Palladium(0) complexes of N-heterocyclic carbene ligands with dangling NMeC=O functionalities: Synthesis, reactivity and application in Mizoroki-Heck reactions

Article abstract of DOI:10.1016/j.poly.2015.07.027

Abstract A series of new Pd(0) complexes of N-heterocyclic carbene (NHC) ligands with dangling NMe(CO) functionalities (L^Me) is reported, with yields in the range 56-73%. The reactivity of one of the Pd(0) complexes, bearing bulky N-mesityl groups, towards the oxidative addition of pentafluoroiodobenzene was studied, revealing the formation of decafluorobiphenyl, a diiodo Pd(II) NHC complex and an intriguing Pd(II) complex with four carbon-donor ligands coordinated to the metal center. The latter complex contains two new NHC ligands derived from L^Me. One of the NHC ligands is a bidentate ligand, resulting from an addition reaction between L^Me and the coordinated maleic anhydride. The other NHC ligand is derived from the N-protonation of L^Me. A pentafluorophenyl ligand completes the fourth coordination site. A mechanism for the formation of this intriguing Pd(II) complex is proposed. The structures of these two new Pd(II) complexes were established by single-crystal X-ray diffraction studies. The catalytic activities of the new Pd(0) complex with L^Me in the Mizoroki-Heck coupling reaction between 4-chloroacetophenone and styrene were compared and they contrast with those reported for the Pd(0) NHC complex with dangling NH(CO) groups (L^H).

Full text of DOI:10.1016/j.poly.2015.07.027

Polyhedron 100 (2015) 28–35
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Palladium(0) complexes of N-heterocyclic carbene ligands with dangling
NMeC@O functionalities: Synthesis, reactivity and application in
Mizoroki–Heck reactions
⇑
Yang-Ming Jhou, Debkumar Nandi, Jhen-Yi Lee, Ru-Jiun Tzeng, Hon Man Lee
Department of Chemistry, National Changhua University of Education, Changhua 50058, Taiwan, Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of new Pd(0) complexes of N-heterocyclic carbene (NHC) ligands with dangling NMe(C@O) func-
tionalities (LMe) is reported, with yields in the range 56–73%. The reactivity of one of the Pd(0) complexes,
bearing bulky N-mesityl groups, towards the oxidative addition of pentafluoroiodobenzene was studied,
revealing the formation of decafluorobiphenyl, a diiodo Pd(II) NHC complex and an intriguing Pd(II)
complex with four carbon-donor ligands coordinated to the metal center. The latter complex contains
two new NHC ligands derived from LMe. One of the NHC ligands is a bidentate ligand, resulting from
an addition reaction between LMe and the coordinated maleic anhydride. The other NHC ligand is derived
from the N-protonation of LMe. A pentafluorophenyl ligand completes the fourth coordination site. A
mechanism for the formation of this intriguing Pd(II) complex is proposed. The structures of these two
new Pd(II) complexes were established by single-crystal X-ray diffraction studies. The catalytic activities
of the new Pd(0) complex with LMe in the Mizoroki–Heck coupling reaction between 4-chloroacetophe-
none and styrene were compared and they contrast with those reported for the Pd(0) NHC complex with
dangling NH(C@O) groups (LH).
Received 25 April 2015
Accepted 7 July 2015
Available online 18 July 2015
Keywords:
Palladium(0) complex
Heck reaction
N-Heterocyclic carbene
X-ray diffraction studies
Catalysis
Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction
However, while the use of palladium(II) NHC precatalysts in C–C
coupling reactions is ubiquitous in the literature, the use of pre-
Nowadays N-heterocyclic carbenes (NHCs) are common ligands
in a wide range of transition-metal catalyzed C–C coupling reac-
tions [1]. In this context, many palladium(II) NHC complexes have
been successfully employed as efficient precatalysts in Mizoroki–
Heck reactions. However, these catalyst systems typically required
harsh reaction conditions, such as high reaction temperature
(>140 °C) and the use of aryl bromides as substrates. The use of aryl
chlorides as coupling partners is still relatively limited [2]. It is
well-accepted that for palladium-catalyzed C—C coupling reac-
tions, the active catalyst is a Pd(0) species which allows the initial
activation of aryl halide substrates via oxidative addition.
Therefore, the use of palladium(II) complexes as precatalysts
should involve an in situ reduction process, generating the active
Pd(0) catalytic species. In this regard, defined palladium(0) com-
plexes should be more desirable for active and productive precat-
alysts compared to the commonly applied in situ generated,
undefined catalyst systems based on palladium(II) complexes [3].
formed palladium(0) NHC complexes is still relatively rare [2c,4].
Previously, we reported a series of Pd(0) complexes featuring N-
heterocyclic carbenes (NHC) and maleic anhydride (MA) ligands,
which were proven to be highly efficient in catalyzing Mizoroki–
Heck coupling reactions using the molten salt of n-
tetrabutylammonium bromide (TBAB) as the solvent (Chart 1)
[5]. A range of activated aryl chlorides could be employed as cou-
pling partners. In this previous work, the NHC ligands with dan-
gling NHC@O groups coordinated to the Pd(0) centers in the
monodentate fashion. As evidenced by electrospray ionization
mass spectrometry, the formation of an anionic Pd(0) complex
with
a
bidentate amidate/NHC ligand, [Pd(0)L]À, was likely
involved in the catalytic process. Herein, we prepared a series of
new Pd(0) complexes with NHC ligands in which the NHC@O
group is replaced by the NMeC@O group. Since the formation of
the aforementioned anionic bidentate [Pd(0)L]À species was
avoided, we envisioned different chemical properties and catalytic
performance of this new set of Pd(0) complexes. It should be noted
that one of the Pd(0) complexes herein was preliminarily applied
to catalyze the decarboxylative coupling of heteroaromatic
⇑
Corresponding author.
E-mail address: leehm@cc.ncue.edu.tw (H.M. Lee).
http://dx.doi.org/10.1016/j.poly.2015.07.027
0277-5387/Ó 2015 Elsevier Ltd. All rights reserved.

