Benzyl group abstraction from Ti(CH2Ph)4 by B(C6F5)3 and [Ph3C][B(C6F5)4] to form Ziegler-Natta and carbocationic alkene polymerization initiators

Article abstract of DOI:10.1016/S0022-328X(00)00552-0

Reaction of Ti(CH₂Ph)₄ with B(C₆F₅)₃ results in the formation of [Ti(CH₂Ph)₃][(η⁶-PhCH₂)B(C ₆F₅)₃], while reactions with [Ph₃C][B(C₆F₅)₄] give either [Ti(CH₂Ph)₃][B(C₆F₅)₄] or [(CH₂Ph)₃Ti-μ-CH₂PhTi(CH₂Ph) ₃][B(C₆F₅)₄], depending on the Ti(CH₂Ph)₄:[Ph₃C] ratio; NMR evidence is presented that all of these compounds, including Ti(CH₂Ph)₄, exhibit multihapto Ti-CH₂Ph bonding. The Ti(CH₂Ph)₄/B(C₆F₅)₃ and Ti(CH₂Ph)₄/[Ph₃C][B(C₆F ₅)₄] systems are assessed for their abilities to induce polymerization of ethylene, 1-hexene, isobutylene and ethyl vinylether, and the Ti(CH₂Ph)₄/[Ph₃C][B(C₆F ₅)₄] system is found to be able to function as both a Ziegler-Natta catalyst and a carbocationic initiator.

Full text of DOI:10.1016/S0022-328X(00)00552-0

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 619 (2001) 62–73
Benzyl group abstraction from Ti(CH2Ph)4 by B(C6F5)3 and
[Ph3C][B(C6F5)4] to form Ziegler–Natta and carbocationic alkene
polymerization initiators
Mei Lin, Michael C. Baird *
Department of Chemistry, Queen’s Uni6ersity, Kingston, ON, Canada K7L 3N6
Received 2 February 2000; received in revised form 26 June 2000
Abstract
Reaction of Ti(CH2Ph)4 with B(C6F5)3 results in the formation of [Ti(CH2Ph)3][(h6-PhCH2)B(C6F5)3], while reactions with
[Ph3C][B(C6F5)4] give either [Ti(CH2Ph)3][B(C6F5)4] or [(CH2Ph)3Ti-m-CH2PhTi(CH2Ph)3][B(C6F5)4], depending on the
Ti(CH2Ph)4:[Ph3C] ratio; NMR evidence is presented that all of these compounds, including Ti(CH2Ph)4, exhibit multihapto
Ti–CH2Ph bonding. The Ti(CH2Ph)4/B(C6F5)3 and Ti(CH2Ph)4/[Ph3C][B(C6F5)4] systems are assessed for their abilities to induce
polymerization of ethylene, 1-hexene, isobutylene and ethyl vinylether, and the Ti(CH2Ph)4/[Ph3C][B(C6F5)4] system is found to
be able to function as both a Ziegler–Natta catalyst and a carbocationic initiator. © 2001 Elsevier Science B.V. All rights
reserved.
Keywords: Benzyl group abstraction; Ziegler–Natta catalyst; Carbocationic initiator
may also behave as initiators of carbocationic polymer-
izations of e.g. styrene, isobutylene (IB) and vinyl
ethers [3a]. The first such multitalented initiator system
to be recognized was Cp*TiMe2(m-Me)B(C6F5)3,
formed on treating Cp*TiMe3 with B(C6F5)3 [3b–e],
which behaves as a source of the highly electrophilic
cation [Cp*TiMe2]+. Consistent with carbocationic
polymerizations by conventional Lewis acid initiators1,
initiation of, e.g. isobutylene polymerization by
[Cp*TiMe2]+, is believed to involve h1-coordination of
the alkene as in A, where a carbocationic center is
stabilized by interaction with a weakly nucleophilic
anion and the substituents on the cationic carbon atom.
Propagation then involves successive additions of
monomer molecules at the carbocationic center, which
migrates away from the metal atom. Thus, the metal
does not behave as a catalyst, but rather as an initiator
of polymerization.2
The roles of metallocene complexes of the Group 4
elements as the active species in the homogenous cata-
lytic polymerization of ethylene and l-alkenes via
Ziegler–Natta catalytic processes are now well estab-
lished [1], while more recently monocyclopentadienyl
and related ‘constrained geometry catalysts’ have also
been the subject of intense investigations [2]. In both
mono- and dicyclopentadienyl Ziegler–Natta systems,
the active species are believed to be cationic alkyl-
metal(IV) complexes, of the types [Cp%MR2L]+ or
[Cp%MRL]+ (Cp%=substituted cyclopentadienyl, M=
2
Ti, Zr, Hf; R=alkyl group; L=labile ligand), respec-
tively. Initiation and propagation steps involve
displacement of L and coordination of monomer to
give a cationic h2-alkene intermediates which are subse-
quently involved in repeated migratory insertion steps
[1,2].
While these families of catalysts show great commer-
cial promise, however, it is also becoming ever clearer
that the same types of Ziegler–Natta catalyst systems
1 For general reviews of carbocationic initiation, see [4].
2 Use of the terms catalyst and initiator is confused in the polymer-
ization literature and probably elsewhere; while all catalysts are
initiators, not all initiators are catalysts.
* Corresponding author. Tel.: +1-613-5452614; fax: +1-613-
5456669.
E-mail address: bairdmc@chem.queensu.ca (M.C. Baird).
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(00)00552-0

