Synthesis of palladium complexes with bis(diphenylphosphinomethyl)amino ligands: A catalyst for the Heck reaction of aryl halide with methyl acrylate

Article abstract of DOI:10.1016/j.jorganchem.2007.01.003

A series of ligands, (Ph₂PCH₂)₂NR (R = -CH₃) (1), -C(CH₃)₃, (2) -m-C₆H₄SO₃Na (3), and their Pd(II) complexes have been synthesized under nitrogen atmosphere

Full text of DOI:10.1016/j.jorganchem.2007.01.003

Journal of Organometallic Chemistry 692 (2007) 1951–1955
www.elsevier.com/locate/jorganchem
Synthesis of palladium complexes
with bis(diphenylphosphinomethyl)amino ligands: A catalyst
for the Heck reaction of aryl halide with methyl acrylate
*
Mustafa Keles, Ziya Aydin, Osman Serindag
Department of Chemistry, Faculty of Science and Letters, University of C¸ ukurova, 01330 Adana, Turkey
Received 23 August 2006; received in revised form 1 December 2006; accepted 5 January 2007
Available online 14 January 2007
Abstract
A series of ligands, (Ph2PCH2)2NR (R = –CH3) (1), –C(CH3)3, (2) -m-C6H4SO3Na (3), and their Pd(II) complexes have been synthe-
sized under nitrogen atmosphere using Schlenk method. All compounds were characterized using elemental analysis and spectroscopic
techniques (AAS, NMR (1H, 31P)). Based on the analysis the complexes have been proposed as in square planar geometry. The Pd(II)
complexes were applied to the Heck reaction of aryl halide (Br, Cl) with methyl acrylate. The results have exhibited that complexes
[PdCl2((Ph2PCH2)2NCH3)] (4) and [PdCl2((Ph2PCH2)2NC(CH3)3)] (5) have shown higher turnover numbers (TON) than complex
[PdCl2(Ph2PCH2)2N-m-C6H4SO3Na] (6).
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Bis(diphenylphosphinomethyl)amino; Pd complex; Heck reaction; Aryl halide
1. Introduction
ingly important due to their improved catalytic activity [6–
8]. The nature of the phosphine ligand on complexes influ-
ences on the stability of the catalysts and on the rate of cat-
alyzed reactions [9]. Several nitrogen containing
phosphine–palladium complexes have been studied in a
variety of catalytic systems to reach higher yields in Heck
reactions [10].
In the present paper, some bis(diphenylphosphinom-
ethyl)amino (P–C–N) type ligands and their Pd(II) com-
plexes have been synthesized in order to test in the Heck
reactions of bromobenzene and chlorobenzene with methyl
acrylate. Although tert-butyl substituted aminomethyl-
phosphine used in this study was reported previously, the
synthetic route was slightly different [11].
Heck reactions, palladium-catalyzed cross coupling
reactions, have been most powerful tools for generating a
new C–C bond in organic synthesis; thus, a variety of pal-
ladium–phosphine complexes have been developed [1–3].
Aryl halides (I, Br and Cl) and activated alkenes are typical
reactants for the Heck reactions. Although iodide deriva-
tives have a high reactivity than bromide and chloride
derivatives, the latter ones are preferred in synthetic routes
due to their cheapness and availability [4]. Cinnamic esters
as the resulting compounds of Heck reaction derived from
aryl halides and alkyl acrylates have found application as
UV absorbers, antioxidants in plastics, and intermediates
in pharmaceuticals [5].
In recent years nitrogen containing chiral phosphines
and their transition metal complexes have become increas-
2. Results and discussion
A series of ligands, (Ph2PCH2)2NR (R = –CH3, –
C(CH3)3, -m-C6H4SO3Na), have been by treating phospho-
nium salt, ([PPh2(CH2OH)2]Cl) with appropriate primary
amines according to the Mannich reaction in the presence
*
Corresponding author. Tel.: +90 322 338 6542; fax: +90 322 338 60
70.
E-mail address: osmanser@cu.edu.tr (O. Serindag).
0022-328X/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.01.003

