Coordination compounds of chromium (+3) and vanadium (+3) and (+5) with 2,6-bis(diphenylhydroxymethyl)pyridyl ligand: Synthesis and study of catalytic activity in the polymerization of ethylene

Article abstract of DOI:10.1016/j.ica.2012.11.006

Coordination compounds of chromium (III) and vanadium (III) and (V) with 2,6-bis(diphenylhydroxymethyl)pyridine have been synthesized. Composition and properties of the obtained complexes have been evaluated by NMR, IR spectroscopy, X-ray diffraction, and elemental analysis. Depending on the nature of the metal and the synthesis conditions, there are various types of coordination state of the terdentate pyridine ligand in the obtained complexes: it can bind either as a dianionic, bisalkoxide, terdentate (compounds 5, 7, 8, 9), monoanionic, alkoxide/alkohol, terdentate (compounds 4, 6) or as a neutral, bisalkohol, terdentate ligand (compound 3). Coordination compounds of chromium were found to be incapable of catalytic polymerization of ethylene. Catalytic activity of vanadium (III) and (V) complexes in this reaction varied between 85 and 578 kg PE/mol V h after activation with diethylaluminum chloride, depending on the V/Al ratio.

Full text of DOI:10.1016/j.ica.2012.11.006

Inorganica Chimica Acta xxx (2013) xxx–xxx
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Coordination compounds of chromium (+3) and vanadium (+3) and (+5) with
2,6-bis(diphenylhydroxymethyl)pyridyl ligand: Synthesis and study of catalytic
activity in the polymerization of ethylene
Dmitry A. Kurmaev a, Nicolai A. Kolosov a, Svetlana Ch. Gagieva a, Alexandra O. Borissova b,
Vladislav A. Tuskaev a, , Natalya M. Bravaya c, Boris M. Bulychev a
⇑
a Faculty of Chemistry, Moscow State University, Leninskie Gory, Moscow 119991, Russia
b Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, ul. Vavilova 28, Moscow 119991, Russia
c Institute of Problems of Chemical Physics, Russian Academy of Sciences, pr. Akademika Semenova 1, Chernogolovka, Moscow oblast 142432, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 July 2012
Received in revised form 1 November 2012
Accepted 3 November 2012
Available online xxxx
Coordination compounds of chromium (III) and vanadium (III) and (V) with 2,6-bis(diphenylhydroxym-
ethyl)pyridine have been synthesized. Composition and properties of the obtained complexes have been
evaluated by NMR, IR spectroscopy, X-ray diffraction, and elemental analysis. Depending on the nature of
the metal and the synthesis conditions, there are various types of coordination state of the terdentate
pyridine ligand in the obtained complexes: it can bind either as a dianionic, bisalkoxide, terdentate (com-
pounds 5, 7, 8, 9), monoanionic, alkoxide/alkohol, terdentate (compounds 4, 6) or as a neutral, bisalkohol,
terdentate ligand (compound 3). Coordination compounds of chromium were found to be incapable of
catalytic polymerization of ethylene. Catalytic activity of vanadium (III) and (V) complexes in this reac-
tion varied between 85 and 578 kg PE/mol V h after activation with diethylaluminum chloride, depend-
ing on the V/Al ratio.
Keywords:
Olefin polymerization
Vanadium
Chromium
Tridentate ligands
X-ray structure
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
activity in the di-, tri- and oligomerization of olefins [16]. Such
their property makes it possible to use chromium complexes as a
The vast majority of post-metallocene catalysts of olefin poly-
merization are coordination compounds of titanium, zirconium
[1,2], iron, cobalt, or nickel [3,4]. Very little is known of the cata-
lytic activity of vanadium and chromium derivatives for this reac-
tion, but overall properties of such complexes deem to be very
promising [5–17]. The interest in studying these complexes stems
from their overall similarities to the coordination compounds of
group IV metals. Indeed, the compounds of vanadium (III) are iso-
electronic analogs of Ti, Zr, and Hf (IV) compounds which show the
highest activity in polymerization. Although activity of vanadium
catalysts is generally found to be lower than that of other systems,
their major advantage is in their ability to form high- and ultra-
high-molecular-weight polymers, as well as copolymers of ethyl-
component of the tandem catalytic systems used in synthesis of
elastomers.
Most works evaluating polyolefin synthesis used phenoxyimine
[12,17] and 2,6-bis(imino)pyridyl [3] ligands, while systems with
oxygen chelating ligands have been studied to much lesser extent
[18–22].
