Synthesis of water soluble Pd-Piperidoimidazolin-2-ylidene complexes and their catalytic activities in neat water

Article abstract of DOI:10.1002/aoc.5499

Through the strategy of water soluble N-heterocyclic carbene (NHC) ligand, Pd-catalyzed reactions were developed in aqueous media. Therefore, four new piperidoimidazolinium salts (1a-d) consisting of sulfonate (a), esther (b, c) and carboxylic acid (d) functionalities and their water-soluble Pd-NHC complexes (2a-d) were synthesized. The new compounds were characterized by elemental analysis, FTIR, TGA, UV–vis and NMR spectroscopy. The catalytic activities of water soluble Pd-NHC complexes (2a-d) were investigated using the Suzuki-Miyaura (S-M) reaction and the reduction of nitroarenes. We found that the water-soluble polar or ionic groups on piperidoimidazolin-2-ylidine had an effect on the catalytic activity. The water-soluble catalyst can be recycled efficiently and reused six times with only a very slight loss of catalytic activity.

Full text of DOI:10.1002/aoc.5499

Received: 2 August 2019
DOI: 10.1002/aoc.5499
Revised: 11 December 2019
Accepted: 12 December 2019
F U L L P A P E R
Synthesis of water soluble Pd-Piperidoimidazolin-2-ylidene
complexes and their catalytic activities in neat water
Sinem Çakır
| Hayati Türkmen
Department of Chemistry, Ege University,
35100 Bornova-Izmir, Turkey
Through the strategy of water soluble N-heterocyclic carbene (NHC) ligand,
Pd-catalyzed reactions were developed in aqueous media. Therefore, four new
piperidoimidazolinium salts (1a-d) consisting of sulfonate (a), esther (b, c) and
carboxylic acid (d) functionalities and their water-soluble Pd-NHC complexes
(2a-d) were synthesized. The new compounds were characterized by elemental
analysis, FTIR, TGA, UV–vis and NMR spectroscopy. The catalytic activities of
water soluble Pd-NHC complexes (2a-d) were investigated using the Suzuki-
Miyaura (S-M) reaction and the reduction of nitroarenes. We found that the
water-soluble polar or ionic groups on piperidoimidazolin-2-ylidine had an
effect on the catalytic activity. The water-soluble catalyst can be recycled effi-
ciently and reused six times with only a very slight loss of catalytic activity.
Correspondence
Hayati Türkmen, Department of
Chemistry, Ege University, 35100
Bornova-Izmir, Turkey.
Email: hayatiturkmen@hotmail.com
Funding information
Ege University, Grant/Award Number:
FYL-2018-20197
K E Y W O R D S
N-heterocyclic carbene, reduction of nitroarene, Suzuki-Miyaura reaction, water soluble complex
1 | INTRODUCTION
that are efficient in a large number of homogeneous pro-
cesses. However, when the literature is examined, infor-
The development of green chemistry through organic
reactions using environmentally benign solvents, biode-
gradable and non-toxic chemicals has become one of the
most exciting areas of chemical research.[1,2] Since it is
the cheapest, safest (nonflammable and nontoxic), envi-
ronmentally benign (nonvolatile) and most abundant sol-
vent in nature as a chemical reaction medium, water has
attracted great interest particulary for potential applica-
tions in the field of catalysts. The development of its use
as a solvent for metal catalysts has increased and this
enormous potential of water for developing new catalytic
transformations has emerged. Water-soluble NHC com-
plexes have been synthesized and used in different cata-
lytic process;[3,4] for example Glorius,[5] Beller,[6]
Herrmann,[7] Nolan,[8] Organ,[9] and Thiel[10] have
achieved significant improvements in Suzuki–Miyaura
reactions. N-Heterocyclic carbene ligands bearing hydro-
philic substituents are attractive candidates for aqueous-
phase catalysis as a consequence of their ability to ther-
mally form air-, and moisture-stable metal complexes
mation on the synthesis of water-soluble annulated
imidazole ring-linked substituents and their catalytic
activities is scarce. Among the transition metal catalysts,
palladium-based catalysts appear superior on one of the
S-M cross-coupling reaction between an arylboronic acid
and an aryl halide. On the other hand, the most popular
and promising route for the transformation of nitro
groups to amine groups is the transition metal-catalyzed
hydrogenation reaction.[11,12] The reduction of
nitroarenes is also an important process since the prod-
ucts of aromatic amines bearing groups, such as chloro-,
carbonyl, cyano are important intermediates in the syn-
thesis of chemicals.[13–15] They form the basis of multi-
component agricultural chemicals, medicines, natural
products, polymers, liquid crystals and ligands.[16,17]
Studies to develop catalytic systems that perform in a
short time at room temperatures using low catalyst load-
ing is an ongoing effort. Recently, we prepared Pd (II)-
piperidoimidazolin-2-ylidene complexes bearing alkyl,
benzil and aryl group.[18–21] The catalytic properties of
Appl Organometal Chem. 2020;e5499.
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https://doi.org/10.1002/aoc.5499

