An effective route for the synthesis of cationic palladium complexes of general formula [(Acac)PdL1L2]+A-

Article abstract of DOI:10.1016/j.jorganchem.2007.12.019

A series of palladium complexes of general formula [(Acac)PdL¹L²]⁺A^-, where L¹, L² = phosphines and A = BF₄, CF₃SO₃, were synthesized. Preliminary studies show that the complexes are active in selective dimerization of styrene and addition polymerization of norbornene.

Full text of DOI:10.1016/j.jorganchem.2007.12.019

Available online at www.sciencedirect.com
Journal of Organometallic Chemistry 693 (2008) 2069–2073
www.elsevier.com/locate/jorganchem
Communication
An effective route for the synthesis of cationic palladium complexes
of general formula [(Acac)PdL1L2]+Aꢀ
a,
a
b
a
*
Vitalii S. Tkach , Dmitrii S. Suslov , Gomboo Myagmarsuren , Gennadii V. Ratovskii ,
Alexander V. Rohin a, Tuczek Felix c, Fedor K. Shmidt a
a Irkutsk State University, K. Marks 1, Irkutsk 664003, Russia
b National University of Mongolia, Ulaanbaatar 210646, Mongolia
c Institute of Inorganic Chemistry, Christian-Albrecht University, Kiel D-24098, Germany
Received 1 October 2007; received in revised form 29 November 2007; accepted 11 December 2007
Available online 23 December 2007
Abstract
A series of palladium complexes of general formula [(Acac)PdL1L2]+Aꢀ, where L1, L2 = phosphines and A = BF4, CF3SO3, were syn-
thesized. Preliminary studies show that the complexes are active in selective dimerization of styrene and addition polymerization of
norbornene.
Ó 2008 Published by Elsevier B.V.
Keywords: Boron trifluoride; Dimerization; Palladium; Styrene; Norbornene; Polymerization
1. Introduction
key intermediate complex 1 was identified by IR spectros-
copy (Scheme 1).
The cationic palladium complexes are widely used as
catalyst precursors in a variety of olefin transformations.
For more than 50 years palladium compounds have been
intensively used as active and selective catalysts for reac-
tions of many difficult substrates providing new useful syn-
thetic organic products. Halide abstraction from neutral
halide complexes in the presence of a weakly coordinating
ligand represents a general route to palladium cationic
complexes [1]. Systems on the basis on Pd(Acac)2 (where
Acac = acetylacetonate) and BF3OEt2 are very promising
catalysts for selective oligomerization of vinyl monomers,
the polymerization of norbornene and its derivates as well
as the telomerization of butadiene with amines [2–4].
We recently reported on mechanistic study on the inter-
actions between Pd(Acac)2, PPh3 and BF3OEt2 [5]. As a
The existence of intermediate 1 is supported by isolation
of complex [(Acac)Pd(PPh3)2]BF4 (2) which is produced
upon addition of 1 equiv. of PPh3 to the reaction mixture.
We report here the synthesis of the cationic palladium
complexes of general formula [(Acac)PdL1L2]+Aꢀ, where
L1, L2 = phosphines and A = BF4 or CF3SO3. The com-
1
pounds were characterized by elemental analysis, IR, H,
13C, 11B, 19F and 31P NMR spectroscopy. Preliminary
studies showed that the complexes are active in polymeriza-
tion of norbornene and selective dimerization of styrene.
2. Results and discussion
2.1. Complexes
The basic principle for the synthesis of the cationic pal-
ladium complexes are shown in Eqs. (1)–(4) presented in
Scheme 2.
*
In Scheme 2, HBF3OH, the product of intermolecular
interaction between BF3 and trace amounts of water in
Corresponding author. Tel./fax: +7 3952425935.
E-mail address: tkach_vk@bk.ru (V.S. Tkach).
0022-328X/$ - see front matter Ó 2008 Published by Elsevier B.V.
doi:10.1016/j.jorganchem.2007.12.019

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V.S. Tkach et al. / Journal of Organometallic Chemistry 693 (2008) 2069–2073
+
O
O
PPh3
O
O
PPh3
C6H6
O
O
PPh3
O
-AcacBF2
-2OEt2
-
Pd
BF4
Pd
(1)
+ 2BF3OEt2
+ C6H6
Pd
- C6H6
F
BF3
O
Scheme 1.
