Assembly of a multilayer film and catalytic application in Suzuki cross-coupling reaction based on synergistic effects of a conjugated organometallic pyridyl Pt(C≡C)2 moiety with palladium

Article abstract of DOI:10.1039/c3cc44439b

A multilayer film (Pd²⁺/1)_n was prepared with Pd(ii) ions and platinum compound trans-[Pt(PPh₃)₂(CCPy) ₂] (1) using the layer-by-layer self-assembly method. The film loaded on the quartz slide could be capable of discharging high catalytic active Pd species to promote the C-C coupling reaction, with extraordinarily low Pd-loading (6.6 × 10^-6 mol%) and high catalytic efficiency in H₂O-EtOH solution.

Full text of DOI:10.1039/c3cc44439b

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DOI: 10.1039/C3CC44439B
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ARTICLE TYPE
Assembly of a Multilayer Film and Catalytic Application in Suzuki
Cross-coupling Reaction Based on Synergistic Effect of a Conjugated
Organometallic Pyridyl Pt(C≡C)2 Moieties with Palladium
Xing Li,*a Xiuhua Zhao,a Jie Zhangb and Yayun Zhaoa
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
(Pd2+/1)n multilayer films from 1 and PdCl2 by layer-by-layer
40 technique, in which pyridine N atoms can bond palladium
centers well. The multilayer film as a catalyst reservoir was
performed the application to the Suzuki coupling reaction. To
the best of our knowledge, this is rare case so far that the
pyridyl-acetylene platinum compound as the organometallic
45 liangd is employed to prepare the multilayer film loaded on
the quartz slides for the catalytic reaction.
A multilayer film (Pd2+/1)n was prepared with Pd(II) ions and
platinum compound trans-[Pt(PPh3)2(CŁCPy)2] (1) through
layer-by-layer self-assembly method. The film loaded on the
10 quartz slide could be capable of discharging high catalytic
active Pd species to promote the C-C coupling reaction, with
extraordinarily low Pd-loading (6.6 x 10-6 mol%) and high
catalytic efficiency in H2O / EtOH solution.
The assembly of ordered architectures by using the linear
15 organometallic motifs to modify the solid surfaces, was much
less explored in the previous studies.1 Fortunately, The
transition metal coordination chemistry provids new
opportunities for preparing the multilayer film, in which metal
particles are orderly deposited on the surfaces of the films,
20 and can gradually release high catalytic active nanoparticles
in the reactions.2-4 It has been documented that rigid pyridyl
Pt-acetylide blocks may be useful potential candidates for
assembly of the functionalized multilayer film.5
Fig. 1 (a) A plot of trans-[Pt(CŁCPy)2(PPh3)2] (1), the hydrogen atoms
are removed for clarity; (b) its space-filling mode.
As part of our interest in the preparation of organometallic
25 compounds with unsaturated groups, we have carried out a study
of the pyridyl-acetylene platinum featuring large conjugated
electronic groups, which can offer both alkyne-ʌ system and
pyridine nitrogen atoms readily bonding to metal centers and
should be useful in the assembly of metal-ligand multilayer films.
