.; Millar, J . M. J . Am. Chem. Soc. 1990, 112, 5634.
(e) Brookhart, M.; Green, M. L. H. J . Organomet. Chem. 1983, 250,
395. (f) Dawoodi, Z.; Green, M. L. H.; Mtetwa, V. S. B.; Prout, K. J .
Chem. Soc., Chem. Commun. 1982, 1410. (g) Ivin, K. J .; Rooney, J . J .;
Stewart, C. D.; Green, M. L. H.; Mahtab, R. J . Chem. Soc., Chem.
Commun. 1978, 604.
(2) For reviews of metallocene-catalyzed alkene polymerization,
see: (a) J ordan, R. F. Adv. Organomet. Chem. 1991, 32, 325. (b)
Brintzinger, H. H.; Fisher, D.; Mu¨lhaupt, R.; Rieger, B.; Waymouth,
R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143. (c) Marks, T. J .
Acc. Chem. Res. 1992, 25, 57. (d) Bochmann, M. J . Chem. Soc., Dalton
Trans. 1996, 255. (d) Alt, H. G.; Koppl, A. Chem. Rev. 2000, 100, 1205.
(e) Resconi, L.; Cavallo, L.; Fait, A.; Pietmontesi, F. Chem. Rev. 2000,
100, 1253. (f) Chen, E. Y.-X.; Marks, T. J . Chem. Rev. 2000, 100, 1391.
(g) Rappe, A. K.; Skiff, W. M.; Casewitt, C. J . Chem. Rev. 2000, 100,
1435.
(5) For a preliminary communication, see: Casey, C. P.; Carpenetti,
D. W., II; Sakurai, H. J . Am. Chem. Soc. 1999, 121, 9483.
(6) Wu, Z.; J ordan, R. F.; Petersen, J . L. J . Am. Chem. Soc. 1995,
117, 5867.
(7) Galakhov, M. V.; Heinz, G.; Royo, P. Chem. Commun. 1998, 17.
(8) Temme, B.; Karl, J .; Erker, G. Chem. Eur. J . 1996, 2, 919.
(9) Inversion at the metal center has also been called site epimer-
ization and back slipping.
(10) (a) Coughlin, E. B.; 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. (c) Cavallo, L.; Guerra, G.; Vacatello, M.;
Corrandini, P. Macromolecules 1991, 24, 1784. (d) Busico, V.; Corran-
dini, P.; De Biasio, R.; Landriani, L.; Segre, A. L. Macromolecules 1994,
27, 4521.
(3) Casey, C. P.; Hallenbeck, S. L.; Pollock, D. W.; Landis, C. R. J .
Am. Chem. Soc. 1995, 117, 9770.
(4) (a) Casey, C. P.; Hallenbeck, S. L.; Wright, J . M.; Landis, C. R.
J . Am. Chem. Soc. 1997, 119, 9680. (b) Casey, C. P.; Fisher, J . J . Inorg.
Chim. Acta 1998, 270, 5. (c) Casey, C. P.; Fagan, M. A.; Hallenbeck,
S. L. Organometallics 1998, 17, 287. (d) Casey, C. P.; Fisher, J . J .;
Fagan, M. A. J . Am. Chem. Soc. 2000, 122, 4320.
10.1021/om9908940 CCC: $19.00 © 2000 American Chemical Society
Publication on Web 08/19/2000

Measurement of Barriers for Alkene Dissociation
Organometallics, Vol. 19, No. 19, 2000 3971
Sch em e 1
Sch em e 2
Sch em e 3
formed using Cs-symmetric metallocenes, such as Ewen’s
(6).11 In this case, the stereochemistry of the polymer
depends on the relative rates of insertion of the next
monomer and inversion at the metal center (Scheme 2).
One of our main goals in studying models of intermedi-
ates related to these polymerization catalysts is to
measure the barrier associated with this process of
inversion at the zirconium center.
Our early attempts to produce a zirconium-alkyl-
alkene complex focused on protonation routes employing
ammonium cations, noncoordinating anions, and Stryk-
er’s â-allyl zirconacyclobutane, Cp* Zr[CH2CH(CH2CHd
2
cationic zirconium-alkyl-alkene complex Cp* Zr[η1,η2-
CH2)CH2] (7).12 An early protonation route employing
[C6H5(CH3)2NH][B(C6F5)4] failed to give the desired
alkene complex. Instead, coordination of the conjugate
base to zirconium occurred to give [Cp*2Zr{N(CH3)2-
C6H5}CH2CH(CH3)CH2CHdCH2][B(C6F5)4] in quantita-
tive yield. Attempts to generate a zwitterionic zirconium-
alkyl-alkene complex using the alkyl group abstractor
B(C6F5)3 were successful and allowed the observation
2
CH2CH(CH3)CH2CHdCH2][B(C6F5)4] (8a and 8b). These
isomers proved to be more thermally stable than the
corresponding zwitterions 2a and 2b and allowed the
measurement of both the barrier decomplexation as well
as the additional barrier corresponding to inversion at
the zirconium center.9
of two isomers of Cp* Zr[η1,η2-CH2CH[CH2B(C6F5)3]-
Resu lts
2
CH2CHdCH2] (2a and 2b). We were able to measure
the barrier for alkene decomplexation, but the thermal
instability of 2 prevented measurement of the barrier
to inversion at zirconium, an important factor in the
production of syndiotactic polypropylene.
In the work reported here, we have protonated
Stryker’s â-allyl zirconacyclobutane 714 with [(C6H5)2-
CH3NH][B(C6F5)4], which generates a more sterically
inaccessible and much less Lewis basic amine conjugate
base. This procedure produced two isomers of the
Alk yl Gr ou p Abstr a ction Rou te to Zir con iu m -
Alk yl-Alk en e Com p lexes.5 The reaction of Stryker’s
â-allyl zirconacyclobutane complex 712 with B(C6F5)3
13
generated the first example of a stable zwitterionic d0
zirconium-alkyl-alkene complex with an all-carbon
chelate backbone (Scheme 3). Reaction of 7 with B(C6F5)3
in CD2Cl2 at -78 °C led to the formation of a bright
orange solution containing a 1.8:1 mixture of two
diastereomers of the zwitterionic d0 zirconium(IV)-
alkyl-alkene chelate complex Cp* ZrCH2CH[CH2B-
2
(C6F5)3]CH2CHdCH2 (2a and 2b).
