Polyphenylenepyridyl Dendrons with Functional Periphery and Focal

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Polyphenylenepyridyl Dendrons with Functional Periphery and Focal Points: Syntheses and Applications Nina V. Kuchkina,† Ekaterina Yu. Yuzik-Klimova,† Svetlana A. Sorokina,† Alexander S. Peregudov,† Dmitri Yu. Antonov,† Samuel H. Gage,‡ Bethany S. Boris,‡ Linda Z. Nikoshvili,§ Esther M. Sulman,§ David Gene Morgan,‡ Waleed E. Mahmoud,∥ Ahmed A. Al-Ghamdi,∥ Lyudmila M. Bronstein,*,‡,∥ and Zinaida B. Shifrina*,† †

A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilov St., Moscow, 119991 Russia Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States § Department of Biotechnology and Chemistry, Tver State Technical University, 22 A. Nikitina St., 170026, Tver, Russia ∥ Department of Physics, King Abdulaziz University, Faculty of Science, Jeddah, Saudi Arabia ‡

S Supporting Information *

ABSTRACT: For the first time we report syntheses of a family of functional polyphenylenepyridyl dendrons with different generations and structures such as focal groups, periphery, and a combination of phenylene and pyridyl moieties in the dendron interior using a Diels−Alder approach and a divergent method. The dendron structure and composition were confirmed using NMR spectroscopy, MALDI-TOF mass spectrometry, FTIR, and elemental analysis. As a proof of concept that these dendrons can be successfully used for the development of nanocomposites, synthesis of iron oxide nanoparticles was carried out in the presence of thermally stable dendrons as capping molecules followed by formation of Pd NPs in the dendron shells. This resulted in magnetically recoverable catalysts exhibiting exceptional performance in selective hydrogenation of dimethylethynylcarbinol (DMEC) to dimethylvinylcarbinol (DMVC).

1. INTRODUCTION Dendrons and dendrimers are unique macromolecules with a regular and highly branched three-dimensional architecture and defined molecular structure.1−8 They have been used in catalysis,9−11 nanomedicine,12−14 sensing,15,16 and optoelectronic applications.17 They were also chosen as templates or capping molecules for inorganic nanoparticle (NP) formation.16 Nowadays, dendritic polymers are important tools for new inventions in materials chemistry, medicine, and nanotechnology. Dendrons and dendrimers are obtained in iterative sequences of reaction steps, in which each additional iteration leads to a higher generation molecule. At the present time a wide assortment of repetitive reaction sequences are known for construction of dendrimers.1,18−20 Among numerous dendrimers synthesized to date, dendrimers consisting of exclusively aromatic moieties have a number of advantages.21−23 In contrast to conformationally flexible dendrimers containing single bonds, aromatic dendrimers are shape persistent with a rigid structure as only confined rotations around the biaryl bonds are possible. These dendrimers possess a high chemical and thermal stability and demonstrate solubility in a wide range of organic solvents. However, if such dendrimers consists of only phenylene rings, they rarely can be used as capping molecules for © 2013 American Chemical Society

nanoparticle formation due to functionality limitations. Earlier we reported syntheses of the family of polyphenylenepyridyl dendrimers (PPPDs) and the formation of Pd NPs stabilized by such dendrimers.24 These systems showed excellent stability and promising catalytic properties in selective hydrogenation of dehydrolinalool (a C10 acetylene alcohol).25 Although numerous active and selective homogeneous catalysts are described in the literature, less than 20% of such processes are used in industry, where heterogeneous catalysis is clearly prevailing.26 This should be ascribed to a complexity of separation of homogeneous catalysts from reaction solutions. The separation can be performed, as is thoroughly discussed in a recent review,15 but it is time- and energy-consuming. In recent years catalytic species and magnetic NPs were combined in one material to allow robust and inexpensive magnetic separation of a catalyst.27−34 The flexible chain dendron functionalized/stabilized magnetic NPs have been synthesized and employed for biomedical35−37 and catalytic applications.38−40 In the latter case, a metallodendron shell was built on a presynthesized magnetic NP either through dendron Received: May 19, 2013 Revised: July 4, 2013 Published: July 24, 2013 5890

