Suzuki cross-coupling in aqueous media catalyzed by a 1,1′-N-substituted ferrocenediyl Pd(II) complex

Article abstract of DOI:10.1016/j.jorganchem.2003.09.028

Reaction of PdCl₂(MeCN)₂ with Fe[η-C₅H₄NC(H)Ph-N]₂ (1) at MeOH at r.t. gives air-stable PdCl₂Fe[η-C₅ H₄NC(H)Ph-N]₂ (2; yield 84%). X-ray single-crystal diffraction analyses show that 2 is a Pd(II) square planar complex with N,N^′ chelation of the ferrocenediyl ligand, without Fe-Pd bond. It effectively catalyzes Suzuki cross-coupling reactions of aryl iodides and bromides with aryl boronic acids in aqueous media under non-homogeneous conditions in which the products can be easily isolated. The reaction conditions including choice of base, catalytic load and catalyst recoverability have been investigated and reported.

Full text of DOI:10.1016/j.jorganchem.2003.09.028

Journal of Organometallic Chemistry 689 (2004) 18–24
www.elsevier.com/locate/jorganchem
Suzuki cross-coupling in aqueous media catalyzed by
a 1,10-N-substituted ferrocenediyl Pd(II) complex
b
b,
*
Zhiqiang Weng a,b, Lip Lin Koh , T.S. Andy Hor
a
Institute of Chemical and Engineering Sciences, Singapore 139959, Singapore
b
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Kent Ridge, Singapore 119260, Singapore
Received 24 July 2003; accepted 15 September 2003
Abstract
Reaction of PdCl2(MeCN)2 with Fe[g-C5H4NC(H)Ph–N]2 (1) at MeOH at r.t. gives air-stable PdCl2Fe[g-C5H4NC(H)Ph–N]2
(2; yield 84%). X-ray single-crystal diffraction analyses show that 2 is a Pd(II) square planar complex with N,N0 chelation of the
ferrocenediyl ligand, without Fe–Pd bond. It effectively catalyzes Suzuki cross-coupling reactions of aryl iodides and bromides with
aryl boronic acids in aqueous media under non-homogeneous conditions in which the products can be easily isolated. The reaction
conditions including choice of base, catalytic load and catalyst recoverability have been investigated and reported.
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: 1,10-Ferrocenediyl; Palladium; Catalysis; Aqueous; Suzuki coupling
1. Introduction
conversion to 1,10-N-substituted ferrocenediyl ligand, 1,
caught our attention. The latter ligand, although water-
insoluble, fulfils at least three of our criteria, namely,
phosphine-free, ferrocenyl-base, and coordinatively
mobile. We therefore set out to examine its coordination
ability to Pd(II) and the catalytic potential of the resul-
tant complex.
Palladium-catalyzed Suzuki cross-coupling of haloa-
renes with arylboronic acid has wide-ranging applications
in organic synthesis [1]. In most cases, phosphine-sta-
bilized catalysts are used. These are largely toxic [2] and
organic-based. We are interested in developing a new
generation of environmentally friendly water-based and
phosphine-free catalysts of high efficiency. Since the
products from these reactions are essentially water-
insoluble, their separation and isolation as well as the
catalyst recover process can also be conveniently
achieved. Similar Pd-initiated reactions in aqueous
media began to appear in the literature only very recently
[3].
We have been focusing on the chemistry of 1,10-
bis(diphenylphosphino)ferrocene (dppf) (see for example
[4]) largely because of its coordinative adaptability [5]
and catalytic potential [6]. The recent elegant works
of Arnold et al. on the improved synthesis of 1,10-
diaminoferrocene [7] and Gibson et al. [8] on its
2. Results and discussion
2.1. Structure, efficiency and advantages
The reaction between PdCl2(MeCN)2 and 1 proceeds
smoothly at MeOH at r.t. giving air-stable PdCl2Fe[g-
C5H4NC(H)Ph–N]2, 2 (Scheme 1).
Single-crystal crystallographic analysis of 2 revealed
the expected Pd(II) square planar complex with N,N0
chelation of the ferrocenediyl ligand. The unit cell of 2
contains two independent molecules (A and B). The
molecule A is shown in Fig. 1. Selected bond parameters
are given in Table 1. The benzaldehyde imine function-
ality is within proximity but shows no apparent inter-
action with the metal center (the C–N bonds (1.290(15)
* Corresponding author. Tel.: +65-6874-3057; fax: +65-6779-1691.
E-mail addresses: weng_zhiqiang@ices.a-star.edu.sg (Z. Weng),
andyhor@nus.edu.sg (T.S.A. Hor).
ꢀ
and 1.309(15) A) retain their double bond character).
0022-328X/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2003.09.028