+
O
O
PR3
PR3
O
O
PR3
O
+ PR3
O
O
-
(1)
+2OEt2
+AcacBF2
BF4
Pd
Pd
+
Pd
PR3
+
2BF3·OEt2
O
O
O
-OEt2
H+[BF3OH]- (2)
BF3·H2O
+
H2O
BF3·OEt2
+
O
O
PR3
O
PR3
O
+ PR3
H+[BF3X]-
(3)
BF3X-
Pd
+
+
AcacH
Pd
PR3
O
O
-AcacBF2
-OEt2
+
(4)
HBF4
2BF3·OEt2
AcacH
Scheme 2. R = Ph, o-CH3C6H4, p-CH3C6H4, o-CH3OC6H4; X = F, OH.
the solvent (Eq. (2)), initiates the protolysis of the Pd–C
bond (Eq. (3)). The formed AcacH reacts with BF3OEt2
to give HBF4 (Eq. (4)) which continues the further protol-
ysis (Eq. (3)). The procedure of protonation and removing
of acetylacetonate ligand from co-ordination sphere of pal-
ladium is not new. This process has been already observed
during kinetic studies [6] and in preparation of some com-
plexes [(Acac)PdL2]BF4 by interaction of Pd(Acac)2 with
[Ph3C]BF4 [7] or with HBF4OEt2 [8] followed by addition
of L.
In our case the general synthetic procedure includes
addition of 2 equiv. of BF3OEt2 to a benzene solution of
1 equiv. of Pd(Acac)2 and 2 equiv. of PR3. Solid complexes
2, 3, 4, 5 were isolated as yellow or orange crystals in about
90% yields.
A route for the synthesis of complexes with diverse
ligands and anions is shown in Eq. (5). Complexes 5 and
6 were synthesized in this manner and obtained as powders
in nearly 95% yield. One equivalent of a Brønsted acid
(HBF4OEt2 or HSO3CF3) was added into a benzene
solution of an equimolar mixture of Pd(Acac)2 ꢁ PPh3 and
PR03.
2.2. Catalytic activity of the complexes
Complexes 2, 3, 4, 5 activated with boron trifluoride
etherate were used as catalysts in the addition polymeriza-
tion of norbornene and selective styrene dimerization. Pre-
liminary experiments were made to check the activity for
each of two components of the system, namely complex 2
and BF3OEt2 at 60 °C. Complex 2 showed no activity in
styrene and norbornene transformations. Run with an
amount of BF3OEt2 corresponding to B/Pd = 25 ratio at
standard conditions resulted in the complete conversion
of styrene to polymer (Mv = 31500). NMR analysis
showed the product to be atactic polystyrene typical for
cationic initiators. No polymeric products were observed
when norbornene was used as substrate at the same
conditions.
Styrene could be dimerized to a mixture of trans- and
cis-1,3-diphenyl-1-butenes in the presence of the [(Acac)-
Pd(PR3)2]BF4 + 5BF3OEt2 catalyst system. GC analysis
showed that the dimers consist of the trans-stereoisomer
up to 95% and the content of the cis-isomer is up to 5%.
IR spectroscopy analysis also proved the existence of both
isomers exhibiting bands at 965 and 910 cmꢀ1 attributable
to dC–H vibrations at the double bond in trans and cis struc-
tures, respectively [9]. The molar mass of the dimer product
(m/z = 208) was confirmed by the mass spectrometry. The
1H and 13C NMR spectra of the dimer products are iden-
tical to those described in Ref. [10], indicating 1,3-diphe-
nyl-1-butene is the main product.
+
O
O
PR3
-AcacH
-2OEt2
A-
O
O
PR3
O
(5)
Pd
+ HA
+ PR'3
Pd
PR'3
O
R'=Ph, cyclohexyl; A=BF4, CF3SO3

V.S. Tkach et al. / Journal of Organometallic Chemistry 693 (2008) 2069–2073
2071
Norbornene (bicyclo[2.2.1.]hept-2-ene) and its deriva-
tives can be polymerized via ring-opening metathesis
(ROMP), olefin addition polymerization and also cationic
or radical polymerization [11]. The increasing interest in
the addition polymerization of norbornene (bicy-
clo[2.2.1]hept-2-ene) and its derivatives is due to the attrac-
tive properties of these polymers, which show high glass
transition temperatures, high optical transparency, low
dielectric constant and low birefringence [12]. Therefore
the addition polymers of norbornene and its derivatives
are attractive materials for the manufacture of microelec-
tronic and optical devices. The [(Acac)Pd(PR3)2]BF4 +
25BF3OEt2 system catalyzes the polymerization of nor-
bornene via the vinyl-addition route:
metal center, thus disfavoring the coordination and trans-
formation of substrate. In the case of 5 in the presence of
an excess of BF3OEt2 a formation of cationic Pd complexes
with chelate P(o-CH3OC6H4)3 ligands is also possible and
is a subject of special studies.
