New Pd(II) complexes of sulfur ylides; Synthesis, X-ray characterization, a theoretical study and catalytic activity toward the Mizoroki-Heck reaction

Article abstract of DOI:10.1039/c6ra01390b

The reaction of sulfur ylides (L) SMe₂C(H)C(O)R (R = 4-nitrophenyl, phenyl, and 3-nitrophenyl) with the dichloro(1,5-cyclooctadiene)palladium(II) complex, [PdCl₂(cod)], in a 2:1 ratio gives the new Pd(II) complexes of type cis- and trans-[PdCl₂(SMe₂C(H)C(O)R)₂] (R = phenyl (1), 3-nitrophenyl (2), and 4-nitrophenyl (3)). Characterization of the obtained complexes was performed by elemental analysis, IR, ¹H, ¹³C NMR and mass spectroscopies. Also, the structure of complex trans-[PdCl₂(SMe₂C(H)C(O)C₆H₄-p-NO₂)₂] (3) was characterized by single crystal X-ray analysis. The X-ray crystallography results reveal that the structure of complex 3 contains two Cα-coordinated sulfur ylide ligands in trans geometry. These air/moisture stable complexes were also employed as efficient catalysts for the Mizoroki-Heck cross-coupling reaction of several aryl halides with olefins. The coupled products of these reactions were obtained in good to excellent yields and purity, short reaction times and low catalyst loading. Also a theoretical study on the structure of the complexes 1-3 have been investigated at the BP86/def2-SVP level of theory. The bonding situation between the [PdCl₂] and L₂ fragments [(Ylide)₂] in the [L₂PdCL₂] complexes, were carried out by NBO and energy-decomposition analysis (EDA), as well as their natural orbitals for chemical valence variation (EDA-NOCV). The results confirmed that the contribution of the electrostatic interactions in the Pd-C bond in the complexes is about 50%.

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New Pd(II) Complexes of Sulfur Ylide; Synthesis, X-ray
Characterization, Theoretical Study and Catalytic Activity toward
Mizoroki-Heck Reaction
Received 00th January 20xx,
Accepted 00th January 20xx
S. J. Sabounchei,a M. Ahmadianpoor,a A. Yousefi,a A. Hashemi,a M. Bayat,a A. Sedghi, a F. Akhlaghi
Bagherjeri,b R. W. Gableb
DOI: 10.1039/x0xx00000x
www.rsc.org/
The reaction of sulfur ylides (L) SMe2C(H)C(O)R (R = 4-nitrophenyl, phenyl, and 3-nitrophenyl) with dichloro(1,5-
cyclooctadiene)palladium(II) complex, [PdCl2(cod)], in 2:1 ratio gives the new Pd(II) complexes of type cis- and trans-
[PdCl2(SMe2C(H)C(O)R)2] (R = phenyl (1), 3-nitrophenyl (2), and 4-nitrophenyl (3)). Characterization of the obtained
complexes was performed by elemental analysis, IR, 1H ,13C NMR and mass spectroscopies. Also, the structure of complex
trans-[PdCl2(SMe2C(H)C(O)C6H4-p-NO2)2] (3) was characterized by single crystal X-ray analysis. The X-ray crystallography
results reveal that the structure of complex 3 contains two Cα-coordinated sulfur ylide ligands in trans geometry. These
air/moisture stable complexes were also employed as efficient catalysts for the Mizoroki-Heck cross-coupling reaction of
several aryl halides with olefins. The coupled products of these reactions were obtained in good to excellent yields and
purity, short reaction times and low catalyst loading. Also a theretical study on structure of the complexes 1-3 have been
investigated at the BP86/def2-SVP level of theory. The bonding situation between the [PdCl2] and L2 fragments[(Ylide)2] in
[L2PdCL2] complexes, were carried out by NBO and energy-decomposition analysis (EDA), as well as its natural orbitals for
chemical valence variation (EDA-NOCV). The results confirmed that the contribution of the electrostatic interactions in the
Pd-C bond in the complexes is about 50%.
structure modes. When a metal ion such as Pd(II) interacts
only with the Cα atoms of two ylidic groups, these ligands
Introduction
can be placed in the cis or trans positions to form the
mononuclear structure [8]. Dinuclear structure was also
observed in case of Hg(II) halides, in which two ylides were
coordinate from the Cα atoms to dimeric Hg2X4 (X=Cl, Br, I)
unit [9-10]. Finally, the polymeric structure was seen when
metal ions have bridging ligands. For example, it was
reported a polymeric structure of sulfur ylide with AgNO3
by our group and the X-ray analysis shows a structure with
a metallic skeleton based on the C-coordinated sulfur ylide
and bridged nitrate group [11].
