Effect of phosphonium ionic liquid/Pd ratio on the catalytic activity of palladium nanoparticles in Suzuki cross-coupling reaction

Article abstract of DOI:10.1016/j.jorganchem.2020.121454

Phosphonium salts were synthesized and used as support for palladium nanoparticles. It was demonstrated that the catalytic behavior of palladium nanoparticles in Suzuki cross-coupling reaction is determined by the ratio of the phosphonium ionic liquid to

Full text of DOI:10.1016/j.jorganchem.2020.121454

Journal Pre-proof
Effect of phosphonium ionic liquid/Pd ratio on the catalytic activity of
palladium nanoparticles in Suzuki cross-coupling reaction
D. Arkhipova , V. Ermolaev , V. Miluykov , G. Gaynanova ,
L. Zakharova , G. Wagner , O. Oeckler , E. Hey-Hawkins
PII:
S0022-328X(20)30356-9
DOI:
Reference:
https://doi.org/10.1016/j.jorganchem.2020.121454
JOM 121454
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Journal of Organometallic Chemistry
Received date:
Revised date:
Accepted date:
5 March 2020
19 June 2020
21 July 2020
Please cite this article as: D. Arkhipova , V. Ermolaev , V. Miluykov , G. Gaynanova , L. Zakharova ,
G. Wagner , O. Oeckler , E. Hey-Hawkins , Effect of phosphonium ionic liquid/Pd ratio on the cat-
alytic activity of palladium nanoparticles in Suzuki cross-coupling reaction, Journal of Organometallic
Chemistry (2020), doi: https://doi.org/10.1016/j.jorganchem.2020.121454
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Palladium nanoparticles (PdNPs) stabilized with phosphonium ionic liquid (PIL) effectively catalyze
cross-coupling reaction of 1,3,5-tribromobenzene and phenylboronic acid
PIL/Pd ratio influences on catalytic activity of PdNPs
Transmission electron microscopy (TEM) and Dynamic light scattering (DLS) methods were used for
catalytic system characterization
1

Effect of phosphonium ionic liquid/Pd ratio on the catalytic activity of
palladium nanoparticles in Suzuki cross-coupling reaction
a
a
a
a
a
b
b
D. Arkhipova, V. Ermolaev, * V. Miluykov, G. Gaynanova, L. Zakharova, G. Wagner , O. Oeckler,
c
E. Hey-Hawkins
Abstract
Phosphonium salts were synthesized and used as support for palladium nanoparticles. It was
demonstrated that the catalytic behavior of palladium nanoparticles in Suzuki cross-coupling reactions is
determined by the ratio of the phosphonium ionic liquid to palladium source and the nature of the
stabilizer. Transition electron microscopy and dynamic light scattering techniques allowed to determine
the most effective range of stabilizer concentration in the reaction media.
Key words: phosphonium ionic liquids, palladium nanoparticles, catalytic system tuning, TEM, DLS
1
. Introduction
In the last two decades, ionic liquids (ILs) have received particular interest due to their attractive
features such as limited volatility and flammability, recyclability, broad temperature range of the liquid
state, high thermal and chemical stability, wide electrochemical potential window, solubility and
miscibility depending on the choice of cations and anions [1,2,3]. In addition, ILs form preorganized
structures through ionic interaction and hydrogen bonding that induce structural directionality. Such a
supramolecular network is an “entropic driver” for the synthesis of well-defined nanosized objects [4].
For a good reason, the strategy of IL applications became a widely used approach [5] for design
and stabilization of highly efficient transition metal nanocatalysts for organic synthesis [6,7]. Due to its
unique solvent properties and wide range of miscibility, ILs have the additional function as a reaction
media [8,9]. The need to reduce the use of toxic volatile organic compounds in production processes,
improve atom efficiency and minimize waste became the driving force of the explosive growth in the
number of publications on the use of ionic liquids as solvents [10,11].
In spite of the fact that ILs are often claimed as an environmentally friendly alternative to
common organic solvents [97,12,13] there are many factors that should be considered before calling ionic
liquids “green” solvents [43]. The hazards of IL itself must be taken into account [14] as well as the
negative environmental impact of its manufacturing [15]. Despite the superior efficiency of ILs as a
nanoparticle (NP) stabilizer, it is essential to include a rational approach in their synthesis and
applications [16]. Along with usage of the supported ionic liquids [17,18], one of the ways to partially
avoid hazards caused by ILs is decreasing their quantity in reaction media. Instead of using “neat” ILs
[
19] for the synthesis of the nanocatalyst, a co-solvent approach was proposed. For example, a co-solvent
reduces the viscosity of the reaction media [20], in other cases, it allows the effective separation of the
reaction products from the catalyst [108].
a)
b)
c)
Arbuzov Institute of Organic and Physical Chemistry, FRC Kazan Scientific Center, Russian Academy of Sciences, ak. Arbuzov str, 8, 420088,
Kazan, Russia. Fax: +7 (843) 273-18-72; Tel: +7 (843) 273-93-65; E-mail: ermolaev@iopc.ru.
Leipzig University, Faculty of Chemistry and Mineralogy, Institute of Mineralogy, Crystallography and Materials Science, Scharnhorststr. 20, D-
04275 Leipzig, Germany, Tel.: +49 341 9736250; E-mail: gewagner@rz.uni-leipzig.de.
Leipzig University, Faculty of Chemistry and Mineralogy, Institute of Inorganic Chemistry, Johannisallee 29, D-04103 Leipzig, Germany. Tel:
+
49 341 9736151; Fax:+49 341 9739319; E-Mail: hey@uni-leipzig.de.
2

