Synthesis, Structures, and Norbornene Polymerization Behavior of

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Synthesis, Structures, and Norbornene Polymerization Behavior of Palladium Complexes Bearing Tridentate o‑Aryloxide-N-heterocyclic Carbene Ligands Dandan Yang,† Yungang Tang,† Haibin Song,† and Baiquan Wang*,†,‡,§ †

State Key Laboratory of Elemento-Organic Chemistry, College of Chemistry, and ‡Collaborative Innovation Center of Chemical Science and Engineering, Nankai University, Tianjin 300071, People’s Republic of China § State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, People’s Republic of China S Supporting Information *

ABSTRACT: A series of new pincer-type tridentate o-aryloxide-N-heterocyclic carbene ligands 2a−d were synthesized. Treatment of the proligands with Ag2O and (COD)PdCl2 afforded the desired o-aryloxide-NHC tridentate palladium complexes 3a−d in high yields (NHC = N-heterocyclic carbene). In comparison with the above tridentate complexes, bidentate bis(aryloxide-NHC) palladium complex 3e was also synthesized. All of these complexes were fully characterized by 1H and 13C NMR spectroscopy, high-resolution mass spectrometry, and elemental analysis. The molecular structures of 3a,b,d,e were determined by single-crystal X-ray diffraction analysis. On activation with either methylaluminoxane (MAO) or diethylaluminum chloride (Et2AlCl), all palladium complexes exhibited excellent activities of up to 5.99 × 107 g of PNB (mol of Pd)−1 h−1 toward norbornene addition polymerization, and the monomer conversion is up to 99.9%. Notably, the tridentate palladium complexes show better activities than the corresponding bidentate bis(aryloxide-NHC) palladium complexes in the presence of MAO. The resulting polymers were soluble in CHCl3 when the reactions were conducted in the presence of Et2AlCl and were characterized by gel permeation chromatography (GPC).

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INTRODUCTION N-heterocyclic carbenes (NHCs) as well as their transitionmetal complexes have been applied in many types of homogeneous catalytic reactions since the isolation and characterization of the first stable imidazol-2-ylidene by Arduengo and co-workers in 1991.1 The introduction of an NHC ligand into transition-metal complexes can generally enhance catalytic activities.2 Examples of this improved reactivity include ruthenium-based olefin metathesis catalysts3 and palladium-based catalysts for C−C and C−N coupling reactions.2,4 Another attractive feature of NHCs is the potential variability in the substituents at the nitrogen atoms, which allows for a wide range of asymmetric,5 steric,6 and chelating7 features. Recently, a great deal of research effort has been devoted to the study of transition-metal complexes with anion-tethered NHC ligands.8 Our group is interested in o-aryloxide-Nheterocyclic carbene ligands, which are capable of binding through the carbene carbon and the phenoxide oxygen to provide an [O,C]-type chelate. These ligands are analogous to the salicylaldimine framework (Chart 1), a common motif in organometallic chemistry that has been extensively applied in catalyzing organic reactions and olefin polymerization.9 In our previous work we synthesized a series of o-aryloxide-Nheterocyclic carbene as well as NHC-sulfonate transition© XXXX American Chemical Society

Chart 1

metal complexes, which exhibited high activities for the polymerization of norbornene (NBE).10 In order to accomplish high catalytic activity and stability for palladium(II) complexes, we aimed to design and synthesize new pincer complexes containing o-aryloxide-NHC ligands. While pincer ligands are known to give stable complexes, there has been only two reports for the palladium(II) complexes with tridentate carbene ligands containing aryloxy groups. It must be said, though, that the above tridentate Pd complexes have been respectively applied to catalyze Heck reactions7e and hydroamination,7l Special Issue: Organometallics in Asia Received: December 11, 2015

