Bis(imino)pyridine iron(II) alkyl cations for olefin polymerization

Article abstract of DOI:10.1021/ja0524447

Treatment of the five-coordinate ferrous dialkyl complex, (ⁱPrPDI)Fe(CH₂SiMe₃)₂ (ⁱPrPDI = ((2,6-CHMe₂)₂C₆H₃N=CMe)₂C₅H₃N), with [PhMe₂NH][BPh₄] in the presence of diethyl ether or tetrahydrofuran furnished the corresponding alkyl cations, where the donor ligand is coordinated in the basal plane of a distorted square pyramidal iron(II) alkyl cation. Performing the same reaction with the neutral Lewis acid, B(C₆F₅)₃, induced methide abstraction from a silicon atom followed by rearrangement to afford the base free ferrous alkyl cation, [(ⁱPrPDI)Fe(CH₂SiMe₂CH₂SiMe₃)][MeB(C₆F₅)₃]. This complex is active for the polymerization of ethylene and yields polymers that are of higher molecular weight and narrower polydispersity than traditional methylalumoxane-activated catalysts. Copyright

Full text of DOI:10.1021/ja0524447

Published on Web 06/16/2005
Bis(imino)pyridine Iron(II) Alkyl Cations for Olefin Polymerization
Marco W. Bouwkamp, Emil Lobkovsky, and Paul J. Chirik*
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell UniVersity, Ithaca, New York 14853
Received April 14, 2005; E-mail: pc92@cornell.edu
When activated with an excess of methylalumoxane (MAO), bis-
(imino)pyridine iron and cobalt dichloride complexes are highly
active for the polymerization and oligomerization of ethylene and
R-olefins.1 Despite intense interest from both academic1 and
industrial2 laboratories, the mechanism of the activation process
and the nature of the propagating species are not well understood.
Recent studies with the cobalt catalysts are consistent with an MAO-
induced reduction to Co(I) rather than formation of a discrete Co-
(II) alkyl cation.3 Isolated bis(imino)pyridine cobalt(I) methyl and
chloride complexes, when treated with MAO or a borate activator,
are active for ethylene polymerization, suggesting that preformed
cobalt alkyls are not a prerequisite for catalytic activity.3-5
For iron, the identity and oxidation state of the propagating
species are more controversial. Initial experimental6 and compu-
tational7 results supported cationic iron(II) alkyls and hydrides upon
activation with MAO. However, a combined Mo¨ssbauer and EPR
spectroscopic study suggested that all of the iron is oxidized to the
ferric form under catalytic conditions.8 Here we describe the
synthesis and crystallographic characterization of bis(imino)pyridine
iron(II) alkyl complexes and report their activity for ethylene
polymerization. These results establish that ferrous alkyl cations
are indeed catalytically competent.
1
pyramidal, similar to 1-Cl2 and 1-(CH2SiMe3)2,9 with the donor
ligand coordinated in the basal plane and the alkyl group occupying
the apical site. For 2-Et2O, the iron atom lies 0.660(1) Å out of
the ligand plane, and a similar value of 0.680(1) Å is found for
2-THF. The iron-carbon bond lengths of 2.087(5) and 2.034(3)
Å for 2-Et2O and 2-THF are comparable to the value of 2.062(3)
Å for the apical alkyl group in 1-(CH2SiMe3)2.9
Attempts to induce alkyl abstraction in 1-(CH2SiMe3)2 with the
neutral borane, B(C6F5)3, resulted in silicon methide abstraction
followed by rearrangement. The base-free cationic alkyl complex,
[(iPrPDI)Fe(CH2SiMe2CH2SiMe3)][MeB(C6F5)3] (3), was isolated
as a red solid in 39% yield (eq 3). A similar abstraction-
We previously reported the divergent alkylation chemistry
associated with the five-coordinate ferrous dichloride precursor,
(((2,6-CHMe2)2C6H3NdCMe)2C5H3N)FeCl2 ((iPrPDI)FeCl2, 1-Cl2).9
Treatment with 2 equiv of small alkyllithiums such as MeLi yielded
1-Me, the product of reductive alkylation, while larger reagents
such as LiCH2SiMe3 furnished the ferrous dialkyl, 1-(CH2SiMe3)2
(eq 1).
rearrangement sequence has been observed with a titanium bis-
(trimethylsilyl)methyl complex.10 In the present case, the preference
for silicon methide over iron alkyl abstraction is most likely a result