M. Lin, M.C. Baird / Journal of Organometallic Chemistry 619 (2001) 62–73
63
are much less susceptible to carbocationic initiation
because of the lower stabilities of the carbocations
which would be produced; thus ethylene polymerization
probably implies invariably the relevance of a Ziegler–
Natta process.
We have begun this investigation with the long
known but, in fact, little studied homoleptic tetrabenzyl
compound Ti(CH2Ph)4 [6]. It has been shown previously
that this neutral compound and its zirconium analogue
Zr(CH2Ph)4 are essentially unreactive with ethylene,
styrene and a-alkenes, but that both promote the poly-
merization of styrene, ethylene and propene in the
presence of conventional activators such as MAO,
[NHMe2Ph][B(C6F5)4], [Ph3C][B(C6F5)4] and B(C6F5)3
[7]. We describe here an investigation of the properties
of Ti(CH2Ph)4 activated with B(C6F5)3 and
[Ph3C][B(C6F5)4] as initiators for polymerization of eth-
ylene, 1-hexene, IB and ethyl vinyl ether (EVE). Al-
though it is clear from our results that Ti(CH2Ph)4/
B(C6F5)3 and Ti(CH2Ph)4/[Ph3C][B(C6F5)4] can behave,
as expected, as Ziegler–Natta catalysts, the system also
appears to behave as an active carbocationic initiator.
The polymerization processes presumably involve co-
ordinatively unsaturated, cationic benzyl species of the
type [Ti(CH2Ph)3]+, but the reactions of Ti(CH2Ph)4
with B(C6F5)3 and [Ph3C][B(C6F5)4] seem not to have
been unexplored in any detail. We therefore also present
here a variable temperature NMR investigation of the
very unstable complexes formed when Ti(CH2Ph)4 is
treated with B(C6F5)3 and [Ph3C][B(C6F5)4].
Ziegler–Natta catalysts and carbocationic initiators
have generally been used to induce polymerization of
rather different classes of alkenes – the key differences
being the nature and number of the substituents on the
carbon–carbon double bond – and the fields of carbo-
cationic and Ziegler–Natta polymerization have devel-
oped almost completely independently. However,
successful polymerizations of alkenes via both processes
have the same requirements, specifically a vacant site on
a highly Lewis acidic metal complex in combination
with poorly coordinating solvents and counteranions
which do not compete effectively for the metal inner
coordination sphere. Thus it might be anticipated that
some putative Ziegler–Natta polymerizations may in-
volve, at least in part, carbocationic processes.
Indeed, it has recent been shown that the cationic
complexes [Cp%MMe]+ (M=Zr, Hf) [5a,b], as well as
2
the complexes obtained on treating [Cp%ZrH2]2 [5c],
2
[Cp2%YMe]2 [5d] and ZrMe(N{SiMe3}2)3 [5e] with
B(C6F5)3 do in fact behave as initiators of carbocationic
polymerization of isobutylene. These results may have
implications for Ziegler–Natta catalyst systems, al-
though carbocationic polymerization or oligomerization
of 1-alkenes has as yet not been observed. However,
polymers containing vinylidene end groups of the type
CH2ꢀCMeꢀpolymer are often formed during Ziegler–
Natta processes and might well re-coordinate to the
metal cation in h1 fashion as in A. If this were to occur,
the catalyst sites would be converted to neutral metal
species, cationic metal-centered sites would no longer be
available for initiation of the Ziegler–Natta process,
and the end result would be cessation of the Ziegler–
Natta processes via de facto poisoning of the catalysts.
Alternatively, depending on the nature of the
monomer(s), carbocationic polymerization could be ini-
tiated as in A, and possibly a block of a different kind
of polymer would be added to any polymer already
formed.
In an effort to extend our knowledge of systems
which may behave as both Ziegler–Natta and carboca-
tionic initiators, we are investigating a number of puta-
tive Ziegler–Natta catalysts for their capacity to also
initiate polymerization of, e.g. IB and vinyl ethers. The
latter monomers are, for a number of reasons, not
generally susceptible to Ziegler–Natta processes [4], and
thus successful polymerization of these alkenes may be
taken as a signature of a carbocationic polymerization
process. In contrast, ethylene, especially, and a-alkenes
1. Experimental
All operations were performed under purified argon
using normal Schlenk techniques or a Vacuum Atmo-
spheres Dry Lab. Solvents were purified by standard
methods, and distilled and degassed before use. All
1D-1H-, 13C-, 13C{1H}- and 19F-NMR spectra and 2D-
COSY and HMQC spectra were recorded using Bruker
Avance 400 or 500 spectrometers, chemical shifts being
1
determined by reference to residual H and 13C solvent
1
peaks for H and 13C{1H} studies, external CFCl3 for
19F studies. Gel permeation chromatography (GPC)
experiments with the polymers were carried out at room
temperature (r.t.) using a Waters Model 440 liquid
chromatograph with THF as eluant with polystyrene
standards. The compounds Ti(CH2Ph)4 [6], B(C6F5)3
[8a,b] and [Ph3C][B(C6F5)4] [8c] were prepared according
to published procedures.
Ethylene was purified by passing through a column
,
containing 3 A molecular sieves. Isobutylene (IB) was
purified by passing through a barium oxide column and
,
two columns containing 3 A molecular sieves. 1-Hexene
and EVE were purified by stirring over CaH2 followed
by vacuum distillation.