30 For instance, Pd-organic ligand films were successfully
assembled by alternating adsorption organic ligands and Pd(II)
ions onto solid slides,6 and Pd-pyridyl complexes were loaded
within the multilayer films, which as catalysts in the coupling
reaction showed considerable high catalytic activity.4,7,8
50
Single X-ray diffraction analysis showed that 1 is a mono-
nuclear (Fig. 1). The Pt(II) ion is a four coordinated geometry
comprised trans-oriented C2P2 donors from ı-coordinated
acetylides and PPh3 ligands. The Py–CŁC–Pt–CŁC–Py array is
quasi-linear in 1. Triple bond is confirmed by the feature peak of
55 2110 cm-1 (Q(CŁC)) in IR spectra with the bond distance being
1.195(11) Å (ESI). All crystallized data with bond lengths and
angles are listed in table S1 and Table S2. The absorption spectra
of 1 at 220 nm due to the níS* transitions, and the broad
absorption band 300í400 nm is tentatively attributed to the S-S*
60 transitions (Fig. S4), probably along with some d(Pt)ĺS* MLCT
(Metal-to-ligand charge transfer) character.9,10
The growth process of the film was monitored with UV-vis
spectroscopy. As shown in Fig. 2, the maximum absorption peak
centered at 350 nm for the film PEI-(Pd2+/1)n (n = 1-10, PEI =
65 polyethylenmine) was consistent with one of 1 with no position
change of the absorption peak during the film assembly process,
but Pd2+ ions adsorption resulted in the absorbance increase upon
adding deposition layers. The absorbance increases was close to
linear with R2 = 0.9824 between the absorption at Ȝmax = 350 nm
70 and the number of bilayers, suggested that 1 was incorporated in
the films PEI-(Pd2+/1)n for the first ten layers. Thus, metal-pyridyl
coordination interactions were responsible for the LbL deposition
35
Here, we prepared a bridging organometallic ligand trans-
[Pt(PPh3)2(CŁCPy)2 (1, py = pyridyl), and characterized by IR,
UV-vis, NMR, XPS, and single X-ray diffraction (ESI). The
compound above was acted as a linear motif to assemble
a Faculty of Materials Science and Chemical Engineering, Ningbo
University, Ningbo, Zhejiang 315211, China.
Fax: (+8657487609987); E-mail: lixing@nbu.edu.cn
b College of Chemistry, Xiangtan University, Xiangtan, Hunan 411105,
P. R. China.
† Electronic Supplementary Information (ESI) available: IR, UV-vis,
NMR, XPS, preparation of the materials, crystallographic data and
catalytic experiments. See DOI: 10.1039/b000000x/
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

Page 3 of 4
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between metal ions and 1 by using its pyridyl groups. The
desorption experiments were also prove that the assembly of the
among particles. As shown in Fig. 4a, some larger aggregated
particles as islets were unevenly distributed on the surface of
multilayer film was in layer-by-layer manner, monitored by UV- 25 the film PEI-(Pd2+/1)5 with root-mDeOanI:-1s0q.u10aV3rie9ew/CA3(rCRtiCcMl4e4SO4n)3li9nBe
vis spectra (Fig.S5). The amount of Pd in multilayer film PEI-
(Pd2+/1)10 is 7.0 μg (6.6 × 10-6 mol%), determined by UV-vis
spectra and inductively coupled plasma-atomic emission
spectroscopy (ICP-AES) techniques (Details, see ESI). The XPS
roughness being 5.677 nm. Upon increasing the film number,
more particle aggregates were observed and the island-shaped
nanostructures were dispensed on the film surface with the
RMS roughness 72.945 nm for ten layer film PEI-(Pd2+/1)10
5
results confirmed the presence of all the proleptic elements in the 30 (scan area 6.0 × 6.0 ȝm2). (Fig. 4b).
films (Fig. 3, Fig. S6),11,12 in the other words, PdCl2/1
The Suzuki cross-coupling reactions were carried out by
10 components were deposited on the substrates.
using the slide coated with PEI-(Pd2+/1)10 films as catalysts, in
which the coated slides were immersed into the reaction
mixtures. After the reaction finished, the desired products
35 were separated by extraction and purification. As shown in
Table 1, the para-substituted aryl bromides with electron-
withdrawing groups (e.g., CN, CF3) gave the relevant biaryls
in high yields at 50 °C for 4h (Table 1, entry 2 and 3). The
electron-donating group (e.g., 4-MeO) of aryl bromide also
40 offered the high yield in the same reaction condition (Table 1,
entry 4). However, the ortho -substituent (2-MeO) gave rise to
the low yield of the biaryl products due to the steric hindrance
effect in the reaction (Table 1, entry 5), even though longer
reaction time and higher temperature were executed in the
45 reaction system, it was very difficult to reach considerably
high yield (Table 1, entry 6), comparing with one of entry 4 in
table 1. The para-substituted arylboronic acids with electron-
donating substituents (e.g., Me, 4-MeO) favored the coupling
reactions (Table 1, entry 7 and 8). Even for the ortho-
50 substituted arylboronic acid (e.g., 2-MeO), it also gave high
yield (Table 1, entry 9), in which the steric hindrance little
affected the result of the reactions (Details in Table S3).
