Synthesis and Ethylene Polymerization Capability of
Metallocene-like Imido Titanium Dialkyl Compounds and
Their Reactions with AliBu3
Paul D. Bolton,† Nico Adams,† Eric Clot,‡ Andrew R. Cowley,† Paul J. Wilson,§
Martin Schro¨der,§ and Philip Mountford*,†
Chemistry Research Laboratory, UniVersity of Oxford, Mansfield Road, Oxford OX1 3TA, U.K.,
Laboratoire de Structure et Dynamique des Syste`mes Mole´culaires et Solides (UMR 5636 CNRS-UM2),
Institut Charles Gerhardt, cc 14, UniVersite´ Montpellier 2, 34095 Montpellier Cedex 5, France, and
School of Chemistry, UniVersity of Nottingham, UniVersity Park, Nottingham NG7 2RD, U.K.
ReceiVed July 7, 2006
A series of alkyl- and aryl-imido titanium dialkyl compounds Ti(NtBu)(Me3[9]aneN3)R2 (R ) Me (1),
CH2SiMe3 (3), CH2tBu (4), CH2Ph (5)), Ti(NR)(Me3[9]aneN3)Me2 (R ) iPr (6), Ph (7), 3,5-C6H3(CF3)2
(8), 2,6-C6H3iPr2 (9), 2-C6H4CF3 (10), 2-C6H4tBu (11)), and Ti(NR)(Me3[9]aneN3)(CH2SiMe3)2 (R ) iPr
(12), ArF (13)) were prepared and crystallographically characterized in the case of 1, 6-9, and 11 (Me3-
[9]aneN3 ) 1,4,7-trimethyl triazacyclononane; ArF ) C6F5). These compounds, isolobal with the titanocenes
Cp2TiR2, were thermally stable at elevated temperatures except for 4. Reaction of 7 with [Ph3C][BArF4]
(TB) and diisopropylcarbodiimide in CH2Cl2 gave the Ti-Me insertion product [Ti(NPh)(Me3[9]-
aneN3){MeC(NiPr)2}][BArF4] (15-BArF4). The corresponding reaction of 7 in the absence of organic
substrate gave [Ti2(í-NPh)2(Me3[9]aneN3)2Cl2][BArF4]2 via a solvent activation reaction. The room-
temperature ethylene polymerization capabilities of the dialkyl compounds were evaluated using TB
cocatalyst in the presence of AliBu3 (TIBA). Among the dimethyl precatalysts, only the systems 1 and
11, with the bulkiest imido groups, showed high productivities (6230 and 1210 kg mol-1 h-1 bar-1,
respectively). The productivites of the other tert-butyl imido precatalysts 3 and 4 (130 and 120 kg mol-1
h-1 bar-1, respectively) were substantially lower than that of 1. The catalyst system 1/TIBA (2500 equiv,
no added TB) was also active for ethylene polymerization (225 kg mol-1 h-1 bar-1). The less productive
imido dialkyl precatalysts all formed complex mixtures on exposure to TIBA. The polyethylenes produced
with 1, 3, and 5-11 generally had Mw/Mn values in the range 2.6-3.0. The PE formed with 1/TB/TIBA
was terminated only by methyl end groups, consistent with chain transfer to TIBA followed by subsequent
â-H transfer by the resultant titanium isobutyl cation. The alkyl cations [Ti(NtBu)(Me3[9]aneN3)R]+ (R
) Me or CH2SiMe3) reacted rapidly with TIBA in C6D5Br at -30 °C, forming isobutene. DFT calculations
found that TIBA adducts of the model methyl cation [Ti(NMe)(H3[9]aneN3)Me]+ were energetically
favorable by ca. -80 to -110 kJ mol-1. Whereas 1 alone or with AlMe3 present has been shown to form
only Ph3CMe on reaction with [Ph3C]+, 1:1 mixtures of 1 and TIBA gave Ph3CH as the only trityl-
containing product, suggesting a key role for transient [AliBu2]+ in the activation process for these catalysts.
Overall, the imido group in the Ti(NR)(Me3[9]aneN3)Me2/TB/TIBA catalysts systems appears to have
two roles: to stabilize the dialkyl precatalyst toward degradation by the TIBA itself prior to activation,
and to inhibit the formation of catalytically inactive hetero- or homo-bimetallic complexes.
Introduction
The development of nonmetallocene, transition metal olefin
polymerization catalysts continues to be an area of considerable
academic and industrial activity.1-8 Complexes of a wide range
of heteroatom-donor supporting ligands have been prepared and
assessed, including transition metal imido compounds, LnM-
(NR)mX2 (Ln ) additional ligand or ligand set; X ) alkyl or
halide typically).8 The formally dianionic imido ligand [NR]2-
is isolobal with cyclopentadienide,9-12 a conceptual feature that
has been used in the design of catalysts containing these N-donor
groups. Group 6 bis(imido) and group 5 cyclopentadienyl-imido
catalysts of the type M(NR)2X2 and CpM(NR)X2, respectively,
* Corresponding author. E-mail: philip.mountford@chem.ox.ac.uk.
† University of Oxford.
‡ Universite´ Montpellier 2.
§ University of Nottingham.
