Chromium complexes bearing pyrrolide-imine N,N-chelate ligands: Synthesis, structures and ethylene polymerisation behaviour

Article abstract of DOI:10.1039/b204568k

Chromium(II) and chromium(III) complexes bearing two bulky monoanionic N,N-chelating pyrrolide-imine ligands, {ArN=CH(C₄H₃N-2)}₂ Cr (1) and {ArN=CH(C₄H₃N-2)}2Cr(μ-Cl)₂ Li(THF)₂ (2), have been prepared in high yield by treatment of the sodium and lithium salts of ArN=CH(C₄H₃NH-2) (Ar=2,6-i-Pr₂C₆H₃) with CrCl₂(THF) and CrCl₃(THF)₃, respectively. The molecular structure of 1 reveals a square planar geometry about chromium with the pyrrolide-N units disposed mutuallytrans. Conversely, an octahedral geometry is exhibited by the chromium centre in 2 with mutually cis-oriented pyrrolide-N units and cis-chloride ligands which, in turn, link to a Li(THF)₂ unit. Complex 2 can be alkylated with AlR₃ to give the five-coordinate mono-alkyl species {ArN=CH(C₄H₃N-2)}₂CrR (3a R=Me;3b R=Et), in which two pyrrolide-imine ligands occupy the square base of the square pyramidal chromium centre. Complexes 1-3 behave as ethylene polymerisation catalysts upon treatment with an excess of a range of aluminium activators including MAO, Et₂AlCl and Me₂AlCl, converting ethylene to high molecular weight polyethylene. In all cases the dialkylaluminium chlorides are superior co-catalysts to MAO, affording acitivities up to 120 g mmol^-1 h^-1 bar^-1. The coordinatively unsaturated bis(pyrrolide-imine) chromium alkyl complexes 3a and 3b are inactive for the polymerisation of ethylene in the absence of activator.

Full text of DOI:10.1039/b204568k

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Chromium complexes bearing pyrrolide-imine N,N-chelate ligands:
synthesis, structures and ethylene polymerisation behaviour†
Vernon C. Gibson,* Claire Newton,‡ Carl Redshaw,§ Gregory A. Solan,¶ Andrew J. P. White
and David J. Williams
Department of Chemistry, Imperial College, South Kensington, London, UK SW7 2AY.
E-mail: V.Gibson@ic.ac.uk
Received 13th May 2002, Accepted 30th July 2002
First published as an Advance Article on the web 24th September 2002
Chromium() and chromium() complexes bearing two bulky monoanionic N,N-chelating pyrrolide-imine ligands,
{ArN᎐CH(C H N-2)} Cr (1) and {ArN᎐CH(C H N-2)} Cr(µ-Cl) Li(THF) (2), have been prepared in high yield
᎐
᎐
4
3
2
4
3
2
2
2
by treatment of the sodium and lithium salts of ArN᎐CH(C H NH-2) (Ar = 2,6-i-Pr C H ) with CrCl (THF) and
᎐
4
3
2
6
3
2
CrCl3(THF)3, respectively. The molecular structure of 1 reveals a square planar geometry about chromium with the
pyrrolide-N units disposed mutually trans. Conversely, an octahedral geometry is exhibited by the chromium centre
in 2 with mutually cis-oriented pyrrolide-N units and cis-chloride ligands which, in turn, link to a Li(THF)2 unit.
Complex 2 can be alkylated with AlR to give the five-coordinate mono-alkyl species {ArN᎐CH(C H N-2)} CrR
᎐
3
4
3
2
(3a R = Me; 3b R = Et), in which two pyrrolide-imine ligands occupy the square base of the square pyramidal
chromium centre. Complexes 1–3 behave as ethylene polymerisation catalysts upon treatment with an excess of a
range of aluminium activators including MAO, Et2AlCl and Me2AlCl, converting ethylene to high molecular weight
polyethylene. In all cases the dialkylaluminium chlorides are superior co-catalysts to MAO, affording acitivities up to
120 g mmolϪ1 hϪ1 barϪ1. The coordinatively unsaturated bis(pyrrolide-imine) chromium alkyl complexes 3a and 3b
are inactive for the polymerisation of ethylene in the absence of activator.
ligands, along with the results of ethylene polymerisation
studies.
Introduction
Recent years have seen growing interest in the development of
new highly active, well-defined catalysts for olefin polymeris-
ation based on both early and late transition metals.1 Central to
these developments has been the design of new ligand frames
that can be readily tailored to influence the properties of
the resultant polymers. Some notably successful ligands are
those containing the unsaturated imino donor functionality.
Examples include α-diimines (Ni, Pd),2 bis(imino)pyridines
(Fe, Co)3 and phenolate-imines (Ni4 and Group 4 metals.5)
Notwithstanding the industrial significance of chromium in
heterogeneous polymerisation catalysis,6 the development of
molecular-based chromium catalysts for olefin polymerisation
has progressed more slowly than for other metals and is based
principally on low valent half-sandwich chromium() species7
along with a limited number of non-cyclopentadienyl chrom-
ium()8 and chromium() systems.9–11 We have been interested
in the use of sterically demanding mono-anionic bidentate
ligands as supports for chromium catalysts in low oxidation
states.10 In an earlier communication we described chrom-
ium complexes containing two N,N-chelating pyrrolide-imine
ligands as ethylene polymerisation catalysts.11 Such ligands have
more recently been reported to give rise to active polymeris-
ation catalysts when attached to Group 4 metals.12,13
Results and discussion
The pyrrole-imine, (2,6-i-Pr C H )N᎐CH(C H NH-2), was
prepared in high yield as a white solid by the condensation
reaction of pyrrole-2-carboxaldehyde with one equivalent of
2,6-diisopropylaniline in refluxing diethyl ether in the presence
of a catalytic amount of glacial acetic acid (Scheme 1). Com-
᎐
2
6
3
4
3
pound (2,6-i-Pr C H )N᎐CH(C H NH-2) has been character-
᎐
2
6
3
4
3
1
ised by microanalysis, H, 13C NMR spectroscopy along with
mass spectrometry (see Experimental section).
