Imidazole containing palladium(II) complexes as efficient pre-catalyst systems for Heck and Suzuki coupling reaction: Synthesis, structural characterization and catalytic properties

Article abstract of DOI:10.1016/j.ica.2012.08.003

A series of new palladium(II) complexes of the general formula trans-[PdX₂L₂] [X = Cl, Br; L = 1-phenylimidazole (PI), 1-(4-methoxyphenyl)-1H-imidazole (MPI), 4(5)-(hydroxymethyl)-imidazole (HMI), 1-(4-cyanophenyl)-imidazole) (CPI)] [X = Cl; L = PI(1), X = Cl; L = MPI(2), X = Cl; L = HMI(3), X = Cl; L = CPI(4), X = Br; L = PI(5), X = Br; L = MPI(6), X = Br; L = HMI(7), X = Br; L = CPI(8)] have been synthesized. Resulting complexes have been characterized by elemental analyses, IR, ¹H and ¹³C NMR, FAB-MS and electronic spectral studies. The molecular structures of 1, and 6 have been determined by single crystal X-ray analyses. The catalytic activities of all the Pd complexes (1-8) in coupling reactions such as the Mizoroki-Heck and Suzuki-Miyaura coupling reaction have also been studied. Among the 1-8 complexes in our investigations, 1, 2, 5, and 6 exhibited highest catalytic activity as compared to 3, 4, 7, and 8 complexes which is due to formation of Pd nanoparticles that might be actually the true active species.

Full text of DOI:10.1016/j.ica.2012.08.003

Inorganica Chimica Acta 394 (2013) 107–116
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Inorganica Chimica Acta
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Imidazole containing palladium(II) complexes as efficient pre-catalyst systems
for Heck and Suzuki coupling reaction: Synthesis, structural characterization
and catalytic properties
a
a
b,
Manoj Trivedi a, , Gurmeet Singh , R. Nagarajan , Nigam P. Rath
⇑
⇑
a Department of Chemistry, University of Delhi, Delhi 110007, India
b Department of Chemistry & Biochemistry and Centre for Nanoscience, University of Missouri-St. Louis, One University Boulevard, St. Louis, MO 63121-4499, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of new palladium(II) complexes of the general formula trans-[PdX2L2] [X = Cl, Br; L = 1-phenyl-
imidazole (PI), 1-(4-methoxyphenyl)-1H-imidazole (MPI), 4(5)-(hydroxymethyl)-imidazole (HMI), 1-(4-
cyanophenyl)-imidazole) (CPI)] [X = Cl; L = PI(1), X = Cl; L = MPI(2), X = Cl; L = HMI(3), X = Cl; L = CPI(4),
X = Br; L = PI(5), X = Br; L = MPI(6), X = Br; L = HMI(7), X = Br; L = CPI(8)] have been synthesized. Resulting
complexes have been characterized by elemental analyses, IR, 1H and 13C NMR, FAB-MS and electronic
spectral studies. The molecular structures of 1, and 6 have been determined by single crystal X-ray anal-
yses. The catalytic activities of all the Pd complexes (1–8) in coupling reactions such as the Mizoroki–
Heck and Suzuki–Miyaura coupling reaction have also been studied. Among the 1–8 complexes in our
investigations, 1, 2, 5, and 6 exhibited highest catalytic activity as compared to 3, 4, 7, and 8 complexes
which is due to formation of Pd nanoparticles that might be actually the true active species.
Ó 2012 Elsevier B.V. All rights reserved.
Received 19 March 2012
Received in revised form 7 August 2012
Accepted 7 August 2012
Available online 17 August 2012
Keywords:
Palladium(II) complex
X-ray structure
Cross-coupling reaction
Palladium nanoparticle
ˇ
1. Introduction
as the P,O-ligands for coupling reactions [34–37]. Cermák et al.
have reported diphosphinoazine-Pd(II) complexes that feature a
novel tridentate ligand for Mizoroki–Heck reactions with high
TON and TOF (hÀ1) [38,39]. Various attempts have been adopted
to improve the Mizoroki–Heck reaction by activating aryl chloride
using Pd compounds such as (CH3CN)2APdCl2APPh4Cl [40],
Pd(OAc)2 with excess P(OEt)3 [41] and heterogeneous Pd/C as cat-
alysts [42,43]. In contrast to phosphine-type ligands, the low tox-
icity and stability of nitrogen-based ligands, such as imidazole
derivatives, have attracted the interest of synthetic organic chem-
ists. Togni and Venanzi [44] exhaustively discussed the relevance
of N-donors in organometallic chemistry. Although, to date, a large
number of N-ligand derivatives have found for practical applica-
tions but the search for new, more effective and/or selective spe-
cies is still in progress. However, there are few reports of highly
catalytic reactive complexes bearing nitrogen ligands, although
there are many reports of nitrogen ligands [45–61]. We are inter-
ested in the possibility of using imidazole derivatives as ligands be-
cause they are structurally simple, readily available, and
inexpensive, and they allow for simplistic introduction of various
substituents into their structure. Herein, we report the synthesis,
spectroscopic characterization of eight new imidazole based
mononuclear palladium(II) complexes (1–8). We also report the
crystal structure of the complexes, dichlorobis(1-phenylimidazole)
palladium(II) (1) and dibromobis(1-(4-methoxyphenyl)-1H-imid-
azole)palladium(II) (6) using single-crystal X-ray diffractometry
(XRD). We also discuss their comparative catalytic activities in
Remarkable progress has been made in the area of palladium
complexes due to their promising properties in applied sciences,
mostly owing to high efficiency as homogenous catalysts in a vari-
ety of coupling reactions in organic synthesis [1–5]. Currently,
well-designed ligands have contributed significantly to improving
catalytic activity. The majority of them are phosphine based li-
gands, because of increasing bulkiness and also introducing elec-
tron-donating functions increase the reactivity of aryl halides,
enabling their use as substrates [6–8]. There are, however, a num-
ber of problems that are frequently encountered in the use of phos-
phine ligands in catalysis. For example, the degradation of PAC
bonds is notorious and can result in deactivation of the catalyst
as well as the scrambling of the coupling partners with the phos-
phine substituents [9,10]. This, in addition to their sensitivity to
moisture and aerial oxidation, their high toxicity, often laborious
synthesis and loss during product extraction, has driven research
into alternatives to phosphines. Furthermore, because of recent
development of palladacyclic catalysts [11–19] and N-heterocyclic
carbene palladium catalysts [20–33] should be mentioned as well.
With regard to other types of ligands in coupling reactions, Dai
et al. have focused on the development of amide-based phosphines
⇑
Corresponding authors. Tel.: +91 0 9811730475 (M. Trivedi), +1 314 516 5333
(N.P. Rath).
E-mail addresses: manojtri@gmail.com (M. Trivedi), rathn@umsl.edu (N.P. Rath).
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.08.003

108
M. Trivedi et al. / Inorganica Chimica Acta 394 (2013) 107–116
the Mizoroki–Heck reaction [62,63] and the Suzuki–Miyaura cou-
pling reaction [64–67].
(8404). FAB-MS m/z 465(464), [M]+; 429(430), [M]+ACl;
393(391), [M]+ACl2.
