High catalytic activity in aqueous heck and Suzuki-Miyaura reactions catalyzed by novel Pd/Ln coordination polymers based on 2,2′-bipyridine-4,4′-dicarboxylic acid as a heteroleptic ligand

Article abstract of DOI:10.1016/j.poly.2016.04.032

Four novel heterobimetallic Pd/Ln coordination polymers, [LnPd(BPDC)_5/2(H₂O)·4H₂O]_n (Ln = Nd (1), Sm (2), Eu (3) and Dy (4); H₂BPDC = 2,2′-bipyridine-4,4′-dicarboxylic acid), have been synthesized und

Full text of DOI:10.1016/j.poly.2016.04.032

Polyhedron 115 (2016) 47–53
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
High catalytic activity in aqueous heck and Suzuki–Miyaura
reactions catalyzed by novel Pd/Ln coordination polymers
0
0
based on 2,2 -bipyridine-4,4 -dicarboxylic acid as a heteroleptic ligand
a
a
a
a
a
a
b
Lixin You , Wenli Zhu , Shuju Wang , Gang Xiong , Fu Ding , Baoyi Ren , Ileana Dragutan ,
b
a,
⇑
Valerian Dragutan , Yaguang Sun
a
The Key Laboratory of Inorganic Molecule-Based Chemistry of Liaoning Province, Shenyang University of Chemical Technology, Shenyang, China
Institute of Organic Chemistry, Romanian Academy, Bucharest, Romania
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Four novel heterobimetallic Pd/Ln coordination polymers, [LnPd(BPDC) (H O)ꢀ4H O] (Ln = Nd (1), Sm
5
/2
2
2
n
Received 18 November 2015
Accepted 15 April 2016
Available online 28 April 2016
0
0
(
2
2), Eu (3) and Dy (4); H BPDC = 2,2 -bipyridine-4,4 -dicarboxylic acid), have been synthesized under
hydrothermal conditions and characterized by X-ray crystallography, thermogravimetric (TG) analysis,
powder X-ray diffraction (PXRD) spectroscopy, elemental analysis and infrared spectroscopy (IR).
Single-crystal X-ray diffraction analysis revealed that compounds 1–4 are isomorphous, having a 2D layer
structure with a triclinic crystal system. The new Pd/Ln coordination polymers displayed impressively
high activity in Heck and Suzuki–Miyaura cross-coupling reactions under eco-friendly, aqueous
conditions.
Keywords:
Coordination polymers
Catalysis
Heck reaction
Suzuki–Miyaura reaction
Pd catalyst
Ó 2016 Elsevier Ltd. All rights reserved.
1
. Introduction
Over the past few decades, research in the field of coordination
organic synthesis [18]. Improvements in C–C cross-couplings have
been implemented using green solvents to reduce the generation of
environmentally hazardous waste [19]. Water is especially attrac-
tive as a green solvent because it is largely available, non-toxic,
non-flammable, inexpensive and can be easily separated from
organic products [20]. Among the different approaches aimed at
reaching environmentally-friendly reaction conditions, the most
often used C–C coupling procedures are based on mixtures of
organic solvents and water [21–23].
