Transition metal imido catalysts for ethylene polymerisation

Article abstract of DOI:10.1016/S0022-328X(99)00452-0

The imido complexes CpV(N-2-MeC₆H₄)Cl₂ (1), Cr(N-^tBu)₂Cl₂ (2), CpNb(N-2-^tBuC₆H₄)Cl₂ (3) and Mo(N-^tBu)₂Cl₂ (4) have been tested as procatalysts for the polymerisation of ethylene in combination with diethylaluminium chloride or methylaluminoxane (MAO) co-catalysts. The vanadium precursors give the highest activities but are short-lived, while the chromium system gives a long-lived catalyst of moderate activity. The niobium and molybdenum derivatives gave relatively low activities under all test conditions. The polyethylene generated by the vanadium and chromium catalysts is of high molecular weight with little branching. The dialkyl complexes Cr(N-^tBu)₂(CH₂Ph)₂ (5), Cr(N-2,6-Prⁱ₂C₆H₃) ₂(CH₂Ph)₂ (6), Cr(N-2,6-Prⁱ₂C₆H₃) ₂Me₂ (7), Mo(N-^tBu)₂(CH₂Ph)₂ (8), Mo(N-2,6-Prⁱ₂C₆H₃) ₂(CH₂Ph)₂ (9), (C₅Me₅)Nb(N-2,6-Prⁱ₂C ₆H₃)Me₂ (10) and (C₅Me₅)Ta(N-^tBu)(CH₂Ph)₂ (11) have been prepared by treatment of the dihalide precursors with appropriate alkylating reagents and investigated as precursors to well-defined cationic alkyl catalysts. Treatment of (5) with [Ph₃C][B(C₆F₅)₄] affords the cationic η²-benzyl species [Cr(N-^tBu)₂(η²-CH₂Ph)][B(C ₆F₅)₄] (12) while its reaction with [PhNMe₂H][B(C₆F₅)₄] liberates toluene to give a mixture of the mono and bis(dimethylaniline) adducts [Cr(N-^tBu)₂(η²-CH₂Ph)(NMe ₂Ph)][B(C₆F₅)₄] (14) and [Cr(N-^tBu)₂(η²-CH₂Ph)(NMe ₂Ph)₂][B(C₆F₅)₄] (15). Complex 12 reacts with trimethylphosphine to give the mono-phosphine adduct [Cr(N-^tBu)₂(η²-CH₂Ph)(PMe ₃)][B(C₆F₅)₄] (13). Solutions containing cationic species (12, 14, 15) are active for ethylene polymerisation in the absence of co-catalyst, affording high-molecular-weight polyethylene with relatively broad molecular-weight distributions.

Full text of DOI:10.1016/S0022-328X(99)00452-0

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 591 (1999) 78–87
Transition metal imido catalysts for ethylene polymerisation
Martyn P. Coles a, Christopher I. Dalby a, Vernon C. Gibson a,*, Ian R. Little b,
Edward L. Marshall a, M. Helena Ribeiro da Costa a, Sergio Mastroianni a
a Department of Chemistry, Imperial College, Exhibition Road, London SW7 2AY, UK
b BP Amoco Chemicals, Sunbury Research Centre, Chertsey Road, Sunbury-on-Thames, Middlesex TW16 7LN, UK
Received 22 June 1999; accepted 22 July 1999
Abstract
The imido complexes CpV(N-2-MeC6H4)Cl2 (1), Cr(N-tBu)2Cl2 (2), CpNb(N-2-tBuC6H4)Cl2 (3) and Mo(N-tBu)2Cl2 (4) have
been tested as procatalysts for the polymerisation of ethylene in combination with diethylaluminium chloride or methylaluminox-
ane (MAO) co-catalysts. The vanadium precursors give the highest activities but are short-lived, while the chromium system gives
a long-lived catalyst of moderate activity. The niobium and molybdenum derivatives gave relatively low activities under all test
conditions. The polyethylene generated by the vanadium and chromium catalysts is of high molecular weight with little branching.
The dialkyl complexes Cr(N-tBu)2(CH2Ph)2 (5), Cr(N-2,6-Pr2i C6H3)2(CH2Ph)2 (6), Cr(N-2,6-Pri2C6H3)2Me2 (7), Mo(N-
tBu)2(CH2Ph)2 (8), Mo(N-2,6-Pr2i C6H3)2(CH2Ph)2 (9), (C5Me5)Nb(N-2,6-Pr2i C6H3)Me2 (10) and (C5Me5)Ta(N-tBu)(CH2Ph)2 (11)
have been prepared by treatment of the dihalide precursors with appropriate alkylating reagents and investigated as precursors to
well-defined cationic alkyl catalysts. Treatment of (5) with [Ph3C][B(C6F5)4] affords the cationic h2-benzyl species [Cr(N-tBu)2(h2-
CH2Ph)][B(C6F5)4] (12) while its reaction with [PhNMe2H][B(C6F5)4] liberates toluene to give a mixture of the mono and
bis(dimethylaniline) adducts [Cr(N-tBu)2(h2-CH2Ph)(NMe2Ph)][B(C6F5)4] (14) and [Cr(N-tBu)2(h2-CH2Ph)(NMe2Ph)2][B(C6F5)4]
(15). Complex 12 reacts with trimethylphosphine to give the mono-phosphine adduct [Cr(N-tBu)2(h2-CH2Ph)(PMe3)][B(C6F5)4]
(13). Solutions containing cationic species (12, 14, 15) are active for ethylene polymerisation in the absence of co-catalyst,
affording high-molecular-weight polyethylene with relatively broad molecular-weight distributions. © 1999 Elsevier Science S.A.
All rights reserved.