(b) It was also prepared by the same method as described above
except that [PdCl2(C6H5CN)2] (0.383 g, 1 mmol) was used in place
of [PdCl2(CH3CN)2]. Yield: (0.372 g, 80%).
2. Experimental
2.2.2. trans-[PdCl2(MPI)2] (2)
2.1. Materials and physical measurements
This complex was prepared by the following two methods: (a)
1-(4-methoxyphenyl)-1H-imidazole (0.348 g, 2 mmol) was added
slowly to a solution of [PdCl2(CH3CN)2] (0.259 g, 1 mmol) in
30 mL CH3OH:CH2Cl2 (50:50 V/V) mixture at room temperature.
The resulting mixture was refluxed with stirring at room tempera-
ture for 24 h, during this the color of the solution changed from
light yellow to dark yellow. The resulting solution was filtered
and left at room temperature for slow evaporation crystallization.
After 2 weeks, yellow color powder was separated, washed with
diethyl ether and dried. Yield: (0.383 g, 73%). Anal. Calc. for C20H20-
Cl2N4O2Pd: C, 45.71; H, 3.81; N, 10.67. Found: C, 45.80; H, 3.96; N,
All the synthetic manipulations were performed under ambient
atmosphere. The solvents were dried and distilled before use
following the standard procedures. 1-(4-Methoxyphenyl)-1H-imid-
azole (Aldrich), 1-phenyl imidazole (Aldrich), 4(5)-(hydroxy-
methyl)-imidazole (Aldrich), Palladium chloride (Aldrich) and
Palladium bromide (Aldrich) were used as received. [PdX2(MeCN)2]
and [PdX2(PhCN)2], were prepared by Kharash’s method and puri-
fied following the literature procedure [68]. The ligand 1-(4-cyano-
phenyl)-imidazole was prepared and purified following the
literature procedures [69]. Elemental analyses were performed on
a Carlo Erba Model EA-1108 elemental analyzer and data of C, H
and N is within 0.4% of calculated values. IR(KBr) and electronic
spectra were recorded using Perkin-Elmer FT-IR spectrophotometer
and Perkin Elmer Lambda-35 spectrometer, respectively. FAB mass
spectra were recorded on a JEOL SX 102/DA 6000 mass spectrome-
ter using Xenon (6 kV, 10 mA) as the FAB gas. The accelerating volt-
age was 10 kV and the spectra were recorded at room temperature
with m-nitrobenzyl alcohol as the matrix. The 1H and 13C NMR
10.80. IR(cmÀ1
,
Nujol):m = 3424, 3129, 3092, 2933, 2834,
1609(C@N), 1564, 1519, 1440, 1413, 1381, 1361, 1337, 1307,
1251, 1184, 1133(CAN), 1099, 1067, 1027, 955, 831, 810, 797,
730, 690, 666, 615, 524. Far-IR:
m .
(PdACl) = 354 cmÀ1 1H NMR (d
ppm, 400 MHz, CDCl3, 298 K): 7.65–7.74(m, 6H, J = 6.5 Hz, Imd),
7.39–7.44(m, 8H, J = 7.5 Hz, Ph), 3.49(s, 6H, OCH3). 13C{1H} NMR
(CDCl3):
158.80(CAPh),
136.10(CAImd),
129.30(CAPh),
124.80(CAImd), 123.10(CAImd), 122.60(CAPh), 114.20(CAPh)
54.60(CAOCH3). UV–Vis: kmax (DMSO,
e
[dm3 molÀ1 cmÀ1]) = 343
spectra were recorded on
a JEOL DELTA2 spectrometer at
(10630), 260 (8449). FAB-MS m/z 525(524), [M]+; 489(491),
400 MHz using TMS as an internal standard. The chemical shift val-
ues were recorded on the d scale and the coupling constants (J) are
in Hz. GCMS studies were done with the Shimadzu-2010 instru-
ment containing a DB-5/RtX-5MS-30Mt and 60Mt column of
0.25 mm internal diameter. M+ is the mass of the cation. The struc-
tural characterization of the recovered catalyst after one catalytic
cycle was done using X-ray diffraction (XRD) measurements using
Bruker D8 Discover X-ray diffractometer, with Cu Ka1 radiation
(k = 1.5405 Å). Small quantities of the recovered catalyst after one
catalytic cycle were dispersed in ethanol by sonicating for about
30 min. 5 ml of the suspension was put on copper grids using a
microliter pipette for TEM measurements that was carried
[M]+ACl; 453(451), [M]+ ACl2.
(b) It was also prepared by the same method as described above
except that [PdCl2(C6H5CN)2] (0.383 g, 1 mmol) was used in place
of [PdCl2(CH3CN)2]. Yield: (0.367 g, 70%).
2.2.3. trans-[PdCl2(HMI)2] (3)
This complex was prepared by the following two methods: (a)
4(5)-(hydroxylmethyl)imidazole (0.196 g, 2 mmol) was added
slowly to a solution of [PdCl2(CH3CN)2] (0.259 g, 1 mmol) in
30 mL CH3OH:CH2Cl2 (50:50 V/V) mixture at room temperature.
The resulting mixture was refluxed with stirring at room tempera-
ture for 24 h, during this the color of the solution changed from
light yellow to light greenish yellow. The resulting solution was fil-
tered and left at room temperature for slow evaporation crystalli-
zation. After two weeks, yellow color powder was separated,
washed with diethyl ether and dried. Yield: (0.280 g, 75%). Anal.
Calc. for C8H12Cl2N4O2Pd: C, 25.73; H, 3.22; N, 15.01. Found: C,
out using
a FEI TECNAI G2 200 kV transmission electron
microscope.
2.2. Synthesis of complexes
2.2.1. trans-[PdCl2(PI)2] (1)
25.80; H, 3.36; N, 15.30. IR(cmÀ1, Nujol):
m = 3420, 3100(OH),
This complex was prepared by the following two methods: (a)
1-phenylimidazole (0.288 g, 2 mmol) was added slowly to a solu-
tion of [PdCl2(CH3CN)2] (0.259 g, 1 mmol) in 30 mL CH3OH:CH2Cl2
(50:50 V/V) mixture at room temperature. The resulting mixture
was refluxed with stirring at room temperature for 24 h, during
this the color of the solution changed from light yellow to dark yel-
low. The resulting solution was filtered and left at room tempera-
ture for slow evaporation crystallization. After three days, yellow
color crystals were separated, washed with diethyl ether and dried.
Yield: (0.325 g, 70%). Anal. Calc. for C18H16Cl2N4Pd: C, 46.45; H,
3.44; N, 12.04. Found: C, 46.66; H, 3.56; N, 12.34. IR(cmÀ1, Nu-
2926, 2850, 1640, 1590(C@N), 1458, 1380, 1304, 1284, 1150,
1130(CAN), 1090, 1040, 1006, 968, 820, 770, 740, 691, 660, 650,
615, 516. Far-IR:
m .
(PdACl) = 353 cmÀ1 1H NMR (d ppm,
400 MHz, DMSO-d6, 298 K): 7.57(s, 2H, Imd), 6.90(s, 2H, Imd),
4.43(s, 4H, CH2OH). 13C{1H} NMR (CDCl3): 136.80(CAImd),
135.40(CAImd), 120.80(CAImd), 62.80(CACH2OH). UV–Vis: kmax
(DMSO, e
[dm3 molÀ1 cmÀ1]) = 366 (7688), 260 (7814). FAB-MS m/z
373(371), [M]+; 337(336), [M]+ACl; 301(307), [M]+ACl2.