Z. Weng et al. / Journal of Organometallic Chemistry 689 (2004) 18–24
19
CH
N
N=CHPh
N=CHPh
Cl
Cl
RT, 24 h
Methanol
Pd
+
PdCl2(MeCN)2
Fe
Fe
N
CH
Ph
1
Scheme 1.
The sp2 characteristic of the N donor also minimizes the
interference of its substituents on the Pd(II) sphere.
This, coupling with the weaker trans-influence of the
imine compared to phosphine resulted in the signifi-
cantly shorter, and presumably stronger, Pd–Cl bonds
ꢀ
(2.284(4), 2.291(3) A) compared to those in PdCl2(dppf)
ꢀ
(2.333(4) and 2.350(9) A) [9].
Complex 2 is a highly active catalyst in the Suzuki
cross-coupling of aryl iodides and bromides with aryl
boronic acids (Table 2). For example, in a suspension of
2 (2 mol%) in aqueous Cs2CO3 (2.8 equivalents) 4-bro-
mobenzonitrile couples with phenyl boronic acid gives
near-quantitative yield of 4-phenylbenzonitrile at 80 °C
after 2 h (Entry 1). Not surprisingly, the product yield
depends on the para-substituents of the aryl halides.
Activated, electron-poor aryl halides react near-quanti-
tatively (Entry 1, 2, 3, 6 and 7) whereas deactivated,
electron-rich ones give only moderate yields (Entry 4
and 5).
Fig. 1. Molecular structure A of complex 2. Thermal ellipsoids are
drawn at the 50% probability level.
Table 1
ꢀ
Selected bond lengths (A) and angles (°) for 2
Complex 2 is effective towards both aryl iodides and
bromides under similar operational conditions, but
inactive towards aryl chlorides (Entry 10). This activity
is comparable with PdCl2(dppf) which effectively cat-
alyzes the reaction between 4-bromoacetophenone and
phenyl boronic acid in toluene at 70 °C giving 94% yield
[10]. Catalyst 2 is fairly easy to prepare and with an
excellent shelf-life. Its high stability towards oxygen and
water and adaptability in aqueous media without the
need to achieve full homogeneity are additional ad-
vantages.
Mol. A
Mol. B
Pd(1)–N(1)
Pd(1)–N(2)
2.056(10)
2.023(11)
2.284(4)
2.291(3)
1.290(15)
1.309(15)
1.409(16)
1.443(17)
90.27(14)
84.5(4)
2.066(11)
2.077(12)
2.301(4)
2.287(4)
1.228(15)
1.237(17)
1.436(16)
1.440(17)
90.90(13)
84.5(4)
Pd(1)–Cl(1)
Pd(1)–Cl(2)
C(1)–N(1)
C(2)–N(2)
C(3)–N(1)
C(8)–N(2)
Cl(1)–Pd(1)–Cl(2)
N(1)–Pd(1)–N(2)
Cl(1)–Pd(1)–N(1)
Cl(2)–Pd(1)–N(2)
Two CpÕs dihedral
93.1(3)
91.8(3)
7.3(3)
91.1(3)
93.0(3)
5.7(3)
2.2. Effect of the base on coupling between 4-bromo-
acetophenone and phenylboronic acid
The ferrocenediyl moiety is significantly skewed, with the
Cp rings tilted inward at 7.3(3)°, in order to adapt to the
chelate structure with short Pd–N bonds (2.056(10),
The coupling between 4-bromoacetophenone and
phenylboronic acid illustrates the importance of the
base, and the choice of it, on the catalytic activity in
terms of yield (Table 3). It is generally accepted that the
basic nucleophile would assist the departure of halide on
Pd(II), thus facilitating the subsequent transmetallation
with the boron substrate. Nolan et al. [11] recently
demonstrated the highly effective use of Cs2CO3 in a
palladium/imidazolium salt system in Suzuki catalyzed
reactions. Unfortunately, the more common and less
expensive inorganic bases, such as Na2CO3 and K2CO3
ꢀ
2.023(11) A).
Since PdCl2(dppf) is one of the most established ac-
tive C–C coupling catalysts and that much of its cat-
alytic functions could be related to its stereogeometric
properties, a careful comparison between PdCl2(dppf)
and 2 is pertinent. The coordination sphere of the latter
is sterically more favored because of the smaller donor
atoms (N) and the absence of the diphenyl in proximity.