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Scheme 1. Divergent Synthesis of Dendrons with Anhydride, o-Dicarboxyl (DC), and o-Dicarboxylate (DCNa) Focal Groups

grafting38 or via a multistep synthesis of a dendron on the magnetic NP core followed by the complexation with catalytic species.39,40 In neither case were magnetic NPs synthesized in the presence of thermally stable, rigid functional dendrons allowing incorporation of catalytic species. In this paper for the first time we report syntheses of a family of functional polyphenylenepyridyl dendrons with various focal groups, periphery, and a combination of phenylene and pyridyl moieties in the dendron inner shell using a universal, wellestablished Diels−Alder approach and a divergent method. The focal groups and the shells were varied to create multiple functionalities in a single dendron. Multiple functionalities allow different functions. In particular, such focal groups as carboxyl, dicarboxyl, and dicarboxylate have high affinity to metal oxide surfaces (e.g., magnetic iron oxide NPs). Pyridyl groups bind to catalytic metal. When these two functions are combined with rigid structure as well as high chemical and thermal stability of polyphenylenepyridyl dendrons, the richness of novel nanocomposites can be created. As a proof of

concept, synthesis of iron oxide nanoparticles was carried out in the presence of thermally stable dendrons as capping molecules followed by formation of Pd NPs in the dendron shells. This led to magnetically recoverable catalysts showing exceptional performance in selective hydrogenation of dimethylethynylcarbinol (DMEC) to dimethylvinylcarbinol (DMVC).

2. EXPERIMENTAL PART 2.1. Materials. The materials used are described in the Supporting Information. 2.2. Synthetic Procedures. 2.2.1. Synthesis of Tetraarylcyclopentadienones. The tetraarylcyclopentadienones 1, 2, 4, 5, and 6 were synthesized starting from the corresponding 1,2-diketones in a Knoevenagel condensation (see Supporting Information). 2,3,4,5Tetraphenylcyclopenta-2,4-dienone (3) was obtained from Aldrich and used as received. 2.2.2. General Procedure for Diels−Alder Reaction between Tetraarylcyclopentadienones and Arylethynyl Derivatives. In a typical procedure, the reaction Schlenk flask, equipped with a condenser and a magnetic stir bar and containing 1.5−2 mol of cyclopentadienone per 1 ethynyl group and arylethynyl derivatives in 5891

