Synthesis, characterization and catalytic activity of three palladium(II) complexes containing Schiff base ligands

Article abstract of DOI:10.1007/s11243-012-9635-y

Three new Pd(II) complexes of Schiff base ligands, namely, [Pd4(L ¹)4] (1), [Pd²(L²)₂Cl₂] (2) and [Pd(L³)₂ Cl2] (3) [HL1 = N-(benzylidene)-2- aminophenol; L² = N-(2,4-dichlorobenzylidene)-2,6-diethylbenzenamine, L³ = 4-(2,4-dichlorobenzylide-neamino)phenol] have been synthesized using solvothermal methods and characterized by elemental analysis, spectroscopy and single crystal X-ray diffraction. The crystal structures of the free ligands were also determined. The l-oxygen-bridged tetranuclear cyclometallated Pd(II) complex (1) contains four nearly planar units, in which PdII is four-coordinate. Complex 2 is a l-chloro-bridged dinuclear cyclometallated Pd(II) complex, whereas complex 3 is mononuclear. The Heck reactions of bromobenzene with acrylic acid catalyzed by complexes 1-3 have also been studied. Springer Science+Business Media B.V. 2012.

Full text of DOI:10.1007/s11243-012-9635-y

Transition Met Chem (2012) 37:661–669
DOI 10.1007/s11243-012-9635-y
Synthesis, characterization and catalytic activity of three
palladium(II) complexes containing Schiff base ligands
•
Hai-Fu Guo Yong Pan De-Yun Ma
•
•
•
Kuan Lu Liang Qin
Received: 26 May 2012 / Accepted: 16 July 2012 / Published online: 3 August 2012
Ó Springer Science+Business Media B.V. 2012
Abstract Three new Pd(II) complexes of Schiff base
ligands, namely, [Pd4(L1)4] (1), [Pd2(L2)2Cl2] (2) and [Pd(L3)2
Cl2] (3) [HL1 = N-(benzylidene)-2-aminophenol; L2 =
N-(2,4-dichlorobenzylidene)-2,6-diethylbenzenamine, L3 =
4-(2,4-dichlorobenzylide-neamino)phenol] have been synthe-
sized using solvothermal methods and characterized by ele-
mental analysis, spectroscopy and single crystal X-ray
diffraction. The crystal structures of the free ligands were also
determined. The l-oxygen-bridged tetranuclear cyclometal-
lated Pd(II) complex (1) contains four nearly planar units, in
which PdII is four-coordinate. Complex 2 is a l-chloro-bridged
dinuclear cyclometallated Pd(II) complex, whereas complex 3
is mononuclear. The Heck reactions of bromobenzene with
acrylic acid catalyzed by complexes 1–3 have also been
studied.
chemistry for the formation of carbon–carbon bonds. Com-
pared to homogeneous catalysis for such coupling reactions,
heterogeneouscatalysis involvingsupportedmetal complexes
offersseveral advantages, suchas robustnessand increasedair
and moisture stability [4–7]. Various supports, including sil-
ica, sepiolites, zeolites and polymers have been used to het-
erogenize the homogeneous catalysts [8–11]. Palladium(II)
complexes derived from Schiff base ligands have been
extensively studied over the past few decades in light of their
applications as heterogeneous catalysts in organic reactions
[12]. Most Pd(II) complexes have been prepared by the
reaction of Pd(OAc)2 or Na2PdCl4 with Schiff base ligands.
Examples of Pd(II)complexes obtained bythe combination of
PdCl2 with Schiff base ligands are much rarer. Here, we report
the synthesis and structures of three new Pd(II) complexes
obtained by the reaction of PdCl2 and Schiff base ligands
(HL1, L2 and L3) under solvothermal conditions. The Pd(II)
atoms can be anchored by the three new Schiff base ligands to
generate heterogeneous catalysts. These Pd(II) Schiff base
complexes show moderate catalytic activity in the Heck
coupling of bromobenzene with acrylic acid.
Introduction
The palladium-catalyzed formation of biaryls from aryl
halides with arylboronic acids (the Suzuki reaction) [1, 2] and
the arylation and alkenylation of olefins (the Heck reaction)
[3] have become mainstays of modern synthetic organic
Experimental
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-012-9635-y) contains supplementary
material, which is available to authorized users.