1952
M. Keles et al. / Journal of Organometallic Chemistry 692 (2007) 1951–1955
Table 1
of triethylamine as a base (Scheme 1). The Pd(II) com-
plexes of the bis(diphenylphosphinomethyl)amino ligands
were prepared under nitrogen atmosphere using Schlenk
techniques as shown in Scheme 1.
31P NMR data for ligands and metal complexes
Compounds
dp (ppm)
Dd (ppm)a
1
2
3
4
5
6
a
À27.2
À26.36
À27.53
7.4
–
–
–
2.1. Characterization of bis(diphenylphosphino-
methyl)amino ligands and their Pd(II) complexes
34.6
34.48
35.74
8.12
8.21
Bis(diphenylphosphinomethyl)amino ligands and their
31P chemical shifts Dd = d (complex) À d (free ligand) [17].
1
Pd(II) complexes were characterized using H NMR, 31P
NMR, and elemental analysis.
1
Assignment of H NMR spectra of ligands and their
Elemental analysis for C, H and N of ligands and AAS
for metal contents have indicated that the metal–ligand
ratio of complexes were 1:1.
Pd(II) complexes showed that the aromatic protons of
the phenyl ring had the chemical shift value (d) in a range
of 6.9–7.52 ppm. The spectra of P–CH2–N protons of
ligands and their Pd(II) complexes were observed around
3.42–3.62 ppm which is in agreement with reported studies
[12,13]. N–C–(CH3)3 proton signals of ligand 2 and com-
plex 5 appeared in 1.21 and 1.23 ppm, respectively. Assign-
ment of 1H NMR spectra showed no remarkable
differences between free ligands and their Pd(II) complexes.
The P–CH2–N resonance was slightly shifted to a low field
compared to the free ligands, the consequence of which
shows that N is not coordinated Pd. This observation is
an additional evidence that the NCH2 protons are situated
away from the coordination sphere around the metal as
reported in the literature [11–14].
2.2. Heck reaction
Pd(II) complexes 4, 5, and 6 were applied to the Heck
reaction of methyl acrylate with bromobenzene and chloro-
benzene respectively. The reactions were carried out in the
NMP solvent and using K3PO4 as a base at 140 ꢁC for 14 h
since these aryl halides are often unreactive towards oxida-
tive addition under mild conditions (Scheme 2).
The percentage of conversions of the aryl halides and
methyl acrylate to the methyl cinnamate was determined
based on the amount of the product by GC and GC–MS.
Product yields and TON’s corresponding to the catalysts
are presented in Table 2. The Heck reaction of bromoben-
zene with methyl acrylate gave better yields of the desired
product than that of chlorobenzene. Additionally, the pal-
ladium complexes exhibited a better catalytic activity for
the Heck reaction of bromobenzene than that of chloro-
benzene with methyl acrylate in the catalytic experiments
under the studied conditions. These results are in agree-
ment with the reported studies [3,20].
It was found that 31P NMR spectra of all complexes
gave more shielded signals compared with the
uncoordinated aminomethyldiphosphine ligands as shown
in Table 1. The results are in consistent with those of the
reported studies giving coordination shifts in a range of
34.6–35.74 ppm [12,15,16]. The coordination shift values
of the complexes (D), depending on the metal centers and
the chemical structures of the ligands showed that the
ligands coordinated to Pd(II) via phosphorus atoms to give
chelated complexes [17]. Based on 31P NMR data, Pd(II)
complexes have been proposed as in square planar geome-
try as those of the reported studies [12,13,15]. Although
nitrogen is seemed as a potential coordinating site to the
metal center, many studies reported including single crystal
X-ray diffraction studies clearly showed that there was no
close interaction between the nitrogen atom of the amine
group and the metal center [11,15,18,19].
It is well known that the catalytic activity depends on
the type of aryl halides. While electron-withdrawing groups
on the aryl ring increase the reaction rate [21], electron-
donating substituent on the aryl group make the oxidative
addition more difficult and, as a result, electron-poor aryl
halides are often referred to as ‘‘activated’’, and electron-
rich aryl halides as ‘‘deactivated’’ [20].
From the obtained results it can be concluded that the
presence of Pd(II) complexes of bis bis(diphenylphosphi-
H2O/EtOH
2[Ph2P(CH2OH)2]Cl + RNH2 + NEt3
(Ph2PCH2)2NR + Et3NHCl + HCHO
reflux
Ph2
P
Cl
Cl
CH2Cl2
Pd
[PdCODCl2] + (Ph2PCH2)2NR
Ph2P
R
N
R= -CH3, -C(CH3)3, -m-C6H4SO3Na
Scheme 1. Synthesis of bis(diphenylphosphinomethyl)amino ligands and their Pd(II) complexes.

