Pendant arm Schiff base complexes of aluminium as ethylene polymerisation catalysts

Article abstract of DOI:10.1039/a905120a

The potentially tridentate Schiff base ligands [3,5-Bu(t)₂-2-(HO)C₆H₂CH=NL] 1, on reaction with Me₃Al at room temperature, afford the complexes [(3,5-Bu(t)₂-2-(O)C₆H₂CH=NL)AlMe₂] [L = CH₂CH₂NMe₂ 2a, (2-PhO)C₆H₄ 2b, 2-CH₂C₅H₄N 2c and 8-C₉H₆N (quinoline) 2d], 2a and 2c have been characterised crystallographically; further reaction of the dimethyl compounds with B(C₆F₅)₃ affords the cationic systems [(3,5-Bu(t)₂-2-(O)C₆H₂CH=NL)-AlMe]⁺ 3a-d of which 3a and 3b are ethylene polymerisation catalysts.

Full text of DOI:10.1039/a905120a

Pendant arm Schiff base complexes of aluminium as ethylene polymerisation
catalysts
Paul A. Cameron, Vernon C. Gibson,* Carl Redshaw, John A. Segal, Michael D. Bruce, 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 (in Cambridge, UK) 25th June 1999, Accepted 9th August 1999
The potentially tridentate Schiff base ligands [3,5-But2-
dures. Treatment of these potentially tridentate ligands with
Me3Al in toluene at room temperature readily affords the
corresponding dimethyl complexes [3,5-But-2-(O)C6H2CHNN-
L]AlMe2 2a–d with concomitant elimination of methane
(Scheme 2).†
Crystals of 2a suitable for an X-ray structure determination‡
were grown from MeCN at room temperature. Deprotonated 1a
is seen to act as a tridentate ligand, binding to aluminium via the
oxygen and the imino and amino nitrogen atoms [Fig. 1(a)]. The
geometry at aluminium can best be described as distorted
trigonal bipyramidal though with the bond to the axial amino
nitrogen N(1) [2.413(5) Å] being substantially longer than that
to the equatorial imino centre N(4) [1.998(4) Å]. With the
exception of Al–N(1), the Al–X bonds are all clearly single in
nature. These bonds, however, are noticeably longer than the
2-(HO)C6H2CHNNL] 1, on reaction with Me3Al at room
temperature,
afford
the
complexes
[(3,5-But2-
2-(O)C6H2CHNNL)AlMe2] [L
=
CH2CH2NMe2 2a,
(2-PhO)C6H4 2b, 2-CH2C5H4N 2c and 8-C9H6N (quinoline)
2d], 2a and 2c have been characterised crystallographically;
further reaction of the dimethyl compounds with B(C6F5)3
affords the cationic systems [(3,5-But2-2-(O)C6H2CHNNL)-
AlMe]+ 3a–d of which 3a and 3b are ethylene polymerisation
catalysts.
Recent reports by Coles and Jordan1 and ourselves2 have
revealed the potential for cationic aluminium alkyls as ethylene
polymerisation catalysts. The aluminium procatalysts reported
thus far are bi- and tri-dentate chelates (L) of the general form
[{L}AlMe2], which are synthesised by the reaction of the
relevant ligand system with Me3Al. To obtain an active catalyst
system, cationic species are generated on further reaction with
B(C6F5)3 in hydrocarbon or chlorocarbon solvents. The cationic
aluminium alkyls so derived display low activities as ethylene
polymerisation catalysts and they produce polymers of high1
(Mw 176000–272000) or moderate2 (Mw 13000–23000) molec-
ular weight. In studies directed at extending the family of
cationic aluminium catalysts to N,O-Schiff base chelate ligands,
we first targeted bidentate chelates e.g. 3,5-But2-
2-(HO)C6H2CHNNR (R = alkyl or aryl), which led to the
isolation of a range of systems of the type [(3,5-But2-
2-(O)C6H2CHNNR)AlMe2].3 However attempts to obtain clean
alkylaluminium cations by way of reaction with B(C6F5)3 (in
toluene or dichloromethane) were unsuccessful, always leading
to a mixture of species. 1H NMR spectra of the product mixtures
show inter alia the presence of high field methyl resonances that
are characteristic of (ligand)AlMe(C6F5) species4 formed by
arylation of the Al centre by the boron reagent. In view of this
undesired reactivity, the investigation was extended to Schiff
base chelates bearing a pendant donor arm which it was
envisaged might, through coordination, lend stability to the
cationic methyl product. Here we show not only that such
ligands afford stable cationic alkyl species but also that these
cationic alkyls are active catalysts for ethylene polymerisation.
