Sustainable carbonecarbon bond formation catalyzed by new oxamate-containing palladium(II) complexes in ionic liquids

Article abstract of DOI:10.1016/j.jorganchem.2013.06.041

New and versatile bis(oxamato)palladate(II) complexes of formula (n-Bu ₄N)₂[Pd(2-Mepma)₂]·4H₂O (1a) and (n-Bu₄N)₂[Pd(4-Mepma)₂]·2H ₂O·MeCN (1b) (n-Bu₄N+ = tetra-n-buthylammonium, 2-Mepma = N- 2-methylphenyloxamate and 4-Mepma = N-4-methylphenyloxamate) have been synthesized and characterized by spectroscopic methods and single crystal X-ray diffraction. Each palladium(II) ion in 1a and 1b is four-coordinate with two oxygen and two nitrogen atoms from two fully deprotonated oxamate ligands building a centrosymmetric square planar surrounding. Their catalytic role has been investigated for both Heck and Suzuki coupling reactions using a series of aryl iodide/bromide derivatives in tetra-n-butylammonium bromide (n-Bu ₄NBr) as ionic liquid, i.e. molten salt. These precatalysts appear as highly efficient, easily recovered and reused at least eight times without any drastic loss of their exceptional reactivity or leaching from the ionic liquid medium.

Full text of DOI:10.1016/j.jorganchem.2013.06.041

Journal of Organometallic Chemistry 743 (2013) 102e108
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Sustainable carbonecarbon bond formation catalyzed by new
oxamate-containing palladium(II) complexes in ionic liquids
a
a
a
b,
Francisco Ramón Fortea-Pérez , Isabel Schlegel , Miguel Julve , Donatella Armentano **,
b
a,
*
Giovanni De Munno , Salah-Eddine Stiriba
Instituto de Ciencia Molecular (ICMol), Universidad de Valencia, C/.Catedrático José Beltran, 2, 46980 Valencia, Spain
Centro di Eccelenza CEMIF.CAL, Dipartimento di Chimica e Tecnologie Chimiche, Università della Calabria, Cosenza 87030, Italy
a
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 April 2013
Received in revised form
New and versatile bis(oxamato)palladate(II) complexes of formula (n-Bu N) [Pd(2-Mepma) ]$4H O (1a)
4
2
2
2
þ
and (n-Bu
4
N)
2
[Pd(4-Mepma)
2
]$2H
2
O$MeCN (1b) (n-Bu
4
N
¼ tetra-n-buthylammonium, 2-Mepma ¼ N-
2-methylphenyloxamate and 4-Mepma ¼ N-4-methylphenyloxamate) have been synthesized and
25 June 2013
characterized by spectroscopic methods and single crystal X-ray diffraction. Each palladium(II) ion in 1a
and 1b is four-coordinate with two oxygen and two nitrogen atoms from two fully deprotonated oxa-
mate ligands building a centrosymmetric square planar surrounding. Their catalytic role has been
investigated for both Heck and Suzuki coupling reactions using a series of aryl iodide/bromide de-
Accepted 26 June 2013
Keywords:
Palladium(II) oxamates
Ionic liquids
CeC cross-coupling
Homogenous catalysis
Sustainable chemistry
4
rivatives in tetra-n-butylammonium bromide (n-Bu NBr) as ionic liquid, i.e. molten salt. These pre-
catalysts appear as highly efficient, easily recovered and reused at least eight times without any drastic
loss of their exceptional reactivity or leaching from the ionic liquid medium.
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
The design and use of simple, cheap, and recyclable active cat-
alysts together with environment-friendly green solvents are
Palladium-catalyzed cross-coupling coupling of aryl halides
with olefins or aryl boronic acids, known as Heck and Suzuki re-
actions respectively, are among the most versatile and efficient
methods for the formation of carbonecarbon (CeC) bonds in
organic synthesis [1,2]. Their use in the synthesis of pharmaceuti-
cals, fine chemicals, natural products and conducting polymers is
motivated by their versatility and tolerance towards different
functional groups [3]. Many palladium-based systems with excel-
lent catalytic performances have been developed and largely
employed, such as those with phosphine ligands [4], palladacycle
catalysts [5], N-heterocyclic carbene-containing catalysts (NHCs)
highly desirable to set up greener catalytic protocols [2c]. In this
context, the performance of Heck and Suzuki coupling reactions in
alternative green solvents such as ionic liquids by phosphine- and
carbene-free palladium catalysts containing inexpensive N,O li-
gands [8], should be advantageous as environmentally friendly
organic reaction methodology and also as a strategy for recycling
catalysts [9]. Furthermore, it has been shown that chelating ligands
containing O or N donor atoms result in more stable and efficient
palladium catalysts, a feature which prevents their deactivation
after the reductive elimination step [10].
