Flavone complexes of Ti and Zr active in ethylene polymerization

Article abstract of DOI:10.1016/j.apcata.2013.08.015

Abstract Two new complexes of Ti and Zr were synthesized with 3-hydroxyflavone bidentate ligand and investigated in homogeneous ethylene polymerization. Both complexes were characterized by UV-VIS, ¹H and ¹³C NMR, and electrochemical

Full text of DOI:10.1016/j.apcata.2013.08.015

Applied Catalysis A: General 467 (2013) 439–449
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Flavone complexes of Ti and Zr active in ethylene polymerization
Grasiela Ghenoa, Nara Regina de Souza Bassob, Marco Antônio Ceschia, Paolo R. Livottoa,
Alanjone Azevedo Nascimentoc, Zênis N. da Rochac, Griselda Barrera Gallanda,∗
a Instituto de Química, Universidade Federal do Rio Grande do Sul, Avenida Bento Gonc¸ alves n◦ 9500, 91501-970 Porto Alegre, Brazil
b Faculdade de Química, Pontifícia Universidade Católica do Rio Grande do Sul, Avenida Ipiranga n◦ 6681, 90619-900 Porto Alegre, Brazil
c Instituto de Química, Universidade Federal da Bahia, Campos de Ondina, 40170-290 Salvador, Bahia, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new complexes of Ti and Zr were synthesized with 3-hydroxyflavone bidentate ligand and investi-
gated in homogeneous ethylene polymerization. Both complexes were characterized by UV–VIS, 1H and
13C NMR, and electrochemical studies. The geometries and energies of all possible isomeric species were
studied by full unconstrained optimizations performed at Density Functional Theory (DFT) level. The
polymerization reactions were performed at different experimental conditions with methylaluminox-
ane (MAO), as the cocatalyst. Both complexes were active in ethylene polymerization in all the conditions
used. However, zirconium complex showed the best activity comparing to the titanium complex at 2500
Al/M ratio. The polymers obtained were high density polyethylene with ultra high molecular weights.
© 2013 Elsevier B.V. All rights reserved.
Received 28 April 2013
Received in revised form 7 August 2013
Accepted 9 August 2013
Available online xxx
Keywords:
Synthesis
Complex
3-Hydroxyflavone
Catalyst
Polyethylene
1. Introduction
titanium complexes of methylmaltol and guaiacol and verified their
catalytic activity in the polymerization of ethylene and propylene
The polyolefin production grows, continuously, since the dis-
covery of Phillips and Ziegler–Natta catalysts in the early 1950s
[1,2] and this drives the study for new catalysts and new meth-
ods for improving the synthesis of these polymers. Polyolefins
are cheap and versatile and present good properties that make
them widely used in various segments, such as, packaging, build-
ing and construction, transportation, medical and health, electrical
and electronic devices, agriculture, sport, leisure and design. Global
industrial activity in the polyolefin production has reached around
125 millions of tons in 2011 and only in Europe were produced
22 million of tons. More than a half of this total amount was
dedicated to manufacturing polyethylenes, including low density
polyethylene (LDPE), linear low density polyethylene (LLDPE) and
high density polyethylene (HDPE) [3,4].
Intense research has involved the catalysis area and differ-
ent complexes types for olefin polymerization were investigated,
including post-metallocene complexes. Post-metallocene systems
can provide some advantages over conventional metallocenes, as
for example, the chemical synthesis of complexes can be sim-
pler and more scalable. Some post-metallocene catalysts based on
alkoxides bidentate ligands have been proposed in the literature.
Sobota synthesized and characterized the crystal structure of the
[5]. Due to the flexibility of these ligands they easily form complexes
with transition metals. Our research group has synthesized various
analogs complexes of Ti and Zr based in methylmaltol and ethyl-
maltol ligands. The synthesized complexes were tested in ethylene
polymerization showing activity in homogeneous and heteroge-
neous conditions [6–9].
The coordination of alcoxide bidentate ligands derivates from
flavonoids with transition metal ions has been also investigated.