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ÇAKIR AND TÜRKMEN
SCHEME 1 Synthesis of palladium complexes of 2a-d
these complexes were investigated in the different cata-
lytic reactions. In this work, we report the synthesis of a
series of water-soluble PEPPSI (pyridine-enhanced pre-
catalysts, preparation, stabilization, initiation) type Pd
(II)-piperidoimidazolin-2-ylidene complexes containing
the sulfonate (a), ester (b, c) or carboxylate (d) group.
Their catalytic applications were investigated in the
Suzuki-Miyaura cross-coupling reaction and the reduc-
tion of nitro compounds in water.
as reported in the literature (Scheme 1).[18] All salts (1a-
d) were obtained in the form of a yellow, ionic liquid and
found to be soluble in DMSO, H2O and polar solvents.
The NMR spectroscopic data of 1a-d were consistent with
1
the proposed structures. In the H NMR spectrum of 1a-
d, the piperidoimidazolinium protons appeared at δ 8.37,
8.05, 8.82 and 8.88 ppm, respectively. The chemical shift
of the NCN sp2 carbon atoms in the 13C NMR spectrum
of salts (1a-d) appear at δ 155.9, 162.2, 162.5 and
155.3 ppm, respectively (Supporting Information).
The synthesis of water-soluble PEPPSI-type Pd-NHC
complexes (2a-d) was readily obtained from
piperidoimidazolinium salts by heating them with Pd
(OAc)2 and NaBr in pure pyridine. The complexes were
purified by column chromatography on silica gel. All new
complexes (2a-d) were isolated as stable solids in air and
also showed good solubility in DMSO and polar solvents.
Complexes (2a-d) exhibited moderate solubility in water.
Complexes (2a and 2d) were soluble up to ꢀ20 mg/ml, and
2 | RESULTS AND DISCUSSION
2.1 | Synthesis of piperidoimidazolinium
salts and their palladium complexes
A number of piperidoimidazolinium salts (1a-d) func-
tioned with substituted water soluble group (sulfonate
(a), esters (b, c) and carboxylic acid (d)) were synthesized
TABLE 1 Melting points, yields and selected 13C NMR data of 2a-d
Entry
Complex
Yield (%)
m.p (ꢁC)
168
υ(C=N)(cm−1
1481, 1523
1446, 1527
1482, 1566
1486, 1616
)
DTG (ꢁC)
221
13C NMR
177.6
1
2
3
4
2a
2b
2c
2d
72
82
80
84
172
210, 276
292
169.2
177
171.6
170
211, 289, 336
172.4

WATER SOLUBLE PD-PIPERIDOIMIDAZOLIN-2-YLIDENE COMPLEXES
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whereas Complexes (2b and 2c) were slightly soluble up to
ꢀ10 mg/ml. When the aqueous solutions of complexes
(2a-d) were extracted with organic solvents, 2a and 2d
remained in the aqueous phase, while 2b and 2c remained
in organic solvents. Structures of the PEPPSI complexes
confirm the typical trans arrangement of the NHC ligand
to the coordinated pyridine.[19,20] The characteristic signal
for the NCHN+ proton in the piperidoimidazolinium salts
selected in the presence of catalyst 0.5 mol%, K2CO3 as
the base, chlorobenzene or bromobenzene as model sub-
strates for the Suzuki-Miyaura cross-coupling reaction
and phenylboronic acid in the water (Figure 1). When
the cataltytic activities of Complexes 2a-d were compared
in the S-M reaction, 2a and 2d showed a higher level
than the others complexes (2b and 2c). This can be attrib-
uted to the ionic group which was attached to the NHC
ligand, and thus increased the solubility in water, posi-
tively affecting the catalytic activity.
1
(1a-d) dissappared in the H NMR spectrum of the Pd
1
(II) complexes(2a-d) (Supporting Information). The H-
NMR spectra of these complexes showed some differences
from their ligands, particularly the CH2 protons bound to
piperidoimidazole. They were observed at lower fields
compared to their respective ligands. The imine streching
vibration band C=N of Complexes 2b-d appeared at 1446–
1616 cm−1. The carbonyl stretch C=O of the esters of the
complexes was also seen at 1728 (for 2b) and 1637 (for 2c)
cm−1 (Supporting Information). The process of the ther-
mal degradation of the water-soluble complexes was inves-
tigated using thermogravimetric analyses (TGA)
techniques (Supporting Information). The DTG curve of
the complexes (2a, 2b and 2d) show sharp peaks around
A number of observations during the initial screen-
ings lead us to believe that Pd(0) nanoparticles (PdNPs)
were formed during the reaction, a common factor in
NHC-complex catalysis in aqueous media.[23] From the
color change in Suzuki-Miyaura reaction in the first
hour, it was understood that palladium (II) was reduced
to palladium(0) (Figure 2). From the formulation of the
black precipitate during the reaction we considered that
PdNPs had been formed (Figure 2).
In order to confirm this supposition, transmission
electron microscopy (TEM) analysis was performed on
the precipitate isolated from cross-coupling of 4-
chloroacetophenone with phenylboronic acid using pre-
catalysts 2d (Figure 3, above). The black precipitate was
isolated by centrifugation and washing with IPA and
water. TEM shows the formation of Pd(0) nanoparticles
ꢁ
100 C which was attributed to the loss of coordinated
water and at 221 ꢁC considered to be related to the decom-
position of Complex 2a. TheꢁDTG curve showed short and
ꢁ
broad merged peaks at 211 C and 289 C attributable to
ꢁ
the loss of both pyridine and decomposition of NHC
ligand, and another sharp peak around 336 ꢁC due to the
decomposition of the complex 2d.
(2d') taking place. An endothermic peak at 217 C of 2d'
in TGA is considered to be associated with its NHC
ligand, which leads to the final decomposition of the
ligand moiety around the metal ion at 335 ꢁC (Supporting
Information).
2.2 | Catalytic studies
An XPS analysis of the 3d levels in the
palladium(0) complex is a powerful tool to evaluate the
ligand donor ability, due to the strict relationship
between the electron bonding energies and the electron
density on the metal centre.[24] To confirm the reduction
of the Pd (II) to Pd(0) species, an XPS analysis of the
Pd(0) nanoparticles (2d') was performed. The XPS Pd 3d
spectrum of the nanocatalyst revealed that the Pd atom
S-M cross-coupling reactions in water offer additional
benefits in terms of designing safer processes. This appli-
cation also facilitates the separation of the apolar biaryl
products possessing low solubility in water. We previ-
ously reported a number of water-soluble Pd-NHC cata-
lysts for Suzuki-Miyaura cross-coupling reactions.[22] We
FIGURE 1 Catalytic Suzuki-
Miyaura cross coupling reaction of
(left) chlorobenzene (1.0 mmol),
(right) bromobenzene (1.0 mmol)
and phenlyboronic acid (1.5 mmol),
K2CO3 (1.5 mmol), TBAB
(1.5 mmol), 1 ml H2O, 0.5 mol % cat