(11) (a) Ewen, J . A.; J ones, R. L.; Razavi, A.; Ferrara, J . D. J . Am.
Chem. Soc. 1988, 110, 6255. (b) Ewen, J . A. Makromol. Chem.,
Macromol. Symp. 1995, 89, 181.
(12) Tjaden, E. B.; Stryker, J . M. J . Am. Chem. Soc. 1993, 115, 2083.
(13) Massey, A. G.; Park, A. J . J . Organomet. Chem. 1964, 2, 245.
(14) These compounds were synthesized by a procedure similar to
that of: Tjaden, E. B.; Swenson, D. C.; J ordan, R. F.; Petersen, J . L.
Organometallics 1995, 14, 371.
There are two diastereomers of 2 due to the presence
of two stereogenic centers, the â-carbon of the chelate
chain and the re or si face of the alkene. The two
diastereomers were identified by 1H and 13C NMR
spectroscopy at -82 °C. The major diastereomer is
assumed to have the alkyl substituent in the pseu-

3972 Organometallics, Vol. 19, No. 19, 2000
Casey and Carpenetti
Sch em e 4
Sch em e 6
Sch em e 5
vinyl hydrogen and the other Cp* ligand (HZ-Cp* δ
1.98), and between the secondary vinyl proton and the
Cp* at δ 1.98. In contrast, for the nonchelated THF
adduct 9, no NOE enhancements of Cp* peaks were
detected upon irradiation of the vinyl protons.
doequatorial position. The most striking feature in the
1H NMR spectrum of these complexes is the high-
frequency chemicals shifts of the secondary vinyl pro-
tons of the coordinated alkene compared to those of 7.
Upon coordination, the secondary vinyl proton experi-
ences a large shift to higher frequency from δ 6.29 in 7
to 7.92 in the major isomer 2a (+1.63 ppm) and to δ
7.54 ppm in the minor isomer 2b (+1.25 ppm). Similar
shifts to higher frequency upon alkene coordination to
a d0 metal are seen for 1 (+1.04 ppm), 3 (+1.64), and 4
(+1.58). This shift to higher frequency can be attributed
to a resonance contributor that allows delocalization of
positive charge from zirconium to the internal vinyl
carbon (Scheme 4). Greatly increased separation of the
terminal vinyl proton resonances also support alkene
coordination. 7 shows these resonances at δ 5.25 and
5.14 (∆δ ) 0.09 ppm). For major chelate isomer 2a ,
these resonances are observed at δ 5.85 and 2.44 (∆δ )
3.41 ppm); for 2b, they are observed at δ 5.73 and 2.27
(∆δ ) 3.46). The coupling constants between the vinyl
protons of 2a and 2b are similar to those of free alkenes.
In the 13C NMR spectrum of 2a and 2b, the ZrCH2
resonances at δ 75.7 (2a ) and 73.2 (2b) confirm the
presence of an alkyl group bound to a zirconium cation.
The shifts to higher frequency of the internal vinyl
carbon resonances of 2a (∆δ ) +22.4) and 2b (∆δ )
+25.5) relative to those of 7 provide further evidence
for alkene coordination and polarization of the alkene
double bond.
Upon slow warming of the solution to -42 °C, 2a and
2b interconvert rapidly and the individual resonances
of 2a and 2b coalesce into one set of weighted average
resonances (Scheme 6). The four original Cp* reso-
nances converge to two distinct resonances at δ 2.03 and
2.01; each new resonance corresponds to the weighted
average of two of the original four Cp* resonances. The
interconversion of 2a and 2b is consistent with a
dynamic process that involves decomplexation of the
bound face of the alkene and recoordination to the other
enantioface. This process renders the secondary vinyl
proton resonances of 2a and 2b equivalent. All temper-
ature-dependent changes are reversible up to -35 °C,
but significant decomposition occurs at higher temper-
atures. Line shape analysis of the coalescence of the
secondary vinyl hydrogen resonances gave ∆Gq ) 10.5
kcal mol-1 for 2a to 2b and ∆Gq ) 10.3 kcal mol-1 for
2b to 2a .
The barrier to alkene dissociation from zirconium in
2 is significantly higher than from similar neutral
yttrium chelates (∆Gqdiss ) 7.5 kcal mol-1 for Cp*2YCH2-
CH2CH2CHdCH2), presumably due to stronger alkene
binding to a cationic zirconium center than to a neutral
yttrium center. The alkene dissociation barrier of 2 is
similar to that of other cationic zirconium chelates
(∆Gq
) 10.7 kcal mol-1 for J ordan’s compound 3).6
diss
Addition of THF to the solution of 2a and 2b at -78
°C in CD2Cl2 led to the immediate displacement of the
coordinated alkene and formation of a bright yellow
solution of a single 1:1 THF adduct, Cp*2Zr(THF)CH2-
The combined barrier for alkene dissociation and zir-
conium inversion from Royo’s compound 4 is 11.7 kcal
mol-1 7
.
Two distinct Cp* resonances for 2 are observed up to
temperatures where decomposition starts to occur; this
is consistent with slow inversion at zirconium. Line
shape analysis of the Cp* resonances observed before
1
CH[CH2B(C6F5)3]CH2CHdCH2 (9) (Scheme 5). The H
NMR chemical shifts of the vinyl hydrogens (δ 5.73,
4.92, 4.89) and the 13C NMR chemical shifts of the vinyl
carbons (δ 114.9 and 138.1) of 9 are very similar to those
of 7 and very different from those of 2a and 2b,
providing evidence that coordination of THF displaces
the alkene from zirconium.