Flavonoids are natural substances of low molecular weight iso-
lated from a wide variety of plants. The basic structure of these
compounds consists of 15 carbon atoms arranged into three rings
(C6–C3–C6), two substituted phenolic rings (A and B) and a pyrone
ring (C) coupled to ring A. The carbon atoms are numbered 2–10
in A and C rings and 1ꢀ to 6ꢀ in B ring [10]. There are several
classes of flavonoids differing in the oxidation state and position
of hydroxyl and methoxy groups in the two phenyl rings. Most
flavonoids contain one or more ortho hydroxy phenolic groups, or
one phenolic group with a nearby carbonyl group [11]. Flavonoids
are used against various diseases due to their antioxidant activity
and pharmaceutical properties as antiinflammatory, antiallergic,
anticancer and antimicrobial activities [12]. The 3-hydroxyflavone
(3HF) ligand (Fig. 1) has a hydroxyketone functional group present
on ring C that is capable of forming complexes with a vari-
ety of metals [13]. Several coordination reactions with different
metals have been investigated for biomedical purposes, such as
complexation of flavonoids with aluminum [13], vanadium [14],
∗
Corresponding author. Tel.: +55 51 33087317.
E-mail addresses: griselda.barrera@ufrgs.br, griselda@iq.ufrgs.br (G.B. Galland).
0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.08.015

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G. Gheno et al. / Applied Catalysis A: General 467 (2013) 439–449
washed twice with diethyl ether, dissolved in dichloromethane
and recrystallized in hexane. The complex was dried under vac-
uum. The dichlorobis(3-hydroxyflavone)titanium(IV), complex 1,
brown solid, yield 70%. Elemental analysis: % theoretical calculated
for C30H18O6TiCl2 (M = 592.77 g/mol): C 60.73%, H 3.04%, found:
C 60.44%, H 3.08%. 1H NMR (300 MHz, CDCl3), ı (ppm): 8.52 (d,
ꢀ
ꢀ
J = 7.4 Hz, 2H, H6 ,2 ); 8.08 (d, J = 7.8 Hz, 1H, H5); 7.81 (dd, J = 1.4 and
ꢀ
ꢀ
ꢀ
8.6 Hz, 1H, H8); 7.77 (t, J = 8.7 Hz, 1H, H7); 7.6–7.4 (m, 3H, H3 ,4 ,5 );
7.42 (t, J = 6.5 Hz, 1H, H6). 13C NMR (75 MHz, CDCl3), ı (ppm): 171.4
ꢀ
(C4); 164.9 (C9); 155.6 (C2); 147.1 (C3); 134.9 (C7); 131.9 (C4 ); 129.7
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
(C1 ); 129.3 (C3 ,5 ); 129.0 (C2 ,6 ); 125.90 (C5); 125.6 (C6); 118.3
(C8); 117.5 (C10). The dichlorobis(3-hydroxyflavone)zirconium(IV),
complex 2, yellow solid, yield 77%. Elemental analysis: % theoret-
ical calculated for C30H18O6ZrCl2 (M = 636.12 g/mol): C 56.59%, H
2.83%, found: C 53.03%, H 3.09%. 1H NMR (300 MHz, DMSO-d6), ı
ꢀ
ꢀ
(ppm): 8.25 (d, J = 8.3 Hz, 2H, H6 ,2 ); 8.16 (d, J = 8.1 Hz, 1H, H5); 7.95
(m, 2H, H7,8); 7.65 (td, J = 1.7 and 6.2 Hz, 1H, H6); 7.39 (t, J = 8.4 Hz,
1H, H4 ); 7.30 (t, J = 6.8 Hz, 2H, H5 ,3 ). 13C NMR (75 MHz, DMSO-d6),
ı (ppm): 179.3 (C4); 154.6 (C9); 148.6 (C2); 146.2 (C3); 134.9 (C7);
ꢀ
ꢀ
ꢀ
Fig. 1. Structure and atomic numbering of 3HF.
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
130.5 (C1 ); 130.3 (C4 ); 128.3 (C3 ,5 ); 127.2 (C2 ,6 ); 125.9 (C5); 123.7
(C6); 118.8 (C10); 117.7 (C8).
nickel (II) [15], iron (III) [16], copper [17], manganese and cobalt
[18].
Based on our previous works with the pyrone heterocyclic,
we studied the coordination of 3-hydroxyflavone (3HF) bidentate
chelating ligand with Ti and Zr and tested the behavior of the
two new complexes, dichlorobis(3-hydroxyflavone)titanium(IV)
and dichlorobis(3-hydroxyflavone)zirconium(IV) in ethylene poly-
merization. The advantage in using 3-hydroxyflavone ligand is its
provenance from renewable resources and its ease complexation
with transition metals. To the best of our knowledge there are not
works in the literature reporting those 3-hydroxyflavone-M (M = Ti
and Zr) complexes and their activity in ethylene polymerization.