The 1H NMR spectrum of (2,6-i-Pr C H )N᎐CH(C H -
᎐
2
6
3
4
3
NH-2) in CDCl3 exhibits a resonance at δ 7.91 for the imine CH
proton, with the corresponding 13C NMR signal occurring at
δ 153.2. Compound (2,6-i-Pr C H )N᎐CH(C H NH-2) also
᎐
2
6
3
4
3
1
displays three signals between δ 6.41 and 5.97 in the H NMR
spectrum characteristic of the pyrrole ring protons, whilst the
NH proton of the pyrrole ring appears downfield at δ 11.59.
The infrared absorption band of the imine is clearly visible at
1623 cmϪ1, and a molecular ion peak is observed in the EI mass
spectrum at 254 amu.
Reaction of two equivalents of the sodium salt of (2,6-i-
Pr C H )N᎐CH(C H NH-2) (prepared by treatment with NaH)
᎐
2
6
3
4
3
In this article we present full synthetic and characterisation
details for chromium pre-catalysts containing pyrrolide-imine
with CrCl2ؒTHF in THF at ambient temperature afforded, after
work-up, air sensitive orange crystalline {(2,6-i-Pr C H )N᎐
᎐
2
6
3
CH(C4H3N-2)}2Cr (1) in high yield (Scheme 1). Crystals of 1
were grown from petroleum ether (bp 40–60 ЊC) at 253 K. The
molecular structure is shown in Fig. 1; selected bond lengths
and angles are listed in Table 1. The molecule is monomeric
and has non-crystallographic C2 symmetry. The geometry at
chromium is distorted square planar with cis angles in the range
81.8(2) to 98.4(2)Њ, the coordination sphere being planar to
within 0.002 Å; the five-membered chelate rings are co-planar
to within 0.012 Å. The pyrrolide-N units of each ligand are
† Electronic supplementary information (ESI) available: a plot of
the molecular structure of 3a. See http://www.rsc.org/suppdata/dt/b2/
b204568k/
‡ Present address: Millennium Pharmaceuticals, Inc., 270 Albany
Street, Cambridge, MA 02139, USA.
§ Present address: Wolfson Materials and Catalysis Centre, School of
Chemical Sciences, University of East Anglia, Norwich, UK NR4 7TJ.
¶ Present address: Department of Chemistry, University of Leicester,
University Road, Leicester, UK LE1 7RH.
DOI: 10.1039/b204568k
J. Chem. Soc., Dalton Trans., 2002, 4017–4023
This journal is © The Royal Society of Chemistry 2002
4017

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Scheme 1 Reagents and conditions: (i) 2,6-i-Pr2C6H3NH2, Et2O, Hϩ, 308 K; (ii) NaH, THF; (iii) 1/2 CrCl2ؒTHF, THF, 293 K; (iv) n-BuLi, THF,
195 K; (v) 1/2 CrCl3ؒ3THF, THF, 195 K; (vi) AlR3 (R = Me, Et), THF, 293 K.
observed at 1591 cmϪ1. Microanalysis and FAB mass spectro-
metry of 1 are consistent with its formulation.
The preparation of {(2,6-i-Pr C H )N᎐CH(C H N-2)} -
᎐
2
6
3
4
3
2
Cr(µ-Cl)2Li(THF)2 (2) was carried out in THF by reacting
CrCl ؒ3THF with the lithium salt of (2,6-i-Pr C H )N᎐CH-
᎐
3
2
6
3
(C4H3NH-2) in a 1 : 2 molar ratio at 195 K (Scheme 1). Com-
plex 2 was isolated in good yield and in crystalline form as an
air sensitive compound. Although elemental analyses on the
bulk sample indicate some contamination by the monochloride
species, orange/red crystals of 2, suitable for X-ray diffraction,
can be grown from a pentane solution at ambient temperature.
The molecular structure is illustrated in Fig. 2; selected bond
lengths and angles are listed in Table 2.
Fig. 1 The molecular structure of 1.
Table 1 Selected bond lengths [Å] and angles [Њ] for complex 1
Cr–N(27)
Cr–N(21)
N(21)–C(25)
C(6)–N(7)
C(5)–C(6)
2.069(4)
2.078(4)
1.389(6)
1.291(7)
1.413(7)
Cr–N(1)
Cr–N(7)
N(1)–C(5)
C(26)–N(27)
C(25)–C(26)
2.073(4)
2.085(4)
1.382(7)
1.281(7)
1.423(7)
N(27)–Cr–N(1)
N(27)–Cr–N(7)
N(1)–Cr–N(21)
98.4(2)
179.7(2)
179.5(2)
N(27)–Cr–N(21)
N(1)–Cr–N(7)
N(21)–Cr–N(7)
82.1(2)
81.8(2)
97.7(2)
oriented mutually trans and the Cr–N distances are comparable
with those of related Cr() complexes containing chelating
nitrogen donor ligands.14 The imino linkages within the chelate
rings retain their double bond character [C(6)–N(7) 1.291(7) Å,
C(26)–N(27) 1.281(7) Å]. The 2,6-diisopropyl rings are oriented
virtually orthogonally (ca. 86Њ) to the planes of their associated
five-membered metallocyclic rings.
Fig. 2 The molecular structure of 2. The transannular Cr ؒ ؒ ؒ Li
distance is 3.26(2) Å.
Complex 1 is paramagnetic with a magnetic moment of 4.81
µB (Evans Balance at ambient temperature) indicating a high
spin d4 electronic configuration for the divalent chromium
centre. As with most divalent chromium compounds, 1 showed
a poorly resolved and uninformative EPR spectrum. Although
The structure of 2 consists of an octahedral Cr() centre
coordinated to two N,NЈ-chelating pyrrolide-imine ligands and
two cis-chloride groups. Unlike in 1, here the pyrrolide func-
tionalities of each ligand are oriented mutually cis with the
trans sites occupied by an imine-N unit and a chloride ligand.
