Group 4 metal bis(chelate) complexes of 2-anilidomethylpyridine ligands: Synthesis and catalytic activity for olefin polymerization

Article abstract of DOI:10.1016/j.molcata.2011.01.004

A series of monoanionic, bidentate 2-anilidomethylpyridine ligands were synthesized and used to prepare bis(chelate) Zr(IV) and Hf(IV) bis(dimethylamido) complexes_. All complexes were characterized by NMR spectroscopy and elemental analysis. The solution structures of the complexes were more or less fluxional, depending on the substituent on the ligands. The complexes were tested as catalysts for the polymerization of ethylene and propene, in combination with different activators. Use of AlⁱBu ₂H - methylaluminoxane as the co-catalyst resulted in the generation of catalysts producing high molecular weight polyethylenes with good activities, but yielding only stereoirregular and poorly regioregular polypropylenes.

Full text of DOI:10.1016/j.molcata.2011.01.004

Journal of Molecular Catalysis A: Chemical 337 (2011) 1–8
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Group 4 metal bis(chelate) complexes of 2-anilidomethylpyridine ligands:
Synthesis and catalytic activity for olefin polymerization
Liana Annunziata, Gang Li, Claudio Pellecchia∗
Dipartimento di Chimica, Università di Salerno, Via Ponte don Melillo, I-84084 Fisciano (SA), Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of monoanionic, bidentate 2-anilidomethylpyridine ligands were synthesized and used to pre-
pare bis(chelate) Zr(IV) and Hf(IV) bis(dimethylamido) complexes. All complexes were characterized
by NMR spectroscopy and elemental analysis. The solution structures of the complexes were more or
less fluxional, depending on the substituent on the ligands. The complexes were tested as catalysts for
the polymerization of ethylene and propene, in combination with different activators. Use of AliBu2H
– methylaluminoxane as the co-catalyst resulted in the generation of catalysts producing high molecu-
lar weight polyethylenes with good activities, but yielding only stereoirregular and poorly regioregular
polypropylenes.
Received 14 September 2010
Received in revised form
17 December 2010
Accepted 6 January 2011
Available online 15 January 2011
Keywords:
Amidopyridine
Bis(dimethylamido)
Polymerization
Fluxionality
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
The design of new olefin polymerization catalysts remains a
topic of interest in both academic and industrial research. A large
variety of metal-ligand combinations has been explored, resulting
in the development of several classes of highly active homoge-
neous catalysts able to produce polyolefins with different and
controlled features (including stereoregularity, molecular weight,
MWD, comonomer incorporation, etc.) [1]. In particular, Group 4
metal complexes bearing polydentate nitrogen-based ligands, e.g.
amidinate [2], ␣-diimine [3], iminopyridine [4], iminopyrrolide [5],
amidopyridine [6–8], amidopyrrolidepyridine [9], have recently
emerged as promising catalysts. Simple aminopyridinato ligands
were widely employed, providing a variety of mono-, bis-, tris-
and tetrakis-aminopyridinato complexes [10–12]. In this frame-
work, we recently reported some bis(chelate) Zr(IV) complexes
bearing pentafluoro-N-((pyridin-2-yl)methyl)aniline ligands fea-
turing an increased size of the chelating ring with respect to
aminopyridinato complexes [13]. In this paper we report the syn-
thesis and the characterization of some new suitably substituted
N-((pyridin-2-yl)methyl)aniline ligands and their bis(chelate) zir-
conium and hafnium complexes, as well as the investigation of their
performances as catalysts for the polymerization of ethylene and
propene.
2.1. General procedures
Manipulation of sensitive materials was carried out under
a nitrogen atmosphere using Schlenk or glove box techniques.
Hexane, benzene and toluene were dried by distillation over
sodium/benzophenone; methylene chloride was dried by dis-
tillation over calcium hydride. CDCl3, CD2Cl2 and C6D6 were
dried over calcium hydride, distilled prior to use and stored
on molecular sieves. Methylaluminoxane (MAO) was purchased
from Sigma–Aldrich as a 10 wt.% toluene solution; the resid-
ual AlMe3 contained in it was removed by distilling the volatile
under reduced pressure, washing the resulting solid with dry hex-
ane and drying the obtained white powder in vacuo. Ethylene
and propene were purchased from SON and used without fur-
ther purification; 1-hexene was distilled over calcium hydride
prior to use. NMR spectra were recorded on a Bruker Avance
400 MHz, 300 MHz and 250 MHz spectrometers. Chemical shift
(ı in ppm) are referenced vs. tetramethylsilane (TMS). 13C NMR
polymer spectra were recorded on an Bruker AM-250 spectrome-
ter (62.5 MHz) in 1,1,2,2-tetrachloroethane-d2 (C2D2Cl4) at 100 ◦C
and reported vs. hexamethyldisiloxane (HDMS). Elemental anal-
ysis were measured on a Thermo Finnigan Flash EA 1112 series
C,H,N,S Analyzer. Molecular weights (Mn and Mw) and polydisper-
sities (Mw/Mn) of polyethylene and polypropylene samples were
determined by high-temperature gel permeation chromatogra-
phy (HT-GPC) using PL-GPC210 with PL-Gel Mixed A Columns,
∗
Corresponding author. Tel.: +39 089 969576; fax: +39 089 969603.