polymers (CPs) has become one of the fastest growing areas of
chemistry as these hybrid materials have intriguing structures
and are endowed with unique properties, recommending them
for multifarious applications [1]. Recent examples of such frame-
works centre on the development of functional materials capable
of inducing photocatalysis [2], photoluminescence [3], proton con-
duction [4] or which are suitable for chemical sensing [5], gas stor-
age [6], adsorptive separation [7], drug delivery, etc. [8]. On the
other hand, CPs have recently emerged as a versatile platform for
creating heterogeneous catalysts, e.g. based on Pd(II), Ru(II) and
Rh(II)-complexes [9,10], used in organic synthesis [11]. Palla-
dium-containing coordination polymers (Pd-CPs) have been inten-
sively investigated, owing to their powerful synthetic function and
key role in industrial processes [12]. Pd-CPs and other metal-CPs
catalysts have been used to trigger a range of reactions including
alcohol oxidation [13,14], olefin hydrogenation [15], in photocatal-
ysis [16] and carbon–carbon cross-couplings [17]. Of these, palla-
dium-catalyzed cross-coupling reactions, the most popular
method for forming C–C bonds, have become convenient tools in
In the present work, we disclose the synthesis and full struc-
tural characterization of four novel heterobimetallic coordination
polymers, namely [LnPd(BPDC)5/2(H
2
O)
2
ꢀ4H
2 n
O] (Ln = Nd (1), Sm
(2), Eu (3) and Dy (4)), which coordinate Pd(II) and Ln(III) ions with
0
0
2
the heteroleptic ligand 2,2 -bipyridine-4,4 -dicarboxylic acid (H -
BPDC). Moreover, we focus on the catalytic performance of the
newly synthesized compounds 1–4 in Heck and Suzuki–Miyaura
cross-couplings, proceeding in an environmentally benign reaction
2
medium (DMF-H O).
2
. Experimental
2.1. Materials and physical–chemical measurements
⇑
Corresponding author. Tel./fax: +86 24 89389359.
E-mail address: sunyaguang@syuct.edu.cn (Y. Sun).
All chemicals were purchased commercially and used without
further purification. The syntheses of 1–4 were carried out in
http://dx.doi.org/10.1016/j.poly.2016.04.032
277-5387/Ó 2016 Elsevier Ltd. All rights reserved.
0

4
8
L. You et al. / Polyhedron 115 (2016) 47–53
2
0 mL Teflon-lined autoclaves under autogenous pressure. The reac-
radiation (k = 0.71073 Å). An empirical absorption correction was
applied to the data using the SADABS program [24]. All the structures
were solved by direct methods and refined by full-matrix least
tion vessels were filled to approximately 50% volume capacity.
Water used in the synthesis was distilled before use. The C, H and
N elemental analyses were performed on a Perkin-Elmer 240C ele-
mental analyzer. FT-IR spectra, using KBr pellets, were recorded
with a Nicolet IR-470 spectrometer. TG analysis was carried out on
2
squares on F using the SHELXTL crystallographic software package
[25]. All non-hydrogen atoms were refined anisotropically. Hydro-
gen atoms were placed in calculated positions and refined using a
riding mode. Crystal and structural refinement data for 1–4 are dis-
played in Table 1 while important bond lengths and angles are
listed in Table S1.
ꢁ1
a SDT Q600 instrument with a heating rate of 10 °C min . Powder
X-ray diffraction (PXRD) patterns of the samples were collected on
an X-ray diffractometer (BRUKER D8 ADVANCE) with Cu K
tion. GC analyses were performed on an Agilent Technologies
890A gas chromatograph fitted with an HP-5 column
m). 1H NMR spectra were recorded on a
a radia-
7
2.4. Catalytic reactions
(
30 m ꢂ 320
l
m ꢂ 0.25
l
Bruker BioSpin GmbH AVANCE III 500 MHz spectrometer operating
2.4.1. General experimental procedure for the Heck reaction
at 500 MHz.
A
mixture of aryl halide (1.0 mmol), styrene (1.5 mmol),
OK (2.0 mmol), DMF-H O (1:1, 6 mL) and 0.4 mol% of
t-C
4
H
9
2
catalyst (1–4) was heated at 90 °C in air for 9 h. The mixture was
cooled to room temperature, filtered, washed with a saturated
NaCl aqueous solution (10 mL) and extracted with ethyl acetate
2
.2. Synthesis of complexes 1–4
The complexes 1–4 were synthesized under hydrothermal condi-
0
0
2 4
(20 mL). The organic phase was separated and dried over Na SO .
