Group 4 complexes bearing alkoxide functionalized N-heterocyclic carbene ligands as catalysts in the polymerization of olefins

Article abstract of DOI:10.1002/pola.26183

Novel octahedral zirconium complexes bearing alkoxide functionalized N-heterocyclic carbene ligands have been synthesized and characterized. NMR analysis showed that more than one species was obtained during synthesis. The synthesized complexes were able to polymerize ethylene and propylene giving rise to a linear polyethylene with high M_w and polydispersity index often bimodal with molecular weight distribution (MDI) > 2, and highly isotactic polypropylene together with an atactic fraction. Density functional theory (DFT) studies on the complex stability indicate the possible species active during the polymerization reaction.

Full text of DOI:10.1002/pola.26183

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Group 4 Complexes Bearing Alkoxide Functionalized N-Heterocyclic
Carbene Ligands as Catalysts in the Polymerization of Olefins
Chiara Costabile, Carmen Bocchino, Mariagrazia Napoli, Pasquale Longo
ꢀ
Dipartimento di Chimica e Biologia, Universita di Salerno, Via Ponte don Melillo, 84084 Fisciano (SA), Italy
Correspondence to: P. Longo (E-mail: plongo@unisa.it)
Received 5 April 2012; accepted 12 May 2012; published online
DOI: 10.1002/pola.26183
ABSTRACT: Novel octahedral zirconium complexes bearing alk-
oxide functionalized N-heterocyclic carbene ligands have been
synthesized and characterized. NMR analysis showed that
more than one species was obtained during synthesis. The
synthesized complexes were able to polymerize ethylene and
propylene giving rise to a linear polyethylene with high Mw
and polydispersity index often bimodal with molecular weight
distribution (MDI) > 2, and highly isotactic polypropylene to-
gether with an atactic fraction. Density functional theory (DFT)
studies on the complex stability indicate the possible species
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active during the polymerization reaction. 2012 Wiley Periodi-
cals, Inc. J Polym Sci Part A: Polym Chem 000: 000–000, 2012
KEYWORDS: heterocyclic carbene complexes; isotactic; polypro-
pylene; metal-organic catalysts/organometallic catalysts; NMR;
polyethylene; zirconium catalyst
INTRODUCTION N-heterocyclic carbene (NHC) are widely
used as ancillary ligands in late transition metal complexes,
giving strong metal-carbene bonds and slow dissociation
rate.1,2 Due to their excellent r and p-donating properties and,
as theoretical and structural studies suggest, p-backbonding
ability,3,4 complexes bearing NHC ligands have shown often
enhanced catalytic performance compared with their phos-
phine analogues.2 Although NHC complexes of late transition
metal were extensively used in catalysis,1,2 as well as by some
of us,5 on the contrary, NHC complexes of early transition
metals were less developed, despite their relevant potentiality
as catalysts.6 This is above all due to propensity at dissocia-
tion of the NHC ligand from the electron poor metal center.7
To prevent dissociation, NHC having a pendant amine-, amide-
, alkoxide-, and cyclopentadienyl-group have been recently
synthesized. The presence of a strong electron-donating yli-
dene carbon with an anionic group generates a chelating
ligand [NHC-X] able to produce complexes with higher stabil-
ity and constrained and possibly chiral geometry.7–9
tactic and an atactic fraction. Characterization of the complex
by NMR analysis showed that more diastereoisomers were
obtained during the synthesis. DFT calculations located all
possible diastereoisomeric structures of the alkylated Zr com-
plex [NHC-O]2Zr(CH3)2, showing which species could be
involved in the polymerization of ethene or propene and, in
particular, in the formation of isotactic polypropilene.10
Herein, the synthesis of two novel alkoxide-functionalized N-
heterocyclic carbene ligands [NHC-O] and the relative octa-
hedral zirconium complexes are reported. In particular,
[NHC-O] presenting ethenyl and cyclohexenyl alkoxide
groups as N-substituents (2 and 3 in Chart 1, respectively)
were obtained, and the corresponding complex behaviors in
catalysis were compared with that of previously reported
complex bearing cyclopentenyl alkoxide NHC ligand.10
[NHC-O] with ethenyl alkoxide group present higher flexibil-
ity with respect to the constrained cyclopentenyl and cyclo-
hexenyl alkoxide groups. This characteristic can lead to dif-
ferent ways the ligand wraps around the metal. Moreover,
differently from cyclohexenyl and cyclopentenyl alkoxide,
ethenyl [NHC-O] ligand does not show any stereogenic cen-
ter. Chirality of the corresponding complexes can only arise
from a suitable coordination of the ligand on metal.