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palladium complex 3e were synthesized by the reactions of proligands 2a−e with 3 equiv of Ag2O and 1 equiv of (COD)PdCl2 in good yields (Scheme 2). All of these palladium complexes 3a−e are air and moisture stable. They are all soluble in CH2Cl2, CHCl3, and 1,2dichlorobenzene and insoluble or only partially soluble in diethyl ether, toluene, and chlorobenzene. Complexes 3 have been fully characterized by 1H and 13C NMR, HRMS, and elemental analysis. In their 1H NMR spectra the signals of the phenol and imidazole protons at the C-2 position for the proligands disappeared completely. The chemical shifts of the pyridine protons were shifted downfield in comparison with those in the proligands owing to the coordination to the metal (pyridyl-H of 3a, from 8.57 to 9.32 ppm in DMSO-d6; pyridylH of 3b, from 8.62 to 9.57 ppm in CDCl3). The chemical shifts of the N substituents were slightly shifted upfield (im-CH2 of 3a, from 5.70 to 5.62 ppm in DMSO-d6; im-CH2 of 3b, from 5.73 to 5.40 ppm in CDCl3; im-CH2 of 3c, from 4.83 to 4.34 ppm in DMSO-d6; im-CH2 of 3d, from 4.86 to 4.31 ppm in CDCl3; im-CH2 of 3e, from 5.55 to 5.01 ppm in CDCl3). The signals of the carbene carbons in 13C NMR spectra (154.7 ppm for 3a; 155.3 ppm for 3b; 153.4 ppm for 3c; 152.8 ppm for 3d; 168.2 ppm for 3e) further supported their structures. The molecular structures of 3a,b,d,e were determined by single-crystal X-ray diffraction analysis. As shown in Figures 1−4, they all have distorted-square-planar palladium centers. Ligands 2a−d acted as tridentate ligands, where nitrogen atoms on the side arms, carbene carbon, and aryloxy oxygen atoms were bonded to the metal centers. The two six-membered rings in 3a,b,d all adopt twist-boat conformations. It should be noted that the Pd−C(carbene) bond distances for the tridentate palladium complexes 3a (1.912(4) Å), 3b (1.944(4) Å), and 3d (1.946(4) Å) were evidently shorter than those for the bidentate palladium complex 3e (2.0179(18) and 2.0245(18) Å). Notably, the Pd−N bond distances are lengthened (2.046(4) Å for 3a; 2.073(3) Å for 3b; 2.106(3) Å for 3d) relative to the previous results.7e The Pd−Cl bond distances for 3a,b,d are 2.3820(11), 2.1010(15), and 2.3804(12) Å, respectively. The ranges for the C(carbene)−Pd−O, C(carbene)−Pd−N, C(carbene)−Pd−Cl, O−Pd−Cl, N−Pd− Cl, and N−Pd−O angles are 87.19−88.77, 87.98−93.38, 172.27−174.61, 84.62−91.74, 91.79−94.79, and 172.37− 178.86°, respectively. Norbornene Polymerization. Vinyl polynorbornene has received considerable attention owing to its good mechanical strength, heat resistivity, and optical transparency.11 Recently, some N-heterocyclic carbene nickel and palladium complexes

rather than polymerization. In this paper, we report an efficient method to synthesize pincer-type tridentate o-aryloxide-Nheterocyclic carbene ligands, as well as their palladium complexes. Upon activation with either diethylaluminum chloride (Et2AlCl) or methylaluminoxane (MAO), all palladium complexes exhibited high activities of up to 5.99 × 107 g of PNB (mol of Pd)−1 h−1 toward norbornene polymerization. It is interesting to note that the resulting polymers are soluble in CHCl3 when the reactions are conducted in the presence of Et2AlCl. Accordingly, the molecular weights and molecular weight distribution can be obtained on GPC analysis. To the best of our knowledge, this is the first report of tridentate salicylaldimine-like NHC palladium complexes for the addition polymerization of norbornene.

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RESULTS AND DISCUSSION Synthesis of o-Hydroxyaryl Imidazolium Proligands 2a−e. Recently, we developed a simple and efficient method to synthesize the o-hydroxyaryl imidazoles 1a,b.10j As shown in Scheme 1, the reactions of 1 with halohydrocarbons in refluxing Scheme 1. Synthesis of o-Hydroxyaryl Imidazolium Proligands 2a−e

acetonitrile produced the desired o-hydroxyaryl imidazolium salts 2a−d in high yields. These compounds can act as [O,C,N]-tridentate ligands upon deprotonation. For comparison, the [O,C]-bidentate proligand 2e was also synthesized.10j Proligands 2a−e are soluble in CH2Cl2, CHCl3, and CH3OH but insoluble in hexane and diethyl ether. All of the compounds have been fully characterized by 1H and 13C NMR spectra and high-resolution mass spectrometry (HRMS). All of the 1H NMR spectra of 2a−e exhibited a characteristic singlet at 9−10 ppm for the imidazolium proton, consistent with those reported in the literature for similar imidazolium salts.10 Synthesis of o-Aryloxide-NHC Palladium Complexes. A series of tridentate palladium complexes 3a−d and bidentate

