Ni(II) and Pd(II) complexes bearing novel chiral bis(β-ketoamino) ligand and their catalytic activity toward copolymerization of norbornene and polar norbornene derivatives

Abstract

A series of novel Ni(II) and Pd(II) complexes bearing chiral bis(β-ketoamino) ligand were synthesized and characterized. The Ni(II) and Pd(II) complexes exhibited high activities for norbornene homopolymerization with only B(C₆F₅)₃ as cocatalyst, for Ni(II) complex even up to 1.38 ×10⁶ g_polymer/mol_Ni·h. All the complexes had good thermal stability. Most importantly, the complexes also displayed high activities of 10⁵ g_polymer/mol_metal·h toward the copolymerization of norbornene and 5-norbornene-2-yl acetate with high molecular weights and tunable norbornene and 5-norbornene-2-yl acetate content. The copolymerization activity, molecular weight, and norbornene and 5-norbornene-2-yl acetate feed ratio of the Ni complex were higher than those of the Pd complex.

Introduction

The addition of polynorbornene (PNB) with its derivatives causes a great deal of attention worldwide over the past decade, because of its excellent dielectrics, optical transparency, and high thermal stability.^1–3 The PNB is synthesized through the vinyl-addition polymerization of norbornene (NB). Since the vinyl-addition polymerization of NB was first catalyzed by modified Ziegler catalysts in 1963, ⁴ the vinyl-addition polymerization of NB has already been reported using many transition metal precursors, including titanium,^5,6 zirconium,^7,8 iron, ⁹ nickel,^10–13 palladium,^14–17 cobalt, ¹⁸ and chromium. ¹⁹ Due to the lower oxophilicity and resistance toward deactivation by polar functionalities, the late transition metal catalysts Ni(II) and Pd(II) complexes have received renewed interest.^20–25 The catalytic features of catalysts Ni(II) and Pd(II) complexes rely on the electronic or steric influence of the ligand used, so substantial work has been conducted on the substituent variation of ligands set for new model precatalysts.

Among various ligands nickel and palladium complexes, the utilization of bis(β-ketoamino) complexes of nickel(II) or palladium(II) for NB copolymerization has attracted much interest in both academic and industrial fields over the past dozen years.^26,27 The extensive investigations have favored the nickel derivatives, due to their good catalytic performances and illustrated reaction mechanism. It is generally known that the catalytic features rely on the electronic and steric influence of ligand use. Some efforts have been made to exploit new bis(β-ketoamino) ligands metal catalyst and the variations of bis(β-ketoamino) ligands metal complexes have been studied in the latest years.

It is well known that substituted bulky ligands can enhance the metal complexes’ stability by hindering the rotation, which leads to the increase of catalytic properties.^28,29 Meanwhile, the chiral ligands can promote the rate of chain propagation.^30,31 Yuan et al. ³⁰ reported a series of nickel complexes with chiral naphthylamine. The chiral nickel complexes exhibited high activity toward ethylene polymerization.

To sum up, bulky chiral bis(β-ketoamino) ligands are selected. The syntheses and characterizations of Ni(II) and Pd(II) complexes bearing novel chiral bis(β-ketoamino) ligand are reported along with a detailed investigation of their catalytic performance in NB polymerization.

Results and discussion

Synthesis of the ligand and complexes

The stoichiometric condensation reaction of 2-acetyl-1-tetralone and 2-(1-phenylethyl)-4-methylaniline produced a ligand. Crystal of the ligand (CCDC: 2170956) suitable for single crystal X-ray diffraction analysis was obtained by slow evaporation from toluene solution. The molecular structure was shown in Figure 1, and the crystallographic data were summarized in Table 1.

Table 1.

Crystallographic data for the ligand.