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ÇAKIR AND TÜRKMEN
FIGURE 2 The different colours of
the reaction system within 6 hr
FIGURE 3 TEM images taken
from isolated black precipitate during
screening the nanocatalyst of complex
2d’ in catalytic Suzuki-Miyaura crossing-
coupling reaction (above) and in
nitroarene reduction (below)
was present in the 0 oxidation state, corresponding to the
binding energies of 334.96 and 340.26 eV for the Pd
(0) 3d5/2 and 3d3/2 levels, respectively (Figure 4, left).
On the basis of the results, various reaction parame-
ters such as base and temperature were scanned on 2a
which was the most active catalyst for the optimization
process (Table 2). Table 2 shows that, bases, such as
KOH, K2CO3, Na2CO3, Cs2CO3 and NaOH were scanned
to increase the activity of the system and the highest cou-
pling efficiency among the scans was provided by K2CO3.
The catalyst showed very low activity in the presence of
KOH and Na2CO3. pH optimization conditions were also
investigated. Optimal pH value was found to be 9.0 in pH
optimization studies for Complex 2a (Figure 5, below).
The temperature in the Suzuki-Miyaura coupling
reaction is one of the most important parameters
required to create the C-C bond; therefore, low product
conversion was observed at room temperature duriꢁng
the experimental reactions. After experiments at 100 C
shoꢁwed low reaction yields, the ideal temperature of
60 C was determined and only the results of the test
reactions at this temperature are given below. Tetra-n-
butylammonium bromide (TBAB) is
a quaternary
ammonium salt with a bromide counter ion that is gen-
erally used as a phase transfer catalyst. It is known that
the addition of tetra-n-butylammonium bromide
(TBAB) may increase the reaction rate of the aqueous
Suzuki-Miyaura cross coupling reactions.[25] The role of

WATER SOLUBLE PD-PIPERIDOIMIDAZOLIN-2-YLIDENE COMPLEXES
5 of 14
FIGURE 4 XPS spectra of the
nanocatalyst (2d’) in catalytic Suzuki-
Miyaura cross-coupling reaction (left)
and in nitroarene reduction (right)
TABLE 2 Screening of reaction conditions
Entry
1
Solvent
H2O
H2O
H2O
H2O
IPA/H2O
IPA
Base
Temperature(ꢁC)
Additive
Yield (%)
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
KOH
100
60
40
25
60
60
60
60
60
60
60
60
60
60
60
60
60
-
8
2
-
48
44
6
3
-
4
-
5
-
77
61
29
25
78
24a
20b
19c
17d
48
98
38e
57f
6
-
7
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
-
8
Na2CO3
Cs2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Pd (OAc)2
Pd (OAc)2/1a
-
9
-
10
11
12
13
14
15
16
17
Hg
Hg
Hg
Hg
ODPB
TBAB
TBAB
TBAB
Reaction conditions: chlorobenezene (1.0 mmol), phenylboronic acid (1.5 mmol), 2a (0.5 mol), base (1.5 mmol), H2O (1.0 mL), TBAB (1.5 mmol), a-d Hg
(0.01 ml; 0.03 ml; 0.05 mL; 0.07 mL), e Pd (OAc)2 (0.5 mol %). f Pd (OAc)2 (0.5 mol %) and 1a (0.5 mol %).
the TBAB is thought to be 3-fold. First they facilitate
solvation of the organic substrates in the aqueous
medium. Second they are thought to enhance the rate
of the coupling reaction by activating the arylboronic
acid to reaction by formation of a aryl boronate com-
plex [ArB (OH)3]−[Bu4N]+. Finally, they stabilize the
PdNPs in reaction media. Thus we used TBAB in our
catalytic systems. During the reaction, by the addition
of tetra-n-butylammonium bromide (TBAB) the yield
was increased from 48% to 98%, which facilitated the
dissolution of organic substrates in the solvent media.
TBAB was added at different period of time (1–5 hr)
to test the stability of PdNPs (Figure 5, above). This
experiment indicates that the TBAB is
stabilising agent which prevents agglomeration of
the nanoparticles. The catalytic activity of
a crucial

