Showcasing research from Sang Woo Han’s group, Department of Chemistry and KI for the NanoCentury, KAIST, Daejeon, Korea.
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The facet-dependent enhanced catalytic activity of Pd nanocrystals The catalytic activity of Pd nanocrystals toward heterogeneous Buchwald-Hartwig amination strongly depends on their facets, and rhombic dodecahedral Pd nanocrystals enclosed exclusively by {110} facets are the most desirable catalyst among the various types of nanocrystals.
See Woo Youn Kim, Sang Woo Han et al., Chem. Commun., 2014, 50, 9454.
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Cite this: Chem. Commun., 2014, 50, 9454 Received 4th April 2014, Accepted 14th May 2014
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The facet-dependent enhanced catalytic activity of Pd nanocrystals† Minjune Kim,‡ab Yeonjoon Kim,‡a Jong Wook Hong,‡a Seihwan Ahn,a Woo Youn Kim*a and Sang Woo Han*ab
DOI: 10.1039/c4cc02494j www.rsc.org/chemcomm
A systematic study of heterogeneous Buchwald–Hartwig amination using shape-controlled Pd nanocrystals with distinctly different surface facets is presented.
Noble metal nanocrystals (NCs) have had a profound impact in the field of catalysis due to their substantially different characteristics compared to their homogeneous counterparts and their more flexible catalytic functions compared to conventional heterogeneous catalysts, which are due to their structural diversity.1–3 Indeed, the structural diversity of NCs can open a new door to the development of novel catalysts that could replace conventional ones with better performance or even show an unexpected selectivity. However, it simultaneously leads to difficulty in engineering an optimal structure for a specific catalytic application, because the catalytic properties of NCs strongly depend on their size, shape, and composition.4–12 Fortunately, thanks to enormous previous efforts towards the development of synthetic techniques for metal NCs, it is now possible to finely control such structural variables as desired.13–16 Despite immense advances in this field, it is a great challenge to elucidate the chemical dynamics that occur in NC catalysts under operating conditions due to the lack of a direct monitoring tool with a suitable spatiotemporal resolution. For example, it is still under debate whether catalytic reactions occur on the surface of NCs or on atoms/clusters leached from them, which is essential knowledge to devise a design principle for NC-based catalysts. In this regard, we present a systematic study on the facet-dependent catalytic activity of Pd NCs, which provides a clue to resolve the above debate. The remarkable catalytic capability of Pd has been well-known in various organic cross-coupling reactions, such as Heck,17 Suzuki,18 Stille,19 Hiyama,20 Sonogashira,21 Negishi coupling reaction,22 and Buchwald–Hartwig amination.23,24 It has been accepted that the oxidative addition of Pd into the carbon–halogen bond of aryl halide a
Department of Chemistry and KI for the NanoCentury, KAIST, Daejeon 305-701, Korea. E-mail: [email protected], [email protected] b Center for Nanomaterials and Chemical Reactions, Institute for Basic Science (IBS), Daejeon 305-701, Korea † Electronic supplementary information (ESI) available: Experimental and calculation details and additional data. See DOI: 10.1039/c4cc02494j ‡ These authors contributed equally to this work.
9454 | Chem. Commun., 2014, 50, 9454--9457
is the rate-determining step for these Pd-catalyzed cross-coupling reactions.17–24 If the catalytic event occurs on the surface of Pd NCs, the reaction rate should show a strong dependence on the type of facets by which NCs are enclosed because the adsorption habit of an aryl halide relies on the crystallographic property of the metal surface. In fact, recent scanning tunneling microscopy studies have shown that an aryl halide covalently adsorbed on a metal surface forms a reactive intermediate to the cleavage of the carbon–halogen bond.25–28 Among the various Pd-catalyzed cross-coupling reactions, we chose Buchwald–Hartwig amination as a model reaction to investigate the catalytic activity of Pd NCs because it is of immense importance in the bond formation between C and N. Furthermore, heterogeneous catalysis for this reaction has been relatively unexplored compared to the C–C coupling reactions.29,30 The facet-dependent catalytic function of Pd NCs was examined through comparative studies on the catalytic activity of Pd NCs with octahedral, cubic, and rhombic dodecahedral (RD) shapes, which are representative NC morphologies enclosed exclusively by the {111}, {100}, and {110} facets. Since such low-indexfaceted Pd NCs have well-defined morphologies and have only terraces on their surfaces with different atomic arrangements, we expect that mechanistic evidence can readily be acquired through the comparative study. In addition, we have performed density functional theory (DFT) calculations for each step in the catalytic reaction of different facets to understand the facet-dependent catalytic activity of Pd NCs. From the results, we could propose a plausible reaction pathway with strong evidence that the reaction occurs on the surface of NCs. The synthesis of Pd NCs with distinct morphologies was achieved through one-pot methods (see ESI†). Fig. 1 shows representative scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the prepared Pd NCs and the corresponding fast Fourier transform (FFT) patterns, demonstrating the production of single-crystalline Pd NCs with uniform shape and size. The octahedral and cubic Pd NCs were prepared according to previously reported procedures.15 The average edge lengths of the octahedral and cubic Pd NCs were 25.3 3.7 and 27.3 4.2 nm, respectively (Fig. 1a–d and Fig. S1a and b, ESI†). In contrast, the synthesis of RD Pd NCs enclosed predominantly by the {110} facets is quite challenging due to their high surface energy. Although a seed-mediated
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Table 1 Yields and TOFs for Buchwald–Hartwig amination catalyzed by various Pd catalysts
Fig. 1 Representative SEM (top panel, scale bar = 400 nm) and TEM (bottom panel, scale bar = 10 nm) images of (a,b) octahedral, (c,d) cubic, and (e,f) RD Pd NCs. The corresponding FFT patterns are shown in the inset of each TEM image.