Some parameters such as ethylene pressure, cocatalyst/catalyst
ratio and reaction temperature were studied in the aim to improve
the catalytic activity.
2.2.1. Complex characterization
The 1H NMR spectra of the complexes were recorded on a Var-
ian Inova 300 Spectrometer, using DMSO-d6 or CDCl3 as solvents.
The UV–VIS absorption spectra of the ligand and the complex were
recorded on a Varian Cary 100 spectrophotometer with quartz cells
of 1 cm path length at room temperature. The analyses were per-
formed using acetonitrile as solvent. Elemental analysis (C, H) was
performed in a 240 Perkin-Elmer.
2.2.2. Theoretical calculations
The geometries and energies of all possible isomeric species
were obtained by full unconstrained optimizations performed at
Density Functional Theory (DFT) level using the B3LYP hybrid
functional obtained by the three parameter fit of the exchange-
correlation potential suggested by Becke [19] and the gradient
corrected correlation functional of Lee et al. [20]. The polarized
Dunning-Huzinaga DZ basis set [21,22] was used for the hydrogen,
carbon, chloride and oxygen atoms. For the titanium and zirconium
atoms the inner shell electrons were represented by the Los Alamos
effective core potential (LANL2) of Hay and Wadt [23,24] and the
valence electrons were explicitly included using the associated DZ
basis set. All calculations were performed with the Gaussian 03
program using standard procedures and parameters [25].
2. Experimental
2.1.1. Reagents and materials
All experiments were performed under argon atmosphere using
the Schlenk technique. The 3-hydroxyflavone ligand (Aldrich) and
ZrCl4 (Merck) were used without purification. All the other reagents
used in the present study were of analytical grade. All the solvents
were dried by usual methods existing in the literature. MAO (Witco,
10% (w/w) Al in toluene solution) was employed as received. For
the Zr complex synthesis it was prepared the ZrCl4·2THF adduct.
2.2.3. Cyclic voltammetry
The electrochemical measurements, cyclic voltammetry (CV),
differential pulse voltammetry (DPV) and electrolysis were taken
with a potentiostat EG and G, 273A of Princenton Applied Research.
The electronic spectra were obtained with Hewlett-Packard 8453
Spectrometer. All experiments were carried out using a conven-
tional three electrodes cell: glassy carbon was used as working
electrode for CV and platinum gauze for electrolysis. An Ag/AgCl
electrode was used as the reference electrode and a platinum wire
as the auxiliary electrode. Ferrocene (+0.50 V versus Ag/AgCl), was
employed as internal standard in acetonitrile solution. A tetrabuty-
lammoniun tetrafluoroborate in acetonitrile solution was used as
supporting electrolyte. Successive spectra were recorded during
the redox process of the complexes. The supporting electrolyte does
not show redox process in the potential range evaluated.
2.1.2. Synthesis of ZrCl4·2THF adduct
The adduct of ZrCl4 and THF was prepared in a Schlenk under
inert atmosphere in the following way: 10 mL of THF was added
drop wise, at room temperature, to a stirred suspension of 8.8 g
(37.8 mmol) of ZrCl4 in 100 mL of dichloromethane. After 2 h, the
pink solution was transferred with a syringe to other Schlenk with
a fritted disk. 80 mL of hexane was added to the filtered solution
that was filtered again to obtain a white solid that was washed tree
times with 10 mL of hexane and dried under vacuum.
2.2. Synthesis of the complexes
100 mg of the ligand 3-hydroxyflavone (0.42 mmol) was dis-
solved in dichloromethane using a Schlenk. Another solution of
MCl4 (0.21 mmol, TiCl4 or ZrCl4·2THF) into dichloromethane was
drop wise into the solution containing the ligand. The mixture
was stirred at room temperature for 1.5 h. Then, the solid was
2.3. Polymerization reactions
Ethylene polymerizations were performed in a PARR 4843
reactor with 100 mL capacity. Into the reactor was added

G. Gheno et al. / Applied Catalysis A: General 467 (2013) 439–449
441
Scheme 1. Synthesis of complexes 1 and 2.
