Tetranuclear assembly of palladium(II): Catalyst for C-C coupling reactions

Article abstract of DOI:10.1016/j.poly.2013.07.020

The 2-((2-benzylamino)phenyldiazenyl)phenol, H₂OL [where H ₂OL = ArOHN = NC₆H₄N(H)CH₂(C ₆H5); Ar = C₆H₅ (for H2OL¹), p-MeC₆H₄ (for H₂OL²) or p-ClC ₆H₄ (for H₂OL³)], ligands were prepared by the reaction of the appropriate 2-((2-hydroxyaryl)azo)anilines with benzyl bromide. The reactions of Na₂PdCl₄ with H ₂OL afforded tetranuclear Pd(II) complexes of the composition [Pd(OL)]₄. The ligands bind palladium(II) in a tridentate (N, N, O) fashion. The X-ray structure of [Pd(OL¹)]₄ was determined to confirm the characterization. The newly synthesized [Pd(OL¹)] ₄ complex was utilized as a catalyst for the Suzuki and Heck reaction for a variety of substrates.

Full text of DOI:10.1016/j.poly.2013.07.020

Polyhedron 63 (2013) 133–138
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Tetranuclear assembly of palladium(II): Catalyst for C–C coupling
reactions
Poulami Pattanayak a, Jahar Lal Pratihar a,b, Debprasad Patra a,c, Chia-Her Lin d, Surajit Chattopadhyay a,
⇑
a Department of Chemistry, University of Kalyani, Kalyani 741235, India
b Department of Chemistry, Kandi Raj College, Murshidabad 742137, India
c Department of Chemistry, East Calcutta Girls’ College, Kolkata 700089, India
d Department of Chemistry, Chung Yuan Christian University, Chung-Li 320, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
The 2-((2-benzylamino)phenyldiazenyl)phenol, H2OL [where H2OL = ArOHN = NC6H4N(H)CH2(C6H5);
Ar = C6H5 (for H2OL1), p-MeC6H4 (for H2OL2) or p-ClC6H4 (for H2OL3)], ligands were prepared by the reac-
tion of the appropriate 2-((2-hydroxyaryl)azo)anilines with benzyl bromide. The reactions of Na2PdCl4
with H2OL afforded tetranuclear Pd(II) complexes of the composition [Pd(OL)]4. The ligands bind palla-
dium(II) in a tridentate (N, N, O) fashion. The X-ray structure of [Pd(OL1)]4 was determined to confirm
the characterization. The newly synthesized [Pd(OL1)]4 complex was utilized as a catalyst for the Suzuki
and Heck reaction for a variety of substrates.
Received 18 May 2013
Accepted 3 July 2013
Available online 24 July 2013
Keywords:
2-((2-Benzylamino)phenyldiazenyl)phenol
Palladium(II)
Tetranuclear chelate
Suzuki and Heck reaction
Ó 2013 Published by Elsevier Ltd.
1. Introduction
attracted attention in current chemical research [19–53]. As a re-
sult we have designed and synthesized the appropriate mononu-
The appropriate ligands have been designed and used for the
synthesis of structurally diverse tetranuclear palladium(II) com-
plexes. Different types of bridging to hold the four square planar
palladium(II) units were found to be the basis of structural diver-
sity within the tetranuclear assemblies [1–10]. A structure contain-
ing bridging phenoxo oxygens and two nitrogen coordinations is
only known in one case [11]. Earlier, we have shown that azo li-
gands of type 1 afforded only mononuclear complexes of Pd(II)
coordinating through (C, N, N) or (N, N, N) modes [12–15].
clear palladium(II) complexes incorporating azo ligands which
exhibited satisfactory results toward catalytic C–C coupling reac-
tions [19–53]. Both challenges, i.e., to prepare tetranuclear palla-
dium(II) complexes and to examine their effectiveness as
catalysts toward C–C coupling reactions, encouraged us to carry
out the present work. Therefore, we contemplated to design and
synthesize (N, N, O) coordinating ligands, 2, where the phenoxo
oxygen may be available as a bridging atom, and to study the Heck
and Suzuki coupling reactions using one of the newly synthesized
tetranuclear complexes as a catalyst.