This is a fact that the sulfur ylides R2S=C(R')(R") are interesting
ligands in organometallic chemistry; because these compounds
are powerful nucleophilic reagents and provide versatile ligands
for metals in their various oxidation states [1-6]. The
coordination chemistry of some sulfur ylide complexes have
been investigated extensively and α-keto stabilized sulfur ylides,
because of their resonating character, show more interesting
properties such as high stability and ambidentate character as
ligands. Due to the proximity of the keto group and carbanion in
these ylides, resonance delocalization of the ylidic electron
density was observed, providing additional stabilization and
leading to form some useful chelates and complexes [7].
Coordination modes of these compounds in several
metal complexes have been studied and it was found that
these ylides can coordinate to metal ions in three modes as
depicted in Chart 1: mononuclear, dinuclear and polymeric
Chart 1. The possible bonding modes of sulfur ylides to metal
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It was reported that some of phosphorus ylide palladium
Journal Name
crystal was kept at 130.00(10) K during data collection.
Using Olex2 [31], the structure was solved with the ShelXT
[32] structure solution program using Direct Methods and
refined with the ShelXL [33] refinement package using Least
Squares minimisation. All non-hydrogen atoms were
refined with anisotropic displacement parameters; H-atoms
were constrained to geometrical estimates, with an
isotropic displacement parameter of 1.5 times (Me) or 1.2
times (other) the parent carbon atom.
complexes have useful applications in catalytic C-C coupling
reactions [12-15]. Although these complexes have some
advantages such as high efficiency, low catalyst loading and
moisture/air stability, but most of them are expensive and
have environmental considerations. In the past years,
phosphine free catalytic reactions were developed to
eliminate or reduce costs, operational hazards and
environmental pollution [16-19]. Among these, there are
some catalytic reactions that sulfur containing ligands and
their Pd(II) complexes were employed as catalyst in the
reaction [20-22].
Synthesis of the Pd(II) complexes
General procedure: To
a [PdCl2(cod)] (0.5 mmol)
dichloromethane solution (5 ml), a solution of ylide (1
mmol) (5 ml, CH2Cl2) was added. The resulting solution was
stirred for 2 h at room temperature and then concentrated
to a ca. 2 ml under reduced pressure and treated with n-
hexane (15 ml) to afford the Pd(II) complex of desired sulfur
ylide.
Heck reaction is one of the most widely adopted
methods for the construction of carbon-carbon bonds in
modern organic chemistry and it is commonly carried out in
the presence of a palladium complex [23-28]. For example,
in the reported work by the group of Grubber et al.
palladium catalysts containing modified sulfur ligands could
activate the coupling reactions with less reactive aryl
bromides and aryl chlorides as well as nonactivated alkenes
[29]. It appears that the release of Pd(0) species in Heck
reactions is depended on the ligands around the palladium
complex. Thus, organochalcogen ligands such as sulfur
ylides, show rivalry with their respective phosphorus ylide
analogues for releasing of Pd(0) species in the Heck C–C
coupling reaction.
Data for [PdCl2((Me)2SC(H)C(O)C6H5)2] (1)
Yield: 0.26 g (75%). Anal. Calc. for C20H24Cl2O2PdS2 (%): C,
44.66; H, 4.60; S, 11.92. Found: C, 44.95; H, 4.77; S, 12.28.
M.p. 189-191 °C. Selected IR absorption in KBr (cm-1) ν(CO):
1
1646. H NMR (250.13 MHz, DMSO-d6) δH (ppm): 2.46-2.74
(m, 12H, SMe2) (trans isomer), 2.67-3.12 (m, 12H, SMe2) (cis
isomer), 4.93 (s, 1H, SCH, trans major diastereoisomer),
5.15 (s, 1H, SCH, trans minor diastereoisomer), 5.62 (s, 1H,
SCH, cis major diastereoisomer), 5.69 (s, 1H, SCH, cis minor
diastereoisomer) 7.30–8.30 (m, 20H, Ph). 13C NMR (62.90
MHz, DMSO-d6) δC (ppm): 194.48 (s, CO), 128.30-156.40 (m,
Ph), 49.94 (s, SCH) (cis isomer), 49.39 (s, SCH) (trans
isomer), 27.31-27.79 (d, SMe2) (cis isomer), 26.51-26.73 (d,
SMe2) (trans isomer).
Although it has been published the synthesis of
palladium complexes with sulfonium ylides by Bravo et al.
[8], but only the trans structure of complexes was reported.
