Synthesis of 6,13-dihydro-6,13-ethanopentacene-15,16-diimine palladium(II) complex and steric effect of the backbone polycyclic aromatic system on palladium-catalyzed olefin polymerization

Abstract

The synthesis and characterization of a novel 6,13-dihydro-6,13-ethanopentacene-15,16-diimine palladium(II) complex are described. The effect of the backbone framework of the α-diimine metal catalyst on the polymerization reaction of norbornene and 1-octene is investigated. Compared to the 9,10-dihydro-9,10-ethanoanthracene-11,12-diimine palladium(II) complex (C1), the 6,13-dihydro-6,13-ethanopentacene-15,16-diimine palladium(II) complex (C2) with a polycyclic aromatic system on the backbone exhibits relatively high activities and leads to high molecular weights as well as narrow molecular weight distributions depending on the variation in feed ratios for copolymerization of norbornene and 1-octene. Moreover, the copolymers obtained using the 6,13-dihydro-6,13-ethanopentacene-15,16-diimine palladium(II) complex (C2) with large steric hindrance show high thermal stability and high glass transition temperatures.

Introduction

Since Brookhart and colleagues discovered aryl-substituted α-diimine nickel and palladium complexes for olefin polymerization,^1–3 late-transition-metal catalysts have attracted increasing attention due to their lower oxophilicity and resistance toward polar functionalities.^4–13A series of breakthroughs have been made in this field, mainly involving other late transition metal α-diimine catalysts.

The catalytic features of the late-transition-metal α-diimine catalysts rely on the electronic or steric influence of the ligand used.^14–18 However, the current reported catalysts have some defects, such as poor thermal stability and short catalytic lives, which limit their applications in industrial polymerization. As far as we know, bulky substituents can hinder the rotation of aniline moieties and protect the metal center effectively, which leads to the enhancement of the thermal stability of the catalyst. α-Diimine nickel catalysts with a bulky camphyl backbone were synthesized by Wu and colleagues,^19,20 which showed good thermal stability toward olefin polymerizations and enabled special branched polyolefins to be obtained. Sun and colleagues^21–24 have reported a series of Ni(II) complexes bearing unsymmetrical α-diimine ligands that exhibit high activity and thermal stability in olefin polymerization. Chen and colleagues ²⁵ showed that a naphthyl-α-diimine nickel complex bearing chiral bulky sec-phenethyl groups at the o-naphthyl position exhibited good catalytic activity and resulted in branched polymers (42–88/1000C) with high molecular weights (M_n: (4.3–15.2) × 10⁴ g·mol⁻¹) and narrow molecular weight distributions (MWDs; M_w/M_n = 1.13−1.29, room temperature (RT)). From the aforementioned results, the incorporation of bulky axial substituents in the α-diimine ligand is crucial for the enhancement of the thermal stability of the catalyst.

There are a large number of papers on α-diimine nickel and palladium complexes containing bulky substituents at the ortho-position of the aniline rings. However, very little work has been performed on studies of the backbone steric effects of α-diimine metal complexes for olefin polymerization. Recently, we successfully explored a type of thermostable and highly active 9,10-dihydro-9,10-ethanoanthracene-11,12-diimine Pd(II) catalysts with three-dimensional geometry and large steric hindrance on the backbone toward norbornene (NB) polymerization. ²⁶ In this context, we became very interested in the influence of the backbone steric effects of such α-diimine palladium complexes on olefin polymerization. Herein, we report the application of a 6,13-dihydro-6,13-ethanopentacene-15,16-diimine palladium(II) complex (C2) with larger steric hindrance as a result of the polycyclic aromatic system for the polymerization of the 1-octene and NB. Meanwhile, the catalytic activities of the 9,10-dihydro-9,10-ethanoanthracene-11,12-diimine palladium (C1) and 6,13-dihydro-6,13-ethanopentacene-15,16-diimine palladium(II) (C2) complexes with different polycyclic aromatics on the backbone will be compared with an emphasis on the steric effects of the backbones.