(b) It was also prepared by the same method as described above
except that [PdCl2(C6H5CN)2] (0.383 g, 1 mmol) was added in place
of [PdCl2(CH3CN)2]. Yield: (0.223 g, 60%).
jol):m = 3422, 3142(CAH), 3011, 2923, 2853, 2359, 2341,
1636(C@N), 1597, 1513, 1457, 1384, 1306, 1277, 1233, 1159,
2.2.4. trans-[PdCl2(CPI)2] (4)
1129(CAN), 1095, 1062, 1000, 966, 819, 766, 738(CAH), 692,
This complex was prepared by the following two methods: (a)
CPI (0.338 g, 2 mmol) was added slowly to a solution of [PdCl2(CH3-
CN)2] (0.259 g, 1 mmol) in 30 mL CH3OH:CH2Cl2 (50:50 V/V) mix-
ture at room temperature. The resulting mixture was stirred at
room temperature for 24 h, during this the color of the solution
changed from light yellow to dark turbid yellow. Finally, the yellow
color solid precipitated out, and was filtered, washed with diethyl
668, 653, 610, 530. Far-IR:m .
(PdACl) = 353 cmÀ1 1H NMR (d ppm,
400 MHz, CDCl3, 298 K): 8.40(s, 2H, Imd), 7.60(d, 2H, J = 1.3 Hz,
Imd), 7.37–7.53(m, 10H, J = 6.7 Hz, Ph), 7.15(t, 2H, J = 2.0 Hz,
Imd). 13C{1H} NMR (CDCl3): 137.60(CAImd), 135.90(CAPh),
129.10(CAPh), 128.60(CAPh), 125.60(CAImd), 122.30(CAImd).
UV–Vis: kmax (DMSO,
e
[dm3 molÀ1 cmÀ1]) = 342 (10976), 260

M. Trivedi et al. / Inorganica Chimica Acta 394 (2013) 107–116
109
ether and dried. Yield: (0.412 g, 80%). Anal. Calc. for C20H14Cl2N6Pd:
C, 46.60; H, 2.72; N, 16.31. Found: C, 46.90; H, 2.96; N, 16.46.
IR(cmÀ1
Nujol): = 3407, 3147, 2922, 2225(CN), 1608(C@N),
1522, 1384, 1306, 1102(CAN), 1059, 962, 835, 723, 643, 622,
573, 547. Far-IR:
= 341 cmÀ1 1H NMR (d ppm, 400 MHz, CDCl3,
2.2.7. trans-[PdBr2(HMI)2] (7)
This complex was prepared by the following two methods: (a)
4(5)-(Hydroxyl methyl)imidazole (0.196 g, 2 mmol) was added
slowly to a solution of [PdBr2(CH3CN)2] (0.348 g, 1 mmol) in
30 mL CH3OH:CH2Cl2 (50:50 V/V) mixture at room temperature.
The resulting mixture was stirred at room temperature for 24 h,
during this the color of the solution changed from greenish yellow
to orange. The resulting solution was filtered and left at room tem-
perature for slow evaporation crystallization. After two weeks,
mustard yellow color powder was separated, washed with diethyl
ether and dried. Yield: (0.300 g, 71%). Anal. Calc. for C8H12Br2N4O2-
Pd: C, 20.78; H, 2.59; N, 12.12. Found: C, 20.80; H, 2.86; N, 12.36.
,
m
m
.
298 K): 8.38(s, 2H, Imd), 8.16(d, 2H, J = 8.7 Hz, Imd), 7.83(d, 2H,
J = 4.5 Hz, Imd), 7.73(d, 4H, J = 7.5 Hz, CNPh), 7.48(d, 4H,
J = 6.0 Hz, CNPh). 13C{1H} NMR (CDCl3): 136.40(CAImd), 132.90
(CACNPh), 129.30(CACNPh), 127.20(CACNPh), 124.40(CACNPh),
124.20(CAImd), 123.60(CAImd), 113.12(CNACNPh). UV–Vis: kmax
(DMSO, e
[dm3 molÀ1 cmÀ1]) = 335 (23902), 265 (16643). FAB-MS
m/z515(516), [M]+; 479(480), [M]+ACl; 443(441), [M]+ACl2.
(b) It was also prepared by the same method as described above
except that [PdCl2(C6H5CN)2] (0.383 g, 1 mmol) was added in place
of [PdCl2(CH3CN)2]. Yield: (0.283 g, 55%).
IR(cmÀ1
1590, 1458, 1380, 1304, 1284, 1150, 1130(CAN), 1090, 1040,
1006, 968, 820, 770, 740, 691, 660, 650, 615, 516. Far-IR:
, Nujol):m = 3420, 3100(OH), 2926, 2850, 1640(C@N),
m
(PdABr) = 245 cmÀ1 1H NMR (d ppm, 400 MHz, DMSO-d6, 298 K):
.
2.2.5. trans-[PdBr2(PI)2] (5)
7.56(s, 2H, Imd), 6.91(s, 2H, Imd), 4.44(s, 4H, CH2OH). 13C{1H}
This complex was prepared by the following two methods: (a)
1-phenylimidazole (0.288 g, 2 mmol) was added slowly to a solu-
tion of [PdBr2(CH3CN)2] (0.348 g, 1 mmol) in 30 mL CH3OH:CH2Cl2
(50:50 V/V) mixture at room temperature. The resulting mixture
was stirred at room temperature for 24 h, during this the color of
the solution changed from light yellow to greenish yellow. The
resulting solution was filtered and left at room temperature for
slow evaporation crystallization. After two weeks, greenish yellow
color powder was separated, washed with diethyl ether and dried.
Yield: (0.470 g, 85%). Anal. Calc. for C18H16Br2N4Pd: C, 38.98; H,
2.88; N, 10.11. Found: C, 39.10; H, 2.94; N, 10.26. IR(cmÀ1, Nu-
NMR (CDCl3): 136.30(CAImd), 135.10(CAImd), 120.50(CAImd),
62.30(CACH2OH). UV–Vis: kmax (DMSO, e
[dm3 molÀ1 cmÀ1]) = 354
(15460), 261 (7855). FAB-MS m/z 462(466), [M]+; 382(384),
[M]+ABr; 302(307), [M]+ABr2.
(b) It was also prepared by the same method as described above
except that [PdBr2(C6H5CN)2] (0.472 g, 1 mmol) was added in place
of [PdBr2(CH3CN)2]. Yield: (0.446 g, 65%).
2.2.8. trans-[PdBr2(CPI)2] (8)
This complex was prepared by the following two methods: (a)
jol):
1500, 1430, 1380, 1304, 1260, 1150, 1120(CAN), 1080, 1000, 946,
888, 750, 725(CAH), 680, 670, 643, 600, 510. Far-IR:
(PdABr) = 245 cmÀ1 1H NMR (d ppm, 400 MHz, CDCl3, 298 K):
m = 3418, 3112(CAH), 3010, 2926, 2851, 1610(C@N), 1580,
CPI (0.338 g, 2 mmol) was added slowly to
a solution of
[PdBr2(CH3CN)2] (0.348 g, 1 mmol) in 30 mL CH3OH:CH2Cl2
(50:50 V/V) mixture at room temperature. The resulting mixture
was stirred at room temperature for 24 h, during this the color of
the solution changed from light orange to dark orange. Finally,
the orange color solid precipitated out, and was filtered, washed
with diethyl ether and dried. Yield: (0.483 g, 80%). Anal. Calc. for
m
.