In this paper, we report the synthesis and theoretical study
of the both cis- and trans-[Pd((Me)2SCHC(O)R)2] (R = phenyl,
3-nitrophenyl and 4-nitrophenyl) complexes
(
1
-3). In
Data for [PdCl2((Me)2SC(H)C(O)C6H4-m-NO2)2] (2)
addition, the X-ray crystal structure of complex
3
and
Yield: 0.27 g (70%). Anal. Calc. for C20H22Cl2N2O6PdS2 (%): C,
38.26; H, 3.53; N, 4.46; S, 10.21. Found: C, 38.52; H, 3.80; N,
4.75; S, 10.55. M.p. 195-198 °C. Selected IR absorption in
KBr (cm-1) ν(CO): 1630. 1H NMR (500 MHz, DMSO-d6) δH
(ppm): 2.55-2.79 (m, 12H, SMe2) (trans isomer), 2.69-3.05
(m, 12H, SMe2) (cis isomer), 5.02 (d, 1H, SCH, trans major
diastereoisomer), 5.11 (s, 1H, SCH, trans minor
diastereoisomer), 5.66 (s, 1H, SCH, cis major
diastereoisomer), 5.73 (s, 1H, SCH, cis minor
diastereoisomer), 7.40–8.20 (m, 16H, Ph). 13C NMR (125.77
MHz, DMSO-d6) δC (ppm): 192.17-192.21 (d, CO), 119.40-
149.20 (m, Ph), 50.08 (s, SCH) (cis isomer), 49.65 (s, SCH)
(trans isomer), 28.08-28.39 (d, SMe2) (cis isomer), 27.10-
28.00 (d, SMe2) (trans isomer).
further spectroscopic characterization of complexes was
performed. Finaly, Mizoroki-Heck reactions of various aryl
halides with styrene and ethyl acrylate using these
complexes as efficient catalysts were performed, giving
good to excellent yields of products under aerobic
conditions.
Experimental
Materials and methods
The
sulfur
ylides[Me2SCHC(O)C6H5)2]
(Y1),
[Me2SCHC(O)C6H4-m-NO2] (Y2) and [Me2SCHC(O)C6H4-p-
NO2](Y3) and complex [PdCl2(cod)] were prepared
according to the previously published procedures [11, 30].
Dichloromethane was used as reagent grade and dried over
CaCl2. Elemental analysis was performed on a Leco, CHNS-
Data for [PdCl2((Me)2SC(H)C(O)C6H4-p-NO2)2] (3)
Yield: 0.32 g (82%). Anal. Calc. for C20H22Cl2N2O6PdS2 (%): C,
38.26; H, 3.46; N, 4.75; S, 10.21. Found: C, 38.50; H, 3.81; N,
4.78; S, 10.35. M.p. 199-201 °C. Selected IR absorption in
KBr (cm-1) ν(CO): 1637. 1H NMR (250 MHz, DMSO-d6) δH
(ppm): 2.62-2.86 (m, 12H, SMe2) (trans isomer), 2.78-3.14
(m, 12H, SMe2) (cis isomer), 5.10 (s, 1H, SCH, trans major
diastereoisomer), 5.24 (s, 1H, SCH, trans minor
diastereoisomer), 5.71 (d, 2H, SCH, cis two
diastereoisomer), 7.90–8.40 (m, 16H, Ph). 13C NMR (62.90
MHz, DMSO-d6) δC (ppm): 192.25-193.87 (d, CO), 123.90-
1
932 apparatus. The H and 13C NMR spectra were recorded
on BrukerAvance 500 MHz, 400 MHz, 250 MHz and Jeol 90
MHz spectrometers in CDCl3 or DMSO-d6 as solvents at 25
°C. IR spectra were recorded on KBr pellets using a
Shimadzu 435-U 04 spectrophotometer in the region of
4000-400 cm–1.
Crystallography
A
suitable crystal was selected and mounted on a
SuperNova, Dual, Cu at zero, Atlas diffractometer. The
2 | J. Name., 2012, 00, 1-3
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150.10 (m, Ph), 55.89 (s, SCH) (cis isomer), 55.33 (s, SCH)
(trans isomer), 28.20-29.41 (d, SMe2) (cis isomer), 27.37-
27.61 (d, SMe2) (trans isomer).