R
R
N
N
H
OH
N
N
/
R
N
H
N
1
Recently, it was shown that (C, N, O) coordinating Schiff base li-
gands afforded tetranuclear palladium complexes [16–18]. The
use of palladium(II) mononuclear complexes as efficient catalysts
in homogenous and heterogeneous C–C coupling reactions has
2
In this paper, the syntheses of the newly designed ligands and
corresponding tetranuclear palladium(II) complexes, including
characterization, have been described. The catalytic behavior of
one of the newly synthesized complexes toward C–C coupling
reactions has also been described.
⇑
Corresponding author. Tel.: +91 33 25828750; fax: +91 33 25828282.
E-mail address: scha8@rediffmail.com (S. Chattopadhyay).
0277-5387/$ - see front matter Ó 2013 Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.poly.2013.07.020

134
P. Pattanayak et al. / Polyhedron 63 (2013) 133–138
NMR (400 MHz, CDCl3) d(ppm): 13.20 (s, OH), 9.50 (s, NH) 8.91
(d, 1H), 7.68–7.54 (m, 3H), 7.43–7.35 (m, 2H), 7.27–7.21 (m, 1H),
7.10 (d, 1H), 6.98 (d, 1H), 6.93–6.89 (m, 2H), 4.54 (d, 2H).
2. Experimental
2.1. Materials
The solvents used in the reactions were of reagent grade (E.
Merck, Kolkata, India) and were purified and dried by the reported
procedures [54]. Palladium chloride was obtained from Arora Mat-
they, Kolkata, India. Na2[PdCl4] was prepared following a reported
procedure [55]. The 2-((2-hydroxyaryl)azo)anilines were prepared
according to the reported procedures [56,57]. Benzyl bromide and
potassium carbonate were purchased from E. Merck, Kolkata, India.
Phenylboronic acid, iodobenzene, bromobenzene, 1-iodo-3,5-
dimethylbenzene, 1-iodo-3,4-dimethylbenzene, 1-iodo-2-nitro-
benzene, 1-iodo-3-nitrobenzene, 1-bromo-3,5-dimethylbenzene,
1-bromo-2-nitrobenzene and styrene were purchased from
Aldrich.
2.4. Syntheses of the complexes
The complexes, [Pd(OL1)]4, [Pd(OL2)]4 and [Pd(OL3)]4 were pre-
pared following similar procedures. A representative procedure for
[Pd(OL1)]4 is given below.
2.4.1. Syntheses of [Pd(OL1)]4
A solution of H2OL1 (0.146 g, 0.52 mmol) in 15 cm3 methanol
was added to a solution of Na2PdCl4 (0.153 g, 0.52 mmol) in
10 cm3 methanol. The mixture was stirred for 12 h under slightly
warm conditions. The dark solid precipitate was separated by fil-
tration and purified by column chromatography using silica gel
(60–120 mesh). The eluent was benzene:acetonitrile (95:5 v/v)
mixed solvent. Upon evaporation of the solvent, a blue–violet solid
of pure [Pd(OL1)]4 was obtained. Yield: 50%. Anal. Calc. for C76H60-
N12O4Pd4: C, 55.9; H, 3.6; N, 10.3. Found: C, 56.0; H, 3.6; N, 10.3%.
2.2. Physical measurements
Microanalysis (C, H, N) was performed using a Perkin-Elmer
2400 C, H, N, S/O series II elemental analyzer. Infrared spectra were
recorded on a Perkin-Elmer L120-00A FT-IR spectrometer with the
samples prepared as KBr pellets. Electronic spectra were recorded
on a Shimadzu UV-1800 PC spectrophotometer. 1H NMR spectra
were obtained on Brucker 400 spectrometers in CDCl3.
