The role of the ionic radius in the ethylene polymerization catalyzed by new group 3 and lanthanide scorpionate complexes

Article abstract of DOI:10.1016/j.molcata.2009.10.021

A series of new monomeric group 3 and lanthanide [N,N,N]-heteroscorpionate triflate-complexes [Ln(OTf)₂(cybpamd)(THF)] {Ln = Sc (2), Y (3), La (4), Nd (5), Sm (6), Dy (7), Yb (8); OTf = SO₃CF₃; cybpamd = N,N′-dicyclohexyl-2,2-bis-(3,5-dimethyl-pyrazol-1-yl)-acetamidinate} has been synthesized and characterized. The behavior of 2-8 as catalysts in olefin polymerization was investigated after proper activation with methylaluminoxane and the comparative results are reported. The activity of the catalytic systems towards ethylene polymerization is affected by the nature of the metal center and linearly grows with the ionic radius, with the exception of the scandium derivative. From DFT calculations it was possible to correlate the activity data with computed properties of the metal-alkyl bonds of the catalytically active species. The very narrow polydispersivities showed that all the considered systems act as single-site catalysts and high-weight linear polyethylene polymers were always obtained.

Full text of DOI:10.1016/j.molcata.2009.10.021

Journal of Molecular Catalysis A: Chemical 317 (2010) 54–60
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
The role of the ionic radius in the ethylene polymerization catalyzed by new
group 3 and lanthanide scorpionate complexes
Gino Paoluccia,∗, Marco Bortoluzzia, Mariagrazia Napolib,
Pasquale Longob,∗∗, Valerio Bertolasic
a Dipartimento di Chimica, Università Ca’ Foscari di Venezia, Dorsoduro 2137, 30123 Venezia, Italy
b Dipartimento di Chimica, Università di Salerno, Via Ponte don Melillo, 84084 Fisciano, SA, Italy
c Dipartimento di Chimica e Centro di Strutturistica Diffrattometica, Università di Ferrara, via Borsari 46, 44100 Ferrara, 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 new monomeric group 3 and lanthanide [N,N,N]-heteroscorpionate triflate-complexes
[Ln(OTf)2(cybpamd)(THF)] {Ln = Sc (2), Y (3), La (4), Nd (5), Sm (6), Dy (7), Yb (8); OTf = SO3CF3; cyb-
pamd = N,Nꢀ-dicyclohexyl-2,2-bis-(3,5-dimethyl-pyrazol-1-yl)-acetamidinate} has been synthesized and
characterized. The behavior of 2–8 as catalysts in olefin polymerization was investigated after proper
activation with methylaluminoxane and the comparative results are reported. The activity of the cat-
alytic systems towards ethylene polymerization is affected by the nature of the metal center and linearly
grows with the ionic radius, with the exception of the scandium derivative. From DFT calculations it was
possible to correlate the activity data with computed properties of the metal–alkyl bonds of the catalyt-
ically active species. The very narrow polydispersivities showed that all the considered systems act as
single-site catalysts and high-weight linear polyethylene polymers were always obtained.
© 2009 Elsevier B.V. All rights reserved.
Received 4 September 2009
Accepted 20 October 2009
Available online 30 October 2009
Keywords:
Scandium
Yttrium
Lanthanum
Lanthanides
Olefin polymerization
Scorpionate
DFT
1. Introduction
ands [3] and the expected electrophilicity of group 3 metals makes
them attractive as homogeneous catalysts for Ziegler-Natta poly-
In addition to the first examples of scandium, yttrium, lan-
thanum and f-block metals homoleptic hydrotris(pyrazolyl)borate
complexes, MTp3 [1], homo- and heteroscorpionate ligands have
shown to be very interesting species for the synthesis of a wide
range of stable group 3 elements and lanthanide derivatives [2]. As
a result of the variable size of the Ln(III) ions, the predominant ionic
bonding and the well-known oxofilicity of these metals, the usual
coordinative unsaturation of their complexes, the presence of hard
donor atoms and the ligand charge are critical factors in control-
ling the coordination number, the geometry and the architecture
of their complexes and the isolation of well-defined molecular
species. Scorpionates represent an attractive and versatile choice,
due to the fine-tuning of the electronic and steric properties of these
ligands and consequently the control of the metal coordination
sphere.
merization. From a catalytic point of view the preparation of new
scorpionate derivatives of early-d and f-block elements is a cur-
rently active field of research, as this type of complexes showed
to be, among all, potentially interesting non-cyclopentadienyl
Ziegler-Natta homogeneous catalysts in olefin polymerization, as
observed for Sc, Y and lanthanides derivatives with substituted
tris(pyrazolyl)methane and tris(pyrazolyl)borate ligands [4].
Our recent research interest in the fields of organometallic
chemistry and olefin polymerization has been mainly devoted to
group 3 and lanthanide-based catalysts with polydentate ligands
containing nitrogen-donor groups [5]. We have currently extended
our studies to heteroscorpionate bis-pyrazol-1-yl-acetamidinate
anionic ligands, whose lithium, magnesium and zinc derivatives
have recently shown to be active initiators for the ring-opening
polymerization of cyclic esters [6].