30 mL of toluene, methylaluminoxane (MAO) as co-catalyst (co-
catalyst/catalyst ratio: 1000, 1500 and 2500) and 10−6 M of catalyst.
The polymerization reactions were performed at 2.8, 4 and 6 bar
of ethylene at 40 and 60 ◦C during 30 min. Acidified ethanol with
chloride acid was used to quench the process, and reaction prod-
ucts were separated by filtration, washed with ethanol and acetone,
and finally dried.
HT4, HT3). The mobile phase used was trichlorobenzene (TCB) with
0.1% of BHT at 150 ◦C at a flow rate of 1 mL/min.
3. Results and discussion
The complexes dichlorobis(3-hydroxyflavone)titanium(IV) (1)
and dichlorobis(3-hydroxyflavone)zirconium(IV) (2) were synthe-
sized from 3-hydroxyflavone ligand, as it is shown in Scheme 1.
Fig. 2 shows the 1H NMR spectra of 3HF ligand and complex
1. The resonance of the hydroxyl group 3HF ligand at 7.07 ppm
disappears in the complex 1 spectrum showing the deprotonation
of the 3HF ligand due to the titanium metal insertion. It can be
observed in the complex spectrum, the shift of most of the pro-
ton signals to higher frequencies in relation to the ligand spectrum,
indicating the deshielding effect of the aromatic protons due to
the donation of electronic density to the metal. The 13C chemical
shifts of 3HF ligand have already been reported in the literature
2.3.1. Polymer characterization
The melting points (Tm) and crystallinities (Xc) of the polymers
were determined using a Differential Scanning Calorimeter DSC
Q20 TA Instruments with heating of 20–160 ◦C and heating rate
of 10 ◦C/min with 50 mL/min of N2 flow rate. The heating cycle
was performed twice, but only the results of the last scan were
considered. The molar masses and molar mass distributions were
measured by Gel Permeation Chromatography GPC PL 220 Polymer
with RI and VI detector equipped with Water columns (HT6, HT5,
Fig. 2. 1H NMR spectra of the 3HF ligand and complex 1 in CDCl3.

442
G. Gheno et al. / Applied Catalysis A: General 467 (2013) 439–449
Fig. 3. 13C NMR spectra of 3HF ligand and complex 1 in CDCl3.
[13,26] and our data is in accordance. Comparing 13C NMR spec-
tra of the free ligand and complex 1 at 180–110 ppm region is also
observed the displacement of some carbon signals in the complex
spectrum, caused by the coordination of the ligand to the metal
(Fig. 3).
The 1H NMR spectra of 3HF ligand and complex 2 are shown
in Fig. 4. The signal at 9.64 ppm in the 3HF ligand spectrum dis-
appears in the complex 2 spectrum showing the deprotonation of
hydroxyl group due to the zirconium metal insertion. In addition
it was observed the shift of all the protons signals in complex 2
Fig. 4. 1H NMR spectra of the 3HF ligand and complex 2 in DMSO.

G. Gheno et al. / Applied Catalysis A: General 467 (2013) 439–449
443
Comparing the relative energy and population percentage of all
isomers shown in Fig. 6, it is observed that the cis structures of
both complexes have the lowest relative energy and are therefore
more stable than the trans structures (Table 1). Among the cis con-
figurations, cis 2 is the structure of the lowest energy and greatest
population percentage for both complexes 1 and 2, therefore the
most stable and most likely to exist. This theoretical study is consis-
tent with the results obtained in 1H NMR spectra of the complexes,
which show signs of a main isomer, probably cis 2, in greater pro-
portion. Furthermore, the higher possibility of obtaining complexes
with cis structure is favorable for our purposes because only cis
isomers are capable of polymerizing ethylene [8].
3.1. Electrochemical behavior
The electrochemical behavior of complexes 1 and 2 was inves-
tigated by cyclic voltammetry (CV), differential pulse voltammetry
(DPV) and controlled potential electrolysis (CPE). Cyclic and pulse
voltammograms for uncoordinated 3HF ligand and complexes were
recorded in the potential range between 1.8 V and −2.2 V with a
scan rate of 100 mV s−1 for DPV. The uncoordinated ligand was also
studied using CV at scan rates from 0.05 to 0.3 V s−1
.
