Encaging palladium(0) in layered double hydroxide: A sustainable catalyst for solvent-free and ligand-free Heck reaction in a ball mill

Article abstract of DOI:10.3762/bjoc.13.160

In this paper, the synthesis of a cheap, reusable and ligand-free Pd catalyst supported on MgAl layered double hydroxides (Pd/MgAl-LDHs) by co-precipitation and reduction methods is described. The catalyst was used in Heck reactions under highspeed ball milling (HSBM) conditions at room temperature. The effects of milling-ball size, milling-ball filling degree, reaction time, rotation speed and grinding auxiliary category, which would influence the yields of mechanochemical Heck reactions, were investigated in detail. The characterization results of XRD, ICP-MS and XPS suggest that Pd/MgAl-LDHs have excellent textural properties with zero-valence Pd on its layers. The reaction results indicate that the catalyst could be utilized in HSBM systems to afford a wide range of Heck coupling products in satisfactory yields. Furthermore, this catalyst could be easily recovered and reused for at least five times without significant loss of catalytic activity.

Full text of DOI:10.3762/bjoc.13.160

Encaging palladium(0) in layered double hydroxide:
A sustainable catalyst for solvent-free and
ligand-free Heck reaction in a ball mill
Wei Shi, Jingbo Yu, Zhijiang Jiang, Qiaoling Shao and Weike Su*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2017, 13, 1661–1668.
National Engineering Research Center for Process Development of
Active Pharmaceutical Ingredients, Collaborative Innovation Center of
Yangtze River Delta Region Green Pharmaceuticals, Zhejiang
University of Technology, Hangzhou, 310014, Zhejiang, China
doi:10.3762/bjoc.13.160
Received: 22 April 2017
Accepted: 18 July 2017
Published: 14 August 2017
Email:
Weike Su* - pharmlab@zjut.edu.cn
This article is part of the Thematic Series "Mechanochemistry".
Guest Editor: J. G. Hernández
* Corresponding author
Keywords:
© 2017 Shi et al.; licensee Beilstein-Institut.
License and terms: see end of document.
ball milling; Heck reaction; layer double hydroxides; solvent-free;
supported Pd catalyst
Abstract
In this paper, the synthesis of a cheap, reusable and ligand-free Pd catalyst supported on MgAl layered double hydroxides
(Pd/MgAl-LDHs) by co-precipitation and reduction methods is described. The catalyst was used in Heck reactions under high-
speed ball milling (HSBM) conditions at room temperature. The effects of milling-ball size, milling-ball filling degree, reaction
time, rotation speed and grinding auxiliary category, which would influence the yields of mechanochemical Heck reactions, were
investigated in detail. The characterization results of XRD, ICP–MS and XPS suggest that Pd/MgAl-LDHs have excellent textural
properties with zero-valence Pd on its layers. The reaction results indicate that the catalyst could be utilized in HSBM systems to
afford a wide range of Heck coupling products in satisfactory yields. Furthermore, this catalyst could be easily recovered and
reused for at least five times without significant loss of catalytic activity.
Introduction
High-speed ball milling (HSBM)-assisted transition metal-cata- The homogeneous palladium salts along with phosphine- or
lyzed cross-coupling reactions such as Heck, Suzuki, Sono- nitrogen-based ligands were employed as the traditional cata-
gashira and Glaser reactions are still unusual methods for the lyst systems not only in solution-based C–C cross coupling [13-
formation of C–C bonds [1-7], but the method arouse consider- 16] reactions but also in mechanically activated Heck [4,17-22],
able attention because of an environmentally benign and sol- Suzuki [23-26], and Sonogashira [5,27,28] coupling reactions.
vent-free synthesis approach as well as high efficiency and The limitations of which are obviously unstable ligands and
good atom economy, which is desirable in the fields of chem- expensive Pd catalysts. Furthermore, the contamination of the
istry, materials science, biology, pharmaceutical, dyestuff, agri- coupled products with unacceptable Pd species led to a hard
culture and so forth [8-12].
