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Efficient PEPPSI-Themed Palladium N‑Heterocyclic Carbene Precatalysts for the Mizoroki−Heck Reaction Yong-Chieh Lin, Hsin-Hsueh Hsueh, Shanker Kanne, Li-Kuang Chang, Fu-Chen Liu,* and Ivan J. B. Lin* Department of Chemistry, National Dong Hwa University, Hualien 974, Taiwan, Republic of China

Gene-Hsiang Lee and Shie-Ming Peng Department of Chemistry, National Taiwan University, Taipei 106, Taiwan, Republic of China S Supporting Information *

ABSTRACT: Two sets of six PEPPSI (pyridine enhanced precatalyst preparation, stabilization, and initiation) themed palladium complexes of amide-functionalized N-heterocyclic carbenes [PdBr2(NHC)(Py)], where NHC = 1-acetamido-3R-imidazolin-2-ylidene, 1-acetamido-3-R-benzimidazolin-2-ylidene and Py = pyridine, were prepared. Solid-state structures of all these complexes were determined by single-crystal X-ray diffraction methods. All complexes under scrutiny were found to adopt a trans arrangement with square-planar geometry. These complexes showed excellent catalytic activity toward the Mizoroki−Heck cross-coupling reaction of aryl chlorides and styrene. Benzimidazole-derived complexes exhibited better catalytic activity than imidazole-based complexes. Formation of palladium nanoparticles in the reaction mixture was evidenced by dynamic light scattering and transmission electron microscopy studies and a mercury poisoning experiment, suggesting the possible involvement of palladium nanoparticles in the catalytic reactions.

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INTRODUCTION

reactions, their application in the Heck reaction especially for aryl chlorides has not been explored much.11−14 Our research has been aimed at exploring the utility of NHCs15 in catalysis with special emphasis on developing userfriendly and highly efficient precatalysts for the Pd-mediated C−C coupling reactions.16 In this contribution, we report the synthesis of a series of new PEPPSI-motif [PdBr2(NHC)(Py)] (Py = pyridine) complexes of imidazole- (7−9) and benzimidazole-based (10−12) N-heterocyclic carbenes (Scheme 1) and their catalytic properties in Mizoroki−Heck coupling reactions. Our interest also dwells in studying the bonding properties of polydentate ligands; hence, an Nacetamido side arm was chosen in the ligand design. This functionalized side arm may engage in chelation to a metal center or bridge between two metal centers, thereby opening up possibilities for a variety of structural motifs. Although these possible coordination modes are not found in our study, the intermolecular hydrogen bonding between the amide groups plays an interesting role in the formation of dimeric and polymeric structures.

After their first isolation in free form, N-heterocyclic carbenes (NHCs) have not only played an outstanding role as ligands in transition-metal catalysis2 but have also found applications as organocatalysts,3 in drug development,4 and in materials science5 in the past two decades. The strong σ-donating properties of these ligands can stabilize metals in different oxidation states and support catalytically active intermediates that are coordinatively unsaturated.6 Remarkable activities in palladium-catalyzed cross-coupling reactions have been recently demonstrated by palladium complex of NHCs (Pd-NHCs) in the presence of ancillary nitrogen ligands. Some worthy examples are the N,Cpalladacycles described by Nolan,7 [Pd(NHC)(dmba)Cl] (dmba = N,N-dimethylbenzylamine) reported by Ying,8 [Pd(NHC)(Et3N)Cl2] developed by Navarro,9 [Pd(NHC)(Im)Cl2] (Im = imidazole) reported by Shao,10 and [Pd(NHC)(3-Cl-pyridine)Cl2] precatalysts, represented as [PdPEPPSI-NHC], introduced by Organ11 (PEPPSI is an acronym for pyridine enhanced precatalyst preparation, stabilization, and initiation). The last family appears to be the most popular nitrogen-stabilized [Pd-NHC] complexes due to the combination of versatility and efficiency.12 In a recent review, Organ highlighted that bulky yet flexible NHC ligands could circumvent various limitations of cross-coupling reactions.13 Although many reports have already shown that these PEPPSI motif complexes performed well in various C−C coupling 1

© 2013 American Chemical Society

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RESULTS AND DISCUSSION Synthesis of Imidazolium and Benzimidazolium Bromides (1−6). The synthetic routes developed by us17 were followed to prepare imidazolium and benzimidazolium Received: April 17, 2013 Published: June 28, 2013 3859

dx.doi.org/10.1021/om4003297 | Organometallics 2013, 32, 3859−3869

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Scheme 1. Synthetic Route to Pd−Pyridine N-Heterocyclic Carbene Complexes

bromides and are summarized in Scheme 1. 1-Acetamido-3-Rimidazolium bromides (R = Me (1), Et (2), nBu (3)) and 1acetamido-3-R-benzimidazolium bromides (R = Me (4), Et (5), n Bu (6)) were obtained in good yields by reacting Nsubstituted imidazoles/benzimidazoles in acetonitrile with 2bromoacetamide, respectively. The 1H NMR spectra of 1−6 exhibit a characteristic downfield resonance at δ 9.08−10.03 ppm for the NCHN proton, indicating the formation of azolium salts.18 The amide protons, appearing around 7.51 and 7.90 ppm for 1−3 and around 7.68 and 8.10 ppm for 4−6, are not equivalent due to the partial double-bond character of the C(O)−NH2 bond. Owing to the presence of Br···H−N type hydrogen bonding in solution between the Br− anion and the amide protons of the cation, these chemical shifts are slightly downfield in comparison to those of 2-bromoacetamide (7.25 and 7.62 ppm).17,19 The formation of NHC ligand precursors (1−6) is also supported by the results from 13C NMR spectra, in which a signal in the range of 137.2−144.1 ppm, assignable to the NCHN carbon, is always observed.17,20 In the infrared spectra, a band appearing at ∼1685 cm−1 is attributed to carbonyl absorption of the amide group.21 Synthesis of [PdBr2(NHC)(Py)] Complexes (7−12). As depicted in Scheme 1, the NHC complexes [PdBr2(NHC)(Py)] (NHC = 1-acetamido-3-R-imidazolin-2-ylidene, R = Me (7), Et (8), nBu (9); NHC = 1-acetamido-3-R-benzimidazolin2-ylidene, R = Me (10), Et (11), nBu (12)) were prepared as yellow solids in good yields by the reaction of 1-acetamido-3-Rimidazolium bromide and 1-acetamido-3-R-benzimidazolium bromide, respectively, with PdBr2 in the presence of sodium acetate using pyridine as a solvent as well as a reagent. All of these complexes are neither air nor light sensitive. In addition to the aforementioned complexes, some quantity of [PdBr2(Py)2] is always noticed as a byproduct, which was removed by column chromatography.14c