The main distortions from octahedral geometry arise from the
bite angles of the two chelating ligands [N(1)–Cr–N(7) 78.5(3),
N(21)–Cr–N(27) 78.8(3)Њ], and a noticeable asymmetry in the
1
1 is paramagnetic the H NMR spectrum was recorded and
broad unassignable resonances could be observed in the range
δ 14 to Ϫ1. The infrared absorption for the imine C᎐N stretch is
᎐
4018
J. Chem. Soc., Dalton Trans., 2002, 4017–4023

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Table 2 Selected bond lengths [Å] and angles [Њ] for complex 2
Table 3 Selected bond lengths [Å] and angles [Њ] for complexes 3a and
3b
Cr–N(21)
Cr–N(7)
2.005(7)
2.188(6)
2.349(2)
1.371(10)
1.380(10)
1.287(9)
1.91(2)
Cr–N(1)
2.011(7)
2.149(6)
2.354(3)
1.394(11)
1.425(11)
1.286(9)
1.92(2)
Cr–N(27)
Cr–Cl(2)
C(5)–C(6)
C(25)–C(26)
C(6)–N(7)
Li–O(50)
Li–Cl(2)
3a
Cr–Cl(1)
Cr–C
Cr–N(7)
N(1)–C(5)
2.037(9)
2.073(4)
1.382(7)
Cr–N(1)
C(6)–N(7)
C(5)–C(6)
2.026(5)
1.318(7)
1.410(8)
N(1)–C(5)
N(21)–C(25)
C(26)–N(27)
Li–O(40)
Li–Cl(1)
N(1)–Cr–N(1Ј)
N(1)–Cr–N(7Ј)
N(1)–Cr–N(7)
173.2(4)
97.9(2)
80.9(2)
N(1)–Cr–C
N(7)–Cr–N(7Ј)
C–Cr–N(7)
93.4(2)
159.2(3)
100.39(14)
2.35(2)
2.31(2)
N(21)–Cr–N(1)
N(21)–Cr–N(7)
N(1)–Cr–N(27)
N(21)–Cr–Cl(1)
N(7)–Cr–Cl(1)
N(27)–Cr–Cl(2)
N(27)–Cr–Cl(1)
O(40)–Li–O(50)
O(40)–Li–Cl(1)
O(50)–Li–Cl(2)
Cr–Cl(1)–Li
88.7(3)
88.5(2)
166.6(3)
93.2(2)
166.7(2)
94.2(2)
87.7(2)
108.6(8)
119.0(8)
114.7(8)
87.8(4)
89.87(9)
N(21)–Cr–N(27)
N(1)–Cr–N(7)
N(27)–Cr–N(7)
N(1)–Cr–Cl(1)
N(21)–Cr–Cl(2)
N(7)–Cr–Cl(2)
N(1)–Cr–Cl(2)
O(40)–Li–Cl(2)
O(50)–Li–Cl(1)
Cl(2)–Li–Cl(1)
Li–Cl(2)–Cr
78.8(3)
78.5(3)
105.5(2)
88.4(2)
172.2(2)
90.2(2)
98.6(2)
112.2(8)
110.9(8)
90.7(6)
88.6(4)
3b
Cr–C(20)
Cr–N(7)
N(1)–C(5)
2.02(2)
2.059(10)
1.42(2)
Cr–N(1)
C(6)–N(7)
C(5)–C(6)
2.054(10)
1.27(2)
1.42(2)
C(20)–Cr–N(1)
N(1)–Cr–N(7Ј)
C(20)–Cr–N(7)
92.9(4)
98.0(5)
99.7(3)
N(1)–Cr–N(1Ј)
N(7)–Cr–N(7Ј)
N(1)–Cr–N(7)
174.1(8)
160.6(6)
81.0(5)
Cl(1)–Cr–Cl(2)
ambient temperature alkylation proceeded smoothly to afford
{(2,6-i-Pr C H )N᎐CH(C H N-2)} CrMe (Scheme 1). Com-
᎐
2
6
3
4
3
2
Cr–N distances with those to the pyrrolide nitrogen atoms
being significantly shorter [Cr–N(1) 2.011(7), Cr–N(21)
2.005(7) Å], than those to the imino-nitrogens [Cr–N(27)
2.149(6), Cr–N(7) 2.188(6) Å]. The two five-membered chelate
rings have different conformations with that containing N(1)
having an envelope conformation and a ca. 11Њ fold about the
N(1) ؒ ؒ ؒ N(7) vector whereas that containing N(21) is essen-
tially planar. The two chlorines are further coordinated to a
solvated Li centre, the CrLiCl2 ring having a slightly folded
geometry with the Li atom lying 0.53 Å out of the CrCl2 plane.
The geometry at Li is distorted tetrahedral with angles ranging
between 90.7(6) and 119.0(8)Њ, the most ‘acute’ angle being that
subtended by the two Cl atoms. The only intermolecular pack-
ing contact of note is a C–H ؒ ؒ ؒ π interaction (H ؒ ؒ ؒ π 2.68 Å,
C–H ؒ ؒ ؒ π 155Њ) between C(23)–H in one molecule and the
C(8)-containing 2,6-diisopropylphenyl ring of another.
It would appear that the steric congestion imposed by the
bulky aryl substituents in the pyrrolide-imine ligands in 2 is
responsible for preventing the binding of three ligands in a
manner that has been observed in related complexes.15 Surpris-
ingly, the preferred octahedral geometry for Cr() is achieved
by the inclusion of a molecule of lithium chloride (as a THF
solvate).16 Presumably, the bulky ligand substituents prevent
dimerisation from occurring. It is worthy of note that, for the
plex 3a was isolated in moderate yield and in crystalline form as
an air sensitive compound. Recrystallisation from petroleum
ether (bp 40–60 ЊC) at 243 K gave green crystals of 3a. The
molecular structure of 3a has been reported previously11 and is
illustrated in Fig. S1 (see ESI†); selected bond lengths and angles
are listed in Table 3.