Further evidence for coordination of the alkene was
obtained from 1D NOE difference spectra of the major
isomer 2a . Due to the presence of other resonances in
the same region of the spectrum as the resonances of
the Cp* protons, the only unambiguous NOE experi-
ments involved the unfavorable NOE transfer from the
vinyl protons of the chelate to the Cp* methyl protons.
Approximately 1% positive enhancements were ob-
served between one terminal vinyl hydrogen and one
Cp* ligand (HE-Cp* δ 1.96), between the other terminal
decomposition leads to an estimate that ∆Gq
≈ 15
invers
kcal mol-1 for 2 in CD2Cl2.
P r oton a tion Rou te to Zir con iu m -Alk yl-Alk en e
Com p lexes. In an initial attempt to generate a cationic
d0 zirconium-alkyl-alkene complex by protonation, we
treated Stryker’s â-allyl zirconacyclobutane 712 with
[C6H5(CH3)2NH][B(C6F5)4]14 at -78 °C in CD2Cl2. How-
ever, instead of intramolecular alkene coordination, the
amine conjugate base N,N-dimethylaniline coordinated
to the electrophilic zirconium center to give {Cp*2Zr-
[N (CH 3)2C6H 5][η1-CH 2CH (CH 3)CH 2CH dCH 2]}[B-
(C6F5)4] (10). Evidence for amine coordination came from
1H NMR chemical shifts; the N-methyl groups of amine
complex 10 appeared at δ 3.65 compared with δ 3.19

Measurement of Barriers for Alkene Dissociation
Organometallics, Vol. 19, No. 19, 2000 3973
Sch em e 7
NOE was observed between the Cp* and vinyl reso-
nances of the THF complex 12.
Upon slowly raising the temperature from -78 to -38
°C, the resonances of 8a and 8b broadened and coa-
lesced to give one set of weighted average resonances
(Scheme 6). The four original Cp* resonances converge
to two distinct resonances at δ 2.02 and 2.00; each new
resonance corresponds to the weighted average of two
of the original four Cp* resonances. The interconversion
of 8a and 8b occurs by alkene dissociation and recoor-
dination of the opposite alkene enantioface.
The internal vinyl proton resonance of 8b was par-
tially obscured by the aryl resonances of diphenyl-
(methyl)amine, making simulation of this coalescence
impossible. Therefore, the rate of interconversion of 8a
and 8b was measured by following the coalescence of
the resonances corresponding to the tertiary proton on
the â-carbon of the alkyl chain. The baseline was flat
in the area of these resonances, and line shapes were
accurately simulated using the DNMR program.15 Coa-
lescence was observed at -38 °C, and the resonances
became relatively sharp at -28 °C. Line shape analysis
gave ∆Gq ) 10.7 ( 0.2 kcal mol-1 for 8b to 8a and ∆Gq
) 11.1 ( 0.2 kcal mol-1 for 8a to 8b.16 These barriers
are similar to those observed for zirconium alkene
for the free amine. Similar amine complexes were
formed upon protonation by analogous tributylammo-
nium and (diisopropyl)ethylammonium salts.
In an effort to avoid amine complexation, we employed
the conjugate acid of a less basic and more sterically
crowded amine. Reaction of [(C6H5)2(CH3)NH][B(C6F5)4]14
with 7 led to the formation of a yellow-brown solution.
1H and 13C NMR spectra at -78 °C provided evidence
for a 2.2:1 mixture of two isomers of Cp*2Zr[η1,η2-CH2-
CH(CH3)CH2CHdCH2][B(C6F5)4] (8a and 8b) and free
diphenyl(methyl)amine (11) (Scheme 7). The ratio of
diastereomers of 8 was similar to the 1.8:1 ratio seen
for the zwitterionic analogue 2. Perturbation of the vinyl
hydrogen chemical shifts compared to those of 7 pro-
vided evidence for alkene complexation. The secondary
vinyl hydrogen of 8a appears at δ 7.89, shifted 1.60 ppm
to higher frequency than in 7 and that of 8b is shifted
1.20 ppm to δ 7.49. In contrast with the small differ-
ences between the chemical shifts of the terminal vinyl
hydrogens of the uncomplexed alkene of 7 (δ 5.25, 5.14,
∆δ ) 0.09 ppm), large chemical shift differences are seen
for the complexed alkene in both 8a [δ 5.43 (J trans ) 17.1
Hz), 2.44 (J cis ) 10.1 Hz), ∆δ ) 2.99 ppm] and 8b [δ
5.71 (J trans ) 17.8 Hz), 2.18 (J cis ) 12.5 Hz), ∆δ ) 3.53
ppm]. At low temperatures, four Cp* resonances are
observed, two for 8a and two for 8b.
Further evidence for alkene coordination in 8a and
8b comes from 13C NMR spectroscopy. The internal
vinyl carbon resonance of 8a at δ 163.0 was shifted 21.8
ppm to higher frequency than in 7 and that of 8b at δ
166.1 was shifted 24.9 ppm to higher frequency. As in
the case of the zwitterionic chelates 2a and 2b, the shifts
of the terminal vinyl carbons of 8a and 8b are similar
to those of the uncomplexed alkene of 7. Characteristic
high-frequency resonances at δ 75.1 8a and 72.6 8b
provided evidence for Zr+CH2 groups.