64
M. Lin, M.C. Baird / Journal of Organometallic Chemistry 619 (2001) 62–73
1.1. General polymerization procedures
ethane at l 6.9–7.22 (m, CH2-m-Ph and CPh3), 6.92
(t, CH2-m-Ph), 6.55 (d, 2H, CH2-o-Ph) and 3.90 (s, 2H,
CH2), and to unreacted [Ph3C][B(C6F5)4] (l 8.24 (t,
p-H), 7.85 (t, m-H) and 7.64 (d, o-H). 19F-NMR (400
MHz, CD2Cl2, −78°C): l −166.8 (br m, 6F, m-C6F5),
−163.0 (br s, 3F, p-C6F5), –134.1 (br m, 6F, o-C6F5).
The stoichiometry of the overall reaction was deter-
mined on the basis of integrations relative to the reso-
nance of the residual protons of the solvent, and it was
found that the reaction proceeded essentially quantita-
tively and without loss of overall intensity.
The monomers were either bubbled through or, alter-
natively, measured aliquots were added to stirred solu-
tions of Ti(CH2Ph)4 (25 mg, 0.06 mmol) in 5 ml of
toluene or methylene chloride at the desired tempera-
tures. Polymerizations were normally initiated by rapid
addition of B(C6F5)3 (25.6 mg, 0.05 mmol) or
[Ph3C][B(C6F5)4] (46 mg, 0.05 mmol) in the same solvent.
In addition, some polymerization experiments were car-
ried out using different initiator:co-initiator ratios. Reac-
tions were terminated after varying periods of time by
the addition of methanol, the residual initiators were
removed by column chromatography with pentane as
eluant, and the polymers were characterized by NMR
spectroscopy and GPC analysis.
The 1H spectrum also exhibited weak resonances
attributable to bibenzyl (l 2.85) and toluene (l 2.31)
respectively. These become stronger at the expense of the
resonances attributed to [Ti(CH2Ph)3]+ when the tem-
perature of the solution was raised above −50°C. How-
ever, if the 1H-NMR spectrum of the initially formed red
solution was examined at −78°C, weak, ephemeral res-
onances of [Ti(CH2Ph)3(m-CH2Ph)Ti(CH2Ph)3]+ (see be-
low) were also observed.
1.2. Preparation of Ti(CH2Ph)3[(p6-PhCH2)B(C6F5)3]
An NMR tube containing 17 mg (0.04 mmol) of
Ti(CH2Ph)4 and 21 mg (0.04 mmol) B(C6F5)3 was cooled
to −78°C and 0.5 ml of pre-cooled CD2Cl2 was added.
All the material dissolved at −78°C to give a dark-red
solution of Ti(CH2Ph)3[(h6-PhCH2)B(C6F5)3], which
was characterized spectroscopically at −78°C. 1H-
NMR (500 MHz, CD2Cl2, −78°C): l 7.00–7.33 (m, 9H,
TiCH2-m, p-Ph), 6.76 (m, 4H, BCH2-o, m-Ph), 6.69 (br,
1H, BCH2-p-Ph), 6.62 (d, 6H, TiCH2-o-Ph), 2.98 (br s,
2H, BCH2Ph), 2.79 (s, 6H, TiCH2Ph). 13C-NMR (125
1.4. Preparation of
[Ti(CH2Ph)3(v-CH2Ph)Ti(CH2Ph)3][B(C6F5)4]
An NMR tube containing 21 mg of Ti(CH2Ph)4 (0.05
mmol) and 23 mg of (0.025 mmol) [Ph3C][B(C6F5)4] was
cooled to −78°C and 0.5 ml of pre-cooled CD2Cl2 was
added. All of the material dissolved at −78°C to give
a dark-red solution of [Ti(CH2Ph)3(m-CH2Ph)Ti(CH2-
1
Ph)3]+, characterized spectroscopically. H-NMR spec-
1
MHz, CD2Cl2, −78°C, H coupled): l 130.0 (d, J 160
trum (400 MHz, CD2Cl2, −78°C): l 7.10–7.45 (m,
TiCH2-m, p-Ph, Ph3C of Ph3CCH2Ph, traces of bibenzyl
and toluene), 6.82 (br s, 2H, m-CH2-m-Ph), 6.61 (br s,
7H: 6H of m-CH2-o-Ph, 1H of m-CH2-p-Ph partially ob-
scured by Ph3CCH2-o-Ph at 6.52), 6.38 (br s, 6H, TiCH2-
o-Ph), 6.23 (br s, 2H, m-CH2-o-Ph), 2.77 (s, 6H, TiCH2),
2.29 (s, 6H, TiCH2), 0.97 (s, 2H, m-CH2). In addition,
resonances were observed of 1,1,1,2-tetraphenylethane
(one equivalent; l 7.02–7.22, 6.92, 6.52, 3.88) and traces
of bibenzyl (l 2.82) and toluene (l 2.31), but not of
[Ph3C][B(C6F5)4] (l 8.24, 7.85, 7.64). Assignments of the
resonances of [Ti(CH2Ph)3(m-CH2Ph)Ti(CH2Ph)3]+ were
confirmed by 2D-COSY and HMQC experiments and
by the multiplicities exhibited on warming (see below),
while the stoichiometry of the overall reaction was deter-
mined using integrations relative to the resonance of the
residual protons of the solvent. It was found that the re-
action proceeded essentially quantitatively and without
loss of overall intensity.
Hz, TiCH2-m-Ph), 129.7 (d, J 157 Hz, TiCH2-o-Ph),
127.3 (d, J 154 Hz, TiCH2-o-Ph), 105.5 (t, J 137 Hz,
Ti-CH2). Resonances of the BCH2Ph and C6F5 groups
were observed, but were relatively weak and could not
1
be identified unambiguously. The H and 13C assign-
ments made were confirmed by complementary 2D-
COSY and HMQC experiments, while integrations
relative to the resonance of the residual protons of the
solvent showed that the reaction proceeded essentially
quantitatively and without loss of overall intensity.
19F-NMR (400 MHz, CD2Cl2, −50°C): l −168.6 (br
s, 6F, o-C6F5), −138.8 (t, J 22 Hz, 3F, p-C6F5), −134.8
(t, J 22 Hz, 6F, m-C6F5).
1.3. Preparation of [Ti(CH2Ph)3][B(C6F5)4]
An NMR tube containing 11 mg (0.025 mmol) of
Ti(CH2Ph)4 and a slight excess of [Ph3C][B(C6F5)4] (23
mg, 0.025 mmol) was cooled to −78°C, and 0.5 ml of
pre-cooled CD2Cl2 was added. All the material dissolved
at −78°C to give a dark-red solution of [Ti(CH2Ph)3]-
[B(C6F5)4], which was characterized spectroscopically.
1H-NMR (400 MHz, CD2Cl2, −50°C): l 7.06 (t, J=7.0
Hz, m-Ph; p-Ph resonance obscured), 5.49 (d, J=7.2
Hz, 6H, o-Ph), 3.54 (s, 6H, CH2Ph). In addition, there
were resonances attributable to 1,1,1,2-tetraphenyl-
2. Results and discussion
2.1. NMR spectrum of Ti(CH2Ph)4
The variable temperature 1H-NMR spectrum of
Ti(CH2Ph)4 does not appear to have been reported