Fig. 2 UV-vis spectra of the multilayer film of PEI-(Pd2+/1)10.
Table 1 Suzuki cross-coupling reactions of aryl halides with arylboronic
acids using (Pd2+/1)10 films as catalysts
H2O/EtOH, K2CO3
B(OH)2
X
+
Catalyst, 6.6x10-6 mol%
R1
R2
R2
R1
Entry
R1
R2
X
T/°C, hour Yield TON x 104
1
2
3
4
5
6
7
8
9
H
4-CN
4-CF3
4-MeO
2-MeO
2-MeO
H
H
H
H
H
H
Br
Br
Br
Br
Br
Br
Br
50, 4h
50, 4h
50, 4h
50, 4h
50, 4h
100, 24h
50, 4h
50, 4h
50, 4h
98
99
98
95
38
86
92
95
91
1.49
1.50
1.49
1.44
0.58
1.31
1.40
1.44
1.38
H
4-Me
4-MeO Br
2-MeO Br
Fig. 3 XPS view of the film (PdCl2/1)10 deposited on the single crystal
15 silicon substrate. (a) Survey. (b, Pd3d; c, Pt4f in ESI).
H
H
General procedure: 1.0 mmol of aryl halide, 1.05 mmol of
arylboronic acid, 3.0 mmol of K2CO3, in H2O / EtOH (4:3, V/V)
under ambient atmosphere. TON = mol product / mol Pd. IR, 1H and
13C NMR Spectra of the products in supplementary materials
55
In order to further investigate the catalytic activity of the
(Pd2+/1)n films,
a series of parallel experiments were
performed in the similar conditions. It was found that PdCl2
rapidly formed Pd-black in the reaction, exhibiting low
activity (Table 2, entry 2). Similarly, 1 was low active as a
Fig. 4 AFM height images of the films. (a) PEI-(Pd2+/1)5 (Rrms = 5.667
nm). (b) PEI-(Pd2+/1)10 (Rrms = 72.945 nm).
60 catalyst in the absence of a palladium source (Table 2, entry
1). A reaction of PdCl2 and 1 produced grayer precipitates,
which acted as catalysts for the Suzuki reaction in the first
cycle, displaying high catalytic activity (Table 2, entry 3).
After reaction, the precipitates were collected, and reused in a
65 second reaction sequence, very low activity was found (Table
2, entry 4). However, (PdCl2/bpy)10 film showed low catalytic
Atomic force microscopy images of the multilayer films
20 was shown in Fig. 4. The height images of PEI-(Pd2+/1)n
showed that aggregated nano-particulates were compactly
distributed on the quartz slides with the vague boundaries
2
| Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

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activity in the first cycle, and lower activity in the second 45 from the film could be controlled by the releasing time. In
cycle (Table 2, entry 5 and 6 ).
addition, the catalytic slides can be easily removed from the
reaction system at any time, and reducDeOthI:e10w.1a0s3t9e/Ca3mCoCu44n4t3o9fB
the catalysts. The palladium components as the catalysts were
embed on the quartz slide surfaces, gradually released
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In contrast, the PEI-(Pd2+/1)10 film could be successfully
applied in multiple times successive coupling reactions. The
procedure that utilized the PEI-(Pd2+/1)10 film as a Pd-catalyst
5
reservoir was as follows: The quartz slide coated with the 50 palladium particulates at the molecular level in the reaction
PEI-(Pd2+/1)10 film was immersed into a solution of 4-
methoxy-aryl bromide, arylboronic acid, K2CO3 in EtOH and
H2O (step 1). After the mixture was stirred for 20 min at
and plausibly performed homogeneous reaction, so it showed
considerably high catalytic activity.13 Additionally, the
concave in the surface of the film could provide appropriate
interspace effect to promote the C-C bond formation.14,15
10 50 °C, the quartz slide was removed from the mixture. The
mixture was allowed to react for a further 10 h at 50 °C. The 55 Otherwise, the catalytic reactions were performed in the other
Pd-1 complex released from the film is highly active as a
catalyst under mild reaction condition, giving a high yield of
the coupling product (Step 2). In the second catalytic cycle,
15 the same coated quartz slide was then immersed into the next
solvents, and the results were listed in Table S5 (ESI).
