Pd-Salen and Pd-Salan complexes: Characterization and application in styrene polymerization

Article abstract of DOI:10.1016/j.inoche.2011.01.028

Two new Pd²⁺ complexes based on the Salen-type H ₂L¹ or Salan-type H₂L² ligand (H₂L¹ = N,N′-Bis(3-methoxy-salicylidene)phenylene-1, 2-diamine), H₂L² = 1,2-Bis(2-hydroxyl-3-methoxy-phenyl) methylamino-benzene), were synthesized and characterized by FT-IR, ¹H NMR and X-ray crystallography. They were shown to catalyze the polymerization of styrene, giving the syndio-enriched atactic polystyrenes with moderate molecular weights and narrow molecular weight distributions. The detailed stereoregular analysis of the polymeric products shows that the slight increase in electrophilicity or Lewis acidity of active species by the reduction from Salen-type to Salan-type ligand endows the distinctive increase of the stereoselectivities besides the increase of the catalytic activities.

Full text of DOI:10.1016/j.inoche.2011.01.028

Inorganic Chemistry Communications 14 (2011) 573–577
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Pd-Salen and Pd-Salan complexes: Characterization and application in
styrene polymerization
a
a
a
d,
Liqin Ding a,b, Zeng Chu a, Leilei Chen a, Xingqiang Lü a,c, , Biao Yan , Jirong Song , Daidi Fan , Feng Bao
⁎
⁎
a
Shaanxi Key Laboratory of Degradable Medical Material, Shaanxi Key Laboratory of Physico-inorganic Chemistry, Northwest University, Xi'an 710069, Shaanxi, China
b
School of Chemistry & Chemical Engineering, Xi'an Shiyou University, Xi'an 710065, Shaanxi, China
c
Fujian Institute of Research on the Structure of Matter, Chinese Academy of Science, Fuzhou 350002, Fujian, China
d
College of Chemistry, Central China Normal University, Wuhan 430079, Hubei, China
a r t i c l e i n f o
a b s t r a c t
Two new Pd2+ complexes based on the Salen-type H2L1 or Salan-type H2L2 ligand (H2L1 =N,N′-Bis(3-
methoxy-salicylidene)phenylene-1,2-diamine), H2L2 =1,2-Bis(2-hydroxyl-3-methoxy-phenyl)methyla-
mino-benzene), were synthesized and characterized by FT-IR, 1H NMR and X-ray crystallography. They
were shown to catalyze the polymerization of styrene, giving the syndio-enriched atactic polystyrenes with
moderate molecular weights and narrow molecular weight distributions. The detailed stereoregular analysis
of the polymeric products shows that the slight increase in electrophilicity or Lewis acidity of active species by
the reduction from Salen-type to Salan-type ligand endows the distinctive increase of the stereoselectivities
besides the increase of the catalytic activities.
Article history:
Received 14 December 2010
Accepted 25 January 2011
Available online 3 February 2011
Keywords:
Pd-Salen or Pd-Salan complexes
Syndio-enriched atactic polystyrene
Electrophilicity or Lewis acidity of active
species and catalytic performance
© 2011 Elsevier B.V. All rights reserved.
Styrene, as one of the few monomers, can be polymerized through
one of the known possible polymerization mechanisms, such as
radical, cationic, anionic or coordination–insertion mechanism [1]. In
the viewpoint of the obtained styrene polymers, isotactic, syndiotactic
or atactic polystyrenes [2], exhibiting different stereoregular con-
formations and physical properties [3], can be obtained from the
special design of needed catalysts and/or co-catalysts and the control
of the polymerization conditions [4]. Compared with the metallocenes
based on late transition metal catalysts preferably in production of
atactic polystyrene [5], early transition metal catalysts for stereospe-
cific polymerization of styrene have shown the superiority [6], in
which, syndiotactic or isotactic polystyrenes are obtained with
different molecular weight sizes and narrow molecular weight
distributions, and the coordination–insertion polymerization of
styrene is proposed for the possible control of stereoselectivity [7].