method for the production of RD Pd NCs is available,14 the synthesis of a sufficient amount of homogeneous RD Pd NCs for catalysis study is problematic because it requires multiple steps, including seed preparation and an intermediate workup process. Therefore, in the present work, we developed for the first time a facile one-pot synthetic procedure for the preparation of RD Pd NCs. Single-crystalline RD Pd NCs with an average size of 52.3 10.5 nm were successfully prepared (Fig. 1e and f and Fig. S1c, ESI†). All the Pd NCs were synthesized by using the identical capping reagent and reductant to exclude any undesirable influence of adsorbates in the subsequent comparative study of catalytic reactions. The catalytic activities of the shape-controlled Pd NCs were investigated and the results were compared with those of commercially available homogeneous (Pd(PPh3)4, PdCl2, K2PdCl4) and heterogeneous (Pd black, Pd/C) Pd catalysts. Commercial heterogeneous Pd catalysts have irregular shapes and rough surfaces (Fig. S2, ESI†), whereas the prepared Pd NCs featured uniform shapes and welldefined surfaces, manifesting the capability of the Pd NCs for elucidating the facet-dependent catalytic activity. Following the typical protocol of Buchwald–Hartwig amination,23,24 we carried out the catalytic reaction using bromobenzene (4 mmol), morpholine (1.25 equiv.), potassium tert-butoxide, and 1 mol% of the Pd catalyst in 1,4-dioxane at 100 1C. All the catalytic reactions were conducted at the identical stirring rate (650 rpm). In fact, the reaction rate was not significantly affected by the stirring rate for the supported catalysts, such as Pd/C, under our experimental conditions (Fig. S3, ESI†). Isolation yields of the product (N-phenylmorpholine) and turnover frequencies (TOFs) for the different Pd catalysts are listed in Table 1. The TOF of each heterogeneous catalyst, the conversion of bromobenzene per surface Pd atom, was calculated according to the total number of surface Pd atoms employed in the synthesis (see ESI† for details). The product selectivity was B100% under our experimental conditions. Notably, the shape-controlled Pd NCs showed significantly higher catalytic activity than the commercial homogeneous and heterogeneous Pd catalysts, and their activities strongly depend on their shapes (facets). The catalytic activity of the three different Pd NCs follows the order RD 4 cubic 4 octahedral NCs. This was also reflected in kinetic data for the catalysis (Fig. S4, ESI†). The activation
This journal is © The Royal Society of Chemistry 2014
Entry
Pd catalyst
Yield (%)
TOF (h 1)
1 2 3 4 5 6 7 8 9
— Pd(PPh3)4 PdCl2 K2PdCl4 Pd black Pd/C Octahedral Pd NCs Cubic Pd NCs RD Pd NCs
0 12 15 20 15 21 70 78 92
0 0.500 0.624 0.833 4.63 5.74 45.7 80.3 410
energies of the catalytic reactions for the octahedral, cubic, and RD Pd NCs, and Pd/C, which were obtained from the Arrhenius plot for each catalyst (Fig. S4, ESI†), were 44.8 4.38, 37.3 1.57, 31.3 3.09, and 47.0 2.12 kJ mol 1, respectively, which are in good agreement with the activity trend across different catalysts determined in terms of the reaction yield. The activity trend has no correlation with the uncontrolled presence of stabilizing/capping agents on the NC surfaces (Fig. S5, ESI†). Since an appropriate synthetic ligand system is indispensable for efficient homogeneous Buchwald–Hartwig amination,23,24 the absence of ligands in the homogeneous catalysts tested in the present work caused poor reactivity. We also tested the substrate tolerance with the RD Pd NCs, and the results are summarized in Table 2. The reactions of various primary and secondary amines with bromobenzene successfully yielded desired aryl amine products. Notably, 2,5-dimethylaniline, which contains a sterically hindered amine functional group, gave a product (1c) in a high-yield. Diverse aryl bromide substrates also successfully facilitated Buchwald–Hartwig amination. Interestingly, Table 2 Yields of Buchwald–Hartwig amination with various amine and aryl bromide catalyzed by the RD Pd NCs
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the ortho-substituted aryl bromide, o-bromotoluene (1f), was found to be less reactive than p- (1h) and m-bromotoluene (1g), which is in sharp contrast to the case of the amine. This indicates that the methyl group adjacent to Br in o-bromotoluene might cause lower reactivity due to its stronger steric hindrance than in p- and m-bromotoluene, which can only be explained by the proposition that the adsorption of the aryl bromide reactant, the initial step of the reaction process, directly occurs on the surface of NC catalysts (vide infra). To decipher the close relationship between the catalytic activity and the shape of NCs, we considered the possibility that the reaction is catalyzed by Pd leached from the NCs. The amount of leached Pd in the reaction solution was measured to be less than 0.05% for repeated catalytic reactions using inductively-coupled plasma-mass spectrometry (ICP-MS). Considering the low-yields of the product from Pd complexes (Table 1), such a small amount of leached Pd does not seem to have given rise to the observed high catalytic activity of the Pd NCs. We further monitored changes in the reaction rate to evaluate the contribution of the leached Pd to the catalytic reaction. Interestingly, the product concentration remained constant after the