Labile Imidazolium Salt Protected Palladium Nanoparticles - Langmuir

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Labile Imidazolium Salt Protected Palladium Nanoparticles Jung-Tang Lu, Joseph C. Y. Lin, Meng-Che Lin, Nageshwar D. Khupse, and Ivan J. B. Lin* Department of Chemistry, National Dong-Hwa University, Shoufeng, Hualien 974, Taiwan S Supporting Information *

ABSTRACT: An imidazolium (Im) salt with two long alkyl substituents at N atoms is employed to prepare cubelike palladium nanoparticles (PdNPs). The bilayer nature of the capped Im salts is characterized by thermogravimetric analysis and NMR studies. These capped Im salts are labile, as evidenced by their displacement reaction with dimethylaminopyridine, and the observation of fast exchange between those free and capped Im salts on the NMR time scale. NMR results also show that these capped Im salts exhibit different diffusion rates, and interesting spinning rate dependent chemical shifts. These cubelike PdNPs could catalyze the Suzuki coupling of aryl chlorides and boronic acids with high yields in 10 min, even at room temperature.

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INTRODUCTION Pd-catalyzed Suzuki cross-coupling reaction between aryl halide and boronic acid is an important tool in organic synthesis.1−5 Over the past decade, colloidal PdNPs are considered to be a promising catalyst for the Suzuki reaction.6−8 It has been well recognized that the catalytic activity of a nanomaterial depends primarily on the particle size and shape.8−11 How these crosscoupling reactions occur in the presence of PdNPs is also an important issue that has been intensively discussed. The reactions have been proposed to operate at the NP surface,11,12 and on the other hand, homogeneous reactions occurring in solution via leaching palladium(0) or dissolution of the activated palladium(II) species from PdNPs have also been proposed.13 Moreover, the influence of capping agents on the catalytic activity has been noticed and reviewed.14,15 An inverse correlation of stability and catalytic activity is often observed, and therefore, it is crucial to acquire a balance between the stability and catalytic activity in performing optimum nanoparticle catalysis.13 Various capping agents have been used to stabilize PdNPs for C−C coupling reactions,7 including the use of Im salts for PdNP-catalyzed Suzuki coupling reaction.16−26 Calculations have been performed to understand the mode and energy of the interaction between metal nanoparticales and Im salts. Results suggest that it is the ring head of the Im salt that sits on the metal surface,27,28 and the interaction energy between the Im salt and metal surface is smaller than that between the quaternary ammonium salt and metal surface. Indeed a better catalytic efficiency for the Im salt stabilized Au nanorods has been found for the reduction of nitro compounds.26 High-resolution magic angle spinning spectroscopy (HRMAS) NMR has been shown as reliable in resolving the inhomogeneous system with liquidlike dynamics,29 and successful in the characterization of bionanomaterials,30,31 gold nanoparticles,32 and magnetic nanoparticles.27,33 When © XXXX American Chemical Society

the sample is spun at the magic angle (54.7° relative to the direction of the main magnetic field), the line-broadening effects, due to the magnetic susceptibilities and chemical shifts anisotropy, are significantly diminished.34 Additionally, 2D diffusion-ordered spectroscopy (DOSY) NMR, which combines both chemical shift and diffusion coefficient scale, is able to obtain different diffusion rates of mixtures with highly overlapped NMR spectra35 and to discriminate the organization of the orgnaic ligands in colloidal metal NPs.36−38 In this work, we demonstrate a simple method to prepare cubelike colloidal PdNPs, which are capped by 1,3-di(dodecyl)imidazolium chloride, [C12,C12−Im]Cl. Thermogravimetric analysis (TGA), HRMAS, and DOSY NMR are used to characterize the structure of Im salts on the PdNPs. Various interactions between Im salts and PdNPs could be differentiated. The labile nature of this capping agent is also studied by the displacement reaction with dimethylaminopyridine. These PdNPs show excellent catalytic activity toward Suzuki cross-coupling reaction under ambient conditions.

