The surprisingly beneficial effect of soft donors on the performance of early transition metal olefin polymerisation catalysts

Article abstract of DOI:10.1039/b409870f

Group 4 metal complexes containing phenoxy-amide ligands bearing soft pendant donors are shown to give more highly active ethylene polymerisation catalysts than counterparts containing hard donors or systems without a pendant donor.

Full text of DOI:10.1039/b409870f

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The surprisingly beneficial effect of soft donors on the performance of
early transition metal olefin polymerisation catalysts{
Daniel C. H. Oakes, Brian S. Kimberley, Vernon C. Gibson,* David J. Jones, Andrew J. P. White and
David J. Williams
Department of Chemistry, Imperial College London, Exhibition Road, London, UK SW7 2AZ.
E-mail: v.gibson@imperial.ac.uk; Fax: 144 (0)20 7594 5810; Tel: 144 (0)20 7594 5830
Received (in Cambridge, UK) 29th June 2004, Accepted 13th July 2004
First published as an Advance Article on the web 23rd August 2004
Group 4 metal complexes containing phenoxy-amide ligands
C(37) respectively, values typical of g2 interactions in related
systems8–10 and distinctly different to those for g1-bound benzyl
bearing soft pendant donors are shown to give more highly
active ethylene polymerisation catalysts than counterparts
containing hard donors or systems without a pendant donor.
units.8,11 The short Zr–N(7) bond length of 2.025(3) A clearly
˚
reflects the formal anionic nature of this donor atom which adopts
a markedly distorted trigonal planar geometry; though N(7) lies
˚
only 0.09 A out of the plane of its substituents, the angles at this
centre range between 106.6(2) and 137.5(2)u, a consequence of the
formation of the six-membered chelate ring. This ring has a twisted
˚
conformation with {Zr, O(1), C(1), N(7)} coplanar to within 0.01 A,
˚
C(6) and C(7) lying 0.49 and 0.99 A out of this plane (on the same
In recent years, there have been tremendous advances in the design
of non-metallocene catalysts for olefin polymerisation.1,2 While
ligands containing relatively hard nitrogen and oxygen donors have
featured in catalyst systems across the transition series, reports of
highly active catalysts incorporating soft tertiary phosphine donors
have, for understandable reasons, been largely restricted to the mid-
to-late transition metals. Notable examples include nickel-based
catalysts supported by bidentate [P,O]2 and [P,P]3 ligands. The
hard–soft mis-match of the early transition metal–phosphine
combination, although occasionally broached,4 has not to date
held any general promise of affording highly active catalyst
systems. Here, we report the first examples of Group 4 metal
catalysts whose activities are dramatically enhanced upon inclusion
of soft neutral donor groups, tertiary phosphines in particular.
Monoanionic phenoxy-imines are amongst the most versatile
ligands to have emerged in olefin polymerisation catalysis over the
past 5 years or so.5–7 A key attraction of these ligands is their ready
availability and amenability to modification via straightforward
Schiff-base condensation procedures. We became attracted to
building on this platform to prepare a family of dianionic chelate
ligands derivable by simple reduction of iminophenol precursors.
The starting point for our studies were bidentate ligand systems.
As an example, the 2,6-dimethylphenyl aminophenol was
synthesised in good yield by LiAlH4 reduction of the imino-
phenol precursor (Scheme 1). Its treatment with Ti(CH2Ph)4 or
Zr(CH2Ph)4 in toluene gave the mononuclear dibenzyl complexes 1
and 2, respectively.
side) respectively. Preliminary ethylene polymerization tests
revealed that 1 and 2 were not particularly active catalysts,
affording activities of 22 and 32 gPE mmol21 h21 (gPE ~ grams of
polyethylene), respectively, in 1 atmosphere ethylene pressure tests.
We therefore decided to extend the study to tridentate deriva-
tives containing hard and soft pendant donors, viz. where R ~
o-C6H4(OPh), quinolyl, o-C6H4(SPh), and o-C6H4(PPh2). Treat-
ment of these aminophenols with Ti(CH2Ph)4 gave the mono-
nuclear products 3–6 in good yields. We were unable to obtain
crystals of 3–6 suitable for X-ray analysis, but a closely related dime-
thylamide derivative 3b, synthesised by treatment of Ti(NMe2)4
with the aminophenol, has been structurally characterised.{ The
geometry at titanium is distorted trigonal bipyramidal (Fig. 2) with
˚
the metal centre lying ca. 0.24 A ‘‘below’’ the equatorial N3 plane
˚
in the direction of the phenoxide oxygen O(1) [Ti–O(1) 1.885(2) A];
the phenylether oxygen O(14) is comparatively loosely bound
˚
[Ti–O(14) 2.244(2) A] and the axial O(1)–Ti–O(14) angle is
161.27(8)u.