removal of the NCs, whereas that of the typical reaction continuously increased (Fig. S6a, ESI†). On the other hand, Hg treatment for the NCs showed a significant suppression in the catalytic reactivity (Fig. S6b, ESI†). These results clearly indicate that the catalytic reaction occurs on the surface of Pd NCs and the contribution of leached Pd to the catalytic reaction is negligible under our experimental conditions.31,32 On the basis of the above findings, we can infer that the reactions would proceed directly on the surface of the NC catalysts. In this regard, we analyzed the high-angle annular dark field-scanning TEMenergy dispersive X-ray spectroscopy (HAADF-STEM-EDS) mapping images of the RD Pd NCs to check any surface alteration after the catalytic reaction. As shown in Fig. S7 (ESI†), a certain amount of Br was detected in the area where Pd NCs reside after the reaction with bromobenzene, providing evidence that the Br atoms in bromobenzene were adsorbed on the NC surface during the reaction. To clarify the pathway of Br adsorption, we carried out a control experiment to verify whether the Br atoms were dissociated from the reactant directly on the surface or came from the reaction solution. The HAADFSTEM-EDS mapping images of the RD Pd NCs after mixing with KBr (4 mmol) in the presence (Fig. S8, ESI†) or the absence (Fig. S9, ESI†) of morpholine (5 mmol) for 24 h showed that the amount of Br atoms adsorbed on the NC surface was less than the detection limit for each case. This demonstrates that the well-defined NC surface is indeed involved in the catalytic dissociation of aryl bromide. Taken together, we can propose the reaction pathway of the heterogeneous Buchwald–Hartwig amination as follows: (i) oxidative addition of aryl bromide, (ii) formation of aryl–amide intermediate, and (iii) reductive elimination of product. To initiate the catalytic reaction, aryl bromide should be adsorbed on the surface of Pd NCs covered by amines. The adsorbed aryl bromide is then dissociated and subsequently diffused away until it forms the aryl–amide intermediate. Finally, the product is dissociated from the surface, while the Br atom is left. This process is schematically illustrated in Fig. 2. In this proposed mechanism, Pd NCs play a crucial role in boosting the oxidative addition of aryl bromide, which has been considered to be the rate-determining step of the cross-coupling reaction. Since different NC facets have different geometric and electronic structures,
9456 | Chem. Commun., 2014, 50, 9454--9457
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Fig. 2 Proposed reaction pathway for Buchwald–Hartwig amination on the Pd surface.
the strong dependence of the reaction rate on the shape of Pd NCs is naturally expected, as was observed in experiments. To elucidate the origin of the facet-dependent catalytic activity of the Pd NCs, we theoretically studied the energetics of each step along the reaction pathway for the three different facets. The DFT method with Perdew–Burke–Ernzerhof exchange–correlation functional implemented into VASP package has been used for this purpose (see ESI†).33,34 In the DFT calculations, morpholine was employed as an amine substrate. In addition, o- and p-bromotoluene were employed as aryl bromides to verify not only the facet-dependent catalytic activity but also the effect of the steric hindrance of the substrate. Fig. 3 summarizes the DFT calculation results. The driving force of the oxidative addition step is the adsorption energy (Eads) of the aryl bromide to the surface. Under our experimental conditions, the competitive adsorption of aryl bromide against amines has to be considered because amine molecules readily occupy the binding sites on the surface due to their excess amount. The relative Eads values of p-bromotoluene with respect to that of morpholine to Pd(110), Pd(100), and Pd(111) are 16.753, 12.458, and 8.817 kcal mol 1, respectively (Fig. 3 and Table S1, ESI†), indicating that the oxidative addition is favored in the order of Pd(110), Pd(100), and Pd(111). This trend is the same for o-bromotoluene, but it shows less negative Eads values than p-bromotoluene due to the steric hindrance induced by the methyl group (Table S1, ESI†). These results are highly consistent with the experimental observations. On the other hand, the relatively unfavorable adsorption of bromotoluene to Pd(111) surface can be understood by the fact that the relatively dense surface atoms on the Pd(111) surface hinder the tight adsorption of Br as well as C on the same atom (Fig. S10 and S11, ESI†). After the oxidative addition of aryl bromide on the Pd surface, efficient cleavage of Br progresses to generate the aryl–amide intermediate. The calculated cleavage energies of Br on the different facets were between 1 and 4 kcal mol 1 (Fig. 3 and Table S1, ESI†), which facilitate the formation of the aryl–amide intermediate. This is another advantage of the heterogeneous Buchwald–Hartwig amination. Finally, the reductive elimination of the aryl–amide intermediate generates the product. The calculated energies of the reductive elimination of p-bromotoluene were 19.60, 37.40, and 46.79 kcal mol 1
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Fig. 3
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The calculated energy diagram of p-bromotoluene at each step along the proposed reaction mechanism.
