Isospecific polymerization of propene by new indolyl-pyridylamido Zr(IV) catalysts

Article abstract of DOI:10.1016/j.molcata.2012.12.008

New zirconium complexes bearing indolyl-pyridylamido ligands [ ^-NNN^-] have been synthesized and used as catalysts in ethylene and propene polymerization in the presence of AlⁱBu ₂H/MAO as cocatalytic system. Complexes 1 and 2, bearing ligands with a different steric hindrance on the carbon bridging the pyridine and the aniline moieties, exhibited remarkable catalytic activity for ethylene and propene polymerization, affording ultrahigh molecular weight polymers with monomodal molecular weight distributions. Moreover, the presence of the 2-isopropyl phenyl substituent on the bridging carbon (complex 2) resulted in higher stereoselectivity and regioselectivity providing a polypropylene sample with an [mmmm] content up to 96% and 1.3% of regioinversion. On the other hand, the change of the substituents of the aniline moiety (complex 3) gave a zirconium complex which resulted completely inactive both in the ethylene and propene polymerization.

Full text of DOI:10.1016/j.molcata.2012.12.008

Journal of Molecular Catalysis A: Chemical 370 (2013) 28–34
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Isospecific polymerization of propene by new indolyl-pyridylamido Zr(IV)
catalysts
Gang Lia, Marina Lambertia, Mina Mazzeoa, Daniela Pappalardob, Claudio Pellecchiaa,∗
a Dipartimento di Chimica e Biologia, Università di Salerno, via Ponte don Melillo, I-84084 Fisciano, SA, Italy
b Dipartimento di Scienze per la Biologia, la Geologia e l’Ambiente, Università del Sannio, Via dei Mulini 59/A, 82100 Benevento, Italy
a r t i c l e i n f o
a b s t r a c t
New zirconium complexes bearing indolyl-pyridylamido ligands [−NNN−] have been synthesized and
used as catalysts in ethylene and propene polymerization in the presence of AliBu2H/MAO as cocatalytic
system. Complexes 1 and 2, bearing ligands with a different steric hindrance on the carbon bridging
the pyridine and the aniline moieties, exhibited remarkable catalytic activity for ethylene and propene
polymerization, affording ultrahigh molecular weight polymers with monomodal molecular weight dis-
tributions. Moreover, the presence of the 2-isopropyl phenyl substituent on the bridging carbon (complex
2) resulted in higher stereoselectivity and regioselectivity providing a polypropylene sample with an
[mmmm] content up to 96% and 1.3% of regioinversion. On the other hand, the change of the substituents
of the aniline moiety (complex 3) gave a zirconium complex which resulted completely inactive both in
the ethylene and propene polymerization.
Article history:
Received 25 October 2012
Received in revised form
10 December 2012
Accepted 12 December 2012
Available online 21 December 2012
Keywords:
Zirconium
Olefin polymerization
Isospecific catalysts
Homogeneous catalysis
Propene
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
resulted in stereospecific catalysts, producing high molecular
weight polypropylenes (Mw up to 106 g/mol) with high isotacticity
Recent research in the field of olefin polymerization has been
focused on a variety of polydentate ligand frameworks based on
nitrogen and/or oxygen and/or sulfur donors, resulting in the
discovery of catalysts displaying excellent performance and pecu-
liar features, such as living polymerization at high temperature
and chain shuttling between two different active species, allow-
ing the synthesis of novel macromolecular architectures [1]. One
of the most successful class of stereoselective catalysts recently
introduced is based on arylpyridylamido Hf(IV) complexes, dis-
covered and optimized by researchers at Dow and Symyx using
high throughput screening technologies and currently used in
commercial production of some polypropylene-based resins [2].
The excellent performance of these catalysts for the production
of highly isotactic, high molecular weight polypropylene at high
temperature in solution processes depends on in situ arylcyclomet-
allation reactions, changing a [NN ] monoanionic ligand into a
([mmmm] up to 95%), and high melting temperatures (Tm > 150 ◦C).
The same class of ligands were used for the synthesis of group
3 and aluminum complexes active in the ring-opening polymer-
ization of cyclic esters. [5] We report here the synthesis of some
related Zr(IV) complexes bearing tridentate [−NNN−] ligands, in
which the pyrrolide group was replaced by the bulkier and more
electron donating indolyde group (Chart 1, complexes II), and their
use as precatalysts for the polymerization of ethylene and propene.