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RESULTS AND DISCUSSION [C12,C12−Im]Cl (structure formula in Scheme 1) is a nearroom temperature ionic liquid crystal having a melting temperature of 47 °C and is an amphiphile, soluble in both aqueous and many polar nonaqueous solvents.39 Previously, we reported that the reaction of [C12,C12−Im]Cl with PdCl2 in a molar ratio of 2:1 could produce [C12,C12−Im]2[PdCl4], a liquid crystal compound, in which the imidazolium cations adopt a U-shape interdigitated bilayer packing in the solid state.40 Although this palladate salt could be used as precursor to prepare PdNPs, a colloidal solution of PdNPs is produced easily through reduction of a mixture of [C12,C12−Im]Cl and Received: April 27, 2014 Revised: August 6, 2014

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dx.doi.org/10.1021/la501569e | Langmuir XXXX, XXX, XXX−XXX

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Scheme 1. Proposed Form of the Im Salts Capped on the Surface of PdNPs

To examine the surface structure of the capped Im salts, three different sample treatments were performed. The first sample denoted as PdNPs-1 was generated via rinsing the freshly obtained solid Pd@[C12,C12−Im]Cl with DI water, ethanol, and acetone sequentially. This process was repeated twice. To prepare PdNPs-2, Pd@[C12 ,C12 −Im]Cl was dissolved in dichloromethane (CH2Cl2), then washed several times with DI water to remove the free Im salts. After that, the CH2Cl2 solvent was removed under vacuum. For PdNPs-3, a CH2Cl2 solution of Pd@[C12,C12−Im]Cl was first treated with ethanol, followed by centrifugation. The structure of the capping agents on PdNPs 1−3 was first examined via TGA, which has been often employed to investigate the surface structure of surfactants capped on metal nanoparticles.44−47 Plots of TGA and their derivatives are given in Figure 2. The

PdCl2 (5:1 molar ratio) in deionized (DI) water/p-dioxane (1:1 by volume) by hydrogen gas. The slight excess of [C12,C12−Im] Cl employed in the process is to enhance the stability of the PdNPs, and the use of this solvent system is because it has been commonly used in the Suzuki cross coupling reactions.41,42 Solid sample of PdNPs could be obtained via centrifugation from the DI water/p-dioxane solution and is denoted as Pd@[C12,C12−Im]Cl. The size and morphology of this solid sample as observed under TEM are shown in Figure 1. These

Figure 1. A TEM image of the cubelike Pd@[C12,C12−Im]Cl with an average length of ca. 7 nm. The inset (upper right) shows the particle size distribution from the experiment of DLS. The upper left inset shows the ED pattern taken from an individual PdNP.

particles display cubelike morphology with an average length of ca.7 nm, consistent with the size distribution obtained via dynamic light scattering (DLS) study (upper right inset Figure 1). The {100} facets of the cubelike PdNPs are evidenced by the electron diffraction pattern shown in the upper left inset in Figure 1 and further by high-resolution TEM and powder X-ray diffraction in Figure S1 in the Supporting Information. For comparison, [C1,C12−Im]Cl, which has only one long alkyl chain, was also employed to study its ability to stabilize PdNPs. Under similar reaction conditions, agglomeration occurred in a short time. Very recently, [C6,C6−Im] cation with two hexyl substituents was used to stabilize the PdNPs and no long-term stability was reported.43 In the present study, the Im salt with two symmetrical dodecyl chains was found to be a good protecting agent for the fabrication of PdNPs.

Figure 2. Thermogravimetric profiles of (a) the PdNPs-1, (b) PdNPs2, (c) PdNPs-3, and their derivatives (DTGA).

thermogram of PdNPs-1 shows broad peaks between 150 and 200 °C (a total weight loss of 15%), followed by two consecutive major peaks of roughly equal weight losses at 211 °C (20%) and 230 °C (23%). The TGA profile of PdNPs-2 shows a trace of weight loss around 178 °C, together with two major equal weight loss peaks at 212 °C (29%) and 233 °C B


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Figure 3. 1H-HRMAS spectra for (a) free [C12,C12−Im]Cl, (b) PdNPs-1, (c) PdNPs-2, and (d) PdNPs-3 at MAS rates of 3 kHz in benzene-d6. The asterisks indicate the residual solvents and spinning side bands.

