Co-ligand effects in the catalytic activity of Pd(II)-NHC complexes

Article abstract of DOI:10.1016/j.jorganchem.2010.08.017

Three cis-chelating di-N-heterocyclic carbene palladium(II) complexes [PdX₂(diNHC)] (X = I, 1; X = SCN, 2; X = CF₃CO ₂, 3) bearing different anionic co-ligands were synthesized and fully characterized. A comparison of thei

Full text of DOI:10.1016/j.jorganchem.2010.08.017

Journal of Organometallic Chemistry 696 (2011) 112e117
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Co-ligand effects in the catalytic activity of Pd(II)eNHC complexes
*
Andrew Dahao Yeung, Pearly Shuyi Ng, Han Vinh Huynh
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history:
Three cis-chelating di-N-heterocyclic carbene palladium(II) complexes [PdX2(diNHC)] (X ¼ I, 1; X ¼ SCN,
2; X ¼ CF3CO2, 3) bearing different anionic co-ligands were synthesized and fully characterized. A
comparison of their catalytic activities in the MizorokieHeck reaction and conjugate addition of aryl-
boronic acids to cyclic enones revealed increasing efficiency in the order SCNꢀ < Iꢀ < CF3CO2ꢀ. The di
(trifluoroacetato) complex 3 showed the best activity in both transformations highlighting the impor-
tance of co-ligands effects in catalysis. In addition, the molecular structure of an unusual poly-hetero-
nuclear complex salt 4 is reported, which has been isolated as a byproduct in the synthesis of complex 3.
Ó 2010 Published by Elsevier B.V.
Received 15 July 2010
Received in revised form
10 August 2010
Accepted 12 August 2010
Available online 20 August 2010
Keywords:
N-Heterocyclic carbenes
Palladium
Co-ligands effects
MizorokieHeck coupling
Conjugate addition
Boronic acids
1. Introduction
their catalytic activities in the conjugate addition of arylboronic
acids to cyclic enones and the MizorokieHeck reaction. In addition,
The use of N-heterocyclic carbenes (NHCs) as ligands in organ-
ometallic chemistry is nowadays well established [1,2]. NHC ligands
are generally stronger donors [3] compared to phosphines and have
found applications as phosphine substitutes in Pd-mediated reac-
tions including carbonecarbon bond formations [4], which are key
synthetic steps in a variety of industrial processes [5]. These ligands
have several advantages over the commonly utilized phosphines:
we report the unusual molecular structure of a poly-heteronuclear
complex salt that has been isolated as a byproduct.
2. Results and discussion
2.1. Synthesis and characterization
(i) The strong
s-donating ability of NHCs, coupled with its minimal
The preparation of the palladium(II)ediNHC complexes is
summarized in Scheme 1. Diiodo-(1,10-dimesityl-3,30-methyl-
enediimidazolin-2,20-diylidene)palladium(II) (1) was straightfor-
wardly synthesized by in situ deprotonation of the imidazolium salt
precursor (A) with palladium acetate in DMSO. A base peak at m/
z ¼ 617 for the [M ꢀ I]þ complex fragment in the positive mode ESI
mass spectrum indicates formation of a carbene complex. The 1H
NMR spectrum of the yellow complex 1 shows generally broad
ligand signals and geminal coupling of the methylene bridge was
not resolved due to the fluxional behavior of the boat-shaped six-
membered palladacycle [9].
Reaction of the air-stable complex 1 with the appropriate silver
salts gave the analogous di(isothiocyanato) (2) and di(tri-
fluoroacetato) (3) derivatives via a metathesis reaction [10]. To the
best of our knowledge, 2 is the first reported mixed NHCedi(iso-
thiocyanato) Pd(II) complex. These two complexes were isolated as
white and air-stable solids. Even in DMSO solutions, they remain
stable and did not decompose to give palladium black after pro-
longed standing. The successful replacement of the anionic co-
p-back bonding capabilities results in strong stabilizing effects and
resistance to dissociation from the Pd center; (ii) the strong
PdeNHC bond and limited catalyst decomposition pathways
ensures high thermal stability of PdeNHC complexes, hence
allowing for their use as catalyst at higher temperatures to increase
reaction rates. Compared to monodentate NHCs, chelating dicar-
benes are expected to be more stable as the reductive elimination
of carbene, a possible decomposition pathway, should take place
at a slower rate for conformationally restricted systems. The
importance of ancillary ligands in the design of transition metal
catalysts is well established. On the other hand, the influence of co-
ligands ebeyond that of simple halidese on the catalytic activities
has been less studied [6e8]. Herein, we report the synthesis and
structural characterization of three cis-chelating dicarbene Pd(II)
complexes bearing different anionic co-ligands and a comparison of
* Corresponding author. Tel.: þ65 6516 2670; fax: þ 65 6779 1691.