Results and discussion

Synthesis of the polycyclic aromatic palladium complexes

6,13-Dihydro-6,13-ethanopentacene-15,16-diimine formed the corresponding palladium complex (C2) in good yield (Scheme 1) on reaction with (COD)PdMeCl in dichloromethane. The structure of complex C2 was determined by elemental analysis and nuclear magnetic resonance (NMR) spectroscopy.

Scheme 1.

Synthesis of the polycyclic aromatic palladium complexes.

Copolymerization of NB with 1-octene

To investigate the influence of the backbone framework of the catalysts on the polymerization activity, the copolymerization of NB with 1-octene using different co-monomers and monomer feed ratios in the presence of complex C1 or C2/B(C₆F₅)₃ under the same conditions was studied. The results of the copolymerization of NB and 1-octene with the C1 or C2/B(C₆F₅)₃ catalytic systems and the yields and activities are summarized in Table 1. The activity of the C1/B(C₆F₅)₃ system in the polymerization of NB and 1-octene was somewhat lower in comparison with that of the C2/B(C₆F₅)₃ system. The reason for this difference in activity was ascribed to the rigid and bulky backbone of the 6,13-dihydro-6,13-ethanopentacene-15,16-diimine palladium complex.

Table 1.

Copolymerization of NB with 1-octene catalyzed by polycyclic aromatic palladium complexes/B(C₆F₅)₃ systems. ^a .

Cat.	NB/1-octene (mol/mol)	Yield (%)	Activity b	Mw c	Mw/Mn c	1-Octene (mol%) d	Tg (°C)	Td (°C)
C1	100/0	76.6	3.6 × 105	– e	– e	0	280.2	429.6
C1	90/10	70.8	3.4 × 105	3.1 × 105	1.6	8.9	258.6	408.7
C1	70/30	63.2	3.1 × 105	0.7 × 105	1.4	12.1	250.6	399.8
C1	50/50	50.6	2.6 × 105	0.3 × 105	1.5	15.6	243.5	391.2
C1	0/100	– e	– e	– e	– e	– e	– e	– e
C2	100/0	86.7	4.1 × 105	– e	– e	0	276.4	416.4
C2	90/10	81.4	3.9 × 105	3.4 × 105	1.4	9.1	254.2	399.5
C2	70/30	75.6	3.8 × 105	0.9 × 105	1.5	18.7	233.8	383.4
C2	50/50	63.5	3.3 × 105	0.4 × 105	1.4	19.5	230.4	379.8
C2	0/100	– e	– e	– e	– e	– e	– e	– e

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

a

Conditions: n[Cat.] = 1.0 × 10⁻⁵ mol, cocatalyst is B(C₆F₅)₃, B(C₆F₅)₃/complex/monomer (n/n/n) is 10/1/5000, polymerization temperature is 60 °C, the solvent is toluene, the total volume is 22 mL, and reaction time is 1 h.

b

In units of g_polymer/mol_Pd h.

c

Determined by GPC versus polystyrene standards in THF.

d

Determined by ¹H NMR spectroscopy in CDCl₃.

e

Not determined.

Besides the activity, the backbone framework of catalyst also affected the incorporation rate of the monomer as well as the yield. As shown in Table 1, the 1-octene content in the copolymers was 8.9%–15.6% for C1/B(C₆F₅)₃ and 9.1%–19.5% for C2/B(C₆F₅)₃ by varying the comonomer feed ratios from 10% to 50%. For both catalytic systems, the yield decreased on increasing the amount of 1-octene.

All the copolymers of NB and 1-octene were soluble in common organic solvents (such as CHCl₃, CH₂Cl₂, cyclohexane, and tetrahydrofuran (THF)) as well as chlorobenzene at ambient temperature, which exhibited better solubility than polynorbornene (PNB).

Characterization of the copolymers

The obtained copolymers were characterized by gel permeation chromatography (GPC) and the results are shown in Table 1. The molecular weights of the obtained copolymers were up to 10⁵, and the MWDs of the obtained polymers were all relatively narrow (MWD < 1.6). The molecular weights of the copolymers prepared using the C2/B(C₆F₅)₃ catalytic system were somewhat higher in comparison with those of the copolymers prepared using the C1/B(C₆F₅)₃ catalytic system.