M. Lin, M.C. Baird / Journal of Organometallic Chemistry 619 (2001) 62–73
65
Fig. 1. 1H-NMR study of the reaction of Ti(CH2Ph)4 with an equimolar amount of B(C6F5)3 in CD2Cl2 at (a) −78°C, (b) −50°C.
previously. In contrast to Sn(CH2Ph)4, which exhibits a
regular tetrahedral structure about the tin, essentially
identical h1-Sn-CH2Ph bonds and Sn–CH2–Ph bond
angles all ꢀ112°, the analogous, homoleptic tetraben-
zyl compounds of titanium, zirconium and hafnium
exhibit varying degrees of h2-metal-CH2Ph bonding in
which the compounds appear to be stabilized by over-
lap of the ring p electron cloud on a phenyl ipso-carbon
atom with a vacant orbital on the metal [9]. The result
is contraction of the metal–CH2–Ph bond angles to
ꢀ90° and much reduced M-ipso-carbon atom internu-
clear distances, as represented by B [9].
in NMR spectra of the compound, we carried out
1
variable temperature H and 13C{1H} in CD2Cl2. The
1H-NMR spectrum of a solution of Ti(CH2Ph)4 in
CD2Cl2 at −50°C exhibits resonances attributable to
methylene (l 2.32, s), o-phenyl (l 6.47, d, J 0.7 Hz) and
m-, p-phenyl (7.1–7.3, m) hydrogen atoms. On warm-
ing to 27°C, the methylene and o-phenyl resonances
shift to l 2.64 and l 6.54, respectively, while the m-,
p-phenyl multiplet remains essentially unchanged. The
change in methylene chemical shift in particular is
consistent with a subtle variation in the position of
equilibrium between various h1- and multihapto struc-
tures [9]. Unfortunately the methylene carbon reso-
nance (125 MHz) did not undergo decoalescence, but
remained a sharp singlet l 94.4 even at −50°C and
thus the ground state structure in solution cannot be
ascertained.
Such h2-coordination of benzyl groups to transition
metals is also known for other types of compounds3,
1
and generally results in both increases of JCH from
2.2. NMR in6estigation of the reaction between
Ti(CH2Ph)4 and B(C6F5)3; formation of
Ti(CH2Ph)3[(p6-PhCH2)B(C6F5)3]
about 120–125 to \130 Hz and shifting of the o-H
resonance upfield of the normal aromatic region, l
6.8–7.5 [10d], i.e. 0.5–1 ppm upfield of the correspond-
ing m-H and p-H resonances [11b]. In cases of com-
pounds for which both h1 and h2 structures are present
in solution, exchange between the two types of sites is
generally rapid on the NMR time scale [10d].
In the case of Ti(CH2Ph)4, the structure in the solid
state contains a range of Ti–CH2–Ph bond angles and
Ti-ipso-C distances [9a], suggesting varying degrees of
Ti-ipso-C interactions within the molecule. Anticipating
that changes in the mode of bonding might be reflected
The resonances of Ti(CH2Ph)4 disappear on addition
of an equimolar amount of B(C6F5)3 at −78°C, and
the methylene resonance (l 2.32) in particular is re-
placed by a narrow singlet at l 2.75 (6H) and a broad
singlet at l 2.98 (2H) (Fig. 1). The latter resonance is
presumably broadened because of the quadrupolar
boron, and thus the data are consistent with the antici-
pated abstraction of a benzyl group from titanium by
boron. As the analogous reaction of Zr(CH2Ph)4 results
in a zwitterionic h6-arene structure as in C (M=Zr)
[11], we expect that an analogous complex (C, M=Ti)
is obtained with Ti(CH2Ph)4.
3 References for other types of h2 benzyl complexes, NMR, see
[10].

66
M. Lin, M.C. Baird / Journal of Organometallic Chemistry 619 (2001) 62–73
Fig. 2. 1H-NMR spectrum of mer-[Ti(CH2Ph)3(THF)3]+ from the reaction of Ti(CH2Ph)4 with B(C6F5)3 and THF at −78°C.
phenyl ring of [B(CH2Ph)(C6F5)3]− counterion. The
chemical shifts of the titanium- and boron-bonded
methylene resonances vary somewhat with temperature,
suggesting a degree of fluxionality of the groups. The
complex decomposes completely at −10°C, a reso-
nance attributable to toluene appearing at l 2.31.
The [Ti(CH2Ph)3+]–[B(CH2Ph)(C6F5)−3 ] interaction is
weak since addition of THF results in the displacement
Unfortunately, the compound formed decomposes in
solution on warming to ꢀ −10°C, and identification
therefore rests completely on the NMR spectroscopic
data. In addition to the two resonances attributable to
the methylene protons of the benzyl groups bound to
titanium and boron, the spectrum at −78°C exhibits
resonances at l 6.62 (6H, d, J 7.2 Hz), attributable to
the TiCH2-o-Ph hydrogens, at l 7.22 (6H, t, J 7.4 Hz)
and l 7.28 (3H, t, J 6.4 Hz), attributable to the
TiCH2-m-Ph and the TiCH2-p-Ph hydrogens, respec-
tively. Thus, the ortho hydrogen resonance lies about
0.6 ppm upfield shift from the resonances of the corre-
sponding meta and para hydrogens, similar to observa-
tions on the zirconium analogue [11b]. In the latter
case, the chemical shift difference was attributed to
interactions of the phenyl p-system with a metal vacant
metal d orbital. The Ti–CH2 13C resonance is shifted
significantly downfield, from l 94 in the precursor
Ti(CH2Ph)4 to l 105.5. There were also observed a
broad four hydrogen resonance at l 6.76 and a broad
one hydrogen resonance at l 6.69, attributable to the
ortho and meta hydrogens and the para hydrogen of the
benzyl group bound to boron, respectively. The pattern
of arene chemical shifts of the BCH2Ph group, with the
meta and para hydrogen resonances shifted upfield rela-
tive to the resonance of the ortho hydrogens, is consis-
tent with structure C. The electronic deficiency of the
titanium is thus relieved by the coordination of the
of anion by THF coordination; a similar result has
been reported for the zirconium analogue [11b]. In an
NMR experiment carried out in CD2Cl2 at −78°C,
addition of a slight excess of THF resulted in complete
1
disappearance of the H resonances of [Ti(CH2Ph)3]–
[(h6-PhCH2)B(C6F5)3]. Resonances for coordinated
THF were observed at l 4.20, 3.26, 2.07, 1.84 at
−78°C, with approximate intensity ratios of 1:2:1:2
(Fig. 2), respectively, consistent with a six-coordinate
meridional structure D.
In addition, resonances attributable to the Ti–CH2
(l 2.99) and m-H (l 7.49) positions of the benzyl
groups were shifted significantly (ꢀ0.2 ppm) downfield
from those of Ti(CH2Ph)3[(h6-PhCH2)B(C6F5)3], consis-
tent with the weak donor abilities of the THF ligands
compared with the h6-borate ligand. In contrast the
o-H resonance (l 6.86) was relatively little shifted,
possibly suggesting a greater degree of h2 bonding in D
than in [Ti(CH2Ph)3][(h6-PhCH2)B(C6F5)3].

