Inorganica Chimica Acta 359 (2006) 1837–1844 www.elsevier.com/locate/ica
Optically active amphiphilic hyperbranched polyglycerols as templates for palladium nanoparticles Yu Chen
a,b
, Holger Frey
b,*
, Ralf Thomann c, Salah-Eddine Stiriba
d,*
a Department of Chemistry,Tianjin University, 300072 Tianjin, PR China Institut fu¨r Organische Chemie, Johannes Gutenberg-Universita¨t, Duesbergweg 10-14, 55099 Mainz, Germany Institut fu¨r Makromolekulare Chemie und Freiburger Materialforschungszentrum FMF Albert-Ludwigs-Universita¨t Freiburg, Stefan-Meier Str. 21/31, D-79104 Freiburg, Germany d Instituto de Ciencia Molecular, Universidad de Valencia, Vicent Andre´s Estelle´s s/n, Burjassot, 46100 Valencia, Spain b
c
Received 12 June 2005; accepted 29 June 2005 Available online 10 August 2005 Special ICA issue to celebrate Prof. Gerard van KotenÕs scientific career. Dedicated to our friend Professor Gerard van Koten in recognition of his excellent scientific career.
Abstract We report a systematic study on the encapsulation of palladium nanoparticles in optically active amphiphilic hyperbranched polyglycerols with different optical signs and different degrees of polymerization, namely ()-P(G40C160.5) 1 and (+)P(G73C160.5) 2. Several issues have been addressed here: (a) relatively wide size distributions (1–5 nm) of palladium nanoparticles have been achieved, (b) a remarkable template effect (1, DPn = 40, 1.2 ± 0.1 nm; 2, DPn = 73, 2.3 ± 0.1 nm average particle size) has been observed using TEM technique, as shown by the particle size dependent on the degree of polymerization of the polymers, (c) NaBH4 is found to be a convenient reducing agent to produce small particle size compared with gaseous hydrogen, (d) catalytic Heck reaction of 2,3-dihydrofuran and aryl triflate has been tested successfully without enantiocontrol. 2005 Elsevier B.V. All rights reserved. Keywords: Chiral hyperbranched; Core–shell structure; Palladium; Nanoparticles; Catalysis
1. Introduction The last two decades have witnessed a great interest in the field of nanoscopic metal particles due to their large potential applications for the development of high performance materials such as plastics, electronic, optical devices, magnetic recording media, quantum dots, fuel catalysts as well as nanocatalysts in both homoge*
Corresponding authors. Tel.: +49 6131 39 24078; fax: +49 6131 39 26106 (H. Frey), Tel.: +49 96 354 3052; fax: +34 96 354 4939 (S.-E. Stiriba). E-mail addresses:
[email protected] (H. Frey), salah.stiriba@ uv.es (S.-E. Stiriba). 0020-1693/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2005.06.067
neous and heterogeneous phases [1]. The high performances of nanosized metal particles are heavily related to their unique electronic motion in matter and their extremely large surface areas, flexibility and hardness [2]. Of particular interest is the nanocatalysis field, which involves catalysis of organic reactions by means of nanoparticles [3]. In connection with the development of this field, the studies have been devoted to: (a) the support of the nanoparticles on solid surfaces resulting in heterogeneous noble catalysts for gas-phase reactions, a process used mainly to overcome aggregation phenomenon [4], (b) the preparation and stabilization of catalytic nanoparticles in homogeneous solutions using the method of ‘‘polymer nanotemplating’’, which
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consists of inorganic metal ions encapsulation followed by conventional controlled reduction [5]. Whilst the first concept provides solution to the aggregation problems; mass transport issues and limited control over metal nanoparticle size and dispersion arise, limiting thereby the efficiency of the catalytic system. The second approach using polymeric materials can lead to homogeneously stable, well-defined and controllable nanoscale particles in the range of 1–10 nm [5,6]. Recently, dendrimers and hyperbranched polymers have become powerful model systems to stabilize and template inorganic crystals and nanoparticles [7,8]. Some representative examples of these systems are poly(amidoamine) (PAMAM) dendrimers used for palladium nanoparticle (1–2 nm) stabilization and templating as well as for platinum inorganic nanocrystal catalysts applied in hydrogenation reactions of alkenes in organic, aqueous as well as fluorous media [5,9]. A second example concerns the use of polyaryl ether disulfide