E-mail address: cpellecchia@unisa.it (C. Pellecchia).
a
RALLS detector (Precision Detector, PD2040 at 800 nm), a
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.01.004

2
L. Annunziata et al. / Journal of Molecular Catalysis A: Chemical 337 (2011) 1–8
H502 viscometer (Viscotek,), a refractive detector and a DM400
data manager. The measurements were recorded at 150◦ using
1,2,4-trichlorobenzene as solvent and narrow molecular weight
distribution polystyrene standards as reference. Some GPC mea-
surements were performed on Waters GPC-V200 RI detector at
135 ◦C using 1,2-dichlorobenzene as solvent and Styragel columns
(range 107–103). Every value was the average of two indepen-
dent measurements. Polymer melting points (Tm) were measured
by differential scanning calorimetry (DSC) using a DSC 2920 TA
instrument in nitrogen flow with a heating and cooling rate of
10 ◦C min−1. Melting temperatures were reported for the second
heating cycle.
(50 ml) at 0◦ C, was added dropwise a solution of trimethylalu-
minium (23.6 mmol) in toluene (20 ml). The mixture was stirred
for 30 min at 0 ◦C, then warmed to room temperature and stirred
overnight. The reaction was quenched by adding a sodium hydrox-
ide water solution. The organic phase was separated, while the
aqueous phase was extracted with chloroform (2× 50 ml). The
organic phases were combined, dried over MgSO4 and the solvent
was removed by rotary evaporation. The crude product was dried
in vacuum resulting in a pale yellow powder (2.24 g, 96%).
1H NMR (CDCl3, 400 MHz, 298 K): ı 1.53 (d, J = 6.8 Hz, 3H, –CH3),
4.63 (q, 2H, –CH), 4.56 (br s, 1H, NH), 6.55 (d, J = 7.9 Hz, 2H, ArH), 6.58
(t, J = 7.6 Hz, 1H, ArH), 7.17 (m, 3H, ArH), 7.42 (d, J = 7.9 Hz,1H, ArH),
7.64 (m, 1H, ArH), 8.58 (d, J = 4.8 Hz, 1H, o-PyH). 13C NMR (CDCl3,
100 MHz, 298 K): ı 23.43 (CH3), 54.92 (CH), 113.59, 117.60, 120.53,
122.20, 129.36, 137.11, 147.26, 149.45, 164.07 (Ar–C).
2.2. Synthesis of the ligands and of the complexes
Anal. found (calcd.) for C13H14N2 (%): C, 78.72 (78.75); H, 7.05
(7.12); N, 14.16 (14.13).
N-((pyridin-2-yl)methyl)aniline (1). To a solution of 2-pyridine-
carboxaldehyde (1.50 g, 14 mmol) and aniline (1.49 g, 16 mmol) in
Complex 1a. 0.500 g of ligand 1 (2.7 mmol) were dissolved
in 20 ml of benzene. To this solution was added dropwise
a toluene solution (5 ml) of tetrakis(dimethylamido)zirconium
(0.362 g, 1.35 mmol). The resulting dark solution was stirred for 1 h
at room temperature. The solvent was then distilled off in vacuo
and the resulting powder washed twice with hexane (2× 5 ml). The
complex was recrystallized from toluene (0.652 g, 88%).
1H NMR (CDCl3, 400 MHz, 298 K): ı 2.80 (s, 6H, –N(CH3)2),
4.60 (AB pattern, J = 19.1 Hz, 2H, –CH2), 6.66 (t, J = 7.1 Hz, 1H, ArH),
7.00–7.24 (m, 6H, ArH), 7.61 (t, J = 7.8 Hz, 1H, ArH), 8.16 (d, J = 4.8 Hz,
1H, o-PyH). 13C NMR (CDCl3, 100 MHz, 298 K): ı 44.47 (N(CH3)2),
59.47 (CH2), 115.78, 116.99, 121.51, 121.89, 128.82, 129.25, 137.73,
148.10, 156.78, 164.39 (Ar–C).
´
˚
toluene (100 ml), containing 3 A molecular sieves, was added p-
toluenesulfonic acid (200 mg) at room temperature. The resulting
solution was refluxed for 18 h. After filtration, the solvent was
distilled off by rotary evaporation. The crude product was puri-
fied from dichloromethane/hexane obtaining a dark yellow solid
(2.19 g, 85%). Reduction of the imine function was achieved by using
NaBH3CN in methanol, following a previously reported procedure
[13], yielding the ligand as a light yellow powder (2.10 g, 95%).
1H NMR (CDCl3, 400 MHz, 298 K):ı 4.46 (br s, 2H, –CH2), 4.78
(br s, 1H, NH), 6.66 (d, J = 8.4 Hz, 2H, ArH), 6.70 (t, J = 7.4 Hz, 1H,
ArH), 7.16–7.20 (m, 3H, ArH), 7.33 (d, J = 7.8 Hz, 1H, ArH), 7.63 (t,
J = 7.8 Hz, 1H, ArH), 8.58 (d, J = 5.5 Hz, 1H, o-PyH). 13C NMR (CDCl3,
100 MHz, 298 K): ı 49.54 (CH2), 113.28, 117.83, 121.80, 122.30,
129.46, 136.82, 149.45 (Ar–C).
Anal. found (calcd.) for C28H34N6Zr (%): C, 61.34 (61.61); H, 6.18
(6.28); N, 15.19 (15.40).
Anal. found (calcd.) for C12H12N2 (%): C, 77.99 (78.23); H, 6.48
(6.57); N, 15.01 (15.21).
Complex 1b. 1b was obtained using a procedure similar to that
described for the synthesis of 1a, allowing to react 0.368 g of lig-
and 1 (2.0 mmol) and tetrakis(dimethylamido)hafnium (0.358 g,
1.0 mmol) in 10 ml of benzene (0.601 g, 95%).
N-((6-methylpyridin-2-yl)methyl)aniline (2). The imine ligand
2 was obtained using a procedure similar to that described
for the synthesis of 1, allowing to react 6-methyl-2-pyridine-
carboxaldehyde (2.30 g, 18 mmol) and aniline (1.76 g, 19 mmol)
(3.14 g, 88%). Subsequent reduction reaction with NaBH3CN gave
the amino ligand as a light brown powder (3.10 g, 98%).