All chemicals were commercially available and used as
received without further purification. Elemental analyses for
C, H and N were carried out using a Vario EL III Elemental
H.-F. Guo (&) ꢀ Y. Pan ꢀ D.-Y. Ma ꢀ K. Lu ꢀ L. Qin
School of Chemistry and Chemical Engineering, Zhaoqing
University, Zhaoqing 526061, People’s Republic of China
e-mail: guohaifu@zqu.edu.cn
Analyzer. Infrared spectra were recorded (4000–400 cm-1
)
as KBr disks on a Bruker 1600 FTIR spectrometer.
Preparation of HL1
Y. Pan
College of Chemical Engineering, Inner Mongolia University
of Technology, Inner Mongolia 010051, People’s Republic
of China
HL1 was prepared by the condensation of benzaldehyde
(3.18 g, 30 mmol) with 2-aminophenol (3.27 g, 30 mmol)
123

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Transition Met Chem (2012) 37:661–669
Table 1 Crystallographic data for the free ligands
Compound
HL1
L2
L3
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
C13H11NO
197.23
C17H17Cl2N
306.22
C13H9Cl2NO
266.11
296(2)
296(2)
296(2)
Orthorhombic
Pca21
Triclinic
P-1
Orthorhombic
Pccn
˚
a (A)
12.304(2)
6.0943(11)
13.967(2)
90.00
8.0047(8)
10.0032(11)
10.8476(11)
98.810(2)
103.431(2)
101.424(2)
809.83(15)
2
13.6884(2)
14.399(2)
12.4291(2)
90.00
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
90.00
90.00
90.00
90.00
3
˚
V (A )
1,047.3(3)
4
2,449.7(6)
8
Z
D (Mg m3)
1.251
1.256
1.443
Limiting indices
-14 B h B 11, -7 B k B 7,
-16 B l B 16
-9 B h B 8, -11 B k B 9,
-12 B l B 12
-16 B h B 9, -11 B k B 17,
-14 B l B 14
Reflections collected/unique
4985/1801
0.0210
4226/2883
0.0138
7590/2190
0.0314
Rint
F(000)
416
320
1088
h (°)
2.92–25.17
1.042
1.97–25.20
1.029
2.63–25.20
1.077
Goodness-of-fit on F2
R(I [ 2r)
R1 = 0.0342
wR2 = 0.0861
R1 = 0.0403
wR2 = 0.0912
0.16, -0.12
0.3(18)
R1 = 0.0527
wR2 = 0.1419
R1 = 0.0840
wR2 = 0.1672
0.41, -0.37
R1 = 0.0460
wR2 = 0.1249
R1 = 0.0709
wR2 = 0.1480
0.34, -0.38
R (all data)
-3
˚
Largest diff. peak and hole (A
)
Flack
P
P
R = (kFo| – |Fck)/ |Fo|
P
P
wR = [ w(F2o - Fc2)2/ w(Fo)2]1/2
in ethanol (20 mL) as the reaction medium. The solution
was refluxed for 3–4 h and then allowed to cool to room
temperature. The yellow precipitate was recrystallized
from ethanol to give HL1 as colorless crystals. Yield
0.4259 g (72 %). Anal. For C13H11NO (%): Calcd. C 79.1,
H 5.6, N 7.1; Found C 79.2, H 5.7, N 7.0. FTIR (KBr,
cm-1): 3448(s), 2366(m), 1622(s), 1577(w), 1476(s),
1380(s), 1288(w), 1246(s), 1171(w), 1147(w), 1067(w),
1026(m), 967(m), 936(w), 873(w), 850(m), 788(m), 761(s),
736(w), 684(s), 640(m), 564(w), 498(m), 426(w). MP:
91–94 °C.
0.689 g (75 %). Anal. For C17H17Cl2N (%): Calcd. C 66.6,
H 5.6, N 4.6; Found C 66.4, H 5.8, N 7.5. FTIR (KBr,
cm-1): 3430(m), 3068(w), 2972(m), 2927(w), 2359(m),
1639(s), 1580(s), 1558(w), 1463(s), 1384(s), 1270(w),
1249(m), 1180(s), 1136(m), 1098(s), 1049(s), 984(w),
963(w), 867(vs), 836(m), 798(m), 767(w), 736(s), 677(w),
565(w), 454(m), 423(m). MP: 72–75 °C.