However, the amount of IL applied for NP stabilization is often not discussed and the exact
quantity seems to be random [86,21]. We suppose that the concentration of the stabilizing agent is worth
being considered since it can have a crucial influence on the catalytic activity and stability of NPs.
Previously, it was found that increasing the concentration of stabilizing polymer (poly(N-vinyl-2-
pyrrolidone)) in 40% ethanol solution resulted in reduction of the size of the NPs [22]. Another study
displayed that an increase in the PEG/Pd ratio in water led to a decrease in both the size and the catalytic
activity of the PdNPs in the Hiyama cross-coupling reaction [23].
There are a few similar studies for ILs. One of them is devoted to the Heck reaction catalyzed by
PdNPs stabilized with N-containing ILs as was reported by Roszak and co-workers [24]. Increasing the
IL/Pd ratio gave different effects in the catalytic activity of the PdNPs depending on the structure of the
IL. It should be noted that DMF which was used as reaction medium could serve as an extra stabilizer for
NPs [25]. In a preliminary study we have shown the same trends in the behavior of PdNPs coated with
phosphonium IL (PIL) [26] in Sonogashira [27] and Suzuki [28] reactions.
As the amount of the supporting agent in the solution has a significant influence on the size and
catalytic activity of PdNPs, the present study focusses on controlling the catalytic activity of PdNPs
through varying the concentration of PIL in a wide range to determine the minimum amount required as
stabilizing agent.
2
. Experimental
2
.1. Materials and methods
All procedures associated with the preparation of the starting materials, synthesis, and isolation of
products were carried out under inert atmosphere using standard Schlenk techniques.
1
31
13
NMR spectra were recorded on a Bruker MSL-400 instrument ( H 400 MHz, P 161.7 MHz, C
1
1
00.6 MHz). SiMe was used as internal reference for H chemical shifts, and 85% H PO as external
4
3
4
31
reference for P NMR.
The ESI MS measurements were performed using an AmazonX ion trap mass spectrometer (Bruker
Daltonic GmbH, Germany) in positive (and/or negative) mode in the mass range of 70–3000. The
-1
capillary voltage was 3500 V, nitrogen drying gas 10 L·min , and desolvation temperature 250 °С. A
-1
methanol/water solution (70:30) was used as a mobile phase at a flow rate of 0.2 mL·min by binary
pump (Agilent 1260 chromatograph, USA). The sample was dissolved in methanol to a concentration of
-6
-1
1
0 g·L . The instrument was calibrated with a tuning mixture (Agilent G2431A, USA). For instrument
control and data acquiring the TrapControl 7.0 software (Bruker Daltonik GmbH, Germany) was used.
Data processing was performed by DataAnalysis 4.0 SP4 software (Bruker Daltonik GmbH, Germany).
TEM images of the catalyst surface before and after the reaction were obtained using a
PHILIPS/FEI CM20 transmission electron microscope. The particle size distribution was determined on a
Veeco MultiMode V scanning probing microscope using the intermittent-contact atomic force method.
The conversion of 1,3,5-triphenylbenzene was estimated by gas chromatography coupled with
mass spectrometry on a DFS Thermo Electron Corporation instrument (Germany) using electron
ionization (70 eV), temperature of the ion source was 250 °C, an ID-BP5Х capillary column (analog of
DB-5MS, length 50 m, diameter 0.32 mm, thickness of the phase layer 0.25 μm), and helium as a carrier
gas. The mass spectrometric data were processed using the Xcalibur program. Prior to injection, a sample
–
3
of the studied substance was dissolved in chromatographically pure ethanol in a concentration of ~10
–
1
g·μL (volume of the injected sample 0.1 μL). The chromatographic regime was as follows: temperature
of the injector 250 °C, flow split 1:10; temperature-programmed column: initial temperature 120 °C (1
–1
min), then heating with a rate of 20 °C·min to 280 °C, and final temperature 280 °C (15 min); the flow
3