Nevertheless, late transition metal catalysts are less oxophilic, not as
sensitive as early transition metal catalysts to function groups, and
thus applicable to efficiently catalyze the homo- [8] or copolymeri-
zation [9] of styrene; especially, some interesting performances have
been noted for late transition metal catalysts systems by different
polymerization mechanisms, giving the isotactic-enriched oligo- and
polystyrenes. For example, the first styrene oligomerization to
isotactic-enriched (up to mm=90%) oligomers was reported with
the cationic η3-benzylic Ni2+ complexes associated with phosphorus
ligands, in which, the stereoregulation was proposed by the cationic
mechanism [10]. Neutral anilido–imino Ni2+ complexes/MAO could
produce non-stereospecific polystyrene, and a possible coordination–
insertion mechanism was proposed [11]. As to the radical polymer-
ization of styrene, biocompatible Cu2+ or Fe2+ complexes bearing α-
diimine or bis(ozazoline) ligands were reported as the efficient
catalysts [12].
Salen-type or Salan-type ligands are easily synthesized and formed
complexes with almost all metal ions [13]. Compared to the
homogeneous or heterogeneous catalysis of their metal complexes
in various chemical reactions [14], efforts on their use for the olefin
polymerization are relatively limited [15] due to the difficulty on the
control of stereoselectivity for the resulting polymers, which has been
assigned to be relative to the site complication (such as coordination
geometry [16], oxidation state [17] or ligand-to-metal distance [18])
on the active species of the selected catalysts. On the other hand, the
recent studies [19] on the comparison of catalytic behaviors for the
polymerization from the metal (early or late transition metal)
complexes with Salen-type ligands or Salan-type ligands, show that
the slight differences in electrophilicity or Lewis acidity of the central
metal ions and steric hindrance of the catalyst system result in the
distinctly different stereoregular polymers besides the differences of
the catalytic activities during the polymerization process. This
reminds the detailed effect of the polymerization behaviors on the
electrophilicity or Lewis acidity of the active species necessary,
especially on the similar condition of other factors. Herein, by the
reaction of deprotonated Salen-type ligand H2L1 (H2L1 =N,N′-Bis(3-
methoxy-salicylidene)phenylene-1,2-diamine) or Salan-type ligand
H2L2 (H2L2 =1,2-Bis(2-hydroxyl-3-methoxy-phenyl)methylamino-
benzene) with anhydrous PdCl2, the Pd2+ complex [PdL1] (1) or
⁎
Corresponding authors. Tel./fax: +86 29 88302312.
E-mail addresses: lvxq@nwu.edu.cn (X. Lü), polymerbaofeng@yahoo.com.cn
(F. Bao).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.01.028

574
L. Ding et al. / Inorganic Chemistry Communications 14 (2011) 573–577
[PdL2] (2) was obtained, respectively. In the viewpoint of the
structures of the two complexes, the reduction from Salen-type to
Salan-type ligand, endows the difference in electrophilicity or Lewis
acidity of Pd2+ ions while keeping the similarity of other characters.
Through the point, the specialty of their catalytic behaviors on the
polymerization of styrene was examined.
As shown in Scheme 1, the Salan-type ligand H2L2 was obtained
from the reduction of Salen-type Schiff-base ligand H2L1 with
excessive KBH4 in good yield of ca 72%. The two ligands are very
soluble in CH2Cl2 and CHCl3, but moderately soluble in hot EtOH and
MeOH. When an equimolar amount of deprotonated H2L1 or H2L2 was
allowed to react with anhydrous PdCl2, the Pd2+ complex [PdL1] (1)
or [PdL2] (2) was obtained in 60–69% yields, respectively. Moreover,
the Pd2+ complex [PdL2] (2) based on Salan-type ligand H2L2 was also
directly obtained from the reduction of the [PdL1] (1) based on Salen-
type Schiff-base ligand H2L1 despite the slightly lower yield of ca 51%.
The 1H NMR spectrum of the Salan-type ligand H2L2 in DMSO-δ6
exhibits a sharp singlet at δ=4.23 ppm for the O―H protons, without
the typical chemical shift of the resonance-assisted hydrogen bonded
(RAHB) protons (δ=12.99 ppm) of the type O―H⋯N C in Salen-type
ligand H2L1. Furthermore, the single crystal X-ray analysis (as shown
in Fig. 1s) of H2L2 reveals that it lies in the trans-conformation, loses
the planar character of the typical Salen-type Schiff-base ligands (like
H2L1), in which, the two “Sal” rings twists with an angle of 120.9°.