1H NMR (CDCl3, 400 MHz, 298 K): ı 2.58 (s, 3H, –CH3), 4.36 (br
s, 1H, NH), 4.43 (s, 2H, –CH2), 6.65 (d, J = 8.4 Hz, 2H, ArH), 6.72 (t,
J = 7.3 Hz, 1H, ArH), 7.05 (d, J = 7.6 Hz, 1H, ArH), 7.14–7.19 (m, 3H,
ArH), 7.54 (t, J = 7.6 Hz, 1H, ArH). 13C NMR (CDCl3, 100 MHz, 298 K):
ı 24.46 (CH3), 49.38 (CH2), 113.20, 117.73, 118.75, 121.93, 129.39,
137.34, 148.01, 158.01 (Ar–C).
1H NMR (CDCl3, 400 MHz, 298 K): ı 2.74 (br s, 6H, –N(CH3)2),
4.56 (AB pattern, J = 18.0 Hz, 2H, –CH2), 6.59 (t, J = 7.3 Hz, 1H, ArH),
6.96–7.03 (m, 4H, ArH), 7.15–7.19 (t, J = 7.6 Hz, 2H, ArH), 7.54 (t,
J = 8.0 Hz, 1H, ArH), 8.15 (d, J = 5.1 Hz, 1H, o-PyH). 13C NMR (CDCl3,
100 MHz, 298 K): ı 44.37 (N(CH3)2), 59.87 (CH2), 115.93, 117.56,
121.59, 122.06, 128. 74, 137.94, 148.34, 157.17 (Ar–C).
Anal. found (calcd.) for C28H34N6Hf (%): C, 53.06 (53.12); H, 5.18
(5.41); N, 13.27 (13.19).
Complex 2a. 2a was obtained using a procedure similar to that
described for the synthesis of 1a, allowing to react 0.500 g of lig-
and 2 (2.5 mmol) and tetrakis(dimethylamido)zirconium (0.332 g,
1.25 mmol) in 15 ml of benzene (0.681 g, 95%).
Anal. found (calcd.) for C13H14N2 (%): C, 78.69 (78.75); H, 7.08
(7.12); N, 13.96 (14.13).
N-((6-bromopyridin-2-yl)methyl)aniline (3). The imine ligand
1H NMR (CDCl3, 400 MHz, 298 K): ı 2.33 (s, 3H, CH3), 3.11 (br
s, 6H, –N(CH3)2), 3.74 (d, J = 18.9 Hz, 1H, –CH2), 4.25 (d, J = 18.9 Hz,
1H, –CH2), 6.64 (t, 1H, ArH), 6.71 (d, J = 7.7 Hz, 1H, ArH), 7.01 (m,
3H, ArH), 7.22 (d, J = 7.2 Hz, 2H, ArH), 7.43 (t, J = 7.2 Hz, 1H, ArH).
13C NMR (CDCl3, 100 MHz, 298 K): ı 22.54 (CH3), 45.35 (N(CH3)2),
58.33 (CH2), 115.76, 116.78, 117.95, 122.30, 128.75, 137.32, 157.52,
159.74, 163.75 (Ar–C).
Anal. found (calcd.) for C30H38N6Zr (%): C, 62.57 (62.79); H, 6.55
(6.67); N, 14.49 (14.64).
Complex 2b. 2b was obtained using a procedure similar to that
described for the synthesis of 1a, allowing to react 0.260 g of lig-
and 2 (1.31 mmol) and tetrakis(dimethylamido)hafnium (0.232 g,
0.65 mmol) in 15 ml of benzene (0.410 g, 95%).
was obtained using
a procedure similar to that described
for the synthesis of 1, allowing to react 6-bromo-2-pyridine-
carboxaldehyde (1.2 g, 6.8 mmol) and aniline (0.67 g, 7.2 mmol).
The crude product was purified by column chromatography on
neutral alumina, using hexane/diethyl ether (1.35 g, 76%). The sub-
sequent reduction reaction with NaBH3CN gave the amino ligand
(1.07 g, 79%).
1H NMR (CDCl3, 400 MHz, 298 K): ı 4.45 (br d, 2H, –CH2), 6.62
(d, J = 7.7 Hz, 2H, ArH), 6.73 (t, J = 7.1 Hz, 1H, ArH), 7.17 (dd, 2H,
ArH), 7.30 (d, J = 7.7 Hz, 1H, ArH), 7.38 (t, J = 7.7 Hz, 1H, ArH), 7.48
(t, J = 7.6 Hz, 1H, ArH). 13C NMR (CDCl3, 100 MHz, 298 K): ı 50.44
(CH2), 113.5, 120.64, 123.6, 127.22, 139.30, 142.20, 147.5, 159.13
(Ar–C).
Anal. found (calcd.) for C12H11BrN2 (%): C, 54.59 (54.77); H, 3.81
(4.21); N, 10.68 (10.65).
N-(1-(pyridin-2-yl)ethyl)aniline (4). To a stirred solution of the
imine ligand (2.15 g, 11.8 mmol, obtained as above) in dry toluene
1H NMR (CDCl3, 400 MHz, 298 K): ı 2.37 (s, 3H, CH3), 3.14 (br
s, 6H, –N(CH3)2), 3.77 (d, J = 18.8 Hz, 1H, –CH2), 4.42 (d, J = 18.8 Hz,
1H, –CH2), 6.64 (t, J = 7.2 Hz, 1H, ArH), 6.72 (d, J = 7.7 Hz, 1H, ArH),
7.01–7.06 (m, 3H, ArH), 7.25 (m, 2H, ArH), 7.44 (t, J = 7.7 Hz, 1H,
ArH). 13C NMR (CDCl3, 100 MHz, 298 K): ı 22.51 (CH3), 45.10

L. Annunziata et al. / Journal of Molecular Catalysis A: Chemical 337 (2011) 1–8
3
2.4. 2D DOSY -1H NMR experiments details
(N(CH3)2), 58.66 (CH2), 113.25, 115.79, 117.24, 117.92, 122.49,
128.65, 137.38, 157.99, 160.14 164.03 (Ar–C).