Polyolefins can be obtained in the presence of several neutral
and cationic alkyl complexes of the rare-earth metals, stabilized
by both cyclopentadienyl- and non-cyclopentadienyl ancillary lig-
In the last few years group 3 metal triflates have been received
great attention for their ability to promote a wide variety of
organic reactions [7]. In this paper we report the synthesis of a
series of new neutral group 3 and lanthanide triflate-complexes
with the [N,N,N]-scorpionate ligand N,Nꢀ-dicyclohexyl-2,2-bis-
(3,5-dimethyl-pyrazol-1-yl)-acetamidinate (cybpamd), together
with the comparison of their catalytic behavior towards ethylene
polymerization.
∗
Corresponding author. Tel.: +39 0412348563; fax: +39 0412348684.
Corresponding author. Tel.: +39 089969564; fax: +39 089969603.
∗∗
E-mail address: paolucci@unive.it (G. Paolucci).
1381-1169/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2009.10.021

G. Paolucci et al. / Journal of Molecular Catalysis A: Chemical 317 (2010) 54–60
55
2. Experimental
at a 10 ◦C/min heating rate, under a flowing nitrogen atmosphere.
Specimens were sealed in aluminum pans.
2.1. Materials and methods
2.3. Synthesis of N,Nꢀ-dicyclohexyl-2,2-bis-(3,5-dimethyl-
pyrazol-1-yl)-acetamidine (cybpamd-H) (C24H38N6,
All inorganic manipulations were carried out under oxygen-
and moisture-free atmosphere in a Braun MB 200 glove-box with
a purifying unity G-II and equipped with apparatus for high
and low temperature reactions. All the solvents were thoroughly
deoxygenated and dehydrated under argon by refluxing over
suitable drying agents, while NMR deuterated solvents (Euriso-
Top products) were kept in the dark over molecular sieves. The
anhydrous triflate salts Ln(OTf)3 {Ln = Sc, Y, La, Nd, Sm, Dy, Yb;
OTf = SO3CF3} and YCl3 (Strem, Aldrich) and the organic com-
pounds 3,5-dimethylpyrazole and N,Nꢀ-dicyclohexylcarbodiimide
(Aldrich) were used as received. Butyllithium (1.6 M solution in
hexanes) was purchased from Aldrich. Bis(3,5-dimethyl-pyrazol-
1-yl)methane was synthesized from 3,5-dimethylpyrazole and
dichloromethane on the basis of a reported procedure [8] and puri-
fied by crystallization from hot cyclohexane solutions.
MW = 410.60)
A solution of bis(3,5-dimethyl-pyrazol-1-yl)methane (2.000 g,
9.8 mmol) in anhydrous THF (50 mL) was cooled to −70 ◦C, then
a 1.6 M solution in hexanes of butyllithium (6.1 mL) was added
dropwise during about half an hour by maintaining the temper-
ature as constant as possible. The resulting solution was allowed
to slowly reach −10 ◦C and then maintained at this temperature
for 20 min. The reaction mixture was cooled again at −70 ◦C and
a solution of N,Nꢀ-dicyclohexylcarbodiimide (2.02 g, 9.8 mmol) in
20 mL of THF was slowly added. Once the addition was ended, the
system was allowed to reach room temperature and left 4 h under
stirring. Cold water (30 mL) was added to quench the reaction and
THF was quite completely removed by evaporation under reduced
pressure. The crude product was extracted with diethylether (3×
50 mL) and the resulting organic fraction was dried over MgSO4.
The solvent was then removed under reduced pressure and the
residual oil was purified by chromatography on silica gel, using
a 1:1 mixture of hexane–ethyl acetate as eluent. After in vacuo
removal of the solvents the residue was dissolved in pentane
(20 mL) and the resulting solution was passed on filter paper to
remove eventual traces of unreacted bis(3,5-dimethyl-pyrazol-1-
yl)methane. Pentane was finally removed by evaporation under
reduced pressure and the product was collected as white micro-
crystals. Yield = 3.550 g, 88%.
All the polymerization operations were carried out under nitro-
gen atmosphere by using conventional Schlenk-line techniques.
Methylaluminoxane (10% in toluene, Witco) was used as a solid
after distillation of solvent. Ethylene (>98%), was purchased from
Aldrich.
2.2. Characterizations
Microanalyses (C, H, N, Cl) of ligands and complexes were
made at the Istituto di Chimica Inorganica e delle Superfici, CNR,
Padova. 1H NMR, homodecoupled 1H NMR, 1H COSY, 1H NOESY,
1
13C { H} NMR, 13C APT, HSQC and HMBC spectra were recorded at
Elemental analysis: found (%): C 69.9, H 9.30, N 20.4. Calcd. for
298 K on a Bruker Avance 300 spectrometer operating at 300 MHz
(1H) and 75 MHz (13C) and referred to internal tetramethylsilane.
The SwaN-MR and MestRe-C software packages were used for
NMR spectroscopic data treatment [9]. Mass spectra (E.I., 70 eV)
of lanthanum derivative and of the paramagnetic compounds were
recorded on a Finnigan Trace GC–MS equipped with a probe con-
troller for the sample direct inlet. The assignments were done by
comparison between theoretical and experimental isotopic clus-
ters and the most intense signals of each characterized cluster are
reported.
C34H38N6 (%): C 70.20, H 9.33, N 20.47.
