Discovery and optimization of new chromium catalysts for ethylene oligomerization and polymerization aided by high-throughput screening

Article abstract of DOI:10.1021/ja0518171

High throughput screening (HTS) of a 205 member Schiff base salicylaldimine ligand library derived from salicylaldehydes bearing bulky ortho-substituents, i.e., 9-anthracenyl, 1,4,5,8-tetramethylanthracenyl or triptycenyl, reacted in-situ with (p-tolyl)CrCl₂(thf)₃, identified two new classes of highly active chromium based systems for the Oligomerization and polymerization of ethylene, respectively. The polymerization system comprises bidentate ortho-substituted anthracenyl Schiff bases bearing small primary or secondary alkyl imine substituents. The Oligomerization catalysts are based upon tridentate orthotriptycenyl-substituted Schiff bases with pyridylmethyl or quinolyl substituents. Validation tests confirmed polymerization productivities of up to 3000 g·mmol^-1h^-1bar^-1 for the polymerization catalyst systems while the oligomerization catalysts gave productivities up to 10 000 g·mmol^-1h^-1bar ^-1. Key catalyst precursors have been characterized by X-ray crystallography.

Full text of DOI:10.1021/ja0518171

Published on Web 07/16/2005
Discovery and Optimization of New Chromium Catalysts for
Ethylene Oligomerization and Polymerization Aided by
High-Throughput Screening
David J. Jones,† Vernon C. Gibson,*,† Simon M. Green,‡ Peter J. Maddox,§
Andrew J. P. White,† and David J. Williams†
Contribution from the Imperial College London, Exhibition Road, London, United Kingdom SW7
2AZ, BP Exploration, Sutton, Surrey, United Kingdom SM1 4TY, and InnoVene, Kingston Road,
Staines, Middlesex, United Kingdom TW18 1DT
Received March 22, 2005; E-mail: v.gibson@imperial.ac.uk
Abstract: High throughput screening (HTS) of a 205 member Schiff base salicylaldimine ligand library
derived from salicylaldehydes bearing bulky ortho-substituents, i.e., 9-anthracenyl, 1,4,5,8-tetramethylan-
thracenyl or triptycenyl, reacted in-situ with (p-tolyl)CrCl2(thf)3, identified two new classes of highly active
chromium based systems for the oligomerization and polymerization of ethylene, respectively. The
polymerization system comprises bidentate ortho-substituted anthracenyl Schiff bases bearing small primary
or secondary alkyl imine substituents. The oligomerization catalysts are based upon tridentate ortho-
triptycenyl-substituted Schiff bases with pyridylmethyl or quinolyl substituents. Validation tests confirmed
polymerization productivities of up to 3000 g‚mmol-1h-1bar-1 for the polymerization catalyst systems while
the oligomerization catalysts gave productivities up to 10 000 g‚mmol-1h-1bar-1. Key catalyst precursors
have been characterized by X-ray crystallography.
Introduction
by Schiff base ligands for the syndiotactic polymerization of
propylene,22 while workers at Symyx Technologies Inc. have
Recent spectacular improvements in activities for nonmetal-
locene olefin polymerization systems have spurred efforts to
develop new commercially viable catalyst systems for the
production of polyolefins.1 Among the transition metals,
chromium holds special attraction due to its role in the
commercial production of polyethylene using silica-supported
Phillips2 and Unipol3 catalyst systems. In recent years, a number
of molecular chromium catalysts have been reported, both for
the oligomerization and polymerization of ethylene4-18 as well
as for ethylene trimerization,19 and recently tetramerization.20
The rate of discovery and optimization of new catalyst
systems has been enhanced significantly by the introduction of
High Throughput Screening (HTS) methodologies.21 For the
Group 4 metals Tian and Coates applied a combinatorial
‘catalyst pool’ approach to discover a titanium catalyst supported
disclosed stereoselective propylene polymerization catalysts
based on dianionic tridentate chelate ligands,23 as well as olefin
polymerization catalysts based on monoanionic bidentate ligands.24
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‡ BP Exploration.
§ Innovene.
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9
10.1021/ja0518171 CCC: $30.25 © 2005 American Chemical Society
J. AM. CHEM. SOC. 2005, 127, 11037-11046
11037

A R T I C L E S
Jones et al.
More recently, Mountford and co-workers have utilized a HTS
approach to discover imido-titanium catalysts for ethylene
polymerization,25 and GPC has been used as a screening method
to discover hetero-ligated phenoxy-imine catalysts for syndio-
tactic propylene polymerization.26 For the Group 6 metal
chromium, we have communicated the discovery of new
polymerization and oligomerization catalyst families assisted
by an HTS approach.12 Here, we describe the rationale behind
the choice of ligand library, the synthesis and characterization
of key pro-ligands and metal complexes, pre-screen validation
chemistry as well as post-validation of hits from the library
screen.
pro-ligand library, specially its ease of synthesis and its
amenability to variation at key substituent sites, combined with
the availability of straightforward complexation procedures and
catalyst screening protocols. Ligands such as phenoxy-imines,
derivable by simple Schiff base condensation chemistry, hold
special attraction for HTS programs, and have already been
shown to afford highly active olefin polymerization catalysts
for Group 422,26,27 and Group 1028 metal systems. The produc-
tivities of the Group 10 metal system have been found to be
enhanced dramatically by incorporating bulky ortho-anthracenyl
substituents in the phenoxide donor.28 The increased activities
are attributed to steric protection of the salicylaldiminato oxygen
by the ortho-anthracenyl group while the isopropyl substituents
of the arylimino donor are positioned above and below the
square plane of the nickel disfavoring associative displacement
of olefin from the metal center. Contrastingly, the Group 4 metal
systems employ salicylaldiminato (phenoxy-imine) ligands
bearing small imine substituents and bis(chelation) is required
to afford high activities. In our earlier work on salicylaldiminato
chromium complexes, we found that bis(chelate) derivatives
containing bulky imine substituents are not particularly active
for ethylene polymerization, with productivities typically <150
Ligand Library Design, Choice of Metal Precursor, and
Initial Validation Experiments. An important consideration
for high throughput catalyst screening is the accessibility of the
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g‚mmol-1h-1bar-1 11e
We reasoned that the relatively poor
.
performance of the chromium system was most likely due to
the unfavorable coordination environment afforded by binding
two bulky N-O chelate ligands and that more highly active
systems may be attainable by binding just one N-O chelate
ligand decorated with judiciously selected ligand substituents.