separation and recycling of homogeneous catalyst systems. In
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our previous study [4] we reported a ball-milling Heck reaction in Suzuki reactions. Jiang et al. [47] demonstrated that LDH-
catalyzed by Pd(OAc)2. Although the catalyst showed the satis- supported on alkaline materials performed higher catalytic
factory reactivity, it was difficult to recover. Thus, Pd catalysts activity in coupling reactions than that on acidic-supported
anchored on heterogeneous solid support materials such as materials. In the present work, co-precipitation was used
MCM-41 [29], alumina [30], silica [31], carbon nanotubes [32], for fabricating MgAl-LDHs with nitrate anions, followed by
microporous polymers [33], SBA-15 [34], or some dendrimers introducing disodium tetrachloropalladate (Na2PdCl4) into the
[35] were preferred to develop a ligandless and recyclable cata- LDH interlayer by the ion exchange method. The prepared
lyst system. However, to the best of our knowledge, only a few hybrid LDHs were then reduced by hydrazine hydrate
of supported Pd catalysts were used in mechanochemically (N2H4·H2O) to obtain the Pd catalyst supported on MgAl-LDHs
assisted coupling reactions because of the low mechanical (Pd/MgAl-LDHs). The as-prepared Pd/MgAl-LDH catalyst was
strength of the catalysts, the active component of which is easy further applied in representative cross-coupling Heck reactions
to leach and deactivate under HSBM conditions. Mack and under HSBM conditions (Scheme 1) by using a planetary ball
co-workers [36] reported a kind of polymer supported mill (Pulverisette 7, Fritsch, Germany). The influence of
Pd(PPh3)4 catalyst with high activity in a Glaser reaction milling-ball filling degree (ΦMB), reaction time (t), milling-ball
(Scheme 1). They found that the catalyst could only be recy- size (dMB) and rotation speed (n), along with catalyst loading,
cled twice without the addition of the PPh3 ligand and the Pd alkaline type and grinding auxiliary category were further in-
component was significantly leached from polymer support vestigated in detail.
after each run. Cravotto et al. [37] used an ultrasound-assisted
Results and Discussion
method to prepare Pd catalysts immobilized on modified
chitosan (Scheme 1). Although these catalysts were found to be Characteristics of prepared materials
effective in the Suzuki reaction after three cycles, the We initially prepared the Pd/MgAl-LDHs catalyst as described
modification conditions of chitosan were rigorous.
in the Experimental section (see Supporting Information File 1).
Figure 1 shows the powder XRD patterns of MgAl-LDHs,
As catalyst-supported material, layered double hydroxides MgAl-LDHs-PdCl42− and Pd/MgAl-LDHs at 2θ = 5–80°. All
(LDHs) have received much attention in the organic catalysis samples have diffraction peaks located around 10°, 20°, 33°,
for its excellent properties such as low costs, high specific sur- 38°, 60°, indexing to (003), (006), (009), (015), (110) reflec-
face area, double-layered structure, anion exchange capacity, tions, which indicates the highly neat degree and well-crys-
high mechanical stability and chemical stability [38-43]. Our tallinity structure of LDH materials without phase impurities
previous studies have proved that LDH catalysts could be suc- apparent. Moreover, the MgAl-LDHs presents an interlayer dis-
cessfully applied in the degradation of organic pollutants tance of 0.82 nm from the basal spacing of d003, which matches
[44,45]. Bai and co-worker [46] synthesized Pd/SDS–LDHs by the results well for the intercalation of nitrate (NO3−) into
using an ultrasonic method, which exhibited excellent activity MgAl-LDHs in literature [48]. In the MgAl-LDHs-PdCl42−
Scheme 1: Supported catalysts in cross-coupling reactions. MM represents mixer mill; PM represents planetary mill.