complexes 2a and 2b (∆Gq ) 10.3 and 10.5 kcal mol-1 5
)
and 3 (∆Gq ) 10.7 kcal mol-1)6 and higher than those
observed for the related neutral yttrium compound 1
(∆Gq ) 8.2 kcal mol-1).4d
Upon further warming, the two Cp* resonances of the
rapidly equilibrating mixture of 8a and 8b broadened
and coalesced into a single resonance at 2 °C. None of
the other resonances in the spectrum were affected. This
coalescence is consistent with dissociation of alkene from
zirconium, inversion at zirconium, rotation about the
Zr-C bond, and recoordination of the alkene (Scheme
8). All temperature-dependent changes are reversible,
but decomposition begins to occur at -5 °C and is rapid
at +5 °C.17 Line shape analysis of this process yielded
∆Gq ) 14.4 ( 0.9 kcal mol-1 (∆Hq ) 14.0 ( 0.7 kcal
mol-1; ∆Sq ) -1 ( 2 eu). Previously, we observed a 9.6
kcal mol-1 barrier for alkene dissociation and inversion
at yttrium for Cp*2YCH2CH2CH(CH3)CHdCH2.4c Marks
found that for [(1,2-Me2C5H3)2ZrCH3][CH3B(C6F5)3] the
barrier for ion pair dissociation and inversion at zirco-
nium was strongly solvent dependent and was 11 kcal
mol-1 in C6D5Cl.18
The possible involvement of associative processes in
alkene decomplexation and inversion was investigated.
The possibility that (C6H5)2CH3N (11) or borate ion pairs
might displace the alkene was investigated by increas-
ing the concentration of both species. Changing the
concentration of the solution containing zirconium-
alkyl-alkene complexes 8a , 8b, and amine 11 did not
change the barriers for alkene decomplexation or inver-
sion measured by dynamic NMR spectroscopy. This
finding rules out any contribution from associative
pathways involving interaction between chelate com-
plexes or interaction with free amine. Added [Ph-
Displacement of the coordinated alkene was observed
upon addition of THF to a CD2Cl2 solution of 8a and
8b at -78 °C, yielding a yellow solution of the single
THF adduct Cp*2Zr(THF)[CH2CH(CH3)CH2CHdCH2]+-
1
[B(C6F5)4]-(12). The H and 13C NMR chemical shifts
of the vinyl group of 12 are consistent with a noncoor-
dinated alkene.
Coordination of the alkene to the zirconium center is
further supported by 1D NOE spectroscopy of 8a . The
Cp* resonance at δ 1.98 received a 3.0 ( 0.5% NOE
enhancement when either the secondary vinyl hydrogen
or the terminal vinyl hydrogen cis to it (δ 2.44) was
irradiated. The Cp* resonance at δ 1.96 received a 2.8
( 0.5% NOE enhancement when the other terminal
vinyl hydrogen at δ 5.43 was irradiated. In contrast, no
(16) ∆Hq ) 14.0 ( 0.2 kcal mol-1 and ∆Sq ) 14 ( 1 eu for 8b to 8a ;
∆Hq ) 14.0 ( 0.2 kcal mol-1 and ∆Sq ) 12 ( 1 eu for 8a to 8b.
(17) The half-life of 8 was 60 min at -5 °C, 17 min at 5 °C, and 5.4
min at 15 °C. This instability prevented us from obtaining 13C NMR
spectra at these temperatures.
(15) Reich, H. J . J . Chem. Educ. 1995, 72, 1086.
(18) Deck, P. A.; Marks, T. J . J . Am. Chem. Soc. 1995, 117, 6128.

3974 Organometallics, Vol. 19, No. 19, 2000
Casey and Carpenetti
Sch em e 8
(CH3)3N][B(C6F5)4] was also found to have no effect on
the barriers for alkene decomplexation and inversion.
When solvent was changed from CD2Cl2 to C6D5Cl, the
barriers were the same, implying that solvent is not
involved in either process.19
of the alkene π-bond. For the cationic and therefore
more electrophilic zirconium center, these high-fre-
quency shifts were of greater magnitude, implying
greater polarization of the alkene π-bond. In agreement
with the hypothesis that alkenes should bind tighter to
a more electron-deficient cationic zirconium center, 3
kcal mol-1 higher kinetic barriers for alkene dissociation
were observed for the zirconium chelates compared with
neutral yttrium chelates. Thermodynamic alkene bind-
ing energies to zirconium are expected to be greater
than to yttrium. We have determined thermodynamic
binding energies for alkene complexation in yttrium
chelates by observing an equilibrium between bound
and free alkene. We are searching for similar zirconium
systems that will permit determination of alkene bind-
ing equilibrium constants. If successful, this will allow
direct comparison of alkene binding energies in related
zirconium and yttrium systems.
The barrier for site epimerization at zirconium (14.4
kcal mol-1) was also found to be higher than in the
corresponding yttrium complexes (9.6 kcal mol-1). This
can be partially attributed to alkene dissociation being
the first step in this process, but the difference between
the barriers for alkene dissociation and inversion is also
greater for the zirconium (3.2 kcal mol-1) complexes
than the yttrium analogues (2.0 kcal mol-1). This
implies that the second step in site epimerization,
inversion of stereochemistry at the zirconium center,
also has a higher barrier than its yttrium relative
(Figure 1). The barrier for site epimerization may arise
from several different sources: (1) there may be an
intrinsic barrier to inversion;22 (2) inversion may require
the loss of a stabilizing agostic interaction;23 (3) rotation
about the M-CH2 bond may be inhibited by steric
repulsion between the alkyl chain and the Cp* ligands;
and in the case of cationic zirconium, (4) inversion may
require the loss of stabilizing interactions with solvent
or tight ion-pairs.
