Palladium(II) complex with a potential N4-type Schiff-base ligand as highly efficient catalyst for Suzuki-Miyaura reactions in aqueous media

Article abstract of DOI:10.1016/j.tetlet.2012.08.026

Two new air-stable palladium(II) complexes containing a N ₄-Schiff-base ligand have been synthesized and investigated as catalysts for the Suzuki-Miyaura reactions. The binuclear complex 2 has proven to be an excellent catalyst for additive-free Suzuki-Miyaura reactions of aryl bromides in neat water at room temperature and aryl chlorides in aqueous-glycerol at 80 °C. Satisfactory to excellent yields of biaryls are obtained with a wide range of substrates with relatively low loading of catalyst.

Full text of DOI:10.1016/j.tetlet.2012.08.026

Tetrahedron Letters 53 (2012) 5627–5630
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Palladium(II) complex with a potential N4-type Schiff-base ligand as highly
efficient catalyst for Suzuki–Miyaura reactions in aqueous media
Biplab Banik a,b, , Archana Tairai a, Nasifa Shahnaz a, Pankaj Das a,
⇑
⇑
a Department of Chemistry, Dibrugarh University, Dibrugarh 786004, Assam, India
b Department of Chemistry, Tinsukia College, Tinsukia 786125, Assam, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new air-stable palladium(II) complexes containing a N4-Schiff-base ligand have been synthesized
and investigated as catalysts for the Suzuki–Miyaura reactions. The binuclear complex 2 has proven to
be an excellent catalyst for additive-free Suzuki–Miyaura reactions of aryl bromides in neat water at
room temperature and aryl chlorides in aqueous-glycerol at 80 °C. Satisfactory to excellent yields of biar-
yls are obtained with a wide range of substrates with relatively low loading of catalyst.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 9 June 2012
Revised 4 August 2012
Accepted 6 August 2012
Available online 11 August 2012
Keywords:
Suzuki–Miyaura reaction
Schiff-base ligand
Water
Aqueous-glycerol
Room temperature
Aryl chlorides
During the last two decades, ligand promoted palladium-medi-
ated Suzuki–Miyaura cross-coupling reaction has exemplified one
of the most important processes in organic chemistry.1 Among
the different ligands, phosphine-based ligands are still considered
to be the most preferred ligand system for Suzuki-reaction.2 How-
ever, the phosphine ligands are toxic and sensitive to air and mois-
ture which pose significant limitations on their uses. On the
contrary, nitrogen-based ligands are generally non-toxic, robust
in nature, insensitive to air/moisture, easy-to-handle, and have
the potentiality to overcome some of the drawbacks faced by tradi-
tional phosphine ligands. Hence, the recent endeavors to search for
nitrogen based ligands developed several attractive alternatives
such as N-heterocyclic carbenes,3 amines,4 oxime-based palladacy-
cles,5 etc. Although significant progress has been achieved in devel-
oping efficient catalysts that worked under mild reaction
conditions with challenging substrates including aryl chlorides, it
still remains a major challenge to carry out the Suzuki–Miyaura
reactions in water. Due to environmental and economical concerns,
many research groups,6 including the groups of Leadbeater,6a,6b
Shaughnessy,4c Eppinger,2j Zhang,6c Vanelle,6d and Turkman,3e,6e
have developed several catalytic systems for Suzuki reactions in
water. However, majority of these catalytic systems required either
elevated temperatures4c,6g,6h and/or addition of a organic co-sol-
vent,6i and/or the addition of tetrabutylammoniumbromide (TBAB)
as phase transfer catalyst,6a–6d even with aryl bromides or iodides
as substrates. It may be noted that the use of TBAB not only makes
the work-up procedure more complicated but also produces waste
when it is used in excess.6b Thus, from academic and industrial view
points, the development of new catalytic systems that can promote
the Suzuki–Miyaura reactions in aqueous medium under mild reac-
tion conditions without using any additive would be highly
advantageous.