tions. K
2
PdCl
4
(0.145 mmol, 0.0473 g), 2,2 -bipyridine-4,4 -dicar-
ꢀ6H O (Ln = Nd
0.09 mmol, 0.0395 g), Sm (0.09 mmol, 0.0401 g), Eu (0.09 mmol,
All coupling products were purified by column chromatography
on silica gel and identified by Agilent 7890A-5975C GC–MS and
boxylic acid (0.153 mmol, 0.0374 g) and Ln(NO
(
3
)
3
2
1
H NMR spectra. Yields calculated from GC were based on the
0
.0403 g) or Dy (0.09 mmol, 0.0402 g) were mixed in Teflon-lined
O and 6 mL NaOH
0.1 mol L ); the mixture was placed in an oven and heated at
5 °C for 96 h, then cooled to room temperature. The resulting light
amount of aryl halide employed.
vessels filled with a solution of 6 mL H
(
9
2
ꢁ
1
2.4.2. General experimental procedure for the Suzuki coupling reaction
yellow diamond crystals were filtered, washed thoroughly with dis-
tilledwateranddriedinair. Theyieldsofthecomplexes, basedonthe
rare earth nitrate, are: 1, 37%; 2, 41%; 3, 38%; 4, 37%. Elemental Anal.
A
mixture of aryl halide (1.0 mmol), arylboronic acid
(1.2 mmol), K CO (2.0 mmol), DMF-H O (1:1, 6 mL) and 0.4 mol%
2
3
2
of catalyst was stirred at 80 °C in air for 8 h. The mixture was
Calc. for 1 (C30
3
3
H
25
N
5
O
15PdNd): C, 38.08; H, 2.66; N, 7.40. Found: C,
8.05; H, 2.69; N 7.39%. Anal. Calc. for 2 (C30 15PdSm): C,
7.84; H, 2.65; N, 7.35. Found: C, 37.86; H, 2.59; N, 7.37%. Anal. Calc.
15PdEu): C, 37.77; H, 2.64; N, 7.34. Found: C, 37.83;
H, 2.69; N, 7.37%. Anal. Calc. for 4 (C30 15PdDy): C, 37.36; H,
extracted with diethyl ether (20 mL), washed with water and dried
over anhydrous Na SO . All coupling products were purified by
2 4
H
25
N
5
O
column chromatography on silica gel and identified by gas
chromatography-mass spectrometry (Agilent 7890A-5975C GC–MS)
25 5
for 3 (C30H N O
1
H
25
N
5
O
and from H NMR spectra. GC–calculated yields were based on
ꢁ1
2
1
3
3
1
.61; N, 7.26. Found: C, 37.39; H, 2.69; N, 7.29%. IR (KBr, cm ) for
the amount of aryl halide employed.
: 3422vs, 1638vs, 1541w, 1366vs, 1242w, 783w, 703 m; for 2:
414vs, 1638 s, 1555w, 1384.vs 1244w, 784 m, 704 m; for 3:
423vs, 1638vs, 1541w, 1384 s, 704 m; for 4: 3400 s, 1615 s,
551 m, 1408w, 1384vs, 1291w, 1244w, 1065w, 778 m, 702 m.
3
. Results and discussion
3.1. Crystallographic structure and characterization of compounds 1–4
2
.3. X-ray crystallography
X-ray crystal structure analysis revealed that compounds 1–4
ꢀ
are isomorphous and crystallize in the space group P1. Therefore,
only the structure of 2 will be described in detail. As illustrated
in Fig. 1a, the unit cell of 2 contains one Sm(III) ion, one Pd(II)
Crystallographic data were collected on a Bruker Smart Apex
CCD diffractometer applying graphite-monochromated Mo K
a
Table 1
Crystal data and structure refinement for 1–4.