One of the problems encountered when studying the vana-
dium-containing catalysts is that the metal easily converted to
lower oxidation states; as a consequence, the catalytic activity
may decline or disappear [8,23–27]. Nevertheless, it may be
reasonably expected that the type of ligand environment, the
number, size, and electronic properties of both a ligand and a
central atom will affect not only the catalytic characteristics
of the complexes but also their ability to withstand metal
reduction.
In this study we use a ligand 1 combining pyridine core and two
diphenylcarbinol fragments. Ligand 1 has very limited use in coor-
dination chemistry. It have been used to prepare complexes of
hypervalent pentacoordinated dimethyltin (IV) [28], germylenes
and stannylenes [29], different titanium complexes [30]. Only in
our previous work [31] complexes of titanium and zirconium-
based on ligand 1 have been proposed as catalysts for olefin
polymerization.
ene with a-olefins containing high amounts of the comonomer.
Application of currently available organoaluminum compounds
as activators of vanadium-containing catalysts is yet another area
worth investigating.
Chromium complexes demonstrate weak catalytic activity in
common polymerization reactions; however they have a notable
⇑
Corresponding author. Fax: +7 495 9393316.
E-mail address: tuskaev@yandex.ru (V.A. Tuskaev).
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.11.006
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2
D.A. Kurmaev et al. / Inorganica Chimica Acta xxx (2013) xxx–xxx
In this study we report synthesis of a new group of chromium
2.2. Polymerization of ethylene
(III) as well as vanadium (III) and (V) coordination compounds
based on ligand 1; and the investigation of their catalytic proper-
ties in polymerization of ethylene, using methylaluminoxane
(MAO) and diethylaluminum chloride (DEAC) as catalyst
activators.
All operations on equipment preparation, the order and tech-
niques of loading of gaseous ethylene and complexes, and the mea-
surement of kinetic parameters of polymerization are similar to
those described in [31].
The polymerization of ethylene was performed at a total pres-
sure of ethylene and toluene vapors of 1.7 atm. Polymerization
was carried out in a 100-ml reactor (PARR) equipped with a mag-
netic stirrer and inlets for loading components of catalytic systems
and ethylene. Toluene (50 ml) and the necessary amount of a
cocatalyst (DEAC or MAO) in the form of toluene solutions were
successively loaded in the reactor together with the cocatalyst.
The required amount of the precatalyst in the form of toluene solu-
tion was placed into a special syringe connected with the reactor.
The reactor was heated to a specified temperature, and the reaction
mixture was saturated with ethylene. Polymerization was started
by precatalyst loading to the reaction mixture. The pressure of eth-
ylene was maintained constant during polymerization. Polymeri-
zation was stopped through the addition of 10% HCl solution in
ethanol to the reactor. The polymer was filtered off, washed several
times with water–ethanol mixture, and dried under vacuum at 50–
60 °C until a constant weight was achieved.
2. Experimental
2.1. General methods
All manipulations were performed under an argon atmosphere
by using standard Schlenk techniques. Toluene and THF were dis-
tilled from Na/benzophenone prior to use. Dichloromethane was
distilled over calcium hydride. The water contents of these sol-
vents were periodically controlled by Karl-Fischer coulometry by
using a Methrom 756 KF apparatus. Argon and ethylene of spe-
cial-purity grade were dried by purging through a column filled
0
with 5 ÅA molecular sieves.
Ligand 2,6-bis(diphenylhydroxymethyl)pyridine was synthe-
sized as described in [31]. Its physicochemical characteristics
and 1H and 13C NMR spectra match the published data. Pheny-
limidovanadium (V) chloride was prepared as described in [32].
VOCl3, VCl3⁄3THF, and CrCl3⁄3THF as well as solutions of dieth-
ylaluminum chloride and butyllitium (Aldrich) were used.
Polymethylaluminoxane (Witco) was used as a 10% solution in
toluene.
NMR spectra were recorded on Bruker WP-600 and Bruker
AMX-400 instruments. Deuterated solvents (CD2Cl2, CDCl3) were
degassed by freeze–thaw–vacuum cycles and stored over 3 Å
molecular sieves. Chemical shifts are reported in ppm vs. SiMe4
and were determined by reference to the residual solvent peaks.
All coupling constants are given in Hertz.
2.3. Synthesis
2.3.1. [2,6-(CPh2OH)2Py]CrCl3 (3)
A two-necked flask equipped with a magnetic stirrer was suc-
cessively charged in a flow of argon with ligand 1 (221.5 mg,
0.5 mmol), toluene (10 ml), and CrCl3⁄3THF (187 mg, 0.5 mmol).
The reaction mixture was stirred at 40 °C for 100 h. The precipi-
tated green crystals were filtered off and washed with methylene
chloride and toluene.