Among the variety of potential ligands having N and O as donor
atoms, the inexpensive and readily prepared chelating and bis-
chelating oxamate type ligands have been extensively used for the
[6] and immobilized phosphine-free catalysts [7], in both homog-
enous and heterogenous phases, conducting largely the catalytic
reactions in polar aprotic solvents, such as dimethylformamide
II
II
synthesis of complexes with divalent (Cu and Ni ) and trivalent
III
III
III
(
DMF), N,N-dimethylacetamide (DMA) and N-methylpyrrolidone
(Co , Fe and Mn ) transition metal ions having interesting mag-
netic properties or catalytic performance in oxidation and epoxi-
dation processes [11,12]. However, to the best of our knowledge, only
the synthesis and structural details of two anionic oxamate-
containing mononuclear palladium(II) complexes and a neutral
oxamate-bridged Pd(II)Co(II) chain are known [13]. The catalytic
activity of anionic bis(oxamato)palladate(II) complexes in the CeC
coupling reactions and their immobilization in ionic liquids, fol-
lowed by their separation from reagents and products as well as
(
NMP). However, organic solvents not only enhance the reaction
rate but also result in environmental pollution due to their toxicity.
*
Corresponding author. Tel.: þ34 963544445; fax: þ34 963543273.
*
* Corresponding author.
E-mail addresses: donatella.armentano@unical.it (D. Armentano), salah.stiriba@
uv.es (S.-E. Stiriba).
0
022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.06.041

F.R. Fortea-Pérez et al. / Journal of Organometallic Chemistry 743 (2013) 102e108
103
ꢀ
recycling and reuse in further catalytic cycles have never been
addressed. Herein, we describe the synthesis and characterization of
two structurally well-defined oxamate-containing palladium(II)
value of 90 for the square planar surrounding. The average PdeN
ꢀ
and PdeO bond lengths for 1a/1b are 2.0209(11)/2.0207(13) A and
2.0384(13)/2.0214(14) A, respectively (see Table 1). These values
ꢀ
complexes of formula (n-Bu
n-Bu N) [Pd(4-Mepma) ]$2H
buthylammonium, 2-Mepma ¼ N-2-methylphenyloxamate and
-Mepma ¼ N-4-methylphenyloxamate). Their efficiency as cata-
lysts in the cross-coupling reactions of aryl halides with olefins or
aryl boronic acids in tetra-n-butylammonium bromide (n-Bu NBr)
as the ionic liquid solvent (i.e. molten salt) and triethylamine (Et N)
4
N)
2
[Pd(2-Mepma)
2
]$4H
2
O (1a) and
¼ tetra-n-
agree with those found in the mononuclear species Na[Pd(Hpba)]$
þ
ꢀ
(
4
2
2
2
O$MeCN (1b) (n-Bu
4
N
2H
2
O [H
4
pba ¼ 1,3-propylenebis(oxamic acid)] [PdeN ¼ 1.97 A and
ꢀ
PdeO ¼ 2.04 A] and (PPh
4 2
)
[Pd(opba)]$2H
2
O [H
4
opba ¼ 1,2-
þ
4
phenylenebis(oxamic acid) and PPh
4
¼ tetraphenylphosphonium
ꢀ
ꢀ
cation] [PdeN ¼ 1.93 A and PdeO ¼ 2.05 A] [13]. The values of the
4
dihedral angle between the basal plane at the palladium(II) ion and
ꢀ
3
the mean plane of the oxamate groups for 1a/1b are 7.4(1)/3.9(1) ,
as the base without any other additive is also explored and reported
herein Ref. [14].
whereas those between the square plane and the phenyl ring are
ꢀ
51.1(1)/52.2(1) . The values of the peripheral C(1)eO(2) and C(2)e
ꢀ
O(3) bond distances [1.2413(18) and 1.2543(19) A (1a) and 1.217(2)
ꢀ
and 1.241(2) A (1b)] are somewhat shorter than the inner C(1)eO(1)
ꢀ
2
. Results and discussion
bond [1.309(2) (1a) and 1.300(2) A (1b)] in agreement with the
greater double bond character of the former carbonyl groups.
þ
The bis(oxamate)palladate(II) complexes 1a and 1b were pre-
The distances and angles of each tetrahedral [(n-C H ) N]
4
9 4
pared in a straightforward one step reaction of K
equivalents of the corresponding proligand in an acetonitrile/water
2
PdCl
4
with two
cation in 1a and 1b are quite as expected. The little discrepancy in
the NeC and CeC distances and in the NeCeC and CeCeC angles
arises from crystallographic disorder, which is a quite common
feature observed in most of the crystals containing this organic
cation.
mixture and an excess of NBu
4
OH dissolved in methanol under
ꢀ
aerobic conditions at 60 C. 1a and 1b were obtained as pale yellow
solids which are soluble in polar solvents such as dichloromethane,
chloroform, methanol and water. Both complexes were fully char-
The complex anions of 1a and 1b are well separated from each
1
þ
acterized by elemental analysis, IR, H NMR and single crystal X-ray
other by the bulky Bu N cations in the resulting three-
4
diffraction.