Keywords: Transition metals; Imido complexes; Ethylene polymerisation
ship can be drawn with the mono-anionic cyclopentadi-
enide moiety [2]. Examples of 1s,2p ligands include
borollide [3,4], dicarbollide [5,6], imide [7–10] and bu-
1. Introduction
In recent years there has been increasing interest in
tadienide [11–15]. It is thus possible using these ligands
to synthesise isolobal Group 5 and Group 6 metal
analogues of the Group 4 metallocenes, thereby offer-
ing an opportunity to extend the horizon of metal-
locene-like reactivity further to the right in the
transition series.
the discovery and development of new families of cata-
lysts for a-olefin polymerisation [1], driven by a desire
to access polyolefinic materials with new or improved
performance parameters. A wide variety of ligand types
have been investigated, ranging from mono through to
tetradentate coordination and bearing either neutral or
charged donor groups.
Over the past few years we have been investigating
the isolobal relationship for half-sandwich metal imido
Dianionic ligands (L2−) that can bind to a metal
using a combination of 1s and 2p orbital interactions
present an interesting case, since an isolobal relation-
complexes of the Group
5 metals [16–22] and
bis(imido)metal complexes of the Group 6 metals [23–
27] — the isolobal relationship between imido com-
plexes and Group 4 metallocenes is shown in Scheme 1.
Within this series, chromium holds a special place due
to its widespread use in the commercial production of
* Corresponding author.
E-mail address: v.gibson@ic.ac.uk (V.C. Gibson)
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00452-0

M.P. Coles et al. / Journal of Organometallic Chemistry 591 (1999) 78–87
2. Results
79
2.1. Dichloride procatalysts
The prototypical Group 5 and Group 6 metal imido
dichloride compounds 1–4 were selected for initial
screening. The results of Schlenk-scale 1 atm ethylene
polymerisation tests using dual component catalyst sys-
tems (metal dihalide procatalyst+co-catalyst) are col-
lected in Table 1. It should be noted that these tests
were employed primarily to establish the presence of
activity for the imido procatalyst and to identify cata-
lysts for a more in-depth study; the results reported in
Table 1 were reproduced several times, though not
optimised beyond the conditions reported. First, it is
evident that the half-sandwich vanadium tolylimido
procatalyst is intrinsically the most active of these metal
imido systems when either diethylaluminium chloride
or methylaluminoxane (MAO) is employed (runs 1 and
2, cf. 3–6). It is also evident that a large excess of MAO
does not lead to a vast increase in activity for the
vanadium system and MAO leads to a decrease in
activity for the chromium procatalyst. Although the
niobium and molybdenum procatalysts showed some
activity under the same conditions, they were far infe-
rior to their first row congeners.
In the case of vanadium, the polyethylene forms
rapidly over the first few minutes of the polymerisation
after which the catalyst is inactive, whereas for the
chromium catalyst the flocculent polyethylene product
is formed over the 60 min duration of the run. Indeed,
filtration of the supernatant solution from this poly-
merisation run followed by exposure to ethylene (1
atm) over a further 60 min period gave a similar
quantity of polyethylene. GPC analysis of the PE gen-
erated from run 1 showed a broad molecular weight
distribution with a main peak (Mp) value of 2×105.
Scheme 1.
polyethylene [28–31]. An additional attraction of this
study, therefore, was the possibility for developing a
homogeneous, well-defined chromium polymerisation
catalyst system. Molecular chromium systems have re-
cently been reviewed [32,33].
Here we describe the ethylene polymerisation charac-
teristics of a series of transition metal imido procata-
lysts, both as conventional dual component systems
and in the case of chromium a more well-defined sys-
tem. Aspects of this work have been communicated
[7,10].
Table 1
Results of 1 atm ethylene polymerisation tests using dichloride procatalysts
Run
Procatalyst (mmol)
Activator (mmol equiv.−1
)
Yield PE (g)
0.44
Activity (g mmol−1 h−1 b−1
)
a
1
2
3
4
5
6
VCp(Ntol)Cl2
Et2AlCl
(3.50/20)
MAO
(12/1715)
Et2AlCl
(3.86/20)
MAO
(12/300)
Et2AlCl
(3.60/30)
Et2AlCl
(3.20/20)
15.1 b
27.1 b
9.0
1 (0.175)
VCp(Ntol)Cl2
a
0.19
1 (0.007)
Cr(N-tBu)2Cl2
2 (0.007)
1.73
Cr(N-tBu)2Cl2
2 (0.040)
0.19
4.4
NbCp(NAr c)Cl2
3 (0.120)
0.06
0.6
Mo(N-tBu)2Cl2
4 (0.160)
B0.03
B0.2
a Ntol=N-2-MeC6H4.
b Catalyst deactivation within minutes.
c Ar=2-tBuC6H4 [41].

80
M.P. Coles et al. / Journal of Organometallic Chemistry 591 (1999) 78–87
The polymer derived from the chromium catalyst (run
3) gave a main peak (Mp) value of 1.2×106. In both
cases there was little evidence of branching (B1 branch
per 1000 carbons) by NMR spectroscopy.
2.2. Dialkyl procatalysts
Having identified promising candidates for ethylene
polymerisation, particularly the vanadium and
chromium procatalyst systems, our next objective was
to synthesise dialkyl versions of these complexes and to
investigate them as precursors to well-defined ‘single-
site’ cationic alkyl catalysts. Attempts to synthesise
dialkyl derivatives of the vanadium system were unsuc-
cessful due to a propensity for reductive dimerisation
[34–36], an observation that is most likely relevant to
the short lifetime of the vanadium system. Teuben and
co-workers [34,35] have isolated and structurally char-
acterised the binuclear V(IV) species, [CpV(m-N-2-
MeC6H4)Me]2, and in a separate study on the sterically
hindered imido complex, CpV(N-2,6-Pri2C6H3)Cl2, we
have isolated two other highly unusual divanadium
species that arise from reductive dimerisation [36]. Sup-
porting the vanadium procatalyst on a polystyrene
support has been shown to increase the lifetime of the
system [37].