Yield 0.09 g (32%); Anal. Calc. for C31H25Cl3NO2Cr: C, 61.80; H,
4.10; Cl, 17.69; N, 2.33; Cr, 8.64. Found: C, 61.75; H, 3.95; Cl,
IR spectra were recorded on a Magna-IR 750 spectrophotome-
ter. Elemental analysis was performed on Carlo Erba-1106 and Car-
lo Erba-1108 instruments.
17.73; N, 2.32; Cr, 8.66%. IR,
and (Cr–N) 583.
m
, cmꢀ1: (OHbound) 3480, (Cr–O) 657
The gel chromatograms of polymer samples were analyzed on a
Waters GPCV-2000 chromatograph with the use of a PLgel 5 lm
2.3.2. [2,6-(CPh2O)2Py]Cr[2-(CPh2O)-6-(CPh2OH)-Py] (4)
MIXED-C column in 1,2,4-trichlorobenzene at 135 °C. Molecular
masses were estimated using the universal calibration curve plot-
ted relative to polystyrene standards.
The thermogravimetric analysis of samples was performed on a
NETZSCHSTA-Jupiter449 C instrument. Measurements were car-
ried out in a flow of argon (100 ml/min) in the temperature range
40–300 °C. The heating rate was 5 °C/min.
A two-necked flask equipped with a magnetic stirrer was suc-
cessively charged in a flow of argon with ligand 1 (221.5 mg,
0.5 mmol) dissolved in toluene (5 ml) and CrCl3⁄3THF (93.5 mg,
0.25 mmol) dissolved in tetrahydrofuran (5 ml). The reaction mix-
ture was stirred at 40 °C for 100 h. Precipitated blue crystals were
filtered off and washed with tetrahydrofuran and toluene.
Yield, 0.21 g (45%); Anal. Calc. for C62H47N2O4Cr: C, 79.55; H,
5.06; Cr, 5.55; N, 2.99. Found: C, 79.27; H, 5.42; Cr, 5.29; N,
The experiments on the polymerization of ethylene were per-
formed in a 100-ml reactor (Parr Instrument Co.).
2.44%. IR,
m
, cmꢀ1: (OHbound) 3440, (O–Cr–O) 750, (Cr–O) 620,
X-ray diffraction data for the single crystals of 3, 4, 6, and 8
were collected using a ‘‘Bruker SMART APEX2’’ CCD diffractome-
ter. The obtained images were integrated [33]. The precise unit
cell dimensions and errors were determined. The absorption cor-
rection was applied semiempirically using the SADABS program
[34]. The details of X-ray data collection in the subsequent
refinement are listed in Table 2. Initially spherical atom refine-
ments were undertaken with SHELXTL PLUS 5.0 [35] using the
full-matrix least-squares method. All non-hydrogen atoms were
allowed to have an anisotropic thermal motion. Atomic coordi-
nates, bond lengths, angles, and thermal parameters have been
deposited at the Cambridge Crystallographic Data Center with
numbers CCDC 878807–878810. X-ray diffraction analysis of
and (Cr–N) 560.
2.3.3. [2,6-(CPh2O)2Py]CrCl (5)
A two-necked flask equipped with a magnetic stirrer was
charged in a flow of argon with compound 1 (221.5 mg, 0.5 mmol)
and toluene (10 ml); then, at ꢀ78 °C, 2.5 M solution of butyllithium
in n-hexane (0.44 ml, 1.1 mmol) was added dropwise. The reaction
solution was slowly heated to room temperature, stirred for 4 h
and cooled to ꢀ78 °C. CrCl3, 3THF (187 mg, 0.5 mmol) dissolved
in tetrahydrofuran (3 ml) was added, and the resulting mixture
was stirred for 96 h at room temperature. The brown precipitate
was filtered off, the organic solvents were evaporated, and the
product formed was recrystallized from methylene chloride.
Yield, 0.11 g (80%); Tmelt = 307 °C; Anal. Calc. for C31H23NO2CrCl:
C, 70.39; H, 4.38; Cl, 6.70; Cr, 9.83; N, 2.65. Found: C, 69.50; H,
polymers was performed on
radiation, Ni filter, a scan rate of 1 deg (2h)/min). The degree
of crystallinity of samples was estimated from the ratio of
a DRON-2 instrument (Cu Ka
v
the integral intensity of crystalline constituent and the total
intensity.
5.03; Cl, 6.15; Cr, 9.40; N, 2.50%. IR,
N) 547.
m
, cmꢀ1: (Cr–O) 689 and (Cr–
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D.A. Kurmaev et al. / Inorganica Chimica Acta xxx (2013) xxx–xxx
3
Table 1
Crystallographic data and structure refinement details for 3, 4, 6 and 8.