Complexes 7−12 were characterized by NMR spectroscopy in DMSO-d6. Successful deprotonation is confirmed from the absence of a 2H-imidazolium proton signal and by the comparison of azolylidene signals with those of related complexes.14c,19a,22 The signals corresponding to pyridine ligands are observed in the downfield region (7.37−9.06 ppm) in all complexes, confirming the coordination of the pyridine ligand to the metal center.14 All the proton signals are shifted upfield by 0.3−0.8 ppm upon coordination to palladium(II), as compared to those corresponding signals in azolium salts. Remarkably, C(O)−NH2 signals are also influenced greatly by the complexation, with an upfield shift of 1.03−2.25 ppm in comparison to the ligand precursors. The 13 C NMR signals for the Pd−Ccarbene carbons in 7−9 and 10− 12 appear at ∼150.0 and ∼163.0 ppm, respectively, which is 9.3−19.7 ppm downfield in comparison to the corresponding signals in the free ligand.20 Small downfield shifts of 1.0−2.3 ppm are also noticed for the carbonyl group. Single crystals for the solid-state structure determination of 7 and 10 were obtained by slow evaporation of their 1/1 hexane/ ethyl acetate solutions at room temperature, while single crystals of complexes 8, 11, and 12 were obtained by slow diffusion of ether into their concentrated chloroform solutions. Crystals of complex 9 were grown by slow diffusion of ether into a DMSO/THF (1/1) solution. Two molecules of 9 with minor variation of bond parameters and three DMSO solvent molecules are found in the unit cell. Figures 1a−6a show the molecular structures of 7−12 with selected bond lengths and bond angles given in the captions; only one molecule of 9 is depicted in Figure 3a. The crystallographic data are presented in the Supporting Information (Tables S1, S6, S11, S16, S21, and S26). The X-ray diffraction study confirms the presence of the metal-bound pyridine moiety in complexes 7−12. The Pd− Npyridine distances in 7 (2.095(2) Å), 8 (2.081(3) Å), 9 (2.115(6), 2.100(6) Å), 10 (2.092(3) Å), 11 (2.084(4) Å), and 3860


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Figure 1. (a) Crystal structure of 7 showing 50% probability thermal ellipsoids. Hydrogen atoms are omitted for clarity. (b) 1D polymeric structure of 7 showing weak N−H···O interactions. Selected bond lengths (Å) and angles (deg): Pd(1)−C(1), 1.951(3); Pd(1)−N(4), 2.095(2); Pd(1)−Br(2), 2.4167(4); Pd(1)−Br(1), 2.4413(4); C(1)− N(1), 1.342(4); C(1)−N(2), 1.348(4); C(1)−Pd(1)−N(4), 179.01(12); C(1)−Pd(1)−Br(1), 88.03(9); C(1)−Pd(1)−Br(2), 88.28(9); N(4)−Pd(1)−Br(1), 92.07(7); N(4)−Pd(1)−Br(2), 91.62(7); Br(2)−Pd(1)−Br(1), 176.261(15); N(1)−C(1)−N(2), 105.5(2).

Figure 2. (a) Crystal structure of 8 showing 50% probability thermal ellipsoids. Hydrogen atoms are omitted for clarity. (b) 1D polymeric structure of 8 showing weak N−H···O interactions. Selected bond lengths (Å) and angles (deg): Pd−C(1), 1.957(3); Pd−N(4), 2.081(3); Pd−Br(2), 2.4230(4); Pd−Br(1), 2.4410(4); N(1)−C(1), 1.337(4); N(2)−C(1), 1.352(4); C(1)−Pd−N(4), 179.16(12); C(1)−Pd−Br(1), 90.06(9); C(1)−Pd−Br(2), 88.01(9); N(4)−Pd− Br(1), 89.55(8); N(4)−Pd−Br(2), 92.33(8); Br(2)−Pd−Br(1), 176.737(17); N(1)−C(1)−N(2), 105.6(3).

12 (2.094(3) Å) are comparable to those in related PEPPSI analogues reported by Organ and co-workers (2.086−2.137 Å)11 and recently reported analogues by Gosh et al. (2.092(8)− 2.107(2) Å)14e and Huynh et al. (2.094(2)−2.154(3) Å).14a It is worth noting that, owing to the strong trans influence of the NHC ligand, the Pd-bound pyridine moiety is expected to be weakly bonded. Hence, quite expectedly, the Pd−Npyridine distances (2.081(3)−2.115(6) Å) in complexes 7−12 are longer than those in the complexes without the NHC ligand: e.g., 2.040(3) Å in [2,6-bis(2′-indolyl)pyridine]Pd(pyridine)],23 2.037(3) Å in [bis(bis(2-pyridylmethyl)amineN,N′,N″)]Pd(pyridine)](ClO4 −) 2 ,24 and 2.038(4) Å in [(2,2′:6′,2″-terpyridine)Pd(pyridine)](ClO4−)2.25 It is important that a weakly bound pyridine moiety is a strategic hallmark of a PEPPSI-themed precatalyst. The pyridine moiety is considered to be a “throwaway” ligand, as it paves way for the incoming substrate.11−14 The molecular structures of 7−9 have a nearly square planar geometry with a palladium center surrounded by a carbene ligand, two bromide ligands in a trans configuration, and a pyridine. The Ccarbene−Pd−Npyridine angle is nearly linear: 179.01(12)° for 7, 179.16(12)° for 8, and 179.2(3), 176.7(3)° for 9. The azole plane diverts from the coordination plane of the molecule by angles of 81.66° for 7, 79.36° for 8, and 84.92, 85.40° for 9 (according to the dihedral angle

between the planes defined by Ccarbene−Br(1)−Npyridine−Br(2) and N(1)−C carbene −N(2)). The Pd−C carbene distances (1.951(3) Å (7), 1.957(3) Å (8), 1.954(8), 1.959(8) Å (9)) are similar to those of other palladium-related species.26 The azole rings are tilted with respect to their pyridine rings with a dihedral angles of 17.62, 34.90, and 36.94, 49.59° for 7−9, respectively. In complexes 7−9, the molecules contain intermolecular hydrogen bonds between the oxygen of the carbonyl group and protons of the amide group of the adjacent molecule, resulting in a 1D polymeric structure in each complex (N···O bond distances are 2.954 Å for 7, 2.904 Å for 8, and 2.928, 2.935 Å for 9, less than the sum of the van der Waals radii of N and O atoms (3.05 Å)).27 In addition to the intermolecular hydrogen bonding between adjacent amide groups, multiple weak noncovalent interactions are also observed between the DMSO molecules and complex 9. The 1D polymers, which are formed by the intermolecular hydrogen bonds, are further connected to the solvent molecules in a parallel fashion to form a double-layered 1D polymeric structure. The DMSO molecules offer two types of interactions; one is a SO···S mutual interaction (the O···S distance is 3.052 Å) from the adjacent DMSO molecules, and the second one is a SO···H−N intermolecular hydrogen 3861


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Figure 4. (a) Crystal structure of 10 showing 50% probability thermal ellipsoids. Hydrogen atoms are omitted for clarity. (b) Dimeric structure of 10 showing weak N−H···O interactions. Selected bond lengths (Å) and angles (deg): Pd(1)−C(1), 1.946(3); Pd(1)−N(4), 2.092(3); Pd(1)−Br(2), 2.4185(5); Pd(1)−Br(1), 2.4437(5); C(1)− N(1), 1.341(4); C(1)−N(2), 1.353(4); C(1)−Pd(1)−N(4), 173.99(13); C(1)−Pd(1)−Br(1), 87.42(10); C(1)−Pd(1)−Br(2), 88.45(10); N(4)−Pd(1)−Br(1), 92.97(9); N(4)−Pd(1)−Br(2), 91.65(9); Br(2)−Pd(1)−Br(1), 173.576(17); N(1)−C(1)−N(2), 107.4(3).