Homogeneous olefin polymerisation catalysts hitherto de-
scribed which are derived from N,O-Schiff bases are limited to
complexes of transition metals, namely Ti,5,6, Zr,6,7 Cr8 and
Ni.9
corresponding linkages in the simple chelate analogues, e.g.
3
{3,5-But2-2-(O)C6H2CHNN(2,6-Me2C6H3)}AlMe2
[Al–O
1.773(3), Al–N 1.972(3), Al–Me 1.948(5), 1.959(5) Å] reflect-
ing the presence of the additional donor in 2a with subsequent
competition for electron density. It is interesting that the
aluminium atom is displaced significantly out of the equatorial
plane (0.16 Å) in the direction of the ligand oxygen. The six-
membered N,O chelate ring adopts a sofa conformation, the
aluminium lying ca. 0.5 Å out of the plane of the other five
atoms (which are co-planar to within 0.03 Å). The ring CNN
bond has clearly retained its double bond character [N(4)–C(5)
1.294(6) Å], there being no evidence of significant delocalisa-
tion into any of the adjacent linkages. The structure of complex
2c [Fig. 1(b)] is very similar to that of 2a, in particular with
respect to the geometry around Al, the only significant
difference of note being a substantial reduction in the bond
distance between the aluminium and the pyridyl nitrogen [Al–
N(1) 2.254(2) Å cf. the dimethylamino nitrogen in 2a [Al–N(1)
2.413(5) Å]. The remaining bond lengths and angles at
aluminium do not differ significantly from those in 2a.
The cationic complexes [(3,5-But2-2-(O)C6H2CHNNL)-
AlMe]+ 3a–d are readily generated on treatment with 1 equiv. of
[B(C6F5)3] in CD2Cl2 or toluene at ambient temperature
(Scheme 2) as illustrated by the 1H NMR spectrum of 3a (Fig.
2); the systems exist as free cations as opposed to the ion pairs
(i.e. with Al...Me–B association) observed by Coles and
Jordan.1 The pendant arm is likely to stabilise the cationic
aluminium centre; in its absence, as noted above, the analogous
reaction with [B(C6F5)3] gave complicated NMR spectra
corresponding to multiple products.
The Schiff base ligands 1a–d (Scheme 1) are accessible in
good yields ( > 80%) via standard imine condensation proce-
Complexes 2a–d were combined with 1 equiv. of B(C6F5)3 to
test for ethylene polymerisation activity. Polymerisations were
run with 0.25 mmol catalyst in 200 mL toluene solution under
5 bar ethylene pressure at 50 °C for 60 min. Procatalysts 2a and
Scheme 1 Ligands 1a–d.
Scheme 2 Reagents: i. AlMe3, toluene; ii, B(C6F5)3, CD2Cl2 or toluene.
Chem. Commun., 1999, 1883–1884
1883

In summary, we have shown that Schiff base ligands are
capable of stabilising cationic methylaluminium centres only
when they possess an additional donor arm. We reasoned,
however, that it might be possible to form cations from the
analogous Schiff bases without the donor arm if a free donor
ligand, e.g. THF, were added to stabilise the system. This is
indeed the case, and our subsequent study of the formation of
these closely related cationic species will be published else-
where.
We thank the EPSRC for a studentship (to P. A. C.), BP-
Amoco Chemicals and ICI Acrylics for financial support, and
the Royal Society and the Leverhulme Trust for Fellowships (to
J. A. S. and C. R.). Mr G. Audley is gratefully acknowledged for
GPC measurements.