Anal. found (calcd.) for C30H38N6Hf (%): C, 54.46 (54.50); H, 5.38
(5.79); N, 12.69 (12.71).
2D DOSY NMR experiments for diffusion measurements were
performed for complex 1a and complex 3a (C6D6, 6.0 mM) on a
Bruker Avance 400 spectrometer, equipped with a 5 mm Multinu-
clear inverse Z-grad z8202/104 probe, at 298 K without spinning
of the sample. The measurements of diffusion has been car-
ried out by using 2D sequence for diffusion measurement using
stimulated echo (STE) with bipolar gradients (BPP) pulses and
longitudinal eddy current delay (LED) [15]. The shape of the gra-
dients was rectangular, their duration (ı) was 1.1 ms and their
strength (G) has been varied during the experiments. All spec-
tra have been obtained using 32 K points and a spectral width of
5000 Hz. Tetrakis-(trimethylsilyl)silane (TMSS; rH (hydrodynamic
Complex 3a. 3a was obtained using a procedure similar to that
described for the synthesis of 1a, allowing to react 0.500 g of lig-
and 3 (1.9 mmol) and tetrakis(dimethylamido)zirconium (0.253 g,
0.95 mmol) in 18 ml of benzene. The complex was recrystallized
from CH2Cl2 (0.528 g, 79%).
1H NMR (CDCl3, 400 MHz, 298 K): ı 2.97 (br s, 6H, –N(CH3)2),
4.59 (br s, 2H, CH2), 6.62 (t, J = 7.0 Hz, 1H, ArH), 6.93 (d, J = 7.8 Hz,
2H, ArH), 7.11 (d, J = 7.4 Hz, 1H, ArH), 7.13 (m, 2H, ArH), 7.35 (m,
2H, ArH). 13C NMR (CDCl3, 100 MHz, 298 K): ı 43.93 (N(CH3)2),
58.40 (CH2), 113.24, 116.93, 117.97, 120.14, 127.00, 128.32, 129.51,
138.82, 141.85, 148.29, 156.23, 166.19 (Ar–C).
Anal. found (calcd.) for C28H32Br2N6Zr (%): C, 47.72 (47.80); H,
4.31 (4.58); N, 12.00 (11.94). C, 47.80; H, 4.58; N, 11.94%.
Complex 3b. 3b was obtained using a procedure similar to that
described for the synthesis of 1a, allowing to react 0.427 g of lig-
and 3 (1.62 mmol) and tetrakis(dimethylamido)hafnium (0.287 g,
0.81 mmol) in 18 ml of benzene. The complex was recrystallized
from CH2Cl2 (0.468 g, 73%).
˚
radius) ≈ rvdW (van der Waals radius) = 4.28 A) was added as inter-
nal standard (6.0 mM) to account for possible changes in solution
viscosity, temperature, and gradient strength. The most intense
signals (methyl protons of the dimethylamide ligands) were inves-
tigated for both complexes. The dependence of the resonance
intensity (I) on the gradient strength (G) is described by the fol-
lowing equation:
ꢀ
ꢁ
ꢂꢃ
ı
1H NMR (CDCl3, 400 MHz, 298 K): ı 3.10 (br s, 6H, –N(CH3)2),
4.69 (s, 2H, –CH2), 6.64 (t, J = 7.2 Hz, 1H, ArH), 6.96 (d, J = 7.7 Hz,
2H, ArH), 7.05 (m, 1H, ArH), 7.16 (m, 2H, ArH) 7.36 (m, 2H, ArH).
13C NMR (CDCl3, 400 MHz, 298 K): ı 43.66 (N(CH3)2), 58.76 (CH2),
117.21, 118.71, 120.09, 127.20, 128.23, 138.82, 156.43, 166.36
(Ar–C).
I = I0 exp −Dꢂ2Gı2 ꢃ −
3
where I is the observed intensity (attenuated signal intensity), I0
is the reference intensity (unattenuated signal intensity), D is the
diffusion coefficient, ꢂ is the nucleus gyromagnetic ratio, G is the
gradient strength, ı is the gradient duration, and ꢃ is the diffu-
sion delay. The parameters ı and ꢃ were kept constant during
the experiments, whereas G varied from 2 to 95% in 16 steps.
The values of ꢃ were 1300 and 1200 ␮s for 1a and 3a, respec-
tively.
Anal. found (calcd.) for C28H32Br2N6Hf (%): C, 42.46 (42.52); H,
3.98 (4.08); N, 10.69 (10.63).
Complex 4a. 0.700 g of ligand 4 (3.52 mmol) were dissolved in
20 ml of benzene. To this solution was added dropwise a ben-
zene solution (5 ml) of tetrakis(dimethylamido)zirconium (0.470 g,
1.76 mmol). The resulting solution was stirred for 5 h at room
temperature. The solvent was then distilled off in vacuo and
the resulting powder was washed twice with hexane (2× 5 ml).
The complex was recrystallized from dichloromethane/hexane
(0.500 g, 50%).