M. Lin, M.C. Baird / Journal of Organometallic Chemistry 619 (2001) 62–73
67
Fig. 3. 1H-NMR spectrum of the reaction of Ti(CH2Ph)4 with one molar equivalent of [Ph3C][B(C6F5)4] in CD2Cl2 at −78°C.
2.3. NMR in6estigation of the reaction between
Ti(CH2Ph)4 and [Ph3C][B(C6F5)4]
Ti–CH2Ph resonance at l 3.54, integrating to about
6H atoms relative to the 2H atoms of the Ph3CCH2Ph
resonance at l 3.904 and shifted downfield ꢀ1.2 ppm
from the chemical shift of the neutral precursor as a
consequence of cation formation. The spectrum also
exhibited a broad resonance at l 5.49, also integrating
for 6H and attributable to the ortho-Ph hydrogen
atoms of the newly formed cationic species. The ortho-
Ph chemical shift of the latter is almost 1 ppm upfield
from that of the ortho-Ph hydrogen resonance of
Ti(CH2Ph)4, and presumably implies strong hn interac-
tion of the benzyl ligands with the highly electrophilic
titanium in the cation [12]. The chemical shifts of the
methylene and ortho-Ph resonances were temperature
dependent, moving downfield l 0.1 ppm and ꢀ0.3
ppm, respectively, on changing the temperature from
−78°C to −50°C to −30°C (Fig. 4). The ortho-Ph
resonance also decoalesced to a doublet as the tempera-
ture was increased, and both the broadening and the
chemical shift changes were found to be reversible
between −78°C and −30°C in CD2Cl2. Our investiga-
tion was hampered, however, by slow decomposition
which was suggested by slight increases in the intensities
of the bibenzyl methylene and toluene methyl reso-
nances during the course of the variable temperature
NMR experiments. Thus, the product could not be
isolated as the dark-red solution turned black and a
dark-brown precipitate formed.
The Ti(CH2Ph)4/[Ph3C][B(C6F5)4] system does not
appear to have yet been investigated even for its cata-
lytic properties, and while the analogous zirconium
system, Zr(CH2Ph)4/[Ph3C][B(C6F5)4], has been shown
to induce polymerization of ethylene [7l], the nature of
the species in solution has not been reported. The
reaction of Ti(CH2Ph)4 with [Ph3C][B(C6F5)4] is ex-
pected, by analogy with metallocene systems [1,2], to
proceed as in Eq. (1) and to involve abstraction of a
benzyl anion and formation of Ph3CCH2Ph and the
electron deficient species [Ti(CH2Ph)3]+. The latter
might be expected to be stabilized via multihapto bond-
ing of one or more of the benzyl groups and/or by h6
coordination of either the Ph3CCH2Ph or the
counteranion.
Ti(CH2Ph)4+[Ph3C][B(C6F5)4]
“[Ti(CH2Ph)3][B(C6F5)4]+Ph3CCH2Ph
(1)
Low temperature NMR scale reactions between
Ti(CH2Ph)4 and [Ph3C][B(C6F5)4] were carried out sev-
eral times in CD2Cl2, at various temperatures and with
varying ratios of reagents, and the formation of about
one equivalent Ph3CCH2Ph was confirmed generally by
1H-NMR spectroscopy and GC–MS analyses. Small,
variable resonances of presumed decomposition prod-
ucts bibenzyl and toluene were also identified, and
sometimes rendered satisfactory spectral integrations
difficult.
The NMR results suggest that a single major species
was formed, although clearly a reversible exchange
between h1- and h2- and/or h3-benzyl ligands occurs in
solution. The resonances of the primary product
Ph3CCH2Ph [13] and of the secondary products biben-
zyl and toluene are identical to those of the pure
materials, while the addition of excess toluene has no
The 1H-NMR spectrum of a solution containing
Ti(CH2Ph)4 and a slight molar excess of [Ph3C][B-
(C6F5)4] at −78°C exhibited relatively weak resonances
of unreacted trityl cation at l 8.24, 7.85 and 7.64 (see
Section 1), in addition to methylene or methyl singlets
attributable to Ph3CCH2Ph (l 3.90), bibenzyl (l 2.85)
and toluene (l 2.31) (Fig. 3). Replacing the methylene
resonance of Ti(CH2Ph)4 at l 2.32 was a new singlet
4 The ratio of resonances was generally B3:1, but inclusion of the
intensities of the resonances of the bibenzyl and toluene, presumed
decomposition products during the abstraction reaction, brought the
ratio close to 3:1.

