Models of intermediates in metallocene-catalyzed alkene polymerizations: Observation of a d0 cationic titanium-alkyl-alkene complex and decomposition by β-allyl elimination

Article abstract of DOI:10.1016/S1387-7003(03)00252-1

Low temperature activation of Cp₂*Ti[η¹, η¹- CH₂CH(CH₂CH=CH₂)CH ₂] (3) with [HN(CH₃)(C₆H₅) ₂] [B(C₆F₅)₄] led to the formation of Cp₂*Ti[η¹,η² -CH ₂CH(CH₃)CH₂CH=CH₂][B(C ₆F₅)₄] (6) as determined by ¹H NMR spectroscopy. Cp₂*Ti[η¹,η ²-CH₂CH(CH₃)CH₂CH=CH ₂][B(C₆F₅)₄] undergoes rapid quantitative β-allyl elimination at temperatures as low as -140 °C. The resulting cationic titanium allyl complex [Cp₂*Ti(η ³-CH₂CHCH₂)][B(C₆F₅) ₄] (4) exhibits a static structure at low temperatures, but interconversion of η³-η¹ binding modes can be observed at higher temperatures. Lineshape analysis of this process yielded Δ?-10 °C) = 13.7 ± 0.6 kcal mol^-1, ΔH? = 9.8 ± 0.6 kcal mol^-1, and ΔS? = -15 ± 3 eu. The use of neutral borane B(C₆F ₅)₃ also resulted in [β-allyl elimination with the formation of [Cp₂*Ti(η³-CH₂CHCH ₂)][CH₂ = CHCH₂B(C₆F ₅)₃] (8).

Full text of DOI:10.1016/S1387-7003(03)00252-1

Inorganic Chemistry Communications 6 (2003) 1287–1290
www.elsevier.com/locate/inoche
Models of intermediates in metallocene-catalyzed alkene
polymerizations: observation of a d0 cationic titanium–alkyl-alkene
complex and decomposition by b-allyl elimination
*
Donald W. Carpenetti II
Department of Chemistry, Trinity University, San Antonio, TX 78212, USA
Received 25 July 2003; accepted 2 August 2003
Published online: 29 August 2003
Abstract
Low temperature activation of Cp2ꢀTi[g1; g1- CH2CH(CH2CH@CH2)CH2] (3) with [HN(CH3)(C6H5)2] [B(C6F5)4] led to the
formation of Cpꢀ2Ti[g1; g2-CH2CH(CH3)CH2CH@CH2][B(C6F5)4] (6) as determined by 1H NMR spectroscopy. Cpꢀ2Ti[g1; g2-
CH2CH(CH3)CH2CH@CH2][B(C6F5)4] undergoes rapid quantitative b-allyl elimination at temperatures as low as )140 °C. The
resulting cationic titanium allyl complex [Cpꢀ2Ti(g3-CH2CHCH2)][B(C6F5)4] (4) exhibits a static structure at low temperatures, but
interconversion of g3–g1 binding modes can be observed at higher temperatures. Lineshape analysis of this process yielded DGz()10
°C) ¼ 13.7 ꢁ 0.6 kcal molꢂ1, DHz ¼ 9.8 ꢁ 0.6 kcal molꢂ1, and DSz ¼ )15 ꢁ 3 eu. The use of neutral borane B(C6F5)3 also resulted in
b-allyl elimination with the formation of [Cpꢀ2Ti(g3-CH2CHCH2)][CH2 ¼ CHCH2B(C6F5)3] (8).