A nonlinear regression on I and G2 data was performed to obtain
the coefficients D for both the samples and the corresponding
internal standard signals (Dsample and DTMSS, respectively). The fol-
lowing expression (on the basis of the Stokes–Einstein equation)
was applied and numerically resolved to get the hydrodynamic
radius of each sample
1H NMR (CDCl3, 400 MHz, 298 K): ı 1.45 (d, J = 6.3 Hz, 3H, CH3),
2.51 (br s, 6H, –N(CH3)2), 5.21 (m, 1H, –CH), 6.57 (t, J = 7.0 Hz, 1H,
ArH), 6.78 (t, J = 6.5 Hz, 1H, ArH), 6.94 (d, J = 8.0 Hz, 2H, ArH), 7.09
(dd, J = 7.0 Hz, 2H, ArH), 7.18 (d, J = 7.5 Hz, 1H, ArH), 7.49 (t, J = 7.5 Hz,
1H, ArH), 8.05 (d, J = 4.2 Hz, 1H, o-Py). 13C NMR (CDCl3, 400 MHz,
298 K): ı 22.75 (CH3), 44.53 (N(CH3)2), 64.05 (CH), 115.98, 119.31,
121.73, 128.10, 137.81, 149.05, 156.37, 168.64 (Ar–C).
Anal. found (calcd.) for C30H38N6Zr (%): C, 62.69 (62.79); H, 6.58
(6.67); N, 14.69 (14.64).
Dsample
DTMSS
cTMSSrH
csamplerH
SS
=
sample
The coefficients csample and cTMSS can be estimated from the
semiempirical equation [16]
6
cx
=
2.234
x
(1 + 0.695rsolv/rH
)
where x is the sample or TMSS, and rsolv ≈ van der Waals radius
˚
of the solvent (2.7 A for C6D6). Hydrodynamic volumes were calcu-
2.3. Line shape and Eyring analysis
lated from the respective radii: VH = 4/3ꢄ (rH)3.
The van der Waals volumes (VvdW) [17] for 1a and 3a were com-
puted for DFT optimized structures using the software package DS
Viewer Pro 5.0 (Table 1).
Variable-temperature 1H NMR experiments were performed on
a Bruker AVANCE 400 in CD3C6D5 using NMR tubes equipped with
J Young valves. NMR spectral simulations were performed using
the gNMR Simulation Package by Budzelaar [14]. The activation
parameters for the processes were obtained using the linear form
of the Eyring equation:
2.5. Polymerizations
Ethylene and propene polymerizations were performed into
a 500 ml Büchi glass autoclave. The reactor vessel was charged
sequentially with MAO and a solution of the pre-catalyst in toluene
(2 ml), pre-aged for 10 min with a toluene solution of AliBu2H. The
mixture was thermostated at the required temperature and the
monomer gas feed was started and maintained at constant pressure
during the run. After the required polymerization time, the mixture
was poured into acidified ethanol. The polymers were recovered
by filtration, and dried at 40 ◦C under vacuum. The polymers were
recovered by filtration, and dried at 40 ◦C under vacuum. The main
k
T
−ꢀH =/
1
T
kB
h
ꢀS =/
R
ln
=
·
+
+
R
where k is the exchange rate constant, kB is the Boltzmann
constant and h is the Plank constant. Estimated standard devi-
ation (ꢁ) in the slope and
y
intercept of the Eyring plot
respectively. The
determined the error in ꢀH =/ and ꢀS =/
,
standard deviation in ꢀG =/ was determined from the formula
ꢁ2(ꢀG =/ ) = ꢁ2(ꢀH =/ ) + T2ꢁ2(ꢀS =/ ) − 2Tꢁ(ꢀH =/ )ꢁ(ꢀS =/ ).

4
L. Annunziata et al. / Journal of Molecular Catalysis A: Chemical 337 (2011) 1–8
Table 1
Calculated van der Waals volumes (VvdW) for 1a and 3a.
Complex
ꢃ (P30)
D1a (×10−10 m2/s)
DTMSS (×10−9 m2/s)
rH (Å)
VH (Å3)
1a
3a
1300
1200
7.03
7.07
1.12
1.17
5.86
5.85
840.38
836.08
NH
R1
N
R2
(1) R1= H
(2) R1= CH3
(3) R1= Br
(4) R1= H
R2=H
R2=H
R2=H
R2=CH3
Chart 1. Schematic representation of ligands 1–4.
Scheme 1. General synthetic scheme for complexes 1a–4a and 1b–3b.
conditions and results of ethylene polymerizations are reported in
Table 4, while those of propene polymerizations are reported in the
supplementary materials.
1H NMR data measured over the temperature range 298–353 K.
The free energy of activation associated to the fluxional process
was calculated to be ꢀG =/ = 16.39 0.04 kcal mol−1 at 298 K. Acti-
vation parameters resulted to be ꢀH =/ = 14.6 0.3 kcal mol−1 and
ꢀS =/ = −6.1 0.5 cal mol−1 K−1. Exchange phenomena in octahe-
dral complexes may occur either via a dissociative mechanism,
involving metal-donor atom bond rupture, or via a non-dissociative
mechanism, involving a trigonal twist with retention of the six-
coordinated geometry [18]. For instance, a dynamic interchange
mechanism, involving N-pyridine bond cleavage and formation
of a configurationally labile five-coordinate intermediate, was
previously proposed for bis(8-quinolato) group 4 metal alkox-
ide [19] and alkyl [20] complexes, and, more recently, for
bis(amidopyridine)zirconium complexes [12b]. In the latter case,
N-pyridine dissociation is promoted by the presence of trans
dimethylamide, a strong trans directing ligand [12b]. On the other
hand, for several other bis(chelate) octahedral complexes a non-
dissociative mechanism was proposed on the basis of the observed
negative values of the entropy of activation [21–23]. The latter
mechanism could be operative also for complex 1a, based on
the negative ꢀS =/ value. The different behavior with respect to
the above mentioned closely related bis(amidopyridine)zirconium
complexes [12b] could be justified hypothesizing the preferen-
tial formation of isomer A, having mutually trans pyridine ligands
(which would also agree with the higher stereo rigidity at room
temperature).