UV–Vis spectrum (CH2Cl2) kmax, nm (e
, Mꢀ1 cmꢀ1): 556 (27245),
421 (24311), 343 (17283), 273 (49264), 228 (100000). IR (KBr,
cmꢀ1): 1337 (N@N). 1H NMR (400 MHz, CDCl3) d(ppm): 7.59 (d,
1H), 7.53 (d, 1H), 7.39 (d, 1H), 7.09–6.99 (m, 3H), 6.74 (t, 1H),
6.66–6.60 (m, 2H), 6.37 (d, 1H), 4.83 (d, 1H), 3.15 (d, 1H).
2.3. Syntheses of the ligands
2.4.2. Syntheses of [Pd(OL2)]4 and [Pd(OL3)]4
All the ligands, H2OL1, H2OL2 and H2OL3, were prepared follow-
ing similar procedures. A representative procedure for H2OL1 is gi-
ven below.
The complexes [Pd(OL2)]4 and [Pd(OL3)]4 were prepared using
H2OL2 and H2OL3 in place of H2OL1, respectively.
[Pd(OL2)]4. Yield 60%. Anal. Calc. for C77H62N12O4Pd4: C, 56.2; H,
3.7; N, 10.2. Found: C, 56.2; H, 3.7; N, 10.1%. UV–Vis spectrum
2.3.1. Syntheses of H2OL1
(CH2Cl2) kmax, nm (e
, Mꢀ1 cmꢀ1): 557 (25030), 417 (18696), 345
A
mixture
of
2-((2-hydroxyphenyl)azo)aniline
(0.3 g,
(15222), 272 (41140), 226 (71185). IR (KBr, cmꢀ1): 1333 (N@N).
1H NMR (400 MHz, CDCl3) d(ppm): 7.33–7.29 (m, 3H), 7.01–6.96
(m, 2H), 6.93 (t, 1H), 6.60 (d, 2H), 6.53 (t, 1H), 6.38 (t, 1H); 6.31
(d, 1H), 4.72 (d, 1H), 3.75 (d, 1H), 2.34 (s, 3H).
1.52 mmol), benzyl bromide (0.27 g, 1.52 mmol), pyridine and a
catalytic amount of KI in 30 cm3 dry acetonitrile was refluxed for
10 h. The orange liquid mass that was obtained after evaporation
of the solvent afforded the crude ligand, H2OL1, which was isolated
by column chromatography on silica gel (60–120 mesh) using the
eluent petroleum ether:benzene (9:1 v/v). Upon evaporation of the
solvent after chromatography, the orange–red solid of pure H2OL1
was obtained. Yield: 50%. Anal. Calc. for C19H17N3O: C, 75.2; H, 5.6;
N, 13.8. Found: C, 75.2; H, 5.5; N, 13.8%. UV–Vis spectrum (CH2Cl2)
[Pd(OL3)]4. Yield 60%. Anal. Calc. for C76H59N12O4Pd4Cl: C, 54.8;
H, 3.5; N, 10.1. Found: C, 54.9; H, 3.5; N, 10.0%. UV–Vis spectrum
(CH2Cl2) kmax, nm (e
, Mꢀ1 cmꢀ1): 556 (29805), 417 (24436), 343
(18640), 273 (51766), 228 (103398). IR (KBr, cmꢀ1): 1334 (N@N).
1H NMR (400 MHz, CDCl3) d(ppm): 7.58 (d, 1H), 7.52 (d, 1H),
7.44–7.38 (m, 2H), 7.35 (s, 1H), 7.13–7.06 (m, 3H), 6.73 (d, 1H),
6.68–6.62 (m, 1H), 6.45 (d, 1H), 4.80 (d, 1H), 3.34 (d, 1H).
kmax, nm (e
, Mꢀ1 cmꢀ1): 478 (3700), 318 (3616), 251 (3926), 230
(4120). IR (KBr, cmꢀ1): 3381 (OH), 3031 (NH), 1454 (N@N). 1H
NMR (400 MHz, CDCl3) d(ppm): 12.95 (s, OH), 9.56 (s, NH) 8.04
(s, 1H), 7.95 (d, 1H), 7.68 (d, 1H), 7.52–7.49 (m, 1H), 7.45–7.41
(m, 1H), 7.40–7.37 (m, 1H), 7.35–7.32 (m, 1H), 7.28–7.22 (m,
2H), 6.99–6.94 (m, 2H), 6.84–6.78 (m, 2H), 4.55 (d, 2H).