Preparation of L3
L3 was prepared by the same procedure as for HL1 except
that benzaldehyde and 2-aminophenol were replaced by 2,
4-dichlorobenzaldehyde and 4-aminophenol. Yield 0.689 g
(75 %). Anal. For C13H9Cl2NO (%): Calcd. C 58.6, H 3.4,
N 5.3; Found C 58.5, H 3.3, N 5.4. FTIR (KBr, cm-1):
3427(m), 3082(m), 2810(m), 2676(m), 2600(m), 2465(m),
2362(m), 1622(s), 1584(s), 1508(s), 1470(s), 1450(s),
Preparation of L2
L3 was prepared by the same procedure as for HL1 except
that benzaldehyde and 2-aminophenol were replaced by
2,4-dichlorobenzaldehyde and 2,6-diethylaniline. Yield
123

Transition Met Chem (2012) 37:661–669
663
Table 2 Crystallographic data for complexes 1–3
Compound
1
2
3
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
C52H36 N4O4Pd4
1,206.45
296(2)
C34H32Cl6N2Pd2
894.12
C26H18Cl6N2O2 Pd
709.52
147(2)
296(2)
Monoclinic
P21/c
Triclinic
P-1
Monoclinic
P21/c
˚
a (A)
12.6285(8)
11.7305(8)
29.4443(19)
90.00
8.5323(3)
10.6883(4)
10.9243(4)
62.8600(4)
76.7300(10)
76.2680(10)
852.69(5)
1
8.4560(4)
21.3107(11)
7.5767(4)
90.00
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
99.1270(10)
90.00
107.1280(10)
90.00
3
˚
V (A )
4,306.6(5)
4
1,304.79(11)
2
Z
D (Mg.m3)
1.861
1.553
1.806
Limiting indices
-15 B h B 14, -14 B k B 14,
-35 B l B 35
-8 B h B 10, -12 B k B 12,
-13 B l B 11
-10 B h B 10, -17 B k B 25,
-8 B l B 9
Reflections collected/unique
23304/7689
0.0516
4445/3049
0.0095
6688/2344
0.0286
Rint
F(000)
2368
444
704
h (°)
1.87–25.20
1.032
2.12–25.20
1.031
1.91–25.20
1.077
Goodness-of-fit on F2
R(I [ 2r)
R1 = 0.0357
wR2 = 0.0868
R1 = 0.0698
wR2 = 0.1073
0.68, -0.79
R1 = 0.0182
wR2 = 0.0460
R1 = 0.0199
wR2 = 0.0470
0.34, -0.37
R1 = 0.0251
wR2 = 0.0548
R1 = 0.0324
wR2 = 0.0577
0.37, -0.35
R (all data)
-3
˚
Largest diff. peak and hole (A
)
P
P
R = (kFo| - |Fck)/ |Fo|
P
P
wR = [ w(F2o - Fc2)2/ w(Fo)2]1/2
Preparation of [Pd2(L2)2Cl2] (2)
1370(s), 1274(s), 1239(s), 1174(s), 1104(s), 1047(s), 981(w),
863(m), 843(s), 743(s), 714(w), 664(w), 570(s), 536(m),
456(m), 423(m). MP: 161–164 °C.
Complex 2 was prepared by the same procedure as 1 except
that HL1 was replaced with L2. Yield 0.1 g (45 %). Anal.
For C34H32Cl6N2Pd2 (%): Calcd. C 45.6, H 3.6, N 3.1;
Found C 45.5, H 3.7, N 3.2. FTIR (KBr, cm-1): 3450(m),
3232(s), 2970(s), 2941(m), 1647(w), 1599(s), 1577(s),
1545(m), 1458(s), 1375(m), 1344(w), 1292(w), 1228(w),
1172(m), 1095(s), 1062(m), 916(w), 858(w), 815(w),
763(s), 721(w), 576(m), 521(w), 459(w).