Notes and references
† Satisfactory microanalyses were obtained for compounds 2a–d, cations
3a–d were characterised spectroscopically. Selected spectroscopic data (J/
Hz): for 2a: 1H NMR (250 MHz, C6D6, 298 K): d 7.66 [d, 1H, 4J(HH) 2.4,
C6H2], 7.39 (s, 1H, CHNN), 6.80 [d, 1H, 4J(HH) 2.6, C6H2], 2.86 [t, 2H,
3J(HH) 6.8, CH2CH2], 2.08 [t, 2H, 3J(HH) 6.2, CH2CH2], 1.85 [s, 6H,
N(CH3)2], 1.63, [s, 9H, C(CH3)3], 1.30 [s, 9H, C(CH3)3], 20.26 (s, 6H,
AlCH3). For 2c: 1H NMR (C6D6, 298 K): d 8.31–8.28 (m, 1H, C5H4N), 7.72
4
4
[d, 1H, J(HH) 2.6, C6H2], 7.38 [t, 1H, J(HH) 1.3, CHNN), 6.83 [d, 1H,
4J(HH) 2.5, C6H2], 6.80 [dt, 1H, 3J(HH) 7.7, 4J(HH) 1.7, C5H4N],
6.50–6.39 (m, 1H, C5H4N), 6.22–6.14 (m, 1H, C5H4N), 3.81 (br s, 2H,
CH2), 1.76 [s, 9H, C(CH3)3], 1.38 [s, 9H, C(CH3)3], 20.15 (s, 6H, AlCH3).
For 3a: 1H NMR (CD2Cl2, 298 K): d 8.55 (s, 1H, CHNN), 7.76 [d,
1H, 4J(HH) 2.6, C6H2], 7.27 [d, 1H, 4J(HH) 2.6, C6H2], 4.0 (br, CH2CH2),
3.0 (br, CH2CH2), 2.77 [br s, 6H, N(CH3)2], 1.41 [s, 9H, C(CH3)3], 1.31 [s,
9H, C(CH3)3], 0.47 (s, 3H, BCH3), 20.31 (s, 3H, AlCH3). For 3c: 1H NMR
(CD2Cl2, 298 K): d 8.46 (s, 1H, CHNN), 8.05 [dt, 1H, 3J(HH) 7.9, 4J(HH)
1.5, C5H4N], 8.00 [d, 1H, 4J(HH) 2.5, C6H2], 7.82 [d, 1H, 3J(HH) 5.3,
C5H4N], 7.51 [d, 1H, 3J(HH) 8.0, C5H4N], 7.20 (m, 1H, C5H4N), 7.20 [dt,
1H, 3J(HH) 6.3, 4J(HH) 1.1, C5H4N], 7.07 [d, 1H, 4J(HH) 2.5, C6H2], 5.15
[AB q, 2H, 3J(HH) 1.5, CH2CH2], 4.98 [AB q, 2H, 3J(HH) 1.5, CH2CH2],
1.75 [s, 9H, C(CH3)3], 1.26 [s, 9H, C(CH3)3], 0.48 (s, 3H, BCH3), 20.47 (s,
3H, AlCH3).
Fig. 1(a) The molecular structure of 2a, showing the trigonal bipyramidal
geometry at aluminium and the very long Al–N(amino) linkage. Selected
bond lengths (Å) and angles (°): Al–N(1) 2.413(5), Al–N(4) 1.998(4), Al–
O(12) 1.854(4), Al–C(13) 1.978(6), Al–C(14) 1.976(5), N(4)–C(5)
1.294(6), O(12)–Al–C(14) 96.6(2), O(12)–Al–C(13) 98.3(2), C(14)–Al–
C(13) 123.7(3), O(12)–Al–N(4) 88.2(2), C(14)–Al–N(4) 116.0(2), C(13)–
Al–N(4) 118.4(2), O(12)–Al–N(1) 163.0(2), C(14)–Al–N(1) 91.4(2),
C(13)–Al–N(1) 89.5(2), N(4)–Al–N(1) 74.9(2). (b) Complex 2c.
