Homogeneous catalytic dimerization of propylene with bis(imino)pyridine vanadium(III) complexes

Article abstract of DOI:10.1016/j.molcata.2010.02.013

A series of new bis(imino)pyridine vanadium(III) complexes was synthesized. They were tested for the homogeneous catalytic dimerization of propylene after activation with MAO. The activity and selectivity depend on the ligand structure of the corresponding organic coordination compound. The influence of PPh₃ as an additive was investigated and high dependency could be observed.

Full text of DOI:10.1016/j.molcata.2010.02.013

Journal of Molecular Catalysis A: Chemical 322 (2010) 45–49
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Homogeneous catalytic dimerization of propylene with bis(imino)pyridine
vanadium(III) complexes
Julian R.V. Lang, Christine E. Denner, Helmut G. Alt∗
Laboratorium für Anorganische Chemie, Universität Bayreuth, Universitätsstraße 30, D-95440 Bayreuth, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 January 2010
Received in revised form 4 February 2010
Accepted 7 February 2010
Available online 13 February 2010
A series of new bis(imino)pyridine vanadium(III) complexes was synthesized. They were tested for the
homogeneous catalytic dimerization of propylene after activation with MAO. The activity and selectivity
depend on the ligand structure of the corresponding organic coordination compound. The influence of
PPh3 as an additive was investigated and high dependency could be observed.
© 2010 Elsevier B.V. All rights reserved.
Dedicated to Professor Uwe Rosenthal on
the occasion of his 60th birthday.
Keywords:
Homogeneous catalyst
Vanadium
Bis(imino)pyridine
Propylene
Dimerization
1. Introduction
and selectivities for the oligo- and polymerization of ethylene
after activation with methyl aluminoxane (MAO). Several com-
Unsaturated short chained hydrocarbons are low priced educts
for polymerization, oligomerization and metathesis application,
produced by unselective thermal cracking processes [1]. Espe-
cially the dimerization of propylene plays an important role for
the formation of gasoline with a high octane number. As a result
branched hexenes can be obtained and used as gasoline blending
compounds. The Research Octane Number (RON) rises with the
number of branching [2–6], from RON = 96–99 for methylpentenes
to 101 for dimethylbutene [2,7,8]. Linear hexenes are in the range
from 73 to 94 and play no role as additives for gasoline improve-
ment. With the ban of lead-alkyl compounds and methyl-tert-butyl
as anti-knocking agent ether from gasoline, branched hydrocar-
bons represent a very important class of compounds for gasoline
reformulation [9].
plexes with various metal centers and different ligand structures
were published and many studies have reported the effects of
ligand substitution patterns on activity and selectivity [19–23].
Bis(imino)pyridine vanadium(III) complexes were found to be
selective for the oligomerization of ethylene to give linear olefins
[13,24–26]. These facts underline the importance of such cata-
lysts.
Here we report the application of bis(imino)pyridine vana-
dium(III) complexes combined with MAO as cocatalyst in the
selective dimerization of propylene. The influence of phosphorous
containing additives is another aspect in this work.
2. Results and discussion
The invention of highly active iron- and cobalt-based olefin
polymerization catalysts in the late 1990s has led to much
interest in the chemistry of transition metal complexes bear-
ing tridentate bis(imino)pyridine ligands [10–21]. These types of
complexes were applied by Gibson and Brookhart in 1998 and
great progress has been achieved since then. It is well established
that bis(imino)pyridine iron(III) complexes show high activities
2.1. Synthesis of the bis(imino)pyridine compounds 1a–d
The bis(imino)pyridine ligand precursors were synthesized via
a condensation reaction (Scheme 1) of 2,6-diacetylpyridine with
the respective aniline according to the literature [27].
The yields of the compounds 1a–d are generally high (up to 94%).
2.2. Synthesis of the complexes 2–5
∗
The complexes were synthesized via an addition reaction
(Scheme 2) of the vanadium(III)trichloride THF adduct and the
Corresponding author. Tel.: +49 921 55 2555; fax: +49 921 55 2044.