Recently, we reported on the synthesis of an octahedral zir-
conium complex with two alkoxide-functionalized N-hetero-
cyclic carbene ligands [NHC-O] and two amide groups [bis
(imidazolidene-N-methyl-N0-cyclopentenyl-2-alkoxy)zirconium-
diamide] (1 in Chart 1).10 The complex, after activation by
methylaluminoxane (MAO), was able to polymerize ethene
producing linear polyethene with high-molecular weight and
large, and often bimodal, molecular weight distribution. On
the other hand, polymerization of propene gave a highly iso-
Cyclohexenyl [NHC-O] ligand is slightly less constrained with
respect to cyclopentenyl but more hindered. It also bears
stereogenic centers, making it for this characteristic similar
to the cyclopentenyl [NHC-O] ligand.
Additional Supporting Information reporting cartesian coordinates and energies of calculated structures may be found in the
online version of this article.
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Synthesis of Complex 2
Imidazolium salt 2a of Scheme 1 was prepared by reacting
ethylenoxide with imidazole to form imidazole-N-ethan-2-ol,
which was subsequently reacted in acetonitrile with iodo-
methane. After distillation of the solvent, the product was
purified by crystallization in acetone, obtaining the imidazo-
lium salt as a white solid in moderate yield. Elemental analy-
sis (C, H, and N) is in agreement with the proposed formula-
tion. 1H COSY and NOESY experiments allowed to assign all
proton resonances of the 1H NMR spectrum, whereas
resonances of 13C NMR spectrum were attributed using sup-
port of DEPT and HMQC experiments. Structure of imidazo-
lium salt 2a was confirmed by diagnostic proton signals, as
the singlet at 8.74 ppm relative to the imidazolium H, and
two multiplets at 4.24 and 3.84 ppm attributable to methyl-
enes bonded to heteroatoms, CH2AO and CH2AN, respec-
tively. As for 13C NMR spectrum, signals of methine carbo-
cation at 137.7 ppm and methylene carbons bonded to oxy-
gen and to nitrogen atoms at 60.9 and 53.4 ppm, respec-
tively, support the reported structure as well. Reaction of
imidazolium salt 2a with a slight excess of potassium-hexam-
ethyldisilazide produces potassium alkoxide carbene, that
was added to a solution of Zr(NEt2)4 to obtain the desired
neutral octahedral complex [NHC-O0]2Zr(NEt2)2 (2 of Chart
1). 2 was isolated as air- and moisture sensitive solids and
characterized by 1H and 13C NMR, elemental analysis and
mass spectroscopy. Elemental analysis (C, H, and N) of 2 is
in agreement with the proposed formulation (see Experi-
mental section). The 1H and 13C NMR spectra provide evi-
dences for the presence of at least two isomers in solution
at RT for Complex 2, and by COSY, NOESY, HMCQ, and DEPT
experiments, it was possible to fully attribute resonances
(see Experimental section). As for the 1H spectrum, two set
of peaks for the protons of the unsaturated methine of the
backbone of heterocyclic, between 6.08 and 5.95 ppm and a
quartet attributable to methylene protons next to alkoxide at
4.28 ppm can be observed. 13C NMR spectrum shows a diag-
nostic signal at 196.3 ppm related the carbenic carbon and
those of methylene carbons bonded to oxygen and to nitro-
gen atoms at 66.6 and 54.1 ppm, respectively. It is worth
noting that next to these main resonances, other signals of
modest intensity can be observed either in the aliphatic
region that at highest field. For example, next to the carbene
signals at 196.3, a smaller signal at 192.2 also attributable to
a carbene carbon can be noticed. This is indicative of the
presence of at least another species in solution. No structural
CHART 1 Complexes 1, 2, 3.
The aim of this work was the study of the effect of structural
variations of the [NHC-O] ligand on properties and catalytic
activity in olefins polymerization of the corresponding
complexes.
RESULTS AND DISCUSSION
Synthesis of bis(imidazolidene-N-methyl-N0-ethenyl-2-alkox-
y)zirconium-diamide 2 and bis(imidazolidene-N-methyl-N0-
cyclohexenyl-2-alkoxy)zirconium-diamide 3 (Chart 1) was
achieved as follows.