Fig. 7 illustrates the DPV with cathodic and anodic scan for the
3HF ligand and complex 1 which demonstrated the reversible of
each reduction process. The 3HF ligand shows cathode signals at
+1.1, −1.43 and −1.85 V, a shoulder at −1.56 V vs Ag/AgCl (curve a,
Fig. 7a). In the anodic scan (curve a, Fig. 7b) there are some signs cor-
responding to the cathode signals with small displacements due to
the existence of a chemical reaction coupled with the redox process.
The existence of chemical reaction coupled to 3HF ligand can
be proven by recording the cyclic voltammograms. Fig. 8 (curve
a) shows the potential sweep begin at +0.5 V followed by nega-
tive potential cathodic signs at −1.50 and −1.90 V and the sweep
reverse that illustrates anodic signals at −1.0 and −0.6 V. Moreover,
an initial scan at +0.5 V followed by positive potential and further
cathodic scan, indicates a lower current intensity of the cathode
signals (−1.50 and −1.90 V) and the existence of cathodic signals
around −0.1 and −1.0 V (curve b, Fig. 8). In curve b can also be seen
a significantly alteration of the current intensities of the anodic
signals −1 and −0.6 V and the appearance of an anodic signal at
0.1 V vs Ag/AgCl. Therefore, modification of the curves profile cur-
rent × potential is indicative that the oxidation and reduction of the
3HF ligand are followed by coupled chemical reaction.
Fig. 5. 13C NMR spectra of 3HF ligand and complex 2 in DMSO.
in relation to the 3HF ligand spectrum. Fig. 5 shows the 13C NMR
expanded spectra of the 3HF ligand and complex 2. In the 1H NMR
spectra of both complexes, it was observed the formation of iso-
mers in small quantities at 8.0 and 7.7 ppm (complex 1) and around
8.5 and 7.8 ppm (complex 2). The hypothesis for the formation of
isomers was also justified by the DFT calculations, which shows
the possibility of existence of several isomers for each complex
synthesized.
The theoretical calculations show that complex 1 can present
five isomers, three with cis configuration and two with trans con-
figuration related to the position of the chlorine atoms. On the other
hand, complex 2 can have three cis and three trans configurations.
Fig. 6 shows the structures of these isomers in which the green
balls represent chlorine atoms; red balls, oxygen atoms; white balls,
hydrogen atoms; ash balls, carbon atoms and the larger balls, the
metal (Ti or Zr). The trans 2 structure of the complex 1 is not present
in Fig. 6 because this structure is unstable and it is unlikely to exist.
The curves of Fig. 7 show the 3HF ligand cathodic and anodic
processes at +1.1 V and shoulder around +0.55 and +0.76 V (anodic
scan) vs Ag/AgCl. The complex 1 shows curves of redox processes
displaced to more positive values at 1.43 V and shoulders around
0.76 and 1.0 V (anodic scan), which is indicative of the change in
electron density in the 3HF ligand when coordinates to the [TiCl2]2+
fragment. The relocation of the relative values of potential to more
positive values indicates greater resistance of the ligand to be oxi-
dized. The comparison between the current × potential curves of
Table 1
Molecular energies and energies relating to more stable configurations of complexes 1 and 2.
Isomer
Complex 1
Energya
Complex 2
Energya
Relative energyb
Population (%)c
Relative energyb
Population (%)c
Trans 1
Trans 2
Trans 3
Cis 1
Cis 2
Cis 3
−1693.8065209
–
−1693.8113058
−1693.8190431
−1693.8221281
−1693.8124134
9794
–
6791
1936
0000
6096
0.00
–
0.00
3.67
96.33
0.00
−1682.3552703
−1682.3507412
−1682.3594538
−1682.3656809
−1682.3681339
−1682.3616324
8072
10,914
5447
1539
0000
0.00
0.00
0.01
6.92
92.98
0.09
4079
a
Hartree.
kcal/mol.
