Synthesis and characterization of triphenylphosphine adducts of ferrocene-based palladacycles and their performance in the suzuki and sonogashira reactions with bromo- and chloroarenes

Article abstract of DOI:10.3390/molecules17055532

A new triphenylphosphine adduct of cyclopalladated ferrocenylpyridazine containing a chloride anion, 2a, has been synthesized from the reaction of the chloride-bridged palladacyclic dimer 1a with triphenylphosphine. The corresponding adducts 3a,b containi

Full text of DOI:10.3390/molecules17055532

Molecules 2012, 17, 5532-5543; doi:10.3390/molecules17055532
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Synthesis and Characterization of Triphenylphosphine
Adducts of Ferrocene-Based Palladacycles and Their
Performance in the Suzuki and Sonogashira Reactions
with Bromo- and Chloroarenes
1
2,
2
2
2
Su-Zhen Bai , Chen Xu *, Hong-Mei Li , Zhi-Qiang Wang and Wei-Jun Fu
1
College of Chemistry and Chemical Engineering, Pingdingshan University, Pingdingshan,
Henan 467002, China
2
College of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang,
Henan 471022, China
*
Author to whom correspondence should be addressed; E-Mail: xubohan@163.com;
Tel.: +86-379-6551-5113.
Received: 12 April 2012; in revised form: 3 May 2012 / Accepted: 4 May 2012 /
Published: 9 May 2012
Abstract: A new triphenylphosphine adduct of cyclopalladated ferrocenylpyridazine
containing a chloride anion, 2a, has been synthesized from the reaction of the
chloride-bridged palladacyclic dimer 1a with triphenylphosphine. The corresponding
adducts 3a,b containing iodide anion have been readily prepared through anion exchange
reactions of 2a,b with NaI in acetone. These complexes were characterized by elemental
1
analysis, IR and H-NMR. Additionally, their crystal structures have been determined by
X-ray diffraction and intermolecular C–H···X (Cl, Br, I) bonds were found in the crystals.
The use of these palladacycles as catalysts for the Suzuki and Sonogashira reactions was
examined. The complexes 2a,b exhibited higher catalytic activity than the corresponding
3
a,b in the Suzuki reaction. However, the order of activity of adducts with varying halogen
anions is 3a~3b > 2a~2b in the Sonogashira reaction.
Keywords: adduct; palladacycle; crystal structure; Suzuki reaction; Sonogashira reaction

Molecules 2012, 17
. Introduction
Palladium-catalyzed coupling reactions such as the Suzuki and Sonogashira reactions have become
5533
1
an extremely powerful method for the formation of carbon-carbon bonds in organic synthesis [1–3]. A
number of Pd catalyst precursors, usually simple palladium salts or complexes associated with
appropriate ligands, can catalyse these reactions under various reaction conditions. Among them,
palladacycles have been extensively applied in coupling reactions as effective catalyst precursors, due
to their ready preparation and modification, high activity and relative stability [4–6]. In the
cyclopalladation reactions, owing to the poor solubility of the dimers, they are usually subjected to a
bridge-splitting reaction with monophosphines to produce the corresponding adducts.
In recent years, we have reported a variety of cyclometalated complexes containing N-donor
ferrocenyl ligands and found that palladacycle adducts were far more active than the corresponding
dimers in coupling reactions [7–9]. In view of these findings and our continuing interest in the
synthesis of palladacycles and the applications of these systems, we have prepared a new chloride
anion-containing palladacycle adduct 2a and two corresponding adducts 3a,b containing iodide anion
(
Scheme 1). Their application to the Suzuki and Sonogashira reactions were investigated since we
were interested to see the effect of halogen anions on the catalytic activity. The results are described in
this paper.
Scheme 1. Synthesis of triphenylphosphine adducts of palladacycles 2–3.
Y
Y
Y
N
X
N
X
Fe
PPh3
N
X
I
Pd Cl
Fe
NaI
Fe
Pd
Cl
Pd
acetone
2
PPh3
PPh3
(a,b)
a: X=N; Y=Cl
b: X=C; Y=Br
1(a,b)
2
(a,b)
3
2
. Results and Discussion
.1. Synthesis and Characterization of Complexes 2–3
2
The chloride-bridged palladacyclic dimers 1a,b were prepared according to the published
procedures [10,11]. Then each of them was treated with 1.1 equivalent of triphenylphosphine per
palladium in CH Cl at room temperature for 30 min to afford the corresponding adducts 2a,b as red
2
2
solids. The iodide anion-containing palladacycle adducts 3a,b have been readily prepared through
anion exchange reactions of 2a,b with NaI in acetone (Scheme 1). These complexes are air- and
moisture-stable, both in the solid state and in solution. They are very soluble in chloroform,
dichloromethane and acetone, but insoluble in petroleum ether and n-hexane. These complexes were
1
characterized by elemental analysis, IR and H-NMR. They exhibit similar peaks for the Cp ring, with
a proton ratio of 1:1:1:5, clearly showing that they are both ortho-cyclopalladated complexes. In
general terms, the spectra of 2a,b were similar to those of the corresponding 3a,b. In order to further