NHC-alkoxy derivatives were obtained with a very conven-
ient and flexible process through the use of epoxides. In fact,
the reaction between an epoxide and an imidazole produces
the epoxy ring opening with formation of alkoxide-tethered
NHCs, which by reaction with an alkyl halide forms the cor-
responding imidazolium salt in good yields. The synthetic
pathways are reported in Scheme 1.10,11
Conversion of the imidazolium pro-ligands to potassium NHC
alkoxy derivatives is readily accomplished by sequential
deprotonation using two equivalents of KN(SiMe3)2. The
reaction between potassium alkoxy NHC and Zr(NEt2)4
affords the final complex.
1
modifications were revealed by variable-temperature (VT) H
SCHEME 1 Synthesis of Complexes 2 and 3.
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TABLE 1 Polymerizations of Ethylene in the Presence of
Catalytic Systems 2 and 3 Activated by MAO
nal of carbene carbon at 198.6 ppm, as well as signals at
79.6 and 69.0 ppm due to methine carbons bonded to oxy-
gen and to nitrogen atoms, respectively, can be observed.
NMR signals of modest intensity next to the main ones indi-
cate, as previously reported for 2, the presence of more than
one species in solution also for Complex 3. No significant
structural variation was observed by VT 1H NMR experi-
Runa Catalyst [Al]/[Zr] Activityb (103) Mw (105) Mw/Mn
c
c
1
2
3
4
5
6
7
8
9
a
2
2
2
2
2
3
3
3
3
600
1.2
4.3
4.1
4.0
4.2
0.3
1.5
4.1
7.3
6.4
7.5
2.7
2.6
2.5
2.3
3.4
2.3
2.4
2.5
2.8
1000
1500
2000
3000
500
9.0
ꢁ
ꢁ
ments, performed in the range ꢀ40 C < T < 80 C.
37.7
11.5
6.9
Polymerization Reactions
Complexes 2 and 3 were tested in the polymerization of eth-
ylene after activation by MAO. The reactions were performed
1000
2000
3000
6.8
ꢁ
in a glass autoclave at 50 C under an ethylene pressure of 6
7.5
bar. As shown in Table 1, Complexes 2 and 3 polymerize
ethylene, producing linear polyethylenes with a melting point
higher than 135 ꢁC and a high-molecular weight (between
6.4ꢂ105 and 3.7ꢂ106 dalton). The molecular weight distribu-
tion is quite large (from 2 to 5) and quite often bimodal,
revealing that polymer chains are not generated by a single
site catalyst. Catalyst activities have been correlated to Al/Zr
mole ratio and, as reported in Figure 1, the activities
increase by increasing the mole ratio of Al/Zr. As for Com-
plex 2, a plateau is rapidly reached over ratio ¼ 1000, while
in case of Complex 3 activity continues to enhance up to
ratio 3000, probably because the maximum concentration of
active species is achieved at higher MAO/Zr ratio.
11.4
Polymerization conditions: temperature 50 ꢁC, solvent toluene, total
volume 150 mL, time 20 min; complex ¼ 8.0 ꢄ 10ꢀ6 mol; cocatalyst:
MAO (based on Al); ethylene concentration in the feed ¼ 1.05 ꢄ 10ꢀ3
mol/L.
b
Activity in Kg polymer/[complex] ꢄ time (h) ꢄ [monomer].
c
Molecular weight and polydispersity index (Mw/Mn) determined by gel
permeation chromatography versus polystyrene standard.
NMR experiments in the range ꢀ40 ꢁC < T < 80 ꢁC.
Unfortunately, despite all the attempts, we were unable to
grow crystals suitable for the X-ray analysis.
Synthesis of Complex 3
Indeed, increasing the concentration of MAO, the equilibrium
Imidazolium salt 3a was prepared by reacting cyclohexen-
oxide with imidazole to form imidazole-N-cyclohexan-2-ol,
and following afterward the procedure reported in ref. 10.
Elemental analysis (C, H, and N) and mass spectroscopy are
in agreement with the proposed formulation. 1H COSY and
NOESY experiments allowed to assign all proton resonances
of the 1H NMR spectrum, whereas resonances of 13C NMR
spectrum were attributed using support of DEPT and HMQC
experiments. Complete attribution of the resonances is
reported in the Experimental section. 1H NMR spectrum
shows as diagnostic signals the singlet at 9.51 ppm of the
carbo-cationic methine, and two multiplet at 4.26 and 3.81
ppm of methine protons of cycloexane bonded to heteroa-
toms, CHAO and CHAN, respectively, in trans configuration.
