Efficient ethylene polymerisation catalysis by a cationic benzyl hafnium complex containing pyrrolide-imine ligands

Article abstract of DOI:10.1039/b209582c

A study was performed on efficient ethylene polymerization catalysis. A dibenzyl hafnium(IV) complex containing pyrrolide-imine ligands was synthesized and investigated as an ethylene polymerization catalyst. It was found that the complex can be converted into the benzyl cation, which showed high ethylene polymerization activity.

Full text of DOI:10.1039/b209582c

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Efficient ethylene polymerisation catalysis by a cationic benzyl
hafnium complex containing pyrrolide-imine ligands†
Shigekazu Matsui,a,b Thomas P. Spaniol,b Yukihiro Takagi,a Yasunori Yoshidaa
and Jun Okuda*b
a Catalysis Science Laboratory, Mitsui Chemicals, Inc., 580–32 Nagaura, Sodegaura,
Chiba 299-0265, Japan. E-mail: SKMatsui@aol.com
b Institut für Anorganische Chemie, Johannes Gutenberg-Universität Mainz,
Duesbergweg 10-14, D-55099, Mainz, Germany. E-mail: okuda@mail.uni-mainz.de
Received 30th September 2002, Accepted 12th November 2002
First published as an Advance Article on the web 20th November 2002
A dibenzyl hafnium(IV) complex containing pyrrolide-
imine chelate ligands was synthesized and investigated as
an ethylene polymerisation catalyst.
an X-ray structure determination were obtained by cooling a
toluene–pentane (1 : 10) solution of 3 to Ϫ78 ЊC. ‡
The 1H NMR spectrum of 3 shows the presence of only one
isomer in solution (benzene-d6, toluene-d8 and bromobenzene-
d5). Cooling or warming a bromobenzene-d5 solution of 3 (Ϫ30
Recent attempts to develop alternatives to metallocene-type
single-site catalysts have resulted in the creation of numerous
non-metallocene catalysts based on Group 4 transition metals,
containing non-cyclopentadienyl ligands.1 Among these,
pyrrolide-imine catalyst systems exhibited intriguing catalytic
performance in the preparation of new polymers, such as the
living catalysis of ethylene–norbornene copolymerisation as
well as the preparation of conventional polyolefins with high
activity.2–6 As part of our research aimed at investigating
the utility of the pyrrolide-imine ligand in transition metal
chemistry, we were interested in preparing dialkyl complexes as
well as cationic alkyl complexes as catalyst precursors for olefin
polymerisation. In this paper, we describe the synthesis and
characterisation of benzyl complexes with pyrrolide-imine
ligands and with hafnium() as the central metal. This
hafnium complex gave ethylene polymerisation results that
demonstrate the high potential of pyrrolide-imine catalysts in
olefin polymerisation.
1
to 75 ЊC) does not result in changes in the spectra. The H
NMR resonances of the benzyl methylene moieties attached
to the central hafnium atom are found as an AB quartet at
2.59 and 2.80 ppm, and the corresponding 13C NMR resonance
is recorded at 84.23 ppm. 2-D 1H NMR (COSY) analysis
confirmed that complex 3 possesses C2-symmetry on the NMR
time scale. Irradiation of the methylene protons resulted in the
disappearance of the peak for the 5-proton in the pyrrole ring
(NOE). These results suggest that the Hf complex 3 adopts
configuration C, with cis-benzyl groups, in solution. Complex
3 is stable at room temperature both in the solid state and
in solution (benzene, toluene, bromobenzene, diethyl ether).
Moreover, warming a bromobenzene-d5 solution of 3 to 75 ЊC
for 15 min and monitoring the solution by NMR spectroscopy
showed that 3 has a relatively high thermal stability.