Molecules 2012, 17
5534
investigate the structures of these complexes and the effect of halogen anions on the structure, their
crystal structures have been determined by X-ray diffraction.
2
.2. Crystal Structures of the Complexes 2–3
2 2
Single crystals of these complexes were obtained by crystallization from CH Cl -petroleum ether at
room temperature. The molecular structures are shown in Figures 1–3. The crystal structure of 3b
reveals two molecules in the asymmetric unit with different geometry (corresponding values for the
second structure are given in brackets). Selected bond lengths and angles are listed in Table 1. The
single-crystal X-ray analysis reveal that all complexes display trans configurations in the solid state.
The Pd atom in each cyclopalladated unit is in a slightly distorted square-planar environment, being
coordinated to phosphorus, chlorine (iodine), nitrogen and the carbon atom of the ferrocenyl moiety.
The Pd–N [2.144(7)–2.160(6) Å] and Pd–P [2.236(2)–2.246(2) Å] bond lengths of 3a,b are similar
to those of the corresponding 2a,b [2.140(3)–2.1418(19) and 2.2322(11)–2.2369(7) Å], while
Pd–I [2.6491(10)–2.6752(12) Å] bond lengths of 3a,b are obviously longer than the
Pd–Cl [2.3393(8)–2.3642(2) Å] bond lengths in the corresponding adducts 2a,b [11].
Figure 1. Molecular structure of complex 2a. H atoms are omitted for clarity.
Figure 2. Molecular structure of complex 3a. H atoms are omitted for clarity.

Molecules 2012, 17
Figure 3. Molecular structure of complex 3b. H atoms are omitted for clarity.
5535
Table 1. Selected bond lengths (Å) and angles (°) for complexes 2a–3b.
Compound
2a
3a
3b
Pd(1)-C[(1) or (5)]
Pd(1)-N(1)
Pd(1)-P(1)
Pd(1)-[Cl(2) or I(1)]
C[(1) or (5)]-Pd(1)-N(1)
C[(1) or (5)]-Pd(1)-P(1)
N(1)-Pd(1)-[Cl(2) or I(1)]
P(1)-Pd(1)-[Cl(2) or I(1)]
1.995(2)
2.1418(19)
2.2369(7)
2.3393(8)
80.87(9)
92.30(8)
92.94(6)
94.17(3)
2.003(9)
2.144(7)
2.246(2)
2.6752(12)
80.3(3)
91.8(3)
94.79(19)
95.20(6)
1.997(7)[2.011(8)]
2.160(6)[2.160(6)]
2.238(2)[2.236(2)]
2.6491(10)[2.6574(10)]
81.0(3)[81.1(3)]
90.2(2)[91.0(2)]
93.46(17)[93.23(17)]
95.52(6)[96.40(6)]
The most striking common feature of the structures of the three palladacycles is the intermolecular
C–H···X (Cl, Br, I) hydrogen bonds (Figures 4 and 5). The C–H···X–M hydrogen bonding can be used
to assemble transition metal based building blocks into supramolecular structures since the H-bonding
acceptor capability of terminal metal-bound chlorine (M–X) is stronger than their C–X analogues in
organometallic compounds [12,13]. Basically, the crystal structures of the two complexes 2a and 3a
are quite similar, with chlorine (iodine) anion forming C–H···X(Cl, I) hydrogen bonds (Cl···H = 2.441 and
I···H = 3.061 Å), however, the chlorine atom in the pyridazine ring does not participate in hydrogen
bonding. It is noteworthy that intermolecular C–H···Pd hydrogen bonds are present in the crystals of
2
a (Pd···H = 2.861 Å) and 3a (Pd···H = 2.736 Å). Although these interactions have been
extensively studied [14], there are a few reports concerning palladacycles [15–17]. Owing to the
C–H···X(Cl, I) and C–H···Pd hydrogen bonds, the crystal structures of the 2a and 3a are extended to a
2D network architecture (Figure 4). Unlike 2a and 3a, the complex 3b has a one-dimensional chain
structure formed by C–H···I hydrogen bonds between an iodine atom and the adjacent C–H group of
the Cp ring. This is different from that of 2b which presents C–H···Cl hydrogen bonds between a
chlorine atom and the adjacent C–H group of the pyridine ring [11]. In addition, C–H···Br hydrogen