13C NMR spectrum shows the signal of the NACAN sp2
methine carbon at 136.4 ppm, and of methine carbons
bonded to oxygen and to nitrogen atoms at 72.1 and 66.4
ppm, respectively.
ðLÞ ZrX2 þ MAO Ð ½ðLÞ ZrCH3ꢃþ þ ½MAOX2ꢃꢀ
2
2
of formation of the active species is shifted to right.
As reported in Table 1, the molecular weight of polymers,
increases by increasing MAO/Zr molar ratio, in the presence
of both 2 and 3. To rationalize these data, the possible mech-
anisms of termination reactions of the polymerization have
to be considered: (i) chain transfer from zirconium to alumi-
num, (ii) chain transfer to the monomer, and (iii) b-hydrogen
Reaction of imidazolium salt 3a with a slight excess of potas-
sium-hexamethyldisilazide produces potassium alkoxide car-
bene, which was added to a solution of Zr(NEt2)4 to afford
the desired neutral octahedral Complex 3, that was isolated
as air- and moisture sensitive solid. 3 was characterized by
NMR analysis, and 1H and 13C NMR attributions were
assigned on the basis of COSY, NOESY, and HMQC experi-
ments (see Experimental section for full attribution of the
resonances). 1H NMR spectrum shows two set of peaks for
the protons of the unsaturated methine of the backbone of
heterocyclic, between 6.37 and 6.15 ppm and two quartet
attributable to methine next to alkoxide at 4.19 and
3.45 ppm. As for the 13C NMR spectrum, the diagnostic sig-
FIGURE 1 Plot of the activity in ethylene polymerization with
Complexes 2 and 3 versus mole ratio [Al]/[Zr]. Activity in Kg
polymer/[complex] time (h) [monomer] 103.
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TABLE 2 Polymerizations of Propylene in the Presence of
Zirconium Based Catalysts
Runa
Catalyst
[Al]/[Zr]
Activityb
Mw (105)
Mw/Mn
c
c
10
11
12
13
a
2
2
3
3
600
0.3
1
17.5
12.1
3.1
3.0
3.5
2.3
2.5
1000
1000
2000
3
31
6.3
Polymerization conditions: temperature 50 ꢁC, solvent toluene, total
volume 150 mL, time 24 h; complex ¼ 8.0 ꢄ 10ꢀ6 mol; cocatalyst: MAO
(based on Al); propylene concentration in the feed ¼ 5.48 ꢄ 10ꢀ3 mol/L.
b
Activity in Kg polymer/[complex] ꢄ time (h) ꢄ [monomer].
c
Molecular weight and polydispersity index (Mw/Mn) determined by gel
permeation chromatography versus polystyrene standard; Molecular
weight and polydispersity index of the whole polymers.
pylene (ca. 70% w/w, boiling hexane insoluble fraction). An
example of GPC analysis of a polypropylene sample (Run 11,
Table 2) is reported in Figure 2(b).
Molecular Modeling Studies
Experimental results are consistent with the presence of at
least two catalytic species active for the polymerization reac-
tions. Unfortunately, NMR characterization of complexes
[NHC-O]2Zr(NEt2)2 did not allow to assign the exact geome-
try of the most stable species in solution.
FIGURE 2 (a) GPC analysis of polyethylene obtained with 2
(Run 5, Table 1). (b) GPC analysis of polypropylene obtained
with 2 (Run 11, Table 2).
To shed light on the species possibly involved in the poly-
merization promoted by Complex 2, the different diaster-
eoisomers of the alkylated Zr complex [NHC-O]2Zr(CH3)2
were modeled by DFT calculations both in gas phase and in
benzene.
elimination. As for the first mechanism, a higher amount of
MAO would lead to lower molecular weights, giving an oppo-
site trend with respect to the one observed. When the sec-
ond mechanism is active, molecular weights are influenced
only by the concentration of ethylene, which in all reported
tests is constant. As for the third mechanism, molecular
weights could be influenced by the concentration of MAO,
and in particular of the [MAOX2]ꢀ that could be coordinated
to the catalytic sites, preventing the b-hydrogen elimination
and be moved away from the catalytic site only by the ar-
rival of another monomer unit.