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ÇAKIR AND TÜRKMEN
FIGURE 5 Suzuki-Miyaura cross
coupling reaction yields for catalyst 2a
pH-dependent (above) and TBAB
addition (below)
Pd(0) nanoparticles is related to their stability and this
often depends on the TBAB used and PdNPs may form
dispersity, consequently with bigger surface and higger
activity. Mercury poisoning test is a type of metal poi-
soning due to exposure to mercury. The suppression of
catalysis by Hg(0) is evidence for a heterogeneous cata-
lyst. We conducted this test in our reaction system to
determine whether the reaction was homogeneous or
heterogeneous. The yield was significantly reduced and
the activity of 2a dropped from 98% to 19%. These
experiments confirmed that 2a is a heterogeneous cata-
lyst and resistant to leach in solution phase.
bromobenzaldehyde,
chlorotoluene, 4-chloroanisole, 4-bromoanisole) using
K2CO3 as base at 60 C (Table 3). While 4-chloroanisole
1-bromo-4-nitrobenzene,
4-
ꢁ
and 4-chlorotoluene gave yields close to each other, the
highest yield was obtained from 4-chloroacetophenone.
Under these conditions, we also investigated phe-
nylboronic acid with 2-subtituted aryl halides such as 2-
chloro-p-xylene, 2-chlorobenzaldehyde, 2-chloroanisole,
2-bromomesitylene (Table 3, entries 5–8). The ortho-
substituted biaryls were obtained with moderate yields.
Reacting phenylboronic acid with 3-chloropyridine
formed 78% of the expected product. The phenylboronic
acid in combination with 3-chlorothiophene resulted in
cross-coupling products with very good yields.
The series of additional experiments were undertaken
to extend the catalytic activity coverage using both elec-
tron-rich and electron-deficient arylboronic acid deriva-
tives and 4-chloroacetophenone (Table 4). The effect of
the locations of the additional groups on the arylboronic
acid derivatives was also investigated. Those carrying an
electron withdrawing group in the meta-position were
less active than those carrying the electron withdrawing
group in the orto or para -position.
Our complexes displayed a comparable catalytic activ-
ity to the sulfonated water soluble PEPPSI-Pd-NHC com-
plexes described by Kühn and co-workers.[26] Using 4-
chloroacetophenone and phenylboronic acid with
0.5 mol% cat, KOH, at 100 ꢁC over a period of 24 hr, they
obtained with 98% yield of the catalytic coupling product.
Esposito and co-workers synthesized amino acid-derived
N-heterocyclic carbene palladium (II) complexes and
ꢁ
obtained good yields at 60 C within 24 hr for the cou-
pling reaction of arylboronic acid with aryl halides.[27]
Encouraged by the very good results obtained with our
catalyst (2a), we decided to investigate in activated and
deactivated aryl halides (4-chlorobenzaldehyde, 4-
The Pd complexes are effective catalysts for the reduc-
tion of nitroarenes.[16,28,29] However, to the best of our

WATER SOLUBLE PD-PIPERIDOIMIDAZOLIN-2-YLIDENE COMPLEXES
7 of 14
TABLE 3 The Suzuki-Miyaura cross-coupling reaction of aryl halides with phenylboronic acid
Entry
Aryl halide
Time(h)
Yield (%)
TON
196
198
186
190
104
122
116
158
192
164
184
160
156
128
1
4-chloroasetophenone
4-bromoasetophenone
4-chlorobenzaldehyde
4-bromobenzaldehyde
2-chloro-p-xylene
2-chlorobenzaldehyde
2-chloroanisole
6
98
99
93
95
52
61
58
79
96
82
92
80
78
64
2
0.5
6
3
4
0.5
6
5
6
6
7
6
8
2-Bromomesitylene
1-bromo-4-nitrobenzene
4-chloroanisole
0.5
1
9
10
11
12
13
14
6
4-bromoanisole
0.5
6
4-chlorotoluene
3-chloropyridine
8
3-chlorothiophene
8
Reaction conditions: Aryl halide (1.0 mmol), phenylboronic acid (1.5 mmol), K2CO3 (1.5 mmol), H2O (1.0 ml), TBAB (1.5 mmol), 2a (0.5 mol %), 60 ꢁC,
TON = [yield]/[cat.].
knowledge, the catalytic properties of PEPPSI-type Pd (II)
NHC complexes on the reduction of nitro compounds is
yet to be explored. Therefore, in this study, the %
conversion of nitrobenzene to aniline was investigated
using different water-soluble palladium complexes (2a-
d). Pd(0) nanoparticles were formed during the
TABLE 4 Suzuki−Miyaura Cross-Coupling Reaction of 4-chloroacetophenone with arylboronic Acids
(Continues)

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ÇAKIR AND TÜRKMEN
TABLE 4 (Continued)
Reaction conditions: 4-chloroasetophenone (1.0 mmol), arylboronic acid (1.5 mmol), K2CO3 (1.5 mmol), TBAB (1.5 mmol), H2O (1.0 ml), 2a (0.5 mol %), 60 ꢁC,
6.0 hr.
nitroarene reduction reaction. They were supported by
TEM (Figure 3, right) and XPS analysis (Figure 4, bot-
tom). Initally, the most active catalyst was determined
using 3 ml of H2O, nitrobenzene and NaBH4 at room
temperature under argon gas. As a result of this screen-
ing, the most effective catalyst (2d) gave the desired prod-
uct with 98% conversion in two hours (Figure 6).
The effects of controlling reaction parameters such
as Pd loading, different weights of NaBH4, solvents, and
reaction time were also investigated in detail in this
work (Table 5). When the different reducing reagent
such as N2H4, SiH (Et)3, NH (SiMe3)2 used for the
reduction of nitrobenzene to aniline, lower yields were
obtained (entries 2–4). No significant change in activity
FIGURE 6 Screening of palladium complexes catalyst for
was observed when the 2d catalyst was reduced to
nitrobenzene reduction. Reaction conditions: 0.1 mmol
0.5 mol %. However, the conversion of the desired prod-
uct was determined to be 59% when it was reduced to
0.2 mol % (entry 6). We also examined the effect of dif-
nitrobenzene , 8 equiv NaBH4, palladium catalyst (1.0 mol %), H2O
(3.0 ml), room temperature (RT), room temperature (RT), 2 hr
ferent NaBH4 molecular weights(8.0, 6.0 and 4.0 mmol)
(entries 5, 7, 8). This finding indicates that the chemical
reduction of nitrobenzene is a very slow process at such
a low concentration of NaBH4. When EtOH was used
as solvent, the conversion was 73% and it was 77% in
the EtOH/H2O (1.0:1.0) mixture (entry 12). In the blank