3. Experimental
3.1. Materials
All reactions were carried out under argon or nitrogen
using standard Schlenk techniques. Reagent grade chemi-
cals were purchased from commercial suppliers and used
as received unless otherwise stated. Pd(Acac)2 was addi-
tionally recrystallized from acetone. Pd(Acac)2PPh3 was
prepared according to the literature [14]. Inert gases were
purified before feeding to the reactor by passing them
through columns packed with oxygen scavenger (Fisher
REDOX) and molecular sieves 5A (Aldrich), respectively.
Styrene (99%, Aldrich) was purified by distillation under
reduced pressure over calcium hydride, CaH2. Boron tri-
fluoride etherate (Aldrich, 99%) was distilled over CaH2
prior to use. Benzene and norbornene (99%, Aldrich) were
distilled over sodium/potassium alloy (NaK). Methanol
was dried over Mg.
[(Acac)Pd(PR3)2]BF4/BF3OEt2
n
n
The 13C NMR and IR spectroscopy data of the polynor-
bornene obtained showed 2,3-enchained repeating units of
polymer backbone.
The data on the dimerization of styrene and polymeriza-
tion of norbornene using palladium cationic complexes 2–5
as catalyst precursors are shown in Table 1.
The phosphine ligand has a profound influence on both
the activity and selectivity of transition metal catalyst sys-
tems. Tolman [13] reviewed the effects of phosphine ligands
on reactions or properties of metal complexes for the first
time and they were rationalized in terms of electronic and
steric effects. Quantitative measures of electronic and steric
effects were based on A1 carbonyl stretching frequencies (m)
in Ni(CO)3L complexes and ligand cone angles (h) of
space-filling CPK models [13]. The values of TON and
TOF in the styrene dimerization and norbornene polymer-
ization (Table 1) are better correlated with the electronic
properties of phosphines than with their steric properties.
Reasonable explanation of this phenomenon might be that
the phosphines with higher electron-donor properties, such
as P(o-CH3OC6H4)3, enhance the electron density on the
3.2. Physical measurements
The IR spectra of suspensions of the complexes in Nujol
were recorded on the Specord M-80 spectrometer in KBr
cell. The IR spectra for polymer samples were recorded
using a KBr pellet technique with a Nicolet Fourier trans-
form infrared (FT-IR) spectrometer. The NMR spectra
were recorded on a Varian VXR-500S spectrometer.
NMR tubes were filled in argon atmosphere. Viscosity
measurements were carried out in 1,2,4-trichlorobenzene
at 25 °C using Ubbelohde viscometer. Calorimetric analy-
sis was done using a DSC Q100 instrument. Gas chroma-
tography (GC) analyses were performed on a ‘‘Chrom-5”
instrument (3.7 m column, SE-30 phase, nitrogen carrier
gas, T = 240 °C).
Table 1
Dimerization of styrene and polymerization of norbornene over [(Acac)Pd(PR3)2]BF4 + nBF3OEt2 catalyst system
Complex Styrene dimerizationa Norbornene polymerizationb
Electronic and steric
properties of PR3
TOF (hꢀ1
)
TON (mol Pd/mol styrene) Composition
(mass %)
Activity (kg NB/(mol Pd ꢁ h)) Yield (g) [g] (dL/g) m (cmꢀ1
)
h (°)
Dimers Trimers
93
93
2
3
4
5
13500
12900
11200
66225
62690
55670
7
7
5500
5300
4000
2.8
2.7
2.05
Traces
0.68
0.63
0.50
2068.9
2066.6
2066.7
2058.6
145
194
145
194
93
7
Traces
Traces
a
Experimental conditions: [Pd] = 3.7 ꢂ 10ꢀ6 mol, B/Pd = 5, styrene/Pd = 120000, 6 h, 70 °C.
b
Experimental conditions: [Pd] = 1 ꢂ 10ꢀ6 mol, B/Pd = 25, norbornene/Pd = 38000, 60 °C, 3.8 g of norbornene, total volume of 7 ml, 30 min.

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V.S. Tkach et al. / Journal of Organometallic Chemistry 693 (2008) 2069–2073
3.3. Preparation of [(Acac)Pd(PPh3)2]BF4 (2)
OC6H4)3 (1.1569 g; 3.282 mmol) in 40 ml of benzene. An
orange-yellow crystalline solid began to precipitate during
the addition. The solid was filtered, washed with benzene
and dried in vacuo (89 % yield). M.p. 125–127 °C. 1H
NMR (acetone-d6), d 1.5 [s, 6H, –CH3 of Acac], 3.8 [s,
18H, –OCH3 in benzene ring], 5.5 [s, 1H, –CH of Acac],
7.2–7.6 [m, 24H, protons in benzene ring]. Anal. Calc. for
C47H49BF4O8P2Pd: Pd, 10.7; C, 56.6; H, 5.0. Found: Pd,
10.4; C, 57.1; H, 4.8%.