20
Z. Weng et al. / Journal of Organometallic Chemistry 689 (2004) 18–24
Table 2
Suzuki cross-coupling of aryl halides with aryl boronic acids catalyzed by 2 in an aqueous medium
Entry
1
Aryl halide
Aryl boronic acid
Product
Isolated yielda
95
CN
B(OH)2
NC
Br
94
93
O
O
B(OH)2
B(OH)2
2
3
Br
H3C
C
H3C
C
O
O
H
C
Br
Br
Br
H
C
67
51
B(OH)2
CH3O
4
5
CH3O
B(OH)2
B(OH)2
CH3O
CH3O
OCH3
CH3O
CH3O
NC
94
92
Br
6
7
OCH3
NC
O
O
B(OH)2
CH3O
Br
OCH3
H3C
C
H3C C
95
I
I
HO
HO
8
B(OH)2
94
0
B(OH)2
CH3O
HO
OCH
HO
3
9
CN
Cl
NC
10
B(OH)2
a Reaction duration generally not optimized.
Table 3
Effect of the base on the coupling reactiona
Entry
Base
Yield (%)
1
2
3
4
5
6
7
Cs2CO3
K2CO3
Na2CO3
NEt3
94
96
98
93
67
50
22
KOH
NaOH
KF
a Reaction conditions: 0.5 mmol of 4-bromoacetophenone, 0.67 mmol of phenylboronic acid, 1.4 mmol of base, 0.01 mmol of complex 2, 5 ml of
water, 80 °C, 2 h.
or an organic base, Et3N, are less effective. In our case,
such differentiation is less obvious whereas all these
could lead to higher yields. The basic strength is not a
gauge of the catalytic yield, as illustrated by the lower
yields from the use of strong bases KOH or NaOH. This
could be understood in terms of their high nucleophilic

Z. Weng et al. / Journal of Organometallic Chemistry 689 (2004) 18–24
21
Table 4
Influence of low catalyst loading on the coupling reactiona
Entry
Cat 2 (mol%)
Yield (%)
1
2
3
4
5
6
2
94
94
88
87
47
<5
1
0.5
0.25
0.05
0
a Reaction conditions: 0.5 mmol of 4-bromoacetophenone, 0.67 mmol of phenylboronic acid, 1.4 mmol of Cs2CO3, 5 ml of water, 80 °C, 2 h (for
entry 5, 1 h).
effect which could impede the transmetallation process.
Accordingly, although KF is one of common and ef-
fective bases for Pd2(dba)3/phosphine Suzuki-type cat-
alytic reaction [12], it is not effective in our case
presumably because the strength of the Pd–F bond.
Other special methods include the use of a ligand-free
microwave-mediation in water [15]. It gains from
low catalyst loadings and rapid reaction times but is
dis- advantaged by the use of strenuous conditions (150–
175 °C and under pressure). Other ‘‘ligandless’’ ap-
proach [using Pd(OAc)2] is also efficient in water, but
only in the presence of Bu4NBr [16]. This however is
limited by generally poor conversions for aryl iodides.
Complex 2 was retrieved and recovered from the
2.3. Catalytic load
It is important to achieve good yields using minimum
catalysts, particularly so for the use of 2, as opposed to
commercially available reagents such as Pd(OAc)2/PR3.
We therefore examined the effect of catalyst loading on
a convenient coupling between 4-bromoacetophenone
and phenylboronic acid (Table 4). We have observed
high yields (>85%) even at catalyst loadings as low as
0.25 mol%. These are indications of an effective catalytic
system that has commercial potential. Further reduction
to loads to, for example, 0.05 mol% would result in a
moderate yield of 47% and a TON of 940, this is the
lower limit of the use of 2 in these reactions. Absence of
catalyst generally leads to negligible yields.
1
mixture, verified by H NMR and tested on the 4-bro-
moacetophenone and phenylboronic acid for its activity
(Table 5). The reusable ability of the catalyst is valuable
especially in the current case when the catalyst can be
conveniently retrieved and reused at the end of a cat-
alytic run. The yields remain good (>85%) after four
runs (including original). Even for the fourth and fifth
batches, the yields remain at >80%. These are indica-
tions that the present catalyst has potential for com-
mercial developments at minimum cost with reasonable
effectiveness in terms of yields.
For comparison, we carried out the coupling between
4-bromoacetophenone couple and phenylboronic acid
using commercial Pd/C reagent (10 wt%, from Aldrich),
which gave unattractive yield (36%). This finding sup-
ports that colloidal palladium does not play a key role in
this reaction. Similar experiments using 1/Pd(OAc)2,
however resulted in good yield (90%). This is consistent
with the supporting role of 1,10-N-substituted ferro-
cenediyl Fe[g-C5H4NC(H)Ph–N]2 (1) in promoting high
efficiency.
2.4. Recoverability, reusability and recyclability
An advantage of this method is its simple experimental
procedure and the ease of product isolation. Addition of
Et2O/H2O mixture to the mixture at the conclusion of the
reaction easily leads to product separation under the bi-
phasic conditions. The organic product could be con-
veniently isolated from Et2O, analyzed by GC/MS and
purified by chromatography. Our approach comple-
ments some literature examples in Suzuki cross-coupling
in aqueous media, which are mainly on the development
of water-soluble phosphine ligands [3,13], and use of
water as a co-solvent in biphasic reactions [13]. There is
also a heterogeneous example in the use of Pd/C as a re-
usable catalyst for Suzuki reaction of iodophenols in
aqueous K2CO3 solution, which also offers a simple
high-yield method applicable to water-soluble iodophe-
nols with phenyl boronic acids. However, the yield is
significantly lower when bromophenol is used [14].
2.5. Mechanistic considerations
It has been well established that the general catalytic
cycle for the coupling reaction of organoboron reagents
with aryl halides involves an oxidative-addition of the
aryl halide, transmetalation and reductive-elimination
steps (Scheme 2) [1].
Attempts to isolate the intermediate Pd(0) species has
not been successful. This is perhaps not surprising in
view of its instability and high reactivity. Reaction be-

