Synthesis, Structures, and Norbornene Polymerization Behavior of Bis

ARTICLE pubs.acs.org/Organometallics

Synthesis, Structures, and Norbornene Polymerization Behavior of Bis(aryloxide-N-heterocyclic carbene) Nickel Complexes Yong Kong,† Man Cheng,‡ Hongping Ren,† Shansheng Xu,† Haibin Song,† Min Yang,‡ Binyuan Liu,‡ and Baiquan Wang*,† †

State Key Laboratory of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071, People’s Republic of China ‡ Institute of Polymer Science and Engineering, Hebei University of Technology, Tianjin 300130, People’s Republic of China

bS Supporting Information ABSTRACT: Treatment of the o-hydroxyaryl imidazolinium proligands (2-OH-3,5-tBu2C6H2)(R)(C3H3N2)þBr- [R = iPr (1a), t Bu (1b), Ph (1c), Mes (1d)] with NiL2Cl2 (L = PPh3, Py) or Ni(OAc)2 3 4H2O afforded the corresponding cis bis(aryloxideNHC) nickel complexes 2-5. Notably, the products were independent from the pro-ligands/Ni ratios. The same complexes were obtained with the pro-ligands/Ni ratio of 1:1 or 2:1. All the complexes were characterized by 1H and 13C NMR, high-resolution mass spectrometry (HRMS), and elemental analysis. The molecular structures of the aryloxide-NHCligated nickel complexes 2-5 were determined by single-crystal X-ray diffraction analysis. With methylaluminoxane (MAO) as cocatalyst, the nickel complexes showed moderate catalytic activities (106 g of PNB (mol of Ni)-1 h-1) in the addition polymerization of norbornene.

’ INTRODUCTION Since the discovery of the free N-heterocyclic carbene (NHC),1 tremendous efforts have been devoted to the synthesis and utilization of NHCs and their transition metal complexes.2 Among the transition metal complexes, nickel NHC complexes have been extensively studied and employed in a vast number of organic reactions.3 Recently, much research effort has been directed toward the study of transition metal complexes with anion-tethered NHC ligands.4 As an anchor, the introduced anion groups can reduce the tendency of ligand dissociation, via enhancing the bond between the NHCs and metal centers. However, only a few nickel complexes ligated by anion-tethered NHCs ligands have been synthesized and investigated.5 More recently, we reported a series of o-hydroxyaryl-substituted unsaturated NHCs ligands and their palladium complexes.6 The scaffold of the pro-ligands is analogous to the salicylaldimine framework, a common motif in organometallic chemistry and extensively utilized in catalytic organic reactions and olefin polymerization.7 Compared with the expensive jewelry metal palladium, convenient nickel complexes tethered by salicylaldimine ligands have attracted a considerable amount of interest in olefin polymerization.7a,7b,8 In this study, we report the synthesis and structures of a series of bis(aryloxide-NHC) nickel complexes. The addition polymerization of norbornene with these complexes is also studied by treatment with methylaluminoxane (MAO). To the best of our knowledge, this is the first report of this kind of bis(aryloxide-NHC) nickel complexes applied to the addition polymerization of norbornene. r 2011 American Chemical Society

’ RESULTS AND DISCUSSION Synthesis of Nickel Complexes. Following the synthetic route reported previously by our group,6 a series of o-hydroxyaryl imidazolinium pro-ligands can be facilely synthesized by the reactions of 4-bromo-2,4,6-tri-tert-butyl-2,5-cyclohexadien-1-one with different N-substituted imidazoles. Reactions of the pro-ligands 1a-e with 0.5 equiv of NiL2Cl2 (L = PPh3, Py) in the presence of 2 equiv of n BuLi afforded the cis bis(aryloxide-NHC) nickel complexes 2-5 in over 90% yields (Scheme 1). However, without any monoligand complexes, reactions with a pro-ligands/Ni molar ratio of 1:1 also provided the bis-ligand complexes 2-5 in about 45% yield. Furthermore, following the method for the synthesis of bis(alkoxide/ aryloxide-N-heterocyclic carbene) palladium complexes,6,9 the cis bis(aryloxide-NHC) nickel complexes 2-5 could also be obtained via treatment of the pro-ligands 1a-e with 0.5 or 1 equiv of Ni(OAc)2 3 4H2O in the presence of K2CO3 (Scheme 2). Complexes 2-5 are air and moisture stable. They are soluble in CH2Cl2, DME, THF, dioxane, acetone, and toluene, but insoluble in diethyl ether and hydrocarbon solvents. They are also very thermally stable and will melt over 300 °C. In their 1H NMR spectra the signals of the imidazole and phenol protons for the pro-ligands disappeared completely, and the characteristic signals of the carbene carbons in the 13C NMR spectra10 (159.1 ppm for 2, 156.2 ppm for 3, and 161.1 ppm for 5) were comparable to those observed in Ni(II)-based NHC complexes.5c It was worthy of noting that only Received: December 18, 2010 Published: February 14, 2011 1677