–1
rate of the carrier gas through the column was 2 mL·min . The temperature of the communication device
with the mass spectrometer was 280 °C.
Dynamic light scattering measurements were performed with a Malvern Instrument Zetasizer
Nano. A He-Ne gas laser with 4 mW power and 633 nm wavelength was used as a source of radiation.
The measurements were carried out at 173° scattering angle. Dynamic light scattering (DLS) is a well-
established technique for determining the effective radius (R) and size distribution of aggregates that is
based on spherical approximation and the Stokes-Einstein equation: D = kT/6πηR, where D is the
diffusion coefficient, k the Boltzmann constant, T the absolute temperature, η the solvent viscosity, and R
the hydrodynamic radius. Data were analyzed by second-order cumulant expansion method. All
measurements were performed at least five times for each sample. Results of dynamic light scattering is
averaged by the number of particles.
Tri-n-butylphosphine (Sigma-Aldrich), tri-tert-butylphosphine (Dal-Chem), n-decyl bromide
(
Sigma-Aldrich), phenylboronic acid (Alfa Aesar), 1,3,5-tribromobenzene (Alfa Aesar), Pd(OAc) (Alfa
2
Aesar), KOH, NaBF , LiNTf (analytical grade, without additional purification) were used in the work.
4
2
For additional information and spectra see supplementary material.
2
.2. IL synthesis
2
4
.2.1 Synthesis of tri-tert-butyl(decyl)phosphonium bromide (1a)
.39 g (21.68 mmol) tri-tert-butylphosphine was added to 1.8 ml (21.68 mmol) 1-bromodecane without
any solvent, and the resulting mixture was stirred for 5 hours at 120 °С. After completion of the reaction
the mixture became a colourless solid. It was suspended in light petroleum ether (2x10 ml) and filtered.
The solvent was dryied in vacuo. The product was obtained as a white powder; yield 8.14 g (88.7%), m.p.
1
22 °С.
3
1
1
1
P{ H} NMR (161.7 MHz, CDCl ): δ (ppm) 49.5 (s). H NMR (400 MHz, CDCl ): δ (ppm) 2.67 (m,
3
3
2
H, P-CH ), 1.94 (m, 2H, P-CH -CH ), 1.70 (d, 27H, P(C(CH ) ) ), 1.40-1.22 (m, 14H), 0.89 (t, 3H, CH -
2 2 2 3 3 3 2
+
+
CH ). MS [ESI+] found MH 343.4; the phosphonium cation in C H PBr (C H P ) requires 343.59.
3
22 48
22 48
2
1
.2.2 Synthesis of tri-tert-butyl(decyl)phosphonium tetrafluoroborate (1b)
.37 g (12.45 mmol) sodium tetrafluoroborate was added to a solution of 3.53 g (8.3 mmol) 1a dissolved
in 5 ml of water. A white solid precipitated. The solid was filtered from the reaction mixture and washed
with 25 ml of water to remove inorganic salts. Then 1b was dissolved in 10 ml CH Cl and dried over
2
2
MgSO during 12 h. The solution was filtered and the solvent evaporated in vacuo. The product was
4
obtained as a white powder; yield 3.07 g (85.6%), m.p. 106 °C.
3
1
1
1
P{ H} NMR (161.7 MHz, CDCl ): δ (ppm) 48.9 (s). H NMR (400 MHz, CDCl ): δ (ppm) 2.35 (m,
3
3
3
2
0
H, P-CH ), 1.91 (m, 2H, P-CH -CH ), 1.65 (d, J = 13.7 Hz, 27H, P(C(CH ) ) ), 1.37-1.25 (m, 14H),
2 2 2 PH 3 3 3
.88 (t, J = 6.6 Hz, 3H, CH -CH ). C{ H} NMR (100.6 MHz, CDCl ): δ (ppm) 39.3 (d, J = 29.5
3
13
1
1
HH
2
3
3
PC
2
Hz, C(CH ) ), 31.8 (d, J = 12.8 Hz, P-CH -CH ), 31.8 (s, CH ), 29.9 (s, C(CH ) ), 29.5 (s, CH ), 29.33
(
(
3
3
PC
2
2
2
3 3
2
3
s, CH ), 29.2 (s, CH ), 29.2 (s, CH ), 25.1 (d, J = 6.6 Hz, P-CH -CH -CH ), 22.6 (s, CH -CH ), 18.7
2 2 2 PH 2 2 2 2 3
d, J = 34.7 Hz, P-CH ), 14.1 (s, CH -CH ). MS [ESI+] found MH 343.4; the phosphonium cation in
1
+
PC
2
2
3
+
C H BF P (C H P ) requires 343.59.
2
2
48
4
22 48
2
1
.2.3 Synthesis of tri-tert-butyl(decyl)phosphonium bis(trifluoromethanesulfonyl)imide (1c)
.93 g (6.73 mmol) lithium bis(trifluoromethanesulfonyl)imide was added to 1.90 g (4.49 mmol) tri-tert-
butyl(decyl)phosphonium bromide (1a) in 5 ml of water resulting in a two-layered mixture. 10 ml CH Cl
2
2
was added to extract the product. The organic layer was washed with water twice and dried over MgSO4
overnight. Then the solution was filtered and the solvent evaporated in vacuo. The product was obtained
as a colorless viscous liquid; yield 2.22 g (79.3%).
3
1
1
1
P{ H} NMR (161.7 MHz, CDCl ): δ (ppm) 49.3 (s). H NMR (400 MHz, CDCl ): δ (ppm) 2.21 (m,
3
3
3
2
H, P-CH ), 1.90 (m, 2H, P-CH -CH ), 1.64 (d, J = 14.0 Hz, 27H, P(C(CH ) ) ), 1.40-1.24 (m, 14H),
2 2 2 PH 3 3 3
4