dendrons assembled around gold (2–4 nm) and palladium nanoparticles (average diameter 2 nm) [10]. The latter nanoparticles were efficiently tested in Heck and Suzuki coupling reactions, inhibiting agglomeration without affecting the catalytic reactivity of the nanoparticles [10b]. A third dendritic template was the covalently derivatized poly(propylene imine) (PPI) dendrimers, which have been utilized for palladium nanoparticle (2–3 nm in diameter) preparation and effectively catalyzed carbon–carbon bond coupling reactions [11]. On the other hand, only very recently, few hyperbranched polymers have been employed as encapsulating nanoreactors for metal nanoparticles [12,13]. Examples of such application are illustrated in the use of amphiphilic hyperbranched polyglycerols with core–shell structure for the encapsulation and ‘‘templating’’ of catalytically active palladium nanoparticles with an average size between 2 and 5 nm [12a]. The resulting encapsulated nanosize particles have been applied in a model catalytic hydrogenation reaction of cyclohexane. In addition, their recovery using membrane filtration also has been studied [12b]. A second example consisted of the stabilization of catalytically active palladium metallic solutions by means of hyperbranched aramids [13a] and their successful application in unsaturated substrate hydrogenation [13b]. In contrast to dendrimers, hyperbranched polymers are straightforwardly prepared in only one-step process. However, they lack structural perfection and narrow polydispersity like dendrimers. To circumvent this issue, it was found that controlled polymerization of glycidol by ring-opening multibranching polymerization (ROMBP) under slow monomer addition results in the formation of highly hydrophilic flexible aliphatic polyether-polyols with narrow polydispersity (1.2 < Mw/ Mn < 1.5) [14]. Partial esterification of the hydroxyl
groups (40–60%) of these hyperbranched polymers with fatty acids yields organic soluble amphiphilic hyperbranched polymers ‘‘molecular nanocapsules’’ with an inverted micelle-type architecture, capable of irreversibly encapsulating various guests such as water-soluble dye molecules and polar transition metal complexes in their hydrophilic interior in liquid–liquid phase transfer [15,16a,16b]. Our ongoing program aimed to design new easily accessible and recoverable catalyst containing (optically active) hyperbranched polyglycerols [16] and the versatility of amphiphilic hyperbranched nanocapsules to act as nanoreactors in their racemic forms [12a,16a,16b,16c] prompts us to systematically investigate the preparation of catalytically active palladium nanoparticle, using optically active esterified hyperbranched polyglycerols and to study the size control of these palladium nanoparticles as a function of the nanocapsuleÕs molecular weight (i.e., the degree of polymerization of polyglycerol cores). Yet, because of the lack in comparative studies between different reduction conditions (i.e., reducing agent) for the preparation of palladium nanoparticles using essentially dendritic backbone, we decided to address the variation of the reduction conditions on the particle size control. Here, we systematically describe the encapsulation and size control of palladium nanoparticles using optically active esterified hyperbranched polyglycerols with core–shell structures displaying a hydrophilic core and hydrophobic shell. We also report the effect of the variation of the reduction conditions on particle size control as well as preliminary catalytic results of the performance of the palladium nanoparticles in a model Heck reaction. To the best of our knowledge, this is the first report on metallic nanoparticles encapsulated in amphiphilic optically active hyperbranched polymers and the optimization of reduction conditions to control their particle size and distribution.
2. Experimental 2.1. Syntheses of amphiphilic hyperbranched polyglycerols The optically active esterified hyperbranched polyglycerols ()-P(G40C160.5) 1 and (+)-P(G73C160.5) 2 were prepared starting from ()-PG40 (Mn = 3000, Mw/ Mn = 1.3; initiator: bis(2,3-dihydroxypropyl)undecenylamine) and (+)-PG73 (Mn = 5500, Mw/Mn = 1.6; initiator: trimethylolpropane (TMP)), respectively, as described in [16c]. P(G106C160.6) was prepared as reported elsewhere [16a]. PdCl2 and NaBH4 were purchased from Aldrich. All solvents were dried and distilled prior to use.