As well known, octahedral complexes with a chelating ligand,
as in the case of [NHC-O]2Zr(CH3)2, show five possible dia-
stereoisomers, that are depicted in Scheme 2. In particular,
C, D, and E isomers can also have a K and a D configura-
tions. Being, for complex [NHC-O]2Zr(CH3)2, C, D, and E-K
The coordination of the MAO to the catalytic center was also
considered, in our previous article,10 to rationalize the
abnormal activity trend of 1 versus the MAO/Zr ratio, that
showed a maximum for Al/Zr ¼ 2500, and then rapidly
decreased for higher ratios.
Gel permeation chromatography (GPC) analysis of products
suggests the presence for both catalytic systems of more
than one active site. An example of GPC analysis of a poly-
ethylene sample (Run 5, Table 1) is reported in Figure 2(a).
This hypothesis was confirmed by polymerizations of propyl-
ene performed at 50 ꢁC under a pressure of 6 bar. Indeed,
with both catalytic systems we obtained polypropylenes hav-
ing bimodal distribution of molecular weight and polydisper-
sity index (Mw/Mn) higher than 2 (see Table 2). Fractiona-
tion of the whole polymers by boiling hexane gave rise to an
atactic polypropylene (ca. 30% w/w for both catalysts, boil-
ing hexane soluble fraction) and a highly isotactic polypro-
SCHEME 2 Stereoisomers for an octahedral complex with two
chelating ligands of the type [NHC-O] and two methyl groups.
For stereoisomers C, D, and E only, the K configuration is
depicted.
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˚
FIGURE 3 Minimum energy structures of [NHC-O]2Zr(CH3)2 possible diastereoisomers. All distances are in A. Zr-C(NHC) distances
are in black, Van der Waals distances are in red.
enantiomers of the corresponding D structures, from now on
only K configurations will be considered in our discussion.
NHC and alkoxide groups. Such a ligand coordination mini-
mizes sterical interactions, being, in turn, not suitable for po-
lymerization. The formation of C and E isomers, presenting
cis methyl groups on the metal, is thermodynamically unfa-
vored due to the high internal and free energies. Indeed, N-
methyl group seems to play a crucial role on the stability of
these complexes, due to the Van der Waals interactions with
other groups (see red distances in Fig. 3). On the other
hand, D isomer can be considered as stable as A, presenting
a free energy in benzene only 0.1 kcal/mol higher than A. It
is worth noting that Isomer D not only is suitable for poly-
merization, bearing cis methyl groups on the metal, but also
shows a C2 symmetry, which is consistent with the fraction
of isotactic polypropylene obtained during the propylene
polymerization.
Minimum energy structures for all five diastereoisomers
(Fig. 3) were located by DFT calculations, and the corre-
sponding internal and free energies in gas phase and ben-
zene are reported in Table 3. Lowest energy structure A
shows trans methyl groups on the metal, as well as the two
TABLE 3 Internal and Free Energies in Gas Phase and Benzene
of Minimum Energy Structures Reported in Figure 3
E(gas)a
G(gas)a
E(benz)a
G(benz)a
A
B
C
D
E
a
0
0
0
0
2.7
3.4
1.7
1.7
3.7
3.2
0.9
3.4
2.4
2.5
1.0
1.1
3.4
2.3
0.1
2.7
CONCLUSIONS
The synthesis, characterization, and catalytic behavior of
octahedral zirconium catalysts bearing [NHC-O] ligands have
Energies are in kcal/mol.
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been reported. In particular, complexes with [NHC-O] pre-
senting ethenyl and cyclohexenyl alkoxide groups as N-sub-
stituents were obtained, and their behavior in polymeriza-
tion were compared with that of previously reported
complex bearing cyclopentenyl alkoxide NHC ligand.10
product was precipitated as white solid from cold acetone
and isolated in good yield. Yield ¼ 40% (10.1 g).
ELEM. ANAL. of [2a]: found (%): C 28.3 , H 4.2, N 11.1, I 50.0.
Calc. for C6H11IN2O (%): C 28.4, H 4.3, N 11.0, O 6.3, I 50.0.
1H NMR (CD3CN, 298 K): 8.74 (s, 1H, NCHN); 7.46 (s, 1H,
NCHCH); 7.37 (s, 1H, NCHCH); 4.24 (m, 2H, OCH2); 3.84 (s,
3H, NCH3); 3.74 (m, 2H, NCH2).