Table 1. Relative Intensities of 1H-HRMAS NMR of Imidazolium-Based PdNPs at 3 kHz in Benzene-d6 by Taking the Signal Integration of C2−H as 1.0

(30%). Because both the profiles of PdNPs-1 and -2 have a set of two major equal weight loss peaks between 210 and 240 °C, and these two sets of peaks happen at almost identical temperatures, we propose that these desorption events are from the bilayer [C12,C12−Im]Cl capped on the PdNPs with an equal molar ratio for the outer and inner salts. Those desorption peaks occurring at lower temperatures (150−200 °C) are attributed to the suspended Im salts, which sit topside and interact weakly with the bilayer. The sample of PdNPs-1 has 15% of the suspended salts, but only a trace for the PdNPs2. The structure of Im salts in PdNPs-1 is similar to that observed for the cetyltrimethylammonium bromide (CTAB) capped on gold nanorod, in which three types of surfactants are observed: the inner and outer layers of the bilayer surfactants and the suspended surfactants on top of the bilayer.44 On the other hand, the sample of PdNPs-3 has only a major desorption peak between 150 and 185 °C (8%), presumably due to the monolayered [C12,C12−Im]Cl on the PdNP surface; the desorption band at ∼100 °C could be due to the trapped solvent. The small percentage of the monocapped [C12,C12− Im]Cl in PdNPs-3 suggests that the PdNP surface is not completely covered by the stabilizer. The long-term stability of PdNPs-3 is expected to be poor. Indeed dispersion of PdNPs-3 in CH2Cl2 is only stable for 4 h. HRMAS NMR was also utilized to study these PdNPs. Spectra of the free [C12,C12−Im]Cl and PdNPs 1−3 in benzene-d6 are given in Figure 3. We will first compare the spectra between the free [C12,C12−Im]Cl and PdNPs-1, shown in Figure 3, a and b, respectively. In Figure 3b the signals for the Im salts in PdNPs-1 are slightly broad and shifted upfield relative to that of the free salts in Figure 3a. The relative intensity of the signals in Figure 3b needs additional comments. First, we notice that the intensities of the aliphatic protons are higher than that of the ring protons. For example, the ratio of the signals of the two terminal CH3 protons to that of the ring C2H proton is 7.0:1.0. This value is compared to that of the 6.0:1.0 found and expected in the free [C12,C12−Im]Cl (Table 1). Further, the intensity of the NCβH2 signal is greater than

sample [C12,C12-Im] Cl PdNPs-1 PdNPs-2 PdNPs-3

aromatic C2H

aromatic C4,5H

NCαH

NCβH

aliphatic terminal CH3

1.0

2.0

4.0

4.0

6.0

1.0 1.0 not found

2.0 2.1 −

4.0 4.0 −

4.3 5.1 −

7.7 8.2 −

that of the NCαH2 (4.3 to 4.0), which should otherwise be equal. The spectrum of PdNPs-2 in Figure 3c indicates that the C2,4,5,α,β protons are further shifted upfield and broadened, and an increase in the intensity of (1) the aliphatic to ring protons (8.2:1.0, Table 1), and (2) the NCβH2 to NCαH2 signals (5.1 to 4.0). For PdNPs-3, the protons at the aromatic ring cannot be observed, while those of the alkyl chains are very broad. To explain these observations we will begin from the simple PdNPs-3, which has a monolayer of [C12,C12−Im]Cl. NMR studies suggest that the monolayered [C12,C12−Im]Cl has its Im ring head sitting parallel on the PdNP surface, while with the alkyl chains tilt away from the surface to form a U-shaped geometry (Scheme 1). The anisotropic nature of the adsorbed molecule causes the absence of the ring proton signals and gives broad alkyl chain signals.48−50 Further, because the NCαH2 groups are closer to the NP surface than those of the NCβH2 groups, the relative intensity of NCαH2/ NCβH2 would be lower than the expected value of 1. This proposed mode of interaction is consistent with those reported based on theoretical25 and experimental studies.51−53 In PdNPs-2, a bilayer of interdigitated U-shaped Im salts is proposed to act on the surface (Scheme 1). The geometry of this bilayer structure is very similar to that found in [C12,C12−Im]2[PdCl4].40 Since the outer layer of the salts has an ionic head exposed outward, these ring protons are less influenced by the PdNP, and thus show slightly broadened signals. Also shown in Scheme 1 for PdNPs-1, some suspended Im salts locate further away from C