The Ti–NMe2 bond lengths of 1.892(3) and 1.886(3) A to N(29)
˚
Crystals of 2 were grown from a saturated pentane solution at
230 uC.{ Considering the benzyl units as monodentate, the
geometry at zirconium is distorted tetrahedral (Fig. 1) with angles
in the range 88.76(10)–123.33(13)u, the most acute angle being
associated with the bite of the chelating [N,O] ligand. The Zr–C–Ar
angles at each benzyl CH2 moiety are distinctly contracted from
ideal, being 87.0(2)u at C(24) and 91.0(2)u at C(31), with associated
˚
Zr–C(Ar) separations of 2.643(3) and 2.730(4) A to C(30) and
˚
Fig. 1 The molecular structure of 2. Selected bond lengths (A) and angles
(u): Zr–O(1) 1.961(2), Zr–N(7) 2.025(3), Zr–C(24) 2.277(4), Zr–C(30)
2.643(3), Zr–C(31) 2.282(4), Zr–C(37) 2.730(4), C(24)–C(30) 1.465(5),
C(31)–C(37) 1.460(5), O(1)–Zr–N(7) 88.76(10), O(1)–Zr–C(24) 108.75(12),
O(1)–Zr–C(31) 102.53(12), N(7)–Zr–C(24) 123.33(13), N(7)–Zr–C(31)
108.42(13), C(24)–Zr–C(31) 118.64(13), C(30)–C(24)–Zr 87.0(2),
Scheme 1
C(37)–C(31)–Zr 91.0(2). The intramolecular p–p interaction
centroid–centroid and mean interplane separations of ca. 3.67 and 3.37 A
respectively; the rings are inclined by ca. 3u.
a has
˚
{ Electronic supplementary information (ESI) available: crystal structures
of 2 and 3b. See http://www.rsc.org/suppdata/cc/b4/b409870f/
2 1 7 4
C h e m . C o m m u n . , 2 0 0 4 , 2 1 7 4 – 2 1 7 5
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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more active catalyst system under the same conditions, giving
an activity of 3 530 gPE mmol21 h21. This was surpassed by
the diphenylphosphine complex 6 which gave an activity of
19 500 gPE mmol21 h21. A comparison of the effect of temperature
on catalyst performance for 4 versus 6 revealed that the activity for
the hard donor ligand falls off rapidly above room temperature. By
contrast, the activity for catalyst 6/MAO increases with tempera-
ture, to an optimum ca. 70 uC. A possible explanation for this is
binding of the hard aluminium centres within MAO to the hard O
and N centres of the ether and quinolyl pendant groups leading to
their dissociation from the metal centre. This would be anticipated
to be less favourable for the soft thioether and phosphine donors,
and would be further lowered for the phosphine derivative as a
result of the additional steric protection afforded by its two phenyl
substituents. Examination of the 31P NMR spectrum of a sample of
6 treated with excess MAO revealed a new tertiary phosphine
resonance at 19.64 ppm, consistent with a bound phosphine in the
active species. Although there is no evidence for a hemilabile effect
from the available data, further studies of the active species will be
required for a full assessment of this possibility.
BP Chemicals Ltd. is thanked for financial support of this work.
˚
Fig. 2 The molecular structure of 3b. Selected bond lengths (A) and angles
(u): Ti–O(1) 1.885(2), Ti–N(7) 1.986(2), Ti–O(14) 2.244(2), Ti–N(29)
1.892(2), Ti–N(32) 1.886(3), O(1)–Ti–N(7) 88.86(8), O(1)–Ti–O(14)
161.27(8), O(1)–Ti–N(29) 99.52(9), O(1)–Ti–N(32) 103.36(10), N(7)–Ti–
O(14) 73.52(8), N(7)–Ti–N(29) 123.29(10), N(7)–Ti–N(32) 117.41(11),
N(29)–Ti–O(14) 85.43(9), N(32)–Ti–O(14) 90.66(10), N(29)–Ti–N(32)
114.77(12).