3. Results and discussion
The ligands 1–4 were prepared by condensation reactions
between ortho-substituted anilines and the proper pyridinecar-
boxyaldehyde, followed by either reduction (NaBH3CN) or
reductive methylation (AlMe3), and then hydrolysis (Chart 1).
The corresponding bis(dimethylamido) zirconium (IV)
and hafnium(IV) complexes were prepared by reaction of 2
equiv. of the ligand and tetrakis(dimethylamido)zirconium or
tetrakis(dimethylamido)hafnium in benzene, producing com-
plexes 1a–4a and 1b–3b in good yields (60–90%) as yellow-brown
powders (Scheme 1). The ligands and the complexes were charac-
terized by elemental analysis and 1H and 13C NMR spectroscopy.
For octahedral bis(chelate) complexes 1a–3a and 1b–3b five iso-
mers are possible, three of which (A-C) are chiral while the other
two (D, E) are not (Scheme 2). 1H NMR analysis provided significant
information on the solution structures of the complexes.
The 1H NMR spectrum of 1a showed, at room temperature, a
pattern of signals attributable to a C2-symmetric species (isomer
A or B of Scheme 2), as indicated by the presence of one sin-
glet at 3.14 ppm for the two dimethylamido groups and one set
of signals for the remaining protons of the two ligands, with the
Py-CH2-NAr methylene resonance appearing as a pair of AB dou-
blets. Variable temperature NMR analysis between 298 and 353 K
showed coalescence of the AB pattern of the methylene group
in a singlet, while all other resonances sharpened, indicating the
occurrence of a fluxional equilibrium involving enantiomer inter-
conversion at high temperature (see Fig. 1). Kinetic parameters
were calculated for complex 1a using line shape analysis of the
The solution structure of hafnium complex 1b is similar to that
of 1a, as indicated by VT 1H NMR analysis: in particular, the Py-CH2-
NAr methylene resonance appears as a pair of AB doublets at room
temperature and becomes a sharp singlet upon increasing the tem-
perature up to 353 K, with ꢀG =/ = 14.6 0.2 kcal mol−1 at 298 K.
Calculated activation parameters are ꢀH =/ = 12.4 0.3 kcal mol−1
and ꢀS =/ = −7.5 0.5 cal mol−1 K−1
.
Scheme 2. Representation of the five possible isomers for octahedral complexes 1a–3a and 1b–3b.

L. Annunziata et al. / Journal of Molecular Catalysis A: Chemical 337 (2011) 1–8
5
Fig. 1. 1H NMR spectra of the complex 1a at variable temperature (CD3C6D5).
The room temperature 1H NMR spectra of compounds 2a and 2b
also showed patterns of signals indicative of C2 symmetric struc-
tures (A or B isomers), with the Py-CH2-NAr methylene resonance
appearing as AB systems. In these cases, however, the 1H NMR
spectra remain substantially unchanged also increasing the tem-
perature up to 353 K, suggesting a higher stereorigidity of 2a and
2b vs. 1a and 1b, reasonably as a consequence of the introduction
of a methyl substituent in the ortho-position of the pyridine ring.
In agreement with this hypothesis, the 1H NMR spectra of both 2a
and 2b resulted unchanged at low temperature (193 K).
equilibria [24–26]. The 2D DOSY 1H NMR method assesses trans-
lational motion of the analyte in solution through determination
of the diffusion coefficient, a quantity inversely related to its effec-
tive hydrodynamic volume. The diffusion coefficients (D) can be
related to the hydrodynamic radius (r) of the species in solution by
the Stokes–Einstein equation (Eq. (1)) where T is the temperature,
ꢅ is the viscosity of the solvent, r is the hydrodynamic radius of the
molecule or assembly, and K is the Boltzmann constant.
KT
D =
(1)
6ꢄꢅr
The room temperature 1H NMR spectra of compounds 3a and 3b
showed, in both cases, a single set of sharp resonances. In particular,
The Py-CH2-NAr methylene signals appeared as sharp singlets at
4.6 and 4.7 ppm, respectively. Variable-temperature NMR analysis
between 193 and 353 K did not show any significant change in the
spectra, suggesting that 3a and 3b are in the fast exchange regime
in this range of temperatures, or, less probably, that symmetric iso-
mers are preferentially formed. The same behavior was previously
observed for the analogous bis (perfluorophenyl)methylamido-6-
bromopyridine)zirconium(IV) bis(dimethylamide) complex [13],
and could be related to the lower basicity of bromo-pyridine.
Following the suggestion by a reviewer that the fluxional pro-
cesses observed in solution may involve the formation of dimers (or
higher aggregates), we analyzed complexes 3a (which is fluxional)
and, for comparison, complex 1a (which is stereorigid at room tem-
perature) by 2D DOSY 1H NMR measurements. This NMR technique
provides information of the translational diffusion coefficient of the
molecular species in solution and it has been successfully used for
characterizing various organometallic systems and supramolecular
structures in solution, including ones involved in monomer/dimer
The diffusion coefficients of both species (3a and 1a) have been
determined in analogous 2D DOSY 1H experiments in C6D6 with
identical solvent viscosity and at the same temperature. Since
the other parameters are identical, the size of the corresponding
species can be compared based on their diffusion coefficients. The
experimentally determined values of D and rH and the calculated
ones for both complexes are listed in Table 2.