2.4. Synthesis of [YCl3(cybpamd-H)] (1) (C24H38Cl3N6Y,
MW = 685.86)
A solution of cybpamd-H (0.410 g, 1.0 mmol) in THF (15 mL) was
added at room temperature to a suspension of anhydrous YCl3
(1.0 mmol, 0.195 g) in 15 mL of THF. The resulting reaction mix-
ture was allowed to react overnight at room temperature, then the
solvent was removed by in vacuo evaporation and dichloromethane
(30 mL) was added. The CH2Cl2 solution was centrifuged and subse-
quently concentrated to ca. 10 mL under reduced pressure. Hexane
was slowly added until the product separated as solid, which was
collected by filtration after about 1 h under stirring, washed with
n-hexane and dried in vacuo. Yield = 0.522 g, 86%.
Semi-empirical computational geometry optimizations were
carried out with the MOPAC2007 software package [10]. In all the
calculations the PM6 Hamiltonian [11] was used and the param-
agnetic lanthanides ions were simulated using the sparkle model
[12]. Calculations were carried out without symmetry constrains.
Scandium, yttrium and lanthanum derivatives were also optimized
with the restricted EDF1 DFT functional [13] in combination with
the ECP-based LACVP* basis set [14]. Charges derived from Mul-
liken population analysis [15]. The software used was Spartan ‘08
[16]. All the calculations were carried out on a Intel Core I7-based
x86-64 computer.
Elemental analysis: found (%): C 47.4, H 6.30, N 13.8, Cl 17.5.
Calcd. for C24H38N6 (%): C 47.58, H 6.32, N 13.87, Cl 17.55.
2.5. Synthesis of [Ln(OTf)2(cybpamd)(THF)] {Ln = Sc,
C30H45F6N6O7S2Sc, MW = 824.79 (2); Ln = Y, C30H45F6N6O7S2Y,
MW = 868.74 (3); Ln = La, C30H45F6LaN6O7S2, MW = 918.74 (4);
The samples of polyethylene for 13C NMR analysis were
prepared by dissolving polymer sample (40 mg) into tetrachloro-
dideutero-ethane (0.5 mL). The spectra were recorded at 100 ◦C
using hexamethyldisiloxane (HMDS) as internal chemical shift ref-
erence. The Gel Permeation Chromatography (GPC) analysis of the
samples were carried out at 135 ◦C by Waters instrument GPCV
2000 equipped with refractive index and viscosimeter detectors,
Ln = Nd, C30H45F6N6NdO7S2, MW = 924.08 (5); Ln = Sm,
C30H45F6N6O7S2Sm, MW = 930.20 (6); Ln = Dy,
C30H45F6N6O7S2Dy, MW = 942.34 (7); Ln = Yb, C30H45F6N6O7S2Yb,
MW = 952.88 (8); OTf = SO3CF3}
The same synthetic approach was applied for the prepara-
tion of all the 2–8 complexes. In a typical synthesis a solution of
cybpamd-H (0.410 g, 1.0 mmol) in THF (15 mL) was added at room
temperature to a THF solution (15 mL) containing 1.0 mmol of the
proper anhydrous triflate salt Ln(OTf)3 {Ln = Sc, Y, La, Nd, Sm, Dy,
Yb}. After 20 min 12.5 mL of a 0.08 M solution of butyllithium in
hexane and THF, prepared by diluting with THF the commercial
solution, was added dropwise in about 15 min. The resulting mix-
ture was allowed to react at room temperature for 12 h, then the
´
˚
using four PSS columns set consisting of, 105, 104, 103, 102 A (pore
size) – 10 ␮m (particle size). o-Dichlorobenzene was the carrier
solvent used with a flow rate of 1.0 mL/min. The calibration curve
was established with polystyrene standards. Differential Scanning
Calorimetry analysis have been carried out on a DSC 2920 appara-
tus manufactured by TA Instruments, calibrated against an indium
standard (Tm = 156.6 ◦C), with heating scans from −10 to 200 ◦C,

56
G. Paolucci et al. / Journal of Molecular Catalysis A: Chemical 317 (2010) 54–60
solvent was removed by in vacuo evaporation and dichloromethane
(30 mL) was added to the residue. The CH2Cl2 solution was cen-
trifuged to remove the solids and then concentrated to ca. 5 mL
under reduced pressure. n-Hexane was slowly added until the
crude product started to separate as solid, which was collected by
filtration after about 1 h under stirring. The product was purified by
dissolving the crude solid in 50 mL of a 20:1 diethylether–THF solu-
tion. The resulting solution was filtered and then concentrated to
about 10 mL under reduced pressure. By slow addition of hexane
the product separated out as microcrystals, which were filtered,
washed with hexane and dried in vacuo. Yields: (2) 0.558 g, 68%;
(3) 0.621, 71%; (4) 0.644 g, 70%; (5) 0.605 g, 65%; (6) 0.613 g, 65%;
(7) 0.615 g, 65%; (8) 0.701 g, 74%.
Elemental analysis for 2: found (%): C 43.5, H 5.45, N 10.15.