At room temperature, 298 K.
b
c

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G. Gheno et al. / Applied Catalysis A: General 467 (2013) 439–449
Fig. 6. Structures of the possible isomers of (a) complex 1 and (b) complex 2.
the 3HF ligand and complex 1, in the range from 0.0 to −2.0, sug-
gests that the cathodic signals −0.37, −0.81 and −1.22 V vs Ag/AgCl
(curve b, Fig. 7a) correspond to redox processes centered on the
metallic center, as illustrated in Scheme 2. The proposed electrode
process was based on the followed by analogous complexes with
pyrone ligands [8,9]. These signals are not present in the DPV pro-
file of 3HF ligand. It was also found that there are shoulders at
−1.52 and −1.85 V, which due to the proximity of the uncoordi-
nated ligand signs (curves a, cathodic and anodic sweep) can be
assigned to redox processes centered in the coordinated ligand,
redox process centered at ligand and metallic center. Comparing
the curves current vs potential of the Lewis base (3HF ligand), unco-
ordinated and coordinated to the fragment [ZrCl2], lead to assign
that the cathodic sign of the complex 2 at −0.85 (curve b, Fig. 9a)
is a Zr(IV)/III process. Moreover, there are signs at −1.28 and −1.56
in the same curve, which due to the proximity to signals from the
uncoordinated ligand can be attributed to redox processes centered
in the ligand, L/L− and L−/L2−, and also a contribution of redox
process centered on the metal center, illustrated in Scheme 2.
Comparing complex 2 with complex 1, it is apparent that the
redox processes centered in the ligand on positive potential around
1.2 V and shoulder at 1.6 (cathodic) show no significant change
in displacement with the complex 2, the same occurs for redox
L/L− and L−/L2−
.
The profiles of the pulse voltammogram for complex 2 when
compared with the free ligand, as occurs for complex 1, illustrated

G. Gheno et al. / Applied Catalysis A: General 467 (2013) 439–449
445
Fig. 7. Pulse voltammograms of 3HF ligand (curve a) and complex 1 (curve b): (a) cathodic scan and (b) anodic scan. v= 100 mV s−1
.
Scheme 2. Redox process of the complexes 1 and 2 [MCl2(3HF)] in acetonitrile solution.
negative potentials. These results are indicative of greater binding
strength between Ti(IV)-3HF compared to Zr(IV)-3HF, thus leading
to greater changes in the electron density of the donor atoms of the
coordinated ligand to Ti(IV), therefore there is less tendency for the
3HF ligand to be oxidized.
The results presented for 3HF ligand lead us to propose that
the electrode process has a coupled chemical reaction, so for the
complexes [MCl2(3HF)] it can be said that they are reversible with
a coupled chemical reaction.
The data indicates that after reduction of Ti(IV) and Zr(IV) there
is a labilization of the 3HF ligand, confirmed by the studies of
controlled potential electrolysis that support the electrode pro-
cess proposal shown in Scheme 2. Profiles of the electronic spectra
of the original and electrolyzed complexes also appoint to ligand
labilization.
Fig. 10 shows the results of controlled potential electrolysis
(CPE) at −1.3 V for complex 1 and at −1.4 V vs. Ag/AgCl for complex
2, with appropriate load for engagement of 1, 2 and 3 electrons,
which were crucial for confirming the attributed mechanism of
electrode. The pulse voltammogram of titanium(IV) complex elec-
trolyzed, with sufficiently load for engaging consumption of ca
1 F mol−1 (1 electron) illustrated the disappearance of the redox
process metal centered at −0.37 V, as shown in Fig. 10a, as a result
of chemical reaction subsequent to the reduction of Ti(IV) in the
original complex. With the involvement of 3 electrons the profile
curve illustrates the existence of cathodic signals corresponding to
the reduction of uncoordinated and electrolyzed ligand. For com-
plex 2 analogously to complex Ti(IV) it occurs the disappearance
of the cathodic signal at −1.27 V attributed to process Zr(IV)/III,
Fig. 10b. Changes in the profile of the curve with the passage of
charge up to 3 electrons lead to propose the existence of a coupled
chemical reaction.
The electronic spectra of the 3HF ligand and the complexes 1
and 2 are illustrated in Fig. 11. Generally flavonoid compounds
have two major absorption bands in the ultraviolet/visible spec-
tra corresponding to the B-ring Band I between 300 and 500 nm
(cinnamoyl system) and A-ring, Band II <280 nm (benzoyl system)
*
due to ␲ → ␲ electronic transition of an electron from the high-
Fig. 8. Cyclic voltammograms of 3HF ligand: (a) cathodic scan, (b) anodic scan.
v= 100 mV s−1
.
est occupied molecular orbital (HOMO) to the lowest unoccupied

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G. Gheno et al. / Applied Catalysis A: General 467 (2013) 439–449
Fig. 9. Pulse voltammograms of 3HF ligand (curve a) and complex 2 (curve b): (a) cathodic scan; (b) anodic scan; v= 100 mV s−1
.