Having ruled out the involvement of B(C6F5)4- in the
dynamic processes involving zirconium-alkyl-alkene
chelates, we turned our attention to the CH3B(C6F5)3
anion. This anion is of considerable interest, because
addition of B(C6F5)3 to methyl-zirconocenes usually
results in the formation of Zr-CH3-B(C6F5)3-bridged
species, even in the presence of bulky Cp* ligands.20 To
determine whether the CH3B(C6F5)3- would displace the
chelated alkene in favor of an ion-pairing interaction,
we synthesized [(C6H5)2CH3NH][MeB(C6F5)3] (13). Pro-
tonation of zirconacycle 7 with 13 gave two isomers of
Cp*2Zr[η1,η2-CH2CH(CH3)CH2CHdCH2][MeB(C6F5)3]
(14a and 14b). The 13C NMR chemical shift of the
methyl group attached to boron was 10 ppm, charac-
teristic of the free anion, with no interaction between
the methyl group and the zirconium center. All other
aspects of the NMR data are similar to those of the
related B(C6F5)4 ion pairs 8a and 8b. The barriers
associated with alkene decomplexation and inversion
were found to be unaffected by the change in counter-
anion. The binding of the chelated alkene in preference
to the methylborate anion for our Cp*2Zr systems
contrasts with a preference for Zr-CH3B ion-pairing
over alkene complexation in the sterically less crowded
Cp2Zr system, which was predicted on the basis of DFT
calculations.21
Discu ssion
Our earlier direct observation of d0 yttrium-alkyl-
alkene chelates established the viability of d0 metal-
alkyl-alkene complexes, which have been proposed as
intermediates in metallocene-catalyzed alkene polymer-
izations. The current observations of cationic d0 zirco-
nium-alkyl-alkene chelates are more closely related
to catalytic systems. For the yttrium-alkyl-alkene
chelates, alkene coordination resulted in shifts of the
secondary vinyl 1H and 13C NMR chemical shifts to
higher frequency, which were attributed to polarization
(22) DFT and ab initio calculations predict a planar structure for
methyl complexes of d0 metallocenes such as Cp2ScCH3. For ethyl and
propylmetallocene complexes, the d0 metal center is predicted to adopt
a
pyramidal geometry in order to accommodate an energetically
favorable â-CH agostic interaction. (a) Ziegler, T.; Folga, E.; Berces,
A. J . Am. Chem. Soc. 1993, 115, 636. (b) Woo, T. K.; Fan, L.; Ziegler,
T. Organometallics 1994, 13, 2252. (c) Yoshida, T.; Koga, N.; Moro-
kuma, K. Organometallics 1995, 14, 746. (d) Weiss, H.; Ehrig, M.;
Ahlrichs, R. J . Am. Chem. Soc. 1994, 116, 4919. (e) Kawamura-
Kuribayashi, H.; Koga, N.; Morokuma, K. J . Am. Chem. Soc. 1992,
114, 8687.
(23) (a) Brookhart, M.; Green, M. L. H. J . Organomet. Chem. 1983,
250, 395. (b) Brookhart, M.; Green, M. L. H.; Wong, L. Prog. Inorg.
Chem. 1988, 36, 1. (c) Grubbs, R. H.; Coates, G. W. Acc. Chem. Res.
1996, 29, 85.
(19) An attempt to use a nonchlorinated solvent, toluene-d8, was
unsuccessful. We believe that 8a and 8b form, but are insoluble in
toluene at low temperature, where only 11 is observed by NMR.
(20) Yang, X.; Stern, C. L.; Marks, T. J . J . Am. Chem. Soc. 1994,
116, 10015.
(21) Chan, M. S. W.; Vanka, K.; Pye, C. C.; Ziegler, T. Organome-
tallics 1999, 18, 4624.

Measurement of Barriers for Alkene Dissociation
Organometallics, Vol. 19, No. 19, 2000 3975
F igu r e 1. Free energy diagram for the barrier to alkene dissociation and site epimerization for [Cp*2ZrCH2CH(CH3)CH2-
CHdCH2][B(C6F5)4] (8a and 8b).
Sch em e 9
Cs-symmetric metallocene catalysts produce highly
syndiotactic polypropylene by a proposed mechanism in
which stereospecific propene insertion is much more
rapid than site epimerization (Scheme 2). Detailed
understanding of syndiotactic polymerizations requires
knowledge of the relative rates of alkene insertion and
site epimerization. We have now determined that the
barrier for site epimerization in our zirconium com-
plexes is 14.4 kcal mol-1. In future studies, we will
attempt to measure the barrier for insertion of propene
into a related cationic zirconium alkyl complex so that
a quantitative comparison of the relative rates can be
made.24
To understand ligand control of stereochemistry in
metallocene-catalyzed polymerizations, it is important
to know whether stereochemistry is controlled by ir-
reversible alkene coordination or by rate-determining
plexation is at least 104 faster than reversible alkene
alkyl migration to a reversibly bound alkene. In earlier
insertion to give a strained cyclobutylmethyl zirconium
intermediate.
studies of yttrium complexes, we used isotopic scram-
bling of Cp*2Y(η1,η2-CD2CH2CH2CHdCD2) to determine
the rate of reversible alkene insertion to give a strained
cyclobutylmethyl yttrium intermediate. The barrier for
this process was 14.4 kcal mol-1, indicating that alkene
insertion was 108 times slower than alkene dissociation.
We would like to know the related relative rates for the
zirconium complexes reported here. In 8, intramolecular
insertion processes interchange the environments of the
terminal vinyl hydrogens (Scheme 9). However, in-
tramolecular alkene insertion was too slow to give rise
to line broadening of the terminal vinyl 1H NMR
resonances. Saturation transfer experiments, which are
capable of measuring slower rates than coalescence
techniques, showed no evidence for interconversion of
the terminal vinyl proton resonances at -10 °C. This
implies an insertion rate of less than 0.35 s-1. At this
point, we are only able to estimate that alkene decom-
Alkene binding to the d0 metal center is fairly weak
as a result of the inability of the metal to back-donate
electron density into the π* orbitals of the alkene. As a
result, many σ donors can compete with the alkene for
binding to the metal center. We have found that THF,
dimethylaniline, tributylamine, and diisopropyl(ethyl)-
amine are all better donors to the electrophilic zirconium
center than the alkene chelate. In contrast, diphenyl-
(methyl)amine, a more sterically hindered and less
Lewis basic amine, did not displace the alkene chelate.
For future studies of the thermodynamics of alkene
binding, we are seeking a Lewis base that binds
competitively with the alkene.
