Article pubs.acs.org/Organometallics
Ruthenium Hydride Complex Supported on Gold Nanoparticle Cored Triazine Dendrimers for C−C Coupling Reactions Anahita Daneshvar, Majid Moghadam,* Shahram Tangestaninejad,* Valiollah Mirkhani, Iraj Mohammadpoor-Baltork, and Akram Khalili Department of Chemistry, Catalysis Division, University of Isfahan, Isfahan 81746-73441, Iran S Supporting Information *
ABSTRACT: In this work, the unusual ability of a ruthenium hydride catalyst, [RuHCl(PPh3)3CO], supported on gold nanoparticle cored triazine dendrimers in the Suzuki−Miyaura cross-coupling reaction and also in the synthesis of diaryl ketones is reported. [Ru-H@AuNPs-TD] was characterized by Fourier transform infrared spectroscopy, CHNS, TEM, SEM, ICP, and TGA analyses. The ruthenium hydride catalyst was used as a heterogeneous catalyst for the C−C coupling reactions of aryl halides with phenylboronic acids, and the biphenyl derivatives were produced in good to excellent yields. On the other hand, this catalytic system was applied for synthesis of diaryl ketones by the reaction of phenylboronic acids with substituted benzaldehydes. Moreover, this catalyst can be well-dispersed in the reaction medium, conveniently separated from the reaction mixture, and reused several times without significant loss of its activity.
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INTRODUCTION
Dendrimers are treelike, hyperbranched, highly symmetrical polymeric structures and have emerged as promising supports for catalysis. The regular branched structure, monodispersity, and desired solubility in organic/aqueous media are three main advantages that make these compounds a bridge between homogeneous and heterogeneous catalytic systems.1−5 Metal-containing dendrimers are a class of materials that have attracted much attention in recent years. Dendrimer-encapsulated nanoparticles (DENs), dendrimer-stabilized nanoparticles (DSNs), and nanoparticle-core dendrimers (NCDs) are three general categories of metal ion containing dendrimers (Figure 1).6,7 AuNPs can be synthesized by the reduction of HAuCl4 with NaBH4 and stabilized by thiols (Figure 2).9 The Brust−Schiffrin method for AuNP synthesis allowed the facile synthesis of heat- and air-stable AuNPs with a narrow particle size distribution for the first time. Afterward, more investigations opened an avenue to the synthesis of AuNPs
Figure 2. Synthesis of AuNPs and their stabilization with thiols.
stabilized by a variety of ligands bearing thiol groups.9 For example, Astruc et al. investigated the synthesis of gold nanoparticles with both dodecanethiolate and the dendronized thiolate either by the ligand-substitution procedure or by direct synthesis from mixtures of functional and nonfunctional thiols.10 NCDs with a gold nanoparticle core and thiol-functionalized dendrons at the focal point were synthesized by the Brust− Schiffrin method.9 However, this approach required a large excess of dendrons and had poor control of the nanoparticle core dimension. Therefore, a new synthesis methodology is highly desirable for a basic understanding of the structure/property relationships of these nanostructures.11 In an alternative synthetic method, the synthesis of monolayerprotected nanoparticles is followed by building the dendrimer architecture on functionalized nanoparticles. A high surface to volume ratio of the nanoparticles and surface effects increase the catalytic efficiency of these catalysts.10,12 The development of catalytic reactions that use transitionmetal complexes as catalysts under neutral and mild reaction Received: February 26, 2016
Figure 1. Metal-containing dendrimers.8 © XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.6b00163 Organometallics XXXX, XXX, XXX−XXX
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Organometallics conditions is particularly important, and catalysis with metallodendritic complexes has attracted much attention in recent years. Ruthenium-based complexes are generally considered to be efficient catalysts due to their high activity and electron transfer features.13 Among these complexes, ruthenium hydride catalysts have received considerable attention due to their ability in the catalysis of reductive carbon−carbon bond formations,14 postpolymerization functionalization of conjugated polyisoperene,15 synthesis of (E)-styryl ketones from styrene,16 and conversion of dialdehydes and keto aldehydes to lactones.17 Recently, we reported the