2.5. General procedures for the Suzuki cross coupling reaction and
isolation of the catalyst
In an oven dried round bottomed flask a mixture of phenyl
boronic acid (0.244 g, 2.0 mmol), aryl halide (2.0 mmol), the palla-
dium complex [Pd(OL1)]4 (0.002 mmol) and potassium carbonate
(0.145 g, 2.5 mmol) in THF (10 cm3) was heated to reflux for 2 h,
as mentioned in Table 2. After the reaction was completed, the sol-
vent was evaporated and the reaction mixture was extracted with
diethyl ether. The ether solution was dried over Na2SO4 and fil-
tered. The ether solution, containing the reaction mixture, was
passed through a 30.48 cm silica column (60–120 mesh), the com-
plex was not separated out and it remained trapped in the column.
After the desired compound was extracted from the column, the
complex was extracted using dichloromethane. The unchanged
complex was used twice. After evaporation of the ether, solids of
the pure products were obtained. The yields of the products ob-
tained from all the reactions were determined after isolation, and
the products were characterized by 1H NMR spectra.
2.3.2. Syntheses of H2OL2 and H2OL3
The ligands H2OL2 and H2OL3 were prepared using 2-((2-
hydroxytolyl)azo)aniline and 2-((2-hydroxy-4-chloro)azo)aniline
in place of 2-((2-hydroxyphenyl)azo)aniline, respectively.
Yield: 55%. Anal. Calc. for C20H19N3O: C, 75.7; H, 5.9; N, 13.2.
Found: C, 75.7; H, 6.1; N, 13.3%. UV–Vis spectrum (CH2Cl2) kmax
,
nm (e
, Mꢀ1 cmꢀ1): 476 (4240), 326 (3913), 256 (4000), 226
(4720). IR (KBr, cmꢀ1): 3370 (OH), 3031 (NH), 1453 (N@N). 1H
NMR (400 MHz, CDCl3) d(ppm): 12.99 (s, OH), 9.47 (s, NH), 7.65
(d, 1H), 7.39–7.37 (m, 5H), 7.33–7.31 (m, 1H), 7.25 (t, 2H), 6.82–
6.77 (m, 3H), 4.54 (d, 2H), 2.35 (s, 3H).
Yield: 50%. Anal. Calc. for C19H16N3Cl: C, 67.5; H, 4.7; N, 12.4.
Found: C, 67.6; H, 4.7; N, 12.5%. UV–Vis spectrum (CH2Cl2) kmax
nm (
, Mꢀ1 cmꢀ1): 482 (3500), 325 (3296), 250 (3460), 227
(3966). IR (KBr, cmꢀ1): 3380 (OH), 3039 (NH), 1506 (N@N). 1H
,
e

P. Pattanayak et al. / Polyhedron 63 (2013) 133–138
135
R
N
R
2.6. General procedures for the Heck reaction
R
OH
In an oven dried round bottomed flask a mixture containing sty-
rene (2.5 mmol), aryl halide (2.5 mmol), the palladium complex
[Pd(OL1)]4 (0.005 mmol) and potassium carbonate (10.0 mmol) in
methanol (10 cm3) was heated to reflux for 4 h, as mentioned in
Table 3. After the reaction was completed, the solvent was evapo-
rated and the reaction mixture was extracted with diethyl ether.
The ether solution was dried over Na2SO4 and filtered. The ether
solution, containing the reaction mixture, was passed through a
12.48 cm silica column (60–120 mesh), the complex was not sep-
arated out and it remained trapped in the column. After he desired
compound was extracted from the column, the complex was
eluted using dichloromethane. Upon evaporation of the ether, sol-
ids of the pure products were obtained. The yields of the products
obtained from all the reactions were determined after isolation,
and the products were characterized by 1H NMR spectra.