The activity of these new complexes in ethylene polymeriza-
tion depends on the Al/Zr ratio. Indeed, activity of Catalyst 2
reaches a plateau over Al/Zr ratio ¼ 1000, whereas activity
of Catalyst 3 linearly increases at least until ratio Al/Zr ¼
3000. Moreover, polyethylene molecular weights increase, for
both catalysts, by increasing the ratio Al/Zr. This is possibly
a consequence of higher [MAOX2]ꢀ concentration that could
be coordinated to the catalytic sites, preventing the b-hydro-
gen elimination. Bimodal distribution of molecular weights
of all obtained polyethylenes suggests the presence for both
catalytic systems of more than one active site.
13C {1H} NMR (CD3CN, 298 K): 137.7 (NCHN), 124.7
(NCHCH), 124.2 (NCHCH), 60.9 (OCH2), 53.4 (NCH2), 37.4
(NCH3).
Synthesis of Imidazolium Salt
(Imidazolium-N-methyl-N0-cyclohexyl-2-hydroxy-iodide)
(C10H17IN2O FW 5 308.03) [3a]
The synthesis of imidazolium salt was performed in the
same way as compound [2a]. A high pressure ampoule was
charged with 0.1 mol of cyclohexene oxide (10.1 mL) and a
stoichiometric amount of imidazole (7.8 g, 0.1 mol), then the
Both catalysts also promote the propylene polymerization
prevailingly giving a highly isotactic polymer fraction. Molec-
ular modeling studies on the stability in solution of diaster-
eoisomers of the alkylated form of Zr Complex 2, [NHC-
O]2Zr(CH3)2, indicated that the presence of one of the most
stable species, bearing cis methyl groups on the metal and
showing a C2 symmetry (Isomer D), is consistent with the
fraction of isotactic polypropylene obtained during the
polymerization.
ꢁ
reaction mixture was left to react at 50 C overnight. Aceto-
nitrile dry (30 mL) and 0.1 mol of iodomethane (7.2 mL)
were added, and the mixture was stirred for 2 h at 80 ꢁC.
The solvent was concentrated to yield a pale yellow oil; the
product was precipitated as white solid from cold acetone.
Yield ¼ 50% (15.4 g).
ELEM. ANAL. of [3a]: found (%): C 39.1, H 5.4, N 9.0, I 41.1.
Calc. for C10H17IN2O (%): C 39.0, H 5.5, N 9.1, O 5.2, I 41.2.
EXPERIMENTAL
Spectroscopic Measurements
1H NMR (CDCl3, 298 K): 9.51 (s, 1H, NCHN); 7.38 (d, 1H,
NCHCH); 7.30 (d, 1H, NCHCH); 4.26 (m, 1H, OCH); 4.02 (s,
3H, NCH3); 3.81 (m, 1H, NCH); 2.14 (m, 2H, OCHCH2); 1.81
(m, 2H, NCHCH2); 1.60 (b, 4H, CH2CH2CH2CH2).
ESI-MS spectra were performed on a QuattroMicro triple
quadrupole mass spectrometer equipped with an electro-
spray ion source.
13C {1H} NMR (CDCl3, 298 K): 136.4 (NCHN), 123.4
(NCHCH), 121.3 (NCHCH), 72.1 (OCH), 66.4 (NCH), 37.5
(NCH3), 34.5 (OCHCH2), 31.6 (NCHCH2), 24.8 (AOCHCH2CH2-
CH2CH2), 24.3 (AOCHCH2CH2CH2CH2).
Chemistry
All manipulations were performed under oxygen- and mois-
ture-free atmosphere in an MBraun MB 200 glove-box. All
the solvents were thoroughly deoxygenated and dehydrated
under nitrogen by refluxing over suitable drying agents;
while NMR deuterated solvents (Euriso-Top products) were
kept in the dark over molecular sieves.
Synthesis of Bis(imidazolidene-N-
methyl-N’-ethilenyl-2-alkoxy)zirconium-diamide [2]
In glove box, a Schlenk ampoule was charged with imidazo-
lium salt [1a] (0.48 g, 1.9 mmol) and then, under an inert
atmosphere, THF (20 mL) at ꢀ50 ꢁC was added. With the
same procedure, a solution of potassium hexamethyldisila-
All manipulations were performed under oxygen- and mois-
ture-free atmosphere in a MBraun MB 200 glove-box. 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. Zirconium-tetraethylamide,
imidazole, cyclohexene oxide and ethylene oxide, hexamethyl-
disilazide, and iodomethane (Strem, Aldrich) were used as
received.