on Pd(110), Pd(100), and Pd(111), respectively (Fig. 3 and Table S1, ESI†). Since the amine molecules in the reaction solution might re-occupy the binding sites immediately after the dissociation of the product due to their dominant concentration, the energy value was taken as the sum of the reductive elimination energy of the aryl– amide intermediate and the Eads of the amine. The negative energy values indicate that the reductive elimination step is favorable for all the three different facets. We believe that the oxidative addition step should be more important in determining the overall reaction rate than the other steps because once the addition occurred against the competitive binding of amines on the surface, the diffusion of Br and the elimination of products readily followed. Taken together, the DFT calculations show that the finely tuned surface of Pd NCs not only enhances the oxidative addition of aryl bromide, but also makes the cleavage of Br atoms and the elimination of products easier. The different facets of NCs distinctively play a pivotal role in each step due to their unique properties. These results clearly explain the experimental trend in the reactivity of Pd NCs. In summary, through a comparative study of heterogeneous Buchwald–Hartwig amination using the shape-controlled Pd NCs, we found strong facet-dependence of catalytic reactivity. Further investigations showed that the contribution of leached Pd is negligible, indicating that the reaction propagates directly on the surface of the finely-tuned Pd NCs. Theoretical investigations unequivocally revealed that the oxidative addition of aryl bromide to the metal surface with a well-defined facet, regarded as the rate-determining step, is favored in the order of Pd(110), Pd(100), and Pd(111), which could explain their relative catalytic reactivities observed in the experiments. These results emphasize that controlling the morphology of metal NCs is explicitly a key step toward the rational design of heterogeneous catalysts. This work was supported by Basic Science Research Programs (2010-0029149, 2012R1A1A1004154), EPB Center (2008-0061892), and MIR Center (2009-0083525) through the NRF funded by the Korea government (MSIP), and was also supported by Institute for Basic Science (IBS) [CA1301] and KAIST High Risk High Return Project (HRHRP). W. Y. K. is also grateful for financial support from a Chung-Am Fellowship.
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Notes and references 1 K. An and G. A. Somorjai, ChemCatChem, 2012, 4, 1512. 2 Y. Xia, Y. Xiong, B. Lim and S. E. Skrabalak, Angew. Chem., Int. Ed., 2009, 48, 60. 3 A. R. Tao, S. Habas and P. Yang, Small, 2008, 4, 310. 4 K. Zhou and Y. Li, Angew. Chem., Int. Ed., 2012, 51, 602. 5 J. W. Hong, M. Kim, Y. Kim and S. W. Han, Chem. – Eur. J., 2012, 18, 16626. 6 C.-W. Yang, K. Chanda, P.-H. Lin, Y.-N. Wang, C.-W. Liao and M. H. Huang, J. Am. Chem. Soc., 2011, 133, 19993. 7 C.-Y. Chiu, P.-J. Chung, K.-U. Lao, C.-W. Liao and M. H. Huang, J. Phys. Chem. C, 2012, 116, 23757. 8 Y. Wu, S. Cai, D. Wang, W. He and Y. Li, J. Am. Chem. Soc., 2012, 134, 8975. 9 F. Wang, C. Li, L.-D. Sun, H. Wu, T. Ming, J. Wang, J. C. Yu and C.-H. Yan, J. Am. Chem. Soc., 2010, 133, 1106. 10 R. Narayanan and M. A. El-Sayed, Nano Lett., 2004, 4, 1343. 11 M. Crespo-Quesada, A. Yarulin, M. Jin, Y. Xia and L. Kiwi-Minsker, J. Am. Chem. Soc., 2011, 133, 12787. 12 M. M. Telkar, C. V. Rode, R. V. Chaudhari, S. S. Joshi and A. M. Nalawade, Appl. Catal., A, 2004, 273, 11. 13 B. Lim, M. Jiang, J. Tao, P. H. C. Camargo, Y. Zhu and Y. Xia, Adv. Funct. Mater., 2009, 19, 189. 14 W. Niu, L. Zhang and G. Xu, ACS Nano, 2010, 4, 1987. 15 J. W. Hong, S. W. Kang, B.-S. Choi, D. Kim, S. B. Lee and S. W. Han, ACS Nano, 2012, 6, 2410. 16 Y. W. Lee, D. Kim, J. W. Hong, S. W. Kang, S. B. Lee and S. W. Han, Small, 2013, 9, 646. 17 W. Cabri and I. Candiani, Acc. Chem. Res., 1995, 28, 2. 18 N. Miyaura and A. Suzuki, Chem. Rev., 1995, 95, 2457. 19 J. K. Stille, Angew. Chem., Int. Ed., 1986, 25, 508. 20 Y. Hatanaka and T. Hiyama, J. Org. Chem., 1988, 53, 918. 21 K. Sonogashira, Y. Tohda and N. Hagihara, Tetrahedron Lett., 1975, 16, 4467. 22 E. Negishi, Acc. Chem. Res., 1982, 15, 340. 23 J. P. Wolfe, S. Wagaw, J.-F. Marcoux and S. L. Buchwald, Acc. Chem. Res., 1998, 31, 805. 