E-mail address: helmut.alt@uni-bayreuth.de (H.G. Alt).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.02.013

46
J.R.V. Lang et al. / Journal of Molecular Catalysis A: Chemical 322 (2010) 45–49
Scheme 1. Synthesis of the bis(imino)pyridine compounds 1a–d.
Scheme 2. Synthesis of the bis(imino)pyridine vanadium(III) complexes 2–5.
respective bis(imino)pyridine compound in diethyl ether. The
resulting complexes were obtained in good yields (65–87%).
The listed complexes 2–28 were all tested for their catalytic
activities in dimerization reactions (Table 1).
The activity was determined by the weight increase of the reaction
vessel after removing the propylene. While high activities for the
oligo- and polymerization of ethylene were achieved with this type
of catalyst [25,28], the results with propylene varied in the range
of 95–215 kg/mol h (TOF = 1130–2560 h−1). For our application, it
is more important to have a look at the selectivities and product
distributions.
2.3. Catalytic dimerization reactions of propylene
The dimerization of propylene can lead to 12 hexene iso-
mers via coordination, double insertion and elimination reactions
(Scheme 3). The names and abbreviations of all isomers are listed
in Table 2.
Various bis(imino)pyridine vanadium(III) compounds were
tested for the dimerization of propylene after activation with MAO
(V:Al = 1:500) to give hexene isomers. The catalytic activities and
selectivities of the corresponding catalysts are important aspects.
It is obvious that complexes 14–17 with bulky ligands like
alkyl/aryl substituents on positions 2 or 6 (ortho position) of the
imine fragment (Scheme 4), achieve high selectivities up to 95%
(16). Bulky substituents on both sides have a negative effect. The
selectivity falls from 90% to 81% with the replacement of methyl
(17) to iso-propyl (18). Moreover, steric hindrance in ortho posi-
tion has an influence on the product distribution. While complexes
11–13 produce 4-methyl-1-pentene as main product, bulky sub-
stituents shift it to 2-methyl-1-pentene. These bulky groups favor
1,2-insertion as an initial step.
A substitution with halides on the para position has a great influ-
ence on the formation of hexenes. Compared to complex 11 (main
product 4-methyl-1-pentene with a selectivity of 62%), a halide
substitution gives 4-methyl-1-pentene with selectivities between
74% (25) and 82% (9).
The selectivity of the formation of hexene isomers decreases
in the following manner F (93%) (2) > Cl (87%) (3) > Br (83%) (4)
on the meta position (Scheme 4). The ˇ-hydrogen elimination is
favored by electron withdrawing groups compared to the heav-
ier homologue halides. The distribution of the dimeric products is
nearly the same for all three halide substituted complexes with
4-methyl-1-pentene as main product and selectivities up to 90%
are observed (Table 3). With the high dimer and product selectiv-
ity of 2, 4-methyl-1-pentene is produced with a total amount of
83%.
Table 1
Synthesized complexes 2–28. Complexes 6–28 were already reported in the litera-
ture [25]. Compounds 2–5 are new.
V(III)
complex no.
R
X
Y
Z
Rꢀ
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
H
H
H
H
H
H
H
F
Methyl
Methyl
Methyl
Methyl
Cl
I
NO2
I
Methyl
H
H
H
H
H
H
H
H
H
H
Br
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Cl
Br
Br
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Methyl
H
H
H
H
H
H
Cl
Methyl
Methyl
Methyl
Ethyl
iso-Propyl
tert-Butyl
Propyl
Benzyl
iso-Propyl
iso-Propyl
Methyl
H
Methyl
Methyl
Methyl
H
Methyl
Methyl
H
Methyl
H
H
H
H
H
H
Methyl
iso-Propyl
H
H
H
Cl
H
H
H
Methyl
Butyl
H
H
H
Br
Cl
H
F
H
H
H
H
H
Methyl
H
H
H
H
Electron withdrawing or pushing groups on position 4 of the
imine fragment have no influence on the dimer selectivity (6–8, 20,
24 and 27). The difference is obvious in product distribution. Com-
plex 20 with a withdrawing group produces 2-methyl-1-pentene
H
Methyl
H
H
Methyl
H

J.R.V. Lang et al. / Journal of Molecular Catalysis A: Chemical 322 (2010) 45–49
47
Scheme 3. Dimerization products of propylene and catalytic cycles.
