Mechanistic study of β-methyl and β-hydrogen elimination in the zirconocene compounds Cp′2ZrR(μ-CH3)B(C 6F5)3 (Cp′ = Cp, Cp*; R = CH 2Me3, CH2CHMe2)

Article abstract of DOI:10.1021/om0504710

A kinetics study of β-methyl elimination reactions of the compounds Cp₂ZrNp(μ-Me)B-(C₆F₅)₃ (in various solvents) and Cp*₂ZrNp(μ-Me)B(C₆F ₅)₃ (in CD₂Cl₂) shows that the reactions are accelerated by polar solvents and by steric crowding in the Cp* system, results consistent with previous findings and conclusions that the methyl migrations are accompanied in a concerted process by borate anion departure from the inner coordination sphere of the metal ion. Surprisingly, however, the isobutyl compound Cp*₂Zr(i-Bu)(μ-Me)B(C ₆F₅)₃, which is expected to undergo /?-methyl elimination by analogy with an extensive literature on chain transfer processes during propylene polymerization, undergoes very rapid β-hydrogen elimination only.

Full text of DOI:10.1021/om0504710

Organometallics 2005, 24, 6005-6012
6005
Mechanistic Study of â-Methyl and â-Hydrogen
Elimination in the Zirconocene Compounds
Cp′2ZrR(µ-CH3)B(C6F5)3 (Cp′ ) Cp, Cp*; R ) CH2CMe3,
CH2CHMe2)
Ping Yang and Michael C. Baird*
Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada
Received June 9, 2005
A kinetics study of â-methyl elimination reactions of the compounds Cp2ZrNp(µ-Me)B-
(C6F5)3 (in various solvents) and Cp*2ZrNp(µ-Me)B(C6F5)3 (in CD2Cl2) shows that the reactions
are accelerated by polar solvents and by steric crowding in the Cp* system, results consistent
with previous findings and conclusions that the methyl migrations are accompanied in a
concerted process by borate anion departure from the inner coordination sphere of the metal
ion. Surprisingly, however, the isobutyl compound Cp*2Zr(i-Bu)(µ-Me)B(C6F5)3, which is
expected to undergo â-methyl elimination by analogy with an extensive literature on chain
transfer processes during propylene polymerization, undergoes very rapid â-hydrogen
elimination only.
There has in recent years been explosive growth in
the use of zirconocene compounds as homogeneous
catalysts for coordination (Ziegler-Natta) polymeriza-
tion of alkenes. Among the most successful catalysts are
the formally 14-electron, cationic complexes [Cp′2ZrR]+X-
(Cp′ ) functionalized cyclopentadienyl, R ) alkyl, H;
X- ) weakly coordinating anion), which contain a very
labile anionic ligand and an alkyl group.1 A great deal
of progress has also been made in the development of
theoretical models of alkene polymerization, with con-
siderable interest being shown in the individual steps
involved in the initiation, chain propagation, termina-
tion, and chain transfer steps.2 However, while it is clear
that good computational models have greatly enhanced
our mechanistic understanding of metallocene-induced
polymerization reactions, there remains a need to gain
clearer insight into the complex solution equilibria and
processes which are often involved. There is therefore
also a very great need for more extensive experimental
thermodynamic and kinetic data for purposes of calibra-
tion and confirmation of computational models,2 includ-
ing activation energies for â-hydrogen1 and the much
rarer â-methyl elimination reactions,2h,3,4 important in
chain transfer and chain termination processes.
While â-hydrogen migration to metal (â-elimination)
or directly to incoming monomer molecule is normally
the predominant mode of chain transfer during metal-
locene-catalyzed alkene polymerization processes,1 â-
methyl elimination is generally dominant during, for
example, propylene polymerization by sterically crowded
catalysts such as the bis-η5-C5Me5-metal (Cp*2M) sys-
tem.3 The reasons for the variations in behavior are not
clear, as â-hydrogen elimination seems clearly to be
significantly preferred thermodynamically.5 However,
(1) For useful reviews see: (a) Mo¨hring, P. C.; Coville, N. J. J.
Organomet. Chem. 1994, 479, 1. (b) Gupta, V. K.; Satish, S.; Bhardwaj,
I. S. J. Macromol. Sci., Rev. Macromol. Chem. Phys. 1994, C34, 439.
(c) Brintzinger, H. H.; Fischer, D.; Mu¨lhaupt, R.; Rieger, B. Waymouth,
R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143. (d) Bochmann, M.
J Chem. Soc., Dalton Trans. 1996, 255. (e) Kaminsky, W.; Arndt, M.
Adv. Polym. Sci. 1997, 127, 143. (f) Chien, J. C. W. In Topics in
Catalysis; Marks, T. J., Stevens, J. C., Eds.; Baltzer AG, Science
Publishers, 1999; Vol. 7, p 23. (g) Alt, H. G.; Ko¨ppl, A. Chem. Rev.
2000, 100, 1205. (h) Coates, G. W. Chem. Rev. 2000, 100, 1223. (i)
Resconi, L.; Cavallo, L.; Fait, A.; Piemontesi, F. Chem. Rev. 2000, 100,
1253. (j) Angermund, K.; Fink, G.; Jensen, V. R.; Kleinschmidt, R.
Chem. Rev. 2000, 100, 1457. (k) Scheirs J., Kaminsky, W., Eds.
Metallocene-based Polyolefins; John Wiley and Sons: U.K, 1999. (l)
Chen, E. Y.-X.; Marks, T. J. Chem. Rev. 2000, 100, 1391.
(2) (a) Rappe´, A. K.; Skiff, W. M.; Casewit, C. J. Chem. Rev. 2000,
100, 1435. (b) Lohrenz, J. C. W.; Bu¨hl, M.; Weber, M.; Thiel, W. J.
Organomet. Chem. 1999, 592, 11. (c) Vanka, K.; Chan, M. S. W.; Pye,
C. C.; Ziegler, T. Organometallics 2000, 19, 1841. (d) Chan, M. S. W.;
Ziegler, T. Organometallics 2000, 19, 5182. (e) Lanza, G.; Fragala`, I.
L.; Marks, T. J. Organometallics 2001, 20, 4006. (f) Nifant’ev, I. E.;
Ustynyuk, L. Y.; Laikov, D. N. Organometallics 2001, 20, 5375. (g)
Vanka, K.; Ziegler, T. Organometallics 2001, 20, 905. (h) Moscardi,
G.; Resconi, L.; Cavallo, L. Organometallics 2001, 20, 1918. (i) Borrelli,
M.; Busico, V.; Cipullo, R.; Ronca, S.; Budzelaar, P. H. M. Macromol-
ecules 2002, 35, 2835. (j) Talarico, G.; Blok, A. N. J.; Woo, T. K.; Cavallo,
L. Organometallics 2002, 21, 4939. (k) Borrelli, M.; Busico, V.; Cipullo,
R.; Ronca, S.; Budzelaar, P. H. M. Macromolecules 2003, 36, 8171.