3
4
6
8
Formula
M
C
39H41Cl3CrNO4
C135H110Cr2N4O9
2036.27
C39H40Cl2NO4V
708.56
C31H23ClNO3V
204.06
746.08
T (K)
Space group
100(2)
P21/c
100(2)
P1
100(2)
P1
100(2)
C c
ꢀ
ꢀ
a (Å)
b (Å)
c (Å)
13.172(2)
17.525(4)
16.410(3)
12.7934(5)
12.8886(5)
16.6146(7)
103.038(2)
98.875(2)
96.789(2)
2603.45(18)
2
15.050(2)
21.834(3)
22.090(3)
83.905(3)
70.256(3)
89.896(3)
6789.0(16)
8
10.5834(10)
15.1756(10)
16.0379(12)
a
(°)
b (°)
111.640(5)
102.386(2)
c
(°)
V (Å3)
3521.4(11)
4
1.407
0.595
2515.9(3)
4
1.436
0.535
Z
q
l
(g cmꢀ3
(Mo K
)
1.299
0.275
1.386
0.492
a
), (mmꢀ1
)
F(000)
1556
1068
2960
1120
Absorption correction (Mo K
a)
semiempirical from
equivalents
semiempirical from
equivalents
semiempirical from
equivalents
semiempirical from
equivalents
Scan technique
hmax
x
27
-scan with 0.5 step
x
29
-scan with 0.5 step
x
29
-scan with 0.5 step
x-scan with 0.5 step
29
Number of collected reflns
Number of independent reflections (Rint
Number of observed reflections with
20254
7667 (0.2029)
2605
31409
13822 (0.0423)
8976
79971
35931 (0.2249)
10222
14763
6610 (0.0389)
4766
)
[I>2
wR2
R1
Goodness-of-fit (GOF) on F2
a(I)]
0.1089
0.0542
0.703
0.430 and ꢀ0.582
0.0962
0.0550
1.091
0.356 and ꢀ0.735
0.2432
0.1050
0.872
0.898 and ꢀ1.154
0.1546
0.0576
1.000
0.596 and ꢀ0.748
qmax.
/
qmin (e Åꢀ3
)
Table 2
Catalytic activity of vanadium complexes with 2,6-bis(diphenylhydroxymethyl)pyridyl ligand activated by different cocatalysts (toluene solvent, an ethylene pressure of 1.7 atm,
30 °C, and a time of reaction of 30 min).
Activity [kg of PE (mol of V)ꢀ1 hꢀ1
]
Tg (°C)
Crystallinity degreea (%)
a
Run
Pre-catalyst
Cocatalyst, reactivator [V]/[Al]/[R-Cl]
1
2
3
4
5
6
7
8
6
7
7
7
7
7
7
7
8
8
8
8
8
8
9
9
9
9
Et2AlCl (1/300)
MAO (1/300)
Et2AlCl (1/100)
Et2AlCl (1/300)
0
trace
trace
139
153
187
544
579
trace
trace
255
442
306
499
102
85
148
60
Et2AlCl (1/500)
Et2AlCl (1/1000)
Et2AlCl/MTCA (1/300/35)
Et2AlCl/MTCA (1/300/300)
MAO (1/300)
Et2AlCl (1/100)
Et2AlCl (1/300)
150
139
54
60
9
10
11
12
13
14
15
16
17
18
140
138
45
45
Et2AlCl (1/500)
Et2AlCl (1/1000)
Et2AlCl/MTCA (1/300/300)
Et2AlCl(1/300)
Et2AlCl (1/500)
Et2AlCl (1/1000)
Et2AlCl/MTCA (1/300/300)
85
360
a
Melting temperatures and crystallinities determined by DSC at second heating of PE samples.
2.3.4. [2-(CPh2O)-6-(CPh2OH)-Py]VCl2⁄THF (6)
and toluene (10 ml); then, at ꢀ78 °C, 2.5 M solution of butyllithium
in n-hexane (0.44 ml, 1.1 mmol) was added dropwise. Then the
reaction solution was slowly heated to room temperature, the
resulting mixture was stirred for 4 h and cooled to ꢀ78 °C, and
VCl3⁄3THF (187 mg, 0.5 mmol) was added. The reaction mixture
was stirred at room temperature for 20 h. Inorganic products were
filtered off, and the organic solvents were evaporated. The green
complex formed was recrystallized from toluene.
A two-necked flask equipped with a magnetic stirrer was suc-
cessively charged in a flow of argon with ligand 1 (221.5 mg,
0.5 mmol), toluene (10 ml), and VCl3⁄3THF (187 mg, 0.5 mmol).
