Highly efficient ethylene polymerisation by scandium alkyls supported by neutral fac-κ3 coordinated N3 donor ligands

Article abstract of DOI:10.1039/b310867h

Reaction of [M(CH₂SiMe₃)₃(THF) ₂] (M = Sc or Y) with the neutral fac-K³ N₃ donor ligands (L) Me₃[9]aneN₃ or HC(Me₂pz) ₃ gave the corresponding tr

Full text of DOI:10.1039/b310867h

Highly efficient ethylene polymerisation by scandium alkyls supported by
neutral fac-k3 coordinated N3 donor ligands†
Sally C. Lawrence, Benjamin D. Ward, Stuart R. Dubberley, Christopher M. Kozak and Philip
Mountford
Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, UK OX1 3QR.
E-mail: philip.mountford@chem.ox.ac.uk
Received (in Cambridge, Uk) 8th September 2003, Accepted 8th October 2003
First published as an Advance Article on the web 28th October 2003
Reaction of [M(CH2SiMe3)3(THF)2] (M = Sc or Y) with the
Me2[9]aneN3)X2] (MAO for IV, [CPh3][B(C6F5)4], B(C6F5)3
or [PhNMe2H][B(C6F5)4] for V) yielded intractable and
inactive (towards PE production) mixtures. Mindful of re-
ports8a,b of Y and cationic lanthanide dialkyl complexes
stabilised by aza-[18]-crown-6 (deprotonated) and [n]-crown-m
(n, m = 12, 4; 15, 5; 18, 6) we decided to target “simple”
complexes of the type [M(L)(CH2SiMe3)3] (M = Sc, Y; L =
neutral fac-k3 N3 donor ligands (L) Me3[9]aneN3 or
HC(Me2pz)3
gave
the
corresponding
trialkyls
[M(L)(CH2SiMe3)3]; activation of the scandium congeners
with B(C6F5)3 in the presence of ethylene afforded highly
active polymerisation catalysts (Me3[9]aneN3 = 1,4,7-tri-
methyltriazacyclononane).
3
fac-k N3 donor ligand) as shown in Scheme 1 (we reasoned that
Much of the recent interest in “post-metallocene” (in general,
non-cyclopentadienyl) chemistry of the Group 3 elements1 is by
its very nature linked with the academic and industrial quest for
novel types of new olefin polymerisation catalysts.2 Neutral
alkyls of these elements typically have poor olefin polymer-
isation capability2,3 and so attention has naturally turned to
cationic compounds. Of the few very active Group 3, non-
cyclopentadienyl olefin polymerisation precatalysts (giving rise
to cationic active species), the only scandium system of merit is
tridentate ligands – as opposed to the 4- to 6-coordinating ones
used in previous studies – ought to allow for a more reactive
metal centre in the target cationic complexes, especially for
scandium).8c
The new compounds† [M(Me3[9]aneN3)(CH2SiMe3)3] (M =
Sc 1 or Y 2) and [M{HC(Me2pz)3}(CH2SiMe3)3] (M = Sc 3 or
Y 4) are easily prepared in reasonable yields from the
corresponding readily available [M(CH2SiMe3)3(THF)2]
(Scheme 1). The compounds 1–4 are all obtained as spec-
troscopically pure compounds but give consistently low %C by
combustion analysis. The compounds 1 and 2 are analogues of
Bercaw’s [M(Me3[9]aneN3)Me3] (M = Sc , Y). To gain further
support for 3 and 4 the former was treated with 3 equivs of
Piers’
[Sc{ArNC(tBu)CHC(tBu)NAr}Me2]
(Ar
=
i
2,6-C6H3 Pr2; the activity for polyethylene production has been
classified as “moderate”2b for borane- or trityl-activated
dialkyls)4a and the only significantly active systems for yttrium
are
[Y{(tBuNCH2CH2)R2[9]aneN3}(CH2SiMe3)2]
and
ArAOH (ArA
=
2,6-C6H3Me2) forming [Sc{HC(Me2p-
[Y{PhC(NAr)2}(CH2SiMe3)2(THF)n] (n = 1 or 2).4b,c All three
polymerisation systems feature chelating monoanionic ligands.