WATER SOLUBLE PD-PIPERIDOIMIDAZOLIN-2-YLIDENE COMPLEXES
9 of 14
TABLE 5 Screening of reaction conditions
Entry
Cat. (% mol)
Solvent
H2O
Reducing agent/mmol
NaBH4/8
Yield (%)
TON
98
1
1.0
1.0
1.0
1.0
0.5
0.2
0.5
0.5
0.5
0.5
0.5
0.5
-
98
39
57
-
2
H2O
N2H4/8
39
3
H2O
SiH (Et)3/8
NHSi (Me3)2/8
NaBH4/8
57
4
H2O
-
5
H2O
93
59
79
49
73
8
186
295
158
98
6
H2O
NaBH4/8
7
H2O
NaBH4/6
8
H2O
NaBH4/4
9
EtOH
IPA
NaBH4/8
146
16
10
11
12
13
NaBH4/8
IPA/H2O
EtOH/H2O
H2O
NaBH4/8
7
14
NaBH4/8
77
Trace
154
-
NaBH4/8
Reaction conditions: nitrobenzene (1.0 mmol), RT, solvent (3.0 ml), 2d, 2 hr, TON = [yield]/[cat.].
TABLE 6 Screening of Substrates for Nitro Compound
Reduction Catalyzed by Complex 2d
Yield
Entry Substrate
Product
(%)
1
83
2
3
4
98
90
73
FIGURE 7 XRD spectra of the nanocatalyst (2d’) in
nitroarene reduction
experiment, the desired product was not observed under
the same conditions without the palladium complex
(entry 13).
The powder XRD pattern of palladium nanoparticles
2d’ is shown in Figure 7. The size of the particles can also
be obtained from the XRD spectra. This shows the prefer-
ential orientation of the facet {111} in Pd
nanoparticles.[30]
The nitroarenes were chosen to explore the elec-
tronic effect and the variations in the substrate back-
bone (Table 6). The nitro group was chemoselectively
reduced in the presence of bromine group with excel-
lent yield (98%) without dehalogenation (entry 2). The
methyl substituent at the para position gave the desired
5
56
(Continues)

10 of 14
ÇAKIR AND TÜRKMEN
TABLE 6 (Continued)
investigated (entry 6). The nitro group in the N-hetero-
cyclic compound was also suitable for this catalytic sys-
tem, giving the desired product with 87 and 90% yields,
respectively (entries 7, 8).
Recovery and catalyst reuse are important issues for
catalytic reactions. Easy catalyst separation and recycling
in successive batch operations can greatly increase the
efficiency of the reaction. When we examined the reuse
of the nanocomposite Pd catalyst in the Suzuki-Miyaura
coupling reaction and the reduction of nitrobenzene (Fig-
ure 8), there was a small decrease of the activity after
each recycling of the catalyst. After each reaction run,
the organic products were extracted into dic-
hloromethane followed by simple decantation. Based on
the above observations, the catalyst recyclability was
more efficient in case of the reduction of nitrobenzene
compared to the Suzuki-Miyaura reaction. The main rea-
son may be related to the different degrees of Pd aggrega-
tion in both reactions. The catalyst 2d in the reduction of
nitrobenzene was reused up to 6 times with a small
decrease in activity, affording aniline in quantitative yield
for each of the 6 runs. The ICP-MS was used to control
the amount of Pd in each cycle of the reusability test in
the reduction of nitrobenzene. The PdNPs solids resulting
from the recycling experiments were analyzed by means
of ICP-MS. The ICP-MS analysis after the first recycling
experiments accounts for 9.0 wt % of palladium, indicat-
ing a total loss of 1.0 wt %. The palladium contents after
the other recycling experiments are 8.0, 7.1, 5.2, 4.6,
3.5 wt %, respectively. However, this little loss of percent
Yield
(%)
Entry Substrate
Product
6
52
7
8
86
45
Reaction conditions: nitroarene (1.0 mmol), NaBH4 (8.0 mmol), H2O
(3.0 ml), 2d (0.5 mol %), RT, 2 hr.
products with decreased yield due to the electronic
effect (entry 3). Apart from the electronic features, the
impact of steric hindrance nitrobenzene was
FIGURE 8 Recycling of catalyst
used for the reduction of nitrobenzene
and Suzuki-Miyaura reaction
FIGURE 9 Numbering of
salts 1a-d