1
2
3
4
Formula
Fw
T (K)
C
30
H
25
N
5
O
15NdPd
C
30
H
25
N
5
O
15SmPd
C
30
H
25
N
5
O
15EuPd
C
30 25 5
H N O15DyPd
946.19
293
952.30
293
954.09
293
964.45
293
Cryst. system
Space group
a (Å)
triclinic
P1ꢀ
9.677(5)
triclinic
P1ꢀ
9.657(4)
triclinic
P1ꢀ
9.652(4)
triclinic
P1ꢀ
9.665(6)
b (Å)
c (Å)
13.760(6)
13.946(8)
116.77(2)
94.29(4)
106.85(3)
1540.2(13)
2
13.729(5)
13.939(7)
13.716(4)
13.913(5)
116.54(2)
94.69(3)
106.526(19)
1532.2(10)
2
13.728(7)
13.892(10)
116.31(2)
94.77(4)
106.56(3)
1535.9(16)
2
a
(°)
b (°)
(°)
116.537(14)
94.59(3)
106.64(2)
1536.7(11)
2
c
3
V (Å)
Z
q
l
calc (g cmꢁ3)
(mm
2.040
2.338
2.058
2.565
2.068
2.703
2.085
3.087
ꢁ
1
)
F(000)
932.0
936.0
938.0
944.0
Reflns collected/unique
15874/7005
0.808
0.0532
15925/7033
1.045
0.0420
15852/7030
1.042
0.0388
0.0920
15934/7027
1.078
0.0628
Goodness-of-fit (GOF)
a
R
1
(I > 2
r
(I))
b
wR
2
0.1077
0.0930
0.1252
a
|. bwR
2
2
2
2
)2]}1/2.
R
1
=
R
||F
o
| ꢁ |F
c
||/
R
|F
o
2
= {
R
[w(F
o
ꢁ F
c o
) ]/R[w(F

L. You et al. / Polyhedron 115 (2016) 47–53
49
ion, 5/2 BPDC2 ligands, one coordinated water molecule and four
lattice water molecules. The Pd(II) ion connects four N atoms from
the bipyridine rings of two ligands in a square planar coordination
ꢁ
Table 2
Optimization of the reaction conditions for the Heck cross-coupling of 4-bromoace-
tophenone with styrene, catalyzed by 2.
a
2
mode to form a Pd(BPDC) building block, with an average Pd–N
O
O
2
,Solvent
o
bond distance of 2.046(4) Å. Each Sm(III) ion is coordinated by nine
O atoms in a distorted monocapped square antiprismatic geome-
try, one being from the coordinated water and eight from the car-
boxyl groups of four different BPDC2 ligands, with Sm–O
distances around 2.35–2.65 Å. Two neighbouring Sm(III) ions are
bridged by two carboxyl oxygen atoms (from two ligands) to form
Br +
T/ C,Base
ꢁ
Yield (%)b
Entry
Solvent
Toluene
Base
T (°C)
Time (h)
1
2
3
4
5
6
7
8
9
t-C
t-C
t-C
t-C
4
H
4
H
4
H
4
H
9
9
9
9
OK
OK
OK
OK
90
90
90
90
90
90
90
90
90
100
50
90
90
9
9
9
9
9
9
12
6
3
9
21
9
2
H O
a dimer (Sm
Sm 16 dimers are connected alternatively by two Pd(BPDC)
one BPDC
Fig. 1b). The 1D chains are further connected by the carboxyl
groups of Pd(BPDC) to construct a 2D layer (Fig. 1c). In addition,
2
O
16) with an Sm–Sm distance of 3.9809(13) Å. The
and
ligand to form a 1D linear chain along the c axis
DMF
32
92
85
78
93
72
51
87
21
0
2
O
2
DMF-H
DMF-H
DMF-H
DMF-H
DMF-H
2
2
2
2
2
O
O
O
O
O
2ꢁ
K
Cs
2
CO
CO
3
2
3
(
t-C
t-C
4
H
H
9
OK
OK
2
4
9
the 2D layers are assembled into a 3D framework via hydrogen
bond interactions (Fig. S1).
DMF-H O
t-C H OK
2
4
4
4
9
9
9
10
DMF-H
DMF-H
DMF-H
DMF-H
2
2
2
2
O
O
O
O
t-C
t-C
H
H
OK
OK
1
1
1
1
2
3
9
9
9
None
t-C
c
4
H
9
OK
79
3.2. Thermogravimetric (TG) analysis and powder X-ray diffraction
(
PXRD)
a
Reaction conditions: 4-bromoacetophenone (1.0 mmol), styrene (1.2 mmol),
catalyst 2 (0.4 mol%).
b
Yields were determined by GC analysis.
