One ligand fits all: Cationic mono(amidinate) alkyl catalysts over the full size range of the group 3 and lanthanide metals

Article abstract of DOI:10.1021/ja0475297

Using a sterically demanding amidinate ancillary ligand and an in situ alkylation procedure, neutral mono(amidinate) dialkyl and cationic mono(amidinate) monoalkyl complexes were prepared for metals spanning the full size range of the group 3 and lanthanide metals. The activity of the cationic monoalkyls in catalytic ethene polymerization was found to vary by over 2 orders of magnitude depending on the metal ionic radius, the intermediate metal sizes being found to be the most effective. Copyright

Full text of DOI:10.1021/ja0475297

Published on Web 07/07/2004
One Ligand Fits All: Cationic Mono(amidinate) Alkyl Catalysts over the Full
Size Range of the Group 3 and Lanthanide Metals
Sergio Bambirra, Marco W. Bouwkamp, Auke Meetsma, and Bart Hessen*
Center for Catalytic Olefin Polymerization, Stratingh Institute for Chemistry and Chemical Engineering,
UniVersity of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
Received April 28, 2004; E-mail: hessen@chem.rug.nl
Cationic d0 group 4 metal alkyl complexes have been the subject
of intensive investigations over the past decade and a half, mainly
because of their high efficiency in catalytic olefin polymerization.1
In contrast, cationic alkyl species of trivalent group 3 and lanthanide
metals have become available only very recently.2-8 For transition-
metal catalysts, the influence of the electronic configuration of the
metal on the availability of accessible metal-centered valence
orbitals causes a specific ancillary ligand type to yield active
catalysts only for a limited range of metals. Due to the noninvolve-
ment of the 4f electrons in ligand or substrate bonding, this is not
a limiting factor for the group 3 and lanthanide metals. The relative
reactivity of these compounds is expected to be determined mainly
by the metal ionic radius. These range from 0.89 Å for the smallest
metal in the series (Sc3+) to 1.17 Å for the largest metal (La3+).9
Thus far, a comparison of catalytic properties of such cationic alkyl
complexes over the whole range of metal ion sizes has been
precluded by the fact that the trialkyl complexes M(CH2SiMe3)3-
(THF)x (x ) 2, 3), used as precursors for the various derivatives,
are only available for a limited set of metals (Sc-Dy),6d,10 thus
excluding the largest metals of the series. Nevertheless, early results
by Okuda et al. on M(alkyl)n(THF)m cations showed that the metal
ionic radius strongly affects the reactivity of these species.6d
Recently, we described the synthesis of the mono(amidinate)
yttrium dialkyl complexes (NCN)Y(CH2SiMe3)2(THF)q (NCN )
PhC(NAr)2; Ar ) 2,6-diisopropylphenyl; q ) 1, 2), which could
be converted to a cationic monoalkyl species that catalyzes the
polymerization of ethene.11 Here we report that, using an in situ
alkylation procedure, these compounds can now be obtained over
the full size range of the group 3 and lanthanide metals. From
single-crystal X-ray structure determinations, it is seen that the
cationic monoalkyl derivatives [(NCN)M(CH2SiMe3)(THF)r]+ bind
an increasing number of THF molecules (r ) 2-4) with increasing
metal size. Their activities in catalytic ethene polymerization were
found to be highly dependent on the metal ionic radius, with best
activities being obtained for intermediately sized metals.
Following the procedure described previously for Y,11 the
reaction of the known trialkyl complexes M(CH2SiMe3)3(THF)2 (M
) Sc, Lu)10a with the amidine PhC(NAr)(NHAr) yielded the
corresponding amidinate dialkyl complexes (NCN)M(CH2SiMe3)2-
(THF) (M ) Sc, Lu; Scheme 1). These were characterized by
elemental analysis, NMR spectroscopy, and a crystal structure
determination for M ) Sc. For the larger lanthanides Gd, Nd, and
La, suspensions of MX3(THF)p (M ) La: X ) Br, p ) 4; M )
Gd, Nd: X ) Cl, p ) 3) in THF were stirred with 3 equiv of
Me3SiCH2Li for several hours, followed by addition of 1 equiv of
the amidine (Scheme 1). Subsequent extraction with and crystal-
lization from pentane yielded the salt-free amidinate dialkyl
complexes (NCN)M(CH2SiMe3)2(THF)2 (M ) Gd, Nd, La) as
analytically pure crystalline solids in moderate yields (from 33%
for La to 61% for Gd). A crystal structure determination for M )