Preparation of [Pd4(L1)4] (1)
Palladium chloride (0.0887 g, 0.5 mmol) was dissolved in
methanol (5 mL). HL1 (0.236 g, 1 mmol) was added and
the mixture stirred at room temperature for 5 h under an
anhydrous atmosphere. The resulting mixture was filtered
under reduced pressure. The collected solid was washed
with ethyl ether and dried in air to give red crystals that
were purified by recrystallization from methylene chloride
(11 mL) and hexane (8 mL). Yield 0.0754 g (50 %). Anal.
For C52H36N4O4Pd4 (%): Calcd. C 51.7, H 3.0, N 4.6;
Found C 51.5, H 3.1, N 4.7. FTIR (KBr, cm-1): 3442(s),
1633(w), 1600(s), 1573(m), 1490(m), 1473(s), 1398(m),
1302(w), 1269(s), 1203(w), 1114(m), 1043(w), 954(w),
868(w), 750(s), 619(w), 576(m) 526(w), 460(w).
Preparation of [Pd(L3)2Cl2] (3)
Complex 3 was prepared by the same procedure as 1 except
that HL1 was replaced with L3. Yield 0.1 g (45 %). Anal.
For C26H18Cl6N2O2Pd (%): Calcd. C 44.0, H 2.5, N 3.9;
Found C 44.2, H 2.4, N 3.8. FTIR (KBr, cm-1): 3461(m),
3292(s), 1623(s), 1581(s), 1545(m), 1510(s), 1458(m),
1398(w), 1355(w), 1284(s), 1220(m), 1174(m), 1101(m),
123

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Transition Met Chem (2012) 37:661–669
Table 3 Selected bond length
and angles of complexes 1–3
1
Pd1–N1
1.968(5)
1.959(6)
1.969(6)
1.967(5)
1.967(6)
2.148(4)
1.969(7)
2.042(4)
82.3(2)
Pd1–O1
2.167(4)
2.064(4)
2.149(4)
2.051(4)
1.956(5)
2.054(4)
1.957(5)
2.136(5)
179.23(2)
82.7(2)
Pd1–C3
Pd1–O3
Pd2–C16
Pd2–N2
Pd2–O2
Pd2–O1
Pd3–C29
Pd3–O3
Pd3–N3
Pd3–O4
Pd4–C42
Pd4–O2
Pd4–N4
Pd4–O4
C3–Pd1–N1
O3–Pd1–O1
N2–Pd2–O1
N3–Pd3–C29
O4–Pd3–O3
N4–Pd4–O2
2
N1–Pd1–O3
N2–Pd2–C16
O1–Pd2–O2
N3–Pd3–O4
N4–Pd4–C42
O2–Pd4–O4
99.66(2)
97.3(2)
98.80(2)
176.0(2)
82.5(3)
82.0(2)
98.36(2)
179.2(2)
98.03(2)
Pd1–C13
Pd1–Cl5
1.978(2)
2.4404(5)
81.429(8)
176.79(5)
97.20(5)
Pd1–Cl5i
2.3291(5)
2.0319(2)
95.41(6)
Pd1–N1
C13–Pd1–N1
N1–Pd1–Cl5i
N1–Pd1–Cl5
3
C13–Pd1–Cl5i
C13–Pd1–Cl5
Cl5i–Pd1–Cl5
177.74(6)
85.987(2)
Pd1–N1
N1–Pd1–N1i
N1–Pd1–Cl3i
2.033(2)
180.0
Pd1–Cl3
2.3101(6)
88.51(6)
180.0
Symmetry codes: 2, i: 1 - x,
2 - y, -z. 3, i: 1 - x, 1 - y,
1 - z
N1–Pd1–Cl3
Cl3i–Pd1–Cl3
91.49(6)
Table 4 Hydrogen bond
geometries for HL1, L3, 2, 3
(A, °)
D–HꢀꢀꢀA
HL1
d(D–H)
d(HꢀꢀꢀA)
d(DꢀꢀꢀA)
\D–HꢀꢀꢀA
˚
O1–H1ꢀꢀꢀN1
0.82
2.13
2.614(8)
118
L3
O1–H1ꢀꢀꢀN1i
C2–H2ꢀꢀꢀO1ii
C7–H7ꢀꢀꢀCl1
C13–H13ꢀꢀꢀO1ii
2
0.82
0.93
0.93
0.93
1.98
2.57
2.66
2.58
2.801(7)
2.320(3)
3.057(4)
3.356(7)
173
138
106
142
C7–H7BꢀꢀꢀN1
C9–H9AꢀꢀꢀN1
C14–H14ꢀꢀꢀCl5i
3
0.99
0.99
0.95
2.53
2.51
2.76
2.882(7)
2.884(4)
3.309(2)
101
102
118
O1–H1ꢀꢀꢀCl3ii
C6–H6ꢀꢀꢀCl3
C10–H10ꢀꢀꢀO1iii
C13–H13ꢀꢀꢀCl3
0.84
0.95
0.95
0.95
2.24
2.79
2.53
2.76
3.076(3)
3.498(1)
3.432(7)
3.642(5)
174
132
159
155
Symmetry codes: L3, (i) 1.5
minus; x, y, -0.5 ? z; (ii) 1.5
- x, y, 0.5 ? z. 2, (i) 1 - x,
2 - y, -z. 3, (ii) -1 ? x, y, -1
? z; (iii) -x, 0.5 ? y, 0.5 - z