The reaction mixture was stirred at 40 °C for 70 h. The green pre-
cipitate was filtered off and recrystallized from methylene
chloride.
Yield 0.14 g (43%); Anal. Calc. for C35H32NO3VCl2: C, 66.05; H,
5.07; Cl, 11.14; N, 2.20; V, 8.00. Found: C, 66.02; H, 4.92; Cl,
11.07; N, 2.15; V, 7.91%. 1H NMR (400 MHz, CDCl3, 298 K):
d = 1.69 [m, 4H, –CH2 THF], 3.52 [m, 4H–CH2 THF], 3.70 [s, 1H],
Yield, 0.14 g (52%); Anal. Calc. for C31H23NO2VCl: C, 70.53; H,
4.39; Cl, 6.72; N, 2.65; V, 9.65. Found: C, 69.02; H, 4.27; Cl, 6.29;
N, 2.49; V, 9.50%. 1H, (CDCl3) 7.25–7.94 (m, 23HAr). IR,
(V–O) 657 and (V–N) 583.
m :
, cmꢀ1
7.03–7.42 [m, 22H], 7.95 [t, J = 7.8 Hz, 1H]. IR, m )
, cmꢀ1: (OHbound
3517, (V–O) 630, and (V–N) 575.
2.3.6. [2,6-(CPh2O)2Py]V(O)Cl (8)
2.3.5. [2,6-(CPh2O)2 Py]VCl (7)
A two-necked flask equipped with a magnetic stirrer was
charged in a flow of argon with ligand 1 (221.5 mg, 0.5 mmol)
A two-necked flask equipped with a magnetic stirrer was
charged in a flow of argon with ligand 1 (221.5 mg, 0.5 mmol)
and toluene (10 ml); then at ꢀ78 °C a solution of butyllithium
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4
D.A. Kurmaev et al. / Inorganica Chimica Acta xxx (2013) xxx–xxx
(2.5 M in n-hexane, 0.44 ml, 1.1 mmol) was added dropwise. Then
the reaction solution was slowly heated to room temperature, the
mixture was stirred for 4 h and cooled to ꢀ78 °C, and VOCl3
(0.05 ml, 0.5 mmol) was added. The resulting mixture was stirred
at room temperature for 12 h, inorganic products were filtered
off, and the organic solvents were evaporated. The yellow complex
formed was recrystallized from methylene chloride.
Distorted tetragonal bipyramidal environment of the chromium
atom consists of three chlorine atoms and pyridyl nitrogen atom in
its equatorial plane and two oxygen atoms in axial position (see
Fig. 2). CrCl3N fragment is almost plane; the maximum shift of
atoms from the plane does not exceed 0.01 Å. The polyhedron dis-
tortion can be characterized by O(1)–Cr(1)–O(2) angle of
155.58(14)°, which significantly deviates from 180°. Chromium-
chlorine bonds are similar to each other: their lengths vary be-
tween 2.3022(17) and 2.3225(18) Å, which is a mean value for
Cr–Cl bonds for this class of compounds. The Cr–N bond between
the metal atom and pyridine ligand (2.004(4) Å) is among the
shortest of its kind according to Cambridge Structural Database
data. The shortening of this bond is due to polydentate chelating
character of the ligand. Cr–N distance in the complexes of ‘‘free’’
pyridine is about 2.10–2.15 Å [35]. Both Cr–O bonds have a length
of 2.013(4) Å which is in good agreement with the equal degree of
oxygen atoms protonation. Moreover, both OH groups are involved
into similar H-bonds with outer-sphere tetrahydrofuran solvent
molecules (Oꢁ ꢁ ꢁO distances are 2.640(5) and 2.555(5) Å, and OHO
angles are 155° and 174° for O(1)–H(1O)ꢁ ꢁ ꢁO(2S) and O(2)–
H(2O)ꢁ ꢁ ꢁO(1S), respectively).
Yield, 0.10 g (32%); Anal. Calc. for C31H23ClNO3V: C, 68.45; H,
4.26; Cl, 6.52; N, 2.85; V, 9.37. Found: C, 68.67; H, 4.27; Cl, 6.51;
N, 2.86; V, 9.34%. 1H, (CDCl3) 7.19–7.70 (m, 22H), 8.05 (t,
J = 7.7 Hz, 1H–Ar).IR, m
, cmꢀ1: (V–O) 630, (V–N) 521, and (V–Cl)
450.