Notes and references
{ Crystal data for 2: C37H45NOZr, M ~ 610.96, monoclinic, P21/n (no. 14),
˚
a ~ 13.2413(15), b ~ 14.9424(15), c ~ 16.766(2) A, b ~ 101.462(8)u V ~
3
3251.2(6) A , Z ~ 4, Dc ~ 1.248 g cm23, m(Cu–Ka) ~ 2.969 mm21, T ~
˚
193 K, pale yellow blocks; 4818 independent measured reflections, F2
refinement, R1 ~ 0.039, wR2 ~ 0.090, 3938 independent observed
absorption-corrected reflections [|Fo| w 4s(|Fo|), 2hmax ~ 120u], 362
parameters. CCDC 236411. Crystal data for 3b: C31H43N3O2Ti?0.5C5H12,
and N(32) respectively are noticeably shorter than that to the amide
˚
nitrogen N(7) [1.986(2) A], though in each case the nitrogen centre
adopts an essentially trigonal planar geometry with N(7), N(29)
˚
and N(32) lying ca. 0.11, 0.08 and 0.01 A out of the planes of their
respective substituents. The 5-membered [N,O] chelate ring adopts
˚
an envelope conformation with the metal lying ca. 0.40 A out of
˚
the C2NO plane (which is coplanar to better than 0.01 A). The
6-membered [N,O] chelate ring has a boat conformation with O(1)
˚
and C(7) lying ca. 0.56 and 0.37 A respectively out of the {Ti, C(1),
M ~ 573.66, monoclinic, C2/c (no. 15), a ~ 28.1195(8), b ~ 12.6087(7),
3
˚
˚
c ~ 18.9179(13) A, b ~ 94.379(4)u V ~ 6687.8(6) A , Z ~ 8, Dc ~
1.139 g cm23, m(Cu–Ka) ~ 2.401 mm21, T ~ 183 K, yellow/orange
prisms; 4970 independent measured reflections, F2 refinement, R1 ~ 0.047,
wR2 ~ 0.108, 3921 independent observed absorption-corrected reflections
[|Fo| w 4s(|Fo|), 2hmax ~ 120u], 368 parameters. CCDC 236412. See http://
www.rsc.org/suppdata/cc/b4/b409870f/ for crystallographic data in .cif or
other electronic format.
˚
C(6), N(7)} plane (which is coplanar to within 0.09 A).
The 31P NMR spectrum of 6 gave a singlet signal at 16.70 ppm
(cf. 219.94 ppm for the pendant phosphine in the free amino-
phenol), consistent with a coordinated tertiary phosphine unit. This
signal remained unchanged in VT NMR spectra recorded to 90 uC.
It is therefore reasonable to conclude that 6 is thermally robust with
a closely related structure to 3b, i.e. with a tridentate [O,N,P]
ligand. The results of ethylene polymerization tests on compounds
3–6 (Table 1), activated by methylaluminoxane (MAO), revealed
marked activity enhancements for catalysts containing the soft
donor substituents. The phenyl ether adduct 3 and the quinolyl
adduct 4 afforded activities v100 gPE mmol21 h21, values
comparable to those obtained for the bidentate derivative 1. By
contrast, the phenylthioether derivative 5 afforded a much
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Table 1 Polymerisation results for complexes 3–6a
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Pre-catalyst
(amount/
mmol)
Activity/g
mmol21
Yield
PE/g
Mw/
Mn
h21 bar21
Mn
83 700
17 400
163 250
466 700
Mw
306 500
617 500
594 700
1 803 000
3 (10.0)
4 (10.0)
5 (2.0)
6 (0.2)
0.44
0.48
3.53
1.95
88
96
3530
19 500
3.7
35.5
3.6
3.8
a Conditions: 400 ml Fisher–Porter glass reactor, mechanical stirring,
25 uC, 2000 equiv. MAO, heptane solvent (200 ml), 1 bar ethylene
pressure, 30 min.
10 S. L. Latesky, A. K. McMullen, G. P. Niccolai, I. P. Rothwell and
J. C. Huffman, Organometallics, 1985, 4, 902.
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C h e m . C o m m u n . , 2 0 0 4 , 2 1 7 4 – 2 1 7 5
2 1 7 5