2. Experimental
2.1. General procedures
All manipulations of air- and/or water-sensitive compounds
were carried out under nitrogen atmosphere using standard
Schlenk or glovebox techniques. All solvents, purchased from
Carlo Erba, were refluxed over sodium/benzophenone or calcium
hydride (CaH2) and then distilled under nitrogen atmosphere
before use. 1-Bromo-2-isopropylbenzene was purchased from Alfa
Aesar and used as received. All other chemicals were purchased
from Sigma–Aldrich and used as received. Solid MAO (10 wt%
solution in toluene) was obtained by distilling off the volatile mate-
rials under reduced pressure and excess of trimethylaluminum
was removed by washing the resulting solid with dry hexane,
and drying under vacuum the obtained white powder. Ethylene
[
CNN ] dianionic tridentate one and, possibly, by further ligand
modifications, such as monomer insertion into a Hf Caryl bond
[2,3]. In this respect, we have previously reported penta-coordinate
group 4 metal complexes bearing dianionic pyrrolylpyridylamido
[−NNN−] tridentate ligands [4] (Chart 1, complexes I) which
∗
Corresponding author. Tel.: +39 089 969576; fax: +39 089 969603.
E-mail address: cpellecchia@unisa.it (C. Pellecchia).
1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.12.008

G. Li et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 28–34
29
Chart 1. Pyrrolylpyridylamido group 4 complexes (I) previously reported and indolylpyridylamido zirconium complexes (II) synthesized in this work.
and propene were purchased from SON and used without fur-
ther purification. N-(t-butoxycarbonyl)-indole-2-boronic acid [6]
and 4,6-dimethyl-2-(2-phenylethyl)aniline [7] were prepared by
literatures procedures.
silica gel (1.60 g, 10 equiv.). The mixture was refluxed and stirred for
1 h and then cooled to room temperature. 2,6-Diisopropylaniline
(0.50 g, 2.82 mmol) and molecular sieves were added and the
resulting mixture was refluxed for 1 h. The solvent was distilled
off by rotary evaporation. The crude product was purified via col-
umn chromatography on silica gel using 9:1 hexane/ethyl acetate
as eluent. The removal of the solvent by rotary evaporation afforded
a light yellow solid (0.79 g, yield: 78%). 1H NMR (400 MHz, CDCl3,
25 ◦C): 1.20 (d, J = 6.9 Hz, 12H, CH(CH3)2), 3.00 (sept, 2H, CH(CH3)2),
7.09–7.24 (m, 6H, Ar H), 7.45 (dd, J = 8.1 Hz, 1H, Ar H), 7.67 (d,
J = 8.1 Hz, 1H, Ar H), 7.90 (m, 2H, Ar H), 8.14(dd, J = 7.2 Hz, 1H,
Ar H), 8.36 (s, 1H, HC N), 9.61 (s, 1H, indole NH). 13C NMR
(100.62 MHz, CDCl3, 25 ◦C): 23.67, 28.18, 101.29, 111.65, 119.70,
120.45, 121.47, 121.56, 123.27, 123.62, 124.68, 129.29, 136.30,
136.71, 137.42, 137.52, 148.72, 150.52, 153.91, and 163.16.
The 1H and 13C NMR spectra were recorded on a Bruker Avance
spectrometer operating at 400 MHz and 100.6 MHz, respectively.
13C NMR spectra of polymers were recorded on a Bruker AM-250
spectrometer operating at 62.5 MHz in 1,1,2,2-tetrachloroethane-
d2 (C2D2Cl4) at 100 ◦C and referenced vs hexamethyldisiloxane
(HMDS).
Molecular weights (Mn and Mw) and polydispersity indexes
(PDI) of polymers were determined by high-temperature GPC using
a Waters GPC-V200 RI detector. The measurements were recorded
at 135 ◦C using 1,2-dichlorobenzene as a solvent and Styragel
3
6
˚
columns (range 10 –10 A). Every value was the average of two
independent measurements.
Polymers melting points (Tm) were measured by differential
scanning calorimetry using a TA Instruments DSC 2920 in nitro-
gen flow with a heating and cooling rate of 10 ◦C/min. Melting
temperatures were reported for the second heating cycle.
2.2.3. Synthesis of H2L1
To a solution of compound B (250 mg, 0.66 mmol) in 20 mL of
methanol, 3 drops of CHOOH (88%) were added. Then, NaBH3CN
(78 mg, 1.24 mmol) was added into the solution. The yellow solu-
tion turned to colorless. The solution was refluxed for one hour
and then cooled to room temperature. After removal of the sol-
vent by rotary evaporation, the solid was dissolved in diethyl ether
and water. The organic phase was separated and reserved. The
aqueous phase was extracted with diethyl ether (3 × 30 mL). The
combined organic phases were washed with water (2 × 30 mL) and
brine (1 × 30 mL). The organic phase was dried over Na2SO4. The
solvent was distilled off by rotary evaporation. The crude prod-
uct was purified by flash column chromatography on silica gel
using hexane/diethyl ether (9/1) as eluent. The colorless oil was
concentrated under vacuum, affording a colorless solid (215 mg,
85%). 1H NMR (300 MHz; CDCl3, 25 ◦C): 1.29 (12H, d, J = 6.9 Hz,
CH(CH3)2), 3.45 (2H, CH(CH3)2), 4.25 (2H, s, CH2N ), 7.04–7.24
(7H, m, Ar H), 7.43 (1H, d = 8.4 MHz, Ar H), 7.66–7.72 (3H, m,
Ar H), 9.43 (1H, s, indole H). 13C NMR (100.62 MHz; CDCl3, 25 ◦C):
24.42, 28.11, 56.68, 100.86, 111.47, 118.47, 120.41, 121.44, 123.45,
123.85, 124.07, 129.37, 136.60, 136.81, 137.40, 142.57, 143.50,
149.93, and 158.54.