OH
H2OL1
H2OL2
H2OL3
2a
H
PhCH2Br
N
...
1
CH3
N
ð1Þ
N
2b
2c
H
Pyridine, KI
Dry CH3CN
Reflux
N
NH2
Cl
2
All the H2OL, 2, ligands, were characterized by the usual spec-
troscopic methods. The purified ligands were used for the reactions
with Na2PdCl4. Upon reaction with Na2PdCl4 the H2OL ligands
afforded a dark colored compound as the major product. The com-
positions of these compounds corresponded to ‘‘[Pd(OL)]’’, accord-
ing to spectral and analytical data. Interestingly, the ‘‘[Pd(OL)]’’
compounds were actually tetranuclear complexes having the com-
position [Pd(OL)]4, as determined by X-ray studies (see below).
R
2.7. Crystallography
N
N
N
N
R
N
O
N
A single crystal of [Pd(OL1)]4 was grown by slow diffusion of
petroleum ether into a dichloromethane solution at 298 K. Data
were collected by the
N
Pd
Pd
2-
PdCl4
O
OH
x
-scan technique on a Bruker Smart CCD dif-
radiation, monochromated by graphite
R
R
N
...
fractometer with Mo K
a
O
2
H
N
Pd
N
crystal. Structure solution was done by the direct method with
the SHELXS-97 program [58–59]. Full matrix least square refine-
ments on F2 were performed using the SHELXL-97 program [58–
59]. All non-hydrogen atoms were refined anisotropically using
N
Pd
O
N
N
N
N
2
R
reflections I > 2r(I). All hydrogens were included in calculated
positions. The data collection parameters and relevant crystal data
3
ð2Þ
are collected in Table 1.
3. Results and discussion
3.2. Spectral characterization and solution structures
3.1. Syntheses
The H2OL ligands displayed characteristic UV–Vis spectra with
absorption maxima near 478 and 325 nm, whereas the violet solu-
tions of the [Pd(OL)]4 complexes in dichloromethane exhibited
characteristic low energy absorption maxima near 555 and
422 nm. Representative UV–Vis spectra of the H2OL1 ligand and
[Pd(OL1)]4 complex are given in Fig. 1. Relevant data are collected
in Section 3.
The reaction of 2-(2-aminoarylazo)phenol with benzyl bromide
in the presence of pyridine and a catalytic amount of KI in dry ace-
tonitrile afforded a reddish yellow mass, from which the orange
colored N-benzylated product H2OL, 2, was separated and isolated
by preparative TLC. The relevant chemical equation and com-
pounds prepared are shown in Eq. (1).
In the IR spectra, mOH and mNH appeared near ꢁ3283 and
ꢁ3030 cmꢀ1 respectively, and these bands are absent in the com-
,
plexes [12–15,56,57,60]. The mN
@
N
band of the H2OL ligands
(ꢁ1460 cmꢀ1) was shifted to lower frequency (ꢁ1337 cmꢀ1) after
Table 1
Crystallographic data for [Pd(OL1)]4, 3a.
Chemical formula
Formula weight
Crystal system
Space group
a (Å)
C80.20H64.20N12O4Pd4
1685.64
triclinic
ꢀ
P1
14.8428(2)
14.8930(2)
17.4782(3)
93.0960(10)
110.7710(10)
98.7860(10)
0.71073
3545.57(9)
952
b (Å)
c (Å)
a
(°)
b (°)
(°)
c
k (Å)
V (Å3)
F(000)
Z
2
T (K)
295(2)
1.579
1.059
D (mg/mꢀ3
)
l
(mmꢀ1
)
R1 (all data)
wR2[I > 2 (I)]
Goodness-of-fit
0.0486
0.1118
1.065
r
Fig. 1. UV–Vis spectra of H2OL1 (—) and [Pd(OL1)]4 (—)) in dichloromethane.

136
P. Pattanayak et al. / Polyhedron 63 (2013) 133–138
Fig. 2. Molecular structure of [Pd(OL1)]4 with the atom numbering scheme. Hydrogen atoms are omitted for clarity.
formation of the Pd(II) complexes, which is consistent with coordi-
nation of the azo nitrogen [12–15,56,57,60].