‡ Crystal data for 2a: C21H37N2OAl, M = 360.5, monoclinic, space group
P21/c (no. 14), a = 10.332(2), b = 24.982(4), c = 9.729(2) Å, b =
115.98(1)°, V = 2257.4(7) Å3, Z = 4, Dc = 1.061 g cm23, m(Mo-Ka) =
1.00 cm–1, F(000) = 792, T = 293 K, 2934 independent reflections. For 2c:
C
23H33N2OAl, M = 380.5, monoclinic, space group P21/c (no. 14), a =
15.009(2), b = 12.207(2), c = 14.129(1) Å, b = 116.36(1)°, V = 2319.5(4)
Å3, Z = 4, Dc = 1.090 g cm23, m(Mo-Ka) = 1.01 cm–1, F(000) = 824, T
= 293 K, 5299 independent reflections. Data were collected on Siemens P4/
PC diffractometers using w-scans, and the non-hydrogen atoms were
refined anisotropically using full matrix least squares based on F2 to give R1
= 0.067 (0.054), wR2 = 0.156 (0.136) for 1646 (3526) independent
observed reflections [∫Fo∫ > 4s(|Fo|), 2q ≤ 45° (45°)] and 226 (245)
parameters for 2a (2c) respectively.
Fig. 2 1H NMR spectrum (250 MHz) of 3a.
CCDC 182/1366. See http://www.rsc.org/suppdata/cc/1999/1883/ for
crystallographic files in .cif format.
2b produced solid polyethylene but procatalysts 2c and 2d did
not display any polymerisation activity. Procatalyst 2a gave a
productivity of 50 g(PE) mol21 h21 bar21 and yielded
polyethylene with Mw = 172000, Mn = 2400, whilst procata-
1 M. P. Coles and R. F. Jordan, J. Am. Chem. Soc., 1997, 119, 8125.
2 M. Bruce, V. C. Gibson, C. Redshaw, G. A. Solan, A. J. P. White and
D. J. Williams, Chem. Commun., 1998, 2523.
3 P. A. Cameron, V. C. Gibson, C. Redshaw, G. A. Solan, A. J. P. White
and D. J. Williams, manuscript in preparation.
lyst 2b gave 110 g(PE) mol21 h21 bar21 with polymer Mw
218000, Mn = 5200.
=
In contrast to our previous tridentate aluminium alkyls,2 the
present system affords higher molecular weights, as exempli-
fied by the much higher Mw values, although the activities are
similar. The results suggest that lability of the pendant donor
arm, as in the cations derived from 2a, 2b, is an important
feature of the polymerisation catalysis mechanism, providing a
pathway for ethylene to approach the aluminium centre. The
observation that 2c and 2d, which have N-heterocycles as the
donor arms, did not provide active systems accords with this
view as these compounds are likely to form cations with
stronger donor to metal bonds (as evidenced by the significantly
shorter Al–N(1) bond length in 2c cf. 2a) thereby reducing the
propensity for dissociation of the coordinating arm to generate
an active centre.
4 B. Qian, D. L. Ward and M. L. Smith III, Organometallics, 1998, 17,
3070.
5 S. Doherty, R. J. Errington, A. P. Jarvis, S. Collins, W. Clegg and M. R. J.
Elsegood, Organometallics, 1998, 17, 3408; J. P. Corden, W. Errington,
P. Moore and M. G. H. Wallbridge, Chem. Commun., 1999, 323.
6 T. Fujita, Y. Tohi, M. Mitani, S. Matsui, J. Saito, M. Nitabaru, K. Sugi,
H. Makio and T. Tsutsui (Mitsui Chemicals), Eur. Pat., 0874005A1,
1998.
7 E. B. Tjaden and R. F. Jordan, Macromol. Symp., 1995, 89, 231.
8 V. C. Gibson, C. Newton, C. Redshaw, G. A. Solon, A. J. P. White and
D. J. Williams, J. Chem. Soc., Dalton Trans., 1999, 827.
9 C. Wang, S. Friedrich, T. R. Younkin, R. T. Li, R. H. Grubbs, D. A.
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Communication 9/05120A
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Chem. Commun., 1999, 1883–1884