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The Heck coupling reaction under HSBM
sample, the (003) plane shifted to the lower position of 8.8°, re-
sulting to an expansion of interlayer spacing of 1.01 nm from conditions
0.82 nm. These phenomena suggest that PdCl42− successfully m-Bromoacetophenone (1a) and styrene (2a) were chosen as the
intercalated into the MgAl-LDHs interlayers. As compared with model reactants (Scheme 2), catalyzed by Pd/MgAl-LDHs
MgAl-LDHs and MgAl-LDHs-PdCl42−, the catalyst of Pd/ under ball-milling conditions with silica gel (5 g) and stainless-
MgAl-LDHs exhibited a lower intensity pattern except for the steel balls (ΦMB = 0.2, dMB = 14 mm) at 800 rpm.
diffraction peaks at 38° and 44°, which was due to the random
dispersion of the Pd component on the Pd/MgAl-LDHs surface. Based on our previous researches [4,51], it is found that the
The Pd loading of catalyst was 8.5 wt %, and the molar ratio of bases used have a significant influence on the yields of the reac-
Mg and Al in LDH layers were 2.97, which is in accordance tion. Thus, several bases such as NaOH, KOH, Cs2CO3,
with the ratio of 3.00 employed in the synthesis step (see Table K2CO3, t-BuOK, Et3N and DBU were investigated and the
S1 in the Supporting Information File 1). Furthermore, the results are shown in Table 1. It is notable that both inorganic
binding energy of Pd 3d5/2 and Pd 3d3/2 in LDH layers approxi- and organic bases could facilitate the reaction successfully.
mately centered at 334.7 eV and 340.2 eV, respectively, K2CO3 exhibited the best yield of 72% (Table 1, entries 1–7) as
assigning to the existence zero oxidation state of Pd bulk compared with other bases for the reaction. In further tests, dif-
(around 335.0 eV and 341.0 eV [49,50]), verified that PdCl42− ferent loadings of Pd/MgAl-LDHs were employed in the model
had been reduced to zero-valence Pd from interlayers and reaction (Table 1, entries 8–10) in order to optimize the usage
loaded on MgAl-LDH surface successfully (see Figure S1 in of catalyst. The results show that the reaction yield kept un-
Supporting Information File 1).
changed when the Pd/MgAl-LDHs loading was reduced to
2.5 mol % (Table 1, entry 9).
After getting access to the optimal reactant system, we shifted
our focus on the mechanochemistry parameters of mill-ball size
and its filling degree. The milling-ball filling degree (ΦMB)
represents the volume of the milling balls relative to the beaker
volume, which is calculated as the ratio of the overall milling
ball volume (VMB) to the total beaker volume (VBV):
This parameter is proved to be the essential factor not only on
the occurrence of collision and friction, but also on the energy
distribution and product yield [52]. In Figure 2, we chose four
types of milling-balls with diameters of 5 mm, 8 mm, 10 mm
and 14 mm in the model reaction under four kinds of filling
degrees (ΦMB = 0.15, 0.2, 0.25, 0.3). It could be found that no
matter which kind of the milling-ball diameter is, the tendency
of the product yield is similar under different filling degrees. A
maximum yield (84%) was obtained by using 5 mm milling
balls at 0.25 filling degree. In addition, the 14 mm milling balls
Figure 1: The XRD patterns for the samples of MgAl-LDHs, MgAl-
LDHs-PdCl42− and Pd/MgAl-LDHs.
Scheme 2: Selected model reaction.
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Because the ball-milling time and the rotation speed have a
strong influence on the energy input, which directly regulates
the product structure and yield during the mechanochemical
process, the combined effect of ball-milling time and rotation
speed was investigated systematically. The results are summa-
rized in Figure 3. It can be seen that with increasing rotation
speed, the yield of 3aa increased first, but decreased at the
highest speed of 1000 rpm. This is mainly due to the overabun-
dant energy input resulting in side products. Furthermore,
prolonging the reaction time over 60 min did not help improv-
ing the product yield, the reactants had all been consumed after
60 min. Therefore, 800 rpm together with 60 min is regarded as
the optimum condition for the maximum yield.
Table 1: Optimization of Heck reaction conditions.a
Entry
Base
Pd (mol %)
Yield (%)b
1
2
3
4
5
6
7
8
9
10
NaOH
KOH
10
10
10
10
10
10
10
5
56
64
59
54
60
72
43
71
71
54
t-BuOK
Et3N
Cs2CO3
K2CO3
DBU
K2CO3
K2CO3
K2CO3
2.5
1.25
aReaction conditions unless otherwise noted: 1a (1.5 mmol), 2a
(2.1 mmol), Pd/MgAl-LDHs, TBAB (1.5 mmol), base (3.6 mmol), and
5 g silica gel were placed in a 80 mL stainless-steel vessel (ΦMB = 0.2,
dMB = 14 mm). HSBM conditions: 60 min at 800 rpm. bIsolated yield.