Fig. 1. The 13C CP MAS solid-state NMR spectrum of 1: (a) without
non-quaternary suppression; (b) with non-quaternary suppression.
By contrast, the bis(imido)chromium system is well
behaved towards alkylation. The chromium dibenzyl
complex Cr(N-tBu)2(CH2Ph)2 (5) can be prepared in
good yield by treatment of Cr(N-tBu)2Cl2 with two
equivalents of benzylmagnesium chloride. Despite gen-
eral difficulties in obtaining crystalline samples of di-
alkyl complexes of this type [38] 5 can be crystallised
from cold pentane. Its molecular structure has been
described previously [10] and reveals h1- and h2-benzyl
ligands. The 13C CP MAS solid-state NMR spectrum of
5 is shown with and without non-quaternary suppres-
sion in Fig. 1 and clearly shows the two different
ipso-carbon resonances at l 158 and 124 ppm at-
tributable to the h1- and h2-bonding modes, respec-
tively. The solution 13C-NMR spectrum shows an
averaged resonance at l 140 ppm. The analogous
arylimido complex, Cr(N-2,6-Pri2C6H3)2(CH2Ph)2 (6),
can be prepared by a closely related procedure.
chromium analogue, characteristic shifts for the ipso-
carbons of h1- and h2-benzyl ligands are observed, this
time at l 156 and 125 ppm, respectively, the solution
NMR spectrum giving an averaged signal at ca. l 140.
In earlier alkylation studies on the half-sandwich
niobium and tantalum system [20], we found that it was
necessary to use bulky cyclopentadienyl ligands in or
The methyl analogue of 5 has been reported by
Schaverien and co-workers [38] as an impure oil. In
order to obtain a crystalline species we turned our
attention to the bulky bis(arylimido)chromium system
[39]; treatment of Cr(N-2,6-Pri2C6H3)2Cl2 with two
equivalents of methylmagnesium bromide afforded
Cr(N-2,6-Pri2C6H3)2Me2 (7) as a green crystalline solid
in reasonable yield.
The molybdenum analogue of 5 was prepared in a
straightforward procedure from Mo(N-tBu)2Cl2(dme)2
[40]. A comparison of the 13C solution and CP MAS
solid-state NMR spectra are shown in Fig. 2. As for its
Fig. 2. 13C-NMR spectra for complex 4: (a) in solution; (b) in the
solid state, with non-quaternary suppression.

M.P. Coles et al. / Journal of Organometallic Chemistry 591 (1999) 78–87
81
Table 2
Results of high pressure/temperature ethylene polymerisation tests using dialkyl procatalysts
Run Catalyst precursor
(mmol)
Activator
(equiv.)
Temp (°C)
Solvent
Pressure (bar)
C2H4
Yield PE (g)
Activity
(g mmol−1 h−1 br−1
)
1
(5)
(0.133)
(5)
(0.133)
(5)
(0.133)
(5)
(0.133)
(5)
(0.133)
(5)
(0.021)
(5)
(0.021)
(5)
(0.133)
(6)
(0.051)
(7)
(0.12)
(8)
[PhNMe2H]
[B(C6F5)4] (1)
[PhNMe2H]
[B(C6F5)4] (1)
[PhNMe2H]
[B(C6F5)4] (1)
[PhNMe2H]
[B(C6F5)4] (1)
[Ph3C]
[B(C6F5)4] (1)
[PhNMe2H] a
[B(C6F5)4] (1)
[PhNMe2H] b
[B(C6F5)4] (1)
B(C6F5)3 (1)
30
50
75
50
50
50
50
50
50
50
50
50
50
50
Toluene/iC4
Toluene/iC4
Toluene/iC4
CH2Cl2/iC4
CH2Cl2/iC4
Toluene/iC4
Toluene/iC4
CH2Cl2/iC4
CH2Cl2/iC4
CH2Cl2/iC4
CH2Cl2/iC4
CH2Cl2/iC4
CH2Cl2/iC4
CH2Cl2/iC4
10
10
10
10
10
10
10
10
10
10
15
15
10
10
65.2
50
57
32
58
57
32
20
–
2
75.2
3
42.1
4
77.2
5
75.4
6
6.7
7
2.5
8
Negligible
33.8
9
[PhNMe2H]
[B(C6F5)4] (1)
[PhNMe2H]
[B(C6F5)4] (1)
[PhNMe2H]
[B(C6F5)4] (1)
[PhNMe2H]
[B(C6F5)4] (1)
[PhNMe2H]
[B(C6F5)4] (1)
[PhNMe2H]
[B(C6F5)4] (1)
44
66
3
10
11
12
13
14
77.0
3.1
(0.071)
(9)
(0.079)
(10)
(0.087)
4.8
4
Negligible
Negligible
–
(11)
(0.087)
–
a Ten equivalents PhNMe2 added.
b 25 equivalents PhNMe2 added.
der to stabilise dialkyl derivatives. Thus, rather than
employing the cyclopentadienyl niobium complex used
for the screening reactions outlined in Table 1, the
study was continued using a pentamethylcyclopentadi-
enyl analogue. (C5Me5)Nb(N-2,6-Pri2C6H3)Me2 (10) [20]
and (C5Me5)Ta(N-tBu)(CH2Ph)2 (11) [41] can be syn-
thesised straightforwardly from their dichloride
precursors.
The results of polymerisation tests on 5–11 using
boron activators are shown in Table 2. These runs were
carried out in a 1 l autoclave fitted with temperature
control and a mass-flow sensor to allow ethylene up-
take to be monitored. Results for the chromium procat-
alysts are reported using the three boron activators
[PhNMe2H][B(C6F5)4], [CPh3][B(C6F5)4] and B(C6F5)3.
