Benzylation of α-diimine ligands bound to zirconium and hafnium. A new convenient route to olefin polymerization catalysts

Article abstract of DOI:10.1246/cl.2007.1420

Reactions of M(CH₂Ph)₄ (M = Zr and Hf) with α-diimine ligands 1 at -78°C selectively afford tribenzyl amido-imino complexes of zirconium 4 and hafnium 5, whose treatment with B(C ₆F₅)₃ gave the corresponding cationic dibenzyl complexes 6 and 7 capable of polymerizing 1-hexene. Copyright

Full text of DOI:10.1246/cl.2007.1420

1420
Chemistry Letters Vol.36, No.12 (2007)
Benzylation of ꢀ-Diimine Ligands Bound to Zirconium and Hafnium.
A New Convenient Route to Olefin Polymerization Catalysts
Kazushi Mashima,Ã Ryuji Ohnishi, Tsuneaki Yamagata, and Hayato Tsurugi
Department of Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531
(Received August 22, 2007; CL-070900; E-mail: mashima@chem.es.osaka-u.ac.jp)
Reactions of M(CH2Ph)4 (M = Zr and Hf) with ꢀ-diimine
Ph
R
ligands 1 at À78 ꢀC selectively afford tribenzyl amido–imino
complexes of zirconium 4 and hafnium 5, whose treatment with
B(C6F5)3 gave the corresponding cationic dibenzyl complexes 6
and 7 capable of polymerizing 1-hexene.
R
N
N
R
N
N
R
Toluene
M
1
- 78 °C to rt
Ph
Ph
Ph
M(CH2Ph)4
M = Zr, Hf
: M = Zr
3: M = Hf
2
In the last two decades, development of homogeneous olefin
polymerization catalysts bearing metallocene fragments has
been remarkable, and recent effort has aimed at developing
the non-metallocene catalysts.1 Among these non-metallocene
catalysts, different types of imine-based ligands have attracted
special interest because these nitrogen-based ligands have been
flexibly designed and their complexes have exhibited unique
catalytic performance for olefin polymerization.2,3 We already
reported the benzyl transfer reaction of tetrabenzylzirconium
with iminopyrroles, giving amido–pyrrolyl zirconium catalysts
for olefin polymerization.4 Similar alkylation of the imine
moiety of phenoxy–imine ligands was reported by Scott and
his co-workers: the reaction of Zr(CH2Ph)4 with phenoxy–imine
ligands afforded the corresponding amido–phenoxy complexes.5
In addition, reduction and alkylation of the imine group has been
observed for imine-based olefin polymerization catalysts com-
bined with any alkylaluminum cocataysts.6–10 As our continuous
interest in polymerization catalysts of early transition metals
bearing such the imine-based ligands,4,11 we focused our atten-
tion to the reaction between alkyl–metal compounds with ꢀ-dii-
mine ligands, whose possible transformations (A–D) are illus-
trated in Scheme 1. Herein, we communicate our preliminary re-
sults on the systematic reaction of various ꢀ-diimine ligands
with M(CH2Ph)4 (M = Zr and Hf), selectively producing two
complexes A and B as olefin polymerization catalysts, by vary-
ing the substituents on the nitrogen atoms of the ꢀ-diimine li-
gands and kind of metal center, though the benzylation reaction
was independently reported by the group of Dow Chemical
Company.12
a: R = 2,6-Me2C6H3
Ph
N R
i
b
: R = 2,6- Pr2C6H3
c: R = 2-MeC6H4
d: R = 2-tBuC6H4
e: R = 4-MeC6H4
f : R = Cy
R
N
M
Ph
Ph
Ph
g: R = tBu
4: M = Zr
5: M = Hf
Scheme 2.
tributable to the benzylated dissymmetric amido–imino ligand
together with two singlets in a 3:1 ratio due to two kinds of meth-
ylene protons of the benzyl groups bound to the metal and the
ligand, respectively. The observation of only one singlet signal
for benzyl groups bound to the metal indicates a rapid exchange
of three benzyl groups on the NMR time scale. Figure 1 shows
the crystal structure of 4a,15 whose structure is essentially the
same as that of the reported compound 4b,12 adopting a distorted
trigonal bipyramidal with N1, C26, and C33 in trigonal planar
(sum of the angles around the zirconium atom = 359.94ꢀ) and
N2 and C40 at apical positions. Dissymmetric ligation is re-
˚
vealed by a significantly longer Zr–N (imino) bond (2.445(3) A)
˚
than the Zr–N (amido) bond (2.121(3) A) and a shorter C(1)=N
˚
(imino) bond (1.301(4) A) compared to the C(2)–N (amido)
˚
(1.450(4) A) bond. In solid state, one benzyl group coordinates
in an ꢁ2-fashion to the zirconium atom, having a rather short
˚
Zr–C(27) (2.651(3) A) bond and an acute angle Zr–C26–C27
(87.06(18)ꢀ).
Treatment of 1,4-dixylyl-1,4-diaza-1,3-butadiene (1a) with
Zr(CH2Ph)4 and Hf(CH2Ph)4 at À78 ꢀC selectively afforded
the corresponding amido–imino complexes 4a and 5a,13,14 which
are products of benzyl transfer from metal to one of C=N bonds
of the ꢀ-diimine ligand 1a, respectively, giving intermediate spe-
cies 2a and 3a, followed by the hydrogen transfer (Scheme 2).
The 1H NMR spectra of 4a and 5a display one set of signals at-
The reaction pattern of such the benzylation of the ligand
highly depends on the substitutents on the nitrogen atoms of
the ligand as well as the kind of metal.13 Hafnium complexes