Figure 3: Examination of ball-milling time and rotation speed on the
yield of 3aa. Reaction conditions: 1a (1.5 mmol), 2a (2.1 mmol), Pd/
MgAl-LDHs (2.5 mol %), TBAB (1.5 mmol), K2CO3 (3.6 mmol), and 5 g
silica gel were placed in a 80 mL stainless-steel vessel (ΦMB = 0.25,
dMB = 5 mm). HSBM conditions: 60 min at 800 rpm.
Figure 2: Examination of the milling-ball filling degree (ΦMB) and
milling-ball sizes on the yield of 3aa. Reaction conditions: 1a
(1.5 mmol), 2a (2.1 mmol), Pd/MgAl-LDHs (2.5 mol %), TBAB
(1.5 mmol), K2CO3 (3.6 mmol), 5 g silica gel were placed in a 80 mL
stainless-steel vessel. HSBM conditions: 60 min at 800 rpm.
In the ball-milling process, the grinding auxiliary is found to be
an efficient transfer medium between energy and reactant
[1,2,55,56]. Additional investigations on the effects of the
grinding auxiliaries were carried out. The results shown in
Table 2 indicate that 5 g silica gel is considered as the most
exhibited the higher yields than 5 mm, 8 mm and 10 mm effective choice for the reaction (Table 2, entry 1), but MgAl-
milling balls under the low filling degrees (ΦMB = 0.15, 0.2). LDHs gave also a good result (Table 2, entry 5). With NaCl,
And then, with the filling degree increased to the value of 0.25, α-Al2O3 and γ-Al2O3, the yields were unsatisfactory (Table 2,
the movement space for 14 mm milling balls was hindered in entries 2–4). Increasing or decreasing the amount of silica gel
the ball-milling jar, resulting in the apparent decrease in the would led to a reduction of the yield of 3aa (Table 2, entries 6
yield of 3aa. On the contrary, 5 mm, 8 mm and 10 mm milling and 7), which might be due to the uneven distribution of the
balls had sufficient collision to produce enough energy under reactants.
the filling degree of 0.25, leading to the high yields. Further-
more, the sharp decrease in the yield could be also observed in After having the optimum reaction conditions in hand, the Pd/
5 mm, 8 mm and 10 mm milling-ball systems with a filling MgAl-LDH catalyst was evaluated to expand the generality and
degree over 0.25, which might be due to the overfull ball- substrate scope in Heck reactions, the results are presented in
milling jar and the overabundant energy input. These results Figure 4 and Scheme 3. As we expected, both with electron-
mentioned above are consistent with the previous studies re- withdrawing and electron-donating groups substituted
ported by us [53] and the others [52,54].
bromobenzenes (1a–l) and styrenes (2a–e) react with each other
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Table 2: Examination of grinding auxiliaries on yield of 3aa.a
Entry
Grinding auxiliary
Weight (g)
Yield (%)
1
2
3
4
5
6
7
silica-gel
5
5
5
5
5
3
7
84 (n.r.)b
NaCl
54
α-Al2O3 (base)
γ-Al2O3(neutral)
MgAl-LDHs
silica-gel
68
61
72 (n.r.)c
74
silica-gel
70
aReaction conditions unless otherwise noted: 1a (1.5 mmol), 2a (2.1 mmol), Pd/MgAl-LDHs (2.5 mol %), TBAB (1.5 mmol), K2CO3 (3.6 mmol),
grinding auxiliary were placed in a 80 mL stainless-steel vessel (ΦMB = 0.25, dMB = 5 mm). HSBM conditions: 60 min at 800 rpm. bSilica gel used as
grinding auxiliary without Pd/MgAl-LDHs catalyst. cMgAl-LDHs used as grinding auxiliary without Pd/MgAl-LDHs catalyst.