0022-328X/$ e see front matter Ó 2010 Published by Elsevier B.V.
doi:10.1016/j.jorganchem.2010.08.017

A.D. Yeung et al. / Journal of Organometallic Chemistry 696 (2011) 112e117
113
which has previously been observed for diphosphine analogues
[12,13] and dicarbene-nickel(II) complexes [14].
Mes
Mes
Mes
Mes
N
N
N
N
N
N
N
N
N
Pd(OAc)2
2 AgX
I
I
X
X
2 I
Pd
Pd
MeCN
- 2 AgI
DMSO
- 2 HOAc
2.2. Molecular structures
N
N
N
Mes
Mes
Single crystals of complex 1 and 3 were obtained easily by the
slow diffusion of diethyl ether into a saturated acetonitrile solution,
while single crystals of complex 2 were obtained from the slow
diffusion of diethyl ether into a saturated DMF solution. Selected
A
1
2: X = NCS
3: X = CF3COO
Scheme 1. Synthesis of Palladium(II)eNHC Complexes 1e3.
Table 1
ꢁ
ꢀ
Selected bond lengths (A) and angles ( ) for complexes 1e4.
1
2$DMF
3$CH3CN
4$2(CH3)2CO
Pd(1)eC(1)
Pd(1)eC(14)
Pd(1)eI(1)
Pd(1)eI(2)
Pd(1)eN(5)
Pd(1)eN(6)
Pd(1)eO(1)
Pd(1)eO(3)
C(1)ePd(1)eC(14)
C(1)ePd(1)eI(1)
C(1)ePd(1)eN(5)
C(1)ePd(1)eO(1)
PdC2X2/NHC dihedral angle
2.018(5)
2.029(5)
2.6465(6)
2.6380(6)
e
1.997(4)
1.967(4)
e
1.969(4)
1.969(4)
e
1.977(6)
1.967(6)
e
2.046(3)
2.032(4)
e
e
e
e
2.068(3)
2.083(3)
86.18(16)
e
2.082(4)
2.069(4)
86.9(2)
e
86.60(2)
92.11(16)
86.07(15)
e
e
95.38(13)
e
e
e
e
93.06(14)
31.2(1), 37.2(1)
95.1(2)
26.0(2), 30.6(2)
47.5(2), 47.5(2)
32.0(1), 37.5(1)
ligands is supported by isotopic envelopes in the ESI mass spectra
of 2 and 3 at m/z ¼ 548 for [M ꢀ SCN]þ and 604 for [M ꢀ CF3CO2]þ,
respectively. Furthermore, substitution of the iodo ligands in
complex 1 with isothiocyanato ligands (2) decreased the complex’s
solubility in DMSO, whereas substitution by trifluoroacetate
ligands (3) had the opposite effect [8,11].
bond parameters are summarized in Table 1, and some important
crystallographic data are listed in Table 2. Complexes 1e3 (Figs.
1e3) were crystallized as mononuclear complexes, in which the
palladium center is coordinated by the chelating dicarbene ligand
and two other co-ligands in an essentially square planar geometry
(sum of angles around Pd is 359.33ꢁ for 1, 360.21ꢁ for 2 and 360.34ꢁ
for 3). The dihedral angles between the NHC and PdC2X2
coordination planes are markedly smaller in complexes 2 and 3
compared to those in 1, which is likely due to the increasing steric
bulk of the anionic co-ligands. In all three complexes, the dicarbene
bite angles are approximately 86ꢁ. This deviation from the perfect
90ꢁ angle may be due to the short methylene bridge. However, it
has also been demonstrated that the length of flexible alkyl bridges
has almost no effect on such dicarbene bite angles, but would
significantly influence the dihedral angle between the NHC and
coordination planes instead [14,15]. The averaged Pdecarbene
The 1H NMR spectra of complexes 2 and 3 agree with the
expected structures. Similar to that of 1, they exhibit broad signals,
and the expected diastereotopy of the methylene bridge protons
was not observed. Due to the limited solubility of all complexes 1e3
in most organic solvents including DMSO, we were unable to obtain
well-resolved 13C NMR spectra despite prolonged acquisition time.