A general feature of all the runs is a remarkably stable
rate of ethylene uptake for the chromium systems,
affording stable kinetic profiles for in excess of 3 h. The
effect of temperature on activity using [PhNMe2H][B-
(C6F5)4] as the activator can be seen from runs 1–3.
While there is a modest increase upon raising the
temperature from 30 to 50°C, the activity falls off at
75°C, presumably due to catalyst deactivation. For runs
1–3, the chromium catalyst mixture was injected into
the reactor as an immiscible suspension in toluene. The
catalyst is fully miscible in methylene chloride and this
can also be used as the injector solvent without loss of
activity (run 4). The trityl activator [CPh3][B(C6F5)4]
gave an activity comparable to that found for the
anilinium reagent (run 5) but all attempts to activate
the chromium procatalyst using the neutral borane,
B(C6F5)3 (run 8), were unsuccessful. The arylimido
dibenzyl complex (6) gave a slightly lower activity
compared to its t-butyl analogue (run 9) while the
dimethyl procatalyst (run 10) gave an activity figure
comparable to 5. Procatalyst 7, however, can also be
activated by B(C6F5)3 (see Section 2.3). The molybde-
num, niobium and tantalum procatalysts were investi-
gated using [PhNMe2H][B(C6F5)4]. The molybdenum
procatalysts, Mo(N-tBu)2(CH2Ph)2 (8) and Mo(N-2,6-
Pri2C6H3)2(CH2Ph)2 (9) (runs 11 and 12) gave much
lower activities compared to chromium, with the
arylimido derivative giving a slightly higher activity
than its t-butylimido counterpart. The half-sandwich
niobium and tantalum imido species (C5Me5)Nb(N-2,6-
Pri2C6H3)Me2 (10) and (C5Me5)Ta(N-tBu)(CH2Ph)2 (11)
gave negligible amounts of polymer (typically B100
mg). A report by Stryker and coworkers indicates that
silyl-substituted imido ligands attached to niobium or
tantalum are capable of giving higher activities [8].

82
M.P. Coles et al. / Journal of Organometallic Chemistry 591 (1999) 78–87
The polyethylene generated using the chromium cata-
show a coupling of 2 Hz to the adjacent phosphorus,
while the methylene carbon appears as a doublet reso-
nance at l 52.9 (2JCP=4.5 Hz). The ipso-carbon reso-
nance at l 128.8, by contrast, shows no coupling to
phosphorus, though a small triplet splitting (5.3 Hz) due
to the methylenic hydrogens is observable in the proton
coupled spectrum. These observations are consistent with
the benzyl methylene group lying adjacent to the PMe3
ligand by comparison with the spectral features of
[Mo(N-tBu)2(L)(PMe3)] complexes [25], where L is an
alkene or alkyne.
lysts was found to be similar to that obtained from the
conventional dual component catalyst system, i.e. high
molecular weight (105–106) and with very little branching
(B1 per 1000 carbons by 13C-NMR spectroscopy).
2.3. Cationic alkyl complexes of chromium
In order to obtain a better understanding of the species
that form upon treatment of Cr(N-tBu)2(CH2Ph)2 with
the anilinium and trityl boron activators, these reactions
were studied by high-field 1H- and 13C-NMR spec-
troscopy. The product is an air- and moisture-sensitive
red oil that is immiscible in hydrocarbon solvent, but
readily miscible in methylene chloride and therefore
amenable to NMR spectroscopic studies in this solvent.
1
The H-NMR spectrum resulting from an equimolar
mixture of Cr(N-tBu)2(CH2Ph)2 and [CPh3][B(C6F5)4]
(recorded in CD2Cl2) reveals a clean transformation to
a single new chromium species and the singlet methylene
resonance of the 1,1,1,2-tetraphenylethane by-product
can be observed at l 3.98. For the chromium species, a
singlet resonance at l 1.59 is observed attributable to the
hydrogens of the t-butyl groups (shifted from l 1.20 for
the neutral dibenzyl precursor), and a new singlet signal
at l 4.21 is assignable to the metal-bound methylene
protons of a new benzyl ligand (shifted from l 2.49 for
5). Integration of these signals versus the methyl hydro-
gens of the t-butylimido ligand indicates that one benzyl
ligand is attached to the metal centre.
(2)
The reaction of Cr(N-tBu)2(CH2Ph)2 with one equiva-
lent of [PhNMe2H][B(C6F5)4] gives a more complicated
product mixture. One equivalent of toluene is generated
via protonation of one of the benzyl ligands; free
dimethylaniline, however, is not observed. The 1H-NMR
spectrum reveals a major species 14 containing reso-
nances attributable to the t-butyl hydrogens of the imido
ligands (l 1.67), a benzyl-methylene signal (l 3.84) and
a resonance at l 2.87 which integrates for the methyls of
one metal-bound dimethylaniline. A second series of
resonances, at l 1.55, 3.64 and 2.97 in the ratio 9:1:6 are
assignable to a bis(dimethylaniline) adduct 15 (Eq. (3)).
Due to the multiplicity of signals arising from this
mixture it did not prove possible to assign the 13C-NMR
spectrum and so we are unable to conclude anything
about the bonding mode of the remaining chromium-
bonded benzyl group. Addition of one equivalent of
dimethylaniline to the cationic benzyl species formed
1
The H decoupled 13C-NMR spectrum shows a reso-
nance at l 59.3 due to the metal-bound methylene carbon
and significantly a resonance at l 128.8 attributable to
the ipso-carbon of an h2-benzyl ligand. This compares
with a value of l 124.4 recorded in the solid state
spectrum of Cr(N-tBu)2(CH2Ph)2. These data are consis-
tent with the formation of the cationic h2-benzyl species
(12) shown in Eq. (1). As drawn, 12 may be regarded as
a three-coordinate chromium species and therefore it is
possible that coordinated NMR solvent or the counter-
ion may play a role in stabilising the cationic chromium
centre.
upon
treatment
of
Cr(N-tBu)2(CH2Ph)2
with
[CPh3][B(C6F5)4] affords this same product mixture.
