New phenylene bridged binuclear bis (imino-quinolyl) palladium (II) complexes: Synthesis, characterization and Heck reactions

Article abstract of DOI:10.1016/j.inoche.2012.07.018

Two new phenylene bridged binuclear bis (imino-quinolyl) palladium (II) complexes were prepared from the reaction of the ligand 1,4-bis (imino-quinolyl) benzene, L1 with either Pd (cod) Cl₂ or Pd (cod) ClMe. The molecular structure of L1 was co

Full text of DOI:10.1016/j.inoche.2012.07.018

Inorganic Chemistry Communications 24 (2012) 221–224
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Inorganic Chemistry Communications
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New phenylene bridged binuclear bis (imino-quinolyl) palladium (II) complexes:
Synthesis, characterization and Heck reactions
William M. Motswainyana, Martin O. Onani ⁎
Chemistry Department, University of the Western Cape, Private Bag X17, Bellville, 7535, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new phenylene bridged binuclear bis (imino-quinolyl) palladium (II) complexes were prepared from
Received 23 April 2012
Accepted 3 July 2012
Available online 25 July 2012
2
the reaction of the ligand 1,4-bis (imino-quinolyl) benzene, L1 with either Pd (cod) Cl or Pd (cod) ClMe.
The molecular structure of L1 was confirmed by single crystal X-ray diffraction analysis. The structure
shows two planar molecules (A and B) in the asymmetric unit. The complexes were evaluated as catalysts
for Heck coupling reactions of methyl and butyl acrylate with iodobenzene, and showed high activity. The re-
actions proceeded at a faster initial rate and there was good selectivity for the formation of the trans- isomers.
Keywords:
Bis (imino-quinolyl)
Palladium
©
2012 Elsevier B.V. All rights reserved.
X-ray diffraction
Heck reactions
Imino-pyridyl ligands are a popular class of N-N ligands [1,2]. They
are best applied in the preparation of air stable complexes which
show hemilability due to weakly coordinating nitrogen atoms [3].
Much of the literature work on these imino-pyridyl ligands is based
on the mononucleating ligands because of the ease of their prepara-
tions [3–5]. However, attention has shifted to binucleating ligands
with a rigid support because they bring two metal centres into closer
proximity, resulting in a bimetallic complex which possesses unique
reactivity patterns and unusual catalytic properties [6–8]. The two
metal centres could exhibit inter-metallic cooperative effect, and
thus be used to activate inert chemical bonds, which is difficult to
achieve by mononuclear metal complexes [6].
Specifically, nickel bimetallic complexes supported by an aryl-linked
iminopyridines have been reported and evaluated in ethylene transfor-
mations giving high activities [7,9]. The intermetallic cooperative effect
between the two metal centres is believed to be the reason for the mod-
ified catalytic performance [7,8]. Traditionally, Heck coupling reactions
have been achieved through the less stable and air sensitive phosphine
based complexes. Investigations are underway for cheap and stable cat-
alysts to replace the above [10,11]. Mononuclear palladium complexes
of carbenes [12], benzoazoles [3,13], thiosemicarbazones [14] and
pyridylimines [5] have been evaluated as catalyst precursors in Heck
coupling reactions. However, bimetallic complexes containing an aryl
support in the ligand structure have not been explored in Heck reac-
tions. In this work, we report the synthesis and characterization of
new phenylene bridged bis (imino-quinolyl) binuclear palladium (II)
complexes. The new complexes were evaluated as catalysts in the
model Heck coupling reactions of iodobenzene with methyl and butyl
acrylate and the preliminary results are herein discussed. To the best
of our knowledge, this work represents the first report on the use of
bridged binucleating complexes in Heck coupling catalysis.
The preparation of L1, 1 and 2 was as follows (Scheme 1): 1,4-
phenylenediamine and 2-quinolinecarboxaldehyde were added to a
Schlenk tube charged with dry MeOH and anhydrous magnesium
sulphate. The reaction was stirred at room temperature for 24 h. After
filtration and evaporation of solvents, an orange solid was obtained,
2 2 6
which was recrystallized from a mixture of CH Cl :C H14 to give single
crystals suitable for X-ray crystallography. (Yield: 80%). The ligand has
been reported before [15] but it was independently prepared by us.
The palladium complexes 1 and 2 were prepared from the subsequent
2
reaction of L1 with either PdCl (cod) [16] or PdClMe (cod) [17] in a
CH Cl /Et O mixture. The reactions were stirred at room temperature
2
2
2
for 6 h and the complexes obtained in high yields (75%) after filtration
and evaporation of solvents.
The IR spectrum of 1,4-bis (imino-quinolyl) benzene ligand
−
1
−1
showed three strong absorption bands at 1621 cm , 1597 cm
−
1
and 1559 cm , which were assigned to the (C_N) imine, (C_N)
and (C_C) quinolyl vibrations respectively [1,15,18]. The other band
−
1
was assigned to the (C_C) phenyl vibration. In the 1
H
at 1504 cm
NMR spectrum of the ligand, a strong evidence of Schiff base condensa-
tion reaction was confirmed by an observed singlet at 7.48 ppm due to
the hydrogen atom in the imine (HC_N) moiety [18,19]. The four
phenyl protons also appeared as a singlet at 6.75 ppm, which is not
surprising because the protons are equivalent due to the symmetry
of the molecule. In the 13C NMR, the imine carbon (C_N) appeared
at 161.85 ppm, which is in agreement with the literature [15]. The
molecular structure of the ligand was solved using X-ray diffraction
studies (Fig. 1, Table 1).
⁎
Corresponding author. Tel.: +27 21 959 3050; fax: +27 21 959 3055.
E-mail address: monani@uwc.ac.za (M.O. Onani).
1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2012.07.018