3
13
1
1
0
.89 (t, J = 7.02 Hz, 3H, CH -CH ). C{ H} NMR (100.6 MHz, CDCl ): δ 120.0 (d, J = 322.0 Hz,
HH 2 3 3 FC
1
2
CF ), 39.3 (d, J = 29.3 Hz, C(CH ) ), 32.8 (s, CH ), 31.8 (d, J = 12.2 Hz, P-CH -CH ), 29.8 (s,
3
PC
3 3
2
PC
2
2
3
C(CH ) ), 29.4 (s, CH ), 29.3 (s, CH ), 29.2 (s, CH ), 29.0 (s, CH ), 25.0 (d, J = 6.6 Hz, P-CH -CH -
3
3
2
2
2
2
PC
2
2
1
+
CH ), 22.6 (s, CH -CH ), 18.7 (d, J = 34.9 Hz, P-CH ), 14.1 (s, CH -CH ). MS [ESI+] found MH
43.4; the phosphonium cation in C H F NO S P (C H P ) requires 343.59.
24 48 6 4 2 22 48
2
2
3
PC
2
2
3
+
3
2
1
.2.4 Synthesis of tri-n-butyl(decyl)phosphonium bromide (2a)
5.0 ml (60 mmol) tri-n-butylphopsphine was added to 12.5 ml (60 mmol) 1-bromodecane without any
solvent, and the resulting mixture was stirred for 2 hours at 80 °С. After completion of the reaction the
mixture was suspended in light petroleum ether (2x50 ml) and decanted. The solvent was dryied in vacuo.
The product was obtained as a colorless viscous liquid; yield 21.5 g (84.5%).
3
1
1
1
P{ H} NMR (161.7 MHz, CDCl ): δ (ppm) 32.8 (s). H NMR (400 MHz, CDCl ): δ (ppm) 2.53-2.39
3
3
3
(
0
m, 8H, P-CH ), 1.54 (m, 16H, P-CH -CH -CH ), 1.25 (m, 12H), 0.97 (t, J = 7.05 Hz, 9H, CH -CH ),
2 2 2 2 PH 2 3
3
.87 (t, J = 6.87 Hz, 3H, CH -CH ).
PH 2 3
2
3
.2.5 Synthesis of tri-n-butyl(decyl)phosphonium tetrafluoroborate (2b)
.89 g (35.4 mmol) sodium tetrafluoroborate was added to a solution of 10.0 g (23.6 mmol) 2a dissolved
in 50 ml of water at room temperature. 50 ml CH Cl was added to extract the product. The organic layer
2
2
was collected and dried over MgSO for 12 h. The solution was filtered and the solvent evaporated in
4
vacuo. The product was obtained as a transparent viscous liquid; yield 8.26 g (81.4%).
3
1
1
1
P{ H} NMR (161.7 MHz, CDCl ): δ (ppm) 33.2 (s). H NMR (400 MHz, CDCl ): δ (ppm) 2.18 (m,
3
3
3
8
(
H, P-CH ), 1.51 (m, 16H, P-CH -CH -CH ), 1.23 (m, 12H), 0.95 (t, J = 6.38 Hz, 9H, CH -CH ), 0.85
2 2 2 2 HH 2 3
3
t, J = 6.57 Hz, 3H, CH -CH ).
HH 2 3
2
2
.2.6 Synthesis of tri-n-butyl(decyl)phosphonium bis(trifluoromethanesulfonyl)imide (2c)
.24 g (7.79 mmol) lithium bis(trifluoromethanesulfonyl)imide was added to a solution of 2.2 g (5.19
mmol) 2a dissolved in 10 ml of water at room temperature. 15 ml CH Cl was added to extract the
2
2
product. The organic layer was collected and dried over MgSO for 12 h. The solution was filtered and
4
the solvent evaporated in vacuo. The product was obtained as a transparent liquid; yield 2.82 g (87 %).
3
1
1
1
P{ H} NMR (161.7 MHz, CDCl ): δ (ppm) 33.1 (s). H NMR (400 MHz, CDCl ): δ (ppm) 2.13 (m,
3
3
3
8
H, P-CH ), 1.52 (m, 16H, P-CH -CH -CH ), 1.28 (m, 12H), 0.99 (t, 9H, CH -CH ), 0.90 (t, J = 6.38
2 2 2 2 2 3 HH
Hz, 3H, CH -CH ).
2
3
2
0
1
.3. Typical procedure for preparation of PdNPs
.45 mg (0.002 mmol) palladium acetate and an appropriate amount of phosphonium salt given in Tables
and 2 were dissolved in 10 ml ethanol and stirred for 20 minutes at room temperature. The color of the
solution changed from transparent to light brownish grey. TEM samples were prepared as follows: one
drop of the solution was put on a copper grid and dried in vacuo.
Table 1. The amount of PILs 1b, 2b
PIL amount
PIL/Pd ratio
1
b, g (mmol)
2b, g (mmol)
0.02 (0.046)
0.06 (0.139)
0.10 (0.232)
0.25 (0.580)
0.50 (1.161)
1.00 (2.323)
1.50 (3.485)
1
6
3 (26 for 2b)
5 (78 for 2b)
0.01 (0.023)
0.05 (0.116)
0.10 (0.232)
0.25 (0.580)
0.50 (1.161)
1.00 (2.323)
1.50 (3.485)
1
3
6
31
26
53
1
1
305
958
5