The structure of 3a reveals a distorted square pyramidal
Cr() centre, coordinated basally to two chelating pyrrolide-
imine ligands and apically to a terminal methyl group at
2.037(9) Å. The complex has crystallographic C2 symmetry
about an axis passing through the Cr–Me bond, and the
two independent Cr–N distances differ significantly, with
that to the pyrrolide [Cr–N(1) 2.026(5) Å] being shorter than
that to the imine [Cr–N(7) 2.073(4) Å], a pattern of bond-
ing that is also observed in 2 but not in 1 where all four
Cr–N distances are essentially the same (av. 2.076 Å). Although
the reason for the absence of any difference between the
Cr–N(pyrrolide) and Cr–N(imino) distances in 1 is not
immediately apparent, it may be a consequence of the degree
of Cr–pyrrolide π-bonding which is maximised in 2 and 3
due to the higher oxidation state of the metal centres, i.e. Cr()
vs. Cr().
Throughout the three structures discussed (1, 2 and 3a) and
also in structure 3b (vide infra) the conformation of, and the
pattern of bonding within, the pyrrolide-imine ligand is
remarkably consistent. For example, the double bond character
related titanium complexes {RN᎐CH(C H N-2)} TiCl (R =
᎐
4
3
2
2
Ph, Et, Cy), the only isomers isolated are those in which the
pyrrolide moieties are mutually trans.12 Indeed, for {PhN᎐
of the imino C᎐N linkage is retained and ranges between
᎐
᎐
CH(C4H3N-2)}2TiCl2 this configuration has been shown by
calculations (DFT) to be more stable than the corresponding
configuration adopted by 2. It is likely that steric hindrance
disfavours formation of a similar isomer for 2.
1.27(2) Å (in 3b) and 1.318(7) Å (in 3a) as does the single bond
imine-pyrrolide linkage with a range between 1.394(11) Å
(in 2) and 1.423(7) Å (in 1). Similarly, the pyrrolide and imine
units are coplanar to within 0.13 Å. In all cases the 2,6-diiso-
propylphenyl ring plane is steeply inclined to the pyrrolide-
imine plane, with interplanar angles ranging between ca. 70
and 86Њ. This orthogonal conformation has been observed
previously and has been attributed to a stabilisation by weak
C–H ؒ ؒ ؒ N(pπ) interactions between the isopropyl methine
hydrogen atoms and the non-bonding p orbitals of the ring
nitrogen atom.10c,17
1
The H NMR spectrum of 2 displays broad and uninform-
ative signals. The magnetic susceptibilty measurement reveals a
magnetic moment of 4.11 µB (Evans Balance at ambient tem-
perature) which is consistent with three unpaired electrons (d3).
This is further supported by the X-band EPR spectrum in tolu-
ene at room temperature which shows a sharp signal with a
g-value of 2.0 which is typical of chromium(), there is no
observable hyperfine splitting. In the infrared spectrum of 2 an
The alkylation of 2 with trimethylaluminium is readily
absorption band for the imine C᎐N stretch is observed at 1595
extendable to other alkylaluminium reagents, and we have syn-
᎐
cmϪ1, and noticeably lower in magnitude than that for the free
ligand. In the FAB mass spectrum of 2 no molecular ion peak is
observed but a fragmentation peak corresponding to the loss of
Li(THF)2 and two chloride ligands is seen.
thesised the ethyl derivative {(2,6-i-Pr C H )ArN᎐CH(C H N-
᎐
2
6
3
4
3
2)}2CrEt (3b) from the reaction of 2 with triethylaluminium in
THF at ambient temperature (Scheme 1). Crystals of 3b were
of poor quality, but sufficient to establish that the complex
adopts a structure that is isomorphous and virtually identical to
that of 3a11 with an ethyl group occupying the axial site but
with the terminal methyl group disordered about the crystallo-
graphic 2-fold axis (Fig. 3). Bond lengths and angles for 3b are
listed in Table 3 for comparison with 3a. It is noteworthy that
With the intent of generating a well-defined single compon-
ent chromium catalyst for olefin polymerisation we set out to
explore the alkylation chemistry of complex 2. We anticipated
that a five-coordinate coordinatively unsaturated alkyl complex
could be accessible. On treatment of 2 with AlMe3 in THF at
J. Chem. Soc., Dalton Trans., 2002, 4017–4023
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Table 4 Results of ethylene polymerisation runs with pre-catalysts 1–3a
Run
Pre-catalyst/mmol
Cocatalystb (mmol/ equiv.)
Yieldc/g
Activity/g mmolϪ1 hϪ1 barϪ1
Mn
Mw
Mw/Mn
1
2
3
4
5
6
7
8
9
1 (0.015)
1 (0.015)
1 (0.015)
2 (0.015)
2 (0.015)
2 (0.015)
3a (0.015)
3a (0.015)
3a (0.015)
3b (0.010)
3b (0.010)
3b (0.010)
ClAlEt2 (0.45/30)
ClAlMe2 (0.45/30)
MAO (6.0/400)
ClAlEt2 (0.45/30)
ClAlMe2 (0.45/30)
MAO (6.0/400)
ClAlEt2 (0.45/30)
ClAlMe2 (0.45/30)
MAO (6.0/400)
ClAlEt2 (0.30/30)
ClAlMe2 (0.30/30)
MAO (4.0/400)
0.20
1.20
0.06
1.04
3.00
0.08
1.05
1.31
0.05
0.55
1.20
0.02
13
80
4
69
113
5
70
87
3
55
119
2
22 000
336 000
–
1 043 000
1 377 000
–
47.4
4.1
–
133 000d
500 000
–
293 000d
1 742 000
–
2.2d
3.5
–
79 000
610 000
–
1 306 000
1 958 000
–
16.6
3.2
–
10
11
12
22 000
1 168 000
53.1
e
e
e
–
–
–
a General conditions: Schlenk tests carried out at 1 bar ethylene in toluene (40 mL) at ambient temperature, over 60 min, reaction quenched with
dil. HCl and the solid PE washed with methanol (50 mL) and dried in a vacuum oven at 323 K. b MAO = methylaluminoxane. c Solid polyethylene.
d Care should be taken in the interpretation of this result as the polymer derived is not fully soluble in the 1,2,4-trichlorobenzene GPC solvent even
upon heating at 433 K for several hours. e Insoluble in 1,2,4-trichlorobenzene GPC solvent even upon heating at 433 K for several hours.
systems, nor has there been any evidence for low molecular
weight oligomers.