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Figure 4. Selected region of 1H-HRMAS spectra for (a) free [C12,C12−Im]Cl and (b−d) PdNPs-1 at MAS rates of 3, 9, and 12 kHz in benzene-d6, respectively. (e−g) Sample PdNPs-2 at spinning rates of 3, 9, and 12 kHz, respectively.

fully understood at present. One possible explanation is that there is a structural change upon increasing the spinning rate, a scenario similar to thermally treated liquid crystals. For example, the liquid crystalline [C12,C12−Im]2[PdCl4] has an interdigitated U-shaped [C12,C12−Im]+ cation at the crystal phase, which changes to a none or partially interdigitated structure upon melting to mesophase.40 Another possibility is that the exchange between the suspended and bilayer Im salts is suppressed under high spinning rates such that they become differentiable. Further information could be obtained from DOSY study, which provides a two-dimensional spectrum correlating the chemical shift with the logarithmic diffusion coefficient (log D) (Figure 5). The DOSY spectrum of [C12,C12−Im]Cl is given in Figure 5a, in which a diffusion coefficient with a log D value of −8.4 is found. The spectrum for PdNPs-1 (Figure 5b) exhibits a diffusion coefficient of an averaged log D value of −9.5. The distinct difference between these two species suggests a large disparity in the overall size and mass. As expected, simple Im

the PdNP surface. Therefore, the Im ring signals in PdNPs-1 are sharper and shifted more downfield. In addition, the presence of only one set of signals suggests that the exchange between the suspended and bilayer Im salts is fast on the NMR time scale. 1 H-HRMAS NMR spectroscopy operating at different spinning rates also provides interesting insights about the Im salts anchored on the PdNPs. In Figure 4, the spectra taken at different MAS rates for the free [C12,C12−Im]Cl, PdNPs-1, and PdNPs-2 in the region of chemical shift from 6 to 11 ppm are shown (the ring protons of PdNPs-3 are not observable, therefore will not be discussed). Panel (a) shows that the chemical shifts of the C2 and C4,5 ring protons for the free [C12,C12−Im]Cl appear at 10.60 and 8.01 ppm, respectively, and are independent of the MAS rates. For PdNPs-1, at the spinning rate of 3 kHz, the C2 and C4,5 proton signals are at 9.82 and 7.62 ppm, respectively (panel (b)). Under the spinning rate of 9 kHz, the C2 proton signal splits to two signals at ca. 9.92 and 9.81 ppm, while those of the C4,5 protons are at 7.68 and 7.60 ppm (panel (c)). At the spinning rate of 12 kHz, the split signals are further apart: 10.02 and 9.89 ppm for the C2 proton, and 7.74 and 7.62 ppm for the C4,5 protons (panel (d)). In panel (e), the spectrum of PdNPs-2 taken at 3 kHz shows that the corresponding Im ring proton signals are at 9.89 and 7.44 ppm, and are relatively broad compared to the free salt. Increasing the spinning rate to 9 kHz, signals for the ring protons become sharper and are downfield shifted to 9.96 and 7.52 ppm (panel (f)). At a spinning rate of 12 kHz, the corresponding signals are further downfield shifted to 10.34 and 7.66 ppm (panel (g)). These results are reversible: decreasing the spinning rate, spectra (b) and (e) could be restored. The nature of the interaction between the inner layered Im salts and PdNPs is presumably electrostatic as often proposed.7,15 On the other hand, the interaction between outer and inner layers is van der Waals force through chains. The suspended Im salts interact weakly mainly via electrostatic forces with the outer Im salts. For PdNPs-2, a higher spinning rate tends to force the outer layer of the Im salts to be pulled away, thus the ring protons are shifted to lower fields and peaks sharpened. For PdNPs-1, the observation of two sets of ring protons could be attributed to the poorly interacting outer and suspended Im salts. This spinning rate dependent behavior has never been reported. The exact cause of this interesting observation is not