(1)
Some support for the chromium centre in 12 being able
to accommodate another ligand is provided by treatment
of 12 with one equivalent of trimethylphosphine. This
gives a PMe3 adduct whose NMR data support the
four-coordinate, pseudo-tetrahedral structure shown in
Eq. (2). The methylene hydrogens of the h2-benzyl ligand
(3)

M.P. Coles et al. / Journal of Organometallic Chemistry 591 (1999) 78–87
83
1
polyethylene generated by heterogeneous chromium
catalysts [28–31]. The molecular-weight distributions of
the products, however, are generally broad and often
bi-modal indicating that these are not ‘single-site’ cata-
lysts. The reaction of 5 with the dimethylanilinium
borate may hold the answer to the other active sites
that might be present in the reaction mixture. These
possibilities are outlined in Scheme 2.
In the presence of a base such as dimethylaniline,
mono(aniline) and bis(aniline) adducts are generated.
Experiments in which excess dimethylaniline is added to
the catalyst mixture expectedly lowers the activity
(Table 2, runs 6 and 7), and also changes the molecular-
weight distribution from essentially broad unimodal
when no excess base is present (run 2), to progressively
bimodal as base is added (runs 6 and 7). In the absence
of a base, such as arises in the reaction of 1 with the
tritylborate reagent, it is not possible to rule out solvent
adducts even though they are not observable. It also
has to be recognised that agostic CꢀH···Cr interactions
involving the growing polymer may also be involved
since the h2-benzyl species is only relevant to the initiat-
ing species.
We have also used H-NMR spectroscopy to investi-
gate the reactions of these mixtures towards ethylene.
Notably, exposure of the cationic alkyl species to 1 atm
pressure of ethylene did not result in formation of
polyethylene. It was found necessary to increase the
ethylene pressure to ca. 2–3 atm before any
polyethylene was generated; the solid polymer, how-
ever, quickly obscured and broadened the NMR signals
preventing further information from being obtained.
Reaction of Cr(N-2,6-Pri2C6H3)2Me2 and B(C6F5)3 un-
der an atmosphere of ethylene produces rapid conver-
sion to polymer, qualitatively faster than the
benchmark Cr(N-tBu)2(CH2Ph)2/[PhNMe2H][B(C6F5)4]
catalyst system. For comparison purposes, the previ-
ously reported molybdenum analogue, Mo(N-2,6-
Pri2C6H3)2(CH3)2, was studied with all three boron
species. In all cases, dark red solutions are formed
identical in appearance to the chromium systems, but
the resulting spectra exhibit several septet signals at-
tributable to the isopropyl methine hydrogens of a
number of different unidentified arylimido products.
Dimeric [Mo(N-tBu)2(CH3)2]2 also shows a reaction
with all three boron species and when formed in the
presence of ethylene in an NMR tube, polymer is
produced, but only very slowly (days). Given the
propensity of molybdenum to form imido-bridged
dimeric species, this may account for the extremely low
catalytic activities generally observed for the molybde-
num systems. Neither the chromium nor the molybde-
num catalysts showed activity for propylene
polymerisation under analogous conditions.
In summary, transition metal imido complexes of the
Group 5 and 6 metals are active for ethylene polymeri-
sation and the chromium system is surprisingly robust.
Scope may remain for improving the performance of
these imido catalyst systems through judicious further
modifications to the imido ligand substituents.
Of the Nb and Ta complexes, (C5Me5)Nb(N-2,6-
Pri2C6H3)Me2 (10) and (C5Me5)Ta(N-tBu)(CH2Ph)2 (11)
did not react with [PhNMe2H][B(C6F5)4], even after 24
h, but they did react swiftly with [Ph3C][B(C6F5)4] to
give a complicated mixture of products that could not
be identified. A similarly complicated product mixture
was obtained using B(C6F5)3. These systems were not
investigated further.
3. Discussion
A significant and surprising feature of the well-
defined chromium system is the remarkably robust
nature of the catalyst. Although the absolute activities
are not particularly high, the catalyst system does not
appear to undergo significant deactivation over a 3 h
polymerisation run. Interestingly, the activity drops to
zero upon exposure of the catalyst solution to an
atmosphere of carbon monoxide and no CO is incorpo-
rated into the product. However, once the CO has been
purged from the system, the activity recovers to its
former level. The polyethylene generated has very little
branching and, in its general properties, resembles the
Scheme 2. Possible initiating species arising from the reaction of
(RN)2Cr(CH2Ph)2 with [PhNMe2H][B(C6F5)4].

84
M.P. Coles et al. / Journal of Organometallic Chemistry 591 (1999) 78–87
4. Experimental
4.3. High-pressure polymerisation procedure
The rig consists of a 1 or 3 l stainless steel autoclave
with a water-fed temperature regulation system, gas
feed lines, an overhead stirrer and a liquid injection
system. Pressure and temperature sensors allow specific
conditions to be maintained throughout the test and a
mass-flow controller monitors the flow of ethylene into
the autoclave, ensuring that the required overpressure is
maintained.
The autoclave is dried by raising the temperature of
the water jacket to 95°C and purging with a flow of 1
l min−1 of N2 for at least 1 h. The temperature of the
water is then set to that required for the experiment and
the jacket allowed to cool. Fifty percent of the required
co-catalyst and ca. 1.5 l of iso-butane (iC4) is intro-
duced to the autoclave, with stirring, via the liquid
injection system.