Figure 3. (a) Crystal structure of 9·3/2DMSO showing 50% probability thermal ellipsoids. Hydrogen atoms and solvent molecules are omitted for clarity. (b) 1D polymeric structure of 9·3/2DMSO showing weak N−H···O interactions. Selected bond lengths (Å) and angles (deg): Pd(1)−C(1), 1.954(8); Pd(1)−N(4), 2.115(6); Pd(1)− Br(2), 2.4193(11); Pd(1)−Br(1), 2.4379(11); N(1)−C(1), 1.349(10); N(1)−C(2), 1.397(10); Pd(2)−C(15), 1.959(8); Pd(2)−N(8), 2.100(6); Pd(2)−Br(3), 2.4259(12); Pd(2)−Br(4), 2.4282(12); N(5)−C(15), 1.342(10); N(5)−C(16), 1.384(10); C(1)−Pd(1)−N(4), 179.2(3); C(1)−Pd(1)−Br(1), 87.7(2); C(1)− Pd(1)−Br(2), 86.6(2); N(4)−Pd(1)−Br(1), 93.07(19); N(4)− Pd(1)−Br(2), 92.68(19); Br(2)−Pd(1)−Br(1), 173.59(4); N(2)− C(1)−N(1), 105.5(7); C(15)−Pd(2)−N(8), 176.7(3); C(15)− Pd(2)−Br(3), 88.9(3); C(15)−Pd(2)−Br(4), 87.6(3); N(8)− Pd(2)−Br(3), 91.86(19); N(8)−Pd(2)−Br(4), 91.77(19); Br(3)− Pd(2)−Br(4), 175.69(4); N(6)−C(15)−N(5), 105.5(7).

of N-acetamido side arms also exist in these structures (the N···O bond distances are in the range 2.879−2.921 Å); however, they form a dimeric construction instead of polymeric structures (Figures 4b−6b). The difference in the arrangement may be due to the offset π stacking between two benzimidazole rings, which inclines the molecules to arrange in opposite directions (the interplanar distances are in the range 3.326− 3.511 Å).28 Mizoroki−Heck Coupling Reaction. The palladiumcatalyzed Mizoroki−Heck reaction is a powerful tool for the preparation of arylated alkenes in fine chemical synthesis.29 In comparison to aryl bromides and iodides, aryl chlorides are attractive substrates for Heck reactions in industrial applications because of their low cost and wide availability.30 However, these chloride substrates are reluctant to undergo catalytic reactions, due to their strong C−Cl bonds.30,31 Recently, PdPEPPSI catalysts have been demonstrated to be among the most reactive and general catalysts for Suzuki−Miyaura, Negishi, and Stille−Migita couplings toward carbon−carbon bond formation due to the beneficial effect of hemilabile ligands, whose role would be to open up a coordination site on the Pd center and facilitate binding of the substrate.13 Still, their application in Mizoroki−Heck coupling, particularly for less reactive substrates such as aryl chlorides, has been less studied.14g Hence, here we mainly focus on the catalytic performances of complexes 7−12 in Heck reactions of aryl chlorides with styrene.

bond with adjacent amide protons (the O···N bond distances are 2.845 and 2.948 Å). The 1D polymeric structures of complexes 7−9 are presented in Figures 1b−3b. Complexes 10−12 were also structurally characterized. As expected, the geometries around the Pd centers are found to be square planar (Figures 4a−6a), in which the Ccarbene−Pd− Npyridine angles in 10 (173.99(13)°), 11 (177.48(16)°), and 12 (173.00(12)°) are slightly deviated from linearity. The Pd− Ccarbene bond distances (1.946(3)−1.959(4) Å) in these complexes are similar to those found in complexes 7−9 and are also in good accord with that in the related transdibromo(diisopropylbenzimidazolin-2-ylidene)(pyridine)palladium(II) complex (1.953(4) Å) reported by Huynh et al.14a The dihedral angle between the azole plane and coordination plane in the range of 73.86−83.15° is in agreement with complexes 7−9. However, the azole and pyridine rings are nearly coplanar, and the dihedral angles are in the range of 3.38−7.19°. Similar to the case for complexes 7−9, the hydrogen-bonding interactions between the amide groups 3862


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Figure 5. (a) Crystal structure of 11 showing 50% probability thermal ellipsoids. Hydrogen atoms are omitted for clarity. (b) Dimeric structure of 11 showing weak N−H···O interactions. Selected bond lengths (Å) and angles (deg): Pd−C(1), 1.959(4); Pd−N(4), 2.084(4); Pd−Br(1), 2.4207(6); Pd−Br(2), 2.4433(6); N(1)−C(1), 1.337(5); N(2)−C(1), 1.350(5); C(1)−Pd−N(4), 177.48(16); C(1)−Pd−Br(1), 87.82(11); C(1)−Pd−Br(2), 89.00(12); N(4)− Pd−Br(1), 91.29(11); N(4)−Pd−Br(2), 92.18(11); Br(1)−Pd− Br(2), 172.20(2); N(1)−C(1)−N(2), 107.9(3).

Figure 6. (a) Crystal structure of 12 showing 50% probability thermal ellipsoids. Hydrogen atoms are omitted for clarity. (b) Dimeric structure of 12 showing weak N−H···O interactions. Selected bond lengths (Å) and angles (deg): Pd−C(1), 1.950(3); Pd−N(4), 2.094(3); Pd−Br(2), 2.4241(4); Pd−Br(1), 2.4426(4); N(1)−C(1), 1.347(4); N(1)−C(2), 1.385(4); C(1)−Pd−N(4), 173.00(12); C(1)−Pd−Br(1), 88.10(9); C(1)−Pd−Br(2), 89.40(9); N(4)−Pd− Br(1), 92.91(8); N(4)−Pd−Br(2), 90.40(8); Br(2)−Pd−Br(1), 172.633(15); N(1)−C(1)−N(2), 106.6(3).

At the outset, the reaction between bromobenzene and styrene was chosen as the benchmark to investigate the best catalyst loading (Table 1). We screened the reactions using complex 7 as the catalyst and tetra-n-butylammonium bromide (TBAB) as a cocatalyst. Reactions were carried out in the commonly used solvent N,N-dimethylformamide (DMF). On the basis of our earlier results, potassium carbonate was used as a base without further optimization.16a Entries 1−5 show that the catalyst loading of 0.1 mol % (entry 3) is enough to produce an excellent yield (>99%) of the coupled product in 2 h at 140 °C. Decreased catalyst loading produces lower yields even at longer reaction times, as reflected by entries 4 and 5. Other [PdBr2(NHC)(Py)] complexes (8−12) also generate a quantitative yield of stilbene (Table 1, entries 6−10) under similar conditions; trans product is formed exclusively in all the investigated complexes. To check the catalytic activity of these complexes towards the chloro-substituted compounds, 1-chloro-4-nitrobenzene was chosen as a model substrate to perform the coupling reaction. Under similar conditions (0.1 mol % of catalyst and 2 h of reaction time at 140 °C) complex 7 only produces traces of coupled product (Table 2, entry 1). With a prolonged reaction time of 15 h, a moderate yield (51%) is obtained (Table 2, entry 2). All other complexes (8−12) give moderate to good yields under these conditions. Especially, complexes 10−12 are found to be more active, suggesting that benzimidazole-derived complexes are more efficient catalysts than those imidazole

Table 1. Mizoroki−Heck Reaction of Bromobenzene and Styrenea

entry

catalyst

amt (mol %)

time (h)

yield (%)b

1 2 3 4 5 6 7 8 9 10

7 7 7 7 7 8 9 10 11 12

1 0.5 0.1 0.05 0.01 0.1 0.1 0.1 0.1 0.1

2 2 2 2 15 2 2 2 2 2

98 >99 >99 83 54 >99 >99 >99 >99 >99

a

Conditions: 1 mmol of bromobenzene, 1.5 mmol of styrene, 1.5 mmol of TBAB, 2.0 mmol of K2CO3, and 2 mL of DMF. bAverage isolated yield of two runs.

derivatives, perhaps due to the rich electron density of the former.32 It is worth mentioning that the catalytic activity increases with increasing alkyl chain length at the heterocycle 3863