On analysis of the data contained within Table 4, several
points emerge. The Cr() complexes 2 and 3b are the most
active of all the pyrrolide-imine containing chromium com-
plexes, with activities of 113 and 119 g mmolϪ1 hϪ1 barϪ1 (Table
2, see runs 5 and 11), respectively. The bis(pyrrolide-imine)
chromium alkyl complexes 3a and 3b are inactive as single
component catalysts despite the availability of a vacant
coordination site. This may be due to ethylene binding trans to
the alkyl group or, alternatively, it is possible that cis binding
occurs but the enhanced stability of the resultant octahedral
Cr() species may lead to an energetically prohibitive barrier to
ethylene insertion. In almost all cases, the most compatible
co-catalysts are the dialkylaluminium chlorides, Me2AlCl and
Et2AlCl, rather than MAO which affords in all cases only trace
amounts of polyethylene.
Fig. 3 The molecular structure of 3b.
The polyethylene generated by catalysts 1–3 has very little
branching (by 13C NMR) and, in its general properties,
resembles the polyethylene generated by heterogeneous chro-
mium catalysts.6 For example, gel permeation chromatography
(GPC) analysis (in 1,2,4-trichlorobenzene) of the polyethylene
formed shows the Mw’s ranging from 293 000 (run 4) to 1 958
000 (run 8), with polydispersities (Mw/Mn) in the range 2.2–
53.1. Indeed, in some cases (see run 11), the molecular weight
was too high to be determined, even upon heating at 433 K in
1,2,4-trichlorobenzene for several days. It is noteworthy that
control polymerisation tests undertaken using the low valent
chromium precursors, CrCl3ؒ3THF and CrCl2ؒTHF, gave only
trace amounts of polyethylene.
the stability of these alkyl complexes (3a and 3b) lies in contrast
to the related β-diketimininato complexes, [(Ph)2nacnac]2-
CrR (R = alkyl), where (Ph)2nacnac is the N,N’-diphenyl-2,4-
pentanediiminato anion, which are unstable and decompose
via orthometallation.9c Furthermore, 3a and 3b provide
rare examples of structurally characterised mononuclear five-
coordinate trivalent chromium alkyl complexes.18
As with the precursor Cr() complex 2 the 1H NMR spectra
of 3a–3b are broad and uninformative. Compounds 3a and 3b
exhibit magnetic moments of 3.61 and 3.67 µB consistent with
the complexes being mononuclear and possessing d3 electronic
configurations. The X-band EPR spectrum of 3a recorded at
ambient temperature in toluene displays broad features at low
field at ca. g = 4.4 and 3.3 along with several weaker features at
higher field at ca. g = 1.2; no hyperfine structure is evident.
Similar features have been observed for the rhombically dis-
torted d3 species (dmpe)2VX2 (X = Cl, Me) reported by Wilkin-
son and Girolami.19 It is noteworthy that for 3b these features
are only observed at 140 K. In the infrared spectra of 3a and 3b
In conclusion, chromium complexes in oxidation states ()
and () supported by two pyrrolide-imine ligands, [(2,6-i-
Pr C H )N᎐CH(C H N-2)], have been prepared and evaluated
᎐
2
6
3
4
3
as ethylene polymerisation catalysts. All of the complexes give
active catalysts on treatment with alkylaluminium activators.
While MAO behaves as a poor activator, dialkylaluminium
chloride activators perform significantly better leading to
more productive systems. The coordinatively unsaturated bis-
(pyrrolide-imine) chromium alkyl complexes 3a and 3b are
inactive catalysts by themselves in the presence of ethylene.
the imine C᎐N stretches are observed at ca. 1595 cmϪ1. In add-
᎐
ition, the microanalytical and FAB mass spectrometric data are
in accord with the structures of 3a and 3b.
All the new complexes, 1–3, have been screened as pre-
catalysts or catalysts for the polymerisation and oligomeris-
ation of ethylene at 1 bar ethylene and room temperature. Three
types of aluminium co-catalyst have been employed to activate
pre-catalysts 1–3 namely, methylaluminoxane (MAO), Et2AlCl
and Me2AlCl (see Table 4; runs 1–12).
All the dual-component systems are active, converting ethyl-
ene to high molecular weight linear polyethylene. In no case has
high molecular weight polyethylene been obtained using the
bis(pyrrolide-imine) chromium complexes as single component
Experimental
General
All manipulations were carried out under an atmosphere of
nitrogen using standard Schlenk and cannula techniques or in a
conventional nitrogen-filled glove-box. Solvents were refluxed
over an appropriate drying agent, and distilled and degassed
prior to use. Elemental analyses were performed by the micro-
analytical services of the Department of Chemistry at Imperial
4020
J. Chem. Soc., Dalton Trans., 2002, 4017–4023

View Article Online
College and Medac Ltd. NMR spectra were recorded on a
Bruker spectrometer at 250 MHz (1H) and 62.9 MHz (13C) at
293 K; chemical shifts are referenced to the residual protio
impurity of the deuterated solvent. IR spectra (Nujol mulls)
were recorded on Perkin-Elmer 577 and 457 grating spectro-
photometers. Mass spectra were obtained using either fast atom
bombardment (FAB), electron impact (EI) or chemical ionis-
ation (CI). Gel permeatation chromatographs (GPCs) were
obtained using a Waters 150CV (columns are supplied by
Shodex (807, 806 & 804) (BP Chemicals Ltd)). Analysis for
oligomeric products was performed using GCMS, employing a
25 m BPX-5 column, injector temperature 513 K and a Micro-
mass Autospec-Q mass spectrometer. Magnetic susceptibility
studies were performed using an Evans Balance (Johnson
Matthey) at room temperature. The magnetic moment was
calculated following standard methods20 and corrections for
underlying diamagnetism were applied to the data.21
Pr C H )N᎐CH(C H NH-2) (1.356 g, 5.34 mmol) in THF
᎐
2 6 3 4 3
(40 mL) at 195 K. The solution was allowed to warm to room
temperature and stirred for a further hour. The pale yellow
solution was re-cooled to 195 K and filtered into a second reac-
tion vessel containing CrCl3ؒ3THF (1.00 g, 2.67 mmol) in THF
(10 mL) at 195 K. The solution was allowed to warm to room
temperature and stirred overnight. The volatiles were removed
under reduced pressure to give an orange/green residue which
was extracted into pentane (60 mL). The solution was filtered
and the filtrate concentrated to ca. half volume and left to stand
overnight a room temperature to give 2 as orange/red crystals
(1.56 g, 75%). FAB mass spectrum, m/z 558 [Mϩ Ϫ 2Cl Ϫ
Li(THF)2]. IR (cmϪ1) ν(C᎐N) 1595. Anal. Calcd. for C H -
᎐
34 42
N4CrClؒ0.4LiCl(THF)2: C, 66.82; H, 7.24; N, 8.38. Found: C,
66.71; H, 7.16; N, 8.59%. µeff 4.11 µB.