2
611
2.00 (4.647)
2.00 (4.647)
Table 2. The amount of PILs 1c, 2c
PIL amount
c g, (mmol)
PIL/Pd ratio
1
2c g, (mmol)
0.02 (0.032)
0.06 (0.096)
0.10 (0.160)
0.25 (0.401)
0.50 (0.802)
1.00 (1.603)
1.45 (2.325)
-
1
4
8
6
8
0
0.02 (0.032)
0.06 (0.096)
0.10 (0.160)
0.25 (0.401)
0.50 (0.802)
1.00 (1.603)
1.45 (2.325)
1.76 (2.822)
2.20 (3.527)
2
4
8
00
01
02
1
1
1
162
411
764
-
2
0
.4 General procedure for the Suzuki cross-coupling reaction
.18 g (0.57 mmol) 1,3,5-tribromobenzene, 0.32 g (2.6 mmol) phenylboronic acid, and 0.15 g (2.6 mmol)
potassium hydroxide were added to a fresh suspension of PdNPs (see 2.3). The reaction mixture was
stirred for 16 hours at 30 °С. Organic compounds were extracted with 10 ml toluene and analyzed by GC-
MS without product isolation. The conversion was calculated by using the correction factor which was
obtained by GC-MS analysis of an equimolar mixture of 1,3,5-tribromobenzene and 1,3,5-
triphenylbenzene.
3
. Results and discussion
The phosphonium salts under investigation (Figure 1) were synthesized according to previously
described methods (1a-c [29], 2a-b [30], 2c [31]).
Figure 1. The structure of PILs.
PdNPs were obtained by reduction of palladium acetate in ethanol in the presence of the
corresponding PIL. No additional reducing agent was used. The quantity of PIL was varied while the
amount of palladium acetate and ethanol remained constant.
In preliminary studies, sterically hindered PILs were found to support the formation of PdNPs
which were the effective catalyst in Suzuki [32] and Sonogashira [2721] cross-coupling reactions. The
wide scope of substrates involved in the reaction resulted in high conversion, e.g. chloroarenes, and
successful recycling of the catalytic system was achieved [33].
Fo
En
The cross-coupling reaction of 1,3,5-tribromobenzene and phenylboronic acid (Figure 2) was
carried out using in situ prepared PdNPs under mild conditions (30 °C, 0.36 mol% [Pd]). Tri-substituted
benzene was formed predominantly; products of mono- and di-substitution were detected as traces. The
6