Y.-M. Jhou et al. / Polyhedron 100 (2015) 28–35
29
2.2. Electrospray ionization mass spectrometry (ESI-MS)
O
O
Ph
N
Ph
N
N
N
R
R
N
H
N
ESI-MS involving a mild ionization procedure permits the
observation of solution state structures in the gas phase [7]. In
the previous study with the Pd(LH)2(MA) complex, a bidentate
[Pd(0)L]À species was detected by ESI-MS. Due to the absence of
such an acidic NH proton in the LMe ligand, other reactive species
would be expected in the mass spectra of 3a–d. As a representative
example, a DMF solution of 3c was analyzed by positive mode
ESI-MS, revealing a base peak at m/z 775.4 corresponding to a
[Pd(0)LM2 eÀH]+ species, in which the Pd(0) center is coordinated by
two LMe ligands. The absence of maleic anhydride in this species
reflects that the olefinic ligand could be easily replaced and such
ligand de-coordination is essential for the coordination of substrates
in catalysis.
Me
potential
deprotonation
LMe
(this work)
LH
(previous work)
Chart 1. The N-heterocyclic carbene ligands used for the formation of the Pd(0)
complexes.
carboxylic acids (imidazo[1,2-a]pyridine and isoxazole) with aryl
halides, showing promising catalytic properties [6]. For carboxyin-
doles, either decarboxylative coupling or tandem C–H arylation
and decarboxylation occurred, leading to the formation of
C2-monoarylated indoles.
2.3. Reactivity of the Pd(0) complex towards oxidative addition
2. Results and discussion
To understand the role of the Pd(0) complexes in the activation
of aryl halides, we carried out the stoichiometric reaction between
3e and pentafluoroiodobenzene in DMF at 80 °C overnight
(Scheme 2). Pentafluoroiodobenzene is an electron-deficient com-
pound which has been successfully used in oxidative addition reac-
tions with Pd(0) complexes [8]. The reaction yielded a mixture of
products from which pure compounds were successfully isolated
after column chromatography. The expected product resulting
from oxidative addition (3e1) was, however, not observed. The
diiodopalladium(II) complex with two trans coordinated NHC
ligands (6) together with decafluorobiphenyl (DFBP, 5) were iso-
lated as the major products. The structure of 6 was confirmed by
single-crystal X-ray diffraction analysis (vide infra). The formation
of 6 could be explained by the existence of 3e1 which undergoes
ligand exchange to form the diiodopalladium complex and an
unobserved palladium diaryl complex as well. Subsequent reduc-
tive elimination affords DFBP. Interestingly, a minute amount of
7 was also obtained and initially the structure of 7 was ambigu-
ously based on only NMR data. Pleasingly, however, the structure
of 7 was successfully established by single-crystal X-ray diffraction
analysis (vide infra). Complex 7 is a highly intriguing Pd(II) com-
plex featuring four coordinated carbon atoms. It contains two
new NHC ligands derived from LMe. One of the NHC ligands is a
bidentate ligand, resulting from an intramolecular addition reac-
tion between the coordinated NHC and maleic anhydride. The
other NHC ligand is derived from the N-protonation of LMe, forming
a protic NHC with a NH substituent on the heterocyclic ring. A
pentafluorophenyl ligand completed the fourth coordination site.
In the 1H NMR spectrum, the characteristic Csp3H protons of the
oxygen heterocycle were observed at d 2.30 and 2.33 ppm, being
significantly upfield compared with the olefinic CH protons at d
3.11–3.25 ppm for 3a–d. A proposed mechanism for the formation
of 7 is given in Scheme 3. Complex 3e1, from the oxidative addition
of pentafluoroiodobenzene, contains acidic methylene protons
next to the NMe(C@O) group and upon deprotonation the carban-
ion could attack the olefinic carbon of maleic anhydride, leading to
the de-coordination of iodide and the formation of the CC-type pal-
ladacycle 3e2. The acidic nature of the methylene proton next to
the amido group had been demonstrated by reports from our
group with related complexes, in which after deprotonation of
such a proton, Pd—C bonds could be formed [9]. Due to the steric
crowding in this complex, subsequent N-protonation by the strong
HI acid affords 7, bearing a protic NHC ligand. Metal complexes
with protic NHC ligands are known in the literature [10]. It is
noteworthy that the analogous Pd(0) complex 4e with the LH
ligand and 3a–d with less sterically hindered N-substitution on
the carbene moieties did not exhibit the above-described reactivity
towards pentafluoroiodobenzene. The reaction was also highly
2.1. Synthesis of the Pd(0) complexes
The new ligand precursors 1a–d and 2e could be prepared read-
ily by simple quaternization reactions between the appropriate
imidazole derivatives with 2-chloro-N-methyl-N-phenylacetamide
or 2-chloro-N-phenylacetamide (see Scheme 1). In general, white
solids were obtained with yields in the range 56–73%. In the 1H
NMR spectra, the NCHN resonances for 1a–d were downfield
shifted, at ca. d 10.37–10.68 ppm, reflecting the acidic nature of
these H atoms. The corresponding signal for 2e was relatively
upfield at d 9.74 ppm. The methylene H atoms of the NHC ligands
in 1a–d were observed as singlets at d 5.04–5.12 ppm, whereas
that in 2e was observed at a slightly downfield position of d
5.88 ppm. The new Pd(0) complexes 3a–d were then prepared fol-
lowing our reported procedure for similar compounds, employing
compound A as the Pd(0) precursor [5], which afforded yellow
solids with decent yields (58–89%) (Scheme 1). Complexes 3e [6]
and 4b [5] were reported by us previously. For a comparative
purpose, we also synthesized complex 4e, featuring dangling
NH(C@O) and N-mesityl groups on the carbene moieties. The
complex was prepared in a similar manner with a decent 77% yield.