Spectroscopy
The structure of complexes
successfully by H and 13C NMR spectroscopic methods and
other conventional techniques as well as IR and elemental
analysis. Table s1 shows the brief summary of these
collected data sets (see supplementary data). Trans
1-
3
was characterized
1
Computational studies
The geometries of complexes 1-3 were fully optimized at
BP86 [34-35] /def2-SVP [36] level of theory. It has been
shown that BP86 is suitable level for calculation of bonding
situation between the M→L in such as these complexes
[37-42]. The geometry of the metal complex 3, as
determined by X-ray crystal structure analysis (Fig. 1), was
fully optimized at above mentioned level of theory. The
structure of complex 3 was being unequivocally determined
by single crystal X-ray analysis. The CHNS elemental analysis
of palladium complexes indicates 1:2 stoichiometry
between the PdCl2 and sulfur ylide. Also, the mononuclear
structure of these complexes is supported by ESI- mass
spectra, with the molecular ion peaks at m/z 537, 628 and
observed geometry of compound
3
was used as a basis for
. All calculations were
DFT calculations of compound 1 and
2
629, respectively, for complexes 1, 2 and
3.
performed using the Gaussian 09 set of programs [43]
Natural bond orbital (NBO) analysis [44] were also carried
out with the internal model GAUSSIAN 09.
In the far IR spectra of complex , two Pd-Cl stretching
3
band was observed in the 327 and 258 cm-1 that is in accord
with both cis- and trans-dichlorobis(sulfur ylide) structure
in the solid state. IR spectra of complexes 1-3 show a
significantly frequency shift of ν(CO) than those of the
related sulfur ylides. An increase in ν(CO), indicates the
coordination of the ylide was occurred through carbon;
whereas decrease of ν(CO) was expected when the ylide
coordinates to metal from oxygen atom. The observed
Typical procedure for the Mizoroki-Heck reaction
Palladium complex 3 (0.005 mmol), olefin (0.75 mmol), aryl
halide (0.5 mmol), Cs2CO3 (1.5 mmol) were added to 2 ml of
solvent in a small tube and the mixture was heated to reflux
temperature for 4 h in the presence of air. The reactions
were monitored by thin-layer chromatography (TLC). At the
end of the cross-coupling reactions, the mixture was
cooled, extracted with n-hexane:EtOAc (80:20), filtered and
purified by recrystallization from ethanol and water or
purified by silica gel column chromatography (n-
hexane:EtOAc, 80:20).
absorption bands about 1514-1540 cm-1 in ylides Y1
were shifted to 1629-1647 cm-1 in complexes
, indicating
-Y3,
1-3
the coordination of ylide through the Cα atoms. The ν (S+–
C) band of the ylides is also sensitive to the coordination. It
was seen that a lower frequency shift was occurred to
about 836-860 cm-1 in comparison to the parent ylides
(810-823 cm-1), that is in consistent with some removal of
electron density in the S–C bonds. However, infrared
spectroscopy could not differentiates between geometric
isomers (cis and trans) of each complexes.
Results and discussion
Synthesis
According to our previously report, SMe2 reacts with 1
equiv. of the proper ketone BrCH2C(O)R (R = phenyl, and 3-
nitropheny, 4-nitrophenyl forming the desired sulfonium
salts. Further treatment of these salts with NaOH 10% led
to elimination of HBr, giving the desired sulfur ylides.
Reaction of these ligands with [PdCl2(cod)] in 2:1 ratio
yielded the both cis- and trans-[Pd((Me)2SCHC(O)R)2] (R =
The 1H NMR signal for the SCH group of the complexes 1-
3
shows a downfield shift compared to those of the free
ylide. The coexistence of the cis and trans isomers is
confirmed by the 1H NMR spectra. Complexes
in
1-3
dimethyl sulfoxide displayed two doublets for
SMe2 and two doublet of methine signals. The higher field
pair of SMe2 and methine proton signals may be assigned to
the trans isomer. Since methinic carbons are chiral, two
(RR/SS or RS/SR) are possible for each cis- and trans-
structure of these complexes. In addition, integrated
intensity of the SMe2 signals revealed that the ratio of the
cis to trans isomer was about 1:2. The fact that PdCl2(ylide)2
has a larger portion of the trans isomer would be mainly
attributed to a smaller steric repulsion between ylide
ligands.
phenyl, 3-nitrophenyl and 4-nitrophenyl) complexes
80-90% yields (Scheme 1).
1-3 in
S
Me
Me
Me
S
O
+
R
1.Aceton, 24h
2. NaOH 10%
Me
O
R
BrH2C
L
In the 13C NMR spectra, a similar downfield shift for
characteristic groups SCH and carbonyl was observed.
Coordination of ylide through the Cα causes more electron
density on this atom, led to the appearance of related 13C
signal in higher field. In addition, presence of two geometric
isomers in these complexes was also confirmed by 13C
NMR. Observation of four CH3 signals for the two SMe2
groups and two methinic carbon signals indicates the
presence of such geometrical and optical isomers.
However, since carbonyl carbons do not have a hydrogen
R=H, m-NO2 and p-NO2
R
Cl
Cl
C
CH
Cl
O
Me
Me2S
Pd
SMe2
CH
CH
Cl
S
Me
Me
Pd
Me
CH
C
S
PdCl2(COD)
CH2Cl2, 4 h
O
O
O
R
R
R
Scheme 1: Synthesis of Pd(II) complexes 1-3
.