Calcd. for C30H45F6N6O7S2Sc (%): C 43.69, H 5.50, N 10.19. Ele-
mental analysis for 3: found (%): C 41.3, H 5.20, N 9.60. Calcd. for
C30H45F6N6O7S2Y (%): C 41.48, H 5.22, N 9.67. Elemental analysis
for 4: found (%): C 39.1, H 4.90, N 9.05. Calcd. for C30H45F6LaN6O7S2
(%): C 39.22, H 4.94, N 9.15. Elemental analysis for 5: found (%): C
38.8, H 4.85, N 9.00. Calcd. for C30H45F6N6NdO7S2 (%): C 38.99, H
4.91, N 9.09. Elemental analysis for 6: found (%): C 38.6, H 4.80, N
8.95. Calcd. for C30
mental analysis for 7: found (%): C 38.1, H 4.85, N 8.90. Calcd. for
C30 45DyF6N6O7S2 (%): C 38.24, H 4.81, N 8.92.
Characterization data for 8. Elemental analysis: found (%): C
37.7, H 4.75, N 8.80. Calcd. for C30H45F6N6O7S2Yb (%): C 37.81, H
4.76, N 8.82.
H
45F6N6O7S2Sm (%): C 38.74, H 4.88, N 9.03. Ele-
Scheme 1. cybpamd-H and [Ln(OTf)2(cybpamd)(THF)] 2–8 syntheses and number-
ing.
H
ment of all the proton resonances of the 1H NMR spectrum, with
the exception of the –CH2– fragments of the cyclohexyl groups far
from the nitrogen atoms. The 13C APT (attached-proton-test), HSQC
and HMBC spectra allowed to separate primary and quaternary
aromatic carbons and to assign the carbon resonances. NMR data
are reported in Table 1.
2.6. Polymerization runs
Polymerizations of ethylene were performed in a 250 mL
glass-autoclave introducing the amount of catalyst and cocata-
lyst dissolved in 150 mL of toluene. The mixture was fed with the
monomer and kept under magnetic stirring over the runs. The
autoclave was vented and the polymerization mixture was poured
in acidified ethanol. The polymers were recovered by filtration,
washed with fresh ethanol and dried in vacuo at 60 ◦C.
Polymerizations of propylene were performed in a 250 mL
glass-autoclave introducing 1 × 10−5 mol of catalyst (4 or 2) and
8.6 × 10−3 mol of MAO (based on Al) dissolved in 150 mL of toluene.
The polymerization mixture was fed with propene (5 bar) and kept
under magnetic stirring for 20 h. The autoclave was vented and the
polymer was recovered as described for polyethylene. The polymer
yield was 30 mg using catalyst 4, while only traces of polymer were
obtained using catalyst 2.
The neutral triflate-complexes [Ln(OTf)2(cybpamd)(THF)]
{Ln = Sc (2), Y (3), La (4), Nd (5), Sm (6), Dy (7), Yb (8); OTf = SO3CF3}
were all prepared in ca. 70% yield by allowing to react cybpamd-H
with a stoichiometric amount of anhydrous Ln(OTf)3 salt in THF
and subsequent addition of diluted butyllithium (see Scheme 1).
The direct synthesis of these products from the lithium salt of the
ligand Li[cybpamd] led always to the formation of mixtures of
hardly separable products and a global lowering of the yields.
The use of stoichiometric amounts of bases such as potassium
tert-butoxide, less strong than butyllithium, did not lead to the
complete deprotonation of the coordinated ligand. Moreover, pre-
liminary studies on the reactivity of cybpamd-H with group 3
chlorides allowed to isolate and characterize by NMR spectroscopy
1
the complex YCl3(cybpamd-H) (1). The 1H NMR and 13C{ H} NMR
spectra of the yttrium complex (1) display the same patterns of
those of cybpamd-H, with moderate variations of the chemical shift
values. In the 1H NMR spectrum of 1 the NH signal falls at 5.09 ppm,
a chemical shift which is very near to the resonance of the free lig-
and NH proton (5.21 ppm). These data suggest that the coordination
of cybpamd-H on the Ln(III) metal ions, fundamental to increase the
selectivity towards the formation of the [Ln(OTf)2(cybpamd)(THF)]
derivatives, does not however strongly improve the acidity of the
ligand N-bonded hydrogen atom. NMR data for compound 1 are
reported in Table 1.
3. Results and discussion
3.1. Synthesis and characterization of N,Nꢀ-dicyclohexyl-2,2-bis-
(3,5-dimethyl-pyrazol-1-yl)-acetamidine (cybpamd-H),
[YCl3(cybpamd-H)] (1) and [Ln(OTf)2(cybpamd)(THF)] complexes
{Ln = Sc (2), Y (3), La (4), Nd (5), Sm (6), Dy (7), Yb (8);
OTf = SO3CF3}
The
organic
compound
N,Nꢀ-dicyclohexyl-2,2-bis-(3,5-
Elemental analyses (C, H, N) of 2–8 complexes agree with the
proposed formulations. The diamagnetic compounds 2–4 have
been characterized by NMR spectroscopy. The 1H NMR spectra
show downfield the signals due to the acetamidinate proton and the
pyrazole-H4. The cyclohexyl hydrogen atoms of the N-bonded CH
groups fall in the range 4.0–3.0 ppm. Upfield, two sharp resonances
due to the pyrazole methyl groups and the complex multiplet of
the cyclohexyl methylene groups are observable. Two multiplets
around 3.7 and 1.8 ppm indicate the presence of a THF molecule.