It is relevant at this point to consider the relative merits of
small versus large ligand substituents in salicylaldiminato
ligands. It has generally been recognized that a bulky substituent
such as anthracenyl positioned ortho to the phenoxide O-donor
is beneficial since it disfavors interactions with the Lewis acidic
centers of the activator or possibly other catalyst centers, both
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11038 J. AM. CHEM. SOC. VOL. 127, NO. 31, 2005

Chromium Catalysts via High-Throughput Screening
A R T I C L E S
where the steric conflict would be anticipated to be similarly,
if not more, severe.
For the metal precursor, several key properties are required:
(a) ease of synthesis, (b) clean reactions with pro-ligands to
generate complexes of targeted stoichiometry, (c) no reactive
byproducts, d) good solubility in a nonreactive solvent, and (e)
good stability in RT solution to allow standard solutions to be
generated. Pre-evaluation tests may be needed to ensure pre-
catalysts are formed from reaction of the pro-ligand and metal
precursor complexes for the major classes of ligands tested; it
also has to be recognized that complexation may occur upon
treatment of the pro-ligand/metal precursor mix with activator,
e.g. MAO. After examination of known alkyl- or aryl-chromium
complexes it was decided that for this study the known complex,
(p-tolyl)CrCl2(thf)3 (I),29 matched the criteria, with the exception
that it carries a coordinated donor solvent molecule which, if
not removed, may suppress activities in the high throughput
screen. The stability of alternative alkyl-chromium complexes
has been reported to be significantly lower with t1/2 values <
60 min at RT in noncoordinating solvents.30 Similarly, potential
alternative homoleptic chromium alkyls or aryls are poorly
stable, more problematic to synthesize, stabilized by bulky
ligands potentially reducing their reactivity,31 retain coordinated
solvent32 or, as for tetravalent homoleptic chromium(IV) species,
are often unreactive toward protic reagents.33
Since it seemed highly likely for chromium systems that
trigonal bipyramidal or octahedral, but certainly not square-
planar, coordination geometries would be preferred for the
precatalysts and possibly the putative active sites, ligands with
small, as well as large, substituents need to be considered. In a
preliminary study, the three ortho-anthracenyl-salicylaldimines
highlighted in the modeling study (Figure 1) were targeted to
validate the synthetic approach to the pro-ligands, as well as to
assess the potential for the new metal-ligand combinations to
support polymerization-active metal centers.
Figure 1. Comparison of energy-minimized structures for anthracenyl-
salicylaldiminato ligands bearing different sized groups attached to the imine
nitrogen donor: (a), (c), and (d) are modeled as (L)CrCl2; (b) is modeled
as (L)NiCl(NCMe).
of which may lead to catalyst deactivation. For the imine donor,
the optimal size of the substituent would be expected to be
dependent upon the coordination geometry. Thus, for square
planar complexes based on the Group 10 metals, large 2,6-
disubstituted aryl groups are advantageous for protecting the
‘axial’ coordination sites from associative displacement pro-
cesses.
Results and Discussion
Preliminary Validation Studies. A preliminary testing
protocol was followed whereby the ligand and metal precursor
(30 µmoles) were dissolved in 10 mL of toluene and the solvent
immediately removed under vacuum to remove excess thf,
Scheme 1. The residue was slurried/dissolved in toluene and
activated with a methylalumoxane (MAO) solution in toluene
(total volume 30 mL, 1 µmolmL-1) from which an aliquot of
the required volume was taken. Results for this preliminary
screen are collected in Table 1; the ligands are numbered
according to their position in the main ligand libraryssee
Supporting Information. The metal precursor alone did not
support significant catalysis (productivity 2.3 g‚mmol-1h-1bar-1),
while the bulky diisopropylphenyl substituted imine (28A) also
For tetrahedral, trigonal bipyramidal, and octahedral geom-
etries, the steric requirements for protection of the active site
will be substantially different and bulky substituents will not
necessarily be required for optimal catalyst performance. The
effect of large and small ligand substituents within a tetrahedral
metal environment is illustrated using the energy-minimized
MacSpartan Pro models shown in Figure 1, and compared with
the square planar environment that predominates in divalent
Group 10 metal systems. In the case of the tetrahedral
environment containing bulky 2,6-diisopropylphenyl substituents
(Figure 1a), the ligand isopropyl groups come into much closer
contact with the metal-bonded chlorine atoms than for the square
planar arrangement (Figure 1b). Indeed, for the tetrahedral
coordination geometry, the Cl‚‚‚H contacts are shorter than the
Cl‚‚‚H van der Waals contact of 2.95 Å. The steric conflict is
removed for ligands containing smaller phenylimine and iso-
propylimine substituents (Figure 1c,d). Thus, the square planar
environment is able to accommodate bulky groups at the imino
nitrogen donor more readily than complexes or catalysts that
adopt a tetrahedral coordination geometry. Such arguments also
hold for trigonal bipyramidal and octahedral metal geometries,
(29) Daly, J. J.; Sneeden, R. P. A. J. Chem. Soc. A 1967, 736.
(30) Nishimura, K.; Kuribayashi, H.; Yamamoto, A.; Ikeda, S. J. Organomet.
Chem. 1972, 37, 317.
(31) (a) Barker, G. K.; Lappert, M. F.; Howard, J. A. K. J. Chem. Soc., Dalton
Trans. 1978, 734. (b) Barker, G. K.; Lappert, M. F. J. Organomet. Chem.
1974, 76, C45.