8.38(s, 2H, Imd), 7.50(d, 2H, J = 3.0 Hz, Imd), 7.33–7.48(m, 10H,
J = 7.5 Hz, Ph), 7.20(t, 2H, J = 4.5 Hz, Imd). 13C{1H} NMR (CDCl3):
136.40(CAImd), 135.20(CAPh), 129.05(CAPh), 128.40(CAPh),
125.30(CAImd), 122.10(CAImd). UV–Vis: kmax (DMSO,
e
[dm3 -
C
20H14Br2N6Pd: C, 39.74; H, 2.32; N, 13.91. Found: C, 39.82; H,
2.46; N, 13.90. IR(cmÀ1, Nujol):
= 3407, 3148, 2916, 2225(CN),
1608(C@N), 1522, 1493, 1428, 1372, 1338, 1306, 1254, 1185,
1131(CAN), 1103, 1059, 1027, 962, 836. Far-IR:
= 341 cmÀ1 1H
molÀ1 cmÀ1]) = 389 (6172), 260 (15781). FAB-MS m/z 554(555),
m
[M]+; 474(476), [M]+ABr; 394(395), [M]+ABr2.
(b) It was also prepared by the same method as described above
except that [PdBr2(C6H5CN)2] (0.472 g, 1 mmol) was added in place
of [PdBr2(CH3CN)2]. Yield: (0.415 g, 75%).
m
.
NMR (d ppm, 400 MHz, CDCl3, 298 K): 8.40(s, 2H, Imd), 8.20(d,
2H, J = 8.7 Hz, Imd), 7.81(d, 2H, J = 4.5 Hz, Imd), 7.71(d, 4H,
J = 7.5 Hz, CNPh), 7.45 (d, 4H, J = 6.0 Hz, CNPh). 13C{1H} NMR
(CDCl3): 136.10(CAImd), 132.60(CACNPh), 129.10(CACNPh),
127.40(CACNPh), 124.30(CACNPh), 124.10(CAImd), 123.30
2.2.6. trans-[PdBr2(MPI)2] (6)
This complex was prepared by the following two methods: (a)
1-(4-methoxyphenyl)-1H-imidazole (0.348 g, 2 mmol) was added
slowly to a solution of [PdBr2(CH3CN)2] (0.348 g, 1 mmol) in
30 mL CH3OH:CH2Cl2 (50:50 V/V) mixture at room temperature.
The resulting mixture was stirred at room temperature for 24 h,
during this the color of the solution changed from reddish orange
to dark orange. The resulting solution was filtered and left at room
temperature for slow evaporation crystallization. After two weeks,
orange color crystals were separated, washed with diethyl ether
and dried. Yield: (0.368 g, 60%). Anal. Calc. for C20H20Br2N4O2Pd:
(CAImd), 112.90(CNACNPh). UV–Vis: kmax (DMSO, e -
[dm3 molÀ1
cmÀ1]) = 340 (3550), 273 (27109). FAB-MS m/z 604(604), [M]+;
524(525), [M]+ABr; 444(446), [M]+ABr2.
(b) It was also prepared by the same method as described above
except that [PdBr2(C6H5CN)2] (0.472 g, 1 mmol) was added in place
of [PdBr2(CH3CN)2]. Yield: (0.392 g, 65%).
C, 39.08; H, 3.26; N, 9.12. Found: C, 39.28; H, 3.26; N, 9.10. IR(cmÀ1
Nujol): = 3309, 3220, 1625(C@N), 1585, 1497, 1449, 1385, 1353,
1273, 1240, 1088(CAN), 1003, 832, 749, 725, 660, 640, 610, 515.
Far-IR:
(PdABr) = 244 cmÀ1 1H NMR (d ppm, 400 MHz, CDCl3,
,
2.3. X-ray crystallographic study
m
Intensity data for 1, and 6 were collected on X-calibur S oxford,
Bruker APEXII and Enraf–Nonius MACH3 CCD area detector diffrac-
m
.
298 K): 7.68–7.73(m, 6H, J = 5.5 Hz, Imd), 7.38–7.43(m, 8H,
J = 7.1 Hz), 3.49(s, 6H, OCH3). 13C{1H} NMR (CDCl3): 157.40(CAPh),
136.30(CAImd), 129.10(CAPh), 124.40(CAImd), 123.10(CAImd),
122.40(CAPh), 114.10(CAPh) 54.30(CAOCH3). UV–Vis: kmax
tometers using graphite monochromatized Mo Ka radiation at
293(2) and 100(2) K and 150(2) K (6). SMART and SAINT software
packages [70] were used for data collection and data integration
for 1, and 6. Structure solution and refinement were carried out
using the SHELXTL-PLUS software package [71,72]. The non-hydrogen
atoms were refined with anisotropy thermal parameters. All the
hydrogen atoms were treated using appropriate riding models.
The computer programme PLATON was used for analyzing the inter-
action and stacking distances [71,72].
(DMSO, e
[dm3 molÀ1 cmÀ1]) = 340 (23162), 261 (15253). FAB-MS
m/z 614(616), [M]+; 534(533), [M]+ABr; 454(453), [M]+ABr2.
(b) It was also prepared by the same method as described above
except that [PdBr2(C6H5CN)2] (0.472 g, 1 mmol) was added in place
of [PdBr2(CH3CN)2]. Yield: (0.429 g, 70%).

110
M. Trivedi et al. / Inorganica Chimica Acta 394 (2013) 107–116
2.4. Catalytic reactions
(553 mg, 4.0 mmol), and complexes 1–8 (0.001 equivalent) in
DMF (10 mL) at t = 0, 24 h, respectively.
2.4.1. General procedure for Mizoroki–Heck reaction
K2CO3 (553 mg, 4.0 mmol) was added to a 100 ml three-necked
flask with a stirring bar and the flask was dried under vacuum and
then filled with nitrogen, aryl halide or 4-halotolune (2.0 mmol)
and n-butylacrylate or styrene (2.0 mmol) in DMF (10 mL). Then
0.001 equivalent palladium complex in DMF was added via a syr-
inge under nitrogen atmosphere. The mixture was stirred at
140 °C for the indicated reaction time. After the completion of
the reaction, the mixture was cooled, the precipitate was removed
by filtration and the product was extracted from the filtrate with
diethyl ether. The combined organic layer was dried over anhy-
drous Na2SO4 and filtered. After evaporation, the obtained residue
was purified by silica-gel column chromatography to give the
Mizoroki–Heck product. Product identities were confirmed by 1H
and 13C NMR and GC–MS.
3. Results and discussion
3.1. Synthesis
Reaction of the [PdX2(RCN)2] [where X = Cl, Br; R = CH3A,
C6H5A] with L in 1:2 stoichiometric ratio in a mixture of dichloro-
methane and methanol (1:1v/v) under stirring at RT afforded neu-
tral mononuclear trans-complexes with the formulations [PdX2L2]
(1–8) in good yield (Scheme 1).