2.3.7. [2,6-(CPh2OH)2Py]V(NPh)Cl (9)
A two-necked flask equipped with a magnetic stirrer was
charged in a flow of argon with ligand 1 (221.5 mg, 0.5 mmol)
and toluene (5 ml); then, at ꢀ78 °C a solution of butyllithium
(2.5 M in n-hexane, 0.44 ml, 1.1 mmol) was added dropwise. Then
the reaction solution was slowly heated to room temperature, the
mixture was stirred for 4 h and cooled to ꢀ78 °C, and VCl3NPh
(124 mg, 0.5 mmol) dissolved in toluene (5 ml) was added. The
resulting mixture was stirred at room temperature for 20 h, inor-
ganic products were filtered off, and the organic solvents were
evaporated. The green product formed was recrystallized from
toluene.
The IR spectrum of complex 3 exhibits bands at 3404 cmꢀ1 due
to the stretching vibrations of coordinated hydroxyl groups. The
resulting bands for stretching vibrations of Cr–O and Cr–N groups
manifest themselves at 640 and 594 cmꢀ1, respectively.
The DSC-TGA study of the thermal properties of compound 3 re-
vealed weight losses at 137 and 175 °C which, in principle, corre-
spond to the stepwise elimination of two HCl molecules.
Yield, 0.13 g (42%); Anal. Calc. for C37H28ClN2O4V: C, 71.79; H,
4.56; Cl, 5.73; N, 4.53; V, 8.23. Found: C, 71.60; H, 4.55; Cl, 5.74;
N, 4.52; V, 8.24%. IR, m
, cmꢀ1: (V–O) 630, and (V–N) 548.
The interaction of two equivalents of the ligand with one equiv-
alent of CrCl3, 3THF yielded coordination-unsaturated complex 4,
which structure was studied by X-ray diffraction analysis (Fig. 3).
In this complex the tetragonal bipyramidal environment of the
chromium atom is significantly distorted: the shift of chromium
atom from the equatorial plane O(1)O(2)N(1)N(2) is about 0.1 Å.
O(3)–Cr(1)–O(4) axial angle is 156.54(6)°. Geometrically, two types
of Cr–O bonds can be detected in complex 4: Cr(1)–O(1) and Cr(1)–
O(4) bond are 0.1 Å shorter (1.9099(13) Å and 1.9167(14)°) than
Cr(1)–O(2) and Cr(1)–O(3) bonds (2.0310(13) and 2.0205(14) Å,
respectively). The above value of 2.013(4) Å in 3 is somewhat be-
tween two types of Cr–O bonds in 4. At this moment we cannot
unambiguously explain this phenomenon. One possibility is the
difference in bond length can be attributed to the general weak-
ness of coordinate bonds in the complexes with high coordination
numbers. It was previously postulated that the variation of metal–
ligand bond lengths in such complexes may be chaotic and to some
extent unpredictable [36]. In contrast, the Cr–N bond lengths in the
complex are virtually identical with the lengths of 1.9930(15) and
1.9976(15) Å for Cr(1)–N(2) and Cr(1)–N(1), respectively.
3. Results and discussion
3.1. Synthesis and structure of chromium and vanadium complexes
Direct interaction of ligand 1 with tetrahydrofuran complex of
chromium (III) chloride leads to either chelate 3 or to the coordina-
tion-saturated octahedral complex 4 (Fig. 1), depending on the
amount of metal halide.
The structure of chelate 3, in which protons of the hydroxyl
groups are retained, is shown in Fig. 2.
Unlike the spectrum of the free ligand, the IR spectrum of the
complex 4 exhibits bands at 750 and 620 cmꢀ1 due to the stretch-
ing vibrations of O–Cr–O and of Cr–O, respectively; the bands cor-
responding to Cr–N bonds are observed at 560 cmꢀ1. The band
corresponding to the stretching vibrations of the bound hydroxyl
group appears at 3340 cmꢀ1. The obtained complex is thermally
stable, and its decomposition begins only at 130 °C.
The use of the ligand preliminary deprotonated by butyl lithium
in complex formation yielded compound 5. The structure of this
compound was studied by elemental analysis and IR spectroscopy.
As in the case of chromium chloride, the direct interaction of li-
gand 1 with VCl3, 3THF yields proton-containing complex 6 (Fig. 5).
The structure of this complex is shown in Fig. 3a. The reaction of
dilithium salt of ligand 2 with vanadyl chloride yields complex 8.
Its structure is presented in Fig. 5.
Coordination polyhedron of vanadium atom in compound 6 is
that of a distorted tetragonal bipyramid. In this complex two chlo-
ride ligands in equatorial plane show considerably weaker bonding
Fig. 1. General synthesis of chromium (III) complex compounds with 2,6-bis(diph-
enylhydroxymethyl)pyridyl ligand.