Thermogravimetric (TG) analysis was performed in a nitrogen
c
Pd(H
2
BPDC)Cl
2
2
ꢀH O (0.4 mol%) was used to replace 2.
atmosphere on polycrystalline samples of compounds 1–4; the
TG curves are shown in Fig. S2. As the curves of the four com-
pounds are similar, only that of catalyst 2 will be discussed in
detail as a representative example. The first weight loss of 2, occur-
ring from 50 to 180 °C, is about 9.37%, corresponding to the loss of
coordinated and free water molecules (calcd: 9.45%). This demon-
strates the high stability of 1–4, degrading only at considerably
high temperatures.
indeed representative for the main constituent of the correspond-
ing sample and that 1–4 retain considerable stability up to 250 °C.
3.3. Catalytic activities
Powder X-ray diffraction (PXRD) analysis of compounds 1–4
3.3.1. Heck cross-coupling reaction
was performed at room temperature, 50, 90, 180 and 250 °C
The performance of the Heck cross-coupling reaction is depen-
dent on a variety of parameters, such as the substrate nature, the
catalyst type and its structure, the solvent, base, and temperature.
Complex 2 was chosen as a model to investigate the catalytic activ-
(Figs. S3 and S4). The patterns obtained for 1–4 are in good agree-
ment with the calculated ones resulting from the single crystal
structures; this indicates that every single crystal structure is
Pd(BPDC)2
(a)
(b)
Sm O
2
16
(c)
Fig. 1. (a) View of the coordination environments of the Sm(III) and Pd(II) ions in 2. All hydrogen atoms and free lattice water molecules are omitted for clarity. Symmetry
codes: (A) ꢁx, ꢁy, ꢁ1 ꢁ z; (B) x, y, ꢁ1 + z; (C) ꢁ1 + x, y, ꢁ1 + z; (D) 1 ꢁ x, ꢁy, ꢁz; (E) ꢁx, ꢁy, ꢁz; (F) ꢁ1 ꢁ x, ꢁy, ꢁ1 ꢁ z. (b) One-dimensional chain of 2. (c) Two-dimensional
layer of 2.

5
0
L. You et al. / Polyhedron 115 (2016) 47–53
ity in the Heck cross-coupling reaction. The reaction conditions
were systematically optimized in the standard reaction of 4-bro-
moacetophenone with styrene; the results are presented in Table 2.
Optimization of the solvent was first performed using toluene,
DMF and water under identical reaction conditions. It was found
that among the tested solvents unsatisfactory yields were obtained
with single solvents (entries 1–3). However, a 1:1 mixture of DMF-
increase when the reaction time was prolonged from 3 to 9 h
(entries 4, 8 and 9), but the yield improvement was almost negli-
gible when the time was lengthened to 12 h (entry 7). An advanta-
geous cooperation operating between palladium and the
lanthanides is apparent when comparing the yields of the Heck
reactions catalyzed by 2 and Pd(H
2
BPDC)Cl
2
2
ꢀH O (92 versus 79%;
Table 2, Entries 4 and 13). The substantially better performance
H
2
O proved an efficient system for the cross-coupling, making the
yields improve significantly (entry 4). We further investigated the
effect of different bases (Cs CO , t-C OK and K CO ) using DMF-
O (1:1, vol.), which was found in this research to be the best sol-
vent combination. The results indicated that t-C OK was the
most efficient base (entries 4–6), and that the reaction cannot be
carried out in the absence of t-C OK (entry 12). On the other
of catalyst 2 is still more evident if we take into account that 2
(PdSm(BPDC)5/2(H
2
O)ꢀ4H
2
O) has less than half the Pd weight con-
O. That lanthanides play a significant role
2
3
4
H
9
2
3
tent of Pd(H BPDC)Cl
2
2
ꢀH
2
H
2
in C–C cross-couplings promoted by 3D Pd/Ln coordination poly-
mers (Ln = Pr, Gd, Tb) has already been demonstrated in our recent
study on Suzuki–Miyaura, Heck and Sonogashira reactions [17b].