3.2. Characterization
All the complexes were isolated as air-stable, non-hygroscopic
solids and soluble in dimethylformamide, dimethylsulfoxide and
halogenated solvents like chloroform but insoluble in petroleum
ether and diethyl ether and do not show any signs of decomposi-
tion in solution upon exposure to air for days. Information about
the composition of the complexes and structure and bonding has
been derived from the analytical spectral studies. Analytical data
of the complexes conformed well to their respective formulations.
More information about composition of the complexes was also
obtained from FAB-MS. Resulting data is recorded in the experi-
mental section and representative FAB-MS spectrum of the neutral
complex 3 and 7 are shown in F-1, Supporting material. The posi-
tions of different peaks and overall fragmentation patterns in the
FAB-MS of the respective complexes are consistent with their
formulations.
2.4.2. General procedure for Suzuki coupling reaction
K2CO3 (553 mg, 4.0 mmol) was added to a 100 ml three-necked
flask with a stirring bar and the flask was dried under vacuum and
then filled with nitrogen, aryl halide or 4-halotolune (2.0 mmol)
and phenylboronic acid (244.0 mg, 2.0 mmol) in DMF (10 mL).
Then 0.001 equivalent palladium complex in DMF was added via
a syringe under nitrogen atmosphere. The mixture was stirred at
120 °C for the indicated reaction time. The mixture was cooled
and the precipitate was removed by filtration and the product
was extracted from the filtrate with diethyl ether. The combined
organic layer was dried over anhydrous Na2SO4 and filtered. After
evaporation, the obtained residue was purified by silica-gel column
chromatography to give the coupling product. Product identities
were confirmed by 1H and 13C NMR and GC–MS.
The IR spectra of all mononuclear complexes 1–8, exhibited
identical bands in the region 3300–3100 cmÀ1
[v(NAH) and/or
v
(CAH)], 1580–1500 cmÀ1 [ring stretching or d(NAH)], 1290–
2.4.3. Procedure of Hg poisoning experiments for Mizoroki–Heck
reaction
1260 cmÀ1 (in plane CAH bending), 912–888 cmÀ1 (ring vibration),
750–725 cmÀ1 (out-of plane CAH bending), 680–670 cmÀ1 (ring
stretch/bending) and 630–600 cmÀ1 (ring deformation) (See F-2,
Supporting material). The mononuclear complexes 4 and 8, dis-
played sharp and intense bands around 2225–2227 cmÀ1 corre-
An excess of Hg(0) (300 equivalent relative to trans-[PdX2L2])
was added into the reaction mixture of aryl halide or 4-halotolune
(2.0 mmol), n-butylacrylate or styrene (2.0 mmol), K2CO3 (553 mg,
4.0 mmol), and complexes 1–8 (0.001 equivalent) in DMF (10 mL)
at t = 0, 24 h, respectively.
sponding to
m(C„N) (See F-3, Supporting material). The position
of the (C„N) remained unaltered, compared to that in the ligand
m
itself, it suggested linkage of CPI with the palladium centre in the
respective complexes through imidazole nitrogen. The Far-IR spec-
tra of all the complexes reveal a strong absorbtion band at 353–
354 cmÀ1 for PdL2Cl2 compounds and at 244–245 cmÀ1 for PdL2Br2
2.4.4. Procedure of Hg poisoning experiments for Suzuki coupling
reaction
An excess of Hg(0) (300 equivalent relative to trans-[PdX2L2])
was added into the reaction mixture of aryl halide or 4-halotolune
(2.0 mmol), phenylboronic acid (244.0 mg, 2.0 mmol), K2CO3
compounds. The former band may be assigned to
v(PdACl) (See F-
4, Supporting material) while the latter to
v(PdABr) stretch. These
X
CH3OH+CH2Cl2
Stirring/r.t./2:1
[PdX2(RCN)2]
L
L
Pd
L
+
N
X
X=Cl, Br
R= CH3-; C6H5-
1-8
= L
R2
L
PI;1
N
X
Cl
MPI;2
HMI;3
CPI;4
PI;5
MPI;6
HMI;7
CPI;8
Cl
Cl
Cl
Br
Br
Br
Br
R1
L1(PI) = R1(C6H5); R2(H)
L2(MPI) = R1(CH3O-C6H4); R2(H)
L3 (HMI) = R1(H); R2(CH2OH)
L4(CPI) = R1(CN-C6H4); R2(H)
Scheme 1. Graphical representation for synthesis of 1–8.

M. Trivedi et al. / Inorganica Chimica Acta 394 (2013) 107–116
111
Fig. 1. Molecular structures for (a) 1, and (b) 6 (30% thermal ellipsoids are shown).
Table 1
Table 2
Crystallographic data for Complexes 1 and 6.
Selected bond lengths (Å), and bond angles (°) for complexes 1 and 6.
1
6
Complex-1
Complex-6
Formula
Formula weight
T (K)
Crystal system
Space group
a (Å)
C
18H16Cl2N4Pd
C20H20Br2N4O2Pd
614.62
150(2)
monoclinic
P21/c
11.1921(3)
12.3378(3)
7.7358(2)
90
91.103(2)
90
1068.01(5)
2
1.911
4.634
1.121
Pd(1)ACl(1)
2.2954(2)
2.2954(2)
2.0088(7)
2.0088(7)
1.3809(11)
1.3199(11)
1.3483(11)
1.3822(11)
1.4263(11)
180.00(6)
89.94(2)
Pd(1)ABr(1)
2.3842(10)
465.65
100(2)
triclinic
Pd(1)ACl(1)#1
Pd(1)AN(1)
Pd(1)ABr(1)#1
Pd(1)AN(1)
2.3842(10)
2.007(5)
2.007(5)
1.378(8)
1.405(10)
1.330(9)
1.358(9)
1.346(9)
1.371(9)
1.433(9)
180.000(1)
90.56(17)
89.44(17)
89.44(17)
90.56(17)
180.0
Pd(1)AN(1)#1
N(1)AC(1)
N(1)AC(3)
N(2)AC(3)
N(2)AC(2)
Pd(1)AN(1)#1
O(1)AC(7)
O(1)AC(8)
N(1)AC(1)
N(1)AC(2)
N(2)AC(1)
N(2)AC(3)
N(2)AC(4)
ꢀ
P1
7.0872(4)
7.8015(5)
9.4730(6)
106.659(3)
99.131(3)
115.376(3)
428.72(5)
1
1.804
1.402
1.072
0.0232
b (Å)
c (Å)
N(2)AC(4)
a
(°)
N(1)#1APd(1)AN(1)
N(1)#1APd(1)ACl(1)
N(1)APd(1)ACl(1)
b (°)
c
(°)
90.06(2)
N(1)#1APd(1)AN(1)
N(1)#1APd(1)ABr(1)
N(1)APd(1)ABr(1)
N(1)#1APd(1)ABr(1)#1
N(1)APd(1)ABr(1)#1
Br(1)APd(1)ABr(1)#1
C(7)AO(1)AC(8)
C(1)AN(1)AC(2)
C(1)AN(1)APd(1)
V (Å3)
N(1)#1APd(1)ACl(1)#1 90.06(2)
Z
N(1)APd(1)ACl(1)#1
Cl(1)APd(1)ACl(1)#1
C(3)AN(1)APd(1)
C(1)AN(1)APd(1)
C(3)AN(1)AC(1)
89.94(2)
Dcalc (g cmÀ3
)
(mmÀ1
)
180.000(1)
125.85(6)
127.30(6)
106.83(7)
107.63(7)
l
Goodness-of-fit (GOF) on F2
R1 all
0.0609
0.0518
0.1757
0.1705
1.121
117.5(6)
106.6(6)
127.2(5)
126.1(5)
R1 [I > 2
r
(I)]
0.0221
0.0483
0.0478
1.072
C(3)AN(2)AC(2)
C(3)AN(2)AC(4)AC(9) 43.91(13)
C(2)AN(2)AC(4)AC(9) À132.50(10) C(2)AN(1)APd(1)
wR2
wR2 [I > 2
r(I)]
GOF on F2
C(1)AN(2)AC(4)AC(10) À35.3(10)
C(3)AN(2)AC(4)AC(5)
À33.5(10)
bands agree well with the trans-orientation of the halogens in the
square planar palladium complexes [73].