Empirical formula	C27 H27 N O	Absorption coefficient (mm)-1	0.070
Formula weight	381.49	F (000)	1632.0
Crystal color	yellow	Crystal size (mm)	0.24 × 0.22 × 0.19
Temperature (K)	298 K	θ range for data collection (deg)	3.27–25.096
Wavelength (Å)	0.71,073	Limiting indices	–27 ⩽ h ⩽ 27, –9 ⩽ k ⩽ 9, –27 ⩽ l ⩽ 27
Crystal system, space group	Monoclinic C 2/c
Unit cell dimensions		Max. and min. transmission	0.789 and 1.000
a (Å)	23.1866 (17)	Refinement method	Full-matrix least-squares on F 2
b (Å)	8.2605 (6)
c (Å)	22.9788 (18)	Data/restraints/parameters	3853/0/265
α (deg)	90
β (deg)	100.248 (7)	Goodness-of-fit on S (F2)a	1.096
γ (deg)	90	Final R indices [I>2ó(I)]	R1 = 0.0923, wR2 = 0.2073
Volume (Å)3	4331.0 (6)
Z, Calculated density (Mg m)−3	8, 1.170	R indices (all data)	R1 = 0.0728, wR2 = 0.1962

Figure 1.

ORTEP plots of {C₁₀H₈(O)C[HN(Ar)]CH₃}. (Ar=N-2-(1-phenylethyl)-4-methylaniline) showing the atom-labeling scheme. Hydrogen atoms are omitted for clarity.

The novel synthesized ligand formed nickel(II) and palladium(II) complexes in good yields (Scheme 1) on reaction with [Et₄N]₂MtBr₄ (Mt = Ni, Pd) in n-heptane/toluene. Due to low solubility in solution, the crystal structure of metal complexes could not be obtained, and its structure was determined by elemental analysis, Nuclear Magnetic Resonance (NMR) spectroscopy, and high-resolution mass spectrometry.

Scheme 1.

Synthesis route of the ligand and complexes.

Homopolymerization of NB

To investigate the NB polymerization activities of the bis(β-ketoamino) nickel(II) and palladium(II) complexes, two complexes were tested as catalysts with B(C₆F₅)₃ as cocatalyst in the toluene as the reaction solvent. It is generally known that the catalytic properties of the catalyst were dependent on the reaction conditions. To explore the most suitable NB polymerization condition, the initial investigations of NB homopolymerization catalyzed by bis(β-ketoamino) nickel(II) complex/B(C₆F₅)₃ were conducted with various conditions, such as different temperatures, NB/Ni ratios, and B/Ni ratios. The data of NB polymerization activities of the bis(β-ketoamino) nickel(II) complex were shown in Table 2. The Ni complex exhibited low activity with the NB/Ni ratio of 2000 at 60 °C (entry 1, Table 2). As the NB/Ni ratio increased from 2000 to 5000, the activity of Ni complex increased significantly from 0.25 to 1.38 × 10⁶ g_polymer/mol_Ni·h (entry 2, Table 2) and kept the same value with further increase of the NB/Ni ratio (entries 3 and 4, Table 2). The reason was probably that the possibility of excitation and encounter between NB is great with high NB concentration, which will accelerate the reaction, but the excessive NB/Ni ratio can lead to the uncontrolled polymerization reaction. Herein, the NB/Ni ratio should be within a suitable range. In a similar vein, the B/Ni ratio can also affect the catalytic activity. In the absence of B(C₆F₅)₃, the Ni complex had no activity toward NB homopolymerization (entry 5, Table 2). The B(C₆F₅)₃ is a very strong Lewis acid and as such is effective in a number of chemical transformations. B(C₆F₅)₃ could realize C₆F₅ transfer from B(C₆F₅)₃ to nickel in NB polymerization.^32,33 The increase of the B/Ni ratio enhanced the activity of the Ni complex. When the B/Ni ratio reached the value of 20, the catalytic activity of Ni complex remained stable (entries 6, 7, and 8, Table 2). The effect of polymerization temperature on the activity was investigated (entries 9, 10, 11, and 12, Table 2). The highest activity was observed at 60 °C and the catalyst system still kept high activity even at 120 °C. Therefore, the condition at 60 °C with the NB/Ni ratio at 5000/1 and B/Ni ratio at 20/1 would be the most effective condition.

Table 2.

Norbornene polymerization on bis(β-ketoamino) nickel, palladium complexes ^a .