WATER SOLUBLE PD-PIPERIDOIMIDAZOLIN-2-YLIDENE COMPLEXES
11 of 14
FIGURE 10 Numbering of Pd(NHC) complexes (2a, 2b, 2c and 2d)
palladium content may be due to mass gain during subse-
quent cycles due to the adsorption of substances from cat-
alyst reactions; desorption or leaching of the palladium
complex.
analysis data were recorded with CHNS Elemental Anal-
1
ysis. H NMR and 13C NMR spectra were recorded on a
Varian AS 400 Mercury instrument. FTIR spectra were
recorded on a Perkin Elmer Spectrum 100 series. The
XPS analyses were performed with a surface sciences
instruments spectrometer (model Thermo Scientific K-
Alpha) using focused monochromatised AlKα radiation
(1486.68 eV). The diameter of the irradiated area was
400 μm. Peaks were recorded with a constant pass energy
of 50 eV. The XRD analyses were investigation of the
crystal structure of P3HT nanowires was performed by a
high resolution Rigaku Ultima IV x-ray Difractometer.
The wavelength of CuK-α x-rays was 1.542 Å. The energy
of focused beam was 0.8 keV and the detector was scintil-
lation counter. Measurements were taken in out of plane
mode by using cross-beam optics. ICP–MS data were
recorded on an Agilent ICP-MS 7900. For calibration, Pd-
containing standard solutions (10% HNO3, Merck) con-
taining 0.00, 0.10, 0.22, 0.49, 0.86, 1.02 and 9.98 mg kg−1
of Pd were used.
3 | CONCLUSIONS
In this study, four different NHC ligands incorporating
sulfonate (a), ester (b, c) and carboxylic acid (d) func-
tionalities were prepared and used in the synthesis of
palladium complexes. The complexes were successfully
tested in the Suzuki-Miyaura cross coupling reactions.
It was observed that the catalytic activity on the
Suzuki-Miyaura reactions exhibited
a
pronounced
dependence on the water-soluble polar or ionic
piperidoimidazoline-2-ylidene groups, thereby increas-
ing the catalytic efficiency in the coupling reaction. The
sulfonate (a) and carboxylate group (d) had the stron-
gest influence on the rate of the Suzuki-Miyaura reac-
tion. Furthermore, the detailed spectroscopic properties
and catalytic activities of water soluble complexes for
the reduction of nitroarenes were discussed in terms of
the modification of the water-soluble ligands. A detailed
study of catalytically active species in the two reaction
solutions was examined by XPS, XRD and TEM ana-
lyzes and mercury drop tests. Further catalytic applica-
tions of novel water-soluble NHC complexes are
currently being investigated.
4.2 | General procedure for preparation
of 1a-d
The compound (A) was prepared according to
the
literature
procedure.[31]
The
1,5,6,7,8,8a-
hexahydroimidazo[1,5-a]pyridine (4.31 mmol) and alkyl
halide (4.31 mmol) was refluxed in toluene (10.0 ml) for
12 hr. The solvent was removed under vacuum. The
resulting product was washed with Et2O. The ionic liquid
was dried under vacuum. They were characterized by
NMR spectroscopy and atomic numbers of ligand are
shown in Figure 9.
4 | EXPERIMENTAL
4.1 | Materials
Unless otherwise noted, all operations were performed
without taking precautions to exclude air and moisture.
All solvents and chemicals were used without any further
treatment. Starting compounds and reagents were
obtained from Merck, Fluka, Alfa Aesar and Acros
Organics; pyridine, 2-aminomethylpiperidine, Pd (OAc)2
were obtained from Alfa Aesar and dichloromethane,
acetonitrile, ethanol, diethyl ether, toluene, pyridine were
obtained from Merck and Ridel de Haen. Elemental
4.2.1 | Compound 1a
\Yield 0.46 g, 81%. 1H NMR (400 MHz, DMSO-d6): δ 8.37
(s, 1 H, H2), 4.09 (m, 1 H, H5), 3.84 (d, J = 16 Hz, 2 H,
H4), 3.51 (m, 4 H, H10–11), 3.20 (m, 2 H, H9), 2.45 (m, 2 H,
H12), 1.89 (m, 2 H, H8), 1.70 (m, 2H, H6), 1.45 (m, 2 H,
H7). 13C NMR (100 Hz, DMSO-d6): δ 155.9 (C2), 59.1 (C4),
53.8 (C5), 48.1, 46.6, 45.3 (C10–12), 31.3 (C9), 25.6 (C8),