Preparation of {(2,6-i-Pr C H )N᎐CH(C H N-2)} CrR
᎐
2
6
3
4
3
2
Compounds CrCl2ؒTHF22 and CrCl3ؒ3THF23 were prepared
according to established procedures. The reagents AlMe3
(2.0 M solution in toluene), AlEt3 (1.9 M solution in toluene),
Et2AlCl (1.8 M solution in toluene), Me2AlCl (1.0 M solution
in hexane), MAO (10% solution in toluene), and pyrrole-2-
carboxaldehyde were purchased from Aldrich Chemical Co.
All other chemicals were obtained commercially and used as
received unless stated otherwise.
(R ؍ Me 3a, Et 3b)
3a R = Me. The lithium salt of 1 was prepared by addition of
n-BuLi (3.34 mL, 5.34 mmol) to 1 (1.356 g, 5.34 mmol) in THF
(40 mL) at 195 K. The solution was allowed to warm to room
temperature and stirred for a further hour. The pale yellow
solution was re-cooled to 195 K and added dropwise to a solu-
tion of CrCl3ؒ3THF (1.00 g, 2.67 mmol) in THF (10 ml) at
195 K. The solution was allowed to warm to room temper-
ature and stirred for 1 h. AlMe3 (2.7 ml, 5.34 mmol) was then
added at room temperature and the resulting solution left to
stir. After one night at room temperature the volatiles were
removed under reduced pressure to give a green residue. Extrac-
tion of the residue into petroleum ether (bp 40–60 ЊC) (60 ml)
gave an olive green solution which, on concentration to half
volume and cooling to 253 K overnight, afforded green crystals
of 3a (0.78 g, 51%). FAB mass spectrum, m/z 574 [Mϩ], 558
Preparation of (2,6-i-Pr C H )N᎐CH(C H NH-2)
᎐
2
6
3
4
3
To a solution of pyrrole-2-carboxaldehyde (1.50 g, 15.8 mmol)
in diethyl ether (30 mL) was added 2,6-diisopropylaniline (2.98
mL, 15.8 mmol). After addition of a few drops of glacial acetic
acid the solution was heated to reflux for 6 h. Upon cooling to
room temperature the product crystallised. After filtration the
white solid was washed with cold diethyl ether (3 x 20 mL) and
dried overnight to give 3.25 g (81%) of (2,6-i-Pr C H )N᎐
[Mϩ Ϫ CH ], 303 [Mϩ Ϫ CH Ϫ ArN᎐CHC H N-2]. IR (cmϪ1
)
᎐
3 3 4 3
ν(C᎐N) 1594. Anal. Calcd. for C H N Cr: C, 73.30; H, 7.85;
᎐
35 45 4
᎐
2
6
3
CH(C4H3NH-2). 1H NMR (CDCl3): δ 11.59 (s, 1H, C4H3N–H),
7.91 (s, 1H, NCHAr), 7.2–7.1 (m, 3H, Ar–H), 6.41 (s, br 1H,
C4H3NH), 6.00 (s, br 1H, C4H3NH), 5.97 (s, br, 1H, C4H3NH),
N, 9.77. Found: C, 73.01; H, 7.67; N, 9.51%. µeff 3.61 µB.
3b R = Et. Complex 3b was prepared in moderate yield
(0.71 g, 45%) by employing a procedure similar to that outlined
above but with AlEt3 (2.8 ml, 5.34 mmol) in place of AlMe3.
3.07 (sept, 2H, 3J(HH) 6.9, CHMe2), 1.08 (d, 12H, CHMe2). 13
C
1
NMR (CDCl3, H gated decoupled): δ 153.2 (s, N=C), 148.9
(s, pyrrole C–N), 139.5 (s, pyrrole C–N), 130.3 (s, Ar), 125.0 (s,
Ar), 124.8 (s, Ar), 123.7 (s, Ar), 117.3 (s, pyrrole), 110.3
(s, pyrrole), 28.4 (s, CHMe2) and 24.0 (s, CHMe2). EI mass
FAB mass spectrum, m/z 558 [Mϩ Ϫ H5], 303 [Mϩ Ϫ C2H5 Ϫ
2
ArN᎐CHC H N-2]. IR (cmϪ1) ν(C᎐N) 1594. Anal. Calcd. for
᎐
᎐
4
3
C36H47N4Cr: C, 73.60; H, 8.01; N, 9.54. Found: C, 73.47; H,
8.18; N, 9.41%. µeff 3.67 µB.
spectrum, m/z 254 [Mϩ]. IR (cmϪ1) ν(N–H) 3453, ν(C᎐N) 1623.