22
Z. Weng et al. / Journal of Organometallic Chemistry 689 (2004) 18–24
Table 5
Recyclability of 2 on the coupling reactiona
Yield (%)
Original use
First reuse
94
93
93
86
84
83
Second reuse
Third reuse
Fourth reuse
Fifth reuse
a Reaction conditions: 0.5 mmol of 4-bromoacetophenone, 0.67 mmol of phenylboronic acid, 1.4 mmol of Cs2CO3, 5 ml of water, 80 °C, 2 h.
tween 2 and K2CO3 in MeOH at r.t. only resulted in
1,10-N-substituted ferrocenediyl (1) and a black insol-
uble precipitate, presumably metallic Pd. The use of a
CO reaction atmosphere helped to prevent decomposi-
tion to Pd metal but could not yield any isolable species.
Attempts to isolate the oxidative addition product from
a mixture of 2 with aryl halide and K2CO3 or Cs2CO3 in
MeOH at 80 °C resulted in a fairly stable complex (with
1 and other decomposition products), which is presently
unidentified.
3. Experimental
Conventional Schlenk techniques were used unless
otherwise indicated. NMR spectra were measured on
Bruker ACF300 300 MHz FT NMR spectrometers (1H
at 300.14 MHz). Mass spectra were obtained on a
Finnigan Mat 95XL-T spectrometer. Elemental analyses
were performed by the microanalytical laboratory in-
house. Complex 1 was prepared according to literature
procedures [8].
Our focus is not to study the mechanistic details,
which have been reported elsewhere, but we would
continue to try to isolate intermediates that are mecha-
nistically significant. The strength of Pd–Cl bonds (in 2)
does not appear to impede the catalytic function. This is
attributed to the assistance from the base, which fosters
the formation of the key intermediate moiety [Ar–Pd-
base]. We are particularly interested to examine if the
imine benzaldehye pendant, through electron donation
from the C@N or phenyl functionalities, can protect and
hence stabilize the unsaturation sites of the catalytically
active species. Work is also underway to expand the
scope of N-derivatized ferrocendiyl catalysis.
3.1. Synthesis of 2
To a red solution of 1 (118 mg, 0.3 mmol) in MeOH
(8 ml) was added PdCl2(MeCN)2 (78 mg, 0.3 mmol), the
red mixture was stirred at r.t. for 24 h. The resultant
dark red precipitate was isolated by filtration, washed by
toluene and Et2O, and dried in vacuo to afford dark-red
solid of 2 (143 mg, 0.25 mmol, 84% yield). Anal. Found:
C, 49.9; H, 3.4; N, 4.8. Calc. for C24H20N2Cl2FePd: C,
50.6; H, 3.5; N, 4.9%. 1H-NMR (CD2Cl2): d 6.46 (1H, t,
C5H4), 4.83 (1H, t, C5H4), 4.62 (1H, t, C5H4), 4.29 (1H,
t, C5H4), 8.02 (1H, s, CH), 8.65 (2H, d, Ph), 7.49 (1H, t,
Ph), 7.36 (2H, t, Ph). FAB-MS (+ve): m/z: 569 [M]þ, 392
[M–2Cl]þ.
3.2. General procedure for the coupling reactions
Base (Z_ )
Complex 2 (5.6 mg, 0.01 mmol), aryl halide (0.5
mmol), and aryl boronic acid (0.67 mmol) were intro-
duced to a flask under air. The flask was evacuated and
refilled with nitrogen. Water (5 ml) was added by
syringe, followed by Cs2CO3 (1.4 mmol). The mixture
was stirred at 80 °C for 2 h, under ambient pressure of
N2. The resultant mixture was diluted with H2O (10 ml)
and ether (10 ml), and the aqueous layer extracted twice
by Et2O. The ethereal extract was stripped of solvent
under vacuum and the products purified by column
chromatography on silica. The residual catalyst, which
is insoluble in both ether and water, distributed on the
wall of the separatory funnel, could be collected and
reused (simply as a suspension in water).
LnPd0
Ar
Ar
Ar'
Y
Ar
Y
Ar
LnPd
LnPd
Ar'
Z_
B(OH)2
Z
Ar
_
Ar'
B(OH)2
LnPd
Scheme 2.
Y

