Catalytic activity of some palladium complexes of a phosphorus ylide and the structure of a 2-phenylaniline-based palladacycle complex

Article abstract of DOI:10.1016/j.poly.2013.04.055

A six-membered N,C-palladacycle bearing the 2-phenylaniline bidentate ligand (1) and three other palladium complexes of a phosphorus ylide (2, 3 and 4) were utilized in Suzuki reactions between phenylboronic acid and a number of aryl bromides and chloride

Full text of DOI:10.1016/j.poly.2013.04.055

Polyhedron 59 (2013) 133–137
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Polyhedron
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Catalytic activity of some palladium complexes of a phosphorus ylide
and the structure of a 2-phenylaniline-based palladacycle complex
Kazem Karami a, , Nasser Rahimi , Corrado Rizzoli b
⇑
a
a
Department of Chemistry, Isfahan University of Technology, Isfahan 84156/83111, Iran
Department of General and Inorganic Chemistry, University of Parma, I-43124 Parma, Italy
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A six-membered N,C-palladacycle bearing the 2-phenylaniline bidentate ligand (1) and three other pal-
ladium complexes of a phosphorus ylide (2, 3 and 4) were utilized in Suzuki reactions between phenyl-
boronic acid and a number of aryl bromides and chlorides. The complexes were found to be capable
homogeneous catalysts for a variety of substrates, affording the coupled products in good to excellent
yields. The structure of compound 1, determined by X-ray diffraction, is also reported.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 26 November 2012
Accepted 27 April 2013
Available online 11 May 2013
Keywords:
Cyclopalladated complex
Aryl halide
Suzuki reaction
Crystal structure
1
. Introduction
The palladium-catalyzed Suzuki reaction for the formation of
ligands. Palladacycles 2, 3 and 4 (Fig. 1) were found to be active
precatalysts for Suzuki reactions. However, phosphorus-based
ancillary catalysts are frequently air and moisture sensitive, toxic
and need an inert atmosphere and gentle reaction conditions.
On the other hand, besides diverse aryl bromides, employing
aryl chlorides for this reaction has been a focus because aryl chlo-
rides are cheaper and more accessible than their bromides and io-
dide counterparts [35]. Therefore, the investigation for novel
palladium catalysts has received much attention, mainly for the
use of less reactive aryl chlorides as substrates under mild reaction
conditions. In continuation of our interest in the development of
employing new palladium complexes in C–C bond forming reac-
tions [36,37], we report here the efficiency of palladacycle 1 [38]
and some other cyclopalladated phosphorus ylide complexes, 2–
4, in Suzuki C–C cross-coupling reactions for both aryl bromides
and chlorides. Furthermore, the detailed structure of 1, a highly
air and moisture stable palladium complex, was determined by
X-ray single crystal analysis.
C–C bonds has appeared as an extremely powerful method for
the preparation of unsymmetrical biaryls [1–5]. Indeed, various
proficient Pd catalyst precursors have been developed in recent
years that allow aryl halides to be effectively coupled with aryl
boronic acids under aerobic conditions [2,4,5] or even in aqueous
solutions [3,5]. For this reason, palladacycles were found to be
the most talented catalysts in this regard as they have been exten-
sively studied during the last few decades [6]. They have signifi-
cant application in organic and organometallic synthesis, chiral
recognition and in homogeneous catalysis [7–10]. Since the first
report on the use of a palladacycle for the Suzuki reaction [11], a
broad diversity of novel palladacycles resulting from the cyclopal-
ladation of phosphines, phosphites, phosphinites, amines, imines,
oximes and thioethers have been effectively employed in this reac-
tion [12–17].
The use of nitrogen-based ligands has been reported to demon-
strate the catalytic activity in Suzuki and Heck reactions [18–29],
which drew our attention to study the catalytic activity of 1
2
. Experimental
(
Fig. 1) in homogenous media. Also, electron-rich, bulky phos-
2
.1. Materials and techniques
phines [30–33] and phosphine oxides [34] have been reported to
be successful ligands. Moreover, among the several classes of
phosphine ligands described in the literature for Pd-catalyzed
Suzuki reactions, we became interested in phosphorus ylide
All chemicals and solvents were purchased from Merck and
Aldrich. Complexes 2–4 were prepared by the method reported
in our previous work [41]. Conversions were monitored using an
Agilent 6890N gas chromatograph equipped with a capillary HP-
+
⇑
Corresponding author. Tel.: +98 3113912351; fax: +98 3113912350.
E-mail address: karami@cc.iut.ac.ir (K. Karami).
5
column, based on aryl halides. The column properties were:
m film thick.
30 m length, 0.32 mm in inner diameter and 0.25
l
0
277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.04.055