PdNPs showed catalytic activity and considerable stability. No “Pd black” precipitation was observed in
the system before and after the reaction.
Figure 2. Suzuki cross-coupling of 1,3,5-tribromobenzene and phenylboronic acid.
The results of the catalytic reactions are presented in Tables 3 and 4. The conversion was
estimated by GC-MS without product isolation. It should be noted that increasing the temperature up to
5
0 °C allowed to achieve almost total conversion of 1,3,5-tribromobenzene to 1,3,5-triphenylbenzene
(
Table 3, entry 4). The control experiment without stabilizing agent gave only 6% conversion; moreover,
palladium black formation was observed.
Table 3. The conversion of 1,3,5-tribromobenzene to 1,3,5-triphenylbenzene (%) for 1b, 2b.
PIL/Pd ratio
Conversion Conversion
Entry
for 1b, %
for 2b, %
17.9
21.6
45.8
26.8
29.2
31.7
7.7
1
.
.
.
.
.
.
.
.
12 (23 for 2b)
58 (70 for 2b)
116
14.3
31.2
2
3
4
5
6
7
67.4
76.5 (94.5 )
54.3
*
290
581
1162
1743
58.9
11.6
8
2323
19.4
2.2
*
Reaction was conducted at 50 °C.
0
1
.45 mg (0.002 mmol) palladium acetate and an appropriate amount of phosphonium salt 1b or 2b (Table 1), 0.18 g (0.57 mmol)
,3,5-tribromobenzene, 0.32 g (2.6 mmol) phenylboronic acid, 0.15 g (2.6 mmol) potassium hydroxide, 10 ml EtOH as solvent.
Table 4. The conversion of 1,3,5-tribromobenzene to 1,3,5-triphenylbenzene (%) for 1c, 2c.
Conversion Conversion
Entry
PIL/Pd ratio
for 1c, %
48.5
64.6
53.4
3.1
for 2c, %
12.7
19.8
30.0
13.2
24.3
10.5
17.3
-
9
.
16
48
80
200
401
802
1162
1411
1764
1
1
1
1
1
1
1
1
0.
1.
2.
3.
4.
5.
6.
7.
8.8
4.3
2.0
23.2
2.7
-
0
.45 mg (0.002 mmol) palladium acetate and an appropriate amount of phosphonium salt 1c or 2c (Table 2), 0.18 g (0.57 mmol)
1
,3,5-tribromobenzene, 0.32 g (2.6 mmol) phenylboronic acid, 0.15 g (2.6 mmol) potassium hydroxide, 10 ml EtOH as solvent.
As shown in Figure 3, the appropriate amount of PIL for the maximum conversion in the Suzuki
reaction is quite small and slightly varies depending on the structure of the stabilizing agent. The increase
of the amount of stabilizing agent in the reaction mixture leads to slow deactivation of the system
followed by a second lower maximum. Accordingly, the use of „neat‟ ILs as stabilizer for NPs and
reaction medium is inexpedient.
7

