Chromium(III) complexes bearing N,N-chelate ligands as ethene polymerization catalysts

Article abstract of DOI:10.1039/a802797h

Novel chromium(III) ethene polymerization catalysts bearing bulky monoanionic N,N-chelate ligands are described.

Full text of DOI:10.1039/a802797h

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Chromium(iii) complexes bearing N,N-chelate ligands as ethene
polymerization catalysts
Vernon C. Gibson,*a† Peter J. Maddox,b Claire Newton,a Carl Redshaw,a Gregory A. Solan,a Andrew J. P.
Whitea and David J. Williamsa
a Department of Chemistry, Imperial College, Exhibition Road, London, UK SW7 2AY
b BP Chemicals, Sunbury Research Centre, Chertsey Road, Sunbury on Thames, Middlesex, UK TW16 7LN
Novel chromium(III) ethene polymerization catalysts bearing
bulky monoanionic N,N-chelate ligands are described.
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 [2.026(5) Å] markedly
shorter than that to the imine [2.073(4) Å] reflecting the formal
anionic nature of N(1). The double bond character of the imine
has been retained [1.318(7) Å] though there is some evidence for
delocalisation between the imine and pyrrolide systems,
C(5)–C(6) being short at 1.410(8) Å. As in 2, the 2,6-diisopro-
pylphenyl rings are steeply inclined (ca. 65°) to the basal plane.
It is noteworthy that both of these structures provide rare
examples of five-coordinate chromium(iii) alkyls, only two
other examples having been found on the CCDC database [to
March 1998, 175093 entries].11
A summary of the ethene polymerization tests for 1–4 is
shown in Table 1. Solid polyethylene is obtained in all cases with
samples displaying high molecular weights¶ and virtually no
branching (by NMR). A comparison of the polymerisation runs
1–12 shows that similar activities are found for procatalysts
bearing either b-diketimate (1 and 2) or pyrrolide-imine ligands
(3and4), the highest activity being75g mmol21 h21 bar21 using
1 and diethylaluminium chloride (Et2AlCl) activator (see run
2).
Chromium supported on silica plays a central role in the world-
wide production of polyethylene.1 As heterogeneous catalysts
they have not proved amenable to intimate study, and even to this
day there remains an on-going debate about the oxidation state
of the active chromium centres.2 The development of homoge-
neous molecular chromium catalysts is therefore an important
objective, since these offer the potential for understanding the
modus operandi of supported chromium catalysts and may
provide new opportunities for tuning activity and selectivity. A
number of reports of molecular chromium catalysts have
appeared in the recent literature,3–6 the majority on half-
sandwich chromium species as models for the active sites of
chromocene-derived systems.3,4
Here we describe a series of coordinatively unsaturated
chromium(iii) ethene polymerization procatalysts bearing either
b-diketimate or pyrrolide-imine ligands. A general feature of
these ligands is the presence of bulky aryl substituents which
offer protection to the active centre, a strategy that has proved
successful for the stabilisation of new N,N-chelate catalysts
based on early7 and late transition metal systems.8 A similar
strategy has also recently been applied to a new catalyst system
based on iron and cobalt.9
The nature of the activator is seen to have an important
influence on activity; for example, alkylaluminium chloride
activators are found to be more compatible with these
The chromium chloride complexes, 1 and 3, were prepared in
high yield by the treatment of [CrCl3(thf)3] with the lithium salts
of the b-diketimate ligand or the pyrrolide-imine ligand,
respectively (Scheme 1).‡ The structures of 1 and 3 were
confirmed by X-ray structure determinations.10 Interestingly,
only one b-diketimate ligand can be coordinated to chromium in
1, while in 3 the less sterically demanding pyrrolide-imine ligand
allows bis-chelation. It is also of note that the vacant sixth
coordination site in 3 is unexpectedly occupied by a molecule of
lithium chloride (as a thf solvate).
R
R
R
Me
Me
Me
Me
R
Me
Me
R
Cl
N
N
N
Cl
Cl
N
i, ii
R
R
Cr
H
Cr
Cl
R
N
R
N
R
R
R
1
iii
Complex 2, the dimethyl derivative of 1, has been prepared in
good yield by treatment of 1 with trimethylaluminium (TMA).
Crystals of 2 suitable for an X-ray structure determination§ were
grown from a concentrated pentane solution. The structure is
dimeric, the two crystallographically independent molecules
both having C2h symmetry and comprising in each case two
slightly distorted square pyramidal CrIII centres linked by
chloride bridges, the remaining basal sites being occupied by
bidentate b-diketimate ligands (Fig. 1). The apical position on
each chromium centre is filled by a terminal methyl group with
Cr–C distances of 2.037(7) and 2.042(8) Å for the two
independent molecules. The 2,6-diisopropylphenyl rings are
oriented almost orthogonally (ca. 87°) with respect to the basal
plane.
In a similar way, complex 4 was obtained by treatment of 3
with TMA (Scheme 1). Crystals of 4 suitable for X-ray analysis
were grown from a concentrated light petroleum (bp 40–60 °C)
solution. The structure again reveals a five-coordinate square
pyramidal CrIII centre, this time coordinated basally to two
chelating pyrrolide-imine ligands and apically to a terminal
methyl group at 2.037(9) Å (Fig. 2). The complex has
R
R
Me
Me
R
Me
Me
R
Me
N
Cl
N
N
N
Cr
H
Cr
N
R
Cl Me
R
N
R
R
R
R
2
i, ii
R
R
R
N
R
N
iii
Me
Cr
N
N
N
N
Cl
R
Cr
N
(thf)2Li
Cl
N
R
R
R
R = Pri
3
4
Scheme 1 Preparation of chromium complexes 1–4. Reagents and
conditions: i, BunLi, 278 °C, thf; ii, [CrCl3(thf)3], 278 °C, thf; iii, AlMe3,
thf.
Chem. Commun., 1998
1651