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Table 2. Mizoroki−Heck Reaction of Aryl Chlorides and Styrenea

entry

R

catalyst

amt (mol %)

temp (°C)

time (h)

yield (%)b

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 C(O)Me C(O)Me C(O)Me C(O)Me C(O)Me C(O)Me H H H H H H CH3 CH3 CH3 CH3 CH3 CH3

7 7 8 9 10 11 12 7 8 9 10 11 12 7 8 9 10 11 12 7 8 9 10 11 12 7 8 9 10 11 12

0.1 0.1 0.1 0.1 0.1 0.1 0.1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

140 140 140 140 140 140 140 140 140 140 140 140 140 140 140 140 140 140 140 140 140 140 140 140 140 140 140 140 140 140 140

2 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15

trace 51 55 68 67 72 81 92 95 97 >99 >99 >99 90 95 97 97 99 >99 42 48 55 54 68 77 5 7 11 12 16 25

a

Conditions: 1 mmol of aryl chloride, 1.5 mmol of styrene, 1.5 mmol of TBAB, 2.0 mmol of K2CO3, and 2 mL of DMF. bAverage isolated yield of two runs.

Table 3. Mizoroki−Heck Reaction of Activated Aryl Chlorides and Styrene Using Catalyst 12a

entry

R

amt of catalyst (mol %)

temp (°C)

time (h)

yield (%)b

1 2 3 4 5 6c

o-CHO m-CHO p-CHO p-CF3 m-C(O)CH3 p-NO2

1 1 1 1 1 1

140 140 140 140 140 140

15 15 15 15 15 15

68 85 96 94 77 31

a

Conditions: 1 mmol of aryl chloride, 1.5 mmol of styrene, 1.5 mmol of TBAB, 2.0 mmol of K2CO3, and 2 mL of DMF. bAverage isolated yield of two runs. cAdded 5 mol % of Hg to poison the reaction.

(Table 2, entries 2−7). Finally, increasing the catalyst loading of complexes 7−12 to 1 mol % improves the reaction yield and produces quantitative yields for the coupling of 1-chloro-4nitrobenzene and styrene (Table 2, entries 8−13). In the case of 4-chloroacetophenone, excellent yields are also observed for all the complexes at 1% loading (entries 14−19).

When chlorobenzene is employed in the reaction, different catalytic activities are found for the six complexes. The benzimidazole-based complexes again showed better activity than the imidazole-based complexes (Table 2, entries 20−25). Unfortunately, the performance of these complexes decreased substantially when deactivated aryl chloride (4-chlorotoluene) 3864


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Figure 7. (a) Particle size distribution by intensity of Pd nanoparticles (12) in the catalytic reaction. (b) TEM image of 12 (×30K; 200 nm scale bar is shown). The inset depicts selected area electron diffraction.

is employed (Table 2, entries 26−31). Organ et al. proposed a possible catalytic mechanism which involves an initial activation of metal via oxidative addition of aryl halide. Then, the loss of pyridine ligand would free the position for incoming styrene to deliver the coupled product efficiently via reductive elimination.11h In consideration of this catalytic cycle, the activated or nonactivated substrates are more facile than the deactivated substrates in the oxidative addition step to insert the Pd catalyst into an aryl−chloride bond. Finally, 12 was utilized to catalyze other activated aryl chloride substrates (Table 3). Indeed, the catalyst system is capable of delivering excellent trans product yield with all aryl chlorides (Table 3, entries 1−5). Especially, the reactions of styrene with p-chlorobenzaldehyde and p-(trifluoromethyl)chlorobenzene produce quantitative yields (Table 3, entries 3 and 4). The steric effect of the substrate plays a role in the activity of o-chlorobenzaldehyde, m-chlorobenzaldehyde, or mchloroacetophenone, giving moderate to good yields (Table 3, entries 1, 2, and 5). Considering all these facts, the newly synthesized PEPPSI-themed catalysts are effective for Heck coupling toward para-substituted activated aryl chlorides at low catalyst loading. Crudden and co-workers demonstrated that some similar Pd PEPPSI complexes also show good activity toward the Mizoroki−Heck reaction; however, the catalysts decompose at higher temperature and hence are not suitable for use with chloro substrates.14g

Recently, palladium-catalyzed C−C coupling reactions showed the participation of nanoparticles.14g,33 We therefore attempted to see if there are any palladium nanoparticles formed during the reactions. Initial dynamic light scattering (DLS) studies witnessed the presence of Pd nanoparticles with a wide range of sizes (∼50−1000 nm) in the reaction mixture (Figure 7a). Further, the sample was analyzed by transmission electron microscopy (TEM); metal particles in the range ∼50− 800 nm are found. Most of the particles are agglomerated into microstructures, where the boundaries between the individual subunits are not clear. Selected area electron diffraction (SAED) reveals a face-centered cubic structure, as has been commonly observed.34 The TEM and SAED pictures are shown in Figure 7b. The mercury test has been largely exploited to identify heterogeneous catalysts due to its ability to poison metal(0) heterogeneous catalysts by formation of an amalgam or adsorption on the metal surface.14g Therefore, the catalytic reaction (Table 3, entry 6) with 1-chloro-4-nitrobenzene was repeated in the presence of mercury (5 mol %); only 31% of the coupled product is observed, supporting the involvement of a heterogeneous catalyst. In an earlier report, the heterogeneous nature of a PEPPSI type catalyst was suggested on the basis of a mercury poisoning experiment.14g In this work, results of DLS, TEM, and mercury poisoning studies all strongly support the participation of palladium nanoparticles in the catalytic reaction. 3865