24 J. F. Hartwig, Acc. Chem. Res., 2008, 41, 1534. ´ndez-Torres, 25 M. M. Blake, S. U. Nanayakkara, S. A. Claridge, L. C. Ferna E. C. H. Sykes and P. S. Weiss, J. Phys. Chem. A, 2009, 113, 13167. ¨rk, F. Hanke and S. Stafstro ¨m, J. Am. Chem. Soc., 2013, 135, 5768. 26 J. Bjo 27 L. Grill, M. Dyer, L. Lafferentz, M. Persson, M. V. Peters and S. Hecht, Nat. Nanotechnol., 2007, 2, 687. 28 D. F. Perepichka and F. Rosei, Science, 2009, 323, 216. 29 Y.-H. Chen, H.-H. Hung and M. H. Huang, J. Am. Chem. Soc., 2009, 131, 9114. 30 F. Wang, C. Li, L.-D. Sun, C.-H. Xu, J. Wang, J. C. Yu and C.-H. Yan, Angew. Chem., Int. Ed., 2012, 51, 4872. 31 J. A. Widegren and R. G. Finke, J. Mol. Catal. A: Chem., 2003, 198, 317. 32 N. T. S. Phan, M. Van Der Sluys and C. W. Jones, Adv. Synth. Catal., 2006, 348, 609. ¨ller, Phys. Rev. B: Condens. Matter Mater. 33 G. Kresse and J. Furthmu Phys., 1996, 54, 11169. 34 J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett., 1996, 77, 3865.
Chem. Commun., 2014, 50, 9454--9457 | 9457
As featured in:
The facet-dependent enhanced catalytic activity of Pd nanocrystals The catalytic activity of Pd nanocrystals toward heterogeneous Buchwald-Hartwig amination strongly depends on their facets, and rhombic dodecahedral Pd nanocrystals enclosed exclusively by {110} facets are the most desirable catalyst among the various types of nanocrystals.
See Woo Youn Kim, Sang Woo Han et al., Chem. Commun., 2014, 50, 9454.
www.rsc.org/chemcomm Registered charity number: 207890
ChemComm Published on 15 May 2014. Downloaded by CAL POLY POMONA UNIV LIB/PER on 22/10/2014 11:47:00.
COMMUNICATION
Cite this: Chem. Commun., 2014, 50, 9454 Received 4th April 2014, Accepted 14th May 2014
View Article Online View Journal | View Issue
The facet-dependent enhanced catalytic activity of Pd nanocrystals† Minjune Kim,‡ab Yeonjoon Kim,‡a Jong Wook Hong,‡a Seihwan Ahn,a Woo Youn Kim*a and Sang Woo Han*ab
DOI: 10.1039/c4cc02494j www.rsc.org/chemcomm
A systematic study of heterogeneous Buchwald–Hartwig amination using shape-controlled Pd nanocrystals with distinctly different surface facets is presented.
Noble metal nanocrystals (NCs) have had a profound impact in the field of catalysis due to their substantially different characteristics compared to their homogeneous counterparts and their more flexible catalytic functions compared to conventional heterogeneous catalysts, which are due to their structural diversity.1–3 Indeed, the structural diversity of NCs can open a new door to the development of novel catalysts that could replace conventional ones with better performance or even show an unexpected selectivity. However, it simultaneously leads to difficulty in engineering an optimal structure for a specific catalytic application, because the catalytic properties of NCs strongly depend on their size, shape, and composition.4–12 Fortunately, thanks to enormous previous efforts towards the development of synthetic techniques for metal NCs, it is now possible to finely control such structural variables as desired.13–16 Despite immense advances in this field, it is a great challenge to elucidate the chemical dynamics that occur in NC catalysts under operating conditions due to the lack of a direct monitoring tool with a suitable spatiotemporal resolution. For example, it is still under debate whether catalytic reactions occur on the surface of NCs or on atoms/clusters leached from them, which is essential knowledge to devise a design principle for NC-based catalysts. In this regard, we present a systematic study on the facet-dependent catalytic activity of Pd NCs, which provides a clue to resolve the above debate. The remarkable catalytic capability of Pd has been well-known in various organic cross-coupling reactions, such as Heck,17 Suzuki,18 Stille,19 Hiyama,20 Sonogashira,21 Negishi coupling reaction,22 and Buchwald–Hartwig amination.23,24 It has been accepted that the oxidative addition of Pd into the carbon–halogen bond of aryl halide a
Department of Chemistry and KI for the NanoCentury, KAIST, Daejeon 305-701, Korea. E-mail: [email protected], [email protected] b Center for Nanomaterials and Chemical Reactions, Institute for Basic Science (IBS), Daejeon 305-701, Korea † Electronic supplementary information (ESI) available: Experimental and calculation details and additional data. See DOI: 10.1039/c4cc02494j ‡ These authors contributed equally to this work.