90
80
70
60
50
40
30
20
10
0
1b
1c
2b
2c
0
500
1000
1500
2000
2500
PIL/Pd ratio
Figure 3. The influence of the structure and quantity of PIL on the conversion in the Suzuki coupling
reaction (Figure 2).
A similar dependence of the conversion on the PIL/Pd ratio for phosphonium salts with different
structures is quite unusual and requires a detailed investigation. The key factors determining the
efficiency of nanocatalysis are the size and surface area of the NPs. TEM measurements of PdNPs in 1b
were performed (Figures 4-7).
45
40
35
30
25
20
15
10
5
0
4
.06±1.07 nm
544 NPs included
1
2
3
4
5
6
7
8
9
10
a
b
PdNP size, nm
Figure 4. TEM image (a) and size distribution (b) of PdNPs in the absence of PIL.
8

30
25
20
15
10
5
0
5
.30±1.64 nm
565 NPs included
1
2
3
4
5
6
7
8
9
10
a
b
PdNP size, nm
Figure 5. TEM image (a) and size distribution (b) of PdNPs at a ratio 1b/Pd = 12/1.
25
20
15
10
5
0
9
.56±3.76 nm
181 NPs included
2
4
6
8
10 12 14 16 18 20 22 24
a
b
PdNP size, nm
Figure 6. TEM image (a) and size distribution (b) of PdNPs at a ratio 1b/Pd = 58/1.
60
50
40
30
20
10
0
2
.62±0.64 nm
519 NPs included
a
1
2
3
4
5
6
7
8
9
10
b
PdNP size, nm
Figure 7. TEM image (a) and size distribution (b) of PdNPs at a ratio of 1b/Pd = 116/1.
Whilst small PdNPs with quite narrow size distribution (4.06±1.07 nm) are formed in the absence
of PILs (Figure 4), they exhibit low catalytic activity. “Pd black” precipitation indicates fast PdNPs
aggregation, and the detected product is formed after a few minutes of the reaction.
The increase of the 1b/Pd ratio (Fig. 5-7) leads to an increase of the conversion of 1,3,5-tribromobenzene;
however, no straight correlation between the size of PdNPs and their catalytic activity is observed. When
the ratio changes from 12/1 to 58/1, the average size of NPs grows. Despite the reduced surface Pd atoms,
an increase in the conversion is observed (Table 5). Noteworthy, at a ratio of 116/1, the size of the PdNPs
drops to 2.62±0.64 nm and the number of surface Pd atoms quadruples, although the conversion only
doubles. Definitely, some more important factors which influence the nanocatalyst activity must exist,
9