Scheme 2. Synthesis of o-Aryloxide-NHC Palladium Complexes 3a−e

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Figure 3. ORTEP diagram of 3d. Thermal ellipsoids are shown at the 30% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Pd(1)−C(17) 1.946(4), Pd(1)− O(1) 1.991(3), Pd(1)−Cl(1) 2.3804(12), Pd(1)−N(1) 2.106(3); C(17)−Pd(1)−O(1) 87.19(13), C(17)−Pd(1)−N(1) 93.38(15), O(1)−Pd(1)−N(3) 176.61(12), C(17)−Pd(1)−Cl(1) 174.61(11), O(1)−Pd(1)−Cl(1) 87.56(8), N(3)−Pd(1)−Cl(1) 91.79 (11).

Figure 1. ORTEP diagram of 3a. One of six independent molecules in the unit cell is shown. Thermal ellipsoids are shown at the 60% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Pd(1)−C(13) 1.912(4), Pd(1)− O(1) 2.012(3), Pd(1)−Cl(1) 2.3820(11), Pd(1)−N(3) 2.046(4); C(13)−Pd(1)−O(1) 88.05(15), C(13)−Pd(1)−N(3) 87.98(16), O(1)−Pd(1)−N(3) 172.37(13), C(13)−Pd(1)−Cl(1) 172.27(13), O(1)−Pd(1)−Cl(1) 91.74(8), N(3)−Pd(1)−Cl(1) 93.07(10).

Figure 2. ORTEP diagram of 3b. Thermal ellipsoids are shown at the 60% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Pd(1)−C(9) 1.944(4), Pd(1)− O(1) 1.988(3), Pd(1)−Cl(1) 2.4010(15), Pd(1)−N(1) 2.073(3); C(9)−Pd(1)−O(1) 88.77(14), C(9)−Pd(1)−N(1) 91.83(15), O(1)−Pd(1)−N(1) 178.86(13), C(9)−Pd(1)−Cl(1) 173.28(12), O(1)−Pd(1)−Cl(1) 84.62(9), N(1)−Pd(1)−Cl(1) 94.79 (11).

Figure 4. ORTEP diagram of 3e. Thermal ellipsoids are shown at the 60% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Pd(1)−C(9) 2.0179(18), Pd(1)− O(1) 2.0138(14), Pd(1)−C(33) 2.0245(18), Pd(1)−O(2) 2.0303(15); O(1)−Pd(1)−C(9) 85.56(7), O(1)−Pd(1)−C(33) 95.17(6), C(9)−Pd(1)−C(33) 170.46(8), O(1)−Pd(1)−O(2) 177.22(5), C(9)−Pd(1)−O(2) 94.68(7), C(33)−Pd(1)−O(2) 85.05(6).

have been extensively applied to catalytic addition polymerization of norbornene with excellent activities.12 Our group has reported the addition polymerization of norbornene with bis(aryloxide-NHC) palladium complexes in the presence of MAO.10b We envisioned that the tridentate palladium complexes could improve the activities of polymerization. In order to explore the potential applications of the tridentate ohydroxyaryl-N-heterocyclic carbene palladium complexes, the vinyl polymerizations of norbornene with these complexes were

studied in the presence of MAO. Interestingly, on activation with Et2AlCl, all palladium complexes also exhibited high activities of 1.23 × 107 g of PNB (mol of Pd)−1 h−1 toward norbornene polymerization and the conversion was up to 99.9%. The polymerization results are given in Tables 1 and 2. Regarding the catalytic system with MAO, complex 3d was chosen as precatalyst for the study of the polymerization in detail, and the results are given in Table 1. As a control C

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Organometallics Table 1. Vinyl Polymerization of Norbornene with Complexes 3 Activated by MAOa entry

cat.

T (°C)

t (min)

[cat] (μmol)

Al/Pd

PNB (g)

conversn (%)

activityb

1 2 3 4 5 6 7 8 9 10 11 12 13 14

3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3a 3b 3c 3e

20 40 60 80 40 40 40 40 40 40 40 40 40 40

2 2 2 2 2 2 2 2 2 1 1 1 1 1

1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.8 0.7 1.0 1.0 1.0 1.0 1.0

5000 5000 5000 5000 2000 2250 2500 2250 2250 2250 2250 2250 2250 2250

1.0253 1.0797 0.9781 0.7765 0.9099 1.0060 1.0150 0.8229 0.6267 0.9680 1.1069 1.0200 0.9903 0.8548

99.4 99.9 95.1 74.5 88.6 99.4 99.5 79.9 60.8 92.2 99.9 99.1 96.1 83.0

29.82 29.97 28.53 22.53 26.58 29.82 29.85 29.95 26.08 55.31 59.99 59.47 57.69 49.97

a Polymerization conditions: solvent, 1,2-dichlorobenzene; Vtotal, 10 mL; norbornene, 1.0 g, being subject to the weighing data; MAO, 1.4 M in toluene. bIn units of 106 g of PNB (mol of Pd)−1 h−1.