Exp er im en ta l Section
[(C6H5)2CH3NH][B(C6F 5)4]. A solution of [(C6H5)2CH3NH]-
Cl (2.56 g, 11.7 mmol) in 70 mL of CH2Cl2 was added via
cannula to a solution of Li[B(C6F5)4] (11.8 g, 17.0 mmol) in 50
mL of CH2Cl2. The resulting white slurry was stirred for 3 h,
(24) For a related yttrium system, we have measured the relative
barriers of site epimerization and alkene insertion. Casey, C. P.; Lee,
T.-Y.; Carpenetti, D. W., II. Manuscript in preparation.

3976 Organometallics, Vol. 19, No. 19, 2000
Casey and Carpenetti
filtered, and washed with CH2Cl2. Solvent was evaporated
from the combined filtrate under vacuum. The resulting solid
was dissolved in a minimal amount of CH2Cl2 (50 mL), and
hexane was added in small increments to give a light gray
precipitate, which was isolated by filtration and dried under
vacuum at 100 °C overnight to give [(C6H5)2CH3NH][B(C6F5)4]
(8.0 g, 85%). 1H NMR (500 MHz, CD2Cl2): δ 3.84 (s, 6H, CH3),
7.41 (m, 4H, Ph), 7.65 (m, 6H, Ph), 8.61 (br s, 1H, NH).
Cp *2Zr [CH2CH(CH2CHdCH2)CH2] (7) was prepared as
reported by Stryker.12 1H NMR (500 MHz, CD2Cl2): δ -0.23
(m, ZrCH2CH), 0.69 (dd, J ) 11.6, 10.6 Hz, ZrCHH), 1.11 (dd,
J ) 11.6, 9.2 Hz, ZrCHH), 1.66, 1.80 (s, C5Me5’s), 2.41 (tm, J
) 7.0 Hz CH2CHdCH2), 5.14 (ddt, J ) 10.1, 2.5, 1.2 Hz, CHd
CHH), 5.25 (ddt, J ) 17.1, 2.5, 1.2 Hz, CHdCHH), 6.29 (ddt,
J ) 17.1, 10.1, 7.0 Hz, CHdCH2). 13C NMR (gated decoupled,
126 MHz, CD2Cl2): δ 11.0 (q, J ) 125 Hz, C5Me5), 12.0 (q, J )
125 Hz, C5Me5), 14.4 (br d, J ) 127 Hz ZrCH2CH), 44.5 (t, J )
127 Hz, CH2CHdCH2), 60.3 (t, J ) 129 Hz, ZrCH2), 113.9 (t,
J ) 153 Hz, CHdCH2), 115.5 (C5Me5), 116.2 (C5Me5), 141.2
(d, J ) 149 Hz, CHdCH2).
°C. The tube was shaken briefly at -78 °C to give a bright
orange solution and was inserted into the precooled probe of
the NMR spectrometer. The NMR spectra showed a 2.2:1
mixture of isomers of Cp* Zr[η1,η2-CH2CH(CH3)CH2CHdCH2]-
2
[B(C6F5)4] (8a and 8b) and (C6H5)2CH3N (11), whose NMR
spectra at various temperatures are given below. More dilute
CD2Cl2 solutions and a solution in C6D5Cl were also prepared.
Cp *2Zr [η1,η2-CH2CH(CH3)CH2CHdCH2][B(C6F 5)4] (8a ,
m a jor isom er ). 1H NMR (500 MHz, CD2Cl2, -78 °C): δ -0.83
(br d, ZrCH2CH), 0.85 (br d, J ) 7 Hz, ZrCH2), 1.96 (br s,
C5Me5), 1.98 (br s, C5Me5), 2.44 (br d, J cis ) 10.1 Hz, dCHH),
5.43 (br d, J trans ) 17.1 Hz, dCHH), 7.89 (m, J ) 17.1, 10.1
Hz, CHdCH2); resonances for CH3 and CH2CHdCH2 were
observed by TOCSY1D NMR at 1.70 and 2.20 but were not
assigned. 13C NMR (gated decoupled, 126 MHz, CD2Cl2, -78
°C): δ 10.7 (q, J ) 127 Hz, C5Me5), 10.8 (q, J ) 127 Hz,
C5Me5), 21.3 (q, J ) 122 Hz, CH3), 26.4 (t, J ) 123 Hz,
CH2CHdCH2), 31.2 (d, J ) 120 Hz, ZrCH2CH)), 75.1 (t, J )
119 Hz, ZrCH2), 113.7 (t, J ) 155 Hz, CHdCH2), 123.8 (s,
C5Me5), 123.9 (s, C5Me5), 163.0 (d, J ) 153 Hz, CHdCH2), 121.0
(br s, BCipso), 135.1 (d, J CF ) 238 Hz, CF), 137.0 (d, J CF ) 243,
CF), 146.8 (d, J CF ) 239 Hz, CF). 19F NMR (416.5 MHz, CD2-
Cl2, -78 °C): δ -135.9 (s, 8F), -166.2 (t, J FF ) 19.7 Hz, 4F),
-170.2 (s, 8F).