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attached to them, they appear less intense than other
carbons in the spectra and the signals were not seen as two
separate peaks.
The Pd atom is in a square planar arrangement,
coordinated by two ligands and two chlorides, both ligands
and both chlorides are in a trans configuration. The ylide
aromatic ring is almost perpendicular to the Pd Cl2 C2
plane. The nitro group and the keto group are oriented
slightly out of the aromatic ring, the dihedral angles being
13.7(3)° and 7.2(7)° respectively. There are π…π
interactions between the aromatic rings of two adjacent
molecules, the interaction being such that C5 is
superimposed over C7 (C5 - C7': 3.550(8) Å, C4 - C5 - C7’:
86.3(3)°) and C6 is superimposed over the center of the
other aromatic ring. The molecules are linked together by
weak C-H…X interactions into a 3D network.
Crystallography
Yellow-orange single crystals of C20H22Cl2N2O6PdS2 were
grown by vapour diffusion of Methanol into DMSO. The
molecular structure of
3 was shown in Fig. 1. Relevant
parameters concerning data collection and refinement
were given in Table 1. Selected bond distances and angles
for the unit cell of complex 3 are displayed in Table s2.
Theoretical studies
A theoretical study on structures and nature of bonds for
complexes 1-3 have been reported at the BP86/def2-SVP
level of theory. The X-ray crystal structure confirmed that
the complex
3 exhibit the square planar, structure. Thus we
can consider two isomers both cis like and trans like for
latter complexes. The structures and relative energy for
both cis and trans like complexes are presented in Fig. 2.
The result in good agreement with experimental data show
that, the trans like structure is more stable than cis like
structure for all complexes investigated here (Fig. 2). Thus
further calculations including structure and nature of bond
in the complexes were performed on trans like structure.
Fig. 1. ORTEP view of X-ray crystal structure
3.
Table 1. Crystal data and structure refinement for 3.
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
a (Å)
C20H22Cl2N2O6PdS2
627.81
130.01(10)
Triclinic
P-1
Trans-like
Cis-like
7.1934(5)
7.5432(5)
11.4096(8)
94.723(6)
103.384(6)
91.985(6)
599.32(7)
1
b (Å)
c (Å)
α (°)
β (°)
γ (°)
V (Å3)
1
Z
Dcalcg/cm3
1.739
b
Absorption coefficient (mm-1)
10.279
a
F(000)
316.0
E= -2770.7699962
∆E=28.96 kcalmol-1
E=-2770.8161515
Crystal size (mm3)
θ Range for data collection (°)
Reflections collected
Index ranges
0.1272×0.0933×0.0452
CuKα (λ = 1.54184)
8 to 154.03
-9 ≤ h ≤ 7, -9 ≤ k ≤ 8, -
11 ≤ l ≤ 14
Independent reflections
5152
Data/restraints/parameters
2488 [Rint=0.0419,
Rsigma = 0.0562]
2488/0/153
2
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
Final R indices (all data)
d
1.090
c
R1=0.0475,
E=-3179.6537652
E=-3179.699935
wR2=0.1268
∆E= 28.97 kcalmol-1
Largest diff. peak and
hole (e Å-3)
R1=0.0532,
wR2=0.1331 1.96/-1.09
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acceptor interactions on the base of NBO analysis. The
values of donor-acceptor interactions in the compounds are
given in Table s4. In these compounds, the important
donor–acceptor interactions in the case of Pd–C bonds in
complexes 1-3 were carry out from the σ (Pd─C) and σ
(Pd─Cl) as donors to σ* (Pd─C), σ* (Pd─Cl), σ*(S– C) and
σ*(O– C) as acceptors(Table s4).
3
The calculated energies for HOMO and LUMO of 1, 2 and
3
complexes are also given in Table 3. The result showed
that the calculated energy gap (ΔE) between the latter
orbitals for the complex containing NO2 as a substituent
f
e
3
E=-3179.6526175
E=-3179.7005363
∆E= 30.06 kcalmol-1
in para position is smaller than those for other complexes.
Also, the latter complex is softer than the other complexes
described here. The HOMO and LUMO for all three
mononuclear complexes are illustrated in Fig. s1 (see
supplementary data).
Fig. 2. Optimized structures of trans-like(a, c, and e) and
cis-like(b, d, and f) of complexes 1-3 as well as the
energy of cis-like isomer relative to trans-like isomer at
BP86/SVP level of theory.