Nd showed that these are essentially isostructural to the yttrium
dialkyl bis-THF adduct reported previously,11 with the alkyl groups
occupying axial positions relative to the (amidinate)M plane. This
in situ alkylation procedure can also be applied to the smaller metals,
e.g., for Sc, which afforded the dialkyl complex in 70% yield, as
compared to 76% when starting from the trialkyl precursor.
These neutral dialkyl complexes could be converted to their
cationic monoalkyl derivatives [(NCN)M(CH2SiMe3)(THF)r] [BPh4]
(M ) Sc, r ) 2; M ) Lu, Y, Gd, r ) 3; M ) Nd, La, r ) 4) by
reaction with [PhNMe2H][BPh4] in THF solvent, followed by
crystallization from THF/alkane mixtures (Scheme 1). The com-
pounds were characterized by elemental analysis, NMR spectros-
copy (for the diamagnetic derivatives), and crystal structure
determinations (M ) Sc, Y, Gd, Nd, La). The structures of the Sc,
Gd, and La derivatives are shown in Figure 1, illustrating the
increasing coordination number of the metal by the binding of
additional molecules of THF with increasing metal ionic radius. In
all complexes, the remaining alkyl group occupies an axial position
relative to the (amidinate)M plane, which can accommodate up to
three molecules of THF (in the case of Nd and La). The increased
steric crowding with increasing the number of bound THF
molecules is also reflected in the alkyl M-CH2-Si angle, which
increases from 135.26(10)° for Sc to 155.98(17)° for Nd. In the
13C NMR spectra of the cations, the alkyl CH2 resonance is found
downfield from that in the neutral dialkyls and shows a smaller
1
1JCH (for La, δ 70.7 ppm and JCH 90 Hz, versus 52.9 ppm and
102 Hz in the dialkyl).
Thus, the dialkyl complexes (NCN)M(CH2SiMe3)2(THF)q and
their monoalkyl cations are now accessible for the full range of
group 3 and lanthanide metals. This allows for the first time a
comparison of the performance of cationic group 3 metal and
lanthanide alkyls in catalytic ethene polymerization over this full
metal size range. Ethene polymerization experiments were per-
formed at 30 °C and 5 bar ethene pressure, in toluene solvent, by
reacting the dialkyls (NCN)M(CH2SiMe3)2(THF)q (q ) 1 for Sc
and Lu, q ) 2 for the other metals) with the Brønsted acid activator
[PhNMe2H][B(C6F5)4]12 in the presence of 20 equiv of isobutyl
alumoxane (TIBAO) scavenger. The results (Table 1) show that
the catalyst activity is strongly dependent on the metal ionic radius.
Whereas the smallest (Sc) and largest (La) metals in the series show
very low activity, activities in the order of 3000 kg mol-1 h-1 bar-1
are observed for the metals of intermediate size (Y, Gd). The
polyethene produced by these catalysts has a high molecular weight
(Mw ≈ 1.5‚106) and a Mw/Mn ≈ 2, characteristic for single-site
catalyst behavior. For the Y system, ethene polymerization in the
absence of alkylaluminum scavengers was found to have living
character, as seen by the very narrow polydispersities (1.1-1.2).11
The present data indicate that alkyl transfer to Al is the main chain
transfer mechanism in operation here. In this respect it is of interest
that the smallest metal to show reasonable activity (Lu) produces
9
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J. AM. CHEM. SOC. 2004, 126, 9182-9183
10.1021/ja0475297 CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1
Figure 1. Representative molecular structures of [(NCN)M(CH2SiMe3)(THF)r] cations (Sc, r ) 2, left; Gd, r ) 3, middle; La, r ) 4, right).
Table 1. Ethene Polymerization with [(NCN)M(CH2SiMe3)2(THF)q]a
plexes of Sc and Nd and ionic monoalkyl complexes of Sc, Y, Gd,
Nd, and La, as well as positional and thermal parameters and bond
distances and angles (PDF, CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
M
R (Å)
yield (g)
productivityb
M (x 10-3
)
M /M
w
w
n
Sc
Lu
Y
Gd
Nd
La
0.89
1.00
1.04
1.07
1.12
1.17
0.20
2.82
24.80
23.50
16.40
0.12
24
342
3006
2848
1988
14
93
496
1666
1753
1596
470
1.6
1.4
2.0
2.1
2.2
2.5
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Acknowledgment. We thank A. Jekel for polymer GPC analysis
and ExxonMobil Chemical Company and NRSC-Catalysis for
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Supporting Information Available: Text giving full experimental
and characterization data; crystallographic data for the dialkyl com-
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