In summary, the optimal conditions found for the Heck cross-
4
H
9
4
H
9
hand, studies on the effects of reaction temperature and time, con-
ducted using complex 2 under the optimal conditions, revealed
that the yield was only 21% after 9 h of reaction at 50 °C (entry
4 9 2
coupling reaction using 2 are t-C H OK as the base, DMF-H O
(1:1) as the solvent, a temperature of 90 °C and a 9 h reaction time.
With the optimized reaction conditions in hand, the limitations
of this transformation were studied (Table 3). A series of aryl
halides and styrene have been employed as reactants in the Heck
coupling to evaluate the catalytic performance of the Pd/Ln-CPs.
1
1), as compared with a yield of 92% at 90 °C (entry 4). However,
the yield was found to decrease to 87% when the temperature
was raised to 100 °C (entry 10). The yields had a tendency to
Table 3
Heck cross-coupling reaction of aryl halides and styrene using Pd/Ln-CPs 1–4.a
Pd/Ln, t-C
4 9
H OK
R
X
+
R
DMF-H
2
O
Entry
1
Catalystb
Aryl halide
Time (h)
9
Product
Yield (%)c
98
1
O
O
O
O
O
I
2
3
4
5
6
7
8
9
2
3
4
2
2
2
2
2
2
2
2
9
9
99
98
96
99
83
87
90
58
9
O
O
O
I
I
9
I
I
9
9
I
9
Br
O
9
O
Br
Br
9
10
11
12
12
9
Cl
O
12
Trace
O
Cl
Cl
12
a
b
c
Reaction conditions: aryl halide (1.0 mmol), styrene (1.5 mmol), K
[Pd/Nd-CPs], 2 [Pd/Sm-CPs], 3 [Pd/Eu-CPs], 4 [Pd/Dy-CPs].
Yields were determined by GC analysis.
2 3 4 9 2
CO (2.0 mmol), t-C H OK (2.0 mmol), DMF-H O (1:1, 6 mL), catalyst (0.4 mol%), 90 °C.
1

L. You et al. / Polyhedron 115 (2016) 47–53
51
Table 4
Suzuki cross-coupling reaction of aryl halides and arylboronic acids using catalyst 2.
a
Catalyst, Base
R
X +
B(OH)
2
R
2
DMF-H O
Entry
1
Catalystb
Aryl halide
Time (h)
7
T (°C)
Product
Yield (%)c
97
1
O
O
O
O
80
O
I
2
3
4
2
3
4
7
7
7
80
80
80
O
O
O
98
97
95
I
I
I
5
6
7
8
9
2
2
2
2
2
7
9
9
9
9
80
80
80
80
80
98
I
82
I
73
Br
Br
62 d
78
O
O
Br
Br
Cl
10
11
12
2
2
2
9
12
12
80
90
90
62
18
31
O
O
Cl
Cl
1
3
2
12
90
7
a
b
c
Reaction conditions: aryl halide (1.0 mmol), arylboronic acid (1.2 mmol), K
[Pd/Nd-CPs], 2 [Pd/Sm-CPs], 3 [Pd/Eu-CPs], 4 [Pd/Dy-CPs].
Yields were determined by GC analysis.
2
CO
3
(2.0 mmol), DMF-H
2
O (1:1, 6 mL), catalyst (0.4 mol%).
1
d
4 9 2
Reaction conditions: bromobenzene (1.0 mmol), t-C H OK (2.0 mmol), DMF-H O (1:1, 6 mL), catalyst 2 (0.4 mol%).
As shown in Table 3, complexes 1–4 displayed approximately the
same catalytic activity, leading to similar yields (Entries 1–4).