dx.doi.org/10.1021/om1011825 | Organometallics 2011, 30, 1677–1681

Organometallics

ARTICLE

Scheme 1

Scheme 2 Figure 2. ORTEP diagram of 3. Thermal ellipsoids are shown at the 60% probability level. Hydrogen atoms have been omitted for clarity.

Figure 3. ORTEP diagram of 4. Thermal ellipsoids are shown at the 20% probability level. Hydrogen atoms have been omitted for clarity.

Figure 1. ORTEP diagram of 2 (showing one of two independent molecules in the unit cell). Thermal ellipsoids are shown at the 30% probability level. Hydrogen atoms have been omitted for clarity.

one singlet was observed for the carbene carbon in each 13C NMR spectrum for these bis(aryloxide-NHC) complexes, which was different from Shen’s result.5f HRMS analysis further confirmed the assignment by showing molecular ion peaks at m/z 685.3983 ([M þ H]; calcd for 2: 685.3986), m/z 713.4308 ([M þ H]; calcd for 3: 713.4299), m/z 753.3681 ([M þ H]; calcd for 4: 753.3673), and m/z 837.4613 ([M þ H]; calcd for 5: 837.4612), respectively. The molecular structures of 2-5 were established by singlecrystal X-ray diffraction studies (Figures 1-4, Table 1). In each

molecule of 2-5, the nickel atom coordinates with two aryloxide-NHC ligands. The two aryloxide-NHC ligands adopt a cis arrangement around the nickel atom with the two aryloxide and two NHC ligands in the same position, respectively. The carbene ligand’s five-membered ring topology varies slightly for different carbenes. The ligand retains the bond characteristic angles for a singlet carbene (∼104° for the imidazol-2-ylidenes). The NiC(carbene) bond lengths [1.8420-1.8587 Å for 2; 1.866(2), 1.873(2) Å for 3; 1.838(2) Å for 4; 1.86(5), 1.847(9) Å for 5] are much shorter than those in the bis(aryloxide-NHC)Pd complexes [1.946-2.023 Å]6b and analogous to that in the salicylaldiminato-functionalized NHC-Ni complex reported by Shen et al.5c The Ni-O bond lengths [1.8728-1.9030 Å for 2; 1.9145(15), 1.8875(14) Å for 3; 1.8830(16), 1.8831(16) Å for 4; 1.901(14), 1.90(4) Å for 5] are much shorter than the Pd-O bond lengths (2.017-2.096 Å) in the bis(aryloxide-NHC)Pd complexes.6b The ranges for the C(carbene)-Ni-C(carbene), O-Ni-O, cis O-Ni-C, and trans O-Ni-C angles are 94.3099.61°, 87.55-90.61°, 86.63-90.33°, and 159.27-168.82°, 1678

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Table 2. Addition Polymerization of Norbornene with 2-5 Activated by MAOa [cat]

MAO

entry

catalyst

R

T (°C)

(μmol)

(Al/Ni)

PNB (g)

activityb

1 2

5 5

Mes Mes

20 40

0.20 0.20

2500 2500

0.2956 0.1087

8.87 5.42

3

5

Mes

60

0.20

2500

0.0726

2.18

4

5

Mes

80

0.20

2500

0.0038

0.11

5

5

Mes

20

0.20

1500

0.0156

0.47

6

5

Mes

20

0.20

2000

0.2548

7.46

7

5

Mes

20

0.20

3000

0.2788

8.36

8

5

Mes

20

0.20

3500

0.2739

8.22

9 10

5 2

Mes i Pr

20 20

0.20 0.20

4000 2500

0.2704 0.2155

8.11 6.46

11

3

t

Bu

20

0.20

2500

0.1057

3.17

12

4

Ph

20

0.20

2500

0.1902

5.71

a

Figure 4. ORTEP diagram of 5. Thermal ellipsoids are shown at the 30% probability level. Hydrogen atoms have been omitted for clarity.