Table 2
All hexene isomers with full names and abbreviations.
Isomer
Name/abbreviation
Isomer
1-Hexene/1-hex
cis-2-Hexene/c-2-hex
trans-2-Hexene/t-2-hex
cis-3-Hexene/c-3-hex
Name/abbreviation
trans-3-Hexene/t-3-hex
2-Methyl-1-pentene/2-MP-2
4-Methyl-1-pentene/4-MP-1
trans-4-Methyl-2-pentene/t-4-MP-2
Isomer
Name/abbreviation
cis-4-Methyl-pentene/c-4-MP-2
2-Methyl-2-pentene/2-MP-2
2,3-Dimethyl-1-butene/DMB-1
2,3-Dimethyl-2-butene/DMB-2
with 47%. On the other side, electron pushing groups generate 4-
methyl-1-pentene with an amount of up to 75%.
complex 20 which shows a selectivity for 2-methyl-1-pentene of
47%. The formation of 4-methyl-1-pentene shows its highest selec-
tivity (90%) (2) in contrast to the formation of 2-methyl-1-pentene
by the reaction of complexes with a −J-effect at the ligand precursor
like Cl, Br or I.
These two products are generated by different first insertion
steps (Scheme 3), and are caused by the electronic influence of
both substituents. Complex 5 is the only complex that produces 2,3-
dimethylbutene in satisfying yields (25%) with medium selectivity
towards dimerization products (Table 3).
The kind of substitution at the meta position of the
bis(imino)pyridine complex has no influence on the selectivity of
the dimers, but it effects the distribution of the dimers immensely.
Complexes 6–9, 24, 25 and 27 with a −J-effect at the meta position
of the phenyl group give a maximum selectivity of 2-methyl-1-
pentene of 13%. A ligand with a +J-effect at the same position gives
2.4. Influence of PPh3 as an additive
In the late 60s Wilke recognized the influence of additives in
catalytic reactions [29]. Phosphanes are widely used additives and
a positive influence on selectivity and activity was observed during
dimerization of propylene [30].
The relevant complexes were dissolved in toluene, PPh3 was
added in a ratio of metal:additive = 1:1 (2, 2.5, 3 and 4). The solu-
tions were stirred for 30 min and activated with MAO.
The addition of the additive had a positive influence on the
dimer selectivity (90%) with the use of 2 equiv. PPh3 for 17. The
selectivity could be improved up to 95%. For all other amounts no
improvement could be observed. In contrast, the use of additive
had a great influence on the product distribution (Scheme 5). With
Scheme 4. Catalysts 2–4, 8, 12, 14–18, 20, 23, 26 and 27 with the highest selectivity
towards dimerization products of propylene.

48
J.R.V. Lang et al. / Journal of Molecular Catalysis A: Chemical 322 (2010) 45–49
Table 3
Selectivity of dimerization products and product distribution within hexene isomers for the vanadium(III) complexes 2–28.
V(III) complex no.
Selectivity to dimers (%)
Products within the dimers (%)
4-MP-1
2,3-DMB-1
c-4-MP-2
t-4-MP-2
2-MP-1
t-2-hex
2-MP-2
c-2-hex
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
93
87
83
60
70
72
83
75
55
55
80
60
85
85
95
90
81
75
83
76
70
80
77
40
83
84
77
90
85
89
24
68
71
73
82
73
62
68
55
5
36
7
3
11
8
19
34
32
25
70
74
41
75
72
1
2
1
25
5
2
5
–
–
2
2
–
–
–
–
–
–
–
5
1
2
–
–
1
–
3
2
4
6
7
45
14
13
3
–
3
1
0
3
2
3
3
–
4
5
3
11
6
6
5
7
9
7
8
4
5
2
4
3
3
6
5
4
2
6
9
8
13
6
20
14
10
26
75
46
80
88
77
76
47
45
52
63
6
–
–
–
–
–
–
–
–
–
–
1
3
1
1
–
–
–
–
1
1
–
–
–
–
–
–
–
–
–
–
–
1
4
3
–
–
–
–
–
–
1
–
–
–
–
6
1
–
–
5
–
–
4
–
–
–
–
–
–
–
–
–
–
–
–
1
–
–
–
–
–
–
1
1
–
–
–
–
–
–
–
9
7
18
14
13
8
10
7
4
5
7
15
10
10
7
17
13
5
8
51
6
9
14
6
accrue the 1,2-propylene insertion. Different halide groups as sub-
stituents on the para position have no influence on the product
distribution and selectivity. Effects can be obtained when elec-
tron withdrawing and donating groups are introduced. The first
ones generate 4-methyl-1-pentene as main product. Electron push-
ing substituents give 2-methyl-1-pentene. The octane numbers
of the main products are between 94% and 99%. It is obvious,
that the structure of the precatalyst, in particular the substitu-
tion pattern of the organic compound has a great influence on
the product distribution, but not on the selectivity. No depen-
dence for dimer selectivity is obvious from the insertion pathway.