(3) Watson, P. L.; Roe, D. C. J. Am. Chem. Soc. 1982, 104, 6471. (b)
Eshuis, J. J. W.; Tan, Y. Y.; Teuben, J. H. J. Mol. Catal. 1990, 62,
277. (c) Mise, T.; Kageyama, A.; Miya, S.; Yamazaki, H. Chem. Lett.
1991, 1525. (d) Yang, X.; Stern, C. L.; Marks, T. J. Angew. Chem., Int.
Ed. Engl. 1992, 31, 1375. (e) Resconi, L.; Piemontesi, F.; Franciscono,
G.; Abis, L.; Fiorani, T. J. Am. Chem. Soc. 1992, 114, 1025. (f) Eshuis,
J. J. W.; Tan, Y. Y.; Meetsma, A.; Teuben, J. H.; Renkema, J.; Evans,
G. G. Organometallics 1992, 11, 362. (g) Yang, X.; Stern, C. L.; Marks,
T. J. J. Am. Chem. Soc. 1994, 116, 10015. (h) Repo, T.; Jany, G.;
Hakala, K.; Klinga, M.; Polamo, M.; Leskela¨, M.; Rieger, B. J.
Organomet. Chem. 1997, 549, 177. (i) Resconi, L.; Camurati, I.;
Sudmeijer, O. Top. Catal. 1999, 7, 145. (j) Weng, W.; Markel, E. J.;
Dekmezian, A. H. Macromol. Rapid Commun. 2000, 21, 1103. (k)
Camurati, I.; Fait, A.; Piemontesi, F.; Resconi, L.; Tartarini, S. ACS
Symp. Ser. 2000, 760, 174. (l) Yang, P.; Baird, M. C., following paper
in this issue.
(4) Guo, Z.; Swenson, D. C.; Jordan, R. F. Organometallics 1994,
13, 1424. (b) Hajela, S.; Bercaw, J. E. Organometallics 1994, 13, 1147.
(c) Horton, A. D. Organometallics 1996, 15, 2675. (d) Beswick, C. L.;
Marks, T. J. J. Am. Chem. Soc. 2000, 122, 10358. (e) Lin, M.; Spivak,
G. J.; Baird, M. C. Organometallics 2002, 21, 2350. (f) Chirik, P. J.;
Dalleska, N. F.; Henling, L. M.; Bercaw, J. E. Organometallics 2005,
24, 2789. (g) Lin, M. Ph.D. Thesis, Queen’s University, 2002. (h) The
kinetics treatment in ref 4e unfortunately utilized an inappropriate
rate law, not that used here. The data presented in Table 2 for Cp2-
Hf(Np)(µ-Me)B(C6F5)3 have been recalculated using the correct rate
law and fortunately differ very little from those originally presented.
(5) Schock, L. E.; Marks, T. J. J. Am. Chem. Soc. 1988, 110, 7701.
10.1021/om0504710 CCC: $30.25 © 2005 American Chemical Society
Publication on Web 10/21/2005

6006 Organometallics, Vol. 24, No. 24, 2005
Yang and Baird
suggestions3c,e that steric interactions of the bulky Cp*
ligands with the â-methyl group of a growing polymer
chain might force the â-methyl group of the polymeryl
chain into the plane perpendicular to the Cp(centroid)-
Zr-Cp(centroid), as in A, seem generally to have found
acceptance.3f,g,4a
although ∆Hq for Cp2Hf(Np)(µ-Me)B(C6F5)3 is lower in
the more polar chlorobenzene. It was tentatively con-
cluded that the â-methyl migration reactions involve
concomitant loosening of the metal-borate linkages, but
clearly more information is needed. While the current
work was being completed, Chirik et al. reported kinetic
isotope effects in the range 1.28-1.49 for â-methyl
elimination reactions of a series of deuterated neopentyl
compounds, including Cp2Zr{CH2C(CD3)(CH3)2}(µ-CH3)B-
(C6F5)3 and Cp*2Zr{CH2C(CD3)(CH3)2}(µ-CH3)B(C6F5)3.4f
On the basis of these results, they deduced a transition
state γ-agostic interaction during the methyl migration
reaction, as might be anticipated in the orientation
shown in A, above. Interestingly, for all compounds
studied evidence was found of reversibility in the
deinsertion process.
We have expanded our earlier kinetics study of the
hafnium neopentyl compound Cp2Hf(Np)(µ-Me)B(C6F5)3,
opting to utilize zirconium compounds in this study
because of the extensive literature on the synthesis of
relevant zirconocene-type compounds,6 and because of
the preeminent position of zirconocene systems in
propylene polymerization.1 We wished to carry out a
broader study in which we would vary the substituents
on the cyclopentadienyl ring and also substitute the
neopentyl group with iso- and sec-butyl groups, in which
there would be a competition between â-hydrogen and
â-methyl elimination reactions. As mentioned above,
â-hydrogen elimination is generally the preferred mode
of chain transfer during metallocene-catalyzed alkene
polymerization processes,1 but â-methyl elimination
becomes dominant during, for example, propylene po-
lymerization by metallocene catalysts containing heavily
substituted cyclopentadienyl rings.3 We hoped to be able
to shed light on the factors controlling the mode of
migratory â-elimination reaction taken by various zir-
conocene systems, and although we were not completely
successful in our endeavors, we have extended the work
of refs 4d and 4e and have gained a clearer picture of
the activation barriers to â-methyl elimination. Inter-
estingly, we have also found conditions under which
chain transfer processes during propylene polymeriza-
tion by the Cp*2ZrMe(µ-Me)B(C6F5)3 catalyst system
preferentially involves â-hydrogen elimination rather
than the often-reported â-methyl elimination. This
aspect of our study is addressed separately.3l
In this conformation, the â-methyl group is in close
proximity to the LUMO, an orientation that presumably
would facilitate, via a γ-agostic interaction, â-methyl
rather than â-hydrogen migration. However, both com-
putational and experimental data for such processes are
in short supply. Indeed, while there have been reported
numerous experimental and computed activation energy
data for reversible migratory insertion reactions involv-
ing, for example, metallocene hydrides and alkenes,1
there has been reported very little information about
the analogous reactions involving alkyl groups.2h
A number of stoichiometric â-methyl elimination
reactions from neopentyl (Np) groups have been de-
scribed,4 and we4e and Beswick and Marks4d have
reported kinetics studies of a small number of â-methyl
elimination reactions of related zirconocene and hafno-
cene systems in toluene and chlorobenzene. In both
cases the net chemistry, of a type originally observed
by Horton,4c is illustrated in eq 1.