Table 2. Vinyl Polymerization of Norbornene with Complexes 3 Activated by Et2AlCla entry

cat.

T (°C)

t (min)

[cat] (μmol)

Al/Pd

PNB (g)

conversn (%)

activityb

Mnc (×103)

PDIc

1 2 3 4 5 6 7 8 9 10 11 12 13 14

3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3a 3b 3c 3e

20 40 60 80 40 40 40 40 40 40 40 40 40 40

8 8 8 8 8 8 8 8 8 5 8 8 8 8

1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.8 0.6 0.8 0.8 0.8 0.8 0.8

2250 2250 2250 2250 1000 1250 1500 1500 1500 1500 1500 1500 1500 1500

0.7577 0.9989 0.7654 0.6352 0.7761 0.8015 0.9999 1.0060 0.5712 0.8193 0.5369 0.8446 0.8328 0.8494

72.2 99.9 74.3 60.5 75.3 76.3 99.9 99.9 57.1 81.9 54.0 82.8 82.5 80.9

5.412 7.492 5.573 4.537 5.651 5.725 7.499 9.374 7.140 12.29 4.074 7.611 6.184 6.064

164 87 44 39 63 59 38 45 64 67 86 125 86 75

1.64 1.86 1.82 1.78 2.05 1.84 2.00 1.89 1.83 1.85 1.71 1.66 1.86 2.16

a

Polymerization conditions: solvent, 1,2-dichlorobenzene; Vtotal, 10 mL; norbornene, 1.0 g, being subject to the weighing data; Et2AlCl, 1.1 M in hexane. bIn units of 106 g of PNB (mol of Pd)−1 h−1. cGPC data in CHCl3 vs polystyrene standards.

comparison to 3e would be due to an increase in the stability of the catalytically active species. The polymers obtained are insoluble in most organic solvents, such as hexane, chloroform, tetrahydrofuran, benzene, acetone, dioxane, methanol, chlorobenzene, dichlorobenzene, and tetrachloroethane. Therefore, we cannot measure the molecular weights of the polymers by GPC. Regarding the catalytic system with Et2AlCl, complex 3d was chosen as a model precatalyst to optimize the polymerization parameters such as temperature, molar ratio of Al/Pd, amount of catalyst, and reaction time, and the results are given in Table 2. The reaction temperature was changed between 20 and 80 °C (entries 1−4, Table 2) with the highest activity appearing at 40 °C. Then the temperature was fixed at 40 °C and the Al/Pd molar ratio was changed from 1000 to 1500. The optimum ratio was observed for 1500 (entries 5−7, Table 2), and the monomer conversion was 99.9%. The Mn values decreased upon increasing the polymerization temperature (entries 1−4, Table 2) and Al/Pd molar ratios (entries 5−7, Table 2), probably due to an increased degree of chain transfer to Al. Reducing the amount of catalyst enhanced the catalytic activity; however, the catalytic activity decreased with further reduction from 0.8 to 0.6 μmol (entries 7−9, Table 2). The molecular

experiment, polymerization was attempted without cocatalyst, but no polymer was produced. At temperatures of 20 and 40 °C, the monomer conversions were both nearly 100%. The catalytic activity decreased with a further increase from 40 to 80 °C (entries 1−4, Table 1). Therefore, we chose 40 °C as the optimal polymerization temperature. Reducing the Al/Pd ratio from 5000 to 2000 caused the activity to decrease significantly (entry 5, Table 1). When the Al/Pd ratio was 2250, the system remained highly viscous and turned into gel-like product mixtures, and the yield was nearly 100%. However, the activity was not improved obviously when the Al/Pd ratio was further increased (entries 6 and 7, Table 1). Reducing the amount of catalyst led to higher activity, but the monomer conversion decreased significantly (entries 8 and 9, Table 1). When the time was shortened to 1 min, the reaction exhibited the highest polymerization activity as well as a high yield of 92.2%. Similarly, complexes 3a−e also exhibited excellent catalytic activities (107 g of PNB (mol of Pd)−1 h−1) in the polymerization of norbornene (entries 10−14, Table 1). It is clear that tridentate o-hydroxyaryl-N-heterocyclic carbene palladium complexes 3a−d showed higher activities than the bidentate palladium complex 3e (conversions: 92.2−99.9% vs 83.0%). The observed higher catalytic activities by 3a−d in D

