Cationic scandium methyl complexes supported by a β-diketiminato ( Nacnac ) ligand framework

Article abstract of DOI:10.1021/ja017128g

The base-free dimethyl scandium complex supported by the bulky β-diketiminato ligand ArNC(^tBu)CHC(^tBu)NAr (Ar = 2,6-ⁱPr₂C₆H₃, 1) reacts with various equivalencies of the strong organometallic Lewis acid B(C₆F₅)₃ to give scandium alkyl cations. With 0.5 equiv, a monocationic μ-methyl dimer (2) was observed spectroscopically. Reaction with a further 0.5 equiv of borane gives the monomeric methyl cation 3, which was fully characterized, including via X-ray crystallography. This compound is fluxional on the NMR time scale via a ligand flip mechanism. Reaction with another equivalent of borane gives the unique dication 4, which exhibits a static structure on the NMR time scale. Dimethyl compound 1 is a highly active catalyst precursor for ethylene polymerization under borane or MAO-type activation. Activities for this group 3 metal based catalyst approach those observed for group 4 based metallocene systems. Copyright

Full text of DOI:10.1021/ja017128g

Published on Web 02/16/2002
Cationic Scandium Methyl Complexes Supported by a â-Diketiminato
(“Nacnac”) Ligand Framework
Paul G. Hayes,† Warren E. Piers,*,† and Robert McDonald‡
Department of Chemistry, UniVersity of Calgary, 2500 UniVersity DriVe N.W., Calgary,
Alberta, Canada T2N 1N4, and X-ray Structure Laboratory, Department of Chemistry, UniVersity of Alberta,
Edmonton, Alberta, Canada T6G 2G2
Received September 21, 2001
Alkyl cations of group 4 metals play a crucial role in olefin
polymerization catalysis,1 and the cationic nature of these species
is critical to their productivity in this process. This is underscored
by the fact that their neutral group 3 metal congeners are generally
much less active catalysts.2 While this has been recognized for some
time, suitable ligand modifications to allow for generation of well-
defined group 3 metal cations have been slow to develop.
Nonetheless, there are a few recently reported systems for which
cationic alkyl scandium3 or yttrium3c,4 complexes have been
implicated spectroscopically and by higher polymerization activities.
We have recently reported a family of base-free â-diketiminato,
“nacnac”, supported dialkyl scandium complexes5 and shown that
one of the dibenzyl derivatives may be activated with B(C6F5)3 to
form an ion pair.2a The X-ray structure of this material shows that
the [PhCH2B(C6F5)3]- counteranion is strongly η6 coordinated to
the cationic scandium center, a feature that is manifested in this
ion pair’s complete lack of reactivity toward olefins. Using a
t
sterically more bulky nacnac donor with Bu groups in the ligand
backbone allows for the isolation of the THF-free dimethyl
Figure 1. Molecular structure of ion pair 3; aryl groups have been removed
for clarity. Selected bond distances (Å): Sc-N(1), 2.060(5); Sc-N(2),
compound 1; here we discuss this material’s reactions with varying
2.093(5); Sc-C(1), 2.221(5); Sc-C(2), 2.703(6); Sc-F(42), 2.390(4);
6
amounts of the activator B(C6F5)3 and report the first structural
characterization of a scandium methyl cation, along with a
preliminary assessment of its solution dynamic behavior. In addition,
we show that 1 has ethylene polymerization activities comparable
to those of metallocenes under borane activation.
B-C(2), 1.643(9). Selected bond angles (deg): N(1)-Sc-N(2), 95.06(19);
Sc-C(2)-B, 138.72(18); Sc-F(42)-C(42), 153.7(4).
Scheme 1
Dimethyl scandium compound 1 reacts with varying equivalen-
cies of B(C6F5)3 to form different ion pairs as shown in Scheme 1.
Upon reaction with 0.5 equiv of borane, a µ-methyl dimer (2)
is formed,7 as indicated by a characteristic set of resonances for
the terminal Sc-Me groups (-0.29 ppm, 6H) and the bridging
1
Sc-Me-Sc group (0.10 ppm, JCH ) 132(1) Hz, 3H). Dimer 2
could not be isolated as a well-behaved solid and slowly evolves
CH4 in bromobenzene solution,7 yielding an as yet uncharacterized
product. When 1 is treated with a full equivalent of B(C6F5)3,
however, a monomeric ion pair [LtBuScCH3]+[H3CB(C6F5)3]-, 3,
is produced in excellent isolated yield when precipitated from
hexane.8
As a yellow crystalline solid, 3 is stable for long periods of
time at -30 °C. Its solid-state structure is shown in Figure 1 along
with selected metrical data. The scandium atom sits 1.232 Å out
of the plane defined by the C3N2 ligand atoms (mean deviation
from plane ) 0.053 Å) mainly for steric reasons and not because
of any bonding interactions with the ligand backbone CH moiety
(Sc-C(5) ) 2.773(6) Å).5 The Sc-CH3 group occupies the endo
coordination site (pointing in toward the ligand) while the methyl-
borate counteranion is situated in the sterically more open exo site.
The methyl group is more closely associated with the boron atom