Cp′2MMe(Np) + B(C6F5)3 f
Cp′2M(Me)(µ-Me)B(C6F5)3 + CH2dCMe2 (1)
The reactions involve initial methyl abstraction from the
compounds Cp2HfMe(Np) and (1,2-Me2C5H3)2MMe(Np)
(M ) Zr, Hf) by B(C6F5)3 to give the zwitterionic inter-
mediates Cp2M(Np)(µ-Me)B(C6F5)3 and (1,2-Me2C5H3)2M-
(Np)(µ-Me)B(C6F5)3, and these exhibit varying degrees
of thermal stability depending on the metal, the nature
of the cyclopentadienyl ligand, and the solvent. All are,
however, readily induced to undergo â-methyl elimina-
tion to form Cp2MMe(µ-Me)B(C6F5)3 or (1,2-Me2C5H3)2-
MMe(µ-Me)B(C6F5)3, presumably via an undetected η2-
isobutene intermediate as in eqs 2 and 3.
Experimental Section
Cp2M(Np)(µ-Me)B(C6F5)3 f
General Comments. Syntheses were carried out under
purified argon using standard Schlenk line and glovebox
techniques. The deoxygenated solvents toluene, dichloro-
methane, diethyl ether, hexanes, and tetrahydrofuran were
dried by passing through activated Al2O3 columns, toluene-d8
and benzene-d6 by refluxing over sodium, and dichloromethane-
d2 and chlorobenzene-d5 by refluxing over calcium hydride. 1H
NMR spectra were run on Bruker AV400 or AV500 spectrom-
eters, chemical shifts being referenced using the residual
[Cp2MMe(η2-CH2dCMe2)][BMe(C6F5)3] (2)
[Cp2MMe(η2-CH2dCMe2)][BMe(C6F5)3] f
Cp2MMe(µ-Me)B(C6F5)3 + CH2dCMe2 (3)
The kinetics studies indicated that the chemistry
summed in eqs 2 and 3 is first order in the product of
eq 1 and is reversible for Cp2HfMe(µ-Me)B(C6F5)3 but
not for the 1,2-Me2C5H3-Zr and 1,2-Me2C5H3-Hf com-
pounds. In addition, the reaction of Cp2Hf(Np)(µ-Me)B-
(C6F5)3, at least, is seriously inhibited by ligands L,
which can coordinate preferentially to give complexes
of the type [Cp2M(L)Np][BMe(C6F5)3]; similar results
had been observed previously.4a,c The activation param-
eters for all three compounds are similar in toluene,
(6) (a) Negishi, E.; Nguyen, T.; Maye, J. P.; Choueiri, D.; Suzuki,
N.; Takahashi, T. Chem. Lett. 1992, 2367. (b) Labinger, J. A. In
Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Eds.;
Pergamon Press: Oxford, 1991; Chapter 3.9. (c) Chirik, P. J.; Day, M.
W.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1999, 121, 10308.
(d) Wendt, O. F.; Bercaw, J. E. Organometallics 2001, 20, 3891. (e)
Pool, J. A.; Bradley, C. A.; Chirik, P. J. Organometallics 2002, 21, 1271.
(f) Pool, J. A.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2003,
125, 2241.

Zirconocene Compounds Cp′2ZrR(µ-CH3)B(C6F5)3
Organometallics, Vol. 24, No. 24, 2005 6007
proton signals of the deuterated solvents. Molecular mechanics
calculations were carried out using PCModel V. 8.0 (Serena
Software, Bloomington, IN). Cp2ZrCl2 and Cp*2ZrCl2 were
purchased from Strem Chemicals; [Ph3C][B(C6F5)4] was pur-
chased from Asahi Glass Co. (Cp2ZrCl)2O,7a Cp2ZrMeCl,7b
neopentyllithium (NpLi),8 B(C6F5)3,9 Cp*2ZrMe2,10 and Cp*2-
ZrMe(i-Bu)11 were prepared via published procedures.
Synthesis of Cp2ZrMe(Np). To a mixture of 583 mg of Cp2-
ZrClMe (2 mmol) and 156 mg of NpLi (2 mmol) at -78 °C was
added 20 mL of THF. The solution was stirred at -78 °C for
a few minutes and allowed to warm to room temperature, and
then the solvent was removed. The resulting solid was
recrystallized from hexanes to give 385 mg of pale yellow,
crystalline product. Yield: 62%. 1H NMR (C6D6): δ 5.78 (s,
10H, Cp), 1.07 (s, 9H, CH2C(CH3)3), 0.48 (s, 2H, CH2C(CH3)3),
-0.06 (s, 3H, ZrCH3). Lit.4c (C6D6Br): δ 5.92 (Cp), 0.98 (CH2C-
(CH3)3), 0.42 (CH2C(CH3)3), -0.17 (Zr-CH3).
(CD2Cl2): δ 1.89 (Cp*), 0.61 (CH2CH(CH3)2), -0.33 (CH2CH-
(CH3)2), -0.83 (Zr-CH3).