The two Pd2+ complexes 1–2 are insoluble in water while soluble
in common organic solvents. These complexes were characterized by
EA, FT-IR, 1H NMR, and X-ray quality crystals were obtained, with the
tables of selected crystal properties of complexes 1·3H2O or 2·4H2O
given in Tables 1 and 2s. The asymmetric unit of complex 1·3H2O is
composed of one neutral [PdL1] molecule and three H2O solvate
molecules. As shown in Fig. 1, the Pd2+ ion is located in the inner cis-
N2O2 core of the (L1)2− Salen-type Schiff-base ligand, and adopts a
perfectly square planar geometry, where the N–Pd–N and O–Pd–O
angles are 84.0(2)° and 88.1(2)°, respectively, and the dihedral angle
of the two delocalized six-membered chelate rings (PdOCCCN) is 2.0°.
The H2O solvate molecules do not appear to interact with the host
structure. Interestingly, The change of ligands from Salen-type H2L1 to
Salan-type H2L2 did not lead to a significant change of the molecular
structure of the monomer Pd2+ complex 2·4H2O in spite of the
distinctive structural difference between the two ligands. As shown in
Fig. 2, the coordination of Pd2+ ion makes the deprotonated Salan-
type ligand (L2)2− twisted in the conformation of cis-N2O2 mode,
similar to that of (L1)2− in complex 1·3H2O while not in the
conformation of trans-N2O2 mode for the free ligand H2L2. In the
Fig. 1. View of X-ray structure of 1. Thermal ellipsoids are drawn at the 25% probability
level. The Solvate molecules were omitted for clarity.
neutral monomer [PdL2] unit of 2·4H2O, the four-coordinate Pd2+ ion
is arranged in an almost square planar geometry, where the N–Pd–N
and O–Pd–O angles of 83.7(3)° and 88.5(3)°, and the small dihedral
angle of 1.6° between the two delocalized six-membered chelate rings
(PdOCCCN) are observed. It is worth noting that the similarity in the
structures of the two Pd2+ complexes is observed, while the
difference in electrophilicity or Lewis acidity of Pd2+ ions arising
from the reduction from Salen to Salan, should play the important role
in catalysis.
Assisted by the co-catalyst of Azobisisbutyronitrile (AIBN) the
polymerizations of styrene using the Pd2+ complexes 1–2 as catalysts
in a stipulated molar ratio ([2]:[1]) of [Pd]/[AIBN] were carried out in
different molar ratios of [M]/[Pd] at the scope of temperature from 80
to 130 °C for 17 h, and the medium catalytic activities were obtained.
As shown in Table 1, for the Pd-Salen (1) and Pd-Salan (2) complexes,
H
H
N
N
N
N
KBH4
OH
HO
OH
HO
O
O
O
O
-H
PdCl2
PdCl2
-H
H
H
N
O
N
N
N
Pd
Pd
KBH4
O
O
O
O
O
O
O
Fig. 2. View of X-ray structure of 2. Thermal ellipsoids are drawn at the 25% probability
level. The solvate molecules were omitted for clarity.
Scheme 1. Controlled design of Pd-Salen (1) and Pd-Salan (2) complexes.

L. Ding et al. / Inorganic Chemistry Communications 14 (2011) 573–577
575
rr
Table 1
Catalytic data in the polymerization of styrene by the different Pd2+ complexes 1–2.
Entry [Pd]
T
[M]/[Pd] Yield Activity
Mbn Mcw
PDI
(°C) ratio
(g)
(104 g/mol h) (×105) (×105) (Mn/Mw
)
1
2
3
4
5
6
7
8
9
–
1
1
1
1
2
2
2
2
130
∞
0.01
–
0.011
0.501
1.052
1.269
1.516
0.645
1.462
1.803
2.065
0.041
1.390
2.471
2.653
2.805
2.059
3.304
3.534
3.676
3.73
2.77
2.35
2.09
1.85
3.19
2.26
1.96
1.78
mr
130 1000/1
100 1000/1
80 1000/1
80 2000/1
130 1000/1
100 1000/1
80 1000/1
80 2000/1
0.24 0.082
0.46 0.157
0.39 0.133
0.18 0.123
0.35 0.119
1.68 0.574
1.57 0.536
0.73 0.499
mm
2.2
2.1
2.0
1.9
1.8
1.7
1.6
a
General polymerization conditions: 8 ml xylene, 2 ml styrene, [Pd]/[AIBN]=2:1,
reaction for 17 h.