dimensional ionic lattices (Fig. 2) and no
pep stacking in-
teractions are present. The values of the shortest Pd/Pd distances
2
-
ꢀ
are 10.829(1) (1a) and 11.855(1)
A
(1b) [Pd(1)/Pd(1b);
O
O
(
b) ¼ ꢁ1 þ x, y, z]. Very weak but non-negligible CeH/O type
Complex
R
ꢀ
interactions [C/O ¼ 3.5e3.6 A] involving the methyl groups from
N
O
1
1
a
b
o-CH3
p-Ca3
þ
the acetonitrile molecules and NBu
4
cations with the peripheral
R
Pd
R
oxygen atoms of the oxamate groups contribute to the stabilization
of the structure. Furthermore very weak CeH/N interactions
O
N
ꢀ
[
C/N ¼ 3.6e3.8 A] take place between the acetonitrile molecules
þ
O
O
and the methyl groups from the Bu N cations.
4
X-ray diffraction on single crystals of 1a and 1b shows the
The applicability of the synthesized anionic complexes 1a and
2ꢁ
occurrence of centrosymmetric mononuclear [PdL
2
]
units
1b in palladium-catalyzed Heck and Suzuki coupling reactions was
next investigated by using a variety of aryl iodide and less-reactive
aryl bromide derivatives in ionic liquids as solvents. These type of
solvents, i.e. molten salts with a melting point below ambient,
seemed to be promising over conventional reaction conditions,
making easier the separation of the product by simple extraction
or distillation from the ionic “solvent”, a feature which allows
þ
(
L ¼ bidentate oxamate ligand) (Fig. 1) and n-Bu
4
N
as counter
cations. Each palladium(II) ion is four-coordinate with two amidate-
nitrogen and two carboxylate-oxygen atoms from the two oxamate
ligands in a trans arrangement building a distorted square-planar
surrounding. The reduced bite of the bidentate oxamate [82.14(5)
ꢀ
1a) and 81.89(6) (1b)] accounts for the deviations of the ideal
(
Fig. 1. Ortep diagrams of the anionic entity of 1a (left) and 1b (right). Thermal ellipsoids are drawn at the 30% probability level.

104
F.R. Fortea-Pérez et al. / Journal of Organometallic Chemistry 743 (2013) 102e108
Table 1
Selected bond lengths (A) and angles (deg) for 1a and 1b.b
ꢀ
a
1
a
1b
2.021(1)
2.021(1)
1.300(2)
1.217(2)
1.557(3)
1.241(2)
1.341(2)
1.433(2)
81.89(6)
Pd(1)eO(1)
Pd(1)eN(1)
C(1)eO(1)
C(1)eO(2)
C(1)eC(2)
C(2)eO(3)
C(2)eN(1)
N(1)eC(3)
2.021(1)
2.038(1)
1.309(2)
1.240(2)
1.566(2)
1.253(29)
1.350(2)
1.435(2)
82.13(5)
97.87(5)
O(1)ePd(1)eN(1)
O(1)ePd(1)eN(1a)
O(1)ePd(1)eO(1a)
N(1)ePd(1)eN(1a)
Pd(1)eO(1)eC(1)
Pd(1)eN(1)eC(2)
Pd(1)eN(1)eC(3)
C(2)eN(1)eC(3)
98.11(6)
180.00(1)
180.00(5)
114.45(10)
113.92(10)
125.83(10)
119.71(13)
180.00(6)
180.00(1)
114.75(12)
114.42(12)
126.24(12)
119.17(14)
a
Symmetry code for 1a: (a) ¼ ꢁx þ 2, ꢁy þ 2, ꢁz þ 2.
Symmetry code for 1b: (a) ¼ ꢁx þ 2, ꢁy, ꢁz þ 2.
b
recycling the whole catalyst-solvent system. Bearing this concept in
mind, we first screened several ionic liquids with 1a as catalyst for
the coupling of iodobenzene with either phenylboronic acid or
ethyl acrylate by using 5 mol% or 1 mol% of catalyst, respectively.
In terms of productivity, stability and recyclability of the cata-
lysts, n-Bu
4
NBr provided the best results as compared to other
NCl)
tested salts such as tetra-n-butylammonium chloride (n-Bu
4
and other imidazolium salts (Table 2). Contrary to the tetrahedral
shape of the tetraalkylammonium cation, the imidazolium one has
a planar structure, a feature that makes easier its aggregation with
the planar complex anions. This would decrease the anion avail-
ability for the stabilization of the catalytically active Pd(0) species,
which may be formed in the reaction mixture either by thermal
reduction of Pd(II) precatalyst or by an excess of the olefin itself as
well as triethylamine employed as base, during the catalytic pro-
4
cess [15]. n-Bu NBr was described as an excellent reaction medium
for the stabilization of catalytically active palladium and copper
species for CeC coupling reactions via electrostatic interactions
[
16]. Catalyst loading of 5 mol% (1a) and 1 mol% (1b) for Suzuki and
Heck reactions respectively, which was carried out with iodo-
benzene and Et N as base, afforded the highest yields in short time
reaction (ca. half an hour). However, lower catalyst loadings of
mol % (1a) gave lower yields for the Suzuki reactions. Interest-
3
1
ingly, while catalyst 1a showed a higher catalytic performance in
Suzuki coupling of aryl iodide with phenylboronic acid compared to
Fig. 2. Crystal packing of 1a (top) and 1b (bottom) showing the relative positions of
the anionic complexes (red) and tetra-n-butylammonium cations (blue). The solvent
molecules have been omitted for clarity). (For interpretation of the references to colour
in this figure legend, the reader is referred to the web version of this article.)