2.2. Synthesis of the proligands
2.2.1. Compound A
Palladium(II)acetate
(50 mg,
0.2 mmol),
2-
dicyclohexylphosphino-2,6-dimethoxybiphenyl
(0.186 g,
0.4 mmol) N-(t-butoxycarbonyl)-indole-2-boronic acid (1.24 g,
4.75 mmol), 6-bromo-2-pyridine-carboxaldehyde (0.71 g,
3.82 mmol), and K3PO4 (2.233 g, 10.5 mmol) were mixed in
10 mL of distilled n-butanol in a 50 mL Schlenk flask under nitro-
gen atmosphere. The resulting mixture was heated to 80 ◦C and
stirred for 2 h. The reaction mixture was allowed to cool to room
temperature, then, it was filtered. The solvent was distilled off by
rotary evaporation. The crude product was purified via column
chromatography on silica gel using 9:1 hexane/ethyl acetate as
the eluent, obtaining the product as white solid (1.06 g, 86%).
1H NMR (400 MHz, CDCl3, 25 ◦C): 1.35 (s, 9H, CH3), 6.87 (s, 1H,
indole H), 7.29 (d, J = 7.3 Hz, 1H, Py H), 7.40 (t, 1H, Py H), 7.62
(d, J = 7.3 Hz, 1H, Py H), 7.75 (m, 1H, indole H), 7.95 (m, 2H,
indole H), 8.19 (d, J = 8.4 Hz, 1H, indole H), 10.11 (s, 1H, HC O).
13C NMR (100.62 MHz, CDCl3, 25 ◦C): 27.90 (CH3), 84.01 (C(CH3)3),
112.31, 115.34, 120.08, 121.42, 123.38, 125.66, 127.64, 137.34,
138.00, 150.07, 152.28, 154.10, 193.68 (CO).
2.2.4. Synthesis of H2L2
n-Butyl lithium (1.45 mL, 3.63 mmol, 2.5 M in hexane) was
added dropwise to a solution of 1-bromo-2-isopropylbenzene
(678 mg, 3.41 mmol) in dry diethyl ether (10 mL) at 0 ◦C. The color-
less solution was warmed to room temperature and stirred for 3 h.
Then, the solution was added dropwise to a dry diethyl ether (5 mL)
solution of compound B (434 mg, 1.14 mmol) at −78 ◦C. The yellow
solution was warmed to room temperature and stirred for 30 min.
2.2.2. Compound B
To a solution of tert-butyl 2-(6-formylpyridin-2-yl)-1H-indole-
1-carboxylate (0.86 g, 2.67 mmol) in toluene (20 mL) was added

30
G. Li et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 28–34
The color turned to red. The reaction was followed by TLC and then
quenched with NH4Cl (aq) at 0 ◦C. The organic phase was separated
and reserved. The aqueous phase was extracted with diethyl ether
(3 × 30 mL). The combined organic phases were washed with water
(2 × 30 mL) and brine (1 × 30 mL). The organic phase was dried over
Na2SO4. The solvent was distilled off by rotary evaporation. The
crude product was purified by flash column chromatography on sil-
ica gel using hexane/diethyl ether (20/1) as eluent. The product was
isolated as a yellow solid (120 mg, yield: 88%). 1H NMR (400 MHz,
CDCl3, 25 ◦C): 0.96 (9H, CH(CH3)2), 0.98 (3H, d, J = 6.8 Hz, CH(CH3)2),
1.05 (6H, d, J = 6.8 Hz, CH(CH3)2), 2.89 (2H, sept, CH(CH3)2), 3.02 (1H,
sept, CH(CH3)2), 4.19 (1H, br, NH), 5.52 (1H, s, NCH), 6.96 (2H, m,
Ar H), 7.07 (3H, s, Ar H), 7.11 (1H, d, J = 1.2 Hz, Ar H), 7.21 (1H,
m, Ar H), 7.28–7.39 (3H, m, Ar H), 7.37 (1H, d, J = 7.6 Hz, Py H),
7.57–7.64 (3H, m, Ar H), 7.68–7.71 (1H, m, Ar H), 8.97 (1H, s,
indole NH). 13C NMR (100.62 MHz, CDCl3, 25 ◦C): ı = 23.97, 24.01,
24.05, 24.30, 28.00, 28.82, 66.18, 100.43, 111.54, 117.97, 119.74,
120.26, 121.33, 123.27, 123.63, 123.71, 125.81, 126.17, 127.47,
127.72, 129.31, 136.45, 136.87, 137.30, 139.43, 137.30, 139.43,
142.56, 142.96, 146.56, 149.53, and 162.48.