M. Keles et al. / Journal of Organometallic Chemistry 692 (2007) 1951–1955
1953
O
Palladium catalysts
X
+
O
K3PO4, NMP, 140 oC
OCH3
OCH3
Scheme 2. Heck reaction of aryl halide with methyl acrylate.
Table 2
Catalytic activities of Pd(II) complexes have been tested
for the Heck reaction. The results show that, these com-
plexes catalyze the reaction of methyl acrylate with bromo-
benzene and with chlorobenzene to give methyl cinnamate
in good yields. The Heck reaction of bromobenzene with
methyl acrylate is better catalyzed than chlorobenzene.
Complexes 4 and 5 were found to be better catalysts than
complex 6 which contained the deactivated group of –SO3.
Heck reactiona of aryl halide with methyl acrylate catalysed by palladium
complex
Entry
X
Catalysts
Conversion
(%)b,c
TON
1
2
3
4
5
Br [PdCl2((Ph2PCH2)2NCH3)]
Cl [PdCl2((Ph2PCH2)2NCH3)]
Br [PdCl2(Ph2PCH2)2NC(CH3)3]
Cl [PdCl2(Ph2PCH2)2NC(CH3)3]
Br [PdCl2(Ph2PCH2)2N-m-
C6H4SO3Na]
94
65
96
68
87
5220
3600
5330
3780
5560
4. Experimental
6
Cl [PdCl2(Ph2PCH2)2N-m-
C6H4SO3Na]
53
4000
a
4.1. General
Reaction conditions: aryl halide (0.56 mmol), methyl acrylate
(0.67 mmol) potassium phosphate (0.67 mmol) (0.018 mmol) catalyst in
1 mL NMP, 140 ꢁC, 14 h.
All reactions were carried out under nitrogen atmo-
sphere using conventional Schlenk glassware. All solvents
and 1-methyl-2-pyrrolidone (NMP) were dried using estab-
lished procedures and then immediately distilled under
nitrogen atmosphere prior to use. The phosphonium salt
([PPh2(CH2OH)2]Cl) and [PdCI2(COD)] were prepared
described in the literatures [23,24].
b
GC yields are based on the amount of aryl halide and methyl
cinnamate.
c
Determined by GC and GC–MS.
nomethyl)amino ligands were found to be catalysis Heck
reaction with bromobenzene and chlorobenzene. The
results are consistent with the literature [10].
In addition, the electron-withdrawing ability of the P–
C–N linkage increases the p-acidity character of the phos-
phine ligands. Thus, it can be assumed that the pre-activa-
tion of the metal, Pd(II) to the active Pd(0) species is a low
energy process since the low oxidation state of the metal
[22].
The catalytic activity of the Pd(II) complexes depending
on the nature of the nitrogen substituent, and increasing in
order to: tert-butyl (5) > methyl (4) > m-C6H4-SO3Na (6).
The trend seems to fit the electronic nature of the substitu-
ents, while the most electron-donating group (tert-butyl)
compared to the most electron-withdrawing group (-phe-
nyl) [8]. The sulfonated derivative of the phosphine ligand
and its metal complex were prepared to find out catalytic
activity for further study on some reactions in the polar
solvent system.
The metal contents of the complexes were determined
using a Hitachi 180–80 Polarized Zeiman atomic absorp-
tion spectrometer. Elemental analysis was performed on a
1
LECO CHNS 932. H NMR and 31P NMR spectra were
recorded at 25 ꢁC in DMSO-d6 and CDCl3 on a Varian
Mercury 200 MHz NMR spectrometer. 31P NMR spectra
were recorded with complete proton decoupling and
reported in ppm using 85% H3PO4 as an external standard.
The catalyst products were analyzed on Shimadzu GC 14 A
with a 30 m · 0.25 mm ZB-5 column and Thermo Finnigan
GC–MS Electron impact (70 eV) with a 60 m · 0.25
mm · 0.25 lm ZB-5 column.
4.2. Synthesis of bis(diphenylphosphinomethyl)amino
ligands and their Pd(II) complexes
4.2.1. [(Ph2PCH2)2NCH3] (1)
Bis(diphenylphosphinomethyl)aminomethane ([(Ph2P-
CH2)2NCH3]) were prepared according to the literature
methods [16].
3. Conclusion
The complexes of Pd(II) with bidentate tertiary
bis(diphenylphosphinomethyl)amino ligands have been
synthesized and characterized by using spectroscopic tech-
niques. 31P NMR spectra of the complexes have indicated
that coordination of aminomethyldiphosphine ligand to
metal center gives deshielded chemical shift value com-
pared with free ligand.
4.2.2. [(Ph2PCH2)2NC(CH3)3] (2)
Phosphonium salt (0.260 g, 0.92 mmol) was dissolved in
2:1 H2O:EtOH (15 mL) solvent system. NEt3 (0.5 mL,
3.65 mmol) was added to a stirred solution of [Ph2P-
(CH2OH)2]CI, followed by the addition of tert-butylamine
(46 lL, 0.46 mmol). The mixture was refluxed for 1 h and