Table 1. Selected Bond Lengths (Å) and Angles (deg) for 2-5 parameter Ni-C(NHC)

2a

3

1.8476(19)

1.866(2)

1.8536(19)

1.873(2)

4 1.838(2)

5 1.86(5) 1.847(9)

1.8420(18) 1.8587(18) Ni-O

1.9030(13)

1.9145(15)

1.8728(13)

1.8875(14)

1.8831(16)

1.901(14) 1.90(4)

1.8908(12) C-Ni-C

1.8794(12) 98.24(8)

99.61(9)

94.30(15)

98(2)

89.84(6)

90.61(10)

88(3)

88.55(7)

86.63(8)

88.64(9)

89(2)

90.03(7)

87.88(8)

95.85(8) O-Ni-O

88.59(6) 87.55(5)

cis-O-Ni-C

89(3)

88.34(7) trans-O-Ni-C

90.33(7) 163.63(7)

164.00(8)

159.27(7)

164.23(8)

168.65(10)

164.5(8) 164.1(4)

168.82(7) 167.60(7) a

There are two independent molecules in the unit cell.

respectively. The dihedral angles of the two planes defined by the O-Ni-C are 25.7°, 15.7°, 21.9°, 16.0°, and 21.3°, respectively. These parameters indicate that the Ni atom takes a distorted square-planar coordination geometry in 2-5. Norbornene Polymerization. Vinyl polynorbornene has received considerable attention owing to its dielectric and mechanical properties for technical application as an interlevel dielectric in microelectronics applications.11 Recently, some N-heterocyclic carbene nickel and palladium complexes have been utilized in the addition polymerization of norbornene with excellent activities.12 More recently, we have reported the addition polymerization of norbornene with the aryloxide-NHC palladium complexes

Polymerization conditions: in 15.5 mL of toluene; norbornene 1.0 g; MAO (1.4 M) in toluene; t = 10 min. b In units of (106 g of PNB) (mol of Ni)-1 h-1.

in the presence of MAO.6b,6c Inspired by the previous results, we envisioned this kind of salicylaldimine-like NHC nickel complex would also have potential uses in catalysis, because the salicylaldimino nickel complexes have been extensively utilized in catalytic olefin polymerization.7a,7b,8,13 So we studied the addition polymerization of norbornene with these bis(aryloxide-NHC) nickel complexes by treatment with MAO. The results are listed in Table 2. Analogous to the palladium complexes,6b,6c complex 5 was chosen as the precatalyst for the study of the polymerization in detail. It was found that the activity decreased with increasing temperature from 20 to 80 °C (entries 1-4). The color of the PNB obtained at 80 °C is slightly black, indicating that the active species is unstable at high temperature and decomposes, similar to the bis(aryloxide-NHC) palladium complexes.6b The activity increased with increasing the Al/Ni ratio from 1500 to 2500 (entries 1, 5, 6). But the activity decreased when the Al/Ni ratio further increased (entries 7-9). In the absence of 5 or MAO no polymer was obtained. The optimal polymerization conditions for the catalytic system are at 20 °C and with an Al/Ni molar ratio of 2500. Similarly, complexes 2-4 also exhibited moderate catalytic activities [106 g of PNB (mol of Ni)-1 h-1] in the polymerization of norbornene (entries 10-12). For the different substituted group, the steric effect resulted in different activities for the precatalysts (5 > 2 > 4 > 3). Although the phenomena of the polymerization are similar to those with the (aryloxide-NHC)Pd complexes, the nickel precatalysts show lower activities. The activities of 2-4 are also lower than those of the (pyridyl-NHC)Ni complexes reported by Jin et al.,12a perhaps due to the firm bonding between the two aryloxide-NHC ligands and Ni atom, which obstructs the generation of the active species. The polymers obtained are insoluble in most organic solvents, such as cyclohexane, chloroform, benzene, chlorobenzene, acetone, dioxane, methanol, and tetrachloroethane. Therefore, we cannot measure the molecular weights of the polymers. The missing absorption of a double bond at 1600-1700 cm-1 in the IR spectra of the polymers (see the Supporting Information) indicates that the polymerization initiated by the bis(aryloxideNHC) nickel complexes/MAO system adopts a vinyl-type 1679

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Organometallics addition manner. DSC analysis shows multiple transitions of the PNB derived from the Ni precatalysts, and it is impossible to determine the glass transition temperatures. The difficulty of determining the glass transition temperature of vinyl homopolynorbornene has been attributed to the fact that it is located close to the temperature range where decomposition tends to set in.14 According to the TGA study (see the Supporting Information), the polymers are thermally stable up to 400 °C.