As expected, the 19F NMR spectrum of 3 shows a singlet at 2.75 ppm
for the trifluoroacetato ligands [6].
Finally, we did not observe any spectroscopic evidence for the
formation of cationicbis (dicarbene) palladium(II) species as
a consequence of autoionization or ligandd is proportionation,
ꢀ
ꢀ
bonds of complexes
2 [1.982(4) A] and 3 [1.969(4) A] are
Table 2
Selected X-ray crystallographic data for complexes 1e4.
1
2$DMF
3$CH3CN
4$2(CH3)2CO
Formula
Formula weight
Color
C
25H28I2N4Pd
C30H35N7OPdS2
680.17
Colorless
100(2)
0.76 ꢂ 0.14 ꢂ 0.08
Monoclinic
P21/c
13.1304(14)
16.3002(17)
14.6497(16)
90
90.869(2)
90
3136.1(6)
4
1.441
1.55 to 27.50
19231
C31H31F6N5O4Pd
758.01
Colorless
90(2)
0.60 ꢂ 0.12 ꢂ 0.08
Monoclinic
P21/c
12.9184(7)
16.1789(8)
15.7207(8)
90
93.1840(10)
90
3280.6(3)
4
1.535
1.58 to 27.50
22942
C79H86Ag3F21N8O19Pd2
2386.97
Orange
744.71
Yellow
296(2)
0.70 ꢂ 0.10 ꢂ 0.04
Orthorhombic
P212121
7.6244(9)
13.6590(14)
25.328(3)
90
90
90
2637.7(5)
4
1.875
Temperature (K)
Crystal size (mm)
Crystal system
Space group
223(2)
0.10 ꢂ 0.06 ꢂ 0.04
Monoclinic
C2/c
16.7465(15)
18.9407(18)
30.621(3)
90
100.386(2)
90
9553.6(15)
4
ꢀ
a (A)
ꢀ
b (A)
ꢀ
c (A)
a
b
g
(ꢁ)
(ꢁ)
(ꢁ)
ꢀ
V (A3)
Z
Dc (g cmꢀ3
)
1.66
1.64 to 25.00
8398
q
range (ꢁ)
1.61 to 27.48
18819
Unique data
Final R indices [I > 2
s(I)]
R1 ¼ 0.0390,
R1 ¼ 0.0515,
R1 ¼ 0.0599,
R1 ¼ 0.0588,
wR2 ¼ 0.0833
wR2 ¼ 0.1068
wR2 ¼ 0.1166
wR2 ¼ 0.1281

114
A.D. Yeung et al. / Journal of Organometallic Chemistry 696 (2011) 112e117
Fig. 1. Molecular structure of complex 1 showing 50% probability ellipsoids. Hydrogen
atoms are omitted for clarity.
Fig. 3. Molecular structure of complex 3$CH3CN showing 50% probability ellipsoids.
Hydrogen atoms and the solvent molecule are omitted for clarity.
ꢀ
significantly shorter than that of the diiodo complex 1 [2.024(5) A].
ꢀ
Similar reduced averaged PdeC bond lengths (w1.9773 A) were
Attempts to obtain single crystals from a crude mixture of
complex 3 in acetone yielded a few single crystals of an unusual
multinuclear byproduct 4 (Fig. 4), and its peculiar molecular
structure determined by X-ray analysis is shown in Fig. 5.