catalytic activity of ruthenium complexes in the alkylation of 1,3-dicarbonyl compounds,18 direct oxidation of alcohols to carboxylic acids,19,20 epoxidation of alkenes,21,22 and catalytic C−C bond formation.23 In organic chemistry, the C−C coupling reaction refers to a variety of reactions in which two hydrocarbon fragments are coupled with the aid of a metal catalyst. The Heck, Suzuki− Miyaura, Sonogashira, Stille, and Buchwald−Hartwig reactions are different common reactions that are catalyzed by transition metals and are widely used in the synthesis of pharmaceuticals, biologically active molecules, natural products, and polymeric materials.24,25 The Suzuki−Miyaura cross-coupling reaction, which is an increasingly popular method for the synthesis of unsymmetrical biaryl compounds, is largely dominated by palladium phosphine complexes. Therefore, the development of new catalytic systems using other transition-metal species such as nickel26,27 and gold28,29 is of great importance. Chang and coworkers reported their investigations on the Suzuki-type crosscoupling reactions of aryl iodides with phenylboronic acids using Ru/Al2O330 as catalyst. In another example, a ruthenium-grafted triazine functionalized mesoporous polymer was reported as an efficient catalyst in Suzuki−Miyaura cross-coupling reactions.31 In two other examples, biphenylene-substituted ruthenocenyl phosphine32 and ruthenium phosphido complexes33 as supporting ligands for palladium-catalyzed Suzuki−Miyaura crosscoupling reactions were reported. Cross-coupling reactions of aryl aldehydes with arylboronic acids in Rh-, Pd-, and Pt-catalyzed reactions have already been reported as an approach to access diaryl ketones. In 2011, Fukuyama and co-workers reported [RuHCl(CO)(PPh3)3] to be an efficient homogeneous catalyst for the coupling reaction of arylboronic acids with aryl aldehydes to give the corresponding diaryl ketones.14 By combination of the advantages of CNDs and the [RuHCl(PPh3)3CO] complex, herein we wish to report the preparation and characterization of a new thermally stable, reusable, and efficient ruthenium hydride catalyst supported on gold nanoparticles functionalized with dendrimers. In this manner, a gold nanoparticle cored triazine dendrimer bearing thiol groups as surface ligands was prepared and [RuHCl(PPh3)3CO] was attached to the thiol groups. The catalytic activity of this new heterogeneous catalyst was investigated in Suzuki−Miyaura C−C coupling reactions and also in the synthesis of diaryl ketones (Scheme 1).
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Scheme 1. C−C Coupling Reactions Catalyzed by [Ru-H@ AuNPs-TD]
under an oxygen flow at a uniform heating rate of 20 °C min−1 in the range of 25−600 °C. Scanning electron microscope measurements were carried out on a Hitachi S-4700 field emission-scanning electron microscope (FE-SEM). Transmission electron microscopy (TEM) was carried out on a Philips CM10 transmission electron microscope operating at 100 kV. The Ru content of [Ru-H@AuNPs-TD] was determined by a Jarrell−Ash 1100 ICP analysis. The complex [RuHCl(CO)(PPh3)3] was prepared according to the method reported in the literature.34 The substances were identified and quantified by gas chromatography (GC) on an Agilent GC 6890 instrument equipped with a 19096C006 80/100 WHP packed column and a flame ionization detector (FID). Synthesis of Triazine Dendrimers Supported on Gold Nanoparticles. Preparation of Gold Nanoparticles Modified by 4Aminothiophenol. A solution of HAuCl4·H2O (500 mg) in CH3OH (62.5 mL) was added to a solution of 4-aminothiophenol (0.4687 mg) in CH3OH (62.5 mL), and the mixture was stirred for 20 min. Then, a freshly prepared solution of sodium borohydride (0.356 mg) in deionized water (30 mL) was added to the vigorously stirred reaction mixture. Stirring was continued for 1 h. Finally, the AuNPs stabilized with 4-aminothiophenol, AuNPs-ATP, were separated by centrifugation and purified by successive washing with CH2Cl2 and deionized water.35 Preparation of AuNPs-ATP-CC1. AuNPs-ATP (1 g) was added to a solution of cyanuric chloride (0.29 g, 1.57 mmol) and N,N-diisopropylN-ethylamine (DIPEA, 1 mL) in THF (5 mL). The reaction mixture was stirred at 0 °C for 18 h. The solid was separated by centrifugation, washed with ethanol several times, and then dried in a vacuum oven.25 Preparation of the First-Generation Dendrimer (G1). To a slurry of AuNPs-ATP-CC1 (1 g) in DMF (10 mL) were