Z. Weng et al. / Journal of Organometallic Chemistry 689 (2004) 18–24
23
Table 6
Selected crystal data, data collection and refinement parameters for compound 2
Formula
Formula weight
Color, habit
C24H20Cl2FeN2PdꢀCH2Cl2
654.50
Dark red block
0.16 ꢁ 0.06 ꢁ 0.04
223(2)
Crystal size (mm)
Temperature (K)
Crystal system
Space group
P2ð1Þ=c
Monoclinic
14.6596(19)
16.554(2)
ꢀ
a A
ꢀ
b A
ꢀ
c A
19.643(2)
90
a (°)
b (°)
c (°)
92.744(5)
90
3
ꢀ
V (A )
4761.2(10)
8
1.826
Z
Dc (g cmꢂ3
)
Radiation used
l (mmꢂ1
Mo Ka
1.832
)
h range (°)
1.86 to 22.50
21,559
No. of unique reflections measured
Max., min. transmission
Final R indices ½I > 2rðIÞꢃ
R indices (all data)
Goodness-of-fit on F 2
ꢀ
Large diff. peak and hole (e A
0.9303, 0.7582
R1 ¼ 0:0792, wR2 ¼ 0:1530
R1 ¼ 0:1381, wR2 ¼ 0:1756
0.975
ꢂ3
)
2.274 and )0.898
3.3. Structure determinations
Acknowledgements
Single-crystals of 2 were obtained by slow evapo-
ration of a CH2Cl2 solution of 2 at ambient temper-
ature. A suitable crystal was mounted on quartz fibers
and X-ray data collected on a Bruker AXS APEX
diffractometer, equipped with a CCD detector, using
We are grateful to the Agency for Science, Technology
& Research (Singapore), the Institute of Chemical and
Engineering Sciences (Singapore) (Grant 012-101-0035)
and the National University of Singapore for support.
ꢀ
Mo Ka radiation (k 0.71073 A). The data were cor-
rected for Lorentz and polarisation effects with the
SMART suite of programs [17] and for absorption
effects with SADABS [18]. Structure solution and re-
finement were carried out with the SHELXTL suite of
programs [19]. The structure was solved by direct
methods to locate the heavy atoms, followed by dif-
ference maps for the light non-hydrogen atoms. Due to
the small crystal size and the small number of reflec-
tions, the C atoms are not refined anisotropically. The
data collection and processing parameters are given in
Table 6.
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