No.	Cat.	NB/Ni (mol/mol)	B/Ni (mol/mol)	T, °C	PNB Yield,%	Activity b
1	Ni	2000	20	60	43.62	0.25
2	Ni	5000	20	60	97.68	1.38
3	Ni	10,000	20	60	46.43	1.30
4	Ni	15,000	20	60	23.54	1.00
5	Ni	5000	0	60	– c	– c
6	Ni	5000	10	60	43.85	0.62
7	Ni	5000	40	60	85.43	1.20
8	Ni	5000	60	60	73.87	1.04
9	Ni	5000	20	0	5.56	0.08
10	Ni	5000	20	30	47.78	0.67
11	Ni	5000	20	90	94.56	1.33
12	Ni	5000	20	120	87.72	1.24
13	Pd	5000	20	60	80.62	1.13
14	Pd	5000	20	120	65.81	0.93

PNB: polynorbornene.

a

Reaction conditions: toluene, 20 mL; Ni complex, 5 × 10⁻⁶ mol; reaction time, 20 min.

b

In units of 10⁶ g_polymer/mol_metal·h.

c

Not determined.

To verify the activity and stability of the Pd complex, the catalytic behavior of the Pd complex for the NB polymerization was studied at the above optimality conditions. The polymerization activity of Pd complex was slightly lower than the Ni complex (entry 13, Table 2). Similarly, the Pd complex also exhibited good thermal stability that the activity at 120 °C could reach 0.93 × 10⁶ g_polymer/mol_Pd·h (entry 14, Table 2).

Copolymerization of NB and NB-OCOCH₃

To verify whether the Ni and Pd complexes are active for copolymerization of NB and polar NB derivatives, a series of copolymerizations of NB with 5-norbornene-2-yl acetate (NB-OCOCH₃) were investigated with the catalyst system of Ni/B(C₆F₅)₃ and Pd/B(C₆F₅)₃ in toluene at 60 °C under nitrogen atmosphere (Table 3). Both of the catalytic systems exhibited high activity toward copolymerization. However, polar NB derivatives NB-OCOCH₃ could not be homopolymerized under the same conditions. The increase of NB-OCOCH₃ concentration in the feed led to the decrease of the activity and molecular weight of copolymer obtained in each catalytic system. Because the oxygen atom of NB-OCOCH₃ was competed with the double bond for the coordination, high concentration of oxygen atom would impede double-bond coordination.^34,35 The copolymerization activity, molecular weight, and NB-OCOCH₃ feed ratio of Ni complex were higher than those of Pd complex. Compared to achiral ligands, ³⁶ the complexes bearing bulky chiral ligand, displayed the higher catalytic activities, molecular weights, and NB-OCOCH₃ feed ratio.

Table 3.

Copolymerization of NB and NB-OCOCH₃ catalyzed by Ni or Pd complex combined with B(C₆F₅)_3. ^a .

No.	Cat.	NB/NB-OCOCH3 (mol/mol)	Reaction time (min)	Yield (%)	Activity b	Mw (g/mol) c	Mw/Mn c	NB-OCOCH3 (mol %) d	Td (°C)	Tg (°C)
1	Ni	100/0	20	97.68	1.38	– e	– e	– e	412.3	301.8
2	Ni	70/30	60	45.14	0.25	1.8 × 105	1.5	10.54	320.4	264.9
3	Ni	50/50	60	25.81	0.16	1.3 × 105	1.2	14.86	318.6	256.4
4	Ni	0/100	60	Trace	– e	– e	– e	– e	– e	– e
5	Pd	100/0	20	80.62	1.13	– e	– e	– e	419.5	304.8
6	Pd	70/30	60	36.83	0.21	1.4 × 105	1.6	10.14	326.7	272.5
7	P d	50/50	60	21.92	0.13	1.2 × 104	1.3	13.34	319.5	261.8
8	Pd	0/100	60	Trace	– e	– e	– e	– e	– e	– e

NB: norbornene; GPC: gel permeation chromatography; NMR: nuclear magnetic resonance; THF: tetrahydrofuran.

a

Conditions: c[Cat.] = 5.0 × 10⁻⁶ mol; cocatalyst is B(C₆F₅)₃, B/complex/monomer (n/n/n) is 20/1/5000, polymerization temperature is 60 °C, the solvent is toluene, the total volume is 22 mL.

b

In units of 10⁶ g_polymer/mol_metal·h.

c

Determined by GPC vs polystyrene standards in THF.

d

Determined by 1H NMR spectroscopy in CDCl₃.

e

Not determined.