added diethylenetriamine (DETA, 1 mL) and DIPEA (1.4 mL). The reaction mixture was stirred at 85 °C for 16 h. The solid part of the reaction mixture was separated by centrifugation, washed with ethanol several times, and then dried in a vacuum oven.25 Preparation of AuNPs-ATP-CC2. G1 (1 g) was added to a solution of cyanuric chloride (1.28 g) and DIPEA (1.56 mL) in THF (10 mL). The reaction mixture was stirred at 0 °C for 18 h. The solid was separated by centrifugation, washed with ethanol several times, and then dried in a vacuum oven.25 Preparation of the Second-Generation Triazine Dendrimer (G2). To a slurry of AuNPs-CC2 (1 g) in DMF (12 mL) were added 2aminoethanethiol (9.2 mmol, 0.71 g) and DIPEA (1.4 mL). The reaction mixture was agitated at 85 °C for 16 h, and then the solid was separated by centrifugation, washed with ethanol several times, and dried in a vacuum oven.25 Preparation of the [RuHCl(CO)(PPh3)3] Catalyst Supported on a Gold Nanoparticle Cored Dendrimer, [Ru-H@AuNPs-TD]. A mixture of [RuHCl(CO)(PPh3)3] (650 mg, 0.74 mmol) and the gold nanoparticle supported triazine dendrimer G2 (1 g) in toluene (15 mL) was refluxed for 72 h, and then the solid was filtered, washed with toluene, and dried under vacuum.23 General Procedure for Suzuki−Miyaura Cross-Coupling Reactions Catalyzed by [Ru-H@AuNPs-TD]. In a screw-capped test tube, a mixture of aryl halide (1 mmol), phenylboronic acid (1.1 mmol), K2CO3 (1.5 mmol), and the [Ru-H@AuNPs-TD] catalyst (0.011 mmol, based on the amount of Ru as determined from the ICP value of 0.293 mmol of Ru/g of [Ru-H@AuNPs-TD]) in toluene/H2O (2/1) was prepared. The test tube was purged with argon and sealed. The reaction mixture was stirred at 80 °C. The reaction progress was
EXPERIMENTAL SECTION
The chemicals used in this work were purchased from Fluka and Merck. FT-IR spectra were recorded on a Jasco 6300 spectrophotometer. 1H and 13C NMR (400 and 100 MHz) spectra were recorded on a Bruker Avance 400 MHz spectrometer using CDCl3 as solvent. Elemental analysis was performed on a LECO CHNS-932 analyzer. Thermogravimetric analysis (TGA) was carried out on a Mettler TG50 instrument B
DOI: 10.1021/acs.organomet.6b00163 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Scheme 2. Preparation of the [Ru-H@AuNPs-TD] Catalyst
monitored by GC. After completion of the reaction, the reaction mixture was cooled to room temperature, and [Ru-H@AuNPs-TD] was separated by centrifugation and washed with EtOH. The solvent was evaporated under reduced pressure, and the residue was recrystallized from ethyl acetate and ether (1/3) to afford the pure product. In all cases, the isolated yields matched well with GC yields with only 2−7% difference. General Procedure for Synthesis of Diaryl Ketones Catalyzed by [Ru-H@AuNPs-TD]. In a screw-capped test tube, a mixture of phenylboronic acid (1 mmol), aryl aldehyde (2 mmol), [Ru-H@ AuNPs-TD] (0.023 mmol, based on the amount of Ru as determined from the ICP value of 0.293 mmol of Ru/g of [Ru-H@AuNPs-TD]), toluene (3 mL), K2CO3 (2 mmol), and H2O (1 mmol) was prepared. The test tube was purged with argon and sealed. The reaction mixture was stirred at 110 °C for 8 h. After the reaction was completed, [Ru-H@ AuNPs-TD] was separated by centrifugation and washed with EtOH.
The solvent was evaporated under reduced pressure, and the residue was purified by chromatography on a silica gel plate (petroleum ether/ethyl acetate 20/1) to give the desired product. Catalyst Recycling Procedure. The reusability of [Ru-H@AuNPsTD] was checked using sequential Suzuki−Miyaura cross-coupling reactions of 4-iodoanisole with phenylboronic acid and the reaction of 4nitrobenzaldehyde with phenylboronic acid in the synthesis of diaryl ketones. The reaction procedures were as described above. After the reaction was completed, [Ru-H@AuNPs-TD] was easily recovered from the reaction mixture by centrifugation, washed with EtOH, and dried at 40 °C. The filtrates were used for the determination of Ru leached by ICP analysis, and the recovered [Ru-H@AuNPs-TD] catalyst was used in the next catalytic cycle using fresh 4-iodoanisole and phenylboronic acid in the Suzuki−Miyaura cross-coupling reaction and C
DOI: 10.1021/acs.organomet.6b00163 Organometallics XXXX, XXX, XXX−XXX
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Organometallics in the reaction of 4-nitrobenzaldehyde and phenylboronic acid in the synthesis of diaryl ketones in the presence of K2CO3 as base.