dimethyl-pyrazol-1-yl)-acetamidine (cybpamd-H) has been
prepared according to Otero et al. [17], i.e. by reacting bis(3,5-
dimethyl-pyrazol-1-yl)methane with butyllithium in THF at low
temperature to form in situ the corresponding lithium salt, which
was subsequently reacted with one equivalent of dicyclohexylcar-
bodiimide. The quench of the reaction mixture with cold water
allowed the formation of the final product, which was purified
following common procedures and isolated in high yield (see
Scheme 1). Elemental analysis (C, H, N) agreed with the proposed
formulation. 1H COSY and NOESY experiments allowed the assign-
The 13C{ H} NMR spectra show in the range 160–140 ppm the sig-
1
nals due to the acetamidinate-Ca, the pyrazole-C3 and pyrazole-C5.

G. Paolucci et al. / Journal of Molecular Catalysis A: Chemical 317 (2010) 54–60
57
Table 1
Characterization data. Refer to Scheme 1 for NMR numbering.
cybpamd-H
1H NMR (CDCl3, 298 K): 7.14 (s, 1H) acetamidine-Hb; 5.77 (s, 2H) pyrazole-H4; 5.21 (d, 1H, 3JHH = 6.7 Hz) NH; 3.69 (m, 1H) cyclohexyl-NCH˛;
3.09 (m, 1H) cyclohexyl-NCHˇ; 2.38 (s, 6H) pyrazole-Me5; 2.16 (s, 6H) pyrazole-Me3; 2.04–1.00 (m, 20H) cyclohexyl-CH2
1
13C{ H NMR} (CDCl3, 298 K): 149.2 pyrazole-C3; 146.4 acetamidine-Ca; 141.2 pyrazole-C5; 106.6 pyrazole-C4; 67.5 acetamidine-Cb; 56.3
cyclohexyl-NCHˇ; 48.0 cyclohexyl-NCH˛; 35.0, 31.8, 26.0, 25.8, 24.8, 24.1 cyclohexyl-CH2; 13.6 pyrazole-Me3; 11.6 pyrazole-Me5
1H NMR (CD2Cl2, 298 K): 6.99 (s, 1H) acetamidine-Hb; 6.05 (s, 2H) pyrazole-H4; 5.09 (d, 1H, 3JHH = 9.0 Hz) NH; 3.89 (m, 1H)
cyclohexyl-NCH˛; 3.58 (m, 1H) cyclohexyl-NCHˇ; 2.61 (s, 6H) pyrazole-Me5; 2.47 (s, 6H) pyrazole-Me3; 2.08–1.05 (m, 20H) cyclohexyl-CH2
1
2
1
13C{ H NMR} (CD2Cl2, 298 K): 155.4 pyrazole-C3; 154.9 acetamidine-Ca; 141.5 pyrazole-C5; 109.0 pyrazole-C4; 60.5 acetamidine-Cb; 58.4
cyclohexyl-NCHˇ; 54.8 cyclohexyl-NCH˛; 35.3-24.9 cyclohexyl-CH2; 15.0 pyrazole-Me3; 11.8 pyrazole-Me5
1H NMR (CD3CN, 298 K): 7.18 (s, 1H) acetamidinate-Hb; 6.02 (s, 2H) pyrazole-H4; 3.78 (m, 1H) cyclohexyl-NCH˛; 3.69 (m, br, 4H) THF; 3.43
(m, 1H) cyclohexyl-NCHˇ; 2.38 (s, 6H) pyrazole-Me5; 2.16 (s, 6H) pyrazole-Me3; 1.84 (m, br, 4H) THF; 1.78–1.02 (m, 20 H) cyclohexyl
1
13C { H NMR} (CD3CN, 298 K): 157.3 acetamidinate-Ca; 152.1 pyrazole-C3; 143.2 pyrazole-C5; 108.8 pyrazole-C4; 68.4 THF; 65.6
acetamidinate-Cb; 56.4 cyclohexyl-NCHˇ; 51.7 cyclohexyl-NCH˛; 32.9 cyclohexyl; 30.8 cyclohexyl; 26.1 THF; 25.6 cyclohexyl; 25.5 cyclohexyl;
25.4 cyclohexyl; 23.9 cyclohexyl; 13.5 pyrazole-Me3; 11.3 pyrazole-Me5
3
4
1H NMR (CD3CN, 298 K): 6.88 (s, 1H) acetamidinate-Hb; 5.96 (s, 2H) pyrazole-H4; 3.70 (m, 4H) THF; 3.43 (m, 1H) cyclohexyl-NCH˛; 3.21 (m,
1H) cyclohexyl-NCHˇ; 2.41 (s, 6H) pyrazole-Me5; 2.22 (s, 6H) pyrazole-Me3; 1.83 (m, 4H) THF; 1.80–1.09 (m, 20 H) cyclohexyl
1
13C { H NMR} (CD3CN, 298 K): 155.3 acetamidinate-Ca; 151.2 pyrazole-C3; 142.0 pyrazole-C5; 107.4 pyrazole-C4; 68.5 THF; 62.4
acetamidinate-Cb; 55.2 cyclohexyl-NCHˇ; 54.4 cyclohexyl-NCH˛; 35.3 cyclohexyl; 33.8 cyclohexyl; 26.4–25.2 cyclohexyl + THF; 13.3
pyrazole-Me3; 11.3 pyrazole-Me5
1H NMR (CD3CN, 298 K): 6.86 (s, 1H) acetamidinate-Hb; 5.94 (s, 2H) pyrazole-H4; 3.68 (m, br, 4H) THF; 3.41 (m, 1H) cyclohexyl-NCH˛; 3.18
(m, 1H) cyclohexyl-NCHˇ; 2.40 (s, 6H) pyrazole-Me5; 2.20 (s, 6H) pyrazole-Me3; 1.81 (m, 4H) THF; 1.78–1.01 (m, 20H) cyclohexyl