(32) (a) Herwig, W.; Zeiss, H. H. J. Am. Chem. Soc. 1957, 79, 6561. (b) Hein,
F.; Markert, E. Ber. 1928, 61B, 2255. (c) Brock, J. R.; Odom, A. L.; Klei,
S. R.; Cummins, C. C. Organometallics 1999, 18, 1360. (d) Stolze, G. J.
Organomet. Chem. 1966, 6, 383.
(33) (a) Schulzke, C.; Enright, D.; Sugiyama, H.; LeBlanc, G.; Gambarotta, S.;
Yap, G. P. A.; Thompson, L. K.; Wilson, D. R.; Duchateau, R. Organo-
metallics 2002, 21, 3810. (b) Poli, R.; Smith, K. M. Sci. Synth. 2003, 2,
283.
9
J. AM. CHEM. SOC. VOL. 127, NO. 31, 2005 11039

A R T I C L E S
Jones et al.
Scheme 1. Protocol for in Situ Catalyst Testing of New Schiff
ortho-substituents; therefore a simplified general route was
investigated with a view to generating large quantities of the
required ligand precursors35 (Scheme 2). Backbone A was
readily formed from anthraquinone by reaction with ortho-
lithiated PhOMOM (or the related Grignard reagent MOMOPh-
MgBr generated from the reaction of MOMOPhLi with MgBr2;
OMOM ) OCH2OCH3).
Base Ligandsa
The substituted anthraquinone 1 could be reduced using Zn
dust in HOAc/H2O at 60 °C without deprotection of the acid
sensitive MOM group, leading to a general route to large
quantities (100-200 g) of 2. Ortho-lithiation of 2 to form 3,
and reaction of 3 with a range of electrophiles generated, after
deprotection, substituted phenols including the required back-
bones A (4a) and I (4b). Reaction of 2 with benzyne resulted
in the triptycenyl derivative (7) which could be functionalized
by ortho-lithiation and reaction with an appropriate electrophile
generating the backbones D (9a) and J (9b). Substituted phenols
underwent similar reactions allowing the synthesis of B (13)
containing an electron withdrawing CF3 group para to the
hydroxyl functionality (Figure 2), as well as presenting a
simplified route to large quantities of salicylaldehydes from
commercially available ortho-substituted phenols, e.g., the
synthesis of F from 2-phenylphenol.
a (i) (p-tolyl)CrCl2(thf)3, vacuum, (ii) MAO/ethylene.
Table 1. Summary of Ethylene Polymerization Results for
Salicylaldimines 28A, 2A, and 25A Using the in Situ Ligand
Screening Protocol Shown in Scheme 1a
ligand
Cr (mmol)
Cr:Al
T (°
C)
P (bar)
yield (g)
prod.
no lig
28A
2A
0.030
0.030
0.015
0.002
1:100
1:400
1:100
1:200
RT
RT
RT
RT
1
1
1
1
0.07
0.20
1.42
0.88b
2.3
6.7
95
1760
25A
a Standard conditions: Schlenk test, 1 bar ethylene, 100 mL toluene, 60
b
min, productivity g.mmol-1h-1bar-1. run time 15 min.
Formation of the tetramethylanthracenyl-salicylaldehyde, C,
required more forcing conditions during reduction resulting in
Lewis-Acid catalyzed deprotection of the phenol, Scheme 3.
Ortho-formylation can be achieved by literature methods36 or
by reprotection and formylation using the conditions described
above.
Crystals of C and D suitable for structural determination were
isolated by vapor diffusion of hexane into benzene solutions of
C or D. The views shown in Figure 3a,b illustrates the
constrained nature of C with the salicylaldehyde wedged
between the methyls C(22) and C(25) of the tetramethylanthra-
cenyl group, forcing each of these units significantly away from
their ideal positions. The steric protection of the phenolic oxygen
afforded by the ortho-triptycenyl substituent is apparent from
Figure 3c,d. Crystal data for C and D can be found in the
Supporting Information along with data for the structure of 4e.
Amines were selected to give salicylaldimines with a diverse
range of steric and electronic properties, as well as additional
donor functionalities. The ligands were synthesized using
standard condensation chemistry, combining 55 amines with
salicylaldehydes A-D to give a total of 205 salicylaldimines.
The full list of pro-ligands is collected in the Supporting
Information.
Figure 2. Salicyladehyde backbones used as part of this study. Backbones
A-D used to generate the 205 member salicylaldimine library.
gave low productivity, 6.7 g‚mmol-1h-1bar-1. Phenyl substitu-
tion (2A) resulted in modest productivity, 95 g‚mmol-1h-1bar-1
,
Screening Results. The ligands were screened under standard
polymerization conditions in a 24 well reactor with each plate
containing a solvent blank and two standard tests, ligand 25A
and the complex [N,N′-dimesityl diiminopyridine]iron dichlo-
ride.37 The measured productivities were normalized to the
average of the standard tests while the variation in the standard
tests was used as a measure of error to aid in the identification
of hits.
comparable to values found for previously reported Schiff base
chromium systems.11e Much higher productivity was found for
the isopropyl derivative (25A), an optimal catalyst loading
affording a productivity of 1760 g‚mmol-1h-1bar-1
.
These preliminary tests validated the reaction protocol and
generated an initial lead for the ligand library. The success of
the preliminary screening tests also made it possible to select a
subset of small amines within the ligand library to allow a
targeted ligand screen around this hit.
Ligand Synthesis. The salicylaldimine pro-ligand library
required access to large quantities of the ortho-substituted
salicylaldehydes shown in Figure 2. An earlier synthesis of
A28b,34 followed a multistep route with low yields for bulky
(35) An alternative synthesis has been communicated for A.28e During the course
of our ligand development program, we also examined a variation of this
route. However, in our hands, we found that reaction of MOMOPhM (M
) Li or MgBr) with anthrone gave yields varying between 0 and 90%,
presumably due to the ability of MOMOPhM to act as a base or a
nucleophile, and the sensitivity of its mode of action to minor changes in
the experimental conditions.