᎐
Anal. Calcd. for C17H22N2: C, 80.31; H, 8.66; N, 11.02. Found:
C, 80.56; H, 8.91; N, 10.91%.
Polymerisation procedure
Results of Schlenk-line tests (1 bar) are listed in Table 4; poly-
merisation procedure is described as follows. The pre-catalyst
was dissolved in toluene (40 mL) and the co-catalyst (MAO,
Et2AlCl or Me2AlCl) added. The Schlenk tube was purged with
ethylene and the contents magnetically stirred and maintained
under ethylene (1 bar) for the duration of the polymerisation.
After 1 h the polymerisation was terminated by the addition
of dil. HCl/MeOH. The solid polyethylene was recovered by
filtration, washed with methanol (50 mL) and dried (vacuum
oven at 323 K). The presence of lower molecular weight poly-
ethylene was screened using GCMS of the separated organic
layer.
Preparation of {(2,6-i-Pr C H )N᎐CH(C H N-2)} Cr (1)
᎐
2
6
3
4
3
2
The sodium salt of (2,6-i-Pr C H )N᎐CH(C H NH-2) was pre-
᎐
2
6
3
4
3
pared by addition of NaH (0.129 g, 5.34 mmol) to (2,6-i-
Pr C H )N᎐CH(C H NH-2) (1.356 g, 5.34 mmol) in THF
᎐
2
6
3
4
3
(40 mL) at room temperature. The solution was left to stir over-
night and then filtered, at room temperature, into a second flask
containing CrCl2ؒTHF (0.521 g, 2.67 mmol) in THF (10 mL).
The resulting red/brown solution was allowed to stir overnight
at room temperature. The volatiles were removed under reduced
pressure and petroleum ether (bp 40–60 ЊC) (60 mL) intro-
duced. The solution was then stirred, filtered and the filtrate
concentrated to half volume. Orange air-sensitive crystals of 1
were obtained upon standing of the solution at 253 K over-
night (1.14 g, 77%). FAB mass spectrum, m/z 558 [Mϩ], 303
[Mϩ Ϫ ArN᎐CHC H N-2]. IR (cmϪ1) ν(C᎐N) 1591. Anal.
X-Ray crystal structure determinations
Table 5 provides a summary of the crystal data, data collection
and refinement parameters for compounds 1, 2, 3a and 3b. All
four structures were solved by direct methods and refined by
full matrix least-squares based on F 2.
᎐
᎐
4
3
Calcd. for C34H42N4Cr: C, 73.12; H, 7.53; N, 10.03. Found: C,
72.91; H, 7.41; N, 9.91%. µeff 4.81 µB.
CCDC refernce numbers 185874–185877.
See http://www.rsc.org/suppdata/dt/b2/b204568k/ for crystal-
lographic data in CIF or other electronic format.
Preparation of {(2,6-i-Pr C H )N᎐CH(C H N-2)} Cr(ꢀ-Cl) -
Li(THF)2 (2)
᎐
2
6
3
4
3
2
2
The lithium salt of (2,6-i-Pr C H )N᎐CH(C H NH-2) was pre-
In 2 the carbon atoms in the 3 and 4 positions of both of the
᎐
2
6
3
4
3
pared by addition of n-BuLi (3.34 mL, 5.34 mmol) to (2,6-i-
THF units were found to be disordered; in each case two partial
J. Chem. Soc., Dalton Trans., 2002, 4017–4023
4021

View Article Online
Table 5 Crystal data, data collection and refinement parameters for complexes 1, 2, 3a and 3ba
1
2
3a
3b
Formula
Solvent
M
C34H42N4Cr
—
558.7
C42H58N4O2Cl2LiCr
0.5 C5H12
816.8
C35H45N4Cr
—
573.8
C36H47N4Cr
—
587.8
Colour, habit
Crystal system
Space group
a/Å
b/Å
c/Å
Orange prisms
Monoclinic
P21/n (no. 14)
17.249(2)
10.570(1)
17.731(2)
107.93(1)
3075.5(5)
4
Orange/red prisms
Monoclinic
P21/c (no. 14)
17.598(4)
17.638(4)
15.151(4)
102.06(1)
4599(2)
Green blocks
Orthorhombic
Fdd2 (no. 43)
20.280(2)
34.103(4)
9.420(1)
—
Dark green prisms
Orthorhombic
Fdd2 (no. 43)
19.820(4)
34.304(9)
9.969(3)
—
β/Њ
V/Å3
6515(2)
6778(3)
Z
4
8b
8b
Dc/g cmϪ3
1.207
1.180
1.170
1.152
F(000)
1192
1744
Mo-Kα
0.40
2456
Mo-Kα
0.38
2520
Mo-Kα
0.37
Radiation used
Cu-Kαc
3.26
µ/mmϪ1
Unique reflections measured
observed, |Fo| > 4σ(|Fo|)
4533
3427
0.086
0.254
0.201, 2.237
0.84, –1.58
5949
2736
0.074
0.133
0.058, 0.000
0.33, –0.31
1530
1193
0.049
0.094
0.036, 0.000
0.23, –0.27
1576
1064
0.099
0.256
0.214, 0.000
0.95, –0.64
d
R1
wR2
e
Weighting factorsa, bf
Largest diff. peak, hole/e ÅϪ3
a Details in common: graphite monochromated radiation, ω-scans, Siemens P4 diffractometer, 203 K, refinement based on F 2. b The molecule has
2
2
crystallographic C2 symmetry. c Rotating anode source. d R1 = Σ Fo| Ϫ |Fc /Σ|Fo|. e wR2 = [Σw(Fo2 Ϫ Fc2)2/Σw(Fo )2]1/2. f wϪ1 = σ2(Fo ) ϩ (aP)2 ϩ bP.