1
13C { H NMR} (CD3CN, 298 K): 152.2 acetamidinate-Ca; 151.2 pyrazole-C3; 141.9 pyrazole-C5; 107.4 pyrazole-C4; 68.4 THF; 62.5
acetamidinate-Cb; 54.3 cyclohexyl-NCHˇ; 53.8 cyclohexyl-NCH˛; 35.3 cyclohexyl; 33.8 cyclohexyl; 26.2 cyclohexyl; 26.1 cyclohexyl; 25.8
cyclohexyl; 25.7 cyclohexyl; 25.5 THF; 13.3 pyrazole-Me3; 11.3 pyrazole-Me5
Mass data (E.I., 70 eV, m/z): 810 [M•+–THF–F–F]+, 679 [M•+–THF–SO3CF3–F]•+; 564 [M•+–THF–SO3CF3–SO2CF3]•+
Mass data (E.I., 70 eV, m/z): 686 [M•+–THF–SO3CF3–F]•+, 571 [M•+–THF–SO3CF3–SO2CF3]•+
5
6
7
8
Mass data (E.I., 70 eV, m/z): 823 [M•+–THF–F–F]•+, 577 [M•+–THF–SO3CF3–SO2CF3]•+; 561 [M•+–THF–SO3CF3–SO3CF3]•+
Mass data (E.I., 70 eV, m/z): 702 [M•+–THF–SO3CF3–F]•+, 588 [M•+–THF–SO3CF3–SO2CF3]•+
Mass data (E.I., 70 eV, m/z): 730 [M•+–THF–SO2CF3–F]+, 599 [M•+–THF–SO3CF3–SO2CF3]•+
Pyrazole-C4 signal falls around 107–109 ppm, while acetamidinate-
Cb shows a resonance between 66 and 62 ppm. The cyclohexyl
N-bonded carbon atoms fall in the range 57–51 ppm, while the
other cyclohexyl 13C NMR resonances are comprised between 36
and 23 ppm. The most upfield signals correspond to the pyrazole
4 was collected for comparison. All the MS spectra show signals
assignable to the molecular ions after the loss of the coordinated
THF and a number of fragments from the triflate groups, in par-
ticular F, SO2CF3 and SO3CF3, in agreement with the proposed
formulations.
1
methyl substituents. The 13C{ H} NMR spectra of 2–4 confirm
1
the presence of a THF molecule. The described 1H and 13C{ H}
3.2. Polymerizations
NMR spectra show a simple set of resonances for the pyrazole
rings, indicating that both the groups are equivalent. These data
suggest an octahedral disposition for the metal atoms with ␬3-
NNN-coordination of the heteroscorpionate ligand, a situation
where a plane of symmetry exists and contains the amidinate group
and the metal center. The NMR signals due to the amidinate moiety
show two sets of resonances for the NCH protons in the 1H NMR
Cationic alkyl complex of group 4 metals are the most common
single-site olefin polymerization catalysts [18]. Isoelectronic neu-
tral alkyl complexes of group 3 metals, due to lower electrophilicity,
generally show much lower polymerization activity [3]. One pos-
sible approach to improve the catalytic performances involves the
generation of cationic alkyl species by a suitable activator, therefore
all synthesized compounds (2–8) were tested in the polymeriza-
tion of ethylene after activation by methylaluminoxane (MAO).
Moreover, triflate anions must be necessary removed from the
coordination sphere of the rare-earths metal complexes. In order
to give polymerization of ethylene the catalyst must have, over the
ancillary ligands, a vacant site where it coordinates the monomer
and an alkyl group where the insertion can take place. It is com-
monly accepted that the role of MAO consists in the alkylation and
1
spectra and eight cyclohexyl resonances in the 13C{ H} NMR spec-
tra. This observation is indicative of a monodentate binding of the
amidinate group to the metal center, which has already observed
for complexes of elements of groups 1, 2, 4 and 12 [6,17], as depicted
in Scheme 1. Despite all the attempts we have been unable to isolate
suitable crystals for the X-ray analyses.