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D.A. Kurmaev et al. / Inorganica Chimica Acta xxx (2013) xxx–xxx
5
Fig. 2. The structure of complex 3 according to single-crystal X-ray diffraction study.
Fig. 3. The structure of complex 4 according to single-crystal X-ray diffraction study.
than in 8 (2.294(2) and 2.322(2) Å for Cl(1) and Cl(2) in 6 versus
2.1858(14) Å in 8), which is in agreement with different types of
coordination polyhedra, as well as a higher coordination number
of vanadium in 6. V–N bonds in both complexes are ca. 2.05 Å.
Equatorial V–O(THF) bond is considerably weaker than two axial
ones (2.150(4) Å versus 1.812(4) and 1.798(4) Å), which is in agree-
ment with additional coordination character of THF.
Therefore, compound 7 is difficult to experiment with. As such,
we could not perform high-quality structural experiments, and all
experiments on the catalytic activity of 7 were carried out in situ;
that is, complex 7 was not isolated from the reaction mixture.
As known from the literature [37,38], the replacement of V@O
group by V@NPh group leads to an increase in stability and activity
of vanadium-containing catalytic systems. In order to check the ef-
fect of such modification, compound 9 was prepared by the inter-
action of dilithium salt of 2,6-bis(diphenylhydroxymethyl)pyridine
2 with phenylimido vanadium (V) chloride (Fig. 4). Composition of
compound 9 was evaluated only by elemental analysis and IR
spectroscopy.
According to the IR data, the stretching vibrations of the coordi-
nated hydroxyl group in complex 6 show a low-frequency shift rel-
ative to the unbound group and are observed at 3364 cmꢀ1
.
The reaction of vanadium (III) tetrahydrofuran complex with
dilithium salt of the ligand produces compound 7. No hydroxyl
groups absorption bands are observed in the IR spectrum of this
complex. Compound 7 is very sensitive to oxygen. During storage
even in the crystalline state, it gradually transforms into com-
pound 8.That is evident from the crystals color change from green
to yellow, which is typical for V (V) compounds.
3.2. Polymerization of ethylene
Catalytic activity of obtained complexes was investigated in the
model reaction of ethylene polymerization (toluene was used as a
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D.A. Kurmaev et al. / Inorganica Chimica Acta xxx (2013) xxx–xxx
solvent, monomer pressure of 1.7 atm, temperature of 30 °C, and
time of polymerization of 30 min). Precatalysts were activated
using toluene solutions of MAO and DEAC.
considerably higher activating ability than MAO. The value of cat-
alytic activity of all the precatalysts depends on the AlDEAC/V molar
ratio.
The experiments showed that all the chromium complexes
were not effective in ethylene polymerization experiments, per-
formed with either type of activator ([Cr]/[Al] = 1: 300). The only
exception was in the case of compound 5, where trace amounts
of polymer were observed. An increase of the reaction temperature
to 70 °C, use of methylene chloride as a solvent, or the preactiva-
tion of catalyst for 1 h with a small excess of the organoaluminum
cocatalyst did not increase its catalytic activity.
Data on vanadium precatalysts are listed in Table 2. Complex 6,
wherein the metal is linked to the ligand via coordination bonds
solely, does not polymerize ethylene when DEAC used as an activa-
tor. Precatalysts 7–9 exhibit only minor activity, when activated by
MAO (AlMAO/V = 300 mol/mol). DEAC, which is a conventional
cocatalyst for Ziegler homogeneous vanadium systems, exhibits
Precatalyst 7 tends to gradually increase its catalytic activity to
the maximum value of 187 kg/(mol V h) at AlDEAC/V = 1000 mol/
mol ratio. Precatalyst 8 demonstrates its maximum catalytic activ-
ity of 442 kg/(mol V h) at AlDEAC/V = 500 mol/mol ratio. A further
increase in AlDEAC/V to 1000 mol/mol results in a drop of the activ-
ity by a factor of 1.4. The maximum catalytic activity of precatalyst
9 is rather low (102 kg/(mol V h) at AlDEAC/V = 300), and tends to
decrease to 85 kg/(mol V h) at AlDEAC/V = 500 and 1000 mol/mol ra-
tios. All precatalysts show the trace activity at AlDEAC/V = 100. Con-
siderable differences in activities of precatalysts 7–9 are possibly
due to the differences in their reduction abilities resulting in spe-
cies inactive in polymerization reactions. At the same time, it
should be noted that the presence of hydroxypyridyl ligands in
complexes 7–9 makes vanadium atoms more resistant to
Fig. 4. Synthesis of complex compounds of vanadium (III) and (V) with 2,6-bis(diphenylhydroxymethyl)pyridyl ligand.