Formation of the Complexes [Cp′2ZrR(µ-CH3)B(C6F5)3]
(Cp′ ) Cp, Cp*; R ) Np, i-Bu) and Studies of Their Thermal
Degradation. These compounds were typically formed in situ
by treating Cp′2ZrMeR with a slight excess of B(C6F5)3 in a
deuterated solvent at low temperature and were characterized
and their chemistry studied by VT NMR spectroscopy. As an
example, an NMR tube containing 14 mg of B(C6F5)3 (0.027
mmol) in 0.25 mL of C6D5Cl was cooled to -40 °C. A solution
of Cp2ZrMe(Np) (7.7 mg, 0.025 mmol) in 0.25 mL of C6D5Cl
was added slowly to the above solution, which turned pale
yellow during the addition. The sample was quickly placed in
an NMR probe precooled to -40 °C, and 1H NMR spectra (500
MHz) of Cp2ZrNp(µ-Me)B(C6F5)3 were obtained at various
temperatures. 1H NMR (C6D5Cl, -10 °C): δ 5.95 (s, 10H, Cp),
0.87 (s, 9H, Zr-CH2C(CH3)3), 1.30 (s, 2H, Zr-CH2C(CH3)3),
0.30 (br, 3H, CH3B). Lit.4c (toluene-d8, -25 °C): δ 5.50 (Cp),
1.08 (Zr-CH2C(CH3)3), 0.80 (ZrCH2C(CH3)3), 0.16 (CH3B). [Cp2-
ZrNp(µ-Me)B(C6F5)3] is thermally unstable above -10 °C,
Synthesis of Cp*2ZrMe(Np). To a mixture of 1 g of Cp*2-
ZrCl2 (2.3 mmol) and 0.45 g of NpLi (5.7 mmol) in a Schlenk
flask at -78 °C was added 40 mL of ethyl ether. The resulting
brownish suspension was allowed to slowly warm to room
temperature and turned yellow after stirring overnight. The
solvent was then removed under reduced pressure, and the
resulting solid material was extracted with 40 mL of hexanes
and filtered. The filtrate was pumped to dryness to give Cp*2-
ZrClNp as a yellow powder. A solution of the latter was
dissolved in 25 mL of THF at -78 °C, and 1 mL of a 3 M
solution of MeMgBr in ethyl ether (3 mmol) was added. The
solution was slowly warmed to room temperature and was
stirred overnight, the solvent was removed under reduced
pressure, and the resulting material was extracted with
hexanes and filtered. The volume of the filtrate was reduced
and the solution was cooled to -78 °C to give 0.89 g of yellow
product, which contained (NMR) Cp*2Zr(Np)2 and Cp*2ZrMe-
4c
giving Cp2ZrMe(µ-Me)B(C6F5)3 and isobutene.
The compound Cp*2ZrNp(µ-CH3)B(C6F5)3 was prepared
similarly. 1H NMR (CD2Cl2, -70 °C): δ 1.99 (s, 30H, Cp*), 1.11
(s, 9H, Zr-CH2C(CH3)3), 1.04 (s, 2H, Zr-CH2C(CH3)3), 0.38
(br, 3H, CH3B). Cp*2ZrNp(µ-CH3)B(C6F5)3 was found to be
thermally unstable above -70 °C, giving [Cp*2ZrMe(µ-Me)B-
(C6F5)3]4c and isobutene.
The compounds Cp2Zr(i-Bu)(µ-CH3)B(C6F5)3 and Cp*2Zr-
(i-Bu)(µ-CH3)B(C6F5)3 were prepared similarly, from Cp2ZrMe-
(i-Bu) or Cp*2ZrMe(i-Bu), with B(C6F5)3 in CD2Cl2 at -78 °C.
1H NMR for Cp2Zr(i-Bu)(µ-CH3)B(C6F5)3 (CD2Cl2, -40 °C): δ
6.35 (s, 10H, Cp), 2.29 (m, 1H, CH2CH(CH3)2), 1.23 (d, 2H,
CH2CH(CH3)2), 0.75 (d, 6H, CH2CH(CH3)2), 0.19(br, 3H, BMe).
1H NMR for [Cp*2Zr(i-Bu)(µ-CH3)B(C6F5)3] (CD2Cl2, -80 °C):
δ 1.90 (s, 30H, Cp*), 2.00 (m, 1H, CH2CH(CH3)2), 1.16 (d, 2H,
CH2CH(CH3)2), 0.62 (d, 6H, CH2CH(CH3)2), 0.30(br, 3H, BMe).
[Cp2Zr(i-Bu)(µ-CH3)B(C6F5)3] was found to be unstable above
-40 °C, giving isobutylene and, presumably, [Cp2ZrH(µ-CH3)B-
(C6F5)3] (δZr-H δ 7.31 in methylene chloride-d2), and [Cp*2Zr-
(i-Bu)(µ-CH3)B(C6F5)3] was found to be unstable above -60 °C,
1
(Np) in a 56:43 ratio. H NMR of Cp*2ZrMe(Np) (CD2Cl2): δ
1.89 (s, 30H, Cp*), 0.77 (s, 9H, CH2C(CH3)3), -0.13 (s, 2H,
CH2C(CH3)3), -0.74 (s, 3H, Zr-CH3). Lit.4c (C6D6): δ 1.80
(Cp*), 1.12 (CH2C(CH3)3), 0.03 (CH2C(CH3)3), -0.39 (Zr-CH3).
1H NMR of Cp*2ZrNp2 (CD2Cl2): δ 1.96 (s, 30H, Cp*), 1.02 (s,
18H, CH2C(CH3)3), -0.13 (s, 4H, CH2C(CH3)3). Attempts to
separate these two compounds failed because of close similari-
ties in solubilities.
giving isobutylene and [Cp*2ZrH(µ-CH3)B(C6F5)3]3d (δZr-H
7.30 in CD2Cl2; lit. 7.70 in benzene-d6).
δ
Synthesis of Cp2ZrMe(i-Bu). This compound has been
previously reported but with little information included,6a and
we have used a modified synthetic procedure. A solution of
1.2 mL of i-BuMgBr (2 M in Et2O, 2.4 mmol) was added to a
stirred and cooled (-78 °C) solution of 0.67 g of Cp2ZrMeCl
(2.4 mmol) in 25 mL of Et2O. The solution was then stirred
for 1 h as it warmed to room temperature, and the solvent
was removed under reduced pressure. The resulting sticky
solid was washed with hexanes and filtered, and the solvent
was removed from the filtrate under reduced pressure to give
Cp2ZrMe(i-Bu) as a thick yellow oily material. 1H NMR
(C6D6): δ 5.72 (s, 10H, Cp), 2.14 (m, 1H, CH2CH(CH3)2), 0.96
(d, 6H, CH2CH(CH3)2), 0.30 (d, 2H, CH2CH(CH3)2), -0.14 (s,
3H, Zr-CH3). The product could not be purified because of
thermal instability and contained up to ∼20% Cp2ZrMe2.
Results and Discussion
Syntheses and Attempted Syntheses of Cp′2ZrMe-
(R) (Cp′ ) Cp, Cp*; R ) CH2CMe3, CH2CHMe2,
CHMeEt). The compounds Cp2ZrMe(Np) and Cp*2-
ZrMe(Np) have been prepared previously,4c but we have
utilized somewhat modified synthetic procedures; both
1
exhibit H NMR spectra consistent with the literature
values, although the synthesis of Cp*2ZrMe(Np) was
accompanied by the formation also of Cp*2Zr(Np)2,
presumably because of a redistribution reaction. The
compound Cp*2ZrMe(i-Bu) has also been prepared
1
previously,11 and the H NMR spectrum confirmed its
identity. The compound Cp2ZrMe(i-Bu) was prepared
similarly and identified unambiguously by 1H NMR
spectroscopy, but it is thermally labile and as well could
not be separated from the byproduct Cp2ZrMe2. All
attempts to prepare the sec-butyl compounds Cp2ZrMe-
(sec-Bu) and Cp*2ZrMe(sec-Bu) and mixed Cp-Cp*
compounds of any kind resulted in mixtures that could
not be utilized.