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polydispersity were determined by gel permeation chromatography (GPC: Waters 410) at 35 °C with a flow rate of 1 mL min−1 (column: Waters HT4, 3). General Procedures for Synthesis of Proligands 2a−d. A mixture of o-hydroxyarylimidazole (5.0 mmol) and 1.5 equiv of halohydrocarbon was refluxed for 36 h in 30 mL of acetonitrile. The mixture was then refrigerated, and the insoluble solid was separated. Recrystallization from CH2Cl2/Et2O gave the product as a white solid. Compound 2a. Yield: 1.375 g (4.0 mmol, 80%). Mp: 203−205 °C. 1 H NMR (400 MHz, DMSO): δ 11.01 (s, 1H, Ar-OH), 9.82 (s, 1H, NCHN), 8.60 (d, 1H, J = 4.00 Hz, pyridyl-H), 8.13 (s, 1H, pyridyl-H), 8.00 (s, 1H, pyridyl-H), 7.92 (m, 1H, pyridyl-H), 7.58 (d, 1H, J = 2.40 Hz, Ar-H), 7.51 (s, 1H, im-H), 7.44 (m, 2H, Ar-H), 7.23 (d, 1H, J = 4.80 H, im-H), 5.70 (s, 2H, NCH2), 1.24 (s, 9H, C(CH3)3) ppm. 13C NMR (100 MHz, DMSO): δ 152.6, 149.7, 148.1, 143.3, 137.7, 136.5, 128.3, 124.2, 124.0, 122.7, 122.5, 121.2, 120.9, 118.4, 54.0, 34.2, 31.3 ppm. HRMS (ESI, m/z): calcd for C19H22ClN3O [M − Cl]+ 308.1757, found 308.1763. Compound 2b. Yield: 1.780 g (4.45 mmol, 89%). Mp: 238−239 °C. 1H NMR (400 MHz, CDCl3): δ 9.90 (s, 1H, Ar-OH), 9.12 (s, 1H, NCHN), 8.62 (d, 1H, J = 4.80 Hz, pyridyl-H), 7.85 (s, 1H, pyridyl-H), 7.76 (m, 2H, pyridyl-H), 7.47 (s, 1H, im-H), 7.32 (m, 2H, Ar-H), 7.05(s, 1H, im-H), 5.73 (s, 2H, NCH2), 1.45 (s, 9H, C(CH3)3), 1.31 (s, 9H, C(CH3)3) ppm. 13C NMR (100 MHz, CDCl3): δ 152.6, 149.7, 147.7, 143.6, 142.2, 138.1, 137.7, 126.2, 125.3, 124.6, 124.0, 123.3, 122.7, 120.0, 54.1, 35.7, 34.5, 31.4, 29.8 ppm. HRMS (ESI, m/z): calcd for C23H30ClN3O [M − Cl]+ 364.2383, found 364.2379. Compound 2c. Yield: 1.133 g (3.50 mmol, 70%). Mp: 258−260 °C. 1 H NMR (400 MHz, D2O): δ 9.32 (s, 1H, NCHN), 7.79 (s, 2H, ArH), 7.50 (d, 1H, J = 8.62 Hz, Ar-H), 7.47 (s, 1H, im-H), 7.07 (d, 1H, J = 8.55 Hz, im-H), 4.82 (d, 2H, J = 6.70 Hz, im-CH2), 3.79−3.83 (m, 2H, NCH2), 2.99 (s, 6H, N(CH3)2), 1.25 (s, 9H, C(CH3)3) ppm. 13C NMR (100 MHz, D2O): δ 146.9, 144.7, 137.2, 128.9, 124.4, 122.5, 122.0, 121.9, 121.6, 116.8, 55.6, 55.4, 44.2, 43.3, 33.7, 30.4 ppm. HRMS (ESI, m/z): calcd for C17H27Cl2N3O [M − 2Cl − H]+ 288.2070, found 288.2075. Compound 2d. Yield: 1.311 g (3.45 mmol, 69%). Mp: 292−295 °C. 1H NMR (400 MHz, D2O): δ 9.27 (s, 1H, NCHN), 7.84 (s, 1H, Ar-H), 7.59 (s, 1H, Ar-H), 7.48 (s, 1H, im-H), 7.35 (s, 1H, im-H), 4.83−4.86 (m, 2H, im-CH2), 3.81−3.85 (m, 2H, NCH2), 3.01 (s, 6H, N(CH3)2), 1.32 (s, 9H, C(CH3)3), 1.20 (s, 9H, C(CH3)3) ppm. 13C NMR (100 MHz, D2O): δ 145.9, 145.0, 141.2, 137.5, 126.0, 125.4, 124.8, 122.6, 121.3, 55.6, 55.5, 44.2, 43.3, 35.0, 34.1, 30.6, 29.2 ppm. HRMS (ESI, m/z): calcd for C21H35Cl2N3O [M − 2Cl − H]+ 344.2696, found 344.2705. General Procedures for Synthesis of Complexes 3a−e. A solution of 2 (0.5 mmol for 3a−d, 1.0 mmol for 3e) and 3 equiv of Ag2O (0.347 g, 1.5 mmol) in 20 mL of CH2Cl2 was stirred at room temperature in the dark for 12 h. The reaction mixture was filtered through Celite, and (COD)PdCl2 (0.104 g, 0.5 mmol) was added. The resultant yellow solution was stirred for 48 h under exclusion of light. After filtration over a pad of Celite and removal of the solvent under reduced pressure the crude product was obtained. Recrystallization from CH2Cl2/hexane gave compounds 3 as yellow crystals. Compound 3a. Yield: 205 mg (0.46 mmol, 92%). Mp: >300 °C. Anal. Calcd for C19H20ClN3OPd: C, 50.91; H, 4.50; N, 9.37. Found: C, 50.83; H, 4.55; N, 9.35. 1H NMR (400 MHz, DMSO): δ 9.32 (s, 1H, pyridyl-H), 8.29 (s, 1H, pyridyl-H), 8.13 (s, 1H, pyridyl-H), 7.79 (s, 1H, pyridyl-H), 7.76 (s, 1H, Ar-H), 7.59 (s, 1H, Ar-H), 7.46 (s, 1H, Ar-H), 7.05 (d, 1H, J = 8.00 Hz, im-H), 6.83 (d, 1H, J = 8.00 Hz, imH), 5.62 (s, 2H, NCH2), 1.27 (s, 9H, C(CH3)3) ppm. 13C NMR (100 MHz, DMSO): δ 154.7, 153.6, 152.8, 146.6, 140.1, 137.3, 125.7, 124.8, 124.4, 123.1, 120.0, 117.1, 115.4, 53.3, 33.6, 31.3 ppm. HRMS (MALDI, m/z): calcd for C19H20ClN3OPd [M − Cl]+ 412.0641, found 412.0640. Compound 3b. Yield: 239 mg (0.47 mmol, 95%). Mp: >300 °C. Anal. Calcd for C23H28ClN3OPd: C, 54.77; H, 5.60; N, 8.33. Found: C, 54.73; H, 5.75; N, 8.35. 1H NMR (400 MHz, CDCl3): δ 9.56 (d, 1H, J = 5.54 Hz, pyridyl-H), 7.80−7.84 (m, 1H, pyridyl-H), 7.53 (d, 1H, J = 7.64 Hz, pyridyl-H), 7.45 (s, 1H, pyridyl-H), 7.33 (s, 2H, Ar-