Fig. 5. The structure of complexes 6 (a) b 8 (b) according to single-crystal X-ray diffraction study.
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D.A. Kurmaev et al. / Inorganica Chimica Acta xxx (2013) xxx–xxx
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Fig. 6. Thermograms of PE sample obtained with 7/DEAC/MTCA system (experiment 7) at first (solid line) and second (dashed line) heating.
reduction. Thus, in the case of ethylene polymerization with homo-
geneous VOCl3/AlEt2Cl catalytic system the catalyst is deactivated
within a few minutes even at low molar ratio AlDEAC/V = 20 due
to reduction to V (II) species [24]. In our case complexes 7 and 8
are working much longer.
It was empirically demonstrated that activity of vanadium
based Ziegler systems can be recovered by addition of small
amounts of perchlorinated organics, specifically perchloropentadi-
ene [39], and esters of trichloroacetic acid [40]. Most likely, these
compounds act as oxidative reagents in the system which convert
inactive species of V (II) to active ones containing V (III), V (IV), and
V (V).
In order to verify the efficiency of post-metallocene systems
reactivation, methyl trichloroacetate (MTCA) was added to the
reaction solution. Approximately fourfold increase in the activity
was observed for catalytic system 7/DEAC, when 35 or 300 equiv-
alents of MTCA relative to vanadium were added (cf. experiments 4
and 7, 8, Table 1). For complexes 8 and 9 the catalytic activity in-
creased by factors of 2 and 4, at a molar ratio of MTCA/V = 300
(experiments 14, 11 and 18, 15), respectively. It is noteworthy that
Fig. 7. Thermograms of PE samples obtained with (1) 7/DEAC, (2) 8/DEAC, and (3)
in the presence of MTCA the differences in catalytic activities of
complexes 7, 8, and 9 are significantly smaller (578, 499 and
360) kg PE/(mol [V] h) than in the absence of reactivator (139,
255 and 102) kg PE/(mol [V] h). However, the introduction of
phenylimido group introduces a significant loss in catalysts activ-
ity both in the presence of the activator and in its absence.
Ultra-high-molecular-weight PEs insoluble in 1,2,4-trichloro-
benzene at 135 °C were obtained with all the vanadium catalysts.
All PE samples are linear in structure. The absence of branches is
evident from the absence of 1378 cmꢀ1 absorption band in FTIR
spectra of polymer films. The band refers to symmetric bending
of methyl groups in branches. As a result, these linear ultra-high-
molecular-weight PE samples demonstrate very high melting
temperatures and crystallinity. Thus PEs prepared with the system
7/DEAC both in the absence (experiment 4) and in the presence of
35 equivalents of TMCA (experiment 7) show very high melting
temperatures (150–152 °C) and crystallinity which are not consid-
erably changed at both first and second heating as demonstrated
for PE obtained in experiment 7 (Fig. 6). An increase in the content
of MTCA to 300 equivalents leads to some decrease in the PE
melting temperature to 139 °C, with crystallinity standing high
(experiment 7).
9/DEAC catalytic systems (experiments 4, 11, and 16 in Table 1).
The morphology of nascent polymers depends on the nature of
precatalysts. Thus, the DSC curves of PEs prepared with precata-
lysts 7 and 8 demonstrate single melting peaks at 148 and 140 °C
(Fig. 7, curves 1, 2), respectively; while the DSC curve of PE ob-
tained with precatalyst 9 reveals two pronounced peaks at 145
and 135 °C (curve 2). After recrystallization during the second
heating, the thermogram of the polymer exhibits only one peak
at 138 °C (experiment 16).
4. Conclusion
New chromium (III), vanadium (III) and (V) complexes were
synthesized and characterized. Depending on the nature of the me-
tal and the synthesis conditions, there are various types of coordi-
nation state of the terdentate pyridine ligand 1 in the obtained
complexes: it can bind either as a dianionic, bisalkoxide, terdentate
(compounds 5, 7, 8, 9), monoanionic, alkoxide/alkohol, terdentate
(compounds 4, 6) or as a neutral, bisalkohol, terdentate ligand
(compound 3).
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D.A. Kurmaev et al. / Inorganica Chimica Acta xxx (2013) xxx–xxx
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performed to evaluate their performances as ethylene polymerisa-
tion catalysts. All chromium complexes were inactive. Vanadium
(III) and (V) complexes act as rather stable and efficient catalysts
for ethylene polymerization. It was demonstrated that small ex-
cesses of a reactivator – methyl trichloroacetate, can significantly
increase the activity of catalytic systems. With all other factors
being the same, the vanadium (V) complexes have the superior
activity when compared to vanadium (III) complexes. However,
application of a reactivator levels off this difference.
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