NMR (400 MHz; CD2Cl2, 25 ◦C): 1.17 (12H, CH(CH3)2), 2.79 (12H,
s, NMe2), 3.45 (2H, sept, CH(CH3)2), 4.89 (2H, s, CH2N), 6.92–7.16
(7H, Ar H), 7.45 (1H, d, J = 8 Hz, Ar H), 7.57 (1H, d, J = 8 Hz, Ar H),
7.72 (1H, d, J = 8 Hz, Ar H), 7.85 (1H, t, J = 8 Hz, Ar H). 13C NMR
(100.62 MHz; CD2Cl2, 25 ◦C): 24.14 (CH(CH3)2), 26.79 (CH(CH3)2),
28.12 (CH(CH3)2), 40.68 (N(CH3)2), 66.05 (NCH2), 101.74, 116.51,
117.11, 119.44, 121.10, 122.99, 123.65 (2C), 124.57, 128.71, 131.90,
140.42, 146.48, 146.98, 147.06, 149.28, 155.10, and 164.64. Elemen-
tal analysis: Calcd for C30H39N5Zr (%): C, 64.24; H, 7.01; N, 12.49.
Found (%): C, 63.92; H, 6.95; N, 12.62.
2.3.2. L2Zr(NMe2)2 complex 2
A solution of Zr(NMe2)4 (160 mg, 0.600 mmol) in benzene (5 mL)
was added dropwise into a stirred solution of H2L2 (310 mg,
0.600 mmol) in benzene (5 mL). The solution was stirred for
30 min at room temperature. All volatiles were removed under
vacuum to yield a yellow solid that was washed with pentane.
The light yellow product was obtained in 93% yield (380 mg,
0.56 mmol). 1H NMR (400 MHz; CD2Cl2, 25 ◦C): 0.09 (3H, d,
J = 9.6 Hz, CH(CH3)2), 0.57 (3H, d, J = 8.8 Hz, CH(CH3)2), 1.04–1.11
(9H, CH(CH3)2), 1.31 (3H, d, J = 9.2 Hz, CH(CH3)2), 2.57 (1H, sept,
CH(CH3)2), 2.65 (6H, s, Zr N(CH3)2), 2.95 (1H, sept, CH(CH3)2), 3.00
(1H, s, Zr N(CH3)2), 3.33 (1H, sept, CH(CH3)2), 6.06 (1H, s, NCHAr),
6.70 (1H, m, Ar H), 6.95–7.19 (9H, m, H Ar), 7.49 (1H, d, J = 11.2 Hz,
2.2.5. Synthesis of H2L3
Molecular sieves were added to a solution of 6-bromo-2-
pyridine-carboxaldehyde (0.205 g, 1.1 mmol), 2, 4-dimethyl-6-(1-
phenylethyl)benzenamine (0.225 g, 1 mmol) and p-toluenesulfonic
acid. The resulting mixture was refluxed for 3 h. After cooled to
room temperature, the solvent was distilled off by rotary evapora-
tion. The crude product was purified via column chromatography
on silica gel using 9:1 hexane/diethyl ether as the eluent. The
removal of the solvent by rotary evaporation afforded light yel-
low solid (yield: 87%). 1H NMR (400 MHz; CDCl3, 25 ◦C): 1.58 (3H,
d, J = 7.2 Hz, ArCHCH3), 2.09 (3H, s, CH3), 2.33 (3H, s, CH3), 4.22 (1H,
t, J = 7.2 Hz, Ar CH CH3), 6.96 (2H, d, J = 13.2 Hz, Ar H) 7.09–7.19
(4H, m, Ar H), 7.58 (1H, d, J = 7.8 Hz, Ar H), 7.68 (1H, t, J = 7.8 Hz,
CH N), 7.99 (1H, s, Ar H), 8.14 (1H, d, J = 6.9 Hz, Ar H). 13C NMR
(100.62 MHz; CDCl3, 25 ◦C): 18.67, 21.36, 21.81, 39.78, 120.07,
125.81, 125.88, 126.02, 127.90, 128.40, 129.75, 129.90, 133.98,
136.04, 139.09, 141.95, 146.65, 147.13, 155.81, and 162.66.