(36) Wang, R. X.; You, X. Z.; Meng, Q. J.; Mintz, E. A.; Bu, X. R. Synth.
Commun. 1994, 24, 1757.
(34) (a) Rice, J. E.; Cai, Z. W. Tetrahedron Lett. 1992, 33, 1675.
9
11040 J. AM. CHEM. SOC. VOL. 127, NO. 31, 2005

Chromium Catalysts via High-Throughput Screening
A R T I C L E S
Scheme 2. Synthesis of Sterically Protected Salicylaldehydesa
a (i) MOMOPhM (M ) Li or MgBr) in thf added over 3 h; (ii) Zn/cat. ZnCl2 in AcOH/H2O (60/40) 60 °C, 16 h; (iii) BuLi 1.1 eq., ether 4 h; (iv) -78
°C DMF; Ac2O, PhCONMe2, (EtO)2CO or CO2, excess Ph2PCl or [Me2NCH2]I 1.1 eq, then deprotection with thf/6M HCl reflux 4 h, (v) MgBr2 (from Mg
and BrC2H4Br), (vi) Br-Py/cat (dppe)NiCl2, reflux 4 h then deprotection; (vii) diazobenzenecarboxylate (from anthranilic acid and isoamylnitrite) thf,
reflux; (viii) BuLi (2.1 eq.), ether, RT, 4 h, -78 °C DMF or Ac2O, then deprotection.
Scheme 3. Synthesis of Salicylaldehyde Ca
productivities comparable to the standard isopropyl-amine
derivative (25) are apparent. It is interesting to note that only
primary or secondary alkyl substituents afford catalysts with
significant productivities; neither tertiary alkyls, nor substituents
with pendant O, N, or S donor units gave active systems.
Substitution of the salicylaldehyde in the para-position with
the electron withdrawing CF3 substituent, backbone B, gave hits
for the same series of imine substituents (Figure 4b), but no
improvement in productivity was observed. Productivities are
generally slightly lower than for the parent ligands based on
backbone A.
For the severely hindered ligands containing the tetramethyl-
anthracenyl substituent (C), no productivity for ethylene po-
lymerization or oligomerization was found. However, changing
the ortho substituent to a triptycenyl unit gave the screen results
shown in Figure 4c where, quite remarkably, a single active
a (i) MOMOPhM (M ) Li or MgBr), thf added over 3 h; (ii) LiAlH4/
BF3.OEt2 in ether reflux, 4 h; (iii) EtMgBr (1.0 eq.), (CH2O)n (2.5 eq),
Et3N (1.5 eq), toluene, 80 °C, 3 h then 2 M HCl.
system, one containing a pyridylmethyl substituent, was identi-
fied.
Hit Validation. The six most active ‘hits’ from the library
screen using salicylaldehyde A, i.e., ligands 6A (3,3-diphenyl-
propyl), 8A (cyclopentyl), 24A (cyclobutyl), 25A (isopropyl),
27A (hexyl), and 31A (cyclopropyl), were repeated on a
preparative scale and the resultant polyethylene products
analyzed by GPC. The results are collected in Table 2.
The isopropyl (25A), hexyl (27A) and cyclopropyl (31A)
The HTS results for amines condensed with anthracenyl-
salicylaldehyde (A) are shown in Figure 4a. Several hits with
(37) (a) Britovsek, G. J. P.; Bruce, M.; Gibson, V. C.; Kimberley, B. S.; Maddox,
P. J.; Mastroianni, S.; McTavish, S. J.; Redshaw, C.; Solan, G. A.;
Stromberg, S.; White, A. J. P.; Williams, D. J. J. Am. Chem. Soc. 1999,
121, 8728. (b) Britovsek, G. J. P.; Gibson, V. C.; Kimberley, B. S.; Maddox,
P. J.; McTavish, S. J.; Solan, G. A.; White, A. J. P.; Williams, D. J. Chem.
Commun. 1998, 849.
derivatives gave activities between 1000-3000 g‚mmol-1
-
9
J. AM. CHEM. SOC. VOL. 127, NO. 31, 2005 11041

A R T I C L E S
Jones et al.
in low productivity indicating a sensitivity of catalyst productiv-
ity to substitution at the imine carbon atom. Replacement of
the imine by diphenylphosphino (4e), dimethylaminomethyl (4f)
or pyridine (6) donor groups also resulted in no catalytic activity.
On the basis of these results, it would appear that a combination
of ortho-anthracenyl and imino donors bearing primary or
secondary alkyl groups is optimal for catalyst performance.
The N-isopropyl and N-cyclopentyl derivatives of o-anthra-
cenyl-salicylaldimine (complexes II & III) were readily formed
by reaction of 25A or 8A with p-tolylCrCl2(thf)3 (I) in toluene
at RT (according to Scheme 1). Analytically pure complexes
were recovered by solvent removal and washing with hexane.
The pale yellow-green complexes have been fully characterized
as the mono-thf adducts, LCrCl2(thf).
Crystals of III suitable for an X-ray structural analysis were
grown from thf/benzene and show octahedral coordination at
the chromium center with two cis-coordinated thf molecules
and a single bidentate salicylaldiminato ligand (Figure 5). The
anthracenyl moiety is oriented approximately orthogonally to
the phenoxy ring (ca. 85°). The cyclopentyl substituent on N(7)
is orientated such that the proton attached to C(8) is directed
toward the metal center, with a [Cr,N(7),C(8),H(8A)] torsion
angle of 18°.