1
b, the Heck coupling of aryl iodide with olefins was efficiently
promoted by using 1b compared to 1a (Entry 1e4, Table 2). In the
Heck process, the rate of reaction and regioselectivity are sensitive
to the steric hindrance about C]C bond of the vinylic partner. In
fact, steric effects play a major role in directing the attack by the
metal i.e. palladium in the insertion step. Catalyst 1a features
pronounced steric effects around the palladium catalytic centre due
to the presence of a methyl group in the ortho-position to the
oxamate ligand, which would cause a lowering of its reactivity in
the Heck reactions.
4
black. This feature would suggest that n-Bu NBr stabilizes the
catalytic active species from precipitation into the insoluble and
inactive black palladium(0) [17]. Under the same conditions, the
coupling of a wide variety of acceptor and donor-substituted aryl
4
iodide with phenylboronic acid mediated by 1a in n-Bu NBr as
solvent, efficiently yielded the corresponding asymmetrically
substituted biaryl derivatives, demonstrating the greater activity/
performance of these new catalyst (Entry 2e6, Table 3). Interest-
ingly, the coupling of the less reactive bromobenzene with phe-
nylboronic acid was also achieved in good yield (Entry 7e8,
Suzuki coupling of aryl halide (0.5 mmol) and boronic acid
(
0.75 mmol), and Et
3
N (1 mmol) with 1a (5 mol%) in 2e3 g of
ꢀ
n-Bu
4
NBr at 120 C led to the dissolution of the anionic palladium
complex and the formation of diphenyl with a yield of 78% after
20 min (Entry 1, Table 3). The only side product observed was from
1
2 2
Table 3). Well known catalysts such as Pd(OAc) and Pd(dba)
deboronation. Interestingly, the catalytic reaction did not lead to
visible palladium black formation, neither during the catalytic
process nor after completion of the catalytic reaction, implying that
the reaction does not occur at the surface of inactive palladium
employed for Suzuki reaction resulted in similar catalytic activity
and identical lower turnover numbers (TONs) like 1a with the
formation of palladium black, which could hamper recycling of
these catalysts (Fig. 3S, Supporting information).

F.R. Fortea-Pérez et al. / Journal of Organometallic Chemistry 743 (2013) 102e108
105
Table 2
4
dissolved in n-Bu NBr was achieved by the extraction of the re-
Screening of Suzuki (1a) and Heck (1b) reactions in different ionic liquids.
agents and products with n-pentane. The catalytic activity/recy-
clability of 1a and 1b was tested by reaction of iodobenzene with
phenylboronic acid mediated by 1a (5 mol%) and iodobenzene with
ethyl acrylate mediated by 1b (1 mol%). The catalytic protocol of
recovery/reuse was performed until six (1a)/eight (1b) runs with
good yields, as shown in Fig. 3.
ꢀ
Entry
Ionic liquid
T ( C)
Yield (%)
Suzuki /1a
Time
(min)
Yield (%)
Heck /1b
Time
(min)
a
b
1
2
3
4
n-Bu
n-Bu
BMIMBr
BIMIMPF
4
NBr
120
120
120
120
78
46
23
33
120
120
120
120
99
88
85
66
30
30
30
30
4
NCl
c
d
6
No leaching of the palladium(II) compound employed was
observed, as evidenced by the analysis of the final organic products
a
Reaction conditions: 0.5 mmol of aryl iodide, 0.75 mmol of phenylboronic acid,
N, 5 mol % 1a. Determined by GCeMS using diethyleneglycol-di-n-
2
equiv of Et
3
1
using both H NMR spectroscopy and SEM-Edax microscopy. In fact,
butyl ether as internal standard.
1
b
the analysis of the isolated product by H NMR and FT-IR spectro-
Reaction conditions: 0.5 mmol of aryl iodide 0.75 mmol of ethyl acrylate, 2 equiv
of Et
3
N, 1 mol % 1b. Determined by GCeMS using diethyleneglycol-di-n-butyl ether
scopic techniques showed no signal of the palladium(II) precatalyst.
This was also confirmed by the microscopic analysis that points out
the absence of palladium(II) traces. In addition, during both recy-
cling protocols, no palladium black was visually detected. The fact
that the catalytic yields decreased during the protocol of recycling
and reuse may be caused by the decomposition of the catalyst.