The Suzuky–Miyaura cross coupling reaction was carried out
by the same procedure described for the synthesis of com-
pound A (yield: 78%). 1H NMR (400 MHz; CDCl3): 1.59 (3H, d,
J = 7.81 Hz, ArCHCH3), 2.15 (3H, s, CH3), 2.21 (3H, s, CH3), 4.51 (1H,
m, ArCHCH3), 6.77 (2H, d, J = 10.7 Hz, Ar H), 6.87(2H, s, Ar H),
6.97–7.36 (10H, m, Ar H), 7.73 (1H, d, J = 9.7 Hz, Py H), 8.03(1H,
t, Py H), 8.17 (1H, s, CH N), 9.30 (1H, s, indole H). H2L3 was
then synthesized by reduction of the imino moiety as described for
ligand H2L1 (yield: 40%). 1H NMR (400 MHz; C6D6, 25 ◦C): 1.49 (3H,
d, J = 7.2 Hz, ArCHCH3), 2.19 (3H, s, CH3), 2.29 (3H, s, CH3), 3.89 (1H,
br, NH ), 4.07 (2H, s, Py CH2N ), 4.43 (1H, q, J = 7.2 Hz, ArCHCH3),
6.69 (1H, d, J = 8 Hz, Ar H), 6.90 (2H, s, Ar H), 6.98 (1H, t, J = 7.2 Hz,
Ar H), 7.00–7.36 (10H, m, Ar H), 7.73 (1H, d, J = 7.8 Hz, Py H),
9.21 (1H, s, indole NH); 13C NMR (100.62 MHz; C6D6, 25 ◦C): 18.94,
21.17, 23.01, 40.04, 54.96, 101.33, 111.86, 118.16, 119.93, 120.71,
121.69, 123.67, 126.36, 127.82, 128.06, 128.30, 128.89, 129.86,
130.56, 131.47, 132.13, 132.17, 136.94, 137.10, 138.53, 143.68,
147.04, 150.39, and 159.47.
H
Ar), 7.58 (1H, d, J = 10.8 Hz, H Ar), 7.72–7.77 (2H, H Ar).
13C NMR (100.62 MHz; CD2Cl2, 25 ◦C): 22.31 (CH(CH3)2), 23.96
(CH(CH3)2), 25.22 (CH(CH3)2), 25.79–25.90 (3C, CH(CH3)2), 28.03
(CH(CH3)2), 28.58 (CH(CH3)2), 28.79 (CH(CH3)2), 40.92 (N(CH3)2),
42.41 (N(CH3)2), 75.25 (NCH), 101.88, 116.74, 117.03, 119.17,
119.63, 121.16, 123.10, 123.96, 124.54, 124.63, 125.59, 126.53,
127.71, 128.74, 130.48, 132.05, 140.71, 141.81, 146.23, 146.91,
147.11, 147.26, 147.48, 147.95, 154.84, and 169.66. Elemental anal-
ysis: Calcd for C39H49N5Zr (%): C, 68.98; H, 7.27; N, 10.31. Found (%):
C, 69.21; H, 7.53; N, 10.12.
2.3.3. L3Zr(NMe2)2 complex 3
Complex 3 was synthesized following a similar procedure to
that used for complex 2 (yield: 78%). 1H NMR (400 MHz; C6D6,
25 ◦C): 1.60 (3H, d, J = 7.2 Hz, CH3CHAr), 2.14 (3H, s, CH3 Ar), 2.27
(3H, s, CH3 Ar), 2.70 (6H, s, NMe2), 2.83 (6H, s, NMe2), 4.11 (1H,
d, J = 20.5 Hz, NCHH Py), 4.33 (1H, d, J = 20.5 Hz, NCHH Py),
4.99 (1H, m, CH3CHAr), 6.03 (1H, J = 7.7 Hz, Ar H), 6.83 (1H, t,
Ar H), 6.96–7.26 (10H, m, Ar H), 7.42 (1H, t, J = 7.4 Hz, Py H),
7.83 (2H, d, J = 8.6 Hz, Ar H), 7.88 (2H, d, J = 7.8 Hz, Ar H). 13C NMR
(100.62 MHz; C6D6, 25 ◦C) 18.77, 21.38, 22.71, 24.05, 39.20, 40.56,
40.87, 63.57, 102.86, 115.75, 116.39, 116.75, 120.19, 121.66, 123.77,
125.65, 126.32, 129.75, 132.50, 133.05, 136.24, 139.41, 143.33,
146.57, 147.62, 147.68, 149.44, 155.22, and 165.27.