The microstructure of the obtained copolymers was characterized by 1H NMR analyses, and the typical spectrum was illustrated in Figure 2. The obtained polymers are random copolymers. The absence of the resonance of the proton hydrogen connected to the double bond at 5.3–5.9 ppm proved that the polymers were vinyl-addition type. The signals in the range of 0.9–1.3 ppm were assigned to the hydrogen corresponding to 5′/6′ /6; the signals in the range of 1.5–1.7 ppm were assigned to the hydrogen corresponding to 7′ /7; the signals in the range of 1.9–2.2 ppm were assigned to the hydrogen corresponding to 1′/2′ /3′/4′/1/2/3/4/8; and the peaks at 5.0 ppm assigned to the hydrogen corresponding to 5. The conomer contents in the copolymers were then calculated from the intensities from 1H NMR according to the literature. ³⁷ The content of NB-OCOCH₃ in copolymers was calculated from 0.00% to 14.86% by varying the comonomer feed ratios (NB-OCOCH₃/NB) from 0% to 50%.

Figure 2.

The 1H NMR spectra of NB/NB-OCOCH₃ copolymer obtained by Ni/B(C₆F₅)₃ with 10.54% of NB-OCOCH₃ molar ratio.

The decomposition temperatures (T_d)s and glass transition temperatures (T_g)s of the copolymers were analyzed by TGA and DSC, respectively, and also summarized in Table 3. All the copolymers showed single distinct (T_d) and (T_g) values, and the values decreased by the increase of NB-OCOCH₃ content, indicating the formation of the uniform copolymer of NB with NB-OCOCH₃ in each copolymerization.

Experimental

Materials

All reactions were performed under an inert atmosphere. Solvents were purified using standard procedures. Toluene was dried over sodium/benzophenone and distilled under nitrogen before use. Tris(pentafluorophenyl)borane (B(C₆F₅)₃, 95%) and NB (NB, 98%) were all purchased from Aldrich Chemical company, and was purified by drying over sodium and distilling at 106 °C under N₂ atmospheres, used as a solution 0.4 g/mL (4.25 mol/L) in toluene. Nickel(II) bromide (Acros) and palladium(II) bromide (Acros) were used without further purification.

General

Elemental analyses (EAs) were characterized by means of elemental analysis with Vario Elementar III. The 1H and 13C NMR spectra of the ligand, and Mt(bchkni)₂ complexes, and copolymers were obtained on a Bruker ARX 600 NMR spectrometer at room temperature with CDCl₃ as solvent and tetramethylsilane (TMS, δ = 0) as an internal reference. Mass spectra were recorded by ESI methods. HRMS (ESI) was measured on a Bruker Daltonics APEXIII 7.0 TESLA FTMS. The gel permeation chromatography (GPC) was conducted with a Breeze Waters system using polystyrenes as the standard and tetrahydrofuran as the eluent at a flow rate of 1.0 mL/min. Thermogravimetric analysis (TGA) was performed on a TA Q600 SDT for thermogravimetry at a heating rate of 10 °C/min under nitrogen with a sample size of 8–10 mg. The differential scanning calorimetry (DSC) measurements were obtained on a Shimadzu DSC-60 with a heating/cooling rate of 10 °C/min under nitrogen atmosphere.

Ligand and complexes syntheses

The Ni(II) and Pd(II) complexes bearing novel chiral bis(β-ketoamino) ligand were synthesized according to the method reported. ²⁹ The synthetic route was shown in Scheme 1.

Synthesis of ligand {C₁₀H₈(O)C[HN(Ar)]CH₃} (Ar = N-2-(1-phenylethyl)-4-methylaniline)