weights of the resultant polymers decreased with an increasing amount of catalyst. The influence of the polymerization time was also investigated. A shorter reaction time results in higher catalytic activity but lower conversion (entries 8 and 10, Table 2). Taking account of the conversion of NBE, the optimal polymerization conditions for the catalytic system are 40 °C with a Al/Pd ratio of 1500 for 8 min. Similarly, complexes 3b,c,e also exhibited high catalytic activities ((6.064−7.611) × 106 g of PNB (mol of Pd)−1 h−1) and conversions (80.9− 82.8%) for the polymerization of norbornene (entries 12−14, Table 2), while complex 3a showed slightly lower activity (4.074 × 106 g of PNB (mol of Pd)−1 h−1) and conversion (54.0%) (entry 11, Table 2). No evident difference in catalytic activity was found for the tridentate o-hydroxyaryl-N-heterocyclic carbene palladium complexes 3a−d and the bidentate palladium complex 3e in the presence of Et2AlCl (conversions: 54.0−99.9% vs 80.9%). The polymers displayed glass transition temperatures in the range of 38.8−46.4 °C, which were determined by means of differential scanning calorimetry (DSC) techniques at a heating rate of 10 °C/min. According to the TGA study, the polymers are thermally stable up to 400 °C (see the Supporting Information). From this result, the cationic or radical polymerization could be excluded. The missing absorption of a double bond at 1600−1700 cm−1 in the IR spectra (see the Supporting Information) of the polymers indicates that the polymerization is not ROMP type. Therefore, we can presume that the polymerization initiated by these palladium complexes in the presence of MAO or Et2AlCl system adopts a vinyl-type addition manner.