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2.2. UV–Vis measurements
3. Results and discussion
UV–Vis characterizations were recorded on a Varian Cary I spectrophotometer.
3.1. On the chiral amphiphilic hyperbranched polyglycerols
2.3. Thermal analysis
Partial esterification of hydroxyl groups (50%) of the optically active polar hyperbranched polyglycerols ()PG40 (Mn = 3000, Mw/Mn = 1.3; initiator: bis(2,3dihydroxypropyl)undecenylamine) and (+)-PG73 (Mn = 5500, Mw/Mn = 1.6; initiator: trimethylolpropane (TMP)) with palmitoyl chloride results in amphiphilic chiral hyperbranched polyglycerols, namely ()P(G40C160.5) 1 and (+)-P(G73C160.5) 2, respectively, as illustrated in Fig. 1. The resulting polymers are soluble in apolar solvents such as toluene and chloroform. Both polymers 1 and 2 have shown optical activity as demonstrated by chiral dichroism traces and specific rotation measurements (Table 1). Thermal properties of polymers 1 and 2 have also been studied, using differential scanning calorimetry (DSC) as shown in Table 1. The solid state properties of all samples were similar. The melting points of polymers 1 and 2 were in the range of 47–48 C, with similar melting enthalpies DH = 41–47 J/g; glass transitions were observed at 15 to 25 C. Hyperbranched polymers 1 and 2 exhibit similar thermal bulk properties. This is expected, since the alkyl side chains determine the crystallization behavior [15].
DSC measurements were carried out on a Perkin– Elmer 7 series thermal analysis system in the temperature range 100 to 100 C at a heating rate of 10 K/min. The melting point of indium (156 C) was used for calibration. 2.4. Palladium nanoparticle preparation Palladium (II) chloride (3.2 mg, 17.9 lmol; 4OH groups/Pd) was dissolved in a toluene solution (5 mL) of partially esterified chiral hyperbranched polyglycerol (+)-P(G73C160.5) 2 (Mn = 15 270 g/mol) with continuous stirring. Once the solution becomes homogeneously yellow, NaBH4 (1 M, 500 ll) in methanol was added dropwise. The solution turns brown, then dark in 10–30 min and was kept under nitrogen for further 30 min. The reduction was monitored by UV–Vis spectroscopy, using quartz cuvettes. The methanol solvent was removed under vacuum and dried, resulting in a black residue, which was washed several times with ethanol (2 · 5 mL) and diethyl ether (2 · 5 mL) to remove NaCl. The same procedure was used for the preparation of ()-P(G40C160.5) 1/Pd and P(G106C160.6)/Pd. Palladium nanoparticles were very soluble in THF, toluene, chloroform and dichloromethane and are insoluble in ethanol and pentane. 2.5. TEM characterization TEM analyses were carried out on a LEO 912 Omega apparatus using an acceleration voltage of 120 kV. Samples were prepared by applying a drop of the toluene palladium nanoparticle encapsulated in polymers 1 and 2 to a carbon-coated grid, followed by drying the sample in air. 2.6. Catalytic Heck reaction The reaction was carried out in toluene under nitrogen atmosphere using: PhOTf/2,3-dihydrofuran/ i Pr2NEt/(polymer (+)-2/Pd)/ = 1.0/5.0/3.0/0.01 at 30 C. The product formation was followed by TLC over the course of the reaction, using ethyl acetate/hexane (2/1, v/v) mixture as eluent. Dialysis (MWCO 1000, Sigma) was carried out using chloroform as solvent to separate the Heck reaction product from the palladium nanoparticles containing amphiphilic hyperbranched polyglycerol.