BF3OEt2 (0.41 ml, 3.282 mmol) was dropped into a solu-
tion of Pd(Acac)2 (0.5000 g, 1.641 mmol) and PPh3
(0.8611 g, 3.282 mmol) in 40 ml of benzene. A lemon-yellow
crystalline solid began to precipitate during the addition.
The solid was filtered, washed with benzene and dried in
1
vacuo (91% yield). M.p. 174–176 °C. H NMR (acetone-
d6), d 1.5 [s, 6H, –CH3 of Acac], 5.6 [s, 1H, –CH of Acac],
7.2–7.8 [m, 30H, –PPh3]; 13C NMR(acetone-d6), d 27.5
[–CH3 of Acac], 102.3 [–CH of Acac], 128–136 [–PPh3],
188.4 [–C@O]; 19F{1H} NMR (CD2Cl2)d ꢀ153.51 [s, 1F],
ꢀ153.56 (s, 4F) [BF4, the integral ratio of the two signals
is 1:4 equal to the 10B/11B isotopic ratio, natural isotopic
abundance: 10B/11B = 19.4%/80.6%]; 11B NMR (acetone-
d6) d ꢀ0.56 [BF4, quintet, 1:4:6:4:1, JB–F = 1.1 Hz];
31P{1H} NMR (acetone-d6) d 36.0 [–PPh3]. IR(cmꢀ1) 1523,
1565 [C@C and C@I bonds in Acac group]; 1590 [C@C bond
of benzene ring]; 1465, 1478 [CH3– of Acac group]; 1020–
1150 [BF4]; 730; 750; 960; 980 [C–H bonds of benzene ring];
511; 522; 543 [deformation vibrations of P–C bonds and
benzene ring]. Anal. Calc. for C41H37BF4O2P2Pd: Pd, 13.0;
C, 60.3; H, 4.5; P, 7.6; B, 1.35; F, 9.3. Found: Pd, 12.8; C,
61.2; H, 4.3; P, 7.1; B, 1.1; F, 8.25%.
3.7. Preparation of [(Acac)Pd(PPh3)2]CF3SO3 (6)
HCF3SO3 (0.144 ml, 1.641 mmol) was dropped into a
solution of Pd(Acac)2 ꢁ PPh3 (0.9303 g, 1.641 mmol) and
PPh3 (0.4306 g, 1.641 mmol) in 40 ml of benzene. An
orange crystalline solid began to precipitate during the
addition. The solid was filtered, washed with benzene and
dried in vacuo (95% yield). M.p. 165–167 °C. 1H NMR
(acetone-d6), d 1.5 [s, 6H, –CH3 of Acac], 5.6 [s, 1H, of
–CH Acac], 7.2–7.8 [m, 30H, –PPh3]; 19F{1H} NMR
(CDCl3)
d
ꢀ78.5 [s, CF3SO3]. Anal. Calc. for
C42H37F3O5P2SPd: C, 57.4; H, 4.2. Found: C, 57.9; H,
4.8%.
3.8. Preparation of [(Acac)Pd(PPh3)(PCy3)]BF4 (7)
3.4. Preparation of [(Acac)Pd(P(o-CH3C6H4)3)2]BF4 (3)
HBF4 ꢁ OEt2 (0.225 ml, 1.641 mmol) was dropped into a
solution of Pd(Acac)2 ꢁ PPh3 (0.9303 g, 1.641 mmol) and
PCy3 (0.4600 g, 1.641 mmol) in 40 ml of benzene. A yellow
crystalline solid began to precipitate during the addition.
The solid was filtered, washed with benzene and dried in
vacuo (93% yield). 1H NMR (CD2Cl2), d 1.0–2.2 [m,
33H, P(C6H11)3], 1.3 [s, 3H, –CH3 of Acac], 2.1 [s, 3H,
–CH3 of Acac], 5.5 [s, 1H,–CH of Acac], 7.2–7.8 [m,
BF3OEt2 (0.41 ml, 3.282 mmol) was dropped into a solu-
tion of Pd(Acac)2 (0.5000 g, 1.641 mmol) and P(o-
CH3C6H4)3 (0.9991 g, 3.282 mmol) in 40 ml of benzene.