The surface morphology of the nanocatalyst [Ru-H@AuNPsTD] was directly visualized by FE-SEM (Figure 3). As can be seen, [Ru-H@AuNPs-TD] particles are aggregated and have formed a porous structure.
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RESULTS AND DISCUSSION Synthesis and Characterization of the Ruthenium Hydride Catalyst Supported on a Gold Nanoparticle Cored Triazine Dendrimer, [Ru-H@AuNPs-TD]. The preparation route for the [Ru-H@AuNPs-TD] catalyst is shown in Scheme 2. First, the AuNPs were prepared by reduction of HAuCl4 with NaBH4 and then coated with 4-aminothiophenol.35 The reaction of cyanuric chloride (CC) with the amines of the AuNPs-ATP led to substitution of one chlorine atom in cyanuric chloride. Then, AuNPs-ATP-CC1 was reacted with diethylenetriamine, to give G1, which in turn converted to AuNPs-CC2 upon reaction with cyanuric chloride. Finally, AuNPs-ATP-CC2 was reacted with 2-aminoethanethiol to produce the gold nanoparticle cored triazine dendrimer (G2 or AuNPs-TD) as support. These processes were monitored by FT-IR, elemental, and thermogravimetric analyses (TGA). The presence of bands in the FT-IR spectra at 1563−1608 cm−1 (CN) and 1408 and 2932 cm−1 (C−Haliph) confirms the synthesis of the support. The broad bands at 3300−3500 cm−1 are due to primary and secondary amines in the structures of AuNPs-ATP, AuNPs-ATP-CC1, G1, AuNPs-ATP-CC2, and G2 (Figure S1 in the Supporting Information). The weight loss of the dendrimer between 25 and 600 °C as a function of temperature was determined using TGA, which is an irreversible process because of the thermal decomposition. The TGA plots of AuNPs-ATP, AuNPs-ATP-CC1, G1, AuNPs-ATPCC2 and G2 depict a two-step thermal decomposition (Figure S2 in the Supporting Information). The first weight loss step corresponds to the removal of physically adsorbed water, and the main weight loss in the second step is due to the removal of organic moieties on the surface. The observed total weight loss is 14%. A summary of the elemental analysis and TGA data is included in Table 1.
Figure 3. FE-SEM image of [Ru-H@AuNPs-TD].
The energy dispersive X-ray (EDX) results for AuNPs-TD and [Ru-H@AuNPs-TD], shown in Figure 4, clearly showed the presence of Ru in [Ru-H@AuNPs-TD]. The XPS spectra of the Ru 3d region are shown in Figure 5. The Ru 3d3/2 peak could not be clearly observed, because the C 1s peak of the organic moieties overlaps with the Ru 3d3/2 peak. The binding energy of the Ru 3d5/2 peak is 281.4 eV, which clearly showed the presence of Ru species in [Ru-H@AuNPsTD]. The elemental mapping for Ru and Au together with an SEM image is shown in Figure 6. The resulting images clearly showed the distribution of Au and Ru elements in the texture of the [RuH@AuNPs-TD]. Further characterization of [Ru-H@AuNPs-TD] was performed by transmission electron microscopy (TEM). The TEM images of [Ru-H@AuNPs-TD] showed well-defined spherical Au nanoparticles dispersed in a dendrimer matrix (Figure 7). The average sizes of the Au nanoparticles are in the range of 15−22 nm (Figure 8). The larger particle sizes of the Au nanoparticles prepared by this method in comparison to those in dendron synthesis followed by AuNP coordination can be attributed to the fragility of the Au−S bonds. This leads to Oswald ripening, and a large dispersity of particle sizes from 5 to 60 nm was obtained. The ruthenium content of [Ru-H@AuNPs-TD], measured by ICP, showed a value of about 0.293 mmol/g. From the TEM size distributions, one can obtain the average core diameter (D) of the nanoparticles. The average number of gold atoms in a cluster can then be calculated using eq 1, in which d is the density of gold (59 atoms nm−3).36−39
Table 1. TGA and Elemental Analysis Results for Syntheses of Gold Nanoparticle Cored Triazine Dendrimers elemental analysis (%) sample
TGA organic wt (%)
C
H
N
S
AuNPs-ATP AuNPs-ATP-CC1 G1 AuNPs-ATP-CC2 G2
5 7 10 13 14
5.36 8.44 10.13 14.51 17.66
0.66 0.99 1.28 1.65 2.43
0.25 0.62 1.19 1.79 2.38
2.28 2.22 2.25 2.24 3.18
An increase in the amount of carbon, hydrogen, and nitrogen in each step and sulfur in the last step confirmed the successful synthesis of the dendrimer. After preparation and characterization of AuNPs-TD, the ruthenium hydride catalyst was immobilized on it to produce [Ru-H@AuNPs-TD] (Scheme 2). The FT-IR spectrum of the complex [RuHCl(CO)(PPh3)3] showed characteristic bands at 2013 cm−1 (Ru−H) and 1922 cm−1 (CO) (Figure S3 in the Supporting Information). According to Figure S4 in the Supporting Information, the FT-IR spectrum of AuNPs-TD showed no aforementioned band in this region. The appearance of bands at 2052 and 1947 cm−1 in the FT-IR spectrum of [Ru-H@AuNPs-TD] indicates that the ruthenium complex has been successfully supported on the dendrimer.