7
6
5
4
3
2
1
0 ppm
b
Activity in g of polymer/(mol of Pd h).
c Mn is the relative number-average molecular weight.
Fig. 3. View of typical 1H NMR spectrum of polystyrene prepared by 1/AIBN or 2/AIBN
d
Mw is the relative weight-average molecular weight.
system.
their catalytic activities obviously depend on the choice of the
polymerization temperature: With increase of the polymerization
temperature from 80 to 100 °C, the catalytic activity increases first
Functional Theory (DFT) method [21]. Obviously, the slight increase of
the electrophilicity of Pd2+ ion in Pd-Salan (2) should be beneficial to
increase the efficiency of catalysis in polymerization of styrene at mild
reaction conditions. In addition to that, the catalytic systems from 2/
AIBN, in contrast to the polymeric products with lower molecular
weights from 1/AIBN systems, afforded the polystyrenes with higher
molecular weights under the same polymerization condition, in
which an elucidation is that the propagating polymeric chains easily
produce from the more electrophilic active species.
The slight differences in electrophilicity or Lewis acidity of the
central Pd2+ ions also result in the distinct difference to the regio-
and/or stereoslective polymerization of styrene. With the obtained
polymers from the two Pd2+ complexes at different reaction
conditions, the results of FT-IR spectra, the 1H NMR and the 13C
NMR spectra consistently indicate that the two catalytic systems can
efficiently polymerize styrene to give the atactic polystyrene: in the
FT-IR spectra, the absorption band at about 1071 cm−1, assigned to
the characteristic signal of the atactic polystyrene [22], is observed;
The typical 1H NMR spectra (as shown in Fig. 3) show the
characteristically broad signals of atactic polystyrene in the region
of ca. 1.70–1.96, 1.96–2.06 and 2.06–2.25 ppm, which can be assigned
to the syndiotactic (rr) triad, the heterotactic (mr) triad and the
isotactic (mm) triad of the methine carbon hydrogen proton
resonances [23], respectively; the identical 13C NMR spectra (as
shown in Fig. 4) in the region of ca. 144.5–146.5 ppm also show the
characteristically broad signals of the Phenyl C1 of the atactic
(0.123–0.157 × 104 g/mol h for
1 in entries 3–5 and 0.499–
0.574×104 g/mol h for 2 in entries 7–9), while higher temperature
(130 °C) leads to the decrease of the activities (0.082×104 g/mol h for
1 in entry 2 and 0.119×104 g/mol h for 2 in entry 6), which shows
that the active species loss rate obviously increases at 130 °C, resulting
in the decrease of high molecular weight polystyrenes and the
increase of low molecular weight styrene oligomers; Moreover, the
polymerization yields and the catalytic activities of complex 2 are
apparently higher than those of complex 1 below 100 °C, and the
highest activity value can be reached 0.157×104 g/mol h for 1/AIBN
system and 0.574×104 g/mol h for 2/AIBN system at 100 °C, respec-
tively. In addition, the molecular weights and the molecular weight
distributions of the obtained polymers are also strongly affected by
the polymerization temperature: For the two Pd2+ catalysts, the
molecular weight (Mw or Mn) of the polymers decreases monoto-
nously with polymerization temperature increasing, while the lower
temperatures could afford the polymers with narrow molecular
weight distributions (PDI=2.09–2.35 for 1 and PDI=1.96–2.26 for
2). It may be due to the significant increase of the rates of the chain
transfer and the chain termination at higher temperature. It is worth
noting that the polymerization behaviors of styrene are also affected
by the [M]/[Pd] ratio: for both of the two catalytic systems, the
increase of [M]/[Pd] ratio leads to the distinctly increase in molecular
weights (Mw or Mn) in spite of the slight decrease of the polymer
yields and the catalytic activities. This fact shows that the lower
initiator concentration should be helpful for the chain growth of the
polymeric products, but unfavorable for the monomer conversion
with less initiation sites.
rr
Quite significantly on correlation of the molecular structures of the
two Pd2+ catalysts versus their catalytic behaviors, a fact is disclosed
that on the similar condition of the controllable polymerization, the
catalytic activity of Pd-Salan (2) is about 3.7–4.0 times that of the Pd-
Salen (1), which is comparable to the result of the copolymerization of
epoxides and CO2 catalyzed by Cr-Salan and Cr-Salen systems [20]. As
a matter of fact, from Salen-type H2L1 to Salan-type H2L2, the
reduction of C=N double bonds leads to the sp3-hydrization of the
two amino groups with the change of planar to non-planar character.