According to the literature, 2-acetyl-1-tetralone (4 g, 0.0213 mol), 2-(1-phenylethyl)-4-methylaniline (4.5 g, 0.0213 mol), and a catalytic amount of p-toluene-sulfonic acid in toluene (23 mL) were combined and heated to reflux for 3 h, while H₂O removed as a toluene azeotrope at 125–130 °C using a water separator. The resulting solution was evaporated under vacuum to remove the residual toluene and recrystallization from hexane yielded benzocyclohexan-ketonaphthylimine in yellow crystals. Yield: 76 % (m.p.: 171–173 ^oC). C₂₇H₂₇NO elemental analysis (%), found: C, 85.02; H, 7.10; N, 3.70; calcd: C, 85.00; H, 7.13; N, 3.67. 1H NMR (CDCl₃, δ, ppm): δ 14.01 (s, 1H), 8.10–8.04 (m, 1H), 7.39–7.31 (m, 2H), 7.24 (s, 1H), 7.19 (d, J = 7.2 Hz, 1H), 7.17–7.12 (m, 2H), 7.12–7.07 (m, 3H), 7.03 (d, J = 8.0 Hz, 1H), 6.91 (d, J = 8.0 Hz, 1H), 4.44 (q, J = 7.2 Hz, 1H), 2.84 (t, J = 6.8 Hz, 2H), 2.56–2.42 (m, 2H), 2.39 (s, 3H), 1.60 (d, J = 7.2 Hz, 3H), 1.38 (s, 3H). 13C NMR (CDCl₃, δ, ppm): 184.1, 162.5, 145.5, 142.3, 141.2, 136.7, 136.0, 135.0, 130.7, 128.1, 128.0, 127.9, 127.3, 127.1, 126.6, 126.4, 125.7, 101.5, 40.3, 29.2, 24.6, 21.5, 21.4, 15.7. HRMS (ESI): m/z [M+H]⁺ calcd for C₂₇H₂₇NO: 381.21; found: 381.11.

Synthesis of {C₁₀H₈(O)C[N(Ar)CH₃]}₂Ni

A 0.273 g (0.007 mol) sample of potassium was added to 25 mL of dried ^tBuOH. After the potassium had dissolved completely, the solution was heated to 50 °C and 2.654 g (0.007 mol) of ligand added. The solution changed to yellow-orange as ligand completely reacted with ^tBuOK and was kept stirring for 30 min. The solution was cooled slowly to room temperature, and 2.236 g (0.0035 mol) of [Et₄N]₂NiBr₄ was introduced in; the reacting mixture immediately formed a gray-green precipitate. After it was stirred vigorously at room temperature for several hours, the residual ^tBuOH was removed by evaporating in vacuum. The residue slurry was then extracted successively with enough hot n-heptane/toluene, the filtrate was collected by fast hot filtering and the resulting hot filtrate was induced to crystallize by cooling slowly overnight. Then, the product was isolated by filtration and drying under reduced pressure. One or two further recrystallizations from n-heptane/toluene mixture solution resulted in dark green block crystals. Yield: 47.1% (m.p.: 203–205 ^oC). C₅₄H₅₂N₂O₂Ni elemental analysis (%), found: C, 79.19; H, 6.36; N, 3.39. calcd. C, 79.12; H, 6.39; N, 3.42.1H NMR (CDCl₃, δ, ppm): 7.62 (d, J = 6.8 Hz, 1H), 7.50 (s, 1H), 7.26 (d, J = 6.2 Hz, 2H), 7.121–7.17 (m, 2H), 7.13-7.08 (m, 2H), 6.97–6.90 (m, 6H), 6.89–6.81 (m, 6H), 6.50 (d, J = 7.6 Hz, 1H), 6.46 (d, J = 7.2 Hz, 1H), 5.92 (d, J = 7.2 Hz, 1H), 5.79 (d, J = 7.2 Hz, 1H), 2.65–2.55 (m, 4H), 2.47 (s, 3H), 2.38 (s, 3H), 2.24–2.10 (m, 3H), 2.02 (d, J = 7.2 Hz, 3H), 1.97–1.86 (m, 2H), 1.67–1.59 (m, 1H), 1.56 (d, J = 7.2 Hz, 3H), 0.68 (s, 3H), 0.61 (s, 3H). 13C NMR (CDCl₃, δ, ppm): 166.9, 166.8, 163.9, 163.8, 146.3, 146.0, 145.6, 145.6, 139.0, 138.6, 137.8, 137.7, 133.9, 133.9, 128.3, 128.2, 127.9, 127.9, 127.8, 127.6, 127.2, 127.1, 126.9, 126.3, 126.2, 125.5, 125.4, 125.4, 125.3, 125.2, 125.2, 106.8, 106.8, 41.9, 41.1, 29.0, 28.9, 25.7, 22.8, 22.4, 21.7, 21.6, 20.8, 20.7. HRMS (ESI): m/z [M+H]⁺calcd for C₅₄H₅₂N₂O₂Ni [M]⁺ 818.34; found 818.36.