The crystal structure of the C2-symmetric hafnium complex
3 reveals that the pyrrolide-imines are bound to the central
hafnium atom in an η2 fashion and that the Hf–N(a) bond
has σ-character, as shown in Fig. 1. The overall structure is
similar to dichloro titanium() complexes with pyrrolide-imine
Pyrrole-imine (HL1–3)† used in this study can be prepared
from the reaction of pyrrole-2-carboxyaldehyde with a corre-
sponding amine in dry ethanol (yield: HL1; 57%, HL2; 72%,
HL3; 71%). Pyrrole-imine HL1, with an aromatic group
attached at the imine position, reacted with 0.5 equiv. of
Hf(CH2Ph)4 at Ϫ78 ЊC in toluene to give an orange solid.7 In
the complicated 1H- and 13C-NMR spectra of the reaction
mixture at Ϫ30 ЊC, a singlet could be assigned to equivalent
benzyl groups [2.73 ppm (1H), 82.70 ppm (13C)]. However, the
typical AB quartet pattern due to cis-benzyl groups could not
be found. Isolation of the complex failed, because its low
stability resulted in its decomposition during the purification
step.
The reaction of 2 equiv. of HL2 with Hf(CH2Ph)4 in toluene
at Ϫ78 ЊC afforded an orange solution of (L2)2Hf(CH2Ph)2 (2).8
1
According to the H NMR spectra of the reaction mixture at
Ϫ30 ЊC, a singlet was observed for the benzyl groups at 2.83
ppm. 13C NMR spectra showed a methylene peak at 82.15 ppm.
These results suggest that complex 2 possesses trans-benzyl
groups, so that, of the five idealised octahedral structures A–E,
shown in Scheme 1, configuration A or B is adopted in solution.
The Hf complex 2 gradually decomposed at room temperature
even in the solid state.
In contrast, dibenzyl hafnium complex cis-(L3)2Hf(CH2Ph)2
(3) (R = t-Bu) was successfully obtained in 84% yield as an
orange powder from the reaction of 2 equiv. of HL3 with
Hf(CH2Ph)4 in toluene.9,10 Single crystals of 3, suitable for
Fig. 1 Molecular structure of 3 (hydrogen atoms omitted for clarity,
thermal ellipsoids drawn at the 30% probability level). Disordered
carbon atoms of the tert-butyl group C(16)–(18) as well as the imine
group C(4) are shown in one split position. Selected interatomic distances
(Å) and angles (Њ): Hf–N(1) 2.201(6), Hf–N(3) 2.196(6), Hf–C(19)
2.25(1), Hf–C(26) 2.270(9), Hf–N(2) 2.354(6), Hf–N(4) 2.363(6);
N(1)–Hf–N(3) 178.7(3), N(2)–Hf–N(4) 90.4(2), C(19)–Hf–C(26)
96.4(4), C(20)–C(19)–Hf 116.3(7), C(27)–C(26)–Hf 114.6(6).
† Electronic supplementary information (ESI) available: experimental
and spectroscopic details. See http://www.rsc.org/suppdata/dt/b2/
b209582c/
DOI: 10.1039/b209582c
J. Chem. Soc., Dalton Trans., 2002, 4529–4531
This journal is © The Royal Society of Chemistry 2002
4529

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Table 1 Ethylene polymerisation with 3
b
Entry
Cocatalyst
Yield/g
Activitya
Mw b/104
Mw/Mn
T m/ЊC
1c
2c
3d
4c
Boranee
Boratef
Boratef
MAOg
0.934
0.123
1.277
0.530
2242
294
3064
1273
8.8
2.8
89.6
8.7
9.19
2.07
21.15
6.05
132
131
134
132
Conditions: 25 ЊC, 1 bar, polymerisation time; 5 min, 3; 5 µmol.a In (g PE) (mmol Hf )Ϫ1 hϪ1 barϪ1 b Determined by GPC using polystyrene standard.
.
Solvent (250 mL).c Toluene. d n-Heptane. e B(C6F5)3; 10 µmol/i-Bu3A1; 0.25 mmol. f [Ph3C][B(C6F5)4]; 10 µmol/i-Bu3A1; 0.25 mmol. g MAO(A1);
1.25 mmol.