Figure 5. 1H DOSY 2D spectra of (a) [C12,C12−Im]Cl and (b) PdNPs-1 acquired in benzene-d6. Inset: the corresponding DOSY spectra to C2 and C4,5 protons. D is expressed in m2 s−1. D


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Figure 6. 1H NMR spectra of PdNPs-2 and its reaction with DMAP in D2O: (a) PdNPs-2, (b) free DMAP, (c) 10 min after adding 100 μL of 1.5 M DMAP to PdNPs-2, (d) 20 min, and (e) 30 min. Enlarged signal for C4,5 proton of Im salt is given on top of each of the original signals. Inset: Schematic drawing for displacement of PdNPs-2 by DMAP with labelings.

on PdNPs albeit slowly. The use of DMAP for displacing the capping agent of tetraoctylammonium bromide on gold and palladium NPs in order to transfer NPs from organic solvent to water has been reported.54 Further, during the displacement reaction, while two sets of DMAP signals corresponding to the free and bound DMAPs are observed, only one set of C4,5 proton signal is found. This observation additionally suggests that the exchange between the free and capped DMAP is slow on the NMR time scale, whereas the exchange between the capped and those displaced Im salts is fast. In short, (1) DMAP has a greater affinity toward PdNPs than that of the Im salt, and (2) the salt of [C12,C12−Im]Cl on the PdNPs-2 is labile. The results therefore indicate that [C12,C12−Im]Cl on the PdNP surface is labile but is capable of stabilizing PdNPs in solution. This feature should be advantageous for use as a catalyst.55 Therefore, Pd@[C12,C12−Im]Cl prepared in situ in a mixed solvent of water/p-dioxane (1:1 by volume) was employed to catalyze Suzuki cross-coupling reaction of aryl halide and phenylboronic acid at room temperature with K2CO3 as a base. The progress of the reaction was followed by thin layer chromatography (TLC) and gas chromatography (GC), and the results are shown in Tables 2 and 3.

salt diffuses faster, while those in PdNPs-1 diffuse slower. Interesting results could be further obtained via careful inspection of the spectrum of PdNPs-1. There are two major sets of C2 protons with averaged log D values of −9.3 and −9.8. So is true for those of C4,5 protons: −9.4 and −9.5. We propose that the observation of two sets of diffusion coefficients arises from the suspended and outer [C12,C12−Im]Cl. The faster diffusion rate is presumably attributed to the suspended salts. This observation is consistent with the HRMAS experiments. Although DOSY NMR has been employed to evaluate the interaction between NPs and ligand,34,35 this is the first report of using this technique to differentiate two different stabilizer on the NPs. To evaluate the lability of [C12,C12−Im]Cl in PdNPs-2, we studied its displacement reaction by dimethylaminopyridine (DMAP). The displacement reaction was carried out via adding 100 μL solution of 1.5 M of DMAP in D2O to PdNPs-2 (0.017 mg, 2 mL of D2O), and the reaction was monitored by 1H NMR at a time interval of 10 min. Since for a free Im salt, the C2 proton at the Im ring undergoes facile H/D exchange reaction in D2O and is silent in the spectrum, only regions covering the chemical shifts of the C4,5 protons of PdNPs-2 and the ring protons of DMAP were monitored as shown in Figure 6. 1 H NMR spectrum of PdNPs-2 in D2O shows that the C4,5 protons is at 7.52 ppm (Figure 6a); this is compared to the value of 8.01 ppm found for the free [C12,C12−Im]Cl in D2O. Free DMAP exhibits two doublets at δ = 7.93 and 6.55 ppm, corresponding to ring protons of Ha and Hb, respectively, as in Figure 6b. At the first 10 min of reaction, as shown in Figure 6c, two new doublets appear at δ = 7.77 ppm (as Ha′) and δ = 6.42 ppm (as Hb′), assignable to DMAP capped. At the same time, the chemical shift for the C4,5 protons is moved downfield to a′