Further investigations are in progress to elucidate the structure of
the palladium species which remains in the ionic liquid medium.
as internal standard.
c
1
1
-n-Butyl-3-methylimidazole bromide.
-n-Butyl-3-methylimidazole hexafluorophosphate.
d
Heck coupling of a variety of para-susbtituted aryl halide with
donor- and acceptor substituents and different olefins was carried
out under the same conditions like Suzuki reaction, by using lower
loading of 1b as catalyst (1 mol%). The coupling took place in a short
time (ca. 30 min), in a quantitative yield (99%) without observation of
the insoluble palladium black (Entry 1, Table 3). The extension of this
method towards a wide variety of activated and deactivated aryl
iodide derivatives and different olefins led to good and quantitative
yields (60e99%) for both donor- and acceptor-substituted aryls
3. Conclusions
In summary, oxamate-containing palladium(II) complexes are
very stable and efficient catalysts in palladium-catalyzed Suzuki and
Heck reactions, by using both active and less reactive aryl halides.
The anionic character of the catalytic units permits their facile
immobilization in the ionic liquid, which has been found to influ-
ence positively their reactivity, stabilization and recycling [18]. Such
stabilization may be due to the formation of individual catalytically
(
Entry 2e9, Table 4). The efficiency of the catalysts used here is also
supported by the very good yields obtained by coupling the less
reactive bromobenzene with two different olefins (99%, Table 4). The
Precatalyst 1b was found to be better than PdCl
Pd(dba) in coupling bromobenzene with ethyl acrylate in terms of
yield of product and TONs. One again, formation of palladium black
was observed when using Pd(OAc) and Pd(dba) . This new approach
demonstrates the versatility of the oxamate-containing palladiu-
m(II) complexes toward the coupling reactions of different aryl ha-
lides with olefinic substrates in ionic liquids as reaction media.
The immobilization of the 1a and 1b precatalysts in molten n-
2 2
, Pd(OAc) and
4
active palladium(0) species, surrounded by a bulky n-Bu NBr salt
2
[19]. This would impose a Coulomb barrier for collision which would
prevent the precipitation of inactive palladium black and conse-
quently extend the catalyst life for further catalytic runs. Research
work about the influence of the steric and electronic nature of the
substituents of the oxamate ligand on the Suzuki and Heck reaction
mechanisms as well as the scope of oxamate-containing palladiu-
m(II) complexes in coupling other bromo- and chloro-aryl de-
rivatives with different electronic demands are underway.
2
2
4
Bu NBr, driven by their ionic interaction with this ionic liquid
suggested the possibility of their reusability. Recycling of 1a and 1b
4. Experimental
Table 3
Suzuki coupling reactions of aryl halides and phenylboronic acid mediated by 1a in
4.1. Materials and methods
n-Bu
4
NBr.
X
B(OH)2
Palladium(II) acetate and palladium chloride were purchased
5
mol % Pd-catalyst
from Aldrich. Palladium dibenzylacetone Pd(bda)
2
(dba ¼ dibenz-
R
1
+
ylideneacetone) was prepared as reported [20] NMR spectra ( H and
Et N / 120 min /120ºC
3
13
C) were recorded on an Avance DRX 300 Bruker instrument at
room temperature and referenced to residual protons in the solvent
R
1
13
13
X= I, Br
( H) or the solvent C signal ( C). The elemental analysis (C, H and
N) were carried out on a EuroEA3000 analyzer by the Servei Central
d’Instrumentació Científica at the University of Jaume I. FT-IR
Entrya
Pd-catalyst Yieldb (%) TON (mol product mol Pd)
ꢁ1
ꢁ1
R
X
spectra (4000e450 cm ) were recorded with a Nicolet 5700 FT-
1
2
3
4
5
6
7
8
9
1
1
H
CN
CH
COCH
OCH
COOCH
H
CH
H
H
I
I
I
I
I
I
1a
1a
1a
1a
1a
1a
78
99
99
47
78
99
65
60
5
16
20
20
10
16
20
14
12
1
IR instrument. GCeMS spectra were recorded on an Agilent 5973N
mass spectrometer equipped with capillary columns (split/splitless,
pulsed split and pulsed splitless) at the SCSIE (Servicio Central de
Soporte a la Investigación Experimental) of the University of
Valencia. The SEM-Edax microscopy analysis was performed on a
Philips mod. XL 30 ESEM with microanalysis EDAX mod. PV 9760
3
3
3
3
Br 1a
Br 1a
Br PdCl
Br Pd(OAc)
Br Pd(dba)
ꢀ
3
and HOT STAGE until 1000 C.
2
0
1
2
60
65
14
15
4.2. General procedure for the preparation of the N-phenyloxamate
H
2
proligands
a
Reaction conditions: 0.5 mmol of aryl halide, 0.75 mmol of phenylboronic acid,
N, 5 mol % 1a, 2 g of n-Bu NBr.