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163 °C. 1H NMR (DMSO-d6, 400 MHz): δ 9.33 (s, 1H, NCHN), 7.97 (s, 1H, NH2), 7.87 (s, 1H, CHCH), 7.74 (s, 1H, CHCH), 7.51 (s, 1H, NH2), 5.03 (s, 2H, NCH2), 4.23 (q, J = 7.3 Hz, 2H, NCH2CH3), and 1.35 ppm (t, J = 7.3 Hz, 3H, NCH2CH3). 13C NMR (DMSO-d6, 100 MHz): δ 167.2 (CO), 137.2 (NCHN), 124.2, 121.9 (CHCH), 50.9 (NCH2), 44.7 (NCH2CH3), and 15.7 ppm (NCH2CH3). IR (KBr): 3390 (w), 3324 (vw), 3307 (vw), 3263 (vw), 3231 (vw), 3148 (w), 3115 (w), 2989 (w), 2912 (w), 2846 (vw), 2802 (w), 2731 (w), 2588 (vw), 2538 (vw), 2495 (vw), 2456 (vw), 2418 (vw), 2379 (vw), 2352 (vw), 2054 (w), 1989 (w), 1940 (vw), 1920 (vw), 1904 (vw), 1885 (vw), 1866 (vw), 1737 (w), 1693 (vs), 1649 (vw), 1613 (w), 1564 (s), 1506 (vw), 1460 (w), 1438 (m), 1399 (s), 1353 (w), 1339 (w), 1309 (m), 1248 (vw), 1226 (vw), 1204 (w), 1169 (s), 1122 (m), 1095 (m), 1021 (m), 971 (w), 958 (w), 881 (m), 862 (w), 785 (m), 771 (m), 753 (w), 669 (m), 647 (s), 620 (m), 603 (m), 538 (w), 516 (vw), 499 (vw), 483 (vw), and 466 (vw) cm−1. Anal. Calcd for C7H12BrN3O: C, 35.92; H, 5.17; N, 17.95. Found: C, 35.70; H, 5.12; N, 17.90. Synthesis of 1-Acetamido-3-butylimidazolium Bromide (3). The preparation of compound 3 was similar to that of compound 2. From 124.1 mg of 1-butylimidazole (1.0 mmol) and 137.9 mg of 2bromoacetamide (1.0 mmol), 241.1 mg of colorless crystals was obtained (92% yield). Mp: 122−124 °C. 1H NMR (DMSO-d6, 400 MHz): δ 9.32 (s, 1H, NCHN), 7.98 (s, 1H, NH2), 7.87 (s, 1H, CH CH), 7.75 (s, 1H, CHCH), 7.51 (s, 1H, NH2), 5.03 (s, 2H, NCH2), 4.22 (t, J = 6.9 Hz, 2H, NCH2CH2−), 1.72 (m, 2H, NCH2CH2−), 1.19 (m, 2H, NCH2CH2CH2−), and 0.83 ppm (t, J = 6.8 Hz, 3H, NCH2CH2CH2CH3). 13C NMR (DMSO-d6, 100 MHz): δ 167.2 (CO), 137.6 (NCHN), 124.3, 122.2 (CHCH), 50.9 (NCH2), 48.9 (NCH2CH2−), 31.8 (NCH2CH2−), 19.1 (NCH2CH2CH2−), and 13.7 ppm (NCH2CH2CH2CH3). IR (KBr): 3462 (w), 3379 (vw), 3308 (w), 3264 (w), 3143 (w), 3066 (w), 2984 (w), 2956 (w), 2929 (w), 2863 (vw), 2791 (vw), 2714 (w), 2643 (vw), 2599 (vw), 2478 (vw), 2412 (vw), 2220 (vw), 2071 (vw), 2000 (vw), 1693 (vs), 1619 (s), 1564 (s), 1460 (m), 1446 (m), 1399 (s), 1347 (m), 1309 (m), 1289 (m), 1273 (m), 1204 (m), 1172 (s), 1103 (m), 1037 (w), 968 (w), 867 (w), 856 (m), 790 (m), 765 (m), 713 (vw), 637 (s), 620 (m), 598 (m), and 538 (w) cm − 1 . Anal. Calcd for C9H16BrN3O·1/3H2O: C, 40.31; H, 6.26; N, 15.67. Found: C, 40.50; H, 6.26; N, 15.80. Synthesis of 1-Acetamido-3-ethylbenzimidazolium Bromide (5). A mixture of 1-ethylbenzimidazole (146.2 mg, 1.0 mmol) and 2bromoacetamide (137.9 mg, 1.0 mmol) in acetonitrile was heated to reflux under nitrogen overnight to form a white precipitate. After completion, the solvent was removed, and the solid was washed with ether repeatedly. The resulting white solid was crystallized by slow diffusion of ether into a methanol solution of the compound, and colorless crystals were obtained (198.9 mg, 70% yield). Mp: 209−211 °C. 1H NMR (DMSO-d6, 400 MHz): δ 10.03 (s, 1H, NCHN), 8.18 (s, 1H, NH2), 8.10 (m, 1H, C6H4), 7.94 (m, 1H, C6H4), 7.71 (s, 1H, NH2), 7.65 (s, 2H, C6H4), 5.40 (s, 2H, NCH2), 4.59 (q, J = 6.5 Hz, 2H, NCH2CH3), and 1.52 ppm (t, J = 6.5 Hz, 3H, NCH2CH3). 13C NMR (DMSO-d6, 100 MHz): δ 167.0 (CO), 143.3 (NCHN), 132.1, 130.9, 127.1, 126.9, 114.2, 114.0 (C6H4), 48.9 (NCH2), 42.7 (NCH2CH3), and 14.7 ppm (NCH2CH3). IR (KBr): 3467 (w), 3418 (vw), 3313 (w), 3253 (w), 3137 (m), 3038 (w), 2978 (w), 2934 (w), 2863 (vw), 2824 (vw), 2758 (vw), 2714 (vw), 2538 (vw), 2373 (vw), 2341 (w), 2027 (w), 1992 (w), 1964 (vw), 1937 (vw), 1920 (vw), 1896 (vw), 1882 (vw), 1830 (vw), 1794 (vw), 1772 (vw), 1687 (vs), 1610 (m), 1567 (s), 1490 (m), 1465 (m), 1449 (m), 1432 (m), 1396 (s), 1385 (m), 1364 (m), 1292 (m), 1268 (w), 1254 (w), 1221 (m), 1196 (m), 1169 (w), 1144 (w), 1133 (w), 1086 (w), 1037 (m), 1015 (w), 982 (w), 944 (w), 892 (w), 853 (m), 798 (w), 749 (s), 713 (w), 669 (w), 653 (w), 617 (m), 598 (m), 578 (w), 565 (w), 554 (w), 516 (w), and 480 (vw) cm−1. Anal. Calcd for C11H14BrN3O: C, 46.50; H, 4.97; N, 14.79. Found: C, 46.65; H, 4.79; N, 14.40. Synthesis of 1-Acetamido-3-butylbenzimidazolium Bromide (6). The preparation of 6 was similar to that of compound 2. From 174.2 mg of 1-butylbenzimidazole (1.0 mmol) and 137.9 mg of 2bromoacetamide (1.0 mmol), 287.2 mg of colorless crystals was

CONCLUSIONS In conclusion, the synthesis of six PEPPSI motif complexes, [PdBr2(NHC)(Py)] (7−12), from substituted imidazolium or benzimidazolium salts are described. All complexes exhibit square-planar geometry with a trans arrangement of carbene and pyridine ligands. Intermolecular hydrogen bonding between adjacent amide groups results in a 1D polymeric structure for 7−9 and a dimeric structure for 10−12. In complex 9, the 1D polymers are further connected to the solvent molecules to form a double-layered 1D polymeric structure. We further illustrated the use of these complexes as catalysts in the Mizoroki−Heck reaction. All complexes show excellent activity for the substrate of bromobenzene at low catalyst loading (0.1 mol %). These complexes are also highly effectual toward activated aryl chlorides. Among the Pd complexes used, the benzimidazole-based complexes (10−12) are more effective to some extent than imidazole-based complexes (7−9). The complexes may serve not only as a source of soluble Pd species but also as a source of Pd nanoparticles. Further studies of their applicability in other synthetic transformations are currently underway.