The catalyst is pre-weighed into a Schlenk tube and
dissolved in ca. 50 ml solvent, and the remaining 50%
of co-catalyst is added. The autoclave is then charged
with (typically) 10 bar overpressure of ethylene and
allowed to reach equilibrium, and the data collection
system is activated. The catalyst/co-catalyst system is
passed via syringe into the injection system and then
injected into the autoclave using an over pressure of N2.
After the desired time (typically 1 h) the polymerisation
is terminated by closing the ethylene supply and venting
the autoclave to atmosphere.
4.1. General
All manipulations were performed under an atmo-
sphere of dry nitrogen using standard Schlenk and
cannula techniques, or in a conventional nitrogen-filled
glovebox. Solvents were dried by reflux and distillation
from an appropriate drying agent and degassed prior to
use. Elemental analyses were performed by the micro-
analytical services of the Department of Chemistry at
Durham. NMR spectra were recorded on Varian VXR
400 S and Bruker WM250 spectrometers at 400.0 MHz
(1H); 100.6 MHz (13C) and 250.0 MHz (1H), respec-
tively; chemical shifts are referenced to the residual
protio impurity of the deuterated solvent; IR spectra
(Nujol mulls, CsI windows), Perkin–Elmer 577 and 457
grating spectrophotometers; mass spectra, VG 7070E
[70 eV (ca. 1.12×1017 J), 100 mA emission]. Cr(N-
tBu)2Cl2 was synthesised via the procedure described by
Wilkinson and co-workers [42]. Mo(N-2,6-Pri2C6H3)2-
Cl2(dme) [25] was prepared from Na2MoO4. VCp(N-2-
MeC6H4)Cl2 [43], VCp(N-tBu)Cl2 [43], Mo(N-tBu)2Cl2
[44], Cr(N-2,6-Pr2i C6H3)2Cl2 [39] and the boron reagents
[PhNMe2H][B(C6F5)4], [CPh3][B(C6F5)4] and B(C6F5)3
[45–47] were prepared by literature procedures.
CrO2Cl2, BCl3 (1.0 M solution in heptane), MgCl-
CH2Ph (1.0 M solution in diethyl ether), MgBrCH3 (3.0
M solution in diethyl ether), Et2AlCl (1.8 or 1.0 M
solution in toluene) and 2,6-diisopropylaniline were
purchased from Aldrich Chemical Co.; the aniline was
distilled under a dinitrogen atmosphere prior to use. All
other chemicals were obtained commercially and used
as received unless stated otherwise.
The isolated polymer is washed in turn with MeOH–
dil. HCl, EtOH–water and dried overnight in vacuo.
4.3.1. Cr(N-tBu)2(CH2Ph)2 (5)
Cr(N-tBu)2Cl2 (1.0 g, 3.76 mmol) was dissolved in
diethyl ether (75 ml) and cooled in a dry-ice–acetone
slush bath. To this very dark red–black solution was
then added two equivalents of MgClCH2Ph (1.0 M
Et2O, 7.52 ml, 7.52 mmol) dropwise via syringe over 10
min. The solution became progressively more red dur-
ing the addition. The mixture was allowed to warm to
room temperature (r.t.) and stirred for 18 h. The resul-
tant blood-red solution was filtered from the white
precipitate and the volatile components were removed
under reduced pressure to give a red oily solid. Extrac-
tion with pentane (30 ml) followed by concentration
and cooling of the filtrate to ca. −30°C afforded
crystalline Cr(N-tBu)2(CH2Ph)2. Yield 1.08 g, 76%.
Anal. Calc. for C22H32N2Cr: C, 70.18; H, 8.57; N, 7.44.
Found: C, 70.49; H, 8.66; N, 7.42%. IR: 1593m,
1352m(sh), 1260w, 1223s(br), 1173w, 1092m(br),
1019m(br), 888w, 806s, 759m, 690vs, 597w, 534m,
513m, 466m cm−1. NMR (C6D6): 1H (400 MHz), l
6.99 (m, 10H, CH2Ph), 2.49 (s, 4H, CH2Ph), 1.20 (s,
4.2. Ambient temperature/pressure polymerisation
procedure
Ethylene was dried by passing through a column of
CaCl2, molecular sieves and P2O5, which had previously
been flushed with N2 for at least 1 h. Further drying
was achieved by passing through a bubbler containing
silicone oil (30 ml) to which 2 ml of 1.8 M deac in
toluene had been added.
In a typical polymerisation procedure, diethylalu-
miniumchloride (deac, 20 equivalents) was added via
syringe to a stirred solution of the catalyst (50 mg) in
toluene (ca. 75 ml), and the mixture was left to stir for
10 min. The catalyst solution was then purged with
ethylene for 1 h, after which time the polymerisation
was terminated by purging with N2. Any polymer
formed was isolated, washed with in turn MeOH, dil.
HCl–MeOH (ca. 1:10), toluene, filtered and dried in
vacuo.
18H, C(CH3)3); 13C (100.6 MHz),
C6H5ꢀCipso), 131.60 (d, JCH 157.1 Hz, C6H5ꢀCortho),
128.94 (d*, C6H5ꢀCmeta), 125.65 (d, JCH 160.5 Hz,
l 140.34 (s,
1
1

M.P. Coles et al. / Journal of Organometallic Chemistry 591 (1999) 78–87
85
1
C6H5ꢀCpara), 71.97 (s, C(CH3)3), 44.50 (t, JCH 141.9
933w, 795m, 722s, 515m(sh), 446w(br). NMR (C6D6):
1
3
Hz, CH2Ph), 31.63 (q, JCH 127.0 Hz, C(CH3)3). (*
1H (400 MHz), l 6.91 (m, 6H, C6H3), 3.73 (sept, JHH
1JCH value obscured by overlap with solvent reso-
nance). MS (CI+, m/z, 35Cl) 377 [M+H]+, 303 [M−
N-tBu−2H]+, 286 [M−CH2Ph+H]+.