Table 3. Calculated energies for HOMO and LUMO molecular
orbitals, hardness and energy gap(∆E) between the HOMO
and LUMO of trans like structures of complexes 1-3.
The optimized structures of both trans like and cis like
complexes 1-3 at the BP86/def2-SVP level of theory are
shown in Fig. 2, and trends for the variation of the
corresponding bond lengths and bond angles extracted
from optimizing geometries and are given in Table s3. The
LUMO
(hartree)
-0.0989
-0.1261
-0.1364
HOMO
(hartree)
-0.1700
-0.1836
-0.1886
Compound
η (eV)
ΔE (eV)
1
2
3
0.97
0.78
0.71
1.94
1.56
1.42
calculated bond lengths and bond angles of complex
3 are
in good agreement with the experimental values (Table s3 ).
The computed Pd–C and Pd-Cl bond distances are about 0.1
Å longer than the X-ray values. Also the result showed that
the R substituent has an insignificant effect on both the Pd–
C and Pd-Cl bond distances and C-Pd-C, C-Pd-Cl and Cl-Pd-Cl
bond angles in the complexes 1-3. (Table s3).
To better understand the nature of the Pd–C bonding
between the L2 fragments[(Ylide)2] and [PdCl2] fragment
(See Scheme 1) in 1-3 complexes, we carried out quantum
chemical calculations in the terms of energy-decomposition
analysis at the BP86/TZ2P(ZORA)//BP86/def2-SVP with C1
symmetry with the program package ADF2009.01. The ∆Eint
between two investigated fragments in complexes 1-3 (see
Scheme 1) are about 149.4, 145.83 and 139.26kcal mol-1
respectively (Table 4).
For studying the nature of metal-ligand bonds and/or
possible interactions in these complexes, the natural bond
orbital (NBO) analysis was performed and used to estimate
the delocalization of electron density between occupied
Lewis-type orbitals and formally unoccupied non-Lewis
NBOs (antibonding or Rydberg), which corresponds to
stabilizing donor-acceptor interaction [45]. The values of
the partial charge on the Pd, C and Cl atoms involved in
interaction between the PdCl2 and L2 in of the complexes 1-
Table 4. EDA analysis (BP86/TZ2P(ZORA)//BP86/def2-
SVP) of the [L2PdCl2] complex with the C1 symmetry.
compound
1
2
3
3
were investigated at BP86/def2-SVP level of theory (Table
∆Eint
∆EPaul
∆Eelst
∆Eorb
∆Edisp
2). The results show that the Pd and C atoms of the Pd−C
bonds carry positive and negative partial charges and this
confirm that there is σ donation between the lone paire of
C atoms and empty orbitals of Pd in Pd−C bonds (Table 2).
-149.38
-145.83
-139.26
384.16
378.20
367.60
-264.61(49.6%)
-246.98(46.3%)
-21.94(4.1%)
-261.42(49.9%)
-240.51(45.9%)
-22.09(4.2%)
-257.42(50.8%)
-227.31(44.8%)
-22.13(4.3%)
Table 2. Wiberg bond indices(WBI) of Pd−C and Pd-Cl bonds
and natural charges of Pd, C and Cl atoms.
WBIs
Pd-C
0.406 0.6453 0.067
NPA
C
Compound
The results also show that there is a decreasing trend in
the interaction energies in the complexes as following
P-Cl
Pd
Cl
-0.67
-0.397
-0.390
-0.390
1
2
3
0.410 0.6751 0.034 -0.634
0.402 0.6488 0.067 -0.671
1>2>3. Thus the interaction energies of complex
substituent is greater than complex with NO2 as
substituent in meta position and both of them are greater
than complex with NO2 as substituent in para position.
1 with H as
2
3
Also the results show that the R substituent has an
insignificant effect on the values of WBIs for Pd−C bonds
(Table 2). A second-order perturbation theory analysis of
the Fock matrix was carried out to evaluate the donor–
The breakdown of the ΔEint values into the Pauli repulsion
ΔEPauli and the three attractive components shows that
roughly 50-51% comes from the electrostatic attraction
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ΔEelstat while ~45-46% comes from the orbital term ΔEorb
and the remaining concern to ΔEdispersion. Thus the values of
∆Eelstat The covalent bonding between the [PdCl2] fragment
and L2 fragments[(Ylide)2] in 1-3 complexes becomes visible
by the calculated deformation densities Δρ. Fig. 3 show the
important deformation densities Δρ and the associated
energy values which provide about 94 % of the overall
Mizoroki-Heck cross-coupling reactions of aryl
halides
The use of Pd(II) complexes 1-3 as a catalyst for Mizoroki-
Heck cross-coupling reaction was then examined where
both solvent and base effects were carefully examined in
the reaction of styrene with bromobenzene under ambient
atmosphere. We chose the complex 3 as default catalyst for
orbital interactions for the complex
Fig. 3a indicates the Δρ1, which show the mainly
contribution to ΔEorb and comes from the Cl2Pd←L2
1
. Visual inspection of
the optimization of Mizoroki-Heck reaction. Various
reaction conditions including base, solvent, temperature
and catalyst loading were examined to achieve the best
condition (Table s5). First, reaction of styrene with
bromobenzene in DMF at 130°C for 4 h in the presence of
σ
donation. Fig. 3c displays the deformation density Δρ3
which come from charge transfer from L2 to [PdCl2]
fragment. Also Fig. 3b, d and e display the deformation
densities Δρ2, Δρ4 and Δρ5 which comes from the [PdCl2]
fragment to Y2 ligands σ-back donation.
