J.-Y. Lee et al. / Inorganica Chimica Acta 464 (2017) 74–80
75
utilizing MoKa radiation (k = 0.71073 Å). The unit cell parameters
were obtained by least-squares refinement. Data collection and
reduction were performed using the APEX2 and SAINT software
[
10]. Absorption corrections were performed using the SADABS
program [11]. All the structures were solved by direct methods
and refined by full-matrix least squares methods against F2 with
the SHELXTL software package [12]. All non-H atoms were refined
anisotropically. All H-atoms were fixed at calculated positions and
refined with the use of a riding model. Crystallographic data are
listed in Table 1. CCDC files 1477204 (1) contains the supplemen-
tary crystallographic data for this paper. These data can be
Chart 1. Zwitterionic palladium(II) complex bearing an imidazolium-based carbon
donor and tricyclohexylphosphine.
spectrometer. Transmission electronic microscopy (TEM) was per-
formed on a Jeol JEM-2010 at 200 kV. Inductively coupled plasma
optical emission spectrometer (ICP-OES) was carried out using an
Agilent 725. Complex 1 was prepared according to the literature
procedure [7].
3
. Results and discussion
3.1. Zwitterionic palladium complex
Zwitterionic complex 1 is a dichloropalladium(II) complex fea-
2.2. Suzuki-Miyaura cross-coupling reactions
turing tricyclohexylphosphine and an imidazolium-based carbon-
donor as ligands [7]. Although 1 contains an electron-donating
phosphine ligand, which is normally air-sensitive, the complex is
in fact robust and exhibits a high stability in air. As a yellow solid,
it dissolves readily in common organic solvents and is insoluble in
water at room temperature. But at elevated temperature, it dis-
solves in aqueous solution. Similar complexes have been reported
by us previously [7]. The structure of 1 was successfully estab-
lished by X-ray crystallography (Fig. 1). The Pd atom adopts dis-
torted trigonal planar coordination geometry. Due to the cis
disposition of the imidazolium-based carbon donor and bulky tri-
cyclohexylphosphine ligands, the CAPdAP bond angle of 96.16
For a typical reaction, a Schlenk tube was charged with aryl
halide (0.5 mmol), phenylboronic acid (0.6 mmol), KOH (2 equiv),
Pd catalyst precursor (2.0 mol %), TBAB (1 g), and H O (5 mL). The
2
o
reaction mixture was stirred at 80 C for 2–12 h. After cooling to
room temperature, the reaction mixture was extracted with
diethyl ether (3 Â 10 mL). The organic solvent was evaporated to
dryness under vacuum to give the crude product, which was either
analyzed by GC using benzophenone as internal standard or puri-
fied by column chromatography.
2
.3. Recycle experiments
(
14)° is larger than the ideal 90°. The PdAC distance of 2.078
(
5) Å is slightly shorter than that found in similar zwitterionic pal-
In the first run, a Schlenk tube was charged with 4-bromoace-
ladium complexes (2.094(6)–2.108(7) Å) [7]. In contrast, the PdAP
bond distance of 2.2766(13) Å is much longer than that in similar
complexes with triphenylphosphine ligands (2.2370(16)–2.2515
tophenone (0.5 mmol), phenylboronic acid (0.6 mmol), KOH (2
equiv), 1 (2.0 mol %), TBAB (1 g), and H O (5 mL). The reaction mix-
2
ture was stirred at 80 °C for 2 h. After cooling to room temperature,
the reaction mixture was extracted with diethyl ether (3 Â 10 mL)
and the organic layer was then transferred to another Schlenk tube.
The organic solvent was evaporated to dryness under vacuum to
give the crude product, which was purified by column chromatog-
raphy. The original Schlenk tube was recharged with aryl halide,
phenylboronic acid, and KOH for the next catalytic run.
(
13) Å) [7]. The carbonyl oxygen atom of the ketone group is
exposed to potential hydrogen bonding interactions in water.
3
.2. Catalysis
We explored the use of zwitterionic complex 1 as a viable
catalyst precursor for Suzuki-Miyaura coupling reactions in water.
Initially, the coupling reactions of phenylboronic acid with
4
2
.4. Procedure for TEM measurement
-bromo- or 4-chloroacetophenone were investigated in the pres-
ence of KOH at 80 °C for 2 h using 2 mol% of Pd loading (Table 2). A
From the catalytic solution, a drop of the aqueous solution or
the ionic liquid layer was placed in a carbon grid, pumped under
vacuum for a week, and then analyzed by TEM.
Table 1
Crystallographic data.
Empirical formula
Formula weight
Crystal system
Space group
a, Å
C
38
H
53Cl
2
N
2
O
2 2 2
PPdÁCH Cl
2
.5. Procedure for ICP-OES measurement
863.02
Monoclinic
P2 /n
15.2741(4)
14.6897(3)
18.9122(4)
90
108.6410(10)
90
4020.76(16)
150
4
Initially, a portion of the aqueous solution (1 mL) from the cat-
1
alytic solution was transferred to a flask and it was diluted with
water (2 mL). The mixture was stirred for 10 min and it was further
diluted with water (12 mL). The solution was then analyzed by ICP-
OES. After phase separation, the TBAB layer was separated. It was
diluted to 2 mL with water. A portion of this solution (1 mL) was
dissolved in 1:6 nitric acid hydrochloride/H
for 10 min. The mixture was further diluted to 12 mL with water
and then analyzed by ICP-OES.
b, Å
c, Å
a
, deg
b, deg
, deg
c
V, Å
3
2
O (2 mL) and stirred
T, K
Z
No. of unique data
8784
No. of params refined
445
0.0742
0.2079
a
2.6. X-ray diffraction studies
R
1
[I > 2
r
I]
b
wR
2
(all data)
Samples were collected at 150(2) K on an X-ray diffractometer
equipped with a CCD area detector and a graphite monochromator
a
R
wR
1
=
R
= [ (|F
(||F
R
o
| À |F
c o
||)/R |F |.
À |F
b
2
c o
| ) /R (F .
)]1/2
2
2
2
2
o
|