2.4. General procedure for ethylene and propene polymerization
The polymerization experiments were carried out in a magnet-
ically stirred flask (250 mL) or in a Büchi glass autoclave (500 mL).
Under nitrogen atmosphere, the required equivalents of AliBu2H
were added to a solution of the precatalyst in toluene (2 mL) and
then stirred for 10 min at room temperature. The reactor vessels
were charged sequentially with toluene, dried MAO and a solution
of the precatalyst in toluene. The stirred mixture was thermostated
at the required temperature and then the monomer gas feed was
started. After the prescribed time, the polymerization mixture was
poured into acidified ethanol. The polymers were filtered, washed
with ethanol, and dried in vacuum oven at 40 ◦C overnight.
2.3. Synthesis of the complexes
2.3.1. L1Zr(NMe2)2 complex 1
A solution of Zr(NMe2)4 (122 mg, 0.456 mmol) in benzene (5 mL)
was added dropwise into a stirred solution of H2L1 (175 mg,
0.456 mmol) in benzene (5 mL). The solution was stirred for 30 min
at ambient temperature. All volatiles were removed under vac-
uum to yield a yellow solid which was washed with pentane.
The light yellow product was obtained in 70% yield (180 mg). 1H

G. Li et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 28–34
31
1H NMR analysis of complex 1 indicated a “time-averaged” Cs-
symmetry in solution, as suggested, e.g., by the presence of one
singlet for the CH2Namido methylene protons at 4.89 ppm and of
one sharp singlet for the Zr NMe2 amido protons. In contrast, the
1H NMR spectra of complexes 2 and 3 suggested a C1-symmetry
in solution. For complex 2, three septets were observed for the
methine protons of isopropyl groups and two sharp singlets for the
Zr NMe2 protons (2.65 and 3.00 ppm). In the spectrum of com-
plex 3 the two protons of the methylene bridge appear as an AB
pattern, and two sharp singlets for Zr NMe2 (2.85 and 2.70 ppm)
were observed. The 1H NMR spectrum of complex 3 showed the
presence of two species at a ratio of ca. 9:1. A plausible explana-
tion is the coexistence of two rotational diasteroisomers due to the
hindered rotation around the C N bond of the 4,6-dimethyl-2-(2-
phenylethyl)anilido moiety [9].
3. Results and discussion
3.1. Synthesis and characterization of proligands and complexes
Indolyl-pyridylamine proligands H2L1 and H2L2 were syn-
thesized via the Suzuky-Miyaura cross-coupling reaction [4a,8]
of 6-bromo-2-pyridine-carboxaldehyde with N-Boc-indole-2-
boronic acid (Boc = tert-butyloxycarbonyl), affording tert-butyl
2-(6-formylpyridine-2-yl)-1H-indole-1-carboxylate (A) as yellow
solid in 86% yield (see Scheme 1). A condensation reaction between
A and 2,6-diisopropylaniline gave the indolylpyridylimine com-
pound B as light yellow solid (yield: 78%); subsequent reduction
with NaBH3CN or, alternatively, alkylation with 2-iPrC6H4Li, gave
indolyl-pyridylamine proligands (yields: 85% for H2L1, 88% for
H2L2, respectively). H2L3 was synthesized via the condensation
reaction of 4,6-dimethyl-2-(2-phenylethyl)aniline with 6-bromo-
2-pyridine-carboxaldehyde and subsequent Suzuky-Miyaura
cross-coupling reaction with N-Boc-indole-2-boronic acid [4a],
affording B as a yellow solid (71% yield). After reduction with
NaBH3CN and Boc removal by silica gel 60, H2L3 was obtained in
40% yield. All the compounds were fully characterized by NMR
spectroscopy.
3.2. Ethylene and propene polymerization
Indolylpyridylamido complexes 1–3 were tested as precata-
lysts for ethylene polymerization in combination with AliBu2H as
alkylating agent (in view of the well-known difficult alkylation of
dimethylamido group 4 complexes by MAO alone) [10] and MAO
as ionizing activator. As explored in previous papers [4] this is the
most efficient cocatalytic system for this class of complexes. The
main polymerization data are summarized in Table 1, where the
results previously reported [4] for the analogous pyrrolylpyridy-
lamido zirconium complexes (see Chart 1) are also displayed for
Complexes 1–3 were synthesized in benzene by treat-
ment of the corresponding proligands with one equivalent of
tetrakis(dimethylamido)-zirconium(IV) at room temperature, and
isolated as light-yellow solids in high yield (70% for 1, 91% for 2,
78% for 3, respectively).
Scheme 1. Synthesis of proligands H2L1–3 and complexes 1–3. General conditions: (i) K3PO4, Pd(OAc)2/sphos (1:2), n-Butanol, 80 ◦C; (ii) refluxing toluene; (iii)
NaBH3CN/HCOOH for H2L1 and H2L3; 2-iPrC6H4Li for H2L2. In the case of ligand H2L3 step (ii) preceded step (i).