The proposed geometry was confirmed by computational stud-
ies on the [Ln(OTf)2(cybpamd)(THF)] derivatives {Ln = Sc (2), Y (3),
La (4), Nd (5), Sm (6), Dy (7), Yb (8)}, whose structures were initially
optimized with the PM6 Hamiltonian. The obtained ground-state
geometries confirmed that cybpamd acts as a tridentate ligand
with only one amidinate nitrogen atom strongly bonded to the lan-
thanum center. The singlet-state complexes of Sc, Y, and La 2–4
have been subsequently optimized by DFT EDF1/LACVP* calcula-
tions. As for the semi-empirical calculations, also the DFT results
indicated that the scorpionate ligand is quite strongly bonded to
the metal ions with three nitrogen atoms. The coordination mode
Table 2
Computed EDF1/LACVP* metal–ligands bond lengths for the complexes 2–4.
Bond
Bond
length (Å)
length, Å
[Sc(OTf)2(cybpamd)(THF)]
Sc-N(pyrazole1)
Sc-N(pyrazole2)
2.380
2.366
2.084
Sc-O(triflate1)
Sc-O(triflate2)
Sc-O(THF)
2.043
2.044
2.335
Sc-N(acetamidine)
1
of the two triflate groups is ␬ and the sixth position of the octa-
[Y(OTf)2(cybpamd)(THF)]
Y-N(pyrazole1)
Y-N(pyrazole2)
hedric coordination sphere is occupied by a THF molecule. The
DFT-computed bond lengths for the complexes 2–4 are reported
in Table 2, while their optimized geometries are depicted in Fig. 1.
The paramagnetic complexes 5–8 have been characterized by
means of elemental analyses and mass spectrometry by comparing
the theoretical and experimental isotopic patterns. The mass spec-
trum of the completely NMR characterized lanthanum derivative
2.518
2.532
2.250
Y-O(triflate1)
Y-O(triflate2)
Y-O(THF)
2.210
2.210
2.473
Y-N(acetamidine)
[La(OTf)2(cybpamd)(THF)]
Y-N(pyrazole1)
Y-N(pyrazole2)
2.704
2.712
2.413
Y-O(triflate1)
Y-O(triflate2)
Y-O(THF)
2.385
2.390
2.681
Y-N(acetamidine)

58
G. Paolucci et al. / Journal of Molecular Catalysis A: Chemical 317 (2010) 54–60
Fig. 1. DFT-optimized geometries of [Ln(OTf)2(cybpamd)(THF)] {Ln = Sc (2), Y (3), La (4)}. Hydrogen atoms were omitted for clarity.
cationization of the catalytic precursor, producing the suitable cat-
alytic site. We have recently reported as the role of MAO is essential
to have group 3 complexes with fair activity [5b] and in the lit-
erature are reported a number of cationic alkyl complexes of the
rare-earth metals able to polymerize ethylene [3].
Data reported in the Table 3 show that all the complexes 2–8
are able to polymerize ethylene, producing linear polyethylenes
having a melting point higher than 135 ◦C and a high molecular
weight (>6 × 105 Da). Moreover, the molecular weight distribution
is in all the cases close to 2, revealing that the polymer are generate
on single-site catalysts.
The most active catalysts in ethylene polymerization were those
based on lanthanum (4), neodymium (5) and scandium (2) com-
pounds. Complexes 2 and 4 were therefore tested also in the
polymerization of propylene (see experimental part). Polypropy-
lene having a substantially isotactic microstructure (mm triads
concentration = 81%) was produced by the lanthanum deriva-
tive 4, although with very low activity. Complex 2 was instead
practically inactive. Probably, the high steric hindrance of the
[N,N,N-scorpionate ligand] prevents the coordination of propene
to the catalytic site.
The activities of the catalysts in the ethylene polymerization
have been correlated to the ionic radii r of the metals [19,20]
and the series Yb–Y–Dy–Sm–Nd–La follows the linear relation-
ship (1), depicted in Fig. 2. The scandium-based catalyst gives
instead a higher activity than that expectable on the bases of this
activity–radius relationship.
Fig. 2. plot of the activity in ethylene polymerization of complexes 2–8 against Ln3+
ionic radius.
These findings can be rationalized by considering that the poly-
merization of ethylene was reported to occur as sequence of the
following events: (i) coordination of monomeric unit to the metal
bearing the ancillary ligand and the growing chain (ii) inser-
tion of the activated monomer into the growing chain [21]. DFT
EDF1/LACVP* calculations have been carried out on models for the
active species of the type [Ln(CH3)(CH2 CH2)(cybpamd)]+ {Ln = Sc,
Y, La} and a selection of computed data is reported in Table 4. The
DFT-optimized geometries are reported in Fig. 3.
activity=mr + q {activity = gpolymer/(molcat [ethylene] h); r = Å }
(1)
In all the three models the ancillary ligand cybpamd results
bonded to the metal center with three nitrogen atoms. The ethylene
molecule appears weakly coordinated, being the Ln–olefin bond
m = 63,000 4000 gpolymer/(molcat [ethylene] h Å);
q = −61,000 4000 gpolymer/(molcat [ethylene] h).
Table 3
Polymerizations of ethylene in the presence of complexes 2–8.