Catalysts Containing Two Salicylaldiminato Ligands. For
completeness, we decided to specifically target a bis(bidentate)
chromium pre-catalyst containing small imine substituents as a
direct analogue of the highly active Group 4 metal pre-catalysts
of the type L2MCl2 (M ) Ti, Zr) described by Fujita and co-
workers.27 In earlier work, we described bis(bidentate) deriva-
tives of chromium containing bulky N-2,6-diisopropylphenyl
substituents, but had found only modest ethylene polymerization
activities.11e The bis(ligand) complex, (L)2CrCl (IV), containing
salicylaldiminato ligands with an o-tert-butylphenoxy/N-phen-
ylimino donor combination was prepared by treatment of the
sodium salt of 2G with CrCl3(thf)3 in thf to give a yellow-brown
thf-free solid. Crystals of the six-coordinate acetonitrile adduct
(V), suitable for an X-ray structure determination were obtained
by slow cooling of a hot acetonitrile solution of IV. The
molecular structure is shown in Figure 6; selected bond lengths
and angles are given in the Figure caption. The positioning of
the two N,O chelate ligands, with trans-O,O and cis-N,N
dispositions, is virtually identical to that seen in the dichloro-
titanium24a and dichlorozirconium precatalysts.38
Figure 3. The molecular structures of C (a and b) and D (c and d). View
(b) shows distortions in C arising from steric constraints imposed by the
tertramethylanthracenyl substituent; view (d) shows the molecular structure
of D viewed down the C(2)‚‚‚C(15) vector.
h-1bar-1. In all cases, high molecular weight linear polyethylene
was formed with, in the two cases that could be determined, a
narrow molecular weight distribution; in the case of 25A, the
polymer was insoluble indicating a MW > 2M Da. The
cyclobutyl and cyclopentyl derivatives, 24A and 8A, gave
activities comparable to each other, but with much lower
molecular weight products than for their acyclic relatives. For
the 3,3-diphenylpropyl derivative 6A, polyethylene of interme-
diate molecular weight was produced and with a significantly
broadened molecular weight distribution. Importantly, the
relative activities of the catalysts obtained in the high throughput
screen were reproduced in the preparative scale runs, validating
the robustness of the HTS screen data.
Bidentate Salicylaldimines with Small Alkyl Groups. The
success of the HTS screen and validation study led us to target
a small, but more focused, ligand library to probe structure-
activity relationships around the observed hits. Added diversity
was achieved by reacting isopropylamine (25) with 3,5-di-tert-
butyl-saliylaldehyde (E), phenyl-salicylaldehyde (F), and 3′-
(9-anthracenyl)-2′-hydroxyacetophenone (I). The polymerization
test results on in situ generated catalysts (Table 3) confirmed
the requirement for a small alkyl group attached to the N-imino
donor. Ligands based on the commercially available backbone
E gave no active species (see also ref 39) while the phenyl
substituted salicylaldimine 25F gave low productivity (<10
g‚mol-1h-1bar-1). The ketimine derivative (25I) also resulted
IV was screened for ethylene polymerization using, sepa-
rately, MAO and diethylaluminum chloride (DEAC) as activa-
tors. MAO afforded a low productivity of 6 g‚mmol-1h-1bar-1
,
whereas DEAC gave a value of 70 g‚mmol-1h-1bar-1. The more
efficient activating effect of DEAC is consistent with our
findings for bulky bis(ligand)Cr systems, and provides an
indication that a neutral Cr-alkyl catalyst is formed. The clearly
inferior productivity of the putative L2CrR species versus [L2-
TiR]+ may be attributed, in part, to the lower electrophilicity
of the neutral chromium center relative to the cationic, and
therefore more electrophilic, titanium centers, but another
important effect in Cr(III) systems is the kinetic stability of the
d3 configuration for octahedral (or pseudo-octahedral) species.
This may lead to a large energetic barrier to ethylene insertion
for a six-coordinate ethylene adduct of the type [L2CrR(C2H4)].
(38) (a) Matsui, S.; Tohi, Y.; Mitani, M.; Saito, J.; Makio, H.; Tanaka, H.;
Nitabaru, M.; Nakano, T.; Fujita, T. Chem. Lett. 1999, 1065. (b) Matsui,
S.; Mitani, M.; Saito, J.; Tohi, Y.; Makio, H.; Tanaka, H.; Fujita, T. Chem.
Lett. 1999, 1263.
(39) Ittel, S. D.; Wang, Y. E. I. Dupont De Nemours & Co., USA, WO 0144324,
2001.
Ligands with Pendant π-Arene Groups. The initial series
of ‘hits’ from the library screen revealed a beneficial effect of
9
11042 J. AM. CHEM. SOC. VOL. 127, NO. 31, 2005

Chromium Catalysts via High-Throughput Screening
A R T I C L E S
Figure 4. Ethylene polymerization screen using the ligand generated from 55 commercial amines and (a) o-anthracenyl-salicylaldehyde (A); (b) o-anthracenyl-
CF3-salicylaldehyde (hits for (A) shown in green for comparison) (B); (c) triptycenyl-salicylaldehyde (D). Conditions: metal precursor I (0.005 mmol),
ligand (0.005 mmol), Al:Cr 200:1, toluene 5 mL, RT, 10 min.
Table 2. Summary of Productivity and Polymer Data for Selected
Anthracenyl-salicylaldimines were synthesized containing one
(69A), two (70A) and three (71A) methylene units separating
Catalyst Systems Containing N-alkyl Salicylaldiminato Ligandsa
ligand
prod. HTS
prod. in-situ
Mn
Mw
Mw/Mn
a phenyl group from the imine nitrogen donor. Productivities
for catalyst systems containing these salicylaldimines are
collected in Table 4. A significant increase in productivity is
found upon lengthening the link from zero (95 g‚mmol-1h-1bar-1)
or one (94 g.mmol-1h-1bar-1) to two methylene units (420
g‚mmol-1h-1bar-1). Also, the polymer obtained using 70A is
insoluble indicating very high molecular weight, and a signifi-
cant increase over the molecular weight of PE isolated using
ligand 2A (Mw 93 4000, PDI 4.4). The productivity is raised
further, to 650 g‚mmol-1h-1bar-1, for ligand 71A containing a
6A
8A
24A
25A
27A
31A
51.6
54.6
70.0
55.4
78.4
75.8
672b
920
1288
1760
2960
2380
81000
26000
30000
$
728000
1370000
611000
85000
75000
$
1550000
2360000
7.6
3.2
2.5
$
2.1
1.7
a Standard conditions: Schlenk test, 1 bar, 100 mL toluene, 15 min,
productivity g.mmol-1h-1bar-1. brun time 60 min. $ insoluble polymer, MW
> 2M Da.
pendant arene substituents. High productivities were found for
1-indanyl (38A), 3,3-diphenylpropyl (6A), 1-methyl-3-phenyl-
propyl (41A), 2-CF3-benzyl (47A), and p-tert-butylbenzyl (11A)
amines. Potentially stabilizing π-arene interactions have recently
been identified in other systems,40 leading to substantially
improved catalyst lifetimes and altered product distribu-
tions.