Kinetics Studies of â-Methyl Elimination Reac-
tions of Cp′2ZrNp(µ-CH3)B(C6F5)3. Horton has previ-
ously shown that the compounds Cp2ZrNp(µ-CH3)B-
(C6F5)3 and Cp*2ZrNp(µ-CH3)B(C6F5)3 undergo â-methyl
elimination in toluene-d8 as in eqs 1-3, the former
Synthesis of Cp*2ZrMe(i-Bu). This compound was pre-
pared as in the literature.11 1H NMR (CD2Cl2): δ 1.86 (s, 30H,
Cp*), 1.67 (m, 1H, CH2CH(CH3)2), 0.60 (d, 6H, CH2CH(CH3)2),
-0.34 (d, 2H, CH2CH(CH3)2), -0.85 (s, 3H, Zr-CH3). Lit.11
(7) (a) Wailes, P. C.; Weigold, H. J. Organomet. Chem. 1970, 24,
405. (b) Wailes, P. C.; Weigold, H.; Bell, A. P. J. Organomet. Chem.
1971, 33, 181.
(8) Feldman, D.; Barbalata, A. Synthetic Polymers Technology,
Properties, Applications; Chapman and Hall: New York, 1996.
(9) Massey, A. G.; Park, A. J. J. Organomet. Chem. 1964, 2, 245.
(10) Manriquez, J. M.; McAlister, D. R.; Sanner, R. D.; Bercaw, J.
E. J. Am. Chem. Soc. 1978, 100, 2716.
(11) Casey, C. P.; Carpenetti, D. W. J. Organomet. Chem. 2002, 642,
120.

6008 Organometallics, Vol. 24, No. 24, 2005
Yang and Baird
Figure 1. Decay of Cp2ZrNp(µ-Me)B(C6F5)3 in C6D5Cl at 10 °C. *: Cp2ZrNp(µ-Me)B(C6F5)3, x: Cp2ZrMe(µ-Me)B(C6F5)3,
o: isobutylene.
reversibly at temperatures above 0 °C, the latter
instantaneously and apparently completely at -78 °C.4c
Somewhat surprisingly, perhaps, we were able to obtain
1
for the first time the H NMR spectrum of the inter-
mediate, Cp*2ZrNp(µ-CH3)B(C6F5)3, in CD2Cl2 at -70
°C; the previous study of the reaction of Cp*2ZrMe(Np)
with B(C6F5)3 resulted in the observation only of Cp*2-
ZrMe(µ-CH3)B(C6F5)3 (toluene-d8 at -75 °C).4c For
purposes of comparison, we note that the earlier kinetics
study on (1,2-Me2C5H3)2Zr(Np)(µ-Me)B(C6F5)3 and (1,2-
Me2C5H3)2Hf(Np)(µ-Me)B(C6F5)3 was carried out4d in
2
Figure 2. Plots of [(Ao - Ae)/(Ao + Ae)] ln[(Ao - AtAe)/
(AoAt - AoAe) vs time for the decomposition of Cp2ZrNp-
(µ-Me)B(C6F5)3 in C6D5Cl.
toluene-d8 in the temperature ranges -5 to 25 °C and
-15 to 15 °C, respectively, while Cp2Hf(Np)(µ-Me)B-
(C6F5)3 was studied in the temperature range 0 to 20
°C in toluene-d8 and in the temperature range -5 to 10
°C in C6D5Cl.4e,h
at δ 5.97 (Cp), 1.31 (CH2), and 0.86 (CMe) and the
appearance of the resonances of Cp2ZrMe(µ-Me)B(C6F5)3
at δ 5.85 (Cp) and 0.45 (Zr-CH3) and isobutene at δ
4.73 (CH2) and 1.60 (Me); the broad resonance of the
µ-Me group remained constant at δ 0.33. A rate study
of the â-Me elimination reaction was carried out in C6D5-
Cl by carefully monitoring the decay of the Cp signals
in the 1H NMR spectrum at the temperatures 5, 10, 15,
and 20 °C. Although the chemical shifts are slightly
temperature dependent, the reactions were quite clean,
as shown in Figure 1.
The kinetics studies carried out here all involved
NMR scale reactions in C6D5Cl, toluene-d8, or CD2Cl2,
depending on solubilities and the temperature range
required. As outlined in the Experimental Section, the
general procedure involved reacting solutions of each
of the compounds Cp′2ZrMe(R) (Cp′ ) Cp, Cp*; R ) CH2-
CMe3, CH2CHMe2) with 1 equiv of B(C6F5)3 to give the
compounds Cp′2Zr(R)(µ-Me)B(C6F5)3, which were identi-
fied by NMR spectroscopy. Decay of the µ-Me com-
1
pounds was then monitored by H NMR spectroscopy
Interestingly, we found that at all temperatures the
â-Me elimination reactions in C6D5Cl appeared to
eventually reach equilibrium, linear plots of [(Ao - Ae)/
(Ao + Ae)] ln[(Ao2 - AtAe)/(AoAt - AoAe) vs time (Figure
2)12 confirming that the reactions are reversible and
first-order with respect to the concentration of Cp2ZrNp-
(µ-Me)B(C6F5)3. The slopes give k1 for the â-Me elimina-
tion reaction, and rate constant data are provided in
Table 1. On the basis of the temperature dependence of
k1, an activation energy Ea of 19.4(0.9) kcal/mol was
calculated. Activation parameters for the â-Me elimina-
tion reaction were determined utilizing an Eyring plot,
shown in Figure 3; values for ∆Hq, ∆Sq, and ∆Gq (0 °C)
at four different temperatures. The temperature ranges
studied were unfortunately rather narrow, in all cases
only about 15 degrees, but formation of other decom-
position products became a factor during the longer
times required at lower temperatures and the reactions
were too rapid to be monitored satisfactorily at higher
temperatures.