⎛π ⎞ NAu = d⎜ ⎟D3 ⎝6⎠
(1)
In this work, the average core diameter (D) is about 33 nm. Therefore, the value of NAu is 1.11 × 106 atoms in a hypothetical cluster. D
DOI: 10.1021/acs.organomet.6b00163 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
Figure 6. SEM image and elemental mapping of Au and Ru.
Figure 4. SEM-EDX spectra of (a) AuNPs-TD and (b) [Ru-H@AuNPsTD].
Figure 5. XPS spectra of the Ru 3d region showing the peaks corresponding to Ru 3d5/2 (orange line) and C 1s (pink line) in [Ru-H@ AuNPs-TD]. The black line is the average of these two peaks.
The ruthenium and gold contents of [Ru-H@AuNPs-TD], measured by ICP, showed values of about 0.293 mmol of Ru/g (1.76 × 1020 atoms/g) and 0.84 mmol of Au/g (5.05 × 1020 atoms/g). By consideration of the results of ICP analysis for Au and NAu in a hypothetical cluster, the number of Au particles is
Figure 7. TEM images of the [Ru-H@AuNPs-TD] at different scales.
E
DOI: 10.1021/acs.organomet.6b00163 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
Table 2. Optimization of the Conditions in the Suzuki− Miyaura Cross-Coupling of 4-Iodoanisole with Phenylboronic Acid Catalyzed by [Ru-H@AuNPs-TD]a
entry
Figure 8. Histogram of Au particle size distribution.
about 4.55 × 1014. Therefore, the number of Ru atoms per Au nanoparticle is 3.9 × 105. Catalytic Experiments. Suzuki−Miyaura Cross-Coupling Reactions of Aryl Halides with Arylboronic Acids. First, the reaction conditions such as amount of catalyst, kinds of solvent and base, and temperature were optimized in the Suzuki− Miyaura cross-coupling reaction of 4-iodoanisole with phenylboronic acid in the presence of [Ru-H@AuNPs-TD]. The results are summarized in Table 2. In the optimization of the amount of [Ru-H@AuNPs-TD] catalyst, the highest yield was obtained in the presence of 0.011 mmol of [Ru-H@AuNPs-TD] on the basis of the Ru content. Smaller amounts of [Ru-H@AuNPs-TD] gave lower yields, while when the amount was increased to 0.014 mmol, no significant increase was observed in the yield (entries 2−5). It is noteworthy that in the absence of [Ru-H@AuNPsTD] no product was detected in the reaction mixture (entry 1). Among the solvents used, aqueous toluene (toluene/H2O 2/1 v/ v) was chosen as the reaction medium (entries 4 and 6−11). It seems that the water attacks phenylboronic acid to form Ph− B(OH)3 and facilitates the elimination of B(OH)3. In addition, the model reaction was carried out at different temperatures; on the basis of the obtained results, the optimum reaction temperature was found to be 80 °C (Table 2, entries 4 and 12−14). Then, the same reaction was carried out in the presence of different organic and inorganic bases; K2CO3 was found to be the most effective base (entries 4 and 15−17). Therefore, it was concluded that the optimum reaction conditions involved 4-iodoanisole (1 mmol), phenylboronic acid (1.1 mmol), K2CO3 (1.5 mmol), and the [Ru-H@AuNPsTD] catalyst (0.011 mmol, based on the Ru content) in toluene/ H2O (2/1) at 80 °C under an Ar atmosphere. In order to show the role of the ruthenium species, G2 (AuNPs-TD) was used as a catalyst in the model reaction. The results showed that the amount of biphenyl derivative was as same as that in a blank experiment in the absence of [Ru-H@ AuNPs-TD] catalyst. This showed that the catalytically active species is Ru and the Au nanoparticles play a supporting role. The generality of this method was investigated in the Suzuki− Miyaura cross-coupling of different aryl halides with arylboronic acids (phenyl and 4-methoxyphenylboronic acids) in the presence of [Ru-H@AuNPs-TD] under the optimized reaction conditions. The results, which are summarized in Table 3, showed that in this catalytic system the biphenyl derivatives were produced in good to moderate yields. On the basis of these