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o-xylene (or diphenyl ether) (4.5 × 10−3 mol/L), was evacuated and filled with argon three times using a Schlenk line and then heated to 144 °C (bp of o-xylene) or 175 °C in the case of diphenyl ether (for high-generation dendrons, the diphenyl ether was used instead of oxylene). The reaction was stirred for a desired time. The reaction was controlled by TLC, and the reaction time was dependent on the generation and varied for dendrons from 4 h for G1 to 12 h for G2 and to 24 h for G3. After cooling the reaction solution was precipitated by acetonitrile. Then the solid was separated by filtration, washed with acetonitrile, reprecipitated twice from CH2Cl2 by addition of acetonitrile, and dried in vacuum at 100 °C (for details see Supporting Information). 2.2.3. Desilylation of Triisopropylsilyl (TiPS)-Substituted Dendrons. In a typical procedure, to 2.3 × 10−4 M solution of triisopropylsilyl-substituted dendron in a mixture of THF and benzene (ratio of THF/benzene was 3.33/1) 1 M solution of tetrabutylammonium fluoride trihydrate (3 mol of Bu4NF per 1 triisopropylsilyl group) in THF was added. After 8 h stirring, CH2Cl2 (5 mL) was added, and the organic phase was washed with water and dried over MgSO4. After complete evaporation of the solvent under reduced pressure, the crude product was purified by reprecipitation from CH2Cl2 to hexane, separated by filtration, washed, and dried. The procedure of desilylation of anhydride derivatives was shortened to 4 h to compare with the general procedure for this process to avoid the possible hydrolysis of an anhydride group in the presence of tetrabutylammonium fluoride trihydrate. Syntheses of tetrasubstituted cyclopentadienones, dendrons with anhydride, o-dicarboxyl (DC-dendrons), o-dicarboxylate (DCNadendron), and monocarboxyl focal groups are described in the Supporting Information. The NMR, MALDI-TOF, and elemental analysis data for all compounds are also presented in the Supporting Information. 2.2.4. Synthesis of 16-Fe3O4. The synthesis of iron oxide nanoparticles in the presence of dendron 16 (Scheme 1) was carried out in the following way. The three-neck round-bottom flask (with elongated necks) equipped with a magnetic stir bar, a reflux condenser, and two septa, one of which contains a temperature probe protected with a glass shield, was loaded with 0.353 g (1 mmol) of Fe(acac)3, 0.3 g (0.22 mmol) of 16, 0.8252 g (3 mmol) of 1,2-hexadecandiol, and 7 mL of benzyl ether. The flask was placed in a Glas-Col heating mantle attached to a digital temperature controller which in turn was placed on a magnetic stirrer. The flask was degassed five times using “evacuation-filling with argon” cycles with filling with argon afterward. The temperature was raised at 5 °C/min to 200 °C. Upon reaching this temperature, the flask was heated for 2 h to better solubilize the sodium salt dendron. The temperature was then raised at 5 °C/min to 300 °C, and upon reaching this temperature, the flask was heated for 2 h. The flask was then removed from the heating mantle and allowed to cool to room temperature. The reaction solution was precipitated into ethanol, washed several times with ethanol and acetone, until the supernatant was colorless, and then dissolved in chloroform. 2.2.5. Interaction of NPs Prepared in the Presence of Dendrons with Palladium Acetate. The NP solution (0.5 mg/mL) was placed in a flask and purged with argon under stirring. The 1 mg/mL solution of Pd acetate in chloroform was also purged with argon and added to the NP solution dropwise. The weight ratio of Pd acetate to NPs was 2/3. After addition the flask was closed and left stirring overnight. The sample was separated with a rare earth magnet and washed with chloroform 2−3 times, each time using magnetic separation. Then the sample was dried for 30 min in vacuum and dispersed in 5 mL of ethanol using sonication. 2.2.6. Reduction of Pd Complexes with Hydrogen. Ethanol dispersion (5 mL) of Pd-containing sample (20 mg) was placed in a three-neck round-bottom flask equipped with a stir bar, a septum with a long needle, and a reflux condenser. The ethanol dispersion was diluted with 5 mL of distilled water. Then the reaction mixture was purged with argon under stirring for 1 h and then bubbled with hydrogen under stirring for 2 h. After that the solution was transferred to a vial, the catalyst was separated with a magnet and washed several

times with ethanol. Then the sample was dried in a vacuum oven overnight and dispersed in chloroform. 2.3. Characterization. 1H NMR and 13C NMR spectra were recorded on a Bruker Avance 600 spectrometer and Avance 400 spectrometer. Chemical shifts are given in parts per million (ppm), using the solvent signal as a reference. Mass spectral analyses were carried out on the ZAB2-SE-FPD (VG Analytical) and Bruker Biflex III MALDI-TOF instruments. MALDI-TOF mass spectra were measured by using a 337 nm nitrogen laser and tetracyanoquinodimethane (TCNQ) as a matrix. Electron-transparent NP specimens for transmission electron microscopy (TEM) were prepared by placing a drop of dilute solution onto a carbon-coated Cu grid. Images were acquired at an accelerating voltage of 80 kV on a JEOL JEM1010 transmission electron microscope. Images were analyzed with the National Institute of Health developed image-processing package ImageJ to estimate NP diameters. Between 150 and 300 NPs were used for this analysis. Scanning TEM (STEM) images and energy-dispersive X-ray spectroscopy maps (EDS) were acquired at accelerating voltage 300 kV on a JEOL 3200FS transmission electron microscope equipped with an Oxford Instruments INCA EDS system. The same TEM grids were used for both analyses. Fourier transform infrared (FTIR) spectra were recorded on a Nicolet 510P FT-IR spectrometer. The samples were prepared by evaporating their chloroform solutions on a KBr disk. X-ray diffraction (XRD) patterns were collected on a Scintag theta− theta powder diffractometer with a Cu Kα source (0.154 nm). The sample was prepared by evaporation of the chloroform solution on a silicon sample holder. Thermal gravimetric analysis (TGA) was performed on TGAQ5000 IR manufactured by TA Instruments. About 2−3 mg of a dendron was placed into a 100 μL platinum pan. The experiments were carried out upon heating to 1000 °C at a rate 10.0 °C/min. 2.4. Catalytic Studies. Catalytic testing was carried out in a 60 mL isothermal glass batch reactor installed in a shaker and connected to a gasometrical buret (for hydrogen consumption control). The total volume of liquid phase was 30 mL. Reaction conditions were ambient hydrogen pressure, stirring rate 850 shakings per minute, and temperature 90 °C. Toluene was used as a solvent. The catalyst was separated after the reaction using a rare earth magnet. Samples were periodically taken and analyzed via GC-MS (Shimadzu GCMS-QP2010S) equipped with a capillary column HP1MS (30 m × 0.25 mm i.d., 0.25 μm film thickness). Helium was used as a carrier gas at pressure of 53.6 kPa and linear velocity of 36.3 cm/s. Analysis conditions: oven temperature 60 °C (isothermal), injector temperature 280 °C, ion source temperature 260 °C, a range from 10 up to 200 m/z.