(40) (a) Sassmannshausen, J.; Powell, A. K.; Anson, C. E.; Wocadlo, S.;
Bochmann, M. J. Organomet. Chem. 1999, 592, 84. (b) Moody, L. S.;
MacKenzie, P. B.; Killian, C. M.; Lavoie, G. G.; Ponasik, J. A., Jr.; Smith,
T. W.; Pearson, J. C.; Barrett, A. G. M. Eastman Chemical Company, USA,
WO. 2002022694, 2002. (c) Deckers, P. J. W.; Hessen, B.; Teuben, J. H.
Organometallics 2002, 21, 5122. (d) Deckers, P. J. W.; Hessen, B.; Teuben,
J. H. Angew. Chem., Int. Ed. 2001, 40, 2516. (e) Gibson, V. C.; Tomov,
A.; Wass, D. F.; White, A. J. P.; Williams, D. J. J. Chem. Soc., Dalton
Trans., 2002, 2261.
9
J. AM. CHEM. SOC. VOL. 127, NO. 31, 2005 11043

A R T I C L E S
Jones et al.
Table 3. Synthesis of Further Salicylaldimines for Examination of
Structure-Activity Relationships for Bidentate Schiff Base Ligands
on Chromiuma
A (R1 & R3 ) H, R2 ) Anthracenyl). E (R1 ) R2 ) tBu, R3 ) H),
I (R1 ) H, R2 ) Anth, R3 ) Me), J (R1 ) H, R2 ) Tript, R3 ) Me).
Salicylaldimine
ligand
prod.
A
R4 ) -tButyl
60A
61A
62A
63A
64A
65A
66A
67A
68A
25E
2E
70
<10
14
76
80
40
26
94
65
<10
<10
<10
<10
-cyclohexyl
-CH(CH2CH2Ph) 2
-NH2
-CHMeCH2CHPh
-CH2CH2NMe2
-(2,5-dimethylpyrrolyl)
-CH2CHMe2
Figure 6. Molecular structure of V. Selected bond lengths (Å) and angles
(°): Cr-Cl(1), 2.3395(11), Cr-O(1), 1.913(2), Cr-O(21), 1.923(2), Cr-
N(7), 2.035(3), Cr-N(27), 2.057(3), Cl(1)-Cr-O(1), 92.21(8), Cl(1)-Cr-
O(21), 88.49(8), Cl(1)-Cr-N(7), 90.42(9), Cl(1)-Cr-N(27), 175.68(9),
Cl(1)-Cr-N(40), 91.76(9), O(1)-Cr-N(7), 88.82(11), O(1)-Cr-N(27),
91.76(11), O(1)-Cr-N(40), 85.98(11), O(1)-Cr-O(21), 173.47(11),
N(7)-Cr-O(21), 97.67(11), N(7)-Cr-N(27), 87.96(12), N(7)-Cr-N(40),
174.43(12), O(21)-Cr-N(27), 87.76(11), O(21)-Cr-N(40), 87.52(11),
N(27)-Cr-N(40), 90.22(11).
-adamantyl
E
R4 ) -iPr
-Ph
R4 ) -iPr
R4 ) -iPr
F
I
25F
25I
a Standard Schlenk polymerization conditions; RT, Cr:Al 1:200 (MAO),
1 bar ethylene, 100 mL toluene, 60 min.
Table 4. Summary of Polymerization Tests for Catalysts
Containing Pendant Arene Substituted
Anthracenyl-Salicylaldimines with Zero (2A), One (69A), Two
(70A), and Three (71A) Methylene Spacers between the Imine
Donor and Phenyl Substituenta
ligand
Cr (mmole)
CH2’s
yield (g)
prod.b
2A
0.015
0.005
0.005
0.002
0
1
2
3
1.42
0.42
2.1
95
94
420
650
69A
70A
71A
1.3
a Standard Schlenk polymerization conditions; RT, Cr:Al 1:200 (MAO),
b
1 bar ethylene, 100 mL toluene, 60 min. g‚mol-1h-1bar-1
.
Crystals of 32D and 73C suitable for structural analyses were
isolated during purification and their structures are provided in
the Supporting Information.
Ethylene polymerization tests on 32D confirmed the results
of the high throughput screen, i.e., that the triptycenyl analogue
Figure 5. Molecular structure of III. Selected bond lengths (Å) and angles
(°): Cr-Cl(1), 2.3266(16), Cr-Cl(2), 2.3402(16), Cr-O(1), 1.907(3), Cr-
N(7), 2.038(4), Cr-O(30), 2.062(5), Cr-O(40), 2.071(4), Cl(1)-Cr-Cl-
(2), 90.95(7), Cl(1)-Cr-O(1), 92.02(12), Cl(1)-Cr-N(7), 94.69(15), Cl(1)-
Cr-O(30), 176.59(13), Cl(1)-Cr-O(40), 91.68(14), Cl(2)-Cr-O(1),
175.59(12), Cl(2)-Cr-N(7), 95.01(13), Cl(2)-Cr-O(30), 89.78(14), O(1)-
Cr-N(7), 88.00(16), O(1)-Cr-O(30), 87.07(17), O(1)-Cr-O(40), 87.13-
(15), N(7)-Cr-O(30), 88.56(18), N(7)-Cr-O(40), 172.1(2), O(30)-Cr-
O(40), 85.00(18).