Table 2. Suzuki Coupling Reaction of Chlorobenzene and Phenylboronic Acid Catalyzed by Pd@[C12,C12−Im]Cl

entry

catalyst (mol %)

1

10.0 mol % Pd@[C12,C12Im]Cl 5.0 mol % Pd@[C12,C12Im]Cl 2.5 mol % Pd@[C12,C12Im]Cl 1.0 mol % Pd@[C12,C12Im]Cl 0.5 mol % Pd@[C12,C12Im]Cl

7.54 ppm. The integration ratio for H /H (or H /H ′) is 3.6, meaning that the molar ratio of the free and absorbed DMAP is 3.6. Subsequently, after 20 min (Figure 6d), there is a decrease a

b

b

2 3

in the integration ratio of Ha/Ha′ (or Hb/Hb′) to 1.3, whereas the C4,5 ring proton signal shifts to 7.56 ppm. After 30 min (Figure 6e), the ratio further decreases to 0.5, and the C4,5 ring proton signal further increases to 7.58 ppm. The gradual decrease on the signal intensity of the free DMAP concomitant with an increase of the bounded ones in a duration of 30 min suggests that these PdNPs interact stronger with DMAP than with Im salt, and therefore, DMAP could displace those Im salts

4 5

time (min)

yield (%)a,b

TON

TOF (h−1)

10

99

15.8

95.0

10

94

30.1

180.5

90

53

33.9

22.6

120

17

27.2

13.6

150

9

28.8

11.5

a

The reaction was carried out with chlorobenzene (1.0 equiv), phenylboronic acid (1.5 equiv), K 2 CO 3 (2.0 equiv), and Pd@[C12,C12−Im]Cl in 2.0 mL of H2O/p-dioxane (1:1) at room temperature. bYield determined by GC. E


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in 5 min (entry 9, Table 3). In the literature, examples of PdNPs-catalyzed Suzuki reactions with high yields under ambient conditions are known.13,56−58 However, most of them are aryl bromide or iodide with prolonged reaction time.13,56−58 In a report, thiolate-stabilized PdNPs have also been used for the coupling of chlorobenzene and phenylboronic acid at 20 °C to produce a modest 52% yield after 5 days.56 Despite the high catalytic activity found in entry 9, the colloidal system of Pd@[C12,C12−Im]Cl is good for only two runs (see entry 9; 95, 90, and 26% for the first, second, and third runs, respectively). To understand this poor recyclability, morphologies of the PdNPs after each run are monitored by TEM. After the first run, cubelike PdNPs with some truncations at corners and edges are observed (Figure S2(a) in the Supporting Information). Thus, before and after the first run, there is a transformation from cube to truncated cube. It indicates the dissolution of the more reactive sites of the corners and edges during the reaction, and supports the possible participation of the homogeneous nature in our PdNPcatalyzed Suzuki reactions. In the second run, the slight decrease of the product yield is consistent with the diminishing corners and edges of the PdNPs. After the second run, a small amount of black precipitate is formed, whereas in the remaining colloidal solution, fused NPs are observed (Figure S2(b)). The sudden decrease in the catalytic activity in the third run (26% yield) is also consistent with the change of morphology. It has been noticed that in a Suzuki cross-coupling reaction a shape transformation for the PtNP catalyst from tetrahedron to sphere rendered a sudden decrease in the catalytic activity in the second cycle.59 Thus, while the labile nature of the cubelike PdNPs could lead to a high catalytic activity, it could also cause the aggregation of NPs and lose its activity. The catalytic activity of this work appears to be limited by approximately 30 turnovers, as suggested by the TON of 30 often observed in this work. Although the catalytic system prepared in this work is only good for two cycles and the Pd loading is high, this system is capable to catalyze the coupling reaction of aryl chloride and boronic acids at room temperature with high yield in a short time.