Compound 4 can be described as a salt consisting of a hetero-
trinuclear [Pd2Ag]-cation and a dinuclear [Ag2]-anion. The complex
cation consists of two Pd(II)edicarbene units that are both con-
reported to for the dicationic complex [Pd(CH3CN)2(diNHC)](PF6)2
bearing the same dicarbene ligand [16]. This is expected as the
lower electron donating ability (Iꢀ > NCSꢀ > (CH3CN) > CF3CO2ꢀ) of
the trifluoroacetato and isothiocyanato ligands would induce
a higher Lewis acidity on the palladium(II) center, which in turn
leads to a higher electron donation of the dicarbene ligand to the
metal center. This claim is further supported by the observation
that the averaged Pdecarbene bond for the least donating di(tri-
fluoroacetate) complex 3 is shorter in comparison to that of the di
(isothiocyanato) complex 2. Surprisingly, the overall charge of the
complex seems to have less influence on the averaged Pdecarbene
bonds compared to that of the co-ligands. The ambidentate iso-
thiocyanato ligands in complex 2 are both coordinated through the
hard N-atom with PdeN distances of 2.046(3) A and 2.032(4) A,
although the Pearson concept would predict an S-coordination to
the soft Pd(II) center. This behavior can be explained by anti-
symbiosis, which was defined as the preference of a hard donor
trans to a soft donor bound to a soft metal center [17]. Nevertheless,
it has been reported for diphosphine analogues that steric effects
can enforce N- over S-coordination [18]. The PdeO bonds in
complex 3 are in the range reported for other mixed carbenee
carboxylato Pd(II) complexes [6,10]. Further structural parameters
are unexceptional and require no further comment.
nected to one Ag(I) center through two bridging m-trifluoroacetato
ligands. The silver atom, which is also the center of inversion, is
coordinated by four oxygen donors in a slightly distorted square
planar geometry. Each of the palladium centers is coordinated by
a cis-chelating dicarbene and two carboxylato ligands in a square
planar fashion. The dicarbene bite angles [86.9(2)ꢁ], the palla-
ꢀ
diumecarbene [1.967(6) and 1.977(6) A] and the palladiume
ꢀ
ꢀ
ꢀ
oxygen [2.082(4) and 2.069(4) A] bond lengths are essentially the
same as observed in complex 3. However, the dihedral angle
between the NHC and the PdC2X2 coordination planes has further
decreased from 3 again due to increased steric crowding of the
additional Ag-complex fragment. The silverepalladium separation
ꢀ
is 2.8438(4) A, which is significantly shorter than the sum of their
ꢀ
van der Waals radii (3.35 A). Nevertheless, the short distance is
expected to be a result of the bridging trifluoroacetato ligands. More
studies are required to reveal the true nature of such potential
PdeAg interactions.
The dinuclear silver anion adopts a three-bladed propeller
structure, with the trifluoroacetato ligands forming the blades
about the hub comprising the two silver atoms and two terminally
coordinated acetone molecules. The three blades with OeAgeO
bond angles of 94.0(3), 133.7(3), and 126.1(3)ꢁ are not spaced
evenly apart. Each silver atom adopts a distorted trigonal bipyra-
ꢀ
midal structure. The inter-silver distance of 2.8415(11) A is signif-
ꢀ
icantly shorter than the sum of their van der Waals radii (3.44 A)
suggesting a ligand-supported argentophilic interaction, which can
CF3
O
F3C
O
Mes
O
Mes
O
N
N
N
N
O
Ag
O
CF3
N
N
N
N
O
Ag
Pd
Pd
O
F3C
O
Ag
O
O
Mes
O
O
O
Mes
O
O
F3C
CF3
F3C
Fig. 2. Molecular structure of complex 2$DMF showing 50% probability ellipsoids.
Hydrogen atoms and the solvent molecule are omitted for clarity.
Fig. 4. Structural diagram of complex salt 4 (cation: left; anion right).

A.D. Yeung et al. / Journal of Organometallic Chemistry 696 (2011) 112e117
115
Fig. 5. Molecular structure of complex salt 4$2(CH3)2CO (cation: left; anion right) showing 50% probability ellipsoids. Hydrogen atoms, solvent molecules and most atoms of the
mesityl substituents are omitted for clarity.
also occur in the absence of supporting ligands [19,20], similar to
the well-studied aurophilic interactions [21,22].
effective catalysts in the conjugate addition of arylboronic acids to
cyclic enones [25]. Hence, we setout totest the co-ligand effects using
the synthesized complexes in this reaction. The results summarized
in Table 4 indicate an even greater co-ligand effect in this reaction.