Here we describe preliminary studies of the synthesis and
z)3}(OArA)3] 5 in 81% isolated yield. The molecular structure of
3
5 has been determined and confirms the fac-k coordination of
HC(Me2pz)3.†
The
corresponding
trichlorides
3
ethylene polymerisation capability of neutral fac-k coordinated
[M{HC(Me2pz)3}Cl3] (M = Sc 6 or Y 7) are also readily
accessible in ca 70% isolated yield and the X-ray crystal
structure of 7 has been determined9 confirming those proposed
in Scheme 1. The compounds 3–7 are the first Group 3
derivatives of any tris(pyrazolyl)methane ligand.
Me3[9]aneN3 and HC(Me2pz)3 tris(trimethylsilymethyl) com-
plexes of scandium and comparative studies for yttrium.
We have recently been interested in the potential applications
of triazacyclononane5a–c I (Chart 1) and tris(pyrazolyl)me-
thane5d ligands II in early transition metal organometallic
Reaction of [Sc{HC(Me2pz)3}(CH2SiMe3)3]
3
with
3
chemistry. These latter k -coordinating N3 donor ligands6a are
[CPh3][B(C6F5)4] in CD2Cl2 either in the presence of THF (2
equivs) or followed by addition of THF 30 minutes later,
neutral analogues of the ubiquitous tris(pyrazolyl)hydroborates
III,6b which are immensely “tuneable” to effect desirable
properties at the metal centre. In previous studies5b,c we
reported the syntheses of a series of linked macrocyle-
phenoxide compounds, including [Sc(ArOMe2[9]aneN3)X2] (X
= Cl IV or CH2SiMe3 V) which we hoped would be effective
polymerisation catalysts by analogy with the well-established
titanium constrained geometry catalysts2b and Hessen and
Teuben’s related yttrium system (vide supra).4b,c We were also
aware that Bercaw and Hajela7 had reported that activation of
the trimethyl scandium complex [Sc(Me3[9]aneN3)Me3] with
B(C6F5)3 or [PhNMe2H][B(C6F5)4] gave only very sluggish
olefin polymerisation. However, activation of [Sc(ArO-
quantitatively
afforded
the
cationic
species
[Sc{HC(Me2pz)3}(CH2SiMe3)2(THF)]+ 8+ and a non-coordi-
nated [B(C6F5)4]2 anion. Scale up in CH2Cl2 afforded 8 as a
Chart 1
† Electronic supplementary information (ESI) available: experimental
details, characterising data, crystallographic data for 5 and GPC data. See
http://www.rsc.org/suppdata/cc/b3/b310867h/
Scheme 1
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CHEM. COMMUN., 2003, 2880–2881
This journal is © The Royal Society of Chemistry 2003

reasonably stable compound. A scandium tris(dimethylpyr-
azolyl)hydroborate analogue of 8+ has been described by
Piers10a but was completely unreactive towards ethylene
polymerisation; an yttrium analogue has, however, been
reported by Bianconi to be a moderately active polymerisation
catalyst.10b
Preliminary ethylene polymerisation studies of the well-
defined alkyls [M(Me3[9]aneN3)(CH2SiMe3)3] (M = Sc 1 or Y
2) and [M{HC(Me2pz)3}(CH2SiMe3)3] (M = Sc 3 or Y 4) have
been carried out (Table 1). Activating 20 mmol of scandium
compounds 1 and 3 with one equivalent of B(C6F5)3 in toluene
in the presence of ethylene (5 bar) at either ca 21 or 33 °C gave
a gradual exotherm to over 70 °C during the course of 15–25
minutes. After 60 minutes total run time and standard work-up,
ca. 22–24 g (for 1) and 29 g (for 3) of free-flowing polyethylene
was obtained; these quantities equate to “high”2b activities in
the range ca. 200–300 kg(PE) mol21 h21 bar21, and clearly
establish that (unoptimised) 1 and 3 are already competitive
with the best post-metallocene scandium based polymerisation
catalysts.