68
M. Lin, M.C. Baird / Journal of Organometallic Chemistry 619 (2001) 62–73
Fig. 4. 1H-NMR spectrum of the reaction of Ti(CH2Ph)4 with one molar equivalent of [Ph3C][B(C6F5)4] in CD2Cl2 at (a) −50°C, (b) −30°C.
effect on the resonances of the titanium complex
formed. It is thus clear that these potential aromatic
ligands are not involved in e.g. h6 bonding. The methyl-
ene or methyl resonances of these compounds would be
expected to shift considerably on coordination; for
instance, the toluene methyl resonance of the complex
[Cp*TiMe2(h6-toluene)]+ is deshielded ꢀ0.4 ppm rela-
tive to the methyl resonance of free toluene [14].
The 19F-NMR spectrum of a freshly prepared reac-
tion mixture at −78°C gave no indication that the
borate anion was coordinating via a fluorine to tita-
nium, and thus the complex formed appears to be the
bare [Ti(CH2Ph)3]+ cation. The latter probably con-
tains all three benzyl ligands at least h2-bound, but the
temperature dependence of the methylene and ortho-Ph
resonances suggests strongly an exchange between two
or more isomers such as E.
dicating that the reaction was not instantaneous at this
temperature, the resonances of this starting material did
disappear after a few minutes, to be replaced by strong,
broad phenyl resonances in the region l 7.1–7.45 and
medium, broad phenyl resonances in the region l 6.23–
6.82 (see Section 1). In addition to the methylene singlet
1
of Ph3CCH2Ph at l 3.88 (2H), the H-NMR spectrum
at −78°C also exhibited rather broad, apparent Ti–
CH2Ph singlets at l 0.97 (2H), 2.3 (6H but overlapping
with toluene methyl resonance) and 2.78 (6H but over-
lapping with bibenzyl methylene resonance).
The spectrum may be tentatively interpreted in terms
of dititanium structures F or G, although the observa-
tion of the high field methylene resonance (l 0.97)
argues in favour of the latter (see below).
On the basis of the NMR evidence, it seems likely the
initial benzyl abstraction reaction (Eq. (1)) was fol-
lowed by reaction of the newly formed [Ti(CH2Ph)3]+
with a second mole of Ti(CH2Ph)4 to give a m-benzyl
complex [(CH2Ph)3Ti-m-CH2PhTi(CH2Ph)3]+ (Eq. (2)):
The exchange may involve interchange between ‘up’
and ‘down’ benzyl groups, or between h1, h2 and/or hn
(n\2) benzyl groups [10f,15].
Reaction of two equivalents of Ti(CH2Ph)4 with one
of [Ph3C][B(C6F5)4] in CD2Cl2 at −78°C resulted again
in the formation of one equivalent of Ph3CCH2Ph per
mole of [Ph3C][B(C6F5)4], as well as small amounts of
[Ti(CH2Ph)3][B(C6F5)4]+Ti(CH2Ph)4
“[(CH2Ph)3Ti(m-CH2Ph)Ti(CH2Ph)3][B(C6F5)4]
(2)
1
bibenzyl and toluene. Although the H-NMR spectrum
It is interesting to note that a 1H-NMR spectrum
recorded at −78°C immediately after the combining of
equivalent amounts of Ti(CH2Ph)4 and [Ph3C][B-
(Fig. 5(a)) run immediately after the reactants had been
combined exhibited resonances of [Ph3C][B(C6F5)4], in-

M. Lin, M.C. Baird / Journal of Organometallic Chemistry 619 (2001) 62–73
69
Fig. 5. VT 1H-NMR of the reaction of Ti(CH2Ph)4 with [Ph3C][B(C6F5)4], 2:1 ratio in CD2Cl2 at (a) −78°C, (b) −50°C, (c) −30°C.
1H chemical shifts of the m-CH2 resonances are consis-
(C6F5)4] exhibited temporarily the resonances of
[(CH2Ph)3Ti(m-CH2Ph)Ti(CH2Ph)3]+, in addition to
those of [Ph3C][B(C6F5)4]. After completion of the reac-
tion, only the resonances of [Ti(CH2Ph)3]+ remained,
suggesting that the two cationic titanium complexes are
indeed related and possibly involved in a facile
equilibrium.
tent with assignments to a bridging benzyl group.5
3. Alkene polymerizations
As indicated in Section 1 and Tables 1–3, we have
assessed the ability of Ti(CH2Ph)4, activated by
B(C6F5)3 and [Ph3C][B(C6F5)4], to act both as a
Ziegler–Natta catalyst (with ethylene and 1-hexene)
and as a carbocationic initiator (IB, EVE). As expected,
both B(C6F5)3 and the trityl cation abstract a benzyl
group from Ti(CH2Ph)4, giving Ti(CH2Ph)3[(h6-
PhCH2)B(C6F5)3] in the case of the former, either
[Ti(CH2Ph)3][B(C6F5)4] or [(CH2Ph)3Ti(m-CH2Ph)Ti-
(CH2Ph)3][B(C6F5)4] in the latter. All are potential
sources of the cationic species [Ti(CH2Ph)3]+, which
should in principle be capable of inducing Ziegler–
Natta or carbocationic polymerization processes.
When the temperature was raised to −50°C (Fig.
5(b)), the o-, m- and p-CH2Ph resonances of
Ph3CCH2Ph at l 6.55 (d), 6.93 (t) and 7.03 (t), respec-
tively, narrowed considerably and were readily iden-
tified. However, the remaining phenyl resonances at l
6.24, 6.38, 6.61 and 6.83 broadened at −50°C and
coalesced into a single broad (Dw1/2 ꢀ0.2 ppm) reso-
nance at l ꢀ6.55 at −30°C (Fig. 5(c)). Furthermore,
the methylene resonances at l 2.77 (6H, TiCH2) and
2.29 (6H, TiCH2) had both broadened significantly
(Dw1/2 ꢀ0.2 ppm) at −50°C while that at 0.97 (2H,
m-CH2) had broadened and shifted downfield ꢀ0.17
ppm. On warming to −30°C, the two resonances at l
2.77 and 2.29 appear to have coalesced into a very
broad resonance at l 2.75 (ꢀ12H, Dw1/2 ꢀ0.4 ppm)
while that at l 0.97 had shifted downfield to l 1.23.
Because of continual decomposition, indicated by
growing resonances of toluene and bibenzyl, the system
was difficult to characterize properly. An HMQC ex-
periment at −50°C indicated that the putative Ti–CH2
resonances at l 2.77, 2.29 and 0.97 correlated with
apparent Ti–13CH2 resonances at l 112, 98, 49, and
respectively, confirming the assignments. The high-field
3.1. Ethylene polymerization
Ethylene polymerizations were performed in both
toluene and chlorobenzene, and with results as shown
in Table 1 where comparisons are also made with
5 The
m-methyl
resonance
in
[Cp*TiMe2(m-
Me)TiMe2Cp*][BMe(C6F5)3] is observed ꢀ1.3 ppm upfield of the
terminal methyl resonance, see [16].