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CONCLUSION In summary, we have successfully developed an efficient route to synthesize tridentate o-hydroxyaryl imidazolium proligands and a series of corresponding o-aryloxide-NHC tridentate palladium complexes. These complexes were fully characterized, including by X-ray crystallography analysis for 3a,b,d,e. By treatment with MAO or Et2AlCl, these complexes showed excellent catalytic activities (5.99 × 107 g of PNB (mol of Pd)−1 h−1) in the addition polymerization of norbornene. The tridentate palladium complexes 3b−d showed better activities in comparison with the bidentate bis(aryloxide-NHC) palladium complex 3e in the presence of MAO. The polymers obtained in the presence of Et2AlCl are soluble in CHCl3 and have been characterized by GPC.

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EXPERIMENTAL SECTION

General Considerations. All experiments were carried out under an atmosphere of dry argon using standard Schlenk techniques. All solvents were distilled from appropriate drying agents under argon before use. MAO (1.4 M in toluene) and Et2AlCl (1.1 M in hexane) was purchased from Arbemarle Co. and Alfa Aesar China (Beijing), respectively, and used without further purification. Norbornene (purchased from Alfa Aesar China (Beijing) Chemical Co. Ltd.) was purified by distillation over sodium. 2-Chloromethylpyridine,13 (COD)PdCl2,14 and 2e10j were synthesized according to the literature. Other commercially available reagents were purchased and used without further purification. 1H and 13C NMR spectra were recorded on a Bruker AV400 spectrometer. HRMS measurements were carried out on Agilent 6520 Q-TOF mass spectrometers. Elemental analyses were performed on a PerkinElmer 240C analyzer. IR spectra were recorded as KBr disks on a Nicolet 380 FT-IR spectrometer. TG and DSC data were obtained from TA Instruments SDT-2960 and SC2910 thermal analyzers, respectively. The molecular weight and the E