¹H and ¹³C NMR spectra of the copolymers

The structures of the poly(NB-co-1-octene) polymers obtained using the C1 or C2/B(C₆F₅)₃ catalytic systems were also characterized by ¹H NMR spectroscopy (Figure 1). Four groups of signals appeared between 0.8 and 2.3 ppm in the ¹H NMR spectra of poly(NB-co-1-octene). The peak at 0.8–0.9 ppm could be assigned to the methyl hydrogens at C8′, and the signal at 0.9–1.5 ppm could be attributed to the methylene hydrogens at C4/5/7/1′/3′/4′/5′/6′/7′. The resonance at 1.5–2.1 ppm was assigned to the methine hydrogens at C3/6, and the signal at 2.1–2.3 ppm could be attributed to the methine hydrogens at C1/2/2′, respectively. Although the detailed assignments of the resonances are not clear at present, the shapes of the spectra are slightly different from each other; this implies that the microstructure of the copolymers is dependent on the complex used.

Figure 1.

The ¹H NMR spectra of the poly(NB-co-1-octene) polymers with: (a) 8.9%, (b) 12.1%, (c) 15.6%, (d) 9.1%, (e) 18.7%, and (f) 19.5% of 1-octene. Spectra (a)–(c) are obtained using C1/B(C₆F₅)₃ as the catalyst. Spectra (d)–(f) are obtained using C2/B(C₆F₅)₃ as the catalyst.

From the above, it can be seen that the 1-octene content in the copolymers obtained using C1/B(C₆F₅)₃ could be controlled up to 8.9%–15.6% by varying the co-monomer 1-octene feed ratio from 10% to 50%, while the 1-octene content in the copolymers obtained using C2/B(C₆F₅)₃ could be controlled up to 9.1%–19.5% by varying the co-monomer 1-octene feed ratio from 10% to 50%. ²⁷ The content of 1-octene in the copolymers obtained by using C2/B(C₆F₅)₃ was higher than that of the copolymers obtained from C1/B(C₆F₅)₃. The reason for this was also attributed to the rigid and bulky backbone of C2.

The ¹³C NMR spectra of poly(NB-co-1-octene) containing 19.5% of 1-octene obtained by using the C2/B(C₆F₅)₃ system are shown in Figure 2. The resonances of the methyl C8′ and methylene C7′ carbons of the hexyl branch and the methine carbons 1/2 and the 3/6 carbons of the 2,3-inserted NB unit are observed at 14, 23, 46–48, and 48–55 ppm together with those of the other carbons around 26.0–46.0 ppm, indicating the production of poly(NB-co-1-octene) with all the catalytic systems.

Figure 2.

The ¹³C NMR spectra of poly(NB-co-1-octene) obtained using C2/B(C₆F₅)₃ with 19.5% of 1-octene.

Fourier-transform infrared spectroscopy spectra of the copolymers

The Fourier-transform infrared spectroscopy (FTIR) spectra of poly(NB-co-1-octene) obtained by the C1 and C2/B(C₆F₅)₃ catalytic system are shown in Figure 3. The signal of the double bond at ca. 1710 cm⁻¹, assigned to the C=C bond of the ring-opening metathesis polymerization (ROMP) structure of PNB, was not observed. The signals at about 941 cm⁻¹ were attributed to the ring of the bicyclo[2.2.1]heptanes, further illustrating the monomers via vinyl-type addition polymerization. Meanwhile, there were no absorptions at about 3050 and 2100 cm⁻¹ due to the characteristic stretching of the C=C and C–H bonds of the 1-alkene, which indicated that 1-octene had been inserted into the chains of the copolymers and thus were true copolymers rather than blend polymers.

Figure 3.

The FTIR curves of poly(NB-co-1-octene)s with: (a) 8.9%, (b) 12.1%, (c) 15.6%, (d) 9.1%, (e) 18.7%, and (f) 19.5% of 1-octene. Spectra (a)–(c) are obtained using C1/B(C₆F₅)₃ as the catalyst. Spectra (d)–(f) are obtained using C2/B(C₆F₅)₃ as the catalyst.