The olefinic proton in the free maleic anhydride resonates at d
7.08 ppm. Upon coordination in 3a–d, these olefinic H atoms were
upfield shifted with chemical shift in the range d 3.11–3.25 ppm.
The large coordination-induced shifts (CIS) are similar to those in
Pd(0) complexes with LH [5], Pd(tmiy)2(MA) [2c] (tmiy = 1,3,4,
5-tetramethylimidazol-2-ylidene) and Pd(IMes)(MA)2 (IMes = 1,3-
bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) [4l]. Interestingly,
the corresponding olefinic proton signals in 4e were observed as
two singlets at d 2.86 and 2.93 ppm. The successful formation of
carbene complexes in 3a–d and 4e was also confirmed by the
absence of NCHN protons and the presence of downfield carbon
resonances observed at d 187.3–189.7 ppm.
R
R
N
N
N
N
O
K2CO3
N
N
Pd
+
Ph
N
Pd
N
O
R
O
N
X
Cl
N
O
N
O
Ph
O
O
A
O
Ph
O
1a-e (X = Me)
2b 2e
X
X
and
(X = H)
3a-e (X = Me)
4b 4e
a
: R = methyl
and
(X = H)
b: R = benzyl
c
: R = 4-methoxybenzyl
d
: R = 4-fluorobenzyl
e: R = mesityl
Scheme 1. Preparation of the Pd(0) precatalysts for the Mizoroki–Heck reactions.

30
Y.-M. Jhou et al. / Polyhedron 100 (2015) 28–35
I
F
F
F
F
O
O
N
N
O
F
N
DMF 80°C
overnight
F
F
F
F
N
N
N
N
N
N
Ar =
d
P
N
N
N
F
Pd
O
Ar
I
O
O
O
3e1
3e
F
N
F
N
F
N
N
O
N
N
N
F
O
O
Pd
F
F
F
F
F
F
F
I
Pd I
N
F
NH
+
O
O
F
N
O
F
F
N
6
(48 % yield)
5 (24 % yield)
7
(6 % yield)
Scheme 2. Reaction of Pd(0) complex 3e with pentafluoroiodobenzene.
O
O
O
F
F
F
N
N
N
H
NMe2
Ar =
F
N
N
N
I
N
I
F
N
N
O
N
N
O
N
OH
NMe2
Pd
Pd
H
Ar
Ar
O
I
3e1
O
O
F
F
F
N
F
N
F
N
F
N
N
OH
N
H
NMe2
I
O
O
Pd
O
O
Pd
F
O
F
O
O
F
F
N
NH
N
O
O
O
N
O
O
N
O
H
NMe2
3e2
7
N
I
+
O
Scheme 3. Proposed mechanism for the formation of 7.
solvent-dependent as 7 was not detected when solvents other than
DMF were employed.
atoms in 3d are situated in general positions in the crystal lattice.
The palladium centers in 3c and 3d are formally in a trigonal planar
coordination geometry (Fig. 1). In each of the structures of 3c and
3d, the two NHC ligands adopted an anti-conformation and form
C–Pd–C angles of 101.7(2) and 98.76(19)°, respectively. Owing to
the symmetry operation of the crystallographic twofold axis, the
maleic anhydride ligand in 3c, unlike that in 3d, is disordered with
two orientations. The O-heterocycle is coordinated to the metal
center in a g2 fashion with Pd–C distances of 2.239(17) and
2.151(13) Å in 3c and 2.109(5) Å in 3d. The C—C bond distance
2.4. Crystal structures
The structures of 3c, 3d, 6 and 7 were successfully established
by X-ray diffraction studies. Crystallographic data are listed in
Table 1. Selected bond distances and angles of 3c and 3d are listed
in Table 2. While the palladium center in 3c is situated in a special
position on a crystallographic twofold axis along the c-axis, all the

Y.-M. Jhou et al. / Polyhedron 100 (2015) 28–35
31
Table 1
Crystallographic data.
3c
3d
6
7
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C
44H44N6O7Pd
C42H38F2N6O5PdÁ0.5CH2Cl2
C42H46I2N6O2Pd
1027.05
orthorhombic
Fdd2
21.738(2)
27.507(4)
14.0977(13)
90
C43H38F5N5O4Pd
890.18
monoclinic
P21/n
10.981(17)
15.07(2)
23.14(3)
90
90.23(4)
90
3829(10)
150(2)
4
875.25
tetragonal
I41/a
16.8875(4)
16.8875(4)
29.7431(11)
90
90
90
8482.4(4)
150(2)
8
893.65
triclinic
ꢀ
P1
12.7496(6)
12.9176(7)
13.1877(5)
99.990(4)
97.505(4)
94.126(4)
2110.70(18)
150(2)
b (Å)
c (Å)
a
(°)
b (°)
90
90
c
(°)
V (Å3)
8429.6(18)
150(2)
8
T (K)
Z
2
No. of unique data
4565
9110
4579
7927
No. of parameters refined
294
0.0647
0.1725
525
0.0634
0.0923
244
0.0457
0.0955
530
0.0711
0.1945
a
R1 [I > 2
r
I]
b
wR2 (all data)
a
R1
wR2 = [
=
R
(||Fo| À |Fc||)/
R|Fo|.
b
R
(|Fo|2 À |Fc|2)2/
R .
(F2o)]1/2
Table 2
Selected bond distances (Å) and angles (°) in 3c and 3d.
3c
3d
Pd1–C9
2.048(5)
2.239(17)
2.151(13)
1.52(2)
Pd1–C1
Pd1–C2
Pd1–C39
Pd1–C42
C39–C42
2.058(5)
2.045(5)
2.109(5)
2.109(5)
1.449(8)
Pd1–C21
Pd1–C22
C21–C22
C9–Pd1–C9#1
C9–Pd1–C21
C9#1–Pd1–C22
C9–Pd1–C39
101.7(2)
109.1(4)
106.6(4)
149.1(5)
145.0(5)
C1–Pd1–C2
C1–Pd1–C39
C2–Pd1–C42
C1–Pd1–C42
C2–Pd1–C39
98.76(19)
111.8(2)
109.3(2)
151.7(2)
149.4(2)
C9#1–Pd1–C21
Symmetry code: #1 Àx + 0, Ày + 3/2, z + 0.