For decades, Schiff-bases have played a key role as chelating
multidentate ligands in coordination chemistry and catalysis be-
cause of their easy synthesis, high stability under a variety of oxi-
dative and reductive conditions, etc.7 Moreover, electronic and
steric properties of Schiff-bases could be easily tuned by properly
selecting the condensing partners. Indeed, there have been few re-
ports about employment of Schiff-base ligands in palladium-cata-
lyzed Suzuki–Miyaura reactions.8 However, almost in all the
cases the reactions were carried out in toxic organic solvents such
as DMF,8a–8f toluene,8g THF,8g etc. In addition, these catalysts often
required a temperature in the range of 100–120 °C for activating
aryl bromides–iodides and usually failed to activate aryl chlorides
as substrates.8a–8d,8f To our knowledge, only recently, Bowes et.
al.8h developed a Schiff-base derived catalyst that was used in
water, however only moderate yields were obtained with aryl
bromides or aryl iodides as substrates even after refluxing the reac-
tion mixture for a few hours with 2 mol % catalyst. Thus, in order to
extend the scope of Schiff-base ligands further in Suzuki–Miyaura
⇑
Corresponding authors at present address: Department of Chemistry, Rajiv
Gandhi University, Doimukh, Rono Hills, Itanagar 791111, Arunachal Pradesh, India.
E-mail address: pankajd29@yahoo.com (P. Das).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.08.026

5628
B. Banik et al. / Tetrahedron Letters 53 (2012) 5627–5630
N
N
N
N
Pd(OAc)2 : L = 2:1
Reflux,
Reflux,
acetonitrile, 6h
acetonitrile, 6h
Pd(OAc)2 : L = 1:1
N
N
N
N
N
N
Pd
Pd
Pd
OAc
AcO
OAc
N
AcO
OAc N
AcO
2
1
Scheme 1.
Table 1
Screening of solvents for room temperature Suzuki–Miyaura reactionsa of 4-bromoanisole with phenylboronic acid using complex 1 and 2 as catalysts
Complex 1 / 2 ( 0.2 mol%),
MeO
+
B(OH)2
Br
OMe
Solvent, K2CO3, air,
rt ,4-24 h
Entry
Solvent
Time (h)
Yieldb,c (%)
Complex 1
Complex 2
1
2
3
4
5
6
7
8
9
H2O
12
8
4
12
24
24
24
24
24
12
36
56
66
52
58
26
26
22
98
98
99
99
95
98
76
62
68
iPrOH
iPrOH–H2O (1:1)
iPrOH–H2O (1:1)
n-BuOH
MeOH
DMF
THF
Toluene
a
b
c
Reaction condition: 1.0 mmol 4-bromoanisole, 1.10 mmol phenylboronic acid, 3.0 mmol K2CO3, solvent (6 ml).
Isolated yield.
Yields are of average of two runs.
reactions, we have synthesized and characterized two new palla-
dium complexes with a previously reported9 N4-type Schiff-base li-
gand 1,2-bis(20-pyridylmethylenediamino)benzene (L). It is
noteworthy to mention that the N4-ligand L due to the presence
of multiple bonding sites is expected to increase the steric conges-
tion around the metal center which is considered to be the vital
step facilitating the reductive elimination step in the cross-cou-
pling mechanism. The catalytic activities of the complexes have
been explored for Suzuki–Miyaura reactions of aryl bromides and
iodides in aqueous condition and aryl chlorides in aqueous-
glycerol.