amt of catalyst (mmol)
base
1
0
K2CO3
2
0.005
K2CO3
3
0.008
K2CO3
4
0.011
K2CO3
5
0.014
K2CO3
6 7 8
0.011 0.011 0.011
K2CO3 K2CO3 K2CO3
9 10 11
0.011 0.011 0.011
K2CO3 K2CO3 K2CO3
12
0.011
K2CO3
13
0.011
K2CO3
14
0.011
K2CO3
15
0.011
Na2CO3
16
0.011
NaOH
17
0.011
Et3N
solvent toluene/H2O (2/1) toluene/H2O (2/1) toluene/H2O (2/1) toluene/H2O (2/1) toluene/H2O (2/1) DMF H2O DMF/H2O (2/1) EtOH toluene toluene/H2O (1/1) toluene/H2O (2/1) toluene/H2O (2/1) toluene/H2O (2/1) toluene/H2O (2/1) toluene/H2O (2/1) toluene/H2O (2/1)
T (°C)
yield (%)b
80
0
80
36
80
57
80
85
80
87
80 80 80
34 28 31
80 80 80
35 53 68
room temp 50
21
100
88
80
67
80
46
80
28
54
a
Reactions were performed using 4-iodoanisole (1 mmol), phenylboronic acid (1.1 mmol), [Ru-H@AuNPs-TD], solvent (3 mL), and base (1.5 mmol) under an Ar atmosphere for 3.5 h. bIsolated yield.
studies, under the same reaction conditions, the aryl iodides were found to be more reactive than other aryl halides. Using the same amount of [Ru-H@AuNPs-TD] catalyst, the aryl chlorides needed higher temperature and longer reaction times. To the best of our knowledge, this is the first report on the application of a ruthenium hydride catalyst in the Suzuki−Miyaura C−C coupling reaction. Under the same reaction conditions described for heterogeneous [Ru-H@AuNPs-TD] catalyst, some of the reactions were repeated in the presence of homogeneous [RuHCl(CO)(PPh3)3] catalyst (Table 3, entries 1, 5, and 9). The results showed that the heterogeneous [Ru-H@AuNPs-TD] catalyst is more efficient than the homogeneous catalyst. This can be related to the dispersion on a nanomatrix in which the catalytically active species are isolated. Synthesis of Diaryl Ketones by the Reaction of Arylboronic Acids with Aryl Aldehydes Catalyzed by [Ru-H@AuNPs-TD]. The catalytic activity of this [Ru-H@AuNPs-TD] catalyst was further studied in the synthesis of diaryl ketones by the reaction of arylboronic acids with aryl aldehydes (Ald). Initially, the reaction of phenylboronic acid (PBA) with 4-nitrobenzaldehyde was chosen as a model reaction to optimize the experimental conditions such as ratio of phenylboronic acid to aldehyde, F
DOI: 10.1021/acs.organomet.6b00163 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
presence of 0.023 mmol of [Ru-H@AuNPs-TD] (entries 5 and 7−10). Note that, in the absence of [Ru-H@AuNPs-TD], no appreciable amount of product was obtained (entry 10). Different solvents such as toluene THF, DMF, DMSO, methanol (with trace amounts of water), and water were used as reaction media. As can be seen, the highest yield was obtained in toluene (Table 4, entries 5 and 11−15). Finally, when different bases such as K2CO3, NaOH, KOH, and NEt3 were used in the model reaction, K2CO3 was selected as base. Therefore, the optimized reaction conditions are phenylboronic acid (1 mmol), aryl aldehyde (2 mmol), [Ru-H@ AuNPs-TD] (0.023 mmol, based on the Ru content), toluene (3 mL), K2CO3 (2 mmol), and H2O (1 mmol) at 110 °C under an argon atmosphere for 8 h (the role of water is in the hydrolysis of ruthenium alkoxide, which is an intermediate in the catalytic cycle).14 In order to explore the scope and generality of the reaction, arylboronic acids were reacted with different aromatic aldehydes under the optimized conditions. The results are shown in Table 5. The reactions of arylboronic acids with aliphatic aldehydes were sluggish in giving the corresponding ketones, but aromatic