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5549Organometallics 2006, 25, 5549-5565
10.1021/om0606136 CCC: $33.50 © 2006 American Chemical Society
Publication on Web 10/17/2006

have been assessed as isolobal analogues of the ubiquitious
group 4 metallocene family.13-16 We have extended this
approach by combining imido titanium17 fragments with neutral,
six-electron donor ligands such as Me3[9]aneN3 (1,4,7-
trimethyltriazacyclononane),18-23 tris(pyrazolyl)methanes,24,25
triazacyclohexanes,23,26 and others.18,27 Since both the dianionic
NR2- and conical fac-N3 ligands28 are isolobal with C5H5-, the
compounds Ti(NR)(fac-N3)X2 (X ) halide or alkyl) are also
isolobal analogues of the group 4 metallocenes Cp2MCl2. We
have also described detailed studies of the electronic structures
of Ti(NR)(Me3[9]aneN3)X2, certain well-defined monoalkyl
cations [Ti(NtBu)(Me3[9]aneN3)R]+ (R ) Me or CH2SiMe3),
and their relationships to the isolobal bis(cyclopentadienyl)-
titanium systems.22 The alkyl cations were prepared by reaction
of [Ph3C][BArF4] (TB, ArF ) C6F5) with the corresponding
dialkyls, the syntheses of which are reported for the first time
here.
Of the Ti(NR)(fac-N3)Cl2/MAO catalyst systems studied
(MAO ) methylaluminoxane), those with Me3[9]aneN3 and tris-
(3,5-dimethylpyrazolyl)methane co-ligands gave the best pro-
ductivites and molecular weight distributions. Furthermore, only
compounds with bulky imido R-substituents (e.g., R ) tBu,
adamantyl, 2-C6H4tBu) afforded highly productive catalysts,
especially under more commercially relevant conditions.20,23,25
We recently found analogous structure-productivity trends in
an isostructural series of vanadium(IV) catalyst systems V(NR)-
{HC(Me2pz)3}Cl2/MAO, implying that adequate steric protec-
tion of the TidNR bond is a prerequisite for highly active
systems.29
MAO is one of the most widely used activators in Zieger-
Natta catalysis, especially with regard to metallocene systems.30-34
It fulfills a dual role as both a precatalyst activator and impurity
scavenger, and is typically used in a large Al:M ratio. The active
species in Zieger-Natta systems are cationic alkyl complexes
of the type “[LnM-R]+”.4,32-34 Typically, however, the MAOs
used in these studies contain up to 20-30 wt % AlMe3 (TMA),
and the catalyst resting state in these instances is probably a
bimetallic species of the type [LnM(í-R)2AlR2]+.35-44 Even this
can be a simplistic view, with a range of homo- and hetero-
bimetallic alkyl and alkyl-chloride species potentially being
formed when dichloride precatalysts are activated with an excess
of MAO.37,40,45 For example, the titanium species formed in
the reactions of Cp2TiCl2 with an excess of MAO included Cp2-
TiMe2, Cp2TiMeCl, [Cp2TiMe(í-Cl)Cp2TiCl]+, [Cp2TiMe(í-
Cl)Cp2TiMe]+, [Cp2TiMe(í-Me)Cp2TiMe]+, and [Cp2Ti(í-
Me)2AlMe2]+.40
As mentioned, we have recently reported the well-defined
monoalkyl cations [Ti(NtBu)(Me3[9]aneN3)R]+ (R ) Me or
CH2SiMe3) and some preliminary accounts of their reactions,
including stoichiometric Ti-R bond insertion, Lewis base
addition, and C-H bond and solvent activation processes.20-22
However, the methyl cation [Ti(NtBu)(Me3[9]aneN3)Me]+ also
formed the mono(í-methyl)-bridged homobimetallic species
[Ti2(NtBu)2(Me3[9]aneN3)2Me2(í-Me)]+ with excess Ti(NtBu)-
(Me3[9]aneN3)Me2 (1)22 and the stable bis(í-methyl)-bridged
heterobimetallic [Ti(NtBu)(Me3[9]aneN3)(í-Me)2AlMe2]+ with
TMA.21 The chloride cation [Ti(NtBu)(Me3[9]aneN3)Cl]+ formed
the homobimetallic species [Ti2(NtBu)2(Me3[9]aneN3)2Me2(í-
Cl)]+ on reaction with 1.22 These rather stable bimetallic
complexes are analogues of the titanocenium and zirconocenium
species identified in MAO-activated metallocene catalyst sys-
tems.40,45 Although we have no direct evidence for the presence
of such species in the Ti(NR)(fac-N3)Cl2/MAO catalyst systems,
it is not unlikely they could represent catalyst trapping and/or
deactivation routes. The extent to which each species might form
would likely depend on the imido substituents, which would in
turn influence the overall structure-productivity relationships
in these catalysts.
Preliminary experiments on the dialkyl compounds Ti(NtBu)-
(Me3[9]aneN3)R2 (R ) Me (1) or CH2SiMe3 (3)) as polymer-
ization precatalysts were promising and suggested that they
would offer a way to avoid MAO as an activator-scavenger.20
Reaction of 3 with TB on the NMR tube scale followed by
addition of C2H4 afforded a copious white precipitate of poly-
ethylene (PE). Polymerization of ethylene with 1 using TB
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