The palladium complexes [Pd(OAc)2L](1) and [Pd2(OAc)4L](2)
were prepared10 by treating [Pd(OAc)2] with the ligand L in 1:1
and 2:1 molar ratio, respectively (Scheme 1). The complexes 1
and 2 were isolated as yellow solids which are stable to air both
in solid state and in solution. The identities of the complexes were
fully established by elemental analyses, ESI-mass, FTIR, and 1H and
13C NMR spectroscopy.10 The elemental analyses values and the
appearances of molecular ion peaks in the ESI-mass spectra of
complexes 1 and 2 support their proposed compositions. In the
higher wave numbers indicating coordination of imine nitrogen
with palladium. In addition to imine shifting, the mPy(C@N) stretch-
ing band in the complex 2 has also been shifted about 9 cmꢀ1, indi-
cating the involvement of pyridine nitrogen in coordination. In the
1H NMR spectra, signals due to imine protons appeared as singlets
at d 8.58 and 8.34 ppm, respectively for complexes 1 and 2. The
coordination of pyridine nitrogen in the complex 2 resulted in a
downfield shift of 1.54 ppm of the a-proton resonance of pyridine
compared to the corresponding signal of the complex 1. A similar
downfield shift was also observed in the corresponding carbon of
the 13C NMR spectra of complex 2. However, no prominent shift
was observed in the imine or other aromatic carbon resonances.
To evaluate the efficiencies of the complexes 1 and 2 as cata-
lysts for the Suzuki–Miyaura reaction11
, initially we have
screened a relatively activated substrate, p-bromoanisole with
phenylboronic acid. The reactions were conducted in water at
room temperature with K2CO3 as base with 0.2 mol % catalysts
and the results are depicted in Table 1. We are very much de-
lighted to see that the desired biphenyls were isolated in nearly
quantitative yields within 12 h with complex 2 as catalyst (entry
1). However, under similar experimental condition, complex 1
was found to be almost ineffective in water producing only poor
yields (entry 1). Nevertheless, the yield with complex 1 could be
FTIR spectra, the mC@N stretching bands in complexes 1 and 2 ap-
peared at 1631 and 1664 cmꢀ1 respectively and, in comparison
with the free ligand (1618 cmꢀ1) these values were shifted toward

B. Banik et al. / Tetrahedron Letters 53 (2012) 5627–5630
5629
Table 2
Suzuki–Miyaura cross-coupling reactionsa of various aryl bromides and iodides with arylboronic acids using complex 2 as catalyst
Complex 2 ( 0.2 mol%),
Solvent, K2CO3, air,
rt, 2-24 h
+
R'
B(OH)2
Br
R
R'
R
Entry
R
R0
X
Time (h)
Yieldb,c (%)
1
2
3
4
5
6
7
8
4-NO2
4-NO2
4-NO2
4-COMe
4-CHO
H
H
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
I
12
24
16
6
12
6
98
96
94
98
92
96
98
99
99
92
98
86
92
82
98
98
4-Cl
4-Me
H
H
H
4-Me
H
H
4-Cl
4-Me
H
H
4-Me
6
6
9
4-OMe
4-OMe
4-OMe
2-Me
2-OMe
1,3-Dimethyl
H
12
24
16
24
24
24
2
10
11
12
13
14
15
16
H
H
H
H
4-OMe
I
2
a
b
c
Reaction condition: 1.0 mmol aryl halide, 1.10 mmol arylboronic acid, 3.0 mmol,K2CO3, water (6 ml).
Isolated yield.
Yields are of average of two runs.
Table 3
Suzuki–Miyaura cross-coupling reactionsa of various aryl chlorides with arylboronic acids using complex 2 as catalyst
Complex 2 ( 1 mol%),
glycerol-H2O (1:1),
K2CO3, 80°C, 12 h
+
R'
B(OH)2
Cl
R
R'
R
Entry
R
R0
Solvent
Yieldb,c (%)
1
3
4
5
6
7
8
9
10
11
12
13
14
4-NO2
H
H
H
H
H
4-Cl
4-Me
H
H
H
Me
H
H
H2O
12d
65
56
36
82
56
68
76
86
74
62
Trace
12
4-NO2
4-NO2
4-NO2
4-NO2
4-NO2
4-NO2
4-COMe
H
iPrOH
iPrOH–H2O (1:1)
Glycerol
Glycerol–H2O (1:1)
Glycerol–H2O (1:1)
Glycerol–H2O (1:1)
Glycerol–H2O (1:1)
Glycerol–H2O (1:1)
Glycerol–H2O (1:1)
Glycerol–H2O (1:1)
Glycerol–H2O (1:1)
Glycerol–H2O (1:1)
4-Me
4-OMe
2-OMe
2-Me
a
b
c
Reaction condition: 1.0 mmol aryl chloride, 1.10 mmol arylboronic acid, 3.0 mmol, K2CO3, solvent (6 ml).