In conclusion, PEI-(Pd2+/1)n multilayers-loaded slides were
used as catalyst reservoir to catalyze the Suzuki reaction in
H2O / EtOH solution, with catalyst loading as low as 6.6 × 10-
6
reaction mixture (Step 3). After the mixture was stirred for 20 60 mol% and high catalytic efficiency under the mild condition.
min, the quartz slide was again removed from the reaction
mixture and the second aliquot of released Pd-1 complex used
to catalyze a second Suzuki reaction (Step 4). Here in each
20 immersion step, a certain amount of Pd-1 complex in the film
The PEI-(Pd2+/1)n film as a high active catalyst is expected to
be greatly useful in organic synthesis for potentially Pd-
catalyzed cross-coupling reactions.
Authors acknowledge the financial support by the NSFC
was desorbed into the reaction mixture and showed high 65 (20971075), the NSF of Zhejiang province (LY12B01005),
catalytic activity for Suzuki reactions with high yields (Table
2, entry 7-10). After four cycles, the product yield came down
rapidly, indicating that the catalytic active species into the
25 solution were gradually depleted in the reactions (Table S4).
the State Key Lab. Struct. Chem. (20110010), Fujian Institute
of Research on the Structure of Matter, CAS and sponsored K.
C. Wong Magna Fund in Ningbo University.
Notes and references
Table 2 Suzuki cross-coupling of 4-methoxyaryl bromide with phenyl-
boronic acid using 1, PdCl2, PdCl2/1 precipitate, PEI-(PdCl2/1)10 film and
(PdCl2/bpy)10 film as catalyst (bpy = 4,4’-bipyridine).
70 † Crystal data for 1: C50H38N2P2Pt, Monoclinic, space group P2(1)/c, a
= 12.0599(12), b = 22.474(2), c = 7.8012(8) Å, ȕ = 101.5980(10), T =
293 K, Z = 8, V = 2071.2(4) Å3, Dc= 1.481 g / cm3, Ȝ = 0.71073 Å,
17791 reflections collected, 4744 unique which were used in
calculations. R1 = 0.0276 and wR2 = 0.0717 for I > 2ı (I). CCDC
Catalyst
K2CO3
OMe
B(OH)2
MeO
Br
+
H2O/EtOH
Entry
1
2
3
4
5
6
7
8
Run
1
Catalysts
Yield
Trace
16 %
97 %
Trace
40 %
<30 %
95 %
96 %
94 %
92 %
75
80
85
900608. Crystallographic data for 1 have been deposited at the
Cambridge Crystallographic Data Center.
C. P. Chen, W. R. Luo, C. N. Chen, S. M. Wu, S. C. Hsieh, C. M.
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b
1st
PdCl2
1
2
3
4
1st
PdCl2/1 precipitateb
PdCl2/1 precipitate
(PdCl2/bpy)10 film
(PdCl2/bpy)10-x film
(PdCl2/1)10 film
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1st
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2nd
3rd
4th
(PdCl2/1)10-x film
(PdCl2/1)10-x film
(PdCl2/1)10-x film
9
10
a General procedure: 1.0 mmol of PhBr, 1.05 mmol of PhB(OH)2, 3.0
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atmosphere for 10 h. b Catalyst 3.0 mol%.
5
6
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Concretely, after each reaction step, the resultant mixture
30 was extracted three times with ethyl acetate (10 mL). Then the
water phase was recycled as the reaction medium for a
subsequent reaction. Additional aliquots of aryl halide and
arylboronic acid were added into the recovered water phase,
with refreshing the catalyst from the film. The mixture was
35 allowed to react for 10 h at 50 °C, and the coupling product
was obtained in good yield from these secondary reaction
sequences. Recycling of the solvent in the organic synthesis
has important significance for environmental protection.
In our work above, PEI-(PdCl2/1)10 film with extremely low
40 Pd-loading was used as a catalyst reservoir being able to
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This journal is © The Royal Society of Chemistry [year]
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