An orange crystalline solid began to precipitate during the
addition. The solid was filtered, washed with benzene and
dried in vacuo (90% yield). M.p. 194–196 °C. 1H NMR (ace-
tone-d6), d 1.5 [s, 6H, –CH3 of Acac], 2.4 [s, 18H, –CH3 in
benzene ring], 5.5 [s, 1H, –CH of Acac], 7.2–7.6 [m, 24H,
protons in benzene ring]; Anal. Calc. for C47H49BF4O2P2Pd:
Pd, 11.8; C, 62.7; H, 5.5. Found: Pd, 12.0; C, 63.0; H, 5.8%.
15H, PPh3]; 31P{1H} NMR (CD2Cl2) d 36.1 [d, 1P, JP–P
=
30 Hz, PPh3], 47.1 [d, 1P, JP–P = 30 Hz, PCy3]. Anal. Calc.
for C41H55BF4O2P2Pd: C, 59.0; H, 6.6. Found: C, 59.1; H,
6.6%.
3.5. Preparation of [(Acac)Pd(P(p-CH3C6H4)3)2]BF4 (4)
3.9. Dimerization of styrene
BF3OEt2 (0.41 ml, 3.282 mmol) was dropped into a
solution of Pd(Acac)2 (0.5000 g, 1.641 mmol) and P(p-
CH3C6H4)3 (0.9991 g, 3.282 mmol) in 40 ml of benzene.
A yellow crystalline solid began to precipitate during the
addition. The solid was filtered, washed with benzene and
dried in vacuo (92% yield). M.p. 185–190 °C. 1H NMR
(acetone-d6), d 1.5 [s, 6H, –CH3 of Acac], 2.4 [s, 18H,
–CH3 in benzene ring], 5.5 [s, 1H, –CH of Acac], 7.2–7.6
[m, 24H, protons in benzene ring]; Anal. Calc. for
C47H49BF4O2P2Pd: Pd, 11.8; C, 62.7; H, 5.5. Found: Pd,
11.4; C, 62.2; H, 5.3%.
Dimerizations were carried out in a glass reactor
equipped with a magnetic stirrer under nitrogen or argon
atmosphere without a solvent. The reactor was evacuated
and filled with inert gas, and then styrene and palladium
precursor were added. Dimerizations were initiated by
the injection of the boron compound. The reaction mixture
was kept at room temperature for 30 min to form catalyt-
ically active species and then heated up to the desired tem-
perature. After stirring for a time needed, the reaction was
terminated and styrene dimers were isolated by vacuum
distillation (130 °C/1.3 ꢂ 10ꢀ2 Torr).
3.6. Preparation of [(Acac)Pd(P(o-CH3OC6H4)3)2]BF4
(5)
3.10. Polymerization of norbornene
BF3OEt2 (0.41 ml, 3.282 mmol) was dropped into a solu-
tion of Pd(Acac)2 (0.5000 g, 1.641 mmol) and P(o-CH3-
Polymerizations were carried out in a 10 ml glass reactor
equipped with a magnetic stirrer. The reactor was evacu-

V.S. Tkach et al. / Journal of Organometallic Chemistry 693 (2008) 2069–2073
2073
ated and filled with nitrogen, and then a toluene solution of
norbornene was added. The solution was kept at the
desired temperature for 15 min and a solution of 2 in tolu-
ene:nitromethane (80:20% vol.) was added. Polymeriza-
tions were initiated by the injection of the boron
compound. After stirring for a time needed, the polymers
formed were precipitated in acidified ethanol. The precipi-
tated polymers were washed three times with ethanol, and
dried in vacuo at 80 °C for 6 h. Polymerization runs were
carried out at least three times to ensure reproducibility.
The IR spectra of polymers show the absence of any dou-
ble bond absorbance at 1620–1680 cmꢀ1 and exhibit a
strong absorbance at 1452–1474 cmꢀ1 due to dH–C–H
stretching modes of bridging CH2 groups. Disappearance
of these peaks would definitely indicate the formation of
2,7-enchained polymer units [15,16]. Assignment of methy-
lene and methine 13C NMR resonances of the polymers
was made using DEPT editing of the 13C NMR spectrum.
13C NMR (1,2,4-trichlorobenzene), d 31.0, 32.5, 36 and 38
and [–CH groups], 20, 23.5 and 27 [–CH2 non-bridging
groups], 30–32, 32.6, 36.5 [–CH2 bridging group]. The
DSC and TGA analyses of the polynorbornene were car-
ried out for representative samples with various intrinsic
viscosities. The glass transition temperatures are about
310 °C. The polymer samples decomposed in the range
from 400 to 410 °C.
07-03-90104) and the Grant of the Irkutsk State University
for young scientists (N 111-02-000/7-12).
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