The close agreement between the D and rH data evaluated for
solutions of the two complexes suggests that 3a is very similar in
size to the monomeric species 1a. For comparison, the hydrody-
namic radii determined from experimental 2D DOSY 1H results for
Table 2
2D DOSY 1H experimental results compared to estimated data from DFT optimized
structures.
Complex
D (×10−10 m2 s−1
)
rH (Å)
VH (Å3)
rꢀ (Å)
1a
3a
7.03
7.07
5.86
5.85
842.48
838.18
6.25
6.21

6
L. Annunziata et al. / Journal of Molecular Catalysis A: Chemical 337 (2011) 1–8
Table 3
nated, is 0.35 ppm, while in the case of 3 and 3a resulted to be only
0.19 ppm. For the Hf complexes, ꢀı is 0.43 for 2 vs. 2b, while is only
0.14 for 3 vs. 3b. These findings agree with the hypothesis that com-
plexes 3a and 3b are in fast fluxional equilibrium in solution due
to a weaker coordination of bromopyridine. In general, the trend of
the ꢀı values is in nice agreement with the VT NMR analysis, i.e.
with the stereorigidity increasing in the order 3a,b < 1a,b < 2a,b.
The 1H NMR spectrum of complex 4a, bearing a chiral center on
the ligand framework, showed one singlet at 2.50 ppm for the two
dimethylamide ligands and one set of signals for the protons of the
two chelating ligands. This picture suggests that interconversion
between different isomers of 4a is fast on the NMR timescale at
room temperature. The ꢀı value (0.24 ppm) in this case is slightly
higher than that observed for (fluxional) 3a, but lower than that
of (stereorigid) 1a. Variable-temperature 1H NMR studies between
298 and 193 K showed little change in the main pattern of signals,
consisting in a broadening of the resonances and the appearance
of some additional minor resonances, possibly arising from other
diastereomers.
Chemical shift (ppm) for the aromatic proton H4 in the free ligand and complexes
(CDCl3, 298 K).
N
R
N
H2
H4
H3
.
Ligand
ı (H4)
Complex
ı (H4)
ꢃı (H4)a
1
2
3
7.33
7.05
7.30
1a
1b
2a
2b
3a
3b
4a
7.02
6.97
6.70
6.62
7.11
7.16
7.18
0.31
0.36
0.35
0.43
0.19
0.14
0.24
4
7.42
a
ꢀı(H4) = ı(H4)free ligand − ı(H4)complex.
3.1. Polymerizations
1a and 3a were compared with the molecular radii (rꢀ, Table 2) esti-
mated from DFT optimized structures by measuring the lengths
between centers of distant hydrogen atoms on the molecular
periphery. The good agreement observed supported the hypothe-
sis of the monomeric nature of 3a in solution at room temperature.
This was also confirmed by the comparison of the hydrodynamic
and calculated volumes of complexes 1a and 3a (assuming that they
have a spherical shape) with the volume of an analogous complex
[13] calculated from X-ray crystal structure by dividing the crys-
All the complexes were tested as pre-catalysts for the poly-
merization of ethylene under 6 atm of monomer pressure using
different activators. The obtained polymers were characterized by
NMR spectroscopy, DSC and GPC analyses. The main polymeriza-
tion conditions and results are collected in Table 4.
Polymerization of ethylene resulted in all cases in the produc-
tion of linear polyethylenes (m.p. = 133–137 ◦C), with moderate
to good activities. Several co-catalysts were tested in com-
bination with complex 1a (runs 1–3, 6, Table 4): use of
methylalumoxane (MAO) alone resulted in very poor activ-
ity, probably owing to difficult alkylation of 1a by MAO, as
previously observed for other dimethylamido complexes [28].
On the contrary, a mixture of AliBu2H and MAO was a very
efficient activator. The less reactive dimethylamido zirconium
complexes require the presence of AliBu2H to generate the
Zr–H or Zr–alkyl bonds where the polyinsertion may start.
The reaction between AliBu2H and dimethylamido zirconium
catalysts has been previously studied by Kim for rac-[ethylen-1,2-
bis(1-indenyl)zirconium(IV)bis(dimethylamido)] complex which,
in combination with AliBu2H, produced zirconium hydride com-
plex [28c]. AliBu2H alone and the mixtures of AliBu2H and
[CPh3][B(C6F5)4] did not result in good catalytic activity. Thus, the
tallographic unit cell volume by the number of molecular entities
3
˚
contained in the unit cell (775.93 A ). In conclusion, these experi-
ments suggest that the dynamic processes observed in solution for
complex 3a are not attributable to monomer/dimer equilibration
and more reasonably can be related to intramolecular rearrange-
ments.
An indication of the weaker coordination of the pyridine moiety
to the metal center in complexes 3a and 3b vs. e.g. 2a and 2b came
from analysis of the chemical shift difference, ꢀı, of the aromatic
protons of the pyridine moiety in the free ligands and in the com-
plexes [27]. The calculated values for the aromatic proton H4 are
reported in Table 3. For instance, the chemical shift difference in
the free ligand 2 and in the corresponding zirconium complex 2a,
for which we know that the pyridine nitrogen is strongly coordi-
Table 4
Ethylene polymerization conditions and results.
Runa
Pre-catalyst
Co-catalyst
T (◦C)
Activityb
Tm (◦C)
Mw (×103)
Mw/Mn
1
2
1a
1a
1a
1a
1a
1a
2a
3a
4a
1b
2b
3b
3b
MAO
25
25
25
50
75
50
25
25
25
25
50
50
75
0.4
1.2
78.5
94.3
255
3.0
16.9
15.3
34.8
4.8
133
134
137
136
135
136
134
135
135
134
134
134
134
N.d.
826
N.d.