3. RESULTS AND DISCUSSION 3.1. Syntheses of Dendrons with Different Focal Points and Periphery. Syntheses of pyridyl-containing dendrons with various focal groups were performed by a divergent approach via a repetitive sequence of the Diels−Alder cycloaddition of tetraarylcyclopentadienones to ethynes (a and c) and of the deprotection of triisopropylsilyl (TiPS) substituted alkynes (b) as shown in Schemes 1−3. Syntheses of dendrons with anhydride and o-dicarboxyl focal groups were performed according to Scheme 1 similar to the procedure established by us earlier for pyridyl containing dendrimers (see Supporting Information).24 The anhydride group was chosen as a focal point because of its easy conversion to an o-dicarboxyl group. The control of the reactant conversion was provided by TLC. To generate phenyl or pyridine decorated periphery of the dendron, the reaction with 2,3,4,5-tetraphenylcyclopenta-2,4-dienone (3), 2,5-dipyrid-2-yl3,4-diphenylcyclopenta-2,4-dienone (4) or 2,3,4,5-tetrapyrid-2yl-cyclopenta-2,4-dienone (5) was performed in the final stage 5892

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Scheme 2. Synthesis of Dendrons with a Carboxyl Focal Group

A NMR C−H correlation carried out for some dendrons provided the most reliable information on their structure. Because of partial overlapping of the proton chemical shifts, the assignment of the aromatic protons has been carried from the COSY spectrum (Figure S3 and the text in the Supporting Information). The more detailed assignment of proton and carbon signals in the 1H and 13C NMR spectra were performed using the 2D NMR spectra (Figure S4 and the text in the Supporting Information). The syntheses of family of dendrons with a monocarboxyl focal group were carried out in analogous manner as for other dendrons according to Scheme 2. In this case, 3-ethynylbenzoic acid was chosen as a starting monomer providing the monocarboxyl focal point. Varying the building blocks (1, 2, 3, 4, or 5) through the dendron synthesis, phenyl- and pyridylcontaining dendritic molecules of different composition and generations were achieved (Scheme 2). The solubility of all dendrons with a single carboxyl focal group was similar to that of dendrons with an anhydride focal group. Similarly to other dendrons, the 1H NMR spectroscopy and MALDI-TOF mass spectrometry were applied for characterization. For all dendrons, the chemical shifts of the protons of the pentaphenyl-substituted benzene rings were detected clearly in 1H NMR spectra for each generation layer as well as the signals of α-protons of the pyridine moieties, which are shifted toward a low field. Additionally, a carboxyl group proton signal was detected at low field. As an example, Figure S5 shows the 1H NMR spectrum of COOH-G1-Py-(ethyne)2 (20). Using MALDI-TOF mass spectrometry, we were able to determine the molecular masses of dendrons and to prove that the dendrons obtained are monodisperse. Any incomplete Diels−Alder cycloaddition through the dendron synthesis