Here only complex 3 was able to catalyze the reaction (entry 3) and in
high yields. Complexes 1 and 2 were both ineffective (entries 1 and 2)
in catalyzing the conjugate addition reaction and the phenylboronic
acid was fully recovered. The addition of 4-biphenylboronic acid,
3- and 4-acetylbenzeneboronic acid using catalyst precursor 3
were equally effective giving high yields (entries 4e6). Surprisingly,
2.3. MizorokieHeck coupling reactions
A number of Pd(II) precatalysts with different of ancillary dicar-
bene ligands were investigated for their activities in the Mizor-
okieHeck reaction and gave rise to high TONs for the coupling of aryl
bromides [23,24]. In order to study any potential co-ligand effects on
the catalytic activity, complexes 1e3 were also tested for their cata-
lytic performance using this reaction. The coupling of simple aryl
bromides and chlorides with tert-butyl acrylate in DMF at 1 mol%
catalyst loading over 24 h was used as the standard test reaction.
The results summarized in Table 3 indicate that all three
complexes had some catalytic activity and in all instances the iso-
thiocyanato complex 2 showed the lowest catalytic activity among
the three complexes, while trifluoroacetato complex 3 consistently
outperformed the diiodo complex 1 in the absence of any additives.
The coupling of electron poor and activated substrates such as 4-
bromobenzaldehyde and 4-bromoacetophenone at 120 ꢁC (entries
1e6) proceeded with moderate to high yields, but the coupling of
electron rich 4-bromoanisole resulted in no conversion for
complexes 1 and 2, and an only low yield of 16% for complex 3 (entry
9). Coupling of 4-chlorobenzaldehyde did not occur at 120 ꢁC at all
and gave only low yields with complexes 1 and 3 at 140 ꢁC (entries
13 and 15). With the addition of [N(n-C4H9)4]Br, the reaction yields
were greatly improved (entries 19 and 21). However, the resulting
excess of bromide anions also eliminated any observable co-ligand
effects and catalyst precursor 1 and 3 gave essentially the same
yield. Overall, the performance of complex 3 is slightly inferior to
a related trifluoroacetatoePd(II) complex bearing two monodentate
benzimidazolin-2-ylidenes under the same reaction conditions [6].
Table 3
MizorokieHeck Coupling reactions catalyzed by complexes 1e3.a
O
Cat. 1 mol%
O
O
R
X +
DMF, NaOAc,
-HX
O
R
X=Br,Cl
R=CHO,CH3CO, OCH3
Entry
Catalyst
Aryl halide
Temp (ꢁC)
Yield (%)b
1
2
3
4
5
6
7
8
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
4-Bromobenzaldehyde
4-Bromobenzaldehyde
4-Bromobenzaldehyde
4-Bromoacetophenone
4-Bromoacetophenone
4-Bromoacetophenone
4-Bromoanisole
4-Bromoanisole
4-Bromoanisole
4-Bromotoluene
4-Bromotoluene
120
120
120
120
120
120
120
120
120
120
120
120
140
140
140
140
140
140
89
85
97
79
57
>99
e
e
9
21
6
10
11
12
13
14
15
16c
17c
18c
e
4-Bromotoluene
46
8
4-Chlorobenzaldehyde
4-Chlorobenzaldehyde
4-Chlorobenzaldehyde
4-Chlorobenzaldehyde
4-Chlorobenzaldehyde
4-Chlorobenzaldehyde
e
2.4. Conjugate addition of arylboronic acids to cyclic enones
15
74
e
Encouraged by the interesting co-ligand influence on the catalytic
activities of the complexes exhibited in the MizorokieHeck coupling
reaction, we explored the utility of the complexes in another catalytic
system. Recent work reported by Shi et al. showed that chelating di
(NHC)epalladium complexes bearing carboxylato ligands to be
75
a
Reaction conditions generally not optimized.
b
c
Yields were determined by 1H NMR spectroscopy over an average of two runs.
With addition of 1.5 equiv of [N(n-C4H9)4]Br.

116
A.D. Yeung et al. / Journal of Organometallic Chemistry 696 (2011) 112e117
Table 4
the type [PdX2(diNHC)]. The superiority of the di(trifluoroacetato)
complex 3, and thus the co-ligand effect, was even more
pronounced for the catalytic conjugate addition of boronic acids to
cyclohexenone, in which iodo and isothiocyanato complexes were
completely inactive. These results highlight that the choice of co-
ligand is equally important to that of the ancillary ligands and
should not be neglected in catalyst design. Further work is
underway to extend this study to other anions in order to establish
a set of “privileged” co-ligands.