The 1H NMR spectral data of the complexes along with their
assignments are recorded in the experimental section. In the 1H
NMR spectra of all the complexes 1–8, imidazole protons displayed
down field shift as compared to that in the ligand itself (See F-5
and F-6, Supporting material). Downfield shift in the position of
the imidazole protons (8.40–6.90 ppm) might result from the
change in electron density on the palladium center due to linkage
of PI or MPI or HMI or CPI through its imidazole nitrogen. The con-
jugative and electron withdrawing abilities of the ACN/AC6H5/
AOCH3/ACH2OH groups pulls electron density away from the
imidazole ring towards itself, leading to a decrease of electron den-
sity on the palladium center which in turn, may pull more electron
density away from the imidazole ring, leading to deshielding of
imidazole protons. The 1H NMR spectra of the Br containing com-
plexes show little change on coordination with imidazole based li-
gands. The position and integrated intensity of various resonances
supported well the presence of ligand and formulation of the
respective complexes. In the 13C NMR spectra of the complexes 2,
6, and 3, 7 show peaks for AOCH3 and ACH2OH carbon of the
MPI and HMI at d = 54.30–54.60 and 62.30–62.80 ppm,

112
M. Trivedi et al. / Inorganica Chimica Acta 394 (2013) 107–116
Table 3
1 and 6 show an almost ideal square planar environment, with
N(1)APd(1)ACl(1), N(1)APd(1)ACl(1)#1
,
N(1)APd(1)ABr(1)and
Hydrogen bond parameters for 1 and 6.
N(1)APd(1)ABr(1)#1 angles of 90.06(2)°, 89.94(2)°, 89.44(17)°,
and 90.56(17)°, respectively. The angles between the N(1)#1
DAHÁ Á ÁAAX
d HÁ Á ÁA Å
D DÁ Á ÁA (Å)
h DAHÁ Á ÁA (°)
Complex 1
APd(1)AN(1), Cl(1)APd(1)ACl(1)#1
,
N(1)#1APd(1)AN(1) and
C(3)AH(3)Á Á ÁCl(1)a
2.80
3.1771
105
Br(1)APd(1)ABr(1)#1
bonds
are
180.00(6)°,
180.000(1)°,
Complex 6
C(1)AH(1)Á Á ÁO(1)b
2.47
2.73
3.340(8)
3.636(7)
153
160
180.000(1)° and 180.0°, respectively which indicated that the 1-
phenyl imidazole or 4-methoxyimidazole rings and the two chlo-
ride or bromide ions in the coordination sphere are found to be
trans disposed. The PdACl (2.2954(2) Å), PdABr (2.3842(10) Å)
and PdAN distances (2.007(5)–2.0088(7) Å) in 1, and 6 lie in the
range expected for such bonds. These are comparable to those in
other Pd(II) imidazole complexes [45–57,58–61,74–78]. The CAO
bond distances in 6 are 1.378(8)–1.405(10) Å, respectively. These
are normal and comparable with CAO distances in other
complexes [79]. The ligand 1-phenylimidazole or 4-methoxyimi-
dazole has lost planarity upon coordination with metal center.
The phenyl group of the ligand is not coplanar with imidazole ring
and is tilted with respect to the imidazole ring plane at an
angle of 44.7(3)–43.91(13)° in 1 and 35.3(10)–35.5(10)° in 6.
The inter annular bond distances N(2)AC(4) in 1, and 6 are
1.434(3)–1.426(11) Å and 1.433(9) Å, respectively. These are
slightly shorter than a single CAN bond indicating a double bond
character. Crystal packing in 1 is stabilised by intermolecular
C(5)AH(5)Á Á ÁBr(1)c
Symmetry equivalents:
a
2 À x, 1 À y, Àz.
b
Àx, 1/2 + y,1/2 – z.
c
1 À x, À1/2 + y, 1/2Àz.
respectively. The nitrile carbon of the CPI in the complexes 4 and 8
resonated at 112.90–113.12 ppm, while the other carbons of CPI
resonated in the range 123.30–136.40 ppm.
3.3. Molecular structure determination
Molecular structure of the complexes 1 and 6 with atomic num-
bering scheme is shown in Fig. 1. Details about the data collection,
solution and refinement are presented in Table 1 and selected bond
lengths and bond angles and hydrogen bond parameters are tabu-
lated in Table 2 and 3, respectively. Complex 1 and 6 crystallized in
triclinic and monoclinic system with P1 and P21/c space group. The
asymmetric unit in complex 1 and 6 comprises half of the formula
unit in the solid state. The coordination geometry of the complexes
CAHÁ Á Á
ter-molecular CAHÁ Á ÁX (X = Cl,
6, intra- and inter-molecular CAHÁ Á ÁX (X = Br, O,
p
, inter- and intra-molecular CAHÁ Á ÁCl, and intra- and in-
) and interactions, while in
) and p–p
p
p–p
p
Fig. 2. Parallel channels motif in complex 6 accompanied by CAHÁ Á ÁO interactions along crystallographic ‘c’-axis.
Fig. 3. Straight channels motif in complex 1 accompanied by CAHÁ Á ÁCl interactions.

M. Trivedi et al. / Inorganica Chimica Acta 394 (2013) 107–116
113
Table 4
Results of the Mizoroki–Heck cross-coupling reactions of aryl halides with vinyl compounds using complex 1–8a.