Isolated yield.
Yields are of average of two runs.
Reaction performed at room temperature.
d
i
improved considerably using PrOH as co-solvent with water (in
1:1 proportion) (entry 4). The iPrOH–water (1:1) combination
was also found to be a very effective solvent for complex 2 and
quantitative biphenyl formation was achieved in a much shorter
time when we compared with neat water as solvent (entry 1 vs
entry 3). Outstanding yields with complex 2 were also achieved
electron-donating substituents underwent the coupling reactions
with phenylboronic acid in nearly quantitative yields. No signifi-
cant differences were observed in yield when phenylboronic acid
was replaced by p-chloroboronic acid (entries 2 and 10) and p-
tolylboronic acid (entries 3, 7, and 11), although some differences
in the reaction times were noticed. In general, reactions are much
slower with p-chloroboronic acid (entries 2 and 10) compared to
phenyl (entries 1 and 9) or tolylboronic acid (entries 3 and 11). It
is interesting to note that our catalyst can also tolerate sterically
demanding substrates such as 2-bromotoluene, 2-bromoanisole,
and 2-bromo-1,3-dimethylbenzene (entries 12, 13, and 14), and
gave the desired products in good yields. Similar to aryl bromides,
aryl iodides as substrates react with phenylboronic acid to give
the corresponding coupling products in quantitative yields (en-
with the other protic solvents such as PrOH, nBuOH, and MeOH
i
(entries 2, 5, and 6). However, non-protic solvents such as DMF,
THF, and toluene gave relatively poorer yields (entries 7–9).
Encouraged by the results of 2 for initial screening reaction in
water, the cross-coupling reactions of a wide variety of aryl bro-
mides in the presence of complex 2 were undertaken in water
and the results are displayed in Table 2. In general, bromo ben-
zene and other aryl bromides with electron-withdrawing and

5630
B. Banik et al. / Tetrahedron Letters 53 (2012) 5627–5630
M. Chem. A. Eur. J. 2010, 16, 4075; (j) Marziale, A. N.; Jantke, D.; Faul, S. H.;
Reiner, T.; Herdtweck, E.; Eppinger, J. Green Chem. 2011, 13, 169; (k) To, S. C.;
Kwong, F. Y. Chem. Commun. 2011, 47, 5079; (l) Zapf, A.; Ehrentraut, A.; Beller,
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Organomet. Chem. 2011, 25, 552.
tries 15 and 16). As expected, reactions with aryl iodides are
much quicker compared to the corresponding aryl bromides.