32
G. Li et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 28–34
Table 1
comparison. Complexes 1 and 2 were active ethylene polymeriza-
Ethylene polymerization results.a
tion catalysts (runs 1 and 2), producing highly linear polyethylenes
(Tm: 135–136 ◦C). As previously observed [4], the introduction of
a bulky substituent on the carbon bridging the pyridyl and the
anilido moieties resulted in a more active catalyst (a 2-fold increase
is observed). In comparison with the corresponding pyrrolylpyridy-
lamido zirconium catalysts (runs 4 and 5), the activities of 1
and 2 are comparable. GPC analysis of the polyethylene sam-
ples produced by 1 and 2 revealed ultra high molecular weights
(710 kg/mol for 1; 1900 kg/mol for 2) and slightly broad molec-
ular weight distributions, in line with the results achieved by the
pyrrolylpyridylamidocatalysts. On thecontrary, complex3, bearing
Run
Precatalyst
Yield (g)
Activityb
Tm (◦C)
Mw (kg/mol)
Mw/Mn
1
2
3
4
5
1
2
3
0.42
0.84
Trace
0.54
0.90
1.4
2.9
–
1.8
3.1
135
136
–
137
137
710
1870
–
1850
1410
2.8
3.9
–
2.1
2.1
I-ac
I-b
a
General conditions: catalyst: 2.5 ␮mol; toluene: 100 mL; cocatalyst:
AliBu2H/Zr = 30, Al(dried MAO)/Zr = 1000; ethylene pressure: 1 atm; polymerization
time: 7 min.
b
Activity: kgPE (mmol(Zr) h atm)−1
Ref. [4a].
.
c
Table 2
Propene polymerization results.a
Run
Precatalyst
T (◦C)
Activityb
[mmmm] %
Regioinversion %
Tm (◦C)
Mw (kg/mol)
Mw/Mn
6
7
8
1
1
2
2
25
75
25
75
25
25
25
75
198
40
121
83
Trace
4
98
86
82
96
95
–
73
95
93
5.4
6.3
1.3
3.9
–
3.0
1.4
2.4
130
115
152
143
–
–
150
140
19
15
861
103
–
39
950
16
1.2
1.1
1.6
1.7
–
1.4
3.5
2.2
9
10
11
12
13
3
I-ac
I-bd
I-bd
14
a
General condition: precatalyst: 10 ␮mol; toluene: 100 mL; cocatalyst: AliBu2H/[Zr] = 30, Al(MAO)/[Zr] = 1000; polymerization time: 60 min; propene pressure: 6 atm.
b
c
Activity: kgPP (mol[Zr] h atm)−1
Ref. [4a].
.
d
Ref. [4b].
Scheme 2. Catalytic cycle for the polymerization of propene promoted by complexes 1–2.

G. Li et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 28–34
33
a substituted 4,6-dimethyl-2-(2-phenylethyl)anilido moiety, was
inactive for ethylene polymerization (run 3). A similar finding was
reported by Coates et al. [3], for an arylpyridylamido hafnium cata-
lyst bearing a ligand with two 4-tert-butylphenyl group in the ortho
positions of the anilido moiety, and tentatively explained suggest-
ing a possible ␩6-binding of the 4-tert-butylphenyl group to the
cationic hafnium center precluding monomer binding.
complex Ia, a deuterium labeling experiment ruled out the lat-
ter path [4a]. The ratio between n-propyl and isobutyl end-groups
is about 1:2. The proposed catalytic cycle for the polymeriza-
tion of propene promoted by complexes 1 and 2 is shown in
Scheme 2.
As observed for ethylene polymerization, complex 3 was sub-
stantially inactive also for the polymerization of propene.
Complexes 1–3 were then tested for propene polymeriza-
tion after activation with AliBu2H/MAO under 6 atm monomer
pressure. The main polymerization data are summarized in
Table 2, where the results previously reported [4] for the anal-
ogous pyrrolylpyridylamido complexes (runs 11–13) are also
displayed for comparison. The unsubstituted catalyst 1 promoted
polymerization of propene, producing isotactic polypropylene
([mmmm] = 86%) with rather low MW and quite narrow molecu-
lar weight distribution (PDI = 1.2). Interestingly, the performance in
terms of both activity and stereoselectivity are significantly better
than the corresponding unsubstituted pyrrolylpyridylamido Zr(IV)
catalyst Ia (run 6 vs run 11). Also in the case of Cs-symmetric
complex 1, the polymer microstructure is that expected for an
“enantiomorphic sites” mechanism of steric control, as previously
observed for Ia and for a Cs-symmetric arylpyridylamido Hf com-
plex [3]. In the latter case, a modification of the catalyst structure by
propene insertion into a Hf-aryl bond resulting in a C1-symmetric
active species was suggested. The same mechanism would be
unlikely for complexes 1 and I-a, for which different hypotheses
have been suggested [4a]. The isopropylphenyl-substituted C1-
symmetric indolylpyridylamido zirconium catalyst 2 also showed
better performance than the corresponding pyrrolylpyridylamido
zirconium catalyst I-b (run 8 vs run 12). In both cases, the intro-
duction of a bulky substituent on the carbon bridging the pyridine
and the aniline moieties resulted in higher stereoselectivity and
regioselectivity.