Fig. 2. Molecular structures of 6 with 35% probability ellipsoids. H-atoms are
omitted for clarity.
for the maleic anhydride ligand in 3d is 1.449(8) Å, reflecting sig-
nificant back -bonding from the Pd(0) atom, resulting in the elon-
p
gation of the C@C bond distance.
is quite distorted from linearity. Crystal structures of trans-
diiodopalladium NHC complexes are known in the literature [11].
The palladium center in 7 is in a distorted square planar coordi-
nation geometry, featuring two trans NHC ligands and the
pentafluorophenyl ligand is trans to the carbon-donor moiety
derived from the maleic anhydride (Fig. 3). Selected bond distances
and angles are listed in Table 3. The bite angle of the bidentate NHC
ligand (92.4(3)°) is slightly larger than the ideal value of 90°. The
Pd–C distance from the bidentate ligand is 2.062(9) Å, which is
The structure of 6 is shown in Fig. 2. Selected bond distances
and angles are tabulated in Table 3. The palladium center is situ-
ated in a special position on a crystallographic twofold axis along
the c-axis. The metal center adopts a slightly distorted square-pla-
nar coordination geometry with the two NHC/iodide ligands trans
to each other. In contrast to 3c and 3d, the two NHC ligands in 6
adopt a syn conformation. The Pd—C distance is 2.037(6) Å, while
the C–Pd–C angle is 179.8(5)°. The I–Pd–I angle of 173.09(4)°
Fig. 1. Molecular structures of 3c (left) and 3d (right) with 35% probability ellipsoids. H-atoms are omitted for clarity. For the structure of 3c, the two orientations for the
disordered maleic anhydride ligand are shown.

32
Y.-M. Jhou et al. / Polyhedron 100 (2015) 28–35
Table 3
Table 4
Selected bond distances (Å) and angles (°) in 6 and 7.
Mizoroki–Heck coupling reaction using the Pd(0) complexes as precatalysts.a
6
7
0.5 mol% Pd(0) cat.
1.1 eq NaOAc
Pd1–C3
Pd1–I1
2.037(6)
2.6016(6)
Pd1–C1
2.062(9)
2.173(8)
2.045(7)
2.079(9)
1.537(12)
Cl
Pd1–C17
Pd1– C26
Pd1– C32
C14– C17
+
+
TBAB, 2 h
110-140oC
O
O
O
C3–Pd1–I1#1
C3#1–Pd1–I1#1
C3–Pd1–I1
C3#1–Pd1–I1
C3#1–Pd1–C3
I1–Pd1–I1#1
90.08(16)
89.90(17)
89.91(16)
90.08(16)
179.8(5)
C1–Pd1–C17
C1–Pd1–C26
C1–Pd1–C32
C17–Pd1–C26
C17–Pd1–C32
C26–Pd1–C32
92.4(3)
90.2(3)
176.7(3)
177.5(3)
87.1(3)
90.3(3)
Entrya
Cat. (0.5 mol%)
Temp. (°C)
Yield % (trans:gem)
1
2
3
4
5
6
7
8
3b
3b
3b
4b
4b
4b
3e
3e
3e
3e
4e
140
120
110
140
120
110
140
140
120
110
140
>99 (94:6)
87 (93:7)
49 (90:10)
98 (94:6)
99 (93:7)
94 (94:6)
99 (90:10)
47 (99:1)b
43 (88:12)
11 (82:18)
82 (94:6)
173.09(4)
Symmetry code: #1 = Àx + 2, Ày + 2, z.
comparable to that of 2.079(9) Å in the protic NHC ligand. The
Pd–C distance of 2.173(8) Å from the carbon donor of the
oxygen-containing heterocyclic ring is significantly longer than
that of 2.045(7) Å from the pentafluorophenyl ligand. The
C14–C17 bond in the O-heterocyclic moiety is 1.537(12) Å,
reflecting its C–C single bond character.
9
10
11
a
Reaction conditions (unless otherwise specified): 1.0 mmol 4-chloroacetophe-
none, 1.4 mmol styrene, 1.1 mmol NaOAc, 2 g TBAB, 0.5 mol% Pd(0) cat.,
110–140 °C, 2 h, isolated yields with regioselectivity determined by NMR.
b
0.1 mol% of Pd(0) cat.
2.5. Mizoroki–Heck reaction
Previously, we had shown that Pd(0) complexes with LH in a
molten salt of TBAB were highly active in catalyzing Mizoroki–
Heck coupling reactions and even aryl chlorides could be employed
as substrates. To compare and contrast the catalytic behaviors
lowering the heating temperature to 120 and 110 °C resulted in a
decrease of the product yield to 87% and 49%, respectively (entries
2 and 3). Complex 4b was more efficient at lower reaction temper-
atures, still affording high yields of 99% and 94% at 120 and 110 °C,
respectively (entries 5 and 6). In contrast, in the case of 3e/4e, the
complex with LMe was more efficient than that of LH such that for
the former complex, a near quantitative yield of product was
obtained, whereas 4e delivered only an 82% yield of the product
(entries 7 versus 11). Lowering the reaction temperature and cata-
lyst loading reduced the product yield significantly (entries 8–10).
We attributed the catalytic efficiency of 4b, possessing non bulky
N-substitution on the carbene ligands, to the possible formation
of the highly electron-rich anionic [Pd(0)L]À species. In the case
of 3b with the LMe ligand, such anionic species could not be formed.