Molecules 2012, 17
5536
bonds between bromine atom and the adjacent C–H group of Cp ring are also present in the crystal of
3
b (Figure 5).
Figure 4. Two-dimensional network structure of complex 2a formed by C–H···Cl and
C–H···Pd hydrogen bonds. Non-hydrogen bonding H atoms are omitted for clarity.
Figure 5. One-dimensional chain structure of complex 3b formed by C–H···I(Br) hydrogen
bonds. Non-hydrogen bonding H atoms are omitted for clarity.
2
.3. Application in Suzuki Reaction
Phosphine adducts of palladacycles have been successfully used as precatalysts in Suzuki coupling
reactions [4,5]. The active species was proposed to be a Pd(0) “Pd-PR ” species and it was shown that
3
the cyclopalladated ligand was released from the metal center during activation of the Pd(II)
precatalyst [18,19]. Usually, cyclopalladated ligands have little effect on the catalytic activity, however,

Molecules 2012, 17
5537
variation of the anionic ligand X(Cl, Br, I, OAc, etc.) is found to have some effect on the catalytic
activity [20,21]. In order to further investigate the effect of halogen anions on the catalytic activity, we
studied the Suzuki reactions catalyzed by the palladacycle adducts 2–3 containing chloride/iodide anions.
In the present study, our initial exploration of the reaction focused on the coupling of
4
-bromotoluene with phenylboronic acid. Based on our previous experiments with palladacylic
precatalysts for Suzuki coupling reactions [11,17,22], the reaction was performed under nitrogen
atmosphere in dioxane in the presence of Cs CO as base at 100 °C for 12 h, affording the coupling
2
3
product in good yields (Table 2, entries 1–4). For an ortho-substituent such as 2-bromotoluene, the
yields decreased slightly (entries 5–8). The complexes 2a,b exhibited higher catalytic activity than
those of the corresponding 3a,b under the same reaction conditions. Even o-bromoanisole provide
good yields by using 0.5 mol% of 2a,b (entries 9–10). For electron-deficient aryl bromides such as
4
-bromonitrobenzene, they could be coupled very efficiently with a catalytic loading as low as
.1 mol% (entries 11–14). We next investigated Suzuki coupling of 4-chlorotoluene under the same
0
reaction conditions. In this case, however, the complexes 2a,b were almost inactive (entries 15–16). It
can be seen from the above results that the activity of adducts in the Suzuki reaction falls in the order
2
a~2b > 3a~3b, and the halogen anions have some effect on the catalytic activity, which is consistent
with the literature [18,21].
a
Table 2. Suzuki coupling of aryl halides with phenylboronic acid catalysed by complexes 2 and 3 .
Cat
X
B(OH)2
+
Base
R
R
b
Entry
X
R
Catalyst (mol% Pd) Yield (%)
1
2
3
4
5
6
7
8
9
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
p-CH
p-CH
p-CH
p-CH
o-CH
o-CH
o-CH
o-CH
o-OCH
o-OCH
p-NO
p-NO
3
3
3
3
3
3
3
3
2a (0.5)
2b (0.5)
3a (0.5)
3b (0.5)
2a (0.5)
2b (0.5)
3a (0.5)
3b (0.5)
2a (0.5)
2b (0.5)
2a (0.1)
2b (0.1)
3a (0.1)
3b (0.1)
2a (1)
93
95
84
82
91
92
73
75
88
86
98
97
92
90
trace
trace
3
10
11
12
13
14
15
16
3
2
2
p-NO2
p-NO
2
p-CH
p-CH
3
3
2b (1)
a
Reaction conditions: aryl halides (0.5 mmol), PhB(OH)
2
(0.75 mmol), Cs
2 3
CO (0.75 mmol),
b
dioxane (3 mL), 100 °C, 12 h; Isolated yields (average of two experiments).