6.90 Hz, 4H, CH(CH3)2), 1.83 (s, 6H, Me), 1.12 (d,
3JHH 6.80 Hz, 24H, CH(CH3)2); 13C (100.6 MHz), l
157.81 (s, C6H3ꢀCipso), 145.24 (s, C6H3ꢀCortho), 127.60
(d*, C6H3ꢀCpara), 122.74 (d, 1JCH 156.4 Hz,
NMR characterising data for [Cr(N-tBu)2(h2-
1
CH2Ph)][B(C6F5)4)]
and
[Cr(N-tBu)2(h2-CH2Ph)-
C6H3ꢀCmeta), 49.73 (q, JCH 130.45 Hz, CH3), 28.80 (d,
1
(PMe3)][B(C6F5)4)] have been reported previously [10].
1JCH 125.12 Hz, CH(CH3)2), 23.52 (q, JCH 126.03,
1
CH(CH3)2). (* JCH value obscured by overlap with
solvent resonance). MS (CI+, m/z, 35Cl) 433 [M]+, 418
[M−CH3]+, 402 [M−2CH3]+.
4.3.2. Cr(N-2,6-Pri2C6H3)2(CH2Ph)2 (6)
Cr(NAr)2Cl2 (0.25 g, 0.53 mmol) was dissolved in
diethyl ether (75 ml) and cooled in a dry-ice–acetone
slush bath. To this bright red solution was then added
two equivalents of PhCH2MgCl (1.06 ml, 1.0 M solu-
tion in diethyl ether; 1.06 mmol). The mixture was
allowed to warm to r.t. and stirred for 18 h. The
resultant red–green solution was filtered from the white
precipitate and the volatile components were removed
under reduced pressure. Extraction and recrystallisation
from pentane at −30°C afforded green crystals of
Cr(NAr)2(CH2Ph)2. Yield 0.18 g, 69%. Anal. Calc. for
C38H48N2Cr: C, 78.05; H, 8.27; N, 4.79. Found: C,
78.12; H, 8.39; N, 4.46%. IR: 1590w, 1318m(br),
1266m(sh), 1175w, 1099w(br), 1058w, 1026m, 984w,
933w, 796s, 753vs, 722s, 692s, 530w, 475m, 452m.
4.3.4. Mo(N-tBu)2(CH2Ph)2 (8)
Mo(N-tBu)2Cl2·dme (2.00 g, 5.03 mmol) was dis-
solved in diethyl ether (75 ml) and cooled in a dry-ice–
acetone slush bath. To this deep yellow solution was
then added two equivalents of PhCH2MgCl (10.1 ml,
1.0 M solution in diethyl ether; 10.1 mmol). After
addition of 50% of the Grignard reagent a colour
change from yellow to deep orange was observed. The
mixture was allowed to warm to r.t. and stirred for 18
h. The resultant deep orange solution was filtered from
the white precipitate and the volatile components were
removed under reduced pressure. Extraction and recrys-
tallisation from acetonitrile at −30°C afforded orange
crystals of Mo(N-tBu)2(CH2Ph)2. Yield 1.62 g, 77%.
Anal. Calc. for C22H32N2Mo: C, 62.85; H, 7.67; N,
6.66. Found: C, 62.27; H, 7.67; N, 6.41%. NMR
1
NMR (C6D6): H (400 MHz), l 7.17–6.88 (m, 16H,
3
C6H3 and CH2Ph), 3.85 (sept, JHH 6.60 Hz, 4H,
CH(CH3)2), 3.41 (s, 4H, CH2Ph), 1.12 (d, 3JHH 7.20 Hz,
24H, CH(CH3)2); 13C (100.6 MHz), l 158.00 (s,
C6H3ꢀCipso), 146.03 (s, C6H3ꢀCortho), 140.15 (s,
1
(C6D6): H (400 MHz), l 6.98 (m, 10H, CH2Ph), 2.21
(s, 4H, CH2Ph), 1.24 (s, 18H, C(CH3)3); 13C (100.6
CH2PhꢀCipso),
132.69
(d,
1JCH
158.3
Hz,
1
MHz), l 139.37 (s, C6H5ꢀCipso), 129.55 (d, JCH 157.1
CH2PhꢀCortho), 128.78 (d*, CH2PhꢀCmeta), 127.12 (d*,
Hz, C6H5ꢀCortho), 129.46 (d*, C6H5ꢀCmeta), 125.26 (d,
1
C6H3ꢀCpara), 126.24 (d, JCH 160.2 Hz, CH2PhꢀCpara),
1JCH 160.7 Hz, C6H5ꢀCpara), 67.94 (s, C(CH3)3), 37.65
1
1
122.79 (d, JCH 156.4 Hz, C6H3ꢀCmeta), 59.88 (t, JCH
1
1
(t, JCH 137.7 Hz, CH2Ph), 31.98 (q, JCH 126.8 Hz,
1
139.6 Hz, CH2Ph), 28.48 (d, JCH 129.35 Hz,
1
C(CH3)3). (* JCH value obscured by overlap with
1
CH(CH3)2), 23.87 (q, JCH 125.90 Hz, CH(CH3)2). (*
solvent resonance).
1JCH value obscured by overlap with solvent
resonance).