In the case of 3e/4e, with bulky N-substitution on the ligand sys-
tem, the activity order was somewhat different due to the possible
degradation of the Pd(0) intermediate.
between Pd(0) complexes with the ligands LH and LMe
, we
performed the Mizoroki–Heck coupling reactions between
4-chloroacetophenone and styrene employing 3b/4b and 3e/4e
as Pd(0) precatalysts (Table 4). Employing the previous established
conditions with 1.0 mmol of 4-chloroacetophenone, 1.4 mmol of
styrene, 1.1 mmol of NaOAc, 2 g of TBAB, 0.5 mol% 3b, a heating
temperature of 140 °C and a duration of 2 h, a quantitative yield
of the couple product could be achieved with a high trans/gem
selectivity of 94:6 (entry 1). The reported complex 4b with the
ligand LH gave a similar high yield of 98% (entry 4). For 3b,
3. Conclusions
In summary, we have reported a series of new Pd(0) complexes
with N-heterocyclic carbene (NHC) ligands with dangling
NMe(C@O) functionalities (LMe). The reactivity of 3e, bearing bulky
N-mesityl groups, towards the oxidative addition of pentafluo-
roiodobenzene was studied, revealing the formation of decafluoro-
biphenyl, the Pd(II) diiodide complex 6 and an intriguing Pd(II)
complex with four carbon-donor ligands coordinated to the metal
center. A mechanism for the formation of this intriguing Pd(II)
complex was proposed. The catalytic activities of the new Pd(0)
complex with LMe in the Mizoroki–Heck coupling reaction between
4-chloroacetophenone and styrene were compared and contrast
with those reported for the Pd(0) NHC complex with dangling
NH(C@O) groups (LH). In the case of the Pd(0) complex with the
NHC ligand in which the N-substitution is less bulky, the ligand
system LH delivered a higher yield than LMe, which could be related
to the possible deprotonation of the NH proton in the former ligand
and the formation of the highly electron-rich anionic [Pd(0)L]À
active species. In contrast, in the case of 3e/4e with bulky
N-substitution on the ligand system, the activity order was differ-
ent due to the possible degradation of the Pd(0) intermediate.
Fig. 3. Molecular structure of
omitted for clarity.
7 with 35% probability ellipsoids. H-atoms are

Y.-M. Jhou et al. / Polyhedron 100 (2015) 28–35
33
6.99 (t, J = 9.0 Hz, 2H, Ar H), 7.28–7.49 (m, 9H, Ar H and imi H),
10.39 (s, 1H, NCHN). 13C{1H} NMR (CDCl3) d, ppm: 37.9 (CH3),
51.2 (CH2), 52.3 (CH2), 116.3 (d, J = 21.9 Hz, Ar C), 121.0, 123.9,
127.5, 129.0, 129.2 (d, J = 3.0 Hz, quaternary C), 130.4, 131.0 (d,
J = 8.3 Hz, Ar C), 138.3 (quaternary C), 140.8 (NCHN), 163.0 (d,
J = 249.1 Hz, CF), 164.2 (C@O).
4. Experimental
4.1. Materials and methods
All manipulations were performed under a dry nitrogen atmo-
sphere using standard Schlenk techniques. Solvents were dried
with standard procedures. Starting chemicals were purchased from
commercial sources and used as received. Compounds 1e [6], A
[12], 3e [6] and 4b [5] were prepared according to the literature
procedures. 1H, 13C{1H} and 19F{1H} NMR spectra were recorded
at 300.13, 75.48 and 282.23 MHz, respectively. Elemental analyses
were performed on a Thermo Flash 2000 CHN-O elemental ana-
lyzer. ESI-MS was carried out on a Finnigan/Thermo Quest MAT
95XL mass spectrometer.
4.1.5. Synthesis of 2e
A mixture of 1-(2,4,6-trimethylphenyl)-1H-imidazole (1.27 g,
6.83 mmol) and 2-chloro-N-phenylacetamide (1.16 g, 6.83 mmol)
in DMF (10 mL) was heated at 110 °C for 2 d. The solvent was then
removed completely under vacuum. THF was added to the residue,
giving a white solid which was filtered under nitrogen, washed
with THF and dried under vacuum. Yield: 1.46 g, 61%, mp: 212.5–
214.5 °C (dec). Anal. Calc. for C20H22ClN3O: C, 67.51; H, 6.24; N,
11.80. Found: C, 67.58; H, 6.77; N, 11.73%. 1H NMR (CDCl3) d,
ppm: 2.04 (s, 6H, o-Me), 2.32 (s, 3H, p-Me), 5.88 (s, 2H, CH2C@O),
6.97 (s, 2H, Mes H), 7.01–7.07 (m, 2H, Ar H, imi H), 7.20 (t,
J = 6.0 Hz, 2H, Ar H), 7.72 (d, JHH = 9.0 Hz, 2H, Ar H), 7.79 (s, 1H,
imi H), 9.74 (s, 1H, NCHN), 11.5 (s, 1H, NH). 13C{1H} NMR (CDCl3)
d, ppm: 17.5 (CH3), 21.1 (CH3), 52.7 (CH2), 120.1, 122.3, 124.3 (qua-
ternary C), 124.5 (quaternary C), 128.7, 129.8, 130.6 (quaternary C),
134.3, 138.0, 138.3 (quaternary C), 141.3 (NCHN), 163.0 (C@O).
4.1.1. Synthesis of 1a
A mixture of 1-methylimidazole (0.89 g, 10.8 mmol) and 2-
chloro-N-methyl-N-phenylacetamide (1.99 g, 10.8 mmol) in THF
(30 mL) was heated at 80 °C for 2 d. The white solid formed was fil-
tered under nitrogen, washed with THF and dried under vacuum.
Yield: 2.13 g, 73%, mp: 240.3–242.8 °C (dec). Anal. Calc. for
C
13H16ClN3O: C, 58.75; H, 6.06; N, 15.81. Found: C, 59.22; H,
6.06; N, 15.87%. 1H NMR (CDCl3) d, ppm: 3.26 (s, 3H, PhNCH3),
3.97 (s, 3H, NCH3), 5.12 (s, 2H, CH2), 7.28–7.48 (m, 7H, imi H and
Ph H), 10.37 (s, 1H, NCHN). 13C{1H} NMR (DMSO-d6) d, ppm: 36.2
(CH3), 37.8 (CH3), 50.9 (CH2), 123.1, 124.4, 127.9, 128.9 (quaternary
C), 130.4, 138.2, 141.7 (NCHN), 165.1 (C@O).