Fig. 10. Cyclic voltammogram (a) complex 1 and (b) complex 2: (a) initial; (b) electrolyzed (1 electron); (c) electrolyzed (2 electron); (d) electrolyzed (3 electron). E
appliedcomplex1 = −1.3 V vs Ag/AgCl and E appliedcomplex2 = −1.4 V vs Ag/AgCl.
molecular orbital (LUMO) as a consequence of conjugation of the
double bonds [14,27]. The bands of these systems for 3HF ligand
appear at 340 and 300 nm, Band I, and 240 nm, Band II, Fig. 11a.
Complex 2 shows another absorption band at 400 nm, probably due
to a ligand charge transfer to Zr(IV), absent in the uncoordinated
ligand, Fig. 11c.
complexes show that after the reduction process of Ti the spec-
trum profile does not change as it occurs in the Zr complex,
which is indicative that the Ti(III)-3HF bond strength is stronger
than the Zr(III)-3HF, this also explain the smaller tendency of
the ligand to output the coordination sphere of the Lewis acid
Ti(III).
For the complex 1, Fig. 11b, analogously to the Zr(IV) complex,
the spectrum shows an absorption band due to the charge trans-
fer 3HF-Ti(IV). Studies of reduction of the metal center for both
3.1.1. Electrochemical analysis for [MCl2(3HF)2] system in the
presence of MAO
The cyclic voltammograms of complexes 1 and 2 in the ethylene
and MAO presence were analyzed and illustrated in Fig. 12. The
evaluation of the DPV in the ethylene atmosphere compared to
the original complex illustrates a small change in the range of
−0.8 to −2.0 V, which may result the ethylene coordination. This
minor modification shows little influence of the coordination of
ethylene to Ti(IV) and Zr(IV) in the electron density of the metal
centers. The profiles of the curves recorded after addition of MAO
in the solution (curve c, Fig. 12) illustrate the disappearance of the
anodic and cathodic waves centered on the metal and assigned to
the process M (IV/III). It can be seen in the system containing the
complex 1, cathodic signals at −1.55 and −1.9 V and a shoulder in
−1.75 V. Analogous, the complex 2 shows cathodic waves at −1.54
and −1.98 V. The potential values in both cases are close to those
reported for the 3HF ligand, however in analogy to metallocene
Zr(IV) complexes, it can be proposed the abstraction of a chloride
by the MAO, leading to the formation of the monomethylated
species [(3HF)2MCl(Me)]+ active in ethylene polymerization [28].
The formation of the active species can also be demonstrated
by the existence of activity of both complexes in the ethylene
Fig. 11. UV–vis spectra in acetonitrile at ambient temperature (a) 3HF ligand, (b)
complex 1 and (c) complex 2.

G. Gheno et al. / Applied Catalysis A: General 467 (2013) 439–449
447
Fig. 12. Cyclic voltammogram (a) complex 1 and (b) complex 2: (a) initial; (b) in ethylene atmosphere, (c) in the presence of MAO and in ethylene atmosphere, ratio Al/Ti = 5
and Al/Zr = 2.
polymerization as shown in Table 2. It is worth mentioning that
due to the similarity in DPV change when applying a suitable
potential for MIV/III reduction (Fig. 10) and the addition of MAO,
the possibility of chemical reduction of the metal center cannot
be discarded. However, due to the fact that the catalytic systems
showed to be active in ethylene polymerization the formation of
the methylated species are more probable.