ꢁ
zide (0.76 g, 3.8 mmol) in 10 mL of THF at ꢀ50 C was pre-
pared and added to the other one, to obtain the potassium
alkoxide-carbenes. The mixture was allowed to slowly warm
to room temperature and was stirred for 2 h. After this time,
the solution was added to a suspension of zirconium salt
ꢁ
(0.36 g, 0.95 mmol) in 10 mL of THF at ꢀ50 C. The result-
Synthesis of Imidazolium Salt (Imidazolium-N-methyl-
N0-ethyl-2-hydroxy-iodide) (C6H11IN2O Mw 5 253.99) [2a]
The synthesis of imidazolium salt was performed by reaction
between 0.1 mol of ethylene oxide (5.0 mL) and a stoichio-
metriꢁc amount of imidazole. The addition was performed at
ꢀ10 C, the mixture was stirred at 70 ꢁC for 12 h and then
was added 0.1 mol of iodomethane and 20 mL of acetoni-
trile. The solution was warmed upto 80 ꢁC for 12 h. The
ing mixture was allowed to slowly warm at room tempera-
ture and was stirred for 12 h, then filtered through celite,
and washed with hexane. The solvent was removed under
reduced pressure to yield the zirconium complex [2] (0.7 g,
1.4 mmol, 76% yield).
ELEM. ANAL.: found (%): C 49.2, H 7.3, N 17.6, Zr 19.0. Calc.
for C20H38N6O2Zr (%): C 49.4, H 7.88, N 17.3, O 6.6, Zr 18.8.
6
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1H NMR (300 MHz, CDCl3, 298 K): 6.08 (s, 1H, NCHCH), 5.95
(s, 1H, NCHCH), 4.28 (m, 2H, OCH2), 3.75 (m, 2H, NCH2),
3.69 (s, 3H, NCH3), 3.39 (m, 2H, NCH2CH3), 1.32 (m, 3H,
NCH2CH3). 13C NMR (400 MHz, C6D6, 298 K): 196.3 (NCN),
119.64 (NCHCH), 118.06 (NCHCH), 66.59 (OCH2), 54.07
(NCH2), 46.00 (NCH2CH3), 36.25 (NCH3), 16.44 (NCH2CH3).
Measurements
Ligands and Complexes Characterization
The elemental analyses for C, H, and N were recorded on a
ThermoFinnigan Flash EA 1112 series and were performed
according to standard microanalytical procedures. The ele-
mental analyses for I was performed by atomic absorption
spectrophotometer AAnalyst model 100 (Perkin Elmer)
equipped with hollow-cathode lamp Lumina Ag (Perkin
Elmer) using the software AAwinLabAnalyst. Iodide was
determined indirectly by reaction of AgNO3 with Iꢀ, precipi-
tation of AgI which was dissolved in Na2S2O3. Silver content
in the solution was determined by FAAS, and the iodide con-
tent was calculated using the content of silver.12 1H NMR,
Synthesis of Bis(imidazolidene-N-methyl-N’-cyclohexenyl-
2-alkoxy)zirconium-diamide [3]
In glove box, a Schlenk ampoule was charged with imidazo-
lium salt (2 g, 6.5 mmol, FW ¼ 308.04) and then, under an
inert atmosphere, THF (20 mL) at ꢀ50 ꢁC was added. With
the same procedure, we prepare a solution of potassium hex-
amethyldisilazide (2.6 g, 2 equiv., 13 mmol, FW ¼ 199.49) in
10 mL of THF, and we added this solution to the other one
to obtain the potassium alkoxide-carbenes. Then, the mixture
was allowed to slowly warm to room temperature and was
stirred for 2 h. After this time, this solution was added to a
suspension oꢁf Zirconium salt (1.25 g, 3.3 mmol) in 10 mL of
THF at ꢀ70 C. The resulting mixture was allowed to slowly
warm to room temperature and was stirred for 12 h, then
filtered through celite, and washed with hexane. The solvent
was removed under reduced pressure to yield the zirconium
complex [3]. (1.99 g, 3.4 mmol, 51% yield). ELEM. ANAL.:
found (%): C 56.7, H 8.2, N 14.3, Zr 15.2. Calc. for
homodecoupled H NMR, H COSY, H NOESY, HSQC, and 13C
{1H} NMR spectra were recorded at 298 K on a Bruker
Avance 300 spectrometer operating at 300 MHz (1H) and 75
MHz (13C) and referred to internal tetramethylsilane. ESI-MS
spectra were performed on a QuattroMicro triple quadrupole
mass spectrometer equipped with an electrospray ion
source.