4.1.6. Synthesis of 3a
To a 50 mL Schlenk flask containing A (0.15 g, 0.41 mmol), 1a
(0.22 g, 0.82 mmol) and potassium carbonate (0.12 g, 0.82 mmol)
was added dry DMF (10 mL). The mixture was allowed to stir for
5 h at ambient temperature. The solvent was completely removed
under vacuum. The residue was dissolved in dichloromethane. The
organic layer was then washed twice with water. After drying with
anhydrous magnesium sulfate, the solvent was completely
removed under vacuum. The residue was washed with diethyl
ether. The pale yellow solid was then filtered on a frit and dried
under vacuum. Yield: 0.24 g, 89%, mp: 163.5–165.6 °C (dec). Anal.
Calc. for C30H32N6O5Pd: C, 56.34; H, 4.86; N, 12.67. Found: C,
56.12; H, 4.89; N, 12.43%. 1H NMR (CDCl3) d, ppm: 3.25 (s, 8H,
CONCH3, CCH@CHC), 3.37 (s, 6H, NCH3), 4.69 (AB quartet,
4.1.2. Synthesis of 1b
The compound was prepared following a similar procedure as
for 1a. 1-Benzyl-1H-imidazole (1.84 g, 11.6 mmol) and 2-chloro-
N-methyl-N-phenylacetamide (2.14 g, 11.6 mmol) were used. A
white solid was obtained. Yield: 2.22 g, 56%, mp: 168.3–169.8 °C
(dec). Anal. Calc. for C19H20ClN3O: C, 66.75; H, 5.89; N, 12.29.
Found: C, 66.57; H, 6.02; N, 12.38%. 1H NMR (CDCl3) d, ppm: 3.25
(s, 3H, NCH3), 5.10 (s, 2H, CH2C@O), 5.42 (s, 2H, PhCH2), 7.10 (s,
1H, imi H), 7.27 (s, 1H, imi H), 7.31–7.50 (m, 10H, Ph H), 10.68 (s,
1H, NCHN). 13C{1H} NMR (CDCl3) d, ppm: 37.7 (CH3), 51.1 (CH2),
52.9 (CH2), 121.1, 124.2, 127.5, 128.6, 128.8 (quaternary C),
129.1, 129.2, 130.3, 133.3, 138.0 (quaternary C), 140.8 (NCHN),
164.3 (C@O).
J = 18.0 Hz,
Dm = 65.5 Hz, 4H, NCH2), 6.77 (s, 2H, imi H), 6.88 (s,
3
2H, imi H), 7.27 (d, JHH = 6.0 Hz, 4H, o-Ph H), 7.38–7.48 (m, 6H,
Ar-H). 13C{1H} NMR (DMSO-d6) d, ppm: 37.3 (CH3), 37.5 (CH3),
37.7 (CCH@CHC), 52.3 (CH2), 122.5, 123.3, 127.7, 128.5, 129.0,
129.4, 130.3, 136.6, 142.5 (quaternary C), 166.6 (CH2C@O), 173.6
(CHCO@O), 187.3 (NCN).
4.1.3. Synthesis of 1c
The compound was prepared following a similar procedure as
for 1a. 1-(4-Methoxybenzyl)-1H-imidazole (4.04 g, 21.0 mmol)
and 2-chloro-N-methyl-N-phenylacetamide (3.94 g, 21.0 mmol)
were used. A white solid was obtained. Yield: 4.55 g, 57%, mp:
221.3–225.8 °C (dec). Anal. Calc. for C20H22ClN3O2: C, 64.59; H,
5.96; N, 11.3. Found: C, 64.47; H, 6.03; N, 11.47%. 1H NMR
(CDCl3) d, ppm: 3.26 (s, 3H, NCH3), 3.75 (s, 3H, OCH3), 5.09 (s,
2H, CH2C@O), 5.35 (s, 2H, ArCH2), 6.85 (d, J = 6.0 Hz, 2H, Ar H),
7.07 (s, 1H, imi H), 7.21 (s, 1H, imi H), 7.29–7.51 (m, 7H, Ar H),
10.55 (s, 1H, NCHN). 13C{1H} NMR (CDCl3) d, ppm: 37.8 (CH3),
51.1 (CH2), 52.9 (CH2), 55.3 (OCH3), 114.7, 120.5, 123.7, 124.8,
127.4, 129.0 (quaternary C), 130.4, 130.5, 138.5 (quaternary C),
140.8 (NCHN), 160.3 (quaternary C), 164.1 (C@O).
4.1.7. Synthesis of 3b
The compound was prepared following a similar procedure as
for 3a. Compound A (0.21 g, 0.56 mmol), 1b (0.38 g, 1.13 mmol)
and K2CO3 (0.16 g, 1.13 mmol) were used. A pale yellow solid
was obtained. Yield: 0.26 g, 58%, mp: 166.6–167.5 °C (dec). Anal.
Calc. for C42H40N6O5Pd: C, 61.87; H, 4.94; N, 10.31. Found: C,
62.29; H, 5.40; N, 9.84%. 1H NMR (CDCl3) d, ppm: 3.02 (s, 6H,
NCH3), 3.11 (s, 2H, CCH@CHC), 4.52 (d, J = 18.0 Hz, 2H,
CHaHbC@O), 4.83 (d, J = 18.0 Hz, 2H, CHaHbC@O), 4.95 (s, 4H,
ArCH2), 6.73 (s, 2H, imi H), 6.80–6.82 (m, 4H, Ar H), 6.92 (s, 2H,
imi H), 7.09 (d, J = 9.0 Hz, 4H, Ar H), 7.28–7.35 (m, 12H, Ar H).