K2CO3 (1.5 mmol) and 0.05 mmol of catalyst
3 was chosen
as model reaction. The coupled product of this reaction was
formed in 83% yield (Table s5, entry 1). Then, a series of
experiments was performed to find optimum conditions
(see supplementary data).
Using the optimized reaction conditions (Cs2CO3 (1.5
mmol), DMF (2 ml), catalyst
temperature) we explored the general applicability of Pd–
Sulfur ylide complex with different olefins and aryl halides
3 (0.005 mmol), reflux
3
containing electron withdrawing to electron donating
substituents. It is well known that the catalytic activity was
extensively depends on the type of aryl halides
substituents. Aryl halides with electron-withdrawing groups
on the aryl ring show more reactivity, while electron
donating groups make the aryl halides relatively inactive
[46]. As expected, the Mizoroki-Heck cross-coupling
reaction of aryl iodides with olefins gave better yields of the
desired product than that of aryl bromides and aryl
chlorides (Table 5). Both electron-neutral and electron-rich
aryl iodides were converted efficiently to the desired
coupled products in excellent yields (Table 5, Entries 1, 4
and 7). The high reactivity of aromatic iodides is attributed
to their small C–I bond dissociation energy and easy
oxidative addition to Pd(0) species [47]. As shown in Table
5, reaction of activated aryl bromides such as 4-
a
b
ꢀρ1: ꢀE= -190.02275
(kcal/mol)
ꢀρ2: ꢀE= -16.02387
(kcal/mol)
ν= 1.99499
Counter =0.001
ν= 0.34160
Counter =0.0003
bromonitrobenzene,
4-bromobenzaldeyde
and
4-
bromoacetophenone with styrene and ethyl acrylate afford
the corresponding coupled products in high yields (Table 5,
entries 9, 10, 11, 14 and 17).
c
d
whereas, deactivated aryl bromides such as 4-
Bromoanisole and 4-Bromotoluene gave lower yields (Table
ꢀρ3: ꢀE= -11.91823 (kcal/mol)
ν= 0.33771
Counter =0.0003
ꢀρ4: ꢀE= -10.41849 (kcal/mol)
ν= 0.28572
Counter =0.0003
5
entries 5 and 8). Also, the electronically neutral
bromobenzene produced good amounts of desired coupling
products (Table 5, entry 2) Additionally, trying to apply 1-
bromonaphthalene as efficient aryl bromide resulted
eligible yield of coupling product (Table 5, entry 12 and 16).
The usually unreactive aryl chlorides were further explored
e
ꢀρ5: ꢀE= -4.03754
(kcal/mol)
ν= 0.18120
Counter =0.0003
as the coupling partner with styrene using the complex
3
catalyst system. Although, a number of reported catalysts
in literature need to be used in high loadings, and showed
little or no activity with aryl chloride substrates [48], due to
the economical aspect of aryl chlorides and the challenge to
activate the C–Cl bond, there was much attention from the
academic communities to couple aryl chlorides with olefins
[49]. Both electron rich and electron neutral 4-
Fig. 3. Deformation densities Δρ associated with the most
important orbital interactions in [L2PdCl2]. Note that the colour
in this Figure, denotes the charge flow, which is from the red to
the blue region.
6 | J. Name., 2012, 00, 1-3
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chlorotoluene and chlorobenzene show the low reactivity
in coupling reactions, but we achived more yield by using of
TBAB as additive (Table 5, entries 3 and 6). TBAB is
proposed to increase the solubility of the inorganic base
Cs2CO3 in the organic reaction medium and strengthened
the stabilizations of Pd(0) species [50].
To extend the scope of our work, we next investigated
the coupling reaction of various electron rich and deficient
aryl halides with activated ethyl acrylate olefin. Palladium
complex
3 exhibited a better catalytic activity for the
Mizoroki-Heck reaction of activated ethyl acrylate than that
of inactive styrene under same reaction conditions. As
expected, electron-withdrawing substituent on olefin gives
the desired coupling products in more purity and less
reaction time (Table 5, entry 13-17).