9454 | Chem. Commun., 2014, 50, 9454--9457
is the rate-determining step for these Pd-catalyzed cross-coupling reactions.17–24 If the catalytic event occurs on the surface of Pd NCs, the reaction rate should show a strong dependence on the type of facets by which NCs are enclosed because the adsorption habit of an aryl halide relies on the crystallographic property of the metal surface. In fact, recent scanning tunneling microscopy studies have shown that an aryl halide covalently adsorbed on a metal surface forms a reactive intermediate to the cleavage of the carbon–halogen bond.25–28 Among the various Pd-catalyzed cross-coupling reactions, we chose Buchwald–Hartwig amination as a model reaction to investigate the catalytic activity of Pd NCs because it is of immense importance in the bond formation between C and N. Furthermore, heterogeneous catalysis for this reaction has been relatively unexplored compared to the C–C coupling reactions.29,30 The facet-dependent catalytic function of Pd NCs was examined through comparative studies on the catalytic activity of Pd NCs with octahedral, cubic, and rhombic dodecahedral (RD) shapes, which are representative NC morphologies enclosed exclusively by the {111}, {100}, and {110} facets. Since such low-indexfaceted Pd NCs have well-defined morphologies and have only terraces on their surfaces with different atomic arrangements, we expect that mechanistic evidence can readily be acquired through the comparative study. In addition, we have performed density functional theory (DFT) calculations for each step in the catalytic reaction of different facets to understand the facet-dependent catalytic activity of Pd NCs. From the results, we could propose a plausible reaction pathway with strong evidence that the reaction occurs on the surface of NCs. The synthesis of Pd NCs with distinct morphologies was achieved through one-pot methods (see ESI†). Fig. 1 shows representative scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the prepared Pd NCs and the corresponding fast Fourier transform (FFT) patterns, demonstrating the production of single-crystalline Pd NCs with uniform shape and size. The octahedral and cubic Pd NCs were prepared according to previously reported procedures.15 The average edge lengths of the octahedral and cubic Pd NCs were 25.3 3.7 and 27.3 4.2 nm, respectively (Fig. 1a–d and Fig. S1a and b, ESI†). In contrast, the synthesis of RD Pd NCs enclosed predominantly by the {110} facets is quite challenging due to their high surface energy. Although a seed-mediated
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Table 1 Yields and TOFs for Buchwald–Hartwig amination catalyzed by various Pd catalysts
Fig. 1 Representative SEM (top panel, scale bar = 400 nm) and TEM (bottom panel, scale bar = 10 nm) images of (a,b) octahedral, (c,d) cubic, and (e,f) RD Pd NCs. The corresponding FFT patterns are shown in the inset of each TEM image.
method for the production of RD Pd NCs is available,14 the synthesis of a sufficient amount of homogeneous RD Pd NCs for catalysis study is problematic because it requires multiple steps, including seed preparation and an intermediate workup process. Therefore, in the present work, we developed for the first time a facile one-pot synthetic procedure for the preparation of RD Pd NCs. Single-crystalline RD Pd NCs with an average size of 52.3 10.5 nm were successfully prepared (Fig. 1e and f and Fig. S1c, ESI†). All the Pd NCs were synthesized by using the identical capping reagent and reductant to exclude any undesirable influence of adsorbates in the subsequent comparative study of catalytic reactions. The catalytic activities of the shape-controlled Pd NCs were investigated and the results were compared with those of commercially available homogeneous (Pd(PPh3)4, PdCl2, K2PdCl4) and heterogeneous (Pd black, Pd/C) Pd catalysts. Commercial heterogeneous Pd catalysts have irregular shapes and rough surfaces (Fig. S2, ESI†), whereas the prepared Pd NCs featured uniform shapes and welldefined surfaces, manifesting the capability of the Pd NCs for elucidating the facet-dependent catalytic activity. Following the typical protocol of Buchwald–Hartwig amination,23,24 we carried out the catalytic reaction using bromobenzene (4 mmol), morpholine (1.25 equiv.), potassium tert-butoxide, and 1 mol% of the Pd catalyst in 1,4-dioxane at 100 1C. All the catalytic reactions were conducted at the identical stirring rate (650 rpm). In fact, the reaction rate was not significantly affected by the stirring rate for the supported catalysts, such as Pd/C, under our experimental conditions (Fig. S3, ESI†). Isolation yields of the product (N-phenylmorpholine) and turnover frequencies (TOFs) for the different Pd catalysts are listed in Table 1. The TOF of each heterogeneous catalyst, the conversion of bromobenzene per surface Pd atom, was calculated according to the total number of surface Pd atoms employed in the synthesis (see ESI† for details). The product selectivity was B100% under our experimental conditions. Notably, the shape-controlled Pd NCs showed significantly higher catalytic activity than the commercial homogeneous and heterogeneous Pd catalysts, and their activities strongly depend on their shapes (facets). The catalytic activity of the three different Pd NCs follows the order RD 4 cubic 4 octahedral NCs. This was also reflected in kinetic data for the catalysis (Fig. S4, ESI†). The activation
This journal is © The Royal Society of Chemistry 2014