In less cases the expected multiple branched hexenes could be
obtained. Complex 5 produced 2,3-dimethylbutene in yields of
25% within the dimerization products. The use of additives had
a positive influence on the product distribution and was very
selective for complex 26. Complex 26 and 2 equiv. of the additive
PPh3 produced 90% of 4-methyl-1-pentene within the dimers. In
the case of complex 17 the use of an additive had an enormous
effect on the initial insertion step. It changed from 90% of 1,2-
insertion up to 78% for 2,1-insertion with the use of 2.5 equiv. of
PPh3.
Scheme 5. Product distribution of the reaction of the complexes 17 and 26 and
propylene with a various ratio of the additive PPh3.
the addition of 2.5 equiv. a maximum of 70% for the formation of
4-methyl-1-pentene (17) could be achieved. The absence of PPh3
effects the formation of 2-methyl-1-pentene with a selectivity of
88%. Insertion mechanisms are influenced by the use of phosphine
containing additives, which results in a 1,2-insertion instead of
2,1-insertion. The results of the corresponding reactions of com-
plex 26 (Scheme 5) confirm the additive dependency as discussed
before. A selectivity of 90% was detected for 4-methyl-1-pentene
by the addition of 2–2.5 mol PPh3 in contrast to 51% without an
additive.
4. Experimental
4.1. General considerations
Air- and moisture-sensitive reactions were carried out under an
atmosphere of purified argon using conventional Schlenk or glove
box techniques. The dimerization reactions were performed with
pressure Schlenk tubes.
3. Summary and conclusion
Novel complexes of the type bis(imino)pyridine vanadium(III)
(2–5) were synthesized. Because of the simple synthetic route,
numerous substitution patterns can be performed. Bulky sub-
stituents on the ortho position have positive influence on the
selectivity of the dimer products. Complex 16 with a benzyl
substituent at the ortho position gave a selectivity of 95% for
dimers. Substituents at the 2 and 6 positions of the phenyl group
The products of the dimerization experiments were character-
ized by a gas chromatograph (Agilent 6890) and GC/MS (FOCUS
DSQTM Thermo Scientific). Mass spectra were recorded on a Varian
MAT CH7 instrument (direct inlet system, electron impact ioniza-
tion 70 eV). Elemental analyses were performed with a VarioEl III
CHN instrument. Acetanilide was used as standard. NMR spectra
were taken on a Varian Inova 400 instrument. The samples were

J.R.V. Lang et al. / Journal of Molecular Catalysis A: Chemical 322 (2010) 45–49
49
prepared under argon atmosphere and measured at room tempera-
4.4.1. Spectroscopic data
ture. Chemical shifts (ı, ppm) were recorded relative to the residual
solvent peak at ı = 7.24 ppm for chloroform-d. The multiplicities
2: MS data: 533 (M•+) (8), 497 M–Cl (100), 377 (30), 150 (62),
36 (100). C23H21Cl3F2N3V (533.02): calcd.: C, 51.66%; H, 3.96%; N,
7.86%. Found: C, 49.87%; H, 4.34%; N, 7.02%.