3.2. PdCl2 in the presence of polymers 1 and 2 PdCl2 complex, well known to be insoluble in apolar solvent such as toluene, was easily and homogeneously dissolved in toluene solutions containing polymer 1 or 2 resulting then in yellow solutions. A molar ratio PdCl2/polymer (3.8 · 103 M) corresponding to 4 OH groups of the polymer/Pd was employed. Upon addition of a dilute solution of NaBH4 in methanol (1 M), the yellow solution turned brown, then dark solution after 10–30 min. The reduction process was also monitored by UV–Vis spectroscopy to follow the coordination of OH groups with palladium (II) ions. Indeed, before reduction two absorptions are detected, a pronounced one at 350 nm (O–Pd) and a second slight one at 450 nm assigned to d–d transition of Pd2+. The reduction of palladium (II) is accompanied by the disappearance of the characteristic absorption for Pd2+ ions and the appearance of a smooth increasing absorption at increasing energy (see Fig. 2). Proof of complete reduction of the palladium (II) ions in the hydrophilic interior of either polymer 1 or 2 could be easily judged by the color change from brown to a dark-brown solution [17]. The palladium metal nanoparticle solutions were stable over several months under nitrogen without observation of any kind of precipitation or suspension
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Y. Chen et al. / Inorganica Chimica Acta 359 (2006) 1837–1844 O
O C15H31
O
O
O
O
OH
O
HO
HO O
O X
C15H31
core
On
O
O
C15H31
O
O HO
O
O
O
O O
OH
O
C15H31
O
C15H31
O O
O O
O
C15H31
O
HO
C15H31
O HO
C15H31
O O
OH
(-)-P(G40C160.5) 1 (+)-P(G73C160.5) 2
Core =
8
OH
OH OH
N
OH OH
OH
Fig. 1. Amphiphilic optically active hyperbranched polyglycerols 1 and 2 with their respective cores (see text).
Table 1 Physical properties of the optically active esterified hyperbranched polyglycerols 1 and 2a Polymer
Mn
()-P(G40C160.5) 1 (+)-P(G73C160.5) 2
Mw/Mn
8140 15 270
Tm (C)
1.2 1.4
48 47
Tg (C) b
n.d. n.d.b
DH (J/g)
c ½a25 D
a (%)d
41 47
0.16 +0.72
50 50
a
Nomenclature P(GxCYz): x = DPn of polyglycerol, Y: number of carbon atoms of the palmitic alkyl chain, z: degree of alkyl substitution per hydroxyl group. b n.d. = not determined. c Angle rotations were measured in chloroform at concentration (c = 0.5, CHCl3) and are given in deg dm1 g cm3. d Degree of substitution per hydroxyl group.
(vide infra). The UV–Vis spectra were similar to those recorded for freshly prepared ones. Toluene was pumped out and the resulting oily dark product washed with ethanol and ether, dried under vacuum and dissolved once again in other organic solvents, such as THF, toluene, chloroform and dichloromethane, for further studies.
2.2 2.0
1.2
1.8
1.0
reduction t = 120 min reduction t = 30 min reduction t = 10 min
0.8
Ab s
Absorbance
1.6 1.4
0.6
0.4
1.2
0.2
0.0 300
1.0
400
500
600
λ (nm)
3.3. TEM analysis of palladium nanoparticles
0.8 0.6
after reduction
0.4
before reduction
0.2 0.0 300
400
500
600
λ (nm)
Fig. 2. UV–Vis spectra of toluene solutions containing polymer 1 (3.8 · 103 M) in the presence of PdCl2 before and after reduction. Absorption spectra (inset) of polymer 1/Pd during the reduction process at different times.