Determined by GCeMS using diethyleneglycol-di-n-butyl ether as internal
standard.
2
equiv of Et
3
4
b
The proligands as ethyl esters were prepared through the
following synthetic procedure: the corresponding aniline derivative

106
F.R. Fortea-Pérez et al. / Journal of Organometallic Chemistry 743 (2013) 102e108
Table 4
Heck coupling reactions of aryl halides and olefins mediated by 1b in n-Bu
4
NBr.
X
1
R
1
mol % Pd-catalyst
+
2
R
2
R
Et N / 30 min /120ºC
3
1
R
X= I, Br
Entrya
1
R1
H
X
I
Olefin
Pd-catalyst
1b
Yieldb (%)
99
TON (mol product mol Pd)
ꢁ1
100
64
2
H
I
1b
64
3
4
H
H
I
I
1b
1b
61
69
60
70
5
6
7
8
9
CN
CH
I
1b
1b
1b
1b
1b
1b
1b
1b
1b
99
99
99
99
99
99
99
99
99
0
100
100
100
100
98
3
I
COCH
3
I
OCH
3
I
COOCH
3
I
10
11
12
13
14
15
H
H
Br
Br
Br
Br
Br
Br
100
100
100
100
0
CH
CH
H
3
3
PdCl
2
H
Pd(OAc)
2
15
18
16
H
Br
Pd(dba)
2
74
75
a
Reaction conditions: 0.5 mmol of aryl halide 0.75 mmol of olefin, 2 equiv of Et
Determined by GCeMS using diethyleneglycol-di-n-butyl ether as internal standard.
3
4
N, 1 mol% 1b, 2 g of n-Bu NBr, 30 min.
b
(
83 mmol) was dissolved in THF (250 mL) under a dinitrogen at-
calculated (%) for C11
3
H13NO (207 g/mol): C 63.76, H 6.32, N 6.76;
mosphere in a two round flask equipped with a condenser and
subsequently treated with ethyl chlorooxoacetate (9.3 mL, 83 mmol)
in the presence of triethylamine (12 mL, 83 mmol) at room tem-
perature under continuous stirring for 30 min. The resulting solu-
tion was filtered and the solvent was removed under vacuum to
afford an oily colourless crude, which quickly becomes solid. The
white solid was suspended into water and filtered off, then washed
with a small amount of diethyl ether and dried under vacuum.
found: C 62.48, H 7.64, N 6.71.
4.2.2. N-4-Methylphenyloxamate proligand
ꢁ
1
Yield: 95%; IR (KBr/cm ):
n
¼ 3338 (NeH), 3120, 2978, 2908 (Ce
H), 1732, 1705 (C]O); H NMR (DMSO-d (ppm): 1.28e1.33 (t, 3H,
CH ), 2.27 (s, 3H, CH ), 4.28e4.31 (q, 2H, CH ), 7.14e7.14 (d, 2H, Har-
omatics), 7.61e7.64 (d, 2H, Haromatics), 10.68 (s, 1H, NH); C NMR
(DMSO-d (ppm): 14.19, 20.85, 62.67,120.76,129.50,134.20,135.32,
55.74,161.14; elemental analysis calculated (%) for C11 (207 g/
mol): C 63.76, H 6.32, N 6.76; found: C 63.36, H 6.04, N 6.77.
1
6
) d
3
3
2
1
3
6
) d
1
H13NO
3
4
.2.1. N-2-Methylphenyloxamate proligand
ꢁ1
Yield: 95%; IR (KBr/cm ):
n
¼ 3216 (NeH), 3033, 2983, 2920
CeH), 1735,1699 (C]O); H NMR (CDCl (ppm): 1.35e1.42 (t, 3H,
CH ), 2.31 (s, 6H, CH ), 4.39e4.48 (q, 2H, CH ), 7.11e7.19 (m, 1H,
aromatics), 7.21e7.30 (m, 2H, Haromatics), 7.9e8.0 (d, 1H, Haromatics),
1
(
3
)
d
4.3. General procedure for the preparation of the bis(oxamato)
palladate(II) complexes
3
3
2
H
8
13
.83 (s, 1H, NH); C NMR (CDCl
3
)
d(ppm): 14.41, 17.74, 64.13,122.31,
A general synthetic method for the preparation of the bis(ox-
amato)palladate(II) complexes is described as follows: a 1.0 M
127.31, 128.82, 130.79, 134.78, 154.31, 161.57; elemental analysis

F.R. Fortea-Pérez et al. / Journal of Organometallic Chemistry 743 (2013) 102e108
107
3
(0.50 mmol), NEt (1.00 mmol), the aryl boronic acid (0.75 mmol)
and 2e3 g of the ionic liquid. The mixture was heated under
ꢀ
continuous stirring during 120 min at 120 C for n-Bu
4
NBr and in
ꢀ
the range of temperature 80e120 C for the other ionic liquids,
4 6
namely n-Bu NCl, BMIMBr and BIMIMPF . The reaction was
monitored by using thin liquid chromatography on silica gel. The
reaction mixture was cooled and extracted with 5 mL of n-pentane.