Entry
R1
R2
X
Yield (%)b/(TON)c
1
2
3
4
5
6
7
8
a
b
c
d
e
f
H
CO2nBu
CO2nBu
CO2nBu
CO2nBu
CO2nBu
CO2nBu
C6H5
Br
I
89
(890)
88
(880)
60
(600)
100
(1000)
96
(960)
65
(650)
92
(920)
92
(920)
59
92
(920)
85
(850)
55
(550)
98
(980)
97
(970)
60
(600)
91
(910)
90
(900)
61
(610)
94
(940)
98
(980)
65
53
(530)
60
(600)
36
(360)
70
(700)
65
(650)
40
(400)
68
(680)
47
(470)
37
(370)
62
(620)
63
(630)
40
56
(560)
66
(660)
30
(300)
61
(610)
66
(660)
39
(390)
49
(490)
56
(560)
32
(320)
62
(620)
66
(660)
41
90
(900)
90
(900)
50
(500)
94
(940)
98
(980)
64
(640)
90
(900)
91
(910)
60
(600)
95
(950)
99
(990)
71
91
(910)
87
(870)
48
(480)
96
(960)
96
(960)
58
(580)
91
(910)
92
(920)
55
(550)
97
(970)
93
(930)
66
59
(590)
64
(640)
35
(350)
59
(590)
62
(620)
36
(360)
57
(570)
65
(650)
35
(350)
60
(600)
61
(610)
35
60
(600)
61
(610)
38
(380)
55
(550)
64
(640)
35
(350)
47
(470)
67
(670)
31
(310)
60
(600)
64
(640)
37
H
H
Cl
Br
I
CH3
CH3
CH3
H
Cl
Br
I
g
h
i
H
C6H5
H
C6H5
Cl
Br
I
(590)
93
(930)
95
(950)
70
j
CH3
CH3
CH3
C6H5
k
l
C6H5
C6H5
Cl
(700)
(650)
(400)
(410)
(710)
(660)
(350)
(370)
a
All reactions were carried out using 2.0 mmol activated or non-activated aryl halide, 2.0 mmol vinyl compound, 4.0 mmol K2CO3, 0.001 equivalent Pd complex and 10 ml
DMF at 140 °C for 24 h.
b
Isolated yield.
Mol(substrate) X% yield/mol(Pd).
c
Fig. 5. XRD pattern of the recovered catalyst (1) after one catalytic cycle in the Heck
reaction.
(Fig. 2). Contact distances for CAHÁ Á ÁO interactions are 2.467 Å.
CAH CAHÁ Á ÁCl type intra- and inter-molecular interactions result
in the formation of straight channels motif both in 1 (Fig. 3). The
Fig. 4. TEM image of the recovered catalyst (1) after one catalytic cycle in the Heck
reaction.
inter- and intra-nuclear CAHÁ Á Á
p interactions that are present in
complex 1 result in single helical motif (See F-7, Supporting mate-
interactions are present. Packing in 6 along crystallographic ‘c’-axis
through CAHÁ Á ÁO interactions leading to parallel channels motif
rial). The most noteworthy structural feature of complex 1 is face-

114
M. Trivedi et al. / Inorganica Chimica Acta 394 (2013) 107–116
Table 5
Effect of Hg(0) poisoning in the Mizoroki–Heck coupling reaction.a
Entry
R1
R2
X
Yield (%)b
1
2
3
4
5
6
7
8
a
b
CH3
CH3
CO2nBu
C6H5
Br
Br
2
3
3
4
70
62
61
62
5
4
4
3
59
60
55
60
a
All reactions were carried out using 2.0 mmol activated or non-activated aryl
halide, 2.0 mmol vinyl compound, 4.0 mmol K2CO3, 0.001 equivalent Pd com-
plex + 300 equivalent of Hg(0) and 10 ml DMF at 140 °C for 24 h.
b
Isolated yield.
to-face
p–p interactions between carbon atoms of the imidazole
rings (See F-8, Supporting material). The contact distances for
face-to-face interactions are 3.353–3.361Å. Inter-molecular
CAHÁ Á ÁBr interactions are present in 6 (See F-9, Supporting mate-
rial). The CAHÁ Á ÁCl and CAHÁ Á ÁBr contact distances are in the range
of 2.48–2.94 Å. These distances are within the range reported in
Fig. 6. TEM image of the recovered catalyst (5) after one catalytic cycle in the
the literature [80]. The intra and inter-molecular CAHÁ Á Á
p weak
Suzuki reaction.
interactions are also present in 6 but only intra-molecular CAHÁ Á Áp
ligand dissociation to produce palladium(0) colloids or nanoparti-
cles under these conditions. With these aspects in mind and in
the quest for to investigate whether there is a difference in cata-
lytic activity between all the Pd complexes 1–8. Actually, as shown
in Table 4, we observed a difference in catalytic activity. That is, Pd
complexes 1, 2, 5, and 6 exhibited higher catalytic activity than Pd
complexes 3, 4, 7, and 8 and also compared with the starting com-
plexes [PdCl2(MeCN)2] and [PdCl2(C6H5CN)2]. In the case of the
complexes 1, 2, 5, and 6, TEM pictures clearly show the formation
of Pd(0) nanoparticles, which is the true active species during the
coupling reaction (Figs. 4 and 5 and See F-12, Supporting material).
In all the reactions catalysed by the complexes 1, 2, 5, and 6, yellow
and orange Pd(II) solution rapidly turns black at the end of reac-
tion, which attributed to the formation of palladium(0) nanoparti-
cles which serves as a source of the actual catalyst following
oxidative addition of the activated or non-activated aryl halide
substrate to a Pd atom on the surface of the nanoparticle. On the
other hand, in the case of the complexes 3, 4, 7, and 8, TEM pictures
clearly indicate with no formation of Pd(0) nanoparticles under the
interactions has been present in 1. Contact distances for intra- and
inter-CAH
p
interactions are in the range of 2.51–2.86 and 2.82 Å
stacking
(See F-10, Supporting material). The importance of
interactions between aromatic rings has been widely recognized
in the intercalation of drugs with DNA which lie in the range of
3.4–3.5 Å. Complex 6 exhibit intermolecular face-to-face (
p–p
pphe-
nyl/ phenyl (ct/ct distances 3.382 Å) (See F-11, Supporting
p
material).
3.4. Catalytic application of the complexes
3.4.1. Mizoroki–Heck reaction catalyzed by Pd complexes 1–8
To compare the catalytic activities of complexes 1–8 in the
Mizoroki–Heck reaction, we examined the reaction of activated
or non-activated aryl halides and vinyl compounds in the presence
of 0.001 equivalent Pd catalyst in DMF at 140 °C (Table 4). Recent
discovery by de Vries [81] and Hayashi and co-workers [74] have
proposed the role of Pd species in the Mizoroki–Heck reaction
under high temperature, and they proposed the mechanism of
Table 6
Results of the Suzuki–Miyaura cross-coupling reactions of aryl halides with phenylboronic acid using complex 1–8.a
Entry
R1
X
Yield (%)c/(TON)d
1
2
3
4
5
6
7
8
a
CH3
CH3
CH3
H
I
99
(990)
99
(990)
75
(750)
93
(930)
97
(970)
60
(600)
5
99
(990)
95
(950)
70
(700)
99
(990)
99
(990)
56
(560)
3
50
(500)
58
(580)
43
(430)
55
(550)
43
(430)
26
(260)
58
55
(550)
54
(540)
40
(400)
47
(470)
49
(490)
20
(200)
54
94
(940)
97
(970)
72
(720)
95
(950)
98
(980)
50
(500)
4
98
(980)
96
(960)
68
(680)
98
(980)
99
(990)
58
(580)
5
45
(450)
48
(480)
42
(420)
48
(480)
45
(450)
27
(270)
48
50
(500)
54
(540)
36
(360)
51
(510)
47
(470)
29
(290)
54
b
c
Br
Cl
I
d
e
H
Br
Cl
Br
F
H
gb
CH3
a
All reactions were carried out using 2.0 mmol activated or non-activated, 2.0 mmol phenylboronic acid, 4.0 mmol K2CO3, 0.001 equivalent Pd complex and 10 ml DMF at
120 °C for 12 h.
b
300 Equivalent of Hg(0).
Isolated yield.