12 of 14
ÇAKIR AND TÜRKMEN
23.5 (C6), 22.3 (C7). IR, υmax (CH2Cl2): 3968, 3858, 3440,
3331, 2949, 2842, 2228, 1651, 1519, 1448, 1346, 1291,
1186, 1110, 1048, 1032, 1017, 927, 735, 666, 611,
531 cm−1. Anal. Calcd for C10H19N2NaO3S (M:270.32): C,
44.43; H, 7.08; N, 10.36%; Found: C, 44.40; H, 7.10; N,
10.35%.
1449, 1375, 1275, 1261, 1195, 1102, 1046, 1018, 967, 944,
892, 763, 750, 708, 620, 547, 472 cm−1. Anal. Calcd for
C15H19BrN2O2 (M:338.06): C, 53.11; H, 5.65; N, 8.26%;
Found: C, 53.15; H, 5.69; N, 8.23%.
4.3 | General procedure for the
complexes 2a-d
4.2.2 | Compound 1b
The salt (0.5 mmol), Pd (OAc)2 (0.5 mmol) and NaBr
(0.5 mmol) were suspended in anhydrous pyridine
(10 ml) and stirred at 80 ꢁC for 12 hr. The pyridine
was removed in vacuum at room temparature. The
product was purified by column chromatography on
silica gel. They were identified by NMR spectroscopy
and their atomic complex numbers are shown in
Figure 10.
1
Yield 0.72 g, 54%. H NMR (400 MHz, DMSO-d6): δ 8.05
(s, 1 H, H2), 4.10 (m, 4 H, H15), 3.86 (m, 1 H, H5), 3.63
(m, 1 H, H13), 3.47 (d, J = 8 Hz, 2 H, H4), 3.29 (m, 2 H,
H10), 2.68 (m, 2 H, H9), 2.60 (m, 4 H, H11–12), 1.86 (m,
2 H, H8), 1.70 (m, 2 H, H6), 1.37 (m, 2 H, H7), 1.16 (m,
6 H, H16). 13C NMR (100 Hz, DMSO-d6): δ 169.3 (C14),
162.2 (C2), 61.4 (C15), 55.7 (C10), 50.8 (C13), 49.2 (C4), 45.2
(C5), 29.9 (C9), 26.3 (C8), 25.0 (C6), 22.3 (C11), 21.9 (C12),
20.1 (C7), 14.3 (C16). IR, υmax (CH2Cl2): 3415, 2939, 1729,
1650, 1518, 1447, 1391, 1369, 1260, 1181, 1095, 1027,
855, 749, 611, 479 cm−1. Anal. Calcd for C17H30 ClN2O4
(M:360.18): C, 56.58; H, 8.10; N, 7.76%; Found: C,
56.57; H, 8.12; N, 7.75%.
4.3.1 | Complex 2a
1
ꢁ
Yield 0.59 g, 52%. m.p = 168.3 C. H NMR (400 MHz,
CD3OD): δ 8.88 (d, J = 5.2 Hz, 2 H, H17), 7.87 (t,
J = 5.2 Hz, 1 H, H19), 7.42 (m, 2 H, H18), 4.97 (dd,
J = 5.2 Hz, 1 H, H5), 4.13 (m, 2 H, H4), 3.82 (m, 2 H,
H10), 3.30 (m, 2 H, H11), 3.01 (m, 2 H, H9), 2.27 (m, 2 H,
H12), 1.80 (m, 2 H, H8), 1.57 (m, 2 H, H6), 1.37 (m, 2 H,
H7). 13C NMR (100 Hz, CD3OD): δ 177.6 (Pd C), 151.9
(C18), 138.0 (C17), 124.4 (C19), 59.7 (C4), 54.1 (C5), 48.9,
48.8, 48.3 (C10–12), 38.8 (C9), 31.6 (C8), 25.1 (C6), 22.8
(C7). IR, υmax (CH2Cl2): 3432, 3286, 3174, 3105, 3087,
3060, 3034, 3040, 3002, 2907, 2234, 1997, 1981, 1922,
1850, 1704, 1640, 1557, 1486, 1338, 1327, 1239, 1214,
1153, 1077, 1051, 1017, 944, 927, 882, 801, 762, 690,
642, 522, 464 cm−1. Anal. Calcd for C15H23Br2N3PdNaO3S
(M:613.62): C, 29.31; H, 3.77; N, 6.84%; Found: C,
29.33; H, 3.76; N, 6.88%.
4.2.3 | Compound 1c
1
Yield 0.51 g, 53%. H NMR (400 MHz, DMSO-d6): δ 8.77
(s, 1 H, H2), 4.69 (m, 1 H, H5), 4.17 (d, 2 H, H12), 4.12 (m,
1 H, H10), 4.03 (m, 2 H, H4), 3.50 (m, 2 H, H9), 2.79 (m,
2 H, H8), 1.73 (d, J = 8 Hz, 2 H, H6), 1.46 (d, 2H, H7),
1.22 (m, 3 H, H14), 1.19 (m, 3 H, H13). 13C NMR (100 Hz,
DMSO-d6): δ 170.3 (C11), 162.5 (C2), 62.2 (C12), 59.1 (C10),
55.1 (C4), 52.9 (C5), 45.5 (C9), 31.4 (C8), 25.9 (C6), 22.0
(C7), 15.8 (C14), 14.4 (C13). IR, υmax (CH2Cl2): 3418, 2945,
2865, 2065, 1738, 1650, 1645, 1514, 1448, 1380, 1283,
1208, 1193, 1184, 1183, 1064, 1019, 948, 919, 894, 859,
827, 731, 696, 666, 620, 571, 478 cm−1. Anal. Calcd for
C12H22N2O2 (M:226.32): C, 63.68; H, 9.80; N, 12.38%;
Found: C, 63.65; H, 9.79; N, 12.39%.
4.3.2 | Complex 2b
1
ꢁ
Yield 0.65 g, 42%. m.p = 171.7 C. H NMR (400 MHz,
CDCl3): δ 8.97 (d, J = 4.8 Hz, 2 H, H17), 7.73 (t,
J = 4.8 Hz, 1 H, H19), 7.31 (m, 2 H, H18), 5.04 (dd,
J = 4.8 Hz, 1 H, H5), 4.18 (m, 4 H, H15), 4.05 (m, 1 H,
H13), 3.80 (m, 2 H, H4), 3.44 (m, 2 H, H10), 3.25 (m, 2 H,
H9), 2.02 (m, 4 H, H11–12), 1.88 (m, 2 H, H8), 1.67 (m, 2 H,
H6), 1.42 (m, 2 H, H7), 1.23 (m, 6H, H16). 13C NMR
(100 Hz, CDCl3): δ 177.8 (C14), 169.2 (Pd C), 152.4 (C18),
137.7 (C17), 124.4 (C19), 63.7 (C15), 61.4 (C13), 59.7 (C4),
54.5 (C5), 51.5 (C10), 49.8 (C9), 47.9 (C8), 31.9, 26.3 (C11–
12), 25.3 (C6), 23.1 (C7), 14.1 (C16). IR, υmax (CH2Cl2):
3564, 2937, 1728, 1527, 1446, 1358, 1258, 1154, 1026,
4.2.4 | Compound 1d
Yield 0.89 g, 63%. 1H NMR (400 MHz, CDCl3): δ 10.05 (s,
1 H, H16), 8.88 (s, 1 H, H2), 8.00 (m, 2 H, H11), 7.45 (m,
2 H, H12), 5.26 (s, 2 H, H10), 4.73 (d, J = 4.0 Hz, 1 H, H5),
4.27 (d, J = 4.0 Hz, 2 H, H4), 4.08 (m, 2 H, H9), 3.38 (m,
2 H, H8), 1.89 (m, 2 H, H6), 1.55 (m, 2 H, H7). 13C NMR
(100 MHz, CDCl3): δ 172.4 (C15), 155.3 (C2), 133.1 (C11),
129.8 (C13), 125.9 (C14), 123.9 (C12), 59.4 (C4), 50.4 (C5),
45.8 (C10), 37.9 (C9), 31.4 (C8), 25.5 (C6), 22.3 (C7). IR,
υmax (CH2Cl2): 3417, 2951, 2864, 2051, 1708, 1644, 1524,