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ethene polymerization catalysts5,6 and highlight the importance
of the choice of co-catalyst for optimal catalyst performance.
BP Chemicals Ltd is thanked for financial support. Dr W.
Reed and Dr J. Boyle are thanked for GPC and NMR
measurements, respectively.
Me
Cr′
Cl
Notes and References
Cr
Cl′
† E-mail: V.Gibson@ic.ac.uk
Me′
N
‡ Satisfactory elemental analyses have been obtained.
N′
¶ As a representative example, GPC analysis of the polyethylene obtained
from run 8 afforded Mw = 293,000, Mn = 113,000; Mw/Mn 2.2. Care should
be taken in the interpretation of these values, however, since in general the
polymers derived from these polymerisations are not fully soluble in the
1,2,4-trichlorobenzene GPC solvent, even upon heating at 160 °C for several
hours.
Fig. 1 The molecular structure of 2. Selected bond lengths (Å) and angles (°)
(values for the second independent molecule are in square brackets); Cr–Me
2.037(7) [2.042(8)], Cr–N 2.028(4) [2.031(4)], Cr–Cl 2.395(1) [2.393(1)];
Me–Cr–N 95.4(2) [96.8(2)], Me–Cr–Cl 97.8(2) [96.8(2)], N–Cr–NA 91.3(2)
[90.8(2)], Cl–Cr–ClA 80.3(1) [80.6(1)], N–Cr–Cl 92.7(1) [92.7(1)], N–Cr–
ClA 165.7(1) [165.4(1)], Cr–Cl–CrA 99.7(1) [99.4(1)]. The transannular
Cr···Cr distance is 3.66 Å [3.65 Å] and the chromium atom lies 0.25 Å [0.26
Å] out of its basal plane.
§ Crystal data: for 2: C60H88Cl2Cr2N4·0.5C5H12, M = 1076.3, monoclinic,
space group C2/m (no. 12), a = 19.434(3), b = 21.764(2), c = 15.098(2)
Å, b = 90.58(1)°, U = 6386(1) Å3, Z = 4 (there are two crystallographically
independent C2h symmetric molecules in the asymmetric unit), Dc = 1.120
g cm23, m(Cu-Ka) = 38.5 cm21, F(000) = 2316. A crimson prismatic
needle of dimensions 0.33 3 0.17 3 0.13 mm was used. For 4: C35H45CrN4,
M = 573.8, orthorhombic, space group Fdd2 (no. 43), a = 20.280(2),
b = 34.103(4), c = 9.420(1) Å, U = 6515(2) Å3, Z = 8 (the molecule has
crystallographic C2 symmetry), Dc = 1.170 g cm23, m(Mo-Ka) = 3.79
cm21, F(000) = 2456. A green block of dimensions 0.37 3 0.37 3 0.10 mm
was used. 4866 (1530) independent reflections were measured at 203 K on
Siemens P4(PC) diffractometers with Cu-Ka—rotating anode source—
(Mo-Ka) radiation using w-scans for 2 (4), respectively. The structures were
solved by direct methods and all of the major occupancy non-hydrogen
atoms were refined anisotropically using full matrix least squares based on
F2 to give R1 = 0.065 (0.049), wR2 = 0.164 (0.094) for 3370 (1193)
independent observed reflections [ıFoı > 4s(ıFoı), 2q @ 120(50°)] and
340 (182) parameters for 2 (4), respectively. The polarity of 4 was
Me
N(1′)
N(7)
Cr
N(7′)
N(1)
C(5)
C(6)
+
2
determined by a combination of R-factor tests [R1 = 0.048, R1 = 0.050]
and by use of the Flack parameter [x+ = 0.10(1), x2 = 0.90(1)]. CCDC
182/903.
Fig. 2 The molecular structure of 4. Selected bond lengths (Å) and angles
(°); Cr–Me 2.037(9), Cr–N(1) 2.026(5), Cr–N(7) 2.073(4), C(5)–C(6)
1.410(8), C(6)–N(7) 1.318(7); Me–Cr–N(1) 93.4(2), Me–Cr–N(7) 100.4(1),