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EXPERIMENTAL SECTION

General Procedures. All solvents and chemicals were analytical reagent grade and were used as obtained from the commercial suppliers. The compounds 1-acetamido-3-methylimidazolium bromide (1) and 1-acetamido-3-methylbenzimidazolium bromide (4) were synthesized according to our earlier report.17 Elemental analyses were carried out on a Hitachi 270-30 spectrometer. 1H NMR spectra (δ(TMS) 0.00 ppm) were recorded on either a Bruker Avance DPX300 or a Bruker Avance II 400 spectrometer operating at 300.13 and 400.13 MHz, respectively. 13C NMR spectra were recorded on either a Bruker Avance DPX300 or a Bruker Avance II 400 spectrometer operating at 75.46 and 100.61 MHz, respectively. Infrared spectra were recorded on a Jasco FT/IR-460 Plus spectrometer with 4 cm−1 resolution. Transmission electron micrographs were recorded with a JEOL JEM-3010 transmission electron microscope (TEM). Palladium catalyst 12 colloidal solution from the Heck reaction mixture was collected after 1 h of reaction time and washed with methanol twice. Samples for TEM were prepared by doping the washed colloidal solution onto a 400 mesh copper grid coated with carbon. The DLS studies were performed several times by Zetasizer Nano ZS (Malvern Instrument, Malvern, U.K.) using the same colloidal solution at 25 °C. Each measurement consisted of a minimum of 30 runs, which was calculated automatically by Dispersion Technology Software Version 4.2. X-ray Structure Determination. Crystallographic data collections were carried out on a Nonius Kappa CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 150(2), 200(2), or 298(2) K. Unit cell parameters were retrieved and refined using DENZO-SMN35 software on all reflections. Data reduction was performed with the DENZO-SMN 34 software. An empirical absorption was based on the symmetry-equivalent reflections and was applied to the data using the SORTAV36 program. The structures were solved using the SHELXS-9737 program and refined using the SHELXL-9738 program by full matrix least squares on F2 values. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were fixed at calculated positions and refined using a riding mode. Synthesis of 1-Acetamido-3-ethylimidazolium Bromide (2). A mixture of 1-ethylimidazole (96.1 mg, 1.0 mmol) and 2bromoacetamide (137.9 mg, 1.0 mmol) in acetonitrile was heated to reflux under nitrogen overnight to form a white precipitate. After completion, the solvent was removed, and the solid was washed with 20 mL of dichloromethane. The resulting white solid was crystallized by slow diffusion of ether into a methanol solution of the compound, and colorless crystals were obtained (165.3 mg, 72% yield). Mp: 161− 3866


Organometallics

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obtained (92% yield). Mp: 201−203 °C. 1H NMR (DMSO-d6, 400 MHz): δ 9.99 (s, 1H, NCHN), 8.15 (s, 1H, NH2), 8.13 (m, 1H, C6H4), 7.95 (m, 1H, C6H4), 7.67 (s, 1H, NH2), 7.66 (m, 2H, C6H4), 5.40 (s, 2H, NCH2), 4.57 (t, J = 6.9 Hz, 2H, NCH2CH2−), 1.85 (m, 2H, NCH2 CH2−), 1.30 (m, 2H, NCH2CH2CH2−), and 0.87 ppm (t, J = 7.3 Hz, 3H, NCH2CH2CH2CH3). 13C NMR (DMSO-d6, 100 MHz): δ 166.9 (CO), 143.7 (NCHN), 132.1, 131.1, 127.1, 126.9, 114.2, 114.1 (C6H4), 48.9 (NCH2), 46.9 (NCH2CH2−), 31.0 (NCH 2 CH 2 −), 19.4 (NCH 2 CH 2 CH 2 −), and 13.8 ppm (NCH2CH2CH2CH3). IR (KBr): 3368 (w), 3291 (w), 3269 (w), 3126 (w), 3055 (w), 3032 (w), 3005 (w), 2951 (w), 2934 (w), 2868 (w), 2775 (w), 2753 (w), 2687 (w), 2379 (w), 2341 (w), 2291 (w), 1942 (w), 1909 (w), 1868 (w), 1846 (w), 1827 (w), 1783 (w), 1698 (vs), 1649 (w), 1608 (w), 1558 (s), 1506 (w), 1482 (w), 1460 (m), 1421 (m), 1399 (s), 1364 (m), 1306 (m), 1276 (m), 1246 (w), 1202 (m), 1163 (w), 1141 (w), 1092 (w), 1054 (w), 1026 (w), 1012 (w), 966 (w), 938 (w), 900 (w), 867 (w), 826 (w), 812 (w), 768 (m), 741 (vs), 694 (w), 656 (m), 634 (m), 601 (w), 579 (w), 568 (m), 549 (w), 527 (w), 513 (w), and 486 (vw) cm−1. Anal. Calcd for C13H18BrN3O: C, 50.01; H, 5.81; N, 13.46. Found: C, 50.41; H, 5.55; N, 13.39. Preparation of [Pd(Me-imy-CH2CONH2)(Py)Br2] (7). A 50 mL flask was charged with 1-acetamido-3-methylimidazolium bromide (111.3 mg, 0.5 mmol), PdBr2 (133.2 mg, 0.5 mmol), CH3COONa (41.7 mg, 0.51 mmol), and about 15 mL of pyridine as a solvent as well as a reagent. The solution was heated at 100 °C overnight, and pyridine was removed after completion. The residue was washed several times with ether to remove most of the pyridine and finally dissolved in dichloromethane and filtered. The pure compound was obtained by column chromatography using dichloromethane. X-rayquality crystals were isolated from a 1/1 hexane/ethyl acetate (v/v) mixture. Yellow crystals (98.5 mg, 41% yield) were obtained. Mp: 150−151 °C. 1H NMR (DMSO-d6, 300 MHz): δ 8.99 (m, 2H, o-Py), 7.79 (m, 1H, p-Py), 7.37 (m, 2H, m-Py), 7.08 (d, J = 2 Hz, 1H, CH CH), 7.03 (d, J = 2 Hz, 1H, CHCH), 6.83 (br s, 1H, NH2), 5.55 (br s, 1H, NH2), 5.28 (s, 2H, NCH2), and 4.14 ppm (s, 3H, NCH3). 13C NMR (DMSO-d6, 75 MHz): δ 168.9 (CO), 152.5, 138.1, 124.7 (Py), 150.3 (NCN), 124.5, 122.4 (CHCH), 54.4 (NCH2), and 38.6 ppm (NCH3). IR (KBr): 3404 (vw), 3316 (m), 3267 (m), 3195 (w), 3164 (w), 3131 (m), 3114 (w), 3042 (vw), 2978 (vw), 2923 (vw), 2789 (vw), 2716 (vw), 2482 (vw), 2359 (vw), 2339 (vw), 1673 (vs), 1604 (s), 1573 (m), 1474 (m), 1447 (s), 1435 (m), 1404 (m), 1343 (w), 1306 (m), 1244 (m), 1212 (w), 1209 (w), 1154 (vw), 1131 (vw), 1092 (vw), 1069 (w), 1048 (vw), 1016 (vw), 965 (vw), 941 (vw), 877 (vw), 846 (vw), 800 (w), 759 (m), 732 (m), 703 (m), 675 (m), 644 (vw), 604 (vw), 550 (w), 520 (vw), 461 (vw), 446 (vw), and 418 (vw) cm−1. Anal. Calcd for C11H14Br2N4OPd: C, 27.27; H, 2.91; N, 11.56. Found: C, 27.29; H, 2.96; N, 11.65. Preparation of [Pd(Et-imy-CH2CONH2)(Py)Br2] (8). Complex 8 was prepared in analogy to complex 7 from 2 (118 mg, 0.5 mmol), PdBr2 (133.2 mg, 0.5 mmol), and CH3COONa (41.7 mg, 0.51 mmol). The compound was purified by column chromatography using a 25/1 dichloromethane/methanol (v/v) mixture. Slow diffusion of ether into a chloroform solution of the compound afforded 152.0 mg (61% yield) of yellow crystals. Mp: 231−235 °C dec. 1H NMR (DMSO-d6, 400 MHz): δ 9.00 (m, 2H, o-Py), 7.79 (m, 1H, p-Py), 7.37 (m, 2H, m-Py), 7.09 (d, J = 2 Hz, 1H, CHCH), 7.06 (d, J = 2 Hz, 1H, CHCH), 6.84 (br s, 1H, NH2), 5.56 (br s, 1H, NH2), 5.29 (s, 2H, NCH2), 4.64 (q, J = 7.4 Hz, 2H, NCH2CH3), and 1.64 ppm (t, J = 7.4 Hz, 3H, NCH2CH3). 13C NMR (DMSO-d6, 100 MHz): δ 168.9 (CO), 152.5, 138.1, 124.7 (Py), 149.4 (NCN), 122.6, 122.4 (CHCH), 54.6 (NCH2), 46.5 (NCH2CH3), and 15.4 ppm (NCH2CH3). IR (KBr): 3418 (m), 3313 (w), 3269 (w), 3170 (w), 3137 (w), 3115 (w), 3071 (w), 3038 (vw), 3027 (vw), 2977 (w), 2951 (w), 2924 (w), 2875 (vw), 2795 (vw), 2704 (vw), 2482 (vw), 2374 (w), 2361 (w), 2346 (vw), 2340 (vw), 2332 (vw), 2302 (vw), 2225 (vw), 1985 (vw), 1968 (vw), 1944 (vw), 1922 (w), 1910 (vw), 1870 (vw), 1848 (vw), 1830 (vw), 1810 (vw), 1802 (vw), 1782 (vw), 1773 (vw), 1759 (vw), 1750 (vw), 1732 (w), 1705 (m), 1698 (m), 1681 (vs), 1651 (w), 1639 (w), 1604 (s), 1571 (w), 1560 (w), 1542 (w), 1538 (w), 1527 (vw), 1516 (vw), 1508 (w), 1501 (vw), 1490 (vw), 1472 (m), 1448 (vs), 1439 (s),