of the dendron synthesis (Scheme 1). To convert the anhydride group to the o-dicarboxyl (DC) one, alkaline hydrolysis was carried out according to ref 41. The completeness of the hydrolysis was confirmed by FTIR. The spectrum of the DC dendron contains the bands characteristic of a carboxyl group: OH at 3200−3300 cm−1 and CO at 1724 cm−1. At the same time, the FTIR spectrum does not contain the bands at 1780 and 1830 cm−1 characteristic of CO of the anhydride cycle (Figure S1). The both type of dendrons are soluble in THF, benzene, and chlorinated hydrocarbons. In addition, the odicarboxyl dendrons are soluble in alcohols while the anhydride dendrons are not. The further transformation of dicarboxyl groups to dicarboxylate ones (DCNa) was performed according to Scheme 1. The reaction occurred smoothly at room temperature with a high yield and was executed for dendrons of the second (16) and third (17, 18) generations with different periphery. The completeness of the conversion was verified by FTIR and mass spectrometry. The FTIR spectrum of DCNa contains two bands assigned to a carboxylate anion at 1421 cm−1 (symmetrical stretching vibration) and 1572 cm−1 (asymmetrical stretching vibration). At the same time, the bands from CO of the carboxyl group at 1720 cm−1 and from the carbonyls of anhydride at 1780 and 1830 cm−1 are absent in the spectra of DCNas (see text and Figure S2 (red curve)). These data allowed us to confirm the formation of carboxylate derivatives of the dendrons. The solubility of the dendrons with DCNa focal groups are close to that of the dendrons with DC groups. The structure and purity of the intermediates and target compounds were confirmed by NMR, mass spectrometry data, elemental analysis, and FTIR (Supporting Information). 5893

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Figure 1. MALDI-TOF mass spectrum of the third-generation dendron, COOH-G3-Py-Py-PyPh (25).

Scheme 3. Synthesis of Dendrons Decorated with Dodecyl Groups in the Periphery

would result in a mass difference of 719 or a multiple of that (in the case of building block 1 the mass of a building block unit minus the molecular mass of CO) and might be easily detectable by this method. Figure 1 shows the example of MALDI-TOF mass spectrum of COOH-G3-Py-Py-PyPh (25). The molecular peak at 2798.14 g/mol is in a good agreement with the mass calculated for 25 (2799.41 g/mol). As an example of rigid core−flexible shell dendritic architecture phenylenepyridyl dendrons with long hydrocarbon (C12H25) tails in the periphery were also synthesized. For this, the building block 6 was employed in the final step of the monocarboxyl dendron synthesis (Scheme 3). The reactions went smoothly, and the target products were obtained with high yields. The dendron characterization was performed by NMR and mass spectrometry as well as by elemental analysis (see Supporting Information). The combination of analytical data confirmed the structure and composition of the products

synthesized. As an example, Figure 2 shows the MALDI-TOF mass spectrum of COOH-G3-C12H25 (28). The molecular peaks 4146 g/mol are in good agreement with the mass calculated for 28 (4146.01 g/mol). Unlike the other dendrons synthesized in this work, the dodecyl-decorated dendrons were soluble in nonpolar solvents, such as hexane and heptane. The coexistence of polar (pyridine, carboxyl) and nonpolar (phenyl, dodecyl) moieties in one dendritic molecule may allow micelle-like behavior in selective solvents, thus extending possibilities for ordered nanocomposites when combined with inorganic nanoparticles. 3.2. Syntheses of Iron Oxide Nanoparticles and Magnetically Recoverable Catalysts. For a proof of concept that functional polyphenylenepyridyl dendrons can be employed for syntheses of iron oxide nanoparticles and magnetically recoverable catalysts, we used second-generation dendrons with the mixed phenylene−pyridyl periphery and 5894

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Figure 4. Schematic representation of the adsorption of 16 on the iron oxide NP surface.