H. Tanaka, N. Kashiwa and T. Fujita, J. Am. Chem. Soc., 2001, 123,
6847.
occupancy orientations were identified with only the major
occupancy carbon atoms being refined anisotropically (the
others were refined isotropically). In 3b the metal-bonded ethyl
moiety was found to be disordered about the crystallographic
C2 axis; two 50% occupancy orientations related by the twofold
axis were identified and refined isotropically. The remaining
non-hydrogen atoms in all four structures were refined aniso-
tropically. The C–H hydrogen atoms for all of the structures
were placed in calculated positions, assigned isotropic thermal
parameters, U(H) = 1.2Ueq(C), [U(H) = 1.5Ueq(C–Me)], and
allowed to ride on their parent atoms. The polarity of 3a was
6 For background to Phillips catalyst see: (a) J. P. Hogan and
R. L. Banks, US Pat. 2,825,721, 1958; (b) E. A. Benham,
P. D. Smith, E. T. Hsieh and M. P. McDaniel, J. Macromol. Sci.,
Chem., 1988, A25, 259; (c) M. P. McDaniel, Adv. Catal., 1985, 33,
47; (d ) H. L. Kraus, J. Mol. Catal., 1988, 46, 97; (e) G. Ghiotti,
E. Garrone and A. Zecchini, J. Mol. Catal., 1988, 46, 61;
( f ) A. Clark, Catal. Rev., 1969, 31, 123.
7 See for example and refs. therein: (a) K. H. Theopold, Chem. Eur. J.,
1998, 15; (b) K. H. Theopold, CHEMTECH, 1997, 27, 26–32;
(c) R. Emrich, O. Heinemann, P. W. Jolly, C. Krüger and G. P. J.
Verhovnik, Organometallics, 1997, 16, 1511; (d ) D. A. Gohre,
P. W. Jolly, B. Kryger, J. Rust and G. P. J. Verhovnik, Organo-
metallics, 2000, 19, 388; (e) V. R. Jensen, K. Angermund, P. W. Jolly
and K. J. Borve, Organometallics, 2000, 19, 403.
8 (a) M. P. Coles, C. I. Dalby, V. C. Gibson, W. Clegg and M. R. J.
Elsegood, J. Chem. Soc., Chem Commun., 1995, 1709; (b) F. J.
Feher and R. L. Blanski, J. Chem. Soc., Chem Commun., 1990,
1614; (c) M. P. Coles and V. C. Gibson, Polym. Bull., 1994, 33,
529.
9 (a) R. D. Köhn, M. Haufe, S. Mihan and D. Lilge, Chem. Commun.,
2000, 1927; (b) W.-K. Kim, M. J. Fevola, L. M. Liable-Sands,
A. L. Rheingold and K. H. Theopold, Organometallics, 1998, 17,
4541; (c) L. A. MacAdams, W. K. Lin, L. M. Liable-Sands,
I. A. Guzei, A. L. Rheingold and K. H. Theopold, Organometallics,
2002, 21, 952; (d ) T. Ruther, N. Braussaud and K. J. Cavell,
Organometallics, 2001, 20, 1247–1250; (e) H. Ikeda, T. Monoi,
Y. Nakayama and H. Yasuda, J. Organomet. Chem., 2002, 642, 156–
162; ( f ) H. Ikeda, T. Monoi, K. Ogata and H. Yasuda, Macromol.
Chem. Phys., 2001, 202, 1806–1811; (g) P. Wei and D. W. Stephan,
Organometallics, 2002, 21, 1308–1310.
10 (a) V. C. Gibson, C. Newton, C. Redshaw, G. A. Solan, A. J. P.
White and D. J. Williams, J. Chem. Soc., Dalton Trans., 1999,
827; (b) V. C. Gibson, S. Mastroianni, C. Newton, C. Redshaw,
G. A. Solan, A. J. P. White and D. J. Williams, J. Chem. Soc.,
Dalton Trans., 2000, 1969; (c) V. C. Gibson, C. Newton, C. Redshaw,
G. A. Solan, A. J. P. White and D. J. Williams, Eur. J. Inorg. Chem.,
2001, 1895; (d ) V. C. Gibson, C. Newton, G. A. Solan, (BP
Chemicals Ltd.), WO 99/19335, 1999 [Chem. Abstr., 1999, 130,
297099p].
11 V. C. Gibson, P. J. Maddox, C. Newton, C. Redshaw, G. A. Solan,
A. J. P. White and D. J. Williams, Chem. Commun., 1998,
1651.
12 Y. Yoshida, S. Matsui, Y. Takagi, M. Mitani, T. Nakano, H. Tanaka,
N. Kashiwa and T. Fujita, Organometallics, 2001, 20, 4793.
13 D. M. Dawson, D. A. Walker, M. Thornton-Pett and M. Bochmann,
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ϩ
determined by a combination of R-factor tests [R1 = 0.0488,
R1Ϫ = 0.0497] and by use of the Flack parameter [xϩ = 0.10(13),
xϪ = 0.90(13)]. The polarity of 3b could not be reliably deter-
mined. All computations were carried out using the SHELXTL
PC program system.24
Acknowledgements
BP Chemicals Ltd is thanked for financial support. Drs J. Boyle
and G. Audley are thanked for NMR and GPC measurements.
Dr D. Oswald (Queen Mary and Westfield College, University
of London) is thanked for ESR measurements.
References and notes
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J. Chem. Soc., Dalton Trans., 2002, 4017–4023

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16 For related complexes containing a Cr(µ-Cl)2Li(THF)2 unit see:
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20 M. B. Mabbs and D. J. Machin, Magnetism and Transition Metal
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21 (a) C. J. O’Connor, Prog. Inorg. Chem., 1982, 29, 203; (b) Handbook
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22 L. F. Larkworthy and M. H. O. Nelson-Richardson, Chem. Ind.,
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23 J. Shamir, Inorg. Chim. Acta, 1989, 156, 163.
18 For examples of five-coordinate Cr() alkyl complexes see:
(a) M. D. Fryzuk, B. B. Leznoff and S. J. Rettig, Organometallics,
24 SHELXTL, PC version 5.03, Siemens Analytical X-Ray Instru-
ments Inc., Madison, WI, 1994.
J. Chem. Soc., Dalton Trans., 2002, 4017–4023
4023