Rea ction of Cp *2Zr [CH2CH(CH2CHdCH2)CH2] (7) w ith
B(C6F 5)3. CD2Cl2 (0.5 mL) was condensed into a resealable
NMR tube containing 7 (20 mg, 0.045 mmol) and B(C6F5)3 (23.0
mg, 0.045 mmol) at -78 °C. The tube was shaken briefly at
-78 °C to give a bright orange solution and was inserted into
the precooled probe of the NMR spectrometer. The NMR
Cp *2Zr [η1,η2-CH2CH(CH3)CH2CHdCH2][B(C6F 5)4] (8b,
m in or isom er ). 1H NMR (500 MHz, CD2Cl2, -78 °C): δ -1.12
(br d, ZrCH2CH), 0.83 (br d, J ) 7 Hz, ZrCH2), 1.96 (br s,
C5Me5), 1.98 (br s, C5Me5), 2.18 (br d, J cis ) 12.5 Hz, dCHH),
5.71 (br d, J trans ) 17.8 Hz, dCHH), 7.49 (m, J ) 17.8, 12.5
Hz, CH)CH2); resonances for CH3 and CH2CHdCH2 were
observed by TOCSY1D NMR at 2.25 and 1.85 but were not
assigned. 13C NMR (gated decoupled, 126 MHz, CD2Cl2, -78
°C): δ 11.0 (q, J ) 127 Hz, C5Me5), 11.6 (q, J ) 127 Hz, C5Me5),
22.0 (q, J ) 128 Hz, CH3), 26.1 (t, J ) 122 Hz, CH2CHdCH2),
29.5 (d, J ) 120 Hz, ZrCH2CH), 72.6 (t, J ) 129 Hz, ZrCH2),
110.6 (t, J ) 157 Hz, CHdCH2), 124.2 (s, C5Me5), 124.6 (s,
C5Me5), 166.1 (d, J ) 152 Hz, CHdCH2), 121.0 (br s, BCipso),
135.1 (d, J CF ) 238 Hz, CF), 137.0 (d, J CF ) 243, CF), 146.8 (d,
J CF ) 239 Hz, CF). 19F NMR (416.5 MHz, CD2Cl2, -78 °C): δ
-135.9 (s, 8F), -166.2 (t, J FF ) 19.7 Hz, 4F), -170.2 (s, 8F).
(C6H5)2NCH3 (11). 1H NMR (500 MHz, CD2Cl2, -78 °C): δ
3.18 (s, NCH3), 7.09 (m, 4H, Ph), 7.33 (m, 6H, Ph). 13C NMR
(gated decoupled, 126 MHz, CD2Cl2, -78 °C): δ 41.5 (q, J )
139 Hz, NCH3), 119.4 (br, m-Ph), 123.1 (br, p-Ph), 128.9 (dd,
J ) 159, 7 Hz, o-Ph), 145.3 (s, ipso-Ph).
spectra showed a 1.8:1 mixture of isomers of Cp* Zr[η1,η2-CH2-
2
CH[CH2B(C6F5)3]CH2CHdCH2] (2a and 2b) whose NMR spec-
tra at various temperatures are given below.
Cp*2Zr [η1,η2-CH2CH[CH2B(C6F5)3]CH2CHdCH2] (2a, m a-
jor isom er ). 1H NMR (500 MHz, CD2Cl2, -82 °C): δ -0.84
(br d, ZrCH2CH), 0.85 (br d, J ) 7 Hz, ZrCH2), 1.96 (br s,
C5Me5), 1.98 (br s, C5Me5), 2.44 (br d, J cis ) 10.1 Hz, dCHH),
5.85 (br d, J trans ) 17.1 Hz, dCHH), 7.92 (m, J ) 17.1, 10.1
Hz, CH)CH2); resonances for CH2B and CH2CHdCH2 were
observed by TOCSY1D NMR at 2.18 (1H), 2.04 (1H), and 1.72
(2H) but were not assigned. 13C NMR (gated decoupled, 126
MHz, CD2Cl2, -82 °C): δ 11.3 (q, J ) 127 Hz, C5Me5), 11.4 (q,
J ) 127 Hz, C5Me5), 26.9 (t, J ) 122 Hz, BCH2), 27.7 (d, J )
127 Hz, ZrCH2CH), 43.0 (t, J ) 135 Hz, CH2CHdCH2), 75.7
(t, J ) 119 Hz, ZrCH2), 114.9 (t, J ) 154 Hz, CH)CH2), 124.3
(s, C5Me5), 124.4 (s, C5Me5), 163.6 (d, J ) 153 Hz, CHdCH2),
118.8 (br s, BCipso), 136.1 (d, J CF ) 247 Hz, CF), 138.6 (d, J CF
) 262, CF), 146.9 (d, J CF ) 238 Hz, CF).
Cp*2Zr [η1,η2-CH2CH[CH2B(C6F5)3]CH2CHdCH2] (2b, m i-
1
n or isom er ). H NMR (500 MHz, CD2Cl2, -82 °C): δ -1.13
Cp *2Zr [η1,η2-CH2CH(CH3)CH2CHdCH2] (r a p id ly equ ili-
br a tin g m ixtu r e of 8a a n d 8b a t -38 °C). 1H NMR (500
MHz, CD2Cl2, -38 °C): δ -0.85 (br, ZrCH2CH), 0.84 (d, J ) 7
Hz, ZrCH2), 2.00 (s, C5Me5), 2.02 (s, C5Me5), 2.40 (br d, J )
10.0 Hz, CHdCHH), 5.82 (br d, J ) 17.3, CHdCHH), 7.84 (br,
CHdCH2). Resonances corresponding to CH2CHdCH2 and CH3
were observed at δ 2.33 and 1.84 but were not assigned. 1H
NMR (500 MHz, C6D5Cl, -38 °C): δ -0.92 (br, ZrCH2CH), 1.08
(d, J ) 7 Hz, ZrCH2), 1.80 (s, C5Me5), 1.82 (s, C5Me5), 2.63 (br
d, J ) 10.0 Hz, CHdCHH), 5.27 (br d, J ) 17.3, CHdCHH),
7.56 (br, CHdCH2). Resonances corresponding to CH2CHdCH2
and CH3 were observed at δ 2.24 and 1.92 but were not
assigned.