The zwitterionic complex Cp2ZrNp(µ-Me)B(C6F5)3
formed cleanly and instantly when Cp2ZrMeNp was
reacted with a slight excess of B(C6F5)3 in C6D5Cl at -10
°C, at which temperature Cp2ZrNp(µ-CH3)B(C6F5)3 is
stable. Above -10 °C, however, Cp2ZrNp(µ-Me)B(C6F5)3
decomposed via â-Me elimination to form Cp2ZrMe(µ-
Me)B(C6F5)3, as indicated at, for example, 10 °C by
weakening of the resonances of Cp2ZrNp(µ-Me)B(C6F5)3
(12) Capellos, C.; Bielski; B. H. J. Kinetic Systems; Wiley-Inter-
science: New York, 1972; pp 41-43.

Zirconocene Compounds Cp′2ZrR(µ-CH3)B(C6F5)3
Organometallics, Vol. 24, No. 24, 2005 6009
Figure 3. Eyring plot for the decomposition of Cp2ZrNp-
(µ-Me)B(C6F5)3 in C6D5Cl.
Figure 5. Eyring plot for the decomposition of Cp2ZrNp-
(µ-Me)B(C6F5)3 in toluene-d8.
to the concentration of Cp2ZrNp(µ-Me)B(C6F5)3. Rate
constant data are presented in Table 1. On the basis of
the temperature dependence of k1, an activation energy
Ea of 22.9(0.9) kcal/mol was calculated. Activation
parameters for the â-Me elimination reaction were
determined utilizing an Eyring plot, shown in Figure
5; ∆Hq, ∆Sq, and ∆Gq (0 °C) were found to be 22.3(0.9)
kcal/mol, 8.5(3.0) cal/mol‚deg, and 20.0(0.9) kcal/mol,
respectively. Similar chemistry was observed when Cp2-
ZrNp(µ-CH3)B(C6F5)3 was treated with 1 equiv of
B(C6F5)3 in CD2Cl2, but side reactions precluded further
work.
2
Figure 4. Plots of [(Ao - Ae)/(Ao + Ae)] ln[(Ao - AtAe)/
(AoAt - AoAe) vs time for the decomposition of Cp2ZrNp-
(µ-Me)B(C6F5)3 in toluene-d8.
Table 1. First-Order Rate Constants k1 for the
â-Me Elimination of Cp2ZrNp(µ-Me)B(C6F5)3 in
C6D5Cl and Toluene-d8, and of
As mentioned above, the kinetics runs in both toluene-
d8 and C6D5Cl appeared to reach equilibrium such that
reasonable values of the equilibrium concentrations
could be obtained in order to utilize the rate law for the
processes. However, we did not attempt to determine
the equilibrium constants precisely by, for example,
adding excess isobutene at the end of each kinetics run,
as this approach with Cp2HfNp(µ-Me)B(C6F5)3 resulted
in the formation of polyisobutene,4e presumably via
carbocationic initiation by the cationic hafnium species
[Cp2HfNp]+ or [Cp2HfMe]+. Either could be a good
initiator of isobutene polymerization,13a and we had good
reason to anticipate that the analogous zirconium
compounds would be also.13b-f That said, assuming that
reasonable values of Keq could be obtained at least
approximately from plots of the kinetics data, Keq in
C6D5Cl increased from ∼0.5 mol‚L-1 at 5 °C to ∼1.8
mol‚L-1 at 20 °C in C6D5Cl, and from ∼4.1 mol‚L-1 at
24 °C to ∼17.3 mol‚L-1 at 34 °C in toluene-d8. We
hesitate to calculate thermodynamic parameters, given
the possible uncertainties in values of Keq, but the â-Me
elimination process summed in eqs 2 and 3 appears to
be moderately endothermic. We are unaware of analo-
gous conclusions being reached elsewhere for this type
of reaction.
Cp*2ZrNp(µ-Me)B(C6F5)3 in CD2Cl2
temperature
(°C)
k1 × 102
compound (solvent)
(min-1
)
Cp2ZrNp(µ-Me)B(C6F5)3 (C6D5Cl)
5
10
1.35 ( 0.02
2.57 ( 0.05
4.46 ( 0.12
8.25 ( 0.28
1.10 ( 0.02
1.71 ( 0.03
3.32 ( 0.07
6.55 ( 0.10
0.83 ( 0.01
1.73 ( 0.02
3.38 ( 0.06
5.74 ( 0.22
15
20
Cp2ZrNp(µ-Me)B(C6F5)3 (toluene-d8)
Cp*2ZrNp(µ-Me)B(C6F5)3 (CD2Cl2)
20
24
29
34
-68
-63
-58
-53
were found to be 18.8(0.9) kcal/mol, 0.6(3.2) cal/mol‚deg,
and 18.6(0.9) kcal/mol, respectively.
The zwitterionic complex Cp2ZrNp(µ-Me)B(C6F5)3 also
formed cleanly and instantly when Cp2ZrMeNp was
reacted with a slight excess of B(C6F5)3 in toluene-d8 at
-10 °C, although concentrations had to be kept lower
than in C6D5Cl in order to prevent precipitation. The
room-temperature resonances of Cp2ZrNp(µ-Me)B(C6F5)3
in toluene-d8 were δ 5.74 (s, 10H) 0.78 (s, 9H), 1.11 (s,
2H), and 0.19 (s, 3H), assigned to the Cp, neopentyl
methyl, Zr-CH2, and B-Me groups, respectively. This
compound was stable below 10 °C in toluene-d8, but
decomposed above 10 °C to give Cp2ZrMe(µ-Me)B(C6F5)3
(δ 5.44 (Cp), 0.26 (Zr-Me), 0.10 (B-Me)) and isobutene
(δ 4.72 (CH2), 1.60 (Me)).
An attempt to study the kinetics of the â-Me elimina-
tion process in the presence of the more weakly coor-
dinating anion [B(C6F5)4]-, utilizing the reaction of
Cp2ZrMeNp and [Ph3C][B(C6F5)4] in C6D5Cl, failed
(13) (a) Baird, M. C. Chem. Rev. 2000, 100, 1471. For examples of
cationic zirconium complexes as isobutene polymerization initiators,
see: (b) Carr, A. G.; Dawson, D. M.; Bochmann, M. Macromolecules
1998, 31, 2035. (c) Carr, A. G.; Dawson, D. M.; Thornton-Pett, M.;
Bochmann, M. Organometallics 1999, 18, 2933. (d) Song, X.; Thornton-
Pett, M.; Bochmann, M. Organometallics 1998, 17, 1004. (e) Carr, A.
G.; Dawson, D. M.; Bochmann, M. Macromol. Rapid Commun. 1998,
19, 205. (f) Garrat, S.; Carr, A. G.; Langstein, G.; Bochmann, M.
Macromolecules 2003, 36, 4276.