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Metallocene; Alkyl-alkene Complex; Titanium; b-Allyl elimination
1. Introduction
The commonly accepted mechanism for metallocene-
catalyzed alkene polymerization involves a d0 metal–
alkyl-alkene intermediate [1]. The absence of d electrons
to stabilize the interaction between the alkene and the
metal by d to pꢀ back-bonding and the high kinetic re-
activity of these intermediates are believed to be re-
2. Results and discussion
sponsible for these compounds having thus far avoided
In order to extend work on zirconium–alkyl-alkene
detection [2,3]. Model systems related to this interme-
chelates [6,7] to titanium, the analogous b-allyl substi-
diate that are stabilized by chelation between the alkyl
tuted titanacyclobutane complex, Cpꢀ2Ti[g1; g1-CH2CH
group and the alkene moiety have been studied. Previ-
(CH2CH@CH2)CH2] (3) was prepared. This compound
ous work has focused on neutral yttrium and cationic
has been prepared by Stryker from Ti(III) precursors
zirconium alkyl-alkene-chelates like 1 and 2 [4–9]. This
[10]. Owing to an abundance of Ti(IV) compounds in
report details attempts to extend this chemistry to cat-
inventory the synthesis was attempted in an analogous
ionic titanium analogues.
manner to that published for the zirconium analogue
Cp2ꢀZr[g1; g1-CH2CH(CH2CH@CH2)CH2]. [11] Addi-
tion of an excess of allylmagnesium chloride to a slurry
of Cpꢀ2TiCl2 in THF at 0 °C led to the formation of 3 in
moderate yields if the reaction temperature was kept at
0 °C for 12 h.
* Corresponding author. Tel.: +1-210-999-7322; fax: +1-210-999-
7569.
E-mailaddress:Donald.Carpenetti@Trinity.edu(D.W. CarpenettiII).
1387-7003/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S1387-7003(03)00252-1

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D.W. Carpenetti II / Inorganic Chemistry Communications 6 (2003) 1287–1290
Protonation of 3 with [HN(CH3)(C6H5)2][B(C6F5)4]
energy barrier for g3–g1 interconversion in 4 is 2.1 kcal
molꢂ1 higher than the corresponding barrier for neutral
scandium. This difference is very similar to those
observed between neutral yttrium and cationic zirco-
nium–alkyl-alkene chelates featuring five carbon tethers.
Cationic zirconium–alkyl-alkene chelates have an alkene
face dissociation barrier ranging from 10 to 11 kcal
molꢂ1 [6,7]. The corresponding barrier for neutral yt-
trium–alkyl-alkene chelates ranges from 7.5 to 8.5 kcal
molꢂ1 [4,5].
Although some interesting comparisons can be made
by analyzing the rate of g3–g1 interconversion for allyl
complexes, more information could be gathered by ob-
serving the titanium–alkyl-alkene chelate with a five
carbon tether. With this goal in mind, 3 was treated with
[HN(CH3)(C6H5)2] [B(C6F5)4] at )140 °C in CDCl2F/
CDClF2 [13]. Much lower temperatures were employed in
an attempt to observe the titanium–alkyl-alkene complex
Cpꢀ2Ti[g1,g2-CH2CH(CH3)CH2CH@CH2][B(C6F5)4] (6)
before it could b-allyl eliminate (Scheme 2).
at )78 °C in CD2Cl2 led to the formation of [Cpꢀ2Ti(g3-
CH2CHCH2)][B(C6F5)4] (4) and propene. The g3-allyl
ligand can be clearly identified in 1H NMR by the triplet
of triplets resonance at d 7.91 ppm and the two doublet
resonances at 4.99 (J ¼ 15:0 Hz) and 1.57 ppm (J ¼ 9:7
Hz) characteristic of a static g3-allyl structure.