TEM-images of the palladium nanoparticles encapsulated in chiral polymers 1 and 2 were obtained by depositing a dilute solution on carbon-coated electron microscopy grid, followed by evaporation of the solvent. The presence of the palladium metal nanoparticle surfaces in the hyperbranched materials renders them directly visible by TEM and leads to the possibility to study their shape and size distribution. Whilst several dendrimers stabilizing metal colloids and metal
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nanoclusters have been extensively reported, images from hyperbranched polymers containing metal nanoparticles are less investigated. In the case of metal particles stabilized in chiral polymers, no report including TEM images has been published until now, except a protocol reported several years ago on platinum particles protected by the protonated chiral alkaloid dihydrocinchonidine molecules [18]. Fig. 3(a) and (b) show high-resolution TEM images of ()-P(G40C160.5) 1/Pd and (b) (+)-P(G73C160.5) 2/ Pd, and their histograms, respectively. It can be seen from the TEM images that the particles exhibit a relatively wide size distribution (1–5 nm). The particle average sizes obtained from the histogram in both cases were found to be 1.2 ± 0.1 nm for ()-P(G40C160.5) 1/Pd and 2.3 ± 0.1 nm for (+)-P(G73C160.5) 2/Pd. The standard deviations r which indicate the magnitude of dispersity were calculated from the histograms, corresponding to 1.20 and 0.95 nm for ()-P(G40C160.5) 1/Pd and (+)P(G73C160.5) 2/Pd, respectively, a further proof of the relative large size distribution. Of interest was the palladium particle size change upon changing the molecular weight of the polymer. Then, whereas polymer (+)-
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P(G73C160.5) 2 with a degree of polymerization DPn = 73 results in 2.3 ± 0.1 nm particle average size, polymer ()-P(G40C160.5) 1 with low degree of polymerization DPn = 40 gives rise to small palladium particles with 1.2 ± 0.1 nm average size. The templating of metal nanoparticles observed with polymers 1 and 2 is an evidence of the unimolecular micellar structure, featuring esterified amphiphilic hyperbranched polyglycerol polymers. Dendrimers have displayed such template effect as demonstrated in various studies [5,7a,9]. The size of several metal nanoparticles was varied as a function of the dendrimer generations. Concerning hyperbranched polymer, only two reports by Mecking et al. [12a,12c] have reasonably claimed such property using racemic esterified hyperbranched polyglycerol polymers. 3.4. Effect of the reducing agents In the course of our experimental work on templating palladium nanoparticles in chiral amphiphilic hyperbranched polyglycerols, we kept in mind the results obtained in previous work using racemic esterified hyperbranched polyglycerols [12a]. Accordingly, the
Fig. 3. TEM images and histogram of optically active amphiphilic hyperbranched polyglycerols from a toluene solution, illustrating the size and shape distribution of the encapsulated metal nanoparticles of: (a) ()-P(G40C160.5) 1/Pd, and (b) (+)-P(G73C160.5) 2/Pd.
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Table 2 Comparison of different reducing agents on palladium particle sizea Polymer
DPn
Reducing agent
Average particle size (nm)
Reference
()-P(G40C160.5) 1 (+)-P(G73C160.5) 2 (+)-P(G73C160.5) 2 P(G106C160.6)b P(G63C160.6)
40 73 73 106 63
NaBH4 NaBH4 H2c NaBH4 H2c
1.2 ± 0.1 2.3 ± 0.1 7.7 ± 0.2 2.4 ± 0.1 5.2 ± 1.8
this work this work this work this work [12a]
a
All reductions have been performed in toluene. Methanol was used to prepare NaBH4 solution (1 M). b For synthesis of this polymer, see reference [16a]. c Schlenk tube was filled with hydrogen (1.5 bar) then the solutions were kept stirring at room temperature overnight.
esterified hyperbranched polyglycerol P(G63C160.6) with DPn = 63 encapsulated palladium nanoparticles with (2–10 nm) size distribution and 5.2 ± 1.8 nm average size [12a], whereas in our cases, polymer (+)P(G73C160.5) 2 with higher degree of polymerization (DPn = 74) stabilizes particles in the range of 1–5 nm with an average size of 2.3 ± 0.1 nm. This unusual difference should certainly be attributed to the difference of the reducing agents, since the palladium nanoparticles in the present work have been prepared using NaBH4 while those previously reported using P(G63C160.6) (DPn = 63) have been performed under gaseous hydrogen conditions [12a] (see Table 2). It is well documented that metal nanoparticles were obtained mainly by conventional chemical reduction of metal salts [1d]. In the case of silver, for example, it has been experimentally verified that stronger reducing agents produce smaller nuclei [19], which grow during the ripening process to yield colloidal metal particles in the size range of (1–50) nm because of the agglomeration of zerovalent nuclei or collisions of already formed nuclei with reduced metal ions [20]. Exposure of poly-