N(1)–Cr–N(7) 80.9(2), N(1)–Cr–N(7A) 97.9(2), N(1)–Cr–N(1A) 173.2(4),
N(7)–Cr–N(7A) 159.2(3).
1 F. J. Karol, G. L. Garapinka, C. Wu, A. W. Dow, R. N. Johnson and
W. I. Carrick, J. Polym. Sci., Part A, 1972, 10, 2621; J. P. Hogan,
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Table 1 Results of ethene polymerisation runs using procatalysts 1–4a
3 For recent reviews, see: K. H. Theopold, Chem. Eur. J., 1998, 3, 15;
K. H. Theopold, CHEMTECH, 1997, 27, 26.
4 R. Emrich, O. Heinemann, P. W. Jolly, C. Kru¨ger and G. P. J.
Verhovnik, Organometallics, 1997, 16, 1511.
Activityd/
Proctalyst
Activatorb
Run (0.015 mmol) (mmol/equiv.)
Yield
g mmol21
PEc/g h21 bar21
5 F. J. Feher and R. L. Blanski, J. Chem. Soc., Chem. Commun., 1990,
1614.
6 M. P. Coles, C. I. Dalby, V. C. Gibson, W. Clegg and M. R. J. Elsegood,
J. Chem. Soc., Chem. Commun., 1995, 1709.
7 J. D. Scollard and D. H. McConville, J. Am. Chem. Soc., 1996, 118,
10008; V. C. Gibson, B. S. Kimberley, A. J. P. White, D. J. Williams and
P. Howard, Chem. Commun., 1998, 313.
8 L. K. Johnson, C. M. Killian and M. Brookhart, J. Am. Chem. Soc., 1995,
117, 6414; L. K. Johnson, S. Meeking and M. Brookhart, J. Am. Chem.
Soc., 1996, 118, 267; C. M. Killian, D. J. Tempel, L. K. Johnson and
M. Brookhart, J. Am. Chem. Soc., 1996, 118, 11664; L. K. Johnson,
C. M. Killian, S. D. Arthur, J. Feldman, E. F. McCord, S. J. McLain,
K. A. Kreutzer, M. A. Bennett, E. B. Coughlin, S. D. Ittel,
A. Parthasarathy, D. J. Tempel and M. S. Brookhart, Pat. Appl., WO
96/23010, 1996, DuPont.
9 G. J. P. Britovsek, V. C. Gibson, B. S. Kimberley, P. J. Maddox,
S. J. McTavish, G. A. Solan, A. J. P. White and D. J. Williams, Chem.
Commun., 1998, 849; B. L. Small, M. Brookhart and M. A. Bennett,
J. Am. Chem. Soc., 1998, 120, 4049.
1
2
3
4
5
6
7
8
9
1
1
1
2
2
2
3
3
3
4
4
4
MAO (6.0/400)
0.12
2.25
0.69
0.15
0.80
0.48
0.08
1.04
4
75
23
10
54
32
5
69
27
3
Et2AlCl (0.45/30)
Et2AlCl·EtAlCl2 (0.3/20)
MAO (6.0/400)
Et2AlCl (0.45/30)
Et2AlCl·EtAlCl2 (0.3/20)
MAO (6.0/400)
Et2AlCl (0.45/30)
Et2AlCl·EtAlCl2 (0.45/30) 0.40
MAO (6.0/400)
Et2AlCl (0.45/30)
10
11
12
0.04
1.05
70
15
Et2AlCl·EtAlCl2 (0.45/30) 0.23
a
General conditions: 1 bar ethene Schlenk test carried out in toluene (40
cm3) at 25 °C, over 60 min, reaction quenched with dil. HCl and the solid
PE washed with methanol (50 cm3) and dried in a vacuum oven at 40 °C.
b MAO = methylaluminoxane. c Solid polyethylene. d Activity reported per
chromium centre.
10 X-Ray data for 1 and 3 will be published elsehwere.
11 M. D. Fryzuk, D. B. Leznoff and S. J. Rettig, Organometallics, 1997, 16,
5116; Y. Liang, G. P. A. Rheingold and K. H. Theopold, Organome-
tallics, 1996, 15, 5284.
procatalysts than methylaluminoxane (MAO). Moreover, di-
ethylaluminium chloride (Et2AlCl) is superior to aluminium
sesquichloride (Et2AlCl·EtAlCl2). Notably, the alkyl procata-
lysts 2 and 4 are inactive in the absence of activator.
The new catalyst types described herein represent a notable
addition to the limited list of non-cyclopentadienyl chromium
Received in Exeter, UK, 14th April 1998; 8/02797H
1652
Chem. Commun., 1998