13C{1H} NMR (CDCl3) d, ppm: 37.2 (NCH3), 39.0 (CCH@CHC), 52.9
(CH2C@O), 54.1 (PhCH2), 120.8, 123.0, 126.9, 127.1, 127.8, 128.6,
130.3, 136.8 (quaternary C), 141.4 (quaternary C), 166.8
(CH2C@O), 174.4 (CHCO@O), 189.3 (NCN).
4.1.4. Synthesis of 1d
The compound was prepared following a similar procedure as
for 1a. 1-(4-Fluorobenzyl)-1H-imidazole (6.71 g, 38.0 mmol) and
2-chloro-N-methyl-N-phenylacetamide (6.99 g, 38.0 mmol) were
used. A white solid was obtained. Yield: 9.18 g, 67%, mp: 142.3–
144.8 °C (dec). Anal. Calc. for C19H19ClN3OF: C, 63.42; H, 5.32; N,
11.68. Found: C, 63.74; H, 5.51; N, 11.55%. 1H NMR (CDCl3) d,
ppm: 3.23 (s, 3H, CH3), 5.04 (s, 2H, CH2C@O), 5.46 (s, 2H, PhCH2),
4.1.8. Synthesis of 3c
The compound was prepared following a similar procedure as
for 3a. Compound A (0.35 g, 0.94 mmol), 1c (0.70 g, 1.88 mmol)
and K2CO3 (0.26 g, 1.88 mmol) were used. A pale yellow solid
was obtained. Yield: 0.65 g, 80%, mp: 171.2–172.9 °C (dec). Anal.

34
Y.-M. Jhou et al. / Polyhedron 100 (2015) 28–35
Calc. for C44H44N6O7Pd: C, 60.37; H, 5.06; N, 9.60. Found: C, 60.63;
H, 5.04; N, 9.50%. 1H NMR (CDCl3) d, ppm: 3.02 (s, 6H, NCH3), 3.14
(s, 2H, CCH@CHC), 3.74 (s, 6H, OCH3), 4.55 (d, J = 15 Hz, 2H,
CHaHbC@O), 4.80–4.87 (m, 6H, CHaHbC@O, Ar CH2), 6.69 (s, 2H,
imi H), 6.81–6.91 (m, 10H, Ar H, imi H), 7.05 (d, J = 9.0 Hz, 4H, Ar
H), 7.34–7.36 (m, 6H, Ar H). 13C{1H} NMR (CDCl3) d, ppm: 37.2
(NCH3), 38.8 (CCH@CHC), 52.7 (CH2C@O), 53.5 (PhCH2), 55.2
(OCH3), 113.9, 120.5, 122.9 (quaternary C), 126.9, 128.6, 130.2,
141.3 (quaternary C), 159.2 (quaternary C), 166.7 (CH2C@O),
174.3 (CHCO@O), 188.6 (NCN). Crystals suitable for X-ray diffrac-
tion studies were obtained by vapor diffusion of diethyl ether into
a dichloromethane solution containing 3a.
H), 6.99 (s, 2H, imi H), 7.32–7.40 (m, 10H, Ar H). 13C{1H} NMR
(DMSO-d6) d, ppm: 20.9 (CH3), 21.2 (CH3), 38.1 (CH3), 53.9 (CH2),
123.7, 124.5 (quaternary C), 126.7, 127.7, 129.3, 129.9 (quaternary
C), 135.2, 135.6, 137.3 (quaternary C), 143.4 (quaternary C), 166.0
(NCN), 166.8 (C@O). Crystals suitable for X-ray crystallographic
analysis were obtained by vapor diffusion of diethyl ether into a
dichloromethane solution containing the compound.
Compound 7 was obtained from the first band eluted with
n-hexane/ethyl acetate (60:40).
A white solid was obtained.
Yield: 27 mg, 6%. Anal. Calc. for C43H38F5O4Pd: C, 58.01; H, 4.30;
N, 7.86. Found: C, 58.20; H, 4.50; N, 7.85%. 1H NMR (CDCl3) d,
ppm: 1.97 (s, 6H, Mes CH3), 2.30 (s, 9H, Mes CH3), 2.30 (br s,
10H, O-heterocyclic CH and Mes CH3), 2.31 (s, 3H, Mes CH3), 2.33
(s, 1H, O-heterocyclic CH), 3.35 (s, 3H, NCH3), 5.22 (s, 1H,
O@CCHN), 6.70 (s, 1H, imi H), 6.85 (s, 1H, imi H), 6.92 (s, 2H,
Mes H), 6.93 (s, 2H, Mes H), 7.33–7.38 (m, 4H, Ar H, imi H),
7.54–7.57 (m, 3H, Ar H, imi H), 7.88 (s, 1H, NH). Due to the minute
quantity of the compound, the 13C NMR spectrum was not
acquired. Colorless crystals suitable for X-ray diffraction studies
4.1.9. Synthesis of 3d
The compound was prepared following a similar procedure as
for 3a. Compound A (0.40 g, 1.07 mmol), 1d (0.77 g, 2.15 mmol)
and K2CO3 (0.29 g, 2.15 mmol) were used. A pale yellow solid
was obtained. Yield: 0.53 g, 58%, mp: 168.5–169.1 °C (dec). Anal.
Calc. for C42H38F2N6O5Pd: C, 59.26; H, 4.49; N, 9.87. Found: C,
59.26; H, 4.59; N, 10.08%. 1H NMR (CDCl3) d, ppm: 3.03 (s, 6H,
NCH3), 3.14 (s, 2H, CCH@CHC), 4.54 (d, J = 18 Hz, 2H, CHaHbC@O),
4.84 (d, J = 15.0 Hz, 2H, CHaHbC@O), 4.91 (s, 4H, ArCH2), 6.71 (s,
2H, imi H), 6.87–7.12 (m, 16H, Ar H, imi H), 7.35–7.38 (m, 4H, Ar
H). 13C{1H} NMR (CDCl3) d, ppm: 37.2 (NCH3), 39.0 (CCH@CHC),
52.8 (CH2C@O), 53.3 (PhCH2), 115.5 (d, J = 21.9 Hz, Ar C), 120.7,
123.1, 126.9, 128.6, 128.9 (d, J = 7.5 Hz, Ar C), 130.3, 132.5 (d,
J = 3.0 Hz, quaternary C), 141.3 (quaternary C), 162.3 (d,
J = 247.5 Hz, CF), 166.7 (CH2C@O), 174.2 (CHCO@O), 189.0 (NCN).