mers 1 and 2 encapsulating palladium (II) precursors (E0 = +0.62 V) to gaseous hydrogen (E0 = +0.00 V) at 1.5 bar pressure and the maintenance of the reaction solution later under hydrogen atmosphere (1 atm.) for quite a long time (overnight – 2 days) seem to produce large size palladium nanocrystals (2–10 nm) [12a]. Very earlier reports have demonstrated the close relationship between the particle size and rate of hydrogenation, particularly for palladium colloidal catalysts stabilized by polyvinyl alcohols [21a]. A further recent example involving the well defined homopolymer, poly(vinylpyrrolidone) (PVP), stabilizing palladium colloids has also reported previously (1–3 h) and the reduction pressure (1–4 bar) remarkably affect the palladium particle size [21b]. Furthermore, dendrimer-encapsulated nearly monodisperse palladium nanoparticles have been achieved using mainly NaBH4 (E0 = 0.481V). In order to compare the effect of the variation of reducing agents on the particle size and dispersity, amphiphilic hyperbranched polyglycerol stabilizing palladium particles (+)-P(G73C160.5) 2/Pd were prepared using hydrogen rather than NaBH4. Interestingly, we observe the same tendency for the palladium particle size and distribution with those previously reported using P(G63C160.6)/Pd. Fig. 4 and the corresponding histogram show large particle size displaying an average size of 7.7 nm (DPn = 73). The feasibility of NaBH4 to produce controllable small size palladium nanoparticles leads to the extension of our investigation to other amphiphilic hyperbranched polyglycerol with a high degree of polymerization. To this end, molecular nanocapsules P(G106C160.6) (DPn = 106) were chosen, and effectively small palladium nanoparticles with an average size of 2.4 ± 0.1 nm were obtained as illustrated in the TEM image and histogram (Fig. 5). Interestingly, it seems that NaBH4 is rather convenient than H2 to produce small size palladium particles encapsulated in the amphiphilic hyper-
Fig. 4. TEM images and histogram of (+)-P(G73C160.5) 2/Pd nanoparticles using hydrogen as reducing medium.
Y. Chen et al. / Inorganica Chimica Acta 359 (2006) 1837–1844
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Fig. 5. TEM image and histogram of P(G106C160.6)/Pd nanoparticles using NaBH4 as reducing medium.
+ PhOTf
polymer(+)-2 / Pd i
Pr2NEt, Toluene 30C, 4d
O
Ph
O
Scheme 1. Catalytic Heck reaction.
branched polyglycerols in both racemic and chiral forms. Catalysis stands at the front of the most promising and main potential application of palladium nanoparticles stabilized by polymer backbone. With chiral polymers containing palladium nanoparticles in hand, one expects to achieve enantioselective control in asymmetric organic methodologies. Very preliminary tests applying (+)-P(G73C160.5) 2/Pd in the intermolecular Heck reaction of 2,3-dihydrofuran and an aryl triflate result in the formation of the 2-aryl-2,3-dihydrofuran at moderate yield (35%) without any enantiomeric excess. The moderate yield may be attributed to the mass transport to the particle surface due to the long hydrophobic chain. On the other hand, rigid chiral backbones are likely required in the polymer to cooperate in the chiral induction. Further developments including time reactions, solvent effect, change of degree of functionalization as well as modification of hyperbranched polyglycerols with short aliphatic chains are under study (see Scheme 1).
4. Conclusions In summary, several directions have been achieved in this investigation and are presented as follows: two optically active amphiphilic hyperbranched polyglycerols have been used to produce palladium nanoparticles, namely ()-P(G40C160.5) 1/Pd and (+)-P(G73C160.5) 2/ Pd, with a remarkable average particle size ranging from 1.2 to 2.3 ± 0.1 nm. Particle size was found to be degree
of polymerization dependent, indicating that molecular nanocapsules 1 and 2 display a template effect. A clear relationship between particle size and reducing agent was verified. NaBH4 was found to be a very convenient reducing agent, acting at room temperature and 1 atm, to afford small particle size than reduction with hydrogen (1–1.5 bar) followed by long time exposure (1 atm.). Finally, the palladium particles were catalytically effective in Heck reaction with the absence of enantioselective control. To the best of our knowledge, this is the first report on the use of chiral amphiphilic hyperbranched polymer with core–shell structure to template palladium nanoparticles and the demonstration of the effect of different reducing agents on palladium particle size.
Acknowledgments This work was supported by the Fonds der Chemischen Industrie in Germany (H.F.) and by the Ministerio de Educacio´n y Ciencia (MEC) in Spain for (S.-E.S.) in the context of ‘‘Ramo´n y Cajal’’ national program. Alexander von Humboldt Stiftung is also aknowledged for support to (S.E.S.) in the context of the follow-up program.
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