1434 (s), 1410 (s), 1385 (m), 1364 (w), 1353 (w), 1308 (m), 1264 (m), 1237 (s), 1212 (w), 1197 (w), 1152 (vw), 1134 (w), 1111 (w), 1091 (w), 1069 (s), 1049 (w), 1038 (w), 1020 (w), 985 (vw), 952 (w), 940 (vw), 862 (vw), 820 (w), 803 (w), 791 (m), 760 (s), 725 (s), 685 (vs), 663 (w), 645 (w), 601 (vw), 567 (vw), 531 (vw), 519 (w), and 461 (w) cm−1. Anal. Calcd for C12H16Br2N4OPd: C, 28.91; H, 3.24; N, 11.24. Found: C, 28.91; H, 3.31; N, 11.30. Preparation of [Pd(Bu-imy-CH2CONH2)(Py)Br2] (9). Complex 9 was prepared in analogy to complex 7 from 3 (132 mg, 0.5 mmol), PdBr2 (133.2 mg, 0.5 mmol), and CH3COONa (41.7 mg, 0.51 mmol). The compound was purified by column chromatography using a 25/1 dichloromethane/methanol (v/v) mixture. Slow diffusion of ether into a DMSO/THF (1/1) solution of the compound afforded 174.0 mg (54% yield) of yellow crystals. Mp: 265−268 °C dec. 1H NMR (DMSO-d6, 400 MHz): δ 8.99 (m, 2H, o-Py), 7.78 (m, 1H, p-Py), 7.37 (m, 2H, m-Py), 7.07 (d, J = 2.0 Hz, 1H, CHCH), 7.03 (d, J = 2.0 Hz, 1H, CHCH), 6.85 (br s, 1H, NH2), 5.49 (br s, 1H, NH2), 5.29 (s, 2H, NCH2), 4.54 (t, J = 7.6 Hz, 2H, NCH2CH2−), 2.09 (m, 2H, NCH2CH2−), 1.50 (m, 2H, NCH2CH2CH2−), and 1.04 ppm (t, J = 7.3 Hz, 3H, NCH2 CH2CH2CH3). 13C NMR (DMSO-d6, 100 MHz): δ 168.3 (CO), 152.5, 139.1, 125.4 (Py), 147.0 (NCN), 124.5, 122.7 (CHCH), 52.8 (NCH2), 50.3 (NCH2CH2−), 32.1 (NCH2CH2−), 19.7 (NCH2CH2CH2−), and 14.1 ppm (NCH2CH2CH2CH3). IR (KBr): 3456 (m), 3428 (w), 3324 (w), 3286 (w), 3258 (w), 3159 (w), 3126 (w), 3104 (w), 3066 (w), 3033 (w), 2956 (m), 2923 (w), 2857 (w), 2368 (vw), 2341 (w), 2330 (w), 2308 (w), 2016 (w), 1992 (w), 1967 (w), 1942 (w), 1923 (w), 1909 (w), 1890 (w), 1868 (w), 1846 (w), 1830 (w), 1811 (w), 1794 (w), 1772 (w), 1759 (vw), 1745 (w), 1731 (w), 1693 (vs), 1679 (vs), 1649 (w), 1646 (w), 1635 (vw), 1599 (m), 1569 (w), 1558 (w), 1542 (w), 1522 (w), 1517 (w), 1490 (w), 1465 (m), 1457 (m), 1446 (s), 1432 (s), 1405 (m), 1375 (w), 1350 (w), 1339 (w), 1309 (m), 1287 (w), 1259 (w), 1240 (m), 1213 (m), 1182 (w), 1147 (w), 1136 (w), 1106 (w), 1092 (w), 1067 (m), 1045 (w), 1015 (w), 982 (w), 963 (w), 944 (w), 875 (w), 862 (w), 804 (w), 765 (m), 733 (w), 719 (w), 697 (m), 678 (m), 639 (w), 604 (w), 565 (w), 516 (w), and 475 (w) cm−1. Anal. Calcd for C14H20Br2N4OPd: C, 31.93; H, 3.83; N, 10.64. Found: C, 31.82; H, 3.83; N, 10.66. Preparation of [Pd(Me-bimy-CH2CONH2)(Py)Br2] (10). Complex 10 was prepared in analogy to complex 7 from 4 (135.2 mg, 0.5 mmol), PdBr2 (133.2 mg, 0.5 mmol), and CH3COONa (41.7 mg, 0.51 mmol). The pure compound was obtained by column chromatography using dichloromethane. X-ray-quality crystals were isolated from a 1/1 hexane/ethyl acetate (v/v) mixture. Yellow crystals were obtained (125.8 mg, 47% yield). Mp: 274−278 °C dec. 1H NMR (DMSO-d6, 300 MHz): δ 9.04 (m, 2H, o-Py), 7.83 (m, 1H, p-Py), 7.51 (m, 1H, C6H4), 7.39 (m, 5H, C6H4, m-Py), 6.92 (br s, 1H, NH2), 5.57 (s, 2H, NCH2), 5.52 (br s, 1H, NH2), and 4.36 ppm (s, 3H, NCH3). 13C NMR (DMSO-d6, 75 MHz): δ 168.6 (CO), 163.9 (NCN), 152.5, 138.3, 124.8 (py), 135.2, 134.2 124.1, 110.7, 110.2 (C6H4), 52.7 (NCH2), and 35.3 ppm (NCH3). IR (KBr): 3417 (w), 3356 (vw), 3284 (vw), 3155 (w), 3056 (vw), 2964 (vw), 2928 (vw), 2848 (vw), 2763 (vw), 1689 (vs), 1603 (m), 1480 (w), 1447 (s), 1402 (m), 1384 (m), 1342 (w), 1321 (w), 1261 (w), 1237 (vw), 1218 (vw), 1199 (w), 1148 (vw), 1125 (w), 1100 (w), 1073 (w), 1036 (vw), 1011 (vw), 978 (vw), 949 (vw), 914 (vw), 876 (vw), 849 (vw), 814 (w), 796 (m), 754 (s), 696 (m), 671 (w), 647 (vw), 618 (vw), 602 (vw), 582 (vw), 563 (w), 541 (vw), 499 (m), 433 (vw), and 422 (vw) cm−1. Anal. Calcd for C15H16Br2N4OPd: C, 33.70; H, 3.02; N, 10.48. Found: C, 33.73; H, 3.06; N, 10.56. Preparation of [Pd(Et-bimy-CH2CONH2)(Py)Br2] (11). Complex 11 was prepared in analogy to complex 7 from 5 (142 mg, 0.5 mmol), PdBr2 (133.2 mg, 0.5 mmol), and CH3COONa (41.7 mg, 0.51 mmol). The compound was purified by column chromatography using a 25/1 dichloromethane/methanol (v/v) mixture. Slow diffusion of ether into a chloroform solution of the compound afforded 143.0 mg (52% yield) of yellow crystals. Mp: 270−274 °C dec. 1H NMR (DMSO-d6, 400 MHz): δ 9.06 (m, 2H, o-Py), 7.82 (m, 1H, p-Py), 7.52 (m, 1H, C6H4), 7.46 (m, 1H, C6H4), 7.41 (m, 2H, m-Py), 7.36 (m, 2H, C6H4), 6.96 (br s, 1H, NH2), 5.58 (s, 2H, NCH2), 5.46 (br s, 1H, NH2), 4.97 (q, J = 7.3 Hz, 2H, NCH2CH3), and 1.76 ppm (t, J = 7.3 Hz, 3H, 3867