32D gave a catalyst with good productivity (1574 g‚mmol-1h-1
-
bar-1 under the test conditions employed), while the anthracenyl
derivative 32A is inactive. The more rigid ligands 73C and 73D
gave productivities of 80 and 1858 g‚mmol-1h-1bar-1, respec-
tively, again showing the more beneficial effect of the triptycenyl
substituent. The products afforded by these catalysts are typically
low molecular weight solids (Mw 1700-1900; PDI 1.7-1.8)
with 10-15 wt % being toluene soluble; GPC traces are shown
in Figure 7. 13C NMR analyses of the solid fractions show the
presence of linear R-olefins while GC analysis on the soluble
fraction indicates predominantly linear R-olefins, along with a
small percentage of alkanes (depending on the Cr:Al ratio)
which arise by chain transfer to aluminum.
trimethylene spacer group. These results indicate a potential
π-arene stabilization for >1 methylene linkers, affording higher
productivities as a result of longer catalyst lifetimes, and
significantly enhanced molecular weights, the latter arising from
a lowering of the rate of â-H transfer due to competition with
π-arene coordination.
Tridentate Salicylaldimine Ligands. The screen of trip-
tycenyl-substituted salicylaldimines (Figure 4c) revealed a
unique ‘hit’ for the pyridylmethylamine derivative 32D. This
ligand and its chromium complex were therefore scaled-up,
along with its anthracenyl relative 32A, and nonbulky 3,5-di-
tert-butylphenoxy (32E) and 3-phenoxy (32F) derivatives. In
addition 8-aminoquinoline (73), a rigid variation of 32, was
reacted with backbones C and D to generate 73C and 73D.
Complexes were synthesized by reaction of 32D and 73D
with metal precursor I in toluene. The poorly soluble red-brown
solids were isolated as the thf-free species LCrCl2, VI and VII.
Complex VI dissolved in hot acetonitrile to give the crystalline
acetonitrile adduct (VIII) which was found to be inactive for
ethylene polymerization. Complex VII dissolved in hot pyridine
to give the pyridine adduct (73D)CrCl2(py) (IX) which was used
in polymerization tests with no obvious influence on reactivity
9
11044 J. AM. CHEM. SOC. VOL. 127, NO. 31, 2005

Chromium Catalysts via High-Throughput Screening
A R T I C L E S
Figure 9. Ethylene uptake profiles for Complex IX activated with MAO
at different temperatures.
Figure 7. GPC traces of polyethylene samples generated by reacting 32D
(A&B) and 73D (C&D) with (p-tolyl)CrCl2(thf)3 [the trace obtained using
25A (E) is shown for comparison].
VIII and IX, the steric protection of the salicylaldiminato
oxygen by the triptycene is evident, though the orientations of
the three C6 rings in VIII and IX are slightly different, possibly
a consequence of an intramolecular π-π stacking interaction
between the coordinated pyridine and one of the triptycene C6
rings in IX.
The isolated complexes have been screened and all show high
productivities with the exception of the acetonitrile complex.
The presence of pyridine however has no detrimental affect on
productivity (Table 5). The use of TiBAl as a scavenger led to
poorer catalyst performance (cf. entries 10 and 12) while
attempted activation with DEAC does not lead to an active
system, unlike for previously reported Schiff base chromium
complexes.11e Under optimized conditions, productivities of up
to 7000 g‚mmol-1h-1bar-1 are observed for complexes contain-
ing ligand 32D while productivities in excess of 10 000
g‚mmol-1h-1bar-1 were recorded for catalysts containing the
quinolyl substituted ligand 73D; these are among the highest
productivities reported for homogeneous chromium based
systems.
Figure 8. Molecular structure of IX. The intramolecular π-π stacking
interaction (a) has centroid‚‚‚centroid and mean interplanar separations of
ca. 3.71 and 3.64 Å respectively; the rings are inclined by only ca. 1°.#
Kinetic profiles of ethylene uptake for IX/MAO, at differing
reaction temperatures are shown in Figure 9. Performance
optimizes ca. 50 °C and declines rapidly at higher temperatures.
At 30-40 °C ethylene uptake continues for at least 1 h
indicating a thermally stable, long-lived system. The catalysts
exhibit first order dependence on ethylene concentration (as
approximated by pressure) over a range of 1-5 bar, Figure 10.
Unlike the bidentate systems described earlier, the tridentate
system incorporates hexene to moderate levels (Table 6),
reaching 3.7 butyl branches per 1000 carbons at high 1-hexene
concentrations.
or polymer distributions due to the coordinated pyridine. Crystals
of IX suitable for X-ray structural analysis were grown by slow
vapor diffusion of benzene (over 5-6 weeks) into a saturated
pyridine solution of VII. The molecular structure (Figure 8)
shows meridional binding of the tridentate ligand with the two
chloride ligands occupying mutually trans positions. The
coordinated pyridine lies trans to the imino nitrogen donor. In
the closely related complex VIII, the acetonitrile ligand occupies
a similar position between the two chloride ligands (see
Supporting Information, Figure S9). Since VIII is inactive in
catalysis, the acetonitrile ligand may interfere with the activation
process or may even be retained by the metal center. In both
To explore further the potential of the tridentate triptycenyl-
salicylaldimine ligand family, a number of derivatives closely
Table 5. Ethylene Polymerization Results Using Triptycenylsalicylaldiminato Chromium Catalystsa
entry
complex
Cr (mmole)
Cr:Al
T (
°C)
reactor
P (Bar)
yield (g)
prod.b
1
2
3
4
5
6
7
8
9
32DCrCl2 (VI)
32DCrCl2 (VI)
32DCrCl2 (VI)
32DCrCl2 (VI)
32DCrCl2(MeCN)2 (VIII)
73DCrCl2 (VII)
73DCrCl2 (VII)
0.001
0.001
0.001
0.005
0.005
0.005
0.0005
0.0005
0.001
0.001
0.0005
0.001
1:200
1:2200
1:2200
1:15c
1:600
1:200
1:2000
1:4000
1:700
1:2200
1:4000
1:2200d
RT
50
50
RT
50
RT
RT
50
50
50
50
50
Schlenk
FP
1
1
4
1
4
1
1
4
4
4
4
4
3
3000
6050
6968
0
6.05
27.87
0
Large
Schlenk
Large
Schlenk
Schlenk
FP
FP
FP
FP
FP
1.49
10.9
5.06
18.7
12.25
29.79
20.32
1.17
75
2180
10120
9350
3063
7448
10161
292
73DCrCl2 (VII)
73DCrCl2(Py) (IX)
73DCrCl2(Py) (IX)
73DCrCl2(Py) (IX)
73DCrCl2(Py) (IX)
10
11
12
a Standard tests except entry 5, 6, 9, and 10 activated in situ by injecting the catalyst solution into the reactor scavenged with the required amount of
b
c
MAO; g‚mol-1h-1bar-1
,
catalyst activated with DEAC, 15 equiv., d TiBAl used as scavenger, 2 mmol.