70
M. Lin, M.C. Baird / Journal of Organometallic Chemistry 619 (2001) 62–73
Table 1
Ethylene polymerization of Ti(CH2Ph)4 and Zr(CH2Ph)4 with various co-catalysts
a
Run
Catalyst d
(mmol)
Co-catalyst d
(mmol)
Solvent
Time (min)
Temperature
(°C)
Yield (g)
Activity (kg PE
(mol·cat·h)−1
)
1 b [7j]
2 b [7j]
ZrBz4 (0.035)
ZrBz4 (0.2)
MAO (10.0)
Toluene
Toluene
Toluene
Toluene
30
60
15
15
15
30
30
8
50
50
50
50
50
50
26
50
25
3.2
5.0
1.8
0.2
0.004
0.048
Trace
0.078
Trace
18.3
28.6
14.4
1.6
0.64
1.9
–
AFPB (0.035)
B(C6F5)3 (0.1)
B(C6F5)3 (0.1)
B(C6F5)3 (0.025) Toluene
B(C6F5)3
B(C6F5)3
[Ph3C][B(C6F5)4] Toluene
[Ph3C][B(C6F5)4] Toluene
(0.05)
[Ph3C][B(C6F5)4] Chlorobenzene 15
(0.05)
3 b [7k] ZrBz4 (0.15)
4 b [7k] TiBz4 (0.15)
5 a
TiBz4 (0.025)
6 b [12] ZrBz4 (0.05)
Toluene
Toluene
7 a [12] ZrBz4 (0.025)
8 a
9 a
ZrBz4 (0.05)
TiBz4 (0.06)
28
15
–
10 a
11 c
TiBz4 (0.06)
TiBz4 (0.06)
25
50
0.056
0.012
4.48
0.96
[Ph3C][B(C6F5)4] Toluene
(0.05)
15
a P(C2H4)=1 atm.
b P(C2H4)=5 atm.
c P(C2H4)=3 atm.
d Bz, benzyl, MAO, methylaluminoxane, AFPB, N,N-dimethylanilinium tetrakis(pentafluorophenyl)borate.
Table 2
1-Hexene oligomerization initiated by Ti(CH2Ph)4/[Ph3C][B(C6F5)4]
a
Run
Solvent
Time (time)
Temperature (°C)
Yield (g)
Conversion (%)
Mw
Mw/Mn
1 b
2 b
3 c
4 b
5 b
6 b
7 d
8 b
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
Toluene
Toluene
Toluene
30
60
60
60
60
30
30
30
50
25
25
0.28
2.0
0.08
2.2
Trace
0.23
0.67
Trace
16.5
66.7
B1
73.3
–
854
1200
–
1320
–
1000
1250
–
1.10
1.20
–
1.24
–
1.10
1.14
–
0
−30
0
8
22
0
−50
–
a 3 g of 1-hexene.
b 0.06 mmol Ti(CH2Ph)4; the ratio Ti(CH2Ph)4:[Ph3C][B(C6F5)4]=1.2:1.
c 0.05 mmol [Ph3C][B(C6F5)4].
d Ratio Ti(CH2Ph)4:[Ph3C][B(C6F5)4]=2:1.
Table 3
Polymerization of IB and EVE with Ti(CH2Ph)4 activated by [Ph3C][B(C6F5)4]
a
Run
Alkene Solvent
Time (min)
Temperature
(°C)
Monomer weight (g) Yield (g)
Conversion (%)
Mw
Mw/Mn
1
2
3
4
5
6
7
8
9
IB
IB
IB
IB
IB
IB
IB
IB
CH2Cl2
CH2Cl2
CH2Cl2
Toluene 10
Toluene 10
Toluene 10
CH2Cl2
Toluene
CH2Cl2
20
20
10
25
0
−15
25
–
–
2.7
–
–
2.5
3.1
2.7
2.0
2.0
3.8
3.4
2.4
Trace
Trace
0.4
2.9
0.26
2.0
–
–
979
1.19
1.78
1.84
1.27
1.48
1.51
1.38
1.89
1.82
2.06
19 400
20 800
47 800
50 500
203 000
4300
230 800
33 900
73 100
90
–
–
0
−15
−50
−50
25
16
92
10
5
5
10
EVE
EVE
ꢀ100
ꢀ100
10
Toluene 10
25
2.0
a 0.06 mmol Ti(CH2Ph)4; ratio Ti(CH2Ph)4:[Ph3C][B(C6F5)4]=1.2:1, except for runs 7, 8, for which Ti(CH2Ph)4:[Ph3C][B(C6F5)4]=2:1.

M. Lin, M.C. Baird / Journal of Organometallic Chemistry 619 (2001) 62–73
71
Fig. 6. 1H-NMR spectrum of oligomer of 1-hexene.
similar systems involving Zr(CH2Ph)4. While compari-
sons with investigations carried out elsewhere are
difficult, the catalytic activities of the Ti(CH2Ph)4/
B(C6F5)3 and Ti(CH2Ph)4/[Ph3C][B(C6F5)4] systems for
ethylene polymerization are both very low, indeed
lower than activities of the similar zirconium systems
[7j,k,12]. While the Ti(CH2Ph)4/B(C6F5)3 system also
appears to be much less active than the Ti(CH2Ph)4/
[Ph3C][B(C6F5)4] system, consistent with observations
that the poorly coordinating anion [B(C6F5)4]− gener-
ally gives more active catalysts [4,11], ethylene polymer-
ization clearly does not warrant further comment
except to note that the activity of the Ti(CH2Ph)4/
[Ph3C][B(C6F5)4] system is higher in chlorobenzene than
in toluene, possibly because of toluene coordination
[14] which would hinder monomer coordination. The
polyethylene products were in all cases too insoluble for
molecular weights to be determined, and thus were
presumably essentially linear.
the polymerizations were not induced by protons result-
ing from reaction of the trityl cation with adventitious
water [4a,b].
The oligomers obtained were characterized by their
13C{1H}-NMR spectra (Fig. 6), which were found to
differ greatly from the much simpler spectra of atactic
poly-1-hexenes formed via normal 1,2-inserted 1-hexene
units, obtained using Ziegler–Natta catalysts such as
Cp*TiMe3/B(C6F5)3 [17]. Although the latter catalyst
can also produce polymers containing significant
amounts of regioirregularities arising from 2,1-inser-
tions, the 13C{1H}-NMR spectra of such materials
nonetheless indicate that they are essentially linear
polymers containing almost exclusively butyl branching
[17]. In contrast, the 13C{1H}-NMR spectra of the
materials formed by the Ti(CH2Ph)4/[Ph3C][B(C6F5)4]
initiator system indicate that they contain branches of
varying lengths [18], and are in fact rather similar to the
low molecular weight, highly branched products of
carbocationic oligomerization of 1-hexene induced by
AlCl3 [18a]. Our finding that the Ti(CH2Ph)4/
[Ph3C][B(C6F5)4] initiator system is significantly more
active for 1-hexene than for ethylene polymerization is
also consistent with carbocationic rather than Ziegler–
Natta polymerization of the 1-hexene; while 1-hexene is
not readily polymerized via a carbocationic mechanism,
it is more amenable to this type of process than is
ethylene [3a,4].
3.2. 1-Hexene polymerization
The results of polymerizations of 1-hexene by the
Ti(CH2Ph)4/[Ph3C][B(C6F5)4] system are shown in
Table 2; as with ethylene, the Ti(CH2Ph)4/B(C6F5)3
system was relatively inactive and was little studied. In
a typical experiment, a solution of [Ph3C][B(C6F5)4] was
added to a mixture of Ti(CH2Ph)4 and 1-hexene in
methylene chloride or toluene; there was invariably an
immediate color change accompanied by evolution of
heat. Termination of the reaction after 30–60 min
resulted in moderate conversions (ꢀ70%) at both room
temperature and 0°C, the products formed being
oligomeric and exhibiting narrow molecular weight dis-
tributions. Conversion decreased significantly at tem-
peratures \50°C and B −30°C in CH2Cl2.
Polymerizations in toluene resulted in lower conver-
sions compared with methylene chloride, while
[Ph3C][B(C6F5)4] alone was relatively ineffective. Thus,
1
Interestingly, the H-NMR spectra of the oligomers
also exhibited very weak phenyl resonances, indicating
the presence of benzyl end groups. The latter could be
incorporated via the initial insertion step of a Ziegler–
Natta process (as in Eq. (1)), or via either intra- or
intermolecular transfer of benzyl groups from a tita-
nium to a carbocationic center of intermediates of the
type [(PhCH2)3Ti(C6H12)nCH2–+CH(n-Bu)]. To test
for the possibility that a neutral titanium species can
transfer a benzyl group to a distant carbocationic cen-
ter, we carried out oligomerization experiments in the