’ CONCLUSION In summary, we have successfully synthesized a series of cis bis(aryloxide-NHC) nickel complexes. The complexes could be obtained either by reactions of the pro-ligands with NiL2Cl2 (L = PPh3, Py) in the presence of 2 equiv of nBuLi or via treatment of the pro-ligands with Ni(OAc)2 3 4H2O in the presence of K2CO3. Notably, the pro-ligands/Ni ratio had no effect on the products. In the presence of MAO, these nickel precatalysts showed moderate activities [106 g of PNB (mol of Ni)-1 h-1] in the addition polymerization of norbornene. The steric effect of N-substituents at the NHCs slightly influenced the catalytic activities. ’ EXPERIMENTAL SECTION General Considerations. All experimental manipulations 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 (10% solution in toluene) was purchased from Arbemarle Co. 1H and 13C NMR spectra were recorded on a Bruker AV400 or Varian AS-400, while ESI mass spectra and HRMS were performed on Thermo Finnigan LCQ Advantag and Varian 7.0 T FTICR mass spectrometers, respectively. IR spectra were recorded as KBr disks on a Nicolet 380 FT-IR spectrometer. Elemental analyses were performed on a PerkinElmer 240C analyzer. TG and DSC data were obtained from TA Instrument SDT-2960 and SC-2910 thermal analyzers, respectively. The aryloxide-NHC pro-ligands 1a-d6 were prepared according to the literature procedures. General Procedures for Preparation of the Bis(aryloxideNHC) Nickel Complexes 2-5. Method A. A solution of nBuLi (1.7 M, 2.0 mmol) in hexane was added to the suspension of the imidazolium salt (1.0 mmol) in THF. After stirring for 2 h, NiL2Cl2 (L = PPh3, Py) (0.5 or 1 mmol) was added and the resulting orange solution was stirred overnight. After removal of solvent, the residue was chromatographed on an alumina column. Elution with petroleum ether/CH2Cl2 (2:1) gave an orange or red band, which afforded the corresponding nickel complex as orange or red crystals. Method B. A mixture of o-hydroxyaryl-substituted imidazolium salts 1 (1 mmol), Ni(OAc)2 3 4H2O (0.5 or 1 mmoL), K2CO3 (3 mmol, 0.42 g), and dioxane (20 mL) was heated under reflux for 24 h. It was then allowed to cool to room temperature. After removal of solvents under vacuum, the residue was extracted with CH2Cl2 and filtered through a pad of Celite. After the volatiles were removed, the corresponding nickel complexes were obtained as orange or red solids. Compound 2 (R = iPr): yield 97%, mp >300 °C. Anal. Calcd for C40H58N4O2Ni: C, 70.07; H, 8.53; N, 8.17. Found: C, 69.98; H, 8.63; N, 7.99. 1H NMR (CDCl3): δ 7.28 (s, 1H, Ar-H), 7.27 (s, 1H, Ar-H), 7.22 (s, 1H, Ar-H), 7.21 (s, 1H, Ar-H), 7.09 (d, J = 2. 40 Hz, 2H, im-H), 6.96 (d, J = 1.98 Hz, 2H, im-H), 3.98 (m, 2H, CH(CH3)2), 1.63 (d, J = 6.64 Hz, 6H, CH-CH3), 1.56 (s, 18H, C(CH3)3), 1.36 (s, 18H, C(CH3)3), 1.05 (d, J = 6.70 Hz, 6H, CH-CH3) ppm. 13C NMR (100 MHz, CDCl3): δ 159.1, 154.9, 141.1, 134.3, 128.5, 121.7, 118.8, 117.9, 112.6, 51.6, 36.1, 34.1, 31.8, 29.9, 25.6, 21.3 ppm. HRMS (MALDI, m/z): calcd for C40H58N4O2Ni (M þ H) 685.3986, found 685.3983.