Figure 4: Substrate scope of Pd/MgAl-LDHs catalyzed Heck reactions. Reaction conditions unless otherwise noted: 1a (1.5 mmol), 2a (2.1 mmol),
Pd/MgAl-LDHs (2.5 mol %), TBAB (1.5 mmol), K2CO3 (3.6 mmol), and 5 g silica gel were placed in a 80 mL stainless-steel vessel (ΦMB = 0.25,
dMB = 5 mm). HSBM conditions: 60 min at 800 rpm.
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Scheme 3: Pd/MgAl-LDHs catalyzed Heck reactions of heteroaryl bromides. Reaction conditions unless otherwise noted: 1a (1.5 mmol), 2a
(2.1 mmol), Pd/MgAl-LDHs (2.5 mol %), TBAB (1.5 mmol), K2CO3 (3.6 mmol), and 5 g silica gel were placed in a 80 mL stainless-steel vessel
(ΦMB = 0.25, dMB = 5 mm). HSBM conditions: 60 min at 800 rpm.
successfully to afford the coupling products in satisfactory regardless of the substrate type, the catalyst system could be
yields. The electron-deficient bromobenzenes (1a–c, 1e–i) or reused at least five times efficiently without significant loss in
styrenes (2d, 2e) show slightly higher yields than the electron- catalytic activity, resulting in almost no change in the yields.
rich substrates. The ketone group at ortho-, meta- and para-po- Hence, the reusability of Pd/MgAl-LDHs is one of the major
sitions (1a–c) were chosen to examine the steric hindrance for advantages for Heck reactions under HSBM conditions.
this reaction. To our surprise, the position of the ketone group
had a little effect on the yields and the larger sterically hindered
substrate 1c led to a higher yield as compared with 1a and 1b,
which is contrary to Li’s study [57] in solution-based Heck
reactions. This might be because of the lone pairs of the oxygen
atom in the keto group at the ortho-position could coordinate
with Pd/MgAl-LDHs under HSBM conditions and promote the
reaction efficiently. Furthermore, the couplings of heteroaryl
bromides (1m–o) and styrene (2a) as well as substituted
bromobenzene (1i) and butyl acrylate (2f, 2g) were investigated
to extend the scope and generality of the reaction. The results
clearly demonstrate that all the substrates are well tolerated to
give the corresponding coupling products smoothly with yields
of 60–80%.
Figure 5: Recycling studies of the Pd/MgAl-LDH catalyst for Heck
reactions. Reaction conditions: 1i or 1m (1.5 mmol), 2a (2.1 mmol),
Pd/MgAl-LDHs (2.5 mol %), TBAB (1.5 mmol), K2CO3 (3.6 mmol), and
silica gel 5 g were placed in a 80 mL stainless-steel vessel
Finally, the coupling reactions of aryl bromide 1i and styrene
(2a) as well as heterocyclic bromide 1m and styrene (2a) were
chosen as the model reactions under the optimized conditions to
investigate the reusability of the Pd/MgAl-LDH catalyst. The
(ΦMB = 0.25, dMB = 5 mm). HSBM conditions: 60 min at 800 rpm.
catalyst together with the grinding auxiliary are recovered by a Conclusion
simple rinse after each run, which is more convenient com- In summary, a supported and recyclable Pd catalyst
pared to other methods [36,37]. As can be seen in Figure 5, (Pd/MgAl-LDHs) was designed and synthesized by co-precipi-
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14.Amatore, C.; Jutand, A. Acc. Chem. Res. 2000, 33, 314–321.
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Heck reactions under HSBM conditions. The results indicate
that the Pd is successfully dispersed on the surface of Pd/MgAl-
LDHs, and a small quantity of Pd/MgAl-LDHs (2.5 mol % of
Pd) shows the remarkable activity in Heck reactions with a wide
range of aryl bromides and olefins under mild conditions. In
these cases, toxic solvents, expensive ligands and inert atmo-
sphere were efficiently avoided. Furthermore, the Pd/MgAl-
LDH catalyst can be recycled for at least five times without sig-
nificant loss in coupling product yields.
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Supporting Information
Supporting Information File 1
Details of experimental procedures and characterization
data of prepared compounds, 1H, 13C NMR, and MS
spectra of all coupling compounds.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-13-160-S1.pdf]
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Acknowledgement
This work was supported by the National Natural Science Foun-
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