1
were assigned as follows: s, singlet; m, multiplet; t, triplet. 13C { H}
NMR spectra were fully proton decoupled and the chemical shifts
3: MS data: 565 (M•+) (13), 531 M–Cl (100), 406 (18), 396 (10).
(ı, ppm) are relative to the solvent peak (77.0 ppm).
C23H21Cl5N3V (564.96): calcd.: C, 48.67%; H, 3.73%; N, 7.40%. Found:
C, 48.97%; H, 3.55%; N, 7.13%.
4.2. Materials
4: MS data: 653 (M•+) (7), 619 (37), 541 (10), 187 (63), 36 (100).
C23H21Cl3Br2N3V (652.86): calcd.: C, 42.08%; H, 3.22%; N, 6.40%.
Found: C, 42.61%; H, 3.33%; N, 6.42%.
All solvents were purchased as technical grade and purified by
distillation over Na/K alloy under an argon atmosphere. All other
chemicals were purchased commercially from Aldrich or Acros or
were synthesized according to literature procedures. The methyl
aluminoxan solution (MAO, 30 wt.% in toluene) was obtained from
Albemarle, USA.
5: MS data: 808 (M•+) (4), 772 (100). C23H19Cl3Br4N3V (808.68):
calcd.: C, 33.92%; H, 2.35%; N, 5.16%. Found: C, 33.45%; H, 2.30%; N,
4.89%.
4.5. Homogeneous dimerization of propylene
4.3. General procedure for the synthesis of the preligands
The respective complex was dissolved in toluene and activated
with MAO solution (V:Al = 1:500) and transferred into a 400 ml
pressure Schlenk tube. The pressure Schlenk tube was filled with
50 ml liquid propylene and closed, warmed to room temperature
with an external water bath and stirred. After the reaction time of
1 h, the Schlenk tube was opened and the solution was analyzed by
GC.
10 g mol sieves (4 Å) and 0.5 g of catalytically active SiO2/Al2O3
pellets were added to a solution of 0.49 g (3.0 mmol) diacetylpyri-
dine in toluene. After addition of 7.0 mmol of the respective
aniline, the solution was heated at 45 ◦C for 24 h. After filtration
over Na2SO4 and evaporation to dryness, the products were pre-
cipitated as yellow solids from methanol over night at −20 ◦C
(73–94%).
Acknowledgement
4.3.1. Spectroscopic data
We gratefully acknowledge financial support from Cono-
coPhillips, Bartlesville, USA.
1a: 1H NMR (400 MHz, CDCl3): 8.30 (d, 2H, Py–Hm), 7.85 (t, 1H,
Py–Hp), 7.15 (t, 2H, Ph–H), 6.53 (m, 4H, Ph–H), 2.39 (s, 6H, N CMe),
2.26 (s, 6H, Ph–CH3). 13C {1H} (100.5 MHz, CDCl3): 167.9 (Cq),
163.1 (Cq), 159.9 (Cq), 155.3 (Cq), 150.4 (Cq), 136.9 (CH), 131.6 (CH),
122.4 (CH), 114.8 (CH), 106.6 (CH), 16.2 (CH3), 14.1 (CH3). MS data:
377 (M•+) (88), 362 (12), 150 (100).
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136.8 (CH), 131.0 (CH), 123.0 (CH), 122.4 (CH), 118.4 (CH), 22.2
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151 (Cq), 132.0 (Cq), 125.2 (Cq), 137.0 (CH), 129 (CH), 122.7 (CH),
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M–CH3C NAr (100).
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Tanaka, E. Steinrucke, D. Walter, H. Zimmermann, Angew. Chem. Int. Ed. 5
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4.4. Synthesis of the bis(imino)pyridine vanadium(III) complexes
An amount of 0.22 mmol of the respective bis(imino)pyridine
compound was dissolved in 20 ml diethylether and stirred. A
stoichiometric amount of vanadium trichloride–tetrahydrofuran
adduct was added at room temperature. Stirring was continued
overnight. Pentane was added to precipitate the product, which
was subsequently collected by filtration, washed with pentane and
dried in vacuo. The resulting solids were obtained with an overall
yield of 65–87%.
[30] K. Schneider, Dissertation Thesis, University Bayreuth, 2006.