Scheme 1 Reagents and conditions: (i) RNH2, ethanol; (ii) 0.5 equiv. Hf(CH2Ph)4, toluene.
ligands,2c displaying an octahedral structure with trans-
pyrrolide nitrogens [anionic nitrogen: N(a)] [N(1)–Hf–N(3):
178.7(3)Њ] and cis-imine nitrogens [neutral nitrogen: N(n)] [N(2)–
Hf–N(4): 90.4(2)Њ] to one another, respectively. The cis
configuration of the two benzyl groups is evident from the
C(19)–Hf–C(26) angle of 96.4(4)Њ. Both benzyl groups are
bound in η1 fashion to the metal. The Hf–C–C angles [116.3(7)
and 114.6(6)Њ] of the benzyl groups in 3 are slightly smaller
than the theoretical value of 120Њ, suggesting the partial flow of
electron density from the benzyl moieties to the Hf centre.
Addition of strong Lewis acid B(C6F5)3 to 3 in toluene-d8
at Ϫ25 ЊC resulted in the formation of a yellow solution
from which some red–brown oil precipitated immediately.
The corresponding cationic species was not observed in the
attached to the Hf atom and the B atom at 2.54 and 3.66 ppm,
respectively. The anion shows ∆δ(m,p-F) = 2.78 ppm; this is
indicative of a solvent-separated ion pair.12 It is noteworthy that
the cationic hafnium complex 4 possesses high thermal stability
in bromobenzene-d5, showing almost no decomposition, even
at 75 ЊC over a period of at least 15 min.
Subsequently, the catalytic performance of the cationic
complex derived from complex 3 was investigated. Cationic
complex 4, which was prepared in situ, resulted in ethylene
polymerisation with the high activity of 2242 (g PE) (mmol
Hf )Ϫ1 hϪ1 barϪ1 and with an Mw value of 8.8 × 104 in the
presence of triisobutylaluminium (i-Bu3Al) in toluene (entry 1).
When [Ph3C][B(C6F5)4] was used as a cocatalyst in the presence
of i-Bu3Al, rapid heat generation was observed at the beginning
of the reaction; however, deactivation was observed immedi-
ately in toluene (entry 2). The molecular weight distribution
(Mw/Mn) of the polymer thus obtained is 2.07, as expected for
a polymer produced by a single-site catalyst, with an Mw of
2.8 × 104. Interestingly, in n-heptane solvent (entry 3), the
catalyst system complex 3/[Ph3C][B(C6F5)4]/iBu3Al resulted in a
continuous exothermic reaction for at least 5 min and a high
activity [3064 (g PE) (mmol Hf )Ϫ1 hϪ1 barϪ1] with a high Mw
value of 89.6 × 104. In the case of association with methyl-
aluminoxane (MAO) in toluene (entry 4), the activity of com-
plex 3 was 1273 (g PE) (mmol Hf )Ϫ1 hϪ1 barϪ1, (Table 1) which
is the same order of magnitude as that found with titanium
complexes with pyrrolide-imine ligands under the same poly-
merisation conditions.2a–c The results reported here, along with
those reported recently for titanium complexes with chelating
pyrrolide-imine ligands, suggest that pyrrolide-imine catalyst
systems based on Group 4 transition metals are promising
post-metallocene catalysts.
1
toluene-soluble part by H NMR analysis. Alternatively, acti-
vation of 3 in bromobenzene-d5 at Ϫ25 ЊC with 1 equiv. of
B(C6F5)3 yielded a homogeneous red–brown solution con-
taining the ion pair [(L3)2Hf(CH2Ph)][B(C6F5)3(CH2Ph)] (4)
(Scheme 2).11 The 1H NMR spectrum (Ϫ25 ЊC, bromobenzene-
d5) for 4 shows the resonances for the methylene groups
In conclusion, we could show that a thermally robust
dibenzyl hafnium complex containing a pyrrolide-imine ligand
can be made accessible by the proper choice of the imine
substituent. Moreover, the complex can be converted into
the benzyl cation, which shows high ethylene polymerisation
activity. Detailed investigation of the polymerisation catalysis
of the pyrrolide-imine Group 4 metal complexes and mech-
anistic studies are now in progress.
We thank Drs B. Mathiasch, C. Capacchione, and T. Fujita
for useful discussions.