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NCH2CH3). 13C NMR (DMSO-d6, 100 MHz): δ 168.5 (CO), 163.1 (NCN), 152.6, 138.2, 124.8 (Py), 134.6, 134.1, 124.0, 110.9, 110.4 (C6H4), 52.9 (NCH2), 44.1 (NCH2CH3), and 14.3 ppm (NCH2CH3). IR (KBr): 3407 (m), 3291 (w), 3149 (w), 3059 (vw), 2973 (w), 2930 (w), 2871 (w), 2850 (w), 2764 (vw), 2527 (vw), 2475 (vw), 2427 (vw), 2358 (w), 2346 (w), 2324 (vw), 2209 (vw), 1994 (vw), 1963 (vw), 1944 (vw), 1911 (vw), 1870 (w), 1848 (vw), 1803 (vw), 1812 (vw), 1796 (vw), 1773 (vw), 1750 (vw), 1718 (w), 1704 (m), 1690 (vs), 1651 (w), 1638 (vw), 1604 (m), 1575 (vw), 1560 (w), 1544 (w), 1538 (vw), 1523 (vw), 1518 (vw), 1508 (w), 1498 (vw), 1489 (w), 1480 (w), 1466 (m), 1447 (m), 1424 (m), 1418 (m), 1385 (w), 1357 (vw), 1340 (vw), 1324 (w), 1278 (w), 1262 (vw), 1246 (w), 1214 (w), 1198 (m), 1149 (w), 1131 (w), 1104 (w), 1089 (w), 1071 (m), 1041 (w), 1016 (w), 981 (vw), 938 (vw), 873 (w), 846 (vw), 831 (w), 805 (w), 779 (w), 753 (s), 692 (s), 670 (w), 664 (w), 645 (w), 621 (m), 605 (w), 588 (w), 567 (w), 548 (w), 504 (m), and 473 (w) cm−1. Anal. Calcd for C16H18Br2N4OPd: C, 35.03; H, 3.31; N, 10.21. Found: C, 35.04; H, 3.30; N, 9.99. Preparation of [Pd(Bu-bimy-CH2CONH2)(Py)Br2] (12). Complex 12 was prepared in analogy to complex 7 from 6 (156 mg, 0.5 mmol), PdBr2 (133.2 mg, 0.5 mmol), and CH3COONa (41.7 mg, 0.51 mmol). The compound was purified by column chromatography using a 25/1 dichloromethane/methanol (v/v) mixture. Slow diffusion of ether into a chloroform solution of the compound afforded 148.0 mg (52% yield) of yellow crystals. Mp: 180−189 °C dec. 1H NMR (DMSO-d6, 400 MHz): δ 9.04 (m, 2H, o-Py), 7.82 (m, 1H, p-Py), 7.53 (m, 1H, C6H4), 7.47 (m, 1H, C6H4), 7.43 (m, 2H, m-Py), 7.37 (m, 2H, C6H4), 6.95 (br s, 1H, NH2), 5.59 (s, 2H, NCH2), 5.46 (br s, 1H, NH2), 4.84 (t, J = 7.8 Hz, 2H, NCH2CH2−), 2.24 (m, 2H, NCH2CH2−), 1.60 (m, 2H, NCH2CH2CH2−), and 1.10 ppm (t, J = 7.4 Hz, 3H, NCH2 CH2CH2CH3). 13C NMR (DMSO-d6, 100 MHz): δ 168.6 (CO), 163.3 (NCN), 152.5, 138.2, 124.8 (Py), 134.7, 134.4, 123.9, 110.9, 110.5 (C6H4), 52.9 (NCH2), 48.8 (NCH2CH2−), 31.0 (NCH2 CH2 −), 20.3 (NCH2 CH2CH2 −), and 13.8 ppm (NCH2CH2CH2CH3). IR (KBr): 3412 (m), 3352 (vw), 3280 (vw), 3159 (w), 3055 (vw), 3038 (vw), 3000 (vw), 2951 (m), 2929 (w), 2868 (w), 2764 (vw), 2473 (vw), 2357 (vw), 2341 (vw), 1992 (vw), 1967 (vw), 1942 (vw), 1920 (vw), 1909 (vw), 1890 (vw), 1868 (vw), 1846 (vw), 1827 (vw), 1811 (vw), 1797 (vw), 1781 (vw), 1748 (vw), 1690 (vs), 1651 (w), 1638 (vw), 1602 (s), 1558 (vw), 1542 (vw), 1523 (vw), 1518 (vw), 1510 (w), 1499 (vw), 1479 (m), 1461 (s), 1449 (m), 1424 (s), 1418 (s), 1384 (m), 1376 (w), 1361 (vw), 1340 (w), 1325 (w), 1293 (vw), 1260 (vw), 1239 (vw), 1214 (m), 1193 (w), 1150 (w), 1134 (w), 1104 (w), 1073 (m), 1046 (w), 1017 (w), 980 (vw), 951 (vw), 940 (vw), 873 (vw), 849 (vw), 826 (w), 801 (w), 754 (vs), 692 (s), 674 (w), 645 (w), 631 (w), 608 (w), 594 (w), 570 (w), 551 (w), 504 (w), and 474 (vw) cm−1. Anal. Calcd for C18H22Br2N4OPd: C, 37.49; H, 3.85; N, 9.72. Found: C, 37.45; H, 3.87; N, 9.69%. General Procedure for the Mizoroki−Heck Coupling. In a typical run, a sealed tube was charged with aryl halide (1.0 mmol), styrene (1.5 mmol), potassium carbonate (2 mmol), palladium precatalyst, TBAB (1.5 mmol), and 2 mL of DMF. The tube was locked up and then placed in a preheated oil bath at the appropriate temperature. After the mixture was cooled, it was diluted with water (10 mL) and extracted with diethyl ether (3 × 10 mL). The combined organic portions were then dried over anhydrous magnesium sulfate. After filtration, the solvent was removed completely under vacuum. Isolated yields were obtained after purification with column chromatography on silica gel using a 20/1 n-hexane/EA (v/v) mixture. Products were identified by comparison of their NMR data with those in the literature.39

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AUTHOR INFORMATION

Corresponding Author

*F.-C.L.: e-mail, [email protected]; fax, 886-3-8633570. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Council of the ROC through Grant NSC 101-2113-M-259-005-MY3. REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic data (Tables S1−S30) for all of the complexes and crystallographic information files (CIF) of complexes 7−12. This material is available free of charge via the Internet at http://pubs.acs.org. 3868


Organometallics

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