1
34
K. Karami et al. / Polyhedron 59 (2013) 133–137
Fig. 1. Palladacycles employed in Suzuki reactions.
2.2. X-ray crystal structure determination
Crystals suitable for X-ray molecular structure determination of
1
were obtained by the diffusion method for a solution of 1 in ace-
tone/n-hexane (1:3 v/v). Intensity data were collected at ambient
temperature (295(2) K) using graphite-monochromated MoK
a
radiation (k = 0.71073 Å) on a Bruker APEX-II CCD diffractometer.
Data were corrected for absorption using the SABABS program [39].
All non-hydrogen atoms were refined anisotropically. The amine
H atoms were located in a difference Fourier map and refined
freely. All other H atoms were calculated geometrically and refined
using a riding/rotating model, with C–H = 0.93–0.96 Å and with
Uiso(H) = 1.2 Ueq(C) or 1.5 Ueq(C) for methyl H atoms. The F1, F2
and F3 atoms of the tetrafluoroborate anion were disordered over
two orientations (called A and B) with refined occupancies of
0
.621(9) and 0.379(9) for the major and minor components,
respectively. During the refinement, the geometry of these atoms
was restrained to be similar by the SAME instruction of SHELXL-97
[
40].
2.3. General protocol for the Suzuki reaction
Fig. 2. X-ray molecular structure of 1, with displacement ellipsoids drawn at the
0% probability level. Only the major components of the disordered fluorine atoms
are shown.
A round bottom flask was charged with the aryl halide
0.5 mmol), phenylboronic acid (0.75 mmol), base (1.5 or 3 mmol),
5
(
catalyst (0.01 mmol) and solvent (7 mL), although the amount of
the reagents and catalysts was diminished to half when catalysts
2–4 were utilized. The mixtures were heated for the times 2, 4, 8
Table 1
and 24 h and at the temperatures 85 and 100 °C given in the tables
using the bases and catalysts there indicated. The products were
characterized by GC.
Selected bond distances (Å) and angles (°) for 1.
Pd1–C1
2.007 (3)
Pd1–N1
Pd1–N3
Pd1–N2
N1–Pd1–N3
C1–Pd1–N2
C1–Pd1–N1
C1–Pd1–N3
N1–Pd1–N2
N3–Pd1–N2
2.054 (2)
2.061 (2)
2.125 (2)
177.89 (10)
177.83 (10)
82.92 (11)
99.18 (10)
99.21 (10)
78.68 (8)
3
. Results and discussion
3.1. Molecular structure of 1
The single crystal X-ray diffraction study confirmed the details
of the structure proposed. Fig. 2 shows the ORTEP plot of compound

K. Karami et al. / Polyhedron 59 (2013) 133–137
135
Table 2
The asymmetric unit of 1 consists of a complex cation and a tetra-
fluoroborate anion rotationally disordered over two orientations.
The palladium metal exhibits a slightly distorted square-planar
geometry (maximum displacement 0.007(3) Å for atom C1), as
indicated by the angles subtended by the ligands at the Pd(II) cen-
tre, varying from 78.68(8)° to 99.21(10)° and from 177.83(10)° to
Crystallographic data and structure refinement details for 1.
Empirical formula
Formula weight
T (K)
Radiation (k, Å)
Crystal system
Space group
a (Å)
C
22
H
18
N
3
PdꢀBF
4
517.60
295
Mo Ka (0.71073)
Orthorhombic
Pbca
10.4380 (19)
19.695 (4)
20.790 (4)
4273.9 (14)
8
1
77.89(10)°. The summation of the bond angles around the palla-
dium is 359.99°. The metal protrudes by 0.0018(2) Å from the
mean coordination plane. The Pd1–C1 bond distance (2.007(3) Å)
is similar to that found in a related orthopalladated complex
b (Å)
c (Å)
3
V (Å )
[
38], and the Pd1–N3 distance also falls in the common range
Z
3
D
calc (Mg/m )
1.609
0.92
found for this bond. In contrast to that observed in a related Pd
ꢁ
1
0
l
(mm
)
complex containing the 2,2 -bipyridine ligand [41], the Pd1–N1
3
Crystal size (mm )
0.13 ꢂ 0.09 ꢂ 0.05
(
(
2.054(2) Å) bond length is significantly shorter than the Pd1–N2
2.125(2) Å) distance. The dihedral angles formed by the aromatic
No. of reflections measured
No. of independent reflections
No. of restraints/parameters
40765
3876
3/316
0.026
0.073
0.037
1.02
rings of the biphenyl and bipyridine ligands are 31.20(9)° and
12.08(9)°, respectively. The six-membered chelate ring assumes a
half-chair conformation, with the atoms Pd1 and N1 displaced by
2
2
R[F > 2(F )]
2
wR(F )
R
S
int
0
.8029(2) and 0.512(3) Å, respectively, from opposite sides of the
C1/C6/C7/C12 mean plane. The five-membered chelate ring adopts
a flattened envelope conformation, with the atom N3 displaced by
0
.172(2) Å from the Pd1/N2/C17/C18 mean plane (maximum devi-
1
. Selected bond distances and angles are listed in Table 1; crystal-
ation 0.002(2) Å for atom C17). As illustrated in Fig. 3, in the crystal
the cations and anions are linked into a three-dimensional network
by N–Hꢀ ꢀ ꢀO and C–Hꢀ ꢀ ꢀO hydrogen bonds.
lographic data and parameters concerning the data collection and
structure solution and refinement are summarized in Table 2.
Fig. 3. Packing diagram of 1. Dashed lines represent the hydrogen bonding interactions. The minor components of the disordered fluorine atoms are omitted.