To quantitatively determine the amount of dendrons on the NP surface, we carried out TGA (under inert atmosphere) of pure dendron 16 and 16-Fe3O4 NPs (Figure S7). The weight loss below 150 °C (3%) is associated with absorbed water and/ or solvents and is not taken into account in assessment of total weight losses. For 16, the 5% weight loss is observed at 500 °C, revealing a remarkably high thermal stability of this dendron (Figure S7a). The total weight loss upon heating to 1000 °C is 35%, indicating formation of 62% of coke for the pure dendron 16. The TGA trace of the 16-Fe3O4 NPs (Figure S7b) shows 26% weight loss in the same conditions. Then the dendron content is between 26% and 42%. The latter number is obtained with the assumption that the dendron coke formation on iron oxide NPs is the same (which is probably not the case) as that of pure dendron. In any case, TGA data confirm that dendrons are present on the NP surface, but their IR vibrations are limited due to adsorption of pyridyl groups. The 16-Fe3O4 NPs were reacted with palladium acetate in a chloroform solution. This leads to formation of aggregates due to interparticle interactions as was reported by us for magnetic NPs coated with linolenic and linoenic acids.43 After hydrogen reduction, Pd NPs are formed within magnetic aggregates presented in Figure 5. The aggregates formed do not exceed 500 nm but allow fast and efficient magnetic separation. The dark-field image in Figure 6a clearly shows bright small dots laying on the top of grayer and larger spots. Bright dots should belong to more electronically dense particles (Pd in this case), while grayer spots presumably belong to Fe3O4 NPs. The elemental Fe and Pd EDS maps (Figures 6b and 6c) confirm this assumption and clearly show that Pd NPs have been formed in the shells of iron oxide NPs. The Pd NPs measure 1.6 nm in diameter with a standard deviation of 16% 3.3. Catalytic Behavior. The magnetic nanocomposites based on 16-Fe3O4 and containing Pd NPs have been tested in a model reaction of selective hydrogenation of DMEC to DMVC (Scheme 4), which is an intermediate in syntheses of fragrant substances and vitamins E and K.44 Results of the testing of the 16-Fe3O4-PdAc and 16-Fe3O4Pd-NP catalysts are shown in Table 1. In the case of the unreduced 16-Fe3O4-PdAc catalyst, the selectivity was only 92.8% and the activity was rather low. The kinetic curve (Figure 7) shows a long induction period which may be due to catalytic site formation. After the reaction, the catalyst was completely separated from the reaction mixture using a rare earth magnet, washed with toluene and chloroform, and placed back in the reactor. In the repeated use, the

Figure 2. MALDI TOF spectrum of the third-generation dendron COOH-G3-C12H25 (28) decorated with dodecyl groups in the periphery.

sodium o-dicarboxylate group in the focal point ((COONa)2G2-PhPy, 16) as capping molecules. Figure S6 shows a TEM image of NPs with a mean diameter 16.6 nm and 19.9% standard deviation obtained by thermal decomposition of iron acetylacetonate in the presence of 16. The XRD profile of these NPs is shown in Figure 3. The positions and intensity of the

Figure 3. XRD profile of the 16-Fe3O4 NPs.

signals are typical for those of magnetite;42 however, considering the similarity of XRD profiles of magnetite (Fe3O4) and maghemite (γ-Fe2O3) NPs due to line broadening, this is a tentative assignment. The FTIR spectrum of the second-generation polyphenylenepyridyl dendrons with a disodium salt group in the focal point (16) shows a set of bands which can be associated with polyaromatic structure (Figure S2 and the text in the Supporting Information). In the FTIR spectrum of 16-Fe3O4 NPs the strongest band at 574 cm−1 belongs to Fe−O species. On the other hand, the bands belonging to dendrons are strongly reduced in intensity or merely absent. This suggests two possible outcomes: either the dendron coverage is low or dendrons are adsorbed on the NP surface not only via their focal groups but also through their phenylenepyridyl moieties as shown in Figure 4. 5895

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Scheme 4. Hydrogenation of DMEC to DMVCa

2-Methylbutane-2-ol is a side product. “Cat” stands for the catalyst of hydrogenation.

a

Table 1. Results of DMEC Hydrogenation Using PdContaining Magnetically Recoverable Catalysts sample 16-Fe3O4-PdAc 1st use 16-Fe3O4-PdAc 2nd use 16-Fe3O4-Pd-NP 1st use 16-Fe3O4-Pd-NP 2nd use 16-Fe3O4-Pd-NP 3rd use 16-Fe3O4-Pd-NP 1st use 2% Pd/CaCO3