Molecules 2012, 17
.4. Application in Sonogashira Reaction
The high activity of triphenylphosphine adducts of palladacycles 2–3 in the Suzuki coupling of
5538
2
aryl bromides encouraged us to examine their activity and the effect of halogen anions in the
Sonogashira reaction. Usually, the Sonogashira reaction is mediated by a palladium catalyst with
copper cocatalyst and amine base. However, many phosphine-palladium complexes are sensitive to
both air and moisture, and are both costly and toxic. Moreover, the use of a copper reagent results in
contamination of the coupling products with metal residues. Thus a lot of efforts are underway toward
developing new palladium catalysts and copper- and amine-free conditions for the Sonogashira
reaction [23–26]. Among them, a wide variety of palladacycles have been reported and successfully
used in the Sonogashira reaction [5,27–30]. To the best of our knowledge, there was no report
concerning the effect of halogen anions on the catalytic activity of palladacycle for the Sonogashira
reaction.
The adducts of palladacycles 2–3 were evaluated in arylation reaction of phenylacetylene with aryl
halides under copper- and amine-free conditions (Table 3). The model reaction of bromobenzene with
phenylacetylene produced maximum yields when CsOAc was used as a base, dimethylacetamide
(
DMA) as a solvent, and the reaction occurred at 120 °C for 24 h.
a
Table 3. Sonogashira reaction of aryl halides with phenylacetylene catalysed by complexes 2 and 3 .
Cat
X
+
Base
R
R
b
Entry
X
R
Catalyst (mol% Pd) Yield (%)
1
2
3
4
5
6
7
8
9
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
H
H
H
H
2a (1)
2b (1)
3a (1)
3b (1)
2a (1)
2b (1)
3a (1)
3b (1)
3a (1)
3a (1)
2a (0.1)
2b (0.1)
3a (0.1)
3b (0.1)
3a (2)
3b (2)
3a (2)
3b (2)
88
90
96
97
81
80
92
94
89
87
91
92
96
98
35
37
46
49
o-CH
o-CH
o-CH
o-CH
o-CH
o-CH
p-COCH
p-COCH
p-COCH
p-COCH
p-COCH
p-COCH
3
3
3
3
3
3
10
11
12
13
14
15
16
17
18
3
3
3
3
3
3
p-NO
p-NO
2
2
a
Reaction conditions: aryl halides (0.5 mmol), arylacetylene (0.6 mmol), CsOAc (0.75 mmol),
b
DMA (3 mL), 120 °C, 24 h; Isolated yields (average of two experiments).