222
W.M. Motswainyana, M.O. Onani / Inorganic Chemistry Communications 24 (2012) 221–224
Scheme 1. Preparation of bis (imino-quinolyl) palladium (II) complexes 1 and 2.
The molecule crystallizes in the monoclinic space group C2/c, with
Coordination of the ligand to the palladium metal centre could be
two planar molecules (A and B) in the asymmetric unit. The molecular
structure consists of two terminal imino-quinolyl units bridged by
a phenylene moiety, with an inversion centre located within the
phenylene ring. Each imino-quinolyl unit is almost planar (C (3)–
N(1)–C(4)–C(5) 177.1(2) ), and the adjacent imine and quinolyl
nitrogen atoms are disposed in a trans- configuration, resembling a
related bis (imino-pyridyl) ligand [20]. The phenylene ring adopts a
quasi-orthogonal arrangement with respect to the imino-quinolyl
confirmed from the IR spectrum of complexes 1 and 2 which showed
−
1
lower absorption bands between 1597 and 1599 cm
compared to
their corresponding ligand [22]. Elemental analysis data was consistent
with the proposed formulas of the bimetallic complexes. The 1H NMR
data of the complexes could not be obtained due to the solubility prob-
13
lems of the complexes. However, C NMR of the complexes was mea-
sured in a solid phase NMR. In the 13C NMR spectra, the imine carbons
in 1 and 2 appeared downfield at 167.49 and 165.77 ppm respectively,
compared to 161.85 ppm in the free imine ligand. A similar trend has
been reported in related imino-pyridyl palladium (II) complexes [1,23].
The new binuclear complexes 1 and 2 were investigated as cata-
lysts in the Heck coupling reactions of iodobenzene with methyl
5 4 2 6 4
units in a way similar to the compound 1,4-[(2-C H N) HC_N] C H
[
20] and 1,4-[(2-C N) HC_N] -2,3,5,6-Me [21]. These structural
5
H
4
2
4 6
C
features are similar to those of the reported ligand except the two
planar molecules per asymmetric unit observed in this work [15].
Fig. 1. X-ray crystal structure of ligand L1.