of the steric protection imparted by the bulky aryl groups that
impede the approach of the Lewis acid to the metal center.
Previously, we reported that activation of the corresponding four-
coordinate, R-diimine ferrous dialkyl with B(C6F5)3 resulted in initial
alkyl group abstraction, followed by aryl group transfer from boron
to iron, affording a neutral Fe(II) aryl alkyl compound.11 These
results highlight the importance of terdentate ligands for catalysis
and are consistent with the reduced polymerization activities
observed with bidentate supporting ligands.12
Solid-state magnetometry on powdered samples of 3 yielded a
magnetic moment of 4.6 µB, consistent with the spin-only value
for four unpaired electrons. Accordingly, the benzene-d6 1H NMR
spectrum of 3 exhibits the number of resonances for a C2V-
symmetric compound that are paramagnetically shifted over a 250
ppm chemical shift range. The hydrogens attached to the carbon
bound to iron were not located. Three singlets are observed for the
[MeB(C6F5)3] anion by 19F NMR spectroscopy; significantly, the
para resonance is shifted upfield to -172.5 ppm.
Treatment of 1-(CH2SiMe3)2 with [PhMe2NH][BPh4] in toluene
resulted in protonation of one of the alkyl ligands to furnish a red-
brown solid, tentatively identified as the base-free alkyl cation,
[(iPrPDI)Fe(CH2SiMe3)][BPh4] (2). To obtain more crystalline
products, the protonation reaction was repeated in the presence of
neutral donors. In this manner, both the diethyl ether and THF
adducts, [(iPrPDI)Fe(CH2SiMe3)(L)][BPh4] (L ) Et2O, 2-Et2O;
THF, 2-THF), were prepared and isolated in 54 and 74% isolated
yields, respectively (eq 2). Solid-state magnetometry (Gouy method)
produced magnetic moments of 4.8 and 4.6 µB, consistent with
high-spin, iron(II) complexes with four unpaired electrons.
Both 2-Et2O and 2-THF have been characterized by X-ray
diffraction, and the molecular structures of the compounds are
presented in Figure 1 and are topologically similar. There are no
obvious interactions between the iron cations and the [BPh4] anions.
The geometry about iron is best described as distorted square
The solid-state structure of 3, determined by X-ray diffraction,
is presented in Figure 1. The geometry about iron is best described
as distorted square planar where the metal lies 0.316(1) Å out of
9
9660
J. AM. CHEM. SOC. 2005, 127, 9660-9661
10.1021/ja0524447 CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Figure 1. Molecular structures of 2-Et2O, 2-THF, and 3 at 30% probability ellipsoids; anions and hydrogen atoms are omitted for clarity.
Table 1. Ethylene Polymerization Dataa
as a propagating species. Importantly, these compounds will allow
further investigation into both the kinetics and the mechanism of
iron-catalyzed carbon-carbon bond-forming reactions.
productivityb
Mn
c
1
time
yield
(g)b
(g
‚
mmol-
‚
(103 g/
melting
temp ( C)
1
1
compound
(min)
h-
‚
bar-
)
mol)
PDIc
°
Acknowledgment. We thank the Research Corporation (Cottrell
Scholar) and the Packard Foundation for financial support. M.W.B.
thanks the Netherlands Organization for Scientific Research for a
postdoctoral fellowship.
2-Et2O
5
30
30
5
trace
0.43
trace
0.38
0.79
2-Et2O
2-THF
3
86
317
2.5
133
218
942
199
172
1.6
2.3
132
129
1-Cl2/MAOd
5e
Supporting Information Available: Experimental procedures and
crystallographic data for 2-Et2O, 2-THF, and 3 (PDF, CIF). This
material is available free of charge via the Internet at http://pubs.acs.org.
a Polymerizations were carried out using 10 µmol of catalyst in 10 mL
of toluene at 23 °C with 1 bar of ethylene. b Mean values over two runs.
c Determined by high-temperature GPC in 1,2,4-trichlorobenzene. d Fe:Al
) 1:600. e Mass transfer problems after approximately 2 min.
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those found in 2-Et2O and 2-THF, demonstrating a significant
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