72
M. Lin, M.C. Baird / Journal of Organometallic Chemistry 619 (2001) 62–73
presence of excess Ti(CH2Ph)4 (two equivalents per
[Ph3C][B(C6F5)4]) in toluene at 0°C (Table 2, compare
runs 6, 7). The products formed exhibited essentially
the complex [Ti(CH2Ph)3]+, and the latter appears to be
capable of existence as such, stabilized by multihapto
benzyl coordination. In the presence of excess
Ti(CH2Ph)4, however, a m-benzyl dititanium complex is
formed, apparently with the structure [(CH2Ph)3Ti(m-
CH2Ph)Ti(CH2Ph)3]+. All of these species are thermally
unstable above about −10°C in methylene chloride,
forming dark solutions and precipitates. However, the
species [Ti(CH2Ph)3]+ is sufficiently stable in the pres-
ence of alkenes in toluene that solutions do not darken
or throw precipitates, but instead can initiate oligomer-
ization or polymerization of ethylene, 1-hexene,
isobutylene and ethyl vinyl ether over a range of temper-
atures. An alternative possibility, that proton sources
such as residual water could be the actual initiator, is
quite unlikely since the benzyl-titanium complexes react
rapidly with water. NMR studies of the oligomeric
poly-1-hexene show that this material is highly
branched, similar to the similar material produced via a
carbocationic process, but contains minor amounts of
benzyl end groups; thus it may be formed in part via a
Ziegler–Natta process. In contrast, the polyisobutylene
and polyethyl vinyl ether products do not contain
benzyl end groups, consistent with the anticipated car-
bocationic process for these polymerizations.
1
identical molecular weight distributions and H-NMR
spectra, suggesting that the presence of excess
Ti(CH2Ph)4 had little effect on the oligomerization
process. Unfortunately, the relative intensities of the
weak phenyl resonances varied erratically and we were
unable to confirm or deny a correlation with the pres-
ence of excess Ti(CH2Ph)4. Thus, the possibility that a
Ziegler–Natta process pertains to a slight degree re-
mains moot.
3.3. Polymerization of IB and EVE
These two monomers are prototypical alkenes for
carbocationic processes [4], and no unambiguous exam-
ples of their homopolymerization via Ziegler–Natta
processes are known. Polymerizations of the two alke-
nes by the Ti(CH2Ph)4/[Ph3C][B(C6F5)4] system were
carried out largely as described for 1-hexene, although
in some cases IB was just bubbled through a solution of
Ti(CH2Ph)4 as co-initiator was added; the results are
indicated in Table 3. Addition of [Ph3C][B(C6F5)4] to the
reaction mixtures resulted in all cases in rapid polymer-
ization of IB and EVE, with concurrent heating and
immediate color changes. High conversions are ob-
tained in polymerization of IB in CH2Cl2 and of EVE in
both solvents (ꢀ90–100%). Polymerizations of IB in
toluene give only trace yields at temperatures higher
than 0°C and moderate to low conversions at low
temperature, although reasonably high molecular
weights and quite narrow molecular weight distributions
were obtained. Rather lower molecular weights were
obtained in methylene chloride for both monomers.
That the polymerizations of IB and EVE proceed by
Acknowledgements
We thank the Natural Sciences and Engineering Re-
search Council of Canada (Research and Strategic
Grants to M.C.B.) and Queen’s University (Queen’s
scholarship to M.L.) for financial support of this re-
search, and to Dr. M. Parent for assistance with GPC
measurements, Dr. F. Sauriol and Ms S. Blake for
assistance with NMR measurements.
1
carbocationic processes was confirmed by the H-NMR
spectra of the products, as no phenyl resonances were
observed in any cases. The observations of higher activ-
ities and molecular weights for PIB formation with
decreases in temperature are also consistent with carbo-
cationic processes [4], and suggest a different polymer-
ization mechanism from that which applies for
1-hexene.
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Treatment of Ti(CH2Ph)4 with an equimolar amount
of B(C6F5)3 results in benzyl group abstraction to give
Ti(CH2Ph)3[(h6-PhCH2)B(C6F5)3], in which the result-
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reactions involving one and two molar equivalents of
the trityl salt, [Ph3C][B(C6F5)4], also result in benzyl
group abstraction to give 1,1,1,2-tetraphenylethane and

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