Mol(substrate) X% yield/mol(Pd).
c
d

M. Trivedi et al. / Inorganica Chimica Acta 394 (2013) 107–116
115
Fig. 7. Yield vs reaction time for Heck coupling of bromobenzene and n-butylacrylate using 1–8.
same conditions (See F-13, Supporting material). This observation
suggested that the ease of dissociation of the ligand promoted
the formation of the Pd(0) nanoparticles occur more easily in com-
plexes 1, 2, 5, and 6 in comparison to the 3, 4, 7, and 8 complexes. A
high isolated yield to the Mizoroki–Heck coupling product was ob-
tained for activated aryl iodides or bromides (Table 4, entry d, e, j
and k) than activated aryl chlorides (Table 4, entry f and l) for 1, 2,
5, and 6 complexes in comparison to 3, 4, 7, and 8 complexes.
Extending the reaction time up to 48 h did not give better results
for activated aryl chlorides. However, for non-activated iodo or
bromo or chlorobenzene, the desired coupled product yield was
obtained in 48–92%, corresponding to TON of 480–920 in 1, 2, 5,
and 6 complexes as compared to 30–68% yield with TON of 300
to 680 in 3, 4, 7, and 8 complexes (Table 4, entry a, b, c, g, h and
i). The low yields obtained with aryl chloride or non-activated aryl
chlorides are due to the stronger Csp2ACl bonds of aryl chlorides
than those of the heavier congeners. Moreover, the observed cata-
lytic activity of our pre-catalysts (1, 2, 5, and 6) are better than al-
ready reported Pd–imidazole complexes in the literature [45–60].
It has been known that Hg(0) can poison the catalytic property of
a metal(0) species by amalgamating the metal or absorbing on
the metal surface in a heterogeneous catalysis [82,83]. In our study
we also found that the addition of excess Hg(0) (Hg:Pd = 300:1) in-
deed totally deactivated the coupling reaction in 1, 2, 5, and 6 com-
plexes, while the addition of excess Hg(0) did not deactivated the
coupling reaction in 3, 4, 7, and 8 complexes (Table 5, entry a, b).
This result might suggest that the actual catalytic species is Pd(0)
species as reported [84]. We assume that the imidazole containing
Pd(II) complexes might be reduced to a stable palladium(0) nano-
particles which in an active species for the catalytic cycle. The rea-
son for the induction period of pre-catalysts 1–8 as shown in Fig. 7
is also not clear at present. However, we believe the difference in
the rate of ligand dissociation to produce the real catalytic species
causes the different induction periods between Pd pre-catalysts 1–
8. Further investigation of the true activation mechanism for imid-
azole containing Pd(II) complexes and its practical applications are
undergoing in our laboratory.
6) in DMF at 120 °C for 12 h gave 50–99% yield to the Suzuki–Miya-
ura coupling product, corresponding to TON of 500–990, whereas
use of Pd complex 3, 4, 7, and 8 under the same conditions afforded
the product in 20 to 58% yield with TON of 200–580. The results
presented in Table 6 are in agreement with the earlier observation
that imidazoles incorporating an NAH bond do not catalyze the
coupling reaction [85,86]. However, neither of the imidazole com-
plexes studied, 1–8, obeyed this rule, affording 20–99% product
yield. The higher catalytic activity of these complexes, which con-
tains imidazole based ligands, had been shown better results in
comparison to Hayashi et al. [74] and Trzeciak et al. [61]. To ex-
plain the reason behind the higher catalytic activity of 1, 2, 5,
and 6 complexes compared to 3, 4, 7, and 8 is due to the easily dis-
sociation of the ligand from the respective complex results into for-
mation of Pd nanoparticles (Fig. 6) that activate substrates of the
Suzuki–Miyaura reaction. We also carried out the Hg poisoning
experiments. In our study, we found that the addition of excess
Hg(0) (Hg:Pd = 300:1) indeed deactivated the reaction in 1, 2, 5,
and 6 complexes, although no activity has been diminished in com-
plexes 3, 4, 7, and 8 as shown in Table 6 (entry g).
4. Conclusion
In this work we have synthesized and characterized a series of
new palladium(II) complexes with different imidazole derivatives.
We also determined the structures in the solid state of the com-
plexes dichlorobis(1-phenylimidazole)palladium(II) (1) and dib-
romobis(1-(4-Methoxyphenyl)-1H-imidazole)palladium(II)
(6)
using single-crystal X-ray diffractometry. The newly synthesized
complexes were successfully used in the Mizoroki–Heck reaction
and the Suzuki–Miyaura coupling reaction. While all eight of the
complexes screened were found to be catalytically active, the best
results were obtained for the complexes 1, 2, 5 and 6 in comparison
to the complexes 3, 4, 7, and 8, which is due to ease of dissociation
of the ligand lead to Pd nanoparticles that assist higher catalytic
activity of these complexes. Further investigations towards other
coupling reactions using our catalytic system and catalytic mecha-
nism are currently in progress in our laboratory.
3.4.2. Suzuki–Miyaura coupling catalyzed by Pd complexes 1–8
The Palladium–imidazole complexes (1–8) were tested as cata-
lysts in the model Suzuki–Miyaura reaction of selected activated or
non-activated aryl halides with phenylboronic acid (Table 6). Here
as well, complexes 1, 2, 5, and 6 exhibited higher catalytic activity
than 3, 4, 7, and 8 (Table 6) and also compared with the starting
complexes [PdCl2(MeCN)2] and [PdCl2(C6H5CN)2]; that is, the reac-
tion of activated or non-activated aryl halides with phenylboronic
acid in the presence of 0.001 equivalent of Pd complex (1, 2, 5, and
Acknowledgements
We gratefully acknowledge financial support from University
Grants Commission, New Delhi (Grant No. F.4–2/2006(BSR)/13–
76/2008(BSR)) and DST, New Delhi (SR/FT/CS-104/2011). We also
thank Dr. S. Uma, and the Head, Department of Chemistry, Univer-
sity of Delhi and Professor D.S. Pandey, Department of Chemistry,
Faculty of Science, Banaras Hindu University, Varanasi, India for

116
M. Trivedi et al. / Inorganica Chimica Acta 394 (2013) 107–116
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their kind encouragement. Special thanks are due to Professor P.
Mathur, Department of Chemistry, Indian Institute of Technology,
Mumbai and National Single Crystal X-ray Diffraction Facility, In-
dian Institute of Technology, Mumbai and The Director, USIC, Uni-
versity of Delhi for providing single crystal X-ray data and GC-MS
facility from AIRF center of JNU, New Delhi. We acknowledge fund-
ing from the National Science Foundation (CHE0420497) for the
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Appendix A. Supplementary material
CCDC 784764 & 828055 (complex 1) and 784765 (complex 6)
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_re-
quest/cif. Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.ica.2012.08.003.
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