123

Transition Met Chem (2012) 37:661–669
665
Scheme 1 Synthetic route for the
free ligands. R1 = OH,
R2 = R3 = R4 = R5 = H, HL1;
R1 = R2 = C2H5, R3 = H,
R4 = R5 = Cl, L2; R1 = R2 = H,
R3 = OH, R4 = R5 = Cl, L3
1053(m), 973(w), 919(w), 883(m), 841(s), 744(m), 624(m),
580(w), 503(w), 462(w), 424(w).
Catalytic reactions
A mixture of bromobenzene (1.0 mmol), acrylic acid
(1.3 mmol), triethylamine (2.0 mmol), DMF (6 mL) and
0.5 mol% of catalyst was stirred at 80 °C under air. Fol-
lowing the reaction, the catalyst was separated by filtration.
The filtrate was dried over Na2SO4 and filtered. The
products were quantified by GC–MS analysis (Shimadzu
GCMS-QP5050A equipped with a 0.25 mm 9 30 m DB-
WAX capillary column). The typical GC–MS analysis
program was as follows: initial column temperature 100 °C,
hold 2 min, ramp temperature to 280 °C at 15 °C/min, and
hold for 5 min.
Fig. 1 The molecular structure of compound HL1 with numbering
scheme
X-ray crystallographic determination
Single crystal X-ray diffraction analyses of the free ligands
and their complexes were performed on a Bruker Apex II
CCD diffractometer operating at 50 kV and 30 mA using
Fig. 2 The molecular structure of compound L2 with numbering
scheme
˚
MoKa radiation (k = 0.71073 A). Data collection and
reduction were performed using the APEX II software [13].
Multi-scan absorption corrections were applied to all the
data sets using SADABS, as included in the APEX II pro-
gram [13]. All structures were solved by direct methods and
refined by least squares on F2 using the SHELXTL program
package [14]. All non-hydrogen atoms were refined with
anisotropic displacement parameters. Hydrogen atoms
attached to carbon and oxygen were placed in geometrically
idealized positions and refined using a riding model. Crys-
tallographic data are listed in Tables 1 and 2. Selected bond
lengths and angles, and H-bonding parameters for the
compounds are given in Tables 3 and 4, respectively.
Fig. 3 The molecular structure of compound L3 with numbering
scheme
123

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Transition Met Chem (2012) 37:661–669
Fig. 4 a A perspective view of 1 with numbering scheme; b the packing of 1 formed by C–Hꢀꢀꢀp stacking interactions with turquoise dashes
Results and discussion
centroid of the C8–C13 ring. Symmetry code: (iii) x,
0.5 - y, -0.5 ? z]. All the three of these Schiff bases
adopt the E configuration with respect to the C=N bond,
and all the bond lengths are within normal ranges [17].
Structures of the free ligands
The Schiff bases were prepared by condensation of benz-
aldehyde or 2,4-dichlorobenzaldehyde with aromatic
amines in absolute ethanol (Scheme 1). The reaction pro-
gress was monitored by TLC; the reactions were termi-
nated once the TLC showed no further reactants present.