Conjugate addition reactions catalyzed by complexes 1e3.a
O
O
HO
Cat. 1 mol%
B
R
KOH
THF/H2O (10:1)
HO
R
Entry
1
Catalyst
R
Isolated yield (%)b
4. Experimental section
1
e
4.1. General considerations
2
3
4
2
3
3
e
All manipulations were carried out without taking precautions
to exclude air and moisture unless otherwise stated. All solvents
were used as received. 1H and 19F NMR spectra were recorded on
Bruker 300 MHz FT-NMR spectrometer at 300 and 282 MHz
respectively. The residual protio-solvent or tetramethylsilane
signals were used as the internal standard for 1H NMR spectra;
trifluoroacetic acid was used as the external standard for 19F NMR
spectra. 13C NMR spectra were not obtained due to the poor solu-
bility of the complexes. ESI mass spectra were obtained using
a Finnigan MAT LCQ spectrometer. Elemental analyses were per-
formed on a PerkineElmer PE 2400 elemental analyzer at the
Department of Chemistry, National University of Singapore.
96 (97)
99 (97)
O
5
3
74
4.2. 1,10-Dimesityl-3,30-methylenediimidazolium diiodide (A)
O
Analogous to the previously-reported dibromide salt [9], 1-
mesitylimidazole (1.50 g, 8.05 mmol, 2 equiv) was dissolved in
toluene (10 mL) and diiodomethane (0.32 mL, 4.03 mmol, 1 equiv)
was added. The resultant solution was heated at 150 ꢁC for 48 h. The
white precipitate was filtered off, washed with toluene and tetra-
hydrofuran (10 mL each), and dried in vacuo. Yield: 2.19 g (84%). 1H
6
7
3
3
94
3
NO2
NMR (300 MHz, DMSO-d6):
d 9.76 (s, 2H, NCHN), 8.33 (s, 2H, CH),
8.10 (s, 2H, CH), 7.17 (s, 4H, Ar-H), 6.84 (s, 2H, NCH2N), 2.34 (s, 6H, p-
ArCH3), 2.03 (s, 12H, o-ArCH3). MS (ESI) m/z (%): 513 (60) [M ꢀ I]þ.
8
3
1
S
4.3. Diiodo-(1,10-dimesityl-3,30-methylenediimidazolin-2,20-
diylidene)palladium(II) (1)
a
Reaction conditions generally not optimized.
Isolated yields over an average of two runs. () reported yield.[4].
b
A mixture of A (1.40 g, 2.19 mmol, 1 equiv) and Pd(OAc)2 (0.49 g,
2.19 mmol, 1 equiv) in DMSO (12 mL) was heated at 60 ꢁC for 1 h,
then 80 ꢁC for 16 h to give a bright orange solution. The solvent was
removed via vacuum distillation at 70 ꢁC to afford a bright orange
residue. Dichloromethane (10 mL) was added to the residue and the
mixture was stirred and filtered. The bright yellow residue was
washed with dichloromethane (2 ꢂ 10 mL), and dried in vacuo.
3-nitrophenylboronic acid and thiophene-2-boronic acid failed to
undergo conjugate addition indicating that substrates with donor-
atoms may deactivate the active species by coordination to the metal
center (entries 7 and 8). The results obtained here clearly suggest
a strong co-ligand influence in the conjugate addition of arylboronic
acids to cyclic enones. Similar to the MizorokieHeck reaction, weakly
coordinating carboxylato ligands play a very important role in the
reaction allowing for an easy substrate binding and activation,
whereas halido or pseudo-halido ligands are more difficult to be
displaced leading to low activities.
Yield: 1.22 g (75%). 1H NMR (300 MHz, DMSO-d6):
d 7.90 (s, br, 2H,
CH), 7.45 (s, br, 2H, CH), 7.02 (s, br, 4H, Ar-H), 6.56 (s, br, 2H, NCH2N),
2.29 (s, br, 6H, p-ArCH3), 2.02 (s, br, 12H, o-ArCH3). MS (ESI) m/z (%):
617 (100) [M ꢀ I]þ; 127 (93) [I]ꢀ.