The products were examined by GCeMS, purified by column
1
13
chromatography and finally characterized by H NMR, C NMR and
C NMR-Dept.
13
4.4.2. General procedure for the Heck reaction
A test-tube with screw cap and valve was charged with a
magnetic stir bar, the pre-catalyst 1 mol % Pd, the aryl halide
0.50 mmol), NEt (1.00 mmol), the olefin (0.75 mmol) and 2e3 g of
(
3
the ionic liquid. The reaction was heated under continuous stirring
ꢀ
during 30 min at 120 C for n-Bu
4
NBr and in the temperature range
NCl, BMIMBr
. The reaction was monitored using thin liquid
Fig. 3. Histogram of the recycling experiments of the Suzuki reaction (black) of
iodobenzene with phenylboronic acid (black) and of the Heck reaction of iodobenzene
and ethyl acrylate (grey) by using 1a and 1b, respectively.
ꢀ
80e120 C for the other ionic liquids, namely n-Bu
4
and BIMIMPF
6
chromatography on silica gel. Workup was identical to that
described above for the Suzuki reaction.
4
methanolic solution of n-Bu NOH (1.5 mL, 1.2 mmol) was added
directly to a suspension of the corresponding oxamate-aniline
proligand (0.6 mmol in 10 mL of acetonitrile) in a two-neck round
flask under continuous stirring. Then, an aqueous solution of
4.4.3. Procedure for catalyst recycling
The catalysis was performed as described above. After comple-
tion of the reaction, the mixture was cooled and extracted with
5 mL of n-pentane. The products had to be prevented from early
solidification by using a heat gun. The remaining ionic solvent
containing the palladium(II) catalyst was charged once again with
fresh aryl halide and the corresponding aryl boronic acid or olefin
for further catalytic runs. The products were examined by GCeMS.
Further purification of the products was performed by flash column
chromatography.
K
2
PdCl
4
(100 mg, 0.30 mmol) was added dropwise to the resulting
ꢀ
solution and the reaction mixture was heated at 60 C under N
2
for
10 h. The resulting mixture was filtered and the volume reduced
under vacuum. The remaining concentrated solution was washed
with diethyl ether to remove the unreacted ligand and subsequently
treated three times with dichloromethane to extract the complex
from the aqueous solution. The addition of n-hexane to the
dichloromethane solution affords the complex as a pale yellow
powder which was collected by filtration and dried in the open air.
4.5. Procedure for the analysis of the catalyst leaching
4
.3.1. (n-Bu
Yield: 86%; IR (KBr/cm ):
4
N)
2
[Pd(N-2-methylphenyloxamate)
2
]$4H
2
O (1a)
The isolated products of the first and final catalytic run of each
protocol were analysed by H NMR and FT-IR spectroscopy to check
the presence of typical signals and bands of the palladium(II) pre-
catalysts. The isolated dried products were also analyzed by SEM-
Edax microscopy to verify the absence of palladium traces.
ꢁ
1
1
n
¼ 3423 (OeH), 2961, 2930, 2874 (Ce
1
H), 1669, 1647, 1618, 1590 (C]O); H NMR (CDCl
3
)
d(ppm): 1.00e
þ
1.02 (t, 24H), 1.40e1.42 (m, 16H, n-Bu
4
N ), 1.51e1.59 (m, 16H, n-
þ
þ
Bu
4
N ), 2.17 (s, 6H, CH
3
), 3.18e3.24 (m, 16H, n-Bu
4
N ), 7.16e7.19
(
C
6
m, 8H,
H
aromatics); elemental analysis calculated (%) for
10Pd (1a) (1018 g/mol): C 59.01, H 9.31, N 5.51; found: C
1.18, H 9.96, N 5.44. X-ray quality crystals of 1a of formula (n-
Bu N) [Pd(N-2-methylphenyloxamate) ]$4MeCN were grown by
H
50 94
N
4
O
4.6. Crystal data collection and refinement
4
2
2
X-ray diffraction data on single crystals of 1a and 1b as aceto-
nitrile solvates were collected with a BrukereNonius X8APEXII CCD
slow vapour diffusion of ether into an acetonitrile solution of the
palladium(II) complex.
area detector diffractometer. Graphite-monochromated Mo-K ra-
a
ꢀ
diation (
l
¼ 0.71073 A) was used. The data were processed through
4
.3.2. (n-Bu
Yield: 95%; IR (KBr/cm ):
4
N)
2
[Pd(N-4-methylphenyloxamate)
2
]$2H
2
O$MeCN (1b)
the SAINT [21] reduction and SADABS [22] absorption software. A
summary of the crystallographic data and structure refinement for
the two compounds is given in Tables S1 and S2.