WATER SOLUBLE PD-PIPERIDOIMIDAZOLIN-2-YLIDENE COMPLEXES
13 of 14
799, 759, 691, 627 cm−1
.
Anal. Calcd for
4.5 | General procedure for the
C22H34Br2N3PdO4 (M:670.75): C, 39.39; H, 5.11; N, 6.26%;
Found: C, 39.38; H, 5.15; N, 6.26%.
reduction of Nitroarenes
The catalyst (0.5 mol%), nitro compound (0.1 mmol, 1.0
equiv), NaBH4 (8 equiv) and diethyleneglycol-di-n-
butylether (0.6 mmol, internal standard) in H2O (3.0 ml)
were charged into a two-necked 5.0 ml flask under the
air atmosphere. Subsequently, the flask was sealed and
the resulting mixture was stirred at RT for 2 hr. The con-
version was monitored by gas chromatography. Yields
were determined by gas chromatography for an average
of two runs.
4.3.3 | Complex 2c
1
ꢁ
Yield 0.63 g, 50%. m.p = 176.9 C. H NMR (400 MHz,
CDCl3): δ 8.99 (d, J = 4.0 Hz, 2 H, H17), 7.75 (t,
J = 4.0 Hz, 1 H, H19), 7.39 (m, 2 H, H18), 5.08 (dd,
J = 4.0 Hz, 1 H, H5), 4.24 (m, 2 H, H12), 4.15 (m, 1 H,
H10), 3.90 (m, 2 H, H4), 3.28 (m, 2 H, H9), 1.59 (m,
2 H, H8), 1.47 (m, 2 H, H6), 1.34 (m, 2 H, H7), 1.30 (m,
3 H, H14), 1.27 (m, 3 H, H13).13C NMR (100 Hz,
CDCl3): δ 178.5 (C11), 171.6 (Pd C), 152.5 (C18), 137.7
(C17), 124.4 (C19), 61.4 (C12), 59.8 (C10), 57.2 (C4), 50.7
(C5), 48.2 (C9), 31.9 (C8), 25.2 (C6), 23.1 (C7), 15.5 (C14),
14.2 (C13). IR, υmax (CH2Cl2): 3426, 1637, 1601, 1566,
1482, 1448, 1243, 1202, 1150, 1067, 1014, 942, 763,
4.6 | Recycling of catalyst
The flask was charged with catalyst 2a, 4-
bromoacetophenone (1.0 mmol), 1.5 mmol K2CO3, phe-
nylboronic acid (1.5 mmol), and diethyleneglycol-di-n-
butylether (0.6 mmol, internal standard). The reaction
690 cm−1
.
Anal. Calcd for C17H26Br2N3PdO2
ꢁ
(M:570.64): C, 35.78; H, 4.59; N, 7.36%; Found: C,
35.79; H, 4.57; N, 7.39%.
was carried out in water at 60 C. After cooling to room
temperature, the organic products were extracted by dic-
hloromethane. The aqueous phase was then transferred
to a new reaction flask for the next cycle. Yields were
determined by gas chromatography.
4.3.4 | Complex 2d
The flask was charged with catalyst 2d, nitrobenzene
(0.1 mmol, 1.0 equiv), NaBH4 (8.0 equiv),
diethyleneglycol-di-n-butylether (0.1 mmol, internal stan-
dard) and the reaction was carried out in water at RT. At
the end of 2 hr, the organic products (aniline) were
extracted by dichloromethane. The same amount of
nitrobenzene and NaBH4 were added to the aqueous
phase for the next cycle. The reactions were checked by
gas chromatography.
1
ꢁ
Yield 0.93 g, 55%. m.p = 170.6 C. H NMR (400 MHz,
DMSO-d6): δ 8.80 (d, J = 8.0 Hz, 2 H, H17), 8.68 (m, 1 H,
H19), 7.98 (m, 2 H, H13), 7.49 (m, 2 H, H18), 7.38 (m, 2 H,
H12), 5.73 (s, 2 H, H10), 3.98 (d, J = 8.0 Hz, 1 H, H5), 3.79
(m, 2 H, H4), 3.01 (m, 2 H, H9), 1.56 (m, 2 H, H8), 1.33
(m, 2 H, H6), 1.16 (m, 2 H, H7).13C NMR (100 Hz, DMSO-
d6): δ 177.2 (C15), 172.4 (Pd C), 162.6 (C18), 159.9 (C17),
154.4 (C19), 150.1 (C11), 139.7 (C13), 136.6 (C14), 125.9
(C12), 61.2 (C4), 56.9 (C5), 55.9 (C10), 44.4 (C9), 30.8 (C8),
26.2 (C6), 22.7 (C7). IR, υmax (CH2Cl2): 3414, 2938, 2034,
1719, 1638, 1616, 1486, 1453, 1393, 1271, 1152, 1102,
1077, 1017, 942, 763, 691, 620, 481 cm−1. Anal. Calcd for
C20H24Br2N3PdO2 (M:601.93): C, 39.73; H, 4.00; N, 6.95%;
Found: C, 39.79; H, 4.01; N, 6.93%.
ACKNOWLEDGEMENT
Financial support from Ege University (Project FYL-
2018-20197) is gratefully acknowledged.
CONFLICT OF INTEREST
The authors declare no conflict of interest.
4.4 | General procedure for Suzuki cross-
coupling reaction
ORCID
Sinem Çakır https://orcid.org/0000-0003-1387-9643
Hayati Türkmen https://orcid.org/0000-0001-7411-2652
A two-necked 5.0 ml flask was filled with aryl halide
(1.0 mmol), phenylboronic acid (1.5 mmol), K2CO3
(1.5 mmol) and the catalyst (0.5 mol %) in 1 ml water
(H2O). 1.5 mmol of TBAB as phase transfer catalyst was
added into flask. The mixture was heated to 60 ꢁC at 6 hr
for aryl chloride (0.5 hr for aryl bromide) under an argon
atmosphere. The conversion was monitored by gas
chromatography.
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How to cite this article: Çakır S, Türkmen H.
Synthesis of water soluble Pd-Piperidoimidazolin-
2-ylidene complexes and their catalytic activities in
neat water. Appl Organometal Chem. 2020;e5499.
https://doi.org/10.1002/aoc.5499
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