Table 3. Suzuki Coupling Reactions Using 5.0 mol % Pd@[C12,C12−Im]Cla

entry

X

1 2 3 4 5 6 7

Cl Cl Cl Cl Cl Cl Cl

8

2chlorothiophene Br

9

R H p-CH3 p-CH3O p-CH3CO m-O2N p-OHc pCOOHc

H

time (min)

yield (%)b

TON

TOF (h−1)

10 15 15 12 40 40 40

94 91 90 91 72 83 77

30.1 29.1 28.8 29.1 23.0 26.6 24.6

180.5 116.5 115.2 145.6 34.6 39.8 37.0

20

95

30.4

91.2

5

95 90d 26e

30.4 28.8 8.3

364.8 345.6 99.8

a

Reactions were carried out at room temperature with 5.0 mol % Pd@[C12,C12−Im]Cl, 1.0 equiv aryl halide, 1.5 equiv phenylboronic acid, 2.0 equiv K2CO3 in H2O/p-dioxane (1:1). bIsolated yield (GC yield). c2.5 equiv K2CO3 was used. dSecond cycle. eThird cycle.

Cross-coupling between the least reactive chlorobenzene and phenylboronic acid is examined first. Here we investigate the effect of loading of Pd@[C12,C12−Im]Cl on the promotion of the Suzuki reaction. With respect to chlorobenzene, a loading of 10.0 mol % of Pd@[C12,C12−Im]Cl (based on the mol of PdCl2 used) gives the coupling product of biphenyl with a 99% yield in 10 min at room temperature (Table 2). Subsequently, at a loading of 5.0, 2.5, 1.0, and 0.5 mol % of PdNPs catalyst, yields decrease to 94% (10 min), 53% (90 min), 17% (120 min), and 9% (150 min), respectively (Table 2). Thus, increasing the loading of Pd@[C12,C12−Im]Cl leads to an increase in the product yield. Remarkably, the least reactive aryl chloride produces the high yield over short reaction time even at room temperature, albeit at high Pd@[C12,C12−Im]Cl loading. Aryl chlorides with various electron-donating and -withdrawing substituents are used to investigate the scope of the Suzuki reaction. The results are summarized in Table 3. All the reactions are carried out using 5.0 mol % of Pd@[C12,C12− Im]Cl, since this catalyst loading is sufficient to catalyze the reaction effectively. In general, substituted aryl chloride gives lower yields than that of the unsubstituted chlorobenzene. Aryl chloride with substituents of p-CH3 and p-CH3O gives 91% and 90% product yields, respectively (entries 2 and 3, Table 3). The observation of high yields of the cross-coupling reaction products suggests that the homocoupling reaction is negligible. Aryl chloride with a weak electron-withdrawing group of p-CH3CO yields 91% of the coupling product (entry 4, Table 3). A strong electron withdrawing group −NO2 at meta position gives a 72% yield of the coupling product (entry 5, Table 3). Aryl chlorides with p-OH and p-COOH substituents are acidic; at higher quantity of K2CO3, they react with boronic acid to give 83% and 77% yields of the coupling product (entries 6 and 7, Table 3), respectively. Further, 2-chlorothiophene also reacts smoothly and has promising yields (95% yield, entry 8, Table 3). As expected, aryl bromides react more effectively than aryl chlorides. Indeed, reaction of bromobenzene with boronic acid at room temperature gives a 95% yield of the coupling product

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CONCLUSION Here we report the use of [C12,C12−Im]Cl to fabricate cubelike PdNPs. These PdNPs are capped by a U-shaped bilayer of [C12,C12−Im]Cl with interdigitated alkyl chains and could be slowly displaced by DMAP. The techniques of HRMAS and DOSY NMR are useful to differentiate the outer layer and suspended Im salts in this system. The {100} surface of the Pd nanocubes, and the labile yet stable Im salt capped PdNPs, allow them to show high activities for Suzuki coupling reactions. Im salts are easy to prepare and modify. They therefore are potentially very useful in the preparation of metal nanoparticles with various properties to suit for different purposes. Preliminary study also shows that stable colloidal PtNPs could be prepared with [C12,C12−Im]Cl. We expect that the use of Im salt in nanoscience would flourish in the near future, and the HRMAS and DOSY NMR analyses would be very helpful to understand the nature of the capping agent.