Entry
Pd catalyst
Yield (%)
TOF (h 1)
1 2 3 4 5 6 7 8 9
— Pd(PPh3)4 PdCl2 K2PdCl4 Pd black Pd/C Octahedral Pd NCs Cubic Pd NCs RD Pd NCs
0 12 15 20 15 21 70 78 92
0 0.500 0.624 0.833 4.63 5.74 45.7 80.3 410
energies of the catalytic reactions for the octahedral, cubic, and RD Pd NCs, and Pd/C, which were obtained from the Arrhenius plot for each catalyst (Fig. S4, ESI†), were 44.8 4.38, 37.3 1.57, 31.3 3.09, and 47.0 2.12 kJ mol 1, respectively, which are in good agreement with the activity trend across different catalysts determined in terms of the reaction yield. The activity trend has no correlation with the uncontrolled presence of stabilizing/capping agents on the NC surfaces (Fig. S5, ESI†). Since an appropriate synthetic ligand system is indispensable for efficient homogeneous Buchwald–Hartwig amination,23,24 the absence of ligands in the homogeneous catalysts tested in the present work caused poor reactivity. We also tested the substrate tolerance with the RD Pd NCs, and the results are summarized in Table 2. The reactions of various primary and secondary amines with bromobenzene successfully yielded desired aryl amine products. Notably, 2,5-dimethylaniline, which contains a sterically hindered amine functional group, gave a product (1c) in a high-yield. Diverse aryl bromide substrates also successfully facilitated Buchwald–Hartwig amination. Interestingly, Table 2 Yields of Buchwald–Hartwig amination with various amine and aryl bromide catalyzed by the RD Pd NCs
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the ortho-substituted aryl bromide, o-bromotoluene (1f), was found to be less reactive than p- (1h) and m-bromotoluene (1g), which is in sharp contrast to the case of the amine. This indicates that the methyl group adjacent to Br in o-bromotoluene might cause lower reactivity due to its stronger steric hindrance than in p- and m-bromotoluene, which can only be explained by the proposition that the adsorption of the aryl bromide reactant, the initial step of the reaction process, directly occurs on the surface of NC catalysts (vide infra). To decipher the close relationship between the catalytic activity and the shape of NCs, we considered the possibility that the reaction is catalyzed by Pd leached from the NCs. The amount of leached Pd in the reaction solution was measured to be less than 0.05% for repeated catalytic reactions using inductively-coupled plasma-mass spectrometry (ICP-MS). Considering the low-yields of the product from Pd complexes (Table 1), such a small amount of leached Pd does not seem to have given rise to the observed high catalytic activity of the Pd NCs. We further monitored changes in the reaction rate to evaluate the contribution of the leached Pd to the catalytic reaction. Interestingly, the product concentration remained constant after the removal of the NCs, whereas that of the typical reaction continuously increased (Fig. S6a, ESI†). On the other hand, Hg treatment for the NCs showed a significant suppression in the catalytic reactivity (Fig. S6b, ESI†). These results clearly indicate that the catalytic reaction occurs on the surface of Pd NCs and the contribution of leached Pd to the catalytic reaction is negligible under our experimental conditions.31,32 On the basis of the above findings, we can infer that the reactions would proceed directly on the surface of the NC catalysts. In this regard, we analyzed the high-angle annular dark field-scanning TEMenergy dispersive X-ray spectroscopy (HAADF-STEM-EDS) mapping images of the RD Pd NCs to check any surface alteration after the catalytic reaction. As shown in Fig. S7 (ESI†), a certain amount of Br was detected in the area where Pd NCs reside after the reaction with bromobenzene, providing evidence that the Br atoms in bromobenzene were adsorbed on the NC surface during the reaction. To clarify the pathway of Br adsorption, we carried out a control experiment to verify whether the Br atoms were dissociated from the reactant directly on the surface or came from the reaction solution. The HAADFSTEM-EDS mapping images of the RD Pd NCs after mixing with KBr (4 mmol) in the presence (Fig. S8, ESI†) or the absence (Fig. S9, ESI†) of morpholine (5 mmol) for 24 h showed that the amount of Br atoms adsorbed on the NC surface was less than the detection limit for each case. This demonstrates that the well-defined NC surface is indeed involved in the catalytic dissociation of aryl bromide. Taken together, we can propose the reaction pathway of the heterogeneous Buchwald–Hartwig amination as follows: (i) oxidative addition of aryl bromide, (ii) formation of aryl–amide intermediate, and (iii) reductive elimination of product. To initiate the catalytic reaction, aryl bromide should be adsorbed on the surface of Pd NCs covered by amines. The adsorbed aryl bromide is then dissociated and subsequently diffused away until it forms the aryl–amide intermediate. Finally, the product is dissociated from the surface, while the Br atom is left. This process is schematically illustrated in Fig. 2. In this proposed mechanism, Pd NCs play a crucial role in boosting the oxidative addition of aryl bromide, which has been considered to be the rate-determining step of the cross-coupling reaction. Since different NC facets have different geometric and electronic structures,
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Fig. 2 Proposed reaction pathway for Buchwald–Hartwig amination on the Pd surface.