Molecules 2012, 17
5539
All the tested palladacycle precatalysts 2–3 showed good activity for the above reaction, giving the
coupled product in 88–97% yields (entries 1–4). It was interesting that the complexes 3a,b exhibited
higher catalytic activity than those of the complexes 2a,b in the Sonogashira reaction. This is different
from Suzuki reaction. Then, we investigated the Sonogashira coupling of 2-bromotoluene under the
same conditions to further compare the effect of halogen anions on the catalytic activity (entries 5–8).
Similar to the results of bromobenzene, the activity of adducts falls in the order 3a~3b > 2a~2b. For
electron-rich o-bromoanisole, the yields decreased slightly, but still were very high (89% and 87%,
entries 9–10). In the case of activated 4-bromoacetophenone, it was not surprising that good yields
were obtained with a catalytic loading as low as 0.1 mol% (entries 11–14). Furthermore, Sonogashira
reaction of phenylacetylene with aryl chlorides were also studied in this system. For activated
chlorides such as 4-chloroacetophenone and 4-chloronitrobenzene, the coupled products could be only
produced in 35–49% yields by using 2 mol% of 3a,b (entries 15–18).
3. Experimental
3.1. General Procedures
Solvents were dried and freshly distilled prior to use. All other chemicals were commercially
available expect for the chloride-bridged palladacyclic dimers 1a,b and 2b were prepared according to
published procedures [10,11]. All other chemicals were used as purchased. Elemental analyses were
determined with a Thermo Flash EA 1112 elemental analyzer. IR spectra were collected on a Bruker
1
VECTOR22 spectrophotometer using KBr pellets. H-NMR spectra were recorded on a Bruker
DPX-400 spectrometer in CDCl
3
with TMS as an internal standard.
3
.2. The Synthesis of Complex 2a
A solution of chloride-bridged palladacyclic dimer 1a (0.1 mmol) and triphenylphosphine
0.22 mmol) in CH Cl (10 mL) was stirred at room temperature for 30 min. The product was separated
by passing through a silica gel column with dichloromethane as eluent. The first band was collected to
(
2
2
5
5
afford complex 2a after the evaporation of the solvent. [PdCl{[( -C H )]Fe[( -C H )-N C H -Cl]}PPh ]
5
5
5
3
2
4
2
3
(
2a): Yield 90%. Found (%): C, 55.1; H, 3.4; N, 4.2. Calc. (%) for C32
H
25Cl
2
2
FeN PPd: C, 54.8; H, 3.6;
-1
N, 4.0. IR (KBr, cm ): 3045, 1584, 1533, 1472, 1435, 1392, 1376, 1273, 1161, 1097, 999, 855, 815,
1
7
4
43, 689. H-NMR (CDCl ):  7.84–7.80 (m, 6H, ArH), 7.40–7.33 (m, 11H, ArH), 4.65 (s, 1H, C H ),
3
5
3
.22 (s, 1H, C
5
H
3
5 5 5 3
), 3.75 (s, 5H, C H ), 3.43 (s, 1H, C H ).
3
.3. General Method for the Synthesis of Complexes 3a,b
A solution of the chloride anion-containing triphenylphosphine palladacycle adduct 2 (0.1 mmol)
and NaI (0.2 mmol) in acetone (10 mL) was stirred at room temperature for 3h. The product was
separated by passing through a silica gel column with dichloromethane as eluent. The first band was
collected to afford the corresponding complex 3 after evaporation of the solvent.
5
5
[
PdI{[( -C
5
H
5
)]Fe[( -C
5
H
3
)-N
2 4 2 3
C H -Cl]}PPh ] (3a): Yield 96%. Found (%): C, 48.7; H, 3.1; N, 3.8.
−1
Calc. (%) for C32
H
25ClFeIN
2
PPd: C, 48.5; H, 3.2; N, 3.5. IR (KBr, cm ): 3043, 1582, 1533, 1470,