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EXPERIMENTAL SECTION

Chemicals. PdCl2 were purchased from Aldrich and used as received. Solvents, boronic acids, aryl halides, and K2CO3 were purchased either from Aldrich, Lancaster, or local commercial sources F


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and were used as received. All solvent purchased from local commercial sources and were used as received. All reactions were carried out with double DI water. Preparation of PdNPs. [C12,C12−Im]Cl (176 mg, 0.40 mmol) was mixed with PdCl2 (14 mg, 0.08 mmol) in DI water/1,4-dioxane (10 mL of each) and was stirred for 10 min. Dihydrogen gas was then charged into the solution for 10 min (until the color of solution turned from reddish brown to gray colorless). Thus prepared, the colloidal solution could be used for the Suzuki reaction. Centrifugation (8000 rpm, 20 min) of the colloidal solution produced solid Pd@[C12,C12− Im]Cl. Washing these residues with DI water (1 mL), ethanol (1 mL), and then acetone (1 mL) twice gave PdNPs-1. For PdNPs-2, the virgin solid Pd@[C12,C12−Im]Cl was redissolved in 1 mL of CH2Cl2, and 3 mL of DI water was added to extract the free Im salts. The CH2Cl2 was then removed by an evaporator. The process was repeated three times. PdNPs-3 was obtained by dissolving the Pd@[C12,C12− Im]Cl in 1 mL of CH2Cl2, followed by addition of 3 mL of EtOH and then centrifugation (8000 rpm, 20 min). This process was repeated twice. Characterization of PdNPs. The nanoparticles were studied by TEM, JEOL 3010. TEM samples were prepared by placing a drop of the solution on carbon-coated Cu grids and allowed to air-dry. The particle size and standard deviation were determined by DLS using a Malvern Zetasizer Nano ZS. TGA was performed on a Mettler Toledo TGA851. 1H NMR spectra were recorded on an Avance DPX300 BRUKER and the chemical shifts (δ) were given in ppm and referenced to residual solvent signals. All the 1H HRMAS NMR experiments were acquired on a BRUKER 600 UltraShield equipped with a 4 mm HRMAS 1H/13C Z-GRD probe. Samples were first dispersed or dissolved in deuterated solvents and then loaded to a 50 μL zirconia rotor. HRMAS experiments were accomplished under the spinning rate of up to 12 kHz. 2D DOSY NMR experiments were acquired at 298 K on a BRUKER 600 UltraShield equipped with a 5 mm BBO inverseprobe. All DOSY experiments were performed with a longitudinal eddy-current delay (LED) and two spoil gradients pulse sequence (ledbpgp2s). 32 spectra were acquired using a linear gradient ramp from 2 to 95% using sine-shaped gradients. A gradient recovery delay of 100 μs and an eddy current delay of 5 ms were used. The diffusion time (Δ) was set between 50 to 55 ms and the magnetic field pulse gradient (δ) was set between 0.9 to 1.6 ms depending on the sample. PdNPs-Catalyzed Suzuki Corss-Coupling Reaction. Aryl halide (1.6 mmol), phenylboronic acid (2.4 mmol), and K2CO3 (3.2 mmol) were added into the preprepared catalystic system (Pd@[C12,C12− Im]Cl, 5 mol % loading of Pd with respect to aryl halide) at room temperature. The extent of reactions was monitored by TLC, GC, and 1 H NMR. The reaction mixture was extracted with petroleum ether and the organic layers were combined and dried with anhydrous Na2SO4. Purification by column chromatography (silica gel/petroleum ether) gave the desired products confirmed by 1H NMR. The other reactions followed similar procedures.

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Dong Hwa University Nanotechnology Research Center for providing research facilities.

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ASSOCIATED CONTENT

S Supporting Information *

Further experimental data, including PXRD pattern and image of HRTEM for PdNPs, are provided. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Council (NSC 102-2113-M259-002; Taiwan, ROC) for financial support and National G


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