Cp *2Zr [η1,η2-CH2CH(CH3)CH2CHdCH2] (r a p id ly equ ili-
br a tin g m ixtu r e of 8a a n d 8b a t 2 °C). 1H NMR (500 MHz,
CD2Cl2, 2 °C): δ -0.85 (br, ZrCH2CH), 0.84 (br, ZrCH2), 2.05
(s, C5Me5), 2.46 (br, CHdCHH), 5.78 (br, CHdCHH), 7.79 (br,
CHdCH2). Resonances corresponding to CH2CHdCH2 and CH3
were obscured by decomposition products and were not ob-
served. 1H NMR (500 MHz, C6D5Cl, 2 °C): δ -0.92 (br,
ZrCH2CH), 1.08 (br, ZrCH2), 1.90 (s, C5Me5), 2.67 (br, CHd
CHH), 5.20 (br, CHdCHH), 7.49 (br, CHdCH2). Resonances
corresponding to CH2CHdCH2 and CH3 were obscured by
decomposition products and were not observed.
(br d, ZrCH2CH), 0.83 (br d, J ) 7 Hz, ZrCH2), 1.94 (br s,
C5Me5), 1.96 (br s, C5Me5), 2.47 (br d, J cis ) 12.5 Hz, dCHH),
5.73 (br d, J trans ) 17.8 Hz, dCHH), 7.54 (m, J ) 17.8, 12.5
Hz, CHdCH2); resonances for BCH2 and CH2CHdCH2 were
observed by TOCSY1D NMR at 2.45 (1H), 2.16 (1H), and 1.68
(2H) but were not assigned. 13C NMR (gated decoupled, 126
MHz, CD2Cl2, -82 °C): δ 11.6 (q, J ) 127 Hz, C5Me5), 12.0 (q,
J ) 127 Hz, C5Me5), 30.0 (t, J ) 135 Hz, CH2B), 31.7 (d, J )
124 Hz, ZrCH2CH), 38.7 (t, J ) 135 Hz, CH2CHdCH2), 73.2
(t, J ) 119 Hz, ZrCH2), 114.2 (t, J ) 158 Hz, CH)CH2), 124.7
(s, C5Me5), 125.1 (s, C5Me5), 166.7 (d, J ) 151 Hz, CHdCH2),
118.8 (br s, BCipso), 136.1 (d, J CF ) 247 Hz, CF), 137.6 (d, J CF
) 262, CF), 146.9 (d, J CF ) 238 Hz, CF).
Cp *2Zr [η1,η2-CH2CH[CH2B(C6F5)3]CH2CHdCH2] (2, r a p -
id ly equ ilibr a tin g m ixtu r e of 2a a n d 2b a t -32 °C). 1H
NMR (500 MHz, CD2Cl2, -32 °C): δ 7.79 (m, J ) 17.3, 10.0
Hz, CHdCH2), 5.82 (br d, J ) 17.3, CHdCHH), 2.47 (br d, J
) 10.0 Hz, CHdCHH), 2.03 (s, C5Me5), 2.01 (s, C5Me5), 0.85
(d, J ) 7 Hz, ZrCH2), -0.86 (br d, J ) 10.6 Hz, ZrCH2CH).
Resonances corresponding to CH2CHdCH and BCH2 were
2
observed at δ 2.33, 2.11, and 1.74 but were not assigned.
Rea ction of Cp *2Zr [CH2CH(CH2CHdCH2)CH2] (7) w ith
[(C6H5)2(CH3)NH][B(C6F 5)4]. CD2Cl2 (0.5 mL) was condensed
into a resealable NMR tube containing 7 (20 mg, 0.045 mmol)
and [(C6H5)2(CH3)NH][B(C6F5)4] (39.0 mg, 0.045 mmol) at -78
{C p *2 Zr [N (C H 3 )2 C 6 H 5 ]C H 2 C H (C H 3 )C H 2 C H dC H 2 }-
[B(C6F 5)4] (10). CD2Cl2 (0.5 mL) was condensed into a

Measurement of Barriers for Alkene Dissociation
Organometallics, Vol. 19, No. 19, 2000 3977
resealable NMR tube containing 7 (20 mg, 0.045 mmol) and
[C6H5(CH3)2NH][B(C6F5)4] (36 mg, 0.045 mmol) at -78 °C. The
tube was shaken briefly at -78 °C to give a yellow solution of
10. 1H NMR (500 MHz, CD2Cl2, -78 °C): δ 0.18 (br, ZrCH2CH),
0.66 (br d, J ) 7 Hz, ZrCH2CHCH3), 0.82 (br d, ZrCH2), 1.86
(s, C5Me5), 2.16 (m, CH2CHdCH2), 3.65 (s, N(CH3)2), 4.82 (br
d, J cis ) 10.1 Hz, CHdCHH), 4.84 (br d, J trans ) 17.1 Hz, CHd
CHH), 5.70 (dd br t, J ) 17.1, 10.1, CH)CH2).
Su p p or tin g In for m a tion Ava ila ble: General experimen-
tal, preparations of [C6H5(CH3)2NH][B(C6F5)4], [(C6H5)2CH3NH]-
[CH3B(C6F5)3] (13), [C6H5(CH3)3N][MeB(C6F5)3], [C6H5(CH3)3N]-
[B(C6F5)4], Cp*2Zr(THF)[CH2CH(CH2B-(C6F5)3)CH2CHdCH2]
(9), Cp*2Zr(THF)[CH2CH(CH3)CH2CHdCH2][B(C6F5)4] (11),
{Cp*2Zr[N(CH2CH2CH2CH3)3]CH2CH(CH3)CH2CHdCH2}-
[B(C6F5)]4, {Cp*2Zr[N(CHMe2)2Et]CH2CH(CH3)CH2CHdCH2}-
[B(C6F5)]4, reaction of Cp*2Zr[CH2CH(CH2CHdCH2)CH2] (7)
with [(C6H5)2(CH3)NH][MeB(C6F5)3] (13), and temperature-
dependent 1H NMR spectra of 8a and 8b. This material is
available free of charge via the Internet at http://pubs.acs.org.
Ack n ow led gm en t. Financial support from the Na-
tional Science Foundation (CHE-9972183) is gratefully
acknowledged. Grants from the NSF (CHE-9629688) for
the purchase of NMR spectrometers are acknowledged.
We thank Dr. Charles Fry for helpful discussions.
OM9908940