W.M. Motswainyana, M.O. Onani / Inorganic Chemistry Communications 24 (2012) 221–224
223
Table 1
Table 2
Crystallographic data and refinement for ligand L1.
Heck coupling reaction of iodobenzene catalysed by complexes 1 and 2.
Crystallographic data
L1
Entry Catalyst Alkene
Time Amount of
h) catalyst
mol%]
Yield TONb TOFb Selectivity
(
(%)
84
(%)
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
26 18 4
C H N
386.44
173
monoclinic
C2/c
[
1
1
2
Methyl
1
0.1
940 940 90
acrylate
2
3
8
1
0.1
0.1
96
85
990 124 92
950 950 92
Methyl
Unit cell dimensions
acrylate
a (Å)
b (Å)
c (Å)
β (°)
41.356(8)
3.8939(8)
24.169(5)
100.967(4)
3820.9(13)
8
1.344
1616
0.08
0.42×0.03×0.03
3505
4
5
6
8
8
8
0.1
0.1
0.3
98
98
98
1000 125 94
1000 125 94
990 124 93
a
2
Methyl
acrylate
3
V (Å )
7
8
9
2
2
2
1
8
8
8
1
0.08
0.05
0.01
0.1
82
74
60
76
900 113 90
830 104 90
800 100 82
860 860 85
Z
D
cal (Mgm−3)
F(000
)
10
Butyl
acrylate
Absorption coefficient (mm−1)
3
Crystal size (mm )
Number of data collected
11
12
8
1
0.1
0.1
84
77
940 118 88
890 890 86
2
Butyl
Number of parameters refined
272
0.050
0.129
acrylate
2
2
R[(F >2σ(F )]
13
8
0.1
87
960 120 88
2
wR(F )
Reaction conditions: PhI >ml)/>mmol); Meac (11 mmol); Buac (12 mmol); Et
(10 mmol); Temp: 80 °C; Solvent: DMF (10 ml);
3
N
Theta range for data collection
1.8 to 25.5°
0.94
2
Goodness-of-fit on F
a
0.21 and −0.23 e.Å−
3
Mercury drop test,
Largest diff. peak and hole
b
Determined by GC; Internal standard: mesitylene; TON: mol of substrate converted/mol
of catalyst; TOF: TON/h.
3
and butyl acrylate using DMF as solvent, Et N as base and a tempera-
ture of 80 °C (Scheme 2). The optimized molar ratio Pd:PhI:Alkene:
Et N=1:1000:1100:1000 was maintained in all the catalytic experi-
the major coupling product, with selectivity ranging from 85 to 88%.
The initial rate was also observed to be significantly higher (over 75%
yield) after only 1 h (Table 2, entries 10 and 12). It is worth mentioning
here that our proposed work was aimed at synthesizing new complexes
whose ligand backbone would stabilize intermediate oxidation state of
active species Pd (0), while promoting the conversion of aryl halides
into Heck products [24]. This objective is confirmed by the high activities
observed in the initial stages of the reactions.
The effect of catalyst concentration on the performance of the cata-
lysts was investigated using catalyst 2 (Table 2, entries 6–9). The cata-
lytic results showed that smaller amounts of the catalyst were capable
to effectively catalyse the arylation of methyl acrylate at 80 °C [25]. At
a low catalyst loading of 0.01%, catalyst 2 exhibited significant activity,
giving 60% yield. When the catalyst concentration was increased to
0.3%, the catalyst did not show any difference in activity from catalyst
loading of 0.1% (compare Table 2, entries 4 and 6). These observations
are consistent with research findings by de Vries et al. on the appropri-
ate palladium concentration needed to effectively convert substrates
into Heck products [26]. Mercury drop experiments (Table 2, entry 5)
showed no significant difference in the conversions between the exper-
iments with or with no mercury suggesting that no heterogeneous Pd
(0) was involved in the catalytic process.
3
ments. A summary of the catalytic results is given in Table 2. The op-
timization experiments showed that the catalysts were active, with
the reactions almost getting to completion in the first 8 h, without
formation of palladium black. There was no attempt to recover the
catalysts at the end of the reactions despite them showing high de-
gree of insolubility. The accepted mechanism for palladium catalysed
Heck coupling reactions is where palladium is first reduced from Pd
(
II) to the active catalytic species Pd (0), after which the ligand stabi-
lizes the Pd (0) species [24]. Complexes 1 and 2 successfully catalysed
arylation of methyl acrylate, giving yields of over 95% after 8 h. The
major coupling product was observed to be trans- methyl cinnamate
1
as confirmed by H NMR spectroscopy. A typical singlet chemical
shifts of the products were observed at δ 7.83 (HC_C) and δ 3.63
(
3
CH ) respectively. There was good selectivity for the formation of
trans- methyl cinnamate in the range 90–94%. The catalysts also
showed a faster initial rate with over 80% yields obtained after 1 h
(
Table 2, entry 1 and 3). The high activities observed, without any de-
composition suggest that the phenyl bridged ligand acts as an excel-
lent support for the active species and also prevents precipitation of
the metal from solution [22].
The complexes also successfully catalysed the Heck coupling reac-
tions of iodobenzene with butyl acrylate, giving yields of over 85%. On
analysis, again the Heck product showed the trans-butyl cinnamate as
In conclusion, the new phenylene bridged binuclear bis (imino-
quinolyl) palladium (II) complexes 1 and 2 have been successfully syn-
thesized and characterized [27–30]. The complexes were evaluated for
Scheme 2. a: Heck coupling reaction of iodobenzene with methyl acrylate. b: Heck coupling reaction of iodobenzene with butyl acrylate.