Scheme 2
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J. Chem. Soc., Dalton Trans., 2002, 4529–4531

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7 NMR data for 1 (mixture): 1H NMR (C6D6): δ 1.05–1.28 (m, CH3),
2.61–2.80 (m, CH), 2.73 (s, CH2), 6.35–7.49 (m, Ar–H ϩ pyrrole-H),
Notes and references
‡ Crystal data for 3: C32H40N4Hf; M = 659.19, monoclinic, P21/c,
a = 13.602(1), b = 9.7110(5), c = 22.397(2) Å, β = 95.228(7)Њ, U =
7.72–7.73 (m, CH᎐N).
᎐
8 NMR data for 2: 1H NMR (C6D6): δ 0.63 (d, J = 6.8 Hz, 12H, CH3),
2.83 (s, 4H, CH), 3.17 (q, J = 6.8 Hz, 2H, CH2), 6.46 (m, 2H, pyrrole-
4-H), 6.69 (m, 2H, pyrrole-3-H), 6.80–7.24 (m, 20H, Ar–H), 7.51 (s,
2946.1(4) Å3, T = 293(2) K, Z = 4, Dc = 1.486 Mg mϪ3, µ = 3.567 mmϪ1
,
2θmax = 54Њ, 12320 collected reflections, 6401 unique (Rint = 0.0736), 427
parameters, R1 = 0.0525 [I > 2σ(I )], wR2 = 0.0772. CCDC reference
number 194574. See http://www.rsc.org/suppdata/dt/b2/b209582c/ for
crystallographic data in CIF or other electronic format.
2H, pyrrole-5-H), 7.57 (s, 2H, CH᎐N). 13C-NMR (C6D6): δ 22.47,
᎐
54.50, 82.15, 113.48, 119.48, 121.43, 125.23, 128.87, 138.99, 140.01,
146.28, 158.30.
9 NMR data for 3: 1H NMR (C6D6): δ 0.83 (s, 18H, t-Bu), 2.59, 2.80
(AB q, J = 12.0 Hz, 4H, CH2), 6.49 (dd, J = 3.3 and 1.2 Hz, 2H,
pyrrole-4-H), 6.72 (dd, J = 3.3 and 1.0 Hz, 2H, pyrrole-3-H), 6.85
(t, 2H, J = 7.6 Hz, Ar–H(p)), 6.94 (d, 4H, J = 7.2 Hz, Ar–H(o)), 7.22
(t, 4H, J = 7.6 Hz, Ar–H(m)), 7.71 (br s, 2H, pyrrole-5-H), 7.84
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᎐
Hz, CH2), 113.91, 120.92, 121.46, 126.48, 127.84, 138.66, 139.98,
149.98, 158.44.
10 (L3)2Zr(CH2Ph)2 (5) can be prepared from the reaction of
Zr(CH2Ph)4 and 2 equiv. of ligand HL3 in a manner analogous
to that used to prepare complex 3. The configuration of 5 was
similar to that of 3, with C2-symmetry and two cis-benzyl groups, as
determined by 1H and 13C NMR. (see ESI).
11 NMR data for 4: 1H NMR (C6D5Br, Ϫ25 ЊC): δ 0.90 (s, 18H, t-Bu),
2.54 (s, 2H, CH2Hf ), 3.66 (s, 2H CH2B), 6.45–6.48 (d, J = 7.2 Hz,
2H, pyrrole-4-H), 6.58 (br s, 2H, pyrrole-3-H), 6.97–7.46 (m, 10H),
7.50 (br s, 2H, pyrrole-5-H), 8.27 (s, 2H, CH᎐N). 13C-NMR
᎐
(C6D5Br, Ϫ25 ЊC): δ 29.86, 61.11, 83.12, 117.48, 134.86, 141.87,
149.70, 159.42, (145–150). 19F-NMR (C6D5Br, Ϫ25 ЊC): δ Ϫ130.25
(o-F), Ϫ162.93 (p-F), Ϫ165.71 (m-F).
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J. Chem. Soc., Dalton Trans., 2002, 4529–4531
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