As Bercaw and co-workers [12] have recently ob-
served, the g3–g1 transition of an allyl ligand is a special
case of alkene dissociation from the metal center and
represents the upper limit for the strength of the metal–
alkene interaction (Scheme 1). The allyl ligand can be
thought of as a three carbon alkyl-alkene chelate. Ob-
servation of the temperature dependence of the allyl
resonances of 4 revealed that the resonance of the internal
hydrogen at d 7.91 ppm coalesced from a triplet of triplets
to a pentet at )20 °C (Fig. 1). The resonances of the syn
and anti hydrogens (d 1.57 and 4.99 ppm) broadened
significantly as the temperature was raised, but due to
the extreme separation in frequency coalescence was
not observed. Lineshape analysis of the temperature
dependent broadening of these resonances yielded
rate constants over a 70 °C temperature range. An Eyring
plot of the data yielded DHz ¼ 9.8 ꢁ 1.2 kcal molꢂ1
and DSz ¼ )15 ꢁ 4 eu, corresponding to DGz()20 °C) ¼
Unfortunately, at )140 °C the rate of b-allyl elimi-
nation is still fairly rapid (t1=2 < 60 s) [14]. Complex 6
1
was partially identified by H NMR at )140 °C. The
high frequency shift of the resonance assigned to the
secondary vinyl proton of 6 (d 7.96 ppm) is character-
istic of an alkene bound to a d0 metal center (Fig. 2). 1
and 2 feature similar resonances at d 6.78 and 7.89 ppm,
respectively [4,7]. The difference in chemical shift of the
resonances assigned to the two terminal vinyl protons [d
5.46 and 2.42 ppm, Dd( ¼ CH2) ¼ 3.04 ppm] is similar to
those observed for related cationic zirconium–alkene
chelates [Dd( ¼ CH2) ¼ 2.99 ppm for 2] [7] and more
pronounced than those observed for yttrium–alkene
chelates [Dd( ¼ CH2) ¼ 1.38 ppm for 1] [4]. Another
characteristic of alkyl-alkene chelates is a low frequency
shift of one of the diasteriotopic a-methylene protons.
In 6, the broad doublet resonance at d )0.65 ppm is
reminiscent of those observed for more thoroughly
13.6 ꢁ 0.6 kcal molꢂ1
.
The dynamic NMR behavior of Cpꢀ2Sc(g3-
CH2CHCH2) (5), the neutral group III analogue of 4,
was reported by Bercaw and co-workers [12]. Activation
parameters for its g3–g1 interconversion were found to
be DHz ¼ 8.0 ꢁ 0.6 kcal molꢂ1 and DSz ¼ )14 ꢁ 4 eu
[DGz()20 °C) ¼ 11.5 kcal molꢂ1] in ether. The free
Scheme 1.
Fig. 1. Interconversion of the internal allyl hydrogen 1H NMR resonance of 4 from a triplet of triplets to a pentet, a consequence of the syn and anti
hydrogens of the allyl ligand becoming equivalent on the NMR time scale.

D.W. Carpenetti II / Inorganic Chemistry Communications 6 (2003) 1287–1290
1289
Scheme 2.
Fig. 2. 1H NMR spectrum of a mixture of Cpꢀ2Ti[g1,g2-CH2CH(CH3)CH2CH@CH2][B(C6F5)4] (6) (broad multiplet centered at d 7.96 ppm) and
[Cpꢀ2Ti(g3-CH2CHCH2)][B(C6F5)4] (4) (triplet of triplets at d 7.91 ppm) at )140 °C in CDCl2F/CDClF2.
characterized complexes 1 (d )0.44 ppm) and 2 (d )0.83
ppm). Due to the short lifetime of 6 at )140 °C it
could not be fully characterized by NMR, but all
evidence from H NMR is consistent with the assigned
structure.
were obtained using a Varian Inova 400 spectrometer.
Spectrometer temperatures were measured after a 15-
min equilibration, using a methanol standard containing
0.03% HCl. CD2Cl2 (Cambridge Isotopes) was dried
over P2O5, then distilled from CaH2. CDCl2F/CDClF2
was prepared by a published procedure and stored over
CaH2 [13]. Tetrahydrofuran and pentane were dried
over sodium then distilled. Allylmagnesium chloride was
obtained from Aldrich as a THF solution and used as
received. [HNCH3(C6H5)2][B(C6F5)4] was prepared by a
known procedure [7].