Compound HL1 crystallizes in orthorhombic space
group Pca21. As depicted in Fig. 1, the molecular structure
is almost planar, with a dihedral angle between the ben-
zylidene (C1–C7) and 2-(amino)phenol rings (C8–C13/N1/O1)
of 1.11(2)°. In the crystal of 1, an infinite chain is formed
through C–Hꢀꢀꢀp stacking interactions. The H-to-centroid
Structures of the complexes
To prepare complexes 1–3, the Schiff bases were treated
with PdCl2 in equimolar amounts at room temperature in
methanol. The reactions proceeded without reflux under
mild conditions, thus preventing oxidation of Pd(II) to
Pd(IV) [18].
The asymmetric unit of complex 1 consists of one formula
unit, in which all palladium atoms are crystallographically
inequivalent. As shown in Fig. 4a, the core of complex 1
consists of an eight-membered ring of alternating Pd and O
atoms. This structure is similar to that of [Pd{C6H4–CH=N–
(C6H4–2–O)}]4ꢀ2CHCl3, but the two complexes are not iso-
morphous [19]. The coordination environment about each
palladium atom can be described as square planar, having
two oxygen, one imine nitrogen and one aryl carbon donor
atoms. Consequently, ligand HL1 acts as a C, N, O-chelator.
The Pd–C, Pd–N and Pd–O bond distances and angles range
i
˚
distance H6ꢀꢀꢀCg1 is 2.90(2) A [Cg1 is the centroid of the
C8–C13 ring. The angle C6–H6ꢀꢀꢀCg1 is 125°. Symmetry
Code: (i) 0.5 ? x, 1 - y, z]. Intramolecular O–HꢀꢀꢀN
hydrogen bonds are also observed (Table 4). The melting
point of HL1 is in good agreement with literature values
[15, 16].
In L2 (Fig. 2), the dihedral angle between the mean
planes of the benzylidene (C11–C17) and benzenamine
rings is 84.37(2)°. In the crystal of L2, there are no direc-
tional interactions but only van der Waals interactions
between neighbouring molecules.
˚
from 1.956(5) to 2.167(4) A and 81.25(18)° to 179.23(18)°,
respectively, all of which are within normal ranges [19]. The
Pd–Ochelating distances trans to carbon are longer than the Pd–
Obridging distances trans to nitrogen [average Pd–Ochelat-
The dihedral angle between the benzene rings of com-
pound L3 is 24.24(2)° (Fig. 3). In the crystal of L3,
hydroxyl-H (H1) and phenyl-H (C2 and C13) are involved
in intermolecular O–HꢀꢀꢀN and C–HꢀꢀꢀO hydrogen bonds
(Table 4), linking the molecules into an infinite chain
parallel to the b axis. This is further interconnected into a
doubles-chain by C–Hꢀꢀꢀp stacking interactions. The H-to-
˚
= 2.150(2) A and average Pd–Obridging = 2.053(3) A],
˚
ing
which is attributed to the trans influence of the coordinated
carbon of the cyclometalated ligand. The Pd1–Pd4 and Pd2–
˚
Pd3 distances are 3.349(4) and 3.376(3) A, respectively,
suggesting the absence of any Pd–Pd bonding. These are
close to the corresponding distances [3.340(2) and 3.390
centroid distance H6ꢀꢀꢀCg2iii is 2.93(2) A [Cg2 is the
(4) A] found in [Pd{C6H4CH=N(C6H4–2–O)}]4ꢀ2CHCl3
˚
˚
123

Transition Met Chem (2012) 37:661–669
667
Fig. 5 a The molecular
structure of compound 2 with
numbering scheme; b view of
an infinite chain of 2 formed by
C17–Cl1ꢀꢀꢀp stacking
interactions with turquoise
dashes. Symmetry codes:
(i) 1 - x, 2 - y, -z
[19]. The packing shows C–Hꢀꢀꢀp stacking interactions. The
i
˚
H-to-centroid distances of H33ꢀꢀꢀCg1 = 2.78(2) A, H19ꢀꢀꢀ
i
ii
iii
˚
˚
Cg2 = 2.93(4) A, H12ꢀꢀꢀCg1 = 2.87(3) A, H18ꢀꢀꢀCg1
=
˚
2.84(5) A [where Cg1, Cg2, Cg3, Cg4 and Cg5 are the
centroids of the C15–C20, C21–C26, C34–C39, C47–C52
and C28–C33 rings, respectively. Symmetry codes: (i) -x,
-0.5 ? y, 0.5 - z; (ii) x, 1 ? y, z; (iii) -1 ? x, y, z]. A weak
pꢀꢀꢀp stacking interaction between the parallel phenyl rings
within ligand HL1 is also observed [the centroid–centroid
˚
distance is 3.733(3) A] (Fig. 4b).