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Compound 3 (R = tBu): yield 90%, mp >300 °C. Anal. Calcd for C42H62N4O2Ni: C, 70.68; H, 8.76; N, 7.85. Found: C, 70.47; H, 8.95; N, 7.62. 1H NMR (CDCl3): δ 7.20 (s, 1H, Ar-H), 7.19 (s, 1H, Ar-H),), 7.12 (s, 1H, Ar-H), 7.11 (s, 1H, Ar-H), 7.04 (d, J = 1.80 Hz, 2H, im-H), 7.02 (d, J = 2.25 Hz, 2H, im-H), 1.66 (s, 18H, C(CH3)3), 1.54 (s, 18H, C(CH3)3), 1.37 (s, 18H, C(CH3)3) ppm. 13C NMR (100 MHz, CDCl3): δ 156.2, 154.7, 141.7, 135.0, 131.3, 121.6, 120.6. 117.8, 113.1, 57.8, 36.0, 34.1, 31.9, 31.1, 29.7 ppm. HRMS (MALDI, m/z): calcd for C42H62N4O2Ni (M þ H) 713.4299, found 713.4308. Compound 4 (R = Ph): yield 94%, mp >300 °C. Anal. Calcd for C46H54N4O2Ni: C, 73.31; H, 7.22; N, 7.43. Found: C, 73.21; H, 7.30; N, 7.40. 1H NMR (CDCl3): δ 8.00 (s, 4H, Ph-H), 7.26 (s, 2H, Ar-H), 7.20 (s, 6H, Ph-H), 6.88 (s, 2H, Ar-H), 6.81 (s, 2H, im-H), 6.71 (s, 2H, imH), 1.63 (s, 18H, C(CH3)3), 1.43 (s, 18H, C(CH3)3) ppm. HRMS (MALDI, m/z): calcd for C46H54N4O2 Ni (M þ H) 753.3673, found 753.3681. Compound 5 (R = Mes): yield 95%, mp >300 °C. Anal. Calcd for C52H66N4O2Ni: C, 74.55; H, 7.94; N, 6.69. Found: C, 74.28; H, 8.11; N, 6.53. 1H NMR (CDCl3): δ 7.25 (s, 1H, Ar-H), 7.24 (s, 1H, Ar-H), 6.91 (s, 1H, Ar-H), 6.90 (s, 1H, Ar-H), 6.76 (d, J = 2.23 Hz, 2H, im-H), 6.75 (d, J = 1.78 Hz, 2H, im-H), 6.65 (s, 4H, Ar-H), 3.53 (s, 6H, Ar-CH3), 2.26 (s, 6H, Ar-CH3), 1.80 (s, 6H, Ar-CH3), 1.56 (s, 18H, C(CH3)3), 1.44 (s, 18H, C(CH3)3) ppm. 13C NMR (100 MHz, CDCl3): δ 161.1, 156.0, 140.4, 137.0, 136.7, 134.9, 134.2, 131.7, 130.6, 129.5, 128.0, 123.6, 121.7, 118.3, 113.8, 36.0, 34.1, 32.0, 29.7, 21.1, 20.9, 20.3 ppm. HRMS (MALDI, m/z): calcd for C52H66N4O2 Ni (M þ H) 837.4612, found 837.4613. Crystallographic Studies. Single crystals suitable for X-ray diffraction were obtained from toluene for 2 and CH2Cl2/hexane for 3-5. Data collections were performed on a Rigaku Saturn 994 for 2 and a Rigaku Saturn 70 for 3-5. Semiempirical absorption corrections were applied for all complexes. The structures were solved by direct methods and refined by full-matrix least-squares. All 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. The molecular structure of 2 contains one toluene molecule of solvation, the molecular structure of 3 contains two CH2Cl2 molecules of solvation, and the molecular structure of 5 contains three CH2Cl2 molecules of solvation, respectively. Selected bond lengths and angles are listed in Table 1. Norbornene Polymerization. In a typical procedure, 1.00 g of norbornene in 13.1 mL of toluene and 0.25 mL of MAO (1.4 N) were added into a flask (100 mL) with stirring under an Ar atmosphere. After the mixture was kept at the desired temperature for 2 min, 0.20 μmol of the nickel complex in 2 mL of toluene was injected into the flask via syringe, and the reaction was started. Three minutes later, 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 the polymerization procedures, the total reaction volume was 15.5 mL, which can be achieved by variation of the added toluene when necessary.

’ ASSOCIATED CONTENT

bS

Supporting Information. CIF files giving X-ray structural information for 2-5, 1H and 13C NMR spectra of complexes 2-5, and IR, TGA spectra of the PNB. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel and fax: þ86-22-23504781. E-mail: bqwang@nankai. edu.cn. 1680

dx.doi.org/10.1021/om1011825 |Organometallics 2011, 30, 1677–1681

Organometallics

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dx.doi.org/10.1021/om1011825 |Organometallics 2011, 30, 1677–1681