Cationic benzyl zirconium heteroscorpionates: Synthesis and characterization of a novel ethylene polymerisation catalyst showing an unusual temperature dependent polymerisation mechanism

Article abstract of DOI:10.1039/b301997g

The reaction of (bpzmp)Zr(CH₂Ph)₃ with B(C₆F₅)₃ produces the active ethylene polymerisation catalyst [(bpzmp)Zr(CH₂Ph)₂] ⁺[PhCH₂B(C₆F₅) ₃]^- which showed a temperature dependent polymerisation mechanism identified by variable temperature ¹H NMR analysis of the catalyst solution.

Full text of DOI:10.1039/b301997g

Cationic benzyl zirconium heteroscorpionates: synthesis and
characterization of a novel ethylene polymerisation catalyst showing an
unusual temperature dependent polymerisation mechanism
Stefano Milione,a Cosimo Montefusco,a Tomas Cuencab and Alfonso Grassi*a
a Università di Salerno, Dipartimento di Chimica, 84081 Baronissi, Italy. E-mail: agrassi@unisa.it;
Fax: +39 089 965296
b Departamento de Quimica Inorganica, Universidad de Alcala, Campus Universitario, 28871 Alcala de
Henares, Spain
Received (in Cambridge, UK) 19th February 2003, Accepted 31st March 2003
First published as an Advance Article on the web 16th April 2003
The reaction of (bpzmp)Zr(CH2Ph)3 with B(C6F5)3 produces
quantitative in few minutes. The addition of one more
equivalent of bpzmp to 1 did not yield the bis-phenoxy
derivative probably as a result of the relative bulkiness of the
ligand and complex 1. The molecular structure of 1 was defined
by means of 1H NMR, TOCSY, 1D NOE, 13C NMR and
gHMQC experiments.‡ 1 exhibits a pseudo-octahedral sym-
the
active
ethylene
polymerisation
catalyst
[(bpzmp)Zr(CH2Ph)2]+[PhCH2B(C6F5)3]2 which showed a
temperature dependent polymerisation mechanism identi-
fied by variable temperature 1H NMR analysis of the catalyst
solution.
3
metry and the bpzmp ligand is k -N,N,O co-ordinated to the
One of the most attractive subjects in the organometallic
chemistry of the group 4 metals is the synthesis of non-
cyclopentadienyl complexes showing a chemical structure
closely reminiscent of the active sites in the classical Ti based
heterogeneous Ziegler–Natta catalysts.1 Ligands containing
nitrogen and/or oxygen donors have proved to be suitable and
group 4 metal complexes bearing bis-phenoxyimine (FI)
ligands are described as extremely active ethylene homoge-
neous polymerization catalysts with living properties at room
temperature.2
Aiming to design novel homogeneous olefin polymerisation
catalysts with octahedral co-ordination symmetry we con-
sidered the (3,5-tBu2-2-hydroxyphenyl)bis(3,5-Me2-pyrazolyl)-
methane tridentate ligand (bpzmp). The corresponding mono-
anion is a six electron donating system with a phenoxydiimine
skeleton and could be considered a suitable alternative to the
classical cyclopentadienyl ligand where the Lewis basic
properties of the nitrogen donors can be tuned and the bulkiness
of the ligand and the symmetry properties of the resulting
complexes modified introducing appropriated substituents in
the 3-position via well established procedures.3
metal centre. The single set of 1H resonances observed for