1
In contrast to related zirconium–alkyl-alkene chelate
2 [7] where two diastereomers are observed at low
temperature, only one diastereomer of 6 is observed.
This could be due to a very low barrier for alkene dis-
sociation from the sterically hindered titanium center
resulting in an averaging of the resonances of the two
diastereomers on the NMR time scale, but is more likely
due to a steric interaction between the b-methyl group of
the alkyl chain and one of the Cpꢀ ligands which leads to
observation of only one orientation which features the
methyl group in a pseudo-equatorial position [5].
The reaction of 3 with B(C6F5)3 was attempted to see if
the zwitterionic titanium–alkyl-alkene complex Cpꢀ2Ti
{g1; g2-CH2CH[CH2B(C6F5)3]CH2CH@CH2} (7) would
be more thermally stable. This compound also quickly
and quantitatively undergoes b-allyl elimination forming
[Cpꢀ2Ti(g3-CH2CHCH2)][CH2 ¼ CHCH2B(C6F5)3] (8).
Variable temperature NMR analysis of 8 showed that
changing the anion from B(C6F5)4 to CH2 ¼ CHCH2B
(C6F5)3 had no effect on the g3–g1 interconversion of the
allyl ligand.
3.2. Preparation of Cpꢀ2Ti[g1; g1-CH2CH(CH2CH@CH2)
CH2] (3)
Cpꢀ2TiCl2 (1.0 g, 2.57 mmol) was slurried in THF (40
ml). Allylmagnesium chloride (10 ml of a 2.0 M THF
solution, 20 mmol) was added by syringe at 0 °C. The
reaction was stirred for 12 h at 0 °C and then solvent
was removed under reduced pressure. Resulting material
was triturated with pentane, chilled to )30 °C and fil-
tered. Removal of solvent from the filtrate yielded 3 as a
red powder (0.43 g, 42%). NMR was consistent with
literature data [10].
3.3. Reaction of 3 with [(C6H5)2(CH3)NH] [B(C6F5)4] at
)78 °C
3. Experimental
CD2Cl2 (0.5 ml) was condensed into a resealable
NMR tube containing 3 (0.018 g, 0.044 mmol) and
[(C6H5)2(CH3)NH][B(C6F5)4] (0.038 g, 0.044 mmol) at
)78 °C. The tube was shaken briefly at )78 °C to give a
red solution and was inserted into the precooled
probe of the NMR spectrometer. The NMR spectrum
shows a mixture of [Cpꢀ2Ti(g3-CH2CHCH2)][B(C6F5)4]
4, (C6H5)2(CH3)N and propene, details are given
below.
3.1. General considerations
All reactions were carried out in an inert atmosphere
glovebox or using standard high-vacuum line tech-
niques. All NMR experiments were carried out in 1.9 ml
medium-walled resealable NMR tubes that were
flamed–dried under vacuum prior to use. NMR spectra

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D.W. Carpenetti II / Inorganic Chemistry Communications 6 (2003) 1287–1290
3.3.1. NMR Data for [Cpꢀ2Ti(g3-CH2CHCH2)][B(C6F5)4]
(4) at )78 °C
BCH2), 113.8 (t, J ¼ 150 Hz, CH@CH2), 121.1 (br s,
ipso-BC), 133.4 (d, J ¼ 154 Hz, CH ¼ CH2) 135.3 (d,
JCF ¼ 238 Hz, CF), 137.2 (d, JCF ¼ 245 Hz, CF), 146.8
(d, JCF ¼ 240 Hz, CF).