Unfortunately, the reaction condition was not compatible with
aryl chloride as substrate as only very poor yield was obtained in
the reaction between p-chloronitrobenzene with phenylboronic
acid even after increasing the catalyst loading up to 1 mol % (Table
3, entry 1). However, a search for an alternative condition revealed
that use of glycerol as co-solvent with water (in 1:1 proportion) at
a relatively elevated temperature (80 °C) significantly improved
the product formation (Table 3, entry 6). It may be mentioned that
glycerol has already been recognized as a sustainable solvent as it
has all the desirable properties of a green solvent such as low flam-
mability, high availability, low toxicity, biodegradability, obtained
from renewable feedstock, etc.12 Indeed, chlorobenzene (entry 10)
or aryl chlorides containing electron withdrawing substituents
such as p-chloronitrobenzene/p-chloroacetophenone (entries 6
and 9) and electron donating substituents such as p-chloroani-
sole/p-chlorotoluene (entries 11 and 12) reacted smoothly with
phenylboronic acid affording the desired biphenyls in moderate-
to-good yields. However, aryl chlorides with electron donating
substituent at the ortho-position are reluctant to react with phen-
ylboronic acid and very poor yield was obtained (entries 13 and
14). Noteworthy to mention that although we have obtained rela-
tively less yields of coupling products with aryl chlorides com-
pared to aryl bromides as substrates, these results are quite
significant as we are able to use aryl chlorides as substrates in
the Suzuki–Miyaura reaction using environmentally-benign reac-
tion media using reasonably low catalyst loading (1 mol %). In gen-
eral aryl chlorides are difficult substrates for coupling reactions
because of stronger C–Cl bond strength. To activate such chlorides
most of the existing catalytic systems still rely on phosphines as li-
gands and usually operate at drastic reaction conditions (e.g., up to
130 °C, Pd: up to 4 mol %) in environmentally least preferred reac-
tion media such as DMF, toluene, etc.2a,2e,2h,2l Nevertheless, there
also exist very few outstanding catalysts derived from N-based li-
gands that smoothly performed cross-coupling reactions with aryl
chlorides.3b,3f,13
3. (a) Singh, R.; Viciu, M. S.; Kramareva, N.; Navarro, O.; Nolan, S. P. Org. Lett. 1829,
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Lei, X.; Yin, Z. Green Chem. 2010, 12, 1072; (g) Ogo, S.; Takebe, Y.; Uehara, K.;
Yamazaki, T.; Nakai, H.; Watanabe, Y.; Fukuzumi, S. Organometallics 2006, 25,
331; (h) Ines, B.; SanMartin, R.; Churruca, F.; Dominguez, E.; Urtiaga, M. K.;
Arriortua, M. I. Organometallics 2008, 27, 2833; (i) Liu, C.; Ni, Q.; Bao, F.; Qiu, J.
Green Chem. 2011, 13, 1260.
7. (a) Gupta, K. C.; Sutar, A. K. Coord. Chem. Rev. 2008, 252, 1420; (b) Cozzi, P. G.
Chem. Soc. Rev. 2004, 33, 410; (c) Kasselouri, S. J. Coord. Chem. 1998, 1–2, 55.
8. (a) Rao, G. K.; Kumar, A.; Kumar, B.; Kumar, D.; Singh, A. K. Dalton Trans. 1931,
2012, 41; (b) Kostas, I. D.; Steele, B. R.; Terzis, A.; Amosova, S. V.; Martynov, A.
V.; Makhaeva, N. A. Eur. J. Inorg. Chem. 2006, 2642; (c) Kylmälä, T.; Kuuloja, N.;
Xu, Y.; Rissanen, K.; Franzén, R. Eur. J. Org. Chem. 2008, 4019; (d) Tas, E.; Kilic,
A.; Durgun, M.; Yilmaz, I.; Ozdemir, I.; Gurbuz, N. J. Organomet. Chem. 2009,
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Zhang, M.; Zhang, Y. Inorg. Chem. Commun. 2010, 13, 81; (g) Lai, Y.-C.; Chen, H.-
Y.; Hung, W.-C.; Lin, C.-C.; Hong, F.-E. Tetrahedron 2005, 61, 9484; (h) Bowes, E.
G.; Lee, G. M.; Vogels, C. M.; Decken, A.; Westcott, S. A. Inorg. Chim. Acta 2011,
377, 84.