ꢁ
1
n
¼ 3422 (OeH), 2961, 2930, 2874 (Ce
1
H), 1658, 1617, 1588, 1540 (C]O); H NMR (CDCl
0
Bu
3
)
d(ppm): 0.92e
þ
.95 (t, 24H), 1.33e1.36 (m, 16H, n-Bu
4
N ), 1.48e1.52 (m, 16H, n-
The structures were solved by Patterson and subsequently
completed by Fourier recycling using the SHELXTL software pack-
age [23]. All non-hydrogen atoms were refined anisotropically. The
hydrogen atoms of the counter ions as well as those of the solvent
molecules were set in calculated positions and refined as riding.
þ
þ
4
N ), 1.89 (s, 6H, CH
3
), 3.07e3.13 (m, 16H, n-Bu
4
N ), 6.32e6.35
(
d, 2H, Haromatics), 6.43e6.46 (d, 2H, Haromatics), 6.87e6.90 (d, 2H,
aromatics), 7.00e7.02 (d, 2H, Haromatics); elemental analysis calcu-
lated (%) for C52 10Pd (1b) (1023 g/mol): C 61.07, H 9.17, N
.85; found: C 61.55, H 10.31, N 6.35. X-ray quality crystals of 1b of
formula (n-Bu N) [Pd(N-4-methylphenyloxamate) ]$4MeCN were
H
93 5
H N O
2
6
The final full-matrix least-squares refinements on F , minimizing
P
2
4
2
2
the function w(jF j ꢁ jF j) , reached convergence with the values
o
c
grown by slow vapour diffusion of ether into an acetonitrile solu-
tion of the palladium(II) complex.
of the discrepancy indices given in Tables S1 and S2. In particular,
there is a high electronic residual density for 1a which is located
þ
near the carbon atoms of the [(n-C
4
H
9
)
4
N] cations as a conse-
4
4
.4. General catalytic procedures and recycling of catalysts
quence of a static disorder that affect them.
The final geometrical calculations were carried out with the
PARST [24] program whereas the graphical manipulations were
performed with the XP utility of the SHELXTL system. Interatomic
bond lengths and angles are listed in Tables S3 and S4.
.4.1. General procedure for the Suzuki reaction
A test-tube with screw cap and valve was charged with a
magnetic stir bar, the pre-catalyst 5 mol% Pd, the aryl halide

1
08
F.R. Fortea-Pérez et al. / Journal of Organometallic Chemistry 743 (2013) 102e108
[7] (a) B.R. Buckley, S.P. Neary, Tetrahedron 66 (2010) 7988.
Acknowledgements
[
[
8] I. Suresh, M.K. Girish, C. Ramesh, Tetrahedron 60 (2004) 2163.
9] (a) R. Sheldon, Chem. Commun. (2001) 2399;
This work was supported by the Generalitat Valenciana
(
b) T. Welton, Coord. Chem. Rev. 248 (2004) 2459.
[10] (a) A. Corma, H. García, A. Leyva, Tetrahedron 60 (2004) 8553;
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(
GV-Prometeo2009/108) and MCIIN (CTQ2010-15364). R.F. thanks
(
the MCIIN for a predoctoral FPU grant. Prof. G. D. M. thanks the
Universidad de Valencia for a visiting professorship grant.
[11] (a) E. Pardo, R. Ruiz-García, J. Cano, X. Ottenwaelder, R. Lescouëzec,
Y. Journaux, F. Lloret, M. Julve, Dalton Trans. (2008) 2780;
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Appendix A. Supplementary material
[
[
12] J. Estrada, I. Fernández, J.R. Pedro, X. Ottenwaelder, R. Ruiz, Y. Journaux,
Tetrahedron Lett. 38 (1997) 2377.
13] (a) R. Kivekäs, A. Pajunen, A. Navarrete, E. Colacio, Inorg. Chim. Acta 284
CCDC 902499, 902500 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
(
(
1999) 292;
b) W.X.C. Oliveira, M.A. Ribeiro, C.B. Pinheiro, W.C. Nunes, M. Julve,
Y. Journaux, H.O. Stumpf, C.L.M. Pereira, Eur. J. Inorg. Chem. 34 (2012) 5685.
[14] Generally, ionic liquids (ILs) are defined as those fused salts with a melting
ꢀ
point less than 100 C, with salts with higher melting points referred to as
Appendix B. Supplementary data
molten salts. See: J. Ranke, S. Stolle, R. Störmann, J. Arning, B. Jastorff, Chem.
Rev. 107 (2007) 2183. However, others defined ILs as salts with low melting
ꢀ
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2013.06.041.
points less than 150 C. See: In Ionic Liquids, J.D. Holbrey, R.D. Rogers ACS
Symposium Series; ACS; Washington, DC, 2002, Chapter 1, p. 4. TBAB has
largely been used as ILs, see: V.P.W. Böhm, W.A. Hermann Chem. Eur. J. 6
(
2000) 1017.
[15] (a) T.J. Gannon, G. Law, P.R. Watson, A.J. Carmichael, K.R. Seddon, Langmuir 15
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