224
W.M. Motswainyana, M.O. Onani / Inorganic Chemistry Communications 24 (2012) 221–224
[
[
19] Y. Song, Z. Xu, Q. Sun, B. Su, Q. Gao, H. Liu, J. Zhao, J. Coord. Chem. (2007) 2351.
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the first time as catalysts for Heck reaction of iodobenzene with methyl
and butyl acrylate under mild reaction conditions and low catalyst
loading. The preliminary results showed that the complexes were high-
ly active, with good selectivity towards formation of the trans- isomers.
2
[
[
3
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Acknowledgements
[24] N.J. Whitcombe, K.K Hi, S.E, Tetrahedron 57 (2001) 7449–7476.
[
[
25] Y. Cui, L. Zhang, J. Mol. Catal. A Chem. 237 (2005) 120–125.
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We are grateful for the financial support from University of the
Western Cape Senate Research and National Research Foundation
[27] Preparation of compound L1: 1,4-phenylenediamine (0.0751 g, 0.694 mmol) and
2-quinolinecarboxaldehyde (0.2182 g, 1.388 mmol) were added to a Schlenk tube
charged with 20 ml dry MeOH and anhydrous magnesium sulphate (0.3 g). The
reaction was allowed to proceed at room temperature for 24 h. A light brown mixture
was obtained. The mixture was filtered under vacuum and the solvent evaporated
under reduced pressure to obtain an orange solid, which formed light orange crystals
suitable for X-ray crystallography after recrystallization from a mixture of CH2Cl2 and
C6H14. Yield: 0.2145 g (80%), mp: 162 °C; IR (nujol cm-1); 1621 (C=N), 1597, 1559,
(
NRF, THUTHUKA).
Appendix A. Supplementary material
Crystallographic data for the structural analysis have been deposited
with the Cambridge Crystallographic Data Centre, CCDC No. 856305 for
compound L1. Copies of this information may be obtained free of charge
from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK
1
504; 1H NMR (200 MHz, CDCl3): δ 8.85 (dd, 1H, J=7.8, quinolyl); 8.35 (d, 1H,
J=8.0, quinolyl); 8.07 (t, 1H, J=8.2, quinolyl); 7.88 (t, 1H, J=7.2, quinolyl); 7.48
s, 1H, CH=N); 8.21 (t, 1H, J=7.4, quinolyl); 7.63 (t, 1H, J=7.8,
(
[
28] Preparation of compound 1: PdCl2(cod) (0.0921 g, 0.323 mmol) was added to a
Schlenk tube charged with CH2Cl2/Et2O (1:2) (15 ml) mixture. The ligand L1
(0.0488 g, 0.1566 mmol) was dissolved separately in CH2Cl2 (4 ml) and the resultant
orange solution was added dropwise to the Schlenk tube. The reaction was stirred at
room temperature for 6 h, resulting in a light brown precipitate. The precipitate was
filtered and dried under reduced pressure to obtain light brown solid. Yield: 0.0316 g,
(
fax: +44-1223-336033; e-mail: deposit@ccdc.cam.ac.uk or www:
http://www.ccdc.cam.ac.uk). Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/10.
1
016/j.inoche.2012.07.018.
13
(75%); mp: 174 °C; IR (nujol cm-1); 1599 (C=N), 1563, 1502; C NMR (125.65 MHz,
References
CDCl3): δ 167.49, 160.89, 152.22; Anal. Calcd for C26H18Cl4N4Pd2: C, 42.14; H, 2.45;
N, 7.56; Found: C, 42.20; H, 2.73; N, 7.70
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[
1
3
431–455.
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1
confirmed by H NMR spectroscopy. Chemical shifts of the products:
[
. trans methyl cinnamate: 1H NMR (200 MHz, CDCl
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1
(
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[
[
[
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.34(m, 3H, aromatic)
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);
).
(d, 1H, J=16.2, C_CH); 7.55–7.50 (m, 2H, aromatic); 4.09 (t, 2H, J=3.8, CH
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[
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