DOI: 10.1021/acs.organomet.5b01006 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics H), 7.21 (s, 1H, im-H), 7.04 (s, 1H, im-H), 5.40 (s, 2H, NCH2), 1.55 (s, 9H, C(CH3)3), 1.30 (s, 9H, C(CH3)3) ppm. 13C NMR (100 MHz, CDCl3): δ 155.3, 152.4, 151.1, 148.3, 141.2, 138.3, 136.0, 126.1, 124.3, 124.1, 121.8, 120.8, 116.2, 111.9, 53.8, 35.0, 33.2, 30.6, 28.8 ppm. HRMS (MALDI, m/z): calcd for C23H28ClN3OPd [M − Cl]+ 468.1267, found 468.1266. Compound 3c. Yield: 150 mg (0.35 mmol, 70%). Mp: >300 °C. Anal. Calcd for C17H24ClN3OPd: C, 47.68; H, 5.65; N, 9.81. Found: C, 47.46; H, 5.49; N, 9.90. 1H NMR (400 MHz, DMSO): δ 8.20 (s, 1H, Ar-H), 7.67 (s, 1H, Ar-H), 7.38 (s, 1H, Ar-H), 6.99 (d, 1H, J = 8.00 Hz im-H), 6.77 (s, 1H, J = 8.40 Hz, im-H), 4.33 (t, 2H, J = 4.00 Hz, im-CH2), 2.79 (m, 2H, NCH2), 2.85 (s, 6H, N(CH3)2), 1.26 (s, 9H, C(CH3)3) ppm. 13C NMR (100 MHz, DMSO): δ 153.4, 136.7, 125.5, 124.0, 123.1, 119.8, 116.7, 115.5, 63.3, 51.4, 45.3, 33.7, 31.4 ppm. HRMS (MALDI, m/z): calcd for C17H24ClN3OPd [M − Cl]+ 392.0954, found 392.0948. Compound 3d. Yield: 212 mg, (0.44 mmol, 88%). Mp: >300 °C. Anal. Calcd for C21H32ClN3OPd: C, 52.07; H, 6.66; N, 8.68. Found: C, 52.13; H, 6.65; N, 8.57. 1H NMR (400 MHz, CDCl3): δ 7.38 (s, 1H, Ar-H), 7.17 (s, 1H, Ar-H), 7.00 (d, 2H, J = 14.37 Hz, im-H), 4.31 (s, 2H, im-CH2), 3.00 (s, 6H, N(CH3)2), 2.72 (s, 2H, NCH2), 1.51 (s, 9H, C(CH3)3), 1.30 (s, 9H, C(CH3)3) ppm. 13C NMR (100 MHz, CDCl3): δ 152.8, 148.4, 142.1, 136.6, 126.5, 122.5, 121.7, 116.7, 112.9, 64.6, 52.3, 46.3, 35.9, 34.2, 31.6, 29.7 ppm. HRMS (MALDI, m/z): calcd for C21H32ClN3OPd [M − Cl]+ 448.1580, found 448.1588. Compound 3e. Yield: 165 mg, (0.20 mmol, 40%). Mp: > 300 °C. Anal. Calcd for C48H58N4O2Pd: C, 69.51; H, 7.05; N, 6.75. Found: C, 69.63; H, 7.15; N, 6.57. 1H NMR (400 MHz, CDCl3): δ 7.67 (s, 4H, Ar-H), 7.37−7.35 (m, 6H, Ar-H), 7.30−7.29 (m, 2H, Ar-H), 7.12− 7.10 (m, 2H, Ar-H), 7.04−7.02 (m, 2H, im-CH), 6.98−6.97 (m, 2H, im-CH), 6.58 (d, 1H, J = 7.20 Hz, NCH2), 6.55 (d, 1H, J = 7.20 Hz, NCH2), 5.08 (d, 1H, J = 5.20 Hz, NCH2), 5.05 (d, 1H, J = 7.20 Hz, NCH2),1.37 (d, 18H, J = 7.20 Hz, C(CH3)3), 1.31 (d, 18H, J = 7.20 Hz, C(CH3)3). 13C NMR (100 MHz, CDCl3): δ 168.2, 155.8, 140.8, 137.3, 136.8, 131.0, 128.8, 128.4, 127.9, 122.0, 121.0, 118.3, 115.5, 53.4, 35.4, 34.1, 31.3, 29.3 ppm. HRMS (MALDI, m/z): calcd for C48H58N4O2Pd [M + H]+ 829.3673, found 829.3685. Crystallographic Studies. Single crystals of 3a,b,d,e suitable for X-ray diffraction were obtained from CH3CN/hexane. Data collections were carried out on a Rigaku Saturn 724 CCD (for 3a,b,e) or Rigaku Saturn 70 (for 3d) diffractometer equipped with a rotating anode system at 113(2) K or 293(2) K by using graphite-monochromated Mo Kα radiation (ω−2θ scans, λ = 0.71073 Å). Semiempirical absorption corrections were applied for all complexes. The structures were solved by direct methods and refined by full-matrix least squares. Calculations were performed by using the SHELXL-97 program system. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were assigned idealized positions and were included in structure factor calculations. Norbornene Polymerization. In a typical procedure, 1.00 g of norbornene in 5.0 mL of 1,2-dichlorobenzene and the exact amount of MAO or Et2AlCl solution were placed in a flask (100 mL) with stirring under an Ar atmosphere. After the mixture was kept at the desired temperature for 3−5 min, the right amount of the palladium complex dissolved in dichlorobenzene was injected into the flask with a syringe, and the reaction was started. After the desired time, the polymerization was terminated by addition of 10% HCl in ethanol. The precipitated polymer was washed with ethanol and water and dried at 60 °C in vacuo to a constant weight. For all of the polymerization procedures, the total reaction volume was 10 mL, which can be achieved by variation of the added dichlorobenzene when necessary.

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Crystallographic data for 3a,b,d,e (CIF)

AUTHOR INFORMATION

Corresponding Author

*B.W.: e-mail, [email protected]; tel/fax, +86-2223504781. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Science Foundation of China (Grants 21421062) and Specialized Research Fund Program of Higher Education of 20110031110009).

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National Natural 21174068 and for the Doctoral China (Grant

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b01006. NMR spectra for 2a−d and 3a−e and GPC curves, IR, DSC, and TGA spectra of the PNB obtained (PDF) F

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DOI: 10.1021/acs.organomet.5b01006 Organometallics XXXX, XXX, XXX−XXX