1
1
1
Polymers Analysis
The samples of polyethylenes and polypropylenes for 13C
NMR analysis were prepared by dissolving polymer sample
(40 mg) into tetrachloro-dideuterio-ethane (0.5 mL). The
C28H50N6O2Zr (%): C 56.6, H 8.48, N 14.1, O 5.4, Zr 15.4.
ꢁ
spectra were recorded at 100 C using hexamethyldisiloxane
(HMDS) as internal chemical shift reference.
1H NMR (300 MHz, C6D6, 298 K): 6.37 (s, 1H, NCHCHN),
6.15 (s, 1H, NCHCHN), 4.19–3.77 (m, 1H, OCH), 3.86 (s, 3H,
NCH3), 3.45–3.38 (m, 1H, NCH), 3.77–3.61 (m, 2H,
NCH2CH3), 1.93 (m, 2H, OCHCH2), 1.69 (m, 2H, NCHCH2),
1.51 (m, 4H, CH2CH2CH2CH2), 1.10–0.94 (m, 3H, NCH2CH3).
13C NMR (400 MHz, DMSO-d-, 298 K): 198.6 (NCN), 118.6
(NCHCHN), 116.8 (NCHCHN), 79.6 (OCH), 66.0 (NCH), 47.4
(NCH2CH3), 34.8 (OCHCH2), 33.2 (NCH3), 31.4 (NCHCH2),
25.2 (CH2CH2CH2), 24.8 (CH2CH2CH2), 17.42 (NCH2CH3).
The GPC analyses of the samples were performed at 135 ꢁC
by Waters instrument GPCV 2000 equipped with refractive
index and viscosimeter detectors, using four PSS columns set
consisting of 105, 104, 103, 102 Å (pore size) ꢀ 10 lm (parti-
cle 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 analyses have been per-
formed on a DSC 2920 apparatus manufactured by TA
Polymerization
Run 1–9
Instruments, calibrated against an indium standard (Tm
¼
Polymerizations of ethylene were performed in a 250 mL
glass-autoclave introducing the amount of catalyst and coca-
talyst dissolved in 150 mL of toluene, as reported in Table 1.
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 etha-
nol, the polymers were recovered by fiꢁltration, washed with
fresh ethanol, and dried in vacuo at 60 C.
156.6 ꢁC), with heating scans from ꢀ10 to 200ꢁC, at a 10
ꢁC/min heating rate, under a flowing nitrogen atmosphere.
Specimens were sealed in aluminum pans.
Computational Details
The density functional calculations were performed on all
the systems with the Gaussian09 set of programs.13 BP86
was used as a functional and gradient corrections were
taken from the work of Becke14 and Perdew.15,16 The elec-
tronic configuration of the molecular systems was described
by the split-valence basis set with polarization functions of
Ahlrichs and coworkers17 (standard SVP basis set in Gaus-
sian09), for H, C, N, and O. For Zr, we used the small-core,
quasi-relativistic Stuttgart/Dresden effective core potential
basis set (standard SDD basis set in Gaussian09).18 Minimum
free energy structures were characterized by the presence of
zero imaginary frequency. Solvent effects have been esti-
mated in calculations based on the polarizable continuous
solvation model PCM. Benzene was chosen as model solvent.
Run 10–13
Polymerizations of propylene were performed in a 250 mL
glass-autoclave introducing the amount of catalyst and coca-
talyst dissolved in 150 mL of toluene, as reported in Table 2.
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 etha-
nol, the polymers were recovered as usual.
Fractionations
Polypropylenes were fractionated by exhaustive extractions
with boiling heptane in a Kumagawa extractor.
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8 (a) Downing, S. P.; Danopoulos, A. A. Organometallics 2006,
25, 1337–1340; (b) Liddle, S. T.; Edworthy, I. S.; Arnold, P. L.
Chem. Soc. Rev. 2007, 36, 1732–1744; (c) Downing, S. P.; Gua-
dano, S. C.; Pugh, D.; Danopoulos, A. A.; Bellabarba, R. M.;
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ACKNOWLEDGMENTS
The authors thank Dr. Patrizia Oliva, Dr. Patrizia Iannece, and
Dr. Ivano Immediata for technical assistance. Computational
support from CINECA—High Performance Computing Portal
and financial support of ‘‘Ministero dell’Universita e della
Ricerca Scientifica e Tecnologica’’ are gratefully acknowledged.
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