4.3.5. Mo(N-2,6-Pri2C6H3)2(CH2Ph)2 (9)
4.3.3. Cr(N-2,6-Pri2C6H3)2(CH3)2 (7)
Mo(NAr)2Cl2·dme (2.00 g, 3.3 mmol) was dissolved
in diethyl ether (75 ml) and cooled in a dry-ice–acetone
slush bath. To this burgundy solution was then added
two equivalents of PhCH2MgCl (6.6 ml, 1.0 M solution
in diethyl ether; 6.6 mmol). After addition of 50% of
the Grignard reagent a colour change from burgundy
to deep orange was observed. The mixture was allowed
to warm to r.t. and stirred for 18 h. The resultant deep
orange solution was filtered from the white precipitate
and the volatile components were removed under re-
duced pressure. Extraction and recrystallisation from
acetonitrile at −30°C afforded orange crystals of
Mo(NAr)2(CH2Ph)2. Yield 1.45 g, 60%. Anal. Calc. for
C38H48N2Mo: C, 72.59; H, 7.69; N, 4.46. Found: C,
Cr(NAr)2Cl2 (0.390 g, 0.83 mmol) was dissolved in
diethyl ether (75 ml) and cooled in a dry-ice–acetone
slush bath. To this bright red solution was then added
two equivalents of an MeMgBr solution (0.58 ml of a 3
M solution in diethyl ether; 1.65 mmol). The mixture
was allowed to warm to r.t. and stirred for 18 h. The
resultant green solution was filtered from the white
precipitate and the volatile components were removed
under reduced pressure. Extreme solubility precluded
recrystallisation from pentane. However, green micro-
crystalline Cr(NAr)2(CH3)2 was isolated from a concen-
trated acetonitrile solution at −30°C. Yield 0.18 g,
51%. Anal. Calc. for C26H40N2Cr: C, 72.19; H, 9.32; N,
6.48. Found: C, 71.70; H, 9.24; N, 6.49%. IR: 1919w,
1583w, 1271m, 1157w(br), 1110m(sh), 1058w, 994w,
1
72.36; H, 7.61; N, 4.20%. NMR (C6D6): H (250 MHz),
l 7.03–6.93 (m, 16H, C6H3 and CH2Ph), 3.81 (sept,

86
M.P. Coles et al. / Journal of Organometallic Chemistry 591 (1999) 78–87
3JHH 6.9 Hz, 4H, CH(CH3)2), 2.83 (s, 4H, CH2Ph), 1.13
Acknowledgements
3
(d, JHH 6.87 Hz, 24H, CH(CH3)2); 13C (100.6 MHz), l
153.35 (s, C6H3ꢀCipso), 143.26 (s, C6H3ꢀCortho), 136.74
(s, CH2PhꢀCipso), 132.69 (d, 1JCH 158.3 Hz,
CH2PhꢀCortho), 129.39 (d*, CH2PhꢀCmeta), 126.51 (d*,
This investigation was supported by the Engineering
and Physical Sciences Research Council, UK (a post-
doctoral fellowship to C.I.D. and a studentship to
M.P.C) and BP Chemicals Ltd. M.H.R.C. thanks the
European Commission for a TMR-Madame Curie fel-
lowship. Professor R.K. Harris and Dr T. Thompson
are thanked for solid-state 13C-NMR measurements,
and Dr M. Chan is thanked for providing a sample of
CpNb(2-tBuC6H4)Cl2.
1
C6H3ꢀCpara), 125.59 (d, JCH 160.69 Hz, CH2PhꢀCpara),
1
1
122.42 (d, JCH 157.55 Hz, C6H3ꢀCmeta), 47.49 (t, JCH
1
135.75 Hz, CH2Ph), 28.27 (d, JCH 128.21 Hz,
1
CH(CH3)2), 23.35 (q, JCH 125.24 Hz, CH(CH3)2). (*
1JCH value obscured by overlap with solvent
resonance).
4.3.6. (C5Me5)Nb(N-2,6-Pri2C6H3)Me2 (10)
A THF solution of methylmagnesium chloride (20
ml, 3.0 M) was reduced to dryness to give a white solid.
(C5Me5)Nb(N-2,6-Pr2i C6H3)Cl2 (2.50 g, 5.27 mmol) was
then added and Et2O (30 ml) condensed onto the vessel
at −196°C. The mixture was then allowed to warm to
0°C and stirred for 4 h. The volatile components were
then removed under reduced pressure and the resultant
residue was extracted with pentane (3×30 ml) to give
an orange solution. This solution was left to stand for
6 h to allow magnesium residues to precipitate. After
filtration, concentration to ca. 20 ml and cooling
to −78°C, yellow needle crystals of (C5Me5)Nb-
(NAr)Me2 were precipitated. Yield 1.49 g, 72%. Anal.
Calc. for C24H38NNb: C, 66.50; H, 8.84; N, 3.23.
Found: C, 66.33; H, 8.86; N, 2.96%. For NMR data:
see Ref. [20]
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2.46. Found: C, 58.58; H, 6.88; N, 2.52%. IR: 3075w,
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1305w, 1265vs, 1205s, 1180w, 1095m, 1055m, 1030m,
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1
430w, 405w. NMR (C6D6): H (400 MHz), l 7.20 (t,
3
3
4H, JHH 7.2 Hz, m-Ph), 7.05 (d, 4H, JHH 7.2 Hz,
3
o-Ph), 6.93 (t, 2H, JHH 7.2 Hz, p-Ph), 2.07 (d, 2H,
2JHH 12.0 Hz, CH2Ph), 1.77 (s, 15H, C5Me5), 1.38 (d,
2
2H, JHH 12.0 Hz, CH2Ph), 1.09 (s, 9H, CMe3); 13C
1
(100.6 MHz), l 149.35 (s, ipso-Ph), 128.85 (d, JCH 160
1
Hz, o-Ph), 128.05 (d, JCH 158 Hz, m-Ph), 122.82 (d,
1JCH 158 Hz, p-Ph), 115.91 (s, C5Me5), 71.28 (t, JCH
1
1
119 Hz, CH2Ph), 65.17 (s, CMe3), 33.20 (q, JCH 126
1
Hz, CMe3), 11.36 (q, JCH 127 Hz, C5Me5). MS (EI,
m/z): 570 [M]+.

M.P. Coles et al. / Journal of Organometallic Chemistry 591 (1999) 78–87
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