Therefore, complex 2 was chosen as a model catalyst for further
Heck reactions. The coupling of styrene with iodobenzene afforded
the expected trans-stilbene in very high yield (98%; Entry 5) while
the reaction with bromobenzene also led to a good yield (87%;
Entry 7). However, a pronounced drop in activity was observed
for the sluggish chlorobenzene (9%; Entry 10), even after a reaction
time of 12 h. As should have been expected, the catalytic activity
decreases from iodobenzene to bromobenzene and chlorobenzene
because of the ever enhanced difficulty in activating the C–halide
bond which is essential in the oxidative addition step of the cat-
alytic cycle. The presence of a methyl group on the aromatic halide
has a negative effect on the reaction yield (Entries 6 versus 5; 9
versus 7; 12 versus 10), suggesting that electron-donating sub-
stituents impede C–halide bond activation, thus resulting in a poor
reactivity. Conversely, the presence of electron-withdrawing sub-
stituents on the aromatic halide greatly promotes the cross-cou-
pling reaction leading to almost quantitative yields (99% for
acetyl on 4-iodoacetophenone and 90% for acetyl on 4-bromoace-
tophenone; entries 2 and 8, respectively). Yet the effect of such
substituents is much diminished in the case of aromatic chlorides
the oxidative addition step from the palladium catalytic cycle of
1–4. On the contrary, the reductive elimination step appears to
be greatly influenced by the base, the aqueous solvent and also
the lanthanide, endowed with a strong reducing ability stemming
from its electropositive nature.
3.3.2. Suzuki–Miyaura cross-coupling reaction
To our delight, catalysts 1–4 were also active in promoting the
Suzuki–Miyaura cross-coupling reaction, displaying similar cat-
alytic profiles (Table 4, Entries 1–4). As shown in this table, com-
pound 2 readily catalyzes Suzuki–Miyaura coupling reactions in
DMF-H O (1:1 vol.), at a low catalyst loading (0.4 mol%). In the case
2
of this cross-coupling we found that K
2 3
CO is a much better base
than t-C H OK, a result contrasting with data from the Heck reac-
4
9
tion with the same catalyst 2 (Entries 7 versus 8). Then, reaction
parameters such as substrate, time and temperature were varied
to establish their influence on the product yields. The nature of
the aryl halide plays a key role. The highest yields were obtained
for aryl iodides (98%; entries 2 and 5 for iodobenzene and 4-
iodoacetophenone after 7 h at 80 °C). However, for bromobenzene
and 4-bromoacetophenone a longer time (9 h) was needed to
obtain substantial yields (73% and 78%; entries 7 and 9, respec-
tively). A similar trend of the electronic effects played by sub-
stituents on the aryl halides as in the Heck reactions was
observed in Suzuki cross-couplings catalyzed by 2. Electron-with-
(
just 12% yield versus 9% yield; entry 11, versus 10).
The above results on the substrate behaviour can be rational-
ized by considering the involvement of the aryl halide bond in

5
2
L. You et al. / Polyhedron 115 (2016) 47–53
drawing substituents vigorously promote the reaction (exception-
ally high yields of 98%) while the reverse holds true for electron-
releasing substituents. Overall, substrate reactivity decreases
in the following order of the substituents on the aryl halide:
Appendix A. Supplementary data
CCDC 1432828 (1), 1432829 (2), 1432827 (3) and 1432826 (4)
contains the supplementary crystallographic data. 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 the online version, at
http://dx.doi.org/10.1016/j.poly.2016.04.032.
–
3 3
CH CO > –H > –CH . Notwithstanding, the type of halide prevails
over the role of the substituent with which it is equipped (biaryl
yields of 98% for iodobenzene, after 7 h, and 73% for bromoben-
zene, after 9 h, whereas the difference in yields is smaller for the
methyl-substituted counterparts: 82% vs. 62% for iodotoluene vs.
bromotoluene. Table 4). Consequently, the electronic effect of the
substituent is more easily noticeable on the less reactive aryl
halides (yields of 31% for 4-acetyl-chlorobenzene, 18% for
chlorobenzene and just 7% for 4-methyl-chlorobenzene, at the
same temperature and reaction time; entries 11–13).
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This work was conducted in the framework of a project spon-
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