Crystals suitable for X-ray diffraction studies were obtained by
vapor diffusion of diethyl ether into a dichloromethane solution
containing 3d.
were obtained by vapor diffusion of diethyl ether into
dichloromethane solution containing the compound.
a
4.3. Mizoroki–Heck reaction
In a typical run, a Schlenk tube was charged with aryl halide
(1.0 mmol), alkene (1.4 mmol), anhydrous sodium acetate
(1.1 mmol) and the palladium(0) precatalyst (0.5 mol%). The flask
was thoroughly degassed, TBAB (2 g) was added and then the flask
was placed in a preheated oil bath at the appropriate temperature
as listed in Table 3. After the mixture was cooled, the mixture
was diluted with water (10 mL) and extracted with ether
(3 Â 10 mL). The combined organic portions were dried over anhy-
drous MgSO4. After filtration, the solvent was removed completely
under vacuum. The coupled products were identified by
comparison of NMR data with those in the literature [14]; yields
and regioselectivity were determined by integration ratios using
1,3,5-trimethoxybenzene as an internal standard or after
purification with column chromatography on silica gel.
4.1.10. Synthesis of 4e
The compound was prepared following a similar procedure as
for 3a. Compound A (0.26 g, 0.69 mmol), 1f (0.5 g, 1.39 mmol)
and K2CO3 (0.19 g, 1.39 mmol) were used. A pale-yellow solid
was obtained. Yield: 0.45 g, 77%, mp: 195.2–195.9 °C (dec). Anal.
Calc. for C44H44N6O5Pd: C, 62.66; H, 5.25; N, 9.96. Found: C,
62.79; H, 5.34; N, 9.50%. 1H NMR (CDCl3) d, ppm: 2.03 (s, 12H,
o-Mes CH3), 2.34 (s, 3H, p-Mes CH3), 2.86 (s, 1H, maleic anhydride
CH), 2.93 (s, 1H, maleic anhydride CH), 5.68 (s, 4H, CH2), 6.82–7.12
(m, 10H, imi H, Ar H), 7.25–7.33 (m, 4H, Ar H), 7.68–7.74 (m, 4H, Ar
H), 11.5 (s, 2H, NH). 13C{1H} NMR (DMSO-d6) d, ppm: 17.5 (CH3),
17.8 (CH3), 20.8 (CH3), 36.2 (maleic CH), 38.7 (maleic CH), 53.3
(CH2), 119.9, 123.2, 123.6, 124.0, 129.1, 134.7 (quaternary C),
135.5 (quaternary C), 137.0 (quaternary C), 138.1, 139.2, 140.7
(quaternary C), 162.7 (CH2C@O), 165.8 (CH2C@O), 173.2
(CHCO@O), 189.7 (NCN).
4.4. X-ray diffraction studies
Compounds 6 and 7 were collected on a Bruker APEX II
equipped with a CCD area detector and a graphite monochromater
utilizing Mo K
a radiation (k = 0.71073 Å). Data collection and
reduction were performed using APEX2 and SAINT software [15].
Absorption corrections were performed using the SADABS program
[16]. For compounds 3c and 3d, data were collected at 150(2) K
on an Oxford Xcalibur single crystal diffractometer mounted with
a fine focus molybdenum enhance X-ray source (k = 0.71073 Å)
and a Sapphire charge couple device detector. Data collection and
reduction and absorption corrections were performed using
CrysAlisPro software [17]. All the structures were solved by direct
methods and refined by full-matrix least squares methods against
F2 with the SHELXTL software package [18]. All non-H atoms were
refined anisotropically. All H-atoms were fixed at calculated posi-
tions and refined with the use of a riding model. Crystallographic
data are listed in Table 1.
4.2. Reaction of 3e with pentafluoroiodobenzene
A mixture of 3e (0.44 g, 0.51 mmol) and pentafluoroiodoben-
zene (0.30 g, 1.02 mmol) in DMF (10 mL) was stirred at 80 °C over-
night under a nitrogen atmosphere. After cooling down to ambient
temperature, the solvent was removed completely under vacuum.
The residue was purified by silica gel column chromatography.
Decafluorobiphenyl [13] (DFBP) was obtained from the second
band eluted with n-hexane/ethyl acetate (50:50). Yield: 40 mg,
24% (based on 3e). 19F{1H} NMR (CDCl3) d, ppm: À120.5, À153.4,
À160.6. ESI-MS: m/z Calc. for C12F10 334.1, found 334.3%.
Compound 6 was obtained from the third band eluted with
n-hexane/ethyl acetate (30:70). A pale yellow solid was obtained.
Yield: 125.8 mg, 48%. Anal. Calc. for C42H46I2N6O2Pd: C, 49.11; H,
4.51; N, 8.18. Found: C, 49.60; H, 4.50; N, 7.85%. 1H NMR (CDCl3)
d, ppm: 1.92 (s, 12H, Mes CH3), 2.42 (s, 6H, Mes CH3), 3.35 (s, 6H,
NCH3), 5.11 (s, 4H, CH2C@O), 6.67 (s, 2H, imi H), 6.77 (s, 4H, Mes
Acknowledgements
We are grateful to the Ministry of Science and Technology of
Taiwan for financial support of this work. We also thank the
National Center for High-performance Computing of Taiwan for
computing time and financial support of the Conquest software.

Y.-M. Jhou et al. / Polyhedron 100 (2015) 28–35
35
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uk/conts/retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
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