Synthesis of {C₁₀H₈(O)C[N(Ar)CH₃]}₂Pd

In a similar manner to the above nickel complex, the palladium complex was prepared as a red/brown solid. Yield: 51% (m.p.: 235–237 °C). C₅₄H₅₂N₂O₂Pd elemental analysis (%), found: C, 74.76; H, 6.07; N, 3.25; calcd. C, 74.77; H, 6.04; N, 3.23. 1H NMR (CDCl₃, δ, ppm): 7.56–7.46 (m, 2H), 7.27–7.24 (m, 1H), 7.19–7.15 (m, 5H), 7.13–7.10 (m, 2H), 7.01–6.98 (m, 4H), 6.97–6.94 (m, 6H), 6.32–6.15 (m, 4H), 2.65–2.60 (m, 4H), 2.56 (s, 3H), 2.50 (s, 3H), 2.36–2.31 (m, 1H), 2.24–2.17 (m, 2H), 2.13–2.03 (m, 2H), 1.1.82–1.76 (m, 4H), 1.41–1.35 (m, 3H), 0.82 (s, 3H), 0.80 (s, 3H). 13C NMR (CDCl₃, δ, ppm): 165.3, 165.1, 146.3, 145.9, 145.2, 139.3, 139.0, 138.6, 138.4, 134.8, 134.8, 134.6, 134.5, 128.5, 128.4, 128.3, 128.3, 128.2, 128.1, 128.1, 128.0, 127.9, 127.6, 127.4, 127.2, 127.1 126.9, 126.9, 126.8, 126.4, 125.6, 125.5, 125.5, 125.3, 125.2, 104.8, 104.8, 41.1, 40.5, 29.2, 29.1, 26.6, 26.5, 22.4, 22.1, 21.7, 21.7, 21.0. HRMS (ESI): m/z [M+H]⁺calcd for C₅₄H₅₂N₂O₂Pd [M]⁺ 866.31; found 866.04.

Polymerization

According to the typical procedure, the appropriate B(C₆F₅)₃ solid, a certain amount of a toluene solution of NB and NB-OCOCH₃, and quantitative metal complex solution were introduced into the round-bottom glass flask in order, and the reaction mixture was continuously stirred for a designed period at the polymerization temperature. Polymerizations were terminated by the addition of the acidified ethanol (v/v, 10/1, ethanol/HCl) and stayed overnight. The polymers were then obtained through filtration and washed with ethanol several times and then dried at 40 °C to a constant weight. This fact also confirms the copolymerization procedure, as shown in Scheme 2.

Scheme 2.

Copolymerization of NB and NB-OCOCH₃ catalyzed by Ni and Pd complexes.

Conclusion

The synthesis of a series of novel Ni(II) and Pd(II) complexes bearing chiral bis(β-ketoamino) ligand and their application in the polymerization of NB 5-norbornene-2-yl acetate(NB-OCOCH₃) were investigated. The Ni(II) and Pd(II) complexes with the B(C₆F₅)₃ as cocatalyst exhibit high activities up to 1.38 × 10⁶ g_polymer/mol_Ni·h for the NB polymerization to produce vinyl-addition-type polynorbornene. And all the catalysts showed good thermal stability. Toward the copolymerization of NB and NB-OCOCH₃, the complexes also displayed high activities. Although the addition of NB-OCOCH₃ could decrease the activity, the catalysts conducted copolymerization with activity of 10⁵ g_polymer/mol_metal·h and afforded high-molecular-weight copolymers with high incorporation of polar monomer.

Supplemental Material

sj-pdf-1-chl-10.1177_17475198231187525 – Supplemental material for Ni(II) and Pd(II) complexes bearing novel chiral bis(β-ketoamino) ligand and their catalytic activity toward copolymerization of norbornene and polar norbornene derivatives

Supplemental material, sj-pdf-1-chl-10.1177_17475198231187525 for Ni(II) and Pd(II) complexes bearing novel chiral bis(β-ketoamino) ligand and their catalytic activity toward copolymerization of norbornene and polar norbornene derivatives by Wanyun Liu, Ping Huo, Lin Yuan, Bowen Zhao and Junying Ge in Journal of Chemical Research