Wide-angle X-ray diffraction analyses of the copolymers

Partial information on the conformation of the copolymers prepared using the C1 and C2/B(C₆F₅)₃ catalytic systems were obtained by means of wide-angle X-ray diffraction (WXRD), as shown in Figure 4. There was no trace of Bragg reflection in the characteristic crystalline regions, so the polymers were noncrystalline. The occurrence of two halos at 2θ values of 10.04°–10.32° and 18.08°–18.84° was characteristic for PNB. ²⁸ The packing density became smaller with increase in the 1-octene content in the copolymers, and the different catalytic systems resulted in little difference in the packing density of the copolymers.

Figure 4.

WXRD curves of poly(NB-co-1-octene)s with: (a) 8.9%, (b) 12.1%, (c) 15.6%, (d) 9.1%, (e) 18.7%, and (f) 19.5% of 1-octene. Spectra (a)–(c) are obtained using C1/B(C₆F₅)₃ as the catalyst. Spectra (d)–(f) are obtained using C2/B(C₆F₅)₃ as the catalyst.

TGA and DSC analyses of the copolymers

The decomposition temperatures (T_ds) of the copolymers with different 1-octene contents prepared using the C1 and C2/B(C₆F₅)₃ catalytic systems were investigated by TGA analyses. The results are shown in Table 1. The T_ds of the polymers were as high as 429.6 °C and decreased with increasing 1-octene content in the copolymers. The T_ds of the copolymers obtained with C2/B(C₆F₅)₃ were lower than those of the copolymers obtained using C1/B(C₆F₅)₃.

As shown in Table 1, the glass transition temperatures (T_gs) of the copolymers occur at high temperatures. The T_g values of the copolymers decreased as the 1-octene molar ratio increased in the copolymers, and the T_gs of the polymers obtained with the C2/B(C₆F₅)₃ system (T_gs = 230.4 °C–276.4 °C) were also lower than those of the polymers obtained with the C1/B(C₆F₅)₃ system (T_gs = 243.5 °C–280.2 °C).

Conclusion

A novel 6,13-dihydro-6,13-ethanopentacene-15,16-diimine palladium(II) complex (C2) with large steric hindrance due to the polycyclic aromatic system was synthesized and characterized. Compared to the 9,10-dihydro-9,10-ethanoanthracene-11,12-diimine palladium(II) complex (C1), the 6,13-dihydro-6,13-ethanopentacene-15,16-diimine palladium(II) complex (C2), with larger steric on the backbone, exhibited higher activity toward the polymerization of 1-octene and NB in the presence of B(C₆F₅)₃. Meanwhile, the backbone framework of the catalyst also affected the incorporation rate of the monomer and the yield. The copolymers obtained by using the 6,13-dihydro-6,13-ethanopentacene-15,16-diimine palladium(II) complex (C2) exhibited better properties. These findings identify a promising strategy to explore high reactivity catalysts by designing α-diimine metal catalytic systems with polycyclic aromatic backbones on the framework.

Experimental

Materials

All manipulations of air-sensitive and water-sensitive compounds 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, 97 %) was purchased from J&K (Tianjin, China). NB (98%) was purchased from Sigma-Aldrich (Shanghai, China) and purified by drying over sodium and distilling at 106 °C under N₂, and used as a 0.4 g/mL (4.25 mol/L) solution in toluene. 1-Octene was purchased from Sigma-Aldrich (Shanghai, China) and purified by washing twice with aqueous sodium hydroxide (5.0 wt%) and twice with water to remove inhibitors. This was followed by drying over anhydrous CaCl₂ and distillation over CaH₂ under an argon atmosphere at reduced pressure.