A kinetics study of â-Me elimination in toluene-d8 was
carried out as above at 20, 25, 30, and 35 °C, and linear
plots of [(Ao - Ae)/(Ao + Ae)] ln[(Ao2 - AtAe)/(AoAt - AoAe)
vs time (Figure 4)12 again showed that the â-Me
elimination was reversible and first-order with respect

6010 Organometallics, Vol. 24, No. 24, 2005
Yang and Baird
Scheme 1
because of the preferential formation of the bridged,
dinuclear species [(Cp2ZrNp)2(µ-Me][B(C6F5)4] (δ 6.22
(Cp), 1.15 (C-Me), 0.96 (CH2), -0.59 (µ-Me)) even in the
presence of an excess of [Ph3C][B(C6F5)4]. {[Cp2ZrNp]2-
CH3}{B(C6F5)4} was found to undergo â-Me elimination
to give first [(Cp2ZrMe)(µ-Me)(Cp2ZrNp)][B(C6F5)4]
(δ 6.16 (Cp), 1.15 (C-Me), 0.98 (CH2), -0.64 (µ-Me)) and
then [(Cp2ZrMe)2(µ-Me)][B(C6F5)4] (δ 6.05 (Cp), 0.33
(Zr-Me), -0.77 (µ-Me)), as observed previously for the
analogous reaction of Cp2HfMeNp.4g While the various
intermediates could not be isolated, the changes in the
NMR spectra were consistent with this sequence of
events, and the presence of three resonances at negative
chemical shifts shows unequivocally that three distinct
µ-Me species are present (see Scheme 1).14 In view of
the complications, our attempt to study this system was
abandoned.
Figure 6. Plot of ln At vs T for the decomposition of Cp*2-
ZrNp(µ-Me)B(C6F5)3 in CD2Cl2.
In contrast to Cp2ZrNp(µ-CH3)B(C6F5)3 but in general
agreement with a previous report,4c we find Cp*2ZrNp-
(µ-CH3)B(C6F5)3 to be extremely unstable when gener-
ated in C6D5Cl, toluene-d8, and CD2Cl2. Indeed, it was
found that â-Me elimination occurred in CD2Cl2 even
at -70 °C, below the freezing point of C6D5Cl and a
temperature at which solubility in toluene-d8 was
minimal. A possibly complicating factor was the pres-
ence of a noticeable amount of Cp*2ZrNp2 in the
material used, but this compound was found to be inert
to B(C6F5)3 at the temperatures used, and thus its
presence was not a problem.
A kinetics study of â-Me elimination from [Cp*2ZrNp-
(µ-Me)B(C6F5)3] was successfully carried out in CD2Cl2
at the temperatures -68, -63, -58, and -53 °C.
However, in contrast to the situation with Cp2ZrNp(µ-
Me)B(C6F5)3, establishment of an equilibrium was not
observed during â-methyl elimination of Cp*2ZrNp(µ-
Me)B(C6F5)3, and linear plots of ln At vs time (Figure
6) confirmed that the â-Me elimination reaction of this
compound is irreversible and first-order with respect to
the concentration of Cp*2ZrNp(µ-Me)B(C6F5)3. Rate
constant data are given in Table 1; on the basis of the
temperature dependence of kobs, an activation energy
Ea of 11.6(0.4) kcal/mol was calculated. An Eyring plot
(Figure 7) resulted in ∆Hq, ∆Sq, and ∆Gq (0 °C) values
of 11.2(0.4) kcal/mol, -12.7(2.0) cal/mol‚deg, and
14.7(0.7) kcal/mol, respectively.
Figure 7. Eyring plot for the decomposition of Cp*2ZrNp-
(µ-Me)B(C6F5)3 in CD2Cl2.
complex and, in addition, solvent polarity clearly plays
an important role. We have previously suggested that
the â-methyl migration reactions under consideration
here are accompanied by concomitant, solvent-assisted
dissociation of [BMe(C6F5)3]- anion from the metal, in
essence an intramolecular, nucleophilic displacement
process as in B.4e The data in Table 2 are certainly
consistent with this proposal since, where comparisons
are possible, the values of ∆Hq are lower in the more
polar solvent C6D5Cl than they are in toluene-d8.
This proposal is also clearly consistent with the recent
results of Chirik et al.,4f who have interpreted signifi-
cant kinetic isotope effects for â-methyl elimination
reactions of a series of deuterated neopentyl zirconoce-
nium compounds in terms of a transition state γ-agostic
interaction during the methyl migration reaction. In-
terestingly, the â-methyl elimination reactions studied
by Chirik et al. were all found to be reversible,4f as are
the analogous reactions of the compounds Cp2ZrNp(µ-
Me)B(C6F5)3 (this work) and Cp2HfNp(µ-Me)B(C6F5)3.4e
In contrast the compounds (1,2-Me2Cp)2MNp(µ-Me)B-
(C6F5)3 (M ) Zr, Hf) are reported to undergo irreversible
â-methyl elimination reactions, possibly because of
steric effects resulting from the presence of the four
methyl groups on the rings, although this rationale does
not seem overly convincing.4d
The activation parameters for all of the â-methyl
elimination reactions studied here and reported in refs
4d and 4e are listed in Table 2. The data for Cp2ZrNp-
(µ-Me)B(C6F5)3 in toluene-d8 compare gratifyingly well
with Marks’ data for (1,2-Me2Cp)2ZrNp(µ-Me)B(C6F5)3.4d
Where comparisons are possible, hafnium complexes
seem somewhat more labile than their zirconium analo-
gies, but the interplay between ∆Hq and ∆Sq is clearly
(14) Bochmann, M.; Lancaster, S. J. Angew. Chem., Int. Ed. Engl.
1994, 33, 1634.

Zirconocene Compounds Cp′2ZrR(µ-CH3)B(C6F5)3
Organometallics, Vol. 24, No. 24, 2005 6011
Table 2. Activation Parameters for â-Methyl Elimination Reactions
∆Hq
(kcal/mol)
∆Sq
(cal/mol‚K)
∆Gq (0 °C)
(kcal/mol)
complexa
solvent
ref
[1,2-Me2Cp2ZrNp]+
[1,2-Me2Cp2HfNp]+
[Cp2HfNp]+
toluene-d8
toluene-d8
toluene-d8
C6D5Cl
toluene-d8
C6D5Cl
CD2Cl2
22.5 ( 0.9
17.3 ( 0.9
21.4 ( 1.1
16.5 ( 5
22.3 ( 0.9
18.8 ( 0.9
11.2 ( 0.4
4.3 ( 3.3
-11.9 ( 3.4
8.0 ( 4.0
21.2 ( 0.2
20.7 ( 0.2
19.2 ( 1.1
18.4 ( 5
20.0 ( 0.9
18.6 ( 0.9
14.7 ( 0.7
4d
4d
4e
4e
this work
this work
this work
[Cp2HfNp]+
-7.0 ( 18
8.5 ( 3.0
[Cp2ZrNp]+
[Cp2ZrNp]+
0.6 ( 3.2
-12.7 ( 2.0
[Cp*2ZrNp]+
a All with [µ-MeB(C6F5)3]- as counteranion.