The molecular structure of complex 2 is shown in Fig. 5a.
Selected bond distances and angles are given in Table 3. In
this complex, each Schiff base ligand (L2) is bonded to the
di-l-chloro-bridged unit through nitrogen atoms and an
aromatic carbon atom, providing two equivalent five-mem-
bered N–C–Pd–C–chelate rings. The geometry at the Pd(II)
center in 2 is square planar, with the two cyclometallated
ligands in a trans arrangement with respect to the PdꢀꢀꢀPd
Fig. 6 The molecular structure of compound 3 with numbering
scheme. Symmetry codes: (i) 1 - x, 1 - y, 1 - z
by the phenyl carbon and nitrogen atoms. In the five-
membered chelate rings, the five atoms (Pd1, N1, C11, C12
and C13) are essentially planar. The bite angle [C1–Pd1–
N1 = 81.41(7)°] is in good agreement with those found in a
structurally related l-Cl dimer [21]. In the crystal of com-
plex 2, an infinite chain is formed through C17–Cl1ꢀꢀꢀp
stacking interactions (Fig. 5b). The Cl1-to-centroid distance
˚
axis. The Pd1–C13 bond [1.979(2) A] is shorter than the
˚
expected value of 2.08 A based on the sum of the covalent
radii of carbon and palladium, but consistent with those
found for related complexes where partial multiple-bond
character of the Pd–C was assumed [20, 21]. The Pd1–N1
ii
˚
for Cl1ꢀꢀꢀCg1 is 3.274(7) A [where Cg1 is the centroid of
˚
bond distance [2.032(4) A] is in agreement with the sum of
the C1–C6 ring, symmetry code: (ii) -x, 1 - y, 1 – z].
Intramolecular C–HꢀꢀꢀN and C–HꢀꢀꢀCl hydrogen bonds are
also observed.
covalent radii for nitrogen and palladium [22], and similar to
values reported earlier [20, 21]. The lengths of the Pd–Cl
˚
bonds trans to C [2.440(2) A] and the Pd–Cl bonds trans to
˚
N [2.329(3) A], reflect the different trans influences exerted
The structure of complex 3 is shown in Fig. 6. The
molecular structure is trans-[Pd(L3)2Cl2], and the palladium
123

668
Transition Met Chem (2012) 37:661–669
Fig. 7 a An infinite chain of 3 formed by O–HꢀꢀꢀCl hydrogen bonding interactions; b view of a 2D sheet of 3 with cavities formed by C–HꢀꢀꢀO
hydrogen bonding interactions; c view of the 3D supramolecular network of 3 formed through C–Hꢀꢀꢀp stacking interactions
atom lies on a crystallographic inversion center such that the
angles N–Pd–N and Cl–Pd–Cl are both 180°. The planes of
the two phenyl rings are inclined by 71.07(3) and 79.53(2)°
to the PdN2Cl2 coordination plane, respectively. The phenyl
rings of the Schiff base ligand (L3) in 3 are not coplanar, but
subtend a dihedral angle of 15.24(3)°, which is similar to
that in the free ligand [24.24(2)°]. The Pd–N/Cl distances
intermolecular C–HꢀꢀꢀO hydrogen bonding interactions, as
shown in Fig. 7b. The 2D sheets are connected by C–Hꢀꢀꢀp
stacking interactions, resulting in a 3D supramolecular net-
work (Fig. 7c). The H-to-centroid distance of H7ꢀꢀꢀCg1i is
˚
2.69(2) A [Cg1 is the centroid of the C1–C6 ring. Symmetry
Code: (i) -x, 1 - y, 1 - z].