postmetallocene complexes [30]. However, complex 2 presented a
quite high catalytic activity when polymerized with a Al/Zr = 2500
and 6 bar of ethylene pressure (entry 18). One of the factors that
could explain this difference in catalytic activity between both com-
plexes, it is the difference between oxidation potential of 3HF and
reduction potentials of Ti(IV) and Zr(IV), assigned as electrochem-
ical gap (G (V)). The anodic potential for 3HF is around 1.6 V vs
Ag/AgCl and cathodic potentials for TiIV/III and ZrIV/III occur at −0.37
and −1.27 V vs Ag/AgCl, that results in electrochemical values of
1.97 V and 2.87 V, respectively.
3.2. Ethylene polymerization
Catalyst activity was shown to depend on the complex’s metal
center and reaction conditions. For titanium complex, the increase
in the Al/Ti ratio from 1000 to 1500 increased the catalyst activity
in all the different monomer pressures studied, as illustrated
in Fig. 13. However the catalytic activity of complex 1 drops
dramatically with the increase of the Al/Ti till 2500 at 2.8 and 4 bar
of ethylene pressures, remaining constant at 6 bar. This decrease in
catalytic activity with increasing Al/Ti ratio may be related to steric
hindering caused in the complex due to the excessive amount of
MAO used, which can hinder the insertion of ethylene. Studies on
Ti complexes of methylmaltol ligand also showed that the catalytic
Catalytic activities of the complexes 1 and 2 were evaluated in
homogeneous ethylene polymerization in different experimental
conditions. In fact, the effect of Al/M ratio (Al/M = 1000, 1500 and
2500) and ethylene pressure (2.8, 4 and 6 bar) in catalytic activity
were studied. The polymerization reactions with complex 1 were
performed at 40 ◦C while the reaction with complex 2 was made
at 60 ◦C. At high temperatures the Zr complexes are more stable
and active than complexes based on Ti [8,29]. Table 1 shows the set
of evaluated polymerization conditions as well as polymer melt-
ing temperature and crystallinity. The complexes showed low and
moderate catalytic activity in accordance with literature data for
Table 2
Catalytic activity and properties of the homogeneous ethylene polymerization catalyzed by complexes 1 and 2 at different conditions.
Entry
Complex
Al/M
Ethylene pressurea
Activityb
Tm(◦C)
Tc(◦C)
Xc(%)
Mwc ×103
Mw/Mn
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
1
1
1
2
2
2
1
1
1
2
2
2
1
1
1
2
2
2
1000
1000
1000
1000
1000
1000
1500
1500
1500
1500
1500
1500
2500
2500
2500
2500
2500
2500
2.8
4
6
2.8
4
6
2.8
4
6
2.8
4
6
2.8
4
6
2.8
4
6
18.1
10.4
15.6
35.5
37.4
49.2
45.6
24.6
30.6
31.0
29.1
24.0
4.5
136
135
136
135
135
136
137
135
136
135
137
135
134
133
133
136
136
136
116
119
117
117
118
116
116
118
118
117
117
118
119
119
119
116
118
118
54
61
61
52
55
62
45
62
54
47
56
48
53
18
33
61
56
52
1358
nd
nd
nd
nd
636
394
>3000
nd
nd
nd
604
nd
nd
nd
nd
3.2
nd
nd
nd
nd
2.3
2.4
12.8
nd
nd
nd
2.2
nd
nd
nd
nd
nd
2.6
25.4
4.8
47.8
36.4
94.2
nd
692
Reaction conditions: solvent: toluene; time: 30 min; Al: Methylaluminoxane; [M]: 10−6 mol; temperature: complex 1 at 40 ◦C and complex 2 at 60 ◦C. nd: not determined;
ꢀc: crystallinity.
a
Bar.
kg/mol h bar.
g/mol.
b
c

448
G. Gheno et al. / Applied Catalysis A: General 467 (2013) 439–449
analysis and electrochemical studies. Results from UV–VIS and
NMR showed complexation of the ligand to the metal salt, MCl4.
NMR spectra also showed the presence of one main isomer. Den-
sity Functional Theory confirms the presence of mainly one isomer
with the higher probability for the cis isomers. The results of the
electrochemical study showed that the 3HF ligand is capable of
binding to the metal salt due to changes in electron density when
it coordinates to the [MCl2]2+ fragment. Studies of reduction of the
metal center for both complexes show that after the reduction pro-
cess of Ti the spectrum profile does not change as it occurs in the
Zr complex, which is indicative that the Ti(III)-3HF bond strength
is stronger than the Zr(III)-3HF. Furthermore, it was observed that
the active species are stabilized in the presence of MAO and also
in ethylene atmosphere. Both complexes were active in ethylene
polymerization at different reaction conditions. However, the Zr
complex showed a better catalytic activity in comparison with the
Ti complex, which it could be explained by the different electro-
chemical behavior.
Fig. 13. Influence of the Al/Ti molar ratio on the catalyst activity of complex 1.
Acknowledgements
We thank CAPES, FAPERGS-PRONEX and CNPq for financial sup-
port.
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Two new complexes of titanium and zirconium with the natural
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