Figure 5. TEM image of the 16-Fe3O4-Pd-NP sample.

induction period was eliminated, indicating that the catalytic sites were already formed, but the selectivity and activity remained low. In contrast to the catalyst containing unreduced Pd acetate species, the catalyst after reduction when the Pd NPs were formed (Figure 6) demonstrated the activity increase by a factor of 4 and the increase of selectivity to 98.2% (Figure 7). Figure 7 also demonstrates the absence of the induction period. In the repeated use, the activity decreased by about 30%, but selectivity slightly increased to 98.6%. Further catalyst reuse (third cycle) resulted in no activity or selectivity loss (Table 1), revealing good recyclability of the catalyst. For this catalytic system, we also explored the increase of the DMEC loading (Table 1). Surprisingly, the increase of the DMEC loading allowed the 1.3-fold increase of the activity, while the selectivity remained unchanged. This can be associated with disappearance of diffusion limitations for the DMEC mass transport with the higher DMEC loading. It is noteworthy that 16-Fe3O4 NPs containing no Pd species showed no activity in the DMEC hydrogenation.

g DMEC/ g Cat

activity, g DMEC/ (g Cat·s)

selectivity (%)

conv (%)

476.2

0.047

92.8

97.1

476.2

0.051

93.5

97.1

476.2

0.196

98.2

97.6

476.2

0.131

98.6

98.7

476.2

0.130

98.6

98.6

952.4

0.250

98.0

99.0

200.0

0.040

94.6

99.0

Figure 7. Kinetic curves of the DMVC accumulation with the 16Fe3O4-PdAc (unreduced) catalyst in the first (1) and second (2) uses and with the 16-Fe3O4-Pd-NP (reduced) catalyst in the first (3) and second (4) uses.

Figure 6. STEM dark-field image (a) and elemental maps for Fe (b) and Pd (c) obtained by EDS. 5896

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Comparison of the 16-Fe3O4-Pd-NP catalyst and the commercial hydrogenation Lindlar catalyst, 2%-Pd/CaCO3, demonstrates that the catalyst reported in this work is more active and selective and allows easy magnetic separation, thus making it promising for application in selective hydrogenation of fine organic synthesis.

4. CONCLUSION Novel aromatic dendrons of the first, second, and third generations with an anhydride, o-dicarboxyl, o-dicarboxylate, and carboxyl focal groups, different periphery, and a combination of phenylene and pyridine moieties in the dendron interior were synthesized via a divergent method and characterized by NMR spectroscopy, MALDI-TOF mass spectrometry, FTIR, and elemental analysis. Using a universal Diels−Alder approach with different starting compounds, determining the nature of the focal group, and different building blocks, we established control over multiple functionalities of the dendrons which are essential for creation of a new family of nanocomposites. As a proof of concept, we demonstrated that the second-generation dendrons with the mixed phenylene−pyridyl periphery and sodium o-dicarboxylate group in the focal point (16) can serve as capping molecules in magnetite NP formation and stabilize Pd NPs in the dendron shell. The catalysts formed exhibited excellent performance in hydrogenation of DMEC and easy magnetic recovery for repeated use, thus demonstrating both fundamental importance and applicability of the dendrons synthesized. In our continuing work we will carry out a comparative study of the influence of dendron structure, such as focal points, periphery, middle layer, and generations on the iron oxide NP and catalyst formation and properties.

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ASSOCIATED CONTENT

S Supporting Information *

Synthetic procedures, NMR, MALDI-TOF, elemental analysis, a TEM image, and FTIR data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Z.B.S.); [email protected] (L.M.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of this work was provided in part by funding from the European Community’s Seventh Framework Programme [FP7/2007-2013] under grant agreement CP-IP 246095, the Ministry of Education and Science of Russia and the Russian Foundation for Basic research under grants 11-0300064 and 12-03-31057, and the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under grant GR-33-7. L.B., W.M., and A.A., therefore, acknowledge with thanks DSR technical and financial support.

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