(B-C(2) ) 1.643(9) Å) than the scandium atom (Sc-C(2) )
2.703(6) Å), but is not linearly bridged between the two as is
† University of Calgary.
‡ University of Alberta.
9
2132 VOL. 124, NO. 10, 2002 J. AM. CHEM. SOC.
10.1021/ja017128g CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
Table 1. Ethylene Polymerization with 1 and Various Cocatalysts
E
cat.
CoCat
act.a
M (×10-3
)
M /M
w
w
n
1b,c
2b,c
3b,d
4b,d
LScCl2
PMAO-IP
PMAO-IP
B(C6F5)3
TBe
9.9 × 104
1.2 × 106
3.0 × 105
4.8 × 105
1357
2.2
1.98
1.7
1
1
1
1866
1051
851
2.48
a Activity in g PE/mol Sc‚h. b Polymerization conditions: 50 °C, 300
psi, cyclohexane/toluene, [Sc] ) 300 µM, stir rate ) 2000 rpm. c Al/M )
20. d [Cocatalyst] ) 315 µM, [PMAO-IP] ) 1 mM as scavenger.
e [Ph3C][B(C6F5)4].
commonly observed in metallocene structures7 (Sc-C(2)-B )
138.72(18)°). The bending at this angle allows for a weak stabilizing
interaction between an ortho fluorine group and the scandium center
(Sc-F(42) ) 2.390(4) Å); such a bonding mode for the
[H3CB(C6F5)3]- anion has been observed previously in an yttrocene
derivative.9 In 3, we could find no evidence for such a motif in
solution by 19F NMR spectroscopy.
for metallocene and other group 4 based catalysts, demonstrating
that scandium cations can be highly effective catalysts. Notably,
these activities are observed in the presence of the potentially
coordinating solvent toluene,3a,15 which was necessary to solubilize
the catalyst.
In summary, we have prepared a family of highly reactive
organoscandium methyl cations supported by a bulky nacnac ligand
and examined their solution and solid-state structures. These
represent a new class of cationic organometallic compounds whose
rich chemistry we are further exploring.
Ion pair 3 exhibits complex dynamic behavior in solution. The
1H NMR spectrum at room temperature is broad and essentially
featureless, although signals for the bridging and terminal methyl
groups can be identified at 1.40 and 0.28 ppm, respectively. These
signals do not coalesce under any conditions, suggesting that borane
dissociation10 is a high barrier process in this system. When the
sample is cooled to 213 K, the spectrum sharpens into a pattern
consistent with the presence of two diastereomers (ratio 2.2:1.0)
that differ in the endo/exo disposition of the terminal methyl group
and the anion (Scheme 2). ROESY experiments show that the major
diastereomer is the endo-Me, exo-anion isomer found in the solid
state. Analysis of the NMR spectra at various temperatures reveals
that exchange of the isomers occurs with a barrier of 12.4(5) kcal
mol-1 at 263 K. Likely, these diastereomers interconvert via a
ligand flipping mechanism similar to that proposed for the neutral
dialkyl complexes,5 although an associative ion pair reorganization
process11 via ion pair quadruples12 may also be contributing to this
exchange; further experiments are underway to unravel the details
of this dynamic behavior more completely. Although stable as a
solid for long periods, in solution 3 undergoes methane loss via
metalation with a ligand isopropyl group5 over the course of an
hour at room temperature.
Acknowledgment. Financial support for this work was provided
by Nova Chemicals Ltd. (Calgary, Alberta) and NSERC of Canada
in the form of a CRD Grant. NSERC is also acknowledged for an
E. W. R. Steacie Memorial Fellowship to W.E.P. (2001-2003) and
a PGS A fellowship to P.G.H. Dr. Q. Wang of Nova performed
the polymerization experiments.
Supporting Information Available: Experimental details, tables
of crystal data, atomic coordinates, bond lengths and angles, and
anisotropic displacement parameters for 3 (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
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Although borane dissociation appears not to occur in these
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analytically pure white solid. This is indicated by the absence of
1
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appearance of signals for two diastereotopic [MeB(C6F5)3]- anions
1
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system,14 but unlike this system, compound 4 is moderately active
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temperature, 1 atm).
Compound 1 is an effective catalyst for ethylene polymerization
under B(C6F5)3, trityl borate, or PMAO-IP activation (Table 1) in
a slurry batch reactor at 50 °C. Molecular weights are relatively
high, and the polydispersities are consistent with a single site
catalysis model. Activities are somewhat lower when the dichloride
precursor is employed under MAO type activation, indicating that
alkylation of the scandium center by organoaluminum reagents is
slow. The activities in general, however, approach those observed
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