Table 3. Suzuki−Miyaura Cross-Coupling of Aryl Halides with Arylboronic Acid Catalyzed by the [Ru-H@AuNPs-TD] Recyclable Nanocatalysta
entry
R′
R″
X
time (h)
yield (%)b,c
1 2 3 4 5 6 7 8 9 10
MeO H MeO H H H Me MeO H H
H H 4-MeO 4-MeO H 4-MeO H H H 4-MeO
I I I I Br Br Br Br Cl Cl
3.5 3 4.5 5 7 8.5 9.5 10 16d 20d
85 (68) 84 80 81 78 (31) 75 71 75 67 (16) 65
a Reactions (unless specified otherwise) were performed using aryl halide (1 mmol), phenylboronic acid (1.1 mmol), K2CO3 (1.5 mmol), [Ru-H@AuNPs-TD] (0.011 mmol, based on the Ru content), and toluene/H2O (3 mL) at 80 °C under an Ar atmosphere. bIsolated yield. cThe yields in parentheses refer to the homogeneous [RuHCl(CO)(PPh3)3] catalyst using the same amount of [Ru-H@ AuNPs-TD] catalyst. dThe reaction was performed under reflux conditions.
Table 5. Cross-Coupling Reaction of Aromatic Aldehydes with Phenylboronic Acid Catalyzed by [Ru-H@AuNPs-TD]a
amount of [Ru-H@AuNPs-TD] catalyst, and kind of solvent and base. The results are summarized in Table 4. First, the PBA/Ald ratio was optimized and the best results were detected with a 1/2 PBA/Ald ratio (Table 4, entries 1−6). Then, the amount of [Ru-H@AuNPs-TD] was also optimized in the model reaction and a higher yield was produced in the Table 4. Optimization of the Conditions in the CrossCoupling Reaction of 4-Nitrobenzaldehyde with Phenylboronic Acid Catalyzed by [Ru-H@AuNPs-TD]a
entry PBA/Ald 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
1/1 1.5/1 2/1 1/1.5 1/2 1/2.5 1/2 1/2 1/2 1/2 1/2 1/2 1/2 1/2 1/2 1/2 1/2 1/2
amt of catalyst (mmol)
solvent
base
yield (%)b
0.023 0.023 0.023 0.023 0.023 0.023 0.014 0.008 0.038 0 0.023 0.023 0.023 0.023 0.023 0.023 0.023 0.023
toluene toluene toluene toluene toluene toluene toluene toluene toluene toluene THF DMF DMSO methanol H2O toluene toluene toluene
K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 NaOH KOH NEt3
61 54 42 74 85 88 58 24 90 0 66 45 38 34 26 51 42 36 a
Reaction conditions: phenylboronic acid (1 mmol), aldehyde (2 mmol), [Ru-H@AuNPs-TD] (0.023 mmol, based on the Ru content), K2CO3 (2 mmol), and H2O (1 mmol) at 110 °C under an argon atmosphere for 8 h. bIsolated yield.
a
Reaction conditions: phenylboronic acid, 4-nitrobenzaldehyde, [RuH@AuNPs-TD], base (2 mmol), H2O (1 mmol), and solvent (3 mL) at 110 °C under an argon atmosphere for 8 h. bIsolated yield. G
DOI: 10.1021/acs.organomet.6b00163 Organometallics XXXX, XXX, XXX−XXX
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Organometallics aldehydes gave good yields. Nitro-substituted aryl aldehydes reacted with arylboronic acids more easily, and the corresponding biaryl ketones were produced in high yields; under the same reaction conditions, phenylboronic acids bearing a methoxy group gave higher yields. The possibility of the presence of an induction period in both reactions was investigated. The reaction profiles (Figure 9) showed no obvious induction period in the synthesis of diaryl ketones (Figure 9A) and in the Suzuki−Miyaura cross-coupling reaction (Figure 9B).