1
36
K. Karami et al. / Polyhedron 59 (2013) 133–137
3.2. Suzuki reaction for aryl bromides
showed promising catalytic activity in the reactions of 1-bromo-
naphthalene (entry 1) and 3-bromotoluene (entry 7) with phenyl-
The catalytic applicability of Pd(II) complexes 1 and 2 was eval-
boronic acid. In spite of using 0.02 mmol of 1 and 3 mmol of
uated for the Suzuki reaction for aryl bromides. The efficiency of 1
in the Suzuki reactions of a variety of arylbromides and chlorides
with phenylboronic acid was compared with the palladacyclic
complexes bearing phosphorus ylide. The optimized reaction con-
ditions obtained in our previous investigation [36] were utilized
within this work. However, because of the low to moderate yields
of the reactions using 1 as catalyst, the reaction time and temper-
ature were increased to 2 h and 85 °C. The couplings took place
K
3
PO
4
ꢀ3H
2
O, the coupling of 2-bromo-1, 4-dimethylbenzene was
poor (entry 9). 1 Showed low and moderate catalytic activities in
the reactions of 2-bromopyridine and 2-bromothiophene with
phenylboronoc acid, respectively (entries 3 and 5). The rate of cou-
pling for each of these reactions was higher than that observed
when complex 2 was used (entries 4 and 6). Generally, 2 was found
to be more effective than 1 in all reactions, especially in the cou-
plings of less reactive aryl bromides with phenylboronic acid at
low catalyst loading (entries 8 and 10). This distinction may be
due to the stability of the five membered ring of the palladacycle
in 2 and/or the enhanced solubility of the reagents and reduction
with 1 and 2 employing Na
2 3
CO as a base for neutral substrates
and K PO O for electron-deficient and rich substrates. Table 3
3
4
ꢀ3H
2
summarizes our results for the Suzuki reactions. Complex 1
Table 3
a
The Suzuki reaction of aryl bromides with phenylboronic acid .
Entry
Aryl bromide
Catalyst
Base
Product
Yield (%)
1
2
1
2
2
Na CO
3
90.6
99
3
4
1
2
Na
2
CO
CO
3
3
26.5
85.1
5
6
1
2
Na
2
58.3
96.3
7
8
1
2
K
3
PO
PO
4
ꢀ3H
ꢀ3H
2
O
O
82.2
90.3
9
0
1b
2
K
3
4
2
19.6
91.1
1
a
Reaction conditions: aryl bromide (0.5 mmol), phenylboronic acid (0.75 mmol), catalyst (0.01 mmol), base (1.5 mmol), MeOH (7 ml), 2 h, 85 °C.
Catalyst (0.02 mmol), base (3 mmol).
b
Table 4
a
Suzuki reactions of aryl chlorides with phenylboronic acid.
Entry
Aryl chloride
Catalyst
Base
Na CO
T (°C)
t (h)
Product
Yield (%)
1
2
3
4
5
6
7
8
9
1
2
3
4
1
2
3
4
2
3
100
85
85
85
100
85
85
85
100
85
85
8
2
4
4
8
4
4
4
24
4
33.6
47.5
60.4
48.9
19
35.8
9.3
1.9
73.8
71
75.7
76.5
50.6
67.8
K
K
K
3
PO
3
PO
3
PO
4
4
4
ꢀ3H
2
O
2
O
2
O
b
1
ꢀ3H
ꢀ3H
10
11
12
13
14
2
3
4
2
3
4
4
4
4
85
85
85
a
Reaction conditions: aryl chloride (0.5 mmol), phenylboronic acid (0.75 mmol), catalyst (0.01 mmol), base (1.5 mmol), MeOH (7 ml).
Catalyst (0.02 mmol).
b

K. Karami et al. / Polyhedron 59 (2013) 133–137
137
of Pd(II) to Pd(0), thus providing a simpler mechanism to the cata-
lytic cycle.
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.
3.3. Suzuki reaction for aryl chlorides
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Acknowledgments
Funding of our research from the Isfahan University of Technol-
ogy is gratefully acknowledged. Also, the authors are grateful to the
Department of General and Inorganic Chemistry, University of Par-
ma for X-ray analysis.
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Appendix A. Supplementary data
CCDC 892995 contains the supplementary crystallographic data
for complex 1. These data can be obtained free of charge via http://