Table 5. Mizoroki-Heck cross-coupling reaction of aryl
halides catalyzed by Pd complexa
.
To evaluate the presence of Pd(0) nanoparticles (NPs) in
catalytic system, we performed the mercury drop test [51],
was first introduced by Whitesides. Since mercury leads to
amalgamation of the surface of a heterogeneous catalyst,
and in contrast, Hg(0) is not expected to have a poisoning
effect on homogeneous palladium complexes, the results of
mentioned test help us to determine the nature of the
active species. Addition of 100 equiv. Hg(0) to the reaction
solution at t=0, was not affected the conversion of the
reaction, which suggests that the catalyst is homogeneous
in nature.
time
(h)
4
Yieldd
(%)
Entry
arylhalide
0lephin
Product
1
2
Styren
Styren
92
89
Iodobenzene
1g
1g
5
6
5
Bromobenzene
60
3
4
Styren
Styren
86b
Chlorobenzene
4-Iodotoluene
1g
2g
88
Also, we analyzed an aliquot of the reaction mixture by
SEM analysis. After completion of the reaction, 0.5 ml of
the solution was separated and evaporated under reduced
pressure. The crude residue was washed with water to
remove base and then analyzed by SEM. As expected, the
SEM images does not showed presence of any Pd
nanoparticles in recovered catalyst (see supplementary
data).
4-
5
6
5.5
7
Styren
Styren
85
2g
2g
Bromotoluene
4-
58
83 b
Chlorotoluene
7
8
5
5
Styren
Styren
89
88
4-Iodoanisole
3g
3g
4-Bromoanisole
4-
9
4
4.5
5
Styren
Styren
Styren
Styren
90
82
86
bromonitroben
4g
5g
6g
Conclusions
zene
In summary, we report the synthesis, characterization,
theoretical study on structures and nature of bonds and
application of cis- and trans-[PdCl2(SMe2C(H)C(O)R)2] (R =
4-
10
11
Bromobenzalde
hyde
phenyl
(1), 3-nitrophenyl (2), and 4-nitrophenyl (3)).
4-
bromoacetophe
none
Complexes toward Mizoroki-Heck reaction. These
complexes were characterized fully by spectroscopic
methods as well as X-ray crystallography method. Results
showed that the cross-coupling reaction of various aryl
halides with olefins was performed in good to excellent
yields. The ease of preparation of the complexes, low
catalyst loading and stability toward air and moisture make
these complexes as an ideal catalytic system for Mizoroki-
Heck cross-coupling reactions. The results of theoretical
study in good agreement with experimental data show that,
the trans like structure is more stable than cis like structure
for all complexes investigated here. The NBO analysis show
that Pd and C atoms of the Pd−C bonds carry positive and
negative partial charges and this confirm that there is σ
donation between the lone paire of C atoms and empty
orbitals of Pd in Pd−C bonds. Also the results showed that
the important donor–acceptor interactions in the case of
Pd–C bonds in complexes 1-3 were carry out from the σ
(Pd─C) and σ (Pd─Cl) as donors to σ* (Pd─C), σ* (Pd─Cl),
1-
12
13
14
15
16
16
3.5
3
81
92
93
90
85
bromonaphthal
ene
7g
8g
Ethyl
Bromobenzene
acrylat
4-
Ethyl
bromonitroben
9g
acrylat
zene
4-
Ethyl
4
10g
11g
Bromotoluene
acrylat
1-
bromonaphthal
ene
Ethyl
12
acrylat
4-
Ethyl
4
17
Bromobenzalde
hyde
12g
88
acrylat
a
Reaction conditions for Mizoroki-Heck cross-coupling reaction:
Arylhalide (0.5 mmol), olefin (0.75 mmol), Cs2CO3 (1.5 mmol),
DMF (2 ml), catalyst 3 (0.005 mmol), in the air.
σ*(S– C) and σ*(O– C)
as acceptors. The ETS-NOCV
analyses confirm that the mainly contribution to ΔEorb in
Pd–C bonding in [L2PdCl2] complexes comes from Cl2Pd←L2
σ-donation.
b TBAB (1 mmol) was added
c
Refrence for melting point all of products
d All are isolated yields
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Acknowledgements
Funding of our research from the Bu-Ali Sina University is
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Supplementary material
[19]
M.
M.
Ebrahim,
H.
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Physical measurements and selected 13C and 1H NMR
spectra of some compounds can be found in the online
version. CCDC 1438729 contains the supplementary
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