Characterization

The NMR spectra of the copolymers were recorded on a Bruker ARX 600 NMR spectrometer at ambient temperature, with CDCl₃ as the solvent. GPC was carried out on a Breeze Waters system using polystyrenes as the standard and THF as the eluent at a flow rate of 1.0 mL/min. The infrared (IR) spectra were recorded on a Shimadzu IR Prestige-21 FTIR spectrophotometer. Thermal gravimetric analysis (TGA) measurements were performed on a Perkin-Elmer TGA 7 instrument from RT to 600 °C at a rate of 20 °C/min under a nitrogen atmosphere. The differential scanning calorimetry (DSC) measurements were obtained on a Shimadzu DSC-60 with a heating/cooling rate of 10 °C/min under a nitrogen atmosphere. The WXRD curves were recorded on a Bruker D8 Focus X-ray diffractometer, operating at 40 kV and 40 mA with a copper target (λ = 1.54 Å) and at a scanning rate of 2°/min.

Synthesis of the 9,10-dihydro-9,10-ethanoanthracene-11,12-diimine methyl palladium chloride complex (C1): The 9,10-dihydro-9,10-ethanoanthracene-11,12-diimine methyl palladium chloride complex (C1) was synthesized according to the method reported in our previous work ²⁶ and fully characterized by NMR and elemental analysis. Anal. calcd for C₃₃H₃₁ClN₂Pd: C, 66.34; H, 5.23; N, 4.69. Found: C, 66.31; H, 5.25; N, 4.67. ¹H NMR (400 MHz, CDCl₃): δ = 0.43 (s, 3H), 2.05 (s, 6H), 2.07 (s, 6H), 4.80 (s, 1H), 4.81 (s, 1H), 7.16–77.2 (m, 14H). ¹³C NMR (100 MHz, CDCl₃): δ = 1.11, 17.90, 18.32, 50.31, 51.15, 125.77, 125.93, 126.68, 127.49, 128.15, 128.30, 128.60, 129.09, 129.22, 129.39, 136.49, 137.24, 142.43, 142.96, 167.19, 172.19.

Synthesis of the 6,13-dihydro-6,13-ethanopentacene-15,16-diimine methyl palladium chloride complex (C2): 6,13-Dihydro-6,13-ethanopentacene-15,16-diimine 0.2 mmol and (1,5-cyclooctadiene) methyl palladium(II) chloride ((COD)PdMeCl) 0.25 mmol were added to a Schlenk tube together with dried dichloromethane 20 mL. The reaction mixture was then stirred for 12 h at RT. The solution was filtered through Celite, and absolute hexane 10 mL was added. The resulting yellow crystal complex was crystallized in 78% yield from a mixture of dichloromethane and hexane. Complex C2 was characterized by elemental analysis and NMR spectroscopy. Anal. calcd for C₄₁H₃₅ClN₂Pd: C, 70.59; H, 5.06; N, 4.02. Found: C, 70.56; H, 5.08; N, 4.03. ¹H NMR (400 MHz, CDCl₃): δ = 0.44 (s, 3H), 2.04 (s, 6H), 2.06 (s, 6H), 4.80 (s, 1H), 4.81 (s, 1H), 7.05–77.2 (m, 10H), 7.46–757 (m, 8H). ¹³C NMR (100 MHz, CDCl₃): δ = 1.13, 17.92, 18.34, 50.35, 51.22, 125.61, 125.77, 125.93, 126.68, 127.49, 128.15, 128.30, 128.60, 128.92, 129.19, 129.32, 129.44, 135.94, 136.69, 137.24, 137.43, 140.02, 140.58, 167.41, 172.35.

Polymerization

All procedures were carried out under a nitrogen atmosphere. A typical copolymerization procedure was as follows: a toluene solution of NB and 1-octene was added via a syringe into a 100 mL two-necked round-bottom flask containing a magnetic stir bar. The toluene solution of palladium complex was then added followed by toluene solution of B(C₆F₅)₃. The total volume was kept constant at 22 mL. The polymerization reaction was performed for 1 h at 60 °C. The reaction was terminated by addition of acidic ethanol (90:10, ethanol/HCl). The resulting precipitated polymers were collected by filtration and washed with ethanol several times and then dried under vacuum at 60 °C.