Table 6. Results of the [Ru-H@AuNPs-TD] Catalyst Recoverya Suzuki−Miyaura reaction
diaryl ketone synthesis
run
yield (%)b
Ru leached (%)
yield (%)b
Ru leached (%)
1 2 3 4
85 78 72 68
4 2 1 0
85 74 68 62
6 4 1 0
a Conditions for the Suzuki−Miyaura reaction: 4-iodoanisole (1 mmol), phenylboronic acid (1.1 mmol), K2CO3 (1.5 mmol), [RuH@AuNPs-TD] (0.011 mmol, based on the Ru content), and toluene/H2O (3 mL) at 80 °C under an Ar atmosphere. Conditions for the diaryl ketone synthesis: phenylboronic acid (1 mmol), 4nitrobenzaldehyde (2 mmol), [Ru-H@AuNPs-TD] (0.023 mmol, based on the Ru content), K2CO3 (2 mmol), and H2O (1 mmol) at 110 °C under an Ar atmosphere for 8 h. bIsolated yield.
Miyaura reaction and 0.251 mmol/g (about 11% of the initial Ru is leached) in the synthesis of diaryl ketones. The decrease in the yield during the consecutive cycles can be attributed to the leaching of ruthenium hydride catalyst during each cycle. In both reactions, the hot-filtration test was also performed. In the synthesis of diaryl ketone, [Ru-H@AuNPs-TD] was filtered off in the model reaction after 3 h (33% conversion), and the filtrate was then stirred at 110 °C. The results showed that the reaction progress was only 4%. For the Suzuki−Miyaura crosscoupling reaction, [Ru-H@AuNPs-TD] was filtered off after 90 min (31% conversion) and the filtrate was then stirred at 80 °C. In this case, the reaction progress was only 3%. The nature of the recovered [Ru-H@AuNPs-TD] catalyst was studied by FT-IR spectroscopy. The presence of bands at 2052 and 1946 cm−1 in the FT-IR spectrum of the recovered [Ru-H@ AuNPs-TD] catalyst (Figure S5 in the Supporting Information) confirmed that the ruthenium complex retained its initial structure during the catalytic experiments.
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CONCLUSION In summary, we introduced a new heterogeneous catalytic system based on a ruthenium hydride complex immobilized on gold nanoparticles functionalized with triazine dendrimers and investigated its catalytic activity in Suzuki−Miyaura crosscoupling reactions of phenylboronic acids and aryl halides and syntheses of diaryl ketones by the reaction of arylboronic acids and aromatic aldehydes. Under the optimized reaction conditions, all reactions afforded the related biphenyl derivatives and diaryl ketones in good to excellent yields. Moreover, this catalyst, [Ru-H@AuNPs-TD], can be well-dispersed in the reaction medium, conveniently separated from the reaction mixture, and reused several times without significant loss of its activity.
Figure 9. Reaction profile for (A) synthesis of diaryl ketones by the reaction of phenylboronic acid (1 mmol), 4-nitrobenzaldehyde (2 mmol), [Ru-H@AuNPs-TD] (0.023 mmol, based on the Ru content), K2CO3 (2 mmol), and H2O (1 mmol) at 110 °C under an Ar atmosphere and (B) Suzuki−Miyaura cross-coupling reaction of 4iodoanisole (1 mmol), phenylboronic acid (1.1 mmol), K2CO3 (1.5 mmol), [Ru-H@AuNPs-TD] (0.011 mmol, based on the Ru content), and toluene/H2O (3 mL) at 80 °C under an Ar atmosphere.
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ASSOCIATED CONTENT
S Supporting Information *
Catalyst Reusability. The reusability of [Ru-H@AuNPsTD] was studied as described in the Experimental Section. The results showed that [Ru-H@AuNPs-TD] could be reused several times with a slight loss in its activity (Table 6). The amount of [Ru-H@AuNPs-TD] catalyst leached was measured by ICP. The results showed that the amount of ruthenium in the recovered [Ru-H@AuNPs-TD] catalyst after the fourth run was 0.263 mmol/g (about 9% of the initial Ru is leached) in the Suzuki−
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00163. FT-IR spectra of [AuNPs-TD], [RuHCl(CO)(PPh3)3], AuNPs-TD, [Ru-H@AuNPs-TD], and recovered catalyst and a thermogravimetric analysis curve of the catalyst (PDF) H
DOI: 10.1021/acs.organomet.6b00163 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
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AUTHOR INFORMATION
Corresponding Authors
*E-mail for M.M.:
[email protected]. *E-mail for S.T.:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
This paper is dedicated to Professor Nasser Iranpoor on the occasion of his 62th birthday and his honorable retirement. The authors are grateful to the Research Council of the University of Isfahan for financial support of this work.
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DOI: 10.1021/acs.organomet.6b00163 Organometallics XXXX, XXX, XXX−XXX