1H NMR (400 MHz, CD2Cl2, )78 °C) d 1.54 (d,
J ¼ 9:6 Hz, CHHCHCHH), 1.99 (s, C5Me5), 5.02 (d,
J ¼ 15 Hz, CHHCHCHH), 7.91 (tt, J ¼ 15, 9.6 Hz,
CHHCHCHH). 13C NMR (gated decoupled, 100.5
MHz, CD2Cl2, )78 °C) d 12.8 (s, C5Me5), 13.8 (s,
C5Me5), 91.6, (dd, J ¼ 156, 151 Hz, CHHCHCHH),
124.1 (s, broadened by JBC, ipso-C6F5), 129.6 (br s,
overlapping C5Me5), 136.9 (d, JCF ¼ 245 Hz, C6F5),
139.0 (d, JCF ¼ 247 Hz, C6F5), 148.3 (d, JCF ¼ 245 Hz,
C6F5), 156.3 (d, J ¼ 155 Hz, CHHCHCHH).
3.5. Reaction of 3 with [(C6H5)2(CH3)NH][B(C6F5)4] at
)140 °C
CDCl2F/CDClF2 (0.5 ml) was condensed into a
resealable NMR tube containing 3 (0.018 g, 0.044
mmol) and [(C6H5)2(CH3)NH] [B(C6F5)4] (0.038 g,
0.044 mmol) at )176 °C. The tube was inserted directly
into the pre-cooled probe of the NMR spectrometer at
)140 °C. The solution was allowed to thaw for 5 min,
ejected from the NMR probe, shaken briefly, and rein-
serted. The initial NMR spectrum shows a mixture of
[Cpꢀ2Ti(g3-CH2CHCH2)][B(C6F5)4] 4, Cp2ꢀTi[g1,g2-CH2
CH(CH3)CH2CH@CH2][B(C6F5)4], 6, (C6H5)2(CH3)N
and propene, NMR data for 6 is given below.
3.3.2. NMR Data for [Cpꢀ2Ti(g3-CH2CHCH2)][B(C6F5)4]
(4) at )0 °C
1H NMR (400 MHz, CD2Cl2, 0 °C) d 1.57 (br, CH
HCHCHH), 1.99 (s, C5Me5), 4.99 (br, CHHCHCHH),
7.90 (q, J ¼ 12 Hz, CHHCHCHH). 13C NMR (gated
decoupled, 100.5 MHz, CD2Cl2, 0 °C) d 13.6 (s, C5Me5),
91.6, (t, J ¼ 152 Hz, CHHCHCHH), 124.1 (s, broad-
ened by JBC, ipso-C6F5), 129.6 (br s, overlapping
C5Me5), 136.9 (d, JCF ¼ 245 Hz, C6F5), 139.0 (d,
JCF ¼ 247 Hz, C6F5), 148.3 (d, JCF ¼ 245 Hz, C6F5),
156.0 (d, J ¼ 155 Hz, CHHCHCHH).
3.5.1. NMR Data for Cpꢀ2Ti[g1,g2-CH2CH(CH3)CH2
CH@CH2][B(C6F5)4], 6
1H NMR (400 MHz, CDCl2F/CDClF2, )140 °C) d-
0.65 (br d, J ¼ 10 Hz, TiCHH), 1.35 (d, J ¼ 7 Hz,
TiCH2CH(CH3)), 1.95 (s, C5 Me5), 1.97 (s, C5 Me5), 2.42
(br d, J ¼ 9 Hz, CH@CH H), 5.46 (br d, J ¼ 17 Hz,
CH@CHH), 7.96 (br m, CH ¼ CH2). Resonances were
observed at 1.43 (TiCH2CH), 1.85 (TiCHH), 2.21 and
2.38 (CH2CH@CH2) ppm, but could only be tentatively
assigned.
3.3.3. NMR Data for (C6H6)2NCH3
1H NMR (400 MHz, CD2Cl2, )78 °C) d 3.31 (s,
NCH3), 7.03 (m, 4H, Ph), 7.26 (m, 6H, Ph). 13C NMR
(gated decoupled, 100.5 MHz, CD2Cl2, )78 °C) d 41.1
(q, J ¼ 139 Hz, NCH3), 121.6 (br, m-Ph), 129.9 (br,
p-Ph), 130.1 (dd, J ¼ 159, 7 Hz, o-Ph), 149.4 (s, ipso-Ph).