9. Kasselouri, S.; Garoufis, A.; Kalkanis, G. Trans. Metal Chem. 1993, 18, 531.
10. (a) Synthesis of complex 1: A solution of the ligand L (0.075 g, 2.62 mmol) in
20 ml acetonitrile was added dropwise to a solution of [Pd(OAc)2] (0.117 g,
2.62 mmol) in 15 ml acetonitrile. After refluxing the reaction mixture for 6 h,
the yellow precipitate was filtered. The residue was washed with hexane and
recrystallized from dichloromethane and finally complex 1 was obtained as a
bright yellow solid. Yield: 82%; Anal. Calcd for C22H20N4O4Pd: C: 51.72; H:
3.94; N: 10.96. Found: C, 51.91; H, 3.91; N, 10.92. MS-ESI (MeOH): m/z: 510
In conclusion, we have developed a simple and highly efficient
phosphine-free catalytic system that can perform the Suzuki–
Miyaura cross-coupling reactions of aryl bromides and iodides in
aqueous media at room temperature, and for aryl chlorides in
aqueous-glycerol at 80 °C. High product yields, use of environ-
ment-friendly reaction media, relatively mild reaction condition,
use of nitrogen-based ligand, no TBAB was required, etc. are some
of the most significant advantages of our present catalytic system.
[M]+; Selected IR frequencies (cmꢀ1, KBr): 1631 (
mC@N: imine), 1589 (mC@N:
pyridine), 1676 (
m
COO: acetate); 1H NMR (400 MHz, CDCl3) d/ppm: 2.18 (s,
6H,CH3) 8.58 (s, 2H, CH@N), 7.47 (d, 2H, Py, J = 8.0 Hz), 7.89–6.91 (m, 10H,
Ph+Py). 13C NMR (100.62 MHz, CDCl3) d/ppm: 182.01 (COO), 152.85 (CH@N),
23.54 (CH3), 124.59–150.61 (Ph+Py); (b) Synthesis of complex 2: Complex 2 was
prepared by following the same procedure for synthesizing complex 1 using
ligand
L and Pd(OAc)2 in 1:2 molar ratio. Yield: 86%; Anal. Calcd for
C
26H26N4O8Pd2: C: 42.46; H: 3.56; N: 7.61. Found: C, 42.26; H, 3.53; N, 7.58.
MS-ESI (MeOH): m/z: 734 [Mꢀ1]+; Selected IR frequencies (cmꢀ1, KBr): 1664
Acknowledgements
(mC@N: imine), 1581(mC@N: pyridine), 1698 (m
C@O: acetate); 1H NMR (400 MHz,
CDCl3) d (ppm): 2.21 (s, 6H, CH3), 2.14 (s, 6H, CH3) 9.01 (d, 2H, Py, J = 5.2 Hz),
8.34 (s, 2H, CH@N), 7.30–8.24 (m, 10H, Ph+Py); 13C NMR (100.62 MHz, CDCl3) d
(ppm) 198.01 (COO), 172.60 (CH@N), 29.70 (CH3), 22.60 (CH3) 125.60–158.01
(Ph+Py).
We gratefully acknowledge the University Grants Commission
(UGC), New Delhi for a research Grant (No: 36-73/2008 (SR)).
The SAIF, NEHU, Shillong is gratefully acknowledged for various
analytical services.
11. General procedure for the Suzuki–Miyaura reaction: A 50 ml round bottom flask
was charged with
a mixture of aryl halide (0.5 mmol), arylboronic acid
(0.55 mmol), K2CO3 (1.5 mmol), and Pd catalyst (0.2 mol% for aryl bromide or
1 mol% for aryl chloride) and the mixture was stirred for required times at
room temperature in water (6 ml) for aryl bromides/at 80 °C in aqueous-
glycerol (6 ml) for aryl chlorides. After completion, the reaction mixture was
diluted with water (20 ml) and extracted with ether (20 ml ꢁ 3). The combined
extract was washed with brine (20 ml ꢁ 3) and dried over Na2SO4. After
evaporation of the solvent under reduced pressure, the residue was
chromatographed (silica gel, ethyl acetate/hexane 1:9) to obtain the desired
product. The products were confirmed by comparing the 1H NMR and mass
spectral data with authentic samples.
References and notes
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