Unusual Enhancement of Ethylene Polymerization Activity of Benzyl Zirconium Complexes by Benzylation of the Imino Moiety of 2-(N-Aryliminomethyl) pyrrolyl Ligand

Article abstract of DOI:10.1246/cl.2003.756

Reaction of Zr(CH₂Ph)₄ (1) with one equiv. of 2-(N-aryliminomethyl)pyrrolyl ligands (2a-e) in toluene afforded two types of products, depending upon the bulkiness of the aryl moiety. The reactions of 1 with two equiv. of the most ste

Full text of DOI:10.1246/cl.2003.756

756
Chemistry Letters Vol.32, No.8 (2003)
Unusual Enhancement of Ethylene Polymerization Activity of Benzyl Zirconium Complexes
by Benzylation of the Imino Moiety of 2-(N-Aryliminomethyl)pyrrolyl Ligand
Hayato Tsurugi, Tsuneaki Yamagata, Kazuhide Tani, and Kazushi Mashimaꢀ
Department of Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531
(Received May 14, 2003;CL-030418)
Reaction of Zr(CH2Ph)4 (1) with one equiv. of 2-(N-arylimi-
nomethyl)pyrrolyl ligands (2a–e) in toluene afforded two types of
products, depending upon the bulkiness of the aryl moiety. The
reactions of 1 with two equiv. of the most sterically bulky 2e af-
forded complex 5e, whose pseudo octahedral geometry around
the zirconium atom and totally dissymmetric cis-dibenzyl struc-
ture were elucidated. The high catalytic activity for ethylene
polymerization was obtained by using complexes 3a and 3b.
Homogeneous polymerization catalysis has been extensively
studied in recent years.1 In addition to metallocene-based cata-
lysts of Group 4 metals, various transition metal complexes with
nitrogen-based ligands such as 1,4-diaza-1,3-dienes,2 bis(imino)-
pyridines,3–5 phenoxy-imines,6–9 and pyrrolyl-imines10–13 have
attracted much interest in terms of tunable and more flexibly de-
signable ancillary ligands. It has recently been highlighted that
phenoxy-imine complexes of titanium and zirconium have exhib-
ited ultra-high catalytic activity6 and also catalyzed the living
Scheme 1.
polymerization of ꢀ-olefins.6 Some of these catalysts bearing
atom.
The reactions of 1 with two equiv of 2 were also conducted
in similar condition. The most sterically bulky ligand 2e afforded
a complex 5e (Eq 1), otherwise the reactions with 2a–d did not
give any isolable compounds due to the formation of the compli-
cated reaction mixture. The complex 5e was characterized by
imino-derivatives, however, have been reported to show shorter
lifetime due to the deactivation of the catalysts through the mi-
gratory insertion reaction of alkyl group bound to the metal cen-
ter to the C=N moiety of the ligand.5,9 We herein report unique
enhancement, the inverse tendency, of catalytic activity for ethyl-
ene polymerization by the alkylation of the imino moiety of 2-
(N-aryliminomethyl)pyrrolyl ligand bound to the zirconium cen-
ter.
spectral data and X-ray analysis.14 The H NMR spectra of the
1
complex 5e showed one singlet due to benzyl group at room tem-
perature;however, two broad signals were observed below
263 K, indicating that dissymmetrical structure. The coalescence
of the benzyl resonances allowed to estimate the energy parame-
ters for the interconversion process to be ꢂGz ¼ 12:6 kcal/mol.
Figure 1 shows the pseudo octahedral geometry around the zirco-
nium atom and a cis-arrangement of two benzyl groups suitable
for the catalyst of the ethylene polymerization. It is of particular
interest that one benzyl group is trans to the nitrogen atom of the
imino group, while the other is trans to the nitrogen atom of the
pyrrolyl moiety, being consistent with NMR spectroscopy. This
is in contrast to the reported C2-symmetric geometry found for
dichloro complexes of titanium13 and zirconium,11 a diamido
complex of zirconium,12 and a dibenzyl hafnium complex13 bear-
ing pyrrolyl-imine ligands. The ipso carbon of one benzyl group
interacted with the zirconium atom as evident from the shorter
Scheme 1 shows the preparation of zirconium pyrrolyl-imine
complexes starting from Zr(CH2Ph)4 (1). Two types of zirconium
complexes were prepared by the reactions of 1 with one equiv. of
2-(N-aryliminomethyl)pyrrolyl ligands (2a–e) in toluene. Com-
plexes 3 and 4 were selectively obtained depending on the bulki-
ness of aryl group of 2. Reactions of 1 with less bulky ligands
such as 2a and 2b gave dibenzyl complexes 3a and 3b, respec-
tively, with the benzylation of the C=N moiety and the release
of one equiv. of toluene. On the other hand, the reactions of 1
with 2,6-dialkyl substituted ligands 2d and 2e afforded tri(ben-
zyl) complexes 4d and 4e, respectively, with the release of tolu-
ene. The substituenets at ortho-positions prevented the alkylation
of the imino group. In the case of o-tolyl ligand 2c, the reaction
resulted in the complicated mixture, from which any product was
not isolated. The complexes 3a and 3b were characterized by
ꢀ
distance of Zr-C2 (2.760(2) A) and the smaller angle of Zr-C1-
C2 (91.9(1)ꢁ).
1
their H NMR spectra, which displayed three sets of the benzyl
signals. One ABX signal (ꢁ 2.03 and 3.18 for 3a; ꢁ 2.04 and
3.15 for 3b) is assignable to the PhCH2CH group, while the other
two sets are observed as ABq pattern at higher field (ꢁ 1.31 and
1.96; ꢁ 2.45 and 2.79 for 3a: ꢁ 1.30 and 1.96; ꢁ 2.47 and 2.80 for
3b) due to the benzyl groups bound to the zirconium atom. In
1
sharp contrast, the H NMR spectra of 4d and 4e showed only
one singlet at ꢁ 2.13 for 4d and ꢁ 2.18 for 4e due to magnetically
equiv. three CH2 of the benzyl groups bound to the zirconium
Copyright ꢀ 2003 The Chemical Society of Japan