3.9
3.4
5.5
6.6
2.5
2.6
2.1
2.4
3,6
2.4
9.1
N.d.
AliBu2H
3c
4c
5d
6e
7
AliBu2H/MAO
AliBu2H/MAO
AliBu2H/MAO
AliBu2H/[CPh3][B(C6F5)4]
AliBu2H/MAO
AliBu2H/MAO
AliBu2H/MAO
AliBu2H/MAO
AliBu2H/MAO
AliBu2H/MAO
AliBu2H/MAO
1324
2113
831
1057
1019
2492
2356
1466
3052
580
8
9
10
11
12
13f
2.4
1.2
64.6
N.d.
a
Conditions: pre-catalyst = 20 ␮mol; AliBu2H/Mt = 30; Al (MAO)/Mt = 1000; ethylene pressure = 6 atm; toluene = 90 ml; time = 60 min. MAO dried obtained by distilling off
the solvent from the commercial solution.
Activity = kg of polymer × (mol of Mt × h × atm)−1.
b
c
Time = 30 min.
d
Time = 10 min.
Time = 90 min, [CPh3][B(C6F5)4] = 1.1 equiv.
Pre-catalyst = 10 ␮mol; time = 10 min.; n.d. = not determined.
e
f

L. Annunziata et al. / Journal of Molecular Catalysis A: Chemical 337 (2011) 1–8
7
AliBu2H/MAO co-catalyst system was used to activate all the com-
plexes.
bly due to a weaker coordination of bromopyridine. All complexes
after activation with AliBu2H/MAO efficiently promoted the poly-
merization of ethylene, producing polymers with generally broad
molecular weight distributions. The molecular weight distributions
of the polymers are narrower for the more stereorigid complexes.
In general, hafnium complexes resulted less active than their zir-
conium analogues. The activities of the bis(chelate) Zr complexes
reported here are significantly higher than those of the previously
reported complexes bearing perfluorophenylamidopyridine lig-
ands [13], indicating a deleterious effect of electron-withdrawing
substituent on the activity of this class of catalysts, as previously
observed for metallocene catalysts. The same catalytic systems
were tested also in the polymerization of propene, resulting in
rather poor performance in terms of activities, stereo- and regio-
specificities. Extension of these studies to other complexes bearing
ligands of this class for the polymerization of olefins as well as of
other unsaturated hydrocarbon monomers are in progress and will
be reported subsequently.
The activity of the system 1a/AliBu2H/MAO was tested at dif-
ferent temperatures: the activity increases while the temperature
increases from 25 to 75 ◦C (runs 3–5, Table 4).
The activities of the complexes are significantly affected by the
substituents (Scheme 1, R1 and R2) of the ligand: zirconium com-
plexes 2a and 3a, bearing ortho substituents on the pyridine moiety,
afforded less active catalyst than 1a (cf. runs 3, 7, 8, Table 4). Com-
pound 4a, which differs from 1a only for the presence of a methyl
group on the carbon in ␣ position respect to the amido nitrogen of
the ligand, also showed a lower activity (cf. runs 3 and 9, Table 4).
Anyway, all complexes resulted more active (40–50% increased
activity) than the previously reported octahedral zirconium com-
plexes bearing perfluorophenylaminomethylpyridine ligands [13].
This finding has precedent in the behavior of metallocene cat-
alysts for which a reduced activity was generally observed as
a consequence of introducing electron-withdrawing substituents
in cyclopentadienyl (Cp) ligands [29]. On the contrary, for
bis(phenoxy-imine) group 4 metal complexes the opposite effect
was often observed [30].
Acknowledgments
The bis(amidomethylpyridine)hafnium(IV) complexes 1b–3b
afforded much less active catalysts than the corresponding
zirconium complexes 1a–3a; however, complex 3b becomes sig-
nificantly more active at 75 ◦C (run 13, Table 4).
The authors are grateful to Dr. Patrizia Oliva, Dr. Mina Mazzeo,
Dr. Stefano Milione for some NMR experiments and to Dr. Patrizia
Iannece for elemental analyses and Dr. Sebastiano D’Amora for
some polymerization experiments. The authors gratefully thank
Prof. Daniela Pappalardo (Università del Sannio) for useful discus-
sions. This work was supported by the University of Salerno (FARB).
Analysis of the polymer samples by GPC revealed in any case
high molecular weights with molecular weight distributions rang-
ing between those expected for a single site catalyst (cf. runs 7–9,
Table 4) and much broader ones (cf. runs 5, 10, 12, Table 4), depend-
ing on the pre-catalyst and the polymerization temperature. This
behavior could be related to the fluxional character of the com-
plexes, leading to different catalytic species in solution, depending
on the reaction conditions [13,23,29c], and/or to chain transfer to
Al proceeding with different rates under different conditions [31].
Polymerizations of propene were also carried out at a monomer
pressure of 6 atm, by using AliBu2H/MAO as the activator. All the
complexes resulted poorly active, affording substantially stereoir-
regular, high molecular weight polymers. E.g., catalyst 1a produced
at 25 ◦C a stereoirregular, slightly syndiotactic-enriched ([rr] = 36%)
polypropylene, having 16% regioinversions [32]. Complexes 2a
and 3a afforded even less active catalysts than 1a, producing
more regioregular (3% regioinverted units) atactic polypropylene,
and slightly isotactic-enriched, very regioregular polypropylene
([mm] = 40%), respectively. The hafnium complexes 1b–3b also
showed very low activities, producing, in all cases, regioregular
atactic polymers (see supplementary material). The poor activities
with respect to ethylene polymerizations, as well as the different
molecular weight distributions, could be also related to the poor
regiospecificity of propene insertion, resulting in a significant frac-
tion of “dormant” or less reactive propagating species.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molcata.2011.01.004
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