Molecules 2012, 17
5540
1
1
433, 1392, 1374, 1268, 1158, 1091, 998, 838, 824, 749, 680. H-NMR (CDCl
3
):  7.85–7.81 (m, 6H,
), 3.59
ArH), 7.44–7.38 (m, 11H, ArH), 4.57 (s, 1H, C ), 4.19 (s, 1H, C ), 3.70 (s, 5H, C H
5
H
3
5
H
3
5 5
(
[
5 3
s, 1H, C H ).
5
5
PdI{[( -C H )]Fe[( -C H )-NC H -Br]}PPh ] (3b): Yield 97%. Found (%): C, 56.9; H, 3.9; N, 4.4.
5
5
5
3
5
3
3
−1
Calc. (%) for C35
H
31Cl
2
FeN
2
PPd: C, 56.5; H, 4.2; N, 4.7%. IR (KBr, cm ): 3066, 2922, 1594, 1494,
1
1
479, 1432, 1386, 1152, 1107, 1090, 1024, 998, 820, 746, 668. H-NMR (CDCl
3
):  9.87 (s, 1H, PyH),
7
.83–7.78 (m, 6H, ArH), 7.75 (d, 1H, PyH), 7.45–7.39 (m, 9H, ArH), 7.17 (d, 1H, PyH), 4.57 (s, 1H,
C H ), 4.05 (s, 1H, C H ), 3.65 (s, 5H, C H ), 3.46 (s, 1H, C H ).
5
3
5
3
5
5
5
3
3
.4. General Procedure for the Arylation Reactions
A prescribed amount of catalyst, aryl halide (0.5 mmol), phenyl boronic acid (0.75 mmol) or alkyne
0.6 mmol) and the selected base (1.5 mmol) in solvent (3 mL) were placed in a Schlenk tube under
(
nitrogen. The reaction mixture was heated at 100 °C–120 °C for 12–24 h, then cooled and quenched
with water. The reaction mixture was extracted three times with CH Cl , then the combined organic
2
2
layers were washed with water, dried (MgSO
4
), and evaporated to dryness. The products were isolated
1
by flash chromatography on silica gel using petroleum ether as eluent and analyzed by H-NMR.
3
.5. Crystal Structure Determination
Crystallographic data for complexes 2a and 3a,b were collected on a Bruker SMART APEX-II CCD
diffractometer euuipped with a graphite monochromator at 296 K using Mo-Ka radiation
(
λ = 0.071073 Å). The data were corrected for Lorentz polarization factors as well as for absorption.
2
Structures were solved by direct methods and refined by full-matrix least-squares methods on F with
the SHELX-97 program [31]. All non-hydrogen atoms were refined anisotropically, while hydrogen
atoms were placed in geometrically calculated positions. Crystal data and structure refinements are
summarized in Table 4. CCDC reference numbers 875474–875476 conta1n the crystallographyic data
for 2a and 3a,b, respectively. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif.
Table 4. Crystallographic data and structure refinement for 2a–3b.
Compound
Elemental formula
Formula mass
Crystal system
Space group
Crystal size/mm
a / Å
2a
3a
3b
C
32
H25Cl
2
FeN
2
PPd
C
32
H
25ClFeIN
2
PPd
33
C H26BrFeINPPd
836.58
Triclinic
P-1
0.34 × 0.26 × 0.17
10.410(3)
16.185(4)
17.981(4)
85.597(3)
86.449(3)
81.452(3)
701.66
Monoclinic
P2(1)/c
0.38 × 0.27 × 0.16
9.2765(7)
15.8968(13)
19.7419(16)
90
793.11
Monoclinic
P2(1)/c
0.35 × 0.27 × 0.18
8.835(3)
16.756(6)
20.546(7)
90
b / Å
c / Å
o



/
/
/
o
90.8120(10)
90
90.789(4)
90
o

Molecules 2012, 17
5541
Table 4. Cont.
Compound
2a
3a
3b
3
V / Å
2911.0(4)
1.601
4
3041.4(18)
1.732
4
2983.0(12)
1.863
4
−3
D
Z
c
/ g cm
Data/restraints/
parameters
5419/0/352
5623/0/352
10997/0/703
a
R
1
, wR
2
[I > 2σ(I)]
0.0251, 0.0551
0.0595, 0.1755
0.0525, 0.1108
a
2
2 2
c
2 2 1/2
R
1
= ||F | − |F || / |F
o
c
o
|, wR
2
= [ (F
o
− F
o
) / w(F ) ]
4
. Conclusions
In summary, we have prepared and characterized three new triphenylphosphine adducts of
palladacycles containing chloride/iodide anions. Single-crystal X-ray analysis confirms that there are
intermolecular C–H···X(Cl, Br, I) hydrogen bonds in the crystals of 2a–3b. Their performance and the
effect of halogen anions were evaluated in arylation reactions. Currently, further investigation into the
mechanism of these reactions, and other applications involving this class of complexes in other
coupling reactions are in progress.
Acknowledgments
We are grateful to the National Natural Science Foundation of China (No. 20902043) and the
Natural Science Foundation of Henan Province (No. 102300410220) and Luoyang Tackle Key
Problem of Science and Technology (No. 1001060A) for financial support of this work.
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