bpzmp in the temperature range 25–100 °C suggests an elevated
co-ordination energy of the ligand to the metal centre and the
absence of any fluxional equilibrium of the two N,N donors.
The two cis methylene groups in the N,N plane produce an AB
pattern consisting of two doublets at 2.78 and 2.67 ppm with a
1JHH of 11 Hz§ whereas the methylene protons of the benzyl
trans to the oxygen are observed as a singlet at 2.84 ppm. No
site-exchange reaction between the two types of benzyl groups
was observed at room temperature or after heating of the toluene
solution of 1 up to 100 °C.
Treatment of 1 with B(C6F5)3 or MAO produces active
ethylene polymerisation catalysts with activity values in the
range 100–700 gPE mmolCAT h21 atm21.¶ These values are
21
5
4
similar to those reported for (h -C5Me5)ZrCl3 and Zr(CH2Ph)4
but higher than the heteroscorpionate complex (bdmpza)TiCl3
(bdmpza = bis(3,5-dimethylpyrazol-1-yl)acetate) by Otero et
al.5 Note that the molecular weight distributions of the
polyethylene samples (see Table 1) are monomodal and the
polydispersity (Mw/Mn) is significantly lower than that found
for the polyethylene samples by CpAZrX3 (CpA = C5H5;
C5(CH3)5, X = Cl; Bz) and ZrBz4.6 Interestingly the Mw/Mn
ratios decrease with polymerisation temperature and rank from
4.2 (at 50 °C) to 2.3 (at 240 °C); the latter value is very close
to those found for single site catalysts.
Here we report on the synthesis and solution structure of the
zirconium complex (bpzmp)Zr(CH2Ph)3 1 and of the corre-
sponding
ionic
complex
[(bpzmp)Zr(CH2Ph)2]+-
[PhCH2B(C6F5)3]2 2. The latter was found to be active in
ethylene polymerisation and a peculiar temperature dependent
behaviour of the polydispersity of the polymer products versus
polymerization temperature is herein presented and discussed in
the light of the solution structure of 2.
To identify the active species responsible for this behavior
the reaction of 1 with B(C6F5)3 was monitored by variable
temperature 1H NMR spectroscopy. The formation of the
expected ion pair [(bpzmp)Zr(CH2Ph)2]+[PhCH2B(C6F5)3]2
2
The ligand bpzmp was synthesised according to the lit-
erature3 and the benzyl zirconium derivative readily obtained in
good yields by reacting bpzmp with Zr(CH2Ph)4 (1+1 molar
was readily observed in CD2Cl2 at 280 °C where one set of 1H
signals for the bpzmp ligand and two sets of 1H signals for two
inequivalent benzyl groups bound to the zirconium were
1
ratio) in toluene at room temperature (Scheme 1). † H NMR
monitoring of the reaction showed that this is fast and
Table 1 Ethylene polymerization catalyzed by 2
c
Runa
Temp./°C
Yield/g
Activityb
Mw/Mn
1
2
3
4
50
25
220
240
0.14
0.17
0.77
0.44
230
196
340
125
4.2
3.5
2.8
2.3
a Polymerization conditions: (bpzmp)Zr(CH2Ph)3 (38 mg, 50 µmol);
B(C6F5)3 (25 mg, 50 µmol); 40 ml of toluene; 0.3 mmol of AlMe3 used as
scavenger; polymerization time: 5 min; ethylene pressure: 1 bar. b Activity
= gPE mmolCAT21 h21 [C2H4]21 c Determined by GPC.
.
Scheme 1
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CHEM. COMMUN., 2003, 1176–1177
This journal is © The Royal Society of Chemistry 2003