˚
are 2.032(2) and 2.3101(6) A, and the N–Pd–Cl angles are
Heck reaction catalysis
88.52(6) and 91.48(6) °, respectively, all of which are within
the range observed for analogous Pd complexes (Table 3)
[23, 24]. In the crystal structure of complex 3, intermolec-
ular O–HꢀꢀꢀCl hydrogen bonding interactions (Table 4) form
infinite chains involving the hydroxyl groups as donors
and the chlorine atoms as acceptors (Fig. 7a), which are
further self-assembled into a 2D sheet with cavities by
The base and solvent for the Heck reaction greatly influ-
ences catalytic activity. The results for these complexes are
summarized in Table 5. DMF is the best solvent for this
catalytic system. In other organic solvents, e.g., methanol,
acetonitrile or toluene, low yields of coupling products
were obtained. Among four different bases investigated for
Table 5 The effect of base and solvent on the complexes 1–3 catalyzed Heck reaction of bromobenzene with acrylic acid
Entry
Base
Solvent
Temperature (°C)
Atmosphere
Time (h)
Yields (%)
1
2
3
4
5
6
7
8
Et3N
DMF
80
75
Air
Air
Air
Air
Air
Air
Air
Air
12
12
12
12
12
12
12
12
52/48/45
32/27/25
21/19/16
15/11/8
24/22/17
19/16/12
9/6/5
Et3N
Methanol
ACN
Et3N
80
Et3N
Toluene
DMF
100
80
K2CO3
Na2CO3
NaOAc
KF
DMF
80
DMF
80
DMF
80
–/–/–
123

Transition Met Chem (2012) 37:661–669
669
4. Sarkar K, Nandi M, Islam M, Mubarak M, Bhaumik A (2009)
Appl Catal A Gen 352:81
5. Polshettiwar V, Len C, Fihri A (2009) Coord Chem Rev
253:2599
6. Polshettiwar V, Varma RS (2008) Tetrahedron 64:4637
7. Isiam SM, Mondal P, Tuhina K, Roy AS, Mondal S, Hossain D
(2010) J Inorg Organomet Polym 20:264
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these reactions, Et3N was found to be the most effective
(Table 5, entry 1); NaOAc, K2CO3 and Na2CO3 were
substantially less effective. KF failed to promote the
reaction (Table 5, entry 8). Compared to other Pd(II)
complexes containing Schiff bases, the catalytic activities
of the present complexes for the Heck reaction proved to be
less effective [7].
9. Shimizu K, Koizumi S, Hatamachi T, Yoshida H, Komai S,
Kodama T, Kitayama Y (2004) J Catal 228:141
10. Bulut H, Artoka L, Yilmaz S (2003) Tetrahedron Lett 44:289
11. Kim JW, Kim JH, Lee DH, Lee YS (2006) Tetrahedron Lett
47:4745
Conclusion
In summary, the synthesis and crystal structures of three
new Schiff base ligands and their Pd(II) complexes have
been reported in this article. The complexes show moderate
catalytic activity in the Heck coupling reaction of bromo-
benzene with acrylic acid, which is also very sensitive to
the choice of base and solvent.
12. Jain VK, Jain L (2005) Coord Chem Rev 249:3075
13. Bruker (2004) APEXII software, Version 6.3.1. Bruker AXS Inc.,
Madison
14. Sheldrick GM (2008) Acta Cryst A 64:112
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83:3782
16. Tandon JP, Gupta SR, Prasad RN (1976) Monatsh Chem
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Taylor R (1987) J Chem Soc Perkin Trans 2:1
18. Naeimi H, Moradian M (2010) J Coord Chem 63:156
Acknowledgments We are thankful for financial support of this
work provided by the Science and Technology Planning Project of
Zhaoqing City (2009G013) and Zhaoqing University with doctoral
start-up funds. Thanks to Dr. Sean Parkin for all his help with the
suggestions for the paper.
´
´
´
19. Lopez C, Caubet A, Perez S, Solans X, Font-Bardıa M (2003)
J Organomet Chem 681:82
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Nagaoka J 580:273
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Chem 427:409
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