Table 2
Selected bond distances (Å) and angles (°) for the compound [Pd(OL1)]4, 3a.
The compositions of the ligands H2OL and the corresponding
palladium complexes matched well with the C, H, N analytical data
and 1H NMR spectral data. The 1H NMR data are given in Section 3.
The H2OL ligands exhibited a sharp singlet at d 13.00 ppm for the
phenolic OH group [58–60]. The N–H signals (near d 9.5 ppm) ap-
peared as broad signals with a triplet-like structure due to coupling
with two benzyl protons [12]. This N–H resonance is absent in the
1H NMR spectra of the complexes, signifying the dissociation of
this proton upon complexation [12,14–15]. The methylene proton
resonances of the H2OL ligands appeared as doublet near d 4.59
(5.6 Hz) due to the coupling with the adjacent –NH proton. This
doublet for two equivalent of protons split into two sets of dou-
blets each for one equivalent proton near d 4.80 and d 3.15 in the
spectra of the [Pd(OL)]4 complexes due to the non-equivalent
shielding of the two methylene protons. The methyl signal of
H2OL2 was observed at d 2.35. The aromatic proton resonances ap-
peared in the region d 8.04 to 6.35 for the ligands and d 7.60 to 4.79
for the complexes. All the NMR data above are consistent with the
molecular structures obtained from the X-ray studies (see below).
Distances
Pd(1)–N(1)
Pd(1)–O(1)
O(1)–C(1)
N(1)–C(6)
N(2)–C(7)
C(2)–C(3)
C(4)–C(5)
1.933(5)
2.060(3)
1.360(5)
1.419(6)
1.337(6)
1.343(9)
1.391(7)
Pd(1)–N(3)
Pd(1)–O(2)
N(1)–N(2)
C(1)–C(2)
N(3)–C(12)
C(3)–C(4)
C(5)–C(6)
1.970(3)
2.107(3)
1.278(6)
1.383(7)
1.339(7)
1.369(8)
1.387(8)
Angles
N(1)–Pd(1)–N(3)
N(3)–Pd(1)–O(1)
N(2)–N(1)–C(6)
C(6)–N(1)–Pd(1)
C(12)–N(3)–Pd(1)
O(1)–C(1)–C(6)
C(1)–C(2)–C(3)
N(2)–C(7)–C(12)
N(2)–C(7)–C(8)
92.06(17)
175.26(16)
117.3(4)
112.3(3)
124.2(3)
118.3(4)
120.0(5)
127.7(5)
112.9(4)
N(1)–Pd(1)–O(1)
N(1)–Pd(1)–O(2)
N(2)–N(1)–Pd(1)
N(1)–N(2)–C(7)
O(1)–C(1)–C(2)
C(2)–C(1)–C(6)
C(4)–C(3)–C(2)
N(2)–C(7)–C(8)
C(12)–C(7)–C(8)
83.42(13)
171.55(13)
130.4(3)
122.5(4)
122.2(5)
119.5(5)
122.1(5)
112.9(4)
119.4(4)
The remaining two coordination sites on each palladium are
coordinated by two bridging phenolato oxygens. Thus each pal-
ladium is within an (O, O, N, N) coordination sphere. A portion
of the molecular structure containing four Pd and four bridging
phenolic O atoms and eight coordinated N atoms is shown in
Fig. 3.
3.3. X-ray structure determination of 3a
Suitable crystals of [Pd(OL1)]4, were grown by slow diffusion
of a dichloromethane solution into petroleum ether. A perspec-
tive view of the molecule is shown in Fig. 2 and selected bond
distances and angles are collected in Table 2. The X-ray struc-
ture shows that the composition of the molecule is [(OL)Pd]4,
where the four palladium atoms are held together by bridging
phenolic oxygen atoms. The geometry about each palladium is
distorted square planar. The amido and azo nitrogens bind each
palladium centre from each of the four dianionic OL1 ligands.