Steric factors do appear to be important with the
compound Cp*2ZrNp(µ-Me)B(C6F5)3, which exhibits the
lowest activation energy, 11.6 kcal/mol in CD2Cl2, of all
complexes studied. It may be that the presence of the
bulky Cp* ligands helps ease the [BMe(C6F5)3]- ligand
from the inner coordination sphere, thus leading to a
lowered value of the activation energy. Consistent with
the above rationalization of irreversibility of the â-me-
thyl elimination reactions of the compounds (1,2-
Me2Cp)2MNp(µ-Me)B(C6F5)3 (M ) Zr, Hf), â-methyl
elimination from Cp*2ZrNp(µ-Me)B(C6F5)3 is also ir-
reversible.
In an effort to extend our kinetics studies, we syn-
thesized the isobutyl compounds Cp2Zr(i-Bu)Me and
Cp*2Zr(i-Bu)Me as precursors to Cp2Zr(i-Bu)(µ-Me)B-
(C6F5)3 and Cp*2Zr(i-Bu)(µ-Me)B(C6F5)3. The compounds
are of interest because of the possibility of a competition
between â-hydrogen and â-methyl elimination from Cp2-
Zr(i-Bu)(µ-Me)B(C6F5)3 and Cp*2Zr(i-Bu)(µ-Me)B(C6F5)3
and because these compounds provide the simplest
models of zirconocenium ions containing polypropylene
chains. On the basis of the known modes of chain
transfer during propylene polymerizations by the Cp2-
Zr1b,i,k and Cp2*Zr3 catalyst systems, one might antici-
pate predominantly â-hydrogen elimination from Cp2-
Zr(i-Bu)(µ-Me)B(C6F5)3 and â-methyl elimination from
Cp*2Zr(i-Bu)(µ-Me)B(C6F5)3.
Although Cp2Zr(i-Bu)Me is a thermally unstable oil
at room temperature and could not be purified, reactions
of both Cp2Zr(i-Bu)Me and Cp*2Zr(i-Bu)Me could be
carried out with B(C6F5)3 in CD2Cl2 and satisfactorily
monitored at -40 and -60 °C, respectively; in both cases
the only olefinic product was isobutene. Careful inspec-
tion of the NMR spectra indicated that neither free
propylene (multiplets at δ ∼5.75, 5.0) nor polypropylene
(δ ∼1.75, ∼0.75) were formed, and thus the compounds
Cp2Zr(i-Bu)(µ-Me)B(C6F5)3 and Cp*2Zr(i-Bu)(µ-Me)B-
(C6F5)3 both undergo only â-hydrogen elimination. This
result for Cp*2Zr(i-Bu)(µ-Me)B(C6F5)3 is consistent with
reports of similar chemistry elsewhere.11 Unfortunately,
kinetics studies could not be carried out because the
â-hydrogen elimination reactions of both compounds
were too rapid in CD2Cl2 and both compounds oiled out
in the less polar solvent toluene-d8. However, isobutene
is nonetheless the only olefinic product formed when
Cp*2Zr(i-Bu)(µ-Me)B(C6F5)3 is generated in toluene-d8
at temperatures ranging from -35 to +10 °C, and thus
â-hydrogen elimination is a general result with this
compound and not an artifact derived from the nature
of the solvent.
erization by the Cp2*Zr catalyst system. Why is it that
â-methyl elimination is the dominant mechanism of
chain transfer during propylene polymerization while
only â-hydrogen elimination is observed during the
thermal degradation of Cp*2Zr(i-Bu)(µ-Me)B(C6F5)3? Is
it that the steric factors implicit in A do not apply when
the polymeryl chain is very short?
To assess the latter possibility, molecular mechanics
calculations were carried out on the cationic species
[Cp*2Zr(isobutyl)]+, and the minimized structure is
shown in Figure 8. As is indicated, the preferred
conformation is indeed that in which one of the â-methyl
groups lies in the plane perpendicular to the Cp-
(centroid)-Zr-Cp(centroid), while the â-hydrogen atom
eclipses a Cp* ring, as depicted above in A. One of the
hydrogen atoms on the â-methyl group, pictured in red,
is situated about 3.08 Å from the zirconium, more than
the 2.24 Å calculated for full γ-agostic interactions2h but
less than the sum of the zirconium and hydrogen van
der Waals radii (∼3.5 Å).15 Interestingly, this γ-hydro-
gen atom is much closer to the zirconium than is the
isobutyl â-hydrogen atom (deep blue), which is about
3.41 Å from the zirconium. Thus the complex cation
appears to prefer the conformation believed3c,e to be
prerequisite to â-methyl elimination, which does not
occur.
Conclusions. Our kinetics study on â-methyl elimi-
nation reactions of the compounds Cp2ZrNp(µ-Me)B-
(C6F5)3 (in various solvents) and Cp*2ZrNp(µ-Me)B-
(C6F5)3 (in CD2Cl2) shows that the reactions are
accelerated by polar solvents and by steric crowding in
the Cp* system. These results are consistent with
previous findings and conclusions and strengthen the
case for methyl migration being accompanied in a
concerted process by borate anion departure from the
inner coordination sphere of the metal cation. Of some
The fact that no propylene was detected in these
experiments, only isobutene, led us to reconsider the
mechanism of chain transfer during propylene polym-
Figure 8. Optimized structure of [Cp*2Zr(isobutyl)]+.

6012 Organometallics, Vol. 24, No. 24, 2005
Yang and Baird
surprise, however, is the finding that the isobutyl
compound Cp*2Zr(i-Bu)(µ-Me)B(C6F5)3, which is ex-
pected to undergo â-methyl elimination by analogy with
an extensive literature on chain transfer processes
during propylene polymerization, in fact undergoes only
â-hydrogen elimination. This apparent conundrum will
be addressed in the following paper in this issue.3l
Acknowledgment. Financial support from Queen’s
University and the Natural Sciences and Engineering
Research Council made this research possible.
(15) Batsanov, S. S. Inorg. Mater. 2001, 37, 871.
OM0504710