the strong dependence of the reaction rate on the shape of Pd NCs is naturally expected, as was observed in experiments. To elucidate the origin of the facet-dependent catalytic activity of the Pd NCs, we theoretically studied the energetics of each step along the reaction pathway for the three different facets. The DFT method with Perdew–Burke–Ernzerhof exchange–correlation functional implemented into VASP package has been used for this purpose (see ESI†).33,34 In the DFT calculations, morpholine was employed as an amine substrate. In addition, o- and p-bromotoluene were employed as aryl bromides to verify not only the facet-dependent catalytic activity but also the effect of the steric hindrance of the substrate. Fig. 3 summarizes the DFT calculation results. The driving force of the oxidative addition step is the adsorption energy (Eads) of the aryl bromide to the surface. Under our experimental conditions, the competitive adsorption of aryl bromide against amines has to be considered because amine molecules readily occupy the binding sites on the surface due to their excess amount. The relative Eads values of p-bromotoluene with respect to that of morpholine to Pd(110), Pd(100), and Pd(111) are 16.753, 12.458, and 8.817 kcal mol 1, respectively (Fig. 3 and Table S1, ESI†), indicating that the oxidative addition is favored in the order of Pd(110), Pd(100), and Pd(111). This trend is the same for o-bromotoluene, but it shows less negative Eads values than p-bromotoluene due to the steric hindrance induced by the methyl group (Table S1, ESI†). These results are highly consistent with the experimental observations. On the other hand, the relatively unfavorable adsorption of bromotoluene to Pd(111) surface can be understood by the fact that the relatively dense surface atoms on the Pd(111) surface hinder the tight adsorption of Br as well as C on the same atom (Fig. S10 and S11, ESI†). After the oxidative addition of aryl bromide on the Pd surface, efficient cleavage of Br progresses to generate the aryl–amide intermediate. The calculated cleavage energies of Br on the different facets were between 1 and 4 kcal mol 1 (Fig. 3 and Table S1, ESI†), which facilitate the formation of the aryl–amide intermediate. This is another advantage of the heterogeneous Buchwald–Hartwig amination. Finally, the reductive elimination of the aryl–amide intermediate generates the product. The calculated energies of the reductive elimination of p-bromotoluene were 19.60, 37.40, and 46.79 kcal mol 1
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Fig. 3
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The calculated energy diagram of p-bromotoluene at each step along the proposed reaction mechanism.
on Pd(110), Pd(100), and Pd(111), respectively (Fig. 3 and Table S1, ESI†). Since the amine molecules in the reaction solution might re-occupy the binding sites immediately after the dissociation of the product due to their dominant concentration, the energy value was taken as the sum of the reductive elimination energy of the aryl– amide intermediate and the Eads of the amine. The negative energy values indicate that the reductive elimination step is favorable for all the three different facets. We believe that the oxidative addition step should be more important in determining the overall reaction rate than the other steps because once the addition occurred against the competitive binding of amines on the surface, the diffusion of Br and the elimination of products readily followed. Taken together, the DFT calculations show that the finely tuned surface of Pd NCs not only enhances the oxidative addition of aryl bromide, but also makes the cleavage of Br atoms and the elimination of products easier. The different facets of NCs distinctively play a pivotal role in each step due to their unique properties. These results clearly explain the experimental trend in the reactivity of Pd NCs. In summary, through a comparative study of heterogeneous Buchwald–Hartwig amination using the shape-controlled Pd NCs, we found strong facet-dependence of catalytic reactivity. Further investigations showed that the contribution of leached Pd is negligible, indicating that the reaction propagates directly on the surface of the finely-tuned Pd NCs. Theoretical investigations unequivocally revealed that the oxidative addition of aryl bromide to the metal surface with a well-defined facet, regarded as the rate-determining step, is favored in the order of Pd(110), Pd(100), and Pd(111), which could explain their relative catalytic reactivities observed in the experiments. These results emphasize that controlling the morphology of metal NCs is explicitly a key step toward the rational design of heterogeneous catalysts. This work was supported by Basic Science Research Programs (2010-0029149, 2012R1A1A1004154), EPB Center (2008-0061892), and MIR Center (2009-0083525) through the NRF funded by the Korea government (MSIP), and was also supported by Institute for Basic Science (IBS) [CA1301] and KAIST High Risk High Return Project (HRHRP). W. Y. K. is also grateful for financial support from a Chung-Am Fellowship.
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