Polymerization at higher temperature (run 7) resulted in a 4 to
5-fold decrease of activity of unsubstituted catalyst 1. However,
only a slight decrease of activity was observed for polymeriza-
tion at 75 ◦C using substituted catalyst 2 (run 9), while a greater
loss of activity was observed for the corresponding pyrrolyl cata-
lyst I-b (run 13). The features of the polypropylene sample derived
from catalyst 2 at high temperature (run 9), in terms of molecular
weight (>100 kg/mol), melting point (143 ◦C) and narrow molecu-
lar weight distribution (PDI = 1.7), are comparable to those of the
polymers obtained by the highly optimized metallocene catalysts
(Mw = 95 kg/mol, Tm = 142 ◦C, PDI = 2, polymerization performance
at 90 ◦C) [1a].
4. Conclusions
We have reported the synthesis and characterization of three
new zirconium complexes bearing dianionic indolyl-pyridylamido
[−NNN−] tridentate ligands. All complexes have been tested as cata-
lysts for ethylene and propene polymerization after activation with
AliBu2H/MAO.
Complexes 1 and 2 were able to catalyze the ethylene poly-
merization under mild conditions (25 ◦C and 1 atm of monomer
pressure), yielding ultrahigh-molecular-weight PE’s (Mw up to
1900 kg/mol).
They also promote polymerization of propene, producing iso-
tactic polypropylene ([mmmm] up to 96%) with narrow molecular
weight distribution (PDI = 1.2).
13C NMR analysis of the obtained polypropylenes revealed
a microstructure in agreement with the “enantiomorphic sites”
mechanism of the stereoselective propagation. The analysis of the
chain end groups suggests that 1,2 insertion is the main regio-
chemistry of propagation. The analysis of the regioinverted units in
the polymer chains indicate that primary and secondary monomer
insertions occur with the same enantioface selectivity.
Both catalysts show, in the propene polymerization, higher
activity and stereoselectivity than the corresponding pyrrolyl-
pyridylamido Zr(IV) catalysts suggesting that the bulkier and more
electron donating indolide group has a beneficial effect on the per-
formance of this class of complexes.
On the other hand, complex 3 bearing the substituted 4,6-
dimethyl-2-(2-phenylethyl) anilido moiety was inactive for both
ethylene and propene polymerization. A plausible explanation is
the ␩6-binding of the phenyl group to the cationic zirconium center
which may preclude monomer binding [3] although steric effects
cannot be excluded.
These results confirm that the effects of variation of structural
parameters on reactivity and stereoselectivity are difficult to pre-
dict also for the members of a single class of catalysts.
Acknowledgements
For all the polypropylene samples, some regiodefects, deriving
from head-to-head or tail-to-tail misinsertions, were detected by
NMR analysis [11,12]. As previously observed for arylpyridylamido
[2c] and other “post-metallocene” [4,11b] catalysts, regioinverted
units with vicinal methyls in threo configuration in the Fischer pro-
jection were exclusively present, indicating that primary (1,2) and
secondary (2,1) monomer insertion occur with the same enantio-
face selectivity.
The authors are grateful to Dr. Mariagrazia Napoli for GPC anal-
yses and to Dr. Patrizia Oliva for some NMR experiments. This work
was supported by the University of Salerno (FARB Grant).
Appendix A. Supplementary data
In the 13C NMR spectrum of the low molecular weight
polypropylene sample produced by catalyst 1, additional low
intensity resonances were detected (Fig. S4 in the supporting infor-
mation). According to the literatures [13], low intensity resonances
at 45.4, 23.7, 21.8, 20.5 ppm are attributable to isobutyl end groups,
while resonances at 37.6, 28.4, 17.9, 12.4 ppm are attributable
to n-propyl end groups, reasonably arising from two consecu-
tive primary (1,2) propene insertions into Zr hydrogen bonds,
generated from the reaction of amido Zr(IV) complexes with
AliBu2H. An alternative path producing n-propyl end groups
would involve termination by hydrolysis of a growing chain
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j
.molcata.2012.12.008.
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