Runa
Precatalyst
Ionic radiusb (Å)
Yield (g)
Activityc
MW (×105)
MW/Mn
d
d
2
3
4
5
6
7
8
Sc
Y
La
Sm
Nd
Dy
Yb
0.89
1.04
1.17
1.10
1.12
1.05
1.01
1.43
0.62
1.93
1.14
1.47
0.85
0.45
9669
4192
13052
7708
9939
5747
3043
7.3
7.7
6.3
8.8
6.9
9.1
7.5
2.2
2.0
1.7
1.9
1.9
2.2
2.3
a
Polymerization conditions: solvent toluene = 150 ml; precatalyst = 1.0 × 10−5; cocatalyst: MAO (based on Al) = 8.6 × 10−3 mol; ethylene concentration in the feed = 0.87 M;
temperature = 50 ◦C; time = 17 h.
b
Ionic radii determined by the method of Shannon and Prewitt [19] and referred to Ln3+ hexacoordinated ions, taken from Ref. [20].
Activity = gpolymer/(molcat [ethylene] h).
Weight and polydispersity index (MW/Mn) determined by gel permeation chromatography versus polystyrene standard.
c
d

G. Paolucci et al. / Journal of Molecular Catalysis A: Chemical 317 (2010) 54–60
59
Fig. 3. DFT-optimized geometries of the [Ln(CH3)(CH2 CH2)(cybpamd)]+ {Ln = Sc, Y, La} active species models. Hydrogen atoms were omitted for clarity.
Table 4
the metal, should therefore be scarcely dependent upon the ionic
radius. The second event of the polymerization reaction, i.e. the
insertion of the monomer into the growing chain, depends besides
all upon the strength of the Ln–alkyl bond. The linear growth of
the Ln–CH3 on increasing the ionic radius reported in Fig. 4 can
therefore explain the linear trend of Fig. 2, i.e. the activity growth
along the series Yb–Y–Dy–Sm–Nd–La. To a greater ionic radius cor-
responds in fact a longer Ln–CH3 bond and the alkyl migration on
the coordinated olefin should therefore be favored for the greatest
Ln(III) ions.
For the Sc-based catalytic system we have observed an activ-
ity value that is not correlated to the ionic radius, as observable
in Fig. 2. It can be tentatively proposed that the really high hard-
ness of Sc3+ with respect to those of the Ln3+ ions could cause a
greater activation of the coordinated alkyl group, thus the [Ln]–CH3
Mulliken charges were computed to support such an hypothesis.
The CH3 change varies from −0.304 to −0.319 on changing the
metal ion from Y lo La, but this is an expectable behavior asso-
ciated to the increase of the bond length and the growth of the
Ln–C ionic character. In the [Sc(CH3)(CH2 CH2)(cybpamd)]+ model
the coordinated alkyl charge is instead not related with the Sc–C
bond length. Despite this bond is short (2.181 Å), the –CH3 group
has in fact a negative partial charge of −0.318, which is compara-
ble to that of the lanthanum derivative (−0.319). This means that
scandium(III) polarizes the bond with the alkyl group more than
the expected only on the basis of the ionic radius, making there-
fore faster the olefin insertion. The particular electronic features of
Sc(III) with respect to the other metal centers studied in this work
are the causes of the different catalytic activity observed towards
ethylene polymerization.
Computed EDF1/LACVP* metal–ligands bond lengths and CH3 Mulliken charge for
the [Ln(CH3)(CH2 CH2)(cybpamd)]+ {Ln = Sc, Y, La} active species models.
Bond length (Å)
CH3 Mulliken charge
[Sc(CH3)(CH2 CH2)(cybpamd)]+
Sc-N(pyrazole1)
Sc-N(pyrazole2)
Sc-N(acetamidine)
Sc-ethylene
2.251
2.241
2.020
3.234
2.181
−0.318
Sc-CH3
[Y(CH3)(CH2 CH2)(cybpamd)]+
Y-N(pyrazole1)
Y-N(pyrazole2)
Y-N(acetamidine)
Y-ethylene
2.457
2.414
2.184
3.094
2.355
−0.304
−0.319
Y-CH3
[La(CH3)(CH2 CH2)(cybpamd)]+
La-N(pyrazole1)
La-N(pyrazole2)
La-N(acetamidine)
La-ethylene
2.673
2.601
2.356
3.337
2.522
La-CH3
length always greater than 3 Å. The Ln–CH3 bond length is linearly
related with the Ln3+ ionic radius, as depicted in Fig. 4.
The great Ln–ethylene bond lengths make the olefin poorly
affected by the nature of the metal center. The first step of the poly-
merization reaction, i.e. the coordination of the monomeric unit to
4. Conclusions
In this paper the synthesis and characterization of a series of
new group 3 and lanthanide [N,N,N]-heteroscorpionate triflate-
complexes has been reported and the coordination geometry
has been suggested both on the basis of the analytical, spectro-
scopic, MS data and by optimized structural calculations. Catalytic
tests towards ethylene polymerization highlighted the correlation
between the properties of the metal center and the catalytic activity
of the corresponding complexes. DFT calculations on active species
models allowed proposing an explanation for the observed catalytic
activities.
The very low activities showed in the polymerization of propy-
lene by the catalytic systems based on compounds 4 and 2 could
probably be improved using precatalysts having ancillary ligands
with lower steric hindrance. Work is currently in progress in order
to verify this hypothesis.
Fig. 4. Relationship between metal center ionic radius and Ln–CH3 bond length in
[Ln(CH3)(CH2 CH2)(cybpamd)]+ active species models.

60
G. Paolucci et al. / Journal of Molecular Catalysis A: Chemical 317 (2010) 54–60
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