As distinguished from the nature of active species in Pd-Salan (2) or
Pd-Salen (1), the presence of sp3-hydrized amino donors in 2
effectively increases the Lewis acidity of the central Pd2+ ion, despite
no distinctive difference from coordination geometries, oxidation
states or ligand-to-metal distances of the Pd2+ ions, which can further
be proved by the mulliken net charges of Pd2+ ion in the two
complexes (1.2309 e in 1 and 1.2914 e in 2) from the single point
energy calculation based on their obtained XRD data by Density
mr
mm
ppm
146
145
160
140
120
100
80
60
40
20
0 ppm
Fig. 4. View of typical 13C NMR spectrum of polystyrene prepared by 1/AIBN or 2/AIBN
system.

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L. Ding et al. / Inorganic Chemistry Communications 14 (2011) 573–577
polystyrene nature [24]. It is interesting to notice that from the stereo-
trial distributions of mm, mr and rr calculated from the triad
resonance integral (as shown in Table 3s), the [rr]=51.0–54.8%, the
[mr]=33.4–38.2% and the [mm]=9.6–15.6% for 1/AIBN system,
while the [rr]=62.0–68.4%, the [mr]=20.2–28.9% and the [mm]=
7.4–17.8% for 2/AIBN system are observed, respectively. The results
show the higher stereoregularity in polymerization of styrene by the
2/AIBN system than that by 1/AIBN system, although the obtained
polystyrenes from the two catalytic systems were the same syndio-
enriched atactic. Furthermore, from the investigation of the micro-
structure analysis of the polystyrenes catalyzed by the two catalytic
systems at various polymerization conditions, lower polymerization
temperature and higher catalyst concentration were in favor of the
well sterospecific control, which was consistently relative to the
catalytic activity. In addition, the TGA data further indicate that these
polystyrenes could be stable up to 370 °C. In fact, similar to the
polymerization behaviors from the reported late transition metal
catalysts [8,10–12], the resulting polystyrenes catalyzed by the two Pd
(II) complexes are essentially atactic, despite the possibly different
polymerization mechanisms between the catalytic systems. Further
comparing our results with the data in the literature for the
polymerization of styrene mediated by the Ni2+ β-ketoamine [25]
or N–O–chelate catalysts/MAO [26], it can be seen that the lower
catalytic activities while narrower molecular weight distributions are
observed, which should be due to the perfectly square planar
geometry of the active species in catalysts 1–2, especially in the
present of AIBN co-catalyst, while there are important differences in
the polymerization products. Depending on the adjustment of
electrophilicity or Lewis acidity of active species by the reduction
from Salen-type to Salan-type ligand used, the modulation to the
stereo-regularity of the polystyrenes in a certain degree is effective,
endowing the syndio-enriched atactic, instead of the typically atactic
polystyrenes from the Ni2+ catalysts.
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This work is funded by the National Natural Science Foundation
(20871098), the State Key Laboratory of Structural Chemistry
(20100014), the Provincial Key Item of Shaanxi and Graduate Cross-
discipline Funds (09YJC23) of Northwest University.
Appendix A. Supplementary material
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The syntheses and characterization of H2L1, H2L2 and complexes 1–
2 are founded in the Supporting Information. The micro-structure
analysis of the obtained polystyrenes is shown in Table 3s. The
Crystallographic data for H2L2 and complexes 1·3H2O and 2·4H2O are
founded in Tables 1 and 2s, and have been deposited with the
Cambridge Crystallographic Data Centre (CCDC reference numbers:
H2L2 and complexes 1·3H2O and 2·4H2O: 794142–794144, respec-
tively). Copies of the data can be obtained free of charge on
application to The Director, CCDC, 12 Union Road, Cambridge CB2
1EZ, UK (fax: int.code +(1223)336-033; e-mail:teched@chemcrys.
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