Scheme 2
actually detected.∑ In particular the methylene protons of the
Notes and references
ZrCH2Ph groups produce two broad signals at 2.37 and 2.47
ppm; the methylene protons of the benzylborate anion are
observed as a broad signal at 2.76 ppm and the corresponding
ortho, para and meta protons are found at 6.60, 6.76 and 6.82
ppm, respectively. When the temperature is raised to 220 °C, a
rapid exchange of the two inequivalent benzyl groups becomes
active producing a unique pattern of broad resonances for these
† Synthesis of 1: bpzmp (0.450 g, 1.1 mmol) dissolved in 50 ml toluene was
added to a solution of ZrBz4 (0.502 g; 1.1 mmol) in 20 ml of toluene at room
temperature. The resulting yellow solution was stirred for 30 minutes, then
concentrated to 10 ml and kept overnight at 220 °C yielding 0.450 g of a
pale yellow solid (yield 53%).
‡ 1H NMR data (400 MHz, CD2Cl2, 25 °C) for 1: d 1.34 (s, 9H, 5-tBu-Ar),
1.52 (s, 9H, 3-tBu-Ar), 2.05 (s, 6H, 3-CH3-Pz), 2.48 (s, 6H, 5-CH3-Pz), 2.67
(d, 2H, J = 11 Hz, eq-CH2Ph), 2.78 (d, 2H, J = 11 Hz, eq-CH2Ph), 2.84 (s,
2H, ax-CH2Ph), 5.85 (s, 2H, Pz-H), 6.36 (d, 2H, ax-o-Ph), 6.58 (t, 1H, ax-p-
Ph), 6.75 (t, 2H, ax-m-Ph), 6.79 (t, 2H, eq-p-Ph), 6.84 (d, 4H, eq-o-Ph), 7.03
(t, 4H, eq-m-Ph), 7.10 (d, 1H, J = 2 Hz, 6-H-Ar), 7.14 (s, 1H, NN)2-
CH(ArO)-), 7.47 (d, 1H, J = 2 Hz, 4-H-Ar).
1
groups and a significant broadening of the H signal corre-
sponding to the methyls of the pyrazolyl groups facing the metal
center and the anion. Furthermore a small downfield shift of the
aromatic protons of the anion resulting from the increased
dissociation of the ion pair was observed with the ortho, para
and meta protons actually detected at 6.69, 6.78 and 6.86 ppm,
respectively.7 We interpreted these results assuming that at low
temperature (in the range from 280 to 240 °C) 2 adopts an
octahedral geometry in which the anion is co-ordinated in one
out of the two sites in the N,N plane. This hampers the
isomerization process of the two inequivalent benzyl groups
which, in contrast, becomes active at 220 °C as a result of the
anion displacement. The olefin complex, assumed as the
intermediate species in ethylene polymerisation, can thus exist
in three forms: (3a) in which ethylene is in the N,N plane and the
polymer chain in the axial site (opposite to the oxygen); (3b)
with ethylene trans to the oxygen and polymer chain in the N,N
plane and finally (3c) with ethylene and the growing polymer
chain both in the cis sites of the N,N plane. Correspondingly two
different pathways for the growth of the polymer chain can be
drawn: in the pathway A of Scheme 2 the propagation step
includes the polymer chain flipping between the axial and the
equatorial sites (comprising the 3a and 3b intermediate species)
whereas in pathway B the polymer chain flips between the two
equatorial sites in the N,N plane.
At room or higher temperature both pathways are possible
and a broader molecular weight distribution of the polymer
products is obtained. At 240 °C the coordination of the anion
produces an octahedral species and the incoming ethylene can
replace the anion in the same N,N plane site followed by fast
chain migratory insertion. Later on, the vacant site is readily
occupied by the anion producing a resting state of the catalyst.
In the frame of this picture only the pathway B is active at 240
°C or lower temperature producing a polymer with a narrow
molecular weight distribution.
1
§ The methylene carbons of three benzyl groups show a JCH coupling
constant of 114 Hz, in the range expected for carbon atoms with sp3
hybridisation.
¶ The highest activity value was achieved with the MAO activated catalyst
(A = 700 gPE mmolCAT21 h21 atm21) under the following polymerisation
conditions: 1 (7.7 mg, 10 µmol); MAO (5 mmol, Al/Zr molar ratio = 500);
toluene (100 ml); 0.3 mmol of AlMe3 used as scavenger; polymerization
temperature = 50 °C, ethylene pressure = 5 bar; polymerization time: 5
min. Under similar conditions the 1–B(C6F5)3 catalyst (1 = 50 µmol; B/Zr
21
molar ratio = 1+1) showed an activity value of 250 gPE mmolCAT h21
atm21 and the corresponding polyethylene a polydispersity index (Mw/Mn)
of 4.2.
1
∑ H NMR data (400 MHz, CD2Cl2, 280 °C) for 2: d 1.28 (s, 9H, 5-tBu-Ar),
1.42 (s, 9H, 3-tBu-Ar), 1.92 (s, 6H, 3-CH3-Pz), 2.42 (s, 6H, 5-CH3-Pz), 2.37
(s, 2H, CH2Ph), 2.47 (s, 2H, CH2Ph), 2.76 (s, 2H, B-CH2Ph), 5.91 (s, 2H,
Pz-H), 6.36 (d, 2H, ax-o-Ph), 6.60 (d, 2H, BCH2-o-Ph), 6.74 (d, 2H, o-Ph),
6.76 (t, 1H, BCH2-p-Ph), 6.82 (t, 2H, BCH2-m-Ph), 7.15 (m, 10H, Zr-
CH2C6H5, 6-H-Ar, NN)2-CH(ArO)-), 7.47 (s, 1H, 4-H-Ar).
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The authors wish to acknowledge the financial support from
the Ministero dell’Università e della Ricerca Scientifica
(MURST, Roma, Italy; PRIN-2002: “Fine tuning by organome-
tallic catalysts of microstructure and chemical and physical
properties of hydrocarbon homopolymers and copolymers”).
6 Unpublished data from our laboratory.
7 For comparison, the chemical shifts of the aromatic protons in the “free”
anion [PhCH2B(C6F5)3]2 were observed at 6.88 (m), 6.78 (p), 6.74 (o)
ppm (CD2Cl2, 25 °C). See e.g. A. D. Horton and J. de With, Chem
Commun., 1996, 1375.
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