3.3.4. NMR data for propene
References
1H NMR (400 MHz, CD2Cl2, )78 °C) d 1.72 (d, J ¼ 7
Hz, CH3), 4.91 (d, J ¼ 10 Hz, CH@CHH), 4.94 (d,
J ¼ 15 Hz, CH@CHH), 5.83 (ddq, J ¼ 15, 10, 7 Hz).
[1] For a review of metallocene catalyzed alkene polymerization, see:
L. Resconi, L. Cavallo, A. Fait, F. Piemontesi, Chem. Rev. 100
(2000) 1253.
[2] C.P. Casey, T.-Y. Lee, J.A. Tunge, D.W. Carpenetti, J. Am.
Chem. Soc. 123 (2001) 10762.
3.4. Reaction of 3 with B(C6F5)3 at )78 °C
[3] C.P. Casey, J.A. Tunge, T.-Y. Lee, D.W. Carpenetti, Organo-
metallics 21 (2002) 389.
CD2Cl2 (0.5 ml) was condensed into a resealable
NMR tube containing 1 (0.018 g, 0.044 mmol) and
B(C6F5)3 (0.023 g, 0.044 mmol) at )78 °C. The tube was
shaken briefly at )78 °C to give a red solution and was
inserted into the precooled probe of the NMR spec-
[4] C.P. Casey, S.L. Hallenbeck, D.W. Pollock, C.R. Landis, J. Am.
Chem. Soc. 117 (1995) 9770.
[5] C.P. Casey, J.F. Klein, M.A. Fagan, J. Am. Chem. Soc. 122
(2000) 4320.
[6] C.P. Casey, D.W. Carpenetti, H. Sakurai, J. Am. Chem. Soc. 121
(1999) 9483.
trometer. The NMR spectrum shows
a mixture
of [Cpꢀ2Ti(g3-CH2CHCH2)][CH2 ¼ CHCH2B(C6F5)3] 8,
and (C6H5)2(CH3)N. The NMR of 8 differs from that of
4 only in the resonances corresponding to the anion,
which are given below.
[7] C.P. Casey, D.W. Carpenetti, Organometallics 19 (2000) 3970.
[8] C.P. Casey, D.W. Carpenetti, H. Sakurai, Organometallics 20
(2001) 4262.
[9] Other complexes featuring an interaction between a d0-metal and
a chelated alkene: E.J. Stoebenau, R.F. Jordan, J. Am. Chem. Soc.
125 (2003) 3222, references therein.
3.4.1. Anion resonances of [CH2 ¼ CHCH2B(C6F5)3]
1H NMR (400 MHz, CD2Cl2, )78 °C) d 0.47 (br d,
J ¼ 7 Hz, BCH2), 5.00 (d, J ¼ 10 Hz, CH@CHH), 5.04
(d, J ¼ 15 Hz, CH@CHH), 5.52 (ddt, J ¼ 15, 10, 7 Hz,
CH ¼ CH2). 13C NMR (gated decoupled, 100.5 MHz,
[10] G.L. Casty, J.M. Stryker, J. Am. Chem. Soc. 117 (1995) 7814.
[11] E.B. Tjaden, J.M. Stryker, J. Am. Chem. Soc. 115 (1993) 2083.
[12] M.B. Abrams, J.C. Yoder, C. Loeber, M.W. Day, J.E. Bercaw,
Organometallics 18 (1999) 1389.
[13] J.S. Siegel, F.A.L. Anet, J. Org. Chem. 53 (1988) 2629.
[14] C.P. Casey, D.W. Carpenetti, J. Organomet. Chem. 642 (2002)
120.
CD2Cl2, )78 °C) D 25.2 (br, JCH obscured by JBC
,