5b–5e were obtained by the reactions of 1b–1e with
Hf(CH2Ph)4. When cyclohexyl group was introduced as the
substituents of the ꢀ-diimine ligand, the benzylation of 1f by
Hf(CH2Ph)4 resulted in the selective formation of an amido–imi-
no complex 3f, in which a simple benzylation of the C=N bond
of 1f proceeded and no intramolecular hydrogen transfer occur-
red. Similarly, tert-butyl ligand 1g reacted with Hf(CH2Ph)4 to
give 3g. At room temperature, complex 3f is thermally stable,
while 3g decomposes gradually. Notable spectral feature of 3f
and 3g is that benzyl methylene protons are observed as an
ABX pattern coupled with the methin proton. The reactions of
Zr(CH2Ph)4 with 1b and 1d, respectively, afforded 4b and 4d,
R2
N
R2
N
R2
N
R2
N
R1
N
R1 R1
N
R1
R1
R1
R1
N
N
R1
M
M
M
M
R2
R2
R2
R2
R2
R2
R2
R2
R2
R2
A
B
C
D
Scheme 1.
Copyright ꢀ 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.12 (2007)
1421
Table 1. Polymerization of 1-hexene by complexes 3–5a
by Hf(CH2Ph)4, giving another amido–imino hafnium com-
plexes 3 (Type A). Complexes 4a, 4b and 5a, 5b serve as catalyst
precursors for 1-hexene polymerization upon activated by
B(C6F5)3. Thus, the convenient one-pot synthesis of catalyst
precursors for olefin polymerization not only extends the flexi-
bility in the design of precatalysts for olefin polymerization
but also provides a new insight into possible side reaction of
the catalysts bearing imine-based ligands because of easy alkyl-
ation of the imino moiety upon coordinated to early transition
metals.
c
Mw
(103)
c
Entry Cat. Activityb
Mw=Mn
Tacticityd
1
2
3
4
5
4a
4b
5a
5b
3f
3.6
2.9
3.4
5.6
bimodal
2.29
7.84
4.39
—
—
—
1.99
2.27
2.03
—
atactic
atactic
atactic
—
trace
aPolymerization conditions: cat. 10 mmol; B(C6F5)3 10 mmol;
[1-Hexene] = 2.5 M; [Cat.] = 5.0 mM in chlorobenzene,
time 6 h; temperature 25 ꢀC. bKg-polymer/mol-catÁh. cDeter-
mined by GPC. dDetermined by 13C NMR spectroscopic
data.17
This work was supported by a Grant-in-Aid for Scientific
Research on Priority Areas ‘‘Advanced Molecular Transforma-
tions of Carbon Resources’’ from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
References and Notes
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Figure 1. Drawing of the molecular structure of 4a. Solvent
molecule and all hydrogen atoms are omitted for clarity.
4
5
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while the reaction with less bulky substituted ligands 1c, 1e, 1f,
and 1g resulted in the decomposition, in consistent with the less
stable Zr–C bond compared with Hf–C bond.2a
Zirconium and hafnium amido–imino complexes 4a, 4b and
5a, 5b together with 3f were tested as catalyst precursors for 1-
hexene polymerization upon treated with B(C6F5)3 as cocatalyst,
and results are shown in Table 1.13 Complexes 4a, 4b and 5a, 5b
showed moderate catalytic activity, and the polymerization pro-
ceeded through cationic dibenzyl species; the reactions of these
tribenzyl complexes with B(C6F5)3 in bromobenzene-d5 afford-
ed the corresponding zwitterionic dibenzyl complexes 6a, 6b
and 7a, 7b, whose 19F NMR data for ꢀꢂ(m, p-F) indicated the
formation of the separated anion of [PhCH2B(C6F5)3]À.16 In
contrast, complex 3f showed no activity because of thermal
unstability of cationic species derived by 3f and B(C6F5)3.
6
7
8
9
M. Zimmermann, K. W. Tornroos, R. Anwander, Angew. Chem., Int. Ed.
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¨
10 D. Reardon, F. Conan, S. Gambarotta, G. Yap, Q. Wang, J. Am. Chem.
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Ph
R
Ph
R
B(C6F5)3
C6D5Br
R
N
N
R
N
N
PhCH2B(C6F5)3
(1)
M
M
Ph
Ph
Ph
Ph
Ph
13 Experimental details are given in Supporting Information, which is
available electronically on the CSJ-Journal web site, http://www.csj.
jp/journals/chem-lett/.
4: M = Zr a: R = 2,6-Me2C6H3
5: M = Hf b: R = 2,6-iPr2C6H3
6: M = Zr
7: M = Hf
14 On monitoring the reaction, we observed the formation 2 and 3 as
transient species, which gradually disappeared. In contrast, the same re-
action carried out at room temperature afforded ene–amido dibenzyl
complexes (Type D) as by-roducts, whose formation was in consistent
to the observation reported in Ref. 12.
In conclusion, we demonstrated that the substituent of the
nitrogen atom of the ꢀ-diimine ligand controls the reaction pat-
tern with M(CH2Ph)4 (M = Zr and Hf): (1) benzylation of
1,4-diaryl-1,4-diaza-1,3-butadienes followed by the hydrogen
transfer within the ligands, giving amido–imino complexes 4
and 5 (Type B) through the intermediate complexes 2 and 3
and (2) benzylation of the dialkyl-substituted ꢀ-diimine ligands
15 Crystal data for 4a is given in CIF format.
16 A. D. Horton, J. de With, A. J. van der Linden, H. van de Weg,
Organometallics 1996, 15, 2672.
17 T. Asakura, M. Demura, Y. Nishiyama, Macromolecules 1991, 24, 2334.
Published on the web (Advance View) November 3, 2007; doi:10.1246/cl.2007.1420