3.4. Catalytic reactions (C–C coupling): Suzuki cross-coupling and
Heck reactions
Suzuki cross-coupling and Heck coupling reactions can be rep-
resented as given in Scheme 1. Quite a large number of catalysts
based on palladium metal and palladium complexes have been

P. Pattanayak et al. / Polyhedron 63 (2013) 133–138
137
Table 3
Suzuki cross coupling reaction with the catalyst [Pd(OL1)]4.a,b,c
Aryl halide
Product
Isolated yield
95
I
92
I
90
87
I
O2N
O2N
I
Fig. 3. Arrangement of Pd4O4.
88
85
Br
PhB(OH)2
Ph-CH=CH2
Ar-Ph
Ar-X
X=IorBr
Scheme 1. Suzuki and Heck reactions.
Ar-CH=CH-Ph
Br
a
b
c
Solvent, THF.
Base, K2CO3.
Time, 2 h.
developed for such reactions [19–53]. Since the advantages of
using polynuclear complexes are yet to be identified, we contem-
plated examining the activity of
a new palladium complex,
and iodoaryls) and styrene [42–53]. We have studied the catalytic
activity of [Pd(OL1)]4 toward conversions of arylhalides to the
corresponding stilbene by Heck coupling reactions in refluxing
methanol and in the presence of K2CO3 (Eq. (4)). The results are gi-
ven in Table 4. All the reactions were carried out under ambient
conditions. Although the catalysis occurred smoothly, at the end
of each reaction the [Pd(OL1)]4 catalyst could not be isolated as
[Pd(OL1)]4, as a catalyst for Suzuki and Heck reactions in THF sol-
vent (Eqs. (3) and (4)).
THF / DMF
Reflux
X
B(OH)2
+
K2CO3
R
R
Catalyst 3a
X = I or Br
R = H, Me, NO2
Table 4
Heck reaction with the catalyst [Pd(OL1)]4.a,b,c
ð3Þ
ð4Þ
Aryl halide
Product
Isolated yield
91
MeOH
Reflux
I
X
+
K2CO3
Catalyst 3a
R
88
85
R
I
X = I or Br
R = H, Me, NO2
I
All the Suzuki reactions, by coupling arylhalides and phenylbo-
O2N
ronic acid, occurred in refluxing THF (ꢁ66 °C) using K2CO3 as a
base, keeping the reaction time constant, i.e., two hours in all cases.
The isolated yields and results are collected in Table 3. All the reac-
tions were carried out under a non-inert atmosphere.
O2N
90
I
Further, we examined the amount of unchanged [Pd(OL1)]4 cat-
alyst after the reaction was stopped (after two hours). Spectropho-
tometrically (kmax = 556 nm) we could recognize that 68% of
[Pd(OL1)]4 remained unchanged. Though this observation did not
provide any clue to differentiate between a Pd(0)–Pd(II) and
Pd(II)–Pd(IV) mechanism, the issue of a Pd(II)–Pd(IV) cycle could
not be discarded [45]. Moreover, the observation did not indicate
any particular advantage on using the polynuclear palladium com-
plex as a catalyst. More experimental data are necessary to come to
a definite conclusion in this regard.
82
80
Br
Br
a
b
c
Solvent, MeOH.
Base, K2CO3.
Time, 2 h.
Heck coupling is another important palladium catalyzed C–C
coupling reaction where the substrates are arylhalides (bromoaryls

138
P. Pattanayak et al. / Polyhedron 63 (2013) 133–138
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Acknowledgements
We are thankful to the D.S.T (New Delhi) for funding and a fel-
lowship to P.P. under DST-WOS-A project (No. SR/WOS-A/CS-140/
2011). The necessary laboratory and infrastructural facilities are
provided by the Department of Chemistry, University of Kalyani.
The support of DST under the FIST program to the Department of
Chemistry, and PURSE to the University of Kalyani is
acknowledged.
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Appendix A. Supplementary data
CCDC 935349 contains the supplementary crystallographic data
for [Pd(OL1)]4. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: +44 1223-336-033 or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found, in
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