1954
M. Keles et al. / Journal of Organometallic Chemistry 692 (2007) 1951–1955
then the formed product was extracted with CH2CI2 and
dried over Na2SO4. The solvent was removed under vac-
uum. Yield: 0.173 g (80%). Elemental analysis calculation
for [(Ph2PCH2)2NC(CH3)3]: C, 76.75; H, 7.04; N, 2.98%.
Found: C, 77.14; H, 7.15; N, 3.07. 1H NMR (CDCI3,
25 ꢁC): d 7.34–7.18 [m, 20H, 4Ph], 3.42 [d-br, 4H, 2 (P–
CH2–N)], 1.21 [s, 9H, 2 (N–C–(CH3)3)] ppm 31P NMR
(CDCI3, 25 ꢁC): d À27.2 [s, PPh2–CH2] ppm.
Elemental analysis calculation for [Pd(Ph2PCH2)2N-m-
C6H4SO3Na]Cl2: C, 49.97; H, 3.64; N, 1.82%. Found: C,
50.6; H, 3.85; N, 2.02%. 1H NMR (CDCI3, 25 ꢁC): d
7.83–7.42 [m, 20H, 4Ph], 6.87 [m, 4H, N–Ph] 3.62 [d, 4H,
P–CH2–N], ppm. 31P NMR (CDCI3, 25 ꢁC): d 8.12 [br,
Pd–PPh2] ppm.
4.3. Heck reactions
4.2.3. [(Ph2PCH2)2N-m-C6H4SO3Na] (3)
An oven-dried Schlenk flask was charged with potas-
sium phosphate (K3PO4) (0.67 mmol) and NMP (1 mL)
under N2 atmosphere. Bromobenzene (0.56 mmol), meth-
ylacrylate (0.67 mmol) and complex 4 (0.018 mmol) were
added into the flask. The flask was then sealed under
N2 atmosphere and placed in an oil bath preheated to
140 ꢁC. The reaction mixture was stirred for 14 h and
then allowed to cool to room temperature. The reaction
mixture was poured into water (20 mL) and extracted
with ethyl acetate (3 · 20 mL). The extracts were washed
with brine, and dried over MgSO4. Similar procedures
were used for complexes 5, 6 and for chlorobenzene with
each complexes.
NEt3 (0.8 mL, 5.84 mmol) was added to a stirred solu-
tion of [Ph2P(CH2OH)2]Cl (0.5 g, 1.77 mmol) in 2:1
H2O:MeOH (15 mL) solvent system followed by the addi-
tion of sodium 3-aminobenzenesulfonate (0.175 g,
0.9 mmol) dissolved in CH2Cl2. The mixture was refluxed
for 2 h and then allowed to cool to room temperature.
The formed product was extracted with CH2Cl2 and dried
over Na2SO4. The solvent was removed under vacuum.
Yield: 0.346 g (66%). Elemental analysis calculation for
[(Ph2PCH2)2N-m-C6H4SO3Na]: C, 64.97; H, 4.74; N,
2..37%. Found: C, 65.40; H, 4.57; N, 2.53%. 1H NMR
(CDCI3, 25 ꢁC): d 7.52–7.32 [m, 20H, 4P–Ph], d 6.90 [m,
4H, N–Ph], 3.50 [m, 4H, P–CH2–N] ppm. 31P NMR
(CDCI3, 25 ꢁC): d À26.36 [s, PPh2–CH2] ppm.
Acknowledgments
4.2.4. [PdCl2((Ph2PCH2)2NCH3)] (4)
We thank Dr. Ozgur So¨nmez and Dr. Arif Hesenov for
¨
Ligand 1 (0.28 g, 0.655 mmol) was added to a stirred
solution of [PdCl2(COD)] (0.185 g, 0.21 mmol) in CH2Cl2
(10 mL). The mixture was stirred for a further 1 h at room
temperature. The addition of diethylether gave the pale yel-
low solid which was then filtered and dried. Yield: 0.353 g
(88%). Elemental analysis calculation for [PdCl2((Ph2-
PCH2)2NCH3)]: C, 53.6; H, 4.21; N, 2.03%. Found: C,
52.6; H, 4.58; N, 2.36%. 1H NMR (CDCI3, 25 ꢁC): d
7.37–7.87 [m, 20H, 4Ph], 3.24 [d, 4H, P–CH2–N], 2.52 [s,
3H, N–Me] ppm. 31P NMR (CDCI3, 25 ꢁC): d 8.21 [Pd–
PPh2] ppm.
GC–MS and GC analysis. The authors also thank Re-
search Fund of C¸ ukurova University for financial support
to this project.
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1
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