3. Conclusion
4.4. Di(isothiocyanato)(1,10-dimesityl-3,30-methylenediimidazolin-
2,20-diylidene)palladium(II) (2)
Three cis-chelating di-N-heterocyclic carbene palladium(II)
complexes [PdX2(diNHC)] (X ¼ I, 1; X ¼ SCN, 2; X ¼ CF3CO2, 3)
bearing different anionic co-ligands were synthesized and fully
characterized. An interesting polynuclear complex salt (4) was
obtained as a byproduct of in the synthesis of trifluoroacetato
complex 3. The identities of all complexes 1e4 have been
confirmed by X-ray diffractions studies. Catalytic studies using the
MizorokieHeck coupling reaction revealed a positive anionic co-
ligand effect in the order of SCNꢀ < Iꢀ < CF3CO2ꢀ for complexes of
A mixture of 1 (1.00 g, 1.34 mmol,1 equiv) and silver thiocyanate
(0.45 g, 2.68 mmol, 2 equiv) in acetonitrile (15 mL) was shielded
from light and heated at 65 ꢁC overnight to give a light yellow
suspension. The mixture was filtered, and the white residue was
washed with acetonitrile (20 mL). The white residue was further
stirred with DMSO (20 mL) at room temperature overnight, and
filtered again. Solvents were removed from the combined filtrates
to give a white solid, which was washed with diethyl ether (20 mL)

A.D. Yeung et al. / Journal of Organometallic Chemistry 696 (2011) 112e117
117
and dried in vacuo to give the desired product. Yield: 0.52 g (64%).
1H NMR (300 MHz, DMSO-d6):
7.91 (s, 2H, CH), 7.50 (s, 2H, CH),
6.99 (s, 4H, Ar-H), 6.55 (s, 2H, NCH2N), 2.26 (s, 6H, p-ArCH3), 1.96 (s,
12H, o-ArCH3). MS (ESI) m/z (%): 548 (100) [M ꢀ SCN]þ; 58 (10)
[SCN]ꢀ.
programs [29]. The structure was solved by direct methods to locate
the heavy atoms, followed by difference maps for the light, non-
hydrogen atoms. All hydrogen atoms were put at calculated posi-
tions. All non-hydrogen atoms were generally given anisotropic
displacement parameters in the final model. A summary of the most
important crystallographic data is given in Table 2.
d
4.5. 1,10-Dimesityl-3,30-methylenediimidazolin-2,20-diylidene-di
(trifluoroacetato)palladium(II) (3)
Acknowledgements
A mixture of 1 (2.418 g, 3.25 mmol, 1 equiv) and silver tri-
fluoroacetate (1.43 g, 6.50 mmol, 2 equiv) in acetonitrile (30 mL)
was shielded from light and heated to reflux overnight to give
a grey-green precipitate in an orange solution. The solids were
filtered off, and the orange filtrate was dried in vacuo to give an
orange residue. The residue was suspended in dichloromethane
(15 mL) and filtered to give the desired product as a white solid,
which was washed with diethyl ether (20 mL) and dried. Yield:
The authors thank the National University of Singapore for
financial support (Grant No. R 143-000-407-112) and especially Ms
Geok Kheng Tan and Prof. Lip Lin Koh for determining the X-ray
molecular structures.
Appendix A. Supplementary material
CCDC-783145 (for 1), -783146 (for 2$DMF), -783147 (for
3$CH3CN), and -783148 (for 4$2(CH3)2CO) contain the supple-
mentary crystallographic 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. Supplementary
data associated with this article can be found, in the online version,
at doi:10.1016/j.jorganchem.2010.08.017.
1.39 g (60%). 1H NMR (300 MHz, DMSO-d6):
(s, 2H, CH), 7.01 (s, br, 4H, Ar-H), 6.57 (s, 2H, NCH2N), 2.28 (s, 6H, p-
d 7.96 (s, 2H, CH), 7.50
ArCH3), 2.01 (s,12H, o-ArCH3). 19F NMR (282 MHz, DMSO-d6):
d 2.75
(CF3). MS (ESI) m/z (%): 548 (100) [M ꢀ SCN]þ; 604 (40)
[M ꢀ CF3CO2]þ; 113 (100) [CF3CO2]ꢀ.
4.6. General procedure for the MizorokieHeck reaction
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in vacuo. The residue was analyzed by 1H NMR spectroscopy, and
conversion was calculated by comparing the integrals of the aryl
halide starting material, and the MizorokieHeck product.
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