[Sc(Me3[9]aneN3)Me3], but one reason could be the greater
ease with which methyl groups bridge metal centres compared
to the bulkier CH2SiMe3 (thereby leading to catalyst deactiva-
tion for the cationic complexes); another contribution could be
the favourable stabilisation of electron-deficient cationic metal
centres by b-Si–C agostic interactions12 from the CH2SiMe3
ligands in either the initiating or/and propagating catalytic
species derived from 1 and 3. The failure of the apparently more
promising [Sc(ArOMe2[9]aneN3)(CH2SiMe3)2] systems to act
as ethylene polymerisation catalysts might be accounted for by
dimerisation side-reactions of the desired cations via m-OBr
bridges, or by metallation of one of the aryl ring ortho-tBu
groups as postulated for a C5Me5-supported scandium aryloxide
system.13
In conclusion, we believe that there is considerable scope for
development of the title compounds and the unsaturated
substrates which they can activate. Studies of this type on the
effects of varying metal (transition and f element), solvent, co-
catalyst, reaction time and temperature are currently in
progress.
The yttrium triazacyclononane compound 2, in contrast,
afforded an activity of only ca, 10 kg(PE) mol21 h21 bar21 at
33 °C, and the tris(pyrazolyl)methane homologue 4 afforded no
solid ethylene products under the conditions studied to date.
This is in contrast to Hessen and Teuben’s results for yttrium
with their anionic, chelating (tBuNCH2CH2)R2[9]aneN3 ligand
system (ca. 700 kg(PE) mol21 h21 bar21 at 30 °C) which might
point to an importance of having at least one anionic ligand
donor group for the larger metals.4b
GPC analysis† of the polymers produced revealed a critical
dependence on metal, supporting ligand and activation tem-
perature. For precatalyst 1 activated at 21 °C the polyethylene is
approximately bimodal with a very dominant higher molecular
weight fraction having Mp = 575 3 103 and a minor lower
fraction having Mp = 1070 (estimated Mw and Mw/Mn for these
being 1310 and 1.2, and 936,000 and 15.1 for the minor and
major components, respectively). Polymer obtained on activat-
ing 1 at 33 °C had a more dominant lower molecular weight
fraction and a much broader higher molecular weight fraction,
both accompanied by a shift to lower overall Mw. In contrast,
polymer from activation of 2 at 33 °C was dominated by a high
molecular weight fraction with little low molecular weight
material; compound 3 on the other hand (activated at 33 °C)
afforded two main fractions with the lower molecular weight
one being the dominant. These preliminary experiments
indicate that significant control over polymer structure might be
gained by simple variation of N3 donor ligand, ligand
substituents and polymerisation conditions, especially for the
highly active scandium systems. The multiple factors that can
influence polymer polydispersities and modalities have been
well discussed in the recent literature.11
We thank the EPSRC, Leverhulme Trust, Royal Society and
NSERC (Canada) for support, and S Jones and Dr S. Holding
(RAPRA Technology Ltd.) for recording the GPC data. SCL is
the recipient of a European Scatcherd Scholarship.
Notes and references
1 For recent reviews and leading references see: W. E. Piers and D. J. H.
Emslie, Coord. Chem. Rev., 2002, 233, 131; P. Mountford and B. D.
Ward, Chem. Commun., 2003, 1797.
2 For recent reviews and leading references see: (a) Z. Hou and Y.
Wakatsuki, Coord. Chem. Rev., 2002, 231, 1; (b) V. C. Gibson and S. K.
Spitzmesser, Chem. Rev., 2003, 103, 283.
3 Compare reference 4c with F. G. N. Cloke, B. R. Elvidge, P. B.
Hitchcock and V. M. E. Lamarche, J. Chem. Soc., Dalton Trans., 2002,
2413.
4 (a) P. G. Hayes, W. E. Piers and R. McDonald, J. Am. Chem. Soc., 2002,
124, 2132; (b) S. Bambirra, D. van Leusen, A. Meetsma, B. Hessen and
J. H. Teuben, Chem. Commun., 2001, 637; (c) S. Bambirra, D. van
Leusen, A. Meetsma, B. Hessen and J. H. Teuben, Chem. Commun.,
2003, 522.
5 (a) N. A. H. Male, M. E. G. Skinner, S. Y. Bylikin, P. J. Wilson, P.
Mountford and M. Schröder, Inorg. Chem., 2000, 39, 5483; (b) S. Y.
Bylikin, D. A. Robson, N. A. H. Male, L. H. Rees, P. Mountford and M.