9
J. AM. CHEM. SOC. VOL. 127, NO. 31, 2005 11045

A R T I C L E S
Jones et al.
resulted in a decrease in productivity (cf. entries 2, 4, and 5,
Table 7). Changing the pendant nitrogen donor to a dimethyl-
amino (65D) or pyrrolidino (76D) moiety gave catalysts with
productivities approximately one-third the level achieved using
32D. Altering the donor to a diphenylphosphino moiety (entry
8) resulted in a lower productivity catalyst. It is thus apparent
that the pyridylmethyl and quinolyl donors afford the best
catalyst performance for these tridentate Schiff base ligands.
Summary
In summary, we have used High Throughput Screening (HTS)
methods to examine the catalytic performance for ethylene
polymerization of a library of salicylaldiminato ligands attached
to chromium. Systems based on bidentate 6-anthracenyl-
salicylaldiminato ligands bearing small 1° or 2° alkylimino
donors gave linear high molecular weight polyethylene with
productivities of up to 3000 g‚mmol-1h-1bar-1. Contrastingly,
ligands bearing large groups at the imine nitrogen donor afforded
poor catalyst productivities, highlighting the importance of
matching the ligand substituent size to the likely coordination
geometry of the active species. Ligands containing pendant
π-arene groups were found to exert a beneficial effect on catalyst
performance, consistent with stabilization of the active site by
weak π-arene-metal interactions. A bis(chelate) chromium
analogue of the highly active titanium phenoxy-imine catalyst
system was found to afford only low productivity which may
be a consequence of the lower electrophilicity of the postulated
neutral active site in the chromium system, and possibly the
high kinetic stability of pseudo-octahedral Cr(III) (d3) com-
plexes. A second category of catalyst was discovered based on
tridentate bulky ortho-triptycenyl substituted salicylaldimines
containing pendant pyridylmethyl or quinolyl donors. These
thermally robust catalysts oligomerize ethylene with produc-
tivities up to 10 000 g‚mmol-1h-1bar-1 to give predominantly
linear R-olefins.
Figure 10. Pressure response for pre-catalyst IX activated with MAO
showing a first order dependence on ethylene.
Table 6. Summary of 1-Hexene Incorporation Data for Complex
73DCrCl2‚y (IX)a
Hexene
(mL)
Butyl
prodb
Mn
Mw
Mw/Mn
(/1000 C’s)
0
8
16
32
9987
7685
6575
6005
1000
1000
1000
1000
1500
1500
1500
1400
1.5
1.5
1.5
1.4
0
0.8
2.3
3.7
a Polymerization conditions; Fischer Porter reactor, solvent ) toluene
200 mL, Cr 0.0005 mmol, 2.0 mmol MAO scavenger (in situ activation),
b
T ) 50 °C, P ) 4 bar, t ) 60 min. g‚mol-1h-1bar-1
.
Table 7. Summary of Polymerization Tests on Catalysts Bearing
Triptycenyl-Salicylaldiminato Ligandsa
prod.
mol-1h-1bar-
1
entry
ligand
Cr (mmol)
yield (g)
g
‚
1
2
3
4
5
6
7
8
80D
32D
79D
74D
75D
65D
76D
72D
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.59
7.87
2.23
5.9
0.576
2.84
2.86
1.64
118
1574
446
1180
115
568
572
328
Experimental Section
Full experimental details are provided in the Supporting Information
package.
a Standard Schlenk polymerization conditions; RT, Cr:Al 1:200 (MAO),
1 bar ethylene, 100 mL toluene, 60 min.
Acknowledgment. Innovene Ltd. is thanked for financial
support. Tripos Receptor Research Ltd is thanked for assistance
with the ligand library synthesis. Drs G. Audley and J. Boyle
are thanked for GPC and NMR measurements, respectively.
related to 32D were targeted. These encompass a series of
ligands in which the linker to the pyridine donor is systemati-
cally lengthened, derivatives containing methyl and phenyl
substituents attached to the methylene linker of 32D, and
derivatives containing other nitrogen donors and a phosphine
donor (Table 7). Increasing the length of the spacer between
the imine and the pyridine does not result in improved catalyst
performance. The 2-pyridyl derivative 80D gave a productivity
of 118 g‚mmol-1h-1bar-1, which compares with a productivity
of 1574 g‚mmol-1h-1bar-1 for the pyridylmethyl derivative 32D
while the pyridylethyl derivative gave a productivity 446
g‚mmol-1h-1bar-1 (entries 1-3, Table 7). Replacing the meth-
ylene bridge protons with either methyl or phenyl substituents
Supporting Information Available: Complete listing of
amines used to generate the ligand library; tables listing
polymerization screen results and polymer data; GC data for
the soluble oligomer fraction arising from tridentate ligand
polymerization tests; full experimental details for ligand and
complex syntheses and experimental data for ligands formed
for in situ screening purposes; crystallographic data files for
all reported structures. This material is available free of charge
via the Internet at http://pubs.acs.org.
JA0518171
9
11046 J. AM. CHEM. SOC. VOL. 127, NO. 31, 2005