such as the lifetime of NPs and their surrounding in solution. Furthermore, it is impossible to obtain high
quality images of PdNPs at ratios above 116/1 due to the opaqueness of the samples with a high content
of PILs. The TEM method is, therefore, not suitable for a comprehensive explanation of the catalytic
activity of PdNPs.
Table 5. Parameters of PdNPs.
1
b/Pd ratio
0
12/1
5.30 nm
~ 5273
58/1
9.56 nm
~ 30928
116/1
2.62 nm
~ 623
Mean diameter of PdNPs
Number of Pd atoms in NP
Quantity (proportion) of
surface atoms
4.06 nm
~ 2369
~
711 (30%)
~ 1212 (23%)
~ 3942 (13%)
~ 291 (47%)
Earlier, quantum-chemical calculations demonstrated a moderate influence of the cation structure
variation of PILs on their interaction with Pd clusters and Pd atoms [34]. However, the results obtained
6
in our research show the key role of the cation in the catalytic process. These results led us to apply
dynamic light scattering (DLS) as a method that allows to investigate a catalytic system including PdNPs,
PIL, and solvent altogether. The hydrodynamic diameters of the catalytic compositions based on 1c and
2
b are given in Fig. 8-10. Comparison of the dependences of catalytic activity of PdNPs and the
hydrodynamic diameter of aggregates on the PIL/Pd ratio reveals a correlation between them.
900
800
700
600
500
400
300
200
100
0
90
80
70
60
50
40
30
20
10
0
0
500
1000
1500
2000
2500
3000
PIL/Pd ratio
Hydrodynamic diameter
Conversion
Figure 8. The conversion and the hydrodynamic diameter of aggregates formed by PIL 1b and PdNPs
depending on the Pd/PIL ratio.
1
0

900
850
800
750
700
650
600
70
60
50
40
30
20
10
0
0
500
1000
1500
2000
2500
PIL/Pd ratio
Hydrodynamic diameter
Conversion
Figure 9. The conversion and the hydrodynamic diameter of aggregates formed by PIL 1c and PdNPs
depending on the Pd/PIL ratio.
1
1
1
200
100
000
50
45
40
35
30
25
20
15
10
5
0
9
8
7
6
5
00
00
00
00
00
0
500
1000
1500
2000
2500
3000
PIL/Pd ratio
Hydrodynamic diameter
Conversion
Figure 10. The conversion and the hydrodynamic diameter of the aggregates formed by PIL 2b and
PdNPs depending on the Pd/PIL ratio.
Fig. 8-10 show an inversely proportional dependence of the hydrodynamic diameter of the
aggregates and the catalytic activity of PdNPs on the Pd/PIL ratio. However the maximum of conversion
for salt 1b was achieved with aggregates of 200 nm, and for the two another salts, this value is about 600
nm.
1
1

The maximum conversion for the catalytic system containing 1b is 76.5% at a PIL/Pd ratio of
2
90/1. It corresponds to the minimum of the hydrodynamic diameter of aggregates, that is 200 nm. A
conversion of 64.6% of 1,3,5-tribromobenzene in the case of 1c is reached at a Pd/PIL ratio of 48/1 and
corresponds to the minimum of hydrodynamic diameter of aggregates, i.e. about 600 nm. Finally, the
catalytic system including 2b demonstrates the maximum conversion of 45.8% at Pd/PIL ratio of 116/1
with an average size of aggregates of 600 nm. Although the structures of the investigated PILs are similar,
the highest activity of each catalytic system has been achieved at different PIL/Pd ratio.
According to TEM and DLS data, the size of PdNPs and PIL/Pd aggregates are changing
unrelated with increasing amounts of PILs in the reaction mixture. With TEM, only the PdNPs are
observable, whereas DLS allows to explore the whole catalytic system consisting of both PdNPs and
surrounding PIL molecules. Thereby, the DLS method is more suitable for characterization of the
catalytic system and allows to choose the most effective concentration range of the stabilizers.
Conclusions
We have found that small amounts of PILs are sufficient for the formation and stabilization of
PdNPs and result in a high catalytic activity in Suzuki cross-coupling reactions. To the best of our
knowledge, this is the first investigation of the influence of different quantities of PILs on the formation
and stabilization of PdNPs. The PIL/Pd ratio influences the size, the hydrodynamic diameter of
aggregates and the catalytic behavior of the resulting PdNPs. Noteworthy, the highest conversion was
achieved with sterically demanding PILs as stabilizers. Therefore, the right selection of PIL and PIL/Pd
ratio allows to obtain the desired products in high yields meeting the principles of green chemistry.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors gratefully acknowledge the CSF-SAC FRC KSC RAS for providing all necessary
facilities to carry out this work. Transition electron microscopy analyses of palladium nanoparticles were
carried out with financial support of DAAD (Leonhard Euler programm).
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