Schröder, J. Chem. Soc., Dalton Trans., 2001, 170; (c) M. E. G. Skinner,
B. R. Tyrrell, B. D. Ward and P. Mountford, J. Organomet. Chem.,
2002, 647, 145; (d) S. A. Lawrence, M. E. G. Skinner, J. C. Green and
P. Mountford, Chem. Commun., 2001, 705.
6 (a) D. L. Reger, Comments Inorg. Chem., 1999, 21, 1; D. L. Reger, T.
C. Grattan, K. J. Brown, C. A. Little, J. J. S. Lamba, A. L. Rheingold and
R. D. Sommer, J. Organomet. Chem., 2000, 607, 120; (b) S.
Trofimenko, Scorpionates: the coordination chemistry of polypyr-
azolylborate ligands, Imperial College Press, London, 1998.
7 S. Hajela, W. P. Schaefer and J. E. Bercaw, J. Organomet. Chem., 1997,
532, 45.
It is not clear at this time why our tris(CH2SiMe3)3 scandium
complexes 1 and 3 are so much more active than Bercaw’s
8 (a) L. Lee, D. J. Berg, F. W. Einstein and R. J. Batchelor,
Organometallics, 1997, 16, 1819; (b) S. Arndt, T. P. Spaniol and J.
Okuda, Chem. Commun., 2002, 896; (c) Both [Sc([12]-crown-
4)(CH2SiMe3)2]+ and [Sc(CH2SiMe3)3(THF)2] show negligible activity
towards ethylene polymerisation: S. Arndt, T. P. Spaniol and J. Okuda,
Angew. Chem., Int. Ed. Engl., 2003, 42, 5075.
9 S. E. Brown, S. C. Lawrence, S. R. Dubberley, J. W. Steed, D. A.
Tocher, P. Mountford and A. Sella, manuscript in preparation.
10 (a) J. Blackwell, C. Lehr, Y. Sun, W. E. Piers, S. D. Pearce-Batchilder,
M. J. Zaworotko and V. G. Young Jr., Can. J. Chem., 1997, 75, 702; (b)
D. P. Long and P. A. Bianconi, J. Am. Chem. Soc., 1996, 118, 12453.
11 Y. Tohi, H. Makio, S. Matsui, M. Onda and T. Fujita, Macromolecules,
2003, 36, 523; B. L. Small, M. Brookhart and A. M. A. Bennett, J. Am.
Chem. Soc., 1998, 120, 4049; S. Murtuza, O. L. Casagrande and R. F.
Jordan, Organometallics, 2002, 21, 1882; D. Reardon, F. Conan, S.
Gambarotta, G. Yap and Q. Wang, J. Am. Chem. Soc., 1999, 120,
9318.
Table 1 Ethylene polymerisation with compounds [Sc(ArOMe2[9]a-
neN3)(CH2SiMe3)2] V, [M(Me3[9]aneN3)(CH2SiMe3)3] (M = Sc 1 or Y 2)
and [Sc{HC(Me2pz)3}(CH2SiMe3)3] (M = Sc 3 or Y 4)a
Pre-
(1)
(2)
catalyst Yield (g) Activityb Mw
Mp
Mp
V
1
1
2
3
4
0c
0
220
240
10
290
0
—
—
—
22.3c
23.6d
0.97d
29.4d
0d
8.47 3 105
3.53 3 105
1.18 3 106
1.92 3 105
—
1070
560
–
2750
—
575 3 103
70.8 3 103
851 3 103
537 3 103
—
a Conditions: steel autoclave equipped with removable glass liner, magnet-
ically coupled mechanical stirrer (stirring rate 750 rpm); 20 mmol scandium
precatalyst, 20 mmol B(C6F5)3 co-catalyst; 250 cm3 total volume toluene; 5
bar ethylene pressure delivered on request; run time 60 min; 250 equivs
.
AliBu3. b In kg(PE) mol(Sc)21 h21 bar21 c Activation temperature 21 °C;
d Activation temperature 33 °C.
12 For a review see: G. I. Nikonov, J. Organomet. Chem., 2001, 635,
24.
13 W. E. Piers, E. E. Bunel and J. E. Bercaw, J. Organomet. Chem., 1991,
407, 51.
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