Chemistry Letters Vol.32, No.8 (2003)
757
species, but the replacement of the chelating ligand might be the
first step to initiate the polymerization.
In summary, we demonstrated that 2-(N-aryliminomethyl)-
pyrrolyl ligands reacted with one equiv of Zr(CH2Ph)4 (1) to give
two kinds of pyrrolyl-amido complexes 3 and pyrrolyl-imine
complexes 4, while the reaction of 1 with two equiv. of 2e result-
ed in the formation of a bis[(iminomethyl)pyrrolyl]zirconium
complex 5e. In the case of complexes 3, the benzylation of the
imino moiety of the ligand dramatically enhanced the ethylene
polymerization activity. Investigation on unique catalyst activa-
tion by the alkylation of the imino moiety is in progress.
References and Notes
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Figure 1. Crystal Structure of 5e.
Table 1 summarizes the results of ethylene polymerization
using mono- and bis[2-(N-aryliminomethyl)pyrrolyl] zirconium
complexes 3–5 as catalyst precursors. All these zirconium com-
plexes exhibited catalytic activity for ethylene polymerization
under atmospheric pressure of ethylene in the presence of excess
(1000 equiv.) amounts of MMAO. The catalyst precursor with
the highest activity was the complex 3b;the activity of ethylene
polymerization within 5 min at 0 ꢁC was found to be
1:08 Â 103 kg-PE/mol-catÁh (Run 2). Comparable high activity
(0:8 Â 103 kg-PE/mol-catÁh) was obtained for the polymerization
using 3a (Run 1). It is noteworthy that these complexes are pyr-
rolyl-amido complexes, whereas 4d and 4e, which have the tri-
benzyl zirconium center supported by one (iminomethyl)pyrrolyl
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in sharp contrast to the reported deactivation caused by the alky-
lation of the imino moiety of phenoxy-imine9 and by the alkyla-
tion of pyridine ring of bis(iminomethyl)pyridine5 bound to the
catalyst center. Thus, this is the first example of the enhancement
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and 60 ꢁC, Run 3 and 4) resulted in the lower activities, indicat-
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tion condition due to coordinative unsaturation around the zirco-
nium center. We assumed that high unsaturation around the
zirconium center was compensated by the interaction with the
phenyl group of the armed-pendant in 3a and 3b, as similar to
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2
3
4
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5
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9
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The dibenzyl zirconium complex 5e exhibited a low catalytic
activity equal to those found for 4d and 4e, but the obtained poly-
ethylene had a very broad Mw=Mn value, presumably due to the
gradual formation of catalytic active species (Run 7). This sug-
gested that the cis-dialkyl zirconium center having bulky ligands
did not readily generate a catalytically active cationic monoalkyl
Table 1. Polymerization of ethylene catalyzed by zirconium complexesaÞ
Time
/min
5
5
5
Temp.
/ꢁC
25
0
25
60
25
25
25
Mw
(Â103)
3.0
3.8
7.9
4.9
—
—
94
Run
1
2
3
4
5
6
7
Cat.
3a
3b
3b
3b
4d
4e
AbÞ
810
1084
916
419
19
Mw=Mn
2.5
2.2
2.8
2.2
—
—
28.4
14 Crystal data for 5e: C48H56N4Zr, Orthorhombic, space group Pna21 (No. 33),
ꢀ
ꢀ
ꢀ
ꢀ 3
a ¼ 23:1021ð5Þ A, b ¼ 17:3933ð4Þ A, c ¼ 10:5947ð3Þ A, V ¼ 4257:2ð2Þ A ,
Z ¼ 4, Dcalcd ¼ 1:217g cmꢂ3, T ¼ 233ð1Þ K, Rigaku R-AXIS RAPID, Mo Ka
radiation (ꢃ ¼ 0:71069), ꢄ ¼ 0:295 mmꢂ1, numerical absorption correction
(0.9586–0.9862). The full-matrix least squares refinement on F2 with all
13510 reflections and 645 variables converged to R1 ¼ 0:0576 (all data),
wR2 ¼ 0:0881 (all data), Flack parameter (ꢅ) = ꢂ0:01ð2Þ, GOF = 1.081,
and ꢂꢆ(max) ¼ 0:004.
5
60
60
90
51
21
5e
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40, 2516 (2001).
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(2002).
a) Conditions: [cat.] = 1 mM in toluene, ethylene pressure = 1 atm,
and [cocatalyst] = 1000 equiv of MMAO. b) A = Activity (kg-PE/
mol-catÁh).
Published on the web (Advance View) July 21, 2003;DOI 10.1246/cl.2003.756